Xuan Zhang,Junqing Pan*,Yanzhi Sun,Yongjun Feng,Huixia Niu

State Key Laboratory of Chemical Resource Engineering,Beijing Engineering Center for Hierarchical Catalysts,Beijing Advanced Innovation Center for Soft Matter Science and Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Lead refining Perchloric acid Fluoride-free Electrorefining process

ABSTRACT The present paper reports a new fluoride-free and energy-saving lead electrolytic refining process in order to solve the serious problems of the existing Betts lead electrorefining process,such as low production efficiency,high energy consumption and fluorine pollution.In the process,a mixed solution of perchloric acid and lead perchlorate(HClO4-Pb(ClO4)2)with the additives of gelatin and sodium lignin sulfonate is employed as the new electrolyte.The cathodic polarization curves show that HClO4 is very stable,and there is no any reduction reaction of HClO4 during the electrolytic process.The redox reactions of lead ions in HClO4 solution are very reversible with an ultrahigh capacity efficiency,so the HClO4 acts as a stable support electrolyte with higher ionic conductivity than the traditional H2SiF6 electrolyte.The results of the scale-up experiments show that under the optimal conditions of 2.8 mol·L-1 HClO4,0.4 mol·L-1 Pb(ClO4)2 and electrolysis temperature of 45°C,the energy consumption is as low as 24.5 kW·h·(t Pb)-1,only about 20%of that by Betts method at the same current density of 20 mA·cm-2,and the purity of the refined lead is up to 99.9992%,much higher than that specified by Chinese national standard(99.994%,GB/T 469-2013)and European standard(99.99%,EN 12659-1999).

1.Introduction

As one of the non-ferrous metals with the largest output and highest consumption,lead is the main raw material of lead acid batteries[1,2].In 2015,about 82.9%global production of refined lead was consumed to produce lead acid batteries[3].Because of the rapid development of automobile industry, the production of lead acid batteries increases sharply.Until now,lead acid batteries have become the consumable secondary batteries of maximum quantity,leading to the rapid development of refined lead production[4,5].

Refined lead is usually obtained by electrolytic refining and pyrometallurgical refining of crude lead(95wt%-99 wt%).The latter refers to the production by pyrometallurgical smelting of lead paste and lead ore(e.g.,galena)[6-9].The purity of the refined lead produced by electrolytic refining method is much higher than that by pyrometallurgical refining method[10].Because of the poor solubility of lead in sulfuric acid and hydrochloric acid as well as other strong acids,the option of electrolyte is greatly limited for lead refining process.In 1901,Betts proposed the fluosilicic acid and lead fluosilicate(H2SiF6-PbSiF6)solution as the suitable electrolyte for lead refining method.In 1902-1903,this process was successfully applied in industrial production at Trail Lead Factory,Canada.Since then,Betts method[11,12]has become a classical electro-refining technology of crude lead in the past 110 years.Therefore,Betts method plays a prominent role in the existing refining techniques in Japan,Canada and China.

In recent years,lots of serious problems have been exposed in the traditional Betts electrorefining process, involving high energy consumption(＞120 kW·h·(t Pb)-1),low current density(20 mA·cm-2)[13],low space-time yield efficiency,severe volatilization of fluoride(HF and SiF4)[14,15]and emission of fluorine during the electrolytic process at high operating temperature of 50°C.The pollution is particularly severe in summer.To solve these problems,many researchers have carried out numerous studies on developing new electrorefining processes,including the electrorefining in chloride system[16,17],alkaline xylitol system[18],NaOH-PbO system[19-22],tartaric acid system[23]and citric acid system[24].Unfortunately,some of the reported electrolytes are hard to reuse and recycle,some are easy to carbonate when exposed in the air,leading to high over-potential and low electrolytic efficiency[25].So it is quite urgent and important to develop a new green acidic and fluorine-free lead electrorefining method.

Perchloric acid(HClO4)is a kind of super strong acid.According to the standard electrode potential in acidic solution,the reduction reaction of HClO4can be expressed as follows[26]:

Based on the traditional viewpoint,HClO4is usually defined as a kind of strong acid with strong oxidizability for its high standard potential and the application of HClO4solution as electrolyte was rarely reported in the past.HClO4and related chemicals are considered as a kind of dangerous and explosive chemical reagents by traditional view.In fact,a large number of studies have shown that HClO4solution,even concentrated HClO4solution is stable within a certain temperature range.Recently, it has been found that the strong oxidizability of ClO4-is proved to be very weak for its stable tetrahedron structure of Cl=O bonds under the condition of high concentration and high temperature(80°C)[27].It is also found that HClO4solution is stable and safe at a high concentration of 12.4 mol-1at 70°C[28].

Therefore,we propose a new lead electrorefining process,in which HClO4-Pb(ClO4)2with some electroplating additives are employed as the electrolyte,designated as“perchloric acid system”.The experimental results show that more than 63%electric energy has been saved in the new process.The current density of the new method can reach up to 50 mA·cm-2,which can greatly increase the space-time yield of Pb and shorten the production period. The refined lead with a purity of 99.9992%is obtained through high electrochemical reversibility and selectivity of HClO4. So it will be an excellent raw material of highperformance lead acid batteries[29,30].This process is also very useful for the enrichment and recycle of valuable metals from crude lead,such as copper,silver,tin and antimony.

The schematic diagrams of the lead electrolytic refining process are shown in Fig.1.As can be seen in Fig.1,during the electrolytic process,the lead atoms on the anodic surface lose electrons,becoming lead(II)ions and dissolving in the electrolyte.The metals with more positive electrode potential like arsenic and antimony remain in the anode to form the slime mud.The lead(II)ions close to cathodic surface obtain electrons,becoming metallic lead.The metals with more negative electrode potential remain in the electrolyte,thus high purity lead is obtained on the cathode. In order to obtain a smooth and dense electrodeposited lead layer, the electrolyte is continually circulated throughout the whole process to minimize concentration polarization.

2.Experimen tal

2.1.Preparation of electrolyte

4.51 g PbO (AR, 99%, Beijing Huaye Chemicals) was dissolved in 50 ml 3.6 mol·L-1HClO4(71 wt%,Tianjin Eastern Chemicals)to form the electrolyte containing HClO4(2.8 mol·L-1) and Pb(ClO4)2(0.4 mol·L-1).Electrolytes with different concentrations of HClO4and Pb(ClO4)2were also prepared in the same way.The dissolving reaction takes place in accordance with the following equation:

The traditional electrolyte of Betts method consisting of H2SiF6(1.3 mol·L-1) and PbSiF6(0.5 mol·L-1) was provided by Henan Haoda Lead Electrolyte Group.

2.2.Preparation of electrodes

The refined lead samples (Minshan Non-Ferrous Industry Group)with areas of 2×2 cm2and 4×5 cm2were washed by 10 ml 10 vol%HCl, and dried in a vacuum oven at 60 °C to be used as the cathode.The crude lead plates(Minshan Non-Ferrous Industry Group)with dimensions of 2×2×0.6 cm3and 18×26×1.6 cm3were polished with abrasive papers of 48 μm （grain size） followed by 13 μm （grain size),washed with 10 ml 10 wt%NaOH,and rinsed by deionized water until the rinsing water became neutral. Finally, the lead plates were dried in a vacuum oven at 60°C for 1 h to be used as anode.

Fig.1.Schematic diagrams of the lead electrolytic refining process.

2.3.The electrochemical tests

The potentiodynamic scanning tests were carried out by using a CS300 electrochemistry workstation(Wuhan Corrtest Instrument)in 1 mol·L-1HClO4,1 mol·L-1HNO3and 0.5 mol·L-1H2SiF6respectively at a scan rate of 1 mV·s-1. The temperature was changed from 30 to 70 °C. A refined lead plate with an area of 1 × 1 cm2was used as working electrode, a platinum plate with an area of 1 × 1 cm2as counter electrode and a Hg/Hg2Cl2electrode as reference electrode.The CV tests, Tafel tests and AC Impedance tests were carried out with a CHI760D electrochemistry workstation (Shanghai CH instrument) in HClO4solution. In these experiments, two refined lead plates with an area of 1 × 1 cm2were used as working electrode and counter electrode, respectively. The reference electrode was a Hg/Hg2Cl2electrode. The constant current charging test was carried out with a LAND CT2001A cell test instrument (Wuhan Land instrument).

2.4.Morphological and structural characterization

The X-ray powder diffraction(XRD)patterns of crude l ead and refnied lead were collected by means of a Rigaku D/max 2500VB2+/PCX diffractometerwith a Cu anticathode(40 kV,200 mA)at a scan rate of 10(°)·min-1in a scan range (2θ)from 10°to 90°. The morphologies and structures were examined by a feild emission scanning electron microscopy(FSEM,Hitachi S-4700).The energy-dispersive X-ray microanalysis(EDX,Oxford EDS Inca Energy Counter 300,operated at 10 kV)was conducted to analyze the element compositions of crude lead,refnied lead and lead anode slime.The contents of Pb and other elements in the samples of crude lead,refnied lead,lead anode slime and electrolyte were measured with an Inductively Coupled Plasma(ICP)analyzer(Agilent7700).

3.Results and Discussion

The cathodic polarization curves of Pt electrode in nitric acid(HNO3),H2SiF6and HClO4with 1 mol·L-1H+at 30°C are shown in Fig.2(a).It can be seen that no reduction peak appears for H2SiF6and HClO4in the scan range from-0.3 to 0.7 V.There is only one reduction peak at 0.1 V for HNO3, representing the reduction of NO3-. The cathodic polarization curves indicate that diluted HClO4,similar to diluted H2SiF6,only shows the oxidizability of H+at room temperature.Fig.2(b)shows the cathodic polarization curves for HClO4(1 mol·L-1)at different temperatures.As can be seen from Fig.2b,there is no reduction peak of ClO4-appearing between-0.3 and 0.7 V in the range from 25 to 65°C,and only a series of peaks exist at-0.73 V,indicating that ClO4-is stable except that the cathodic current of hydrogen evolution rises with increasing temperature.Fig.2(c)shows the cathodic polarization curves of the electrode in the HClO4of different concentrations at 55 °C. The results show that the curves keep falt and no characteristic reduction peak is observed in the scan range from -0.35 to 0.70 V when the concentration of HClO4changes from 1 to 5 mol·L-1.Fig.2(d)shows the cathodic polarization curves of the electrode in HClO4with lead (II) ion of different concentrations.Results indicate that there is no reduction peak appearing between-0.3 and 0.7 V for pure HClO4solution,only a reduction peak obviously appears at-0.76 V,which is attributed to the reduction reaction of Pb2+. It is also observed that the reduction current increases with increasing the concentration of lead ions. Due to the high overpotential of hydrogen evolution on metallic lead surface,the selfdischarge of lead(II)in HClO4solution hardly occurs.Therefore,HClO4solution is suggested here as the appropriate electrolyte for the new lead electrorefining process owing to its stable chemical properties and ultra-strong ion conductive ability that can reduce the polarization and increase electrochemical reaction rate,thus saving electric energy.

Fig.2.Cathodic polarization curves of Pt electrode in(a)nitric acid,fluosilicic acid and perchloric acid,(b)perchloric acid containing 1 mol·L-1 hydrogen ion at different temperatures,(c)perchloric acids of different concentrations at 55°C and(d)perchloric acids with different lead(II)ion contents.

Panel labels(a)and(b)in Fig.3 respectively show the electrochemical impedance spectra and Tafel curves of lead in H2SiF6and HClO4solutions with the same H+concentration of 1 mol·L-1and the same lead(II)ion concentration of 0.3 mol·L-1at 30°C.According to the Lange's Handbook of Chemistry,HClO4,as a typical super strong acid,has stronger acidity and higher ion conductivity than H2SiF6and other acids,which is beneficial to reducing polarization and solution resistance,thus decreasing cell voltage of electrolytic process.

As is seen in Fig.3(a),the solution resistance of the HClO4solution is 0.67 Ω while that of the H2SiF6solution is 1.24 Ω, indicating that the ionic conductivity of HClO4solution is much higher than that of H2SiF6solution at the same concentration of H+. The EIS show regular semi-circles in low frequency domain, and the radius of the semicircle in H2SiF6solution is almost two times of that in HClO4solution,indicating that the reaction process in the HClO4solution is faster than that in the H2SiF6solution. In order to better explain the effects of different acids on the electrodeposition process of lead, the Tafel experiments of Pb electrode in different acids of the same concentration were carried out. The exchange current densities extracted from the Tafel curves in Fig. 3(b) are 0.0592 A·cm-2and 0.0384 A·cm-2in HClO4solution and H2SiF6solution, respectively. It is meant that the HClO4solution is beneficial to the increase in exchange current and reaction rate of lead electrorefining process and decrease in polarization.

Fig.3.(a)Electrochemical impedance spectra(EIS)and(b)Tafel curves of the lead sample in fluosilicic acid and perchoric acid with same H+concentration of 1 mol·L-1 and same lead(II)ion concentration of 0.3 mol·L-1 at 30 °C; (c) constant current electrolysis curves of crude lead in 1.3 mol·L-1 fluosilicic acid plus 0.4 mol·L-1 lead (II) ion at 35 °C and in 2.4 mol·L-1perchloric acid plus 0.4 mol·L-1 lead (II)ion at 45°C for the electrode distances of 20 mm;(d)concentrations changes of fluosilicic acid solution and perchloric acid solution with the same initial H+concentration of 2 mol·L-1 at different temperatures for 3-day-electrolysis.

The constant current electrolysis curves of crude lead in H2SiF6and HClO4solutions are presented in Fig.3(c).The cell voltage of electrolytic process in HClO4solution keeps 32 mV at 20 mA·cm-2while that in H2SiF6solution is as high as 88 mV.It is calculated that the current efficiencies in the H2SiF6solution and in the HClO4solution are 97.8%and 99.5%,respectively.As a result,the energy consumption in H2SiF6solution is 23.81 kW·h per tonne Pb while that in HClO4solution is only 7.80 kW·h per tonne Pb.It is obvious that the energy consumption in HClO4solution is much lower than that in H2SiF6solution.That is also consistent with the results from EIS and Tafel tests.In order to reveal the volatilization characteristic of H2SiF6and HClO4solutions of the same concentration under different temperatures,titration tests were performed.It is observed that the evaporation speed of H2SiF6obviously increases with increasing temperature(Fig.3(d)),meanwhile the concentration decreases gradually when the electrolyte is above 30 °C,and almost 1/4 of H2SiF6losses when temperature increases to 60°C.Besides,it is known that H2SiF6easily decomposes at high temperature above 60°C in accordance with the following equation:

However,Titration results reveal that the concentration of HClO4is very stable and keeps between 1.95 and 1.98 mol·L-1,indicating that HClO4is hardly decomposed and volatilized in the temperature range from 30 to 70°C.It can be concluded that the HClO4is more suitable as the non-fluoride electrolyte for lead electrorefining process than the traditional H2SiF6when the electrolyte temperature is above 30°C.

Fig.4 presents the influences of the concentration of(a)HClO4and(b)lead(II)ion and(c)temperature on Tafel tests.The corresponding exchange current are calculated and presented in Fig.4(d).In the aspect of H+influence,the exchange current increases from 0.064 to 0.082 A as the concentration of H+increases from 1.2 to 2.8 mol·L-1,showing that the reversibility of electrode process is improved by increasing the concentration of HClO4.However,when the concentration of HClO4is over 2.8 mol·L-1, the exchange current of the lead electrode decreases slightly,indicating that exorbitant concentration of HClO4inhibits the electrodeposition of lead. Therefore, the optimal concentration of HClO4solutions will be 2.8 mol·L-1.It is also observed that the increase in Pb2+concentration in the range of 0.1-0.4 mol·L-1can improve exchange current.If the Pb2+concentration is above 0.4 mol·L-1,the increase in exchange current becomes much slower as the concentration increases. Considering the solubility of Pb2+in HClO4solution,0.4 mol·L-1of Pb2+should be the optimal concentration of the electrolyte for lead electrorefining.Fig.4(c)shows the influence of temperature on exchange current.The results indicate that the increase in temperature can promote ionic diffusion and exchange current, beneficial to the decrease in polarization and the electricity consumption.However,too high temperature causes extra high heating load.So,considering total energy consumption of heat and electricity,45-50°C will be chosen as the optical operation temperature range for lead electrorefining.

Panel labels(a)and(b)in Fig.5 are the galvanostatic electrolysis curves of anodic stripping and cathodic depositing of the Pb electrode at different current densities, respectively. As is seen from Fig. 5(a),the anodic potential increases with increasing anodic current density.When the current density is 10 mA·cm-2,the potential is 6.1 mV.However,when the current density reaches 50 mA·cm-2,the potential is only 20.3 mV. However, there appears a big jump of potential when the current density further increases from 50 to 60 mA·cm-2.The reason might be that too high current density limits the lead(II)ion diffusion from the surface of the anode to the electrolyte in the anodic process.The optimal anodic current density thus should be controlled between 10 to 50 mA·cm-2. A similar phenomenon of the cathodic process is also found in Fig.5(b).Compared with the data of the anodic process,the cathodic process shows much higher polarization.The lead cathode gives a potential of 38.5 mV,which is 17.2 mV higher than that of the anode at the same current density of 50 mA·cm-2.

Fig.4.Tafel curves of electro-refining lead in perchloric acid:(a)of different perchloric acid concentrations,(b)with different lead(II)ion concentrations,(c)at different temperatures;(d)the curves of exchange current corresponding to the above conditions.

Fig.5.(a)Anodic potential vs.time curves,(b)cathodic potential vs.time curves,(c)electrolytic cell voltage vs.time curves,and(d)potential of the anode and cathode and cell voltage vs.current density curves.The electrolyte was perchloric acid-lead perchlorate solution.

Panel labels(c)and(d)in Fig.5 display the cell voltages in the whole electrolysis course at different current densities.The increase in cell voltage is approximately directly proportional to the increase in current density when the current density is in the range of 10-50 mA·cm-2.A big jump of cell voltage is also observed in Fig.5(c)when the current density increases from 50 to 60 mA·cm-2.Considering the energy consumption and space time yield,the optimal current density may be controlled lower than 50 mA·cm-2.

Fig.6.(a)Constant current electrolysis curves,(b)photographs and(c)SEM images of the electrodeposited lead obtained in the electrolyte of 2.8 mol·L-1perchoric acid plus 0.4 mol·L-1 lead(II)ion at 50 mA·cm-2 and 45°C after 60 h electrolysis per cycle in the first 6 cycles.

Table 1 ICP analysis results of refined lead after first 6 cycles of the scaled up test

In order to evaluate the quality of deposited lead,the photographs and SEM images of the above cathodic electrodeposited lead at different current densities are presented in Figs.S1 and S2(Supporting Information), respectively. As is seen in Fig. S1, the samples formed in the range from 10 to 50 mA·cm-2show smooth and flat morphology.When the current density is over 50 mA·cm-2,the lead dendrites and non-uniform particles appear on the surface of the cathode plate.The photographs well conform to the SEM images shown in Fig. S2. Extremely high current density causes too fast electrodeposition process of lead particles on the cathodic surface,leading to the instantaneous impoverishment of lead(II)ion and formation of heterogeneous crystals.On the other hand,extremely high current density at high potential leads to the dissolution of impurities from the anode plate,leading to poorer quality of cathodically electrodeposited lead.In order to obtain the electrodeposited lead product with good morphology and quality,the current density should not be higher than 50 mA·cm-2during the electrolytic process.

It is observed that the additives such as bone glue and sodium lignin sulfonate are gradually adsorbed along with the lead deposition proceeding[23].So the additive supplement is needed during the cycling of electrolyte.The effects of cyclic process on the cell voltage and the morphology of the electrodeposited lead were studied.Fig.6(a)presents the cell voltage of the electrolytic process versus cycling number of electrolysis,and panel labels(b)and(c)in Fig.6 show the photographs and SEM images of the electrodeposited lead in the first 6 cycles,respectively.The results indicate that the cell voltages of the first 6 cycles all maintain around 80 mV,meaning that the cell voltages keep steady for the first 6 cycles.From Fig.6(b)and(c),we can also see that both the surface and the micro morphologies are almost consistently smooth and dense,showing the electrolysis process is very stable.In addition, if there are a few metallic impurities of the electrolyte slowly accumulated after the electrorefining processes,sodium lignin sulfonate was regularly added into the electrolyte to adsorb the impurities to form the flocculate of impurities,which can be removed by filtration process. As seen from Table 1, the purities of the refined lead obtained in the first 6cycles are all higher than 99.997%.

Fig.7.(a)Cell voltage of the electrolytic process vs.time,(b)photographs of the cell and(c)photographs of the original crude lead anode(185×260×15.8 mm3)and cathode(185×260×1 mm3,1390 g),residual lead anode and cathodic electrodeposited lead in 2.8 mol·L-1perchloric acid plus 0.4 mol·L-1 lead(II)ion at 20 mA·cm-2 and 45°C after 60 h electrolysis.(d)Comparison of the novel electrolytic process with Betts process:(d1)energy consumption,(d2)lead purity,(d3)F emission and(d4)current density.

The scale-up experiments of the electrolytic process were carried out in the laboratory.Panel labels(a)and(b)in Fig.7 show the cell voltage curves and the photographs of the installation, respectively.The photographs of the original anode crude lead (c1), residual anode(c2),cathode lead(c3)and cathodically electrodeposited lead(c4)are shown in Fig.7(c).As is seen in Fig.7(a),the cell voltage in the whole electrolytic process is very stable and keeps 94-95 mV,which benefits from the favorable conductivity of HClO4. Finally, the obtained net weight of the refined lead is 5570 g. According to Faraday law,the current efficiency is 99.7% and the consumed electrical energy is 24.5 kWh·tPb-1. Fig. 7(d) shows the comparison between the properties of the traditional Betts electrorefining process and our new electrorefining process ((d1) energy consumption, (d2) lead purity,(d3) Fluorine emission and (d4) current density). In Fig. 7(d1), it is clearly seen that the energy consumption in perchloric acid system only accounts for about 20%in Betts system at the same current density.Furthermore, the purity of the refined lead obtained by our method reaches up to 99.9992%, much higher than that by Betts method(99.994%),as seen in Fig.7(d2).It is obvious that there is no fluorine emission in our process while that in Betts electrorefining process reaches up to 4.2 kg·(t Pb)-1in Fig.7(d3).In Fig.7(d4),the current density of our method reaches up to 50 mA·cm-2while that of Betts method is only 20 mA·cm-2,indicating that the production efficiency by our new process is 2.5 times of that by Betts electrorefining process.

Fig.8.EDX spectra of(a1)crude lead,(b1)refined lead,(c1)anode slime;SEM images of(a2)crude lead,(b2)refined lead,(c2)anode slime for the electrolytic process in 2.8 mol·L-1 perchloric acid solution plus 0.4 mol·L-1 lead(II)ion at 50 mA·cm-2 and 45°C.

Table 2 Pb mass balance before and after electrolytic process in certain 3 batches

Fig.8 presents the EDX spectra of crude lead(a1),refined lead(b1)and anode slime(c1),and SEM images of crude lead(a2),refined lead(b2) and anode slime(c2). From Fig.8(a1)and(a2), we can see the anode crude lead with a rough and rugged surface contains 98 wt.%lead and other impurities,such as cadmium,copper,arsenic and antimony.Panel labels(b1)and(b2)in Fig.8 show that cathodically electrodeposited sample only contains metallic lead,presenting a smooth,dense and bright surface without irregular corners. Panel labels(c1)and(c2)in Fig.8 indicate that the main impurities in the anode slime are cadmium,copper,arsenic and antimony and the sample shows a series of spherical particle structure.During the electrolytic process,the impurities are removed from crude lead and into anode slime.Finally,not only the refined lead with good morphology and quality but also the anode slime containing noble metals can be obtained.

The ICP analysis results of the impurities of the original crude lead(a), the refined lead (b), the anodic slime (c) and the electrolyte(d)are shown in Table S1.It can be seen that the purity of the refined lead exceeds 99.9992%,which is higher than that specified by 1#standard metallic lead (GB/T 468-2005, Chinese national standard,99.994%)and PB990R grade Pb(EN 12659-1999,European standard,99.99%).The refined lead will be excellent raw materials for the manufacture of advanced lead-acid batteries and special lead alloys.

The recovery efficiency of Pb=the gain mass of cathode/(The loss mass of anode-the mass of anodic slime in electrolyte)×100%.

The concentration of Pb2+was titrated by EDTA method.Table 2 shows the Pb mass balance before and after electrolytic process in certain 3 batches.The volume of the solution is 100 ml.Calculated from Table 2, the recovery efficiencies of Pb of the above 3 batches are 99.57%,99.65%,99.53%,respectively.

4.Conclusions

A lead electrorefining process with a new fluoride-free electrolyte system, perchloric acid system, has been proposed in the present paper.It is proved that the HClO4solution is stable without the reduction reactions of the ClO4-ions in 1-5 mol·L-1HClO4below 65°C,so there is no any fluorine and other toxic gas emission during the electrorefining process.The energy consumption per tonne of lead in perchloric acid system is 24.5 kW·h·(t Pb)-1,which accounts for only about 20% in Betts system at the same current density of 20 mA·cm-2. Moreover, the current density is able to increase to 50 mA·cm-2,meaning the production efficiency can be 2.5 times as compared with the limitation of 20 mA·cm-2in Betts Process. That benefits from superior conductivity and dissolution ability of HClO4.The purity of the refined lead obtained by perchloric acid system can reaches up to 99.9992%, much higher than that specified by the Chinese national standard (99.994%, GB/T 469-2013) and European standard(99.99%,EN 12659-1999),and the recovery efficiencies of Pb reach up to 99.5%.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.09.005.