AU7587981A - Process to produce pulps - Google Patents

Description

PROCESS TO PRODUCE PULPS

This invention refers to a process to produce pulps from vegetable, lignocelllulose-containing fibrous material, e.g. coniferous wood, straw and bagasse.

Traditional processes to produce pulps, and parti cularly the sulphate process which is used to produce almost the total chemical pulp on the world market, have considerable drawbacks. Thus, there is required in order to bring about in the cooking system an even distribution of reaction agents in the starting or raw material and thereby good delignification, plants with pressurized reaction devices of big volume and circulation of remarkable quantities of liquids heated by heat exchangers. Such plants are extremely expensive. The obtained pulp is difficult to bleach for which reason v ery complex and highly expensive bleaching plants become necessary for treatment of the pulp. Furthermore, the cooking and bleaching process causes pollution of air and water which is unacceptable. Therefore, internal treatment plants such as combustion and chemical recovery installations, and external treatment plants such as water purifying basins, are necessary to bring the effluents to acceptable level, which require high initial investment costs, which in turn implies demand for large production units. Air and water pollution are mainly due to the use of chemical reactants which contain sulphur and chlorine and chlorine derivates, the latter ones in large amounts as consequence of the aforesaid low bleachability of the pulps. The presence of sulphur brings about i.a. the characteristic evil smell from plants operating with the sulphate process. The mentioned drawbacks involve also hibh energy consumption due to the plants for impregnation of the raw material with cooking chemicals, for pulp bleaching, for recovery of chemicals and for environment protection.

The main object of the invention is to provide a process for production of pulps wherein the handling of the chemicals is simplified so that the aforestated drawbacks are avoided. A second object of the invention is by the simplified handling of chemicals to obtain lower investment costs which results in that minor plants also become profitable which permits installation of plants nearer the raw material sources with consequent reduction of transport costs, which also involves a generally widened range of raw or starting materials.

A third object of the invention is to render possible in this connection a widened utilization of non-traditional raw materials for production of pulps such as green chips, i.e. twigs, branches, bushes and shrubs and low-grade timber especially from coniferous wood as result from various thinning and cleaning operations, furthermore agricultural wastes like wheat, rice straw and bagasse, but also e.g. banana stems, soybeans and coconut shells, furthermore also from other annual vegetables cultivated for their fibre content more than with regard to their nutritive value, such as cotton, flax, kenaf, but also wild annual vegetables such as certain kinds of grass and reeds, and finally also wastes from traditional woodworking industry in the shape of sawmill waste and especially waste from tropical kinds of wood possessing very high density and therefore difficult to treat with conventional processes of pulp production, and waste from traditional pulp industry in the shape of knot catcher rejects and some kind of fibre sludge.

These and other objects of the invention are obtained by imparting to the process the characterizing features stated in the appended claims.

The use according to the invention of other chemical reactants than sulphur and its compounds and compositions with sodium in an alkaline surrounding, molecular chlorine and, in addition, a significant reduction of the use of chloro-deri vates result in a reduction of environment pollu tion. Thus, i.a. the problem of evil smell can practically be eliminated by the process of the present invention. The use of the proposed chemical reactants allows simplification of the plants for reco v e ry of chemicals and heat and assures also the production of chemical pulps which are easily bleachable, making bleaching plants less complex and less capital-intensive. According to the invention, it is possible to produce, in addition to chemical pulps, also pulps of the so-called chemi-mechanical and semi-chemical types.

The invention will in the following be described in more detail with reference to the embodiment shown in the accompanying drawing.

The figure shows diagrammatically a complete plant for utilization of straw, the process being subdivided into blocks (A, B, C, D, E, F, H, I) and units, each one representing a section of the plant. Following the direction of the main flow represented by double drawn arrows, one will observe:

Block A represents the washing in that section of the plant where the fibrous material is washed in several steps with water and waste liquor from the pulp washing after the first cooking step to remove heavier particles and sand, but also some fine substance of partly colloidal nature, from the fibrous material. The washing can be carried out under some simultaneous disintegration of the fibrous material.

Block B represents that part of the plant where the fibrous material is imbibed with make-up-chemicals ("makeup" = losses in the washing and recovery systems) in the form of sodium hydroxide under continued disintegration, squeezing out of excess of chemicalsand thereby adjustment to the concentration of chemicals to the correct one for the process. The whole process stage is carried out at temperatures between 50 and 110°C and results in addition to impregnation in a first defibration and delignification The impregnated fibrous material is dehydrated in a press to a dry content between 20 and 55%. The dry content in the outlet from the press and the concentration of the circulating liquid in block B renders possible control of the concentration of chemicals in the outlet from the system so that the amount is the necessary one to counterbalance losses in the process and to activate the carbonate according to the principles of the invention.

Block C represents that section of the plant where a continued delignif ication in a pressurized reactor is effected after supply of recycled sodium carbonate (Na₂CO₃) from a chemical recovery system. Block C represents also that section of the plant where the delignif ication is completed by means of oxygen gas. The vegetable raw material coming from block B is added with the cooking chemical Na₂CO₃ in that defibration stage which constitutes the beginning of block C. The delignif icating reaction is completed in a pressurized oxygen gas reactor. The oxygen gas is added to the fibrous material in known manner in a mixer especially constructed for this purpose.

Block D-D₁ represents the different bleaching stages used on the unbleached, easily bleachable pulp which is produced by the process of the invention. Of particular interest is the possibility to use ozone (block D₁ ) which will be described below.

Block E represents the plant section where dehydration, drying and baling following traditional techniques are carried out.

Blocks F, G, H, I, L, M represent auxiliary plants of the main line. The continuous lines coupled with the dotted lines represent alternative flow lines.

Block F represents a section where in case of using raw materials with high silica or silica-compound content, a first treatment of the effluents is obtained to reduce silica. This treatment will also be described nearer more below. Block G represents the section of the plant for use as storage or for production of chemicals necessary for the process and particularly:

G₁ corresponds to storage of reactants and auxiliary additives for the various cooking and bleaching stages,

G₂ corresponds to the section for production of chlorine dioxide ( C10₂),

G₃ corresponds to storage of sodium hydroxide (NaOH),

G₄ corresponds to storage of hydrogen peroxide (H₂O₂), G₅ and G₆ correspond respectively to the sections for production of, respectively, oxygen gas (O₂) and ozone

(O₃ ) .

It is obvious that not all these sections and/or storages of the plant are co-existing; some of them are alternatives to the other sections depending on the bleaching sequency used.

Block H represents the section for the treatment of the gases consisting of a mixture of O₂ + CO₂ + H₂O coming from the relief valve of the oxygen gas reactor (block C). The aim is to separate and recycle O₂ following a technique which will be described hereinafter.

Block I represents the section to remove traces of O₃ from the mixture O₂ + O₃ coming from the bleaching system in case the bleaching is based on ozone (block D₁ ). The corresponding technique is described nearer in the following. Block L represents the section for secondary treatment of effluents coming from the individual sections of the plant: at 1) admission of the effluent from the raw material washing (see block A); at 2) admission of the effluent from the bleaching when the alternative technique chosen originates an effluent containing chlorine dioxide; at 3) admission of effluents from various mill services.

Block M finally represents the plant section for recovery of chemicals and heat as follows: M₁ represents the evaporating system for concentrating black liquors coming from the main pulp production line. M₂ represents the system for concentrated black liquor combustion and heat recovery from the dissolved organic matter of the raw material and recovery of chemicals such as Na₂CO₃.

M₃ represents dissolution and storage of the recovered Na₂CO₃ solution which is recycled to the delignification stages (blocks B and C).

In accordance with the invention sodium carbonate is used as reactant. The reactivity of the mentioned salt is known to be low towards lignine but also towards the cellulosic components and the cellulose and in any case inferior to the reactivity of the correspondent hydrate. Therefore the carbonate is activated, in the two stages where the delignification is accomplished by means of sodium hydrate which is added in that stage, where chips imbibition takes place, in quantities corresponding to the necessary ones to make up the loss of process reactants.

The quantity varies, according to raw materials and final products, between 15 and 30 kgs of hydrate per BDT (bone dry ton) of processed fibrous material.

For the activation O₂ also and possibly other oxidative agents (peroxides) are used in the last phase of delignification where working preassure, temperature and amount of O₂ are varied within very wide limits (10-20 Bar, 110-150ºC and 6-14% of O₂ ) according to raw material and desired final products. The process according to the invention is therefore characterized by the use of carbonate as alkaline salt directly in the process of delignification which develops in one or several stages.

Very good delignification (kappa no between 15 and 8) has been obtained whith the aforedescribed process simultaneously with viscosity levels high enough after the subsequent bleaching process to ensure good characteristics of resistance of the paper pulp together with a brightness of about 50% Scan impossible to be obtained by any known alkaline process. Characteristic for the process according to the invention is also that it yields pulps with low extractive content and with high cleanliness inspite of utilization of low quality raw material containing bark, for example. This is due to the fact that oxygen gas in alkaline medium reacts specifically with bark and fibre bundles which are difficult to become delignified by usual processes.

These properties of the pulps and particularly the high brightness permit v ery simplified bleaching treatments in comparison with usual processes. Particularly, chlorine treatment of the pulp is not required.

In the following table examples are given for proceedings and results in the bleaching of a pulp produced from wheat straw by treatment according to the invention:

As is apparent from this table, pulps produced accord ing to the invention are highly bleachable by ozone which affords a number of advantages. Ozone bleaching is carried out in a single stage at atmospheric pressure. Investment and energy costs of the ozone bleaching are lower than for other comparable processes. The production costs for ozone and chlorine dioxide are on the same level as today's technology, the energy consumption is 10 kWh per kg ozone provided that ozonisers are fed with oxygen, in comparison to 12-14 kWh per kg of chlorine dioxide starting from sodium chlorate. The effluent from the ozone bleaching stage can be conveyed to black liquor evaporation and combustion, for which reason environmental pollution is reduced to a minimum.

The process according to the invention is characterized also by the possibility of utilizing O₂ (moist and not pure) resulting from the ozone bleaching treatment. Industrial ozonisers when fed with oxygen yield a gas mixture where the ozone content is 2.5-2.7 %.

After exhaustion by reaction of ozone in the bleaching process oxygen may be recycled to the second delignification stage. The gas mixture coming from the bleaching stage (block D) is here passed through an ozone purifier in the shape of a coal bed (block I) in the presence of water in order to prevent carbohydrate degradation which is particularly severe at the used high reactor temperature even when traces only of ozone are present.

Pressure control in the oxygen gas reactor (block C) and providing security that any explosive gas mixture of carbon monoxide (CO) and oxygen gas (O₂) cannot be formed are made by continuous venting of the reactor for treatment of the mixture of steam, carbon dioxide and oxygen gas. The treatment is carried out by washing the mixture with a carbonate solution. Sufficiently pure oxygen gas is obtained which is separated and recycled to the reactor after recompression whereas the carbonate is turned into bicarbonate. By heating the bicarbonate solution the carbon dioxide is expelled and the obtained carbonate solution is concentrated and can be used again for washing (block H).

Every process for production of cellulose pulp requires both for economic reasons and environment protection reasons destruction or recovery in some shape of used inorganic chemicals and organic material released in the process.

Every process for recovery of chemicals and heat involves necessarily combustion, mostly after concentration by evaporation, to a dry content of 50-65 % of black liquors collected from the pulp washing during and/or after the del ignification process. Black liquors contain those inorganic chemicals which have been used for the del ignification, chemicals to a small extent as free reactants and for the most part in combination with salts of organic acids, alkal lignine and other compounds of sometimes v e ry complex structure and forming non-eel 1 ulosic wood components.

Combustion of black liquors liberates carbon dioxide (CO₂) and for this reason the alkaline salts are recovered as carbonates in a sulphur-free process. Conventionally, the carbonate is turned to hydrate in order to restore the reactivity and delignifying capacity of the alkali, which procedure is carried out in so-called causticizing plants which involve high investment costs, high energy consumption and cause considerable injury to the environment. The direct utilization of carbonate in the del ignification pro cess according to the invention thus eliminates the causticizing step and the disadvantageous effects thereof (block M in the figure).

Straw and other annual vegetables normally have a high silica content which causes troubles in connection with evaporation and combustion and even results in that the recovered chemicals are contaminated. The process according to the invention renders possible a treatment for reducing the silica content in black liquors. The treatment is based on the fact that the solubility of silica is a function of the pH of the alkaline solutions and that the silica content is higher in the so-called "0" fibre fraction than in other fibre fractions. The treatment is carried out as separation of that fraction of the black liquor from the straw washing stage which has a high content of "0" fibres. The pH reduction of the liquor by means of that CO₂ which is present in the combustion gas enables silica to be precipitated out of the system (block F in the figure).

While one more or less specific embodiment of the invention has been shown and described, it is to be understood that this is for purpose of illustration and that the invention is not to be limited thereby but its scope is to be determined by the appended claims.

Claims (9)

CLAI MS

1. A process to produce pulps from vegetable, lignocellulose-containing fibrous materials such as coniferous and hard-wood, straw, bagasse and the like, c h a r a c t e r i z e d in that the fibrous material is subjected to a sequence of treatment stages, of which in stage 1 the fibrous material is impregnated with a solution of sodium hydroxide in which the material is soaked and partially disintegrated by mechanical working, that in stage 2 excess of alkaline solution is squeezed out from the fibrous material, but so much only that after the squeezing stage the content of sodium hydroxide in the pulp amounts to at least 15 kgs per ton of pulp calculated as dry substance (BD ton of pulp) that in stage 3 the fibrous material is subjected to additional disintegration under supply of a solution of sodium carbonate so that a pumpable pulp suspension having a sodium carbonate content of 150-400 kgs per BD ton of pulp is obtained, that in stage 4 the pulp suspension is subjected to a cooking process at a temperature of at least 100°C during 1-4 hours and under stirring and supply of oxygen, whereafter the cooked pulp is washed.

2. The process according to claim 1, c h a r a c te r i z e d in that the fibrous material, prior to the impregnation with sodium hydroxide, is washed, the material being disintegrated at the same time.

3. The process according to claim 1 or 2, c h a r ae r i z e d in that the material during the impregnation in stage 1 is soaked during a time of from 15 to 45 minutes at a temperature of between 50 and 110°C.

4. The process according to any of the claims 1 - 3, c h a r a c t e r i z e d in that from the fibrous material in stage 4 the excess of alkali solution is squeezed out until the dry content of the pulp amounts to 20 - 55 % .

5. The process according to any of the claims 1 - 4, c h a r a c t e r i z e d in that in stage 4 during the cooking process 5-15 kgs of magnesium carbonate per BD ton of pulp are supplied to the pulp suspension.

6. The process according to any of the claims 1 - 5, applied to fibrous material which has been disintegrated mechanically into piece-shape such as chips, c h a r a c te r i z e d in that the disintegrated fibrous material is impregnated by becoming steamed, compressed and thereafter allowed to expand in the solution of sodium hydroxide and to soak up part thereof.

7. The process according to any of the claims 1 - 6, c h a r a c t e r i z e d in that the cooking process of stage 4 is performed in two separate cooking steps, the first of which is carried out at a temperature of preferably 140-160°C during between 60 and 120 minutes on a pulp suspension coming from stage 3 and containing 150-200 kgs sodium carbonate per BD ton of pulp, that the cooked pulp obtained thereby is washed and solution is squeezed off and thereafter additional 150-200 kgs sodium carbonate per BD ton of pulp are supplied, whereupon the pulp is subjected to a second cooking step at a temperature of preferably 130-150ºC during between 60 and 120 minutes under stirring and supply of oxygen so that the total pressure amounts to 10-20 Bar at preferably 110-150°C.

8. The process according to claim 7, c h a r a c te r i z e d in that the pulp after the first cooking step is liberated from solution by squeezing to a dry content of 30% and thereafter subjected to additional disintegration under simultaneous addition of sodium carbonate.

9. The process according to any of the claims 1 - 7, c h a r a c t e r i z e d in that the finally treated pulp in known manner is bleached by means of ozone, the ozone (O₃) being produced from oxygen gas (O₂) in ozonisers and after the bleaching the oxygen gas is purified from remainders of ozone by being passed through a coal bed in the presence of water and thereafter returned into the process.