Aluminum Fluoride

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Aluminum Fluoride

OVERVIEW

Aluminum fluoride (uh-LOO-min-um FLOR-ide) is a highly stable compound that occurs as a white crystalline solid. It resists the action of even strong solvents, such as hot concentrated sulfuric acid. The compound often occurs as a hydrate containing one or more molecules of water of hydration. The most common of these hydrates has the chemical formula AlF₃·3.5H₂O, meaning that for every two molecules of aluminum fluoride in a crystal, there are seven molecules of water. The major uses of aluminum fluoride are in a variety of applications in the chemical industry.

KEY FACTS

OTHER NAMES:

Aluminum trifluoride

FORMULA:

AlF₃

ELEMENTS:

Aluminum, fluorine

COMPOUND TYPE:

Inorganic salt

STATE:

Solid

MOLECULAR WEIGHT:

83.98 g/mol

MELTING POINT:

1291°C (2356°F; begins to sublime above 1250°C [2280°F])

BOILING POINT:

Not applicable

SOLUBILITY:

Slightly soluble in cold water, soluble in hot water; insoluble in alcohol, acetone, and most organic solvents

HOW IT IS MADE

Three methods are available for the preparation of aluminum fluoride commercially. In the first method, alumina trihydrate (aluminum hydroxide; Al(OH)₃) is treated with hydrofluoric acid (HF). A second method of preparation is almost identical except that it takes place in the dry state between dry alumina trihydrate and gaseous hydrogen fluoride. The third method involves the addition of fluorosilicic acid (H₂SiF₆) to alumina trihdrate. The last of these methods is commercially attractive because fluorosilicic acid is obtained inexpensively as a by-product in the production of fertilizers and phosphoric acid.

COMMON USES AND POTENTIAL HAZARDS

The primary use of aluminum fluoride is in the production of aluminum metal. In that process, aluminum is extracted from one of its compounds (usually aluminum oxide) by passing an electric current through the molten (melted) compound. The addition of aluminum fluoride to the raw materials used in the process reduces the temperature at which the reaction occurs and improves the conductivity of the molten compound.

Some other uses of aluminum fluoride include:

As a catalyst in the synthesis of organic compounds;
To suppress the process of fermentation in the wine- and beer-making industries;

In the manufacture of ceramics and glass; and
As a flux in the preparation of glazes, enamels and in metallurgical processes.

Interesting Facts

Controversy exists about the health hazards of aluminum fluoride. On the one hand, the compound would appear to pose a relatively minor risk when ingested because it is so insoluble in water and other solvents. If it does not dissolve in the body, it can not get into the blood stream. On the other hand, it can cause health problems if inhaled or deposited on the skin. Opponents of water fluoridation argue that aluminum fluoride is sufficiently toxic to humans that it should not be added to public water supplies.

Words to Know

FLUX: A material that lowers the melting point of another substance or mixture of substances or that is used in cleaning a metal.
HYDRATE: A chemical compound formed when one or more molecules of water is added physically to the molecule of some other substance.
SYNTHESIS: A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.
WATER OF HYDRATION: Water that has combined with a compound by some physical means.

Most authorities agree that aluminum fluoride does not pose a health hazard when ingested in moderate amounts because of its low solubility. The compound may pose a threat if inhaled or deposited on the skin, however. In modest amounts, it may produce skin and eye irritations that range from moderate to severe; irritation of the nose, throat, and lungs, that may include nosebleeds; and asthma-like symptoms with coughing, shortness of breath, and tightness in the chest. Long-term exposure may result in weakness of the bones and stiffening of the joints.