JP2003518613A - An electrochemical method for determining the lignin content of pulp - Google Patents

Abstract

(57) Summary The present invention describes a novel method for determining the kappa number or lignin content of kraft pulp based on voltammetric measurements of catalytic reactions involving lignin and redox mediators. The method involves measuring the regeneration rate of a lignin redox catalyst in the presence of pulp following its oxidation at a voltammetric electrode. It has been shown that the intensity of the catalytic current generated at the electrode surface is proportional to the amount of lignin present in the pulp. Within the type of pulp (softwood, hardwood, or oxygen delignified pulp), a linear relationship can be obtained between the intensity of the generated current and the range of kappa numbers. The redox properties and concentration of the vehicle, and the voltage scanning rate of the voltammetry method, are among the parameters that can be tuned to the linear relationships obtained for the kappa numbers of various pulps. Automation of this method can lead to the development of a new on-line electrical kappa number analyzer used for pulping or bleaching process control.

Description

Detailed Description of the Invention

TECHNICAL FIELD The present invention relates generally to the paper and pulp industry, and more particularly to a novel electrochemical method for measuring the lignin content and / or Kappa number of wood pulp to form pulp and bleach spent liquids. Regarding the method.

BACKGROUND ART In order to produce high uniform quality kraft pulp, it is necessary to efficiently control the pulping and bleaching process. Pulp Kappa number measurements are commonly used to monitor and control cooking conditions and to estimate the amount of bleaching chemicals to add to achieve the desired pulp brightness without wasting the chemicals. There is. Furthermore, mills using an oxygen delignification step also require reliable Kappa number measurements to prevent excessive delignification and fiber degradation. Manual measurement of Kappa number using the standard sodium camanganate titration method takes several hours. The delay between Kappa number measurement and process modification is incompatible with efficient process control schemes that require continuous monitoring of pulp quality. An automatic on-line copper analyzer can measure the copper number within minutes and thus process changes in the process quickly. Several automatic copper analyzers have been developed for commercial use.
They all measure the optical properties of pulp fiber suspensions. UV light absorption is used in the "Optical-Kappa Analyser" developed by STFI [Kubulnieks et al., Tappi J., November 198].
7, pp. 38-42], but another device that operates on a similar principle uses reflection of UV light instead of absorption. These are the BTG Company [Agneus and Damlin, 1990, Stockholm, Sweden, Proceedings of the
24th Eucepa Conference, pp. 234-241], Valmet Automation Kajaani Inc. [Ollila and Elkile (
Erkkilae), Jacksonville, Florida, TAPPI Proceedings of the 1997 Proc
ess & Product Quality Conference & Trade Fair, pp. 117-122], and Honeywell-Measurex (Van Fleet)
And Whalley, TAPPI Proceedings of the 1998 Pulping Conferenc
e, pp. 1509-1512] and US Pat. No. 5,953,111. Fluorescence measurements [Berthold et al., US Pat. Nos. 5,216,483 and 5,220,172], and color shifts of fluorescence [Jeffers and Malito, US Pat. No. 5,486. , 915] was applied to determine the lignin content in wood pulp. Measuring devices based on optical methods are generally very sensitive to variations in pulp consistency. Therefore, strict control of pulp consistency is also required to obtain accurate lignin measurements.

Electrochemical sensors are commonly used in a very wide range of applications such as environmental monitoring, industrial quality control, and biomedical analysis. In the pulp and paper industry,
Amperometric / polarographic and potentiometric sensors have been used to measure the concentration of various oxidizing or reducing chemicals in pulping and bleaching liquids. For example, the polarographic method can selectively and quantitatively measure sulfides, sulfites, thiosulfites, and sulfhydryl compounds in Kraft liquids [nell (
Noel), Tappi J. 61 (5): 73-76, 1978], chlorine dioxide and chlorite in a bleached used liquid can be selectively and quantitatively measured [Dence C].
． W. And Reeve D.L. W. Editing, Pulp Bleach-Principle and Practice (Pulp Bl
eaching-Principles and Practice), Tappi Press, 1996, pp. 625-645, Willems (Willems) and Williamson (Williamson)]. However, these sensors cannot measure lignin in the fiber. This is because it is necessary to bring the lignin into close contact with the electrode surface in order to allow free electrons to flow. Recent developments in the field of enzyme bleaching of kraft pulp have shown that the enzyme laccase can delignify kraft pulp and oxidize lignin model compounds in the presence of redox mediators. Shown [Bourbonnais and Paice, FEBS
Lett 267: 99-102, 1990 and Appl. Microbiol. Biotechnol. 36: 823-827, 1992.
]. Redox mediators act as diffusible electrical carriers between the lignin that remains in the fiber wall and the large laccase molecules in solution. Electrochemical studies of the interaction between lignin and redox mediators have shown that electrochemically oxidized mediators are continuously regenerated at the electrode surface after reduction by lignin-type compounds. Shown (Bourbones et al., Biochim. Biophys. Acta. 1379: 381.
-390, 1998). The redox reaction rate has been shown to be related to the concentration of lignin-type compounds.

It is an object of the present invention to apply the voltammetric method to pulp suspension in the presence of soluble lignin redox mediator in order to quantitatively and rapidly determine kraft pulp lignin content.

DISCLOSURE OF THE INVENTION The present invention provides a novel method for determining Kappa number or lignin content of kraft pulp based on voltammetric measurements of catalytic reactions involving lignin and redox mediators. This electrochemical method consists of measuring the rate of regeneration of a lignin redox catalyst in the presence of pulp after it has been oxidized at a voltammetric electrode. The intensity of the catalytic current generated on the electrode surface is proportional to the amount of lignin present in the pulp and the oxidation state. Within a wide range of Kappa number and pulp type, it is possible to obtain a linear relationship between the Kappa number and the strength of the regeneration current. The redox properties and concentration of the mediator and the voltage scanning rate of the voltammetric method are parameters that can be tuned to the linear relationship obtained with different pulps.

According to one aspect of the present invention, in a method of determining the Kappa number or lignin content of pulp, i) the effectiveness of the oxidation state effective to oxidize lignin in the pulp in an electrochemical cell. In the presence of a redox mediator, contacting the pulp sample with a voltammetric electrode to produce a reduced state of the redox mediator, causing a current discharge at the electrode, which discharge oxidizes the redox mediator again,
ii) determining the peak intensity of the current discharge by voltammetry at the electrode, and iii) determining the Kappa number or lignin content of the pulp sample from the peak intensity measured in ii) above. .

As another aspect of the present invention, in a method for analyzing the lignin content of pulp,
a) mixing a sample of pulp with a redox mediator, b) placing the resulting sample in contact with a working electrode in a cyclic voltammetry device, c) measuring the intensity of the peak current generated at said electrode, and d) An analytical method is provided which comprises converting the peak currents into Kappa numbers from calibration data determined by pulps with a known lignin content.

DETAILED DESCRIPTION OF THE INVENTION AND DESCRIPTION OF PREFERRED EMBODIMENTS The principles of the present invention use electroanalytical methods such as cyclic voltammetry (CV) and soluble redox mediators that catalyze lignin oxidation. Based on a combination of doing.

As shown in FIG. 1 a, the basic voltammetric device 10 comprises an electrochemical cell 12, a potentiostat 14, and a recorder 16. The cell 12 has a working electrode 18, an auxiliary electrode 20, a reference electrode 22, a redox mediator solution 24 is placed in the cell 12, and a sample 26 of pulp mixed with the mediator 24 is a working electrode 18.
Supported on the surface of. In use, the potentiostat 14 performs a linear potential scan at the working electrode 18 while measuring the current resulting from the oxidation or reduction of the redox active compound mediator 24 in the electrochemical cell 12.

The intensity of the peak current (I _p ) measured by the recorder 16 at the surface of the working electrode 18 is measured by the concentration of the target analyte as shown by the following Randles-Sevcik equation. It is directly proportional to C: I _p = (2.69 × 10 ⁵ ) n ^3/2 AD ^1/2 ν ^1/2 C (where n is the number of electrons, A is the area of the working electrode, D is the diffusion coefficient of the analyte and ν is the voltage scanning speed).

This relationship applies to soluble redox couples, but cannot be used directly for lignin in pulp fiber. This is because there is no direct contact between the lignin and the electrode surface. However, the addition of soluble redox mediators can effectively couple the electrode reaction to lignin in the lup fiber. During the voltammetric process, the electrooxidized mediator diffuses into the pulp fiber and reversibly reacts with lignin. Reaction with lignin regenerates the reduced mediator at the electrode surface, resulting in an increased current compared to the mediator alone. When a constant concentration of mediator is used, the current (I _k ) measured at the median peak potential in the presence of pulp is proportional to the amount of lignin or kappa number of the pulp (see Figures 2 and 3).

The present invention can be applied to determine the lignin content or Kappa number of hardwood or softwood kraft or sulfite pulps during or after the steaming and bleaching steps. The pulp is preferably washed with water or an electrolyte buffer solution. After pressing or filtering, an amount of pulp is mixed with an amount of mediator solution 24 to form sample 26. To this end, the mediator is any organic or inorganic compound capable of forming a redox pair at a potential of 0.3-1.2 V, as measured against an Ag / AgCl reference electrode (RE) 22. But it's okay. The pulp / medium suspension sample 26 is applied to the surface of the working electrode (WE) 18 and pressed to form a medium solution 24, a reference electrode (R).
E) 22 and an auxiliary electrode (AE) 20 in an electrochemical cell 12 (FIG. 1). BAS CV-50W voltammetric analyzer using Ag / AgCl reference electrode 22, platinum wire auxiliary electrode 20, and 3 mm diameter glassy carbon working electrode 18 [Bioanalytica, USA].
l Systems, Inc.)], and cyclic voltammetric determinations were typically performed. Electrochemical cell 12, electrodes 18, 20, 22 and potentiostat 14
Any combination suitable for the electrochemical analysis of can be used in the present invention.

The redox mediator should suitably be reversible so that it can be repeatedly cycled between reduced and oxidized states. It is most convenient to use redox mediators that are water soluble.

The range and speed of voltage scanning and peak potential measurement can be done automatically by a voltammetric analyzer. Examples of suitable mediators include 2 as an organic mediator.
, 2'-azinobis (3-ethylbenzthiazoline-5-sulfonate) (AB
TS), and potassium octacyanomolybdate [K ₄ Mo (
CN) ₈ ]. ABTS is available _{commercially, K 4 Mo (CN) 8} · 2H 2 O
Furman and Miller (Inorg. Synth. 3,160-163, 1950)
). The preferred organic mediator used to exemplify the present invention is ABTS, which has two stable reversible redox pairs within a potential range suitable for oxidizing lignin. The first anode peak that appears when the electrode potential approaches 0.52V corresponds to the oxidation of ABTS to the cation radical (ABTS. ⁺ ), Whereas the second peak at 0.92V is a divalent cation. Ion (ABTS ²⁺ )
Corresponding to the formation of. FIG. 3 shows a) sodium citrate buffer solution, 0.1M, pH
Cyclic voltammograms obtained at a slow potential scan rate (2 mV / sec) using 0.2 mM ABTS in 4.5 and b) ABTS in the presence of softwood kraft pulp. Is shown. When compared to ABTS alone (diffusion current I _d ), the increase in the anode current peak intensity of ABTS (catalyst current I _k ) in the presence of pulp is due to the two oxidation states of ABTS and in kraft pulp. It illustrates the extent of reaction that takes place with residual lignin. The intensity of the catalytic current at the peak potential of the mediator is proportional to the amount of residual lignin or Kappa number of the kraft pulp.

[0015]

【Example】

Example 1 Electrochemical Measurement of Kappa Number of Softwood Kraft Pulp Using 0.5 mM ABTS Softwood kraft pulp having a Kappa number in the range of 32 to 87, Eastern Canada spruce pine chips were mixed in a pilot plant batch cooker. It was produced by steaming with the H factor of. Furthermore, further delignification of Scots kraft pulp was carried out at various temperatures, reaction times and alkali loadings using a laboratory scale oxygen pressure reactor. The resulting softwood oxygen delignified pulp (S
WO ₂ ) had a Kappa number in the range of 11-29. All pulp Kappa numbers were measured by the camanganate titration method according to the Tappi test method T236.

Cyclic voltammetry of the pulp was performed as follows: After washing the pulp with water, a solution of ABTS (0.5 mM) in sodium citrate (0.1 M) (pH 4.5). A small sample (corresponding to about 10 mg pulp dry weight) was suspended in 1 ml. The pulp was then applied to the surface of the glassy carbon electrode by pressing the pulp against the bottom of the electrochemical cell. Cycling in an electrochemical cell containing 0.5 mM ABTS in citrate buffer solution, platinum wire auxiliary electrode, silver / silver chloride reference electrode, pulp sample immobilized on the surface of working carbon electrode. Formula voltammetry was performed at a scan rate of 2 mV / sec. ABTS / ABT
The current intensity (I _k ) at the anode peak of S. ⁺ (520 mV) and ABTS. ⁺ / ABTS ²⁺ (920 mV) was measured and plotted against the Kappa number of pulp samples. 4a-c show graphs of the variation of the peak current intensity I _k of ABTS (0.5 mM) with the kappa number of softwood kraft pulp. In FIG. 4a, all the softwood pulps from both the Kraft cooker and the oxygen stage are plotted together, and the I _k vs. Kappa number plot is non-linear over the entire range of Kappa numbers. Is. However, softwood pulp from the craft steamer (Fig. 4b
) And the softwood pulp after the O ₂ delignification step (FIG. 4c) are plotted separately, a good linear relationship can be seen between I _k for both ABTS peak potentials and the Kappa number of pulp samples. . The error line shows the standard deviation of three measurements.

Example 2 Effect of changing the concentration of ABTS Oxygen delignified softwood kraft pulp was prepared and treated as described in Example 1. However, the concentration of ABTS used for the electrochemical measurement was lowered to 0.2 mM. Voltammetric determinations were performed as described in the previous example. FIG. 5 shows 9
It shows good linearity between the peak current of ABTS at 20 mV and the Kappa number of all oxygen delignified pulps when ABTS is used at low concentration. However, the linearity of the line showing the peak current intensity at 520 mV is low and it appears to separate as the Kappa number of the pulp becomes lower.

Example 3 Electrochemical Measurement of Hardwood Kraft Pulp Kappa Number Mixed hardwood kraft pulp is cooked in a pilot plant kraft cooker with a mixture of maple, hippo and poplar wood chips to a Kappa number in the range of 16-21. It was manufactured by Pulp samples were prepared as described in Example 1 and cyclic voltammetry of the pulp with 0.2 mM ABTS was performed as described in Example 2. As with softwood pulp, FIG. 6 shows a linear relationship between both catalytic current intensities of ABTS and Kappa number of hardwood kraft pulp.

Example 4 Application of electrical Kappa number determination for the ECF bleaching continuous process Ezo pine kraft pulp with a Kappa number of 31.8 obtained from a pilot plant cooker was treated with chlorine dioxide (D ₁₀₀ stage) and then Alkali extraction (
Step E). 0.05 to 0.20 active chlorine multiple
e multiple) (ACM) by applying different amounts of chlorine dioxide, 8-23.
Partially bleached pulp with an extraction Kappa number in the range of 5 was obtained. Pulp samples were prepared as described in Example 1 and cyclic voltammetry of pulp and ABTS (0.2 mM) was performed as described in Example 2. The results of the linear regression plot of the catalyst current intensity at 520 and 920 mV against the Kappa number of these bleached pulps are shown in FIG.
Shown in.

Example 5 Use of an inorganic redox mediator for electrical Kappa number determination In this example, potassium octacyanomolybdate was used as the inorganic redox mediator to determine the Kappa number of pulp samples. Oxygen delignified softwood kraft pulp was prepared and processed as described in Example 1. However, ABTS was prepared by adding K ₄ Mo (CN) ₈ (0.2 m) in sodium acetate buffer solution (0.1 M, pH 4.5).
It was replaced by the solution containing M). This vector had only one redox pair (E ⁰ = 0.55V) within the range of potentials used to measure Kappa number. A cyclic voltammogram of octacyanomolybdate in the presence of kraft pulp at various kappa numbers is shown in FIG. 8A, and a linear regression of peak current intensity (I _k ) against kappa number for pulp samples is shown in FIG. 8B.

Example 6 Effect of Pulp Sample Size on Voltammetric Response To demonstrate the importance of pulp sample size on the medium peak current intensity,
Cycling voltammetry experiments with ABTS (0.2 mM) were performed in the presence of varying amounts of oxygen delignified softwood pulp (Kappa number = 21.4). As shown in Figure 9, both ABTS peak current intensities decreased only very slightly with the amount of pulp applied to the electrode surface. For a pulp sample size of 8-12 mg, the variation of I _k is within standard deviation.

Example 7 Oxidation State Characterization of Residual Lignin The peak current ratio for two oxidation states of ABTS (ie 520 and 920 mV) can be used as an indication of the oxidation state of residual lignin in pulp. The increase in peak current intensity at 520 mV corresponds to the amount of more easily oxidised residues in the lignin such as phenolic groups, whereas the increase in current at 920 mV leads to a higher redox potential of lignin. It seems to be related to the group that it has. Thus, a pulp with a large ratio of peak current intensity at 520 mV to that at 920 mV shows that the pulp can oxidize even more easily than pulp with a lower peak ratio. This kind of instruction
More rational use and savings of bleaching chemicals can be brought about. 10, the peak current ratio of ABTS (0.2 mM) in the presence of various pulps _{(I k, 520m V / I} k, 920mV) shows a graph of. The large ratios as in hardwood kraft pulp indicate that hardwood kraft pulp at the same Kappa number is more easily oxidized or bleached than softwood pulp. Furthermore, these results show the difference in the oxidation state of softwood kraft pulp when delignified to the same Kappa number by chlorine dioxide followed by extraction stage (DE) or oxygen stage (SWO ₂ ). There is.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing measurement of pulp Kappa number by cycle voltammetry according to the present invention.

FIG. 2 is a schematic representation of an electrochemical method with a mediator pair of the present invention for determining pulp Kappa number.

3 is a graph showing cyclic voltammetry of redox mediator ABTS alone, and FIG. 3b is such a graph in the presence of kraft pulp.

FIG. 4 is a graph showing the relationship between peak current intensity and kappa number for different pulps in FIGS. 4a, 4b, and 4c.

Fig. 5 Kappa number of softwood oxygen delignified kraft pulp and ABTS (0.2 m
It is a graph showing the relationship between the peak current intensities (I _k) of M).

[Figure 6]   Peak current intensity of mixed hardwood kraft pulp and ABTS (0.2 mM) (I _k ) Is a graph showing the relationship with.

FIG. 7: Chlorine dioxide and alkali extraction step (D ₁₀₀ ) of partially bleached kraft pulp.
E) the kappa number after is a graph showing the relationship between the peak current intensities (I _k) of ABTS (0.2 mM).

8 is a graph showing cyclic voltammetry of redox mediator potassium octacyanomolybdate in the presence of softwood kraft pulp, and FIG. 8b is a graph showing pulp Kappa number and peak current. It is a graph which shows a linear relationship with intensity ( _Ik ).

FIG. 9 is a graph illustrating the effect of pulp sample size on ABTS peak current intensity.

FIG. 10 is a graph showing the relationship between the peak current ratio of ABTS (I _{k, 520 mV} / I _{k, 920 mV} ) and the type of pulp or the degree of delignification.

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Claims (12)

[Claims] 1. A method for determining the Kappa number or lignin content of pulp, comprising: i) in an electrochemical cell, the presence of an oxidation state redox mediator effective to oxidize lignin in the pulp, Contacting the sample with a voltammetric electrode to form a reduced state of the redox mediator, causing a current discharge at the electrode and reoxidizing the redox mediator, ii) a voltammetric current discharge at the electrode. Measure the peak intensity,
And iii) determining the Kappa number or lignin content of the pulp sample from the peak intensity measured in ii) above. 2. The redox mediator is a reversible redox mediator, Ag /
The method of claim 1, wherein the redox couple is formed at a potential of 0.3-1.2 V, as measured against an AgCl reference electrode. 3. The method of claim 2, wherein the redox mediator is water soluble. 4. The method according to claim 1, wherein the redox mediator is 2,2′-azinobis (3-ethylbenzthiazoline-5-sulfonate). 5. The method of any one of claims 1-3, wherein the redox mediator is potassium octacyanomolybdate. 6. A method for analyzing the lignin content of pulp, comprising: a) mixing a sample of pulp with a redox mediator; and b) placing the resulting sample in contact with a working electrode in a cyclic voltammetry device, c) measuring the intensity of the peak current generated at the electrode, and d) converting the peak current into kappa number from calibration data determined by pulp having a known lignin content. 7. The mediator is an organic or inorganic compound capable of forming a redox couple at a potential of 0.3 to 1.2 V as measured against an Ag / AgCl reference electrode (RE). Item 6. The method according to Item 6. 8. The method of claim 6 or 7, wherein the redox mediator is a water soluble, reversible redox mediator. 9. The method of claim 6 wherein the mediator is ABTS and the ratio of the two peak currents measured at 0.52V and 0.92V is used to generate information about the oxidation state of lignin in the pulp. 8. The method according to any one of item 8. 10. The method of claim 1, wherein the redox mediator is 2,2′-azinobis (3-ethylbenzthiazoline-5-sulfonate). 11. The method of any one of claims 1-3, wherein the redox mediator is potassium octacyanomolybdate. 12. On-line for determining Kappa number based on the principles of any of claims 1 to 11, in combination with established techniques for sampling and washing pulp samples. Automated method.