referring to the drawings , reference numeral 51 in fig1 designates a source for carbon dioxide which , by the intermediary of a conduit 57 , is in communication with a rotameter 52 , a manometer 53 and a vessel 54 . the bottom 58 of the vessel 54 is a sintered body which finely divides the gas entering from the source 51 . during the carbon dioxide addition , the contents of the vessel 54 are agitated by means of an agitator 55 driven by a motor 56 . to 25 g of dry pulp , there was added 200 ml of black liquor . the mixture was left to stand for 48 hours . thereafter , the mixture was diluted with deionised water to one liter , i . e . to a 2 . 5 % pulp concentration . the suspension underwent agitation for one hour before the filtrate was removed . the pulp was then subjected to washing in four stages . in each stage , deionised water was added to a pulp concentration of 2 . 5 %. each washing stage lasted for one hour under agitation . all filtrates were kept for analysis . the amount of sodium in the washing water was determined using atom absorption and cod according to the dr . lange method . in the accounted experimental series , carbon dioxide was added in the second washing stage during 5 , 10 , 20 and 40 minutes , respectively . the carbon dioxide flow was constant and uniform in all experiments . because of the low degree of efficiency on the dissolution of carbon dioxide in these experiments , the quantity of co 2 has not been calculated . in industrial and other plants , equipment well - known to the person skilled in the art may readily dissolve the carbon dioxide in the washing water and pulp suspension . it will be apparent from table 1 that the total amount of washed - out sodium will be higher when co 2 is added in the 2nd washing stage . the effect becomes manifest already in the stage where the carbon dioxide addition took place , i . e . the 2nd stage . na + is more effectively washed - out as a result of the ph reduction which results from addition of co 2 . the cod washing - out procedure is more complicated . total amount of washed - out cod will be higher in an addition of co 2 in the 2nd washing stage . however , the effect is delayed and the improved washing effect does not become apparent until the 3rd washing stage . it is not clear why the cod washing - out improves in this way . one possible explanation is of surface chemical nature . the following will then apply : caco 3 is more sparingly soluble than ( rcoo ) 2 ca . table 1______________________________________co . sub . 2 addition in 2nd washing stage ( co . sub . 2 flow constant onaddition ) co . sub . 2 co . sub . 2 co . sub . 2 co . sub . 2 0 sample 5 min 10 min 20 min 40 min______________________________________na ( tot ) 1 . 14 1 . 26 1 . 23 1 . 18 3 . 28na ( 1 ) na ( 1 + 2 ) 1 . 10 1 . 21 1 . 19 1 . 16 1 . 22na ( 1 ) na ( 1 + 2 + 3 ) 1 . 13 1 . 25 1 . 21 1 . 17 1 . 24na ( 1 ) ph ( 2 ) 10 . 8 7 . 5 7 . 4 7 . 2 5 . 7ph ( 4 ) 9 . 4 7 . 6 7 . 8 7 . 6 7 . 1cod ( tot ) 1 . 20 1 . 29 1 . 26 1 . 32 1 . 32cod ( 1 ) cod ( 1 + 2 ) 1 . 11 1 . 13 1 . 13 1 . 12 1 . 07cod ( 1 ) cod ( 1 + 2 + 3 ) 1 . 17 1 . 21 1 . 19 1 . 23 1 . 25cod ( 1 ) ______________________________________ ## str1 ## relates to the quotient of total washedout na and washedout na in the first washing stage . correspondingly the example below relates to ## str2 ## cod washed out in first plus second stage through cod washed out in stage 1 . no carbon dioxide was added in stage 1 . ph ( 2 ) refers to ph after the second washing stage . calcium carbonate is deposited temporarily , calcium being &# 34 ; inactivated &# 34 ;. calcium soaps are insoluble in character and do not form lamellar phases to the same extent as sodium soaps . the surface activity in the system increases , which gives an improved washing - out of organic substance . a trial has been made to wash birch kraft pulp counter - currently on a laboratory scale . the purpose of this trial is to get closer to full scale conditions by using a counter - current method and to make this washing on birch kraft pulp having a higher extract content than pine kraft pulp . the washing steps are schematically shown in fig2 . during steady state condition a batch of pulp suspension is supplied to the first washing , step 61 , and washed by using the filtrate from the second step 62 . the pulp washed in the first step is thereafter supplied to the second step 62 and washed with filtrate from the third step 63 . the pulp washed in the second step thereafter is supplied to the third step in which said pulp is washed with filtrate from the fourth step 64 . in the fourth step the pulp is washed with filtrate from the fifth step 65 . in the fifth or last step 65 the pulp is washed with deionized water . for the washing of each batch there was used a total amount of 700 ml deionized water as a washing liquid . the washing for each step was made in a graduated cylinder and the pulp suspension was agitated during several minutes . the pulp slurry then was filtered using a buechner funnel and 700 ml of filtrate was taken out . for every batch of the pulp suspension 25 g of dry birch kraft pulp was mixed with 100 ml of black liquor . the suspension was allowed to stand at least 2 days before the washing was started in step 61 . during this time equilibrium conditions were reached or almost reached . two series of trials were made , one with carbon dioxide and one without carbon dioxide . in the first series no carbon dioxide was used . in order to build up a steady state washing system 16 batches were washed in each series . batch no . 1 was first washed in step 61 using 700 ml of deionized water , the obtained filtrate , 700 ml , was discarded . batch no . 1 was then taken to step 62 and washed with 700 ml of deionized water . this obtained filtrate of 700 ml was taken care of and used as washing liquid in step 61 for batch no . 2 . in the third washing step 63 , of batch no . 1 , the pulp was washed again with 700 ml of deionized water , the obtained filtrate , 700 ml , was taken care of and used as washing liquid for batch no . 2 in step 62 . the filtrate from batch no . 2 in step 62 then was used as washing liquid in step 61 for batch no . 3 . the filtrate , 700 ml , from step 1 was discarded . then batch no . 1 was washed in step 64 with 700 ml of deionized water and finally in step 65 with 700 ml deionized water . the filtrate , 700 ml from batch no . 1 in step 65 then was used as washing liquid for the next batch . thereafter deionized water was added only to step 65 and the filtrate from that step was used for washing the next batch in step 64 , etc . the filtrate from step 61 for the 12 first batches was discarded . then the last four filtrates from step 61 were taken care of , brought together to a comparative sample and analyzed . in the second test series carbon dioxide was added to step 65 . the carbon dioxide was added during the washing , which took some minutes and took place during stirring of the pulp suspension deionized water . the steady state condition was built up in the same manner as was done in the reference series without carbon dioxide . the last four filtrates from step 61 out of the 16 filtrates obtained were brought together and analyzed . in addition , the ph of the filtrates from all steps was measured when the steady state condition had been reached . in the following , filtrate 61 designates filtrate from step 61 , filtrate 62 designates filtrate from step 62 , etc . in table 2 the ph values of the filtrates after reaching steady state condition are shown . table 2______________________________________filtrate ph without co . sub . 2 ph with co . sub . 2______________________________________65 10 . 3 6 . 864 10 . 6 9 . 463 11 . 0 10 . 962 11 . 6 11 . 861 12 . 1 12 . 6______________________________________ washed pulp , i . e . pulp from batches 12 - 16 were brought together to one sample , and filtrate 61 was analyzed further . filtrate 61 designates the last four filtrates in each series containing 16 batches taken from step 61 brought together to an average sample . the results from the analysis of the filtrate are shown in table 3 . table 3______________________________________ without co . sub . 2 with co . sub . 2______________________________________na . sup .+ ( g / l ) 3 . 59 3 . 81ca . sup . 2 + ( g / l ) 0 . 033 0 . 033cod ( g / l ) 17 . 1 17 . 9______________________________________ cod = chemical oxygen demand the results from the pulp analysis are shown in table 4 . table 4______________________________________ without co . sub . 2 with co . sub . 2______________________________________na . sup .+ ( g / kg ) 2 . 00 1 . 34ca . sup . 2 + ( g / kg ) 1 . 10 1 . 17cod ( g / kg ) 10 . 19 9 . 37dkm extract (%) 0 . 10 0 . 08______________________________________ dkm = dichloromethane addition of co 2 to the last washing step has resulted in that more sodium ions and cod have been washed out of the pulp . the content of calcium ions has not been affected to any appreciable degree . the extractives content ( dichloromethane ) in the pulp is lower after washing with addition of carbon dioxide . a modified plant for washing of pulp in sulphate cooking is illustrated in fig3 . according to fig3 the pulp comes from a digester ( not shown ) through a conduit 1 to a blow tank 3 . the conduit 2 leads to a blow condensor . the pulp in the blow tank 3 is diluted with weak ( thin ) liquor via a conduit 4 from a weak liquor cistern 19 . an agitator in the blow tank 3 is designated 5 . the pulp from the blow tank 3 is passed through a conduit 29 to a knotter screen 6 . the pulp passes thence to the first washing filter 15 and subsequently to the three following washing filters 16 , 17 and 18 . the filtrate from the first filter 15 is collected in the weak liquor cistern 19 and the filtrate from the other three filters in the washing liquor cisterns 20 , 21 and 22 , respectively . the washed pulp departs from the fourth filter 18 at reference numeral 13 . when the pulp passes from one filter to the next , the filter cake is comminuted by shredders 8 . filtrate from one filter is used as washing liquid in the preceding filter and dilution liquid in the same filter . normally , pure water is added to the last filter 18 as washing liquid , entering through a conduit 12 . according to the present invention , carbon dioxide is introduced into the conduits 31 and / or 32 via conduits 23 and 24 , respectively . the carbon dioxide dissolves at once in the washing liquid and is led to the filters 17 and 18 , respectively through the conduits 31 and 32 and the conduits 10 and 11 . the filtrate from the filter 16 passes through a conduit 26 to the cistern 20 . a ph sensor device is disposed either in the cistern 20 or in the conduit 26 , the sensor controlling , by the intermediary of the control and regulation equipment , the supply of carbon dioxide to the conduit 31 . correspondingly , there is disposed a ph sensor device in the conduit 27 or the cistern 21 , this sensor being connected to control apparatus which maintains the ph at the predetermined level by adjusting the co 2 addition via the conduit 24 . water is added to the last washing filter 18 , this water being admixed with carbon dioxide and / or sulphur dioxide . a ph sensor device may also be disposed in the conduit 28 . it has been found that it is possible to reduce the wash losses , counted as na 2 so 4 , by approx . 1 kg per batched kg of carbon dioxide . a suitable amount of carbon dioxide is approx . 6 kg per tonne of pulp , it being thus possible to reduce the washing loss by approx . 6 kg per tonne of pulp . by further reducing the ph by an addition of a mineral acid , preferably sulphuric acid , and thus lowering the ph to 6 or less , even lower washing losses will be achieved . in the manufacture of ctmp and cmp pulp , the cellulosic material is pre - treated with alkaline treatment liquid and is disintegrated in one or more refiners , normally disc refiners . the pulp suspension passes thence to a screen room . the accept therefrom then passes to a washing plant , for example of the type illustrated in fig3 . using the method according the the present invention , it is possible to improve the washing results of all alkaline , cellulosic pulp , irrespective of whether the pulp is softwood / hardwood pulp or any other type of pulp , for example produced from bagasse . the present invention should not be considered as restricted to that disclosed above and shown on the drawings , many modifications being conceivable without departing from the spirit and scope of the appended claims .