area of the contamination separation opening 4 is attained so that contamination can be better separated out and the area of the edge of the disintegrator roll can be kept free of fiber accumulations in fig2 a presentation of the fastening means 6 is presented in dotted lines the insert 5 can be secured to the side wall 18 of the disintegrator roll housing 1 with the aid of these fastening elements 6 on the left side of the insert as shown in fig2 can be seen the contact surface 53 as well as the outside of the contamination separation wall 42 the limiting wall 43 is designated as an invisible edge by the use of the dotted lines 431 fig2 further makes plain that the location of the contamination separation opening 4 is more or less freely chosen on the insert 5 likewise the length of the contamination separation opening 4 in its circumferential direction is more or less freely chosen fig3 shows a top view similar to fig2 of an insert in accord with the invention whereby however the insert 5 does not encompass the contamination separation 4 from all sides the lateral limitations of the contamination separation 4 take from the insert 5 of fig3 two forklike extensions 51 which carry on their ends contact surfaces 53 with which the insert 5 strikes on the abutment 41 see fig1 of the circumferential wall 11 the fastening not shown of the insert 5 of fig3 is.