Study On Recycling Ammonia Nitrogen In The Coking Wastewater By Dialysis-Precipitation Combined Technology

Sodium tetraphenylborate is a compound with good chemical stability and higher molecular weight. It can react with ammonium ions specifically in neutral or alkaline conditions. In this thesis, polyethylene was selected to develop a diffusion dialysis unit which separated ammonia nitrogen out from coking wastewater via dialysis membrane, filtrating ammonium ions in wastewater, and then ammonia nitrogen in coking wastewater was precipitated and recycled by sodium tetraphenylborate.The main results are as following:Having Compared the separation effect of commercially available cation exchange membrane and homemade cellulose acetate membrane to ammonia nitrogen in coking wastewater, the result showed that under the same operating conditions, cellulose acetate membrane dialysis device has higher removal efficiency to ammonia nitrogen than cation exchange membrane dialysis device, respectively 40.1% and 30.6%.In the cellulose acetate membrane dialysis device, the impact of the concentration of polyethylene glycol 10,000(in composite precipitant) and sodium tetraphenylborate on removal efficiency to ammonia nitrogen in coking wastewater was studied. It showed that, 95.3% ammonia nitrogen in coking wastewater was removed by cellulose acetate membrane device in 10 hours in the conditions of concentration of sodium tetraphenylborate and polyethylene glycol 10,000 being kept within 5-7g/L and 10-14g/L respectively by refilling composite precipitation agent continuously. The membrane diffusion coefficient was 0.59×10-6cm2/s calculated by the Fick’s law.Processed coking wastewater was analyzed by high performance liquid chromatography and sodium tetraphenylborate was undetected in it. This showed that the sodium tetraphenylborate in composite precipitate wouldn’t enter coking wastewater via membrane which avoided the secondary pollution during the process.After separating ammonia nitrogen from coking wastewater by methods of diffusion dialysis and chemical precipitation, the recovery of ammonia nitrogen has been studied by distillation and ion exchange. The result showed that 99.7% ammonia nitrogen has been recycled by distilling tetraphenylborate-PEG 10000 precipitation, however, only 30.5% sodium tetraphenylborate in the remaining could be reused, which means a high operating cost and unfeasible. The recovery rate of ammonia nitrogen can reach 98.3% by dissolving precipitate in acetone and then exchanging cations. The average recovery rate of sodium tetraphenylborate was 96.2% after repeating the dissolving-exchanging process five times.Under alkaline conditions, the ratio of sodium phosphate, disodium hydrogen phosphate and magnesium chloride was made to 8.5:1.5:10, then NH4+ in eluent was recycled in the form of magnesium ammonium phosphate and the recovery rate was 92.5%. Resource recovery of ammonia nitrogen in coking wastewater was well achieved.The study also showed, acetone solvent could be recycled in the experiment by distillation, and alkaline resin elution could be reused, without discharging, which means the process has preferable economic and environmental benefits.