Mushrooms recover gold from mobile scrap
The metals are used get the signal from one chip to another (gold wirebonds, copper traces on printed circuits boards) or to improve contact reliability (gold or silver electrodeposition on connectors), or as minute traces in passive components just to name a few applications.
The e-waste issue is not new, and before it became on the European legislation agenda, it used to be that unscrupulous “recyclers” would ship discarded electronic devices to third world countries where very basic and hazardous metal recovery techniques would be used.
This often includes burning and smelting the metals from cables (toxic fumes including dioxins), or separating gold from burnt PCB ashes using toxic cyanides solutions that then contaminate nearby rivers.
In Europe and the US, several companies have industrialised the recovery of precious metals from e-waste, first crushing the devices and PCB boards, then using various separation methods (magnets to take out the steel, Eddy currents to separate non-ferrous metals from plastic) before smelting again or using toxic chemistries (often sulphuric acid or cyanide solutions) to dissolve the metal particulates and recover them through chemical reactions. The processes are similar, only better managed at industrial scale, but they are still energy intensive and environmentally debatable.
Reportedly, such industrialised processes can yield up to 300 grams of gold per ton of discarded mobile phones, and between 2 and 2.5 kilos of silver. By far, the most aboundant metal in e-waste is copper, making up between 10 and 15% of a mobile phone’s weight.
Searching for non toxic e-waste processing alternatives VTT Technical Research Centre of Finland has developed a biological filter made of mushroom mycelium mats that could recover of as much as 80% of the gold in electronic scrap. The researchers are also looking at ways to extract copper from circuit board waste by flotating the crushed and sieved material rather than indiscriminate smelting.
In VTT experiments, cell phones were crushed and the particles sieved and separated magnetically and by eddy current into circuit board fractions.
Further crushing, sieving and flotation (a separation method that separates hydrophobic particles from hydrophilic particles by blowing air into the sludge) resulted in a fraction with high concentration of valuable metals for solution extraction experiments. The researchers say their flotation technique raised the copper content of circuit board fraction from 25% to 45%, while gold content increased by a factor of 1.5.
Close-up on the biosorben film: courtesy VTT Technical Research Centre of Finland.
“Because it is difficult to remove the components from the circuit boards, the first step in most recycling processes is to crush everything into particulates and that’s how we start too”, explained Jarno Mäkinen, Research Scientist at VTT Technical Research Centre of Finland.
“But then, using non-toxic water-based solutions, we have managed to engineer mycelium-based biomass that acts as a biosorbent specifically targeted at gold complexes”.
Using biosorbents such as fungal and algae biomass, the Finnish lab demonstrated that more than 80% of the gold in the solution adhered to the biomass, compared with only 10 to 20% of gold recovery when using most commonly used harmful chemical preparations.
Different filament structures can be formed, for example, into biological filters, which could make that specially engineered biomass useful to recycle precious metals on an industrial scale.
Mäkinen didn’t want to say more about the biomass engineering tricks used to make the biosorbents more effective for gold or other precious metals. But in principle, the idea would be to engineer various biosorbents targeted at different metals (including rare earth metals) and cascading the e-waste recycling process through different metal absorption steps.
At the end of each step, the collected biomass is burnt or chemically processed to recover the metal complexes inside.
“We have been most successful with gold so far, but we’ll be working to recover other rare metals too”, commented Olli Salmi, Research Professor at VTT, adding that the processes relied on organic chemistry and ionic liquids to dissolve the gold particulates and form complexes.
In other VTT experiments, the researchers were able to recover more than 90% of the metal solution dissolved from a circuit board with the help of functional ionic liquid.
These results stem from the European “Value from Waste” project of the research consortium AERTO (Associated European Research and Technology Organizations), initiated two years ago. The Finnish lab developed both biological and mechanical pre-treatment methods for a more efficient and more sustainable recovery of precious metals from electronic waste. Its findings could enable the metal refining industry to use cleaner electronic waste in larger amounts.
VTT participated in joint technology R&D with the following six European research institutes: Fraunhofer ICT and Umsicht (Germany), CEA (France), TNO (the Netherlands), SINTEF (Norway), Tecnalia (Spain) and SP (Sweden). The project was co-ordinated by SINTEF from Norway.
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