An electrolytic method of manufacture of a coarse tungsten powder

Abstract

the purity and particle size of the starting metal. Tungsten produced by powder metallurgy plants is characterized by a narrow particle size range (0.5-100 ~m), and is therefore not very suitable for direct use in electron-beam and arc furnaces owing to the difficulties arising in the melting of fine powders. Use of bar and rod materials leads to additional contamination of fused tungsten by impurities (particularly carbon) introduced during the powder pressing stage. Of particular interest from the point of view of rational utilization of tungsten waste is a solid-phase process (involving no melting) in which a coarse metal powder is grown by electrolysis in a fused medium. During electrodeposition tungsten is refined -- purified from a number of metallic contaminants and from the majority of gaseous and nonmetallic contaminants. The cathodic metal can, after being purified from electrolyte salts, be employed directly for melting. In this article are presented some results of laboratory investigations and pilot-plant tests of a technique for the manufacture of a coarse tungsten powder by electrorefining in an oxide-~hloride--fluoride fused bath. Under laboratory conditions use was made of a hermetic multicathode electrolytic cell [i] enabling the process to be performed in batches and electrolyte samples to be collected without stopping and dehermetizing the apparatus. Combined with constancy of the inert atmosphere employed, this ensures that cathodic deposits and bath samples of better quality are obtained. The electrolyte was a fused mixture of 60% NaCI, 15% NaF, and 25% WO3 [2]; the process parameters were D c = 0.4-1.0 A/cm 2, D a ! 0.i A/cm 2, and T = 830-930~ The principal factors affecting the quality and structural characteristics of deposits were found to be cathodic current density, temperature, electrolysis time, and purity and degree of utilization of anode material. Tungsten crystallized on the cathode in the form of individual acicular crystals and agglomerates. The acicular crystals were, depending on their internal structure, of two -- single-crytsal and twinned -- types. Their relative amounts in the cathodic deposit varied and depended mainly on the current density, rate of supply of tungsten ions to the cathode, and passivation of the growing faces. All crystals grew in the direction of the close-packed row of the bcc lattice. The faces were clearly defined, but varied in size and type of growth. The needle length attained 20-40 mmo In most cases along the whole needles there formed, without any apparent regularity, buildups of fine crystals and branches of large crystals.