MgO production

Reaction (623 to 703 K) is believed not to proceed through the intervention of Mg(OH)2 and no significant amount of Mg(OH)C is formed. There was no evidence of melting. Yield-time data were well expressed by the contracting volume equation with = 110 5 kJ mol" in dry N2, or 75 kJ moT in 10 Torr H2O. Kinetic characteristics of the reaction in water vapour were closely similar to behaviour reported by Ball [117] for the reaction MgCC2 + ViO MgO + It is concluded [116] that both reactions proceed with common controls based on the increasing stability of the MgO product at the reaction interface. 

The initial and perhaps most critical design assumption is that both the MgO and the fly ash, which are carried out of the calciner and into the MgO product cyclone, have the same particle size distribution and density. This design (i.e., no possibility of physical separation of MgO and fly ash) is a conservative design assumption and adds complexity to the FGD process. It results in the need to recirculate large quantities of the MgO/fly ash mixture through the spray dryer and the calciner. In order to keep the MgO/ash recycle streams to a reasonable size, the mechanical collectors in the main flue gas ducts upstream from the SO2 absorber, which were used in the initial design because they are relatively inexpensive but yet remove only 80% of the fly ash, had to be replaced with the 95% efficient ESP mentioned earlier. 

The production of chemical-grade magnesia or light-burned MgO requires careful control of the calcination temperature to achieve the required specific surface area of the finished product. A furnace well suited to this requirement is the multiple-hearths Herreshoff-type. For the production of dead-burned magnesia typically shaft or rotary kilns are employed. See Chapter 5 for furnaces used in MgO production. 

The temperature at which the magnesium oxide is calcined is important for its performance in rubber. The calcined MgO product must have sufficient activity to be an effective acid acceptor for use in halogenated elastomer compounds. The light burned grades that are calcined between 700 and 1000 °C usually work well for rubber applications. 

Basic properties of product Be(OH)2 amphoteric MgO insoluble slightly MjOH), soluble... 

In earlier studies (24), the reaction was carried out at temperatures above 200°C under autogenous pressure conditions usiag alkaU metal hydroxide or alkoxide catalysts significant amounts of carboxyUc acid, RCH2COOH, were formed as were other by-products. More recent reports describe catalysts which minimize by-products MgO—K CO —CUC2O2 (25), less basic but stiU requiring high temperatures Rh, Ir, Pt, or Ru complexes (26) and an alkaU metal alkoxide plus Ni or Pd (27), effective at much lower temperatures. 

The term lime also has a broad coimotation and frequently is used in referring to limestone. According to precise definition, lime can only be a burned form quicklime, hydrated lime, or hydraiflic lime. These products are oxides or hydroxides of calcium and magnesium, except hydraiflic types in which the CaO and MgO are chemically combined with impurities. The oxide is converted to a hydroxide by slaking, an exothermic reaction in which the water combines chemically with the lime. These reversible reactions for both high calcium and dolomitic types are Quicklime... 

When magnesium oxide is chlorinated in the presence of powdered coke or coal (qv), anhydrous magnesium chloride is formed. In the production of magnesium metal, briquettes containing CaCl2, KCl, NaCl, MgO, and carbon are chlorinated at a temperature such that the electrolyte or cell melt collects at the bottom of the chlorinator, enabling the Hquid to be transferred directly to the electrolytic cells. 

In seawater—dolime and hrine—dolime processes, calcined dolomite or dolime, CaO MgO, is used as a raw material (Table 9). Dolime typically contains 58% CaO, 41% MgO, and less than 1% combined Si02, P O, and CO2 where R is a trivalent metal ion, eg, Al " or Fe " ( 4). Roughly one-half of the magnesia is provided by the magnesium salts in the seawater or brine and the other half is from dolime (75). Plant size is thus reduced using dolime and production cost is probably lower. 

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. 

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

When the Diels-Alder reaction between butadiene and itself is carried out in the presence of alkah metal hydroxide or carbonate (such as KOH, Na2C02, and K CO on alumina or magnesia supports) dehydrogenation of the product, vinylcyclohexene, to ethylben2ene can occur at the same time (134). The same reaction can take place on simple metal oxides like Zr02, MgO, CaO, SrO, and BaO (135). 

Calcination or dead burning is used extensively to dehydrate cements (qv) and hygroscopic materials such as MgO, and to produce a less water sensitive product. Calcination is also used to decompose metal salts to base oxides and to produce multicomponent or mixed oxide powders for... 

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