Comparison of theoretical and measured composition of brine during salt precipitation and resulting co-crystals

The evaporation of sea water reslts in the formation of different precipitaties, usually following the order CaSO4, NaCl, and MgSO4. Depending on the brine composition, NaCl can precipitate as either ‘lamellose’ (flaky) or ‘hopper-faced’ (pyramidal) crystals.

The crystallisation sequence of brine was investigated based on: theoretical ion concentration calculated from specific gravity values and ion concentrations obtained through Ion Chromatography (IC). It was found that as specific gravity of the seawater increased and decreased throughout a production cycle, the ion concentration varied.

It was observed that the concentration of calcium ions decreases during the production process due to the precipitation of calcium carbonate and calcium sulphate. As the calcium ion concentration decreased, the sodium and magnesium ion concentrations were shown to increase and the precipitation of sodium chloride was observed.

Single crystal X-Ray Diffraction (XRD) analysis on both ‘lamellose’ (flaky) and ‘hopper-faced’ (pyramidal) crystals were shown to contain (as co-crystals with NaCl) CaSO4.0.5H2O and MgSO4.7H2O, respectively