Abstract: As one of the principal air pollutants produced by cement industry, the SO2 emission levels have been strictly limi-ted all over the word. However, with the mass adoptions of inferior quality raw materials and alternative fuels in the future, more sulfur will be brought into the cement production process. Hence leading to the increase of the SO2 emission level and aggravate the circulation enrichment of sulfur in the cement kiln, which has an impact on the operation of the kiln and the quality of clinker as well. During the cement production process, SO2 originated from both raw meal and fuel. Among them, part of the sulfur from raw meal was oxidized to SO2 in the preheater system and left with flue gas, then a portion of the SO2 was captured by the raw meal in the grinding and de-dusting system and entered the preheater system again, thus, forming a low-temperature sulfur cycle among the preheater, the raw meal mill and the dust collector. The rest of the sulfur from the raw meal and the fuel was transformed to SO2 in the kiln and calciner under high temperature, and the most of the SO2 was absorbed in the calciner generating the alkali sulfates, afterwards the alkali sulfates would enter the rotary kiln and partially decomposed in the burning zone, the generated SO2 would enter the calciner again and a high-temperature sulfur cycle was established in the kiln and calciner. Extensive research on low-temperature cycle and high temperature cycle of sulfurt in the cement production process has been devoted at home and abroad. For the relative low-temperature sulfur cycle, the emission of SO2 in the cement production was produced through the oxidation of pyrite and organic sulfur. The oxidation temperature of pyrite begins at about 400 ℃, and the reaction pro-ducts mainly consist of FeSX and Fe2O3. According to the diverse reaction conditions, the oxidation mechanisms of pyrite were classified into the direct oxidation and the reoxidation by thermal decomposition to pyrrhotite first. In the preheater, CaCO3 and CaO absorbed a portion of SO2. The SO2 absorption by CaCO3 belonged to direct sulfuration reaction, whose reaction rate highly depended on the temperature, moisture content and CO2 concentration of the flue gas. Some researches deemed that CaO was prior to react with CO2 in the preheater circumstance, which would affect the absorption of SO2. The absorption efficiency of SO2 in the raw meal mill was considered to decrease with the increasing drying temperature or decreasing moisture content of the raw material particles. The reaction product was dominated by CaSO3·1/2H2O, which would form an enrichment on the surface of the raw meal particles. For the relative high-temperature sulfur cycle, the studies indicated that the temperature, O2 concentration, and sulfur/alkali ratio played a pivotal role in the decomposition reaction of sulfate in the rotary kiln. The pyrolysis gas generated when the cement kiln co-disposes the alternative fuel would significantly reduce the decomposition temperature of CaSO4 and promote the release of SO2, which would aggravate with the decrease of the size of alternative fuel and the increase of volatile matter content. The sulfur contained in the clinker was mainly in the form of K2SO4、3K2SO4·Na2SO4、K2SO4·2CaSO4、CaSO4, and other substances in the silicate minerals, wherein increasing the sulfur content in the clinker would stabilize the C2S and inhibite the formation of C3S. This paper reviews the cycle mechanisms of sulfur in the whole process of cement production and emphases the low-temperature and high-temperature cycle of sulfur. Revolving round the low-temperature cycle of sulfur, the writer detailedly expounds the sulfur-containing form of raw meal and its detection methods, the oxidation of pyrites and the characteristics of releasing the SO2, the absorption mechanism of CaCO3 and CaO in the preheater environment, and the sulfur fixation of the raw meal in the grinding process, respectively. Furthermore, the writer elaborates the volatilization and circulation mechanism of sulfur in the cement kiln, the in-fluence of alternative fuels on sulfur release, the form of sulfur in the clinker and its effects on the quality of clinker on the high-temperature cycle of sulfur.
王俊杰, 房晶瑞, 汪澜. 水泥生产全过程硫循环机制的研究进展[J]. 材料导报, 2018, 32(23): 4160-4169.
WANG Junjie, FANG Jingrui, WANG Lan. The Sulfur Cycle Mechanism in the Whole Process of Cement Manufacturing: a Review. Materials Reports, 2018, 32(23): 4160-4169.
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2018, Vol. 32 
