EP0823948B1 - Method of producing a pulp of cellulosic material - Google Patents

Abstract

A process for producing a pulp from cellulose-containing material, wherein the material is reacted with formic acid as the solvent, and cooked at approximately the boiling temperature of the solvent, whereby return condensation is used. Annual plants, deciduous or coniferous wood can be used as the cellulose-containing material. In one variant of the process, the cellulose-containing material is only slightly warmed, whereby backflow cooling is used, and then a precisely predetermined quantity of hydrogen peroxide is slowly added to the liquid at a constant rate. This process variant can be repeated at a lower cooking temperature. The pulp thus obtained is preferably utilized in the production of cellulose, and in particular in the production of paper or cardboard. It is proposed that the lignin, which is isolated from the cellulose-containing material, have further applications, whereby such lignin is, after the pulp has been separated from the solvent, itself precipitated out in water. The lignin thus obtained can be used as a new building material, as a filler material or as an output substance to be used in the manufacture of aromatic products.

Description

The invention relates to a method for producing a pulp from cellulose-containing Material.

Pulp is understood to be a cellulose pulp that is required for the production of cellulose. This pulp is then used to make paper or cardboard, for example.
The pulp-producing industry is finding it increasingly difficult to meet the requirements and requirements relating to environmental protection. In addition, it is no longer permitted, for example in Germany after the environmental regulations of 1990, to use traditional pulp production processes such as the sulfate process. When using sulfur-containing disintegrants in order to extract the lignin that acts as a composite from the cellulose-containing material, only the sulfite process is used in Germany.

As a result of efforts to use greener pulp production processes to develop, the so-called Acetosolv process, developed which the digestion liquid contains at least 50% by weight of acetic acid and additionally a small amount of hydrochloric acid. After this information the pulp thus obtained with sodium hydroxide solution and optionally also with organic Solvents washed to make the lignin more or less complete to remove.

A disadvantage of this process is the relatively high acetic acid consumption and the use of sodium hydroxide solution and optionally organic solvents for washing the pulp.
In accordance with the high demands of the paper industry, the pulp obtained in this way is regularly bleached in a subsequent step in order to achieve kappa numbers of at least below 25. In the Acetosolv process, too, downstream bleaching by means of a peroxide is provided, with a relatively high consumption of this bleaching agent being disadvantageous.
EP-A-0 325 891 has therefore disclosed an improvement in this acetosolve process. The improvement essentially consists in that the pulp is no longer washed with sodium hydroxide solution after digestion, but with a C _1-3 carboxylic acid or a mixture of such acids and that the subsequent bleaching is also carried out in the acidic medium with additional Use of hydrogen peroxide or ozone is carried out. A carboxylic acid ester such as butyl acetate can also be used as the solvent.
In addition, it is known from the cited document that the C _1-3 carboxylic acid used for washing the pulp can subsequently be used again as a digestion liquid.
WO-A-82/01902 also discloses a process for producing a pulp from cellulose-containing material using formic acid. GB-A-850 367 teaches the use of formic acid in the production of cellulose.
WO-A-86/05529 has disclosed a process for producing bleached pulp from cellulose-containing material, which is generally also referred to as the Milox process. In this process, the digestion is carried out in a peroxyformyl, peroxyacetic, peroxypropion or peroxybutyric acid medium, the peroxyacid being obtained by mixing the respective acid with a relatively high proportion of hydrogen peroxide and then carrying out the digestion process. This digestion is followed by bleaching of the pulp, which is carried out in an alkaline solution to which hydrogen peroxide has been added. The disadvantage of this process is again the use of an alkaline solution such as sodium hydroxide solution and the high proportion of hydrogen peroxide required.
Starting from this prior art, the present invention has for its object to provide a method for producing a pulp from cellulose-containing material that works as environmentally friendly as possible and at the same time extremely economical and efficient.
The method according to the invention as claimed in claim 1 is used, for example, when the cellulose-containing material used is not so easy to digest. This is the case, for example, with hardwood or coniferous wood, but also with grain straw, depending on the intended use of the pulp obtained from it. The cellulosic material is mixed with formic acid and water and heated slightly under reflux by an external energy source. Then hydrogen peroxide is slowly added in a constant dosage. Such a procedure has hitherto not been known by any prior art and enables a very economical procedure because the now exothermic reaction itself provides the heat necessary for cooking. This means that after initial activation, no external energy supply is required to a large extent. At the same time, the reaction control is made considerably easier because only a small predetermined amount of hydrogen peroxide is added to the digestion solution. Digestion solution is understood to mean the sum of the solvent, the cellulose-containing material and components, such as lignin and sugar, that may have been extracted therefrom. Furthermore, in the reaction according to the preferred embodiment it is advantageous that the equilibrium in the reaction of the formation of the peracid from formic acid and hydrogen peroxide is continuously shifted to the peracid side by the continuous addition of the hydrogen peroxide.

In the process according to the invention, aqueous formic acid can be combined in one Concentration range between about 60 to 99% by weight can be used. The use of 100% formic acid is not cheap, because at least one certain percentage of water in the manufacture of the pulp, i.e. in the digestion process should be present. The cooking time can be a period of can be given in about 30 to 120 minutes, depending on the cellulose-containing used Material.

The predetermined amount of hydrogen peroxide can be about 1 to 3 % By weight, preferably 1 to 2% by weight, particularly preferably 1% by weight on the total weight of cellulosic material and solvent be. It is particularly advantageous here that only a very small amount of added hydrogen peroxide is necessary, which is sufficient to the one hand Requirements for the whiteness of the pulp so produced for the production of paper and which, on the other hand, is sufficient to withstand the temperature of the To keep the digestion solution approximately at the boiling point of the solvent, without supplying additional energy in the form of heat. So that can good properties of the paper made from the pulp can be combined with a minimal expenditure of energy.

It has proven advantageous to use the liquid-to-material ratio in the Keep range from 20: 1 to 25: 1.

The method according to the invention can further be modified in this regard be that in addition to the solvent a gas such as air, oxygen, ozone or a comparable gas or a mixture of two or more of these Gases is introduced. The oxidative power of these gases is used, which increases lignin degradation and thus whiteness, as well as the kappa number lowers. The required digestion time is also reduced.

According to the invention, the pulp can be easily removed after the cooking time Sieves are separated from the solvent. This will sieve understood in the broadest sense, i.e. a separation by a suitable one Membrane, a filter or a frit is also possible, but then that Disconnection can happen continuously. The pulp so screened can then be washed with water and / or formic acid. Especially when Washing with formic acid becomes the remaining lignin, which is already out the cellulose-containing material is dissolved out. By opening the pulp by means of a stirrer can still flush this out be promoted.

When the pulp has been washed with formic acid, it becomes formic acid together with the solvent from the digestion process by simple Distillation recovered. The proportion of formic acid recovered is usually over 95% by weight. The rest of the Formic acid remains as a residue with the lignin.

If the pulp has been washed with formic acid, it will subsequently washed neutral with water, with this then optionally Wash water of the above distillation can be supplied so also recover the amount of formic acid remaining in the pulp.

With the above-mentioned method according to the invention or its more preferred Embodiment has been a method of making a pulp cellulosic material specified, for example for the production of Paper with sufficient whiteness (e.g. kappa number <10) in one One-step process with minimal energy consumption is suitable.

Depending on the cellulose-containing material used or on the particular requirements for the paper properties of the paper to be made from the pulp, for example, it may be necessary to further increase the degree of whiteness. This is also made possible according to the method according to the invention in a particularly preferred embodiment with minimal energy and at the same time minimal expenditure on apparatus, since it can then be provided that the method according to the invention is repeated. The cooking temperature is reduced. Aqueous formic acid is then used as the solvent, to which hydrogen peroxide, as already described above, is added. The reduced cooking temperature can be around 70 to 80 ° C. The cooking temperature is preferably 70 ° C., since at this temperature there is an optimum of performic acid. At the same time, the cooking time can be extended to up to 5 hours to achieve a higher degree of whiteness. However, the cooking time is then preferably about 3 hours, because it was found that after this time the pulp no longer brightened up appreciably.
It has also proven advantageous to increase the liquid-to-material ratio to over 25: 1.

For example, elephant grass (Miscanthus) can be used as a cellulose-containing material. and / or corn leaves or stems are used. Both materials, in particular Elephant grass can be easily unlocked. It can even the addition of hydrogen peroxide to the solvent can be dispensed with. For example, when using 99% formic acid and one Cooking time of one hour reaches a kappa number of 4.7.

A particular advantage of the method according to the invention is, however, that as cellulosic material basically annual plants, especially cereal straw, can be used. This is especially true for papermaking important, because so far almost exclusively as a cellulosic material Wood is used. Worldwide there are over 200 million a year Tons of paper used. The demand continues to grow. The deforestation large forest areas to cover the need for cellulose-containing material but brings significant environmental problems such as climate change and habitat destruction of flora and fauna with them. Where, if any, Reforestation is usually only rapidly growing Monocultures created, which in turn are very pest-prone and ecological are not useful. In addition, developing countries or Countries of the so-called "third world" an increasing need for paper for the determine various purposes. For the usual production is missing mostly wood as a cellulosic material. Here the grain straw can than in terms of costs and ecological aspects serve as a sensible substitute. Often straw is even considered a nuisance and burned on the fields harmful to the environment. Because in Germany alone In total, around 50 million tons of straw are harvested each time there is a large potential of cellulose-containing material available. This applies even more to countries like the United States or Canada, which are known to have huge grain fields. But also for developing countries or countries of the so-called "third world" can be the cultivation of simple Annual plants an opportunity to use difficult soils and at bad climatic conditions. It is possible where the Planting more demanding cereals due to weather or Soil conditions are no longer possible on management means easier to pass annual plants and thus cellulosic material to produce a pulp, which in turn is used for paper production, provide.

When using grain straw, it has proven to be useful that to use the method according to the invention in its preferred embodiment and the digestion solution a precisely predetermined amount of hydrogen peroxide to be added continuously in constant doses. By means of this Process it is possible to obtain a sufficient degree of whiteness from that obtained Ensure pulp made paper. The repetition of the Method according to the particularly preferred embodiment with reduction the cooking temperature and possibly increasing the cooking time however of course also possible.

However, the process according to the invention is also based on the cellulose-containing processes which have been customary to date Materials such as hardwood or softwood can be used. You know that it is generally more difficult with these materials, an adequate one To provide the whiteness of the paper made from the pulp, it then lends itself, according to the particularly preferred embodiment to proceed and the inventive method while reducing the Repeat cooking temperature and optional extension of cooking time.

Another aspect of the present invention relates to that from pulp the process of the invention lignin. This is after the Recycling of formic acid used as a solvent by simple Distillation obtained from the distillation residue by precipitation in water. This precipitation of the water-insoluble lignin at the same time makes it of the water-soluble sugars also present in the residue separated.

The pulp thus obtained is used for pulp production for example for the paper industry or for the production of Cardboard used. Of course, that is according to the method of the invention manufactured pulp can be used and used wherever pulp is used Starting material for a product is required. For example Chemical cellulose or any other product from regenerated or chemical modified cellulose from the pulp.

Lignin, which by the inventive method is available can be safely processed as it is contains no sulfur or chlorine, as is the case in the prior art known methods for producing a pulp would mostly be the case. The Isolated lignin can be used, for example, as a construction material, in particular as Starting material for pressboard, medium density fibreboard or as filler be used if it decomposes with excess oxalic acid and either melted or boiled in a saturated formic acid solution and then steamed with cellulose fiber. This creates a waterproof black-brown mass.

The uses of lignin are even more diverse. So it can be used as a raw material for the production of flavorings such as vanillin or mulled wine flavor.

In the following, the invention will be explained in greater detail on the basis of exemplary embodiments explained.

1. Making a pulp out of straw

Cereal straw, formic acid and water are transferred into the reaction vessel in accordance with the percentages by weight given below and in accordance with the liquid-to-straw ratio also given there. On a laboratory scale, this is a round bottom flask, combined with a ground thermometer and a dimrotre reflux condenser. The reaction mixture is then heated slightly under reflux and then hydrogen peroxide of the concentration given below is slowly added continuously. The reaction is exothermic, so that the external energy supply, such as here via a heating pad or a heating plate, can at least initially be reduced and then completely eliminated. The digestion time is also given below. After the cooking time has ended and the reaction vessel has cooled, the pulp is separated from the pulping solution by simple sieving and mixed with further fresh formic acid in a concentration of 60 to 80% by weight, so as to flush out the remaining lignin. A paddle stirrer is then inserted into the reaction vessel and stirred for about one minute. This creates a defibrillation of the cellulose dressings and the lignin washout is facilitated. After the formic acid has been separated off, it is recovered with the formic acid from the digestion by simple distillation. Again on a laboratory scale, a Liebig distillation apparatus and a 300 mm column were used. The formic acid used can largely be recovered here. Usually in a percentage of over 95%. What remains is a residue of lignin and sugars or residual carbohydrates, which further contains solvent that is not recovered as residual moisture. The insoluble lignin is separated from the soluble sugars by precipitation in water and can now be processed further.
The pulp is washed neutral with water and air-dried.

Digestion times of 30, 45, 60, 90 and 120 minutes were selected in the experiments. These were combined with formic acid concentrations of 50, 60, 75, 80, 85, 90, 95 and 100%. The amount of hydrogen peroxide added was varied between 1% and 2%. It was found that an addition of more than 1% did not bring any appreciable advantage, but only made the digestion more difficult to control due to the even more violent exothermic reaction. The digestion temperature was always kept at the boiling point of the solvent.
Two experiments are shown below by way of example, experiment 1.a) not according to the invention. The liquid-to-straw ratio is referred to as the liquor ratio. The test parameters shown indicate optimized production processes, although the optimization in test 3.a) was carried out in such a way that not only was a maximum lignin removal achieved, but the lignin itself should also be obtained in as large a quantity as possible. It has been shown that optimal lignin removal is accompanied by an increased breakdown of the lignin to water solubility. The optimization was therefore carried out in such a way that lignin which had not been broken down or altered as far as possible should be obtained in addition to the lignin-free pulp. Test parameters 1.a) liquor ratio 25: 1 Concentration of formic acid 80% Concentration of water 20% digestion time 2h Yield of pulp 56% Yield of lignin 21%

This and all other figures refer to% by weight, unless stated otherwise. The percentages regarding the lignin yield relate to the total mass of water-insoluble residues. Test parameters 1.b) liquor ratio 25: 1 Concentration of formic acid 75% Concentration of hydrogen peroxide 1% Concentration of water 24% digestion time 30 minutes Yield of pulp 56% Yield of lignin 38%

2. Modification of the production of a pulp from straw

Special requirements for the quality of the pulp or paper made from it may require whiteness further increase. Then the digestion procedure mentioned under 1 can be repeated, with the most favorable being essentially concentrations to choose the equivalent to the concentrations mentioned under 1. are.

First the straw is treated as indicated under 1. After the cooking time has ended, a stirrer is used and the pulp obtained is broken down for about 1 minute. Then the pulping solution is separated from the pulp and further processed without drying.
The parameters of an experiment for producing a pulp with a repetition step are given below. The repetition step is labeled 2.b.). Experimental parameters: digestion reaction 2. a) Repetition 2.b) Liquor ratio: 1:25 > 1:25 formic acid concentration 60% 60% Water concentration: 39% 39% Hydrogen peroxide concentration: 1% 1% Cooking temperature: approx. 106 ° C 70-80 ° C Cooking time: 30 min. 3h

Pulpe: 25%

Lignin: 20.2%.

The yields after the completion of both stages were:

Pulp: 25%

Lignin: 20.2%.

3. Production of a pulp from softwood

It is known that paper made from a pulp of softwood exhibits an often insufficient level of whiteness for the requirements of the paper industry. With the process specified under 2., in particular by repeating the digestion process as specified under 2.b), it was possible to produce a pulp which has a sufficient degree of whiteness. The test parameters are given below. 3a.) 3b.) Liquor ratio: 1:25 > 1:25 formic acid concentration 85% 85% Water concentration: 13% 13% Cooking temperature: approx. 106 ° C 70-80 ° C Cooking time: 4h 4h

4. Modification of the production of a pulp after 1st or 2nd

The described manufacturing possibilities of a pulp can still be achieved be improved that a gas is used in addition to the digestion like air, oxygen, ozone or the like, for its oxidative power to exploit in the digestion. Also a mixture of two or more the gases mentioned can be used.

The digestion process described above or one of its variants is then carried out so that additionally into the reaction vessel from below the appropriate gas type or a gas mixture using a wide nozzle is initiated. This can be done continuously or discontinuously. The escaping gas can then be used again or disposed of in an environmentally friendly manner using the known methods become.

5. Recycling of the lignin

After the formic acid from the digestion solution by simple distillation has been recovered, a residue of lignin and sugars remains, or remaining carbohydrates. Failures in water will insoluble lignin separated from soluble sugars. Since it's through that method according to the invention advantageously not by sulfur or Chlorine is contaminated, it can be safely processed. These processing options are extremely diverse.

5.a Modification of lignin extraction

Instead of the previously described procedure for lignin extraction precipitating it in water can also cause continuous removal the lignin, which is dissolved in the solvent by the digestion, respectively. A suction device is attached to the reaction vessel, which continuously pumps out the digestion solution and through a membrane, drives a filter, frit, or similar separator. There will then the lignin was separated from the digestion solution and separated. The remaining digestion solution is pumped back into the reaction vessel, that served the information. This can act as a competitive reaction lignin condensation occurring for lignin cleavage can be suppressed. Moreover the boiling point of the digestion solution lowers because the proportion of Solved drops. Another advantage is the easier recovery the digestion solution and the savings in chemicals required, because an attack on already solved material according to this process variant can be avoided.

5.b Processing into a building material

For this purpose, an excess of oxalic acid is added to the lignin and melted or according to another process variant in saturated formic acid solution cooked and then evaporated with cellulose fibers. there creates a waterproof black-brown mass that can be used as a filler or after Type of pressboard or fiberboard can be used.

Claims (18)

1. Process for producing a pulp from cellulosic material which comprises admixing the material with formic acid as solvent, heating the mixture slightly by means of an external energy source with reflux cooling, and then slowly adding, substantially without supplying any further external energy, hydrogen peroxide at a constant rate so that the material is cooked under reflux at approximately the boiling temperature of the solvent.
2. Process according to Claim 1, wherein the solvent used is aqueous formic acid in a concentration range of between approximately 60 and 99% by weight.
3. Process according to Claim 1 or 2, wherein the cooking time is approximately 30-120 minutes.
4. Process according to Claim 2 or 3, wherein the predetermined amount of hydrogen peroxide is approximately 1-3% by weight, preferably 1-2% by weight, particularly preferably 1% by weight, based on the total weight of cellulosic material and solvent.
5. Process according to any of Claims 1 to 4, wherein the liquid to material ratio is approximately 20:1 to 25:1.
6. Process according to any of Claims 1 to 5, wherein additionally air, oxygen or ozone is introduced into the solvent.
7. Process according to any of Claims 1 to 6, wherein the pulp is separated from the solvent after completion of the cooking time.
8. Process according to Claim 7, wherein the pulp is separated by screening.
9. Process according to Claim 7 or 8, wherein the resulting pulp is washed with water and/or formic acid.
10. Process according to any of Claims 7 to 9, wherein at least some, preferably more than 95% by weight, of the formic acid is recovered by recycling, the recycling being carried out by simple distillation.
11. Process according to any of Claims 1 to 9, which is repeated at a reduced cooking temperature.
12. Process according to Claim 11, wherein the reduced cooking temperature is approximately 70-80°C, preferably 70°C.
13. Process according to Claim 11 or 12, wherein the cooking time is prolonged and is approximately 1-5 hours, preferably 3 hours.
14. Process according to any of Claims 11 to 13, wherein the liquid to material ratio is increased to more than 25:1.
15. Process according to Claim 1, wherein the cellulosic material used comprises elephant grass and/or maize leaves or stalks.
16. Process according to one of Claims 1 to 14, wheerein the cellulosic material used comprises annual plants, in particular cereal straw.
17. Process according to one of Claims 1 to 14, wheerein the cellulosic material used comprises hardwoods or softwoods.
18. Process according to Claim 10, wherein the distillation residue contains lignin which is isolated by precipitation in water.