Applications[ Pickling of steel
Hydrochloric acid is a strong inorganic acid that is used in many industrial processes. The application often determines the required product quality.
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. Technical quality HCl at typically 18% concentration is the most commonly used pickling agent for the pickling of carbon steel grades. Production of organic compounds
Fe2O3 + Fe + 6 HCl → 3 FeCl2 + 3 H2O
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:
4 FeCl2 + 4 H2O + O2 → 8 HCl+ 2 Fe2O3
By recuperation of the spent acid, a closed acid loop is established. The iron(III) oxide by-product of the regeneration process is valuable, used in a variety of secondary industries.
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. Production of inorganic compounds
2 CH2=CH2 + 4 HCl + O2 → 2 ClCH2CH2Cl + 2 H2O (dichloroethane by oxychlorination)
wood + HCl + heat → activated carbon (chemical activation)
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). pH Control and neutralization
Fe2O3 + 6 HCl → 2 FeCl3 + 3 H2O (iron(III) chloride from magnetite)
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.
CaCO3 + 2 HCl → CaCl2 + CO2 + H2O (calcium chloride from limestone)
Hydrochloric acid can be used to regulate the acidity (pH) of solutions. Regeneration of ion exchangers
OH− + HCl → H2O + Cl−
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.
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.
Na+ is replaced by H+
Ca2+ is replaced by 2 H+
Ion exchangers and demineralized water are used in all chemical industries, drinking water production, and many food industries