a new pre-desulphurization process of damped lead...
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Hydrometallurgy 160 (2016) 123–128
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Hydrometallurgy
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A new pre-desulphurization process of damped lead battery paste withsodium carbonate based on a “surface update” concept
Junfeng Zhang, Liang Yi, Liuchun Yang ⁎, Yan Huang, Wenfang Zhou, Wenjing BianDepartment of Environmental Science and Engineering, Xiangtan University, Xiangtan 411105, China
⁎ Corresponding author.E-mail address: [email protected] (L. Yang).
http://dx.doi.org/10.1016/j.hydromet.2015.12.0160304-386X/© 2016 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f oArticle history:Received 22 April 2015Received in revised form 17 December 2015Accepted 28 December 2015Available online 31 December 2015
Recycling of lead-acid batteries is an important sector of the lead-acid battery industry, and green technologieswith low energy consumption and pollutant emission are in urgent demand. A new pre-desulfurization processof damped lead battery paste sodium carbonate based on “surface update” was developed, and the optimumreaction conditionswere investigated. According to the experimental results, the process canmaintain the sulfurcontent of lead paste under 0.5% at the optimum reaction conditions: pH= 8–10 and a molar ratio of Na2CO3 toPbSO4 of 1.00–1.10:1. The desulfurization efficiency increaseswith the reaction temperature and can be qualifiedwithin 40 min in each experiment. The effect of “surface update” contributes to the enhancing effect in thedesulfurization process. The new process has a great advantage in desulfurization efficiency compared withthe traditional processes.
© 2016 Elsevier B.V. All rights reserved.
Keywords:Lead pastePre-desulfurizationSodium carbonateSurface update
1. Introduction
Lead is an important non-ferrousmetal that has beenwidely used inlead-acid batteries, pigments, chemicals, electronics and otherindustries. In 2013, the world's annual refined lead output reached upto 11.12 million tons, of which over 80% was consumed in manufactur-ing of lead-acid batteries. Recycling lead from spent lead acid batteries isimportant not only to sustainable development of the lead-acid batteryindustry but also to the reduction of lead-caused pollution to theenvironment.
Lead-acid batteries are composed of the followingmain parts: anodeplate (lead dioxide, PbO2), cathode plate (spongy lead, Pb) and electro-lyte (dilute sulfuric acid, H2SO4). After each discharging reaction, thelead sulfate exists in the battery in three forms: reversible lead sulfate,insoluble lead sulfate, and irreversible lead sulfate. During each cycle,the battery is charged and discharged, and 1% to 5% insoluble leadsulfate and irreversible lead sulfatewould be produced. This lead sulfatecould attach to the plate to form a layer of solid lead sulfate crystals. Thisphenomenon is called “the sulfation of the lead battery”, with the solidproduct usually referred to as “irreversible sulfide”. This vicious cyclewill increase with each charging and discharging reaction, and thelead sulfate crystals will continue to increase in number during thisprocess. This is the main reason that lead-acid batteries are not of anyuse after being in operation for a period of time.
According to the composition of lead paste in spent lead-acidbatteries after crushing (see Table 1) (Chen and Dutrizac, 1996; Zhuet al., 2013a; Ferracin et al., 2002), a large proportion of lead sulfate
exists that could potentially cause the SO2 release problem in traditionalpyrometallurgical processes for lead reclaiming.
Among several Pb recycling methods, the former pyrometallurgicalhas been abandoned because the decomposition of PbSO4 requireshigh temperatures and SO2 would be produced (Genaidy et al., 2008;Elmer, 1996; Kreusch et al., 2007).
The hydrometallurgical approaches for recycling metallic lead fromleadpaste have beenwidely researched in thepast 20 years. The processmainly comprises steps such as the following: (1) insoluble sulfur in thelead paste is converted to a soluble form by reacting with Na2CO3,NaOH, (NH4)2CO3, or K2CO3 (Yanakieva et al., 2000; Lyakov et al.,2007; Volpe et al., 2009), and PbO2 must be reduced to Pb2+ to aidthe subsequent leaching by using H2O2(aq), FeSO4, or Na2S2O3; (2) thedesulfurized paste after filtering is dissolved in strong leaching agents,such as HCl, fluoroboric acid (HBF4) or H2SiF6(aq) (Andrews et al.,2000; Warlimont and Olper, 1996; Prengaman, 1995); and (3) metalliclead is produced by electro-winning from the aqueous solution ofdissolved Pb2+.
Kumar et al. invented the patented technology of recycling lead-acidbatteries using a citric acid and sodium citrate solution. In this method,lead is recovered in the form of ultra-fine lead oxide powder aftercalcining the lead citrate precursor, and the lead oxide can be directlyused for making new batteries, thus circumventing the oxidation stepfrom metallic lead (Kumar et al., 2006; Sonmez and Kumar, 2008a,b;Li et al., 2013).
Although the SO2 release problem is solved in the hydrometallurgi-cal process, hazardous gases containing fluorine appear in the electro-winning process. Meanwhile, the electro-winning process is capitallyintensive and usually only suitable for large-scale operations. Theprocess using citric acid and sodium citrate provides a convenient and
Table 1Average composition of lead-acid battery paste.
Component PbSO4 PbO2 PbO Sb
Content 50–60% 30–35% 10–15% 0.2–0.7%
Fig. 3. Equipment of desulfurization reaction: (1)inlet, (2) mixing tank, (3) pump,(4) grinding reactor and (5) outlet.
124 J. Zhang et al. / Hydrometallurgy 160 (2016) 123–128
environment-friendly approach for recycling lead paste. However, citricacid is much more expensive than sodium carbonate, which is oftenused in the hydrometallurgical process, and the leaching rate with citricacid is relatively inexpensive, which usually takes more than 8 h toconvert lead sulfate into lead citrate.
Currently, pre-desulfurization-pyrometallurgy technology isbeing widely used in most countries. These methods combine theadvantages of hydrometallurgical and pyrometallurgical processes.Lead sulfate from the lead paste is first transformed into leadcarbonate by reacting with Na2CO3, (NH4)2CO3, or K2CO3, amongother species. Next, the insoluble sludge or filter cake from filtrationunit is charged into smelting furnaces. Theoretically, there is muchless or even no SO2 generated in the process, and decomposingPbCO3 only requires a lower temperature at 350 °C. Sodiumcarbonate has been widely applied in the industry as a desulfurizerdue to its low price and lack of secondary pollution. The sodiumsulfate produced during the reaction can be precipitated in theform of crystals and then separated for commercial use.
Although the process of lead paste smelting at 350 °C afterdesulfurization by Na2CO3 has some advantages, there is still SO2
generated in the practical applications because the desulfurizationefficiency is far inferior to the theoretical level. The reason for thisdifference lies in the fact that the traditional stirred reactors cannotsufficiently promote the reaction of sulfuric acid and sodiumcarbonate. In China, several advanced processes have beenintroduced but are out of operation due to the abovementioneddifficulties. Thus, a better reactor is urgently required fordesulfurization.
Fig. 1. Schematic diagram of lead paste desulfurization.
Fig. 2. Effect diagram o
Because lead-acid batteries lose effectiveness due to the effect of“irreversible sulfide”, it is recognized that lead sulfate will graduallycondense into small particles along with an increase in “irreversiblesulfide”. During the desulfurization process, the surface of PbSO4
particles will gradually be covered with a layer of much fewersoluble PbCO3 crystals, thereby impeding the reaction of desulfuriza-tion. The schematic diagram of this process is shown in Fig. 1.
To address this problem, we designed a new desulfurizationprocess of damped lead battery paste with sodium carbonate thatcan convert the vast majority of lead sulfate to lead carbonatebased on the traditional process and a “surface update” conception.Because the desulfurization of lead paste is a type of solid–liquidmultiphase leaching reaction, it will increase the difficulty ofdiffusion of the desulfurizer in the mass transfer process when leadpaste particles are coated by the product layer. “Surface update”refers to the timely removal of lead carbonate from the surface oflead sulfate particles by mechanical force, such as the collision ofsteel balls and hydraulic shear stress. The desulfurization efficiencycan be greatly promoted in this way. The effect diagram is shownin Fig. 2.
2. Experimental
2.1. Principle and materials
The reaction equations for PbSO4 and Na2CO3 are as follows:
mainreaction : PbSO4ðsÞ þ Na2CO3ðlÞ→PbCO3ðsÞ þ Na2SO4 ðlÞ
f “surface update”.
Fig. 4. Effect of grinding reactor's stirring rate on desulfurization efficiency.
Table 2Relationship between pH and the concentration of Na2CO3.
pH 7 8 9 10 11 12
COH- 10−7 10−6 10−5 10−4 10−3 10−2
CNa2CO3 2.3 × 10−8 2.3 × 10−6 2.3 × 10−4 2.3 × 10−2 2.3 2.3 × 102
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sidereactions : 3PbSO4 þ 2Na2CO3 þ 2H2O→Pb3ðCO3Þ2ðOHÞ2þ 2Na2SO4 þ H2SO4 ð2Þ
Pb3ðCO3Þ2ðOHÞ2 þ 2Na2CO3→3NaPb2ðCO3Þ2OH þ NaOH: ð3Þ
The following values are obtained for Gibbs' energy and the equilib-rium constant of reaction at 298 K and 343 K: G298 =−41.91 kJmol−1,Kc=2.213× 107 (T=298K) andKc=2.174× 106 (T=343K) (Lyakovet al., 2007).
Under the conditions that sufficient Na2CO3 is available and fullreagent contacts could be achieved, PbSO4 will be exhausted at thistemperature range, considering the high equilibrium constant values.
The solubility products of PbSO4 and PbCO3 also have a great differ-ence. At 298 K,Ksp(PbSO4)=1.6 × 10−8 andKsp(PbCO3)=7.5× 10−14.Therefore, it could be inferred that if the reaction includes a solutionphase, then PbCO3 shall precipitate in the solid phase, and CO3
2− ionswill replace SO4
2− ions because PbCO3 has lower solubility.The lead paste used in thework is provided by TianNeng Battery Co.,
Ltd., a company specializing in the production of lead-acid batteries andrecycling of lead resources in Zhejiang, China. Taking into account the
Fig. 5. Effect of particle initial size on desulfurization efficiency.
different sampling batches, the initial sulfur content (S%, wt.%) will bedetermined before each experiment.
2.2. Equipment and methods
The experiments for slurry desulfurization were performed in amixing tank (1500 mm in diameter and 2000 mm in length, seeFig. 3). The effect of “surface update” occurs primarily in the grindingreactor (300 mm i.d., 500 mm long), which contains a certain numberof steel balls (D = 10 mm). First, quantitative water and Na2CO3 wereadded into the mixing tank according to a certain molar ratio ofNa2CO3 to PbSO4 under stirring. Next, the grinding reactor and circula-tion pump were opened. When the slurry entered the tubular reactorfrom the pipe, the lead paste particles became smashed by grindingand collision. The temperature of the slurry was controlled by the hotsteam introduced into the tank.
In the experiment, the sulfur content of the lead paste wasdetermined by the precipitation method of BaSO4. The sulfur content(S%,wt.%) of slurry obtained from themixing tankwas analyzed to eval-uate the efficiency of desulfurization. When the sulfur content wasunder 0.5% (the initial sulfur content is approximately 4.8–5.6%), thedesulfurization efficiency was qualified. The less sulfur the producthas, the better the desulfurization efficiency becomes.
A certain amount of solid (M)was taken from the reacted paste first,and then lead sulfate in the solidwas dissolved by the chemical reagent.Under the action of barium chloride, the sulfur content (S%, wt.%) wascalculated using the following equation.
S% ¼nSO2−
4molð Þ � 32 g=molð Þð
M gð Þ � 100%
3. Results and discussions
3.1. Feasibility analysis of “surface update”
To verify the feasibility of “surface update” by the shear stress ofhydraulic or mechanical force from the steel balls, a series of testswere performed in themixing tank. The initial conditions are as follows:the slurry concentration was 60%, with 20 kg lead paste, the ratioof Na2CO3 to PbSO4 was 1.10:1, the stirring rate of mixing tank was60 r/min and the reaction temperature was 40 °C. Fig. 4 illustrates thechanges in the sulfur content at different stirring rates and the grindingeffects in the grinding reactor. Meanwhile, the effects of differentparticle sizes on changes in the sulfur content under the same experi-mental conditions are shown in Fig. 5.
It can be observed from Fig. 4 that the stirring rate can significantlyaffect the efficiency of desulfurization. When the stirring rate is 0(equivalent to the traditional desulfurization reactor), the desulfuriza-tion efficiency increased with the improvements of grinding rates.Because particle destruction might occur in the reactor with stronggrindings,which resulted in particle size decrease and surface breakage,it is reasonable to anticipate that both the change in the particle sizeand the surface modification could play an important role in the en-hancement of desulfurization. However, it can be seen from further ex-perimental results shown in Fig. 5 that the desulfurization efficienciesexhibit little difference with the change in the particle size of leadpaste. Thus, these results indicate that it is “surface update” by the
Fig. 6. Effect of the ratio of Na2CO3 to PbSO4 on desulfurization efficiency. Fig. 8. Effect of slurry concentration on the desulfurization efficiency.
126 J. Zhang et al. / Hydrometallurgy 160 (2016) 123–128
shear stress of the hydraulic or mechanical force from steel balls thatcontributes to the enhancing effect during the desulfurization process.
3.2. Effect of different dosages of Na2CO3 on desulfurization efficiency
The hydrolysis equation of CO32−is as follows:
CO2−3 þ H2O⇔HCO−
3 þ OH− Kb1 ¼ 4:3� 10−7
HCO−3 þ H2O⇔H2CO3 þ OH− Kb2 ¼ 5:6� 10−11
Kb1 ¼ ½ HCO−3
� �• OH−½ �
CO2−3
h i ¼ 4:3� 10−7 PH ¼ − lgc Hþð Þ ¼ 14−POH
¼ 14þ lgc OH−ð Þ:
The second step of hydrolysis is insignificant relative to thefirst step;thus, it can be neglected. The relationship between pH and the concen-tration of Na2CO3 is as follows (See Table 2):
The maximum solubility of sodium carbonate is 49 g/100 gH2O atthe temperature of 35.4 °C; as a result, the pH of solution cannot reach12. In addition, the concentration of sodium carbonate is found tohave a great influence on pH.
When the pH value is higher than 10.16 (i.e., the concentration ofsodium carbonate is greater than 52 mmol/L), part of lead carbonate
Fig. 7. Effect of temperature on desulfurization efficiency.
will be converted to hydrocerussite (Pb3(CO3)2(OH)2). When the pHreaches 10.23 or the concentration of Na2CO3 is greater than68 mmol/L, lead carbonate will be completely converted toPb3(CO3)2(OH)2 (Zhu et al., 2012, 2013b). In addition, these conditionswill increase the reagent consumption and create difficulties insubsequent processing or use of the final products (Dong et al., 2012).Thus, determining the amount of sodium carbonate is critical to thepre-desulfurization process.
Experiments have been performed to test the effect of the dosageof Na2CO3 on the desulfurization efficiency under the followingconditions: 20 kg of lead paste, 60% of slurry concentration,temperature at 50 °C, stirring rate of the mixing tank of 60 r/minand a grinding reactor stirring rate of 600 r/min. In addition, theratio of Na2CO3 to PbSO4 is between 0.90:1 and 1.20:1. Theexperimental results are shown in Fig. 6.
It can be seen from Fig. 6 that the sulfur content is lower thanothers when the ratio of Na2CO3 to PbSO4 is between 1.00:1 and1.10:1. According to the aforementioned analysis, the excessiveamount of Na2CO3 could cause the generation of hydrocerussite.The reason why the sulfur content increases when the ratio ofNa2CO3 to PbSO4 exceeds 1.10 is that the generation ofhydrocerussite may impact the determination of the sulfur content.Thus, we can determine that the optimum molar ratio range ofNa2CO3 and PbSO4 is 1.00–1.10:1. In addition, the pH value doesnot surpass 10 in this range.
Fig. 9. The change of sulfur content of the two reactors.
Fig. 10. XRD pattern of lead paste (a. the original paste; b. the desulfurized paste by new process; c. the desulfurized paste by traditional process).
127J. Zhang et al. / Hydrometallurgy 160 (2016) 123–128
3.3. Effect of temperature on desulfurization efficiency
Experiments were performed under the same operation conditionsto investigate the effect of temperature change on desulfurization effi-ciency. The ratio of the slurry concentration was kept at 60% with 20 kglead paste, the ratio of Na2CO3 to PbSO4 was set at 1.05:1, the stirringrate of themixing tankwas 60 r/min, and the stirring rate of the grindingreactor was 600 r/min. The experimental results are depicted in Fig. 7.
It can be seen from Fig. 7 that the desulfurization efficiency increaseswith temperature. For example, the sulfur content decreases from theinitial value of 5.6% to 0.60%, 0.25%, and 0.12% after 40 min of operationat temperatures of 40 °C, 60 °C, and 80 °C, respectively. It is wellknown that the higher the temperature is, the greater the energy con-sumption is. In practical production, the slurry temperature range is usu-ally 40 °C–70 °C. There is no requirement to adjust the temperature ofthe reaction substantially.
3.4. Effect of slurry concentration on the desulfurization efficiency
It is of great importance to study the effect of slurry concentration onthe desulfurization efficiency because the slurry concentration often variesinpractical applications. Therefore, a series of experimentswereperformedat different slurry concentrations in themixing tank, with the other condi-tions remaining constant. The molar ratio range of Na2CO3 and PbSO4 was1.05:1, the reaction temperature was 50 °C, and the stirring rates of themixing tank and the grinding reactorwere 60 r/min and600 r/min, respec-tively. The variation in the sulfur content is shown in Fig. 8.
Fig. 8 demonstrates that the sulfur content decreases dramaticallywith time, and this trend is much more distinct at lower slurry concen-trations. However, the desulfurization efficiency is excellent (all below
0.5% after 30–50 min of reaction) when the slurry concentration variesbetween 20% and 80%, which confirms that the process can adapt to themajority of industrial conditions.
3.5. Comparison with traditional process
The operation of a traditional process with commonly used stirringreactors was simulated by closing the tubular reactor shown in Fig. 3,and the desulfurization performance could be tested and thereforecompared under the same conditions: 20 kg of lead paste, a ratio ofslurry concentration of 60%, a temperature of 50 °C, and a stirring rateof the mixing tank of 60 r/min. Fig. 9 exhibits the change of sulfurcontent with time for the operation of the two types of processesunder the same reaction conditions.
It can be seen from Fig. 9 that the traditional process cannot obtain aqualified desulfurization rate to control the sulfur content of the leadpaste below 0.5% after 100 min of reaction. However, the new processequipped with an additional grinding reactor shows a very distinctadvantage over the traditional process. Within 20 min, the sulfur con-tent of the lead paste can be reduced to less than 0.5%. Fig. 10 showsthat PbCO3 and Pb3(CO3)2(OH)2 are identified in the desulfurizedpaste for the new process and NaPb2(CO3)2OH is identified in thedesulfurized paste for the traditional process. The results show thatNa2CO3 does not react completely with PbSO4 in the traditional processbecause the major components of desulfurized paste is NaPb2(CO3)2OHgenerated by Pb3(CO3)2(OH)2with the rest of Na2CO3. In addition, thereare only PbCO3 and Pb3(CO3)2(OH)2 in the desulfurized paste by thenew process. It can be predicted that, when applied in industrialpractice, this process shall significantly promote the development ofthe industry of lead paste recycling in developing countries.
128 J. Zhang et al. / Hydrometallurgy 160 (2016) 123–128
4. Conclusions
To resolve the low desulfurization effect existing in the waste leadpaste recycling process, which now is now frequently encountered indeveloping countries, a novel intensified pre-desulfurization processbased on a surface update concept has been proposed and tested. Thefollowing conclusions are made:
1) The process can significantly promote the conversion of PbSO4 toPbCO3 to maintain the sulfur content under 0.5% for a wide rangeof initial slurry concentration, i.e., 20%–80%.
2) The optimal reaction pH range is at 8–10, and the optimal molarratio of Na2CO3 to PbSO4 is in the range of 1.00:1 to 1.10:1.
3) The desulfurization efficiency increases with the reaction tempera-ture, and 50–60 °C is determined to be the optimum temperature.
4) It takes only 40 min of reaction for the new process to achievequalified desulfurization efficiency under the optimal conditions,exhibiting a very distinct advantage over the traditional process.
Acknowledgments
This paper is supported by the National Natural Science Foundationof China (Grant No. 51574202).
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