Experimental Study On Solidification Of Chromium-bearing Sludge By Circulating Fluidized Bed Combustion Ashes-based Geopolymer

The chromium-bearing sludge was classified as a kind of hazardous waste due to the large quantities of leachable toxic chromium ions.Therefore,the chromium-bearing sludge must be stabilized before the landfill.Geopolymer is inorganic polymeric material with stable performance and formed in cage structure,which show excellent performance in terms of solidification of heavy metals.Immobilization of chromium-bearing sludge with geopolymer made by Circulating Fluidized Bed Combustion ashes(CFAs)has the advantage in reducing the harm to environment and also prevent the chromium ions in the chromium-bearing sludge from diffusing to the environment effectively.According to the analysis of physico-chemical properties of CFAs,this work focused on the experiment and activation mechanism of alkali fusion process.The experimental result showed that,the optimum mass ratio of sodium hydroxide to CFAs was 0.6,and the effective alkali fusion condition could be achieved at 550 ? for 60 min.The activation mechanism of alkali fusion as follows:The crystal of quartz in the CFAs was destroyed;SEM analysis of alkali fusing CFAs showed a large number of internal spaces;Alkali-fusion method could reduce the degree of Si-O-Si or Al-O-Si polymerization and depolymerize the reticular structure of sillico aluminate.After that,alkali fusing circulating fluidized bed combustion ashes(f-CFAs)was used for synthesizing the geopolymer,which was used to solidify the chromium-bearing sludge.The results showed that the optimum mass ratio(chromium-bearing sludge:f-CFAs:MK:sodium silicate)was 5.4:4:1:5.2,in the optimal curing temperature of 25? and curing time for 28d.The CFAs-based Geopolymer could reach 6.52MPa when 52%chromium-bearing sludges were added.The solidification needed lots of chromium-bearing sludges and met the requirements of landfill disposal.The leaehing concentrations of Cr is 0.93mg/L,much lower than the limits specified in the national standard GB 14569.1-2011.Systemic physico-chemical and structure charaterization by methods that included X-ray diffraction(XRD),scan electron microseopy(SEM)and infrared spectroseop(IR).Besides,a mechanism accounting for immobilization of the Cr from the chromium-bearing sludge with geopolymer has been presented.It is concluded that the mechanism accounting for immobilization of the Cr is a combination of physical sealing and ion exchange.