The sequence of salts deposited by the evaporation of sea water is in accordance to the solubility of its several compounds. Thus, the precipitation of salts includes the less soluble compounds in the base to the more soluble at the top of the sequence, in the following order: limestone (CaCO3), gypsum (CaSO4), halite (NaCl), potassium salts sylvinite (NaCl-KCl system), and magnesium salts (bischofite - MgCl2·6H2O); it is also considered the presence of other compounds, according to physical and chemical variations of the brine during the various stages of evaporation .
NaCl crystal (halite) is formed when the total salt concentration reaches value above 300 gL-1. After most of the NaCl precipitates to the bottom of the crystallizer ponds, the remaining concentrated brine (the "bitterns") contains mainly Mg2+, K+, Cl- and SO4
2- (Oren, 2002). The bittern remaining after the crystallization of halite is nutrient-rich, but apparently devoid of life, as no organisms tolerate the extremely high Mg2+ concentration .
When all calcium carbonate, calcium sulfate, and 83% of the halite is crystallized from seawater by solar concentration, a bittern of a specific gravity of about 1.26 is obtained. This bittern with few exceptions is placed back to the sea. In some cases, as in Spain, is used for recovering some epsomite, bischofite, and bromine, but not for the production of potassium salts. On further evaporation, a complex mixture of halite, sylvite, and double salts of potassium, sodium, and magnesium start to crystallize; the recovery of marketable products becomes difficult and inefficient. However, in the absence or near absence of sulfate, the bittern may be readily processed to recover high-purity sylvite and bischofite with excellent efficiency .
The partially desulfated seawater bittern obtained from the epsomite plant is readily amenable for recovering sylvite and magnesium chloride hexahydrate by a combination of solar evaporation and fractional crystallization. Despite the very complex chemical phase of seawater bittern, a simple crystallization method may be employed for the efficient recovery of high-purity epsomite and sylvite .
Magnesium is profusely present in seawater evaporites as chloride (9.44%), sulfate (6.5%) and bromide (0.22%). The raw material of the magnesium industry is, however, magnesium hydroxide. This is then treated with hydrochloric acid to obtain magnesium chloride. The potential value of magnesium chloride as raw material is established, but involves separation of different salts to obtain magnesium chloride in a relatively pure form. Magnesium chloride occurs in the nature as bischofite (MgCl2 • 61120) and as carnallite (KCI • MgC12 • 6H20), both from oceanic origin .
The production of crystalline magnesium chloride hexahydrate by solar evaporation of low-sulfate-containing inland bittern has yielded a product suitable for electrolytic production of magnesium metal. Using the sea bittern for the production of such crystalline magnesium chloride hexahydrate was not attempted, probably due to the high value of sulfate content of about 3.5 per cent at sp. gr. 1.350. In the arid and semiarid tropical regions, solar evaporation of sea bittern reaches the equilibrium density of sp. gr. 1.377, and at equilibrium relative humidity of 32% . This clearly approaches the equilibrium of the pure system of magnesium chloride hexahydrate and water enabling to take advantage of solar evaporation in the process .