According to conventional knowledge in the art of kraft pulping of cellulose, the level of dissolved organic materials (DOM) - - - which mainly comprise dissolved hemi-cellulose, and lignin, but also dissolved cellulose, extractives, and other materials extracted from wood by the cooking process - - - is known to have a detrimental affect in the later stages of the cooking process by impeding the delignification process due to consumption of active cooking chemical in the liquor before it can react with the residual or native lignin in wood. The effect of DOM concentration at other parts of cooking, besides the later stages, is according to conventional knowledge believed insignificant. The impeding action of DOM during the later stages of the cook is minimized in some state-of-the-art continuous cooking processes, particularly utilizing an EMCC.RTM. digester from Kamyr, Inc. of Glens Falls, N.Y., since the counter-current flow of liquor (including white liquor) at the end of the cook reduces the concentration of DOM both at the end of the "bulk delignification" phase, and throughout the so-called "residual delignification" phase.
According to the present invention, it has been found that not only does DOM have an adverse affect on cooking at the end of the cooking phase, but that the presence of DOM adversely affects the strength of the pulp produced during any part of the cooking process, that is at the beginning, middle, or end of the bulk delignification stage. The mechanism by which DOM affects pulp fibers and thereby adversely affects pulp strength has not been positively identified, but it is hypothesized that it is due to a reduced mass transfer rate of alkali extractable organics through fiber walls induced by DOM surrounding the fibers, and differential extractability of crystalline regions in the fibers compared to amorphous regions (i.e. nodes). In any event, it has been demonstrated according to the invention that if the DOM level (concentration) is minimized throughout the cook, pulp strength is increased significantly.
It has been found, according to the present invention, that if the level of DOM is close to zero throughout a kraft cook, tear strength of the pulp is greatly increased, i.e. increased up to about 25% (e.g. 27%) at 11 km tensile compared to conventionally produced kraft pulp. Even reductions of the DOM level to one-half or one-quarter of their normal levels also significantly increase pulp strength.
In state-of-the-art kraft cooks, it is not unusual for the DOM concentration at some points during the kraft cook to be 130 grams per liter (g/l) or more, and at 100 g/l or more at numerous points during the kraft cook (for example in the bottom circulation, trim circulation, upper and main extractions and MC circulation in Kamyr, Inc. MCC.RTM. continuous digesters), even if the DOM level is maintained between about 30-90 g/l in the wash circulation (at later cook stages, according to conventional wisdom). In such conventional situations it is also not unusual for the lignin component of the DOM level to be over 60 g/l and in fact even over 100 g/l, and for the hemi-cellulose component of the DOM level to be well over 20 g/l. It is not known if the dissolved hemi-cellulose component has a stronger adverse affect on pulp strength (e.g. by adversely affecting mass transfer of organics out of the fibers) than lignin, or vice versa, or if the effect is synergistic, although the dissolved hemi-celluloses are suspected to have a significant influence.
According to the present invention it has been recognized for the first time that the DOM concentration throughout a kraft cook should be minimized in order to positively affect bleachability of the pulp, reduce chemical consumption, and perhaps most significantly increase pulp strength. By minimizing DOM levels, one may be able to design smaller continuous digesters while obtaining the same throughput, and may be able to obtain some benefits of continuous digesters with batch systems. A number of these beneficial results can be anticipated by keeping the DOM concentration at 100 g/l or less throughout substantially the entire kraft cook (i.e., beginning, middle and end of bulk delignification), and preferably about 50 g/l or less (the closer to zero DOM one goes, the more positive the results). It is particularly desirable to keep the lignin component at 50 g/l or less (preferably about 25 g/l or less), and the hemi-cellulose level at 15 g/l or less (preferably about 10 g/l or less).
According to the present invention it has also been found that it is possible to passivate the adverse affects on pulp strength of tile DOM concentration, at least to a large extent. According to this aspect of the invention it has been found that if black liquor is removed and subjected to pressure heat treatment according to U.S. Pat. No. 4,929,307 (the disclosure of which is hereby incorporated by reference herein), e.g. at a temperature of about 170.degree.-350.degree. C. (preferably 190.degree. C.) for about 5-90 minutes (preferably about 30-60 minutes) and then reintroduced, an increase in tear strength of up to about 15% can be effected. The mechanism by which passivation of the DOM by heat treatment occurs also is also not fully understood, but is consistent with the hypothesis described above, and its results are real and dramatic.
According to the present invention-various methods are provided for increasing kraft pulp strength taking into account the adverse affects of DOM thereon, as set forth above, for both continuous and batch systems. Also according to the present invention increased strength kraft pulp is also provided, as well as apparatus for achieving the desired results according to the invention.
According to one aspect of the present invention, a method of producing kraft pulp by cooking comminuted cellulosic fibrous material is provided. The method comprises the steps of continuously, at a plurality of different stages during kraft cooking of the material to produce pulp: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength. Step (b) is typically practiced by replacing the withdrawn liquor with liquor selected from the group consisting essentially of water, substantially DOM free white liquor, pressure-heat treated black liquor, washer filtrate, cold blow filtrate, and combinations thereof. For example for at least one stage during cooking, black liquor may be withdrawn, and treated under pressure and temperature conditions (e.g. superatmospheric pressure at a temperature of about 170.degree.-350.degree. C. for about 5-90 minutes, and at least 20.degree. C. over the cooking temperature) to significantly passivate the adverse affects of DOM. The term "effective DOM" as used in the specification and claims means that portion of the DOM that affects pulp strength. A low effective DOM may be obtained by passivation, or by an originally low DOM concentration.
The method according to the invention can be practiced in a continuous vertical digester, in which case steps (a) and (b) are practiced at at least two different levels of the digester. There is also typically the further step (c) of heating the replacement liquor from step (b) to substantially the same temperature as the withdrawn liquor prior to the replacement liquor being introduced into contact with the material being cooked. Steps (a) and (b) can be practiced during impregnation, near the start of the cook, during the middle of the cook, and near the end of the cook, i.e. during substantially the entire bulk delignification stage.
According to another aspect of the present invention a method of kraft cooking is provided comprising the steps of, near the beginning of the kraft cook: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength.
According to another aspect of the present invention a method of kraft cooking is provided comprising the steps of, during impregnation of cellulosic fibrous material: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength.
According to still another aspect of the present invention a method of kraft cooking pulp is provided comprising the following steps: (a) Extracting black liquor from contact with the pulp at a given cooking stage. (b) Pressure-heating the black liquor to a temperature sufficient to significantly passivate the adverse effects on pulp strength of DOM therein. And, (c) re-introducing the passivated-DOM black liquor back into contact with the pulp at the given stage.
The invention also comprises the kraft pulp produced by the methods set forth above. This kraft pulp is different than kraft pulps previously produced, having a tear strength as much as 25% greater at a specified tensile for fully refined pulp (e.g. at 9 km tensile, or at 11 km tensile) (and at least about 15% greater) compared to kraft pulp produced under identical conditions without the DOM maintenance or removal steps according to the invention, or as much as 15% greater (e.g. at least about 10% greater) where passified black liquor is utilized.
The invention is also applicable to kraft batch cooking of cellulosic fibrous material utilizing a vessel containing black liquor and a batch digester containing the material. In such a method of kraft batch cooking according to the invention there are the steps of: (a) Pressure-heating the black liquor in the vessel to a temperature sufficient to passivate the adverse effects on pulp strength of DOM therein. And, (b) feeding the black liquor to the digester to contact the cellulosic fibrous material therein. Step (a) is practiced to heat the black liquor at superatmospheric pressure at a temperature of about 170.degree.-350.degree. C. for about 5-90 minutes (typically at least about 190.degree. C. for about 30-60 minutes, and at least 20.degree. C. over cooking temperature), and step (b) may be practiced to simultaneously feed black liquor and white liquor to the digester to effect cooking of the cellulosic fibrous material.
According to another aspect of the present invention an apparatus for kraft cooking cellulose pulp is provided. The apparatus comprises the following elements: An upright continuous digester. At least two withdrawal/extraction screens provided at different levels, and different cook stages, of the digester. A recirculation line and an extraction line associated with each of the screens. And, means for providing replacement liquor to the recirculation line to make up for the liquor extracted in the extraction line, for each of the recirculation lines. Each recirculatory loop typically includes a heater, and the digester may be associated with a separate impregnation vessel in which removal of high DOM concentration liquor and replacement with lower DOM concentration liquor also takes place (including in a return line communicating between the top of the impregnation vessel and the high pressure feeder).
The invention also relates to a commercial method of kraft cooking comminuted cellulose fibrous material by the step (a) of continuously passing substantially DOM-free cooking liquor into and out of contact with the material until completion of the kraft cook thereof, at a rate of at least 100 tons of pulp per day. This method is preferably practiced utilizing a batch digester, and by the further step (b), prior to step (a), of filling the digester with cellulose material, and the further step (c), after step (a) of discharging kraft pulp from the digester. The invention also relates to a batch digester system for practicing this aspect of the invention.
It is the primary object of the invention to produce increased strength kraft pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.