U.S. patent application number 10/122021 was filed with the patent office on 2003-04-03 for method for brightening mechanical pulps.
Invention is credited to Goda, Rangamannar, Hache, Maurice Joseph Albert.
Application Number | 20030062138 10/122021 |
Document ID | / |
Family ID | 23113814 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030062138 |
Kind Code |
A1 |
Hache, Maurice Joseph Albert ;
et al. |
April 3, 2003 |
Method for brightening mechanical pulps
Abstract
The present invention is directed to an improved method for
brightening mechanical pulp under neutral or alkaline papermaking
conditions. The improvement comprises the steps of: (a) separating
neutral or alkaline pulp dilution water into a high-solids stream
and a neutral or alkaline low-solids stream; and (b) reusing the
neutral or alkaline low-solids stream for pulp dilution purposes
prior to a bleaching process.
Inventors: |
Hache, Maurice Joseph Albert;
(Andover, MA) ; Goda, Rangamannar; (Hampstead,
NH) |
Correspondence
Address: |
ROHM AND HAAS COMPANY
PATENT DEPARTMENT
100 INDEPENDENCE MALL WEST
PHILADELPHIA
PA
19106-2399
US
|
Family ID: |
23113814 |
Appl. No.: |
10/122021 |
Filed: |
April 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60289937 |
May 9, 2001 |
|
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|
Current U.S.
Class: |
162/24 ; 162/83;
162/90 |
Current CPC
Class: |
D21C 9/1026
20130101 |
Class at
Publication: |
162/24 ; 162/83;
162/90 |
International
Class: |
D21B 001/16; D21C
003/04; D21C 009/10 |
Claims
1. An improved method for brightening mechanical pulp under neutral
or alkaline paper making conditions; said improvement comprising
steps of: (a) separating neutral or alkaline pulp dilution water
into a high-solids stream and a neutral or alkaline low-solids
stream; and (b) reusing the neutral or alkaline low-solids stream
for pulp dilution purposes prior to a bleaching process.
2. The method of claim 1 in which said high-solids stream is
treated with at least one chelant and at least one reducing agent
to produce a treated high-solids stream.
3. The method of claim 2 in which the neutral or alkaline pulp
dilution water and the neutral or alkaline low-solids stream have a
pH from 6 to 8.
4. The method of claim 3 in which the treated high-solids stream is
added to mechanical pulp entering a paper making machine.
5. The method of claim 3 in which the treated high-solids stream is
introduced into a bleaching step.
6. The method of claim 3 in which the pulp dilution water contains
suspended calcium carbonate.
7. The method of claim 6 in which said at least one reducing agent
comprises dithionite anion.
8. The method of claim 1 in which the low-solids stream is added to
mechanical pulp entering a paper making machine.
9. The method of claim 8 in which the neutral or alkaline pulp
dilution water and the neutral or alkaline low-solids stream have a
pH from 6 to 8.
10. The method of claim 1 in which the high-solids stream is added
to mechanical pulp entering a paper making machine.
Description
BACKGROUND
[0001] This invention relates generally to a method for brightening
mechanical pulp.
[0002] Paper pulp typically is subjected to a brightening process
prior to paper making. The presence of transition metal ions in
paper pulp is known to be detrimental to the brightening process.
Chelation techniques, also known as Q stage techniques, have been
used to remove transition metal ions from pulp, thereby enhancing
brightness levels. Y. Ni et al., Pulp & Paper Canada, vol. 98,
T285 (1998). Treatment of pulp with sodium hydrosulfite prior to
chelation, known as the Q.sub.y stage technique, is believed to
improve chelation of metals, thereby further enhancing pulp
brightness. However, this technique is less effective at higher pH
values, such as those encountered when precipitated calcium
carbonate ("PCC") is used as a filler in a pulp and paper mill.
[0003] The problem addressed by this invention is to find a more
effective method for brightening mechanical pulp at high pH
values.
STATEMENT OF INVENTION
[0004] The present invention is directed to an improved method for
brightening mechanical pulp under neutral or alkaline paper making
conditions. The improvement comprises the steps of: (a) separating
neutral or alkaline pulp dilution water into a high-solids stream
and a neutral or alkaline low-solids stream; and (b) reusing the
neutral or alkaline low-solids stream for pulp dilution purposes
prior to a bleaching process.
DETAILED DESCRIPTION
[0005] FIG. 1 is a graph showing the effect of differing levels of
hydrosulfite on brightness at varying consistency levels.
[0006] FIG. 2 is a graph showing the effect of white water
components on brightening with 1.2% sodium hydrosulfite.
[0007] FIG. 3 is a graph showing the effect of temperature on
brightness at 60 minutes retention and 3.5% consistency and at
varying hydrosulfite levels.
[0008] FIG. 4 is a graph showing the effect of temperature on
brightness at 10 minutes retention and 3.5 % consistency and at
varying hydrosulfite levels.
[0009] FIG. 5 is a graph showing the effect of retention time on
brightness at 80.degree. C. and 3.5% consistency and at varying
hydrosulfite levels.
[0010] FIG. 6 is a graph showing the effect of retention time on
brightness at 70.degree. C. and 3.5% consistency and at varying
hydrosulfite levels.
[0011] Dilution water (i.e. cloudy white water) typically is added
to mechanical pulp prior to a bleaching step. In an integrated pulp
and paper mill, the dilution water is a recycled stream from the
paper making operations. In acid-based mills, the dilution water
typically is at a pH from 4 to 5, and contains impurities such as
pulp fines, suspended and dissolved solids, fillers and transition
metal ions. In a neutral to alkaline paper making environment,
i.e., one that utilizes PCC as a filler, the dilution water is at a
pH from 6 to 8, and contains impurities such as pulp fines,
suspended calcium carbonate, and transition metal ions. Of these
impurities, transition metals can be especially troublesome as they
can catalyze the decomposition of bleaching chemicals resulting in
reduced bleaching efficiency and lower brightness levels. They also
tend to increase brightness reversion and thus further contribute
to lowering the brightness of bleached pulp. Both reductive and
oxidative bleaching chemicals are affected by transition metals.
The most commonly used oxidative bleaching chemical is hydrogen
peroxide. Reductive bleaching chemicals typically are aqueous
reducing agents, including, e.g., dithionite anion, also known as
hydrosulfite, borohydrides and bisulfites, and formamidine sulfinic
acid.
[0012] The present inventors have determined that use of cloudy
white water from neutral to alkaline processes for dilution of pulp
decreases the brightness level attainable with reducing agents,
such as hydrosulfite. According to this invention, the neutral or
alkaline cloudy white water used by the industry for the dilution
of pulp prior to a bleaching process can be separated into a
high-solids-containing as well as a low-solids-containing stream
for treatment to improve the aforementioned bleach process.
Separation of the cloudy white water is achieved using any of the
methods well-known in the pulp industry for separation of solids,
including, e.g., retaining fines on a paper machine wire,
processing through a saveall or a clarifier, and flotation or
filtration devices. The levels of solids in the low- and
high-solids streams are determined by the initial level of solids
in the cloudy white water and the method of separation. The amount
of solids in the high-solids stream is not critical because this
stream can be handled by solids or slurry handling equipment at a
variety of solids contents. The high-solids stream can be as much
as 30%, or even 40% solids and still be handled as a stream.
Moreover, when the high-solids stream is separated by filtration,
it is a wet filter cake, which may have an extremely high solids
content. The low solids stream contains no more than 5000 ppm of
solids, preferably no more than 2000 ppm, more preferably no more
than 1000 ppm, still more preferably no more than 500 ppm, still
more preferably no more than 250 ppm, and most preferably no more
than 100 ppm.
[0013] According to this invention, the use of the low solids
stream under neutral or alkaline conditions (pH 6 to 8) minimizes
the adverse effects on the brightness of the resulting pulps. The
Examples demonstrate that the reduction of the amount of solids in
the cloudy white water significantly lowers the levels of
transition metals and other impurities in the low solids stream and
thereby improves the efficiency of bleaching. Preferably, the pH of
the dilution water is from 6.5 to 7.5.
[0014] In one aspect of this invention, the low-solids stream is
introduced into a bleaching step. In another aspect of this
invention, the low-solids stream is added to the mechanical pulp
entering a paper making machine. In one aspect of this invention,
the high-solids stream is treated with at least one chelant and at
least one reducing agent to produce a treated high-solids stream.
Suitable chelants include, e.g., DTPA, STPP, EDTA, and
phosphorus-containing chelants, e.g., phosphonate- and
phosphonic-acid chelants. In one aspect of this invention, the
treated high-solids stream from the process is added to the
mechanical pulp entering a paper making machine. In another aspect
of this invention, the treated high-solids stream is introduced
into a bleaching step. In another aspect of this invention, an
untreated high-solids stream is added to the mechanical pulp
entering a paper making machine.
[0015] Chemical treatment of the high solids stream recovered from
cloudy dilution water from neutral or alkaline processes according
to the method of this invention allows recycling of the solids
without adverse effects on brightness of the resulting paper and
pulps. Without being bound by theory, it is believed that addition
of a reducing agent to the solids recovered from the pulp dilution
water reduces the valences of the transition metal ions. The
reduced valences in turn result in better chelation of transition
metals, and treated solids that typically have reduced levels of
transition metals, and thus can be introduced into the bleaching
and paper making process without adversely affecting pulp
brightness.
[0016] It is preferred that the reducing agent is dithionite anion,
i.e., hydrosulfite anion. Examples of other reducing agents are
borohydride ion and bisulfite ion. Most preferably, the reducing
agent is sodium hydrosulfite generated from treatment of sodium
bisulfite with sodium borohydride, the latter preferably in the
form of a strongly basic aqueous solution, e.g., the product
containing 12% sodium borohydride and 40% sodium hydroxide, and
sold by Rohm and Haas Company under the name Borol.TM. solution.
Sodium dithionite produced in this manner is known as
Borol.TM.-solution-generated hydrosulfite ("GH").
EXAMPLES
Example 1
Effect of Process Conditions on BGH Brightening
[0017] Pulp and white water used in this study were obtained from a
North American mill. The pulp was a chemothermomechanical pulp
(cTMP), which was collected after the secondary refiners and prior
to the latency chest. The Precipitated Calcium Carbonate (PCC)
containing white water (WW) was collected just prior to dilution at
the latency chest. Studies were conducted to determine the effect
on BGH bleached pulp brightness levels of the following four
process variables: retention time, bleaching temperature, and
bleaching consistency using either PCC-containing white water (WW)
or deionized (DI) water for dilution of the pulp slurry. The
hydrosulfite dosage (hydro) was 0-24 lbs./ton BGH at a pH of 10.
The raw data for these experiments can be seen in Table 1.
Temperatures are in .degree. C. (Temp.), retention times are in
minutes, consistency ("Consist.") in weight % of pulp in the pulp
slurry, deionized water had a pH of 6.5 and PCC-containing white
water a pH of 7.5, and brightness is given as a percentage ISO. The
pulp was cTMP with a pH of 7.1. Initial pH, (before hydrosulfite
addition) and final pH (after hydrosulfite addition and after
retention time) are also tabulated for each experiment.
1TABLE 1 cTMP Bleaching Results Initial Hydro Final Time Temp.
Consist. Water Bright. pH (lb/ton) pH (min.) (.degree. C.) (%) Type
(% ISO) 6.9 0 6.8 60 80 3.5 WW 47.4 6.9 8 6.7 60 80 3.5 WW 52.3 6.9
16 6.8 60 80 3.5 WW 54.5 6.9 24 6.8 60 80 3.5 WW 55.5 6.5 0 6.5 60
80 3.5 DI 50.4 6.5 8 6.6 60 80 3.5 DI 56.5 6.5 16 6.7 60 80 3.5 DI
57.8 6.5 24 6.7 60 80 3.5 DI 59.9 6.9 0 6.9 60 70 3.5 WW 46.9 6.9 8
6.9 60 70 3.5 WW 51.9 6.9 16 6.9 60 70 3.5 WW 52.8 6.9 24 6.9 60 70
3.5 WW 54.4 6.5 0 6.5 60 70 3.5 DI 49.8 6.5 8 6.7 60 70 3.5 DI 55.3
6.5 16 6.8 60 70 3.5 DI 57.4 6.5 24 6.7 60 70 3.5 DI 58.8 6.9 0 6.8
10 80 3.5 WW 47.1 6.9 8 6.8 10 80 3.5 WW 50.9 6.9 16 6.8 10 80 3.5
WW 52.7 6.9 24 6.7 10 80 3.5 WW 53.2 6.5 0 6.5 10 80 3.5 DI 50.2
6.5 8 6.6 10 80 3.5 DI 55.0 6.5 16 6.5 10 80 3.5 DI 56.9 6.5 24 6.6
10 80 3.5 DI 57.8 6.9 0 6.9 10 70 3.5 WW 47.4 6.9 8 6.9 10 70 3.5
WW 50.2 6.9 16 6.8 10 70 3.5 WW 51.8 6.9 24 6.9 10 70 3.5 WW 52.3
6.5 0 6.5 10 70 3.5 DI 50.0 6.5 8 6.6 10 70 3.5 DI 53.6 6.5 16 6.7
10 70 3.5 DI 55.7 6.5 24 6.7 10 70 3.5 DI 56.1 7.1 0 7.1 10 80 6.5
WW 47.9 7.1 8 7.0 10 80 6.5 WW 52.5 7.1 16 7.0 10 80 6.5 WW 54.1
7.1 24 6.9 10 80 6.5 WW 55.2 6.8 0 6.8 10 80 6.5 DI 49.7 6.8 8 6.8
10 80 6.5 DI 54.8 6.8 16 6.8 10 80 6.5 DI 56.2 6.8 24 6.7 10 80 6.5
DI 56.0 7.2 0 7.2 10 80 10 WW 47.9 7.2 8 6.7 10 80 10 WW 52.7 7.2
16 6.5 10 80 10 WW 54.0 7.2 24 6.5 10 80 10 WW 56.1 6.9 0 6.9 10 80
10 DI 49.1 6.9 8 6.5 10 80 10 DI 53.5 6.9 16 6.3 10 80 10 DI 55.3
6.9 24 6.3 10 80 10 DI 56.1
[0018] FIGS. 1-6 depict the effect on bleached brightness of
various combinations of the process conditions investigated. FIG. 1
shows the effect of bleaching consistency and the type of dilution
water used on BGH bleached pulp brightness. Traditionally, it has
been difficult to obtain a brightness increase at higher
consistencies in laboratory-scale studies, although mill experience
has shown that brightness increases with increasing consistency.
This is believed to be due to the difficulty in effectively mixing
pulp and chemicals at medium consistency in the laboratory.
Therefore, the fact that the present study showed that the
brightness of bleached pulp decreased with increasing consistency
when the pulp was diluted with deionized water was not surprising.
However, the fact that the brightness of bleached pulp increased
with increasing consistency when the pulp was diluted with
PCC-containing white water was unexpected. Without being bound by
theory, it is believed that PCC-containing white water has a large
negative effect on brightness. The decrease in that negative effect
at higher bleaching consistency, where there is less PCC-containing
white water present because less water is used for dilution
relative to low consistency bleaching, has a positive effect on
brightness. This positive effect is larger than the negative effect
from increased consistency that is usually observed due to poor
mixing for laboratory scale bleaching. Table 2 summarizes the
averaged effect of changes in consistency, temperature and
retention time on brightness in pulp diluted with either deionized
water (DI) or PCC-containing white water (WW). The average
brightness gains were calculated by taking the average of the
brightness gains throughout the response curve of FIG. 1, i.e.,
from 8 to 24 pounds of BGH per ton of pulp.
2TABLE 2 Effect of Changes in Process Conditions on Average
Brightness Gains Process Condition Change DI WW Consistency 3.5 to
10.0 -1.8 +1.9 Temperature 70.degree. C. to 80.degree. C. +1.4 +1.0
Retention Time 10 min to 60 min +1.8 +1.8
[0019] Table 3 shows the effect of changing from DI water to
PCC-containing white water at two different BGH levels:
3TABLE 3 Effect on Brightness of Changing from DI to PCC White
Water BGH Level Change in Brightness 8 lbs./ton -3 24 lbs./ton
-5
[0020] Table 4 shows the maximum absolute brightness level achieved
with each process variable combination tested.
4TABLE 4 Maximum Absolute Brightness Level Obtained Brightness
Brightness with DI with PCC Time Temp. Consistency water water 60
80 3.5 59.9 55.6 60 70 3.5 58.8 54.4 10 80 3.5 57.8 53.2 10 70 3.5
56.1 52.3 10 80 6.5 56.0 55.2 10 80 10.0 56.1 56.2
[0021] These results demonstrate that the use of PCC-containing
white water for pulp dilution has a negative effect on BGH
brightening. However, these results also suggest that this effect
is mitigated to some degree by adjustment of process conditions,
for example, by increasing the bleaching consistency.
Example 2
Fines (Solids) Removal and Reuse of Low-Solids White Water
[0022] FIG. 2 compares the BGH-bleached pulp brightness of pulp
diluted with DI water, PCC-containing white water, the filtrate of
PCC-containing white water, and fines that were removed by
filtration of PCC-containing white water and re-suspended in DI
water. The results show that the removal of solids and reuse of low
solids white water for bleaching purposes minimizes the adverse
effects of BGH brightening under alkaline or neutral conditions.
The results also show that it is the fines portion, which consists
of actual pulp fines, undissolved solids, and transition metals in
the white water that is responsible for most of the brightness
loss. Based on these results, further testing of the fines portion
of PCC containing white water was undertaken and is summarized in
the following examples.
[0023] Results of transition metal analysis of the pulp, the white
water filtrate, and the fines are shown in Table 5.
5TABLE 5 Metals Concentration (in ppm) for cTMP &
PCC-Containing White Water Portions Al Ca Cu Fe Mg Mn Pulp 17 1760
0.9 40 190 111 PCC Fines 822 96200 7 667 587 183 High solids PCC WW
2 217 0.1 1.2 9 2 Low solids
[0024] By far the largest concentration of metal is 96,200 ppm of
calcium, almost 10%, in the fines portion of the white water. This
high level results from the presence of precipitated calcium
carbonate (PCC) in the white water. It is believed that the white
water is detrimental to BGH brightening because the white water
introduces large amounts of impurities to the pulp slurry. The high
iron concentration is detrimental to hydrosulfite brightening. The
high manganese concentration is also of concern, especially in the
case of peroxide brightening. Manganese is well known as a catalyst
for decomposition of peroxide.
Example 3
Fines Treatment and Re-use
[0025] To reduce the transition metal concentrations, fines were
treated by the Qy process. The fines first were treated with 0.1%
BGH and then with 0.5% diethylenetriaminepentaacetic acid (DTPA).
Experiments were conducted at a pH of 5.5, a consistency of 3.0%,
and a temperature of 50.degree. C. for 30 minutes. The Qy treatment
is believed to be more effective than the Q treatment, i.e., use of
only chelant, because reduction of transition metal valence state
by BGH renders the transition metal ions more amenable to
chelation. The Qy treatment allows higher brightness levels when
using hydrogen peroxide as a brightening agent. Table 6 shows the
results from Q and Qy treatments on the fines portion of
PCC-containing white water.
6TABLE 6 Metal Levels After Q or Qy Treatment of Fines From
PCC-Containing White Water (levels in ppm, unless otherwise
indicated) Al Ca (%) Cu Fe Mg Mn Q 807 8.23 4.3 668 336 76.1 Qy 726
6.84 3.3 638 312 61.2 Control 821 9.99 3.4 667 504 165
[0026] The results demonstrate that the Qy process enhanced removal
of transition metals from the fines portion of PCC white water.
[0027] Table 7 shows the results from Qy treatment on the pulp
portion with BGH and DTPA, and from Q treatment with DTPA alone.
The metal levels are given in ppm.
7TABLE 7 Treatment of cTMP Pulp by Q and Qy Methods Treatment DTPA,
% BGH, % Al Cu Fe Mg Mn Q 0.5 -- 21 0.7 24 130 9 Q 0.13 -- 13 0.8
43 133 48 Qy 0.5 0.1 14 0.7 18 123 6 Qy 0.13 0.1 11 1.1 19 120 44
Control 0 0 17 0.9 40 190 111
[0028] The table demonstrates that good results are obtained for
reduction of manganese and iron, both of which are associated with
poor brightening with BGH and hydrogen peroxide. Although the
difference in manganese concentration is small, Qy treatment
produces a lower level of manganese than Q treatment. For iron, the
results are more readily apparent. Even at the lower level of DTPA,
the iron level is substantially lower for the Qy treatment. It is
important to note that this pulp sample was taken at the secondary
refiner outlet, prior to addition of mill white water. In reality,
the pulp entering the bleach plant would have higher levels of
metals from the PCC-containing white water dilution.
[0029] Table 8 shows the results from brightening with BGH the
treated pulp described in Table 7. Brightness (B) is given in %
ISO, and levels of iron and manganese in ppm. These results
demonstrate that at relatively low initial levels of transition
metals in pulp, the Qy treatment produces a higher brightness pulp.
Thus, Qy treatment would be more effective on pulps with higher
initial transition metal concentrations.
8TABLE 8 BGH Brightening of Q and Qy Treated cTMP BGH, % Fe Mn B
0.5% DTPA (Q) 1.2 24 9 58.3 0.13% DTPA (Q) 1.2 43 48 57.2 0.5% DTPA
+ 0.1% BGH (Qy) 1.2 18 6 58.5 0.13% DTPA + 0.1% BGH (Qy) 1.2 19 44
57.4
[0030] Tables 9 and 10 show the results of hydrogen peroxide
brightening of pulps treated as shown in Table 7. Peroxide
(H.sub.2O.sub.2) bleaching was carried out with a sodium hydroxide
dosage of 1.5% for 3% peroxide and 2.0% for 5% peroxide. The sodium
silicate dosage was 2.5%, magnesium sulfate dosage was 0.05%, the
consistency was 12.0%, the temperature was 80.degree. C. and the
bleaching time was 2 hours. The results for % ISO brightness (B)
demonstrate that the pulps receiving the Qy treatment display a
greater brightness enhancement along with a higher residual
peroxide level than those subjected to Q treatment. Transition
metal levels are given in ppm, with other measurements given as per
cent values.
9TABLE 9 Hydrogen Peroxide (P) Brightening of Q and Qy treated cTMP
(5.0% H.sub.2O.sub.2) Residual H.sub.2O.sub.2 Fe Mn B Peroxide 0.5%
DTPA (Q) 5.0 24 9 72.9 9.2 0.13% DTPA (Q) 5.0 43 48 71.6 6.3 0.5%
DTPA + 0.1% BGH (Qy) 5.0 18 6 73.5 15.1 0.13% DTPA + 0.1% BGH (Qy)
5.0 19 44 71.9 8.4
[0031]
10TABLE 10 Hydrogen Peroxide Brightening of Q and Qy Treated cTMP
(3.0% H.sub.2O.sub.2) Residual H.sub.2O.sub.2 Fe Mn B Peroxide 0.5%
DTPA (Q) 3.0 24 9 69.5 7.6 0.13% DTPA (Q) 3.0 43 48 67.0 4.3 0.5%
DTPA + 0.1% BGH (Qy) 3.0 18 6 69.6 13.2 0.13% DTPA + 0.1% BGH (Qy)
3.0 19 44 67.8 8.1
* * * * *