Process for obtaining alumina in the manufacture of portland cement

摘要

Sodium aluminate, from which alumina can be recovered conventionally, is obtained in the manufacture of Portland cement by proportioning the starting materials so that the content of Al2O3, CaO and SiO2 are within the range of primary crystallization of calcium disilicate as defined by a phase diagram (not illustrated) and after calcination separating the calcine into a finer and a coarser portion, the latter of which is leached with sodium carbonate solution to form sodium aluminate solution and a residue which is mixed with the finer portion; the mixture is calcined to form Portland cement, if desired after addition of clay. Suitable starting materials are clay and other materials containing alumina, and fluxes such as lime, coke, and gypsum. In the examples, there are employed (1) lime and clay only and (2) anhydrite, aluminium sulphate, clay, coke and sand; in each case a further quantity of clay is added before the second calcination. Lime may be added later, also. The first calcination may be performed, e.g. in a rotary furnace, and in the examples temperatures of 1250-1280 DEG C. and 1300-1350 DEG C. are used. After calcination the mixture may be annealed at 100-150 DEG C. lower than the temperature at which sintering begins. On cooling, the clinkers disintegrate and are separated, e.g. by sieving or air classification, into finer and coarser portions, preferably in the ratio 1 : 3; the fine portion may have a particle size below 25m . If desired the coarse material may be crushed before leaching.ALSO:In the manufacture of Portland cement sodium aluminate is obtained by proportioning the starting materials so that the content of Al2O3, CaO and SiO2 are within the range of primary crystallisation of calcium disilicate as defined by a phase diagram (not illustrated) and after calcination separating the calcine into a finer and a coarser portion, the latter of which is leached with sodium carbonate solution to form sodium aluminate solution and a residue which is mixed with the finer portion; the mixture is calcined to form Portland cement, if desired after addition of clay. Suitable starting materials are clay and other materials containing alumina and fluxes such as lime, coke, and gypsum. In the Examples, there are employed (1) lime and clay only and (2) anhydrite, aluminium sulphate, clay, coke and sand; in each case a further quantity of clay is added before the second calcination. Lime may be added later, also. The first calcination may be performed, e.g. in a rotary furnace and in the Examples temperatures of 1250 DEG -1280 DEG C. and 1300 DEG -1350 DEG C. are used. After calcination the mixture may be annealed at 100 DEG -150 DEG C. lower than the temperature at which sintering begins. On cooling, the clinkers disintegrate and are separated, e.g. by sieving or air classification, into finer and coarser portions, preferably in the ratio 1 : 3 the fine portion may have a particle size below 25m . If desired the coarse material may be crushed before leaching.