The invention relates to small pieces of carpet which are introduced into a first impeller type mixer together with an aqueous solution containing a separating salt in order to obtain a first suspension which is guided to a first mechanical separating step. A high-solid phase is extracted therefrom, as is a second high-solid phase which contains a polymer fibre material, and a liquid phase. The second high-solid phase is mixed with a separating solution containing water in an impeller type mixer and a second suspension is produced which is guided to a second mechanical separating step. A third high-solid phase is extracted therefrom, along with a phase rich in polymer fibre material and a liquid phase. An acid is introduced into the impeller-type mixer along with the separating solution containing water, said acid being stronger than H2 CO3, and the pH value of the liquid phase in the impeller type mixer is adjusted to 2-6.