Occurrence[ edit ] Magnesite occurs as veins in and an alteration product of ultramafic rocks , serpentinite and other magnesium rich rock types in both contact and regional metamorphic terrains. These magnesites are often cryptocrystalline and contain silica in the form of opal or chert. Magnesite is also present within the regolith above ultramafic rocks as a secondary carbonate within soil and subsoil , where it is deposited as a consequence of dissolution of magnesium-bearing minerals by carbon dioxide in groundwaters. Formation[ edit ] Magnesite can be formed via talc carbonate metasomatism of peridotite and other ultramafic rocks. Magnesite is formed via carbonation of olivine in the presence of water and carbon dioxide at elevated temperatures and high pressures typical of the greenschist facies. However, when performing this reaction in the laboratory, the trihydrated form of magnesium carbonate nesquehonite will form at room temperature.
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Applications for sintered magnesite include isostatic pressed shapes and flow control systems sliding gate plates etc. When the grain size of the magnesia crystal is large then its stability is very good. The addition of fused magnesia grains can greatly enhance the performance and durability of basic refractories such as mag carbon bricks. This is a function of a higher bulk specific gravity and large periclase crystal size, plus realignment of accessory silicates. Refractory grade fused magnesia has exacting specifications and is normally characterized by the following.
Lower grade EFM is also used in refractory bricks and shapes. EFM also has high thermal conductivity. Calcined magnesia is normally graded according to purity, sizing and reactivity. Historically the main global producers of high grade DBM have been based on synthetic technology converting magnesium rich seawater or brine into magnesia. The only natural high grade DBM producers are Turkey and Australia which are based on cryptocrystalline magnesite deposits.
Calcining at different temperatures produces magnesium oxide of with different reactivity. High temperatures deg C to deg C produces dead-burned magnesia, an unreactive form used as a refractory. Calcining temperatures deg C to deg C produces hard-burned magnesia which has limited reactivity while lower temperature, deg C to deg C calcining produces light-burned magnesia, a reactive form, which is called calcined magnesia. Magnesite is converted into magnesia by the application of heat which drives off carbon dioxide CO2 , thereby converting the carbonate to the oxide of magnesium MgO.
Magnesite, from both natural sources primarily magnesite and synthetic sources seawater, natural brines or deep sea salt beds , is converted into calcined magnesia by calcining to between deg C and deg C, driving off most of the contained CO2. Calcined magnesia is both an end product and an intermediary step in the chain of magnesia products. Further calcining of magnesite at higher temperatures between deg to deg C results in the largely inert product, dead burned magnesia.
Heating to this level drives off all but a small fraction of the remaining CO2 to produce a hard crystalline non reactive form of magnesium oxide known as periclase. Dead burned magnesia exhibits exceptional dimensional stability and strength at high temperatures.
Fused magnesia is produced in a three phase electric arc furnace. Taking high grade magnesite or calcined magnesia as raw materials, 12 hours is required for the fusion process at temperatures in excess of deg C. The process promotes the growth of very large crystals of periclase greater than microns compared microns for dead burned magnesia with a density approaching the theoretical maximum of 3. In fused magnesia production, the main constraints on capacity are the size and number of electric arc furnaces, and the cost of energy.
Dead Burned MgO