Thermodynamic and experimental study of the fluoride recovery from Spent Pot Lining recycling process by precipitation of calcium fluoride

The material resulting from removing the electrolytic cell in primary aluminium production is called Spent Pot Lining (SPL). SPL is considered to be toxic due to its high concentration of fluorides and cyanides, corrosive and reactive with water. It is estimated that more than 50% of the SPL generated annually is stored indefinitely or deposited in a landfill. Precipitation is considered a simple and cost-efficient method of removing fluoride from aqueous streams producing a valuable product such as fluorite. In fact, fluorite (CaF2) is one of 27 critical raw materials for the EU because the risks of supply shortages and their impact on the economy are high. Its high carbon and fluorine content, as well as CaF2 as the main raw material for almost all fluorochemicals, have made SPL one of the most crucial wastes for recovery, reuse and recycling in a circular economy. This work is focused on the thermodynamic study to predict the behaviour of a stream coming from an SPL recycling process by introducing calcium salts in order to obtain a precipitated product of interest such as CaF2, within a circular economy approach. The thermodynamic of the reaction chemistry was investigated by HSC Chemistry software and the subsequent verification of these results in an experimental way. As a result, the generation of a solid composed of CaF2 has been verified. Among the three precipitation agents studied, it has been found that the one with higher purity values in thermodynamic calculations is CaCl2 (45.1%). For the real sample, the presence of other precipitates such as Na2CO3, CaO, Ca(OH)2, has been observed. A subsequent washing stage is proposed to increase the degree of CaF2 purity. Thus, the results present a reliable and environmentally friendly process that produces a product with a high added value. The research indicates that the process could also be applied for the treatment of wastewater with a high concentration of fluorine. The resulting product has a high economic value and could be applied in the aluminium smelting industry.OutgoingObjectius de Desenvolupament Sostenible::12 - Producció i Consum Responsables::12.4 - Per a 2020, aconseguir la gestió ecològicament racional dels productes químics i de tots els residus al llarg del seu cicle de vida, de conformitat amb els marcs internacionals convinguts, i reduir-ne de manera significativa l’alliberament a l’atmosfera, a l’aigua i al sòl a fi de minimitzar-ne els efectes adversos sobre la salut humana i el medi ambien