METHOD OF MAKING MECHANICAL AND CHEMI-MECHANICAL PAPERMAKING PULP

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: Method of making mechanical and chemi-mechanical papermak ing pulp 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 disinte¬ gration 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 propert¬ ies 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-bea at lower concentrations a TMP manufactured at high concen tration. In Scan Porsk Rapport 40 /198* , 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 l600 to 2300 k h/ton. In Pulp and Paper Magazine of Canada, Vol. 81, No 6, June 1980, page 72-80 . (N.Hartler) experiments are reporte to reduce the energy consumption at the refining of chips One proposal made here is to change the chemical environ¬ ment by the addition of chemicals. By adding sodium hydro ide 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 deteriorate slightly and the ISO-brightness considerably. In Svensk Papperstidning, 1982, page R 132-139 (P.Axelso and R.Simonson) the effect of sulphite impregnation of t chips on the refining process, a.o. the energy consumpti is discussed. At a certain amount of sulphite taken-up, energy consumption diagram showed a minimum. Totally, ho ever, 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 en consumption could be lowered by up to 30%. This, however could be achieved only at the expense of the yield. According to the present invention it has proved possibl to manufacture mechanical papermaking pulp by a substant ally 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 shal be at maximum 800 kWh/ton wood material. The acid groups included in the wood material thereafter shall be neutra ized entirely or partially, and the material be diluted with water of a temperature corresponding to the softeni temperature of the lignin. The dilution water shall have ion strength of at maximum 0.05 mole per litre. The coars -disintegrated material then shall be beaten at a concen¬ tration of 1-10% with an energy input of totally a maximu of 500 kWh/ton material. The present invention is based on the idea that there is a relation between the disintegration of the wood materia to fibres and the way, in which the energy pulses are transferred to the material, i.e. whether the energy puls 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 trans¬ ferred. 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 mecha ical 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 pu and at the same time to bring about improved binding prop erties 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 Impressa- finer or PREX) or by defibering in a refiner. The final beating then takes place in one or several step at low pulp concentration, i.e. at a concentration of 1-10%. At this beating must be observed, that the specifi 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 consump ion of a pulp manufactured according to Fig. 1, Fig. 5 is a flow sheet of a second embodiment of the invention, and Figs. 6-δ show properties and energy consumption at the method according to said second embodiment. EXAMPLE 1 The flow sheet according to Fig. 1 illustrates the manu acture of thermomechanical pulp for newsprint. Chips from spruce were steamed in a first step and preh ed. 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 aci groups included in the wood material. To the defibered material dilution water with a temperature of 80 C and ion strength of 2.0 mmole/1 was added in order to' obtai 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/ 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 con¬ sumption 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 manu¬ factured conventionally and one manufactured according t the invention appears from Table 1. In this connection should be mentioned, that as conventi al TMP-process the refining system was used, which up ti 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. T A B L E T M P entioan 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 bre^k, % 2.0 1,9 Tear index, N /kg 6.5 5.5 Density, kg/m 380 380 S, in /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 com pared in Fig. 2, 3 and 4 with TMP manufactured conventio ally with single-step refining in twin-disc refiner, whi 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 elec energy consumption. It appears clearly from the Figure, the increase in tensile inde at a certain electric ene consumption is considerably greater for TMP manufacture according to the invention. Fig. 3 shows the tear index as a function of the tensil index for TMP according to the invention and convention TMP. It appears that the development of the tear index for the respective TMP is about the same, i.e. at optim ation of the low concentration beating according to the invention the fibre cutting and thereby the serious de¬ crease in tear index are substantially entirely avoided which cutting and decrease occur usually at conventiona low concentration beating of mechanical pulps. Fig. 4 shows how the light-scattering coefficient (s) develops at conventional TMP and TMP manufactured accor ing 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 t a certain s-value is of equal size as the saving of el¬ ectric energy to a certain tensile index value. EXAMPLE 2 This example relates to the manufacture of chemi-mechan al 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 fina 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 carri out in the same way as in Example 1. At the manufacture of chemi-mechanical pulp the washing is very essential, so that according to the invention th beating shall take place at low ion strength. The washin 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 accordi to Example 1, but process temperature and chemical envir ment must be adjusted to the special properties, which t wood polymers have assumed by treatment with impregnatio 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 4θ°C. From a brightness point of view it is advantageous to use the lowest possible temp¬ erature. 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 carboxyl acid groups increases considerably, which implies lower¬ ing 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 solutioninan amount corresponding to a 2% charge, whereafter they were preheated at a temperature of 130 for 3 minutes. The material was then coarse-disintegrat 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 δθ°C for 20-30 minutes. The pulp was pressed to a concentration of 45-50% and again diluted to 3% at a temperature of 70 C. 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 correspond ing 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 convention ally manufactured CTMP-pulp produced in one step with an energy consumption of 1750 kWh/ton. By the method according to the invention, thus, the en¬ ergy 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 vent ional Invention Energy consumption, Wh/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 Nm 2/kg 6.7 5 .5 Density, kg/m 420 450 S, /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 accor ing to the invention and for pulp manufactured convention ally. 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 manuf¬ actured 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 consumpt¬ ion to a certain light-scattering index is substantially lower. EXAMPLE 3 This example relates to the manufacture of highly sulfon- ated 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 C for 10 minutes, whereafter they were coarse-disintegrate 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 6θ°C for 20-30 minutes at a pulp concentration of 3% 3 the pulp was beaten in three steps at an edge load of 0.3 to 0.5 Ws/m and a net energy con¬ sumption of 100 kWh/ton corresponding to a gross energy consumption of l6θ 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/ on. TABLE 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 2 Tear index, Nm /kg 8 . 0 8 .0 Density, kg/πr 440 450 S, m 2 /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 C 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-rscrew. After discharge, the fibre material was diluted to about 5% pulp concen¬ tration at about 70°C, 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 2 55 kNm/kg, and the tear index 6 Nm /kg at a net energy consumption of 150 kWh/ton and a gross energy consumpt¬ ion 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 conventi¬ onal 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: Claims 1. 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, c h a r a c t e r i z e d i n 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, that 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 soften¬ ing temperature of the lignin, i.e. 40-95 C, and with an ion strength of at maximum 0.05 mole per litre, and that the material th ien 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. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that at least 25% of the total energy input is supplied at the beating. 3. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the coarse-disintegrated material after the first step is washed in order to reduce the ion strength. 4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the energy input in each beating step amounts to 50-150 kWh/ on pulp.

Inventors: ENGSTRAND PER OSSIAN, HAMMAR LARS-AAKE, HTUN MYAT THOUNG, PETTERSSON RUNE LENNART, SVENSSON BOERJE NILS, ENGSTRAND, PER, OSSIAN, HAMMAR, LARS-AAKE, HTUN, MYAT, THOUNG, PETTERSSON, RUNE, LENNART, SVENSSON, BOERJE, NILS
Applicants: SVENSKA TRAEFORSKNINGSINST, STFI
Application year: 1989
Main classification: D21B1
IPC codes: D21B1, D21D1