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(a) (b)

**Figure 4-5: Factor interaction plots for acid pre-treatment: (a) Cu extraction and (b) Au extraction **

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(a) (b)

**Figure 4-6: Ammonium thiosulphate leaching [0.1M ammonium thiosulphate, 0.2M ammonia, 0.03M **
**copper sulphate, 0.03M nickel sulphate, pH 10, temperature 25°C, 6h reaction time, 350 rpm **

**stirring speed]: (a) gold extraction and (b) thiosulphate consumption **

Therefore, the synergistic effect of combining these two factors was that the gold exposure to the lixiviant was improved and the Ni-ATS leaching system was more stable and efficient thermodynamically, and thus improved the kinetics of the leaching process.

A more detailed statistical assessment of the factor effect sizes and interaction between PCB pre- treatment and metal oxidant is provided in section 4.4.3.2.

**4.4.2 Ammonium Thiosulphate Consumption **

The ammonium thiosulphate consumption profiles obtained in this research are provided in Figure 4-6b. The first observation was that the copper-thiosulphate leaching system without acid pre- treatment incurred the highest thiosulphate consumption, reaching a peak value of 140.4 kg/ton- PCB before stabilising around 90.9 kg/t-PCB, although the resulting gold extraction reached a maximum of 20.82% only. The thermodynamics of Cu-ATS leaching outlined in chapter two (section 2.6.1) indicated that side reactions involving copper and thiosulphate, leading to the degradation of thiosulphate to tetrathionate, are among the main causes for this phenomenon.

0 10 20 30 40 50 60 70

0 60 120 180 240 300 360

Gold Extractoion (%)

Time (min)

ATS Leaching - Gold Extraction

Cu-ATS w/o AP Cu-ATS with AP Ni-ATS w/o AP Ni-ATS with AP

0 20 40 60 80 100 120 140 160

0 60 120 180 240 300 360

Thiosulphate Consumption (kg/t-PCB)

Time (min)

ATS Leaching - Thiosulphate Consumption

Cu-ATS w/o AP Cu-ATS with AP Ni-ATS w/o AP Ni-ATS with AP

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The ATS consumption was found to improve significantly with the use of Ni(II) ion in place of Cu(II) ion. With untreated PCBs, the stabilised lixiviant consumption was reduced from 90.9 kg/t-PCB to 47.07 kg/t-PCB with an equivalent increase in gold extraction from 18.61% to 46.89%. With acid pre-treated PCBs, the stabilized reagent consumption decreased from 72.6 kg/t-PCB to 61.03 kg/t-PCB with a corresponding Au extraction increase from 36.02% to 65.41%. The thermodynamics of the process had thus shifted to the more stable Ni-ATS system which lessened the decomposition of thiosulphate. Another significant improvement in terms of the ATS consumption was observed in Cu-ATS leaching with acid-pretreated PCBs. The ATS consumption was reduced from a peak value of 140.4 kg/t-PCB to 87.2 kg/t-PCB, and the stabilized consumption was reduced from 90.9 kg/t-PCB to 72.6 kg/t-PCB. This consumption reduction was accompanied by an increase in gold extraction from 18.61% to 36.02% in 5 hours. The combined effect of PCB pre-treatment and nickel oxidant saw a decrease in thiosulphate consumption from a stabilised level of 90.9 kg/t-PCB to 61.03 kg/t-PCB with a resulting gold extraction increase from 18.61% to 65.41% in 5 hours, thus confirming the research hypotheses formulated for this project (section 1.4).

**4.4.3 Statistical Analysis of Ammonium Thiosulphate Leaching Results **

**4.4.3.1 Experimental Repeatability Test **

Based on the p-values, t-statistics and Pearson correlation coefficients of the t-tests tabulated in Appendix D (Table D-5 and Table D-6), there were no statistical grounds for rejecting the null hypothesis, and thus the runs and their respective duplicates were indicative of the same experimental conditions, which confirmed the experimental repeatability of ammonium thiosulphate leaching.

**4.4.3.2 Analysis of Variance (ANOVA) – ATS Leaching **

The ANOVA results for the ammonium thiosulphate leaching experiments are fully provided in Appendix D (Table D-9). The p-values of the effect of PCB pre-treatment variation were extremely small compared to the adjusted alpha value of 0.0253 for both gold extraction (0.000) and ATS consumption (0.002), confirming the statistical significance of PCB pre-treatment. Similarly, the variation of the metal oxidant used in ATS leaching was found to be statistically significant for both gold extraction and ATS consumption based on the low p-values obtained.

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For gold extraction, the effect of PCB pre-treatment and metal oxidant, in terms of the partial eta squared, indicated equal or close significance levels for both factors. However, the F-statistic provided more insight into the significance level of each factor. The F-critical of the metal oxidant (694.8) was greater than that of the PCB pre-treatment (574.9), indicating that the metal oxidant had a higher significance level than PCB pre-treatment with respect to gold extraction. This observation was in agreement with the previous discussion in section 4.4.2 which established that a significant improvement in gold extraction was achieved by first replacing copper(II) ion with nickel(II) ion as the metal oxidant which was found to increase the gold extraction from 18.61% to 46.89% (i.e. 152% increase) in 5 hours, whereas for each metal oxidant used, the PCB pre- treatment improved the gold extraction by a lower amount. Acid pre-treatment increased the gold extraction from 18.61% to 36.02% (i.e. 93.6% increase) in Cu-ATS leaching and from 46.89% to 65.41% (i.e. 39.5% increase) in Ni-ATS leaching. Replacing copper with nickel had a major effect in that it changed the chemistry and thermodynamic stability of the leaching system, as discussed in chapter two (section 2.6.3.3).

The interaction between PCB pre-treatment and metal oxidant were not statistically significant for gold extraction (p-value of 0.788 > 0.0253) but significant for ATS consumption (p-value <<<

0.0253). The interaction plots in Figure 4-7 indicate that the estimated marginal means of gold extraction exhibited parallel lines, indicating no interaction or correlation between the two factors with respect to gold extraction. On the other hand, the estimated marginal means of ATS consumption exhibited nonparallel lines, indicating a possible interaction or correlation between the two factors with respect to ATS consumption. Although no factor interaction was established statistically, the individual factor effects had proven significant in relation to Au extraction. For ATS consumption, since there was an intimate correlation between PCB pre-treatment and metal oxidant, the individual effects of these factors were also interrelated, expectedly in chemical and thermodynamic aspects.

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(a) (b)

**Figure 4-7: Factor interaction plots for ammonium thiosulphate leaching: (a) Au extraction and (b) **
**ATS consumption **