An equilibrium study of the hydrogen reduction of copper sulfates
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N. JACINTO, S. N. SINHA, M. NAGAMORI, and H. Y. SOHN
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The reduction of copper sulfate with hydrogen or carbon monoxide gas was studied by Habashi and Dugdale I as a potential alternative to the electrowinning of metallic copper from aqueous solutions of the salt. The reduction reaction was readily completed at temperatures as low as 573 K. The hydrogen reduction was later applied to other sulfates such as NiSO4, COSO4, PbSO4, ZnSO4, and so forth, where reduction products were observed to vary from sulfides (e.g., Ni3S2, C09S8), oxides (e.g., ZnO), to oxysulfates (e.g., P b O . PbSO4), depending on the reaction temperature. 2 Despite the preliminary attempts made by Vijh, 3 the problem of predicting the compounds formed during the hydrogen reduction of a sulfate remains unanswered to date. The present work was undertaken to solve the problem theoretically and experimentally by taking the hydrogen reduction of CuSO4 as an example. The formation of Cu2SO4 during the hydrogen reduction of CuSO4 was established by X-ray diffraction studies. 4 Nagamori and Habashi 5 subsequently presented the free energy of formation for Cu/SO4 (s, f) together with the predominance area diagram of the Cu-S-O system including the stability zone for Cu2SO4. The stability of various compounds of the Cu-S-O system may be shown in a predominance area diagram as a function of the partial pressure of SO 2 (Ps02) and the ratio ofpH20 to PILE, as shown in Figure 1. The heat of formation of Cu/SO4(s) has been reported to be AH~9s = - 7 5 0 . 8 5 -+ 10.48 kJ/mol. 6 If the value of -761.33 kJ/mol is adopted, the stability zone of Cu2SO4 widens considerably as shown by A ' B ' D ' in Figure 1. On the other hand, if the value of -740.37 kJ/mol is used, the stability zone for CuzSO4 disappears at 623 K, i.e., Cu2SO4 is not stable up to 623 K. 5 For AH~98 = - 7 5 0 . 8 5 kJ/mol, which is used in the present study, the lowest temperature for Cu2SO4 stability is 562 K. Thus, a more reliable value of the heat of formation is desired to define the stability zone of Cu2SO4 more precisely. The free energy of formation of CuSO4 (s, ~) used in this work is as follows: 1 2Cu(s) + ~ S2(g) + 202(g) = CuzSO4(s):
AG ~ = -814185 - 23.26Tlog T + 467.53T J/mol (298 to 696 K) N. JACINTO, formerly Graduate Student in Metallurgy at the University of Utah, is now with University of Oruro, Oruro, Bolivia; S. N. SINHA is Graduate Student in Metallurgy at the University of Utah; M. NAGAMORI, formerly Associate Professor of Metallurgy at the University of Utah, is now with Centre de Recherches Minerales, Quebec Government, 2700 Einstein, Ste-Foy, Quebec G1P 3W8, Canada; and H. Y. SOHN is Professor in the Department of Metallurgy and Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112-1183. Manuscript submitted May 24, 1982. 136--VOLUME 14B, MARCH 1983
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Fig. 1 - - T h e predominance area diagram of
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