| The coal resources are abundant in our country, mainly distributed in Inner Mongolia, Northeast China, Yunnan and other arid regions. And the lignite with high moisture content (20%～50%) has to be dehydrated before burning. In this paper, the water after the lignite dehydration would be collected, detected and purified for secondary industrial water in order to reduce the water consumption for power plant and local residents.In this paper, the evaporation, microwave and hydrothermal methods were used for the dehydration of lignite and recycle of the collected water. After the analyses of coal quality, moisture detection and purification recovery, the three methods were compared from the dehydration efficiency, the modification of lignite and the pollution of the water.In this way, it will find the best method and provide an industrial feasible reference for purification of lignite condensed water.From a series of physical and chemical properties analyses of lignite composition, microstructure, micro pore structure, it can be concluded that the three methods have different dehydration ability. And the dehydration efficiency increases with the increasing of the working temperature or power. From the comprehensive dehydration efficiency and the lignite modification, the hydrothermal and microwave methods were better than evaporation modification method, which was more suitable for lignite modification and water recovery.By detecting the chemical oxygen demand, ammonia nitrogen, total phosphorus, hardness, ions and organic compounds, the quality analysis of lignite water indicated that the water after evaporation and microwave methods was close to industrial standard and the condensed water after hydrothermal method was beyond the standard range. The impurity content under the working condition of250℃reached the maximum. In addition, trace amount of toxic metal elements, toxic carcinogenic polycyclic aromatic hydrocarbons and phenolic compounds were detected in the condensed water after three methods treatment, which will be the focus of subsequent research work.The flocculation precipitation-filtration purification test of mixed water after hydrothermal method was conducted, and the three kinds of flocculants: polyaluminium chloride (PAC), polyaluminium chloride (PAFC), polyacrylamide (PAM) were used. The optimum flocculation conditions were confirmed by flocculation coarsening tests, refining tests and pH adjustment tests. The experiment results shown that the flocculants effect of PAC and PAFC is similar, which was difficult to form solid alum, and the flocculants time was long. In the flocculation process of PAM, the solid alum was easy to form as a compact fine solid, and the alum was easy to separate from water. Therefore the flocculation effect of PAM was better than PAC and PAFC from the comprehensive purification efficiency and economy.From the activated carbon absorption experiments of condensed water after hydrothermal, evaporation and microwave methods, it shown that the activated carbon amount, adsorption time and pH value were vital factors influencing the adsorption. The experiments confirmed that the optimum adsorption time for the mixed water was60min, and the optimum dosage of activated carbon was500g/L for hydrothermal method,50g/L for microwave and evaporation methods.The water after activated carbon adsorption was treated by ultrafiltration filtration, in which the organic content was greatly reduced, the cations and anions changed a little, the contents of COD, NH3-N, TP and turbidity reduced in the standard range. Therefore after a series of purification treatment of flocculation precipitation filtration, activated carbon adsorption and ultrafiltration filtration, the dehydrated water from three different methods reached to the quality standard of secondary industrial water. |
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