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