Evaluation of saltwork ponds operation through brine characterization and geochemical modelling using PHREEQC code integrating the Pitzer correction

Abstract

Seawater represents a potential resource for the extraction of salts and raw materials [1]. About one-third of the global table-salt production is manufactured in solar saltworks [2], being the most representative product of seawater processing. However, other valuable compounds such as Magnesium, Lithium and trace elements belonging to the alkaline/alkaline-earth metals (e.g. Rb, Cs, Sr) and transition/post-transition metals (e.g. Co, Ga, Ge) are present. Many of these elements are included in the EU Critical Raw Materials (CRM) list, grouping natural assets classified as fundamental for the wealth of the socio-economic structure of Europe [3]. In saltworks, natural evaporation of seawater leads to fractional crystallization of Calcium and Sodium salts and the generation of residual brines called “bitterns” with electrolytes concentrations up to 20-40 times higher than seawater. In this complex crystallization process, preliminary precipitation of Calcium based minerals is the basis for the enhancement of sodium chloride recovery and purity. Along the sequence of evaporation and concentration stages, the fate of the minor components (e.g CRMs) has not been extensively studied so far. With this respect, it is fundamental to deeply understand the potential formation of mineral phases involving such minor trace elements. Pitzer model accounting for an equilibrium approach for high ionic strength can be a valuable tool for salterns modelling by allowing a reliable description of the precipitation and crystallization reactions occurring there [4]. In the present work, PHREEQC incorporating the Pitzer model was used as equilibrium computation tool to describe the minerals precipitation pathway along the different ponds of a saltwork. The Trapani saltworks (SOSALT, Italy), where the "Sea Salt of Trapani" is produced was used as case study. The validation of the obtained results was performed by using a monitoring campaign by withdrawing samples from each pond belonging to different evaporation stages of the production process. The salterns have been fully characterized to determine their composition. A good agreement was obtained between the model prediction and the analytical characterization. Finally, an attempt to provide indications for table-salt recovery and purity maximization has been done and insights on trace elements enrichment were given.