Section 4.5 Zone Refining

Forced circulation growth type c/s with pump, drive, heat exchanger, vacuum equipment and crystallizer. FOB cost 675000 for a crystal capacity = 1 kg/s with n = 0.53 for the range 0.1 -10. L+M = 1.7-2.7. L/M = 0.38. Alloy cost factors c/s, X 1.00 cast iron, X 1.0 rubber lined, X 0.75 s/s, X 3.0 copper alloy, X 1.3 nickel alloy, X 2.6 titanium, X 6.0. 

Mechanical vapor recompression c/s. FOB cost 3 400 000 for a crystal capacity = 

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Very pure germanium is made by the method of zone refining, usually achieved by slowly traversing a rod of solid with a molten zone by means of radio-frequency heating. Impurities are concentrated in a short section at one end of the bar, which is discarded. The success of the process depends on the difference between the solid and liquid solubilities of one element in another at the point of solidification. 

Zone refining is often used when extremely pure metals are desired for such applications as solar cells and semiconductors (Section 13-17). An induction heater surrounds a bar of the impure solid and passes slowly from one end to the other (Figure 22-4). As it passes, it melts portions of the bar, which slowly recrystallize as the heating element moves away. The impurity does not fit into the crystal as easily as the element of interest, so most of it is carried along in the molten portion until it reaches the end. Repeated passes of the heating element produce a bar of high purity. 

Pure silicon is then made super-pure (impurities <10-9 atom percent) by zone refining. In this process a rod of metal is heated near one end so that a cross-sectional wafer of molten silicon is produced. Since impurities are more soluble in the melt than they are in the solid they concentrate in the melt, and the melted zone is then caused to move slowly along the rod by moving the heat source. This carries impurities to the end. The process may be repeated. The impure end is then removed. 

The process known as zone refining can further purify the element ( FIGURE 22.33). As a heated coil is passed slowly along a silicon rod, a narrow band of the element is melted. As the molten section is swept slowly along the length of the rod, the impurities concentrate in this section, following it to the end of the rod. The purified top portion of... 

In this chapter we consider the separation of species contained in a homogeneous phase, such as a liquid or gas. The separation is based on exploiting a fundamental difference that exists between the species. Section 4.0 gives some overall guidelines. Methods that exploit differences in vapor pressures are evaporation, in Section 4.1 and distillation, in Section 4.2. Methods that exploit differences in freezing temperature and solubility are freeze concentration. Section 4.3, melt crystallization. Section 4.4 and zone refining. Section 4.5. Methods exploiting solubility are solution crystallization. Section 4.6 precipitation. Section 4.7 absorption, Section 4.8, and desorption. Section 4.9. Solvent extraction. Section 4.10, exploits differences in partition coefficient. 

Mechanism of pH-zone-refining CCC is shown in Figure 13A, which schematically illustrates a cross-sectional view of the separation column where three acidic compounds are eluted with a mobile phase containing ammonia and through a stationary phase containing TFA. Due to its nonlinear isotherm, the retainer (TFA) forms a sharp rear border that moves at a rate considerably lower than that of the mobile phase. Three analytes. Si, S2, and S3, competitively form solute zones behind the sharp TFA border according to their pX and hydrophobicity. Among... 

The continuous Brodie crystallizer (Figure 9.7) consists of three horizontal, scraped-wall chillers, in cascade formation, and one vertical purification column. The crystals are produced in the upper part of the crystallizer (recovery section) they then pass through the middle enrichment zone (refining section) and arrive at the base of the apparatus as molten product. Brodie crystallization is applied for the purification of naphthalene, for example, by Nippon Steel Chemical, Tobata/ Japan. 

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