Researchers find alternative way to extract high purity silver from used solar panels

Scientists from the University of Leicester have discovered an alternative process that recovers silver and aluminium from end-of-life photovoltaic (PV) cells, the functioning units of solar panels. This process uses cheap solvents and is environmentally friendlier than the most common process used at present, which typically involve mineral acids.

Dr Guillaume Zante and his team describe a process using iron chloride and aluminium chloride dissolved in brines to extract the silver and aluminium from solar cells. The process using these solvents retrieves more than 90% of the silver and aluminium in a period of 10 minutes. The silver recovered is high purity, which means that it can be reused in industrial settings.

This process demonstrates one of the first instances of using unusual brines to extract metals instead of using mineral acids. The brines – or salt water – used here are not made of sodium chloride/regular salt table but of choline chloride (chicken feed) and calcium chloride (which makes up grit used on roads to avoid ice).  Brines are cheap and more environmentally friendly whereas mineral acids are hazardous chemicals like nitric acid, which contribute to acid rains, eutrophication and climate change by releasing nitrous oxide (a greenhouse gas) in the environment.

Silver is essential to the functioning of PV cells, helping to transfer energy from the sun's rays to the silicon components which make up the semiconductor part of the panel’s structure. The amount of naturally occurring silver found in silver ores is decreasing, making supply a concern for the future. This not only affects the production of PV cells but other essentials such as LED chips, nuclear reactors, equipment for the medical industry and more.

As we move towards a greener future, relying less on fossil fuels, the uptake of solar energy is set to increase 30-fold in the next 10 years. Where the lifespan of a PV cell is 25-30 years, it is estimated that there will be 80 mega tonnes of waste from solar panels by 2050. Under EU regulations, a minimum of 75% of the material making up solar panels must be recovered, and minimum 65% recycled. Though not all can be recycled, aluminium, silver and copper can.

PV cells in landfill can release toxic materials, volatile organic chemicals, and heavy metals (e.g., lead, tin) which poses a serious threat to the environment and human health.

The solvents were also effective in extracting the silver and aluminium without affecting the silicon in the structure of the PV cell, indicating that the silicon can be reused too.

The solvents used in this process open up new possibilities in managing recovery and recycling of valuable elements used in solar cells affordably and with a low environmental impact.  

Dr Zante, from the Centre for Materials Research at the University of Leicester, said: These new 'unusual' brines offer new possibilities for the processing of metals. Unlike 'regular' salted water made of sodium chloride, we used choline chloride (chicken feed) and calcium chloride (a de-icing agent), but there are many different salts that can be investigated in further studies.

“Brines are a credible alternative to the toxic mineral acids used for metal processing because of their low price. We are now trying to apply the same approach for different metals from different sources of waste, such as smartphones, thermoelectric materials, and magnets.”

Dr Zante and colleagues will be at the British Science Festival hosting an event allowing the public to get hands on with metal extraction activities. This is a drop-in event at The Clock Tower & Humberstone Gate, Leicester, on Saturday 17 September, from 11am to 4pm.

The British Science Festival is hosted by De Montfort University (DMU) and takes place from Tuesday 13 to Saturday 17 September across the DMU campus and Leicester city centre. For more information and to book tickets visit www.britishsciencefestival.org