A novel method for the recovery of silver from thiosulphate leach solutions using trimercapto-s-triazine (TMT)

Abstract

• TMT effectively precipitates Ag from ammoniacal thiosulphate solutions. • Precipitation of Ag with TMT occurs fast. • TMT dosage is the most influential parameter on the recovery of silver. • Efficiency of the method is also demonstrated using real leach solutions. • A comprehensive characterisation of Ag-TMT precipitate is provided. In recent years, the demand for silver has increased due to its use in green technologies such as photovoltaic cells and electric vehicles (EVs). Due to environmental concerns associated with the use of cyanide leaching for gold/silver production, alternative lixiviants have received particular attention with thiosulphate having been often quoted as the most prominent one owing to its environmentally friendly characteristics and some technical advantages. However, thiosulphate leaching suffers from technical difficulties encountered particularly in the metal recovery from pregnant leach solutions. This study presents a novel approach for the recovery of silver from ammoniacal thiosulphate solutions through its precipitation using trimercapto- s -triazine (TMT), which is a non-toxic organo-sulphide reagent. Effects of concentration of TMT (0.07–0.33 mM), thiosulphate (0.20–1.00 M), and ammonia (0.40–1.00 M NH 3 ) on silver precipitation from synthetic Cu(II)–S 2 O 3 2− –NH 3 solutions were investigated by adopting the response surface methodology i.e., central composite design (CCD). The statistical analysis of the experimental data showed that the concentration of TMT had the highest impact on the extent of silver precipitation. Separate tests were also carried out to obtain the kinetic profiles of silver precipitation under varying concentrations of TMT (0.69–6.84 mM), copper (2.00–20.0 mM), and ammonia (0.10–1.00 NH 3 ) over 60 min. These results have also confirmed that silver can be readily and rapidly precipitated to the extent of 96.5% within a minute at a TMT concentration of 0.69 mM. The performance of the proposed method was also tested using real leach solutions derived from ammoniacal thiosulphate leaching of silver ore. It was found that extensive precipitation of silver (i.e., 99.0% Ag at 6.84 mM TMT) could be achieved with co-precipitation of copper (94.7%) and zinc (34.6%). SEM-EDS and FT-IR analyses were performed to characterise the precipitates obtained. This study has demonstrated that TMT can be used as a potent, non-toxic reagent for the recovery of silver from ammoniacal thiosulphate solutions.