Title:
METHOD FOR PRODUCING FIBROUS MATERIAL
Kind Code:
A1


Abstract:
A method for producing fibrous material from softwood, hardwood, or annual plants, including preparing a solution of chemicals having one of: more than 5% of chemicals for softwood, more than 3.5% of chemicals for hardwood, or more than 2.5% of chemicals for annual plants. The method includes: mixing the solution of chemicals with the lignocellulosic material in a predetermined liquor ratio; heating the solution of chemicals and the lignocellulosic material to a temperature above room temperature; and, after the heating, performing one of: removing a free-flowing portion of the solution of chemicals and digesting the lignocellulosic material in a vapor phase, or digesting the lignocellulosic material in a liquid phase and separating the free-flowing portion of the solution of chemicals and the lignocellulosic material. The produced fibrous material has a lignin content of at least 15% for softwood, at least 12% for hardwood, and at least 10% for annual plants.



Inventors:
Aalto, Esa-matti (Ravensburg, DE)
Schubert, Hans-ludwig (Baienfurt, DE)
Application Number:
12/161649
Publication Date:
12/24/2009
Filing Date:
04/07/2007
Primary Class:
Other Classes:
162/63, 162/72, 162/82, 162/83, 162/90
International Classes:
D21C9/02; D21C3/02; D21C3/06
View Patent Images:
Related US Applications:



Primary Examiner:
CALANDRA, ANTHONY J
Attorney, Agent or Firm:
GREENBLUM & BERNSTEIN, P.L.C. (RESTON, VA, US)
Claims:
1. 1-19. (canceled)

20. A method for producing fibrous material from lignocellulosic material, the lignocellulosic material comprising one of softwood, hardwood, and annual plants, the method comprising: preparing a solution of chemicals comprising one of: more than 5% of chemicals for softwood, more than 3.5% of chemicals for hardwood, or more than 2.5% of chemicals for annual plants, wherein percent of chemicals is calculated as NaOH and is based on an oven-dry amount of the lignocellulosic material, mixing the solution of chemicals with the lignocellulosic material in a predetermined liquor ratio; heating the solution of chemicals and the lignocellulosic material to a temperature above room temperature; and after the heating, performing a digesting comprising one of: removing a free-flowing portion of the solution of chemicals and digesting the lignocellulosic material in a vapor phase, or digesting the lignocellulosic material in a liquid phase and separating the free-flowing portion of the solution of chemicals and the lignocellulosic material, wherein the produced fibrous material has a lignin content of at least 15%, based on oven-dry fibrous material, when the lignocellulosic material comprises softwood, a lignin content of at least 12%, based on oven-dry fibrous material, when the lignocellulosic material comprises hardwood, and a lignin content of at least 10%, based on oven-dry fibrous material, when the lignocellulosic material comprises annual plants.

21. The method of claim 20, wherein the lignocellulosic material comprises softwood and the produced fibrous material has a lignin content of at least 18%.

22. The method of claim 20, wherein the lignocellulosic material comprises softwood and the produced fibrous material has a lignin content of at least 21%.

23. The method of claim 20, wherein the lignocellulosic material comprises hardwood and the produced fibrous material has a lignin content of at least 14%.

24. The method of claim 20, wherein the lignocellulosic material comprises hardwood and the produced fibrous material has a lignin content of at least 16%.

25. The method of claim 20, wherein the lignocellulosic material comprises annual plants and the produced fibrous material has a lignin content of at least 12%.

26. The method of claim 20, wherein the solution of chemicals comprises a quinine component.

27. The method of claim 20, wherein the lignocellulosic material comprises softwood and not more than 15% of the chemicals are used for the digesting.

28. The method of claim 20, wherein the lignocellulosic material comprises softwood and between 9% and 11% of the chemicals are used for the digesting.

29. The method of claim 20, wherein the lignocellulosic material comprises hardwood and not more than 10% of the chemicals are used for the digesting.

30. The method of claim 20, wherein the lignocellulosic material comprises hardwood and between 4% and 10% of the chemicals are used for the digesting.

31. The method of claim 20, wherein the lignocellulosic material comprises hardwood and between 6% and 8% of the chemicals are used for the digesting.

32. The method of claim 20, wherein the lignocellulosic material comprises annual plants and not more than 10% of the chemicals are used for the digesting.

33. The method of claim 20, wherein the lignocellulosic material comprises softwood and between 3% and 10% of the chemicals are used for the digesting.

34. The method of claim 20, wherein the solution of chemicals comprises at least one of sulfites and sulfides.

35. The method of claim 34, wherein the solution of chemicals comprises at least one of an acidic and an alkaline component.

36. The method of claim 34, wherein the solution of chemicals comprises at least one of an acid, sulfur dioxide, sodium hydroxide, and a carbonate.

37. The method of claim 20, wherein the digesting uses an alkaline component and an acidic component, the acidic component comprises SO2, and a ratio of the alkaline component to the SO2 is in a range from 4:1 to 1.6:1.

38. The method of claim 20, wherein the digesting uses an alkaline component and an acidic component, the acidic component comprises SO2, and a ratio of the alkaline component to the SO2 is in a range from 2:1 to 1.6:1.

39. The method of claim 20, wherein the solution of chemicals has a pH between 6 and 11.

40. The method of claim 20, wherein the solution of chemicals has a pH between 7 and 11.

41. The method of claim 20, wherein the solution of chemicals has a pH between 7.5 and 10.

42. The method of claim 20, wherein the predetermined liquor ratio is a ratio of the lignocellulosic material to the solution of chemicals and is between 1:1.5 and 1:6.

43. The method of claim 42, wherein the predetermined liquor ratio is between 1:3 and 1:5.

44. The method of claim 20, wherein the heating comprises heating the solution of chemicals and the lignocellulosic material to a temperature less than or equal to 120° C.

45. The method of claim 20, wherein the heating comprises heating the solution of chemicals and the lignocellulosic material to a temperature less than or equal to 110° C.

46. The method of claim 20, wherein the heating lasts for less than or equal to 90 minutes.

47. The method of claim 20, wherein the heating lasts for less than or equal to 60 minutes.

48. The method of claim 20, wherein the heating lasts for less than or equal to 30 minutes.

49. The method of claim 20, wherein the heating lasts for less than or equal to 10 minutes.

50. The method of claim 20, wherein the digesting is carried out at a temperature between 120° C. and 190° C.

51. The method of claim 20, wherein the digesting is carried out at a temperature between 150° C. and 180° C.

52. The method of claim 20, wherein the digesting is carried out at a temperature between 160° C. and 170° C.

53. The method of claim 20, wherein the digesting lasts for less than or equal to 180 minutes.

54. The method of claim 20, wherein the digesting lasts for less than or equal to 90 minutes.

55. The method of claim 20, wherein the digesting lasts for less than or equal to 60 minutes.

56. The method of claim 20, wherein the digesting lasts for less than or equal to 30 minutes.

57. The method of claim 20, wherein the digesting lasts for less than or equal to 2 minutes.

58. The method of claim 20, wherein less than or equal to 80% of the chemicals present at a beginning of the digesting are consumed during the digesting.

59. The method of claim 20, wherein less than or equal to 60% of the chemicals present at a beginning of the digesting are consumed during the digesting.

60. The method of claim 20, wherein less than or equal to 40% of the chemicals present at a beginning of the digesting are consumed during the digesting.

61. The method of claim 20, further comprising: determining a composition of the free-flowing portion of the solution of chemicals; and adjusting the composition of the free-flowing portion of the solution of chemicals for further use in producing other fibrous materials.

62. The method of claim 20, further comprising: washing the produced fibrous material to release another portion of the solution of chemicals; and using the other portion of the solution of chemicals in another process.

63. The method of claim 62 further comprising beating the digested lignocellulosic material before the washing.

Description:

The invention relates to a process for producing fibrous material from wood or annual plants having a lignin content of more than 15% for softwoods, of more than 12% for hardwoods and of more than 10% for annual plants, based in each case on the oven-dry fibrous material produced, the fibrous material having specified strength properties.

Processes which produce fibrous materials having a relatively high lignin content of more than 15% for softwood and of more than 12% for hardwood are known. They give a yield of 70% or more, based on the starting material used. These processes are based on chemical and/or on mechanical disintegration of the wood.

In the case of mechanical defibration of the wood, the latter is separated into fibre bundles in beating units—generally after presteaming. These fibre bundles are then defibrillated into individual fibres by further beating. The yield is very high but so is the quantity of beating energy to be applied. The strength of the wood fibres is very low—even after beating—because the fibres contain a large amount of natural lignin and therefore have little binding potential. They are also strongly degraded by the mechanical defibration, which adversely affects their recyclability.

In the case of chemical digestion of the wood, chemicals act on the wood, generally under high pressure and at elevated temperature. The NSSC process may be mentioned as a typical process for high-yield fibrous materials. However, other processes, such as the kraft or the soda process, can also be modified so that high-yield fibrous materials are produced (cf. “Choosing the best brightening process”, N. Liebergott and T. Joachimides, Pulp & Paper Canada, Vol. 80, No. 12, December 1979). If the degradation of the originally used wood is to be limited to a maximum of 30%, far less chemicals are required and used than in the production of chemical pulps which are to be completely freed from lignin. For the production of high-yield chemical pulps, the amount of chemicals is metered as a function of the desired yield. In order to achieve a yield of about 70%, based on oven-dry wood use, the prior art recommends using up to 10% of chemicals, based on the starting material. In the case of chemical pulps, the use of chemicals is often 30% or more of chemicals, based on the oven-dry wood.

Chemicals play a role in determining the process costs, i.e. they are used as sparingly as possible. CTMP fibrous materials are usually produced with amounts of 3% to 5% of chemicals. In known, industrially established processes for producing high-yield fibrous materials, e.g. the NSSC process, up to 10% of chemicals, based on the starting material, are used. With the use of chemicals limited in this manner, no recovery is as yet installed for recovering the chemicals. In spite of the relatively small amounts of chemicals, this method of fibrous material production leads to considerable pollution of the environment, in particular of bodies of water, not only because of the introduction of chemicals but especially because of the organic load which is released into the main outfall.

Regarding the cost situation, it should be noted that, in the case of mechanically produced fibrous materials, the sharply rising energy prices add to the production costs. In the case of chemically produced high-yield fibrous materials, the production is adversely affected by the costs for the lost chemicals.

High-yield fibrous materials are beaten to high freenesses for the current intended uses. Only then do they reach an acceptable strength level. Here, high freenesses are to be regarded as values of about 300 ml CSF (Canadian Standard Freeness), equivalent to 41° SR (Schopper-Riegler, see below) and 500 ml CSF, equivalent to 26° SR, as described, for example, in “Choosing the best brightening process”, N. Liebergott and T. Joachimides, Pulp & Paper Canada, Vol. 80, No. 12, December 1979, for high-yield fibrous material from softwood. A high freeness is achieved by using mechanical energy. The fibres are rubbed against one another or against a grinder or against a grinding medium and thus changed in their surface properties to achieve better binding behaviour. The high freeness is therefore not an end in itself. Rather, it arises out of the requirements regarding the strength properties of the fibres.

The high-yield fibrous materials produced by a mechanical and/or chemical method are used in particular where high final whiteness and high whiteness stability are not absolutely essential. They could open up numerous further fields of use if the strength level could be increased.

It is therefore an object of the invention to propose a process for producing fibrous material from wood or annual plants having a lignin content of more than 15% for softwoods, of more than 12% for hardwoods and of more than 10% for annual plants, by means of which fibrous materials of high strength can be produced in an economical manner.

This object is achieved by a process for producing fibrous material from wood or annual plants having a lignin content of at least 15% for softwoods and 12% for hardwoods and 10% for annual plants, based in each case on the oven-dry fibrous material, comprising the steps:

    • preparation of a solution of chemicals comprising more than 5% of chemicals (calculated as NaOH) for softwoods or comprising more than 3.5% of chemicals (calculated as NaOH) for hardwoods or comprising more than 2.5% of chemicals (calculated as NaOH), based in each case on the oven-dry amount of the wood used,
    • mixing of the solution of chemicals with the wood or annual plants in a specified liquor ratio,
    • heating of the solution of chemicals and of the wood or annual plants to a temperature above room temperature and then either (1st alternative)
    • removal of free-flowing solution of chemicals and
    • digestion of the wood or annual plants in the vapour phase or (2nd alternative)
    • digestion of the wood or annual plants in the presence of the solution of chemicals in the liquid phase and
    • separation of the free-flowing solution of chemicals and of the wood or annual plants.

The process according to the invention is based on the fact that larger amounts of chemicals are used for producing high-yield fibrous materials than previously customary. More than 5% of chemicals for softwoods are significantly above the previously customary amounts of chemicals for industrial production of fibrous materials, likewise more than 3.5% of chemicals for hardwoods and 2.5% for annual plants. This high use of chemicals gives fibrous materials in a good yield and with excellent strength properties. Thus, for softwood breaking lengths of more than 8 km, but also breaking lengths of more than 9 km and more than 10 km, are measured at freenesses of only 12° SR to 15° SR. For hardwoods, values of more than 5 km, but also breaking lengths of more than 6 km and more than 7 km, are measured at only 20° SR. Thus, the desired high strength level is achieved.

It is to be regarded as an extraordinary advantage of the process according to the invention that these strength values are achieved even at extremely low freenesses, as have not been available to date for high-yield fibrous materials. Fibrous materials according to the prior art have an unacceptable strength level at freenesses of 12° SR to 15° SR for softwood fibrous materials or of 20° SR for hardwood. At these low freenesses, known fibrous materials have to date given fibres which have not had sufficient binding power and which accordingly have not had sufficient strength properties for commercial use of such fibrous materials.

Particularly suitable annual plants are bamboo, hemp, rice straw, bagasse, wheat, miscanthus or the like.

In contrast, the fibrous materials produced by the process according to the invention have breaking lengths of more than 8 km to 11 km and tear resistances of more than 70 cN to more than 110 cN, based on a sheet weight of 100 g/m2, even at freenesses in the range from 12° SR to 15° SR. These low freenesses are moreover achieved with a low specific demand for beating energy, which is less than 500 kWh/t of fibrous material for softwood fibrous materials; in the case of hardwood fibrous materials, the demand for beating energy may even be less than 300 kWh/t of fibrous material. The discovery that the high strength level is achieved even at low freenesses of 12° SR to 15° SR for softwood and at 20° SR or less for hardwood is a substantial part of the invention.

These high strength values have not been known to date for fibrous materials having a lignin content of more than 15% for softwood fibrous materials, of more than 12% for hardwood fibrous materials or of more than 10% for annual plants. The high strength level can, however, also be retained for fibrous materials having an even higher lignin content. The process according to the invention is also suitable for producing softwood fibrous materials having a lignin content of more than 18%, preferably more than 21%, advantageously more than 24%, based on the oven-dry fibre material. Hardwood fibrous materials having a lignin content of more than 14%, preferably more than 16%, particularly preferably more than 18% and annual plants having a lignin content of more than 10%, preferably more than 12%, in particular more than 19%, can likewise be produced by the process according to the invention and show a high strength level.

The composition of the solution of chemicals used for the digestion can be tailored to the wood or the annual plants to be digested and the desired properties of the fibrous material. As a rule, a sulphite component alone is used. Alternatively or in addition, a sulphide component may be added. A digestion with a sulphite component is not disturbed by the presence of sulphide components. Sodium sulphite is generally used industrially, but the use of ammonium or potassium sulphite or of magnesium bisulphite is also possible. Particularly if large amounts of sulphite are used, it is possible to dispense with the use of an alkaline component because a high pH, which promotes the digestion, is established even without addition of alkaline components.

For establishing the pH and for supporting the delignification, an acidic and/or an alkaline component can be metered in. Industrially, sodium hydroxide (NaOH) is generally used as the alkaline component. However, the use of carbonates, in particular sodium carbonate, is also possible. All data are in amounts of chemicals in the digestion process in this document, for example on the total use of chemicals or on the division of the sulphite component and the alkaline component, are in each case calculated and stated as sodium hydroxide (NaOH), unless stated otherwise.

Acids can be metered in as the acidic component in order to establish the desired pH. However, the addition of SO2, if appropriate in aqueous solution, is preferred. It is economically and readily available, particularly when the spent solution of chemicals, for example based on sodium sulphite, is prepared for further use after the digestion.

To have recognized the advantages of using a quinone component for the high-yield digestion according to the invention is regarded as an independent inventive performance. Quinone components, in particular anthraquinone, have been used to date in the production of chemical pulps having a minimum lignin content in order to prevent an undesired attack on the carbohydrate towards the end of the digestion. By adding quinone components, it is possible to continue the digestion of wood until almost complete degradation of the lignin. That quinone components significantly increase the rate of the lignin degradation in the production of high-yield chemical pulps has to date proved to be an unrecognized, unexpected property of said quinone components. For example, in the production of softwood fibrous materials, the duration of the digestion can be shortened by more than a half, depending on digestion conditions by more than three quarters. This remarkable effect is achieved with minimum use of quinone. Use of, for example, anthraquinone in an amount of between 0.005% and 0.5% is optimum. Use of anthraquinone in an amount of up to 1% also has the desired effect. Use of more than 3% of anthraquinone is generally uneconomical.

A solution of chemicals is prepared from individual chemicals or a plurality of chemicals from among the abovementioned chemicals. In general, an aqueous solution is prepared. As an option, the use or the addition of organic solvents can also be provided. Alcohol, in particular methanol and ethanol, gives, as a mixture with water, particularly effective solutions of chemicals for producing high-quality high-yield fibrous materials. The mixing ratio of water and alcohol can be optimized for the respective raw material in a few experiments.

The amount of chemicals to be used according to the invention for producing a fibrous material having a yield of at least 70% is at least 5% for softwood, at least 3.5% for hardwood and at least 2.5% for annual plants, based in each case on the oven-dry wood or annual plant material to be digested. The quality of the fibrous material produced gives the best results when the chemicals used comprise up to 15% for softwood, up to 10% for hardwood and up to 10% for annual plants. Preferably, between 9% and 11% of chemicals, based on the oven-dry wood, for softwood used, are added. For hardwoods, the use of chemicals tends to be lower, preferably between 4% and 10%, particularly preferably between 6% and 8% and, for annual plants, between 3 and 10%.

As already explained above, it is by no means necessary to establish a specific pH. It may be expedient to add acid or an alkaline component before or during the digestion only if, for example, particular properties of the chemical pulps (particularly high whiteness, a certain ratio of breaking lengths and tear resistance) are to be achieved by the digestion. According to an advantageous embodiment of the process according to the invention, a ratio of an alkaline component to sulphur dioxide (SO2) can be established within a wide range regardless of the chosen use of chemicals altogether. SO2 is mentioned here as being representative of the abovementioned acidic component. It is also possible to use an acid instead of SO2. Since the optionally added quinone component is used only in minimum amounts, generally of substantially less than 1%, it is negligible for establishing this ratio. A ratio of alkaline component:SO2 in a range of 4:1 to 1.6:1 is very suitable for carrying out the process according to the invention and for obtaining fibrous materials having good strength properties. A customary, particularly suitable range is between 2:1 and 1.6:1. The adaptation of the proportioned components is effected as a function of the raw material to be digested and the process management chosen in each case (digestion temperature, duration of digestion, impregnation).

The process according to the invention can be carried out in a wide pH range. The ratio of alkaline component to acidic component and the use of an acidic or of an alkaline component can be established to that a pH between 6 and 11, preferably between 7 and 11, particularly preferably between 7.5 and 10, is established at the beginning of the process. The more likely alkaline pH of between 8 and 11, which is advantageous for the process according to the invention, also promotes the action of the quinone component. The process according to the invention is tolerant with regard to the pH; a small amount of chemicals is required for pH adjustment. This has an advantageous effect on the costs for chemicals.

Without further addition of acid or alkaline components, a pH of between 8 and 10, in general between 8.5 and 9.5, is established in the free-flowing solution of chemicals and the organic constituents dissolved therein, which were liquefied by the digestion, for example for softwood, at the end of the digestion. The dissolved organic constituents include in particular lignosulphonates.

The liquor ratio, i.e. the ratio of the amount of the oven-dry wood or annual plants to the solution of chemicals, is adjusted to between 1:1.5 and 1:6. A liquor ratio of 1:3 to 1:5 is preferred. In this range, good and easy mixing and impregnation of the material to be digested in ensured. For softwood, a liquor ratio of 1:4 is preferred. For wood chips having a large surface area, the liquor ratio may also be substantially higher in order to permit rapid wetting and impregnation. At the same time, the concentration of the solution of chemicals can be kept high so that the amounts of liquid to be circulated are not too large.

The mixing or impregnation of the wood or annual plant digestion material is preferably effected at elevated temperatures. Heating of the wood chips and of the solution of chemicals to 110° C., preferably to 120° C., particularly preferably to 130° C., leads to rapid and uniform digestion of the wood. For the mixing or impregnation of the chips, a period of up to 30 minutes, preferably of up to 60 minutes, particularly preferably of up to 90 minutes, is advantageous. The duration which is optimum in each case depends, inter alia, on the amount of the chemicals and the liquor ratio and the method of digestion (liquid or vapour phase).

The digestion of the lignocellulosic material mixed or impregnated with the solution of chemicals is preferably effected at temperatures between 120° C. and 190° C., preferably between 150° C. and 180° C. For most timbers, digestion temperatures between 155° C. and 170° C. are established. Higher or lower temperatures can be established but, in this temperature range, the energy consumption for the heating and the acceleration of the digestion are economically related to one another. Higher temperatures can moreover have an adverse effect on the strengths and the whiteness of the fibrous materials. The pressure generated by the high temperatures can be readily absorbed by appropriate design of the digester. The duration of heating is usually only a few minutes, generally up to 30 minutes, advantageously up to 10 minutes, particularly if heating is effected by means of steam. The duration of heating can be up to 90 minutes, preferably up to 60 minutes, e.g. if digestion is effected in the liquid phase and the solution of chemicals is to be heated together with the chips.

The duration of digestion is chosen in particular as a function of the desired properties of the fibrous material. The duration of digestion can be shortened to 2 minutes, for example for the case of a vapour-phase digestion of a hardwood having a low lignin content. However, it may also be up to 180 minutes if, for example, the digestion temperature is low and the natural lignin content of the wood to be digested is high. Even if the initial pH of the digestion is in the neutral range, a long duration of digestion is required. The duration of digestion is preferably up to 90 minutes, in particular in the case of softwood. Particularly preferably, the duration of digestion is up to 60 minutes, advantageously up to 30 minutes. A duration of digestion of up to 60 minutes is suitable especially in the case of hardwoods.

In the case of annual plants, the duration of digestion is up to 90 minutes. The use of a quinone component, in particular anthraquinone, permits a reduction in the duration of digestion to 25% of the time required without addition of anthraquinone. If the use of quinone components is dispensed with, the duration of digestion increases by more than one hour, for example, by 45 minutes to 180 minutes, for comparable digestion results.

According to an advantageous embodiment of the process according to the invention, the duration of digestion is established as a function of the chosen liquor ratio. The lower the liquor ratio, the shorter can the duration of the process be set.

The production of high-yield fibrous material with the use of a large amount of chemicals of more than 5% for softwood, of more than 3.5% for hardwood and at least 2.5% for annual plants initially appears to be uneconomical. However, experiments have shown that only a part of the chemicals is consumed during the partial digestion of the lignocellulosic material. The predominant part of the chemicals is discharged unconsumed, either before the digestion (vapour-phase digestion) or after the digestion (digestion in the liquid phase). The actual consumption of chemicals is below the amounts used in the digestion solution.

The consumption of chemicals is recorded as the amount of chemicals which—based on the originally used amount of chemicals—is measured after removal or separation of the solution of chemicals and optionally the registration of the solution of chemicals, which is measured after the defibration or in combination with the registration of the solution of chemicals. The consumption of chemicals is dependent on the absolute amount of the chemicals used for the digestion, based on the oven-dry wood material to be digested. The greater the use of digestion chemicals, the lower is the direct conversion of chemicals. With the use of 27.5% of chemicals, based on oven-dry wood material, for example, only about 30% of the chemicals used are consumed. With the use of 15% of chemicals, based on oven-dry wood, however, 60% of the chemicals used are consumed, as could be demonstrated in laboratory experiments. According to a preferred embodiment of the process, the consumption of chemicals for the process according to the invention during the digestion is up to 80%, preferably up to 60%, particularly preferably up to 40%, advantageously up to 20%, particularly advantageously up to 10%, of the chemicals used at the beginning of the digestion.

The consumption of chemicals for the production of a metric ton of fibrous material is about 6% to 14% of sulphite and/or sulphide component and, if appropriate, alkaline and/or acidic component and/or if appropriate, quinone component, based on oven-dry fibrous material (soft- and hardwood or annual plants). According to the invention, this amount of chemicals is sufficient for producing a fibrous material having the specified properties. However, in order to ensure a standard process result and, if appropriate, to obtain particular, desired properties of the fibrous material, it may prove to be expedient to use larger amounts of chemicals for the dilution, for example the abovementioned amounts of up to 30% of chemicals, based on oven-dry wood or annual plant material.

The use of these amounts of chemicals at the beginning of the digestion has an advantageous effect, since the fibrous materials obtained in this manner have properties not available to date, in particular good strength properties and good whitenesses. In particular, no digestion process has been available to date which produces fibrous materials having high strengths over a broad pH range from neutral to alkaline range. That the fibrous materials produced according to the invention can be beaten with a far lower energy demand to specified freenesses than known fibrous materials has proved to be economically particularly attractive. Moreover, they develop the high strengths even at unusually low freenesses of 12° SR to 15° SR for softwood and of 20° SR for hardwood.

An excess of chemicals is present in the free-flowing liquid after the mixing and impregnation of the wood with the solution of chemicals or after digestion. This excess is subtracted before the digestion (1st alternative) or after digestion (2nd alternative). According to an advantageous further development of the process, the composition of the solution of chemicals which is removed is determined and then adjusted to a specified composition for further use for the production of fibres. The solution of chemicals which is removed before or after the digestion of the wood or of the annual plants no longer has the initially established composition. At least a part of the chemicals used for the digestion has—as described above—penetrated into the material to be digested and/or has been consumed during the digestion. The unconsumed chemicals can readily be used again for the next digestion. However, it is proposed according to the invention initially to determine the composition of the removed solution of chemicals and then to replenish the consumed proportions of, for example, sulphite, alkaline component, quinone component or water or alcohol in order to prepare the specified composition again for the next digestion. This replenishment step is also referred to as fortification.

It is to be regarded as a considerable advantage of this measure that the solution of chemicals contains no substances at all or only very few substances which prove to be troublesome in further use of the fortified solution of chemicals for the next digestion only on removal before the digestion but also on removal after the digestion. The process according to the invention which is based on providing an oversupply of digestion chemicals during the impregnation can thus operate extremely economically in spite of the initially apparently uneconomical procedure involving the considerable use of chemicals, since the removal or the separation and the fortification of the solution of chemicals can be carried out simply and economically.

The process according to the invention is controlled in a targeted manner so that only as little as possible of the starting material used is degraded or dissolved. It is desirable to produce a fibrous material which has for softwood a lignin content of at least 15%, based on the oven-dry fibre material, preferably a lignin content of at least 18%, particularly preferably of 21%, advantageously of at least 24%. For hardwood, it is desired to achieve a lignin content of at least 12%, based on the oven-dry fibre material, preferably of at least 14%, particularly preferably of at least 16%, advantageously of at least 18%. In the case of annual plants, the preferred lignin content is between 10 and 28%, in particular between 12 and 26%.

The yield of the process according to the invention is at least 70%, preferably more than 75%, advantageously more than 80%, based in each case on the wood used. This yield correlates with the abovementioned lignin content of the fibrous material. The original lignin content of wood is specific for the type. In the present process, the loss of yield is predominantly a loss of lignin. In the case of unspecific digestion processes, the proportion of carbohydrates is substantially increased, for example because digestion chemicals also bring cellulose or hemicelluloses into solution in an essentially undesired manner.

A further, advantageous measure is to remove the still remaining solution of chemicals after the defibration and, if appropriate, beating of the lignocellulosic material and to reuse it. This reuse may comprise two aspects in a preferred configuration. Firstly, the organic material, predominantly lignin, degraded or brought into solution during the partial digestion is further used. It is, for example, incinerated in order to obtain process energy. Alternatively, it is processed in order to be otherwise used. Secondly, the spent and unconsumed chemicals are reprocessed so that they can be used for a further, partial digestion of lignocellulosic material. This includes the processing of spent chemicals.

According to a particularly preferred variant of the process according to the invention, the solution of chemicals used is extremely efficiently utilized. After the defibration and, if appropriate, beating, the fibrous material is washed in order to displace the solution of chemicals as far as possible by water. The filtrate forming during this washing and displacement process contains considerable amounts of solution of chemicals and organic material. According to the invention, this filtrate is fed into the removed or separated solution of chemicals before the solution of chemicals is fortified and fed to the next digestion. The chemicals and organic constituents present in the filtrate do not disturb the digestion. If they make a contribution to the delignification during the next digestion, their content in the solution of chemicals is determined and is taken into account in the determination of the amount of chemicals which is required for this digestion. The further chemicals present in the filtrate are inert during the upcoming digestion. They do not cause problems. The organic constituents present in the filtrate are likewise inert. They are reused after the next digestion in the processing of the solution of chemicals, either to generate process energy or in another manner.

It is regarded as particularly advantageous that, by this management of the filtrate, less fresh water and fewer chemicals are used for the digestion. At the same time, a maximum of dissolved organic material is detected. This improved use of the organic material which has gone into solution also improves the cost-efficiency of the process according to the invention.

Details of the process according to the invention and of the apparatus are explained in more detail below with reference to working examples.

The following experiments were evaluated according to the following specifications:

    • The yield was calculated by weighing the raw material used and the chemical pulp obtained after the digestion, in each case dried at 105° C. to constant weight (absolutely dry).
    • The lignin content was determined as Klason lignin according to TAPPT T 222 om-98. The acid-soluble lignin was determined according to TAPPI UM 250.
    • The properties relating to paper technology were determined on test sheets which were produced according to Zellcheming data sheet V/8/76.
    • The freeness was determined according to Zellcheming data sheet V/3/62.
    • The bulk density was determined according to Zellcheming method V/11/57.
    • The breaking length was determined according to Zellcheming method V/12/57.
    • The tear resistance was determined according to DIN 53 128 Elmendorf.
    • The determination of tensile, tear and burst index was effected according to TAPPI 220 sp-96.
    • The whiteness was determined by producing the test sheets according to Zellcheming data sheet V/19/63; measurement was effected according to SCAN C 11:75 using a Datacolor elrepho 450× photometer; the whiteness is stated in percent according to ISO standard 2470.
    • The viscosity was determined according to data sheet IV/36/61 of the Association of the Pulp and Paper Chemists and Engineers (Zellcheming).
    • All % data in this document are to be understood as meaning percent by weight, unless specifically stated otherwise.
    • The statement “oven-dry” in this document relates to “oven-dry” material which was dried at 105° C. to constant weight.
    • The chemicals for the digestion are stated in percent by weight as sodium hydroxide, unless explained otherwise.

EXAMPLE 1

Digestion of Softwood in the Liquid Phase

A sodium sulphite digestion solution was added to a mixture of spruce wood and Douglas fir chips with a liquor ratio of wood:digestion solution 1:3, after steaming (30 minutes with saturated steam at 105° C.). The total use of chemicals was less than 15%, based on oven-dry chips. The pH at the beginning of the digestion was adjusted to pH 6 by addition of SO2.

The spruce wood chips impregnated with the solution of chemicals were heated to 170° C. over a period of 90 minutes and were digested at this maximum temperature over 60 minutes.

Thereafter, the free-flowing liquid was removed by centrifuging, collected and analyzed in an arrangement for recycling unconsumed liquid and fortified and thus provided for the next digestion.

The digested chips were defibrated. Portions of the fibrous material thus produced were beaten for different times in order to determine the strength at different freenesses. The energy consumption for defibration of the partly digested chips was less than 300 kWh/t of fibrous material.

In this experiment, the yield was 77%, based on the wood material used.

This corresponds to a fibrous material having a lignin content of well above 20%. The average lignin content for spruce wood is stated as 28%, based on the oven-dry wood material (Wagenfur, Anatomie des Holzes [Anatomy of Wood], VEB Fachbuchverlag Leipzig, 1980). The actual lignin content of the fibrous material is higher than 20% since predominantly, but not exclusively, lignin is degraded during the digestion. Carbohydrates (cellulose and hemicelluloses) are also dissolved in small amounts. The stated values show that the digestion has good selectivity with respect to the lignin and carbohydrate degradation.

The whiteness is unexpectedly high with values above 55% ISO and thus offers a good initial basis for any subsequent bleach in which whitenesses of 75% ISO are achievable.

With an initial freeness of 12° SR, these materials already have a breaking length of 6 km at a density of 1.87 cm3/g.

In order to beat the fibrous materials to a freeness of 15° SR, a duration of beating of 20 to 30 minutes is required. Up to a duration of beating of 20 minutes (freeness 12° SR-15° SR), the freeness develops independently of the pH at the beginning of the digestion (pH 6 to pH 9.4) within a narrow range.

Likewise independently of the initial pH of the digestion and of the duration of beating required for achieving the freeness, a high strength level is achieved at a freeness of 15° SR.

EXAMPLE 2

The fibrous material was produced from spruce chips, the pH at the beginning of the digestion being 9.4.

In addition to the 15% of total chemicals (sulphite and NaOH in specified ratio), 0.1% of anthraquinone, based on the amount of wood used, was added to the solution of chemicals.

The duration of the digestion was 60 minutes.

The following values were obtained:

Yield (%):81.1
Lignin content:22.7
Whiteness (% ISO):53.7
Breaking length (km):9.6
Tear resistance (cN; 100 g/m2):75.0

By the addition of 0.1% of anthraquinone, the duration of the digestion can be reduced from about 180 minutes to 60 minutes under otherwise unchanged digestion conditions. This time gain is valuable especially because the plants for the production of fibrous materials can be designed with small dimensions. The fact that the temperature required for the digestion need be maintained only over a very much shorter period constitutes a further potential saving.

Furthermore, it was determined that, with decreasing use of total chemicals down to values between 5 and 15% in the case of softwood, fibrous material having substantially equally good properties is produced. These results are not dependent on the use of the anthraquinone. The anthraquinone results in an acceleration of the digestion, but the desired fibrous material can also be digested without addition of anthraquinone.

EXAMPLE 3

Digestion of Hardwood in the Liquid Phase

A sodium sulphite digestion solution was added to eucalyptus chips with a liquor ratio of wood:digestion solution 1:3, after steaming. The use of chemicals here was 10.5% (as NaOH), based on oven-dry chips.

In a period of 90 minutes, the material to be digested was impregnated and the material to be cooked was heated to the maximum digestion temperature of 170° C. The duration of cooking was 50 minutes.

Digestions with eucalyptus wood show that these materials can be produced with a specific energy input of less than 250 KWh/t for defibration.

In these experiments, the yield was 77%, based on the wood material used. At an initial freeness of 14 SR, these materials already have a breaking length of 3.5 km at a density of 2.05 cm3/g. These materials can be bleached in the subsequent bleach to whitenesses of 79.6% ISO.

Experiments have shown that the digestions in the vapour phase show a small overall time requirement. Compared with the digestion in the liquid phase, the heating to the maximum digestion temperature is effected very much more rapidly. The actual digestion then requires the same duration as cooking in the liquid phase. During the vapour-phase digestion, no free-flowing solution of chemicals is present; such solution is removed after the impregnation and before the digestion. Less organic material is therefore added to it than the solution of chemicals which is removed after the digestion in the liquid phase. However, this has no significant influence on the quality of the fibrous material produced.

While similar values in the case of the yield are achievable in vapour-phase digestions, the whiteness of the fibrous materials produced in the vapour digestion is, however, substantially lower. The reduction of the maximum digestion temperature from 170° C. to 155° C. has a significant effect: the whiteness increases.

The fibrous materials produced in the vapour phase have outstanding strengths. The breaking length was measured, for example, as 10 km and as 11 km at 15° SR. The tear resistance was measured, for example, as 82.8 cN and as 91.0 cN. These values correspond to the best values for fibrous materials having a high lignin content which were achieved for digestions in the liquid phase or are even higher. For fibrous materials having a high lignin content from the prior art, comparable strength values are not known.

From the examples, it is particularly clear that the fibrous materials according to the invention require only a small amount of energy during the beating in order to establish large breaking lengths, without the tear resistance being reduced. A freeness of 12° SR was reached in each case in 0-10 minutes; a freeness of 13° SR in 5-30 minutes, generally 10-20 minutes. In order to arrive at a freeness of 14° SR, the Jokro beater had to operate for 30-40 minutes, and between 35 and 40 minutes were required for a freeness of 15° SR. It is obvious that beating up to freenesses of about 40° SR would require an enormous amount of beating energy. A particular advantage of the process according to the invention is therefore that fibrous materials to be beaten with a small amount of energy and having high strengths are produced.