With the booming development of electric vehicles and energy storage industries, cobalt (Co) and nickel (Ni) are key metals in lithium-ion battery cathode materials, and their recovery and purification technologies have attracted much attention. Due to the similar chemical properties of cobalt and nickel, especially the similar structures of the complexes formed in acidic solutions, the efficient separation of the two has become a technical difficulty in the field of hydrometallurgy.
At present, solvent extraction is the mainstream method for separating cobalt and nickel, among which Cyanex 272 (di(2,4,4-trimethylpentyl)phosphinic acid) and DEHPA (di(2-ethylhexyl)phosphoric acid) are the two most widely used organophosphorus extractants.
Cyanex 272: Highly selective cobalt and nickel separation expert
pH dependence and separation factor
When using 0.25 mol/L Cyanex 272 (with toluene or similar diluent) for extraction:
The semi-extraction pH of cobalt (pH₁/₂, Co) is about 5.3
The semi-extraction pH of nickel (pH₁/₂, Ni) is about 7.0
The difference between the two is about 1.7 pH units, which means that in the range of 5.3<pH<7.0, selective extraction of cobalt can be achieved while suppressing the co-extraction of nickel.
In practical applications, using 0.8 mol/L Cyanex 272 (toluene diluent, about 30% volume fraction), single-stage extraction at pH=5.1:
Process stability and application range
Cyanex 272 (CAS 83411-71-6) can still maintain high selectivity and stability when treating impure solutions such as high magnesium and high calcium. In addition, it shows good compatibility in different diluents and is suitable for a variety of solvent extraction processes.
However, Cyanex 272 is relatively expensive and has low extraction efficiency under low pH conditions, which may limit its promotion in some industrial applications.
DEHPA: a cost-effective broad-spectrum extractant
pH range and extraction efficiency
In the 0.8 mol/L DEHPA (solvent: kerosene) system, the semi-extraction pH values (pH₁/₂, i.e., the pH at which the extraction rate is 50%) of the three metals were measured as follows:
Mn(II)≈2.20
Co(II)≈3.30
Ni(II)≈3.83
Since the pH₁/₂ difference between Co(II) and Ni(II) is only 0.53 pH units, DEHPA has poor selectivity for cobalt and nickel separation, and it is difficult to achieve high-purity separation through single-stage extraction. However, at the same time, the pH₁/₂ difference between Co(II) and Mn(II) (≈1.10) allows Mn(II) to be preferentially extracted at pH≈2.7, while most of Co(II) is retained in the aqueous phase. This feature makes DEHPA have certain application value in the pre-separation stage of multi-metal coexistence systems containing Mn-Co, such as battery recycling.
Process simplicity and economy
DEHPA (CAS 298-07-7) has good selectivity and stability when treating solutions containing impurities such as iron, calcium, and magnesium, and is suitable for a variety of hydrometallurgical processes. It is relatively cheap and can achieve effective metal extraction under low pH conditions, reducing the use of alkalinity regulators and reducing process costs.
Application scenario comparison: choosing a suitable extractant
High-purity cobalt recovery
In application scenarios that require high-purity cobalt products (such as the production of battery-grade cobalt), Cyanex 272 has a higher separation factor and selectivity, and can achieve efficient separation of cobalt through a multi-stage extraction process, making it a more ideal choice.
Treatment of multi-metal coexistence systems
DEHPA is more suitable for occasions such as waste battery recovery liquid that require preliminary metal separation and recovery due to its good extraction performance and low cost under low pH conditions.
Conclusion: Choose the right extractant based on your needs
Cyanex 272 and DEHPA each have their own advantages, and should be weighed based on specific application requirements and process conditions:
For applications that require high-purity cobalt products, Cyanex 272 is a better choice due to its high selectivity and separation factor
In the treatment of multi-metal coexistence systems, especially cost-sensitive applications, DEHPA has more advantages due to its good extraction performance and economy