Leaching Characteristics Of Heavy Metal In The Coal Ash From West China

The development of western China has accelerated the use of coal resources as well as triggered a serious ecological and environmental problems. The pollutants from coal combustion has caused a direct harm to the health of the western people, and it's has been universal concerned by the researchers. Such a large quantity of coal ash leads to a number of environmental problems, among which is the leaching of heavy metals from fly ash. The adverse impacts of leached heavy metals result from their changing form and not easily breaking down into harmless substances. Though a large number of studies on this subject have been carried out worldwide, the leaching method is still far from the actual situation. No research has been done on the leachability of heavy metals in coal ash from west China. The method suited to national conditions for harmless disposal of fly ash should be proposed. For these reasons, this dissertation concentrates on the leaching characteristics of heavy metals in fly ash from the typical western coal and the factors which can influence the leachability of the heavy metals are studied. Based on the results to be obtained in this dissertation, the theoretical basis for the green-economic coal ash disposal approach and relative law fitting the practical situation in western China could be hopefully obtained.The in situ influences of an ash dump in Guizhou Province to surrounding soil, surface and groundwater have been investigated. The order of the in situ heavy metals mobility in ash dump is Cd> Zn> Cu>Pb> Cr> Co from calculation of mobility index by the contents of heavy metals in storehouse samples and ash dump samples. The ash dump has significant influence on the surrounding soil. The closer soil samples have higher comprehensive pollution indexes. The single pollution index of Cd in some soil samples are even larger than 100. Compared with the concentrations of heavy metals in the upstream water, Cr, Cd, Pb, Zn and Co is enriched in the downstream water. The concentrations of Cd, Cr and Pb are beyond the Grade in Quality Standards for Surface Water Resources. It is noteworthy that the high mobility index of Cd and high content in surrounding soil and water are very closely related.Sequential extraction(SEE) have been performed on fly ashes collected from three electrostatic precipitators(ESP) in a coal-fired power plant in order to in order to determine the binding forms of heavy metals in fly ash. Test results show that the major heavy metals enriched in the residue state in the fly ash. The residue state contents of Cr, Cu and Cd are more than 80%. In the fractionation of ion-exchange and acid-soluble state, the heavy metals in the coal ash from the last ESP have higher contents. And as the load increases, the fractionations of ion-exchange and acid-soluble state have been influenced by the smaller particle size and decrease of unburned carbon.Batch leaching experiments have been carried to investigate the factors for heavy metals leaching. It is shown that, the pH value has the more important influence on the leaching of coal ash than liquid/solid ratio, temperature and leaching time. The results of simulation by Visual MINTEQ show that the precipitation of Cd4(OH)6SO4, Cd(OH)2, Ni(OH)2 and PbO·PbSO4 have related to the leaching pH value. The leachability of heavy metals is increase as the increase of the liquid/solid ratio till 30:1.The leaching temperature during 15℃-45℃have no significant influence on the leachability of Cr and Cd. With the accumulating of leaching time, the leachability of heavy metals has obviously increased. The leachability for Cr and V shows a stable tendency but not for the Cd and Pb in 168 hours.Column leaching tests have been performed on the size-classified fly ash samples. Leaching time intervals are selected ranging from 15 minutes to 40 days. The concentrations of Co and Cd keep stable after 9-16 days for three size-classified fly ash samples. However, the concentrations of Pb and Zn increase from 1 day to 40 days. The leachability of Pb is smaller in the fine ash particle. Compared to the static batch leaching, the column leaching of fly ash is more similar to the situation of ash dump. The mobility index and leachability have the same order as below:Co< Cr< Pb< Zn< Cd.A low-cost and effective in situ chemical treatment technique has been proposed. The accessible ferrous sulfate is used to treat the coal ash. The lower leachability of relevant heavy metals is due to the adsorption of the iron hydroxide and the formation of secondary mineral such as (CrxFe1-x)(OH)3 and CuxFe3-xO4. The optimization conditon for the Cr control is obtained when liquid/solid ration is 5:1 in the treatment. The leachability control ration is smaller in the column leaching because of the restraining of secondary mineral formation. This treatment is not fully applicable to the Cd, Ni and Pb control in leaching of fly ash.