Thermal Separation Technology Of Heavy Metals In MSWI Fly Ash

For the high leachable concentration of heavy metals, the municipal solid waste incineration (MSWI) fly ash was defined as hazardous waste. Heavy metals in fly ash was studied on solidification/stabilization much more than on separation at home and abroad. On the basic of the study on fly ash characteristics, this article had explored how the conditions (temperature, time, thickness of the sample, flux of the gas, atmosphere) influenced the thermal separation of heavy metals in fly ash. The article gave theoretical reference to the thermal separation of heavy metals in fly ash.The fly ash had such characteristics as follows: (1) The moisture content of fly ash was 0.125 0.705%. There were two rising and falling situation on the distribution of particle size, and the fractions of 38⁵3μm accounted for about 50%. (2) The fly ash is mainly composed of CaO,SO₃,Cl, accounted for about 68.49%, and also had some heavy metals such as Cd, Pb. The heavy metal concentrations in fly ash showed the trends: Pb＞Zn＞Cu＞Cr＞Cd＞Ni. (3) The surface appearance of fly ash was various. There were clava, conglobulation, clump and so on. The components were different with the different surface appearance, and .the existent of crystal masses were Chlorine Salt, CaSO₄ and so on. (4) The fly ash has poor heating stability, and it had four endothermic procedures, one exothermal procedures, and four distinct weight-loss, when the temperature was heating from 100℃to 1400℃. They were caused by the evaporation of water, the melting reaction, the decomposition of CaSO₄ and so on.Thermal separation characteristics of fly ash are studied in different condition, the results showed that as follow: (1) the evaporation rate of other heavy metals (Pb,Cd,Cu,Zn) all increased with the rising temperature below 1000℃and above 1150, but all decreased rapidly at 1050¹150℃. Along with the rise of time, the evaporation rate of heavy metals (Cu, Zn, Pb, Cd) increased, and all almost evaporated totally in top rate except for Zn (only 60%). Pb and Cd were not influenced by the atmosphere, while the evaporation rate of Cu and Zn in N₂ was more than in air, The evaporation rates of heavy metals (Cu, Zn, Pb, Cd) decreased as the thickness of the sample increased, but not influenced by the flux of the gas. (2) The reduction rate of volume and weight all increased with the rising temperature, the increasing time or the decreasing thickness of the sample, but only little influenced by the atmosphere or the flux of the gas. (3) The regularity of heavy metals (Pb, Ni, Zn, Cu, Cd) in the leaching concentrations was opposite to the regularity of reduction rate of volume and weight, and the leaching concentrations of Cr was increasing and transnormal easily in the air, with the rising time or temperature, or the decreasing thickness of the sample.In the condition (air, 1000℃, 300min, 17mm, 600ml/min), the evaporation rates of Pb, Cu, Cd were 99.6%, 97.5%, 93.8%, respectively, and the rank of the evaporation rates was Pb＞Cu＞Cd＞Zn; In the recovery rate, Cu was highest (72.4%), and the rank is Cu＞Pb＞Cd＞Zn.