Mechanism Study Of Heavy Metals Adsorption And Stabilization By Wet Milling Fly Ash

Disposal and treatment of coal fly ash and municipal solid waste incineration fly ash is one of the research focus.With the increasing demand for electricity,the corresponding amount of coal fly ash production is huge.But the study of high value utilization of coal fly ash is seldom.At the same time,the launching of municipal solid waste incineration?MSWI?power generation project increased,along with large amounts of fly ash would be produced.Currently,there are two typical ways of waste incineration in China,one is the grate furnace incineration,the other is the fluidized bed incineration.According to the content of heavy metals from low to high,the typical fly ash in China is coal fly ash,fluidized bed incineration fly ash and grate furnace incineration fly ash.As the grate incineration is relatively mature,previous scholars have done more research on the grate furnace incineration fly ash.The fluidized bed incineration fly ash was seldom studied.Aiming at the above problems,different treatment and utilization schemes were developed in this paper based on wet grinding method.For coal fly ash,in which the heavy metal content is low,a new method of zeolite synthesis via coal fly ash under atmospheric pressure was established.Then the synthesied zeolite was used to remove heavy metal Cr???,Pb???and Cd???in waste water.Further,a high iron fly ash was used to adsorb Cr₂O₇^2-after modified by wet milling to improve the removal effect.The influence factors in the process of adsorption was detailed analysized.For MSW fluidized bed incineration fly ash,aiming at the features of heavy metals leaching and volatile during heat utilization process,a wet milling pretreatment method was developed with water as the medium to reduce the leaching and evaporation of heavy metals.Then a"wet milling-heat treatment"joint processing scheme of MSWI fly ash was established.Firstly,zeolite was synthesized via wet milling under the alkaline medium,and the synthetic parameters were optimized by the method of orthogonal method,in which process cation exchange capacity?CEC?was used as evaluation index.The optimum condition is the concentration of NaOH 4 mol/L,liquid-solid ratio 4,milling time 24 h,rotation speed 350rpm.Through XRD and fourier infrared results,it is found that after wet milling processing,Na-X zeolite(Na_4.43Al₆Si₆O₂₄·?H₂O?_8.882)was generated along with the disappearance of the mullite peak,which means that wet milling treatment under alkaline medium made the mullite dissolved.That is to say,wet milling treatment has a higher utilization of Si and Al in the mullite.After wet milling,the specific surface area of fly ash increased from 3.5 m²/g to 65.0m²/g,and the pore capacity changed from 0.01 cm³/g to 0.13 cm³/g,both of which increased more than 10 times.Then the synthesied zeolite was used to remove heavy metal Cr???,Pb???and Cd???in waste water.The results showed that zeolite synthesized by wet milling process has a good adsorption effect of heavy metals Pb???and Cd???,the removal rate reached more than 90%.This mainly attribute to the strong basicity of the solution after zeolite adsorption,then much OH^-exist,which would combined with Pb???and Cd???,leading to the formation of Pb?OH?₂and Cd?OH?₂,i.e.,sediment trap occured.With the increase of reaction time,solid to liquid ratio,and the initial solution pH value,the removal percentage of heavy metals Pb???and Cd???increased.For the negative surface charge characteristics,the remocal effect of Cr???,which is in the form of Cr₂O₇^2-in the solution,is not ideal.Removal date of Cr???by synthesized zeolite proved that low valance iron played an important role.So high iron fly ash was used to synthesis zeolite via wet milling,then the zenlite was used to remove Cr???in the solution.It turned out that the maximum removal of Cr???was increased to 1.549 mg/g.Comparing with raw coal fly ash,the increase rate has reached 92.90%.This maily owe to the reducing low valance iron,which made the Cr???reduction to Cr???,then Cr???combined with OH^-forming Cr?OH?₃ precipitation.And for zeolite has a better removal effect of cationic Cr???,enhance the adsorption removal effect.About MSWI fluidized bed fly ash,a wet milling pretreatment method was developed with water as the medium.After wet milling,the leaching ratios of heavy metals Cr and Cd in fly ash decreased to 1.50%and 3.04%respectively,the reduction rates reached 77.94%and65.96%respectively.The leaching rates of other three heavy metals also had a certain decrease.After wet milling pretreatment,the evaporation rates of heavy metal Cu at 750?and 1050?,were reduced by 79.35%and 88.06%,respectively.The thermal evaporation rates of heavy metals Pb,Zn and Cd were reduced nearly 100%.