UNDERSTANDING CLINKER LIQUID PHASERicardo A. Mosci INTRODUCTION Clinker liquid phase or clinker melt is the fraction of the kiln feed that melts between the upper transition and the burning zone. The liquid has a critical role in clinker nodulization and clinker mineral development and properties. In the absence of liquid, the conversion of C2S and free lime to C3S would be almost impossible in the kiln. Plant chemists and kiln operators are usually more concerned with the amount of liquid rather than with the rheological1 properties of the liquid. The latter is much more important during clinkering reactions than the former. AMOUNT OF LIQUID PHASE If the raw mix consisted of only four oxides, i.e. CaO, SiO2, Al2O3 and Fe2O3, it would start melting at 1338 oC, the so-called eutectic2 temperature for the system C-S-A-F. At the eutectic temperature, the liquid composition is 55% CaO, 6 % SiO2, 23 % Al2O3 and 16 % Fe2O3. Such composition is saturated in lime and unsaturated in silica. Therefore, it is aggressive to refractory products containing silica or silicates in their composition. Industrial raw mix contains impurities such as MgO, Na2O, K2O and SO3. At certain concentrations, these impurities reduce the eutectic temperature of the system to 1280 oC, thus promoting earlier clinker formation. These oxides act as fluxes3 in the kiln, forming liquid as far up as in the calcining zone. Formulas used to compute the amount of liquid at any given temperature usually take into account these minor oxides. Example: % L.P. at 1450 oC = 3 x A + 2.25 x F + MgO + K2O + Na2O + SO3 (MgO 2) 1 2 Fluid property such as viscosity. lowest melting temperature of a system. 3 substances that reduce the melting temperature of a system. For most commercial clinkers. Liquid phase calculations can be used to predict where in the kiln the stable coating will form. The tendency to coating formation or the coatability of the clinker increases with the amount of liquid. the amount of liquid phase in the burning zone varies between 23 and 29%. A good example is the thin but stable coating encountered in white cement kilns. its elastic modulus4 increases and so does its tendency to spall off. Higher values can be damaging to most refractory bricks in the absence of a stable coating. where the silica ratio of the raw mix is above 10 and the C4AF is 0. alite formation is extremely 4 Ratio between stress and strain. The closer the two lines are to each other. As the brick is infiltrated and saturated with liquid. texture and stability are by far more important than the amount of coating deposited on the lining. . as shown on the following chart. In the absence of liquid. more coating does not necessarily mean better coating. However. Alumina brick is quickly destroyed by clinker melt. Coating refractoriness. IMPORTANCE OF THE LIQUID PHASE The most important clinker mineral C3S (alite) requires the presence of liquid for its formation. The amount of liquid is calculated at 1338 C and at 1450 C and the results are compared. Too much liquid at 1338 ºC is undesirable because of the proximity of the liquid to the alumina brick section. the longer will be the stable coating zone. slow and it would render commercial clinkering impossible. Calcium ions migrate towards C2S through chemical diffusion. C2S and free CaO dissolve in the clinker melt. 2. although both promote clinker formation. whereas MgO and sulphates considerably increase it. one must first understand the alite formation mechanism: 1. where the amount of liquid is at a maximum. such as calcium chloride. at commercial clinkering temperatures. C3S is formed and crystallized out of the liquid. 3. It is important to mention that Na2O and K2O decrease the mobility of Ca ions. and that of C2S almost impossible. PROPERTIES OF THE LIQUID PHASE VISCOSITY Temperature has the most pronounced effect on liquid phase viscosity. feldspars and slags should not be confused with mineralizers. Increasing the burning temperature by 93 oC reduces liquid viscosity by 70% for a regular Type I clinker. Mineralizers are usually transition metals such as copper. To understand why alite formation requires liquid phase. This fact also explains why alite is formed essentially in the burning zone. Fluxes. the addition of metallurgical slags to the raw mix promotes clinker formation. Without liquid phase the diffusion of Ca ions towards C2S would be extremely slow. lead and zinc that reduce the amount of energy required for clinker silicate formation. This is why the addition of gypsum to the raw mix promotes alite formation. This simple fact explains why hotter-than- . Similarly. SO3 and K2O. Free alkali and phosphorus increase liquid phase viscosity. So does temperature.16 N. Therefore. Moreover. would experience the negative effects of high liquid viscosity. Extreme caution should be exerted when insufflating calcium chloride into the burning zone as a way to reduce alkali in the clinker. High surface tension values favor nodule formation and liquid penetration through the pores of the nodules. . and low in MgO. but this effect is offset by MgO and SO3. followed by Sodium and Magnesium. Sulphur and Potassium have the strongest effects.normal temperatures are so beneficial to clinkering and yet so harmful to the refractory lining. MgO. coating adherence to the lining and clinker quality. alkali sulphates. Unfortunately. fluorides and chlorides also reduce liquid phase viscosity. Another important property of the liquid phase is its surface tension. a regular clinker at 1450 ºC has a viscosity of 0. For instance. leading to its premature failure.83. Over-sulfated clinkers are usually dusty as a consequence. Only clinkers with sulphate/alkali ratio lower than 0. Alkali.s/m2. the liquid properties that induce C3S formation are detrimental to the refractory lining and to clinker nodulization. The surface tension has a direct impact on clinker fineness. The liquid phase viscosity increases linearly with the alumina/iron ratio. A liquid phase with high surface tension has less tendency to adhere to the brick surface. to a certain concentration. therefore reducing clinker coatability or adherence to the lining. the liquid phase is considerably less damaging to the refractory lining when the liquid is viscous. Low viscosity liquid infiltrates the refractory lining faster. Adding 2% SO3 to the clinker reduces that viscosity to 0. MgO. The resulting clinker contains less dust (fraction below 32 mesh) and lower free lime content.s/m2. as shown on the photo below. For a given burning temperature. The injection of sodium carbonate into the burning zone is also detrimental to the refractory lining.05 N. are good coating promoters. or its ability to "wet" the lining. MgO and SO3 reduce liquid surface tension. high C3A clinkers tend to nodulize better than low C3A clinkers. kiln dust or solid wastes to the kiln. raw material properties and flame temperature as steady as possible. Brick lining eroded by excessive liquid phase. clinker fineness. cement strength and refractory depth of infiltration. It is then very important to keep fuel. the rheological properties of the melt are even more important. clinker coatability. . The rheological properties of the clinker melt control parameters such as clinker mineral formation. Whenever introducing drastic changes in raw material or fuel properties. the refractory lining must be changed accordingly to meet the differences in clinker coatability and burnability. This proves particularly true when adding slags.CONCLUSIONS Although the amount of liquid phase in the burning and transition zones of the kiln is important to clinker formation and brick performance.