FINAL TECHNICAL REPORT

September1, 1995, through August 31, 1996

Project Title: VALUABLE PRODUCTS FROM UTILITY FLY ASH

ICCI Project Number: 95-1/3.1A-15

Principal Investigator: Joseph A. DeBarr, ISGS

Other Investigators: David M. Rapp, Massoud Rostam-Abadi, John M. Lytle, ISGS, Mark J. Rood, UIUC

Project Manager: Dan Banerjee, ICCI

ABSTRACT

The objective of this project is to investigate the potential of recovering valuable products from utility fly ash. The quantity and quality of recoverable adsorbent carbon, magnetite and cenospheres were determined for two fly ashes from an Illinois utility. Adsorbent carbons sell for $600 to $2,000 per ton, magnetite for approximately $80 per ton and cenospheres (hollow spheres composed mostly of silica, alumina and iron oxides) can sell fro $700 or more per ton. Removing carbon, magnetite and cenospheres will also improve the quality of the remaining fly ash for applications such as blending with cement. Recovery of valuable products from fly ash may help offset in-mine disposal costs, and represents a component of an overall scheme of fly ash processing where fly ash is regarded as a potential resource rather than a waste.

Two class F fly ashes collected from Illinois Power's Baldwin and Wood River power plants were used in this study. Beneficiation of the Wood River fly ash using a multi-step procedure yielded 0.5 wt% cenospheres, 4.5% magnetic components, and 4.0% carbon. Processing of the Baldwin fly ash gave 0.4% cenospheres and q15% magnetic components. Carbon was not recovered from the Baldwin fly ash due to the low LOI.

If cenospheres of higher specific gravities (up to 2.0 g/cm³) are included, the ashes contain up to 22% cenospheres. Concentrates having iron contents of 51% and 74% were prepared from teh Baldwin and Wood River ashes. Concentrates with up to 83% carbon, at recovery rates of 82%, were obtained from the Wood River fly ash. The fly-ash carbons were activated to attain surface areas as high as 350 m²/g. Pellets formed from the carbon appeared quite strong and maintained structural integrity during handling, but were noticeably more friable after activation. The activated pellets had a surface area of about 170 m²/g, but results suggest higher surface area may be attained with further activation, or by altering activation conditions. The fly-ash carbons exhibited SO₂ uptake that suggests they would be suitable for commercial processes to clean flue gases.

A no-cost extension has been granted for this project to allow further characterization of the recovered materials and to address potential new uses for these materials. This will allow reasonable comparison of product quality with commercial alternatives, and give a better understanding of the overall value of the recovered products.

EXECUTIVE SUMMARY

Utilization of wastes from coal combustion is becoming an issue of increasing concern to Illinois coal companies and to the utilities that purchase Illinois coal. The willingness of an Illinois coal company to dispose of the waste generated when its coal is burned is an advantage in a very competitive marketplace. Recovery of valuable products from fly ash may help offset in-mine disposal costs, and represents a component of an overall scheme of fly ash processing where fly ash is regarded as a potential resource rather than a waste. Such a progression has already occurred in the wood products industry where former waste products now are used to produce valuable building materials. If such a progression occurs for fly ash, Illinois coal companies, utilities and the State of Illinois will all benefit.

The objective of this project is to investigate the potential of recovering valuable products from utility fly ash. The quantity of recoverable adsorbent carbon, magnetite and cenospheres were determined for Illinois fly ashes. Adsorbent carbons can be used for, among other things, removing air toxics from waste incinerator and utility flue gas and sell for $600 to $2,000 per ton. Magnetite sells fro approximately $80 per ton and can be used in heavy media coal cleaning equipment to provide a suspension of artificial gravity (i.e., heavy media). Cenospheres are hollow spheres composed mostly of silica, alumina and iron oxides and can sell for $700 or more per ton. Cenospheres are used in applications that take advantage of their high strength, low weight, sphericity, chemical inertness and thermal stability. Such applications include use as mineral fillers or as lightweight refractory materials.

Bulk Fly Ash

Two fly ashes collected from Illinois Power's Baldwin and Wood River power plants were used in this study. The Baldwin and Wood River fly ash samples had similar particle size distributions, but had median particle diameters of about 12 um and 20 um. The compositions of the ashes were typical of class F fly ashes, with the major components of both fly ashes being silica and alumina. The Baldwin fly ash contained about twice the iron and calcium present in the Wood River fly ash. The other major difference between the ashes that was pertinent to this project was the loss on ignition (LOI) which is an indicator of the unburned carbon in the fly ash. The Wood River and Baldwin fly ashes had LOIs of about 5% and 1.2%.

Size classification of the fly ashes was done to determine if sieving could be used to concentrate desirable components in a given size fraction. Except the LOI (carbon content), little difference in composition among the different size fractions was observed. The LOI increased significantly with increasing particle diameter for both fly ashes. For the size fractions studied, the carbon concentrations ranged from about 2.8% to 47.3% for the Wood River fly ash, and from about 0.5% to 17.5% for the Baldwin fly ash. Although the carbon was concentrated in the larger particle diameters, these size fractions represented only a small portion of the total mass of the fly ash. Most of the carbon mass (50-70%) was present in the < 38 um fraction.

The ashes were separated into specific gravity fractions of < 1.0, 1.0-1.5, 1.5-2.0 and >2.0 g/cm³. Both fly ashes had similar mass fractions in the two lower-density classes (< 1.0 and 1.0-1.5 g/cm³), with about 0.5% and 3.5% recovered. The Baldwin fly ash contained almost twice the material as the Wood River fly ash in the class 1.5-2.0 g/cm³. Recent evidence suggests that cenospheres with densities aS high as 2.0 g/cm³ may be found in fly ash. The total weight percents recovered up to 2.0 g/cm³ were about 15% and 22% for the Wood River and Baldwin fly ashes. However, the LOI also increased for the classes 1.0-1.5 and 1.5-2.0 g/cm³. The highest-density class of the fly ashes comprised most of the mass and had a composition similar to the as-received ash.

Ash Beneficiation

A multi-step procedure was used to separate components of over 12 kg of Wood River fly ash and smaller quantities of the Baldwin fly ash. Separation of the Wood River fly ash gave 0.5 wt% cenospheres (< 1.0 g/cm³), 4.5% magnetic components, and 4.0% carbon. Separation of the Baldwin fly ash gave 0.4% cenospheres and 15% magnetic components. Carbon was not recovered from the Baldwin fly ash due to the low LOI.

Cenospheres

Cenospheres recovered at 1.0 g/cm³ from the Wood River fly ash were separated into distinct size fractions by dry sieving. The size distribution of the Wood River cenospheres followed a typical log-normal distribution, skewed slightly to smaller particle diameters. Except for the >212 um size fraction, a significant number of cenospheres were in each size range, which suggests that Wood River fly ash could be used as a resource for size classified cenospheres that may be used in many applications.

Magnetite

The Baldwin fly ash contained four times the magnetic material present in the Wood River fly ash. The composition of the magnetic concentrates and the remaining nonmagnetic materials for Baldwin and Wood River fly ash samples was determined by XRF. The iron content (Fe₂O₃) increased from 17.7% to 46.8% for the Baldwin concentrate, and from 6.9% to 55.6% for the Wood River concentrate. Magnetic concentrates still contained significant amounts of silica and alumina and smaller amounts of other components. Further concentration of the magnetic material using a dry separation technique resulted in removal of about 8% and 25% nonmagnetic material from the Baldwin and Wood River magnetic concentrates. The particle size distribution for the magnetic concentrate prepared from the Wood River fly ash followed a typical log-normal distribution, with the particles 2-300 um in diameter. The magnetic concentrate was easily ground, suggesting that any smaller size distribution could be prepared. Additional improvements in separation techniques may be necessary to prepare a commercially acceptable magnetic concentrate.

Carbon

Carbon was recovered from the Wood River fly ash using several procedures: sieving, froth flotation, and a dry fluidized-bed separation (FBS) technique. FBS of a +150 mesh Wood River fly ash gave a concentrate that had about 69% carbon, however, recovery was low. Further refinement of this technique may result in samples with higher carbon content. Froth flotation of -150 mesh and ground +150 mesh Wood River fly ash gave about 55% and 83% carbon in the concentrates. Overall, about 82% of the carbon present in the bulk Wood River fly ash was recovered during processing by flotation.

Scanning electron microscopy revealed the residual carbon material was significantly macroporous in nature, with many 1-10 um macropores open to the external particle surface. The particles also had significant surface ash present. It is suggested that the highly porous, fragmented particle structures, and visible, fused surface ash was due to significant oxidation of the residual carbon particles. The particles obviously have not completely bypassed the combustion zone.

The fly-ash carbons were activated to attain surface areas as high as 350 m²/g. Pellets formed from the -150 mesh carbon recovered by flotation with starch as a binder appeared quite strong and maintained structural integrity during handling. After activation, the pellets had a surface area of about 170 m²/g. The total weight loss during processing was only 20%. This suggests that pellets with higher surface area may be attained with further activation, or by altering activation conditions. The pellets after activation were noticeably more friable than the original pellets. Preliminary results were promising, and it appears that with further study, pellets with desirable characteristics could be prepared from fly ash carbons.

The kinetics of SO₂ adsorption was determined for the low-surface-area carbons to evaluate application of the carbons in processes for removing air toxics from waste incinerator and utility flue gas. STEAG, a German-based multinational corporation, has licensed technology for carbon-based systems installed on commercial medical, hazardous and municipal waste incinerators in the European Community. The carbon used in the STEAG process, Herdofenkoks, is an activated char produced from lignite, with surface area of 300 m²/g. The SO₂ adsorption capacity of a carbon is reported to be a reliable guide to acceptability in the STEAG process. Although the fly ash-derived carbons have N₂-BET surface areas less than or equal to the of the Herdofenkoks, they have SO₂ adsorption capacities greater than that of the Herdofenkoks. These results suggest that the fly ash-derived carbons may have significant potential for application in the STEAG and other similar processes.