A review of binders in iron ore pelletization.pdf

Mineral Processing & Extractive Metall. Rev., 24: 1 90, 2003 Copyright # Taylor & Francis Inc. ISSN: 0882-7508 print DOI: 10.1080/08827500390198190 A REVIEW OF BINDERS IN IRON ORE PELLETIZATION T. C. EISELE AND S. K. KAWATRA Department of Chemical Engineering, MichiganTechnological University, Houghton, Michigan, USA The majority of iron ores must be ground to a fine particle size to allow the iron oxides they contain to be concentrated, and the concentrate must then be agglomerated back into large enough particles that they can be processed in blast furnaces. The most common agglomeration technique is pelletization, which requires the use of binders to hold the iron oxide grains together so that the agglomerates can be sintered into high-strength pellets. Although bentonite clay is the most commonly used binder, there are many other possibilities that could be competitive in a number of situations. This article reviews the We would like to thank the following sponsors for providing the generous financial support for this work: EVTAC Mines, particularly Bob Anderson; Hibbing Taconite, especially Steven G. Rogers; ISPAT Inland Mining; LTV Steel Mining; National Steel Pellet, especially Jim Wennen, Sarah Blust, and Dennis Murr; Northshore Mining; Svedala; Minnesota Department of Natural Resources, particularly Peter Clevenstine; USX-MN Ore Operations, especially Bob Strukel; and Cleveland Cliffs Iron Co, particularly Paul Rosten, Dick Kiesel, Bob Thiebault, and Ted Seppanen. We would also like to thank Chris Glenn and Franz Reisch of American Colloids, John Engesser of the Coleraine Minerals Research Laboratory, and Dr. Ron Weigel for their invaluable technical advice. Thanks are also due to S. Jayson Ripke of Northshore Mine for his critical analysis, Henry Walqui and Basak Anameric of Michigan Tech, and the following undergraduate students: Katy Marten, Kari Buckmaster, Karla Andrade, Gabriella Ramirez, Toby Lee, Frank Perras, Elise Anderson, and Jamie Krull. Address correspondence to S. K. Kawatra, Dept. of Chemical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931-1295 USA. E-mail: [email protected] 1 2 T. C. EISELE AND S. K. KAWATRA numerous types of binders (both organic and inorganic) that have been considered for iron ore pelletization, including discussion of the binding mechanisms, advantages and limitations of each type, and presentation of actual pelletization results, so that the performance of the various types of binders can be compared and evaluated. Typical iron ores contain a great deal of gangue minerals, particularly silicates, and the iron oxides must be concentrated from these ores before they can be used by the steel industry. In the process of concentrating the iron oxides, the ore is ground into a fine particle size that is not suitable for use in ironmaking, and thus the ore must be agglomerated into larger particles before it is used. The most common agglomeration technique is pelletization, which requires that a small amount of binder be added to the powdered ore to control balling rates and hold the pellets together until they are hardened by sintering. A variety of binders are possible, with the most commonly used being bentonite clay; however, the bentonite contributes silica and other undesirable elements to the ore, and so there is considerable interest in developing binders that have the good qualities of bentonite at a comparable or lower cost without contributing any harmful contaminants to the ore. This review covers the various types of binders that have been investigated for use in iron ore agglomeration. The binding mechanisms, chemistry, pelletization results, and advantages=drawbacks of each are discussed. AGGLOMERATION TECHNIQUES The feed to a blast furnace should form a permeable bed of material, permitting gas to flow through it uniformly at a high rate. Powdered iron ore concentrates are not suitable in their as-produced form, both because fines tend to pack into a nonpermeable bed and because the fine particles are likely to be carried away as dust by the high gas flowrates. The powdered ore must therefore be agglomerated into larger particles that will improve permeability of the furnace burden, increase the rate of reduction, and reduce the amount of material blown out of the furnace as dust. There are four basic methods that have been developed for agglomerating iron ores: sintering, nodulizing, briquetting, and pelletizing. These processes are briefly described below (AISE 1985). A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 3 Sintering The sintering process consists of combining iron-bearing fines with a solid fuel and igniting the mixture on a traveling grate with a downdraft of air. As the fuel burns, the temperature in the bed increases to about 1300 C to 1480 C, sintering the fines into a porous, clinker-like material that is suitable for use as blast furnace feed. The bonding between the particles is by recrystallization and partial melting, and so no additional binder needs to be added in this process. Sinter performs well in the blast furnace, particularly if it is made with fluxes added before sintering and sized to 25 mm  6 mm before charging to the furnace. Because the sinter product is subject to breakage and abrasion during handling, this process is mostly used for processing ores from mines that are very close to the blast furnace operation and for recycling ironbearing fines, such as furnace dusts and mill scale. Nodulizing Like sintering, nodulizing does not require the addition of binders. The process works by charging iron-bearing fines to a rotary kiln and heating to the point of incipient melting. As the charge is tumbled in the kiln, it forms into nodules that are bonded together by the liquified portion of the partially melted fines. The process does have a few advantages, such as insensitivity to feed moisture and particle size and high strength of the nodules; however, its disadvantages of high fuel consumption, operating and control difficulties, nonuniform nodule size, and poor nodule reducibility in the blast furnace have tended to make this process uncompetitive, and it is no longer in general use. Briquetting Briquetting consists of compressing fines into lumps of regular shape using rolls, punches, extruders, and similar devices. Although it is used routinely for many materials, briquetting of unheated iron ores has not been successful because the available binders do not develop sufficient strength. Briquetting is used for some direct reduction processes because the metal produced is ductile enough to bond together by mechanical deformation without the need for binder. usually by heating. Pelletizing Pelletizing differs from the other agglomeration techniques listed in that the powdered ore is first formed into a ‘‘green’’ pellet or ball. The selection of a proper binder type and dosage is of critical importance in producing good quality pellets at a reasonable price. C. pelletization is the one that is both widely used in the iron ore industry and requires a binder to be added to the ore in order to work properly. FUNCTIONS OF A BINDER Out of the four basic agglomeration techniques. Pelletizing has the following requirements:  The ore being pelletized must have a sufficiently fine particle size distribution.’’  A binder is necessary to hold the particle grains together after the pellet is dried and before it is finally hardened. This temperature is lower than the melting point of iron oxides. This review will therefore concentrate on the suitability of various binders for pelletization. and finally heated to approximately 1300 C to harden them. The pelletization process is the primary consumer of binders in the iron ore industry. The pelletization process is very widely used. . and the pellets harden by recrystallization across the particle grain boundaries. K. which is then dried and hardened in a separate step. Green pellets are made by combining moist ore with a binder and rolling it into balls using either a pelletizing disc or a pelletizing drum. with good bed permeability and reducibility. The pellets are then dried. The pellets also perform very well in the blast furnace. particularly when the ore must be shipped great distances between the mine and the blast furnace. because the fired pellets are durable and easy to handle.  Sufficient moisture is needed to make the ore sticky enough to pelletize but not so much moisture that the ore becomes ‘‘muddy.4 T. preheated. KAWATRA This process is generally more expensive than other agglomerating processes due to wear of the briquetting surfaces and the energy required to compress the briquettes. EISELE AND S. so that it will nucleate seeds that grow at a controlled rate into well-formed pellets. Inactive Film: The binder forms a sticky layer on the particles which bind them together. The binding is typically reversible. with one of the most useful being the division of binders into the following five groups (Holley 1982): 1. this capability of bentonite gives a relatively inexpensive method for making small adjustments of feed moisture content after filtration. The binding is typically irreversible. they cannot all be used in all possible applications. 2. Several different classifications have been proposed. . CLASSIFICATIONS OF BINDING SYSTEMS Binders are in general anything that can be used to cause particles to adhere together into a mass. Because not all ore concentrates will filter to the same moisture content. which then undergoes a chemical reaction and hardens. The suitability of a binder is determined by how well it can carry out each of these functions while at the same time not causing contamination or sintering problems. The film can bind by capillary forces or through adhesional or cohesional forces.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 5 Binders accomplish two very important functions in iron ore pelletization:  The binder makes the moist ore plastic. It is therefore useful to categorize binders in some systematic way. This is a valuable feature because pelletization works over a fairly narrow range of feed moisture contents.  During drying. the binder holds the particles in the agglomerates together while the water is removed and continues to bind them together until the pellet is heated sufficiently to sinter the grains together. Chemical Film: The binder forms a film on the particle surface. Since binders can accomplish this in a number of different ways. This makes it possible to control the free moisture content of the pelletization feed by simply adjusting the bentonite addition rate. An additional feature of bentonite binder that is helpful in pelletization is its ability to absorb several times its own weight in water. e d ð1Þ . This is specific to particular types of material. PELLET QUALITY MODELS A number of pellet quality models have been developed for predicting various properties of pellets. 5. 4. depending on its dosage and the details of its application.6 T. pitch. and the bond strength of the binder. examples of each of these types of binder are shown in Table 1. Inactive Matrix: The binder forms a more-or-less continuous matrix that particles are imbedded in. It should be noted that sometimes a single binder can be classified in different ways. KAWATRA 3. Binding may be reversible upon heating. C. and binders of this type have not been developed for iron ores. resulting in a very strong bond. Wet tensile strength is related to the wet crushing strength of a pellet. Rumpf’s formula for the tensile strength of moisture-filled agglomerates (wet balls) is as follows (Rumpf 1962): st ¼ C   1
e g cos y. These models assume that the pellet properties are dependent only on the physical characteristics of the particles being pelletized. EISELE AND S. Chemical Matrix: The binder forms an approximately continuous matrix. K. Iron ore pelletization currently uses ‘‘inactive film’’ binders because they are generally effective at low dosages. which undergoes a chemical reaction that causes it to harden. Binding by this mechanism usually is irreversible. or wax that is heated or emulsified to make it fluid and then hardens upon cooling or drying. Often the binder is a material such as a tar. and typically do not require large compaction pressures in order to work. the viscosity and surface tension of the fluid phase. Binders of this type often require high compaction forces and high binder dosages. bind the particles rapidly without the need to wait for a chemical reaction to be completed. Chemical Reaction: The binder actually undergoes a chemical reaction with the material that it is binding. Magnesium chloride and water (magnesium hydrolizes to oxide) Chemical matrix Quicklime þ water Hydrated lime þ CO2 Lime þ molasses Portland cement þ water Plaster of Paris þ water For the ‘‘inactive film’’ binders. Examples of various binding systems . Dilute phosphoric acid (reacts with semisoluble alkalis). 1981).7 Inactive matrix Coal tar pitch Petroleum asphalt Carnuba wax Paraffin Slack wax Wood tars Gilsonite Resins Bentonite clay Attapulgite clay Corn starch Dry sugars Dry lignosulfonates Colloidal alumina Colloidal silica Metal stearates Chemical film Sodium silicate þ CO2 Sodium silicate þ Dilute acid Sodium silicate þ Lime Chemical reaction Water (partially dissolves or reacts with many materials). and oil require some liquid (usually water) to be added to complete the binding system (Holley. Water Alcohol Oils Bentonite clay Attapulgite clay Sodium silicate Potassium silicate Sodium lignosulfonate Calcium lignosulfonate Ammonium lignosulfonate Polyvinyl alcohol Molasses Corn starch Tapioca starch Wheat flour Potato starch Casein Glucose Dextrin Salts Sulfates Alginates Glues Gum arabic Sodium borate Fuller’s earth Inactive film Table 1. 1980. all except the water. alcohol. Magnetite and water (magnetite oxidizes and recrystallizes). Dilute sulphuric acid (reacts with semisoluble alkalis). K. C. the relevant strength parameter is the dry strength. d2 e3 ð2Þ where Dp ¼ pressure drop resulting from the flow of a fluid through a porous system of equal-sized spheres K ¼ Kozeny-Karman constant Z ¼ viscosity of the liquid phase d ¼ particle diameter e ¼ porosity L ¼ depth of the agglomerate v ¼ velocity of fluid flow (drying rate) Once the pellet has dried. which is a function of the type of binder used and the grain morphology inside the pellet. EISELE AND S. The dependence of impact fracture on fluid viscosity means that the impact resistance can easily be affected by the binder type. KAWATRA where st ¼ tensile strength of an agglomerate C ¼ a constant e ¼ porosity g ¼ surface tension d ¼ mean particle size of grains in the agglomerate y ¼ contact angle at the air=water=solid interface Impact fracture resistance is viscosity controlled because of the high rate of deformation upon impact. which can be estimated as follows (Rumpf 1962): . it is important to predict whether the pellets will remain intact upon drying and heating. The main mode of failure of pellets during heating is thermal spalling. The Kozeny-Karman equation for thermal spalling is as follows (Kater and Steeghs 1984): Dp ¼ K Z ð1
eÞ2 Lv. This is predicted using Wada’s viscocapillary model (Kater and Steeghs 1984).8 T. In addition to the wet ball properties. as binders can strongly affect the fluid viscosity. The strength is related to the dry tensile strength. where pressure buildup inside the pellet due to fluid evaporation causes the outer layers of the pellet to flake off. 1980). as it produces green pellets that can be directly measured to determine quality. Many of these tests are also applicable to other binder types.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION   1
e H st ¼ C e d2 9 ð3Þ where st ¼ tensile strength of an agglomerate C ¼ a constant e ¼ porosity H ¼ mean strength of an interparticle bridge d ¼ mean particle size of grains in the agglomerate IMPORTANT BINDER CHARACTERISTICS There are a number of property tests that have been used. or tire to produce . disc. either historically or currently. These tests are               Batch balling Enslin water absorption Alumina plate water absorption Grit content Moisture content Size distribution Marsh funnel Gel strength Colloid content Chemical analysis Methylene blue uptake Free swell Exchangeable cations by atomic absorption spectroscopy (AAS) Glycolated layer expansion by X-ray diffraction (XRD) Batch Balling This is the most basic measure of the quality of a binder for iron ore pelletization. to measure the quality of bentonite binders (Wakeman et al. For this test. bentonite and moist iron ore concentrate are mixed and pelletized in a small balling drum. wet compression test. and in kilograms=pellet elsewhere.  Screens: 4 mesh (4. C. in order to provide good statistics. The resulting green balls are then sized to between 7=16 inch (1.24 cm)). Equipment:  6. usually 20. Results are reported as three values: average number of drops from a height of 18 inches (45.48 cm)). 13. and dry compression test are repeated for a number of pellets. expressed in force per pellet. . and procedures to produce pellets.00-6 airplane tire (approximately 16 inches (40. and either 10 mesh (2.7 mm). average wet compression strength at failure.6 cm)  6 inches (15. apparatus.11 cm) and 1=2 inch (1.  Atomizer filled with distilled water. the balls can also be sintered for more advanced testing.27 cm) and evaluated for drop number.5 mm.10 T. The conversions between these units are 1 lb ¼ 0:4536 kg. 12. 1 Cincinnati Muller (12 inch (30. It should be noted that since the Newton is the recognized unit of force in SI.2 mm.0 mm) or 12 mesh (1. it is technically more correct to express crushing strengths as Newtons=pellet (N=pellet) or dekanewtons=pellet (daN=pellet). there is no generally agreed-upon standard method for producing pellets in the first place. 1 daN ¼ 10 N: Unfortunately. If desired. S. and therefore it is virtually impossible to make meaningful comparisons of results from different laboratories. K. 6 mesh (3. and average dry compressive strength at failure expressed in force per pellet. which was as follows: 1. All of these units can be found used in the pelletization literature. wet strength. EISELE AND S.35 mm). and dry strength.7 cm) before failure (drop number). ‘‘Standard Test Method for Determination of Crushing Strength of Iron Ore Pellets’’ (ASTM 1997).. rotating at 52 rpm. The drop test. Each iron ore producer uses their own ore. 1 kgf ¼ 9:807 N.75 mm). while there are standard methods for evaluating pellets after they are made. The closest approach to a standard procedure was the procedure developed by the Bentonite Users Committee (1982). KAWATRA green balls.  Model No. 1 lbf ¼ 4:448 N. Pellet strengths are frequently reported in units of pounds=pellet by industry in the U. Pellet crushing tests should be carried out in accordance with ASTM standard method E 382.  Screen mixed product through a 2. Bentonite Addition=Mixing:  Weigh out appropriate amount of bentonite (the Bentonite Users Committee used a level of 0.1 g with 3 kg capacity.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 11  Balance accurate to 0.5% moisture. Concentrate Sample:  2500 g of ore concentrate (dry weight) at 8.  Return the
6 mesh material to the balling tire and add additional feed and water spray until size approaches 4 mesh again.  Add a small portion of feed to the rotating balling tire and use atomizer to spray the material with distilled water to initiate seed formation.  Mix for three minutes.335 cm)). Save the
4=þ6 mesh seeds in a sealed container.  Sealable containers for seeds and balls. 5. .  Means to remove balls from tire.  Distribute bentonite uniformly over top of concentrate.  If necessary. 700 g is used for seed production.  Spread concentrate uniformly in the muller. Discard the þ4 mesh material. moisture content can be adjusted by slowly adding water ahead of the muller wheel after 1 minute of mixing.0 mm screen.  Screen seeds and repeat procedure until a sufficient amount of
4=þ6 mesh seeds have been produced (approximately 34 g).  When top size of seeds approaches 4 mesh (0.475 cm)).7% bentonite in their reproducibility studies). remove from tire and screen at 4 and 6 mesh (0.187 inches (0. 3.132 inches (0. Of this material. Add water spray as required. 2. Green Ball Preparation:  Place 34 g of seeds into balling tire and add concentrate by handfeeding over a 6-minute period. Seed Ball Preparation:  Start with 700 g of the feed material. 4. concentrate from the Minntac plant (Mountain Iron. the plate water absorption Test (PWAT) is used to measure the water absorption capacity of the binder. This is placed onto a porous ceramic plate that is nearly immersed in water and allowed to absorb water for a specified time of up to 24 hours. and the percentage weight gain from absorbed water is calculated and reported. In the experiments that were carried out with the above procedure by the Bentonite Users Committee to determine the reproducibility of the test. The complete procedure is described by ASTM standard E 946 (ASTM 1992b).  Screen balls at 13. and so calibration is a concern. C. The procedure is as follows. This test was developed by the Bentonite Users Committee (1978
1980f) and was specifically designed for the iron ore industry to evaluate binders.2=þ12. EISELE AND S. K. Minnesota) was used as a standard feed material.12 T. Enslin Water Absorption The Enslin test is a measure of water absorption capacity of the binder. weighed. A problem in this test is that there is apparently variability of the results when different alumina plates are used. First. which was originally designed for soil testing (Seger and Cramer 1984). allow one additional minute of re-rolling without additional water spray. and the results are reported as percentage weight gain.5 mm. but it has since been discontinued by ASTM. . KAWATRA  After forming balls. retain the 713. At the end of this time. the filter paper and binder are removed from the plate.5 mm balls for testing. converted to a weight. The definition of this procedure was not completely successful in producing reproducible results between laboratories. accurately weigh 1
2 g of binder onto a circle of filter paper of specified size. In this test. and so it was never made into a formal standard (Bentonite Users Committee 1980d
1982b). 0. Alumina Plate Water Absorption Like the Enslin test.2 g of binder are placed on a glass frit connected to a buret and allowed to absorb water from the buret for a set period of time of up to 24 hours. The volume of water absorbed is measured.2 mm and 12. they will often contain a significant amount of moisture. Sizing This is a measure of the fineness of grind of the material as received from the supplier.  Blend with 220 ml of distilled water in a Waring blender at low speed for 40 seconds. the more rapidly it will disperse or dissolve in water.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 13 Grit Content Grit content of bentonite is the fraction of the material that is retained on a 325 mesh screen during wet screening. and consists of dry screening of the material on a 200-mesh screen.  Allow the suspension to settle for a predetermined length of time. a means for measuring ultrafines can give an estimate of how much binder is actually present in production pellets. The more finely ground the material is. the grit test provides a measure of the amount of inactive material in the binder. even when in a nominally ‘‘dry’’ state. Moisture Content Because binders absorb moisture readily.  Wash suspension into a 250-ml graduated cylinder and dilute with distilled water to 250 ml. . and so this moisture represents material that is being paid for but does not directly benefit the user. Moisture is determined by drying the material at 105 C until it reaches a constant weight. SettlingTest for Ultrafine Particles The presence of ultrafine particles in an ore concentrate affects the properties of the pellets made. The active clay minerals in bentonite disintegrate when wet. and the inactive minerals remain as particles coarse enough to be captured by the screen. Binders are sold by weight. One procedure for measuring ultrafines was devised by Stone and Cahn (1968) as follows:  Weigh material accurately using between 4 and 5 g of material. Because many binders consist of colloidal material. Because coarse mineral particles do not contribute to binding. K. A flat bed of dried iron ore concentrate is then prepared. The ‘‘bonding strength’’ is the percentage of the ore lump weight that remains behind on the screen. Diffusibility and Bonding Strength This is a nonstandard test that has been suggested as a rapid means for evaluating bentonites. These solids are the ultrafine fraction. this is a useful . This procedure is not fully standardized. and the lump volume in cm3 is the diffusibility. The results will vary depending on the specific settling time and decantation procedure used. The volume of each dried lump is determined. and 1 ml of the dispersed bentonite slurry is then placed on the bed in each of several designated spots. KAWATRA  Decant a measured volume of the suspension and accurately record its pH. and weight. It is therefore of little direct interest in iron ore pelletization.  Filter the suspension and determine the weight of the solids by standard gravimetric techniques.14 T. EISELE AND S. temperature. Although this test is much faster and uses less material than a complete pelletization test. Gel Strength This is a measure of the shear strength of a suspension of bentonite in water. as measured by a direct-reading viscometer. The impregnated ore lumps are then shaken by an automatic sieve shaker on a 20-mesh screen for 30 seconds. its reproducibility and relevance to plant performance is not as good. The wetted spots are then dried to form ore lumps and their appearance observed. but is not a good predictor for binding properties. Marsh Funnel The Marsh Funnel is a method for quickly estimating the viscosity of a bentonite-water slurry.  Calculate the quantity of ultrafine material present in the original material. Again. This method consists of preparing a slurry of the bentonite in water and determining the amount of time it takes to flow from a standardized funnel. It is useful for applications where slurry viscosity is important. Diffusibility is determined by first dispersing 10 g of bentonite in 490 ml of water. such as production of drilling muds. C. such as phosphorus and sulfur. dried. The liquid is then decanted off. Analysis can be carried out by any convenient means. The test is carried out by drying and grinding the clay to pass 150 mm and then slowly dusting 2. as well as to determine the important components present. and it is allowed to stand for 16 hours.00 g of the clay over the surface of 90 ml of water in a 100-ml graduated cylinder. A typical value for Methylene Blue uptake is 90 milliequivalents=100 g clay.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 15 measurement for applications where the flow properties are important. The basic procedure is to titrate a suspension of the clay with a solution of Methylene Blue and determine the addition level that allows color to appear in the liquid phase (ASTM 1992a). The volume of the settled material after 16 hours is then measured (ASTM 1995). such as AAS. or X-ray fluorescence spectroscopy (XRF). A typical value for the swell index is 30 ml=2 g. . and re-weighed. Swell Index The swell index is a means of evaluating the degree to which a clay will swell in contact with water. Colloid Content Colloidal material in a bentonite suspension is measured by preparing a suspension of 1
2% by weight bentonite in water and allowing it to settle in a graduated cylinder for 18
24 hours. The weight of solids remaining after evaporation of the water is then taken as the amount of colloidal material in the clay. but it has been found to have little relevance to iron ore pelletization. Chemical Analysis Chemical analysis is used to monitor deleterious impurities in the binder. weighed. the level is brought up to 100 ml. inductively coupled plasma spectroscopy (ICP). Methylene Blue Uptake The quantity of Methylene Blue that can be adsorbed by the clay is a measure of its ion exchange capacity. The cylinder is then carefully washed. It is therefore much more valuable to use a glycol as a standard liquid. The remainder of this review will cover each of these binder classes in detail. there are many variables that can confound the diffraction results. The strength of a pellet is dependent on the type of bonding produced by the binder. which will cause the various clay types to have a reproducible.16 T. Glycolated Layer Expansion by X-Ray Diffraction (XRD) It is possible to measure the amount of expansion of an expandable clay by low-angle XRD. Clays with Naþ as the main cations in the expandable layer are much more able to expand. disperse. This can be a powerful analytical tool for distinguishing grades of expandable clays. These various types of binders can be broadly classified as      Clays and colloidal minerals Organic polymers and fibers Cements and cementitious materials Salts and precipitates Inorganic polymers Each of these classes of binder have inherent advantages and drawbacks and all have been investigated to some extent as iron ore pellet binders. with minimal introduction of contaminants and with minimal inconvenience in processing. K. making them essentially meaningless. and absorb water than clays with Caþ2 as the main cations. TYPES OF BINDER Literally hundreds of materials have been considered for use as binders in iron ore pellets. EISELE AND S. characteristic degree of expansion. These types of clay can be distinguished by atomic absorption spectroscopy. KAWATRA Exchangeable Cations byAtomic Absorption Spectroscopy (AAS) The characteristics of bentonite are controlled by the nature of the exchangeable cations in the expandable layer. if water is used to expand the clay. with the goal of finding the material that will produce the highest-quality final pellet at the lowest possible cost. as shown in Figure 1. however. Virtually any finely divided . C. ferrous sulphate. Binders that can take advantage of adhesional or cohesional forces are therefore needed. CaMg(CO3)2. KCl. Microstructural studies have shown that these effects are linked to the degree to which additives cause quartz dissolution and melt formation. or electrostatic forces. (D) mechanical interlocking. 1974). Al2O3. magnetic. glucose. Studies have been carried out to determine the effects of additives such as NaCl. in addition to affecting the unfired strength of the pellets. but this type of bond is very weak and of only minor importance. CaCl2. Ca(OH)2. whereas others have no effect or even cause a strength decrease. (B) capillary forces from the liquid phase. Capillary forces are stronger but still are not sufficient for finished pellets and additionally require the presence of liquid in the pellet. MgO.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 17 Figure 1. (E) solid bridges formed by sintering or crystallization of dissolved materials (after Sastry 1996). KCl. Some of these additives increase strength up to a certain point. material can contribute to Van der Waals bonding. and bentonite on the fired pellet properties (Ball et al. electrolytes (NaCl. CaCO3. It should be noted that. CaCl2) and alkali calcium compounds (Ca(OH)2. Magnitudes of bond strengths for various classes of interparticle bonds in pellets: (A) van der Waals’. MgCl2. (C) adhesional and cohesional forces. CaCO3) tended to cause an increase in fired . various additives alter the characteristics of fired pellets. In general. due to reaction with the magnetite to produce magnetite=magnesioferrite solid solutions. When a slurry of clay dries. EISELE AND S. These layers are weakly bonded in clays and can be easily separated when wet. Ferrous sulfate had no significant effect on fired pellet strength. arranged in various ways with a variety of counterions that neutralize the excess charge on the sheets and bind them together into layers. Structure and Chemistry of Bentonite Bentonite is formed by hydrothermal alteration of volcanic ash deposits. K. OTHER CLAYS.’’ They are so named because their crystalline structure allows them to cleave in one direction. however. It is generally accepted that a clay that performs well in the plant must have two characteristics: 1. which is derived from the Greek word for ‘‘leaf. KAWATRA strength. These minerals consist of sheets of SiO4 tetrahedra. the plates can attach mechanically or electrostatically to surfaces. A high capacity for absorbing moisture in the balling feed The clays which possess these properties to the greatest degree are the bentonite clays. with relatively little material left over to form a slag. overdosage of any of these additives leads to a decrease in strength. plastic mass or as very finely divided platelets. mainly due to an increase in the amount of slag melt that formed. The effect of additives has not been completely studied. It is actually a mixture of clay minerals. acting as a binder. and many of the members of the group therefore have a plate-like or flaky appearance. therefore it is difficult to be certain in advance whether a particular binder will have undesirable effects on the fired pellet quality. clays can be readily dispersed in water as either a thick. MgO reduced the fired strength. A high degree of dispersion in the plant concentrate 2. AND COLLOIDAL MINERALS Clays are members of a class of minerals known as the Phyllosilicates. which are the most commonly used clay-type binders. Bentonite tended to increase the pellet strength due to increased amounts of slag whereas glucose reduced the strength by increasing the porosity. Some clays are more useful for this purpose than others. BENTONITE.18 T. C. with the primary component . As a result. depending on details of their structures. The basic crystal structure of montmorillonite is shown in Figure 2. causing the clay to expand (after Grim 1968). which has the ideal composition: (Na.Ca)0. In the presence of water.67.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 19 being the smectite class mineral.Mg0. Each clay platelet consists of three layers: two layers of silica tetrahedra and an octahedral alumina=magnesia layer joining them. montmorillonite. . Structure of the smectite crystal.33)Si4O10(OH)2  nH2O.33(Al1. Isomorphic substitution of Alþ3 with Mgþ2 into the tetrahedral SiO4 sheets alters the Figure 2. the counterions hydrate. Platelets are loosely bonded by counterions (typically sodium or calcium) between them. which can be valuable for controlling the moisture content of the finished pellets. EISELE AND S. Calcium ions have a higher charge and smaller diameter than sodium ions. 3. It also increases the viscosity of the fluid between the mineral grains in the pellet. allowing dusting and breakage to be minimized during transfer to the final firing step. reducing the melting point of some of the minerals in the pellet. K. The effect of bentonite platelets on pellets during the drying process is illustrated in Figure 3. One of the features of clay minerals that help in this regard is that the edges of the platelets tend to have an electrostatic charge of the opposite sign from the faces of the platelets. As a result. This causes the clay platelets to bond to each other quite strongly by electrostatic bonding as the slurry dries (Van . When combined with water. Second. leading to a wellrounded. The swelling ability of montmorillonite varies depending on the type of exchangeable cation. sodium montmorillonite hydrates and expands readily on contact with water whereas calcium bentonite expands to a much lesser extent. 2. the sodium and calcium components of the bentonite act as fluxing agents. C. plastic pellet that can be conveniently handled for sizing and transport in the plant.20 T. the clay bonds to the mineral grains and to each other. 4. and as a result the calcium ions tend to interact more strongly with the aluminosilicate platelets. which then attach to the iron ore particles and to each other as they dry. During drying. making them less prone to hydration. During sintering to produce finished high-strength pellets. it absorbs water. First. This helps to strengthen the pellets during the preheating stage. hydration of these exchangeable cations causes the clay mineral to swell. This allows a portion of the pellet to melt before the sintering temperature is reached. the expanded clay is very easily spread through the iron ore upon mixing. giving excellent dry strength to the pellet. This is one of the most important functions of a pellet binder because in the absence of a binder. The traditional view of the behavior of clay as a binder is that the expanded clay disaggregates into submicron platelets. KAWATRA crystal charge balance and requires surface adsorption of exchangeable cations (commonly Naþ and Caþ2) to balance the charge. The expansion of the clay minerals in bentonite when they come in contact with water has three effects that are of interest in pelletization: 1. the pellet will disintegrate after it is dried. particularly when the moisture content of the material being bonded is not sufficient to completely disperse the bentonite (Wenninger and Green 1970. Bonding is enhanced by the electrostatic attraction between the platelet faces (which have a negative charge) and the platelet edges (which are positively charged). Traditional view of how bentonite platelets bind mineral grains in a pellet. Olphen 1987). from deposits where beds of volcanic ash reacted with fresh . it appears that this traditional view may not be entirely correct. however. Platelets are initially dispersed in the liquid. as shown in Figure 4. These platelets can slide relative to one another under shear to form strands.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 21 Figure 3. and the platelets bond to the mineral grains and each other as the liquid dries. 2002a). In low-moisture situations. it has been suggested that. the clay grains expand into a stack of lubricated platelets. Kawatra and Ripke 2001. rather than dispersing. which is shown in the scanning electron micrographs of Figure 5. Sources of Bentonite The best-grade sodium bentonites in North America are mined from Wyoming. This effect is consistent with the fibrous appearance of bentonite binding sand grains. the grain can then spread into a long fiber in an effect similar to spreading a deck of cards across a table (Ripke and Kawatra 2000a). KAWATRA Figure 4. K. as it is in great demand for use in ‘‘clumping’’ cat litter (Rosten 1999). water over time.10=lb ($0. Under shear stress. however.22 T. and so the cost of bentonite is quite reasonable. Since the cat litter market is bidding the price of bentonite up to as high as $0. competition for the highest-grade bentonites has increased. In 1997.055=kg) at the point of production. these high-grade bentonites are becoming much less available for use in the comparatively low-value iron ore pelletization market. the value of bentonite sold for use as cat litter was $840 million and is expected to reach $1 billion by the year . The grain expands when moistened and the platelets are lubricated by the interplatelet water. Lower-grade calcium bentonites that formed from alteration of volcanic ash by seawater are also available. EISELE AND S.025=lb ($0. Behavior of bentonite grains that are not completely dispersed in water. C. Wyoming sodium bentonites cost approximately $0. but these are much less suitable as pellet binders because of their lesser expansion ability.22=kg). all of which result in excellent binding properties.5 minutes with 3. and disperse readily in water.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 23 Figure 5. structure. 55. as these bentonites are highly absorbent. which is consistent with the bonding mechanism described in Figure 4 (Wenninger and Green 1970). When the main counterions are calcium (calcium bentonites). The bentonite formed strands stretching over and between grains. Scanning electron micrographs of silica sand and of the same sand after bonding with bentonite. and history. mulled 1. Using the plate water absorption test (PWAT). It will therefore be necessary to use lowergrade bentonite or alternative binders to keep binder costs down in iron ore pelletization. and . (a) Sand grains. swelling.0% sodium bentonite. with considerable variations depending on details of their composition. 250 magnification. good-grade bentonites can absorb in excess of 900% of their weight in water. AFS Fineness No. 2003. Factors Affecting Bentonite Performance Bentonites from different sources and deposits behave differently. The most important parameters for evaluating a bentonite are as follows Water Absorption Capacity. (b) Sand with 6. The best grades of bentonite are those where the main counterions in the expandable layer are sodium (sodium bentonites). the water absorption.2% water. expand to as much as 14 times their dry volume on contact with water. 250 magnification. and therefore this trend of reduced availability of high-grade bentonite is likely to continue. The most water absorbent and expandable bentonites are those where the exchangeable cations are predominantly sodium. K. The bentonites from source ‘‘A’’ also appear to produce higher pellet quality than bentonites from source ‘‘B’’ with similar PWAT values. generally to such a degree that they are not acceptable as pellet binders.7 cm) drop number and the dried pellet strength. the finer the clay.4 10. As a result. The Wyoming bentonites are well known for their high ratio of sodium to calcium and their resulting high quality. The effects of relatively small variations in water absorbency on pellet strength can be seen in Table 2.5 (42. Effects of bentonite source and PWAT value on pellet properties in an operating ore concentrator Binder source PWAT value. Dry strength. C.7 9. For bentonite from each source. it is apparent that other properties of the bentonite have effects of similar magnitude.5 10.5 18. Particle Size Distribution.9) Supplier B 916 1035 934 10. A low amount of grit and a high quantity of
2 mm material are both correlated with high pellet strength. While the water absorbency does have an effect.3 15. increases in the water absorbency were accompanied by increases in both the 18-inch (45.6) 12.6 15.7 (47.0) dispersion is much reduced. In general. . EISELE AND S.1) 9.9 10.5 10. Bentonites which contain more calcium and magnesium as their exchangeable cations are much less water absorbent. the stronger the pellets will be (Ehrlinger et al.2) 10.5 13.24 T. Calcium=Magnesium=Sodium Content.7) 10.6 (47.3 13.6 (42. there is some question about the importance of variations in water absorbency for iron ore pelletization. % % moisture Drop no. KAWATRA Table 2. with the quantity of
2 mm material being the most important (Volzone and Cavalieri 1996). Lbf=pellet (N=pellet) Supplier A 881 895 1028 10.0 (40. Fine particles are important for a good binder because they increase the available surface area for binding.5 10.8 9. 1966).8 (56. and these bentonites are correspondingly much less effective as binders. It clearly can be seen that increasing sodium carbonate dosage greatly increases the bonding strengths of the two high-gypsum bentonites but has little effect on the low-gypsum bentonite. as can be seen from Table 4. there are some cases where adding sodium carbonate to bentonites with poor water absorption properties causes the properties to improve (Ehrlinger et al. Addition of sources of soluble calcium.44% SO3) High-gypsum bentonite #2 (1.31 31.18 30.00 2.92 31. such as Ca(OH)2 and CaCl2.00 3.50 1.79 34.14 34.04 15. % % Soda ash addition 0.06 23.28 .00 4.02 35. This is apparently because the added sodium displaces part of the calcium from the interplatelet space. For example. Sodium carbonate in solution also helps to prevent the properties of high-grade bentonite from being degraded by soluble sources of calcium. however.23 16.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 25 The properties of bentonite can be changed by combination with chemicals that alter its exchangeable cations. are known to cause the binding properties of bentonite to degrade because the sodium bentonite is being converted into calcium bentonite.00 0.13 16.90 29.02% SO3) Low-gypsum bentonite (0. The effects of adding soda ash (sodium carbonate) on the bonding strengths of high and low gypsum content bentonites are shown in Table 3. although it is also possible that the improved performance is due to the dispersing action of the sodium carbonate.04 15. probably due to alterations in the particle size distribution.99 27. 1966).02 31. converting a calcium bentonite into a sodium bentonite.18 21. such as gypsum.00 12. the effectiveness of bentonite as a binder is apparently not affected by addition of insoluble calcium compounds like limestone (CaCO3). Table 3.40 11. Effect of soda ash on the bonding strength of bentonites with high and low gypsum contents Bonding strength.65 10.25 0.51 30. In fact.00 High-gypsum bentonite #1 (1.62 35. addition of limestone may increase the pellet strength.46% SO3) 11. 0 1.6 27.0 1.9 32.3 5.5 15 27 39 52 31 42 54 65 19 21 21 26 21 27 33 37 4.2 25.4) (49.5) (37.0) (60.7 25. Physical properties of pellets made from Egyptian Baharia iron ore and Gebel El-Rifai limestone (after Abouzeid et al.1 (42. Increasing the mixing time from 30 seconds to 60 seconds resulted in a small but consistent increase in the dry crushing strength of the pellets and . Bentonite Pellet drop strength. Kg=pellet Porosity.0 0.5 13. 13.7) (32. 10% limestone Fluxed pellets.9) (64.3 5.8) (23.0 0.5 1. The effectiveness of bentonite binder appears to increase as its mixing with the iron ore improves. % wt.1 14.3 4.2) (57. An example of this is shown in Table 5. % wt. as can be seen from Figures 6
8.0 3.5 16.9 7.5 16.2 2.5 1.0 7. sometimes quite markedly. EISELE AND S.1 14.1 29.5 16. Chemistry of the plant water can affect the performance of bentonite binders.5 13.0 0.9 27.2 5. magnesium chloride.8 13. Mixing Effects. KAWATRA Table 4.2) (42.4 3.8 14. This effect can be seen in Table 6. number (N=pellet) % vol. Ions in solution affect the Zeta potentials of both the iron oxide grains and the bentonite platelets.3 33.5 13. the washed concentrate produced stronger.5 13.4 33.5 0.5 1.5 1.1 6.1 14.3) (56.4 Chemical Environment.0 1. 20% limestone Dry crushing Water.8 13.2) (52.6 33.8 6.26 T.0) (59.8 5. 1985) Material being pelletized Iron ore Limestone Fluxed pellets.0 0.0 6. K.5 0.5 0. Further tests using washed concentrates and additions of pure salts showed that high levels of calcium chloride.9) (68.0 1.8) 31.7) (41. In each case.9 34.7 33.3 6. where magnetite concentrates from three different sources were first pelletized with their respective plant process waters and subsequently pelletized after washing repeatedly with distilled water. and so can affect the binding action. tougher pellets.1 0.5 16.8 13.7 33.7) (68. and acidic pH caused the dry compressive strength to decrease.7 32.3 33. C.6 4.8) (59.9 33. apparently due to removal of dissolved salts. 1) (29. Pellets have the highest wet strength when their moisture content approaches the value for complete saturation of the voids (Nicol and Adamiak 1973).3) (7.2 9.5 4.4) (7. bentonite tends to slow the growth rate of balls compared to balling without bentonite. During the balling process.6 2.1 2.0 9.3 13.6) (8.5 9.5 10.0) (43.1 1.2 6. Since bentonite absorbs moisture. Using bentonite in this way also varies the pellet properties due to variations in the binder dosage. lbs=pellet (N=pellet) 5.6) (8.9) Dry comp.0 (24.6 15.8 5. it can be used to take up excess water and bring the moisture content down to the saturation point.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 27 Table 5.4 9.8 6.8 5.9) In each case. .4 3. had a similar effect on the wet-drop values. Wet drop 18 in.2) (26. lbs=pellet (N=pellet) 1.6) (28.1 1.3 9.4 8.7 6.0) (40. more uniform ball than would be produced without binder.1 6.0 (6.6 1.3 4.7 2.1 6.1 8.9) (28.5 9.9 9.1) (8.0 9.0 11.4 9.2 Wet comp.3) (8.4) (9. as can be seen in Figure 9.7) (48. the bentonite leads to a smoother.0) (4.7) (20. although the wet crushing strengths showed very little change with the increase in mixing time.7 cm) 9.9) (9.8 3.6 8.9 1. The effects of varying bentonite dosage on pellet properties is shown in Table 7.9) (11. which occurs even before balling begins. (45. the binder was Wyoming bentonite with 78% colloid content and 87% passing 200 mesh.8 5.1 9.54%) (Rice and Stone. Effects of Bentonite in the Pelletization Process The first effect of bentonite in pelletization.0 2. and was added to the concentrate at a dosage of 12 lb=ton (0.4 2. 1972).0) (43.3 6. While this does reduce the capacity of the process.5) (46.7 1.3 9. is control of the moisture content.5 6.2) (12.4) (42.6 9. iron ore concentrators Concentrate source Pea Ridge Grace Cornwall Concentrate treatment Set Set Set Set Set Set Set Set Set Set Set Set 1 — unwashed 1 — washed 2 — unwashed 2 — washed 1 — unwashed 1 — washed 2 — unwashed 2 — washed 1 — unwashed 1 — washed 2 — unwashed 2 — washed % moist. Effect of soluble impurities on the balling properties of magnetite concentrates from three Bethlehem Steel Co.9 2.9 9. wet compressive strength. C. Figure 7. The magnesium chloride was dissolved in the moisture contained in the pellet (Rice and Stone 1972). Effect of magnesium chloride concentration on the drop number. . K. and dry compressive strength of pellets made using Wyoming bentonite and washed Bethlehem Steel Pea Ridge concentrate. EISELE AND S. KAWATRA Figure 6. Effect of calcium chloride concentration on the dry compressive strength of pellets made using Wyoming bentonite and washed Bethlehem Steel Cornwall concentrate (Rice and Stone 1972).28 T. This cost is typically higher than the actual price of the bentonite at the mine because iron ore concentrators are a considerable distance from the sources of high-grade bentonite. and the amount of moisture that can be removed without spalling is reduced. This cost could be greatly reduced if the clay binders could be produced closer to the iron ore producer. Effect of solution pH on the dry compressive strength of pellets made using Wyoming bentonite and washed Bethlehem Steel Cornwall concentrate. Drying rates are reduced by the presence of bentonite. Other ClayTypes One of the major cost items in the use of bentonite clay is the cost of shipping. which tends to cancel out the drying limitations (Nicol and Adamiak 1973). however. both of which are undesirable. Clays that contain large percentages of montmorillonite and other expanding clay minerals can form from other sources than the volcanic ashes that give rise to bentonites. The Illinois Geological Survey . bentonite also increases the drying temperature that can be used without spalling. and so there is always interest in developing binders from clay deposits near the mines.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 29 Figure 8. The pH was adjusted by adding of HCl and KOH. ExpandingClays. 75 9.0 13.45) 16 (0.8 4.1 4.7) (17.8 3.0 (10. and Series C. K.7 cm) 9.1 4.4 19.85 10.27) 3 (0.87 9.3) (79. The clays were dried.49 2.2) (86. C.4 3.55 10.8) (78.0 3.4 5.11 10.13) 0 (0) 18 (0. (45.89) Mixing time (seconds) % moist.1) (15.45) 10 (0.47 9.07 9. Series B.21 1. Effects of bentonite dosage on green ball properties % moisture Bentonite.6 17.09) 2 (0.84 10.9) (121.8 Wet crush lbs=pellet (N=pellet) Dry crush lbs=pellet (N=pellet) 2.8) (17.7) (12.8) (129.41 2.04 10.4 10.3) (15.1%).38 9 11 16 13 23 20 38 53 40 62 Wet crush lbs=pellet (N=pellet) 2.01 9.2) (10.2 4.9 28.9 17.4 10.78 10.9) (13. Three test series were carried out: Series A.89) 20 (0.80) Feed Feed þ Bentonite Finished pellets Wet drop 18 in (45.1 6.8) (11. KAWATRA Table 6.’’ which are clay deposits formed on the surface of glacial till (Ehrlinger et al.1) Dry crush lbs=pellet (N=pellet) 13. lbs=long ton (%) 18 (0.36 10.3 30.64 9.07 10.47 2.9 3.65 9.43 2. with Table 7.5) (126.40) 6 (0.2) carried out a study of a number of different ‘‘accretion-gleys.9) (15.2 16.2 27.30 10.32 10.23 10.8 6.31 2.67) 12 (0.0) (13.90 9.71) 16 (0.5) (81.3) (86.74 2.72 10.78 11.07 11.80) 15 (0. disaggregated. Effect of mixing time and bentonite dosage on the properties of hematite pellets Bentonite dosage lbs=long ton (%) 2 (0.2) .51 10.65 9. Wet drop 18 in.48 15.6 1. which was the same as Series A but with sodium carbonate also added at a rate of 2 lbs per ton (0.7) (88.3 8. screened.0) (25.68 9.7 cm) 30 60 30 60 30 60 30 60 30 60 10.8%). and added to an unidentified iron ore concentrate at a dosage of 16 lbs of clay per ton of concentrate (0.8) (20.0 14.4 (61.04 10.5 29.09) 6 (0.6 9.27) 6 (0.72 10.27) 10 (0.4 3.0) (56.9 3.5 4.4) (135.71) 20 (0.11 10.1) (37.1) (6.30 T.1) (12.90 10.5) (87.1) (16.8 19.0) (18. with clay screened to pass 325 mesh (45 mm).90 9.8 5.5 19.53 15.95 10.0) (8.54) 9 (0. 1966).3) (10.1) (44. EISELE AND S.07 10.4 (12.7) (73. These clay deposits are common throughout the Midwest and are conveniently located relative to the iron ore pelletization plants. however.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 31 Figure 9. all of the pellets made in these experiments tended to be too dry. While NaOH. interestingly. and KCl were ineffective in improving the properties of the clay. clay screened to pass 20 mm and no sodium carbonate addition. as indicated by their low and variable moisture contents and their very low drop numbers. Unfortunately. nearly all of the clays produced markedly higher dry strengths when sodium carbonate was added. some conclusions can be drawn. Attempts to upgrade substandard Wisconsin expanding clays was carried out by Clum et al. Effect of bentonite on batch balling kinetics (after Sastry 1996). however. . Smaller. First. All of the clays produced lower dry strengths than the bentonite sample. The clay studied had a high quartz content in its natural state and had a low Na=Ca ratio that tended to reduce its expansive properties. This clay was first treated by sedimentation to remove a portion of the quartz. The results of the pelletization experiments are given in Table 8. NaCl. it was found that the clay could be made into an effective binder by washing with an 18. KOH. but still significant. followed by washing with various sources of Na and K cations. increases in the dry strength were seen when the size of the clay was reduced. the presence of bentonite also causes the pellets to grow more slowly during the balling process.2% solution of sodium carbonate or potassium carbonate. but. While bentonite results in a stronger final pellet. they tended to produce higher fired strength than the bentonite. (1977). 2.8 3.1 12.0 2.8 Drop no.3 12.48) (4.0 4.8 5.84) (3.3 16.0 16.2 2.48) (3.64) (3.2 5.7 2.81) (3.42) (4.5 2.2 % moist.0 2.4) (14.3) (10.56) (4.73) (3.9 17.8 5.0 2.0 4.3) (12.6 5.70) (3.0 1.32 61 66 55 43 61 61 69 Panama A Rochester Zion Church Lierle Creek Akers School Woodland (lower) %M Funkhouser East Clay source 17 28 27 29 31 25 32 %Mx 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 Clay size and additives 4.9 7.3 2.9 3.0 1.4 2.0 2.2 (8.00) Wet strength Oz (N) 1.8 16.75) (2.8) (13.5 6.9) (15.3 3. Properties of iron ore pellets made using a wide range of glacial clays from Illinois and a typical western bentonite from Wyoming .3 5.1 10.9 2.7) (4.4 6.3 6.95) (3. 12.7 2.4) (13.8 6.6) (6.3) (21.0 2.0 1.8) Dry strength Lbf (N) 1372 1372 1433 1430 1426 1479 1496 1207 1343 1201 1005 1213 1404 1442 1231 1289 1083 1044 922 819 905 (6102) (6102) (6374) (6360) (6343) (6578) (6654) (5369) (5974) (5342) (4470) (5395) (6245) (6414) (5475) (5733) (4817) (4644) (4101) (3643) (4025) Fired strength Lbf (N) Table 8.7) (10.48) (4.3 5.8 1.2) (14.9 15.25) (2.7 7.64) (3.9 6.4 2.7 2.45) (3.48) (2.0 2.9 3.1 13.7 0.0 2.1 2.0 2.0 2.4 10.7) (9.7) (17.4) (9.0 3.1 2.0 2.5 15.8 13.5 5.0) (10.2 5.53) (4.86) (4.9 1.4 1.1) (12.3 10.6 13.4 8.3) (5.9 9.3 9.0) (3.4 3.3 5.7 6.6 2.0 13.42) (2.5 6.5 12.0 2.2) (25.5 15.4) (16.8 (3.1 12.2 3.17) (4.0 1.8 7. 33 55 65 52 31 0 78 99 Hipple School Forreston Mt. Morris Cedarville East River King Mine Olmsted Wyoming bentonite 1 10 47 62 13 23 27 37 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 745 745 720 mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 þ Na2CO3 5.2 8.7 7.1 6.5 6.8 6.4 7.4 6.8 6.3 6.3 6.4 6.1 6.2 6.3 6.2 6.3 6.4 6.0 7.2 6.5 6.5 6.5 7.4 7.0 2.0 2.0 2.0 2.0 2.0 2.0 1.9 2.0 1.9 2.0 2.0 1.9 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.1 2.0 2.7 2.4 11.7 12.2 12.5 12.0 12.8 14.3 13.3 16.8 14.7 13.5 13.9 13.8 14.6 12.9 13.9 10.8 12.0 12.3 12.0 14.0 11.3 13.5 16.7 11.8 (3.25) (3.39) (3.48) (3.34) (3.56) (3.98) (3.70) (4.67) (4.09) (3.75) (3.86) (3.84) (4.06) (3.59) (3.86) (3.00) (3.34) (3.42) (3.34) (3.89) (3.14) (3.75) (4.64) (3.28) 1.8 1.5 2.4 1.2 3.0 1.0 0.7 3.9 3.6 0.9 2.0 1.0 0.8 1.2 0.7 0.5 0.9 0.8 1.1 4.8 1.9 9.3 9.4 8.0 (8.0) (6.7) (10.7) (5.3) (13.3) (4.4) (3.1) (17.3) (16.0) (4.0) (8.9) (4.4) (3.5) (5.3) (3.1) (2.2) (4.0) (3.5) (4.9) (21.4) (8.4) (41.4) (41.8) (35.6) 994 730 1012 1162 1072 1041 1190 1524 1311 1095 986 1276 1049 1064 1040 1005 986 784 760 848 905 868 1144 681 (4421) (3247) (4501) (5168) (4768) (4630) (5293) (6779) (5831) (4870) (4386) (5676) (4666) (4733) (4626) (4470) (4386) (3487) (3380) (3772) (4025) (3861) (5088) (3029) Clay dosage for all tests was 0.8%, and when sodium carbonate was added, its dosage was 0.1%. The columns ‘‘%M’’ and ‘‘%Mx’’ represent the quantity of montmorillonite and mixed-layer expandable clay minerals, respectively, measured in each clay sample by X-ray diffraction (Ehrlinger et al. 1966). 53 Woodland (upper) 34 T. C. EISELE AND S. K. KAWATRA even after treatment, the necessary dosage of this clay was three times higher than the necessary dosage of western bentonite. An interesting possible replacement for bentonite as a pellet binder is nontronite clay. This is an expanding clay, similar to the montmorillonite in bentonites, with the difference being that it is significantly higher in iron (as much as 26% by weight) and lower in silica than montmorillonite. The results of some very preliminary work with nontronite clay in the former USSR has been reported, where it was claimed that nontronite gives satisfactory pellet binding at low temperature, along with high-temperature sintering performance that is superior to that of conventional bentonite clay (Tikhomirov et al. 1988). It is important to find out whether these results can be confirmed. Some very preliminary work has been done in Minnesota with ‘‘paint rock,’’ which is a highly altered product from the slaty iron formation on the Mesabi range. Completely altered paint rock contains mostly hematite, quartz, and kaolinite, with small quantities of nontronite, resulting in a clay-like texture combined with a high iron oxide content (48.7% Fe) and low silica content (3.4% Si). This paint rock was found to be most effective at a dosage of 0.5%, in combination with 0.1% pregelatinized starch, which gave results comparable to those obtained with 0.5% of a typical western bentonite (Haas et al. 1987). The compositions of this paint rock, the nontronites examined in the USSR and a typical Russian low-iron bentonite, are given in Table 9. Very little work has been done to date with high-iron expanding clays, but they are promising due to their high iron content, which allows minimal dilution of the iron ore concentrate when they are used as binder. Further investigation of this type of binder is therefore warranted. Table 9. Compositions of Minnesota paint rock, and Russian nontronites and bentonite (Tikhomirov et al. 1988; Haas et al. 1987) Fe SiO2 CaO Al2O3 MgO Na2O þ K2O Minnesota paint rock Kairaktinsk nontronite Sakharinsk nontronite Sarigyukhsk bentonite 48.7 7.3 0.3 5.1 2.6 0.4 26.0 31.0 3.5 6.0 10.0 0.1 12.0 47.0 0.7 9.0 4.0 0.5 3.4 65.2 2.7 15.2 2.7 4.0 A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 35 Non-Expanding Clays. Nonexpanding clays, like kaolinite, fire clay, attapulgite, and illite, are generally not sufficient binders for producing satisfactory pellets. Their main benefit in pelletization is to add colloidalsized particles that improve the effectiveness of other binders. For example, Haas et al. (1989) found that addition of clays at a rate of 0.5% allowed the dosage of an organic binder (pregelatinized starch) to be reduced from 0.3% to only 0.1%; however, because these clays all contribute silica to the finished pellet, their use in iron ore pelletization is no more attractive than the use of bentonite. Earthy Minerals and Fines In addition to clays, other minerals can also be useful for binding together larger mineral grains in pellets, provided that they are sufficiently finely divided. Fines in general tend to increase the crushing strength of pellets (Stone and Cahn 1968). If the mineral particles are reduced in size to the order of 1 mm, their very high surface area and small size allow them to fit into interstices between the larger grains. If the chemistry is correct, these micron-sized particles can then attach to the grain surfaces by surface-chemical effects, binding them together. Increasing fines content in this way increases both the wet drop number and the dry crushing strength and increases the effectiveness of bentonite binder (Abouzeid and Seddik 1981). The benefit of using finely divided minerals as binder would be that the minerals could be iron rich themselves. This would allow them to be added to the ore mix with minimal problems from contamination, as they could have composition very similar to the rest of the ore. Earthy Iron Minerals. The lowest cost source of finely divided iron-rich particles is earthy iron ores that have already been disintegrated by weathering. This type of material is likely to be associated with some naturally occurring clays, such as the nontronite in the Minnesota ‘‘paint rock’’ (Haas et al. 1987). Apparently it is also possible to increase the binding properties of paint rock by addition of sodium carbonate, although it should be kept in mind that excessive amounts of sodium cause operating problems in blast furnaces. Earthy iron minerals can be disintegrated into fine particles quite easily with a minimum of energy input, thus in the cases where they are available, they are worth 36 T. C. EISELE AND S. K. KAWATRA consideration as either a binder in their own right or as a binder ‘‘booster’’ to improve the performance of other binders. Highly Ground Ores. In cases where there are no naturally occurring, finely divided iron minerals, it may be practical to simply take a portion of the concentrate and grind it to a very fine size. This could be accomplished using attrition mills or tower mills, which can grind to extremely fine sizes with greater efficiency than conventional ball mills. The main drawback of this would be the cost of installing and operating the grinding circuit. Such an operation would need enough capacity to grind a portion of the total concentrate produced into material on the order of a few micrometers in size. Some work has been done in the past to use either plant fines or reground concentrate, with encouraging initial results, although it was not pursued at the time because the replacement of bentonite binder was not a priority. There is significant potential for this approach, and it warrants further study. ORGANIC BINDERS The main disadvantage of clays and other binders based on silicate minerals is that they add silica to the finished pellet. Since the purpose of iron ore processing is to remove silicate minerals from the ore, adding silicates back in the form of binder is counterproductive. This has prompted long-term interest in developing or discovering binders that contain no silica. Particular attention has been devoted to organic binders, which include a variety of carbon-based polymeric or fibrous compounds. Investigators seeking new binders have tried everything from acrylamide polymers to cattle manure, with mixed success (Kramer et al. 1967). Some of the organic binders that have been described in the published literature are listed in Table 10. Manufactured Organic Binders Manufactured organic binders are intentionally synthesized for this purpose. In general, these binders have the advantages that they are highly effective, can be specifically designed for binding particular types of particles, and have highly reproducible characteristics. Their main disadvantage is their comparatively high cost relative to other binder types. 37 Lactose Whey Whey permeate Guar 211 Guar 211 D Guar 416 Xanthan Hemicellulose SW Hemicellulose WW Caustic leonardite Naphthalene sulfonate Ammonium SST Calcium ammonium SFs Calcium ammonium SFT Calcium SG2s Calcium SG Acrylic 125 Acrylic GA Acrylic GJ Dairy wastes Natural gums Starch acrylic copolymers Lignin sulfonates Wood-related products Methyl carboxyl C1 Methyl carboxyl H Methyl carboxyl P1 Celluloses Binder name Table 10. Organic binders that have been considered for iron ore pelletization Extremely high water absorption capacity; comparatively high cost By-products of paper manufacture Synthetic cellulose derivatives, with high water absorption capacity Available at low cost; decompose rapidly; have little water absorption capacity High water absorption capacity; less expensive than synthetic binders Notes (Continued) Haas et al. 1989 Reference Water-soluble resin Starches from a variety of sources. Waxy 09 Potato C10 Potato S14 Wheat P Wheat 20 Wheat 30 Wheat 70 ethylene oxide plastic Nongelled starches Polyox resin WSR N-80 Gelled starches Corn Gs Corn Ps Binder name Alkalized starches Table 10. Waxy 06 Corn. (Continued). all have broadly similar characteristics Notes Ogbonlowo 1989 Reference .38 Corn Dextrin 06 Corn 71 Corn 806 Corn C5A Corn CTS Corn IA Corn. It is interesting to note that the waterabsorption capability of the binders tested was strongly affected by the type of water used. Synthetic binders derived from cellulose have been used commercially. In general. the physical properties of pellets bonded with 0. These include the Peridur binders manufactured by Akzo Nobel and the Alcotac binders manufactured by Allied Colloids.5% bentonite. with the carboxymethyl cellulose binders affected less than bentonite and much less affected than the starch=acrylic copolymers.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 39 A broad range of binders are or have been available from various manufacturers. although they are frequently only identified by name or number. this is an undesirable feature that should be kept in mind when using these binders. tap water. Peridur has been studied extensively for use as an iron ore pellet binder and has been used on an industrial scale. Tests were carried out using three different types of cellulose binders. All of the binders had reduced water absorption when the water had a high ion concentration. In tests carried out at the Bureau of Mines. Many of these organic binders have been tested by one of the local iron ore producers. Several successful organic binders are based on the cellulose and carboxymethyl cellulose structures. for a given ore the required dosage of Peridur is 5%
15% of the required dosage of . The highest values for the wet-drop test were achieved with the CMC binders with the highest PWAT values at about 8% pellet moisture. which are illustrated in Figure 10. The analyses of the three types of water are given in Table 12. and it can be seen that the quantity of dissolved material in the filtrate water is much higher than in the distilled water or tap water. with no identification of their structure. Some of these binders have a substantial sodium content because of their inclusion of sodium in some of their anionic side groups. In the Bureau of Mines study. with the results shown in Table 13. 1989). These binders can be added to the concentrate as either dry powder or as a water slurry while still producing satisfactory pellet quality. and a summary of the results obtained with several binders is given in Table 11. in addition to two starch-acrylic copolymers and bentonite. Because sodium can be troublesome in the blast furnace. and filtrate water made by making a 50% solids slurry of the iron ore concentrate and filtering out the water (Haas et al. Cellulose Derivatives.1% carboxymethyl cellulose (CMC) were comparable to pellets bonded with 0. PWATs were carried out with cellulose binders in distilled water. Results . All three binders gave good performance with fluxed pellets as a total bentonite replacement but poor performance with unfluxed pellet feed. improved reducibility. FE4 satisfactory in minipot but poor balling performance in the plant. No kiln dusting. Minipot tests were successful. Successful as complete replacement for bentonite.40 Carbinder 498 GPC-4 GPC-46 GPC-49 S-7241 S-7242 S-7243 S-7244 S-7245 Cellulon Union Carbide Grain Processing Corp. Binder F is no longer available. GPC-46 gave the best minipot sintering results. When used as partial bentonite replacement. but balling characteristics and green ball quality were variable and generally poor. resulted in poor balling and constant surging.) Nalco Manufacturer Table 11. but minipot results were poor. Slight deterioration in physical fired pellet quality. All five binders gave good minipot results. FE8. American Cyanamid Weyerhauser FE4. FE8 performed well in balling circuit. and combined with soda ash Binder F TX-4712=TX-6326 Peridur 330 Binders Allied Colloids Akzo (Dreeland Colloids. Inc. Organic binder summary review Generally unsuitable because the binder could not be produced with a fine enough size consist. Physical pellet quality was good but ballability was poor and the green balls had excessive surface moisture. Some green ball plasticity occurred at higher binder dosage levels. .41 Mackquadt I-O AQU-D3217-B (sodium carboxymethyl hydroxyethylcellulose) DP 3349-179 (petroleum extracts and paraffinic distillate solvent) Glenn Chemical Aqualon Sherex Generally good fired pellet quality and poor green ball quality. Gave good bench balling and minipot firing results as a total bentonite replacement. EISELE AND S. Peridur is derived from cellulose by substituting some of the OH groups with polar groups to make it water soluble. K. and (C) starch. Proper mixing of the binder with the concentrate is important. it appears that the dissolution of the Peridur during mixing and pelletization gives it excellent dispersion through the material. and because the necessary dosage of Peridur is much smaller than for bentonite. The wet compressive strength of pellets bonded with Peridur is . bentonite (Kater and Steeghs 1984). Differences in characteristics of various starch and cellulose derivatives are due to changes in the side groups and chain lengths. however.42 T. C. KAWATRA Figure 10. (B) carboxymethyl cellulose. Idealized chemical structures of the repeating base units for (A) cellulose. mixing would be expected to be even more important for Peridur. 260 2.290 3.700 42. and the results are given in Table 17. starch-acrylic copolymer (SACP) binders.990 1.6 1.270 3.070 2. in Brazil. The Peridur pellets also have been reported to have better resiliency. better resistance to spalling.100 850 1.6 5.000 50.500 470 1.3 1.4 2. greater resistance to disintegration when wetted. The main limitations that have prevented more widespread use of Peridur as a binder are its relatively high cost compared to bentonite and an increased tendency toward dusting when the pellets are fired. aside from a small increase in dust production. Peridur has been applied with some success on an industrial scale. and 16.500 940 1.550 14. where it can be seen that the pellet properties are comparable to those achieved with bentonite.010 1. and a western bentonite (Haas et al.060 13.390 9.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 43 Table 12. Table 13. Comparisons of Peridur with other selected organic binders are given in Tables 14. 1989). CVRD. 1989) Binder type CMC 1 CMC 2 CMC 3 SACP 1 SACP 2 Bentonite Distilled water Tap water Filtrate water Ratio of filtrate=distilled water values 1. Because . Extensive plant testing at one mine showed that Peridur could produce satisfactory pellets. Some comparative laboratory results for Peridur-bonded pellets and bentonitebonded pellets are given in Figure 11. 15. and higher dry strength. produced pellets on a 6-m disk pelletizer.0 comparable to that with bentonite.200 1 7 160 Values given are parts per million (Haas et al. and the effects of Peridur dosage on batch balling kinetics are shown in Figure 12.3 2. Effects of water composition on PWAT values for carboxymethyl cellulose (CMC) binders.800 5. Analysis of the three types of water used for PWATs at the Bureau of Mines Distilled Tap Filtrate Ca K Mg Na 4 22 90 1 2 76 1 3 1. EISELE AND S. K. KAWATRA Figure 11. Peridur is effective at much lower dosages than bentonite and can reach higher strength without contaminating the pellets with excess silica (after Kater and Steeghs 1984).44 T. Comparison of (A) drop number and (B) dry compressive strength for pellets bonded with Peridur and with bentonite. . C. This problem with the binder burning away is common to the majority of organic binders. The dusting problem is due to the Peridur burning away while the pellets are being preheated.0% limestone along with the peridur (Murr and Englund 1988). Because Peridur begins to burn away at around 250 C and magnetite does not begin to undergo oxidation (with its accompanying recrystallization and oxide bonding) until about 375 C. and it is cheaper to use organic binder than to attempt to reject enough silica to make specification.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 45 Figure 12. The degree to which this is a problem depends on how roughly the pellets are handled during the preheat stage. Interestingly. Peridur and similar organic binders are only competitive when the concentrate has a high silica content. where pellet strengths were measured at different preheat temperatures using Black Hills bentonite and two formulations of Peridur. Effect of Peridur on batch balling kinetics (after Sastry 1996). of the cost. An example of this can be seen in Table 18. there is a range of temperature where the pellet is essentially unbonded and therefore quite fragile (Kater and Steeghs 1984). It has been reported that the dusting problem can be reduced by adding approximately 1. it has been reported that CMC binders give physical and metallurgical results superior to bentonite when petroleum coke is . 8.5 8.3 8.00 2.8 10.4 9.5 1.6 8.1 0.7) (14.3 1.5 1.5 4.7 8.6 1.025 0.5 þ 0.00 8. 16 14 9 4 6 5 7 5 6 3 11 13 9 14 10 13 23 18 11 7 9 12 6 1 Drop no.075 0.7) (11.8 8.8 9. kg=pellet (N=pellet) 7.7) (14.10 1.5 8.46 0.2) (29.) Table 14.100 0.7) (13.8) Wet comp.5 1.7) (33.2) (31.8) (40.7 8.0) (38.2 Pellet % moist.4 1.8 8.7 8.4) (20.7) (15.6) (61.5) (30.1 1.9) (61.7) (14. Feed was a magnetite concentrate from the Iron Ore Company of Canada’s Carol pellet plant (Martinovic et al.3 1.3 8. 1989).0 8.7) (15.025 0.9) (6.7) (14. kg=pellet (N=pellet) The ‘‘Aged’’ Peridur C-10 was combined with the ore and allowed to stand 4 hours before pelletizing.4 7.7) (43.4 8.5 1.2 2.6 1.1 þ 0.7) (12.3) (3.100 0.5 8.7 0.1 2.5 1.4 3.8 8.1 0.9 6.6 6.7) (12.7) (15.1 3.050 0.8 10.7 8.7) (11.9 3.4) (27.100 0.7 3.5) (52.9) Dry comp.1 6.7 8.4 8.4 5.5 8.8 0.6 8.1 þ 0.3 1.000 0.6 1.8) (64.0 8.075 0.4 9.9) (57.2 0.4 (69.7) (12.5 8.7 3.7) (12.075 0.7) (15.6 8.4 8.1 2.6 5.2) (26.5 8. 18 in. (45.10 0.5 9.9) (4.050 0.050 0.100 0.6 8.8) (6.7) (14.3 4.4 1.1 þ 0.6 1.6) (10.040 2. aged 4 hours Alchem 8C26 Peridur C10 þ Na2CO3 (feed washed with deionized water) Peridur C10 þ Na2CO3 Carbobind 135 TACBIND Peridur XC-10 Bentonite (control) Peridur C-10 Binder Dosage (% wt.0 8.9) (11.5 8.6 8.4 8.5 8.7) (14.7) (63.7 cm) 1.1 þ 0.7 .7 1.6 1.5 1.1 þ 0.7 0. Comparative results from several organic binders from bench-scale balling tire experiments .3 6.2 1.8 5.4 9.075 0.0 12.2 9.3 1. No binder Peat moss (70% moisture) þ NaOH Peridur C-10.8) (15.2 9.4 0.7 12.4) (25.5 þ 0.7) (13.9) (16.4) (50.050 0.9) (6.8) (5.5 Feed % moist.67 þ 0.100 0.5 1.5 1.075 0.7 8.6 0.7 8.2 (15.0 2.2 8. 7) (15.5% dolomite Feed mix Binder type and dosage Table 15.2 4.8) Wet comp. kg=pellet (N=pellet) Green balls were subsequently used as feed for the pot-grate experiments shown in Table 16.8% Bentonite 0.7) (11.4 1.9 5.4) Dry comp.7) (16.2 1.7 1.6 8.6 1.7) (13.7 1. Pelletization results for several binders with acid (magnetite only) and fluxed feeds using a 1-m balling disc .8 9.7) (16.7 1. 1989).1% Peridur 0.7 3.5 5.9) (38.1) (52.4 3.2 (80.5% Peat moss (dry) 0.9) Acid Acid þ 1.0 Green ball % moisture 8 7 6 6 7 8 7 7 12 10 3 Drop number 1.4) (44.0% dolomite Acid þ 0.7 1.4) (15.1% NaOH 8.7) (16.7) (12.7 1.0% limestone Fluxed mix (CaO=SiO2 ¼ 0. kg=pellet (N=pellet) 8.7) (31.7 8.5 8.0) (46.5 8.7 5.1 4.8 8.7 8.1% Peridur 0.2) (50.1% Alchem 8C26 0.7) (16.1% Peridur 0.7) (15.7) (16. Feed was from the Iron Ore Company of Canada’s Carol pellet plant (Martinovic et al.6 3.5% dolomite Acid Acid Acid Acid þ 1.1% Peridur 0.8 9. Acid þ 1.5 8.075% Peridur 0.2 (16.1% Peridur 0.3 4.1% Peridur 0.3 1.1) (52.47 0.5% dolomite Acid þ 1.7 1.0) (46.1) (31.075% Alchem 8C26 0.5 8.6 1. 9 29.68 R-40 .0 93.8 Porosity (%) — — — 18.6 96.9 26.1 93.48 Acid Acid Acid þ 1.34 0.5 26.075% Peridur 0.3 21.9) Acid Acid þ 1.5% dolomite Acid þ 1.2 96.1 21.1% Peridur 0.1% Peridur 0.1% Peridur 0.6 96.1% Alchem 8C26 0.70 0.0% dolomite Acid þ 0. Pot-grate firing test results for the pellets described in Table 15 28.6 96.6 25.8% Bentonite 0.5 26.3 22.075% Alchem 8C26 0.3 96.3 Swelling (%) Metallurgical properties — — — 1.7 26.3 31.1% NaOH Tumble þ6.1 21.5% dolomite Acid þ 1.76 0.1 28.1% Peridur 0.5 27.1 96.5 0.3 95.5% dolomite Acid Feed mix 96. kg=pell. (N=pellet) Physical quality Table 16.3 mm 0.7 26.3 96.0% limestone Fluxed mix (CaO=SiO2 ¼ 0.73 0.56 0.5% Peat moss (dry) 0.0 22.1% Peridur Binder type and dosage (3570) (3168) (3030) (3462) (3364) 226 (2216) 265 (2599) 307 (3011) 293 (2873) 347 (3403) 320 (3138) 364 323 309 353 343 Comp.1% Peridur 0.74 0.7 24.5 17.69 0. kg=pellet (N=pellet) % moisture content Peridur Bentonite 0.7) 8.85 (0. 1984) Binder dosage Wet strength.0826) 1.0 kg=tonne (0.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 49 Table 17.2 (31. The highest dry compressive strengths Table 18.7) .3 5.4) 2.4 (32.3) 4..0) 40 (178) 15 (66. and so there is some interest in this procedure.6 kg=tonne (0. Starches generally are less expensive than cellulose derivatives. potatoes.0826) 1.85 (0.07) 24 (1. In the Bureau of Mines organic binder study (Haas et al.9) 20 (89.0826) 1.26 (12. Properties of green pellets produced at CVRD on a 6-m pelletizing disk (de Souza et al. Starch Derivatives.85 (0.85 (0.85 (0.0826) Preheat temp.3 (36. starches with PWAT values less than 100 produced pellets with correspondingly low wet drop values. depending on the source. Strengths of preheated pellets made using Bentonite and Peridur Binder type Black Hills bentonite Peridur XC3 Peridur XC7 Binder dosage lbs=long ton (%) 24 (1.  F ( C) 1800 1800 1400 1600 1400 1600 1800 1600 (982) (982) (760) (871) (760) (871) (982) (871) Crush strength lbs=pellet (N=pellet) 50 (222) 34 (151) 8.3 (12.4 1.96 (9.4) 7. As can be seen from Figure 10. They can be used either dry or ‘‘gelled’’ (cooked in water). Starches are produced from vegetable matter. Such additives to the pellet tend to improve pellet reducibility and also reduce the fuel consumption of the firing kiln. kg=pellet (N=pellet) Drop number Dry strength. or wheat.0826) 1. 1989). starch and cellulose are isomers and as such can be expected to have generally similar but not identical properties.85 (0.6 added to unfluxed magnetite pellets (Hanna 1987).9) 12 (53.4) 8. such as corn.06%) 1.0826) 1. and their properties vary somewhat.5%) 0.07) 1.2 3. EISELE AND S. lignin sulfonates. This tended to produce pellets with a higher moisture content and drop number but lower compressive strength because of starch losses in the filtrate. Results of comparative tests with caustic leonardite and with bentonite are given in Table 19. Caustic leonardite binder produced pellets with a higher dry compressive strength than bentonite produced at similar dosages. The correspondingly large binder shrinkage upon drying apparently tended to tear the binder away from the grain surfaces (Haas et al.1% binder.1% but did not reach the target wet-drop values. Gelled starches. K. the most effective was naphthalene sulfonate slurry. . The lignin sulfonates reached the target dry compressive strengths at a dosage of 0. they are typically inexpensive compared to other manufactured binders. Wood Products. (1989) found that more than 0.1% binder was needed to reach their target values for wet-drop and dry compressive strength. Of these binders. followed by pelletization of the concentrate. With hemicellulose binders. but the leonardite pellets had lower wet-drop values. The lowest wetdrop values were found with the binders that had PWAT values less than 500. Haas et al. and caustic leonardite. which were in excess of 10. generally reached the target wet-drop values and dry compressive strengths at a dosage of 0. including hemicelluloses. This indicates that water absorption by a binder can be excessive. and the ideal value is probably somewhere around 1000%
2000%. KAWATRA were obtained with alkali starches due to their high adhesive strengths. Synthetic Polymers. the dry compressive strength increased more rapidly than the wet-drop value. The low dry strength is probably due to their extremely high PWAT values. Some experimentation was also done where a dry-gelled wheat starch was added to a slurry of iron ore concentrate and then filtered. In tests at the Bureau of Mines. C. and also generally with those where a 6% solids slurry of the binder had a viscosity less than 75 centipoise. which had PWAT values greater than 500.50 T.000% (see Table 13). starch-acrylic copolymers produced green pellets that met the target wet drop values but had extremely low dry compressive strengths. As the dosage level increased. 1989). These materials are produced as by-products of wood processing. particularly paper manufacture. As by-products. because of limited sources. strength. however. with the galactose branches connected by an a1-6 linkage.1 (47.7 9.32 1. these binders are summarized in Table 21.8 5. competition for the materials from other markets.5% Leonardite-lime.93 (10. 2. and similar materials can be made from natural sources. The chemical analysis of a typical Jaguar is given in Table 20.0 7.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 51 Table 19. there is more interest in using it as a pellet binder. It has been reported that several varieties of ‘‘Jaguar’’ guar gums are promising binders (Ogbonlowo 1987). 1989).6) (53.56 1.03% Leonardite-lime.58) Dry comp.51 7.94) (8. tars.3) (6. 1. guar gum was found to be most effective with higher moisture contents. lbs (N) 2.9) (214) % moist. S. (1989) suggested that binder slurry viscosity is directly correlated with wet drop value and adhesive strength is directly correlated with dry crushing strength.0 Wet comp.52% Leonardite-lime.8) (84. such natural binders could be produced at lower cost than synthetic polymers because they do not require as much processing.18) (8. In principle. From this difference. Addition of . 12% lime) as binder Binder and dosage Bentonite. lbs (N) 10. which has lower slurry viscosity and lower adhesive strength than guar gum. The high viscosity of guar gum also led to increased wetdrop values. 9. or shipping costs. produced weaker pellets. Jaguar is a high-molecular-weight. Gums.84 1. Properties of pellets made using a mixture of caustic leonardite with hydrated lime (82% leonardite.55 8. 0.33 8. 0. The mannose units are connected by b1-4 glycosidic linkages. particularly the gums.61 Naturally Occurring Organic Binders A wide range of gums. Haas et al. Ogbonlowo (1987) found that four varieties of Jaguar were suitable for pelletization.1 48.7 12. Xanthan gum. In areas near where guar gum is produced. strength. which increased the wet-drop value relative to other binders. In the organic binder study conducted by the U.1 19. Bureau of Mines (Haas et al. straight-chain material composed of mannan branched at regular intervals with single-membered galactose units on alternate mannose units. in practice some of these materials are comparatively expensive.06% Wet drop number 9. 9 0.0 Jaguar guar gums at levels less than 0.5
0. KAWATRA Table 20. Bitumen is a low-cost heavy hydrocarbon that is available either in naturally occurring forms or as a residue from oil refining. were generally found to be ineffective. K. (1977) examined eight guar gums as binders and ranked them according to the dosage needed to make pellets with properties comparable to 1% bentonite pellets at 9% moisture. The gums listed in Table 21 were the highest viscosity of those tested.0
13.5% .0% 0. Table 21.0
5. Clum et al.5
2. indicating that high viscosity is a desirable feature of a binder. their ranking is given in Table 22. Most of the work that has been done with bitumen binders has been with briquettes.5
0. EISELE AND S. The material is very adhesive and will harden due to a loss of volatiles if it is ‘‘baked’’ at relatively modest temperatures.0 4. Tars and Bitumens.4% 0.0 0.0 1.75 0 0 Trace 10. apparently because the bitumen does not have the proper characteristics to easily form balls in balling drums or discs. They also found that curing time was important. Galactomannan component Protein Crude fiber Ash Ether extract Arsenic Heavy metals Iron Moisture 78. Chemical analysis of a typical Jaguar guar gum (Ogbonlowo 1987) Component % Wt.0
82.9% 0. with increasing green drop results as the curing time increased from 15 minutes to 4 hours.52 T. Description and recommended dosages of Jaguar guar gums (Ogbonlowo 1987) Designation Description Optimum dosage A-40-F C-13 cationic CMHP HP-11 Nonionic guar gum Cationic guar gum Carboxymethyl hydroxypropyl guar gum Hydroxypropyl guar gum 1. C.3% wt. 12% ash.1%.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 53 Table 22.7% All results good at 0. The pellets produced weighed 4 g and were 9 mm thick.8% Green drop. The greatest strength is produced when the pellets are baked at 200 or 250 C for 4 to 16 hours.3% All results good at 0. and fired results all good at 0.2% and 0. By baking in air for extended periods at various temperatures. The mixture was then allowed to cool for 1 hour before pelletizing. fired properties poor Green drop results good at 0. and 20.4%. dry. 1977) Guar gum designation Ranking Comments Jaguar A2S 8 (worst) Jaguar G-12-74 MLP-1 7 Jaguar 36 4408 12 Jaguar MDD Jaguar C-13 cationic 6 Jaguar CHMP 4 Jaguar A-40-F Jaguar HP-11 3 2 Modified guar gum 1 (best) Green drop results only good at 2%. The results of these tests are given in . (1984) produced bitumen-bonded pellets by combining magnetite superconcentrate (97.8% Green drop results good at 0.35% 5 Ansari et al.50% fixed carbon. dry and fired results good at 0. 79.’’ which had a proximate analysis of 0.1% Green and dry properties good. Before baking. was used as a binder at 0. 0. dry and fired results good at 0. the bitumen-bonded pellets had green crushing strengths of 2.0% Fe3O4 þ 2. It was noted that the bitumen had considerable reducing power and largely prevented the oxidation of the pellets up to a temperature of 800 C or more.7 kg=pellet (26. manufactured by Kureha Chemical Co. in dies of either 9 mm or 12.4% and compared with bentonites from three different sources. the results shown in Table 23 were produced.7 mm diameter. Similar experiments under a nitrogen atmosphere produce similar results.3% Fe2O3. dry and fired results good at 0.5%. dry and fired results good at 0.38% volatiles.5 N=pellet). Ranking of guar gums as pelletization binders (Clum et al. Pellets were compacted at specified pressures ranging from 30 to 170 MPa.5% Green drop results good at 0.06% SiO2) with 5% ‘‘blown bitumen. They were mixed by heating the bitumen to above its softening point and mixing it with the superconcentrate in a sigma-blade mixer at 180 C.. A water-soluble sulfonated pitch. bentonite Table 24.3 8.7) (22.7 mm in diameter and compacted at 60 MPa before baking (Ahier and Singer 1981).5) (23.5 (83) 13.  C 150 200 250 350 450 550 650 750 850 950 1050 1100 1 4 8 16 9 (88) 13 (127) 13 (127) 21 (206) 11 (108) 10.6 7.4) .6 8.3 7.3 (13. the wet ball properties with the sulfonated pitch pellets were noticeably inferior to those of the bentonite-bonded pellets.6) (61.5 (780) Pellets were 12. Kg (N) No binder Kanto bentonite Greek bentonite Wyoming bentonite Kureha sulfonated pitch — 1.0 3.7 4. and from the data it appears that the binding properties of the sulfonated pitch are comparable to those of bentonite but require only a fraction of the dosage.6) (78.6) (23. Pelletization tests using three different bentonites and Kureha sulfonated pitch to pelletize iron ore concentrate with a particle size of 70% passing 45 mm % Binder % Wet drop Wet comp. K.5 (132) 19.4 8.15 8.0 4.6) 1.8 5. however.0 (17.4 2.4 1.6 4.5 (740) 46 (451) 48 (470) 14.5 (142) 97 (951) 157 (1540) 67 (657) 8.4 2. dosage moist.0 1. Crushing strengths in Kg=pellet (N=pellet) of bitumen-bonded.0 1.2 8.5) (72. In these experiments. with the results given in Table 25. Dry comp. EISELE AND S. although the dry compressive strengths were similar or superior. hours Temp. mechanically compacted iron ore superconcentrate pellets after a single baking treatment Baking time. KAWATRA Table 23. Subsequently.5 (534) 75. Kg (N) strength.8 6.. number strength.5) (22.5 (132) 16 (157) 36 (353) 72 (706) 77 (755) 59 (579) 79. C.8) (74. additional experiments were carried out with iron ore from three different sources.0 0.5 (191) 30 (294) 65 (637) 75.5 (103) 16 (157) 32 (314) 66 (647) 64 (628) 56 (549) 40 (392) 16 (157) 51 (500) 184 (1804) 50 (490) 10 (98) 11 (108) 15 (147) 23 (225) 54.4 2. Table 24.54 T.5 (740) 46 (451) 52 (510) 20 (196) 221 (2167) 173 (1697) 21 (206) 10 (98) 13.4 8.3 2. 1 3.4 — 0.3) (10.9 9.1 2. although it is possible that it would also be useful in pelletizing operations.5 — — — — — 0.8) (54. 1700 F (927 C) lbs (N) % bent.4 8. and papermill sludges.9 4. and so they are available for little more than the cost of shipping. whey.75 — — 0.7) (61.1 3.8) (57.2 10.6) (26.9 6. lbs (N) 0. Molasses has many characteristics in common with soluble tars or pitches and would be used similarly (Morse 1963).A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 55 Table 25.9) (17.3 (52. Wastes that exhibit binding properties include food and dairy wastes (lactose. Properties of pellets bonded with bentonite and with Kureha-soluble sulfonated pitch binder Ore source Plant 1 Plant 2 Plant 3 Strength after preheating at Wet comp. These have an advantage in that the producer is eager to get rid of them.7 3.8) (13. the producer of the waste may even be willing to pay to get rid of them.8) (9. In some cases. % pitch % moist. strength. Organic Waste Products The lowest-cost organic binders are those that are waste products of one type or another.1 2.3 10. probably due to the pitch burning away at the preheating temperature. pellets showed much higher preheating strengths.0) (139) 104 (462) 49 (218) 55 (244) 32 (142) 16 (71.2 0.1 8. .9) (27.3 4.3) (40.0 11.0 (13. Dry comp. Molasses has been used as a binder.2) (13.2) (18. This decomposition problem can be reduced by either adding a biocide or by drying the material.4 9. mainly in briquetting operations.2 0.2 6. A drawback of all of these materials is that in their asreceived (wet) state. lbs (N) strength.1 2. municipal sewage sludges.8) (12.0 13. and so it can be difficult to find a source of organic wastes with both good binding properties and consistent performance. The disadvantage is that the properties of waste materials are not controlled.4 — 0.3 9.2 9. and whey permeate).9 13.8) 11. they decompose rapidly and are quite unpleasant to work with.0 31.0 12.7) Not measured Not measured Results are given for pellets produced from three mines in the Lake Superior iron district.2) 15 (66. KAWATRA Food and DairyWastes. Godin et al. 1990). but peat moss that had been treated with sodium hydroxide could be used to make reasonably high-quality pellets. whey. as can be seen from the results given in Table 26. 1987. Bureau of Mines. EISELE AND S. such as lactose. The sludges tended to have loss-on-ignition (LOI) values ranging from 42.56 T. These sludges were preserved by either adding 1% of Dowcil 75 or formaldehyde or by refrigeration. Paper mill sludges are readily available in Minnesota. indicating that they can have binding properties. The function of the sodium hydroxide was to ‘‘digest’’ the peat. it could be acceptable because whey permeate is a low-value waste product. These sludges are similar in many respects to peat moss. drying and grinding. K. from both the primary and secondary thickeners (Haas et al. as well as bark. but whey permeate produced satisfactory pellets at a dosage of 1. Lactose and whey pellets did not reach the targets for compressive strength or drop value. and so it has been of interest for mines in these areas. While this dosage is high. In work by the U. sludges were collected from five different papermaking sludge treatment operations. C. and Michigan. largely due to their negative PWAT values (indicating that a portion of their mass was dissolved and lost during the absorption test). shredding. was examined by Vincze (1968). 1989). and wet rod-milling. These sludges are rich in wood fibers that could help to reinforce the pellets. Sludges were prepared in a number of ways. Pellets made using these binders in the Bureau of Mines study (Haas et al.0%. This is particularly true of dairy wastes. and humic acid.1% to . Peat Moss. air drying. converting the humic acids that it contained into a gel that functioned as a binder. Panigrahy et al. A number of investigators have used it as a pellet binder (Vincze 1968. 1989) did not perform well. lignite. including addition as a wet sludge. Wisconsin. The use of peat moss. dispersal and sedimentation in water. Paper Mill Sludge. S. and whey permeate. Peat moss is obtained from bogs in many parts of the world and mainly consists of partially decomposed vegetation. Lignite and bark were not satisfactory binders. Some food waste products have a ‘‘sticky’’ consistency. They also contain clays and organic chemicals that are included in paper during its manufacture and which tend to act as binders (Garvin 1985). 1) (66. lbs=pellet (30.6) (61.1 17.4 9.5 7.1 18.1) (63. which sintered the pellets in an electrically heated furnace.2 13.5 13.71%) Wyoming bentonite Peat moss þ Peat combined with NaOH NaOH diluted to (6:1 ratio) 6.0) (59.7) (11. Properties of pellets bonded with bentonite and with peat moss reacted with sodium hydroxide (Vincze 1968) Binder Bentonite (controls) Binder dosage and treatments 16 lb=ton (0.2 9.4% solids and heated to 90 C for 6 hours.71%) Wet drop Wet comp. Dried sludges also did not have very high PWAT values because they did not rehydrate readily.5 2.4 (15. as compared to almost 100% LOI for most of the synthetic organic binders.60 3. In pelletization experiments.3 8.5 cm) (N=pellet) 17.0 9.55 3.4 2.83 13. mainly because they already contained a great deal of water.06 7.9) (13.1) (10.35 3.0 7.7 2.8 9. To reach the target pellet physical properties.61) (13.9) (46. even when ground to fine sizes.06 3. Raw sludges were found to have very low PWAT values.3 86.6 9.7) % moisture 9.0) (14.6 2. This difference is due to the presence of clays in the paper sludge.8%.5 2. they are able to continue at least some degree of binding up until the pellets reach sintering temperature.3 7.5 (63.3 9.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 57 Table 26.2 26. 12 in. lbs=pellet (N=pellet) 14. Since the sludges do not burn away completely. provided that mixing difficulties can be overcome. then dried and ground.2) (55.20 13. In the original Bureau of Mines study.1 9.61) (11. the weight (dry basis) of sludge that had to be added was about twice as high as the necessary bentonite dosage and about ten times as high as for pure organic polymers.18 13.96 14.3 9. This is not necessarily a problem because of the negligible cost of the papermaking sludges. It may be necessary to use procedures for rehydrating dried sludges that are similar to those needed for producing gelled starch.97 14.3 12.6 8. it was found that wet sludge is more effective as a binder than dried sludge.7 10.2) (62.1) (62.8) (16.5 3.6) (11.8 13. Dosage: 16 lb=ton (0.1) (10.7) Dry comp.4) (60. it was reported that the metallurgical properties of pellets made with raw sludge binder were superior to those . Addition of the sludge as damp filter cake to the filtered ore concentrate is impractical because they cannot be properly mixed. 1989). KAWATRA made with bentonite binder (Haas et al.  The sludge easily becomes moldy. papermill sludges were found to be unattractive for full-scale use for the following reasons (Goetzman 1989):  Proper mixing of the sludge with the iron ore concentrate presents practical problems. a means for grinding it to pass 75 mm and intensive mixing. In this study. If the sludge is added as a wet slurry to the ore before filtration.58 T. particularly phosphorus. it reduces the filter capacity and increases the moisture content of the filter cake. which more closely simulate the operation of full-scale pelletization operations. their reducibility became almost identical to the bentonite-bonded pellets. when the firing cycle was changed for the sludgebonded pellets to improve their physical quality. although they may be useful for other binding applications. and in pot-grate testing the sludge-bonded balls had poor physical quality. C. that would probably make it unacceptable in a full-scale pelletizing plant.  The ISO reducibility of the sludge-bonded pellets was higher than that obtained with bentonite when both types of pellets used the same firing cycle. Subsequent pot-grate tests. however. compression. Municipal Waste Sludge. To be practical as binder. and tumble index values. with a noxious odor. a higher content of colloidal material. K. more variable composition. and a high concentration of undesirable contaminants. it will almost certainly be necessary to dry and grind the sludge before adding it to the iron ore concentrate. were therefore carried out at the Natural Resources Research Institute. Municipal waste sludges generally are similar to papermaking sludges except with a lower fiber content. Therefore they are generally less desirable than papermill sludges and are not suitable for use as iron ore pellet binders. If the sludge is dried so that it can be added in the same manner as bentonite. it requires installation of a drying facility. EISELE AND S. Studies have not yet been done to calculate the costs of such sludge processing or to determine how the performance of paper sludge binders can be improved.  The quality of sludge-bonded green balls is marginal compared to bentonite-bonded balls. . all of which represent significant capital and operating costs. it will not reharden if it is dried. hydrated cement.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 59 Figure 13. It should be noted. It gives properties roughly similar to those of bentonite-bonded pellets. As a result. and the material is normally dried to 50%
100% solids by spray drying. The liquor is the waste resulting when wood is sulfonated with Ca(HSO3)2 solution. however. once a cement is wetted and cured one time. and as a result cement-bonded pellets can easily . Many cements are based on calcium compounds. Spent Sulphite Liquor. Compressive strengths of pellets made with varying amounts of spent sulfite liquor. and with bentonite as a function of baking temperature (after Goksel and Ozturk 1985). The resulting material is easily dissolved in water and has some adhesive properties. as a hardened cement still contains the water of hydration. This is a by-product of the sulphite pulp process for paper manufacture. and rewetted. cementitious binders chemically react with water to form a hard. which is an undesirable contaminant in blast furnace feeds. that sulphite liquor is very high in sulfur. CEMENTITIOUS BINDERS Unlike clays and many organic binders. as can be seen from Figure 13 (Goksel and Ozturk 1985). Their binding action is not reversible. broken up. we find that a significant amount of fly-ash is still being disposed of in . however. C. The typical composition of ordinary Type 1 portland cement is approximately 67% CaO. These cements can be made containing less than 0. K. depending on the purpose of the cement. there also are calcium aluminate cements that may be of interest. The main disadvantage of cementitious binders is that they require some period of time. 5% Al2O3. 1979). They can also be made to harden even more rapidly by addition of hydrated lime (Taylor 1990). in order to completely harden the pellet. aluminate (Ca3Al2O6). the properties deteriorated as the pellets were enlarged. 3% Fe2O3. In addition to typical portland cements. who found that a high water content was needed for the concentrate to ball properly. and 3% other components of various types. with the most readily available type of pozzolan being the fly-ash produced by coal combustion. Pozzolanic Materials A pozzolanic material is a material that will react with lime or other alkalis to form a material similar to cement. Most of this material is in four major phases: alite (Ca3SiO5). A large fraction of fly-ash is used as cement admixtures and fill material (Taylor 1990. Some very preliminary work was done with calcium aluminate cement binders by Clum et al.4% SiO2. typically several hours. and their use would therefore minimize the amount of silica contributed to the pellet by the binder. Seeley et al. Cement The most obvious cementitious binder is ordinary portland cement.60 T. Calcium aluminate cements can be made that harden to a useful strength in only a few hours and that harden as much in 24 hours as portland cement does in 28 days. EISELE AND S. KAWATRA be made self-fluxing. Behr-Andres 1991. From this it is evident that large dosages of cement as binder will tend to contribute a significant amount of silica. and that while it was an effective binder for small pellets. with minor amounts of other phases such as alkali sulfates and calcium oxide (Taylor 1990). belite (Ca2SiO4). and ferrite (Ca2AlFeO5). cementing phase that binds the particles together. (1977). 22% SiO2. which reacts with water to form a hard. Pozzolanic materials include finely divided silicate and aluminosilicate glasses. The reason for this is that many fly-ashes have compositions that are not suitable for the fly-ash markets that have already been developed.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 61 landfills at considerable cost and no benefit. however. temperature. and (2) it assumes complete reaction and no side reactions or intermediate phases. ashes produced by fluidized-bed combustors have no existing markets because they are too different in composition from more ‘‘traditional’’ fly-ashes. In particular. H ¼ H2O. fly-ashes are not simple homogeneous materials. and Class C. and the presence of chemicals that act as hardening accelerators or retarders (Helmuth 1987). The rates of these reactions depend strongly on many factors. which also act as binders upon formation. If fly-ashes were simple homogeneous materials. which already contains enough alkali (usually in the form of CaO) to make it behave as a cementitious material (ASTM 1994). which has significantly different requirements than cement admixtures and can provide a means for utilizing many ashes that are currently unmarketable. Sˆ ¼ SO3. with the most important phases including calcium aluminum hydrate (C4AH19). z is a numerical variable): AS2 þ 3CH þ zH ¼) C-S-Hz
5 þ C2 ASH8 : The products produced by this particular reaction are an amorphous calcium silicate hydrate gel (C-S-H) and gehlenite hydrate (C2ASH8). and calcium monosulfoaluminate hydrate (C3AC SˆH12). and so their utilization is difficult (ASTM 1994). including composition. A ¼ Al2O3. which would be unacceptable in construction (Taylor 1990). and so this is by no means the only reaction occurring in actual fly-ashes for two reasons: (1) it does not account for the presence of sulfur trioxide or iron oxide. Pozzolanic fly-ashes are classified by the ASTM into Class F. which requires the addition of supplemental alkali in order to undergo a pozzolanic reaction. High-carbon flyashes are also generally not marketed because the carbon they contain interferes with the air-entraining chemicals that are added to concrete. S ¼ SiO2. a variety of other phases also form. intrinsic fly-ash characteristics. Their high sulfate content would cause delayed expansion and cracking of concrete. the reaction between high-purity aluminosilicate fly-ash and water would be as follows (using cement chemistry notation where C ¼ CaO. ettringite (C3A3C SˆH32). While ASTM does not include either fluidized-bed . With real fly-ashes. Use of fly-ashes as binders for iron ore pellets is a significantly different application for fly-ashes. 5% fly-ash and no calcium chloride. the results for the dried pellets were very similar to those for the unslaked ash.2 N=pellet (5. which made the fly-ash a more effective binder. C. pellets made with only 1% fly-ash and added calcium chloride were stronger than pellets made with 1. Pellets with dry crushing strengths greater than 22.2 N=pellet (the industry requirement) could be readily produced with this ash (Eisele et al. depending on their calcium content. . 1998). however. The calcium chloride acted as an accelerator for the hardening reactions in the fly-ash.0 lbs force=pellet). Addition of calcium chloride to fluidized-bed combustor (FBC) ash increased the strengths of pellets dried at 105 C at all ash dosages. KAWATRA fly-ashes or high-carbon fly-ashes in their classification system.62 T. these ashes vary similarly in their pozzolanic behavior. The results given in Figure 14 show that pellets can be made with high-calcium fluidized-bed fly-ash binders and that the pellets can reach compressive strengths greater than 22. Figure 14. and so do not require additional CaO to be added. K. When the fly-ash was slaked with water before it was added to the magnetite. the strength of the dried pellets could be considerably increased by adding calcium chloride to the fly-ash. EISELE AND S. Preslaking of the ash produced results comparable to those obtained using unslaked ash. Fluid-bed fly-ashes have a very high calcium content. which meets the industrial requirement for dried pellet strength. As can be seen from Figure 14. but many high-carbon fly-ashes require supplemental CaO before they will exhibit binding properties. In tests 1
8 with ‘‘unslaked’’ ash.94 0.0 0. 1995).50 1. 1998).2 2.26 0. Min et al. the water was added to the fly-ash to form a slurry and hydrate the ash before it was added to the magnetite (Eisele et al.9 28.00 1. the lime will expand and cause pellet cracking (Boynton 1966.00 1.2 0.16 0.5 2.1 19.31 0. This is believed to be due to the presence of reactive calcium compounds in the slag.31 0. An example of this can be seen in Figure 15.66 1.4 20.0 30. 10. as can be seen from Table 27.66 1.5 26.66 1.0 2.50 Dry crush strengths. and it can be used directly as a pellet binder without the need for added calcium compounds.3 2.7 2.16 0. Test conditions and results using fluidized-bed combustor ash as pellet binder Test number 1 2 3 4 5 6 7 8 9 10 11 Fly-ash dosage (% of magnetite weight) 0. N=(pellet) Calcium chloride dosage (% of magnetite weight) Water dosage (% of magnetite weight) Mean P95 0.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 63 It has been determined that in at least one application. .6 1. where the pellets made with 1. the ash was added to the magnetite as a dry powder and extra water was not added to the fly-ash=magnetite mixture until immediately before the high-energy mixing stage.3 1. which undergo a pozzolanic reaction with the fly-ash. and 11 with ‘‘slaked’’ ash. In tests 9.7 32.2 9.9 27. Fluidized-bed combustor ash contains unhydrated lime (CaO) that reacts with water.50 17.8 1.94 0.2 21.66 1. if this unhydrated CaO does not hydrate completely before the pellets are dried or fired.50 1. Kawatra and Ripke 2002b). however.94% fly-ash and no calcium chloride were considerably weaker than the other pellets after Table 27. The reactive calcium does not have any corresponding beneficial effects on bentonite.7 23.10 0.8 31.0 0.00 1.0 All dosages are given as percentage of the total magnetite weight.3 1.00 1.26 0.10 0. pelletization of high-iron steel-making slags for recycling fly-ash is actually superior to bentonite for development of high pellet strengths (Ripke and Kawatra 2002.2 1. Chatterji 1995. as can be seen from Figure 15. Upon firing at 1200 C. the sintered pellets produced using FBC ash were all at or above 1800 N=pellet crushing strength. EISELE AND S. K. A better solution appears to be to add calcium chloride to the fly-ash. . a small tendency for a change in strength with changing carbon contents was observed. which can be seen in Figure 17. Examination of these pellets showed that they had cracked severely during the firing process. The effects of fly-ash carbon content on pelletization was determined by producing mixtures of several fly-ashes with different carbon contents but otherwise similar compositions. if anything. there was. As the carbon content of the ash was increased.11%. which not only increases the strength of the dried pellets. Addition of calcium chloride prevented the pellets from cracking (Eisele et al. The carbon content of the fly-ash was varied over a wide range. up to a maximum of 11.5% of the magnetite weight. This problem can be prevented by adding water to the fly-ash to ‘‘slake’’ it before adding it to the magnetite. the pellets made using slaked ashes tended to be weaker after firing. at higher ash dosages. but also prevents the pellets from cracking upon firing. 1998). KAWATRA Figure 15. however. a slight increase in the strength of the pellets dried at 105 C. however. C. firing. as shown in Figure 16. pellets made with unslaked ash and no calcium chloride showed severe cracking upon firing. although any change was so small that it is not statistically significant. At ash dosages below 1.64 T. leading to low strength. The most likely application for fly-ash binders is therefore the production of fluxed pellets. Intermediate levels of carbon content were produced by mixing the fly-ashes with higher and lower carbon contents. Strengths of pellets after drying at 105ºC. Since fly-ash is readily available from power plants located close to iron ore pelletizing operations. which are too high in carbon to be suitable for other applications. however. The dosage of fly-ash is similar to the dosage of bentonite. in combination with calcium hydroxide. which are pellets with lime or limestone added to act as flux in the . and therefore the use of high-carbon ashes as binder will slightly reduce the fuel requirements for the sintering process. The combustion of the carbon during pellet sintering provides heating value. it can be cheaply obtained at a low shipping cost. The change in strength with increasing carbon content is minimal.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 65 Figure 16. using Class F fly-ashes of varying carbon content as binders. It was originally feared that high carbon content in the fly-ash would cause pellets made with fly-ash to weaken or crack during sintering due to combustion of carbon particles. the fly-ash needs calcium oxide or hydroxide additions in order to act as a binder. and the need to add calcium compounds will increase the cost of this type of binder. which is currently used in industrial practice. however.11%. the magnitude of the change observed was comparable to the random variations in fired pellet strength and was therefore not statistically significant. This indicates that iron-ore pelletization can utilize fly-ashes with carbon contents as high as 11. as they simplify the furnace operation and provide better contact between the pellets and the flux. Since the uncertainty limits for the data points are larger than any evident trends in the data. C. . It has a number of very appealing features for use as a pellet binder:  It contributes no silica to the final product. The use of fluxed pellets is becoming more common. blast furnace.  It acts as a silica flux when the pellet is reduced in the blast furnace and improves the performance of the pellet. this is an application that is well suited for fly-ash binders. as limestone deposits suitable for lime production are widespread and readily accessible. KAWATRA Figure 17. Strengths of pellets after firing at 1200 C using Class F ashes of various carbon contents as binders. Because the flux can provide the alkali needed to produce binding properties in fly-ash. the pellet strength is not affected by the carbon content for all practical purposes. K.  It is readily available.66 T. Lime Hydrated lime (Ca(OH)2) will harden into a cement when mixed with water and allowed to dry and recrystallize. EISELE AND S. 4 5. A major limitation on its success is the moisture content of the feed.2 8.0 3.1 8.2 11. Limestone and dolomite added to a pellet give off carbon dioxide at temperatures of 885 C and 796
895 C.7 . 1.1 6.35% CaO. disciplined plant operating procedures. where pellets were produced containing 66. the flux is added as limestone (CaCO3) or dolomite (CaMg(CO3)2). with sintered crushing strengths of 367þ=743 kg (de Souza 1976).0 7.8 10. and only 2. Comparison of fluxed pellets produced with lime and dolomite hydrates.3 5.4 3. particularly by CVRD in Brazil. however. if either of these flux components is calcined to either CaO or CaMgO2 and then hydrated and added to the pellets.4 8.2 3.8 8.1 16.5% moisture in order to achieve satisfactory binding.4 15. which require several percent CaO and=or MgO to be added to the pellet. with similar pellets produced with uncalcined fluxes and bentonite or Peridur binders (Bleifuss and Goetzman 1991) Additive Lime hydrate Calcined dolomite hydrate Lime hydrate þ calcined dolomite hydrate Bentonite þ uncalcined flux Peridur þ uncalcined flux % moist. The use of hydrated lime as binder is most attractive when fluxed pellets are being produced.6 3. neither of which have any appreciable binding properties. however.67% Fe. then they can be quite effective binders.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 67 Hydrated lime has been used successfully as a binder in a few cases. mainly because it requires precise control of the physical properties of the concentrate and strict. Unfortunately. which takes up energy being supplied to preheat the pellets.6 8. Wet drop no.5 15. Comparative results for pellets using these calcined binders=fluxes and for pellets using either bentonite or Peridur binder with uncalcined fluxes are given in Table 28 (Bleifuss and Goetzman 1991). In traditional fluxed pellets.6 4. An additional benefit of using lime hydrate and calcined dolomite hydrate as flux is that potentially less energy has to be devoted to preheating the pellets. the carbon dioxide is already removed from the fluxes and energy loss due Table 28. respectively. Wet strength Dry strength 7.06% SiO2. If the fluxes are calcined before they are ever added to the pellet. no dosage levels for fluxes and binders were given for these data. Use of this binder has not become more widespread. which must be kept down to 8%
8. until at 4% dosage the hardening can become almost immediate (Taylor 1990). adding 2% calcium chloride to cement will shorten the time for initial set from 3 hours to only 1 hour and double the 1-day compressive strength. it should be kept in mind that as the lime hydrates. The main drawback to using lime hydrate or calcined dolomite hydrate is that. They will therefore have to install their own calcination and hydration plants on site. KAWATRA to calcination during pelletization is reduced. which tends to make the technique much less attractive economically. although CaCl2 is one of the most effective. but for pelletization. Higher dosages increase the setting rate still further. it hydrates and works similarly to hydrated lime. There is also concern about the quality of mixing of the flux with the ore. it is not practical to buy the material precalcined. for most iron ore producers. Unhydrated lime can also be added as a binder. Accelerators One of the greatest problems with using cementitious binders is that they require some time to come to their full strength. This is not a problem for normal cement applications. it also expands. these effects are not of particular concern for pelletization. K. Experience has shown that if more than about 5% CaO is added to the ore and pellets are made before the lime hydrates properly. the expansion of the lime during hydration will cause the pellets to crack or disintegrate (Holley 1985). The mode of action of calcium chloride accelerator is that it increases the rate of the hydration reactions. the hardening should be made very rapid. and so this mixing method would not be appropriate if these binders were used. Ordinary Portland cement requires many hours to harden and takes up to 28 days to reach its full strength. which require that the cement harden slowly enough for it to be transported and poured. C. A great many salts have effects qualitatively similar to those of calcium chloride. however. Typically.68 T. EISELE AND S. The fine particle size of calcined lime or calcined dolomite would be likely to interfere with filtration. Thorough mixing is critical and is currently achieved by mixing ground flux stone with the iron ore concentrate in the slurry tanks before filtration (Ripke 2003). Calcium chloride has long been known to accelerate the setting and hardening of ordinary Portland cement. Both the cation and the anion of the salt have an . When mixed with the moist ore. While high-calcium chloride dosages can have some undesirable effects on the property of structural concrete. binding the mineral grains together. high cost. with the exact ratio for a given application determined by experiment. In the case of calcium salts in general. will delay the loss of water during the pellet drying and preheating stage. the effectiveness of a salt as an accelerator tends to increase with increasing charge and decreasing ionic diameter.
Br
ffi Cl
> SCN
> I
> NO
3 > ClO4 : In general. and so the curing of the pellets should take place at ambient temperatures. such as CaCl2. The ratio of molasses to lime is typically between 2:1 and 4:1. Salts If a soluble salt is dissolved in the water contained in a pellet. The reaction that hardens the binder is the production of calcium sucrate. or pellet contamination problems. Pellets produced using this binder are quite weak initially but cure to their full strength in approximately 1 hour. the increased availability of Ca2þ ions in solution contributes to the increased reaction rate. including general effects of ions on solubility. OTHER BINDERS A number of miscellaneous materials have been suggested as binders but often are not followed up to any great extent due to poor performance. with the relative effects of a number of ions as follows: Ca2þ > Sr2þ > Ba2þ > Liþ > Kþ > Naþ ffi Csþ > Rbþ . allowing the hardening of the cementitious binder to proceed more completely and at higher temperatures than would be practical otherwise. There probably are several mechanisms involved in the acceleration effect. If . which is a reliable binder for agglomerating many ores (Holley 1982). The most common example of this is the combination of lime and molasses. An added benefit for pelletization is that strongly hygroscopic salts.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 69 effect on the acceleration of the reactions. Partially Organic Cements There are a few types of binder that consist of a combination of inorganic and organic materials that react with each other to harden. which is slowed at high temperatures. then the salt will crystallize as the water dries. Increasing temperature increases the binding strength of the pellets up to about 150 C.7 4.1 1. Solutions of ferrous sulfate (FeSO4.7 2.05% FeSO4 0.89) (6. K. they will obviously not be able to supply any water absorbency for control of pellet moisture content.2 1. such as ‘‘pickle liquor’’ from steel treatment. EISELE AND S.1 (5.14% FeSO4 0.34) (6.8 7.56) (9. the urea burns away and its binding strength is lost. At higher temperatures.78) (11.55% FeSO4 0. producing a hard shell that gives good abrasion resistance.5 16. strength.55% FeSO4 2.12) (5.0 1.5 1. Because it is soluble. it contributes no strength until the pellet is dried.1 0.4 2.34) Wet drop no.6 8.8 (4.6% bentonite Plant 2 Wet comp. In general.12) (7. Precipitates As with the salts. it has been found that a large fraction of the salt crystallizes on the pellet surface. allowing the urea to solidify. a lack of salt crystals in the core of the pellet will lead to low crushing strengths.22) (11. however.0 50 Dry comp.4 2.6 28.6 1. KAWATRA Table 29.35) (40.73) no binders other than the salt are used.0) (21.70 T. as can be seen in Figure 18 (Goksel and Ozturk 1985). lbs (N) 1.8 9. Since waste ferrous sulfate liquors are in a liquid form. C.6 8. Situations where ferrous sulfate would be attractive as a binder would be when it is readily available as a waste product.6% bentonite None 0. Urea (CO(NH2)2) is a highly water-soluble organic material that has been considered as a binder.23) (12.48) (2.) have been tested as binders. lbs (N) 1.2 2. 5. other soluble materials can be added to the moist iron ore concentrate that will precipitate out of solution as the pellets are dried. as is .23) (9. strength. Results of laboratory balling tests with ferrous sulfate solutions (Nora 1966) Ore source Binder type and dosage Plant 1 None 0.12) (74. with the results shown in Table 29.34) (7.2 1.5 2.5 7. the ferrous sulfate is inferior to bentonite at comparable dosages. The amount of salt crystallizing in the core can be increased by adding bentonite or other materials to slow the drying rate (Ayers 1976). Compressive strength of urea-bonded pellets at different temperatures (after Goksel and Ozturk 1985). rather. Sodium silicates are a generic class of materials. Inorganic Polymers Inorganic polymers are complex inorganic molecules that can form chains and cross-linked networks that act as binders. typical for organic binders.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 71 Figure 18. The result is a range of silicic acids that are soluble in water. The iron powder then corrodes on contact with air and water during the balling process. Because urea burns away completely. The most common examples of inorganic polymers are sodium silicates. it is even less effective at high temperatures than many of the other available organic binders. which are synthesized by melting quartz sand with a soda-ash flux and then dissolving the resulting molten glass with high-pressure water. leaving behind no carbon deposits. A variation of this is to add iron powder to the iron oxide concentrate. It has been reported that this method can give green ball strengths of almost 15 kg per pellet (147 N=pellet) (Xi-Lun 1983). producing iron hydroxide precipitates that can bond the pellets. their compositions and properties vary depending on the ratio of SiO2 to . Sodium silicates are not a specific composition. The greatest potential for cold-bonding is with . KAWATRA Na2O.’’ whereas smaller ratios (higher quantities of Na2O) are referred to as ‘‘alkaline. As heat is applied. with the understanding that the ratios between the components can vary greatly. In carbon dioxide curing. K. EISELE AND S. The tensile strengths of foundry sand cores bonded with sodium silicates and cured by various methods are shown in Table 30. and silicic acids (Owusu 1982). Sodium silicates can also be polymerized by heating to dehydrate them. in a manner similar to that shown below: : The polysilicic acid polymer then acts as a binder. Sodium silicates are commonly used as binders for foundry sands. the idealized reaction is: Na2 O  SiO2 H2 O þ CO2 þ H2 O¼)SiðOHÞ4 þ Na2 CO3 where the Si(OH)4 undergoes polymerization to form polysilicic acid. an application that has many similarities to iron ore pelletization. the bound water is released and the material cross-links into a glassy form that is very strong. Cold-Bond Processes The sintering step is one of the most expensive and energy-intensive stages of ore pelletization. sodium silicate.72 T. assisted by crystallization of sodium carbonate. therefore substituting a final hardening process that works at a much lower temperature could potentially reduce pelletizing costs a great deal. They act as binders through their ability to form a bond consisting of precipitated silica gel.’’ Sodium silicates are commonly referred to with the formula Na2OSiO2xH2O. A weight ratio of SiO2:Na2O of 3. C.22 is referred to as ‘‘neutral. with the polymerization produced either by heating or by exposure to carbon dioxide. Curing of the sodium silicate is a result of polymerization of the polysilicic acid. 22 3. similar processes (Lotosh 1973. which both increases the binder cost and can increase contamination of the pellets by silica.58 3. 1992). More recently. such as granulated blast furnace slag. The bedding keeps the pellets from being deformed while they are still soft and is separated from the finished pellets by screening (Linder and Thulin 1973). can also be used (Dutta et al. uses cement or a similar binder. The cost of the storage volume generally makes it uneconomical. cement-bonded pellets historically have required approximately 10% binder to harden sufficiently. the Grangcold process (Svensson 1969). including the RRL-Jorhat process (Iyengar et al. which can be up to 28 days to develop full strength or 5
7 days with steam curing (Dutta et al. Effects of curing process on sodium silicate bond strength in foundry sand Sodium silicate type (SiO2=Na2O ratio) 2. 1968).4 2.) þ hot air curing (150 C=5 min. 1992).) 60 50 50 10 0 (413) (345) (345) (69) (0) CO2 curing (2
5 sec. Other types of binder.) 360 340 340 320 290 (2482) (2344) (2344) (2206) (2000) Hot air curing (150 C=5 min) 400 370 360 350 300 (2758) (2551) (2482) (2413) (2068) CO2 curing (10
15 sec. in pounds=square inch (kilopascals). which generally is representative of these processes. Several processes have been developed that use ordinary Portland cement as the binder. added in sufficient quantity that the finished pellets do not need to be fired.5% of the dry weight of the sand. cementitious binders because the cementing reaction proceeds at near room temperature and can reach strengths comparable to sintered pellets with sufficient time or proper processing. The main disadvantage of room-temperature cement bonding is that a considerable volume of storage is needed to provide the necessary curing time. The main innovation in this process is that the wet pellets are bedded in a fine material while they are curing. and several other. The Grangcold method.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 73 Table 30. Also. it has been found that increasing the .) 210 210 50 0 0 (1448) (1448) (345) (0) (0) Sodium silicate dosage was 1. Lotosh and Efimov 1973).0 2. Values given are tensile strengths.75 Microwave curing (6 Kw=5 min. a process that requires hours or days. and so it was not necessary to add supplemental silica. Organic binders could also be added. This resulted in partial dissolution of the silica and lime. KAWATRA surface area of the cement can reduce the binder requirement to only 4%
6% and that the strength of the finished pellets could be increased by addition of amorphous silica. without burning out during hardening. Prior to development of the PTC process. and 100
200 kg=pellet (981
1962 N=pellet) after hardening for 28 days (Dutta et al. Properties of pellets made by this process are shown in Table 31. which precipitated as the calcium silicate hydrates xonotlite. along with the carbonaceous material to be used as a reducing agent. Pellets produced by this means had crushing strengths of 14
18 kg=pellet (137
176 N=pellet) after 1 day.74 T. The basic PTC process used lime at a dosage up to 7% and silica at up to 3% as the main binder components. 1997). The Pellet Technology Corporation (PTC) Process was developed to attempt to take advantage of this and was tested in a 10 ton=hour pilot plant (Goksel et al. This process is no longer being pursued. tobermorite. K. BINDER COMBINATIONS In addition to binders used alone. and similar phases that acted as binders. C. One of the potential advantages of a cold-bond process is the ability to combine the ore concentrate with fluxes and carbonaceous material as a reducing agent and form them all into an integrated pellet that can be readily reduced. which required a process that would be economical on a smaller scale than the typical iron-ore pelletizing operation. This technique was developed for pelletization of chromite fines. 1987). apparently due to mechanical and cost problems associated with the autoclaving step that made it uncompetitive. The process used the silica naturally present in the ore as the siliceous binder component. grolite. EISELE AND S. Pellets in the PTC process were formed by normal methods and were then hardened by autoclave treatment at 300 psig (2068 KPa) and 390 F (200 C) for approximately 1 hour. Avoidance of a high-temperature hardening phase allows the carbonaceous material to remain in the pellets while they are being made. the very similar COBO process had been developed (Doughty 1975). there has been some investigation of the effects of mixing binders of different types. such as clay binders with . 97 15.58 37. Because the use of a mixture of binders would require the addition of storage and dispensing facilities for the second binder. however.0 (151) 563 (2504) 26.0 5. no.95% bentonite 9.2) 17.56 1.8 — — 8.00 1.20 20.00 16% Coke fines 7.00 18% Coke fines 16.00 1.00 1. and all binder combinations tested to date either have properties intermediate between the two binders or perform more poorly than either binder alone.84% Coke fines — — 0. lbs (N) 5.3 (152) 23.95 2.1 (76.00 3.00 16% Coke fines 8.00 16% Coke fines 7.0 (102) 18.00 — 6. Usually this is done in the hopes that there will be some synergetic effect making the mixture stronger than either binder alone.6 (91.0 (191) 718 (3194) 34. Results for a combination of Allied Colloids FE4 organic binder with bentonite are given in Table 32.00 3.0 (249) 758 (3371) 43.30 >25 7.83 >25 5.2 (76.0 (120) 959 (4265) 55.7 (83.5) 20. For example.25 16. such as bentonite.5 Magnetite 4.1) 18.2 (116) 493 (2193) — 516 (2295) organic binders or clay binders with pozzolanic binders.5 7.0 Pyrite cinder 8.6) 17.7 (136) 880 (3914) 27.8 (83. to improve the high temperature performance.00 — 6.0 5. Properties of pellets made using the PTC hydrothermal bonding process (Goksel et al. such a synergetic effect has yet to be seen.9 (249) 696 (3096) 56.00 1.80 11.6) 26. .80 18. One reason for a combination of binders is to attempt to remedy the deficiencies of one binder by addition of another.3 (108) 617 (2744) 30.9 (39.6) — Dry strength lbs (N) Hardened strength lbs (N) 22.8 (119) 34.40 >25 5.8) 413 (1837) 24.00 — 5. the effects of the tendency of organic binders to burn away before the pellet sinters can be reduced by adding a noncombustible binder. these mixtures would have to have a significant benefit before it would be worthwhile to use them.0 (97.00 3.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 75 Table 31.00 — 6.16 11. 1987) Concentrate % % type CaO SiO2 Hematite Additives Wet Wet % drop strength moist. 8) 10.8 (25. wet drop (N=pellet) (N=pellet) 18.5 (37.6 (11.16 18.4 2.0 (0. This effect can be seen in Figure 19.3 (41. I.3 (28.2) 9.7) 9.80) 0.8 16.036) 10.0 (0.13%) could be added as a bentonite extender without compromising the pellet quality.7 (12.4 (10.8 (0. C.4 2.4 (10.36) 0. Drilling Fed.7 (34.2 2.36 11.0 (0) M.4 12. fly-ash does not exhibit any binding properties if no alkali is present. where the pozzolanic binder is fly-ash.7 (12.2 (27.0 (0.39 8. I.2) 5.0 (0.31 11. the strength of pellets made using any of the bentonite=organic combinations are lower than for the pellets bonded with bentonite alone.0 (0) Wyo-Ben 8.0 (0) bentonites 8.7 (12. results of such combinations are shown in Table 33. This was true for both Class C fly-ash and Class F fly-ash.6) 10. Wet comp. Tests conducted by industrial personnel indicated that addition of up to 3 lbs of fly-ash per long ton of concentrate (0.6 17.3 2.8 (0.6) 5. which can act as sources of soluble calcium (in the form of Ca(OH)2 and CaCl2).4 (24. Bentonite binders have been tested in combination with both Class F and Class C fly-ash. Mixing bentonite clay binder with pozzolanic binders.78 10.0) 2. Benefits of using fly-ash .0) 2.0) 2.7 (12.8 (0.8) 6.6) 7.8 (30.036) (PWA ¼ 820) 18. and so in these experiments the strengths of the pellets generally decreased as the fly-ash dosage was increased. Tacbond 850 8.1) 6.09 10.4) 7.7) 8.8) 6.036) Sample A (PWA ¼ 663) 18.8 (0.0) 2. KAWATRA Table 32.81 10.80) 0.36) 0.0) 2. dosage dosage (45.0 (0. In general. Dry comp.4 (10.36) 0. EISELE AND S.7) 7.7 (12.1 (31.0) 10.36) 0. K.80) 0.76 T.80) 0.21 10.0 (0) Wyo-Ben 8.036) Sample D (PWA ¼ 854) 18. Properties of pellets made using mixtures of Allied Colloids FE4 organic binder with bentonite from two sources Bentonite source Bentonite FE4 18 in.0 (0.8 2.036) (PWA ¼ 723) Averages. causes the binding properties to degrade because the sodium bentonite is being converted into calcium bentonite (Ripke and Kawatra 2000b).16 10.0 15. Tacbond 650 8. all 18.7 cm) lbs=pellet lbs=pellet lbs=ton (%) lbs=ton (%) % moist.0 (0) M.0 (0.2 (9.0) It can be seen that.9 (35.0 (0.80) 0.8 (0.5 15. unfortunately. without any supplemental calcium or other alkalis added to activate the pozzolanic properties of the fly-ash.59 7.0 (0.0 (0. Drilling Fed.36) 0. Benefits to the fly-ash producer include reduced disposal cost and a reduction of necessary landfill capacity. In contrast.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 77 Figure 19. Bentonite is still the dominant binder for this application. savings of bentonite by using the ash as an extender. CONCLUSIONS AND RECOMMENDATIONS A wide variety of binders have been considered for use as binders for iron ore pellets. Addition of soluble calcium from the Ca(OH)2 drastically reduced the strength of pellets produced with bentonite. and use of ash as an absorbent of excess water when the concentrate is very wet. however. Effect of calcium hydroxide addition on the binding performance of bentonite and bentonite=fly-ash mixtures. the effectiveness of the fly-ash as a binder was markedly improved by the addition of calcium hydroxide. there are a number of potential avenues for further study that . along with the bentonite were recovery of the fuel value of unburned carbon remaining in the fly-ash. showing that the mechanisms of the two types of binder are fundamentally incompatible (Ripke and Kawatra 2000b). as no other readily available binder has yet been able to equal its combination of effectiveness and low cost for such a broad variety of ore types. Both industries can benefit from the ‘‘green’’ reputation they could gain by utilization of this product. apparently due to conversion of sodium bentonite to calcium bentonite. 36) 0 0 8 (0.8 1.67) 18 (0.54) 16 (0.7 0.67) 0 0 15 (0.8) (9.4) (6.8) 18 (0.65 9.5 (0.8) 18 (0.1 8. lbs=pellet lbs=pellet % moist.6 3.38 11.5 (0.3 5.0 3.8) 4.8) 0 6 (0.8 1.2) (19.0) (7.67) 0 3 (0.6) (7.0) (7.4 2.9) (9. Laboratory bench balling tests with magnetite concentrate and mixtures of bentonite with Class C and Class F fly-ashes Binder dosage.6 4.67) 0 3 (0.9) (19.2) (8.7 3.95 10.8) 15 (0.54) 0 6 (0.96 10.67) 3 (0.29 10.54) 0 9 (0.9 4.0 (21.9 2.9 4.0 2.0 1.3) (4. It appears that the mechanism is different from the simple dispersion mechanism that has been assumed in the past.1 2.1 5. lbs=long ton (%) Bentonite Class C fly-ash Class F fly-ash 18 (0.1 1.33 10.3) (4.3) (9.7) (9.76) 0 1 (0.71) 8 (0.134) 0 12 (0.3) (27.1) (30.8) 18 (0.2 6.09) 0 18 (0.9 1.7) (9.5 3. Study is therefore needed to determine whether modifications to the ore-bentonite mixing step and to the pelletization .8) (20.8) 12 (0.05 10.27) 12 (0.8 7.1 1.78 11.27) 6 (0.0) (7.4 2.0) (4.5 2.045) 0 15 (0.5 8.04 10.7 cm) (N=pellet) (N=pellet) 10.09) 18 (0.8 4.8) 0 0 12 (0.93 11.0) (21.0) (5.8 5.39 10.76) 1 (0.1) (23.8) 18 (0.4 1.8 1.2) (26. EISELE AND S.8) 18 (0.78 T.8) 18 (0.5) (31.1 2.8) 0 0 17 (0.54) 6 (0.045) 15 (0.27) 0 12 (0.1) (22.18) 2 (0.51 10.2 2.8) 18 (0.3 3.1 1.8 7.2 2.7 9.4 1.0 6.8 1.1 1.3 3.54) 0 8 (0.9 0.3) (8.42 9.05 9.3 8.10 10.8) 18 (0.72 10.0) (9.99 11.0) (3.50 10.3) (8.1 6.6) (9.6) (16.8) 18 (0.8) 18 (0.7) (8.4) (9.1) (8.1 6.36) 8 (0. with the bentonite instead forming a fibrous structure through the pellet.6) (8.90 6.1 4.0 3. K.2 0.1 2.9 1.8 7. (45.134) 15 (0.40) 9 (0.7 4.8) 0 0 0 18 (0.78 10.7 1.1 1.6 1.1 2.8 2.27) 17 (0.067) Total binder 18 (0.74) Dry Wet Wet crush crush drop Green ball 18 in. KAWATRA Table 33.0) (8.40) 0 12 (0. the physical mechanisms of bentonite binding in pellets have not been investigated thoroughly.134) 12 (0.8) 16.9 0.60 9.54 9.6 1.67) 0 1.9 6.2) will be of considerable benefit to the iron ore industry in reducing their binder costs and increasing binder effectiveness:  To date.8 0.9 1.4) (4.3 3.79 9.04 10.0) (7.9 5.36) 4 (0.8) 0 0 15 (0.18) 0 0 4 (0.0 5. C.27) 0 18 (0.9) (4.3) (10.0 6.8) 18 (0.9) (5.3) (8.4 5.9) (4.6) (4.18) 4 (0.18) 0 0 2 (0.0 8.7 1.36) 8 (0.36) 0 4 (0.54) 6 (0.1) (6.8) 18 (0.2 (10.8) 18 (0.54) 0 0 18 (0.9) (8. with the advantage that they would contribute less silica and more iron to the finished pellet. Because these materials are pozzolanic. These clays could potentially perform as well as the bentonite clays. A broad range of organic binders. cold-bond processes in their present form are not well suited for large-scale pellet production due to . Certain low-cost industrial wastes. and so it would be valuable to determine whether addition of fines. however. unlike bentonite binders. have been found to work as binders. which act very rapidly. These problems are likely to be solvable with better understanding of the properties of these binders. Aside from bentonite. there are a broad range of chemically bonding and cement-type binders that could be used to cold-bond pellets.A REVIEW OF BINDERS IN IRON ORE PELLETIZATION      79 procedure can take better advantage of this mechanism to improve bentonite effectiveness and reduce the necessary dosage. Binding effectiveness is improved when more ultrafine particles are present. Of particular interest are high-iron expanding clays. improved quality control. and as a result they have found some use in plants where meeting silica specifications is difficult. resulting in pellet breakage and dust problems. Their main drawbacks are cost for the synthetic binders. and therefore it is not practical to use mixtures of these two binder types. which are used to hold pellets together until they can be hardened by sintering. such as nontronite clay. other types of clays have shown some promise as binders. and addition of high-iron slag-bonding materials to reduce the problems with binder burnoff. such as coal fly-ash. Their main advantage is that they do not contribute silica to the finished pellet. either of natural origin or produced from the ore itself. they require a period of time to develop their full binding strength. ranging from synthetic chemicals to low-value waste products. poor quality control for the waste-product binders. 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