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 to 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 to value-added products.

Thermochemical purification of talc with ammonium sulphate as chemical additive

“…It consisted of unreacted, irregularly-shaped talc-rich particles similar to those found in TalcHCl, which were surrounded by numerous globular structures (Figure 5b). These globular particles were different from the perfectly formed hexagonal structures of metal-and ammonium-metal sulphate species reported previously (Doucet et al, 2016;Mohamed et al, 2016Mohamed et al, , 2019. They were most likely precursors to similar hexagonal structures, but had nevertheless developed crystalline lattices that could be identified by XRD; longer thermochemical reactions may have induced the growth and conversion of the globular particles into fully-shaped hexagons.…”

“…Flotation (Ahmed et al, 2007) • No chemical changes occur and Marruzzo, 1992;Hojamberdiev et al, 2010) • Reduces the iron content Chlorination (Orosco et al, 2011) • Removes Fe2O3 efficiently (Wu et al, 2014;Doucet et al, 2016), laterite nickel ore (Li et al, 2018), PGM tailings (Mohamed et al, 2016(Mohamed et al, , 2019, and bauxite residue (Borra et al, 2016). This process can also be successfully applied for the extraction of magnesium from serpentinite rocks in the context of CO2 mineral sequestration (Nduagu et al, 2012).…”

“…The temperature was selected to ensure that AS loss was minimised during thermochemical treatment, allowing AS to be recycled at a subsequent stage. The selected duration of the thermochemical treatment ensured good flowability of the powdered reaction product after thermochemical treatment (Mohamed et al, 2019).…”