The deep treatment of spent NMC batteries is to dissolve and leach the electrode materials after pre-treatment processes, including discharge, dismantling, crushing and separation, so that the metals and compounds enter the leaching solvent in the form of ions, which are to be separated and recovered through leaching and extraction.
1. Leaching
Leaching is a key step in hydrometallurgical recycling. It mainly converts the metal elements in the pretreated cathodes into ions in the solution to facilitate the subsequent separation and recycling process. Commonly used acids include inorganic acids (HCl, H2SO4, HNO3, H3PO4, etc.), and organic acids (oxalic acid, citric acid, malic acid, etc.).
Among traditional inorganic acids, hydrochloric acid has the best leaching effect, but it is volatile and will generate Cl2 during the reaction. Nitric acid is not only volatile, but also highly oxidizing, easily generating toxic nitrogen oxides, and is more expensive than hydrochloric acid and sulfuric acid.
Sulfuric acid is cheap and easy to obtain, and has a high boiling point, which helps increase the leaching speed and dissolution rate. However, the leaching efficiency of sulfuric acid is relatively low, so the reducing agent H2O2 is often added to the sulfuric acid solution during actual operation, which requires a higher leaching temperature and liquid-to-solid ratio. However, due to the poor stability of H2O2 and its easy decomposition, there is currently a lot of research devoted to finding more efficient and stable alternatives.
In addition to inorganic acids, environmentally friendly organic acids are also the current focus.
Compared with inorganic acids, organic acids do not produce toxic gases, and the waste liquid is not strongly acidic, making them easier to handle and less corrosive to equipment. However, the prices of organic acids are higher with a slower leaching speed and higher liquid-to-solid ratio. That is, the organic acids are with poorer efficiency, which is not suitable for large-scale treatment.
If metal aluminium needs to be recovered first, a two-step method can also be used. That is to first dissolve and leach the aluminium metal with alkali solution, followed by acid leaching of other metals. The core of the acid leaching process is reaction temperature, time, acid concentration, solid-liquid ratio and reducing agent content, which directly affect the leaching rate of metal ions.
2. Extraction
After the active substance is leached from the cathodes, the key valuable metals such as cobalt, lithium, nickel, and manganese all exist in the leach solutiont in the form of ions and require further in-depth treatment for thorough separation, purification, and recovery. Mainstream processes include chemical precipitation, solvent extraction, electrochemical deposition, etc.
The chemical precipitation method is to add an appropriate precipitant to the leach solution to react with the metal ions to achieve a separation effect. The key lies in the different solubility of metal compounds at a certain pH value.
This method has high extraction rate, low cost and low requirements for equipment. However, the precipitation conditions of some metals are harsh. If the amount of precipitant and the pH value of the solution are not controlled well, multiple metals may precipitate at the same time, making it difficult to separate, resulting in unnecessary waste of resources.
The solvent extraction method is to use a specific extraction agent or a mixture of several extraction agents, to form a stable complex with the target metal ion, which is used to separate metal ions from the leach solution by the organic phase, and then back-extract the complex to achieve the separation and purification of the metal ions.
The advantages of the solvent extraction method include accurate extraction of target metal ions, low energy consumption, easy operation, and relatively high recovery rate and purity. However, the disadvantage is that it requires a large amount of chemical reagents with risks of polluting the environment. The solvent will also be lost during the extraction process and the cost is high.
Usually, mixed extraction agents have better synergistic effects and the extraction effect is better than that of a single extraction agent.
Written by Aggie Hu, huchenying@mysteel.com
