NZ259595A

NZ259595A - Producing mechanical or chemi-mechanical pulp: two step process, with first step below softening point of lignin

Info

Publication number: NZ259595A
Application number: NZ259595A
Authority: NZ
Inventors: Hans Hoeglund; Roland Baeck; Ove Danielsson; Bo Falk
Original assignee: Sunds Defibrator Ind Ab; Sca Research Ab
Priority date: 1992-12-30
Filing date: 1993-12-08
Publication date: 1997-10-24
Also published as: NO952613D0; DE69320989T2; EP0677122A1; NO312206B1; SE470555B; AU678802B2; CN1092832A; JP3258019B2; NO952613L; CA2151364A1; SE9203943L; SE9203943D0; CA2151364C; FI115844B; ZA939370B; PL309195A1; DE69320989D1; KR950704567A; WO1994016139A1; BR9307748A

Abstract

A method of producing mechanical and chemi-mechanical pulp with a yield above 85 % from lignocellulose-containing material for the manufacture of paper or board products. The material is subjected to mechanical processing in at least two steps. The material at its feed into the first step has a temperature below the softening temperature of lignin. When the material is fed into at least one subsequent processing step, it shall have a temperature above the softening temperature of lignin.

Description

<div class="application article clearfix" id="description"> New Zealand No. International No. 259595 PCT/SE93/01058 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 30.12.1992; Complete Specification Filed: 08.12.1993 Classification:^ D21B1/02.14 Publication date: 24 October 1997 Journal No.: 1421 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: A method of producing mechanical and chemi-mechanical pulp Name, address and nationality of applicant(s) as in international application form: SCA RESEARCH AB, Landsvagsallen 59, S 852 29 Sundsvall, Sweden; SUNDS DEFIBRATOR INDUSTRIES AB, S-851 94 Sundsvall, Sweden WO 94/16139 PCT/SE93/01058 i 5 9 5 < A method of producing mechanical and chemi-mgchanical sulo This invention relates to the production of mechanical and chemi-mechanical pulp with a yield of above 85% from ligno-celiulose-conlaining material for the making of paper or board products. ' ¥ Mechanical pulp (e.g. TMP) or chemi-mechanical pulp (e.g. CTMP) is today produced in several different process variations, where steamed chips are refined in disc refiners of various types. At the production of pulps for different printing papers or wrapping materials, of board type, the refining usually is carried out in one or more steps. The first step normally is pressurized, i.e. the refining takes place at temperatures exceeding 100°C, usually immediately below or at the so-called softening temperature (Tg) of the lignin. Heretofore, it was chosen to hold the pressure and temperature in subsequent refining steps on the same level as in the first refining step, or to carry out later refining steps in systems not pressurized, i.e. at a temperature lower than in the initial step, usually at the softening temperature of the lignin or below the same. The softening temperature of the lignin, which has proved to be an important variable at the refining of chips in mechanical and chemi-mechanical pulp processes, has been determined in the most recent decades in a number of scientific investigations for a plurality of wood types concerned. At the investigations standard equipment and conventional measuring principles for the determination of viscoelastic parameters have been used. For wood, as for other viscoelastic materials, the softening temperature varies with the load frequency at the measurements. At a higher load frequency, the softening temperature increases. At the processing frequencies normally applied in refiners the softening temperature of coniferous wood was determined to be between 125°C and 1£5°C> while it proved to be somewh WO 94/16139 PCT/SE93/01058 lower for our most usual hardwood types. The softening temperature can be shifted by the addition of different chemicals. It can be lowered, for example, after impregnation by usual lignin softening chemicals, of the type sulphite. Relatively high total electric energy amounts are required for producing the aforementioned types of pulp. The production of pulp for newsprint from coniferous wood, for example, can require up to 2000kWh/ton pulp. In many studies carried out recently with the object of trying to lower the electric energy consumption in the TMP-process, it was found that the initial processing phase seemed to be quite essential for the total energy consumption in different process variants and for the character of the resulting pulp. This.; seems to apply in spite of the fact that only a small part of the total electric energy consumption in the refining process is used for the fiber separation proper, i.e. for the conversion from chips to free individual fibers (also called defibering). A fiber separation energy-effective per se as a result of an effective thermal or .chemical softening of the chip areas rich in lignin, however, does not prove to be a garanty that the total energy consumption will be low. On the contrary, it was proved that TMP-process variants, which were initiated with a mild fiber separation poor in energy, often require a high total energy input. This circumstance seems to be caused by the fact, that mildly separated but unprocessed fibers, which were obtained by carrying out the defibering at temperatures above the softening temperature of the wood lignin, are difficult to fibrillate during the continued working in the refining process. This fibrillation is necessary for increasing the flexibility of the fibers to a desired level and bringing about the fine material characterizing a good Twp-qualily. An intensive WO 94/16139 PCT/SE93/01058 processing below the softening temperature of the wood lignin initially and during the continued refining process, on the other hand, easily leads to a deterioration of the long fiber content and thereby of the strength properties of the pulp. This is in many cases unacceptable from a quality point of view. A decrease in the energy consumption from an established levei in the TMP-process, as a rule, has been associated with a deterioration of certain quality properties of the resulting pulp, for example lower long fiber contant, lower tear strength, lower tensile strength and higher shives content. The present high energy consumption in the TMP- and CTMP-process, therefore, has been necessary for achieving the desired puip properties. It was now found by surprise, that it is possible to combine low energy consumption in a mechanical or chemi-mechanical pulp making process with maintained quality properties. The present invention relates to such a method, where the mechanical processing, for example refining, takes place in at least two steps. According to the invention, the material at its feed into the first processing step has a temperature below the softening temperature of the lignin, and at its feed into at least one subsequent processing step has a temperature exceeding the softening temperature of the lignin. The invention is described in greater detail in the following, with reference zo some expedient, embodiments and examples with associated Figures 1-8.. In a TMP-process according to the invention, refining takes place in at least two steps. In the first step the chips are fed into the refiner at a temperature below the softening temperature of the lignin and then are processed under relatively intensive conditions, for example in a double-disc refiner with a speed of at least 1200 rpm or in a single-disc refiner with high relative speed between the refiner:-discs (at ieast 1800 rpr., preferably at least 24.00 rpm). The energy input WO 94/16139 PCT/SE93/Q1058 in the first step is chosen to be on such a low level, that the long fiber content of the pulp which a.o. yields the potential for the later strength development'at the refining, is not deteriorated appreciably. The freenes (CSF) of the pulp after the first step, therefore, shall be high, preferably 500 ml. A subsequent refining step is carried out under conditions where the lignin of the fiber material is well softened. The fiber material then is fed into the refiner at a temperature exceeding the softening temperature of the lignin. In cases when the material consists of coniferous wood not treated with chemicals, the temperature should exceed 150°C, suitably 160°C and preferably 170°C. When the material is treated with chemicals, the temperature should exceed 135°C, suitably 150°C and preferably 160°C. As regards an upper temperature limit, temperatures over 200°C should be avoided, a.o. with regard to dark colouring of the fiber material. The processing frequency preferably can be high (relative speed at least 24.00 rpm) at the processing of the well-eoftened fiber material, which has proved especially favourable from an energy point of view. The temperature difference between the temperatures of the material at its feed into the first and, respectively, a subsequent processing step should be at least 15°C, suitably at least 25°C and preferably at least 35°C. In a process according to the invention fractures and fracture indications in the material initially are guided to layers in the fiber wall- not rich in lignin. During the final refining the known fact then can be utilized, that the fiber material can be separated with low energy inputs in areas rich in lignin at temperatures above the softening temperature of the wood lignin. The fractures initially having been guided to areas not rich in lignin, it is thereby avoided to obtain a fiber material with only lignin-joverea surfaces which are difficult to fibrillaue. This has previously been the great WO 94/16139 PCT/SE93/010S8 problem when it was tried to utilize refining temperatures above the softening temperature of the lignin at the production of mechanical pulps for printing paper or board products. Fine material from areas between the initial fracture zone and the middle lamina of the fiber rich in lignin also is easily released at temperatures above the softening temperature of the lignin in the later refining step, which can explain the low total energy consumption to a certain freeness (CPF) in this process step and in the entire process according to the invention. The production of fine material otherwise is the most energy requiring part of the mechanical pulp process using conventional technique. EXAMPLE Thermomechahical pulp from spruce chips was produced after refining in two steps in a 20" single-disc refiner of a well-equipped test plant. The first refining step (defibering) was carried out after preheating the chips at 115°C for about 3 minutes, i.e. at a temperature below the softening temperature of the lignin. The refiner was driven by a 3000 rpm motor, in order to ensure that the initial defibering should not take place under too mild conditions. The effect input in the first step was 64.O kWh/t, which yielded a pulp with freeness (CSF) 518 ml. In the second refining step the conditions were varied according to the following Table: Test Preheating time Refining temoerature or. x) Motor speed mi n rem A about 1 115 ( < Tg) 1500 B ii i 160 ( > Tg) 1500 C i, •, 160 ( > Tg) 3000 D ii i 170 ( >Tg) 3000 x) Temperature at preheating and at feed into the refiner The effect of where the most and arc coMen the varying conditions essential pulp proper ted cn as follows: is shown ties have in Figs. 1-6 been valued, WO 94/16139 PCT/SE93/01058 Fig. 1 shows freeness as a function of energy consumption. It appears that by carrying out the second refining step at temperatures above the softening temperature of the lignin the energy input at refining to a certain freeness can be reduced considerably compared to conventional second step refining at temperatures below the softening temperature of the lignin (compare Tests A and B).The energy reduction will be still greater.when, in addition, the speed is increased from 1500 to 3000 rpm (compare Test B with Tests C and D). Fig. 2 shows the shives content as a function of"the energy consumption. It appears that second step refining at temperatures above the softening temperature of the lignin yields a clearly lower shives content at a certain energy input than refining at a temperature below the softening temperature of the lignin (compare Test A with Tests B-D). Also in this case the higher speed yields the most favourable values. This proves to be a further advantage by using the conditions according to the invention. Fig. 3 shows the long fiber content as a function of freeness. It appears that the long fiber content of the pulp generally can be maintained all the way down to the freeness range 150-200 ml, in spite of the heavy energy reduction at refining according to the conditions of the invention. Fig. I show,-; the tear index as a function of freeness. It appears that the tear index of the pulp can be maintained all the way down to the freeness range 150-200 ml, in spite of the large energy reduction at refining at the conditions of the invention. WO 94/16139 PCT/SE93/01058 Figs. 5 and 6 show the tensile index and, respectively, light scattering as a function of freeness. It appears that-all tested pulps develop tensile index and, respectively, light scattering coefficient in a similar way when they are valued conventionally against freeness. In parallel with the tests described and with reference thereto it also was investigated, how energy consumption and pulp quality are affected when a refining process contrary to the conditions of the invention was started with a refining step where the temperature at the feed to the first step refiner is higher than the softening temperature of the lignin. Also in this case the thermomechanical pulp was made from .spTuce chips after refining in two steps by single-disc refiners. The first refining step was carried out at temperatures above the softening temperature of the lignin in the same equipment which was used previously in the test. The conditions in the first refining step and the freeness after refining with a certain energy input are described in the following Table: Test Preheating Refining . Motor Energy Freeness o ^ / time, min temp. C speed rpm input kWh/t ml E about 1 150 (>Tg) 3000 F " 1 160 (>Tg) 1500 G '■ 1 160 C>Tg) 3000 940 900 790 450 580 415 x) Temperature at preheating and feed into the refiner In a second refining step which was carried out under atmospheric conditions in a 20" refiner, i.e. at temperatures below the softening temperature of lignin, the freeness was lowered to an interesting range for printing paper pulps. The refiner speed in this case was 1500 rpm. WO 94/16139 PCT/SE93/01058 The effect of the varying conditions on energy consumption and light scattering capacity appears from Figs. 7 and •?, which show freeness as a function of energy consumption and, respectively, light scattering as a function of freeness. Fig. 7 shows that the energy consumption is considerably higher when the TMP-process is initiated with a refining step at a temperature above the softening temperature of lignin than that obtained with conditions according to the invention (compare Fig. 1). Fig- 8 shows that the light scattering coefficient is considerably lower when the TMP-process is initiated with a refining step at temperatures above the softening temperature of lignin than that obtained with conditions according to the invention (compare Fig. 6). The pulps produced, according to the invention, therefore, are clearly most suitable for use as printing paper pulps, where just the light scattering coefficient must be sufficiently high for achieving the desired optical properties. The example described proves clearly, that mechanical pulp can -.be produced with the conditions of the invention at low energy consumption, at the same time as essential properties like shives content, long fiber content,tear strength, tensile strength and light scattering meet high requirements on this type.of pulps. The energy consumption at the production of newsprint, for example, can be reduced by about 40% compared with conventional manufacturing methods. In the process according to the invention, chemicals can be added advantageously after or during the first refining step, in order to avoid dark colouring at the high temperatures above the softening temperature of lignin in subsequent refining steps. The chemicals also can have a bleaching effect. WO 94/16139 PCT /SE93/01058 Examples of such chemicals are sodium sulphite, sodium bisulphite, sodium ditionite, peroxide etc. According to the invention, the initial processing can be carried out, besides in refiners, also in grinders, compressing screws or other mechanical processing equipment. In cases when a reject fraction separated from the processed material is subjected, to additional mechanical processing, this reject with a temperature above the softening temperature of lignin shall be fed into at least one subsequent processing step. The invention, of course, is not restricted to the examples shown, but can be varied within the scope of the invention idea. WO 94/16139 10 PCT7SE93/01058 25 9 5 g j </div>

Claims

<div class="application article clearfix printTableText" id="claims"> Claims

1. A method of producing mechanical and chemi-mechanical pulp with a yield above 85% from lignocellulose-containing fiber material for the manufacture of paper or board .products, which method comprises mechanical processing in at least two steps, wherein the material at its feed into the first processing step has a temperature below the softening temperature of lignin, and at its feed into at least one subsequent processing step has a temperature above the softening temperature of lignin.

2. A method as defined in claim 1 , wherein the temperature difference between the temperatures of the material at its feed into the first step and at its feed into the at least one subsequent step is at least 15°C/ suitably at least 25°C and preferably at least 35°C.

3. A method as defined in claim 1 or 2, where the material consists of coniferous wood not treatedwith chemicals, wherein the temperature of the material at its feed into the at least one subsequent, step exceeds 150°C, suitably 160°C and preferably 170°C.

4- A method as defined in claim 1 or 2, where the material is treated with chemicals, wherein the temperature of the material" at its feed into the at le&st one subsequent step exceeds 1;35°C, suitably 150°C and preferably 160°C.

5- A method as defined in any one of the preceding claims, wherein the mechanical processing in the at least one subsequent step is carried out by a refix WO 94/16139 11 PCT/SE93/01058 25 9 5 9 5

6. A method as defined in any one of the preceding claims, wherein all processing steps.are carried out by refiners.

7. A method as defined in any one of the preceding claims, wherein a reject fraction separated from the mechanically, processed material with a temperature above the softening temperature of lignin is fed into the at least one subsequent processing step.

8. A method as defined in any one of the preceding claims, wherein the first processing step is carried out in a double-disc refiner with a speed of at least 1200 rpm or in a single-disc refiner with a speed of at least 1800 rpm, preferably at least 24.00 rpm.

9. A method as defined in any one of the preceding claims, wherein chemicals maintaining brightness or bleaching chemicals; such as sodium sulphite, sodium bisulphate, sodium ditionite, or peroxide are added after or during the first processing step.

10. A method of producing mechanical and chemi-mechanical pulp substantially as herein described with reference to the Example. end of claims </div>