GB1577233A - Method and apparatus for treating fly ash for the production of bricks or tiles and to bricks or tiles so produced - Google Patents

Description

(54) METHOD AND APPARATUS FOR TREATING FLY ASH FOR THE PRODUCTION OF BRICKS OR TILES AND TO BRICKS OR TILES SO PRODUCED (71) We, STEAG AKTIENGESELLSCHAFT, a German Body Corporate, of Bismarckstrasse 54, D-4300 Essen, German Federal Republic, 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: The invention relates to a method and apparatus for treating fly ash and to a method and apparatus for producing bricks or tiles from a mixture containing fly ash, more particularly that originating from hard-coal fired power stations. The invention furthermore relates to an improved form of fly ash and to bricks or tiles containing fly ash.

It is known to utilise the combustion residue, i.e. ash, which is obtained in power stations by mixing it with clay and moulding the mixture to form bricks or tiles which moulded shapes are then fired. Such a process is particularly used for the manufacture of klinker bricks or clinkers. The combustion residue consists mainly of combustion chamber granulate and fly ash particularly from hardcoal-fired power stations. A comparatively large quantity of this raw material is obtained (between 4 and 6 million tonnes per annum in the Federal German Republic). The conversion of this material into bricks or tiles leads to a comparatively higher degree of value enhancement than its use in other building materials.

The conversion of hard-coal ash into bricks is known (West Virginia University, Coal Research Bureau, Technical Reports Nos. 11,16, 29, 34 and 55). Tests were carried out with a mixture containing approximately two-thirds by weight of fly ash obtained from the exhaust gas dust precipitator and water glass as bonding agent. Semi-commercial tile-making tests with hard-coal ash and clay as bonding agent are also known (published in Brennstoff-WBrme-Kraft, Volume 8 (1956) pp. 584, 587).

It is also known that in some circumstances fly ash contains widely varying quantities of residual carbon. These contents can amount to 8% and more and originate from the unburnt constituents in the raw dust. The carbon must be removed because brick products call for a material free from nuclei of oxidisable particles to provide the necessary mechanical strength.

It was found that the removal of carbon by burning was the most efficient solution but that it was important to incorporate this procedure correctly into the manufacturing process on the one hand to obtain bricks of improved and more uniform quality, and on the other hand to achieve efficient manufacture of such products.

The carbon is removed by oxidation in the course of firing the bricks, but the time required for this procedure amounts to approximately 50 h in the case of brick products of the highest quality with a correspondingly dense structure of the body. Laboratory tests on specimens of 7 x 4 x 4 cm containing 65% by weight of fly ash and varying amounts of carbon showed the following results in the temperature range between 600 and 800"C for the heatingup time which corresponds substantially to the time required for burning out the carbon: C content (%) 0 2 4 6 8 Heating-up 3.0 6.0 7.5 9.5 11.5 time (h) These times are increased by a multiple in a large kiln for bricks of normal size (24 x 11.4 x 7.1 cm).

The substantially longer firing time of carbon-containing bricks compared with carbonfree bricks requires a proportional increase in the size of the kiln. This greatly increases the investment costs of a brickworks and can render the conversion of hard-coal ash into bricks impossible for economical reasons. Conversion into bricks of fly ash is therefore possible in these circumstances only up to a specific maximum carbon content because the firing operation is made extremely difficult by a higher and varying carbon content and can in fact become uncontrollable. There are, however, other reasons why an increase in the size of the brick kiln represents an insurmountable obstacle for converting hard-coal fly ash into bricks.

For economical reasons it is desirable for the brickworks to be erected directly adjacent to the power station as this saves transportation costs for the ash and breakdowns of transportation means can be minimized. The brick kiln should be situated closely adjacent to the boiler house because the exhaust gases of the brick kiln contain sulphur dioxide and volatile fluorine compounds which can be rendered harmless in the interests of environmental protection by means of the power station flue gas purification system and thus without any substantial increase in costs.

However, it is this choice of location in the case of power stations which encounters substantial difficulties in many cases because space in the power station is insufficient to accommodate such large brick kilns.

A varying carbon content in the fly ash also leads to a varying compression density in the blanks. This in turn leads to a varying density of the green bricks. After firing, it causes varying porosity of the body with substantial quality differences in the end product as a result.

It is the object of the invention to eliminate the difficulties resulting from burning out, in the brick kiln, of the carbon in the raw fly ash.

According to one aspect of the present invention, a method of producing carbon-free fly ash comprises the steps of burning off the carbon in the raw fly ash with combustion air and then separating the solids from the combustion products. Thus a carbon-free fly ash is produced which can be used for making bricks or tiles, thereby avoiding the problems described above which arise when carbon-containing fly-ash is employed for brick or tile making.

The invention also includes within its scope a method of producing a brick or tile from a wet mix including fly ash and a binding agent by moulding the mix to produce a green shape which is fired wherein the carbon from raw fly ash is burnt off with combustion air to producefree fly ash before forming the wet mix.

This process substantially shortens the time for burning out the carbon in the raw fly ash and the process can be performed in a separate kiln. The large specific surface area of the finely divided carbon can be utilised by these means and on the other hand it is possible for the size of the kiln to be reduced and the quality of the end product is simultaneously improved while quality fluctuations are reduced to a minimum. It is a consequence of the method in which the carbon is burnt to obtain a carbon-free dust that iron is oxidized into Fe203 due to a specific excess of air in combustion if there is iron in the mixture.

It is another advantage of the method that it yields a fireproof clay (free of carbon, free of water, low shrinkage and pulverized) which can be used as an ideal semi-finished product for brickworks and other ceramic applications and can be used as filler, for example for concrete, mortar, plastics, paper and board.

It is advantageous to perform combustion of the carbon at a temperature which is below the softening point of the fly ash. This defines an upper limit for the combustion temperature.

The lower limit is the ignition temperature of the carbon. In one embodiment of the method according to the invention, the combustion temperature is therefore between approximately 600 and 1000"C. To effect instantaneous ignition of the carbon, preferably the material is pre- - heated to approximately 500"C before combustion of the carbon is performed.

Advantageously, the method according to the invention is also performed so that the quantity of raw fly ash and the amount of combustion air supplied thereto is maintained at a constant value in order to simplify the process. In this case, additional fuel is supplied to the raw material if the carbon content falls.

This fuel can be coal dust or a combustible gas.

Using this method, a fly ash which is practically free of carbon can be produced from the raw dust. The mixture from which the bricks are to be moulded and fired is therefore practically free of carbon. After being fired, these bricks have a lower pore volume, which is a decisive feature for a higher body density and therefore for a higher body quality.

The apparatus for producing carbon-free fly ash may comprise a combustion chamber, means for injecting raw fly ash and combustion air into said chamber and means for separating the solids from combustion products from said chamber.

In one form of construction, the apparatus for producing carbon-free fly ash comprises an upright tubular combustion chamber, an outlet at the top of said chamber, injector means for injecting preheated raw fly ash and combustion air into the bottom of said combustion chamber to be carried upwardly by the air stream to said outlet, a bunker surrounding said combustion chamber, means for feeding raw fly ash into said bunker to be preheated therein by heat from the combustion chamber and feed means for feeding the preheated fly ash from the bunker to said injector means.

The accompanying drawing illustrates, in diagrammatic form, one exemplified form of apparatus suitable for performing the method disclosed in the description hereinbelow.

The apparatus illustrated in the drawing is suitable for a capacity of approximately 25 tonnes/h of fly ash containing approximately 5% of carbon. This fly ash is obtained from the flue gas passing through a dust precipitator plant of a power station powered by hard coal but not shown. The raw dust or ash from the precipitator flows via a pipeline 1 to a treatment apparatus. Initially, the dust passes into an annular bunker 2 around a combustion tube 3.

-The capacity of the bunker 2 is chosen in accordance with the rate of throughput so that the fly ash is held in the bunker for approximately one to two hours. During this time it is preheated to at least 5000C by the hot gases in the combustion tube 3.

Under gravity, the raw ash flows downwardly through the bunker 2 and the heated ash passes into the bunker discharge 4 and from there into a pipeline 5. The pipeline 5 conveys the hot raw ash into the suction entry of an injector, references in its entirety by the numeral 6.

Preheated combustion air is fed at the rate of 12,000 n3NTp/h in the case of the selected numerical example, into the injector 6 to entrain the raw ash and to blow it into the bottom conically flared part 7 of the combustion tube 3. The flow velocities are reduced and the pressure is increased as the air passes upwardly through the conical flaring of the part 7. This ensures that coarse carbon particles dwell for a longer period in the combustion tube 3 than the smaller carbon particles, thus resulting in complete combustion of the carbon.

In cross-section, the combustion tube 3 above the conically flared part 7 has a diameter of approximately 2 m and this part is approximately 2Y4 m long. The dwell time of the dust is therefore approximately 5 seconds.

The preheating of the ash raises the initial temperature of the dust to approximately 500 C. This leads to reliable ignition and complete combustion of the carbon in the tube 3.

The combustion temperature is in the range between 600 and 1,000"C.

These conditions in the combustion tube are maintained constant by maintaining constant amounts of air and dust. If the dust contains 5% of carbon and if approximately 2 kg of dustlm3NTp are blown into the system, the final temperature will be approximately 9000C.

if the carbon content in the raw dust drops to approximately 3% C, a temperature of approximately 600"C will be reached and this will have to be regarded as the bottom limit for the required rapid combustion.

The temperature muust be increased as soon as the C content drops below 3%. Carbon dust, held available in a bunker 8 and passing into the injector 6 through a duct 9, is provided to this end. The system is automatically controlled.

Valve means 10, incorporated into the coal dust supply duct 9, are therefore controlled through a temperature sensor 11 which is installed in a discharge duct 13 that extends from the combustion tube 3. The said temperature sensor 11 controls the supply of coal dust to the injector 6 so that the temperature at the outlet of the combustion tube is at least 6000C.

The amount of air is adjusted at 1 m3NTp per 2 kg of raw dust at 5% C so as to ensure complete combustion into CO2. If the carbon content exceeds 5%, part of the carbon will merely be burnt into CO so that the final temperature does not exceed 9000C despite the increased C content.

In the embodiment illustrated in the accompanying drawing the residuel gases containing CO are oxidized into CO2 in an afterburning chamber 14. However, a maximum temperature, conveniently also 1000 C, should not be exceeded. The combustion chamber 14 is therefore fed with supplementary air. The supplementary air enters at 15 and is controlled by valve means 16. The valve means 16 in turn are controlled by a temperature sensor 17 installed in the exhaust gas duct 18 downstream of the afterburning chamber 14. The valve means 16 therefore regulates the amount of supplementary air so that a temperature of 1,000 C is not exceeded.

If the raw dust has an extremely large C content, for example in excess of 10%, the amount of supplementary air at 15 would become excessive. The supply of raw dust is therefore substantially reduced in such cases. To this end, a regulating element 19 is incorporated into the duct 5 and can be embodied, for example in the form of an apertured slider. The flow rate of dust can be suddenly reduced by means of such a slider. A regulating element 19 with a continuous closing action would not be sufficiently reliable because the hot dust would have acquired a temperature of the order of 500"C.

The combustion air fed to the injector 6 is preheated by a heat exchanger 20 incorporated in the exhaust gas duct 18. This combustion air is supplied at 21 to a compressor 22 for feeding to the heat exchanger 20. A second heat exchanger 23 abstracts heat from the exhaust gases which are discharged from the heat exchanger 20 in duct 18 and this heat can be supplied to a waste heat boiler, not shown. Such a boiler will supply saturated steam at the rate of 15 tonnes/h and a pressure of 8 atm abs if the raw dust contains approximately 5% of carbon.

The saturated steam can be used for drying the green bricks which are moulded from the mixture as described later.

Downstream of the heat exchanger 23 in the illustrated embodiment the exhaust gas has been cooled to approximately 100cC. It then passes into a cyclone separator 24 in which the major part of dust is separated from the gas stream for transfer to a collecting bin 25.

Another dust precipitator 26 in the illustrated embodiment is constructed as a hose filter and delivers the separated fine dust through a duct 27 to the bin 25 while the exhaust gas which has been stripped of dust is discharged from the system at 28.

To start the system from cold, combustion gas is introduced in place of the coal dust which passes via the regulating element 10 to the injector 6, until the temperature at the outlet of the combustion tube 3 has risen to approximately 8000C and the dust in the bunker 2 has been preheated to 50POC. The regulating element 19 initially remains closed. When the stated temperatures are reached, the supply of gas is shut down after the supply of dust has first been released.

In a simplified construction, the supply of coal dust 8-10 is replaced by the gas feed used for starting up the system.

Bricks can be manufactured by the production of a moist mixture from the fly ash at 25, screened combustion chamber granulate, and a bonding agent, from which said mixture the bricks are moulded. After drying, these bricks are fired in a kiln.

In the selected example relating to raw dust containing 5% C the entire firing time for standard format bricks is approximately 60 h.

The method according to the invention results in a reduction to 20 h. This means that in accordance with the invention and given the same output of the brickworks, the brick kiln need only be designed to a third of its normal size.

In the specification of Application No.

7937895 (Serial No. 1577234) which was divided out of the present application, there is claimed apparatus for producing carbon-free fly ash comprising an upright tubular combustion chamber, an outlet at the top of said chamber, injector means for injecting preheated raw fly ash and combustion air into the bottom of said combustion chamber to be carried upwardly by the air stream to said outlet, a bunker surrounding said combustion chamber, means for feeding raw fly ash into said bunker to be preheated therein by heat from the combustion chamber and feed means for feeding the preheated fly ash from the bunker to said injector means.

WHAT WE CLAIM IS: 1. A method of producing carbon-free fly ash comprising the steps of burning off the carbon in the raw fly ash with combustion air and then separating the solids from the combustion products.

2. A method as claimed in claim 1 wherein the combustion of the carbon is performed at a temperature below the softening point of the fly ash.

3. A method as claimed in claim 1 wherein the combustion temperature is maintained between 600"C and about 1,000"C.

4. A method as claimed in any of the preceding claims wherein the raw fly ash is preheated to approximately 5000C before combustion of the carbon is effected.

5. A method as claimed in any of the preceding claims wherein the relative proportion of raw dust and combustion air is maintained constant and wherein fuel is added if the carbon content of the fly ash is below a predetermined value.

6. A method as claimed in claim 5 wherein the added fuel is coal dust.

7. A method as claimed in claim 5 wherein the added fuel is a combustible gas.

8. A method as claimed in any of the preceding claims wherein the raw fly ash is fed continuously into a combustion chamber for burning off the carbon and wherein the raw fly ash before combustion of the carbon is preheated using heat from the burning of the carbon in the combustion chamber.

9. A method as claimed in any of the preceding claims wherein the combustion is effected in an upward stream of combustion air so that coarse carbon particles have a longer dwell time than fine particles.

10. Carbon-free fly ash produced by the method of any of claims I to 9.

11. Carbon-free fly ash comprising a fly ash in which any carbon has been burnt off in combustion air.

12. A method of producing a brick or tile from a wet mix including fly ash and a binding agent by moulding the mix to produce a green shape which is fired, wherein the carbon is burnt off from raw fly ash with combustion air to produce carbon-free fly ash before forming the wet mix.

13. A brick or tile produced by the method

Claims (1)

1. of claim 12.

   14. A brick or tile comprising a fired moulded shape of material including carbonfree fly ash as claimed in claim 10 and a binding agent.

   15. A method of producing carbon-free fly ash substantially as hereinbefore described with reference to the accompanying drawing.

   16. Fly ash produced by the method of claim 15.

   17. A method of producing bricks or tiles using carbon-free fly ash substantially as hereinbefore described with reference to the accompanying drawing.

   18. A brick or tile produced by the method of claim 17.