GB1600372A

GB1600372A - Production of fired materials

Info

Publication number: GB1600372A
Application number: GB898677A
Authority: GB
Original assignee: SHERWEN ENG CO Ltd
Current assignee: SHERWEN ENG CO Ltd
Priority date: 1977-03-03
Filing date: 1977-03-03
Publication date: 1981-10-14
Also published as: ES467488A1; NL7802387A; DE7806480U1; FR2382665A1; BE864502A; DE2809223A1; BE864501A; DK94878A

Description

(54) IMPROVEMENTS IN OR RELATING TO THE PRODUCTION OF FIRED MATERIALS (71) We, SHERWEN ENGINEERING COMPANY LIMITED, a British Company of Mile End Green, Dartford, Kent, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to processes and apparatus for firing materials, in particular mineral products. It is especially concerned with the application of such processes and apparatus to the firing of minerals comprising particles of grain size not substantially greater than 0.06 mm.It is particularly, though not exclusively, concerned with the firing of argillaceous materials, that is to say materials in which clay minerals are present of a character and in proportions that allow the material to be formed into an extrudable mass, with the addition of water if necessary, the material being capable of being fired to form an expanded product with a mass of small cavities within it, a process known as "bloating", and especially with the firing of argillaceous materials in a pelletized form, e.g. for the production of synthetic aggregate for use as a building material.

The production of such synthetic aggregates is known using a cylindrical brick-lined rotary kiln to heat and fire the raw material which is first ground and may be mixed with oil before being extruded and chopped into pellets or particles of a suitable size. The pellets are fed to the entry end of the rotary kiln, which is of considerable length and has a firing zone at its other end with a burner or burners. The kiln cylinder is slightly inclined so that the pellets progress by gravity along its length as they are tumbled by the rotation of the kiln. The combustion gases from the burners in the firing zone are directed along the length of the kiln to act on the material from its entry into the kiln, so that the material is dried and heated steadily towards its firing temperature.

It is found that this process can be difficult to control, mainly because the conditions in the kiln cannot be easily regulated to fulfil the different requirements for processing the material as it progresses from its initial cooled state, still maintaining a significant amount of water, to the final stage where it is subjected to an intense heating temperature of about 1200"C.

In their initial state the pellets will be broken up if heated too rapidly before the moisture is driven off. In the final stage the tumbling action must be able to prevent the pellets accreting into large fused masses of slag or clinker as the temperature rise renders the material plastic, while on the other hand the tumbling action must not be so vigorous as to damage the pellets while they are in a mechanically weak condition between the drying and firing stages. All the different requirements have to be controlled by regulation of the burners and of the rate of progress of the material through the kiln, which may alter conditions along the length of the kiln even though it may be desired to influence the material at one particular stage only.It therefore can take some days after start-up to reach a stable operating condition in which a satisfactory compromise has been met for the different parameters controlling the processing of the material. The high thermal mass of the kiln also adds to the difficulty of controllably adjusting the operating conditions.

In practice, because the firing stage is regarded as the most critical the operation of the kiln is usually regulated with the primary aim of avoiding the formation of slag or clinker, which must be discarded and can be difficult to remove because of the size of the masses that form once the accretion process has started. As a result, other lesser faults have to be tolerated which can make a proportion of the product substandard.

Another disadvantage of the known synthetic aggregate process lies in the bulk of the plant and its high capital cost that, because the aggregate is essentially a lowcost product, may be lost if the local material supply fails or if the local demand for the product falls.

According to the present invention, there is provided a process for firing materials comprising the stages of selecting a particulate starting material of graded particle size, removing moisture from the material while heating it to a temperature below it firing temperature in a first rotary heating chamber, heating the dried material to firing temperature and firing said material in another rotary heating chamber and then cooling the fired particles in a further rotary chamber.

By means of such a process it is possible to maintain a flexible control of the treatment of the material. In particular the heating sources and the manner of movement of the material in the different chambers can be quite independently chosen to suit the requirements of the material at that stage of the process.

In order to conserve the heat generated in the firing and cooling stages, the waste combustion gases from both these stages can be utilised for the drying of the pellets and/or their preheating, if required with the addition of heat from a further source. An advantage of this procedure is that a relatively large hot gas flow is available for the first heating stage, when for example it may be required to remove large quantities of moisture from the material, without there being any significant quantity of free oxygen which might cause undesired chemical reactions that affect the subsequent firing process.

Preferably, the process is carried out in an apparatus wherein said rotary chambers are each of metal construction and the firing chamber has a lightweight sandwich construction with heat-insulating material between an inner heat-resistant metal lining and an outer support structure. It may be arranged, moreover, that respective control means are provided for regulation of the heating in each heat input stage.

The use of such a lightweight construction for the firing kiln avoids the need for a brick lining in the rotary chamber so each unit can be of relatively low weight. This allows a totally different approach to the design of apparatus for producing construction materials such as aggregate, with the result that it is possible to provide apparatus that is transportable from one site to another.

Consequently, the previous need for a high capital cost installation operating on a fixed site is avoided and substantial economies are possible in the construction of the apparatus, with the installation able to be sited in the most advantageous place in each instance to satisfy the demands of the market.

In one preferred form of the apparatus, it may thus be arranged as a series of separately transportable units including respective units for the pelletizing or screening stage, the drying stage, the firing stage and the cooling stage. These units may even be formed as standard container modules so that they can be handled and transported by commercially available means, and at least some of the units, e.g. those for the drying, firing and cooling stages can be arranged to be stacked upon each other with the material being transferred by gravity to each succeeding unit.

The control of the individual units may extend not only to the rate of heat input but it is also possible to adjust individually the rate of material throughput separately.

When a gravity feed is used, it can simply be arranged that means are provided in each unit for adjustment of the inclination of its rotary chamber to alter the time of dwell of the materials therein. In each instance the rotary chamber can be so shaped and divided as to give the material throughput the type of agitation that is most suited to its physical condition at that stage of the process.

The rotary chambers preferably provide non-circular passages and they may comprise a multiple lobed transverse crosssection as described in our concurrently filed patent application No. 7763/78, (Serial No. 1600373), especially in the case of the firing chamber or kiln.

Examples of the invention will be described with reference to the accompanying drawings, from which further features of the invention will be appreciated. The illustrated examples refer to the manufacture of a bloated lightweight product but it should be understood that a denser non-expanded product could equally well be made. In the drawings: Figure 1 is a schematic flow diagram of a process according to the invention for producing lightweight synthetic aggregate, Figures 2 and 3 are side and plan views showing in outline an installation employing the process of Figure 1, Figures 4 and 5 are a side view and a transverse cross-section respectively of the low-temperature dryer in the installation of Figures 2 and 3, Figure 6 is a transverse cross-section of the high temperature dryer, Figure 7 is a schematic end view of the vertical shaft kiln, Figures 8 and 9 are a side view and a transverse cross-section respectively of the rotary firing kiln, Figure 10 is a transverse cross-section of the rotary cooler, Figure 11 is a schematic flow diagram of another process according to the invention for producing aggregate, Figure 12 is a diagrammatic outline of an installation employing the process of Figure 11, Figures 13 to 16 are side views of the respective units in the installation of Figure 12, Figure 17 is an end view of the unit in Figure 13, and Figures 18 to 20 are sectional views on the planes X-X, Y-Y and Z-Z in Figures 14 to 16 respectively.

Referring initially to Figures 1 to 3 of the drawings, the installation comprises an entry conveyor 2 carrying raw clay to a shredder-feeder unit 4 that may be of conventional design and serves to break-up the material and feed it to a rotary, lowtemperature dryer 6 in which the clay is subjected to a first drying operation reducing the moisture content to not more than substantially 18%, e.g. 12% to 14%. A lift conveyor 8 then takes the partly dry materlial to a pelletizing unit 10, not illustrated in any detail, which can comprise conventional machines including a crusher or grinder 10a where oil or other fuel can be added to the clay, and one or more pelletizers 10b.

The substantially uniformly sized pellets are carried by conveyor 12 to a hightemperature rotary dryer 14 to remove all remaining free water and at least some of the chemically combined water in the material. The pellets then fall at a controlled rate through a static shaft kiln 16 to heat the material close to its firing temperature. The static kiln leads to a rotary kiln 18 where the sintering or firing of the material is performed, in this example the material being of such a nature that the bloating effect already referred to occurs. At the exit from the firing kiln the product is entrained by a cooling air flow through a transfer duct 20 to a rotary cooler 22 when the final product is discharged from the cooler onto an exit conveyor 24.

The low temperature dryer 6, the shaft kiln 16 and the rotary kiln 18 each has its own heat source in the form of one or more burners 26, 28, 30 respectively. The hightemperature dryer 14 is heated using the exhaust gases from the shaft kiln, the rotary kiln and the cooler, the gases being led direct from these units to the dryer 14.

Ducting 32, 34 from the rotary kiln and from the cooler is provided for this purpose.

Ignoring radiation and other extraneous heat losses, the internal heat requirements at the different stages of the material being processed are divided in percentage terms at 15% in the low-temperature dryer, 55% in the high-temperature dryer, 17% in the shaft kiln and 11% in the rotary kiln. The cooler can supply approaching 40% of the total internal heat requirement as recirculating heat retained in the process. This goes to the high-temperature dryer two be supplemented by the heat in the exhaust gases from the shaft kiln and the rotary kiln which may make contributions in the ratio of 2:1.

The burner inputs of the low-temperature dryer, the shaft kiln and the rotary kiln are in the approximate ratio of 15:29:16.

The different stages of the process and apparatus will now be described in more detail.

At entry, clay with a moisture content of about 22% by weight is broken up in the shredder-feeder 4 so as to produce a feed of particles less than 1" (2.5 cm) in size to the low-temperature dryer 6 shown in Figures 4 and 5. The dryer comprises a container 42 with an internal heat insulating layer (not shown) and it has an internal transverse cross-section of trefoil shape with deep lobes 44, as can best be seen in Figure 5, described in more detail in our concurrently filed patent application No. 7763/78 (Serial No. 1600373). The container has end rings 45 mounted on rollers 46 for rotation about the axis of symmetry of its cross-section and is driven by motors 48.A forced draft fan 50 at the material exit end and an induced draft fan 51 at the material entry end act in conjunction with the burner 26 at the material exit end to produce an airflow heated to about 300"C through elongate exit slots 52 of a duct 54 that extends along the centre of the container. Control means 55 allow the burner output and/or the fan suction rate to be adjusted for control of this first drying stage independently of later stages of the process.

The moisture-laden air exits at about 90"C through a chimney 56. The clay is fed into the dryer through a feed hopper 58 projecting into the interior of the container and travels the length of the container while being agitated by the rotation of the container to expose it to the counterflow of warm gases that heat and dry the clay to a final moisture content of some 18%.

Because of the lobed cross-section of the container, as it rotates the material in the bottom lobe is lifted to the point at which it exceeds its angle of repose, and a thick upper layer then slips and falls into the succeeding lower lobe followed quickly by the remaining material previously forming the lower layers which now falls on top of the first portion of the material, this process being continuously repeated as each lobe rises from a lowermost position. The material is in effect continually turned over as it falls in a free stream in one localised region of the container cross-section and the slots in the duct directs the warm gas flow into this region, so that all the material is exposed to a fresh heated gas flow three times in every revolution of the container in a concentration similar to that achieved in a fluidised bred.

To obtain the tumbling effect described, the dryer must not rotate so fast as to prevent material from cascading from each rising lobe to the next succeeding lobe, but subject to this limitation increase of the rate of rotation will increase the rate of drying and the rate at which the clay particles are degraded, thus reducing the energy requirement when the dried material is crushed before pelletizing.

The rate of progress of material through the dryer 6 is controlled by altering the downwards inclination of the longitudinal axis of the dryer from the clay entry end, support means 59 at one end of the drum 42 being vertically adjustable. The dryer 6 is shown provided with its own heat source but it can be arranged to utilise the heated exit gases from the high temperature dryer to supply part of the required heat and gas flow input, or, with modifications of the heating arrangements for the high-temperature dryer 14, the dryer 6 can utilise heat in the waste gases from a later stage such as the kiln 18.

As the crushing and pelletizing operations following the first drying stage are performed by conventional equipment they will not be described further. At the output from the pelletizing unit the reformed material, from which dust and small particles have been screened, is raised by bucket elevator 12 to the high-temperature dryer 14. A bucket elevator is preferred because although compacted to be mechanically stable at this stage the pellets cannot withstand high rates of heat input without damage if simultaneously subjected to mechanical shock. For the same reason the dryer 14 should therefore agitate the pellets relatively gently.As can be seen from the cross-sectional view in Figure 6, the drum 62 of the high-temperature dryer has cylindrical inner and outer walls 64, 66 between which extend a series of radial partitions 68 to form a large number of small crosssection passages 70 for the throughput of the pellets. The drum is mounted through end rings 72 and rollers 74 and is driven by motor 76. It also has means 78 for adjusting its inclination to the horizontal.

Material entering the drum through the conically widening entry is distributed among the passages 70 as the drum is rotated and the material progresses along the drum, but at a slower rate for a given inclination of its rotary axis as compared with the low-temperature dryer because of the smaller cross-section of the passages.

These smaller passages also keep the tumbling impact forces low. The pellets form a series of thin streams continuously turned over by gravity as the drum rotates while being heated by conduction from hot gases passing through the drum, with the assistance of fan 59, in counterflow to the material. As already described, heating is provided by the hot exhaust gases taken direct from the vertical shaft kiln, the rotary kiln and the cooler to the material exit end of the high temperature dryer, as well as some additional air heated by convection flow through an outer jacket of the shaft kiln. The gases dry the pellets completely from their initial moisture content of 18% and also raise their temperature to some 500"C.

At their discharge from the hightemperature dryer, the pellets enter the vertical shaft kiln 16 which has a rectangular cross-section chamber across which extends a series of narrow baffles 82 (Figure 6) formed by angle-section bars with their apices uppermost. Pressure jet burners 84 (Figure 2) are provided at opposite sides of the chamber with their nozzles projecting into the chamber immediately under respective baffles so that in each instance the burner flame is directed parallel to an associated baffle in the space immediately below the baffle. There may be apertures (not shown) in the baffles to allow hot combustion gases to escape upwards more easily through the chamber.

The pellets are allowed to fill the chamber but each baffle will keep clear the space immediately beneath it. Thus in operation, there is a series of voids extending across the chamber, the upper sides of each void being defined by the lower faces of a baffle and the lower sides being determined by the angle of repose of the material filling the chamber. The flames of the burners extend into the voids and the burners preferably produce a flame length that extends a substantial length of each void so as to assist uniform heating of the material along the length of the associated baffle. Preferably the air flow associated with a burner is restricted to that necessary for stoichiometric combustion to keep the gas speed low and so reduce the power required and the need for dust extraction, and also to avoid or limit the presence of free oxygen.

For the control of the heating process, thermocouples 86 may be provided above the level of respective groups of burners that they are required to control and automatic control means 88 may be provided to regulate the operation of the burners in dependence upon the signals from the thermocouples. The rate of heating can also be influenced by the shape of the chamber since the material flow through it under gravity and for a given rate of material removal per unit plan area, the taller the chamber the longer the material heating period.

In the discrete spaces within the chamber in which the material is exposed directly to the combustion process there is a radiant heating effect and also a free path for the flow of hot gases upwardly, so that a relatively rapid heating effect can be obtained. It is possible to regulate the temperature conditions closely and quite independently of the other stages of the process. It is also possible to control the temperature profile in the kiln as the burners are disposed at different levels, and each level may be individually controlled, if needed, by respective control means 88.

The shaft kiln has a relatively compact construction which aids its manufacture in a transportable form, although it may be required that the structure is dismantled for this purpose. For example, the burners may be mounted on side walls 90 that are separate from the main heating chamber body and that can be suspended on an overhead roller track 92, a feature that also assists maintainance.

At the exit from the shaft kiln the meaterial must be fed at a sufficiently uniform rate from across its relatively large width to the narrower, conically restricted inlet opening of the rotary firing kiln. To do this the material falls from the bottom of the shaft kiln into a collector box 94 of the same width as the shaft kiln. In the box it settles against an exit barrier 96-at its natural angle of repose. Beyond the barrier there is a tapering chute 98 leading to the rotary kiln entry 99 and immediately preceding the barrier there is a rotary agitator comprising a series of spaced discs 100 mounted on a shaft 102 that extends the width of the exit.

As the agitator is rotated the movement of the trapped pellets lessens their angle of repose and pellets therefore begin to slip past the barrier. The material will be taken off evenly from all points across the width of the box 94 so that there is no danger of a non-uniform removal pattern being created, as would happen if the exit from the shaft kiln was simply a funnel leading directly into the rotary kiln. Drive motor 104 for the agitator controls the rate of material movement.

The rotary kiln is described in more detail in our co-pending patent application No.

7763/78 (Serial No. 1600373). Over its main region it has a similar trefoil form to the low-temperature dryer but without internal transverse baffles. It has a lightweight metal sandwich construction with an intermediate heat-insulating layer or layers of ceramic fibre. The lobed cross-section tumbles the pellets vigorously to allow them to be heated more evenly and also helps to prevent the individual pellets from adhering together as they reach bloating temperatures and pass through a stage which they become sticky and relatively soft.

The kiln burner 30 may be a spear gas burner or thermal lance (Figure 8) the flame of which extends over a substantial part of the length of the kiln, or an elongate radiant plate burner (Figure 9) that is described in our concurrent patent application. Control means 106 for the burner are regulated by a thermocouple 108 in the kiln. The burner is preferably operated under substantially stoichiometric conditions to avoid undesired chemical reactions in the pellets, this leading to a lower gas speed with reduced requirements for dust extraction. Because of the lobed cross-section of the kiln the radiant energy is better focussed onto the material so that it is subjected to a high radiant heat flux density and this together with the nature of the tumbling action created by the lobed cross-section allows the use of a relatively short firing kiln.Means 109 are provided to alter the inclination of the kiln and so adjust the dwell time of the material.

Although the burners in the installation have so far been described as gas burners, it is possible to use any convenient fuel to provide heat for the process.

At the end of the rotary kiln the fired pellets leave with a temperature of some 1150"C through a chute 110 leading to an upwardly inclined air duct 112 from a forced draft fan 114. The airflow at ambient temperature from the fan acts as a jet pump as it passes a restriction in the duct at its junction with the chute so that the pellets are immediately entrained by the airstream.

The pellets are violently agitated in the airstream and are suddenly cooled so that any still plastic material is immediately hardened. This combined cooling and agitation prevents large numbers of pellets fusing together to form unmanageable masses of slag or clinker.

There is still, however, a substantial amount of heat in the pellets and they are therefore passed through the rotary cooling container in counterflow to a larger flow of cooling air from a motorised fan 126. As can be seen from Figure 9 the container has a lobed cross-section 118 also and is in many ways similar to the low-temperature dryer with transverse baffles 120 (Figure 10) that ensure a minimum bed of material in the chamber, and a central axial air duct 112 with longitudinal slots 124 through which the cooling flow from fan 126 is directed.

The mounting and drive of the container is similar to that of the low-temperature dryer and it also has means 128 for independent adjustment of the inclination of its axis.

The tumbling of the pellets will, similarly to the other rotary lobed containers, expose the individual pellets continually to the cooling gas flow and to ensure a maximum rate of heat extraction before the pellets are finally discharged onto an exit conveyor with an exit temperature of about 200"C.

The installation described above is of a semi-permanent nature in that, although a number of the units are mounted on trailer beds, fixed support structures are provided for some of the apparatus and site preparation and construction is therefore required to erect the installation. The plant shown in Figures 11 to 20, however, is intended as a transportable installation manufacturing a similar product but requiring a minimum of site preparation.

The plant comprises four individual units each contained in a standard ISO container frame. The first unit 210 is a preparation module with the equipment for producing pellets from a raw material input. The remaining three units 212, 214, 216 are stacked one upon the other for a gravity feed of the process material through them and a conveyor 218 lifts the pellets from the first unit to the second unit which has a pre-heater that in this instance combines the functions of the high-temperature preheater and the vertical kiln of the first embodiment. The pellets leave this unit dried and heated to 900"C, close to their firing temperature, to pass to the third unit in which there is a rotary kiln generally similar to the rotary kiln of the firstdescribed embodiment, while a rotary cooler is provided in the fourth unit.

In the first unit 210 a double shafted feeder-mixer 222 receives roughly brokenup raw material through a feed hopper 224, and supplies an edge-runner mill 226. The material delivered to the mill should not have a moisture content more than about 18% and it may be required to include an additional drying unit (not shown) in the unit 210, e.g. using waste heat from another part of the process, if the raw material supply has an unacceptable moisture content. If the material is too dry when it reaches the- mill, water or fuel oil can be added at this stage. The mill may be a batch process unit with a non-perforated floor, in which case successive batches of material are each fully ground before they are taken from the mill.Alternatively, it can be a continuous process mill, feeding the product through a perforated floor to exit conveyor 228 that leads to one or more pelletizer units 230 of the type in which the finely divided material is subjected to pressure to compact it into a mechanically stable pelletized form.

Fuel can also be introduced at this stage, in particular solid fuel such as powdered coal from a hopper 232, but the fuel may alternatively be added at the mill. Screening means 234 return any fines from the pelletized product by way of a conveyor 236 to the mill 226. The screened pellets are delivered to a loading hopper 238 in the second module through the rubber belt conveyor 218. All the machines in the unit 210 can be hydraulically driven from a central power pack (not shown) and speed controls may be provided for each.

In the second unit 212, some of the heat for rotary drying and heating chamber 240 is provided by the hot waste gases from the firing and cooling means in the units 214, 216. If sufficient fuel has been mixed with the pelletized material in the first unit, this can provide sufficient supplementary heat for the stage prior to firing with a suitably controlled air inflow, but alternatively material discharge hood 242 of the rotary chamber can be provided with fuel burners 243.

The rotary chamber of the drying and preheating stage is a double-wall construction with insulating jacket and is similar to the high-temperature dryer of the first described embodiment in that a tubular space 244 is provided for the throughflow between inner and outer cylindrical walls 246, 248 and radial partitions 250 divide this space axially so as to provide a relatively gentle tumbling action that is particularly desirable when the pellets have been completely dried. This inner chamber structure is surrounded by a heat-insulating layer within an outer casing 252 and is not mechanically fixed to the outer casing but is simply prevented from moving rotationally or axially relative thereto. The inner chamber structure can therefore be completely withdrawn from the outer casing if required for inspection and repair.

This second unit has a hinge connection 254 to the third unit 214 below it and is supported near its other end by a motorised jack 256. Its inclination can therefore be adjusted to vary the rate of material throughput. The normal operating angle would be some 1" to 2" from the horizontal.

The third module 214 contains the firing kiln 260 which has the lightweight insulated double-shell and lobed construction already referred to and described in more detail in our concurrently filed patent application.

Material from the discharge hood 242 falls into inlet chute 262 to enter the kiln and the hot waste gases from the kiln flow upwards by the same route into the chamber 240. An enclosed firing platform 264 in the unit 214 serves also as the control room for the installation. The individual stage control means are not fully shown as they are similar to those in the first described installation. Like the second unit the third unit has a hinge connection 266 at one end to the unit 216 below it and a motorised jack 268 near the other end to adjust the angle of operation of the rotary kiln.

The fired pellets leave the kiln at a temperature of some 1150"C to fall into entry chute 270 of the fourth unit 216 which contains a rotary dryer chamber 272 the interior of which is of cylindrical form with cruciform axially extending dividing plates 274 to form a series of separate passages 276 leading from the chute 270. A forced draft fan 279 provides cooling air from the exit end at which there is a discharge chute for the delivery of the completed product, and the heated gases from the chamber 272 flow directly through duct 278 to the drying and heating chamber 240.

In this plant the rotary chambers are also shown mounted on rollers but the chambers are each rotated by their own chain drive 286, equivalent to the friction wheel motor drives illustrated in the first-described installation.

The fourth unit 216 is supported on self-elevating legs 280 which can be adjusted for varying both the inclination of the axis of the dryer chamber to control its throughput, and the height of discharge chute 282 from the drum to suit the output conveying means (not shown). Like the rotary chamber of the second module, the internal structure of the dryer can be completely removed from the insulated outer casing if required.

The invention thus provides the advantage of being able to construct a plant for producing synthetic aggregate, whether expanded or non-expanded, in a number of relatively compact modules sufficiently light and small in size to be transported between sites and are sufficiently flexible in their operating conditions to be able to cope with a variety of argillaceous materials, such as clays (including mudstones and shales), soils, dredged river mud, wastes of various kinds such as colliery spoil and china clay waste, and mixtures of materials such as sand and clay, or sand and shale mixtures, incinerator ash and clay mixtures or calcium bauxite and clay mixtures. It will be understood however, that different starting materials may require modifications of treatment, e.g. as regards the type of grinder preceding pelletizing, and pretreatment means may be required before the first dryer.In particular instances there may also be required means for blending additives into the basic feedstock. In all instances, the starting material should be graded to avoid large variations in particle size, whether by screening a mixture of large and small particles or by pelletizing a finely divided material.

The transportability of the equipment described and illustrated above is facilitated by a number of features. Firstly, it will be noted that the process is arranged to reduce the amount of dust generated and so avoid the need for bulky dust precipitation equipment as conventionally employed in conjunction with the firing of products such as synthetic aggregate. In particular there may be noted that violent mechanical shock of the material is avoided until it has first been given a toughening heat treatment.The use of separate burners at different stages of the process, all of which can be automatically controlled, makes it unnecessary to supply a large excess of combustion air so that the reduced gas flow results in lower gas velocities causing less dust entrainment, with the added advantage of requiring smaller fan powers, and avoiding the danger of spoiling the material by the formation of undesired metallic oxides.

Transportability is also facilitated because of the relatively high heating efficiency of the lobed heating chambers with the continual turnover of their contents, which means that these chambers can be very much shorter than a plain cylindrical chamber, particularly as regards the firing kiln.

Also in the case of the kiln the lightweight insulation greatly assists transportability in comparison with a conventional brick-lined kiln.

The division of the process into a number of separately controlled stages, together with the rapid cooling and heating made possible by the lightweight chamber constructions also means that many repairs are facilitated because it would be possible to shut down only the affected stage and hold the material in the remaining stages for a relatively short period, so avoiding the need to shut down the complete plant.

WHAT WE CLAIM IS: 1. A process for firing materials comprising the stages of selecting a particulate starting material of graded particle size, removing moisture from the material while heating it to a temperature below its firing temperature in a first rotary heating chamber, heating the dried material to firing temperature and firing said material in another rotary heating chamber and then cooling the fired particles in a further rotary chamber.

2. A process according to claim 1 wherein the fired particles are cooled by a gas flow and the gas heated thereby is directed to said rotary chamber for the drying stage to heat the material therein.

3. A process according to claim 1 or claim 2 employing a finely divided starting material reformed by pelletising and treated to have a moisture content not substantially greater than 18% before pelletising.

4. A process according to any one of claims 1 to 3 wherein the material is fed by

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. has a hinge connection 266 at one end to the unit 216 below it and a motorised jack 268 near the other end to adjust the angle of operation of the rotary kiln. The fired pellets leave the kiln at a temperature of some 1150"C to fall into entry chute 270 of the fourth unit 216 which contains a rotary dryer chamber 272 the interior of which is of cylindrical form with cruciform axially extending dividing plates 274 to form a series of separate passages 276 leading from the chute 270. A forced draft fan 279 provides cooling air from the exit end at which there is a discharge chute for the delivery of the completed product, and the heated gases from the chamber 272 flow directly through duct 278 to the drying and heating chamber 240. In this plant the rotary chambers are also shown mounted on rollers but the chambers are each rotated by their own chain drive 286, equivalent to the friction wheel motor drives illustrated in the first-described installation. The fourth unit 216 is supported on self-elevating legs 280 which can be adjusted for varying both the inclination of the axis of the dryer chamber to control its throughput, and the height of discharge chute 282 from the drum to suit the output conveying means (not shown). Like the rotary chamber of the second module, the internal structure of the dryer can be completely removed from the insulated outer casing if required. The invention thus provides the advantage of being able to construct a plant for producing synthetic aggregate, whether expanded or non-expanded, in a number of relatively compact modules sufficiently light and small in size to be transported between sites and are sufficiently flexible in their operating conditions to be able to cope with a variety of argillaceous materials, such as clays (including mudstones and shales), soils, dredged river mud, wastes of various kinds such as colliery spoil and china clay waste, and mixtures of materials such as sand and clay, or sand and shale mixtures, incinerator ash and clay mixtures or calcium bauxite and clay mixtures. It will be understood however, that different starting materials may require modifications of treatment, e.g. as regards the type of grinder preceding pelletizing, and pretreatment means may be required before the first dryer.In particular instances there may also be required means for blending additives into the basic feedstock. In all instances, the starting material should be graded to avoid large variations in particle size, whether by screening a mixture of large and small particles or by pelletizing a finely divided material. The transportability of the equipment described and illustrated above is facilitated by a number of features. Firstly, it will be noted that the process is arranged to reduce the amount of dust generated and so avoid the need for bulky dust precipitation equipment as conventionally employed in conjunction with the firing of products such as synthetic aggregate. In particular there may be noted that violent mechanical shock of the material is avoided until it has first been given a toughening heat treatment.The use of separate burners at different stages of the process, all of which can be automatically controlled, makes it unnecessary to supply a large excess of combustion air so that the reduced gas flow results in lower gas velocities causing less dust entrainment, with the added advantage of requiring smaller fan powers, and avoiding the danger of spoiling the material by the formation of undesired metallic oxides. Transportability is also facilitated because of the relatively high heating efficiency of the lobed heating chambers with the continual turnover of their contents, which means that these chambers can be very much shorter than a plain cylindrical chamber, particularly as regards the firing kiln. Also in the case of the kiln the lightweight insulation greatly assists transportability in comparison with a conventional brick-lined kiln. The division of the process into a number of separately controlled stages, together with the rapid cooling and heating made possible by the lightweight chamber constructions also means that many repairs are facilitated because it would be possible to shut down only the affected stage and hold the material in the remaining stages for a relatively short period, so avoiding the need to shut down the complete plant. WHAT WE CLAIM IS:

1. A process for firing materials comprising the stages of selecting a particulate starting material of graded particle size, removing moisture from the material while heating it to a temperature below its firing temperature in a first rotary heating chamber, heating the dried material to firing temperature and firing said material in another rotary heating chamber and then cooling the fired particles in a further rotary chamber.

gravity through said drying, firing and cooling stages.

5. A process according to any one of the preceding claims wherein a heated gas flow from the rotary chamber of the firing stage is directed to the rotary chamber for the drying stage to heat the material therein.

6. A process according to any one of the preceding claims wherein between the drying and firing stages in their respective rotary chambers the material is passed through a further chamber with its own heat source.

7. A process according to claim 5 together with claim 6 wherein the heated exhaust gases from the firing stage are fed directly to the drying stage rotary chamber bypassing said further heating chamber.

8. Apparatus for firing particulate materials of graded particle size comprising a first rotary heating chamber for a heating stage adapted to remove moisture from the material while heating it to a temperature below its firing temperature, a second rotary heating chamber for a further heating stage adapted to heat the dried material to firing temperature and to fire the material, and a further rotary chamber arranged to receive the fired material after firing for cooling the material.

9. Apparatus according to claim 8 wherein said rotary chambers are each of metal construction and the firing chamber has a lightweight sandwich construction comprising an inner heat-resistant metal lining, an outer support structure and a heat-insulating layer therebetween.

10. Apparatus according to claim 8 or claim 9 wherein respective control means are provided for regulation of the material heat input in the respective heating stages.

11. Apparatus according to any one of claims 8 to 10 comprising a series of separately transportable units including respective units for pelletizing the starting material for producing the graded particles, for said moisture removal and heating stage, for said firing stage and for said cooling stage.

12. Apparatus according to claim 11 wherein the units are formed as standard container modules.

13. Apparatus according to claim 11 or claim 12 wherein the units for drying, firing and cooling stages are arranged to be stacked upon each other with the material being transferred by gravity to each succeeding unit of the stack.

14. Apparatus according to any one of claims 8 to 13 wherein means are provided for adjustment of the inclination of said rotary chambers to alter the time of dwell of the material therein.

15. Apparatus according to any one of claims 8 to 10 comprising a further heating means in the form of a shaft kiln disposed between said drying and firing stages.

16. Apparatus according to claim 15 wherein an exit space at the bottom of the shaft kiln is arranged to receive material falling through the kiln, said space having a retaining barrier, and an agitator is provided for operation in the space to reduce the angle of repose of the material whereby to control its rate of flow past said barrier.

17. Apparatus according to any one of claims 8 to 16 wherein the firing chamber has an internal transverse cross-section of non-circular form comprising an equispaced series of lobes.

18. Apparatus according to any one of claims 8 to 17 wherein said drying chamber and/or said cooling chamber has an internal transverse cross-section divided into a plurality of smaller throughpassages by means of axially extending baffles.

19. A process for firing materials substantially as described herein with reference to the accompanying drawings.

20. Apparatus for firing materials constructed and arranged for use and operation substantially as described herein with reference to any of the embodiments illustrated in the accompanying drawings.