Mg(OH)2 Production from Seawater and Saltworks Brines

Desalination and saltworks are widely adopted to produce freshwater and table salt. On the other hand, these processes generate waste saline solutions, named brine or bitterns, that have been managed so far following the linear economy approach. Only in the last 10 years, more attention has been paid on the valorisation of these waste streams for the recovery of high added value compounds, such as magnesium hydroxide, Mg(OH)2. Although several scientific efforts have investigated the precipitation of Mg(OH)2 compounds from brines and bitterns, literature still lacks of comparative studies under similar operating conditions. With this respect, the present work aims at filling this gap by testing two types of real saline waste solutions: (i) a reverse osmosis (RO) brine outcoming from the desalination plant located in the Lampedusa Island (Italy) and (ii) a bittern collected from Margi saltworks (located in Trapani, Italy). Synthetic sodium hydroxide (NaOH) solutions were employed as alkaline reagent. Tests were carried out using a continuous-flow stirred-tank laboratory reactor (CSTR). Mg(OH)2 suspensions synthesized from desalination brines exhibited faster settling velocities, but lower filtration rates with respect to suspensions synthesized from saltworks bitterns. The purity of Mg(OH)2 solids produced from saltworks bittern was > 98.5%, whereas the purity of Mg(OH)2 solids synthesized from desalination brines was below 95%, due to calcium co-precipitation.