Researchers at Chalmers University of Technology in Sweden have developed an environmentally friendly method for recycling and purifying metals. Using gold earrings from a pawnshop in Gothenburg and biodiesel from the nearest filling station, the discovery could change an industry that is currently dependent on large amounts of fossil oil. “Pure metals have a number of uses in a modern society, not least for the development of green technologies. Our research shows how the metal industry can accelerate the transition from fossil to bio-based solvents,” says Mark Foreman, Associate Professor of Chemistry at Chalmers.
Gold is not only a precious metal that is a symbol of wealth in jewellery and gold bars. A regular smartphone contains slightly more than 0.03 grams of gold, and the metal is found in most of the everyday electronics we have around us. It is also important in components for the aerospace industry. For many applications, gold is mixed with other metals, which then need to be removed when the valuable gold sheet is to be recycled. In this process, organic solvents, such as fossil diesel, are used.
“Even if the diesel used in the production and recycling of metals is not incinerated, there are many good reasons to switch to fossil-free alternatives. For example, in the production of oil, methane, which is a worse greenhouse gas than carbon dioxide, often leaks into the atmosphere. A lot of crude oil also contains toxic aromatic hydrocarbons, which damage the nervous system and are therefore dangerous for humans and animals to breathe in,” says Mark Foreman.
Together with his research colleagues at Chalmers, Mark Foreman has found a way to use biodiesel instead of fossil diesel, which can be produced from residual products from the forest and pulp industry, and which is sold commercially as fuel under the brand name HVO100. Biodiesel contains virtually no aromatic hydrocarbons at all.
In the researchers’ method, gold scrap − usually in the form of small earrings that Mark Foreman buys at his local pawn shop − is dissolved in a mixture of hydrochloric acid and nitric acid (aqua regia). The gold found in jewellery is an alloy with other metals, including silver, and this leads to the silver depositing in solid form as silver chloride. In just two more steps, pure gold is then extracted from the solution. Firstly, HVO100 and the chemical malonamide are added, and secondly the entire mixture is shaken with ordinary salt water. The method used by the Chalmers researchers is even more ‘green’, since the malanomide was made from renewable biomass, which replaces more toxic and carcinogenic chemicals that are traditionally used to purify gold scrap.
The scientific article Sustainable solvent extraction of gold and other metals with biomass chemicals has been published in the journal RSC Sustainability. The authors of the study are Mark Foreman, Richard Johansson, Gloria Mariotti, Ingmar Persson, Behabitu Tebikachewa and Mikhail Tyumentsev. The researchers are active at Chalmers University of Technology and the Swedish University of Agricultural Sciences, SLU.
