may be mounted such that it is adjustably rotatably mounted within the housing 12 from a first mounting position wherein the pin rotor is concentric with respect to the interior housing surface 25 as illustrated in fig6 b through a range of mounting positions to a second mounting position wherein the pin rotor is mounted in an extreme eccentric position with respect to the interior housing surface 25 as illustrated in fig6 a as should be understood numerous other variations of the geometry of the relief chamber can be used in place of those described hereinabove such as an elliptical housing or an obround housing providing two relief chambers the apparatus 10 of fig5 may be used as a flail type vertical contactor in a gaseous bleaching process when used in such a configuration the pin rotor 14 may be concentrically mounted within the housing 12 generally vertical contactors are not effective in a gaseous bleaching process because the high consistency pulp tends to fall through the vertical housing at a faster rate than desired to achieve effective bleaching to overcome this shortcoming it has been discovered that if the housing 12 is frusto conically shaped with converging interior wall surfaces 25 and the pin rotor 14 is rotated at a predetermined high velocity the wood pulp is contained within the contactor for a longer desired time period thereby achieving effective bleaching during operation of the apparatus of fig5 the high consistency wood pulp entering the housing 12 is thrown against the interior housing wall 25 and travels at high velocity in a circumferential direction around an upper housing portion the friction of the pulp on the surface 25 quickly decelerates the pulp and the pulp begins to fall such that the pins.