Patent GB-774177-A (GB)
Title: Improvements in or relating to sinter magnesia
IPC: C04B35/03
Filed: 1953-06-22
Abstract: In the production of sinter magnesia by sintering artificially produced magnesium hydroxide with the aid of known sintering agents, the magnesium hydroxide employed is an artificially produced finely crystalline brucite of which at least 90 per cent, by weight has a particle size of up to 100m , which brucite contains 1 per cent by weight or less of impurities other than materials which on sintering are converted to magnesia, and which brucite is so reactive that when heated to a temperature of 950 DEG C., it contains periclase crystals having a diameter of 20-50m .  The starting material is suitably prepared by precipitating magnesium hydroxide from a magnesium salt solution containing an ammonium salt by means of ammonia.  The ammonia may be introduced in a quantity approximately twice the theoretical quantity together with the magnesium salt solution into a neutral or ammoniacal liquor, at a temperature above 40 and below 80 DEG  C. The sintering may be effected with for example 6 to 8 per cent. by weight of ferric oxide, together with calcium preferably such that the molecular ratio of CaO : Fe2O3 in the additive is greater than 2 and preferably greater than 4.  The calcium may suitably be added as calcium phosphate such as Thomas meal, phosphate rock, phosphorites or bone ash.  Magnesium or other alkaline earth metal phosphates or iron or manganese phosphates may also be employed with or without the addition of ferric oxide.  Sintering may also be effected with the addition of clay or chromium ore.  According to examples (1) a solution of ammonia is added at 30 DEG  C. to a solution containing magnesium sulphate with a small proportion of ammonium sulphate.  After about 4 hours at 50 to 70 DEG  C. precipitation of magnesium hydroxide is complete and the hydroxide so produced is filtered, washed and dried at 250 DEG  C.  It contains 94 per cent. by weight of magnesium hydroxide as brucite and 4.2 per cent. by weight of magnesium sulphate.  The magnesium hydroxide may be mixed with the usual sintering agent and the mixture granulated with water or sulphite liquor as a binding agent, calcined at 680 DEG C. and sintered in a rotating shaft furnace at 1600 DEG  C.  At a temperature of 950 DEG  C., that is after calcination but before sintering, the brucite contained periclase crystals having a diameter of 20 to 50m ; (2) Magnesium hydroxide as in (1) above is mixed with 10 per cent. by weight of calcium ferrite and the mixture granulated with water or sulphite liquor after which it is calcined at 690 DEG C. and burnt at 1600 DEG  C. to form sinter magnesia.  The sinter is ground such that 40 per cent. by weight has a granule size less than 0.15 mm.. and 60 per cent. by weight has a size between 0.15 and 3 mm.; the ground material is then mixed with sulphite liquor as a binding agent and pressed into moulds at a 1000 kg/cm2 pressure and burnt at 1500 DEG  C.  The product has a 99 per cent. by volume open pores and 23.7 per cent. by volume of total pores. The product softened at 1730 DEG C.  Further examples relate to the employment as a sintering agent of 5 per cent. by weight of Thomas meal and 5 per cent. by weight of an ore containing lime and phosphate to yield a product after sintering containing 89.2 per cent. magnesia, 4.2 per cent. calcium oxide, 2.9 per cent. ferric oxide and 0.6 per cent. maganese oxide, 0.26 per cent. aluminium oxide and 1.5 per cent. silica, the employment as sintering agent of 15 per cent. by weight of ground Moroccan phosphate, and to sintering with the addition of 10 per cent. by weight of calcium hydogen phosphate.  Specifications 583,009 [Group V] and 693,108 are referred to.ALSO:In the production of sinter magnesia by sintering artificially produced magnesium hydroxide with the aid of known sintering agents, the magnesium hydroxide employed is an artificially produced finely crystalline brucite of which at least 90 per cent by weight has a particle size of up to 100 m , which brucite contains 1 per cent by weight or less of impurities other than materials which on sintering are converted to magnesia, and which brucite is so reactive that when heated to a temperature of 950 DEG C., it contains periclase crystals having a diameter of 20-50 m .  The starting material is suitably prepared by precipitating magnesium hydroxide from a magnesium salt solution containing an ammonium salt by means of ammonia.  The ammonia may be introduced in a quantity approximately twice the theoretical quantity together with the magnesium salt solution into a neutral or ammoniacal liquor, at a temperature above 40 DEG  and below 80 DEG C. The sintering may be effected with for example 6 to 8 per cent by weight of ferric oxide, together with calcium preferably such that the molecular ratio of CaO : Fe2O3 in the additive is greater than 2 and preferably greater than 4.  The calcium may suitably be added as calcium phosphate such as Thomas meal, phosphate rock, phosphorites or bone ash.  Magnesium or other alkaline earth metal phosphates or iron or manganese phosphates may also be employed with or without the addition of ferric oxide. Sintering may also be effected with the addition of clay or chromium ore.  According to examples (1) a solution of ammonia is added at 30 DEG C. to a solution containing magnesium sulphate with a small proportion of ammonium sulphate. After about 4 hours at 50 to 70 DEG C. precipitation of magnesium hydroxide is complete and the hydroxide so produced is filtered, washed and dried at 250 DEG C.  It contains 94 per cent. by weight of magnesium hydroxide as brucite and 4,2 per cent. by weight of magnesium sulphate.  The magnesium hydroxide may be mixed with the usual sintering agent and the mixture granulated with water or sulphate liquor as a binding agent, calcined at 680 DEG C. and sintered in a rotating shaft furnace at 1600 DEG C.  At a temperature of 950 DEG C., that is after calcination but before sintering, the brucite contained periclase crystals having a diameter of 20 to 50m . (2) Magnesium hydroxide as in (1) above is mixed with 10 per cent. by weight of calcium ferrite and the mixture granulated with water or sulphite liquor after which it is calcined at 690 DEG C. and burnt at 1600 DEG C. to form sinter magnesia.  The sinter is ground such that 40 per cent by weight has a granule size less than 0,15 mm. and 60 per cent by weight has a size between 0,15 and 3mm.; the ground material is then mixed with sulphite liquor as a binding agent and pressed into moulds at a 1000 kg/cm2 pressure and burnt at 1500 DEG C. The product has a 99 per cent. by volume open pores and 23,7 per cent. by volume of total pores; the product softened at 1730 DEG C. Further examples relate to the employment as a sintering agent of 5 per cent. by weight of Thomas meal and 5 per cent. by weight of an ore containing lime and phosphate to yield a product after sintering containing 89,2 per cent magnesia, 4,2 per cent. calcium oxide, 2,9 per cent. ferric oxide and 0,6 per cent. manganese oxide, 0,26 per cent. aluminium oxide and 1,5 per cent. silica, the employment as sintering agent of 15 per cent. by weight of ground Moroccan phosphate, and to sintering with the addition of 10 per cent. by weight of calcium hydrogen phosphate. Specifications 583,009 and 693,108 [Group III] are referred to.