AGRICULTURAL AND INDUSTRIAL APPLICATIONS OF THE HELLENIC FLY ASH AND ENVIRONMENTAL IMPACTS

摘要

Research data targeted on the development of new sustainable agricultural and industrial applications of fly and bottom ash from the lignite combustion electricity power plants involve the chemical and mineralogical characterisation, the leaching experiments and bulk sampling on agricultural land, compared to other s oil improvers such as zeolite tuffs and vermiculite. The Hellenic fly ash has an average grain size of a few to 200 μm with glassy and spherical parts. A small portion of solid carbon is also included depending on the completion of the coal combustion process. The petrogenetic minerals found in the fly ash originate from marls, schists, l imestones and dolomites. They are only those which cannot be melted under the combustion process and mainly include quart z, anhydrite, gellenite, portlandite, feldspars, hematite, magnetite and some pyrite and chalcopyrite. Clay minerals are structurally damaged in the high temperatures of the combustion. The Hellenic fly ash can be classified according to the silica (25-59 wt% SiO2), calcium (1.3-32.0 wt% CaO) and iron (5-24 wt% Fe2O3) content, while other elements such as Al, Mg, K, Na, Ti, P, C and S also occur in significant concentrations. Trace elements include Mn, Cr, Ni, Sr, Cu, Zn, Pb, Co, Li and Mo. Radioactive elements (e.g. U, Th, K) are inhibitive f or use in construction materials, while elements such as B, Cr, Ni and Co are “toxic” f or agricultural applications. The sulphate values of the fly ash can be inhibitive f or some industrial applications. Leaching experiments of fly ash reveal that the elements S, Ca, and possibly K, plus traces of Sr, Mo, B, Mn, Zn, Cd, Br, I, Sb and W are the mobile constituents, wit h Mo and Sr and sometimes Mn, Zn, B and Cd being the most mobile. On t he contrary , Si, Fe and Mg, plus the Ag, As, Be, Bi, Ce, Co, Cu , REE, Nb, Nd, Ni, Pb, Th, U and Zr exhibit a low mobility , i.e. restrict ed dissolution and transport by the meteoric waters. Conclusively, even a small proportion of fly ash in t he mixture can dramatically change the concentrations of Ca, Mg, K and Na and considerably increase the pH and the electric conductivity of the soil. Finally , besides the applications of fly ash in massive constructions (walls, dams, blocks, roadworks, concrete, pavement blocks ), it is currently test ed whether the Hellenic fly ash is suitable f or increasing the pH of B-poor acid soils in certain agricultural applications. Preliminary results show that a proportion of 5 wt% fly ash in acid soil can increase the pH to 7,3-7,8. As these pH values are considered satisfactory , the current experiments will be continued on a pi lot plant scale in the areas of Ptolemais-Amynteo and in the Central Macedonia area.