GB2076034A - Process for treating lignocellulosic material - Google Patents

Abstract

A process for treating lignocellulosic material, for example, straw, uses a nitrogen containing acid, a nitrogen containing organic acid or a soluble salt of these acids in conjunction with an accelerating agent which is a strongly ionising water soluble substance to release the cellulosic and ligneous components. The acid may be sulphamic acid used in conjunction with magnesium sulphate as the accelerating agent. In a second stage of the process a neutralising alkaline agent may be used to separate out the ligneous component. The process may be a semi-dry process conducted under atmospheric pressure and at temperatures up to 100 DEG C. End products suitable for use in papermaking and/or for animal feed may be produced by the process.

Description

SPECIFICATION Process for treating lignocellulosic material This invention relates to a process for treating lignocellulosic material in particular for breaking down or pulping lignocellulosic material, for example straw, for various end uses.

Straw has been treated in various ways to enhance its value, for example as a feed material for animals and for paper and board making. In each case the treatment is intended to release the cellulose from the lignocellulose and a wide range of processes using a variety of chemical substances have been used to achieve this. As an example of such processes we refer to British Patent No. 1,366,138 in which sodium hydroxide is used on straw to produce a feed material, and sodium hydroxide has also been used in a number of processes intended to produce fibre for paper making. In European patent application No. 78101794.2 there is disclosed a process in which nitric acid or nitrate salts such as sodium nitrate, potassium nitrate or ammonium nitrate are used in combination with aluminium sulphate.

Sodium sulphite, ammonium sulphite and ammonium sulphate have also been used but each of these processes has suffered from various disadvantages such as expensive and complicated treatment plant, the need for high temperature and/or high pressure treatment steps, the production of unusable waste products with resulting environmental problems, and lengthy, restricted capacity treatment systems.

An object of this invention is to provide a process for treating lignocellulosic material in which treatment is achieved effectively and economically using chemicals which give few environmental problems and result in a product suitable for various uses.

According to the invention a process for treating lignocellulosic material comprises applying a nitrogen containing acid, such as sulphamic acid; nitrogen containing organic acid, such as amino-acetic acid; or compounds of these acids such as ammonium sulphates, ammonium sulphamates or other soluble salts of these acids; together with an accelerating agent in the form of a strongly ionising, water soluble substance, such as ammonium sulphate, ammonium chloride, magnesium sulphate, sodium sulphate, magnesium chloride, sodium chloride, sodium chlorate, calcium sulphate and calcium chloride, to Ihe lignocellulosic material to cause the ceilulosic and ligneous components of the material to be released.

Instead of applying the reagents in their form as acids or salts such chemicals can be formed in situ by reacting suitable chemicals which will form the desired acids or salts. For example, instead of applying sulphamic acid to the material, urea and sulphuric acid may be applied.

Preferably the nitrogen containing acids or their soluble salts are present in the proportion of 0.15-9% dry weight of the lignocellulosic material and the accelerating agent is in the proportion of 0.05 to 0.8% dry weight of the material.

After or simultaneously with the release of the cellulosic and ligneous components a neutralising alkaline agent such as sodium hydroxide, calcium hydroxide, potassium hydroxide, or ammonium hydroxide may be added in the concentration of 115% by dry weight of the lignocellulosic material to separate the ligneous and cellulosic components.

The process may be conducted at atmospheric pressure and at a temperature of 80-1 000C thereby enabling the process to take place in simple plant which avoids the use of pressure vessels and the addition of significant extra heat, the exothermic nature of the action on the material providing some of the heat requirements.

However the process can with advantage take place under pressure to save on the rate of usage of chemicals and to reduce the duration of the process.

It has also been found that the treatment time for the process is not excessive thereby enabling a high throughput of the plant. For example if the treatment is conducted in two stages, a first stage in which the acid or salt and accelerating agent are added, and a second stage in which the alkali is added, the first stage may take from 21 to 20 minutes and the second stage from 5 to 30 minutes depending upon the cellulosic material used.

After such a first stage free acid or salt and accelerating agent may be removed from the material for recycling before adding the alkali and similarly free alkali may be removed after the second stage. Such removal may be by known means such as adding wash water and using a centrifuge, a press or a screen.

The addition of accelerating agent reduces the time that would otherwise be necessary for the acid or salt to dissolve the lignin. The process results in the lignin being extracted in a form in which it may readily be used, for example, as a sizing agent in paper or board making.

The use of sulphamic acid is advantageous in that it is relatively easy to deal with and can be obtained in crystalline form and is without some of the pollution and other hazards of other materials such as sodium hydroxide and nitric acid.

Simple equipment can be used in the process although the process can also be used in existing equipment hitherto used in other processes. As a result of the simplicity of the equipment which can be used relatively small plants local to the supply of material such as straw can be economically employed using the process of the invention.

The ligneous component (black liquor) of the process extracted after the alkali has been added can be discharged or used. The nature of the process enables the black liquor to find a variety of uses either in its orginal form or after extraction and separation of lignin and other substances. In particular the black liquor can be used in paper making as a sizing agent and this is especially useful if the extracted fibre is also used for unbleached paper and board making.

The process provides for a high degree of flexibility in the end uses of the products of the process. Thus the fibre product of straw treatment can have a variety of uses ranging from animal feed of high nutritional value ot its use in paper making processes. The ligneous component, which can take the form of small particles, can also be used in paper making or the various constituent components can be separated out for a variety of uses.

The process of the invention can be a "semi-dry process" and by "semi-dry process" is meant that the mixture of materials process do not become fluid due to a relatively high liquid content. Due to the dry nature of the mixture it is conveyed by mechanical or pneumatic means and does not flow. The process aims to reduce the liquid content to the minimum necessary to incorporate the chemicals into the lignocellulosic material being treated. The amount of added liquid is dependent on the moisture content of the raw lignocellulosic material. With other 'semi-dry processes' the materials used, such as nitric acid, can give rise to hazards such as combustion of the lignocellulosic material.

The basic requirement of the treatment plant is that it should be constructed of acid resistant steel suitable for temperatures up to 1000C.

The process is applicable to shredded cereal straw (including rice straw), sugar cane (bagasse), waste paper, hardwoods (such as willow, salix aquatica gigantea and salix viminalis) and softwoods (such as Douglas fir and Sitka spruce).

A general requirement of the process is that the lignocellulosic material is shredded and various constituents such as stone and metal, leaves, dust grain, ears are removed prior to treatment.

The reagents in the form of the acid or salt are added together with the accelerating agent, either in solution or in powder form.

The mixture is placed in a hydrapulper and steamed at up to 900C for 2 to 20 minutes for a first stage in the operation.

The free liquid is then extracted by a press, screw press, centrifuge or vacuum filter and such free liquid is used as stock liquid in the hydrapulper of the first stage.

The material is then refined by;for example, a disc refiner. The refined material is placed in a further, second stage hydrapulper and alkali in solution is added. The mixture is steamed for 5 to 30 minutes at up to 1000C. The free liquid is then extracted from the material by a press or centrifuge to provide stock liquid for the second stage hydrapulper. The extracted free liquid may also be treated to precipitate the lignin constituent.

Prior to alkali treatment the material may be ground and the alkali treatment may take place simultaneously with pelletisation or cubing of the material. When the product is to be used in board or paper making, the aim of the process is to minimise the damage to the fibre constituent.

If the fibre material is to be used in paper making it may be treated wet in known manner.

The lignin component is precipitated by the addition of paper maker's alum (aluminium sulphate) for removal from the liquid and the resulting clear liquid may be reused in the alkali treatment stage with topping up of alkali as may be necessary. The precipitated lignin may be used for example in paper finishing, animal feed, as a binder for paper board, a soil stabiliser or a fertiliser (when containing potassium).

The fibre product may be used as a constituent in unbleached papers and boards, in combination with waste papers and suitably treated black liquor. In this application the waste paper is upgraded by the action of the lignin part of the black liquor. For use in bleached papers the fibre product must first be put through a bleaching process and the addition of black liquor made with caution so as not to downgrade the shade of the final prodct.

If used for animal feed the fibre may be passed through a screw feeder to reduce moisture and additives such as rolled grain, fines (as extracted from the initial stage of the process) and molasses made to the fibre.

The following examples of the process of the invention are now given by way of illustration only: EXAMPLE 1 Shredded wheat straw was treated in a two stage process to produce fibre suitable for feed or paper making.

After removal of fines and detritus the straw was shredded into lengths of about 5 cm. 2% by dry weight crystalline sulphamic acid and 0.2% by dry weight of magnesium sulphate, as accelerating agent, were added to the straw in solution.

The mixture was agitated thoroughly and heated to 900C for 5 minutes for the first stage of the process.

After removing free liquid for recirculation to the first stage and further mechanical refining, 5% by weight sodium hydroxide was added. The material was hydrapulped and maintained at a temperature of 90--1000C for 20 minutes for the second stage of the process.

At the completion of the second stage the lignin is free from the straw fibre and can be washed out in the form of alkaline black liquor. By adding papermakers' alum to the black liquor the pH value is reduced which causes the lignin content to be precipitated. Lignin-free liquid is produced which can be recirculated to the second stage of the process and the lignin can find other uses, as previously indicated.

In Example 1 as an alternative to the use of sodium hydroxide in stage 2, 6% by weight of calcium hydroxide can be added. The use of calcium hydroxide can be advantageous when the straw fibre is to be used for animal feed. The calcium hydroxide may be formed by adding slaked lime to the straw in the presence of damp material.

EXAMPLE 2 Shredded wheat straw was treated in a two stage process, as for Example 1, except that in the first stage 2% by dry weight of ammonium sulphate was used with 0.2% by dry weight of sodium sulphate or aluminium chloride as the accelerating agent.

The second stage of the process was the same as for Example 1.

EXAMPLE 3 For the semi-dry application, shredded wheat straw (which could be other cereal, rice or oil seed rape) was treated by spray with a solution of sulphamic acid and magnesium sulphate in the quantities set out in Example 1. After thorough agitation the mixture was passed through a cyclone to remove dust and then proceeded as follows: 1. For paper and board making The over-long fibre was screened out and returned to the beginning of the process and the remainder was treated with a solution of sodium hydroxide so that the amount added was between 5 and 6% dry weight. The resulting fibrous mass was compressed for easy transportation.

2. For Animal Feed The fibre was passed through a hammer mill to comminute the mixture after which alkali was added in the manner described above for-paper.

The mass was then pelletised to improve the digestibility of the fibre further. In order to reduce waste, the dust emanating from the cyclone was added to the animal feed fraction prior to alkali treatment.

The process of the invention offers various potential advantages. The chemicals used can be in powder form and the treatment can be a 'semi dry process' thus reducing energy consumption.

The process can be free of waste effluents by continuous reuse of the chemicals and by this means the use of chemicals is kept at a minimum and pollution avoided.

The plant used may take a simple form and the process may be a continuous or a batch process.

Claims (17)

1. A process for treating lignocellulosic material comprising applying a nitrogen containing acid, nitrogen containing organic acid or compounds of said acids which are soluble salts of the acids, together with an accelerating agent in the form of a strongly ionising water soluble substance to the lignocellulosic material to cause the cellulosic and ligneous components of the material to be released therefrom.

2. A process according to claim 1 wherein the nitrogen containing acid is sulphamic acid.

3. A process according to claim 1 wherein the nitrogen containing acid is amino acetic acid.

4. A process according to claim 1 wherein the salt compound is selected from ammonium sulphate and ammonium sulphamate.

5. A process according to any one of the preceding claims wherein the accelerating agent is selected from ammonium sulphate, ammonium chloride, magnesium sulphate, sodium sulphate, magnesium chloride, sodium chloride, sodium chlorate, calcium sulphate and calcium chloride.

6. A process according to any one of the preceding claims wherein the acid or salt is applied by reacting chemicals in situ with the lignocellulosic material to form the desired reagent.

7. A process according to claim 6 wherein the reagent is formed by applying urea and sulphuric acid to the material.

8. A process according to claim 1,2 or 4 wherein the nitrogen containing acid or its soluble salt is present in the proportion of 0.1 5-9% dry weight of the lignocellulosic material.

9. A process according to claim 1 or 5 wherein the accelerating agent is present in the proportion of 0.05 to 0.8% dry weight of the lignocellulosic material.

10. A process according to any one of the preceding claims wherein a neutralising alkaline agent is added after the step of releasing of the cellulosic and ligneous components.

11. A process according to claim 10 wherein the neutralising alkaline agent is selected from sodium hydroxide, calcium hydroxide, potassium hydroxide and ammonium hydroxide.

12. A process according to claim 10 or 11 wherein the neutralising alkaline agent is added in the concentration of 1-1 5% by dry weight of the lignocellulosic material.

13. A process according to any one of the preceding claims wherein the step of releasing the cellulosic and ligneous components is conducted at a temperature of 80-1 000C.

14. A process according to any one of the preceding claims wherein the step of releasing the cellulosic and ligneous components is conducted at or near atmospheric pressure.

15. A process according to claim 13 or 14 wherein said step lasts for 21 to 20 minutes.

1 6. A process according to any one of the preceding claims wherein after the step of releasing the cellulosic and ligneous components free acid or salt and accelerating agent are removed.

17. A process according to any one of the preceding claims wherein the lignocellulosic material is selected from cereal straw, sugar cane, waste paper, hardwood and softwood.

1 8. A process for treating lignocellulosic material substantially as described with reference to any one of examples 1-3.