the upper extraction zone 112 there are two vertically spaced sets of screens 126 through which the liquid i e liquor that has moved with the pulp downwardly through the impregnation zone 118 is drawn by a pump 128 the pump directs this liquor through a first heat exchanger 130 and thence upwardly into an inlet 132 to flow downwardly through a outermost pipe 134 positioned vertically in the center of the upper part of the digester 102 the exit end of the pipe 134 is indicated at 136 which is at the location of the first extraction location 112 thus it can be recognized that the liquid extracted at 112 is moved by the pump through the heat exchanger to raise its temperature and then back into the digester to flow into and through the pulp and thence be recirculated again through the pipe 128 the mixture of liquor and pulp that flows downwardly from the upper extraction location 112 goes through an initial cooking or delignification stage where a substantial portion of the lignin in the pulp is acted upon by the white liquor to cause a substantial portion of the lignin and other organic products to be dissolved in the liquor then when the mixture of pulp and liquor flowing downwardly from the upper extraction location 112 reaches the second extraction location 114 a portion of the black liquor is extracted through the two sets of screens 138 at the intermediate extraction location 114 by the action of a pump 140 the pump directs the extracted liquor through another heat exchanger 142 and the liquor is directed by valves 141a and 141b in either or both of two directions first a portion of the liquor flowing from the heat exchanger 142 is recirculated through a line 144.