The method of multi-stage bleaching of pulp

 

(57) Abstract:

The inventive after sequential processing of the pulp suspension nitrogenous polycarboxylic acid with a pH of 3.1 to 9.0 and a temperature of 10 To 100C, washing with water and processing a compound containing peroxide, it is treated with halogen-containing substance and the exhaust seloc from this stage recycle to one of the previous stages. As the halogenated substance use chlorine dioxide, and as a nitrogen-containing polycarboxylic acid is used diethylenetriaminepentaacetate or ethylenediaminetetraacetic acid. 4 C.p. f-crystals, 7 PL.

The invention relates to the pulp and paper industry.

In the manufacture of pulp obtained by chemical treatment, high degree of whiteness, wood chips are first boiled for separation of cellulose fibers. During the cooking portion of the lignin that holds the fibers together, split and mutate so that it can be removed by successive washing. However, in order to achieve a sufficient degree of whiteness, most of the lignin must be removed along with the worsening white (chromophore) groups. This part of the reach p is a sequence of bleaching for didgeridoonas pulp, containing lignocellulose, for example, from Kraft softwood is (C+D) F1D E2, D, where (C + D) stage chlorine/chlorine dioxide, E. alkaline extraction stage, On stage chlorine dioxide, (C+D) and (E1the stage is defined as the pre-whitening. Sequence D E2D is called the final bleaching.

If you use alkaline oxygen stage to the sequence of pre-whitening in the multistage bleaching, for example, kraftzentrale, it is possible to reduce the discharge of more than half of the original number, because the spent liquor oxygen bleaching, chlorine-free, is returned. However, after stage oxygen delignification of lignin remaining in the pulp mass is about half of the amount remaining after deliveryone in the method of cooking, which, at least in part should gradually disappear from the pulp. This is achieved subsequent bleaching.

The bleaching of pulp obtained by chemical treatment, for the most part carry out chlorine bleaching agents such as chlorine, chlorine dioxide and hypochlorite, resulting in trebst last complicates the creation of a closed loop on otbelivayushe plant, and halogen-substituted organic matter form a plum, harmful to the environment. Therefore, at present there is a tendency in the direction of use in the most possible within bleaches, pure or free from chlorine,to reduce plums and make possible the regeneration of spent liquor. Examples of such bleaching agents are hydrogen peroxide, for example, inorganic peroxides such as hydrogen peroxide and sodium peroxide, and organic peroxides such as peracetic acid. The formation of substances that are harmful to the environment, particularly noticeable in the pre-bleaching, in which the lignin content is high. Therefore, the greatest impact has the transition to a bleaching substances that are less harmful to the environment, such as, for example, peracid hydrogen used in the pre-bleaching. In practice, however, the hydrogen peroxide is not used significantly in the first stage of multi-stage bleaching with obtaining initial reduction of lignin and/or increase the degree of whiteness because of the need to add large quantities of hydrogen peroxide.

Thus, large quantities of peroxide of verslagen lignin, since this treatment gives a high degree of decomposition of hydrogen peroxide, leading to significant cost chemicals. In the acid treatment with hydrogen peroxide the same decomposition of lignin can be obtained as in the alkaline treatment with a lower consumption of hydrogen peroxide. However, acid treatment leads to a significant drop in the viscosity of the pulp, i.e. the decomposition of hydrogen peroxide at low pH values affect not only lignin but also on cellulose that the length of the hydrocarbon chains is reduced, leading to deterioration prognostic properties of pulp.

According to the patent Sweden N 420430 this drop in viscosity in acid treatment hydrogen peroxide can be avoided by performing it in the presence of complexing agents, such as, for example, DTRA (diethylenetriaminepentaacetic acid), pH 0.5-3. At this stage of processing should stage alkaline extraction to remove decomposed lignin without intermediate rinsing.

The purpose of the various stages of pre-processing is to reduce the lignin content in front of a pair of chlorine stage so fast, shigeie needs in practical connections) in spent bleaching liquor. Examples of ways in which the Kappa number (which is a measure of the lignin content is reduced, is a modification of the method of cooking or the use of a combination of oxygen and nitrogen compounds in accordance with the so-called PRENOX-way. However, these methods require uneconomically large investment. Value AOH may be reduced by substitution of (C + D) stage in the usual sequence of bleaching stage D. this changes the number of educated harmful products drain is significantly reduced. This is reasonable, although it usually requires a higher loading of chlorine dioxide per ton of pulp to reduce the lignin content to the required low level before the subsequent bleaching. Possibility of obtaining factory bleaching device, which is more introverted, very limited, formerly known as free chlorine chemicals) ways of pre-processing or include stage acid treatment, or involve unacceptable additives from the point of view of regeneration. To overcome these technical problems in the way you need to install expensive equipment. In the proposed izopet the ordinary to get the lowest possible value Oh and give the product the same or even improved quality.

There is a method of multi-stage bleaching pulp, comprising processing the pulp slurry complexing agents nitrogen-containing polycarboxylic acid at pH 3.1 to 9.0 and a temperature of 10-100aboutWith the wash water and processing a compound containing peroxide at pH 7-13.

However, this method is not effective enough.

An object of the invention is to increase the whiteness of the pulp while reducing the number of adsorbed organic Halogens in waste waters.

This is achieved by the fact that in the method of multi-stage bleaching of pulp after treatment with compound containing peroxide, the pulp suspension is treated with halogen-containing substance and the spent liquor from this stage recycle to one of the previous stages. This method provides significantly fewer discharges from existing plants whitening, because the amount of halogenated chemicals can be reduced while maintaining the quality of pulp in relation to the degree of whiteness, viscosity, Kappa number and strength properties.

The method in accordance with the invention is preferably used in such processing is the processing of complex-forming substance and a substance peroxide, in accordance with the invention, may be either directly after cooking, the pulp, or after the oxygen stage.

In the proposed method, the first stage is conducted respectively at pH from 4 to 8 (preferably 5 to 7) and the second stage preferably at pH 8 to 12.

Used complexing substances mainly include nitrogen polycarboxylic acids, respectively diethylenetriaminepentaacetic acid (DTRA), ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA), preferably DTRA or EDTA, polycarboxylic acid, preferably citric or tartaric acid, phosphinic acid, preferably diethylenetriaminepentaacetic acid or polyphosphates. Used a substance containing the peroxide is preferably hydrogen peroxide or a mixture of hydrogen peroxide and oxygen.

Processing in accordance with the invention preferably includes a step of rinsing between the two stages of processing, so complex associated metals are removed from the pulp suspension prior to the peroxide stage.

The halogenated otbelivaut or alkaline-earth metals and hypochlorites of alkali metals or alkaline-earth metals and substances fluorine, bromine and iodine. Halogen-substituted organic matter related to dissolved organic molecules from wood, in which the halogen is included in the molecule during the processing of halogen-containing bleaching chemicals. Examples of such organic substances are cellulose, hemicellulose, aromatic and alifaticheskii astatkie lignin. Examples galoisienne organic substances are chlorinated lignin residues, where it is especially difficult to degrade aromatic substances.

Final bleaching can be performed with chlorine and/or chlorine dioxide in one or more stages, possibly with an intermediate stage of extraction. For convenience, use only technical chlorine dioxide, as in this case, education AOH per 1 kg of bleach, calculated as active chlorine, is only the fifth part of molecular chlorine. Technical chlorine dioxide refers to the chlorine dioxide obtained by the conventional method, without the external addition of chlorine. In other words, the chlorine dioxide may contain chlorine formed during retrieval and dissolved in water absorbent. One of the examples of industrial processes in which a certain amount of chemiluminescence is how, for example, sulfur dioxide and methyl alcohol, give only a small amount of chlorine. Water with chlorine dioxide from such mainly free from chlorine ways, preferably containing less than 0.5 grams of chlorine per 1 liter, is especially preferred.

Furthermore, the method in accordance with the invention include the recycling of spent liquor from one or more of the final stages of bleaching in pre-whitening, free from halogenated chemicals. It is also possible to recycle the spent liquor from the final stages of bleaching, which is acid, such as stages with chlorine chemicals in the processing of complex-forming substance and the spent liquor from the alkaline extraction stages in ocontainer bleaching in processing peroxide. The combination of pH, temperature and residence time in the processing of complex-forming substance and a substance containing the peroxide, as it turned out, was particularly appropriate for reducing the content being galijasevic organic substances in spent liquor from the final bleaching. Thus, in the proposed method with several advantages for the environment and stage 2 mix before as they will be merged into the receiver. The streams are mixed and then incubated for at least 5 min, preferably from 5 to 180 min) before they will be merged into the receiver. Most preferably the flow of waste water to mix as soon as possible, which makes it possible to benefit from the high temperatures existing in the processing stage of the peroxide. This has a favorable effect on the reduction Oh and reduces the time spent, which can be crucial when dealing with large amounts of waste water.

In the proposed method, the first stage performed at a temperature of from 10 to 100aboutWith (preferably from 40 to 95aboutC) during the time from 1 to 360 minutes, preferably from 5 to 60 min) second stage performed at a temperature of from 50 to 130aboutWith (preferably from 60 to 100aboutC) during the time from 5 to 960 min (preferably from 60 to 360 min). The concentration of the pulp can be from 1 to 50 wt. (preferably from 3 to 30 wt.). In preferred embodiments, including the processing of nitrogenous polycarboxylic acids in the first stage and hydrogen peroxide in the second stage, the first stage is performed with the load (100% product) from 0.1 to 10 kg per 1 ton of pulp (predpochtitel 40 kg per 1 ton). Terms of the way in both stages of processing is adjusted so that you get the maximum whitening effect on 1 kg loaded compounds containing the peroxide.

In the first processing stage, the pH value can be adjusted by sulfuric acid or residual acid from the reactor with chlorine dioxide, while the pH in the second stage, adjust by adding to the pulp mass lye or liquid containing alkali, such as sodium carbonate, acid sodium carbonate, sodium hydroxide or oxidized white liquor.

In a variant of the invention, where the processing performed after the oxygen stage in the bleaching sequence, the processing gives an excellent result for the decomposition of lignin, as processed by the oxygen pulp mass is more sensitive to treatment with hydrogen peroxide, reducing the lignin content and/or increasing the degree of whiteness. This process, used in combination with a complexing agent and performed after the oxygen stage, thus, gives good results, that from the point of view of the environment is significant: improved handling with the more reserved device for posledovateley through the use of two oxygen stages one after the other at the beginning of the bleaching sequence. However, it was found that after initial treatment oxygen is difficult to use re-process oxygen for the removal of such amount of lignin that were based on high capital investment for this stage.

As noted above, the goal of the proposed method is the reduction of discharges Oh (adsorbed organic Halogens), at the same time preserving the quality of the pulp through the use of peroxide and arbitrarily oxygen instead of halogenated bleaching agents in pre-whitening. To obtain the same result with the peroxide, as with chlorine substances in respect of delignification, in accordance with the invention it has been found that the pulp must be pre-processed complex-forming substance with pH in the range from 3.1 to 9.0. Thus, the trace metal profile of the pulp (the position and content of each present metal) can be changed so that the peroxide selectively decompose lignin, at the same time leaving the cellulose chain virtually untouched.

In the processing in accordance with known methods, the goal was only to reduce the total cm that trace metal profile, modified by selective changes in the content and provisions of metals, has a more favorable impact on the quality of pulp. Assume that the processing in accordance with the invention with the first stage of the complex-forming substance at pH from 3.1 to 9.0 means that mainly aktivnye trace metals around the cellulose chains are complexly related, while the corresponding metals in the vicinity of the lignin remains virtually untouched. In the subsequent bleaching peroxide will decompose by these metals and react with the nearest substance, i.e. lignin. Thus, the selectivity of delignification remarkably improved. Examples of metals, especially harmful for the breakdown of cellulose, are manganese, while, for example, magnesium may have a beneficial effect (among other cases) on the viscosity of the pulp. For this reason, among other metals magnesium, it is advisable not to delete.

Using the proposed method provides better or unchanged the quality of the resulting pulp. In the bleaching method in order yavlyaetsyato mass. In addition, the aim is a high viscosity, which means that the pulp contains a long hydrocarbon chain, resulting in a more durable product, and low flow peroxide hydrogen, resulting in lower processing costs. In the proposed method, all four objectives are achieved, which is obvious from the examples. Thus, the low Kappa number, and consumption peroxide hydrogen, and a high degree of whiteness and the viscosity is obtained in the processing of complex-forming substance with pH in the range from 3.1 to 9.0 and subsequent peroxide bleaching. In addition, the combination of high quality pulp and greatly reduced the impact of water flows surrounding bleaching plants, get through recycling of spent liquor from halogenated bleaching stages.

The invention and its advantages are illustrated in more detail by the following examples. The percentages and parts given in the examples, refers to mass% and mass parts, except where otherwise indicated.

P R I m e R 1. Delignification oxygen Kraft pulp from coniferous wood treated in accordance with the invention. In stage 1 and the lo Kappa and viscosity equal to 16.9 and 1040 DM3/kg, respectively, before treatment. In the experiments the pH change in stage 1 between the 1.6 and 10.8. In stage 2 use 15 kg peroxide hydrogen per 1 ton of pulp. pH equal to 11, temperature 90aboutWith the residence time of 240 minutes Consistency pulp 10 wt. in both stages 1 and 2. The Kappa number, viscosity and degree of whiteness of the pulp were determined in the standard way Scandinavia, the consumption of hydrogen peroxide was measured by iodometric titration. The results obtained are shown in table.1.

As can be seen from the table. 1, it is crucial that the processing in stage 1 is performed in the presence of complexing substances in the range of pH in accordance with the invention, to achieve the maximum reduction in Kappa number and flow rate of the hydrogen peroxide, as well as to maximize the degree of whiteness. Selectivity, expressed as the viscosity at the specific Kappa number, higher in the presence of complexing agents. This is true in all the investigated range of pH.

P R I m m e R 2. Delignification oxygen Kraft pulp from pine wood with a Kappa number, 16.9% to processing in accordance with the invention was processed in the following sequence whitening: study the Oia 2 alkaline bleaching peroxide, Daboutand D1the first and second processing technical chlorine dioxide, respectively, EP phase extraction, enhanced peroxide. The total loading of chlorine dioxide and hydrogen peroxide was 35 kg per 1 ton of pulp and 4 kg per 1 ton of pulp, respectively. The final degree of whiteness and viscosity were 89% contact to JSO and 978 DM3/kg respectively. The spent liquor from this experience, containing 0.35 kg AOH per 1 ton of pulp, was recycled from washing of the filter after the Daboutin the inflow to the stage 1. The temperature in stage 1 was changed between 50 and 90aboutC. in Addition, studied the cleansing effect of mixed waste liquor from stage 1 and 2. Everywhere the time spent in stage 1 of 30 minutes In the experience in which mixed waste liquor from stage 1 and 2, the time after mixing was increased by approximately 15 minutes This time is a common time in the neutralization tower. Content galijasevic organic substances designated as Oh (adsorbed organic Halogens), was determined according to SCAN-W9: 89. The sample is acidified with nitric acid and organic constituent parts adsorb periodically on charcoal. Inorganic chlorine ion C. Hydrochloric acid, thus formed, absorb in the electrolytic suspension and determine through microcoulometric titration.

As the law has defined the content AOH as kg AOH per 1 ton of pulp, the experimental values were recalculated by multiplying mg AOH per 1 liter of waste water with liters of waste water per 1 ton of pulp.

The results are shown in table.2.

In factory tests with the same pulp mass and sequence whitening results were obtained, are given in table.3.

As can be seen from the table.2, the content AOH in the waste water is reduced by more than 50% at temperatures above 60aboutWith in stage 1. As this level is very low, on the basis of 0.35 kg per 1 ton of pulp after the Daboutthe result is a plant that is almost completely closed against discharge AOH. This is especially true if waste water from stage 1 and stage 2 mix, which gives a further reduction of 40% compared to the 90aboutWith in stage 1. In addition, the possibility of using existing equipment in bleaching plants DL which may be fully or partially excluded, as the pH in the waste water from stage 1 and/or 2 higher than in spent liquor from the Dabout.

In addition, the higher temperature at stage 1 has a beneficial effect on the lignin content in the pulp mass after stage 2. In respect of Kraft with Kappa number, 21,0 before bleaching Kappa number, 12,3 reach after stage 2 PI 50aboutWith in stage 1. When 90aboutWith the first stage results are 12,0, i.e., a negligible increase in the efficiency of delignification from 41 to about 43%

P R I m e R 3. For comparative purposes pulp mass, used in example 2, bleach according to a known method. The bleaching sequence according to a known method and the present invention was ABOUT(C+D) EP D EP D and On and stage 1, stage 2, D, EP, D, respectively. The content of chlorine dioxide stage (+D) was 50% and 100%, respectively, calculated as active chlorine.

The results are shown in table.4.

As can be seen from the table.4, the method in accordance with the proposed invention makes it possible to obtain pulp with equal final degree of whiteness also, as in the case when using conventional bleaching. In this case, however, the content Oh the it environment whitening method is only with the technical chlorine dioxide. Total content AOH 0.03 kg per 1 ton of pulp was obtained when the spent liquor from stage 1 and stage 2 were mixed at 90about(PL.2).

P R I m e R 4. The example illustrates the experiments conducted at pH values and temperatures specified in paragraph 1 of the claims, in the processing of pulp complexing agent. Table.5 and table.6, below, show the effect of pH and temperature in the processing of complexing agent on some intrinsic properties of pulp after treatment complexing agent, wash with water, peroxide-based processing by the connection processing halogenated substance and recycling the spent solution from the stage, including halogenated substance to one of the previous stages according to the invention. The treated pulp mass, representing oxygen bleached sulphate pulp of softwood, which before treatment had a Kappa number, 15,3, white 36,7% ISO and a viscosity of 1000 DM3/kg.

The machining conditions with varying pH values as follows:

stage 1 (complexing agent): 2 kg complexing agent EDTA 1 t cellulosome): 20 kg of hydrogen peroxide H2ABOUT2per 1 ton of dry pulp; 90aboutWith; 240 min; final pH value of 11; the concentration of mass 10 wt.

step 3 (processing halogenated substance): 15 kg lO21 t dry weight, calculated on the active chlorine; 50aboutC; 120 min; the final pH value of 3.2; the concentration of mass 10 wt.

The machining conditions at varying temperatures:

stage 1 (complexing agent): 2 kg EDTA per 1 ton of dry pulp; 60 min; pH 6.5; the concentration of pulp 10 wt.

stage 2 (processing peroxide-based compound): 20 kg of hydrogen peroxide H2ABOUT21 t dry mass; 90aboutWith; 240 min; final pH value of 11; the concentration of mass 10 wt.

step 3 (processing halogenated substance): 17.5 kg of chlorine dioxide per 1 ton of dry pulp, calculated on active chlorine; 60aboutC; 120 min; the final pH value of 3.2; the concentration of mass 10 wt.

The example also illustrates the experiments were carried out in the range of pH, including the boundary values, when the peroxide-based bleaching compound, as defined in paragraph 1.

Table. 7 shows the results after recirculatory the spent solution from the stage, including the th mass is identical to the above.

Conditions testing at varying pH values:

stage 1 (complexing agent): 2 kg EDTA 1 t dry mass; 90aboutC; 60 min; pH 6.5; the concentration of mass 10 wt.

stage 2 (processing peroxide-based compound): 20 kg of hydrogen peroxide H2ABOUT21 t dry mass; 90aboutWith; 240 min; mass concentration of 10 wt.

step 3 (processing halogenated substance): 13/-24 kg of chlorine dioxide per 1 ton of dry pulp, calculated on active chlorine; 50aboutC; 120 min; the final pH value of 3.0, the concentration of mass 10 wt.

As follows from the table. 5 to 7, selected within the parameters of the method provide a bleached pulp of high quality.

1. The METHOD of multi-stage BLEACHING PULP, comprising the sequential processing of the pulp suspension nitrogen-containing polycarboxylic acid with p 3,1-9.0 and a temperature of 10 100oWith the wash water and processing a compound containing peroxide, at a pH of 7 to 13, characterized in that after treatment with compound containing peroxide, the pulp suspension is treated with halogen-containing substance and the spent liquor from this stage recycle to one of the previous stages.

2. Src> 3. The method according to p. 1, characterized in that as the nitrogen-containing polycarboxylic acid is used diethylenetriaminepentaacetic acid or ethylenediaminetetraacetic acid.

4. The method according to p. 1, characterized in that compounds containing the peroxide, use pereskia hydrogen or a mixture of it with oxygen.

5. The method according to p. 1, characterized in that the processing of the pulp suspension nitrogen-containing polycarboxylic acid is carried out at pH 4-8 and temperature 40-95oWith over 1-360 minutes, and processing a compound containing peroxide, at temperatures 50 130oWith over 5-960 min and the concentration of the pulp suspension 1-50 wt.

 

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EFFECT: improved quality of product, reduced average degree of polymerization, and increased reactivity thereof.

2 tbl, 18 ex

FIELD: pulp-and-paper industry, in particular method for PULP whitening.

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EFFECT: reduced consumption of whitening agents.

1 tbl, 9 ex

FIELD: pulp-and-paper industry, in particular method for pulp whitening.

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EFFECT: pulp with increased whiteness.

1 tbl, 17 ex

FIELD: pulp-and-paper industry.

SUBSTANCE: unbleached sulfate cellulose obtained from hardwood is subjected oxygen-alkali delignification, after which treated with chlorine dioxide in presence of oxyethylidenediphosphonic acid and then consecutively hydrogen peroxide and chlorine dioxide solutions. Process may be used in production of bleached fibrous intermediates.

EFFECT: increased whiteness and strength characteristics of cellulose and improved environmental safety of bleaching process.

2 cl, 1 tbl

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EFFECT: enhanced economical efficiency without loss cellulose characteristics.

2 tbl, 11 ex

FIELD: paper; chemistry.

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EFFECT: higher strength in wet condition and softness of paper.

42 cl, 2 dwg, 1 ex

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7 cl, 2 dwg

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EFFECT: improved quality of bleached cellulose, enhanced efficiency of process and ecological safety.

1 tbl, 6 ex

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EFFECT: improved efficiency of waste paper recycling and control of waste paper quality.

15 cl, 4 dwg

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EFFECT: increased whiteness and optic brightness of paper at decreased utilisation of optic whitener.

22 cl, 11 dwg, 11 tbl, 7 ex

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