*3.3. Leaching of the Washed ZPR*

The next step of the process is to leach the metals out of the ZPR into an aqueous solution from whichi it will be possible to separate the cobalt from the other metals. The leaching tests are conducted in a mechanically agitated beaker for 30 min using a weight L/S ratio of 15/1 at 80 ◦C. Leaching tests are carried out with sulfuric acid as it is the acid used in the zinc plant that provided the ZPR. Table 5 gives the average proportion of metals dissolved (±standard deviation) of three (3) leaching tests and the average metal contents in the liquor. About 65% of the solids in the ZPR are dissolved during the leaching process with 98% of the cobalt effectively leached off the ZPR, a performance similar to that reported in [10].

**Table 5.** Proportion of the metals dissolved during the leaching of the ZPR (Average (±standard deviation) of the results of three (3) leaching tests and the average composition of the liquor (Ratio L/S: 15/1; 100 g/L H2SO4, Eh = 90 mV, pH = 0.2, 30 min, 80 ◦C).


#### *3.4. Selective Precipitation of the Cobalt from the Pregnant Liquor Solution*

The main impurities in the pregnant liquor solution are Zn, Cd, Ni, Fe, and Cu (Table 5). The approach used to selectively separate cobalt from these elements begins with an oxidation of iron and manganese to precipitate of these oxidized elements by increasing the pH near 3.0 while Cu, Zn, Cd, and Co(II) remain in the solution. The next step is to oxidize Co(II) to Co(III) and to precipitate Co(III) by increasing the pH above 3.0 leaving Zn, Cd, Ni and Cu in the solution.

Ammonium PerSulfate or APS ((NH4)2S2O8), as discussed in [1,8], provides the required oxidation potential for iron, manganese and subsequently cobalt. The sequence of the steps followed to precipitate the cobalt is shown in Figure 6.

**Figure 6.** Process for the extraction of cobalt from the ZPR.

#### 3.4.1. Precipitation of Fe-Mn

Sodium hydroxide is firstly added to the pregnant liquor solution to bring the pH to 3.0. APS is then added to the solution to increase the Redox potential from 90 to 650 mV. During the oxidation process, the pH of the solution is maintained at 3.0 by a regular addition of NaOH. Samples of the solution were collected at different Eh values and analyzed for Fe, Mn, and Co. As anticipated [1] Fe and Mn precipitate prior to Co a behavior confirmed by the variations of the metal contents in the solution as shown in Figure 7. According to Figure 7, the optimal Eh for the precipitation of Fe and Mn without precipitation of Co is 650 mV and the minimum Eh for the precipitation of Co is 1000 mV. These Eh values are coherent with those reported in [1]. Table 6 gives the proportions of the metals in the leached solution that are removed by the oxidation to 650 mV. Iron and manganese are almost completely removed from the solution. Ideally, cobalt should not be precipitated at this stage; however, it was observed that a close control of the Eh was not always possible as the Redox probe shows a significant measurement variability causing cobalt losses (~17%) in the Fe–Mn precipitate. The difficulty in the control of the Eh is attributed to the probe and to its sensitivity to the APS addition. Indeed, a small addition of APS can make a large local step in the Redox potential of the solution. It is likely that a process solely based on the use of an APS dosage [1,8] is not recommended to achieve a selective separation. Cleary the oxidant addition should be based on a reliable measurement of the ReDox potential of the solution and not on the dosage of the oxidant.

Once the Redox potential reaches 650 mV, the addition of APS is stopped and the iron and manganese precipitate are removed by filtration (see Figure 6). The filtration of the sludgy Fe-Mn precipitate is difficult and is believed to be one of the reasons for the loss of cobalt at this stage. Ensuring a rapid and adequate filtration of the solution after the end of the reaction seems critical to limit cobalt losses. The solution from that solid/liquid separation advances to the cobalt precipitation step (Figure 6). Results of Table 6 show that Mn and Fe are effectively removed from the solution at the expenses of some cobalt losses. The large standard deviation observed for the fraction of

cobalt precipitated is attributed to Eh measurement problems as discussed above and difficulties in the filtration of the precipitate from one test to another one. Improving on the accuracy of the Eh measurement method and of the solids/liquid separation of the Fe–Mn precipitate could significantly improve the global recovery of cobalt.

**Figure 7.** Precipitation of Fe and Mn with increasing the Eh while maintaining a pH of 3.0 (**a**) Fe, Mn, and Co contents in the solution during the oxidation to 1000 mV. (**b**) Fe and Mn contents in the solution during the oxidation below 1000 mV.

**Table 6.** Metal precipitation during the oxidation for Fe-Mn removal of the leach solution (Average (±standard deviation) results of three (3) tests (Eh = 90 to 650 mV, pH = 3.0, 80 ◦C, 15 min).


#### 3.4.2. Precipitation of Co

The cobalt precipitation is carried out by adding APS to bring the Redox to 1000 mV and keeping the solution at 80 ◦C to accelerate the precipitation [1]. The precipitation of cobalt under the form of CoOOH is slow as shown in Figure 8a. This observed behavior is consistent with previously reported results [1] reproduced in Figure 8b. Table 7 gives the variation of the solution composition from the leach step to the Co precipitation one. Zn, Cd, and Cu remain in the solution while cobalt precipitates with some of the iron and manganese remaining in the solution after the first oxidation step. Table 8 gives the proportions of the different metals precipitated during the cobalt precipitation step. About 2% of the copper precipitates with the cobalt and represents a critical impurity as discussed later. The fact that the observed Co precipitation rate in Figure 8 is significantly less than that reported in [1] is an indication that there is room to improve the proposed processing scheme in order to reduce the precipitation time and subsequently the volume of the precipitation vessels for a continuous process.

**Figure 8.** Cobalt precipitation rates. (**a**) Observed rate of Co precipitation (pH = 3.0, Eh = 1000 mV, 80 ◦C, 1 g/L Co). (**b**) Rates of Co precipitation reported in [1] B-0.



**Table 8.** Metal precipitation during the cobalt precipitation stage (Average ± standard deviation) for three (3) tests (Eh = 1000 mV, pH = 3.0, 80 ◦C, 120 min).

