Method and an apparatus for producing nanocellulose

A method for producing nanocellulose, wherein cellulose based fiber raw material is processed mechanically to separate microfibrils, wherein

• the mechanical processing is performed by feeding a mixture of cellulose based fiber raw material and water at a consistency ranging from 1.5 to 4.5% at a feeding pressure through a ring-shaped refining gap having a width smaller than 0.1 mm and formed between refining surfaces performing a relative movement in the direction of the periphery of the ring, an inner refining surface and an outer refining surface, the diameter of the ring-shaped gap becoming larger in the direction of feeding the mixture;
• in the refining gap, the fiber raw material is subjected to processing forces varying in the direction of introducing said mixture, by means of refining zones provided one after each other in the feeding direction in the gap, whereby the refining surfaces differ in surface patterning and/or surface roughness;
• part of the mixture of fiber raw material and water is guided past the refining surfaces in the feeding direction to different points of the refining zone;
• and
• the width of the refining gap is maintained by the combined effect of the feeding pressure of the mixture of fiber raw material and water fed into the refining gap and an axial force of the inner refining surface.