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Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually defines the capacity of the cement plant. As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of kiln efficiency has been the central concern of cement manufacturing technology. ==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 . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cement kiln」の詳細全文を読む スポンサード リンク
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