**1. Introduction**

The use of electronic compact devices with batteries such as remote controls, watches, electric toys, and pocket lamps has become an integral part of our society. Furthermore, these batteries have a certain lifetime, and the increase in volume of spent batteries over the last few years requires an innovative recycling process. Findings from research into metal recovery in recent years indicate the importance of recycling spent batteries [1–8]. In Canada, Call2recycle collected more than 2.5 kt of batteries for recycling in 2017 and 2.7 kt in 2018, of which 78% consisted of alkaline and Zn–C batteries [9]. However, the collected quantity only represents approximately 20% of all batteries sold in the market [10]. Alkaline batteries consist of a negative zinc metal electrode and a positive manganese dioxide (MnO2) electrode with an alkaline potassium hydroxide electrolyte, instead of the acidic ammonium chloride electrolyte used in zinc–carbon batteries. After collection, batteries are separated by type and sent to appropriate processing plants. Alkaline and carbon zinc batteries are sent to Retriev (Trail, BC, Canada), Inmetco (Elwood City, PA, USA), Raw Materials Company (Port Colbone, ON, Canada), and Battery Solutions Recovery (Brighton, MI, USA), where the batteries

are treated by pyrometallurgical processes [10,11]. These processes separate metals by volatilization and melting, and, therefore, require high-energy consumption and, due to the release of toxic gases, require an additional collection/cleaning system. By contrast, hydrometallurgical processes usually have a lower energy consumption and lower environmental impacts than pyrometallurgical processes. Hydrometallurgical processes consist of metal leaching followed by the separation, purification, and recovery of valuable metals through various techniques including, among others, precipitation, solvent extraction, and electrowinning. Numerous studies have used leaching processes under various conditions for the leaching of Zn and Mn from battery powder (Table 1).


**Table 1.** Leaching yields of Zn and Mn from alkaline battery by acid leaching.

Notably, the respective reaction time, temperature, acid concentration, and solid/liquid (S/L) ratio should be compared, as these parameters vary across the different studies. Sulfuric acid is commonly used either singly or in combination with an auxiliary agent. For example, the addition of H2O2 (4% *v*/*v*) in a sulfuric acid solution was used to remove 100% of Zn and 95.7% of Mn from battery powder [12]. In another study, the combination of ascorbic acid with sulfuric acid led to the dissolving of 99.5% of Zn and 98.8% of Mn [13]. Although the use of an auxiliary agent is key to dissolving Mn in sulfuric acid, results from a study where no auxiliary agents were used indicate a 90% removal of Zn and less than 20% removal of Mn in a sulfuric acid medium [14]. Furlani et al. (2009) studied the use of carbohydrates, primarily lactose, as reducing agents for the leaching of manganese from the zinc alkaline battery powder [15]. The carbohydrates reduced Mn(IV) and Mn(III) oxides to acid-soluble Mn(II), and approximately twice the stoichiometric amount of lactose was used for complete leaching [15]. Gallegos et al. (2018) proposed a process using biogenerated sulfuric acid with 5 vol.% H2O2 or 1 wt.% Na2SO3 for the leaching of Zn and Mn in a single step [16]. In their study, 99% of Zn and 90–98% of Mn was extracted after 2 h of leaching at 30 ◦C and 0.04 g/mL [16]. Sodium metabisulfite (Na2S2O5) was also used as a reducing agent for the dissolution of metals from a mixture of spent batteries. In their research, Tanong et al. (2017) obtained 94 and 99% removal yields for Mn and Zn, respectively, by adding 0.45 g Na2S2O5/g to battery powder in H2SO4 1.34 M in a single leaching step, with an S/L ratio of 10.9% for 45 min at ambient temperature [6]. In other studies, a thermal pre-treatment was added to increase the efficacy of leaching. For example, Petranikova et al. (2018) investigated the effects of a thermal treatment at 300–950 ◦C of battery powder on the acid leaching (0.5 M H2SO4 at 25 ◦C for 60 min) [17]. In general, these studies indicate that sulfuric acid leaching allows for the complete dissolving of Zn and partial extraction of Mn (MnO, Mn2O3, and Mn3O4). The total dissolving of Mn (including MnO2) demands an auxiliary agent to reduce MnO2 to MnO, soluble in sulfuric acid. Therefore, the leaching process can dissolve Zn and Mn

simultaneously in a single step or via selective leaching in a two-step process. Selective leaching allows for the use of different techniques for recovery.

After leaching, the second challenge in the hydrometallurgical process is the efficient recovery of metals at high purities. The metals present in the leachate can be recovered by precipitation in the form of hydroxides, sulfides, or carbonates according to their respective pH and redox potential. Sobianowska-Turek et al. [18] used NH4HCO3 3 M and NH4OH 1 M to recover almost 100% of manganese, iron, cadmium, and chromium, as well as 98.0% of cobalt, 95.5% of zinc, and 85.0% of copper and nickel from the solution after reductive acidic leaching (H2SO4 + C2H2O4) [18]. Furthermore, Sayilgan et al. used KOH 2 M and NaOH 2 M for the selective precipitation of manganese and zinc, with complete precipitation obtained for Zn at pH 7–8 and Mn at pH 9–10 [19].

In industry, zinc is usually recovered in metallic form by electrowinning. In a study by Alfantazi and Dreisinger (2003), electrowinning experiments were conducted at an 80 min plating time, 500 A/m<sup>2</sup> current density, and temperature of 38 ◦C with zinc-containing electrolyte zinc and H2SO4 concentrations of 62 and 170 g/L, respectively [20]. In this study, 90% of zinc was recovered by using an Al cathode and Pb anode. Similar results were recorded by Ivanov (2004) with an electrolyte containing 45–55 g Zn/L, 5–6 g Mn/L, and traces of other metals including, among others, Ni, Sb, and Ge [21].

In our study, in addition to developing an efficient battery recycling process, relevant economic factors were also considered. Economic factors generally include processing and operating costs, as well as transport and residue disposal costs. For example, Gasper et al. (2013) evaluated the economic viability for recycling alkaline batteries using mechanical separation [22]. Although the mechanical process developed in their study was cheaper than other reported processes (\$US529/ton), it was still not economically feasible, due to the low end-product value. The revenue of the end products was \$US 383/ton of batteries that consisted of brass, Zn/ZnO powder, mixed Mn oxides powder, and KOH powder. In our study, we developed and described a complete hydrometallurgical process including the recovery of Zn by electrowinning and Mn by precipitation from alkaline battery powder. The process included dismantling, magnetic separation, leaching, and metal recovery. Furthermore, an economic evaluation was carried out to assess the feasibility on a global scale, especially for applications in Quebec (Canada), where an alkaline battery recycling process is not yet available.

### **2. Materials and Methods**

## *2.1. Process Description*

Figure 1 represents a flow sheet of the entire process for the recovery of Zn and Mn from spent alkaline batteries for this study. The globally relevant process includes two steps, namely, a physical pre-treatment step and a chemical treatment step. After dismantling, a representative sample of batteries is subjected to the attrition process followed by filtration and rinsing to remove any metallic powder attached to the coarse fraction. In addition to the separation of battery products, the removal of alkaline-soluble salts during this process reduces acid consumption in the following step. The coarse fraction is then transferred to the magnetic separator, allowing the magnetic fraction to be recycled as ferrous material and the non-magnetic fraction containing nylon, carton, and plastic to be used as energy sources.

**Figure 1.** Detailed flow sheet of the hydrometallurgical route to treat the spent alkaline batteries.

The fine fraction containing manganese and zinc described as "metallic powder" is transferred to the chemical treatment, where zinc is dissolved during the first leaching step using sulfuric acid (Equation (1)). A portion of manganese, in the form of MnO, Mn2O3, and Mn3O4, is also dissolved according to Equations (2) to (4) as follows:

$$\text{ZnO} + \text{H}\_2\text{SO}\_4 \to \text{ZnSO}\_4 \tag{1}$$

$$\text{MnO} + \text{H}\_2\text{SO}\_4 \rightarrow \text{MnSO}\_4 + \text{H}\_2\text{O} \tag{2}$$

$$\rm Mn\_2O\_3 + H\_2SO\_4 \to MnSO\_4 + MnO\_2 + H\_2O \tag{3}$$

$$\text{Mn}\_3\text{O}\_4 + 2\text{H}\_2\text{SO}\_4 \to 2\text{MnSO}\_4 + \text{MnO}\_2 + 2\text{H}\_2\text{O} \tag{4}$$

As MnO2 is insoluble in sulfuric acid, sodium metabisulfite (Na2S2O5) is added during the second leaching step to reduce Mn(IV) to Mn(II) [6]. During this leaching step, MnO2 in metallic powder is transferred in the leachate solution according to Equation (5):

$$\rm Na\_2S\_2O\_5 + 2MnO\_2 + H\_2SO\_4 \to 2MnSO\_4 + Na\_2SO\_4 + H\_2O \tag{5}$$

After precipitation for the removal of iron, and cementation for the removal of trace metals such as Ni and Cu, the first pregnant leach solution (PLS-1) is treated using sodium persulfate (Na2S2O8) to oxidize manganese for MnO2 recovery. Then, zinc is reduced by electrodeposition, resulting in a high-purity zinc metal. Furthermore, the second pregnant leach solution (PLS-2) is treated for the removal of iron and zinc followed by the precipitation of manganese as MnCO3. While the water used during this process can be reused through a counter-current mode, the final carbon-rich residue can be used for the fabrication of new batteries.

#### *2.2. Physical Pre-Treatment*

#### 2.2.1. Sampling and Pre-Treatment

The sample of spent alkaline Zn-MnO2 and Zn-C batteries was obtained from Laurentide Re-Sources Inc. (Victoriaville, QC, Canada). This company receives used batteries from various collection centers located throughout the Province of Quebec (Canada). During physical pre-treatment, spent alkaline batteries were shredded to approximately 1 cm × 4 cm fractions using a mechanical grinder (Muffin Monster, model 30005, JWC Environmental®, Santa Ana, CA, USA).

#### 2.2.2. Attrition Process

The neutral attrition consisted of three 20 min steps with a solid/liquid (S/L) ratio fixed at 30% (*w*/*w*) and 700 rotations per minute (rpm). The high rotation speed accelerates the separation of fine powder from other parts of the battery including carton pieces, ferrous scraps, and plastic. This step is also useful in removing soluble potassium hydroxide from the battery powder. Attrition experiments were carried out using a 40 L stainless tank reactor equipped with three internal baffles. For each experiment, 3 kg of shredded battery material was combined with 10 L tap water for 20 min at ambient temperature. After each step, S/L separation was carried out using 1.7 mm sieves. The remaining coarser fraction was then rewashed until three attrition stages were complete. The metallic powder was removed from the liquid phase after 2 h of settling. The water used for the washing process was recycled via counter-current mode, the result will show the media after three cycles.

#### *2.3. Chemical Treatment*

#### 2.3.1. Leaching Process

After washing, the metal powder was collected and different conditions were used to leach a maximum amount of zinc and manganese from the metallic powder. The challenge within this process is to obtain a high leaching efficiency and a PLS zinc concentration ≥40 g/L. This is important for avoiding energy loss during the electrowinning process. The selective leaching process was carried out in two stages in a 40 L stainless steel reactor. The water was recycled three times (LC 1 to 3) as presented in Figure 2. During the first cycle (LC 1), leaching was conducted with 4 kg of metallic powder in 2 M H2SO4 and an S/L ratio fixed at 40% (*w*/*v*) for 45 min at ambient temperature. For the second and third cycles (LC 2 and 3, respectively), leaching was conducted with 2 kg of metallic powder, and an S/L ratio of 20% in 1 M H2SO4, using the recycled solution after electrowinning (approximately 35–40 g Zn/L).

**Figure 2.** Schematic representation of the counter-current leaching and recovery process including rinsing steps performed on metallic powder.

During the second leaching step, 0.45 g Na2S2O5 per gram of metallic powder was added to reduce Mn(IV) to Mn(II), soluble in 1.34 M H2SO4, with an S/L ratio of 14% for 45 min at ambient temperature. The PLS-2 obtained from the second leaching step contained a significant proportion of manganese. Following each leaching step, S/L separation was carried out by filtration.

#### 2.3.2. Purification

Both PLS-1 and PLS-2 were purified prior to zinc and manganese recovery. The iron removal from PLS-1 was carried out at pH 4.0–4.5 by the addition of sodium hydroxide (NaOH) and hydrogen peroxide (H2O2). The added H2O2 dose was 1.5 times that of the stoichiometric molar ratio (SMR) of the total iron concentration (approximately 2 mM for PLS-1 and 30 mM for PLS-2).

After iron removal, the solution was purified by cementation using Zn powder to remove metal traces, such as Ni, Cu, and Co via the following equation:

$$\text{Zn}\_{\text{(s)}} + \text{M}^{2+}\text{ (aq)} \rightarrow \text{Zn}^{2+}\text{ (aq)} + \text{M}\_{\text{(s)}}\tag{6}$$

where M represents metal traces such as Ni, Cu, and Co.

In our study, a quantity of Zn metal powder equal to 20 times the impurity of metals (around 200 mg/L) was added, and cementation was conducted at 80 ◦C at pH 4.0–4.5 for 30–120 min.

Using the same principle, iron was also removed from PLS-2. Thereafter, the Zn residual (about 0.1 mol/L) in the PLS-2 was precipitated out using sodium sulfide (Na2S) at pH 4.0–5.0. The SMR values of 1, 2, and 3 were used during these experiments. The ZnS precipitate was then returned to the first leaching step and the PLS-2 was transferred to the MnCO3 recovery step.

#### 2.3.3. Metal Recovery from PLS-1
