Aluminium winning from coal ash as substitute for bauxite - by leaching with hot nitric acid to obtain aluminium nitrate roasted to produce high purity alumina

摘要

The combustion residue or ash, obtd. from burning a solid fuel contg. carbon, contains by wt. max. 4% Fe calculated as Fe2O3, and max. 2% carbon. The ash is treated by the steps: (a) pretreating with excess HNO3 at a temp. (Ta) and removing the HNO3 soln. (a1) thus obtd. (b) the residue from step (a) is leached with excess HNO3 of 30-65 wt.% strength at Tb = 100-225 degrees C., where Tb is higher than Ta. When the ash is obtd. by fluidised bed combustion, step (a) pref. uses 30-65% HNO3 for 1-5 hr. at 40-90 degrees C., whereas step (b) is at 100-130 degrees C. When the ash is obtd. from burning powdered coal, step (a) pref. uses 30-65% HNO3 for 1-5 h. at 100-120 degrees C., whereas step (b) is at 150-225 degrees C. The ash is pref. first heated in air to reduce the carbon to max. 2%; and magnetic sepn. is pref. used to reduce the iron to max. 2% Fe2O3. Many lands contain little or no bauxite, and the latter continues to rise in cost. The invention provides for winning Al, e.g. as Al2O3 with high purity.