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Method of producing mechanical pulp and the mechanical pulp thus producedRelated Patent Categories: Paper Making And Fiber Liberation, Processes Of Chemical Liberation, Recovery Or Purification Of Natural Cellulose Or Fibrous Material, Treatment With Particular Chemical, OrganicMethod of producing mechanical pulp and the mechanical pulp thus produced description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050241785, Method of producing mechanical pulp and the mechanical pulp thus produced. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a method of mechanical pulping based on cellulose fiber material after pretreatment of the fiber material with pectinase prior to mechanical defibration. The invention is also related to the mechanical pulp thus produced. PRIOR ART [0002] In mechanical pulping wood fibers are separated by mechanical defibration, typically in a refiner or a grinder, with or without pretreatment. When pretreatment is applied, the process can be either TMP, where thermal softening is applied; or CTMP, where for instance sulfite is used for the sulfonation of lignin; or APMP, where alkaline peroxide is applied resulting in a combined effect of chemical softening due to alkali and bleaching or brightening due to peroxide. In almost all the mechanical pulping processes, thermal treatment is employed prior to refining to soften the wood material thus leading to reduced energy consumption as well as improved pulp strength. While alkaline treatment is an effective treatment for reducing energy consumption, it also induces darkening reactions that result in poorer optical properties that are essential for paper products based on mechanical pulps. Besides, the use of alkali results in extensive dissolution of wood material, which results in yield loss and necessitates effluent treatment. [0003] In addition to conventional processes, there have been efforts to apply biotechnology to mechanical pulping. In WO 97/40194, a mechanical pulping process using various fungi or enzymes is described. The proposed treatment includes compression of wood chips followed by impregnation with a biological agent prior to refiner treatment. Although a great number of fungi and enzymes are referred to in the patent application, the biological agents used in the given examples are limited either to fungus with ligninase activity, or to enzymes based on xylanase activity aiming for the hemicellulose, and lipase activity specific for the wood resin. The examples show that the enzymatic treatment can reduce the refiner energy by 4 to 10%, in addition to the energy reduction of about 4% due to the compression treatment as compared to the reference. At the same time the strength properties of the pulp are improved when compared at the same pulp freeness. [0004] Another biological pulping process involving pretreatment of wood chips is based on lignin oxidizing fungi treatment (U.S. Pat. No. 5,055,159). The refining energy consumption can be reduced by about 30% at the same fi-eeness. The treatment however requires long incubation time and the pulp produced has a dark color. [0005] Different to the above-mentioned applications, pectinase has been specifically used in post-treatment of mechanical pulps. In U.S. Pat. No. 5,487,812 pectinase is used for alkaline bleached mechanical wood pulp. The purpose of such treatment is however to remove pectin remaining after the pulping process in order to reduce the source of so-called "anionic trash" which is harmful in paper machine operation. SUMMARY OF THE INVENTION [0006] The present invention aims at providing a method of mechanical pulping requiring significantly reduced refining energy consumption to reach a certain pulp freeness, or pulp strength, as compared to prior art. The invention also aims at improved optical properties in the pulp, such as an improved brightness. [0007] This is accomplished by the method according to the invention, as defined in the claims. [0008] According to the invention, the method comprises pretreatment of the comminuted cellulose fiber material with pectinase, alone or in combination with a chelating agent, for instance DTPA and/or sulfite, followed by mechanical defibration and refining to produce a mechanical pulp. [0009] Pectin is a group of amorphous polysaccharide substances in wood. Although the amount of pectin in softwood and hardwood is normally less than 1%, it is predominantly deposited in the compound middle lamella, and the tori of bordered pit-membranes of coniferous species (Hafren J. and Westermark U.: Nordic Pulp and Paper 16 (4), 284-290, 2001). The main component of pectin is polygalacturonan consisting of galacturonic acid units that to various extend are esterified. When in free acid form, the carboxylic groups of the galacturonan having a negative charge can induce local swelling of the fibers in contact with aqueous solutions. The esterified groups, on the other hand, can be de-esterified through for instance alkaline treatment and thus contributing to swelling. [0010] Given the technical background, the present invention is aimed to utilize the specific features of pectin with respect to its specific location and the potential to induce selective weakening in the fiber wall. Enzyme with specific activity towards pectin is used for the pretreatment of cellulose fiber material. Maceration of the fiber material prior to the enzyme treatment proves an efficient way to get the enzyme in place. To render the fiber separation in the region where chemical weakening occurs, the refining condition is optimized in respect to refining intensity and preheating temperature. The results according to the present invention are a significantly reduced refining energy to reach a given pulp freeness and improved strength and optical properties of the pulp. [0011] According to one aspect of the invention, the pretreatment is preferably assisted by a mechanical compression to facilitate liquor uptake and to give a more even treatment of the material. Compression screw device or twin roll presses can be used at a compression ratio of 1:1 to 8: 1. The pretreatment can be further improved if pre steaming is conducted prior to the compression. [0012] Impregnation according to the invention follows immediately after the compression and/or thermal pretreatment. The pectinase is charged in an amount required for sufficient treatment, preferably 2,000,000 to 200,000,000 polygalacturonase units/ton material and even more preferred 10,000,000 to 50,000,000 polygalacturonase units/ton. The retention time of treatment is 3 min to 24 hours, preferably 3 to 300 min, even more preferred 15 to 240 min and most preferred 30 to 120 min at a temperature of 20 to 100.degree. C., preferably 35 to 70.degree. C., and more preferably about 50.degree. C.. The treatment condition can be adjusted depending on the charge of the pectinase to allow for a sufficient hydrolysis of the pectins. Other pectinase preparations can be used provided that the pectolytic activity is satisfied. [0013] Defibration and refining of the pectinase treated material can be carried out either at conventional TMP conditions and high intensity conditions. A surprising energy reduction effect is obtained. To reach a given pulp freeness of 100 ml CSF, the energy reduction is 400 kWh/t with TMP condition, from 2500 kWh/t without pectinase treatment to 2100 kWh/t with pectinase treatment, or by about 16%. The energy reduction is 150 kWh/t with high intensity condition, from 2150 kWh/t without pectinase treatment to 2000 kWh/t with pectinase treatment, or by about 7%. The combination of high intensity and pectinase treatment gives a total reduction of 500 kWh/t, from 2500 kWh/t to 2000 kWh/t at a pulp freeness of 100 ml CSF, which is 20% in reduction. The shives content decreases more rapidly after the pectinase treatment. [0014] The strength properties remain the same as compared to conventional TMP, and improved by about 10% as compared to high intensity TMP. The brightness of the pulp is also increased by the enzymatic treatment. [0015] While the addition of DTPA or sulfite doesn't contribute to further energy reduction, it increases the brightness by 2 to 5 brightness units. The improved brightness is maintained after peroxide bleaching, together with the benefit of an increased amount of residual peroxide for recycling. [0016] The surprising effects resulting from the pectinase treatment could be explained by a selective weakening of the pectin-enriched region in the fiber wall due to hydrolysis of the pectins, with the consequence of a more efficient fiber separation in defibration and refining of the material. It is evident to those skilled in the art that this process can bring about a significant economical benefit, in teens of improved pulp properties and reduced energy cost. It is also evident to those skilled in the art that any enzyme preparation that contains sufficient pectolytic hydrolysis activity, alone or in combination with other chemicals, can be used for the treatment according to the present invention. [0017] The present invention describes a mechanical pulping process comprising: DETAILED DESCRIPTION OF THE INVENTION [0018] A) Mechanical pretreatment of the comminuted fiber material with suitable mechanical means, for instance compression with screw device or roll press device, to enable an efficient uptake of aqueous solution into the fiber material during an impregnation immediately following the pretreatment. Replacement of resin-containing liquor in natural cellulose fiber material with added aqueous solution is also positive in a process point of view. The compression ratio is preferably in the range of 1:1 to 8:1, preferably 2:1 to 5:1, where 1:1 means a screw conveyer transporting the fiber material into the impregnation bin. In combination with compression treatment, an initial thermal heating of the fiber material may further facilitate an efficient uptake of the aqueous solution, preferably by use of fresh or recycled steam at atmospheric pressure, for 1 to 30 min, preferably 10 to 20 min. As an alternative, thermal heating followed immediately by impregnation may be used instead of compression treatment when no such facility is available. The pretreatment procedure can also be repeated in one to several stages with the freedom of applying various impregnation chemicals in different stages. [0019] B) Impregnation of the fiber material with pectinase-containing aqueous liquid. Added in connection with or immediately after the pretreatment stage, the enzyme can more easily reach the sites of reaction. In addition to the pectinase enzyme, other chemicals for instance chelating agents such as sulfite, preferably at a charge of 5 to 50 kg/ton, and/or DTPA, preferably at a charge of 1 to 10 kg/ton may be added in the impregnation to further improve the process. The pectinase-containing liquid may comprise an enzymatic preparation with pectolytic activity for both un-esterified pectins and esterified pectins. Moreover, the aqueous liquid may comprise two or more enzymatic preparations wherein at least one of the preparations has pectinase activity; or the pectinase may be added as a biological agent comprising one or more fungi or bacteria, at least one of which having pectolytic activity. The charge of enzyme is subject to process conditions and cost effectiveness, and lies in the range of 2,000,000 to 200,000,000 polygalacturonase units/ton fiber material preferably 10,000,000 to 50,000,000 polygalacturonase units/ton. The pH of the impregnation liquor is adjusted by alkali or acid, preferably by caustic soda or mineral acid, to a value optimal for enzymatic reactions, suitably in the pH range of 3 to 10, preferably 4 to 7 and more preferably about 5. The retention time and temperature are also adjusted depending on the process setup and reaction requirements, preferably being 3 to 300 min at a temperature of 20 to 100.degree. C.. It is evident to those skilled in the art that any enzyme that contains sufficient pectolytic activity, alone or in combination with other chemicals, can be used in the treatment as described. [0020] C) Defibration and refining of the impregnated fiber material is carried out under optimized conditions. The fiber material is preferably preheated prior to feeding into the pressurized primary refiner, to allow for thermal softening of the fiber wall. The pulping conditions can comprise a refiner rotation speed of 1000 to 3000 rpm, preferably 1500 to 2600 rpm, using either conventional TMP or high refining intensity. Conventional TMP conditions may comprise preheating at 0 and up to 4 or 5 bar with a retention time of 2-10 min in the preheater and a refiner rotation speed of 1200 to 1800 rpm. The pressure 0 bar means atmospheric refining. High intensity conditions may comprise preheating at above 4 or 5 bar and up to 8 bar with a retention time of 3 to 30 sec and a refiner rotation speed above 2000 rpm but usually not above 3000 rpm. The retention time should be matched against the preheating temperature (steam pressure) as high preheating temperature requires shorter retention time. After the primary refining, a secondary refining stage can be used to reach the required pulp freeness. The seconddary refining stage may have the same conditions as the primary stage. 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