UBC Theses and Dissertations
Oxidative pressure leaching of chalcocite by INCO Ltd.’s second stage leach process Saito, Benjamin
The INCO Ltd. Copper Refinery Electrowinning Department (CRED) in Copper Cliff, Ontario, Canada, processes a Cu, Ni, Co, Fe, As, S and precious metals containing residue produced by the INCO Pressure Carbonyl (IPC) plant. This residue is treated in two successive leaching stages. The First Stage batch leach extracts most of the Ni, Co, Fe and As. During the Second Stage batch leach process, the oxidative pressure leaching of a predominantly Cu₂S feed is effected under acid deficient conditions. The main process objective is to preferentially extract the copper, leaving behind a residue rich in precious metals. Several physico-chemical parameters hinder the copper extraction by limiting oxygen mass transfer. Part I of this investigation focuses specifically on the impact of temperature and total arsenic content. Laboratory experiments were conducted at temperatures ranging from 100 to 135°C with different plant feed solids having arsenic concentrations ranging from 0.37 to 1.4 wt.%. Additional tests were performed with controlled arsenic additions to the feed electrolyte. The results showed that conditions of high temperature and high arsenic were detrimental to overall copper extraction rates. Suppression of leach kinetics was attributed to the formation of high viscosity basic copper sulphate slurries which inhibited oxygen mass transfer. High viscosity slurries resulted from the preferred precipitation of sub-micron acicular basic copper sulphate crystals over large platelet shaped crystals. The observed effects were believed attributable to two main factors: elevated supersaturation levels from increased initial rates of chalcocite leaching, and the formation of a heterogeneous nucleation substrate, possibly, basic copper arsenate, Cu₂As0₄(OH). In part II of this investigation, the oxygen mass transfer/mixing performance of the Second Stage leach process was examined. This was done in scaled-model tests (125 L). Improvements in surface aeration rates through changing impeller systems were sought. Results from the scaled model tests showed that converting from the current dual axial down-draft impeller system to a radial disk impeller system resulted in improvements in mass transfer rates by as high as 200% at 221 rpm. For a dual radial disk system, mass transfer rate increases came at the expense of significantly higher power draw. For a radial(top)/axial system, high mass transfer rates were achieved with only marginal power draw increase. Fill level sensitivity tests revealed that the axial impeller system was most sensitive to overfilling, resulting in near zero mass-transfer rates from only a +5% increase in fill height above the scaled normal fill height. Similar decreases in mass-transfer rates for both radial impeller systems came after +10% increase. Based on the test-work performed, the radial/axial system was judged to be the most successful in overall kLa and power draw performance. The internal cooling coil system was found to offer a partial baffling effect, sufficient to prevent gross swirling yet low enough prevent central vortex collapse. This condition was believed to be critical in achieving high gas-liquid mass transfer performance in the scaled model.
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