Note: Descriptions are shown in the official language in which they were submitted.
~L1 ~3~
Background of the Invention
This invention relates to a process for the production of a
high-yield, high-strength bleached chemimechanical pulp from hard-
wood chips using a treatment with an alkaline peroxide liquor prior
to defibration of the chips.
3~i~3~1
The use of bleached hardwood chemical pulps in the ~anufacture
of papers, in particular printing papers, has increased ;n recent
years. This increase in demand for this relatively short fiber pulp
has been due to the fact that this pulp, which is used in conjunct;on
with long ~iber pulps, has relatively high strength, fold, tear,
burst and tensile, as compared to previously used refiner pulps, and
thereby contributes increased strength to the finished paper. Manu-
facture of such hardwood chemical pulps by the kra~t process has the
disadvantage of malodorous emissions from the pulping process. The
odor contributes heavily to pollution of the atmosphere. An additional
disadvantage oF the hardwood chemical pulping process is the relatively
low yield, being in the range of 40% to 50% on the dry chips depending
on the wood and the processing steps. A further disadvantage is the
high capital cost of producing the hardwood chemical pulps.
Using the process of the instant invention, a relatively high-
strength, high-yield pulp can be produced. This pulp can be used to
essentially replace bleached hardwood chemical pulps in various types
o~ white papers without the aforementioned disadvantages in the manu-
facture of the bleached hardwood chemical pulps. The process of this
invention comprises the steps of: (l) impregnating wood chips with
an alkaline peroxide liquor o~ high alkalinity, ~2) holding the
impregnated chips for a time suFficient to bleach the impregnated
chips, (3) acidifying the bleached chips, and thereafter, (4) de-fiber-
ing the chips by passing through a reF;ning appara~us under atmospheric
conditions. A discussion of the prior art relevant to the instant
invention is as follows:
U. S. Patent No. 3,023,1~0, issued February 27, 1962, to Textor,
and U. S. Patent No. 3,069,309, issued December 18, 1962, to Fennell,
disclose refiner bleaching processes wherein bleaching of wood chips
is accomplished by treatment at the refiner with alkaline peroxide
liquors. In each process, sodium silicate and magnesium sulfate are
added to -the bleaching liquor and the alkalinity of the liquor is low,
thus minimi~ing any pulping effect the liquor might have on the chips.
U. S. Patent No. 2,958,622, issued November 1, 1960, to Sparrow,
discloses a pulping process for cellulose containing materials, such
as wood shavings, us;ng an alkaline oxidizing bleaching agent, such
as a hypochlorite solution, for treating the wood shavings prior to
mechanical disintegration of same. Sodium peroxide is suggested as
an alternate oxidizing bleaching agent. U. S. Patent No. 3,0167324,
issued January 9, 1962, to Textor, discloses a pulping process and
apparatus for wood chips wherein the wood chips may be impregnated
with sodium sulfite liquor by squeezing the chips in a screw press
and spraying the chips emerging from the press with sulfite liquor.
Sodium peroxide, together with additives such as sodium silicate and
epsom salts, is disclosed as an alternative to the sodium sulfite.
French Patent 1,238,412, gran~ed July 4, 1960, discloses the
impregnation of wood chips with alkaline peroxide plus a stabilizer,
such as an alkali silicate, for not more than an hour during which
time the temperature of the chips may be raised to 100C, defibering
the chips and storing the defibrated chips for two minutes to two
hours before refining and washing. The alkaline peroxide is sodium
peroxide.
U. S. Patent No. 3,558,428, issued January 26, 1971, to Asplund~
discloses a chemimechanical pulping process wherein lignocellulose
material is ;mpregnated with alkaline hydroxides or alkaline carbonates
at a temperature below 100C. The impregnated chips are cooked in a
digester in an atmosphere of saturated steam at a temperature above
100C for a time period ranging from 1 to 20 minutes. During this
cooking period, sulfur clioxide S02 yas is adcled with the steam to
react with the alkaline impregnating materials to Form alkali sulfite
compounds. The cooked chips are then defibrated under the pressure
and temperature prevailing in the digester.
U. S. Patent No. 4,187,181, issued ~ebruary ~9 1980, to Ahrel,
discloses a process of producing bleached mechanical pulp of increased
brightness and light scattering properties. In this process, screw
pressed wood chips are impregnated with an alkaline peroxide solution
and surplus impregnating solution is removed by compression of the
chips. The impregnated chips are introduced into a pressur~ vessel
and ground between a pair of rotating discs. This prior ar-t discloses
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3L~L7 3~j~3~
that in the absence of peroxide, the optica1 properties o~ the fibers
deteriorates by storage o~ the alkalized raw fibers. Thus, temperature
and time of retention prior to defibration and temperature of defibra-
tion must be controlled or restricted to insure retention of peroxide
before and during defibration. To do this, compressed air is added
to the pressure vessel to maintain the chips at a temperature below
90C while the chips are being passed to the grinding zone. The im-
pregnating solution is a weakly alkaline solution and may contain 0.5%
to 1.5% sodium hydroxide, based on dry wood. The impregnating solution
may also contain, if necessary, the usual chemical adjuvants, such as,
e.g., water glass (sodium silicate~, a magnesium salt or a complex
forming substance.
As can be noted from the cited prior art, the alkaline peroxide
liquors for bleaching and pulping normally contain an alkali silicate,
generally sodium silicate, to stabilize the peroxide. Using such
silicates, silica will gradually build up on the refiner plates and
drastically shortens plate life. Also silica will deposit in the
evaporator system during known chemical recovery processes. The silica
is difficult to remove and, thus, adds to the cost of the pulping
process. Similar deposition of difficult-to-remove silica deposits
also occurs to a lesser extent in the furnace where the concentrated
liquor, which contains waste wood products from the pulping process,
is burned. As a result oF this deposition of silica, pulpin~ eF-fluents
containing sodium silicate are diFFicult to dispose of and sometimes
are sent to the sewer, thus contaminating rivers and streams below the
effluent outfall. Alternatively, such effluents are subjected to
costly treatments which either remove and/or destroy the organic con-
tent of the e-Ffluent.
Further, it is known from the cold soda process that under certain
conditions of high alkalinity, hardwood chips and/or hardwood pulp
will o-Ften turn yellow to light orange. Subsequent bleaching of the
discolored pulp to a higher brightness requires inordinate amounts of
bleaching chemicals and time. It has been observed by the inventor
that development of this difficult-to-bleach color in alkaline peroxide
~ 3 6C~
pulping can occur due, in part~ to the high alkalini~y of the impreg-
nating liquor if the peroxide is depleted prior to or during de-Fibration
in a disc refiner. Localized temperatures in the refiner may be as high
as 320F. A combination of: (1) high alkalinity of the liquor7 (2) ao-
sence of peroxide, and (3) high temperature of the impregnated chips and
pulp are the conditions conducive ~o formati~n of the yellow to orange
color. ~ontaminating metal ions, in particular iron and manganese,
function as ca~alysts to decompose ~he peroxide9 there~y causing forma-
tion of objectionable color. The source o~ the contaminating me~al
ions may be the wood chips, water, or i~ may be from the manufacturing
apparatus.
It ;s an object of this invent;on to pro~ide a process of chemi-
mechanical pulping using alkaline peroxide liquor, whereby a bleached
hardwood pulp of high-strength, high-yield and high-brightness can be
produced.
It is a further object of this invention to provide a chemi-
mechanical pulping process wherein the pulping chemicals can be readily
recovered and waste wood products can be eliminated from mill efFluents
at minimal cost.
Statement of the Invention
A process is provided for the production oF chemimechanical pulp
in the absence o-F silicate stabil1zers. This chemimechanical pulp is
characterized by (l) a pulp yield oF at least 80% based on dry chips,
(2) streng-th properties as shown by a combination oF tear, burst and
tensile properties which are superior to refiner mechanical pulp, and,
(3) bleaching properties as shown by brightness properties which are
superior to both refiner mechanical pulp and cold soda pulp. The
chemimechanical pulp is prepared by impregnating hardwood chips with
an alkaline peroxide liquor. The alkaline peroxide liquor is an aqueous
solution containing about 0.5% to about 4% hydrogen peroxide and about
2.5% to about 10% sodium hydroxide based on dry ch1ps. The impregna-ted
~L~l7 ~ 3~
chips have a p~l greater than 7.5. The thus impregnated wood chips are
then held For about 3 minutes to about 3 hours to soften and bleach the
chips after wh;ch the chips are acidi-fied to a pH of 7.5 or below by
the addition of an acid material and thereafter defibrated by passing
the acidified chips through refining apparatus operated at atmospheric
pressure. The pulp produced by this def;bration is recovered from the
refining apparatus.
Detailed Description of the Invention
The process of this invention can be used in pulping cellulose
material, in particular hardwood chips such as aspen, oak, maple and
birch chips. Excellent results have been obtained on aspen chips.
The resulting bleached pulp can be used as a substitute for the more
expensive bleached chemical pulps. It is particularly adaptable for
use as the short fiber in printing papers. This pulp could also be
used as a component of, for example, tissue or absorbent papers or
any other papers where hardwood chemical pulps are presently being
used.
The impregnating step uses an alkaline peroxide liquor which con-
tains from about 0.5% to about 4%, by weight, preFerably about 1% to
about 3%, hydrogen peroxide based on the amount o-F dried wood chips.
The arnount of hydrogen peroxide needed for bleaching would vary, de-
pending on the degree of bleaching desired a~d the type and age of
wood chips used. Oak chips, for example, would require a higher amount
of peroxide than, for example, aspen chips. Above 4% peroxide can be
used, but any benefits from the added peroxide would be minimal.
The alkaline peroxide liquor also contains about 2.5% to about
10%, hy weigilt, preferably about 4% to about ~%, sodium hydroxide
based on the amount of dried wood chips. Above 10% sodium hydroxide
may be used, but the benefits from the added sodium hydroxide would
be minimal.
.. . . . . . .. .. ... .. . . . . ...
3~
Stoichiometric amounts of sodium peroxide can be substituted for
all or part of the hydrogen peroxide and all or part of the sodium
hydroxi de, if desired.
The alkaline peroxide liquor may additionally contain a chelating
or otherwise complexing agent to control contamination with metal ions,
particularly iron and manganese ions. Preferred agents are organic
complexing agents, such as die~hylenetriaminepentacetic acid (DTPA),
2-hydroxyethylethylenediaminetriacetic acid ~HEDTA), ethylenediamine-
tetracetic acid (EDTA), diethylenetriaminepenta(methylenephosphonic)
acid, their alkali metal salts, and combinations thereof. The use
of about 0.5% DTPA on a dried wood chip basis has been found to give
bene~icial results. If recovery of the sodium hydroxide is desirable,
inorganic complexing agents, such as, in particular sodium silicate,
are to be avoided. Sodium silicate causes a gradual silica buildup on
the recovery apparatus, primarily on the evaporators. The silica is
difficult, if not impossible, to remove. Thus adding to the cost of
chemical recovery. Sodium silicate also is known to precipita~e sili-
cate on the refiner plates during defibration, resulting in reduced
plate life.
In a preferred form of the invention~ the wood chips are com-
pressed to remove some of the chip water prior to the impregnating
step and then allowed to expand while immersecl in the alkaline peroxide
liquor. This compressing and impregnation is conveniently accomplished
using a Bauer Impressafiner which is disclosed and described in U. S~
Patent No. 2,975,0~6 (1976) to Ginaven et al. Alternately, impreg-
nation may be done using the PREX (pressure-expansion) system developed
by Sunds DeFibrator AB, Stockholm, Sweden.
Impregnation of the chips may be further facilitated by pretreat-
ing the wood chips with steam prior to compressing and/or the chips
may be impregnated in two stages with an optical dewatering (compres-
sing) step in be-tween the two impregnating stages, if desired. The
manner of impregnation or apparatus for accomplishing the same are not
critical to the practice of this invention.
... . , . . .. . .. , , . . , ~ . . .... ~, ....... . . . ... ..
~L~'736~)~
The impregnated wood chips are held for a period oi time
sufficient to bleach the chips un-til they are white in color. This
can occur over a period of time from about 3 minutes to about 3 hours,
depending on the temperature, size and type of wood chips. Oak chips,
for example, would take longer to bleach than the less dense hardwood
chips, for example, aspen chips. Aspen chips are preferably held for
about 10 minutes to about 20 minutes at a temperature of about 120F
to about 160F. During this time, the relatively high alkalinity of
the alkaline peroxide softens the wood chips and thus enables the
fibers to more easily separate during the subse~uent de~ibrating step.
The impregnated chips are then neutralized or made slightly acid
by the addition of an acid material, such as inorganic or organic
acids or acid salts. Typical acid materials are sulfurlc acid,
sodium bisulfite, sulfur dioxide, acetic acid, phosphoric acid and
hydrochloric acid. Combinations of these acid materials may be used !
if desired. The p~ of the acidified bleached chips should be about
7.5 or below, preferably about 6 to about 7.
The acidified bleached chips are defibrated in a refining
appartus, preferably a disc refiner. The disc refiner is operated at
atmospheric pressure. A typical refiner, which can be used in the
process of this invention, is a C-E BaueP Double Disc Refiner.
Control of temperature of the refining i5 not necessary for the
practice of this invention. Temperatures in the refiner are in
excess of 212F.
The consistency of the acidified bleached chips during the
defibrating step is preferably about 20% to about 306, but lower
consistencies in excess o~ about 8% such as consistencies in excess
of about 10% or in excess of about 12% and higher consistencies up
to 35% can be tolerated.
If desired, the defibrated bleached pulp may be fur-ther refined
in one or more known refining steps. If higher brightness is
desirable, the pulp may be additionally bleached by one or more known
bleaching steps.
A comparison of pulps produced by the process of this inven-tion
with a typical refiner mechanical pulp and a typical cold soda pulp
as controls disclosed an unexpected combination of improvement in
brightness, opacity and strength values of the ins-tant pulps over
the control pulps. Aspen chips were used in these comparative
pulping processes. The magnitude of the improvements was also
unexpected.
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The instant pulps were shown to have an ;ncrease o~ about ~ to 15
points in brightness over the refiner mechanical pulp and an increase
of about 20 to 30 points in brightness over the soda pulp. A two-stage
bleaching of the cold soda pulp after the pulping process only increased
the brightness of the soda pulp by about 16 points, to about 64 bright-
ness which is too low for most white printing papers. Standard hand-
sheets produced using the instant pulps were shown to have burst values
of about 2 to 5 times, tear values of about double, tensile values of
about 2 to 3 times and ~old values of about 10 to 50 times as compared
to the typical refiner mechanical pulp. In general, the strength values
o-f the instant pulps were comparable to those of cold soda pulp. Op-
tical scattering values of the instant pulps showed about a 50% to 60%
increase in optical scattering over a typical cold soda pulp.
In the practice of this invention, yields of pulp on wood chips
were in the order of 85% to 90% based on dried wood chips. Color re-
version on aging was minimal. Where the chips were acidified before
the defibrating step, the yellow to light orange, di-fficult-to-bleach
color did not develop. In several instances, where the impregnated
wood chips were not neutralized before the refining step, the yellow
color developed in the re~ined pulp. This was particularly true if
iron and/or manganese were present.
Spent alkaline peroxide liquor can be removed from the system by
washing and/or compressing the defibered pulp. Excess water may be
removed by evaporation from the spent liquor with subsequent recovery
o~ the sodium values, and sulfur values iF present, from the concen-
trate. Alternatively, the spent liquors may be used, in part, in the
preparation of fresh alkaline peroxide liquors or the spent liquors
may be used as wash water in a normal kra-ft pulping process with the
subsequent recovery oF sodium and sulfur values in the kra-ft recovery
system-
The ~ollowing examples further illustrate the preFerred embodi-
ments of this invention and the advantages obtained thereby without
limiting the scope of the invention. All tests ~lere performed accord-
ing to TAPPI standard testing procedures.
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3l~L7 3~g3
Examples 1 and 2
Two runs were made according to the teachings of this invention
using 560-GS Bauer Impressafiner and C-E ~auer 401 A-tmospheric Double
Disc Refiner. The procedure was as follows. About 200 lbs. of aspen
chips (bone dry basis) were presteamed -for about 10 minutes with low
pressure steam. Presteaming was done in a hopper located above the
Impressafiner. Presteamed chips were then fed into the Impressa~iner
having a capacity of about 60 tpd bone-dry chips. As noted supra
the Impressafiner is a screw press with an attached impregnation
vessel. Impregnation liquor containing sodium hydroxide and hydrogen
peroxide was continuously supplied to this impregnation vessel. An
overflow was provided for excess liquor. Compressed chips were allowed
to expand in this impregnation vessel and they absorbed the liquor
while expanding.
The impregnated chips were allowed to drain on the floor ~or
about 25-30 minutes before re-fining them in 401 DD Bauer refiner. Chips
became whiter during th;s storage. Twenty (20) lbs. o~ these chips,
bone-dry basis, were used for each refiner run. S02 dissol~ed in water
was injected into the eye of the refiner during the first pass through
the refiner. The quantity o-F acid was regulated according to the p~l o~
pulp leaving the re-finer. An attempt was made to keep the pH below 7.
Actual pH levels were 5.6 for Example 1 ancl 5.2 -for Example 2. In
Example 2, mater-ial refined in the ~irst pass was additionally refined
during the second pass through the refiner. Example 1 represents one
pass re-fining operation.
Examples 3 and 4
Two prior art pulps were produced -for comparison with pulps from
Examples 1 and 2, using the same source of chips and the same equip-
ment as in Examples 1 and 2. These were a RMP (refiner ~echanical pulp~
and a cold soda pulp. In the production of RMP pulp (Example 3~ there
was no chip presteaming or impregnation prior to defibrating and
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.~L7 3~
refining. Chips as receive~ were fed directly into the Bauer 401
refiner in 20 lb. batches and the resulting pulp was additionally re-
fined in the second pass ~hrough the same refiner. Procedwres in the
production of co1d soda pulp were very similar to those of Example 2,
except that different impregnation chemicals ~caustic soda only) were
used and there was no acidification of chips prior to defibrating.
Pulp testing results are shown in the following table (Table I~.
TABLE I
Prior Art
Chemi- Cold
Process Mechanical RMP Soda
Example No. 1 2 3 4
Number of re~iner passes 1 2 2 2
HPD/T N/A 41.2 82.5 35.5
Chemicals App1ied on Chips (bone-dry basis), X:
NaOH 5 0 5.0 0 7 4
H2o2 1.9 1.8 0 0
DTPA 0.5 0.5 0 0
Pulp Properties:
Brightness, Initial 76.5 74.4 63.3 48.1
Brightness, Aged * 75.0 72.8 61.9 47.6
C.S. Freeness, ml 205 95 90 135
Bulk, cc/g 2.33 2.27 3.08 1.76
Tear Factor 75.3 69.9 36.6 71.9
Burst Factor 20.0 23.8 7.1 29.5
Tensile~ b.l.m. 4435 4680 2080 6100
MIT Fold 9 8 1 30
TAPPI Opacity 81.1 81.5 95.6 86.9
Optical Scattering 3 S .0417 .0425 .0706 .0274
- 30 * Aged for 1 hour in oven at 105C.
... ..... . . . . . .. .. . . .. ..
Chemimechanical pulps made according to the teachings of this
invention were brighter by about 12 points and much stronger than the
conventional RMP pulp. Cold soda process also gave a strong pulp.
However, brightness was very low (48.1). It had a yellow-orange tint
and could not be used in white paper grades where high brightness is
required. Subsequent bleaching of this pulp with 1% hydrogen peroxide
followed by 1% sodium hydrosulfite increased brightness to only 64.3.
Exameles 5-7
Three runs were made to determ;ne the effect oF acidification
level prior to re-Fining on brightness and on other pulp properties.
DifFerent levels oF S02 addition were applied to adjust pulp pH
between 6 and lO. Pulps in Examples 6 and 7 were additionally
acidified with S02 ~o pH 5.5 - 6.0 a-Fter re-Fining. Otherwise, pro-
cedures were identical to those oF Example 2.
Results were as in the Following table (Table II).
.
~736(~
TABLE II
Example No. 5 6 7
Number of refiner passes 2 2 2
Chemicals Applied on Chips (bone-dry basis),%:
NaOH 7.0 7.0 7.0
H22 1~5 1.5 1.5
DTPA 0.5 0.5 0.5
Pulp Properties:
pH, immediately after
refining 5.9 7.2 10.2
Brightness, Initial 71.6 69.9 65.4
Brightness~ Aged * 69.8 68.6 64.4
C.S. Freeness, ml 100 112 155
Bulk, cc/g 1.80 1.76 1.63
Tear Factor 67.5 64.8 64.4
Burst Factor 30.5 31.8 32.2
Tensile, b.l.m. 5900 6120 6030
~IT Fold 34 32 27
TAPPI Opacity 81.1 82.3 82.2
Optical Scattering, S .0402 .0405 .0393
* Aged for 1 hour in oven at 105C.
In these runs the best brightness was obtained when impregnated
chips were fully acidified before refining, to obtain a pulp sf pH
5.9~ Brightness was lower by 6.2 points when chips were only
partially acidified (pH 10.2 after refining). Other pulp properties
were not affected by the acidification level to a noticeable degree.
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Examples 8-13
Six runs were made on Sunds Defibrator pilot plant uni-t con-
taining Defibrator 300 CD disc re-finer for primary refining and
Raffinator R0 20 disc refiner for secondary refining~ both connected
in series by a screw conveyor. Aspen chips were presteamed with low
pressure steam in a hopper and then were compressed by a feed screw
in a continuous operation. Feed screw delivered compressed chips
to a digester containing two chambers. The first chamber was used
for chip impregnation and was partially filled wi$h liqùor. The
second one served as a retention vessel. Bo~h chambers were under
atmospheric pressure. Impregnation chemicals wère continuously
injected into the -first chamber where the compressed chips were
submerged for about 10 minutes. Impregnated chips were lifted out
from the -first chamber and moved into the second chamber for
steeping. From there the chips were screw fed into the primary
re-Finer, followed by secondary refining in Ra-ffinator R0 20. Chip
retention time between impregnation and refining was about 20
minutes. Two variables were explored in these 6 runs: peroxide
addition level ~0.55 to 2.04%) and the location of acicli-fication
step (immediately before versus after refining). Ac;dification
after re-fining was done within 3 minutes a~ter collecting the pulp
from the secondary refiner. Sodium bisulFite solution was used
-for acidifica-tion in all 6 runs. Resul-ts were as Follows:
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TABLE III
Example No. 8 9 10 11 12 13
Chemicals Applied in Impre~nation (% on bone-dry chips):
NaOH 6.90 6.90 6.77 6.77 6.86 6.86
~22 0.55 0.55 1.06 1.06 2.04 2.04
DTPA 0.5 005 0.5 0.5 0.5 0.5
Acidification Point AFter Before After BeFore After Before
Ref. Ref. Ref. Ref. Ref. Ref.
Consistency during refining, %:
Primary refiner 12.0 9.4 13.5 12.4 14.1 14.8
Secondary refiner 10.2 8.4 12.1 12.6 12.2 14.1
pH~ secondary re-Finer 11.9 6.4 11.4 6.5 11.4 6.2
Brightness, Initial 58.0 64.0 63.2 68.5 70.7 73.8
Brightness~ Aged * 56.3 62.1 61.1 66.1 68.9 71.5
15Canadian Standard Freeness, ml:
Before latency removal 60 80 80 70 75 70
AFter latency removal 35 40 60 20 45 10
Handsheet Properties After Latency Removal:
Bulk, cc/g 1.45 1.75 1.52 1.66 1.50 1.59
Tear Factor 55 56 54 55 55 55
Burst Factor 41.0 31.8 38.9 38.2 42.6 40.1
Tensile, b.l.m. 7497 6292 7071 7053 7266 7075
~IT Fold 136 30 77 72 92 55
TAPPI Opacity 83.5 88.9 82.9 84.7 79.3 82.5
Optical Scattering, S .0384 .0492 .0395 .0440 .0376 .0417
* Aged for 1 hour in oven at 105C.
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According to these data, signiFicant gains in pulp brightness
were achieved by acidi~ying before re-fining according to the tech-
niques of this invention, and not after refining which has been the
accepted way in prior art. Other advantages of acidification before
refining is higher opacity and higher bulkg two very desirable pulp
properties in the manufacture of printing papers. Pulp strength
properties were comparable for runs made with acidification before
versus after refining. The only exception was pulp in Example 9
which was somewhat weaker. However, even this pulp is as strong
or stronger than some chemical hardwood market pulps. The apparent
reason for lower strength in Example 9 is lower consistency during
refining (9.4 - 8.4% versus 12.0 - 10.2% in Example 8). This low
consistency was discovered only after the runs were completed.
.
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