WO2014037187A1

WO2014037187A1 - Slurry dewatering device

Info

Publication number: WO2014037187A1
Application number: PCT/EP2013/066700
Authority: WO
Inventors: Joseph Thomson
Original assignee: Siemens Vai Metals Technologies Gmbh
Priority date: 2012-09-06
Filing date: 2013-08-09
Publication date: 2014-03-13
Also published as: GB2505670A; GB201215911D0

Abstract

A slurry dewatering device comprises a filter drum (31) mounted for rotation about an axis of rotation (37) and a conveyor (32) coaxial with the drum. A slurry inlet 36 is provided at an inlet end of the drum; and a slag outlet (33) is provided at an outlet end of the drum. The drum (31) and the conveyor (32) are mounted for rotation independent of one another. The slurry inlet and slag outlet are at the same end of the drum.

Description

SLURRY DEWATERING DEVICE

This invention relates to a device for dewatering slurry, in particular for blast furnace slag slurry, or other non ferrous dewatering including in the fields of dredging, mining, water technologies; fuming, smelt or other type of furnaces; and processing slag from smelters.

Granulating blast furnace slag with high pressure water, known as wet slag granulation, rapidly cools and granulates the slag into a product that has value, for example for use in the cement industry. After the slag has been granulated, hot slurry is formed that typically has a high water to slag ratio in the range of4:l to 10:1, typically 8: 1.

In order to get the slag in a usable form, the water must be removed. This process is known as dewatering. In addition, it is desirable to be able to reuse the water in the process, or other purposes, so an additional filtering stage is also required.

Conventional slag slurry dewatering devices generally have a large footprint due to the number of stages involved.

US2007107466 describes a series of dewatering steps after slag granulation, using drum filters or inclined screw conveyors, where the dewatered granules and removed process water are stored separately along the way.

US4204855 describes dewatering granulated slag by rotating a drum with inwardly projecting vanes to carry granulated slag upwards away from the slurry and subsequently deposit it for removal.

In accordance with the present invention a slurry dewatering device comprises a filter drum mounted for rotation about an axis of rotation; a screw conveyor coaxial with the drum; a slurry inlet at an inlet end of the drum; and a slag outlet at an outlet end of the drum; wherein the drum and the conveyor are mounted for rotation independent of one another; and wherein the slurry inlet and slag outlet are at the same end of the drum.

A screw conveyor inside a filter drum is able to rotate about the axis of rotation independently of the drum and arranging the slurry inlet and slag outlet at the same end allows for a more compact design with the drive mechanism mounted outside the harmful environment of the slurry . Preferably the device further comprises a pipe coaxial with and inside the conveyor.

Preferably, a slag filter bed is formed between the mesh filter of the filter drum and the conveyor.

The mesh drum filter and screw conveyor provide a structure for a slag filter bed to form, further improving the filtering effect.

In one embodiment, the drum and conveyor rotate in the opposite direction.

In an alternative embodiment, the drum and conveyor rotate in the same direction.

Preferably, the conveyor is mounted on the pipe.

Slurry from the inlet passes down the pipe and is then dewatered as it passes up to the outlet between the screw conveyor and the mesh drum.

Preferably, the mesh filter of the filter drum comprises a metal mesh.

Preferably, the speed of rotation of the drum is less than the speed of rotation of the conveyor.

Preferably, the mesh filter is shorter than the drum.

The drum may be formed of a solid section of small diameter and a filter section of larger diameter, in order reduce the overall size.

Preferably, the pipe is shorter than the conveyor.

This ensures that the slurry outlet is within the drum.

Preferably, the drum further comprises vanes mounted to its inner surface.

These reduce filter wear and help in carriage of fines along the device.

Preferably, the filter drum comprises baffles along its length.

These help to promote a water content gradient along the length of the device.

Preferably, at least one of the drum and the conveyor are conical, tapered or staggered in diameter.

Modifying the shape of either the drum or the conveyor in these ways simplifies the mechanical requirements for mounting.

Preferably, the screw conveyor is inclined relative to the top surface of a hot water tank on which the conveyor is mounted.

The present invention enables dewatering of blast furnace slag and filtering of process water in a single stage with a device that can be located above the hot water tank in order to minimize plant footprint and maximize plan layout options An example of a slurry dewatering device will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a typical slag granulation plant, such as in US2007107466;

Figure 2 shows an example of a slag granulation system including a slurry dewatering device according to the present invention;

Figure 3 illustrates in more detail, one embodiment of a slurry dewatering device according to the present invention for use in the system of Fig.2;

Figure 4 illustrates in more detail, another embodiment of a slurry dewatering device for use in the system of Fig.2;

Figure 5 illustrates an enhancement to the embodiments of Figs.3, 4 and 6; and,

Figure 6 illustrates a further embodiment of a slurry dewatering device according to the present invention for use in the system of Fig.2.

Fig.l illustrates an example of a conventional arrangement whereby slag from a blast furnace is passed to a granulation device and cooled by water sprays. Various dewatering stages, using a dewatering screw, then a filter, split the granules from the water and store the granules and the water separately at each stage. The collected water is topped up and cooled, then returned to the granulation device as process water.

However, the multiple stages and sequential processing mean that a large amount of space and materials are required to build a plant in this form.

Hot slag from a blast furnace and/or a smelting reduction plant is passed through a slag channel 1 , in the direction indicated by the arrow, to a granulation device 2, where it is cooled by spraying in water. The granule/water mixture formed passes via a granulation tube 3 into a granulation tank 4 and, from there, through a passage 5 into a granule dewatering installation 6a and 6b, and water basins 7a-7c. In the dewatering installation, the granules are dewatered and the slag sand is stored at storage areas 8a and 8b. The water which is separated off in the water basins 7a-7c, after replacement of the losses and cooling in a cooling tower 11, is returned as process water from the collection tank 10 of the cooling tower 11 via a line 9 to the granulation device 2.

Bucket conveyor technology is effective at dewatering and filtration, but has high service and maintenance requirements. Simpler devices, such as bucket wheels and simple grab systems are good at removing the solids from the water, but not particularly effective at filtering the process water. This has not been considered a problem in systems without cooling towers and condensation systems, because the problem of sprays clogging up does not arise. These low cost systems use more make up water, but require less efficient slurry or gravel pumps and expensively lined pipes.

The design of the present invention has been chosen to produce a compact system for continuously separating water from granulated slag slurry, which also combines the dewatering and filtration stages in one device. This results in the overall footprint and use of concrete for foundations and waterways being reduced. In addition, the design results in lower fines generation and improved filtration of fines. Small fines that pass through the drum filter mesh are separated in the hot well and pumped back into the slurry dewatering device. The small fines are pumped towards the outlet of the drum to allow them to become trapped in the concentrated slag slurry at that end.

An example of a slag granulation system including a dewatering device according to the present invention is illustrated in Fig. 2. Molten slag flows from the blast furnace along slag runners from the cast house towards the blowing box 21 where it is rapidly granulated and cooled by pressurized water jets 20. Further granulation and heat transfer take place in the granulation basin 22 and steam, H₂S and S0₂ are released. A condensation tower 41 primarily helps to reduce the emission of H₂S and S0₂ into the atmosphere while also condensing steam in order to keep make up water to a minimum.

After granulation, hot slurry is formed with a high water to slag ratio. The slurry enters a slurry dewatering drum and filter device, such as any of the examples subsequently described in more detail. In the example shown in Figs.2 and 3, a screw conveyor 32 is mounted within a mesh filter drum 31 , such that both are able to rotate about the same axis 37. Slurry enters 36 the dewatering device under gravity through the centre of the screw 32 from agitation basin 22, typically down a pipe 34. Slurry emerges from an outlet 27 of the screw or pipe, at least part way down the length of the drum. As the screw conveyor 32 rotates, the slag slurry is transported over the mesh filter 31 which allows water to drain into a hot well 26. A gap between the screw conveyor and the outer mesh filter allows a layer of slag to form a filtration bed that helps to remove the fine particles from the cooling water as well as providing stone box effect protection for the filter mesh. The filtration bed allows improved water quality and hence eliminates the need for a complicated hot water tank with associated lamellar separator, recirculation and agitation pumps. The option remains for a fines

recirculation system to be incorporated, if further filtration is required, but the device of the invention operates well without the need for this in most situations. In order to reduce harmful emissions into the atmosphere the device is fully enclosed with a hood 35. A pipe 38 allows steam and H₂S and S0₂ to be captured in the condensation tower 41.

When the granulated slag slurry leaves the screw conveyor 32 through outlet 33, it is pulled over the outer mesh drum 31 towards the mesh drum outlet 24 where it is transported by conveyor 25 to a heap, storage silos or an area where it can be taken away by transport. As the slag slurry is pulled over the mesh drum, the water drains through the slag bed and the filter mesh and into the hot well 26. Hot water is pumped from the hot well 26 to cooling towers 29. Cold water collects in the cold well 28 at the base of the cooling towers and is recycled in the system and pumped to condensation sprays 39 and the blowing box granulation sprays 20. An emergency water tank 40 is located at the top of the condensation tower, if there is a problem in the system, this allows enough water to be able to granulate slag and condense fumes until the flow of slag can be diverted into a slag pit.

It is desirable to achieve low moisture content in the granulated slag product as less additional water, known as make up water, needs to be added into the granulation process to replace water lost due to evaporation or in the slurry mixture. Furthermore, low moisture content means less energy is required to dry the product further downstream making the product more attractive to the customer. It is also desirable to achieve low particle content in the process water, as the cleaner the process water is, the finer (and hence more efficient) the sprays can be within the cooling and

condensation systems. This allows a reduction in the overall size of plant as well as costs associated with ceramic pipe linings, slurry/gravel pump costs & inefficiencies as well as reducing the chances of blockages and cleaning problems within the

granulation, cooling and condensation system sprays, pumps, pipes and tanks.

In one example of the present invention, as illustrated in Fig.3, a slurry dewatering device is mounted above the hot water tank, or hot well 26. Pumps 45 pump the hot water from the hot well to the cooling towers 29. The device is supported on a bearing 30 at one end, and rollers 34 at the other end. A hydraulic drive motor 41 turns the inner screw 32 and a drive motor/mechanism 43 turns the outer drum. A bearing & seal arrangement 42 allow the drum 31 to rotate independently of the conveyor 32, but about the same axis 37, with a seal arrangement to stop the bearing suffering from water/slag damage. The drum comprises a wire mesh filter, which allows water to drain through the mesh, almost immediately, so that pooling of water in this section is minimised. The mesh drum filter 31 is mounted for rotation about the axis of rotation 37 and is able to rotate independently of the rotation of the screw 32.

The screw conveyor 32 comprises a central void, or tube 34 to carry the slag slurry from the inlet 36 to the outlet 27. The tube runs along most of the length of the device and slurry flows into the lower end of the device from an outlet end 27 of the tube. Around the tube 34 and optionally extending beyond the outlet end, is the conveyor 32. The conveyor is typically in the form of flights forming a screw, which may be mounted directly onto the tube, or flights of the screw may be self-supporting, particularly towards the outlet end of the screw. The screw may be open ended to allow slurry flow. Optionally, the tube may also have holes cut for slurry flow in the outlet end 27. The tube may be fixed if the conveyor is self supporting, or may rotate in order to rotate the screw conveyor 32, if the screw is mounted on the tube.

In this example, the device is inclined relative to the top surface of the hot water tank 26, so that the end supported on the water tank is lower than the slurry inlet end 36 of the device. The slurry inlet end is mounted on a support 23 which also allows access for monitoring and maintenance. Preferably, the device is mounted inclined at an angle of about 20 degrees, but any angle in the range of 0 degrees to 25 degrees may be used. The angle used is a trade off between dewatering efficiency and power consumption. Values at the lower end tend not to give such good drying effect because more of the length of the screw is in the region where water is draining out and values at the higher end mean that the screw has to do more work and hence requires much more power. However, the precise angle can be adapted to the specific requirements of the plant, which is particularly useful when retrofitting. Dewatered slag is removed from the device at the same end as the slurry by an outlet in the mesh filter via a chute 24 onto a dry slag conveyor 25.

Fig. 4 illustrates another embodiment of the present invention. In Fig.3, the structure of the drum included the filter mesh along the majority of or all the length, with the option of omitting the mesh right at the outlet end, where the filtering has been completed. In the example of Fig.4, the outer drum comprises a mesh filter section 31 and a solid section 46 towards the slag outlet end 33. The diameter of the mesh drum filter is typically larger than that of the solid casing 46, allowing a replenishable slag filter bed to form around the inner surface of the drum and for water in a lower part of the filter to drain out. Preferably, the filter is also provided with a cleaner 47 fitted externally, for example an array of compressed air jets, or a traversing array of the type described in our co-pending patent application no GB1212819.5, using liquid. In this example, the outer drum 31 rotates at a rate of approximately 1 revolution per hour to 3 revolutions per minute and rotates in the opposite direction to the conveyor 32, which rotates at a significantly higher speed, in the order of 6 revolutions per minute. The precise speed of rotation is chosen according to operational requirements. If the traversing type cleaner array, using liquid, rather than air jets, is in use, then a controller prevents operation of cleaning fluid flow in the section closer to the tube slurry inlet and dewatered slag outlet, so that the spray is only applied between dewatering batches. A gutter arrangement can be used towards the outlet to help catch cleaning to help maintain minimum slag water content. This maintains the moisture content gradient along the length of the drum, preventing the slag closer to the outlet 33 being made wet again. In view of the slow speed of rotation of the mesh drum, the drum is cleaned in bands or sectors parallel to the axis of rotation.

The slag collects on the inner surface of the drum and acts as a wear liner, as well as a continuously replenished filtration bed. This reduces wear on the drum filter mesh. In addition, vanes 49, 50 may be provided on the inner surface of the wire mesh drum filter to help reduce wear and to carry fines to the top of the screw. Fig.5 shows in more detail the provision of vanes or paddles 49, 50 on the inner surface of the drum and between the drum and the screw to reduce mesh wear and to help carry fines to the top of the screw. In this example, the replenishable slag filter bed depth 48 extends between the inner surface of the drum 31 and the outmost edge of the screw conveyor flights. Water drains through the continuously replenishable filter bed and dry slag is creamed off from the top of the bed where it is driest.

The screw conveyor provides dewatering and can be made to be shorter than in prior art screw conveyors, such as used in US20070107466, because the water collection lower down the screw is minimal and the dual dewatering processes by the filter bed and the screw operate in parallel. The outer drum 31 rotates slowly and drops fines that have worked their way towards the bottom filter bed onto the top side of the screw conveyor 32, which are then carried up by the screw to the outlet 33. Low moisture content in the granulated slag product is desirable as less make up water is required as well as less energy to dry the product further downstream.

Fig.6 illustrates a screw conveyor 53 and filter drum 52 which are constructed with a tapered, or a conical shape. The drum 52 and screw 53 rotate independently as in the previous embodiments, but the shape allows them to be mounted about the same horizontal axis 37. This is beneficial as it is mechanically simpler if the mountings and bearings are horizontal, rather than at an angle under load. The tube 53 is illustrated as having a constant outer diameter, but the tube may be tapered on the inside to promote flow of slurry towards the slurry outlet end. The conical, tapered or staggered shape promotes a water content gradient along the length of the drum 52 with saturated slag at the tube outlet 27 and dewatered slag at the drum outlet 33. Water drains through the filter 52, as indicated by the arrows 54 and dewatered slag is removed to the outlet 33 as before. Baffles may be used along the length of the drum to stop water from flowing towards the dry product outlet 33

The invention has a number of advantages over the prior art by virtue of its construction. Locating the rotating filter drum coaxially and on the outside of the dewatering screw eliminates the fixed water level typically maintained at the base of the dewatering screw. Removing the water level helps dehydrate the slag more quickly and allows a shorter length screw to be used and a corresponding reduction in site footprint, as well as keeping the lower bearing out of the slurry. In addition, the combination of this location for the rotating filter drum and removing the water line allows fines and wool that would normally float in the water to be removed by the screw. Furthermore, the location of the filter drum makes possible a compact layout which reduces the amount of concrete required on a RASA system and uses a shorter length of screw and allows the unit to be positioned directly above a single hot water tank for immediate dewatering and filtering.

Using the centre pipe of the screw as the slurry inlet allows the drum to fully enclose the screw which eliminates the need for large seals that would be exposed to a harsh abrasive environment. The abrasive slurry is kept away from bearings and minimises the number and size of exposed seals and bearings required in the plant.

Allowing the filter drum to rotate independently of the screw allows better control for varying flows and varying water to slag ratios and the slower speed of rotation compared with convention drums has the advantage of maintaining a more effective slag filter bed. The filter bed acts as a self filtration bed which helps to filter the fine particles and improves filtration of slag fines and wool and hence water filtration quality, reducing wear on pipes, pumps and reducing the likelihood of blockages in sprays, or the blowing box, with consequent efficiencies in cooling and condensation systems. The fine particles pack together under the screw and as the outer drum rotates and the fines reach the top of the drum they drop under gravity and are carried away by the screw. The dual effect dewatering both by the drum and the screw is not possible in the prior art systems. The screw does the bulk of the work and the drum lifts fines to the top of the screw for removal. In addition, the filter bed protects the filter screen by creating a stone box effect wear lining and hence problems with wear and mesh splitting are minimised.

Claims

1. A slurry dewatering device comprises a filter drum mounted for rotation about an axis of rotation; a screw conveyor coaxial with the drum; a slurry inlet at an inlet end of the drum; and a slag outlet at an outlet end of the drum; wherein the drum and the conveyor are mounted for rotation independent of one another; and, wherein the slurry inlet and slag outlet are at the same end of the drum.

2. A device according to claim 1 or claim 2, further comprising a pipe coaxial with and inside the conveyor.

3. A device according to any preceding claim, wherein a slag filter bed is formed between the mesh filter of the filter drum and the conveyor.

4. A device according to any preceding claim, wherein the drum and conveyor rotate in the opposite direction.

5. A device according to any of claims 1 to 3, wherein the drum and conveyor rotate in the same direction.

6. A device according to at least claim 2, wherein the conveyor is mounted on the pipe.

7. A device according to any preceding claim, wherein the mesh filter of the filter drum comprises a metal mesh.

8. A device according to any preceding claim, wherein the speed of rotation of the drum is less than the speed of rotation of the conveyor.

9. A device according to at least claim 2, wherein mesh filter is shorter than the drum.

10. A device according to at least claim 2, wherein the pipe is shorter than the conveyor.

11. A device according to any preceding claim, wherein the drum further comprises vanes mounted to its inner surface.

12. A device according to any preceding claim, wherein the filter drum comprises baffles along its length.

13. A device according to any preceding claim, wherein the at least one of the drum and the conveyor are conical, tapered or staggered in diameter.

14. A device according to any preceding claim, wherein the screw conveyor is inclined relative to the top surface of a hot water tank on which the conveyor is mounted.