Direct electrolytic preparation of chromium powder

Abstract

Chromium oxide powder (Cr2O3) was slip cast or pressed into small cylindrical pellets which were then sintered in air. The sintered pellets were attached to a current collector to form an assembled cathode. Constant-voltage (2.7 to 2.8 V) electrolysis, with a graphite anode, was performed in molten CaCl2 (950 °C). After electrolysis, the pellets were removed from the molten salt and washed in water. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and fusion elemental analysis all confirmed that, when electrolyzed for a period longer than 4 hours, the Cr2O3 pellets were fully reduced to Cr metal. The oxygen content in the product depended on electrolysis time. Typically, for a 6-hour electrolysis, less than 0.2 wt pct oxygen was found in the product, with the current efficiency and energy consumption being 75 pct and 5 kWh/kg, respectively. The fully reduced pellet had a friable strength and could be manually crushed into a powder composed of cubic crystallites, very uniform in size, that grew with the electrolysis time, up to 50 μm (15 hours). The unique product morphology (cubic crystallites) differs drastically from the nodular morphology observed in other metals prepared by similar methods and is rarely seen among various commercial metal powders. A reduction mechanism is proposed, emphasizing the surface metallization at the early stage of electrolysis through the propagation of the metal-oxide-electrolyte three-phase interline (3PI).