Hydrogen chloride is a gas at room temperature. Solutions of hydrogen chloride in water are known as hydrochloric acid. Hydrogen chloride is widely used in the chemical industry as a reagent in the manufacture of other chemicals. Most of it is produced as a co-product of reactions involving chlorine.
Uses of hydrogen chloride
Among its many uses (Figure 1) in the chemical industries is in the production of 1,2-dichloroethane (ethylene dichloride), fluorocarbons, dyes and synthetic rubber. It is also used to make inorganic chlorides (for example, aluminium chloride).
Figure 1 Uses of hydrogen chloride and hydrochloric acid.
Other uses include the treatment of effluent, to control pH, and to regenerate ion-exchange resins.
Hydrochloric acid, rather than sulfuric acid, is usually used to clean the surface of steel ('pickling') as it leaves the surface of the steel in a better condition for further treatment such as coating and plating. Other acids are sometimes used to pickle stainless steel.
Hydrochloric acid is also used to produce chlorine. This is, in effect, recycling as most of the hydrogen chloride has been produced as a co-product when chlorine reacts with a reactant, for example in the manufacture of chloroethene (vinyl chloride), to make PVC. Acid, surplus to requirements (see below), can be electrolysed to form hydrogen and chlorine. A particular advance for this electrolysis, which is being used by Bayer in China, is the ODC (oxygen-depleting cathode) process.
Manufacture of hydrogen chloride
The vast majority of hydrogen chloride/hydrochloric acid is formed as a co-product. In the US, for example, hydrogen chloride/hydrochloric acid, as a co-product, accounts for about 90% of the total output, much of this is in the production of chloroethene via 1,2-dichloroethane, for the production of poly(chloroethene), PVC. This is then used to produce more 1,2-dichloroethane and is thus unavailable for other processes.
The manufacture of magnesium by electrolysis of magnesium chloride also produces the acid but this is again recycled.
Amongst other processes that produce hydrogen chloride/hydrochloric acid is the chlorination of hydrocarbons, for example the chlorination of methane to form chloromethane, ethane to chloroethane and benzene to chlorobenzene.
Some plants in which hydrogen chloride/hydrochloric acid is generated have been built close to plants which use the acid. For example, the hydrogen chloride/hydrochloric acid produced from the manufacture of isocyanates is then piped across to a PVC plant to produce 1,2-dichloroethane.
The acid is also produced in the manufacture of potassium sulfate (used as a speciality fertilizer, for example for citrus fruits and grapes) from potassium chloride and sulfuric acid.
A small amount of the acid is produced by burning chlorine in hydrogen. Both reactants are produced from the electrolysis of aqueous solutions of sodium chloride (brine). The reactants are heated together. The reaction is highly exothermic and great care is taken to control the reaction, mainly, by controlling the flow rates of the two gases.
Both hydrogen and chlorine are very pure, when manufactured by the electrolysis of brine. The resulting hydrogen chloride is either used as a gas or is absorbed in water in special vessels, usually made of graphite. It is stored as a concentrated solution (about 30% HCl) and is the purest hydrochloric acid produced.
Date last amended: 18th March 2013