Melt zone melting

The partial melt zone (melt and matrix) migrates upwards as a diapir into a hotter... 

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

Hot-melt coatings Hottel zoning method Hot tub disinfection Hot tubs... 

Zone electrophoresis Zone melting Zone refining... 

Ablative materials are classified according to dominant ablation mechanism. There are three groups subliming or melting ablators, charring ablators, and intumescent ablators. Figure 4 shows the physical zones of each. Because of the basic thermal and physical differences, the classes of ablative materials are used in different types of appHcations. 

Phosphoms-containing additives can act in some cases by catalyzing thermal breakdown of the polymer melt, reducing viscosity and favoring the flow or drip of molten polymer from the combustion zone (25). On the other hand, red phosphoms [7723-14-0] has been shown to retard the nonoxidative pyrolysis of polyethylene (a radical scission). For that reason, the scavenging of radicals in the condensed phase has been proposed as one of several modes of action of red phosphoms (26). 

In commercial practice, powdered explosives on an ammonium nitrate basis are used in most cases. Typical detonation velocities are between 1800 and 3500 m/s depending on the metal system to be bonded. The lower detonation velocity range is preferred for many metal systems in order to minimize the quantity of solidified melt associated with the bond-zone waves (12). In addition, subsonic detonation velocity explosives are required for the parallel cladding technique in order to avoid attached shock waves in the coUision region, which preclude formation of a good bond. 

Plutonium trichloride, PUCI3, is the only known binary chloride of plutonium. No higher chloride in the soHd state has been prepared so far. The trichloride is a blue-green soHd and can be prepared from the reaction of Pu metal or PUO2, and HCl gas at elevated temperatures, and can be purified by sublimation and zone melting. The yellow complex chloride, CS2PUCI3, is stoichiometric, stable, and can be used as a primary analytical standard for plutonium (156). 

Any refractory material that does not decompose or vaporize can be used for melt spraying. Particles do not coalesce within the spray. The temperature of the particles and the extent to which they melt depend on the flame temperature, which can be controlled by the fueLoxidizer ratio or electrical input, gas flow rate, residence time of the particle in the heat zone, the particle-size distribution of the powders, and the melting point and thermal conductivity of the particle. Quenching rates are very high, and the time required for the molten particle to soHdify after impingement is typically to... 

Fig. 6. Stmctural zones in condensates at various substrate temperatures. When is the melting point (10) ...

The modification shows that the transition temperature may vary significantly from those stated above and in general shift to higher temperatures as the gas pressure in the synthesis process increases. The transition from one zone to the next is not abmpt, but smooth. Hence, the transition temperatures should not be considered as absolute but as guidelines. Furthermore, not all zones are found in all types of deposit. For example. Zone T (see Fig. 7) is not prominent in pure metals, but becomes more pronounced in complex alloys, compounds, or in deposits produced at higher gas pressures. Zone 3 is not often seen in materials with high melting points. 

W. G. Pfann, Zone Melting, 2nd ed., John Wiley Sons, Inc., New York, 1966. 

One more variation to the many methods proposed for sulfur extraction is the fire-flood method. It is a modem version of the Sickian method, by which a portion of the sulfur is burned to melt the remainder. It would be done in situ and is said to offer cost advantages, to work in almost any type of zone formation, and to produce better sweep efficiency than other systems. The recovery stream would be about 20 wt % sulfur as SO2 and 80 wt % elemental sulfur. The method was laboratory-tested in the late 1960s and patents were issued. However, it was not commercially exploited because sulfur prices dropped. 

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

Zone refining can be appHed to the purification of almost every type of substance that can be melted and solidified, eg, elements, organic compounds, and inorganic compounds. Because the soHd—Hquid phase equiHbria are not favorable for all impurities, zone refining often is combined with other techniques to achieve ultrahigh purity. 

Zone Melting. A similar material balance may be made for a zone of mass moving a short distance in such a manner that the mass dm of sohd is frozen out and an identical mass melts into the zone (Fig. 4). For the first zone pass, it is assumed that the rod is initially at uniform composition i to obtain the following (1,4,8) ... 

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