Unraveling competitive cation-melamine complexation for efficient wet-process phosphoric acid purification via precipitation and gradient alkaline extraction

Abstract

The purification of wet-process phosphoric acid (WPA) was essential for producing high-value phosphate products. Metal impurities (Fe³⁺, Al³⁺, Mg²⁺) in WPA compromised downstream processes, while conventional purification methods suffered from low phosphorus recovery efficiency, inadequate impurity removal, and generation of intractable raffinate streams. Herein, we proposed an integrated strategy combining melamine-induced precipitation with pH-gradient alkaline extraction for selective metal removal and WPA purification. Under optimal conditions (n(ME): n(P) = 1.20, 30 °C, 50 min, liquid-solid ratio = 4 mL/g), phosphate precipitation efficiency reached 90.46 %. Subsequent alkaline extraction (pH 7.8, 25 °C, 20 min) achieved 80.41 % overall phosphorus recovery, with Fe³⁺, Al³⁺, Mg²⁺ removal efficiencies of 93.74 %, 98.56 %, and 99.78 %, respectively. Density functional theory (DFT) calculations established the binding energy hierarchy (Fe³⁺ > Al³⁺ > Mg²⁺), elucidating the selective removal mechanism. Moreover, regeneration experiments demonstrated melamine's recyclability, and the short-flow design minimized waste and energy consumption. This integrated approach provides a sustainable and scalable pathway for high-purity phosphate production, enabling dual valorization of phosphorus and strategic metals from complex raffinate acids.