Integration of ion-exchange and crystallisation processes to recover boric acid and magnesium hydroxide from saltworks bitterns

The lack of primary sources of the so-called Critical Raw Materials within the European Union is directing research towards alternative mining to extract them. Among the different potential alternative sources, the brines generated in traditional saltworks (denominated bitterns) can be a very promising option. In fact, several elements can be up to 50 times more concentrated in bitterns than in seawater. Magnesium, for example, can present concentrations above 50 g/L, and its recovery can be pursued as hydroxide by using crystallization processes. However, the presence of boron even at relatively low concentrations (100 to 150 mg/L) can contaminate the final magnesium hydroxide, thus making it not suitable for certain applications, such as the refractory industry (target <∼ 0.11 mg B/g of magnesium hydroxide). Because of boron speciation (as boric acid), only chelating ion-exchange resins based on N-methylglucamine functional groups can selectively remove boron from aqueous solutions. In this work, the integration of ion-exchange and crystallization processes is carried out to produce pure magnesium hydroxide from real bitterns collected in Trapani (Sicily). Two different bitterns were treated with two commercial B-selective chelating ion-exchange resins (S108 and CRB05), and the boron-free bittern was later used for Mg(OH)₂(s) crystallization. The effect of pH on Mg(OH)₂(s) crystallization was studied and data was compared (in terms of purity) in the cases with or without B-removal pre-treatment. Moreover, once the resin was saturated, elution with HCl allowed to recover H₃BO₃ via evaporative crystallization. Results showed the possibility of recovering pure Mg(OH)₂(s) (>98 %) with low B-content (<0.10 mg B/g), matching the specifications for refractory industry, and H₃BO₃(s) with 95 % purity.