Hannah Cornish, of Balclutha Primary School, asks :-

How do people get salt from the sea?

Robert Kooistra, a chemist at the Grassmere Salt Works, responded.

Salt has been extracted from the sea well before written records began. Salt seems to have been in use almost universally, in all times and places. Certainly men have pursued it by all means necessary, including war, fraud, ingenuity and massive effort.

In principle it is easy to extract salt from the ocean. You boil off the water and the salt remains behind. Indeed this was a common method of obtaining salt for much of history. For example the first expedition to the west coast of America wintered on the coast before their return journey and they needed salt to preserve any game they shot for lean times and to treat animal skin they used to make clothes. One team spent all winter using wood fires to evaporate seawater in a copper kettle.

This method, however, is costly in fuel and produces a very impure salt (about 70 per cent pure).

In New Zealand we use solar and wind evaporation of seawater to obtain the salt. Salt is pumped into a series of ponds where the sun and wind slowly evaporate the water. As the water becomes more concentrated gypsum precipitates on the bottom of the ponds. When the volume of water gets to 10 per cent of the original seawater amount, salt starts to crystallise. The water is then pumped into flat ponds where further evaporation deposits the salt. When three quarters of the salt is crystallized the remaining water is discarded to sea to prevent contamination from other chemicals dissolved in the sea. We then scrape the salt up with a harvester, wash it, and stack it on the heaps at our Lake Grassmere site. This process is known as fractional crystallisation and produces a salt that is 99 per cent pure or greater. It uses natures 'free' energy and so the salt can be extracted economically.

AAS c89wash

Judith Simpson of Springside, RD Oamaru, asked:-

How do you wash salt without the salt dissolving?

Robert Kooistra, a chemist at the Grassmere Salt Works, responded.

Your question about why salt doesnt dissolve when it is washed, is a popular question from visitors to our site. The answer is simple: We wash the salt in water that is already saturated in salt. This stops the salt from dissolving, but will still carry the dirt away. The brine (salt water) is recycled through a series of ponds where the mud is settle out and is then pumped back through the washeries.