The hydrometallurgical extraction of gold from ores, concentrates, and tailings in a cost effective and environmentally safe manner offers an interesting challenge. Conventional gold mining operations rely heavily on cyanide leaching as the predominant method for recovering gold from ores and concentrates. Cyanide has been the leach reagent of choice in gold mining because of its high gold recoveries, robustness and relatively low cost. As a result, over 76% of all gold extracted worldwide is produced by hydrometallurgical extraction with the use of cyanide.
The gold mining sector uses approximately 66,000 tons of sodium cyanide annually worldwide. The use and subsequent disposal of cyanide, present significant safety and environmental challenges. As a result, the safety, environmental and associated permitting issues have been a critical driver for the industry to evaluate effective alternatives.
These alternative lixiviants have to offer comparable leach kinetics, be environmentally friendly and cost effective. Some of the experimental alternative lixiviants have included; thiosulfate, thiocyanate, ammonia, bromine, chlorine, bisulfites, thiourea and others. Some of these chemicals offer comparable leach kinetics under certain conditions, but for a variety of reasons, none are reported to be an effective alternative to cyanide.
Although new processes are being proposed on a regular basis, there have, in fact, been no dramatic changes in the metallurgical techniques for gold extraction since the introduction of the cyanide process (cyanide leaching or cyanidation) by McArthur and Forrester in 1887.
Cyanide and cyanide gas are both toxic and care has to be taken during ore processing to avoid exposure for workers. Solutions containing cyanide have to be carefully monitored and managed to prevent the formation of deadly cyanide gas. In addition, there are significant costs associated with destruction of cyanide in the tailings streams and final disposal.
Internationally, several countries, like the Czech Republic, Greece, Turkey, Germany, Hungary, Costa Rica, Argentina, Ecuador, and some states of the United States (e.g. Montana) have banned cyanide leach technology in gold and silver mining. Therefore an environmental friendly alternative is crucial for the continued development of the mining sector.
The EnviroLeach reagent is ideally suited for the leaching of gold in an agitated or vat leach type process. This includes the treatment of whole ores, gravity concentrates, and flotation concentrates. This segment of the market represents the majority of the worldwide gold produced.
There are numerous potentially economic gold deposits that are unable to secure permitting due to cyanide restrictions. The EnviroLeach process could be an effective alternative for these deposits.
The EnviroLeach process is based on the science of extractive hydrometallurgy which involves the dissolution of the precious metals into the inorganic aqueous solution. The oxidizing solution or lixiviant consists of a base formula of nontoxic dry ingredients which form the complexing agent by mixing with water. To achieve and maintain the desired oxidation level of the complexing agent necessary to form the gold complex, the lixiviant is subjected to a unique electrochemical oxidation process. The ore is introduced into the lixiviant and agitated for a predetermined period of time to produce the pregnant solution. After the pregnant solution is separated from the ore solids, the extract is subjected to filtration before the metals are recovered by conventional electrowinning or carbon absorption and then the barren solution is returned to the process for reuse.
Over 13 months of independent testing and analysis by Met-Solve Labs of Langley, BC, Canada involved the preparation, leaching and assaying of numerous different ores, gravity concentrates, flotation concentrates and tailings. Following the leach cycle, analysis of the aqueous solution was performed by Atomic Absorption spectroscopy, ICP mass spectrometer and the subsequent analysis of the leach residue was done by acid digestion, ICP MS and fire assays.
Extensive test work included the comparison of the leach kinetics of the EnviroLeach process to conventional and high intensive cyanide leaching. Most of the test work was carried out using the bottle roll method, which is employed extensively for cyanide leach test work, however a few tests were done using a stirred reactor and lab scale agitators. The test were done under ambient temperature and atmospheric conditions.
The results of the test work indicated that the EnviroLeach product performed better than cyanide on both kinetics and total amount of precious metal recovered on most materials. The metals are very stable in solution and there are no preliminary indications to a maximum solubility level.
The recovery of gold from the EnviroLeach solution can be accomplished using a number of conventional methods including; carbon absorption and electrowinning. A number of tests were completed to quantify gold recovery from solution with conventional electrowinning (EW) using the Electrometals EMEW® cell. The results indicate that gold in EnviroLeach solutions can be rapidly electrowon to very low tenors.
The results of the extensive testing further supports the objective of replacing cyanide with the EnviroLeach chemistry in that auxiliary processes, e.g. carbon-in-leach and conventional electrowinning, are immediately applicable to the EnviroLeach system, and that the EnviroLeach chemistry may be a viable replacement for cyanide at the commercial and/or industrial level.