Roles of Electrolyte Characterization on Bauxite Electrolysis Desulfurization with Regeneration and Recycling
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INTRODUCTION
IN China, the exploitation of high-sulfur bauxite was highly desirable with the quick development of alumina industry.[1,2] However, during the leaching process of high-sulfur bauxite, the sulfur-containing compounds could result in many disadvantages, such as accelerating the corrosion of equipment, increasing NaOH consumption, deteriorating the equality of Al2O3, and lowering the coefficient of heat transfer during the distillation process.[3] Therefore, it is necessary to conduct the desulfurization before using the minerals, which would not only potentially improve the efficiency of aluminum oxide production, but also increase the applicability of high-sulfur bauxite.[1] Fuel containing sulfur, such as coal[4] and oil,[5] was sometimes electrolyzed to achieve desulfurization.[6–8] S-containing phases (e.g., FeS2) were transformed into SO42 and/or S2O32 by oxidants (e.g., Cl2, HClO and HO[9–13]) during water electrolysis. The solid–liquid mixture could be then easily separated to realize the
XUZHONG GONG and MINGYONG WANG, Associate Professors, ZHI WANG, Professor, SIYUAN ZHUANG and YUHUA WANG, Engineers, and DONG WANG, Assistant Professor, are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China. Contact e-mails: xzgong@ ipe.ac.cn, [email protected] Manuscript submitted January 8, 2016. Article published online October 25, 2016. 726—VOLUME 48B, FEBRUARY 2017
purpose of desulfurization. However, some other S-containing species (such as PbS and CuFeS2) in ores may be converted into S0 or S8[14] that could not be easily oxidized into SO42. Therefore, these ores rich in PbS and CuFeS2 were not suitable for electrolysis desulfurization. As expected, S-containing phase in bauxite was mainly FeS2 which could be oxidized to liquid ions by various oxidants (e.g., OH and/or O2 from water electrolysis[15–17]). FeS2 oxidation reactions during electrolysis in NaCl solution[18,19] are shown in Eqs. [1] and [2]. 2FeS2 þ 15ClO þ H2 O þ 2Hþ ! 2Fe3þ þ 4HSO 4 þ 15Cl ;
þ FeS2 þ 8H2 O 15e ! Fe3þ þ 2SO2 4 þ 16H :
½1
½2
Through the reactions above, FeS2 in bauxite was converted into soluble SO42, and very little solid S-containing phase was produced, such as S0 and CaSO4, and so on. However, the desulfurization ratio would have to be compromised if the formation of vitriol (jarosite) with other water-insoluble species was severe in the condition of the PH value as low as 3 to 5,[20] as shown in Eq. [3]. Actually, in acidic solution (low pH value), solid Fe(OH)3 would not be formed.[21] Instead, the liquid Fe3+ which had strong oxidation activity to react with FeS2 (Eq. [4])[22,23] was substantially available in the solution. Of course, during electrolysis process, Fe2+ was oxidized into Fe3+ (Eq. [5]).[24,25] METALLURGICAL AND MATERIALS TRANSACTIONS B
Naþ þ 3Fe3þ þ 2SO2 4 þ 6H2 O !NaFe3 ðSO4 Þ2 ðOHÞ6 þ 6Hþ ; ½3
þ FeS2 þ 14
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