Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~7~8~
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IMPROVEMENT IN A PROCESS FO~R DELIGNIFYING
LIGNp~ELkULOSIC PULP ~Y MEANS OF OXY~
Technical Field
The present invention refers to an
improvement in a process for delignifying
lignocellulosic pulp hy means of o~ygen, and more
specifically, to an improvement in a process for
delignifying lignocellulosic pulp by means of oxygen
with the addition of ethanol.
Background Art
In conventional processes, bleaching of
chemical pulps is carried out mainly by employing
chlorine-containing compounds such as molecular
chlorine, chlorine dioxide and hypochlorites, thus
resulting in a corrosi~e, chlorine-enriched and
hardly recoverable effluent, which has a high degree
of environmental deterioration.
In view of that and due to more rigorous
environmental controls, there is nowadays a great
interest in employing bleaching agents, which reduce
the amount of chloro-compounds used in bleaching to
the minimum so as to recover the major portion of the
effluent. A bleaching agent e~hibiting such
properties is o~ygPn and, in recent years, its use
has substantially increased. Thr~ugh an initial
delignifying stage with o~ygen in an alkaline medium
in a multistage bleaching sequence of, for example, a
kraft pulp, the discharge of effluent from the
bleaching plant can be reduced to half, due to the
fact that effluent from the oxygen delignifying
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stage, which does not contain chlorides, is entirely
recoverable. Reduction o effluent discharge during
treatment with oxygen would be higher when the amount
of lignin removed during oxygen delignification is
higher than the usual 40-50%. This delignifying
limit is a ~unction of the low selectivity of o~ygen
as a delignifying agent~ which is caused by the
presence of free radicals derived from o~ygen during
reaction, thus reducing the cellulose chain lengths.
Such a reduction is usually characteri~ed by
increased reduction of the pulp viscosity after
o~ygen del;gnification. Formation of these free
radicals is increased by the presence of certain
transition metals ound in the pulp, wherein their
removal or inactivation nhances the process
efficiency.
It is verified from the state of the art
that magnesium salts have a veIy positive eff~ct on
preventing cellulose chain degradation during
treatment with o~ygen possibly due to inactivation of
transition metals, as well as the combination of
magnesium salts with chelating agents,
e.g.triethanolamine plus magnesium, -is also
substantially effective in maintaining the pulp
viscosity. Chelating agents, ~or example, DTPA,
EDTA, HEDTA and NTA, in the absence of magnesium,
have shown to be inefficient in maintaining pulp
viscosity. Removal of transition metals by treating
the pulp with acids or chelating agents prior to
delignifying the pulp with o~ygen renders good
results, but it requires additional installations for
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2 ~
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the bleaching process. The use of other additives,
such as tin and manganese salts have relatively
succeeded.
A typical bleaching se~uence for removing
lignim from lignocellulosic material, for e~ample,
kraft pulp of conifers or foliage, is (C~D) (E+O)
DED, where (C~D) is a chlorine/dioxide stage, (E~O)
is is oxygen-reinforced alkaline e~traction stage, D
is a chlorine dio~ide stage and E is a single
alkaline e~traction stage. In this sequence, the
main o~idizing components are chlorocompounds which
produce dangerous organochloro-compounds in the
effluent from the bleaching process, which are
measured by calculating ~OCl (total organic chlorine)
or AOX (absorbable organic halogens).
The need for chlorine-based compounds to
bleach brown chemical pulps, for example, kraft,
sulfide and organosolvent, and, consequently, the
amount of TOCl and AOX discharged into the effluent
from the bleaching is directly proportional to the
lignin content of the pulp derived from the baking
operation. Thus, the purpose of subjecting the brown
pulp to various treatments prior to the bleaching
operation is to reduce lignin content of the
lignocellulosic material to the ma~imum so that, in
the subsequent bleaching sequences, the smallest
possible amount of chlorine-based compounds is
consumed.
A treatment which has been successfully used
in the brown pulp is thus called oxygen
delignification which due to the low oxygen
selectivity, reduction of the Kappa Number of the
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pulp (indirect measurement of lignin content in the
brown pulp) is generally limited to, e.~. in the case
of kraft pulp of foliage, from 40-50% and of kraft
pulp of conifers, from 45-50~. Increasing
environmental pressure against the use of
chlorine-based compounds has, however, led to several
changes in the convention oxygen delignifying process
aiminy at obtaining a more significant reduction of
the Kappa Number. Thus, when, during the o~ygen
delignifying process, the reaction temperature is
increased from the usual 95-105C to 120~C,
reductions of the Kappa Number higher than 50% can be
achieved for foliage and conifer kraft pulps.
However, this gain in the delignifying rate of
lignocellulosic material occurs in parallel with an
accentuated reduction of the pulp viscosity, i.e., in
a decrease of the process selectivity if carbohydrate
protectiYe additives are not used. Some additives
which are commonly employed ~or maintaining the
viscosity during oxygen delignify:ing processes, for
example, magnesium salts, have been verified to be
inefficient when the process is carried out at
temperatl~res higher than 100C. On the other hand,
some other additives are not compatible with recovery
of the effluent frorn the o~ygen delignifying process,
which is one of the great advantageæ of the process.
Therefore, it is an object of the present
invention to provide an improvement in o~ygen
delignification which, without prejudice to the pulp
amounts, leads to reductions of the lignin content in
the lignocellulosic materials, which are higher than
those attainable with conventional o~ygen
delignifying processes.
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It is another object of the present
invention to provide an improvement in an o~ygen
delignifying process so that pulp viscosity is
maintained even when the o~ygen deliynifying process
is performed at elevated temperatures.
Summary Of The Inven~iQn
These and other objects are achieved by the
present invention through the use of an additive,
which under certain process conditions, during o~ygen
delignification of the lignocellulosic material,
o~ygen is made to act more efficiently, as can be
inferred rom a higher reduction of lignin content in
the lignocellulosic material, i.e. a higher reduction
of the Kappa Number, as compared with conventional
techniques, without, however~ prejudicing the quality
of the pulp whose cellulosic chains are preserved
from the action of free radicals derived from
intermediates of the reaction of oxygen with the pulp
and with transition metals. Consequently, the use of
the invention renders a pulp which in the subsequent
bleaching sequences will need a s~aller amount of
chloro compound to attain the desired whiteness and,
as a function of that, a smaller clischarge of TOCl or
AOX in the effluent from the bleaching plant and will
e~hibit a high viscosity, since it will contain long
chains of carbohydrates, thus resulting in a product
having good resistance properties. Furthermore, as
the employed additives are compatible from the
recovery point of view, the use of the invention
allows a more closed system in the bleaching plant
and the additive can be partially recovered or
reuse, which makes the process more economical. --
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Detailed Description
According to the present invention, the
additive used is ethanol and its dosage may vary from
1 to 130 weight percent, preferably from 10 to 20%,
wherein pre~erably 20 weight percent is added to the
pulp, whose consistency may vary in a range of from
5-30%, preferably in a range of from 8-15%, the
characteristic range of a pulp of middle consistency
prior to the first bleaching stage, possihly
immediately after the brown pulp storage tank,
preferably together with alkali~ which can be sodium
hydro~ide, o~idized white liquor and/or non-oxidi~ed
white liquor, wherein the pH is adjusted to a range
between 7 and 13, more preferably to a range between
11 and 12, the additive-containing pulp being fed to
a mixing apparatus to which oxygen is added at a
pressure varying from 2-6 kgf/cm2, preferably at a
pressure of 4 kgf/cm2, and then the pulp is passed to
a pressure reactor where the delignification in the
presence of oxygen takes place, at a temperature of
from 80-140C, preferably from 100-120C, wherein the
high temperature range of 120C is preferable, for a
period of time that can vary from 10 to 120 minutes,
preferably 6D minutes.
To evaluate the effects of adding ethanol to
the process for delignifying with oxygen a
lignocellulosic material in comparison with the
conventional processes, examples and respective
tables with e~periment results will be shown
hereinbelow, whi~h are intended to illustrate the
application of the invention and not to limit same.
D-16935
hereinbelow, which are intended to illustrate the
application of the invention and not to limit same.
Kappa Numbers, viscosity and whiteness of the pulp
were calculated in accordance with the standard
procedures of the ~Technical Association of Pulp and
Paper Industry tTAPPI)" and delignifying efficiency
values and selectivity coefficient were determined in
accordance with equations 1 and 2, respectively:
. EOUATION 1
Delignifying ~Initial Kappa Number -
Efficiency (~) ~appa Number after treatment) X 100
Initial Kappa Number
EOUATION 2
Delignifying Efficiency (%)
Selectivity (Inltial VlSCoslty -
Coefficient Viscosity after treatment3 X 100
,.= . _
Initial viscosity
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2 ~
Example 1
Th;s e~ample illustrates the temperature
effect on the efficiency of an industrial process for
delignifying, with o~ygen, a kraft pulp wherein the
experiments were carried out without the use of an
additive, with the use of ethanol as an additive and
with the use of magnesium as an additive. The sample
consisted of a kraft pulp of conventional eucalyptus
having a consistency of 12~, which, prior to
treatment, had a Kappa Number of 16.7, viscosity of
30.8 cP and whiteness of 33.8~Iso. The reaction time
was 60 minutes, using 15 kg 02~ton of kraft pulp,
partial pressure of oxygen equal to 4 kgf~cm2, 20
weight percent of ethanol and 0.5 we.ight percent of
MgSO4.7H20. Experimental results are shown in Table
1 as follows:
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P ~ ~ ~ ~ ~ I~ O V~ _ ~ ~ ~ r~
q~
~ O
U~ ~
~ U
0 ~1
a ~3
~o
V~ r~O O ~1 It) ~D ~1 ~ 3 ~ O 0\ 3 ~ O ~i ~ U'l
:~
O ~ ~ l O CO ~ I~ ~ i O\ ~ r I CO O CO
~O 0 C~ ~ O ~ ~ X U') ~ ~ ~ ~ O QO 00 U~ ~ ~ cr~ ~
--I O o o~ i o o o~ ~ o~ oo a~ 1:0 ~ ~\ 00 00
~ æ ~ ~
.J ~ O ~ Tl m C7~ 0 Cl~ ~ 0 0 C~ O ~ `~ ~;
O _I ~ O O ~ ~ ~ O _ C~ ~ O O O~ S~ O _
~ ~OU~U~OU~UlO~n~oU'~u,ou~U~ou~ouo
2 `--
g C~ O O O ~ o o g 3 .
0 ~ ~ ~
v~ ~ a a a a GO a ~ a a a
.,, ~ r ~ ~ r a a
V .u ~ V V Ll V V V V V ~ ~ ~ ~
V ~ ~ W
Q~
18~
-- 10 --
Example 2
This e~ample illustrates the ethanol
feedstock effect on the efficiency of industrial
o~ygen delignification of a kraft pulp in accordance
with the present invention. A sample of kraft pulp
of conventional eucalyptus having a consistency of
12%, which prior to treatment had a Kappa Number of
16.7, a viscosity of 30.8 cP and a whiteness of
33.8Iso was used. The reaction time was 60 minutes,
using 15 kg 02/ton of pulp, 02 partial pressure of 4
kgf~cm2 at a temperature of 120~C. E~perimental
results are shown in Table 2 as follows:
D-16935
v ~
~ ~ ~ O Ul ~ I~ O ~ ~ O 1~ ~ 00 ~ ~ _
c
a
O~
C o
^ c~ ~o ul l~ u~ o . ~ ~o o ~ o r~
r ~ o ~ ~ a~ ~r ~ oo
E~ ~
o ~ ~r ,~ r o oo ~o ~ ~ u~ ~ oo . r o~ u~
3 ~t u~ 00 r~ o~ o o~ ;r cr~ r o oo ~ Ul
~ ~ ~ ~ C~ r ~D eo
3: o~ o o o o o ~ o o o O ~ O o o o o o
o U~ o U~ Ul o U~ U~ o U~ U~ o U~ Ul o U~ U~ o Ul
Z--
o
o 9 0 0 0 0 0 9 0 o o o o O O O O O
~s~
V
~ ~ r u~ o ~ c~l ~ ~ u~
- 12 - 2~711~5
Example 3
This example illustrates the o~ygen partial
pressure effect on the industrial o~ygen
delignification of a kraft pulp in accordance with
the present invention. A sample of conventional
eucalyptus kraft pulp having a consistency of 12%,
which prior to treatment had a Kappa Number of 16.7,
a viscosity of 30.8 cP and a whiteness of 33.8Iso
was used. The reaction time was 60 minutes, using 15
kg 02/ton of pulp, 2 weight percent of sodium
hydro~ide, 20 weight percent of MgSO4.7H20 at a
temperature of 120Co E~periment results are shown
in Table 3 as follows:
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~ V
V
a~
0
O C~
al . . . .
t: rl U~ 3 ~O `D
3 ~ ;~
4,,
~O
O _I cl~ .J ~ r~
O L~ , . , . ~D
~ ~' 00 ~ ~ ~ O
.~ ~ ~ ~ ~ C
p
~3 .
~1 ~ ~ C`~ o cr
y ~
a~ o o~ _~
l 0 3
~ O O O O O
o
~ C~ O O O O O
h ,Y
~ .
.r~ O O O U~
5 J .~::
J- V ~J
W ~ ~ ~:
O
3 3 ':t
~7~18~
- 14 -
Egample 9
This e~ample illustrates the alkali
feedstock effect on the efficiency of the oxygen
delignification of an industrial kraft pulp in
accordance with the present invention in comparison
with conventional processes wherein tr~atment does
not involve the use of magnesium as additive. A
sample of conventional eucalyptus kraft pulp having a
consistency of 12% which prior to treatment e~hibited
a Kappa Number of 16.7, a viscosity of 30.8 cP and a
whiteness of 33.8Iso was used. The reaction time
was 60 minutes, using 15 kg 02Jton of pulp, an 02
partial pressure of 4 kgf/cm2, 20 weight percent of
ethanol and 0.5 weight percent of MgSo4.7H20 at a
temperature of 120C. ~periment results are shown
in Table 4 as follows:
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v c:
`O ~ ~ O .J ~ r~
~1 ~ O~ ~ ~ ~ ~ O
~1
a~ ~
Ql
~ u
)
. ~ . . . . ~ .
F-rl ~ ~ u~3 ~D 00
3 3 3 ~ 3 ~ 3 ~ ;i'
r.o
U~
C O c~
~ U~
:~
U ~ O X ~ ~ ~ ~ 0 5~ ~D
~ ~ ~ O _ ~J O 0~
~ 00 0 ~ _ O O
t~ ~ 3 C~ O
CJ~ O O ~ O O O C: _~
~: n o U~ U~ o n c:, m o
æ ~ê
O O O ~q u~ oq
.C ~ .C ~
V
n ~ r~ oo ~ o ~
~ J J m n u~ m
- 16 - 2~7~8~
As can be seen from the e~perimental results
of E~ample 1 (Table 1), the use of ethanol as
additive in the lignocellulosic pulp oxygen
delignifying process of the present invention,
provides an increase in the delignifying efficiency,
by means of a higher reduction of the Kappa Number
and an increase of the process selectivity allowing
the process to be carried out at higher
temperatures. In experiments in which ethanol was
used as an additive, an increase in the process
temperature of from 80-140C confers a significant
increase on the delignifying efficiency to the same
alkali feedstock, without, however, causing a
significant decrease in the process selectivity. For
an alkali dosage of 2% sodium hydroxide, while the
delignifying efficiency at 100C in the absence of
additives did not reach 38%, in the presence of
ethanol at 120C this eficiency e~ceeded 46~,
wherein the process selectivity coefficients, at
lOO~C and at 120C, were similar. Thus, it is
demonstrated that the use of ethanol in the
lignocellulosic pulp o~ygen delignifying process, in
accordance with the present invention, allows the
ogygen delignifying process to be performed at higher
temperatures resulting in a highex reduction of the
Kappa Number without, however, prejudicing the pulp
quality. It ~hould be pointed out the low values of
selectivity obtained when o~ygen delignifying takes
place at 120C in the absence of additives. In the
e~periments without additives, an increase in the
temperature confers an increase on the delignifying
efficiency for the same amount of alkali but on the
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other hand causes an accentuated decrease of the
process selectivity, and these results are in
conformity with the industrial results where the use
of temperatures above lOO~C for foliage pulp has been
verified to be impracticable due to the low
selectivities. Therefore, delignifying efficiency is
always limited ts a ma~imum of 40% with said pulps.
Th~ use of the present invention enables one to reach
delignifying levels above 46%, with good selectivity
and working at 120~C.
Upon evaluating the ethanol dosage effect on
the delignifying efficiency and on the selectivity,
it can be inferred, from the results refexring to
e3periments of E~ampl~ 2 (Table 2), that a variation
of the ethanol amount of from 10-130 weight percent
causes small increases in the delignifying process
efficiency and for a dosage of 20 weight percent the
best selectivity coefficient for an alkali dosage of
2~ sodium hydroxide was obtained.
Evaluating the o~ygen partial pressure
effect on the delignifying and process selectivity,
it can be verified, from the results of e~periments
of E~ample 3 (Table 3), that in the pressure range of
2-4 kgf/cm2 good selectivity values are obtained,
wherein the best value is achieved at 4 kgf/cm2. A
comparison between the selectivity values obtained
from the use of the process in accordance with the
present invention at 4 kgf/cm2 and processes in the
absence of additives or only using magnesium as
additive illustrates the great superiority of the
process of the present invention.
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As can be seen from the results of
e~periments of Example 4 (Table 4), the use of
ethanol as an additive in the ligocellulosic pulp
oxygen delignifying process, in the present
invention, gives better selectivity results when
compared with conventional technique.s, i.e., o~ygen
delignification in the absence of additives or in the
presence of magnesium. The process of the present
invention rendered delignifying efficiencies abo~e
47%, with selectivity values higher than those
obtained with conventional processes. Choice of the
most favorable alkali dosage for the o~ygen
delignifying process of the present invention will
depend on the minimal viscosity values which are
acceptable by the pulp manufacturing industry.
Results shown in Table 4 certainly point out that an
increase in the alkali amount enhances the
delignifying efficiency with concomitant decrease of
the selectvity process, irrespective of the presence
or absence of additives.
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