Characteristics Of Chromium Contaminated Soil And Ceramsite Sintering

With the rapid development of urbanization, a large number of companies movedout of the city, leaving the contaminated soil as a big problem for city construction.Among the heavy mental pollution, chromium pollution gained more attention becauseof its carcinogenic and teratogenic effects. To date, China is in the “mop-up stage” ofchromium disposal; and the remediation of chromium contaminated sites has just begun.Therefore, it would be of great significance to study the treatment or utilization ofchromium contaminated sites.In this paper, the study centers on the collection of chromium contaminated soilsamples and their utilization. The main contents include: surveying and evaluatingselected sites, analyzing characteristics of typical chromium contaminated soil samples,screening disposal or treatment methods, selecting antidote, making ceramsite,optimizing the firing process, analyzing the firing mechanism, the mid-experiment ofsintering ceramsite, testing and applying the ceramsite products, and so forth.The original site of Mingfeng Chemical Factory was chosen as the research area,and the pollution degree was assessed by verifying the samples collected from the studyarea. The potential ecological risk assessment and health risk assessment wereintroduced to determine the risk degree of the selected site as well as to obtain thelimited values for restoring. Afterwards, high concentrations of chromium contaminatedsoils was selected to delve into their physical and chemical properties and kineticcharacteristics of thermal reaction by way of X-ray diffraction analysis, Fourier infraredspectroscopy analysis, laser particle size analysis, static nitrogen absorption BET andpore size analysis, atomic absorption spectrophotometry, and TG-DSC method.Characteristics of heavy metals were analyzed by TCLP, step by step extraction, andthermal sintering method, thereby determining the appropriate disposal or treatmentmethods for chromium contaminated soils.Fly coal ash, coal gangue, and sludge were chosen as the antidote; and theirphysical and chemical characteristics were analyzed. The single factor experiment wasconducted to analyze raw material ratios, sintering temperature, sintering time, Cr （VI）leaching concentration, particle strength, surface particle density and one-hour waterabsorption, and so on. Sintering process of ceramsite was optimized by orthogonalexperimental design. Thereafter, the second optimization of ceramsite raw material ratio was conducted by uniform design method based on the advantages and disadvantages ofceramsite. The evolution of ceramsite was simulated by a quadratic polynomialnonlinear fitting, according to the most probable mechanism function. The mechanismof ceramsite formation and chromium solidification were studied by observing thesurface and the microscopic structural characteristics with X-ray micro-spectroscopyscanning electron microscope.The results are shown as follows：Most of the research areas were heavily polluted. The soil quality cannot meet theHJ305-2007and also cannot used as stadium land, green land, commercial land, andpublic municipal land. In potential ecological risk assessment,31.3%of soil sampleshad high ecological risks. In health risk assessment, all HQ and CR values exceeded thethreshold, posing a great threat to human health. The repair limit values of Cr （III） andCr （VI） were8621.8and5.3mg/kg, respectively.Most soil in study area was small or mid-sized. The pore was large and had nostrong absorption, and its structure was inconspicuous. Soil was rich in SiO₂, Al₂O₃, andFe₂O₃. The main crystal type was quartz, and did not have thermal properties. The totalchromium content was1726.3mg/kg, and the leaching concentration of Cr （VI） was60.25mg/l, indicating that it is not a hazardous waste. Chromium migrated easily in thesoil, with residual> weak acid extractable> reducible> oxidisable. The dryingdetoxification programs had more advantages than leaching and cement solidificationby comparing the disposal method of soil.Additives of fly coal ash, coal gangue and sludge were rich in SiO₂, Al₂O₃, tonerresidual and organic ingredients, generated CO under high temperature to deoxygenizeCr （VI）, and eventually promote the expansion of ceramsite. The losses of threeadditives were5.97%,28.60%, and33.24%, respectively. TG-DSC curves evinced thatthere was a major phase of thermal loss at atmospheric conditions or different heatingrates.The optimal process conditions for sintering ceramsite of chromium contaminatedsoils and fly coal ash were25%of fly coal ash addition, the temperature of1120℃, andsintering time of10min. The characteristics of ceramsite were as follows: the leachingconcentration of Cr （VI） was0.042mg/l; the strength of particle was600N; the densityof particle was1.19g/cm3; one-hour water absorption was15.2%. Similarly, the optimalprocess conditions for sintering ceramsite of chromium contaminated soils and sludgewere12%of sludge addition, the temperature of1030℃, and sintering time of10min, with0.067mg/l leaching concentration,440N particle strength,1.08g/cm3particledensity, and20.2%one-hour water absorption. Temperature was the main factor forsintering ceramsite of chromium contaminated soils and fly coal ash, while both thecontent of sludge addition and sintering temperature were important for sinteringceramsite of chromium contaminated soils and sludge. Furthermore, ceramsite ofchromium contaminated soils and coal gangue was abandoned for its deliquescent.The constraint mixing method was introduced to optimize the raw material ratios.The results indicated that75.1%of chromium contaminated soil,18.7%of fly ash, and6.2%sludge; the total heat loss rate of ceramsite billet decreased with the heating rateincreased, while the peak temperature of DSC curves increased as the heating rateincreased. The thermal reaction of the billet involved water volatilization, combustionof organic compounds, thermal decomposition of carbonate, oxidation of residualcarbon, and melting process of metal oxides. The increase of heating rate was conduciveto reducing the ceramsite quality. More intense reaction between the components of thebillet was good for firing ceramsite.30K/min was generally considered as the optimalheating rate for sintering ceramisite.The diffraction peaks of ceramsite billet had no significant difference at differenttemperatures, signifying that very few new substances were generated during thesintering process. However, the contents of each phase were quite different as thetemperature increased, and a large number of amorphous materials were produced at1150℃. After sintering, SiO₂, metal or nonmetal oxide, silicon oxide in themontmorillonite clay, and aluminum oxide in ceramsite billet reacted to generate a chainor skeleton-like structure of silicate. Meanwhile, other chemical groups have all beendestroyed. R3mechanism function was the most probable mechanism function forsintering ceramsite. Three kinetic factors of fitting equation were Ea=214.37kJ/mol,A=4.80E+06(s^-1), and G （a）=1-（1-a）^1/3.The mechanism of chromium solidification was as follows. Cr （VI） wasdeoxygenize by C and CO at the temperature of more than800℃. Cr （Ⅲ） may enterSi-Al-O structure of clay when temperature was greater than900℃. Cr （Ⅲ） oxide andsilicon and aluminum oxide in raw material would generate NaCrSi₂O₆or CaCrAlSiO₆.Fe （Ⅲ）, Cr （Ⅲ）, and Al （Ⅲ） have similar radius and same charge. Glassy substances ofCr （VI） can not only play a fixed role, but also prevent re-oxidation of Cr （Ⅲ）substances.Rotary kiln experiment was conducted to determine the optimum sintering conditions for sintering ceramsite. The optimum conditions were as follows:Temperature of kiln preheating is350℃, the sintering temperature was1120℃, and thehold time was2min. The characteristics of ceramsite were0.065mg/l Cr （VI） leachingconcentration,540N particle strength,530kg/m³particle density,7.2%one-hour waterabsorption, meeting the standards of lightweight aggregate. Results of leaching heavymetals shown that heavy metals in ceramsite leaching solution were much lower thanthe hazardous waste identification standards （GB/T5083.3-2007）, wastewater dischargestandard （GB8978-1996）, and landfill standards （GB16889-2008）, revealing theirenvironmental safety.