There are six basic stages of cement manufacturing process.
Stage 1:Raw Material Extraction/Quarry
The raw cement ingredients needed for the production of cement are limestone (calcium), sand and clay (silicon, aluminum, iron), slate, fly ash, inlays and bauxite. The mineral rocks are extracted and crushed into smaller pieces of approximately 6 inches. The secondary crushers or hammer mills reduce them to an even smaller size of 3 inches. After that, the ingredients are prepared for grinding, proportioning and blending.
Stage 2: Grinding, Proportioning and Blending
The crushed raw ingredients are prepared for the manufacturing process of the cement in the kiln by combining them with additives and grinding them to ensure a homogeneous fine mixture. The composition of the cement is provided here as a function of the desired properties of the cement. In general, limestone is 80% and the remaining 20% is clay. In the cement plant, the crude mixture is dried (the moisture content is reduced to less than 1%); the heavy rollers and the rotating tables mix the raw mixture and then the roller crushes it until obtaining a fine powder to store it in silos and feed it into the kiln.
Stage 3: Pre Heating Raw Material
A preheating chamber consists of a series of cyclones that use the hot gases produced in the kiln to reduce energy consumption and make the cement manufacturing process more environmentally friendly. The raw materials are passed through here and converted into oxides to burn in the furnace.
Stage 4: Kiln Phase
The furnace phase is the main stage of the cement production process. Here, clinker is produced from the raw mix through a series of chemical reactions between calcium and silicon dioxide compounds. Although the process is complex, the events of clinker production can be written in the following sequence:
- Evaporation of free water
- Evolution of combined water in the argillaceous components
- Calcination of the calcium carbonate (CaCO3) to calcium oxide (CaO)
- Reaction of CaO with silica to form dicalcium silicate
- Reaction of CaO with the aluminum and iron-bearing constituents to form the liquid phase
- Formation of the clinker nodules
- Evaporation of volatile constituents (e. g., sodium, potassium, chlorides, and sulfates)
- Reaction of excess CaO with dicalcium silicate to form tricalcium silicate
The above events can be condensed into four major stages based on the change of temperature inside the kiln:
- 100°C (212°F): Evaporation of free water
- 100°C (212°F)-430°C (800°F): Dehydration and formation of oxides of silicon, aluminum, and iron
- 900°C (1650°F)-982°C (1800°F): CO2 is evolved and CaO is produced through calcination
- 1510°C (2750°F): Cement clinker is formed
The oven has an angle of 3 degrees to the horizontal to allow the material to pass through it, for a period of 20 to 30 minutes. By the time the raw mix reaches the bottom of the oven, clinker forms and leaves the oven in marble-sized nodules.
Stage 5: Cooling and final grinding
After leaving the oven, the clinker cools rapidly from 2000 ° C to 100 ° C-200 ° C as air passes over it. In this stage, different additives are combined with the clinker that must be ground to produce the final product, cement. The plaster, added and ground with clinker, regulates the setting time and provides the most important property of the cement: the resistance to compression. It also prevents the agglomeration and coating of dust on the surface of the balls and the wall of the mill. Some organic substances, such as triethanolamine (used at 0.1% by weight), are added as grinding aids to prevent dust agglomeration. Other additives that are sometimes used are ethylene glycol, oleic acid.
The heat produced by the clinker is circulated back to the furnace to save energy. The last stage of cement manufacturing is the final grinding process. In the cement plant, there are rotating drums equipped with steel balls. The clinker, after being cooled, is transferred to these rotating drums and ground into a powder so fine that each pound contains 150 billion grains. This powder is the final product, the cement.
Stage 6: Packing and Shipping
The cement is transported from the mills to the silos (large storage tanks) where it is packed in bags of 20 to 40 kg. Most of the product is shipped in large quantities on trucks, trains or ships, and only a small amount is packaged for customers who need small quantities.
Chemical reactions during the cement manufacturing process
The reactions that take place (after the evaporation of free water) between the reagents in the kiln phase of the cement manufacturing process are the following:
- Clay Decomposition:
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:
CaMg(CO3)2 → CaCO3 + MgO + CO2
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:
2 CaCO3 + SiO2 → Ca2SiO4 + 2 CO2
2 MgO + SiO2 → Mg2SiO4
Ca5(PO4)3OH + 0.25 SiO2 → 1.5 Ca3(PO4)2 + 0.25 Ca2SiO4 + 0.5 H2O (vapour)
- Alumina and Oxide Reaction:
12 CaCO3 + 7 Al2O3 → Ca12Al14O33 + 12 CO2
4 CaCO3 + Al2O3 + Fe2O3 → Ca4Al2Fe2O10 + 4 CO2
4 CaCO3 + Al2O3 + Mn2O3 → Ca4Al2Mn2O10 + 4 CO2
- Reaction of Remaining Calcite:
CaCO3 → CaO + CO2
Ca2SiO4 + CaO → Ca3SiO5