GB2373842A

GB2373842A - Heat treatment of expansible materials to form lightweight aggregate

Info

Publication number: GB2373842A
Application number: GB0107827A
Authority: GB
Inventors: Robin Kyffin; Eifion Wyn Williams
Original assignee: FFESTINIOG EXPANDED SLATE COMP
Current assignee: FFESTINIOG EXPANDED SLATE COMP
Priority date: 2001-03-29
Filing date: 2001-03-29
Publication date: 2002-10-02
Also published as: GB0107827D0; WO2002079113A1

Abstract

A method and apparatus of heat treatment of expansible materials to form lightweight aggregate, where the material is first heated within in a kiln and then further heated by the application of microwave energy to complete its expansion, either within or external of the kiln. The use of the microwaves is intended to overcome the problem of the material fusing together or to parts of the furnace, as heating with microwave radiation tends to heat the core of the particles, while the surface of the particles remains cooler. This method may be used with a material such as slate chips, shale or clay. The material may be mixed with a refractory material such as silica sand prior to treatment.

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 4mm to 20mm.

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 oc. 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.

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 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 30 oC below the optimum for expansion By thismeansthesticktion"roblemiseliminatedHoweve the nroduct is then onlv partially expanded Second, a refractory usually coarse-grained powder is mixed with the feed particles This powder typically has a grain size of 0.25mm to 1 mm, and a melting point substantially greater than that of the material under treatment For instance, 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.

An 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, 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 expansion prior to application of microwave energy. For example, the material may be heated in the kiln to a temperature less than 50 duc (for example, in the range of 20 to 30 duc) below the optimum expansion temperature.

In order to further reduce the risk of agglomeration of the material, it is preferably mixed with a refractory material prior to introduction into the kiln. For example, the refractory material could be a refractory sand, for example, silica sand. The material may be added at a rate of less than 20% by mass, and more preferably at a rate of 10% by mass or less.

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, be slate, typically in the form of chippings. In such embodiments, the material may be heated in the kiln to a temperature of approximately 1210 duc. Other materials may be used, such as shale, clay or other minerals, each being heated to a particular temperature to achieve full expansion.

Advantageously, the material is accumulated in a body, microwave radiation being applied to the surface of the body. Material is typically removed from the body at a rate substantially equal to the rate at which the material is accumulated in the body.

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 may be a rotary kiln, or 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

This inventior. also provides 2 method and an apparatus for production of expanded aggregates substantially as herein described with reference to the accompanying zn drawings An 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 embodying the invention The embodiment 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.

In the above it is important that the material is heated sufficiently to seal the surface 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 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 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 embodying 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 progressibely 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 sand move down the kiln chamber 10 at a rate of 7000kg/hour and 700 kg/hour respectively. Both are discharged at a temperature of 1210 duc. 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 duc, separation and the prevention of sticktion is assured by the admix of the sand. 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 duc 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 duc 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.

Claims

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 zn that required for optimum expansion, and then further heated by application of microwave energy.
2. A method according to claim I in which the surface of the particles remains t : D below their full fusion temperature.
3. A method according to claim I 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

ly w'th'n the k'lti 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.

Z-1
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 expansion prior to

application of microwave energy. c
7. A method according to claim 6 in which the material is heated in the kiln to a temperature less than 50 oC (for example, in the range of 20 to 30 OC) 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 a refractory sand such as silica sand.
10. A method according to claim 8 or claim 9 in which the refractory material is added at a rate of less than 20% by mass
11. A method according to claim 10 in which the refractory material is added at a rate of 10% by mass or less.
12. 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.
13. A method according to claim 12 in which microwave radiation is applied immediately after the material has exited the kiln.
14. A method according to any preceding claim in which the material is slate.
15. A method according to claim 14 in which the material is heated in the kiln to a temperature of approximately 1210 oc.
16. A method according to any one of claims 1 to 13 in which the material is shale, clay or another mineral, each being heated in the method to a particular temperature to achieve full expansion.
17. A method according to any preceding claim in which the material is in the form of chippings.
18. A method according to any preceding claim in which the material is accumulated in a body, microwave radiation being applied to the surface of the body.
19. A method according to claim 18 in which material is removed from the body at a rate substantially equal to the rate at which the material is accumulated in the body.
20. A method of heat treatment of expansible materials to form lightweight aggregate substantially as herein described with reference to the accompanying drawings.
21. 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.
22. Heat treatment apparatus according to claim 21 in which the kiln is a rotary kiln.
23. Heat treatment apparatus according to claim 21 or claim 22 in which the kiln

includes one or more of a travelling grate furnace, a fluidised bed furnace or a Z-1 z : l tunnel kiln.
24. Heat treatment apparatus according to any one of claims 21 to 24 suitable for use in performing a method according to any one of claims 1 to 21.
25. Heat treatment apparatus for production of expanded aggregates substantially as L-t : l-1 herein described with reference to the accompanying drawings. p I L