Extraction and Preparation of Raw Materials
The main raw materials are quarried from natural rocks limestone, clay chalk or basalt. They are crushed and transferred to pre-blending storage where additional substances are added to the desired chemical composition such as sand, iron ore, bauxite, shale, slag, fly ash.
Homogenization
The raw materials are ground in the raw mill where the particle size on a 90 um sieve is decreased and then moved to a homogenization silo to ensure standardized and high-quality clinker output.
Pre-Heating
In pre-heat cyclones fitted with a pre-calciner fired with petroleum, natural gas or coal, the homogenous mixture of raw materials is heated and transformed into oxides ready for the burning phase in the kiln.
Kiln Phase
Because clinker is a complex mix of ingredients, a multi-stage heat treatment is needed, which is performed in the kiln. The main part of the clinker’s manufacturing process is in the kiln. The kiln process phases are as follows:
- Free water evaporation: A force above the atmosphere is needed to vaporize the liquid from the raw materials slurry mixture. Air is overheated and when the temperature rises past 120 ° C, the evaporation slowly ceases.
- Tile decomposition: “Tile” minerals contain most of the alkali in the raw materials, the most common of which is kaolinite, Al2Si2O5(OH)4. The separated alkalis interact at high temperature with the acid gases in the kiln. Here the active reactions 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
- Decomposition of the dolomite: Magnesia in the raw-mix mainly exists as dolomite, CaMg(CO3)2, but also as silicate or carbonate. Dolomite has the following reaction: CaMg(CO3)2>→ CaCO3 + MgO + CO2 Again, compounds of non-carbonate magnesium e.g. phlogopite behave like this: KMg3AlSi3O10(OH)2 + 0.5 SO2 + 0.25 O2 → 0.5 K2SO4 + 3 MgO + 0.5 Al2O3 + 3 SiO2 + H2O (vapor)
- Calcium carbonate, present in the raw-mix as calcite, creates carbon dioxide, which consumes half of the finished clinker’s weight. This requires a huge output of energy. One of the considerations deciding the production and energy usage in the kiln is the performance of this reaction. Simple calcite breaks down at about 650 °C in the kiln: 2 CaCO3 + SiO2 → Ca2SiO4 + 2 CO2 Solutions for reactive clay decomposition and small amounts of alkali sulphate / chloride-melt mix the solutions by surface tension and act as an ion transfer medium. CO2 is created here, but there is no free lime (CaO) being formed. In the processes of silicate, magnesium responds in forsterite with silica (which becomes a solid solution through belite): 2 MgO + SiO2 → Mg2SiO4 Phosphorus (as raw-mixed apatite) interacts with a little free silica and creates whitlockite (which is also used in strong belite solution): Ca5(PO4)3OH + 0.25 SiO2 → 1.5 Ca3(PO4)2 + 0.25 Ca2SiO4 + 0.5 H2O (vapor)Once all the silica in the kiln is used up, this phase will end.
- Reacting with alumina and iron oxide: Calcium carbonate begins to react with other oxides and no free lime is produced yet. While tricalcium aluminate is stable here, it appears to be forming poorly crystallized mayenite (Ca12Al14O33). At this point, the reactions are: 12 CaCO3 + 7 Al2O3 → Ca12Al14O33 + 12 CO2
4 CaCO3 + Al2O3 + Fe2O3 → Ca4Al2Fe2O10 + 4 CO2
4 CaCO3 + Al2O3 + Mn2O3 → Ca4Al2Mn2O10 + 4 CO2 - Remaining calcite decomposition: at this point, a small amount of calcite persists and absorbs free lime for the first time: CaCO3 → CaO + CO2
- Sintering: Sintering is the method of compacting or shaping by heat and stress to the extent of liquefaction a solid mass of substance without completely melting it. When the temperature inside the kiln exceeds approximately 1300°C (aluminate, ferrite, and silicate eutectic melting point), clinker flux is made. The rate of melting varies and alite, which becomes stable at above 1250°C, begins to form when the liquid is formed. This stage’s main reaction is as follows: Ca2SiO4 + CaO → Ca3SiO5
- The water produced both serves as an ion-exchange solvent between the solid phases and by surface tension draws the reactant particles together.
Cooling and Storing
The mixture is easily cooled from 2000°C to 100°C-200°C within the moderately inclined kiln. The final product, clinker, is therefore processed and then deposited, prepared for cement manufacturing.