Boric oxide, reduction

Preparation. Boron carbide is most commonly produced by the reduction of boric oxide with carbon in an electric furnace between 1400 and 2300°C. In the presence of carbon, magnesium reduces boric oxide to boron carbide at 1400—1800°C. The reaction is best carried out in a hydrogen atmosphere in a carbon tube furnace. By-product magnesium compounds are removed by acid treatment. 

The borothermic reduction of a metal oxide is easiest when there is no formation of dodecaboride. The boric oxide is volatile and is completely eliminated > 1500°C under vacuum. 

The CaB can be obtained by reduction of boric oxide with calcium aluminide at 800°C... 

Boron carbide is prepared by reduction of boric oxide either with carbon or with magnesium in presence of carbon in an electric furnace at a temperature above 1,400°C. When magnesium is used, the reaction may be carried out in a graphite furnace and the magnesium byproducts are removed by treatment with acid. 

The principal commercial methods in use for production of elemental boron are the Moissan reduction of boric oxide with magnesium for 90% amorphous boron and thermal decomposition of boron trichloride in hydrogen for crystalline... 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

Borides. Zirconium forms two borides zirconium diboride [12045-64-6] ZrB2, and zirconium dodecabotide [12046-91 -2] ZtB 2- Th diboride is synthesized from the elements, by vapor-phase coreduction of zirconium and boron hahdes, or by the carbothermic reduction of zirconium oxide and boron carbide boric oxide is avoided because of its relatively high vapor pressure at the reaction temperature. 

Derivation (1) Sintering mixtures of metal powder and boron at 2000C (2) reduction of mixture of the metal oxide and boric oxide with aluminum, silicon, or carbon (3) fused-salt electrolysis (4) vapor-phase deposition. 

Derivation (1) By heating boric oxide with powdered magnesium or aluminum (2) by vapor-phase reduction of boron trichloride with hydrogen over hot filaments (80-2000C) (3) by electrolysis of fused salts. 

Industrial production of B4C is based on fusion of boric oxide and carbon in electric furnaces operated around 2500 C. This process leads to lumps of coarse-grained material of high purity which has to be milled for sinter powders. Finer powders are obtained by magnesiothermic reduction of the boric oxide at lower temperatures around 1750°C ... 

Secondary alcohols (C q—for surfactant iatermediates are produced by hydrolysis of secondary alkyl borate or boroxiae esters formed when paraffin hydrocarbons are air-oxidized ia the presence of boric acid [10043-35-3] (19,20). Union Carbide Corporation operated a plant ia the United States from 1964 until 1977. A plant built by Nippon Shokubai (Japan Catalytic Chemical) ia 1972 ia Kawasaki, Japan was expanded to 30,000 t/yr capacity ia 1980 (20). The process has been operated iadustriaHy ia the USSR siace 1959 (21). Also, predominantiy primary alcohols are produced ia large volumes ia the USSR by reduction of fatty acids, or their methyl esters, from permanganate-catalyzed air oxidation of paraffin hydrocarbons (22). The paraffin oxidation is carried out ia the temperature range 150—180°C at a paraffin conversion generally below 20% to a mixture of trialkyl borate, (RO)2B, and trialkyl boroxiae, (ROBO). Unconverted paraffin is separated from the product mixture by flash distillation. After hydrolysis of residual borate esters, the boric acid is recovered for recycle and the alcohols are purified by washing and distillation (19,20). 

Phosphoric acid, aside from its acidic behavior, is relatively unreactive at room temperature. It is sometimes substituted for sulfuric acid because of its lack of oxidising properties (see SuLFURic ACID AND SULFURTRIOXIDe). The reduction of phosphoric acid by strong reducing agents, eg, H2 or C, does not occur to any measurable degree below 350—400°C. At higher temperatures, the acid reacts with most metals and their oxides. Phosphoric acid is stronger than acetic, oxaUc, siUcic, and boric acids, but weaker than sulfuric, nitric, hydrochloric, and chromic acids. 

Preparation. It is difficult to prepare very pure boron. The element can be obtained by magnesium reduction of the oxide, B203. The oxide is made by melting boric acid which in turn is obtained from borax. 

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