Study On Precipitation And Membrane Separation Precipitation Treatment Of Ammonia Nitrogen In Coking Wastewater

In this study, sodium tetraphenylborate precipitation method and magnesium ammonium phosphate (MAP)-sodium cobaltinitrite composite precipitation method were respectively used to the treatment of coking wastewater. The results showed that, two methods could quickly remove ammonia nitrogen in coking wastewater, and ammonia nitrogen could be recycled by distillation. The sodium tetraphenylborate precipitation method was better than MAP-sodium cobaltinitrite composite precipitation method in ammonia nitrogen removal efficiency, and it can remove over 98% ammonia nitrogen in coking wastewater that the ammonia nitrogen concentration was 500mg/L, and the reuse rate of precipitating agent was more than 99%. However, the sodium cobaltinitrite precipitating agent can not be reused in MAP-sodium cobaltinitrite composite precipitation method. Therefore sodium tetraphenylborate were chosen to remove and recover ammonia nitrogen in coking wastewater.When direct addition of sodium tetraphenylborate was used to treat coking wastewater, because there were a lot of ions which can cause salt effect in wastewater, the excess precipitating agent was dissolved in wastewater, resulting CODcr of coking wastewater from 5103mg/L elevated to 179010mg/L.To solve this problem, membrane separation techniques and diffusion dialysis principle had been used. In this experimemnt, the ammonia nitrogen existed in the form of NH4+ in coking wastewater migrated to the other side of the diffusion dialysis membrane, then NH4+ reacted with sodium tetraphenylborate and generated precipitation, which can avoid the secondary pollution of coking wastewater caused by direct addition sodium tetraphenylborate.Cellulose acetate acetone solution in 3%,5%,7%(m/v) were respectively acted as the casting solution, and the solvent evaporation method was used to make membrane, then the effect of concentrations of cellulose acetate on membrane performance had been studied. Resulting from membrane toughness and the removal rate of ammonia nitrogen, the content of cellulose acetate in casting solution should be 5%.In this study, PEG-400, NH4Cl and Tween-20 were chosen to be pore-foaming agent.The effect of three pore-forming agents on ammonia nitrogen removal rate and sodium tetraphenylborate leakage rate were studied. The results showed that, pore effect of NH4Cl was negative. Within a certain range, content of PEG-400 and Tween-20 were larger, the pore effect was stronger, the removal rate of ammonia nitrogen was higher but the leakage rate of sodium tetraphenylborate was higher. When the content of PEG-400 was 4%(v/v) or the content of Tween-200 was 0.8% (v/v), the leakage rate of sodium tetraphenylborate was 0.75%. In this condition, PEG-400 as pore forming agent was better in treatment effect, ammonia nitrogen removal rate reached 90%. Acetone solution of cellulose acetate in 5% and PEG-400 in 4% should be used to make dialysis membraneThe effect of the composition of sodium tetraphenylborate solution on diffusion dialysis of NH4+ was studied. The results showed that, when the concentration of sodium tetraphenylborate was 15g/L, the ammonia nitrogen removal rate was 91.5% in coking wastewater. As the increase of sodium tetraphenylborate concentration, the removal rate of ammonia nitrogen decreased, and the concentration in the range of 10g/L to 15g/L had no obvious effect on the removal rate. When the concentration of PEG-10000 was lOg/L in precipitating agent solution, the leakage rate of sodium tetraphenylborate could be reduced to 0.75%, and the removal rate of ammonia nitrogen could be increased to 91.7% in coking wastewater. Thus, the components of mixed precipitant solution should be 15g/L sodium tetraphenylborate and 10g/L PEG-10000 in concentration, then ammonia nitrogen removal rate of the simulated coking wastewater and coking wastewater were 93.4% and 91.7% respectively, meaning that concentration of ammonia nitrogen could be reduced from 500mg/L to 35mg/L and 42mg/L.Calculated in Fick’s law, the diffusion coefficient of NH4+ in membrane respectively were 0.264 X 10-6cm2/s and 0.382 X 10"6cm2/s both in simulated coking wastewater and coking wastewater respectively. Other components in coking wastewater had impact on the diffusion of NH4+ in the membrane. Resulted from HPLC analysis, sodium tetraphenylborate was not detected in coking wastewater, showing that it didn’t leak through the membrane into the coking wastewater, so it would not cause secondary pollution of coking wastewater.