The surface chemistry of leaching coal fly ash
Introduction
The disposal of fly ash as a by-product of incineration of coal, is a more significant problem than ash produced from burning of municipal solid wastes, sugarcane bagasse, rice husks or tea dusts because of its volume. Fly ash finds reuse primarily in cementitious products [1], [2], construction areas such as highway road bases [3] grout mixes [4] stabilizing clay based building materials [5]. Utilization of fly ash in materials other than the construction industry has been reviewed by Iyer and Scott [6]. However despite positive uses, the rate of production is greater than the consumption. For the unused fly ash, disposal practice involves holding ponds, lagoons, landfills and slag heaps. There are coal based power plants situated in places where disposal sites are difficult to locate. The limited availability of space and tightening of regulations of leachate water and the subsequent discharge requires prudent management to satisfy regulatory bodies. The surface chemistry of leaching coal fly ash (CFA) during disposal and value addition is presented in this review.
Section snippets
Size dependent leaching properties of fly ash
Coal fly ash is a heterogeneous material, both between particles and within the particle [7]. The formation of inorganic pollutants during coal combustion has been reviewed [8]. A detailed study on the elements versus particle size distribution shows that there is an inverse dependence on concentration with respect to particle size for some elements. Elements were divided into two groups on the basis of their concentration dependence upon particle size, those with no enrichment in the smallest
Leachate chemistry
There are two types of fly ashes produced from coal combustion, types F and C. Type F is produced when anthracite, bituminous or sub-bituminous coal is burnt and is low in lime (<7%) and contains more silica, alumna and iron oxide. Type C comes from lignite coal and contains more lime (18%). The major oxides of types C and F fly ash is as follows (http://www.cpmash.com/ash/flyash.html ). The minor oxides are not mentioned.
Oxide (%) | Type F | Type C |
SiO2 | 49.90 | 53.79 |
Al2O3 | 16.25 | 16.42 |
Fe2O3 | 22.31 | 5.00 |
TiO2 | 1.09 |
The leaching of CFA during disposal
There are two methods of disposing fly ash, one as a dilute slurry and the other as a dense slurry. For a dilute slurry disposal system effective water management techniques are required. The dilute slurry disposal even though still in use in several countries has been effectively replaced by a dense slurry disposal system at the Stanwell power station in Queensland, Australia [24]. In this process the ash, rather than being sluiced from hoppers with a stream of water, is added to a stirred
Discussion
In this review the surface chemistry and mechanism of leaching of CFA has been presented. The leaching of elements, both toxic and non-toxic during disposal has been a focal point of many publications. The delay in achieving a steady state during disposal and the self-inhibition during value addition by leaching with sulfuric acid has interesting conclusions.
The mechanism of leaching during disposal attributes the retention of ions in the diffuse double layer and the subsequent resistance it
Conclusions
- (1)
The heterogenity leading to the wide range of elements leached from CFA is highlighted in this review.
- (2)
The particle size distribution being constant after the leaching process proves that the surface of fly ash particle, a few microns in thickness is wholly involved in leaching. Therefore the charge on the surface of fly ash particle and formation of the diffuse double layer plays a significant role in leaching.
- (3)
Delayed leaching, the similarity in both the approaches is due to resistance provided
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