Metallurgical and Chemical Applications of Intermetallics
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MRS BULLETIN/MAY 1996
We will review these two classes of applications using representative examples from both process metallurgy and chemistry.
Process Metallurgy Recovery and Reduction We have already cited the ancient practice of using amalgams in the recovery of precious metals, but there are still other examples of the use of intermetallics in this area. Lead. Lead bullion, a product of the lead blast furnace, ordinarily receives further refining, both to recover precious metal values and to remove detrimental impurities. The famous Parkes refining process (1850) depends on intermetallic compound formation for its success. Following a preliminary softening (removal of Ca, As, Sb, Sn, and Cu) by treating the molten lead with sulfur and later air, zinc is added to the base bullion. This process takes advantage of the facts that AuZn (melting point: Tm = 725°C) and Ag2Zn3 (Tm = 661°C) have higher melting points and lower density than lead, and are virtually insoluble in lead saturated with zinc. Furthermore gold combines preferentially with zinc, so a separation of gold and silver is also possible. The molten alloy to which zinc has been added is held at 480°C in kettles of up to 100-ton capacities for several hours. The crust containing the precious-metal intermetallics rises to the surface where it may be skimmed off and the valuables recovered in a separate cupellation. Bismuth. Most bismuth is produced, not from bismuth ores, but as a by-product of the lead-refining process and to a lesser extent from processing residues of cop-
per smelting and refining operations. In the pyrometallurgical Kroll-Betterton process, metallic calcium and magnesium are added to desilverized lead bullion held molten in a kettle at 485°C. The Ca, Mg, and Bi react to form the ternary intermetallic CaMg2Bi2, which has both a higher melting point and lower density than the lead bullion. As the temperature of the kettle is lowered, the Bi-rich IMC floats to the top, where the dross can be removed by skimming. Reduction of the dross with lead chloride or chlorine gas removes the calcium and magnesium to leave a Bi-rich (7-wt% Bi) alloy that is readily processed to 99.999+ pure bismuth. Zinc. Although most commercial zinc today is Special High Grade, produced electrolytically, pyrometallurgical reduction and refining were once important and are still practiced today for lower zinc grades. Crude zinc from the horizontal retort furnace contains significant amounts of lead, iron, and other impurities. Many of these can be removed by simple liquation in which the zinc is held just above the melting point (419°C) for 24-72 hours. Oxides and impurities form a surface dross, removable by skimming. Liquid lead (insoluble in liquid zinc) sinks to the bottom of the furnace and can be drawn off. Just above the lead, a layer of FeZn 13 (Tm = 530°C) forms and effectively extracts all of the iron. In the prepurification of zinc before electrolytic refining, zinc dust is added to remove impurities by cementation. This process is more efficacious if a
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