Magnesium oxide vaporization

The base-catalyzed reaction of acetaldehyde with excess formaldehyde [50-00-0] is the commercial route to pentaerythritol [115-77-5]. The aldol condensation of three moles of formaldehyde with one mole of acetaldehyde is foUowed by a crossed Cannizzaro reaction between pentaerythrose, the intermediate product, and formaldehyde to give pentaerythritol (57). The process proceeds to completion without isolation of the intermediate. Pentaerythrose [3818-32-4] has also been made by condensing acetaldehyde and formaldehyde at 45°C using magnesium oxide as a catalyst (58). The vapor-phase reaction of acetaldehyde and formaldehyde at 475°C over a catalyst composed of lanthanum oxide on siHca gel gives acrolein [107-02-8] (59). 

Processes rendered obsolete by the propylene ammoxidation process (51) include the ethylene cyanohydrin process (52—54) practiced commercially by American Cyanamid and Union Carbide in the United States and by I. G. Farben in Germany. The process involved the production of ethylene cyanohydrin by the base-cataly2ed addition of HCN to ethylene oxide in the liquid phase at about 60°C. A typical base catalyst used in this step was diethylamine. This was followed by liquid-phase or vapor-phase dehydration of the cyanohydrin. The Hquid-phase dehydration was performed at about 200°C using alkah metal or alkaline earth metal salts of organic acids, primarily formates and magnesium carbonate. Vapor-phase dehydration was accomphshed over alumina at about 250°C. 

Isopropyl alcohol can be oxidized by reaction of an a,P-unsaturated aldehyde or ketone at high temperature over metal oxide catalysts (28). In one Shell process for the manufacture of aHyl alcohol, a vapor mixture of isopropyl alcohol and acrolein, which contains two to three moles of alcohol per mole of aldehyde, is passed over a bed of uncalcined magnesium oxide [1309-48-4] and zinc oxide [1314-13-2] at 400°C. The process yields about 77% aHyl alcohol based on acrolein. 

The purity of commercial-grade calcium depends to a large extent on the purity of the calcium oxide used in its production. Impurities such as magnesium oxide, or other alkaline-earth or alkaH metal compounds are reduced along with the calcium oxide, and these metals can contaminate the calcium. In addition, small amounts of aluminum may distill with the calcium vapor, and small amounts of calcium nitride may be produced by reaction with atmospheric nitrogen. 

Metallic magnesium is produced by either chemical or electrolytic reduction of its compounds. In chemical reduction, first magnesium oxide is obtained from the decomposition of dolomite. Then ferrosilicon, an alloy of iron and silicon, is used to reduce the MgO at about 1200°C. At this temperature, the magnesium produced is immediately vaporized and carried away. The electrolytic method uses seawater as its principal raw material magnesium hydroxide is precipitated by adding slaked lime (Ca(OH)2, see Section 14.10), the precipitate is filtered off and treated with hydrochloric acid to produce magnesium chloride, and the dried molten salt is electrolyzed. 

Magnesium oxide powder from magnesium vapor and oxygen,and tungsten carbide. 

Figure 17.8 Effect of vaporization of magnesium on the reducibility of magnesium oxide with coke.

It is therefore possible to reduce magnesium oxide to the metal with coke above about 1600 °C (Fig. 17.8), even though there would be no intersection of the Ellingham lines for the oxidations of solid Mg and coke in an accessible temperature range. The practicability of producing magnesium in this way depends on rapid quenching of the Mg vapor below temperatures at which the reverse reaction... 

Acids such as sulfuric, hydrochloric, nitric, phosphoric, perchloric, formic, acetic, chlorosulfonic, 50% hydrofluoric, and adipic can be treated by a mix of magnesium oxide and other chemical additives. It must be expected that when the dry chemical agent is applied to an acid spill, there will be a momentary increase in the volume of vapor coming off the spill. This puff is caused by the heat generated from the neutralization of the acid. To protect the personnel applying the dry chemical cover from this puff, suitable personnel protective equipment should be worn. 

The direct catalytic, oxidation method of ethylene is described in Ref 17, pp 77—87 Explosibility. Liquid ethylene oxide is stable to detonating agents, but the vapor will undergo explosive decomposition. Pure ethylene oxide vapor will decompose partially however, a slight dilution with air or a small increase in initial pressure provides an ideal condition for complete decomposition. Copper or other acetylide-forming metals such as silver, magnesium, and alloys of such metals should not be used to handle or store ethylene oxide because of the danger of the possible presence of acetylene. Acetylides detonate readily and will initiate explosive decomposition of ethylene oxide vapor. In the presence of certain catalysts, liquid ethylene oxide forms a poly-condensate. 

The deflagration of hydrazine perchlorate, both pure and with fuel and catalyst additives, has been investigated. Hydrazine perchlorate will deflagrate reproducibly if a few percent fuel is present. The deflagration process is catalyzed by copper chromite, potassium dichromate, and magnesium oxide. Deflagration rates have been measured photographically from 0.26 to 7.7 atm. A liquid layer was observed at the surface in these experiments. Vaporization rate measurements from 180°-235° C. have yielded the expression... 

The old method of heating the calcium salts of formic and a second carboxylic acid for aldehyde formation has been modified by the use of a catalytic decomposition technique. By this scheme, the acid vapors are passed over thorium oxide, titanium oxide, or magnesium oxide at 300° or the acids are heated under pressure at 260° in the presence of titanium dioxide. In the latter procedure, non-volatile acids can be used. With aliphatic acids over titanium oxide, reaction occurs only when more than seven carbon atoms are present, the yields increasing with increase in the molecular weight (78-90%). Aromatic-acids having halo and phenolic groups are converted in high yields to aldehydes, e.g., salicylaldehyde (92%) and p-chlorobenzaldehyde (8S>%). Preparation of a thorium oxide catalyst has been described (cf. method 186). 

The phrase boiling point must be used carefully for other reasons. Care must be exercised such that the boiling process is indeed defined. Two examples which illustrate this point are MgO and LiF. Magnesium oxide does not vaporize congruently (so that the total vapor pressure depends on the ambient oxygen potential), whereas LiF vaporizes congruently but monomer, dimer, and trimer species exist in the vapor. 

Methylation of phenol is carried out either in vapor phase or liquid phase. Catalysts employed have been magnesium oxide, mixtures of magnesium oxide and oxides of manganese, copper, titanium, etc. Temperatures have varied from 390° C to 420° C and pressures atmospheric or somewhat higher pressure. Of late, zeolite catalysts have proved to be more effective and eco-friendly. 

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