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– 0.776 mol/L), the logarithm of the gold dissolution rates (Fig. 7.2) was a linear function of the logarithm of bromine concentration with a slope of 0.55, which also confirmed that the isolation method mentioned in Section 7.2.1 was appropriate. However, it was found that as the concentration of bromine exceeded the limit (0.78 mol/L), there was very little increase in the initial gold dissolution rate. Thus, during the study of the effect of sodium bromide, the concentration of bromine was determined to be 1.94 mol/L, which was excessive. It is important to state that in the absence of sodium bromide, bromine alone can dissolve gold present in the shredded waste printed circuit boards, which was in conflict with Pesic and Sergent’s study. In their study, they stated that gold could not be dissolved by bromine in the absence of sodium bromide. The reason could be explained by the presence of other active metals in the waste printed circuit boards, which could donate free electrons and accelerate the reaction (Eq. 7.8) proceeding the right direction, thus forming bromide ions. Br2 + 2e = 2Br- (7.8) 7.2.3 Effect of Sodium Bromide As shown in Fig. 7.3, it was found that the initial gold dissolution rate increased as the concentration of sodium bromide increased. The addition of NaBr in the current concentration range studied significantly increases the initial gold dissolution rate. Based on Pesic and Sergent’s study (1993), beyond a limit of NaBr concentration, there was no further effect on increasing the gold dissolution rate. 7.2.4 Effect of Copper Given that copper is the major metallic element in printed circuit boards and is predominantly present as copper metal, it is of importance to investigate the effect of copper on gold bromide leaching. The results shown in Fig. 7.4 indicated that under the conditions of pH 0.22 and ORP 930 mV (vs SHE), the addition of copper (-45μm) had a dramtically negative effect on the gold dissolution rate. Based on the Pourbaix diagram shown in Fig. 3.7, copper is likely to be dissolved in the Br2-NaBr system to form Cu2+ and CuBr+ in the acidic oxidizing region, which means that copper is a competitor to gold. Moreover, due to the fact that copper is less stable than gold, the gold present as gold-bromine ions in the solution can be reduced to zero valence and 92PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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