Developing an Integrated Process for the Recycling of Tantalum from Electronic Waste

The critical metal tantalum is of high importance to our society. The functioning of many daily and high-tech electronic appliances relies on electronic components made from tantalum that are mounted on printed circuit boards (PCB). Despite its importance, tantalum is not recycled but lost in state-of-the-art recycling process for PCB. Pyrolysis of PCB liberates metals contained in PCB, providing the prerequisite for separating tantalum from other materials and thus the opportunity to recycle it. The current work explores the separation processes sieving, centrifugal density separa-tion and froth flotation to recover it in the form of a tantalum concentrate that can be used as feed-stock in tantalum refineries to produce new tantalum products. Sieving was found to be an effective initial treatment step to recover high shares of tantalum from the solid pyrolysis product but only little share of total mass and impurities. Temperatures of 650 °C and simultaneous mechanical stress during pyrolysis in an auger reactor lead to selective disinte-gration of tantalum capacitors into fine particles. Via sieving the fine tantalum particles are sepa-rated from coarse particles. The sieving fraction 0-0.125 mm contains 40 wt% of the total tantalum but only 3.9 wt% of total mass, resulting in an increase of tantalum concentration from 445 ppm in the solid product to 3,483 ppm in the fine fraction. Centrifugal density separation was found to effectively enrich and recover tantalum contained in the sieving fraction 0-0.063 mm of the solid pyrolysis product. In the process a slurry is fed into a fast-spinning separation bowl effecting the separation of high-density tantalum from low-density materials. Statistical analysis revealed that process parameters percentage of solids (SP) in the feed slurry and mass flow (MF) of feed slurry have a positive effect on the enrichment of tantalum. On the recovery rate of tantalum, the rotational speed (RS) of the bowl has a positive and SP and MF have a negative effect. Tantalum concentration was increased from 4,706 ppm to 11,436 ppm with a tantalum recovery rate of 56 wt%. Simultaneously the concentration of bromine was reduced, and gold was completely removed from tantalum.
During froth flotation hydrophobic particles suspended in a slurry attach to air bubbles and are removed as froth, while hydrophilic particles remain in the slurry. Several flotation collectors were identified that changed the hydrophobicity of particles. Sodium diethyldithiocarbamate trihydrate exhibited highest selectivity in tantalum hydrophobisation. It was determined that the ratio of solids in the slurry (SL), air flow (AF) and stirring rate of impeller (SR) have positive effects on both recovery and enrichment of tantalum in the froth product. The ratio of collector per solid (CR) has a negative effect on tantalum enrichment but a positive effect on tantalum recovery. High enrich-ment of tantalum and simultaneous good removal of impurities was achieved at low SL, SR, and CR, and high AF. At these conditions selective separation due to hydrophobicity is high and unse-lective separation due to entrainment and entrapment is low. An integrated process for the recycling of tantalum was proposed, consisting of (1) pyrolysis to liberate tantalum, (2) sieving to initially enrich tantalum and reduce the amount of mass to be fur-ther treated, (3) froth flotation to enrich tantalum further and simultaneously reduce the concentra-tion of impurities, and (4) centrifugal density separation to strongly enrich tantalum. The tantalum concentrate can be used as feedstock for tantalum refineries, while other by-products can be utilised in secondary copper smelters to recycle other valuable metals. The current research adds to the limited information regarding treatment of the solid pyrolysis product and recovery of tantalum via sieving, centrifugal density separation and froth flotation. Future research should expand on this, optimise individual separation steps, and investigate com-binations of separation steps to further increase concentration, recovery, and purity of tantalum.