WO2013004909A1

WO2013004909A1 - Method of removing hemicelluloses from pulp

Info

Publication number: WO2013004909A1
Application number: PCT/FI2012/050697
Authority: WO
Inventors: Kari Kovasin; Esko Turunen; Ismo Reilama
Original assignee: Metsä Fibre Oy
Priority date: 2011-07-01
Filing date: 2012-07-02
Publication date: 2013-01-10
Also published as: FI20115711L; UY34183A; FI20115711A0

Abstract

The present invention concerns a method of producing dissolving pulp, wherein bleached paper pulp produced by kraft pulping is used as a raw-material pulp, the method being carried out by cold alkali treatment by mixing the raw-material pulp with a concentrated alkali solution such that the mixture of the pulp and the alkali solution contains 50 - 220 g/1 alkali and the solution is allowed to be absorbed into the pulp at a temperature of 20 - 50 °C. The alkali is allowed to act on the pulp in order to dissolve hemi celluloses into it and the hemicelluloses-containing alkali solution is separated from the pulp, i.e. from the dissolving cellulose pulp. Finally, the separated alkali solution is concentrated and recycled to the feed of the cold alkali stage.

Description

METHOD OF REMOVING HEMICELLULOSES FROM PULP

Background of the Invention Field of the Invention

The present invention relates to a method of producing dissolving pulp, wherein the raw- material is cellulose pulp which is subjected to a chemical treatment. The method can also be applied in connection with dissolving pulping processes known in the art.

Description of Related Art

Dissolving pulp is normally used for the manufacture of viscose fibers and other fibers as well as for the production of processed products requiring high chemical purity. This kind of use requires an increase of the actual cellulose content (also known as alpha-cellulose) and reactivity towards various types of derivatization chemicals, such as carbon disulphide.

To provide high alpha cellulose content means that lignin and hemicellulose materials (xylan and glucomannan), in particular, are removed from the cellulosic fibers. Removal of lignin is accomplished in a similar fashion as, generally, in the manufacture of chemical pulp, viz. by cooking of the lignocellulosic raw materials with chemicals which remove lignin at conditions typical for each process. The cooking methods may be subdivided, for example, into alkaline, acid and solvent (organosolv) cooking methods. It is also possible to combine a variety of cooking steps by which lignin is removed to provide a multi-step cook. Removal of lignin is complemented in downstream processing (oxygen

delignification and bleaching) whereby the fibrous suspension obtained from the cooking is treated with a variety of chemicals, such as oxygen, chlorine dioxide and hydrogen peroxide. For the removal of hemicelluloses there have already been employed in industry a number of known processes, which have been described in detail in, inter alia, the Handbook of Pulp (2006) by Herbert Sixta. Alkaline extraction of kraft pulp is also a known method which is described in several patent applications, such as WO 2007/065 969.

Many manufacturing processes of viscose are based on sulphite cooking. The increase of the alpha-cellulose takes place at the sulphite pulping mills in general such that the hemicelluloses are removed in the connection with bleaching during a so-called hot-alkali step using dilute NaOH solutions as reagent and at high temperature.

There are also pulping methods wherein the target alpha-cellulose content can be achieved in connection with the cooking. Such methods are based on a multi-stage sulphite cooking or a prehydrolysis-kraft cooking.

In the sulphite methods the chemicals of the various cooking stages, and in particular the pH, are varied suitably. At the beginning of the prehydrolysis-kraft cooking process there is a specific acid hydrolysis step, in which the pH of the chips is reduced with steam, water or an acid solution to a low level. In the bleaching of the pulp obtained by pre-hydrolysis no separate removal step of hemicelluloses is necessarily needed at all. The prehydrolysis- kraft cook is so close to the basic kraft process used for the production of paper pulp that its application does not require a radically different technology compared to that of the paper pulp production.

The prehydrolysis-kraft process has the disadvantage that when the intensity of the hydrolysis is increased, selectivity towards cellulose is jeopardized. This leads to the fact that the wood consumption per ton of dissolving pulp produced may be uneconomically high. Selectivity can be improved by selecting milder hydrolysis conditions, at which hydrolysis does not remove as much hemicelluloses as would be theoretically possible. A lower rate of removal of hemicelluloses, however, means that in order to reach the desired alpha-cellulose concentration the removal of hemi cello ses has to be continued for example, with a hot-alkali step connected to bleaching.

Xyrofin and Eastman Kodak WO 98/56958 and WO 98/16682 (Al) describe the production of xylose in connection with an alkaline treatment. Xyrofin is the owner of a patent family (WO 01/04 362), which describes a solution in which xylan is removed from kraft pulp in one single step. This step is carried out at acidic conditions using hot formic acid. Cold alkali extraction is a method which is highly effective and selective with regard to the removal of xylan-based hemicelluloses and, thus, for purification of cellulose.

In cold alkali extraction, hemicelluloses are dissolved in polymeric form in a concentrated alkaline solution at a low temperature. The NaOH solution used in the processing should be clearly more concentrated than that used in a hot alkaline step. The solubility of the hemicelluloses into such a solution is limited, and after a time they begin to re-precipitate on the surfaces of the fiber. The other, mainly economic, limiting factor is associated with the closure of the alkali balance at a pulp mill. A known method is to use oxidized white liquor, and to return from the cold alkali step the alkali recovered by washing and to recycle it in concentrated form to the recovery cycle of the white liquor of a pulp mill. The use of a cold alkali step is not widespread due in particular to the difficulties with closing of the alkali recycle. Two features pose challenges with regard to process technology for the cold alkali extraction, viz . the low temperature of the stage requires uneconomical cooling and, after the accomplishment of the cold alkali step, heating in the middle of a process which otherwise is operated at a significantly higher temperature. It is possible to compensate for the uneconomical features by heat recovery, but all of the lost heat cannot be completely recovered.

The most difficult practical problem related to the performance of the cold alkali step is to take care of the further treatment and recycling of NaOH back to the process. A completely open process step, in which NaOH is not recovered but drained, is a completely · unacceptable (economic) alternative as regards the operational economics of the process and processing of the waste water. If the pulp of the cold alkali stage is washed efficiently, almost all the NaOH can be transferred to the washing filtrate, whereby NaOH can in principle be returned to the feed of the cold alkali stage and re-used. The washing step is never, however, 100% efficient, so that part of the NaOH will proceed forward along with the washed pulp through the process. This washing loss may be compensated by adding fresh NaOH to the filtrate recycle which is recirculated to the feed of the cold alkali stage.

More problematic than the loss of alkali is the fact that the same washing filtrate contains dissolved hemicelluloses, which are returned when the filtrates are recycled to the cold alkali stage. When the washing filtrate in this way is recycled, organic matter will accumulate into the circulation. According to literature, the dissolved xylan content must not be more than 10 - 20 g / 1, otherwise xylan will begin to precipitate back onto the fiber surface.

Due to xylan, total recycling of alkaline filtrates is not an option but the filtrate circulation has to be opened. When opening the filtrate circulation, due attention has to be paid to the fact that maintaining the concentration of the dissolved xylan at sufficiently low level will require a substantial amount of the filtrate circulation. In order to keep the xylan concentration below the solubility limit requires that at least half of the filtrate is removed from the filtrate circulation.

It is known that if the source of alkali used in the cold alkali stage is white liquor the NaOH containing solution removed from the filtrate circulation can be taken back to the chemical circulation of the pulping process for use as cooking chemical, e.g. in a prehydroly si s-sulphate cooking. The only NaOH exiting the chemical circulation of the mill is, in this case, formed by the portion leaving the process along with the pulp as a washing loss. Cold alkali extraction of kraft pulp is known, for example, from U.S. 2002/096276. As such this treatment step is not, however, sufficient for reaching the objectives of the present invention.

A similar cold alkali extraction is also described in patent application WO 2007/128026, in which the relevant phase is followed by bleaching, after which the bleached pulp is treated with ozone, chlorite or electron beam radiation, which does not achieve the same advantages as the present invention.

Cold alkali extractions according to the prior art do not correspond to the treatment step of the present invention, and do not afford the same advantages.

After cold alkali treatment, the paper pulp can be used for other chemical processes, which aim may be to lower the degree of polymerization (DP) of the cellulose or at opening of the fiber or at increasing cellulose reactivity. For such purposes there can be used, for example, peroxide treatment and/or treatment with endoglucanase enzymes.

The prior art does not include solutions wherein bleached kraft pulp would be treated first with cold alkali, and then the polymerization degree which describes the length of the cellulose molecules, would be adjusted with hydroxide, and not with endoglucanase enzyme.

The use of peroxide in the DP reduction is, indeed, disclosed in EP 1068376 Bl, but according to the publication hemicellulose is not extracted off before such a treatment nor after it either.

Summary of Invention

The present invention aims at providing a novel method for treatment of bleached kraft pulp.

In particular, it is an object of the present invention to provide a novel manufacturing process of dissolving pulp, which can be carried out in existing kraft mills producing paper cellulose or which is an advantageous way of carrying out cold alkali treatment in connection with prehydrolysis-kraft cooking.

It has now been found that these objectives are achieved using a method that includes a so- called cold alkali step, namely a phase in which the alkaline treatment is carried out at low temperature. This step is technically sensible and economically viable if the washing filtrate is fractionated such that the organic substance is enriched into the concentrate and the small-molecular inorganic substance travels through the membrane at a rate

proportional to the flows.

The present invention therefore relates to a method of manufacturing dissolving pulp, in which method the raw-material is bleached paper pulp obtained by the kraft method and for which material the present chemical treatment is carried out. A cold alkali stage carried out in this way can be equally well be coupled to a prehydrolysis-kraft cooking method of a dissolving pulp mill and with some restrictions even to a dissolving pulp process equipped with a sulphite cooking step. More specifically, the present inventive method is characterized by what is set forth in the characterizing part of claim 1.

The present invention enables the production of modified cellulose, especially dissolving pulp, produced in any mill producing bleached kraft cellulose. In addition, by adjusting filtration and concentration, it is possible by the present invention to handle the sodium supply for the entire pulp mill without there being hardly any additional need for sodium.

Membrane filtration and treatment of the filtrate by evaporation also allow for an increase of the alkali concentration of the cold alkali stage to a higher level than normal. By raising the concentration of the NaOH treatment it is possible to improve the selectivity of treatment with regard to cellulose.

Based on balance calculations carried out in connection with the present invention it has been found that it is preferred to carry out any wash after the cold alkali stage at high consistency. For this reason it is easy after the cold alkali stage to regulate the pH and/or the temperature of the pulp.

The method according to the present invention for separating hemicellulose material from the filtrate of the cold alkali stage also makes it possible to recover the hemicellulose material dissolved during the alkali stage from the pulp and to exploit the material in the cellulose manufacturing process. The hemicelluloses can be conducted for instance to cooking or to oxygen delignification, in which case a part of them can be precipitated on the fibers whereas the rest is taken to the production of energy by the recovery circulation.

Brief Description of the Drawing

Fig 1 shows one preferred embodiment of the method of the present invention. Detailed Description of Preferred Embodiments

The present invention relates to a method of manufacturing dissolving pulp or such modified pulp in which the raw material chosen is a normal hemicellulose containing bleached pulp, in particular paper pulp, cooked by the kraft method. The method can also be applied to dried cellulose pulp.

The raw-material is preferably washed prior to the treatment, after cooking and bleaching, with water or with an aqueous solution, preferably with water. Washing can be carried out in one or more stages.

The present method is carried out by cold alkali treatment which is effected by mixing the raw-material mass into a concentrated alkali solution to give a mixture comprising 50-220 g/1, for example 50 to 120 g/1 of an alkali, and allowing the solution to be absorbed into the mass at a temperature of 20-50 °C, preferably 20-40 °C. The alkali is allowed to act upon the pulp in order to dissolve the hemicelluloses into it, and the thus obtained hemicellulose containing alkali solution is separated from the thus treated pulp, i.e. dissolving pulp.

Finally, the hemicelluloses are separated from the alkali solution used for said treatment, and the solution is concentrated whereby a substantial part of the alkali solution is recycled, for example by recirculating it for the provision of a mixture of the raw-material and the alkali solution. In particular, at least 90% of the alkali solution is recycled.

In the solution according to the present invention it is essential that the hemicellulose concentration of the mass is reduced by employing the cold alkali treatment in a novel way. The cold alkali stage can be followed by further chemical processing to shorten the cellulose chains, for example in a peroxide step, or to improve the reactivity of cellulose using endoglucanase enzymes. The cold alkali treatment carried out in a novel way also makes it possible efficiently to manufacture dissolving pulp also in dissolving pulp mills based on present process solutions.

The absorption of the cold alkali treatment is continued for at least 10 minutes, preferably for 30 minutes - 2 hours.

The alkali used in the actual cold alkali treatment is a strong base, which preferably is the NaOH or white liquor commonly used in the kraft cooking, preferably it is an aqueous NaOH solution, having a concentration of from 10 to 50 % by weight. The alkali concentration is adjusted in the pulp suspension fed to the cold alkali stage to a level of 50 - 120 g NaOH/1. The concentration of alkali may be increased if necessary to improve the selectivity of the method up to the concentration of mercerization liquor which usually is 180 - 220 gNaOH/l.

By this treatment, hemicelluloses are dissolved in polymeric form in the concentrated alkali solution used. The temperature at this stage is from 20 - 40 ° C, preferably close to ambient temperature, that is, close to a value of 25 ° C, typically approximately 20-25° C.

It should be noted that there is no significant hydrolyzation taking place in the cold alkali stage but largely fiber swelling and dissolution. The cold alkali stage in particular is carried out in such a way that the base used in the treatment or the liquor is recycled. For recycling purposes, the base is preferably recovered by filtration, for example, with a semipermeable membrane, in particular by ultrafiltration. Recycling is preferably carried out while maintaining the necessary alkali concentration, for example, by evaporation of water and by removing dissolved organic matter (such as hemicelluloses) from the recycled liquor.

After the treatment the pulp is difficult to wash because it is swollen. Washing is however preferably carried out, whereby the pulp which has passed through the cold alkali stage is washed, for example, with water or aqueous solution, preferably water. In this case, the base used is conducted into the washing filtrate which is then filtered to remove the organic material as described above. Washing can be carried out in one or more stages, preferably in 2 to 3 steps.

It has been found that the performance of the cold alkali stage can be technically particularly meaningful and economical if the filtrate of the optional washing stage is fractionated by filtration such that the organic material, i.e. essentially the hemicelluloses dissolved in the cold alkali stage is enriched in the concentrate and the low molecular weight inorganic matter, i.e. in practice the alkali, is conducted according to the flow rate, through the membrane into the permeate. Washing losses can be, based on literature data, kept at a three-step washing at 20-50 kg NaOH / ton of pulp. Depending on the separation efficiency or the so-called cutoff-value of the filtration membrane used, and depending on the concentration ratio employed, it is possible to optimize the portion of NaOH containing permeate which is returned into the feed of the cold alkali stage such that the recycled concentrate contains an amount of NaOH allowed for by the production of the pulp and the proportion of dissolved organic material in the permeate recycle is kept sufficiently low, i.e. below the precipitation point of xylane which generally is considered to be 10 - 20 g xylan/1.

In the present invention there is preferably used an ultrafiltering membrane having a cutoff values in the range of 1 - 10 kDa.

Thus in one preferred embodiment the filtering is executed so that the hemicellulose is concentrated in the input side of the membrane, in the so called concentration stream, and the alkali passes the membrane essentially without concentrating in the permeate stream. In particular, the filtering is carried out in such a way that the hemicellulose content in the liquor, after filtering, remains in the reactor below the solubility limit, and by using a concentration rati o which enables total recycling of the concentrate as make-up alkali (generally 10 - 14 kg NaOH/ton of pulp, or less) for the cooking or oxygen delignifi cation. It is to be remembered that the ultrafiltration membranes used are not capable of separating 100% of the organic matter and the NaOH from each other, whereby there will be present both NaOH and organic material in the permeate and in the concentrate.

When the filtrate of the cold alkali stage is fractionated by recycling the NaOH containing permeate, as described above, it is also necessary to ensure that the desired alkali concentration is maintained at the cold alkali stage by taking into account the losses resulting from, inter alia, the optional washing after the cold alkali stage. Since the aim is to have a high recycle rate, only very little fresh NaOH needs to be fed into the feed of the cold alkali stage. For example, if the membrane filtration removes from the filtrate cycle to the pulping process 30 kg NaOH/ton of pulp, and if the washing loss of the washing following the cold alkali stage is 20 kg NaOH/ton of pulp, there is a need for fresh feed of NaOH for the cold alkali stage of 50 kg/ton of pulp.

Similarly, if the removal of hemi celluloses takes place during the cold alkali stage at a concentration of 100 g/1 NaOH, and if the fiber consistency of the stage is 10 weight %, then the required dosage of NaOH per ton of pulp is as high as 900 - 1000 kg, expressed as pure NaOH. If the pulp of the cold alkali stage is efficiently washed almost all of the

NaOH can be transferred to the washing filtrate whereby this NaOH can be recycled to the feed of the cold alkali stage and re-used. The washing step is never, however, 100 % efficient, so a part of the NaOH continues with the washed pulp further along the process. This washing loss can, however, be compensated by adding fresh NaOH to the filtrate recycle returned to the feed of the cold alkali stage.

Depending on the efficiency of the washing, the amount of NaOH exiting chemical circulation can amount to 10-100 kg of NaOH/ton of pulp. If it is desired to use fresh NaOH in the cold alkali stage, which is justified by processing efficiency and in order to avoid contaminants appearing in the white liquor, it is possible to recycle to the chemical circulation of the pulping process at the most 10 to 20 kg

NaOH/ton of pulp, whereby the Na balance of the chemical circulation is kept stable. On the other hand, if the filtrate of the cold alkali stage is used in alkaline bleaching steps as a source of NaOH, the need for alkali is of the same magnitude. At maximum it is thus possible to recycle to the pulping process from 20 - 40 kg of NaOH/ton of pulp of an alkali which results from the cold alkali stage.

The filtrate circulation of the cold alkali stage is almost completely closed. To the stage there is fed diluting water along with the pulp stemming from an optional washing stage of the raw-material pulp together with excess water from an optional washing step which is carried out after the phase. Also with the fresh alkali a small amount of water is transferred to the process. The permeate of the membrane filtration is diluted because of these diluting water volumes, and in order to maintain the water balance and in order to achieve a sufficiently high concentration of alkali water needs to be evaporated from the permeate. The evaporated water can be used for washing, for example in washing carried out after the stage or elsewhere in the pulp process. Treatment of the washing filtrate or of a filtrate obtained without washing with membrane filtration and by evaporation also makes it possible to increase the alkali concentration of the cold alkali stage to a higher level than normal. An increase of the concentration of the NaOH improves the selectivity of the treatment with respect to cellulose. As mentioned above, the NaOH concentration can to be increased to a concentration up to that of a mercerization liquor, which is generally at the level of 180 - 220 g NaOH/1.

Based on balance calculations it is preferred to use for the washing after the cold alkali stage a washing device or a washing system consisting of several washing stages from which the pulp is finally removed pressed to high consistency, for example at a consistency of more than 25 %, which is typical for a washing press. Consequently, after the cold alkali stage it is easy to adjust the pH to and/or temperature of the pulp. It becomes necessary to adjust the acidity for example when endoglucanase enzyme is being dosed in the next process step to increase the reactivity of the cellulose. pH adjustment may be needed also if the pulp is conducted to an acid bleaching reaction stage or to a dryer.

By using the method the alkali consumption of the entire mill can be reduced by improving the washing carried out after the cold alkali stage by neutralizing the pulp with S0₂ or with sulfuric acid, in particular to a pH of 5 to 7, prior to the last washing stage. The thus obtained neutralized filtrate can be conducted for example to a washing of the raw-material pulp after bleaching, or to an endoglucanase treatment carried out for opening the fibre wall.

Based on the afore-described, in one embodiment, the raw-material, which is a bleached hemicelluloses containing paper pulp produced by kraft pulping,

- is subjected to a cold alkali treatment by mixing the raw-material pulp with a concentrated alkali solution such that the mixture formed by the pulp and the alkali solution contain 50 - 220 g/1 alkali, e.g. 50 - 120 g/1, and the solution is allowed to absorb into the pulp at a temperature of 20 - 50 °C, - then the alkali is allowed to act on the pulp for dissolving hemicelluloses from it,

- the obtained hemicelluloses containing alkali solution is separated from the thus treated pulp,

- hemicelluloses are separated from the separated alkali solution,

- the alkali solution is concentrated and

- a part of the alkali solution is circulated for renewed use to provide a mixture of a raw-material pulp and concentrated alkali solution. In another embodiment, the alkali solution which contains hemicelluloses and which is obtained from the cold alkali treatment is separated from the treated pulp, and then

• hemicelluloses having a high molecular weight are separated from the alkali

solution by ultrafiltration using an ultrafiltration membrane;

• the alkali solution which passes the ultrafiltration membrane is concentrated by evaporation in order to concentrate the alkali solution so that it has a greater concentration than in the cold alkali treatment;

• the bulk of the thus obtained, concentrated alkali solution is recycled back to use for providing a mixture of the raw-material pulp and a concentrated alkali solution and

· at least a part of the alkali solution is circulated to production of paper pulp.

In both cases, the alkali solution obtained from separation, in particular the concentrate of ultrafiltration, is conducted to cellulose production by kraft pulping, for example to an oxygen delignification step. Preferably the amount corresponding to the make up alkali need of the mill is conducted to the oxygen delignification.

At least a part ((typically 1 to 45 % by weight)) of the hemicellulose containing alkali solution is conducted in one preferred embodiment to the bleaching of a kraft pulping process to replace fresh alkali such that the amount of fresh alkali fed into the cold alkali stage can be kept at 40-90 kg NaOH/ton of pulp.

Furthermore, at least a part of the large molecular hemicellulose of the concentrate stream is circulated to the production of paper pulp and precipitated on the surface of the cellulose fibres. Then at least a part of the circulated hemicellulose ends up in the recovery circulation of the kraft pulping process and is conducted further to combustion in a recovery boiler. The term "high molecular weight" used in respect of hemicellulose means, in the present context, hemicellulose which does not significantly or at all pass an ultrafiltration membrane of the kind described herein.

To be complete, it must be stated that from concentrate stream containing hemicellulose, in which, in a preferred embodiment a part of can be precipitated on the surface of the fibres, for example in oxygen delignification, xylan can also be separated by known methods (for example PCT application publication WO/2007/065969). This type of xylan can be used for example as a barrier material. Figure 1 shows an apparatus suitabl e for use in a specific, preferred embodiment of the method according to the present invention. The apparatus comprises the following parts:

1. Prewasher

2. Cold alkali treatment reactor

3. Post-treatment washer

4. Membrane filtration device

5. Evaporator

6. Pulp feed

7. Reactor 2 outlet

8. Water inlet of washer 3

9. Outlet of treated pulp

10. Feed piping for filtration

11. Outlet for concentrate of filtration

12. Discharge piping for permeate of filtration

13. Alkali circulation piping

14. Water outlet for evaporation

15. Inlet of fresh alkali

16. Sidestream - feed to pulp mill, for example to bleaching Thus, in the process of the present invention, the raw material pulp is conducted through the inlet 6 of the pulp into the cold alkali reactor 2. The pulp can be obtained, for example, from an optional Prewasher 1 which preceeds the cold alkali stage and which has the task of not only reaching the desired washing result but also to regulate the temperature of the pulp suspension conducted to the cold alkali stage to a desired level. To the pulp inlet 6 there is also preferably introduced fresh alkali solution through the inlet 15 of this feed and via the alkali circulation line 13 of the recycled filtrate.

The concentrate 1 1 of the filtration 12 can be conducted to a pulp mill, e.g. to oxygen delignifi cation. Similarly, a part of the concentrated filtrate can be conducted 16 to a pulp mill, e.g. to bleaching.

After a preselected reaction time, the cold alkali treated pulp is drawn off the reactor 2 along outlet 7 and conducted to, for example, an optional postwasher 3. To this washing stage, which is carried out after the cold alkali treatment, a preselected amount of washing water is conducted along water inlet 8. The temperature of the washing water can be adjusted as needed so that the cold alkali treated pulp that enters the next process step is at the desired temperature. Furthermore, in the washing step, the pH of the discharge pulp can be adjusted to a level suitable for a next processing step.

The cold alkali treated and optionally washed pulp is conducted away from the cold alkali stage along the pulp discharge piping 9.

The efficiency of the optional washing stage carried out after the alkali treatment has to be selected such that the alkali and the organic substances dissolved by the alkali stage (mainly hemicellulose material) which are conducted along with the flow of the treated pulp can be transferred to the filtrate of the post washing stage, which filtrate preferably is further conducted to the feed piping 10 of the filtrate. The recovery rate of this washing stage with respect to alkali and dissolved organic material is at least 95 %. The recovery rate can be even higher than that, even in excess of 98 %, if it is desired to recover the alkali as completely as possible or if the next processing step requires significant adjustment of pH.

The filtrate of the washing step is conducted preferably through said filter feed pipe 10 to the membrane filtration device 4, which can be an ultrafiltration device. In the membrane filtration device 4 a desired amount of water and filtrate is transferred through the membrane, which together form the so-called permeate. The cutoff value of the membrane determines how small molecular weight materials will most likely be transferred to the permeate. For example, NaOH has so low molecular weight that its concentration in the permeate is essentially the same as that of the inlet filtration flow. Material with a higher molecular weight, in this case an organic material mainly consisting of dissolved hemicelluloses, is concentrated in the so-called concentrate. It is characteristic to determine for membrane filtration a so-called concentration ratio which is the flow ratio of the feed flow to the flow rate of the concentrate. For example, if the concentration ratio is 20, it means that the flow of the concentrate is 1/20 of the volume of the feed stream. Therefore, 19/20 of the feed stream is transferred to the permeate. The mass flow of the NaOH is divided between the permeate and the concentrate in relation to their flow rates because sodium hydroxide has a molecular weight which is very small compared to the membrane's cutoff value.

A so-called retention value is generally defined for the high molecular material which is intended to be fractionated by the membrane separation. The calculation formula for the retention is R = 1 - C_p / Cf, wherein C_p is the concentration of the material to be fractionated in the permeate and C_f is the corresponding concentration in the feed stream. For example, if it is desired that the concentration of hemicellulose in the permeate is one tenth of that in the feed stream, then the membrane must have a retention of 0.9 for this material.

From the membrane filtration device 4 the concentrate is removed via concentrate discharge 1 1, and the permeate is removed through the permeate piping 12. The concentrate of the membrane separation, into which the bulk of the hemicellulose material dissolved in the cold alkali step and a desired small amount of NaOH has been

concentrated, is removed from the filtrate circulation and conducted preferably back to preceding steps of the cellulose process, for example to cooking, to washing of brown pulp, to oxygen delignification or to bleaching.

Next, the permeate flow is preferably conducted to evaporation in an evaporator 5 which is suitable for that purpose in which the permeate can be concentrated to desired

concentration for example by removing water, preferably by using evaporating water removal 14. Evaporation can be carried out by using different known technologies which are generally in use for aqueous NaOH containing solutions. Depending on the performance of the evaporation stage there may be a need for temperature regulation and heat recovery for adjusting the temperature of the circulating filtrate flow in order to maintain a desired temperature level of the cold alkali stage. For water economy of the process it is preferred to use the evaporated water in a second washing stage as washing water, depending on how that is made possible by the liquid balance. The evaporated water can also be used as washing water also elsewhere in the manufacturing process of cellulose, in washing stages where there is a need for clean washing water or, more generally, fresh water.

The concentrated permeate containing the desired amount of recyclable alkali at the desired concentration and with sufficiently low content of hemicellulose (so that no back- precipitation occurs), is preferably conducted to the pulp flow fed into the cold alkali stage, i.e. into the pulp feed 6. The alkali and liquid balance is thus closed, and the concentration of the dissolved hemicelluloses (mainly xylan) is sufficiently low.

According to a preferred embodiment, one or several chemical further processing steps are carried out after the cold alkali stage, whereby for example the length of the cellulose chains are shortened in a peroxide step or the reactivity of the cellulose is improved by using endoglucanase enzyme.

In the peroxide treatment the peroxide solution is allowed to act for at least 10 minutes, especially at least 30 minutes, for example 45 minutes - 10 hours, preferably 1 - 5 hours, at a temperature which is 80 - 100 ⁰ C, preferably about 90 ° C.

At treatment with endoglucanase enzyme preferably a neutral pH and a temperature range of 50 - 60 ⁰ C is maintained.

The cold alkali extraction is suitable to use in combination with both the sulphite process and a prehydrolysis-kraft process. With regard to the location of the hemicellulose removal, the process is flexible: it can be placed in connection with oxygen delignification, inside bleaching or after bleaching. Implementing the cold alkali stage as a step which is carried out after bleaching for example before a dryer which is operated at a temperature which is significantly lower than that of the bleaching is more advantageous from the point of view of heat economy than the placing of it in the middle of the process.

The prepared dissolving pulp can be used, for example, in viscose manufacture, or in speciality papers, in pressing products or tissue products, in particular in products, such as a plastic laminates, photo papers, filter papers, blotting and absorption papers, egg cartons, or baby diapers or disposable medical supplies, or, after further processing,

microcrystalline cellulose.

The following non-limiting examples illustrate the invention and the advantages thereof.

Examples Example 1 The following calculations are based on an example of a preferred embodiment of the method according to the invention and carried out with an apparatus as shown in Figure 1, wherein (X is for xylan, "tp" for tons of pulp):

Mass in question, 6: consistency 30%

Reactor, 2: consistency 10%, the concentration of NaOH solution 70g/l, dissolved X 50kg/tp, X=20g/1

Inlet of the washing water: 5.33 m³/tp

Washed mass, 9: consistency 30%, NaOH 10 kg/tp, X=2.9 kg/tp

Filtrate of the washer, 10: 12.0 m³/tp, NaOH 620 kg/tp, NaOH 51.7 g/1, X=14.8 g/1 Permeate, 12: 11.42 m³/tp, NaOH 590 kg/tp, NaOH 51.7 g/1, X=l 1.4 g/1

Concentrate, 11 : 0.58 m³/tp, NaOH 30 kg/tp, X=47.1 kg/tp, X=81.2 g 1

Evaporated water, 14: 4.87 m³/tp

Fresh NaOH, 15: 40 kg/tp

Recirculated filtrate, 13. 6.55 m³/tp, NaOH 590 kg/tp, NaOH 90.12 g/1, X=19.9 g/1

Thus

• The reactor feed comprises a pulp which comes from a first washer (wash press or equivalent) at a 30 % consistency;

• After the reaction phase the consistency of the effluent pulp is also 30 %;

· The consistency in the reactor is 10 %;

• In the reactor the prevailing NaOH concentration is 70 g/1;

• In the reactor, hemicellulose (xylan) is dissolved at 50 kg/ton of pulp;

• In the reactor, the allowed xylan concentration is 20 g/1;

• Surplus of washing water after the reactor is 3 m³/tp, i.e. since at 30 %, 2.333 m³/tp of the mass is water, in other words the total amount of wash water is 5.333 mVtp;

• NaOH wash loss of the subsequent wash is 10 kg/tp, with the concentrate 30 kg NaOH/tp is returned to the cellulose pulping process, i.e. the need for fresh-NaOH is 40 kg of NaOH tp. In a preferred embodiment of the method, the following calculations have been carried out based on the above input, which gives the following results:

• In the after-washing 620 kg NaOH/tp is transferred to the filtrate, 590 kg NaOH/tp being returned to the reactor feed;

• The concentration ratio of the membrane filtration is 20.7, because the flow ratio between inlet and concentrate outlet of the membrane filtration is 12/0.58 = 20.7;

• Retention of xylan during membrane filtration is 0.82, in order for the filtrate to be returned to the reactor is sufficiently low in xylane concentration

· 47.2 kg of xylan is transferred to the pulping process, which means that of the

xylan removed from the pulp in the reactor the recovered mass is about 94 % The evaporation need is about 4.9 m3/tp, whereby the NaOH concentration of the permeate increases from 51.7 g/1 to 90.1 g/1. Example 2

68.5 litre of washing filtrate of a cold alkali stage was concentrated by ultrafiltration using an ultrafiltration membrane made of a polysulphone material. The cutoff value of the membrane was 10 kDa.

The test conditions were as follows: temperature 20-25 °C (room temperature) and pressure 0.8-1.2 bar overpressure.

The composition of the filtrate before ultrafiltration was:

NaOH 89 g/1,

carbohydrate concentration 13 g COD/1,

carbohydrate composition: xylan 67 % and glucomannan 33 %

The filtrate was concentrated on the ultrafiltration device at a concentration ratio of 6.9. This means that the portion which passes the membrane in ultrafiltration (permeate) was 58.7 litre and the concentrate was correspondingly 9.8 litre.

The composition of the concentrate was:

NaOH 84 g/1, carbohydrate concentration 54 g COD/1,

carbohydrate composition: xylan 67 %, glucomannan 30 %, glucose 3 % (glucose is apparently derived from partially hydrolyzed glucomannan) The composition of the permeate was:

NaOH 84 g/1 and

carbohydrate composition: 2.2 g COD/1

It can be noted that the carbohydrate composition, i.e. the relative portions of xylan and glucamannan, remains roughly the same in the feed and in the concentrate. Ultrafiltration retains rather well the carbohydrate material: COD-retention was 1-2.2/13 = 0.83 (83 %).

Correspondingly, NaOH is divided in the relationship between the flows, this means that retention of NaOH is in practice zero. Some minor dilution takes place in the system because of leakage of condensation water to the apparatus. The test arrangements were not of a kind which allowed further concentration to be carried out. However, it is evident based on the results already obtained that by further concentrating the streams, a concentration ratio of 10 to 20 could have been attained.

The test results show that an ultrafiltration membrane having a nominal cutoff value of 10 kDa in the present method makes it possible selectively to separate dissolved carbohydrate. Selectivity can be further improved by lowering the cutoff value of the membrane to, for example, a value of about 5 kDa.

Claims

1. Method of producing modified cellulose, such as dissolving cellulose pulp, from a hemicellulose containing cellulose pulp obtained by the kxaft method, in which method the cellulose pulp is subjected to a cold alkali treatment for reducing its hemicellulose concentration, wherein

• a bleached paper cellulose pulp cooked by the kraft method is selected as raw- material;

• the cold alkali treatment is carried out such that the raw material pulp is mixed into a concentrated alkali solution so that the pulp and alkali solution mixture contain 50 - 220 g/1 alkali, and the solution is allowed to absorb into the pulp at a temperature of 20 - 50 °C;

• the alkali is allowed to act on the pulp for dissolving hemicelluloses into said alkali;

• the thus obtained hemicelluloses-containing alkali solution is separated from the thus treated pulp;

• hemicelluloses having high molecular weight are separated from the alkali solution by ultrafiltration using an ultrafiltration membrane;

• the alkali solution which has passed through the ultrafiltration membrane is

concentrated by evaporation in order to increase the concentration thereof;

• the bulk of the thus obtained, concentrated alkali solution is recycled for renewed use in forming a mixture of the raw-material and a concentrated alkali solution; and

• at least a part of the alkali solution is recycled to the production of cellulose paper pulp.

2. The method according to claim 1, wherein the concentrate of the ultrafiltration is conducted to an oxygen delignification stage of the production process of the kraft cellulose pulp.

3. The method according to claim 2, wherein the amount conducted to the oxygen- delignification stage corresponds to the requirement of make up alkali of the pulping mill.

4, The method according to any of the preceding claims, wherein at least a part of the hemicelluloses having a large molecule size of the concentrate flow are recycled to the production of paper pulp and precipitated on the surface of the cellulose fibres, whereby at least a part of the recycled alkali ends up in the recovery circulation of the production process of the kraft pulp and is then conducted further for combustion in a recovery boiler.

5. The method according to any of the preceding claims, wherein a part of the alkali solution containing hemi celluloses is conducted to the bleaching of the production process for the kraft pulp for replacing fresh alkali such that the amount of fresh alkali fed into the cold alkali stage is kept at 40-90 kg NaOH/ton of pulp.

6. The method according to any of the preceding claims, wherein hemicelluloses are separated from the alkali solution using an ultrafiltration membrane wherein

hemi cell uloses are concentrated on the inlet side of the membrane, into the so-called concentrate flow, and the alkali passes through the membrane essentially unconcentrated into the permeate flow, the ultrafiltration membrane having a cutoff value of 1-10 kDa.

7. The method according to any of the preceding claims, wherein over 90 % of the alkali solution is recycled for use in the provision of a mixture of the raw-material pulp and the concentrated alkali solution.

8. The method according to any of the preceding claims, wherein the alkali solution is concentrated by evaporating off water from it.

9. The method according to any of the preceding claims, wherein at least a part of the hemicellulose containing concentrate flow is conducted to the cooking of kraft cellulose pulping process for replacing fresh alkali or white liquor, at least a part of the

hemicellulose being precipitated on the surface of the fibre.

10. The method according to any of the preceding claims, wherein the raw-material is washed before the alkali treatment with cold water of a temperature of 10 - 40 °C in one or more stages.

1 1. The method according to any of the preceding claims, wherein the alkali used is a strong base, which preferably is NaOH or white liquor, in particular an aqueous NaOH solution.

12. The method according to claim 11, wherein an aqueous NaOH solution is used having a concentration of alkali of 10 - 50 weight-%.

13. The method according to any of the preceding claims, wherein the alkali solution is allowed to act on the pulp for a sufficiently long to dissolve hemicellulose for at least 10 minutes, preferably 30 minutes to 2 hours.

14. The method according to any of the preceding claims, wherein the treated pulp is washed in one or more stages.

15. The method according to claim 14, wherein the washing after the treatment is carried out for the pulp while simultaneously pressing it to a consistency of 25 to 45 %.

16. The method according to claim 14 or 15, wherein the pulp is neutralized with acid, in particular to a pH of 5 - 7, before the last washing stage.

17. The method according to any of the preceding claims, wherein the organic matter concentration of the permeate is maintained at a value which is below 20 g xylan 1, preferably at a value below 10 g xylan/1.

18. The method according to any of the preceding claims, wherein evaporated water is circulated to the raw-material pulp to for use as washing water for the treated pulp.

19. The method according to any of the preceding claims, wherein the cold alkali treated pulp is conducted to the peroxide stage of the kraft pulping in order to shorten the cellulose chains and reduce the degree of polymerization of cellulose.

20. The method according to any of the preceding claims, wherein the pulp obtained from the cold alkali treatment and any following peroxide treatment is conducted to

endoglucanase treatment for opening of the fibre wall and for improving the reactivity of the cellulose.

21. The method according to any of the preceding claims, wherein it is carried out by using an apparatus which contains a cold alkali reactor (2), a pulp inlet (6) for feeding raw- material pulp into the reactor (2), an inlet (15) for feeding fresh alkali into the reactor (2) and a pulp outlet (9) for removing treated pulp from the cold alkali stage.

22. The method according to claim 21, comprising using an apparatus which contains a washer (3) for washing of the treated pulp.

23. The method according to claim 21 or 22, comprising using an apparatus, which contains a membrane filtration apparatus (4) used for separating alkali from organic matter.

24. The method according to claim 23, wherein the recovery degree for the alkali and the dissolved organic matter is at least 95 %, preferably in excess of 98 %.