Recovery Of Electrode Materials From Spent Lithium-ion Batteries By Grinding Flotation

Due to the excellent electrochemical characteristics,such as high energy density,high voltage and no memory effect,lithium-ion batteries（LIBs）have replaced nickel-metal hydride batteries and nickel cadmium batteries,and become a stable power source for convenient portable electronic devices,and even for electric vehicles.The annual output of lithium-ion batteries in China shows an increasing trend year by year.From the 2.687 billion annual production in 2010,it has rapidly increased to7.842 billion in 2016.However,lithium-ion batteries can usually only be charged and discharged for 1000 times.The limitation of the cycle life will result in the generation of a large number of waste batteries every year.There are a lot of heavy metal and organic electrolyte in waste lithium ion batteries,which will pose a serious threat to the ecological system and human health.On the other hand,the metal grade of spent LIBs is much higher than that of natural ores or even concentration ores.Therefore,both on the basis of resource preservation and environmental protection,the responsibly recycling of the vaulable components from spent LIBs is imperative and significant.In this paper,the electrode materials of spent LIBs,lithium cobalt oxide（LiCoO₂）and graphite,are selected as objects.Through the mutual confirmation of laboratory grinding flotation experiments and modern analytical technique characterization,the synergistic effects of grinding characteristics between LiCoO₂and graphite are systematically studied,and the dry surface modification mechanism in grinding process is revealed in detail.First of all,with the help of the modern analytical instruments of phase composition and element quantitative analysis,the quality percentage of LiCoO₂ and graphite in cobalt-riched crushed prodution are 66.75%and 33.25%,respectively.At the same time,the chemical composition of the characteristic elements from the Polyvinylidene Fluoride（PVDF）binder was analyzed by the analytical equipments of the surface elements and the functional group to determine the valence state of fluorine element on the surface of electrode particles.In combination with the distribution of fluorine element,it is clear that a layer of organic film adhered to the surface of the LiCoO₂ and graphite particles,resulting in their similar surface hydrophobicity.Thus,conventional flotation is difficult to effectively separate positive and negative electrode materials.Based on the experiments of single mineral grinding and mixed mineral grinding of LiCoO₂ and graphite,the variation of size composition and contact angle during grinding were analyzed.The stripping behavior during the grinding process of graphite particles was captured by scanning electron microscopy technology.The results show that the mixed mineral grinding between LiCoO₂ and graphite successively undergoes the peeling stage of the graphite lamellar structure,the abrasion state of the Li CoO₂ particles,and the bending fracture stage of graphite sheets.Short-time mixed mineral grinding can only cause cracking,sliding and stripping of the layer structure of graphite,but it will not break the graphite sheet.This means that moderate grinding pretreatment will exponentially produce new graphite surface,significantly improve the wettability differences between LiCoO₂and graphite,without excessive reduction of graphite particle size to guarantee the lower size limit.In order to explore the grinding effects on electrode particles from the microscopic level,the physical and chemical properties of LiCoO₂ and graphite particles before and after grinding were analyzed in detail.With the help of XPS,SEM,TEM and EPMA,the surface morphology,chemical state and element distribution of the particles were systematically obtained.Finally,the surface modification mechanism based on mechanical grinding was revealed.Under the horizontal shear force produced by the grinding media,the lamellar structures of graphite are sliding and flaking,exposing a large number of new hydrophobic surfaces.On the other hand,the organic film coating LiCoO₂ particles are partially worn down to restore the original hydrophilic surface.With the action of vertical rolling pressure,the adhesion of LiCoO₂ and graphite will occur and gradually become serious.Although the adhesion behavior in grinding process will lead some LiCoO₂ concentrate to follow graphite into the foam layer to reduce the flotation recovery rate,the great wettability difference results in an extremely good flotation concentrate grade.With the help of Design-Expert analysis software,the response surface design of flotation condition optimization was carried out.Under the condition of 5-min grinding,the optimal flotation results show that the grade of LiCoO₂ concentrate is the highest,reaching 96.11%,when the aeration volume is 80 m L/min,the dosage of collector and frother is 150 g/t,and the impeller speed is 2000 r/min.When the aeration volume is 80 mL/min,the dosage amount is 250 g/t,and the impeller speed is2500 r/min,the recovery rate of LiCoO₂ concentrate is the highest,reaching 73.36%.In addition,if the aeration volume is A,the dosage amount is B,and the impeller speed is C,the two regression equation based on the LiCoO₂ concentrate grade is shown.Y=-218.24325+4.10349A-2.64750×10^-3AB-0.015924A².And the two regression equation based on the recovery rate of LiCoO₂ concentrate is shown.Y=166.48923-1.58481A-0.23701B-0.036207C+2.61625×10^-3AB+5.32750×10^-4AC+3.21000×10^-5BC.