WO1989006717A1

WO1989006717A1 - Method of making mechanical pulp

Info

Publication number: WO1989006717A1
Application number: PCT/SE1989/000004
Authority: WO
Inventors: K. Ove Danielsson; Bo G. S. Falk; Michael Jackson
Original assignee: Sunds Defibrator Industries Aktiebolag
Priority date: 1988-01-22
Filing date: 1989-01-11
Publication date: 1989-07-27
Also published as: CA1315583C; SE459924B; AU2925089A; NZ227679A; SE8800200D0; US5145010A

Abstract

The making of mechanical pulp from softwood intended for coated light-weight paper (LWC), for uncoated calendered magazine paper (FSC) or the like is carried out in several steps as follows: a) impregnation (2) of chips with water in combination with complex-forming agent; b) refining of the chips with double-disc refiner (3); c) fractionating screening (6-10) of the pulp; d) refining (12, 14) of the screen reject in two steps, at high and, respectively, low concentration; e) screening (15) of the refined reject and combining with the accept from the screening (6-10) according to step c).

Description

Method of making mechanical pulp

This invention relates to the making of mechanical pulp from softwood intended for coated paper with low gramm- age, so-called L C-paper (light weight coated), uncoated calendered magazine paper, so-called FSC-paper (filled supercalendered) or similar paper qualities.

This type of.paper makes very high demands on the prop¬ erties of the pulp , because the paper must have high strength and low roughness. Coated qualities, besides, require low porosity. It is particularly important, that these papers have a smooth surface structure.

This type of paper normally contains both chemical and mechanical pulp. The mechanical pulp component traditi¬ onally has been groundwood pulp..In recent years also thermomechanical pulp (TMP) has been used as an altern¬ ative to groundwood, but with limited success, because the energy consumption is relatively high compared with the manufacture of groundwood. There exist, furthermore, several examples that the use of TMP has resulted in unevenesses in the surface structure of the paper, which in its turn has implied poor coating and poor printab- ility. These problems could be avoided only in cases when the paper manufacturer had taken special measures capable to modify or eliminate the negative effects of the long fibre fraction in the thermomechanical pulp. This long fibre fraction affects in a negative way the smoothness of the paper, because it causes poor forming and also because it contains some long stiff fibres and has poor binding strength.

The present invention implies, that the energy consumpt¬ ion at the manufacture of TMP is reduced and at the same time the pulp quality is improved, thereby rendering it possible to increase the amount of mechanical pulp in the stock and the amount of chemical pulp be decreased compared with the traditional formulas for the differ¬ ent paper qualities. TMP for use in LWC-paper and the like usually is manuf¬ actured by refining in two or more steps and subsequent screening and reject processing, bleaching and post-re¬ fining.

According to the present invention, the softwood in the form of chips is impregnated with water in combination with complex-forming agent, preferably within the pH range 5-9• The impregnation is followed by a re ining under pressure in a doubl_.-disc refiner, i.e a refiner with two counter-rotating discs. The energy input at the refining shall be 1300-2300 k h/ton, preferably 1900-2100 kWh/ton. Thereafter a fractionated screening of the pulp is carried out, preferably in two steps with re-screening of the reject. After dewatering, the screen reject is refined in two steps. The first step takes place at high concentration under pressure, preferably in a disc-refiner of single-disc type, i.e. with one stationary and one rotating disc. This refin¬ ing suitably is carried out with a specific energy input of 1000-2000 kWh/ton reject, preferably 1200-1500 kWh/ ton. The second step is carried out at low concentrat¬ ion, preferably in a pump-fed disc-refiner of the same type as in step one. The energy input here should be 50-500 kWh/ton, preferably 100-200 kWh/ton.

The invention implies, that the development of light- -scattering coefficient can be maximized, and that the long fibre proportion can be minimized in an energy-saving way in the first and only refining step before screening. It is generally known that double-disc refiners yield a higher light-scattering coefficient and a lower long fibre proportion than single-disc re¬ finers. It also is known that refining with high spec¬ ific energy input in a single refining step results in a pulp with shorter fibres than refining in two steps, unless special measures are taken to prevent the same. It further is known that chips having a low temperature at the passage into a refiner, and thereby during the defibering phase, yield a pulp with shorter fibres and with higher light-scattering than preheated chips with the same refining energy.

The special design of the screen room implies that a large proportion of the fibres in the pulp can be con¬ centrated in the reject circuit. The reject fraction, thus, shall be 15-35*, preferably 18-25* of the total pulp flow. Owing to the refining of the reject at high concentration,the long stiff fibres become flexible. The subsequent refining at low concentration has the object to reduce the amount of long fibres in the pulp. It is generally known that refining at high concentrat¬ ion develops the binding strength of the pulp and increases its density, but reduces its long fibre content only to a small extent. This applies especially to high-concentration refining of a long-fibre reject. It also is generally known that refining at low concen¬ tration of long-fibre reject results in substantial shortening of the fibres, unless special measure are taken for preventing this.

The invention implies that the total energy consumption for the refining is reduced not only, because the re¬ fining is carried out in a single step in adouble-disc refiner, but also because the final refining of the pulp takes place on the smaller amount of the pulp containing the long fibres, which are to be made flexible and shorter.

The characterizing features of the invention appear in greater detail from the attached claims.

The invention is described in the followingwith refer¬ ence to an embodiment thereof, which is shown in the accompanying Figure by way of a flow chart. The pretreatment of the raw material in the form of spruce chips is carried out by washing and atmospheric steaming 1 followed by water impregnation 2 with com¬ plex-forming agent present within the pH range 5-9. The material thus pretreated is refined under pressure in adouble-disc refiner ^~ . The refining in this first step with 2000 kWh/ton yields a pulp with a freeness value according to CSF of 95 ml and a fibre length distribution according to Bauer Mc Nett characterizat¬ ion as follows

After steam separation in a pressure cyclone 4, latency is removed in a vat 5. Thereafter a fractionating screening of the pulp is carried out in two steps in primary screens 6,7, which are pressure screens with mesh size 1,8 mm and, respectively, 1,6 mm. The reject withdrawal amounts to 20* and, respectively, 25*.

The resulting accept has a freeness value according to CSF of -40 ml and a fibre distribution according to Bauer Mc Nett characterization as follows

+ 16 0 - 2 *

16/30 20 - 22 * -200 38-40 *

The screen rejects combined in vat 8 are re-screened in two steps in secondary screens 9_S10, which are pressure screens with mesh size 2,2 mm and, respectively, 2,0 mm. The reject withdrawal amounts to 25* and, respectively, 30*.

The combined reject from these secondary screens 9,10 amounts to 19* of the entire pulp flow and has the characteristics as follows: Freeness 50 ml CSF

+ 16 40 - 44 *

16/30 26 - 28 *

-200 4 - 6 *

This reject is passed through a dewatering press 11 where the concentration is increased to 20-35 *, where¬ after the reject is refined in a pressure refiner 12 with an energy input of 1250 kWh/ton. The resulting characteristics are as follows:

Freeness 110 ml CSF

+ 16 25 - 28 *

16/30 25 - 28 *

-200 δ - 11 *

This refined reject is diluted in a vat 13 to a con¬ centration of about 5* and refined in a single-disc refiner 14, which renders possible precision adjustment of the gap. With an energy input of 150 kWh/ton a reduction of the long fibre content by about 70* is obtained, and the reject shows the characteristic as follows:

Freeness 80 ml CSF

+ 16 8 - 10 *

16/30 25 - 27 *

-200 9 - 12 *

The refined reject is screened in one step with a reject screen 15, which is a pressure screen with mesh size 1,8 mm with a reject withdrawal of 10 *.

The accept from this reject screen 15 and from the two secondary screens 9,10 combined with the accept from the two primary screens 6,7 constitute the final pulp, which is dewatered to be bleached with ditionite or peroxide to suitable diffuse blue reflectance. The final unbleached pulp manufactured with a total refining energy of 2250 kWh/ton has the fibre charact¬ eristics as follows:

Freeness 50 ml CSF + 16 - L 1 %

+ 30 i 21 * -200 3 * and the pulp characteristics according to TAPPI test standard as follows:

The invention, of course, is not restricted to the embodiment described above, but can vary within the scope of the invention, especially as regards the design of the screen room and the location of the bleaching operation in the process scheme. The bleach¬ ing, for example, advantageously can be carried out on the refined pulp prior to the screening.

Claims

1. A method of making mechanical pulp from softwood intended for coated light weight paper (LWC), for un- coated calendered magazine paper (FSC) or the like, c h a r a c t e r i z e d i n a combination of the steps a) impregnation of the material in the form of chips with water in combination with complex-forming agent within the pK range 5- 5 b) a first refining of the water-impregnated chips under pressure in a disc-refiner with two counter-

-rotating discs (double-disc refiner) to a long fibre content measured according to Bauer'Mc Nett fraction- _. ation to i-8 * on + 16 mesh and to 5-.26 % on 16-30 mesh; c) fractionating screening of the pulp so that a greater proportion of the long and stiff fibres of the pulp is found in the reject fraction, which amounts to 15-35*, preferably 18-25* of the pulp flow; d) refining of this reject in two steps, the first one under pressure at high concentration, and the second one at low concentration to a fibre length distribution of the reject measured according to Bauer Mc Nett characterization to _ 10 % on + 16 mesh and —27 * on 16-30 mesh; e) screening of the refined reject and combining of the accept from this screening with the accept from the screening according to step c.

2. A method as defined in claim 1,c h a r a c t e r ¬ i z e d i n that the final pulp has a long fibre content measured according to Bauer Mc Nett character¬ ization to —1 * on + 16 mesh and .21* on l6-30 mesh.

3. A method as defined in claim 1 or 2, c h a ¬ a c t e r i z e d i n that an energy input is made in the first refining of 1800-2300 kWh/.on, preferably 1900-2100 kWh/ton.

4. A method as defined in any one of the claims 1-3, c h a r a c t e r i z e d i n that a specific energy input is made at the reject-refining in high concentration of 1000-2000 kWh/ton reject, preferably 1200-1500 kWh/ton reject.

5. A method as defined in any one of the claims 1-4, c h a r a c t e r i z e d i n that a specific energy input is made at the reject-refining in low concentration of 50-300 kWh/ton, preferably 100-200 kWh/ton reject.