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Chapter 2 Literature review and theory

2.3 Cement manufacturing process

2.3.2 Clinker production

The proportioned raw materials, or the raw mix, is thereafter fed into a rotary kiln, a long cylindrical shaft that slopes downward and rotates slowly as illustrated in Figure 2.1.The purpose at this stage is to convert the raw mix into granular materials referred to as cement clinker. This single process requires maximum temperatures that are high enough to partially melt the raw mix. The latter is constantly agitated to ensure the homogenisation of the chemical composition within the clinker.

Figure 2.1: Typical rotary kiln for the cement clinkering process (Jennings & Thomas, n.d.) Generally, the raw mix enters at the upper end of the kiln and moves slowly downward to the hottest area of the kiln within a couple of hours (60-90 minutes). Materials undergo different chemical reactions as they gradually approach the end of the kiln, a progressive allowing each reaction to be completed at its appropriate temperature. However, it is difficult to reach these designated temperatures for each specific reaction due to their endothermicity properties. This is the reason why it is challenging to bring a reaction at a particular zone in the kiln to its completion. In fact, there are four different reaction zones that can be found in the kiln depending on the prevailing temperature (Jennings & Thomas, n.d.).

First thermic zone

This zone in the kiln is referred to as the dehydration section with ambient temperatures of up to 450 oC. The aim of this zone is simply to allow for the evaporation and removal of free water since,

regardless of the selected process (dry or wet process), adsorbed moisture can be found in the raw mix. The time and energy needed for this task depend on the moisture content of the raw mix. However, this moisture is only partially removed due to the fact that the ambient temperatures in this zone are not high enough to accomplish this work. In the wet process, this zone takes half of the kiln length, whereas in the dry process only short distances are required.

Second thermic zone

This is the calcination zone. The ambient temperatures in this region vary between 450-900 oC.

Normally, at around 600 oC, the bound water is driven out of the clay while the decomposition of

limestone or calcium carbonate occurs at about 900 oC and carbon dioxide is released. At the

end of this zone, the mix consists mostly of oxides of the four main raw materials. At this stage, the mix is still a free-flowing powder since calcination does not involve melting.

Third thermic zone

This zone is referred to as the solid-state zone with ambient temperatures between 900-1300 oC.

This region sometimes overlaps with the calcination zone since the onset of the solid-state reactions occurs at 900 oC. Calcium oxides (CaO) combine with reactive silica and form small

crystals of dicalcium silicate (C2S). In the same way, the formation of intermediate calcium

aluminates and calcium ferrites takes place. These chemical compounds are very important in the clinkering process because they play the role of fluxing agent and can melt at relatively low temperature of about 1300 oC, increasing the reaction rate and aiding the formation of calcium

silicate cement compounds. As the mix goes through this zone, adjacent particles tend to fuse together and the products become quite sticky.

Fourth thermic zone

This constitutes the clinkering zone, the hottest zone of the kiln with ambient temperatures varying between approximately 1300-1500 oC. The formation of tricalcium silicate (C

3S) occurs in this

zone. The beginning of this zone is characterized by the melting of the intermediate calcium aluminate and ferrite phases. These melt phases activate the agglomeration of the mix into large nodules that consist of many small solid particles joined together by a thin liquid layer. Inside this liquid phase, the reaction between C2S crystal and CaO takes place and C3S is formed. As the

process evolves, the crystals of solid C3S grow within the liquid while those of C2S decrease in

number but grow in size. The clinkering process is completed when all the silica is in the C3S and

C2S crystals and the amount of free lime, CaO, is reduced to a minimal level, normally below an

admissible value of 1%

.

Figure 2.2: Reactions within the kiln for the formation of Portland cement clinker phases (Newman & Choo, 2003)

The cooling zone

This region is located at the bottom part of the kiln behind the heating source. As the mix moves past this hot point, the temperature drops rapidly and the liquid phase solidifies, allowing the complete formation of C3A and C4AF. The dissolved sulphate and alkalis (Kand Na) in the liquid

phase also combine to form K2SO4 and Na2SO4. The final products, the nodules formed, are hard

after being through this zone. The reactivity of the cement in the future depends strongly on the actual rate of cooling of the clinker. Rapid cooling has been found to provide a more reactive cement, which is why the clinker is rapidly cooled down by either air blowing or water spraying as it exists the kiln (Shafeek et al., 2017). Without this rapid cooling, the prevailing temperature in the cooling zone (1100 oC) can cause the C

3S to degenerate back into C2S and CaO.

In modern cement plants, there is another section placed at the upper end of the kiln and referred to as suspension preheaters or calciners. In fact, the reaction that happens inside the kiln is mostly endothermic. This suggests that the raw mix particles will require a consistent and permanent input of energy for the completion of a particular reaction at a specific zone. This is difficult to achieve as the raw mix piles up in the kiln. Consequently, the rate at which the heat can be transferred into a large mass of particles decreases and the rate of reaction will be limited. A remedy to this issue is to use suspension preheaters that allow the hot gas from the kiln to enter from the bottom, moving upward, thereby dehydrating and partially calcining the mix particles as they enter and settle in the kiln in less than a minute. Similarly, some plants can have precalciners

in which some of the fuel is burnt directly and completely calcine the mix particles as they enter the kiln.