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3/3/04

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Page 197

The Solubility of Cr2O3 in Calcium Ferrite Slags at 1573 K N.P. FAHEY, D.R. SWINBOURNE, S. YAN, and J.M. OSBORNE Continuous converting of copper matte based on calcium ferrite slag has many attractions for the copper smelting industry. However, a serious drawback is that this slag leads to shorter furnace campaigns because it is aggressive toward the magnesia-chromia refractories that form the furnace lining. As part of an investigation into the causes of this aggressiveness with a view to devising strategies to mitigate it, the solubility of Cr2O3 in calcium ferrite slag has been determined. The standard drop-quench experimental technique was employed at a temperature of 1573 K and a relatively high oxygen partial pressure of 3.7
104 atm, conditions similar to those used in continuous converting. It was found that approximately 2 wt pct of Cr2O3 can dissolve in calcium ferrite slag under these conditions. The Cr2O3 solubility was found to be unaffected by the Cu2O content of the slag, but may decrease as CaO content decreases. The implications of these findings on the mechanism of attack of magnesia-chromia refractories by calcium ferrite slag are discussed.

I. INTRODUCTION

CONTINUOUS converting is an attractive technology for the copper industry. There are significant environmental and capital cost benefits of installing or retrofitting continuous converters into industrial copper smelting circuits.[1,2,3] There are two continuous processes for producing blister copper that have enjoyed commercial success, the Mitsubishi[2] and the Kennecott-Outokumpu[4] processes, while the Isasmelt process is reported to have been successfully demonstrated at the pilot scale.[5,6] These processes all use magnesia-chromia refractories to contain the molten slag and copper. Magnesiachromia refractories are made from a mixture of magnesia and chromium ores, which contain both chromium oxide and iron oxide. After firing, these refractories contain two main phases, periclase (almost pure MgO) as large grains and chromite spinel containing Cr2O3, Fe2O3, and a small amount of MgO, which surrounds the periclase grains and acts as a bonding phase for them.[7] The continuous converting processes use a lime-fluxed (calcium ferrite) slag rather than the traditional silica-fluxed slag. This has been found necessary because a high oxygen potential is required to produce copper metal, but this oxygen potential is also high enough to produce a large amount of magnetite in iron silicate slags. This leads to an unworkable slag. In contrast, calcium ferrite slag containing approximately 20 wt pct CaO does not saturate with magnetite, even under pure oxygen.[8,9] A drawback of calcium ferrite slag is that it leads to shorter furnace campaigns due to its aggressiveness toward the magnesia-chromia refractory bricks.[2,4,10–12] The problem is exacerbated by the fact that calcium ferrite slags also have much lower viscosities than iron silicate slags,[13,14] so dissolution products are more quickly transferred away N.P.