Hydration of Magnesium Oxide

Another common method of producing magnesium hydroxide slurry is by reacting magnesium oxide in water in excess of that require for theoretical hydration  

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Kato, Y., J. Nakahata, and Y. Yoshizawa, 1999. Durability characteristics of the hydration of magnesium oxide under repetitive reaction, J. Mater. Sci., 34, 475-480. 

The other main approach is to use the controlled hydration of magnesium oxide to precipitate the hydroxide in the desired form [105]. This has the advantage of having no co-product salt to consider, but obtaining the oxide in the desired form at a reasonable cost is a problem that can only be overcome in special circumstances. Despite this there are at least two significant commercial producers who use this route. 

Magnesium hydroxide may be precipitated with alkali from solutions of magnesium salts. It may also be obtained by hydration of magnesium oxide and by the reaction of water with magnesium amalgams. 

The data on the interference line width for magnesium hydroxide samples s 50 m /g), obtained in the course of the hydration of magnesium oxide under various conditions, are summarized in Table 6.11. In one case, the hardening process was carried out in a sample with high water content (W/S = 50) with a continuous abrasion to prevent bridging. In the other case, the process was carried out in samples with low water content (W/S = 1.0 and 0.8) without any abrasion, so that hardened samples could be obtained. 

Magnesium hydroxide can also be produced by slaking or pressure hydrating various reactive grades of magnesium oxide. The reaction is highly exothermic (AH gg = —40.86 kJ/mol (—9.77 kcal/mol)) to produce crystalline form at stoichiometric water addition ... 

Magnesium trisilicate hydrate (Mg2Si30s xH20, CAS No. 39365-87-2) is constituted of magnesium oxide and silicon dioxide with varying proportions of water. It should contain not less than 20% of magnesium oxide and not less than 45% of silicon dioxide and can be prepared from sodium silicate and magnesium sulfate. It also occurs in nature as the minerals meerschaum, parasepiolite, and sepiolite. 

There may be simultaneous or preliminary dehydration processes for hydrated salts. Group 1 metal perchlorates, as well as calcium, barium and silver perchlorates [59] are represented by reaction (2.13). Aluminium and iron(IIl) perchlorates are represented by reaction (2.14). The solid product of decomposition of magnesium perchlorate is a mixture of magnesium oxide and magnesium chloride. 

The direct observation of radiation-desorbed gas has sometimes been sought in irradiated sealed ampules containing a catalyst, usually with negative results. Thus no gas was observed in ampules of magnesium oxide or nickel oxide which had received as much as 6 x lO nvt of fast neutrons (57). A surprising observation 152) was that of drops of water in the cold parts of a system in which quartz or aerosyl had been irradiated to about 7 X IQi ev/gm with y-rays at room temperature. The fact that water itself was observed indicates removal as water rather than as products of radiolysis. Measurement of the degree of hydration confirmed that detectable amounts of water had been removed. 

Rate of solution. The rate of solution depends on the particle size distribution of the hydrated lime. Three test methods are described in section 20.7.1. The presence of magnesium oxide and other impurities can markedly reduce the rate of solution of the calcium hydroxide component [19.4]. 

The calcium and magnesium oxides that hydrate after cutting cause expansion of the blocks in the autoclave. While a small expansion can be tolerated, excessive expansion causes the material to exceed the specified dimensions [26.50, 26.51] and can reduce strength through cracking of the mass, or micro-cracking of the walls between the voids. It is, therefore, essential to limit the amounts of overburned calcium oxide and of magnesium oxide, in the lime. 

Smoke suppressants are also important additives in fire retardancy. Resulting from an incomplete combustion, opaque smoke may evolve that leads to panic and slow the rescuers progress. Additives, such as antimony oxides, metal borates, hydrates of magnesium or aluminium and magnesium oxychlorides, are used as fillers and flame retardants, but they are also good smoke reducers. 

Light Magnesia. A form of magnesium oxide normally made by extraction from sea-water (see sea-water magnesia) the precipitated hydrate is subsequently purified by conversion to the carbonate which is then calcined at approx. 800°C. The bulk density is 0.1. On exposure to the air it takes up water and COj to form a basic carbonate. Light magnesia finds use as a heat insulator and as a source of magnesia in various chemical processes. 

The samples of calcium hydroxide used in these studies were commercially available and while every effort was made to protect them from exposure to the atmosphere, no purification was undertaken. Commercial calcium hydroxide is obtained by the hydration of calcium oxide, which in its turn had been obtained from the thermal decomposition of limestone -calcium carbonate-. Small amounts of impurities are known to be present, and include magnesium hydroxide (magnesium is always present in small amounts in calcium ores), undecomposed calcium carbonate and traces of quartz. This procedure yields a calcium hydroxide with a Cdlj-like layer hexagonal structure, of the brucite [Mg(OH)2] type. This was confirmed by powder X-ray diffractometry. 

Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

These can be prepared by electrolytic oxidation of chlorates(V) or by neutralisation of the acid with metals. Many chlorates(VII) are very soluble in water and indeed barium and magnesium chlorates-(VII) form hydrates of such low vapour pressure that they can be used as desiccants. The chlorate(VII) ion shows the least tendency of any negative ion to behave as a ligand, i.e. to form complexes with cations, and hence solutions of chlorates (VII) are used when it is desired to avoid complex formation in solution. 

Mag nesia. ndAlumina. Suspension. A mixture of salts, available as Maalox, Mylanta, Gelusil, and Aludrox, contains magnesium hydroxide [1309-42-8] Mg(OH)2, and variable amounts of aluminum oxide in the form of aluminum hydroxide and hydrated aluminum oxide, ie, 2.9—4.2% magnesium hydroxide and 2.0—2.4% aluminum oxide, Al O, for a mixture of 4.9—6.6% combined magnesium hydroxide and aluminum oxide. This mixture may contain a flavoring and antimicrobial agents in a total amount not to exceed 0.5% (see Aluminum compounds, aluminum oxide). 

Mir-slaked lime contains various proportions of the oxides, hydroxides, and carbonates of calcium and magnesium which result from excessive exposure of quicklime to air that vitiates its quaUty. It is partially or largely decomposed quicklime that has become hydrated and carbonated. 

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