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Hydrochloric acid (HCl) is a strong mineral acid. It is probably the acid that is used the most by refiners. It is easy to get in most places.
As this is such a versatile tool for the refiner it is used in more places than is described here, for example in making stannous chloride. Here are some of the main usages in refining.
(Main article : Dissolving gold with aqua regia)
Together with nitric acid it makes up aqua regia that is used to dissolve gold. The classical formula is three parts HCl and one part HNO3, but often a mixture with less nitric is used an excess of hydrochloric acid doesn't affect precipitation of gold.
Hydrochloric acid and bleach
(Main article : Dissolving gold with hydrochloric acid and bleach)
An alternative to aqua regia is to use hydrochloric acid and bleach, the bleach breaks down in the acidic mixture and gives off chlorine gas. This has the power to dissolve gold but not as fast as aqua regia. Often used on gold foils or powders.
Hydrochloric acid and hydrogen peroxide
(Main article : Dissolving gold with hydrochloric acid and hydrogen peroxide)
Another alternative to aqua regia is to use hydrochloric acid and hydrogen peroxide, the hydrogen peroxide is a strong enough oxidizer to have the power to dissolve gold but not as fast as aqua regia. Often used on gold foils or powders.
Copper chloride process
(Main article : Copper chloride process)
In the copper chloride process (also called acid peroxide or AP) the acid plays a double role. It both dissolves the [[copper(I) chloride created in the leach and provides chloride ions when the copper (I) chloride is oxidized into copper (II) chloride)
(Main article : Washing gold powder)
After gold has been precipitated it usually requires washing. Concentrated HCl dissolves copper chlorides and when warm and concentrated also some silver chloride. By boiling the gold in HCl most of the contaminants are dissolved and can be washed away.
HCl is used to lower the pH to neutral after metals have been filtered off as hydroxides at basic pH
Hydrochloric acid is sold as several different products, for example concrete cleaner. Sometimes it can be found under the name muriatic acid, an old name and often with quite a lot of contaminants. It is still usable for refining, especially in the copper chloride process.
Maximum concentration that is commonly sold is 30%, lower concentrations are sold as low fume versions.
Making hydrochloric acid
Since hydrochloric acid is often easily available in most parts of the world there is seldom a need to make it yourself. But in the rare cases where there is no HCl available there are a number of processes that can be used for making HCl. Some are hard to do for an amateur, some are easier.
- Industrially HCl is produced in several processes that are hard to replicate in a small scale.
- In the chlor-alkali industry NaCl is electrolytically split up into H2, Cl2 and NaOH. The chloride and hydrogen is recombined in a "HCl burner" and absorbed into water, creating hydrochloric acid.
- In organic chemistry industries HCl is often created as a by-product while making chlorinated and fluorinated organic compounds.
- By mixing a chloride salt like NaCl or CuCl2 with sulfuric acid. The released HCl gas is absorbed into cold water.
- By mixing salt with sodium bisulfate and heating the mixture to 200 °C : NaCl + NaHSO4 → HCl + Na2SO4 The released HCl gas is absorbed into cold water.
A video of HCl production on a lab scale.
- Nurdrage : Making hydrochloric acid from from sodium bisulfate (or sulfuric acid) and table salt
- Nurdrage : Recovering HCl and copper from copper chloride etchant