Effective magnesium recovery via seawater brines mining

In the last couple of decades, a rising interest has been focused on the research of novel and unconventional sources for mining minerals, such as industrial waste brines. As an example, brines are produced by desalination plants and discharged directly, or after being treated, into receiving water bodies possibly causing environmental concerns and renouncing the possibility to recover minerals [1,2]. Within this framework, the Water Mining WU-H2020 project aims at implementing an advanced Zero Liquid Discharge (ZLD) demo systems for the extraction of minerals and valuables products from industrial waste brines through a circular approach. The ZLD demo plant consists of: (i) Nano Filtration, (ii) Multi effect distillation, (iii) Evaporative ponds, (iv) NaCl Thermal crystallizer, (v) Mg-Ca crystallizer and (vi) Electro-Dialysis with Bipolar Membrane (EDBM). This work presents the development, construction and testing of an innovative key technology of such ZLD treatment chain: the Mg-Ca crystallizer. Also called MF-PFR, “Multi Feed – Plug Flow Reactor” [3], the crystallizer is capable of selectively recovering magnesium and removing calcium in the form of hydroxides from real industrial brine or seawater. More specifically, the precipitation of the magnesium hydroxide occurs mixing directly the feed with an alkaline solution (such as Sodium Hydroxide - NaOH) produced within the treatment chain itself by the EDBM unit. The selective recovery of magnesium and calcium as hydroxides is achieved by an accurate control of the reaction pH. An experimental campaign has been carried out via long-run tests to investigate the stability of MF-PFR in terms of pH, which affects the recovery and purity of Magnesium Hydroxide. The results show that a purity higher than 90% with a recovery higher than 90% of magnesium hydroxide can be achieved using industrial waste brine or seawater as the feed to the demo plant.