SE452896B - METHOD OF TREATING CELLULOSAMASSOR - Google Patents

Description

To overcome these problems, a process has been proposed for bleaching pulp with oxygen, in which the pulp at high pressure (15 bar) is first added with foaming agent and then oxygen gas. The mixture is stirred to a foam, which is allowed to expand by depressurizing to 3 bar over a valve.

Through the foaming, the oxygen binds more strongly to the seagull, so that the above-mentioned inconveniences are reduced to a certain extent. However, this method also has its drawbacks. Thus, the sharp increase in pressure is partly energy-intensive and partly difficult to achieve with normally available pumps. Furthermore, the rapid and sharp decrease in pressure leads to the oxygen bubbles in the foam tending to burst, whereby the oxygen loss is still significant. The object of the present invention is to provide an oxygen treatment of cellulose pulp, in which problems described above in connection with the prior art have been eliminated. This has been achieved by means of a process of the kind mentioned in the introduction, which according to the invention is characterized mainly in that the oxygen gas is converted by means of a foam part into a foam, the foam formation being carried out in a foam generator from which the finished foam is fed to the pulp before the treatment step.

For foam formation, for example, a foam generator of the type described in Swedish patent specification 86586 is used.

The foaming agents used are, for example, a mixture of nonionic and anionic surfactants, stabilizers (eg fatty alcohols) and solvents (eg glycol ethers). The foam is mixed with water or baking soda or other suitable liquid in a mixing vessel and pumped to the foam generator.

At the same time, oxygen gas is led to the foam generator from a storage tank, whereby the overpressure in the tank is used as a drive source.

The foam formed in the foam generator is allowed to expand slowly by successive pressure drop in a relatively long pipeline, before it is mixed into the pulp suspension, which takes place by means of a stirrer arranged before the current treatment step, which is included in the normal process equipment. Due to the successive pressure drop, the foam expands relatively slowly, so that the bubbles do not burst. The energy required for foaming is insignificant and the pulp suspension does not need to be pressurized. In addition, it is easier to check the properties of the foam when it is produced in an apparatus designed for the purpose.

With this method, the oxygen can be distributed evenly in the mass in the form of small bubbles, so that a large contact area is obtained. It is also possible to add relatively large amounts of oxygen and to keep this floating in the pulp suspension for a sufficiently long time for the reaction, whereby the losses are reduced to a minimum. Even in downstream-type reaction vessels, where the pulp is fed to the upper end of the vessel, the oxygen supply can take place without significant losses, since the oxygen is kept bound in foam form. Therefore, in such cases it is also not necessary to arrange next to the reaction vessel any further reaction vessel, where the oxygen reaction takes place in upstream, as previously sometimes required.

Example. Comparative experiments were carried out with both direct conventional oxygen gas supply and supply of oxygen in foam form according to the invention. In the experiments, softwood sulphate pulp with a kappa number of 33.1 and a viscosity of 1179 dma / kg was bleached with a mixture of chlorine and chlorine dioxide, namely a total of 70 kg aCl per tonne of dried pulp, of which 5, d kg as chlorine dioxide. The chlorine bleached mass was then extracted in three different ways as shown below. With NaOH only. With NaOH + O 2, where the oxygen was added in the form of foam. With NaOH + O 2, where the oxygen was added as pure gas.

All experiments were performed at atmospheric pressure and the other conditions are shown in the table below.

Temp (° c) ss ss ss Concentration (%) 8 10.3 10.3 Time (min) 80 80 80 Qxygen - Foam Gas 452 896 Chemicals and results are shown in the following table.

Oxygen- Alkali- _ viscose- additive additive Kappa- Brightness Bleaching sequence (kg / h) (kg / h) number (% ISO) (dm3 / kg) - Unbleached mass - - 33.1 - 1179 1 (C + D) E - 35.0 8.4 29.8 1046 2 (C + D) (EO) 5.7 29.8 5.5 38.1 1027 3 (C + D) (EO) 5.7 30.0 6.7 35.1 - The tahellen shows that the addition of oxygen to the extraction step gives a considerable reduction in kappa number and an increase in brightness.

It also appears that the effect is greatest in the case where the oxygen is added in foam form. The result is remarkable given the low pressure.

Examples of apparatus arrays in pulp mills are shown in Figures 1 - 3, where 1 is a reaction vessel, 2 is a washing filter, 3 is a stirrer and 4 are pumps. In all cases, only one treatment step is shown, which is part of a so-called fiber line.

Pig. 1 shows a downstream-type reaction vessel with a filter and stirrer arranged above it. The mass is led from the previous treatment step via a washing filter and stirrer to the upper end of the vessel 1. After the reaction, the pulp is pumped from the bottom of the vessel via a washing filter to subsequent treatment steps.

In Fig. 2, washing filters and stirrers are arranged next to the vessel 1, which is also in this case of the downstream type. The mass is led from the stirrer 3 via the pump 4 and a pipeline 5 to the upper end of the reaction vessel. Fig. 3 shows reaction vessels of the upstream type, while the apparatuses are otherwise arranged according to Fig. 2. The mass is fed from the pump 4 to the bottom of the vessel 1 and departs at its upper end. 452 896 (n In all the cases shown, NaOH is added to the pulp in connection with the discharge from the washing filter, after which oxygen in foam form is supplied to the stirrer 3 from a foam generator (not shown).

Claims (2)

A process for carrying out in a process for the production of cellulosic pulp a treatment step which consists of a delignification or bleaching step, in which oxygen gas is included as a reactant, characterized in that the oxygen gas is converted into a foaming agent by means of a foaming agent. a foam, the foaming being carried out in a foam generator from which the finished foam is fed to the pulp before the treatment step. 2. A method according to claim 1, characterized in that the foam formed in the foam generator expands slowly by successive pressure drop in a pipeline, through which it is led to the treatment step.