Salt Production | How It’s Made
July 30, 2019 Lottie Bingham By Lottie Bingham

Salt Production | How It’s Made

Salt is used across industries and cultures, and has held an important place in society for over four millennia. But where does this versatile mineral come from?

An Age Old Love

With its capacity to enhance flavour and mask bitterness, combined with its relative abundance and low cost, it should come as no surprise to learn that salt is the most commonly used seasoning across the globe.1

Surprisingly, however, despite being best known for its culinary uses, human consumption accounts for just 6% of the >250 million metric tonnes of salt that is used worldwide every year.2 Of the remaining 94%, a small proportion is used for softening water, de-icing roads and in agriculture, with the largest volume being used for industrial processes including the manufacture of PVC, plastics and paper pulp.3,4

Our love of salt is far from being a modern phenomenon. In fact, it has been integral to societies for many millennia, with the earliest record of salt usage dating back to before 2000 B.C.E., and production being recorded for the first time about 1000 years later. However, despite this age-old love for the mineral and its near-ubiquitous presence across the globe, few of us really know where salt actually comes from.3

Sourcing Salt

Salt can be found across the globe, but there is no one way in which it can be obtained. Today we rely on 3 main methods to source salt:

  1. Through the evaporation of sea water,
  2. Mining rocky-salt formations within the earth,
  3. Creating salt brines.

Common table-salt is largely derived from salt brines, speciality or gourmet salts will more often come from seawater evaporations, whereas the majority of salt produced through mining is industrially used.5,6

Sea Salt: How it’s Collected

3.5% of the world’s oceans are salt. If shallow ponds or bays are left to naturally evaporate and dry up, salt crystals are left behind. The resulting crystals are harvested and, depending on the requirements, may simply be packaged, ready to be sold, or they may be subject to further rounds of processing such as washing, sifting and grading. This natural process is the oldest method of salt production, and whilst some salt is still produced according to ancient methods, new faster and less expensive methods have been developed and are now more widely utilised.7,8

Modern Sea Salt Harvesting

These modern methods place the sea water in specially designed ‘concentrating ponds’ which enable the most efficient rate of evaporation from the sun and the wind. The resulting product is a highly concentrated brine, which passes continuously to a ‘crystallising pond’ where the final salt grains are formed. These final crystallising ponds range from 20 to 400 acres in size, with a floor of salt about a foot deep resulting from years upon years of deposition. The depth and salinity of the brine within these ponds is tightly regulated, and tailored to the changing environmental conditions. Due to the requirement for complete evaporation, this technique is only effective in areas with low rainfall. Thus, the majority of sea salt is produced in dry climates such as the Mediterranean and Australia.8, 9

On a smaller scale, sea salt can also be produced following ancient methods. Fleur de sel is one such example: a light, flaky salt manufactured in small shallow ponds in France, during the months of May to September.2

Rock Salt: Is it edible?

As is true of all salts, rock salt is derived from a body of water, only in this instance, the water has long since dried up. Whether it be found beneath the rocky underlayers of the Earth’s surface, or deep within a mountain range, at some point in history, that salt was once part of the sea, or a salt-water lake. These large salt deposits developed over time as ancient waterways underwent intense periods of evaporation, followed by decades of geologic ageing and tectonic movement, and can really only be accessed through mining.10

Salt Minecraft

Dry mining is typically carried out in much the same way as the mining of other minerals: specialised equipment cuts the salt beds into large blocks, before explosives are used to break the blocks up into small enough fragments which can be transported either to the surface, or edge of the mountain. Most commonly, the ‘room and pillar’ technique is utilised, which extracts large ‘rooms’ of salt from the salt beds, with unscathed ‘pillars’ being left behind in a regular pattern to support the roof.11,12,13

Rock Salt Mines

The Sifto Salt Mine in Ontario, Canada is the largest salt mine in the world. At 2000 feet deep, it produces over 7 million tonnes of salt per year, ranging from 92-98% purity. It is these impurities which often give rock salts their grey, pink or brown hue. Whilst Sifto salt largely fails to meet the 97% sodium chloride requirement to be used in culinary pursuits, meaning that (like most rock salt) it is used in highway de-icing and industrial purposes, a small proportion of rock salts do make the grade, such as the somewhat infamous Himalayan Pink Salt.2, 14

Salt Brines & Extraction

Surprisingly, the same salt beds which give rise to this relatively impure rock salt, also give rise to the majority of what we know as table salt. The difference lies is in the way in which the salt is extracted. Rather than blasting the salt beds in their solid form, hydraulic or solution mining of salt involves pumping water beneath the earth’s surface where it will dissolve the salt deposits, forming a salt brine in the process. This brine is then pumped back to the surface where it will be subject to evaporation processes.14, 15

Before drying, the brine is typically transported to a purification plant where impurities such as magnesium and calcium are removed, eventually leaving a near-pure sodium chloride crystal as the end product.14,15

Salt for Cleaning Products?

In hot countries the brine may be left to evaporate naturally, whereas in cooler countries such as the UK, a process known as Vacuum Evaporation is used. A vacuum plant consists of a series of closed cylindrical vessels containing steam chambers of decreasing temperatures. As the brine boils and evaporates an increasingly thick brine-slurry begins to form. Slurry from the final vessel is sold in bulk for the chemical industry, where it will often be electrolysed to produce chlorine and caustic soda, both of which are then used in an array of processes including in the production of other chemicals and plastics, in the treatment of water, and as a disinfectant.

For culinary uses a drier salt is required. In these instances the slurry is subject to a final round of drying, utilising a system not dissimilar to a hair dryer, before being sieved, graded and stored ready for distribution. As a whole, this method is very inexpensive, has a high yield and produces a very clean salt.14,15, 16

Salt Flavours Today

Whilst almost all salt production around the world relies on one of these three methods, harvesting the mineral is just the beginning. Once out of the ground (or sea), sodium chloride can be subject to various rounds of processing, resulting in salts with differing textures, flavours and properties – and thus different uses when it comes to the kitchen.

Even back when the salt was first recorded nearly 5 millenia ago, there were more than 40 varieties available. Today, from the subtly indulgent truffle salt, or to the smoky flavoured Sal de Guasano, a Mexican salt with added dried worm larvae, the means by which basic sodium chloride can become be transformed into something much more exciting, are as numerous as they are varied.6

What is the most surprising salt you have come across, and have you ever thought of flavouring your own?

References

  1. The Salt Association (2019). “Food Manufacturing” Accessed 27 June 2019.
  2. Ruth; Dekker; Brouwer; Rozijin; Erasmus; Fitzpatrick (2019). “The sound of salts by Broadband Accoustic Resonance Dissolution Spectroscopy” Accessed 25 June 2019.
  3. The Freedonia Group (2014). “World Salt: Industry Study with Forecasts for 2018 & 2023” Accessed 27 June 2019.
  4. Food Executive (2016). “Global Salt Consumption to reach 335 Million Tons”. Accessed 27 June 2019.
  5. Loh (2008). “Know the Common Substance: Table Salt (Sodium Chloride, NaCl) Accessed 16 June 2019.
  6. McDowell (2017). “Mineral History, The Early Years’. Accessed 23 June 2019.
  7. Williams; Guilyardi; Madec; Gualdi; Scoccimarro (2010). “The role of mean ocean salinity in climate” Accessed 28 June 2019.
  8. Morton Salt (2019). “Salt Production and Processing” Accessed 26 June 2019.
  9. Zhiling; Guangyu (2008). “The Promotion of Salt Quality Through Optimising Brine Concentration a New Technique ‘”Bidirectional Brine Concentration”” Accessed 6 July 2019.
  10. Maraenco (2019). “Rock Salt Mining Process” Accessed 26 June 2019.
  11. Harraz (2014). “Underground Mining Methods: Room and Pillar method” Accessed 26 June 2019.
  12. Compass Minerals (2019). “How We Do It”. Accessed 26 June 2019.
  13. Bingham; Cohrssen; Powell. (2001). “Patty’s Toxicology Volumes 1-9, 5th Edition” Accessed 29 June 2019.
  14. Hudson Institute for Minerology (2019). “Sifto Salt Mine (Goderich Mine), Goderich, Huron Co., Ontario, Canada” Accessed 6 July 2019
  15. The Salt Association (2019). “White Salt Production” Accessed 29 June 2019.
  16. Encyclopaedia of Occupational Health and Safety (2011). “Chlorine and Caustic Production” Accessed 6 July 2019.