Double alkali charging for chip impregnation

Abstract

The invention relates to a method and an impregnation vessel where the bulk volume of impregnation liquor added is in form of hot black liquor having a residual alkali content. Hot black liquor is added in an upper central pipe supplying the hot black liquor as impregnation liquor and flashing off steam for driving off air from the comminute cellulose material. During this first steaming is the wood acidity released from the material and spent impregnation liquor with wood acidity content is withdrawn from an upper withdrawal screen to recovery, a lower withdrawal screen recirculates the remaining impregnation liquor to a second lower central pipe in order to equalize the alkali content well before the remaining retention time in the impregnation vessel. According to the invention is additional alkali added to the hot black liquor and the recirculated impregnation liquor respectively dependent on established alkali level in the liquor withdrawn from the upper withdrawal screen and the alkali level established below the lower withdrawal screen.

1 . 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wood acidity from chip steaming to recovery;   d) measuring the alkali content in the spent impregnation liquor withdrawn by the upper withdrawal screen ( 3 ) and comparing this measured alkali content with a target value for the alkali content;   e) adjusting the alkali content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 ).   
     
     
         2 . The method according to  claim 1 , wherein the first charge of impregnation liquor comprises additional charge of alkaline liquor with an alkali concentration higher than the residual alkali concentration of the hot black liquor. 
     
     
         3 . The method according to  claim 2 , wherein the additional charge of alkaline liquor comprises white liquor and wherein the amount of white liquor added into the first charge of impregnation liquor is regulated to achieve the target value for the alkali content withdrawn by the upper withdrawal screen ( 3 ). 
     
     
         4 . The method according to  claim 1 , wherein the second charge of impregnation liquor comprises additional charge of alkaline liquor with an alkali concentration higher than the residual alkali concentration in the impregnation liquor withdrawn by the lower withdrawal screen ( 4 ). 
     
     
         5 . The method according to  claim 4 , wherein the alkaline liquor comprises white liquor or any liquor with higher alkali concentration than the impregnation liquor withdrawn by the lower withdrawal screen ( 4 ) and wherein the amount of alkaline liquor added into the second charge of impregnation liquor is regulated to achieve the target value for the alkali content below the lower withdrawal screen ( 4 ). 
     
     
         6 . The method according to  claim 5 , wherein the alkali content established in the impregnation liquor by said second charge of impregnation liquor is measured in the flow of impregnation liquor withdrawn from the lower withdrawal screen ( 4 ). 
     
     
         7 . The method according to  claim 5 , wherein the alkali content established in the impregnation liquor by said second charge of impregnation liquor is measured in or at an outlet of the impregnation vessel ( 1 ). 
     
     
         8 . The method according to  claim 1 , wherein essentially all of the spent impregnation liquor withdrawn by the upper withdrawal screen ( 3 ) is sent to recovery (REC). 
     
     
         9 . The method according to  claim 1 , wherein essentially all of the liquor withdrawn by the lower withdrawal screen ( 4 ) is collected in a second collection volume ( 6 ) and is returned to the impregnation vessel ( 1 ) via a pipe ( 12 ) connected to the second collection volume ( 6 ) and the second central pipe ( 9 ). 
     
     
         10 . An atmospheric impregnation vessel ( 1 ) for the impregnation of chips during the manufacture of chemical pulp, comprising:
 an upper withdrawal screen ( 3 ) arranged at a first level in a vessel wall ( 2 ) of the impregnation vessel ( 1 );   a first central pipe ( 7 ) connected in one end to a source supplying hot black liquor and having an outlet ( 8 ) at the other end located at the first level of the upper withdrawal screen ( 3 );   a pipe ( 15 ) connected in one end to an alkali source (WL) and with an outlet connected in the other end to the first central pipe ( 7 ) and provided with a second regulating valve ( 16 ) for regulating an additional charge of alkali into the first charge of impregnation liquor charged to the impregnation vessel ( 1 ) via the first central pipe ( 7 );   a lower withdrawal screen ( 4 ) is arranged at a second level in the vessel wall ( 2 ), the second level being below the first level of the first withdrawal screen ( 3 ); and   a second central pipe ( 9 ) connected in one end to the lower withdrawal screen ( 4 ) and with an outlet ( 10 ) arranged at the level of the lower withdrawal screen ( 4 ), characterized in that   one pipe ( 15 ) is connected in one end to an alkali source (WL) and with an outlet connected in the other end to the first central pipe ( 7 ) and provided with a second regulating valve ( 16 ) for regulating an additional charge of alkali into the first charge of impregnation liquor charged to the impregnation vessel ( 1 ) via the first central pipe ( 7 );   one pipe ( 17 ) is connected in one end to an alkali source (WL) and with an outlet connected in the other end to the second central pipe ( 9 ) and provided with a third regulating valve ( 18 ) for regulating an additional charge of alkali to the second central pipe ( 7 );   means for detecting the alkali content in the spent impregnation liquor withdrawn by the upper withdrawal screen ( 3 ) and adjusting the additional charge of alkali into the first charge of impregnation liquor by regulation of the second regulating valve ( 16 ); and   means for detecting the alkali content in the impregnation liquor below the lower withdrawal screen ( 4 ) and adjusting the additional charge of alkali into the second charge of impregnation liquor by regulation of the third regulating valve ( 18 ).