Process Selection and Design for the Palmarejo Silver MineJ.P. Errey ABSTRACT Silver and gold bearing ore from the Palmarejo mine, located within the Mexican state of Chihuahua, contains a complex mix of electrum, native silver, silver sulphides and silver chlorides. This paper presents a case study that describes the selection, development and engineering of a tailored process plant for the recovery of silver and gold from this ore. Feasibility study modelling, metallurgical testwork and engineering studies are outlined. Options considered to optimise the recovery of the contained silver and gold are compared, based upon cost, operability, recovery and engineering aspects. Concentration by flotation was found to be the preferred primary recovery route. Intensive leaching of the produced concentrate resulted in solution grades sufficient to allow direct electrowinning to be evaluated against the more traditional Merrill Crowe process. However a flotation tail of sufficiently low economic value was unable to be produced requiring the addition of stages to allow metal extraction from this stream. The processing flow sheet, which is designed to maximise the recovery from both the flotation concentrate and tailings, and to utilise a common metal production facility, is presented. INTRODUCTION The Palmarejo Silver Gold Project is being developed by Coeur d’Alene Mines through their wholly owned subsidiary, Coeur Mexicanna S.A. de C.V. The project consists of an open pit and underground mine along with associated milling and processing facilities with a Run of Mine (ROM) throughput of 2,000,000 tonnes per annum. The project is located 265 km southwest of Chihuahua, in the southwest corner of Chihuahua State deep in the Sierra Madre Occidental Mountain Range of Mexico. It is 420 km by road from Chihuahua City. Most of the route is along the sealed Highway 127 to the township of Creel, followed by a gravel road to Palmarejo via Temoris. The Chihuahua-Pacifico rail service operates between Chihuahua and Los Mochis on the south west coast. The railway passes through the town on Temoris, which is 35 km from Palmarejo by the government maintained gravel road. This road is an extension of Highway 127 and continues through to the town of Chinipas. Airstrips for light aircraft are located at Temoris and Chinipas. A map showing the location of the Palmerjo project relative to other projects in the region is shown in Figure 1. Mining operations commenced in Palmarejo in 1818. Later in the 19th century the British constructed a railway between Palmarejo and Chinipas for the transportation of concentrate from the Palmarejo mine. Both the British mining and railway operations ceased in 1910, around the time of the Mexican revolution. Mineralogy was completed on five drill hole head samples and on one concentrate sample produced from the master composite. native silver and copper-silver sulphides.Ca)Mn3O7·3H2O). Electrometals Technology Limited provided the direct electrowinning technology via the use of their EMEW powder cells. . The structures are filled by a quartz-vein breccia unit as a result of several phases of hydrothermal activity. Analysis identified that the contained silver predominantly occurred as either electrum or silver sulphide (acanthite) with sparodic occurrences of aurorite (Mn2+. La Prieta trends WNW-ESE and dips 48 degrees to the SW. Intermet Engineering Limited commenced metallurgical testwork supervision in early 2005. This paper describes the process selection and engineering of a process plant with the emphasis on treating high sulphide silver ores by utilising direct electrowinning. Gold occurs mainly as electrum. Testwork carried out has concentrated on drill holes that have intersected either the La Prieta or La Blanca structures or both. this mill last operated in 1989. Intense stockwork has developed within the rifted block at the intersection of the two faults. The veining within the La Prieta structure is considered the richer due to a high content in silver represented by bands of fine grained black sulphides (acanthite and galena). Samples were combined to create master composites from a number of different drill holes to include both structures as well as testing the variability of individual holes. with engineering of the process plant beginning in late 2005.Ag. Engineering was undertaken in parallel with ongoing exploration drilling and project development and as such resulted in changes to the initial design plant throughput during 2006. The La Blanca structure trends NW-SE and dips 60 degrees to the SW. PALMAREJO MINERALISATION Mineralisation at the Palmarejo Project occurs at the intersection of the 'La Prieta' (the black one) and 'La Blanca' (the white one) structures. No modern exploration programs were undertaken in the district until drilling commenced in late 2003. This is similar to the structural setting and controls on mineralisation at other deposits in the Sierra Madre Occidental Mountains. The majority of earlier underground mining was concentrated within the La Prieta structure. A small milling facility capable of processing 120 tonnes per day was utilised to produce flotation concentrate. The La Prieta and La Blanca veins occur as polymetallic Au-Ag vein/breccias with higher grade epithermal Au-Ag overprint that adds to the formation of ore shoots within steeper dipping portions of a normal (listric) fault.Small scale mining was intermittent throughout the 20th century with more continuous efforts made in the 1980’s. Direct Electrowinning Technology versus Merrill Crowe Technology . covellite. resulting in poor recoveries in typical cyanide leaching practices. Comminution testwork identified three main rock types to be treated. Despite devoting considerable effort in refining the flotation techniques. particularly when treating oxide ores. the required leaching conditions for the two separate flotation streams are different. requiring the installation of two leaching circuits. greatly reducing the project capital cost. This ensured adequate information was available for the comminution circuit design. chalcocite. Flotation Concentrate Sale Preliminary terms for the sale of flotation concentrate were discussed with existing smelters in Mexico. chalcopyrite. Pretreatment of the ore indicated high recoveries could be achieved from flotation. However. and • direct electrowinning recovery versus Merrill Crowe recovery. Flotation Concentrate Leaching Only Treatment of a flotation concentrate at site had the advantage of requiring a small leaching and recovery facility. • leaching of both flotation concentrate and tailings streams. allowing models for each type to be established. as it relied heavily on the flotation response of the ore. These terms suggested downstream treatment of the flotation concentrate at site would be more economical. treatment of flotation tailings was also economic and as such the option to sell concentrate was not pursued further. Each rock type has been tested separately. offering the highest recovery from the resource and ultimately the greatest return to the project. A number of treatment options for the resulting flotation concentrate were investigated including: • sale of flotation concentrate. namely quartz vein breccia. amygdaloidal andersite and footwall sediments. being able to cope with variations in flotation response. Leaching of Both Flotation Concentrate and Tailings Streams Leaching of both flotation streams offered the most robust solution. sphalerite. • leaching of flotation concentrate only at site. producing a high grade concentrate. Additionally. Subsequent risk analysis indicated this route was a high risk.Mineralogical analysis of the flotation concentrate confirmed that the sample is predominately a pyrite concentrate with other base metal sulphides including galena. TREATMENT OPTIONS Palmarejo ore contains significant amounts of silver sulphide and electrum which are slow leaching. bornite and marcasite. the flotation route was unable to produce a tailings stream containing grades that were uneconomic to leach. between 2003 and 2005. • crushing work index. • autogenous media competency. and • the availability of potential second hand mills. • ore SG. an engineering study was completed to compare direct electrowinning versus Merrill Crowe process options.The use of direct electrowinning to treat solutions produced from the leaching circuits was considered early in the project engineering phase. There was a significant operating cost saving with direct electrowinning. • ball mill work index. or • three stage crushing and ball milling. with the direct electrowinning option having a slightly lower cost. The top ball size and charge levels for the SAG mill is anticipated to eliminate any likelihood of critical size build up in the mill. However. • rod mill work index. • SABC circuit. The study involved developing comparative nominal capital and operating cost estimates. and • abrasion index. Capital costs were very similar. Electrometals Technology Limited (Electrometals) was involved in early discussions to determine the treatment options and direction for testwork programs. . METALLURGICAL TESTWORK AND PROCESS DESIGN Six testwork campaigns were conducted over a two year period. with the estimated operating cost approximately half that for a Merrill Crowe circuit. • JK drop weight test parameters. Selection of the comminution circuit for the plant considered the following: • capital and operating costs. SAG. These campaigns included standard bench scale testing through to two pilot plant campaigns. ball and crushing milling circuit. • risk of critical size build-up in the SAG mill circuit with some ores. • possibility of future plant capacity expansion. A summary of the results is given below. Comminution Comminution testwork was undertaken on samples from all ore types and comprised: • unconfined compressive strength tests. A summary of the comminution data is shown in Table 1. Readers should note that these were comparative costs and do not represent current market costs. Single stage crushing followed by an open circuit SAG mill and a ball mill closed by hydrocyclones was selected as the preferred circuit configuration. provision was made in the plant layout for a recycle crusher should it be required. As testwork progressed and plant selection and sizing became available. It was concluded that Palmarejo ores would be amenable to any one of the three circuits: • SAB circuit. • JK SMC testing. semi autogenous grinding (SAG) and ball milling circuit. A master composite sample prepared from three drill holes was used for this test. This pilot produced a high mass pull to concentrate (17. with the resulting leach liquor separated from the solids and sent for electrowinning testing. The addition of cleaner flotation stages indicated that the mass pull could be reduced to between 1. While the addition of cleaning lowered the recovery of both silver and gold. Subsequent testwork campaigns concentrated on optimising the flotation circuit configuration and reagent selection. Early flotation testwork indicated high flotation recoveries into a rougher concentrate with a mass pull to concentrate of between 6% to 10% (average 9. Two pilot plant flotation runs were then conducted.3%. The first was undertaken on four RC drill hole samples.9% and 79. Other tests included the use of gravity separation. the settling properties of the concentrate were found to have significantly improved.6%). The flotation circuit for this run consisted of rougher . followed by flotation and also controlled potential sulphidisation flotation. • flotation followed by leaching of the flotation concentrate and tailings.1% respectively) compared to rougher flotation. The second pilot test was carried out on combined samples from diamond drill hole and bulk underground samples. • gravity concentration followed by leaching of the concentrate and tailings.1% and 5.1% respectively. with no cleaner stage. The flotation circuit included only roughing and scavenging. flotation tests were carried out to try to obtain a ‘throw away tail’ from flotation. (averaging 80. at 25 g/t and 2. The test was carried out for seven cycles and gave a mass pull of 5. The primary objective of the pilot plant run was to produce a flotation concentrate that could be leached. Due to these high recoveries. These tests failed to produce a tail that was of a low enough grade to be discarded. Locked-cycle testing was carried out following the batch flotation testwork to allow optimisation of the circuit configuration and reagent selection and dosage.297 g/t respectively.5%) with recoveries of between 77% to 89% for both silver and gold.3% to concentrate with gold and silver recoveries of 92. i. Flotation testwork then concentrated on the best option for handling the rougher concentrate following cyanide leaching.e.0 and 85. with considerable improvement in obtained grade.2%) with relative low gold and silver grades. (average of 3. A summary of the different process routes tested is provided in Table 2.Flotation Initial testwork programs investigated the different circuit configurations to maximise silver and gold recovery and included: • whole ore leaching. so these approaches were not pursued further. the flotation tailings could be sent straight to the tailings dam. The results indicated that the best recovery was achieved by flotation followed by separate leaching of both product streams. It was found with the early rougher concentrates that a considerable quantity of fine material was present that gave difficulties with settling and filtration. Recoveries were quite variable which is partly due to the head grades treated and varied between 55% and 96. The addition of air to the leaching slurry was seen to have little impact on the cyanide requirement.4% for silver and 94. the average reagent consumption was 19. The leaching of the flotation tailings samples generally yielded lower leach recoveries than achieved from the concentrate samples.9 kg/t cyanide and 0.0% for gold and 91. made up from drill holes 078D. Leaching of the concentrates indicated that high cyanide levels (initially 5%) were required to ensure high silver and gold recoveries. although one variability test on drill hole 078D only achieved concentrate leach recoveries of 44. the solids removed and the clear solution sent for electrowinning testing. cleaner flotation tests. This resulted in a high mass pull of cleaner concentrate which was of low grade. 115D and 125D achieved 97.9% silver recovery on the concentrate leach. This cleaning increased the concentrate grades to 23. This is supported as the master composite.31 kg/t . Additionally. however after a number of hours it became apparent that a circulating load of fine gangue material had built up in the circuit which finally reported to the cleaner concentrate. combined leach recoveries from both the concentrate and tailings were 93. it was decided to re-clean the cleaner concentrate by pumping the concentrate through the cleaner cell again.4 % for gold. however oxygen sparging was found to lower the cyanide concentration requirement to 1%.170 g/t silver.3% for silver.7% for silver if the 078D variability sample is excluded. Overall. This highlighted the need to monitor the circulating load in the cleaner circuit at site. As such a higher cyanide concentration may be required to increase the leaching rate for this sample. the recovery of silver and gold by leaching was in excess of 97%. As the primary objective of the pilot program was to again produce a leached solution for electrowinning testing.7 g/t gold and 3. locked cycle testing and pilot plant tests are given in Table 3. The flotation circuit initially ran well with control samples indicating high grade concentrate and low tailings grade.2% for gold and 87.6%. after the pilot trial was complete. In general.flotation followed by a cleaner stage.59 kg/t lime. For flotation concentrate leaching. Cyanide consumption was seen to vary between 0. the plant design includes options to feed the cleaner tail to different points within the rougher circuit. Cyanide leach Leaching testwork has been carried out on all flotation concentrate and tailings samples resulting from the major testwork campaigns. these averaging 84% for gold and 72. This was a high grade sample and the leach curves from this test indicate that silver leaching was not complete at the end of the test.9% for silver. A summary of the leaching results for the flotation concentrate and tailings is given in Table 4. This concentrate was leached. This was seen to improve to 94. A summary of results for the batch rougher tests. 2 ppm gold. followed by powder production as the silver concentration dropped below 300 ppm. The leach solution contained approximately 1.16 kg/t lime. as well as minimising the workforce required to work in the refinery area.71 kg/t cyanide and 1.33 kg/t and 1. An economic evaluation of the capital cost versus projected operating cost savings from cyanide recovery and detoxification reagent savings was undertaken. Due to the high free cyanide levels contained in the CIL residue. The average combined reagent consumption for the concentrate and tailings leach was 1. followed by dilution of the thickener underflow with tailings return water prior to feeding the detoxification circuit. Electrometals Technologies used the results from the second solution test to size the required electrowinning circuit.64 kg/t. Cyanide consumption varied between 0. The first solution was produced from a rougher concentrate product that was leached with a 5 % cyanide concentration leach in April 2005. The solution produced silver powder for the full range of testing and indicated a higher production rate per cell than the first solution tested. The average cyanide consumption decreased in the tests using oxygen to approximately 10 kg/t. • The cyanide concentration of the leach was at the planned level of 1%. Direct electrowinning Two solutions were produced from the two pilot plant trials that were conducted at the SGS Lakefield Oretest laboratory. Design discharge limits for the CIL residue for WAD cyanide were set at 50 ppm. The second solution produced is expected to be more representative of the full scale leaching circuit product.000 ppm silver and 10. That being: • The concentrate used to produce the second solution was cleaned in the flotation circuit to remove excess gangue material prior to leaching.and 54.2 kg/t. as the process used closely represented the final plant flowsheet. A CIL tailings thickener was proposed to recover some of this solution. . The advantage of being able to use the powder cells is that this style of cell can be automated and the powder can be collected in a closed filter.26 kg/t cyanide and 1. This helps in securing the product. Detoxification The CIL residue will contain high levels of free and weak acid dissociable (WAD) cyanide when compared to plants treating oxide gold ores. Reagent consumption for the flotation tailings leaching averaged 0. The electrowinning test showed silver plate was initially produced at higher solution tenors.21 kg/t lime. The second leach solution was produced from a flotation cleaner sample from the second pilot trial and leached at 1% cyanide in November 2005.3 ppm gold. This solution contained approximately 1.900 ppm silver and 18. the recycling of some of this solution to the cyanide leach circuits was investigated. Feed slurry to the detoxification testwork therefore set at a cyanide concentration of 240 mg/l WAD CN. 150% and 165% of the stoichiometric requirement.0 and metabisulphite additions of 115%. which resulted in a detoxification feed containing 870 mg/l WAD CN and a total CN of 1. • addition of carbon to the leach in the last 4 hours. with two tests at a target density of 40% solids and one at 35% solids. however all three tests decreased the WAD cyanide level below the target value of 10 mg/l. The batch testing was done in two separate phases. This optimisation resulted in the addition of oxygen to the concentrate and a reduction in the cyanide concentration to 1%. Excess clear solution was decanted from the slurry and fresh water added to achieve the density required for detoxification testwork. at 42. The two tests at 40% solids used a metabisulphite target of 150% and 200% of the stoichiometric requirement. cyanide optimisation testwork had been carried out on the flotation concentrate sample. The method included: • leaching a sample of flotation concentrate for 48 hours.100 mg/l. Subsequent to the first phase of testing. with the pH maintained at 9. The results of all of the cyanide destruction testwork are summarised in Table 5. with the aim being to define the conditions required for the semi-continuous tests. These tests reduced the WAD cyanide level to 126 mg/l and as such did not achieve the target level of 10 mg/l. while the test at 35% solids used 150%. • leaching of the combined slurry for 24 hours. Three tests were conducted.5% and 37. It was also felt that a lower slurry pH would aid the process. Cyanide detoxification testwork included six batch tests and two semi-continuous tests. . • combining of the concentrate leach tailings and flotation tailings in a ratio representative of the final circuit. The density of the tests was found to be higher than planned. • addition of carbon to the leach in the last 4 hours. and • removal of carbon at the end of the leach. The testing of this slurry was undertaken at 45% w/w solids. • removal of carbon at the end of the leach.5%. The method used to prepare the sample for detoxification testing was quite involved due to the nature of the circuit design. The second phase of detoxification testing was done using the lower cyanide level in the concentrate leach and also was planned to study the affect of pulp density on cyanide destruction.Cyanide destruction testing evaluated the Inco SO2/air system using 100 kg of master composite sample treated through a 7 kg batch flotation cell. The first phase of batch testing was conducted with conditions simulating a cyanide concentration in the concentrate leach of 5%. The final leach tailings were then flocculated and left to settle overnight. It is believed that this had a significant impact on the effectiveness of the detoxification process. It was noted during the tests that the viscosity of the slurry increased dramatically during testing. Miscellaneous A number of other metallurgical tests were carried out to collect data required for the plant design and as alternative process route. The unit rate for the flotation tailings was found to be best at 0. 0. Final tailings samples following cyanide detoxification were sent to the consulting geotechnical laboratories for testing.0286 mg/l/min. . These tests were done to confirm the conditions determined from the batch testwork and to confirm final cyanide levels in the resultant slurry. although some minor redissolution was seen over one hour. Both tests were conducted at 42% solids. Both campaigns indicated that the targeted underflow densities should be achieved. Thickening Two settling testwork campaigns were done by Outokumpu Technologies which concentrated on flocculant screening and dynamic settling tests.0 and 8. • Oxygen Uptake Testwork.A final large slurry sample was produced to allow two semi-continuous tests to be carried out.85:1 and 1.3207 mg/l/min for the first six hours and then slowly reduces over the next 18 hours. This is especially true for the concentrate sample that is at a high pH and has high sodium content due to the high cyanide levels.5. A summary of results for the oxygen uptake testwork is given in Table 7. A summary of results for the settling testwork is given in Table 6. Solution tenors dropped from 2. The tailings sample oxygen demand was quite low with a peak of 0. Merrill Crowe testwork was done on high grade flotation concentrate leach liquors as an alternative process route to direct electrowinning. with the pH maintained between 8. This was done to determine the oxygen requirement for each slurry type and in turn allow for sizing of the oxygen plant. The concentrate slurry has a high oxygen demand.76 t/m²/h and for the concentrate 0. 0.28 t/m²/h. The testwork indicated high silver and gold precipitation rates from the solution. Both tests resulted in WAD cyanide levels of less than 2 mg/l.15:1. These have included the following: • Rheology Testwork. • Merrill Crowe Testwork. Further testwork on optimum flocculants will be done on site during commissioning. however overflow clarity may not be as good as expected. The results from these tests will be used for the final tailings dam design by the consultant. Oxygen uptake tests were done on both flotation concentrate and tailings samples. This data has been used in pump and agitator designs. had a residence time of 70 minutes and tested the metabisulphite stoichiometry at 150% and 165%.360 ppm silver to approximately 7 ppm in 15 minutes at a zinc stoichiometry of 1:1. • Tailings Testwork Rheology testwork has been carried out on flotation concentrate and tailings samples at different densities. Flowsheet The process developed for the Palmarejo ore is shown in Figure 2. cyanide destruction and disposal in a tailings facility. This gives the advantage of: • • • allowing a high cyanide concentration to be utilised to treat the slow leaching silver and gold minerals in the concentrate. flotation. Blending will also be important to ensure that silver and gold feed grades are maintained at a relative stable grade to minimise process disruptions through the flotation and electrowinning stages of the plant. electrowinning. however a two stage milling circuit is required to process the ore. PLANT DESIGN AND ENGINEERING CONSIDERATIONS Process Plant The testwork carried out has allowed the design of the process plant for Palmarejo to proceed. The plant is to operate on a continuous 24 hour per day. leaching. CCD washing. The overflow solution from the CCD circuit is pumped to the electrowinning circuit. The circuit design utilises conventional mineral processing techniques and consists of SAG and ball milling. but has incorporated them into novel flow sheet that utilises direct electrowinning as the primary recovery of silver in a large scale process plant. Blending of the ore types will be important to maintain a steady throughput as well as to ensure that the mills are not damaged during changes in ore hardness being fed to the plant. The comminution testwork indicates that the ore is amenable to SAG milling. The Palmarejo processing plant is required to process a range of ores with varying hardness. The slurry will then be sent to a three stage CCD washing circuit to wash the high grade solution from the leached solids. the flotation tails and final CCD underflow are . allows the production of a high grade solution that can be sent directly to electrowinning. CIL. This is done to allow additional leaching of this slurry to ensure complete leaching of the concentrate. The underflow from the final CCD thickener is pumped to the head of the CIL circuit. The concentrate will then be diluted to 50% solids w/w before it is leached at 1% cyanide concentration for 48 hours. The flowsheet has utilised conventional mineral processing techniques. As such. The flotation concentrate will be thickened to remove excess water and so minimise the amount of flotation reagents going forward to the leaching circuit. friable quartz vein breccia and competent footwall sediments. seven days per week basis. The flotation tailings still contain sufficient silver and gold grades to warrant the leaching of this product. The milling circuit has to handle a blend of hard competent amygdaloidal andesite. and minimises the tonnage of material that must be processed through a solids – liquids separation process. The process plant was designed with a flotation circuit to concentrate the majority of the silver and gold minerals into a low weight concentrate. The EMEW circuit is designed to produce 1. This solution will be returned to both the concentrate leach circuit and leach circuits as dilution water. The study indicated the lowest cost and most effective solution for the SAG mill was to install variable speed drives for the two SAG mill motors. as the power supply in Mexico is 60 Hz. The Electrometals electrowinning circuit consists of 380 EMEW Powder Electrowinning Cells. a study was carried out to determine the most effective solution for modifying the drives to meet the power supply requirements. Additionally. girth gear and gearbox. The incorporation of a CIL circuit into the plant design adds considerable flexibility to the plant operation. The underflow solution is then diluted with tailings return water and sent to the cyanide detoxification circuit incorporating the Inco SO2/air system. It also can handle oxide ores that may have lower flotation recoveries. concentrate leach and electrowinning circuits. The feed grade of solution to this circuit is expected to vary between 1. These are a fully automatic cell that produces a silver and gold powder from the feed solution. The circuit consists of one leach tank with a residence time of 4 hours and seven carbon adsorption tanks with a combined residence time of 20 hours. An inspection of these mills indicated that they were in extremely good condition and comminution analysis indicated they would be suitable for the project. No other second hand equipment was sourced for the project. The carbon stripping circuit consists of a 10 tonne split Anglo American (AARL) stripping circuit.500 ppm and 2. These were selected with a slower speed so that the mill would operate at 68% of critical speed. Tailings from the CIL circuit will be pumped to the tailings thickener to recover excess water and the contained cyanide. The mills were fitted with 50 Hz motors and. Flotation rougher and cleaner banks were purchased along with the two mills. Second hand equipment The use of high quality second hand equipment was considered a priority for the project to help with the development schedule. The design of this circuit allows for three strips per day to be completed.000 ppm precious metal (silver plus gold). including pinion. This was done as the configuration of the mill was not ideal for ball milling.250 kg/t of precious metal powder. the comminution study had indicated that the SAG mill would need to be variable speed to handle the variation in ore hardness.5 million tonnes per annum. however it is only planned to do 13 strips per week. . The pregnant solution produced by the carbon stripping circuit is sent to the electrowinning circuit along with the overflow from the CCD circuit. with a possible production throughput of 2.combined in a conventional CIL circuit. flotation cells were available at the same location and could be transported at the same time as the mills. The best solution for the ball mill was to purchase two new 60 Hz motors. An extensive search in 2006 for available equipment found two 5 MW autogenous grinding (AG) mills in Spain. Additionally. plus numerous mill spares. This circuit acts as the surge control within the circuit and will smooth process upsets from the flotation. while the barren solution tenor is 50 ppm precious metal. A single stage pumping station was found to offer the lowest capital and operating costs. an economic analysis was undertaken into adding a final thickener after the detoxification circuit. Water supply The original plan for the process plant water supply was to build an emergency containment dam (ECD) in one of the valleys below the mine site. The study indicated that the pay back period was less than 12 months. the mills required additional modification to enable delivery of the mill shells to site. Water from the ECD will then be pumped in two stages to supply water to the process plant. The second pump station will deliver water to the main process plant area which is 1 km further up the valley with a static lift of 140 metres. The Chinipas River is at an altitude of 440 metres. The first pumping station will deliver water to a lower water tank farm.Site terrain The Palmarejo mine site is located in an extremely mountainous region of the Sierra Madre Occidental Mountain Range. The pipeline route between the two is 17 km long and an engineering study was done to investigate the most effective pumping system for this duty. however ongoing investigations were required to determine if sufficient room was available at the process plant site for the addition of another 22 metre thickener. Due to difficulties in expected dam construction and variability in rainfall. the plant design is extremely compact in an attempt to minimise the amount of earthworks required to provide suitable flat areas for the plant layout. it was also decided to secure water from the Chinipas River. some 5 km from the ECD with a static lift of 220 metres. This would reduce the amount of water required to be returned to the plant. Limits on the largest individual equipment piece were imposed by restricted access to the site caused by narrow and steep mountain roads. Due to this restriction. Power supply . This pumping system consists of a two stage pumping station that delivers water from the tailings dam 3 km to the process plant. The static lift required for this duty is 255 metres. The original schedule was to have this complete before the last wet season prior to plant commissioning. approximately 1000 m. Tailings return water will be pumped back from the tailings dam to a separate storage tank so this water can be distributed as dilution water for cyanide detoxification and also as make-up water in the milling circuit. Whilst the altitude of the final plant site is not extremely high. the actual site consists of steep mountain slopes with very little flat ground. Even with the road improvements. Due to the high static head of returning tailings water to the plant. alongside the town of Chinipas. while the ECD is at an altitude of 790 metres. Considerable effort was expended on improving the road access from Creel to site to allow the delivery to site of the large mechanical equipment. Ammtec Ltd Campaign – A9848. SGS Lakefield Oretest Campaign – 9632. Diesel will be trucked to site for the power house as well as for use by the mining fleet.5 Mt/a.5 Mt/a. REFERENCES Ammtec Campaign – January 2004. consisting of 12 x 1 MW caterpillar generators. May 2005. December 2005. Outokumpu Technologies Pty Ltd – S559TA. Provision for future expansion An integral component of the Palmarejo project strategy is further exploration at the Palmarejo site as well as exploration on leases in the immediate area around Palmarejo. Outokumpu Technologies Pty Ltd – December 2005. Electrometals Technologies Ltd – November 2005. and • Electrometals EMEW circuit has additional area available to add 26% more cells. SGS Lakefield Oretest Campaign – 9745. Provision has been included in the design of the plant as follows: • crushing circuit has been sized for throughput increase to 2.5 Mt/a. • Milling circuit has been designed for the addition of a pebble crushing circuit to be added.5 Mt/a. • Milling circuit has been sized for throughput increase to 2. SGS Lakefield Oretest Campaign – 9609.Power will initially be supplied from an onsite 12 MW diesel power house. September 2005. The rectifiers have also been oversized for potential high grade solution treatment. July 2005. December 2004. ACKNOWLEDGEMENTS Coeur D’Alene . December 2005. • Flotation circuit has been sized for throughput increase to 2. • Thickeners have been sized for throughput increase to 2. An overland power supply will be constructed once the process plant is completed to supply power from the Mexican power grid. SGS Lakefield Oretest Campaign – 9772. It is expected that additional resources will be identified that may justify an expansion in capacity.
Report "Process Selection and Design for the Palmarejo Silver Mine Rev A"