The effect of pulp density was investigated at 20, 60 and 100 g/dm3 for low sulfur tailings samples dissolution in sulfuric acid. The changes in recoveries can be seen in Figure 21. Increasing the pulp density decreased the recovery of nickel, copper and iron. The largest difference in recovery between 20 and 100 g/dm3 being approximately 6 % points in case of nickel and iron. This seems not significant, but in case of nickel this actually means a difference of about 15 % in recovery.
FIGURE 21. The effect of changing pulp density on the recoveries on Fe, Ni and Cu. Leaching experiments were done with 0.3 mol/dm3 sulfuric acid for low sulfur tailings samples for a leaching time of 72 hours at a temperature of 22 °C.
The recoveries at smaller pulp density can be explained by the surface area available for leaching. A mixing speed of 500 rpm was used for every leaching experiment and hence better contact between the valuables and the leaching
0 10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45
Pulp density, g/dm3
Recovery, %
Fe Ni Cu
agents can be reached with lower pulp density as described in Chapter 2.4. The dissolution behavior and the change in pH during 72 hour leaching experiments in 0.3 mol/dm3 sulfuric acid at a temperature of 22 °C and a changing pulp density are shown in Figure 22.
FIGURE 22. Dissolution of Fe, Ni and Cu from low sulfur tailings samples during 72 hour leaching tests in 0.3 mol/dm3 sulfuric acid at different pulp densities with a constant temperature of 22 °C. In addition on below right the change in pH represented by normalized pH values for the same leaching experiments
The changes in pH, shown in Figure 22, indicate that changing pulp density does not have significant difference on the change in pH of the solutions. However, the shape of the pH curves are somewhat different depending on the pulp density. The higher the pulp density, the faster the pH rises. This can also be shown by the dissolution curves of copper, nickel and iron. The higher the pulp density the steeper the increase in concentration during the first hours of leaching. This would mean that the kinetics of leaching were faster at higher pulp density.
Both the experiments conducted with 60 g/dm3 and 100 g/dm3 seemed to be
density had reached equilibrium at 72 hours. In addition to this also the fact that the recovery was highest at 20 g/dm3 would indicate that leaching would be preferred to conduct with a low pulp density. This would however mean an increase in leaching agent consumption and hence an increase in operational costs.
A pulp density of 100 g/dm3 is closest to the actual pulp density of tailings in the process when compared to the other densities studied. The initial pulp density of low sulfur and high sulfur tailings were different, being approximately 26 % and 9
%, respectively. Of the densities chosen, the highest pulp density would after all be preferred since no extra dilution of high sulfur tailings is needed. Investigating if an even higher pulp density could be used is recommended. Optimization of leaching conditions for both tailings samples should be done separately.
The process should be further optimized in terms of pulp density, temperature and leaching time for both tailings samples in order to find the most optimal leaching conditions. Also investigating the effect and finding the most suitable redox potential would benefit the recovery of nickel and copper.
9 KINETIC MODELING
The most suitable kinetic models for the leaching of two tailings samples in sulfuric and citric acid were found out by fitting 15 different models shown in Table V to the experimental data obtained. Complete dissolution was not achieved during the experiments conducted and hence the kinetic models represent the first 72 hours of leaching. Modeling was done for each valuable separately even though leaching of constituents had an effect on leaching of other valuables as well. Focus was laid on finding the best fits for nickel and copper leaching. The two best fits for each leaching experiment excluding the experiments in which effects of temperature and pulp density were investigated are shown in Table XII.
TABLE XII Best fitting kinetic models for each leaching agent at different concentrations and with low sulfur (LS) and high sulfur (HS) tailings samples.
Leaching R2 values for Fe, Ni and Cu were 0.99, 0.99 and 0.98, respectively. These fits for
each valuable are represented in Figure 23. Table XIII shows the fit of each equation for this experiment.
TABLE XIII The best fits of each kinetic model for 1.0 mol/dm3 72 h citric acid leaching experiments at a pulp density of 100 g/dm3 and a temperature of 22 °C.
Equation no. Cu Ni Fe and in some cases even a nucleation model fitted poorly, since the R2 values for these models were very poor (R2<0.75 in most of the cases).
Fitting of models for nickel using data from the high sulfur tailings leaching, gave acceptable fits only with the Kabai model. Also in the case of sulfuric acid leaching of low sulfur tailings samples at lowest acid concentration only the Kabai model fitted. All in all it can be concluded that the leaching of nickel in sulfuric and citric acid can only be described by the Kabai model since all the other models gave R2 values smaller than 0.53.
The best fitting models in case of copper were obtained for high sulfur tailings leaching with 1.0 and 0.1 mol/dm3 citric acid. Again, the Kabai model fitted best, but also other diffusion models gave R2 values above 0.9. In all other citric acid
leaching experiments, the R2 values obtained were lower than 0.78 and for sulfuric acid leaching, only the Kabai model fitted the data.
The dissolution of the two tailings samples seem to be controlled through the diffusion of valuables through a product layer formed. The Kabai (1973) model describes diffusion when the constant a in equation 35 in Table V is smaller than 1 which is the case in all conducted experiments. The constant for leaching of iron is however about double compared to the constant for nickel and copper leaching.
This might arise from the fact that iron affects the leaching at the surface of the particles and hence might affect the surface reaction control.
As shown in Figure 23 the best fitting models describe the data very well since the values for the extent of reaction calculated by the models (equations shown in Figure 23) are close to the measured values for the extent of reaction. All the t-cFe,Ni,Cu curves show deceleratory behavior as do the experimental results.
Previous research on chalcopyrite and pentlandite leaching showed dissolution often is described by the shrinking core model through surface reaction control.
Many of the previous investigations (Aydogan et al., 2006; Bredenhann and Van Vuuren, 1999; Mbaya et al., 2013; Sokić et al., 2009) however only examined equations based on the shrinking core model. The obtained second best fitting equations describing 3D-diffusion are both derived from the shrinking core model and hence the conducted experiments are in line with previous research.
FIGURE 23. The experimental results for concentration of different valuables, the calculated extent of reaction and the best fitting models for 72 hour 1.0 mol/dm3 citric acid leaching for low sulfur tailings samples at 22 °C and a pulp density of 100 g/dm3.
10 CONCLUSIONS
Leaching of sulfides is very demanding because of the complex nature of the minerals. The leaching of tailings is in addition difficult to predict since the composition of tailings varies greatly between different mines. This study will serve as a base for future studies on the recovery of metal valuables from the
The two tailings samples showed that significant value is lost to the tailings in form of valuable metals. Through preliminary leaching tests, sulfuric acid and citric acid were shown to be the most promising leaching agents and were chosen for further study. Also sodium dithionite showed great potential in the leaching of nickel and copper during preliminary leaching tests. Leaching with sodium dithionite is recommended as further study, but the disadvantages should be noted before applying sodium dithionite leaching at industrial scale. Sodium dithionite is unpleasant and demanding to handle because of the odour and the reactivity.
Sulfuric acid and citric acid would hence be more preferable at industrial scale since no demanding material for process equipment and pipelines is needed and the cost of the leaching agent is lower.
Nickel was more efficiently recovered by sulfuric acid than by citric acid from low sulfur tailings samples whereas for high sulfur tailings samples the change in recovery was not significantly different depending on the acid. A similar trend could be seen for copper recovery however in the case of sulfuric acid leaching of high sulfur tailings samples some precipitation occurred and hence citric acid gave a higher recovery after 72 hours of leaching. The results may however not be applicable to all tailings samples since the mineralogy differs greatly depending on the mining process. Also the effect of pH is highly dependent on the mineralogy of the tailings and on the leaching mechanism of the dissolution reagent.
The experiments of this thesis show that leaching of the tailings studied at atmospheric pressure and ambient temperature resulted in moderate recovery of Ni and Cu. However, the recovery of valuables was significantly enhanced by increasing temperature giving recoveries as high as 57 %. Also extending the leaching time was shown to have a positive effect on the recovery. Increasing temperature and leaching time however results in increased operational costs.
Extending the leaching time might also be hard to carry out in practice especially since large production capacities would be needed. The change in stirring speed has not been investigated in this study and hence conclusions can not be made on the effect of stirring speed on the kinetics. An optimal temperature, stirring speed
and leaching time should thus be found out to find out the optimal process conditions. If a decrease in stirring speed would not hinder the leaching of nickel and copper, even the use of a settling tank could be considered when leaching at industrial scale.
The operational costs are also influenced by the concentration and consumption of the acid. Increasing the concentration of sulfuric and citric acid mainly increased the recovery of valuables. Some precipitation in case of sulfuric acid leaching could be observed for tailings samples containing high concentrations of sulfur and iron. Similar behavior was not observed for leaching with citric acid at higher concentrations and hence citric acid might be a more attractive leaching agent if applied at industrial scale. Citric acid also leached less iron and since leaching of iron was not a target in this experiment and the dissolution of iron might even be a hindrance in the following steps of the recovery of metals, citric acid would be preferable.
Kinetic modeling of the experimental results show that the rate determining step is the diffusion presumably through a product layer formed. Hence as discussed above, the optimization of stirring speed should be further investigated. When the rate determining step is the diffusion to and from the solid surface, the rate of reaction can be increased by liquid agitation. Hence the use of settling tanks at larger scale might be questionable. The best kinetic fits were obtained for the Kabai equation and in cases of iron dissolution also for the 3D diffusion models.
The conducted experiments are in line with previous research.
A number of possible future studies using similar setup are suggested. The effect of leaching with a solution of both sulfuric and citric acid could be investigated.
Factors that affect leaching need further, more comprehensive research. For example controlling the redox potential and investigating the effect of leaching different particle size fragments could further optimize leaching conditions. A possible increase in the revenue of the mine could also be gained by investigating the possibility of recovering platinum group metals from the tailings samples.
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APPENDICES
APPENDIX I Experimental results
Experiment no. 1 Tailings sample low sulfur
Leaching agent H2SO4 Mass of sample, g 100.1
Concentration, mol/dm3 0.5 Volume of concentrated acid, ml 27.8
Temperature, °C 22
Experiment no. 2 Tailings sample low sulfur
Leaching agent H2SO4 Mass of sample, g 100.0
Concentration, mol/dm3 0.3 Volume of concentrated acid, ml 16.7
Temperature, °C 22
Experiment no. 3 Tailings sample low sulfur
Leaching agent H2SO4 Mass of sample, g 100.1
Concentration, mol/dm3 0.1 Volume of concentrated acid, ml 5.5
Temperature, °C 22
Experiment no. 4 Tailings sample high sulfur
Leaching agent H2SO4 Mass of sample, g 100.0
Concentration, mol/dm3 0.5 Volume of concentrated acid, ml 27.8
Temperature, °C 22
Experiment no. 5 Tailings sample high sulfur
Leaching agent H2SO4 Mass of sample, g 100.1
Concentration, mol/dm3 0.3 Volume of concentrated acid, ml 16.7
Temperature, °C 22
Experiment no. 6 Tailings sample high sulfur
Leaching agent H2SO4 Mass of sample, g 100.1
Concentration, mol/dm3 0.1 Volume of concentrated acid, ml 5.5
Temperature, °C 22
Experiment no. 7 Tailings sample low sulfur
Experiment no. 7 Tailings sample low sulfur