![]() Graphical AbstractĮlectronic waste (e-waste) covers all manner of electronic devices from mobile phones and tablets through to the larger appliances such as fridges and washing machines that have ceased to be of value to their users or no longer satisfy their original purpose. The complementary methods applied for gold retrieval from the pregnant solution are also described with a focus on sustainable methods that have the potential to provide a closed-loop system, the key objective for material recovery in a circular economy. This review aims to provide an up-to-date evaluation of thiourea-gold leaching studies from electronic waste, with emphasis on the recent progression from the classic chemical method to a more sustainable hybrid bioleaching-based system, while its challenges are highlighted. Thiourea has received attention as an alternative lixiviant for gold leaching due to its fast reaction kinetics and less harmful nature. Although pyrometallurgy and hydrometallurgy processing are still the preferred modes of recovery for gold, the use of high-energy consuming pyro-methods, and the use of gold cyanidation that uses harmful lixiviants are increasingly discouraged. The recovery of gold, due to its high value, is one of the main motivations for recycling e-waste. ![]() Gold is widely used in electronics in numerous applications, although principally for contact points and external connectors. Still, as an amalgamation of precious and rare raw materials, electronic waste is a considerable economic resource with the most valuable components located on the printed circuit boards. In general, the results obtained were very promising and showed that gold can be successfully cemented from selected organic solvents by galvanic stripping using less noble solid metal reductants.Electronic waste is a dominant global issue with over 50 million tons generated annually. Activation energies were calculated and possible reaction mechanisms are discussed. The gold morphology was usually powdery or dendritic in nature but continuous films were obtained in some instances. The reaction rates were studied as a function of the variables zinc particulate size, oxygen and nitrogen atmosphere, water content in the organic phase, organic ratios and temperature. more » The gold reduction rates were relatively fast even though the conductivity of the organic solutions is very low. The direct precipitation of metallic gold from the loaded organic phase using zinc powder and iron, aluminum and copper slabs at 70 C was successfully demonstrated. = O solution into a mixed organic 40 vol.% TBP, 10 vol.% D2EHPA in kerosene. ![]() The significant advances that have been made in research on the hydrated electron, along with data indicating that it is feasible, at least in principle, to use the hydrated electron for industrial purposes, have been the stiumlus for setting up the studies that are reported here. An interesting approach to the solution of this problem is the use of so-called ''reagentless'' precipitation methods, among which we may class the reduction of gold and silver to the metallic state in cyanide solutions by hydrated electrons generated more » by ionizing radiation. ![]() All of these factors make it attractive to seek new methods for processing cyanide solutions that do not have these shortcomings. Hence, greater quantities of reagents are required, the process conversion becomes more complicated, and the cost of producing the metals is higher. However, the precipitation of gold and silver is accompanied by contamination of the solutions with zinc ions, which makes it difficult to recycle the cyanide solutions also, additional treatment of the precipitates is required before they are directed to the refining process. This process has certain advantages, and it is easy to carry out under plant conditions with high indices of efficiency. The classical method for precipitation of gold and silver from cyanide process solutions is reduction by metallic zinc. Redox reactions are widely used in chemistry and chemical engineering for the precipitation of noble metals, since this general class of reactions offers the possibility of selective recovery of these metals from solutions that are complex in composition.
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