CA2024935A1 - Modified process for preparing pulp for paper making - Google Patents

Abstract

ABSTRACT

This invention relates to an improved process preparing pulp suitable for papermaking, consisting of short time saturated steam cooking of chips impregnated with alkaline solutions of sulphites in the temperature range of 180 to 210°C, instantaneous pressurizing the reactor with cool nitrogen after cooking to super atmospheric pressure from 25 to 50 atm followed by explosive decompression and fiber refining.

Description

6~ r E3ACKGROUND OF THE iNVENTlON

Ingruber ~t al., Pulp and Paper Manufacture, Volume 4, TAPPI, CPPA, p. 160 (1985) define tha~ convention conventional ultra-high-yield chemithermomechanical or lO chemimechanical pulping is pref~rably conducted at a pH level between 4 and 9, and involves either liquid or vapor phase cooking with sodium sulphite-bisulphite solutions for about 10 to 30 minutes at a temperature between ~o and 175C. It is generally accepted that the chemical treatment is mainly responsible for permanent fibre sof~ening, increase in long fiber content, fibre specific surface and conformability, as demonstrated by ~leitner et al., Pulp and Paper Can., (84)11:T252-T257 (1983).
There is another softening approach which consists of a steam treatment of chips at high temperatures followed by explosive decompression.
The production of pulp using high-pressure and high s~eam chip softening well above glass transition temperatures of lignin should ~heoretically lead to lowerenergy consumption in subsequent refining stages.
The initial research in the field of high-pressure steam cooking, followed by defibration by explosion, was made by Mason, U S. Pat. 1 824 221; 2 64~ 623; 2 4g4 545; 2 379 8290. The masonite pulp obtained according to a two stage Sprout-Waldron refining procedure showed weak physical strength, dark color and yield loss of 16% to 20%, and revealed itself simply unsuitable for the production of paperaccording Koran et al., Pulp and Paper Can., 79(3): T1û7-T-113 (1978). Mamers and al., TAPPI, 64(7): 93-96 (1981); APPITA, 29(5): 356-362 (1976) investigated explosion pulping of pinus elliotti wood chips with the help of high pressure carbon dioxide solutions and bagasse of wheat straw explosion pulping under high pressure of nitrogen. Paper properties which were obtained wers simllar to that of CTMP/CMP pulps, but at the expense of brightness. The major problem to overcome ars oxidation, as w011 as hydrolytic degradation of flbers leading to ~2~
orightness and yield loss.
It has been suggested by Vit et Kokta, Vit et al., Can. Pat. 1 212 505 (1986) that the ultra-high-yield (90%+) pulp sui~able for papermaking can be produced by vapor phase steam explosion cooking. The initial propertiss of pap0rs made from exploded softwood chips were similar to those of TMP. However, refining energy was about 20% to 25% lower. Recently, a pulping process entitled "Process for Preparin~ Pulp for Paper Making~, Kokta B.V., Can. Pa~. 1 230 208 (1987); U.S.
Pat. 4 798 651 (1989); Can. Pat. Appl. #542 643 (May 1~87), referred to as "Steam Explosion Pulping Process" or "S-pulping~' has been proposed both for softwoods and hardwoods. In this process, impregnation and cooking conditions were aimed at minimizing yield and brightness loss, maximizing resulting paper properties and decreasing specific refining energy. The steam expiosion pulping process consists of the chemical impregnation of chips, short duration saturated steam cooking attemperatures varying from 180C to 210C, pressure release, refining and bleaching (if necessary).
Kokta et al., Paperi Ja Puu - Paper and Timber, 9, 1044-1055 (1989), have shown that the specific refining energy of aspen explosion pulps is at least 50%- lower than that of CMP pulp of similar yield and ionic content level, while paper strength increases by up to 50%. Compare at similar CSF levels, explosion hardwood pulps (i.e. aspen, maple, hardwood mixtures, eucalyptus) at 90% yield provide similar or higher paper properties then commercial low yield (- ~0%) bleached hardwood pulps.

OBJECTS

The object of this invention is to provide a process in which additional energy saving and improved properties are obtained when compared to previous invention 60 of Kokta, Can. Pat. 1 230 208 (1987) by pressurizing the reactor by nitrogenbefore explosive decompression. 2~d. ~

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 properRes and loss of yield. The approach adopted by this invention is therefore to attempt to curtail hydrolytic and oxidative wood degradation and thereby 20 to protect against loss of yield, brightness and fiber strength. The loss of flber strength will be particularly great if the degree o~ polymerization of ~he 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.
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 potontial sites for hydrogen bonding.
The conditions for the achievement of the foregoing objects in accordance with 40 the prooess of this invention are as follows:
1) The wood fragments, havin~ fibers suitable for paper rnaking, such as chips, are in a form in which thorough chemical impregnation can be achieved in a reasonable time.

2) There is an initial thorough impr~gnation of ~he chips or other wood fragments by an alkaline aqueous liquor having at least one agent acting to produce hydrophilic groups and as an antioxidant which is capable of protecting the chips against oxidation and develops hydrophilic 0roups during the cooking stage. The same chemical may act as both an agent to produce hydrophilic groups and as an 60 antioxidant or these functisns may be p~rformed by separate chernicals. At the encl of cooking the pH should not be lower than about 6.0, so that acids released during cooking will be neutralized. Pre~erably a swelling a~ent is also used in the case of high density wood.

3) The impregnated chips are cooked using sa~urated steam in the substantial absence of air at high temperature and pressure.

4) After cooking reactor jc, pressuri7ed with cool nitrogen ~he chips that have been steam cooked and pressurized are subjected to explosive decompression to result in chips which are softened and mostly defibrated.

5) The defibrated chips are preferably washed and then, without undue delay, and pr~ferably immediately, refined to provide pulp.
The steps of the process of this invention which will for convenience be referred to as the improved explosion process, will now be considersd in more detaii.

The wood fra~ments The starting material will normaly be chips in chich the fibars are of a length suitable for paper making. Shavings could also be used but sawdust would be undesirable exGept as a minor part of the total furnish as the fibers are partially cut.
The chips would also, as is well known, be suitable in the sense of being free from bark and foreign matter.
It is desirable for the purpos0s of this invention that coarse chips be avoided as otherwise the subsequent impregnation may deposit chemicals only on the chip surface, unless impre~nation 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 shraddad or thin chips of a 4-8 mm thickness. It has been found that this process is applicable to hardwoods, jack pine and larch, black spruce, doublas fir giving strong~r papers at lower rafining energy comparGd with convantional chsmo-thermomechanical or chemi-mechanical pulping.

2 ~ a ~
m~reqnation The purpose of impregnation is to protect the chips against oxidation during cooking and during transfer from tho cooking vessel to the refiner. It is also an objective to provide a positive increase in strength by developing hydrophylic groups 10 on the fiber surface during steam treatment. This will then provide additional sites for hydrogen bonding.
The preferred anti-oxidant is sodium sulphite Na2SO3 which also forms hydrophilic groups, and which is available at a low cost. It is used to provide a concentration of absorbed chemical of about 1 to 15%. Concentrations below 4%
20 would ba used where brightness pro~ection is unimportant and high strength is not required. Where, however, bri0htness is important the sodium sulphite should be at least 4%. If physical properties are important thesR will be irnprav~d by using a concentration of at least 4% sodium sulphite and will be fur~her improved as the concentration is further increased towards 12%. The concentration of the solution is preferably about the same as percent of chemical to be absorbed where there are equal quantities of chips and liquor. For example, a ton of chips of 50%
consistency mixed with one ton of 8% solution will result in about 8% absorbed on the pulp. Of impo~tanc~ is thorough impregnation to distribute the antioxidant avenly 40 rather than depositing it just on the surface. Other antioxidants that can ba used are potassium sulphite or magnesium sulphite. Ammonium sulphite could be us~d if cooking conditions are not severe or with a buffer. Complexing agents such as ethylene diamine tetracetic acid (EDTA), sodium diethylene triaminepentacetate (DTPA), sodium tripolyphosphate (TPF) and other complexing agents known in the 50 art as being usable under alkaline conditions may be added to minimize the catalytic effect of metals such as iron on oxidative degradation.
It is desirable also to use a swelling agent to assist the antioxidant or hydrophilic agent in penetrating the wood and this contributes also to softening th0 chip. This is of particular value in the case of high density wood. Suitable swelling 2 ~

agents are sodium or potassium hydroxide or ammonium hydroxide or sodium carbonate or sodium bicarbonate which will contribute also to providing hydrophilic groups. Other swelling agents that can be used and which may be desirable as auxiliary swelling agents for high density wood are zinc chloride, sodium chloride, 10 sodium bromide, calcium isocyanate, Schweitzers solution, cupriethylenediamne (C.E.D.) tetraethylammonium hydroxide, dimethyldib0nzylammonium hydroxide. The concentration of swelling ayent and conditions of swelling must be controlled in such a way as to avoid any dissolution of the hollocellulose. Thus the percentage of swelling agent in the impregnating solution will be in the range of about 1 to 4%
20 dep~nding on the agent and the conditions.
The impregnating solution must be alkaline and have enou~h free hydroxyl to be able to neutralize the liberated wood acids such as formic acid and acetic acid.
Normally the starting pH is about 7.~ 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 hours to 24 hours at a temperature of about 30C to 60C.
Approximately equal weights of chips and of aqueous impregnating solution can be used. For industrial purposes, however, the time may be shortened to an hour or to 40 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 temperatura of about 100C to 110C. To improve impregnation the chips should be compressed in advance of impregnation in cool solutions of chemicals. Under these conditions, penetratlon will be achieved in a shorter time, but penetration is what 50 predominantly occurs. There is no significant cooking~

Steam cook n The impregnated chips are steann cooked at a high temperature and pressure.

, Q ~ -i3 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 the chips under conditions of explosive decompression and 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 Technology Ltd. The equiprnent used in the examples was acquired from that compagny.
The temperature of cooking should be wi~hin the range of abou~ 180C to 21 0C and preferably within the range 1 90-200C, which is in excess of the temperatures considered possible according to the publications of Asplund and Higgins previously referred to. These temperatures correspond with a pressure of 10 atmospheres for 1 80C and 15.5 atmospheres for 200C. It is these high pressures which make a very important contribution to ensuring excellellt penetration of the chips by the cooking liquor.

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, in that with a batch reactor the cooking vessel is initially open to the atmosphere, to eliminate air. This air would be disadvantageous in ~hat it would result in oxidation if it were trapped in the cooking vessel. Additional antioxidant may if desired be added at this stage. 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 ~o 6 minutes and preferably about 1 to 4 minutes.
it has been found that within reasonable limits there is a property irnprovement by increasing the time - temperature (K). By increasing this constant from 285 to 760 in the case of biack spruce at about the same freeness (157-167) the burst index increased ~rom 3.15 to 4.41 and breaking length from 6.3 to 7.6 and tear from 5~6 to 5.B. Refining energy dropped from 3.2 to 3.1 and brightness dropped from 2 ~

~3.7 to 49.1 (equivalent to 59.7 to 55.1). These figures are adjusted to those that ordinarily would be obtained by using an industrial refiner in place of a laboratory refiner. Impregnation was with 8% sodium sulphite and 1/2% of DTPA.

Pressurization After cookinig, reactor is pressurized instantaneously with inert gas like nitrogen. The pressurization is conducted after cooking with saturated steam is accomplished in order to obtain optimum results as far as paper properties, yield and brightness is concerned. The nitrogen ~emperature being well below ~he cooking temperature is not like to increase hydrolysis and decrease cooking yield.
Nitrogen pressure used, varying from 25 to 50 atm contributes to even more efficient chip defibration in the subsequent step of pressure release.

Explosive decom~ression Afeer cooking the pressure is instantaneously released and the chips are exploded into a release vessel. If there is ~o be a delay between release of thechips and refining it is important to cool the chips down by washing them. VVashing may also be desirable for the purpose of chemical recovery.
l~ is desirable immediately to rsfine the chips after explosive decompression.
Otherwise, if the chips are stored, some oxidation will occur with resultant loss of brightness. The rapidity with which this will 020cur depends on how much residlJal antioxidant is present at that time and on ths temperature of the chips and the extent of exposure to oxygen. Preferably, therefore, refining is immediate so that it 5Q is unnscessary to incur the cost of excess antioxidant. In any event, unduc delay should be avoided. Such delay is regarded as being undue if oxidalion takes place to an extent that will materially affect brightness.
The chips resulting from the explosive decompression are softened and partially defibrated-, 3"~

~efinin~
Refining in the experiments described below standards using an atmospheric laboratory refining was conducted at 2% consistency ievel 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 with industrial refiners. Properties were evaluated after preparing paper sheetsaccording to standard CPPA testing methods.
Refining energies are usually low and can be expected to be in the range of 2.6 to 4 MJ/kg, hardwoods, CSF - 100 ml, which is cnnsiderably lower than that of conventional CMP and about 20% lower than that described in Kokta, Can. Pat. 1 230 208 and U.S. Pat. 4 798 651 (1989).
The present invention is described in the enclosed example.

EXAMPLE
Chips Freshly cut and naturally grown aspen trees from the Joliette region of Quebec were debarked, chipped and screerled at La Station Forestière Duchesnay, Quebec.Average chip size after screening, was as follows: length 2.5 to 3.75 cm; width:to 2 cm; thickness: 1 to 9 mm with maximum distribution at 5 mm.

Impre~na~ on 150 g of chips (= ~0% siccity) were mixed in plastic ba~s alon~ with 375 q of a solution made up of 8% Na2SO3. Time of impregnation: 24 hours; temperature of impregnation: 60C. Liquid/chip ratio during impregnation was 0qual to 6.
In addition, 0.5% DTPA was usad in applied cooking iiquors.

2 ~ r~
~,ookinc3 Explosion pulps have been prepared using vapor phase team cooking of sulfite pretreated aspen woad chips. Pulps have been prepared with the same 90% yielcl by using cooking temperatures 1 90C, 1 95C and 200C with cooking times 2 10 minutes, 1.5 minute and 1 minute respectively.
Cooking took place using saturated steam in a laboratory batch reactor build by Stake Tech. Co., Tha temperature and time of cooking are presented in Table 1.
Cooking was preceded by one minute steam flushing at atmospheric pressure. Aftercooking, the pressure was instantaneously released and chips which exploded into20 the release vessel were washed and cooled down wi$h one liter of tap water, and subsequently refined after being stored in a cold room. The reported amount of steam used for cooking varied from 0.5 to 1 kg of steam for 1 kg of chips. Yieldwas measured as follows: exploded chips ~75 Çl) were washed with one liter of tap water and subsequently defibrated for 90 seconds in a laboratory blender at 2%
consistency. The pulp was washed again with one liter of water, dried at 10SC to constant weight and the resulted weights were compared to the initial O.D. weight of chips.
Two types of explosion have been used:
a) Ordinary explosion: the ~hips were exploded after vapor phase cooking.
The pressure was proportional to the saturated steam pressure at ~he cooking t~mperature (Table 1).
b) Nitrogan explosion: nitrogen had been added to the reactor 5 seconds before finishing the cookin~ process. When the pressure reached 25 atm, the chips were exploded into the releas~ vessel (Table 2).

Refininq Laboratory refining was also done using a domastic blend~r Osterizer B-8614 at a consistency level of 2%. Defibration and r~fining ener~y was measured using a 2 ~
rlIOKI model 3181-01 powermeter with an integra~or. Specific refining energy was calculated by substracted blending en~rgy of fully beated pulp frorn the total energy needed to defibrate and blend the fiber suspension.

10 Property evalua-ion Paper sheets were prepared and tested according to standard CPPA testing methods on 1.2 g sheets. Brightness (Elrepho) was evaluated on sheets made with deinosized water.

20 Blcachina Bleaching was carried out using 4% of hydrogen peroxide.
In Table 1, cooking conditions as weil as paper properties are provided for ordinary explosion process as defined in Kokta, Can. Pat. 1 230 208 and U.S. Pat.

4 798 651 (1982).

In Table 2, cooking conditions as well as paper properties are provided using the pressurization by nitrogen bsfore explosion, conditions subject of present invention.
The yield of nitrogen expioded pulps has not been decreased by more than 40 o 4% compared to ordinary exploded pulps.
As describad in Tables 1 and 2, refining time and refining energies of nitrogen exploded pulps showed significant savings (about 20%) compared to ordinary exploded pulps.
At the same freeness lev~ls nitrogen exploded pulps showed better mechanical properties (breaking length, tear, burst) as presented in Figures 1, 2, and 3.
The brightness of nitrogen exploded pulps in both cases, unbleached and bleached, has not been lower by more than 1% compared to ordinary exploded pulps.

The nitrogen explosion seems to be very promisin~ field in the explosion pulping processes. It allows to prepare very high yield pulps which are easy to refine, they can be easily bleached to more than 80% MgO brightness by single stage peroxide bleaching and their properties are superior to conventional l0 CMP/CTMP with the same yield.

_ O P~ D ~ li A Pl Y E X P L O S I O N

COOKING COOKFREENESS BREAKING TEAR BURST
TEMPERATURETIME LENGTH
(kPa m2/g) (C) (min) (ml) (km)(mNm2/g) _ 190 2 500 4.50 4.50 1.85 190 2 300 ~.05 4.55 2.20 190 2 100 6.45 4.70 3.10 .

195 1.5 500 3.90 4.50 1.60 195 1.5 300 4.60 4.55 2.05 195 1.5 100 6.60 4.75 3.05 . _ 200 1 500 3.45 4.20 1.40 200 1 300 4.65 4.40 1.95 200 l 100 6.45 4.60 2.85 2 ~ f'~

Nl TROG E N EXPLOSION

COOKING COOK FREENESS BREAKING TEAR BURST
TEMPERATURE TIME LENGTH
(kPa m2/g) (C) (min) (ml) (km)(mNm2/g) , . _ _ _ . _ _ _ 190 2 500 4.05 4.60 1.65 1 90 2 300 5.3~; 4.70 2.30 190 2 100 7.30 4.85 4.00 195 1.5 500 4.40 5.2û 1.95 195 1.5 300 6.05 4.g5 2.70 195 1.5 100 7.35 5.ûO 3.65 _ _ . . . _ . _ _ . .

200 1 500 4.10 4.95 1.60 200 1 300 5.10 5.00 2.40 200 1 100 6.95 5.10 3.65 ..... ___ .

Claims (6)

1. A process for ultra-high-yield pulping providing yield in excess of 85% to produce pulp suitable for making paper, which comprises the steps of thoroughly impregnation wood fragments with an alkaline aqueous liquor including a soluble sulphite or other hydrophylic agents capable of acting to provide hydrophylic groups and/or act as an antioxidant, for example water soluble sulphite, sodium sulphite, potassium sulphite, magnesium sulphite alone or in combination with sodium hydroxide, sodium carbonate, sodium bicarbonate, preferably in an amount of 1-16% absorbed by the wood fragments, or other hydrophylic agents, steam cooking the impregnated wood fragments with saturated steam at superatmospheric pressure and at an elevated temperature, sebsequent instant pressurizing of reactor with room temperature nitrogen; subjecting the cooked wood fragments to explosive decompression to partially defibrate same;
transferring the partially defibrated fragments to a refiner and refining the softened and defibrated fragment to provide pulp, characterized in that the steam cooking is conducted at the cooking temperature in the range of about 180°C to 210°C, preferably 190°C to 200°C whereby the cooking pressure is about 10 atm to about 15.5 atm.

2. The process of claim 1, wherein the step of impregnation wood fragments is also carried out in the substantial absence of air achieved by replacing air with saturated steam.

3. The process as claimed in claim 1 or claim 2, wherein the time of cooking is in the range of about 30 seconds to about 6 minutes, preferably 1 minute to 4 minutes.

4. The process as claimed in claim 1 or claim 2 or claim 3 wherein the pressure of nitrogen introduced after cooking is about 25 to 50 atm.

5. The process as claimed in previous claims wherein the temperature of nitrogen is about 10 to 50°C.

6. The process as claimed in previous claims wherein the time of pressurizing is in the range of about 5 seconds to about 2 minutes, preferably 15 seconds to 45 seconds.