Fluoride is a naturally occurring ion of the element fluorine, widely distributed in the Earth’s crust, soils, and water supplies. At low concentrations, the addition of fluoride to public drinking water is a long-standing public health measure intended to help prevent tooth decay. This practice has led to a common belief that the fluoride compounds added to water are simply waste products from aluminum manufacturing. This article clarifies the industrial sources of the fluoride compounds used in municipal water systems and explains the chemical relationship between fluoride and the aluminum production process.
Defining the Relationship Between Fluoride and Aluminum
The fluoride compounds currently used for municipal water fluoridation are generally not sourced directly from the waste streams of modern aluminum production facilities. The historical confusion stems from the fact that early water fluoridation efforts, such as the one started in Grand Rapids in 1945, often relied on sodium fluoride. At the time, sodium fluoride was a byproduct of aluminum smelting. This connection established the long-held association, but the dominant industrial source has since shifted.
The aluminum industry is actually one of the largest industrial consumers of fluoride compounds, rather than a primary supplier of the specific chemicals used in water treatment. Aluminum smelting requires fluoride compounds as a processing agent, and this use leads to the generation of fluoride emissions, which must be captured to meet environmental standards. These captured emissions are a type of fluoride byproduct, but they are chemically distinct from the compounds primarily used in water fluoridation today.
The Primary Industrial Source of Fluoride
The vast majority of industrial fluoride compounds used for water fluoridation come from the phosphate fertilizer industry, not aluminum manufacturing. Phosphate rock, the raw material for fertilizer, naturally contains a high concentration of fluoride, primarily in the mineral fluoroapatite. This phosphate ore typically holds between 2 and 4 percent fluoride by weight.
The process of creating fertilizer begins with the acidulation of the phosphate rock, most commonly using sulfuric acid. This chemical reaction is necessary to create water-soluble phosphate for agricultural use. During this “wet-acid” process, the fluoride content is volatilized, or released as a gas, in the form of highly toxic compounds like hydrogen fluoride (\(\text{HF}\)) and silicon tetrafluoride (\(\text{SiF}_4\)).
Because these gases are significant air pollutants, environmental regulations require that they be captured, or “scrubbed,” from the exhaust stacks before release. The water-based scrubbing system captures the volatile fluorides, which then react to form a concentrated liquid solution of fluorosilicic acid (\(\text{H}_2\text{SiF}_6\)). This recovery process is highly efficient and converts a hazardous waste into a marketable product.
Fluorosilicic acid is the main chemical compound that is purchased and used by water utilities across the United States for community water fluoridation. It is sometimes converted into its salt form, sodium silicofluoride (\(\text{Na}_2\text{SiF}_6\)), which is also used. The recovery of these silicofluorides from phosphate processing plants is the primary source of the specific fluoride compounds added to public drinking water supplies.
How Aluminum Production Uses Fluoride
Aluminum metal is produced almost exclusively through the Hall-Héroult process, which uses a high-temperature electrolytic reduction to separate aluminum from its oxide, alumina (\(\text{Al}_2\text{O}_3\)). This process requires the use of fluoride compounds to function efficiently. Specifically, the compound cryolite (\(\text{Na}_3\text{AlF}_6\)) is utilized as the primary electrolyte.
The purpose of the cryolite is to dissolve the alumina and significantly lower the melting point of the mixture. Pure alumina melts at over 2000°C, which would be prohibitively expensive to maintain, but the cryolite bath allows the smelting to occur at a much lower temperature of approximately 940 to 980°C. This reduction in operating temperature drastically lowers the energy consumption of the entire operation.
During the intense heat of the smelting process, some of the cryolite is consumed, and gaseous fluoride compounds are released. These emissions consist mainly of hydrogen fluoride and perfluorocarbons, which are strictly regulated air pollutants. Modern aluminum smelters employ highly sophisticated dry scrubbing systems that capture up to 99% of these fluoride emissions to comply with environmental laws.
While these captured materials are indeed fluoride byproducts of aluminum production, they are chemically complex and are generally recycled back into the smelting process or managed as industrial waste. They are not the primary feedstock for municipal water fluoridation, which relies overwhelmingly on the fluorosilicic acid recovered from the phosphate fertilizer industry.