Stage of Cement Manufacture
There are six main stages of cement manufacturing process.
Raw Material Extraction/Quarry
The raw cement ingredients needed for cement production are limestone (calcium), sand and clay (silicon, aluminum, iron), shale, fly ash, mill scale and bauxite. The ore rocks are quarried and crushed to smaller pieces of about 6 inches. Secondary crushers or hammer mills then reduce them to even smaller size of 3 inches. After that, the ingredients are prepared for pyroprocessing.
Grinding, Proportioning and Blending
The crushed raw ingredients are made ready for the cement making process in the kiln by combining them with additives and grinding them to ensure a fine homogenous mixture. The composition of cement is proportioned here depending on the desired properties of the cement. Generally, limestone is 80% and remaining 20% is the clay. In the cement plant, the raw mix is dried (moisture content reduced to less than 1%); heavy wheel type rollers and rotating tables blend the raw mix and then the roller crushes it to a fine powder to be stored in silos and fed to the kiln.
Pre-Heating Raw Material
A pre-heating chamber consists of a series of cyclones that utilizes the hot gases produced from the kiln in order to reduce energy consumption and make the cement making process more environment-friendly. The raw materials are passed through here and turned into oxides to be burned in the kiln.
The kiln phase is the principal 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. Though the process is complex, the events of the 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 kiln is angled by 3 degrees to the horizontal to allow the material to pass through it, over a period of 20 to 30 minutes. By the time the raw-mix reaches the lower part of the kiln, clinker forms and comes out of the kiln in marble-sized nodules.
Cooling and final grinding
After exiting the kiln, the clinker is rapidly cooled down from 2000°C to 100°C-200°C by passing air over it. At this stage, different additives are combined with the clinker to be ground in order to produce the final product, cement. Gypsum, added to and ground with clinker, regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid and dodecyl-benzene sulphonate.
The heat produced by the clinker is circulated back to the kiln to save energy. The last stage of making cement is the final grinding process. In the cement plant, there are rotating drums fitted with steel balls. Clinker, after being cooled, is transferred to these rotating drums and ground into such a fine powder that each pound of it contains 150 billion grains. This powder is the final product, cement.
Packing and Shipping
Cement is conveyed from grinding mills to silos (large storage tanks) where it is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities by trucks, trains or ships, and only a small amount is packed for customers who need small quantities.
Chemical Reactions during Cement Manufacturing Process
The reactions that take place (after evaporation of free water) between the reactants in the kiln phase of cement making process are as follows:
- 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