AU616999B2 - Method of making chemi-mechanical pulp from hardwood - Google Patents

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OPI DATE 02/04/90 PCT AOJP DATE 10/05/90 APPLN. ID 40374 89 PCT NUMBER PrT/SE89/00395 INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT CO "RATION TREATY (PCT) (51) International Patent Classification 4: (11) International Publication Number: WO 90/02835 D21B 1/02, D21C 3/26 Al (43) Inte on Pubi on 2(jrch 1990 (22.03.90) (21) International Application Number: PCT/SE89/00395 (74) Agent: SUNDQVIST, Hans; Svenska Cellulosa Aktiebolaget SCA, Kungsgatan 33, 7tr, S-111 56 Stockholm (22) International Filing Date: 7 July 1989 (07.07.89) (SE).

Priority data: (81) Designated States: AT (European patent), AU, BE (Euro- 8803244-6 14 September 1988 (14.09.88) SE pean patent), BR, CH (European patent), DE (European patent), FI, FR (European patent), GB (European patent), IT (European patent), LU (European patent), NL (71) Applicant (for all designated States except US): SUNDS DE- (European patent), NO, SE (European patent), US.

FIBRATOR INDUSTRIES AKTIEBOLAG [SE/SE]; S-851 94 Sundsvall (SE).

Published (72) Inventors; and I ith international search report.

Inventors/Applicants (for US only) DANIELSSON, 1' Ove [SE/SE]; Nybergsgatan 8, S-114 45 Stockholm FALK, Bo, S. [SE/SE]; Avstyckningsvigen 46, S-175 43 Jiirfiilla JACKSON, Michael [GB/CA]; 1045 Marigold Avenue, North Vancouver, British Columbia V7R 2E1 (CA).

(54)Title: METHOD OF MAKING CHEMI-MECHANICAL PULP FROM HARDWOOD (57) AbStract Mtnufacture of chemi-mechanical pulp from hardwood intended for wood-containing printing paper, especially woodcontaining fine paper. The wood material in the form of chips is impregnated in one or two steps at a temperature below with 15-40 kg NaOH and 0-30 kg Na 2

SO

3 per ton pulp. Thereafter the chips are refined under pressure, so that the temperature of the chips prior to the refining does not exceed 80 0 C for a time longer than ten seconds. Bleaching of the pulp with peroxide, at which bleaching at least 15 kg NaOH per ton pulp is charged additionally, so that the total amount of NaOH charge is 30-80 kg per ton pulp. After the bleaching a second refining step is carried out.

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WO 90/02835 PCr/SE89/00395 1 Method of making chemi-mechanical pulp from hardwood This invention relates to the making of chemi-mechanical pulp from hardwood for wood-containing printing paper, especially wood-containing fine paper.

Fine paper traditionally is manufactured from a mixture of short-fibred and long-fibred chemical pulp. In order to improve the opacity of the paper and to increase its bulk, conventionally a certain amount of bleached groundwood pulp was mixed into the stock. The term wood- -containing fine paper is used when the amount of mechanical pulp exceeds 107.

In recent years a new type of pulp, peroxide-bleached chemi-mechanical pulp of hardwood, so-called CTMPH, has implied an interesting alternative as a replacement of both the groundwood pulp and the chemical hardwood pulp at the manufacture of wood-containing printing papers. Especially asp and poplar have been used therefor.

For this type of paper very high requirements are made on the properties of the CTMPH-pulp, because the paper in addition to high ISO-brightness and good opacity preferably also should have high bulk and simultaneously high stremgth and low roughness. For coated papers also low porosity is required. It is especially important, that these papers have a uniform surface structure.

For being able to use CTMPH-pulp in wood-containing fine paper, it must be refined to a drainage resistance of 100-200 ml CSF, and it must be possible to bleach it to a brightness of 82-85% ISO, which is a very high brightness for a mechanical pulp. The energy consumption at the refining thereby is relatively high, and so is the chemical consumption at the bleaching. This applies particularly to dark hardwood with high density and various types of eucalyptus. An extensive equipment WO 90/02835 PC/SE89/00395 2 in the form of means for drainage and washing is required for the bleaching.

The present invention implies, that this equipment can be simplified and the consumption of chemicals and electric energy be reduced without deteriorating the pulp quality.

According to the invention, the hardwood in the form of chips is impregnated in one or two steps at a temperature below 8 0 °C with 15-40 kg NaOH and 0-30 kg Na 2

SO

3 per ton pulp. Thereafter a refining under pressure is carried out in such a way, that the temperature of the chips prior to the refining does not exceed 80 0 C for a period longer than 10 seconds. Subsequent bleaching of the pulp is carried out with peroxide in one or two steps, and additionally a minimum of 15 kg NaOH is charged so that the total amount of NaOH charged is kg per ton pulp. Thereafter refining of the pulp is carried out in a second step.

The invention implies that the brightness and light-scattering coefficient of the pulp is maximized and at the same time the energy consumption at the refining is minimized.

It is generally known that a high alkali charge during the impregnation step prior to the refining increases the light absorption coefficient and decreases the light-scattering coefficient of the pulp, i.e. decreases the brightness, but improves the strength and reduces the shives content.

According to the invention, the strength of the pulp is determined by the total of the alkali charge during the impregnation and peroxide bleaching steps.

According to the invention, the alkali charge during the impregnation step shall be determined only with regard being paid to the shives content and fibre distribution number and disregarding the pulp-:strength obtained by WO 90/02835 PCT/SE89/00395 the alkali charge during the bleaching step. As a result, the reduction in brightness and light scattering coefficient can be minimized. The refining in the second step is carried out with partially washed pulp, i.e. pulp containing residual alkali and residual peroxide from the bleaching, in order to minimize the energy demand at the refining and to be able to utilize the refining step as a third bleaching step carried out at a very high temperature, whereby it shall be possible to obtain highest brightness with a certain peroxide charge. The total peroxide charge at the bleaching shall be 30-50 kg peroxide per ton pulp, preferably 30-40 kg for light wood types and 40-50 kg for dark ones.

The characterizing features of the invention become apparent from the attached claims. The invention is described in the following with reference to an embodiment thereof and experiment results shown in the accompanying Figures, of which Fig. 1 shows the ISO-brightness after peroxide bleaching as a function of the NaOH-charge during the impregnation step, and Fig. 2 shows the tensile strength of the pulp as a function of the total NaOH-charge.

The pretreatment of the hardwood in the form of chips of eucalyptus saligna is carried out by washing, atmospheric steaming and so-called PREX-impregnation with a NaOH/Na 2 SO -solution with pH 13. By the impregnation, kg NaOH and 20 kg Na 2

SO

3 per ton are supplied to the chips. The chemicals are allowed to react for minutes with the chips which hold 50 0

C.

The chemicals and solved substance are thereafter pressed out of the chips, which are pressure refined with 1000-1200 kWh/ton at a steam pressure of 3,5 kg/cm 2 in the refiner house to a dewatering number measured as 350 ml CSF. The energy input at this refining should WO 90/02835 PCT/SE89/00395 be 800-1400 kWh/ton The pulp is diluted and dewatered to 35% pulp concentration and thereafter mixed with complex former, preferably EDTA, at pH 5, thereafter diluted and again pressed to 35% dry matter content.

The pulp is thereafter peroxide bleached in two steps, the first one at 15-18% pulp concentration with residual peroxide from the washing following after the second bleaching step. After the first bleaching step, which is carried out at 700C for 30 minutes, the pulp is again washed and pressed to 35% dry matter content, at which content the fresh bleaching chemicals are mixed into the pulp. The pulp is bleached for 120 minutes at 7000 with 40 kg peroxide, 25 kg NaOH and 30 kg sodium silicate Na 2 SiO 3 per ton pulp, whereafter the pulp is washed by dilution with 4,5 parts washing water and pressing to 35% dry matter content. The pulp is refined in a pressurized disc refiner,together with reject from the screen room, with 600 kWh/ton, with 3.5 kg/cm 2 steam pressure and simultaneous addition of 5 kg peroxide per ton pulp to the refiner inlet. The energy input at this refining Bhbuld be 400-800 kWh/ton. The pulp is latency treated at 3.5% pulp concentration and 70 C, screened in pressurized screens in two steps and thereafter pumped to storage before being used in a paper mill.

After the different reaction steps, the pulp has the properties as follows: After the first refining step Freeness 350 ml CSF Fibre length acc. to Bauer Mc Nett 16 1% 16/30 8% 200 Light scattering coefficient 48 m2/kg ISO-brightness WO 90/02835 PCT/SE89/00395 After the secordbleaching step Freeness Light scattering coefficient ISO-brightness After the second refining step Freeness Fibre length acc. to Bauer Mc Nett +16 16/30 -200 Light scattering coefficient ISO-brightness After the screening room Freeness Fibre length acc. to Bauer Mc Nett 16 16/30 -200 Tensile index Tear index Light scattering coefficient ISO-brightness 325 ml CSF 48 m2/kg 160 ml CSF 0% 83% 50 m2/kg 82% 125 ml CSF 0% 2% 24% 40 Nm/g 4.5 mNm2/g 50 m /kg 82.5% Different NaOH-charges during the impregnation step result in different ISO-brightness of the pulp after the peroxide bleaching.

A lower.NaOH-charge implies, that it is possible to obtain a higher final ISO-brightness of the pulp. In Fig. 1 is shown how the brigltness varies with the NaOH- -charge during the impregnation step at three different peroxide charges.

Pulp I was manufactured from chips, which had been impregnated with 25 kg NaOH per ton pulp. With a peroxide charge of 45 kg per ton pulp it is then possible to obtain a brightness of above 80% ISO.

WO 90/02835 PCT/SE89/00395 6 Pulp II was manufactured from chips impregnated with kg NaOH per ton pulp.In spite of bleaching with a peroxide charge as high as 45 kg per ton pulp, it is not possible to increase the brightness higher than to about 77% ISO.

At the embodiment described above the original NaOH- -charge was 20 kg per ton pulp. The brightness could there be increased by peroxide bleaching to 82.5% ISO.

A lower NaOH-charge at the chip impregnation, however, results in a decrease of the strength properties of the pulp. It was found, however, that the pulp strength within certain limits depends only on the total NaOH-charge during the impregnation and peroxide bleaching steps. During the impregnation step, therefore, NaOH is charged in an amount of 15-40 kg per ton pulp in order to ensure the necessary shives content and fibre distribution number.

Additional NaOH in an amount of at least 15 kg per ton pulp to a total amount of 30-80 kg per ton pulp is charged first in connection with the peroxide bleaching after the first refining. As regards the NaOH-charge can be said, that heavy and dark wood types, such as eucalyptus, require more alkali for the development of their strength than wood types, which are lighter and brighter, such as asp and poplar. Against this background, the NaOH-charge for the heavier and darker wood types should be 20-40 kg and for the lighter and brighter ones 15-25 kg per ton pulp. The total NaOH-charge should be 40-80 and, respectively, 30-50 kg per ton pulp.

In Fig. 2 is shown how the tensile strength of the pulp changes with the total amount of NaOH charged and peroxide bleaching with 45 kg peroxide per ton pulp. Freeness 125 ml CSF.

It is, thus, possible at the peroxide bleaching by means of an additional NaOH-charge to increase the tensile -i i i- WO 90/02835 PCT/SE89/00395 7 strength of the pulp I to a level very close to the tensile strength of pulp II, i.e. about 45 Nm/g.

One prerequisite for being able to increase the strength properties of the pulp in this way, hovever, is that the original NaOH-charge during the impregnation step is at least 15 kg NaOH per ton pulp, see above.

The invention, of course, is not restricted to the embodiments described above, but can be varied within the scope of the invention idea.

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Claims (2)

1. A method of manufacturing chemi-mechanical pulp from hardwood for wood-containing printing paper, especially wood-containing fine paper, c h a r a c t e r i z e d i n that it comprises the combination of the steps a) impregnation of the wood material in the form of chips in one or two steps at a temperature below 80 C with 15-40 kg NaOH and 0-30 kg Na 2 SO 3 per ton pulp; b) refining of the wood chips in a first step under pressure, so that the temperature of the chips prior to the refining does not exceed 8000 for a time longer than 10 seconds; c) bleaching of the pulp with peroxide in one or two steps, at which at least 15 kg NaOH is charged addition- ally so that the total amount of NaOH charged is

30-80 kg/ton pulp; d) refining of the pulp in a second step. 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 peroxide bleaching is carried out with a total charge of 30-50 kg peroxide per ton pulp. 3. A method as defined in claim 1 or 2 c h a r a c t e r i z e d i n that 5 kg peroxide per ton pulp of the total peroxide charge is added during the second refining step. 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 the first refining step is carried out with an energy input of 800-1400 kWh/ S f ton pulp, and the second one with 400-800 kWh/ton pulp.