CN114108353A

CN114108353A - Screening system

Info

Publication number: CN114108353A
Application number: CN202111508852.8A
Authority: CN
Inventors: 侯焕涛; 秋增昌
Original assignee: Andritz China Ltd
Current assignee: Andritz China Ltd
Priority date: 2021-12-10
Filing date: 2021-12-10
Publication date: 2022-03-01
Anticipated expiration: 2041-12-10
Also published as: CN114108353B

Abstract

The present invention relates to a screening system consisting of one or more screening sections, at least one of which comprises: a screen having an inlet and an outlet; a circulation tank having an input side at one end in a length direction for receiving a slurry to be screened and an output side at the other end in the length direction; a pulp pump for pumping the pulp to be screened contained in the circulation tank from the output side to the inlet of the screen, the reject of the screen being re-feedable to the input side of the circulation tank or out of the screening system according to the selection; a diluted white water supply device for adjusting the inlet concentration of the screen machine by adding an appropriate amount of diluted white water to the circulation tank or the pipeline in front of the screen machine.

Description

Screening system

Technical Field

The invention relates to a screening system, in particular to a screening system used in a pulping process.

Background

Screening in the pulping process can be divided into a coarse screening system, a classifying screening system and a fine screening system. The fine and coarse screen systems are typically multi-stage screens, most commonly three or four stage screening designs. The purpose of multi-stage screening is to reduce fiber loss, wherein "stage" refers to the number of times the reject from the screen is screened. Classifying screens are usually single-stage screens, which are used to separate long and short fibers from each other; or two stage screening, where the purpose is to separate the fibers into long/medium/short tertiary fibers. The long fibers can be regarded as the reject from the classifying screen.

If the system contains two stages of classification and three stages of fine screening, the final stage of fine screening will have the solid weight of the discharged slag accounting for 1.0-2.0% of the input raw material, thus 30-60% of the good fiber is contained in the discharged slag, in fact the discharged slag will pass through five stages (or stages) of fiber screening process (i.e. the screening process will leave the fiber in the good pulp relative to the pulp slag). If the system yield is 1000 tons/day, the fiber loss of fine screening slag discharge is 3-5 tons/day. With the increase in the number of (fine) stages, it is still possible to screen out fibres, but less and less can be screened out per increased stage because of two difficulties: on the one hand, the remaining fibers are larger in size, less flexible, and more free fibers than the screened-out fibers, which themselves are difficult to pass through the holes or slots of the screen cylinder. On the other hand, after multi-stage screening, the impurities which can not pass through the screen holes or screen slits are the main components in the material, and the fibers which can pass through the screen holes or screen slits are changed into the minor components. The slurry is a two-phase fluid of water and suspended solids, with the solids that cannot pass through the screen deck being the major constituent and the screen deck being a concentrating effect on these components. The concentration of the slurry running in the slurry screening machines with different designs has an upper limit, generally ranges from 3% to 5%, and when the concentration is reached, suspended matters can be pasted on the surface of the sieve plate and instantly blind the sieve plate to cause blockage, so that the necessary premise of continuously increasing the number of the screening sections for screening is to maintain high slag discharge flow rate, thereby ensuring that the slag discharge concentration is not too high to block the slurry screen. Along with the reduction of the proportion of the fiber to the solid, the residue discharge rate is higher and higher, and the pulp screen can normally operate. The high slag discharge rate is contradictory to the requirement of less loss of final screening slag discharge fiber.

The reason why the content of fiber in the slag discharged by the fine screening system in the prior art is high is that: the fine screen is usually a 0.15-0.25 mm wide slit screen, compared with the coarse screen, the slit width is very small, the dewatering and thickening effects of the screening area are stronger, and the 1 st section, the 2 nd section, the 3 rd section and the 4 th section are successively strengthened. For the 3 rd and 4 th stages of the fine screen, a very low feed consistency is selected in order to reduce the reject rate while avoiding plugging of the screen. But the concentration of the good pulp is reduced at the same time, and the productivity of the unit screening area is also reduced. For example, the pulp feed concentration in stage 3 is reduced to 0.7%, 50% of which is that it can pass through the screen openings or slots, i.e., 0.35% of the solids in 0.7% total solids concentration can be screened to the accepts side under overfeeding conditions, but these solids (which can pass through the slots and can be considered approximately all fibers) are more difficult to pass through than water. That is, the concentration of the fiber in the accept pulp is lower than 0.35% of the feed pulp by a discount, if the discount is 70%, the accept pulp concentration can only reach 0.245%, and if the reject concentration of 3% is to be maintained, the solid reject rate is 71% and the volume reject rate is 16.5% according to the balance calculation of the materials. The components passing through the screen slots in the slag discharge are reduced from 50 percent to 29.4 percent. And 4, screening the slag discharge section, if the discount of the concentration of the good pulp is 60%, calculating that the concentration of the good pulp is 0.12%, 23% of fiber residues still exist in the slag discharge, and the solid slag discharge rate is up to 88% and the volume slag discharge rate is 21%. The efficiency is lower by continuing to reduce the residual amount of the fiber discharged by 23%. If the pulp making system uses a filtrate (white water) concentration and accepts concentration of 0.12% equivalent, the fourth stage of fine screening is not meaningful. The above equilibrium calculation is for simplicity the filtrate concentration for dilution in the fine screen system is neglected to be zero. At present, the residue discharged by a fine selection system consisting of three sections of fine sieves in actual production is more than 30 to 60 percent of the fiber content. In order to reduce the fiber loss of the screening system, the paper industry typically presses the screen reject rate as low as possible to near the edge of the screen plugging. Even many paper making enterprises return the slag discharged from the system to reduce fiber loss, which results in dirtier finished paper surface and normal operation influenced by the accumulated stickies on the paper machine net and drying cylinder.

It would therefore be desirable to provide a screening system that reduces energy consumption to a reasonable level and ensures pulp quality while increasing fiber recovery efficiency.

Disclosure of Invention

In view of the above technical problem, the present invention provides a screening system, which is composed of one or more screening sections, wherein at least one screening section comprises: a screen having an inlet and an outlet; a circulation tank having an input side at one end in a length direction for receiving a slurry to be screened and an output side at the other end in the length direction; a slurry pump by which slurry to be screened contained in the circulation tank is pumped from an output side of the circulation tank to an inlet of the screen, a reject of the screen being resuppliable to an input side of the circulation tank or discharged from the screening system according to selection; a dilution water supply that adjusts the inlet consistency of the screen by adding an appropriate amount of dilution water into the screening system (e.g., in the recycle tank or elsewhere prior to entering the screen or on a reject line between screen and tank). The dilution water supplied by the dilution water supply is typically white water, but the use of other low-consistency pulps or fresh water is not excluded.

Preferably, the length of the circulation tank is greater than the diameter to such an extent that the reject flows, which enter the circulation tank from the input side one after the other and which have been subjected to different screening times, are kept flowing one after the other to the output side.

The ratio of the length to the diameter of the circulation tank is designed such that the circulation tank is formed in an elongated shape, so that the reject flows which successively enter the circulation tank and which have been subjected to different screening times are as little mixed as possible in the circulation tank. For this reason, considering the outlet flow that can be provided by the existing screening section, it is preferred that the length to diameter ratio of the circulation tank is greater than 2.5.

According to another preferred embodiment of the invention, the screening system may comprise two circulation tanks, but also more circulation tanks. The slurry pump pumps slurry to be screened contained in each circulation tank from an output side to an inlet of the screen in turn, the reject of the screen is supplied to an input side of the same or another circulation tank, and the slurry in each circulation tank is screened in turn in a circulating manner until all the slurry in the circulation tank is discharged out of the screening system.

After the slurry to be screened in each circulation tank passes through the pulp screening machine, the discharged slag of the pulp screening machine can return to the same circulation tank to realize repeated screening, and can also be returned to another circulation tank. It is obviously also conceivable that reject from the screening of the pulp to be screened in one circulation tank is returned to a plurality of circulation tanks. In any of the above-described embodiments, the slag discharge is divided into a plurality of portions circulating in the respective circulation tanks due to the presence of the plurality of circulation tanks, and the slag discharge is isolated from each other by different screening times, so that the effect of isolating a single elongated tank by controlling the flow state in the tank is achieved by the tank rotation, and the slag discharge from the pulp screening machine is repeatedly screened until the slag discharge in all the circulation tanks reaches the condition of being discharged from the screening system.

In embodiments employing multiple circulation tanks, it is not necessary that the reject flow be maintained sequentially from the input side to the output side. Here, the circulation tank may be a long and thin tank body having a size such that slag discharged successively thereinto through different screening times are not mixed as much as possible, or a tank body having a general shape. Under the condition of selecting an ordinary tank body, although no stagnant flow type slurry motion is realized in the circulating tank, the beneficial technical effect of repeatedly screening the slag discharge of the pulp screening machine is still kept due to the adoption of the plurality of slurry storage tanks. For a slender tank body, the flow in the circulation tank is equivalent to the flow in a pipeline, and additionally, a better fiber recovery effect is obtained, because the size of the circulation tank is matched with the flow and the flow speed of the slurry, and the slag discharged into the circulation tank successively and through different screening times is not mixed in the circulation tank as far as possible.

According to a preferred embodiment of the invention, the screening system further comprises a stock tank for storing stock to be screened and for feeding the stock to be screened in batches into the circulation tank, wherein the stock to be screened comes from a previous process in the pulping process or from another screening section upstream of the at least one screening section in the present screening system. The pulp to be screened is continuously collected by a pulp storage tank and is intermittently discharged in batches after being accumulated to a certain volume, and the circulating tank receives the pulp to be screened from the pulp storage tank through a pipeline or an opening. For example, after the circulation tank is emptied by discharging residue through the pulp screening machine in the initial operation, and after the liquid level of the pulp storage tank is checked to be a certain height, the valve at the bottom of the pulp storage tank is opened, and the pulp to be screened in the pulp storage tank can enter the empty circulation tank from any position above, below or in the middle of the circulation tank, even enter from the pump port of the pulp pump and reversely flow into the circulation tank. The bottom valve of the pulp storage tank is closed immediately after the pulp storage tank discharges the preset pulp amount. The opening and closing action of the valve at the bottom of the slurry storage tank can be completed quickly, for example, within 5 seconds. The pulp in the circulation tank is then started to circulate through the screen by the pulp pump.

In this way, the reject from the screen is resupplied to the input side of the circulation tank, flows in the circulation tank to the output side in a stagnant manner, and is gradually replaced from the input side to the output side, and the replaced reject is pumped to the inlet of the screen and is screened again. Thus, the reject is continuously circulated in the loop consisting of the circulation tank, the pulp pump and the pulp screen, and the accept from the pulp screen is continuously removed from the loop consisting of the circulation tank, the pulp pump and the pulp screen. The residue discharged by the pulp screening machine which is circularly sent back to the circulating tank contains less fiber than the existing material in the circulating tank, so that the sieve residue which enters the tank body of the circulating tank later and the material which contains relatively higher fiber in the previous circulating tank are prevented from being mixed with each other, and the aim of reducing the fiber content in the tank as soon as possible is achieved. Since the reject consistency of the screen is higher than the pulp consistency at the inlet of the screen, the thickening effect caused by the consistency difference between the two is overcome by adding dilution water to the circulation tank, in particular from the input side of the circulation tank or elsewhere before entering the screen. That is, the dilution water supply device determines the amount of dilution water in accordance with the flow rate of accepted pulp screened by the screen so that the pulp concentration at the inlet of the screen is maintained in a range in which the pulp yield and the fiber recovery rate can be optimized. In practice, because the solids flow rate of the reject from the screen is lower than the solids flow rate at the inlet of the screen, the dilution water is added in an amount lower than the accept flow rate of the screen to ensure that the concentration at the inlet of the screen does not rise. Thus, the liquid level in the circulation tank is gradually lowered, and the time required for once circulation of the slag discharge in the loop composed of the circulation tank, the slurry pump and the pulp screening machine is less and less. The circulation times of the discharged slag in a loop consisting of a circulating tank, a slurry pump and a slurry screening machine can be determined according to the required content of the fiber of the screened slag, and the discharged slag is discharged out of the system through the arrangement of a valve and the like at proper time. The next batch of slurry to be screened is then received from the holding tank and a new batch of screening is performed.

In the prior art, multi-stage screening consists of multiple screen machines, for example, the reject from one stage of the screen machine is provided to the second stage of the screen machine for re-screening, while the reject from the second stage of the screen machine is provided to the third stage of the screen machine for re-screening, and so on if the system has four or even five stages. Because the slag discharged by the front section screen is processed, the screening area of the rear section screen of the screening equipment is smaller than that of the front section screen. In contrast, the screening system of the invention realizes circular screening in the screening loop consisting of the circulating tank, the slurry pump and the slurry screening machine, each circulation corresponds to a section of screening in the prior art, and the time for re-screening the discharged slag is less and less.

One of the characteristics of the invention is that the residue discharged by the pulp screening machine returns to the pulp inlet circulation tank for circular screening, thus creating conditions for improving the residue discharge rate of the sieve. In the prior art, the fiber loss is increased by simply increasing the slag discharge rate of the sieve or ensuring the fiber loss not to be increased by using more sieving sections. The present invention will now be further analyzed with respect to differences from the prior art on the basis of the above exemplary analysis with respect to the prior art. In the present invention, in stage 3 fine screening, 50% of the solids in the slurry to be screened are screened out through a given fine screen into accepts. In the invention, the higher concentration of the entering pulp is adopted, the entering pulp concentration is changed from 0.7 percent to 1.5 percent, 50 percent of the entering pulp concentration can pass through the good pulp (can be approximately regarded as fiber), and the fiber content of the entering pulp is improved from 0.35 percent to 0.75 percent. The fiber concentration discount from pulp feed to accepts is conservatively assumed from 70% to 50% of the prior art analysis, the accept flow is unchanged, the concentration is increased from 0.245% to 0.375%, and the calculated volume reject rate needs to be increased from 16.5% to 42.8%, keeping the reject concentration at 3% and the same accept flow. The solid slag discharge rate is improved from 71 percent to 86 percent. The screening can be carried out without blocking the pulp screen, namely, the fibers are flocculated into a net shape and pasted on the screen surface, and the concentration in the screen is ensured not to be as high as the degree of forming flocculation through higher slag discharge rate of slag discharge. In the above example analysis, the slag discharge rate is enlarged, the volume flow pumped to the pulp screening machine is improved by 46.1% compared with the prior art, the yield of good pulp is improved by 53% compared with the prior art, and the slag discharge achieves the effect of more sections of screening through multiple times of circular screening, which actually brings about more reduction of the fiber content in the slag discharge.

Addition of dilution water from other locations of the circulation tank than from the inlet side of the circulation tank is also a conceivable solution, as long as the addition of dilution water does not cause strong up-down convection of the slurry flowing from top to bottom in the circulation tank. If the screen apparatus is selected with dilution nozzles, all or part of the dilution water required can be added to the screen.

In the case of a feed to the circulation tank, in the embodiment with a stock tank, the circulation tank receives the stock to be screened from the stock tank via a pipe or an open mouth. In embodiments with multiple circulation tanks, the circulation tanks may also alternately receive pulp to be screened from a previous process in the pulp making process or from another screening section upstream of the at least one screening section in the present screening system, either through pipes or open mouths.

Whereas in the case of the circulation tank itself, the circulation tank itself may be closed or open when it is arranged upright or inclined. The closed circulation tank receives the slurry to be screened and the slag of the pulp screening machine through a pipeline. The open circulation tank may receive the pulp to be screened and the reject from the screen through a pipe or open. The circulation tank arranged obliquely may be site-restricted in use, but has a better effect of avoiding turbulence and impact. When the circulation tank is placed obliquely, the advantages of horizontal and vertical are combined, so that not only an open design can be adopted, but also the interference of the slurry flow at the input side of the circulation tank to the internal slurry can be reduced.

When the circulation tank is arranged horizontally, the circulation tank itself can only be closed and the pulp to be screened and the reject of the pulp screen can only be supplied to the input side of the circulation tank via a pipeline.

By "closed" is meant that the circulation tank forms a closed space and that the feeding takes place only through a pipe. In this case the circulation tank may be arranged either vertically or inclined or horizontally, and the slurry in the circulation tank may also be operated under pressure.

When the circulation tank is arranged vertically or obliquely, the pulp to be screened and the reject of the pulp screening machine flow into the circulation tank along the tangential direction of the horizontal cross section of the circulation tank, so that the disturbance effect on the pulp in the circulation tank caused by the downward flow of the inflowing pulp and reject along the tank wall is minimized.

When the circulation tank is arranged upright, it preferably has a conical shape narrowing from said input side to said output side, which is more favorable for the slurry or reject to flow from above down the tank wall to the surface of the liquid level rather than falling in a free-fall manner into the circulating reject or dilution water causing local turbulence.

According to a preferred embodiment of the invention there are several screening sections in the screening system, the last screening section having a screen, the accept of which is supplied to the input of the other screening section upstream in the screening system or directly to the equipment downstream in the screening system. Optionally, the accepts of the screen are first collected by an accepts stabilization tank, eliminating accept concentration cycling under batch screening operating conditions, and then supplied to the input of other screening sections upstream in the screening system or directly to equipment downstream of the screening system.

According to another preferred embodiment of the invention, the only screening section of the screening system has a screen. The accepts of the screen are optionally collected from an accepts stabilization tank or directly supplied to equipment downstream of the screening system.

The screening section with the circulating tank in the screening system can be used independently or can be used as the tail screening section of a multi-stage screening system, the effect of multi-stage screening is realized by one screen, the conventional thinking and limitation that the fiber content of the screen slag is reduced by increasing the number of the screening sections are broken through, and the content of the slag-off fiber in the screening section is reduced to be extremely low. It is of course possible to use the screening section with the recycle tank described above as an intermediate stage of a multi-stage screening system, although this arrangement is not preferred.

For example, in a small-scale pulping system with the yield of below 50 tons/day, multi-stage screening of a plurality of pulp screens is too complex, and by using the scheme, one pulp screen can replace multi-stage screening consisting of three pulp screens, the fiber content of the screen residue can be reduced to a very low degree, and meanwhile, the process is simplified, and the investment of equipment, electric appliances, instrument control pipelines and the like is saved.

The technology can be used on a new project to achieve the effects of improving fiber recovery and improving the competitiveness of a system, and can also be used for reforming the existing production line to save fibers and reduce the cost of raw materials. Compared with the original production line, the circulating tank, the pulp storage tank, the good pulp stabilizing tank and the like can be conveniently provided in a form of supplement or external hanging of the pulp screening machine shell, and can be flexibly configured in space. The technical effect of augmenting the volume of the intermittent sieve housing with the volume of the circulation tank, which can be built on site very large, for example tens or tens of cubic meters, is actually achieved. The cost of building the circulation tank is low, the large volume of the circulation tank enables the pulp to be screened in each batch to be circulated in the screening system for a long time, the reasonable residence time of 2-10 minutes can be designed, the automatic valve does not need to be frequently opened and closed, and the fibers are thoroughly screened. In contrast, most of the current pressure sieves or other sieves on the market are designed in a continuous working mode, and the shell is small and can not carry out intermittent deslagging. The coarse screen system has equipment suppliers still providing intermittent deslagging screens with shell volume less than 1 cubic meter.

In conventional intermittent screening apparatuses (e.g. defibrator, hydrotrash separator for pulped pulp, etc.), the flow of pulp in the casing is realized by the rotor producing strong turbulence while screening, and the rotor must ensure that enough turbulence is still maintained in the casing at the position farthest from the rotor to generate no dead angle, for example, the pulp screening machines such as hydrotrash and defibrator are cavity casings, the casing volume is from small size less than 0.5 cubic meter to larger 5 cubic meters, the intermittent deslagging can be performed, the linear speed of the rotor is more than 12-22m/s, the high-speed running motion around the rotor, and the moving speed of the pulp from the rotor is attenuated with the distance. In the screening system, the pulp in the circulating tank flows from one end to the other end in a stagnation state, the screening system is designed to reduce the mixing of the pulp entering the circulating tank and the pulp entering the circulating tank previously as much as possible, the moving speed can be constant at 0.1-0.5m/s, the pulp enters the pulp screening machine in a queue mode in the circulating tank, and the pulp is circularly screened for one time. Thus, this approach has a higher throughput rate than a completely mixed screen within the equipment housing.

Drawings

The present invention will be described in detail with reference to the accompanying drawings. In the drawings:

FIG. 1 schematically illustrates a three-stage fine/coarse screen system of the prior art;

figure 2 schematically shows a screening circuit consisting of a circulation tank, a pulp pump, a screen, according to the screening system of the invention;

FIG. 3A schematically illustrates a screening system according to the present invention wherein a third section employs a screening loop consisting of a recycle tank, a slurry pump, and a screen;

figure 3B schematically shows a variant of the screening system according to the invention in which the accept stabilizing tank for the tail screen is omitted and the accept of the screening section consisting of circulation tank, pulp pump, pulp screen is fed directly to the input of the upstream screening section;

figure 4 schematically shows a screening system according to the invention, in which no stock tank is arranged but two circulation tanks, one pulp pump, one screen are used;

fig. 5 schematically shows another embodiment of the screening system according to the invention, in which the circulation tank is not an elongated tank as shown in fig. 2 to 4, but is replaced by two tanks of conventional shape;

fig. 6 shows schematically another embodiment of the screening system according to the invention, in which the circulation tank is arranged inclined.

Detailed Description

Figure 1 schematically illustrates a three stage fine/coarse screen system of the prior art. As shown, accepts from the secondary screen 180 may optionally be fed back to the input end of the primary screen 170, as required by the design, and combined with upstream slurry to be treated and pumped by the pump 130 to the inlet of the primary screen 170, but this is not required. The accept from the third screen 190 is fed back to the input of the second screen 180 and pumped to the inlet of the second screen 180 by the slurry pump 130, while the reject from the third screen 190 is discharged directly to the outside of the system. Dilution water is added before the pump 130 of the first screen 170, the second screen 180, and the third screen 190, respectively.

Figure 2 schematically shows a screening section of a screening system according to the invention consisting of a circulation tank 120, a pulp pump 130, a screen 140. In the case of other screening sections upstream of the screening system, the slurry to be screened (reject from the upstream screening section) is fed into the circulation tank 120 from the input side 121 of the circulation tank 120, joins with dilution water and reject recovered after screening by the screen 140, and flows down to the output side 122 of the circulation tank 120. The slurry pump 130 pumps the slurry from the output side 122 of the recycle tank 120 to the screen 140, and the screen 140 screens out accepts and delivers the accepts back to the upstream screening section. In a three stage screen, either the first or second screening stage may be used, although transport back to the second screening stage is preferred.

In the case where this screening section is the only screening section of the screening system, the stock to be screened is fed into the circulation tank 120 from the inlet side 121 of the circulation tank 120, and the accept screened by the screen 140 is either fed out directly or via an accept stabilising tank, not shown here, after equalizing the periodic fluctuations in the concentration of the circularly screened accept over time.

Figure 3A schematically shows a screening system according to the invention, wherein the third screening section is a screening section consisting of a circulation tank 120, a pulp pump 130, a screen 140. As shown, the screen 140 screens out the input end of the accept being sent back to the upstream second screening stage. Figure 3B schematically shows a variant of the screening system of the embodiment shown in figure 3A, in which the accept stabilizing tank for the tail screen is omitted and the accept of the screening section consisting of circulation tank 120, pulp pump 130, and pulp screen 140 is fed directly to the input of the upstream screening section.

Both fig. 3A and fig. 3B show a stock tank connected upstream of the circulation tank. The slurry storage tank is used for storing slurry to be screened. The stock tank has approximately the same volume as the circulation tank, e.g. 1-20 cubic meters, and is located above the circulation tank so that the stock in the stock tank can flow directly into the circulation tank under the influence of gravity. The slurry storage tank is connected with the circulating tank through a valve V1, and the circulating tank is quickly filled from the lowest liquid level to the highest liquid level after the valve V1 is opened. At the moment, the slurry pump and the screen are operated, a valve V2 on a discharge pipeline of the screen is opened, and a valve V3 is closed. The accept from the screen is continuously returned to the input of the upstream screening stage in the screening system under the control of the regulating valve, while the reject from the screen is fed back to the upper part of the circulation tank via the open valve V2. Since the circulation tank is of an elongated shape, i.e. its size and flow rate of the pulp are designed such that the screened reject and the mixed pulp of the pulp to be screened and dilution water from the stock tank flow downward in a stagnant manner from top to bottom in the riser to displace the pulp in the circulation tank, the pulp from the circulation tank is pumped to the inlet of the screen and screened again, whereby each displacement displaces one screening stage equivalent to the prior art. One sieve realizes the multistage screening effect that could be realized by many pulp screens, and the screening area of this scheme sieve, if the total area that adds is the same with several sieve areas, this scheme still has higher output. The reason is that under the same good pulp flow rate, the scheme can increase the slag discharge rate of the sieve without limit, and the concentration factor of the sieve is reduced under the high slag discharge rate, so that the pulp inlet concentration can be greatly increased and the blockage of the pulp screening machine can be still ensured, the good pulp concentration is correspondingly increased when the pulp inlet concentration is high, and more fibers are discharged to downstream processing equipment, which means that the yield is higher, the slag discharge is more clean, and the fiber loss is reduced.

The accept continuously leaves the circulation loop consisting of the circulation tank, the pulp pump and the pulp screen. The dilution water is used for compensating the concentration and thickening of the discharged slag. However, because the solid flow at the outlet of the pulp screening machine is lower than the solid flow at the inlet, the concentration at the inlet of the pulp screening machine can be ensured not to rise by adding the dilution water in an amount lower than the good pulp flow of the pulp screening machine, so the liquid level in the circulating tank is gradually reduced, and the time required for circulating once is less and less. After 4-6 times of circulation, the fiber content in the screen slag can reach nearly 10-15%, V3 is opened, V2 is closed, the slag discharge of the screen pulp machine is discharged out of the system, the liquid level in the vertical pipe is reduced to the lowest, and the screening of one batch of pulp to be screened is finished. The valve V1 is opened and the screening of the next batch of slurry to be screened, which has now been stored in the stock tank, is started.

Fig. 4 schematically shows another embodiment of the screening system according to the invention, in which no stock tank is arranged but two

circulation tanks

1201, 1202 are used for alternately collecting the stock to be screened which is continuously fed in. Valves V11, V12 on the input side of the circulation tank and valves V31 and V32 on the output side of the circulation tank are controlled to be alternately opened and closed, so that the two circulation tanks are alternately connected into the circulation loops of a slurry pump and a slurry screening machine to screen the slurry in the circulation tanks. First, the valve V11 is opened (and V12 is in a closed state), slurry to be screened is made to enter the circulation tank 1201, when the liquid level of the slurry with the screen in the circulation tank 1201 reaches a certain liquid level height, V11 is closed, V12 is opened, V32 is closed, and the slurry to be screened is collected by the circulation tank 1202. Meanwhile, V21 and V31 are opened, so that the circulation tank 1201 is communicated with a pulp pump and a pulp screening machine to form a circulation loop, and pulp in the circulation tank 1201 is screened. When the slurry to be screened collected in the circulation tank 1202 reaches a certain liquid level height, the circulation screening of the slurry in the circulation tank 1201 is stopped, V3 is opened, V21 and V22 are closed, the slag of a pulp screening machine is discharged outside the system, when the lowest liquid level value in the circulation tank 1201 is reached, V11, V32 and V22 are opened, V12, V31 and V3 are closed, a slurry pump and the pulp screening machine are connected to the circulation tank 1202 to form a new circulation loop, and the slurry in the circulation tank 1202 is screened. After the pulp in the circulation tank 1202 is screened, or the liquid level in the circulation tank 1201 reaches a certain height, V3 is opened, V21 and V22 are closed, and the residue discharged from the pulp screening machine is discharged out of the system. And the above steps are repeatedly carried out, so that the two circulation tanks receive the slurry in turn and screen the slurry in the two circulation tanks in turn. Dilution water may be added before the pump or at other locations in the circulation path during the screening process. The amount of good pulp, slag and dilution water needs a flow meter and an automatic valve to control the flow.

Fig. 5 schematically shows another embodiment of the screening system according to the invention, in which the circulation tank of the elongated-shaped tank of the previous embodiment is not used, but is replaced by two tanks of general shape. That is, in this embodiment, it is not necessary to design the length and diameter of the circulation tank such that the reject flows, which enter it one after the other from the input side and pass through different screening times, are kept flowing one after the other to the output side. In the implementation form, the stagnant flow type slurry motion is not required to be realized in the circulating tanks, after the slurry in one circulating tank is screened by the pulp screening machine, the discharged slag is sent to the other circulating tank to realize the separation of the discharged slag after different screening times, the effect of a single long and thin tank body is achieved in a circulating tank alternating mode, and the discharged slag of the pulp screening machine is repeatedly screened.

Similar to the embodiment shown in fig. 4, which employs two recycle tanks, multiple automatic valve switching is also required here. As shown in fig. 5, after a valve V11 above the circulation tank 2201 is opened to fill the circulation tank with slurry to be screened, a valve V31 communicated with the slurry pump 130 is opened at the bottom of the circulation tank 2201, the slurry is continuously diluted in a pipeline in front of the slurry pump 130 and then is fed to the pulp screening machine 140, after screening, the flow of discharged slag is controlled by a flow control unit, so that the valve V22 is in an open state, and the valve V21 is in a closed state, and thus the discharged slag flows into the circulation tank 2202; when the liquid level in the circulation tank 2201 is reduced to be close to an empty tank, the valve V31 between the circulation tank 2201 and the slurry pump 130 is closed, the communicating valve V32 between the circulation tank 2202 and the slurry pump 130 is opened at the same time, the valve V22 is in a closed state, and the valve V21 is in an open state, so that the discharged slag in the circulation tank 2202 flows into the circulation tank 2201 after being screened again by the slurry screening machine, and the corresponding valve is switched until the liquid level in the circulation tank 2202 is reduced to be close to the empty tank, so that the tank body into which the discharged slag flows is switched, and the switching is repeated.

In this embodiment, only two circulation tanks are shown, and in fact, it is also conceivable to use a larger number of circulation tanks and to switch the circulating flow of the slag discharge between these circulation tanks. The sizes of these circulation tanks may be the same as or different from each other as in the present embodiment. Although circulation tanks of different sizes or shapes may result in the necessity of introducing somewhat complex valves and flow control, it will be apparent to those skilled in the art that this does not go beyond or depart from the scope of the invention.

The size of the pulp screening machine is selected according to the pulp amount to be screened and the required deslagging circulation number during design. After the last deslagging cycle is finished, the valves V21 and V22 are closed, the valve V3 is opened, and then the slurry is supplied to the pulp screening machine 140 from the circulating

tank

2201 or 2202 participating in the last cycle through the correspondingly opened valve V31 or V32, so that the deslagging is carried out of the system. When the slurry in the

circulation tank

2201 or 2202 is emptied, the valve V11 or V12 at the bottom of the slurry storage tank is opened to quickly place the slurry accumulated in the slurry storage tank 110 into the

circulation tank

2201 or 2202, and the screening of the next batch is started.

Fig. 6 shows a schematic illustration of a further embodiment of the screening system according to the invention, in which the circulation tank is arranged at an angle and the inlet side is designed open. The motors of the slurry pump 130 and the screen 140 are continuously operated, the accept and reject of the screen are continuously discharged by flow control, and the dilution water is continuously added before the slurry pump 130.

When it is detected that the liquid level in the circulation tank 120 is lower than the predetermined value, indicating that the slag discharge of the previous batch is completed, the valve V1 and the valve V2 are opened and the valve V3 is closed. The valve V1 and the pipeline are designed to be large enough so that the slurry to be placed in the high-level slurry tank 110 falls rapidly. Here, since the slurry tank 110 is disposed at a certain height and the valve V1 and the pipeline are designed to have a flow rate much greater than the inlet flow rate of the slurry tank 110, although the inlet slurry of the slurry tank 110 continuously flows in, when the valve V1 is opened, the liquid level in the slurry tank 110 rapidly decreases and the slurry flows into the circulation tank 120. When the level in the stock tank 110 is nearly as low as 0%, valve V1 is closed. The amount of dilution water is controlled to an optimized value between 20% and 90% of the accept flow so that a portion of the slurry passing through valve V1 is mixed with dilution water and pumped to the screen 140 and the remainder of the slurry is directed back to the recycle tank 120.

Since the flow rate of the accept is greater than the dilution water, the liquid level in the recycle tank 120 is gradually decreased, and when the liquid level approaches the set value (e.g., 30%), the valve V3 is opened and then the valve V2 is closed, and the reject is discharged out of the system.

When the discharge is deemed complete when the low set point (e.g., 0-5%) is approached in the recycle tank 120, valve V2 is again opened, valve V3 is closed, and valve V1 is then opened until the level of liquid in the stock tank 110 is nearly 0% low.

The screening process of the various embodiments described above requires the addition of dilution water, preferably at the inlet of the pump 130, with the amount of water being controlled by the flow control unit.

In addition to the above-described embodiments, it is also possible to consider improvements in the placement of the screen, for example, in that two screens are connected in parallel or are combined into two sections instead of the screen according to the invention. A larger number of screens and their more complicated connection are also alternatives falling within the scope of the invention.

Furthermore, it is also conceivable to insert a refiner, fluffer, desander or the like upstream of the screen or on the line where the screen is tapped to the circulation tank.

Although the invention has been described with reference to specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. Those skilled in the art can develop further embodiments and specific applications from the teachings of this invention that fall within the spirit and scope of this invention. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

List of reference numerals

100 screening system

110 slurry storage tank

120. 1201, 1202, 2201, 2202 circulation tank

Input side of 121 circulation tank

122 output side of the circulation tank

130 slurry pump

140 pulp screen

150 dilution water supply device

160 good pulp stabilizing tank

170 first section sieve

180 two-section sieve

190 three-section sieve

A fine pulp

R pulp slag

V1, V2, V3, V11, V12, V21, V22, V31, V32 valves.

Claims

1. A screening system (100), characterized in that the screening system (100) is composed of one or more screening sections, at least one of which comprises:

a screen (140) having an inlet and an outlet;

a circulation tank (120), the circulation tank (120) having an input side (121) at one longitudinal end for receiving the slurry to be screened and an output side (122) at the other longitudinal end;

-a pump (130), the pump (130) being adapted to pump the slurry to be screened contained in the circulation tank (120) from an output side (122) to an inlet of the screen (140), the reject of the screen (140) being re-feedable to the input side (121) of the circulation tank (120) or exiting the screening system according to choice;

a dilution water supply device (150), the dilution water supply device (150) being used for adding an appropriate amount of dilution water to adjust the inlet concentration of the screen (140).

2. A screening system according to claim 1, characterized in that the length of the circulation tank (120) is greater than the diameter to such an extent that reject flows from the input side (121) into the circulation tank successively and through different screening times are kept flowing successively to the output side (122).

3. A screening system according to claim 1, characterized in that the screening system (100) comprises a plurality of circulation tanks (120), that the pump (130) in turn pumps the pulp to be screened contained in each circulation tank (120) from an output side (122) to an inlet of the screen (140), that the reject of the screen (140) is fed to an input side (121) of the same or another circulation tank (120), and that the pulp in the respective circulation tank (120) is in turn screened cyclically until all the pulp in the circulation tank (120) is discharged from the screening system.

4. A screening system according to claim 2, characterized in that the screening system (100) comprises a plurality of circulation tanks (120), that the pump (130) in turn pumps the pulp to be screened contained in each circulation tank (120) from an output side (122) to an inlet of the screen (140), that the reject of the screen (140) is fed to an input side (121) of the same or another circulation tank (120), and that the pulp in the respective circulation tank (120) is in turn screened cyclically until all the pulp in the circulation tank (120) is discharged from the screening system.

5. A screening system according to claim 1, characterized in that the screening system (100) further comprises a stock tank (110), the stock tank (110) being adapted to store and feed stock to be screened into the circulation tank (120) either once or in batches, wherein the stock to be screened comes from a previous process of the screening system (100) or from another screening section of the screening system (100) which is located upstream of the at least one screening section.

6. A screening system according to any one of claims 1 to 5, wherein the circulation tank (120) is arranged upright or inclined.

7. A screening system according to claim 6, characterized in that the pulp to be screened and the reject of the screen (140) both flow into the circulation tank (120) in a tangential direction of the horizontal cross-section of the circulation tank (120).

8. A screening system according to claim 6, wherein the circulation tank (120) has a conical shape narrowing from the input side (121) to the output side (122).

9. A screening system according to claim 6, characterized in that the circulation tank (120) is closed, and that the pulp to be screened and the reject of the screen (140) are supplied to the circulation tank (120) through pipes.

10. A screening system according to claim 6, characterized in that the input side (121) of the circulation tank (120) is open, adapted to receive the pulp to be screened and the reject of the screen (140).

11. A screening system according to any one of claims 1-5, characterized in that the circulation tank (120) is closed and arranged horizontally, and that the pulp to be screened and the reject of the screen (140) are supplied to the input side (121) of the circulation tank (120) through a pipe.

12. A screening system according to any one of claims 1 to 5, characterized in that the dilution water supply (150) is arranged to determine the amount of dilution water in dependence on the accept flow screened by the screen (140) in order to maintain the pulp consistency at the inlet of the screen (140) in a range that optimizes the reject rate and the fibre recovery of the screening system (100).

13. A screening system according to any one of claims 1-5, characterized in that the screening system has a plurality of screening sections, and that accept of the screen (140) is supplied to the input of other screening sections located upstream in the screening system or directly to equipment located downstream in the screening system.

14. A screening system according to claim 13, characterized in that the accept of the screen (140) is first collected by an accept stabilization tank (160) and then supplied to the input of other screening stages located upstream in the screening system or directly to equipment located downstream of the screening system.

15. A screening system according to claim 13, characterized in that the screen (140) is the last screening section of the screening system (110).

16. A screening system according to any one of claims 1-5, characterized in that the screen (140) is the only screening stage the screening system (110) has, and that the accepts of the screen (140) are collected by an accepts stabilising tank (160) and supplied to downstream equipment.

17. A screening system according to any one of claims 1-5, characterized in that the screen (140) is the only screening stage the screening system (110) has, the accepts of the screen (140) being supplied directly to the equipment located downstream of the screening system.

18. A screening system according to any one of claims 1 to 5, wherein the length to diameter ratio of the circulation tank (120) is greater than 2.5.