FI66032C - FOERFARANDE VID DELIGNIFIERING AV CELLULOSAMASSA - Google Patents

Description

1 6-5032

This invention relates to a process for delignifying cellulose pulp with a base in the presence of oxygen, ozone, air or a mixture thereof. In particular, the invention relates to the dissolution of lignin in a chemically cooked cellulosic pulp by means of oxygen gas and sodium hydroxide solution.

Today, almost all oxygen processing plants operate in the so-called at a high consistency, i.e. in the range of 22 to 33%, and the reactors 10 are gas phase reactors. The pressure is in the range of 5-7 bar and the temperature is 100 - 125 ° C. Alkali and oxygen are dosed on average about 1 kg for each kappa unit to be dropped, i.e. if the kappa reduction is about 17 units, this means a dose of 17 kg of oxygen and white liquor. The use of preservatives 15 (.Mg compounds) in this type of plant is necessary to avoid adverse reactions to the pulp.

Recently, however, there is a shift in the performance of oxygen bleaching so-called. to the medium consistency process, where the consistency-20 range is B - 15 ί. The reactors are then usually bottom-up and have no free gas spaces. The conditions of the medium consistency process are otherwise the same in the high-consistency process of the species. Such reactors have a lower potential for pulp-destroying reactions because, for example, 25 local overheating points do not occur in the pulp at all due to the lower consistency, which means a higher amount of solution around the fibers, which effectively binds the heat of reaction.

However, since cellulosic pulps contain heavy metals such as manganese and iron, which catalyze adverse reactions, it is good to use preservatives (Mg compounds).

It is known that when the cellulosic pulp is oxidized under temporal conditions, the cleavage of the carbohydrates and the decrease in the degree of polymerization of the cellulose occur at the same time. This cleavage is promoted (catalyzed) by sensations. _ rr 66032 heavy metal cations contained in pulp .and water, such as manganese, cobalt, iron, nickel and copper.

This side effect has generally been counteracted by carrying out Hanni bleaching by adding various lal-5 magnesium salts to the pulp entering the bleaching stage as a preservative, e.g. immediately before the addition of the alkali. It has also been proposed to simultaneously add magnesium salts and organic complexing agents to the acid-bleached pulp to make the magnesium more evenly distributed in the alkaline pulp.

However, the protective effect of magnesium salts is limited, especially in the presence of inactive heavy metal catalysts. In addition, .Mg is not desirable nor: "without adverse effects in the alkali salt recovery process-15 cycle.

It has now surprisingly been found that magnesium salts and / or complexing agents are not required in oxygen treatment if the entry of heavy metal ions catalyzing the oil coupling of carbohydrates into the oxygen treatment is prevented.

The process according to the invention is characterized in that the pulp is pretreated with salts of aminopolycarboxylic acids, for example the Na salts of diethylenetriaminepentaacetic acid or ethylenediaminetetraacetic acid, before the deliomization and that the delignification is carried out without the use of a preservative.

Sodium salts of aminopolycarboxylic acids such as diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid react with metal ions to form stable and soluble commands with them. In this way, harmful metals can be removed from the pulp before the pulping step and it is not necessary to use Mg compounds as a preservative at all.

According to the invention, the pretreatment can be carried out in a mass wash, sorting or in a separate pretreatment step before the oxygen step.

The consistency should preferably be low, °, 5 to 5 "so that the fibers are completely surrounded by liquid.

t 3 6 '5032 In this case, the pretreatment chemicals, the sodium salts of diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA) react rapidly to bind the heavy metals, which are subsequently washed away from the pulp stock.

The amount of treatment chemicals varies within wide limits, depending on the amount of heavy metals contained in the pulp, water, and chemicals used. Generally, the dose will range from 0.05 to 0.5% by weight.

10 The treatment temperature is mainly determined by other process temperatures, but in the case of high-density oxygen bleaching it is in the range of 70 to 85 ° C, and in the case of medium-density oxygen bleaching in the range of 80 to 98 ° C, respectively.

The pretreatment time depends mainly on the consistency and temperature, so that low consistency and high temperature are advantageous in terms of delay.

In the process according to the invention, after cooking the cellulose pulp, the waste solution can first be separated from the pulp in the usual way by displacement washing. Before the last 20 washing filters, a complexing agent, e.g. sodium salt solution of DTPA, for example 0.1 to 0.5% by weight, is added to a dilute stock which may have a consistency of 0.5-5%. The temperature of the stock can then be normal in the washing system, e.g. 70-95 ° C, and the exposure time e.g. 3-15 min. The treatment solution is then filtered off from the stock or can also be displaced by the solution separated after the oxygen bleaching step itself. The solution can also be pressed from the cellulose pulp if oxygen bleaching occurs at a consistency of more than 20%. Alkali 30 is added to the pulp thus treated and oxygen gas is mixed in a known manner.

Example An unbleached sulphate pulp with a kappa number of 40 and a viscosity of 1200 m 2 / kg was treated as follows:.

Experiment 0. The pulp was subjected to oxygen treatment at 105 ° C for 35 minutes, with a dose of 2.5% NaOH, a consistency of 10 # 66032 and a pressure of 6 bar, and the result thus obtained was called the O-test.

Experiment 1. A comparative experiment was performed using MgSO 4 · 7H 2 O, 0.5% by weight, as a protective agent, otherwise oxygen treatment as above.

Experiment 2. The third pulp was pretreated by adding 0.2% DTPA Na salt, at 95 ° C, for 15 minutes, at a consistency of 3%, and after the treatment, the stock was washed and oxygen bleached according to Experiment 0.

10 In addition to the above, experiments 1 and 2 underwent exactly the same treatments, but the pulps contained 3> 5% dry matter in the waste liquor. Experiment 1 corresponds to Experiment 3 and Experiment 2 to Experiment 4.

15 Table

Kappa Viscosity (m ^ / kg)

Unbleached 40 1200 0 test 22.0 938 20 Test 1 24.8 1001

Experiment 2 22.9 1031

Experiment 3 22.5 971

Experiment 4 22.6 994 25 The results of the experiment show that the use of Mg compounds improves the viscosity of the oxygen-alkali delignified pulp, but an even better result was obtained by DTPA pretreatment. In addition, there is a considerable effect of the waste liquor, which, on the contrary, reduced the viscosity of 3Q, as claimed in Finnish patent no. 59273. For this reason, the pulp should be as pure as possible before it is subjected to oxygen-alkali delignification.

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Claims (12)

5,60032 A process for delignifying a cellulose pulp with a base in the presence of oxygen, ozone, air or a mixture thereof, characterized in that the pulp is pretreated with salts of aminopolycarboxylic acids, for example diethylene triamine pentaacetic acid or as Mg compounds. Process according to Claim 1, characterized in that the pulp has been washed and / or pressed before oxygen-alkali delignification, containing not more than 3.5% by weight of dry matter from the cooking. Process according to Claim 1 or 2, characterized in that the pretreatment lasts 2 to 30 minutes, preferably 5 to 10 minutes. Process according to one of Claims 1 to 3, characterized in that the pretreatment consistency is 0.5 to 3% t, preferably 2 to 5%. Process according to one of Claims 1 to 4, characterized in that temperatures of 60 to 100 ° C, most preferably 80 to 95 ° C, are used in the pretreatment. Process according to one of Claims 1 to 5, characterized in that the salts of aminopolycarboxylic acids are metered in for pretreatment from 0.05 to 0.5% »preferably 0.10 to 0.30% by weight per tonne. Process according to one of Claims 1 to 6, characterized in that, after the pretreatment, the chemicals added to it and the cations bound to them are washed or pressed from the pulp in one or more stages before the oxygen-alkali delignification. Process according to one of Claims 1 to 7, characterized in that the pretreatment is carried out by sorting the pulp before oxygen-alkali delignification. - Γ 6 66032 Method according to one of Claims 1 to 3, characterized in that the pretreatment is carried out on a so-called pulp. in countercurrent washing by adding pretreatment chemicals to the washing solution. Process according to one of Claims 1 to 9, characterized in that the oxygen bleaching is preceded and followed by washing and / or pressing of the pulp. Method according to one of Claims 1 to 10, characterized in that the pure or condensate water used for washing the pulp is introduced only to the last scrubber after oxygen-alkali delignification, from which the filtrates are always pumped as wash water from the previous washing and / or pressing step. Process according to one of Claims 1 to 11, characterized in that the oxygen-alkali delignification of the pulp is carried out at a consistency of 10 to 18%, at a temperature of 30 to 110 ° C, at an average pressure of 2 to 6 bar and from 20 to 120 bar. with a reaction time of one minute. 7 66032