Extraction and Recovery of Gold from both Primary and Secondary Sources by Employing A Simultaneous Leaching and Solvent Extraction Technique and Gold Leaching In Acidified Organic Solvents

Abstract

Gold extraction processes from both primary and secondary sources have changed little over the past centuries with cyanide remaining the preferred leach reagent used in 90% of gold mines, while aqua regia, which has been used for more than 1000 years, remains an unmatched gold leaching reagent for secondary sources. However, due to their extremely toxic nature, their continued use presents significant environmental risks and safety hazards for the work place. Therefore, considerable efforts have been made to find an alternative to cyanide and/or aqua regia, and variety of leaching reagents in aqueous media have been studied and proposed. Besides the reported leaching reagents in aqueous media, recently, a few preliminary investigations have also been done on non-aqueous solutions and in some cases, promising results have been achieved. However, among all reported leaching systems, none have been widely implemented on an industrial scale. The discovery of more efficient and benign gold extraction technologies is the main problem facing the gold industry today. This thesis describes the discovery of two new recovery methods for selective extraction of gold from both primary and secondary sources which are promising for implementation on industrial scales. These two new methods are (i) simultaneous leaching and solvent extraction of gold and (ii) gold leaching in acidified water-miscible organic solvents. Gold recovery from gold ore by the simultaneous technique proved to be fast, efficient, and selective. In this method, the gold ore was contacted with a biphasic solution including an acidified aqueous phase and a water- immiscible organic solvent containing different derivatives of dithiobiuret-based ligands, and the resulting mixture was stirred for a short time to both leach and purify gold at the same time. It was demonstrated that the efficiency of this system was dependant on the concentrations of both acid and oxidizing agent in the aqueous phase, the type and concentration of ligand in the organic phase, ii and the type of organic solvent. More than 94% of gold could be selectively extracted and recovered after 9 hours which was significantly faster than the current cyanidation process with 35 hours for just the leaching step.