Application of a Sulfur Removal Hydrometallurgical Process in a Lead-Acid Battery Recycling Plant in Costa Rica

This study presents the implementation of a desulphurization process for lead recycling under different chemical and physical conditions using pyro-metallurgical processes. Desulphurization was done using a hydrometallurgical process using sodium carbonate as a desulphurization agent and different leadbearing loads compositions. Waste characterization included: SO2 concentrations in the stack emissions, total lead content in the furnace ash, the total lead content in the slag, and the toxicity characteristic leaching procedure (TCLP). A significant reduction in SO2 emissions was achieved (~55% reduction) where mean SO2 concentrations changed from 2193 ± 135 ppm to 1006 ± 62 ppm after the implementation of the modified processes. The desulfurized lead paste (i.e. the metallic fraction lead of the battery) of the modified process exhibited an improvement in the concentration of the lead in the TCLP test, with an average value of 1.5 ppm which is below US EPA limit of 5 ppm. The traditional process TCLP mean value for the TCLP was 54.2 ppm. The total lead content in the bag house ashes shows not significant variations, when comparing the desulphurization (67.6% m/m) and non-desulphurization process (64.9% m/m). The total lead mean content in the slag was higher in the desulphurization process (2.49% m/m) than the traditional process (1.91% m/m). Overall, the implementation of a new desulphurization method would potentially increase the operation costs in 10.3%. At the light of these results, a combination of hydrometallurgical and pyro-metallurgical processes in the recycling of lead-acid batteries can be used to reduce the environmental impact of these industries but would increase the operational costs of small lead recyclers. How to cite this paper: Navarro-Monge, M., Esquivel-Hernández, G., Brenes, J.P.S., Mora-Barrantes, J.C., Sánchez-Murillo, R., Valdés-González, J. and Bolaños-Ulloa, P. (2017) Application of a Sulfur Removal Hydrometallurgical Process in a Lead-Acid Battery Recycling Plant in Costa Rica. Open Journal of Air Pollution, 6, 18-26. https://doi.org/10.4236/ojap.2017.61002 Received: January 29, 2017 Accepted: March 14, 2017 Published: March 17, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Lead-acid batteries belong to a group of commercial products that can become a potential hazardous waste after their use.Lead recovery from lead-acid batteries is the type of business that combines environmental protection and economic profits.The largest component of the lead-acid battery is the so-called metallic fraction lead or lead paste (60% -75%) that is obtained during the processing of battery scrap.The other components are comprised of electrolytes and various types of plastics.Lead paste is primarily a mixture of PbSO 4 , PbO 2 , Pb 2 O, Pb 2 O 3 and metallic lead.PbSO 4 is the main lead carrier in the battery paste (40%).It is formed during the battery operation as a result of the reaction of metallic Pb and PbO 2 with the electrolyte (i.e.aqueous solution of sulphuric acid IV) [1] [2] [3].
Raw lead is obtained by melting the metallic fraction to achieve a reduction of the lead compounds (i.e.mainly PbSO 4 and PbO) [1] [4] [5].At the smelting step, named pyro-metallurgical process, the lead compounds are reduced to metallic lead by smelting the battery paste (i.e.lead oxides with a small amount of lead sulfate) using reducing agents rich in carbon, cast iron (Fe), and sodium carbonate (Na 2 CO 3 ) for the removal of other metals (e.g.Cu, Sn, As, Ag) in the oxide form.This reduction process leads to the formation of sulfides, named lead slag, as well as other gases and particulate matter emissions.The equipment used is a rotary furnace, short or long, to minimize the amount of waste, along with emissions control systems, such as bag houses for the filtration and recovery of the lead-rich particulate matter [6].
As a result of the battery paste melting process, a fraction of the sulfur is transferred into the slag in the form of sulfides, and the remaining large portion is oxidized to sulfur dioxide.Recently, national and international environmental agencies have updated the environmental legislation and have implemented new emissions limits that seek a reduction of the SO 2 emissions into the atmosphere.
For example, the Basel Convention has enforced secondary lead producers to take actions in order to decrease SO 2 emissions.One available method to prevent large SO 2 emissions relies on the desulphurization of the battery paste before melting [1] [2] [3] [4].The most common process in desulphurization of battery paste includes the application of caustic soda (NaOH) or sodium carbonate (Na 2 CO 3 ) solutions.[1] [5]- [10].
The sodium carbonate method is commonly used for desulphurization of battery paste when the sulfur content is less than 1%.This method converts the sulfur present in the paste into sodium sulphate using sodium carbonate as a desulphurization agent.Consequently, a desulphurized paste and soluble sodium sulphate is obtained.Equation (1) describes the chemical reaction in the desul-phurization step [1] [4] [5]: To achieve high efficiency in the desulphurization process, the reaction must be carried out using an excess of carbonate.Therefore, an excess of sodium carbonate is added and later neutralized according to Equation ( 2 ii) to make a waste characterization of the traditional process (i.e. the pyro-metallurgical process) and the hydrometallurgical modified process which includes total lead content in the sulphidic slag, the lead content in the bag house ashes, the toxicity characteristic leaching properties of the slag using the TCLP test, and the SO 2 concentrations in the stack emissions, iii) to estimate the change in operational costs after the implementation of the new hydrometallurgical process (Figure 1).

Materials and Methods
The experimental approach of this work was similar to that reported by Lewis and Beautement (2002) [11] that used a characterization of the waste to facilitate the overall process understanding and to aid the identification of process defi- Typically, the composition of each charge was at least 79% in lead bearing materials.The optimization of the process was carried out by varying the amount of these materials in order to achieve a more efficient reduction process in the furnace (i.e. a lesser amount of lead lost during the process).PB Metals operates under a batch production system, where every batch requires 4 hours to be completed.In this study, we had only one furnace available.Desulphurization of the lead paste was achieved by using a hydrometallurgical process and sodium carbonate as the desulphurization agent.The waste characterization of the traditional process (pyro-metallurgical only) and the hydrometallurgical modified process included the total lead content in the sulphidic slag, the lead content in the bag house ashes, the TCLP test, and SO 2 concentrations in the stackremissions.The analyses of the bag house ashes and the slag were done by acid digestion and atomic absorption (AA).In each case, 0.5 -1.0 g of the solid material was homogenized and acid digested in a microwave digestion system CEM MARS 5 (CEM, USA).An atomic absorption spectrophotometer ZEEnit 700P was used for the lead quantitative analysis (Analytical Jena, Germany).The bag house dust and the slag were collected after every batch was completed.The toxicity characteristic leaching procedure (TCLP) was applied to the slag according to United States Environmental Protection Agency (US-EPA) Method 1311 [12].
The stack gas analyses were done according to US-EPA stationary source emissions test methods [12].A TESTO 350 (Testo AG, Germany) flue gas analyzer was used for the analysis of SO 2 concentrations and flue gas concentrations, whereas an APEX XC-522 isokinetic sampler (Apex Instruments, USA) was used for the total suspended particulate matter (TSP) sampling.Stack gas and particulate matter sampling was carried out during the furnace operation and until every batch was completed.The lead content in the particulate matter (P-Pb) matter was extracted by acid digestion and analyzed by atomic absorption (AA) as described above.

Results and Discussion
Table 1 shows the results the SO 2 analysis in the stack emissions for each process, namely for the processes using desulfurized lead paste and non-desulfurized lead paste.Table 2 shows the total lead content in the bag house ashes.There was not a significant variation of the total lead content when comparing the 3 furnace charge compositions for both processes (with desulphurization and without desulphurization).However, the total lead content in the bag house were greater for those furnace charges that used desulfurized lead paste.The small increase of the lead content when using the desulfurized lead paste for the individual  charges could be related with the increase of the lead oxides content during the desulphurization stage.In general the lead concentration was stable (Δ% Pb < 5%) in every single charge.
For pyro-metallurgical processes, the Basel Convention has set a total lead content in the ash of approximately 65% m/m [13].In the case of the furnace charge composition [7.4]- [7.4]- [5.3] with and without the desulfurized paste, our results are very close to the Basel Convection limit of 65% m/m.The [8]- [8]- [5] furnace composition with the non-desulfurized paste presents the lowest lead concentration in the ash, with a valued of 55.2%.In the case of the formula [5]- [5]- [3], the concentrations are over the limit for both processes, with 74.1% (without desulphurization) and 76.2% (with desulphurization).It's important to point out that the small increase of the lead content when using the desulfurized paste could be related with the increase of the lead oxides during the desulphurization process [14].
Table 3 shows the total lead concentrations in the slag.In the non-desulfurized lead paste, the lead concentrations were 1.95%, 1.92%, and 1.87, for the formulas [5]-[5]- [3], [7.4]- [7.4]- [5.3], and [8]-[8]- [5], respectively.In the case of the desulfurized lead paste, the values between each single formula used were also very similar.In general, there is a small increase in the lead concentration when using the desulfurized paste.
Calvo et al. [14] concluded that a slag with high content of Fe 2 O and Fe 3 O (named as magnetite) increases the slag viscosity making the lead decantation less efficient, therefore the lead remains in the slag.According to the authors, one reason of this phenomenon could be associated with the furnace operation, related with a low reduction potential in the furnace, increasing the content of magnetite and hence the slag lead content.However, Calvo et al. [14] recommended values between 2% and 3% of lead concentration in the slag as a permissible mean concentration values.Based on this premise, PB Metals furnace operation has an acceptable performance.In the case of the desulfurized paste, the TCLP values obtained for the 3 furnace charges compositions were under the US-EPA limit of 5 ppm.The difference between both processes could be associated to the decrease of sulfate content ( 2 4 SO − ) during the desulphurization process as well as the decrease of FeS.
This situation allows the slag to tolerate more easily acid conditions.Once the FeS is solubilized, the lead retained in the slag will be released.Therefore when using desulfurized paste, a low concentration of FeS in the slag is obtained (comparing with the non-desulfurized paste), hence a greater lead concentration is retained in the slag, providing a lower lead lost, because there is a small quantity of lead in the cationic form to be lixiviated.
Finally, an economic evaluation of both processes was done in order to establish the feasibility of using the new hydrometallurgical process in PB Metals.
Using the furnace charge composition recovery efficiency of 70%.We estimated that the desulphurization process increased this efficiency in only 1% when the same formula was used meaning that the increment in the operational costs cannot be compensated directly from the increment in the lead production.Another issue is the increment in the processing time of the lead bearing materials.The desulphurization process takes about 5 hours for completion, which means that PB Metals would need to increase the existing lead inventories to be able to feed the furnace at the same rate they do now.Nevertheless, the hydrometallurgical process implementation could also provide environmental benefits for PB Metals, for example, a decrease in the treatment costs of the sulphidic slag, the largest contributor to waste material [11].

Conclusions
Based on the analysis of SO 2 of the stack emissions, the desulphurization process are according to the permissible limits (2500 mg/Nm 3 ).Nevertheless, for the case of the non-desulfurized lead paste, the values are very close to the limit for second and third furnace charge composition.The results of the total lead content in the baghouse ash indicate that there was not a significant variation of the total lead content when comparing the selected furnace charge compositions for both processes (with desulphurization and without desulphurization).However, the total lead content in the bag house was greater for each furnace charge with the desulfurized lead paste.For similar processes, the Basel Convention recommends a total lead content in the ash of approximately 65% (m/m).In the case of

Figure 1 .
Figure 1.Schematic diagram of the experimental process in PB Metal S.A.

SO 2
emissions were reduce 47%, 86% and 21% for the furnace charge compositions[5]-[5]-[3],[7.4]-[7.4]-[5.3]and[8]-[8]-[5], respectively, when using the desulfurized lead paste.The results indicate an acceptable efficiency for the implemented desulphurization process.The second furnace charge composition (i.e.[7.4]-[7.4]-[5.3])showed a major decrease in the SO 2 concentrations related with a better furnace efficiency.This composition was formerly identified by PB Metals as the most efficient composition to recover lead.For the[8]-[8]-[5] charge composition, the SO 2 emissions decrease was close to 20%, which could be explained by a deficient Na 2 CO 3 agitation process or because an excess of lead paste in the furnace charge.The excess of lead paste in the initial charge leads to a less PbCO 3 formation and an increase of the SO 2 content[14].According to Costa Rica's environmental regulations, the SO 2 concentration in the stack emissions from furnace charge composition[7.4]-[7.4]-[5.3]and[8]-[8]-[5] (under desulphurization conditions) are according to the permissible limits (maximum value 2500 mg/Nm 3 ); nevertheless, in the second and third furnace charge composition using the non-desulfurized lead paste the values are close to the limit (2500 mg/Nm 3 ).In the case of the charge composition[5]-[5]-[3], only when the desulfurized lead paste was used, the country regulatory standards were met.

Table 1 .
SO 2 analysis in the stack emissions for each evaluated process and furnace charge composition.

Table 4
presents the lead concentrations in the slag lixiviate (TCLP test).The

Table 3 .
Total lead content in the sulphidic slag expressed in % m/m.

Table 4 .
Lead concentration in the slag lixiviates based on the TCLP test.