Wettability and reaction of copper slag with MgO, Al2O3, ZrO2, MgCr2O4, MgAl2O4, ZnAl2O4 and Mg2SnO4

Abstract

Using a copper slag containing 63.1 wt% Fe₂O₃, 23.0 % SiO₂ and 1.9–3.5 % each of PbO, CuO, Al₂O₃, CaO and MgO, corrosion resistance of > 97.2 % pure refractory aggregates sintered to 0.4–4.7 % apparent porosity were evaluated by hot stage microscope. At a fixed temperature of 1425°C, MgCr₂O₄ displayed the greatest wetting angle (62°) and MgO had the largest relative height for the slag (91 % of starting 3 mm cube) demonstrating their superior corrosion resistance. MgO-MgCr₂O₄ bricks are the prevalent refractory used in copper converters despite studies that warn that spent materials may contain carcinogenic hexavalent chromium that pose health hazards. MgO was found to be superior to Al₂O₃ as base composition because of higher temperature of attainment of 90 % relative height of the slag (1425°C and 1412°C, respectively) and 30° contact angle (>1455°C and 1419°C). ZrO₂ and Mg₂TiO₄ had the lowest temperatures for 90 % slag height (1405°C and 1396°C) and 30° contact angle (1402°C and 1406°C), revealing their poor corrosion resistance. Mg₂SnO₄ formed a thick reaction layer of SnO₂, Mg₂SiO₄ and Fe₃O₄ containing 5–5.1 at% each of Mg and Sn indicating severe corrosion. MgAl₂O₄ and ZnAl₂O₄ had 90 % slag height temperatures of 1417°C and 1413°C, and 30° contact angle temperatures of 1426°C and 1420°C. The most corrosion resistant chromium-free refractory for copper converter is expected to be MgO-MgAl₂O₄ brick as both the phases exhibited delayed softening and spreading of slag. Upon reaction with MgAl₂O₄, low-melting fayalite-rich olivine in the starting slag was eliminated, magnetite solid solution remained along with a glass phase containing dissolved Al₂O₃, whereas MgO aggregate had Mg-rich glass and additional small amounts of high melting point compounds, Mg₂SiO₄ and MgO-FeO solid solution at the reaction interface.