| During past years,the water quality has been improved with the application of dredging technique in the restoration of contaminated waters.However,an increasing awareness of a series of problems associated with the dredged sediments has been observed at the same time.The problems include a great quantity of sediments produced in dredging operation,the risks that dredged sediments may have on both the human health and the environment,shortage of land resources for sediment stacking,second pollution may be caused by contaminants in an unsuitable stocking process,and wastes of plant nutrient elements P and N and other resources,those problems get worse as the sediments dredged from eutrophic lakes or other contaminated waters.The problems derived from dredged sediment have contributed to the developments of its remediation strategies.Different approaches can be used to deal with dredged sediments depending on site-specific conditions,which including agricultural use,filling materials,as a resource for production of end products such as ceramics and glass tiles,bricks,soils,cement,lightweight aggregates,ceramsite.Among all the utilization approaches,the dredged materials used as a certain rate replacement for natural clay to make ceramsite is probably one of the most promising utilization strategies since most heavy metals existed in steady forms and immobilized within ceramsite,and organic matters were decomposed during the sintering process.On the basis of reviewing much literature,dredged sediments from Dianchi was selected as main raw material to study the preparation of ceramsite,and application of the cermasite in waste water purification and lightweight concrete preparation was investigated as well.The study contents are as follows:(1)Analysis and determination of the physical and chemical properties of dredged sediment,including the main chemical composition,the nutrient elements,particle size distribution,mineral composition,main heavy metals contents and thermal properties.(2)On the basis of understanding of sediment physicochemical properties,the research of ceramsite formula was studied by the single factor experiment,sintering system of preparing ceramsite was investigated by the orthogonal experiment.(3)The main characteristics of ceramsite were determined,including density,water absorption,broken rate,compressive strength,porosity,specific surface area,hydrochloric acid soluble rate,mineral composition,morphologystructure,heavy metal leaching rate.Removal of phosphate in waste water and lightweight concrete experiment were conducted using ceramsite with good performance.(4)In order to explore the characteristics of reaction kinetics in the ceramsite sintering process,a set of non-isothermal TG data at various heating rates in an air atmosphere was obtained to simulate the sintering process of ceramsite.The Coats-Redfern method was applied to treat non-isothermal TG data in order to calculatethe pre-exponential factor and activation energy of sintering process of lightweight aggregates.Through the above research on various aspects,the conclusions are drawn as followings:(1)Two types of sediments used in this study come from Lake Dianchi.The first one(labelled as A)was dredged from Panlong river which is one of rivers enter into Dianchi,and the second(labeled as B)one was taken from Caohai which is a part of Dianchi lake.A mainly contains SiO2 and Al2O3,B contains a lot of combustible organic matter.The composition of A is much closer to the composition requirements of ceramsite,and B can be used as pore-forming agent.Combining the composition requirements of ceramsite and experiments,A and B are slected as the main raw material,Na2CO3,Al2O3 as auxiliary materials for preparing ceramsite.(2)Results of the single factor experiment shows thattheraw material ratio A:B:Na2CO3:Al2O3 5:1:0.5:0.5 and 4:1,the performance of ceramsiteis good,the utilization ratio of sediment reaches 100%at 4:1,which is beneficial to sediment utilization.The results of orthogonal experiment and poor and variance analysis show that the best sintering system is thatsintering temperature is 1120℃,sintering time is 15 mins,preheat temperature is 450℃,the preheating time is 60 mins.(3)After optimization of ceramsite formula and sintering system,the basic properties of ceramsite meet "Water treatment with artificial ceramsite filter material" requirements.At the same time,through SEM observation,it can be seen that the ceramsite internal space is rich,appears caking phenomenon,which make the ceramsite have certain compressive capacity.EDS test shows that ceramsite mainly contains Si,Al,Na,Ca,Mg,Fe,K and other elements,and Si and Al content is relative higher than others,which is conducive to the formation of the strength of ceramsite.(4)Combining the experiments and analyses,it can be obtained the following conclusions:(1)the heating rate had an effect on the weight loss rate of the sample,total weight loss rate increaseswith the increase of heating rate,when weight loss rate reaches the maximum at heating rate of 20℃·min-1,at heating rate of 40℃·min-1,the total weight decreases slightly.(2)The Coats-Redfem method was useful in analyzing the reaction mechanism of the sample sintering process for all the fitting level was high.The obtained values of E are between 12.59 kJ·mol-1～427.63 kJ·mol-1,the values of A are between 2.23 s-1～9.13×1026 s-1.(3)The combustion process of the sample at lower heating rates(5 ℃C·min-1 and 15℃ min-1)was assumed to be simple diffusion reaction,The combustion process of the sample at lower heating rates(20 ℃·min-1 and 40℃ ·min-1)was assumed to be a combination of a series of complicated reaction including diffusion reaction,nucleation and reduction chemical reaction.(5)After ceramsite used in lightweight concrete and waste water purification experiment shows that when the ceramsite raw materials ratio is 4:1,the strength grade of the lightweight concrete accord with requirement of LC30,the dry apparent density of 1754.96 kg/m3 conforms to the requirements of lightweight aggregate concrete,and can be used for bearing component or structure.(6)It can be seen from ceramsite purifying the wastewater experiment,and the effect of removing phosphate in the waste water is better,and ceramsite raw material ratio at 5:1:0.5:0.5,removal efficiency can reach 72.808%,so the ceramsite can be used in the purification of the wastewater containing phosphorus.In this study,dredged sediment from Dianchi was used as the main raw material,adding proper amount of admixture to prepare ceramsite with high specific surface area,high surface roughness,and wide application,which provide a theoretical basis and direction for treatment and disposal of dredged sediment. |
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