Solubility behaviors of (Na+, K+or NH4+)hexafluoridosilicates in H2SO4, HF, H2SiF6, H3PO4 acidic aqueous solutions at T=353.15 K

Abstract

The solubility equilibria of sodium, potassium or ammonium hexafluoridosilicates in hydrofluoric, sulfuric, hexafluoridosilicic, and phosphoric acids aqueous solutions were investigated at T=353.15 K. The experimental solubilities were performed at various molalities ranging from dilution to concentrated solutions of acid. The dissolution of these salts in acidic solutions could be one of the solutions to overcome the problem of fouling during the production of phosphoric acid. The knowledge of the behaviors and solid-liquid equilibria of these compounds in acid aqueous solutions is essential to understand the mechanisms governing such complex thermodynamic equilibria in phosphate industry. The formed solid phases of (Na+,K+,orNH4+) hexafluoridosilicates were characterized using X-ray diffraction and IR spectroscopy. The obtained solids show only a single crystalline form corresponds to malladrite, hieratite, or cryptohalite respectively. The solubility behaviors of these compounds in acidic aqueous solutions are different over the molality range of each acid. The exact treatments of these systems can be complicated according their trends. They may be different from that expected and it's depending on the molality range of acid (pH) and species in solutions at temperature 353.15K.