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Description
FIELD OF THE INVENTION
The
present invention relates to a method for
increasing the efficiency of pulp washing by
decreasing the tendency for lignin to remain in
the fibrous mat after washing.
BACKGROUND OF THE INVENTION
The
manufacture of paper from wood requires many
complex steps, including the formation of pulp
fiber from wood chips. This process takes place
in a digester, where wood chips are cooked at
high temperature with sodium sulphide and sodium
hydroxide in order to break down and solubilize
the lignin, so that it can be separated from the
wood pulp. The most prominent by-product of the
process is kraft lignin, a complex
three-dimensional material based on repeating
phenol propane units.
The lignin and
spent cooking chemicals are contained in the
liquid fraction, often referred to as black
liquor, of the brown stock. Additional
by-products found in the black liquor include
wood pitch and hemicelluloses (low molecular
weight polysaccharides). When pine is used,
crude tall oil and turpentine become very
important by-products.
Following the
digester, the black liquor (containing organics,
mostly lignin, and inorganic spent cooking
chemicals) is separated from the wood pulp in a
process commonly known as brown stock washing.
Rotary drum washers placed in series are
commonly used to wash brown stock. Generally,
these drums are made up of different washing
zones. The first washing step within a drum is
usually dilution/thickening, where the brown
stock is diluted with liquid which is cleaner
than the liquid within the brown stock. After
the stock is thickened on the vacuum drum, a
second washing step of displacement is usually
conducted. In the displacement phase, liquid
which is cleaner than the mat of pulp is applied
to the mat surface via showers and pulled
through the pulp mat to displace the dirty
liquid held within it. Kraft brown stock washing
can also be conducted with variations of this
washing technique. Other washing methods include
pressure washers, which use pressure rather than
vacuum, and belt washers, which use
displacement.
Brown stock washing is
important to the pulp mill operation. Digester
cooking chemicals are recovered for reuse during
washing. Pulp mills also burn the organics for
their heating value. Therefore, the efficient
collection of organics from the pulp is very
important to an effective pulp mill operation.
Bleaching, which Often follows brown stock
washing, is more efficient when the brown stock
washers remove the most by-product sol ids
possible.
The brown stock washing phase
is also especially important environmentally.
The effluent from bleaching is discharged from
the mill; this effluent contains chlorinated
organics, which can be toxic. Toxic substances
which are currently of concern include dioxins
and furans, specifically
2,3,7,8-tetradichlorodibenzo-p-dioxin and
2,3,7,8-tetrachlorodibenzofuran, absorbable
organic halogens, and color. Increased organics
removal in brown stock washing has been shown to
decrease the environmental impact of bleaching.
Brown stock washing is an important
aspect of pulp mill operation. Specifically, the
washing of organics from pulp is becoming
increasingly important. In bleached processes,
enhanced organic removal would reduce bleaching
chemical consumption, costs, and environmental
problems associated with effluent discharge of
chlorinated organics. In unbleached processes,
enhanced organics removal in washing should
decrease runnability problems associated with
excess lignin in pulp, such as reduced retention
aid performance.
DETAILED DESCRIPTION OF
THE INVENTION
The present invention
relates to a process for enhancing pulp washing
efficiency by decreasing the tendency of lignin
to remain with the pulp fraction during washing.
In this method, anionic surfactants are added
within the washing or pulping operation to
enhance the removal of lignin. These surfactants
are anionic sulfonate compounds of the following
general structure:
R--SO3 M
where M is H or a water soluble cation
such as Na+,
NH4+, K+,
Ca2+ or Mg2+. R may be
alkyl, aryl or alkylaryl, and may be
unsubstituted or substituted (with e.g.,
hydroxyl groups) and can contain various
linkages such as ether, ester, amide or urethane
linkages.
Furthermore, a multisulfonate
can be used, where the sulfonate groups are
separated by alkyl, aryl, or alkylaryl groups
which may or may not be substituted and may also
contain any of the above linkages.
Preferred compounds include sodium alkyl
taurine or alkyl diphenol oxide disulfonate,
with alkyl chain lengths of C8
-C18.
The treatment may be
added at any point from the digester to the
brown stock washers (and the decker, which is a
washer that follows but is usually separated
from the brown stock washers) in Kraft, or
sulphate systems for both hardwood and softwood.
In the digester, the temperature of treatment is
from about 200° to 350° F., with a pH of about
12 -13. In the washers, the temperature range of
treatment is from about 100°-200° F., with a pH
of about 8-13. It is expected that the method of
the present invention would also be effective in
the washing processes that occur within a
bleaching plant.
Treatment levels of
from about 0.1 to 1000 parts of anionic
sulfonate surfactant per million parts of pulp
are expected to be effective. Chemically
prepared pulp (e.g., sulphate, sulfite) as well
as mechanically and semi-chemically prepared
pulp may all benefit from the present invention.
The invention will be further understood
by reference to the following examples.
Kraft black liquor and unbleached Kraft
pulp were collected from a softwood brown stock
washer and mixed so that the pulp constituted
0.75% of the mixture (based on oven dry fiber).
The stock was divided into separate samples, and
the pH of each sample was adjusted to the
desired level. Following an incubation period of
30 minutes at 71° C., the samples were filtered.
The absorbance of the filtrate was measured at a
wavelength of 700 nm, (chosen to be able to
measure the broadest array of concentrations of
black liquor with minimal dilution) and the
Kappa number of the pulp mat was measured as
well.
The absorbance was used to measure
the solution color, a high color relating to
more lignin remaining in the filtrate. The Kappa
number measurement is a well-established test
method used in the paper industry to determine
the lignin content of pulp. In this method, pulp
is bleached with an excess and known quantity of
potassium permanganate. The unused permanganate,
determined with a titration using thiosulfate,
is used to report the Kappa number, which is
directly related to the level of lignin
remaining with the pulp.
TABLE 1
______________________________________
Effect of pH on Pulp Mat Kappa Number and
Filtrate Absorbance Using Softwood Kraft Pulp
and Black Liquor at 71° C.
Filtrate
pH Absorbence
Kappa Number
______________________________________
12 11.3 57
11 10.5 76
10 7.1 211
9 4.0 271
______________________________________
The above results demonstrate that
the amount of lignin remaining with the mat (as
shown by Kappa number) increases with decreasing
pH. The filtrate absorbance decreased as the
Kappa number of the mat increased since the
lignin, the main color-producing substance in
black liquor, remained with the mat instead of
the liquid phase. Therefore, filtrate absorbance
may be used in place of mat Kappa number to
determine where the lignin is, either in the
fiber mat or with the liquid phase.
The
above experiment was repeated replacing the
black liquor with a solution containing 5000 ppm
Kraft lignin (Indulin AT, by Westvaco Corp.),
100 ppm Ca+2 and enough caustic to
raise the pH to 12. The results are found in
Table 2.
TABLE 2
______________________________________
Effect of pH on Mat Kappa Number and
Filtrate Absorbance Using Kraft Lignin
(Indulin AT) at 71° C.
pH Absorbence
Kappa Number
______________________________________
12 2.80 13
9 0.38 120
______________________________________
Table 2 illustrates that the use
of Indulin AT is an acceptable model for testing
in place of black liquor as the lignin also has
a tendency to remain with the fiber when the pH
is decreased. The Indulin AT is also more
consistent than black liquor, as black liquor
may vary with age and sample location.
In order to determine whether the
addition of pulp was needed for testing,
solutions containing 5000 ppm Indulin AT and 100
ppm Ca+2 which did not contain pulp
fiber were brought up to pH 12 to dissolve the
Indulin AT. The pH was then lowered to the
desired level, and the sample was incubated for
30 minutes at the desired temperature. Following
incubation, the samples were filtered, and the
filtrate brought back up to pH 12 prior to
measuring its absorbance at 700 nm.
TABLE 3
______________________________________
Effect of Temperature on Lignin Washability
(Filtrate Absorbance vs. pH and Temperature)
71° C. 50° C. 23° C.
pH Blank pH Blank pH Blank
______________________________________
12.0 2.35 10.0 2.15 9.2 2.33
10.0 2.52 9.0 2.18 7.5 2.40
9.8 2.52 8.5 1.24 6.5 1.46
9.5 0.74 8.0 0.96 6.0 0.53
9.0 0.58 7.0 0.30 5.0 0.07
8.0 0.27 6.0 0.16 4.0 0.07
______________________________________
By comparing the results in Table
3 for 71° C. with those of Table 2, it is
apparent that the inclusion of the fiber is not
necessary to measure the reduction of lignin in
the filtrate with decreasing pH.
EXMAPLE
1
Based on the above results, a test
method was developed to screen materials in
order to determine if they could decrease the
tendency of the lignin to be filtered out of
solution. The procedure consisted of making a
solution of 5000 ppm Indulin AT, 100 ppm
Ca+2, 1000 ppm treatment actives and
enough sodium hydroxide to bring the pH to 12
and dissolve the Indulin AT. The solution pH was
then decreased with hydrochloric acid to pH 6
and allowed to incubate at room temperature for
30 minutes prior to filtration. After
filtration, the filtrate pH was raised to 12 and
the absorbance was measured at 700 nm. The
materials used in the following examples are
described in Table 4. The taurines (sulfoalkyl
amides) and isethionate (sulfoalkyl ester) are
available from Rhone Poulenc, Inc., the
benzenesulfonate is available from Witco
Chemical, and the disulfonate is available from
Dow Chemical.
TABLE 4
______________________________________
Products Tested
Tradename Description
______________________________________
Witconate 1260
sodium dodecyl benzenesulfonate
Dowfax .RTM. 2A1
alkyl diphenol oxide disulfonate
Geropon TN74
N-methyl-N-palmitoyl-taurine, sodium salt
Geropon T33
N-methyl-N-oleyl-taurine, sodium salt
Geropon TC42
N-methyl-N-(coconut oil)-taurine, sodium salt
Geropon TK32
N-methyl-N-(tall oil acyl)-taurine, sodium salt
Geropon AS 200
coconut acid ester of sodium isethionate
Witconate D510
sodium 2-ethylhexyl sulfate
Poly-Tergent .RTM.
alkoxylated linear alcohol carboxylic acid
CS-1
Emcol .RTM. CBA50
poly(oxy-1,2-ethanediyl), -(carboxymethyl)-
(tridecyloxy)-branched, sodium salt
Emcol CNP120
poly(oxy-1,2-ethanediyl), -(carboxymethyl)-
(nonylphenoxy)-, sodium salt
Gafac .RTM. RE610
polyoxyethylene nonyl phenyl ether phosphate
Pluronic .RTM. F108
ethoxy/propoxy/ethoxy block copolymer
Tergitol .RTM. 15-S-7
secondary alcohol ethoxylate, 7 mole EO
Igepal C0530
nonyl phenol ethoxylate, 6 mole EO, HLB =
10.5
Igepal C0880
nonyl phenol ethoxylate, 30 mole EO, HLB =
17.2
Floerger .RTM. 45.20
80% dimethyldiallyl ammonium chloride
(DMDAAC)/20% acrylamide copolymer
Goodrite .RTM. K732
polyacrylate, MW = 5100
Carbopol 941
polyacrylate, MW = 1,250,000
Foam-Trol .RTM. 275
defoamer, containing PEG, ethylenebis
stearamide, oil and silicone oil
Polyox .RTM. N60K
polyethylene oxide, MW = 2,000,000
______________________________________
TABLE 5
______________________________________
Effect of Anionic Surfactants
(Filtrate Absorbance)
Material Absorbence
______________________________________
Blank 0.53
Witconate 1260 2.53
Dowfax 2A1 2.34
Geropon TN74 2.60
Geropon T33 2.26
Geropon TC42 2.66
Geropon TK32 2.65
Geropon AS200 1.07
Witconate D510 0.87
Polytergent CS1 1.05
Emcol CBA50 0.32
Emcol CNP120 0.18
Gafac RE610 2.19
______________________________________
The data in Table 5 show that the
majority of anionic surfactant types decreased
the tendency of lignin to remain with the filter
paper (as shown by high liltrate absorbances).
The sulfonates were unexpectedly superior to the
alkyl sulfate and the carboxylated surfactants.
Table 6 contains the results of using
nonionic surfactants as well as typical
materials that may be found in brown stock
washers. The effect of a combination of
alkylphenol ethoxylate and polyacrylate (Igepal
C0530/Goodrite K732), a combination taught by
Freis et al. U.S. Pat. No. 4,810,328, was also
tested at a 1:1 ratio.
TABLE 6
______________________________________
Effect of Other Materials
(Filtrate Absorbance)
Material Absorbence
______________________________________
Blank 0.53
Pluronic F108 0.12
Tergitol 15-S-7 0.06
Igepal C0530 0.13
Igepal C0880 0.13
Floerger 45.20 0.03
Polyox N60K 0.06
Igepal C0530/Goodrite K732
0.95
Foamtrol 275 1.61
Goodrite K732 2.03
Carbopol 941 1.59
______________________________________
As shown in Table 6, the
combination found in Freis et al. '328 gave
relatively little improvement as compared with
the untreated sample.
EXAMPLE 2
In this example, the same procedure was
followed as for Example 1, with the exception
that the pH was decreased to various levels in
order to determine the pH range where the lignin
began staying with the filter paper. Materials
which had shown some effect in Example 1 were
used for this testing. The results are reported
in Table 7.
TABLE 7
______________________________________
pH Range in which Lignin remains with Filter Paper
(Filtrate Absorbance)
pH
Product 6.0 5.5 5.0 4.5 4.0
______________________________________
Untreated 0.53 0.16 0.07 0.04 0.07
Goodrite K732
2.03 0.98 0.25
Carbopol 941
1.59 0.10
Foamtrol 275
1.61 0.33
Witconate 1260
2.53 1.50 0.22
Dowfax 2A1 2.34 1.65 0.88 0.13
Geropon TN74
2.60 1.16 0.61
Geropon T33
2.26 0.96 0.32 0.13
Geropon TC42
2.66 1.30 1.27 0.35
Geropon TK32
2.65 1.95 0.87 0.17
Polytergent CS1
1.05 0.21
Gafac RE610
2.19 0.31 0.09 0.08
______________________________________
Table 7 demonstrates the relative
ineffectiveness of the polyacrylates, defoamer
and surfactants which were carboxylated or
phosphated as compared to the sulfonates.
While this invention has been described
with respect to particular embodiments thereof,
it is apparent that numerous other forms and
modifications of this invention will be obvious
to those skilled in the art. The appended claims
in this invention generally should be construed
to cover all such obvious forms and
modifications which are within the true spirit
and scope of the present invention.
* *
* * *
Other References
- Blomberg, L., et al., "Organic Carry-Over in
Kraft Pulp and Bleaching Discharges",
Proceedings from TAPPI Pulping Conference, pp.
217-226 (1990).
- Hise et al., "The Effect of Brown Stock
Washing on the Formation of Chlorinated Dioxins
and Furans During Bleaching", TAPPI Journal,
73(1):185-190 (1990).
- Laxen, T., "Dynamic and Physical-Chemical
Aspects of Sulphate Pulp Washing", Pulp and
Paper Canada, 87(4):49-53 (1986).
- Wilkes et al, "New Surface Active
Chemicals", Paper Trade Journal, Aug. 17, 1939,
pp. 63-66.
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United States Patent 5464502
