The present invention is concerned with a method for filling a batch digester for pulp as well as with a batch digester to be used in the method. The invention can be utilized in prior-art batchwise pulp cooking processes.

Batch cooking of pulp is the oldest mode in use for chemical defibration of chips or of other material.

Among the prior-art modes of filling a batch digester for pulp with chips, the commonest one is to allow the chips to fall through the deck hole of the digester into the empty digester until the digester is filled. In order that more chips could be placed into the digester, it is possible to use, e.g., a steam packer or equivalent.

In that method, large chips conveyors and distributor means are required for the transfer and distribution of the chips. As an alternative, it is possible to use pneumatic transfer.

In any case, the digesters require expensive decks and, in the upper part or at the top of the digester, operating levels and support structures supporting the distributor equipment. For winter con¬ ditions, a building must usually be constructed for the top portion of the digester.

After the filling of the chips, the cooking liquor is added into the digester, and the contents in the digester are heated to the cooking temperature. After the cooking stage, the digester is emptied through a so-called blow valve placed at the bottom. The power required for the removal of the pulp is obtained either from the pulp being removed as hot and pressurized, or the digester is pressurized, e.g., by means of compressed air or steam before blowing, or the contents of the digester are pumped from the

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digester into a blow tank. In connection with the pumping, it is possible to use additional dilution.

In the Letters Patent SE 64,488, a method is suggested for packing a batch digester so that a liquid flow is introduced into the digester and the chips are combined with the liquid inside the digester. Liquid is removed out of the digester through screens as it is flowing in, whereat the chips are packed tightly into the digester. After the packing, the cooking liquor used in the cooking process is introduced into the digester.

In the Letters Patent FI 21,905, a method is suggested in which chips and black liquor are fed into the digester through its top end by using a chips filler in which the black liquor acts as the pressure medium. The chips and the black liquor are supplied as separate flows to the top end and mixed in the chips packer. In this method, large mechanical or pneumatic chips conveyors and distributor means are required for the transfer of the chips to the top plane of the cooking plant and for their feeding into the digesters.

For example, from the Letters Patent SE 137,753, a continuous pulp cooking process is known in which cooking liquor is removed during the cooking through screens placed in the top portion of the digester, and material to be defibrated is combined with the removed liquid flow under pressure, whereupon the liquid flow is returned into the digester into its top. The requirements and problems of a continuous cooking process are, however, in principle, to such an extent different that the devices or principles used therein cannot be considered applicable to batch cooking.

In spite of the fact that pulp cooking pro¬ cesses have been developed in recent years, in particu- lar in respect of the recovery of heat, the construc¬ tion and the mode of filling of the digester itself have remained almost unchanged.

The object of the present invention is to provide a new system for batch cooking of pulp, wherein the filling of the digester has been arranged in a more simple way than in prior-art methods. According to the present invention, the material to be defibrated in a batch process is mixed with a liquid outside the digester, and the mixture obtained in this way is introduced into the digester as one flow. The mixing liquid consists in particular of the liquor that is used in the first stage of the cook. The mixture of the material to be defibrated and the liquid is introduced into the digester prefer¬ ably so that most of the feed takes place to under- neath the liquid level formed in the digester.

The batch digester in accordance with the invention is a digester that is provided with a filling opening through which the material to be defibrated, mixed with liquid, is introduced into the digester. The opening is located in particular below the liquid level formed in the digester, and thereat most appropri¬ ately so that the filling opening and the opening for the removal of the digested pulp form one opening.

The chips and the liquid are preferably fed into the digester through its bottom end. At the filling stage, the chips are usually of lower weight than the liquid, whereat the digester starts being filled so that the chips are packed as a column first in the top portion of the digester and from there downwards as more chips are coming in. If the chips are heavier than the liquid, the flow can be controlled so that it takes the chips along with the flow into the top portion of the digester. During the filling, the evacuation of air from the top end of the digester is natural as the filling takes place from the bottom end. In some processes, the iquid used for filling is also readily foaming, whereby the filling through the bottom

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end is a good way of avoiding foaming. The require¬ ment of energy for filling from the bottom upwards is also lower than if the filling took place in the opposite direction. In certain special processes, such as, e.g., in prehydrolysis cooking, wherein the liquid consists mainly of water, or in cookings in which the chips are heavier than the liquid, filling from the top may also be used very well. The top end of the digester is usually also provided with a screen, through which liquid can be removed during the filling.

The removal of the pulp from the digester is simplest and most reliable when it is accomplished through the bottom part of the digester, as it is also accomplished to-day.

Certain embodiments of the invention will be illustrated in more detail by means of the attached drawings. Figure 1 shows a digester to be used in the method. Figure 2 shows a detail of an alternative digester, and Figures 3 and 4 show certain cooking processes to which the invention has been applied.

At the top end of the body 8 of the digester shown in Fig. 1 , there is a screen 9 for the removal of any excessive filling liquid and for the suction of the cooking circulation 10. The excessive filling liquid is preferably removed through the same opening as the cooking circulation. The gases are removed through the opening 11, which may also be used for the removal of the excessive liquid. The bottom end is provided with a blowing bow 12, to which the supply 13 of the ^' chips into the same opening 14 is also intro¬ duced.

The mixture of chips and liquid may also be introduced into the digester by means of a separate connection, located at a position different from that • of the conduit for emptying, preferably, however, in

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the bottom portion of the digester. In such cases, there is, however, no reason why it could not be intro¬ duced into the digester through the same conduit through which the pulp is removed. On the contrary, if the chips are not introduced into the digester as mixed with liquid or if the liquid is used as an aid for packing by mixing the liquid with the chips falling down into the digester, it is necessary that the filling opening is placed at the top portion of the digester. In such a case, it is necessary that there is a sepa¬ rate opening for the removal of the pulp at the bottom of the digester.

Thus, in a digester in accordance with the invention, necessarily only one such opening 14 is needed through which the chips and the pulp pass, and, if required, all the other connections can be placed behind the same screen plate or behind separate screen plates.

By the said openings and conduits is meant the openings and conduits on the digester that are required by the process.

The openings required by maintenance work and regulations, such as manholes, have, not been considered as conduits or openings required by the process, and, when required, they may be placed at appropriate points.

Fig. 2 shows a second mode of locating the screens 15 intended for the removal of excessive liquid. The mixture 16 of chips and liquid is fed into the digester through the bottom of the bottom cone 17, and the excessive liquid 18 is recirculated in order to fetch fresh chips. The chips are pushed into the digester, by-passing the screens.

In the method in accordance with Fig. 3, the warm black liquor 5 is mixed with the chips 6 in a separate tank 3 and pumped in through the bottom end of the digester 1. When the pumping is continued, the digester is filled with a mixture of chips and liquor,

whereat any excessive liquor starts flowing out through the screens 2 at the top end and is passed back into the storage tank 4 for warm liquor. The pumping is con¬ tinued until the digester has been filled with chips and reached an appropriate degree of packing with chips. Hereupon the black liquor is displaced through the screens at the top end by means of a mixture of hot black liquor and white liquor, and the cooking process is con¬ tinued in the desired way (see, e.g., Swedish published Patent Application No. 82-013,343).

In the method of Fig. 4, the prehydrolysis ^" water, to which chemicals may have been added, and the chips 6 are mixed 3 outside the digester 1 and the mixture is pumped into the digester through its bottom end. Any excessive water is returned through the screens 2, upon filling of the digester, in order to fetch fresh chips. After the filling, the prehydrolysis cooking is performed, the hydrolysate is removed, and the cooking process is continued in accordance with the desired sequence.

The method of the invention is also excellent¬ ly suitable for cooking processes in which, in the first stage, an absorption or heating treatment of the chips is performed at the normal atmospheric pressure. Accord- ing to the present invention, this treatment can be performed in an ordinary vessel outside the digester, whereat only the cooking proper at a high temperature and under high pressure remain to be performed in the digester. In this way, the overall duration of the cooking cycle in the high-cost digester can be reduced.

Even though the method of the invention is best suitable for such processes,of the sort shown in Figures 3 and 4, in which the first step of the cooking consists of a treatment with some solution, whose quan- titative dosage per one cooking batch is not necessarily very precise, the claims are by no means confined to

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the examples or to processes similar to the examples. Nor do the shape of the digester, the size, quality, or location of the screens, or the direction of filling restrict the present invention.

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