ES2208445T3 - ELCTROCHEMICAL PROCEDURE TO DETERMINE THE LIGNIN CONTENT OF THE CELLULOSE PASTE. - Google Patents

Abstract

A method for determining the kappa index or lignin content of a cellulose pulp, which comprises: i) contacting a sample (26) of the cellulose pulp with a vathometric electrode (18) within an electrochemical cell ( 12) in the presence of a reduction-oxidation mediator (24) in the oxidation state that is effective to oxidize lignin within the cellulose pulp, with formation of a reduced state of said reduction-oxidation mediator (24) and a current of discharge to said electrode, which re-oxidizes the reduction-oxidation mediator, ii) the measurement of the peak intensity of the discharge current (Ip) by means of wattmetric means in said electrode (18), and iii) the determination of the kappa index or the lignin content of the cellulose pulp sample (26) based on the peak intensity (Ip) measured in ii).

Description

Electrochemical procedure to determine the lignin content of cellulose pulp.

Technical field

The present invention relates, in general, to the pulp and paper industry and, in particular, to new electrochemical procedure to measure the content of lignin and / or the kappa index of cellulose pastes and Lyes used in the manufacture and bleaching of pulp.

Prior art

In order to produce a kraft paste of excellent and uniform quality it is necessary to regulate efficiently the manufacturing processes and bleaching of pulp. The measurement of the cellulose pulp kappa index is currently used to monitor and regulate cooking conditions, and to calculate the amount of bleaching chemicals that are they have to add to get the reference point brightness of Cellulose pulp, without waste of chemical products. In addition, paper mills that employ a stage of oxygen delignification also require reliable measurements of the kappa index in order to avoid a delignification excessive and fiber degradation. Index measurement kappa, with the standard valuation procedure with sodium permanganate, takes several hours. The time that elapses between the measurement of the kappa index and the modification of the process no It is compatible with an efficient process control strategy, which requires uninterrupted monitoring of the quality of cellulose pulp Automatic analyzers of the Kappa index in the production line they are able to measure the kappa index in a few minutes, so that changes in the process are can run quickly for commercial applications have developed several automated indica analyzer systems kappa. They all measure the optical properties of the fibers of cellulose pulp In the Opti-Kappa analyzer, discovered by STFI (Kubulnieks et al., Association Magazine Pulp and Paper Technique (TAPPI), November 1987, pp. 38-42) UV light absorption is used, while in other systems, which work under a similar principle, the reflection of UV light is used instead of absorption. These systems were devised by BTG Inc. (Agnéus and Damlin, Meetings of the XXIV Eucepa Conference, Stockholm, Sweden, 1990, pp. 234-241), Valmet Automation Kajaani Inc. (Olilla and Erkkilä, TAPPI Meetings in the Conference on Quality and Products and Exhibition Industrial of 1997l, Jacksonville, Fl, pp. 177-122), and Honeywell-Measurex (Van Fleet and Whalley, TAPPI Meetings at the Conference on the Paper Pulp Manufacturing 1998, pp. 1509-1512) and US Pat. number 5,953,111. The fluorescence measurement (Berthold et al, U.S. Patents: Nos. 5,216,483 and 5,220,172), and change of fluorescence color (Jeffers and Malito, patent of the USA no. 5,486,915), were applied to measure the content of lignin in cellulose pulp. In general measurement systems based on optical devices are very sensitive to variations in the consistency of cellulose pulp. In this way, in order to obtain an accurate measurement of lignin, it is also required Strict control of the consistency of cellulose pulp.

The use of electrochemical sensors, within a wide range of applications, such as monitoring Environmental, industrial quality control and analysis Biochemical, they are common. In the pulp and paper industry ammeter / polarographic sensors and sensors are used potentiometric detectors to measure the concentration of various oxidizing or reducing chemicals in the bleach manufacture and bleaching of pulp. That way, with the Polarographic procedures can be measured, in a way selective and quantitative, sulfur compounds, sulfite, thiosulfite and sulfihydrile in the lyes of the kraft paste (Noel, Tappi Magazine 61 (5), pp. 73-76, 1978) chlorine dioxide and chlorine in bleach spent on bleaching (Williams and Williamson, in: Dence, C.W and Reeve, D.W., editors, Bleaching of cellulose pulp, Principles and Practice, Tappi Press, 1996, pp. 625-645). However these sensors are not able to measure lignin inside the fiber, because there needs to be intimate contact between lignin and the electrode surface in order to allow free passage to the electrons Recent discoveries in the field of money laundering enzymatic kraft paste have shown that the enzyme laccasa can delignify the kraft paste and oxidize the compounds of the sample of lignin, when a mediator of the reduction-oxidation is present (Borbonnais and Paice, FEBS Lett 267: 99-102, 1990, and App. from Microbiological Biotechnology 36: 823-827, 1992). The reduction / oxidation mediator acts as a carrier of diffusible electrons between the residual lignin in the wall of the fiber and the large laccasa molecule in solution. The studies electrochemicals of the interaction between lignin and mediators of oxidation reduction have shown that the mediator, oxidized, by electrochemical means regenerates continuously on the electrode surface following its reduction by means of lignin type components (Bourbonnais et al., Proceedings of Biof. And Biochemistry, 1379: 381-390, 1998). It was shown that the kinetics of the oxidation reduction reaction was related to the concentration of the sample compound of the lignin

In the document DE-A-1974278 describes the use of ABTS reduction-oxidation mediator in combination with cellulose pulp to increase the delignification of cellulose pulp, resulting in bleaching reinforced through the use of a mediator of the oxidation reduction

In the document WO-A-99 54545 discloses a transition metal complex as mediator of the oxidation reduction, which can be added to a cellulose pulp

The purpose of the present invention is, without However, the application of wattmeter procedures to a cellulose pulp suspension, in the presence of a mediator reduction-oxidation of soluble lignin, for Measurement, quantitative and rapid lignin content in pasta Kraft

Description of the invention

An embodiment of this invention provides a new procedure to measure the kappa index or the content of lignin in kraft pastes, based on the wattmeter measurement of catalytic reactions that involve the use of lignin and mediators of reduction-oxidation. This procedure electrochemical comprises measuring the speed of regeneration of a catalyst reduction-oxidation of lignin in the presence of cellulose pulp, following its oxidation in an electrode Vathometric The intensity of the catalytic current, generated in The electrode surface is proportional to the quantity and the oxidation state of lignin present in cellulose pulp. Linear relationships between kappa indices and intensities of the current that is generated can be obtained within various ranges of kappa indexes and types of cellulose pulp. The reduction-oxidation characteristics and the concentration of the meter, and the scan rate of the voltage Vathometric procedure are parameters that can be tuned with the linear relationships obtained from the various pastes of cellulose.

According to one aspect of this invention, it is provided a procedure to find out the kappa index or content of lignin in a cellulose paste, which comprises: i) contact of a sample of cellulose pulp with a wattmeter electrode within an electrochemical cell, in the presence of a mediator of effective reduction-oxidation in a state that is effective to oxidize lignin inside cellulose pulp, the formation of a reduced state of said mediator of reduction-oxidation and a discharge current of said electrode, which reoxidizes the mediator of oxidation reduction; ii) the vaimetric measurement of the peak intensity of the discharge current in said electrode, and iii) the determination of the kappa index or lignin content of the cellulose pulp sample starting from the peak intensity at ii).

A procedure is also described for analyze the lignin content of a cellulose paste, which comprises: a) mixing a sample of the cellulose pulp with a reduction-oxidation mediator; b) the placement of the resulting sample in contact with an electrode of work within a cyclic vatimetry apparatus, c) measurement of the intensity within the peak current that is generated in the electrode, and d) the conversion of the peak current into the index kappa based on the calibration data determined with Cellulose pastes whose lignin content is already known.

Brief description of the drawings

Figure 1 illustrates, schematically, the measurement of the cellulose pulp kappa index by cyclic vatimetry, according to this invention.

Figure 2 illustrates, schematically, a electrochemical procedure of coupled mediator, to find out the kappa index.

Figures 3a and 3b illustrate, graphically, the cyclic vatimetry of an ABTS mediator of oxidation reduction only (figure 3a) and in the presence of a kraft paste (3b).

Figures 4a, 4b and 4c illustrate, in a way graph, the relationship between the intensities of the peak current and the kappa index, corresponding to various pastes of cellulose.

Figure 5 is a graphic illustration of the relationship between the kappa index of cellulose kraft pastes of conifers, oxygen delignified, and the intensities of the peak current (Ik) of ABTS (0.2 mM).

Figure 6 is a graphic illustration of the relationship between kraft pastes, mixed hard cellulose, and peak current intensities (Ik) of ABTS (0.2 mM).

Figure 7 is a graphic illustration of the relationship between the kappa index of bleached kraft pastes, in part, after the chlorine dioxide extraction stages and alkaline (D 100 E) and peak current intensities (I_ {k}) of ABTS (0.2 mM).

Figure 8a illustrates, in a graphic way, the cyclic vatimetry of a mediator of oxidation reduction, potassium octacyanomolibdate, in presence of coniferous cellulose kraft pastes;

Figure 8b illustrates the linear relationship between the Kappa index of cellulose pulp and the intensities of peak current (Ik);

Figure 9 illustrates, in a graphic way, the effect of cellulose pulp size on the intensities of the peak current of ABTS;

Figure 10 illustrates, in a graphic way, the relationship between the proportion of the peak current (I_ {k ,. 250 mV} / I_ {k, 920 mV}) of ABTS and the type of cellulose pulp or degree of delignification.

Detailed description of the invention and description of the preferred embodiments

The principle of this invention is based on the combination of an electrochemical procedure, such as that of the cyclic vatimetry (CV), with the use of soluble mediator of oxidation reduction that catalyzes the oxidation of the lignin

As illustrated in Figure 1a, the apparatus Basic of the vatimetry 10 includes an electrochemical cell 12, a potentiometer 14 and a recorder 16. Cell 12 has a working electrode 18, an auxiliary electrode 20 and an electrode of reference 22; a solution of a mediator of reduction-oxidation 24 is housed in cell 12 and a sample 26 of cellulose pulp, mixed with mediator 24 is held on the surface of the working electrode 18. During use, potentiometer 14 simultaneously generates a linear potential scan at work electrode 18 and measures the current resulting from oxidation or reduction of the mediator 24, which is an active compound of reduction-oxidation inside the cell electrochemistry 12.

The intensity of the peak current ( I p ), measured on the surface of the working electrode 18, by means of the recorder 16, is directly proportional to the concentration C of the objective analytic, as shown by the equation from Rancles-Sevcik:

I_ {p} = 2.69 x 10 5) n 3/2 AD 1/2 v 12 C

in which n is the number of electrons, A is the area of the working electrode, D is the diffusion coefficient of the analytical, and v is the speed of tension exploration.

This relationship applies to soluble reduction-oxidation pairs but cannot be used directly in lignin within the cellulose pulp fibers because there is no direct contact between the lignin and the electrode surface. However, by adding a soluble reduction-oxidation mediator, the electrode reaction can be effectively coupled to the lignin within the cellulose pulp fibers. During the vatometric process, the electroxidized mediator diffuses into the cellulose pulp fibers to react reversibly with lignin. The reaction with lignin regenerates the reduced mediator on the electrode surface, resulting in an increase in current compared to that of the mediator alone. Using a fixed concentration of mediator, the current measured at the peak potential of the mediator in the presence of pulp (I {k}) is proportional to the amount of lignin or the kappa number of the pulp ( see figures 2 and 3).

This invention can be applied to measure the lignin content or the kappa index of cellulose kraft pastes of conifers, hardwood or sulphite, either during the stage of cooking or bleaching. It is preferable to wash the paste Cellulose with water or with an electrolytic buffer. After the Pressed or filtered a fixed amount of pasta is mixed cellulose with a fixed amount of mediating solution 24 to form sample 26. For this purpose, the mediator may be a organic or inorganic compound which can form a pair reduction-oxidation in a potential included between 0.3 and 1.2 volts, measured compared to the electrode of reference (RE) of Ag / AgCI 22. Sample 26 of the suspension of Cellulose paste and mediator is applied and pressed on the surface of the working electrode (WE) 18 and is soaked inside the cell electrochemistry 12 containing the mediating solution 24, the reference electrode (RE) 22 and auxiliary electrode (AE) 20 (Figure 1). Typically, determinations through cyclic vatimetry are carried out with a wattmeter analyzer BAS CV-50W (Bioanalytical Systems, Inc., Indiana. USA) with a reference electrode 22 of Ag / AGCl, an electrode auxiliary platinum wire 20 and a working electrode 18 of 3 mm diameter vitreous carbon. It can also be used, for this invention, any combination of electrochemical cells 12, electrodes 18, 20 and 22 and potentiostats 14, suitable for electrochemical analysis

The mediator of oxidation reduction should be conveniently reversible so that it can be changed repeatedly between the reduced state and the oxidized state. The most convenient is to employ reduction-oxidation mediators that are soluble in water.

The wattmeter analyzer can automatically perform measurements of the range and speed of the voltage and peak potential scans. As an example, the appropriate meters include 2,2'-azinobis (3-ethylbenciazoline-5-sulfonate) (ABTS), as organic mediator, and potassium octacyanomolibdate [K4 Mo (CN) 8 ], as inorganic mediator. ABTS is commercially available and K4 Mo (CN) 8 .2H2 O can be prepared according to Furman and Miller (Sint. Inorganic 3,160-163, 1950). The organic mediator that has been preferred to use to illustrate this invention is ABTS which has two stable and reversible oxidation reduction pairs within a potential range of stresses suitable for oxidizing lignin. The first anodic peak, which appears when the electrode potential reaches 0.52 V, corresponds to the oxidation of ABTS to its cationic radical (ABTS. +), While the second peak at 0.92 V corresponds to the formation of the dication (ABTS 2+). Figure 3 shows cyclic vatigrams at a slow rate of potential exploration (2 mV / sec.) With, a) 0.2 mM ABTS in 0.1M sodium citrate buffer, pH 4.5, and b) ABTS in the presence of coniferous cellulose kraft paste. By comparing the increase in peak intensities of the anodic current of ABTS in the presence of cellulose pulp ( I k , catalytic current) when with ABTS alone ( I d, diffuser current) the extent of the reactions is illustrated. they occur between the two oxidized forms of ABTS and the residual lignin or the kappa index of the kraft paste.

Examples Example 1 Measurement of the kappa indica of cellulose kraft pulp from conifers with 0.5 mM ABTS

Kraft pulp of cellulose was prepared conifers with a kappa index in the order of 32 to 87 by the cooking of black spruce shavings from eastern Canada to various H factors in a discontinuous installation bleach experimental. Additional delignification of kraft pastes from black spruce was carried out with a chemical reactor pressed with oxygen at laboratory scale, at various reaction times, temperatures and alkaline charges. The pulp of oxygen delignified conifers (SWO2) that were obtained they had kappa indices ranging from 11 to 29. All Kappa indices of cellulose pastes were measured by volumetric analysis with permanganate according to the procedure of Tappi T236 test.

The cyclic vatimetry of the pastes was carried out as follows: After washing the cellulose pulp with water, a small sample (equivalent to approximately 10 mg of dried pasta in the oven) was suspended in a solution of 1 ml of ABTS (0.5 mM) in 0.1M sodium citrate, pH 4.5). The paste was then applied to the surface of a vitreous carbon electrode by pressing the paste against the bottom of the electrochemical cell. Cyclic vatimetry was performed at a scanning speed of 2 mV / sec. within an electrochemical cell containing a solution of ABTS (0.05 mM) in a citrate buffer, a platinum wire auxiliary electrode, a silver / plat chloride reference electrode, and a fixed cellulose paste sample on the surface of the working carbon electrode. Current intensities ( I x ) were measured at the anodic peaks of ABTS / ABTS. + (520 mV) and ABTS. + / ABTS 2+ (920 mV) and they were plotted compared to the kappa indices of the cellulose pulp samples. Figures 4a to 4c show graphs of the variation of the intensities of the peak current Ik of ABTS (0.5 mM) with the kappa index of the coniferous cellulose kraft pastes. In Figure 4a, where all the coniferous cellulose pastes, from the stages of the kraft pulp bleach and oxygen, have been plotted together, the plot of the I_ {k}, in comparison with the kappa index, does not It is linear across the entire range of the Kappa index. However, when coniferous cellulose pulps from the kraft pulp bleach (Figure 4b) and coniferous pulp pulp are drawn separately after the delignification of the O2 stage (Figure 4c), You can see that there is a good correlation between Ik for both the potential ABTS peak and the kappa index of cellulose pulp samples. Error bars indicate the standard deviation of triplicate measurements.

Example 2 Effect of ABTS concentration changes

Cellulose kraft pastes were prepared and treated of conifers, delignified with oxygen, as described in Example 1, except that the ABTS concentration that was used for electrochemical measurements it was lowered to 0.2 mM. The Vimetric determinations were carried out as indicated in The previous example. A good linearity is shown in figure 5 between the peak current of ABTS at 920 mV and the kappa index of all cellulose pulps delignified with oxygen when Use ABTS with a lower concentration. However the curve which describes the intensities of the peak current at 520 mV is less linear and seems to be uneven for cellulose pastes with lower kappa.

Example 3 Electrochemical measurement of the kappa kraft pulp index of coniferous cellulose

A kraft paste of cellulose mixture of conifers was prepared by cooking a chopped mixture Maple, birch and poplar loggers in kraft bleach experimental installation at kappa indices that varied from 16 to 21. Cellulose pulp samples were prepared as described in example 1 and paste vatimetry was carried out and of ABTS 0.2 as described in example 2. So with respect to coniferous cellulose pastes in figure 6 is shows a linear relationship between both the intensities of the ABTS catalytic current as the kappa index of kraft pastes of hard cellulose.

Example 4 Electro-kappa measurement application for ECF bleaching sequences

A black spruce kraft paste with an index 31.8 kappa, obtained from an experimental installation bleach, it was treated with chlorine dioxide (step D 100), followed by alkaline extraction (stage E). Various amounts of chlorine dioxide equivalent to multiples of active chlorine (ACM) between 0.05 and 0.20 to obtain partially cellulose pastes bleached with extracted kappa indices that varied from 8 to 23.5. Cellulose pulp samples were prepared as indicates in example 1 and the cyclic vatimetry of the paste and the ABTS (0.2 mM) was performed as described in example 2. The results of the linear regression curves of the intensities of the catalytic current at 520 and 920 mV compared to the index Kappa of these bleached pastes are shown in Figure 7.

Example 5 Use of the oxidation reduction mediator for electro-kappa measurement

In that example, potassium octacioanomolibdate was used as a reduction-oxidation mediator to measure the kappa index of cellulose pulp samples. Prepared and treated, as described in Example 1, coniferous cellulose kraft pastes, delignified with oxygen, except that the ABTS was exchanged for a solution of K4 Mo (CN) 8 (0, 2 mM) in a sodium acetate buffer (0.1M, pH 4.5). This mediator was only a reduction-oxidation pair (Eº = 0.55 V) within the order of the potential used to measure the kappa index. Cyclic octacyanomolibdate vatigrams in the presence of kraft pulp with various kappa indices are shown in Figure 8A, and the linear regression of peak current intensities ( I x ) compared to pulp samples is illustrated in Figure 8B .

Example 6 Effect of paste sample size on response vammetric

In order to verify the importance of the cellulose pulp sample size in the intensities of the mediator current, cyclic wattmeter experiments with ABTS (0.2 nM) were carried out in the presence of various amounts of a cellulose pulp of conifers that are delignified with oxygen (kappa index = 21.4). As shown in Figure 9, both intensities of the ABTS peak current only decrease very slightly with the amount of paste applied to the electrode surface. The variations of the I_ {x} are within the standard deviation for the sizes of cellulose pulp samples between 8 and 12 mg.

Example 7 Characterization of the oxidation state of lignin residual

The relationship between peak currents for two ABTS in oxidation state (i.e. 520 and 920 mV) can be use of oxidation status indicator of residual lignin in cellulose pulp. The increase in current intensity peak at 520 mV corresponds to the amount of waste plus easily oxidizable in lignin, such as groups phenolic, while the increase in current at 920 mV is most likely related to groups of reduction-oxidation of higher potential. This way, a pulp of cellulose with a high ratio of the peak current intensity at 520 mV, above that of 920 mV, indicates that cellulose pulp can be oxidized with more ease than a paste that has lower peak ratios. East type of indication can lead to more rational employment and to saving of bleaching chemicals. In figure 10 it illustrates a graph of the ABTS peak current ratio (0.2 mM) (I_ {k, 520mV} / I_ {k, 920mV}) in the presence of several pastes cellulose. A high ratio, such as kraft pasta from hard cellulose, indicates that with the same kappa index, a paste hard cellulose kraft oxidizes or bleaches more easily than a pulp of coniferous cellulose. In addition, these results also indicate that there is a difference in the oxidation state of a coniferous cellulose kraft paste when it is delignified, at the same kappa index, with chlorine dioxide, followed by a stage of extraction (DE) or with an oxidation stage (SWO2).

Claims (9)

1. A procedure to determine the index kappa or the lignin content of a cellulose paste, which understands:

i)

put in touch a sample (26) of the cellulose pulp with an electrode vammetric (18) inside an electrochemical cell (12) in presence of a reduction-oxidation mediator (24) in oxidation state that is effective to oxidize lignin within cellulose pulp, with formation of a reduced state of said reduction-oxidation mediator (24) and a current  discharge to said electrode, which reoxidizes the mediator of oxidation reduction,

ii)

measuring the peak intensity of the discharge current (I_ {p}) by means wattmeters in said electrode (18), and

iii)

the determination of the kappa index or the lignin content of the pasta sample of cellulose (26) starting from the peak intensity (I_ {p}) measured in ii).

2. A method according to claim 1, in which said reduction-oxidation mediator (24) it is a reversible oxidation reduction mediator and forms a reduction-oxidation pair to a potential between 0, 3 and 1.2 volts, when measured in comparison with the Ag / AgCl reference electrode (22). 3. A method according to claim 2, in which said reduction-oxidation mediator (24) It is soluble in water. 4. A method according to claim 1, 2 or 3, wherein said mediator of oxidation reduction (24) is 2,2'-anobis (3-ethylbenciazoline-5-sulfonate). 5. A method according to claim 1, 2 or 3, wherein said mediator of oxidation reduction (24) is octacyanomolibdate potassium 6. A method according to claim 1, in which the reduction-oxidation mediator (24) is an organic or inorganic compound which can form a pair oxidation reduction at a potential between 0.3 and 1.2 volts when measured compared to the electrode of reference (RE) of Ag / AgCl (22). 7. A method according to claim 6, in which said reduction-oxidation mediator (24) is a oxidation reduction mediator reversible. 8. A method according to claim 6 or 7, in which the oxidation reduction mediator (24) is ABTS and in which a relationship of two currents is used peak (Ip) measured at 0.52 V and 0.92 V to generate information on the oxidation state of lignin in cellulose pulp. 9. An automated online procedure for determination of the kappa index based on the principles that indicated in any of claims 1 to 8, with technology the one established for sampling and washing the paste of cellulose.