Research On The Pyrolysis And Flotation Recovery Process Of Waste Lithium Iron Phosphate Battery Electrode Material

Lithium-ion batteries are widely used in the field of new energy vehicles and energy storage due to their high energy density,long life,low self-discharge and high safety performance.China’s new energy automobile industry continues to grow rapidly,is facing a large number of lithium-ion battery retirement and no efficient,economic recovery technology and other problems.In this paper,the mixed electrode powder of positive and negative electrodes of spent lithium-iron phosphate battery was taken as the research object,and the organic binder on the surface of electrode material was removed by pyrolysis to achieve the efficient flotation separation of positive and negative electrode particles.Flotation concentrate is directly used to prepare cathode materials.Flotation tailings are leached by acid to recover valuable elements such as P,Fe and Li.The leaching residue further prepares high-purity graphite by sulfuric acid roasting-water leaching process,which greatly reduces the waste residue discharge and improves resource utilization.The main research contents of this paper are as follows:（1）The elemental composition,phase composition and occurrence state of each element of the mixed electrode powder of spent lithium-iron phosphate battery were identified by means of process mineralogy analysis,which provided theoretical guidance for the recycling of spent electrode powder by pyrolysis and flotation.The results show that the spent electrode powder contains 67.60 wt.%LiFePO₄,and the crystal structure of LiFePO₄ particles is perfect.However,the particle surface is covered by PVDF,and the F element content is 5.72 at.%,which makes the surface show strong hydrophobicity.It is difficult to separate LiFePO₄ and graphite directly.（2）Pyrolysis-enhanced flotation was used to separate LiFePO₄ and graphite in spent electrode powder,and the pyrolysis characteristics of spent electrode powder were explored to examine the flotation separation efficiency under different pyrolysis conditions.The pyrolysis characteristics of waste electrode powder show that the pyrolysis process is divided into four stages.The first stage is the volatilization of water and electrolyte.The second stage is the decomposition of organic binder PVDF.The mass of the third stage is almost unchanged.The fourth stage is the decomposition of LiFePO₄.The recovery of LiFePO₄ in the flotation process increases with the increase of pyrolysis temperature,and the recovery of LiFePO₄increases first and then decreases with the increase of pyrolysis time.Under the optimal pyrolysis conditions of 600°C and 30 min,the recovery grade of LiFePO₄in concentrate was 87.88%and the recovery rate was 95.17%.Flotation concentrate is directly used as cathode material to prepare new batteries,and the discharge capacity is 133.65 m Ah/g at 0.2C charge-discharge rate,which indicates that LiFePO₄ particles have the potential of direct recycling after pyrolysis flotation.（3）The effects of pyrolysis on the surface wettability,element occurrence,morphology and phase of the mixed electrode powder of spent lithium-iron phosphate battery were studied to reveal the mechanism of pyrolysis on the surface modification of electrode materials.The results showed that the hydrophilicity of the electrode material surface increased with the increase of pyrolysis temperature.With the increase of holding time,the hydrophilicity of the electrode material increased first and then decreased.At the pyrolysis temperature of 375°C,PVDF began to undergo macromolecular chain-（CH₂CF₂）-n fracture and de HF reaction,and the decomposition products of single molecular structure were obtained.At 500°C-（CH₂CF₂）-n was completely broken,and single molecule continued to decompose at 500–600°C,leaving pyrolysis carbon.This process shows that with the increase of pyrolysis temperature,the original hydrophilic area of LiFePO₄ particle surface continues to expand,but the holding time is too long,the pyrolysis carbon of LiFePO₄particle surface appears sintering phenomenon,which reduces the surface hydrophilicity.（4）After extracting valuable elements such as P,Fe and Li from flotation tailings in sulfuric acid system,the leaching residue was purified by sulfuric acid roasting-water leaching process to obtain high purity graphite.The results show that under the conditions of H₂SO₄ concentration 12.1 wt.%,liquid-solid ratio 4:1（g/g）,leaching temperature 40°C and leaching time 90 min,the leaching rate of Li,Fe and P is above 97%.After sulfuric acid roasting-water leaching of the leaching residue,the total contents of eight impurities Li,Fe,P,Cu,Al,Ni,Co and Mn were 998 ppmw,and the purity of graphite was above 99%.The sulfuric acid roasting-water leaching filtrate can be returned to the leaching flotation tailings process,further improving the recovery rate of Li,Fe and P,and reduce the discharge of acidic wastewater.