Magnesium Ammonium Phosphate Reuse Process For Ammonia Nitrogen Removal In Coal Gasification Wastewater

The development of coal gasification technique is rapid in China. Given the situation that China is lack of oil and gas, but rich in coal as well as policy adjustment on energy structure,in the next few decades, coal gasification technique will play an important role in the new energy resource market. However, during the coal gasification process, a typical coal chemical wastewater, coal gasification wastewater (CGW), was produced which containing high concentration of toxic pollutants and ammonia nitrogen. Thedevelopment of coal gasificationenterprisesare hindered by the treatment of CGW. Ammonia removal from the CGW without environmental hazard is significant.Magnesium Ammonium Phosphate (MAP)method has been found to be an effective process for theammonium nitrogen removal due to its high reaction rateand low remanent ammonia nitrogen concentration. The MAP method is conducted by adding phosphate and magnesium salt into the wastewater to form MAPcrystal which can remove ammonium nitrogen effectively. The operating conditons, influence fator and the chemical sluge were the key points investigated in this study. The response surface methodology, crystallization, crystal structure, molecular weight distribution and reuse of MAP with electrodialysis were applied to investigate the ammonia nitrogen removal in coal gasification wastewater combining with the use of high performance liquid Chromatograph (HPLC), X ray diffracmeter (XRD)and Fourier transform infrared spectroscopy (FTIR). In this study, experiments for operating conditons of MAP, influences of organics and molecular weight distribution on MAP and recycle use of MAP with electrodialysis were carried out and the results were as following:(1)The optimization of operation conditions for ammonia removal with MAPThe optimum operating conditons were as following:pH=9.5, Mg:N=1.1～1.2, P:N=0.8～1.1. the range analysis indicated that Mg:N was more positive for ammonia removal than P:N. The characterization of the precipitation showed that the main phase was Magnesium Ammonium Phosphate.The microscopy showed that the shape of the crystal was rhombus.(2)Exploring the influence of organics on ammonia removal in CGW with MAPHeterocyclic and phenolic compounds suppressed ammonium nitrogen removal by MAP crystallization. with the addition of pyridine, quinoline, m-cresol, dimethylphenol and phenol. The suppression of heterocyclic compounds was more significant than that of phenolic compounds. The complexation and adsorption result in the suppression of heterocyclic and phenolic compounds on the ammonium nitrogen removal by MAP crystallization.(3)Investigating the effects of organic constituent on ammonium nitrogen removal in MAPIn terms of this coal gasification wastewater, the main dissolved organics concentrated on the MW>500D constituent,which characterized by COD and UV254 take account for 55% and 65%,respectively.The MW>500D constituents in coal gasification wastewater can act as the inducer for the formation of MAP crystal, which will promote the ammonia removal by MAP. The organics in the fraction of MW<500D will change the shape of MAP crystal, resulting in the effect of ammonia removal by MAP decreased.Due to the adsorption and homogeneous phase crystal formation, the organics in the gasificationwastewater of MW<500D fraction.(4)Recycle Use of Magnesium Ammonium Phosphate to Remove Ammonium Nitrogen from coal Gasification WastewaterThe operating conditions are as following:the current density ranging from 70 to 80 mA/cm’2. operating time from 2 to 3h, the conductivity of cathode chamber beyond 1900 ms/cm. The limiting current density of the CEM is enough for the normal operating conditions, avoiding the concentration polarization and water decomposition, resulting in the economy of this technology.The removal ratio of ammonium nitrogen remained stable with the rise of the circulation times. The magnesium in the MAP solution can avoid the formation of CaCO3 on the surface of CEM. Another advantage of the novel electrodialysis with CEM technique is that the cost of electric power is much less than that of the MAP pyrogenation technique.