Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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
11 11
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
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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.
1 1
2~3!~7
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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
