PURPOSE: To prevent mist from being allowed to flow out to a vent line by connecting the bent line to a casing which surrounds a rotary bowl and connecting a demister to the bent line and also equipping a recovery line of clarified liquid to the casing.

CONSTITUTION: Dissolving liquid is supplied to the inside of a rotary bowl 4 via a supply line 6 from a supply pot 2 and separated into solid and a soln. in the rotary bowl 4. Clarified liquid is discharged as a foggy state into a casing 3 from a discharge nozzle 5. The discharged clarified liquid is conveyed on the inner face of the casing 3 as liquid state and stored in a clarified liquid reservoir 15. Then this clarified liquid is passed through a discharge line 11 and recovered in a receiving tank 13. Sludge S is discharged from a discharge line 12 and the clarified liquid is allowed to flow through a vent line 7 and introduced into a demister 8 together with exhaust gas. Thereby enlargement of pollution is prevented.