Evaluation of the activity and molecular form of bi in cu smelting slags: Part I. ternary silicate slags
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I.
INTRODUCTION
ENVIRONMENTAL contamination by minor elements associated with copper-bearing ores and concentration of those elements in blister copper are problems of increasing severity as ore quality declines. Antimony, arsenic, bismuth, selenium, and tellurium enter smelting processes in the form of sulfides, oxides, intermetallics, or as complex refractory compounds. Those elements are either volatilized, representing an air quality problem, dissolved in the discard slag, or are retained in the blister copper and thus must be removed during electrolytic refining. Selenium and tellurium are not soluble in electrolytes used in copper electrochemical refining cells, eventually being removed in the cell bottom slime. Antimony, arsenic, and bismuth are not so readily eliminated. These elements dissolve in the electrolyte as they are less noble than copper. This results in electrolyte poisoning unless careful monitoring and chemical control are exercised. These elements also will contaminate cathode copper, reducing the metal purity. Contamination of cathode copper occurs by occlusion of impure electrolyte as copper is plated out of solution at the cathodes. The presence of trace quantities of minor elements in copper significantly reduces its ductility, electrical conductivity, and lowers its thermal conductivity. Embrittlement of copper also occurs due to impurity-phase precipitates at the copper grain boundary interfaces. These issues point out the need for an understanding of the behavior of minor elements during smelting if their concentration in blister copper is to be minimized. In this study the behavior of bismuth in silica-saturated slags was examined.
S. C. MARSCHMAN is Senior Research Engineer with Battelle Pacific Northwest Laboratories, Richland, WA 99352. D. C. LYNCH is Professor in the Department of Materials Science and Engineering at the University of Arizona, Tucson, AZ 85721. Manuscript submitted October 21, 1987.
METALLURGICAL TRANSACTIONS B
II.
REVIEW OF THE LITERATURE
The interaction of bismuth with slags, mattes, white metal, and blister copper has been examined by several investigators. In lieu of restricting the review to only Bi-slag interactions and for the sake of brevity, the results of those earlier investigations are summarized in Table I. Further review of those results is presented, as needed, in the discussion. III.
EXPERIMENTAL
A. lsopiestic Experimental Technique An isopiestic technique was used to evaluate the solubility of bismuth in slag. This technique eliminates the loss of bismuth through volatilization and it allows the experimentalist to set the activity of bismuth in the system. Furthermore, as other condensed phases are not present, the isopiestic technique has the advantage of eliminating analytical errors associated with entrapment of particles of one phase in another. During an experiment a 0.5 g slag specimen was saturated with Bi vapor at 1458 K and 1523 K using the assembly shown in Figure 1. The assembly consisted of a series of tubes and crucibles posit
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