Removal of iron, aluminum and magnesium ions from the wet-process phosphoric acid solutions via Donnan dialysis

Removing iron, aluminum, and magnesium ions from the primary wet-process phosphoric acid solution (WPA) is a promising strategy for mitigating scaling in downstream processing. Here, Donnan dialysis using Nafion membranes, with HNO₃ as the stripping solution, was proposed and evaluated. The species in WPA as a function of temperature were modeled. The influence of HNO₃ molarity (1–5 M), temperature (25–65 °C), and Nafion membrane type on the metal ion mass transport was systematically studied. The effect of H₃PO₄ molarity on the metal ion mass transport was analyzed with synthetic WPA solutions, considering the form (free cation or complexed with anions) in which these metal ions transport through the membrane by analyzing the ionic composition of membranes in equilibrium with WPA. The coupled cation diffusion coefficients in membranes are modeled via the Nernst - Planck flux equation. It is found that the cation flux always follows this order: J_Mg > J_Al > J_Fe for WPA. These factors can be attributed to: 1) in WPA Mg appears mostly as Mg²⁺, while almost all Fe ions and around half the Al ions are in a paired state, giving the bulky Fe(H₂PO₄)²⁺, Al(SO₄)⁺ and Al(SO₄)₂^-, respectively; 2) only solution viscosity influences the Mg²⁺ transport, while both the solution viscosity and the complexation with anions influence Fe and Al transport; 3) Mg²⁺ ions have the largest diffusion coefficient in the membrane phase. These metal ions transport in the form of free cations through the membranes. It is concluded that Donnan dialysis is suitable for removing the fast-diffusing Mg²⁺ ions from diluted WPA, and increasing the solution temperature could enhance the Mg²⁺ transport rate.