Refining, zone

One possible mechanism for such a process is that of zone refining (Harris, 1974), analogous to a metallurgical industrial process, in which superheated migma consumes several times its own volume during melt migration and thus becomes enriched in incompatible elements. The equation for the enrichment of a trace element by zone refining is 

Dynamic models (a) Dynamic melting To improve upon earlier static models of partial melting, Langmuir et al. (1977) proposed a model of partial melting in which a number of melting processes take place continuously and simultaneously. The main feature of 

Equation (9.3.22) can be used when volume to mass-fraction conversion is needed. In the right-hand side braces, the first term corresponds to the amount of element i entering the molten zone, the last two terms to the amount left behind at z. Assuming 

The characteristic length over which concentration of element i in the liquid changes by a factor e because of zone melting is (ktL/kfR)L. If the distribution prior to melting is constant and such as C0 z+L)=Co independent of the depth z, equation (9.4.25) is integrated as 

The value of C,iq at z=0 can be the concentration of a liquid generated by batch- or fractional melting from the same source or that of an exotic liquid introduced at the 

We recognize in equation (9.4.29) the batch equilibrium melting equation (9.2.2) with 

Inserting this expression into (9.4.29) leads to the integral equation 

The distribution of impurity along a bar subjected to this process of directional solidification can be calculated from equation 7.6. The greater the deviation of k from unity, the greater the concentration gradient along the bar. 

This condition, known as constitutional supercooling, represents an instability and the advancing face usually breaks up into finger-like cells, which progress in a more or less regular bunched array. In this manner heat of crystallization is more readily dissipated and the tips of the projections advance 

Directional solidification is not readily adaptable for purification purposes, because the impurity content varies considerably along the bar at the end of the operation. The end portion of the bar, where the impurity concentration is highest, could be rejected and the process repeated after a remelting operation, but this would be extremely wasteful. 

Different methods of heating may be used to produce molten zones the choice of a particular method is usually governed by the physical characteristics 

The speed of zoning is a factor that can have a considerable effect on the effieiency of zone refining. The correct speed is that which gives a uniform zone passage and at the same time permits impurities to diffuse away from the 

Operates below the freezing temperature. Solid feed with input concentration of 20 to 70%, but usual is 95 to 99.9% w/w, can obtain 99.999999% purity. Temperatures 480 to 600 °C. Impurity must lower the freezing temperature for method to be effective. Particulate impurities must not be encapsulated by the solidification front. 

Design based on heat transfer with AT = 50 to 100 °C from the freezing temperature and stop before the eutectic temperature. Ultra-purity of solids, low throughput at 10 to 10 kg/s. Multi-cycles might be needed to yield improved purity. 

Design (velocity of freezing front) (liquid boundary layer thickness that depends on mixing)/(diffusivity of solute impurity in the liquid) = 1. 

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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. 

The element is a gray-white metalloid. In its pure state, the element is crystalline and brittle, retaining its luster in air at room temperature. It is a very important semiconductor material. Zone-refining techniques have led to production of crystalline germanium for semiconductor use with an impurity of only one part in lOio. 

Zone electrophoresis Zone melting Zone refining... 

Selenium purification by zone refining is not feasible. At practical zone-refining speeds, crystallization does not occur and impurities do not segregate. However, a controlled differential thermal treatment of selenium in a long vertical glass tube has been described (45). The treatment time is several weeks to several months. 

TJItrahigh (99.999 + %) purity tellurium is prepared by zone refining in a hydrogen or inert-gas atmosphere. Single crystals of tellurium, tellurium alloys, and metal teUurides are grown by the Bridgman and Czochralski methods (see Semiconductors). 

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. 

Continuous zone-refining techniques have been developed, both theoretically and experimentally (1,4,10—11). 

Early zone-refining theory attempted to correlate the concentration of impurities with location. 

Electric heaters have also been directly immersed in the molten zone. Zone refining has been accompHshed with a single hehcal heater rotating in an annular sample space (71). 

The primary application for floating-zone melting is crystal growth rather than purification. Semiconductor-grade siUcon is not purified by zone refining siUcon chlorides are distilled and then reduced with hydrogen. 

Approximately 1 kg of biphenyl per 100 kg of benzene is produced (6). Because of the large scale, HD A operations provide an ample source of cmde biphenyl from which a technical grade of 93—97% purity can be obtained by distillation (35). Zone refining or other crystallization techniques are requited to further refine this by-product biphenyl to the >99.9% purity requited for heat-transfer appHcations. 

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