Study On Neutral Roasting And Non-cyanide Leaching Of Gold-bearing Sulfides

Considering the risk of severe environmental pollution caused by traditional cyanide leaching technology, a novel approach of neutral roasting and non-cyanide leaching of gold-bearing sulfides was systematic studied. The sample was first subjected to roasting treatment during which the sulfide minerals were transformed into elemental sulfur. The elemental sulfur then reacted with lime added, producing gold leaching reagents for the following non-cyanide leaching process.The thermodynamics and kinetics of reaction process of pyrite during roasting was investigated. The results showed that pyrite could be transformed into elemental sulfur under neutral roasting atmosphere. The decomposition of pyrite followed the Mample principle and the corresponding mechanism was a random nucleus and the consequent growth. The kinetic equation could be expressed as followsIn this paper, gold flotation concentrate was used as the gold-bearing sulfides sample for the neutral roasting and non-cyanide leaching study. Au and S content of the sample was 62.68 g·t-1 and 30.52%, respectively. It showed that 42.53% of sulfur phase could be converted to elemental form after roasting treatment. The experimental conditions were nitrogen flow 1.0 L·min-1, sample mass 50g each, roasting temperature 800℃ and roasting time 60 minutes. The elemental sulfur was then mixed with lime and water and boiled for 45 minutes to produce sulfur-containing reagent, which was a leaching reagent for the following gold leaching process. The mass ratio of generated elemental sulfur, lime and water was 2:1:(50～75). The best gold leaching conditions were found to be:grinding fineness 90% -0.038 mm, reagent concentration 15%, Na2SO3 0.14 mol·L-1, NH3·H2O 2.1 mol·L-1, CuSO4 0.06 mol·L-1, Na2CO3 0.12 mol·L-1, solution temperature 60℃, leaching time 7 hours, ration of liquid to solid 6:1, stirring rate 550 r·min-1. Under these conditions, gold leaching rate of 96.31% was obtained, which was 5.41 percent points higher than the leaching process without roasting pre-treatment using sulfur-containing reagent.The neutral roasted products of gold-bearing sulfides were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that the pyrite in gold-bearing sulfides transformed to elemental sulfur and pyrrhotite through thermal decomposition during roasting. And the pyrrhotite decomposed to elemental sulfur and ferrous sulfide upon further desulfurization. Dense pyrite particles became porous, and gold exposed fully, which was beneficial to grinding and gold leaching process.The sodium hydroxide could react with the gold-bearing sulfide during the roasting process, producing leaching reagent. Thus it could achieve the leaching of gold in calcine but the gold leaching rate was 89.96%. It was 6.35 percent points less than the process of neutral roasting and non-cyanide leaching of gold-bearing sulfides. There were two reasons of the low leaching rate. On one hand, the produced erdite (NaFeS2·2H2O) and gangue minerals during the process could encapsulate the pyrrhotite and thus hindered gold leaching. On the other hand, the effective leaching chemical thiosulfate was decomposed under high temperature and thus affected the gold extraction performance of the leaching reagent.The approach of neutral roasting and non-cyanide leaching provides a high-efficient and environmental way to recycle gold from gold-bearing sulfides, and the sulfur in gold-bearing sulfur minerals can also be utilized effectively. The research results of this article provide a new reference for resources utilization of sulfide-bearing gold ores.