<p>The development and utilization of mineral resources are accompanied by the production of a large number of solid wastes such as tailings and smelting slag. Bayan Obo tailings and blast furnace slag were used as the main raw materials. Coal gangue was used as pore-forming agent to prepare ceramsite which can efficiently treat ammonia nitrogen wastewater. The optimum preparation process parameters were obtained. The mineral evolution process of ceramsite prepared by smelting solid waste during roasting was clarified. The effects of sintering process parameters on the properties of ceramsite and its removal of ammonia nitrogen wastewater were revealed. The results show that, the optimum proportion of raw materials for preparing ceramsite was: 25% Bayan Obo tailings, 65% blast furnace slag and 10% coal gangue. The reasonable process for preparing ceramsite was: temperature of 400 °C, preheating for 20 min, heating rate of 10 °C/min, calcination at 1090 °C for 15 min, and cooling with the furnace. With the increase in calcination temperature, the main crystal phase changes from dolomite, kaolinite, fluorite and calcite to melilite and Fe₂O₃. Finally, the ceramsite with porosity of 48.13%, specific surface area of 2.44 m²/g and soluble rate of hydrochloric acid of 1.88% was prepared. The removal rate of ammonia nitrogen wastewater by the ceramsite was 54.13%.</p>