Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 2 4 from the reactor bottom and recycled using conventional magnesium-reduc- tion methods. Batches of titanium up to 10 metric tons are produced. Sponge product is further processed to remove unreacted titanium chlorides, magne- sium, and residual magnesium chlo- rides. These impurities, which can be as much as 30 wt%, are removed by acid leaching in dilute nitric and hydrochlo- ric acids at a low energy requirement of 0.3 kWh/kg of sponge, but effluent pro- duction of 8 L/kg of sponge; vacuum distillation at a temperature between 960° and 1020°C for up to 60 h; and ar- gon sweep at 1000°C used by Oremet, a subsidiary of Allegheny Technolo- gies Inc. After purification, sponge is crushed, screened, dried, and placed in airtight, 23-kg drums to await consoli- dation. Energy required to convert TiCl 4 to sponge, which is ready for further processing by the leaching routes, is 37 kWh/kg of sponge [25] , of which 97% is required for magnesium production. SODIUM-REDUCTION PROCESS The sodium-reduction process was used in the U.S., UK, and Japan for several years as an alternative to mag- nesium reduction. However, the last large production plant was closed in the early 1990s, although a small plant is still in operation in Salt Lake City under Honeywell ownership. Even though the process was more costly than magne- sium reduction, the product contained less metallic impurities (Fe, Cr, and Ni). Desirable for the electronics and computer indus- tries, high-purity titanium is pro- duced by The Alta Group Inc., Fombell, Pa., now part of Honeywell. Hardness, which indicates the de- gree of purity, is affected both by inter- stitial impurities (O, N, and C) and by the noninterstitial impurity Fe. Hard- ness numbers range from 80-150 HRB; 110-120 HRB for typical commercial sponge. Some developmental process- es, such as electrolysis reduction, pro- duce sponge with a hardness of 60-90 HRB. Iron impurities in Kroll sponge are difficult to control due to diffusion into the sponge from the reactor wall. In the sodium-reduction process, sponge is protected from the wall by sodium chloride. Other impurities originate Fig. 5 − Schematic of conventional two-station vacuum arc melting of titanium. Fig. 6 − Schematic of electron-beam cold hearth melting (top) and plas- ma cold hearth melting (bottom) of titanium.