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Process Description 3 Fig. 2.1. Steps in the manufacture of Portland cement
4 Technology, Energy Efficiency and Environmental Externalities of the Cement Industry The largest volume of raw material is CaCO3 or comparable materials (such as oyster shells in locations where appropriate). The CaCO3 as mined is often in chunks up to 750 mm in diameter. These must be crushed to about 10 mm, and then mixed with the sand, shale, and other ingredients for further grinding to about 60µm in diameter. Frequently, the initial crushing is done at the quarry, prior to transport to the cement plant. After grinding, depending upon the exact process, water may be added. The mixture is then taken to some high-temperature processing unit known as rotary kiln for conversion to cement clinker. The clinker must be cooled before further processing. Then it is either stored, sold or transported (to individual grinding mills), or is ground at the plant with gypsum and other possible additives to a fine powder of finished cement. The cement is either packaged or sold in bulk to the distributors. 2.1 Cement Kiln The Cement kilns are large; up to 230 m in length and 8 m in diameter; inclined at an angle of three to six degrees, and lined with temperature-resistant refractory brick. They rotate at about 50 to 70 revolutions per hour in the older generation plants, and 170 to 180 in the more modern ones. The feed material is introduced at the elevated end and is moved slowly by the rotation of the kiln down towards the firing end, where heat is applied by a flame of coal, gas, oil or a combination of these fuels. Coal is most widely used as the kiln fuel nowadays. Several distinct thermal zones exist in the kiln. At the elevated end where the feed is introduced, is a drying and preheating zone in which the material reaches a temperature of about 800 o C. This is followed by the calcining zone where carbon dioxide is driven off the limestone, converting it to free lime at a material temperature close to 1000 o C. The chemical reaction taking place in this zone is as follows: CaCO3 ⇑ CaO + CO2 By the time the calcination is complete, the free lime enters the intermediate zone where temperature prevails in the range of 1000 - 1200 o C and the basic oxide (CaO) reacts with silica (SiO2) and alumina (Al2O3) as shown: Al2O3 + CaO ⇑ CaO.Al2O3 (mono-calcium aluminate) 2CaO + CaO.Al2O3 ⇑ 3CaO.Al2O3 (tri-calcium aluminate) SiO2 + 2CaO ⇑ 2CaO.SiO2 (di-calicium silicate) 2CaO + Fe2O3 ⇑ 2CaO.Fe2O3 2CaO.Fe2O3 + 2CaO + Al2O3 ⇑ 4CaO.Al2O3.Fe2O3
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The Asian Institute of Technology (
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