Double alkali charging for chip impregnation

A method for the impregnation of chips during the manufacture of chemical pulp, comprising the following steps:

• a) continuously feeding chips without preceding steam treatment to the top of an atmospheric impregnation vessel (1), from the bottom of which impregnated chips are fed out;
• b) charging a first charge of impregnation liquor comprising hot black liquor to the centre of the impregnation vessel via a first central pipe (7) having an outlet (8) located at the level of an upper withdrawal screen (3) located close to the impregnation liquor level established in the impregnation vessel and releasing steam from the hot black liquor for steaming the chips located above the impregnation liquor level;
• c) withdrawing an amount of spent impregnation liquor from the impregnation vessel (1) by the upper withdrawal screen (3) and sending the spent impregnation fluid with its content of released wood acidity from chip steaming to recovery;
• d) measuring the alkali content in the spent impregnation liquor withdrawn by the upper withdrawal screen (3) and comparing this measured alkali content with a target value for the alkali content;
• e) adjusting the alkali content of the first charge of impregnation liquor to the centre of the impregnation vessel (1) via the first central pipe (7) in accordance with the comparison between the measured alkali content and the target alkali content, to achieve a minimum target value for the alkali content in the liquor withdrawn by the upper withdrawal screen (3) said minimum target value avoiding precipitation in the spent impregnation liquor;
• f) withdrawing an amount of remaining impregnation liquor from the impregnation vessel by a lower withdrawal screen (4) and returning withdrawn impregnation liquor to the impregnation vessel via a second central pipe (9) having an outlet located at level of the lower withdrawal screen (4);
• said lower withdrawal screen located below the upper withdrawal screen at a distance shorter than the diameter of the impregnation vessel, establishing a retention zone for chips in impregnation liquid below said lower withdrawal screen well over 70% of the total retention time in the impregnation vessel;
• g) measuring the alkali content in the impregnation liquor withdrawn by the lower withdrawal screen (4) and comparing this measured alkali content with a target value for the alkali content;
• and
• h) adjusting the alkali content of the impregnation liquor withdrawn by the lower withdrawal screen (4) in accordance with the comparison between the measured alkali content and the target alkali content, to achieve a target value for the alkali content in the liquor withdrawn by the lower withdrawal screen (4), said target value providing sufficient alkali level for the remaining impregnation process below the lower withdrawal screen (4).