(74d) Coal Fly Ash: from Waste to Industrial Product

Most fly ashes currently produced in coal-fired power stations with and without co-combustion contain leachable trace elements in amounts higher than allowed by most prevailing environmental standards for its free application as granular building material. Forced leaching using water, citrate-, oxalate-, EDTA- and carbonate-solutions were successful in removing substantial amounts of the most problematic components (Mo, Se, Sb, Cr, V and SO42-). Although the environmental quality of the washed fly ash was improved, remaining extractants in the moisture after filtration, and cleaning of the extraction solutions, may lead to new environmental problems. Also the economics of the proposed process seems not favourable. Therefore, fly ashes are still mainly being used in bound form in which the leachable components are immobilized, either by physical encapsulation or by chemical incorporation. One of the obvious matrices are available in the cement and concrete industry.

However, coal fly ash is a beautiful material in the sense of being mainly composed of nearly perfect spheres of Al-silicate glass that disserves applications making use of such properties, rather than to be used as an inert filler. Further, it could be a major source of Si and Al for the synthesis of industrial minerals. Sphericity in many cases is an important feature for fillers in the polymer industry, therefore opening new and exciting opportunities to the application of fly ash products in the area of rubber, PVC, PE, PP, polyester resins and paint. Although incidental success stories are published, a systematic approach on investigating the influence of ash properties on polymer properties for the various different polymer types and the enormous amount of possible formulations has not yet been undertaken. A few attempts into this direction are shown and the way how to proceed to come to the establishment of fly ash specifications applicable for a particular polymer product is shown. Up to now, such clear specifications only exist for the cement and concrete industry as a result of a very long history of trial and error.

Zeolites may be synthesised by reacting ash directly with NaOH solutions under hydrothermal conditions. However, zeolites grow on the surface of ash particles thereby encapsulating these particles, resulting the reaction to cease before it comes to completion. The proposed alternative is to perform the conversion in two steps, first Si-Al extraction and solid-liquid separation followed by crystallisation of zeolites from the extracts. In this way a pure zeolite without residual ash is produced and zeolite species can be tailored by adjusting the Si:Al ratio in the extracts through minor additions.

By performing the alkaline activation reaction with fly ash in such a way that zeolite crystallisation does not occur, but instead only a minor skin of particles is initially dissolved to form an interstitial gel that hardens at low temperature, a new binder material can be produced. The gel polymerises into a geopolymer, an inorganic polymeric material with a chemical composition similar to zeolite but possessing an amorphous structure. In a simple way matrices bound by geopolymeric cements may be seen as man-made rocks. Usually, geopolymers are synthesized at relatively low temperatures from meta-kaolinite, whereby the geopolymerization reaction is favoured by its amorphous state. The Si/Al ratio, the predominant amorphous structure, and the fine particle size of fly ashes, are appropriate for the synthesis of geopolymers. In general, the smaller the particle size of the starting material the higher the reactivity and the higher the geopolymerization rate will be. Therefore, particle size separation and micro-grinding techniques are use as a pre-treatment. Due to their low porosity highly-packed microstructure, high-temperature resistance, water resistance, acid resistance, low thermal expansion and fire-proofness, geopolymers can be used in many applications, among which binders in certain speciality cements and for immobilization, stabilization and solidification of a large number of materials, are best known. In this research, the aim is to produce geopolymeric matrices for the immobilisation of hazardous waste such as Electric Arc Furnace dust. Leachable amounts of trace metals from both ash and dust are being chemically bound into the geopolymeric structure. First results show compressive strengths of up to 90 MPa after 28 days with high early strengths as well. The relation between ash characteristics and geopolymer properties is now the main subject of research in order to obtain a basic understanding of the geopolymeric reactions. For the future, geopolymers produced from coal fly ashes may become serious competitors for Portland cement and concrete, because of improved mechanical properties and the huge resources of ash available. The fact that CO2 emissions from their production amounts to only 10% of that produced by the manufacturing of Portland cement, will enhance further development.