Aluminum vapor pressure, high temperature

Carbothermic reduction in the presence of an alloying element, such as copper, iron, or silicon, to decrease aluminum vapor pressures decreases volatility problems but requires a further stage to recover aluminum from the alloy product. It may be selectively dissolved from the alloy with a more volatile metal, such as mercury, lead, or zinc, and then the aluminum recovered by distillation. Or, the tendency for aluminum halides to form more volatile monohalides at high temperatures, which revert to the trihalides at lower temperatures (Eq. 12.25) may be employed. 

Many elements evaporate, but many such as chromium (Cr), cadmium (Cd), magnesium (Mg), arsenic (As), and carbon (C) sublime, and many others such as antimony (Sb), selenium (Se), and titanium (Ti), are on the borderline between evaporation and sublimation. For example, chromium, which has a vapor pressure of 10 Torr 600°C below its melting point, is generally vaporized by sublimation. Carbon cannot be melted except under high hydrostatic pressure. Materials such as aluminum, tin, gallium, and lead have very low vapor pressures at temperatures above the points at which they are just-molten. For example, tin has a vapor pressure of lO Torr 1000°C above its melting point. Aluminum and lead have vapor pressures of 10 Torr at about 500°C above their melting points. 

Significant vapor pressure of aluminum monofluoride [13595-82-9], AIF, has been observed when aluminum trifluoride [7784-18-1] is heated in the presence of reducing agents such as aluminum or magnesium metal, or is in contact with the cathode in the electrolysis of fused salt mixtures. AIF disproportionates into AIF. and aluminum at lower temperatures. The heat of formation at 25°C is —264 kJ/mol(—63.1 kcal/mol) and the free energy of formation is —290 kJ/mol(—69.3 kcal/mol) (1). Aluminum difluoride [13569-23-8] h.3.s been detected in the high temperature equihbrium between aluminum and its fluorides (2). 

Chlorofluorocar- bon (CFC) (e.g Halo-Vac ) Extremely nonreactive and will only ignite under extreme circumstances. Less expensive than perfluoropoly-ethers. Should not be used in pumps with aluminum parts. Heating over 280°C will produce HF gas. Difficult to clean. Viscosity varies with temperature. Has a rather high vapor pressure. 

---