FEEDER FOR PARTICULATE MATERIAL

Abstract: A feeder for particulate material for connecting a mechanical feeding device (14) to a pneumatic conveying line comprises a funnel (7) formed of a gas pervious material which is mounted in a closed outer housing (1) formed of an impervious material wherein the walls of the funnel and the outer housing together defining a plenum chamber (10) which is provided with an inlet (11) for connection to supply of conveying gas under pressure. The funnel (7) is adapted to form a sealing engagement with the outlet of the mechanical feeding device (14) and the funnel (7) at or towards its narrow end communicates with the pneumatic conveying line. In use, conveying gas, under pressure, enters the plenum chamber (10) and then passes through the walls of the funnel (7). The particulate material delivered to the funnel (7) by the mechanical feeding device is flushed out of the funnel and into the conveying line by the conveying gas. The feeder enables a uniform flow of particulate material using one pressurized gas supply for conveying particulate material from bulk storage to discharge.

Description: Feeder for Particulate Material The present invention relates to a feeder for particulate material , for instance material in the form of a powder or fibres, filaments or whiskers. More particularly, it relates to a feeder for connecting a mechanical feeding device to a pneumatic conveying line. Feeders of this type, generally known as "particulate feeders", are used when it is desired to change the means of transporting a particulate material in a conveying line from a mechanical means (e.g. a screwfeeder) to a gas driven means (e.g. fluidised bed). Many known particulate feeders are effective only when using large volumes (high velocities) of conveying gas and/or connecting feeder outlet and conveying line diameters of essentially similar size. The feeder of the invention is particularly useful as part of a controllable feeder system for conveying particulate material from a hopper to a pneumatic conveying line for controllably supplying said particulate material for incorporation into metals by spray co-deposition in the production of metal matrix composites. A typical spray co-deposition method of making metal matrix composites comprises the steps of atomising a stream of molten metal to form a spray of hot metal particles by subjecting the metal stream to relatively cold gas directed at the stream, feeding a stream of particulate ceramic material in a fluidising gas to the atomising zone where said particulate material becomes incorporated into the metal particles and co-depositing the metal and the particulate material onto a collecting surface. Conventionally, the particulate ceramic material is conveyed pneumatically from a hopper to the atomising zone. However, the ambient pressure conditions at the discharge point of the particulate feed tube a re variable because of the highly turbulent gas jet flows present in the atomising region. Because of this variable pressure at discharge, any powder feeding device for use in this method which relies on a gas stream to control particulate feed rate tends to be unreliable, particularly in view of the low conveying line pressures (e.g. 0.3. bar g) and conveying line flowrates (e.g. 80 dm /min) used. A conventional feeding system for transporting a particulate material from bulk storage in a hopper to the atomising zone uses two gas streams: one for introducing the particulate material into the conveying line from bulk storage and one for conveying the particulate material to the atomising zone. Such a system dictates that the two gas streams meet at equal pressure at some point. A change in conditions at this point will obviously cause changes to occur in all gas " flows including the feedrate of the particulate material. The use of higher gas flows which might otherwise overcome such a problem is not desirable for a number of reasons. For instance, because of the nature of the spray co-deposition method, it is desirable to keep the flow rate of the gas used to convey the particulate material to the atomising zone as low as possible in order that the conveying gas stream does not affect the gas stream used to atomise the molten metal at the atomising zone. Furthermore, high flow rates for the conveying gas are not desirable since equipment life can be greatly reduced because of the effects of abrasion caused by high speed particulate material which abrasion could also result in the degradation of the particulate material itself by size reduction and/or contamination. If higher conveying gas flows and larger pipe bore sizes could be used then it might be possible to maintain the conveying velocity at its original level. However, the geometry and dimensions of the atomising zone a re such that the particulate feed entry has to be relatively small (e.g. 7mm diameter) to ensure maximum delivery of the particulate material to the atomising region accurately. In view of the above, it is clear that there is a need for an improved system of feeding particulate material from a hopper to a pneumatic conveyer for use in metal matrix composite production by the spray co- deposition process which system allows for greater control over the feed rate of the particulate material than achieved previously. According to one aspect, the present invention provides a particulate feeder for connecting a mechanical feeding device to a pneumatic conveying line which comprises a funnel formed of a gas pervious material mounted in a closed outer housing formed of an impervious material, the walls of the funnel and the outer housing together defining a plenum chamber which is provided with an inlet for connection to a supply of conveying gas under pressure, wherein the housing at or towards the wide end of the funnel is adapted to form a sealing engagement with the outlet of the mechanical feeding device and wherein the funnel at or towards its narrow end communicates with the pneumatic conveying line. According to another aspect, the present invention provides a feeder system for conveying particulate material from a hopper to a pneumatic conveying line for use in a spray co-deposition process of producing metal matrix composites which feeder system comprises (1) a mechanical feeder device for moving particulate material from a bulk storage hopper to an outlet and (2) a funnel formed of a gas pervious material mounted in a closed outer housing formed of an impervious material, the walls of the funnel and the outer housing together defining a plenum chamber which is provided with an inlet for connection to a supply of conveying gas under pressure, said housing at or towards the wide end of the funnel being engaged with the outlet of the mechanical feeder device to form a gas-tight seal therewith and said funnel at or towards its narrow end being in communication with the pneumatic conveying line. Thus, the feeder system of the invention makes use of a mechanical feeder device to move the particulate material from a bulk storage hopper to a particulate feeder wherein the particulate material is fluidised for introduction to the pneumatic conveying line. The use of the mechanical feeder device overcomes the above-mentioned problems arising from the use of a gas stream to move the particulate material from a storage hopper. This is because, in the case of a mechanical feeder device, the particulate solids feed rate from the hopper is independent of other process conditions and is essentially dependent only on the speed of the motor driving the mechanical feeder device. Mechanical feeder devices are, of course, well known and include screwfeeders , vibrating conveyor feeders and rotary valve feeders. We have obtained good results using a screwfeeder in the present invention. In ideal circumstances with a free flowing powder, the solids feedrate delivered by a mechanical feeder device is proportional to the speed of the drive motor. However, in normal circumstances, particulate materials are far from ideal in their flow behaviour and handeabi 1 ity , especially if the particulate material is fine in size, irregular in shape, damp and cohesive. Usually such factors are responsible for variations in powder "bulk density". When a mechanical feeder device operating at a fixed speed is used, fluctuations in the mass flow rate of particulate delivered by the mechanical feeder device may be experienced. It is, therefore, preferable in the present invention for the rate of delivery of particulate by the mechanical feeder device be controlled by a feedback system. To do this, the mechanical feeder device is suspended or loaded on a weighing device (for example, a load cell). The change in weight of the feeder device under operation as a function of time is monitored and automatically compared with the change in weight that would be expected for a desired feedrate of particulate. If the actual rate of weight decrease in the system being monitored is greater than that expected, the system compensates by reducing the speed of the feeder device accordingly. Alternatively, if the actual rate of weight decrease is less than expected for the particulate feedrate desired, the system automatically increases the speed of the mechanical feeder device accordingly. Such controlled feeder devices are known generally as "Loss in Weight" feeders. The process of sampling the feeders weight, calculating the resulting feedrate and effecting the appropriate motor speed control action is almost continuous during feeder operation and allowance is made for the finite time required for data sampling and microprocessor program calculation time (typically milliseconds per cycle) which is negligible in terms of system (motor) response time. By using a "Loss in Weight" feeder of this kind, particulate materials of different kinds can easily and accurately be fed using the same feeding device and change from one particulate material to a different particulate material to be fed to the feeding device can be effected easily and rapidly. The particulate feeder of the invention for connecting the mechanical feeder device to the pneumatic conveying line typically comprises a funnel made of a porous material which is pervious to th conveying* gas which, when the feeder is in operation, is supplied to the plenum chamber which lies between the funnel and the external gas impervious housing in which the funnel is mounted. In order that the speeds of the mechanical feeder device involved are acceptable to the particulate materials involved, the discharge part of the mechanical feeder is typically an order of magnitude larger than the diameter of the pneumatic conveying tube. Thus, the funnel in the particulate feeder has to achieve a transition from an inlet diameter of, for instance, 100 mm to an outlet diameter of, for instance, 10 mm with a minimum hold-up volume being created whilst, at the same time, without the transition being made so rapidly that blockages of particulate material are caused in the pneumatic conveying line. The funnel in the particulate feeder of the invention is preferably a conical funnel. However, a non-conical funnel, suclras one having a bowl shape wherein the sides curve inward towards the narrow end which communicates with the pneumatic conveying line, can also be used in the present invention. In such a case, the inwardly curving sides of the bowl-shape funnel will preferably not be so great a s to present a surface where any build-up of particulate material could occur during operation of the particulate feeder. We have found that the transition is preferably effected by using a conical funnel having a vertical axis and walls at an angle of from 30°to 60° to the vertical axis, and more preferably between 30 and 45°. The shape of the external housing is not critical although, preferably, it will be large enough to provide uniform filling of the plenum chamber that surrounds the funnel. The funnel used in the present invention is formed of a material which is pervious to the conveying ' gas that will be supplied under pressure to the plenum chamber during operation of the particulate feeder. Gas- pervious materials such as sintered plastics, filter cloths and woven wire meshes have been used successfully in the present invention for forming the funnel. However, because the conveying gas will be supplied under pressure, during operation, to the plenum chamber, the walls of the funnel should have sufficient mechanical rigidity so that the pervious funnel has sufficient dimensional stability to withstand this pressure. Preferably then, we use material such as sintered metal or perforated metal for forming the funnel for use in the present invention. In the case of a funnel formed from a perforated metal sheet, the metal sheet immediately at the periphery of the perforations may advantageously be deformed away from the plane of the metal sheet so as to shield the particulate material (when the feeder is in operation) from the perforations. Preferably, the internal surface of the funnel should be sufficiently smooth so as not to allow the build-up of any particulate material thereon. In operation, as the particulate material free-falls into the funnel from the mechanical feeder device, the conveying gas which passes through the bowl from the plenum chamber flushes it towards and into the pneumatic conveying line. In the accompanying drawings:- Figure 1 is a section through a preferred particulate feeder embodying the present invention; Figure 2 schematically illustrates the operation of the particulate feeder shown in Figure 1; and Figure 3 is a diagrammatic representation of a feeder system in accordance with the invention. In Figure 1, a housing 1 formed of a gas impervious material such as stainless steel or aluminium has cylindrical sides 2 and an upper radial flange 3. The housing has a base 4 which opens into an exit pipe 5 leading to a pneumatic conveying line (not shown). The flange 3 is adapted to abut the base of a mechanical feeder device outlet (not shown) and be fixed thereto by means of bolts placed through holes 6 provided in said flange. Contained inside the housing 1 is a truncated conical funnel 7 having sides at an angle of 45 to the vertical and formed of a gas pervious material . The funnel is located in the housing so as to provide a smooth transition in size from its wide end 8 positioned near the top opening of the housing to its narrow end 9 which communicates with the exit pipe 5. A plenum chamber 10 is defined by the sides 2 and base 4 of the housing together with the sides of the conical funnel 7. The housing is provided with an inlet 11 for a supply of a conveying gas introduced under pressure when the particulate feeder is in operation. As can be seen from Figure 2, during operation, particulate material 12 free-falls into the particulate feeder from a mechanical feeding device (not shown). Conveying gas supplied under pressure to the inlet 11 in the housing 1 enters the plenum chamber 10 from where it passes through the walls of the conical funnel 7. The particulate material 12 falling into the conical funnel meets the conveying gas at or near to the funnel walls from where it is carried by the gas flow down the interior of the funnel and flushed into the, exit pipe 5 to the pneumatic conveying line (not shown). In Figure 3 a particulate feeder of the type shown in Figures 1 and 2 is engaged by flange 3 at the outlet 13 of a mo .or driven screwfeeder 14. The screwfeeder 14 in operation feeds particulate material 12 from a sealed bulk storage hopper 15 to the outlet 13 from where the particulate -materia 1 free-falls into the conical funnel 7 of the particulate feeder. In order that the conveying gas introduced into the plenum chamber 10, lying between the housing 1 and the funnel 7, via gas inlet 11 flows in the desired direction and at the desired flowrate, a pipe 16 is preferably provided to connect the screwfeeder outlet region \ 1 to the airspace 18 above the level of particulate material in the hopper. Thus, when the conveying gas is flowing, the gas pressures at 17 and 18 will be equalized so that particulate feed is not adversely affected by any pressure build-up in the system. Typically, the pipe 16 will contain or be provided with a device to restrict the flow of particulate material and conveying gas through the pipe but which still allows the equalisation of pressure between the bulk storage hopper and the particulate feeder. For example, the pipe 16 may contain or be fitted with a pressure equalisation valve which only need be opened periodically to relieve any pressure buildup in the system. Such a feature would be advantageous at low flow rates. Alternatively, instead of providing a return line 16, with or without a control valve, to connect the funnel inlet (17) to the top space (18) of the hopper, the top space of the hopper may be connected to a separate gas feed (not shown) which would be controlled automatically to maintain the gas pressure at the top of the hopper at a value equal to the gas pressure at 17. In the case where the volume of gas in the hopper is large, such a system having a separate gas feed to the top space of the hopper would be preferable to a system having a return line 16 since in the latter system pressure equalisation would cause the flow of gas to be diverted in the funnel with the result that the uniform transport of solids in the conveying line would be disrupted. As mentioned earlier, spray co-deposition processes require a maximum amount of particulate material with a minimum amount of carrier gas. This ratio is termed "phase density" and is given the symbol "μ", where mass flowrate of solids mass flowrate of gas Some powders will convey in "dense phase", i.e. μ= 50-200 (approx) at velocities of about 1 m/s. Experience of- handling such powders in this mode is essential as operating conditions are approaching conditions in blocked pipes. However, to ensure accurate operation of a "Loss in Weight" feeder, it is desirable to operate the conveying line at the opposite end of the phase density range (i.e. μ= 1-10 and v = 20-40 m/s) which conditions are analogous to conditions in an open pipe. The other advantage of this regime is that even small discontinuities in powder flow within the conveying line (causing line pressure fluctuations) can be measured easily relative to the normally low line pressure. In "dense" phase conveying, large fluctuations in solids flowrates can go unnoticed since they are masked by the normally high line pressures required. The feeder system of the present invention canbe made to work quite satisfactorily using phase densities of 20 to 50 at velocities of 10 m/s. This mode of transport (i.e. moving/sliding beds and dunes), however, may not give desirable results in terms of metal matrix composite product structure. The present invention has been used successfully to convey SiC powder (F230 grit, F600 grit and F1000 grit) over a range of feedrates from 0.5 to 3.5 kg/min using transport gas flows of from 45 x 10~ 3 to 85 x 10~ 3 Nm^/min through nominal 8 mm diameter tubing. The terms "F230, 600 and 1000 grits" are described in FEPA Standard 42-GB-1984 and US Standard ANSI B.74.12-1976. As soon as the feeder motor is turned off, the conveying line clears rapidly rather than undergoing any gradual reduction in powder level concentration in the conveying line. EXAMPLE The feeder system as described above was used to convey various particulate materials at various flow rates. The system was found to work at high solids densities and yet still maintain uniform flow. The results are shown in the following Table. TABLE Particulate Grade/ Flowrate Gas Flow Sol ids Conveyi ng Type ^50% (kg/min) (Ndm 3 / Density ( 1 ) Line size min) (kg/Nm 3 ) SiC 11.1 3m x 8mm diam 15.4 12m x 10mm diam (3) X 21.1 3m x 8mm diam K B 4 C 12.5 2m x 10mm diam 17.1 SiC F230(53μm) 1.5 87.5 17.1 F400(17μm) F600 F800(6μm) F1000 (4μm) Table-continued Particulate Grade Flowrate Gas Flow Solids Conveying Type α " 50% (kg/ in) (Ndm 3 / Density ( 1 ) Line size min) (kg/Nm 3 ) 09 C CD 2} Zr0 2 (5) H I C H Si 3 N4 m w x 3 Notes to Table (1) The solids density values are obtained by dividing "Flowrate" by "Gas Flow" x10 3 . (2) 50:50 wt% mixture (3) Screwfeeder pressure 1.0 bar g α 2 kg/min (4) Screwfeeder pressure 3.0 bar g α 3.5 kg/min (5) Screwfeeder pressure 2.5 bar g The first six runs shown in the Table, i.e. those for SiC (F600) at 0.5, 2.0, 3.0 and 3.5 kg/min demonstrate that high solids densitites can be achieved. The other runs in the Table demonstrate the ability of the feeder of the invention to handle a range fo materials and particle sizes. In all cases, a satisfactory uniform flow was achieved.

Claims: C L A I M S 1. A particulate feeder for connecting a mechanical feeding device to a pneumatic conveying line which comprises a funnel formed of a gas pervious material mounted in a closed outer housing formed of an impervious material, the walls of the funnel and the outer housing together defining a plenum chamber which is provided with an inlet for connection to a supply of conveying gas under pressure, wherein the housing at or towards the wide end of the funnel is adapted to form a sealing engagement with the outlet of the mechanical feeding device and wherein the funnel at or towards its narrow end communicates with the pneumatic conveying line. 2. A feeder according to claim 1, wherein the funnel is a conical funnel. 3. K feeder according to claim 2 wherein the sides of the conical funnel are at an angle of from 30° to 60° to the vertical axis. 4. A feeder system for conveying particulate material from a hopper to a pneumatic conveying line for use in a spray co-deposition process of producing metal matrix composites which feeder system comprises (1) a mechanical feeder device for moving particulate material from a bulk storage hopper to an outlet and (2) a funnel formed of a gas pervious material mounted in a closed outer housing formed of an impervious material, the walls of the funnel and the outer housing together defining a plenum chamber which is provided with an inlet for connection to a supply of conveying gas under pressure, said housing at or towards the wide end of the funnel being engaged with the outlet of the mechanical feeder device to form a gas-tight seal therewith and said funnel at or towards its narrow end being in communication with the pneumatic conveying line. 5. A feeder system according to claim 4, wherein the mechanical feeder device is a motor-driven screwfeeder. 6. A feeder system according to claim 4 or claim 5, wherein the operation of the mechanical feeder device is regulated by an automatic control means to ensure a constant average mass flow rate of particulate material. 7. A feeder system according to any one of claims 4 to 6, wherein the funnel is a conical funnel. 8. A feeder system according to claim 7, wherein the sides of the conical funnel are at an angle of from 30° to 60°C to the vertical axis. 9. A feeder system according to any one of claims 4 to 8, wherein the outlet of the mechanical feeder device has a diameter of about 100 mm and wherein the outlet of the feeder system to a pneumatic conveying line has a diameter of about 10 mm. 10. A feeder system according to any one of claims 4 to 9, wherein the outlet region of the mechanical feeder device is adapted to communicate with the air space above the level of particulate material in the hopper by means of a connectable pipeline. 11. A feeder system according to any one of claims 4 to 9, wherein the air space above the level of particulate material in the hopper is connected to a supply of gas under pressure which supply is controlled to maintain a pressure in the air space equal in value to the pressure existing at the outlet region of the mechanical feeder device.

Inventors: DARBY NEIL ANDREW, DARBY, NEIL, ANDREW
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1990
Main classification: B65G53
IPC codes: B65G53