Mining minerals and critical raw materials from bittern: Understanding metal ions fate in saltwork ponds

Seawater represents a potential resource for raw materials extraction. Although NaCl is the most representative mineral
extracted other valuable compounds such as Mg, Li, Sr, Rb and B and elements at trace level (Cs, Co, In, Sc, Ga and
Ge) are also contained in this “liquid mine”. Most of them are considered as Critical Raw Materials by the European
Union. Solar saltworks, providing concentration factors of up-to 20 to 40, offer a perfect platform for the development
of minerals and metal recovery schemes taking benefit of the concentration and purification achieved along the evaporation
saltwork ponds.
However, the geochemistry of these elements in this environment has not been yet thoroughly evaluated. Their knowledge
could enable the deployment of technologies capable to achieve the recovery of valuable minerals. The high ionic
strengths expected (0.5–7 mol/kg) and the chemical complexity of the solutions imply that only numerical geochemical
codes, as PHREEQC, and the use of Pitzer model to estimate the activity coefficients of the different species in solution
can be adopted to provide valuable description of the systems.
In the present work, for the first time, PHREEQC Pitzer code database was extended to include the target minor and
trace elements using Trapani saltworks (Sicily, Italy) as a case study system. The model was able to predict: i) the purity
in halite and the major impurities contained, mainly Ca,Mgand sulphate species; ii) the fate of minor components as B,
Sr, Cs, Co, Ge and Ga along the evaporation ponds. The results obtained pose a fundamental step in critical raw materials
mining from seawater brine, for process intensification and combination with desalination.