Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~;)7~
This invention relates to a process for the
delignification of lignocellulosic material such as wood
pulp, bagasse and straw, employing treatment with nitrogen
dioxide and oxygen in sequence.
For many of the applications of lignocellulosic
material a bleached product is required. Bleaching is
basically a delignification process. commonly ligno-
cellulosic material such as sulphate (kraft) wood pulp
is bleached by treatment with chlorine and chlorine-
containing compounds such as chlorine dioxlde or
hypochlorite. A typical bleaching sequence for sulphate
wood pulp is chlorination,caustic extraction, chlorine
dioxide treatment, caustic extraction, and chlorine
dioxide treatment, abbreviated C-E-D-E-D. The use of
chlorine compounds, however, introduces difficulties in
handling the effluent from the bleaching process. The
chlorine-containing waste cannot be discharged into bodies
of water because of its adverse effect on aquatic organisms.
If the chlorine-containing waste is recycled through the
; 20 sulphate process black liquor recovery system, damage to
the evaporator and recovery furnace may result. In addition,
a build-up of sodium chloride occurs in the recovery system,
lowering the efficiency of the furnace. There is thus an
advantage in the avoidance of chlorine compounds in bleaching
processes,
It is known to replace the chlorine ingredient of a
pulp bleaching sequence by nitrogen dioxide, N02. In an
article by G. L. Clarke in Volume 118 of the Paper Trade
~ournal, February 24, 1944 at page 62 entitled "The Action
of Nitrogen Dioxide on Unbleached Pulp, Part I" a bleaching
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sequence employing a first treatment with nitrogen dioxide
followed by caustic extraction and treatment with hypochlor-
ite is disclosed. This bleaching sequence had the disadvan-
tage that it caused severe degradation of the pulp as
indicated by a much lower viscosity value when compared
with an analogous bleaching sequence, ch:Lorine, caustic
extraction, hypochlorite. When applied to Western hemlock
pulp the above nitrogen dioxide bleaching sequence gave a
bleached product having a viscosity of 39 centipoise
whereas with the analogous chlorine bleaching sequence a
viscosity of 67 centipoise was achieved.
It has now been found that lignocellulosic material
can be delignified to give a product of acceptable viscosity
employing nitrogen dioxide as a first step in a sequence
followed by alkali treatment and treatment with oxygen under
supra atmospheric pressure, abbreviated N02-0. The novel
sequence has an especial advantage in the abatement of
pollution. It is capable of removing 75% to 90% of the
lignin residue of a conventional kraft pulp and, because of
the chloride-free nature of the effluent, the latter is
capable of being recycled to the pulping chemicals recovery
furnace in amounts related to the washing efficiencies after
the oxygen stage.
Thus the main object of the invention is to provide a
process for delignifying lignocellulosic material wherein the
pollution load is reduced. Additional objects will appear
hereinafter.
The novel process for the delignification of
lignocellulosic material comprises the steps of
~l~7~909
(l) treating lignocellulosic material in aqueous
medium with nitrogen dioxide,
(2) washing the nitrogen dioxide-treated material
with water,
(3) treating the washed material in aqueous medium
with alkaline material, and
(4) treating -the alkaline treated material with
oxygen or an oxygen-containing gas at supra
atmospheric pressure.
In the first step the lignocellulosic material is
treated in aqueous medium at a consistency in the range
of 3.0% to 50%, preferably 30% to 50% by weight. The
temperature employed in the first step is in the range
of ambient to 120C., preferably 70C. to 90C. The
concentration of nitrogen dioxide employed is 0O50/o to 10.0%
by weight, preferably 1.5% to 6.0% by weight. The first
step treatment is carried out for a period of 5 to 60
minutes.
After the first step treatment with nitrogen dioxide
the lignocellulosic material is diluted with water to a low
consistency e.g. 3.0%. At this point the pH of the aqueous
lignocellulosic pulp will usually be in the range 1.0 to 3Ø
The lignocellulosic material is then washed thoroughly with
water. The lignocellulosic pulp is then pressed to a high
consistency e.g. 35%.
Then in the third step of the process the
lignocellulosic material is mixed with 1.0% to ~0.0% by
weight of an alkali or alkaline earth base. Preferably
there is also added at this stage 0.1% to 1.0% by weight
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of a magnesium salt such as magnesium chloride or
magnesium sulphate.
Then in the fourth step of the process the
lignocellulosic material in aqueous suspension at a
consistency of 3.0% to 35.0% by weight i5 treated in a
reactor with oxygen or an oxygen-containLng gas at a
partial pressure of oxygen of 30 to 200 pounds per
square inch at a temperature in the range 80C. to
200C. for a period of lO to 60 minutes. The ligno-
cellulosic material is then diluted to low consistency
and washed with water.
The above steps may be followed by conventional
bleaching steps such as C-E-D or D-E-D. Employing
bleaching sequences N0z-O-C-E-D or NO2-O-D-E-D final
pulp properties can be obtained equivalent to or better
than those fr~m a conventional sequence C-E-D-E-D. When
this is done the conditions are as follows:
Chlorine treatment of C-E-D sequence. The lignocellulosic
.
material in aqueous suspension at a consistency of 2.0%
to 35.0% by weight is treated with 0.5% to 5.0% by weight
of chlorine. The treatment lasts from l to 60 minutes at
a temperature of 15C. to 60C. The material is then
washed with water.
First chlorine dioxide treatment of _-E-D sequenc_. The
lignocellulosic material in aqueous suspension at a
consistency of 3.0% to 35.0% by weight is treated with
0.5% to 1.5% by weight of chlorine dioxide and, optionally
with sufficient base or other alkaline buffering agent, to
give a pH of about 4 at the end of the chlorine dioxide
- 30 treatment. The treatment lasts from 60 to 300 minutes at
-- 4
1~7~09
a temperature of 60C. to 80C. The material is then
washed with water. f
Caustic extraction (C-E-D or D-E~D). The washed material
in aqueous suspension at 3.0% to 35.0% consistency by
weight i5 treated with 0.25% to 2.0% by weight of base for
60 to 120 minutes at 40C. to 70C. The material is
then washed with water.
Chlorine dioxide treatment.__(Final step of C-E-D or D~E-D).
The washed material in aqueous suspension at 3.0% to 35.0%
consistency by weight is treated with 0.1% to 1.5% by weight
of chlorine dioxide for 60 to 300 minutes at 60C. to 80C.
The material is then washed with water.
The base employed in the present process usual:Ly is
sodium hydroxide. However, other materials such as potassium
hydroxide, calcium hydroxide, sodium carbonate or potassium
carbonate may be used. -
The base employed in the third step of the present
process, prior to treatment with oxygen, may be white liquor
rom the alkaline recovery system of a wood pulping plant.
Instead of using fresh water as a washing medium
"white water" available in pulp mills may be substituted.
The process of this invention has the advantage that
the effluent from nitrogen dioxide and alkaline oxygen
stages contains no chlorine compounds and so can be recycled
through the recovery system of a pulp mill. This results in
a reduction of polluting effluent from a pulp mill employing
the process.
The N02-0 partial sequence of this invention has been
found suitable for delignifying high yield kraft and soda
pulps, resulting in yield advantayes normally associated
with oxygen delignification but with added pollution
abatement b~eits. Whexeas a single oxygen stage will
delignify a high yield (50 to 100 Kappa number) pulp to
a Kappa number in the vicinity of 20 to 30, the NO2-O
sequence delignifies to 10 to 15 Kappa numbex at viscosity
levels equivalent to or better than those observed for
single oxygen stage pulps. ~ larger portion of pulluting
material can thus be returned to the recover~ system.
High yield pulps partially delignified by the
N02-O partial bleaching sequence can be brought -to a
fully bleached level by means of a C-E-D sequence. ~pplica-
tion of the bleaching sequence N02 0-C-E-D to a high yield
(Kappa number 50 to 100) pulp results in a fully bleached
pulp with strength properties equivalent to or greater
than those observed for the bleaching sequence C-E-D-E-D
on conventional kraft pulp of Kappa number 30.
In this specification all proportions and percentages
are by weight of oven dried material, unless otherwise
stated.
The tests characterizing the treated product of this
invention were carried out by the following standard methods.
Kappa number TAPPI Method T-236 M-60
Viscosity TAPPI Method T-230 SU-66
Brightnass TAPPI Method T-217m, 218m
Handsheet
For~ation TAPPI T-205 M-58 (Hot disintegration)
Freeness TAPPI T-227 M-58
Tear Factor TAPPI T-220 M-60
Burst Factor TAPPI T-220 M-60
Breaking Length TAPPI T-220 M-60 (Tensile)
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Colour of the process effluent was measured by
comparison wi-th colour standards prepared from potassium
chloroplatinate and cobaltons chloride according to
ASTM D-1209. The effluent colour is expressed as pounds
of platinum per ton of ton.
The invention is illustrated additionally by the
following Examples but its scope is not limited to the
embodiments shown therein.
EXAMPLE 1
50 grams, oven dried basis, of a softwood ~mainly
spruce species) kraft pulp of Kappa number 32.4 and
viscosity 34.7 centipoise at 40.0% consistency were placed
in a rotating reaction vessel. The pulp was preheated in
the vessel to a temperature of 75C. Nitrogen dioxide
(3.0% on pulp) was then distilled into the rotating pulp.
The reaction was allowed to proceed for an additional
30 minutes, after which the reactor was evacuated to
remove any unreacted nitrogen dioxide. The pulp was then
diluted to a consistency of 4.0% and the pH was found to
; 20 be 2.3. After thorough washing with distilled water, the
pulp was pressed to 35% consistency in preparation for the
oxygen stage.
To the washed pulp was added 2.6% by weight sodium
hydroxide and 0.1% magnesium ion as magnesium sulphate and
sufficient water to give a consistency of 27% while stirring
in a Hobart mixer. The pulp was then transferred to an
oxygen reactor. In the reactor the alkaline pulp was treated
with oxygen gas at a pressure of 90 pounds per square inch
gauge at 120C. temperature for a period of 30 minutes.
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9~9
The pulp was then diluted to a consistency of 4% and the
pH was found to be 10Ø After washing with water the
pulp had the following properties:
Kappa number 5.1
Visc05 ity 10.8 centipoise
The oxygen treated pulp was then further bleached
by the D-E-D sequence, the chemical charges and conditions
being as follows:
D1 stage: 1.2% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
E staye: 0.6% sodium hydroxide at 65C. for 1.5 hour
on pulp at 12.0% consistency.
D2 stage: 0.4% chlorine dioxide at 70C. for 3 hours
on pulp at 6~0% consistency.
The pulp bleached by the ~02-O-D-E-D sequence had
the follo~ing properties:
Brightness 90.1
Viscosity 9.7 centipoise
Strength properties are shown in the accompanying
Table.
Assuming return of the nitrogen dioxide and oxygen
stage effluents to the kraft recovery system and an oxygen
stage washing efficiency of 85% the combined D-E-D bleach
effluent has the following properties:
Colour (pH 7) 17.7 lb/ton of pulp
Total organic carbon 16.3 lb/ton of pulp
Chlorine ion 14.0 lb/ton of pulp
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The same softwood kraft pulp of Kappa number 32.4
and viscosity 34.7 centipoise was bleached by a conventional
C-E-D-E-D bleaching sequence. The chemical charges and
conditions of the various stages were as follows:
C stage: 6.9% chlorine at 21C. for 1 hour on pulp
at 3.0% consistency.
El stage: 3.5% sodium hydroxide at 65C. for 1.5 hour
on pulp at 12.0% consistency.
Dl stage: 1.0% chlorine dioxide at 70C. for 3 hours
on pulp at 6~0% consistency.
E2 stage: 0.8% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
D2 stage: 0.3% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The pulp after the final chlorine dioxide stage had
the following properties:
Brightness go,~
Viscosity 11.0 centipoise
The stxength properties are shown in the Table. The
combined effluents from the C-E-D-E-D bleaching sequence had
the following properties:
Colour (p~ 7) 502 lb/ton of pulp
Total organic carbon 53.3 lb/ton of pulp
Chlorine ion 135 lb/ton of pulp
It can be seen that by using the NO2-O-D-E-D rather
than the C-E-D-E-D sequence there results a reduction of 96%,
70% and 90% in effluent colour, total organic carbon and
chlorine ion respectively.
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EXAMPLE 2
The softwood pulp of Example 1 was subjected to a
NO2-O bleaching sequence as in Example 1 except that the
nitrogen dioxide concentration was 1.5% on pulp. The
bleached pulp had the following properties:
~appa number 8.0
Viscosity 18.6 centipoise
The NO2-O treated pulp was bleached further by a
C-E-D sequence with chemical charges and conditions as
follows:
C stage: 1.6% chlorine at 21C. for 1 hour on pulp
at 3.0% consistency.
E stage: 0.8% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
D stage: 0.7% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The bleached pulp had the following properties:
Brightness 88.5
Viscosity lS.l centipoise
The strength properties are shown in the Table. The
combined C-E-D effluent from the bleach plant had the
following properties:
colour (pH 7) 60 lh/ton of pulp
Total organic carbon 21.4 lb/ton of pulp
Chlorine ion 34~8 lh/ton of pulp
Thus reductions of 88%, 61% and 74% in colour, total
organic carbon and chlorine ion respectively were realized
by using the sequence NO2-O-C-E-D rather than the sequence
C-E-D-E-D.
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EXAMPLE 3
A hardwood (mainly oak species) kraft pulp having a
Kappa number of 19.0 and a viscosity of 25.7 centipoise
was bleached by the NO2-0 sequence using the same conditions
as in Example 1 except that -
Nitrogen dioxide 2.0% on pulp
0xygen treatment
Temperature 130C.
; Time 15 minutes
% sodium hydroxide 2.0
The NO2-O bleached pulp had the following properties:
: Kappa number 5.8 ~: :
: Viscosity 19.1 centipoise ~-~
The hardwood pulp bleached by the N02-O pa~tial
sequence was bleached further by a D-E-D sequence with
chemical charges and conditions as ollows:
Dl stage: 1.2% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
E stage: 0.6% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistencyO :
: D2 stage: 0.4% chlorine dioxide at 70C. for 3 hours
: on pulp at 6.0% consistency.
The resulting bleached pulp had the following properties~
Brightness 89.4
Viscosity 16.8 centipoise
The same hardwood kraft pulp of Kappa number 19.0 ~ :~
and viscosity 25.7 centipoise was bleached by a conventional : .
C-E-D-E-D sequ~nce. The chemical charges and conditions of
the various stages were as follows:
. .
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~L(3i7~9~ ~
C stage: 3.6% chlorine at 21C~ for 1 hour on a
pulp at 3.0% consistency.
El stage: 2.5% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
Dl stage: 0.7% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
E2 stage: 0.5% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
Dz stage: 0.28% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The bleached product had the following properties:
Brightness 88~6
Viscosity 18.2 centipoise
EXAMPLE 4
The same hardwood kraft pulp of Example 3 having a
Kappa number of 19.0 and a viscosity of 25.7 centipoise was
subjected to a N02-0 bleaching sequence as in Example 3
except that -
Nitrogen dioxide 1.5% on pulp
Oxygen treatment
Temperature 130C.
Time 15 minutes
% sodium hydroxide 2.0
The bleached product had the following properties: - -
Kappa number 6.0
Viscosity 19.8 centipoise
The pulp bleached by the N02-0 partial sequence was
bleached further by the sequence C-E-D.
,, ', ,.
~ - 12 -
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The chemical charges and conditions of the stages
were as follows:
C stage: 1.24% chlorine at 21C. for l hour on
pulp at 3.0% consistency.
E stage: 0.8% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
D stage: 0.5% chlorine dioxlde at 70C. for 3 hours
on pulp at 6.0% consistency.
The bleached product had the following properties:
~rightness 88.8
Viscosity 16.5 centipoise
E~MPLE S
A softwood (pine) kraft pulp of Kappa number 50
and viseosity 28 eentipoise was subjeeted to a N02-0
bleaching sequence as in Example l except that -
Nitrogen dioxide 3.0% on pulp
Oxygen stage
Sodium hydroxide 3.0%
The bleached product had a Kappa number of 9.3 and
a viseosity of 15.2 centipoise.
The product from the above N02~0 partial hleaching
sequence was subjected to further bleaching by the sequence
C-E-D. The chemical charges and conditions of the stages ~`
were as follows:
C stage: 1.9% chlorine at 21C. for 1 hour at a
- pulp consistency of 3.0%.
E stage: 0.9% sodium hydroxide at 65C. for 1.5 hours
at a pulp consistency of 12.0%.
D stage: 008% chlorine dioxide at 70C. for 3 hours
at a pulp eonsistency of 6.0%.
- 13 -
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The bleached product had the following properties:
Brightness 88.1
- Viscosity 13.7 centipoise
The same softwood kraft pulp was subjected to a single
stage oxygen treatment under conditions identical to those
given above in this example. The resulting oxygen treated
pulp had a Kappa number of 22.4 and a viscosity of 17.2
c~ntipoise.
Thus it can be seen that the N02-0 sequence in this
case allows 58% greater delignification than 0 alone.
The pulp treated by the single oxygen stage was
subjected to the additional bleaching sequence C-E-D. The
chemical charges and conditions of the stages were as
follows:
C stage: 4.6% chlorine at 21C. for 1 hour on
pulp at 3.0% consistency.
E stage: 3.3% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
D stage: 1.4% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The bleached product had the following properties:
Brightness 86.5
Viscosity 14.5 centipoise
It can be seen that in order to attain a similar level
of brightness the bleaching sequence 0-C-E-D requires the use
of substantially more chlorine and chlorine dioxide than the
sequence N02-0-C-E-D.
- 14 _
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)9~9
EXAMPLE 6
A softwood (spruce-balsam fir) soda pulp having a
Kappa number of 93 was refined to fiberiæe the rejects and
then subjected to a N02-0 partial sequence as in Example 1
except that - :
Nitrogen dioxide 6.0% on pulp
Oxygen stage
% sodium hydroxide 7.5
The bleached product had a Kappa number of 17.G
and a viscosity of 16~6 centipoise. ~:
The N02-0 bleached material was then subjected to
the further bleaching sequence C-E-D. The chemical charges ~ .
and conditions of the stages were as follows:
; C stage: 3.2% chlorine at 21C. for 1 hour on
pulp at 3.0% consistency.
E stage: 1.8% sodium hydroxide at 65C. for 1.5 hours ~
on pulp at a consistency of 12.0%. ~;;
D stage: 0.6% chlorine dioxide at 70C. for 3 hours
on pu}p at a consistency of 6.0%. :
The bleached product had the following properties:
Brightness 85.8
Viscosity 16.7 centipoise
The same softwood soda pulp was subjected to a single
stage oxygen treatment under conditions identical to those
given above in this example. The resulting oxygen-treated
pulp had a Xappa number of 32.4 and a viscosity o~ 11.8
centipoise.
15 _
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~L~7~9~9
The above soda pulp treated by the sinyle oxygen
stage was subjected to the additional bleaching sequences
C-E-D. The chemical charges and conditions of the stages
were as follows:
C stage: 6.1% chlorine at 21C. for 1 hour on
pulp at a consistency of 3O0%.
E stage: 3.4% sodium hydro~ide at 65C. for 1.5 hours
on pulp a~ a consistency lf 12.0%.
D stage: 0.8% chlorine dioxide at 70C. for 3 hours
on pulp at a consistency of 6.0%.
The bleached product had the following properties:
Brightness 84.8
Viscosity 11.2 centipoise
EXAMPLE 7
A mixed hardwoods kraft pulp of ~appa number 46 w~s
refined to fiberize rejects and then subjected to a ~2-
partial bleaching sequence as in Example 1 except that -
~itrogen dioxide 3.0% on pulp .
Oxygen stage
% sodium hydroxide 4.0 on pulp
The resulting bleached product had the Kappa number 8.2
and viscosity of 25.1 centipoise.
The above bleached pulp was subjected to the additional
bleaching sequence C-E-D. The chemical charges and conditions
of the stages were as follows:
C stage: 1.6% chlorine at 21C. for 1 hour on
pulp at 3.0% consistency.
: E stage: 0.9% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
,
- 16 -
. . .
~7~
D stage: 0.4% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The resulting product had the foLlowing properties:
Brightnass 88.8
Viscosity 24.3 centipoise
Overall pulping and bleaching yield - 51.1% on wood.
The strength properties of this pulp are shown in the Table.
The above mixed hardwood kraft pulp was subjected to
a single stage oxygen treatment under conditions identical
to those given above in this example. The resulting oxygen-
treated pulp had a Kappa number of 15.8 and a viscosity of
29.9 centipoise. ~
The above kraft pulp treated by the single oxygen -
stage was subjected to the additional bleaching sequence
; C-E-D. The chemical charges and conditions of the stages
were as follows:
C stage: 2.9% chlorine at 21C. for 1 hour ~
on pulp at 3.0% consistency. -
E stage: 1.6% sodium hydroxide at 65C. for 1.5 hours
on pulp at 12.0% consistency.
D stage: 0.4% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The resulting bleachecl pulp had the following properties:
Brightness 89.0
Viscosity 20.7 centipoise
Overall pulpins and bleaching yield - 51.4% on wood.
The strength properties are shown in the Table.
Another portion of the mixed hardwood chips used in this
example was pulped to a Kappa number of 14.8 and bleached by
_ 17 - `
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~C~7~3~
a conventional C E~D E-D sequence. The chemical charyes
and conditions of the stages were as follows:
C stage: 3.1% chlorine at 21C. for 1 hour
on pulp at a consistency of 3.0%~
El stage: 1.7% sodium hydroxide at ~5C. for 1.5 hours
on pulp at a consistency of 12~0%~
Dl stage: 0.7% chlorine dioxide at 70C. for 3 hours
on pulp at a consistency of 6.0%.
E2 stage: 0.5 sodium hydroxide at 65C. for 1.5 hours
on pulp at a consistency of 12.0%.
Dz stage: 0.3 chlorine dioxide at 70C. for 3 hours
at a consistency of 6.0%.
The bleached pulp had the following properties:
Brightness 89.4
Viscosity 28.3 centipoise
Overall pulping and bleaching yield - 47.8% on wood.
Strength properties are given in the Table.
EXAMpLE 8
A softwood (mainly spruce species) kraft pulp of
Kappa number 32 was subjected to a NO2-O partial sequence
as in Example 2 but in distinction to said Example, a low
consistency pulp was employed in the oxygen stage. The
conditions of treatment were as follows:
~itrogen dioxide stage: 1.5% NO2 at 7SC. for 30 minutes
on pulp at 40% consistency.
Oxygen stage: oxygen gas at 90 pounds per square inch
pressure for 15 minutes at 130C. on pulp
at 3.0% consistency containing 3.0% sodium
hydroxide.
- læ -
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The product had the Kappa number of 9.8 and a
viscosity of 23.1 centipoise.
The oxygen treated pulp was then fully bleached by
a C-E~D sequence. The conditions and results were as
follows:
C stage: 2.0% chlorine at 2LC. for 1 hour on
pulp at 3.0% consistency.
E stage: 1.0% sodium hydroxide at 55C. for 1.5 hours ~ -
on pulp at 12.0% consistency.
- 10 D stage: 0.6% chlorine dioxide at 70C. for 3 hours
on pulp at 6.0% consistency.
The bleached product had the following properties:
Brightness 88.2
Viscosity 16.5 centipoise
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