Study On Preparation Of Light Calcium Carbonate With Municipal Solid Waste Incineration Fly Ash And Migration Of Heavy Metals

The urban waste discharge of China increased rapidly,and incineration treatment has developed rapidly in recent years in China due to its outstanding advantages of heat recovery and power generation and reduction of MSW volume and quantity.However,in the process of waste incineration,a large amount of incineration fly ash will be discharged,which is designated as hazardous solid waste by the state,because the fly ash will be enriched with dioxins,heavy metals and other harmful substances.The solidified landfill of fly ash will occupy a large amount of land and cause potential long-term pollution to the environment.Therefore,the safe disposal and resource utilization of fly ash has become a new challenge for national urban development.In this paper,the fly ash of a municipal waste incineration power plant is used as raw material,and ammonium chloride is adopted as leaching agent to extract calcium ions from fly ash.Then light calcium carbonate is prepared by gas-liquid reaction.The influencing factors in leaching and carbonization process are explored through experiments.The process parameters are optimized and the appropriate process conditions are determined.Also the influence of leaching on heavy metal leaching is explored.In the fly ash leaching experiment,the effects of NH₄Cl solution concentration,leaching liquid-solid ratio,leaching temperature and leaching time on the leaching of Ca?OH?₂ and Mg²⁺in fly ash were studied.The optimum process conditions for the leaching of fly ash are as follows:liquid-solid ratio 3:1,leaching temperature 25 ^oC,leaching time 40 min,concentration of ammonium chloride 2.49 mol/L.Under the optimal condition,the leaching rate of Ca?OH?₂ is 98.4%and the concentration of Mg²⁺is 510 mg/L.Through the detection of heavy metals,it can be found that during ammonium chloride leaching process,the leaching of heavy metals Cr and Mn is less,and that of Cu,Cd and Pb is more;The concentration of ammonium chloride,liquid-solid ratio and leaching temperature has a great influence on the leaching of Cu and Pb;When the leaching time is from 20 min to 50 min,the leaching amount of heavy metal Cu is the highest,and that of heavy metal Mn is the lowest.The process parameters of the preparation of CaCO₃ by introducing CO₂ gas into the leaching solution were investigated,including reaction termination pH,the amount of ammonia,reaction temperature,flow rate of carbon dioxide,and stirring rate,etc.The optimum conditions are determined as follows:under the conditions of temperature of 50 ^oC and a rotation speed of 400 r/min,CO₂ gas was introduced into the leaching solution containing 21%ammonia water at a flow rate of 0.24 L/min,and react to about6.5 pH.Under these conditions,the yield of CaCO₃ is 94.87%and the purity is 97.12%.The CO₂ gas flow rate has the most obvious effect on the agglomeration phenomenon and particle size of CaCO₃,followed by the stirring rate,and finally the amount of ammonia water added.