Description

In chemistry, hydrochlorides are salts resulting, or regarded as resulting, from the reaction of hydrochloric acid with an organic base (mostly amines). This is also known as muriate, derived from hydrochloric acid's ancient name: muriatic acid.

For example, reaction of pyridine (C5H5N) with hydrochloric acid (HCl) yields pyridine hydrochloride (C5H5N·HCl). Even though this style of formula is often used for denoting the hydrochlorides, the dot incorrectly implies that the two molecules are weakly bonded together; rather, what is present is the salt C5H5NH+ Cl- with the correct chemical name pyridinium chloride.

Pickling of steel

One of the most important applications of hydrochloric acid is in the pickling of steel, to remove rust or iron oxide scale from iron or steel before subsequent processing, such as extrusion, rolling, galvanizing, and other techniques.[6][*8] Technical quality HCl at typically *8% concentration is the most commonly used pickling agent for the pickling of carbon steel grades.

The spent acid has long been re-used as iron(II) chloride (also known as ferrous chloride) solutions, but high heavy-metal levels in the pickling liquor have decreased this practice.

The steel pickling industry has developed hydrochloric acid regeneration processes, such as the spray roaster or the fluidized bed HCl regeneration process, which allow the recovery of HCl from spent pickling liquor. The most common regeneration process is the pyrohydrolysis process, applying the following formula:[6]

By recuperation of the spent acid, a closed acid loop is established.[*8] The iron(III) oxide by-product of the regeneration process is valuable, used in a variety of secondary industries.[6]

Another major use of hydrochloric acid is in the production of organic compounds, such as vinyl chloride and dichloroethane for PVC. This is often captive use, consuming locally produced hydrochloric acid that never actually reaches the open market. Other organic compounds produced with hydrochloric acid include bisphenol A for polycarbonate, activated carbon, and ascorbic acid, as well as numerous pharmaceutical products.[*8]

Numerous products can be produced with hydrochloric acid in normal acid-base reactions, resulting in inorganic compounds. These include water treatment chemicals such as iron(III) chloride and polyaluminium chloride (PAC).

Both iron(III) chloride and PAC are used as flocculation and coagulation agents in sewage treatment, drinking water production, and paper production.

Other inorganic compounds produced with hydrochloric acid include road application salt calcium chloride, nickel(II) chloride for electroplating, and zinc chloride for the galvanizing industry and battery production.[*8]

Hydrochloric acid can be used to regulate the acidity (pH) of solutions.

In industry demanding purity (food, pharmaceutical, drinking water), high-quality hydrochloric acid is used to control the pH of process water streams. In less-demanding industry, technical quality hydrochloric acid suffices for neutralizing waste streams and swimming pool treatment.[*8]

High-quality hydrochloric acid is used in the regeneration of ion exchange resins. Cation exchange is widely used to remove ions such as Na+ and Ca2+ from aqueous solutions, producing demineralized water. The acid is used to rinse the cations from the resins.[6] Na+ is replaced with H+ and Ca2+ with 2 H+.

Ion exchangers and demineralized water are used in all chemical industries, drinking water production, and many food industries.[6]

Hydrochloric acid is used for a large number of small-scale applications, such as leather processing, purification of common salt, household cleaning,[*7] and building construction.[*8] Oil production may be stimulated by injecting hydrochloric acid into the rock formation of an oil well, dissolving a portion of the rock, and creating a large-pore structure. Oil well acidizing is a common process in the North Sea oil production industry.[6]

Many chemical reactions involving hydrochloric acid are applied in the production of food, food ingredients, and food additives. Typical products include aspartame, fructose, citric acid, lysine, hydrolyzed vegetable protein as food enhancer, and in gelatin production. Food-grade (extra-pure) hydrochloric acid can be applied when needed for the final product.[