The production of chemical pulp consisting of cellulosic fibres is performed by cooking wood chips or any similar comminuted wood raw material, in which the cooking chemicals contained in the cooking liquor are dissolved in the lignin con- tained in the wood. The cooking chemicals and the lignin terminate in the residual liquor removed from the cooking vessel, which is evaporated in order to recover the material dissolved into it as a fuel. Continuous cooking, in which the cooking vessel is continuously supplied with wood raw material and cooking liquor and from which is also continuously removed cooked pulp and residual liquor, has typically been combined with counter-current washing of cooked pulp in the same vessel.

Then the upper part of the cooking vessel forms a cooking zone, where the pulp and the cooking liquor move from the top downwards, and the lower part of the cooking vessel forms a washing zone, where the pulp moves further downwards and the cooking liquor moves upwards concurrently with pulp. At the zone boundary, the cooking liquor and the washing liquor join as residual liquor, which is discharged from the cooking vessel and led to an evaporation plant.

To enhance pulp washing, the washing zone of the cooking vessel is usually equipped with a"washing cycle", where a sieve or any similar separating means placed in the vessel is used to separate the liquid phase and transfer it above the separating point, where it is applied on the transverse surface of the vessel in order to equalise the flow and temperature profiles. By these means, irregular washing caused by ducting of the incoming washing liquor flow is avoided.

In current processes, cooked pulp is removed into an exhaust pipe, usually by means of a scraper mounted at the lower end of the vessel. Pulp agglomeration at the lower end of the vessel exerts stress on the scraper and hampers the expulsion of

pulp into the exhaust pipe. It is particularly detrimental if the pulp is simultaneously subjected to strong scuffing by remaining fibre agglomerations corresponding chiefly to chip particles in the raw material being scuffed against each other. This causes damage to the fibres, appearing as crimped fibres with reduced strength. The technical properties of paper produced from this pulp deteriorate accordingly.

Fibre agglomeration problems could be eased by increasing the washing liquor flow supplied for counter-current washing of the pulp at the lower end of the cooking vessel. The washing liquor would then dilute the pulp so as to diminish scuffing and to ease pulp removal from the lower end of the vessel. However, this solution in- volves the problem of excess washing liquor terminating in the residual liquor, thus increasing the amount of this, so that its treatment would require additional evapora- tion capacity and would cause higher cost.

The purpose of this invention is to resolve problems caused by fibre agglomeration in a manner that is substantially more advantageous than the one described above.

The method of the invention is characterised in the liquid phase being separated at the separation point in the washing zone of the cooking vessel, the liquid phase be- ing then transferred as washing liquor upstream from the pulp above the separation point, and in the separated liquid phase being additionally transferred below the separation point to form a diluting liquor that reduces pulp consistency.

By means of the inventive solution, the cooked pulp formed of fibres is diluted, re- sulting in easier removal from the lower end of the cooking vessel, reduced scuffing and fibre damage entailed by this, and improved pulp strength properties. The es- sential feature of the invention is that dilution is performed in combination with the wash cycle known per se, by means of liquor transfer within the cooking vessel, raising the dilution factor at the lower end of the vessel without changing the overall flow degree of the cooking process; when the cooking vessel is not supplied with fresh diluting liquor, there will consequently be no increase in the amount of resid- ual liquor to be evaporated.

Conducting diluting liquor to the lower end of the vessel in accordance with the in- vention can be carried out by branching off the washing cycle. Wash liquor circula- tion and diluting liquor transfer can be provided with the use of a common pump disposed in the circulating channel system.

In the wash zone, the separated diluting liquor is circulated in accordance with the invention so as to lift the counter-current at the lower end of the vessel, and it can

be completely or partly combined with the wash liquor flow supplied to the lower end of the vessel, so that the diluting liquor returns to the vessel along with fresh wash liquor. With an increase of the counter-current amount, the diluting liquor will decrease pulp consistency between the lower end of the vessel and the diluting liq- uor separation point, which is at a higher location in the wash zone.

The continuous pulp digester of the invention, by means of which the method de- scribed above can be implemented, comprises as elements known per se a vertical cooking vessel, feed pipes for feeding ligno-cellulose material and wash liquor to the upper end of the vessel, a feed pipe for feeding wash liquor to the lower end of the vessel, an exhaust pipe for removing cooked delignified pulp from the lower end of the vessel, and an exhaust pipe for removing residual cook liquor from the area between the upper end and the lower end of the vessel. In accordance with the in- vention, the cooking vessel is characterised in that the wash zone of the digester is equipped with a means for separating the liquid phase, from where by-pass ducts are led to transfer the separated liquor as was liquor upstream from the pulp, above the separation point, and also below the separation point, to form a diluting liquor reducing pulp consistency.

The invention is explained in greater detail below by means of an example and with reference to the accompanying drawing (figure 1), which illustrates a continuous pulp digester of the invention comprising washing and counter-current washing zones.

The pulp digester illustrated in the drawing consists of a vertical, principally cylin- drical cooking vessel 1 including associated flow pipes. In the cooking vessel 1, the cellulose-based pulp is continuously cooked in the cooking zone 2 at the upper end of the vessel and the cooked pulp is continuously counter-current washed in the wash zone 3 at the lower end of the vessel. The ligno-cellulose material forming raw material and the cooking liquor containing cooking chemicals are supplied to the upper end of the cooking vessel 1 through a common feed duct 4.

The pulp and the cooking liquor move downwards in the cooking zone 2 of the ves- sel, where fibres separate as cooked pulp when lignin is dissolved into the cooking liquor. The cooking liquor and the material dissolved by this terminate in the resid- ual liquor, which is separated at the boundary between the cooking and wash zones 2,3 with a sieve 5 and is removed to an exhaust duct 6 leading to an evaporation plant (not shown).

For counter-current washing of cooked pulp in the washing zone 3 at the lower end of the cooking vessel, wash liquor is conducted to the lower end of the vessel as a continuous flow from the feed duct 7, the wash liquor being a filtrate in the follow- ing washing process. The pulp moves downwards in the washing zone 3 towards the lower end of the vessel, which is equipped with a scraper 8, which expels the cooked and washed pulp from the lower end of the vessel into the exhaust duct 9.

The wash liquor flows from the washing zone 3 upwards against the pulp flow di- rection and joins the cooking liquor at the boundary between the cooking and wash- ing zones 2,3, and eventually terminates in the residual liquor to be discharged from the vessel into the duct 6.

The continuous process described above is suitable for e. g. hardwood or softwood Kraft cooking, in which wood chips and white liquor is brought from the feed duct 4 to the upper end of the cooking vessel 1 and wash water is brought from the duct 7 to the lower end of the vessel. The sulphate cellulose pulp obtained by cooking is discharged in washed state into the duct 9, e. g. for transfer to the following wash process, and the black liquor obtained as residual liquor is conducted through the duct 6 to the evaporation plant.

In the continuously operating digester described above, the ratio of liquor to fibres at the lower end of the vessel, i. e. the amount of liquor flowing upwards to the amount of liquor flowing downwards, is too low, and for this reason the fibres tend to agglomerate, resulting in mutual scuffing that damages the pulp and deteriorates its strength. In the digester of the invention described above, this problem has been overcome by diluting the pulp at the lower end of the cooking vessel 1 by means of diluting liquor conducted from a separation point 10 located higher in the digester.

The cooking vessel comprises a sieve 10 disposed in the washing zone 3 and serv- ing the internal wash cycle arranged in the wash zone. In this cycle, wash liquor separated from the pulp by the sieve 10 is transferred with a pump 11 through a cir- culating duct 12 to a level located downstream with the vessel relative to the wash liquor and upstream relative to the pulp, above the sieve 10, and by means of which the liquor transferred as a by-pass flow is evenly applied on the cross-surface of the vessel in order to equalise the wash liquor flow and the temperature of the wash liq- uor. A duct 13 has been branched off from the wash cycle 12 for circulating wash liquor separated by the sieve 10 by means of the pump 11, also concurrently with the pulp, to form diluting liquor at the lower end of the vessel. As shown in the drawing, the diluting liquor circulating duct 13 is bifurcated, joining the inlet branches 14,15 of the wash liquor feed duct 7, with dilution directed partly to the

scraper 8 at the lower end of the vessel and partly slightly above this. Regarding these circulating facilities, however, there are optional variations of the construc- tion, which are obvious to those skilled in the art.

In other respects as well, it is obvious to those skilled in the art that the applications of the invention are naturally not limited to the example particularised above, but may vary within the scope of the accompanying claims. Thus, diluting liquor may be independently fed to the lower end of the vessel, independently of the wash liq- uor supply. The diluting liquor circulating duct 13 may be located, as shown in the drawings, outside the jacket of the cooking vessel 1, or also inside the jacket. The number of circulating ducts 13 may also vary and they may start from liquor separa- tion points located at varied heights in the vessel.