Fluorine (F) is a naturally occurring element found in our air, soil, and water. Fluorine is among the most reactive elements on the periodic table and is rarely, if ever, found as elemental fluorine in our environment and is more known and observed as a fluoride in many different naturally occurring compounds.
Fluorides can be found in relative abundance in our daily lives, most commonly in our drinking water either added artificially as a dental aid or naturally occurring in some areas of the country. Most toothpastes are sold with sodium fluoride (NaF) added as a dental aid. The addition of fluoride to our drinking water supply and to toothpaste has been a source of well published controversy.
Fluoride is used abundantly in industry especially the semiconductor and nano-technology industries as a glass and silica etchant. The most common source of fluoride in industry is Hydrofluoric Acid (HF) although buffered oxide etch (BOE) containing ammonium bifluoride (NH4-HF) is relatively common as well.
Fluoride has a very high affinity for calcium (Ca) owning to its use as a dental aid and fundamental to its concern to the health of aquatic and terrestrial life. One of the most commonly occurring elements within our own bodies is calcium found in many forms including the primary structure for our skeletal system and as an essential electrolyte in our blood stream.
Bonding Energies (kJ mol-1)
CRC 74th edition
Exposure to excessive free fluorides can be fatal to humans and all life forms. In our body fluoride has the same natural affinity for calcium as it does in the environment and will form preferentially with calcium over all other cations, likewise calcium will form preferentially with fluorides over all other anions lending to its health hazard. Attacking bone structure fluoride will leave our bones a weak porous mass; in our blood stream fluorides will rapidly tie up the free calcium electrolytes leading to rapid heart failure.
Regulatory limits for fluoride in industrial wastewater streams range from a stringent 10ppm to no limit with no limit being not nearly as prevalent as in the years past. In areas where fluoride occurs naturally in the ground water or in areas in which fluoride is commonly used in industry fluoride limits tend to be more prevalent and tighter.
Digital Analysis Corp. manufactures a line of fluoride removal systems ranging in size from 50 gallons per day (GPD) to well over 300,000 GPD. Our fluoride removal systems are easily capable of reducing fluorides to under 10ppm and have the ability to meet a limit as low as 2ppm; although typical limits range from 10 to 25 ppm of total fluoride.
Total fluoride is a key metric in the design of any fluoride reduction system. A common mistake is to assume that if free fluoride is low that one has met their discharge permit criteria. A discharge permit will always state total fluoride with free fluoride being a metric that is rarely if ever used. Most fluoride analyzers on the market measure free fluoride only and not total fluoride as total fluoride is a difficult measurement to make in the field.
Fluorides can be removed from industrial wastewater streams, depending on their form using one of many different methods including:
Ca++ + 2F- ↔ CaF2 Ksp(250C) 3.45 x 10-11
Digital Analysis Corp. offers a detailed white paper thoroughly describing our fluoride reduction process in detail. For a copy of the paper send an email to: [email protected] and we will email you a copy upon request.
The fluoride removal process requires an abundant source of calcium (Ca); in industry the two most economical forms of Ca are lime [Ca(OH)2] or calcium chloride [CaCl2). Both can be used and both have merit. However, lime can be difficult to handle and, at best, is supplied as an emulsified slurry that tends to plug pipes, pumps, and settle in tanks. Most typically lime is supplied as a dry and somewhat corrosive powder that is difficult to handle. Additionally, as a result of the chemistry (see our white paper referenced above), the hydroxide demand of the fluoride reduction system is often met long before the Ca demand is met which means the pH ends up much higher than allowed for discharge and post pH adjust is required. Such is not the case for calcium chloride which is purchased as a liquid and is far easier to handle and deliver. Additionally CaCl2 will not drive up the pH and post pH adjustment is not necessary. For large applications lime has merit as the cost of operation will be less and liquid / solid separation is easier.