How to purify table salt from rock salt

Rock salt is the common name for the sedimentary mineral halite . This mineral is composed mainly of sodium chloride or table salt, but it is also associated with other minerals such as silvina (potassium chloride), and carnallite (double chloride of potassium and magnesium hexahydrate).

This form of salt is found in mineral deposits throughout the world. It is a common source of table salt, especially in countries that do not have access to the sea or other sources of salt water from which to obtain the indispensable seasoning. However, because it is extracted directly from the ground, it is very common for rock salt to be contaminated with various unwanted minerals, as well as water-insoluble rocks that can make it dangerous to use in cooking.

Common Rock Salt Contaminants

As previously mentioned, halite or rock salt is naturally associated with other minerals such as silvina and carnallite. For this reason, rock salt is almost always contaminated with potassium and magnesium chloride. In addition to these salts, it is common to also find other water-soluble salts such as sodium sulfate and magnesium sulfate, among others.

It is possible to find other less soluble and even insoluble contaminants such as other minerals and rocks, including silica (main component of sand) and gypsum (hydrated calcium sulfate).

In view of the above, it is important to purify rock salt before consuming it. Below is a simple procedure by which we can purify rock salt, turning it into table salt suitable for consumption.

The procedure consists of two stages, one to remove insoluble impurities and the other, which is optional, to further purify the sodium chloride for better quality table salt. Both processes are based on the difference in solubility of the different components of rock salt.

First stage: removal of silica and other insoluble impurities

Necessary materials and equipment

• 2 large beakers of at least 1L capacity or, failing that, a large saucepan.
• Coffee filters.
• Funnel.
• Funnel holder.
• Large mortar or coffee grinder.
• Wide flat skillet (a large paella pan or something similar would be ideal).
• Lighter, gas stove or other heat source.
• Kitchen scale.
• Big spoon.
• Kitchen gloves.
• Safety glasses

Reagents or chemicals

• 250 g sample of rock salt to be purified.
• 1 L of pure water, preferably distilled.

Procedure

1. With the help of the mortar, crush the rock salt until it is reduced to relatively small pieces. It is not necessary to bring the sample to a very fine powder. Getting chunks around 1-5mm would be enough.
2. Weigh 250 g of the crushed salt on the balance and transfer it to the beaker or saucepan.
3. Measure approximately 700 mL of distilled water and add it to the rock salt.
4. Heat the mixture with the burner or the gas stove. With the help of the spoon, stir frequently to speed up the dissolution of the salt. A nearly saturated solution should be obtained.
5. While the mixture is heating, prepare the funnel with the coffee filter and place it in the funnel support and place the second beaker below it to collect the filtrate.
6. Once the water is boiling and with the help of kitchen gloves to avoid burns, pour the solution through the funnel. Carry out this step slowly to avoid splashing and to prevent the solution from overflowing the filter paper.
7. Once all the solution has been filtered, the hot filtered liquid is transferred to the pan (or paellera) to evaporate all the water present and thus recover the purified salt. The filter paper with the insoluble solids can be discarded.
8. The pan is heated with the burner or on the stove to a gentle boil and boiled for long enough for all the water to evaporate. At a certain point before all the solvent is gone you should see salt crystals forming around the edges of the pan. This is perfectly normal.
9. Once the water has evaporated, let it rest until it cools completely, then, with the help of a spatula, remove all the salt, crush it in a clean and very dry mortar. Weigh the solid with the balance to determine the performance of the purification process and store the purified rock salt in an airtight container.

Second stage: further purification by recrystallization

The above procedure makes it possible to remove all insoluble impurities such as any trace of gypsum, silica or sand and any other rock or solid that could contaminate the original rock salt. However, it does not allow to eliminate those impurities consisting of salts soluble in water.

However, sodium chloride, which is the main component of halite, is actually less soluble than some of the more common soluble contaminants in rock salt. This allows them to be separated by the recrystallization process.

Necessary materials and equipment

• 2 large beakers of at least 1L capacity or, failing that, a large saucepan.
• Coffee filters.
• Funnel.
• Funnel holder.
• Lighter, gas stove or other heat source.
• Kitchen scale.
• Big spoon.
• Kitchen gloves.
• Safety glasses.

Reagents or chemicals

• Sample of approximately 250 g of purified rock salt (by the above procedure).
• A tablespoon of activated charcoal.
• 1 L of pure water, preferably distilled.

Procedure

1. Weigh 250 g of purified rock salt and transfer it to the beaker.
2. In the second beaker, heat approximately 700 mL of water to a boil.
3. Slowly and using oven mitts to avoid burns, pour small portions of hot water over the rock salt and stir with the help of the large spoon. Add just enough water to completely dissolve all the salt, avoiding adding too much excess water.
4. Once the salt is completely dissolved, heat the solution to a boil, remove from the heat and add the activated carbon. Stir with the help of the spoon.
5. Filter while hot, collecting the filtrate in the second beaker. If activated carbon is found to have passed through the filter, it should be filtered again, this time using two filters simultaneously .
6. Boil the solution to reduce the solvent to the point where you observe the formation of small salt crystals around the edges of the container.
7. At this point, add a small additional amount of pure water and continue heating until all the salt is dissolved again (5 to 10 mL of water should be sufficient).
8. Remove from the heat, cover the beaker to prevent any dust particles from contaminating the solution, and allow to stand until completely cool. On cooling, the solution should become supersaturated, causing crystallization of sodium chloride. It is important not to disturb the dissolution during the crystallization process, since the slower it is done, the purer the crystals obtained will be.
9. Once at room temperature, carefully take the container to the refrigerator and let it rest for at least 24 hours (although longer is better). This is done to cool the solution and also to allow the water to slowly evaporate, further reducing the solubility of the salt in water. This increases the amount of purified salt that we can obtain.
10. Using a new, clean, dry coffee filter, filter the cold solution to recover the pure sodium chloride crystals. These should be completely colorless crystals, shaped like transparent cubes. Do not discard the filtrate as it contains a considerable amount of the dissolved salt. This can be recovered by completely evaporating all the water as in the previous procedure.
11. Allow the solid to dry on the filter paper in a cool place in the open air. Once no moisture is evident, and if desired, the sodium chloride can be dried further in an oven at medium temperature.

This procedure allows obtaining a small sample of sodium chloride of very good purity, but most of this compound remains in solution along with the other contaminants. In fact, generally less than 10% of the solid that was initially dissolved is recovered as crystals unless the solution is allowed to evaporate very slowly over a long period of time.

However, if high purity is what you are looking for and quantity is not that important, this method of purifying rock salt is ideal.

References

Merida, K. (nd). Material Properties . Your Virtual Teacher. https://www.guao.org/

Web minerals. (n-a). carnalita . Mineralesweb.ES. https://www.mineralesweb.es/halogen/carnalit.htm

Web minerals. (nd-b). Silvina . Mineralesweb.ES. https://www.mineralesweb.es/halogen/silvina.htm

What is rock salt? Is rock salt better than normal salt? (2020, April 15). Ittefaq Salt. https://www.ittefaqsalt.com/en/what-is-rock-salt-rock-salt-is-better-than-normal-salt/

Rocksalt Online. (2021, September 6). What Is Rock Salt? Online Rock Salt. https://www.onlinerocksalt.co.uk/knowledge-base/salt-basics/what-is-rock-salt

Salt Institute. (n.d.). Solubility of Salt at Various Temperatures . saltinstitute.org. http://www.chlorates.exrockets.com/nacl.html