Mineral Recovery from Brine
The process of producing table salt involves seawater. This is collected and evaporated until the dissolved salt within it reaches saturation and begins to precipitate. Seawater contains many other minerals that are extremely important for recent technological developments. Once the seawater has been concentrated and as much salt as possible has been produced, the residual concentrated solution, brine, is discarded because it can no longer be used for further production. Brine is therefore a waste saline solution, but thanks to the enormous quantity of minerals it contains, it can be used as a raw material in plants specifically designed for their recovery.
Pre-treatment of Brine
As part of the MareMag LIFE project, the brine is pre-treated in an ultrafiltration system to remove most of its organic components. This organic matter is generated by the extremely saline initial environment, which leads to the emergence and proliferation of particular life forms, microalgae and extremophilic bacteria, which are responsible for the pink color of the brine itself. When stressed by increased temperature, salinity, and/or solar radiation, these life forms produce carotenoids as a survival mechanism. These, by interfering with light, generate the typical pink color of the brine and its main macrofauna: flamingos.
Absorption and Crystallization Processes
Once the purification step is complete, the actual mineral recovery phase can begin, starting with boron. Boron is trapped in chelating resins through an adsorption process and, once the resin is saturated, it is recovered by flushing the column with an acid solution. Within the eluent solution, the boron is recovered in the form of boric acid, and this solution, following further purification and crystallization processes, allows the boron to be recovered. The boron-free solution is sent to a crystallization reactor where, thanks to the further addition of an alkaline solution, the magnesium is recovered in the form of hydroxide. The resulting product, an aqueous suspension of magnesium hydroxide solids, is separated by gravity in settlers, resulting in a clarified saline solution and a more concentrated suspension of solids. The thickened solution, once washed and purified of other salts, is filtered to completely separate the magnesium hydroxide solid. The clarified saline solution is then sent to a bipolar membrane electrodialysis (EDBM) unit, where the basic and acidic solutions are produced by applying an electric field and introducing fresh water. The basic solution produced is precisely the one used primarily to precipitate magnesium from brine.
Between Osmosis and Electrodialysis
The remaining saline solution is then concentrated in a reverse osmosis unit, which recovers water for the plant, while the concentrated saline solution feeds a small salt pan designed to recover the remaining salts in solution (for example, Na2SO4). Another type of concentrator, this time an electrodialysis one, is used to increase the acid content of the hydrochloric acid solution exiting the process to the levels required by the detergent market.
