FI78326C - Process for treating cellulose pulp - Google Patents

Description

1 78326

The present invention relates to a process for carrying out a treatment step in a process for the production of cellulose pulp in which oxygen gas is the reaction medium.

Oxygen delignification of cellulosic pulp today tends to move from a high-density pulp with a pulp concentration of 25-30% to a medium-density pulp with a concentration of 8-15%. The reason for this is mainly to reduce investment costs. However, the addition of oxygen to such a medium-density pulp presents problems because the amounts required are considerably greater than what can be dissolved in the liquid.

The first problem is to distribute the oxygen evenly in the mass in the form of small bubbles to provide a large contact surface.

Another problem is to keep the bubbles floating in the pulp suspension long enough for the reaction to proceed.

A third problem is the ability to add, if necessary, amounts greater than 4-5 kg of oxygen per tonne of pulp, calculated on a dried pulp with a dry matter content of about 90%.

With those stirrers, so-called mixers, currently used to mix oxygen into the pulp, it has been found that the bubbles do not become as small as would be desirable, resulting in a large proportion of the oxygen rising through the pulp to the top of the reaction vessel without being consumed. The desired reaction therefore requires a relatively high pressure, especially * at the beginning of the reaction. As a result, an ascending flow method has been resorted to in which the hydrostatic pressure at the bottom of the vessel is high. In addition to the low pressure at the beginning of the reaction, the downflow process also has the disadvantage that there is an empty space above the pulp at the top of the vessel, through which the fed pulp falls freely, whereby a large part of the oxygen is removed.

2 78326 To solve these problems, a method has been proposed for bleaching the pulp with oxygen, in which a foaming agent is added first to the pulp under high pressure (15 bar) and then oxygen gas. The mixture is mixed into a foam which is allowed to swell at a pressure drop to 3 bar through the valve.

Through foaming, oxygen binds more strongly to the pulp, so that the above-mentioned disadvantages are reduced to some extent. However, there are drawbacks to this method as well. The strong pressure reduction is thus partly energy-intensive and partly difficult to achieve with normally available pumps.

The rapid and strong pressure drop further results in the oxygen bubbles in the foam tending to break, in which case the oxygen loss becomes considerable in any case.

The object of the present invention is to provide an oxygen treatment of a cellulose pulp in which the above-mentioned problems in connection with the prior art have been eliminated. This is achieved by a method of the type mentioned in the introduction, which according to the invention is mainly characterized in that the oxygen gas with the blowing agent is converted into a foam and that the oxygen-containing foam is mixed into the mass before the treatment step.

For foam formation, for example, a foam generator of the type described in SE patent 85586 is used. The blowing agent used is a mixture of nonionic and anionic surfactants, stabilizers (e.g. fatty alcohols) and solvents (e.g. glycol ethers). The blowing agent is mixed with water or sodium hydroxide solution or other suitable liquid in a mixing vessel and pumped to the foam generator. At the same time, oxygen gas is fed to the foam generator from the storage tank, whereby the overpressure of the tank is utilized as a source of propulsion.

The foam formed in the foam generator is allowed to swell slowly by successively reducing the pressure in the relatively long pipeline, before mixing it with the pulp suspension, which

II

3 78326 takes place with a mixer arranged in front of the actual processing 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 break. The energy required for foaming is negligible and the pulp suspension does not need to be pressurized. In addition, it is easier to control the properties of the foam when it is manufactured in equipment built for this purpose.

With this method, oxygen can be evenly distributed in the mass in the form of small bubbles, so that a large contact surface is obtained.

It is also possible to add relatively large amounts of oxygen and keep this floating in the pulp suspension long enough for the reaction to minimize losses. Also in downflow type reaction vessels, where the pulp is fed to the upper end of the vessel, the addition of oxygen can take place without significant losses because the oxygen is bound in the form of a foam.

Therefore, in these cases, it is also not necessary to fit any additional reaction vessel next to the reaction vessel, in which the oxygen reaction would take place in an ascending flow, as has been necessary in the past.

Example

Comparative experiments were performed partly by direct conventional addition of oxygen gas and partly by adding oxygen in the form of a foam according to the invention. In the experiments, a softwood sulphate pulp with a kappa number of 33.1 and a viscosity of 1179 dm / kg was bleached with a mixture of chlorine and chlorine dioxide, namely a total of 70 kg of aCl per tonne of dried pulp, of which 5.6 kg as chlorine dioxide. The chlorine bleached pulp thus extracted was then extracted in three different ways as below.

* 1. With NaOH only.

2. NaOH + CH 2 Ha, where oxygen was added in the form of a foam.

3. NaOH + Ojilla / where the acid was added as a pure gas.

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

4 78326 __1_ 2 3 Temperature (° C) 55 55 55

Concentration (%) δ 10.3 10.3

Time (min) 80 80 80

Oxygen - foam gas

Chemical additions and results are shown in the table below.

Oxygen- Alkali- Viscose- Addition to the order

Bleaching Kappa Brightness _ sequence (kg / t) (kg / t) number (% ISO) (dnr / kg) unbleached mass - - 33.1 - 1179 1 (C + D) E - 35.0 8.4 29.8 1076 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 table shows that the addition of oxygen to the extraction step significantly lowers the kappa number and increases the brightness. It also appears that the power is greatest in the case where oxygen is added in the form of a foam. The result is significant in terms of low pressure.

Examples of equipment arrangements in pulp bleaching plants are "shown in Figures 1-3, where 1 is a reaction vessel, 2 is a wash filter, 3 is a mixer and 4 is a pump. In all cases, only one treatment step is included in the so-called fiber line.

Figure 1 shows a downstream type reaction vessel with filters and stirrers arranged above it. The pulp is passed from the previous treatment step through a wash filter and mixer to the top of the vessel 1. After the reaction, the pulp is pumped from the bottom of the vessel through a wash filter to the next treatment step.

In Fig. 2, the washing filter and the mixer are arranged next to the vessel 1, which in this case is also of the downflow type. The mass 5 78326 is led from the mixer 3 through the pump 4 and the pipeline 5 to the top of the reaction vessel.

Figure 3 shows an upflow type reaction vessel with the devices otherwise arranged in accordance with Figure 2. The pulp is led from the pump 4 to the bottom of the vessel 1 and exits from the top.

In all the cases shown, sodium hydroxide is added to the pulp at the same time as it is removed from the wash filter, after which oxygen is added in the form of a foam to the mixer 3 from a foam generator (not shown).

Claims (2)

A process for carrying out a treatment step in a process for producing a cellulosic pulp, consisting of a delignification or bleaching step in which oxygen gas is included as a reaction medium, characterized in that the oxygen gas is converted to a foam by a blowing agent. A method according to claim 1, characterized in that the foam formed in the foam generator is slowly expanded by periodically reducing the pressure in the pipeline through which it is passed to the treatment stage. Λ