High-temperature Enhanced Selective Recovery Of Valuable Metals From Spent Lithium-ion Batteries

Lithium-ion batteries（LIBs）have recently gained popularity as energy storage devices due to their excellent electrochemical performance.The vast amount of the spent LIBs reaching their end of life calls for effective recycling processes to guarantee the sustainable supply of relevant strategic metals and mitigate environmental pollution.This thesis firstly comprehensively reviews the current status of pyrometallurgical options for recycling LIBs.Emphasis is placed on pyrometallurgy,considering it is a dominant and mature process.Lastly,the thesis provides experimental evidence to prove the effectiveness of carbothermic reduction in recycling LIBs.Carbothermic reduction was chosen because of its mild recycling conditions,selective Li recovery,the ability to recover ignoble metals,and the elimination of reductants during the subsequent leaching.However,the low Li₂CO₃solubility exacerbates evaporative crystallization costs.Thus this research aims to enhance selective Li extraction and subsequent recovery of heavy metals.In this work,carbothermic reduction was employed to selectively recover Li from the black mass of spent LIBs.The black mass comprised various cathode material chemistries to enable the development of a versatile recycling process.The influence of several factors,such as roasting temperature and time,grinding time,CO_2（g）flowrate,sonication,and water leaching temperature,on the leaching efficiency of metals was investigated.Secondly,the influence of hydrochlorination on the enhanced selective extraction of metals was also explored.The results are summarized as follows:（1）Systematic studies were conducted to determine the effect of roasting temperature（500-1000℃）and roasting time（15-120 minutes）on the recovery of target metals.The leaching results illustrate that effective control of the roasting temperature and time was imperative.Although higher roasting temperatures resulted in more reduction,it adversely affected the subsequent leaching efficiencies.Likewise,prolonged reduction had the same adverse effects.The optimum reduction condition was roasting at 600℃for 30 minutes.SEM-EDS and XRD results depict that the principal reduction products were Li₂CO₃,Ni,Co O,Co,and Mn O.This moderate roasting temperature resulted in lower energy requirements yet provided≥84.75%Li water recovery and>99%Ni,Mn,and Co acid recovery.（2）A vast content of residual graphite was noted after carbothermic reduction,and it is susceptible to absorbing the Li⁺leading to lower leaching efficiencies.Hence,ultrasonic-assisted water leaching was employed to cause desorption of the absorbed Li⁺.The Li water leaching efficiency reached 85.60%,employing ultrasonic-assisted water leaching.（3）Higher roasting temperatures resulted in larger particle sizes.Thus,intensive grinding was conducted to reduce the particle size,which increased the active spots of the leaching reaction.The Li water leaching efficiency reached 87.14%after the ground mixture underwent ultrasonic-assisted water leaching.（4）Bubbling CO_2（g）through the leaching solution was adopted to convert the less soluble Li₂CO₃（13.3 g/L at 20℃）into more soluble Li HCO₃（57.4 g/L at 20℃）.The Li recovery rose to 89.13%after adopting carbonated ultrasound-assisted water leaching to the ground calcine.（5）The influence of grinding time,leaching duration,leaching temperature,and CO_2（g）flow rate on selective Li recovery was investigated.The optimum conditions were:grinding time of 2.5 minutes,leaching time of 3 hours,leaching temperature of 50℃,and CO_2（g）flow rate of 100m L/minute.A water leaching recovery of up to 92.25%Li was attained under the optimized conditions.The as-obtained Li bearing solution was subjected to evaporative crystallization to attain high-purity Li₂CO₃（≥99.2%）.（6）The drawback of low Li₂CO₃ solubility was further catered for by hydrochlorination conversion of the Li compound to Li Cl（824.5 g/L at25℃）.Synergetic carbothermic reduction and hydrochlorination resulted in targeted directional conversion.The targeted products have distinctive water solubility and magnetism（water-soluble Li Cl and Mn Cl₂,water-insoluble Cu Cl,magnetic Ni-Co alloy,and non-magnetic Al₂O₃）,allowing maximum separation.The experiments were carried out at 1000℃,and the effect of hydrochlorination time and material bed thickness was studied.The optimum conditions are as follows:30 minutes of carbothermic reduction followed by another 30 minutes of hydrochlorination at 1000℃,using a 9 mm bed thickness.The recovery rates reached 97.28%Li,98.13%Mn,93.03%Ni,91.37%Co,and 95.28%Al.（7）This work also provided pathways for obtaining recycled graphite,given its increasing demand from various applications.The obtained recycled graphite had a similar resemblance to commercial graphite.