Study On Preparation And Drying Mechanism Of Municipal Sludge Dryer

In recent decades,with the continuous improvement of economic level and urbanization level in China,the discharge of municipal sludge is also rising.The sludge discharged by mechanical dewatering treatment still has a moisture content of about 80%,which can not be directly disposed of or utilized.Therefore,it is urgent to develop a kind of desiccant with high efficiency,low cost and good stabilization effect of heavy metals and to study its desiccation and stabilization mechanism.In this paper,a sewage treatment plant in Zhengzhou City,Henan Province,was selected as the research object.The basic physical and chemical properties of the dewatered sludge were tested through a series of experiments.The microstructure of the dewatered sludge was analyzed by means of SEM(Scanning Electron Microscope)、XRD(X-Ray Diffraction)、XRF(X-Ray Fluorescence)and TG-DTG(Thermal Gravity AnalysisDifferential Thermal Gravity).In order to treat wastes with waste,a series of industrial wastes(slag and fly ash)and cementitious materials(cement and quicklime)were used as raw materials to carry out preliminary experiments on the drying effect of sludge.Mixture experimental design method with upper and lower constraints was adopted.Finally,slag,quicklime and fly ash were selected as raw materials to develop sludge drying agent.At the same time,different factors were discussed.The mechanism of sludge drying was studied by XRD,SEM and TG-DTG.The leaching toxicity of heavy metals and the morphological distribution of heavy metals in sludge before and after drying were compared and analyzed.Through the above research,the main conclusions are as follows:(1)The moisture content of single material drying sludge decreases with the increase of drying material content and curing time.The drying effect of these four drying materials on sludge is in the order of quicklime > slag > fly ash > cement.The drying process of sludge water content by quicklime,fly ash and cement mainly occurs in the pre-curing range.In this range,the water content of sludge after drying of the three materials decreases significantly more than that of the original sludge.By analyzing the water resistance test of sludge after drying of a single material,it was found that physical evaporation method could not change the structure and morphology of sludge,and the change of hydrophilicity of sludge needed to be realized by chemical methods such as hydration reaction.By comparing and analyzing the drying effect of these four drying materials on sludge and the change of hydrophilicity of sludge,it is determined that the sludge drying agent is prepared by mixing slag,quicklime and fly ash.(2)The test scheme is designed by using the mixed material test module with upper and lower restrictions in DPS(Data Processing System).Considering the drying effect and economy,the optimum proportion of sludge drying agent is slag: quicklime:fly ash = 0.5567: 0.386: 0.0573.The sludge desiccant prepared with the optimum proportion was used to dry the sludge,and the results were compared with the quicklime with better drying effect in a single material.When the curing time is the same,the drying effect of desiccant on sludge has been greatly improved compared with that of quicklime.When the content of material is in the range of 15%-30%,the drying efficiency of desiccant is higher than that of raw lime.When the dosage of material is constant,the drying effect of quicklime on sludge mainly concentrates in one day,and the drying rate is almost the same as the natural evaporation rate of original sludge in the later period.The drying rate of sludge by desiccant is not only higher in the earlier period,but also increases with the increase of curing time,and then tends to be flat.Through the water resistance test of drying sludge,it can be seen that the water resistance of drying sludge increases gradually with the increase of curing time.Sludge desiccant will react with water in sludge to change the granular structure of sludge,reduce the hydrophilicity of sludge and increase the strength of sludge after drying.(3)The results of micro-test of desiccant slurry and desiccant desiccant sludge show that Ca(OH)2 is formed from quicklime in desiccant when it meets water.Si O2 and Al2O3 in slag glass react with Ca(OH)2 and water molecules under alkaline conditions.The main reaction products are ettringite and calcium silicate hydrate crystal.Drying agent will react with water with weak binding ability in sludge.The hydrated products include ettringite crystal and calcium silicate hydrate.The formation of these products will fill the pore in sludge,and the sludge will be firmly linked into a whole,thus improving the hydrophilic characteristics of sludge,increasing the hardness of original sludge and contributing to the further utilization of sludge resources.(4)Compared with the original sludge,the leaching concentration of heavy metals(Zn,Cu,Cr,Cd,As and Pb)in the drying sludge is obviously lower.The drying agent has better solidification effect on the heavy metals in the original sludge.Moreover,when the curing time is fixed,the leaching concentration of heavy metals in the drying sludge decreases gradually with the increase of the dosage of the drying agent,and the solidification effect is more obvious.Drying agent has the best curing effect on zinc and the worst curing effect on As.The leaching concentration of these six heavy metals decreases rapidly when the content of desiccant is between 5% and15%,and then decreases slowly.(5)The results of speciation test of heavy metals in dry sludge show that Zn and Pb in sludge dried by desiccant mainly exist in the form of residue and have low potential migration.The effect of desiccant on the solidification of these two heavy metals in raw sludge is obvious.The forms of Cu and Cr are mainly residue and potential migration.Compared with the original sludge,the extractable state of copper in the drying sludge is obviously lower.Cd and As exist mainly in extractable state and have strong potential migration.Compared with other heavy metals,Cd and As are easily transformed into the environment,so their potential risks should be paid attention to.