WO2003061872A1

WO2003061872A1 - Recovery of particulates using microwave energy

Info

Publication number: WO2003061872A1
Application number: PCT/AU2003/000063
Authority: WO
Inventors: Brett David Reich; Leonard Robert Lindsay
Original assignee: Munck, Gregory, Ross; Gregory, Phillip, James
Priority date: 2002-01-22
Filing date: 2003-01-22
Publication date: 2003-07-31
Also published as: AUPS007702A0

Abstract

There is provided apparatus for the recovery of foundry sands including a fluidized-bed preheater (14) having a controlled inlet (16) for comminuted waste foundry sand and heated air supply annulus (17), preheating material to between 200 & 300 °C using recycled heat. The controlled inlet (16) is operable to allow air entraining dust to pass to a cyclone separator (21), air heat values being recovered into the feedstock. Preheated materials pass through outlet (23) to a main reaction assembly (24) comprising a microwave containment chamber (25) having microwave input assemblies (26) and containing an open-topped, annular vessel provided with vibratory means for circulating the material about the annulus. An overflow lip allows treated material pass through a controlled treated-material outlet (27) from the chamber (25) to a finishing assembly (30) comprising a fluidized bed finishing/cooling chamber (31). Control of the treated material outlet (27) permits heated air and dust to pass into an exhaust riser (34) including a diverter directing air at greater than 400 °C to the preheating chamber (15), and directing air at less than 400 °C to the cyclone separator (21), heat values in the separated cyclone air being recovered into the feedstock. Finished material passes to storage. Application of microwave energy to the pre-heated material circulating in the annulus increases its temperature to the ignition temperature of the resin coating.

Description

RECOVERY OF PARTICULATES USING MICROWAVE ENERGY

This invention relates to methods and apparatus for the recovery of particulates using microwave energy.

This invention has particular application to methods and apparatus useful in the reclamation of waste and recycled foundry sand, and the application will be described hereinafter with reference to this application. However, it will be envisaged that this invention has other applications such as the beneficiation of any particulate substance formed of a microwave-permeable substance from heat-labile and microwave-absorbent contaminants or adjuncts. Foundry sand is a specialised particulate material having a selected grading as to particle size, shape and distribution, and comprising a selected material such as silica or other suitable sand or grit. As a consumable, the foundry sand is a significant contributor to foundry costs. In addition, the binders used to produce the green moulds usually include or comprise a phenolic resin or other condensation polymer, and such resins may include volatile monomers such as formaldehyde. The presence of these organic resins and their breakdown products create an environmental issue out of disposal and/or recycling of sand casting moulds.

Accordingly, it has long been recognized as desirable to reclaim and/or recycle foundry sand waste. The usual manner of recovery comprises comminution of the used sand casting moulds by mechanical means to an approximate consistency for reuse. The presence of the binder in the mix means that the grain surface actually carries a resin layer, which results in a different grain form and size compared to the original sand, thus impacting on properties when reused. The comminution process, usually comprising the crushing and milling of the waste, also results in grain fracture, resulting in as much of 25% by weight of fines that must be removed prior to reuse.

Calcining of the comminuted product to remove binder results in caking that necessitates further crushing and fines separation. The calcining process does not remove refractory salts and sulphur and its compounds which tend to agglomerate on the particle surfaces. The dynamics of the calcining process also contributes to grain fracture, thus increasing the fines to be removed and altering the particle size distribution of the recycled product as opposed to the original product.

German patent application DE 19914118365 describes a process of recycling used foundry sand by treating in a thin-layer microwave heater at 1800 to 3000MHz for up to 5 minutes at a rate of 1kW/kg in a continuous furnace. However, the product produced is not suitable for reuse as foundry sand and instead is intended for low-value end use as a feedstock for building materials, ceramics, or road construction material. The present invention in one aspect resides broadly in a method of recovery of particulates from a composition including particulate material and an organic adjunct and including the steps of: heating said composition with microwave energy to an oxidizing temperature; subjecting said heated composition to an oxidizing atmosphere to oxidize said organic adjunct; agitating said composition to dislodge adherent materials from said particulates; and separating a recovered particulate product from said adherent materials. The particulate material is preferably of relatively microwave transparent

material and the organic adjunct is preferably of a relatively microwave absorbent

material. In the context of foundry sands, this has the advantage of the heating of

the composition being by microwave absorption by the binders. This results is less

thermal shock to the particles, and in addition means that for short residence times

the oxidation temperature may be greater than the mean temperature within the

particles. However, it is envisaged that the composition may be heated by absorbance by the particles and/or the organic adjunct. Further the composition may

be formulated with an added microwave-absorbent compound or composition to

effect the heating. In all cases it will be realized that the particulate must be refractory to the heating temperatures.

The composition may be comminuted to a selected degree prior to heating. For example the foundry sand compositions of the preferred embodiment may be

comminuted to substantially discrete particles prior to heating. However, since the microwave energy essentially and selectively destroys the binder, the applicant has

determined that the process works practically with a significantly lesser degree of

comminution. For example, the foundry waste may be comminuted by crushing or the like to particle sizes of greater than 5 mm, although it is preferred that the

crushing be to less than 5 mm.

The heating process may be done in a batchwise manner or may be

substantially continuous. The microwave energy may be applied continuously or may

be applied intermittently under the control of control means in order to maintain a

preselected temperature. In the case of substantially continuous processes, the control means may control both feed rate of material and microwave energy to

control process temperature.

The oxidizing atmosphere may be supplied during the heating step in a batch

or substantially continuous heating process, or may be applied after heating in a

batch process. The oxidizing atmosphere may comprise entrained air or other

oxidizing gas, or may comprise the ambient atmosphere at heating, The process may

be continuous in that the oxidizing atmosphere is supplied by way of a countercurrent injection of the oxidant. The injection of the countercurrent of oxidizing atmosphere

may be under the control of the same or different control means as described above.

The oxidizing atmosphere may comprise air or other oxidizing atmosphere and will be determined at least in part by the economics of the process and the respective

natures of the respective particulate and adjunct. In the case of foundry sands it is preferred to use air as the oxidizing atmosphere. The oxidizing atmosphere may be

preheated to assist in elevating the temperature of the process to an optimal oxidation temperature. Preheating may be by way of heat recovery in the process.

Vented oxidation product gases may be recycled to the mixture to preheat

before processing or admitted to the reacting composition to recover process heat

and/or to contain entrained fines. Alternatively, the vented gases may be precipitated and vented.

The agitation of the particles may be by any means selected to result in

dislodgment of ash and other adherent refractories and oxidation products from the surface of the particles. For example, the particles may be agitated in bulk by means

of a vibratory vessel whereby the particles may work together to dislodge the

adherent materials. If desired, the particles may be admixed with an optimized further particulate for agitation, whereafter the optimized further particulate material may be separated from the product. The agitation may be done by the use of a fluidized bed type agitator or other controlled vibration. In one preferred embodiment the agitation is performed in an essentially open topped, annular, vibratory vessel, for reasons that will become apparent hereinafter.

The separating of the particulate product may be by any means determined at least in part by the nature of the particulate. For example the grading may be screening such as vibrating screens, rotating screens, curtain screening or the like, gravimetric grading such as cyclone separation, or chemicophysical methods such as flotation.

In a further aspect this invention resides broadly in apparatus for recovery of particulates from a composition including particulate material and an organic adjunct, and including: a chamber having an oxidizing atmosphere, an inlet adapted to receive said composition and an outlet; a microwave source supplying microwave energy to composition in said chamber; agitation means for composition being processed by said chamber; and separating means adapted to receive agitated composition from said agitation means and separate therefrom said particulates.

The chamber may be adapted to receive and process the composition in a batch process or continuously. The chamber is preferably configured whereby microwave energy cannot significantly escape its confines once admitted thereto. The continuous-process chamber may comprise a chamber assembly including a preheat chamber in addition to the main chamber. The preheat chamber may itself be supplied with microwave energy. The preheat chamber may be heated by process gases or by externally supplied energy. The preheat chamber may form part of the microwave containment system for the chamber assembly. The chamber assembly may also comprise a heated finishing chamber.

The chamber may comprise one or more chambers arranged to receive the particulate material in sequence and to each at least partially process the material. By this means the parameters of throughput, applied energy at each chamber and oxidizing gas supply may be optimized to produce a selected quality of end product. The chamber preferably takes the form of an open topped annular vibratory vessel as mentioned above. It has been determined that such a vessel may be configured whereby the contents of the vessel will migrate around the annulus while being treated. By this means, the contents may be continuously treated while passing from the inlet to the vessel, usually by gravity feed, to the outlet, which may comprise a gravity overflow outlet, vacuum or scoop.

The preheat chamber may be adapted to preheat the incoming waste sand to a degree determined by the nature of the composition. In the case of foundry waste this may be from 50 to 500°C and preferably from 200 & 300°C. Preferably, the bulk of the preheat is by way of recycled heat generated throughout the processing of the composition, with microwave energy also being introduced into the chamber at controlled intervals to maintain the temperature in the chamber. The preheated composition may then be fed into the main chamber.

The main chamber may comprise a simple microwave containment chamber. However, it is preferred that the main chamber comprise a multiwall arrangement. The multiple walls may advantageously include a refractory lining such as a ceramic lining and preferably includes one or more microwave containment walls such as stainless steel. The lining and or walls may be separated by insulating layers.

In the case of batch mode processes, the main chamber may be of generally a bowl shaped form having the preheat chamber above and the finishing chamber below. The main chamber is preferably provided with transport means such as augers, rakes, controlled vibration or the like and adapted to progress the composition around the main chamber. It is preferred that the main chamber be oriented horizontally with the preheat chamber on top such that feed of the composition is gravity assisted and whereby process gases then to rise from the main chamber up through the composition as a countercurrent thereto.

For continuous processes, the chamber may be configured to have an inlet and an outlet such as an overflow outlet, wherein the material may pass continuously from the inlet, to reside in the chamber for a selected residence time, thereafter to pass in processed form to the outlet. As described above, this residence time may be achieved by vibratory migration about the aforementioned annular chamber. However it will be appreciated that the chamber may comprise any suitable elongate guide or path and having means to continuously progress the material therealong.

The microwave energy may be applied to the pre-heated sand in the main chamber under temperature responsive control means . For example, in the case of foundry waste the temperature of the pre-heated sand may be increased to the ignition temperature of typical resin coatings, usually between 600 and 700°C. The speed of the transport means may also be controlled to accommodate a variety of sand/resin combinations. The source of oxidizing gas may comprise the ambient atmosphere at the oxidizing stage, or entrained air or other oxidizing gas. There may be provided a compressed reservoir or compressor, depending on the selection of oxidant. In the case of air being the oxidant of choice, a compressor may be used having delivery means associated therewith and under control of control means for supply of the air to the main chamber.

The thermal removal and destruction of unwanted binder compounds may result in a partially reclaimed sand which may be fed into the preferred lower finishing chamber through a distribution sieve or other outlet. The preferred heated finishing chamber preferably maintains the partially reclaimed sand at approximately the ignition temperature of the resin coating. Further microwave or other energy may be introduced as required to maintain temperature. The finishing chamber may be in addition to or incorporate the agitating means functioning as described above. For example, the finishing chamber may comprise fluidised bed apparatus or other vibratory apparatus.

The separating means may take any form as described above. Preferably the grading means removes excess heat from the sand (with or without heat recovery) and separates undesirable particulate matter, usually of less than 10 microns, and ash from the recycled sand. Advantageously the separating means may comprise curtain separator. The separating means may be incorporated with, or include an additional, agitation means.

Preferably, the apparatus in accordance with the present invention is configured to be installed into a standard shipping container to make it portable if required. The invention will be further described with reference to the figures illustrating an embodiment of the invention, and wherein:

FIG. 1 is an elevation of apparatus for use in the present invention; and

FIG. 2 is a plan view of the apparatus of FIG. 1. In the figures there is provided processing apparatus comprising spaced frames 10 interconnected by upper 11 , intermediate 12, and lower 13 plant support members. The upper plant support members 11 support a preheater assembly 14.

The preheater assembly 14 comprises a fluidized-bed preheating chamber 15 having a controlled inlet 16 adapted to receive comminuted waste foundry sand from a supply hopper (not shown). The fluidization of the comminuted material and the preheating thereof is attained by a heated air supply annulus 17 to the preheating chamber 15. The controlled inlet 16 is operable to allow closure of the comminuted waste supply, allowing air entrained dust to pass through conduit 20 to a cyclone separator 21. Heat values in the separated air are recovered by allowing exhaust air to pass through outlet 22 to the supply hopper.

Preheated materials pass through a controlled preheater outlet 23 to a main reaction assembly 24 supported on the intermediate plant support members 12. The main reaction assembly 24 comprises a microwave containment chamber 25 having microwave input assemblies 26 mounted thereon. The microwave containment chamber 25 contains an open-topped annular vessel into which microwaves are directed, the vessel being provided with vibratory means, whereby material passing through the inlet to the vessel progressively passes about the annulus under treatment. An overflow lip allows treated material to drop towards a controlled treated-material outlet 27 from the microwave containment housing. The lower plant support members 13 support a finishing assembly 30 comprising a fluidized bed finishing/cooling chamber 31 adapted to receive treated materials from the controlled treated-material outlet 27. Cooling and fluidizing air is supplied via an annular air manifold 32 which accepts a pressurized external supply (not shown) via supply flange coupling 33. Control of the treated material outlet 27 permits heated air and dust to pass into an exhaust riser 34. The exhaust riser 34 includes a temperature controlled diverter (not shown) that directs air at greater than 400°C to the heated air supply annulus 17 and thence to the preheating chamber 15. The diverter directs air at less than 400°C to the cyclone separator 21 , whereafter the heat values in the separated air are recovered by allowing exhaust air to pass through outlet 22 to the supply hopper. Finished foundry sand product passes from the fluidized bed finishing/cooling chamber 31 via controlled outlet 35 supplying a mini pot conveyor 36, which has compressed air operating supply inlet 37 and a product outlet 40 for delivering product to a storage silo (not shown). The lower plant support members further support a dust tank 41 adapted to receive dust from the cyclone separator 21 via dust conduit 42, the dust being conveyed from dust outlet 43 to the foundry baghouse (not shown).

In use, initial breakdown of the recycled sand is achieved using traditional mechanical attrition. Conglomerates of sand are reduced to less than 5mm in diameter prior to being fed into the abovedescribed apparatus.

This apparatus achieves thermal reclamation of sand particles. The reclamation process can be divided up into three separate process components - a preheat process, a reaction process and a finishing process. The incoming waste sand is pre-heated in the preheat chamber to between

200 and 300X primarily by way of recycled heat generated throughout the process.

The preheated sand is then fed into the main reaction chamber.

By use of selected microwave frequencies, generally about 915 MHz to about

2450 MHz depending on the binder and/or processing rate, applied to the pre-heated

sand within the annular bowl shaped vessel, the temperature of the pre-heated sand

is increased to the ignition temperature of the resin coating (between about 450 and

700°C).

The bowl shaped vessel consists of an open-topped, annular stainless steel

bowl around which the sand flows through vibration of the vessel. The speed of the

flowing sand can be controlled to accommodate a variety of sand/resin combinations.

Microwave energy is introduced to the sand as it moves around the bowl resulting in

an increase in the temperature of the sand/resin towards the ignition temperature of

the resin.

This increase in temperature results in the thermal removal and destruction of

unwanted binder compounds. A microwave detection device is also installed to

ensure safe operation of the unit. Depending on the sand/resin type undergoing

reclamation the sand may be only partially reclaimed at this point. If this is the case

then the partially reclaimed sand is then fed into the finishing process.

The finishing process may at first maintain the partially reclaimed sand at

approximately the ignition temperature of the resin coating by autooxidation.

Thereafter or otherwise the product is cooled as described above.

This process simultaneously removes excess heat from the sand and

separates particulate matter (less than 10 microns) and ash from the recycled sand utilizing a fluidised bed process and curtain separator. The reclaimed sand can then be directed away from the reclamation unit for reuse in the foundry process. The resultant sand grain shape, size and distribution remain essentially the same as the original sand prior to recycling. The air heated in the fluidised bed is contained and piped to the preheat chamber to preheat the incoming sand/resin.

Apparatus in accordance with the foregoing embodiment has the ability to recycle waste sand at far lower cost than current alternative processes. The low energy consumption, low waste heat evolution, portability, and environmentally desirable aspects of this invention are of advantage to the casting industry. The apparatus and method achieves a reclaimed sand with minimal "loss on ignition", minimal thermal shock damage, or segregation of sand grain particle size.

Energy required to achieve the above result is significantly less than alternative methods due to the ability of microwave "targeting", hence minimal energy input. The waste heat and emissive by products are reintroduced back into the incoming unprocessed sand at the preheat stage. This further increases the thermal efficiency of the apparatus whilst reducing the environmental impact.

It will of course be realised that while the foregoing description has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as claimed in the claims appended hereto.

Claims

Claim

1. A method of recovery of particulates from a composition including particulate material and an organic adjunct and including the steps of: heating said composition with microwave energy to an oxidizing temperature; subjecting said heated composition to an oxidizing atmosphere to oxidize said organic adjunct; agitating said composition to dislodge adherent materials from said particulates; and separating a recovered particulate product from said adherent materials.

2. A method according to claim 1 , wherein said particulate material is a relatively microwave transparent material and the organic adjunct is a relatively microwave absorbent material.

3. A method according to claim 1 , wherein said particulate material is a foundry sand and said organic adjunct is a foundry sand binder.

4. A method according to claim 3, wherein said composition comprises comminuted foundry cores or moulds.

5. A method according to claim 1 , wherein said composition includes a microwave-absorbent compound or composition to effect the heating.

6. A method according to claim 4, wherein said composition is comminuted to a particle size of less than 5 mm.

7. A method according to claim 6, wherein the heating by microwave energy is incorporated in a substantially continuous process.

8. A method according to claim 7, wherein the feed rate of material and microwave energy is controlled to provide a process temperature of from about 450 to 700°C.

9. A method according to claim 8, wherein said oxidizing atmosphere is air.

10. A method according to claim 8, wherein said air is preheated to assist in elevating the temperature of the process to an oxidation temperature.

11. A method according to claim 1 , wherein vented oxidation product gases are recycled to the composition to preheat before heating or admitted to the reacting composition to recover process heat and/or to contain entrained fines.

12. A method according to claim 1 , wherein said agitation of the composition is by means of a vibratory vessel whereby the particles may work together to dislodge adherent materials.

13. A method according to claim 12, wherein said vibratory vessel comprises an essentially open topped, annular, vibratory vessel.

14. A method according to claim 13, wherein said heating of the composition with microwave energy to an oxidizing temperature is performed in said essentially open topped, annular, vibratory vessel.

15. A method according to claim 1 , wherein said separating of the particulate product is by cyclone separation.

16. Apparatus for recovery of particulates from a composition including particulate material and an organic adjunct, and including: a chamber having an oxidizing atmosphere, an inlet adapted to receive said composition and an outlet; a microwave source supplying microwave energy to composition in said chamber; agitation means for composition being processed by said chamber; and separating means adapted to receive agitated composition from said agitation means and separate therefrom said particulates.

17. Apparatus according to claim 16, wherein said chamber is an open topped annular vibratory vessel.

18. Apparatus according to claim 16, further including a preheat chamber adapted to preheat said composition.

19. Apparatus according to claim 18, wherein said composition is foundry sand waste said preheat chamber heats the waste to between 200 to 300°C.