Waste gas contg. hydrofluoric acid - purified by scrubbing with caustic soda soln. from which sodium fluoride crystals are recovered

摘要

Exhaust gases contg. F are finally purified by scrubbing with an alkaline absorbent, esp. NaOH- or KOH- soln. to obtain crystalline prods. contg. F2. A single process circuit is used for both scrubbing and crystallisation with a single scrubber soln. and the supernatant liq. remaining after crystallisation is completely returned to the scrubbing tower. From the tower, the liq. travels through an intermediate cooler to a crystallising vessel, and/or fresh NaOH or KOH soln. is added to the liq. before crystallisation so a constant pH is maintained in the alkaline region. Fresh water is pref. added continuously to the upper part of the scrubbing tower to saturate the gas and also undertake a final cleaning operation. The waste gas entering the environment after treatment. contains less than 1 mg F2/m3 of gas; and no waste water is generated in the process. The fluoride crystals obtd. are commercially useful.