Oxygen deleading of lead-bismuth alloys
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I.
INTRODUCTION
B I S M U T H is removed from lead in the penultimate refining process of lead metal by the Kroll-Betterton process. This process utilizes calcium and magnesium metals which react with bismuth to form the solid intermetallic compound CaMg2Bi: which rises to the surface of the liquid metal. The optimum masses of the metals, men and mMg (kg), required per tonne of bullion to b e t r e a t e d a r e [I]
2BiC13 + 3Pb = 3PbC12 + 2Bi
mca = 0.959{[Bi]i- [Bi]y} + O.022r2/3[Bi]f2/3 mMg = 1.163{[Bi]i- [Bi]f} +
O.044r-1/3[Bi]f2/3
[1] [2]
where [Bi]i and [Bi b represent the initial and final (specified) concentrations (weight percent) of bismuth in the bullion and r represents the specific cost ratio of magnesium to calcium. The solid compound is subsequently skimmed from the surface of the liquid and contains entrained lead. A flow diagram of the current scheme for recovery of pure bismuth from this material is shown in Figure 1. The material streams are numbered and identified in Table I; the composition of streams (weight percent Bi) is also indicated. The skimmed material (bismuth dross at stream 4, 5.2 pct Bi) is passed to the liquation kettle in which some physical separation of liquid lead and the solid is achieved. The liquation lead (stream 3, - 0 . 0 2 pct Bi) is recycled to the debismuth kettle. The liquation blocks (stream 6, 6.2 pct Bi) pass to the lead-chloride upgrade kettle in which calcium and magnesium associated with the intermetallic compound together with the calcium and magnesium dissolved in the entrained lead metal are reacted with lead chloride (stream 7) from the chlorination kettle. The reactions are Ca + PbC12 ~ CaC12 + Pb
[3]
Mg + PbC12--~ MgC12 + Pb
[4]
The reactions proceed essentially to completion. The exchange slag (stream 9, t 0 pct Bi) comprising CaC12, FUNSHO OJEBUOBOH, Senior Research Engineer, is with ASARCO, Salt Lake City, UT. DAVID R. MORRIS, Professor, is with the Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada. Manuscript submitted October 26, 1992. Paper based on presentation at The Metals Society of CIM Meeting.
Nonferrous Pyrometallurgy: Trace Metals, Furnace Practices and Energy Efficiency. Proc. Symp., R. Bergman et al., eds., The Metals Society, CIM, Montreal, 1992 pp. 135-47. METALLURGICAL TRANSACTIONS B
MgC12, and excess PbC12 is used in the exchange kettle to remove dissolved calcium and magnesium from the de-Bi lead (stream 5, 0.02 pct Bi) by the same reactions. The chloride slag (stream I 1) is a waste stream. The high-bismuth metal (stream 8, 5.7 pct Bi) is passed to the chlorination kettle in which chlorine gas reacts preferentially with lead to form lead chloride. Consider the reaction [5]
At T = 800 K (527 ~ AG ~ - - 2 8 8 kJ t2] and the equilibrium lies well to the right permitting essentially complete elimination of lead. The process temperature is determined by the melting temperature of lead chloride (504 ~ At an intermediate stage of the chlorination process, the metal is treated with zinc for
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