Cobalt recovery from old Nkana Copper Slag via solid state carbothermic reduction and sulphation

Large quantities of old copper smelting slag containing cobalt exist at Nkana Division of Zambia Consolidated Copper Mines. Recovery of cobalt from the slag would contribute significantly to the economy of Zambia. A literature review, however, indicated that although processes have been developed for the recovery of cobalt from primary ores and converter slags, none have been for dump slags. The potential economic benefits of cobalt recovery and non¬existence of a process for recovery of the metal from dump slags stimulated interest in undertaking the study reported in this thesis. Thermodynamic considerations indicated that cobalt could be recovered from old Nkana copper slag via solid state carbothermic reduction and sulphation. Consequently, sulphation and carbothermic reduction experiments were carried out in the temperature range 600 to 1000°C for times ranging from 1 to 6 hours. The experiments were aimed at determining whether the two processes could be suitable as pretreatment steps in the recovery of cobalt from old Nkana copper slag. In addition to reaction temperature and duration, other experimental parameters investigated were; particle size of the slag, relative amounts of the reactants and for sulphation only, the effect of air flowrate.The extent of carbothermic reduction and sulphation were determined by evaluating the percentage of cobalt solubilized from reacted slag samples leached in sulphuric acid to a terminal pH of 1.4-2.0. Solid state carbothermic reduction was carried out in an inert nitrogen atmosphere with charcoal as a reducing agent. The results obtained indicate that solid state carbothermic reduction produces a partially reduced product which can be selectively leached for cobalt. A fair amount of cobalt was leached (52%) from the -0.60 + 0.150mm fraction of slag reduced with 20 % charcoal at 950 °C for two hours. Difficulties of obtaining higher percentages of cobalt leached were due possibly to the formation, during reduction, of non-leachable cobalt ferrites and carbides. Solid state sulphation of old Nkana copper slag was carried out using pyrite and cobalt sulphide concentrates as sulphating agents. The highest amount of cobalt leached at 72% was obtained from the -0.045mm fraction of slag sulphated with pyrite. A 1:1 slag to pyrite ratio was found to be suitable for the solid state sulphation of old Nkana copper slag. The work reported in this thesis consists of four chapters, which are preceded by an introductory section. Chapter one gives the literature review. Chapters two and three are preceded by theoretical considerations, and discuss the results of solid state carbothermic reduction and sulphation, respectively. Each chapter ends with its own references. Chapter four gives the general conclusions.