Removal of copper from carbon-saturated iron with an aluminum sulfide/ferrous sulfide flux
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[1]
Starting from the equilibrium constant for the reaction in Eq. [1], we can derive an equation for the distribution coefficient of copper (LCu) between the matte and the metal; i.e., LCu
WCu+ XCu+ gCu 5 5 k 5 kK1 WCu XCu gCuS
0.5
=
aFeS aFe
[2]
where k is a constant used to convert mole percent to weight percent, K1 is the equilibrium constant for the reaction in Eq. [1], ai is the activity of species i, gi is the activity coefficient of species i, Xi is the mole fraction of species i, and Wi is the weight percent of species i. Equation [2] indicates the variables that are available to maximize LCu. The activity coefficient of copper in iron can be raised by increasing the carbon content or decreasing the silicon content in the metal[1] or by lowering the melt temperature, but such changes have little effect or applicability in a practical sense. Raising the activity of ferrous sulfide increases the sulfur content of the metal and is therefore undesirable. The activity of iron is essentially fixed because the metal is iron based. Reduction of the activity coefficient of the cuprous sulfide remains the best way to significantly increase the distribution coefficient of copper between the matte and metal phases. The most recent research on slagging for copper removal has concentrated on binary fluxes of ferrous sulfide, combined with sodium sulfide or other alkali or alkaline earth sulfides;[2,3,4] however, the resulting distribution coefficients
ADAM COHEN, Engineering Assistant, is with the Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439-4838. MILTON BLANDER, formerly Senior Scientist, Chemical Technology Division, Argonne National Laboratory, is retired. Manuscript submitted September 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
have been too low for commercial processing. Work done to determine the enthalpies of mixing (DHm) of binary mixtures of molten salts with a common anion, both charge symmetrical and asymmetrical systems, provides a possible basis for reducing the activity coefficient of cuprous sulfide. The activity coefficients of each compound can be decreased in binary mixtures; the change is a function of both relative cation size and charge. Many of the results from molten salt studies have been summarized by Kleppa[5,6] and Blander.[7] This article presents results from a study in which a binary flux of ferrous sulfide and aluminum sulfide was used. The use of aluminum sulfide improves the thermodynamics over that in other ferrous sulfide-based fluxes and therefore increases the distribution coefficient of copper. Each test used '90 g of carbon-saturated iron and 22.5 g of the flux of interest; a patent[8] describes experiments that used 10 g of carbon-saturated iron and 2.5 g of flux, but we chose the larger samples to allow for continuous stirring of the sample during the test. Although an iron alloy saturated in carbon is not equivalent to any typical scrap grade, the carbon-saturated iron was used to facilitate comparisons with other published result
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