GB910330A - Improvements in or relating to the manufacture of dead-burned magnesia and dead-burned dolomite - Google Patents

Abstract

Low porosity dead-burned oxides of Ca and/or Mg are prepared by heating Mg(OH)2, Ca(OH)2, CaCO3, MgCO3, magnesium carbonate hydrates or dolomite below sintering temperature of the resulting oxides, pressing the calcined material into pieces at a pressure of at least 5 tons/sq. in. and then dead-burning the pieces at a temperature greater than 1400 DEG C. The initial calcining should be for not more than 4 hours and at a sufficiently low temperature to ensure that the resulting dead-burned oxide has a porosity of less than 10%. The preferred starting material is a slurry of hydrated magnesia and magnesium hydroxide obtained from brine by the action of an alkali, e.g. calcined dolomite or limestone. The filter cake after filtration is calcined to convert Mg(OH)2 to MgO and to sinter the MgO into a dense crystalline product. The filter cake may be washed before it is calcined in, for example, a rotary kiln to give a product in the form of grain. The dry pressing of the oxide may be carried out in a mechanical or hydraulic press or pelletizing apparatus. It is advantageous to press the pellets at high p temperatures which are below that at which the oxide begins to sinter appreciably. The dead-burning of the pressed oxide may be carried out in a rotary kiln, a travelling grate kiln or a shaft kiln, the temperature generally being from 1400 DEG C. to 2200 DEG C. According to the examples (1) A slurry of hydrated magnesia is prepared from calcined dolomite and sea-water and the precipitated magnesium hydroxide is calcined at 900 DEG C. for 3 hours. Samples of the oxide were then pelletized in a hydraulic press at pressures of 5, 10, 20 and 40 tons per square inch and the pellets were dead-burned at 1500 DEG C. for 4 hours, the resulting porosities being from 3 to 4%. (4) Hydrated magnesia of very high quality is prepared by dissolving pure magnesium metal in dilute nitric acid and adding 1 cone prepared from pure calcium carbonate to precipitate the magnesia: thereafter the magnesia is treated according to the invention. (7) Samples of dolomite were calcined at temperatures of 900 DEG to 1400 DEG C., pressed into pellets, and then dead-burned for 2 hours at 1500 DEG C. and the porosities of the treated samples varied from 3,5 to 12%. (8) Limestone samples were calcined at temperatures from 900 DEG to 1100 DEG C. for 2 hours and then dead-burned at 1500 DEG C. for 2 hours and the porosities were found to be 3 or 4%. Specifications 482,339, 482,340, 539,027, 540,139, 615,782 and 635,781 are referred to.ALSO:Low porosity dead-burned oxides of calcium and/or magnesium are prepared by heating Mg(OH)2, Ca(OH)2, CaCO3, MgCO3, magnesium carbonate hydrates or dolomite below sintering temperature of the resultant oxides, pressing the calcined material into pieces at a pressure of at least 5 tons/sq. in. and then dead-burning the pieces at a temperature greater than 1400 DEG C. The initial calcining should be for not more than 4 hours and at a sufficiently low temperature to ensure that the resulting dead-burned oxide has a porosity of less than 10%. The preferred starting material is a slurry of hydrated magnesia and magnesium hydroxide obtained from brine by the action of an alkali e.g. calcined dolomite or limestone. The filter cake after filtration is calcined to convert Mg(OH)2 to MgO and to sinter the MgO into a dense crystalline product. The filter cake may be washed before it is calcined in, for example, a rotary kiln to give a product in the form of grain. The dry pressing of the oxide may be carried out in a mechanical or hydraulic press or pelletising apparatus. It is advantageous to press the pellets at high temperatures which are below that at which the oxide begins to sinter appreciably. The dead-burning of the pressed oxide may be carried out in a rotary kiln, a travelling grate kiln or a shaft kiln, the temperature generally being from 1400 DEG C. to 2200 DEG C. According to the examples: (1) a slurry of hydrated magnesia is prepared from calcined dolomite and sea-water and the precipitated magnesium hydroxide is calcined at 900 DEG C. for 3 hours. Samples of the oxide water then pelletised in a hydraulic press at pressures of 5, 10, 20 and 40 tons per square inch and the pellets were dead-burned at 1500 DEG C. for 4 hours, the resulting porosities being from 3 to 4%; (4) hydrated magnesia of very high quality is prepared by dissolving pure magnesium metal in dilute nitric acid and adding lime prepared from pure calcium carbonate to precipitate the magnesia which is thereafter treated according to the invention; (7) samples of dolomite were calcined at temperatures of 900 DEG to 1400 DEG C., pressed into pellets, and then dead-burned for 2 hours at 1500 DEG C. and the porosities of the treated samples varied from 3,5% to 12%; (8) limestone samples were calcined at temperatures from 900 DEG to 1100 DEG C. for 2 hours and then dead-burned at 1500 DEG C. for 2 hours and then porosities were found to be 3 or 4%. Specifications 482,339, 482,340, 539,027, 540,139, 615,782 and 635,781 are referred to.