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 an inclined drum with inwardly projecting vanes to carry granulated slag upwards away from the slurry and subsequently deposit it for removal.

JP54104500 describes an inclined dewatering drum which relies on gravity for transporting the slag to the exit. However, this arrangement may suffer in high flow volumes, so that if the drum is not rotated sufficiently quickly, both water and slag will exit the far end of the drum without effecting separation.

In accordance with the present invention a slurry dewatering device comprises a drum mounted for rotation about an axis of rotation; one or more helical baffles mounted on an inside surface of 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 is mounted such that its axis of rotation is level; and wherein the device further comprises a drum rotation mechanism mounted outside the drum. The present invention is able to dewater the blast furnace slag and filter the process water in a single stage with a device that can be located above the hot well in order to minimize plant footprint and maximize plan layout options, whilst the drum rotation and support mechanism is protected from contamination.

Preferably, the drum rotation mechanism comprises a driven wheel coupled to the drum and outside one of the inlet end or outlet end of the drum and a corresponding drive wheel.

The drive wheel may be sited at either end of the device and the support wheel mechanism at the other end is not driven.

Preferably, the support is mounted on a hot well.

Preferably, the device further comprises inlet baffles at the inlet end of the drum.

These protect the drum mesh from impact damage due to the slurry falling through a long drop at the inlet end.

In one embodiment, the pitch of the helical baffle varies along the length of the baffle.

Preferably, the pitch of the helical baffle reduces from the inlet end to the outlet end of the drum.

Preferably, the pitch of the helical baffle varies over a range of 3m to 0.5m Preferably, an external thread diameter of the helical baffle remains constant along the length of the baffle

Preferably, an internal thread diameter reduces along the length of the baffle. In another embodiment, the pitch of the helical baffle remains constant along the length of the baffle.

Preferably, an external thread diameter of the helical baffle varies along the length of the baffle.

The helical baffle may comprise a continuous helix, but alternatively the helical baffle further comprises slots along its length.

Preferably, the slots are positioned between each change in pitch of the helical baffle.

The device may comprise one or more baffles and for a baffle with slots, there may be multiple baffles, e.g. 10 or more, but preferably, the device further comprises a second helical baffle forming a double helix. Preferably, the drum further comprises a mesh screen.

Preferably, the mesh screen comprises a pair of parallel meshes of different thread count.

This enables fines to be collected and retained during rotation of the drum. Preferably, the device further comprises a collection tank beneath the drum.

This allows filtered water to be collected and recycled.

Preferably, the device further comprises a ramp at the slag outlet whereby dewatered granulated slag is output to a conveyor.

Preferably, the device further comprises a variable speed drive.

In normal operation the drive speed is constant, but in periods of high slurry flow, the drive speed can be increased to increase the speed of rotation of the drum.

Preferably, the device further comprises an array of mesh cleaning nozzles mounted outside the drum.

In accordance with a second aspect of the present invention, a slag granulation system comprises a slurry dewatering device according to the first aspect.

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;

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

Figure 3 illustrates a first example of a slurry dewatering device according to the present invention;

Figure 4 illustrates part of the slurry dewatering device of Fig.3 in more detail; Figure 5 illustrates another part of the slurry dewatering device of Fig.3 in more detail;.

Figure 6 illustrates an alternative embodiment of a slurry dewatering device according to the present invention;

Figure 7 shows part of the slurry dewatering device of Fig.6 in more detail; Figure 8 shows another view of part of the slurry dewatering device of Fig.6 in more detail;

Figures 9a and 9b show more detail of part of the slurry dewatering device of

Fig.6; Figure 10 is a partial cross-section through the drum of a slurry dewatering device according to the present invention; and,

Figure 11 shows another view of part of the slurry dewatering device of the present invention.

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.

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 casthouse towards the blowing box 20 where it is rapidly granulated and cooled by pressurized water jets 21. Further granulation and heat transfer take place in the granulation basin 22 and steam, H2S and S02 are released. A condensation tower 23 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 1 according to the present invention under gravity by an inlet 6. As the device 1 rotates, the slag slurry is transported over the mesh filter which allows water to drain into the hot well 24. Any fines that have passed through the filter mesh are separated in the hot well 24 and reintroduced back into the device 1 by a fines return line and pump 25. Fines are introduced back into the outlet half of the device 1. In order to reduce harmful emissions into the atmosphere the device 1 is fully enclosed with a hood 26. A pipe 27 allows steam and H2S and S02 to be captured in the condensation tower 23.

When the dewatered granulated slag product leaves the device through outlet 12, it is transported by conveyor 28 to a heap, storage silos or an area where it can be taken away by transport.

Hot water is pumped from the hot well 24 to cooling towers 29. Cold water collects in the cold well 30 at the base of the cooling towers and is recycled in the system and pumped to the condensation sprays 31 and the blowing box granulation sprays 21. An emergency water tank 32 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.

Fig.3 illustrates an example of a device according to the present invention, with more detail in Figs. 4 and 5. Unlike the prior art systems which use inclined screws and drums, the device 1 of the present invention is designed to be mounted

substantially horizontally, preferably above a hot well 24, with access for water removed by the dewatering process to pass into the hot water tank through a shell of a rotating drum 3. The filtered water is pumped back around the system into cooling towers for reuse in the slag granulation process. In normal operation, the drum is rotated at a fixed speed, but this can be adjusted to cope with variations in the throughput rate. A typical speed of operation is anything up to 7 rpm to accommodate varying slag flows from 1 or 2 tapholes. Slag surges can be addressed by increasing the speed of rotation for a period.

The drum is fabricated from metal, such as steel. Mounted on the inside of the rotating drum and attached to the drum is a helical baffle 9, co-axial with the drum. The baffle preferably comprises double helix flights, although single or triple helix flights could also be used. Following the slag granulation process, the granulated slag slurry enters the drum at an inlet end 5 of the device through an inlet pipe 6, arranged substantially co-axially with the drum. At this stage the water content is approximately 70% to 90% of the slurry and solid granulate and fines only makes up 10%> to 30%> of the mixture.

In the example of Fig.3, the inlet end 5 of the drum 3 and the inlet pipe 6 are supported on mounts 41 on the structure 40 of the hot well 24. A driven wheel 42 is also mounted on the mounts 41. The driven wheel 42 engages with teeth on a ring 43 around the inlet pipe 6 and coupled to the drum 3 to cause the drum to rotate when the driven wheel 42 of the drive mechanism is operated. A corresponding arrangement may be provided at the outlet end, but typically drive is only applied at one end, so in this example, at the outlet end, the wheel 44 and ring 45 are not driven. More detail of the structure of the drum and its operation is shown in Figs.4 and 5.

The slurry is directed to a lower surface 7 of the drum 3 onto a mesh screen 8 fitted between adjacent baffle flights 9, or alternatively the drum itself is constructed from a mesh and the baffle is fitted to reinforced sections along the drum. The mesh is typically stainless steel although other materials, such as mild steel or non metallic membranes could be used. The mesh screen panels may be constructed from spaced parallel meshes with a lower thread count on the inner panel in order to trap a layer of slag and filter the water through the layer before reaching the standard mesh screen. This arrangement assists in holding the layer of slag in place as the drum rotates and so also reduces wear from tumbling and allows more small fines to be captured. Another aid to filtering is reducing the pitch to maintain a height of slag in the drum to reduce the surface area of slag exposed to the mesh to give the small fines a better chance of being caught up in the rest of the slag. The inlet pipe 6 may be angled downwards for the purpose of directing the slurry to the first mesh screen 8 closest to the inlet end 5. The slurry sits between the flights 9, and as the drum rotates, the flights direct the slurry along the length of the baffle in the drum. As the slurry is moved, water is drained from the system and the transported slag slurry volume is reduced. At the far end of the drum, the dewatered granulated slag exits the drum at the outlet end 11 via a ramp onto a conveyor system 12 for transport.

As the drum rotates and moves the slurry, the slurry is dewatered and the water drains through the mesh 8 into the hot well 10 which is located directly below the drum. The mesh filters the fines from the water as it passes through. The sizing of the mesh is chosen to keep the fines content which drains from the system to a minimum. In the example with two mesh layers, the slag may pack between the mesh layers and restrict the throughput of water. This problem is addressed using an array of nozzles 13 mounted outside the drum 3 in order to clean the mesh 8 regularly, so that the slag cannot pack sufficiently to prevent passage of water, as shown in Fig.5. These nozzles provide high pressure jets, either compressed air blown at high pressure from the outside of the drum, or fluid jets, which dislodge fines which otherwise clog the mesh. This cleaning is applicable to any embodiment of the invention.

An alternative embodiment is shown in Fig.6, with the inlet pipe 6 provided with supports 41 beyond the structure 40 of the hot well. The drum is supported on the hot well 24. The driven wheel 44 is mounted at the outlet end of the drum and engages with teeth in the ring 45 on the outlet end of the drum. As with the arrangement of Fig.3, typically drive is only applied at one end, so the inlet end support wheel is not driven. The illustrations of Figs 8, 9a,9b, 10 and 11 show more detail for the examples driven at the outlet end 11.

The discharge opening at the outlet end 11 forms part of the main drum and allows for the drum to be supported by wheels mounted externally of the drum itself. The advantage of this design of support and drive mechanism is that the moving parts are shielded from contamination and are more easily accessible for maintenance. Conventionally, support wheels for rotating drums of this design for dewatering slag are mounted in an area inboard of the discharge/outlet end of the drum and so suffer from dirt and liquid contamination.

A further problem addressed by the present invention is that as slurry is fed into the drum 3 via the inlet pipe 6, the slurry has some distance to fall onto the mesh screen 8 which may result in damage to the mesh. To reduce the likelihood of this, the present invention further comprises inlet baffle plates 46 at the inlet end of the drum to deflect the slurry and slow the fall of the slurry onto the mesh 8, without unduly restricting the volume of material that can be handled.

In any of the embodiments of the invention, the pitch of the flights of the baffle varies over its length, with a reduced pitch further from the inlet end, so that the transported volume remains the same as the water drains. The internal thread diameter may remain the same, or may reduce along the length as the volume being processed is reduced. Variable height or tapered baffles with a tapered inner diameter may be beneficial in stopping water from flowing out of the end of the drum, during slag surges, which is a well known problem with some conventional systems. An alternative, although more complex, arrangement, is to reduce the thread diameter along the length of the helical baffle and maintain a constant pitch, but this would require an overall greater length of drum to give sufficient time for dewatering.

The purpose of decreasing the pitch of the flights along the length of the drum, whilst maintaining a constant diameter of the helix is in order to maintain the volume of the slurry as it is dewatered, so with a lower percentage of water in the mix. At the inlet of the drum, when the water to slag ratio is high (typically in the region of 8: 1) the required conveying capacity of the helical baffles is high and hence the pitch needs to be large in order to minimize drum size and speed. As the slurry travels through the drum and baffles, water is drained through the mesh which reduces the water to slag ratio, increasing the concentration of granulated slag and decreasing the slurry volume.

For example, a slag flow of 6m ³ /min at the inlet can be considered as a slag flow of 6m ³ /min at the outlet, less any small fines that have passed through the filter mesh. The one or more helical baffles are sized according to the desired product moisture content at the outlet. Thus, if a 10% product moisture content at the outlet is desired and the volume of water arriving at the inlet is 48m ³ /min, the volume of water must be reduced from 48m ³ /min at the inlet to 0.6 ³ /min by the time it exits the drum.

To achieve minimum water content in the granulated slag, maximum draining time is required and to achieve this while maintaining a relatively small plant, the pitch of the helical baffle is reduced in stages, or progressively in a gradient, which reduces the velocity of the slurry as it travels through the drum whilst maintaining a uniform depth of slag.

Maintaining a constant depth of slag has the benefit of allowing the water to filter through the slag as it tumbles in a self filtering effect which helps keep more fine particles from passing through the mesh filter. A further advantage of a smaller pitch and shorter length of helical baffle in the part of the drum nearer the outlet is reduced power consumption as both pitch and length are directly proportional to the power required to drive the drum.

The size of the helical baffle is chosen for the expected slag flow requirement, but a typical example is approximately 10m long, 4m diameter with a pitch ranging from 3m to 0.5m.