A mathematical model of ionic transport in a porous diaphragm of a chrome-alum cell
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I.
INTRODUCTION
T H E electrowinning of chromium from aqueous chrome-alum solutions (i.e., electrolytes containing chromic and ammonium sulfate) is an attractive alternative to extraction from chromic acid. m Such a process is employed industrially in Marietta, Ohio by the Elkem Metals Company. t~a,3~ The use of chrome-alum solutions reduces the current consumption since deposition occurs from its trivalent (as opposed to hexavalent) oxidation state. Chrome-alum solutions are also less toxic than chromic acid. The process was developed in 1939 by the United States Bureau of Mines. In 1950, the Bureau, in collaboration with the Union Carbide Corporation, conducted a pilotplant study to determine present industrial operating conditionsJ 11 Although the industrial process has been in operation for several decades, there is little fundamental understanding of the electrochemistry and transport phenomena in the cell. In an attempt to improve present performance and to determine the feasibility of applying similar technology to the removal of chrome ions from wastewater concentrates, laboratory-scale studies were performed at Columbia University by Arslan t41 and Vidal. [5] At the cathode of the chrome-alum electrowinning cell, chromic ions are reduced to chromous ions, Cr +3 + e- ~ Cr +2
[l]
which accumulate in the catholyte and are further reduced to elemental chromium: Cr +2 + 2e- ---> Cr ~
[2]
These reactions compete with the discharge of hydrogen ions, 2H + + 2e- --->H2
[3]
which leads to a loss in cathodic current efficiency. The ROBERTO VIDAL, Graduate Student, PAUL DUBY, Professor, and ALAN C. WEST, Assistant Professor, are with the Department of Chemical Engineering, Materials Science, and Mining Engineering, Columbia University, New York, NY 10027. Manuscript received October 1, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
Bureau's pilot-plant study reported cathodic current efficiencies of 60 pct. tL21 Industrially, current efficiencies in the range of 40 pct are common, tr~ The catholyte pH, optimally around 2.5, is crucial: if the catholyte is more acidic, the current efficiencies are reduced as a result of hydrogen evolution. If the catholyte becomes basic, insoluble precipitates form, suppressing chrome reduction. The steady-state concentration of Cr(II) ions is also important. To attain acceptable current efficiencies, ratios of Cr(II)/Cr(III) greater than 1 are required. L1,2,31The catholyte must be a strongly reducing medium to favor the buildup of chromous ions. The following oxidation reactions prevail on the anode: 2Cr +3 + 7H20 ~ Cr207 -2 + 14H + + 6e-
[4]
2 H 2 0 ~ 4H + + 02 + 4e-
[51
Since the purpose of the electrowinning cell is to reduce chromic ions, Reaction 141is undesired; nevertheless, this reaction may consume more than 90 pct of the anodic current. The hydrogen ions produced in both reactions result in a steady-state anolyte pH in the range of 0 to 1.[4.5] Thus, the anolyte is a highly acidic and strongly oxidizing medium. Since the chemical environments differ dramatica
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