The simulation of a transient leaching circuit

[摘要] The hydrometallurgical leaching of sulphide concentrates was introduced in the1950's. Generally the leaching mechanisms of these processes are not understoodfundamentally. The reasons for this are the inherently complex nature of sulphidechemistry and that the sulphide concentrates usually consist of highly intergrownsulphide minerals. The leaching kinetics of sulphide concentrates where only onemetal-sulphide mineral occurs have been investigated intensively, but not forsulphide concentrates with more than one metal-sulphide mineral. The behaviour ofthese mixed metal-sulphide minerals has mostly been investigated on plant scale toqualitatively determine the leaching trends of the process. The consequence of therelatively unKnown leaching mechanism and kinetics is that these processes are notcontrolled efficiently.This study was conducted on the acid-oxygen pressure leaching of Ni-Cu matte (thefirst stage leach process at the Ni-Cu refinery of Impala Platinum Ltd.). As a first stepto improve the control efficiency of the process, the process must be stabilised.Therefore, an off line computer simulation program is proposed to control therepulping section of the plant that has previously been controlled solely by anoperator. Controlling the repulping section is very important, because conditions existin the repulping tanks for leaching to occur. This causes perturbations in the pulpentering the pressure leach autoclave. Due to the fast reaction kinetics of the mattein the pressure leach autoclave the perturbations entering the autoclave will influencethe performance of the acid-oxygen pressure leach process. The simulation programwas tested on the plant and indicated that considerable improvement in the stabilityof the operation could be achieved.In obtaining a better understanding of the behaviour of this process, it is essentialthat key variables and trends are identified. A methodology is proposed to analyseand model this ill-defined and poorly understood process from historical data byvartificial neural networks (ANN), inductive learning by decision trees and statisticaltechniques. The back propagation neural network, learning vector quantization neuralnetwork and the decision trees yielded comparable classification rates between 73%and 84%, and could serve as a basis for the adjustment of operating conditions toimprove the efficiency of the process. The relative importance of the processvariables is determined by a method of sensitivity analysis and together with thestatistical mean, the effect of an increase or decrease in the variable on the processis quantified. These results are substantiated by experimental findings.A leaching mechanism for the acid-oxygen pressure leach of Ni-Cu matte ispostulated. The leaching sequence of the nickel and copper sulphides is Ni3Sr Ni7SNiS-Ni3S4, and CU2S-CU31SWCU1.BS-CUS, respectively. Ni7Sa and CU31 S1a areintermediate nickel and copper sulphide phases that form during the leachingprocess. Ni alloy has a galvanic effect on the sulphide minerals which inhibits theoverall leaching rate and results in the formation of H2S and the intermediate nickeland copper sulphides (Ni7Sa and CU31 S1a). A semi-empirical kinetic model wasdeveloped based on the chemical reaction rate expressions of the leachingmechanism. This model can accurately simulate the batch leaching process forvariations in the oxygen partial pressure, oxygen flowrate, temperature, particle size,initial acid concentration and pulp density. A sensitivity analysis on the modelindicated that for a matte with a lower initial Ni alloy content the leaching rate ofnickel is much faster.

[发布机构] Stellenbosch University