NZ206982A - Method for manufacturing improved groundwood pulp and producing a long-fibre fraction - Google Patents

Description

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Complete Specification Filed: I • ^m a Class: Publication Date: P.O. Journal, No: .... Q? NEW ZEALAND PATENTS ACT, 1953 206982 COMPLETE SPECIFICATION A METHOD FOE MANUFACTURING IMPROVED GROUNDWOOD PULP */We, MO OCH DOMSJO AKTIEBOLAG, a Swedish company, of Horneborgsvagen 6, 89200 DOMSJO, Sweden hereby declare the invention for which / we pray that a patent may be granted to 0ap/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - Followed by la t' -la- -806982 A method for manufacturing improved groundwood pulp Technical Field The present invention relates to a method for manufacturing improved groundwood pulp from wood in the form of logs or chips.

The ISO Brightness is determined according to SCAN-C11:75 (=ISO 2469 Measurement of diffuse reflectance factor) and gives the brightness of each sample in per cent as compared with a perfect reflecting diffuser which has 100% brightness in the reflectometer apparatus used. The per cent opacity is thus the ratio of the light flux reflected from a single sheet of paper placed over a black background to that reflected from the same sheet placed over an opaque pad of the paper - the latter representing 100% opacity.

Background Art It is known to produce groundwood pulp by bringing logs or wood chips into contact with a rotating grindstone. The resultant fibre suspension is normally treated in a bull screen, to remove coarse particles, which are treated in a defibrating apparatus, e.g. a disc refiner, and returned to the fibre suspension, while the accept is passed to a primary screening department. The reject obtained in the primary screening department is normally returned to the disc refiner, while the accept is conducted to a wet machine or paper machine, optionally after being purified in a vortex cleaner and optionally after being bleached.

Disclosure of the Invention Technical Problem other types of printing paper, and also to manufacture soft crepe paper. In the manufacture of these paper qualities, high requirements are placed on a low shive Groundwood pulp is normally used to manufacture newsprint and 2 206982 content, i.e. the content of partially defibrated wood residues. In the manufacture of paper, high shive contents result in web breakages, impart to the paper a high degree of surface roughness, and give rise to printing irregularities. Consequently, a serious problem associated with the manufacture of groundwood pulp is to bring the shive content down to a desirable low level; The pulp used in the manufacture of the aforesaid paper qualities is ground to a relatively low freeness, i.e. 70-200 ml C.S.F. Groundwood pulp can also be used for manufacturing cardboard and paperboard, in which manufacture it is also desirable to obtain a pulp having-a low shive content. Groundwood pulp used to produce cardboard should have a relatively high freeness, i.e. 250-400 ml C.S.F. One disadvantage with grinding to high freeness, however, is that the pulp will have a relatively high shive content and will be relatively weak.

In recent years there has been developed a chemi-mechanical pulp, designated Chemithermomechanical j>ulp, abbreviated to CTMP, which has a very high freeness, i.e. 400-700 ml C.S.F., and also a low shive content, which is highly suitable for the manufacture of absorption products. It is not possible to produce a usable pulp having a freeness above 500 ml C.S.F. in a groundwood mill using grindstones and present day techniques. Groundwood pulp having such high freeness comprises but a small percentage of fibres, and consists mainly of shives and splinters, and cannot be used to manufacture absorption products.

Solution The aforementioned problems are solved by means of the present invention, which relates to a method for manufacturing improved groundwood pulp, in which method after being passed through a bull screen, the fibre suspension leaving the grinder is screened in a primary screening department and the resultant screen reject is treated in a r% e o defibrating apparatus and returned to the primary screening department, the method being characterized in that in the primary screening department there is intentionally talcen out from the bull-screened fibre suspension arriving from the grinder a larger quantity of screen reject than is normal, i.e. more than 30%, to form a reject pulp fraction which is treated in-a first separating means, in which shives and splinters having a length exceeding 4 mm, preferably 8 mm, are removed to form a reject fraction which is defibrated and returned to the primary screening department, while the accept from the first separating means is treated in a second separating means having the form of a fractionating means, to form A) a long fibre fraction at least 801 of which contains fibres retained on a screen according to Bauer McNett having 59 meshes/cm, said fraction being de-watered and removed from the process for special purpose use, and B) a fine fibre fraction, 15-601 of which contains fibres which pass through a screen according to Bauer McNett having 59 meshes/cm, said fraction being mixed with the accept arriving from the primary screening department.

Advantages By means of the proposed method there is obtained with a low energy consumption a practically shive-free groundwood pulp of low surface roughness and of extremely high opacity, suitable for producing for example, LWC-naper (LWC=lightweight coating) and suitable for admixing with other high quality printing papers. The separate, removed long-fibre fraction, which is produced with a very low energy -982 consumption, has a low resin content, a high freeness (about 200-700 ml C.S.F.), and can be very suitably admixed with pulps intended for use as absorption products of high bulk, good absorption rates and extremely high absorption capacities, and 5 for use in pulps intended for the manufacture of cardboard or paperboard. By means of the method according to the invention it is possible to bring the properties of the groundwood pulp up to the level of CTMP-pulps, while keeping to a lower energy consumption.

Brief Description of the Drawings Figure 1 is a block schematic illustrating the manufacture of groundwood pulp in accordance with known techniques, while Figure 2 is a block schematic illustrating the method according to the invention.

Preferred Embodiment of the Invention With the known technique illustrated in Figure 1, logs or woodchips are ground in a grinder 1. The fibre suspension leaving the grinder contains knots, splinters and other coarse wood residues, and is passed through a conduit 2 to a coarse screening 20 department in the form of a bull screen 3 (knotter), which may comprise a so-called vibratory screen, i.e. a screen arrangement provided with vibrating screen plates provided with holes or slots. The bull-screened coarse material, which normally has a length exceeding about 50 mm, is passed through a conduit 4 to a 25 defibrating means 5, e.g. a disc refiner, and the defibrated material is then returned through a conduit 6 to the fibre suspension leaving the grinder, for example to the conduit 2. The O fibre suspension which had been freed from coarse wood residues is conducted away from the bull screen through a conduit 7 and 30 passed to a primary screening department 8, which in its simplest form may comprise a pressurized screen having perforated screen plates and hole diameters of, for example, 1.75 mm. Up to about 201 by weight of the fibre suspension entering the primary screening department is separated as reject, and is returned in the circuit through conduit 6, optionally after having passed through the de-fibrating means 5, as illustrated in Figure 1. The accept obtained in the primary screening department is taken out through a conduit 10, and normally has a freeness of 70-200 ml C.S.F. and a shive content of 0.08-0.20 %. The accept is passed to a wet machine, optionally after being cleansed in a vortex cleaner.

When manufacturing groundwood pulp in accordance with the invention (c.f. Fig. 2) logs or woodchips are ground in a grinder 1. The fibre suspension leaving the grinder is passed through conduit 2 to a bull screen 3, and the wood residues separated out by the screen 3 are passed through a conduit 4 to a defibrating means 5, e.g. a disc refiner, from where the defibrated material is returned to the suspension leaving the grinder through conduit 6, in the same manner as that illustrated in Figure 1. In the primary screening department 8, which in its simplest form may comprise a pressurized screen, the amount of reject is increased, e.g. by increasing the throughflow (increasing the opening of valve 17) or by reducing the hole diameters of the screen plates, or by adopting both measures, so that 30-851 by weight of the fibre suspension arriving through the conduit 7 is separated out as reject. The resultant reject pulp fraction is passed through the conduit 9 to a first separating means 11 for separating residual shives and splinters having a length above 4 mm, preferably above 8 mm. The first separating means may comprise, for example, a vibratory screen having smaller hole diameters than the vibratory screen 3. The reject fraction from the first separating means 11 is returned, through a conduit 12, to the defibrating means 5, from where said fraction is returned to the suspension leaving the gTinder, e.g. to the conduit 2, while the accepted reject 6 206 9»2 pulp fraction is passed, through a conduit 13, to a second separating means 14, which may comprise, for example, a centrifugal screen or a curved screen, for fractionating said accepted fraction into a long-fibre fraction at least 80 % of which contains fibres which are retained on a screen according to Bauer McNett having 59 meshes/cm, and a fine-fibre fraction, 15-60 % of which contains fibres which pass through a screen according to Bauer McNett having 59 meshes/cm. The long-fibre fraction is taken out through a conduit 16, and is dewatered and used for special purposes, while the fine-fibre fraction is taken out through a conduit 15 and is passed to the accept flow leaving the primary screening department in conduit 10, to form a second fine-fibre fraction.

In order to produce the effect sought for in accordance with the invention, the outgoing material flows B and C in the method should be so dimensioned that the weight ratio between the long-fibre fraction and the fine-fibre fraction is caused to lie within the range of from 1:3 to 3:1.

In accordance with a particularly suitable embodiment of the invention, the white water obtained when dewatering the long-fibre fraction is returned as dilution water to the first separating means 11, and therewith has a fibre content lower than 200 mg/1, which, if so desired, can be produced by incorporating a separate filter in the circuit.

Because more reject than normal is taken out from the primary screening department 8, the fine-fibre fraction obtained through the conduit 10 in accordance with the invention will have an extremely low shive content, lying within the range of 0.0-0.05$. Corresponding known groundwood pulps of comparable freeness have a shive content of 0.08-0.20 %. A shive -content of this last mentioned order of magnitude considerably impairs the usefulness of the pulp in the manufacture of printing paper, and contributes to imparting a surface roughness to the paper, and also causes the paper to have nonuniform ink absorbency. In addition, the fine-fibre fraction - • . - -< • * - /v J <■'. obtained according to the invention has a different fibre distribution to conventional groundwood pulp, which affords the advantage of a higher tensile index and opacity in printing paper produced from said fraction, than paper produced from conventional groundwood pulp. Consequently, it is highly suited for the manufacture of printing paper of the highest quality.

The long-fibre fraction according to the invention obtained through the conduits 16 has a high freeness (200— 750 ml C.S.F) and a low resin content, less than 0.3 % DKM, (after bleaching less than 0.15 % DKM), and 80-100 I of said fraction comprises fibres which are retained on a screen according to Bauer McNett, having 59 meshes/cm. The said long-fibre fraction has extraordinary properties for the manufacture of absorption products, and provides high bulk, good absorption rates and an extremely good absorption capacity. In the method according to the invention, 15-75 % of the incoming wood is recovered as a long-fibre fraction.

Thus, instead of producing a single product which is inferior to a chemithermomechanical pulp (CTMP) there can be produced when practicing the method proposed in accordance with the invention two products each of which has extremely good properties, said products being produced in a higher yield and while using less energy, since the total energy consumed when producing the long-fibre fraction according to the invention is 300-500 kWh/ton of dry pulp, while the corresponding value in respect of chemithermomechanical pulp is about 1000 kWh/ton of dry pulp. When manufacturing the fine-fibre fraction, the energy consumption is 1300-1500 kWh/ton of dry pulp, while corresponding values for CTMP of corresponding quality is about 2000 kWh/ton of dry pulp. When practicing the present invention, at least 96 % of the wood is converted to pulp (92-94 % with CTMP).

The long-fibre fraction produced in accordance with the invention is well suited for admixture with other pulps, such as sulphite pulp, sulphate pulp and chemimechanical pulp. 8 2 0' 9 ? 2 It is also highly suited for the manufacture of cardboard and paperboard, and also for the manufacture of absorption products. Other fibre materials, such as return fibres, peat fibres and synthetic fibres can also be mixed -with the long-fibre fraction.

The method according to the invention can also be applied, with good results, in the manufacture of pressure-ground pulp, in which case the energy consumption is still lower, about 10 9» lower.

The invention is illustrated in the following Example. Example 1 Groundwood pulp was manufactured from spruce in accordance with known techniques (c.f. Figure 1). The spruce was ground in a grinder (1) and the resultant fibre suspension, which had a concentration of 2.1 I, was passed through the conduit 2 to a Jonsson-type vibratory screen 3, having screen plates perforated with holes of 6 mm diameter, where the coarse wood residues were screened off. The reject obtained in the vibratory screen 3 and comprising coarse splinters and shives and constituting about 3 I by weight of the ingoing fibre suspension, was passed through the transport conduit 4, where the pulp concentration was 5%, to a disc refiner 5 for defibrating the coarse material to separate fibres. The defibrated reject pulp was passed through the conduit 6 and the conduit 2, back to the vibratory screen 3. The accept from the vibratory screen 3 had a concentration of 1.3 % and was passed through the conduit 7 to a pressurized screen 8, i.e. a screen having a fixed cylindrical screen basket, to the inner cylindrical surface of which the pulp suspension was passed under an overpressure, the screen also being provided with an internal, rotating scraper means. The holes in the perforated screen plates of the pressurized screen had a diameter of 1.75 mm. The flow of fibre suspension to the pressurized screen was regulated so that 20 $ by weight of the fibre content of the fibre suspension supplied remained on the screen plates, and was passed further in the form of reject pulp, via valve 17, through the conduit 9 to the conduit 4, for further treatment in the disc refiner 5, and return to the conduit 2. The pulp consistency in the conduit 9 was 1.9 %. Accept from the pressurized screen had a pulp consistency ef 1.1 % and was removed through the conduit 10 and cleansed further in a vortex cleaner (not shown in the Figure). Samples, here designated A, were removed for determining the shive content and fibre composition.

In accordance with the invention, the groundwood pulp manufacturing process was modified, as illustrated in Figure 2. Thus, the fibre suspension arriving from the grinder 1 through conduit 2, said suspension having a concentration of 2.1 %, was coarsely screened in the vibratory screen 3, having a hole diameter of 6 mm, and the reject, which comprised 3 % by weight of the ingoing fibre suspension, was passed through the conduit 4, in which the pulp consistency was 5 %, to the disc refiner 5, whereafter the defibrated reject pulp, which had a concentration of 5 %, was returned to the vibratory screen 3, through the conduit 6 and the conduit 2. The accept from the vibratory screen 3 had a concentration of 1.3 % and was passed through the conduit 7 to the pressurized screen 8, the screen plates of which were changed to a hole diameter of 1.60 mm instead of 1.75 mm, as was previously the case. At the same time, the valve 17 was further opened, so that the amount of reject pulp rose to 70 % by weight of the fibre content of the ingoing fibre suspension. The reject-pulp fraction obtained in the pressurized screen 8 had a freeness of 245 ml C.S.F., a shive content of 2.95 % according to Sommer-ville, and a fibre composition according to Bauer McNett of 33.1 % retained with 7.9 meshes/cm, 41.5 % retained with 59 meshes/cm and 25.4 % passing through 59 meshes/cm. The reject pulp was given a pulp consistency of 1.5 % and transferred, through the conduit 9, to a first separating means having the form of a vibratory screen 11 provided with screen plates having a hole diameter of 3.0 mm. The vibratory screen 11 was adjusted so that the amount of reject was 0.8 % by weight of the incoming fibre suspension. The reject pulp *7 O Q ft ? obtained in the vibratory screen 11 had a shive content of 73 % according to Sommerville, and was passed through the conduit 12 to the conduit 4, for further treatment in the disc refiner 5, and was returned via the conduit 6 to the conduit 2. The pulp consistency in the conduits 6 and 2 was the same as that in the test carried out in accordance with the known method. The accept pulp from the vibratory screen 11 was given a pulp consistency of 1.2 %, and was passed through the conduit 13 to a second separating means 14, where said pulp was fractioned. The fractioning means comprised a Cowan-type centrifugal screen, i.e. a screen having a stationary, cylindrical screen basket, against the inner cylindrical surface of which the fibre suspension was slung by means of a rotating device. As distinct from vibratory screens, centrifugal screens are constructed to utilize the mutual autogenous effect of the fibres at the screen surfaces and have larger hole diameters. The centrifugal screen 14 had a hole diameter of 1.50 mm. The accept fraction obtained from said screen had a freeness of 140 ml C.S.F. and a shive content of 0.04 % according to Sommerville. 3.1 % of the accept fraction was retained on a screen according to Bauer McNett having 7.9 meshes/cm, 82 % with 59 meshes/cm, while 14 % passed through 59 meshes/cm. The accept fraction was given a pulp consistency of 0.9 %, and was passed, via the conduit 15, to the accept flow from the pressurized screen 8, this accept flow having a freeness of 90, a shive content of 0.01, and a fibre distribution according to Bauer McNett of 2.3 % retained at 7.9 meshes/cm, 63.7 % retained at 59 meshes/cm and 34 I passed 59 meshes/cm. The mixed accept from the pressurized screen 8 and the centrifugal screen 14, the fine-fibre fraction, was removed through the conduit 10 and further cleansed in a vortex cleaner (not shown in the Figure). 20 I of the fibre suspension leaving the vibratory screen 11 through the conduit 13 was removed as accept in the centrifugal screen 14. Samples, here designated B, were taken from the fine-fibre 11 206982 fraction in the conduit 10, for determining shive content and fibre composition. The amount of fine-fibre fraction calculated on incoming wood material was 44 %, of which 31.2 % was in the conduit 15. The reject fraction from the centrifugal screen 14 comprised 80 % of the fibre suspension leaving the vibratory screen 11 through the conduit 13 and was given a pulp consistency of 1.9 % and removed as long-fibre fraction through the conduit 16. The ratio of fine-fibre fraction to long-fibre faction was 1:2.2. Samples, here designated C, were removed through the conduit 16, for determining shive content and fibre composition.

Table I below sets forth the freeness, shive content and fibre distribution of the pulp samples taken, A representing known groundwood pulp and B and C represerlting groundwood pulp according to the invention, B being the fine-fibre fraction and C being the long-fibre fraction. Representative values for chemithermomechanical pulp D (CTMP) are also given by way of comparison. " i Table I Sample Freeness Shive Content Fibre Distribution (SCAN-C21:65) acc. acc. Bauer McNett C.S.F, ml Sommerville (SCAN-M6:69) % by weight % by weight +7.9 meshes/cm +59 meshes/cm -59 meshes/cm (=+ 20 mesh) (=+ 150 mesh) (=- 150 mesh) A 120 0.06 8.5 58.4 33.1 B 115 0.03 2.7 62.3 35.0 C 622 0.40 48.1 45.2 6.7 D 600 0.35 35.2 . 46.1 18.9 It will be seen from the Table that the fine-fibre fraction produced in accordance with the invention has a very low shive content. The long-fibre fraction produced in accordance with the invention has a very high freeness, low shive content and a very high content of long fibres. " * The meshes referred to are Tyler meshes. i \ ■i 12 206982 Some of the samples taken were bleached with 3 % by \ireight hydrogen peroxide calculated per ton of dry pulp, were washed, dewatered and dried to form laboratory sheets, which were analyzed with respect to extract content and brightness. Part of the laboratory sheets were disintegrated in a disc refiner to form a fluff pulp, the bulk, absorption rate and absorption capacity of which were examined. The results obtained are set forth in Table 2 below, sample E relating to a chemical pulp, sulphite pulp: Table II Sample Extract ISO Content Brightness (SCAN-C7:62) (SCAN-C11:75) DCM % by weight Z Bulk cm3/g Absorption Time (SCAN-C33:80) sec.

Absorption Capacity (SCAN-C33:80) g H20/g pulp A 0.95 80.0 12.5 4.6 9.9 C 0.08 78.2 17 . 7 6.0 .7 D 0.19 74.6 17 .9 7.3 .9 E 0.32 92.5 16 . 2 .0 9.2 It will be seen from Table 2 that the long-fibre fraction (C) produced in accordance with the invention had very high values with respect to bulk and absorption capacity, which is in parity with the high yield pulp, chemithermomechanical pulp (CTMP) considered best hitherto for the manufacture of absorption products. The properties of the long-fibre fraction are markedly better than those of the sulphite pulp. The long-fibre fraction also has a far lower resin content and, at the same time, a far greater brightness than CTMP. This high brightness is surprising, since the pulp has a low light-scattering coefficient.

Paper was produced from the samples A and B and the technical properties of the paper evaluated. The resuljtju-ax.e set forth in Table 3 below. 2MARW87 >"'// . ..~ct*?*,£,1 13 206982 Table III Sample A B Tensile index Ntn/g 31.8 34. 2 Tear index-mN "m /g 3.7 3.3 Light-scattering coefficient (SCAN-C27:69) m /kg 59.5 63.5 Opacity (SCAN-P8:75) % 92.5 95.6 The fine-fibre fraction produced in accordance with the invention thus had a higher tensile index and a considerably higher opacity than conventional groundwood pulp.

Consequently, when practicing the method according to the present invention it is possible through groundwood pulp manufacturing processes to produce improved products for widely separated purposes, such as finer pulps for the manufacture of printing paper of high quality, and coarse pulps for the manufacture of fluff and cardboard, while consuming less energy than normal.

Claims (5)

-"t- 20698fc WHAT VQfiS. CLAW <*»«

1. A method for manufacturing improved groundwood pulp, in which the fibre suspension leaving the grinder, after coarse-screening, is screened in a primary screening department and the screen reject therefrom is treated in a defibrating means and returned to the primary screening department, characterized in that in the primary screening department there is intentionally removed from the coarsely-screened fibre suspension arriving from the grinder a greater quantity of screen reject than normal, i.e. more than 30% by weight, to form a reject pulp fraction which is treated in a first separating means, in which shives and splinters having a length exceeding 4 mm, are separated to form a reject fraction, which is defibrated and returned to the primary screening department, while the accept from the first separating means is treated in a second separating means in the form of a fractionating means, to form A) a long-fibre fraction at least 80% by weight of which contains fibres retained on a screen according to Bauer McNett having 59 meshes/cm (Tyler mesh No. 150), said fraction being dewatered and removed from the process for special purpose use, and B) a fine-fibre fraction 15-60% by weight of which contains fibres passing a screen according to Bauer McNett having 59 meshes/cm (Tyler mesh No. 150), said fraction being mixed with the accept arriving from the primary screening department.

2. A method according to Claim 1, characterized in that the weight ratio between the long-fibre fraction and the fine-fibre fraction is caused to reach to from 1:3 to 3:1. ^ - V 2H6982

3. A method according to Claims 1-2, characterized in that 15-75% by weight of incoming wood material is taken out as a long-fibre fraction.

4. A method according to Claims 1-3, characterized in that the screening and defibrating operations are carried out in a manner such that the shive content of the fine-fibre fraction is less than 0.05% by weight.

5. A method for manufacturing improved groundwood pulp as herein described with reference to the accompanying Figure 2. fib OC H £ S~3£> A(CTP?e>cc.rtCr by r£*/tr.sir authorises AgedO* A. J. PA&K & SON* TT