Enhancement Of Leaching Of Cobalt And Lithium From Spent Lithium-ion Batteries By Mechanochemical Process

Recently,lithium ion battery（LIB）has attracted great interest because of its advantages such as light weight,long cycle life,high specific energy,wide working temperature and good environmental property.The market share of LIB has increased year by year since its birth.However,with the retirement of LIBs,the number of spent LIBs will continue to increase in the future.It will pose a serious threat to the ecosystem and human health if without recycled in time.Moreover,the spent LIBs contain a lot of high-grade valuable metals,which have high resource value.Therefore,the recycling of spent LIBs is of great significance on environmental protection and resources recycling.This study proposes a simple,efficient and environmentally friendly treatment method of spent LIBs.The method of mechanochemical grinding combined with hydrometallurgy was employed to enhance the leaching efficiency of Co and Li.In this way,two kinds of grinding aids with different properties were selected to have co-grinding with LiCoO₂.The mechanism of mechanochemical grinding on the surficial physical and chemical properties changes of LiCoO₂ was fully explored.The effects of grinding and leaching parameters on the leaching efficiency of metal were investigated,and meanwhile the leaching kinetics mechanism was further discussed.Finally,leach solutions are collected and reacted with oxalic acid to generate cobalt oxalate,which forms a complete recovery process.Firstly,the cathode LiCoO₂ material was ground with SiO₂ powder simultaneously.Crystal structure changes caused by mechanochemical were studied by characterizing the changes of surface morphology,crystal structure and element valence state of LiCoO₂ before and after mechanochemical grinding.Scanning electron microscopy combined with energy dispersive spectroscopy（SEM-EDS）,X-ray diffractometry（XRD）,X-ray photoelectron spectroscopy（XPS）and transmission electron microscopy（TEM）were accordingly used in this study.Results indicate that the additional grinding of LiCoO₂ can be carried out and the agglomeration of fine particles can be avoided due to the high hardness and dispersing effect of the additive SiO₂ in the grinding process,which makes the particles more fine in the grinding process and constantly exposes a large amount of new surface;The increase of grinding speed causes the increase of extrusion pressure and sliding friction force on the particles,which leads to the gradual transformation of crystal structure of LiCoO₂ to amorphous state;It is found that the crystal structure of LiCoO₂ was destroyed after mechanochemical grinding characterized by TEM.The atomic arrangement of Li（Co）appeared to be incomplete coordination and partial defects of lattice streaks.The elemental chemical state analysis of LiCoO₂ shows that carbon black plays a reducing role in the grinding process,which reduces Co³⁺to Co²⁺.Meanwhile,Co²⁺is found to be more easily leached in acid solution than Co³⁺.The results show that when ratio of SiO₂ and LiCoO₂ was 1:1,grinding speed was 500rpm and grinding time was 30 min,the leaching rates of Co and Li reached 94.91%and 97.22%,respectively.On the basis of fully mastering the mechanochemical grinding mechanism of SiO₂ and LiCoO₂,the effects of citric acid concentration,solid-liquid ratio,leaching time and temperature on the leaching efficiency of Co and Li were explored.The results show that the optimal leaching efficiencies of 94.91%Co and 97.22%Li were obtained under 1.25 mol/L citric acid concentration,20 g/L solid-liquid ratio,30minutes leaching time and 80°C temperature.The leaching kinetic analysis shows that the leaching reaction of Li and Co is controlled by diffusion and surface chemical reaction.The activation energy of Co and Li is determined to be 38.1745 kJ·mol^-1 and35.8765 kJ·mol^-1 respectively.According to the value of activation energy of metal leaching reaction,Li is easier to be leached than Co.The purity of cobalt oxalate obtained from the recovery of leaching solution and oxalic acid precipitation is higher.The cathode material LiCoO₂ was co-grinding with amino triacetic acid（NTA）combined with modern analytical methods such as SEM,XRD and FT-IR to study the chemical reactions caused by mechanochemistry methods.In order to clarify the effect of PVDF on the mechanchemical grinding,a reference group was designed to remove the PVDF by roasting,and the change of metal leaching rate after the material and NTA being ground together before and after calcination was compared.The experimental results show that the leaching efficiencies of Co and Li changed minimally after roasted,Therefore,the mechanochemical effect can overcome the influence of binder covered on metal leaching process,and produce the same effect as roasting for removal of PVDF,so this method can omit the roasting process.The mechanism study of mechanochemical reaction by surface morphology,phase and surface chemical bond changes of LiCoO₂ showed that the mixture of NTA and LiCoO₂ grinding caused the decreased particle size,irregular sheet shape and rough surface.The XRD spectrum obviously showed that the peaks of NTA and LiCoO₂became wider and the intensity was reduced,indicating that the crystal structure was destroyed,and the lattice defects were intensified.During the leaching process,LiCoO₂ reacts with NTA to form water-soluble Li-NTA and Co-NTA cyclic chelates.When the ratio of LiCoO₂ and NTA was 1:5,grinding speed was 400 rpm and grinding time was 60 min,the best leaching efficiencies of Co and Li in water walis92.26%and 94.12%,respectively.The precipitation diffraction peak of cobalt oxalate recovered from this process is sharp,with good crystal shape and high purity.This thesis concludes 38 pictures,8 tables,and 97 references.