The recovery of precious metals from acidic effluents using sodium formate

At Western Platinum Refinery in South Africa, zinc was used for the reduction of the platinum group metals (PGMs) in acidic effluent (palladium filtrate). Owing to the increasing cost of zinc and the risk of zinc pollution, sodium formate was investigated as an alternative reductant. It was found that pH 1.5 was the optimum starting pH for sodium formate reduction. The optimum concentration of sodium formate was found to be 18 g/dm³ at a temperature of approximately 100°C where the process time was 5 hours. The addition of sodium formate increased the pH of the final reaction mixture to approximately pH 4.5. Palladium was the most effectively reduced PGM, exhibiting an average precipitation efficiency of 98%. Difficulty was experienced with the precipitation of platinum (average precipitation efficiency of 47%). The precipitated PGMs were readily dissolved in hydrochloric acid (6 M) and sodium chlorate (2%). A reduction in costs resulted from the discontinuation of the use of zinc for reduction purposes. An additional advantage was that zinc was no longer introduced into the PGM refinery circuits. This effectively reduced the pollution potential of the acidic effluent.