Gold mining can be a dirty business. Even with relatively rich deposits, usually found in remote areas, it takes giant excavators, huge crushers, lots of water and highly toxic chemicals, such as acids and cyanide, to extract just ten grams of gold from one ton of ore (there are 31 grams in a troy ounce). At current rates that is worth over $800.
However, another type of payment waste offers the prospect of much greater returns for urban miners: the printed circuit boards (PCBs) found in the fast-growing mountains of electronic waste. Estimates vary, but a ton of PCBs can contain 150 grams or more of pure gold, which, because it does not tarnish, provides stable electrical connections. In addition to gold, other valuable materials are also used, including silver, palladium and copper, which, if recovered, could bring total yields well above $20,000 per tonne.
According to the UN, around 62 million tonnes of electrical appliances were thrown away worldwide in 2022, ranging from household goods to computers and mobile phones. . than a quarter is expected to be recycled, at least in any formal way. Typically, the PCBs are removed and ground before being burned in a furnace to melt metals or treated with chemical solvents, such as strong acids. Because these processes produce large carbon emissions and have toxic byproducts that are difficult to clean up, companies are developing a number of cleaner recycling methods. One of the most intriguing applications is the use of bacteria to perform the actual extraction of metals.
Bio-leaching, as this process is called, is an old idea: more than 2,000 years ago, metal leaching was seen to turn water in copper mines blue. But it was not until the 1950s, after bacteria were found to be responsible for the phenomenon, that the process was commercialized to recover leftover material in tailings, the liquid and solid waste from mining operations. Bioleaching relies on the metabolism of certain naturally occurring bacteria, such as Acidithiobacillus ferrooxidans, which produce oxidants that dissolve metals in solution. The metals can then be recovered using various separation and filtration methods. When used outdoors on tailings, the process can take months and is not very efficient.
However, within a custom-built modern processing plant, bioleaching can be accelerated and improved through the use of a combination of similar bacteria. These also occur naturally and are safe to work with because they are non-pathogenic. According to Bioscope Technologies, an urban mining company building a bio-leaching plant in Cambridge, England, the trick is to keep the bacteria in the desired conditions. This includes a carefully controlled acidic environment, a warm temperature below 50°C and a good supply of oxygen bubbled into their breeding tanks. Once these spoiled bacteria are mixed with ground PCBs in a reaction chamber, they digest many of the metals within a day or two.
The resulting liquid is then treated in a number of ways to recover the metals. Gold precipitates quickly when a little water is added. An electric current passed through the liquid in a closed system recovers copper in a manner similar to electroplating. After they have done their work, the bacteria are returned to their culture tanks before being used again.
The idea is to create a closed, circular recycling system that accelerates a natural process and is sustainable, says Jeff Bormann, CEO of Bioscope. Trial production runs are already underway and full production will start in January. The Cambridge factory has a capacity to process 1,000 tonnes of PCBs per year, although plans to build a much larger factory are already being discussed.
Bioscope was established as a separate entity by N2S, a sister company that specializes in the recycling of IT equipment and which supplies Bioscope with the crushed PCBs. Before being treated, Bioscope mechanically separates plastics and fiberglass, the base material on which the circuits are built. Plastics can be recycled and the company extracts silica from the fiberglass for use in specialist ceramics.
The process can recover almost all of the gold, silver, copper and palladium from the ground-up PCBs, although the exact amount depends on the type of circuitry being recycled. Servers and telecom equipment usually contain the most precious metals, household appliances less so. The recovered metals are pure enough to be used again in electronics.
At the end of all treatments, nothing is thrown away for good. This is because new bio-leaching methods are being developed to obtain some of the non-precious, but still valuable, metals in PCBs. Tin mining has recently been added, Mr Bormann says, while zinc, gallium and tantalum are planned for next year. The gold bugs still have a lot of work to do.
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