Salt from the sea
Sea water was the long row in the very first table of this chapter — water and half a dozen salts. A saltern takes them out one at a time, using evaporation and crystallisation and nothing else.
Where a saltern can be put
Siting one depends on geography and climate as much as on chemistry. Four conditions have to be met:
- A flat place in a coastal area from which sea water can easily be obtained
- Soil with clay in it, so that the loss of water by seepage is minimal
- A dry, warm climate with strong sunlight and wind throughout the year
- A region with minimal rainfall
Figure to be added
Three kinds of pond, three salts
The structure of a saltern has three kinds of pond in it, and the sea water flows from one to the next as it grows more concentrated. What crystallises depends on how concentrated the water has become.
Step 1 — the large shallow ponds
Sea water is filled into large shallow ponds by pumping or with the help of the tide, and is left to evaporate by solar heat. When the concentration has risen to about twice that of the original water, calcium carbonate (CaCO3) crystallises in this first pond and is deposited at the bottom.
Step 2 — the medium ponds
This water is then let flow into medium-sized ponds, where the water in the solution evaporates further. When the salt concentration has risen to about four times that of the original water, the calcium sulphate (CaSO4) in it crystallises and is deposited at the bottom.
Step 3 — the small ponds
After the calcium sulphate has been deposited, this solution is let flow from the medium ponds to the third, small ponds and the water is evaporated further. When a concentration about ten times that of the starting sea water is reached, salt (NaCl) crystallises and is deposited at the bottom.
What comes after the salt
While the salt is being deposited the concentration goes on rising. Before the deposition of sodium chloride has finished, magnesium chloride (MgCl2) and magnesium sulphate (MgSO4) begin to be deposited. Because these salts get mixed in, the salt acquires a bitter taste. The concentrated solution left after the salt has been deposited is called the mother liquor, or the bittern.
Six months in a heap
Pure sodium chloride is not hygroscopic. But the magnesium chloride and magnesium sulphate present as impurities in salt obtained this way are deliquescent — that is, they absorb water vapour and dissolve. The salt taken from the third pond is carried away, heaped into prisms and kept for six months. During that storage time those two salts absorb water vapour from the air, dissolve and are removed. What remains is a solid mass of salt.
Nothing new was needed
A saltern uses two techniques and both are from this chapter: evaporation to concentrate the solution, and crystallisation to take each salt out as its own saturation point is passed. The order the salts come out in is simply the order of their solubilities.