CA2026102A1 - Non-sulfur process for preparing pulp for paper making - Google Patents

Abstract

ABSTRACT

This invention relates to a non-sulfur process for preparing pulp suitable for paper making.

Description

I d:~
eACKGROUND OF THE INVENTION

The important factors considered in process for preparing pulp includes:
1. Physical properties of the fiber which carryover to the paper product to give 0 satisfactory stren~th. These are conventionally evaluated in terms of burst, tear and breaking length.

2. Freeness which is related to dewatering on conventional paper making equipment.

3. Brightness is desirable for most purposes. The paper made from the pulp be white or at least light colored. The greater the brightness of the pulp the lessthe cost of chemicals for bleaching.

4. The higher the yield, the greater cost efficiency in the utilization of forest products.

30 5. Chemicals are required for pulping process but these are costly and create environmental pollution problem. It is therefore desirable to minimize chemical consumption and also to avoid the chemicals responsible for major pollution problem.
6. Time of pulping. This also affects cost in that it involves the use of costly equipment and energy in terms of heat input to maintain the cooking temperatures.
7. Refining energy. The cost of the energy required for pulping processes that include mechanical refining is an important cost factor.
8. Pollution: The cost of effluent treatment required for pulping process is an important cost factor. In this context, the use of non-sulfur chemicals that canbe recycled easily in pulping is very important.
Chemical pulping leads to strong papers, but is expensive due to low yield, high chemical consumption, chemicals recovery system, effluent treatment and emission control. There are also accompanying problems of pollution abatement.

Mechanical pulping provides good yields but the refining costs are high, especially in the case of thermomechanical pulping (TMP) and refiner mechanical pulping (RMP), and the strength of paper produced is rather low. In the case of groundwood, even though the defibrating energy is low, the pulp and paper properties are so low that it can be used only in admixture with other pulps.
There has been increased interest in recent years in so called chemither-momechanical, chemimechanical or semi-chemical processes which provide pulps of a strength that is adequate for most purposes and in which the yield is of the order of 90% or more. The drawback is, however, the high power requirements for the 20 mechanical refining part of these processes due to the higher percentage of lignin and fiber stiffness. The chips are not as soft as those produced by chemical pulping.
An alternative to high energy mechanical refining is to soften wood chips with steam under high pressure followed by explosive decompression. This was indeed the process invented by mason in the 1920's and used for hardboard manufacture.
Chips were steamed at low pressure for about one minute, then at high pressure for about two minutes and then brought to an even higher pressure followed by discharge of superheated chips to atmospheric pressure to explode the chips into a 40 pulp called gun stock which was then further refined. Although the pulp rasulting from the Mason process had high freeness and bulk and although the step of explosive decompression resulted in saving of power need for further refining, the physical strength, as evaluated in terms of burst, tear and breaking length, was low.
The fibers were therefore, unsuitable for paper making. Another problems was therelatively dark color which would have required excessive chemical consumption for bleaching. There was also considerable yield loss due to acidic hydrolytic degradation due to the wood acids liberated at the high temperature used.
According to Asplund svensk papperstid (1953) 56,550 pulp with good paper 60 making properties can be produced by a process involving explosive decompression -" 2 ~ 2 if the steam temperature is controlled between 100 and 160C. Higgins et al in Appita 32(3) 187-200 (November 1978) suggested that the Asplund process could be improved if the chips were chemically pretreated and the steam temperature was limitted to less than 130C. In Higgins~ modification of the Asplund process, the pressure at a temperature of 130C will be about 1.5 atmospheres.

OBJECTS

The object of this invention is to provide a process in which the energy saving advantage of explosive decompression is achieved but in which good brightness, high yield, and good fiber strength are also maintained.
It is also an object to provide a process tha~ is conducted at higher temperatures than those considered to be desirable according to the publications of 30 Asplund and Higgins referred to above. The higher temperatures enable higher pressures to be used, thereby greatly improving steam penetration along with pretreatment chemicals inside the fibers and softening of the hydrogen bonds in the mainly crystalline region of the fibers.
The most important object includes the utilization of impregnation chemicals 40 ;hat are non-sulfur and environment friendly. Moreover, the chemicals can be recouvered by simple distillation. -~

THE INVENTION

The major problems accompanying previous processes using explosive decompression are believed to have been the degradation due to the oxidation of wood and acid hydrolysis leading to loss in brightness, deterioration of fiber and paper properties and loss of yield. The approach adopted by this invention is 60 therefore to attempt to curtail hydrolytic and oxidative wood degradation and thereby 2 ~ 0 2 to protect against loss of yield, brightness and fiber strength. The loss of fiber strength will be particularly great if the degree of polymerization of the cellulose falls below the critical value which is about 500-600. Hydrolytic degradation will also cause yield loss due mainly to degradation of hemi-cellulose.
lG The process of this invention tries to achieve a positive improvement in the strength of the paper that will be produced from the fibers by increasing the number of hydrophilic groups on the fiber surfaces thereby adding to the potential sites for hydrogen bonding.
The conditions for the achievement of the foregoing objects in accordance with the process of this invention are as follows:
1. The wood fragments, having fibers suitable for paper making, such as chips, are in a form in which thorough chemical impregnation can be achieved in a reasonable time.

30 2. There is an initial thorough impregnation of the chips or other wood fragments by an organosolvent with or without the presence of alkaline aqueous solution and also with or without the presence of catalyst based on acid or base.
3. The impregnated chips are cooked using saturated steam in the substantial absence of air at high temperature and pressure.
40 4. The chips that have been steam cooked are subjectad to explosive decompression to result in chips which are softened and partially defibrated.

5. The softened chips are preferably washed and then, without undue delay, and preferably immediately, refined to provide pulp.
The steps of the process of this invention which will for convenience be referred to as the non-sulfur process, will now be considered in more detail.

The wood fraaments The starting material will normally be chips in which the fibers are of a length 60 suitable for paper making. Shavings could also be used bu~ sawdust would be ~-` 2~2~2 undesirable except as a minor part of the total furnish as the fibers are partially cut.
The chips should also, as is well known, be suitable in ~he sense of being free from bark and foreign ma~ter.
It is desirable for the purposes of this invention that coarse chips be avoided 10 as otherwise the subsequent impregnation may deposit chemicals only on the chip surface, unless impregnation is carried out for a very long time. Another problem with coarse chips is that cooking would not be complete. It is best to use shredded or thin chips. In the examples, except where otherwise stated, aspen hardwood chips were used. These were shreeded, the energy for which was of the ord0r of 0. 1 MJ/kg.

Impreqnation The purpose of impregnation is to protect the chips against oxidation, to 30 preserve the fiber length and to provide a positive increase in strength by developing hydrophylic groups on the fiber surface during steam treatment. This will then provide additional sites for hydrogen bonding. The impregnation chemicals used in this invention are non-sulfur chemical and organo-solvent that can be recycled easily. The preferred organo-solvents are low molecular weight aliphatic 40 alcohol and aromatic alcohol. These are methanol, ethanol, butanol and propanol and phenol. The different alcohol can be used alone or in combination of chemicals such as sodium hydroxide, sodium chlorite, sodium carbonate, sodium bicarbonate,calcium chloride, magnesium chloride, calcium nitrate, magnesium nitrate, sodium chloride, ammonium hydroxide or potassium hydroxide. The concentration of chemicals varies from 0 to 8 percent (based on oven dry wood) depending on steam cooking temperature and types of wood species. The percentage of alcohol in pretreatment 1i4uor varies from 20 to 80. The impregnating solution must be alkaline and have enough free hydroxyl ion to be able to neutralize the liberated 60 wood acids such as formic acid and acetic acid. Normally, the starting PH is about ~2;~02 7.5 or higher and the final PH after steam cooking should be at least 6 or higher.
The time of impregnation at atmospheric pressure in holding tanks typically ranges from about 12 to 24 hours at a temperature of about 25 to 60C. The liquor/chips ratio should be between 3 to 6, preferably 5. For industrial purposes, 0 however, the time may be shortened to an hour or to minutes by impregnating with steam under pressure and at a higher temperature. The pressure should be up to about 1 atmospheric extra pressure at a temperature of about 100 to 110C. To improve impregnation, the chips should be compressed in advance of impregnation.Under these conditions, penetration will be achived in a shorter time. Ther0 is no significant cooking.
In the examples, unless otherwise stated, 75g of chips (based on oven dry basis) were mixed in plastlc bag with a mixture of alcohol and aqueous solution as if the ratio of liquor/chips become 5. The time of impregnation was 24 hours and the temperature of impregnation was 25C. The foregoing is applicable only on a laboratory scale. In industry, the impregnation time would be shortened as described above.

Steam cookina The impregnated chips are steam cooked at a high temperature and pressure.
Equipment and methods that can be used for preliminary compacting of the impregnated chips, for cooking the chips with steam and for the discharge of thechips under conditions of explosive decompression are described in canadian patent 1,070,537 dated January 29, 1980; 1,070,646 dated January 29, 1980; 1,119,033 dated March 2, 1982 and 1,138,708 dated January 4, 1983, all of which were granted to Stake Technolgy Ltd. The equipment used in the examples was acquired from the company.
The temperature of cooking should be within the range of about 160 lo 210DC
and preferably within the range 185-200DC, which is in excess of the temperatures :

2~2~
considered possible according to the publications of Asplund and Higgins previously referred to. These temperatures correspond with a pressure of 7.9 atmospheres for 170C and 15.5 atmospheres for 200C. Following the high temperature csoking, the increased pressure applied by means of pressurized nitrogen are found to be 10 beneficial in terms of fiber liberation.
The cooking may be preceded by steam flushing under low pressure steam at 100C for a short period such as one minute. This is a matter of convenience, inthat with a batch reactor the cooking vessel is initially open to the atmosphere, to eliminate air. This air would be disadvantageous in that it would result in oxidation if it were trapped in the cooking vessel. Steam flushing is desirable with a batch reactor but would not be necessary for a continuous reactor.
This preliminary treatment is then followed by cooking for about 30 seconds to 6 minutes and preferably about 1 to 4 minutes. It has been found that within 30 reasonable limits there is a property improvement by increasing the time-temperature factor.

Exploslve decompressiol~
After cooking the pressure is instantaneously released and the chips are 40 exploded into a release vessel. After cooking the pressure was also released slowly to compare the properties of the resulting pulp obtained by two methods of chips discharge. If there is to be a delay between release of the chips and refining it is important to cool the chips down by washing them. Washing may also be desirable for the purpose of chemical recovery.
It is desirable immediately to refine the chips after the discharge. Otherwise, if the chips are stored, some oxidation will occur with resultant loss of brightness. In any event, undue dalay should be avoided. Such delay is regarded as being undue if oxidation takes place to an extent that will materially affect brightness.

The chips resulting from the explosive decompression are softened and partialiy defibrated, whereas the chips coming through slow pressure release arealso softened but are not defibrated.

o Refinin~
Refining in the experiments described below was conducted at 2%
consistency level using a blender coupled with an energy meter model EW 604.
According to A.C. shaw "Simulation of Secondary Refining" Pulp and Paper Canada 85(6) T152-T155(1984), the blender results closely match those obtained ~o with industrial refiners. Properties were evaluated after preparing paper sheetsaccording to standard CPPA testing methods.
Refining energies are unusually low and can be expected to be in the range 3 to 4 MJ/kg to provide a freeness of about 700 and about 4.5 to 5 MJ/kg for a 30 freeness of 100 which is about one half of the energy demand of refiner mechanical pulp (RMP) or thermo-mechanical pulp (TMP).
Table 1 shows the characteristics of explosion pulp of aspen pretreated with sulfur (Na2SO3) and non-sulfur base chemicals. In both cases, the specific refining energy of the pulp is similar. Explosion pulp corresponding to sulfite pretreatment 40 yield pulp of superior brightness in comparison to that of explosion pulp corresponding to non-sulfur base chemicals. However, the physical properties of both pulps are comparable.
Table 2 shows the characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (20:80). The effects of the variation of the concentration of NaOH in impregnation solution on the resulting explosion pulp physical properties, optical properties, yield and refining energy are clearly shown.
For similar purposes, the characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (50:50) and (80:20) are respectively shown in Table 3 6 o and 4.

2~ 2 Tables 2, 3 and 4 show the effects of methanol concentration and sodium hydroxide concentration applied during pretreatment on the resulting explosion pulp yield, physical and optical properties and specific refining energy.
Figure 1 shows ~he effect of the concentration of methanol in impregnation 10 solution on the resulting explosion pulp yield and tear index. Similarly, Figure 2 shows the effects of the concentration of methanol in impregnation solution on the resulting explosion pulp yield and breaking length.
In conclusion, it may be expected that the non-sulfur explosion process will provide a product having a yield in the range 88 to 94% and an energy of defibration of 2 to 4 MJ/kg. Hardwood (aspen) w!ll have a brightness without bleaching in the range 40-45%. The physical properties of hardwood (aspen) non-sulfur explosion pulp are comparable with those of explosion pulp obtained from the same species pretreated with sulfite. The additional benefit is that the non-sulfur 30 base chemicals (mainly alcohol) are environment friendly and can be recycled easily by simple distillation.
It is reasonable to expect that by applying the principles disclosed herein further optimization will result in even better results.

Characteristics of explosion pulp of aspen pretreated with sulfur and non-sulfur base chemicals.

Code no. 1 2 3 4 5 6 7 8 Chemicals Na2SO3 (absolute) % 24 48 24 48 96 NaOH
(absolute) % 0 0 3 3 3 4 5 4 Methanol-NaOH
solution - - - - - 20-80 50-50 80-20 Liquor/chips ratio 3 3 3 6 6 5 5 5 CSF, ml 100 100 100 100 100 100 100 100 Specific Ref.
Energy, 6.6 1.2 2.8 1.66 1.16 2.5 1.75 3.1 MJ/kg Breaking length, km 5.4 7.7 8.2 8.97 10.1 5.8 7.4 5.3 Burst index, kPa.m2/g 2.9 4.2 3.7 4.4 4.9 2.7 4 2.8 mN m2/gde ' 5 4 6.2 5.8 5.2 4.8 6.7 6.4 6.7 Brightness (with tap water), % 54 56 50 48 47 39 39 38 Opacity, (%) 88 85 86 84 77 98 99 99 Yield, (%) 93.6 89 94.7 85.3 92 91.6 88.2 88 _ _ 2~2~1~2 TABLE 2a -Characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (20:80).

_ _ Code MN9 MN10 NaOH (%) o (Based on O.D.chips) Liquor/chips ratio 5 5 Cooking temperature C 190 190 Cooking time, min. 4 4 CSF, ml 125 222 477 110 375 500 Specific Ref. Energy, MJ/kg 2.3 1.4 0.5 3.9 2.3 1.5 Porosity, mVmin. 105 240 1510 200 1750 2225 Bulk, cm3/g 2.8 2.89 3.62 2.44 2.8 3.37 Burst index, kPa.m2/g 2.1 1.8 1.1 1.960.87 0.75 Breaking length, km 4.3 4.0 2.55 4.2 2.4 2.3 ~ ~-Stress (%) 1.77 1.55 1.05 1.681.01 1.11 Tear index, mN.m2/g 6.0 5.8 4.9 5.0 4.1 4.4 Brightness, (%) 34 33 32 39 38 38 `~
Opacity (%) 99 99 99 99 99 99 Yisld (%) 86.2 94.7 ~ 2~2~ 0~
TABLE 2b Characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (20:80).

Code MN11 MN12 :

NaOH (%) 2 4 (Based on O.D.chips) Liquor/chips ratio 5 5 Cooking temperature C 190 190 Cooking time, min. 4 4 ..
CSF, ml 213 295 436 152 300 530 Specific Ref. Energy, MJ/kg 3.6 2.85 1.5 2.25 0.6 0.6 Porosity, ml/min. 540 850 2040 133 350 Bulk, cm3/g 2.68 2.79 3.07 2.672.75 3.3 Burst index, kPa.m2/g 1.59 1.49 0.95 2.6 2.14 1.4 Breaking length, km 3.7 3.45 2.67 5.5 4.8 3.3 Stress (%) 1.5 1.54 1.25 1.971.74 1.3 Tear index, mN.m2/g 4.9 4.8 4.3 6.7 6.5 6.9 Brightness, (%) 40 40 39 39 39 37 Opacity (%) 99 99 98 98 98 98 Yield (%) 94.6 91.6 2~2 TABLE 3a Characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (50:50).

Code MN5 MN6 NaOH (%) 0 1.25 (Based on O.D.chips) Liquor/chips ratio 5 5 Cooking temperature C 190 190 Cooking time, min. 4 4 ;-CSF, ml 60 247 506 101 255 533 Specific Ref. Energy, MJ/kg 2.24 1.5 0.64 3.6 2.1 1.75 Porosity, ml/min. 100 550 2440 200 850 1330 Bulk, cm3/g 2.88 3.03 3.59 2.4 2.78 3.57 Burst index, kPa.m2/g 2.1 1.29 0.73 1.98 1.26 6.79 Breaking length, km 4.57 3.17 2.07 4.38 3.14 2.22 Stress (%) 1.69 1.28 0.99 1.67 1.24 1.04 Tear index, mN.m2/g 5.4 4.9 4.07 4.69 4.04 4.37 Brightness, (%) 35 35 34 39 39 38 Opacity (%) 99.5 99 99 99.5 99.6 99 Yield (%) 91 92.3 ~:

-TABLE 3b Characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (50:50).

Code MN7 MN8 NaOH (%) 2.5 5 (Based on O.D.chips) Liquor/chips ratio 5 5 Cooking temperature C 190 190 Cooking time, min. 4 4 CSF, ml 144 334 547 49 152 380 Specific Ref. Energy, MJ/kg 2.3 1.26 0.9 2.1 1.4 0.6 Porosity, ml/min. 210 833 2950 23 Bulk, cm3/g 2.48 2.72 3.63 1.77 2.3 2.5 Burst index, kPa.m2/g 2.13 1.5 0.83 4.23 3.1 2.1 Breaking length, km 4.86 3.59 2.4 8.15 6.9 5.3 Stress (%) 1.72 1.38 1.1 2.3 2.10 1.67 Tear index, mN.m2/g 5.87 5.42 5.72 6.48 6.4 6.4 Brightness, (%) 39 39 38 34 34 33 Opacity (%) 99.5 99 98 99 99 98 Yield (%) 91 88.2 _ 2-~`~`$~ 2`
TABLE 4a Characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (80:20).

Code MN1 MN2 : --NaOH (%) 0 1 ::(Based on O.D.chips) Liquor/chips ratio 5 5 Cooking temperature C 190 190 Cooking time, min. 4 4 CSF, ml 113 371 518 163 355 557 Specific 2.7 1.2 1.1 3.9 2.0 1.3 :
Porosity, ml/min. 308 1760 3350 3851800 3640 Bulk, cm3/g 2.37 2.41 ~.49 2.5 2.9 3.2 Burst index, :~:
kPa.m2/g 1.38 0.87 - 1.260.76 Breaking length, km 3.4 2.4 1.6 3.1 2.1 1.5 Stress (%) 1.27 0.97 0.71 1.260.89 0.8 TmeNarmi2n~9eX' 5 4 4.2 2.9 5.2 3.9 4.3 Brightness, (%) 37 36 36 41 40 39 Opacity (%) 99 99 98 99 98 98 Yield (%) 89.2 92.1 .. .. . _ _ _ . .

~ '61~ 2 TABLE 4b Characteristics of explosion pulp of aspen pretreated with methanol-NaOH solution (80:20).

_ _ _ Code MN3 MN4 NaOH (%) 2 (Based on O.D.chips) Liquor/chips ratio 5 5 Cooking temperature C 190 190 Cooking time, min. 4 4 CSF, ml 72 230 433 33 143 449 Specific Ref. Energy, MJ/kg 4.3 2.5 1.7 4.5 2.36 1.4 Porosity, mUmin. 80 400 1850 34 - -Bulk, cm3/g 2.17 2.49 3.43 1.782.20 2.6 Burst index, kPa.m2/g 1.99 1.42 0.83 2.94 2.1 1.5 Breaking length, km 4.7 3.5 2.5 6.2 4.9 3.7 Stress (%) 1.66 1.30 1.08 ?.151.66 1.28 Tear index, mN.m2/g 6.4 5.6 5.4 6.7 6.8 7.4 Brightness, (%) 42 40 41 38 38 37.2 Opacity (%) 98 98 98 99 98.7 98 Yield (%) 91.0 88.1

Claims (16)

1. A process for preparing pulp suitable for making paper comprising the step of1. Thoroughly impregnated wood fragments having fibers suitable for paper making with an alkaline aqueous liquor in combination with aliphatic or aromatic alcohol with or without the presence of catalyst based on acid or base;
II. Steam cooking of impregnated chips with saturated steam in the substantial absence of air at superatmospheric pressure and a temperature within the range of about 170 to 210°C;
III. Subjecting the wood fragments to explosive decompression to give wood fragments that are softened and partially defibrated;
IV. Without undue delay that would result in brightness loss, refining the softened and defibrated chips to provide pulp.

2. A process as in claim 1 in which the temperature of steam cooking is in the range 160 to 200°C.

3. A process as in claim 1, in which the liquor used for impregnating is at a PHof at least 7.5 and the final PH following steam cooking is at least 6.

4. A process as in claims 1, 2 or 3, in which the wood fragments are chips.

5. A process as in claims 1, 2 or 3, in which the wood fragments are shredded chips.

6. A process as in claims 1, 2 or 3 in which the aqueous portion of the liquor used for impregnating may include a swelling agent.

7. A process as in claims 1, 2 or 3, in which the liquor consists of alcohol andwater, used for impregnating includes one or two chemicals selected from the group consisting of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium chlorite, sodium chloride, calcium chloride, magnesium chloride, calcium nitrate, magnesium nitrate, ammonium hydroxide and potassium hydroxide.
The concentration of chemicals varies from 0 to 8% (based on oven dry wood) depending on steam cooking temperature and types of wood species.

8. A process as in claims 1, 2 or 3 in which the organosolvent used is methanol.

9. A process as in claims 1, 2 or 3, in which the organosolvent used is selectedfrom the group consisting of methanol, ethanol, butanol, propanol and phenol.
The amount of organosolvent used in liquor varies from 20 to 80%.

10. A process as in claims 1, 2 or 3 in which the liquor comprises one or more than one catalyst based on acid or base.

11. A process as in claim 1, 2 or 3 in which the liquor includes only the non-sulfur base chemical.

12. A process as in claims 1, 2 or 3, in which the time of cooking is in the range 30 seconds to 10 minutes.

13. A process as in claims 1, 2 or 3, in which the time of cooking is in the range of 1 to 4 minutes.

14. A process as in claim 1, in which the temperature of cooking is in the range180 to 195°C and the time of cooking is in the range 1 to 4 minutes.

15. A process as in claim 1, 2 or 3, in which both hardwoods and softwood can be used.

16. A process in which non-woody plants such as bagasse, wheat straw, rice strew, jute, bamboo etc. can be used.