Influence of Silica on the Lime-Free Roasting of Chromium-Containing Slag
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NTRODUCTION
ALKALI roasting-water leaching technique is used worldwidely for extracting chromium from chromiumcontaining minerals and industrial solid wastes. In the alkali roasting process, chromium-containing minerals and alkali were mixed with each other homogeneously and heated in a rotary kiln or other furnace. Once the temperature reached as high as enough, the trivalent chromium begins to be transformed into water-soluble sodium chromate in the presence of oxygen. For the
KAI-PING YU, M.S. Student, is with the College of Zijin Mining, Fuzhou University, Fuzhou 350108, Fujian, China, and with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, and also with the Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, Beijing 100190, China. BO CHEN, Associate Professor, is with the College of Zijin Mining, Fuzhou University, Fuzhou 350108, Fujian, China. HONG-LING ZHANG, Associate Professor, SHAO-YONG DUAN, Research Assistant, HONG-BIN XU, Professor, and YI ZHANG, Academician of the Chinese Academy of Engineering, Professor, are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China, and also with the Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, Beijing 100190, China. Contact e-mail: [email protected] YUAN-YUE LIU, M.S. Student, is with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, and with the Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, Beijing 100190, China, and also with the School of Chemical Engineering and Technology, Hebei University of Science and Technology, Shijiazhuang 050018, Hebei, China. Manuscript submitted December 1, 2014. Article published online November 5, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
nearly half a century, the lime-free alkali roasting technique has been employed to produce sodium chromate instead of lime-based alkali roasting due to its less generation of chromite ore processing residue (COPR) and few emission of hazardous Cr6+ compounds.[1] During the roasting reaction process, the sodium chromate and sodium carbonate form a binary eutectic liquid phase with a low melting point of 928 K (655 °C).[2,3] The objective particles are surrounded by the generated binary eutectic liquid phase of Na2CO3Na2CrO4 and the transportation of oxygen from all around to the reaction interface is greatly impeded. As a result, the oxidation rate of trivalent chromium decays sharply.[4] The oxygen fugacity of the roasting atmosphere also plays an important part in the lime-free roasting[5,6] and excess sodium carbonate decreases the fractional volume of the sodium chromate in the plural liquid phase that lowers the oxidation rate of trivalent chromium.[7] Besides that, the impurit
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