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, comprising the following steps:

• i) an absorption step, in which hydrogen cyanide from a synthesis gas containing hydrogen cyanide is absorbed in an aqueous alkali metal hydroxide solution to produce an aqueous alkali metal cyanide solution, wherein cyanide-containing waste gases arise from the absorption step;
• ii) a step of processing the cyanide-containing waste gases arising in step i), wherein
• iia) in a first combustion step, the cyanide-containing waste gases arising in step i) are subjected to a sub-stoichiometric combustion, in the sub-stoichiometric combustion the oxygen component in the combustion chamber being less than stoichiometrically required;
• iib) in a cooling step, a reaction mixture obtained in step iia) is cooled by introducing an aqueous liquid;
• iic) in a second combustion step, a reaction mixture obtained in step iib) is combusted by supplying further oxygen under super-stoichiometric conditions, in the super-stoichiometric conditions the oxygen component in the combustion chamber being greater than stoichiometrically required;
• iii) a crystallization step, in which the alkali metal cyanide solution is introduced into an evaporative crystallizer, which is heated by heating and in which a pressure below 1013 mbar is provided, wherein cyanide-containing vapors arise from the crystallization step;
• iv) a step of condensation of the cyanide-containing vapors arising in step iii) to form a cyanide-containing vapor condensate;
• v) a step of recirculation, during which the cyanide-containing vapor condensate obtained in step iv) is used as an aqueous liquid in step iib).