Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Author "Albertyn, Pierre Wouter"

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    Ammonium thiosulphate leaching of gold from printed circuit board waste
    (Stellenbosch : Stellenbosch University, 2017-03) Albertyn, Pierre Wouter; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Technological innovation leads to a reduced lifespan of older electrical and electronic equipment, which in turn leads to the generation of vast quantities of electronic waste (e-waste). The recycling of e-waste is becoming increasingly important as it provides certain economic benefits apart from the obvious environmental benefits. Printed circuit boards (PCBs) are found in most forms of e-waste and contain especially high concentrations of base and precious metals. Hydrometallurgy is one of the major processing routes for the recovery of valuable metals from e-waste. This processing route normally implements several leaching stages to selectively recover certain metals. A two-step base metal leaching stage was implemented that utilized two different lixiviants. The first step used nitric acid to mainly recover Pb and Fe, while the second step used sulphuric acid in combination with hydrogen peroxide to mainly recover Cu, Zn and Ni. The Au and Ag were subsequently recovered in an additional leaching stage with ammonium thiosulphate in the presence of copper(II) sulphate. This study focused on the use of a less environmentally hazardous lixiviant than the traditional alternative, cyanide, to promote the development of a more sustainable recovery process. The primary objective of this study was to determine how the variation of copper in the first stage residue will affect the gold leaching in the second stage. The extent of interactions between process conditions was also studied. These process conditions included temperature, thiosulphate concentration, ammonium concentration, cupric ion concentration, pH and pulp density. The secondary objective of this study was to determine how the degradation of thiosulphate was affected by the change in certain process conditions. The screening phase determined that only a change in S2O3 concentration, pH range and pulp density had a statistically significant effect on the Au extraction. Statistically significant interactions existed between the Cu on the PCBs and Cu(II) concentration; and the Cu on the PCBs and pulp density. These results were used together with recommendations from literature to determine what factors to include in the full factorial design. The S2O32- concentration (0.1 and 0.2 M), NH3 concentration (0.2 and 0.4 M), pH range (9 – 9.5 and 10 – 10.5) and pulp density (25 and 50 g/L) were chosen. The investigation of the S2O32- and NH3 concentrations determined that Au leaching was dependent on the S2O32-/NH3 ratio. S2O32- concentrations that were too high relative to NH3 resulted in the Cu(S2O32-)35- complex becoming more prominent, which hindered Au dissolution. NH3 concentrations that were too high resulted in a decrease in the oxidation potential of the Cu(II)-Cu(I) couple, which in turn reduced the driving force for the Au leaching reaction. NH3 concentrations that were too low reduced the amount of Cu(NH3)42+ (oxidizing agent for gold) that was available, which in turn also reduced Au leaching. The optimum S2O32-/NH3 ratio for the range of parameters that were investigated was found to be 0.5. A change in NH3 concentration was found to have a more significant effect on Au extraction at the lower pH range of 9 – 9.5. This was believed to be due to a higher concentration of NH4+ relative to NH3 being present at lower pH values, which caused faster Au leaching. The lower pH range of 9 – 9.5 also generally produced better Au leaching. An increase in pulp density from 25 to 50 g/L resulted in a decrease in Au extraction, which could be attributed to the fact that the amount of reagent per unit weight of PCB decreased. The importance of the interactions between S2O32- and NH3; and pH range and NH3 were confirmed in the statistical analysis of the full factorial design. The statistical analysis produced a model with a R2 value of 0.94 that predicted an optimum Au extraction of 78.04 % at the same conditions that produced the optimum Au extraction during testing. The predicted an optimum compared well with actual value of 78.47 %, which was obtained at 0.2 M S2O32-, 0.4 M NH3, 0.02 M Cu(II), 25 g/L, 25°C, 1 – 10 % leftover Cu and pH range of 9 – 9.5. The optimum conditions were used to determine the effect of a variation in Cu in the first stage residue, temperature and Cu(II) concentration. Au extraction decreased with an increase in Cu leftover content, temperature and Cu(II) concentration. Increased amounts of Cu inhibited Au leaching through the dissolution of Cu to Cu(NH3)2+ with the consumption of Cu(NH3)42+. Increased rates of thiosulphate consumption/degradation were encountered at higher temperatures, Cu(II) concentrations and leftover Cu.