PURPOSE: To recover NH4HF2 or NH4F as products from an etching waste liquid containing fluorine compds. by adding hydrofluoric acid or ammonia to the waste liquid to effect the reaction with the fluorine compds. in the liquid, crystallizing, and then separating the crystal from the liquid.

CONSTITUTION: An aq. soln. of HF or NH4OH is added to an etching waste liquid containing fluorine compds. or hydrofluoric acid produced in the production process of semiconductors to produce NH4HF2. Then this aq. soln. is heated, evaporated, concentrated and cooled to precipitate crystals of NH4HF2, which is then separated from the soln. with a separator. Thus, NH4HF2 is obtained as products. Or otherwise, NH3 gas is added by the amt. more than the equiv. amt. of the reaction to the HF component in the etching waste liquid to produce NH4F. Then this soln. containing NH4F is heated at rather low temp. as about 80°C under reduced pressure, evaporated, and concentrated to precipitate crystals of NH4F, which is then separated and recovered from the soln. Thus, harmful F is recovered in the form of NH4HF2 or NH4F from an etching waste liquid without producing sludge, while the residual liquid is made harmless.