Molten metal purification

Consequently, the maximum deepness of molten metal purification from oxygen can be determined from Equation (4.122) with Equation (4.129). 

The word foam refers to a dispersion of gas bubbles in a liquid, but can also be used to describe a uniform dispersion of a gaseous phase in a sobd matrix. Open-celled foams are, in fact, sponge-like strucmres made of intercoimected solid struts which enclose cavities (or pores) communicating through windows. They are commercially available in a variety of metallic and ceramic materials well-established industrial apphcations include thermal insulation, energy adsorption, fabrication of noise-reduction devices, filtration of molten metals, purification of hot gases and others. 

Slag A mixture of materials that separates from a metal during its purification and floats on top of the molten metal. 

Commercially, however, it is obtained as a by-product of the manufacture of iron. Slag and fly ash are purified to remove the vanadium metal contained within them. Slag is a mixture of materials that separates from iron and floats on top of the molten metal. Fly ash is a powdery material produced during the purification of iron. 

Examples of mass transfer under high vacuum are distillation of thermally unstable organic compounds, high-vacuum freeze drying, vacuum concentration of fruit juices, vacuum drying of coffee concentrate, vacuum purification of molten metals, etc. The mechanism of mass transfer in the concentration of fruit juices can be adequately described by the Gilliland-Sherwood equations (Gl), since the mean free path is negligible relative to the dimensions of the vessel at the pressures used. These equations show that the mass transfer coefficient is inversely proportional to the pressure. By extrapolation to very low pressures, it should be possible... 

Silicon dioxide, one of the products of this interaction, is insoluble in pure alkali metal halides and separates from the molten medium owing to the difference in densities. Thermodynamic analysis of the processes of molten iodide purification with different halogenating agents shows that their effectiveness reduces in the sequence SH4 > HI >h [294], An obvious advantage of silicon halides for the purification of halide melts used for singlecrystal growth is the fact that their use does not result in the appearance of additional impurities in the purified melts, since these processes are usually performed in quartz (Si02) vessels-reactors. 

The application of zone melting to the purification of semiconductor materials has been well established. In this technique a molten zone is passed along the length of a solid rod in one direction several times. Impurities more soluble in the molten metal i ill move in the direction in which the molten zone is moved while impurities less soluble in the liquid metal will be deposited in the solid metal and will tend to move in the opposite direction. Since the degree of purification depends on the solubility of the impurities in the solid metal, those impurities which are soluble in the solid cannot be removed to below the equilibrium concentration. Thus the interstitial impurities which are quite soluble in the solid rare earths just below their melting point cannot be completely removed. 

A two-stage, countercurrent molten-salt extraction process is used to extract Am from many kilograms of aged plutonium metal in which Am has grown-in by beta decay of Pu. The purification scheme, based partly on molten-halide/molten-metal studies at the Los Alamos Scientific Laboratory and Argonne National Laboratory, ronoves about 90% of the americium from plutonium metal, typically containing 200 2000 ppm Am [16]. 

For primary Al, the molten metal of a slightly different composition from different cells is mixed in the holding furnace. The mixing (together with purification and degassing) is done in certain proportions in order to obtain metal of a... 

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. 

Iron Precipitation. Rich sulfide ore or Hquated antimony sulfide (cmde antimony) is reduced to metal by iron precipitation. This process, consisting essentially of heating molten antimony sulfide ia cmcibles with slightly more than the theoretical amount of fine iron scrap, depends on the abihty of iron to displace antimony from molten antimony sulfide. Sodium sulfate and carbon are added to produce sodium sulfide, or salt is added to form a light fusible matte with iron sulfide and to faciHtate separation of the metal. Because the metal so formed contains considerable iron and some sulfur, a second fusion with some Hquated antimony sulfide and salt foHows for purification. 

Production of A1 metal involves two stages (a) the extraction, purification and dehydration of bauxite, and (b) the electrolysis of AI2O3 dissolved in molten cryolite Na3AlF6. Bauxite is now almost universally treated by the Bayer process this involves dissolution in aqueous NaOH, separation from insoluble impurities (red muds), partial precipitation of the trihydrate... 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

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