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Technical aspects of water fluoridation
 
The manufactuing process of fluoride compounds used in the UK for fluoridation of public water supplies
 
Introduction

Fluorine is the thirteenth most abundant element found in the earth's crust. Fluorine is never found in a free state in nature, but is always in combination with other elements as fluoride compounds. When dissolved in water, these compounds dissociate into ions. Thus fluoride in water is present as the fluoride ion which is identical regardless of its origin.

There are two compounds of fluoride permitted for fluoridation of water supplies in the UK1:

  • hexafluorosilicic acid (H2SiF6) also known as fluorosilicic acid
  • disodium hexafluorosilicate (Na2SiF6) also known as sodium fluorosilicate

Fluoride chemicals for water fluoridation in the UK must meet the specification laid down in the Department of the Environment's Code of Practice on Technical Aspects of Fluoridation of Water Supplies.

 
Raw materials

Fluoride can be found in a solid form in minerals such as fluorspar, cryolite and apatite. Fluorspar (also called fluorite) is found in most parts of the world. It is a mineral containing between 30 to 98 percent calcium fluoride. Cryolite is a compound of aluminium, sodium, and fluoride. Because of its low melting point, it is preferred for industrial use, but is not a major source of fluoride for water fluoridation. Apatite is a deposit of a mixture of calcium compounds, including calcium phosphates, calcium fluorides, and calcium carbonates. Apatite contains between 3 to 7 percent fluoride and is the main source of fluorides used in water fluoridation.

Trace elements such as lead and arsenic are present in minute quantities in fluoride compounds. But, because of the very high dilution factor, fluoridation makes no measurable contribution to the concentration of these substances in the water supplies.

 
Manufacturing process

hexafluorosilicic acid (H2SiF6) [liquid]

  • Apatite is ground up and treated with sulphuric acid forming fluorine gas as a co-product.
  • The fluorine gas is reacted with water, forming a weak hexafluorosilicic acid (H2SiF6). The acid is then concentrated to 30% strength.

disodium hexafluorosilicate (Na2SiF6) [powder]

Hexafluorosilicic acid can readily be converted into various salts; one of these, disodium hexafluorosilicate, is the result of neutralising hexafluorosilicic acid with sodium carbonate, and precipitating the solid matter.

 
Common misconceptions
It is often claimed by opponents of fluoridation that the aluminium and phosphate fertiliser industries have a vested interest in promoting water fluoridation. It is worthwhile addressing those claims here.

The phosphate fertiliser industry
Apatite is the raw material used in the manufacture of phosphate fertilizers. Fluorine for the manufacture of fluorides for water fluoridation is recovered from this process. The manufacturing process of fluorosilicic acid for water fluoridation involves recovery of the product from a vapour phase, which ensures a high degree of purity. The product is therefore manufactured in tandem with the production of phosphoric acid, and can be described as a co-product. It would be equally valid to describe phosphate fertilizers as co-products of the manufacture of fluorosilicic acid, since each are valuable products.

Prior to 1987 when hexafluorosilicic acid was produced in the UK for water fluoridation, manufacture was intermittent to match market need and represented only about 10% of the total fluorine which could be extracted from the process. Since production in the UK ceased, supplies of hexafluorosilicic acid have been imported by chemicals companies to service the established market. The service is Quality Assured to BS5750, and the specification conforms to that given in the Department of the Environment's Code of Practice on Technical Aspects of Fluoridation of Water Supplies.

The aluminium industry

The basis of the claims that the aluminium industry promotes fluoridation in order to dispose of its toxic waste seems to be based on a tenuous link more than 40 years ago in the USA. In the 1940s, The Aluminium Company of America (ALCOA) acquired a chemical manufacturing plant which produced fluoride compounds by the method described above (not as a waste product of aluminium production). ALCOA was then using sodium fluoride as a catalyst in the aluminium smelting process, but soon replaced sodium fluoride with less costly fluoride compounds. ALCOA sold the chemical plant in the early 1950s; their last sales of sodium fluoride were in 19522. Nowadays, the aluminium industry is the largest user of fluoride compounds. Thus, the aluminium industry has no interest in promoting water fluoridation whatsoever.
 
Summary

Only two compounds of fluoride are permitted for artificial fluoridation in the UK: fluorosilicic acid (H2SiF6) and sodium fluorosilicate (Na2SiF6). These compounds are manufactured to exacting quality standards and must meet Department of the Environment purity specifications. Fluoride compounds used for water fluoridation are not now, and have never been, obtained as industrial waste.

 
References:
  1. Water Industry Act 1991
  2. Source NIDR US Public Health Service 1993
 
 
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The British Fluoridation Society provides evidence based information on fluoride and all aspects of water fluoridation including water fluoridation and infant formula, the extent of water fluoridation in the UK and worldwide, legal aspects of water fluoridation in the UK, the ethics of water fluoridation, support for water fluoridation including public opionon surveys, the cost and cost effectiveness of water fluoridation, water fluoridation and the environment, UK Politics and water fluoridation, the dental benefits of water fluoridation for both children and adults, the safety of fluoridation, dental health, health inequalities, and dental fluorosis.

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