Co-hydrothermal Processing To Stabilize Of Fly Ash Of Different Waste Incinerators And Its Mechanism

MSWI fly ash was classified as hazardous waste in China.Due to the complex and variable chemical composition of fly ash,containing heavy metals and organic pollutants,the proper disposal of large quantities of MSWI fly ash has become a social problem.After the domestic and international waste incineration fly ash treatment methods were summarized and compared,the hydrothermal method was chosen as the fly ash disposal technology,which could not only deal with heavy metals in fly ash harmlessly,but also realize the recycling of fly ash without causing secondary pollution.According to the complementarity of the characteristics of fly ash from two different waste incinerators,the synergistic hydrothermal synthesis of aluminosilicate with two kinds of fly ash was realized at 200℃for 24 h without additional additives and hence the leaching toxicity of heavy metals in fly ash was reduced.The mechanism research on the stabilization of heavy metals of fly ash was carried out.The main results and conclusions were summarized as follows:（1）The characteristics of fly ash from two different types of waste incinerations were studied.The CaO content in both fly ash was very high,up to 48.45%in the grate furnace fly ash,and 26.29%in the fluidized bed fly ash.A large amount of Si and Al elements was found in the fluidized bed fly ash where SiO₂ and Al₂O₃ accounted for 22.5%and 17.93%,respectively.However,the content of Si and Al in the grate furnace fly ash was small.Ca（OH）₂ and CaClOH were present in the grate furnace fly ash resulting in a strong basicity.（2）Hydrothermal processing for stabilization of heavy metals in two typical fly ash were studied.In the absence of additives,the tobermorite and hibschite were formed in the hydrothermal product,which inhibited the migration of Cd,Cr,Cu,Pb and Zn in the fly ash into the liquid phase.When the mixing ratio of fluidized bed fly ash and grate furnace fly ash was7:3,heavy metals in fly ash were more stable,and the leaching toxicity of Cd,Cr,Cu,Pb and Zn was 0.560,0.011,1.507,70.095,18.780 mg/L respectively.After hydrothermal processing,the fly ash could be used as a stabilizing agent for the grate furnace fly ash.When the mixing ratio of the hydrothermal products and grate furnace fly ash was 1:3,the concentration of heavy metals leaching reached a low level.The washing experiment of the two fly ash was used as a control.When the mixing ratio of the two fly ash was 4:6,the heavy metal leaching toxicity was the lowest,and the leaching toxicity of Cd,Cr,Cu,Pb and Zn was 0.003,0.087,0,0,0.011 mg/L,respectively.Under the hydrothermal condition of adding alkali activator,katoite,tobermorite and a small amount of analcime were formed.（3）The formation of hydrothermal products from fly ash and the mechanism of heavy metal adsorption under the experimental conditions were studied.In the hydrothermal reaction without the alkali activator,hibschite would be first form,and then continue to react to form the tobermorite under the suitable conditions of Ca/Si.In the hydrothermal reaction of the alkali activator,katoite would be first form when n（CaO）/n(SiO₂+AlO_1.5)was about 0.7¹.1.Part of the formed katoite would be further converted into tobermorite.The activation energy of the tobermorite crystals was reduced and the formation of tobermorite was accelerated by the synthetic route.When n（CaO）/n(SiO₂+AlO_1.5)was close to 0.6,a small amount of analcime was formed before the formation of katoite.The ratio of n（CaO）/n(SiO₂+AlO_1.5)in the system is an important factor determining the hydrothermal reaction product.With this ratio,materials with different silicon and aluminum contents could be added to efficiently dispose of different characteristics of fly ash hydrothermally.According to the crystal structure of the tobermorite,there were free Ca²⁺and water molecules in the middle of the interlayer.As the water molecules enter and exit,the external heavy metal ions was transported into the interior of the tobermorite lattice where the Ca²⁺was replaced by which heavy metals were adsorbed.