MICROWAVE TREATMENT OF BULK PARTICULATE MATERIAL

THIS INVENTION relates to the microwave treatment of bulk particulate material, in particular bulk multi-phase composite material. Specifically, the invention relates to a method of treating bulk particulate material with microwaves and to bulk particulate material microwave treatment apparatus.

The use of microwaves to treat bulk particulate material, such as ores, is known. The Applicant is aware of a material handling system as disclosed in WO2006030327 in which the bulk particulate material is gravity fed to fall freely through a microwave reactor or cavity where the bulk particulate material is irradiated with microwaves, e.g. to liberate minerals from an ore. Such a free falling system however has the disadvantage that the bulk density of the free falling bulk paniculate material is lowered, resulting in arcing and plasma formation in the microwave reactor or cavity due to the large air gaps between the falling particles.

According to one aspect of the invention, there is provided a method of treating bulk particulate material with microwaves, the method including feeding microwaves into a microwave treatment zone defined between horizontally spaced microwave reflective side walls to generate a microwave field which is uniform across the width of the treatment zone between the side walls; and feeding the bulk particulate material in the form of a bed of the bulk particulate material, on an inclined vibrating or oscillating base or support through said microwave treatment zone thereby to irradiate the moving bed of bulk particulate material with microwaves in the treatment zone.

Typically, the base or support is microwave reflective. Thus, the base or support may be of, or may include a layer of, a microwave reflective material, e.g. steel.

The microwaves are typically fed from above into the treatment zone through a microwave reflective roof of the treatment zone so that the bed of paniculate material is irradiated from above. As will be appreciated, any microwave radiator or the like which is configured to feed a uniform field into the treatment zone can be used. Examples of microwave radiators suitable for feeding a uniform microwave field into the treatment zone are rectangular or circular waveguide radiators, pyramidal horns, E or H plane sectoral horns or slotted waveguide radiators.

Preferably, the treatment zone is defined by a non-resonating microwave cavity. Thus the width of the treatment zone may be less than the wavelength of the microwaves, e.g. about half the wavelength of the microwaves. The width of the treatment zone may however be up to ten times the wavelength of the microwaves.

The bed of bulk particulate material may have a height or depth which is less than half the wavelength of the microwaves. Preferably, the bed depth is then less than the width of the treatment zone.

The bed of paniculate material may instead be at least one wavelength deep, in which case the width of the treatment zone is less than 10 times the depth of the bed, preferable less than 2 times the width of the treatment zone, e.g. about 0.5 times the width of the treatment zone.

The method of the invention ensures a high bulk density in the bed of particulate material and maximum interaction between the microwaves and the bulk particulate material in the treatment zone. By restricting the width of the material bed in the treatment zone, maximum field intensity in the bulk particulate material is induced. Using a uniform microwave field across the width of the treatment zone ensures all of the bulk particulate material is uniformly treated. Selecting the appropriate width to depth ratio for the bed of bulk particulate material in the treatment zone is important, to prevent breakdown of air and formation of arcing on sharp edges of particles.

The bed of bulk paniculate material may have a depth between about 50mm and about 150mm, such as between about 75mm and about 125mm, e.g. about 100mm for a particle size less than 42mm. Microwave frequency is an important operational

parameter determining the thickness of the bed that may be used and the area of the bed illuminated by microwave irradiation and therefore affects the microwave exposure time of the bulk particulate material. By selecting a microwave frequency and bed thickness such that the treatment zone is less than a half-wavelength high, a treatment zone is created that does not allow propagation of microwave energy in the direction of movement of the material bed. In this case the dimensions of the treatment zone are below cut-off and thus are too small to support fundamental mode resonance. Microwave heating of the bulk particulate material bed occurs in the high field intensity zone underneath the microwave outlet of the microwave radiator typically located above the bed.

Instead, the paniculate material bed may have a thickness greater than one wavelength. The microwave energy is then partially propagated through the bulk particulate material bed to a microwave reflective surface of the vibrating base or support in the treatment zone where it is reflected, setting up standing waves in the bulk particulate material bed. The treatment zone is restricted to approximately the length of the microwave waveguide radiator located above the material bed in the treatment zone. The distance between the bed surface and the roof of the treatment zone may be optimized to achieve maximum energy transfer from microwaves to the bulk particulate material bed.

A downwardly depending microwave choking structure or shield may be provided on the roof of the microwave treatment zone, spaced from the microwave outlet of the microwave radiator, upstream and/or downstream of the microwave outlet of the microwave radiator. These chokes prevent propagation of microwaves along the length of the bulk particulate material bed. This concentrates the microwave field in a small volume portion of the bulk particulate material bed.

Feed rates through the microwave treatment zone are, for example, of the order of 70 to 760 metric t/hr. Feed rates through the microwave treatment zone and hence microwave exposure times in the microwave treatment zone depend on the following factors: the inclination angle of the vibrating base, the particulate bed depth and width, the phase variation of motors of the vibrating base, particulate material bulk density and particulate size.

Preferable, the bulk particulate material has a residence time in the microwave treatment zone of less than 2 seconds, more preferably less than 1 second.

The microwave field may have a power density of at least 10 ⁷ W/m ³ .

The radiator preferably has a rectangular microwave outlet arranged above the microwave treatment zone, the length dimension of the outlet corresponding in direction to the direction of travel of the moving bed of bulk particulate material. Thus, the width of the outlet is in a direction which is transverse to the direction of travel of the bed of bulk paniculate material. The spacing between the side walls of the treatment zone is chosen according to the width of the outlet to ensure a constant microwave field across the width of the bulk particulate material bed.

The method may include generating the microwaves with a microwave pulse generator, thereby to achieve high peak power and thus a high heating rate for the bulk particulate material.

The bulk particulate material may be an ore, and may in particular be a multiphase composite material or ore such as banded iron ore. The bulk particulate material may have an average particle size of less than about 50mm, such as less than

40mm or less than 35mm. Typically, the bulk particulate material has an average particle size which is larger than 1 micron.

The method may include spacing the moving bed of particulate material in the treatment zone from the microwave reflective floor or base, to ensure that the microwave field extends to below a bottom of the moving bed of paniculate material so that all particles move through the microwave field.

According to another aspect of the invention, there is provided bulk particulate material microwave treatment apparatus, the apparatus including a microwave cavity having a microwave reflective base or support which defines a support surface and which is operable to vibrate or oscillate to feed a bed of bulk

particulate material over the support surface, the microwave cavity having a width defined between laterally spaced microwave reflective side walls; and a microwave radiator adapted to feed microwaves into said microwave cavity to generate a microwave field which is uniform across the width of the microwave cavity, i.e. in use transverse to the direction of travel of the bed of bulk particulate material.

Preferably, the microwave cavity is a non-resonating microwave cavity.

The apparatus may include a microwave generator, and in particular a microwave pulse generator operable to feed microwaves into the microwave radiator.

Instead, as will be appreciated, microwaves may be generated at a location remote from the apparatus and guided to the microwave radiator for feeding into a moving bed of bulk paniculate material on the base or support.

The microwave radiator may have a rectangular microwave outlet arranged above the support surface, to feed microwaves from above into the microwave cavity and hence in use into the bed of bulk paniculate material. Typically, the microwave outlet is in a plane which is parallel to the support surface. The microwave radiator may be as hereinbefore described.

The microwave radiator may be spaced from the base, with no contact, and in particular no electrical contact, between the microwave radiator and the base.

The apparatus may include a microwave choke through which the microwave radiator passes. Typically, there is no contact between the microwave choke and the base.

Typically, the microwave cavity includes a microwave reflective cover or roof over the base, an opening in the roof being provided underneath the microwave outlet of the microwave radiator, or such that the microwave radiator passes through the opening.

The microwave choke through which the microwave radiator passes may be circular in outline in plan view, defining an upwardly projecting annular choke ditch or structure which surrounds the microwave radiator.

A circular in outline in plan view choke element may be provided on the base, to be more or less in register with, but vertically spaced from, the microwave choke through which the microwave radiator passes.

A downwardly depending microwave choking structure or shield may be provided on the roof or cover, spaced from the microwave outlet of the microwave radiator, upstream and/or downstream of the microwave outlet of the microwave radiator. These chokes, if present, prevent propagation of microwaves in a lengthwise direction in the bed of paniculate material on the base or support thus enhancing the intensity of the microwave field in the microwave cavity immediately below the outlet of the microwave radiator.

The microwave generator may be configured to generate microwaves in a narrow band of wavelengths, e.g. 322 to 333mm, corresponding to a microwave frequency of 915MHz ±15MHz. The width of the microwave cavity may be less than the wavelength of the microwaves, e.g. half the wavelength of the microwaves. The width of the treatment zone may however be up to ten times the wavelength of the microwaves.

The microwave cavity may have a height which is less than half the wavelength of the microwaves. Preferably, the height is then less than the width of the treatment zone.

The distances between the aperture of the waveguide radiator, the microwave chokes depending from the roof and the surface of the material bed are optimised in use to maximise the microwave field strength inside the material bed.

The base or support may define or include a bottom zone penetrable by microwaves but through which particulate material does not move in use, the particulate

material in use passing over the bottom zone. In other words, a microwave reflective surface of the base may be below the support surface defined by the base, or below an upper boundary of the bottom zone. For example, the base may define a bottom zone in the form of a recess or lowered area filled by a microwave transparent material, such as a microwave transparent or dielectric ceramic material, so that in use microwaves enter the recess or lowered area but particulate material moves over the recess or lowered area. Another example is a bottom zone defined by a raised microwave transparent area on the base, with particulate material moving over the raised area and microwaves passing through the microwave transparent raised area to a microwave reflective surface of the base where it is reflected. The raised area may be defined by a dielectric layer on the base. Instead, the base may define a recess filled by particulate material and defining a dead zone, with microwaves in use entering the recess but with particulate material in the bed in use passing over the particulate material in the dead zone. Advantageously, the bottom zone ensures that in use the microwave field extends to below a bottom of the moving bed of particulate material so that all particles must move through the microwave field.

The invention extends to the use of the method and apparatus as hereinbefore described, to treat a bulk paniculate material which is a multiphase composite material or ore.

The ore may be as hereinbefore described.

The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which

FIGURE 1 shows a three-dimensional view of bulk particulate material microwave treatment apparatus in accordance with the invention;

FIGURE 2 shows a three-dimensional view of a lower, vibrating portion of the apparatus of Figure 1 ; FIGURE 3 shows a three-dimensional view of an upper, stationary portion of the apparatus of Figure 1 ;

FIGURE 4 shows the predicted microwave field distribution in the apparatus of Figure 1 where the treatment zone has a height less than 0.5 times the wavelength of the microwaves being used; and

FIGURE 5 shows another embodiment of bulk particulate material microwave treatment apparatus in accordance with the invention.

Referring to Figure 1 of the drawings, reference numeral 10 generally indicates bulk particulate material microwave treatment apparatus in accordance with the invention. The apparatus 10 includes, broadly, a slightly inclined vibrating or oscillating base 12 and a stationary waveguide radiator 14 above the base 12.

The base 12 includes a U-shaped channel member 16 of a microwave reflective material (e.g. steel or stainless steel) which defines a channel 18 with an inclined floor 20 and side walls 22. The side walls 22 are spaced about 150mm from one another and the channel 18 has a depth of about 140mm.

The base 12 includes a cover 24 over the U-shaped channel member 16. The cover 24 is also of a microwave reflective material, e.g. steel or stainless steel. A circular in outline in plan view choke element 26 is welded to the cover 24. A rectangular opening 28 (see Figure 2) is provided centrally in the choke element 26. The waveguide radiator 14 passes through the opening 28 with a 40mm clearance on all sides. A longitudinal central axis of the opening 28 is in the same vertical plane as a longitudinal central axis of the channel 18.

The apparatus 10 includes a microwave generator or microwave pulse generator (not shown) operable to feed microwaves into the waveguide radiator 14. The waveguide radiator 14 is rectangular in transverse cross-section, and has a rectangular microwave outlet 30 (see Figure 3). The microwave outlet 30 is in a plane which is parallel to the floor 20. Long sides of the outlet 30 are parallel to the channel 18, with short sides of the microwave outlet 30 being arranged transversely to the channel 18. The outlet 30 has a length of about 260mm and a width of about 136mm.

A 2mm thick polyethylene skirt 32 extends downwardly from the waveguide radiator 14.

The waveguide radiator 14 passes through a circular in outline microwave choke 34. The choke 34 comprises two annular radially spaced rings 36 with an

upwardly projecting annular choke ditch or structure 38 being located between the annular rings 36. The waveguide radiator 14 passes through the radially inner annular ring 36, the choke ditch 38 thus surrounding the waveguide radiator 14. A vertical spacing of about 40mm is provided between the choke element 26 and the microwave choke 34.

Two downwardly depending microwave choking shields 40 are provided inside the cover 24 (only one of which is shown in Figure 1 ) to concentrate the microwave field between them. The shields 40 are on opposite sides of the waveguide radiator 14, one shield 40 in use being upstream of the waveguide radiator 14 and one shield 40 in use being downstream of the waveguide radiator 14.

The apparatus 10 includes a Faraday cage (not shown) around the base 12 and waveguide radiator 14 to protect operating personnel from residual microwave leakage. The Faraday cage is of expanded metal mesh of 25x12x3mm or 35x12x1 .6mm.

The waveguide radiator 14 is of aluminium. At least around the microwave outlet 30, the aluminium has a thickness of 6mm that is chamfered to reduce microwave field intensity on the edges of the waveguide radiator 14, thereby reducing the chances of arcing between the waveguide radiator 14 and the base 12.

The apparatus 10 can be used to treat a multi-phase composite material, for example, banded iron ore, with a particle size of say, 35mm, with microwaves in order to weaken the bond strength between minerals contained in the ore for further downstream liberation of the minerals from the ore, provided that the power density is higher than 10 ⁷ W/m ³ . The ore is fed in the form of a 100mm thick bed 42 along the U- shaped channel member 16, by vibrating the base 12 in an oscillating fashion. The bed 42 thus passes underneath the microwave outlet 30 where continuous wave or pulsed microwaves from the microwave generator, fed by means of the waveguide radiator 14, are radiated into the bed 42. With the waveguide radiator 14, an electric field 50 is generated across the width of the channel 18. Importantly, the electric field 50 is uniform across the width of the channel 18. In a longitudinal direction, ie in the direction of the movement of the bed 42, the electric field 50 has a maximum underneath the

microwave outlet 30. When the shields 40 are present, they may cause another set of standing waves between the shields 40 and the microwave outlet 30. Typically, the bed of bulk paniculate material is displaced at such a rate that the material particles remain for less than 2 seconds in the treatment zone underneath the microwave outlet 30 and between the choking shields 40.

The floor 20, side walls 22 and cover 24 define a treatment zone or microwave cavity which acts as a non-resonant microwave cavity at the operational microwave frequency of the apparatus 10. Typically, a microwave frequency of 915 MHz is used, meaning that the microwaves have a wavelength of about 327mm. The spacing between the side walls 22, and thus the width of the treatment zone, is accordingly less than half the wavelength of the microwaves, rendering the treatment zone non-resonating. Also, the depth of the bed 42 is less than half the wavelength of the microwaves. The defined microwave treatment zone thus does not allow the propagation of the microwave energy in the direction of the bulk particulate material flow through the treatment zone

The electric field 50 is partially absorbed as it penetrates into the bed 42 and the residual field is reflected by the metal floor 20, setting up a standing wave. The electric field 50 is therefore zero against the floor 20, while the magnetic field is at a maximum against the floor 20. The lower 15mm of the bed 42 is not substantially heated by the electric field, with a local field strength of less than 20% of the maximum of the field strength. However, with a particle size of about 35mm, it is probable that most ore particles lying on the floor 20 will be at least partially heated. Furthermore, since hematite has some magnetic absorption properties, the ore will also be heated by the magnetic field as well as the electric field.

An acceptable microwave reflection coefficient of about -7dB (20%) is obtained by correctly positioning the microwave outlet 30 above the bed 42. With an auto tuner, it should be possible to reduce this reflection to approximately 5%.

Advantageously, the use of a vibrating or oscillating base 12 increases the bulk density of the particulate material in the moving bed, reducing the risk of arcing and plasma formation in the treatment zone.

Referring to Figure 5 of the drawings, another embodiment of a bulk particulate material microwave treatment apparatus in accordance with the invention is shown and indicated by reference numeral 100. The apparatus 100 is similar to the apparatus 10 and unless otherwise indicated, the same reference numerals are used to indicate the same or similar parts or features.

The base 12 defines a bottom zone 102 penetrable by microwaves but through which particulate material does not move. Thus, the base 12 defines a recess filled by a microwave transparent ceramic plate 104, so that in use microwaves enter the recess but paniculate material moves over the recess. Advantageously, the bottom zone 102 ensures that in use the microwave field extends to below a bottom of the moving bed of particulate material, into the ceramic plate 104, so that all particles moving over the ceramic plate 104 are forced to move through the microwave field.

The Applicant expects that the use of the apparatus 10, as illustrated, will avoid, or will at least reduce, the problem of arcing and plasma formation in the microwave reactor or cavity defined by the U-shaped channel 16 and the cover 24.