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What is the waste flouride derivitive?
The term "waste fluoride derivative" typically refers to hydrofluorosilicic acid (also known as fluorosilicic acid or HFSA), which is a byproduct collected from the air pollution control systems—specifically wet scrubbers—of the phosphate fertilizer industry.
During the production of phosphate fertilizers, toxic fluoride gases such as hydrogen fluoride and silicon tetrafluoride are released when phosphate rock is processed.
To prevent environmental damage, these gases are trapped using wet scrubbers, resulting in the formation of hydrofluorosilicic acid, a liquid waste product.
Rather than being discarded, this industrial byproduct is purified and used for community water fluoridation in many areas, particularly in large cities.
The U.S. Environmental Protection Agency has supported this practice, stating that using hydrofluorosilicic acid for fluoridation helps minimize water and air pollution while providing a low-cost source of fluoride for public water systems.
Approximately 63% of the U.S. population receiving fluoridated water gets it from this source, with sodium fluorosilicate and sodium fluoride used less frequently.
How is fluorosilicic acid purified
Fluorosilicic acid purification in industrial processes often begins with its recovery from phosphate fertilizer production off-gases, where silicon tetrafluoride and hydrogen fluoride are absorbed using circulating fluorosilicic acid in a scrubber system, yielding a solution typically between 18% and 25% concentration.
This recovered fluorosilicic acid can then be further processed for purification and conversion into high-value fluoride products.
One method involves neutralizing fluorosilicic acid with ammonia or a potassium base, which hydrolyzes the acid and precipitates silica (as white carbon black) that is subsequently removed by filtration.
For instance, Do-Fluoride New Materials Co., Ltd. reacts fluorosilicic acid with a potassium base at 90°C, followed by ammoniation to pH 8–8.5, enabling silica removal and yielding an ammonium fluoride solution.
This solution is further processed by adding potassium fluoride, followed by concentration, crystallization, and thermal decomposition of potassium hydrogen fluoride to produce crude hydrogen fluoride, which is then purified into electronic-grade hydrofluoric acid meeting SEMI UPSS standards.
Another approach, used by Yunnan Yuntianhua International Chemical Co., Ltd., involves ammoniating fluorosilicic acid to form ammonium fluoride and precipitating silica, followed by reaction with fluorogypsum to produce high-purity calcium fluoride, which is then used in the conventional fluorspar process to generate hydrofluoric acid.
An alternative method described in German Patent 1,811,178 involves neutralizing fluorosilicic acid with ammonia, filtering out silica, precipitating ammonium fluoride using a metal salt, and then producing anhydrous HF via high-temperature decomposition.
Similarly, Picardie Aluminium Company in France produced calcium fluosilicate by reacting calcium chloride with hydrogen hexafluorosilicate, followed by thermal decomposition at 300°C–400°C to yield calcium fluoride and silicon tetrafluoride, with the calcium fluoride used for hydrofluoric acid production.
For waste streams containing fluorosilicic acid, a patented method (US5876685A) involves adjusting the pH to 8–9 to hydrolyze fluorosilicic acid into silica and free fluoride ions, followed by anion exchange using a Type 2 strong base resin in hydroxide form to adsorb fluoride.
The resin is then stripped, and the resulting anion mixture is distilled with sulfuric acid to produce ultrapure hydrofluoric acid (30–100% concentration), effectively separating fluoride from silicon and other contaminants.
This method is particularly suited for semiconductor industry waste, where ultrapure HF (metals <1 ppb, anions <10 ppb) is required.