There is a lot of gold in the urban garbage heap. peppermint

Gold mining can be a dirty business. Even with relatively rich deposits, which are usually found in remote areas, you need huge excavators, huge crushing machines, lots of water and acids and cyanide to extract just ten grams of gold from a tonne of ore. Such highly toxic chemicals are required (there are 31 grams in one troy ounce). At current rates, it is worth more than $800.

However, a different type of pay dirt offers the potential for much higher returns for urban miners: printed circuit boards (PCBs) found in rapidly growing mountains of electronic waste. Estimates vary, but a ton of PCB may contain 150 grams or more of pure gold, which does not tarnish and produces stable electrical connections. Along with the gold, other valuable materials are also mined, including silver, palladium and copper, which, if recovered, could bring the total haul to upwards of $20,000 per ton.

According to the United Nations, about 62 million tonnes of electrical goods, from household goods to computers and mobile phones, were to be disposed of globally in 2022. Less than a quarter is considered to be recycled, at least in any formal way. Typically, PCBs are removed and crushed or treated with chemical solvents such as strong acids before being burned in a furnace to melt the metals. Because these processes produce large carbon emissions and contain toxic by-products that are difficult to clean up, companies are developing a number of clean recycling methods. One of the more interesting is using bacteria to do the actual extraction of metals.

Bioleaching, as the process is called, is an old idea: More than 2,000 years ago, leaching of the metal in copper mines turned water blue. However, it was not until the 1950s, when bacteria were found to be responsible for this phenomenon, that the process was commercialized to recover materials, liquid and solid waste left over from mining operations. Bioleaching relies on the metabolism of certain naturally occurring bacteria, such as Acidithiobacillus ferrooxidans, which produce oxidizing agents that dissolve metals in solution. The metals can then be recovered by various separation and filtration methods. When used outdoors on sewing this process can take months and is not very efficient.

Contained within a custom-built modern processing plant, however, bioleaching can be accelerated and improved by the use of a combination of similar bacteria. These are also naturally occurring and are safe to work with as they are non-pathogenic. The trick is to keep the bacteria in their preferred conditions, according to Bioscope Technologies, an urban-mining company that builds bioleaching plants in Cambridge, England. This includes a carefully controlled acidic environment, warm temperatures below 50 °C and a good supply of oxygen in their breeding tanks. Once these pampered bacteria are mixed with crushed PCBs in a reaction chamber they digest many metals within a day or two.

The resulting liquid is treated in several ways to recover the metals. By adding a little water the gold comes out faster. Electric current flowing through a liquid in a closed system recovers copper in a manner similar to electroplating. After completing their work, the bacteria are returned to their breeding tanks before being used again.

Jeff Borman, chief executive of Bioscope, says the idea is to create an enclosed, circular recycling system that speeds up the natural process and is sustainable. Trial production runs are already underway with full production set to begin in January. The Cambridge plant has the capacity to process 1,000 tonnes of PCBs per year, although plans to manufacture even larger PCBs are already being discussed.

Bioscope was founded as a separate entity by N2S, a sister company that specializes in recycling IT equipment, and which supplied Bioscope with crushed PCBs. Before being treated, the bioscope mechanically separates the plastic and fiberglass, the base material on which the circuits are built. The plastic can be sent for recycling and the company extracts silica from fiberglass for use in specialty ceramics.

This process is able to recover almost all the gold, silver, copper and palladium from crushed PCBs, although the exact amount depends on the type of circuit being recycled. Servers and telecommunications equipment contain the most precious metals, home appliances contain less. The recovered metals are so pure that they can be used again in electronics.

At the end of all the treatments, nothing is still being thrown away for good. This is because new bioleaching methods are being developed to recover some of the non-precious, though still valuable, metals in PCBs. Mr Borman says tin recovery has recently been added, with zinc, gallium and tantalum planned for next year. The Goldbugs have a lot of work ahead of them.