Electroanalytical study of the reduction of chromate in molten lithium chloride-potassium chloride eutectic

Abstract

Chronopotentiometry of chromate in molten LiCI-KCI eutectic revealed a single, irreversible, diffusioncontrolled, three-electron reduction step. The diffusion coefficient of chromate ion at 450° is estimated to be 1.7 x 10-6 cm2 sec-1. Current-reversal chronopotentiometry revealed abnormally short reverse transition times, which decreased markedly with rising temperature, but increased somewhat in the presence of added oxide ion. It is concluded that the primary reduction product is the ion Cr04+, which decomposes to yield Cr03-3 and O-2. At a sufficiently high surface Concentration of CrO4-5, solid Li,Cr04 is deposited.In the presence of MgClz or CaCI,, the reduction potential is shifted 0.8 and 0.6 volt, respectively, in the positive direction.Extraordinarily stable deposits are produced. No reverse electrochemical oxidation of these deposits occurs.

Electroanalytical Chemistry in Molten Salts—A Review of Recent Developments

“…Laitinen and co-workers (70)(71)(72)(73) examined the reduction of the chromate ion in this melt. A three-electron reduction of CrOl-to CrO~-was observed.…”

Direct electro-deposition of metallic chromium from K2CrO4 in the equimolar CaCl2-KCl molten salt and its reduction mechanism

“…Interestingly, with the addition of a small amount of CaCl2 into the LiCl-KCl molten salt, the reduction potential of chromate shifted sharply toward the positive direction [13,14,16,17]. It indicates that Ca(II) plays an important role in the reduction of chromate in molten chloride salts.…”

“…Obviously, chromate is closer to the crude ore than Cr2O3. In addition, chromates are stable in air and can be easily dissolved into the molten salts [4,13,14]. It is well known that high solubility of reactant in electrolyte contributes to fast mass transfer and promotes the metal electrodeposition.…”

“…It will be a new route for the safe detoxifying action of chromium slag if metallic chromium can be directly prepared from chromates. Some researchers have studied the electrochemical behavior of chromate such as K2CrO4 in the equimolar LiCl-KCl molten salt [13,14,16,17]. It was found that Li5CrO4 was the main electrolysis product by a three-electron-transfer process [13,14].…”

“…It indicates that Ca(II) plays an important role in the reduction of chromate in molten chloride salts. Although some researches have been carried out in the last decades of the 20 th century [13,14,16,17], the disagreement about the cathodic reduction products still exists. Especially, the direct preparation of metallic chromium from the chromates by molten salt electrolysis was not reported.…”