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Waste salt is any chemical salt which results as an undesired byproduct of an industrial process. It may also refer specifically to common salt as a component of industrial waste or pollution.

Sodium chloride

Due to the prolific industrial and chemical applications of sodium, chlorine, and their many chemical derivatives, many industrial processes and chemical syntheses yield a NaCl byproduct. This is economically undesirable, as NaCl is the raw material for the energy-intensive chloralkali process which is the primary source of industrial sodium and chlorine.

Waste salt is so uneconomical that for many commodity chemicals, avoiding it was a major impetus for the development of alternative production routes. Some examples of chemical processes:

• For phenol, the Dow process produced one equivalent of NaCl per equivalent of phenol. It was replaced by the cumene process, which coproduces acetone.
• For hydrazine, the Olin Raschig process produced one equivalent of NaCl per equivalent of hydrazine. It was replaced by the peroxide process, which only has water as a byproduct after regenerating its catalyst.
• For titanium, the Hunter process produced four equivalents of NaCl per equivalent of titanium metal. Its replacement, the Kroll process, instead produces two equivalents of magnesium chloride, a different waste salt which is easier to process.

For instance, the Dow process for phenol and Olin Raschig process for hydrazine both produce an equivalent of sodium chloride for each equivalent of the primary product, and have largely been phased out in favour of the salt-free cumene process and peroxide process. Such developments are often marketed as a form of green chemistry.

Other notable waste salts

The Grignard reaction typically requires work-up by aqueous acid, which results in an aqueous solution of various magnesium salts, typically halides.

The Kroll process reduces titanium tetrachloride to metallic titanium by using magnesium metal, producing two equivalents of magnesium chloride. Compared to the sodium chloride byproduct of the Hunter process, magnesium chloride is easier to separate from the titanium metal and has a lower decomposition potential. As both magnesium and chlorine are used in the production of titanium from ore, the salt is almost always electrolysed immediately upon production, with the chlorine being fed back into the chloride process and the magnesium reused in the next cycle of the Kroll process.