Cement Kiln - The Manufacture of Cement Clinker

The Manufacture of Cement Clinker

A typical process of manufacture consists of three stages:

• grinding a mixture of limestone and clay or shale to make a fine "rawmix" (see Rawmill);
• heating the rawmix to sintering temperature (up to 1450 °C) in a cement kiln;
• grinding the resulting clinker to make cement (see Cement mill).

In the second stage, the rawmix is fed into the kiln and gradually heated by contact with the hot gases from combustion of the kiln fuel. Successive chemical reactions take place as the temperature of the rawmix rises:

• 70 to 110 °C - Free water is evaporated.
• 400 to 600 °C - clay-like minerals are decomposed into their constituent oxides; principally SiO₂ and Al₂O₃. Dolomite (CaMg(CO₃)₂) decomposes to calcium carbonate, MgO and CO₂.
• 650 to 900 °C - calcium carbonate reacts with SiO₂ to form belite (Ca₂SiO₄).
• 900 to 1050 °C - the remaining calcium carbonate decomposes to calcium oxide and CO₂.
• 1300 to 1450 °C - partial (20–30%) melting takes place, and belite reacts with calcium oxide to form alite (Ca₃O·SiO₄).

Alite is the characteristic constituent of Portland cement. Typically, a peak temperature of 1400–1450 °C is required to complete the reaction. The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1–10 mm. This is called clinker. The hot clinker next falls into a cooler which recovers most of its heat, and cools the clinker to around 100 °C, at which temperature it can be conveniently conveyed to storage. The cement kiln system is designed to accomplish these processes .