Sustainable recovery of metals from waste printed circuit boards

Abstract

The continuous progression of technology and ever-increasing demand for the latest products has led to an increase in electronic waste (e-waste), causing a major global waste issue. This is also an increase in secondary sources of materials presenting an ideal opportunity to utilise e-waste for material recovery as part of the transition from a linear to a circular economy. Pyrometallurgy methods, utilising high temperatures and hydrometallurgical leaching processes are the traditional routes used for metal recovery from e-waste. The pyrometallurgical processes use lots of energy and emit harmful gases detrimental to the environment. Large volumes of lixiviant are often necessary to obtain high recovery yields which require additional processing and treatment steps to ensure safe disposal. Global climate issues are leading to more sustainable processes being preferred. In this work, bioleaching, the use of microorganisms to facilitate metal recovery was tested as an alternative method for metal leaching as it is considered a greener process. The aim was to develop and assess processes that could be viable for industrial use. The bioleaching work focussed on a consortium of acidophiles utilising redox and acid leaching mechanisms through iron and sulphur oxidation. The initial process looked at direct bioleaching and compared one and two-step methods of material addition using a serial acclimatisation process. The next stage in this process was to assess an indirect leaching method using biogenic lixiviants for base metal recovery, produced from a solid support bioreactor. The most efficient pulp density was determined, and the effect of temperature was also assessed. The final stage of the process was focused on developing a hybrid bio- method for gold leaching using the alternative lixiviant, thiourea. The direct bioleaching system showed that a two-step process was preferential to a one-step process, highlighting lower bacterial activity due to harsh changes in the system media caused by the addition of the PCB material. The twostep system was less affected up to pulp densities of 1.0%(w/v) however the use of this method industrially was deemed inappropriate due to long growth periods. The indirect system developed had a better suitability to scale up and showed effective base metal recovery. Gold leaching was achieved using an indirect hybrid method showing that careful control of the process can allow for reduced chemical usage.

Date of Award	May 2024
Original language	English
Awarding Institution	Coventry University
Sponsors	Network 2 Supplies (N2S) Ltd & Bio Scope Technologies Ltd
Supervisor	Sebastien Farnaud (Supervisor), John Graves (Supervisor) & Alan Greenwood (Supervisor)