FACTS ABOUT FLUORIDE

GENERAL FACTS

• Fluoride is a form of the element fluorine, one of the most common substances found in nature.
• It is the 13th most abundant element found in the earth's crust and is also present in significant concentrations in some foods.
• Tea and sardines are especially rich in fluoride.

FLUORIDE IN THE ALUMINIUM SMELTING PROCESS

Two of around 100 naturally occurring minerals in which fluoride is a major component are cryolite and fluospar. Both are important in the aluminium smelting process.

The introduction of fluoride was a key factor in the success of the original Hall-Heroult method of extracting aluminium metal from alumina. Despite intensive research since the Hall-Heroult method was discovered in 1886, no commercially feasible process of smelting aluminium has been discovered that does not rely on the use of fluoride.

Due to the high temperatures of over 900 degrees Celsius involved in the smelting process, some gaseous fluoride evaporates from the surface of the cryolite bath. To minimise heat loss, the surface of the molten cryolite is covered so that a solid semi-permeable crust forms over the molten bath. The crust assists in the minimisation of fluoride emissions.

But even with this in place, some fumes can escape, especially if the crust is broken. This can occur when anodes are being charged or when aluminium is being tapped. Specially designed hoods fitted to each pot capture more than 95% of these fumes. The uncollected fumes are released in to the potroom building. Here, the fumes are diluted through ventilation before being discharged through the potroom roof vents.

The captured fume is conveyed to pot fume scrubbers where more than 99% of the collected fluoride is removed, using alumina as the scrubbing agent. The alumina and absorbed fluoride is recycled as part of the alumina feed for the pots.

Fluoride is also a component of the spent anodes, which are used in the manufacture of anodes. Some of this fluoride is released during the anode baking process. This fluoride is scrubbed from the exhaust gas stream using carbon bake scrubbers similar to those used to remove the fluoride from the pot fumes. Again, alumina is used as the scrubbing agent and once it is saturated with fluoride it is recycled to the aluminium smelting process.

The Impact of Fluoride on the Environment

In certain circumstances, fluoride in the air can damage vegetation. Some types of plants are more susceptible than others. Cattle grazing on grass subjected to fluoride emissions can damage their teeth, especially when they’re young, and can also accumulate fluoride in their bones.

Cattle are generally regarded as the animal species most sensitive to fluoride. Many other living creatures - including man - are much more tolerant of fluoride.

Like many other minerals, fluoride at low levels can be beneficial.

It is generally accepted by public health authorities that one of the most effective public health measures ever adopted has been the program of controlling the level of fluoride in drinking water at the level of one part per million (1 ppm). This is about the same concentration of fluoride that’s found in a cup of tea.

In areas where fluorinated water is supplied, children's teeth have suffered fewer cavities. The fluoridation of drinking water is practised by most Australian cities, including Newcastle. Fluoride is used as a medication to treat bone deficiency diseases like osteoporosis.