Combined stripping-struvite precipitation process for treating high ammonia-nitrogen wastewater generated by the craft of vanadium extraction

Abstract

The direct discharge of high ammonia–nitrogen (NH₃–N) wastewater generated during vanadium extraction can cause serious pollution of ecosystems. The results of this study indicate that the optimal process conditions are as follows: NH₃–N wastewater pretreated by the stripping method in the first stage, with an initial pH of 11, stripping temperature of 40 °C, and gas–liquid ratio of 3600 (L/L); followed by a second stage involving the struvite precipitation method, with a reaction temperature of 25 °C, pH 9.5, n(Mg²⁺) : n(NH₄⁺) ratio of 1.15, n(PO₄³⁻) : n(NH₄⁺) ratio of 1.05, and a precipitation time of 25 min. Under these optimal conditions, the recovery of ammonia–nitrogen in the first stage could reach 90.61% (reduced from 23 941 mg L⁻¹ to 2249 mg L⁻¹). In the second stage, using a response surface methodology, the main influencing factors were optimized, and a high-fitting quadratic response surface model was obtained. This optimization led to an ammonia–nitrogen recovery of 95.5% (reducing the ammonia–nitrogen content from 2249 mg L⁻¹ to 101 mg L⁻¹). The final precipitate analysis showed that struvite (magnesium ammonium phosphate, MAP) crystals were the main component, with higher purity, which could be used as a compound fertilizer. The study showed that the combined process of stripping and struvite precipitation could effectively remove ammonia–nitrogen from high-concentration ammonia–nitrogen wastewater generated during vanadium extraction and that the reaction conditions optimized by response surface methodology ensured good operability and accuracy.