The coal-fired utility companies produce fly ash as a by-product from the combustion of pulverized coal and, in many instances, including petroleum coke. The material making up the fly ash includes primarily devolotized mineral matter which had been trapped or loosely associated with the coal and incombustible components and elements of the coal, and also of the petroleum coke where that fuel is present. In addition to incombustible minerals, the fly ash also contains discrete carbon rich particles which have not been completely combusted, due to the inefficiency of the boiler design or other related conditions.

Fly ash as collected throughout the United States can have carbon contents of as little as about 0.5% up to 20% or more. Fly ashes may be utilized as an acceptable mineral admixture for use in portland cement concrete under the standards set out in ASTM C-6 18 which limits loss on ignition (LOI) to 6%. The LOI value generally equates to the carbon content of the fly ashes.

As a practical matter, carbon content is known to be undesirable in any case because its presence changes the demand for air entraining admixtures that are used to entrain air into concrete. Carbon causes an undesirable reduction in the entrained air, and in many countries, entrained air is the only real protection which concrete has against the freeze-thaw/wet-dry phenomena. Therefore, the smaller the carbon content in any pozzolan, the better the pozzolan from an air entrainment perspective. Finally, an excessive amount of carbon can reduce the appearance quality of concrete as it can

provide an oily or dark residue on finished concrete surfaces, since the carbon component tends to float to the surface during finishing. Also, a high carbon content causes fly ash to exhibit a reduction in pozzolanic activity.

The problem of carbon loading in the fly ash product has been exacerbated by the use, by certain public utilities, of lower grade coals which do not tend to burn completely, by the use of petroleum coke as part of the fuel mix and/or the need to change burners and burning practices to control nitrous oxide emissions. In such instance, the burning efficiency can be degraded with the result that the resulting fly ash has a substantially greater LOI component, in the form of unburned carbon and particles entrapped within the carbon matrix. Such burning conditions can degrade the fly ash component so as to render it useless as a pozzolan in Portland cement concrete mixes.

A number of prior attempts have been made to salvage a marketable carbon product from fly ash by wet processing, as described in the patents of Ashworth et al. U.S. 4,652,433, Aunsholt U.S. 4,426,282 and Hurst et al. U.S. 4,121,945. While carbon purities greater than 70% have been alleged (for example 80 to 95% in 4,652,433) it is believed that these findings are in error and have actually resulted in a lower than expected carbon purity for reasons not previously fully understood, for reasons which are developed below.

Further, following flotation separation of a carbon fraction from the fly ash, attempts to dry the wet fly ash using conventional dryer techniques such as rotary dryers, fluidized beds, and the like, have resulted in the tendency toward the formation of lumps or aggregations of clumps of the dried fly ash particles. This resulted in a certain coarseness to the product which can cause the fly ash to fail the ASTM fineness test i.e., more than 34% retained on a wet 325 mesh screen, and can cause handling and flow problems in pneumatic conveying systems.

There is accordingly a need for an improved beneficiation system and method for processing carbon laden fly ash for removing a truly refined and enhanced carbon fraction and for creating an enhanced and improved pozzolan of a fineness greater than that of the fly ash source, free of clumps and aggregations of fly ash particles, with improved pozzolanic qualities.

SUMMARY OF THE INVENTION This invention relates to a wet process for beneficiating fly ash and providing a relatively high purity carbon. The carbon content is predominately devolatized and exhibits a porous surface area with pore sizes up to 20 microns. As a result, this carbon has a very high total surface area.

In addition to carbon, one can find high silica content quartz crystals (sand that has been partially fused) and hollow glassy cenospheres in the fly ash as well as tiny slag particles (or scoria). These types of particles interfere with the pozzolan reaction by not contributing to the cementitious reaction at all and because scoria is porous and has a high surface area similar to carbon, it will tend to negatively impact any chemical reaction by action like a "sponge" to retain any surface modifiers one might add to a slurred aqueous mixture.

It has been found that carbon particles, as liberated by flotation in a wet process, contains very tiny fly ash spheres encapsulated or bound within the carbon vesicles and the matrix of particles. These spheres, which are usually less than 1 micron in size, tend to occupy surface pores in the devolotized porous carbon particle. The presence of the spheres are important for at least two reasons. First, the carbon purity has been found to be dramatically impacted by the presence of these spheres. Second, such extremely small spheres represent some of the most reactive pozzolan particles when exposed to free alkalies and moisture.

An important aspect of the method of this invention includes the recognition of the cause of degradation of carbon purity and the value of such entrapped particles, and provides processing steps which free the particles from the carbon flotation fraction by attrition scrubbing and by reflotation of such fraction into a carbon fraction of reduced size and enhanced purity, and an underflow fraction of extremely fine sub-micron spheres which may be combined with the pozzolan fraction to increase pozzolanic activity. A further benefit to the process is the fact that the carbon is broken down into even a finer particle size with increased surface area, so that the carbon product can be more competitive as a filler for products such as rubber and plastic, and as a purifying agent for aqueous filters and the like.

The combined fly ash fraction is treated and dried in such a way that the end product is essentially free of combined agglomerations and clumps greater than 325 mesh. The handling and drying process of this invention not only prevents the coarseness which has been the result of conventional drying but maintains the pozzolanic activity of the product by assuring the freeness of even the smallest particles. This is accomplished by feeding the dewatered and combined pozzolan fractions through an enclosed dryer system including a screw-type feed with a heated blade or blades. This drying in such a heated blade-type conveyor controls the escape of volatiles and provides a means by which any remaining volatile organic compounds (VOC) may be properly collected and handled. It also enhances the freeness of the particles one from the other by constantly causing these particles to move at differential speeds along the surfaces of the flighting and the enclosing barrel in a manner which resembles the circular flow of a fluidized material between the flights of a screw-type conveyor. Such continued agitation accompanied by drying, along a linear length, has been found to be effective in maintaining the fluidity of the product during drying.

Even after such drying in a heated flighting screw type dryer, smaller lumps or agglomerations while bone dry nevertheless may have chemical or mechanical bonds lightly holding them together. Therefore, the enhanced dried pozzolan is subjected to a final disaggregation in a high speed type mixer which has the energy necessary to separate the agglomerations into individuals particles so that the final product is essentially free of clumps which would prevent it from passing through a 325 mesh screen.

The pozzolan fraction may also be enhanced by adding an alumina clay and/or lime in the mixer to increase early and overall strength of a cementitious product, as set out in co-owned U.S. patent of Styron et al.

5,484,480. Such additive, in aggregate, may comprise between about 3 to 5% by weight of the enhanced pozzolan.

The invention may be considered as a method of making an enhanced pozzolan from fly ash containing carbon, in which the carbon is in the form of carbon particles which contain non-carbon fly ash microspheres, including the steps of screening the fly ash to obtain a screened ash which is free of larger particles, such as in the size of about +50 mesh or more. This step has the advantage of removing scoria, larger ferrite particles, and particles which have fused together to form hard clumps. These types of particles interfere with the pozzolan reaction by not contributing to the cementitious reaction and because scoria is porous and has a high surface area and negatively impacts any chemical reaction by retaining surface modifiers which may be added. Further, pre-screening substantially reduces the demand for conditioning chemicals which are required in the subsequent flotation steps.

Water is added to the screened fly ash to form a fly ash slurry and flotation and conditioning chemicals may be added at that time. The conditioned fly ash is then applied to flotation tanks from which a carbon rich fraction and a non-carbon pozzolan fraction are separated.

The carbon rich fraction, described above, contains impurities primarily in the form of tiny microspheres in the porous surface which impurities have significant value for the final pozzolan product. These microspheres are removed, in substantial part, by delivering the carbon rich fraction to an attrition mill or other commutation equipment in which the carbon flotation fraction is subjected to conditions in which a substantial portion of the microspheres are released from the remaining carbon particles.

At the same time, the carbon particles are broken down into even smaller particles increasing the surface area of the carbon fraction. At this time, further flotation reagents are re-added thereby forming a carbon rich slurry which contains freed microspheres.

A particularly effective and preferred method of breaking down the carbon rich fraction includes applying the fraction to an attrition mill in which a counter flow caused by opposing impellers, combined with steel or ceramic scrubbing balls in the liquid matrix, the balls operate to increase the impingement contact with the particles. Such attrition scrubbing also reduces the size of the carbon component, thereby facilitation floatation separation from the matrix.

This carbon rich slurry is then floated in a second bank of flotation cells and a second carbon rich fraction and a second pozzolanic non- carbon fraction is formed, consisting primarily of tiny microspheres. The second non-carbon fraction may then be combined with the first pozzolanic fraction to form an enhanced pozzolan which may then be dewatered and dried. The second carbon fraction forms an improved carbon product.

The preferred drying procedure comprises applying the combined wet fraction to a heated screw-type linear dryer for driving off water and volatiles, thereby having less than 0.3% moisture non-carbon product.

This product, in spite of the agitation received in the screw-type dryer, may yet

contain small agglomerations of weakly bonded together fly ash particles which may exceed the opening of a 325 mesh screen. Therefore, the dried product from the linear screw-type conveyor is preferably applied to a high speed mixer which frees up any bonds between such particles and breaks up agglomerations to provide a final pozzolan which consists of a non-carbon fraction of fly ash combined with the microspheres extracted from the carbon rich fraction. At this time, the pozzolan may be further enhanced by the addition of lime and/or an alkaline clay, as explained above.

It is accordingly an important object of this invention to provide an improved process for removing a useful carbon rich fraction from fly ash and to provide a wet process for utilizing fly ash which otherwise could not be utilized because of an excessive carbon content expressed as LOI, in excess of 6%.

A further object of the invention is to provide a super enhanced pozzolan by separating microspheres as small as 1 micron or less from the matrix of a carbon rich flotation fraction and recombining such microspheres into the non-carbon fraction and similarly providing an enhanced carbon fraction which has been reduced in size and from which has been extracted a non-carbon impurity.

A still further object of the invention is to provide a wet-type enhancement process for carbon laden fly ash which is economical to practice and which provides a valuable useful end products.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING The figures of the drawing Figs. 1A and 1B represent a diagram of the steps of the method and process in accordance with this invention. In the interest of clarity, the process water recirculation and water make-up connections are omitted.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to the figure 1A of the drawing, a source of dry fly ash having a carbon content is illustrated at 10. This source may have been electrostatically collected from a public utilities electric power generation station, for example, and may contain a significant component of carbon particles, usually in excess of 6%, and for some fly ashes may exceed 20%. A first desirable step in the beneficiation of such fly ash is to remove that portion of the source 10 which exceeds about 50 mesh by screening on a dry or a wet screen 11. This screening step has the effect of removing larger aggregates, clumps, and scoria commonly contained in raw fly ash and which would provide an undesirable load on the system and on the reagents which are required in the flotation processes.

The underflow from the screen 11 is directed into a conditioner or blender in which process water is added to provide a relatively high consistency slurry of fly ash and water in the order of about 65% solids. The conditioner 12 provides a thorough mixing and blending of the screened ash, and flotation reagents are added at this stage. The conditioning in the conditioner 12 need only extend until there has been a thorough mixing and blending of the solids with the liquid and reagents suitable for application to a first bank of flotation cells.

The combined conditioned ash and reagents are then withdrawn from the conditioner 12 by a slurry pump 14 and applied to the inlet of a bank

15 of flotation cells 15a-15d. While four cells are shown, more or fewer may be used depending upon the capacity required. Also, at this time, dilution process water is added to reduce the relatively high consistency to a consistency ideal for proper flotation. The solids content is influenced by particle size, the percentage of carbon content, the retention time, and the pH.

Typically, the solids content will be in a range of about 5 to 20% solids. The process water used at the bank 15 of flotation cells and at the conditioner 12 may be recycled from a system clarifier common to the various thickener and dryer positions within the system, with suitable make-up water added as required at the clarifier.

The mixture of conditioned and screened fly ash, flotation reagents, and dilution water is agitated and aerated, according to conventional flotation practice in the cell bank 15 to form a first carbon rich flotation fraction at the top and a non-carbon fraction at the bottom. The non-carbon fraction, following flotation, is removed on a line 16 for further thickening and processing.

The first carbon rich floatation fraction from the cells 15 in line 17 consists primarily of carbon and other elements which have attached to the froth. This fraction is applied to an attrition mill 20.

The carbon rich fraction is, in affect, a carbon which is contaminated with a non-carbon component. As previously mentioned, the carbon particles tend to be porous and vesicular, and have entrapped within them and/or bonded to them non-carbon particles primarily in the form of very small microspheres, in the order of one micron or less in size. This microsphere component may comprise a substantial portion of the mass of the fraction such as in the range of approximately 20% to 40%. Although a particle has been separated because of its hydrophobic nature and its attachment to an aeration bubble, it is insufficiently pure to be used

effectively, and at the same time, it contains material which, when separated, has a high pozzolanic activity, and therefor has value independent of the carbon content.

A particularly useful attrition mill 20 is a two-stage attrition scrubber of the kind commonly used in clay processing operations, such as the variable pitch axial flow two-stage Attrition Scrubber made by Denver Equipment Company, Div. of the Denver Solo group of companies, Colorado springs, Colorado 80901. The preferred attrition mill employs helix variable- pitch reversed pitched impellers 20A,20B mounted in pairs on common parallel shafts to provide counterflow between the opposing impellers. The attrition process is augmented by the inclusion of ceramic or steel balls 20C in the matrix, steel balls being preferred, to enhance particle breakdown by impingement of particles between the balls. It has been found that the balls significantly increase particle size reduction and surface area. At the same time, the microspheres are released from entrapment. The percentage of such recovered non-carbon solids in the first carbon floatation fraction typically may run between about 25 to 35% of the total weight of the solids.

The preferred attrition mill with ceramic or metals balls as a grinding media has been found to operate over a wide range of matrix consistencies or percentages of solid material. In view of this versatility, the first float may be taken directly from the cells 15 and applied to the mill 20 without pre-thickening although pre-thickening could be inserted, if desired.

Further, floatation reagents may be added in the mill 20 so that the conditioner will come into intimate contact with the carbon particles.

As stated above, the addition of grinding balls in the mill 20 has a significant effect on the increase in the specific surface area of the carbon component. A preferred loading of grinding balls consists of the use of between about 25 to 40 percent displacement, by volume, by balls in the diameter of about between one-quarter and three-eighths of an inch into matrix, which balls are

retained within the mill 20 by a retention screen at the outlet. In a specific example, about 310 pounds of steel balls are used in an approximately two cubic foot diameter attrition mill of the kind described above and shown at reference numeral 20 and the through-put rate at about 16 percent solids, may be in the order of about nine gallons per minute.

The carbon-rich fraction following scrubbing in the mill 20 is diluted as necessary and applied to a second bank 22 of flotation cells, illustrated as comprising cells 22a-22d. Obviously, these cells may be of considerably lower capacity than the cells of bank 15 since the quantity of product handled may be anywhere between 10% to 30% of the original input to bank 15. The milled carbon rich fraction may be diluted with process water and further reagents may be added at the cells 22. The dilution is to a solids consistency which is most efficient for further separation, in the order of about 5 to 10%. In the second bank 22 of flotation cells, a second carbon rich fraction is formed since the froth has a purity substantially greater than that of the first fraction, while a second non-carbon fraction is formed consisting primarily of fly ash microspheres released from the first carbon fraction, as an underflow from the cells. The increased fineness of the carbon component facilitates its flotation. The non-carbon fraction from the cells is removed on a line 24 for further processing.

In a test, under the conditions described above, the feed float on line 17 to the mill in a particular instance was measured as having a specific surface area (CM2CM3) of 9,888. Conventional milling in the mill 20, without balls, provided a specific surface area of 10,140, as measured on the carbon overflow from the bank 22 of the floatation cells. When the same first carbon overflow on line 17 was subjected to milling using steel balls 22C of a size and loading as previously described, the carbon overflow from the bank 22 produced a specific surface area of 11,942, indicating an increase in surface area of about 18 percent over milling without using steel balls.

The second carbon rich fraction is, therefore, of relatively high purity, and is applied to a thickener 25 where it is thickened to a consistency of approximately 50%, and applied by pump 26 to a final drum-type filter 27 and further thickened. Then it is dried in an enclosed linear heated flighting screw- type dryer 30 for storage in a container 32. The heated flighting screw-type dryer 30 may be a thermal processor sold under the HOLO-FLIGHT trademark by Denver Equipment Company, Colorado Springs, Colorado 80901. Filtrate from the thickener 25 and filter 27 is returned to the clarifier.

The underflow fractions from lines 16 and 24 are combined and dried, see Fig. 1B. The product may be applied to a thickener 40 where a substantial portion of the fluid content is removed to provide a thickened pozzolan, and to collect the cenospheres. The cenospheres rise to the surface, in the thickener vat, in which the fly ash component and microspheres are allowed to settle to the bottom. If the cenospheres are collected at the thickener effluent, they may be collected as a float in the clarifier.

The underflow from the thickener 40 may be applied by a slurry pump 41 to a drum-type filter 44 in which further liquid content is removed and the contents are then preferably applied to a delumper 45. The delumper 45 is a moderate speed mixer which reduces the larger aggregations of the material when it is in a partially dried state from the drum-type filter such as wet sand. This product is then delivered to a linear heated screw-type processor 50 which may also be a HOLO-FLIGHT thermal processor with heated screw vanes and heated walls. The screw vanes re hollow and the vanes and walls may be oil heated, such as by oil at 6000 F. The pozzolan product will leave the processor 50 at about 390" F and at a moisture content of less than 0.3%. This enclosed processor releases a water fraction as well as any volatile organic compounds (VOCs) and controls their release for proper

disposal. The dried product is delivered at the elevated temperature from the thermal processor 50 to a high speed pin-type mixer 52.

The output from the linear processor or dryer 50 is a fine product but may contain aggregations of weakly bonded ash particles which are not capable of passing through a 325 mesh screen. The high speed pin-type mixer 52 breaks down these weak bonds and frees the particles one from the other.

This is particularly advantageous since the pozzolan is enhanced by the microspheres from the second flotation step, and these microspheres must be free of bonds to provide the maximum pozzolanic activity. At this stage, supplemental materials may be added such as an alkaline clay as described in the previously referenced U.S. patent no. 5,484,480 and/or lime to produce the final product and stored in a receptacle or container 55.

The reagents which are added to effectuate the frothing and collecting functions preferably consist of a combined frother and collector, although these elements may be separately added. The frother/collector reagent may consist of the non-aromatic reagent S-8259 of Cytec Industries Inc., Five Garret Mountain Plaza, West Paterson, New Jersey 07424, comprising a emulsion of a hydrotreated light petroleum distillate, CAS registration number 064742-478 and a sulfonic acid, petroleum, sodium salt, CAS registration number 68608-26-4, which will further include a suitable frothing agent such as "Aerofroth" 88, CAS registration number 104-76-7 or frother S-7904, provided by Cytec Industries Inc., identified above.

Other examples of collectors may be used are described in Australian Patent 572344 granted September 29, 1988 and include a suitable oil such as crystal free neutral oil or fuel or, an acid or an acid ester, as described in the examples, and a sulfonated hydrocarbon compound, as described in the examples. However, this non-aeromatic reagent S-8259 is preferred for VOC control.

The reagent dosage may be about 2 to 3 lbs. per ton in the primary floatation which includes about 2 lbs. of frother such as S-7904 or aerofroth 88 and about A to 1 lb. of collector. In the second stage at the bank of cells 22, the amount of reagents may be decreased to about 1 to 1 A lbs. per ton including about A lb. per ton of frother and about A to 1 Ib. per ton of collector.

Tests were run to confirm the increase in pozzolanic activity that is be found using as a pozzolan the microsphere component separated by the second float, on line 24, and comparing its performance against that of the raw fly ash (5% carbon) from which it was derived following course screening by the screen 11. Cubes were made and 28 day strengths were obtained and compared to control strengths for cubes made of 100% portland cement and cubes made of 80% cement, 20% fly ash.

The cubes and tests were according to ASTM C-109. The 28 day strength (in psi) of the (I) control, the (II) raw fly ash mix, and the (III) mix using 80% cement and 20% pozzolan from line 24 were respectively as follows: 5466; 4767, and 6513. This test showed that the mix in which 20% of the portland cement was substituted with the fine microsphere component from line 24 resulted in a 28 day strength was 119% of the control.

On the second test, cubes were made using an ash which originally contained 11% carbon and the above test was duplicated with cubes in which 20% of the portland cement was replaced by screened but untreated fly ash and 20% was replaced by the underflow fraction from the second float, on line 24. The respective 28 day strengths (in psi) were as follows: 6161; 5150; and 5625. The cubes made with the raw fly ash exhibit about 84% of the strength of the control whereas cubes made from the pozzolan from line 24 achieved 91% of the strength of the control.

These test results confirm the highly reactive nature of the microsphere component which is separated in the floatation in the bank of cells 22 including the attrition ball milling of the first carbon fraction prior to such floatation with the release of the microspheres from the carbon matrix.

The invention, therefore, provides a valuable process by which troublesome fly ash can be beneficiated to yield an enhanced pozzolan and a improved-purity carbon under economical conditions. The step of attrition scrubbing the first carbon float and releasing from that float a pozzolan- enhancing fraction which is heretofore been discarded, permits the manufacture of a vastly improved fly ash component which can be further enhanced by additions of clays and/or limes, as described. The drying process has been improved to control volatiles and to provide an end product which is essentially free of +325 mesh aggregations. A devolatized carbon product, reduced in size, and reduced in non-carbon content, is also produced.

While the methods herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise methods, and that changes may be made without departing from the scope of the invention, which is defined in the appended claims.