# Effect of monothiophosphinic acid and phosphine sulfide impurities on the solvent extraction behavior of bis(2,4,4-trimethylpentyl)dithiophosphinic acid from sulfate and methanesulfonate media

## TL;DR

This study compares a synthesized dithiophosphinic acid extractant to a commercial one, finding it effective for metal extraction with improved stability.

## Contribution

The study evaluates a synthesized DTPhos mixture as a stable and selective alternative to Cyanex® 301 for metal extraction.

## Key findings

- Synthesized DTPhos mixtures extracted Ni(ii), Co(ii), and Zn(ii) efficiently without third-phase formation.
- DTPhos showed better Mn(ii) extraction at lower pH compared to Cyanex 301.
- DTPhos exhibited comparable cobalt extraction but reduced separation from nickel in less enriched formulations.

## Abstract

A crude, synthesized mixture of bis(2,4,4-trimethylpentyl)dithiophosphinic acid (DTPhos), was evaluated as a functional alternative to the discontinued Cyanex® 301 extractant. This study enabled the assessment of the influence of common impurities, such as bis(2,4,4-trimethylpentyl)monothiophosphinic acid (MTPhos) and phosphine sulfides, on metal extraction performance from methanesulfonate (MSA) and sulfate media. Both extractants exhibited nearly quantitative extraction of Ni(ii), Co(ii), and Zn(ii) under non-saponified conditions. In contrast to Cyanex 301, synthesized DTPhos-based mixtures did not give rise to third-phase formation at elevated pH values in methanesulfonate media. Additionally, Mn(ii) was extracted at lower pH values with the DTPhos mixtures than with Cyanex 301. Regression analysis of distribution ratios confirmed comparable cobalt(ii) extraction efficiencies and revealed a moderate reduction in separation from nickel(ii) for the least-enriched DTPhos formulation. Overall, the effect of the DTPhos impurity on the extraction properties was limited under the investigated conditions. Furthermore, variations in extraction mechanisms were suggested by pH-dependent behavior, particularly in sulfate-based systems. These findings support the use of synthesized DTPhos as a viable alternative to Cyanex 301, offering practical benefits such as stability against third-phase formation and flexibility in extractant composition.

In-depth study of the extraction behavior of commercial versus synthesized dithiophosphinic acid extractants reveals significant differences in Co(ii)/Ni(ii)/Mn(ii) selectivity and the physical stability of the system.