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Wet grinding of hard minerals is usually much more efficient than dry grinding. When slurry is dried in the kiln, it forms a granular crumble that is ideal for subsequent heating in the kiln. In the dry process, it is very difficult to keep the fine powder rawmix in the kiln, because the fast-flowing combustion gases tend to blow it back out again.
The Kilns of modern cement plants are running at 4 to 5 rpm. The bearings of the rollers must be capable of withstanding the large static and live loads involved and must be carefully protected from the heat of the kiln and the ingress of dust. Since the kiln is at an angle, it also needs support to prevent it from walking off the support rollers.
Portland cement clinker is made by heating a homogeneous mixture of raw materials in a rotary kiln at high temperature. The products of the chemical reaction aggregate together at their sintering temperature, about 1,450 °C (2,640 °F).
In limestone calcination, a decomposition process that occurs at 900 to 1050 °C, the chemical reaction is CaCO 3 (s) → CaO(s) + CO 2 (g) Today, this reaction largely occurs in a cement kiln. The standard Gibbs free energy of reaction in [J/mol] is approximated as ΔG° r ≈ 177,100 J/mol − 158 J/(mol*K) * T. [4]
The waste streams also often contain combustible materials which allow the substitution of part of the fossil fuel normally used in the process. Waste materials used in cement kilns as a fuel supplement: [32] Car and truck tires – steel belts are easily tolerated in the kilns; Paint sludge from automobile industries; Waste solvents and lubricants
Roller kiln: A special type of kiln, common in tableware and tile manufacture, is the roller-hearth kiln, in which wares placed on bats are carried through the kiln on rollers. In the intermittent kiln , the ware is placed inside the kiln, the kiln is closed, and the internal temperature is increased according to a schedule.
This cement is made by heating limestone (calcium carbonate) with other materials (such as clay) to 1,450 °C (2,640 °F) in a kiln, in a process known as calcination that liberates a molecule of carbon dioxide from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials in the mix to ...
Fluorine is beneficial to the kiln process in that it allows alite to form at lower temperature. However, at levels above 0.25% in the clinker, delayed and erratic cement setting time results. Alkali metals (primarily sodium and potassium) cause processing problems because they form volatile salts in the kiln system. These evaporate in the kiln ...