Liquid extraction of metals from sulphate solutions by alkylphosphoric acids. I. Equilibrium distributions of copper, cobalt and nickel with di‐(2‐ethyl hexyl) phosphoric acid

Liquid‐liquid equilibrium data were obtained at 25° for the distribution of copper, cobalt and nickel between aqueous sulphate solutions and kerosene solutions of di‐(2‐ethyl hexyl) phosphoric acid (EHPA). Two mechanisms of extraction were observed. When the ratio (L) of organic phase equilibrium metal concentration to initial EHPA concentration was less than 0·1, the metals were extracted by the reaction: equation image and a correlation for the equilibrium distribution coefficients was obtained from the thermodynamic equilibrium relationship for this reaction, taking into account metal ion complexing in the aqueous phase. Ammonium hydroxide and sodium hydroxide were used as alkalis for pH adjustment and similar distribution coefficients were obtained with both when L was less than 0·1. For L greater than 0·1, metal‐EHPA polymers were also formed in the organic phase, a typical step in their formation being: equation image Equilibrium relationships were obtained from consideration of the equilibrium constants of the typical step and the reaction for monomer formation. The nature of the alkali used for pH adjustment influenced the distribution coefficients when L exceeded 0·1, higher distribution coefficients being obtained with sodium hydroxide than with ammonium hydroxide.