Hydrofluorosilicic acid, or H2SiF6, is a challenging chemical because it has properties that pose a danger and specific storage concerns. With its common use in water treatment, it’s important that you’re aware of the risks of improperly storing this 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 gases — hydrogen fluoride and silicon tetrafluoride — are released. The condensation from this hydrogen fluoride is collected, then scrubbed with water.
The liquid collected in these scrubbers is hydrofluorosilicic acid. The 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. 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. But 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 ceramics (to increase hardness), disinfecting copper and brass vessels, hardening cement, etc., wood preservative and impregnating compounds, electroplating, manufacture of aluminum fluoride, synthetic cryolite and hydrogen fluoride, sterilizing bottling and brewing equipment (1-2% solution) and to etch glass. We’ll get into storage options in the next section but because it is used to etch glass, fiberglass tanks are not good storage solutions.
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, 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, 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 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 can be 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 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.
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) 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 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 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.
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?
- 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.