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, 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. Based on the second gold recovery system, tiny amounts of HCl and an oxidant in some specific water-miscible organic solvents can effectively dissolve metallic gold. During this project, a comprehensive set of common water-miscible organic solvents in combination with different acids and oxidizing agents were investigated for extraction and recovery of gold from both gold ore and electronic wastes. It was found that among different organic solvents, a mixture of acetic acid and tiny amounts of HCl, CaCl2 (0.1 – 1.5 M) and some specific oxidants (such as H2O2 and Ca(ClO)2) can dissolve gold with the fastest rate reported to date. Along with an unmatched gold dissolution rate, this new leaching system was also highly selective for gold over base metals such that in the case of electronic scraps, more than 99% of gold was extracted from printed circuit boards (PCBs) in less than 20 seconds (which is the fastest known gold recycling process from e-waste ever reported) while less than 1.5% and 0.3% of nickel and copper, and 0.01% of all of the other base and heavy metals were leached during the same contact time. The efficiency of this new recovery method was also examined on gold ore, resulting in 99% of gold leaching at room temperature in only 25 min which was significantly faster than the cyanidation process with 35 hours leaching time. 1H and 17O NMR studies suggested that secondary interactions specially hydrogen bonding between acetic acid and the oxidizing reagent to generate in-situ chlorine in the first step, and subsequently between acetic acid and the generated Cl2, play significant roles to enhance the efficacy of the system for dissolving gold. This discovery promises to replace highly hazardous chemicals commonly used for gold recovery from both primary and secondary sources in the gold industry. In the last part of this thesis, some studies were done on the recovery and purification of recycled gold from e-waste. It was found that among different inorganic reducing agents, adding zinc powder to the pregnant acetic acid solution could precipitate more than 99% of gold (with more than 84% purity) out of the solution. Then, by re-dissolving the impure gold and selective reduction with sodium metabisulfite or/and solvent extraction of gold by dithiobiuret ligands, more than 99.6% purity was obtained. On the other hand, organic-based reducing agents such as hydrazine and hydroxylamine showed much more efficiency than the zinc powder. In other words, a 99.9% pure gold could be easily precipitated out from the acetic acid leach solution by adding a basic solution of these two organic reductants. Washing the resulting gold precipitate with 2M H2SO4 solution (containing 10% v/v H2O2) could increase the purity of final gold product up to 99.99%. This simple recovery process has a high potential to find industrial applications for refining impure gold products.