Evaluation of the Purity of Magnesium Hydroxide Recovered from Saltwork Bitterns

Magnesium has been listed among the 30 critical raw materials by the European Union. In recent years, many green and sustainable alternative Mg²⁺ sources have been sought to satisfy the EU’s demand and to avoid mineral ore consumption. In this context, saltwork bitterns, the by-products of solar sea salt production, have attracted much attention thanks to their high Mg²⁺ concentrations (up to 80 g/L) and low Ca²⁺ and bicarbonate contents (<0.5 g/L). Although investigations on Mg²⁺ extraction from bitterns in the form of Mg(OH)₂(s) have already been performed, product purity has never been properly addressed. Mg(OH)₂(s) is a chemical compound of great interest and extensive utility in numerous industrial applications only if the powder’s purity is >95% (w/w). This work presents a comprehensive experimental effort of reactive precipitation tests with NaOH solutions at stoichiometric and over-stoichiometric concentrations to: (i) assess the technical feasibility of Mg²⁺ recovery from real bitterns collected in saltworks of the Trapani district (Italy) and, (ii) for the first time, conduct an extensive purity investigation of the precipitated magnesium hydroxide powders as brucite. This experimental investigation demonstrates the possibility of extracting highly valuable compounds from saltwork bittern waste, embracing the water valorization and resource recovery approach.