Gold, the precious metal that built and destroyed empires, was the focus of millions of investors for years. Two years ago, gold prices peaked at around $2,000 per ounce. This ongoing interest in gold not only motivated ancient kings to start wars of conquest but also inspired a team of scientists to search for innovative methods of extracting gold.
In late March of 2015, a team of researchers found that human waste contains minute traces of gold and other precious metals. This waste is carried to wastewater facilities along with other metal-containing products like detergents. By taking waste samples from different communities, scientists have determined that the gold level in the sewage is the same as that of a minimal mineral deposit — an amount that is commercially viable to extract from rocks.
This is not the most recent advance in gold extraction, however. Recently, a research team discovered an efficient method of stripping gold from electronic wastes. Modern electronics contain very small parts that are powered by low currents. As a result, a reliable, corrosion-resistant metal is necessary for the devices to work. Therefore, most electronics contain trace amounts of gold that are bound to their internal components.
However, extracting gold from electronics is difficult, and the amount of recovered gold from a single device is very low. Gold is a very non-reactive metal, which forces companies to use chemicals such as sodium cyanide to dissolve and extract gold. However, sodium cyanide is both environmentally damaging and expensive, making it unsuitable for stripping small amounts of gold from electronics.
To avoid using sodium cyanide, the research team, led by Stephen Foley, an associate professor at the University of Saskatchewan, developed a method that relies on a combination of acids and oxidants. This solution consists of acetic acid mixed with an oxidant which, in the presence of another acid, dissolves gold at a record rate.
In their experiments, gold from electronic circuit boards were dissolved in around 10 seconds. In a large scale study, 100 liters of the solution was capable of dissolving one kilogram of gold. Additionally, the fluid is cheap and more environmentally-friendly than sodium cyanide; a liter of this solution costs 50 cents and can be reused in subsequent extractions.
This novel solvent resembles a more commonly known liquid called aqua regia. Known for being very corrosive, aqua regia consists of a mixture of nitric acid and hydrochloric acid that, in the correct ratio, can dissolve nonreactive elements such as gold and platinum.
Although it can be used to extract gold, there are several disadvantages to using aqua regia. First, the components of the solution react with one another, speeding up the decomposition of the liquid and reducing its effectiveness. Furthermore, only one kilogram of gold can be extracted with 5,000 liters of aqua regia, none of which can be recycled.
As a result, the solution developed by Foley is currently considered one of the best options for commercial extraction of gold from electronic wastes.