Separation of Cr³⁺ and Fe³⁺ in high sulfate systems using P507. Part I: A new extraction mechanism involving sulfate coordination

Solvent extraction is an effective method for addressing the difficult separation of Cr³⁺ and Fe³⁺ in sulfuric acid media. Bis(2-ethylhexyl)phosphate (P204) has been shown to extract Fe³⁺ from systems where Cr³⁺ and Fe³⁺ coexist. However, the issue of co-extraction of chromium remains. Considering that 2-ethylhexyl 2-ethylhexylphosphate (P507) and P204 are the same class of extractants, this study proposes the use of P507 to separate Cr³⁺ and Fe³⁺. Under optimal conditions, the primary extraction efficiency for Fe³⁺ reached 99.65%, with Cr³⁺ being co-extracted at only 5.93%, and Al being co-extracted by 8.66%. The co-extracted Al³⁺ was removed by a three-stage cross-current scrubbing process. After three-stage counter-current stripping, selective stripping of iron was achieved, leading to the effective separation of Cr³⁺ and Fe³⁺. To date, no studies have reported on the role of sulfate ions in the extraction of Fe³⁺ by P507. This study for the first time, reveals a new extraction mechanism in which sulfate ions (SO₄²⁻) play a role in the formation of complexes during the extraction process. It also provides a technological foundation for the further recovery of high-purity Cr³⁺ and offers valuable insights into the behavior of sulfate ions during the extraction process.