Study On The Pretreatment Technology Of Municipal Solid Waste Incineration Fly Ash Prior To Co-processing In Cement Kilns

At present, the rapid expansion of municipal solid waste incineration （MSWI） makes theproduction of MSWI fly ash increase rapidly. Because of the high concentrations of heavymetal and dioxins, the fly ash is classified as hazardous waste and needs to be treatedharmlessly. The technique of cement kiln has become an effective method for theco-processing of MSWI fiy ash because of the advantages fixing heavy metal efficiently anddestroying dioxins completely. But the high content of chlorine salts in fly ash could volatilizein the high-temperature belt of cement kiln and condense in low-temperature belt, whichcauses crust and corrosion easily and affects the cement quality and the normal operation ofcement kiln.Thus, in order to remove the harmful chlorine element from fly ash, improve thedisposal capacity and provide the guaranty for the normal operation of cement kiln, the paperstudied the water-washing pretreatment of Beijing fiy ash and Chongqing fly ashsystematically. The main content of this paper includes the removal chlorine mechanism ofwater-washing process and the determination of the optimum water-washing parameters, theremove characteristics and the determination of the optimum technological parameters ofheavy metal in washing solution, and the screening for removal method of chlorine salts inwashing solution. The main results of research were shown as follows:The water-washing pretreatment could remove thechlorine from fly ash obviously, andthe removal rate of Beijing fly ash and Chongqing fly ash can reach93.71%and97.63%respectively when liquid-to-solid （L/S） was8, water-washing time was10min, water-washingtemperature was40⁵0℃and water-washing frequency was1. By adjusting the additiveamount of fly ash, the technology could reach the required chlorine content of cementproducts. In addition, the removal chlorine mechanism showed that the water-washing processfor removing chlorine included dissolution process and desorption process, when L/S is lessthan20mL/g, dissolution process was the dominant one and when L/S is more than20mL/g,desorption process was primary.The partial macro-elements and heavy metal were also washed out in water-washingprocess and the percentage of Na and K decreased significantly after water-washing, but Cawas opposite. The elution rate and concentration of Pb in washing solution were both high and the concentration had exceed the comprehensive sewage discharge standard. Thus, thestudy needs to take comprehensive treatment for washing solution before discharge.CO₂, Na₂CO₃, Na₂S all had good removal efficiencies for heavy metal in washingsolution of Chongqing fly ash. With30mL washing solution as the research object: theremoval rate of Pb, Zn, Cu, Cr can reach98.6%,99%,95%,45%respectively when the initialconcentration of Pb, Zn, Cu, Cr were18.95,5.50,0.56,0.37mg/L respectively （L/S=8mg/L）,CO₂concentration was10%, flow rate was2mL/min, feeding time was80s, CO₂amount was0.27mL and the system reaction temperature was about40℃. When the initial concentrationof heavy metal increased, increasing the CO₂concentration, feeding time or flow rate allcould reach the same treatment efficiency. In addition, the removal rate of Pb, Zn, Cu, Cr canreach97%,99%,84%,30%and99.9%,99%,95%,20%respectively when the dosage ofNa₂CO₃and Na₂S were1mmol and0.08mmol, reaction time was5min. The content of heavymetal in washing solution all could reach the comprehensive sewage discharge standard aftertreatment.The three kinds of the precipitation agent all make the heavy metal had a significanttreatment effect, but Na₂CO₃and Na₂S couldn’t decrease the pH of washing solution and hadto add other acid reagent to adjust. If the initial concentration of heavy metal in fly ash is veryhigh, increasing the dosage of the precipitation agent or adding several kinds of theprecipitation agent at the same time could reach the same treatment effect.The concentration of chlorine in washing solution was up to10000²0000mg/L andabout90%of chlorine exist in the form of NaCl and KCl, which was very harmful to water.Multilevel distillation, reverse osmosis and cryoapplication were considered some suitablemethods for chlorine salt by analyzing the characteristics of chlorine salt in washing solution,and considering the economic applicability, multilevel distillation and cryoapplication have abetter prospect of application.