FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title:  HOLISTIC APPROACH TO FLY ASH:  BY-PRODUCTS RECOVERY

ICCI Project Number:  96-1/3.1A-24

Principal Investigator:  R. Q. Honaker, Southern Illinois University

Other Investigator:  J. A. L. Campbell, Custom Coals International; B. J. Arnold and G. A. Shirey, CQ Inc.

Project Manager:  D. D. Banerjee, ICCI

ABSTRACT

Less than 10 percent of the three million tons of combustion by-products produced in Illinois each year are used to commercial products.  Realizing that the utilization of these by-products is a high priority for Illinois, a holistic approach to fly ash beneficiation was developed in this study.  The separation and recovery of four valuable by-products:  fly ash-derived magnetite, a pozzolanic portion for cement, cenospheres, and unburned carbon, were investigated using a combination of dry magnetic separation, fluid bed gravity separation, and CarefreeTM cyclone technologies.  An integrated balance flowsheet for the combined processes was developed and an economic analysis of the integrated beneficiation approach was performed.

The results obtained in this study found that the primary step in fly ash beneficiation should be a particle size separation.  The majority of the LOI material in a low NOx fly ash was found to be concentrated in the +325 mesh size fractions.  Also, the cenospheres in low NOx and low LOI PC boiler fly ash were mostly found in the +200 mesh size fraction.  Thus, for the reduction of LOI and cenosphere recovery, a 325 mesh particle size separation is recommended.  The dry fluidized bed gravity separator was found to reduce the LOI content of the +325 mesh fraction in the low NOx fly ash from 11.9% to 6.4%.  Thus, by combining the fluidized bed product with the original -325 mesh material, and overall reduction from 6.9% to 3.7% was achieved while recovering nearly 80% of the fly ash for potential cement applications.  Further treatment of the coarse, high LOI fly ash using the water-based Carefree cyclone was unsuccessful at separating the carbon and cenospheres, although a 57% reduction in LOI content was achieved based on a particle size separation.  To recover the magnetic material, a rougher-cleaner arrangement of dry and wet magnetic drums was successfully used.  The final product from the treatment of the low NOx and PC boiler fly ash samples contained 99% magnetics with nearly 10% of the total feed fly ash material.  An economic analysis of a 25 tph fly ash processing plant designed based on the characterization and unit performance data revealed that the economic viability is driven by the production of the pozzolanic material.  A 50% annual return on investment was projected for the proposed processing plants designed for the low NOx and low LOI PCC fly ash sources.  Thus, the economic recovery of cenospheres and enriched carbon must be based on incremental costs and revenue.

EXECUTIVE SUMMARY

Due to the increased costs associated with the handling and disposal, the development of alternative uses for coal combustion residues has been the subject of many research investigations.  Several methods have been successfully evaluated and commercially applied, of which, the majority are wet-based separators.  Due to handability concerns and high dewatering costs, dry-based separations are preferred for treating the fly ash.  This project attempted to maximize the amount of material being processed by dry-based techniques while using novel wet-processing techniques to achieve the final by-products recovery.  The goals of this project were (1) to develop an integrated flowsheet for separating and recovering the various fly ash by-products using a combination of dry and wet processing and (2) to determine the economics of such a fly ash separation plant.

Southern Illinois University teamed with Custom Coals International, CQ inc., and their subcontractors Eriez Magnetics and Lehigh University to develop an holistic approach to fly ash byproduct recovery.  The specific objectives of this project were:

To determine the revenue achievable for each by product stream and the costs associated with the separations. The fly ash characterization study conducted on fly ash samples from three different sources resulted in important findings related to the ability to achieve effective by-product separations.  One such finding is that the particle size and density distributions of the LOI material in fly ash is highly variable and dependent on boiler type.  As shown in Figure 1, the low NOx boiler obtained from the Gibson Power Station generates fly ash in which most of the LOI material has a particle size greater than 100 mesh (150 um).  In fact, by dry screening the Gibson fly ash at 100 mesh, a sufficiently low LOI material for cement-concrete applications is produced from the screen underflow stream at a mass yield of nearly 80% to the underflow.  On the other hand, the LOI material in the fly ash from the high temperature PC boiler at the Baldwin Power station has a very fine distribution with the majority of the particles having a size less than 325 mesh.  Since the LOI content of both the Baldwin and ADM FBC fly ash are sufficiently low according to ASTM standards, further treatment of reduce LOI content is not needed.

Figures 1 and 2 are available by hard copy.

Following the LOI reduction of the Gibson fly ash achieved through dry screening, the screen overflow can be subsequently treated to concentrate both the cenospheres and carbon-based material, which has potential uses in the activated carbon market.  For both the Gibson and Baldwin fly ash samples, the low density cenospheres were found to be concentrated in the +200 mesh particle size fractions.  Approximately 2.5% of the total mass comprising the Gibson fly ash sample and 1.0% in the Baldwin sample were +200 mesh cenospheres having a density less than 1.3 gm/ml.  In terms of carbon concentration, a dry gravity-based concentration of the +100 mesh Gibson fly ash could result in a substantial increase in the combustible content to about 50% as shown in Figure 2.  Since the carbon-rich product from the gravity-based process would contain the valuable cenospheres, further wet-based processing could be justified to recover cenospheres and enrich the carbon content.  As shown, the process stream from the gravity separator containing the heavier particles could be used as re-burn material in combustion systems.

Utilizing the characterization information described in the above paragraph, the +325 mesh particle size fractions of the Gibson and Baldwin fly ash samples were treated in a dry fluidized-bed gravity separator.  The air-based fluidized-bed density separator developed by Lehigh University was found to provide a significant reduction in LOI content, especially for the +100 mesh size fraction.  An effective separation required the removal of the -325 mesh size fraction prior to treatment in the fluidized-bed.  The coarse LOI material, i.e., +100 mesh, concentrated near the top of the fluidized-bed.  Thus, by removal of the upper layers of the bed, the LOI content of the +325 mesh size fractions in the Gibson fly ash was reduced from 11.94% to 6.44%.  After combining the +325 fluidized-bed product with the original -325 mesh size fraction (LOI content of 2.17%), the effective overall reduction in LOI content was from 6.87% to 3.70% while recovering about 80.7% of the total fly ash.  Thus, by using a combination of pre-screening/classification and dry fluidized bed separation, an effective LOI reduction can be achieved that will yield a majority of the fly ash for use in cement applications.

The recovery of cenospheres from the low NOx and PC boiler fly ash using the dry fluidized-bed density separator was found to be difficult since the cenospheres concentrated in the middle of the suspended particle bed.  This finding is possibly due to the spherical shape, which substantially reduces the drag force.  Thus, since the particle density is less than air, the cenospheres would be more difficult to suspend in comparison to granular particles of equal particle size.  In addition, the average particle size of the cenospheres in the +200 mesh size fractions may be larger than the other material types which will also cause the cenospheres to settle further into the fluidized particle bed.  Thus, cenospheres may be more easily recovered in water-based systems since the density of the medium would provide a buoyancy effect for the cenospheres.

The Carefree cyclone was evaluated for the purpose of separating the carbon-based material from the cenospheres contained within the overflow stream of the dry fluidized bed separator.  However, overall LOI reductions fort he low NOx Gibson fly ash were also investigated.  The Carefree cyclone was found to provide LOI reductions up to about 55%, while yielding approximately 60% to the low LOI process stream.  This reduction was achieved mainly by providing a particle size separation, whereby, the high LOI +100 mesh size fraction was rejected to the underflow stream.  The main goal of separating the cenospheres from the LOI material was not successful.

To evaluate the potential of recovering magnetite, a magnetic separation evaluation was performed using a series of dry and wet magnetic separation devices.  From the characterization study, the majority of the magnetic material in each fly ash sample was found to have a particle size less than 325 mesh and a density greater than 2.4 gm/ml.  Most of the magnetic particles were iron-metal oxide complexes that appear as solid spheres.  A minor amount of magnetic cenospheres is present.  Using dry magnetic separators in a rougher-cleaner arrangement, the iron content of the Gibson fly ash sample was upgraded from 8.37% to 35.06% with a mass yield of 9.1%.  The corresponding improvement in the amount of magnetic material was from 11.5% to 57.7%.  Likewise, the iron content of the Baldwin fly ash was increased from 9.96% to 29.64% while yielding 19.1% of the total mass to the product.  The improvement in yield with the Baldwin sample can be explained to some extent by its corresponding lower LOI value.  Further treatment of the dry magnetic separation product by wet magnetic separators further increased the iron content of the Baldwin fly ash to 45.42% with 69% mass yield, which equates to an overall mass yield (dry and wet separations) of 13.2%.  The total magnetic content, which was determined from a Davis tube test, was 99.0% for the Baldwin fly ash.  A similar treatment of the Gibson fly ash sample resulted in a final product containing 48.36% iron and 98.65% magnetic material while yielding 56.0% of the dry magnetic product to the final wet drum concentrate.  The overall process yield (dry and wet separations) achieved on the Gibson fly ash sample was 5.1%.

Flowsheets for 25 ton/hr processing plants to treat the low NOx Gibson fly ash and the low LOI Baldwin fly ash were developed based on the characterization data and the separation performances achieved from the individual units evaluated in this study.  An economic analysis revealed that the economic feasibility of the processing plants is driven by the production of pozzolanic material.  In other words, the major economic benefits of fly ash beneficiation can be realized by the production of fly ash of sufficient grade for concrete applications.  Thus, the recovery of the cenospheres, carbon-rich material, and magnetite must be economically evaluated based on incremental costs and profits.  The fly ash processing plants were found to provide a 50% annual return on investment.