Patent GB-482326-A (GB)
Title: Improvements in or relating to the treatment of aluminous silicious material
IPC: C01F7/04
Filed: 1937-04-17
Abstract: In treating aluminous silicious material to recover alumina, by furnacing a mixture thereof with alkaline earth metal compound and alkali metal compound and dissolving out the alkali metal aluminate from the sinter, the residue is treated to form hydrated silica and alkaline earth metal bicarbonate which are separated. The bicarbonate may be converted to carbonate and used in the furnace charge.  The aluminous raw material may be felspar, leucite, marl, slag, waste coal products &c. and may include material from the alkali metal aluminate residual liquor.  The alkaline earth metal compound may be in the form of limestone, burnt lime, waste sludges, marl, slag &c., but it may be possible to use only that recovered from the system. The alkali metal compound may be in the form of carbonates or hydroxides of potassium or sodium, or may be produced by furnacing aluminous silicious material with sodium chloride in the presence of steam, or may be partly recovered from the system, or may be derived from the aluminous raw material.  Preferably 2 molecules of calcium oxide to each molecule of silica, and from 1 to 1,7 (usually 1,2 to 1,5) molecules of sodium oxide to each molecule of aluminium oxide, are used.  The finely-mixed charge, dry, moist, or as slurry, is heated to a temperature which may be from 1150 to 1400 DEG  C.  The alkali metal aluminate is dissolved out of the ground sinter by water or by a residual lye of weak aluminate in an autoclave or by lixiviation, and the solution may be purified by heating to precipitate unreacted silica or zeolitic compounds of silica, alumina and alkali, after which aluminium hydrate may be precipitated by the Bayer method and filtered off and dried. The residual liquor may be concentrated and used in the furnace charge; or may be treated with carbon dioxide to precipitate remaining alumina, and the alkali metal carbonate produced used in the furnace charge.  The sinter residue, which may be chiefly di-calcium silicate, is further ground and while suspended in water is treated with carbon dioxide e.g. from the furnace gases or the subsequent heating of the bicarbonate, to form orthosilicic acid or other hydrated silica, and calcium bicarbonate. The hydrated silica may be filtered off through a portion of the ground residue and dried; it may contain dicalcium silicate, which will be beneficial as an addition to cement.  The bicarbonate in the filtrate is converted by heating, to carbonate, which is used in the furnace charge or in the manufacture of cement.  The active silica itself when added to cement and concrete will counteract the ill effects of free lime.ALSO:In treating aluminous silicious material (felspar, leucite, marl, slag, waste coal products, &c.) to recover alumina, by furnacing a mixture thereof with alkaline earth metal compound (limestone, burnt lime, waste sludges, marl, slag, &c.) and alkali metal compound (carbonates or hydroxides of potassium or sodium, or produced by furnacing aluminous silicious material with sodium chloride in the presence of steam, &c.) and dissolving out the alkali metal aluminate from the sinter, the residue is treated to form hydrated silica and alkaline earth metal carbonate which are separated.  The sinter residue, chiefly di-calcium silicate is treated with carbondioxide while suspended in water, forming ortho-silicic acid or other hydrated silica which is filtered off, and calcium bicarbonate.  The hydrated silica may contain di-calcium silicate, which will be beneficial as an addition to cement.  The bicarbonate in the filtrate is converted by heating to carbonate which may be used in the manufacture of cement.  The active silica itself when added to cement and concrete will counteract the ill effects of free lime.