Swedish researchers have developed a method for recycling precious metals from flexible thin-film copper-indium-gallium-diselenide (CIGS) solar cells under mild dissolution conditions. They showed that 100% of silver and 85% of indium can be recovered after 24 h of dissolution at room temperature using two moles of nitric acid.
The academics placed small pieces of PV cells in perfectly sized crucibles to obtain the total amount of silver and indium per cell. They placed the larger piece in the dissolution tanks and filled both tanks with the desired amount of nitric acid solution (HNO3) of a certain concentration. The geometric ratio of surface area to liquid (A:L) also varied between experiments. The team performed all experiments at room temperature and at a stirring speed of 200 rpm.
After demonstrating that both silver and indium were completely dissolved during digestion, and that the dissolution experiments would be performed with the complete amount of each element in the cells, the researchers decided to test which concentration of nitric acid and which ratio of geometric surface area to liquid resulted in the highest leaching.
The results show that a 2 molar concentration of nitric acid and a 1:3 A:L ratio lead to very high silver and indium recovery rates. After four to six hours of extraction, approximately 90% of the silver is recovered, and after 24 hours the recovery rate is 100%. About 20-25% of the indium is recovered during the first hour of dissolution, and the maximum yield of 85% is reached after 28 hours.
“Comparing all the results of the different experimental conditions, it can be said that high yields of Ag and In are always accompanied by a high level of contamination from many other elements, and (zinc) is always present,” the researchers noted, adding. Molybdenum contamination can also be problematic.
They then showed that cleaner streams could be obtained using a selective leaching process. Using 0.5 molar nitric acid concentration and an A:L ratio of 1:3, the silver recovery rate reaches approximately 85% after 24 hours at lower contamination levels. Nevertheless, the indium recovery rate drops to only 30 percent.
The scientists also noted that their method is not yet suitable for industrialization because the concentrations of silver and indium are too low to be used directly in a potential purification process that uses more than 1 gram per liter. “Some means of increasing concentration, for example reuse of the same leachate in several cells, should be considered in the context of industrialization,” they concluded.
The team shared their findings on “Precious metal recycling from thin-film CIGS solar cells by dissolution under mild conditions,” published recently Solar energy materials and solar cells.