Comparison of Ferrous Calcium Silicate Slag and Calcium Ferrite Slag Interactions with Magnesia-Chrome Refractories
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TRODUCTION
THE Pierce-Smith converter has served the copper industry for 100 years, but increasingly strict environmental regulations and higher energy costs have set the trend toward continuous copper converting. A major consideration with continuous converting is the choice of slag. Acidic iron silicate slag, as used in the Pierce-Smith converter, has many favorable properties, including little attack of magnesia-chrome refractories and a high capacity to absorb basic oxides, such as lead oxide. It also has a satisfactory solubility for magnetite at the high oxygen partial pressures used during copper making as a result of the presence of large amounts of Cu2O in the slag. However, iron silicate slag also has some significant disadvantages. It is relatively viscous, which leads to a risk of foaming and limits the slag tapping rate. Iron silicate slag also has a poor capacity for absorbing acidic oxides such as those of arsenic and antimony; these elements are deleterious contaminants in copper. Despite these problems, iron silicate slag is used in the Noranda converting process,[1] Ausmelt batch converting,[1] and direct-to-blister flash converting.[2] R.R. KAUR, Engineer, and C. NEXHIP, Operations Manager, are with Rio Tinto, Kennecott Utah Copper, Salt Lake City, UT 84044. Contact e-mail: [email protected] D.R. SWINBOURNE, Professor, Deputy Head, and Director, is with the School of Civil, Environmental & Chemical Engineering, RMIT University, Melbourne, Victoria, 3001 Australia. M.W. WADSLEY, Consultant, is with Austherm Pty Ltd, Richmond, Tasmania 7025, Australia. Manuscript submitted November 16, 2009. Article published online March 29, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
Mitsubishi introduced basic calcium ferrite slag for their continuous converting process because it has high magnetite solubility, even in the absence of dissolved Cu2O. Calcium ferrite slag also has a high capacity for absorbing acidic oxides. However, the low viscosity of calcium ferrite slag gives rise to significant refractory attack,[3] although it has the advantage of minimizing the risk of slag foaming. Calcium ferrite slag also has limited silica solubility, so carry-over of silicate slag from the smelting to the converting stage becomes a problem by causing dicalcium silicate precipitation. Finally, calcium ferrite slag has a poor ability to absorb basic oxides, such as lead oxide. Calcium ferrite slag is used in both the Mitsubishi process and the Kennecott flash converting process.[1] Adding CaO to iron silicate slag increases the activity coefficient of Cu2O above that for either iron silicate or calcium ferrite slags.[4] It also reduces the slag volume[5] and viscosity. Thus, Yazawa et al.[4] proposed that a slag whose composition is between that of iron silicate and calcium ferrite could reduce copper loss to the slag, adequately absorb both acidic and basic oxides, and not cause significant refractory wear problems. The anticipated reduced viscosity, compared with iron silicate slag, would also lessen the risk of
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