GB2061241A - Bricks containing pulverised fuel ash and their manufacture - Google Patents

Abstract

Bricks are made from pulverised fuel ash (PFA) by classifying the PFA to remove the large particles and preferably also to remove the fine particles; the classified PFA is then mixed with a predetermined quantity of fuel and sufficient binder clay and moisture to enable a coherent green brick to be formed, e.g. by semi-dry pressing and the green brick is fired in a kiln or clamp. The classifying of the PFA removes the larger particles, commonly containing a high proportion of carbon and facilitates the standardising of the carbon content; the added fuel may be a low-grade fuel, e.g.. sewage sludge, or colliery spoil or may make use of the coarse fraction.

Description

SPECIFICATION Bricks containing pulverised fuel ash and their manufacture This invention relates to bricks containing pulverised fuel ash (PFA) and clay and to the manufacture of such bricks in kilns or clamps.

Bricks may be made from a wide range of earths, clays, mudstones and shales provided the material can be shaped and fired to give a hard structure with the desired physical properties. Naturally occurring sediments contain a proportion of the clay minerals mixed with a variety of other minerals as unweathered or partly weathered rock particles. The clay itself consists of very finely divided crystalline hydrated alumino-silicates in the form of platelike particles smaller than 2 microns in diameter. These platelets are separated by a film of water which confers cohesion and plasticity. Removal of the water occurs on drying and firing of the clay which consequently shrinks. To minimise shrinkage and to reduce stickiness of the clay it is usual to add a considerable proportion of particulate aggregate and for brickworking purpose a material seldom contains more than one third clay.

A wide range of materials including sand, crushed fireclay and ash may be added to clay for brickmaking.

For optimum properties of the fired bricks it is however important that the particle size range of the non-clay components is appropriate. There is an optimum particle size range for a particular clay since if the non-clay particles are either too large or too small, the fired brick will have low strength.

Several different methods of forming bricks are commonly used and this affects the required level of water content in the brick clay. Hand or soft mud moulding needs a soft plastic clay with a relatively high water content. At the other extreme the semi-dry press method uses damp granular clay and this reduces drying shrinkage. Between these extremes fall the extrusion wire-cut and the stiff plastic press processes which require intermediate water content in the clay.

The manufacturing process therefore influences the extent to which water and non-clay materials are used. It also influences the drying process and the drying and firing shrinkage of the brick. In general the higher the water content of the clay the greater the shrinkage and it is therefore advantageous if the non-clay materials used can have the characteristic of improving plasticity since the amount of water in the clay may then be reduced.

The desired physical properties of the brick are obtained by first removing all the water in the green brick and then raising the temperature of the brick to the level at which the more fusible components in the clay melt and bind the structure together by vitrification. The energy required to achieve this therefore depends partly on the water content of the clay and partly on the temperature at which vitrification occurs.

Consequently it is advantageous to add a material to the clay which in addition to having acceptable properties also contains carbon which will oxidise during firing and reduce the heat-input to the kiln. Too high a carbon content would of course be deleterious in that the firing temperature could reach a level high enough to completely vitrify the brick with consequent severe distortion.

It is therefore possible to enumerate the desired properties of the aggregate to be added to clay in brick making. These are: (a} The material should promote flowability during the forming process and should ideally consist of spherical particles.

(b) The particle size range should be suitable preferably 20-200 microns.

(c) Preferably the material should contain sufficient carbon in finely-divided form so that heat energy may be provided to assist the firing process without leaving large pores in the brick after the carbon is oxidised.

(d) The material should allow the minimum quantity of clay to be used and with the minimum water content to reduce the energy input requirement and reduce shrinkage.

A material which is particularly suitable for mixing with clay in brick making is pulverised fuel ash (PFA) since it possesses all the desirable properties which have been defined. PFA is created in coal fired power station steam boilers in which heat is generated by the combustion of finely ground particles of coal. The coal contains carbon and ash and the burning process agglomerates the ash in the form of individual spheres. About 750/D of these spheres are carried away in the flue gases and extracted in cyclone collectors and,or electrostatic precipitators. The chemical composition of PFA is similar to clay and these are compared in table I.

TABLE 1 Chemical Composition of PFA and Brick Making Clays Constituent Brick Clay PFA %wt %wt Silica 43-80 38-58 Alumina 8-35 20-40 Iron Oxide 0-12 6-16 Magnesia 0-13 1-3.5 Soda and Potash 1.5-8 2-5.5 Sulphur compounds 0-5.6 0.5-2.5 Carbon 0-5 2-30 The particle size range of PFA depends on whether it is collected from a cyclone, precipitator or from a lagoon. Typically it varies from 0.5-200 microns. Carbon content depends on the combustion conditions in the particular power station and can vary from day to day and from station to station. In certain combustion conditions the PFA may contain as much as 30% by weight carbon either as comparatively large particles (50-200 microns) or as very fine particulate carbon coating on the surface of the PFA spheres.

According to one aspect of the present invention, a method of making bricks comprises the steps of classifying pulverised fuel ash (PFA) to separate the large grains of unburnt or partly combusted fuel together with other large particles of contaminating mineral matterfrom the finer fused alumino-silicate material, mixing with the finer fraction a predetermined quantity of fuel together with sufficient binder clay and moisture for making a coherent green brick, shaping the mixed material into green bricks and then firing the green brick in a kiln or clamp to produce vitrification.

It is known that durable bricks can be-made from a ceramic body composed mostly of pulverised fuel ash (PFA), using kiln firing; see for example British Patent Specification No. 823688. However, the quality of such PFA based bricks has been found to be variable and for this reason the bricks have not commanded a widespread commercial acceptance.

It has been established that a major factor in the variable quality of PFA bricks heretofore is the variable level of combustible material, mainly residual pulverised fuel, in PFA which is produced in bulk by coal-fired electricity generating stations. Differences in the amount of combustible material in PFA obtained from these sources arises from variations in combustion and power generation load schedules at the.power station, and it is therefore not normally possible to prevent such differences arising.

Carbon is present in PFA in amounts which vary with the quality of coal burned and the efficiency of combustion. Modern base load powerstations may produce PFAcontaining less than 3% byweightcarbon whereas PFA from the older stations may contain more. In some cases the carbon content can be high and PFA from a particular source may contain as much as 30% by weight carbon. If the carbon content in the PFA is variable then it is difficult to arrange for appropriate firing conditions in the brick kiln or clamp. If the firing conditions are not correct not only will the properties of the fired brick be variable but production is lost due to local over or under firing. The type of carbon content variation which is met is illustrated in Table 2 showing percentages by weight of combustible material for PFA from various sources.

TABLE 2 Variation in PFA Carbon Content Power Station % Combustible in PFA A 10.7 B 4.7 C 5.2 D 10.8 E 29.4 F 3.4 G 2.1 In many instances the majority of the carbon content of PFA occurs in the form of relatively large particles.

Where this is the case it offers the opportunity of standardising on carbon content in the PFA by removal of the bulk of the carbon by sieving or air-classifying and adding back sufficient carbon to give the required level of heat release when the bricks are fired. This possibility is illustrated by the analysis (table 3) of both PFA retained on a 150 micron sieve and of the material passing the sieve.

TABLE 3 % Weight of Heat content of % Weight loss on PFA Sample PFA Retained PFA retained on ignition of PFA Number on 150 micron 150 micron sieve passing 150 micron sieve mjikg sieve 1 1.3 7.67 0.2 2 1.5 10.4 0.56 3 12.5 10.22 1.93 4 6.7 14.11 2.17 5 3.9 11.64 1.23 The calorific value of the material retained on the 150 micron sieve is a relatively high proportion of the calorific value (22.5-23.8 mJ'kg) of the coals normally used to fire the station from which the PFA samples were taken and the heating value of this size fraction of the particular PFA is evident. With a high carbon content PFA it becomes even more important to be able to control the carbon content as the Table 4 analysis of a high carbon PFA indicates.

TABLE 4 PFA Carbon Content % by weight from a Particular Station As produced PFA 30.1 After initial classification 38.1 Passing 38 micron sieve 20.8 The carbon in this particular ash is obviously not all present as large particles and a good deal passes both the coarse and fine sieve. In such a case it may be necessary to blend in a low carbon PFA in order to obtain the desired heat release value during firing. Loss on ignition data are therefore important in determining the acceptability of PFA for brick making.

Considered more generally, the carbon content of the classified PFA may be adjusted to a desired value by mixing a high carbon content PFA with a lower carbon content PFA.

The use of a classification process to remove the coarser fractions containing carbon has the effect of raising the proportion of small discrete particles present in the PFA. It has been established that this is particularly beneficial in that it not only allows the brick firing temperature to be lowered but also reduces the length of time required for vitrification of the brick to be completed. Tests indicative of this effect have been carried out on a particular PFA classified to remove particles above 38 microns in size. The effect of this separation on firing temperature is shown in Table 5.

TABLE 5 Effect of PFA Classification on Firing Temperature Size Temperature C Sample No. fraction Hemispherical drop microns Softening formation Fluid 2/792 > 38 1290 1360 1360 < 38 1070 1080 1080 2/793 > 38 1305 1330 1330 < 38 1070 1070 1070 2794 > 38 1205 1320 1320 < 38 1110 1120 1120 The reduction in firing temperature after classification results from the fact that the shales, quartz etc. are removed and mainly a glass is left. Where boron is present to a significant extent (2-3% by weight) the firing temperature may be further reduced.

Another indirect advantage obtained as a result of classifying PFA to reduce particle size is the ability to speed up the vitrifying process by the provision of a greater particle surface area through which heat may be transferred. Table 6 shows the magnitude of the effect on surface area produced by classification.

TABLE 6 Effect of PFA Classification on Surface Area Sample No. Surface Area cm2/gm 18/10 Unclassified 1616 Classified 2040 25/10 Unclassified 1616 Classified 1930 26/10 Unclassified 1441 Classified 1830 In applying the present invention to the making of bricks, the bulk pulverised fuel ash is classified to separate the larger grains of unburnt or partly combusted pulverised fuel together with other large particles of contaminating mineral matter from the smaller particle size fraction of the bulk PFA. This finer fraction is mixed with either a predetermined quantity of fuel or PFA containing a significantly different amount of fuel, together with sufficient binder clay and moisture for making a coherent green brick. The mixed material is shaped into green bricks which are then kiln or clamp fired to produce vitrification.

The PFA particles may be classified, for example to have an upper size limit which may be in the ranges of 35 to 50 microns, conveniently 38 microns. As explained later, a second state of classification may be effected to remove finer particles, e.g. so that the classified particles have a lower size limit in the range of 5 to 15 microns.

The added fuel may partly or wholly be obtained from the coarser fraction removed from the PFA during the classification process. However other forms of fuel, more particularly low grade fuels may be employed for example sewage sludge or colliery spoil or mixtures of such materials. By this technique, it is possible to form PFA Bricks by kiln or clamp firing and to avoid the variation in characteristics which will normally occur when unclassified PFA is used.

The sizing of PFA is conveniently effected using an air classifier. It may be preferable to carry out this process in two stages. In the first stage, coarser particles typically of greater than 30-50 microns in diameter are removed. The finer fraction obtained may then be classified again to remove the very fine particles, for example less than 5 microns, leaving an intermediate fraction which is ideally utilised in the mixture for forming the bricks. The very fine fraction may be suitable for use as a pozzolan. This finest fraction is not necessary for brick making and its removal is primarily to improve the economy of the overall operation.

A suitable equipment for grading PFA into the desired categories of particle size and density range is the British REMA MAC 1 Aerosplit Classifier. In this equipment the PFA is suspended in an air-stream which is passed through rotating blades. Depending on the size and density of a particular PFA particle it is either accelerated out of the forward moving vortex or passes through the rotor. The classification process is therefore based on the combined effect of centrifugal and aerodynamic drag forces on a particular particle.

In operation of the machine dry PFA is pneumatically conveyed into the variable speed screw feed hopper which controls the feed rate of the material into the classifier. From the screw it is passed into an air stream induced by a 20 HP fan located at the outlet of the classifier. The air stream suspends the PFA particles and the air stream then travels through an induction pipe to the rotor chamber where particle sizing and grading of the PFAtakes place underthe combined effects of centrifugal and drag forces.

The finer fraction passes in the air stream through a bag filter which removes the PFA from the Air. The heavier fraction of PFA settles out under gravity into a separate hopper.

The amount of clay required as a binder would typically be of the order of 5% to 20% by weight of the total mix; the preferred amount may readily be determined and depends inter alia on the plasticity of the clay.

Less clay is required if it is very plastic. The mixture may be dry-blended and moisturised by the addition of typically 7 to 10% (by weight of the dry mix) of water. Sufficient water has to be added to allow the body to gain sufficient coherence to enable the green shape to be formed. Preferably the material is semi-dry pressed to form the green brick. It will be appreciated that the water should be kept to the minimum since energy conservation dictates that the green brick should be as dry as possible. For this reason semi-dry pressing is preferably used for forming the brick. Wetter mixes would be required if the bricks were to be formed for example by extrusion.

The open structure of the green material using PFA is particularly advantageous. It is partly because of this open structure that the green bricks can be formed by semi-dry press machines. Conventional bricks are formed by extrusion requiring a much wetter mix.

The amount of fuel required in the bricks to be fired in a particular kiln or clamp can readily be determined empirically; it will depend to a significant extent on the quality of the fuel materials and the firing characteristics of the particular kiln or clamp. Typically using PFA, all the necessary fuel may be obtained if desired from the coarser fraction of the classified PFA except for certain types of PFA having low carbon content.

The invention furthermore includes within its scope a brick formed by the above-described method.

Considered from another aspect, the invention includes within its scope a kiln or clamp fired brick formed of a vitrified mass comprising the finer portion of PFA after removal of the coarser fraction, together with between 5 and 20% of binder clay, for example ball clay.

Either a kiln or a clamp may be used for firing the PFA;clay bricks. In the latter case the carbon content required in the PFA will be appreciably higher since an objective of clamp firing is to maximise the heat provided by combustion of material contained within the brick.

The clamp firing of bricks is a technique which has been practised for a long time in most parts of the world. Although kiln firing of bricks is more commonly employed in this country, clamp firing is still used because of the particular nature and popularity of the brick produced by that technique.

Clamp firing differs from normal kiln firing in that the majority of the fuel required to raise the temperature of the unfired green bricks to a level where vitrification takes place is contained in the individual bricks themselves. Because the fuel is incorporated in the bricks themselves, the firing can be effected by forming the green bricks into a stack (known as a clamp). This stack is encased around its sides and on its top by a layer of fired brick; the fuel within the bricks is ignited and commences to "burn out" giving off heat directly into the mass of the bricks and so raising the temperature to the point where vitrification commences.

One of the problems with clamp firing of bricks in the past has been the wide range of qualities of bricks produced in clamps. Traditionally, the initial heating of the bricks, necessary so that the fuel within the bricks will ignite, has commonly been effected by forming the clamp with a band of coke-breeze or other fuel put down on an open chequer-work of two or three layers of prefired bricks; the clamp of green bricks is then built on top of the coke-breeze layer. Those courses of bricks in close contact with the fuel bed may be overfired, whilst bricks near the outer walls or crown may be under-fired. Furthermore some of the bricks are distorted and have to be rejected because of the collapse ofthe basic coke-breeze bed as it burns out.

More recently these problems have been overcome or substantially ameliorated by the use of gas-firing, conveniently employing a liquified petroleum gas; gas burners are used to throw a long flame deep into the chequer-work of the clamp through passages formed at the base thereof. The gas burners are maintained in position until the bricks flanking the flame are well alight. Thereupon the burners may be removed and the entrance holes for the burners stopped-up to prevent excessive draught thereby leaving the burning zones to develop into a regular fire4ront and so move upward through the mass bf close set green bricks above the basal layers.

The following is a description of one example of the invention applied to the manufacture and clamp firing of PFA/clay bricks.

Pulverised fuel ash from an electricity power station was classified using an air-classifier to remove particles in excess of 38 microns in dimension. The portion of the classified material below 38 microns was then re-classified using an air-classifier to remove particles of less than 5 microns dimension. The finer fraction thus removed was available for sale as a pozzolon. The intermediate fraction was dry-mixed with 15% by weight (of the total mix) of dry ball clay. A predetermined quantity of fuel taken from the coarser fraction of the first classification process was added to the mix. After dry mixing the material was moisturised with between 7 to 100/c by weight of water, this having been found to be sufficient water to give the material coherence, enabling it to be semi-dry pressed to form green bricks.

These green bricks formed by the pressing operation were then put into a clamp for vitrification, the clamp being built in the known way by stacking the bricks in layers on a non-combustible supporting surface, the stack being formed with a plurality of passages extending through it. The stack was encased with pre-fired bricks around its sides and top. The green bricks at the base of the clamp were ignited by gas burners which produced flames extending through the passages in the lower part of the clamp. When the bricks adjacent to each passage had been ignited, the gas burners were removed from the passages and the aperture at the ends of each passage stopped up. The ignition spread through the mass of the green bricks to produce vitrification in the known way. After all the bricks had been burnt and the clamp cooled, the bricks were removed.

The PFA air classification was carried out using a British Rema MAC 1 Aerosplit Classifier. The PFA separation characteristics of this machine are mainly dependent on the rotational speed of the separation rotor and the air volume passing through the plant. Both these parameter can be accurately controlled and PFA size separation efficiences of up to 98% are attainable.

To obtain a separation of the feed PFA into fraction above and below the desired 38 microns a rotor speed of 800 rpm and a main airflow of 1200 SCFM (Standard Cubic Feet/Minute) was employed with the secondary airflow control flap valve set at 10% of the fully open position. The secondary airflow is used largely to minimise carry down of the fine PFA particles in the large fraction falling to the bottom of the classifier and thus improves separation efficiency. A throughput rate of up to 1.2 tonnes/hour is attainable with the MAC 1 Aerosplit. A larger unit (MAC 8) will allowthroughputs of up to 25 tonnes/hour.

The properties of PFA!Clay bricks produced in this manner is compared with a PFAiClay brick made using unclassified material in Table 7. TABLE 7 Properties of PFA Clay Bricks Shrinkage % Crushing Crushing 24 hours Green to Strength Strength Moisture Fired in Green in Fired Absorption Brick State State -% of dry -psi weight -psi Bricks made from Unclassified PFA and Air Classified Calcium Bentonite Clay Trial 1 1.71 188 2096 17 Trial 2 2.23 124 1397 21.7 Trial 3 1.42 276 2096 18.9 Bricks made from Air Classified PFA { < 38 microns) and Air Classified Calcium Bentonite Clay Trial4 2.34 587 5334 16.1 Trial 5 2.13 480 6223 14.7 NB The material used in trial 1-5 consisted of 90% PFA (70% air classified PFA of below 38 microns and 20% Furnace Bottom Ash below 600 microns) plus 10% Bentonite Clay moisturised 6% of dry weight. (All percentages are by weight). The bricks were all semi dry-pressed at 7000 psi.

An additional advantage obtained from the use of PFA/clay material is the reduced bulk density which is obtainable. Table 8 summarises bulk density for a common English brick clay and the reduced density obtainable by use of an 85115 PFA/Clay mixture.

TABLE 8 Bulk Density of Bricks Brick Constituents Forming Bulk Density Method gms/cm3 100% Etruria Marl Stiff Plastic 2.24 (Carboniferous Clay) Repressed - Bricks produced inMidlandsand Wire Cut Extrusion 2.17 N. Wales 85% by volume of Semi Dry Pressed 1.45 Hams Hall PFA 15% Etruria Marl Wire Cut Extrusion 1.60

Claims (21)

1. A method of making bricks comprising the steps of classifying pulverised fuel ash (PFA) to separate the large grains of unburnt or partly combusted pulverised fuel together with other large particles of contaminating mineral matter from the finer fused alumino-silicate material, mixing with the finer fraction a predetermined quantity of fuel together with sufficient binder clay and moisture for making a coherent green brick, shaping the mixed material into green bricks and then firing the green brick in a kiln or clamp to produce vitrification.

2. A method as claimed in claim 1 in which a high carbon content PFA is mixed with a lower carbon content PFA.

3. A method as claimed in either claim 1 or claim 2 wherein the added fuel partly or wholly comprises the coarser fraction removed from the PFA during the classification process.

4. A method as claimed in either claim 1 or claim 2 wherein the added fuel partly or wholly comprises sewage sludge.

5. A method as claimed in either claim 1 or claim 2 wherein the added fuel partly or wholly comprises colliery spoil.

6. A method as claimed in any of the preceding claims wherein the brick firing is effected using a gas-ignited clamp.

7. A method as claimed in any of the preceding claims wherein the classifying of the PFA is effected using an air classifier.

8. A method as claimed in any of the preceding claims wherein the PFA particles, after classifying, have a size not greater than 50 microns.

9. A method as claimed in any of the preceding claims wherein the PFA particles, after classifying, have a size not less than 5 microns.

10. A method as claimed in any of the preceding claims wherein the classifying of the PFA is effected in two stages, to leave a majority of PFA particles within a predetermined range having a lower size limit at a size between 5 and 15 microns and an upper size limit of a size between 35 and 50 microns for mixing with the binder clay and fuel.

11. A method as claimed in claim 10 wherein, in the first stage of classifying, particles greater than 25-50 microns are removed.

12. A method as claimed in either claim 10 or claim 11 wherein, in the second stage of classifying, particles less than 5-15 microns are removed.

13. A method as claimed in any of claims 1 to 7 wherein the classifying of the PFA is effected in two stages, to leave particles not outside the range 5 microns to 38 microns for mixing as PFA with the binder clay and fuel.

14. A method as claimed in any of the preceding claims wherein the amount of clay is from 5 to 20% by weight of the total mix.

15. A method as claimed in any of the preceding claims wherein the classified PFA, binder clay and fuel are blended dry and then moisturised by the addition ofwaterto al low the body to gain sufficient coherence enabling it to be semi dry-pressed.

16. A method as claimed in claim 15 wherein the amount of water is 7 to 10% by weight of the dry mix.

17. A method as claimed in any of the preceding claims wherein the green bricks are formed by semi dry-pressing.

18. A method of making bricks substantially as hereinbefore described.

19. A brick made by the method of any of the preceding claims.

20. A fired brick formed of a vitrified mass comprising the portion of PFA remaining after removal of the coarser fraction of greater than 38-50 micron size, together with between 5 and 20% weight of binder clay.

21. A fired brick formed of a vitrified mass comprising the finer portion of PFA, after removal of the coarser fraction of greater than 38 micron size, together with between 10 and 20% by weight of binder clay.