Efficient method for decontaminating waste gases and waste water containing cyanide in a method for producing alkali metal cyanides

A method for producing alkali metal cyanides as solids comprises (i) an absorption step in the form of an absorption of hydrogen cyanide from a synthesis gas containing hydrogen cyanide in an aqueous alkali metal hydroxide solution to produce an aqueous alkali metal cyanide solution, (iii) a crystallization step in the form of introducing the alkali metal cyanide solution into an evaporative crystallizer, which is heated by heating, and in which a pressure below atmospheric pressure (below 1013 mbar) is provided, and (iv) a step of condensation of the cyanide containing vapors arising in step (iii) to form a cyanide-containing vapor condensate; wherein the step of the condensation (iv) is performed using a multistage steam jet compressor, which suctions the exhaust vapors out of the crystallizer.