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Epri coal ash utilization research
Energy Vol. 11, No. 11/12,pp. 1377-1387,1986 Printed in Great Britain
0360-5442/86 $3.00 + 0.00 Pergamon Journals Ltd
EPRI COAL ASH UTILIZATION
RESEARCH
DEAN M. GOLDEN Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94304, U.S.A. Abstract-This paper describes a five-year Electric Power Research Institute (EPRI) project directed toward increasing ash utilization. EPRI is undertaking this program to promote the bulk sale of coal ash in high-volume applications that do not require a specific quality or class of ash; in medium-technology applications in cement and concrete; and in high-technology uses for valuable products requiring stringent quality control. High-volume applications include the use of ash in highways and railroads, backfills, and pavement base course; medium-technology applications include cement and concrete; and high-technology uses include the extraction of the mineral matter or the metals from ash.
INTRODUCTION
The annual world combustion of coal in electric plants is now about 2500mtce (million metric tonnes coal equivalent), resulting in the production of 250-300 million tonnes of fly ash.? It has been predicted that consumption will increase to about 6500-7000mtce annually in the year 2000, resulting in the collection of some 650-850 million tonnes of fly ash. The current usage of coal by utilities in the United States results in the production of over 65 million tonnes (71 million short tons) of solid combustion by-products each year. A predicted doubling of coal usage in the United States within the next 10yr will only intensify the challenges associated with the proper disposal or utilization of the byproducts. Stringent particulate removal requirements and increased use of desulfurization systems will further increase by-product volumes. The problem of effective utilization of coal ash has been given wide attention for at least 20yr in the United States. Although the effort devoted to the problem has led to some applications of demonstrated usefulness, the full utilization of coal ash has been hindered by lack of effective dissemination of knowledge from field studies to potential users of coal ash, and by the obvious variability of the product. Extensive utilization of coal ash is both technically and economically feasible, yet less than one-quarter of utility coal ash produced in the United States is presently being utilized. Utilization rates in Europe vary considerably by country. In the United States, approximately 80% of the coal ash by-products are in the form of fly ash, which has a lower utilization rate than bottom ash. As shown in Fig. 1, the amount of fly ash produced in the United States by electric utilities has increased significantly over the past 16yr. There has been a relatively steady increase in the percentage of fly ash utilized from 1967 to 1979; the percentage of fly ash utilized since 1979 has, however, been somewhat erratic. As can be seen in Fig. 1, the ash utilization rate for 1983 was only 14.3% of the 52.4 million short tons produced. Figure 2 presents a summary of the various ways in which fly ash was used in the United States in 1983. As indicated, 3.6 million short tons of fly ash-48% of the fly ash utilizedwent into cement and concrete products. The next largest uses are structural fills (1.4 million tons), road base (0.5 million tons), grouting (0.2 million tons), and coal-mining applications (0.2 million tons). These are frequently defined as “high-volume uses”. Thus, in 1983, high-volume uses accounted for 31% of the fly ash utilized. The Electric Power Research Institute (EPRI) initiated an ash utilization research and development program in 1979 to support the increased use of fly ash in the United States. This program has been divided into three principal areas: high-volume uses, mediumtechnology uses, and high-technology uses. High-volume uses include those fly ash products that are either large in quantity or require huge percentages (over 50%) of fly ash. In ?A metric tonne = 22041b. A short ton = 20001b. 1377
1378
DEAN M. GOLDEN
-60
60
Fly (1sh produced
r-
Fly ash utlllzed
,“TL
/
/--
c
c
3
~lO-,__,---\,
- 10
i; I 1969
0 1967
Fig. 1. Summary
1 7971
1 1975
1 1973
of fly ash produced
I 1977
Millions
of
short
States,
-0 1983
1967-83.
tons
N t
I
8
1 1961
and utilized in the United
0 Cement
t 1979
P I
w I
concrete
I
StructuralflllS
I
Road base Asphalt filler Grouting Coal mining Miscellaneous
0
I 2
I
Mullions
Fig. 2. Fly ash utilization:
1 N of
metric
United
1 w tonnes
States,
1983.
addition, these products do not require a specified type or quality of fly ash, but satisfy only performance specifications. High-volume uses are typified by fills, embankments, backfills, highway base courses, and soil stabilization and amendment. Medium-technology uses are those uses that require that the fly ash satisfy a specific specification, i.e. ASTM C618-83, and typically involve only a small percentage (5-20%) of the product produced. Medium-technology uses include cement pozzolans and bituminous fillers. High-technology uses are currently directed at the extraction of the mineral value from fly ash. COAL
ASH
UTILIZATION
RESEARCH:
HIGH-VOLUME
APPLICATIONS
EPRI is undertaking a multi-year program to promote the bulk sale of coal ash in highvolume applications, primarily in roadway construction. The project is specifically directed to applications that do not require a particular class or quality of fly ash. Some of these uses include: fills, embankments, backfills, landfill cover, soil amendments, subgrade stabilization, pavement base courses, grouting, slurry walls, and hydraulic fills. The major components of the project include: (1) documentation of existing ash utilization projects;
EPRI coal ash utilization
research
1379
(2) participation in new demonstration projects sponsored by EPRI and host utilities; (3) documentation of Federal Highway Administration (FHWA) and other non-EPRI sponsored projects; (4) preparation of draft specification in co-operation with state highway and environmental agencies; (5) preparation of a design manual for ash utilization; (6) preparation of a construction manual for ash utilization; (7) development of a generic utility ash marketing program; (8) preparation of brochures, films, and slides promoting ash use; and (9) development of educational seminars. A major objective of the EPRI ash utilization research project (RP 2422) is to promote the use of lly ash in highway construction to contractors, design engineers, and state highway departments. Since many of these groups consider coal ash a new or unconventional construction material, a major effort is under way to document the numerous successful applications of fly ash in general construction. Using a literature review and a telephone survey of utilities known to have used ash in the past, the EPRI contractor (GA1 Consultants, Inc.) has identified more than 200 existing projects using fly ash in highvolume applications in the United States and Canada. Mail surveys were circulated to solicit information on additional projects from state highway agencies and other coalburning utilities. Approximately 25 of these projects will be visited in 1985. Available data on ash characteristics and details of construction, monitoring, and performance are being collected for each project visited. Each project will be documented in summary brochures. All the brochures will be assembled in a single final report. Each project selected for a field visit will be individually documented with photographs and detailed information on design, construction, and performance. In addition, a list of contacts at each utility using ash in high-volume applications will be provided so that future users of the report can obtain relevant up-to-date information on each project. An important component of the EPRI program is the development of demonstration projects, particularly in those areas of the United States where ash has not been widely used but where considerable market potential and interest exists. EPRI expects that through these demonstration projects, potential customers and highway departments will recognize that power-plant ash by-products are acceptable building materials and economic substitutes for other products now in use. The demonstration projects will be structured to show the environmental and technological acceptability of ash utilization in road construction in a controlled and monitored segment of a highway. The demonstrations will also serve as test cases for a draft design manual and specifications developed for highway ash utilization. Near the end of the project, the design manual will be issued as an EPRI final report, and the specifications will be submitted for approval to the state highway agencies in the states where the demonstrations are located. The demonstration projects will be supplemented with information and data obtained from previous highway projects that used coal ash. This will provide information on long-term durability and performance that otherwise would not be available in a 5-year project. Four demonstration projects have been approved to date for construction in 1985-86. Additional projects will be approved in 1986 for construction in 1987788. Figure 3 shows the location of the demonstration projects and the type of application. In order to achieve the primary objective of the project in a particular utility’s service area, the following must be achieved: (1) support by the local utility; (2) acceptance of fly ash as an approved material on a use-by-use basis by the state Department of Transportation (DOT); (3) acceptance by the state environmental regulatory agency; (4) acceptance by private sector owners; (5) acceptance by design professionals; and (6) acceptance by contractors. Each of these has specific concerns about the use of fly ash in high-volume projects. In some cases the concerns evolve around economic feasibility; in others the concerns involve technical feasibility. The EPRI program is designed to address the concerns of each group through the development of specific work products. The work products to be published as EPRI reports are: a summary of existing high-volume ash utilization projects in the United States and Canada; a summary of new EPRI-sponsored high-volume ash use projects in the United States that will be selected to fill data gaps; a
DEAN
1380
M.
Sponsors
GOLDEN
Consumers Rwer Company Detroit Edison Company
Fly Ash-Cement
Colorado
Roedwey
Subgrede
Stabilizetion
Roadway
cement-fly
ash base
Roadway stabilized
embankments end subbase end base courses
Public
Service
Company
Southern Company Services Georgia Rwer Company Empire Srete Corporetion
Electric
Energy
Construction Schedule
Type of Application
Research
Bese
Course
Fig. 3. Locations and descriptions of EPRI field demonstration road construction.
for Reed
Shoulder
1986
1986 courses
Spring,
1985
1986
projects for fly ash utilization in
generic marketing program manual based on successful utility marketing programs in the United States; a design manual for high-volume uses of dry (conditioned) and flowable fly ash; a construction manual for high-volume uses of ash in highways; and a technology transfer program. MEDIUM-TECHNOLOGY
ASH UTILIZATION
RESEARCH
Historically, the use of fly ash in the United States has been directed mainly towards its use in concrete, as noted in Fig. 2. This is an ideal example of waste product utilization since there are both technical and economical benefits from its use. However, the quantity of ash used in concrete is but a small portion of the volume produced. In fact, if ash was used in all the concrete made in the United States, it would still be only a minor percentage of the volume available. In 1983, only 27% of all fly ash produced could have been used in cement at a 20% replacement rate. Only 14% of all fly ash produced in 1983 was utilized in any form, with the remainder being placed in storage or disposal areas. One of the major impediments to increased ash use may be the ASTM specification for fly ash as a mineral admixture in Portland cement concrete. This is inevitable given the inherent nature of specifications defining compositional or property limits within which a fly ash must fall to be used in certain applications. There are, however, a sufficient number of exceptions to the general rule to cast doubt on the validity of the specifications and on the characteristics being specified. Information developed in a number of research programs on fly ash over the past 20 yr (including work under EPRI sponsorship) has led to increasing doubts not only about the ash characteristics being measured but also about the validity of some of the methods used to measure the characteristics.’ The test results have shown that fly ash has several distinct functions in concrete. It is (1) a pozzolan, (2) a workability modifier, (3) a fine aggregate, and (4) an adsorbent of air-entraining agents. In addition,
EPRI coal ash utilization research
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the chemical properties of both fly and bottom ashes make them potentially useful raw materials for the manufacture of Portland cement clinker. The inadequacy of present specifications and test methods for fly ash was identified by the Project Advisory Committee established in 1984 to guide an EPRI research project on ash in cement/concrete products. The objective of the project is to integrate new and existing knowledge on the effects of using fly ash in cement/concrete products. Existing criteria for determining the suitability of fly ash for cement/concrete use will be validated, and new tly ash classification guidelines developed where existing ones are deemed inadequate for the purposes of the electric utility industry. The ultimate aim is to develop a meaningful and reliable method of evaluation that will allow the use of fly ash with confidence. The EPRI ash “classification” project consists of ash sampling and testing to characterize the physical and chemical properties of fly ash. Following this testing, representative ashes will be selected for use as concrete admixtures. Concrete testing will then be conducted to determine the effect of fly ash incorporation on performance of the concrete mixture. Following the completion of the testing, predictive models relating fly ash characteristics to resulting cement/concrete properties will be developed. These models will include chemical kinetics factors and other parameters. The co-ordinating EPRI contractor for the classification work is Baker Engineers. It is anticipated that the results will be published in early 1986.
HIGH-TECHNOLOGY
ASH APPLICATIONS
The recovery of mineral resources from fly ash is inherently attractive because of the prospects for avoiding disposal costs and generating revenue from metal and mineral sales, and for conserving the worlds strategic mineral resources. Of equal importance are the reduced environmental concerns from reduced volumes in disposal facilities. The chemical composition of coal ash is dependent on the composition of the soil strata from which the coal was mined, and therefore is a function of the geology and hydrogeology of the surrounding strata. Generally speaking, coal ash has a chemical composition similar to clay, which has also been evaluated as a non-bauxitic alumina source. Table 1 shows typical levels of the major ash and clay constituents. Silica (SiOJ, alumina (Al,O,), and iron oxide (Fe,O,) represent about 90% of the total. Because of the high temperature at which fly ash is produced in a high-efficiency boiler, the ash consists of glassy particles (generally spherical) of complex silicates of these three elements. In addition to the constituents listed in Table 1, coal ash contains other elements in trace quantities. If the sensitivity of the analytical method were high enough, virtually all the naturally occurring elements on the periodic table might be found. Table 2 lists the typical ranges for the more common trace elements in coal ash. Under existing economic conditions, the resource components of coal ash that may be considered for their intrinsic value can be summarized as follows: carbon; magnetite; cenospheres; and alumina, iron oxide, and other metal values. The DAL process
The recovery of metals from coal ash is not a new idea. Literature citations go back 50 or more years. The direct acid leaching (DAL) process was first studied as a way to remove the leachable heavy metals from fly ash. In the DAL process, fly ash is leached at about 100 “C for 2 hr in hydrochloric acid (HCl) to remove the metals. The resulting chloride leachate contains most of the metals, including trace amounts of strategic metals such as chromium, cobalt, and manganese; the major components of potential value, however, are aluminum and iron.3 The leachate is then passed through a series of anion exchange columns to produce a partially purified solution of aluminum chloride and a very high purity iron chloride. Final purification of the aluminum chloride is by hydrogen chloride gas-sparged crystallization in two stages. Figure 4 is a simplified process flow diagram.
DEAN M. GOLDEN
1382
Table 1. Typical levels of major ash and clay constituents Constituent
Fly ash
Bottom ash
Kaolin clay
SiO Al,&, Fe,& CaO
40-60% 20-35% 3-6% 3-20%
50-60% 15m25% 4&90/c 4.~15%
4&50% 35-40% 3-20%
Table 2. Concentrations Concentration
of common
trace elements in coal ash’
in coal ash @g/kg)
Elements B,Ba, Cu, Mn, Sr As, Cr, La, MO, Ni, Pb, Th, U, Zn Cd, Ga, Sb, Se, Ti, V Hg
100-1000 10~100 I-10
The technique of producing relatively pure compounds by precipitating chlorides from solution with an excess of HCl has been proven on a commercial scale.4 Purified salts like Al&. 6H,O can be prepared by a saturated solution of HCl, which causes the salt to precipitate. In the DAL process, an aluminum chloride hexahydrate product is purified and recovered. Waste liquor from the multi-stage crystallization system goes into a gypsum reactor where it is reacted with 98% H,S04. Gypsum is crystallized and HCl gas generated. The aluminum chloride hexahydrate crystal slurry from the crystallizer is converted to alumina by calcination of the hexahydrate and absorption of the off-gases, which provide the HCl for recycle. EPRI became involved in fly ash metal recovery in 1979 primarily as a method of treating ash prior to disposal. At that time, there was great uncertainty as to how coal ash would be regulated under the federal government’s Resource Conservation and Recovery Act. The EPRI contractor, Oak Ridge National Laboratory, investigated several potential processes and identified the two most promising. The project report3 included process flow charts, preliminary designs for a demonstration plants, cost estimates, and expected byproduct revenues from recovered resources. EPRI contractor Raymond Kaiser Engineers did a detailed engineering, cost, and financial evaluation for a conceptual commercial plant to process fly ash into marketable metal oxides by the DAL process during 1983. During 1984-85, a fly ash resource evaluation and product market assessment are being conducted. Preliminary design of a 5-ton/date pilot plant began in April 1985. The overall goal of
Solids low
for
value
“SC
Fig. 4. Simplified
DAL process
flow diagram.
EPRI coal ash utilization research
1383
EPRI in sponsoring the DAL process technology development is to produce high-quality, attractively priced products with a reliable, long-term availability in environmentally acceptable operations and in grades specifically processed to meet customer needs. DAL process economics
The 1983 study performed for EPRI by Raymond Kaiser Engineers3 used a hypothetical commercial size plant designed to process about 1 million short tons of ash (dry basis)/yr into marketable metal oxides. The hypothetical site was adjacent to the Tennessee Valley Authority (TVA) Kingston Power Plant. The cost estimates included capital, operation, and maintenance. The revenue projections were made based on several selling prices for the principal products. The estimated capital cost of the hypothetical plant was $270 million. The potential products and their estimated revenues are shown in Table 3. In addition to the by-products produced, the co-generation plant produced an excess of 7189 kW of electric power with an estimated value of more than $2 million. Annual operating costs are tabulated in Table 4. It is interesting to note that the process produces more hydrochloric acid than it consumes, due to the vapor recovery system. The additional chlorides are introduced into the process by the chlorine that is used as an oxidizer. The major cost item in raw materials is the 96,000 short tons/yr of sulfuric acid. It is conceivable that the DAL process could be run in conjunction with a regenerable flue-gas desulfurization (FGD) scrubber system that would produce sulfuric acid as a byproduct. This would provide economic benefits to both processes, by avoiding limestone purchases and FGD waste disposal costs. As noted earlier, the rate of return on equity for a privately owned non-utility DAL processing plant is estimated to reach 20% per year. This rate of return is sufficiently high to attract private capital, but probably not until a pilot plant can produce sufficient quantities of product samples to verify market acceptance.
Annual operating costs are tabulated in Table 4. It is interesting to note that the process Annual Product Spent ash Silica pigments Pumice pigments Calcined silica pigments Fillers/extenders Cement admixture Iron oxide Crude pigments Crude magnetics Ferric chloride Irene ore
Production (tons)
Price/ton 0)
Re?Jeolle 0)
25,OCKl 51,300 300,000 300,000 190,000
200 45 80 80 50
25,COO 3000 10,OcO 64,500
40 500 100 30
1,5OQIOo 1,000,OKl 2,000,oOO 44,250,OoO
5,OwOc@ 2,300,OCNl 24,000,000 24,C0L$OOO 9,500,000
LO@WOO
158,000
180
Gypsum Cement Wallboard Agriculture
20,000 5000 20.000
30 50 50
600,OOQ 250,OOiI l,~,~
Alkali sulfates Fertilizer Salt cake
35,000 46,000
50 90
1,750,OoO 4,140,OoO
Titanium chloride Minor by-products
10,000 1200
Alumina
Subtotal Cogenerated
power
Total annual revenues
390 50-340
3,900,ooo 275,COO 126,465,ooO 2,115,OCO 128,580,OMl
DEAN M. GOLDEN
1384 Table 4. Tabulation
of estimated
annual operating
Raw materials
units
$/unit
Flocculents Ion exchange resin Chlorine Hydrochloric acid Limestone Molten salt Sulfuric acid
Ibs Ihs tons tons Ibs tons
0.1 3.0 160 65 8 1.0 60
Units
$/unit
tons MM Btu
50 5.3 3
tons
costs for the hypothetical
study facility
Units/yr
Annual cost (S) 476,ooO 477,000 3,503,ooo (376,OW 406,000 55,000 5,760,OMl 17,296,ooO
2,597,OOo 159,wO 22,400 (5800) 97,000 55,ooo 96,OIXl
Total raw materials Energy and fuels Coal Natural gas Liquid fuels (diesel gasoline)
Units/y1
Annual cost
169,000 1,665,OOO
8,463,ooO 8,736,OCMl 97,000 17,296,ooO
Total fuels and energy Personnel
Number
Operation Maintenance Administration Technical Supervisory
105 50 I6 8 20
Total personnel
18 19 12 20 23
3,629,OOO 1,824,OOO 369,ooO 307,200 883,000 7,012,000
costs
Maintenance materials Supplies Lab materials
6,076,ooO
and contracts
Total annual variable
Potential
Annual cost
%/hr
manufacturing
645,000 445,000 42,000$00
costs
product markets
One of the primary objectives of the EPRI-sponsored DAL project was to generate no waste by-products requiring disposal. The process has been modified from the earlier concept by the addition of a sulfuric acid loop to produce gypsum and alkali sulfates, rather than create a chloride waste. In all, more than a dozen by-products are possible from the DAL process. A summary of the potential products from the hypothetical commercial-scale facility is shown in Fig. 5. The pie-charts shown in Figs. 6 and 7 show the distribution of products on both a tonnage and a dollar basis. It should be noted that neither magnetite nor carbon were produced because of the low levels found in the Kingston Power Plant ash. Spent ash. The spent ash produced in the leaching process is a unique product. Spent
4 6
Slllca 000
2
700
g
600
pigments
Pumice
pigments
5 500 TJ : 400 e a
Ferric
chloride
300
Spent
Fig. 5. Summary
ash
of potential
Alumina
ash products
Iron
oxide
from the hypothetical
All
others
study facility.
EPRI coal ash utilization
OIhers
lO.6%)
Alkali
research
sulfates
Gypsum
1385
(6%)
(3.5
% I
Alumina
Iron
Fig. 6. Distribution
of potential
ash prbducts
0th.u
study facility (total tonnage
sulfof~r
14.6%) (1.4%)
(52%)
Iron
of potential
(6%)
(3%)
Gypsum
Fig. 7. Distribution
oxide
from the hypothetical basis).
Alhdli
ash
(12.5%)
ash products
oxide
(4%)
from the hypothetical basis).
study
facility
(total
dollar
ash has properties that, with proper processing and marketing programs, could become an appreciable revenue source. Spent ash is an inert, stable, opaque aluminum silicate particle with enhanced surface characteristics. One of its principal advantages is that it is uniform in quality. Based on laboratory studies, spent ash has other superior properties such as high surface area (8-12 times original ash), high abrasion resistance, and low density. The spent ash product can be dried and classified, so that five different size fractions can be supplied to different markets. The finer size fractions can be used for pigments and fillers, and the coarse size for pumice pigments, cements, and aggregates. The potential market for spent ash as fillers and extenders is estimated at 12 million short tons/yr in the United States. The average price for these products is estimated at $85 per short ton. Alumina. The alumina production rate from the hypothetical commercial-scale plant is about 158,000 short tons/yr, which is less than 2% of current U.S. consumption. Iron oxide. The ion exchange system included in the DAL process allows for production of a very high purity iron chloride. This is converted into an agglomerated iron oxide particle. The target markets for this product are finished pigments, magnetics, and catalysts in specialty chemical markets. Gypsum. The gypsum produced in the DAL process should be viewed as a chloride recovery and waste disposal product. The crude gypsum produced contains impurities, such as magnesium, strontium, and barium, which limit marketability and may need to be
1386
DEAN M. GOLDEN
recovered separately. The ciritcal requirements in gypsum markets are purity, particle size moisture content, and specific impurities (such as chlorides). Alkali sulfates. The remaining alkali sulfates are the mixed salts of sodium, potassium, and aluminum. The estimated chemical analysis is as follows: Al(SO&, 9. 7%; KzS04, 42.4%; MgSO,, 24.4%; Mn(SO&, 0.7%; Na,S04, 6.5%; and Ti(SO&, 16.3%. The alkali salts are the variable product in the series of products recovered in the DAL process. The heavy metals would also end up in this product unless they are recovered earlier in the process. Alkali salts have value as fertilizer and salt cake. The titanium dioxide could be a valuable by-product. Magnetite. Synthetic magnetite appears to be a relatively small market since the present market as a coal-cleaning medium is relatively limited. 5 Some ashes have considerably higher magnetic ash contents. Magnetic ash is a relatively low-purity, low-value product compared to higher value iron oxide products. Since magnetic ash is mostly silica with some iron oxide, magnetic ash could possibly be a good ferrosilicon raw material. Carbon. For ash resources with a high carbon content, a carbon recovery circuit can be added in front of the DAL process. In this system the difference in the physical characteristics (size and density) of the ash and carbon particles could be utilized to separate the carbon fraction of the ash. The carbon fraction can be further purified using wet flotation techniques to produce a carbon concentrate that could then be dried and ground. The thermal history of the ash is likely to produce a carbon with sufficient surface activity so that the potential exists for entering such traditional activated carbon markets as municipal and industrial waste treatment, dry cleaning, sugar and syrup industries, motor vehicles, and air cleaning equipment. If so, the price per ton can reach several hundred dollars. Cenospheres. Although the percentage of cenospheres in ash varies widely, it generally represents only a fraction of 1% of the total ash. Nevertheless, it can be easily collected in ash ponds with flotation collection devices. The literature indicates a diverse market potential for ash cenospheres in industrial application, primarily as mineral fillers and lightweight refractory materials. The high strength, low weight, chemical inertness, spherical shape, and particle size distribution of cenospheres allow them to serve as low-cost substitutes for manufactured glass microspheres. A great many applications are possible when binders such as organic resins or cement are used. In these applications the cenospheres act as both an extender and a material conferring desirable engineering properties on the finished products. When used as lightweight refractory materials, cenospheres can be used as a refractory aggregate that can be added to refractory cements or sintered without any binders. The market price for cenospheres is also several hundred dollars per ton. It should be noted that the DAL process is still in the early stages of commercialization. Work has just begun on the design of a pilot plant. It is expected that if suitable funding arrangements can be made, the pilot plant would be built in 1986 and operated through 1988. If the pilot plant proves as successful as the bench-scale unit has, a large semicommercial scale unit would be planned for construction in 1990-92. DAL product marketing
strategy
As part of the ongoing EPRI-sponsored research, product samples are being produced in a bench-scale DAL unit at Oak Ridge National Laboratory with ash supplied by two utilities. The samples will be evaluated by test laboratories recommended by potential ash product users. The goal of the testing is to determine whether or not the by-products meet the specifications for use by the target industries. The results of this testing will be available later this year. It is clear, however, that characteristics that should enhance its marketability as a filler or extender are: (1) ash can be produced and brought to market without the penalty of high-energy related costs required for the mining, processing, and size-reduction (grinding) of conventional materials; (2) ash is stockpiled in large quantities (800 million short tons since the end of World War II), ensuring the dependable supply of a process feedstock; (3) the wide geographic distribution of coal-fired power plants provides an advantage to ash resources over existing mineral resources because of proximity to
EPRI coal ash utilization
research
1387
manufacturing centers; (4) fly ash particles are predominantly spherical in shape with a size distribution extending from ~0.5 to >200pm, and it shares with only a few other minerals the unique property of an almost perfectly spherical shape in the submicron sizes, which contributes to improved packing and rheological characteristics; and (5) high compressive strength and good thermal stability make fly ash suitable for applications requiring high production temperatures or when specifications call for high temperature resistance in the finished products. SUMMARY
The Electric Power Research Institute has been seeking to increase the utilization of coal ash through an applied research and development project begun in 1979, and has a multi-faceted project under way over the next 5yr to continue the research in three principal areas. High-volume applications in high construction uses; medium-volume uses in cement and concrete, and high-technology mineral extraction process development make up the triad of ash utilization research at EPRI. REFERENCES 1. University of California at Berkeley, “Pozzolanic Behavior of Fly Ash”, Electric Power Research Institute report no. (38-3314, Palo Alto, Calif. (1984). 2. D. M. Golden, “Water Pollution Arising from Solid Waste (Coal, Fly Ash, Slag) Disposal, and Measures to Prevent Water Pollution”, Water Science and Technology, Vol. 15, Pergamon Press, Oxford (1983). 3. Oak Ridge National Laboratory, “Evaluation of Fly Ash Metal Recovery Processes”, Vol. l-2, Electric Power Research Institute report no. CS-1992, Palo Alto, Calif. (1981). 4. Ames Laboratory, Recovery of Metals from Coal Ash: An Annotated Bibliography, Iowa State University, Ames, Ia. (1983). Electric Power Research Institute report 5. Baker Engineers, “Market Survey of Fly Ash Derived Magnetite”, no. CS-3615, Palo Alto, Calif. (1984).
0360-5442/86 $3.00 + 0.00 Pergamon Journals Ltd
EPRI COAL ASH UTILIZATION
RESEARCH
DEAN M. GOLDEN Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, CA 94304, U.S.A. Abstract-This paper describes a five-year Electric Power Research Institute (EPRI) project directed toward increasing ash utilization. EPRI is undertaking this program to promote the bulk sale of coal ash in high-volume applications that do not require a specific quality or class of ash; in medium-technology applications in cement and concrete; and in high-technology uses for valuable products requiring stringent quality control. High-volume applications include the use of ash in highways and railroads, backfills, and pavement base course; medium-technology applications include cement and concrete; and high-technology uses include the extraction of the mineral matter or the metals from ash.
INTRODUCTION
The annual world combustion of coal in electric plants is now about 2500mtce (million metric tonnes coal equivalent), resulting in the production of 250-300 million tonnes of fly ash.? It has been predicted that consumption will increase to about 6500-7000mtce annually in the year 2000, resulting in the collection of some 650-850 million tonnes of fly ash. The current usage of coal by utilities in the United States results in the production of over 65 million tonnes (71 million short tons) of solid combustion by-products each year. A predicted doubling of coal usage in the United States within the next 10yr will only intensify the challenges associated with the proper disposal or utilization of the byproducts. Stringent particulate removal requirements and increased use of desulfurization systems will further increase by-product volumes. The problem of effective utilization of coal ash has been given wide attention for at least 20yr in the United States. Although the effort devoted to the problem has led to some applications of demonstrated usefulness, the full utilization of coal ash has been hindered by lack of effective dissemination of knowledge from field studies to potential users of coal ash, and by the obvious variability of the product. Extensive utilization of coal ash is both technically and economically feasible, yet less than one-quarter of utility coal ash produced in the United States is presently being utilized. Utilization rates in Europe vary considerably by country. In the United States, approximately 80% of the coal ash by-products are in the form of fly ash, which has a lower utilization rate than bottom ash. As shown in Fig. 1, the amount of fly ash produced in the United States by electric utilities has increased significantly over the past 16yr. There has been a relatively steady increase in the percentage of fly ash utilized from 1967 to 1979; the percentage of fly ash utilized since 1979 has, however, been somewhat erratic. As can be seen in Fig. 1, the ash utilization rate for 1983 was only 14.3% of the 52.4 million short tons produced. Figure 2 presents a summary of the various ways in which fly ash was used in the United States in 1983. As indicated, 3.6 million short tons of fly ash-48% of the fly ash utilizedwent into cement and concrete products. The next largest uses are structural fills (1.4 million tons), road base (0.5 million tons), grouting (0.2 million tons), and coal-mining applications (0.2 million tons). These are frequently defined as “high-volume uses”. Thus, in 1983, high-volume uses accounted for 31% of the fly ash utilized. The Electric Power Research Institute (EPRI) initiated an ash utilization research and development program in 1979 to support the increased use of fly ash in the United States. This program has been divided into three principal areas: high-volume uses, mediumtechnology uses, and high-technology uses. High-volume uses include those fly ash products that are either large in quantity or require huge percentages (over 50%) of fly ash. In ?A metric tonne = 22041b. A short ton = 20001b. 1377
1378
DEAN M. GOLDEN
-60
60
Fly (1sh produced
r-
Fly ash utlllzed
,“TL
/
/--
c
c
3
~lO-,__,---\,
- 10
i; I 1969
0 1967
Fig. 1. Summary
1 7971
1 1975
1 1973
of fly ash produced
I 1977
Millions
of
short
States,
-0 1983
1967-83.
tons
N t
I
8
1 1961
and utilized in the United
0 Cement
t 1979
P I
w I
concrete
I
StructuralflllS
I
Road base Asphalt filler Grouting Coal mining Miscellaneous
0
I 2
I
Mullions
Fig. 2. Fly ash utilization:
1 N of
metric
United
1 w tonnes
States,
1983.
addition, these products do not require a specified type or quality of fly ash, but satisfy only performance specifications. High-volume uses are typified by fills, embankments, backfills, highway base courses, and soil stabilization and amendment. Medium-technology uses are those uses that require that the fly ash satisfy a specific specification, i.e. ASTM C618-83, and typically involve only a small percentage (5-20%) of the product produced. Medium-technology uses include cement pozzolans and bituminous fillers. High-technology uses are currently directed at the extraction of the mineral value from fly ash. COAL
ASH
UTILIZATION
RESEARCH:
HIGH-VOLUME
APPLICATIONS
EPRI is undertaking a multi-year program to promote the bulk sale of coal ash in highvolume applications, primarily in roadway construction. The project is specifically directed to applications that do not require a particular class or quality of fly ash. Some of these uses include: fills, embankments, backfills, landfill cover, soil amendments, subgrade stabilization, pavement base courses, grouting, slurry walls, and hydraulic fills. The major components of the project include: (1) documentation of existing ash utilization projects;
EPRI coal ash utilization
research
1379
(2) participation in new demonstration projects sponsored by EPRI and host utilities; (3) documentation of Federal Highway Administration (FHWA) and other non-EPRI sponsored projects; (4) preparation of draft specification in co-operation with state highway and environmental agencies; (5) preparation of a design manual for ash utilization; (6) preparation of a construction manual for ash utilization; (7) development of a generic utility ash marketing program; (8) preparation of brochures, films, and slides promoting ash use; and (9) development of educational seminars. A major objective of the EPRI ash utilization research project (RP 2422) is to promote the use of lly ash in highway construction to contractors, design engineers, and state highway departments. Since many of these groups consider coal ash a new or unconventional construction material, a major effort is under way to document the numerous successful applications of fly ash in general construction. Using a literature review and a telephone survey of utilities known to have used ash in the past, the EPRI contractor (GA1 Consultants, Inc.) has identified more than 200 existing projects using fly ash in highvolume applications in the United States and Canada. Mail surveys were circulated to solicit information on additional projects from state highway agencies and other coalburning utilities. Approximately 25 of these projects will be visited in 1985. Available data on ash characteristics and details of construction, monitoring, and performance are being collected for each project visited. Each project will be documented in summary brochures. All the brochures will be assembled in a single final report. Each project selected for a field visit will be individually documented with photographs and detailed information on design, construction, and performance. In addition, a list of contacts at each utility using ash in high-volume applications will be provided so that future users of the report can obtain relevant up-to-date information on each project. An important component of the EPRI program is the development of demonstration projects, particularly in those areas of the United States where ash has not been widely used but where considerable market potential and interest exists. EPRI expects that through these demonstration projects, potential customers and highway departments will recognize that power-plant ash by-products are acceptable building materials and economic substitutes for other products now in use. The demonstration projects will be structured to show the environmental and technological acceptability of ash utilization in road construction in a controlled and monitored segment of a highway. The demonstrations will also serve as test cases for a draft design manual and specifications developed for highway ash utilization. Near the end of the project, the design manual will be issued as an EPRI final report, and the specifications will be submitted for approval to the state highway agencies in the states where the demonstrations are located. The demonstration projects will be supplemented with information and data obtained from previous highway projects that used coal ash. This will provide information on long-term durability and performance that otherwise would not be available in a 5-year project. Four demonstration projects have been approved to date for construction in 1985-86. Additional projects will be approved in 1986 for construction in 1987788. Figure 3 shows the location of the demonstration projects and the type of application. In order to achieve the primary objective of the project in a particular utility’s service area, the following must be achieved: (1) support by the local utility; (2) acceptance of fly ash as an approved material on a use-by-use basis by the state Department of Transportation (DOT); (3) acceptance by the state environmental regulatory agency; (4) acceptance by private sector owners; (5) acceptance by design professionals; and (6) acceptance by contractors. Each of these has specific concerns about the use of fly ash in high-volume projects. In some cases the concerns evolve around economic feasibility; in others the concerns involve technical feasibility. The EPRI program is designed to address the concerns of each group through the development of specific work products. The work products to be published as EPRI reports are: a summary of existing high-volume ash utilization projects in the United States and Canada; a summary of new EPRI-sponsored high-volume ash use projects in the United States that will be selected to fill data gaps; a
DEAN
1380
M.
Sponsors
GOLDEN
Consumers Rwer Company Detroit Edison Company
Fly Ash-Cement
Colorado
Roedwey
Subgrede
Stabilizetion
Roadway
cement-fly
ash base
Roadway stabilized
embankments end subbase end base courses
Public
Service
Company
Southern Company Services Georgia Rwer Company Empire Srete Corporetion
Electric
Energy
Construction Schedule
Type of Application
Research
Bese
Course
Fig. 3. Locations and descriptions of EPRI field demonstration road construction.
for Reed
Shoulder
1986
1986 courses
Spring,
1985
1986
projects for fly ash utilization in
generic marketing program manual based on successful utility marketing programs in the United States; a design manual for high-volume uses of dry (conditioned) and flowable fly ash; a construction manual for high-volume uses of ash in highways; and a technology transfer program. MEDIUM-TECHNOLOGY
ASH UTILIZATION
RESEARCH
Historically, the use of fly ash in the United States has been directed mainly towards its use in concrete, as noted in Fig. 2. This is an ideal example of waste product utilization since there are both technical and economical benefits from its use. However, the quantity of ash used in concrete is but a small portion of the volume produced. In fact, if ash was used in all the concrete made in the United States, it would still be only a minor percentage of the volume available. In 1983, only 27% of all fly ash produced could have been used in cement at a 20% replacement rate. Only 14% of all fly ash produced in 1983 was utilized in any form, with the remainder being placed in storage or disposal areas. One of the major impediments to increased ash use may be the ASTM specification for fly ash as a mineral admixture in Portland cement concrete. This is inevitable given the inherent nature of specifications defining compositional or property limits within which a fly ash must fall to be used in certain applications. There are, however, a sufficient number of exceptions to the general rule to cast doubt on the validity of the specifications and on the characteristics being specified. Information developed in a number of research programs on fly ash over the past 20 yr (including work under EPRI sponsorship) has led to increasing doubts not only about the ash characteristics being measured but also about the validity of some of the methods used to measure the characteristics.’ The test results have shown that fly ash has several distinct functions in concrete. It is (1) a pozzolan, (2) a workability modifier, (3) a fine aggregate, and (4) an adsorbent of air-entraining agents. In addition,
EPRI coal ash utilization research
1381
the chemical properties of both fly and bottom ashes make them potentially useful raw materials for the manufacture of Portland cement clinker. The inadequacy of present specifications and test methods for fly ash was identified by the Project Advisory Committee established in 1984 to guide an EPRI research project on ash in cement/concrete products. The objective of the project is to integrate new and existing knowledge on the effects of using fly ash in cement/concrete products. Existing criteria for determining the suitability of fly ash for cement/concrete use will be validated, and new tly ash classification guidelines developed where existing ones are deemed inadequate for the purposes of the electric utility industry. The ultimate aim is to develop a meaningful and reliable method of evaluation that will allow the use of fly ash with confidence. The EPRI ash “classification” project consists of ash sampling and testing to characterize the physical and chemical properties of fly ash. Following this testing, representative ashes will be selected for use as concrete admixtures. Concrete testing will then be conducted to determine the effect of fly ash incorporation on performance of the concrete mixture. Following the completion of the testing, predictive models relating fly ash characteristics to resulting cement/concrete properties will be developed. These models will include chemical kinetics factors and other parameters. The co-ordinating EPRI contractor for the classification work is Baker Engineers. It is anticipated that the results will be published in early 1986.
HIGH-TECHNOLOGY
ASH APPLICATIONS
The recovery of mineral resources from fly ash is inherently attractive because of the prospects for avoiding disposal costs and generating revenue from metal and mineral sales, and for conserving the worlds strategic mineral resources. Of equal importance are the reduced environmental concerns from reduced volumes in disposal facilities. The chemical composition of coal ash is dependent on the composition of the soil strata from which the coal was mined, and therefore is a function of the geology and hydrogeology of the surrounding strata. Generally speaking, coal ash has a chemical composition similar to clay, which has also been evaluated as a non-bauxitic alumina source. Table 1 shows typical levels of the major ash and clay constituents. Silica (SiOJ, alumina (Al,O,), and iron oxide (Fe,O,) represent about 90% of the total. Because of the high temperature at which fly ash is produced in a high-efficiency boiler, the ash consists of glassy particles (generally spherical) of complex silicates of these three elements. In addition to the constituents listed in Table 1, coal ash contains other elements in trace quantities. If the sensitivity of the analytical method were high enough, virtually all the naturally occurring elements on the periodic table might be found. Table 2 lists the typical ranges for the more common trace elements in coal ash. Under existing economic conditions, the resource components of coal ash that may be considered for their intrinsic value can be summarized as follows: carbon; magnetite; cenospheres; and alumina, iron oxide, and other metal values. The DAL process
The recovery of metals from coal ash is not a new idea. Literature citations go back 50 or more years. The direct acid leaching (DAL) process was first studied as a way to remove the leachable heavy metals from fly ash. In the DAL process, fly ash is leached at about 100 “C for 2 hr in hydrochloric acid (HCl) to remove the metals. The resulting chloride leachate contains most of the metals, including trace amounts of strategic metals such as chromium, cobalt, and manganese; the major components of potential value, however, are aluminum and iron.3 The leachate is then passed through a series of anion exchange columns to produce a partially purified solution of aluminum chloride and a very high purity iron chloride. Final purification of the aluminum chloride is by hydrogen chloride gas-sparged crystallization in two stages. Figure 4 is a simplified process flow diagram.
DEAN M. GOLDEN
1382
Table 1. Typical levels of major ash and clay constituents Constituent
Fly ash
Bottom ash
Kaolin clay
SiO Al,&, Fe,& CaO
40-60% 20-35% 3-6% 3-20%
50-60% 15m25% 4&90/c 4.~15%
4&50% 35-40% 3-20%
Table 2. Concentrations Concentration
of common
trace elements in coal ash’
in coal ash @g/kg)
Elements B,Ba, Cu, Mn, Sr As, Cr, La, MO, Ni, Pb, Th, U, Zn Cd, Ga, Sb, Se, Ti, V Hg
100-1000 10~100 I-10
The technique of producing relatively pure compounds by precipitating chlorides from solution with an excess of HCl has been proven on a commercial scale.4 Purified salts like Al&. 6H,O can be prepared by a saturated solution of HCl, which causes the salt to precipitate. In the DAL process, an aluminum chloride hexahydrate product is purified and recovered. Waste liquor from the multi-stage crystallization system goes into a gypsum reactor where it is reacted with 98% H,S04. Gypsum is crystallized and HCl gas generated. The aluminum chloride hexahydrate crystal slurry from the crystallizer is converted to alumina by calcination of the hexahydrate and absorption of the off-gases, which provide the HCl for recycle. EPRI became involved in fly ash metal recovery in 1979 primarily as a method of treating ash prior to disposal. At that time, there was great uncertainty as to how coal ash would be regulated under the federal government’s Resource Conservation and Recovery Act. The EPRI contractor, Oak Ridge National Laboratory, investigated several potential processes and identified the two most promising. The project report3 included process flow charts, preliminary designs for a demonstration plants, cost estimates, and expected byproduct revenues from recovered resources. EPRI contractor Raymond Kaiser Engineers did a detailed engineering, cost, and financial evaluation for a conceptual commercial plant to process fly ash into marketable metal oxides by the DAL process during 1983. During 1984-85, a fly ash resource evaluation and product market assessment are being conducted. Preliminary design of a 5-ton/date pilot plant began in April 1985. The overall goal of
Solids low
for
value
“SC
Fig. 4. Simplified
DAL process
flow diagram.
EPRI coal ash utilization research
1383
EPRI in sponsoring the DAL process technology development is to produce high-quality, attractively priced products with a reliable, long-term availability in environmentally acceptable operations and in grades specifically processed to meet customer needs. DAL process economics
The 1983 study performed for EPRI by Raymond Kaiser Engineers3 used a hypothetical commercial size plant designed to process about 1 million short tons of ash (dry basis)/yr into marketable metal oxides. The hypothetical site was adjacent to the Tennessee Valley Authority (TVA) Kingston Power Plant. The cost estimates included capital, operation, and maintenance. The revenue projections were made based on several selling prices for the principal products. The estimated capital cost of the hypothetical plant was $270 million. The potential products and their estimated revenues are shown in Table 3. In addition to the by-products produced, the co-generation plant produced an excess of 7189 kW of electric power with an estimated value of more than $2 million. Annual operating costs are tabulated in Table 4. It is interesting to note that the process produces more hydrochloric acid than it consumes, due to the vapor recovery system. The additional chlorides are introduced into the process by the chlorine that is used as an oxidizer. The major cost item in raw materials is the 96,000 short tons/yr of sulfuric acid. It is conceivable that the DAL process could be run in conjunction with a regenerable flue-gas desulfurization (FGD) scrubber system that would produce sulfuric acid as a byproduct. This would provide economic benefits to both processes, by avoiding limestone purchases and FGD waste disposal costs. As noted earlier, the rate of return on equity for a privately owned non-utility DAL processing plant is estimated to reach 20% per year. This rate of return is sufficiently high to attract private capital, but probably not until a pilot plant can produce sufficient quantities of product samples to verify market acceptance.
Annual operating costs are tabulated in Table 4. It is interesting to note that the process Annual Product Spent ash Silica pigments Pumice pigments Calcined silica pigments Fillers/extenders Cement admixture Iron oxide Crude pigments Crude magnetics Ferric chloride Irene ore
Production (tons)
Price/ton 0)
Re?Jeolle 0)
25,OCKl 51,300 300,000 300,000 190,000
200 45 80 80 50
25,COO 3000 10,OcO 64,500
40 500 100 30
1,5OQIOo 1,000,OKl 2,000,oOO 44,250,OoO
5,OwOc@ 2,300,OCNl 24,000,000 24,C0L$OOO 9,500,000
LO@WOO
158,000
180
Gypsum Cement Wallboard Agriculture
20,000 5000 20.000
30 50 50
600,OOQ 250,OOiI l,~,~
Alkali sulfates Fertilizer Salt cake
35,000 46,000
50 90
1,750,OoO 4,140,OoO
Titanium chloride Minor by-products
10,000 1200
Alumina
Subtotal Cogenerated
power
Total annual revenues
390 50-340
3,900,ooo 275,COO 126,465,ooO 2,115,OCO 128,580,OMl
DEAN M. GOLDEN
1384 Table 4. Tabulation
of estimated
annual operating
Raw materials
units
$/unit
Flocculents Ion exchange resin Chlorine Hydrochloric acid Limestone Molten salt Sulfuric acid
Ibs Ihs tons tons Ibs tons
0.1 3.0 160 65 8 1.0 60
Units
$/unit
tons MM Btu
50 5.3 3
tons
costs for the hypothetical
study facility
Units/yr
Annual cost (S) 476,ooO 477,000 3,503,ooo (376,OW 406,000 55,000 5,760,OMl 17,296,ooO
2,597,OOo 159,wO 22,400 (5800) 97,000 55,ooo 96,OIXl
Total raw materials Energy and fuels Coal Natural gas Liquid fuels (diesel gasoline)
Units/y1
Annual cost
169,000 1,665,OOO
8,463,ooO 8,736,OCMl 97,000 17,296,ooO
Total fuels and energy Personnel
Number
Operation Maintenance Administration Technical Supervisory
105 50 I6 8 20
Total personnel
18 19 12 20 23
3,629,OOO 1,824,OOO 369,ooO 307,200 883,000 7,012,000
costs
Maintenance materials Supplies Lab materials
6,076,ooO
and contracts
Total annual variable
Potential
Annual cost
%/hr
manufacturing
645,000 445,000 42,000$00
costs
product markets
One of the primary objectives of the EPRI-sponsored DAL project was to generate no waste by-products requiring disposal. The process has been modified from the earlier concept by the addition of a sulfuric acid loop to produce gypsum and alkali sulfates, rather than create a chloride waste. In all, more than a dozen by-products are possible from the DAL process. A summary of the potential products from the hypothetical commercial-scale facility is shown in Fig. 5. The pie-charts shown in Figs. 6 and 7 show the distribution of products on both a tonnage and a dollar basis. It should be noted that neither magnetite nor carbon were produced because of the low levels found in the Kingston Power Plant ash. Spent ash. The spent ash produced in the leaching process is a unique product. Spent
4 6
Slllca 000
2
700
g
600
pigments
Pumice
pigments
5 500 TJ : 400 e a
Ferric
chloride
300
Spent
Fig. 5. Summary
ash
of potential
Alumina
ash products
Iron
oxide
from the hypothetical
All
others
study facility.
EPRI coal ash utilization
OIhers
lO.6%)
Alkali
research
sulfates
Gypsum
1385
(6%)
(3.5
% I
Alumina
Iron
Fig. 6. Distribution
of potential
ash prbducts
0th.u
study facility (total tonnage
sulfof~r
14.6%) (1.4%)
(52%)
Iron
of potential
(6%)
(3%)
Gypsum
Fig. 7. Distribution
oxide
from the hypothetical basis).
Alhdli
ash
(12.5%)
ash products
oxide
(4%)
from the hypothetical basis).
study
facility
(total
dollar
ash has properties that, with proper processing and marketing programs, could become an appreciable revenue source. Spent ash is an inert, stable, opaque aluminum silicate particle with enhanced surface characteristics. One of its principal advantages is that it is uniform in quality. Based on laboratory studies, spent ash has other superior properties such as high surface area (8-12 times original ash), high abrasion resistance, and low density. The spent ash product can be dried and classified, so that five different size fractions can be supplied to different markets. The finer size fractions can be used for pigments and fillers, and the coarse size for pumice pigments, cements, and aggregates. The potential market for spent ash as fillers and extenders is estimated at 12 million short tons/yr in the United States. The average price for these products is estimated at $85 per short ton. Alumina. The alumina production rate from the hypothetical commercial-scale plant is about 158,000 short tons/yr, which is less than 2% of current U.S. consumption. Iron oxide. The ion exchange system included in the DAL process allows for production of a very high purity iron chloride. This is converted into an agglomerated iron oxide particle. The target markets for this product are finished pigments, magnetics, and catalysts in specialty chemical markets. Gypsum. The gypsum produced in the DAL process should be viewed as a chloride recovery and waste disposal product. The crude gypsum produced contains impurities, such as magnesium, strontium, and barium, which limit marketability and may need to be
1386
DEAN M. GOLDEN
recovered separately. The ciritcal requirements in gypsum markets are purity, particle size moisture content, and specific impurities (such as chlorides). Alkali sulfates. The remaining alkali sulfates are the mixed salts of sodium, potassium, and aluminum. The estimated chemical analysis is as follows: Al(SO&, 9. 7%; KzS04, 42.4%; MgSO,, 24.4%; Mn(SO&, 0.7%; Na,S04, 6.5%; and Ti(SO&, 16.3%. The alkali salts are the variable product in the series of products recovered in the DAL process. The heavy metals would also end up in this product unless they are recovered earlier in the process. Alkali salts have value as fertilizer and salt cake. The titanium dioxide could be a valuable by-product. Magnetite. Synthetic magnetite appears to be a relatively small market since the present market as a coal-cleaning medium is relatively limited. 5 Some ashes have considerably higher magnetic ash contents. Magnetic ash is a relatively low-purity, low-value product compared to higher value iron oxide products. Since magnetic ash is mostly silica with some iron oxide, magnetic ash could possibly be a good ferrosilicon raw material. Carbon. For ash resources with a high carbon content, a carbon recovery circuit can be added in front of the DAL process. In this system the difference in the physical characteristics (size and density) of the ash and carbon particles could be utilized to separate the carbon fraction of the ash. The carbon fraction can be further purified using wet flotation techniques to produce a carbon concentrate that could then be dried and ground. The thermal history of the ash is likely to produce a carbon with sufficient surface activity so that the potential exists for entering such traditional activated carbon markets as municipal and industrial waste treatment, dry cleaning, sugar and syrup industries, motor vehicles, and air cleaning equipment. If so, the price per ton can reach several hundred dollars. Cenospheres. Although the percentage of cenospheres in ash varies widely, it generally represents only a fraction of 1% of the total ash. Nevertheless, it can be easily collected in ash ponds with flotation collection devices. The literature indicates a diverse market potential for ash cenospheres in industrial application, primarily as mineral fillers and lightweight refractory materials. The high strength, low weight, chemical inertness, spherical shape, and particle size distribution of cenospheres allow them to serve as low-cost substitutes for manufactured glass microspheres. A great many applications are possible when binders such as organic resins or cement are used. In these applications the cenospheres act as both an extender and a material conferring desirable engineering properties on the finished products. When used as lightweight refractory materials, cenospheres can be used as a refractory aggregate that can be added to refractory cements or sintered without any binders. The market price for cenospheres is also several hundred dollars per ton. It should be noted that the DAL process is still in the early stages of commercialization. Work has just begun on the design of a pilot plant. It is expected that if suitable funding arrangements can be made, the pilot plant would be built in 1986 and operated through 1988. If the pilot plant proves as successful as the bench-scale unit has, a large semicommercial scale unit would be planned for construction in 1990-92. DAL product marketing
strategy
As part of the ongoing EPRI-sponsored research, product samples are being produced in a bench-scale DAL unit at Oak Ridge National Laboratory with ash supplied by two utilities. The samples will be evaluated by test laboratories recommended by potential ash product users. The goal of the testing is to determine whether or not the by-products meet the specifications for use by the target industries. The results of this testing will be available later this year. It is clear, however, that characteristics that should enhance its marketability as a filler or extender are: (1) ash can be produced and brought to market without the penalty of high-energy related costs required for the mining, processing, and size-reduction (grinding) of conventional materials; (2) ash is stockpiled in large quantities (800 million short tons since the end of World War II), ensuring the dependable supply of a process feedstock; (3) the wide geographic distribution of coal-fired power plants provides an advantage to ash resources over existing mineral resources because of proximity to
EPRI coal ash utilization
research
1387
manufacturing centers; (4) fly ash particles are predominantly spherical in shape with a size distribution extending from ~0.5 to >200pm, and it shares with only a few other minerals the unique property of an almost perfectly spherical shape in the submicron sizes, which contributes to improved packing and rheological characteristics; and (5) high compressive strength and good thermal stability make fly ash suitable for applications requiring high production temperatures or when specifications call for high temperature resistance in the finished products. SUMMARY
The Electric Power Research Institute has been seeking to increase the utilization of coal ash through an applied research and development project begun in 1979, and has a multi-faceted project under way over the next 5yr to continue the research in three principal areas. High-volume applications in high construction uses; medium-volume uses in cement and concrete, and high-technology mineral extraction process development make up the triad of ash utilization research at EPRI. REFERENCES 1. University of California at Berkeley, “Pozzolanic Behavior of Fly Ash”, Electric Power Research Institute report no. (38-3314, Palo Alto, Calif. (1984). 2. D. M. Golden, “Water Pollution Arising from Solid Waste (Coal, Fly Ash, Slag) Disposal, and Measures to Prevent Water Pollution”, Water Science and Technology, Vol. 15, Pergamon Press, Oxford (1983). 3. Oak Ridge National Laboratory, “Evaluation of Fly Ash Metal Recovery Processes”, Vol. l-2, Electric Power Research Institute report no. CS-1992, Palo Alto, Calif. (1981). 4. Ames Laboratory, Recovery of Metals from Coal Ash: An Annotated Bibliography, Iowa State University, Ames, Ia. (1983). Electric Power Research Institute report 5. Baker Engineers, “Market Survey of Fly Ash Derived Magnetite”, no. CS-3615, Palo Alto, Calif. (1984).