chip outlet joins up with the feeding line 12 black liquor is fed to the feeding circulation from the last but one flash cyclone 30 which is the first one in the embodiment shown through the line 33 to the level tank 18 and onward through the line 20 to the return line 15 in addition black liquor is transferred from the middle extraction screen 27 of the digester to the impregnation circulation through a line 52 which is provided with a pump 57 and which is coupled between the line 29 and the circulation line 44 the impregnation of the chips in the impregnation vessel 3 takes place in cocurrent the whole time the impregnation liquid fed in at the top consists of warm black liquor and white liquor if so desired warm green liquor modified green liquor or another sulfide containing or sulfur containing solution can also be included in the impregnation liquid the material fed in at the top has a liquid wood ratio of 2 5 4 0 or greater by means of the circulation screen 43 the impregnation vessel 3 is divided up into a first cocurrent impregnation zone a and a second cocurrent impregnation zone b which begins with the circulation screen 43 the dwell time for the chips is at least 15 minutes in the first cocurrent impregnation zone a and at least 10 minutes in the second cocurrent impregnation zone b and so the overall dwell time can be at least 25 minutes the temperatures in the two cocurrent impregnation zones a b can be identical or different and lie within the range from 100 to 140 u00b0 c and 120 to 160 u00b0 c respectively for reasons of heat economy it is advantageous to maintain a higher temperature.