Study On Toxic Reduction And Product Utilization Of Molten Salt Thermal Treatment Of Hazardous Waste Incineration Fly Ash

The incineration of hazardous waste is widely used due to its advantages of volume reduction,energy recovery,and waste reduction.However,the fly ash generated from hazardous waste incineration contains toxic and hazardous substances such as a large amount of heavy metals and dioxins,which are typically classified as hazardous waste and require treatment.Thermal treatment of fly ash is helpful in achieving harmless and resource utilization.High-temperature thermal treatment is a promising technology for fly ash treatment.,but this technology faces problems such as high energy consumption and“secondary fly ash”,which severely restricts the large-scale disposal of hazardous waste incineration fly ash.Based on this,this paper proposes a mild thermal treatment method for fly ash based on high-temperature liquid molten salt by combining high-temperature thermal treatment technology and wet extraction technology.Na Cl-Ca Cl₂,to investigate the salt removal and heavy metal removal characteristics of fly ash under molten salt thermal treatment,examine the heavy metal leaching characteristics of molten salt thermal treatment products,analyze the variation of salt and heavy metals during the molten salt cyclic thermal treatment process,and explore the electrolytic alkali-making process for the molten salt thermal treatment product’s wash solution,ultimately achieving salt removal,detoxification,and resource utilization of fly ash through molten salt thermal treatment.The main research results are as follows.（1）The study on NaCl-CaCl₂molten salt thermal treatment of high-calcium fly ash was conducted to investigate the influence of thermal treatment temperature,time,and molten salt/fly ash ratio on the distribution and leaching of heavy metals in the thermal treatment products.The results showed that Zn and Cr were distributed in the molten salt and slag products at a similar ratio after thermal treatment,while more than 70%of Cu,Pb,and Cd were distributed in the molten salt.The heavy metal leaching rate of slag products under different thermal treatment conditions decreased by over 88%compared with that of the original ash,and the leaching toxicity met the general solid waste identification standard,with the optimal treatment temperature of 600 ^oC,reaction time of 2 h,and ash-to-salt ratio of 1:5.Additionally,the molten salt was used to treat high-sodium hazardous waste incineration fly ash,and the product was transformed into general solid waste,further demonstrating the feasibility of Na Cl-Ca Cl₂molten salt thermal treatment of hazardous waste incineration fly ash technology.（2）To improve the economic viability of molten salt thermal treatment of fly ash,a study on the multiple cyclic thermal treatment of fly ash using molten salt was conducted to investigate the heavy metal removal and salt migration characteristics of fly ash during the cyclic process and examine the environmental risks of the products after cyclic thermal treatment.The results indicated that heavy metals in fly ash were effectively removed after six cycles of molten salt thermal treatment,and the leaching rate of heavy metals showed a trend of decreasing first and then increasing.As the number of cycles increased,the Na⁺,K⁺,and SO₄^2-content in the molten salt increased,while the Ca²⁺content gradually decreased.Washing the waste salt after treatment removed more than 70%of the soluble salts,with the remaining salt residue mainly Ca SO₄.The leaching toxicity of molten slag and washed salt residue after multiple cycles of molten salt treatment was still lower than the toxic leaching limit of hazardous waste.（3）To address the utilization difficulties of waste molten salt after fly ash treatment,this study explored the method of electrolysis using waste molten salt washing solution.The calcium ions removal from the washing solution and their effects on electrolysis were investigated,and the application prospect of the integrated technology route of molten salt heat treatment of fly ash-electrolysis of waste salt washing was evaluated.The findings suggested that high concentrations of Ca²⁺in the waste molten salt washing solution would deposit on the surface of the electrolytic membrane during the electrolysis process,seriously affecting the electrolysis efficiency.However,the electrolysis efficiency could be effectively improved by removing Ca²⁺in advance.Adding desulfurization wastewater rich in SO₄^2-and introducing CO₂gas into the waste solution could effectively remove Ca²⁺in the waste molten salt washing solution,achieving the effect of waste treatment and carbon reduction.Taking a hazardous waste incineration plant with a fly ash output of10 tons/day as an example,an economic analysis was carried out for the integrated process of fly ash molten salt heat treatment,waste salt washing,collaborative disposal of salt water with desulfurization wastewater,and the reuse of electrolysis products.The analysis showed that the incineration plant could achieve an annual profit of 8.74 million yuan,indicating a promising application prospect of this technology.