A low-cost green approach for synthesis of lead oxide from waste lead ash for use in new lead-acid batteries☆

Wei Zhang ,Gang Tang ,Xiaoqin Xiang ,Renyu Wang ,Shuangquan Gao ,Xinfeng Zhu ,Qiting Zuo

1 School of Water Conservancy &Environment,Zhengzhou University,100 Kexue Avenue,Zhengzhou 450001,China

2 Henan University of Urban Construction,Pingdingshan 467000,China

ABSTRACT This paper proposes a novel method relating to the recycling of waste lead ash originated from procedure of lead alloy production.The spent lead ash was first disposed by acetic acid leaching system,where lead ash structure wrapping impurities would be destroyed.The synthesis of lead oxide products was conducted at a lower temperature of 90°C.The effect of molar ratio of CH3 COOH to lead content of the ash on leaching efficiency was studied through the acetic acid leaching system.The results demonstrate that 84.6% of lead could be obtained in the leaching solution,while merely 0.7% of Fe blend in solution within a leaching time of 120 min.In the stage of lead oxide synthesis from leaching solution,the yield of lead oxide products could reach up to 94.4% when the molar ratio of NaOH to lead in filtrate was 2.5.This novel green method could shed light on the reuse of lead from exhausted ash with a much more convenient and environmentally friendly procedure.

Keywords:Waste lead ash Leaching Acetic acid Solution synthesis Lead Oxide products

1.Introduction

Recycling of waste electrical and electronic equipment (WEEE)[1]including spent lead acid battery [2-4],spent lithium battery[5,6],CRT glasses [7],etc.has already been research hotspots and trends for recent years [8].Exhausted lead ash was a sort of secondary lead resources derived from lead smelting procedure,which was enriched with lead (Pb),and other disparate metals,such as iron,zinc,cadmium,aluminum,etc.The annual production of spent lead ash was numerous for nearly 50000 tons[9,10],constituting an important raw material for lead recycling industry.The spent lead ash was mainly PM10 and PM2.5 particles with complex composition,while the leady containing components are chiefly Pb,PbO,PbS,PbSO4or oxysulfates like PbO·PbSO4and(PbO)2·PbSO4[9,10].

The pyrometallurgical approach for secondary lead resources treatment was mostly applied in industry.However,the emission of lead containing particles and harmful atmospheric contaminants such as SO2could not be avoided for the smelting method.Thusly,some hydrometallurgical methods were investigated as an alternative approach for secondary lead resources treatment.The composition of spent lead ash was also varying depending on procedure of lead-acid battery manufacture.In the conventional recycle route,the spent lead ash was commonly mixed with other plumbiferous raw materials,then accompanied by a refining procedure leading to the enrichment of impurity metals in the smelting furnace [11].The issue of lead containing particles emission during the smelting procedure should be considered.Due to the environmental disadvantages in refining method,the adoption of novel hydrometallurgical approaches for lead ash treatment has also been investigated to avoid free lead emission.In the hydrometallurgical process,the lead ash was treated by some leaching reagents,where the lead containing compounds would be converted into lead oxide products [12].This method mainly comprises the following steps:(1)The lead ash was firstly roasted with concentrated sulfuric acid at 500°C at a volume/mass ratio of 0.6 ml·g-1for 120 min,followed by stirring at 60°C for 120 min,while the lead-containing components were mainly converted into lead sulfate remaining in the residue;(2) leaching of the residue was conducted by sodium chloride solution at 90°C for 90 min,followed by crystallization of PbCl2from the leaching solution after cooling at room temperature;(3) the obtained PbCl2crystal was reacted with sodium hydroxide solution at solid/liquid (S/L) ratio of 1:30 at reaction temperature of 90°C for 60 min,while the crystal of Pb(OH)2was obtained and calcined at the temperature of 504°C.The results were indicating that the finally obtained lead oxide products exhibit interconnected porous structure.However,the overall process including multiple reaction steps would inevitably result in loss of leaching reagent and metal ion such as Pb.Another suggested approach for treatment of lead ash by hydrometallurgical method involves a leaching and electrowinning procedure [13].The leaching of the lead ash consisting of PbSO4and Pb2OSO4was conducted by the leaching reagents of NaOH and HNO3,whose leaching efficiency of lead could reach up to 43%and 67%,respectively.Eventually,lead and lead dioxide would deposit on the substrate.Obviously,electrowinning-based technology shows us a much elevated energy consumption rate.

In previous studies,the recovery of lead ash mainly focusses on recycling of lead,the removal of impurities such as iron,zinc metal or its oxides was seldom studied.Related research was indicating that lead-containing part of secondary lead resources would cover the impurity components[14,15].It would be beneficial to separate the lead from the impurities if lead components were converted into lead ion and shifting into solution,while the impurity components were remained in the wrapped structure.Thusly,the chemical issue to be addressed for the recovery of lead ash was the demolition of wrapped lead ash structure,where the impurities such as iron,zinc metal or its oxides could be left in the residue,as illustrated in Fig.1.In this study,acetic acid was introduced into the leaching system,where lead ash structure would be demolished,while the effect of pH on leaching efficiency was investigated.

The schematic of the entire study was illustrated in Fig.2.As shown,two research issues were investigated in this study.Firstly,lead ash originated from lead alloy production procedure was reacted in acetic acid leaching system,where lead-containing components were converted into lead ion.The impurities would be adsorbed in the residual channel structure,which would lead to the preliminary separation of lead ion from impurities.Secondly,the lead oxide products would be crystallized from alkaline solution environment through control of pH and potential.

2.Experimental

2.1.Reactants and raw materials

Fig.1.Proposed model of lead ash structure wrapping impurity.

Fig.2.Proposed schematic of the entire study.

Fig.3.The appearance (a) and XRD pattern (b) of lead ash samples;(c) proposed model of lead containing component wrapping impurities.

Table 1 Concentration of the principal metal elements in lead ash

The lead ash applied in the whole experiments was collected from lead alloy production procedure and supplied by a metallurgical factory in Hubei province.Then the lead ash samples were dried and collected for further treatment.The appearance (a) and XRD pattern (b) of the lead ash samples were displayed in Fig.3,where the lead ash powder is observed to have a consistent particle size.The XRD pattern was indicating that PbO and Pb are the prime phases with solely a handful of FeS crystal.

The essential composition of the lead ash samples was shown in Table 1,in which lead and iron elements are the major constituents(59.81 wt%and 0.24 wt%,respectively),while the concentrations of Cd,Zn and Al are relatively lower.The concentrations of metal elements for the lead ash samples were digested by aqua regia and measured by ICP-OES 5100 (Keysight Technologies).

In this investigation,the used reagents such as acetic acid,sodium hydroxide and hydrogen peroxide solution (AR grade)were all purchased without further treatment from Sinopharm Chemical Reagent Co.,Ltd.

2.2.Leaching of lead ash samples

With a stirring speed of 400 r·min-1,we use acetic acid solutions with adjusted concentration to leach 50 g lead ash samples in a 500 ml plastic beaker immersed in a temperature-controlled water bath at a temperature of 25°C.Then hydrogen peroxide solution of 0.1 mol was added into the leaching beaker to oxidize Pb to Pb2+transferring into leaching solution [16].The detailed experimental conditions of leaching procedure were displayed in Table 2.

From Table 2,the molar ratios of acetic acid(CH3COOH)to lead in lead ash samples were set as 1.5,2,2.5 and 3,thusly the pH is modulated to varied pHs of 2.46,2.39,2.27 and 2.17,correspondingly.The quantity of CH3COOH added in this experiment was settled to guarantee an identical solid/liquid (S/L) ratio.After the reaction starts,approximately 5 ml of leaching solution is withdrawn from the leaching system at leaching time of 0,10,20,30,60,90 and 120 min,respectively,followed with quick filtration by the membrane filter (0.45 μm pore size).The concentrations of lead and impurities in the withdrawn leaching solution samples were all determined by ICP-OES5100 (Keysight Technologies).

After the leaching step,the residual leaching solution would be filtered,while the filter cake would be rinsed and dried at 65°C.The filter cake would be analyzed by the powder X-ray diffraction(XRD) technique by using an X’Pert PRO X-ray diffractometer.

Table 2 Design of the lead ash samples in weak acid leaching experiments

The leaching efficiencies of lead and main impurities were calculated by using Eq.(1).

In the Eq.(1),v2stands for the volume of leaching solution (L);c2stands for the concentration of lead or main impurity ion in the leaching solution(g·L-1);w stands for the mass of lead ash sample(g);c1stands for the percentage of lead or main impurities in the lead ash sample (%).

2.3.Solution synthesis of lead oxide at lower temperature

For the synthesis of lead oxide procedure,an adjusted mass of NaOH reagent was added into the leach solution having maintained at 90°C under a temperature-controlled water bath for 30 min.The molar ratio of added NaOH to Pb in the filter liquor was set at 2,2.25,2.5 and 2.75,respectively.At terminal of reaction,the solutions containing lead oxide products were filtered and the solid lead oxide products were washed and dried at 65°C.The yield of lead (%) versus molar ratio of NaOH/Pb is computed by employing following Eq.(2).

where m represents the mass of lead oxide in the synthetic product(g),v stands for the volume of filtrate for the synthesis procedure(L),and c stands for the concentration of lead in the filtrate (g·L-1).

The lead oxide products obtained through synthesis procedure were also analyzed by powder X-ray diffraction (XRD) technique by using X’Pert PRO X-ray diffractometer.

2.4.Thermodynamic analysis of leaching procedure

The Potential-pH phase diagram and Fraction phase diagram were both conducted by the MEDUSA software [17,18].This program would use an algorithm,where all the conceivable equilibria between components are considered and the Gibb’s free energy is minimized [19].

3.Results and Discussion

3.1.Leaching of lead ash samples

The leaching efficiency of lead along with leaching time for various molar ratios were reflected in Fig.4,where it could be perceived that lead extraction ratios rise as the molar ratio of CH3COOH/Pb increases from 1.5 to 3.0.When the molar ratio is 1.5,the leaching efficiency is merely 51.9% after 120 min;when molar ratio ranges from 1.5 to 3,the leaching efficiency of lead could reach up to 84.6% at molar ratio of 2.5 and 3.0.Under all molar ratios conditions,the leaching efficiency of lead was practically kept constant after merely 20 min,indicating that most part of lead components in lead ash samples were converted within previous 20 min.

The Fraction phase diagram and Potential-pH phase diagram lead in leaching system were exhibited in Fig.5.As we can see,the pH and potential of leaching system would affect the prevailing species distribution of lead compound.The Pb2+was predominant in the pH region of less than 2 and potential greater than 0.In the range between 2.1 and 3.7,the Pb(CH3COO)+is becoming dominant phase at leaching solution potential above～-0.1 V.With the increasing of pH,in the pH region surpassing 4,the solid phase of Pb(CH3COO)2becomes dominant at solution potential above-0.2 V,indicating that the mass of lead ion transferring into liquid phase is becoming less.The dominant phase of the leaching solution was also affected by the potential,for the pH region of less than 2,metallic Pb was predominant in the potential region of less than-0.1 V,which was also indicating that higher potential condition was beneficial to the existence of lead ion.

3.2.Leaching behavior of the principal impurities

The leaching efficiencies of principal impurities with time variation are revealed in Fig.6.The behaviors,with respect to the growing dosage of acetic acid adding into the leaching system,resemble that of observed lead compounds.Compared with other impurities,Fe is more stable and absorbed in the residual structure,which only extracts less than 0.7% of original amount within the 120-minute leaching,which could also be interpreted that Fe2+in the leaching solution would be oxidized to Fe3+and is no longer soluble.In contrast,nearly 72% of Zn could be extracted within 120 min.For various molar ratios of acetic acid to lead in the lead ash,the leaching ratio of Zn was nearly kept constant.The leaching efficiencies of Cd and Al could reach up to 40% and 65%,correspondingly within leaching time of 120 min.At molar ratio of 1.5,the leaching efficiencies of Cd and Al was lower,compared with increasing molar ratio of more than 2.0.Given that Fe would reduce the electrochemical performance of eventually obtained lead oxide products,the limited amount of Fe in the leaching solution would be conducive to the quality of synthesis products.

The potential-pH phase diagrams for main impurities in the leaching system were manifested in Fig.7,in the range pH of less than 3.3,Fe2+is predominant in leaching system at solution potential ranging from-0.6 V to 0.5 V.The Fe(CH3COO)+and Fe2O3were predominant phase with increasing pH above 3.3.This implies that the Fe compound was more easily to remain in the residual phase with increasing pH condition.The diagrams in connection with metal of Zn,Cd and Al illustrate that the three metals are soluble species in the pH region of less than 3.2,2.8 and 2.3,respectively.The thermodynamic diagrams reveal that Zn,Cd and Al are readily reacted in the acid medium.

3.3.Synthesis of lead oxide products

Fig.8 demonstrates the yield of lead (%) versus molar ratio of NaOH/Pb for synthesis procedure.From the Fig.8 (I-IV),it could be induced that the color of synthesized lead oxide products was golden yellow,there was little difference among the synthesized products for various molar ratio of NaOH/Pb.It could be seen that the lead yield (%) was increasing when the molar ratio of added NaOH to lead in the solution is enhanced from 2 to 2.5.This result is logical,because part of the NaOH is neutralized by the excess acetic acid applied in the leaching procedure.

Fig.6.Leaching efficiencies of main impurities in the leaching system:(a) Fe(II);(b) Zn(II);(c) Cd(II);(d) Al(III).Conditions:0.14 mol·L-1 Pb(II),0.36 mol·L-1 CH3-COO-0.14 mol·L-1 Pb(II),pH of 2.27 at 25°C.

Fig.7.Potential-pH phase diagram for main metal elements in the presence of acetic acid in leaching system:(a) Fe(II);(b) Zn(II);(c) Cd(II);(d) Al(III).Conditions:0.14 mol·L-1 Pb(II),0.36 mol·L-1 CH3 COO- at 25°C.Designed with MEDUSA software.

Fig.8.(a)The Yield of lead(%)versus molar ratio of NaOH/Pb for the synthesis procedure at temperature of 90°C for 30 min;(b)the proposed model of lead oxide products dissolution in solution for higher dosage of NaOH;(I-IV) the appearances of the products for the synthesis procedure for molar ratio of 2,2.25,2.5 and 2.75,respectively.

Fig.9.XRD patterns of the synthesized products with disparate NaOH/Pb molar ratios.Other conditions:temperature controlled at 90°C for 30 min.

In view of this,minimum molar ratio should be 2.5,where the yield of lead (%) could reach up to 94.4%.The reaction could be indicated as Eq.(3):

Nevertheless,the yield of lead tends to fall slightly with the rise of NaOH dosage as shown in Fig.8(a).This behavior may be interpreted that the redissolution of a small portion of lead oxide at higher concentrations of NaOH as show in Fig.8(b) [20].And it could be expressed by the following Eq.(4).The appearances of the synthesized products present color of golden yellow and is more accordant when the molar ratio of NaOH/Pb is 2.5,compared with other molar ratios.

Fig.9 presents XRD patterns of the synthesized products with disparate NaOH/Pb molar ratios.The lead products gained at a molar ratio of 2 are primarily composed of PbO.With higher dosages of NaOH,Pb3O2(OH)2is observed in the lead oxide products,indicating that the alkaline aqueous environment for synthesis procedure might not be beneficial to the purity of acquired products.

4.Conclusions

Lead oxide products could be obtained via acetic acid leaching of exhausted lead ash,followed with synthesis procedure at a lower temperature of 90°C.Within a leaching period of 120 min,about 84.6%of lead could be recovered while only 0.7%of Fe enters into solution.Compared with other impurities,the separation of iron was more effective through this method.Thermodynamic analysis results demonstrate to us the dominant species of lead and the principal impurities(Fe2+,Zn2+,Cd2+&Al3+)under leaching conditions.When the molar ratio of NaOH to lead in filtrate was 2.5 during precipitation procedure,up to 94.4% yield of lead oxide could be obtained.

Acknowledgement

The authors express gratitude to the Modern Analysis and Computing Center of Zhengzhou University for various materials analysis offered in this study.