Experimental Study On Thermal Decomposition Of Recycled Lead Carbonate From Waste Lead Acid Battery In Vacuum

Lead acid battery is possesed of advantages like low cost in manufacture,high safety in applications,excellent electrochemical features,as well as good recyclability.According to the status of development of lead mineral resource and recovery of secondary lead all over the world,about 80%of metallic lead is used to produce lead acid batteries,which are widely applied in the field of transportation,communications and medical industries.However nowadays,there is a large amount of used lead acid batteries,which are discarded after their service life.This has caused some severe problems such as environmental pollution and waste of secondary resource.In this context recycling of those discarded lead acid batteries is now a critical aspect and attracts more and more attentions.The direct recycling of metallic lead from discarded lead acid batteries is a process of high energy consumption and cost,and meanwhile may causes serious environmental pollutions.On the other hand,the oxidation degree is found to be low for the lead oxide powder prepared by oxidizing the metallic lead recoverd based on the direct recycling route.This yield poor electrochemical performance of the lead acid battery using this lead oxide powder with the oxidation degree.In this work,a preparation route of lead oxide by thermal decomposition of lead carbonate under vacuum condition was reported,which could proceed at a low temperature,but at a fast chemical reaction rate.At the same time,this route was found to be environmental friendly as well.Besides,it forms a fundmental theoretical basis for applications of green environmental recycling of lead acid battery with high efficiency.Firstly,thermodynamic analysis was carried out on preparation of lead oxide by thermal decomposition of recovered lead carbonate under vacuum condition.This study showed that when the system pressure was kept at 30Pa,PbCO₃·2PbO was generated in the temperature range of230³10°,whileα-PbO was formed between 310 and 450°C.At temperatures higher than 450°C,β-PbO was formed under the same system pressure.Namely,the preparation of lead oxide by thermal decomposition of lead carbonate under vacuum condition was deemed to be divided into two steps:i)PbCO₃→PbCO₃·2PbO and ii)PbCO₃·2PbO→α-PbO→β-PbO.Secondly,the mechanism of molecular heat and mass transfer in the thermal decomposition process of lead carbonate was studied.On one hand,the PbCO₃·2PbO and the PbO layers were found to show a poor heat transfer characteristic.These layers wrapped up the lead carbonate inside,which inhibited absorption of heat for the inner bulk,and acoordingly induce an incomplete thermal decomposition.On the other hand,the vacuum condition was assumed to promote the diffusion of CO₂ in the thermal decomposition process.Moreover,the temperature,the preservation time and the particle size were considered as key factors,which affected the thermal decomposition procedure.Therefore,a systematic experimental study on the thermal decomposition process of lead carbonate under vaccum condition was carried out to study effects of those factors.Furthermore,influence of the system pressure and the mass of raw material on the decomposition process were investigated as well.The results showed that when the system pressure was kept in the range of 10¹00Pa and the temperature was higher than 310°C,the thickness of the lead carbonate layer was reduced with increase of the temperature.Both the prolongation of the preservation time and the reduction of the system pressure were found to enhance the thermal decomposition of lead carbonate as well,yielding a decomposition rate more than 80%.Due to the crystal structural transformation ofα-PbO toβ-PbO at temperatures higher than 450°C,the thermal decomposition rate of lead carbonate could be further improved by a flashing thermal decomposition technology in nitrogen atmosphere.The thermal decomposition rate was proven to be higher than 90%under these conditions.Finally,the mechanism of the mutual transformation betweenα-PbO andβ-PbO was investigated.The transformation ofα-PbO toβ-PbO was correlated with a thermal volume expansion.The study on crystal structure ofα-PbO andβ-PbO bought about knowledge on relation of the bond length with the volume change.The conversion process fromβ-PbO toα-PbO was supposed to be dominated by shear force.