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
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 cloudy white water recycled from neutral or alkaline
paper making operations into a neutral or alkaline low-solids stream; said low-solids
stream containing no more than 5000ppm solids and a high-solids stream, said high-solids
stream being the other component resulting from the separation; and (b) reusing the
neutral or alkaline low-solids stream for pulp dilution purposes prior to a bleaching
process.
Detailed Description
[0005] Figure 1 is a graph showing the effect of differing levels of hydrosulfite on brightness
at varying consistency levels.
[0006] Figure 2 is a graph showing the effect of white water components on brightening with
1.2% sodium hydrosulfite.
[0007] Figure 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] Figure 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] Figure 5 is a graph showing the effect of retention time on brightness at 80°C and
3.5% consistency and at varying hydrosulfite levels.
[0010] Figure 6 is a graph showing the effect of retention time on brightness at 70°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 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™ solution. Sodium dithionite
produced in this manner is known as Borol™-solution-generated hydrosulfite ("BGH").
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 0-12kg/1000kg
(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 °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.
Table 1:
cTMP Bleaching Results |
Initial pH |
Hydro (lb/ton) |
Final pH |
Time (min.) |
Temp. (°C) |
Consist. (%) |
Water Type |
Bright. (% 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 |
Figures 1-6 depict the effect on bleached brightness of various combinations of the
process conditions investigated. Figure 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 Figure 1, i.e., from 4 to
12 kg (8 to 24 pounds) of BGH per ton of pulp.
Table 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°C to 80°C |
+1.4 |
+1.0 |
Retention Time |
10 min to 60 min |
+1.8 |
+1.8 |
[0018] Table 3 shows the effect of changing from DI water to PCC-containing white water
at two different BGH levels:
Table 3:
Effect on Brightness of Changing from DI to PCC White Water |
BGH Level |
Change in Brightness |
4kg/1000kg (8 lbs./ton) |
-3 |
12kg/1000kg (24 lbs./ton) |
-5 |
[0019] Table 4 shows the maximum absolute brightness level achieved with each process variable
combination tested.
Table 4:
Maximum Absolute Brightness Level Obtained |
Time |
Temp. |
Consistency |
Brightness with DI water |
Brightness with PCC 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 |
[0020] 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
[0021] Figure 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.
[0022] Results of transition metal analysis of the pulp, the white water filtrate, and the
fines are shown in Table 5.
Table 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 High solids |
822 |
96200 |
7 |
667 |
587 |
183 |
PCC WW Low solids |
2 |
217 |
0.1 |
1.2 |
9 |
2 |
[0023] 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
[0024] 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°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.
Table 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 |
The results demonstrate that the Qy process enhanced removal of transition metals
from the fines portion of PCC white water.
[0025] 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.
Table 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 |
[0026] 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.
[0027] 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.
Table 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 |
[0028] Tables 9 and 10 show the results of hydrogen peroxide brightening of pulps treated
as shown in Table 7. Peroxide (H
2O
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°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.
Table 9:
Hydrogen Peroxide (P) Brightening of Q and Qy treated cTMP (5.0 % H2O2) |
|
H2O2 |
Fe |
Mn |
B |
Residual 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 |
Table 10:
Hydrogen Peroxide Brightening of Q and Qy Treated cTMP (3.0 % H2O2) |
|
H2O2 |
Fe |
Mn |
B |
Residual 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 |
1. An improved method for brightening mechanical pulp under neutral or alkaline paper
making conditions; said improvement comprising steps of:
(a) separating cloudy white water recycled from neutral or alkaline paper making operation
into a neutral or alkaline low-solids stream said low-solids stream containing no
more than 5000ppm solids and a high-solids stream, said high-solids stream being the
other component resulting from the separation; 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 high-solids stream is added to mechanical pulp
entering a paper making machine.
1. Ein verbessertes Verfahren zum Aufhellen von mechanischer Pulpe bei neutralen oder
alkalischen Bedingungen der Papierherstellung, wobei besagte Verbesserung die folgenden
Verfahrensschritte aufweist:
(a) Auftrennung von trüb-weißem Wasser, das von neutralen oder alkalischen Vorgängen
bei der Papierherstellung recycelt wurde, in einen neutralen oder alkalischen "Wenige-Feststoffe"-Strom,
wobei besagter "Wenige-Feststoffe"-Strom nicht mehr als 5 000 ppm Feststoffe enthält,
und einen "Viele-Feststoffe"-Strom, wobei besagter "Viele-Feststoffe"-Strom die andere
Komponente, wie sie aus der Auftrennung resultiert, darstellt; und
(b) die Wiederverwendung des neutralen oder alkalischen "Wenige-Feststoffe"-Strom
zum Verdünnen von Pulpe vor dem Bleichprozess.
2. Verfahren gemäß Anspruch 1, bei dem besagter "Viele-Feststoffe"-Strom mit mindestens
einem Komplexierungsmittel und mindestens einem Reduktionsmittel behandelt wird, um
einen behandelten "Viele-Feststoffe"-Strom herzustellen.
3. Verfahren gemäß Anspruch 2, bei dem das neutrale oder alkalische Pulpe-Verdünnungswasser
und der neutrale oder alkalische "Wenige-Feststoffe"-Strom einen pH von 6 bis 8 haben.
4. Verfahren gemäß Anspruch 3, bei dem der behandelte "Viele-Feststoffe"-Strom mechanischer
Pulpe, die eine Maschine zur Papierherstellung betritt, zugegeben wird.
5. Verfahren gemäß Anspruch 3, bei dem der behandelte "Viele-Feststoffe"-Strom in einen
Bleichungsschritt eingeführt wird.
6. Verfahren gemäß Anspruch 3, bei dem das Verdünnungswasser für die Pulpe suspendiertes
Calciumcarbonat enthält.
7. Verfahren gemäß Anspruch 6, bei dem besagtes mindestens ein Reduktionsmittel ein Dithionitanion
umfasst.
8. Verfahren gemäß Anspruch 1, bei dem der "Viele-Feststoffe"-Strom mechanischer Pulpe,
die eine Maschine zur Papierherstellung betritt, zugesetzt wird.
1. Procédé amélioré de blanchiment de la pâte mécanique dans des conditions neutres ou
alcalines de fabrication du papier, ladite amélioration comprenant les étapes de :
(a) séparation de l'eau blanche trouble, recyclée à partir d'opérations de fabrication
du papier dans des conditions neutres ou alcalines, en un courant neutre ou alcalin
à faible teneur en solides, ledit courant à faible teneur en solides ne contenant
pas plus de 5000 ppm de solides, et un courant à forte teneur en solides, ledit courant
à forte teneur en solides étant l'autre composant résultant de la séparation, et
(b) réutilisation du courant neutre ou alcalin à faible teneur en solides. dans le
but de diluer la pâte avant une opération de blanchiment.
2. Procédé selon la revendication 1, dans lequel ledit courant à forte teneur en solides
est traité avec au moins un chélatant et au moins un agent réducteur pour produire
un courant à forte teneur en solides, traité.
3. Procédé selon la revendication 2, dans lequel l'eau neutre ou alcaline pour la dilution
de la pâte et le courant neutre ou alcalin à faible teneur en solides ont un pH de
6 à 8.
4. Procédé selon la revendication 3, dans lequel le courant à forte teneur en solides
traité est ajouté à la pâte mécanique pénétrant dans une machine de fabrication du
papier.
5. Procédé selon la revendication 3, dans lequel le courant à forte teneur en solides
traité est introduit dans une étape de blanchiment.
6. Procédé selon la revendication 3, dans lequel l'eau de dilution de la pâte contient
du carbonate de calcium en suspension.
7. Procédé selon la revendication 6, dans lequel ledit au moins un agent réducteur contient
l'anion dithionite.
8. Procédé selon la revendication 1, dans lequel le courant à forte teneur en solides
est ajouté à la pâte mécanique pénétrant dans une machine de fabrication du papier.