Note: Descriptions are shown in the official language in which they were submitted.
1~ 11787~6
1~ ~
BACKGROUND OF THE I~V~NTIO~
The present invention generally relates to a process for the
delignification and bleaching of lignocellulosic pulps. More
particularly, the invention relates to an economical and effici-
ent multi-stage bleaching sequence for the delignification and
bleaching of lignocellulosic pulps.
The continual aevelopment of new and improved processes in
the pulp and paper industry is of major importance due to manu-
facturing and environ~ental considerations. In recent years, the
pulp and paper industry has devoted a great deal of time and ef-
fort to research on bleaching processes which are free of chlo-
rine. In large measure these efforts have been mounted to comply
with increasingly stringent governmental regulations which dic-
tate the reduction or elimination of atmospheric or water-borne
pollutants.
One avenue taken by pulp bleaching researchers has been to
conduct extensive research and development directed to various
oxygen bleaching systems. While this research has proven fruit-
ful, as attested to by the installation of several commerci~l
pulp bleaching facilities employing oxygen, oxygen bleaching
standing by itself cannot produce pulps which are sufficiently
bright for the needs of the pulp industries customers.
Another direction ta~en by investigators nas been to investi-
gate the use of ozone as a bleaching agent. While ozone produces
pulps of high brightness, it, unfortunately, also extensively de-
grades the cellulose portion of the molecule at the te~perat~res
1178756
norma ly encountered in bleach plant operations. Another problem
associated with the use of ozone is its extremely high cost. The
attractiveness of a bleaching process employing ozone would be im-
measurably improved if it were possible to reduce the amount of
ozone consumed during the ozonation stage(s). It would also be
greatly beneficial if a process employing ozone could be found
which would avoid the necessity of preceding the ozonation stage
with the typical oxygen stage used in most bleaching seouences.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pro-
cess for the delignification of lignocellulosic pulps where~n the
amount of ozone consumed is materially reduced from the hereto-
l fore known ozone-based bleaching processes.
i It is another object of the present invention to provide a
process wherein the necessity for an oxygen bleaching stage,
which typically preceeds ozonation, is eliminated.
The foregoing objects and other objects of the present inven-
tion are achieved by a multi-stage seguence which comprises the
following bleaching stages in the sequence in which they are re-
cited:
(a) extracting a lignocellulosic pulp slurry with alkali
while maintaining the slurry pH above about 10;
(b) washing the pulp slurry with an acid at a pH below 3
whereby the heavy metal cations are removed from the lignocel-
lulosic pulp slurry in order to avoid aetrimental effects during
succeeding bleaching stages:
1 1 78756
c) passing 2 stream of ozone through the pulp slurry while
maintaining the slurry pH below about 5;
(d) extracting the pulp slurry with alkali while maintain-
ing the pH of the slurry from about 7 to about 10; and
(e) passing a stream of ozone through the pulp slurry while
maintaining the slurry pH below about 5.
By employing the foregoing multi-stage ble2ching seauence,
and other embodiments thereof which will be described hereafter,
an ozone-based bleaching process has been found which employs
significantly less ozone, i.e., the ozone consumption is reduced
over prior art processes and, in addition, the need to precede
the ozone bleaching stage with the typical oxygen bleaching stage
is avoided due to the significant amount of delignification achi-
eved during the first alkaline extraction stage.
DETAILED DESCRIPTION OF THE INVENTIO~
The lignocellulosic pulp fibers, employed in the present in-
vention can be prepared by chemical, chemimechanical, or mechani-
cal pulping processes. Pulps obtained from the kraft process,
the sulfite process, the soda process, the neutral sulfite chemi-
mechanical process, and the groundwood process are all eminently
suitable for use in the present invention. Pulps derived from
both hardwoods and softwoods can be employed in the present pro-
cess.
It has been found that the present process has application
with low consistency pulps, namely, between 0.5% an~ 10~, based
on oven-dried (O.D.) pulp, and with medium consiStenCy pulps,
I1 1 178756
viz., 10-15~, and also with high consistency pulps, a~ove 15%.
In the first bleaching stage, after the digested pulp has
been screened and washed, the pulp is subjected to an alkaline
extraction, i.e., E stage, with either sodium hydroxide, sodium
carbonate, or a mixture of sodium hydroxide and sodiu~ carbonate.
When sodium hydroxide is employed it can be present in a con-
centration of from about 0.5% to about 4.0%, based on the weight
of oven-dried pulp, with from about 1.0~ to about 2.0% being pre-
ferred. When sodium carbonate is e~,ployed it can be present at a
concentration of from about 1.0% to about 4.0%, based on the
oven-dried weight of the pulp, with from about 2.0% to about 3.0%
being preferred.
The pH of the pulp slurry during the alkaline extraction
first stage should be above about pH 10 and, preferably, between
pH 10 and 12. While the temperature during the reaction can be
between 60C. and 85C., it is preferred that the temperature be
between 70~C. and 77C. The period of reaction can be from about
1 hour to about 24 hours, with from about 2 hours to 7 hours
being preferred.
The beneficial effects associated with an alkali extraction
during the first bleaching stage is believed to be attributable
to the fact that when kraft pulp is washed, thus reducing the p~
of the black liquor, the lignin present in the black liauor pre-
cipitates onto the pulp fibers fro~ which it is readily removed
by alkaline extraction. In addition, limited lignin oxidation
caused by oxygen present in the air and alkali probably also
facilitate lignin removal during extraction.
-4-
,1. I
~ 1~t8756
After completion of the alkaline extraction first stage, the
pulp is then acid washed, i.e., A stage, in a second stage. The
acid washing can be conducted in a tank or other suitable vessel
for periods of from about 1 minute to about 60 minutes with an ir.-
orgznic acid, such as sulfuric acid. This serves to maintain -~he
pH of the slurry below about 3 and, preferably, between DH 2.5
and 3. It is preferred that the washing period be between about
5 minutes and 10 minutes. The acid wash serves the important
function of removing certain heavy metal cations, such as iron,
manganese and copper which are detrimental to effective bleaching
if they are present in the pulp when it is contacted with ozone
in the next succeeding bleaching stage.
In the third bleaching stage of the multi-stage bleaching
sequence of the present invention, the pulp slurry is contacted
with ozone by passing a stream of ozone gas through the slurry,
i.e., Z ~tage, for a period of time sufficient to consume from
about 0.6% to about 1.4% ozone, by weight of oven-dried pulp. It
is preferred that the amount of ozone consumed during the first
ozonation stage be about 1~. Ozone consumption will depend on
the Kappa number of the pulp after the first alkaline extraction
stage. Pulp of about 14 ~appa would reauire about 1~ ozone,
whereas pulp of about 16 Kappa would require 1.1-1.4% ozone. The
pH of the pulp slurry during the ozonation sta~e should be ~ain-
tained below about ~ and, preferably, between pH 2.5 and 3Ø
In the succeeding bleaching stage, which is the fourth stage
of the multi-stage sequence of the present invention, the ozon-
ated pulp is extracted with from about 0.3~ to about 3.0~ of an
1178756
alkali, based on oven-dried pulp. Either sodium hydroxide or
sodium carbonate, or a mixture of sodium hydroxide and sodium
carbonate, can be employed at a slurry pH from about 7 to about
10. The time for this extraction stage is between about 5
minutes and about 60 minutes and the extraction is conducted at 2
temperature between about 40C. and about 70C-
In the next stage, which is the last stage of the seauencein this embodiment of the invention, the pulp is treated 2gain
with ozone in much the same fashion as was done in the third
stage. In this stage, however, unlike the third stage, the re-
action proceeds until from about 0.2% to about 0.6~ of ozone is
consumed, by weight of O.D. pulp. The ozonation is ~arried out
at a pH below about 5 and, preferably, between pH 2.5 and 3. It
is preferred that the incoming pulp to this stage should have a
brightness of 60-65% GE, in order that the ozone consumption
during this stage should not exceed about 0.6%, which would re-
sult in a loss in viscosity.
In another embodiment of the present invention, an alkaline
peroxide stage employing, for example, from about 0.1~ to about
0.4%, by weight of O.D. pulp, of hydrogen peroxide can be used as j
a last and final stage~ following the second ozone stage, if bet- ¦
ter brightness stability is desired. The alkaline peroxide stage
is carried out at a pH between about 9 and 11 with a pH of 10
being preferred. The temperature of the reaction is between
about 50C. to about 70C., with a temperature of about 500C. to
about 60C. being preferred. The time of reaction can be from
about 5 minutes to about 3 hours, but it is preferred that the re-
-6-
ll
Ij 1178756
ll
action take place over a period of from about 20 minutés to about
60 minutes.
In order to disclose more clearly the nature of the present
invention, the following examples illustrating the invention are
qiven. It should be understood, however, that this is done
solely by way of example and is intended neither to delineate the
scope of the invention nor limit the ambit of the appended
claims.
EXAMPLE 1
100 grams of a hardwood, kraft pulp was mixed with an zlka-
line solution consisting of 1% sodium hydroxide and 4% sodium car-
bonate, based on O.D. pulp, and placed in polyethylene bags.
Water was added to achieve a pulp consistency of 10%. The poly-
ethylene bags were squeezed to exclud~ as much air as possible.
Thereafter, the bags were sealed and the contents kneaded for
uniform distribution of alkali. The bags were then placed in a
constant temperature bath which was maintained at 77C. for a
period of ~ hours. Next, the pulp was filtered under suction,
washed with water, followed by a final washing with water contain-
ing traces of SO2 to a pH of approximately 6.
In the next stage, after the extraction stage described
above, the pulp was then acid washed at 4% consistency, based on
O.D. pulp, with sulfuric acid at pH 2.5.
The acid washed pulp was then mixed with distilled water at
40C. in a 21-liter laboratory reactor to bring the pulp consis-
-7-
I 1 1787~6
I .
tency to 0.8%, based on O.D. pulp. The pH was then adjusted to
3.0 and an oxygen-ozone mixture, having a flow rate of 1,000 mls/
min. and an ozone concentration of about 2.6~, was sparged into
the pulp slurry while maintaining ~ppropriute mixing. Ozone con-
sumption, based on O.D. pulp, was monitored by a Dasibi ozone
monitor. At the completion of the ozone stage, which took ap-
proximately 85 minutes to achieve 1~ ozone consumption, the pulp
was washed with water.
The pulp was then alkaline extracted with 2.3% sodium czr-
bonate, based upon O.D. pulp, for a period of 20 minutes at 60C. Il
and at pH 8Ø The consistency of the pulp during the extraction j
was 4~, based on O.D. pulp.
After completion of the extraction, the pulp was filtered,
washed with water, and the pH adjusted to 3.0, with the pulp then
being filtered and fluffed to a consistency of 35%.
The second ozonation stage was conducted in the same manner
as described above, with respect to the first ozonation stage, ex-
cept the ozone consumed amounted to 0.6%.
~ I
¦ EXAMPLE 2
¦ In this example, there was no initial alkali extraction as
there WâS in Example 1. The first stage in this example was an
acid wash with sulfuric acid which was conducted in the same ~an- !
ner as the acid wzsh stage in Example 1.
Thereafter, the pulp was treated in successive stages with
ozone, followed by alkaline extraction, finally followed by ozona-~
.. !1,
~1 1
!!
!l 1 178756
. I,
tion. These three stages were conducted in the same manner and
under ~he same conditions as the corresponding stages in Example
1. ',
EXAMPLE 3
The first and second stages in this example, namely alkaline
extraction and acid washing, were conducted under the same condi-
tions as in Example 1. The only exception was that the alkaline
solution used during the alkaline extraction consisted of 1% sod-
ium hydroxide and 1% sodium carbonate, based on O.D. pulp. I
The acid washed pulp was then filtered to about 20~ pulp con-¦
sistency and was then fluffed to approximately 35~ pulp consis-
tency. I
In a five-liter baffled flask, an oxygen/ozone mixture, at a ¦
flow rate of 1,000 mls/min and having an ozone concentration of
about 2.6%, was sparged through the fluffed pulp. Mixing was
achieved by rotating the flask at an appropriate speed. Ozone
consumption, based on O.D. pulp, was monitored by a Dasibi ozone
monitor. The first ozone stage took about 25 minutes to achieve
1~ ozone consumption, based on O.D. pulp.
In the next stage, the pulp was then alkaline extracted with
2.~% sodium carbonate, based on O.D. pulp, for a period of 20 min-
utes at 60~C. and at a pH of 8. The consistency of pulp during
the extraction was 4%, based upon O.D. pulp.
~ fter completion of the extraction, the pulp was filtered,
washed with water, and the p~ adjusted to 3Ø The pulp was then
fil~ered and fluffed to a consistency of 35~.
ll il78756
The pulp was then subjected to a second ozonation stage in
the same manner as described with respect to the first ozonation
stage in this example.
In the last stage in this example, the pulp was treated with
0.2% hydrogen peroxide at a pH of 10.0 for a period of 1 hour at
a temperature of 50C. The pulp consistency during the alkaline
peroxide treatment was 12%, based on O.D. pulp.
EXAl~PLE 4
The processes and materials used in this example were the
same as in Example 3, except that the starting Kappa No. was
16.4, as opposed to the starting Kappa No. of 17.1 in Example 3.
EXAMPL~ 5
The initial stage in this example was an acid wash, followed
by a first ozonation stage, which was followed by an alkaline ex-
traction stage, followed by a second ozonation and finally an al-
kaline peroxide stage. The procedures and materials employed in
this example were the same as were employed in Example 3, with
the sole exception being, naturally, that there was no initial al-
kaline extraction stage.
t ~78756
~- ~
C ~
~,o~ =o
o
.~
~ ~ ~ ~
~ .C " ",~
o ~ ~¦ ~ r ~
,c ~-i,l x x 0
'~c ~1 x x x ~
o~
.~ ~ 3_~
~cn ~
1 1787~6
It is evident from a comparison Of the pu1p properties ob-
tained in Example 1 versus Example 2 in Table I, that with the
same amount of total ozone Consumption, viz., 1.6%, the bright-
ness of the pulp is 14~ greater when an alkaline extraction is em-
~ployed as the first stage in the bleaching sequence. It is
equally apparent from a comparison of Examples 3 and 4 versus Ex-
ample 5, that when an alkaline eXtraction first stage is em-
ployed, comparable or increased brightness is obtained with muchless ozone Consumption, namely 1.4~ as compared with 1.8~. Thus,
the employment of an alkaline extraction as the first stage in
the seqUence results in a 22~ reduction in ozone consum~tion. In
addition, the pulps which were alkaline extracted in the first
stage maintain higher viscosity levels. A comparison of Example
1 which does not employ a final P stage, with Examples 3-5, which
do employ a final P stage, reveals that a final P stage results
in a higher reverted brightness level.
The terms and expressions which have been employed are used
as terms of description and not of limitation, and there is no in-
tention in ~he use of such terms and expressions of excluding any
equivalents of the features shown and described or portions
thereof, but it is recognized that var ious modifications are pos-
ible within the scope Of the inve~ n claimed.
