Ammonia Nitrogen Removal From Coking Wastewater By Chemical Precipitation And Repeated Use Of MAP

The removal of ammonia-nitrogen from coking wastewater by chemical precipitation is a new technology of rapid reaction and high removal efficiency, especially for getting rid of high concentration of ammonia nitrogen from wastewater and it can be used as an effective supplement of biodenitrification.Using simulative wastewater from laboratory and coking wastewater from WISCO, MgCl₂·6H₂O and Na₂HPO₄·12H₂O were added into these kinds of wastewater as precipitator, and precipitation occured at an suitable condition to produce a sediment of magnesium ammonium phosphate. This way could remove the ammonia-nitrogen in wastewater. The ammonia nitrogen removal efficiency under different rations of material, pH and operation conditions was studied. By orthogonal and single factors experiments the paper drew an optimal reaction condition. At room temperature, ammonia nitrogen could be removed under the optimal reaction conditions: pH, 9 ⁹.4, PO₄^3-: NH₄⁺ molar ratio, about 1.3, Mg²⁺: NH₄⁺ molar ratio, 1.2¹.5. Under these conditions, the removal rate of ammonium-nitrogen is over 89%. Dealing with ammonia vaporizing wastewater by chemical precipitation, pure MAP crystal were obtained, particulate size generally between 5～20μm.The residues of magnesium ammonium phosphate (MAP) decomposed by heating under alkali conditions were repeatedly used as the sole sources of phosphate and magnesium for the removal of high ammonium concentration from wastewater. When MAP were repeatedly used for the first time, up to 93% of ammonium in MAP powder could be released under the following conditions: NH₄⁺: OH- molar ratio, 1:1, temperature,100℃, heating time ,3 h. And in the 6th cycle, ammonium removal of 77% was achieved for wastewater. Scanning electron microscopy analysis of the surfaces of MAP before heating demonstrated that MAP were rhombic crystal, particulate size generally between 5～20μm. Fourier transform infrared spectra analysis of MAP before and after heating demonstrated that MAP was mainly transformed to amorphous magnesium sodium phosphate (MgNaPO₄), which makes it possible for the NH₄⁺ to replace Na⁺ in MgNaPO₄ to form more stable struvite.The chemical precipitation provides a new way to treat wastewater with high strength ammonia nitrogen, and it can be used as a composite fertilizer. Chemical precipitation has broad application prospects.