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Power station fly ash — a review of value-added utilization outside of the construction industry
Resources, Conservation and Recycling 31 (2001) 217–228
www.elsevier.com/locate/resconrec
Power station fly ash — a review of value-added utilization outside of the construction industry R.S. Iyer 1, J.A. Scott * Center for Integrated En6ironmental Protection, Griffith Uni6ersity, Brisbane, Qld 4111, Australia Received 27 August 1999; accepted 20 July 2000
Abstract The disposal of fly ash from coal-fired power stations causes significant economic and environmental problems. A relatively small percentage of the material finds application as an ingredient in cement and other construction products, but the vast majority of material generated each year is held in ash dams or similar dumps. This unproductive use of land and the associated long-term financial burden of maintenance has led to realization that alternative uses for fly ash as a value-added product beyond incorporation in construction materials are needed. Utilization of fly ash in such areas as novel materials, waste management, recovery of metals and agriculture is reviewed in this article with the aim of looking at new areas that will expand the positive reuse of fly ash, thereby helping to reduce the environmental and economic impacts of disposal. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Power station; Fly ash; Utilization; Materials; Waste management; Materials recovery; Agriculture
* Corresponding author. Tel.: +61-7-38753661; fax: +61-7-38755299. E-mail address: [email protected] (J.A. Scott). 1 Present address: Department of Chemical Engineering, University of Queensland, St Lucia, Qld 4068, Australia 0921-3449/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 3 4 4 9 ( 0 0 ) 0 0 0 8 4 - 7
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1. Introduction Disposal of fly ash as a by-product of incineration of coal, municipal solid wastes, sugar cane bagasse, rice husks and tea dusts, is becoming an increasing economic and environmental burden. As a consequence, there is a growing interest in looking for avenues where the material can be used as a potential resource for preparation of value added products. The majority of fly ash is generated by coal fired power stations and a percentage (typically 10–20%) does find reuse, primarily in cementitious (concrete and cement) products (Al-Almoudi and Maslehuddin, 1996; McCarthy and Dhir, 1999), but also in other construction areas, such as highway road bases (Takada et al., 1995), grout mixes (Akram et al., 1994) and stabilizing clay based building materials (Temimi et al., 1995). In UK, where of the 10 MT pa of fly ash produced from power stations, annual consumption in concrete is around 1 MT (http://www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html), which reduces both ash disposal and raw material costs through replacing nearly 30% of the cement. The picture is similar in other countries, such as Australia, where from fly ash production of around 8 MT pa (http://www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html), annual incorporation into cement and concrete accounts for about 1 MT. In USA in 1990, of the 50 MT pa of fly ash produced, around 20% was reclaimed, with almost all going to the construction industry (http://www.civil.nwu.edu/ACBM/Cote/ faprod.htm). However, despite positive uses, the rate of production clearly far outweighs consumption. For the remaining material, disposal practices involve holding ponds, lagoons, landfills and slag heaps, all of which can be regarded as unsightly, environmentally undesirable and/or a non-productive use of land resources, as well as posing an on-going financial burden through their long-term maintenance. Furthermore, for those coal power plants located in urban areas, finding disposal sites is becoming increasingly more difficult. With competition for limited space and tightening of regulations on surface water and ground water discharge, any waste resulting from fly ash disposal sites must be well managed, so that local surface and ground water supplies are protected. This can cause significant economic burden to achieve the necessary water and land management. These factors have prompted researchers to look for alternative usages for fly ash, other than the cement and construction industry. This paper presents a review of the current situation regarding power station fly ash utilization, which include synthesis and application in materials, adsorbents and waste management, materials recovery and agriculture. 2. Structure of power station fly ash The primary components of power station fly ash are silica (SiO2), alumina (Al2O3) and iron oxides (Fe2O3), with varying amounts of carbon, calcium (as lime or gypsum), magnesium and sulfur (sulfides or sulfates). An empirical formula for fly ash based on the dominance of certain key elements has been proposed (http://www.civil.nwu.edu/ACBM/Cote/faprod.htm) as:
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Si1.0
Al0.45 Ca0.51 Na0.047 Fe0.039 Mg0.020 K0.013 Ti0.011
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(1)
Arsenic, mercury and antimony have also been reported, and the mineralogical structure of the ash is a key variable determining reactivity. Fly ashes are considered as pozzolans (substances containing silica and alumina) wherein the silica reacts with calcium hydroxide Ca(OH)2 released by hydration of calcium silicate to produce calcium silicate hydrate (Nonavinakere and Reed, 1995). It is when the silicate phases have an amorphous structure, rather than crystalline, that materials tend to be pozzolanic and contribute to the formation of hydration products when attacked by hydroxides. There are mainly two types of fly ash produced from coal combustion, types F and C. Type F is produced when anthracite, bituminous or sub-bituminous coal is burned and is low in lime (B7%) and contains more silica, alumina and iron oxide. Type C comes from lignite coal and contains more lime (15–30%) (Fischer et al., 1978). Fly ash has a hydrophillic surface and is extremely porous, with particle size the most important physical characteristic determining the reactivity. In general, smaller ash particles tend to be more reactive for two reasons. Firstly, smaller particles have larger specific areas making a large percentage of the particle available to attack by hydroxides. Secondly, and perhaps more significantly, smaller particles cool faster upon exiting the combustor, resulting in a more disordered and, therefore, reactive structure.
3. Zeolites from fly ash Fly ash from burning Amaga coal (originating from Colombia) was treated with sodium hydroxide at different concentrations and various times and temperatures of crystallization. The resulting zeolite produced contained 50–95% of a faujasite type material and had an adsorptive capacity equal to 70–80% of commercial zeolite (Mehta, 1989). Hydrothermal conditions and varying pH have also been used to synthesize zeolites (Mondargon et al., 1990; Lin and Hai, 1995; Shih et al., 1995; Zhao et al., 1997; Belardi et al., 1998; Classification of fly ash, 2000). An ion exchange type of synthesis to produce 45% zeolites from fly ash, resulting in a product suitable for application as an immobilizer for air pollutants (Stenbruggen and Holman, 1998).
4. Production of mullite A 1:1 mixture of pretreated fly ash and gamma alumina was heated to 1400°C and an 80% yield of mullite was achieved (Ohtake et al., 1991). The properties of sintered bodies of this mullite powder were almost comparable to those of a synthetic commercial grade mullite. Mullite synthesized from beneficiated fly ash and alumina has been also reported to be comparable to commercial mullite
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(Hwang et al., 1994). Class F fly ash was sintered at a temperature range of 800 – 1000°C in a microwave and by conventional sintering in normal ovens. Under identical temperature conditions, microwave sintered samples were found to be denser and stronger than the conventionally sintered material. The sintered product was a glass like ceramic material with mullite as the major crystalline phase (Fang et al., 1996). Indialite, a high temperature hexagonal form of cordierite, has been synthesized using fly ash as one of the starting raw materials. The properties of the synthesized material compare well with the values of conventional corderite (Sampathkumar et al., 1995).
5. Glass-like materials Glasses and glass ceramics were obtained by mixing up to 50% of Italian coal fly ash with glass cullet and float dolomite (Barbieri et al., 1999). The behavior of ten compositions was investigated by differential thermal analysis and X-ray diffraction and microstructural (SEM) characterization. It was verified that the contribution of the alkaline earth elements in the original composition was fundamental to obtaining glass ceramics with a fine microstructure that improves mechanical properties. With a small addition of fly ash and without dolomite, very stable glassy materials were obtained (Barbieri et al., 1999). Fly ash and waste glass were used with commercial alumina platelets as the reinforcing component (Boccaccini et al., 1997). For fly ash contents of up to 20% by weight, a dense compact material was fabricated by using relatively low sintering temperatures (650°C). When the fly ash content was increased above 20%, compaction was hindered due to the presence of crystalline particles in the fly ash. Fly ash used in the preparation of ceramic tableware and artware was found to increase the mechanical strength above that of the original products (Mukerji et al., 1993). Fly ash has also been used in the preparation of ceramic filters suitable for hot gas cleaning (Jo et al., 1996).
6. Composite materials The use of fly ash in plastic composites has shown promise (Jarvala and Jarvala, 1996; Verghese and Chaturvedi, 1996; Yildirim et al., 1996), as has application in metal composites, in particular aluminium (Rohatgi et al., 1995; Guo et al., 1996, 1997). Aluminium alloy fly ash composites made by stir casting exhibited similar or higher hardness and elastic modulii, and improved wear abrasive resistance and were considered potential materials for components like pulleys, oil pans, intake manifolds and valve covers (Guo and Rohatgi, 1997). Coatings made from nickel fly ash composites were found to possess better wear resistance than ‘plain’ nickel coating. The higher wear resistance of the composites was attributed to the excellent bonding between fly ash particles and the nickel (Ramesh and Iyer, 1991). Fly ash
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can be used to reduce the density of metal composites, with the cenospheres (hollow spherical particles) within the ash providing added buoyancy, better insulation properties, reduced shrinkage and warpage values (Wandell, 1996). Fly ash filled unsaturated polyester resin has been cast into sheets and tensile strength, flexural strength and flexural modulus compared with calcium carbonate filled polyester resin (Saroja Devi et al., 1998). The fly ash filled polyester resin was found to have a better flexural modulus than the calcium carbonate filled polyester resin. Fly ash from 0 – 50% content has also been mixed with post consumer polyethylene terephthalate (PET) to produce a molded composite material (Yadong et al., 1998). The fly ash reduced the thermal decomposition of PET, expedited the melting and mixing characteristics and reduced the shrinkage of material during the moulding process. The addition of fly ash also increased the compressive strength by 31 – 53% and water absorption was found to be negligible.
7. Adsorbents for waste management Adsorption of arsenic on fly ash was found to conform to Freundlich’s isotherm and the efficiency of adsorption was comparable to activated carbon (Sen and De, 1987). Arsenic was also successfully removed from samples of industrial wastewaters. Adsorption of cadmium and chromium from wastewater by adsorption onto fly ash was investigated to determine the effects of contact time, pH and temperature (Viraraghavan and Rao, 1995). An aqueous medium pH of 7–8 was found optimal for removal of cadmium and a pH of 2–3 for chromium. Batch adsorption experiments conducted at 5, 10 and 21°C showed that the adsorption capacity of fly ash decreased with increase in temperature. The maximum removal levels at 5°C of cadmium and chromium were 93 and 44%, respectively. Fly ash effectively adsorbed mercury from wastewater when the contact time was 2 h and the pH was 5 – 5.5 (Viraraghavan and Kapoor, 1992). When fly ash was treated with radionucleotide containing water, removal of cesium-137 was at a maximum at neutral pH, whereas strontium-90 adsorption increased with pH, especially above pH 8 (Resat et al., 1996). Tobermorites synthesized from oxides and from fly ashes has also been shown as a resource in the separation, immobilization and disposal of radioactive species, such as Cs and Sr (Ma et al., 1995). The acid-base properties of fly ash were found to be suitable for the removal of heavy metals such as nickel, cadmium, chromium, lead, copper, mercury and zinc from industrial wastewaters (e.g. electroplating and battery manufacture) (http:// www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html). Fly ash being readily available and inexpensive was considered an economic alternative to conventional adsorbents such as activated carbon and ion-exchange resins. Fly ashes in an acidified form were studied with respect to removing color and organic materials from the effluent of a municipal wastewater treatment plant (Vandenbusch and Sell, 1992). In the acidified situation, coagulation of colored colloids dissolved in the effluent takes place. Carbon sorption and calcium precipitation of tannins and humics varied with pH and the chemical composition of the
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fly ash used. Removal of phenol from waste water by adsorption on fly ash has been tried and compared with materials like peat and bentonite, the equilibrium time needed for adsorption onto fly ash was reduced by nearly two-thirds (Viraraghavan and Alfaro, 1998). The ionic bonding in the major constituents of fly ash (Al2O3 and SiO2) cause electronegativity to attract polar molecules such as phenols. This suggests a major reuse option for fly ash, as ‘traditional’ disinfection of industrial wastewaters by chlorination may produce chlorophenols if phenol is present in the water. The removal of fluorides with fly ash has been reported to be favorable at high temperature and acidic pH (Chaturvedi et al., 1990) and chrome dyes have been adsorbed onto a mixture of fly ash and coal (Gupta et al., 1990). It was shown that low adsorbate concentration, small particle size of adsorbent, low temperature and acidic medium favored dye removal. For NOx removal, adsorption was closely examined in terms of carbon content and specific surface area (Lu and Do, 1991). It was found that unburnt carbon remaining in the fly ash particles and appearing on the surface could be activated to further improve the adsorption performance. Activation of coarse fly ash particles showed that the adsorption capacity could also be increased through controlled gasification of the unburnt carbon. In laboratory studies, toluene vapor was adsorbed onto fly ash and it was shown that regeneration of saturated samples was possible (Rovati et al., 1988). The adsorptive capacity of fly ash for vapors was increased by means of a new way of aggregation using residues from the food industry, namely the concentrate of vegetation water from olive mills (Rovatti et al., 1992). The solid product obtained by aggregation was pyrolyzed and activated in order to obtain an adsorbent material. The pyrolysis produced a liquid oily fraction, with good calorific value, high hydrogen content gas and a carbonecous dry matrix with good microstructural characteristics. It was subsequently utilized for adsorption of toluene vapor with promising results (Rovatti et al., 1992).
8. Waste stabilization Waste oil and gas well sludges consist of drill mud and formation cuttings, as well as various hydrocarbons. Laboratory tests indicate that an oil and gas sludge in a semi liquid state with solids content greater than 30% could be solidified using cement and fly ash mixtures (15% of each on a dry weight basis) (Joshi et al., 1995). Fly ash has also been used as a liner material in waste disposal sites to augment the containment capability of the existing soil (Morati et al., 1987). The resulting liner material had low permeability and high strength. Initial results indicated that fly ash liners could represent a cost effective alternative for waste disposal site construction where suitable soils are not available. Fly ash as a prefilter material for the retention of lead ions, has been tried for improving the performance of clay liners widely used to contain toxic and hazardous waste materials (Pandian et al., 1996). At a pH greater than 5.5, fly ash
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retained lead ions through precipitation in the pores as well as onto the surface, whereas at a pH less than 5.5, lead ion retention was through adsorption. Fly ash when used as a binder or in combination with cements, helped the agglomeration process in preventing acid mine drainage from mine tailings (Misra et al., 1996). It immobilized the reactive components of the mine tailings to form strong pellets, which remained resistant to weathering and leaching.
9. Materials recovery A study conducted by the US Department of Energy in 1983–1984 produced a conceptual commercial scale design and financial estimate for a direct acid leaching process to recover various raw materials from fly ash, with the rate of return of around 20% of the investment (Golden and Wilder, 1985). It was estimated that by processing 1180 000 t pa of fly ash, 158 000 t of alumina, 102 000 t of ferric oxide, 46 000 t pa of gypsum, 81 000 t pa of alkali sulfate salts and 866 000 t pa of spent fly ash would be obtained. An excess cogeneration of 1940 MWh of power from a commercial scale plant would also be obtained. More recently, it has been reported that with the use of an extended arc reactor furnace, up to 90% of the iron in the fly ash could be recovered and a ferrosilicon alloy produced with a silicon content of up to 40% (Pickles et al., 1990). Hexavalent chromium has been also successfully continuously separated from fly ash at acidic pH values using a packed bed of fly ash pellets mixed with kaolin as the binder (Dasmahaputra et al., 1998). Investigations were carried out on different fly ashes samples (Al2O3/SiO2 weight ratio in the range 0.4 – 0.78 and a Fe2O3 content of 4.8–9.9%) mixed with limestones of different CaO content (51.7, 53.8, and 55.3%) (Konik et al., 1994). A self-disintegrating sinter synthesis was then carried out to obtain SiO2 well bound in 2CaO.SiO2 and Al2O3 in 12CaO.7Al2O3.3Fe2O3. From the dust obtained, alumina was extracted by sodium carbonate water solution. The higher the fly ash Al2O3/ SiO2 ratio and the higher the limestone CaO content, the higher Al2O3/SiO2 level in the self-disintegrated dust. Fly ash could, therefore, be a useful resource where bauxite is not available (O’Connor, 1985). In other work, treatment of fly ashes resulted in reportedly good levels of gallium recovery (Fang and Geaser, 1996) and vanadium, nickel and magnesium (Kuniaki et al., 1998). The study of complex treatment of alumina-silica containing fly ash by chemical enrichment has been reported (Lin et al., 1998). The removal of silica from aluminium containing raw material with high silica content is based on the property of hydroaluminium silicates, basic silica containing materials to undergo intramolecular phase changes by thermal treatment, resulting in the formation of amorphous silica. Leaching the heat-treated raw material with sodium hydroxide solution effected dissolution of silica. Al2O3 was recovered in the solid state in the form of alpha or gamma alumina. The use of chemical enrichment enabled reduction of the silica content in the fly ash and utilization of the resulting material for alumina production.
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10. Agriculture A greenhouse study conducted to evaluate ameliorating the low pH of acidic coal mine soils showed that lime and fly ash significantly increased the soil pH, above ground plant biomass and root biomass (Taylor and Schuman, 1998). Topsoil placement over the spoil also generally increased plant biomass, but root growth in untreated soil was limited to the depth of the topsoil. However, when the soil was amended with either fly ash or lime, root growth occurred throughout the material (Keefer and Singh, 1985). Fly ash has been utilized as a base for construction of research field plots for growing corn (Sajwan et al., 1996). On the top of the base, two mixes were tried: (i) 58% fly ash+ 42% soil and (ii) 79% fly ash+ 21% soil. Individual plots received additives of cow manure at 0, 60 or 120 tonnes/ha, sewage sludge at 0, 60, or 120 tonnes/ha or chicken manure 0, 25, or 50 tonnes/ha. Corn yields and fodder grown in the field on a growth medium with 58% fly ash in the mix, yielded more than the control plots. The greater yield was attributed to the higher water holding capacity of the fly ash. However, there was boron present in corn leaves when 79% fly ash mix was used. In another study, sorghum sudangrass grown on fly ash/organic waste amended soils, showed an increase in yield with applications of up to 50 tonnes/acre, but decreased at higher application levels (Kukier and Sumner, 1994). This decrease in yield was attributed to accumulation of boron and zinc to phytotoxic levels. From studies on the impact of using fly ash on corn growth, an increase in water extractable boron was found with a decrease in pH (Kukier and Sumner, 1994). Clay topsoil that tends to form surface crusts was mixed with unweathered fly ash produced selenium in barley when excess of fly ash was used (Sale et al., 1996). Further, there was a marked increase in molybdenum that can be detrimental to ruminant diet. The incorporation of fly ash on soil properties and growth yield of wheat, mustard, rice and maize was reported (Kalra et al., 1998). The addition of fly ash to rice fields showed no improvement on yield, but there was marked improvement in case of maize (addition of 10 tonne/ha), wheat (addition of 20 tonne/ha) and mustard (addition of 10 tonne/ha). Fly ash addition to soil lowered bulk density, reduced hydraulic conductivity and improved moisture retention. Fly ash with high calcium reduces the release of soil phosphorus to outer surfaces of the soil (Stout et al., 1999). Excessive soil phosphorus levels cause high concentrations of watersoluble phosphorus in soil, thereby increasing the potential for phosphorous export to water-courses. Fly ash has been shown to increase the water holding capacity of soils and hence the amount of available water to the plant (Menzies and Aitken, 1996). The addition of 5% fly ash to soil was also found to significantly increase the growth of tomato plants and reduced the amount of galling on the roots caused by root-knot nematode (Ahmad and Alam, 1997). The use of 30% fly ash was found to reduce the penetration and reproductive potential of root-knot nematode on tomatoes (Khan et al., 1997) and that 40 –50% fly ash not only reduced nematode damage,
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but also dramatically increased the yield of tomato plants relative to unamended soil. Similarly, the use of volcanic ash was able to reduce the amount of damage caused by burrowing nematodes to Anthuriums (Wang et al., 1997). The ash was able to replace organic matter in the potting medium, which was thought to be responsible for increased nematode damage.
11. Conclusions Fly ash from coal fired power stations has proved to have significant value-added potential as an ingredient in cementitious and other construction process, However, in many cases these markets are close to saturation, plus they are vulnerable to economic down-turn in the building industry. Furthermore, in most countries at least 75% the fly ash generated annually is dumped with no subsequent reutilization. This poses a significant economic and environmental burden, but can also be viewed as an untapped resource of huge potential. As a consequence, there exists significant interest in developing other avenues for commercial exploitation of the material, particularly as a substitute for other (dwindling) resources. This review has shown that whilst in most cases reported work to date in this area has been of laboratory scale and further development work is needed, fly ash nevertheless does have a potential role as a value-added product in material preparation, waste management, recovery of materials and agricultural applications. However, whilst there are many promising options, what also arose from the review was that the key issue of critical comparison between fly ash generated products and processes, and those derived from more ‘traditional’ routes has not been extensively tackled. More analysis of long-term economic and environmental impacts, possibly through employing life cycle assessments (LCAs) is needed. In particular, there is a scarcity of information with regards detailed studies on the environmental impact from use of fly ash as an ingredient in the preparation of materials, such as glass and ceramics. Although for example, in describing the synthesis of mesoporous aluminosilicate from fused fly ash solutions (Chang et al., 1999), mention was made of (unspecified) impurities in the end product, neither they, nor their potential environmental impact, were fully detailed or assessed. Without addressing these issues, many promising ideas will not find full-scale application due to restrictions imposed by the regulators.
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Power station fly ash — a review of value-added utilization outside of the construction industry R.S. Iyer 1, J.A. Scott * Center for Integrated En6ironmental Protection, Griffith Uni6ersity, Brisbane, Qld 4111, Australia Received 27 August 1999; accepted 20 July 2000
Abstract The disposal of fly ash from coal-fired power stations causes significant economic and environmental problems. A relatively small percentage of the material finds application as an ingredient in cement and other construction products, but the vast majority of material generated each year is held in ash dams or similar dumps. This unproductive use of land and the associated long-term financial burden of maintenance has led to realization that alternative uses for fly ash as a value-added product beyond incorporation in construction materials are needed. Utilization of fly ash in such areas as novel materials, waste management, recovery of metals and agriculture is reviewed in this article with the aim of looking at new areas that will expand the positive reuse of fly ash, thereby helping to reduce the environmental and economic impacts of disposal. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Power station; Fly ash; Utilization; Materials; Waste management; Materials recovery; Agriculture
* Corresponding author. Tel.: +61-7-38753661; fax: +61-7-38755299. E-mail address: [email protected] (J.A. Scott). 1 Present address: Department of Chemical Engineering, University of Queensland, St Lucia, Qld 4068, Australia 0921-3449/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 3 4 4 9 ( 0 0 ) 0 0 0 8 4 - 7
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1. Introduction Disposal of fly ash as a by-product of incineration of coal, municipal solid wastes, sugar cane bagasse, rice husks and tea dusts, is becoming an increasing economic and environmental burden. As a consequence, there is a growing interest in looking for avenues where the material can be used as a potential resource for preparation of value added products. The majority of fly ash is generated by coal fired power stations and a percentage (typically 10–20%) does find reuse, primarily in cementitious (concrete and cement) products (Al-Almoudi and Maslehuddin, 1996; McCarthy and Dhir, 1999), but also in other construction areas, such as highway road bases (Takada et al., 1995), grout mixes (Akram et al., 1994) and stabilizing clay based building materials (Temimi et al., 1995). In UK, where of the 10 MT pa of fly ash produced from power stations, annual consumption in concrete is around 1 MT (http://www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html), which reduces both ash disposal and raw material costs through replacing nearly 30% of the cement. The picture is similar in other countries, such as Australia, where from fly ash production of around 8 MT pa (http://www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html), annual incorporation into cement and concrete accounts for about 1 MT. In USA in 1990, of the 50 MT pa of fly ash produced, around 20% was reclaimed, with almost all going to the construction industry (http://www.civil.nwu.edu/ACBM/Cote/ faprod.htm). However, despite positive uses, the rate of production clearly far outweighs consumption. For the remaining material, disposal practices involve holding ponds, lagoons, landfills and slag heaps, all of which can be regarded as unsightly, environmentally undesirable and/or a non-productive use of land resources, as well as posing an on-going financial burden through their long-term maintenance. Furthermore, for those coal power plants located in urban areas, finding disposal sites is becoming increasingly more difficult. With competition for limited space and tightening of regulations on surface water and ground water discharge, any waste resulting from fly ash disposal sites must be well managed, so that local surface and ground water supplies are protected. This can cause significant economic burden to achieve the necessary water and land management. These factors have prompted researchers to look for alternative usages for fly ash, other than the cement and construction industry. This paper presents a review of the current situation regarding power station fly ash utilization, which include synthesis and application in materials, adsorbents and waste management, materials recovery and agriculture. 2. Structure of power station fly ash The primary components of power station fly ash are silica (SiO2), alumina (Al2O3) and iron oxides (Fe2O3), with varying amounts of carbon, calcium (as lime or gypsum), magnesium and sulfur (sulfides or sulfates). An empirical formula for fly ash based on the dominance of certain key elements has been proposed (http://www.civil.nwu.edu/ACBM/Cote/faprod.htm) as:
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Si1.0
Al0.45 Ca0.51 Na0.047 Fe0.039 Mg0.020 K0.013 Ti0.011
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(1)
Arsenic, mercury and antimony have also been reported, and the mineralogical structure of the ash is a key variable determining reactivity. Fly ashes are considered as pozzolans (substances containing silica and alumina) wherein the silica reacts with calcium hydroxide Ca(OH)2 released by hydration of calcium silicate to produce calcium silicate hydrate (Nonavinakere and Reed, 1995). It is when the silicate phases have an amorphous structure, rather than crystalline, that materials tend to be pozzolanic and contribute to the formation of hydration products when attacked by hydroxides. There are mainly two types of fly ash produced from coal combustion, types F and C. Type F is produced when anthracite, bituminous or sub-bituminous coal is burned and is low in lime (B7%) and contains more silica, alumina and iron oxide. Type C comes from lignite coal and contains more lime (15–30%) (Fischer et al., 1978). Fly ash has a hydrophillic surface and is extremely porous, with particle size the most important physical characteristic determining the reactivity. In general, smaller ash particles tend to be more reactive for two reasons. Firstly, smaller particles have larger specific areas making a large percentage of the particle available to attack by hydroxides. Secondly, and perhaps more significantly, smaller particles cool faster upon exiting the combustor, resulting in a more disordered and, therefore, reactive structure.
3. Zeolites from fly ash Fly ash from burning Amaga coal (originating from Colombia) was treated with sodium hydroxide at different concentrations and various times and temperatures of crystallization. The resulting zeolite produced contained 50–95% of a faujasite type material and had an adsorptive capacity equal to 70–80% of commercial zeolite (Mehta, 1989). Hydrothermal conditions and varying pH have also been used to synthesize zeolites (Mondargon et al., 1990; Lin and Hai, 1995; Shih et al., 1995; Zhao et al., 1997; Belardi et al., 1998; Classification of fly ash, 2000). An ion exchange type of synthesis to produce 45% zeolites from fly ash, resulting in a product suitable for application as an immobilizer for air pollutants (Stenbruggen and Holman, 1998).
4. Production of mullite A 1:1 mixture of pretreated fly ash and gamma alumina was heated to 1400°C and an 80% yield of mullite was achieved (Ohtake et al., 1991). The properties of sintered bodies of this mullite powder were almost comparable to those of a synthetic commercial grade mullite. Mullite synthesized from beneficiated fly ash and alumina has been also reported to be comparable to commercial mullite
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(Hwang et al., 1994). Class F fly ash was sintered at a temperature range of 800 – 1000°C in a microwave and by conventional sintering in normal ovens. Under identical temperature conditions, microwave sintered samples were found to be denser and stronger than the conventionally sintered material. The sintered product was a glass like ceramic material with mullite as the major crystalline phase (Fang et al., 1996). Indialite, a high temperature hexagonal form of cordierite, has been synthesized using fly ash as one of the starting raw materials. The properties of the synthesized material compare well with the values of conventional corderite (Sampathkumar et al., 1995).
5. Glass-like materials Glasses and glass ceramics were obtained by mixing up to 50% of Italian coal fly ash with glass cullet and float dolomite (Barbieri et al., 1999). The behavior of ten compositions was investigated by differential thermal analysis and X-ray diffraction and microstructural (SEM) characterization. It was verified that the contribution of the alkaline earth elements in the original composition was fundamental to obtaining glass ceramics with a fine microstructure that improves mechanical properties. With a small addition of fly ash and without dolomite, very stable glassy materials were obtained (Barbieri et al., 1999). Fly ash and waste glass were used with commercial alumina platelets as the reinforcing component (Boccaccini et al., 1997). For fly ash contents of up to 20% by weight, a dense compact material was fabricated by using relatively low sintering temperatures (650°C). When the fly ash content was increased above 20%, compaction was hindered due to the presence of crystalline particles in the fly ash. Fly ash used in the preparation of ceramic tableware and artware was found to increase the mechanical strength above that of the original products (Mukerji et al., 1993). Fly ash has also been used in the preparation of ceramic filters suitable for hot gas cleaning (Jo et al., 1996).
6. Composite materials The use of fly ash in plastic composites has shown promise (Jarvala and Jarvala, 1996; Verghese and Chaturvedi, 1996; Yildirim et al., 1996), as has application in metal composites, in particular aluminium (Rohatgi et al., 1995; Guo et al., 1996, 1997). Aluminium alloy fly ash composites made by stir casting exhibited similar or higher hardness and elastic modulii, and improved wear abrasive resistance and were considered potential materials for components like pulleys, oil pans, intake manifolds and valve covers (Guo and Rohatgi, 1997). Coatings made from nickel fly ash composites were found to possess better wear resistance than ‘plain’ nickel coating. The higher wear resistance of the composites was attributed to the excellent bonding between fly ash particles and the nickel (Ramesh and Iyer, 1991). Fly ash
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can be used to reduce the density of metal composites, with the cenospheres (hollow spherical particles) within the ash providing added buoyancy, better insulation properties, reduced shrinkage and warpage values (Wandell, 1996). Fly ash filled unsaturated polyester resin has been cast into sheets and tensile strength, flexural strength and flexural modulus compared with calcium carbonate filled polyester resin (Saroja Devi et al., 1998). The fly ash filled polyester resin was found to have a better flexural modulus than the calcium carbonate filled polyester resin. Fly ash from 0 – 50% content has also been mixed with post consumer polyethylene terephthalate (PET) to produce a molded composite material (Yadong et al., 1998). The fly ash reduced the thermal decomposition of PET, expedited the melting and mixing characteristics and reduced the shrinkage of material during the moulding process. The addition of fly ash also increased the compressive strength by 31 – 53% and water absorption was found to be negligible.
7. Adsorbents for waste management Adsorption of arsenic on fly ash was found to conform to Freundlich’s isotherm and the efficiency of adsorption was comparable to activated carbon (Sen and De, 1987). Arsenic was also successfully removed from samples of industrial wastewaters. Adsorption of cadmium and chromium from wastewater by adsorption onto fly ash was investigated to determine the effects of contact time, pH and temperature (Viraraghavan and Rao, 1995). An aqueous medium pH of 7–8 was found optimal for removal of cadmium and a pH of 2–3 for chromium. Batch adsorption experiments conducted at 5, 10 and 21°C showed that the adsorption capacity of fly ash decreased with increase in temperature. The maximum removal levels at 5°C of cadmium and chromium were 93 and 44%, respectively. Fly ash effectively adsorbed mercury from wastewater when the contact time was 2 h and the pH was 5 – 5.5 (Viraraghavan and Kapoor, 1992). When fly ash was treated with radionucleotide containing water, removal of cesium-137 was at a maximum at neutral pH, whereas strontium-90 adsorption increased with pH, especially above pH 8 (Resat et al., 1996). Tobermorites synthesized from oxides and from fly ashes has also been shown as a resource in the separation, immobilization and disposal of radioactive species, such as Cs and Sr (Ma et al., 1995). The acid-base properties of fly ash were found to be suitable for the removal of heavy metals such as nickel, cadmium, chromium, lead, copper, mercury and zinc from industrial wastewaters (e.g. electroplating and battery manufacture) (http:// www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html). Fly ash being readily available and inexpensive was considered an economic alternative to conventional adsorbents such as activated carbon and ion-exchange resins. Fly ashes in an acidified form were studied with respect to removing color and organic materials from the effluent of a municipal wastewater treatment plant (Vandenbusch and Sell, 1992). In the acidified situation, coagulation of colored colloids dissolved in the effluent takes place. Carbon sorption and calcium precipitation of tannins and humics varied with pH and the chemical composition of the
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fly ash used. Removal of phenol from waste water by adsorption on fly ash has been tried and compared with materials like peat and bentonite, the equilibrium time needed for adsorption onto fly ash was reduced by nearly two-thirds (Viraraghavan and Alfaro, 1998). The ionic bonding in the major constituents of fly ash (Al2O3 and SiO2) cause electronegativity to attract polar molecules such as phenols. This suggests a major reuse option for fly ash, as ‘traditional’ disinfection of industrial wastewaters by chlorination may produce chlorophenols if phenol is present in the water. The removal of fluorides with fly ash has been reported to be favorable at high temperature and acidic pH (Chaturvedi et al., 1990) and chrome dyes have been adsorbed onto a mixture of fly ash and coal (Gupta et al., 1990). It was shown that low adsorbate concentration, small particle size of adsorbent, low temperature and acidic medium favored dye removal. For NOx removal, adsorption was closely examined in terms of carbon content and specific surface area (Lu and Do, 1991). It was found that unburnt carbon remaining in the fly ash particles and appearing on the surface could be activated to further improve the adsorption performance. Activation of coarse fly ash particles showed that the adsorption capacity could also be increased through controlled gasification of the unburnt carbon. In laboratory studies, toluene vapor was adsorbed onto fly ash and it was shown that regeneration of saturated samples was possible (Rovati et al., 1988). The adsorptive capacity of fly ash for vapors was increased by means of a new way of aggregation using residues from the food industry, namely the concentrate of vegetation water from olive mills (Rovatti et al., 1992). The solid product obtained by aggregation was pyrolyzed and activated in order to obtain an adsorbent material. The pyrolysis produced a liquid oily fraction, with good calorific value, high hydrogen content gas and a carbonecous dry matrix with good microstructural characteristics. It was subsequently utilized for adsorption of toluene vapor with promising results (Rovatti et al., 1992).
8. Waste stabilization Waste oil and gas well sludges consist of drill mud and formation cuttings, as well as various hydrocarbons. Laboratory tests indicate that an oil and gas sludge in a semi liquid state with solids content greater than 30% could be solidified using cement and fly ash mixtures (15% of each on a dry weight basis) (Joshi et al., 1995). Fly ash has also been used as a liner material in waste disposal sites to augment the containment capability of the existing soil (Morati et al., 1987). The resulting liner material had low permeability and high strength. Initial results indicated that fly ash liners could represent a cost effective alternative for waste disposal site construction where suitable soils are not available. Fly ash as a prefilter material for the retention of lead ions, has been tried for improving the performance of clay liners widely used to contain toxic and hazardous waste materials (Pandian et al., 1996). At a pH greater than 5.5, fly ash
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retained lead ions through precipitation in the pores as well as onto the surface, whereas at a pH less than 5.5, lead ion retention was through adsorption. Fly ash when used as a binder or in combination with cements, helped the agglomeration process in preventing acid mine drainage from mine tailings (Misra et al., 1996). It immobilized the reactive components of the mine tailings to form strong pellets, which remained resistant to weathering and leaching.
9. Materials recovery A study conducted by the US Department of Energy in 1983–1984 produced a conceptual commercial scale design and financial estimate for a direct acid leaching process to recover various raw materials from fly ash, with the rate of return of around 20% of the investment (Golden and Wilder, 1985). It was estimated that by processing 1180 000 t pa of fly ash, 158 000 t of alumina, 102 000 t of ferric oxide, 46 000 t pa of gypsum, 81 000 t pa of alkali sulfate salts and 866 000 t pa of spent fly ash would be obtained. An excess cogeneration of 1940 MWh of power from a commercial scale plant would also be obtained. More recently, it has been reported that with the use of an extended arc reactor furnace, up to 90% of the iron in the fly ash could be recovered and a ferrosilicon alloy produced with a silicon content of up to 40% (Pickles et al., 1990). Hexavalent chromium has been also successfully continuously separated from fly ash at acidic pH values using a packed bed of fly ash pellets mixed with kaolin as the binder (Dasmahaputra et al., 1998). Investigations were carried out on different fly ashes samples (Al2O3/SiO2 weight ratio in the range 0.4 – 0.78 and a Fe2O3 content of 4.8–9.9%) mixed with limestones of different CaO content (51.7, 53.8, and 55.3%) (Konik et al., 1994). A self-disintegrating sinter synthesis was then carried out to obtain SiO2 well bound in 2CaO.SiO2 and Al2O3 in 12CaO.7Al2O3.3Fe2O3. From the dust obtained, alumina was extracted by sodium carbonate water solution. The higher the fly ash Al2O3/ SiO2 ratio and the higher the limestone CaO content, the higher Al2O3/SiO2 level in the self-disintegrated dust. Fly ash could, therefore, be a useful resource where bauxite is not available (O’Connor, 1985). In other work, treatment of fly ashes resulted in reportedly good levels of gallium recovery (Fang and Geaser, 1996) and vanadium, nickel and magnesium (Kuniaki et al., 1998). The study of complex treatment of alumina-silica containing fly ash by chemical enrichment has been reported (Lin et al., 1998). The removal of silica from aluminium containing raw material with high silica content is based on the property of hydroaluminium silicates, basic silica containing materials to undergo intramolecular phase changes by thermal treatment, resulting in the formation of amorphous silica. Leaching the heat-treated raw material with sodium hydroxide solution effected dissolution of silica. Al2O3 was recovered in the solid state in the form of alpha or gamma alumina. The use of chemical enrichment enabled reduction of the silica content in the fly ash and utilization of the resulting material for alumina production.
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10. Agriculture A greenhouse study conducted to evaluate ameliorating the low pH of acidic coal mine soils showed that lime and fly ash significantly increased the soil pH, above ground plant biomass and root biomass (Taylor and Schuman, 1998). Topsoil placement over the spoil also generally increased plant biomass, but root growth in untreated soil was limited to the depth of the topsoil. However, when the soil was amended with either fly ash or lime, root growth occurred throughout the material (Keefer and Singh, 1985). Fly ash has been utilized as a base for construction of research field plots for growing corn (Sajwan et al., 1996). On the top of the base, two mixes were tried: (i) 58% fly ash+ 42% soil and (ii) 79% fly ash+ 21% soil. Individual plots received additives of cow manure at 0, 60 or 120 tonnes/ha, sewage sludge at 0, 60, or 120 tonnes/ha or chicken manure 0, 25, or 50 tonnes/ha. Corn yields and fodder grown in the field on a growth medium with 58% fly ash in the mix, yielded more than the control plots. The greater yield was attributed to the higher water holding capacity of the fly ash. However, there was boron present in corn leaves when 79% fly ash mix was used. In another study, sorghum sudangrass grown on fly ash/organic waste amended soils, showed an increase in yield with applications of up to 50 tonnes/acre, but decreased at higher application levels (Kukier and Sumner, 1994). This decrease in yield was attributed to accumulation of boron and zinc to phytotoxic levels. From studies on the impact of using fly ash on corn growth, an increase in water extractable boron was found with a decrease in pH (Kukier and Sumner, 1994). Clay topsoil that tends to form surface crusts was mixed with unweathered fly ash produced selenium in barley when excess of fly ash was used (Sale et al., 1996). Further, there was a marked increase in molybdenum that can be detrimental to ruminant diet. The incorporation of fly ash on soil properties and growth yield of wheat, mustard, rice and maize was reported (Kalra et al., 1998). The addition of fly ash to rice fields showed no improvement on yield, but there was marked improvement in case of maize (addition of 10 tonne/ha), wheat (addition of 20 tonne/ha) and mustard (addition of 10 tonne/ha). Fly ash addition to soil lowered bulk density, reduced hydraulic conductivity and improved moisture retention. Fly ash with high calcium reduces the release of soil phosphorus to outer surfaces of the soil (Stout et al., 1999). Excessive soil phosphorus levels cause high concentrations of watersoluble phosphorus in soil, thereby increasing the potential for phosphorous export to water-courses. Fly ash has been shown to increase the water holding capacity of soils and hence the amount of available water to the plant (Menzies and Aitken, 1996). The addition of 5% fly ash to soil was also found to significantly increase the growth of tomato plants and reduced the amount of galling on the roots caused by root-knot nematode (Ahmad and Alam, 1997). The use of 30% fly ash was found to reduce the penetration and reproductive potential of root-knot nematode on tomatoes (Khan et al., 1997) and that 40 –50% fly ash not only reduced nematode damage,
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but also dramatically increased the yield of tomato plants relative to unamended soil. Similarly, the use of volcanic ash was able to reduce the amount of damage caused by burrowing nematodes to Anthuriums (Wang et al., 1997). The ash was able to replace organic matter in the potting medium, which was thought to be responsible for increased nematode damage.
11. Conclusions Fly ash from coal fired power stations has proved to have significant value-added potential as an ingredient in cementitious and other construction process, However, in many cases these markets are close to saturation, plus they are vulnerable to economic down-turn in the building industry. Furthermore, in most countries at least 75% the fly ash generated annually is dumped with no subsequent reutilization. This poses a significant economic and environmental burden, but can also be viewed as an untapped resource of huge potential. As a consequence, there exists significant interest in developing other avenues for commercial exploitation of the material, particularly as a substitute for other (dwindling) resources. This review has shown that whilst in most cases reported work to date in this area has been of laboratory scale and further development work is needed, fly ash nevertheless does have a potential role as a value-added product in material preparation, waste management, recovery of materials and agricultural applications. However, whilst there are many promising options, what also arose from the review was that the key issue of critical comparison between fly ash generated products and processes, and those derived from more ‘traditional’ routes has not been extensively tackled. More analysis of long-term economic and environmental impacts, possibly through employing life cycle assessments (LCAs) is needed. In particular, there is a scarcity of information with regards detailed studies on the environmental impact from use of fly ash as an ingredient in the preparation of materials, such as glass and ceramics. Although for example, in describing the synthesis of mesoporous aluminosilicate from fused fly ash solutions (Chang et al., 1999), mention was made of (unspecified) impurities in the end product, neither they, nor their potential environmental impact, were fully detailed or assessed. Without addressing these issues, many promising ideas will not find full-scale application due to restrictions imposed by the regulators.
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