6.2 Water-Halite-Sylvite

There is a limit to how much halite can dissolve in a liter of water. In the Binary Saturation Diagrams chapter this limit, the saturated solution composition, was found to be about 25 weight percent NaCl, 75 weight percent H₂O at 21°C. What would happen if the water had KCl in it as well as NaCl? It turns out that a brine with KCl in it will hold less NaCl per gram of H₂O at saturation than an H₂O-NaCl-only brine. And the higher the KCl wt% in the solution, the lower the NaCl wt% will be in the halite-saturated solution. To show this graphically, we need to plot both weight percent NaCl and weight percent KCl along with weight percent H₂O. A ternary (three-component) composition diagram is needed.
Figure 6.01 is a ternary composition diagram showing a saturation curve for halite in an H₂O brine at 20°C at NaCl wt% values that vary with the KCl wt% in the brine. Click on the H₂O-NaCl-KCl diagram to see a larger version. The diagram shows possible brine compositions (colored blue) limited by saturation curves (liquidus curves) for both halite (Hl) and sylvite (Syl). The saturation curve for sylvite occurs at KCl wt% values that vary with the NaCl wt% in the brine. The two saturation curves meet at a point that gives the brine composition for a liquid saturated with both halite and sylvite. For bulk compositions more salty than the saturation curves, the system may be a two-phase mixture of brine + halite, a two-phase mixture of brine + sylvite, or a three-phase mixture of brine + halite + sylvite.

If halite is added to a halite-saturated brine, the halite will not dissolve. Instead, the proportion of halite to brine will increase. The two-phase brine + halite region (shaded green) of the diagram is filled with example tie lines, each of which connects the composition of halite to a saturated brine. The weight percent of crystals (Hl) to brine can be determined for each bulk composition using the lever rule. A similar two-phase brine + sylvite region (also shaded green) exists for sylvite-saturated liquids.

If enough sylvite is dissolved in a halite-saturated brine, the brine may become saturated with both sylvite and halite. If enough halite is dissolved in a sylvite-saturated brine, the brine may become saturated with both sylvite and halite. At equilibrium, all bulk compositions in the pink-shaded region are saturated with both halite and sylvite. The proportions of halite, sylvite, and brine for compositions in the pink, three-phase region may be detemined graphically using the ternary lever rule.    Next Page