Extraction and Preparation of Raw Materials
The main raw materials (limestone, clay chalk or basalt) are quarried from natural rocks. They are crushed and transferred to pre-blending storage where other substances (such as sand, iron ore, bauxite, shale, slag, fly ash) are added to get the desired chemical composition.
The raw materials are ground in the raw mill where the particle size is reduced on a 90 um sieve and then transferred to a homogenization silo to ensure the production of uniform and good quality clinker.
The homogenous mixture of raw materials is heated and transformed into oxides (that are ready for the burning phase in the kiln) in pre-heater cyclones fitted with a pre-calciner fired with petroleum, natural gas or coal.
Since clinker is a complex mixture of ingredients, it requires a multi-staged heat treatment, which is done in the kiln. The main part of the manufacturing process of clinker takes place in the kiln. The stages in the kiln phase are as follows:
- Evaporation of free water: A pressure above atmospheric is required to vaporize the water from the slurry mixture of raw materials. Water becomes superheated and the evaporation gradually stops when the temperature rises above 120°C.
- Clay decomposition: “Clay” minerals account for most of the alkalis in the raw materials, the most common of which is kaolinite, Al2Si2O5(OH)4. The detached alkalis react with the acid gases present in the kiln at high temperature. The effective reactions here are:
Si2Al2O5(OH)2→ 2 SiO2 + Al2O3 + 2 H2O (vapor) KAlSi3O8 (orthoclase) + 0.5 SO2 + 0.25 O2 → 3 SiO2 + 0.5 Al2O3 + 0.5 K2SO4
- Dolomite decomposition: The magnesia in the raw-mix exists mainly as dolomite, CaMg(CO3)2, but also as silicate or in carbonate form. Dolomite reacts as follows:
CaMg(CO3)2>→ CaCO3 + MgO + CO2
Again, non-carbonate magnesium compounds (for example, phlogopite) react thus:
KMg3AlSi3O10(OH)2 + 0.5 SO2 + 0.25 O2 → 0.5 K2SO4 + 3 MgO + 0.5 Al2O3 + 3 SiO2 + H2O (vapor)
- Low-temperature calcite decomposition Calcium carbonate, present in the raw-mix as calcite, produces carbon dioxide, the amount of which exceeds half the mass of the finished clinker. This requires a huge heat input. The efficiency of this reaction is one of the factors that determine the output and heat consumption in the kiln. Pure calcite in the kiln decomposes at around 650°C:
2 CaCO3 + SiO2 → Ca2SiO4 + 2 CO2
Reactive clay decomposition products and small amounts of alkali sulphate/chloride-melt draw the products together by surface tension and act as an ion transfer medium. Here, CO2 is produced but no free lime (CaO) is formed. In the silicate phases, magnesium reacts with silica to produce forsterite (which goes into solid solution in belite):
2 MgO + SiO2 → Mg2SiO4
Phosphorus (as apatite in raw-mix) reacts with a little free silica and produces whitlockite (which also goes into solid solution in belite):
Ca5(PO4)3OH + 0.25 SiO2 → 1.5 Ca3(PO4)2 + 0.25 Ca2SiO4 + 0.5 H2O (vapour)
This stage ends when all the silica in the kiln is used up.
- Alumina and iron oxide react: Calcium carbonate continues to react with other oxides and no free lime is yet formed. Though tricalcium aluminate is stable here, poorly crystallized mayenite (Ca12Al14O33) seems to be formed. The reactions at this stage are:
12 CaCO3 + 7 Al2O3 → Ca12Al14O33 + 12 CO2
4 CaCO3 + Al2O3 + Fe2O3 → Ca4Al2Fe2O10 + 4 CO2
4 CaCO3 + Al2O3 + Mn2O3 → Ca4Al2Mn2O10 + 4 CO2
- Decomposition of remaining calcite: A little amount of calcite remains at this stage and forms free lime for the first time:
CaCO3 → CaO + CO2
- Sintering: Sintering is the process of compacting and forming a solid mass of material by heat or pressure to the point of liquefaction without actually melting it. When the temperature inside the kiln reaches about 1300°C (the eutectic melting point of aluminate, ferrite and silicate), clinker flux is produced. The rate of melting varies, and when the liquid is formed, alite, which becomes stable at above 1250°C, begins to form. The principal reaction of this stage is as follows:
Ca2SiO4 + CaO → Ca3SiO5
The liquid formed both acts as a solvent of ion exchange between the solid phases and draws the reactant particles together by surface tension. The powdery or granular mixture quickly solidifies into spherical nodules of clinker.
Cooling and storing
The mixture inside the slightly inclined kiln is rapidly cooled from 2000°C to 100°C-200°C. Thus, the final product, clinker, is produced, and then stored, ready for the production of cement.