*3.2. Lead Electrodeposition and CH3COOH Regeneration*

The electrochemical recovery of lead from the leach solutions involved as main reactions the simultaneous deposition of Pb at the cathode and CH3COOH regeneration at the anode. Considering the position of Pb in the electrochemical series of metals, its formation at the cathode is accompanied by the hydrogen evolution reaction.

Cathode:

$$\text{Pb}^{2+} + 2\text{e}^- \rightarrow \text{Pb} \tag{1}$$

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_2\tag{2}$$

Anode:

$$\text{H}\_2\text{O} \rightarrow 4\text{H}^+ + \text{O}\_2 + 4\text{e}^-\tag{3}$$

Chemical reaction:

$$\text{H}^+ + \text{CH}\_3\text{COO}^- \rightleftharpoons \text{CH}\_3\text{COOH} \tag{4}$$

According to the results, Figure 5, the extraction degree values increase with the increase in electrolyte flow rate and current density reaching the maximum value at 45 mL/min and 12 mA/cm2. Additionally, the results reveal the fact that the extraction degree is more strongly dependent on current density than electrolyte flow rate. It can be observed, Figure 5, that the extraction degree values almost double with the increase in current density by three times while for the same increase in flow rate at constant current density increases the extraction degree by only 32%.

**Figure 5.** Influence of electrolyte flow rate and current density on lead extraction degree.

As can be seen in Figure 6, in contrast to the evolution of extraction degree, the cathodic current efficiency decreases as the current density increases, while the increase in electrolyte flow rate has a positive impact. This can be explained by the fact that high current densities favor the hydrogen discharge reaction, which leads to a decrease in the cathodic current efficiency by 20–23% between the current densities of 4 and 12 mA/cm2. However, the experimental data show that the impact of the secondary cathodic reaction is even lower as the electrolyte flow rate is higher. In view of this tendency the maximum cathodic current efficiency (38.08%) was obtained at the highest flow rate (45 mL/min) and the lowest current density.

**Figure 6.** Evolution of current efficiency with electrolyte flow rate at different current densities.

Since the most important performance criterion in the performance of an electrochemical process is the specific energy consumption, this parameter was evaluated for both lead deposition and regeneration of the leaching agent. From Figure 7 it can be seen that the specific energy consumption for the cathodic process depends more strongly on the current density than on the electrolyte flow rate and varies in the opposite direction with the two operating parameters. Increasing the current density by three times increases the specific energy consumption of the electrodeposition process by 197–231%, while the same variation of the electrolyte flow reduces it by about 100%. The beneficial impact of flow rate increase can be attributed to the more intensive transportation of Pb2+ ions to the cathode surface which reduces the corresponding mass transport potential.

**Figure 7.** Specific energy consumption values for lead electrodeposition (Wc) and CH3COOH regeneration (Wa) at different electrolyte flow rates and current densities.

Similar conclusions can be reached related to the influence of the operating parameters on the evolution of the specific energy consumption for acetic acid regeneration. In contrast,

Figure 7 reveals that the specific energy consumption of the anodic process is almost twice as high as for the cathodic one. This is due to the lower molar mass of acetic acid than lead, which leads to the generation of a lower amount of CH3COOH by consuming the same amount of electricity.

The influence of the operating conditions on the performance of the lead electrodeposition process at the cathode and the CH3COOH regeneration at the anode is also quantified by the thermodynamic parameters, Table 2, of the electrochemical process. Electrode potentials confirm that lead deposition and oxygen discharge are the main electrochemical reactions, and their increase with increasing current density indicates the negative impact of the current density on the electrode over potentials. As can be seen from Table 2, increasing the current density also leads to higher ohmic drops, which leads to the increase in the cell voltage. In contrast, increasing flow rates reduce both electrode potentials and cell voltage, indifferent of the current density value, due to increased transport of electrochemically active species to the reaction surface.


**Table 2.** Thermodynamic parameters of the electrochemical process.

Eb—cell voltage; εc—cathode potential; εa—anode potential; i—current density.

Based on the above discussions, the optimal operating conditions were obtained at a flow rate of 45 mL/min and a current density of 4 mA/cm2, due to the fact that the specific energy consumption for both main electrochemical processes, Figure 7, attain the lowest values.

#### *3.3. Environmental Assessment of the Lead Recovery Process*

The environmental assessment was performed using the Biwer–Heinzle method [32,33] which is easily applicable in the early phases of process development and reveals the contribution of each input and output substance to the overall environmental impact of the lead recovery process.

In accordance with the Biwer–Heinzle method (Figure 8) the environmental factors were obtained from 6 impact groups which contained 14 impact categories. All of the components involved in the lead recovery process were allocated to a class *A*, *B* or *C* in each impact category (*A* = 1—highly toxic substances, *B* = 0.3—less toxic substances, *C* = 0—nontoxic substances) [32]. Next, the input and output environmental indices were determined by combining the obtained environmental factors with the mass indices resulting from the mass balance data corresponding with the processing of 10 kg/h CRT glass in the identified optimal conditions. Finally, the overall environmental impact of the lead recovery process was evaluated based on the General Effect Indices (GEIs) calculated by dividing the sum of environmental indices by the total mass indices [33].

**Figure 8.** Schematic representation of the Biwer–Heinzle method.

Among the input materials (Table 3), water has the lowest environmental impact considering that it was allocated to class *C* in all seven impact categories. In contrast, CH3COOH was assigned to class *B* for its acute toxicity, thermal risk and raw material availability associated with their production. The processed waste CRT glass was assigned to class *B* in impact category 5 and 6 because Pb can cause serious health issues. For the same reason, in the case of the output streams, Pb was assigned to class *B* in impact category 5 and 6 together. An output stream with similar environmental impact is the waste acetic acid solution which was also assigned to class *B* in impact categories 4, 5, 11 and 14 regarding its impact on air, soil and water pollution. Considering the global warming potential of CO2, it was assigned to class *B* in impact category 8. The other output streams, H2O and SiO2 have the lowest environmental impact, considering that they were allocated to class *C* in all 11 impact categories, being valuable secondary products of the developed process together with the resulting lead acetate, calcium acetate and magnesium acetate.

The *GEI*s values from Table 4 indicate that the environmental impact of the output streams is lower than for the input streams, which means that the developed process lowers the environmental impact of waste CRT glass through the recovery of lead.


**Table 3.** Input impact assessment.

**Table 4.** Output impact assessment.


Since the input and output streams have *GEI*s values close to the minimum possible (0), according to the Biwer–Heinzle method, it means that globally the process has low environmental impact. Nevertheless, caution and special protective measures must be applied when handling concentrated CH3COOH solutions.

## **4. Conclusions**

The results demonstrate that the developed combined chemical–electrochemical process can be efficiently applied for the recovery of Pb from waste CRT glass in the form of metallic Pb. It was found that the Pb dissolution process from the pre-treated waste CRT glass samples is most effective at a concentration of 0.6 M CH3COOH, a consideration which ensures equilibrium between the yield of dissolution process and leaching agent consumption. Based on the specific energy consumption, it can be concluded that the electrochemical process is carried out with the highest performance at a flow rate of 45 mL/min and a current density of 4 mA/cm2, leading to the formation of a high purity Pb deposit (99.98 wt.%) and CH3COOH regeneration. In the identified operating conditions, the amount of metallic lead recovered in one hour of processing represents ~10% of the lead present in the treated waste material the rest is in form of dissolved lead acetate.

The environmental impact assessment of the Pb recovery process was performed successfully in the early phases of process development by applying the Biwer–Heinzle method and the corresponding mass balance data for the treatment of 10 kg/h waste CRT glass. Based on the GEIs values obtained for the input and output streams, it can be stated as an overall conclusion that the novel approach for the recovery of lead from waste CRT glass proved to be a promising alternative with low environmental impact. Still, further studies are recommended in order to model, simulate and scale up the process for higher production and assess its economic performance.

**Author Contributions:** Conceptualization S.F., M.F. and Á.I.-L.; methodology S.F. and F.I.-L.; formal analysis M.F. and Á.I.-L.; investigation, F.I.-L., S.F., M.F. and Á.I.-L.; writing—original draft preparation, F.I.-L., S.F.; writing—review and editing, S.F., M.F. and Á.I.-L.; supervision, M.F., S.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Acknowledgments:** We are grateful for the administrative and financial support offered by the Babes-Bolyai University and University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca.

**Conflicts of Interest:** The authors declare no conflict of interest.
