Extraction of trace elements from platinum group metal concentrates in hydrothermal conditions

Abstract

Concentrates of platinum group metals, despite the high proportion of precious metals, contain significant amounts of impurity non-ferrous elements, the presence of which negatively affects the technology of refining and leads to an increase in the cost of marketable products. In this connection, it is promising to develop approaches for the most complete, deep removal of base elements from concentrates received for processing. In doing so, an extremely important requirement is the complete prevention of the noble metals passing into the solution, since otherwise they “get smeared”, and the process chain is complicated. In the present work, there are data on the features of the composition of concentrates of four platinum group metals, and the possibilities of removing nonprecious metals from them under autoclave conditions are studied. It was found that concentrates, in which platinum metals are represented mainly by platinum and palladium (K1-K3), slightly dissolve in water both at 25 оС in open systems and at elevated temperatures (180 оС). They are characterized by rather high silver contents, and their solubility is due to the presence of a water-soluble form of silver sulfate in them. The transition to the solution of other noble metals (NM) was not detected. In the case of a concentrate enriched with rhodium, iridium and ruthenium (K4), anomalous solubility in water was observed, which was 25.1 and 23.8% at 25 оС and 180 оС, respectively. As this takes place, noble metals at 25 оС significantly passed into solution (34.3%), and under autoclave conditions, the degree of dissolution decreased by 15 times. The use of formic acid was supposed, on the one hand, to ensure the dissolution of metals which are placed below hydrogen in the electrochemical series (iron, nickel and cobalt), and on the other hand, to prevent the dissolution of noble metals. The degree of extraction of noble and impurity non-ferrous metals into the solution from rich concentrates at 120 оС and 180 оС during their treatment with aqueous solutions of formic acid was established. The use of 10% formic acid provides a transition into the solution of iron (90%) and nickel (55%), and is accompanied by a decrease in the mass of the C1 concentrate by about 6%. NM do not pass into the solution in this case. For C2, the mass reduction is 10.1% due to the removal of 94% iron and 82% nickel at a temperature of 180 оС. The total concentration of platinum group metals in the solution does not exceed 2 mg/L. When processing C3, among all the noble metals iridium noticeably passes into the solution (3.5 mg/l), and out of impurity elements 45% nickel and 47.5% iron dissolve with a total weight loss of the concentrate of 8%. As for C4, selectivity of separation of NM and impurity base elements was not achieved; significant amounts of rhodium, ruthenium and iridium were found in the solution.This research was partially carried out under a state assignment of the Institute of Chemistry and Chemical Technology of the Siberian Branch of the RAS (Project: 0287-2021-0014) using the equipment of the Krasnoyarsk Regional Shared Knowledge Centre of the Federal Research Center “Krasnoyarsk Scientific Center of the Siberian Branch of the RAS”. The authors would like to thank Ms Zhizhaeva, a holder of PhD degree in Engineering, for conducting electron microscopy and X-ray studies.