GB2236747A - Heat treatment of expansible materials to form lightweight aggregate - Google Patents

Abstract

Adhesion of particulate material (e.g. slate or clay), undergoing heat treatment in the region of its fusion temperature (e.g. 1200 - 1300 DEG C), to an underlying hearth (14) is prevented by providing a thin layer of sand over the hearth before the material is distributed thereover. The hearth (14) is preferably annular and moved successively through a sand supply zone (SZ), a pre-heating zone (PHZ), a charging zone (CZ) (where material to be treated is supplied), a heat treatment zone (TZ) and a discharge zone (DZ). Discharge of the heat-expanded product is facilitated by the presence of the sand layer, and cooling at this stage is no longer necessary. Intermittent supply of sand may be adequate to replenish that removed along with the expanded product. <IMAGE>

Description

TREATMENT OF EXPANSIBLE MATERIALS TO FORM LIGHTWEIGHT AGGREGATE The production of lightweight aggregate having good thermal insulating properties has become extremely important in the building industry, for use, for example, in the production of breeze blocks and more generally as a component added to concrete cast in situ, and also in precast concrete panels, lightweight concrete roof slabs, and as fillings for hollow cavities, lightweight reinforced concrete structures and many other applications where low thermal conductivity and lightness are important.

Attempts have already been made to produce lightweight aggregates from natural materials, such as certain slates, clays and shales, which can be expanded or "bloated" by suitable heat treatment.

In the case of suitable slate wastes, many hundreds of thousands of tons exist, readily available, but until recently few commercially and economically viable processes emerged due to practical difficulties, high fuel consumption and environmental restrictions. Many of the processes proposed in the past did not result in a fully expanded product..

For example, GB 624,032 discloses use of an annular tunnel furnace for production of porous clinker from granulated slate, clay or similar material. The proposed procedure involves pre-heating the material to 8000C for 1 to 2 hours and then supplying it to a rotating slab within the furnace where it is distributed in a layer one particle thick and brought from 800"C to 1200 to 13000C by contact with hot gases. As the surface of the material does not melt and fuse until it reaches 1200 to 13000 C, the preceding prolonged period of elevated temperature drives off much of the gases beforehand so that only partial expansion results.

Other known processes have employed, for instance, a rotary kiln of the inclined tubular kind, slate chips being fed into the upper end of the kiln and progressing therethrough to the other lower end whereat they are discharged. In their progress through the kiln the chips are supposed to tumble over one another so as to be exposed to the heat supplied into the kiln, but in practice this can only be assured if the kiln temperature is restricted below the optimum for expansion. This is because the slate tends to fuse at least partially during the expansion process and adheres around the lining of the kiln; moreover, individual particles adhere to each other. This is known as "ringing", and has the effect of preventing efficient transit, through the kiln, of the material being processed.

Accordingly, with the aforesaid methods, it has been practical, in the past, only to produce partiallyexpanded slate, which is of greater density than fully expanded slate. Moreover, due to difficulties of useful heat recovery, high fuel consumptions have been prevalent.

In the applicant's earlier European Patent Application, Serial No. 0225074, a method and apparatus were disclosed whereby granular or particulate slate, and comparable materials, such as clay, could be expanded to its fullest possible extent, resulting in lightweight aggregate material of correspondingly lowest possible effective density.

In this method the material is supplied to a moving hearth at a charging zone and the hearth is moved through a high temperature (1100 to 1300 C) treatment zone, to a discharge zone where the resulting expanded material is removed. The most significant feature which enables full expansion of the particulate material to take place is the provision of a hearth pre-heating zone (between the discharge and charging zones respectively) where the empty hearth is heated to a temperature at least close to that required for fusion of the particles (1100 to 13000C). Thus, when the material is fed onto the hearth it is immediately subjected to fusion temperature from the hearth below and the furnace space above, without a period of gradual heating which leads to preliminary escape of gases and incomplete expansion.

A further important factor ensuring rapid heating of particles is their even distribution across the hearth in a thin layer, not exceeding three particles in thickness.

Whilst full expansion of particles is achieved with the aforesaid method there is still a problem of adhesion of particles to the surface of the hearth, necessitating special arrangements for removal of expanded particles in the discharge zone. Indeed, satisfactory discharge has hitherto been achieved only by significant cooling of both the hearth and the expanded particles in the discharge zone, e.g. by blowing cool air through jets or hollow tines, along with reciprocation of raking means.

The primary object of the present invention is to minimise, if possible eliminate, the problem of adhesion of expanding particles to the hearth, at the high temperatures necessary for particle fusion. If this could be achieved it should be possible, as a consequence, to reduce the complexity of the arrangement for discharging the material after treatment. Indeed, a secondary, more specific object is to minimise, if possible eliminate, the need for cooling in the discharge zone so as to make the apparatus more economical, more fuel efficient.

According to the present invention, a method of heat treating particulate material, such as slate chips, so as to expand the same and produce a lightweight granular product, comprises supplying the material to a moving hearth at a charging zone in a substantially uniform thin layer and moving the hearth successively through a treatment zone where the material is subjected to an elevated temperature at which at least partial fusion takes place, so as to expand it, a discharge zone, at which the resultant expanded product is removed from the hearth, and finally a hearth pre-heating zone, where the hearth is heated to the required elevated temperature before returning to the charging zone so that, upon charging, material is immediately subjected to said elevated temperature, characterised in that a thin layer of finely divided particulate material of melting point higher than the fusion temperature of the particulate material to be treated (i.e. expanded) is provided on the surface of the hearth before the material to be treated is distributed thereover.

The finely divided, high melting point material is conveniently any readily available sand. This can be spread over the hearth just prior to, or in, the hearth pre-heating zone, or just prior to the charging zone, where the material to be treated is fed in.

The effect of the sand is to provide a friable layer between the surface of the hearth and the particulate material (slate or clay) being heat-treated. This layer may be up to or in the region of lOmm thick, but generally will be much thinner. Because the sand does not itself melt it prevents adhesion of the particulate material to the hearth, and enables the semi-fused and fused particles to be moved about, relative to the hearth, with ease, even at the high operating temperatures concerned.

In this way, discharge of material at the discharge zone can readily be accomplished without any need for scraping the hearth or for cooling to assist release of particles from the hearth.

To counteract the insulative effect of the sand layer, which may inhibit heat transfer from the hearth surface to the material to be treated, the hearth, in the preheating zone, may advantageously be heated to above the likely fusion temperature of the particles to be treated. Adequate heat transfer across the sand layer should then be assured.

Whilst commercially available quartz-type sand will generally be used in the invention, use of more expensive alternatives, such as aluminum (Al2 O3 ), magnesia (MgO) or silicon carbide (SiC) sands, which have higher thermal conductivities could allow some reduction in the temperature necessary in the hearth preheating zone.

As will be appreciated, the hearth used in carrying the method of the invention into effect is of endless construction, so that at any one time different portions thereof are present in the various zones enumerated above. To achieve this, the hearth is conveniently composed of a plurality of adjacent or interconnected tiles, of a suitable heat-resistant and substantially chemically-inert material of high thermal conductivity, such as silicon carbide. In one arrangement, these tiles may, for example,-be square, hingedly connected to one another in an endless-belt-like assembly, and guided around suitable rollers or drums. An upper horizontal run of the assembly, enclosed by a housing or housings, then provides successively the preheating zone, the charging zone, the treatment zone and the discharge zone.

In a preferred arrangement, however, the hearth is of flat disc-like or annular configuration, constructed of adjacent or connected tiles if so desired, and adapted to rotate in a substantially horizontal plane about a substantially vertical axis. A housing or housings enclose the majority of the hearth to provide successively, the preheating zone, the charging zone, the treatment zone and the discharge zone.

At the treatment zone, the relevant housing part advantageously encloses or is in the form of a furnace equipped with regenerative burners whereby the fuel input necessary for heating the material under treatment is kept to a minimum. The preheating zone is preferably of comparable form.

As applied to the expansion of slate to produce a lightweight granular material suitable, for example, for use in the production of high quality low-density concrete, concrete blocks, and similar products, structures and articles,- the material is subjected to a temperature in the range from 1100 to 13000 C, typically approximately 12500 C, in the treatment zone.

In the case of an annular hearth discharge of treated material in the discharge zone can be accomplished using a stationary deflector blade set transversely across the path of the hearth or a reciprocating blade moving in a radial direction. Another alternative might be use of jets of compressed air to blow the material off the hearth, or suction means to draw it off so long as this did not bring about any significant cooling of the hearth. Other alternatives are also possible, for example a radially arranged rotating auger screw so that material is screwed off the hearth continuously.

In the case of an endless belt arrangement all of the sand will be removed along with the expanded material, whereas in the case of an annular hearth only a proportion will be. The sand can be separated from the product by a suitable sieve arrangement and if the volume of sand usage is high it can be recycled, even returned to the hearth while still hot to further reduce heat loss.

With the preferred annular style of hearth it has been found that continuous supply of sand is not essential to maintain an adequate layer upon the hearth. In practice, whilst some sand is removed along with the discharged material, sufficient usually remains for its continued effectiveness as an intermediate friable layer for several revolutions before replenishment is necessary. Thus sand may be supplied only intermittently, e.g. once an hour, or every ten to fifty revolutions or thereabouts. In this respect, annular hearth apparatus may suitably run at a rotational speed in the range of one revolution in 1 to 6 minutes. Alternatively, of course, a continuous low level replenishing supply of sand may be maintained.

In order that the invention may be fully understood, it will be described further, by way of example, with reference to details of a preferred practical example of the process, and the manner in which it is carried into effect, as illustrated in the accompanying drawing, in which: Fig. 1 is a diagrammatic sectional elevation illustrating a practical embodiment of the apparatus of the invention; Fig. 2 is a diagrammatic sectional plan view of the apparatus of Fig. 1, the section being taken approximately on the line II-II of Fig. 1; Fig. 3 is a diagrammatic sectional elevation, to an enlarged scale, taken approximately as indicated by the line Ill-Ill of Fig. 2, illustrating the structure of the apparatus at its charging zone;; Fig. 4 is a fragmentary schematic plan view, to an enlarged scale, illustrating the structure of the apparatus at its discharge zone; Fig. 5 is an enlarged front face view of the blade in the discharge apparatus of Fig. 4; Fig. 6 is a schematic detail along the line VI-VI in Fig. 5; Figs. 7a and 7b are views along the line VII-VII in Fig. 5 showing the blade during its discharge and return stroke respectively; Fig. 8 is a fragmentary schematic plan view, to an enlarged scale, illustrating the structure of the apparatus at its sand supply zone; Fig. 9 is a perspective sketch of the apparatus shown in Fig. 8; and Fig. 10 is an enlarged fragmentary cross-section illustrating an arrangement for reciprocation of the discharge blade shown in Figs. 4 to 7.

In carrying out the preferred process of the invention, as applied to the expansion of particles in the form of slate chips produced, for instance, by crushing waste slate, e.g. that to be found in the Blaenau Ffestiniog region of North Wales, one uses an annular furnace arrangement which is indicated generally by the reference numeral 10 in the figures. This furnace arrangement comprises an annular hearth 12 composed of a plurality of heat-resistant tiles 14 made, for instance, of silicon carbide. The tiles 14 are disposed in an annular array and are supported upon an annular bed 16 of heat-insulating material, which in turn rests upon a ring 18, e.g. of metal, which constitutes a top plate of an annular carrier which is of box-like radial section with intermediate strengthening spacers 20 between such top plate 18 and a bottom plate 22 thereof. The annular carrier 18, 20, 22 is supported upon the outer ends of radial arms 24 of a spider 26 mounted upon a slewing ring 28 carried upon a plinth 30 so as to be rotatable about a vertical axis which coincides with the central axis of the annular hearth 12. A drive gear 32 is provided around the slewing ring 28 and is engaged by pinion 34 on a gearbox 36 driven from an electric motor 38, so that when the motor 38 is energised the hearth 12 is rotated about its central axis at a rotational speed, for example, in the range of one revolution in 1 to 6 minutes.

Supported from above by a superstructure including, for example, cross-members 40 supported by legs 42 (which superstructure has been omitted from Fig. 2 for clarity) is a heat-insulating furnace hood 44 which is inverted U-shaped in radial cross-section and is of a configuration such as to enclose substantially the whole of the hearth 12 leaving gaps for a charging zone and also a discharge zone concurrent with a sand supply zone shown diagrammatically at CZ, DZ and SZ respectively in Fig. 2. This hood 44 may be composed, for instance, of a plurality of ceramic or refractory sections backed by heat-insulating material.The portion of the hearth 12 that it covers between the sand supply zone SZ and the charging zone CZ may be considered as comprising a preheating zone PHZ of extent approximating to about 20% of the whole hearth, and the portion it covers between the charging zone CZ and the discharge zone DZ may be considered as a treatment zone TZ approximating to 60% of the whole hearth. The charging zone CZ approximates to about 5%, the discharge zone DZ approximates to 10% and the sand supply zone approximates to 5% of the whole hearth 12.

In register with the charging zone CZ, the hood 44 has an opening indicated diagrammatically at 46 in Fig. 2 in its top to permit supply therethrough of material to be expanded such as slate chips indicated diagrammatically at 48 in Fig. 3. For achieving such supply, a substantially planar chute plate 50 is provided to project from outside horizontally into the hood 44 to terminate in an oblique weir edge 52 which is disposed substantially radially of the hearth 12 across substantially the entire radial width thereof. This chute plate 50 is mounted on flexible supports shown diagrammatically at 54 and is adapted to be vibrated in its longitudinal direction by a vibrator motor indicated diagrammatically at 56. Supply of the slate chips 48 to the plate 50 is achieved by means of a hopper 58 supported on a carrier plate 60 adapted to be vibrated by a respective vibrator motor 62.This arrangement is particularly effective in ensuring that a uniform layer of the slate chips 48 is supplied to the annular hearth 12. This layer may, for instance, be about three chips in thickness. The rate of feed can, of course, be controlled in any suitable manner, for instance by control of the frequency and/or amplitude of the vibration of the carrier plate 60 and/or the chute plate 50.

The treatment zone To and the preheating zone PHZ of the apparatus are fitted with respective heating means in the form of series of regenerative burner arrangements. In the illustrated case there is a first pair of complementary burners 64, 66 directed towards one another across approximately one half of the treatment zone TZ, a second pair of complementary burners 68, 70 directed towards one another across the other half of the treatment zone TZ, and a third pair of complementary burners 72, 74 directed towards one another across the preheating zone PHZ.As is well known, such regenerative burner arrangements are actuated so that one burner of the pair is operatively supplied with a combustible fuel/air mixture and applies heat to its respective portion of the furnace, the products of combustion, after passing across said portion, being taken up by the complementary burner and passed to equipment which serves to retain the accumulated residual heat contained in such combustion products. Upon the accumulated heat reaching a predetermined level, the roles of the two burners of the pair are reversed, the previously-accumulated heat each time serving to preheat the combustion air being supplied to the operative burner.In the illustrated case, the burner arrangements are controlled to maintain a temperature of approximately 12500C within the treatment zone TZ; and to maintain a temperature within the preheat zone PHZ sufficient to maintain hearth temperature at an optimum consistent with a satisfactory product. When commercial grade quartz sand is used this preheat zone PHZ temperature could be typically 1300 to 1400"C depending on the layer thickness employed.

As shown digrammatically in Figs. 4 to 7, and in Fig.

10, in the discharge zone DZ, a blade 76 is mounted on a radially extending shaft 80 so as to be inclined both to the vertical and to the direction of hearth rotation. The shaft 80 extends through bearings 82, 84 and is axially reciprocated so that the blade'76 it carries is caused to reciprocate across the hearth width. During its operative discharge stroke (Fig. 78 and broken lines in Fig. 10) the blade 76 moves radially outwards whilst resting lightly on the sand bed. During its return stroke (Fig. 7b and solid lines in Fig. 10) it is raised above the sand bed to avoid contact therewith, being lowered again just prior to the commencement of the discharge stroke.One manner of achieving this reciprocation, as illustrated in Fig. 10, is to have the shaft 80 pivotally mounted towards its outer end on a fulcrum block 81 carried upon a wheeled trolley 83. A counterweight 85 at the outer end of the shaft counterbalances the blade 76. The trolley 83 may be guided on rails and is moved radially in and out either hydraulically by a cylinder 87, as indicated, or by a crank arm (not shown) translating rotary motion to linear motion. As the trolley 83 is moved outwards the blade 76 rests upon the hearth 14, its downward pressure being determined by the mass of the counterweight 85, which can be varied, so as to draw product to a discharge channel 90 around the hearth circumference.

Then, as the trolley 83 is moved inwards, the end of the shaft 80 to which the blade 76 is attached is swung upwards by the action of a hydraulic or pneumatic actuator 92 mounted on the trolley 83. Movement of the actuator 92 may be limited by a stop (not shown), which sets the clearance between the blade 76 and the hearth 14 on the return stroke of the shaft 80. The shaft 80 consists of concentric tubes, and is cooled by air flowing up the central tube and back around the annular space to minimise adverse effects of operating in a furnace.

In the sand supply zone SZ the feed and distribution means can be similar to those used for supply of slate chips at the charging zone CZ, i.e. comprising a hopper and vibratory plates. However, an alternative arrangement (Fig. 8 and 9) comprises a V-shaped trough 78 extending radially above the hearth 12 and provided with numerous very small holes (e.g. 0.6 mm diameter) at the bottom. This trough 78 is mounted on a shaft 88 and vibrated by means of a vibrator 90 so that an even layer of sand is sprinkled through a suitable slit in the hood 44. The trough 78 acts as a reservoir and is supplied with sand from a hopper (not shown) via a sluice valve or similar.

The operation of the apparatus, and accordingly the preferred method of the invention, will readily be understood from the foregoing. The furnace is turned on and set to operate to produce a working temperature, in the treatment zone TZ, of the order of 1250"C, which is such as to cause at least partial fusion of slate.

The hearth 12 is driven in rotation, in the direction indicated by arrow 96 (Figs. 2 and 4), and the sand supply trough 78 is operated to spread a thin layer of sand of only a few mm thickness (e.g. 1 to 5mm) onto the surface of the hearth. By the time the sand has passed through the pre-heating zone PHZ and reached the charging zone CZ both it and the hearth have acquired a temperature of approx. 1300"C. At this stage, the vibrator motors 56 and 62 for the slate chute and carrier plates 50, 60, are set into operation, with the result that, at the charging zone CZ, slate chips 48 are spread over this bed of sand in a substantially uniform layer up to about three chips in thickness.

The layer of slate chips 48 upon the bed of sand is then conducted through the treatment zone TZ where the slate chips are caused to expand. In practice, with chips, which may each be initially only two to four millimeters thick, an expansion to as much as four or five times the original thickness, for instance to about eight to twenty millimeters, can be achieved, with the initial specific gravity of the slate of approximately 2.6 to 2.8 being reduced to as little as 0.6.

Virtually maximum chip expansion is achieved because initial heating of the outside of each chip to incipient fusion is very rapid. As a result, each chip is externally sealed, and as heat penetrates, it produces expansion gases whose escape is inhibited and a maximum degree of expansion is achieved. Slow heating would allow gases to escape with loss of expansion and consequently a denser product. This latter is a failing of some prior known methods of expanding slate and similar expansible materials, for example those using rotary kilns as previously referred to.

After passing through the treatment zone TZ, at a speed such as to ensure full expansion, the particles or chips 48 reach the discharge zone DZ, where together with some underlying sand they are removed by the blade 76 and discharged from the circumference of the hearth.

The discharged material may be channelled into a sieve arrangement (not shown).which only the sand passes through, for recycling of the latter.

When the initial segment of hearth again reaches the sand supply zone, fresh or recycled, hot sand may again be distributed thereover by the trough 78. However, continuous supply of sand may not be necessary (as sufficient may remain from the initial circuit), and it may be supplied intermittently, e.g. for only one or two circuits at hourly intervals or at more widely spaced intervals.

The hearth, with the sand layer (whether residual or partly or totally replenished), then again passes through the preheating zone PHZ, where the temperature of the hearth and-sand layer is raised again, preferably to approximately 13000C to allow for the slight insulative effect of the sand, before it passes back to the charging zone CZ to receive further slate chips as above discussed.

It will thus be understood that the method of the invention can be carried out as a continuous process using the apparatus as just described.

Of course, the method of the invention is not confined in its application solely to slate, and it can be applied to other materials desired to be expanded by heat treatment, for example clay, shales and similar materials capable of being expanded. Clay, of course, has to be formed into pellets before treatment.

Moreover, the apparatus of the invention is not confined to the precise details of the example described above with reference to the drawings and variations or modifications may be made thereto.

For example, the method of the invention could, if desired, be carried out using a hearth which is of endless construction, being composed of connected hearth tiles extending in a linear forward run successively through a sand supply zone, a preheating zone, a charging zone, a treatment zone, a discharge zone in a manner somewhat comparable with the described arrangements, the hearth tiles then passing to a return run and returning to the sand supply and preheating zones.

The sand supply could alternatively be effected just prior to charging of material to be treated, or centrally within the hearth pre-heating zone, depending on which is most appropriate or convenient to construction of the furnace, whether of annular or longitudinal type.

It should be noted also that the temperatures used in the treatment zone and preheating zone may vary from those given, for instance by as much as 200 C to each side of the figures given. The material to be treated and the sand may be supplied onto the hearth in any suitable manner; the treatment zone and the preheating zone may be heated otherwise than by regenerative burners (although these latter are most economical); and discharge of the expanded particles from the hearth may be effected differently, for instance by blowing or by suction, or by other mechanical removal means (e.g. a radially reciprocal brush) so long as there is a minimum cooling effect at this stage.

Claims (12)

1. A method of heat treating particulate material, such as slate chips, so as to expand the same and produce a lightweight granular product, comprising supplying the material to a moving hearth at a charging zone in a substantially uniform thin layer and moving the hearth successively through a treatment zone where the material is subjected to an elevated temperature at which at least partial fusion takes place, so as to expand it, a discharge zone, at which the resultant expanded product is removed from the hearth, and finally a hearth pre-heating zone, where the hearth is heated to the required elevated temperature before returning to the charging zone so that, upon charging, material is immediately subjected to said elevated temperature, characterised in that a thin layer of finely divided particulate material of melting point higher than the fusion temperature of the particulate material to be treated (i.e. expanded) is provided on the surface of the hearth before the material to be treated is distributed thereover.

2. A method as claimed in claim 1 wherein the finely divided, high melting point material is sand.

3. A method as claimed in claim 2 wherein the sand is quartz-type sand.

4. A method as claimed in claim 2 wherein the sand is of alumina (Al2 03 ), magnesia (MgO) or silicon carbide (siC) .

5. A method as claimed in claim 2, 3 or 4 wherein the sand is spread over the hearth just prior to or in the hearth pre-heating zone, or just prior to the charging zone, where the material to be treated is fed in.

6. A method as claimed in any preceding claim wherein the layer of finely divided material is up to l0mm thick.

7. A method as claimed in claim 6 wherein the layer of finely divided material is between 1 and 5mm thick.

8. A method as claimed in any preceding claim wherein upon removal of expanded product from the discharge zone sand is separated therefrom by a sieve arrangement and recycled.

9. A method as claimed in claim 8 wherein recycled sand is returned to the hearth while still hot.

10. A method as claimed in any preceding claim wherein the hearth is of annular configuration and sand is supplied thereto intermittently.

11. A method as claimed in any of claims 1 to 9 wherein the hearth is of annual configuration and is run at a rotational speed in the range of one revolution in 1 to 6 minutes.

12. A method of heat treating particulate material substantially as hereinbefore described with reference to the accompanying drawings.