Chromium deportment in copper matte equilibrated with CrxO-containing slag
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An understanding of the behaviour of chromium in mattes equilibrated with chrome-saturated slags is essential for the prediction and control of chromium deportment in these melts. The main ore reserves of South Africa’s platinum group metals (PGM) are associated with the Merensky and UG2 reefs of the Bushveld Complex. The gradual depletion of the pyroxenitic Merensky reef over the years has necessitated the PGM industry to exploit the underlying chromiterich UG2 reef. The problem with UG2 reef is that it contains significant amounts of chromium, this being typically 5% (reported as Cr2O3) against <1% for the Merensky reef (in concentrates). Chromium has a number of deleterious effects on base-metal smelting processes. Under given conditions, it forms chromite spinels, which can accumulate and, over a period of time, form undesirable build-ups resulting in reduced furnace operational volume. The chromite spinels also tend to increase slag viscosity, thereby impacting negatively on the slag/matte separation, which leads to matte entrainment in slag. Moreover, high viscosities lead to problems with tapping of the furnace melts. Finally it can be said that chromium that deports to the matte during smelting will normally tend to precipitate as spinels during subsequent converting, and lead to the formation of very stable and unwanted slag foams in the converter. In literature, there exists very little published work on the behaviour of chromium in sulphur-saturated systems such as matte-smelting furnaces. The few publications that exist in this area do not cover the effect of controlling all the pertinent variables simultaneously on the behaviour of chromium in mattes in equilibrium with chromium-containing slags. Hence the main focus of the oxygen fugacities on the mineralogical and deportment behaviour of chromium in a matte-slag system. All the three variables (temperature, oxygen fugacity (pO2), and sulphur fugacity (pS2)) were investigated at three levels. Temperatures studied were 1300°C, 1400°C, and 1500°C. Oxygen and sulphur fugacities were established by controlling the mix-ratios of purified carbon dioxide, carbon monoxide, sulphur dioxide, and argon. Oxygen fugacities were maintained at 10-10atm, 10-8atm, and 10-6atm; whereas sulphur fugacities were maintained at 10-6atm, 10-4atm, and 10- 2atm. In order to investigate the effect of each of these three variables (temperature, pS2, and pO2) at different levels of the other variables, a completely randomised 33 full factorial experimental design was adopted. The study revealed that chromium is generally present in matte as both dissolved CrS and as precipitated oxidic and sulphidic chromium spinel phases. It was shown that as the conditions become more oxidising (pO2 = 10-6atm), CrS (which is soluble in matte) becomes a predominant phase, and as the conditions become more reducing (pO2 = 10-10atm), the sulphospinel, daubreelite (FeCr2S4), becomes a more predominant phase. Oxidic chromium spinels were found to be present in matte under the more oxidising conditions (pO2 of 10-6atm) of this investigation. The presence of the above-mentioned phases was confirmed using X-ray diffraction. Subject to the experimental conditions employed in this research, chromium was found to partition the least to matte under the conditions of; low temperature, high pO2, and low pS2. Conversely, chromium was found to partition the most to matte under the conditions of; high temperature, low pO2, and high pS2.