Methods Manual for Salt Lake Studies/Sequential crystallisation of brine salts

Authors: PSJ Coleman

Overview

The brines in salt lakes are made up of a variety of dissolved salts derived from the environment in which the lake is located. Salt lakes, even those in extremely cold places such as Antarctica, are usually found in semi arid areas of the earth with low rainfall and high evaporation. Sources of salts to salt lakes are various: they may be the result of the stranding of marine areas in the geological past, or they may be the result of dissolution of nearby rocks over eons into a terminal lake system.

What "type" of lake is it?

Marine-derived brines are characterised by the presence of calcium and magnesium salts. Salt lakes formed from local dissolution may have very different salt compositions.

Sodium chloride is present in all salt lakes. Most lakes fall into four major groups if you are classifying them by their salt composition:

1. Sulfate type lakes that derive their salts from surrounding rocks that contain sodium sulfate
2. Carbonate type lakes (marl, or flos ferri lakes) are alkaline lakes which contain sodium carbonate in addition to sodium chloride and sodium sulfate (gypsum and anhydrite).
3. Bittern lakes like the Dead Sea contain more magnesium than sodium.
4. Sodium chloride type lakes such as the Great Salt Lake of USA and most Australian salt lakes are similar in composition to sea brine of an identical density.

Crystallisation sequence in marine-derived lakes

In sodium chloride type lakes, the crystallisation pattern of the different salts is very similar to that of evaporating seawater brines and occurs as follows (Baseggio 1974, Venkatesh Mannar 1982):

1. The least soluble salts are the ferric oxides, which precipitate out of solution between 30g/L TDS and 70g/L TDS.
2. Calcium carbonate (calcite or flos ferri) starts to deposit at the same salinity as ferric oxide but continues precipitating up to a salinity of about 170g/L TDS.
3. Calcium sulfate, as gypsum or selenite (CaSO₄.2H₂O)starts to precipitate at a salinity of about 143g/L TDS. At about 191g/L TDS the form of calcium sulfate changes and it deposits as anhydrite (CaSO₄). While most of the calcium sulfate is precipitated before sodium chloride starts to precipitate, a small quantity continues to crystallise out of solution up to a salinity of 385g/L TDS.
4. Sodium chloride (common salt) begins to crystallise at a salinity of 318g/L and precipitates rapidly in brines with a salinity up to 360g/L. Thereafter the quantity of common salt precipitating reduces, although some common salt has still been recorded precipitating from seawater-derived brines with a salinity of about 440g/L TDS (Chitnis & Sanghavi, 1993).
5. Magnesium sulfate (epsom salts), potassium sulfates and chlorides, and sodium sulfates (glaubers salts), start to precipite when seawater-derived brines reach a salinity of 390g/L TDS, and finally magnesium chloride and bromine salts will precipitate when salinities exceed 400g/L TDS.