BLEACHING OF LIGNOCELLULOSIC MATERIAL WITH DIOXIRANES

BLANCHIMENT DE MATERIAU LIGNOCELLULOSIQUE AVEC DES DIOXIRANES

English Abstract

A process of bleaching a chemical pulp with a dioxirane produces a pulp of good quality and reduces environmental con-
cerns in so doing. A chemical pulp is contacted with a dioxirane, and either with or without known bleaching agents, in combina-
tion or sequence.

Claims

- 16 -
The embodiments of the invention in which an
exclusive property or privilege is claimed are defined as
follows:
1. A process of bleaching a chemical pulp that comprises
contacting the chemical pulp with a dioxirane.
2. A process as claimed in claim 1 in which the chemical
pulp has a Kappa number in the range 15 to 60 if
derived from a softwood species and 5 to 25 if
derived from a hardwood species.
3. A process as claimed in claim 1 in which the
dioxirane is contacted with the pulp at a temperature
in the range about 5° to about 50°C for a time up to
about 90 minutes.
4. A process as claimed in claim 1 in which the
dioxirane is used in amount sufficient to provide an
active oxygen charge of from 0.2 to 1.0% based on the
oven-dried pulp.
5. A process as claimed in claim 1 including, when the
dioxirane-treated pulp has a residual lignin
corresponding to less than 10 Kappa units, bleaching
the pulp by chlorine dioxide (D) or by the sequence
chlorine dioxide, caustic extraction, chlorine
dioxide (DED).
6. A process as claimed in claim 5 in which the
bleaching is carried out using a D1 E D2 sequence
using chlorine dioxide in the D1 stage in the range
0.5 to 1%, caustic in the E stage in the range 0.2 to
0.7% and chlorine dioxide in the D2 stage 0.2 to
0.7%, all percentages being on the weight of the
oven-dried pulp.

- 17 -
7. A process as claimed in claim 6 in which the chlorine
dioxide in stage D1 is about 0.8%, the caustic in the
E stage is about 0.4% and the chlorine dioxide in the
D2 stage is about 0.4%.
8. A process as claimed in claim 5 in which the
bleaching is carried out with chlorine dioxide in the
range 0.5 to 1% by weight of the oven-dried pulp.
9. A process as claimed in claim 8 in which the chlorine
dioxide is about 0.8% of the oven-dried pulp.
10. A process as claimed in claim 1 in which, when the
dioxirane-treated pulp has a residual lignin content
corresponding to a Kappa number exceeding 10,
bleaching is carried out using the sequence chlorine,
caustic extraction, chlorine dioxide, caustic
extraction, chlorine dioxide (CEDED).
11. A process as claimed in claim 10 in which the
sequence is CE1D1E2D2 using in stage C an amount of
chlorine appropriate for the Kappa number of the pulp
after contact with the dioxirane, in stage E1 the
caustic charge is 60% of the charge used in the C
stage, in stage D1 0.5 to 1.2% chlorine dioxide
charge is used, in stage E2 the caustic charge is 50%
of that used in the D1 stage and in stage D2 0.25 to
0.6% chlorine dioxide charge is used, the percentages
being based on the oven-dried pulp.
12. A process as claimed in claim 1 in which the
dioxirane has the structure
<IMG>

- 18 -
where R1 and R2, which may be the same or different,
is each an aliphatic or aromatic group and may be
linked together to form a ring.
13. A process as claimed in claim 12 in which R1 and R2
are each aromatic groups.
14. A process as claimed in claim 12 in which R1 and R2
are each aliphatic groups.
15. A process as claimed in claim 14 in which the
aliphatic groups are alkyl groups with 1 to 4 carbon
atoms.
16. A process as claimed in claim 12 in which the
dioxirane is dimethyldioxirane.
17. A process for bleaching chemical pulp comprises:
(a) contacting the pulp with a dioxirane having the
formula
<IMG>
in which R1 and R2 is each an aliphatic or aromatic
group and may be linked together to form a ring at an
active oxygen charge of 0.2 to 1% by weight of the
oven-dried pulp;
(b) bleaching the resulting pulp using the sequence
chlorine dioxide (D) or chlorine dioxide, caustic
extraction, chlorine dioxide (DED) when the pulp has

- 19 -
extraction, chlorine dioxide (DED) when the pulp has
a residual lignin content corresponding to a Kappa number
below 10;
(c) when the pulp has a residual lignin content
corresponding to a Kappa number greater than 10
bleaching the pulp with the sequence chlorine,
caustic extraction, chlorine dioxide, caustic
extraction, chlorine dioxide (CEDED) using a
significantly reduced amount of elemental chlorine
charge.
18. A process as claimed in claim 17 in which the
dioxirane is dimethyldioxirane.
19. A process as claimed in claim 17 in which the
dioxirane is contacted with the pulp at a
temperature in the range of about 5° to 50°C for a
time up to about 90 minutes.

Description

2073862 47930-2
BLEACHING OF LIGNOCELLULOSIC MATERIAL WITH DIOXIRANES
This invention relates to a process for bleaching a
chemical pulp.
In the manufacture of paper, wood is converted into a
pulp, usually by chemical means, and then spread on a wire
to be dewatered to form the paper.
Kraft pulping is the most widely practiced form of
chemical pulping in North America. The pulp is relatively
dark in colour and to produce a commercially acceptable
paper, it is generally bleached before being formed into
paper.
A variety of bleaching agents is used. They include
chlorine, chlorine dioxide, hypochlorites, hydrogen
peroxide and oxygen. Typically, more than one of these
chemicals is required and they are used singly or in
admixture in sequence. In the art the various bleaching
compounds, are designated by letter; chlorine is C,
chlorine dioxide is D, caustic extraction is E,
hypochlorite is H, hydrogen peroxide is P and oxygen is O.
The above compounds include chlorine and certain
chlorine containing compounds and there are concerns about
the use of chlorine and chlorine compounds in bleaching
pulp. Chlorine and chlorine containing compounds react
with pulp constituents - mainly lignin, fatty acids and
resin acids - to produce chlorinated organic compounds.
Some of these chlorinated organic compounds are of concern
environmentally and some major pulp producing countries
have restricted the amount that can be discharged from the
bleaching process into waterways. It is reasonable to
assume that most countries will introduce legislation
restricting the discharge of such compounds.

2073862
The elimination of elemental chlorine and its related
compounds in the bleaching process would eliminate the
production of chlorinated organic compounds in the
effluent but efforts along this line have not been wholly
effective.
The use of elemental chlorine and its related
compounds in bleaching may lead to the formation of
tetrachlorinated dioxins (TCDD) and furans (TCDF) which
are contained in the pulp and in the pulp mill effluent.
However, the formation of these two toxins can be
eliminated by reducing the amount of chlorine applied.
Unfortunately, this is difficult to do with existing
technology without sacrificing some pulp quality or
introducing further problems into the pulping process.
Oxygen delignification is a process of treating pulp
under oxygen pressure, about 100 psig, at elevated
temperature, about 80- to 120C, and for extended periods
of time, about 30 to 90 minutes. This oxygen
delignification process tends to produce weak pulp if
delignification is extended too far - i.e. down to about
10 Kappa number. In this regard the Kappa number of a
pulp is a measure of the bleachability of the pulp. It is
a number indicating the number of milliliters of 0.1
Normal potassium permanganate solution that can be
consumed under standard conditions by one gram of pulp
~oven dried basis) corrected to 50% consumption of
permanganate.
United States Patent 4,439,271 to O. Samuelson
discloses a process using nitrogen dioxide pretreatment
before oxygen delignification. The process retards the
depolymerization of cellulose during oxygen bleaching.
However, the emission of nitrogen-containing compounds
from the bleaching effluent may pose another problem and
these nitrogen compounds cannot be recovered by existing
kraft pulping chemical recovery systems.

~ 3 ~ 2 0 73 8 62
U.S. Patent 4,404,061 to J.J. Cael discloses that
pulpQ produced from a standard alkaline pulping process
can be bleached with monoperoxysulphate to produce
papermaking pulps and bleached pulps. Also, U.S. Patent
S 4,756,800 to E.L. Springer discloses a method of producing
salts of monoperoxysulphuric acid and simultaneously
bleaching pulp. Neither patent describes the strength
properties of the fully bleached pulps.
There is a need for a bleaching process that produces
pulp of good quality and which reduces environmental
concerns in so doing.
The dioxiranes are known compounds of the general
structural formula:
o\~f
`''\
in which R1 and R2 may be aliphatic or aromatic groups and
may be linked to form cyclic compounds. The dioxirane
compounds and their preparation are described, for
example, in the Journal of Organic Chemistry, 50 ~847-
2853 (1985) by R.W. Murray and R. Jeyaraman.
The present invention is a process for the bleaching
of chemical pulps without use of elemental chlorine or by
using elemental chlorine at a lower level than in the
prior art, thus avoiding the production of the toxins TCDD
and TCDF.
Accordingly, in its broadest aspect, the present
invention is a process of bleaching a chemical pulp that
comprises contacting the chemical pulp with a dioxirane.

~ 4 ~ 2073862
The dioxirane may be used with known bleaching agents
either in combination or in sequence.
The pulp is desirably one having a kappa number in
the range of 15 to 60 if derived from a softwood ~pecies
and 5 to 25 if derived from a hardwood species. The
dioxirane is contacted with the pulp preferably at a
temperature in the range of about 5- to 50-C for a time up
to about 90 minutes. Preferably the dioxirane is used in
amount sufficient to provide an active oxygen charge, i.e.
one active oxygen atom per dioxirane molecule, of from 0.2
to 1.0% based on the oven-dried pulp. The dioxirane has
the structure:
O O
R, ~2
where R1 and R2, which may be the same or different, is
each an aliphatic or aromatic group and may be linked
together to form a ring. One dioxirane compound that has
proved useful is dimethyldioxirane but any dioxirane
compound is appropriate, for example, those compounds
where R1 and R2 are each alkyl groups with 1 to 4 carbon
atoms.
If a subsequent bleaching step is used, subsequent
that is to the dioxirane treatment then, when the
dioxirane-treated pulp has a residual lignin of less than
10 Kappa number, that subsequent bleaching of the pulp may
be carried out by chlorine dioxide (D) alone or by the
sequence chlorine dioxide, caustic extraction, chlorine
dioxide (DED). When the bleaching is carried out using
the DED sequence the sequence may be set out as D1ED2.
Chlorine dioxide in the D1 stage is used in the range 0.5

2073862
to 1%, preferably about 0.8%, caustic in the E stage is in
the range 0.2 to 0.7%, preferably about 0.4%, and chlorine
dioxide in the D2 stage is in the range 0.2 to 0.7%,
preferably about 0.4%. All the percentages are expressed
as weight of oven-dried pulp. If chlorine dioxide alone
is used, then it is in the range of 0.5 to 1% by weight of
the oven-dried pulp, preferably about 0.8% of the oven-
dried pulp.
When the dioxirane-treated pulp has a residual lignin
content corresponding to a Kappa number greater than 10
the subsequent bleaching may be carried out using the
sequence: chlorine; caustic extraction; chlorine dioxide;
caustic extraction; chlorine dioxide (CEDED). In this
sequence, expressing the sequence as CE1D1E2D2, a
substantially reduced amount of chlorine charge is used in
stage C, in stage E1 the caustic charge is 60% of that in
the C stage, in stage Dl 0.5 to 1.2% chlorine dioxide
charge is used, in stage E2 the caustic charge is 50% of
that used in D1 stage and in stage D2 0.25 to 0.6%
chlorine dioxide charge is used. The percentages are
based on the weight of oven-dried pulp. The chlorine used
in the C stage is generally applied in combination with
varying amounts of chlorine dioxide.
The invention is illustrated in the examples, which
are included for purposes of illustration not limitation.
Example 1
Aspen pulp (50g oven-dried weight), produced by the
kraft process to a Kappa number of 1~.4, at 39% pulp
consistency, was treated with the dimethyldioxirane (DMD)
solution in acetone (280mL, 4.59g/L concentration, i.e.
0.55% active oxygen by weight on oven-dried pulp) at 25C
for one hour. The dimethyldioxirane-treated pulp was then
bleached with chlorine dioxide (0.8% by weight on oven-
dried pulp) at 70-C for three hours.

- 6 ~ 2073862
A second sample of the same aspen kra~t pulp (100g
oven-dried weight) was delignified by molecular oxygen at
10.7% pulp consistency using sodium hydroxide (4% by
weight on oven-dried pulp), magnesium sulphate ~0.5% by
weight on oven-dried pulp), molecular oxygen (100 psig)
at 115-C for one and half hours. This molecular-oxygen-
delignified pulp was then bleached with elemental
chlorine (1.8% available chlorine by weight on oven-dried
pulp) at 20-C for one hour at 3% pulp consistency,
extracted with sodium hydroxide (1.1% by weight on oven-
dried pulp) at 74C for two hours at 12% pulp consistency,
and further delignified with chlorine dioxide (0.5% by
weight on oven-dried pulp) at 74C for three hours at 6%
pulp consistency. This bleaching sequence will be
referred to as OCED.
A third sa~ple of the same aspen kraft pulp (50g
oven-dried weight) was also bleached by a conventional
process using a CED sequence. Elemental chlorine
bleaching was carried out at 3% pulp consistency and 20~C
for one hour using 3.3% available chlorine charge on
oven-dried pulp, extraction at 74-C for two hours using
sodium hydroxide (2.0% by weight on oven-dried pulp) at
12% pulp consistency, and chlorine dioxide (1.0% by
weight on oven-dried pulp) delignification at 74-C for
three hours at 6% pulp consistency.
The results, shown in Table 1, illustrate that aspen
kraft pulp treated with DMD to a level of 80%
delignification has better mechanical properties than
that treated with molecular oxygen to a level of 67%
delignification. This fully bleached pulp at 86.8%
Elrepho, which was DMD-treated, has similar zero-span
tensile strength to that of a pulp, at 89.9% Elrepho,
bleached by a conventional CED bleaching process.

2073862
I~LE 1
Physical and ~ti~Al properties of aspen kraft pulp bleached with
dimethyldioxirane or mDlecular oxygen or by a oonventional bleaching
.
Aspen KaFea Degree of Zero Sean ElreFh~
Pulp Number Delig. (%) Tensile (hm) Brightness (%)
Unbleached 16.4 - 16.3 40.6
Treated 3.4 80 17.0 59.4
~D
Bleached - - 17.4 86.8
02-Delig. 5.4 67 10.6 70.7
a~ED
Bleached 10.0 91.5
aD
Bleached - - 18.4 89.9

- 8 - 2073862
In these Examples Elrepho Brightness was measured
with a Technobrite (trade mark) Micro TB-lC apparatus at
457 nm. The standard test is set out in TAPPI T-452 om-
87 and CPPA E. 1.
Zero span tensile strength was measured according to
TAPPI T231 cm-85 using Pulmac Instruments' The
Troubleshooter (trade mark) zero span tester.
Example 2
A fourth sample of aspen pulp (50g oven-dried
weight), produced by the kraft process to a Kappa number
16.4, at 39% pulp consistency, was treated with DMD
solution in acetone (170 mL, 4.44g/L concentration, i.e.
0.33% active oxygen by weight on oven-dried pulp) and 80
mL of acetone at 25-C for three hours. This DMD treated
pulp was filtered and washed with a large amount of
distilled water. The resulting pulp was then further
delignified with chlorine dioxide (1.0% by weight on
oven-dried pulp) at 74-C for three hours at 6% pulp
consistency, extracted with sodium hydroxide ~0.72% by
weight on oven-dried pulp) at 10% pulp consistency and
74-C for two hours , and bleached by another chlorine
dioxide stage ~0.5% by weight on oven-dried pulp) at 6%
pulp consistency and 74C for three hours.
A fifth sample of the same aspen kraft pulp (lOOg
oven-dried weight) was delignified by molecular oxygen at
12% pulp consistency using sodium hydroxide ~4.0% by
weight on oven-dried pulp), magnesium sulphate (0.5% by
weight on oven-dried pulp), and molecular oxygen (100
psig) at llO-C for one and a half hours. This molecular-
oxygen-delignified aspen pulp was further bleached with
elemental chlorine (2% available chlorine by weight on
oven-dried pulp) at 20C for one hour at 3% pulp
consistency, extracted with sodium hydroxide (1.2% by

- 9 - 2073862
weight on oven-dried pulp) at 12% pulp consistency and
74'C for two hours, and bleached with chlorine dioxide
(0.5% by weight on oven-dried pulp) at 6% pulp
consistency and 74-C for three hours.
In this particular example, the aspen kraft pulp was
delignified to the same extent (63%) by either the DMD
bleaching agent or molecular oxygen. Results, shown in
Table 2, illustrate that the fully bleached aspen pulp
produced from the DMD treatment followed by DED bleaching
without elemental chlorine has better zero span tensile
strength than that produced by molecular oxygen
delignification followed by CED bleaching with 2%
available chlorine charge on oven-dried pulp at the
elemental chlorine stage. On the other hand the DMD-
treated pulp yields a fully bleached pulp with similarzero span tensile strength to that produced by a
conventional process involving the use of elemental
chlorine for bleaching.

-- 10 --
20~3862
l~eLE 2
Zer~ span strengths an~ viso~sities of ~MC-treated, molecular-oxygen-
delignified Fulps an~ those produced by a ~ v~Dional prooess.
Aspen Kapp~ Degree of ZerO-spQn Elreph~
PL1P Number Delig. (%) lensile ~hm) Brightness (%)
Unbleached 16.4 - 16.3 40.6
Treatf~ 6.0 63 15.0 54.0
~DED
Bleached - - 15.9 91.4
02-Delig. 6.1 63 12.7 66.7
a~ED
Bleached - - 12.6 91.0
aD
Bleached - - 18.4 89.9

2073862
Example 3
A sixth sample of aspen pulp (509 oven-dried
weight), produced by the kraft process to a Kappa number
of 16.4, at 35% pulp consistency, was treated with DMD
solution in acetone ~643 mL, 2.0g/L concentration, i.e.
0.55% active oxygen by weight on oven-dried pulp) at 25C
for one hour. The resulting pulp was then washed
thoroughly with a large quantity of deionized water.
A seventh sample of the same aspen kraft pulp (1009
oven-dried weight) was delignified by molecular oxygen at
12% pulp consistency using sodium hydroxide (4% by weight
on oven-dried pulp), magnesium sulphate (0.5% by weight
on oven-dried pulp), molecular oxygen ~100 psig) at 115C
for one and a half hours. The resulting oxygen-
lS delignified pulp was thoroughly washed with a Iargeamount of deionized water.
The results, shown in Table 3, illustrate that the
DMD- treated pulp has a better yield than that bleached
by molecular oxygen to the same degree of
delignification. This illustrates that DMD has better
selectivity toward lignin than does molecular oxygen.

2073862
I~ELE 3
Pulp yields after nMD t~ u~l~ and oxygen delignification.
Aspen Kapp~ Degree of Yield %
Pulp Number Delig. ~%) Unbleached Pulp
Unbleached 16.4 - -
~MD-Treated 5.3 68 98.0
O2-Delignified 5.4 67 92.7

2073862
Example 4
Spruce pulp (50g oven-dried weight), produced by the
kraft process to a Kappa number of 32, at 37% pulp
consistency, was treated with a solution of DMD in acetone
~330 mL, 4.6g/L concentration, i.e. 0.65% active oxygen by
weight on oven-dried pulp) at 25-C for half an hour. The
DMD-treated pulp was further bleached by a CED sequence.
Elemental chlorine bleaching was carried out at a 3% pulp
consistency and 20C for one hour using 2.8% available
chlorine by weight on oven-dried pulp, extraction at 74C
for two hours using sodium hydroxide (1.7% by weight on
oven-dried pulp) at 12% pulp consistency, and chlorine
dioxide (0.8% by weight on oven-dried pulp)
delignification at 6% pulp consistency and 74C for three
hours.
A second sample of the same spruce kraft pulp (50g
oven-dried weight) was also bleached by a conventional
process using CED sequence. Bleaching with elemental
chlorine was carried out at a 3% pulp consistency and 20C
for one hour using 6.4% available chlorine on oven-dried
pulp, extraction at 12% pulp consistency and 74C for two
hours using sodium hydroxide ~3.8% by weight on oven-dried
pulp), and chloride dioxide ~1% by weight on oven-dried
pulp) bleaching at 6% pulp consistency and 74C for three
hours.
Results, shown in Table 4, illustrate that the DMD-
treated pulp can be bleached to the same brightness as
that bleached by a conventional process. The former uses
a significantly reduced amount of elemental chlorine
compared with that required by the conventional process.
The zero span tensile strengths and viscosity of the pulps
bleached by both processes are similar.

2073862
TAELE 4
Physical and oetical properties of spru~e kraft Fulps bleached with nMD or
by a conNentional ~ t~S.
Spruce Kappa Degree of Zero Span Vi~Q~ity ElreFho
PL1P Number Delig. (%) Tensile (hm) (~Pa.s) Brightness (%)
Un
bleached32.0 - 19.4 35.0 32.7
~}
Treated 18.8 41 18.8 23.0 40.2
nMD-CED - - 18.3 18.3 82.5
Bleached
CED - - 19.1 25.0 82.3
Bleached

- 15 - 2073862
Thus, the present invention provides a process for
treating pulp that can be subsequently fully bleached to
90% Elrepho, either without or with only small amounts of
elemental chlorine. The DMD-treated pulps that have a
S residual lignin content corresponding to 5 Kappa number or
less have pulp strengths equivalent to those of the
untreated pulps. The fully-bleached pulps produced by the
process of the invention have strength properties similar
to those produced by conventional bleaching processes
using high elemental chlorine charge.
The bleaching of chemical pulps with the dioxirane
compounds can be modified for a range of bleaching
conditions, bleaching sequences and combinations with
molecular oxygen and other bleaching agents, not
specifically exemplified above, for example, hydrogen
peroxide, oxygen and monoperoxysulphate. The treatment
with dioxirane can also be combined with oxygen
delignification, being placed before or after the oxygen
treatment. The use of two oxygen containing compounds can
have the virtue of eliminating a wash step needed between
stages of a sequence when chlorine and compounds of
chlorine are used.
The pulps used in this invention can be kraft,
sulphite, soda, or other types of chemical pulps from
hardwood and softwood species. The invention extends to
chemical pulps and this term is intended to extend to
pulps produced by the treatment of wood species with
organic solvents to separate the wood components.