Chiata Suni賈達蘇尼
17 minutes
Fluoride Wastewater Treatment: Precipitating Calcium Fluoride with Lime
Fluoride wastewater treatment removes the fluoride left after a solar factory etches and cleans its wafers. HF is used at many steps, and the rinse and scrubber water carry it out. Dissolved fluoride is harmful. At high levels it damages plants and fish, and over time it weakens human bones and teeth. The usual fix is to add lime (calcium hydroxide) or calcium chloride. Calcium ions meet fluoride ions and form calcium fluoride, which barely dissolves: Ca2+ + 2F- -> CaF2(s). The lime also neutralises the acid, raising the pH toward 7. The solid settles as a sludge, which is pressed into a cake. The cleaner water may go through a second step, often with aluminium salts, to reach the local discharge limit. The cake is landfilled, or it can be used as a raw material for cement or for making new HF. The trade-off is volume. Lime makes a lot of sludge, and some fluoride always stays in the water. Plants can cut the load by reusing etch baths longer and by recovering HF. After this Concept you can write the reaction that removes fluoride and judge whether a plant's fluoride report covers both its water and its sludge.