Into the digester of pulp, air enters along with the raw-material. Moreover, gases are formed in the cooking itself. In order that the process could continue undisturbed and the desired cooking result be obtained, it is important that the air and the gases are removed. Also, during cooking, extract materials insol- uble in the cooking liquor are formed, which should preferably be separated at a stage as early as possible.

At present, in the top portion of the digester, a gas phase is maintained, from which steam and gases are removed. This method involves the drawback that a rather large quantity of steam must be removed from the digester and be treated further. An even larger amount of steam is needed in order to remove the less readily volatile extract materials. Moreover, by means of this method, only a little portion of the gases and extract materials formed at the final end of the cooking can be separated.

In the patent SE 311,591, a method is also described in which cooking liquor is passed from the digester into the bottom part of a separate separation tank, into which steam is passed at the same time in order to separate the extract materials.

The object of the present invention is to intensify the separation of the gases and extract materials from the cooking liquor as well as to reduce the primary steam consumption.

In the method in accordance with the invention, the gases and possibly also the extract materials are removed in connection with the liquid circulation of the digester out of a separation space provided behind and above the circulation outlet screens placed on the wall of the digester vessel.

Above all, it is an advantage of the method in accordance with the present invention that, as gases and extract materials are removed at an early stage, concentration of the said substances in the circulations is avoided. This improves the dynamic conduct of the liquid flow in the flow through the layers of chips or pulp. The flows become more uniform and more controlled, and in this way it is possible to have an improving effect on the overall cooking result. The method also improves the operation of the screen itself by equalizing the hydraulic load on the screen face. As the whole space between the screen area and the digester wall is filled with liquid, the flow and the difference in pressure through the screen are uniform over the entire area of the screen, depending on the desired flow quantity and on the properties of the screen and of the liquid only.

Another advantage of the method that improves the operation of the screens is that when the gas space facing the screen area is eliminated, fixing of fibres and other solid materials onto the screen by burning is avoided, whereby the screens remain clean for a longer time.

The method can be used in all pulp cooking processes in which liquid is circulated in, or removed from the digester through screens.

Some preferred embodiments of the invention will be illustrated in the following in more detail with the aid of the accompanying drawings. Figures 1 to 3 are vertical sectional views of a digester, viewed at one separation space in accordance with the invention. Figure 4 is a schematical view of the whole digester with its different circulations.

In the embodiment shown in Fig. 1, a screen band 2 is placed around the inner circumference of the wall 1 of the digester, which said screen band forms an annular separation space 3 on the inner circumference

of the digester, the gases and the extract materials being separated from the cooking liquor in the said separation space 3. The band 2 comprises a screen 4, through which cooking liquor and, along with it, gases and extract materials are transferred into the separa¬ tion space 3. The mantle of the top portion of the band 2 is solid, in order that a gas phase could be formed in the top portion of the annular space 3.

The flow of liquid into the separation space 3 is practically constant at every portion of the circum¬ ference of the digester. In order to intensify the separation, the outflow from the space 3 is also arranged constant at every portion of the circumference by in this space, on the wall of the band 2, below the screen 4, placing an upwardly widening conical partition wall 5 concentric with the digester.

For the purpose of improving the removal of extract materials, there is an annular trough 6 on the inner circumference of the vessel 1 in the separation space 3.

In the separation space 3, the gases are separated as a phase of their own in the upper part of the space, from which they are removed through the pipe 7. Extract materials lighter than the cooking liquor rise onto the liquid surface, and the liquid level is adjusted so that the extract materials move as an over¬ flow into the trough 6, from which they are removed through the pipe 8. The cooking liquor is removed from the bottom part of the separation space through the pipe 9 and passed through a heat exchanger back into the digester.

Figure 2 shows a second preferred embodiment. In a way corresponding to Fig. 1, the equipment includes a screen band 2 and a screen 4, placed on the wall 1 of the digester, a lower partition wall 5, as well as pipes 7, 8 and 9 for the removal of the gases, extract materials, and the cooking liquor. This

equipment further includes a conical upper partition wall 6', concentric with the partition wall 5, the top edge of the said upper wall 6' being fixed to the wall 1 of the vessel above the pipe 8 for the removal of the extract materials and the said upper wall being directed downwards parallel to the lower partition wall 5 at its side facing the screen 4. In this way, an annular slot 10 is formed between the partition walls 5 and 6 ^* . In the upper end of the upper partition wall 6' , there are pressure equalizing openings 11.

The opening 12 of the pipe 8 for the removal of the extract materials is on the wall of the upper partition wall 6 ' .

In the gas removing pipe 7 there is a control valve 13, and in the extract materials removing pipe 8 a control valve 14.

Outside the screen, in the bottom part 3.1 of the separation space, the extract materials are sepa¬ rated on the surface of the cooking liquor as a layer 3.2, and the gases as the topmost layer as a phase 3.3 of their own.

Owing to the pressure equalizing openings 11, the pressure in the gas phase 3.4 formed underneath the upper partition wall 6 ' remains the same as in the phase 3.3, The cooking-liquor flow 3.5 passes through the slot 10 as overflow over the wall 5 into the space 3.6 for the removal of the cooking liquor. The surface level 15 of the cooking liquor in the space 3.6 is arranged so as to control the gas removing valve 13 so that, if the level 15 is lowered, the rate of removal of gases increases. In this way, the pressure in the spaces 3.3 and 3.4 is lowered, which raises the level 15 accordingly. Thus, hereby the surface level 15 can be kept automatically at the desired level, below the upper edge of the partition wall 5 but above the opening 16 for the removal of the cooking liquors. In a corresponding way, the surface level 17 of the

extract materials 3.2 is arranged so as to control the valve 14 in the pipe for the removal of the extract materials so that the level 17 remains below the upper edge of the partition wall 6' but above the opening 12 for the removal of the extract materials. The bottom level 18 of the extract material layer 3.2 must, of course, be kept above the lower edge of the partition wall 6' but below the removal opening 12.

Figure 3 shows another, simplified solution, in which only the gases are separated and the extract materials are allowed to pass along with the gases or with the liquor.

In the device shown in Fig. 3, in the sepa¬ ration space 3 on the wall 1 of the digester, there is an annular support plate 19 attached to the screen 4 by means of support pins 20. The objective of the support plate 19 is to keep the screen 4 in its shape.

The cooking liquor is removed through the pipe 9 and the gases through the pipe 7. By means of the valve 13, the gas removing rate is adjusted equal to the gas separation rate, whereby the level 15 of the liquid surface remains substantially constant. The level 15 is kept slightly above the upper edge of the screen 4. Figure 4 illustrates a continuous digester to which the method of the present invention is applied. The chips and the cooking liquor are supplied into the top end of the digester. The cooked pulp is removed from the bottom end, into which the washing water is also fed. The digester includes four removal or circulation screens 4A, 4B, 4C and 4D in accordance with the invention.

The topmost screen 4A belongs to the cooking circulation, in which the chips and the liquor are heated to the cooking temperature. The separated cooking liquor 9A is circulated through heating 21 back into the top portion of the digester.

Through the screen 4B, the spent liquor of the cooking and the washing water 9B are removed.

In an equalization circulation, the temperature of the cooked pulp is levelled to the temperature of the washing zone by means of the liquid 9C circulated through the screen 4C, and thereby the cooking reaction is stopped.

In the washing circulation, the washing water 9D removed through the screen 4D is heated 22 to the temperature of the washing zone.

In all the stages, gases 7A, 7B, 7C and 7D are also separated and removed through the screens 4A, 4B, 4C and 4D. If desired, it is also possible to separate extract materials through one or several screens 4A, 4B, 4C and 4D.