Research On The Modified Methods Of Fly Ash For Ammonia-nitrogen Wastewater Treatment

Fly ash is a solid waste produced from coal-fired power plants and smelting, chemical industry etc. by combusting coal at high temperature. Currently, fly ash has become one of the major industrial solid wastes in China. Fly ash was applied to wastewater treatment, because of its porous structure and specific surface area. Recently ammonia-nitrogen wastewater was harm to water quality and human health. The treatment technology had been demanded increasingly for low concentration ammonia-nitrogen wastewater as the decrease of emission concentration. So it received extensive attention using fly ash to treat the low concentration ammonia-nitrogen wastewater. At present, the main researches are to modify the fly ash by sodium hydroxide, sulfuric acid, surface active substances etc. to improve the treatment capacity of fly ash for heavy metal wastewater, printing and dyeing wastewater and other wastewater treatment. However, the removal capacity of ammonia-nitrogen was less than 45% in the reports about the research using modified fly ash to treat low concentration ammonia-nitrogen wastewater. How to improve the removal capacity of ammonia-nitrogen by using fly ash had become a problem.In this paper, the fly ash from Taiyuan First Thermal Power Plant was used as a raw material. The physical properties, chemical properties and adsorption properties of the raw material were analysed at first. Then the modification methods and modification process conditions were researched with sodium carbonate and sodium hydroxide as modifiers and low concentration ammonia-nitrogen wastewater as treatment object. The main researches were that:(1) The properties analysis of fly ash. The fly ash from power plant was classified, and the -200 mesh fraction was used as experimental material. The particle size, morphology, mineral composition, chemical composition and adsorption properties of fly ash were analyzed by laser particle size analyzer, scanning electron microscope, x-ray diffraction, x-ray fluorescence spectrometer, surface area analyzer and nessler spectrophotometric. The results showed that:the particle size of fly ash mainly was between 0.77μm-8.05μm, and its spherical particles were more regular; There was a gap between the ball and the ball and it is the material basis of fly ash with adsorption properties; The main mineral composition including the glass phase and crystal material; The crystal material mainly was mullite and quartz, and its main chemical composition was SiO2 and Al2O3 mainly in the form of glass phase, mullite and quartz; The nature of them was stable not to participate in reaction; The fly ash had a certain surface area and small cation exchange capacity, so it could adsorb the ammonia-nitrogen in wastewater, but the adsorption capacity was limited only for 14%.(2) The studies on the modification methods of fly ash. Three alkali modification methods (sodium carbonate calcination-hydrothermal treatment, sodium hydroxide hydrothermal treatment, and sodium hydroxide calcination-hydrothermal treatment) were explored to modify the fly ash. XRD, SEM, BET were used to characterize on the modified fly ash. Results from XRD analysis showed that the fly ash after sodium carbonate calcination-hydrothermal treatment formed nepheline and Na7Al7SiO16 new substances, after sodium hydroxide hydrothermal treatment the peaks of alumina disappeared in the modified fly ash and a number of silica was dissolved to give a porous structure, and after sodium hydroxide calcination-hydrothermal treatment formed a zeolite-like new phase and a porous structure. Observed by SEM the surface of modified fly ash became rough, and results from BET showed that the specific surface area of modified fly ash increased to varying degrees. It respectively increased from 0.16 m2/g to 1.046,7.216,275.708 m2/g. Three modified fly ash was used to treat 100mg·L-1 ammonia-nitrogen wastewater, and removal capacity was respectively 44%,58%,94% higher than 14% of the original fly ash. Experiments showed that fly ash by alkali modification can improve the activity and the ability of ammonia-nitrogen wastewater treatment, and the modified method of sodium hydroxide calcination-hydrothermal treatment was the best.(3) The researches on the modification conditions of sodium hydroxide calcination-hydrothermal treatment. The modification conditions of sodium hydroxide calcination-hydrothermal treatment were studied by method of single factor experiment with the effect of alkali content (0.2-1.2:1), the solid-liquid ratio (1:1-5) in hydrothermal reaction and calcination temperature (200℃,300℃,400℃,500℃,600℃,800℃). The results showed that, in the low alkali content condition of NaOH:ash=0.4:1, the optimal modified conditions for ammonia-nitrogen wastewater treatment was that:hydrothermal reaction of solid to liquid ratio was 1:1, the calcination temperature was 400℃. The modified fly ash formed a zeolite-like new phase. The cation exchange capacity of fly ash increased from 5 mmol/100g to 81 mmol/100g, and the ammonia-nitrogen removal capacity was 90%.In the three modification methods, the sodium hydroxide calcination-hydrothermal treatment was the best. The additive amount of sodium hydroxide could be reduced by the studies on the modification conditions of it, and the modified fly ash with excellent adsorption performance could be prepared at a low calcination temperature. It achieved the purposes of cost-efficient treatment of ammonia-nitrogen wastewater with fly ash and using waste to control waste.