EP0413736B1

EP0413736B1 - Method of making mechanical and chemi-mechanical papermaking pulp

Info

Publication number: EP0413736B1
Application number: EP19890905472
Authority: EP
Inventors: Per Ossian Engstrand; Lars-Ake Hammar; Myat Thoung Htun; Rune Lennart Pettersson; Börje Nils SVENSSON
Original assignee: STFI Skogsindustrins Tekniska Forskningsinstitut AB
Current assignee: STFI Skogsindustrins Tekniska Forskningsinstitut AB
Priority date: 1988-05-06
Filing date: 1989-04-05
Publication date: 1993-09-15
Anticipated expiration: 2009-04-05
Also published as: FI91787C; CA1320067C; SE8801731L; WO1989010998A1; DE68909231D1; JPH03504256A; SE461103B; FI91787B; EP0413736A1; SE8801731D0; DE68909231T2; FI905482A0

Abstract

The invention relates to a method of manufacturing mechanical and chemi-mechanical papermaking pulp with low energy input by disintegrating and beating wood material in at least two steps. According to the invention, the material is coarse-disintegrated in a first step at a concentration exceeding 20 %, acid groups in the wood material are neutralized, the material is diluted to a concentration of 1-10 % and beaten in one or several steps.

Description

This invention relates to a method of making mechanical and chemi-mechanical papermaking pulp by disintegrating and beating wood material in at least two steps.
One object of the invention is to carry out the disintegration and beating in such a manner, that the total energy consumption is substantially reduced, as will be be described in the following.
The beating of cellulose-containing material at low pulp concentration is a method, which has been employed since long in order to improve the paperforming properties of the fibres. This applies, however, only to fibres free of lignin or substantially free of lignin, such as fibres produced according to the sulphate or sulphite method. As regards pulps manufactured mechanically, such as thermomechanical pulp (TMP) or chemi-mechanical pulp (CTMP), beating at low concentration, so-called post-refining, was not considered applicable other than as a method for increasing the light-scattering capacity of the pulps and for reducing slightly the fibre length and thereby improving the formation at the making of paper.
Investigations have been carried out previously to re-beat at lower concentrations a TMP manufactured at high concentration. In Scan Forsk Rapport 409/1984, for example, works are reported concerning energy consumption at re-beating at low concentration compared with refining at high concentration. The results from this investigation show that the freeness of TMP can be lowered by 10-30 ml without essentially deteriorating the strength properties, and that an energy saving of 50-150 kWh/ton could be achieved. The total energy consumption, however, was considerable and of the magnitude 1600 to 2300 kWh/ton.
In Pulp and Paper Magazine of Canada, Vol. 81, No 6, June 1980, page 72-80 (N.Hartler) experiments are reported to reduce the energy consumption at the refining of chips. One proposal made here is to change the chemical environment by the addition of chemicals. By adding sodium hydroxide the energy consumption could be reduced by 30%, but the total consumption yet amounts to about 1300 kWh/ton. At these experiments, however, the yield was deteriorated slightly and the ISO-brightness considerably.
In Svensk Papperstidning, 1982, page R 132-139 (P.Axelson and R.Simonson) the effect of sulphite impregnation of the chips on the refining process, a.o. the energy consumption, is discussed. At a certain amount of sulphite taken-up, the energy consumption diagram showed a minimum. Totally, however, the energy consumption was on a high level of 2000 kWh/ton.
Experiments have been carried out previously to treat thermomechanical pulp with fibre-modifying chemicals. It was then found, that by treating the defibered pulp with ozone prior to the refining in a two-step process the energy consumption could be lowered by up to 30%. This, however, could be achieved only at the expense of the yield.
In WO-A-87106280 the addition of alkali is disclosed.
According to the present invention it has proved possible to manufacture mechanical papermaking pulp by a substantially reduced energy consumption.
This is achieved according to the invention, in that the wood material in a first step is coarse-disintegrated at a concentration of above 20%. The energy input here shall be at maximum 8oo kWh/ton wood material. The acid groups included in the wood material thereafter shall be neutralized entirely or partially, and the material be diluted with water of a temperature corresponding to the softening temperature of the lignin. The dilution water shall have an ion strength of at maximum 0.05 mole per litre. The coarse-disintegrated material then shall be beaten at a concentration of 1-10% with an energy input of totally a maximum of 500 kWh/ton material.
The present invention is based on the idea that there is a relation between the disintegration of the wood material to fibres and the way, in which the energy pulses are transferred to the material, i.e. whether the energy pulses are transferred in liquid phase or steam phase. Attention also is to be paid to the thermal and physical state of the wood material when the energy pulses are being transferred.
One has not been successful previously by beating at low concentration to defibre the wood package in order to reduce the energy consumption at the manufacture of mechanical pulps. The reason is, that one did not know how to avoid the clipping of the fibres and thereby the much too low tensile and tear index of the resulting mechanical pulp and at the same time to bring about improved binding properties of the pulp.
In order to achieve this, it is important to accurately control the temperature and chemical environment of the fibre suspension in connection with the beating.
For obtaining a low total energy consumption, the energy input in the first coarse-defibering step must be low. The first high concentration step can be at atmospheric pressure or pressurized and be carried out by tearing (shredding), chip pressing, plug screwing (type Impressafiner or PREX) or by defibering in a refiner.
The final beating then takes place in one or several steps at low pulp concentration, i.e. at a concentration of 1-10%. At this beating must be observed, that the specific edge load is sufficiently low, and that the temperature and chemical environment of the fibre suspension has been adjusted to the softening and swelling state of the wood polymers. This implies according to the invention, that the temperature at the beating shall be at least as high as the softening temperature of the stiffest amorphous wood polymer, that the acid groups of the wood polymers substantially are ionized, and that the ion strength of the process water is sufficiently low.
The invention is described in the following in greater detail by way of some embodiments and with reference to to the accompanying drawings, in which

Fig. 1 is a flow sheet of an embodiment of the method according to the invention,
Figs. 2-4 are diagrams of properties and energy consumption of a pulp manufactured according to Fig. 1,
Fig. 5 is a flow sheet of a second embodiment of the invention, and
Figs. 6-8 show properties and energy consumption at the method according to said second embodiment.

EXAMPLE 1

The flow sheet according to Fig. 1 illustrates the manufacture of thermomechanical pulp for newsprint.
Chips from spruce were steamed in a first step and preheated. The preheated chips then were disintegrated in a pressurized refiner with an energy consumption of 700 kWh/ton. At this coarse-defibering 3 kg NaOh were added in the beating zone of the refiner for neutralizing acid groups included in the wood material. To the defibered material dilution water with a temperature of 80^oC and an ion strength of 2.0 mmole/l was added in order to obtain a pulp concentration of 3%.
At this concentration the pulp then was beaten in five subsequent steps at a specific edge load of 0.3-0.5 ws/m and a total net energy consumption of 150 kWh/ton pulp corresponding to a gross energy consumption of 250 kWh/ton pulp to a freeness of 150 ml CSF and a mean fibre length (PML) of 1.8 mm, i.e. about equal to TMP-pulp manufactured in conventional manner with an energy consumption of 1750 kWh/ton pulp.
The total energy consumption at the method according to the invention, thus, was reduced from 1750 to 950 kWh/ton pulp.
The yield amounted to about 97%.
A comparison between the properties for a TMP-pulp manufactured conventionally and one manufactured according to the invention appears from Table 1.
In this connection should be mentioned, that as conventional TMP-process the refining system was used, which up till now was known as the least energy requiring one, i.e. a pressurized twin-disc refiner combined with short dwell time at the pressurized preheating.

When two-step processes with single-disc refining are used, in most cases more than 2000 kWh/ton are required for obtaining a pulp with 150 m/CSF.

TABLE 1

	T M P
	conventioan	Invention
Energy consumption,kWh/ton	1750	950
Freeness, ml CSF	150	150
PML x), mm	1.9	1.9
Shives content,Sommerville %	1.3	0.5
Tensile index, kNm/kg	32.0	32.0
Tensile stiffness index	3.4	3.4
Stretch at break, %	2.0	1.9
Tear index, Nm²/kg	6.5	5.5
Density, kg/m³	380	380
S, m²/kg	58.0	58.0
ISO-brightness, %	60	60

x) PML = mean particle length measured according to SIFI pulp measuring system

The manufacture of TMP according to the invention is compared in Fig. 2, 3 and 4 with TMP manufactured conventionally with single-step refining in twin-disc refiner, which is the least energy consuming TMP-process existing with the present state of art.
Fig. 2 shows the tensile index as a function of the electric energy consumption. It appears clearly from the Figure, that the increase in tensile index at a certain electric energy consumption is considerably greater for TMP manufactured according to the invention.
Fig. 3 shows the tear index as a function of the tensile index for TMP according to the invention and conventional TMP. It appears that the development of the tear index for the respective TMP is about the same, i.e. at optimation of the low concentration beating according to the invention the fibre cutting and thereby the serious decrease in tear index are substantially entirely avoided, which cutting and decrease occur usually at conventional low concentration beating of mechanical pulps.
Fig. 4 shows how the light-scattering coefficient (s) develops at conventional TMP and TMP manufactured according to the invention. It appears that the s-development requires as low an electric energy input as the tensile index development, i.e. the saving of electric energy to a certain s-value is of equal size as the saving of electric energy to a certain tensile index value.

EXAMPLE 2

This example relates to the manufacture of chemi-mechanical pulp (CTMP or CMP) according to the flow sheet shown in Fig. 5.
The treatment with impregnation chemicals, which can be sulphites, peroxide, oxygen gas, ozone and/or liquor, can take place prior to the first defibering step, after this step but prior to the final beating, after the final beating, or at combinations of these. In the flow sheet shown the impregnation is carried out prior to the first defibering step.
The first defibering step at high concentration is carried out in the same way as in Example 1.
At the manufacture of chemi-mechanical pulp the washing step is very essential, so that according to the invention the beating shall take place at low ion strength. The washing, therefore, is carried out prior to the final beating. In cases when the chemical treatment is carried out as the last process step, washing takes place even after this step.
The final beating takes place in the same way as according to Example 1, but process temperature and chemical environment must be adjusted to the special properties, which the wood polymers have assumed by treatment with impregnation chemicals. It is essential to pay regard to the number of sulphonic acid groups, which have been introduced by possible sulphite treatment. As an increasing amount of sulphonic acid groups reduces the softening temperature of the lignin, the temperature of the process water can be lower than at the manufacture of TMP-pulp according to Example 1. A sufficiently high temperature at the manufacture of CTMP is 40^oC. From a brightness point of view it is advantageous to use the lowest possible temperature. By sodium sulphite treatment both the sulphonic acid groups and the carboxylic acid groups ate ionized from the beginning.
When at the manufacture of CTMP it is chosen to modify the wood with peroxide, oxygen or ozone, certainly no sulphonic acid groups are obtained, but the content of the wood material, especially of the lignin, of carboxylic acid groups increases considerably, which implies lowering of the softening temperature of the lignin.
In the eaxmple according to Fig. 5 chips from spruce were steamed and impregnated with a sodium sulphite solution in an amount corresponding to a 2% charge, whereafter they were preheated at a temperature of 130^oC for 3 minutes. The material was then coarse-disintegrated with an energy consumption of about 600 kWh/ton in a pressurized chip refiner at high concentration (about 35%). The resulting yield amounted to about 96%. The coarse-defibered material obtained was diluted to about 3% and latency treated at 80^oC for 20-30 minutes. The pulp was pressed to a concentration of 45-50% and again diluted to 3% at a temperature of 70^oC. At this concentration the pulp then was beaten with a specific edge load of 0.3-0.5 Ws/m in five subsequent steps with a net energy consumption of 150 kWh/ton corresponding to a gross energy consumption of 250 kWh/ton for obtaining a pulp with a freeness of 250 ml CSF and a mean fibre length (PML) of 1.7 mm, i.e. as a conventionally manufactured CTMP-pulp produced in one step with an energy consumption of 1750 kWh/ton.
By the method according to the invention, thus, the energy consumption was reduced from the conventional 1750 kWh/ton to 850 kWh/ton.

The properties of the CTMP-pulp obtained compared with conventional CTMP-pulp are shown in Table 2.

TABLE 2

	Conventional	Invention
Energy consumption,kWh/ton	1750	850
CSF ml	250	250
PML, mm	-	1.7
Tensile index, kNm/kg	40	40
Tensile stiffness index	-	4.6
Stretch at break, %	1.9	1.6
Tear index Nm²/kg	6.7	5.5
Density, kg/m³	420	450
S, m²/kg	43	45
ISO-brightness, %	60	60

The manufacture of CTMP according to the invention is compared in Figs. 6,7 and 8 with manufacture according to conventional technique.
In Fig. 6 the tensile index is shown as a function of the energy consumption for a CTMP-pulp manufactured according to the invention and for pulp manufactured conventionally. Compared at a certain tensile index, for example 40 kNm/kg, conventional refining consumes about 1750 kWh/ton pulp while at the method according to the invention only about 850 kWh/ton are consumed.
At a comparison of the method according to the invention with conventional method in a relationship with the tear index as a function of the tensile index, it appears that the relationships are similar, i.e. one avoids the fibre cutting, which is usual at low concentration beating and which gives rise to a substantially reduced tear index. This appears from the diagram in Fig. 7.
The light-scattering coefficient of the CTMP-pulp manufactured according to the invention as a function of the energy consumption is shown in the diagram in Fig. 8 compared with conventionally manufactured pulp. It shows here that according to the invention the energy consumption to a certain light-scattering index is substantially lower.

EXAMPLE 3

This example relates to the manufacture of highly sulfonated CTMP or CMP, i.e pulp containing more than 4 g of bound sulphur per kg wood material.
Chips from spruce were impregnated with a sodium sulphite solution containing about 120 g of sodium sulphite per litre in an amount corresponding to a charge of about 12%. The chips were preheated at a temperature of 140^oC for 10 minutes, whereafter they were coarse-disintegrated with an energy consumption of about 400 kWh/ton wood. The defibration was carried out in a pressurized chip refiner, and the yield obtained was 93-94%. After a latency treatment at 60^oC for 20-30 minutes at a pulp concentration of 3%, the pulp was beaten in three steps at an edge load of 0.3 to 0.5 Ws/m and a net energy consumption of 100 kWh/ton corresponding to a gross energy consumption of 160 kWh/ton.

The properties of the resulting pulp were determined, and in the following Table 3 the values obtained are stated in comparison with a CTMP manufactured in a conventional way in a one-step process with an energy input of 1500 kWh/ton.

TABLE 3

	Conventional	Invention
Energy consumption, kWh/ton	1500	560
CSF, ml	400	400
PML, mm	1.9	1.9
Tensile index, kNm/kg	65	65
Stretch at break, %	2.0	1.8
Tear index, Nm²/kg	8.0	8.0
Density, kg/m³	440	450
S, m²/kg	34	36
ISO-brightness, %	59	59

EXAMAPLE 4

This embodiment is an example of how in the first step extruders can be used for coarse-disintegrating wood material. According to the example an extruder of the type Bivis was used.
Spruce chips were steamed in the usual manner at 100^oC for 10 minutes, whereafter they were fed into a Bivis-machine. At the defibration in the machine 2-3% sodium sulphite solution was charged so that the sulfonation degree of the material amounted to 1.5 g of sulphur per kg wood. The electric energy consumtion was about 400 kWh/ton wood when the material passed through the four compression zones of the twin-screw. After discharge, the fibre material was diluted to about 5% pulp concentration at about 70^oC, whereafter the suspension was pumped to beating in seven steps in a low concentration refiner. After the fifth beating step, the freeness of the pulp was about 250 ml CSF, the tensile index was 55 kNm/kg, and the tear index 6 Nm²/kg at a net energy consumption of 150 kWh/ton and a gross energy consumption of 250 kWh/ton.
The total energy consumption for the manufacture of chemi-mechanical pulp (CTMP) to a freeness value of 250 ml CSF by the method according to the invention, thus, amounts to 650 kWh/ton, which is to be compared with about 1750 kWh/ton according to the best conventional technique for obtaining the same freeness value.
By beating in seven low concentration steps according to the invention, a freeness value of 50 ml CSF can be obtained, and the resulting CTMP-pulp is suitable for use in magazine and LWC-paper. The total energy consumption yet can be kept below about 850 kWh/ton. Pulp of this lastmentioned type cannot be manufactured with conventional technique,because then a sufficiently good smoothness cannot be achieved.
The invention is not restricted to the embodiments described, but can be varied within the scope of the invention idea.

Claims

A method of manufacturing mechanical and chemi-mechanical papermaking pulp with low energy input by disintegrating and beating wood material in at least two steps, characterized in that the material is coarse-disintegrated in the first step at a concentration above 20% with an energy input of at maximum 800 kWh/ton wood material, £hat acid groups existing in the wood polymers are neutralized entirely or partially by the addition of NaOH in an amount of at maximum 9 kg/ton, that the material is diluted by water with a temperature corresponding to the softening temperature of the lignin, i.e. 40-95^oC, and with an ion strength of at maximum 0.05 mole per litre, and that the material then is beaten in one or several steps at a concentration of 1-10% with an energy input of totally at maximum 500 kWh/ton material.
A method as defined in claim 1, characterized in that at least 25% of the total energy input is supplied at the beating.
A method as defined in claim 1, characterized in that the coarse-disintegrated material after the first step is washed in order to reduce the ion strength.
A method as defined in any one of the preceding claims, characterized in that the energy input in each beating step amounts to 50-150 kWh/ton pulp.