CHEMICAL COMPOSITION AND LEACHING OF ASH FROM ASH REPOSITORY PILE OF TRBOVLJE COAL FIRED PLANT IN SLOVENIA

摘要

In the paper the influence on the environment of the ash from the ash repository pile of the Trbovlje coal-fired plant in Slovenia is described. The repository is located in the small valley of Prapreski graben which extends NNE of the Trbovlje power plant. The pile is situated in the medium, relatively gentle sloping part of the valley on Triassic dolomite. The rock is tectonically intensely fractured and permeable. The final capacity of the repository pile is about 12 million cube metres. The slopes of the pile have been revegetated. The surface is covered with a 30 cm thick layer of soil. On it grow orchard trees and clover. In the power plant mostly coal from brown coal pits in Zasavje in Slovenia is fired. Yearly about 280,000 tons of ash are deposited on the repository pile. From the repository pile 11 samples of ash of various ages were collected. Ash samples have been crushed and sieved to -0,063 mm powder. Leaching of ash was carried out by two procedures, by column leaching experiments, and by shake leaching. In the experiment three ash samples from the pile were used: sample 101 (flesh material), 107 (material approx. 10 years old), and 104 (approx. 20 years old), and one sample 112, of fresh fly ash from the electrostatic precipitator, before its contact with water or slag. The amounts of ash and distilled water in leaching tests were deliberately chosen. The glass column was filled with 100 g of ash and washed with distilled water. The ash was permeable for water. Silica wool at the bottom of the column prevented the passing of ash into the leachate. Each 100 ml of leached liquid represent one sample of leachate. In total through the column about 1000 ml of distilled water was passed for each ash sample, and 10 samples of leachate were obtained. In the investigation of adsorption of metals on ash, sample 101 was prepared in the same way as described above, and leached with solutions which contained each 200 mg/l of Cr(III), Cu and Cd in ionic form. Each 100 ml of eluate represented one sample. In a larger closed glass vessel 50 g ash in 1000 ml distilled water were shaken. In distinct time intervals (1 hour, 4 hours, 8 hours, 1 day, 14 days and 30 days) 100 ml of liquid were removed for analysis. In samples of ash Al, Mg, Mn, Ti, Ga, V, and Zr were determined with optical emission spectroscopy (OES), Ca, Fe, Na, Ag, As, Au, Ba, Br, Ce, Cs, Eu, Hf, Ir, Lu, Mo, Nd, Rb, Sb, Sc, Se, Sm, Ta, Tb, Th, U, Yb and W with instrumental neutron activation analysis (INAA), Cu, Pb and Zn with DCP ES, and Hg with flameless atomic absorption spectrometry (AAS). Contents of oxides of major elements in ash were determined with wet chemical analysis. In leachates of ash were determined pH, Ca, Na and K with flame photometry, Cd, Cr, Cu and Zn with AAS, and cyanides with amperometric method. Quality control of analytical methods was carefully carried out. Accuracy of analytical methods was monitored with blind determinations of geological standards, and is considered acceptable. Precision is passable for all chemical analysis. Chemical composition of power plant ash from the repository pile is very similar to the composition of ash of coal from Zasavje collieries Trbovlje (Tab. 1). Ranges of major element oxides in examined ash samples are given in Slovenian text. Means of 5 determinations of ash samples are 13 mg/g U and 19 mg/g Th. Emission of radon at the repository pile was estimated to be 4,000 Bq/m~3. Concentration of radon in air rapidly decreases with distance from the pile, and reaches at several hundred meters the natural background. The aim of leaching experiments of ash was the assessment of the influence of ash from the repository pile of power plant ash on surface and ground water. With leaching on the column conditions of percolating water through the repository pile can be simulated. As shown by experiments (Fig. 1, Tab. 2 and 3), in normal conditions no appreciable short term influence on waters can be expected beside