Thermochemical treatment of platinum group metal tailings with ammonium salts for major element recovery

Abstract

It was recently demonstrated that the extraction of major elements from South African Platinum Group Metals (PGM) tailings can be achieved via thermochemical treatment with ammonium sulphate followed by aqueous dissolution. The current study uses the insight gained from the previous work and investigates and compares major element extraction efficiencies obtained using ammonium sulphate and three other ammonium salts (ammonium bisulphate vs ammonium chloride vs ammonium nitrate) as extracting agents at five different temperatures (350–550 °C). Overall, ammonium sulphate was the most promising reagent for the co-extraction of major elements, with the best extraction efficiencies achieved for aluminium (ca. 60%) and calcium (ca. 80%), alongside chromium (ca. 29%), iron (ca. 35%), magnesium (ca. 25%) and silicon (ca. 32%). In contrast, ammonium chloride and ammonium nitrate extracted smaller quantities of these elements, in particular chromium which was hardly extracted (≤ 1.2%), whereas calcium was nearly equally extracted for all salts (70–80%). It could not be ascertained whether the plagioclase phase, as anorthite, reacted with ammonium salts during the thermochemical step, but it was found to dissolve nearly completely during the acid leaching step in the absence of thermochemical processing. This phase was the main contributor to calcium and aluminium in solution. The greatest effect of temperature was on the flowability characteristic of the reaction product after treatment rather than on elemental extraction efficiencies. Thermochemical treatment using ammonium sulphate represents the most promising route for extracting valuable elements from PGM tailings, which could be subsequently converted into value-added products.