Process for the preparation of magnesia (MgO)

Abstract

The present invention provides an improved process for the preparation of MgO of high purity >99% from salt bitterns via intermediate formation of Mg(OH)2 obtained from the reaction of MgCl2 and lime, albeit indirectly, i.e., MgCl2 is first reacted with NH3 in aqueous medium and the slurry is then filtered with ease. The resultant NH4Cl-containing filtrate is then treated with any lime, to regenerate NH3 while the lime itself gets transformed into CaCl2 that is used for desulphatation of bittern so as to recover carnallite and thereafter MgCl2 of desired quality required in the present invention. The crude Mg(OH)2 is dried and calcined directly to produce pure MgO, taking advantage of the fact that adhering impurities in the Mg(OH)2 either volatilize away or get transformed into the desired product, i.e., MgO.

Description

FIELD OF THE INVENTION[0001]The present invention relates to an improved process for the preparation of magnesia (MgO). More particularly, the invention relates to an improved process for the preparation of MgO of high purity from Mg(OH)2 without the need to wash Mg(OH)2 or MgO.BACKGROUND OF THE INVENTION[0002]Magnesia is an important compound that finds application in various industries. Magnesium oxide has the highest melting point of the moderately priced oxides and is therefore an important raw material for refractory bricks and other materials. It is the only material apart from ZrO2 that can withstand long-term heating above 2000° C.[0003]Reference may be made to Ullmann's Encyclpedia, 6th Edition (electronic version) wherein it is stated that: “The increased demands made on refractory materials as a result of higher operating temperatures and shorter tap to tap times in metallurgical furnaces and reactors can only be met by pure, high-density magnesia sinters. Small quantitie...

AI Technical Summary

Benefits of technology

[0025]The main object of the present invention is to provide an improved process for the preparation of magnesia from magnesium chloride via intermediate formation of magnesium hydroxide wherein no washing is required either of the magnesium hydroxide or of the magnesia while at the same time obtaining MgO of very high purity.

[0032]Yet another object is to recover the ammonium chloride and HCl vapors by absorbing these in the ammonium chloride / ammonia filtrate obtained from the magnesium hydroxide-forming reaction thereby increasing the ammonium chloride concentration.

[0035]Yet another object is to utilize the by-product CaCl2 liquor, with or without clarification, for desulphatation of raw bittern as described in the prior art, to facilitate crystallization of carnallite double salt and also to minimize sulphate impurity in end bittern.

[0037]Yet another object is to obtain high yields MgO having purity 99% and very low B2O3 impurity directly upon calcination, useful for refractory applications.

Problems solved by technology

Small quantities of “contaminants” are disadvantageous if they form low-melting eutectics with MgO (e.g., with CMS at 1485° C. or with C2F at 1200° C. because this leads to deterioration of mechanical properties (e.g., strength and volume stability) at high temperatures.

The main drawback of this method is that magnesite ore can have high levels of impurity.

The presence of accompanying, low-melting minerals can adversely affect the properties of the sintered magnesia.

The drawback of this process is that magnesite ores contain varying amount of silica, iron oxide, alumina, and lime as silicates, carbonates, and oxides.

Due to all this, high purity magnesia is difficult to produce by this process and most such magnesia has less than 95% purity.

The main drawback of the process is that the process is very sensitive to the partial pressure of carbon dioxide and to the temperature.

Moreover, the bulk density of the MgO would be too low for refractory applications.

The main drawbacks of the above process are that a supply of freshwater (>40 m3 per tonne MgO) is required to wash the Mg(OH)2 and to produce the milk of lime.

The resulting slurry is difficult to filter and is washed and dewatered in a two-stage vacuum drum filter.

The main drawback is that the spray calcined product needs to be washed to remove unreacted MgCl2 and soluble salts and once again subjected to calcination.

Spray calcination is an energy intensive process and choking up of nozzles can pose a problem.

The problem of spray calcination, however, remains.

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