Introduction Fluoride limit value
Fluoride is a common component of industrial wastewater, which can come from various sources such as metal processing, fertilizer production or the semiconductor industry. Fluoride in high concentrations can have negative effects on the environment and human health, such as acidification of soil and water, damage to plants and animals or impairment of bone and dental health. There is therefore a fluoride limit value (or more precisely: monitoring value) of 20 and 50 mg/L for direct and indirect dischargers. This article presents some possibilities for fluoride removal in industrial chemical-physical wastewater treatment.
Chemical precipitation
The regular method for fluoride removal is chemical precipitation in a chemical-physical wastewater treatment plant, in which a precipitant such as lime (or Ca compounds), aluminum salts or iron (III) chloride is added to the wastewater to form insoluble fluoride compounds, which can then be filtered off or separated. The advantage of chemical precipitation is that it is simple and inexpensive and can achieve high fluoride removal rates. However, it also requires a high consumption of chemicals, produces large quantities of sludge and can change the pH value and conductivity of the wastewater. Ca compounds (Ca(OH)2, CaCl2) are commonly used in wastewater treatment to precipitate the fluorides as CaF2 after pH adjustment to pH > 7. According to Hartinger, p. 193ff, neutral salts, excessive alkalinity, complex fluorine compounds, etc. can cause CaF2 to redissolve, resulting in higher fluoride levels in the treated wastewater. This method often shows that the regulatory requirements of 50 or 20 mg/L of fluoride limit value cannot (or can no longer) be achieved.
Post-cleaning of fluoride via ion exchangers
One option for post-cleaning to comply with regulatory requirements, i.e. the fluoride limit value, is the ion exchanger. Due to a lack of selectivity, a commercially available anion exchanger does not make sense, as it would load itself inefficiently with all anions. However, in some constellations with a suitable wastewater matrix, there is the option of conditioning an ion exchanger with Al3+ on a case-by-case basis. In this form, the ion exchanger can bind F- with approx. 2-3 g/L resin and it can also be eluted from it again as part of any external ion exchanger regeneration.
However, aluminum and fluoride also have some chemical challenges. If they are added to the ion exchanger in an acidic environment, AlF3 is formed with increasing absorption and cannot be dissolved. There are also indications that alkali fluorides can react with AlF3 to form aluminum-containing fluorine complexes up to the aluminum trifluoride M3AlF6, which are certainly no longer absorbed by a cation exchanger. Furthermore, it can be seen that with increasing fluoride intake there is also an increase in fluoride discharge. The application is therefore advanced application-specific ion exchange technology and therefore individually adapted to the respective wastewater matrix as part of R&D.
Fluoride limit value Conclusion
Fluoride is a problematic pollutant in industrial wastewater that requires various methods of removal in order to comply with the regulatory fluoride limit of 50/20 mg/L. The choice of a suitable method depends on various factors such as fluoride concentration, wastewater composition, costs and environmental compatibility. In this article, several options for fluoride removal in industrial chemical-physical wastewater treatment were presented, each of which has advantages and disadvantages. An optimal solution could be a combination of different methods, such as precipitation and downstream ion exchange, to achieve the best possible fluoride removal.
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