WO2002079113A1

WO2002079113A1 - Heat treatment of expansible materials to form lightweight aggregate

Info

Publication number: WO2002079113A1
Application number: PCT/GB2002/001391
Authority: WO
Inventors: Robin Anthony Kyffin; Eifion Wyn Williams
Original assignee: FFESTINIOG EXPANDED SLATE Co Ltd
Current assignee: FFESTINIOG EXPANDED SLATE Co Ltd
Priority date: 2001-03-29
Filing date: 2002-03-22
Publication date: 2002-10-10
Anticipated expiration: 2003-09-29
Also published as: GB2373842A; GB0107827D0

Abstract

A method and apparatus for heat treatment of expansible materials to form lightweight aggregate is disclosed. The material is heated within a kiln to a temperature less than that required for optimum expansion. Then the material is further heated by application of microwave energy to complete its expansion. Typically, initial heating of the material is performed in a kiln to a temperature below an optimum expansion temperature of the material. Subsequently, microwave energy is applied either within or externally of the kiln. The method and apparatus can be used with a material such as slate chips, shale, clay or other material. It may be mixed with a refractory material that is microwave-transparent, such as alumina or bauxite.

Description

Heat treatment of expansible materials to form lightweight aggregate

This invention relates to heat treatment of expansible materials to form lightweight aggregate.

It is well known, for example from GB-A-2261938, that lightweight aggregate can be produced commercially by suitable heat treatment of certain mineral and rock materials. These include clays, shales and slates of suitable chemical and mineral composition. Before such treatment, in general these materials have to be prepared by size grading and drying. Most usually, the minerals are formed into particles; slates and shales are used in such processes in the form of chippings whereas clays are formed as pellets. Sizes are usually in the range 4 mm to 20 mm.

Treatment involves heating of the particulate material to the point where fusion starts to occur with the material being taken into a plastic state. There is a wide variation of such temperatures, depending on the material under treatment. These are usually in the range 1100 to 1300 °C. At these treatment temperatures, the material under treatment starts to fuse, each particle on its outer surface first. This tends to seal the particle. Then, as heat penetrates to raise the interior temperature, gases are formed within the particle. Such gases cannot easily escape through the sealed surface and they act on the plastic material of each particle, forcing expansion to take place. Once expanded, the material is removed from the heat source as speedily as practicable and cooled to preserve the expanded structure. Expansions of up to five times the original volume are possible and the resulting aggregate has a hard outer skin with a cellular structured interior full of voids. The resulting bulk density can be as low as one fifth of that of the original feed stock.

Heat treatment can be effected in a number of devices including travelling grate furnaces, fluidised bed furnaces, rotary kilns, tunnel kilns and more including any device capable of transporting material through a heat transfer zone at high temperature and heating the same to that temperature, rotary kilns being used most commonly.

Most known treatment methods suffer from a difficulty (the "sticktion problem") which is inherent in the process. As already explained the material has to be heated to a point of fusion at which it is effectively in the plastic state. In this condition the particles become sticky and tend to stick together to form agglomerations and also to stick to the lining or support system within the furnace, be that material metallic, refractory or other. This can cause the treatment process to fail. This problem appears to be most severe when the method is carried out in a rotary kiln.

There are two known approaches taken to address the sticktion problem.

First, the process temperature may be reduced. For example, in the hottest zone, the temperature may be reduced by some 20 to 50 °C below the optimum for expansion. By this means the sticktion problem is eliminated. However, the product is then only partially expanded.

Second, a refractory usually coarse-grained powder is mixed with the feed particles. This powder typically has a grain size of 0.25 mm to 1 mm, and a melting point substantially greater than that of the material under treatment. Fairly pure silica sand could be used. Amounts of powder are typically 20% by weight. However in order to obtain the very best expansions by using a slightly higher temperature again, a greater proportion of powder is needed. The powder acts in two ways, firstly to line the kiln refractory providing it with a non sticky surface at high temperature and secondly to keep the individual feed particles from sticking together by coating them with a non- sticky layer a high temperature. The powder can be separated from the product after cooling and can thus be reused. The method is effective but does carry a penalty in that additional energy is needed merely to heat the powder to temperature.

A principal aim of this invention is to overcome, or at least ameliorate, the sticktion problem in a manner that is compatible with a wide range of treatment apparatus, and with rotary kilns most especially, without encountering disadvantages of known approaches. From a first aspect, this invention provides a method of heat treatment of expansible materials to form lightweight aggregate in which material is heated within a kiln to a temperature less than that required for optimum expansion, and then further heated by application of microwave energy.

Because the outer surface of the material remains below its full fusion temperature within the kiln, it does not have a chance to agglomerate and stick to the lining of the kiln. Heating with microwave radiation tends to heat the core of the particles, providing the internal temperature required for full expansion. The surface of the particles preferably remains below the full fusion temperature.

The material may be heated by microwave energy externally of the kiln or within the kiln, typically at a region near to a discharge of the kiln.

The material is preferably heated in the kiln to a temperature just (a small amount) below that required to achieve full expansion prior to application of microwave energy. For example, the material may be heated in the kiln to a temperature less than 50° or 60 °C (for example, in the range of 20 to 30 °C or 30 to 40 °C) below the optimum expansion temperature.

In order to further reduce the risk of sticktion and agglomeration of the material under treatment, it is preferable to add a proportion (e.g. 10% by weight) of refractory powder along with the material. Both the material and the refractory powder are subject to microwave energy towards the end of the treatment process. Some of the refractory powder may advantageously remain as an adhesion-resistant coating in parts of the apparatus. In such embodiments, the refractory powder is most preferably selected as to be non-absorbent of microwave radiation in order than it is not heated by the microwave energy. Such refractory material may include highly aluminous materials. A material of being largely alumina or of highly aluminous bauxite may be suitable. In general, references to refractory materials in this specification can be taken as typically referring to materials of this type. The same considerations apply to refractory materials that are used within the construction of the apparatus. The region of the apparatus in which microwave energy is applied is most typically enclosed within a microwave containment casing, typically of microwave-reflective metal to prevent leakage of microwave energy into the environment. Any aperture that penetrates the casing must be equipped with suitable seals or filters to prevent leakage.

While it is possible to effect application of microwave energy within the kiln itself, it may be more convenient to provide an external chamber into which hot material can fall upon exiting the kiln.

The temperature (at least within the core of the particles) of the material is preferably raised to the optimum for expansion by the application of microwave energy of suitable frequency either in the kiln close to discharge or after discharge from the kiln. In the latter case, microwave radiation is preferably applied immediately after the material has exited the kiln, in order that it has a minimum amount of time in which to cool.

The material may, for example, include slate, typically in the form of chippings. In such embodiments, the material may be heated in the kiln (or in the kiln and subsequently in a microwave chamber) to a temperature of approximately 1210 °C. Other materials may be used, such as shale, clay or other minerals, each being heated to a particular temperature to achieve full expansion.

From a second aspect, this invention provides heat treatment apparatus for production of expanded aggregates comprising a kiln for heating particles, and a microwave system for supplying microwave energy to material that has exited the kiln. Apparatus embodying this aspect of the invention can typically be used in carrying out a method according to the first aspect.

The kiln is most usually a rotary kiln, but could be any other device suitable for heating the material to the temperature required. For example, it may be a travelling grate furnace, a fluidised bed furnace or a tunnel kiln.

The microwave system may include an elongate chamber through which particles that have exited the kiln can pass under the action of gravity. In preferred embodiments, the chamber rotates axially in use to assist the transport of material through the chamber. In such embodiments, the chamber may formed with a plurality of axially extending pockets between which material falls when the chamber rotates. The effect of this is to tumble the material form one pocket to the next during exposure to microwave energy. In alternative embodiments, the chamber is disposed with its axis approximately vertical. Such a chamber need not rotate to cause the material to pass through it; indeed, it is typically necessary to provide internal formations that control the speed at which material passes through the chamber under the action of gravity.

In typical embodiments of the invention, the chamber has a refractory lining that is highly transparent to microwave radiation.. The refractory material is most usually contained within an enclosure that contains microwave radiation to prevent leakage of microwave energy into the environment. To this end, apertures in the enclosure through which material under treatment pass are advantageously provided with filters that inhibit passage of microwave radiation through the apertures.

This invention also provides a method and an apparatus for production of expanded aggregates substantially as herein described with reference to the accompanying drawings.

Embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawing, in which:

Figure 1 shows diagrammatically a section through the discharge end of heat treatment apparatus being a first embodiment of the invention;

Figure 2 shows diagrammatically a section A-A through the discharge end of heat treatment apparatus being a second embodiment of the invention;

Figures 3 shows a section of a kiln rotor of the embodiment of Figure 2; and

Figures 4 and 5 are diagrammatic elevations of a microwave chamber of a third embodiment of the invention. The embodiments will be described with reference to a system for expanding materials in a rotary kiln and the following discourse on the invention relates to this device. However, the invention has equal application to systems that are based on other types of treatment apparatus.

Kiln chamber lengths and types vary but the sticktion problem is encountered with all of them. Towards discharge, over the final quarter or so of the chamber's length, the material reaches incipient fusion at high temperature and then the transit time to discharge must be sufficiently long for heating of the interior of each particle to take place. The result is that there is then a strong tendency for the particles to stick together and form large agglomerations and also for some sticktion of the material to the lining to take place. This impinges on the reliability of the process and prevents an orderly transit of material through the hottest zone of the kiln.

Even with the lower temperatures at which embodiments of the invention can operate (as compared with known expansion systems) it is important that the material is heated sufficiently to seal the surface and heat each particle through to its core, and it is equally important to keep free of sticktion. These requirements are to an extent mutually conflicting. In order to be certain that the sealing surface temperature has been reached, that the particles have been heated to the same temperature right through, and at the same time ensure a regime within the hot zone that is free from sticktion, some, albeit a smaller proportion, of refractory powder that is, as far as possible, non-absorbent of microwave energy, will need to be admixed. Typically this would be 10% by weight and proportions of this order are preferable to attempting to work the process without any admix at all.

With reference to Figure 1, heat treatment apparatus being a first embodiment of the invention includes a rotary kiln that has an inclined tubular chamber 10 (only a hot discharge end portion of it being shown in Figure 1), usually lined with refractory material, which is made to rotate about an axis that is inclined slightly to the horizontal. Particulate feed material as above described is fed into the upper end of the chamber 10 and is progressively heated as it travels along the chamber 10 as this rotates. Residence time within the chamber 10 is determined by a combination of speed of rotation and angle of inclination. Heat is supplied by a burner 12 at the lower end of the chamber 10, the material being discharged there after having been heated to the required temperature. A hood 14 of refractory material seals the chamber 10 (except for an exit route through a discharge chute 16) and provides a burner aperture.

Heated material exiting the chamber 10 falls into a refractory lined and insulated enclosed discharge chute 16, and from there, enters a conically shaped microwave chamber 18, insulated and refractory lined.

The microwave chamber 18 has an exit that through which material can leave it. Material flow through this exit to a cooler below is induced by a conveyor at the cooler discharge at a rate such that a body 20 of material is formed within the microwave chamber 18, whereby the material has a dwell time within the chamber. A control system is employed to adjust the rate of the conveyor to maintain the body 20 at an optimum level within the microwave chamber 18.

Microwave radiation is introduced into the chamber by an array of several waveguides 22. The waveguides are disposed such that they apply microwave radiation as evenly as possible to the surface of the body 20. They are also constructed to be resistant to heat that will be absorbed from material being treated.

In this example, slate chippings and an admix of refractory powder move down the kiln chamber 10 at a rate of 7000 kg/hour and 700 kg/hour respectively. Both are discharged at a temperature of 1210 °C. By the time the chippings reach discharge they will have been heated right through and their surfaces will have lightly fused providing a sealed surface. During the latter stage of the transit through the chamber 10, they enter a soaking phase after the chippings' surfaces has reached and is maintained at 1210 °C, separation and the prevention of sticktion is assured by the admix of the powder. At this stage, on the point of discharge, the chippings will be sealed but only partially expanded.

Chippings and admix then fall down the exit chute 16 to the microwave chamber 18 to form a cone-shaped body 20 in the microwave chamber 18. The chamber 18 is well insulated as is chute the exit so that heat loss is small in relation to the mass flow rate of materials arriving. This ensures that there will be insignificant loss of particle surface temperature on arrival in the microwave chamber 18. The surface of the body of partially expanded slate chippings will be under the influence of a microwave flux supplied from the waveguides 22. The microwave energy incident on the conical surface then heats the interior of the slate pieces preferentially to a temperature of 1230 to 1240 °C with only a small rise in particle surface temperature. This internal rise of temperature causes maximum gas generation within the particles and thus ensures maximum expansion. The surface of the body 20 will be under continuous renewal by material under treatment falling from above. This being so, the degree of penetration of microwave energy into the bulk of the deposit is not of crucial importance.

The microwave power needed to cause this 20 to 30 °C temperature rise on 7000 kg/hour is expected to be in the order of 40 kW, although experimentation may show that more or less power is required in a particular installation. Calculations suggest that the energy supplied in the form of microwave radiation represents only some 2 to 3 % of the total power requirement., provided that efficient heat recovery and insulation systems are used.

A second embodiment of the invention is shown in Figures 2 and 3.

With reference to Figure 2, heat treatment apparatus embodying the invention shows the discharge trunk of a rotary kiln 110. coupled by a gaiter or packing 112 to an inclined insulated chute 114.

The apparatus includes a microwave chamber 118 that comprises an inclined cylindrical rotary vessel 124 of microwave reflective stainless steel. (Support rollers, thrust and drive arrangements are provided for the camber but these are not shown.) The angle of inclination is typically between 3° and 15° to the horizontal and is adjustable. The rotary vessel 124 is lined with high alumina microwave transparent refractory 120 which is backed with high alumina fibre microwave transparent insulation 122. The refractory lining 120 is in the form of sixteen deep, rounded, concave pockets 150 as shown in Figure 2. The design speed for the rotor is 0 to 10 r. p. m. A stationary end wall 126 is provided to close the feed end of rotary vessel 124. This is of similar construction to the chamber 118 having high alumina refractory backed by high alumina fibre insulation all contained within a microwave-reflective stainless steel casing. Microwave seals 132 are provided between the fixed end walls and the rotor.

A chute 114 feeds material exiting the kiln 110 as far as a stainless steel of the end wall 126 of the vessel 124. At this station there is a microwave filter 116 prevents escape of microwave energy from the chamber 124 where it is penetrated by the chute 114. The filter includes a stainless steel tube or set of tubes with dimensions related to the microwave wavelength. In the case of the normal industrial microwaves of frequency 915 MHz as used in this embodiment, the tube diameter is of the order of 140 mm with a corresponding length of about 450 mm. The filter tube (or tubes) 116 feed into a high alumina, non-absorbent refractory feed chute 128. Also in the end wall 126 there is provided a flue 142, as shown in Figure 2. The flue 142 similarly incorporates a microwave filter similar to that employed at 116.

A waveguide 134 feeds microwave energy at 915 MHz. The waveguide 134 connects accurately with the stainless steel casing and directs the microwave energy through the transparent alumina of the linings.

Optionally, an additional wave guide supply is provided at the other end of the vessel 124. The actual number and disposition of the wave guides to introduce the energy into the chamber can readily be determined by routine experimentation or calculation.

At a lower discharge end of the vessel 124 there is provided a stationary discharge hood 130, again of constructed with a stainless steel casing and alumina refractory and insulation lined. Material falling from the discharge end of the vessel 124 enters the discharge hood 130 and falls through it. Opposite the path taken by the charge of material entering the discharge hood 130, there is provided a low-output burner 136 with a correspondingly positioned flue 142 in the stationary end wall 126 of the vessel 124. Both the flue and burner are provided with microwave filter or barrier arrangements as previously discussed. At the bottom of the end wall approximately opposite the feed chute 114, there is provided a window 138, for viewing the regime within the chamber and for optical or radiative temperature measurements. Again, a microwave barrier or filter is incorporated into the window 138. At the bottom of the discharge hood 130 a set of microwave filter tubes 140 is positioned and the output of expanded aggregate and refractory powder fall out of the system through these tubes 140 to a disposal route incorporating a cooler and thence to stockpiles.

In practice, the entire system of the chamber, rotor and stationary end walls would be fully monitored by instrumentation sensors for temperature and escape of microwave energy for the safety of personnel.

The whole system in its entirety between rotary kiln 110 discharge and disposal route is capable of adjustment in terms of angle of inclination of the rotor axis to the horizontal. Thus by a combination of angle of inclination and speed of rotation the residence time of material within the chamber 118 can be controlled. In order to line up with the rotary kiln 110 discharge, the whole device is capable of being moved bodily in the horizontal plane to allow for misalignment caused by adjustment of angle of inclination. At the chamber discharge at the filter 140, the disposal route is capable of adjustment as necessary to effect alignment

This embodiment has the following important features that make for effective working and operation.

1. The residence time of material within the microwave chamber is entirely adjustable. Operational residence times are typically between 5 and 20 seconds. There has to be a minimum acceptable residence time to enable the material to respond to the energy and to enable a control system to operate in terms of microwave energy supplied in relation to the chamber throughput and temperature rise achieved.

2. The profile of the rotor refractory in the form of sixteen deep, rounded pockets has the effect of dividing the charge into discrete bundles thereby tending to limit the size of any agglomeration that might form. Furthermore the pockets will retain their contents for approximately 120° of rotation and at 6 r. p. m. this equates to some 3 seconds only. During this time the particles will rearrange themselves by reason of expansion. Being stationary in relation to the lining, the expansion will tend to cause rearrangement without compaction and after the short time of approximately 3 seconds the material will fall out.

Agglomeration formation will be unlikely in the time scale and any tend to form will tend to be broken after the free from the chamber. In any event, the regime can be observed and the power level of the microwave energy can be adjusted to ensure a safe working regime. The manner of heating by microwave with the interior of each particle heating preferentially also helps to avoid agglomeration.

3. The heat transfer relationship between lining and charge is different to that existing within a rotary kiln. Instead of transfer from lining to charge the reverse takes place in that the lining loses heat to the casing which is then replenished by heat loss from the charge. If the burner 136 is not brought into play, the lining will settle in equilibrium at a lower temperature than the materials in transit for any set of steady-state conditions. This contributes to the prevention of sticktion. The chamber is also supplied with refractory powder. The risk of sticktion to the lining is therefore small. In any event the regime can be observed and more refractory powder can be supplied in the event of sticktion appearing to be imminent. Additionally, although microwave heating is in progress, heat transfer from the particles in transit to the lining will tend to lower their surface temperature. This reduces the likelihood that agglomeration will occur.

4. The principal purpose of the low output burner 136 is to raise the chamber temperature to that required for operation prior to start-up. However, it can also be used to supply casing losses, and to control the temperature of the lining consistent with safe operation of the chamber. This allows greater control of product quality and particularly the condition of aggregate skin by being able to exert some influence on particle skin temperature. Again, the viewing window 138 allows an operator to judge what is safe. With regard to the profile of the rotary chamber, all manner of rib and pocket designs are possible all within the scope of the invention. This example is intended to demonstrate a particular arrangement of what is possible under the invention.

Typically the chamber dimensions could be approximately as follows:

• Rotary chamber 124 length: 1800 mm;

• Minor diameter (i.e. of a circle described by refractory teeth 150): 600 mm;

• Major diameter (i.e. of a circle described by pocket bottoms): 850 mm;

• Diameter of containment rotary cylinder 124: 1300 mm; and

• Refractory and insulation thickness: 225 mm.

Each pocket 150 when full to the brim can contain at least 18 litres of material (the volume of refractory powder is ignored in this figure, since it will be small and will not affect the overall volume). At the feed end, partially expanded aggregate will arrive with a bulk density of about 650 kg/m³. Therefore, each pocket 150 can contain as much as 11.5 kg. At discharge, the lightest aggregate is expected to be some 280 kg/m³ and at this density each pocket 150 could contain 5 kg. This is the extreme case where the aggregate is at its lightest.

In operation, the rotary kiln 110 is operated in an entirely reliable manner with an operating temperature of 1160 °C. The feed rate is 7000 kg/hr of 4 to 12 mm slate (or other) chippings accompanied by 700 kg/hr (10%) bauxite powder sized 300 to 900 μm. Partially expanded slate with a bulk density of approximately 650 kg/m³ with the bauxite falls out of the kiln and passes into the microwave chamber 118. This partially expanded slate at approximately 1160 °C will be just starting to fuse and will be thus sealed and will have been heated right through.

The speed of rotation of the chamber 118 is set at 6 r. p. m. with an angle of inclination to effect a residence time of 15 seconds. 7000 kg/hour equates to a throughput of approx. 2 kg/s. This means that the chamber will only contain approximately 30 kg of chippings. On balance, with the pockets as shown about 6 pockets 150 (120° of the full circle) would be in use, each one being active in this respect for about 3.5 seconds before tumbling out to be picked up again by a following pocket 150. Thus after approximately 4 to 5 tumbles or 1.5 revolutions the product is discharged from the chamber 118 via the outlet microwave filter 140.

Now to consider the pockets 150. At any time, approximately six pockets 150 contain a total of 30 kg of slate chippings (i.e. approximately 5 kg per pocket). At the lightest envisaged density of around 280 kg/m³, this will not exceed the pocket capacity of 18 litres. Even if this were to happen, some spill-over towards the discharge end, in the time scale of 15 seconds residence time would not matter.

The product leaves the chamber fully expanded at an average temperature of 1200- 1210 °C. In the time scale of 15 seconds, and with the charge divided as described, agglomeration is unlikely to be a problem.

A particular advantage of this embodiment is the degree of control of the process that is possible, and that the control can be judged by observation through the window 138, this being additional to information provided by instrumentation. For any set throughput, there is provided:

• Control over residence time;

• Control over microwave power input;

• Maintenance of residence time, but with variation of speed of rotation by altering the angle to compensate, this affects the amount of material retained in each pocket; and

• Some control of the temperature of the refractory lining by use of the low output burner 136.

A third and simpler embodiment of the invention will now be described with reference to Figures 4 and 5. Figures 4 and 5 are diagrammatic elevations of a microwave chamber that includes a cascade of tiles which slow material in free fall to provide a useable residence time for the microwave energy to effect the desired degree of heating.

A rotary kiln discharge trunk 160 feeds partially expanded slate mixed with refractory powder into a microwave chamber 164 through a microwave filter 162. The chamber 164 has a microwave-reflective stainless steel casing 166 lined with a microwave- transparent refractory and insulating system similar to that described in the second embodiment. A cascade of microwave transparent (aluminous) tiles 170 is incorporated in to the system. These are set at approx. 35° to the horizontal. Exit filter tubes 160 are provided to seal the chamber - a requirement that has already been discussed. Microwave energy is introduced into the chamber via wave guides 172 and the energy passes through the transparent lining. The height and number and exact disposition of the tiles will be determined by experiment and modelling. However given sufficient height a delay in free fall to effect residence times of some 5 to 8 seconds is possible. The operating parameters and results are much in accordance with the second embodiment. Although simpler, this system does not have the same advantages in terms of control as the first or second embodiment. However, this embodiment can offers a valuable alternative for a lower cost.

The microwave power needed to effect 50 °C rise in temperature on 7000 kg/hour is about 80 kW. Allowing for losses in the microwave generator and its system the actual power input would be some 120 kW. Investigations show that at this level the microwave energy would be only some 5-6 % of the total energy requirement of a modern thermally efficient expanded aggregate production plant incorporating all available energy saving measures.

Claims

1. A method of heat treatment of expansible materials to form lightweight aggregate in which material is heated within a kiln to a temperature less than that required for optimum expansion, and then further heated by application of microwave energy.

2. A method according to claim 1 in which the surface of the particles remains below their full fusion temperature.

3. A method according to claim 1 or claim 2 in which the material is heated by microwave energy externally of the kiln

4. A method according to any one of claims 1 to 3 in which the material is heated by microwave energy within the kiln

5. A method according to claim 4 in which the material is heated within the kiln at a region near to a discharge of the kiln.

6. A method according to any preceding claim in which the material is heated in the kiln to a temperature below that required to achieve optimum expansion prior to application of microwave energy.

7. A method according to claim 6 in which the material is heated in the kiln to a temperature less than 60 °C (for example, in the range of 30 to 40 °C) below the optimum expansion temperature.

8. A method according to any preceding claim in which the material is mixed with a refractory material prior to introduction into the kiln.

9. A method according to claim 8 in which the refractory material is relatively transparent to microwave radiation.

10. A method according to claim 9 in which the refractory material is high in alumina or in bauxite.

11. A method according to any one of claims 8 to 10 in which the refractory material is added at a rate of less than 20% by mass

12. A method according to claim 11 in which the refractory material is added at a rate of 10% by mass or less.

13. A method according to any preceding claim in which the temperature within the core of the particles of the material is raised to an optimum temperature for expansion by the application of microwave energy of suitable frequency either in the kiln close to discharge or after discharge from the kiln.

14. A method according to claim 13 in which microwave radiation is applied immediately after the material has exited the kiln.

15. A method according to any preceding claim in which the material is slate.

16. A method according to any preceding claim in which the material is heated in the kiln to a temperature of approximately 1210 °C.

17. A method according to any one of claims 1 to 16 in which the material is shale, clay or another mineral, each being heated in the method to a particular temperature to achieve full expansion.

18. A method according to any preceding claim in which the material is in the form of chippings.

19. A method of heat treatment of expansible materials to form lightweight aggregate substantially as herein described with reference to the accompanying drawings.

20. Heat treatment apparatus for production of expanded aggregates comprising a kiln for heating particles, and a microwave system for supplying microwave energy to material that has exited the kiln.

21. Heat treatment apparatus according to claim 20 in which the kiln is a rotary kiln.

22. Heat treatment apparatus according to claim 20 or claim 21 in which the kiln includes one or more of a travelling grate furnace, a fluidised bed furnace, a tunnel kiln or a rotary kiln.

23. Heat treatment apparatus according to any one of claims 20 to 22 in which the microwave system includes an elongate chamber through which particles that have exited the kiln can pass under the action of gravity.

24. Heat treatment apparatus according to claim 23 in which the chamber rotates axially in use.

25. Heat treatment apparatus according to claim 24 in which the chamber is disposed at an angle in the range of 3° to 15° to the horizontal.

26. Heat treatment apparatus according to claim 23 or claim 24 in which the chamber is formed with a plurality of axially extending pockets between which material falls when the chamber rotates.

27. Heat treatment apparatus according to claim 23 in which the chamber is disposed with its axis approximately vertical.

28. Heat treatment apparatus according to claim 27 having internal formations that control the speed at which material passes through the chamber under the action of gravity.

29. Heat treatment apparatus according to any one of claims 20 to 28 in which the chamber has a refractory lining that is highly transparent to microwave radiation.

30. Heat freatment apparatus according to claim 29 in which the refractory material is contained within an enclosure that contains microwave radiation.

31. Heat freatment apparatus according to claim 30 in which apertures in the enclosure through which material under treatment pass are provided with filters that inhibit passage of microwave radiation through the apertures.

32. Heat treatment apparatus according to any one of claims 20 to 31 suitable for use in performing a method according to any one of claims 1 to 19.

33. Heat treatment apparatus for production of expanded aggregates substantially as herein described with reference to the accompanying drawings.