Refining of Salt

Natural salts are often processed to remove some of their impurities before sale. All food-grade and certain chemical applications require this. In the chlor-alkali industry, it is necessary at some point to convert crade salt as mined or produced into a material suitable for use in electrolysis. Whether this occurs at the salt producer s site or in the chlor-alkali plant is fundamentally irrelevant. For economic or technical reasons, the salt supplier often undertakes a certain amount of upgrading. The most important processes are salt washing, brine evaporation, and salt recrystallization. 

Washing. For reasons explained below, washing salt to remove its impurities is easier with solar salts. While this is an operation that can be performed by the supplier, it is also a frequent practice in the chlor-alkali industry for the user to install and operate salt-washing facilities. 

The last fact means that solar salt will relca.se high concentrations of magnesium when small amounts are dissolved. Rainfall onto a pile of solar salt will produce a high-magnesium runoff, and this along with drainage fiom the pile may be considered part of the first step in refining. 

If a fresh shipment of solar salt is added as a batch to a salt dissolver, there will be a surge in magnesium content of the brine being manufactured. This can upset the performance of the brine treatment system and eventually of the cells. Many plants 

FIGURE 7.7. Countercurrent nature of flows in salt-washing plant. (Numbers refer to equipment in Fig. 7.6.) 

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William Newman pointed out recently that before the end of the seventeenth century, the word [alchemy] was widely used by early modem writers as a synonym for chymistry, a discipline that included iatrochemistry and a host of technologies such as the refining of salts and metals, the production of acids, alcoholic libations, and pigments, and finally, the transmutation of base metals into noble ones. Newman [2004] p. xiii. 

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