Fly ash beneficiation

A schematic flow chart of the beneficiation options that are currently in practice is shown in Fig. 7. These processing schemes are primarily employed for the production of pozzolan, but other products may result, such as carbon fuel and mineral grade filler. The fly ash beneficiation option applicable to a specific site is dependent on many factors, but the primary consideration is whether the fly ash is wet or dry. Once ash has been wetted, flotation is the only practical beneficiation option. It may be technically feasible to use thermal processes on damp ash, but the amount of heat required will be significantly increased, thus decreasing the economic value... 

Boyd, T. J. Cochran, J. 1994. Fly ash beneficiation by carbon burnout. Proceedings of the American Power Conference, 56, 937-942. 

Fisher, B. C., Blackstock, T. Hauke, D. 1997. Fly ash beneficiation using an ammonia stripping process. Proceedings 12th International Symposium on Coal Combustion By-Product (CCB) Management and Use, Vol. 2, Paper 65, pp. 1-8. 

Groppo, J. G., Brooks, S. M. Kreiser, C. 1995. Fly Ash Beneficiation by Air Classification. 124th Annual American Institute of Mining and Metallurgical Engineers Meeting, Denver, CO, 6-9 March 1995, 95-196. 

The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

For the purpose of this discussion, coal combustion ash consists of two distinct products bottom ash and fly ash. The distinction between bottom ash and fly ash is how they form and exit from the boiler. Bottom ash is removed from the bottom of the boiler, whereas fly ash exits in the flue gas, where it is subsequently collected by a variety of devices. This distinction is important because the beneficiation options available for either of these products, as well as potential end uses, are different. 

Perhaps the most significant advantage of using froth flotation to beneficiate fly ash is the ability to process ash that has been impounded in disposal ponds and landfills. This is advantageous for several reasons. First, it effectively de-couples ash beneficiation from ash production. There is no need for the utility to alter disposal practices or install additional ash handling equipment in order to accommodate ash processing since the feed to the processing plant... 

Thermal decomposition of the primary ammonia species present on fly ash occurs at temperatures higher than 434 °C, thus the thermal beneficiation processes described previously (i.e., CBO and MCB) effectively remove ammonia. Several other thermal processes are also described in the literature (Harald Ruegg 1994 Fisher et al. 1997), but unlike CBO and MCB, they address the removal of ammonia only and not the carbon reduction. 

Epperly Sprague 1991 Martin et al. 1993 Gasiorowski et al. 2000). A different chemical approach is to oxidize ammonia by the addition of hypochlorite to the fly ash prior to use as a cement additive (Minkara 2003). Wet beneficiation of fly ash with flotation, although not a specific chemical treatment, will also remove ammonia due to the high solubility of the ammonia salt compounds present. 

Ban, H., Li, T. X., Schaefer, J. L. Stencel, J. M. 1996. Characterizing dry triboelectrostatic beneficiation of coal and fly ash using recovery analysis. In Chiang, S. H. (ed) Coal-Energy and the Environment. Thirteenth Annual International Pittsburgh Coal Conference, September 1996, Pittsburgh, PA, 2, 873-878. 

Grunewald, K. Otterstetter, H. 1989. Investigations into the beneficiation of fly ash from power plants using flotation. Bundesministerium fiir Bildung, Wissenschaft undKultur, 42, 61-63. 

Kim, J.-K., Cho, H.-C., Kim, S.-C. Chun, H.-S. 2000. Electrostatic beneficiation of fly ash using an ejector-tribocharger. Journal of Environmental Science and Health, Part A Toxic/Hazardous Substances and Environmental Engineering, 35, 357-377. 

Levy, E., Herrera, C., Coates, M. Afonso, M. 1999. Beneficiation of fly ash using a fluidized bed separator. Proceedings 13th International Symposium on Use and Management of Coal Combustion Products, American Coal Ash Association, Paper No. 17, January 2003, St. Petersburg, FL. 

Daniels, W. L., Stewart, B., Haering, K. Zipper, C. 2002. The Potential for Beneficial Reuse of Coal Fly Ash in Southwest Virginia Mining Environments. Virginia Cooperative Extension Virginia Tech, Publication number 460-134. 

Wood flour was more beneficial compared to fly ash in terms of notched test of impact resistance of HDPE-fllled materials and showed mixed data in terms of unnotched tests (Table 4.23). 

The above data show that fly ash was not beneficial for mechanical properties of the HDPE-wood flour-based composite. However, the flowability (MFI) of the molten composite was getting higher (viscosity was lower) when fly ash was replacing wood flour. Last but not the least, it was found that fly ash stabilized the HDPE-wood flour composition against heating. The onset of thermal decomposition for the 40% wood-flour-filled HDPE started at about 280°C, while that of 40% fly ash-filled HDPE started at approximately 490°C. The triple (HDPE-wood flour-fly ash) composite was between the two in this regard, showing a thermally more stable behavior than composites with wood flour alone [23]. 

The use of solvent extraction is beneficial in some cases. For example for the separation of Au, Pt, Pd, Ag and Ir (Schutyser et al., 1977) proposed a combination of precipitation scavenging exchange-solvent extraction procedure. Very low detection limit for Be in fly ash (ng/cm ) has been reached after extraction with A-benzoyl-V-phenylhydroxylamine in isobutyl methylketone (IBMK) and measurement by flame atomic absorption (FAAS) (Cresser et ah, 1992). 

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