GB2247306A

GB2247306A - Calcining clay

Info

Publication number: GB2247306A
Application number: GB9117739A
Authority: GB
Inventors: Anthony Railton Windybank; Terry Nicholas Smith
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1990-08-21
Filing date: 1991-08-16
Publication date: 1992-02-26
Anticipated expiration: 2011-08-16
Also published as: GB9018283D0; GB2247306B; GB9117739D0

Abstract

Apparatus for clay calcining includes a vessel 2 in which a fluidised bed of particulate solids is held at an elevated temperature by the combustion of a fuel which may be solid, eg coal, liquid or gaseous. Fine clay particles are injected into the expanded bed region 26 of the vessel 2 to create a transient fluidised bed of clay particles and are calcined therein, the heat treated particles being elutriated from the region 26 by the combustion gases, are cooled by a heat exchanger 50 and are conveyed to a cyclone separator 64. Calcined clay particles are collected in the separator 64 and discharged for use in the manufacture of bricks or tiles <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO CLAY CALCINING This invention concerns improvements in or relating to clay calcining.

In particular the present invention has reference to the calcination of clay utilising a fluidised combustion bed for the generation of the requisite heat.

It is already known from our British Patent No 2 027 859 to employ a bubbling fluidised combustion bed to generate the heat for calcination of gypsum to produce plaster, wherein the gypsum is heated at a temperature in the region of 1500C in a separate bed from the combustion bed but receives heat therefrom by conduction and by the passage of combustion gases therethrough.

The advantage of employing a fluidised bed combustion system for heat treatment purposes is that by designing it for a low excess air requirement, a high thermal efficiency results.

Additionally, lower capital costs are associated with such a system in comparison with the more complex circulating fluidised bed systems or the conventional rotary drum heat exchangers. Fluidised combustion or heat treatment beds require no moving parts and are relatively simple to automate.

An object of the present invention is to provide an improved method of and apparatus for calcining clay.

According to a first aspect of the present invention a method of calcining clay includes the steps of establishing a bed of inert particles, fluidising the bed with a comburent gas, burning fuel within the bed to heat the bed, maintaining the bed at an elevated temperature, introducing clay particles into an upper expanded bed region of the fluidised bed to create a transient bed of fluidised clay particles within said region and to effect calcination thereof, elutriating the hot clay particles from the region in the exhaust gases issuing from the bed, cooling the gases and the clay particles, and separating the elutriated calcined clay particles from the gases.

The temperature of the fluidised bed may conveniently lie in the range 800 to 10000C.

The inert particles may be sand or other suitable material.

The fuel to be combusted within the fluidised bed may be liquid, gaseous or solid.

According to a second aspect of the invention apparatus for calcining clay in accordance with the method of the invention includes a vessel adapted to contain in its relatively lower part a fluidised bed of particulate material, inlet means for a fluidising comburent gas, inlet means for a fuel, an expanded bed region within the vessel, inlet means for introducing ~ clay particles into the expanded bed region, an exhaust gas outlet in a relatively upper part of the vessel, particle separation means, and a heat exchanger interposed between the vessel and the particle separation means.

The heat exchanger may be interposed between the lower part of the vessel and the outlet.

Conveniently, the inlet means for a comburent gas is provided by a plenum chamber defined beneath a gas permeable base and the bottom of the vessel, the gas permeable base being of conventional form, namely a distributor plate with perforated stand pipes. Alternatively, the inlet means may comprise a plurality of sparge pipes provided with bubble caps.

In the case of solid fuel, eg coal, the inlet for fuel may be located above the lower part of the vessel and above or within the expanded bed region. The expanded bed region of the vessel is that region into which upon full flu idisat ion the bed will expand in use. The expanded bed region may advantageously be of greater lateral dimension than the lower part of the vessel in order in use to increase the residence time of the clay particles.

The inlet for clay particles is disposed above the lower part of the vessel and in the expanded bed region, such that in use the clay particles are injected into the expanded fluidised bed.

Conveying means for solid fuel, which is for example coal, is at least one feed screw of conventional form extracting from a suitable storage hopper. The inlet means for the fuel when liquid fuel is used, may be in the form of climbing film nozzles incorporated in the gas permeable base of the inlet means for the comburent gas. The inlet means for a gaseous fuel may be in the form of nozzles in the gas permeable base.

Suitable conveying means for the clay particles are provided for transferring particles from storage hoppers or directly from a grinding system provided to comminute the clay into appropriate particulate form and size.

The heat exchanger may be of any suitable configuration and may be of ceramic material. The heat exchange fluid may be air provided by a forced or induced draft fan, or any other suitable fluid.

The particle separation means may conveniently be a cyclone separator provided with a gas outlet and a material outlet connected to suitable collection means for the calcined clay particles, the inlet to the cyclone being connected by ducting to the exhaust gas outlet of the heat exchanger. In addition to or as an alternative to the cyclone separator, a bag filter may advantageously be employed. A precipitator may be employed as an alternative particle separation means. For acceptable efficiency of the method, it is necessary to achieve maximum retention of the clay particulate product. Furthermore, it is necessary to secure that particulate emissions from the apparatus are kept within the limits prescribed in the impending legislation.

A scrubber may also be employed to constitute at least a part of the particle separation means.

A bursting disc may be associated with the ducting for safety purposes when operating the calcination method using gas as the fuel.

By way of example only, one embodiment of apparatus for calcining clay and a method of calcining clay using such apparatus is described below with reference to the accompanying drawing which is a diagrammatic vertical section through the apparatus.

Referring to the drawing, apparatus for calcining clay is shown generally at 1 and includes a vessel 2 lined with refractory 4 and having a relatively lower part 6, relatively upper part 8 and an intermediate part 10. In the lower part 6 there is provided a distributor plate 12 having a plurality of stand pipes 14 perforated at their upper free ends. A plenum chamber 16 is defined between the plate 12 and the bottom 18 of the vessel 2, and has an inlet 20 connected to a fan (not shown) and a source (not shown) of combustible gas for start-up purposes to be described later in this specification.

The lower part 6 in use accommodates a bed 22 of generally inert particulate material, for example sand, which has a static level at 24. The intermediate part 10 in use accommodates an expanded bed region 26, that is the region into which the fluidised bed expands upon full fluidisation.

An inlet 28 for clay particles is located in the intermediate part 10 and registers with a screw feeder 30 supplied from a hopper 32, the inlet 28 communicating with the expanded bed region 26.

A fuel inlet 34 for coal is positioned above the intermediate part 10 and is connected to a screw feeder 36 supplied from a hopper 38 fed in turn from a bunker 40 by means of an inclined screw 42. A sight glass 44 and a start-up ignition torch 46 are also provided above the intermediate part 10 as shown.

A heat exchanger 50 is disposed within the upper part 8 and comprises a plurality of ceramic tubes 52 arrayed across the exhaust gas duct 54 formed within the part 8. The tubes 52 communicate with an inlet manifold 56 connected to a forced draft fan (not shown) and an outlet manifold 58.

The exhaust gas duct 54 has an outlet 59 coupled to a duct 60 provided with a bursting disc 62 for safety purposes when operating on gas. The duct 60 leads to a cyclone separator 64 of conventional design having an inlet 66, a gas outlet 68 and a solids outlet 70 which communicates with a collection receptacle 72.

In operation, the bed 22 of sand is formed in the lower part 6 of the vessel 2 and is fluidised by air fed via the plenum 16 from the fan (not shown). Start-up gas is also injected into the bed through the plenum 16 and the distributor plate 12 and is ignited by the torch 46. The supply of gas is maintained and combustion continues until the temperature of the bed has reached a level at which coal introduced through inlet 34 will burn satisfactorily. Once this stage has been reached, the supply of gas is stopped.

The temperature of the bed 22 is then elevated by increased combustion to a value at which clay calcination can take place. The temperature of the bed will lie between 8000 and 10000C.

Fine clay particles formed for example by grinding are fed from the hopper 32 through the screw feeder 30 and the inlet 28 into the expanded region 26 of the bed therein to create a transient bed of clay particles and to be subjected to heat treatment which calcines them.

The expanded bed region being of greater dimension than the lower part 6 in which combustion takes place, allows for a greater residence time for the clay particles in the transient bed than with an intermediate part of the same cross section as the lower part 6.

The gases arising from the combustion within the bed 22 elutriate and thus convey the fine calcined clay particles in their upward passage within the vessel 2 to exit through the heat exchanger 50 wherein the particles cool in suspension and thence through the outlet 59 into the ducting 60. The exhaust gases with the entrained calcined clay particles pass into the cyclone separator 64 through the inlet 66. In conventional manner, the solid particles of calcined clay are separated from the gases which exit through the outlet 68, the particles gravitating to the base of the cyclone separator to discharge through the outlet 70 for collection in receptacle 72. The calcined clay so collected may then be employed for brick or tile manufacture.

During the passage of the calcined clay-laden gases through the heat exchanger 50, heat is transferred and is carried away by air flowing through the ceramic tubes 52 between the inlet manifold 56 and the outlet manifold 58. This heated air can be utilised as the combustion/fluidising air, to preheat the incoming raw clay feed in a grinding mill or elsewhere within the overall calcination method and thus the thermal efficiency of the calcination method is optimised.

By removing heat directly from the fluidised bed of inert particles, the excess air required to maintain the bed at a temperature of below 10000C can be reduced to the minimum required for efficient coal combustion, thereby maximising the overall thermal efficiency of the calcination method.

Claims

CLAIMS:

1. A method of calcining clay includes the steps of establishing a bed of inert particles, fluidising the bed with a comburent gas, burning fuel within the bed to heat the bed, maintaining the bed at an elevated temperature, introducing clay particles into an upper, expanded bed region of the fluidised bed thereby to create a transient bed of fluidised clay particles within said region and to effect calcination thereof, elutriating the hot clay particles from the region in the exhaust gases issuing from the bed, cooling the clay particles and the exhaust gases, and separating the elutriated calcined clay particles from the gases.

2. A method according to claim 1 in which the elutriated hot clay particles are passed in heat exchange with a fluid thereby to effect the cooling of the particles prior to their separation from the entraining gases.

3. A method according to claim 1 or 2 in which the fuel is solid, liquid or gaseous.

4. A method according to claim 3 in which the solid fuel is coal.

5. A method according to any one of the preceding claims in which the temperature of the fluidised bed lies in the range 800 to 10000C.-

6 A method of calcining clay substantially as described with reference to the accompanying drawing.

7. Apparatus for calcining clay in accordance with the method claimed in any one of the preceding claims includes a vessel adapted to contain in its relatively lower part a fluidised bed of particulate material, inlet means for a fluidising comburent gas, inlet means for a fuel, an expanded bed region within the vessel, inlet means for introducing clay particles into the expanded bed region, an exhaust gas outlet in a relatively upper part of the vessel, particle separation means, and a heat exchanger interposed between the vessel and the particle separation means.

8. Apparatus according to claim 7 in which the inlet means for a comburent gas is provided by a plenum chamber defined beneath a gas permeable base and the bottom of the vessel.

9. Apparatus according to claim 8 in which the gas permeable base is a distributor plate provided with perforate standpipes.

10. Apparatus according to claim 7 in which the inlet means for a comburent gas is a plurality of sparge pipes arrayed across the base of the vessel.

11. Apparatus according to any one of claims 7 to 10 in which the vessel has an intermediate part between the upper and lower parts, in use an expanded bed region being accommodated therein.

12. Apparatus according to claim 11 in which the inlet means for a solid fuel is located in or above the intermediate part of the vessel.

13. Apparatus according to claim 11 or 12 in which the inlet means for clay particles is located in the intermediate part of the vessel.

14. Apparatus according to any of the preceding claims 7 to 13 in which the heat exchanger is comprised of a plurality of ceramic or metallic tubes, inlet and outlet manifolds being provided for the tubes.

15. Apparatus according to any one of the preceding claims 7 to 14 in which the particle separation means is a cyclone separator.

16. Apparatus according to any one of the preceding claims 7 to 14 in which the particle separation means is a precipitator.

17. Apparatus according to any one of the preceding claims 7 to 14 in which the particle separation means is a bag filter.

18. Apparatus according to any one of the preceding claims 7 to 14 in which the particle separation means is a scrubber.

19. Apparatus according to any one of the preceding claims 7 to 14 in which the particle separation means comprises a cyclone separator in series with a bag filter and/or a precipitator and/or a scrubber.

20 Apparatus for calcining clay substantially as hereinbefore described with reference to the accompanying drawing.