Caustic Seawater and Calcined Magnesia

Industrial preparation. Raw magnesite coming from the run-of-mine is calcined between 700 and 1000°C in a vertical shaft kiln and decomposes yielding magnesium oxide or magnesia (MgO) and giving off carbon dioxide gas  

The product obtained is called caustic-calcined magnesia (CCM), also called natural magnesia. The purity of CCM ranges usually between 75 and 96 wt.% MgO, with most of the impurities (e.g., Fe 03, AI3O3, SiOj, etc.) coming from the raw material used. 

Both grades of caustic magnesia readily react with water to give magnesium hydroxide or brucite [Mg(OH)J, also called slacked or spent magnesia  

Due to its alkaline properties and its poor solubility, when an excess of caustic magnesia is mixed with water, it gives a slurry called milk of magnesia with a pH of 10.25, and hence most heavy metals (e.g., Ni) are precipitated as metal hydroxides and then can be either removed by decantation, centrifugation, or filtration or stabilized in situ after drying of the slurry. 

The building industry consumes large quantities of caustic-calcined magnesite for use as a flooring material, in wall boards, and in acoustic tiles. Worldwide annual production is 1,000,000 tonnes of synthetic magnesia. 

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