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Aside from the achievements in the sporting events, Tokyo 2020 is noteworthy for the spotlight it put on sustainability. Symbolic items – podiums, torches, medals – were made using recycled materials. The Olympic flame was relayed in a torch of recycled aluminium from prefabricated housing units in the aftermath of the 2011 Tohoku earthquake and tsunami. Athletes stepped onto podiums crafted from recycled plastic and were awarded Olympic medals made from recycled small consumer electronics collected all over Japan.
The Olympic medals raise awareness of electronic waste recycling.
With the rapid rise in technology and increasing consumer pressure for upgrades in functionality and design, it is estimated that more than 50 million tonnes of electronic waste (e-waste) was generated in 2019 alone. This figure is predicted to grow by 3-5% annually. Even the most optimistic estimates suggest that global recycling of e-waste is no more than 30%.
An end-of-life printed circuit board (PCB) may have a metal content as high as 40% by weight, which is ten to a hundred times higher than that of conventionally mined ores, so e-waste should be viewed as a valuable secondary source of precious and base metals.
Gold is one of the most valuable components in waste PCBs, with concentrations varying between 140 and 700 g of gold per tonne of PCBs. This is in contrast to 5 to 10 g of gold per tonne of ore.
The processing of e-waste typically begins with a manually intensive dismantling phase, during which the circuit board components and the lithium battery are removed for recycling elsewhere. The remaining PCBs are then categorised according to their metal : plastic ratio and shredded. Shredded PCBs are separated into metallic and non-metallic components, which can be done by a variety of methods. One option is to finely mill the PCBs and use gravity separation to separate the heavier (metallic) and lighter (non-metallic) fractions. Alternatively, electrostatic methods can be used to separate metallic and non-metallic components based on their electrical conductivity. Another option is to use organic solvents to dissolve the resin holding together the metallic and non-metallic layers of the PCB. Pyrometallurgy and hydrometallurgy are currently the dominant methods of gold extraction used in the final phase of recycling. Bioleaching is attracting more interest due to a lower energy requirement than pyrometallurgy and lower reliance on toxic chemicals than traditional hydrometallurgy.
Current industrial processes for metal recovery rely heavily on pyrometallurgy – an economically attractive option due to high throughput rates and ability to deal with a broad range of scrap materials with minimum processing.
Pyrometallurgy processes include roasting, in which waste is heated to sub-melting temperatures and treated in very hot air. This is followed by smelting, which uses heat and a chemical reducing agent to decompose the waste, driving off unwanted elements and leaving behind a mixture of metals. The produce of the smelting is described as a “copper bullion”, due to the high copper content in PCBs. The copper can be purified by leaching and electrowinning, leaving a residue of precious metals (including gold) for further refining.
Due to the high temperatures required, pyrometallurgy carries a heavy environmental burden. This type of recycling also gives poor selectivity for individual elements, meaning that multiple stages are required to recover metals in their pure forms.
Hydrometallurgical processes have a lower capital cost and energy requirement than pyrometallurgy, as well as allowing greater scope for selective metal recovery which simplifies the process of gold extraction.
Hydrometallurgy involves leaching PCBs by a suitable lixiviant (essentially, dissolving the metals in a suitable solvent), followed by separation into metal streams from which pure metals are obtained. It is important to note, that gold is found in its elemental form in e-waste, meaning that in e-waste recycling the gold needs to be oxidised during dissolution. Given its resistance to oxidation (as we all know, gold does not tarnish easily), the options to achieve dissolution are limited.
In current use is cyanide, a cheap but highly toxic reagent that is very effective in leaching gold from low-grade ores to form a solution of the water soluble cyanoaurate [Au(CN)2]-. Due to the toxicity and environmental concerns around the use of cyanide, much research is being undertaken to develop alternatives to cyanide leaching.
Following the leaching process with cyanide, gold must be extracted from the cyanide solution. In commercial mining, cyanoaurate is adsorbed onto activated carbon particles. Gold is subsequently released from the loaded carbon by heat or pH control.
Hydrometallurgy offers the potential of high selectivity recycling with a moderate energy input, however faces the challenges of a highly complex feed stream and high volumes of toxic reagent waste.
Biohydrometallurgy is similar to hydrometallurgy except it utilises biological agents (such as bacteria) to leach out metals from e-waste. Like hydrometallurgy, the energy needed for the process is lower than that required for pyrometallurgy. The hunt is now on to find suitable biological agents and reaction paths that can reduce the toxic waste associated with hydrometallurgy.
Developments are being made in all avenues of recovering valuable metals from e-waste, hopefully paving the way to a more sustainable future. While the exact method used to recover gold from electronics for the Tokyo 2020 medals was not made public, it is nevertheless a great initiative to bring the issue of sustainability to the Olympic podium.
Article by: Ekaterina Kaliteevskaya | 20 September 2021