FIELD OF THE INVENTION

The present invention relates to a method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor.

BACKGROUND OF THE INVENTION

In conventional kraft cooking implemented in the 1960-1970-ies in continuous digesters was the total charge of white liquor added to the top of the digester. It soon emerged that the high alkali concentrations established at high cooking temperatures was detrimental for pulp viscosity.

Cooking methods was therefore developed in order to reduce the detrimental high alkali peak concentrations at start of the cook, and thus was split charges of alkali during the cook implemented in cooking methods such as MCC, EMCC, ITC and Lo-Solids cooking.

Other cooking methods was implemented using black liquor impregnation ahead of cooking stages where residual alkali in the black liquor was used to neutralize the wood acidity and to impregnate the chips with sulfide. One such cooking method sold by Metso is Compact Cooking where black liquor with relatively high residual alkali level is withdrawn from earlier phases of the cook and charged to a preceding impregnation stage. One aspect of alkali consumption during the cooking process, i.e. including impregnation, is that a large part of the alkali consumption is due to the initial neutralization of the wood acidity, and as much as 50-75% of the total alkali consumption is occurring during the neutralization process. Hence, a lot of alkali is needed to be charged to the initial

neutralization. This establish a cumbersome problem as high alkali concentrations had been found to be detrimental for pulp viscosity when charged to top of digesters in conventional cooking. One solution to meet the high alkali consumption and necessity to reduce alkali concentration in top of digester was to charge large volumes of alkali treatment liquors, preferably black liquor having a residual alkali content, but having low alkali concentration, which resulted in presence of relatively large amount of total alkali per kg of wood material but still at low alkali concentration.

IN US7270725 (=EP1458927) Metso disclosed a pretreatment stage using polysulfide cooking liquor ahead of black liquor treatment. In this process was the polysulfide treatment liquor drained after the pretreatment stage and before starting the black liquor treatment. The polysulfide treatment stage was also preferably kept short with treatment time in the range 2- 10 minutes.

In a recent granted US patent, US7828930, is shown an example of a kraft cooking process where 100% of the cooking liquor, in form of polysulfide liquor also named as orange liquor, is charged to top of digester and start of an impregnation stage. Here is also the temperature raised from ΘΟ 'Ό to 120 ^ at start of the polysulfide treatment stage. However, as shown in example 1 is a liquor to wood ratio of about 3.5 established in the top of the digester by adding a proper amount of water. This order of liquor/wood ratio is often perceived as a standard liquor/wood ratio in continuous cooking necessary for a steady process. According to this proposal is a part of the residual polysulfide treatment liquor at relative high alkali concentration withdrawn and replaced with cooking liquor at relative low alkali concentration at start of the cooking stage, and the withdrawn residual polysulfide treatment liquor is added at later stages of the cook.

There has thus been an ongoing development of cooking methods where both alkali concentrations at start of cook is reduced, and increased yield from the cooking process is sought for using among others addition of polysulfide cooking liquor that stabilize the carbohydrates.

SUMMARY OF THE INVENTION The invention is based upon the surprising finding that concentration of polysulfide should be kept high in a low temperature pretreatment stage at relatively long retention time before cooking, using liquor to wood ratios well below that as commonly used. The stabilization effect of carbohydrates, the major objective for polysulfide addition, has shown to be improved dramatically if using a liquor to wood ratio of about 2,9 instead of the conventional liquor to wood ratio of about 3,5, and all other conditions equal. This non proportional effect of low liquor to wood ratio has not been disclosed or realized before despite the numerous proposals for improving cooking yield using polysulfide cooking liquor.

One object of the present invention is to provide an improved method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor, wherein the lignin-containing cellulosic material is heated to a temperature in the range 50-100 °C followed by adding polysulfide cooking liquor to a first impregnation stage which in turn is followed by cooking stages resulting in a kraft pulp with a kappa number below 40, and wherein the impregnation stage is conducted at high alkali concentration, low temperature and high polysulfide concentration using polysulfide cooking liquor at a liquor-to-wood ratio in the range 2.0 to 3.2 , and that the temperature is between 80-120 °C during a retention time resulting in a h-factor in the range 2-20 and preferably 2-10 of the impregnation stage. This low h-factor is indicative for that no cooking or delignification effect is obtained in the first impregnation stage, and hence is no reduction in pulp viscosity seen as could be the case if high alkali concentrations are at hand in cooking stages at higher temperatures.

According to one preferred embodiment of the method is the effective alkali concentration during the impregnation stage above 60 g/l when adding the polysulfide cooking liquor.

According to another preferred embodiment of the method is the polysulfide concentration during the impregnation stage above 3 g/l, or above 0,09 mol/l, when adding the polysulfide cooking liquor.

According to a further embodiment of the method is more than 90% of the total charge of cooking liquor needed for completion of the cooking stages to the intended kappa number below 40 charged to the first impregnation stage, and that at least 175 kg of alkali (EA as NaOH) per ton of chips is charged for softwood and at least 160 kg of alkali per ton of chips for hardwood.

According to yet another embodiment of the method is the alkali concentration reduced by at least 8 g/l by adding additional cooking liquids having less alkali concentration than the alkali concentration prevailing at end of the first impregnation stage when increasing the temperature to cooking temperature, said cooking liquids in at least part thereof includes black liquor.

In a most preferred embodiment of the method is no black liquor added to the first impregnation stage.

When using the inventive method has also preferably the white liquor added to the first impregnation stage an alkali concentration above 100 g/l and a polysulfide concentration above 4 g/l.

The lignin-containing cellulosic materials to be used in the present process are suitably softwood, hardwood, or annual plants. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cooking system capable of implementing the inventive method;

FIG. 2 demonstrate an example of the alkali profile established with the inventive method ; FIG. 3 show the dramatic impact on increased yield when increasing the polysulfide concentration above 0.15 mol/L

FIG. 4 show the relative stabilization of carbohydrates as a function of liquid to wood ratio during the impregnation stage.

DETAILED DESCRIPTION OF THE INVENTION

In figure 1 is shown a 2-vessel kraft cooking system, having a first hydraulic impregnation vessel B and a second steam/liquid phase digester C, wherein the inventive method could be implemented. In this type of system is first the lignin containing cellulosic material Ch fed to a bin A wherein the cellulosic material is heated to a temperature in the range 50-100 °C by using addition of steam St. The lignin containing cellulosic material could preferably be wood chips. From the lower part of the bin A is then the heated chips suspended in treatment liquor in a chute C located above the high pressure sluice feeder SF. The treatment liquor here is preferably only polysulfide cooking liquor, WL, and preferably is the entire charge of cooking liquor needed for the cooking process charged here.

The chips suspended in the treatment liquor are fed to the sluice feeder and displaced liquid is fed out from the bottom outlet of the sluice feeder and returned to the chute in a low pressure circulation. The chips in the sluice feeder is pressurized by the return flow from the vessel B and fed out to the top separator TS in top of the vessel B. Thus, the first impregnation stage is implemented in the vessel B and preferably only with the polysulfide cooking liquor and as small amount as possible of additional liquids such as wood moisture, steam condensates, and especially no black liquor nor additional water or filtrates. The resulting liquor-to-wood ratio established should be in the range 2.0 to 3.2 and the temperature should be in the range 80-120 ^. After the sufficient retention time in vessel B, which should have a retention time resulting in an H-factor in the range 2-20 of the impregnation stage, the impregnated chips will be fed to the steam/liquid phase digester C together with the residual treatment liquor. Here is shown a conventional transfer system with dilution in bottom of the vessel B using withdrawn treatment liquor from the top separator TS in the top of vessel C. At this point is the chip suspension heated to full cooking temperature, in the range 140-170 ^ depending upon type of cellulosic material, and additional liquid is added in order to reduce the alkali concentration at this point. In this embodiment is shown addition of black liquor obtained from a screen section withdrawing black liquor and sending a part of this black liquor to recovery REC. Hence, no detrimental effects upon pulp viscosity would occur by this dilution with black liquor. In this embodiment is shown a digester C with 2 concurrent cooking zones, one cooking zone above the first screen section and a second cooking zone above the final screen section in bottom of digester. In a conventional manner is a final counter current wash zone implemented in bottom of digester by addition of wash water/Wash. The final pulp with a kappa number below 40 is fed out from bottom in flow Pu.

In figure 2 is disclosed the alkali concentration profile that could be established in a system like that disclosed in figure 1 , with alkali consumption of about 1 10 kg/BDT in the

impregnation vessel, 45 kg/BDT in the first cooking zone in vessel C and 15 kg/BDT in last cooking zone in vessel C. In the top of the first impregnation vessel B is an alkali

concentration of about 67 g/l established and this alkali level drops down to about 32 g/l in the bottom of vessel B, where a dilution is made by return flows added to bottom. Combined with the dilution with black liquor in top of digester vessel C the cooking in top of digester starts at an alkali concentration of about 22 g/l. Due to the dilution to a liquor to wood ratio of about 6,5 is however sufficient total amount of alkali present. During the cook the alkali concentration drops evenly, first to a level of about 16 g/l at first withdrawal screen, and finally down to about 8 g/ 1 in final withdrawal screen. It is to be noted that a part of the withdrawn black liquor at concentration of about 16 g/l is recirculated back to top of vessel C. With this alkali profile is an improved usage of the polysulfide obtained as it is used in the first impregnation stage at high alkali concentration, low temperature and high polysulfide concentration.

In figure 3 is disclosed the improved carbohydrate yield as a function of the polysulfide concentration, when about 1 % lignin is still present in the pulp. Here is shown the dramatic increase in yield when increasing the polysulfide concentration above 0,15 mol/L. There is basically a linearly increasing yield when the concentration increases between 0 to 0,15 mol/l. In this initial range is the yield increased from about 45% up to about 46.2%. However, when the concentration reach 0,2 mol/L is the yield increased to about 48,3%.

EXAMPLES

A series of tests has been made simulating a system as that shown in figure 1 using a white liquor having an alkali concentration of about 1 17 g/L and a polysulfide concentration of about 6 g/l. The charges of flows to the first impregnation stage are in tests #1 -7 using part flows a-e. This results in a liquor to wood ratio shown in row L/W. The respective

concentrations established are shown in rows f to j. S _n S ^2" Despite the presence of a number of different polysulfide ions, each polysulfide ion can be considered to consist of one atom "sulfide sulfur", i.e. sulfur in the formal oxidation state S(-ll), and n atoms of polysulfide "excess sulfur", i.e. sulfur in the formal oxidation state S(0).

[S-ll)]=[HS-]+∑[S _n S ² ]

[S(0)]=∑n[S _n S ² ] Finally, the Xs factor has been calculated using the formula:

Xs = [S(0)] / [S(-ll)] and the carbohydrate stabilization has been calculated using the formula ^* :

Log [S(0)] + 1 .7log[OH-] - 1 .6log(1 /Xs - 1 /4)

( ^* see Teder, A. (1965):Svensk Papperstidn. 68:23, 825)

In figure 4 is disclosed the relative carbohydrate stabilization from above examples as a function of liquor to wood ratio during impregnation. The test #3 is used as the reference, i.e. 100%. The relative carbohydrate stabilization is roughly increasing linearly when decreasing the liquor to wood ratio during impregnation from 5,2 down to 3,7. However a dramatic improvement is obtained if the liquor to wood ratio is reduced to and further below 3,2. While the relative carbohydrate stabilization increase from about 19 to about 68 in the liquor to wood ratio from 5,2 down to 3,7, it is increased to astonishing 100 and further to about 134 and up to 220 at liquor to wood ratio of 3,2 , 2,9 and 2,6 respectively.