Patent ID: 8911631

Claim:
A method for recovering fluorine, comprising: setting a first threshold value and a second threshold value lower than the first threshold value as two threshold values of a fluoride ion concentration in a fluoride ion-containing water to be treated; measuring the fluoride ion concentration in the water to be treated and determining whether the fluoride ion concentration is higher than the second threshold value; introducing the water to be treated to a precipitating device if the fluoride ion concentration is determined as equal to or higher than the second threshold value; measuring the fluoride ion concentration in the water to be treated introduced in the precipitating device; precipitating fluorine-containing precipitate in the precipitating device by reacting the water to be treated with a calcium source if the fluoride ion concentration is determined as equal to or higher than the first threshold value; precipitating fluorine-containing precipitate in the precipitating device by reacting the water to be treated with polyaluminum chloride if the fluoride ion concentration is determined as equal to or higher than the second threshold value and lower than the second threshold value; measuring the fluoride ion concentration in the water to be treated in the precipitating device and determining whether the fluoride ion concentration is lower than the second threshold value after the fluorine-containing precipitate is precipitated; mixing a filter aid comprising particles of a magnetic material and having a primary particle diameter of 0.5 to 5 μm with a dispersion medium to produce slurry; feeding the slurry to a solid-liquid separator provided with a filter to deposit a layer of the filter aid on the filter; introducing the water to be treated in the precipitating device to the solid-liquid separator thereby depositing the precipitate on the layer of the filter aid if the fluoride ion concentration in the water to be treated in the precipitating device is determined as lower than the second threshold value; removing the filter aid and the precipitate from the solid-liquid separator; separating the filter aid and the precipitate through magnetic separation; and reusing the separated filter aid.