*3.2. Selective Leaching of Zn and Mn*

### 3.2.1. PLS-1 Leachate

Several groups of researchers have studied the extraction of metals from battery powder using sulfuric acid. In our study, during the first leaching process, metallic powder was leached in 2 M H2SO4 with S/L ratios of 10, 20, 40, and 50% for 90 min at ambient temperature. Our results indicate that the leaching efficiency of Zn was related to the amount of total solids and reaction time (Figure 3).

**Figure 3.** Variation in Zn concentration and the first leaching efficiency (%) with different solid/liquid (S/L) ratios.

The results indicate that an increase in S/L ratio increased leaching up to 90 g Zn/L. Nevertheless, an increase in the S/L ratio also decreased the leaching efficiency. The concentration of Zn in the effluent is an important parameter for electrowinning in this process, and, therefore, an S/L ratio of 40% was chosen for the first cycle of leaching and 20% for the next cycles. A 40% (*w*/*v*) ratio allowed for the dissolving of 87.1% of Zn with a concentration of 83.6 g/L, after 45 min (Table 5). Under these conditions, the concentration of Mn in PLS-1 was 28.5 g/L (22% of Mn in the battery powder).

**Table 5.** Composition of metals in different solutions (pregnant leach solution 1 (PLS-1): 2 M H2SO4, S/L ratio of 40% and PLS-2: 1.34 M H2SO4, S/L ratio of 14.1%, 0.45 g Na2S2O5/g metallic powder for 45 min).


#### 3.2.2. PLS-2 Leachate

Notably, the weight of solids was reduced after the first leaching by the dissolving of metals in PLS-1, with the mass of metal powder decreasing from 400 to 284 g after the first leaching. The addition of Na2S2O5 (0.45 g Na2S2O5 for 1 g metallic powder) in the second leaching resulted in more than 96% of Mn being dissolved. The S/L ratio had a negative impact on the leaching of Mn with a decrease in the S/L ratio from 35 to 9%, resulting in an increase in Mn extraction rate from 72 to 96%. The S/L ratio of 14.1% made it possible to extract 96.5% of Mn (49.1 g/L). Under these conditions, the concentration of Zn in PLS-2 was 5.87 g/L (a 94.5% solubilization of Zn). Table 5 represents the classification of PLS-1 and PLS-2. Our results confirm the important role of Na2S2O5 as the reducing agent for dissolving Mn in the leachate (PLS-2). In an H2SO4 solution, the leaching yield of Mn increased from 22 to 96.5% with the addition of Na2S2O5. The residue mass decreased from 284 to 140 g after the second leaching.

Table 6 represents the mass distribution for the leaching process for 1 kg of metallic powder and also the global efficiency of the leaching process. The composition of metals (in mg/kg of residue) in residue is also represented in Table 6. The combination of Na2S2O5 with sulfuric acid led to the dissolving of not only Zn and Mn, but also other metals (Co, Cu, Ni) of more than 98.4%.

**Table 6.** Composition of metals in different fractions for each step of leaching.


3.2.3. Final Residue

A TCLP test was conducted on the final residue in order to validate the effectiveness of the leaching process and residue valorization capacity. Table 7 presents the TCLP results for the residue obtained in our study across three cycles with the recycling of water. All results were below USEPA limits for the potential hazards of waste [27]. The residue obtained after the second leaching is not considered hazardous and could, therefore, be used for other applications, such as a material for the production of batteries.

**Table 7.** Metal concentration in leachate by toxicity characteristic leaching procedure (TCLP) for final residue.


#### *3.3. Purification and Recovery of Metals*
