The authors used ash from the Reftinskaya state district power plant, Russia’s largest coal-fired power plant, which is located in the Sverdlovsk Region. First, they dissolved ash in a mixture of sulfuric acid and ammonium bisulfate (an oxygen absorber) for three hours at 200 degrees Celsius. After cooling this solution, the authors obtained aluminum ammonium alum, a complex salt of ammonium, aluminum and sulfuric acid.
The scientists removed iron impurities from this salt, dissolved it in water and precipitated it in the form of mineral alunite on the surface of another mineral, boehmite (aluminum hydroxide). After this, the authors washed the alunite with sulfur ammonia and fired it in a furnace at 950 degrees Celsius. As a result, they obtained metallurgical alumina, an aluminum oxide whose chemical composition complies with the technical standards adopted in Russia and China.
In the course of the experiment, the scientists changed the volumes of the reagents and the temperature and duration of precipitation in order to determine which conditions are conducive to obtaining the greatest amount of alunite. They found that the largest amount of alunite, a synthesis intermediate, can be obtained within eight hours by heating the solution to 90 degrees Celsius.
“The proposed method is innovative primarily thanks to the possibility of precipitating aluminum in the form of alunite without using additional reagents like gaseous ammonia. The use of boehmite instead of the traditionally used gibbsite as seed made it possible to reduce the alumina calcination temperature by 300°C. As a result, energy usage was reduced by 30%. In the future, we plan to optimize the duration of the precipitation process and obtain pure boehmite powder without sulfur impurities so as to eliminate the stage of additional purification,” Dmitry Valeev, candidate of technical sciences, is quoted as saying by the Russian Science Foundation.
