Sulfuric acid leaching was studied in three different concentrations 0.1, 0.3 and 0.5 mol/dm3 having a pulp density of 100 g/dm3 and a temperature of 22 °C. Both tailings samples were investigated and the recoveries are shown in Table X. For the low sulfur tailings samples all recoveries seemed to increase with increasing acid concentration whereas the trend was somewhat harder to explain in case of the high sulfur tailings samples. Recoveries are calculated by dividing the amount of dissolved valuable by the amount of valuable introduced in the reaction, which was calculated with the knowledge of the concentration of valuables in the
sample. Highest recoveries for nickel and copper were attained for low sulfur tailings samples at high acid concentration.
TABLE X Recovery of Ni, Cu and Fe in 72 hour sulfuric acid leaching experiments. Pulp density was 100 g/dm3 and temperature 22 °C.
Recovery, % Tailings sample Acid concentration,
mol/dm3
Cu Ni Fe
low sulfur 0.5 17 36 31
0.3 16 27 21
0.1 13 22 11
high sulfur 0.5 0.1 12 22
0.3 0.0 9 29
0.1 2.8 11 7
The reason for such differences in recoveries of the two tailings samples might arise from the change in pH of the solution and the difference in the content of sulfur and iron in both samples. The normalized changes in pH are shown in Figure 10 in which low sulfur tailings are denoted as LS and high sulfur tailings as HS. The pH values are normalized by dividing all pH values by the initial pH of the solution which was measured before introducing the tailings samples into the reactor. Normalizing simplifies the comparison of the experiment data. For the lowest acid concentration the difference in the increase in pH is between 2-5 times the difference in higher acid concentrations for both tailings samples. This is most likely caused by the low acid concentration and the dissolution mechanism.
Sulfuric acid presumably works by protonation and hence enough acid should be available in order for leaching to take place.
FIGURE 10. The change in pH during 72 hour sulfuric acid leaching experiments for two different tailings samples with a pulp density of 100 g/dm3 and temperature 22 °C.
LS stands for low sulfur tailings and HS for high sulfur tailings.
In the leaching of the tailings samples with sulfuric acid, the concentration of iron, nickel and copper can be described by decelerating curves as shown in Figure 11, Figure 12 and Figure 13. For the lowest acid concentration the reaction approached the state of equilibrium after 72 hours of leaching. For the low sulfur tailings samples the equilibrium state might have been reached since the pH as shown in Figure 10 had reached a steady value. However, in case of leaching the high sulfur tailings samples, the pH seemed to be still increasing after 72 hours and this might have been because the concentration of nickel had not reached a steady value yet. There was still some leaching through protonation going on and hence the pH value was not stable. A longer leaching time would have been needed in order to reach equilibrium.
For the leaching experiments conducted with 0.3 mol/dm3 sulfuric acid equilibrium was approached in case of leaching of the low sulfur tailings sample.
This can also be shown in Figure 10 since the pH had reached a steady value. In case of leaching of the high sulfur tailings samples the pH still seemed to be increasing after 72 hours of leaching. This was presumably caused by the continuous increase in iron concentration. For nickel the concentration seemed to
0 20 40 60 80
have reached a steady value and even slight precipitation could have been noticed.
For copper the precipitation was more evident. An expected decelerating behavior could be seen until approximately 12 hours of leaching was achieved after which precipitation of copper could be observed as shown in Figure 13.
In case of leaching with the highest acid concentration, similar precipitation behavior could be observed in case of copper dissolution. The recoveries of copper after 72 hours of leaching were low or even zero as shown in Table X.
Also in case of iron and nickel dissolution the concentration of dissolved valuables and pH seemed to be still increasing, indicating that equilibrium was neither reached nor even approached.
FIGURE 11. Effect of changing sulfuric acid concentration on iron recovery for both of the tailings samples during a 72 hour leaching test with a pulp density of 100 g/dm3 and temperature 22 °C.
FIGURE 12. Effect of changing sulfuric acid concentration on nickel recovery for both of the tailings samples during a 72 hour leaching test with a pulp density of 100 g/dm3 and temperature 22 °C.
FIGURE 13. Effect of changing sulfuric acid concentration on copper recovery using high sulfur tailings sample, with a pulp density of 100 g/dm3 and temperature 22 °C.
0 10 20 30 40 50 60 70
When comparing the copper dissolution curves in Figure 13 precipitation occurred for the lowest acid concentration in case of low sulfur tailings samples and for all acid concentrations in case of high sulfur tailings samples. A linear increase in pH was observed in both of the leaching experiments conducted at 0.1 mol/dm3 until precipitation occurred. When precipitation started (noticed for low sulfur tailings samples at 48 hours of leaching and for high sulfur tailings samples at 12 hours of leaching) also pH began to stabilize to a steady value. The amount of copper leached at the time points when precipitation started differed between the two tailings samples. The high sulfur tailings samples contain ten times the amount of copper compared to the low sulfur tailings samples. About three times as much copper was leached from the high sulfur tailings samples before precipitation occurred. Hence also more copper is available for precipitation in case of the high sulfur tailings samples. The higher the sulfuric acid concentration the earlier precipitation takes place in case of the high sulfur tailings samples. This might be caused by the change in redox potential or by the solubility of copper between the three experiments. For analysis on the redox potential further investigations should be conducted.
The dissolution of copper can also be hindered by the hydrolysis of ferric ions. As was shown, pH is a good indicator of the stage of the reaction. Leaching under suitable pH range is vital in order to avoid the hydrolysis of ferric ions into insoluble iron products that for example harm the dissolution of chalcopyrite (Koleini et al., 2011). Sulfur and the insoluble iron products have been discussed as being possible causes for chalcopyrite passivation. The different composition of the tailings samples might also be a reason for differing recoveries. The high sulfur tailings samples studied contain more than 12 times the amount of sulfur and twice the amount of iron when compared to low sulfur tailings samples. With higher iron and sulfur content in the tailings sample, there is also a higher probability for formation of a passivation layer which hinders copper dissolution.
In addition the tailings samples contained small amounts of pyrrhotite which can cause precipitation of nickel and copper.
There also might be some differences in the mineralogy of the two tailings samples and hence copper liberation might be more difficult after the time point where precipitation occurred. XRD analysis was made for the samples before and after leaching, but the analysis is not covered in this study because of the complexity of the samples.