Designing Proper Hydrofluorosilicic Acid Storage

Hydrofluorosilicic acid is a challenging chemical because it has properties that pose a danger and specific storage concerns. With its common use in water treatment, it is important to be aware of the risks of improperly storing this aggressive chemical.

Let’s take a closer look at the nature of hydrofluorosilicic acid, its applications, the specific storage concerns and solutions, and safety considerations when working with this complicated chemical.

What Is Hydrofluorosilicic Acid?

Hydrofluorosilicic acid is a chemical often known by other names like fluorosilicic acid and fluosilicic acid. It’s often abbreviated to HSA or FSA. The acid is a colorless chemical that is created when you take phosphoric rock from the ground and convert it to soluble fertilizer.

In this process, two very toxic fluoride gasses — hydrogen fluoride and silicon tetrafluoride — are released. The condensation from the hydrogen fluoride is collected and then scrubbed with water. 

The liquid collected in these scrubbers is hydrofluorosilicic acid. The acid is entered into storage tanks and shipped to water treatment facilities throughout the country. 

Depending on the manufacturer, impurities such as arsenic or lead can be found in HFA and are often not removed. While safeguards exist for regulating water safety, over time the contaminants may be a problem for your storage container. The acceptable levels of contaminants are governed by NSF International – ANSI/NSF 60, and American Water Works Association- AWWA B703-00.

How Is Hydrofluorosilicic Acid Used?

The most commonly discussed application for this chemical is water fluoridation at water treatment plants. This process helps prevent periodontal problems and is added to drinking water. Sodium fluoride is also commonly added to drinking water, but it can be five times as expensive. However, because hydrofluorosilicic acid can be more dangerous to store, it’s important to have a reliable and safe storage solution.

Other uses of FSA include:

• Disinfecting copper and brass vessels
• Gardening cement
• Preserving wood and impregnating compounds
• Electroplating
• Manufacturing aluminum fluoride, synthetic cryolite and hydrogen fluoride
• Sterilizing bottling and brewing equipment (1-2% solution)
• Etching glass

Storage Concerns and Solutions

Hydrofluorosilicic acid can be the most dangerous chemical at your local water treatment plant. It can release hydrogen fluoride when it evaporates, it’s corrosive, and it can damage the lungs if breathed in — making it especially dangerous for plant employees if stored incorrectly.

FSA also interacts negatively with metals to produce a flammable hydrogen gas — so a stainless steel chemical storage tank is not a viable option. It eats through concrete and attacks glass, so fiberglass tanks are a poor storage solution as well. 

In short, FSA poses a serious storage concern.

Before rotomolded plastic became a viable storage option, fiberglass tanks, constructed with a resin-rich veil, were often used for storage. The resin-rich veil, however, is often only ⅛” of chemical barrier protection from the incompatible fiberglass (chopped glass) structure itself. Since FSA cuts through glass, it’s actually incredibly dangerous to store FSA in something that only provides a minimal barrier of safety from a glass-made structural support container.

In these cases, a high-density cross-linked polyethylene (XLPE) storage tank is the safest option, and it’s best to choose one with secondary containment in the unlikely event of a leak. With linear polyethylene (HDPE), unzipping or cracking is possible, but with XLPE, the structural integrity of the tank will endure in most cases even if compromised. 

An XLPE tank with secondary containment, like Poly Processing’s SAFE-Tank®, can contain the chemical as well as the outlet to the pump transition from the primary tank. Not containing your fitting, the most vulnerable part of an otherwise robust system, is like having no containment in the first place. 

Enhanced Bellows Transition Fitting

A secure yet flexible, fully contained SAFE-Tank® bottom discharge.

By incorporating an expansion joint, the tank expands freely during loading and unloading, and it virtually eliminates damage from piping vibrations caused by pumps.

With this performance-maximizing fitting:

• Containment of the expansion joint eliminates the threat of uncontained chemical leaks and dangerous “spurts.”

• Piping layouts can be fully contained by connecting a dual-wall piping system onto the fitting. This can mean a safer workplace and less threat to the environment.

• Unsurpassed containment of discharge is allowed on a SAFE-Tank®.

The pressure-tested internal components of the fitting come to you pre-assembled and ready to install.

Another option is to place the pump fitting on top of the tank where the chemical can’t escape if a fitting fails. This, however, requires special design in the pumping system.

With the popularity of fluoridation in American water treatment plants, a tank with NSF-61 certification (and specifically for hydrofluorosilicic acid and not just potable water) is strongly urged. XLPE tanks are available with this certification. Always be sure NSF61 designations are for the specific chemical tested (not just water), as NSF offers certification by exact chemical according to Maximum Allowable Levels (MAL).

Hydrofluorosilicic Acid Storage Tank Requirements

Your chemical storage tank needs a reliable shut-off valve to isolate the pump skid, for regular pump inspection. The tank’s pump needs to be checked several times per year to ensure there is no line corrosion that could break and expose workers to the harmful effects of FSA.

An XLPE tank with a full-discharge IMFO Tank is also a good choice for storage of FSA because it can help prevent a build-up of deposits. One concern in storing FSA is arsenic build-up and other accumulated deposits, as discussed above. Some local EPA authorities will dictate special removal procedures of these tanks because of this. A full-discharge tank, however, will help prevent these deposits from building up.

Be sure that the full-discharge is flush with the bottom of the tank and contains no metal inserts for reasons discussed above.

Is Corrosion A Problem?

Many operators are concerned about HF gas released from concentrated H2SiF6 storage resulting in corrosion since water fluoridation will corrode pipes. Temperatures and concentrations for water fluoridation, however, ensure FSA achieves complete dissociation to fluoride, hydrogen, and silica (sand) and cannot produce HF. Silicates are actually used as a stabilizer for water corrosion. So, in solution, corrosion is not a concern — but venting is.

What Do You Need To Know To Store FSA Safely?

Follow these storage guidelines to ensure that your facility is safely storing FSA.

• All tanks should have a secured lid, so that there is no accidental opening. If someone opens a tank and drops a metal tool, for instance, dangerous hydrogen gas is produced by the reaction of FSA and metal. 

• It is incredibly important to keep the tank vented to the outside of the building. Keep the vent line clear of bird nests or any other blockage via a screen. Since FSA can be diluted by water, and cause metering accuracy issues, make sure the vent and system are water-tight, including containment. Ensure the storage area itself has adequate ventilation or air changes per hour. Seal all electrical and other conduits. 

• FSA is aggressive. Use corrosion-resistant materials. Fittings can be PE such as a B.O.S.S. fitting®, or PVC. Elastomers should be EPDM. Alloys should be C-276. 

• FSA will damage concrete surfaces. A dual application of epoxy undercoat with urethane topcoat provides corrosion resistance. Consult with the coating manufacturer for acceptable products for the surface tank to rest on. 

• Tanks should have good overfill protection to prevent accidents. There are several overfill prevention options commonly found in the market today. 

• Finally, FSA tanks should be marked or stenciled as containing hydrofluorosilicic acid. This can help to prevent any accidental cross-contamination.

For more information on storing this chemical, download the hydrofluorosilicic acid guide.