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Abstract

Apparatus and method for the mechanical refining of high consistency lignocellulosic feed material in a frustroconical refining zone, by facilitating the backflow of steam generated in the refining zone through the upstream feed zone which, despite the backflow of steam, imparts outward force on the feed material sufficient for the material to enter the refining zone and pass therethrough, without blockage or flow interruption. This is accomplished by providing, immediately upstream of the frustroconical refining zone (46), a hybrid feed/grinding zone (104), which acts on the wood chips with relatively high refining intensity at low refining power, so as to reduce the size of the material and outwardly convey the material by centrifugal force into the frustroconical refining zone against the backflowing steam, but without generating steam in the hybrid zone. The hybrid zone receives chips from a feed screw (84) which preferably rotates at the same speed as the rotor (20) portion of the frustroconical refining zone and hybrid zone. The feed screw is shaped (e.g., as a ribbon screw) so that the chip material is easily 'handed-off' to the entry of the hybrid zone, while establishing an open channel (166) between the chip material and the shaft of the feed screw, for conveying backflowing steam away from the hybrid zone. First pressure control means (136) are provided in fluid communication with the backflow steam channel, for removing the backflowing steam from the refiner at a controlled pressure, thereby controlling the retention time of the material in the frustroconical refining zone (146).

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