Masters Degrees (Chemical Engineering)

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Browsing Masters Degrees (Chemical Engineering) by Author "Banda, Wezi"

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    Pyrometallurgical recovery of cobalt from waste reverbaratory furnace slag by DC plasma-ARC furnace technology
    (Stellenbosch : Stellenbosch University, 2001-12) Banda, Wezi; Eksteen, J. J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Slag cleaning has become a common practice at many smelters in the nonferrous industry to maximize recovery of valuable metals. However, during the carbothermic reduction of nonferrous slag to recover cobalt, in particular, iron is recovered predominantly. High iron levels present a problem for the subsequent treatment of the alloy as it may increase the solids loading to the filter and lead to increased reagent consumption during leaching. Finding an appropriate slag modifier that would selectively improve the recovery of cobalt against that of iron to the metallic alloy can solve this problem. In the present study the effects of lime (CaO), rutile (Ti02), and fluorspar (CaF2) on the recovery of cobalt from waste nonferrous slag have been investigated under reducing conditions at 1500°C. The selective recovery of cobalt compared to the recovery of iron at different levels of flux additions is discussed in this study, to show the selectivity of these fluxes. It is also shown in the study that the recovery of cobalt does not only depend on the oxygen partial pressure and temperature but on the slag composition as well. The slags used in the experiments were a) synthetic slag prepared from chemically pure reagents and its composition was derived from that of the industrial nonferrous slag composition typical of reverbaratory furnace and b) actual slag obtained from an old slag dump situated on the Zambian Copperbelt region. The investigation has shown that Ti02 addition leads to the most selective cobalt recoveries in all cases. On the other hand, both CaO and CaF2 lead to higher overall cobalt recoveries. The effect of Ti02 on the slag chemistry leads to the formation of iron titanate compounds in the slag unlike CaO, which displaces "FeO" from the fayalitic slag and thus increases the activity of FeO in the molten slag, which in tum affects the iron recoveries to the alloy product. CaF2 on the other hand, affects the fluidity of the slag leading to improved recoveries by improved settling of metals through the slag to the alloy product. A 44V/I100A DC-plasma arc furnace was used to reduce slag to recover cobalt at about 13.5kV A power input. The major part of the study was conducted in a tube furnace and the findings were applied to the extraction of cobalt from slag using the plasma-arc furnace. It was found that the synthetic slag experiments could be used as a guide to understand the behaviour of cobalt during the carbothermic recovery of cobalt from silica saturated fayalitic slags. Cobalt recoveries were higher in the DC furnace than the corresponding reduction experiments carried out in the tube furnace. However, significant amounts of silicon and carbon were detected in the metallic alloy product of the DC plasma-arc furnace.