The latest research and data show that although the recycling rate of other materials such as lithium and cobalt will increase significantly in the next few years, the potential availability of "Second Life" batteries should not be underestimated.
By 2030, more than 1.2 million tons of discarded lithium-ion batteries will be recycled in the world.” By then, the amount of recyclable lithium that can be used in the global battery supply chain will be equivalent to half of the current lithium mine output, and by 2030 By the year, the amount of recyclable cobalt will be equivalent to about a quarter of the current amount. It is expected that China will dominate the lithium recycling work, and it may account for about 57% of lithium battery waste by 2030, so China may also launch " More stringent" recycling policy.
The recycling process of used lithium-ion batteries mainly includes pretreatment, secondary treatment and advanced treatment. Because there is still some electricity remaining in the waste battery, the pretreatment process includes deep discharge process, crushing, and physical sorting; the purpose of the secondary treatment is to achieve complete separation of the positive and negative active materials from the substrate. Heat treatment and organic solvent dissolution are commonly used. , Lye dissolution method and electrolysis method to achieve the complete separation of the two; advanced treatment mainly includes two processes of leaching and separation and purification to extract valuable metal materials. According to the classification of the extraction process, the recycling methods of batteries can be divided into three categories: dry recycling, wet recycling and biological recycling.
1. Dry recycling
Dry recovery refers to the direct recovery of materials or valuable metals without using media such as solutions. Among them, the important methods used are physical separation and high temperature pyrolysis.
(1) Physical sorting method
The physical separation method refers to the disassembly and separation of the battery, and the battery packs such as electrode active material, current collector and battery casing are broken, sieved, magnetic separation, fine pulverization and classification are carried out to obtain valuable high-content substances . Shin et al. proposed a method for recovering Li and Co from lithium-ion battery waste liquid using sulfuric acid and hydrogen peroxide, including two processes: physical separation of metal-containing particles and chemical leaching. Among them, the physical separation process includes crushing, screening, magnetic separation, fine crushing and classification. The experiment uses a group of rotating and fixed blade crushers for crushing, uses screens with different apertures to classify the crushed materials, and uses magnetic separation for further processing to prepare for the subsequent chemical leaching process.
Based on the grinding technology and water leaching process developed by Zhang et al., Lee et al., and Saeki et al., Shu et al. developed a new method for recovering cobalt and lithium from lithium-sulfur battery waste using mechanochemical methods. The method utilizes a planetary ball mill to jointly grind lithium cobalt oxide (LiCoO2) and polyvinyl chloride (PVC) in the air to form Co and lithium chloride (LiCl) in a mechanochemical manner. Subsequently, the ground product was dispersed in water to extract chloride. Grinding promotes mechanochemical reactions. As the grinding progresses, the extraction yields of Co and Li are both improved. Grinding for 30 minutes resulted in the recovery of more than 90% of Co and nearly 100% of lithium. At the same time, about 90% of the chlorine in the PVC sample has been converted into inorganic chloride.
The operation of the physical separation method is relatively simple, but it is not easy to completely separate the lithium-ion battery, and it is prone to mechanical entrainment loss during sieving and magnetic separation, and it is difficult to achieve complete separation and recovery of metals.
(2) High temperature pyrolysis method
The high-temperature pyrolysis method refers to the high-temperature pyrolysis of lithium-ion battery materials that have undergone physical crushing and other preliminary separation treatments, and the organic binder is removed, thereby separating the constituent materials of the lithium-ion battery. At the same time, the metal and its compounds in the lithium-ion battery can be oxidized, reduced and decomposed, volatilized in the form of steam, and then collected by methods such as condensation.
Lee et al. used a high-temperature pyrolysis method when preparing LiCoO2 from waste lithium-ion batteries. Lee et al. first heat-treated the LIB sample in a muffle furnace at 100-150°C for 1 hour. Second, the heat-treated battery is shredded to release the electrode material. The samples were disassembled by a high-speed crusher specially designed for this study, and classified according to size, ranging from 1 to 50 mm. Then, two-step heat treatment is performed in the furnace, the first heat treatment is performed at 100-500°C for 30 minutes, and the second heat treatment is performed at 300-500°C for 1 hour. The electrode material is released from the current collector through vibration screening. Next, by burning at a temperature of 500 to 900° C. for 0.5 to 2 hours, the carbon and the binder are burned off to obtain the cathode active material LiCoO2. Experimental data shows that carbon and binder are burned off at 800°C.
The high-temperature pyrolysis method has simple processing technology, convenient operation, fast reaction speed in high temperature environment, high efficiency, and can effectively remove the binder; and this method does not require high raw material components, and is more suitable for processing large amounts or more complex battery. However, this method has higher requirements for equipment; during the treatment process, the organic matter of the battery is decomposed and harmful gases will appear, which is not friendly to the environment. It is necessary to add purification and recovery equipment to absorb and purify harmful gases to prevent secondary pollution. Therefore, the processing cost of this method is relatively high.
2. Wet recycling
The wet recycling process is to crush and dissolve the waste batteries, and then use suitable chemical reagents to selectively separate the metal elements in the leaching solution to produce high-grade cobalt metal or lithium carbonate, etc., which are directly recycled. The wet recycling process is more suitable for recycling waste lithium-ion batteries with a relatively single chemical composition, and its equipment investment cost is low, and it is suitable for the recovery of small and medium-sized waste lithium-ion batteries. Therefore, this method is currently widely used.
(1) Alkali-acid leaching method
Since the positive electrode material of the lithium ion battery will not dissolve in the lye, and the base aluminum foil will dissolve in the lye, this method is often used to separate the aluminum foil. Zhang Yang et al. used alkaline leaching to remove aluminum in advance when recovering Co and Li in batteries, and then immersed in dilute acid solution to destroy the adhesion of organic matter and copper foil. However, the alkaline leaching method cannot completely remove PVDF, which has an adverse effect on the subsequent leaching.
Most of the positive electrode active material in lithium ion batteries can be dissolved in acid, so the pre-treated electrode material can be leached with acid solution to separate the active material from the current collector, and then combine the principle of neutralization reaction to target metal Carry out precipitation and purification, so as to achieve the purpose of recovering high-purity components.
The acid solution used in the acid leaching method includes traditional inorganic acids, including hydrochloric acid, sulfuric acid, and nitric acid. However, in the process of leaching with strong inorganic acids, there are often harmful gases such as chlorine (Cl2) and sulfur trioxide (SO3) that have an impact on the environment. Therefore, researchers try to use organic acids to dispose of used lithium-ion batteries, such as lemons. Acid, oxalic acid, malic acid, ascorbic acid, glycine, etc. Li and others use hydrochloric acid to dissolve and recover electrodes. Since the efficiency of the acid leaching process may be affected by the hydrogen ion (H+) concentration, temperature, reaction time and solid-liquid ratio (S/L), in order to optimize the operating conditions of the acid leaching process, an experiment was designed to explore the reaction time and H+ concentration And temperature. The experimental data shows that when the temperature is 80℃, the H+ concentration is 4mol/L, the reaction time is 2h, and the leaching efficiency is the highest. Among them, 97% Li and 99% Co in the electrode material are dissolved. Zhou Tao et al. used malic acid as the leaching agent and hydrogen peroxide as the reducing agent to reduce the leaching of the positive electrode active material obtained by the pretreatment, and studied the influence of different reaction conditions on the leaching rate of Li, Co, Ni, and Mn in the malic acid leaching solution to find out Find the best reaction conditions. The research data shows that when the temperature is 80℃, the malic acid concentration is 1.2mol/L, the liquid-liquid volume ratio is 1.5%, the solid-liquid ratio is 40g/L, and the reaction time is 30min, the efficiency of leaching with malic acid is the highest. Among them, Li, The leaching rates of Co, Ni and Mn reached 98.9%, 94.3%, 95.1% and 96.4%, respectively. However, compared with inorganic acids, the cost of leaching with organic acids is higher.
(2) Organic solvent extraction method
The organic solvent extraction method uses a similar principle of compatibility and uses a suitable organic solvent to physically dissolve the organic binder, thereby weakening the adhesion between the material and the foil and separating the two.
Contestabile et al. used N-methylpyrrolidone (NMP) to selectively separate the components in order to better recover the active material of the electrode when recycling lithium cobalt oxide batteries. NMP is a good solvent for PVDF (solubility is about 200g/kg), and its boiling point is relatively high, about 200°C. The study uses NMP to treat the active material at about 100°C for 1 hour, which effectively realizes the separation of the film and its carrier, and therefore, by simply filtering it out of the NMP (N-methylpyrrolidone) solution, the metal form of Cu is recovered. And Al. Another advantage of this method is that the recovered Cu and Al metals can be directly reused after being sufficiently cleaned. In addition, the recovered NMP can be recycled. Because of its high solubility in PVDF, it can be reused many times. Zhang et al. used trifluoroacetic acid (TFA) to separate the cathode material from the aluminum foil when recycling cathode waste for lithium-ion batteries. The waste lithium ion battery used in the experiment uses polytetrafluoroethylene (PTFE) as the organic binder, and the effect of TFA concentration, liquid-to-solid ratio (L/S), reaction temperature and time on the separation efficiency of cathode material and aluminum foil is systematically studied. . The experimental results show that in a TFA solution with a mass fraction of 15, the liquid-to-solid ratio is 8.0 mL/g, and the reaction temperature is 40°C, reacting for 180 minutes under appropriate stirring, the cathode material can be completely separated.
The experimental conditions of using organic solvent extraction to separate materials and foils are relatively mild, but organic solvents have certain toxicity and may be harmful to the health of operators. At the same time, due to the different manufacturing processes of different manufacturers of lithium-ion batteries, the choice of binder is different. Therefore, for different manufacturing processes, manufacturers should choose different organic solvents when recycling waste lithium-ion batteries. In addition, cost is also an important consideration for large-scale recycling operations at the industrial level. Therefore, it is very important to choose a solvent with a wide range of sources, a reasonable price, low toxicity, harmlessness, and wide applicability.
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