WO2008044218A2

WO2008044218A2 - Microwave treatment of bulk particulate material

Info

Publication number: WO2008044218A2
Application number: PCT/IB2007/054158
Authority: WO
Inventors: Gerrit Coetzer; Mathys Johannes Rossouw
Original assignee: Exxaro Resources Limited
Priority date: 2006-10-13
Filing date: 2007-10-12
Publication date: 2008-04-17
Also published as: BRPI0719812A2; EP2084303A2; WO2008044218A3; CA2666399A1; AU2007305926B2; AU2007305926A1

Abstract

A method of treating bulk particulate material with microwaves includes feeding the bulk particulate material in the form of a bed (32) of the bulk particulate material, on an inclined vibrating or oscillating base or support (12) through a microwave treatment zone. Microwaves are fed from below into the microwave treatment zone thereby to irradiate the moving bed (32) of bulk particulate material in the treatment zone from below with microwaves forming a microwave field.

Description

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 paniculate 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 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 a microwave treatment zone; and feeding microwaves from below into said microwave treatment zone thereby to irradiate the moving bed of bulk particulate material in the treatment zone from below with microwaves forming a microwave field.

The bed of bulk paniculate material may have a depth between about 20 mm and about 150 mm, such as between about 75 mm and about 125 mm, e.g. about 100 mm for a particle size less than 42 mm. 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.

Feed rates through the microwave treatment zone are, for example, of the order of 70 to 175 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.

Typically, the microwave treatment zone is defined between horizontally spaced microwave reflective side walls. Preferably, a microwave field is generated between the side walls which is uniform across the width of the treatment zone between the microwave reflective side walls.

Thus, a microwave radiator or the like which is configured to feed a uniform field into the treatment zone may 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. 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 above a microwave outlet of the microwave radiator located below the bed.

In one embodiment of the invention, the treatment zone has a width which is less than ten times the bed depth, and the width of the treatment zone is at least a quarter of the depth of the bed of particulate material.

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 electrical breakdown of air and formation of arcing on sharp edges of particles under high field intensity conditions.

The bulk particulate material may have a residence time in the microwave treatment zone of less than 2 seconds. Preferably, the residence time is less than 1 second.

The microwave field may have a power density of at least 10⁷ W/m³ in the bed of bulk particulate material.

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.

A microwave radiator with a rectangular microwave outlet, e.g. an open ended waveguide arranged below the microwave treatment zone may be used to feed microwaves from below into the treatment zone. Preferably, the length dimension of the outlet corresponds in direction to the direction of travel of the moving bed of bulk particulate material through the treatment zone. Thus, the width of the outlet is in a direction which is transverse to the direction of travel of the bed of bulk particulate material, setting up an electric field in the outlet of the radiator that is normal to the side walls of the microwave treatment zone. The spacing between the side walls of the microwave treatment zone may be chosen according to the width of the outlet to ensure a constant microwave field across the width of the bulk particulate material bed.

The microwaves may be fed from below into the moving bed of bulk particulate material in a direction which is perpendicular to a direction of travel of the bulk paniculate material.

Instead, the microwaves may be fed from below into the moving bed of bulk particulate material at an included angle of less than 90⁵ to a direction of travel of the bulk paniculate material, in a plane parallel to the direction of travel of the bulk particulate material. Preferably, in such a case, the included angle is less than 60⁵, e.g. between 20⁵ and 50⁵. The microwaves may be fed into the bed of bulk particulate material in a direction which is generally co-current or generally counter-current to the direction of travel of the bulk particulate material. When a bed of particulate material which is relatively thin is used, it may be preferable to feed the microwaves in a direction which is generally counter-current to the direction of travel of the bulk particulate material.

Typically, the microwave treatment zone is bordered by a microwave reflective shield or roof above the bed of paniculate material which serves to increase the microwave field strength inside the treatment zone. Advantageously, a gap between the bed of particulate material and the roof, the thickness of the bed of paniculate material, and the included angle may be selected such that there is a single microwave field maximum in the microwave treatment zone. In this way, microwave power density in the bed of material can be maximised.

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 50 mm, such as less than 40 mm or less than 35 mm. Typically, the bulk particulate material has an average particle size which is larger than 1 micron. Thus, the invention extends to the use of the method as hereinbefore described for treating a bulk paniculate material which is a multiphase composite material or ore.

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, a portion of the base or support being microwave transparent; and a microwave radiator adapted to feed microwaves from below through said microwave transparent portion of the base or support into said microwave cavity and hence into said moving bed of bulk particulate material on the support surface.

The microwave cavity may have a width defined between laterally spaced microwave reflective side walls, with the microwave radiator being adapted 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.

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

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 below the base or support, to feed microwaves from below into the microwave cavity and hence in use into the bed of bulk particulate material. Preferably, the length dimension of the outlet corresponds in direction to a longitudinal axis of the base or support. 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 between the microwave radiator and the base. The apparatus may include a skirt depending from the base and moveable with the base, with the waveguide radiator outlet being located inside the skirt. Typically, the skirt is shaped and sized such that no contact is made between the skirt and the microwave radiator during vibration or oscillation of the base.

The apparatus may include a microwave choke through which the microwave radiator passes, with no physical contact between the microwave choke and the radiator. Typically, the microwave choke is located inside the skirt.

The apparatus may include a microwave generator. The microwave generator may be configured to generate microwaves in a narrow band of wavelengths, e.g. 322 to 333 mm, corresponding to a microwave frequency of 915 MHz ±15 MHz. The width of the microwave cavity may be less than the wavelength of the microwaves, e.g. 1 /10^th 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 five times the wavelength of the microwaves. Preferably, the height is less than half the wavelength of the microwaves and the height is preferably less than the width of the treatment zone.

The microwave transparent portion of the base may be defined by one or more microwave transparent ceramic elements, e.g. alumina tiles.

The microwave radiator may be arranged to feed microwaves from below in a direction which is perpendicular to the base or support.

Instead, the waveguide radiator may be arranged to feed microwaves from below at an included angle of less than 90⁵ to the base or support, in a plane parallel to a longitudinal axis of the base or support. Preferably, in such case, the included angle is less than 60⁵, e.g. between 20⁵ and 50⁵. The waveguide radiator may be arranged to feed microwaves into the bed of bulk particulate material in a direction which is generally co-current or generally counter-current to the direction of travel of the bulk particulate material, in use.

The base may define a channel, e.g. a U-shaped channel, through which the bed of bulk paniculate material travels in use. The base may include a cover or roof over the channel. Typically, the microwave treatment zone is thus bordered by a microwave reflective shield or roof which in use is above a normal level of the bed of particulate material. A gap between the normal level of the bed of particulate material and the roof, the height of the normal level of the bed of particulate material above the base or support, and the included angle may be selected such that there is in use a single microwave field maximum in the microwave treatment zone.

The apparatus may include a downwardly depending microwave choking structure or shield 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 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.

The invention extends to the use of the apparatus as hereinbefore described for treating a bulk particulate material which is a multiphase composite material or ore.

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

Figure 1 shows a vertical section of bulk particulate material microwave treatment apparatus in accordance with the invention;

Figure 2 shows the predicted microwave field distribution in the apparatus of Figure 1 ;

Figure 3 shows a vertical section through another embodiment of bulk particulate material microwave treatment apparatus in accordance with the invention, together with the predicted microwave field distribution in the apparatus; and Figure 4 shows a vertical section through yet another embodiment of bulk particulate material microwave treatment apparatus in accordance with the invention, together with the predicted microwave field distribution in the apparatus.

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, a rectangular in horizontal section skirt 14 fastened to the base 12 and extending from the base 12, and a stationary rectangular in horizontal section waveguide radiator 16 below the base 12.

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

The base 12 includes a microwave reflective cover 24 over the U-shaped channel member. The base 12 and cover 24 together define a microwave treatment zone. Two downwardly depending microwave choking shields 26 are provided underneath the cover 24. The shields 26 are on opposite sides of the waveguide radiator 16, one shield 26 in use being upstream of the waveguide radiator 16 and one shield 26 in use being downstream of the waveguide radiator 16.

An opening is provided in the floor 20 of the channel member, directly above the waveguide radiator 16. The opening is covered by a microwave transparent rectangular ceramic panel or window 28 such that an upper surface of the ceramic window 28 is flush with an upper surface of the floor 20. The ceramic window 28 has the same length and width as the skirt 14. As can be clearly seen in Figures 1 and 2 of the drawings, the waveguide radiator 16 is vertically spaced from the ceramic window 28, leaving an air gap.

The apparatus 10 includes a microwave generator or microwave pulse generator (not shown) operable to feed microwaves into the waveguide radiator 16. The waveguide radiator 16 is rectangular in transverse cross-section and has a rectangular microwave outlet 30. The microwave outlet 30 is in a plane which is parallel to the floor 20. The long sides of the outlet 30 are parallel to a longitudinal axis of the channel 18, with short sides of the microwave outlet 30 being arranged transversely to the channel 18 so that the microwave field in the outlet 30 is polarised normal to the side walls. In the embodiment of the invention shown in Figures 1 and 2, the outlet 30 has a length of about 260 mm and a width of about 136 mm.

The waveguide radiator 16 passes through a microwave choke (not shown) located inside the skirt 14.

The apparatus 10 includes a Faraday cage (not shown) around the base 12 and waveguide radiator 16 to protect operating personnel from residual microwave leakage. Typically, the Faraday cage is of expanded metal mess of 25 x 12 x 3 mm or 35 x 12 x 1 .6 mm.

The waveguide radiator 16 is of aluminium. At least around the microwave outlet 30, the aluminium has a thickness of 6 mm that is chamfered to reduce microwave field intensity on the edges of the waveguide radiator 16, thereby reducing the chances of arcing between the waveguide radiator 16 and the base 12.

The apparatus 10 can be used to treat, for example, banded iron ore, with a particle size of say, 35 mm, with microwaves in order to liberate minerals from the ore. The ore is fed in the form of a 100 mm thick bed 32 along the U-shaped channel member, by vibrating the base 12 in an oscillating fashion. The bed 32 thus passes over the ceramic window 28 and the microwave outlet 30. Continuous wave or pulsed microwaves from the microwave generator, fed by means of the waveguide radiator 16, are radiated into the bed 32 from below. With the waveguide radiator 16, an electric field 34 (see Figure 2) is generated across the width of the channel 18, which is uniform across the width of the channel 18. In a longitudinal direction, i.e. in the direction of the movement of the bed 32, the electric field 34 has a maximum 36 above the microwave outlet 30. The shields 26 also cause another set of standing waves between the shields 26 and the microwave outlet 30, as shown in Figure 2 of the drawings. With reference to Figure 3 of the drawings, the waveguide radiator 16 and the skirt 14 may be arranged at an angle to the vertical, and thus at an acute angle to the floor 20. In the embodiment of the apparatus shown in Figure 3, the microwave radiator 16 and the floor 20 define an acute angle 38 between them of about 32°. Figure 3 also shows the predicted microwave field distribution in such apparatus, which is generally indicated by reference numeral 50.

Compared to the apparatus 10, the microwave transfer volume in the apparatus 50 is larger, causing a smaller field density. A more homogenous field distribution is however obtained in the apparatus 50.

In Figure 4 of the drawings, the waveguide radiator 16 and the skirt 14 are also arranged at an angle to the vertical, and thus at an acute angle to the floor 20. In the embodiment of the apparatus shown in Figure 4, the acute angle 38, the depth of the bed 32, and an air gap 62 between the bed 32 and the cover 24 are selected such that there is a single maximum 36 for the electric field 34, in the bed 32 above the microwave outlet 30. Figure 4 shows the predicted microwave field distribution in such apparatus, which is generally indicated by reference numeral 60. By manipulating the height of the cover 24 above the bed 32, it is also possible to adjust the vertical position of the maximum 36 inside the channel 18, which forms part of a microwave treatment zone.

The applicant expects that, if plasma is formed during use of the apparatus 10, 50, 60 the plasma will move away from the microwave outlet 30, i.e. generally upwards. The plasma will thus not end up in the waveguide radiator 16, which would clearly be undesirable.

Claims

CLAIMS:

1 . A method of treating bulk particulate material with microwaves, the method including 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 a microwave treatment zone; and feeding microwaves from below into said microwave treatment zone thereby to irradiate the moving bed of bulk particulate material in the treatment zone from below with microwaves forming a microwave field.

2. The method as claimed in claim 1 , in which the microwave treatment zone is defined between horizontally spaced microwave reflective side walls and in which the microwave field is generated between the side walls and is uniform across the width of the treatment zone between the microwave reflective side walls.

3. The method as claimed in claim 2, in which the treatment zone is defined by a non-resonating microwave cavity, and in which the width of the treatment zone is less than the wavelength of the microwaves.

4. The method as claimed in any of the preceding claims, in which the bed of bulk paniculate material has a height or depth which is less than half the wavelength of the microwaves.

5. The method as claimed in any of the preceding claims, in which the treatment zone has a width which is less than 10 times the bed depth, and in which the width of the treatment zone is at least a quarter of the depth of the bed of paniculate material.

6. The method as claimed in any of the previous claims, in which the bulk particulate material has a residence time in the microwave treatment zone of less than 2 seconds.

7. The method as claimed in claim 6, in which the residence time is less than 1 second.

8. The method as claimed in any of the preceding claims, in which the microwave field has a power density of at least 10⁷ W/m³ in the bed of bulk particulate material, and in which a microwave radiator with a rectangular microwave outlet arranged below the microwave treatment zone is used to feed microwaves from below into the treatment zone, the length dimension of the outlet corresponding in direction to the direction of travel of the moving bed of bulk particulate material through the treatment zone.

9. The method as claimed in any of the preceding claims, in which the microwaves are fed from below into the moving bed of bulk particulate material in a direction which is perpendicular to a direction of travel of the bulk particulate material.

10. The method as claimed in any of claims 1 to 8 inclusive, in which the microwaves are fed from below into the moving bed of bulk particulate material at an included angle of less than 90⁵ to a direction of travel of the bulk particulate material, in a plane parallel to the direction of travel of the bulk particulate material.

1 1 . The method as claimed in claim 10, in which the included angle is less than 60⁵.

12. The method as claimed in claim 10 or claim 1 1 , in which the microwave treatment zone is bordered by a microwave reflective shield or roof above the bed of particulate material which serves to increase the microwave field strength inside the treatment zone, a gap between the bed of particulate material and the roof, the thickness of the bed of particulate material, and the included angle being selected such that there is a single microwave field maximum in the microwave treatment zone.

13. Use of the method as claimed in any of the preceding claims for treating a bulk paniculate material which is a multiphase composite material or ore.

14. 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, a portion of the base or support being microwave transparent; and a microwave radiator adapted to feed microwaves from below through said microwave transparent portion of the base or support into said microwave cavity and hence into said moving bed of bulk particulate material on the support surface.

15. The apparatus as claimed in claim 14, in which the microwave cavity has a width defined between laterally spaced microwave reflective side walls and in which the microwave radiator is adapted 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.

16. The apparatus as claimed in claim 14 or claim 15, in which the microwave cavity is a non-resonating microwave cavity.

17. The apparatus as claimed in any of claims 14 to 16 inclusive, in which the microwave radiator has a rectangular microwave outlet arranged below the base or support, to feed microwaves from below into the microwave cavity and hence in use into the bed of bulk particulate material, the length dimension of the outlet corresponding in direction to a longitudinal axis of the base or support

18. The apparatus as claimed in any of claims 14 to 17 inclusive, in which the microwave radiator is spaced from the base, with no contact between the microwave radiator and the base, and which includes a microwave choke through which the microwave radiator passes, with no physical contact between the microwave choke and the radiator.

19. The apparatus as claimed in claim 15, which includes a microwave generator, the microwave generator being configured to generate microwaves in a narrow band of wavelengths, the width of the microwave cavity being less than the wavelength of the microwaves.

20. The apparatus as claimed in claim 19, in which the microwave cavity has a height which is less than half the wavelength of the microwaves and in which the microwave cavity height is less than the width of the microwave cavity.

21 . The apparatus as claimed in any of claims 14 to 20 inclusive, in which the microwave transparent portion of the base is defined by one or more microwave transparent ceramic elements.

22. The apparatus as claimed in any of claims 14 to 21 inclusive, in which the microwave radiator is arranged to feed microwaves from below in a direction which is perpendicular to the base or support.

23. The apparatus as claimed in any of claims 14 to 21 inclusive, in which the waveguide radiator is arranged to feed microwaves from below at an included angle of less than 90⁵ to the base or support, in a plane parallel to a longitudinal axis of the base or support.

24. The apparatus as claimed in claim 23, in which the included angle is less than 60⁵.

25. The apparatus as claimed in claim 23 or claim 24, in which the microwave treatment zone is bordered by a microwave reflective shield or roof which in use is above a normal level of the bed of particulate material, a gap between the normal level of the bed of paniculate material and the roof, the height of the normal level of the bed of particulate material above the base or support, and the included angle being selected such that there is in use a single microwave field maximum in the microwave treatment zone.

26. The apparatus as claimed in any of claims 14 to 24 inclusive, in which the microwave cavity includes a microwave reflective cover or roof over the base, and a downwardly depending microwave choking structure or shield 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.

27. Use of the apparatus as claimed in any of claims 14 to 26 inclusive for treating a bulk particulate material which is a multiphase composite material or ore.