Hydrogen fluoride is a gas at room temperature.  Its solution in water is known as hydrofluoric acid.  Anhydrous hydrogen fluoride is the starting material for the production of most fluorine compounds.

Uses of hydrogen fluoride

Its main uses are:

• to manufacture groups of chemicals that can be used in refrigerators and air conditioners and as propellants in inhalers. Originally chlorofluorocarbons (CFCs) such as CCl₂F₂ were made for these purposes but when these very stable compounds escaped into the atmosphere they ended up in the stratosphere where they were photolysed and released chlorine atoms which lead to the decomposition of ozone. This caused depletion of the ozone layer, thus reducing the protection it offers Earth from the sun’s harmful UV rays.  Then hydrochlorofluorocarbons (HCFCs) such as CHClF₂ were made and used. These are less stable and so not as persitant in the atmosphere but some still did reach the stratosphere and damage the ozone layer. There are international agreements leading to the manufacture and use of CFCs and HCFCs being phased out.  Now hydrofluorocarbons (HFCs) such as CF₃CH₂F and CF₃CHF₂, which are not ozone depleting, are manufactured from hydrogen fluoride and used in refrigerators and air conditioners. However, although the amounts escaping into the atmosphere are small, HFC molecules do have a high Global Warming Potential and so do contribute to global warming.  CF₃CHF₂ is the propellant used in inhalers to deliver the drugs for the treatment for asthma attacks.
• to produce chlorodifluoromethane, CHClF₂, and chlorodifluoroethane, CClF₂CH₃, which are used to make tetrafluoroethene and 1,1-difluoroethene.  These alkenes are then used to make the fluorinated polymers, poly(tetrafluoroethene) (PTFE) and poly(1,1-difluoroethene) (polyvinylidene fluoride, PVDF), respectively.

Figure 1  One of the most important uses of PVDF is in the cladding of buildings.  It is used here to coat the Channel Four headquarters in London, designed by Sir Richard Rogers. By kind permission of Arkema.

• to produce fluoroaromatic compounds, e.g. 2-fluoroaminobenzene, 2,4-difluoroaminobenzene and trifluoromethylbenzene.  These compounds are intermediates for a wide range of drugs e.g. the antidepressant Prozac^© and insecticides e.g. Karate^©.  They are also used to make liquid crystals used in LCD television screens, computer monitors and mobile phone displays.

Figure 2  An electrolytic cell is used to manufacture fluorine.  As the fluorine is
produced hydrofluoric acid is added to make up the electrolyte.
By kind permission of Springfields Fuels Limited.

• as a mixture of 60-70% aqueous hydrofluoric acid with sulfuric acid for etching and polishing in the cut-glass industry and for silicon-based integrated circuits in the electronics industry.
• as a mixture of 70-80% hydrofluoric acid with nitric acid in pickling baths for stainless steel sheet and tubing.
• to manufacture alkylbenzene sulfonates, used as a surfactant in anionic detergents.
• to produce uranium(IV) fluoride, UF₄, from uranium oxide, U₂O₃ during the uranium enrichment process.
• to produce synthetic cryolite (sodium hexafluoroaluminate, Na₃AlF₆) which is the molten electrolyte in electrolysis cells used for aluminium production from alumina (Al₂O₃).
• as a catalyst in the manufacture of alkanes from smaller alkenes. Alkenes react with alkanes in the presence of acids, including hydrogen fluoride.

For example,

Manufacture of hydrogen fluoride

Hydrogen fluoride is produced from fluorspar, the commercial name for the mineral fluorite (CaF₂).  The mineral is widely distributed in workable deposits throughout the world - particularly in China, Mexico, Southern Africa and Russia. Annual world production of fluorite exceeds 4 million tonnes.
Fluorspar (20-80% calcium fluoride) is concentrated to 98% purity by flotation techniques to obtain 'acid-grade' material.  Silica is the principal impurity removed.

The acid-grade fluorspar is mixed with preheated concentrated sulfuric acid and the resultant slurry is fed continuously into large, rotary kilns.  The mixture of fluorspar and sulfuric acid is heated to 600 K for several hours.

The gases, mainly hydrogen fluoride, emerge from the end of the horizontal kiln, and are fractionally distilled in a column, termed the pre-scrubber.  Solids and sulfuric acid are removed and the hydrogen fluoride vapour is purified to >99.9% purity by distillation in copper or steel vessels, condensed and stored in steel containers.

The gaseous effluent is largely silicon tetrafluoride, which on reaction with water produces fluorosilicic acid, H₂SiF_6.

Fluorosilicilic acid is an important by-product from this and from the manufacture of phosphoric acid.  It can be neutralised with sodium hydroxide to form sodium hexafluorosilicate, used to fluoridate drinking water.  The acid is also used to make aluminium fluoride, used in turn in the manufacture of aluminium.

Calcium sulfate, generated in the manufacture of hydrogen fluoride, is known as synthetic anhydrite or fluoroanhydrite.  Synthetic anhydrite is now used in a variety of applications, including the manufacture of aerated concrete building blocks, and some cements.

The other major use for calcium fluoride is in the production of iron, steel and other metals.  Metallurgical grade fluorspar is used as a flux to lower the melting point of raw materials and to lower the viscosity of the slag which makes it easier to remove the impurities.

Date last amended: 30th March 2017