EP0060277B1

EP0060277B1 - Process to produce pulps

Info

Publication number: EP0060277B1
Application number: EP81902585A
Authority: EP
Inventors: Franco Nardi
Original assignee: Sunds Defibrator AB
Current assignee: Naco p Srl Te Cervignano Del Friuli Italie
Priority date: 1980-09-22
Filing date: 1981-09-21
Publication date: 1986-03-26
Also published as: IT1209352B; EP0060277A1; DK155192C; IT8024818A0; DK155192B; DK231182A; WO1982001019A1; BR8108805A

Abstract

A process to produce pulp from vegetable, cellulose-containing fibrous material such as coniferous wood, hardwood, straw, bagasse etc., the fibrous material being treated in several stages. In a first stage the fibrous material is impregnated with a solution of sodium hydroxide, in which solution the material is soaked and partly disintegrated by mechanical working. In a second stage excess of alkaline solution is squeezed out to such extent that the content of sodium peroxide in the raw pulp amounts to at least 15 kgs per BD ton of pulp. In a third stage the pulp 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. In a fourth stage 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, whereupon the cooked pulp is washed.

Description

This invention refers to a process to produce pulps from vegetable, lignocellulose-containing fibrous material, e.g. coniferous wood, straw and bagasse.
Traditional processes to produce pulps, (see for example US-A-3769152, US-A-4045279 and US-A-3 691 008) and particularly the sulphate process which is used to produce almost the total chemical pulp on the world market, have considerable drawbacks. Thus, there are 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 very 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 derivatives, 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 high 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 the 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 tradiational 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 providing a process to produce pulps from vegetable, lignocellulose-containing fibrous materials such as coniferous and hard-wood, straw, bagasse and the like, characterized 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 during a time of from 15 to 45 minutes at a temperature of between 50° and 110°C, and simultaneously partially disintegrated by mechanical working, that in stage 2 the excess of alkaline solution is squeezed out from the fibrous material until the dry content of the treated material amounts to 20-55% and its content of sodium hydroxide amounts to at least 15 kgs per ton of treated material calculated as dry substance (BD ton of treated material), that in stage 3 the fibrous material is subjected to additional disintegration under supply of a solution of sodium carbonate . so that a pumpable treated material suspension having a sodium carbonate content of 150-400 kg per BD ton of treated material is obtained, that in stage 4 the treated material suspension is subjected in a pressurized reactor to a cooking process at a temperature of at least 110°C during 1-4 hours and under stirring and supply of oxygen, whereafter the cooked pulp is washed.
Prior to the impregnation with sodium hydroxide, the fibrous material can be washed and simultaneously pre-disintegrated. The finally treated pulp can be bleached in known manner by means of-ozone, the ozone (0₃) being produced from oxygen gas (0₂) 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.
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-derivatives result in a reduction of environment pollution. 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 recovery 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.
In accordance with the invention sodium carbonate is used as a 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 stage 3 after a first delignification has been accomplished by means of sodium hydrate in stage 1, 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 kg of hydrate per BDT (bone dry ton) of processed fibrous material.
For the activation 0₂ also and possibly other oxidative agents (peroxides) are used in stage 3 where working pressure, temperature and amount of O2 are varied within very wide limits (10-20 Bar, 110―150°C and 6-14% of 0₂) 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 (stage 3).
Very good delignification (kappa no between 15 and 8) has been obtained with the aforedescribed process simultaneously with viscosity levels high enough after the subsequent bleaching process to ensure good characteristics of resistance of the paper puIp 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 in spite 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 very 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 according 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 with 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 vol %, the rest being oxygen. After exhaustion by reaction of ozone in the bleaching process oxygen may be recycled to stage 3. The gas mixture coming from the bleaching stage is here passed through an ozone purifier in the shape of a coal bed 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.
Every process for recovery of chemicals and heat involved necessarily combustion, mostly after concentration by evaporation, to a dry content of 50-60% of black liquors collected from the pulp washing during and/or after the delignification process. Black liquors contain those inorganic chemicals which have been used for the delignification, chemicals to a small extent as free reactants and for the most part in combination with salts of organic acids, alkali lignine and other compounds of sometimes very complex structure and forming non-cullulosic wood components.
Combustion of black liquors liberates carbon dioxide (C0₂) 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 delignification process according to the invention thus eliminates the causticizing step and the disadvantageous effects thereof.

Claims

1. A process to produce pulps from vegetable, lignocellulose-containing fibrous materials such as coniferous and hard-wood, straw, bagasse and the like, characterized 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 during a time of from 15 to 45 minutes at a temperature of between 50° and 110°C and simultaneously partially disintegrated by mechanical working, that in stage 2 the excess of alkaline solution is squeezed out from the fibrous material until the dry content of the treated material amounts to 20-55% and its content of sodium hydroxide amounts to at least 15 kgs per ton of treated material calculated as dry substance (BD ton of treated material), that in stage 3 the fibrous material is subjected to additional disintegration under supply of a solution of sodium carbonate so that a pumpable treated material suspension having sodium carbonate content of 150-400 kg per BD ton of treated material is obtained, that in stage 4 the treated material suspension is subjected in a pressurized reactor to a cooking process at a temperature of at least 110°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, characterized in that the fibrous material, prior to the impregnation with sodium hydroxide, is washed, the material being predisintegrated at the same time.

3. The process according to claim 1 or 2, characterized in that the finally treated pulp is bleached in known manner by means of ozone, the ozone (0₃) being produced from oxygen gas (0₂) 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.