Abstract: In order to ensure that the effluent quality is stable and up to standard and solve the prob‐ lem of excessive total nitrogen in the circulating water of mineral processing，an enterprise carries out a study on the improvement path of water quality purification in combination with the water quality character‐ istics and site conditions of the circulating water of mineral processing，aiming at the problems such as the lack of nitrogen removal function and the hardening of the filter material in the sand filter in the existing sewage treatment station. Through the methods of current situation investigation，water quality analysis， process comparison，scheme design and cost-benefit evaluation，it is determined that the two-stage AO pro‐ cess is used to transform the existing regulating tank. The treatment process of grid → two-stage AO tank → coagulation sedimentation tank → sand filter → clear water tank is adopted，and the new carbon source dosing，aeration，reflux and other systems are added. The design treatment scale is 2 000 m3 /d.The results show that the total nitrogen removal rate is 94%，COD removal rate is 75% and suspended solids re‐ moval rate is 92% after the transformation. The effluent quality meets the Discharge Standard of Pollutants for Iron Ore Mining and Dressing Industry（GB28661-2012）and enterprise internal control requirements. The total investment of the project is 1.346 2 million yuan，and the new water operation cost is 1.444 yuan/t. The secondary AO process is suitable for the high nitrogen removal demand and land restriction of the en‐ terprise.The transformation scheme is technically feasible and economically reasonable，which can effec‐ tively solve the problem of total nitrogen exceeding the standard，and has environmental，social and eco‐ nomic benefits. It can provide reference for wastewater treatment of similar mineral processing enterprises.

Key words: circulating water of mineral processing, total nitrogen removal, secondary AO process, process transformation, water purification