Abstract: A multi-stage, especially a two-stage mixing of two media of different viscosity, and a respective mixing nozzle provided therefor. The first medium of higher viscosity is divided into two or more partial streams. The first partial stream is passed through an injector, in which the second medium of lower viscosity is mixed in, in particular tangentially. In an adjoining mixing chamber, the second partial stream of the first medium is admixed into the flowing mixture of the first partial stream and the second medium of lower viscosity and envelops the same, preferably in an opposite or counter-twist to the flowing twist of the first mixture of the first partial stream and second medium. This is especially suitable for bleaching of cellulose with oxygen.

Abstract: Method and apparatus for delignifying chemical pulp by means of oxygen, in which an aqueous slurry of chemical pulp is formed, then mixed with a caustic agent, followed by contact with a delignifying fluid. Water is drained off the slurry without reduction of pressure and while maintaining temperature following which the resulting slurry is maintained under these temperature and pressure conditions for a discrete period of time. The thus-obtained treated slurry is then washed.