Melting Characteristics And Occurrence And Transference Of Heavy Metals During Melting Process Of Fly Ashes From Municipal Solid Waste Incinerator

Fly ashes from municipal solid waste (MSW) incinerator contain a great deal of toxic heavy metals and their compounds. They are defined as hazardous waste because these fly ashes in having high leaching ratios of heavy metals. Melting technology can effectively control emission of heavy metals in fly ashes from MSW incinerator. It is one of the major techniques of fly ashes innocent treatment. The basic properties of fly ashes from MSW incinerator are systematically investigated. The fly ashes are carried out the melting characteristics and the behavior of heavy metals in fixed tube furnace and swirling furnace. At the same time, the factors of the system on coal gasification and fly ashes melting are explored.The fly ashes of bag filters from three typical running municipal solid waste powder plants in east China were studied to explore the chemical composition, physical properties, changes of microstructure, mineralogical composition, leaching characteristics of fly ashes samples. The results show that the compositions of fly ashes are very complexes. The major components contain silicon dioxide, calcium oxide, alumina, ferric oxide, hydrate, and chloride. There are a small amount of heavy metals, such as cadmium, chromium, copper, lead, zinc and so on. The content of zinc and lead are grievously outside of the identification standard for hazardous wastes. The particles size distributions of three fly ashes are approximately normal school. The primary range of particles sizes is 10 to 100 micron. The melting process of fly ash is an endothermic process. It is comprised materials phases transformation and melting reaction. The temperature range, endothermic amount and initiative temperature of melting reaction in each process rest with the compositions of fly ashes. The melting points of fly ash are distinctly affected by the compositions. The temperature of melting point is determined by the content of silicon dioxide and alumina in fly ash. The order of melting point on three fly ashes is FA3> FA2> FA1. The major constitutes of fly ash are composed by plentiful crystal and amorphous materials. They have higher porosity and BET. The fly ash is higher leaching rate of hazardous waste. The mass fractions of cadmium, chromium, lead, copper, zinc are higher than these of soil. The leaching rates of lead, chromium in three fly ashes and zinc in FA1, FA2 go beyond the values of standards. Therefore, it is obligatory for fly ash to immobilize and detoxify. The change rules of pH values in leachate of before and after three fly ashes are very analogical. The pH value of leachate in 5.3 is a critical point. When pH value is less than 5.3, it would sharply raise. On the contrary, the change of leachate pH value becomes smooth. These conclusions will provide the important theory bases for the farther research treatment harmlessly of fly ashes.The fly ashes from MSW incinerator were conducted in a tube melting furnace in this study. The changes of microstructure, mineralogical composition, heavy metals distribution, leaching characteristics of samples before and after melting treatment were explored. The emphasis is on the effects additives on the melting characteristics and heavy metals distribution. Simultaneity, influences of melting temperature, melting time, basicity, additives, different atmosphere on the immobility and volatilization behavior of heavy metals during melting process. The results show that the melting temperature is an important factor on the melting treatment of fly ashes from MSW incinerator. At 1400℃, the molten sample became full smooth, much hard in surface structure, while the section of the molten sample had lustrous or no apparent aperture, the sample fly ash had completely melted. The slag melted is crystalloid. The amounts of crystalsincrease with the melting temperature elevation. The leaching rates of zinc, chromium, lead, copper, cadmium, mercury was very low in the products melted. The gaseous volatile of mercury, cadmium and lead is captured. The effect of heavy metals immobility was in sequence of silicon dioxide>calcium oxide>alumina. Melting temperature has an important impact on the behavior of heavy metals. There is a great difference for the behavior of immobility and volatilization of various heavy metals. The changes of basicity will give birth to the marked influence on the fixation rate of heavy metals. The proper additive rate of silicon dioxide will be propitious to enhance the fixation rate on the heavy metals. The fixation rates of chromium, nickel, copper and arsenic in melted samples would increase with the elevation of their melting point and boiling point at oxidizing atmosphere. The volatilization rates of lead, cadmium and mercury with low boiling points are very high during melting process. The reducing atmosphere would be propitious to enhancing the fixation rates of chromium, nickel, copper and arsenic. It is confirmed that mercury, cadmium, and zinc are more easily vaporized under reducing atmosphere, but the volatilization rate of lead would be inhibited at the same condition.Based on the previous experimental results, the different fly ashes were melted in the self-developed swirling melting furnace system. The melting characteristics and behavior of heavy metals of fly ashes were investigated in detail under different operation runs. Influences of melting temperature, calcium oxide, silicon dioxide, magnesia additional reagent on the melting characteristics and behavior of heavy metals were studies during fly ashes melting process. The dynamic melting treatment characteristics and occurrence and transference of heavy metals in samples were concluded. Experimental results indicate that the fly ashes will transform into vitrification state at 1400℃. The higher melting temperature will redound to fly ashes melting treatment. There are marked differences on the influences of calcium oxide, silicon dioxide, magnesia additional reagent on the melting characteristics. Adding calcium oxide may effectively control melting point of fly ash. If adding less than 15 percent, it would be propitious to melting treatment. The melting-aid effect of calcium oxide should appropriately adjust base on composition of fly ash. Adding silicon dioxide in fly ash would redound to reducing the melting point of fly ash and advance the liquidity of sample. With the adding amount increasing of silicon dioxide, the glassy amorphous materials in the melted sample would enhance. The higher adding amount of silicon dioxide, the better is the stability of melted slag. The netlike structures of silicates in fly ash would be destroyed by the adding magnesia. The glutinosity of melted sample reduces with increasing adding amount of magnesia. The sample would achieve the better melting effect when the adding amount is less than 5 percent in fly ash sample. The glassy materials in slag would enhance and the crystal phases take place transform with adulterating magnesia. The crystals in sample are enveloped by the amorphous slag. The fixation rates of nickel, chromium, copper, cobalt, and manganese in slag increase with melting temperature heightening between 1250 and 1400 degree centigrade. With the exception of mercury, the change of melting temperature would markedly affect on the fixation rates of arsenic, lead, cadmium, zinc. The fixation rates of chromium, copper, manganese are decreased by adding calcium oxide during melting process. The lower is the fixation rate of arsenic, zinc, and lead, the more is adding calcium oxide. It is beneficial for the improvement fixation rate of heavy metals to add silicon dioxide. With the exception of mercury, the fixation rates of residual heavy metals would improve with adding amount increasing. The fixation rates of zinc, arsenic, cadmium, and lead would distinctly enhance. Adding amount being controlled at 10 percent for FA3 would achieve the optimal fixation effect. The volatile heavy metals are sensitive to melting temperature. With melting temperature increasing, the content of them takes on the increasing first and after reducing in flue gas. When calcium oxide adding amount is 5 percent, the content of volatile heavy metals would lessen in flue gas of melting treatment. It would give birth to negative effect for fixation heavy metals once calcium oxide adding exceeding 5 percent. With the increasing of silicon dioxide adding amount, the content of heavy metals in flue gas would reduce, but the content of mercury is ruleless. The adding magnesia in flyash has remarkable influence on the content of heavy metals in flue gas.At last, the gas products of coal gasification were used as the substitutive fuel on melting treatment of fly ashes. The fly ashes were conducted in the experimental system associating coal gasification with melting treatment. The effects of air/coal ratio, steam/coal ratio, bed temperature and additives on the occurrence and transference of heavy metals during melting process on swirling furnace was systemically investigated. The fixation rates of nickel and chromium achieve the maximal value above 90 percent at different air/coal ratio condition. With increasing air/coal ratio, the fixation rates of copper, manganese, and cobalt enhance steadily, the fixation rates of volatile heavy metals take on the ascending. The fixation rates of arsenic and zinc is above 30 percent in slag, the fixation rate of cadmium becomes increasing first and after reducing in slag. That of mercury keeps stable. The fixation rates of cobalt, chromium, copper, manganese, nickel and volatile heavy metals increase first then reduce in slag with increasing steam/coal ratio, and fixation rates of them achieve the maximum at steam/coal ratio 0.41 kg·kg-1, those of chromium and nickel is above 90 percent. With the exception of manganese, the fixation rates of cobalt, chromium, copper, and nickel enhance with increasing bed temperature. The order of fixation rate is Ni>Cr>Cu>Mn>Co. With bed temperature increasing, the fixation rates of lead and cadmium aggrandize, the fixation rate of zinc is reductive, mercury keep stable. The adding magnesia in sample would conduce to increasing the fixation rate of nickel and chromium. The adding calcium oxide, silicon dioxide, magnesia would not be helpful to fixation of manganese. For copper, the fixation effect of silicon dioxide is the best, the magnesia is the better, calcium oxide is the worse. For cobalt, the order of fixation effect is magnesia>silicon dioxide>calcium oxide. The fixation rates of heavy metals would remarkably enhance when adding magnesia and silicon dioxide. For the volatile heavy metals such as arsenic, zinc, cadmium, and lead, the fixation effect of magnesia is better than that of silicon dioxide and calcium oxide. The fixation rates order of heavy metals is magnesia>silicon dioxide>calcium oxide. These conclusions will offer the experimental bases and theoretic references for further comprehending the melting treatment technology of fly ash.Based on the experimental results and existent states of heavy metals in fly ashes and slag, the gas-solid reaction theory during the melting process, heat and mass transfer of particles in fly ashes, chemical dynamics theory such as volatile heavy metals diffuseness, the plentiful experimental datum were analyzed. The melting reaction model and the volatility model of heavy metals in particles during fly ashes melting process have been constructed. The results simulated from the predicted model are basically consistent with the measured values of experiments.