DE3601808A1 - METHOD FOR TREATING A CELLULOSE PULP - Google Patents

Description

KEIL & SCHAAFHAUSEN

PATENT LAWYERS

Frankfurt am Main
January 21, 1986
A 7 P 116

AGA AKTIEBOLAG

S-181 81 Lidingö
Sweden

Method of treating a cellulose pulp

Description:

The invention relates to a method in the production of cellulose pulp with a treatment step, wherein Oxygen gas is used as a reactant.

In the delignification of cellulose pulp, there is currently a tendency to move from a highly concentrated pulp with a pulp consistency of 25 to 30 % to a pulp with an intermediate concentration of a consistency of 8 to 15 % . The main reason for this is to reduce investment costs. However, there are problems with the addition of oxygen to such medium concentration pulps, since the amounts required are considerably larger than they can be dissolved in the liquid.

A first problem is the even distribution of oxygen in the pulp in the form of very small bubbles to achieve a large contact area. A second The problem is that the vesicles remain in the pulp suspension for a long enough time to carry out the reaction

KEIL & SCHAAFHAUSEN

2 patent attorneys

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to keep floating. A third problem arises when greater amounts than 4 to 5 kg of oxygen are to be added per ton of pulp, based on dried pulp having a dry solids content of about 90 %. Experience has shown that using stirrers or mixing devices for mixing the oxygen into the pulp, bubbles are formed which are not as small as desired; therefore a fair amount of oxygen flows through the pulp to the top of the reaction vessel without being consumed. A relatively high pressure is therefore required for the required reaction, especially at the start of the reaction. For this reason, an upward process must be adopted in which the hydrostatic pressure at the bottom of the vessel is high. In a downward process there is an additional disadvantage besides the low pressure at the start of the reaction in the empty space located above the pulp in the upper part of the vessel; the pulp falls freely through this space, whereupon a larger part of the oxygen can escape.

In order to counteract these problems, a method of bleaching pulp with oxygen has been proposed, according to which the pulp is initially supplied with a foaming agent and then with oxygen gas at high pressure (15 bar). the Mixture is stirred into a foam, which is expanded by lowering the pressure to 3 bar via a valve leaves. As a result of the foam formation, the oxygen is bound more strongly to the pulp, so that the aforementioned Disadvantages are somewhat reduced. But even with this method there are still certain disadvantages. For one is the steep rise in pressure is both energy-intensive and difficult to achieve with normally available pumps.

KEIL & SCHAAFHAUSEX

PATENT LAWYERS

In addition, the rapid and steep drop in pressure leads to the bursting of the oxygen bubbles in the foam, which in turn there is a considerable loss of oxygen.

// The object of the present invention is therefore to create V of a process for the oxygen treatment of cellulose pulp, in which the aforementioned problems in connection with a simple design of the method are eliminated.

According to the invention, this object is achieved in a method of Generic type essentially solved in that the oxygen is converted into foam by means of a foaming agent and that the foam containing the oxygen gas is then mixed into the pulp prior to the treatment step.

For example, a foam generator of the type described in Swedish patent specification 86,586 is used for foam formation is described. A mixture of non-ion-active and anion-active surface-active, for example, is used as the foaming agent Substances (foaming agents), stabilizers (e.g. fatty alcohols) and solvents (e.g. glycol esters) are used. The foaming agent is mixed with water or sodium hydroxide or other suitable liquid in a mixing vessel mixed and pumped into the foam generator. At the same time, oxygen is fed into the foam generator from a storage tank conducted using the positive pressure in the tank as a power source.

The foam formed in the foam generator can be gradually reduced by gradually lowering the pressure in a expand a relatively long line before it is mixed into the pulp suspension, which is done with a stirrer, which can be found in normal process equipment

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takes place and is arranged in front of the treatment stage in question is. As a result of the gradual reduction in pressure, the foam expands relatively slowly and consequently the foam bursts Don't blister. The energy required for foam formation is negligible and so is the pulp suspension does not need to be pressured. In addition, it is easier to check the properties of the foam, when it is produced in an apparatus designed for that purpose.

With this method, the oxygen can be evenly distributed in the pulp in the form of very small bubbles, so that a large contact area is obtained. It is also possible to supply relatively large amounts of oxygen and to keep this suspended in the pulp suspension for a sufficiently long time for the reaction to take place, whereby the losses on a To be reduced to a minimum. Even if a reaction vessel is used in which the process is carried out from top to bottom and therefore the pulp at the top of the vessel is filled, the oxygen supply can be without essential Losses occur because the oxygen is kept bound in the form of the foam. That is why it is in such Cases also unnecessary, in addition to the reaction vessel another Provide a reaction vessel where the oxygen reaction occurs Upflow takes place, as has been required heretofore.

Comparative studies were carried out between a direct, conventional oxygen gas supply and one according to the invention Oxygen supply is carried out in the form of foam. With these Investigations were softwood pulp with a kappa number of 33.1 and a viscosity of 1,179 dm / kg with a mixture of chlorine (bleaching acid) and chlorine dioxide, namely a total

Keil & Schaafhausen

PATENT LAWYERS

All three tests were carried out at atmospheric pressure and under conditions otherwise specified in the following table visible are:

Temperature (C) Consistency {%) Time (minutes) Oxygen

55 55 55 8th 10.3 10.3 80 80 80 _ foam gas

The chemical input sizes were as follows:

Pale
episode oxygen
admission alkali
admission Kappa
number Bright
speed Visco
sity - (kg / t) (kg / t) (X ISO) (dm ³ / kg) _ Unbleached
te pulp _ 33.1 1179 1 (C + D) E. - 35.0 8.4 29.8 1046 2 (C + DHEO) 5.7 29.8 5.5 38.1 1027 3 (C + DXEO) 5.7 30.0 6.7 35.1

where C = chlorine treatment stage, E = alkali extraction stage, O = oxygen stage, D = chlorine dioxide stage, C + D = chlorine treatment stage, whereby part of the active chlorine is added in the form of chlorine dioxide (simultaneous addition of the Chemicals), EO = oxygen-enhanced alkaline level.

From this table it can be seen that the addition of oxygen in the extraction step results in a considerable reduction in the kappa number and an increase in brightness. It is It is also evident that the effect is greatest in the case in which the oxygen is added in the form of foam.

,., ..... .. .... ■ "■., 3601803

1 Keil & Schaafhausen

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was given. The result is remarkable considering the low pressure.

Examples of device arrangements for pulp bleaching plants are illustrated in FIGS. There are 1 Reaction vessels, 2 washout filters, 3 mixers and 4 pumps. In all cases there is only one treatment stage, which in one Fiberline is included, illustrates.

Figure 1 shows a reaction vessel 1, which from top to works below and the filters and mixers are located above. The pulp is from the previous treatment stage Passed through the washout filter 2 and mixer 3 into the upper part of the vessel 1. After the reaction, the pulp becomes at the bottom of the vessel 1 through a washout filter 2 to the following Treatment stage pumped.

In Figure 2, the washout filter 2 and mixer 3 are next to the vessel 1 arranged, in which the process direction is also from runs up and down. The pulp is fed from the mixer 3 via the pump 4 and a line 5 to the upper end of the Reaction vessel 1 abandoned.

FIG. 3 shows a reaction vessel 1 in which the process sequence runs from bottom to top; otherwise corresponds to However, the arrangement is that of FIG. 2. The pulp is fed by the pump 4 in the bottom region of the vessel 1 and is on its upper end delivered.

In all cases shown, NaOH is added to the pulp in connection with the discharge from the washout filter 2, whereupon Oxygen is added in the form of foam in the mixer 3 from a foam generator (not shown).

OHiCfNAL. ¹ NSPECTJ = D *

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Claims (2)

Method of treating a cellulose pulp Patent claims: 1 . Process in the manufacture of cellulose pulp with a Treatment stage, which is a deforestation or bleaching stage (Delignification stage), with oxygen gas as Reactant is used, characterized in that the oxygen gas with the aid of a foaming agent in a Foam is converted, whereupon the foaming is carried out in a foam generator, from which the foam to is fed to the pulp prior to the treatment stage. 2. The method according to claim 1, characterized in that the foam generated in the foam generator slowly expands by gradually lowering the pressure in a pipe, through which the foam is fed to the treatment stage.