A METHOD FOR THE PROCESSING OF PULP

Field of the invention

The present invention relates to a method for the processing of pulp to be used in the manufacture of paper or cardboard, in which method the pulp is led from a pulp source to pulp processing and further via a short circulation to the headbox of a paper or cardboard machine.

Background of the invention

Pulp processing is a part of the process that takes place between a pulp mill and a paper or cardboard machine. The purpose of the pulp processing is to manufacture, for the short circulation of a paper or cardboard machine, pulp consisting of one or more partial pulps which are mixed together. The pulp processing consists of several mixing tanks, in which partial pulps are mixed together, the consistency of the pulps and the concentration between the pulps are adjusted, and necessary fillers and other chemicals are added.

In pulp processing systems currently in use, the pulps coming from pulp manufacture are stored in separate storage tanks, from which the pulp is led forward to the pulping process. The pulps are led from the storage tanks to disperser tanks and are simultaneously diluted in the lower part of the storage tank or in a tube between the tanks, or in a disperser tank. Depending on the pulp, pulps can be defibrated or ground in the pulping process. From the storage tank, the pulps are led further to a common mixing tank, in which the different pulp components are mixed together. In addition to different pulp grades, it is possible to admix, among other things, fillers and other necessary additives to the pulp. From the mixing tank, the pulp is fed into a machine chest, in which the pulp is diluted to the final consistency and led further to the short circulation of a paper machine. In the short circulation of the paper machine, the pulp is diluted to the headbox consistency with water recovered from the wire section of the paper machine.

The above-described pulp processing systems currently in use are complex and comprise a large number of various tanks and devices with a significant space requirement. One reason for this complexity of the pulp processing

system is the fact that various intermediate tanks and disperser tanks are considered necessary for pulp processing to keep the pulp quality sufficiently uniform and to keep the process conditions as constant as possible.

However, the successive tanks and dilution locations cause problems in, for example, the process control, because the chain of control becomes long and complex, thereby causing problems in controlling the consistency and the flow of the process, resulting in variations in the quantity of the pulp to be fed into the short circulation and, further, variations in the grammage of the paper to be manufactured. Another problem is irregularities in consistency, caused e.g. by arching in the tanks, wherein pulp pieces of high consistency, detaching from the inner surface of the tanks, cause peaks in the consistency. Abnormalities in the consistency may also be caused by uneven discharging or infiltration from the tanks.

There are solutions, in which the arrangement of feeding pulp to the short circulation and further to the headbox of the paper machine have been simplified; however, no attempts have been made to simplify the whole pulping process from the pulp silo up to the short circulation of the paper machine and to eliminate all the intermediate tanks and disperser tanks in the pulp processing, because the tanks have still been considered necessary to guarantee the pulp quality. The existing solutions relate primarily to simplifying the short circulation. Furthermore, the pulp processing system has been considered a buffer storage between the pulp mill and the paper machine, in case of failures.

Brief summary of the invention

In the invention presented in this application, it has been found that the num- ber of intermediate tanks, mixing tanks and disperser tanks are not necessary in the pulping process, but homogenous pulp of good quality can also be dispensed to the short circulation of the paper or cardboard machine and further to the headbox by means of a simpler process arrangement, by means of which it is possible to solve or at least to reduce substantially the problems related to prior art. It is thus an aim of the present invention to provide a method for the processing of pulp, making it possible to process and feed the pulp to be used for the manufacture of paper or cardboard, from a

pulp source to a paper or cardboard machine in such a way that the pulp is not let to stand in tanks of intermediate storage, mixing tanks or a machine chest. Furthermore, the mixing of the partial pulps and the dilution of pulps are carried out without tanks.

To achieve this aim, the method according to the invention is primarily characterized in that the pulp processing is carried out in a pulp processing pipework so that the pulp flow proceeds continuously at a rate determined by the volume flow.

Thus, the method for the processing of pulp according to the invention does not require dispenser tanks, mixing tanks, machine chests, and the pumps or other actuators related to them, which are conventionally used in the process arrangement. Instead, the invention is based on the idea that the pulp is fed as a steady and continuous volume flow, for example from a pulp silo to a closed pulp processing pipework, in which the pulp flow proceeds continuously all the way from the pulp source to the point of mixing of the partial pulps and further via the short circulation to the headbox. All the actuators required in the pulp processing are selected so that they do not cause a delay in the continuous advancing of the pulp in the pipework. Furthermore, in the pipework and the actuators, the pulp is not exposed to a free air space, which reduces the mixing of air into the mass.

In the method according to the invention, the pulp used for the manufacture of paper or cardboard may consist of chemical, mechanical or mechano- chemical pulp, broke or recycled fibre. The pulp source is preferably a pulp silo or a broke tower, but the pulp source may also be tube pulp coming directly from a pulp factory or pulp coming from pulping. The pulp is diluted to the desired consistency at the same time when it is pumped from the pulp source to the pulp processing pipework, or the dilution is carried out, for example, in the lower part of a silo. The pulp proceeds in this consistency until the pulp is diluted to the required headbox consistency before the pulp is fed into the headbox. Thus, the pulp is not diluted in several different locations, as in the pulp processing systems presently in use. The consistency of the pulp in the pulp processing pipework is normally greater than 4% up to the point of the final dilution to the headbox consistency.

The dilution to the headbox consistency with water recovered from the wire section of the paper machine may take place simultaneously when the partial pulps are mixed together in the pulp processing pipework. Thus, the dilution to the headbox consistency also takes place in the pulp flow, and the pulp proceeds continuously up to the headbox feeding pump of the paper or cardboard machine.

The function of pulp processing is, among other things, the pulping, grinding and refining of pulps. In the method according to the invention, these required pulp processing steps are taken in the pulp processing pipework so that the pulp flow does not stop but it proceeds continuously from one step to another in the process. The process steps and actuators needed in the pulp processing depend on the paper or cardboard grade to be manufactured and on the pulps available; therefore, they vary from one paper machine and par- tial pulp processing to another. However, the method for the processing of pulp according to the invention can be applied in all kinds of paper and cardboard machines.

Normally, various fillers, additives and other chemicals are also added into the pulp in the pulp processing, for example, to improve the quality or printing properties of the finished paper. In the method according to the invention, these substances mentioned above can be admixed to the pulp in the pulp flow.

Depending on the quality of paper or cardboard to be manufactured, the pulp to be fed into the short circulation and further to the headbox of the paper machine consists of one or more partial pulps. In the method according to the invention, the partial pulps are mixed together in the pulp flow, and the mixing can be secured by using a mixing pump. By means of the mixing pump, it is also possible to mix the fillers, additives and other chemicals added to the pulp homogenously in the pulp.

The process arrangement required by the method according to the invention is simple in its apparatus, and it requires fewer actuators and other compo- nents than the pulp processing systems currently in use. Furthermore, the devices required by the method according to the invention are smaller in size, because the volumes of the pulps to be processed are smaller than in prior

art. Moreover, a smaller pipework is required than at present. Therefore, the process arrangement according to the invention reduces the costs of acquisition and maintenance of the process. Furthermore, the process arrangement requires significantly less space. Moreover, the energy consumption of the pulping process is reduced, because the pulps are not, for example, mixed in large thanks and pumped from one tank to another. Also, the water consumption can be substantially reduced in the process.

In the method according to the invention, the pulp is diluted at the stage when the pulp is led from the pulp source to the pulp processing pipework, and for the next time, the pulp is diluted to the headbox consistency either at the location of mixing of the partial pulps or first before the feeding into the headbox. This simplifies significantly the control system required by the process, and variations in pulp consistency can be levelled out and, similarly, problems caused by flow changes can be minimized. Thus, the quality of the paper to be manufactured becomes more uniform. ^'

The method according to the invention also accelerates grade changes in the paper machine, because pulp is not stored in several tanks before the short circulation, and the quantity of broke mass is reduced in connection with grade changes. As there are no intermediate tanks in the pulp processing arrangement according to the invention, no agglomerations are introduced in the pulp which would detach from the walls of the tanks and affect the pulp quality.

Brief description of the drawings

In the following, some advantageous embodiments of the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 is a schematic diagram showing a conventional pulp processing system of a paper machine according to prior art, Fig. 2 is a schematic diagram showing a pulp processing system according to the invention, and Fig. 3 is a schematic view on the pulp processing system according to a second embodiment of the invention.

Detailed description of the invention

Before describing the invention in more detail, the applicant would like to specify some terms used in the description of the invention. "Pulp processing pipework" refers, in this context, to a pipework and the respective actuators, in which the pulp flow proceeds and in which the required pulp processing steps are taken, including the combining and mixing of the partial pulps together. In the pulp processing pipework, the diameter of the pipe may vary, for example, between the actuators, but the pulp flow is not stopped in the actuators and the flow is thus continuously advancing at a rate determined by the volume flow. "Actuators" refer, for example, to a grinder, a pulper, a screen, and pumps, which are required for implementing the different pulp processing steps. "Pulp flow" refers in this application to the pulp advancing in the pulp processing pipework.

In the pulp processing systems shown in Figs. 1 to 3, three partial pulps are illustrated and their processing is shown in a simplified view, but with respect to the method according to the invention, the number of partial pulps may be N, N being a positive integer ≥1. The figures do not show, for example, con- trol systems, dilution water flows, fibre recovery systems, nor all the possible actuators involved in the system.

Figure 1 is a schematic view showing the conventional pulp processing system of a paper or cardboard machine according to prior art.

In Fig. 1 , each partial pulp is fed from its storage tank 1OA, 1OB, 1OC by means of pumps 11 A, 11 B, 11C to dispenser tanks 13A, 13B, 13C, and the partial pulp is diluted for the first time in the lower part of the pulp silo or in the pipe between the tanks with a dilution water flow introduced into it. From the disperser tank 13A, 13B, 13C, each partial pulp is fed further by means of disperser pumps 14A, 14B, 14C into a mixing tank 15, in which the partial pulps are mixed together, and from the mixing tank 15, the pulp is fed further by a pulp feeding pump 16 into a machine chest 17. The pulp is diluted at several different points between the various tanks, and it is diluted to the final dispensing consistency in the machine chest 17, from which the high-consistency pulp is fed further by a pulp feeding pump 18 into the short circulation. In the pulping process, the partial pulps can be pulped or ground, if nee-

essary, for example between the silos and disperser tanks, by actuators 12A, 12B, 12C, as shown in Fig. 1. Furthermore, the necessary fillers and other additives are admixed to the pulp in the machine chest 17. The consistency of the so-called machine pulp to be fed into the short circulation is thus adjusted to be suitable in the machine chest 17 before the pulp is fed into the short circulation, in which the pulp is diluted to the headbox consistency.

Figures 2 and 3 exemplify the process arrangements of the method according to the invention.

Figure 2 is a schematic view of a pulp processing arrangement of the method according to the invention. The partial pulps are led from a pulp source, for example a storage tower 1OA, 1OB, 10C, to the pulp processing pipework, by means of pumps 11 A, 11 B, i1C. The dilution of the partial pulps to the desired consistency takes place in the lower part of the storage towers 10A, 10B, 10C and/or simultaneously when the pulp is pumped into the pulp processing pipework. The pumps 11 A, 11 B, 11C provide a steady and continuous volume flow that proceeds through the required actuators 12A, 12B, 12C and screens 19A, 19B, 19C to a pulp feeding pump 20, in which the partial pulp flows are mixed together and the combined flow is led further as a continuous flow into the short circulation of the paper or cardboard machine. A separate screen may be provided for each partial pulp flow, as shown in the figure, or, alternatively, a screen is placed in the combined pulp flow before or after the pulp feeding pump 20.

Figure 3 shows an alternative process arrangement for the method according to the invention. The partial pulps are dispensed in a desired consistency, for example, from the storage towers 10A, 10B, 10C to the closed pulp processing pipework by means of pumps 11 A, 11 B, 11 C, and immediately after this, the partial pulp flows are combined with each other in the pulp processing pipework. The combined pulp flow is led as a continuously advancing flow via the actuators 12 and the screen 19 to the pulp feeding pump 20, which feeds the pulp flow further to the short circulation.

The actuators 12, 12A, 12B, 12C and the screens 13A, 13B, 13C, 19 shown in Figs. 2 and 3 are not necessary in the pulp processing, but their location may vary, and different actuators may be provided in the pulp flow of various

partial pulps at different locations in the pulp processing pipework, or there may be no said actuators at all. The actuators 12A, 12B, 12C refer primarily to a pulper and/or a grinder. The need for them in the pulping process will depend on the pulp in question. The pump 20 may be a so-called feeder pump for high-consistency pulp, which pumps the pulp further to the short circulation, or the pump 20 may be a so-called mixing pump, in which the dilution takes place simultaneously.

The pulp source is preferably a pulp silo, as shown in the figures. The pulp coming from the pulp manufacture is normally stored in silos before it is lead into the actual pulping process. The function of the pulp silo is to act as a so- called buffer storage between the pulp mill and the paper machine. However, the pulp source may also be pipeline pulp coming directly from a pulp mill, a pulp flow coming from the pulping of cellulose bales, or a broke tower in which pulped broke is stored, or a so-called equalizer tank between the pulp mill and the pulp processing. Thus, the pulp may consist of chemically, mechanically or mechanochemically manufactured pulp, broke or recycled fibre.

The feeding of pulp as a steady and continuous flow from the pulp source into the pulp processing pipework is effected by a pump. The dilution water is admixed to the pulp simultaneously when the pulp is pumped into the pulp processing pipework, or the pulp is diluted already, for example, in the lower part of the storage tank. The consistency of the pulp is maintained at a given constant consistency range in the pulp processing pipework, preferably at about 4 to 6%, but the consistency may also vary within a wider consistency range, depending on the pulp. The pulp can be diluted with water recovered from the wire section of the paper machine to the headbox consistency at the same time when the partial pulps with a high consistency are mixed together by a mixing pump 20, from which the diluted pulp is led further to the head- box via the necessary process steps. Thus, the dilution for the so-called short circulation is carried out already at this stage, and the pulp flow does not need to be diluted any more before the headbox.

One requirement for achieving a uniform consistency in the pulp processing pipework is good mixing in the pulp silo, broke tower or equalizer tank, so that large variations in the consistency take place but the pulp can be diluted

to the desired consistency in the lower part of the tank and/or simultaneously when it is pumped into the pulp processing pipework. Furthermore, discharging from the tank or silo must take place evenly.

A sufficient pressure is developed with the pump so that the pulp volume flow is sufficient in the pipework. The continuous flow in the pipework guarantees that the pulp will not form agglomerations on the walls of the. pipe, as in the conventional method, in which the pulp is let to stand and mixed in tanks, wherein arching and infiltration take place when the tanks are discharged. Thus, with the method according to the invention, it is possible, among other things, to reduce fluctuations and other impurities caused by arching.

The pulp processing pipework is a closed system; in other words, the pulp is not exposed to a free air space in the pipework, which thus reduces the mix- ing of air into the pulp. In the pulp processing pipework, it is possible to remove air from the pulp, if necessary. This is particularly the case, if pulp from direct pulping is used as a pulp source. Air reduction is preferably carried out simultaneously when the pulp is pumped into the pulp processing pipework.

The pulp flow proceeds without stopping, for the processing of the pulp through the required process steps, such as pulping, grinding and screening. This feature is taken into account in the selection and dimensioning of the necessary actuators. Furthermore, no back flow is caused by the actuators. Moreover, in the selection of the actuators, one should take into account that they do not cause excessive resonation in the pulp flow.

The fillers and other additives for the pulp flow are admixed to the pulp flow without separate mixing tanks. The above-mentioned substances are added either into the flows of partial pulps or first into the combined pulp flow. The pulp flow rate in the pulp processing pipework should be designed so that it is sufficient for the above-mentioned substances to be evenly mixed into the pulp by the effect of the flow. In particular, the pulp flow is influenced by the fact that the pulp is led at a sufficient rate from the pulp source into the pulp processing pipework. The pulp flow rate is influenced by many factors, and therefore, it should be adjusted to be suitable for the paper machine in question.

The uniform mixing of fillers and other additives into the pulp is normally secured by a mixing pump 20, through which the pulp flow is led into the short circulation. Also, the partial pulps are mixed together in the pulp proc- essing pipework without separate mixing tanks in the pulp flow, and the mixing is boosted by means of a so-called pumping mixer or a mixing pump 20, as shown in Figs. 2 and 3. Wet broke can be, if necessary, introduced directly to the location of mixing of the partial pulps without circulating it via a broke tower and pulp processing.

In Fig. 3, screening of the pulp flow takes place before the mixing of the pulp flow with the pulp feeding pump 20. Alternatively, each partial pulp flow can be screened separately before the mixing of the partial pulps together, as shown in Fig. 2. The screen 19 may also be placed first after the pump 20.

Figures 2 and 3 show that the pulp flow is mixed by a pulp feeding pump 20, and it has been presented above that the dilution of the pulp to the headbox consistency can be performed at the same time. However, it is possible that the process arrangement does not comprise the pulp feeding pump/mixing pump 20 shown in the figures, but the pulps proceed as a continuous flow directly all the way to the feeding pump of the headbox.

It is also possible that the partial pulp flows are not mixed together but they proceed as continuous separate flows into the short circulation and the headbox. Pulp processing for each partial pulp takes place according to the same principles as presented above.

The system for adjusting the consistency and the pulp flow required by the method is simple, because the pulp is only diluted at the point from which the pulp is led as a steady volume flow into the pipework and, for the next time, first to the required headbox consistency, either in connection with the mixing of the partial pulps or before the headbox. The pulp consistency is adjusted by varying the content of dilution water to be admixed with the pulp; in other words, dilution water is added in a suitable ratio to the quantity and consis- tency of the pulp. As the consistency is adjusted at the early stage of pulp processing and, for the next time, first before the feeding of the pulp into the headbox, it is possible to minimize problems caused by consistency distur-

bances, e.g. fluctuations in the grammage. Among other things, this is affected by the fact that the process arrangement according to the invention involves no tanks that would cause a delay, but fluctuations in the consistency can be fixed quickly.

The consistencies of the partial pulps differ from each other, and depending on the paper to be manufactured, the pulps are mixed to each other in various ratios. Thus, the need for diluting pulps is also specific to the partial pulp in question. The consistency is measured in the pulp processing pipework before the mixing of the partial pulps together, and it is used for adjusting the dilution water flow.

If necessary, the pulp processing pipework may comprise dilution ducts which make it possible to dilute the pulp in the pulp flow, but as a rule in the method according to the invention, the pulp is diluted to the desired consistency simultaneously when it is fed from the pulp source into the pulp processing pipework.

The above-presented process arrangements of the method according to the invention simplify the pulp processing in paper and cardboard machines and partly also the process arrangement of the short circulation in such a way that various intermediate tanks and mixing tanks are no longer needed to secure the uniformity of the pulp, but the pulp processing can be performed in a continuously proceeding pulp flow. The pulp flow led from the pulp source to the pulp processing proceeds quickly through the required processing steps all the way to the headbox of the paper or cardboard machine, and the pulp does not remain standing in intermediate tanks in between.

The above-described embodiments exemplify the solution according to the invention, and a person skilled in the art will understand that the process arrangement according to the invention may involve various process steps, depending on the paper or cardboard grade to be manufactured, and they vary from one paper or cardboard machine to another. The invention is thus not intended to be limited to the embodiments or devices presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea defined in the appended claims.