FLUORIDE for use in New Zealand’s water supply is a byproduct of the fertiliser industry, the New Zealand Institute of Chemistry (NZIC) says.

In a report prepared by the NZIC and available on its website, the extraction and production of the fluroide is explained.

An abridged version is presented below. Water fluoridation is an important preventive measure carried out in much of the western world.

This is usually done with one of three fluorine-containing chemicals (sodium fluoride, sodium fluorosilicate and hydrofluorosilicic acid), but this article focuses on hydrofluorosilicic acid as that is the chemical most commonly used in New Zealand for this purpose.

All manufacturers of superphosphate produce hydrofluorosilicic acid as a by-product.

Hydrofluorosilicic acid manufacture can be viewed as a two-step process, although in reality it is carried out in four steps to ensure that the right concentration of acid is obtained. The superphosphate production process results in the evolution of carbon dioxide, steam and SiF4 [tetrafluorosilane].

This SiF4 is an environmental pollutant and so is removed from the gas stream and used to produce fluorosilicic acid. The SiF4 is removed from the gas stream by contacting the gas with water droplets. This water hydrolyses the SiF4.

The resultant hydrofluorosilicic acid (H2SiF6) is used for fluoridating drinking water. Hydrofluorosilicic acid has several advantages. Being a liquid, it is easy to handle and to meter accurately into the bulk water. This acid is also the cheapest source of fluorine. Its main drawback is that it is a comparatively dilute source of fluoride.