Direct electrolytic preparation of chromium powder
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1/2/04
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Direct Electrolytic Preparation of Chromium Powder GEORGE Z. CHEN, ELENA GORDO, and DEREK J. FRAY 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 m (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).
I. INTRODUCTION
POWDER metallurgy is preferred to traditional melting and casting technologies for producing and processing metal alloys containing metals of very different reactivities, densities, and melting points.[1–6] The success of powder metallurgy relies, to a great degree, on not only the quality of the powder product, but also the relative cost of the production process. The so-called atomization process has been successfully applied to a number of low-melting-point metals on industrial scale, such as aluminum.[5] However, there still remain many challenges for making powders of highmelting-point metals, particularly those having a high reactivity with the air constituents, such as oxygen, nitrogen, and carbon dioxide. An example is chromium powder, which finds many applications in nonferrous high-performance alloys,[6] sputter target material for plasma or spray coating,[7] and cermets, which are metal-ceramic composites of high electric conductivity, thermal stability, and corrosion (oxidation) resistance.[3,4] Chromium has a melting temperature of 1857 °C, and, hence, chromium powder is very difficult, if not impossible, to produce by the atomization method. At present, commercial production of chromium powder involves two main steps: (1) metallothermic (e.g., Al or Ca) or electrolytic (extraction) reduction, and (2) mechanical milling,[8–11] both contributing significantly to the high market price of the material (
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