Advanced Treatment Of The Ammonium-Nitrogen Wastewater From Rare Earth Industry By La-loaded Fly Ash Adsorbent

There is a unique advantage of rare earth resources in China, and the existing rare earth resource in Baotou is accounted for 83 percent in the entire country. With the development of rare earth industry in China, the pollution of the rare earth industry was increasingly prominent.The existing rare earth smelting process was complex and a large number of wastewater was generated by the introduction of chemicals. Ammonia-nitrogen wastewater is one of the three kinds of rare earth industry wastewater. Some untreated rare earth industry wastewater was discharged in Baotou, which caused the water quality of the Yellow River in Baotou deteriorated year after year. Therefore, it is imperative to treat the ammonium-nitrogen wastewater of rare earth industry effectively.On the base of document survey and scene investigation, chemical precipitation-stripping process was chosen as a pretreatment method for the treatment of ammonium-nitrogen wastewater from rare earth industry in this study. Adsorption was chosen as an advanced approach and its' related researches were conducted centralized. Considering treat the wastewater by waste, fly ash was chosen to be the carrier, after the pretreatment of dipping by sulfuric acid, rare earth oxide La2O3 was loaded on fly ash by the method of precipitation to preparate La-loaded fly ash adsorbent. The influence of the preparation conditions on the activity of adsorbent was inspected. The influence of the size of adsorbent, the dosage of adsorbent, the pH value of the wastewater, the adsorption time and the coexistence cation on adsorption was studied by static adsorption experiment. Adsorption thermodynamics and kinetics were researched by the determination of the adsorption isotherm and rate curve, and the performance of the regenerated adsorbent was researched. The physical properties, crystal structure and surface chemical composition of La-loaded fly ash adsorbent were analyzed by SEM, XRD and XPS.On the basis of research above, the mechanism of the ammonium-nitrogen adsorption by La-loaded fly ash was analyzed. Finally, the chemical precipitation-stripping-adsorption combined process for the ammonium-nitrogen wastewater of rare earth industry was proposed. And the technical-economic analysis of the process was conducted.The results shows that:(1)The optimum preparation condition of La-loaded fly ash adsorbent was when the concentration of La3+ was 0.35%, the pH value in the dipping was 11, the microwave irradiation power was 385 W and the irradiation time was 10 min. The impact of various factors on ammonium removal according to the order was the concentration of La3+ dipping> microwave irradiation power> the pH value in the dipping> microwave irradiation time.(2) While the size of adsorbent above 160 sieve, the dosage of adsorbent was more than 50 g·L-1, the pH value of wastewater was higher than 10 and the adsorption time was 120 min, La-loaded fly ash adsorbent had the optimum removal efficiency for ammonia nitrogen. Under alkaline condition, it was uneasy for La2O3 to dissolve out, and helpful for the removal of ammonia nitrogen. During 3 hours'adsorption, the attachment of La2O3 was stabilized on fly ash.(3)La-loaded fly ash adsorbent for the adsorption of ammonia nitrogen was more in line with Freundlich isothermal equation by fitting adsorbent adsorption isotherms, and the adsorption process was easier to carry out. Compared with the fly ash and pretreated fly ash, the La-loaded fly ash adsorbent had better adsorption properties.(4)The adsorbing capacity of La-loaded fly ash adsorbent for ammonia nitrogen changes with time was in line with Bangham equation, which revealed that this adsorption was fit for the first kinetics equation. Through the research of the adsorbent regeneracy, it presented that when pH<6, the adsorption capacity of adsorbent was very low. On the contrary, when pH>6, it was, which indicated that alkaline regeneration conditions was profit to the abatement of ammonia nitrogen. After the first time of regeneration, the adsorption capacity of adsorbent was 8.38 mg·g-1; after six times, the was still in a certain degree; but after 7 times regeneration, the capacity became very low.(5)A preliminary study on the adsorption mechanism showed that the ammonia nitrogen removed by La-loaded fly ash adsorbent was mainly achieved through adsorption and ion exchange, including the physical and chemical adsorption of ammonia nitrogen by the crystal structure of fly ash, the aluminum ion exchange adsorption, the ion exchange adsorption and complex reactions caused by lanthanum oxide surface hydroxyl groups.According to the research, the advantages and disadvantages of the present treatment process were analyzed. Combined with pretreatment process and in-depth treatment process, the chemical precipitation-stripping-adsorption combined technology were finally designed for the treatment of ammonia wastewater of rare earth which was smelted by wet method. To meet the same effluent ammonia concentration, in terms of investment estimation or operating cost, this technique is more practical.