Exploration of a Two-Step Aqueous Process for the Valorization of Sodium Fluorosilicate (Na2SiF6), an Intermediate Product of the Fluorosilicic Acid Conversion
Résumé
The production of phosphoric acid for use in fertilizer generates toxic fluorosilicic acid as by-product, which can have, when disposed in ocean, negative effects on the maritime ecosystem. With the objective of developing a process for the treatment and valorization of this industrial waste, we study a route starting with a reaction between fluorosilicic acid and sodium chloride that produces a hydrochloric acid solution and sodium fluorosilicate powder. In the present work, we focused on the conversion step of sodium fluorosilicate, consisting of a caustification reaction in an aqueous solvent, with the purpose of obtaining valuable products. We identified a reaction scheme based on a preliminary thermodynamic study using the PHREEQC geochemical software. Experimental tests were then carried out in a 2 L stirred batch reactor, with various characterization techniques of the products (ICP-OES, XRD, TGA) allowing to establish mass balances. We propose two new steps in the conversion process, consisting of the aqueous reaction of CaO with Na2SiF6 at 50 °C, with an intermediate filtration for a molar ratio (CaO/Na2SiF6) of 2.77. This process generates two solid products: (i) pure CaF2, which can be used as an additive in various pyrometallurgical processes and (ii) a mixture of calcium silicate hydrates with low content of CaF2, which constitutes a promising resource for construction and building materials.