WO2011078749A1

WO2011078749A1 - Method and system for washing lime mud

Info

Publication number: WO2011078749A1
Application number: PCT/SE2009/000537
Authority: WO
Inventors: Lars Johansson; Magnus Ingelman; Niclas Andersson
Original assignee: Metso Paper Sweden Ab
Priority date: 2009-12-23
Filing date: 2009-12-23
Publication date: 2011-06-30

Abstract

The invention relates to a system and method for washing and thickening a lime mud slurry. A filter 1 is used wherein the liquid volume in the filter vat is divided by at least one partition wall 19 into at least 2 independent vat parts and that these vat parts are fed with lime mud slurry in parallel, by individual supply lines from at least one lime mud storage tank 30a/30b. When a clean out operation of one independent part is activated, is the lime mud vat flushed out to the preceding lime mud slurry container 30a/30b, while all other independent parts are still in operation. By using this parallel operation could lime mud sedimentation in the filter vat be broken up at regular intervals, and a continuous feed of washed and thickened lime mud be made to the lime kiln, maintaining the structural integrity of the kiln.

Description

METHOD AND SYSTEM FOR WASHING LIME MUD

Technical area

The present invention relates to a system and a method for washing and thickening a lime mud slurry obtained in a causticizing process in a pulp mill, which system and method preferably utilize a disc filter.

State of the art

From "Chemical Pulping, Book 6A" (ISBN952-5216-06-03) it is conventionally known to use Lime mud filters in form of vacuum drum filters (see page B171). However in these lime mud filters is the precoat formed on the filter surface gradually blinded. The precoat then had to be completely changed at intervals of 8-24 hours by dropping it into the lime mud filter vat and from there to drain to lime mud storage located in a position before the lime mud filter. These methods significantly disturb the feed of lime mud to the kiln. An even and continuous feed of washed and thickened lime mud to the kiln is essential for maintaining the integrity of the kiln, as sudden decrease of lime mud feed could causes overheating in empty or half-filled zones of the kiln.

The precoat renewal could also be made using high pressure water nozzle that regenerates the precoat partially or totally. By using these high pressure water nozzles could a small portion of the precoat be renewed at the time. One such design is shown in the Squirt™ system sold by Metso Paper, which is shown in SE470227 (=US5759397).

Using disc filters instead of drum filters for washing lime mud is also shown in

SE512666, where the continuous feed of washed and drained lime mud is made trough an elastic chute, which prevents the washed and thickened lime mud from sticking onto the walls of the chute.

The lime mud slurry to be washed and thickened in a disc filter is typically fed into the vat of the lime mud filter, where the disc shaped filter members are located. The filter discs rotate in the vat which is filled with lime mud slurry up to a level of 35-60% and preferably 40-50% of the diameter of the discs. By the applied pressure difference between the vat and the inside of the filter discs, is the lime mud slurry flowing through the discs while initially forming a precoat on the filter surface of the discs, and thereafter forming additional layers of a lime mud cake. The lime mud cake formed on the filter surface is also continuously exposed to a displacement wash by wash nozzles spraying wash water (or filtrate) on the lime mud cake. The thickened and washed lime mud cake is finally and continuously taken off by using scrapers mounted on each side of the disc, and above the lime mud slurry surface in the vat. The scraped off lime mud cake then falls into a chute towards an outlet in the lime mud filter for thickened and washed lime mud.

There are several options for forwarding the washed lime mud to the kiln. In old conventional system is the washed lime mud re slurred again, and stored in a liquid state in intermediate storage reservoirs. Before being fed to the lime kiln is then the re slurred lime mud led to a dewatering apparatus, which in turn feeds the dewatered lime mud to the kiln. Another option is to let the thickened lime mud from the lime mud wash fall onto a conveyor belt that feeds the thickened lime mud directly, or indirectly via dry storage bins, to the kiln. A further improvement of using dry outfeed from pressurized lime mud filter is shown in SE530768, where the washed and thickened lime mud from the lime mud wash is fed out via a elastic tube valve, enabling a pressure lock established by a lime mud plug in the outlet.

The filtrate obtained from the discs is led to a hollow central shaft of the filter and into a storage container. The filtrate stored in this container has normally certain residual alkali content, and is often referred to as weak liquor. This weak liquor is most often used in preceding caustization stages, or used as the driving liquid for the high pressure water nozzles in the lime mud filter, such as shown in SE525450.

The final lime mud wash is implemented in order to recover any residual alkali in the lime mud, thus obtaining a weak liquor to be used in the recaustizing process.

Handling of the thickened lime mud is very problematic as it creates a dust-laden environment if open transport systems are used (for example if open conveyors are used). The thickened lime mud is also causing bridging problems in storage containers, invoking interruption of feed to the lime kiln and exposure of excessive temperatures in the kiln due to inadequate filling. Most washing of lime mud is implemented with the objective of improving the washout of residual alkali. Therefore is also 2-stage washing of the lime mud made in older conventional lime mud washing, often using drum filters in series. Another approach is shown in WO 2206/056649 where a single disc filter is redesigned as two separate halves, and wherein these two halves are arranged such that the lime mud is washed and thickened in a first half, and thereafter re slurred and washed and thickened in the second half. However, arranging filtering in 2 stages in series in one common filter is well known from GB939975 (from 1958), and also shown later for washing pulp in EP683686 where an adjustable partition member could divide the disc filter into several stages.

Another problem when operating lime mud washing filters is the tendency of building up hard cakes of solid lime mud cakes in the vat of the filter, regardless if the filter is a drum or disc filter. These hard mud cakes tend to form in parts of the vat with low flow velocity, i.e. wake areas, as the denser lime mud particles tend to sink and accumulate into these hard mud cakes. Several proposals for solving this problem has been proposed.

In WO9816292 is shown an eductor in the bottom of the vat for introduction of new lime mud slurry to be treated. This eductor is located between each filter disc, but is severely restricted in capacity as the volume of new lime mud slurry to be introduced into the filter is limited by the total outflow from the filter, and thus limits the effect possible.

Another solution is shown in US5849202 where supply of slurry to be filtered is made in between the discs in the filter in a direction downwards, and establishment of a counter flow from the bottom of the vat in this region. The slurry to be filtered is circulated externally in a loop having 2 pumps in series.

However, improving circulation of the lime mud slurry in the vat, especially by using pumps, is not wanted as passage trough pumps may cause the lime mud particles from being crushed, which negatively decrease filterability, and increase content of lime mud particles in the filtrate obtained from the lime mud wash and quicker blinding of the filter media, needing regeneration of the precoat layer and thus an interruption of the filtering process. Yet another system has been put in use where so called "mammoth"-pumps are installed between discs. Here is air continuously blown upwards and into the bottom of a vertical pipe, thus creating an ejector effect on the lime mud at the bottom of the vat. By using the "mammoth"-pumps is not the filterability of the lime mud slurry negatively affected to the same degree as if centrifugal pumps pumping the lime mud slurry where used.

Even if these measures are implemented are mud cakes built up in the vat, and the filter vat need thorough cleaning in order to break up these lime mud cakes. Most often are these vat cleaning procedures implemented at the same time as

regeneration of the precoat layer, and the entire filtering process and feed of thickened lime mud to the kiln is interrupted.

Summary of the invention

The object of the present invention is to provide a system and method for washing and thickening of lime mud which avoids the above mentioned drawbacks. One important objective is to obtain an even feed of washed and thickened lime mud to the kiln in order to maintain the integrity of the kiln and still have a continuous production using only one lime mud filter.

Another objective is to obtain a system and method that do not negatively decrease the filterability of the lime mud slurry. If the lime mud slurry is handled gently could an efficient dewatering and washing process be implemented that could result in very clean lime mud being fed to the kiln, being as clean as could be obtained in any series washing of lime mud handled more aggressively.

According to the invention the lime mud slurry to be dewatered and washed is fed into at least 2 independent compartments of the filter in parallel, and the vat of one of these compartments could be flushed out while the others are still in operation, thus enabling an uninterrupted feed of washed and thickened lime mud to the kiln.

Breif description of the Drawings

In the following, the system and the method according to the invention are described with reference to the enclosed figures, of which Fig. 1 show a preferred embodiment the components of the disc filter system according to the invention, and

Fig. 2 show a schematic layout of a control for the system.

Detailed description of the invention

In figure 1 is an embodiment of the inventive disc filter system shown comprising a disc filter 1 having a vat 16, preferably a pressure vessel, storing a volume of lime mud slurry in the vat. Inside the vat are filter discs 2a/2b arranged on a common drive shaft 3a/3b, rotated by any appropriate propulsion system(not shown). The level of lime mud established inside the vat should typically cover up to 35-60% of the diameter of the discs. The filter discs are hollow and have a filter media covering the discs, and the interior of each hollow disc is connected to evacuation channels inside of the shaft 3a/3b. Lime mud slurry to be treated is conventionally stored in a lime mud storage tank 30 of substantial volume, receiving the lime mud slurry from any preceding white liquor filter PDW in the causticizing process. Lime mud slurry is fed to the vat via inlets 33a/33b, using a pump 3 .

As lime mud is dewatered on the filter media on the discs 2a/2b, is a lime mud cake formed on the discs while a filtrate is led into the interior of the discs, and further to an external filtrate storage container 8a/8b. The lime mud cake formed on the discs are first washed by some washing nozzles 22 arranged on the ascending side of the filter discs as the filter discs during rotation thereof leave the lime mud volume. The washing liquid is supplied from a wash liquid tank via pump 21. The dewatered and washed lime mud cake on the discs is then scraped off by scrapers 5 arranged on both sides of the discs at the descending side of the filter discs above the slurry level in the vat as the rotating discs are about to descend into the pool of lime mud slurry. The scraped off lime mud falls down into a lime mud chute 4. The lime mud chute 4 is in turn connected to any appropriate feed means, which in this embodiment is a first lime mud screw 6a/6b that feeds the lime mud to a pressure lock, here shown as a lime mud plug 14a/14b above a sluice feeder 13a/13b. After passing the pressure lock the lime mud is fed to the lime kiln MO with any appropriate feeding means, here shown as a transport screw 15 but other forms of conveyor belts or chains could be used.

The driving pressure differential between the lime mud slurry side of the filter media and the filtrate side of the filter media is in this embodiment established by a gas pump 9. The suction side of this pump is connected to the gas phase of the filtrate tank 8a/8b, and the pressure side is connected to the gas phase of the vat. Thus is the filter pressurized establishing the necessary driving force for the filtration process. Alternatively could the filtrate side of the filter be subjected to sub-atmospheric pressure by using a conventional drop leg.

In order to counteract settling of lime mud cakes in the vat is also so called

"mammoth"-pumps used, having vertical pipes 1 1 with open ends installed in the lime mud slurry volume of the vat and supply of air blown into the bottom of the pipes. In this embodiment is the same gas pump 9 used as that for pressurizing the filter, feeding air to the "mammoth"-pumps via piping 10b but a separate pump could be used as well. In the drawing is only 4 "mammoth"-pumps shown, but there could also be "mammoth' -pumps located between all discs.

According to the invention the filter vat 16 and filter members 2a/2b are divided by at least one partition wall 19 into at least 2 independent parts and that these parts are fed with lime mud slurry in parallel, by individual supply lines from a common lime mud storage tank 30.

The feed of lime mud slurry to each independent part is preferably controlled by first valve means 32a,32b in the lime mud supply, and filtrate withdrawal from the filter members 2a,2b from each independent part is controlled by second valve means 12a, 12b in the filtrate withdrawal.

Further, each independent part has individual flush nozzle inlets 25a,25b, and individual flush drains 26a,26b, and where the nozzle inlets and drains are located at opposite ends of the vat, said nozzles being supplied with washing liquid from a washing liquid tank 20 via third valve means 24a,24b, and said drains being connectible to the common lime mud storage tank 30 via fourth valve means

27a,27b. The system also includes any appropriate control unit connected to the first, second, third and fourth valve means, and during a steady state filtering operation using the independent parts in parallel, are first and second valve means open, and when a single independent part of the filter is to be cleaned are the first and second valve means closed while opening the third and fourth valve means.

In figure 2 is a schematic control system depicted, having a control unit CPU connected to power supply BAT, and appropriate process parameters PP. In the figure is the function of 1^st to 4th valve means (32a/32b, 12a/12b, 24a/24b, 27a,27b), as well as flushing pumps and drains (23,28) and pressure lock (13a, 13b) shown as relays switches controlled by relay R3 and R4. When associated relay switch is closed is the valve open or function active.

Figure 2 show the system in steady state operation when both independent parts of the filter is in operation.

First independent part in operation

The first independent part, the left hand part in figure 2, has supply of lime mud slurry activated in this state, as the valve 32a is open. The pressure differential over the filter media, and thus withdrawal of filtrate, is activated as the valve 12a is open. And finally the out feed of dewatered lime mud is active as the sluice feeder 13a is in operation. In this state are both valves 24a and 27a closed as well as flushing water pumps 23/28.

Second independent part in operation

The second independent part, the right hand part in figure 2, has supply of lime mud slurry activated in this state, as the valve 32b is open. The pressure differential over the filter media, and thus withdrawal of filtrate, is activated as the valve 12b is open. And finally the out feed of dewatered lime mud is active as the sluice feeder 13b is in operation. In this state are both valves 24b and 27b valves closed as well as flushing water pumps 23/28.

When both independent parts are in operation is the dewatering and washing process in progress in both independent parts, and a high production of dewatered and washed lime mud is possible. First independent part in flushing mode

When a flushing operation of any part is to be activated is the control unit closing relay R1. If the first independent part is due to be flushed out from mud cakes, then relay R2 is not closed by the control unit. As a consequence is relay R3 closing, which then causes the supply of lime mud slurry to be shut off, as the valve 32a is closed. The pressure differential over the filter media, and thus withdrawal of filtrate, is also broken as the valve 12a is closed. And finally the out feed of dewatered lime mud is shut off as the sluice feeder 13a is deactivated. The flushing operation is activated as valves 24a and 27a are opened as well as activating flushing water pumps 23/28.

Second independent part in flushing mode

If instead the second independent part is due to be flushed out from mud cakes, then both relay R1 and R2 are closed by the control unit, and as a consequence is relay R4 closing. The closing of relay R4 causes the supply of lime mud slurry to be shut off, as the valve 32b is closed. The pressure differential over the filter media, and thus withdrawal of filtrate, is also broken as the valve 12b is closed. And finally the out feed of dewatered lime mud is shut off as the sluice feeder 13b is deactivated. The flushing operation is activated as valves 24b and 27b are opened as well as activating flushing water pumps 23/28.

The invention could be modified in a number of ways in relation to the preferred embodiment shown above. As an example could the disc filter be divided into more than 2 independent stages.

Additional system control features could also be implemented. For instance could the out feed rate from the independent part still being in the dewatering and washing mode be increased, while any other independent part is due for flushing operation. This could be implemented by decreasing the distance between the scraper 5 and discs 2a/2b towards a distance equal to the precoat layer. The pressure differential could also be increased, promoting the feed rate of filtrate trough the filter media.

Claims

PATENT CLAIMS

1 . A system in a causticizing process for washing and thickening a lime mud slurry being fed from a lime mud slurry container (30), said system comprising a filter vat (16), at least one inlet (33a,33b) for lime mud slurry in the filter vat establishing a level of lime mud slurry in the vat, at least one hollow filter member (2a,2b) arranged inside the vat, an outlet (3a, 3b) from the hollow filter member for filtrate obtained, at least one washing nozzle (22) for spraying washing liquid on a thickened lime mud cake formed on the filter member, and scraper means (5) arranged over the level of lime mud slurry in the vat for scraping off a washed and thickened lime mud cake from the filter member surface, a chute (4) under each scraper for feeding out thickened and washed lime mud scraped off by said scraper, characterized in that the liquid volume of lime mud slurry in the filter vat is divided by at least one partition wall (19) into at least 2 independent liquid volumes and that these parts are fed with lime mud slurry in parallel, by individual supply lines from at least one lime mud storage tank (30a, 30b).

2. A system according to claim 1 characterized in that the feed of lime mud slurry to each independent part is controlled by first valve means (32a, 32b) in the lime mud supply, and filtrate withdrawal from the filter members (2a,2b) from each independent part is controlled by second valve means (12a, 12b) in the filtrate withdrawal.

3. A system according to claim 2 characterized in that each vat part separated by said partition wall (19) has individual flush nozzle inlets (25a, 25b), and individual flush drains (26a,26b), and where the nozzle inlets and drains are located at opposite ends of each respective vat part, said nozzles being supplied with washing liquid from a washing liquid tank (20) via third valve means (24a, 24b), and said drains being connectible to at least one lime mud storage tank (30a, 30b) via fourth valve means (27a,27b).

4. A system according to claim 2 characterized in that lime mud slurry to each vat part is fed by feeding means (31 a,32a and 31 b, 32b) from an individual lime mud storage tank (30a, 30b), and that the vat part in question is also drained to the same individual lime mud storage tank (30a, 30b) by draining means (26a,27a,28a respectively 26b,27b,28b).

5. A system according to claim 4 characterized in that a control unit is connected to the first, second, third and fourth valve means, and during a steady state filtering operation using the independent parts in parallel, are first and second valve means open, and when a single independent part of the filter is to be cleaned are the first and second valve means closed while opening the third and fourth valve means.

6. A system according to claim 5, characterized in that the hollow filter members are discs arranged on a rotating shaft and that filtrate from the discs of an independent part is drained to an individual filtrate tank compartment (8a, 8b) via the shaft (3a, 3b), and wherein the vat (16) is a pressure vessel surrounding the discs, said vat being pressurized by a gas pump (9) being connected between the gas phase of the filtrate tank compartments (8a, 8b) and the gas phase of the vat, said gas pump being connected to each filtrate tank compartment via fifth valve means (12a, 12b).

7. A system according to claim 6, characterized in that the a pressure lock (13a, 13b/14a, 14b) is arranged below the lime mud chute (4), and before feeding means (15) to the lime kiln (MO).

8. A method in a causticizing process for washing and thickening a lime mud slurry comprising the steps of feeding a lime mud slurry to a filter vat (16) and establishing a level of lime mud slurry in the vat, extracting a filtrate from the lime mud slurry via hollow filter members (2a,2b), forming a thickened lime mud cake on the filter members and washing the lime mud cake formed on the filter member before scraping off the thickened and washed lime mud cake and subsequently feeding out the thickened and washed lime mud, characterized in that the liquid volume of lime mud slurry in the filter vat is divided into at least 2 independent vat parts and that these vat parts are fed with lime mud slurry in parallel, by individual supply lines from at least one lime mud storage tank (30).

9. A method according to claim 8 characterized in that the feed of lime mud slurry to each independent vat part is controlled independently from each other from at least one lime mud supply, and filtrate withdrawal from the filter members (2a,2b) from each independent vat part is controlled independently from each other in the filtrate withdrawal.

10. A method according to claim 9 characterized in that each independent vat part has individual flush liquid supplies (25a,25b), and individual flush drains (26a,26b), and where each flush liquid supply and drains are located at opposite ends of the vat establishing a flushing flow in the vat, said flush liquid supply being made from a washing liquid supply, and said drains being connectible independently to at least one lime mud storage tank.

1 1 . A method according to claim 10 characterized in that the feed of lime mud slurry and filtrate withdrawal is made in parallel to each independent part during a steady state filtering operation, and during a flushing operation of one single independent part of the filter is the feed of lime mud slurry and filtrate withdrawal to the single independent part of the filter interrupted while activating the individual flush liquid supplies and individual flush drains of that single independent part of the filter.

12. A method according to claim 1 1 , characterized in that the filtrate from an independent part is withdrawn via hollow disc filter members arranged on a rotating shaft, said filtrate being drained to an independent filtrate tank compartment, and wherein the vat part of the independent part is pressurized by pressurizing and forwarding gas from the gas phase of the independent filtrate tank compartment to the gas phase of the vat part of the independent part.

13. A method according to claim 12, characterized in that the a pressure lock is established after the lime mud chute, and before feeding the lime mud to the lime kiln (MO).