CA2063547A1 - Steam explosion pulping process for papermaking - 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 and ascorbates in the temperature range of 180 to 210°C, followed by explosive decompression and fiber refining.

Description

2 ~ 7 BACKGRC)UND OF THE IN\/ENTION

Ingruber et al., Pulp and Paper Manufacture, Volume 4, TAPPI, CPPA, p. 160 (1985) define that convention conventional ultra-high-yield chemi~hermomechanical 5 or chemimechanical pulping is preferably conducted at a pH level between 4 and 9, and involves either liquid or vapor phase c:ooking with sodium sulphite-bisulphite solutions for about 10 to 30 minutes at a ternperature between 60 and 175C. It is generally accepted that the chemical treatment is mainly responsible for permanent fibre softening, increase in long fiber content, fibre specific surface and conformability, as demonstrated by Heitner et al., Pulp and Paper Can., (84)11: T252-T2~7 (1983).
There is another softening approach which consists of a steam treatment of chips at high temperatures followed by explosive decorr)pression.
The production of pulp using high-pressure and high steam chip softening well above glass transition temperatures of lignin should theoretically lead to lower energy 15 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; 2 645 623; 2 494 ~45; 2 379 8290. The masonite pulp obtained according to a two stage ~prout-Waldron refining procedure showed weak physical strength, dark color and yield loss 20 of 16% to 20%, and revealed itself simply unsuitable for the production of paper according ~Soran et al., Pulp and Paper Can., 79(3): T107-T-113 (1978). Mamers and al., TAPPI, 64(7): 93-96 (1981); APPITA, 29(5): 3~6-362 (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 25 of nitrogen. Paper propert~es which were obtained were similar to that of CTMP/CMP
pulps, but at the e)(pense of brightness. The major problem to overcome are oxidation, as well as hydrolytic degradation of fibers leading to brightness and yield Ioss.
It has been suggested by Vit et Kokta, Vit et al., C:an. Pat. 1 212 505 (1986) that 30 the ultra-high-yield (90%~) pulp suitable for papermaking can be produced by vapor ~63~

phase steam explosion cooking. The initial properties of papers 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 Preparing Pulp for Paper Making", Kokta B.V., Can. Pat. 1 230 208 (1987); U.S. Pat. 4 798 651 (1989); Can. Pat. Appl. $~542 643 (May 1987), 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 explosion pulping process consists of the chemical impregnation of chips, short duration saturated steam cooking at temperatures varying from 180C to 210(::, 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 rafining 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 (_ 50%) bleached hardwood pulps.
OBJECTS

The object of this invention is to provide a process in which improved properties are obtained when compared ~o previous invention of Kokta, (:~an. Pat. 1 230 208 (1987) by using alcaline ascorbates during impregnation and cooking.

i~

THE INVENTION

The rnajor problems accompanying previous processes using explosive decompression are believed to have been the degradation due to the oxidation of 5 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 therefore to attempt to curtail hydrolytic and oxidative wood degradation and thereby to protect against loss of yield, brightness and fiber strength. The loss of fiber strength will be particularly great if the degree of polyrnerization 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 cellulosa.
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 15 hydrogen bonding.
The conditions for the achievement of the foregoing objects in accordance with the process of this invention are as follows:
1) Th~e 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.
2) There is an initial thorough impregnation of the chips or other wood fragments by an alkaline aqueous liquor having a~ least one agent acting to produce hydrophilic groups and as an antioxidan~ which is capable of protecting the chips against oxidation and develops hydrophilic groups during the cooking stage. ~he 25 same chemical may act as both an agent to produce hydrophilic groups and as an antioxidant or these functions may be performed by separate chemicals. In this - invention sodium ascorbate is used as an powerfull antioxidant. At the end of cooking the pH should not be lower than about 6.0, so that acids released during cooking will be neutralized. Preflerably a swelling agent is also used in the case of high density 30 wood.

2 ~

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

4) After cooking, the chips are subjected to explosive decompression which results in its partial defibration.

5) The defibrated 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 improved explosion process, will now be considered in more detail.

Jhe woo~ fragm~nts The starting material will normaly be chips in chich the fibers are of a length suitable for paper making. Shavings could also be used but sawdust would be undesirable except 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 purposes of this invention that coarse chips be avoided as otherwise the subsequent impregnation may deposit chemicals only on the chip surface, unless impr~gnation 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 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 stronger papers at lower refining energy compared with conventional chemo-thermo mechanical or chemi-rnechanical pulping.

~mpregnatiQn The purpose of impregnation is to protect the chips against oxidation during `~ cooking and during transfer from the cooking vessel to the refiner. It is also an objective 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.

2 ~ L 7 In this invention, the powerfull antioxidant alkaline ascotbates (Vitamin C) areused to protect fibsr surface against oxidation and brightness loss. Furthermore, alkaline ascorbates protects sodium sulfites against excessive oxidation and the loss of their capacity to form hydrophilic groups on fiber surface. The chemical formula of 5 ascorbic acid is as follows(1):

OH OH O O

C = C oxidation C - C

CH C ,~ CH C
HOOH H-C- OH

H2C - o~ H2C - OH

15Ascorbic acid Dehydroascorbic acid ( 1 ) (2) The protective anti-oxidant action of ascorbic acid (or any alcaline salt) is due to ease of being oxidated to dehydroascorbic acid (2).
The amount of alcaline salt of ascorbic acid used during pulping varies from 0,25% to 2% based on ~he amount used in soiution in conjunction with sulfites.
The preferred hydrophylic agent is sodium sulphite Na2SO3 which also act as antioxidant, 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% would be used where ~rightness protection is unimportant and high strength is not required. Where, however, brightness is important the sodium sulphite should be at least 6%. If physical properties are important these will be improved by using a concentration of at least 6% sodium sulphite and wiil be further improved as the concentration isfurther increased towards 16%. Th0 concentration of the solution is preferably about the same as percent of chemical to be absorbed where there are equal quantities of 2~i3~r~

chips ~nd 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 importance is thorough impregnation to distribute the antioxidant evenly rather than depositiny it just on the surface. Other antioxidants that can be used are potassium sulphits or 5 magnesium sulphite. Ammonium sulphite could be usecl if cooking conditions are not severe or with a buffer. Complexing agents such as ethylene diamine tetracetic acicl (EDTA), sodium diethylene triaminepentacetate (DTPA), sodium tripolyphosphate (TPF) and other complexing agents known in the art as being usabla under alkaline conditions may be added to minimize the catalytic effect of metals such as iron on 10 oxidative degradation.
It is dasirable also to use a swelling agent to assist the antioxidant or hydrophilic agent in penetrating the wood and this contributes also to softening the chip. This is of particular value in the case of high density wood. Suitable swelling agents are sodiurn or potassium hydroxide or ammonium hydroxide or sodium 15 carbonate or sodium bicarbonate or magnesium carbonate 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, sodium bromide, calcium isocyanate, Schweitzers solution, cupriethylenediamne (C.E.D.) tetraethylammonium hydroxide, dimethyldibenzylam-20 moniurn hydroxide. The concentration of swelling agent and conditions o~ swellingmust 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% depending on the agent and the conditions.
The impregnating soiution must be alkaline and have enough free hydroxyl to 25 be able to neutralize the liberated wood acids such as formic acid and acetic acid.
Normally the starting pH is about 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 hours to 24 hours at a ternperature of about 30C to 60C.
30 Approximately equal weights of chips and of aqueous impregnating solution can be 2~3~47 used. For industrial purposes, 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 100C to 110C. To improve impregnation the chips should be compressed in 5 advance of impregnation in cool solutions of chemicals. Under these conditions, penetration will be achieved in a shorter time, bu~ penetration is what predominantly occurs. There is no significant cooking.

Steam cook~
lo The impregnated chips are steam cooked at a high temperaiure 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 the chips under condi~ions of explosive decompression and described in Canadian Patent 1 070 537 dated January 29, 1980; 1 070 ~46 dated January 29, 1980; 1 119 033 dated March 2, 1g82 and 1 138 708 dated January 4, 1983, all of which were granted to Stake Technology Ltd. The equipment used in the examples was acquired from that compagny.
The temperature of cooking should be within the range of about 180C to 210C and preferably within the range 190-200C, 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 180C and 15.~ atrnospheres for 200C. It is these high pressures which make a very important contribution to ensuring excellent 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 that it would result in oxidation if it were trapped in the cooking vessel. Additional antioxidant may if desired be added at this ~63~7 stage. Steao 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 reasonable limits there is a prop~rty improvement by increasing the time - temperature (K). By increasing this constant from 285 to 760 in the case of black spruce at about the same freeness (157-167) the burst indexincreased frorn 3.15 to 4.41 and breaking length from 6.3 to 7.6 and tear from 5.6 to 5.8. Refining energy dropped from 3.2 to 3.1 and brightness dropped from 53.7 to49.1 (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 and liquid chip ratio equal 3. By increasing concentration of Na2SO3 to 16% or by the increase 4C ratio to 6 optimal value of K was inferior to 250.
Explosive decornpression After cooking the pressure is instantaneously released and the chips are exploded into a release vessel. 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 mayalso be desirable for the purpose of chemical recovery.
It is desirable immediately ~o refine the chips after explosive decompression.
Otherwise, if the chips are stored, some oxidation will occur wi~h resultant loss of brightness. The rapidi~y with which this will ococur depends on how much residual antioxidant is present at that ~irne and on the temperature of the chips and the extent of exposure to oxygen. Preferably, therefore, refining is immediate so that it is unnecessary to incur the çost of excess antioxidant. In any event, undue delay should be avoided. Such delay is regardecl 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 partially defibrated.

20~3~
R~fining Refining in the experiments described below standards using an atmospheric laboratory refining was conducted at 2% consistency level using a blender couplecl 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 considerably 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.

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

Impreclna~ion 150 g of chips (= 50% siccity) were mixed in ~lastic ba~s alon~ with 3~5 of a solution made up of 8% Na2SC)3 and Na ascorbate varying from 0,25% to 2%.
25 Time of impregnation: 24 hours; temperature of impregnation: 60C. Liquid/chip ratio during impregnation was equal to 6.
In addition, 0.5% I:)TPA was used in applied cooking liquors.

~3~ 7 Cooking Explosicn pulps have been prepared using vapor phase team cooking of sulfite pretreated aspen wood chips. Pulps have been prepared with the same 90%
yield by using cooking temperatures l90C, cooking times 2 minutes.
Cooking took place using saturated steam in a laboratory batch reactor build by Stake Tech. Co., The 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 releas0d and chips which exploded intothe release vessel were washad and cooled down with 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. Yield was measured as follows: exploded chips (75 g) 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, clried at 105C to constant weight and the resulted weights were compared to the initial O.D. weight of chips.

R~ining Laboratory refining was also done using a domestic blender Osterizer B-861~
at a consistency level of 2%. Defibration and refining energy was measured using a HIOKI model 31~1-01 powermeter with an integrator. Specific refining energy was calculated by substracted blending energy of fully beated pulp from the total energy needed to defibrate and blend the fiber suspension.

Property evaluati~n Paper sheets were prepared and tested according to standard CPPA testiny methods on 1.2 9 sheets. Brightness (Elrepho) was evaluated on shee~s made with deinosized water.

2~3~47 R~sul~nd dls~uss3vns In Table 1, the properties of steam explosion pulps (SEP) prepared using the same conditions as defined in Kokta, Can. Pat. 1 230 208 (1987) and U.S. Pat.
4 498 651 (1989) using 8% of Na2SO3 are compared to that with 8% Na2SO3 and 5 Na-ascorbate varying from 0,25% to 1%.
It is quite clear that the present of ascorbates results in Increase of tear resistance as well as in the increase of breaking length, and dramatic decrease of relative specific refining energy.
Results in Table 2 and the Figure 1 and Figure 2 show, that the presence of 10 0,5% or 1% of Na-ascorbate is also improving ths strength of ultra-high-yield chemi-mechanical pups (CMP) comparatively prepared using the same impregnation and cooking reactor like SEP with the only difference being cooking temperature of 130C
and time of 30 minutes. On the other hand, it is quite clear that SEP is considerably stronger than equivalent CMP and uses lower refining energy.

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: 1 2 2~63~7 TABLE :!

C:OMPARISON STAM EXPLOSION AND CHEMIMEC:HANIC:AL
PIJLPING IN Tl IE PRESENCE OF N~-ASCORBATE

Proc~ss Na2SO3 (%) 8 8 8 8 8 Na-Ascorbate (%) 0 0.s 1 0.5 Temperature (C) 190 190 190 150 150 Time (min) 2 2 2 30 :30 UC ~ 6 6 6 6 S:~SF (ml) 100 100 100 100 100 Breaking length (km)5.0 6.9 6.7 4.8 4.6 Tear (mN.m2/g) 4.5 5.3 h.l 4.2 3.9 Brightness (%) 69.5 67 67 67.2 66 Opacity (%) 86.7 84.5 84.7 87.6 89 Ref. energy (MJ/kg)5.2 3.7 3.1 7.7 5.3 Yield (%) 89.6 88.3 89.5 89.2 91.4

Claims (5)

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 ascorbates and 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, magnesium carbonate, 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 pressureand at an elevated temperature; subjecting the cooked wood fragments to explosive decompression to partially defibrate same; transferring the partially defibratedfragments 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 B minutes, preferably 1 minute to 4 minutes.

4. The process as claimed in claim 1 or claim 2 or claim 3 wherein the ascorbates are in the form sodium, magnesium or calcium base

5. The process as claimed in claim 4 wherein the preferred ascorbates amount absorbed on wood fragment is the range of 0.1% to 1%.