FLUORIDE WASTEWATER TREATMENT (FWT)
 
ALSO KNOWN AS HF NEUTRALIZATION OR FLUORIDE REDUCTION

A white paper by Wastech Controls & Engineering, Inc.

HF, or Hydrofluoric Acid, is a high-volume chemical that is extremely corrosive. It is miscible with water with the release of heat and an acrid, irritating odor, forming a clear, colorless liquid. HF is a significant health hazard as a liquid, even in diluted quantities. HF vapor, or Hydrogen Fluoride gas, can cause severe injury through contact, inhalation or ingestion. Free fluoride ions will destroy soft tissue and damage bones.

Wastech offers standard designs for safe HF neutralization and one-step Fluoride Reduction. Fluoride wastewater treatment and fluoride reduction are increasingly important to the discharge authorities, and discharge limits from 7 to 15 ppm are now common. New chemistries, such as MetFloc™, allow for one-step treatments that will remove fluorides to 7-10 ppm. Reductions to 2 ppm are also possible with multiple treatment steps.

HF is used in at least eight industries in applications such as a catalyst in the petroleum industry, as an ingredient in common commercial cleaners to remove rust or spots, as an additive to rocket propellants, as a stripper to etch away silicone dioxide in semiconductor wafer production, as a cleaner to etch glass, quartz products or chrome, as a metal cleaner, as a pickling agent, and for various purposes in laboratories. Typical concentrations run from 2% to 60%, with 100 to 1,000 ppm up to 10,000 ppm seen in industry. Discharge authorities limit fluoride discharge levels to less than 20 ppm, with some localities demanding 10 ppm down to 2 ppm of fluoride.

HF is considered a weak acid, compared to common strong acids such as HCl (hydrochloric acid, HNO3 (nitric acid) or HClO4 (perchloric acid). The rest of the halogens form strong acids, so why doesn’t fluorine? The answer lies in the small size of the fluorine ion, which is a great deal smaller than the chloride ion. This give F an unusually high attraction to the H ion compared to Cl. Since HF eats glass, what do you store it in? Common plastics such as PP polypropylene, PVC, neoprene and polyethylene are compatible. Before plastics, HF was stored glass bottles coated with wax.

Fluorides can be removed from wastewater. A NEW breakthrough chemistry called MetFloc removes fluorides and heavy metals in one pH adjustment. Application experience shows that fluorides were reduced to 7.5 - 10 ppm in one pass. Ion-exchange resins, membrane technology and cross-flow membranes seldom work as satisfactorily. In addition, the sludge is produced in much smaller volumes than with traditional calcium treatment. Wastech Controls offers automated and semi-automated skid mounted packages that will remove fluorides down to between 2 - 15 ppm.

The MetFloc system is a four-stage process that is pH controlled in either a batch or continuous process. The wastewater should have an initial pH below 4. It begins (1) with the injection and mixing of the MetFloc chemistry. MetFloc HM12 is an alkaline blend that will be added until the pH reaches 8-9.5 depending upon the metals involved. (2) A selected polymer is then added to add in the floc formation. (3) The sludge is separated using a clarifier or settling tank and dewatered. (4) Clean wastewater is ready for discharge. Unlike calcium treatments where huge amounts of sludge can be generated, MetFloc chemistry only generates a small amount of sludge, which is easily dewatered. If additional treatment is required for lower discharge limits, an aluminum treatment agent can be added to the wastewater, forming aluminum fluoride. In some cases, calcium chloride is also used; however, aluminum exhibits a higher bonding strength with fluorine than traditional calcium.

Converting fluorides to calcium fluoride is tricky. The solubility of the reactant is low, so large amounts of the reactant are needed. The pH must be controlled, since optimal conversion occurs at a pH of 9.5-10. As the pH rises, large quantities of sludge can be produced with the excess reactant, since smaller amounts of the calcium ion are produced at the high pH levels. Our reactant is added in precise doses based upon pH control and temperature monitoring of the treatment. A two-stage process is required to allow complete coagulation and separation. This further reduces the fluorides to discharge levels.

The Wastech system is a completely automated system that is PLC controlled and monitored in the treatment area with a local HMI touchscreen and remotely using SCADA software. Alarm conditions can be communicated 24/7 to a beeper or cell phone, and historical data is maintained with data logging conforming to 21CFR Part 11.

Special HF electrodes must be used with standard industrial pH controllers, such as George Fischer Signet 8750. Redundant instrumentation insures continuous, accurate operation. Temperature is monitored since the reaction is exothermic. Retention time may vary due to chemistry changes and heat generation. Dual-tank designs and sufficient volumes are required for flexibility. The fluorides are monitored continuously, with an industrial total fluoride analyzer using ion-selective electrodes.

The Wastech HF neutralization or Fluoride Wastewater Treatment (FWT) system using the NEW MetFloc chemistry is skid mounted, easy to maintain, and fully automatic.