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Designing Proper Hydrofluorosilicic Acid Storage

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Hydrofluorosilicic acid, or H2SiF6, is a challenging chemical because it has properties that pose danger and specific storage concerns. With its common use in water treatment, it’s important that you’re aware of the risks that storing this chemical improperly carries.

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

What is Hydrofluorosilicic Acid?

H2SiF6 is a chemical often known by other names, like fluorosilicic acid, fluosilicic acid, silicofluoride, and silicofluoric acid. It 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 gases are released with hydrogen fluoride being one of them; the other is silicon tetrafluoride. The condensation from this hydrogen fluoride is collected, then scrubbed with water.

In the past, the phosphate industry used to let these two gases vent freely into the atmosphere. This, however, caused severe environmental damage to downwind communities, including widespread cattle poisonings, scorched vegetation, and various human health complaints.

Eventually, as a result of both litigation and regulation, the phosphate industry installed “wet scrubbers” to trap the fluoride gases. The liquid collected in these scrubbers (hydrofluorosilicic acid) is entered into storage tanks and shipped to water departments throughout the country. Depending on the manufacturer, impurities (arsenic, lead) can exist and are often not removed. While safeguards exist for regulating water safety, the contaminants can be a factor over time for your storage container which we will address below. The acceptable levels of contaminants are governed by NSF International – ANSI/NSF 60-2002, and American Water Works Association- AWWA B703-00.

Applications of Hydrofluorosilicic Acid

  • 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. Another common chemical added to drinking water for the same purpose is sodium fluoride, but it can be five times as expensive. Hydrofluorosilicic acid (often abbreviated to HFS or FSA) can, however, be more dangerous to store, so it’s important to have a reliable and safe storage solution. 
  • Hydrofluorosilicic acid is also used in the production of the salts that can contain porcelains.
  • Another use of FSA is to etch glass; the extremely corrosive nature of the chemical is effective for this desired application.  We’ll get into storage options in the next section but for this reason, glass or fiberglass tanks are not good storage solutions when eating glass is “not” the intent.

Storage Concerns and Solutions

Hydrofluorosilicic acid is easily the most dangerous chemical at your local water treatment plant. It can release hydrogen fluoride when it evaporates, is corrosive, and 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, meaning a stainless steel chemical storage tank is not a viable option. It attacks glass, eats through concrete, and poses a serious storage concern. Before rotomolded plastic became a viable storage option, fiberglass tanks, constructed with a resin-rich veil, was 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 eats 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 event of an issue. With linear polyethylene (HDPE), unzipping (or a catastrophic tear down the side of the tank) is possible, but with XLPE, the structural integrity of the tank will endure 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. Another option is to place the pump fitting on top of the tank where chemical can not escape if a fitting fails. This, however, requires special design in the pumping system.

With the popularity of fluoridation occurring in most American water treatment plants, a tank with NSF-61 certification (and specifically for hydrofluorosilicic acid and not just potable water) should be included from the tank manufacturer. 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 Considerations

The 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 drain is also a good choice for storage of FSA because it can help prevent build-up of deposits. One concern in storing FSA is arsenic build-up, and accumulated lead as discussed above. Some local EPA authorities will dictate special removal procedures of these tanks because of this. A full drain tank, however, will prevent these deposits from building. Be sure that the full drain is flush with the bottom of the tank and contains no metal inserts for reasons discussed above. Precipitation of silica is a potential problem if dilution ranges get above 10:1. Again, a full drain or IMFO fitting will eliminate this concern.

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

  • A bulk storage container should never be used for additive feed supply. A day tank should always be used instead. For calculating size, a rough rule of thumb is typically around 3-4 gallons of FSA for every million gallons of water to be treated.  
  • It is recommended for tanks to be indoors if possible, often in their own room.
  • It is important to keep at least fifty pounds of hydrated lime on hand to clean up possible small spills. The lime can neutralize the acid.
  • 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 outside 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 coating manufacturer for acceptable products for the surface tank will rest on. 
  • Tanks should have good overfill protection to prevent accidents.Poly Processing has several overfill prevention options. 
  • 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.

Download Hydrofluosilicic Acid Guide

Topics: Chemicals