vessel 1 the black liquor may also be added to other sections of the impregnation vessel such as to an intermediate section of the impregnation vessel the high temperature of the black liquor 100 u00b0 c to 160 u00b0 c preferably exceeding 130 u00b0 c more preferred between 130 u00b0 c to 160 u00b0 c ensures rapid heating of the chips flowing through the impregnation vessel 1 in addition the relatively high ph exceeding ph 10 of the black liquor neutralizes acidic groups in the wood and also any acidic condensate accompanying the chips thereby i e counteracting the formation of encrustation so called scaling an additional advantage of the method of the present invention is that the black liquor supplied into the impregnation vessel 1 has a high content of rest alkali effective alkali ea as naoh at least 13 g l preferably about or above 16 g l and more preferred between 13 30 g l at the top of the impregnation vessel 1 this alkali mainly comes from the black liquor due to the high amount of alkali in the concurrent zone b of the digester 6 furthermore the strength properties of the fibers are positively affected by the impregnation because of the high amount of sulphide a major portion of the black liquor may be directly or via one flash fed into the impregnation vessel 1 a minor amount of the black liquor may be used for transferring the chips from the high pressure feeder 19 to the inlet of the impregnation vessel 1 the minor flow of the black.