The production of salt

Introduction

All salt produced is sea salt; either harvested every year from sea water or from rock salt deposits or former seas, which have evaporated 200 million years ago. There are three types of salt production: rock salt by mining, evaporated salt by solution mining and sea salt. Each one is produced using specific technology.   Countries select the most appropriate technique depending upon their particular topographical and economic conditions.  

About 4,700 years ago a document called the Peng-Tzao-Kan-Mu, already described two methods of extracting salt and putting it in usable form that are amazingly similar to processes used today.

Rock Salt

Rock salt deposits were formed 200 to 250 million years ago as a result of the evaporation of earlier seas.   There are many hundreds of horizontal and vertical salt layers across the world. Mines vary in depth from 100 metres or so, to a mile or more. Within the mines, there are networks of pathways of sometimes up to hundreds of kilometres, formed by the areas from which salt has already been extracted.

There are two main methods of extracting rock salt - Cut and Blast Mining and Continuous Mining.   Under either technique, care must be taken to ensure that the mine is stable by leaving substantial 'pillars of salt' to support the mine roof. This mine layout is called 'Room and Pillar' mining, and mine engineers use the principles of rock mechanics to calculate the optimum size of the pillars, for safety and stability.

Cut and Blast Mining

A slot is cut at the base of the rock face using a machine called an undercutter, with a jib carrying a series of tungsten-carbide picks. The face is then drilled with a series of carefully sited holes, using an electro-hydraulic rotary drill. The holes are charged with explosives and the face is blasted, yielding about 1,400 tonnes of broken rock salt.

The rock is crushed into pieces about the size of a football, using a feeder-breaker. It is then carried on a conveyor belt to the main crusher which breaks it down into smaller pieces, passing through a type of sieve or screen to ensure that it has reached the correct size for use in road de-icing. The salt is then hoisted to the surface in skips.

Continuous Mining

This type of mining produces smaller lumps of rock than the cut and blast technique. A boring machine, similar to a pneumatic drill used in digging up roads, is used. It has a rotating head carrying tungsten-carbide tips, which bores into the salt. The lumps are then taken directly to a crushing and screening plant, without the need to be crushed by a feeder-breaker first.

Evaporated Salt

While the natural energy of the sun can be used to evaporate sea water in hot countries to get salt, in colder countries, salt is produced by evaporating 'solution-mined' brine in pressure vessels. In solution mining, water is forced under pressure into a bore-hole drilled into an underground salt layer. The salt is dissolved, turning the water into brine and creating a cavern in the salt-layer. The saturated raw brine is then withdrawn and pumped to the purification plant where calcium, magnesium and other impurities are removed prior to the evaporation process.

Sea Salt

Producing salt from the sea involves channelling sea water to flow into natural or man-made basins and allowing the water to evaporate using wind and solar energy.   The water evaporates in successive ponds until the brine is fully concentrated and salt crystallizes on the floor of the crystallizing ponds.   Solar salt plans must be located in areas of low rainfall and high evaporation rates.   In the Mediterranean, for example, saltworks succeed because evaporation exceeds rainfall by a factor of 3:1.  

After the salt 'crop' reaches the appropriate density, the salt is harvested, washed and placed on a stockpile to drain.   During this process, salt comes in contact with several impurities and is therefore washed afterwards with brine to get white and pure sea salt with more than 97% NaCl.