Co-hydrothermal Stabilization Of Heavy Metals In Municipal Solid Waste Incineration Fly Ash With Coal Fly Ash

The amount of municipal solid waste incineration fly ash in China is increasing year by year,and the fly ash needs to be safely treated.Hydrothermal method is a potential fly ash disposal technology,and the contents of Si,Al and Ca are important factors that affect the effect of hydrothermal method on stabilizing heavy metals.However,the content of Si and Al in fly ash is insufficient.In the paper,coal fly ash was used as a silicon-aluminum regulator to conduct hydrothermal reaction with fly ash to reduce the leaching toxicity of heavy metals;the strong alkaline remaining hydrothermal liquid was recycled,thereby the addition amount of the alkaline activator and the waste lye in the subsequent hydrothermal reaction were reduced.The main research contents and conclusions are as follows:（1）The physicochemical properties of coal fly ash and fly ash were analyzed,and the heavy metal content,speciation distribution of heavy metals,leaching concentration and the influence of different leaching environments on the leaching concentration were studied.The research results showed that the content of Ca in fly ash was about 28.80%,but the contents of Si and Al were only 1.27%and 0.49%.When fly ash was directly hydrothermal treated,the effect of stabilizing heavy metals was poor.The content of Ca in fly ash was 1.29%,and the contents of Si and Al were as high as 22.79%and 24.22%.The contents of Zn and Pb in fly ash were relatively high.From the leaching behavior of heavy metals in fly ash at different pHs,it can be seen that when the p H was less than 10,the leaching concentration of Zn and Pb decreased with the increase of p H,and when the p H was greater than 10,the leaching concentration curve showed an upward trend;The leaching concentration of Cu and Cd increased as the p H decreased.（2）Coal fly ash was used as a silicon-aluminum regulator to conduct hydrothermal reaction with fly ash.The effects of fly ash mixing ratio,hydrothermal time,hydrothermal temperature,and whether to add alkaline activator on the leaching toxicity,solidification rate and speciation distribution of heavy metals in hydrothermal products were explored.At the same time,the formation and transformation of crystals in hydrothermal products were discussed through phase composition analysis and microscopic morphology analysis.The results showed that the optimal conditions for the hydrothermal reaction of coal fly ash and fly ash used in the research were as follows:coal fly ash to fly ash ratio 7:3,liquid-solid ratio10 m L/g,reaction temperature 160℃,reaction time 48 h,take 0.5 mol/L Na OH as alkaline stimulant.Under these conditions,the solidification rate of heavy metal Zn in the hydrothermal product reached 92.25%,and the solidification rate of Pb,Cu,Cd,Cr,and Ni were all above 99%.The proportion of residue in heavy metals increased,and the migration of heavy metals into the liquid phase was low.This is mainly due to the formation of silicoaluminate minerals in Ca O-Al₂O₃-Si O₂ hydrothermal system,which can adsorb and wrap heavy metals and realized the stability of heavy metals.（3）The influence of alkalinity and salinity on the stabilization effect of heavy metals in hydrothermal products was studied during the recycling and reuse of hydrothermal liquid.The results showed that the direct recycling of hydrothermal liquid was not conducive to the formation of aluminosilicate minerals in the hydrothermal system.With the increasing in the number of recycling and reuse of hydrothermal fluids,the proportion of the acid soluble fraction and reducible fraction of heavy metals increased,and the risk of leaching into the environment increased.When the hydrothermal liquid was recycled and reused with the constant alkalinity,the leaching concentration of heavy metals in the hydrothermal products decreased significantly.At the same time,the accumulation of salt in the hydrothermal liquid will also affect the stability of heavy metals.When the sodium content reached 45-50 g/L,the solidification rate of heavy metals of Zn,Pb,Cu,Cd,Cr,and Ni decreased by 6.66%,1.93%,5.39%,8.93%,5.71%,and 3.25%,respectively.