JP2000220086A - Production of kraft pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the operation load and increase the pulp yield by dividing the white liquor in the production of kraft pulp into the white liquor A and the white liquor B and catalytically oxidizing the white liquor A with air in the presence of a catalyst and feeding it as an oxidized white liquor A to the first pulping step and feeding the white liquor B directly as it is to the second pulping step, and recycling them. SOLUTION: In the step where kraft pulp is produced from lignocellulose through the first and second pulping steps, the white liquor is divided into the liquor A and the liquor B. The white liquor A is air-oxidized in the presence of a catalyst and fed to the first pulping step as an oxidizing white liquor including sodium hydroxide, sodium sulfide and polysulfide sulfur-containing oxidizing white liquor, while the white liquor B is directly fed to the second pulping step and circulated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing kraft pulp. More specifically, the present invention relates to a method for producing kraft pulp that reduces the operation load, improves the pulp yield, and further improves the pulp strength.

[0002]

2. Description of the Related Art Among the methods for producing pulp for papermaking, the method for producing pulp by the Kraft method has been established as a production method capable of recovering calories and alkali components as cooking chemicals from cooking effluent. In addition, it has become a typical method for producing chemical pulp in major paper pulp countries around the world because of the advantage of not selecting a specific tree species as a raw material.

According to the conventional method for producing kraft pulp, lignocellulosic substances such as wood chips are digested in a digestion step with a white liquor containing sodium hydroxide and sodium sulfide as main components. The obtained cooking product is subjected to solid-liquid separation and separated into pulp and cooking waste liquid. The separated pulp is washed with water, and this washing waste liquid is mixed with the cooking waste liquid, and is called a dilute black liquor and contains a large amount of water.
This diluted black liquor is concentrated by vacuum evaporation to become a dark black liquor.
The dark liquor is introduced into a recovery boiler, and the organic components in the black liquor are burned, and the combustion heat generates steam to be recovered as thermal energy.

[0004] In the recovery boiler, a reducing atmosphere is formed by the combustion pyrolysis of the organic matter in the black liquor, and under this reducing atmosphere, sodium sulfide is generated by sulfur and sodium in the black liquor, and excess sodium is formed. The component reacts with carbon dioxide generated by the combustion of the organic component to form sodium carbonate. These alkali-inorganic components accumulate in a sherbet shape at the bottom of the boiler and are called smelt. This smelt is introduced into the water from the furnace bottom and dissolved to become green liquor.

[0005] This green liquor contains a large amount of sodium carbonate, which has insufficient alkali strength for cooking,
Calcium oxide (quicklime) is added to the green liquor, and sodium carbonate is converted to sodium hydroxide by a causticization reaction.
It is used again for cooking as white liquor.

[0006] In the conventional cooking pulp production, in the cooking step, a lignocellulosic substance such as wood chips and white liquor are charged into a pressure cooker, heated by steam heating to maintain the temperature and pressure to a predetermined range. The cooking reaction is stopped by discharging a mixture of the cooking product and the cooking effluent to atmospheric pressure. Important factors in this cooking process are the ratio of alkali to wood, temperature and time.

[0007] Pressure cookers for digestion include a batch system and a continuous system. The batch method is a method in which wood chips are filled in an empty kettle, then the white liquor is charged, the kettle is sealed, and then heated with steam.At the end of the digestion, a mixture of the digestion product and the cooking waste liquid is supplied from the lower part of the kettle. The whole amount is blown at a stretch. The continuous method is a method in which wood chips and white liquor are continuously fed from one end of a kettle provided with a high-pressure rotary valve at both ends of a cylindrical container. The mixture is heated to obtain time, and the mixture of the cooking product and the cooking waste liquid is continuously blown from the other end.

[0008] The reaction between the lignocellulosic substance and the alkali in the cooking step will be described below. Lignocellulosic substances such as wood include cellulose and hemicellulose useful as pulp, as well as lignin and low-molecular polysaccharides that form tree structures, various organic acids, resins, and the like. The main function of alkaline chemicals in kraft cooking is to destroy the high molecular structure of lignin and convert it into low molecular weight compounds that are soluble in aqueous alkaline solutions. In addition, the alkali chemicals also have a function of keeping the solution at a high pH in order to prevent re-polycondensation of the low-molecular compound. Alkaline chemicals are also consumed to neutralize organic acids, hydrolyze low molecular weight polysaccharides, and neutralize the resulting organic acids. That is, an alkaline chemical is essential in the kraft pulp production process, and its consumption is proportional to the pulp production. The effective amount of alkali for wood in cooking differs depending on the tree species, but the weight ratio is about 16% for softwood and about 12% for hardwood.
It is said.

The equipment used in the above-mentioned kraft pulp manufacturing process has its design capability determined in accordance with the planned pulp production volume. Therefore, in the season when pulp demand increases, the pulp production can be increased beyond the design capacity by increasing the amount of alkali added or increasing the cooking temperature in the digester.

However, when increasing pulp production, a recovery boiler having inflexible facility capacity becomes a bottleneck. That is, the upper limit of the capacity of the recovery boiler is often determined by the amount of heat generated, and the amount of heat generated by the recovery boiler increases in proportion to the increase in pulp production. Accordingly, a new method for increasing pulp production by suppressing an increase in heat generation in a recovery boiler has been required.

Various improvements have been proposed for the craft method. Super batch method is used for batch method,
As the continuous method, an MCC method, an EMCC method, an ITC method, and the like have been proposed. All of these improvement methods increase the pulp yield and improve the pulp quality.However, the operation load cannot be reduced significantly, and large-scale remodeling of the equipment is required. There's a problem.

Among these, the MCC method adds the cooking liquor to the continuous digester in three parts, and adds a part of the liquor countercurrently to the chips from the middle or lower stage of the kettle to make the degree of digestion uniform so that the pulp is made uniform. The aim is to improve the strength. This is because the alkali concentration in each stage of the cooking step becomes uniform and the collapse of cellulose in wood due to excessive alkali is suppressed as compared with the conventional cooking liquid batch addition method. However, this method does not contribute much to the improvement of the pulp yield, and when applied to an existing continuous digester, there is a problem that a large-scale modification of equipment is required.

[0013] The super-batch method is a method of pre-treating wood chips to be fed into a kettle using warm black liquor separated from pulp after cooking is completed in a batch kettle. It requires new equipment for transferring and transferring, and it is not expected that the operation load in the pulp manufacturing process will be significantly reduced.

On the other hand, with the aim of improving the pulp yield,
Improvements in cooking liquor and development of delignification aids have also been advanced, and the former includes polysulfide cooking, and the latter includes addition of quinone compounds.

Among these, the polysulfide digestion method oxidizes white liquor to convert sodium sulfide to polysulfide sulfur, or directly adds sulfur to the digestion liquor to oxidize and protect the terminal groups of hemicellulose, and It is said that the pulp yield is improved in order to suppress the elution of cellulose by the alkali.

In general, the polysulfide digestion method has a greater effect on the yield as the rate of addition of polysulfide to chip increases, and it is better to increase the concentration of polysulfide sulfur in the oxidized white liquor. However, while the sodium sulfide concentration in the white liquor after oxidation in order to suppress the reduction in the pulp strength is at even terms of Na ₂ O Low 1
It is said that 0 g / l or more is required. Therefore, the concentration of polysulfide sulfur that can be produced by this method is naturally limited.

Furthermore, the operating temperature of polysulfide sulfur on hemicellulose is around 130 ° C., and at higher temperatures, it decomposes itself. Therefore, in the polysulfide single-stage digestion method in which the temperature of the kettle rises at once, there is a problem that the amount of polysulfide sulfur contributing to the improvement of the pulp yield is further reduced.

[0018]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing kraft pulp which reduces operating load, improves pulp yield, and further improves pulp strength.

[0019]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, have completed the present invention. That is, the method for producing kraft pulp of the present invention comprises:
A method for producing kraft pulp, comprising: a first cooking step of cooking a lignocellulosic material using a first cooking liquor; and a second cooking step of adding a second cooking liquor to the first cooking product and further cooking. B) branching the white liquor into white liquor A and white liquor B; and (II) oxidizing white liquor A with air in the presence of a catalyst to obtain an oxidized white liquor containing sodium hydroxide, sodium sulfide and polysulfide sulfur. This oxidized white liquor is used as the first cooking liquor, and (III) White liquor B is used as the second cooking liquor.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The lignocellulosic material used as a pulp raw material in the present invention includes non-wood such as bagasse, rice straw, kenaf and hemp, in addition to various woods.

The digesting step of the lignocellulosic material of the present invention comprises a first digesting step of infiltrating the lignocellulosic material with an alkaline cooking liquor (oxidized white liquor) containing polysulfide sulfur, sodium hydroxide and sodium sulfide; A second cooking step of cooking using an alkaline cooking liquor (white liquor) containing sodium hydroxide and sodium sulfide in the lignocellulosic material or an alkaline cooking liquor obtained by mixing white liquor and the above-mentioned white oxide liquor. In addition, the polysulfide sulfur referred to in this specification means sulfur generated from sodium polysulfide (Na ₂ S ₂ ) by the following reaction formula 1.

[0022]

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The concentration of polysulfide sulfur in the cooking liquor used in the first cooking step is 4 to 20 g / l, preferably 6 to 15 g / l in terms of sulfur. The sodium hydroxide concentration is 50 to 150 g / l, preferably 90 to 120 g / l in terms of Na ₂ O, and the sodium sulfide concentration is
₂ to 40 g / l, preferably 5 to 20 g in terms of Na ₂ O
/ L.

The concentration of polysulfide sulfur in the cooking liquor used in the second cooking step is 0 to 10 g / l in terms of sulfur.
It is.

The concentration of sodium hydroxide is 40 to 120 g / l in terms of Na ₂ O, preferably 70 to 100 g / l.
1 and the sodium sulfide concentration is 10 in terms of Na ₂ O.
６０60 g / l, preferably 30-50 g / l.

In the first cooking step, the cooking temperature is
The temperature is 90 to 130 ° C, preferably 100 to 120 ° C.
The digestion holding time depends on the type of lignocellulosic substance and the temperature in the kettle, but is usually 20 to 60 minutes, preferably 30 minutes.
~ 50 minutes.

Similarly, in the second cooking step, the cooking temperature is 140 to 180 ° C., preferably 150 to 170 ° C.
℃, the digestion holding time is 40 to 200 minutes, preferably 5
0-150 minutes.

The position of the first cooking step in the present invention is a protection effect on lignocellulosic substances, rather than promotion of the degree of cooking. In the subsequent second cooking step, cooking is mainly performed to pulping.

The degree of digestion of the lignocellulosic material is determined by the sum of the effective alkali of both the first cooking liquor added in the first cooking step and the second cooking liquor newly added in the second cooking step and the second cooking step. And the holding time.

The action in the first digestion step will be further described. In hemicellulose, aldehyde end groups are oxidized by polysulfide sulfur to be converted to aldonic acid and stabilized against alkali. This prevents the breakdown of hemicellulose even in the strong cooking action of sodium hydroxide in the second cooking step. In addition, since polysulfide sulfur is decomposed at a temperature of 130 ° C. or more, it is important that the first digestion step is performed at a lower temperature.

On the other hand, by reducing the amount of white liquor to be oxidized, the processing load in the oxidation step including the filter is reduced, so that it is possible to increase the concentration of polysulfide sulfur. By adding the liquid to the first digestion step and reacting it with the lignocellulose substance, a more efficient action than before can be exhibited.

The ratio of the amount of cooking liquor used in the first and second cooking steps is limited by the amount of circulation in each zone of the digester.
９０90%, preferably 70-80%.

The method for producing kraft pulp according to the present invention can be carried out in a batch mode or a continuous mode. In the case of performing in a batch mode, a digester is filled with a lignocellulosic substance and a first cooking liquor to perform a first cooking, and then a second cooking liquor is added to perform a second cooking. In the case of carrying out in a continuous mode, the first cooking liquor is added at the time when the lignocellulosic substance moving in the digester enters the digester, the first cooking is performed, and the second cooking liquor is added downstream thereof. A second digestion is performed.

The process for producing kraft pulp of the present invention improves the yield of pulp by efficiently reacting polysulfide sulfur with a lignocellulosic substance, thereby reducing the organic content in the black liquor solids and recovering the pulp. Load reduction in the boiler is realized.

[0035]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. The white liquor and the oxidized white liquor used were prepared from commercially available reagents, and the concentrations of the respective components were shown below. White liquor NaOH: 78.0 g / l Na ₂ O equivalent Na ₂ S: 37.0 g / l Na ₂ O equivalent Na ₂ CO ₃ : 20.0 g / l Na ₂ O equivalent Effective alkali: 96.5 g / l Na ₂ O conversion Oxidized white liquor (-1) Assuming 70% oxidation of white liquor NaOH: 99.4 g / l Na ₂ O conversion Na ₂ S: 11.1 g / l Na ₂ O conversion Na ₂ CO ₃ : 20.0 g / l Na ₂ O in terms of effective alkali: 105.0g / l Na ₂ O in terms of polysulfide sulfur: 8.7 g / l S converted oxidized white liquor (-2) assuming the oxidation of white liquor total amount NaOH: 95.2g / l Na ₂ O ₂ conversion Na ₂ S: 14.8 g / l Na ₂ O conversion Na ₂ CO ₃ : 20.0 g / l Na ₂ O conversion Effective alkali: 102.6 g / l Na ₂ O conversion polysulfide sulfur: 6.3 g / l S Example 1 It was assumed that an oxidized white liquor obtained by oxidizing 70% of the total amount of the white liquor was used as a first cooking liquor, and the remaining 30% of unoxidized white liquor was used as a second cooking liquor. However, at this time, the polysulfide sulfur concentration of the oxidized white liquor was improved by 30% as compared with the case where the entire amount of the white liquor was oxidized, and the polysulfide sulfur addition rate was the same as the comparative example.

Subsequently, in Example 2, the oxidized white liquor of Comparative Example 1 was used as the first cooking liquor, and the polysulfide sulfur addition rate was slightly lower than that of Comparative Example 1. In Comparative Example 1, it was assumed that the entire amount of the white liquor was oxidized and used.

Example 1 A eucalyptus chip used in a factory was removed from the eucalyptus chip except for irregularly shaped chips and dust. The eucalyptus chip was stored in a polyethylene bag, a part thereof was taken out, and 500 g of absolute dry weight was accurately weighed. Further, water equivalent to 50% of chip moisture was added, and the mixture was allowed to stand for one day and night, and the chip was impregnated with water and tested. A liquid having a capacity of 1 liter, capable of heating and circulating the liquid, and capable of adding a drug solution heated from another 4 liter fixed pot was used. Put the chips in this digester, then oxidized white liquor (-1) 4
02 ml and 498 ml of water were added, and the lid was sealed. The liquid was circulated, and heating was started with a heater. Starting from the time when the temperature reached 100 ° C., the temperature was raised to 120 ° C. 25 minutes later, and the temperature was maintained for 45 minutes. 1
Cooking was performed. The addition rate of polysulfide sulfur at this time is 0.7%.

Next, 161 ml of the white liquor heated to 160 ° C. in a fixed kettle and 439 ml of water were added to the digester, and the mixture was kept at that temperature for 60 minutes to carry out a second digestion. At this time, the effective alkali addition rate is 11.5%.

Thereafter, the gas in the kettle was evacuated, the pressure was returned to atmospheric pressure, the whole content was taken out, water was added and the mixture was stirred, washed with water, and the cooking waste liquid adhering to the digested pulp was completely removed. Next, using a flat screen (slit width: 0.2 mm), pulp and uncooked matter were separated and dried, and the weight of each was weighed to determine the pulp yield. Also, using a part of the pulp, kappa number, which is an index of the degree of cooking,
Pulp viscosity, an indicator of strength, was measured.

The amounts of the cooking liquor and water added in the first and second digestions were adjusted so that the weight ratio of liquid / chip was 4.0.

Example 2 In Example 1, 476 ml of oxidized white liquor-2 and 424 ml of water were added instead of oxidized white liquor-1 in the first digestion, and 90 ml of white liquor and 510 ml of water were added in the second digestion. An experiment was performed in the same manner as in Example 1 except that the experiment was performed. At this time, the addition rate of polysulfide sulfur was 0.6%.
And the effective alkali addition rate is 11.5%.

Comparative Example 1 An experiment was conducted in the same manner as in Example 1 except that 556 ml of oxidized white liquor-2 and 944 ml of water were added instead of oxidized white liquor-1 in the first digestion. . At this time, the addition rate of polysulfide sulfur was 0.7%, and the effective alkali addition rate was 11.4%.

The following experimental results were obtained from the experiments of the above Examples and Comparative Examples.

[0044]

[Table 1]

The contents of the items shown in Table 1 are as follows. (Yield of pulp) It is the yield of absolutely dry pulp weight based on absolutely dry wood chip weight. (Kappa number of pulp) It is a value representing the index of the degree of cooking in terms of the residual lignin ratio in pulp, and is TAPPI T-236.
hm-85. (Pulp viscosity) A value representing an index of pulp strength.
According to TAPPI T-230om-82.

As can be seen from the experimental results shown in Table 1,
Rather than performing the digestion by adding only the oxidized white liquor at a time, the polysulfide sulfur is made as high as possible in the first digestion step, and then the white liquor is added to the second digestion step. It can be seen that even below that, an improvement in pulp yield and an increase in viscosity are achieved.

Further, by reducing the throughput of the white liquor to be oxidized, the operation load of the white liquor oxidation step is reduced, which means that the economic efficiency in pulp production is improved. In addition, the improvement in the pulp yield also reduces the organic content in the cooking liquor and reduces the load on the recovery boiler for burning black liquor.

[0048]

According to the present invention, the operation load is reduced, the pulp yield is improved, and the pulp strength is further improved as compared with the conventional kraft pulp production method.

Claims (1)

[Claims] 1. Production of kraft pulp in which a first cooking step of cooking a lignocellulosic substance using a first cooking liquor and a second cooking step of adding a second cooking liquor to the first cooking product and further cooking the lignocellulosic material are performed. The method comprises: (I) branching white liquor into white liquor A and white liquor B; (II) oxidizing white liquor A by air oxidation in the presence of a catalyst to contain sodium hydroxide, sodium sulfide and polysulfide sulfur. Obtaining white liquor and circulating and using this oxidized white liquor as the first cooking liquor;
(III) A method for producing kraft pulp, comprising recycling white liquor B as the second cooking liquor.