PURPOSE: To recover high purity ammonium thiocyanate by simple operation by separating ammonium sulfate from waste liquor discharged by the wet desulfurization of coke oven gas, adding water or Ca(OH)2 to the resulting soln., and cooling the soln.

CONSTITUTION: Waste liquor discharged from a coke oven after wet desulfurization is treated by wet oxidation, and the treated liquor 5 is charged into a concentrator 1, concd., and fed to a centrifugal separator 2, where crude crystals 7 of ammonium sulfate are separated and recovered. The separated soln. 8 is introduced into a crystallizer 3, and after adding water, it is cooled with stirring to crystallize ammonium thiocyanate, which is then separated with a centrifugal separator 4 to take out purified ammonium thiocyanate. The separated soln. 10 is returned to the concentrator 1 and reused. The separated soln. 8 from the separator 2 may be mixed with Ca(OH)2, stirred, filtered, and charged into the crystallizer 3.