An alternative approach to recover lead, silver and gold from black gossan (polymetallic ore). Study of biological oxidation and lead recovery stages

Abstract

A novel procedure to recover lead, silver and gold from polymetallic sulphide ores, cleaner than the traditional hydrometallurgical route (hot brine leaching), is proposed. This process consists of a biooxidation stage, where sulphides are oxidised by the action of extremophiles, followed by an acid washing and a citrate leaching in which lead is recovered. The final solid obtained, mainly composed of quartz, is rich in silver and gold. This paper is focused on the biooxidation and the lead recovery of the black gossan, a polymetallic sulphide ore with valuable amounts of lead, silver and gold. Biooxidation performed with a mixed mesophilic culture (mainly Acidithiobacillus ferrooxidans) at 20% pulp density is able to dissolve the sulphide matrix, removing the gold refractory behaviour and producing jarosite, beaverite, gypsum and anglesite. A previous sulphuric acid washing of biooxidation residue greatly improves the lead recovery in the citrate stage. In the sulphuric acid washing, jarosite and beaverite are dissolved, being the most important variables the temperature and sulphuric acid concentration. The jarosite dissolution kinetics shows that the rate controlling step is the chemical reaction, with an activation energy of 86.4 kJ/mol. A linear relationship between the jarosite dissolved and lead recovered is found, reinforcing the necessity of a previous H2SO4 washing. The optimal pH range for lead recovery in the citrate solution is 5–9. Pulp densities higher than 5% produces an unstable solution, precipitating a part of the lead. A negative effect of gypsum has been confirmed through several citrate leaching tests performed with pure anglesite. The solid obtained after the studied stages is suitable to recover gold and silver that contains.