Chlohydric acid


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  • Hydrochloric acid is a strong inorganic acid that is used in many industrial processes such as refining metal. The application often determines the required product quality.

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 extrusionrollinggalvanizing, and other techniques. Technical quality HCl at typically 18% concentration is the most commonly used pickling agent for the pickling of carbon steel grades.
{\displaystyle \mathrm {Fe_{2}O_{3}\ +\ Fe\ +\ 6\ HCl\ \longrightarrow \ 3\ FeCl_{2}\ +3\ H_{2}O} }{\displaystyle \mathrm {Fe_{2}O_{3}\ +\ Fe\ +\ 6\ HCl\ \longrightarrow \ 3\ FeCl_{2}\ +3\ H_{2}O} }
  • The spent acid has long been reused 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:
{\displaystyle \mathrm {4\ FeCl_{2}\ +\ 4\ H_{2}O\ +\ O_{2}\ \longrightarrow \ 8\ HCl\ +2\ Fe_{2}O_{3}} }{\displaystyle \mathrm {4\ FeCl_{2}\ +\ 4\ H_{2}O\ +\ O_{2}\ \longrightarrow \ 8\ HCl\ +2\ Fe_{2}O_{3}} }
  • 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.

Production of organic compounds

{\displaystyle \mathrm {2\ H_{2}C=CH_{2}\ +\ 4\ HCl\ +\ O_{2}\ \longrightarrow \ 2\ ClCH_{2}CH_{2}Cl\ +\ 2\ H_{2}O} }{\displaystyle \mathrm {2\ H_{2}C=CH_{2}\ +\ 4\ HCl\ +\ O_{2}\ \longrightarrow \ 2\ ClCH_{2}CH_{2}Cl\ +\ 2\ H_{2}O} } (dichloroethane by oxychlorination)
wood + HCl + heat → activated carbon (chemical activation)

Production of inorganic compounds

{\displaystyle \mathrm {Fe_{2}O_{3}\ +\ 6\ HCl\ \longrightarrow \ 2\ FeCl_{3}\ +\ 3\ H_{2}O} }{\displaystyle \mathrm {Fe_{2}O_{3}\ +\ 6\ HCl\ \longrightarrow \ 2\ FeCl_{3}\ +\ 3\ H_{2}O} } (iron(III) chloride from magnetite)
{\displaystyle \mathrm {CaCO_{3}\ +\ 2\ HCl\ \longrightarrow \ CaCl_{2}\ +\ CO_{2}\ +\ H_{2}O} }{\displaystyle \mathrm {CaCO_{3}\ +\ 2\ HCl\ \longrightarrow \ CaCl_{2}\ +\ CO_{2}\ +\ H_{2}O} } (calcium chloride from limestone)

pH control and neutralization

  • Hydrochloric acid can be used to regulate the acidity (pH) of solutions.
{\displaystyle \mathrm {OH^{-}\ +\ HCl\ \longrightarrow \ H_{2}O\ +\ Cl^{-}} }{\displaystyle \mathrm {OH^{-}\ +\ HCl\ \longrightarrow \ H_{2}O\ +\ 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 pH control.

Regeneration of ion exchangers

  • High-quality hydrochloric acid is used in the regeneration of ion exchange resinsCation 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 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.


  • Hydrochloric acid is used for a large number of small-scale applications, such as leather processing, purification of common salt, household cleaning, and building construction.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.

Hydrochloric acid has been used for dissolving calcium carbonate, i.e. such things as de-scaling kettles and for cleaning mortar off brickwork, but it is a hazardous liquid which must be used with care. When used on brickwork the reaction with the mortar only continues until the acid has all been converted, producing calcium chloridecarbon dioxide, and water:

{\displaystyle \mathrm {CaCO_{3}\ +\ 2\ HCl\ \longrightarrow \ CaCl_{2}\ +\ CO_{2}\ +\ H_{2}O} }{\displaystyle \mathrm {CaCO_{3}\ +\ 2\ HCl\ \longrightarrow \ CaCl_{2}\ +\ CO_{2}\ +\ H_{2}O} }

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