For our printed circuit board plating plants, please let us know how to treat/recover fluoride-bearing effluents. Thanks. N.M.J.
For such a short question, one could respond with many pages. One of the key factors that you did not identify is the effluent discharge limit for fluoride.
The major reason why fluoride is much more difficult to remove from wastewater is that it forms so many more soluble compounds as compared to other contaminants, such as heavy metals. In many cases, fluoride is removed by precipitation with lime as calcium fluoride, but this would likely result in an effluent concentration of 10-20 mg/liter. If this meets your needs, great. If not, enhanced fluoride removal has been reported by using magnesium hydroxide in conjunction with lime, resulting in effluent fluoride concentrations of less than 1 mg/liter; this is attributed to the adsorption of the fluoride ion into the magnesium hydroxide floc.
Packed beds of activated alumina can be used to remove fluoride. Activated alumina processes are pH sensitive, and fluoride is best adsorbed below pH of 8.2. As a post lime precipitation process, activated alumina has been reported to reduce fluoride concentrations down to 2 mg/liter.
Ion exchange (IX), reverse osmosis (RO) and electrodialysis (ED) have all been used to remove fluoride, but they are quite expensive and are likely not worthwhile unless you need to achieve a very low fluoride limit or desire to achieve zero wastewater discharge and reuse the high-quality water that results from these technologies. However, all three processes produce a regenerant or reject waste stream that must be further processed, such as evaporation to reduce volume, and/or disposed off-site. There are many suppliers of these technologies that can be found in the 2002 Products Finishing Directory And Technology Guide.