METHOD FOR PREPARATION OF A FIBROUS STOCK OUT OF HERB-STEMMED PLANTS

Abstract: A method for preparation of a fibrous stock suitable for manufacture of paper out of herb-stemmed plants, such as, for example, straw or flax. The method comprises steps in which the fibrous raw-material is cut into chips, cooked, and bleached. As the cooking liquor, water is used to which an acid has been added so that the ultimate pH of the cooking is lower than 7 but higher than 3. For regulation of pH, it is possible to use an anorganic acid or some organic acid, such as formic acid or acetic acid or a mixture of same. By means of bleaching, it is ensured that the stock is defibrated. Favourably, peroxide is used as the bleaching chemical.

Description: Method for preparation of a fibrous stock out of herb-stemmed plants The invention concerns a method for preparation of a fibrous stock suitable for manufacture of paper out of herb-stemmed plants, in which method the fibrous raw- material is chipped, cooked, and bleached. Besides forests, straw from grain plants and other annual herb plants are the largest potential fibre resource on earth. In many countries with scarce woods, herb- stemmed plants have already been the principal source of paper pulp for a long time. Practical problems are caused in the utilization of herb-stemmed plants by availability, collecting, transportation, storage, and cleaning of the raw-material. In order that the use of non-wood fibrous raw-material should be economically competitive, it is preferable to carry out the manufacture of pulp in relatively small units, whose annual production is of an order of 30,000 ... 50,000 tons per year. This sets the requirement that the line of pulp production must be simple. In the following, the notion of herb-stemmed plants will be used to cover all those non-wood fibrous raw-materials that can be used for manufacture of fibrous pulp. The properties and the suitability of herb-stemmed plants for preparation of pulp vary. Generally speaking, they contain less lignin than wood raw-material does, but their contents of silicates and ashes can be remarkably high. This imposes specific requirements on the process of manufacture of pulp that is used. For delignification of non-wood fibrous raw-materials, currently, most commonly, cooking processes are employed which have been developed for delignification of wood, such as sulphate and soda cooking and modifications of same. The recovery of the cooking chemicals in these processes is often made difficult by the high content of silicates in the plants. In alkaline cooking, the silicates dissolve in the cooking liquor and are then deposited in the chemical circulation on the heat faces of the evaporation plant unless the process is provided with a separate stage of removal of silicates. It is unfortunate that, in developing countries, the cooking liquor is still often passed directly into waterways, because the recovery of chemicals is difficult and expensive. In alkaline cooking processes, the yield also remains low. For cooking of herb plants, a solution has also been sought in "organosolv" cooking processes based on the use of various organic solvents. In the paper published in Paperi ja Puu, Vol. 78 (1996), No. 10, by E. Rousi, "Muurahaishappo keitto- kemikaalina " ('Formic acid as cooking chemical'), pp. 594...596, one such cooking process is described. In said process, it is, however, necessary to use high acid concentrations, in which case the consumption of acid is high. The process is expensive both in respect of the investment cost and in respect of the cost of operation. In the FI Patent Application 940638, a what is called fertilizer cooking is described, in which the spent liquor from the cooking is suitable to be used directly as a fertilizer in a field. The economic profitability of the process has, however, been questioned. The cooking process is best suitable for areas in which there is an existing water irrigation system. The object of the present invention is to provide a novel method for manufacture of fibrous pulp to be used for manufacture of paper out of herb-stemmed plants, by means of which method the drawbacks related to the prior- art methods are avoided. An object of the present invention is a pulp production process which is favourable in view of the environment and whose technical solutions are simple and which is economically profitable also when the unit is small. In view of achieving the objectives stated above, the method in accordance with the invention is characterized in that the cooking is carried out at a temperature of 60...130 °C and that the cooking liquor that is used is water to which an acid has been added so that the ultimate pH of the cooking is lower than 7 but higher than 3. In prior-art cooking processes, as a rule, high-concentration chemical solutions and high cooking temperatures are employed. When the cooking stage is carried out in accordance with the present invention in acid water and at a relatively low temperature, the loss of yield in the cooking is little, but the pulp can be bleached readily in the bleaching following after the cooking. The main part of the lignin, which is contained in the fibrous material and which binds the fibres together, is not dissolved until during the bleaching. Since, in the cooking, no high dosages of chemicals are employed, it is not necess- ary to arrange a heavy system for recovery of chemicals. The spent liquor from the cooking and the wash waters can be passed into the waste water cleaning plant to be processed by means of known biological cleaning processes. This is why the cost of investment and operation of the process is advantageous, for which reason it is also profitable in small units. The relatively simple pulp preparation process can be inte- grated readily in connection with a paper mill so as to produce, depending on the fibrous raw-material, either reinforcement stock or short- fibre stock for the paper machine. The process produces fibrous pulp of good quality, which is suitable for final products of many sorts. The yield of pulp is somewhat better than in the case of non-wood pulps prepared by means of conventional processes. The water cooking process in accordance with the present invention is suitable for delignification of many different herb-stemmed plants. The requirement of pre- treatment of the fibrous raw-material depends, among other things, on whether the whole plant can be utilized as such, or if it is necessary to separate the fibre fraction from the plant material mechanically. In the case of many grain plants, simple chipping of straw is sufficient as pre-treatment, whereas, for example, flax must be braked and scutched or fractioned mechanically in some other suitable way before the cooking in order that the fibrous material could be separated for cooking. In the case of plants with long fibres, the chips to be used for preparation of pulp must be relatively short in order that bundles of fibres should not be twisted with each other during cooking. For flax, a suitable chip length would seem to be 5...10 mm. In the case of plants with short fibres, the chip length can be up to 50 mm. The fibre chips are cooked in water into which an acid has been added to such an extent that the ultimate pH of the cook will be lower than 7 but higher than 3. The acid that is used can be any anorganic or organic acid whatsoever. For the regulation of pH, favourably organic acids are used, such as formic acid or acetic acid or a mixture of same. The addition of acid to be applied is so little that no "organosolv" cooking is concerned. In "organosolv" cooking, the proportion of the organic solvent in the cooking liquor is, as a rule, 20...100 % . In the water cooking in accordance with the present invention, the addition of acid is always less than 2 % of the whole volume of the cooking liquor, preferably lower than 1 % . Good results have already been obtained even with an acid addition as low as about 0.06 % . The cooking is carried out at a temperature of 60...130 °C, preferably 70...100 °C. After cooking, the spent liquor is separated from the fibre chips and passed to waste water treatment. The loss of yield in acid water cooking is relatively little. Thus, a fibrous raw- material that has been cooked without mechanical pre-treatment is not necessarily defibrated after cooking as yet. Therefore, the water cooking is followed by bleaching, whose object is, besides increasing the brightness of the pulp, also to dissolve lignin and to provide defibration of the pulp. In bleaching, it is possible to use any prior-art bleaching process whatsoever. Preferably, however, chlorine-free bleaching chemicals are used, such as peroxide. When peroxide bleaching is used, the fibre chips must be chelated before bleaching so as to stabilize the ions of heavy metals in order that these ions should not accelerate the decomposition of peroxide during bleaching. For this purpose, the pulp is treated, in a way in itself known, with a suitable chelating agent. Chelating can be carried out either as a separate step before bleaching or in connection with cooking so that a little dose of chelating agent is added to the cooking water. The bleaching is carried out in a conventional way while, however, using such dosages of chemicals and such conditions with which part of the lignin contained in the fibrous material can be dissolved. The unit that produces fibrous pulp out of herb-stemmed plants by means of the method in accordance with the present invention can be placed in connection with a paper mill so that it forms a part of the stock system of the paper machine. In such a case, the fibrous stock produced by the process can be utilized directly in the paper machine as mixed with normal wood pulp. Depending on the fibrous raw-material, it is possible to produce either inexpensive short-fibre filler stock or long-fibre reinforcing stock. Pulps prepared out of herb-stemmed plants do, as a rule, not require highly intensive grinding, but a treatment of defibration type or pulpering can be sufficient. Pulp of short fibres (straw etc.) can be used, among other things, for intermediate layers in multi-layer board to replace recycled fibre or wood fibre. Long-fibre pulp (flax, hemp, kenaf) can be substituted for wood fibre, for example, in the manufacture of recycled paper. By means of the method in accordance with the present invention, it is also possible to prepare paper pulp out of a mixture of short-fibre and long-fibre plant to be used for manufacture of paper as such. The method in accordance with the invention requires low investments, because it is unnecessary to provide for recovery of chemicals. The consumption of chemicals in the cooking is little, but in bleaching, on the contrary, it is necessary to use more chemicals if a fully bleached pulp is aimed at. The method is also favourable in view of the environment, because no sulphur compounds are used in cooking, and preferably no chlorine compounds are used in bleaching. In the following, the method for preparation of pulp in accordance with the present invention will be described further with the aid of three examples, to which the scope of protection and the field of application of the invention are, however, not supposed to be confined. Example 1: Cooking and bleaching of wheat straw Wheat straw was subjected to water cooking and to peroxide/peracetic-acid bleaching under laboratory conditions. The straw was initially chipped to lengths of about 50 mm, and the chips were cooked in revolving electrically heated cookers of 15 litres. The conditions and yields of two cookings performed at different temperatures are given in Table 1. The cooking liquor that was used was water which had been made acid by to it adding a little amount of an acid solution, which had been prepared by mixing concentrated formic acid and concentrated acetic acid at a ratio of 1:3. In addition to the acid solution, the necessary amount of water was added to the cooker in order to reach the desired liquid-to-straw ratio 10: 1. Thus, in both cookings, the addition of acid was 0.064 % of the volume of the cooking liquor. Table 1. Conditions and yields of water cooking of wheat straw. * The acid solution contained concentrated formic acid and concentrated acetic acid at a ratio of 1 :3. After cooking, the straw chips were washed and chelated for an hour with an EDTA dose of 0.3 % at a consistency of 3 % and at a temperature of 70 °C. After chelating, the straw chips still non-defibrated were bleached under the conditions given in Table 2. The bleaching comprised two peroxide stages, a peracetic-acid stage, and a third peroxide stage. The results of the bleachings are given in Table 3. The bleached pulps were ground, and the paper-technical properties were determined from them. Table 2. Bleaching conditions of wheat straw, consistency 10 %. Table 3. Bleaching results of wheat straw cooks 1 and 2. After cooking, the raw-material was still in straw form, but it was defiberized during the second peroxide stage. Based on the yield figures, the loss of material in cooking was about 10 % and in bleaching about 40 % of the weight of the original raw- material. By means of the water cooking method and the peroxide/peracetic-acid bleaching, bleached straw pulp of a brightness level of 80 % was obtained, whose tensile strength was rather high right at the beginning of grinding, and whose tear index was low. During grinding, the properties of the pulp did not develop in a favourable direction, so that it is hardly advisable to grind a straw pulp to a great extent. The water drain capacity of the pulp was poor because of short fibre length and high content of fines. Of the two cooking temperatures that were employed, it seemed that the lower temperature of 100 °C was preferable to 130 °C in respect of yield of pulp, brightness, and paper-technical properties. Example 2: Cooking and bleaching of flax In the manufacture of flax pulp, as the raw-material, flax was used which had been partly soaked on the field, braked and scutched and which was cut off to a length of about 12 mm. The function of the mechanical pre-treatment was to separate the fibre fraction from the rest of the plant material. The flax with long fibres was chipped in order that the fibre bundles should not be twisted together during cooking and during the further treatment of the pulp. Cleaned and chipped fibre fraction of flax was cooked in acid water over a period of 240 minutes at 100 °C. The cooking conditions were substantially equal to those in Cook 1 in Example 1. An acid solution which contained one part of concen- trated formic acid and three parts of concentrated acetic acid was used as an amount of 6.1 ml per 950 grams of flax, in which case, with a liquid-to-fibre ratio of 10: 1, the addition of acid was equal to 0.064 % of the volume of the cooking liquor. The ultimate pH of the flax cook was 5.1, and the yield was 92.0 % . After cooking, washing, and chelating, the flax pulp was bleached in three stages with a sequence peroxide - peracetic acid - peroxide. The bleaching conditions are given in Table 4. The flax pulp was ground as mixed with birch pulp. Table 4. Conditions used in bleaching of flax pulp, consistency 10 %. The yield of bleached flax pulp was 75.0 % of the original fibrous raw-material. The brightness of the pulp was 86.9 , i.e. clearly better than with the straw pulps of Example 1. In order that this level of brightness could be achieved, less bleaching chemicals were needed than with straw pulps. The total consumption of peroxide was 4.25 %, and the consumption of peracetic acid 0.81 % , calculated from the dry solids of the water-cooked flax. In bleaching in laboratory, the flax fibres were twisted into small tight kinks, which made a determination of the paper-technical properties of the pulp in grinding tests difficult. An increase of 20 % of flax improved the tear strength of a sheet made of birch pulp to a considerable extent. Example 3. Mixed cooking and bleaching of straw and flax. A mixed pulp was prepared from wheat straw and flax by cooking them together at a relatively low temperature in the presence of chelate. 645 grams of chipped wheat straw (length 50 mm) and 100 grams of partly soaked, braked, scutched, and chipped fibre fraction of flax (length 12 mm) were cooked over a period of 60 minutes at 70 °C. To the cooking water, for chelating, 0.3 % of EDTA, calculated from the weight of the fibrous raw-material, was added, and for regulation of pH, 6.4 ml of a mixture of concentrated formic acid and concentrated acetic acid (1:3) was added, in which case the concentration of the acid in the cooking liquor was 0.086 % by volume. The ultimate pH of the cook was 4.9. The mixed pulp of flax and straw was bleached by means of the same four-stage sequence and under the same conditions as pure straw pulp was bleached, see Table 2. Owing to different yields of flax and wheat, the ultimate mixing ratio in the bleached pulp was about 20:80. The yield of bleached pulp was 62.7 % of the original raw-material, and the brightness was 84.6 % . The consumption of peroxide in bleaching was 7.2 %, and the consumption of peracetic acid was 0.9 % . The bleachability of the pulp did not have any detrimental effect from the fact that the cooking was carried out at a lower temperature and with a shorter duration than in Examples 1 and 2. Chelating can already be carried out very well in connection with cooking. The test also proved that different fibrous raw-materials can be cooked and bleached together, provided that the requirements of the raw-material whose bleaching is more difficult are taken into account in the choice of the bleaching conditions.

Claims: Claims 1. A method for preparation of a fibrous stock suitable for manufacture of paper out of herb-stemmed plants, in which method the fibrous raw-material is chipped, cooked, and bleached, characterized in that the cooking is carried out at 60...130 °C and that the cooking liquor that is used is water to which an acid has been added so that the ultimate pH of the cooking is lower than 7 but higher than 3. 2. A method as claimed in claim 1, characterized in that, in bleaching, such dosages of chemicals and conditions of bleaching are employed by whose means defibration of the fibrous raw-material is ensured. 3. A method as claimed in claim 1 or 2, characterized in that, in view of regulation of the pH, an anorganic acid is added to the cooking liquor. 4. A method as claimed in claim 1 or 2, characterized in that the acid to be used for regulation of the pH is an organic acid, such as formic acid or acetic acid or a mixture of same. 5. A method as claimed in claim 3 or 4, characterized in that the addition of acid is, at the maximum, 2 % of the volume of the cooking liquor, preferably less than 1 per cent by volume. 6. A method as claimed in any of the preceding claims, characterized in that the cooking is carried out at 70...100 °C. 7. A method as claimed in any of the preceding claims, characterized in that, in the bleaching following after the cooking, chlorine-free bleaching chemicals are used. 8. A method as claimed in claim 7, characterized in that peroxide is used as the bleaching chemical, and the pulp is chelated before bleaching. 9. A method as claimed in claim 8, characterized in that the chelating is already carried out in connection with cooking. 10. A method as claimed in any of the preceding claims, characterized in that, before cooking, the fibrous raw-material is chipped, depending on the plant species, into chips of a length of 5...50 mm. 11. A method as claimed in any of the preceding claims, characterized in that, before cooking, the fibrous material is separated mechanically from the fibrous raw- material. 12. A method as claimed in any of the preceding claims, characterized in that the method is applied in a process placed in connection with a paper mill, and the fibrous pulp produced in said process is utilized in the paper mill as mixed with a traditional wood pulp. 13. A method as claimed in any of the preceding claims, characterized in that fibrous pulp produced by means of the method is used for the desired layers in a multi-layer web. 14. A method as claimed in any of the preceding claims, characterized in that two or more herb-stemmed plants are cooked and bleached as mixed together. 15. A method as claimed in claim 14, characterized in that a herb-stemmed plant with long fibres and a herb-stemmed plant with short fibres are cooked and bleached as mixed together.

Inventors: JOHANSSON ALLAN, LEMINEN ANJA, SIPILAE KAI, JOHANSSON, ALLAN, LEMINEN, ANJA, SIPILAE, KAI
Applicants: JOHANSSON ALLAN, LEMINEN ANJA, SIPILAE KAI, VALMET CORP, JOHANSSON, ALLAN, LEMINEN, ANJA, SIPILAE, KAI, VALMET CORPORATION
Application year: 1999
Main classification: D21C3
IPC codes: D21B1, D21C3, D21C5, D21C9, D21H11