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Urban Mining of Precious Metals with Cyanide as Lixiviant
Published in Sadia Ilyas, Hyunjung Kim, Rajiv Ranjan Srivastava, Sustainable Urban Mining of Precious Metals, 2021
Rajiv Ranjan Srivastava, Sadia Ilyas, Nimra Ilyas, Hyunjung Kim
Dissolved Fe and Cu form different respective cyanide complexes in leach liquors. Similarly, the presence of small amounts of lead also accelerate the leaching. The electrode potential value of these metals in the cyanide solution indicates that gold can displace these metal ions. The accelerating effect in the presence of these metal ions corresponds with the alteration in the surface character of the gold by alloying with displaced ions. A change in surface character may lead to a decreasing thickness of the boundary layer through which the reactants diffuse to reach the metallic surface. Oxygen is necessary for gold cyanidation leaching. Any side reaction that may deprive gold of its oxygen content in cyanide solution will lead to a decrease in the leaching rate. Metal impurities like silver, copper, zinc, and iron associated with gold may dissolve in a cyanide solution, causing depletion of the cyanide content from the lixiviant. Aluminosilicates, if present, form colloidal silica and alumina in alkaline pH as well as precipitating the iron. These are reaction products that have a strong adsorptive capacity for sodium cyanide, thus retarding the gold leaching. A large number of lead ions cause a retarding effect by forming an insoluble film of Pb(CN)2 on the gold surface.
Artisanal Gold Mining and Amalgamation
Published in Sadia Ilyas, Jae-chun Lee, Gold Metallurgy and the Environment, 2018
Tailings generated during amalgamation, which contain ~1 g/t gold with many folds of mercury therein, are treated to enhance the overall gold recovery, usually undergoing a cyanide leaching. By which, the gold along with the mercury and other metals forms various complexes with cyanide. Up to 90% gold from the tailings can be leached in the cyanide solution along with 40%–50% mercury, mainly due to a slower kinetics for mercury complexation with cyanide; hence, it keeps forming as the miners discharge the pulp into the receiving environment. Gold cyanide complexes formed during the leaching step are then adsorbed on activated carbon and removed from the leaching system without the need of filtration. Some species of Hg(CN)2 are more easily adsorbed on the activated carbon than [Hg(CN)4]2− (Adams, 1991). Furthermore, the gold cyanidation process itself occurs at pH levels between 10 and 11. Under this condition, it is expected that little mercury reports to the activated carbon. The tailings from which most of the gold was extracted with carbon are rich in mercury-cyanide complexes.
Extract of cassava waste as a lixiviant for gold leaching from electronic waste
Published in Green Chemistry Letters and Reviews, 2022
Yuranan Photharin, Sirilak Wangngae, Utumporn Ngivprom, Kantapat Chansaenpak, Anyanee Kamkaew, Rung-Yi Lai
After the successful extraction of cyanide from fresh cassava leaves, the pH of the extract was adjusted to 11 for gold cyanidation. Because the pKa value of HCN in water is 9.2, the pH value for efficient gold cyanidation must be greater than 10.5 to produce non-volatile ionic cyanide (19). Certified gold foil (3 mg, 99.99% purity) was added to 20 mL of extract containing 72 ppm cyanide and rapidly agitated at various temperatures for one day. To assess the gold concentration, each mixture was filtered through a 0.22-µm nylon filter before ICP-OES analysis (Figure 6). Gold leaching occurred in all samples. The gold concentration in the sample leached at 25°C was 52.2 ppm, while the gold concentration in the sample leached at 80°C was 40.5 ppm. The leaching activity and rate were reported to increase with increasing temperature (19). However, as temperature increases, gas solubility decreases, causing the amount of oxygen in solution to decrease. Furthermore, higher temperatures also intensify the HCN volatilization in extraction process (43). One option to overcome this issue is to conduct gold leaching at high temperature in a pressure reactor with an oxygen supply (44). In subsequent experiments, a temperature of 25°C was used for simplicity.