ION EXCHANGE MEMBRANE CRYSTALLIZER FOR MAGNESIUM RECOVERY FROM SEAWATER AND INDUSTRIAL BRINE

The treatment and valorisation of waste brines deriving from industrial processes raised in the last years a strong interest in order to promote the whole process sustainability both in terms of minimisation of environmental impacts and raw materials recovery within a circular economy approach. With this respect, magnesium recovery from brines can be an interesting and practical example. In this work, a novel ion exchange membrane crystallizer (CrIEM1) is presented. In a CrIEM reactor, the presence of an Anion Exchange Membrane, separating a brine and an alkaline solutions, allows the passage of hydroxyl ions from the alkaline to the brine compartment, where crystallization of magnesium hydroxide occurs, yet avoiding a direct mixing between the solutions feeding the reactor. This enables the use of low-cost reactants (e.g. Ca(OH)2) without the risk of co-precipitation of by-products and contamination of the final crystals. An experimental campaign was carried out treating two types of feed solutions, namely: 1) Mediterranean seawater, collected from North Sicilian coast (Italy), and 2) a waste industrial brine from the Bolesław S ́miały coal mine in Łaziska Górne (Poland). The Mg2+ concentration in the feed solutions ranges from 0.7 to 3.1 g/L. The CrIEM was tested in two configurations: batch and feed & bleed. The latter being a semi-continuous process. Tests results show that magnesium hydroxide purity between 95 and 99% can be obtained, while the anionic membrane has not suffered performance drops throughout the 80 hours of operation. Furthermore, a mathematical model was purposely developed to simulate the performance of the crystallizer, based on the Donnan Dialysis equilibrium and transport mechanism. The model was implemented using Python and numerical simulations of the experimental runs were successfully carried-out, thus providing a good validation of the modelling tool.