Hydrofluoric Acid and Related Chemicals by: John Rabb

Hydrofluoric Acid and Related Chemicals by: John Rabb

Fluorine (symbol F) is the smallest atom in existence. It does not exist in nature as the fluorine atom but generally as stable, insoluble salts such as calcium fluoride (fluorspar or fluorite), sodium aluminum fluoride (cryolite) and calcium fluorophosphates (fluoroapatite) which is a significant source of phosphate for the fertilizer industry as well as various fluorides for the dental and water treatment industries.

When fluorine combines with hydrogen, hydrogen fluoride is produced. When this is dissolved in water, hydrofluoric acid is generated. Hydrofluoric acid is highly corrosive and is recognized for its ability to dissolve glass and other inorganic silicon-containing compounds. It is a rather weak acid meaning that in solution it does not readily produce free protons but stays associated as the HF molecule. This results in a solution with a pH considerably higher than hydrochloric acid at similar concentrations. This does not make it any less dangerous. HF is a contact poison which readily penetrates skin which can occur via direct contact, swallowing or inhalation. The symptoms of exposure are not immediately evident and if the exposure goes unnoticed causing delay of treatment, the extent and seriousness of the injury will become more severe. Simply flushing the exposure site with water is not enough. Specialized neutralization of the acid and internal treatment are necessary. Both the fluoride ion and hydrofluoric acid are equally dangerous and deserve ultimate care in handling.

Now it’s time to discuss containment of hydrofluoric acid and solutions of fluoride salts. Since both of these will attack both metal and glass, storage in wax and polyethylene containers is necessary for small quantities. Larger vessels require rubber lining. The polymer used in fabricating the lining must not react with HF nor may any fillers, process aids and reinforcers be used in the formulation which may react. These include silica, clay, talc and calcium silicate, among others. Some reinforcers such as barytes (barium sulfate) will react with HF because they may contain trace levels of silicon-containing contaminants while the barium sulfate itself is non-reactive. These strict guidelines have become necessary since the HF currently being stored and transported is now up to 70% concentration. Earlier it was at 40% and was considerably less reactive.

Blair has two linings suitable for HF at all concentrations, VE536BC and CC4624. Both are 100% chlorobutyl linings with no HF-reactive additives. Even though they are the best available, their service life in 70% HF is considerably less than linings in nearly all other services. In transport vessels replacement every two years is not unusual and in storage vessels, a 10 year service life may be expected barring any upset conditions which may weaken the lining. If the lining has a natural rubber tie gum, the need for replacement becomes quite obvious when the HF penetrates the chlorobutyl portion of the lining and contacts the silica-containing tie gum. The HF will react with the silica producing gaseous reaction products which will cause blister formation between the metal and the rubber. Fluoride salt solutions such as sodium fluoride require the same linings and precautions but slightly longer service life may be expected.

Hydrofluorosilicic acid (hydrofluosilcic acid, FSA and HFS) is another commonly encountered commodity. It is much less reactive and toxic than hydrofluoric acid and is much less frightening to handle. It does not react with silicon-containing reinforcers. Soft natural rubber linings were suitable for earlier versions at 23% concentration. Now, 30% and greater levels are available. These require chlorobutyl linings such as VE621BC or LS582.

Occasionally lining for protection against fluorinated hydrocarbons may be requested. These are generally used as refrigerants and are in the Freon grouping. Nitrile or neoprene lining is commonly used for them but more specific information is needed before a suitable recommendation may be made.
In summary it may be stated that lining requests for protection against hydrofluoric acid and related fluorine-containing chemicals may involve some of the most toxic and reactive chemicals known as well some comparatively harmless and inert ones. Because of this, one must understand every aspect of the chemicals involved and not be hesitant to ask questions so that this understanding is complete. We at Blair Rubber are always ready to answer any such questions to the best of our abilities any time they may arise and we encourage our clients to engage us with queries and concerns at any time as it will benefit all parties involved to do so.

(NOTA BENE – This bulletin is intended to educate its readers with descriptions of various fluorine containing compounds and the tank linings needed for their effective containment. Due to the highly toxic nature of hydrofluoric acid and its salts, it is imperative that anyone wishing to use, store, transport or otherwise handle these compounds safely becomes familiar with and fully understands and follows any and all local, state, federal and international legislation pertaining to them.)