Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With washing
Reexamination Certificate
2001-05-15
2004-05-11
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With washing
C162S065000, C162S070000, C162S075000, C162S242000
Reexamination Certificate
active
06733625
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method of and apparatus for treating chemical pulp to optimize the consumption of bleaching chemicals and improve the quality of the pulp. Especially the invention relates to a method and an apparatus, by means of which filtrate obtained from a suitable washing stage of brown stock preferably produced by an alkaline cooking process is treated with an oxidizing chemical prior to the oxygen stage following brown stock washing.
BACKGROUND AND SUMMARY OF INVENTION
In the oxygen stage carried out in medium consistency range, the amount of filtrate per one kg of pulp is 6-9 kg, and thus the properties of the filtrate have an essential effect on reactions which the pulp is subjected to in the oxygen stage, as also in the bleaching later on. So, the properties of the filtrate surrounding the pulp may have a significant effect on the chemical treatments carried out on pulp and also the disadvantageous reactions that the pulp is exposed to.
During the cooking, great amounts of organic material, mainly comprising lignin and carbohydrates originating from hemicellulose are detached off the wood fibers. Each of these organic materials has a chemical composition of its own as a result of the cooking conditions. When passing to the washing and the oxygen stage, these organic materials are carrying chemical compounds and end groups, which react with e.g. oxygen and peroxide. Thus, compounds practically inert in cooking conditions are reactive in new chemical conditions.
In most cases the oxygen stage is connected according the counter-current washing principle so that the object of the so-called brown stock washing located between the cook and the oxygen stage is to replace the liquor passed from the cook with the pulp. This liquor may be referred to e.g. washing loss and/or COD-load and is obtained as filtrate from said last washing stage, with filtrate obtained from the washing in the oxygen stage. The latter filtrate has passed through the oxygen stage with the pulp and due to that has an almost insignificant chemical potential to react with the chemicals in the oxygen stage, so that the chemicals may be used specifically for the desired reactions with the pulp. Nevertheless, some amount of black liquor components is always passed through the washing, which components play a different role than the oxidized filtrate.
In this connection, the oxygen stage refers to an alkaline stage carried out pressurized in the pressure range of 1-17 bar (abg.), and pH-range of 8.5-14, in which stage oxygen is present around the fibers at least part of the reaction time. The oxygen stage may have one, two or even more steps, whereby each reaction step comprises a reaction vessel or reaction retention effected with a tube. In practice, reaction step refers in this connection to adding and mixing some chemical used in the oxygen stage and the following retention at the tube portion. A reaction time short when practiced may thus in mathematical modeling lead to oxygen stages having four or even five steps. Reaction retentions are, depending on the applied method, from 0.1 min to 120 minutes, as the reaction retention is dependent on the desired type of reaction. In this connection, the oxygen stage is identified by a washing stage both prior to and after the oxygen stage and the fact that from the filtrate obtained from the washing after the oxygen stage usually at least part or all the filtrate is introduced to the washing prior to the oxygen stage to be used as washing liquid, so that the oxygen stage is connected countercurrently either completely or at least partially.
Most usually, oxygen and alkali and possibly some inhibitor preventing the deteriorating effect of metals on fibers is dosed into the oxygen stage, or the metals traveling with the fibers are otherwise removed or made non-reactive. The alkali charge is usually 1-60 kg ADMT (air dried metric ton) pulp and the oxygen charge 1-50 kg/ADMT pulp. The alkali that is used is most often sodium hydroxide or oxidized white liquor, but in principle all alkaline compounds containing OH-ion are alkalis which might be used in some conditions in the oxygen stage. The oxygen is dosed in gaseous form, the oxygen content most usually being 75-100% of the specific weight. The temperature in the oxygen stage is 70-120° C. and in most cases 80-105° C. The temperature may be raised utilizing some suitable steam having a pressure of 0.5-20 bar and hot water either via washing or dilution. The steam may be used for heating either mixed directly into the pulp or indirectly.
As to reaction kinetics, the oxygen stage is carried out so that raising the temperature and increasing the alkali charge lead to acceleration of the delignification reaction. The oxygen charge, in turn, is mainly not effected without increasing the amount of alkali. The suppliers of the oxygen stage have their own opinions about which variable is determinant in different steps and thus each supplier regulates the chemical and temperature profile according to his own desire. Nevertheless, as to reaction kinetics, in all applications the kinetics of temperature, oxygen and alkali follow one and the same basic principle.
According to our studies, the chemical reactions of the oxygen stage as a whole proceed essentially so that part of the oxygen reacts directly with the lignin compounds of the pulp and splits lignin by means of a direct reaction. Oxygen in itself is a selective chemical, which does not split carbohydrates. But in alkaline conditions part of the oxygen converts to peroxide which is very quickly decomposed to hydroxyl radicals by the effect of e.g. black liquor compounds originating from the cook. A hydroxyl radical is chemically very reactive, and the reactions thereof are not districted to reacting with lignin only, but it also causes splitting of carbohydrate chains of the pulp. Practice has shown that the selectivity or non-selectivity of a hydroxyl radical may be described e.g. so that a hydroxyl radical splits one cellulose molecule per five lignin molecules. In our experiments especially the presence of black liquor increased the degradation of peroxide and, accordingly, accelerated the forming of hydroxyl radicals at the end of the reaction chain, whereby a bigger portion of the oxygen changes via peroxide to hydroxyl radicals and thus causes damages to the pulp.
When elaborating the oxygen delignification following the washing of chemical pulp, the operation of the brown stock washing line, located in the process order prior to the oxygen stage, is usually determined so that the washing losses have to be adequately low before the oxygen stage in order to obtain a satisfactory selectivity. The term washing loss is used to refer to impurities remaining in the pulp despite the washing, which impurities in this case comprise both different chemicals and organic materials dissolved in the liquid phase during the cook. Various producers of apparatuses have different opinions on an acceptable level of washing losses. Nevertheless, prior art has not earlier performed any systematic reporting about any chemical mechanism or reason to why different washing loss levels have in different mills resulted in contradictory results concerning the effect of the impurity of the pulp on e.g. viscosity and strength properties of the pulp. This invention is based on extensive comparative studies, in which at least one significant reason for the quality losses of pulp has been determined and thus chemical reasons for quality losses of pulp found. According to said studies, the quality losses of pulp are generated as a result of the following kind of process:
The conditions in the oxygen stage generate peroxide as oxygen decomposes in alkaline condition.
Peroxide decomposes to hydroxyl radicals.
The presence of non-oxidized black liquor originating from the cook catalyses and accelerates the forming of hydroxyl radicals.
The hydroxyl radicals, due to their low selectivity, split cellulose molecules and thus cause quality losses.
In mills especi
Henricson Kaj
Kontturi Eero
Pikka Olavi
Vehmaa Janne
Andritz Oy
Chin Peter
Hug Eric
Nixon & Vanderhye P.C.
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