Precious metals recovery from waste printed circuit boards using thiosulfate leaching and ion exchange resin

Precious metals recovery from secondary sources is a novel alternative since they hold higher contents of several metals than average ores. In this research gold and silver were recovered from Waste Printed Circuit Boards (WPCBs) with the use of ammonia-thiosulfate solutions and the MTA 5011 ion exchange resin. First, it was necessary to remove the excess of copper from WPCBs since it could be detrimental for precious metals dissolution during leaching with thiosulfate. Nitric acid was the best reagent for decreasing copper concentration within WPCBs from 27% to 1.5%. Then, precious metals were dissolved with ammonia-thiosulfate solutions during 6 h at room temperature. Since copper was still within WPCBs, there was no need to add copper salts in order to form the copper tetramine complex required for accelerating precious metals dissolution. MTA 5011 ion exchange resin was used for precious metals recovery from leachates. It was determined that at higher resin: solution ratios, gold and silver recovery on the resin increased whereas selectivity towards gold diminished. The 87% of the gold was recovered and only 29% of copper was adsorbed too. Afterwards, a pre-elution stage was performed with sodium thiosulfate for a selective elution of copper. The 83% of the copper was desorbed while 28% of gold was eluted as well. For precious metals elution, several reagents were tested and KSCN was the most efficient of all since it desorbed the 98% of gold and only the 15% of the remaining copper. A combined solution of NaCl and Na2SO3 was assessed as well since they can regenerate the resin during the elution process. Finally, electrodeposition was better with solutions coming from elution with NaCl and NaSO3 (82% of Au and 94% of Ag recovery on cathodes surface) compared to those from thiocyanate elution (62% of Au and 87% of Ag).