Selection of a suitable ligand for the supercritical extraction of gold from a low-grade refractory tailing

Abstract

Reprocessing tailings for gold recovery is generating new low-grade refractory secondary tailings. Unlocking gold trapped within these secondary tailings potentially holds additional economic value. In this study, the use of supercritical carbon dioxide (scCO2) at 100 bar and 40 °C for gold extraction from such a secondary gold tailings sample (0.27 g/t) was investigated. The research identified and screened the following ligands as suitable extractants for gold in scCO2, i.e. 3-(trifluoromethyl)-phenyl-thiourea (TPT), 1,1,1-trifluoro-2,4pentanedione (TFA), betaine bis(trifluoromethylsulfonyl)imide ([Hbet][TF2N]), 4-methyl-4H-1,2,4-triazole-3-thiol (MHTT) and hexafluoroacetylacetone (HFA). Results from screening experiments showed that extraction of gold for all ligands in the presence of a tri-n-butyl phosphate-nitric acid adduct (TBP-HNO3) as oxidant exceeded 50% after 18 h, with the highest extraction offered by [Hbet][TF2N] (82% after 24 h). A comparative study, where no scCO2 was present, showed that the presence of scCO2 offered a distinct advantage in extraction.

Recovery of Precious Metals: A Promising Process Using Supercritical Carbon Dioxide and CO2-Soluble Complexing Polymers for Palladium Extraction from Supported Catalysts

“…The ability of two copolymers, poly(4-vinyl pyridine-co-1,1,2,2tetrahydroperfluorodecylacrylate) (P(4VP-co-FDA)) and poly(diphenylphosphinestyreneco-1,1,2,2-tetrahydroperfluorodecylacrylate) (P(DPPS-co-FDA)), to extract up to 50% of Pd from alumina-supported catalysts was demonstrated [21][22][23]33]. The use of molecular complexing additives, such as dithiocarbamates, beta-diketones, dithizone, and perfluorocarboxylic acids, is also possible and has been reported in the literature [34][35][36][37][38][39]. However, molecular complexing agents appear to be less efficient than complexing copolymers to extract metals, although they allow avoiding the synthesis step of polymers.…”

Synthesis and Phase Behavior of a Platform of CO2-Soluble Functional Gradient Copolymers Bearing Metal-Complexing Units

“…Furthermore, dense CO 2 is a non-polar solvent, unable to solubilize metals; thus, additives are required for metal extractions. So far, different reports have shown the use of low molecular weight compounds, (e.g., dithiocarbamates, beta-diketones, dithizone, perfluorocarboxylic acids) which can act as complexing additives [16,[44][45][46][47][48][49][50][51]. Nevertheless, our group has reported in the last years the possibility to use scCO 2 -soluble metal-complexing polymers to extract metals from solid matrices, without dissolving or destroying the supports [52][53][54][55].…”