Technical field

This invention relates to a method for continuous cooking of pulp, which method improves pulp quality so that sub¬ sequently none chlorine containing bleaching processes can be used in order to reach full brightness for a pulp having desired strength and quality properties.

Background and problem

The environmental authorities are placing ever more stringent demands on the pulp industry to decrease the use of chemicals which can be damaging to the envi¬ ronment, such as, for example, chlorine. Thus, permitted discharges of organic chlorine compounds in the waste water from bleaching plants, following on from the cook¬ ing process, have been decreased progressively and are now at such a low level that pulp factories have in many cases stopped using organic chlorine compounds as bleach¬ ing agents. In addition, market forces are tending pro- gressively to increase the demand for paper products which are not bleached with chlorine or chlorine containing bleaching agents.

The pulp industry is therefore searching for methods which allow bleaching of pulp without using these chemi¬ cals. The lignox method (see SE-A 8902058) , in which, inter alia, bleaching is carried out with hydrogen peroxide, may be mentioned as an example of such a method. Ozone is another interesting bleaching chemical which is also gaining increased application. It is thus possible, using bleaching chemicals of this nature, to

achieve those brightnesses which are required for market¬ able pulp, i.e. 89 ISO and greater, without using chlo¬ rine containing bleaching agents.

There is, however, a problem in using presently known bleaching procedures with these bleaching chemicals which do not contain chlorine, namely that they have a rela¬ tively large effect in diminishing the quality of the pulp fibres.

From EP 476 230 there is known a method for continuous cooking under raised pressure and temperature of fibre material in a vertical digester, where input of fibre ma¬ terial and cooking liquid takes place at the top of the digester, withdrawal of spent cooking liquor is carried out from at least one digester strainer girdle between the top and the bottom of the digester, and fibre mate¬ rial is fed out from the bottom of the digester, wash liquid is supplied at the bottom of the digester, which wash liquid is withdrawn from a lowermost strainer girdle, whereby the digester comprises a digester strainer girdle between said lowest strainer girdle and said strainer girdle for spent liquor, which in-between strainer girdle is attached to a heating device. This known method indicates that the quality of the pulp fibres after continuous cooking can be somewhat improved if cooking liquid is added to the wash circulation in the lower part of the digester and the temperature is raised. In order to be able to bleach pulp to sufficient bright¬ ness without the use of chlorine containing bleaching agents, however, this known method is not always sufficient.

Solution and advantages

By means of experiments which have been conducted under the auspices of Kamyr AB, it has been found, surpris-

ingly, that extremely good results, with regard to delig- nification and strength properties, can be obtained if the pulp is cooked at the same temperature level in prin¬ cipally the whole of the digester, i.e. if essentially the same temperature is maintained in all cooking zones, and if a certain quantity of alkali is also supplied to the lowest zone in the digester, which zone is normally used for counter-current washing. Owing to the fact that essentially the same temperature level is maintained in virtually the whole of the digester, very extensive de- lignification can be achieved at a relatively low tem¬ perature. Besides this, it has been found that the strength properties are affected in a particularly favourable manner, that a higher yield of the crude fibre product is obtained and that the quantity of reject mate¬ rial decreases. These advantages are most clearly appar¬ ent from the diagrams shown in the Figures 1 and 2, which show comparative values between pulp (softwood) which has been cooked using a conventional, modified cooking tech- nique and pulp which has been cooked using the process according to the invention, (in a similar digester, i.e. with a concurrent upper cooking zone, a central counter- current cooking zone and a bottom counter-current washing zone) in which a constant temperature level of about + 155°C has been maintained in the whole digester.

The invention mainly relates to a method for achieving a cooking according to the new process, but also to a pre¬ ferred arrangement for achieving a cooking according to the invention in particular with regard to digesters built according to an older principle and consisting of an upper concurrent cooking zone and a lower counter- current washing zone. Such an arrangement is necessary since certain practical problems arise as a consequence of an isothermal cooking process. The first such problem is the difficulty of efficiently reaching and maintaining

the temperature in the lower part of the digester, i.e. that part which is normally employed for washing. The said problem is solved by creating a more efficient circulation and thus temperature distribution in the lower part (the high-heat or washing zone) of the digester. This is achieved by arranging a strainer girdle adjacent above the lowest strainer girdle, somewhere between the strainer girdle for withdrawal of spent liq¬ uor and said lowest strainer girdle, whereby said in- between strainer is positioned in such a manner that its lowest part is positioned less than 5 meters from the upper part of said lowest strainer and that cooking liq¬ uid is supplied to that liquid flow which has its tem¬ perature controlled by means of the heating device, which flow discharges adjacent the level of said lowest strainer in by means of a central pipe, so that the tem¬ perature in the cooking zone immediately above the said additional digester strainer girdle can be maintained at essentially the same temperature level as the remaining cooking zone or cooking zones of the digester.

In this context it has been found to be advantageous to use digester strainer girdle consisting of oval or circu¬ lar strainers, in particular so-called manhole strainers, in connection with converting existing digesters, both of the modified type and the older type, for operation according to the new process.

Short description of the figures

In Figure sheet 1, a comparison is made in three diagrams between isothermal cooking and so-called modified conven¬ tional cooking (MCC) . Figure sheet 2 shows a diagram which describes degree of delignification and viscosity (the viscosity is normally regarded as indicating the strength properties of the pulp) , and Figure 3 shows how,

in a preferred manner, an existing digester can be con¬ verted, using manhole strainers, to be operated according to the novel process.

Detailed description

The first figure page shows three diagrams which compare different results obtained with isothermal cooking and conventional modified cooking (MCC) . These surprisingly positive results show, according to the upper diagram, that, with a given amount of added alkali, substantially lower kappa numbers are obtained using isothermal cook¬ ing. Furthermore, the second diagram shows that mani¬ festly improved strength properties are obtained when cooking down to the same kappa number. In addition, the third diagram shows that there is also the advantage that the quantity of reject wood (shives) decreases. If the fact is also taken into account that overall substantial energy savings are made when the temperature level is kept constant, it is evident that the results may be re¬ garded as being surprisingly positive. Figure 2 additionally demonstrates that, using the method according to the invention, very low kappa numbers are reached while at the same time retaining good pulp strength (viscosity round about 1000) after oxygen delig- nification. Thus, when employing the method according to the invention, so-called environmentally friendly bleach¬ ing chemicals, such as peroxide and ozone, can be employed in subsequent bleaching stages without risking too low a strength for bleaching up to the level of brightness, and therewith also the level of purity, which the market demands.

Figure 3 shows the lower part of a digester 1, which is intended to represent an existing digester shell (which definitely is higher than 20 metres normally higher than 30 metres and usually about 40 metres and more and having

a diameter of 1/10 of the height)on which has been arranged a new digester strainer girdle 2 in order to be able to raise the temperature in the counter-current zone. The digester has a lowest strainer girdle (IB) and one strainer girdle (ID) for withdrawal of spent liquor and is of the type which has an upper concurrent part and a lower counter-current part. In such a digester, full cooking temperature is normally maintained in the con¬ current zone (i.e. about 162°C for hardwood and about 168 °C for softwood) while in the counter-current part, which in the main is a washing zone, the temperature is about 135°C on a level with the lower strainer. In the text following, the counter-current zone of the digester which has been fitted with a further strainer girdle will be referred to as a cooking zone, even if it is to be considered as a washing zone according to con¬ ventional operation.

The new digester strainer girdle 2 consists of a number of so-called manhole strainers 2A for withdrawal 3 of cooking liquid in the lower part of the digester and is arranged immediately above the lower strainer girdle IB, preferably at most 5 metres above more preferred 1,5 metres above and more preferably at most 1 metre above, measured from the upper edge of the lower digester strainer girdle to the lower edge of the newly fitted digester strainer girdle. Wash liquor is supplied to the lower part of the digester through an inflow arrangement 4 attached in the vicinity of the bottom 1A of the digester and cooking liquid (alkali addition) through the central pipes 5A, 5B. The cooked pulp is taken out from the bottom of the digester via a conduit IE. One of these central pipes, 5A, which belongs to the original system of the digester, penetrates down to the lower strainer girdle IB of the digester, after which the liquid, after heating via the first heat exchanger 6A, discharges through the said pipe on a level with the

latter digester strainer girdle. Subsequently, a part of the liquid flows in a counter-current direction upwards towards the newly fitted digester strainer girdle 2. The liquid withdrawn from this system passes through the said conduit arrangement 3 and is heated via a heat' exchanger 6B to the desired temperature before it discharges, via a second, newly fitted central pipe 5B, immediately above the newly fitted digester strainer girdle 2. A part of the cooking liquid supplied in this manner, which liquid has thus achieved the desired temperature, chemical strength and distribution (spreading) over the whole of the cross-section of the digester, continues to flow up¬ wards in the digester. In order to achieve even distribu¬ tion the flow within each cooking circulation, would nor- mally exceed 1 m ³ /h. In a central digester strainer girdle ID, the spent cooking liquid, together with undis- solved wood material, is drawn off for further treatment. The surface of each strainer element 2A is made rela¬ tively small, preferably less than 0.3 ² . An advantage of strainer elements of small area is that efficient back flushing can be achieved, which is often of great impor¬ tance if the circulation flow is to function efficiently. The new strainer girdle 2 is preferably fitted with ring pipes 2C from which an individual conduit goes to each and every one of the strainer elements 2A. Using such a construction, and a valve arrangement belonging to it, a limited number (for example 4) of strainer units 2A can be efficiently back-flushed at a time. Owing to the rela¬ tively small total strainer surface which is back-flushed under these circumstances (for example 1 ² ) , a very ef¬ ficient back-flushing which cleans the strainers is obtained, thereby ensuring that the circulation is highly efficient. The invention is not limited by that which has been de- scribed above, but can be varied within the scope of the subsequent patent claims. Thus, an existing digester of

the MCC type can also be arranged in accordance with the invention, where, therefore, the digester has an upper concurrent part, a central, mainly counter-current, part and a lower counter-current part, where addition of a part of the cooking liquid takes place in the said lower counter-current part, the so-called high-heat zone. A digester of the so-called hydraulic type, with a lower temperature in the upper part (the impregnation zone) , may also advantageously be fitted with a digester strainer girdle according to the invention for cooking according to the invention, so-called isother ally. Addi¬ tionally the method may be used in connection with all types of cooking liquid, even if the method is princi¬ pally intended for producing sulphate pulp. In addition, it is obvious to the person skilled in the art that the invention is not limited to the above mentioned exempli¬ fying temperature levels. In this connection, however, it is important that the average temperature level in the digester preferably exceeds +150°C but is lower than +165°C, and preferably is between 150-155°C for hardwood and between 160-165°C for softwood, and furthermore that the average temperature in the cooking zone/zones is preferably about +151°C +-1°C, when the wood is hard¬ wood, and that the average temperature in a digester is +159°C +-1°C, when the wood is softwood. In addition, it is understood that strainers deviating from a circular form, for example oval strainers, or even rectangular may also be used, whereby, for technical reasons related to the construction, the smallest radius of curvature should preferably not fall below 0.2 m.

Finally, it is pointed out that new digesters can naturally also be fitted with strainer girdles, and be operated, according to the invention. Furthermore it is understood that more than one strainer girdle can be arranged between the strainer girdle withdrawing spent

cooking liquor and the lowest strainer girdle. It is also evident for the skilled man that more than one strainer girdle may be used for withdrawing spent liquor. This lowest strainer girdle is normally positioned within its lower edge about 1-2 metres above the welded seam that joins the spherical bottom position with the cylindrical digester shell, but in extreme designs the distance might be as short as 0.5 metres and as long as 5 metres.