High-temperature Volatilization Of Sodium Salts And Their Solid-phase Reactions With Cement Raw Materials

During the production of new type dry cement production, the circulation and enrichment of harmful volatile components such as sulfur, chlorine and alkalis can result in the skinning, crusting and blocking problem in kiln end and pre-calcination system. Especially, the use of low-quality cement raw materials and fuels exacerbates the above phenomenon. Finding out heterogeneous reactions mechanism between volatile components and minerals under the condition of thermal system in the rotary kiln is the key to reveal the skinning process. In this work, the model of pure mineral reagents is the research object. The volatilization characteristics of three kinds of sodium salts(NaCl, Na2SO4 and Na2CO3) under different atmospheres and temperatures and the effects of cement raw materials CaO, SiO2, Al2O3 and Fe2O3 on the migration and transformation of sodium were studied by thermodynamic equilibrium calculation. The heterogeneous reactions between sodium salts and minerals were conducted by the combination of high-temperature calcination, XRD, TG-DSC and SEM-EDS. The results are summarized as following:(1) The initial volatilization temperature of three sodiums can be squenced as: Na2SO4> Na2CO3>NaCl. NaCl volatilizes mainly in the form of gaseous NaCl, and Na2CO3 was released in the form of gaseous Na. Moreover, Na2SO4 was volatilized to gaseous Na2SO4 and simultaneously decomposed to Na(g) and NaO(g). In air, SiO2 could react with Na2SO4 and Na2CO3 to form(Na2O)(SiO2)2, and thus hinder the volatilization of Na2SO4 and Na2CO3 by chemical reactions. However, SiO2 will enlarge the temperature interval of liquid phase existence. As a amphoteric oxide, Al2O3 readily reacts with three sodim salts to form NaAl11O17, which could inhibits the volatilization of sodium; Fe2O3 only reacts with Na2CO3 to form stable compound NaFeO2; CaO can suppress the volatilization of the three sodium salts through physical bonding effects.(2) In flue gas atmosphere, the volatization of NaCl decreased with increasing SO2 concentration, no matter what kind of minerals were added into the system. When sodium salts Na2SO4 and Na2CO3 were doped with SiO2, the evaporation of NaCl would not be influenced by the concentration of SO2. With the increas of SO2 content, for three sodium salts, the proportion of liquid sodium gradually increases and liquid region migrates to higher temperature zone.(3) During the calcination process of Portland cement, NaCl is the easist to volatilize, followed by Na2SO4, Na2CO3, in turn. The rapid volatilization temperature zone of three kinds of sodium salts would migrate to higher temperature zone with the increase of salts content, and liquid sodium shares the similar tendency. At high temepratures, NaCl can react with SiO2 to generate Na4SiO4. Unfortunately, Na4SiO4 is not stable, and hence can be decomposed at high temperatures; Na2SO4 can dissolve in solid solution and even react with cement raw materials to form Na2Ca3Si2O8. Na2CO3 will react with clinker mineral C3 A to generate high content of NC8A3.