Several sources claim that, during the refining process for table salt, bleach is used.

Anyway, table salt is refined. That means it’s highly-processed. It is bleached, filtered, and stripped of other naturally occurring trace minerals. Then chemicals are added to keep the salt from absorbing water and clumping.

As most salt in it's natural state is gray or pinkish in color, it probably never occurred to you that your table salt is bleached.

Part of the process for refined salt, or commercial table salt, involves the use of aluminum, ferro cyanide and bleach. [...] You are probably aware of the hazards to human health of chlorine, which is used to bleach the salt.

Heavily processed to eliminate trace elements, table salt is mined from underground salt deposits and is bleached, heated and contains an additive, calcium silicate, to prevent clumping.

On the other hand, Wikipedia doesn't mention any bleach:

The raw salt is refined to purify it and improve its storage and handling characteristics. This usually involves recrystallization during which a brine solution is treated with chemicals that precipitate most impurities (largely magnesium and calcium salts). Multiple stages of evaporation are then used to collect pure sodium chloride crystals, which are kiln-dried.

I interpret the claim to mean that a chemical bleach (e.g. sodium hypochlorite or hydrogen peroxide) is added to the salt during the refining process to whiten the contaminants. Is that true?

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    Given the most common bleach (sodium hypochlorite) is generally manufactured from refined salt (i.e. just sodium chloride rather than other salts), this seems pretty odd. Given that sodium chloride is naturally white, I'm not sure what the point here is. Also the whitening effect bleaches affect organic molecules, not inorganic molecules, however I guess there could be some other legitimate use of a some bleach chemical that I am not aware of. – Jack Sep 14 '16 at 4:57
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    My favorite part of this particular pile of woo is this quote: You are probably aware of the hazards to human health of chlorine, which is used to bleach the salt. Because there's no chlorine in NaCl, apparently. – ReasonablySkeptical Sep 14 '16 at 13:49
  • @ReasonablySkeptical there is no chlorine in NaCl. The chloride ion is a very different chemical from molecular chlorine. Hydride is different from molecular hydrogen, a superoxide radical is different from molecular oxygen, and elemental sodium is different from a sodium ion. In each case, one form is very reactive, the other is relatively inert. Try putting a little molecular sodium in a glass of water, and you'll see the difference... – De Novo yesterday

Technically speaking, refined table salt is not bleached, even if some chemicals, such as chlorine, are used in both the removing impurities from salt and in bleaching.

Bleaching refers strictly to whitening by converting chromophores, which are parts of molecules responsible for color, into substances that do not absorb light.

  • Oxidizing bleaches, which break the bonds in chromophores include:
    • Chlorine-based bleaches that act by releasing chlorine: calcium and sodium hypochlorite
    • Peroxide-based and other bleaches that act by releasing oxygen: hydrogen peroxide, benzoyl peroxide, sodium percarbonate and sodium perborate, peracetic (peroxoacetic) acid, potassium persulfate, potassium permanganate and bromates.
  • Reducing bleaches, which convert double bonds in the chromophore into single bonds, include sulfur dioxide, either as gas or from solutions of sodium dithionite (sodium hydrosulfite), sodium borohydride or sulfuric acid.

Bleaches used as whiteners in food industry, for example, flour-bleaching agents, include benzoyl peroxide, calcium peroxide, chlorine, chlorine dioxide, azodicarbonamide and nitrogen dioxide.

Natural sunlight and atmospheric oxygen have been also used for bleaching:

  • Sunlight: "Bleaching that involves either natural sunlight or artificial light has been used to remove stains from paper artifacts and to treat textiles" (Chemistry Explained).
  • Atmospheric oxygen: "Historically, flour whitening was done ‘naturally’ by allowing the freshly-milled wheat to sit for 1-2 months and get exposed to oxygen" (Bakerpedia).

According to Environmental Science and Pollution Research, 2020:

Refined salt (regular table salt) is usually highly refined—being heavily ground, with most of its impurities and trace minerals removed following treatment with chemicals (e.g., sulfuric acid or chlorine).

In the production of refined table salt, chlorine is used to remove certain impurities and trace elements, not to whiten them by changing chromophores, so, in this case, it is not a bleaching agent. The methods of removing impurities include filtration, evaporation and crystallization, but not bleaching (Thoughtco). The term "bleaching" is virtually never used in association with salt production on any US .gov website or a website of any salt producer.

Apart from sulfuric acid and chlorine, which other chemicals can be used in the production of refined table salt?

1) Other chemicals used in the process of removing impurities (calcium sulfate, magnesium sulfate, magnesium chloride, dirt, sediments, algae...) from salt can include barium, calcium hydroxide (Ca(OH)2), sodium carbonate (Na2CO3) and aluminium chloride (Salt Partners).

2) Chemicals that can be added to refined salt (Wikipedia):

  • potassium iodide as a source of iodine
  • dextrose (glucose) to stabilize the iodide
  • ferrous sulfate or ferrous fumarate as a source of iron
  • fluoride (mainly in South America, Switzerland, Germany)
  • folate (for pregnant women)
  • anticaking agents, such as calcium silicate, sodium ferrocyanide, sodium aluminosilicate and magnesium carbonate, to prevent clumping
  • Is the 'bleaching' that occurs to some substances due to exposure to sunlight and/or UV radiation a misuse of the term? – Roger Jan 21 at 15:41
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    @Jan I think you could make the case that leaving things exposed to UV or sunlight to remove colour would be a very valid use of "bleaching". If I trust wikipedia, the original use of "bleaching" was in reference to the sun doing the work. en.wikipedia.org/wiki/Bleach#History Also see bleachfield. I think calling the sun/UV a bleaching agent is accurate, or at least would have been by the original meaning of the word. – JMac Jan 21 at 20:00
  • @JMac Yes, whitening of fabrics and flour by exposure to sunlight (not only UV) or atmospheric oxygen has been historically used and can be called bleaching. – Jan Jan 22 at 10:19
  • Don't forget the addition of iodine to table salt, which is a legal requirement in some countries and common practice in others. – jwenting Jan 22 at 11:27
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    @Jan which are made from salt. Explaining why large parts of the early bulk chemical industry are located in places with ready access to salt mines: it was a key raw material for the bleach and related products they were making. – matt_black Jan 22 at 11:46

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