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Production

All the salt produced by EuSalt members has its origins in the sea, being either harvested directly from sea water or derived from rock salt deposits (former seas which have evaporated many millions of years ago). There are three types of salt extraction: solar evaporation, solution mining and rock salt mining. Each one involves specific technology and manufacturers select the most appropriate technique depending upon the particular topographical and economic conditions in their country of operation.

Rock Salt Mining

Many rock salt deposits were formed over 200 to 250 million years ago as a result of the evaporation of earlier seas. A typical example is the deposits formed in the Zechstein Sea – these deposits were laid down approximately 250 million years ago in a basin that stretched from the UK to Poland. More recently, around 16 million years ago, the Mediterranean Sea almost dried up, leaving large salt deposits that are now mined in Southern Spain.

There are many hundreds of horizontal and vertical salt layers across the world. Mines vary in depth from 100 metres or so, to 1 ½ kilometres. 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 is taken to ensure that the mine is stable by leaving substantial ‘pillars of salt’ to support the mine roof. This is called a ‘Room and Pillar’ layout, and mine engineers use the principles of rock mechanics to calculate the optimum size of the pillars, for safety and stability.

In ‘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.

This rock is then 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 produces smaller lumps of rock than the cut and blast technique. The machine, similar to that used for building tunnels, has a rotating head carrying tungsten-carbide tips, which bores into the salt. The resulting lumps are then taken directly to a crushing and screening plant, without the need to be crushed by a feeder-breaker first.

Solution Mining (Controlled Brine Pumping)

Depending on the structure and properties of the salt deposits, solution mining is sometimes the preferred method of extraction. In this system salt is produced by evaporating ‘solution-mined’ brine in enclosed vessels. In the process of solution mining, water is forced under pressure into a bore-hole drilled into an underground salt layer. The salt dissolves, turning the water into brine and creating a cavern in the salt-layer. Controlling the amount of water introduced and the rate of extraction ensures that the size of this cavern is fully controlled, minimising the risk of subsidence. 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.

In the first step of the evaporation process, brine is pumped into the first of a series of vessels which also contain special steam chambers. The heat of the introduced steam causes the brine to boil and water evaporates. This in turn produces further steam and causes salt crystals to grow. These salt crystals and associated brine slurry are fed into a second vessel where the process is repeated (using exhaust steam from the first vessel). Pressures (and boiling temperatures) become successively lower through the evaporators. The final ones operate under vacuum and enable the brine to boil at much lower temperatures, which is more fuel-efficient. The salt crystals and slurry from the final vessel is fed into a centrifuge to extract more moisture and the resulting undried vacuum salt is then stored in bulk, for supply to the chemical industry.

Salt for food and other uses which needs to be drier, the undried vacuum salt from the centrifuges is fed into special dryers called fluidized-bed dryers. The salt is then sieved and graded – Iodine can also be added at this stage – before being transferred into large storage hoppers ready for distribution in bulk and bags.

Solar Evaporation

Probably the earliest method of production, 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, this method works well because evaporation exceeds rainfall by a factor of 3:1. However, as in agriculture, storms around harvest time can result in significant losses.

Once 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 containing more than 97% NaCl.

By Product Salt

This is salt which is generated through other industrial processes. Some industries end up with sodium chloride as a waste product and go on to use it for other activities such as de-icing.