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, as well as clinker minerals development and properties. Without the presence of liquid, the conversion of C2S and free lime to C3S at normal clinkering temperatures would be almost impossible.

Plant chemists and kiln operators are usually more concerned with the amount of liquid present rather than with the rheological properties (fluid properties, such as viscosity) of the liquid, although the latter is much more important during the clinkering reactions than the former.

Amount of liquid phase If the raw mix consisted of only four oxides-CaO, SiO2, Al2O3, and Fe2O3, it would start melting at 1,338degrees C (2,440degrees F), the so-called eutectic temperature (the lowest melting temperature of a system) 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 mixes contain impurities, such as MgO, Na2O, K2O, and SO3. At certain concentrations, these impurities reduce the eutectic temperature of the system to 1,280degrees C (2,336degrees F), thus promoting earlier clinker formation. These oxides act as fluxes (substances that reduce the melting temperature of a system) in the kiln, forming liquid as far up as the calcining zone. Formulas used to compute the amount of liquid at any given temperature usually take into account these minor oxides (see Figure 1).

For most commercial clinker, the amount of liquid phase in the burning zone varies between 23% and 29%. Higher values can be damaging to most refractory bricks in the absence of a stable coating. As the brick is infiltrated and saturated with liquid, its elastic modulus (the ratio between stress and strain) increases, as does its tendency to chip off.

The tendency toward coating formation or the coatability of the clinker increases with the amount of liquid. However, more coating does not necessarily mean better coating. Coating refractoriness, texture, and stability are far more important than the amount of coating deposited on the lining. A good example is the thin but stable coating in a white cement kiln, where the silica ratio of the raw mix is more than five and the C4AF is zero.

Importance of the liquid phase The most important clinker mineral, C3S (alite), requires the presence of liquid for its formation. In the absence of liquid, alite formation is extremely slow and would render commercial clinkering impossible. This explains why alite is formed essentially in the burning zone, where the amount of liquid is at a maximum.

To understand why alite formation requires liquid phase, one must first understand the alite formation sequence. First, C2S and free CaO dissolve in clinker melt. Then, calcium ions migrate toward the C2S through chemical diffusion. Finally, the C3S is formed and crystallized out of the liquid.

Without a liquid phase, the diffusion of Ca ions towards C2S would be extremely slow, and that of C2S almost impossible at commercial clinkering temperatures. It is important to mention that Na2O and K2O decrease the mobility of Ca ions, whereas MgO and sulphates increase it considerably. This is why the addition of gypsum to the raw mix promotes alite formation. Similarly, the addition of metallurgical slags to the raw mix promotes clinker formation.

Fluxes, such as calcium chloride, feldspars, and slags should not be confused with mineralizers, although both promote clinker formation. Mineralizers are usually transition metals such as copper, lead, or zinc, which reduce the amount of energy required for clinker silicates to form.

Properties of the liquid phase Temperature has the most pronounced effect on liquid-phase viscosity. Increasing the burning temperature by 93degrees C (199degrees F), reduces liquid viscosity by 70% for a regular Type 1 clinker. This simple fact explains why hotter-than-normal temperatures are beneficial to clinkering yet potentially harmful to the refractory lining, as shown in Photo 1.

MgO, alkali sulphates, fluorides, and chlorides also reduce liquid-phase viscosity. Extreme caution should be exerted when insufflating calcium chloride into the burning zone as a way to reduce alkali in the clinker. The injection of sodium carbonate into the burning zone also is detrimental to the refractory lining.

Free alkali and phosphorus increase liquid-phase viscosity, but this effect is offset by MgO and SO3. Only clinkers with sulphate-alkali ratio lower than 0.83 and low MgO would experience the negative effects of high liquid viscosity.

The liquid-phase viscosity increases linearly with the alumina-iron ratio. For a given burning temperature, high C3A clinkers tend to nodulize better than low C3A clinkers. Moreover, the liquid phase is considerably less damaging to the refractory lining when the liquid is viscous.

Another important property of the liquid phase is its surface tension, or its ability to "wet" the lining. The surface tension has a direct impact on clinker fineness, coating adherence to the lining and clinker quality.

High surface tension values favor nodule formation and liquid penetration through the nodules. The resulting clinker contains less dust (fraction below 32 mesh) and lower free lime content. A liquid phase with high surface tension has less tendency to wet the brick surface, therefore reducing clinker coatability or adherence to the lining.

Alkali, MgO, and SO3 reduce liquid surface tension, as does temperature. Sulphur and potassium have the strongest effects, followed by sodium and magnesium. Therefore, MgO, SO3, and K2O are good coating promoters.

Conclusions Although the amount of liquid phase in the burning and transition zones of the kiln is important to clinker formation and brick performance, the rheological properties of the melt are even more important. The rheological properties of the clinker melt control parameters, such as clinker mineral formation, clinker coatability, clinker fineness, cement strength, and refractory depth of infiltration.

It is then very important to keep fuel and raw materials properties and flame temperature as steady as possible. 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, kiln dust, or solid wastes to the kiln.