GB2109360A - Production of lightweight aggregate material - Google Patents

Abstract

Bloated clay is produced in a fluidised bed furnace by introducing moisture-containing particles of clay into a fluidised bed of inert material (e.g. alumina) in a fluidised bed furnace, the moisture-containing clay particles being of higher density than the fluidised bed of the furnace and the temperature of the furnace being such that the clay particles are caused to bloat and become less dense than the fluidised bed so that the bloated clay particles formed float to the top of the fluidised bed from whence they are removed. In addition formed articles are produced from particles of bloated mineral material, particularly such particles made by the above process, by heating such that their outer walls are soft, pliable and tacky and forming the hot tacky particles together under pressure to the required shape.

Description

SPECIFICATION Production of lightweight aggregate material This invention relates to the production of lightweight aggregate mineral, particularly to lightweight aggregate material produced from clay.

The production of lightweight aggregate by the bloating (or expanding) of clay is well known. Bloated (or expanded) clay is used as a lightweight aggregate in the building industry. In particular the bloated clay may be used together with a binder medium to provide prefabricated building materials. Thus the bloated clay is used together with cement and fines material in the fabrication of rectangular building blocks and is used as a component in concrete structures e.g. pipes, moulded concrete used in the construction industry.

Bloated clay is produced by heating a starting clay with appropriate known properties to a comparatively high temperature (generally 1000 to 1 200 C) such that the caly is caused to expand, and there is formed a bloated product with physical and mechanical properties making it suitable for use as a lightweight aggregate in the building industry.

Bloated clay is produced commercially in rotary kilns where the clay is placed on a conveyor and it is heated from above as it passes below a hood containing heating jets, or batchwise in an enclosed oven. Generally bloated clay is produced commercially in a rotary kiln having a central flame so that some of the clay in the kiln is subjected to the flame itself, some is in the gas of the flame and some contacts the kiln walls. This means that temperature control in the kiln is difficult to achieve. Further with today's fuel costs such kilns are very expensive to run.

There have been proposals to bloat clay using fluidized bed techniques. For example British Patent Specification No. 11 67798 proposes the production of expanded materials by maintaining small granules of the material in a fluidised state when heating by means of an electrical current of frequency in the range of 10 to 80 MHz. British Patent Specification No. 1355031 proposes the production of low density material by introducing a finely divided particulate fuel and finely divided particulate water-containing material e.g. colliery tailings, into or onto a hot fluidised bed of the low density material to be obtained whereby the fuel is burnt within the bed thereby to cause the particulate material to expand.The temperature of the furnace is kept below 1 000 C to prevent sintering and the expanded material obtained is withdrawn from the bottom of the furnace. However no method of making bloat clay using a fluidised bed furnace has been used practically in a commercial production method.

According to a first aspect of the present invention there is provided a process for producing bloated clay, which process comprises introducing particles of clay into a fluidised bed of inert particulate material in a fluidised bed furnace, the clay particles being of higher density than the fluidised bed of the furnace and the temperature of the furnace being such that the clay particles are caused to bloat and become less dense than the fluidisedd bed of the furnace, and removing bloated clay particles from the top of the fluidised bed of the furnace.

In the process of the present invention, the particles of starting clay introduced into the fluidised bed of the furnace are of higher density and accordingly sink towards the bottom of the bed. When the clay particles have picked up sufficient heat and expanded to a lower density than the bed, they will float to the top of the bed from where they can readily be removed. Thus using the process of the present invention, bloated clay can be very simply produced substantially free from starting clay particles. The bloated clay particles can be removed from the top of the of the fluidised bed mechanically or by being allowed to spill over the furnace sides.

Fluidised bed furnces are known see for example R. W. Reynoldson "Development of Submerged Combustion Heating and Use of Inert Atmospheres for Fluidised Bed Furnaces" Paper 3, Heat Treatment '79 Birmingham, 22-24 May, 1979, The Metals Society, American Society of Metals.

The fluidised inert particulate material used as bed carrier according to the present invention can be of any suitable material. In particular the material must be stable at the termperature at which the furnace is operated.

Finely graded alumina is a suitable bed carrier material.

The fluidised bed may be heated by means of the fluidising gas. Thus air and fuel gas in combustible mixture are forced into the furnace below a distributor, e.g. of ceramic porous tile, above which distributor the bed carrier is held. The fluidising gas may provide all the heat to the furnace or only part of it. If desired additional heat may be provided e.g.

by directly heating the bed e.g. from the sides of the furnace. The fluidising gas may be assisted by flue gas recycling. The fuel gas is suitably propane, mains gas or bottled gas.

Once the furnace has been raised to the required temperature the starting clay is introduced into the fluidised bed. The clay may be introduced into the top or at the sides or bottom of the fluidised bed.

Before introduction into the fluidised bed the starting raw clay is destoned and cut into particles e.g. into cubes of suitable size in a material preparation unit. The particles are preferably substantially uniform in size so that a substantially uniform product is obtained.

Before introduction into the fluidised bed, the clay particles may be passed through a preheater section in which some moisture from the clay will be removed. Preferably the preheater is a heat exchanger in which the starting clay is heated by means of heat exchange with hot bloated clay which has just left the fluidized bed furnace.

The so-partially dried clay, generally after having been raised to a suitable height by an elevator, may then be further heated and dried before being introduced into the fluidised bed furnace. This further heating may be effected by allowing the partially dried clay to fall down a heat exchange column in counterflow to a rising hot exhaust gas stream from the fluidised bed furnace; the gas is then exhausted from the top of the column or may be recycled for fluidisation.

Alternatively the clay may be introduced into the fluidised bed cold and moist. This depends on the degree of bloat required.

Such non-heated material with a high moisture content may lead to particles having a very high degree of bloat but which are very friable.

The clay introduced into the fluidised bed furnace, being denser than the bed, sinks towards the bottom. In the furnace the clay is heated to such a temperature that it is caused to bloat. The less dense bloated clay then floats towards the top of the fluidised bed from which it may be removed, passed through the pre-heater heat exchanger to dry incoming raw material and collected as product. In this way bloated clay can be produced very simply and efficiently.

The fluidised bed furnace used according to the present invention may suitably be operated at 1000 to 1 300 C. At temperatures slightly above the bloat temperature (e.g. at above about 1 1 00 C) the outer skin of bloated clay tends to become tacky and the walls of the caly particles themselves become soft and pliable. Care therefore may need to be taken that when in the main part of the fluidised bed the bloated clay does not reach this temperature. Otherwise the bed carrier medium of the fluidised bed may stick to the bloated clay particles, resulting in medium being lost from the system and the product bloated clay being coated with bed carrier medium. Clearly this can be disadvantageous if the bed carrier material is expensive or if it is important that the product bloated clay is free from carrier.However if it is desired to operate the furnace at above 11 00'C, if necessary bed carrier material may be detached from the bloated clay before cooling in a rotating cylinder.

The tackiness which the bloated clay develops can be put to advantage to prepare formed articles e.g. prefabricated building materials, for example building blocks, pipes, without the use of binder materials.

According to a second aspect of the present invention, there is provided a method of producing a formed article which process comprises heating particles of bloated mineral material, preferably of bloated clay, to such a temperature that the outer walls are soft, pliable and tacky, forming a plurality of the hot tacky particles together under pressure to the rquired shape nd then allowing the formed article thereby obtained to cool.

Under the forming pressure voids between adjacent soft, pliable particles close up and when the whole is allowed to cool there is obtained a cohesive prefabricated building material. The forming may be achieved by forcing the heated particles through an appropriately shaped orifice in the floor of a hopper.

If necessary the continuous prefabricated lengths may be cut to length e.g. to give building blocks while for piping long lengths may be desired.

Preferably the provision of heat to the bloated mineral material particles of the first step of the second aspect of the present invention is combined with heating to form the bloated particles themselves. Thus rather than bloating the particles and allowing them to cool, the particles are formed into the required shape, optionally after further heat has been applied to render them sufficiently soft, pliable and tacky, directly after bloating. In this way heating costs are minimised.

The process of the first aspect of the present invention is particularly suitable for use in the production of such preferabricated building materials. Thus the bloated clay removed from the top of the fluidised bed is optionally subjected to additional heating e.g.

by being passed to a further receptacle where it is subjected to fierce flame heat to cause the particles to become tacky and then passed to a hopper for forming. Alternatively the further heating can be applied at the top of the fluidised bed furnace itself and the tacky clay particles then forked off to the hopper for forming. In this later case of course through care should be taken to avoid the grit medium of the bed adhering to the tacky particles or any grit adhering removed if it is desired that the final product should be free from grit.

The invention is further illustrated by way of example in the accompanying drawing which shows diagrammatically apparatus for use in the production of bloated clay.

Referring to the drawing, starting raw clay material 1 is introduced into a material preparation unit 2 where the clay is destoned and cut up into uniform cubes. The cubed clay from unit 2 is then passed along the outer part 3 of the pre-heater heat exchanger 4. In the pre-heater 4 this incoming clay is heated and thus partially dried by means of hot product which flows in counter-current along the inner part 5 of the pre-heater 4. The partially dried product from pre-heater 4 is raised to a suitable height in an elevator 6 and passes then down a short chute 7 into the top of a heat exchanger column 8.

In the column 8 the partially dried clay cubes are allowed to fall down the column in counterflow to a rising hot exhaust gas stream from the fluidised bed furnace below. In this way the clay material is heated and further dried and the exhaust gas stream cooled.

Contact between the falling clay and rising gas stream is improved is improved by means of plates 9 mounted with the column. The gas stream is exhausted from the top of the column at 10.

The heated clay from the bottom of column 8 is then allowed to drop into the hot fluidised bed furnace 11. The fluidised bed furnace 11 is provided with an air source 1 2 and a fuel gas source 1 3. The air and fuel gas sources, controlled by means of valves, serve to provide a combustible air/fuel gas mixture which enters via pipe 14 a gas/air inlet chamber 1 5 in the bottom of the furnace. The gas/air stream then passes through a layer of distributor tiles 1 6 above this chamber and into the main chamber 1 7 of the furnace 11 where it maintains the fluidised bed.

The clay which drops into the fluidised bed furnace 11 from column 8 enters the fluidised bed maintained in the main chamber 1 7. This clay, which is unbloated, is more dense than the bed material and thus sinks towards the bottom of the bed. However while it is in the furnace the caly is sufficiently heated to cause it to become bloated. The bloated clay rises to the top of the fluidised bed from which it is removed to a material extractor 20. From the material extractor 20 the hot bloated clay passes along the inner part 5 of the pre-heater exchanger 4 where it becomes cooled and incoming raw material is heated and partially dried. The bloated clay is then removed from the system as product 21.

Claims (9)

1. A process for producing bloated clay which process comprises introducing particles of clay into a fluidised bed of inert particulate material in a fluidised bed furnace, the clay particles being of higher density than the fluidised bed of the furnace and the temperature of the furnace being such that the clay particles are caused to bloat and become less dense than the fluidised bed of the furnace, and removing bloated clay particles from the top of the fluidised bed of the furnace.

2. A process according to claim 1 wherein the fluidised bed furnace is operated at a temperature of 1000 to 1 300 C.

3. A process according to claim 1 wherein the clay particles are heated by means of exhaust gases from the furnace before being introduced into the furnace.

4. A process for producing bloated clay substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

5. Bloated clay produced by the process claimed in any one of claims 1 to 4.

6. A process for producing a formed article which process comprises heating particles of bloated mineral material to such a temperature that the outer walls are soft, pliable and tacky, forming a plurality of the hot tacky particles together under pressure to the required shape and then allowing the formed article thereby obtained to cool.

7. A process according to claim 6 wherein the particles are formed together by being forced through an orifice of the required shape.

8. A process according to claim 6 or 7 wherein bloated clay particles are used.

9. A process according to claim 8 wherein the hot tacky clay particles are obtained by the process claimed in any one of claims 1 to 4 and wherein bloated clay particles removed from the top of the fluidised bed of the furnace, optionally after additional heating, are formed together.

1 0. A formed article obtained by the process claimed in any one of claims 6 to 9.