RECYCLING OF DISCARDED PRINTED CIRCUIT BOARDS FOR THE RECOVERY OF METALLIC VALUES

Abstract

The discarded printed circuit boards and their components are secondary raw materials for the recovery of metallic values. The printed circuit boards, random-access memory and integrated circuits hold promising potential for recycling base and precious metals. A hot air gun was used to depopulate electronic parts such as connecting ports, diodes, capacitors and others from discarded printed circuit boards. Pre-removal is a necessary step to conserve energy and power during grinding due to hazardous and steel fractions on these components. The depopulated printed circuit boards were grounded using a hammer/cutting mill with a classifying screen. A significant amount (60-65 %) of the non-metallic fraction is present in the feed; therefore, pre separation of the non-metallic and metallic fractions is required for effective recycling. Recovery of metallic values (~ 40 %) from these discarded printed circuit boards and their components were investigated using a combination of air and water fluidization processing followed by thermal exposure or hydrometallurgy route. In comparison to individual air and water fluidization processes, it was revealed that combined fluidization (air followed by water fluidization) processing had the highest separation efficiency. Metallic values were raised by ~ 35 % to ~ 91 % from feed to underflow product of combined fluidization without any chemical reagent or high-temperature treatment. The underflow fraction of the combined fluidization yielded the highest tapped density ~ 3 g/cc while ~ 0.6 g/cc was found for the overflow fraction. The harmful fraction comprising C and Br content is decreased from 40-49 % to 5-7 % and depicting ~ 88 % removal efficiency for underflow fraction of combined fluidization, leading to minimal evaluation of harmful gases during thermal treatment. The generation of fines during size reduction (shredding/crushing/grinding) of printed circuit boards is an unavoidable but essential by-product. However, due to the higher amount of non metallic fraction and poor separation of metallic values, the fines fraction is discarded. The natural hydrophobic response of plastic and organic matter in fines stabilized the froth without additional reagents and separated the metallic values. The recovery of metallic values from concentrate was observed to be ~ 72.3% with a metallic grade of ~ 92%, which consists of 77.9% Cu, 8.3% Sn and 5.5% Pb. Thermal exposure of the underflow product of combined fluidization was investigated with varying time, temperature and particle sizes for the complete removal of plastic/resin values present in the mixed or unliberated particles to enrich the metallic values. Due to solder element, 94 % mixed metallic values such as Cu with Sn and Pb from computer printed circuit board whereas almost 91 % pure Cu can be recovered from random access memory printed circuit boards. Cu and Au dissolution were found about 3 and 7-times higher than feed and physically treated fluidization products. The generation of harmful fumes and acid consumption was relatively low ~ 4.5 l/kg for the treated fraction than the feed sample. The gold was recovered using the leaching and sodium disulfite precipitation route. Cu and Au (as gold dust) with purity 98 % and 89 % were recovered using electrodeposition and precipitation, respectively. It can be said that approximately 0.4 g Au and 0.9 kg Cu can be recovered from 1 kg of physically processed mobile printed circuit boards. The silver value was increased by 4-fold (0.6-2.2 %) from integrated circuits of random-access memory using physical processing followed by thermal exposure. Comparatively, higher dissolution efficiencies (~ 97%) of metallic components were found from underflow products of discarded integrated circuits compared to direct leaching of feed (47 %) because of the successful separation of non-metallic fractions during the physical treatment.