Alumina from coal wastes through the formation of aluminum nitride

The carbothermal reduction of coal wastes under a nitrogen atmosphere to form aluminum nitride and silicon carbide was carried out in the present study. The leaching of the aluminum nitride to subsequently produce aluminum hydroxide was also investigated. Thermodynamic and kinetic factors such as reaction time, temperature, particle size, mixing, pelletizing, excess carbon, and nitrogen flow rate were considered. A minimum temperature of 1350°C and a reaction time of 1-2 hours were needed to produce aluminum nitride. Up to 98% of the alumina originally present was converted to aluminum nitride. When the temperature became too high, or the reaction time too long, the formation of aluminum nitride dropped off. This hindered the ability to completely leach the aluminum nitride. Optimum reaction times at various temperatures were determined. The optimal leaching conditions were found to be one molar sodium hydroxide for two hours at boiling temperature. Approximately 70% of the aluminum nitride could be decomposed by leaching. Before precipitation of aluminum hydroxide, calcium hydroxide was added to remove the iron and silica impurities. The overall recovery of alumina was determined to range from 40-60%. Silicon carbide and ammonia are valuable by-products of the process.