Oxidative Pressure Leaching Of Gold From Refractory Sulfide Gold Concentrate With Thiocyanate Solution

One of the challenges faced by the gold industry worldwide is the extraction of gold economically and efficiently from refractory gold concentrate.The cyanide methods suffer from certain inherent drawbacks such as toxicity, slow leaching rate and inefficiency in treating refractory sulfide gold ores. An oxidative pressure leaching process to extract gold from the refractory sulfide gold concentrate directly with alkaline thiocyanate solution was proposed in this paper. The new process actually consists of two processes, the oxidation of sulphidic minerals by sodium hydroxide solution and the leaching of gold by the stable and non-toxic thiocyanate efficiently simultaneously, cementing gold ions with iron powder in the filtrate solution.The E-pH curves for the Fe-As-S-H2O system and Fe-S-H2O system have been drawn by the thermodynamic calculation and the results indicated that the oxidation decomposition of the sulphidic host minerals under the experimental condition was feasible. Gold can be leached through forming aurothicyanate complex in alkaline Au-SCN--H2O system under oxidative condition, while oxygen could be used as an effective oxidant both in the preatment and leaching process.Based on the theoretical analysis and exploration, A L16(five factors in four levels) standard orthogonal array was employed to evaluate the effects of amount of alkali, sodium thiocyanate concentration, reaction temperature, reaction time, and oxygen pressure on gold leaching rate from refractory gold concentrate. It is showed that the temperature is the most significant factor affecting gold leaching.The optimum conditions were chosed as:the ratio of the addition and the theoretic consumption of sodium hydroxide 0.8, leaching agent concentration 2.0M, reaction temperature 180℃, reaction time of 8 h, pressure in autoclave 1.2MPa, liquid solid ratio 5, rotation speed 550r/min and lignin addition 1.2g. Under these conditions, the average extraction percentage of gold was about 95.29% in the integrated experiments. The XRD and SEM pattern of the gold concentrate before leaching and after leaching revealed that the the sulphide matrix was destroyed and sulfide minerals were oxidized thoroughly. The optimum conditions for the iron powder cementation were determined as follows:iron powder dosage was 2g/L, reaction temperature and time was 25℃and 50min, respectively. Under the optimum conditions, the cement percentage of gold was about 99%.In the leaching process, valuable metals such as gold and silver can be dissolved into the solution, while arsenic turned into scorodite which existed in the slag. The results show that using alkali thiocyanate solution for leaching refractory gold sulfide materials has the following advantages:shorter flowsheet wih a higher efficiency, better selectivity on gold, better stability and lower toxicity.