Method for improving pulp washing efficiency

A process for enhancing pulp washing efficiency is disclosed. An anionic surfactant is added within the washing or pulping operation to enhance the removal of lignin and spent cooking chemicals from pulp.

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 
solids 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 ethosulfate compounds of the following general 
structure: 
EQU R--(OCH.sub.2 CH.sub.2).sub.n OSO.sub.3 M 
where R is alkyl, aryl or alkylaryl, M is H or a water soluble cation, 
(e.g., Na.sup.+, NH.sub.4.sup.+, K.sup.+) and n is from about 1-30. 
The water soluble cation may be any positively charged cation. The R group 
may have from about 8 to 26 carbons when R is alkyl, with about 12-16 
carbons preferred; when R is alkylaryl, the chain length may be from about 
6 to 14 carbons with benzene as the aryl group, with 8-9 carbons 
preferred. 
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.degree. to 350.degree. F., with a pH of 
about 12-13. In the washers, the temperature range of treatment is from 
about 100.degree.-200.degree. 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. 
It will be appreciated that ethosulfate compounds such as alkyl phenol 
ethosulfates and alcohol ethosulfates may be utilized in-the present 
invention. Treatment levels of from about 0.1 to 1000 parts of anionic 
surfactant per million parts of pulp may 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.degree. 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.degree. C. 
pH Filtrate Absorbance 
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.sup.+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.degree. C. 
pH Absorbance 
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.sup.+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.degree. C. 50.degree. C. 23.degree. 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.degree. 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.

EXAMPLE 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.sup.+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 ethosulfates in this table are 
commercially available from Rhone Poulenc, Inc. and Vista Chemical Co. 
TABLE 4 
______________________________________ 
Products Tested 
Tradename Description 
______________________________________ 
Alipal C0433 
nonyl phenol ethosulfate, sodium salt 
Alipal C0436 
nonyl phenol ethosulfate, ammonium salt 
Alipal EP110 
nonyl phenol ethosulfate, ammonium salt 
Alipal EP115 
nonyl phenol ethosulfate, ammonium salt 
Alipal EP120 
nonyl phenol ethosulfate, ammonium salt 
Alfonic .RTM. 1412S 
linear alcohol ethosulfate, sodium salt 
Alipal CD128 
linear alcohol ethosulfate, ammonium salt 
Witconte D510 
sodium 2-ethylhexyl sulfate 
Poly-Tergent .RTM. 
alkoxylated linear alcohol carboxylic acid 
Emcol .RTM. CBA50 
poly(oxy-1,2-ethanediyl), -(carboxymethyl) 
(tridecyloxy)-branched, sodium salt 
Emcol .RTM. 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 .RTM. 941 
polyacrylate, MW = 1,250,000 
Foam-Trol .RTM. 275 
defoamer, containing PEG 100, ethylenebis 
stearamide, oil and silicone oil 
Polyox .RTM. N60K 
polyethylene oxide, MW = 2,000,000 
______________________________________ 
TABLE 5 
______________________________________ 
Effect of Anionic Surfactants (Filtrate Absorbance) 
Material Absorbance 
______________________________________ 
Blank 0.53 
Alipal C0433 2.69 
Alipal C0436 2.50 
Alipal EP110 1.92 
Alipal EP115 1.90 
Alipal EP120 2.14 
Alfonic 1412S 2.37 
Alipal CD128 1.32 
Witconate D510 0.87 
Poly-Tergent 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 filtrate absorbances). The ethosulfates 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 Absorbance 
______________________________________ 
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 
Alipal C0433 
2.69 2.05 0.08 0.06 
Alipal C0436 
2.50 1.65 0.20 
Alipal EP110 
1.92 2.24 0.17 
Alipal EP115 
1.90 1.76 0.04 0.04 
Alipal EP120 
2.14 1.92 0.20 
Alfonic 1412S 
2.37 2.18 1.34 0.58 0.26 
Alipal CD128 
1.32 0.37 0.09 
Polytergent CS1 
1.05 0.21 
Gafac RE610 
2.19 0.31 0.09 0.08 
______________________________________ 
Table 7 illustrates the effectiveness of the ethosulfates as compared to 
the carboxylated and phosphated surfactants. The ethosulfates are also 
more effective as a class than the polyacrylates and defoamer. The above 
table also demonstrates that the sodium salts are more preferred than the 
ammonium salts with similar ethosulfates. For instance, Alipal C0433 and 
Alipal C0436, both nonyl phenol ethosulfates with the same level of 
ethoxylation, behave differently, the sodium salt being the more 
effective. A similar situation exists between two alcohol ethosulfates, 
Alfonic 1412S and Alipal CD128, the sodium salt (Alfonic 1412S) being more 
effective. However, ammonium salts can still be effective, as is apparent 
with a review of ammonium ethosulfate data with greater degrees of 
ethoxylation. 
EXAMPLE 3 
In order to further illustrate the effectiveness of ethosulfates, Alfonic 
1412S was used with fiber. The same test method as Example 2 was used, 
with fiber being added. The experiments were conducted at 71.degree. C. 
and pH 9. The results are contained in Table 8. 
TABLE 8 
______________________________________ 
Results with Fiber 
Treatment Absorbance 
Kappa Number 
______________________________________ 
Untreated 0.4 120 
Alfonic 1412S 0.9 79 
______________________________________ 
These results indicate that Alfonic 1412S decreases the lignin content in 
the fiber mat, as shown by the Kappa number. 
EXAMPLE 4 
In this example, actual black liquor was used in place of Indulin AT, and 
the experiment was carried out as in Example 3, except that the pH set for 
incubation was 10. The results are contained in Table 9. 
TABLE 9 
______________________________________ 
Results with Black Liquor 
Treatment Absorbance 
Kappa Number 
______________________________________ 
Untreated 7 211 
Alfonic 1412S 8 200 
______________________________________ 
The results in Table 9 show that ethosulfates such as Alfonic 1412S gave an 
improvement over an untreated system. 
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.