Calcination of Magnesium Hydroxide

To recap, the major variables influencing the rate of calcination of magnesite are  

Calcining magnesium hydroxide, such as that produced in either a brine or seawater process, involves heating a filter cake that contains between 50 and 72% magnesium hydroxide solids, the balance being water. The thermal decomposition involves the following reaction  

The decomposition reaction begins to take place around 350°C and increases rapidly above this temperature. One kilogram of pure magnesium hydroxide 

In reality, all three processes are occurring to some degree at the same time. It is very difficult to remove the last vestiges of chemically bound water from magnesium hydroxide, unless the kiln temperature is raised above 1000°C. It is believed that this residual water is adsorbed onto the nascent magnesium oxide surfaces in a monolayer (Gregg and Packer, 1955). 

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Recently, the influence of the preparation method of various MgO samples on their catalytic activity in the MPV reaction of cyclohexanone with 2-propanol has been reported 202). The oxides were prepared by various synthetic procedures including calcination of commercially available magnesium hydroxide and magnesium carbonate calcination of magnesium hydroxides obtained from magnesium nitrate and magnesium sulfate sol-gel synthesis and precipitation by decomposition of urea. It was concluded that the efficiency of the catalytic hydrogen transfer process was directly related to the number of basic sites in the solid. Thus, the MgO (MgO-2 sample in Table IV) prepared by hydration and subsequent calcination of a MgO sample that had been obtained from commercially available Mg(OH)2 was the most basic and the most active for the MPV process, and the MgO samples with similar populations of basic sites exhibited similar activities (Table IV). 

Choudhary V. R. and Pandit, M. Y. Surface properties of magnesium oxide obtained from magnesium hydroxide - influence on preparation and calcination conditions of magnesium-hydroxide. Appl. Catal., 1991, 71, 265-274. 

A classic example of a solid—fluid ceramic powder synthesis reaction is that of calcination and dehydration of natural or synthetic raw materials. Calcination reactions are common for the production of many oxides from carbonates, hydrates, sulfates, nitrates, acetates, oxalates, citrates, and so forth. In general, the reactions produce an oxide and a volatile gaseous reaction product, such as CO2, SOg, or HgO. The most extensively studied reactions of this type are the decompositions of magnesium hydroxide, magnesium carbonate, and calcium carbonate. Depending on the particular conditions of time, temperature, ambient pressure of CO2, relative humidity, particle size, and so on, the process may be controlled by a surface reaction, gas diffusion to the reacting... 

The decomposition of magnesium hydroxide has been evaluated as a process of nucleation and growth of MgO crystals within the brucite matrix. The formation of a defect layer of hydroxide structure, which suddenly recrystallizes to the cubic MgO structure when the fracture stress is exceeded in the defect layer, has been postulated (Freund et al., 1975 Guilliat et al., 1970 Garn et al., 1978 Lpnvik, 1978). The MgO crystals formed have an expanded cubic lattice that upon increasing calcination temperature gradually decreases in size until the equilibrium unit cell dimension is reached. 

Solubility in water of magnesium hydroxide and calcined dolomite. Magnesium hydroxide is only sparingly soluble in water (about 0.01 g/1) [19.5]. The magnesium hydroxide present in calcined dolomite is reported not to affect the solubility of calcium hydroxide (but see rate of solution below). 

Fig. 1. MgO prepared from (a) buring magnesium ribbon and collecting the residue, (b) decomposition of magnesium hydroxide and calcining at 800°C, (c) decomposition of magnesium basic carbonate and calcining at 800°C, and (d) decomposition of magnesium basic carbonate and calcining at 1100°C. Reproduced from Ret (8).

Calcination of limestone Decomposition of magnesium hydroxide Production of carbon disulfide Chlorination of rutile to titanium tetrachloride Gasification of carbon Decomposition of ammonium chloride Decompositions of ammonium sulfate Mond process for nickel production Oxidation of silicon tetrachloride to silicon dioxide... 

Figure 10.62 Magnesium oxide (MgO) used in rubber is synthesized from controlled precipitation of magnesium hydroxide, followed by calcination to MgO...

Dead Seas Periclase Ltd., on the Dead Sea in Israel, uses yet another process to produce magnesium oxide. A concentrated magnesium chloride brine processed from the Dead Sea is sprayed into a reactor at about 1700°C (127,128). The brine is thermally decomposed into magnesium oxide and hydrochloric acid. To further process the magnesia, the product is slaked to form magnesium hydroxide which is then washed, filtered, and calcined under controlled conditions to produce a variety of MgO reactivity grades. A summary of MgO purities, for the various processes is given in Table 20. 

Dow seawater A process for extracting magnesium from seawater. Calcined dolomite (CaO-MgO, dololime), or calcined oyster shell, is added to seawater, precipitating magnesium hydroxide. This is flocculated, sedimented, and filtered off. For use as a refractory it is calcined for the manufacture of magnesium chloride for the manufacture of magnesium metal, it is dissolved in hydrochloric acid. Developed by the Dow Chemical Company and later operated by the Steetly Company in West Hartlepool, UK, and Sardinia. 

In most commercial processes, the compound is either derived from the sea water or from the natural brines, both of which are rich sources of magnesium chloride. In the sea water process, the water is treated with lime or calcined dolomite (dolime), CaO MgO or caustic soda to precipitate magnesium hydroxide. The latter is then neutralized with hydrochloric acid. Excess calcium is separated by treatment with sulfuric acid to yield insoluble calcium sulfate. When produced from underground brine, brine is first filtered to remove insoluble materials. The filtrate is then partially evaporated by solar radiation to enhance the concentration of MgCb. Sodium chloride and other salts in the brine concentrate are removed by fractional crystallization. 

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