Understanding the distribution of platinum group metals and gold in the process of copper-nickel slag depletion with gas mixtures

Abstract

This paper describes the results of a study that looked at the distribution of platinum group metals (PGMs) (Pt, Pd, Rh, Ru, Ir) and Au between Cu – Ni – Fe bottom phase and dump slag as a result of pyrometallurgical depletion of slag, a by-product of oxidizing smelting of sulphide copper-nickel concentrate. The pyrometallurgical process was performed in a bubbling mode using reducing gas mixtures of various compositions. The consumption of reducing agent was found to be the key factor governing the output of the bottom phase and hence the chemical composition of the products. For the first time ever, the authors used the results of the chemical analysis of the products to calculate the distribution coefficients for PGMs and gold between the bottom phase versus the slag when the depletion process involved an intensive stirring of the melt. Thus, a variation range was established for the distribution coefficients of noble metals, as well as the threshold concentration of Fe in the melt. Once the latter has been reached, the distribution coefficients remain almost unchanged and the extraction of PGMs into dump slag is minimal. Using scanning electron microscopy and electron probe microanalysis, the authors established how many noble metals get carried away with dump slag. Proneness of Au to get dissolved in silicate melts was revealed, which is also typical of Cu. The conducted study helped establish that only 5% of Pt, Pd and Rh gets carried away with dump slag, which is the least loss. At the same time, about 10% of Ru and Ir goes into dump slag. The data reflecting the behavior of Au during slag depletion with reducing gas mixtures confirm that the maximum recovery of this metal from the slag would only be possible under a high reduction potential of the system. This corresponds to the minimum concentration of Fe in the bottom phase of 50% wt.