CS199637B2 - Dissolving pulp manufacturing method - Google Patents

Description

The invention relates to a process for the production of chemical pulp.

Current processes for producing chemical pulp, i.e. pulps suitable for further chemical processing, from wood always include cooking and bleaching. The cooking is either acidic, i.e. a sulphite batch, or alkaline, preceded by pre-hydrolysis, i.e. a sulphate batch with pre-hydrolysis. The purpose of the acid treatment is to remove a substantial part of the chemicelluloses contained in the wood. Often, chemicelluloses are additionally removed by an alkaline treatment in conjunction with bleaching; this treatment is carried out with dilute alkaline solution, usually 1 to 4% caustic at 70 to 100 ° C (the so-called hot alkaline treatment), or, 9 to 11% caustic at 25 to 40 ° C (the so-called cold alkaline treatment) .

The alkaline treatment, either hot or cold, is usually carried out between pre-bleaching and end-bleaching, with chlorine dioxide, hypochlorite or peroxide generally being used for the final bleaching. In rare cases, a cold alkaline treatment is carried out between cooking and bleaching, with either sodium hydroxide or possibly white liquor from sulphate production, which contains sodium sulfide in addition to sodium hydroxide.

On the other hand, it is not possible to produce pulp that is suitable for further chemical processing, for example into cellulose esters or ethers or viscose products, by alkaline or sulphate cooking and bleaching alone.

Recently, however, remarkable successes have been achieved in the production of cellulose derivatives, in particular viscose and cellulose ethers, which have substantially expanded the raw material base of these processes. Thus, for example, the methods of producing SIN1 viscose based on the so-called double alkalinization described in Czechoslovak patent no. 182757 and in Finnish patents no. 41.543 and 41.544 have proved so effective that it can produce extremely high-quality viscose from paper kraft pulp. However, due to the large amount of hemicelluloses contained in such pulp, the hemicellulosic content of the circulating alkaline liquor rises so strongly that the caustic mercerization ability and the soaking press performance decrease simultaneously. For this reason, papermaking kraft pulp comes into consideration for the technical process only if the hemicelluloses are appropriately removed from the alkalizing liquor. Continuous hemicellulose removal may be, for example

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of course, either by dialysis or by precipitation, but it entails considerable investment and operating costs. Of course, it is also possible to remove the hemicelluloses from the pulp by hot or cold alkaline treatment in connection with bleaching, but in this case the hemicelluloses are transferred to waste water, which must then be costly to be treated.

The process according to the invention is used for the production of chemical pulp, which is particularly suitable as a raw material for the dual alkalinization treatment process, for example for the aforementioned SINI process. In the process according to the invention, the pulp is obtained. with a conventional kraft batch, either prior to bleaching or in conjunction with bleaching, undergo a cold alkaline treatment. It is economically advantageous to carry out this treatment between cooking and bleaching.

According to the process of the present invention, the chemical pulp is produced by alkaline cooking without pre-hydrolysis, the pulp-containing raw material is alkalinized (kraft batch) and bleaching is carried out in a known manner. up to 120 degrees Celsius.

Said alkaline solution may contain additionally varying amounts of sodium sulfide and organic substances, in particular hemicellulose. The ratios at which this treatment takes place are selected according to the existing equipment; the density of the substance during processing is 1 to 20%, preferably 2 to 10%, the temperature is 0 to 120 ° C, preferably 30 to 60 ° C, and the treatment time is 2 to 120 minutes, preferably 10 to 60 minutes .

After such an alkaline treatment, the pulp is bleached by known methods, for example by combining the following stages: chlorination - alkali - hypochlorite - chlorine dioxide - alkali - chlorine dioxide.

The alkaline treatment can be carried out in a number of different ways, for example by the following procedure:

From the substance obtained by continuous or periodic kraft cooking, concentrated hot black liquor is displaced in a known manner with dilute lye. Thereafter, the substance having a density of about 10% is passed through a continuously operating diffuser from bottom to top. In this context, one or more displacement phases take place using white liquor brought to a suitable temperature. The number of displacement phases depends on the desired hemicellulose removal rate and the flow rate of the substance; furthermore, the processing temperature also affects the result. Each displacement stage can be assigned its own separate liquor circulation. Since the sodium hydroxide content of the circulating lye is constantly decreasing, while the hemicellulose content is constantly rising, some of the caustic must be continuously withdrawn from the circulation and replaced by fresh white lye. The effluent is led to the brewing stage, where the energy of the organic substances contained is used in the regeneration of kraft boiling chemicals.

Further, it is possible to sequentially stage the white liquor treatment and operate in the stream so that the purest white liquor flows to the last stage of the diffuser, while the liquor with the highest hemicellulose content flows from the upper stage and is fed either wholly or partially to the brewing stage. Even in this process, the concentration of lye can be increased between the individual stages by supplying fresh white liquor.

After one or more white liquor treatments, the pulp is countercurrently washed. The removed caustic is recovered in a manner suitable for the whole process. The washed pulp is then bleached by conventional means.

It should be noted that the consumption of chlorine for bleaching is substantially reduced as a result of white liquor treatment. The cellulose alpha content of the pulp, which is 84-86% on conventional pulp, can be increased by white liquor treatment up to 93-94%, although this large increase is not always necessary.

White liquor treatment can be carried out in a periodically operating difluoror. For example, if three stages are provided, the plant must contain four lye tanks, one for white liquor and the other three for white liquor containing hemicelluloses. The white liquor from vessel 4, which has the highest concentration of hemicelluloses, is passed through the substance coming from the digester after replacement of the concentrated liquor with diluted liquor. The black liquor flows out of the diffuser, which either evaporates or leads to the brewing stage. This is followed by white liquor mixed with black liquor, into which other pulp hemicelluloses have been passed. This liquor or part thereof is fed to the brewing stage. In the partial introduction, the remaining portion of the liquor is fed back to the tank 4. In the next step, the white liquor from tank 3 having less hemicellulose content than the liquor from tank 4 is passed through the feedstock. In a similar manner, the caustic treatment from tank 2 takes place, and finally pure white liquor from tank 1 is fed through the raw material.

The alkali content of the leaches in reservoirs 1, 2, 3, 4 decreases and the hemicellulose content increases in the same order. If necessary, the concentration of lye in tanks 2 to 4 can be increased by supplying fresh white lye. The white liquor treatment is followed by countercurrent washing and bleaching.

White liquor treatment can also be carried out as a conventional filter wash of kraft pulp using suction or pressure filters.

Since continuous processes are difficult to simulate in the laboratory, the results given in the following examples correspond to the experiments carried out in batches. Experiments carried out in a discontinuous manner give, as is known, generally worse results than the corresponding continuous processes.

Example 1

Unbleached kraft pulp cooked without pre-hydrolysis, with a dry matter content of about 30% and a kappa number of 28.6, was weighed to a dry matter content of 300 g. The pulp samples were dissolved in white liquor at 30 ° C or 50 ° C. so that the solution has a density of 2.5%, 5% or 10%. The white liquor contained 135 g / liter of active alkali and had a sulfldite of 28%. The white liquor treatment time in each experiment was 10, 30 or 60 min. Then the celluloses were washed, bleached under optimal conditions by a combination of chlorination, alkaline treatment, hypochlorite treatment, chlorine dioxide treatment, alkaline treatment and chlorine dioxide treatment, pressed into leaves and dried. Losses and bleaching losses, total chlorine consumption during bleaching, bleach pulp viscosity and alpha number were then determined. Based on the results, the percent recovery of alpha pulp from bleached pulp relative to unbleached pulp was calculated. For comparison, bleaching without white liquor treatment was also performed. The results are shown in the following table:

and

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H

White liquor treatment -r ', bleached pulp

Experiment density temperature duration loss Whitening Loss Yield Yield Viscosity Pulp Yield Compound ° C min. % consumption of% CCA 16 alpha pulp% chlorine% alpha θ '

-CO 05 00, 00 ~ pH CJ oo ti from ó ts ts 03 «W | st> h co o) ro ci ci o ro iri 03 03 03 03 00 OO o C 'cm what co CM CJ CM CM CM CO oo ~ co oo_ cd co cn Ψ TP co 'of ci 00 00 00 00 OO 03 in OO CM <D rH «jt co' eo iri ui ci

U3_03 CO 00 ~ 00 or 00 oo 'of

N O, 0O CM Q ri θ '03' CO 'CO' _I _I *

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CM CM tH t-I t-H CM CO rjf m co

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As shown in the table, 6-11% of the unbleached pulp went into solution in connection with white liquor treatment. In a technical and therefore non-laboratory process, these solutes, in particular organic substances, are fed together with the caustic into a batch and the energy contained therein is used to regenerate cooking chemicals. The chlorine consumption of bleaching after pretreatment with white liquor varied between 7.3 and 8.8%, and the bleaching losses between 3.8 and 6.0%. When bleached without pretreatment with white liquor, chlorine consumption was 9.8 percent and bleaching losses were 7.1%. The cellulose alpha content in the white liquor treated pulp fluctuated between 89.7 and 92.7%, while in the pulp bleached without prior white liquor treatment this content was around 85%. The variation in alpha cellulose yield in the test series was quite minor and independent of whether or not white liquor treatment was performed; Values were between 77.9 and 80.8%.

Example 2

From the pulps 1, 3, 5, 6 of the table in Example 1, viscose was produced by the above SINI process under the following conditions:

I. alkalization:

21% sodium hydroxide solution with a hemicellulose content of 4.3 percent, a temperature of 50 ° C, a density of 4%, a duration of 20 min.

I. Pressing:

pressing ratio 2.8.

Pre-maturation:

Temperature 44 ° C, duration 20 hours.

II. alkalization:

12% sodium hydroxide solution with a hemicellulose content of 2.6 percent, with a temperature of 35 ° C, with a goose of 4 percent, duration of 10 min.

II. odlisování:

pressing ratio 2.6.

Sulfidation:

24% carbon disulphide calculated on the weight of pulp, alkaline pulp, temperature 32 ° C, duration 60 minutes.

Dissolution:

temperature 20 ° C, duration 4 hours.

Viscose:

4.5% NaOH, 10% cellulose.

In the experiments, the soaking losses in both alkalis were determined, the pulp yield from the finished viscose, the viscosity and filterability (K _w ) of the viscose, and the DP value of the precipitated pulp. The results are shown in the table

2. The table below shows the percentage of precipitated cellulose relative to unbleached pulp.

For comparison, corresponding experiments were also performed with a viscose pulp having an alpha pulp content of approximately 90.5%.

Table 2

Sample C. Dipping losses Yield % Viscosity K _w sec DP Total yield % AND and% II % 1 7.8 4,5 87.4 56 - 217 74.1 3 6.9 7.5 86.8 50 620 211 75.3 5 - 38 680 - .— 6 13.4 7.8 80.0 67 -. 74.3 viscose Bunlčina 6.3 8.1 - ^: 50 650 Results summarized in table 22 show that bones like commercial chemical pulp,

in the production of viscose by the above-mentioned SINI process from pulps obtained by the process according to the invention, the soaking losses are of the same order of magnitude, the filterability of this viscose being as good as that of chemical pulp.

Claims (7)

Process for the production of chemical pulp by alkaline boiling without pre-hydrolysis, wherein the pulp-containing raw material is alkalinized (kraft batch) and bleached, characterized in that the pulp is bleached at 0 to 120 ° C with an alkaline solution containing 4 to 13% sodium hydroxide removes hemicelluloses. 2. A process according to claim 1, wherein the alkaline solution is a white liquor for a sulfate batch. 3. The method of claim 1, wherein the alkaline solution comprises hemicelluloses. 4. The process according to claim 1, wherein the alkaline treatment is carried out in one INVENTION of up to four stages, wherein for working in more than one stage a solution having the lowest sodium hydroxide content and the highest hemicellulose content is used for the first step, and a solution having the highest sodium hydroxide content and the lowest hemicellulose content is used for the last step. Method according to claim 1, characterized in that the density of the substance in the alkaline treatment is 1 to 20%, preferably 2 to 10%. 6. Process according to claim 1, characterized in that the alkaline treatment is carried out at a temperature of 0 to 120 ° C, preferably at 30 to 60 ° C. Method according to claim 1, characterized in that the duration of the individual treatments is 2 to 120 minutes, preferably 10 to 60 minutes.