Post-mortem investigation of magnesia-chromium brick corrosion by speiss in a copper smelting furnace

Worn magnesium-chromium type refractory bricks and copper-arsenic metal/speiss/matte deposits recovered from the Rio Tinto Kennecott Copper Flash Smelter were analyzed to evaluate the mechanism of refractory penetration and wear by the high-arsenic copper matte/speiss at elevated temperatures. Microstructural characterization was performed using optical and scanning-electron microscopies, and the compositions of the samples and their constituent phases were measured using micro-X-ray fluorescence, energy dispersive X-ray, and wavelength dispersive X-ray spectroscopies. The observed microstructures and sample/phase compositions were interpreted based on the phase-equilibrium behavior of the complex copper-arsenic-magnesium-sulfur-oxygen slag/metal/speiss/gas system. The initial solidification and/or oxidation of the high-grade, arsenic-rich matte was found to cause the formation of the liquid copper-arsenic speiss with the low solidus temperature, which penetrated deeply into the refractory brick lining of the copper flash smelter. When the penetrating speiss was exposed to oxygen, the arsenic present was selectively oxidized and reacted with magnesio-wüstite from the refractory brick to form magnesium arsenates, degrading the refractory material by intergranular corrosion.