Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Reexamination Certificate
2000-06-02
2002-11-05
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
C356S326000, C356S425000
Reexamination Certificate
active
06475339
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to determining a kappa number of various wood pulps, and more particularly, relates to a system for rapidly determining the pulp kappa number of a pulp sample using a spectrophotometric detector.
BACKGROUND OF THE INVENTION
The kappa number of pulp is an important parameter in pulp manufacturing. It is used for indirectly indicating lignin content, relative hardness, and bleachability of pulp. Once the kappa number is known, the amount of bleaching agent needed for achieving a desired pulp brightness can then be ascertained. The pulp kappa number can be determined by the volume of 0.02 mole/liter (0.1N) potassium permanganate (KMnO
4
) solution consumed through an oxidation reaction by 1 gram of moisture-free or oven dry (O.D.) pulp in an acidic medium, where potassium permanganate, also referred to herein simply as permanganate, is a strong oxidation agent.
Presently, the titration method is a universally known and commonly used method for measuring the kappa number of various pulps. This titration method is described in tappi Test Methods—T236 cm-85, Tappi Press, 1996, which is incorporated herein by reference, and was first proposed in 1934 by Wiler, Paper Trade Journal, 98(11), 1934, and was later developed in the 1950s based on the work of Watson and Stamp, J. Aust. Pulp Paper Ind. Tech. Assoc., 11(1), 1957, Valeur and Torngren, Svensk Papperstidning, 30(22), 1957, and Tasman and Berzins, Tappi J., 40(9), 1957. Using the titration method, the pulp kappa number is calculated using the difference between the initial volume of potassium permanganate blank solution and the final volume of potassium permanganate remaining after the oxidation of lignin in the pulp-permanganate solution. It is known in the art that potassium permanganate blank solution is simply potassium permanganate solution without pulp.
The final volume is determined by titration to determine how much unconsumed potassium permanganate remains after a predetermined time period. In accordance with the method, the final volume is measured after ten minutes, thereby presuming that the oxidation of lignin in a fiber or pulp sample is complete after that time. Also, the titration method is performed wherein the pulp-permanganate solution temperature is maintained at 25° C. and under an initial H
+
concentration of 0.4 mol/L, or pH of about 0.4, acidic conditions,. The following reactions generally occur as a result of such acidic conditions:
MnO
4
-
+
8
H
+
+
5
e
→
E
0
=
+
1.51
V
Mn
2
+
+
4
H
2
O
(1a)
MnO
4
-
+
4
H
+
+
3
e
→
E
0
=
+
1.69
V
MnO
2
+
2
H
2
O
(1b)
where (1a) produces manganese ions, Mn
2+
, and (1b) produces manganese dioxide precipitation, MnO
2
. Finally, the titration method requires that the consumed potassium permanganate volume is approximately 50% of the initial volume after a ten-minute reaction in order to obtain a valid measurement. Such a requirement is difficult to reconcile because the excess permanganate volume is known only after the kappa number is determined.
Under the titration method, a number of assumptions are made which can be problematic, and therefore, may lead to an inaccurate and inconsistent determination of the pulp kappa number. First, it is assumed that all of the lignin in pulp has reacted with the permanganate in the ten-minute time period. In fact, potassium permanganate can also be consumed by decomposition by other organic materials in pulp. Consequently, the final volume of potassium permanganate in the pulp-permanganate solution would not be a reflection of the excess potassium permanganate remaining after oxidation of lignin alone. Therefore, the difference between volumes calculated as the kappa number would be inaccurate.
Secondly, it is assumed that the oxidation reaction is completed in ten minutes. This oxidation reaction time is seemingly arbitrary in light of the fact that actual oxidation reaction times vary from pulp to pulp. Also, the oxidation reaction time is dependent on the mass of the pulp sample, thereby producing inconsistencies in kappa number.
Another assumption is that the effect of the variation in excess permanganate volume is insignificant. Due to its insignificance, the titration method attempts to correct this effect by a non-constant correction factor as tabulated in the Tappi test method. However, the non-constant correction factor is purely empirical through experimental calibration, and therefore, can lead to errors in determining the pulp kappa number.
Besides the problems that can arise due to the foregoing assumptions, the titration method itself is very tedious, time-consuming, and prone to error due to the human element. Titration is performed manually, and as a consequence, titration is very dependent upon the skill of an operator. It also takes about thirty minutes for an operator to complete the tedious titration process. For instance, pulp kappa numbers for the same pulp sample can also vary widely from one operator to the next operator.
Spectrophotometry is another technique that has been attempted to determine the pulp kappa number as described in articles entitled “Kappa Number Determination in Kraft Pulping by FTIR Spectroscopic Measurements on Spent Liquors”, Tappi J., 74(4):235 (1990), by A. J. Michell, “Determination of Total Lignin Content in Fibrous Materials,” Zellstoff Papier, 23(11):327 (1974), by A. M. Plonka et al., and “The STFI OPTI-Kappa Analyzer: Applications and Accuracy,” Tappi J., 70(11):38 (1987), by Kibulnieks et al., which are all incorporated herein by reference in their entireties. Spectrophotometry can provide direct and instantaneous measurements of chemicals in reactions by measuring the absorbance or transmittance of light from chemicals using an optical instrument commonly known as a spectrophotometer.
These methods described in the prior art are capable of ascertaining the lignin content of a pulp sample, but not the lignin reactivity, which can be used to determine the amount of chemicals required in the bleaching process. Consequently, these methods are limited in that they cannot directly ascertain the pulp kappa number, and therefore, use a pre-calibrated linear relationship between the lignin content in pulp and the pulp sample kappa number measured by the traditional titration method. The calibration relationship varies from pulp to pulp due to the variation of lignin reactivity with wood species.
Li and Gellerstedt, Nordic Pulp & Paper Research J., 13(2):147, 1998, reported the results of a study on the kinetics and mechanism of pulp-permanganate oxidation reaction under the conditions suggested in standard kappa testing methods. In the study, the permanganate in pulp-permanganate solutions was directly measured using UV/Vis spectrophotometry under conditions suggested in common kappa number test methods, such as TAPPI Test Method—T236cm-85 and SCAN-C1 Test Method. According to Li and Gellerstedt, the dominant reaction is an overall conversion of permanganate to MnO
2
precipitation according to reaction (1b). It was found that the precipitated MnO
2
strongly interfered with the measured absorption spectrum of the pulp-reacting solution and caused significant measurement difficulties in determining the excess permanganate in the final pulp-reacting solution. It was impossible to obtain meaningful kappa numbers that agree with the kappa numbers obtained by standard titration based methods as they indicated in their work. Nevertheless, they concluded that direct spectrophotometry can be used to calculate the kappa number determination from their experimental data.
One problem with using their proposed method is since it is difficult to determine the amount of precipitated MnO
2
, the amount of permanganate consumed by pulp-permanganate oxidation reactions cannot be quantified for pulp kappa number calculations. In addition, considering the formation of MnO
2
precipitation, one skilled in the art cannot use spectrophotometry to det
Chai Xin-Sheng
Zhu Jun Yong
Alvo Steve
Christmas, Esq. Veda V.
lnstitute of Paper Science and Technology, Inc
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