Extraction of silver from computer printed circuit boards wastes by supercritical fluids: pretreatment study

Electrical and electronic waste is a significant problem threatening countries’ economic, social, and environmental aspects. Electrical waste is toxic but contains valuable substances. With the increasing consumption of electronic devices, large amounts of this waste are generated worldwide. Due to the appearance of poisonous and precious metals in electronic waste, the management policy is different from traditional waste management. Therefore, knowing more about new methods and combining them is necessary. Various methods have been developed to extract these metals, but extraction with carbon dioxide has received particular attention due to its environmental friendliness. This study aimed to increase the extraction efficiency of silver using supercritical carbon dioxide from computer printed circuit boards waste. The use of supercritical water pretreatment was applied to such wastes to enhance the concentration of silver and remove organic matter. Six experiments at the pressure of 250 bar), the temperature of 350–450 °C, and the residence time of 15–45 min were conducted. The pressure of 230 bar, the temperature of 400 °C, and the retention time of 45 min were selected as the optimal point for supercritical water oxidation conditions. Pretreated solids were used as feed for the supercritical carbon dioxide extraction process. Optimal test conditions include the pressure of 217 bar, the temperature of 51 °C, and the residence time of 40 min. The extraction of silver from the residues of printed circuit boards in optimal situations reached 98% without solvents and ligands, which is selected as the best extraction point.