Gold recovery from waste mobile phones PCBs using ammonia thiosulphate leaching and copper cementation process

Abstract

The rate of waste electrical and electronic equipment (WEEE) is growing at an alarming rate, especially in countries where markets are saturated with huge quantities of new electronic goods. Printed circuit boards (PCBs) are a substantial portion of the value contained in waste from WEEE although they are only 6% of the total weight. It is reported that WEEE is currently the fastest growing waste stream in South Africa as the general population’s access to electronic goods in the last decade has increased, especially access to mobile phones. PCBs are found in any piece of electrical or electronic equipment and consist of various metals including precious metals such as gold (Au), silver (Ag) and palladium (Pd). It is reported that gold has the highest economic incentive at 15,200 $ per ton of PCBs. The rapid introduction of new and advanced technology into mobile phones has caused mobile phones to have a relatively short life span, 1 to 2 years to be exact. Mobile phones printed circuit boards (MPPCBs) have more Au content compared to computer circuit boards. They contain 350 g/ton Au whereas computer (PC) PCBs contains 250 g/ton. This research project will recover gold from waste mobile phones PCBs pregnant ammonia thiosulphate leach solution using copper cementation. The cementation process is preferred to all the other technologies of metals extraction from solution due to ultrahigh purity metals that can be obtained and to the less consumption of materials and energy. Electronic parts on the PCBs were manually removed using pliers and screwdrivers. PCBs were then cut to smaller pieces of about 2 x 2 m. The pieces were crushed and milled respectively. Some of the particles were recycled back to the crusher to get finer particles. The particles were separated to particles of sizes between 0 and 1350 μm using a shaker. The comminuted fractions of the PCBs were then used in the leaching step. Batch cementation experiments were performed by bubbling N2 in glass reaction vessel with a working volume of 0.5 L. The reactor was connected to a circulating water bath for temperature control. The recovery percentage of gold at various temperatures, agitation speeds and different amounts of copper powder used, was determined while pH was monitored. The temperature was varied at 30 °C, 40 °C, and 50 °C and the agitation speeds at 300 RPM and 900 RPM. Copper powder was added at 0.5 g/L, 1 g/L, and 1.5 g/L. Gold concentrations were measured by atomic adsorption spectrophotometer (AAS). Scanning electron microscope (SEM) and Energy-dispersive x-ray spectrometry (EDS) analyses of the copper powder after cementation (precipitates) were used to determine the surface morphology and to evaluate the quantitative aspect of the precipitate. It was found that the recovery of gold from ammonia thiosulphate leach solution was greatly affected by agitation speed. At an agitation speed of 900 rpm, 40 °C and 0.5 g of Copper powder, 96% of the gold was recovered from the leach solution. The cementation rate increased as temperature was elevated from 30 to 40 °C, but slightly decreased as the temperature reached 50 °C. The change in experimental conditions affected the gold concentration on the precipitate recovered. This study will provide a possible solution to the WEEE problem and more specifically mobile cell phones, in South Africa.