Researches on preparation and properties of sodium polysulphide as gold leaching agent

Abstract

To date, cyanide-assisted gold leaching has been considered to be the predominant method for gold extraction owing to its simple process and low cost. Increasing concerns on the toxicity of cyanide have triggered the efforts to find the non-cyanide methods to extract the gold from gold ores for the sake of environmental protection. In this work, the leaching behaviors of gold from ores by using alkaline sodium polysulphide solutions were investigated. Sodium polysulphides were prepared by hydrothermal method using sulfur and sodium sulphide nonahydrate (Na2S·9H2O) as precursor under alkaline condition. Our results show that, as-prepared polysulphide showed dual actions: as oxidant and complexant. Accordingly, alkaline sodium polysulphide system was applied to extract the trace-level gold from ores. It demonstrates that, the gold-leaching efficiency is dependent on the concentrations of sodium sulphide, the ratio of sulfur/sodium sulphide, the concentrations of sodium hydroxide, and reaction temperature. The optimized gold-extraction efficiency (85%) was achieved under following conditions: pressure of 1atm; ratio of liquid to solid at 5:1; the concentration of sodium sulphide and sodium hydroxide at 90g/L and 0.8M, respectively; the sulfur/sodium sulphide ratio at 3; reaction temperature at 80°C; reaction time for 1h. To reveal how the composition of sodium polysulphide affects the gold leaching, the phases of polysulphide were characterized by combing X-Ray Powder Diffraction, Laser-Raman Spectrum, and Electrochemical methods. The results indicated that, as-prepared sodium polysulphide mainly comprises of the sulfur, sodium trisulfide, sodium tetrasulfide and sodium pentasulfide, wherein the tetrasulfide and pentasulfide are the main activate reagents for gold leaching. The finding in this work indicates that, the sodium polysulphide could have a good capability for Au extraction from Au-bearing antimony ore.