Recovery of Cobalt and Copper from Zambia Slag via Sulphidisation with Gypsum

Abstract: The main objective of this research was to recover copper and cobalt from slags coming from copper processing on the Copperbelt province of Zambia. Pyrometallurgical processing of copper sulphide concentrate generates huge tonnes of slag wastes. Slag is basically made up of metal oxides and silicates and is less dense than matte, this makes it to float and is separated from matte by skimming. However, the separation is not 100 % efficient. This results in valuable elements such as copper and cobalt being lost to the slag and may require to be recovered. Copper in slag may exist in metallic (Cu), oxide (CuO and Cu2O) and sulphide (Cu2S and Cu5FeS4) forms whilst much of cobalt in the slag exists as an oxide because of the high oxygen potential during conventional copper smelting operations. Copperbelt slags contain about 1.2 wt. % copper and 0.6 wt. % cobalt whereas the slag that was worked on had average grades of 0.997 wt. % for copper and 0.258 wt. % in the case of cobalt. The idea of reprocessing slag dumps is important from both economical and sustainable perspectives, it is also in line with circular economy principles. To find the optimum operating conditions, a series of tests were carried out during sulphidisation and roasting in which the temperature was gradually increased by 50 °C ranging from 1050 °C to 1150 °C in the case of sulphidisation with varying calcium sulphide (CaS) addition of 5 %, 10 %, 15 % and 20 % of the total weight of the slag. This was followed by roasting with a temperature increase of 50 °C ranging from 550 °C to 650 °C. The use of waste gypsum for sulphidisation is advantageous because it is a recycled waste product that comes from the cobalt leach plant. Based on thermodynamic considerations and the optimized parameters, 50 g of slag  was first sulphidised in an electric furnace with CaS ratio of 15 wt. %. In essence, sulphidisation which was conducted at a temperature of 1150 °C for two hours was meant to convert the slag into matte. The matte was then converted to sulphates by roasting at a temperature of 650 °C for three hours followed by leaching of the now acid soluble material using dilute sulphuric acid (H2SO4). Copper and cobalt were captured in solution form. The leach residue was subjected to atomic absorption spectroscopy (AAS)  for the analysis of the levels of metal content retained in the residue. Scanning electron microscopy (SEM) was used to study the microstructure and the mineralogy of the three selected samples namely; the as received slag, the sulphidised slag and the leach residue.