Degradation behavior of tributyl phosphate/trioctyl phosphate and effects of the degradation products on lithium extraction from high Mg/Li ratio brines

Neutral organic phosphate/FeCl 3 systems are widely employed for lithium extraction from high Mg/Li ratio brines; however, extractant degradation under prolonged operational conditions constrains process sustainability. This study investigates the degradation behavior of tributyl phosphate (TBP) and trioctyl phosphate (TOP) during lithium extraction, examining the influence of key process parameters (HCl/NaOH concentration, temperature, and FeCl 3 concentration) and evaluating the impact of degradation products on extraction performance. Single-factor experiments demonstrated that TOP exhibited superior stability compared to TBP, attributable to its longer carbon chains and greater steric hindrance. Both extractants displayed poorer acid resistance, whereas elevated FeCl 3 concentrations significantly suppressed degradation. On the basis of these findings, optimal process conditions are recommended: minimal HCl usage, NaOH concentration ≤ 4.0 mol·L −1 , and temperature ≤ 35 °C. Qualitative and quantitative 31 P NMR analyses confirmed that the primary degradation product of TBP was dibutyl phosphate (DBP), while that of TOP was di-(2-ethylhexyl) phosphoric acid (D2EHPA). Subsequent single-stage extraction experiments revealed that both DBP and D2EHPA diminished lithium extraction efficiency and Li/Mg separation factors, with D2EHPA exerting a more pronounced adverse effect. Across the concentration range of 0–0.5 mol·L −1 , D2EHPA reduced lithium extraction efficiency by 20%, compared to an 11% decrease caused by DBP. This study addresses a critical gap in understanding how extractant degradation affects long-term lithium extraction performance, and supplies feasible operational limits and formulation references for the industrial design and steady running of lithium separation processes.