Chemical Methods for Gold Refining: Safety, Efficiency, and Environmental Impact
Understanding Gold Refining
Gold refining is a crucial step in the gold production process, ensuring the purity and quality of the precious metal. The methods used to refine gold can vary widely, from traditional methods to modern, more precise techniques. This article explores the different chemical methods used in gold refining, their applications, and the associated safety and environmental concerns.
Introduction to Gold Smelting and Chemical Refining
Gold smelting is the process of extracting gold from its ore by heating and melting it to separate impurities. In contrast, chemical refining involves using chemicals to purify gold. While both methods achieve the goal of producing high-purity gold, chemical refining is a more widely employed technique.
Chemical Refining Methods
Chemical Refining Involves:
Leaching with Dilute Cyanide: Mills produce gold by leaching the ore with a dilute cyanide solution and then precipitating the gold from the solution by adding zinc powder. The resulting gold is usually very pure, reducing the need for further smelting. Miller Process: This process utilizes gaseous chlorine to remove impurities from gold when it is at its melting point. The impurities separate into a layer on the surface of the molten, purified gold. The Miller process is rapid and simple but produces gold of only about 99.5 percent purity. Wohlwill Process: Also known as electrolytic refining, this method can achieve a purity of about 99.99 percent. In this process, a casting of impure gold is lowered into an electrolyte solution of hydrochloric acid and gold chloride. Under the influence of an electric current, the gold migrates to a negatively charged electrode (cathode) where it is restored to a highly pure metallic state, leaving the impurities as a separate solution or residue.Mercury Extraction: A Dangerous but Common Method
Mercury extraction is a less common but still used method, particularly in third-world countries. It involves the use of mercury, a highly toxic substance, to dissolve gold from other minerals. The process starts with a mixture of dirt, minerals, and gold. Mercury is added and mixed, then separated from the debris, leaving the gold dissolved in the mercury. The mercury is subsequently boiled off, leaving behind the gold. It is important to note that mercury and mercury vapor are extremely toxic, posing significant health risks to those involved in the process.
Modern Methods and Cleanliness
Modern mills have improved the methods used for gold refining, with cleaner and safer chemicals. The efficiency and safety of these processes continue to evolve. The Miller and Wohlwill processes, for instance, are widely used and offer effective ways to refine gold to different levels of purity.
Conclusion
In conclusion, the process of refining gold involves various methods, each with its own set of advantages and risks. While chemical methods like the Miller and Wohlwill processes are more common and precise, traditional methods like mercury extraction, though still prevalent in some regions, pose significant health and environmental risks. As the demand for high-purity gold continues to grow, ongoing efforts to improve the safety and environmental impact of gold refining methods are essential.