Gold and silver extraction from Hellyer lead-zinc flotation middlings using pressure oxidation and thiourea leaching

A new route for the extraction of gold and silver from three different galena-sphalerite-pyrite flotation middlings (bulk scavenger tailing, zinc middling and pyrite middling) assaying 2.3, 2.6 and 1.5 g/t Au and 82, 53 and 27 g/t Ag, respectively, and containing different proportions of the sulphide minerals was investigated. Each sample was pressure oxidised for 4 h at 200°C or 220°C and 700 kPa oxygen overpressure, at an initial acidity of either 0 or 36.8 g/l H 2SO 4. This resulted in solid products containing, respectively, either hematite and lead jarosite, or iron hydroxy sulphates and lead sulphate. Most of the zinc and copper, together with some iron and a little silver, were dissolved and were removed by filtration. X-ray diffraction (XRD) examination of the solid products from pressure oxidation at high initial acidity confirmed that they contained mainly a basic iron sulphate (Fe(OH)SO 4) with some hydrated iron sulphate (Fe 2S 2O 9·5H 2O) and lead sulphate. Thiourea leaching of these products for 4 h at natural pH (2.3–3.4) and Eh (200–350 mV vs SCE), with a thiourea addition rate of 2 g/l h −1, extracted 83–92% of the gold and 39–96% of the silver, depending upon the sample. The lowest silver extractions were from the pyrite middling. The low silver extractions could have resulted from jarosite formation during thiourea leaching. Some of the hydrated iron sulphate reacted during the leach and there was evidence of silver re-precipitation after 3 h. Thiourea consumptions were 34–73 kg/t. XRD examination of the solid products from pressure oxidation at low initial acidity confirmed that they contained mainly hematite and lead jarosite. Thiourea leaching of these products for 4 h at pH 1.5, with an addition of 17,500 ppm iron(II), an Eh of 225–250 mV, maintained with hydrogen peroxide, and a thiourea addition rate of 2 g/l h −1, extracted 46–88% of the gold but only 2–7% of the silver. The low gold extraction was obtained from the pyrite middling. The low silver extractions are believed to result from silver incorporation into the jarosite lattice during pressure oxidation. Thiourea consumptions were 16–70 kg/t. Gold and silver extractions by thiourea leaching were, in general, inferior to those obtained by cyanidation of similar pressure oxidation products after lime treatment. For cyanidation, gold extractions from all samples were generally around 90%, as was silver extraction from the basic iron sulphate/lead sulphate product. Silver extract