Chromium electrowinning

Anodes. Lead—antimony (6—10 wt %) alloys containing 0.5—1.0 wt % arsenic have been used widely as anodes in copper, nickel, and chromium electrowinning and metal plating processes. Lead—antimony anodes have high strength and develop a corrosion-resistant protective layer of lead dioxide during use. Lead—antimony anodes are resistant to passivation when the current is frequendy intermpted. 

Table 6. Operating Parameters for Electrowinning of Chromium from Chrome Alum...

Table 7. Cell Conditions for Chromic Acid Process for Electrowinning of Chromium ...

A number of electrolytic processes are used for the industrial production of metals. Some metals such as zinc, copper, manganese, gallium, chromium, etc. are electrowon from aqueous baths. Another common electrolytic process used is molten salt electrolysis. The most important application of molten salt electrolysis till now has been in the electrowinning of metals. Today aluminum, magnesium, lithium, sodium, calcium, boron, cerium, tantalum, and mischmetal are produced in tonnage quantities by molten salt electrolysis. As a representative example, the electrowinning process for aluminum is taken up. 

It consists in a deposition of ions from an electrolyte onto the cathode in an electrolytic cell, under the influence of an applied potential. Usually the process is accompanied by material dissolution from the anode. The electrowinning from aqueous solutions is an important commercial method for the production (and/or refinement) of many metals, including, for instance, chromium, nickel, copper, zinc. As for the electrodeposition from non-aqueous solutions, the primary production of aluminium, electrodeposited from a solution of A1203 in molten cryolite, is a typical example. Other metals which may be regularly reduced in a similar way are Li, Na, K, Mg, Ca, Nb, Ta, etc. 

The net result is that metal dissolves from the anode and deposits on the cathode. The phenomenon is the basis of electroplating (e.g., chromium plating of steel), electrowinning, and electrorehning. Also, it is the analytical basis of an electrodeposition method known as electrogravimetry. This involves the separation and weighing of selected components of a sample. Most metal elements can be determined in this manner, usually deposited as the M° species, although some metal elements can be deposited as oxides. The halides can be determined by deposition as the silver halide. Metals commonly determined include Ag, Bi, Cd, Co, Cu, In, Ni, Sb, Sn, and Zn. 

Pure chromium metal is obtained commercially from the aluminothermic or silicother-mic reduction of chromium sesquioxide, or chromia (Cr Oj), and to a lesser extent by electrowinning. In the aluminothermic process, first the chromia is prepared by the soda-ash roasting of chromite ore. After mixing the raw chromite ore with sodium carbonate or soda... 

Finally, chromium metal can be obtained by electrowinning from chromium-rich liquors by two electrochemical processes chromium-alum electrolysis and chromic acid electrolysis. In chromium-alum electrolysis, the chromium-rich liquor is obtained by leaching high-carbon ferrochrome with recycled spent catholyte containing chromium alum [(NHJCr(S0J.12H30] and makeup sulfuric acid. It contains ammonium chromium alum. In chromic acid electrolysis, chromium trioxide is dissolved in deionized water acidified with sulfuric acid. The performance of each electrowinning process is presented in Table 4.85. 

In this paper, results of studies performed in our laboratory, concerning the electrochemistry of hafiiium, molybdenum and chromium in molten alkali chlorides with additions of fluorides will be outlined. The electrochemistry of niobium will not be discussed, because niobium electrowinning is extensively dealt with in this conference. 

From such a concentrated aqueous solution of the metal, the recovery of the metal is achieved in a variety of the ways. The metal may be precipitated as a hydroxide, an oxide, or calcined as an oxide or recovered as a metal via electrowinning (e.g., Cu, Co, Ni), etc. Illustrations of process flowsheets for the recovery of a variety of metals, e.g. copper, zinc, nickel, uranium, chromium, beryllium, etc., are available in Ritcey and Ashbrook (1984b). Benedict et al. (1981) also provide process flowsheets and descriptions of metals relevant to the nuclear power industry. 

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