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By far the greatest use for fluorine is in the nuclear industry, to make uranium(VI) fluoride, needed to produce the U-235 isotope.  Its manufacture is therefore crucial for the nuclear power industry.

 

Uses of fluorine

Fluorine is required for the preparation of uranium(VI) fluoride and subsequent separation of the 235 and 238 uranium isotopes by gaseous diffusion.  The main ore of uranium contains uranium oxide, U2O3.  It is treated with hydrofluoric acid to form uranium(IV) fluoride, UF4.  This is purified and then converted to uranium(VI) fluoride, UF6, with fluorine.

Natural uranium oxide contains two principal isotopes of uranium, U-235 and U-238.  The resulting uranium(VI) fluoride is thus a mixture of 235UF6 and 238UF6.  The former isotope is the one that is required for nuclear power generation but is only 1% of the total. Diffusion is used to enrich the mixture to 3.5-5%.  The enriched gas is collected and converted to uranium oxide.  This oxide, in the form of pellets, is used in nuclear power stations.

This process consumes about 75% of the world fluorine production.  The second most important product is sulfur hexafluoride, SF6, a valuable gaseous insulator in high voltage equipment.

Fluorine is also used to produce fluorocarbons which are chemically inert, thermally stable and non-flammable.  They also have low toxicity, high electrical resistivity, high fluid density, low surface tension, and low thermal conductivity.  Their uses include vapour phase soldering of printed circuit boards, leak testing and thermal shock testing of electronics components. They are also used as refrigerants, dielectric fluids, sterilising agents and lubricants.  Important medical applications rely on their ability to dissolve large volumes of oxygen (organ storage, blood extenders and substitutes, liquid ventilation particularly for premature babies, and eye surgery).

Annual production of fluorine

World 17 000 tonnes1
U.S. 5 000 tonnes2

Data from:
1 Ullmann's Encyclopaedia of Industrial Chemistry 2000
2 Arista Tek

 

Manufacture of fluorine

Fluorine is obtained by the electrolysis of a solution of potassium hydrogendifluoride in anhydrous hydrofluoric acid.  Pure hydrofluoric acid is unsuitable for the process as it does not conduct electricity.  Aqueous acid solutions cannot be used as the hydroxide ions would be preferentially discharged as oxygen.  By using a mixture of potassium hydrogendifluoride and hydrogen fluoride in the ratio of 1 to 2.5-5, a low melting point, 340 K, is obtained, allowing low operating temperatures and thus less cell insulation is needed.

A typical cell has up to 40 anodes (ca 50 x 20 x 5 cm in size) and contains ca 1250 kg of electrolyte, producing 3-4 kg fluorine per hour. A complete installation contains from 6 to 60 cells.  The cell reactions are:

The cell operates at 360 K and cooling is effected by a water jacket operating at 350 K.  The anodes are made of hard carbon since graphite would rapidly disintegrate due to infiltration of the small fluorine atoms between the carbon layers.  The cathodes are made of steel.  The other materials used in the cell are mild steel and a nickel alloy (Monel), which are relatively unaffected by contact with fluorine or hydrofluoric acid.

Both gases leaving the cell contain up to 6% hydrogen fluoride by volume.  In some processes hydrogen fluoride is removed from hydrogen by passing the gases through sodium or potassium hydroxide solution:

The HF content can be reduced to approximately 0.2% by volume in the fluorine gas, by treatment with sodium fluoride:

The electrolyte is replenished by the addition of anhydrous hydrogen fluoride.  The fluorine is usually used immediately. Alternatively, it can be transported as a gas in stainless steel cylinders.

 

Date last amended: 10th October 2016

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