Molten boron

A B-rich liquid forms at the reaction interface with the B4C as a result of this temperature rise. The first B-rich liquid in the system outside of molten boron occurs at 2165°C from eutectic reaction of B4C, C, and ZrCl (l. On further local heating (T > 2220°C), a continuous B-rich liquid can form between pure boron and the eutectic liquid. The exact composition of the B-rich liquid is unknown, but it appears to lie somewhere within the triangle formed by B4C, C, and ZrB2. 

This compound is synthesized by the method of Sliwinski in the apparatus shown in Fig. 243. Before the reaction the apparatus is flushed with dry Hg until free of ejqilosive oxyhydrogen mixture. Amorphous boron is placed in a Vycor tube and heated to a dull red. A stream of dry, COg-free HsS is passed over the boron. Molten boron sulfide condenses close to the point where the heat has been applied (point a, Fig. 243). Upon cooling, the material becomes transparent. Further downstream, at b, porcelainlike sulfide forms, while crystalline B3S3 forms at o. TTie sublimation zone should not be cooled or the ciystals will not be well formed. A steady stream of HgS is maintained throughout the entire reaction and controlled to give a flow of approximately 3 liters/hour. 

Miyake M, Suzuki T (1984) Structural analysis of molten boron oxide (B2O3). J Chem Soc Faraday Trans 1(80) 1925-1931... 

Misawa M (1990) Stracture of vitreous and molten boron oxide (B2O3) measured by pulsed neutron total scattering. J Non-Cryst Solids 122 33-40... 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Development of practical and low cost separators has been an active area of ceU development. CeU separators must be compatible with molten lithium, restricting the choice to ceramic materials. Early work employed boron nitride [10043-11-5] BN, but a more desirable separator has been developed using magnesium oxide [1309-48-4], MgO, or a composite ofMgO powder—BN fibers. Corrosion studies have shown that low carbon steel or... 

Vapor phases ia the B2O3 system iaclude water vapor and B(OH)3(g) at temperatures below 160°C. Appreciable losses of boric acid occur when aqueous solutions are concentrated by boiling (43). At high (600—1000°C) temperatures, HB02(g) is the principal boron species formed by equiUbration of water vapor and molten B2O3 (44). At stiU higher temperatures a trimer (HB02)3(g) (2) is formed. 

Approximately 5% of the U.S. consumption of is in agriculture. Boron is a necessary trace nutrient for plants and is added in small quantities to a number of fertilizers. Borates are also used in crop sprays for fast rehef of boron deficiency. Borates, when apphed at relatively high concentration, act as nonselective herbicides. Small quantities of borates are used in the manufacture of alloys and refractories (qv). Molten borates readily dissolve other metal oxides usage as a flux in metallurgy is an important apphcation. Other important small volume apphcations for borates are in fire retardants for both plastics and ceUulosic materials, in hydrocarbon fuels for fungus control, and in automotive antifreeze for corrosion control (see Corrosion and corrosion inhibitors). Borates are used as neutron absorbers in nuclear reactors. Several borates, which are registered with the Environmental Protection Agency (EPA) can be used for insecticidal purposes, eg, TIM-BOR. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Boron carbide is resistant to most acids but is rapidly attacked by molten alkalies. It may be melted without decomposition in an atmosphere of carbon monoxide, but is slowly etched by hydrogen at 1200°C. It withstands metallic sodium fairly well at 500°C and steam at 300°C (8). 

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

The cubic 2inc blende form of boron nitride is usually prepared from the hexagonal or rhombohedral form at high (4—6 GPa (40—60 kbar)) pressures and temperatures (1400—1700°C). The reaction is accelerated by lithium or alkaline-earth nitrides or amides, which are the best catalysts, and form intermediate Hquid compounds with BN, which are molten under synthesis conditions (11,16). Many other substances can aid the transformation. At higher pressures (6—13 GPa) the cubic or wurt2itic forms are obtained without catalysts (17). 

It is decompd even by weak acids with evolution of brown fumes of nitrous anhydride (Ref 1) KN02 detonates violently when heated in the presence of thiosulfates or cyanides. Ref 1 states that when mixed with K cyanide, the mixt burns, and at 450°, it explds violently. Ref 6 details instances of molten KN02 being violently decompd by boron, and of a wgorous reaction attended by flame caused by the addition of a 0 little amm sulfate to fused KNOa... 

Other useful refractory nitrides for corrosion protection are silicon nitride (Si3N4) and boron nitride (BN). Silicon nitride has good corrosion resistance and is not attacked by most molten metals as shown in Table 17.6 (see Ch. 10). 

Boron nitride is one of the most outstanding corrosion-resistant materials. It is inert to gasoline, benzene, alcohol, acetone, chlorinated hydrocarbons and other organic solvents. It is not wetted by molten aluminum, copper, cadmium, iron, antimony, bismuth, silicon, germanium, nor by many molten salts and glasses. It is used extensively as crucible material, particularly for molten metals, glasses and ceramic processing. 

Boron reacts with more difficulty. When it is heated to 600°C with molten sulphur it becomes incandescent. 

Chlorine has caused numerous accidents with metals. Beryllium becomes incandescent if it is heated in the presence of chlorine. Sodium, aluminium, aluminium/titanium alloy, magnesium (especially if water traces are present) combust in contact with chlorine, if they are in the form of powder. There was an explosion reported with molten aluminium and liquid chlorine. The same is true for boron (when it is heated to 400°C), active carbon and silicon. With white phosphorus there is a detonation even at -34°C (liquid chlorine). 

A number of electrolytic processes are used for the industrial production of metals. Some metals such as zinc, copper, manganese, gallium, chromium, etc. are electrowon from aqueous baths. Another common electrolytic process used is molten salt electrolysis. The most important application of molten salt electrolysis till now has been in the electrowinning of metals. Today aluminum, magnesium, lithium, sodium, calcium, boron, cerium, tantalum, and mischmetal are produced in tonnage quantities by molten salt electrolysis. As a representative example, the electrowinning process for aluminum is taken up. 

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