Evaporate to dryness is worse salt

A common instruction in household salt making is to evaporate seawater completely and use everything that remains. This paper shows, by a fractional crystallisation model, that the instruction produces worse salt. The model starts from 1000 g of seawater containing 965 g of water and 35.0 g of dissolved salts, of which 27.0 g is common salt (NaCl). Each salt is held in solution until the remaining water falls below the amount its stated solubility requires. Calcium sulphate saturates early, when 500.0 g of water remains. NaCl begins to crystallise when 75.0 g of water remains, after a fraction of 0.9223 of the water has been removed, and it crystallises through a window that ends when magnesium chloride begins, at 7.04 g of water remaining. Stopping at 7.0 g of water remaining recovers 24.48 g of NaCl, a recovery of 90.7 percent, at a dry-crystal purity of 0.9532, or 0.9163 after allowing for retained mother liquor. Evaporating to complete dryness recovers the full 27.0 g of NaCl but delivers 35.0 g of mixed salts at a purity of 0.7714. Going to dryness therefore gains only 2.52 g of additional NaCl, which is 9.33 percent of the total NaCl present, while purity falls by 18.18 percentage points on a dry basis. The recommended stopping rule is to harvest when about 7.0 g of water remains for each 1000 g of seawater started — a fraction of 0.9927 of the original water removed — and to discard or separately use the remaining bitter liquor, known as bittern.