Efficient Recovery and High-Valued Utilization of Aluminum and Magnesium from Raffinate Acid via Stripping and Crystallization

Abstract

Solvent extraction is a broadly employed method for removing impurities from wet-process phosphoric acid (WPA). However, the raffinate acid generated by this method remains underutilized due to its high viscosity and elevated metal impurity (Fe, Al, Mg) content and nonmetallic impurities (S, F), resulting in resource waste. This study employs P-15 as the extractant to purify raffinate acid and introduces an innovative stripping–crystallization process for the separation and recovery of Al³⁺ and Mg²⁺, yielding high-value products. A comparative analysis of various stripping agents identified a sulfuric acid solution containing ammonium sulfate as the optimal system. Under the optimized conditions (temperature: 303.15 K, organic–aqueous phase ratio (O/A): 1:1, sulfuric acid concentration: 30 wt.%, reaction time: 30 min), the stripping efficiency reached its optimal value. Theoretical stage calculations using the McCabe–Thiele method and cascade simulation determined that a two-stage countercurrent operation is required. To address crystallization inhibition caused by aluminum–fluoride complexes, silicon or boron (introduced as sodium silicate or borax) was incorporated to form more stable SiF₆²⁻or BF₄^- complexes, facilitating Al³⁺ release and its subsequent precipitation as NH₄Al(SO₄)₂·12H₂O. Further addition of ammonium sulfate enabled the formation of (NH₄)₂Mg(SO₄)₂·6H₂O. This study provides an efficient and environmentally friendly process for the valorization of raffinate acid from WPA production, offering significant industrial application potential and environmental benefits.