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

Spongiform metallic chromium was obtained directly from K2CrO4 in the equimolar CaCl2-KCl molten salt by electrolysis at 1173 K and this reduction mechanism was discussed. The chemical composition and microstructure of the products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The reduction mechanism was analyzed by cyclic voltammetry, square wave voltammetry and potentiostatic electrolysis. K2CrO4 is soluble in the molten chlorides. The soluble CaCrO4 was easily formed in the molten CaCl2-KCl before electrolysis and dissociated to Ca2+ and CrO42−. The CrO42− was firstly electro-reduced to CaCr2O4 by a three-electron-transfer process and CaO was released simultaneously. The generated CaO with a high solubility was dissolved into the CaCl2-KCl molten salt and electro-reduced to metallic calcium. As the electrolysis proceeded, the amount of released CaO increased and more CaO was electro-reduced to Ca. CrO42− can be reduced to CaCr2O4 by electrolysis and calcium reduction simultaneously. Then, CaCr2O4 was further reduced to metallic chromium by the dissolved Ca in the near-cathode region. However, the inclusion of a little CaCr2O4 brought oxygen into metallic Cr powders.