Solidification zone melting

How do we actually use eqn. (5.17) to calculate driving forces in materials processes A good example to begin with is solidification - most metals are melted or solidified during manufacture, and we have already looked at two case studies involving solidification (zone refining, and making bubble-free ice). Let us therefore look at the thermodynamics involved when water solidifies to ice. 

General Reeerences Van t Land, Industrkd CrystaUizaUon ofMdts, Taylor Francis, New York, 2004. Mullin, CrystaiUzathn, 4th ed., Butterworth-Heinemann, 2001. Myerson, Handbook of Industrkd CrystaUizaUon, 2d ed., Butterworth-Heinemann, 2001. Pfann, Zone Melting, 2d ed., Wiley, New York, 1966. U.S. Patents 3,621,664 and 3,796,060. Zief and Wilcox, Fractional Solidification, Marcel Dekker, New York, 1967. 

Zone melting is a possibly generic approach to IL purification. The solidification of ILs often resulfs in the formation of glass. However, it is possible to determine/choose conditions under which single crystals of ILs with a melting point down to -25°C (but not all) can be grown [39]. Where crystallization is seen, then separation of impurities can be demonstrated. 

Fractional solidification and its applications to obtaining ultrapure chemical substances, has been treated in detail in Fractional Solidification by M.Zief and W.R.Wilcox eds, Edward Arnold Inc, London 1967, and Purification of Inorganic and Organic Materials by M.Zief, Marcel Dekker Inc, New York 1969. These monographs should be consulted for discussion of the basic principles of solid-liquid processes such as zone melting, progressive freezing and column crystallisation, laboratory apparatus and industrial scale equipment, and examples of applications. These include the removal of cyclohexane from benzene, and the purification of aromatic amines, dienes and naphthalene. 

As shown by Fig. 22.16, the concentrations of solute atoms are significantly reduced in the material that is solidified early in the solidification process when k < 1. One-dimensional plane-front solidification can therefore be used as a method of purification. However, purification is carried more effectively out by modifying the process and adopting a zone-melting technique. 

Considerable purification is achieved during zone melting. The final transient at the end begins when the leading end of the zone reaches the end of the specimen. At that point, the solidification becomes very similar to plane-front solidification. Additional passes produce further purification and very small solute concentrations in the first part of the specimen. An asymptotic limit exists, however, as taken up in Exercise 22.2. 

E.F.G.Herington, Zone Melting of Organic Compounds, Wiley Sons, NY, 1963 W.Pfann, Zone Melting, 2nd edn, Wiley, NY, 1966 H.Schildknecht, Zonenschmelzen, Verlag Chemie, Weinheim, 1964 W.R.Wilcox, RPriedenberg et al. Chem Rev 64 187 1964 M.Zief and WR.Wilcox (Eds), Fractional Solidification, Vol 1, M Dekkerlnc.NY, 1967.]... 

F Weight fraction of melt produced In partial melting in fractional crystallization, the fraction of melt remaining f Fraction of melt allocated to the solidification zone in in situ crystallization which is returned to the magma chamber f A function of F, the fraction of melt remaining In AFC processes... 

Mq is the initial mass of the magma chamber and Mi is the mass of liquid so that the ratio Mi /Mq is equivalent to the term F — the fraction of melt remaining and used in equilibrium and fractional crystallization models /is the fraction of magma allocated to the solidification zone which is returned to the magma chamber. Equation [4.21] is similar in form to the Rayleigh fractionation equation but with a more complex exponent. 

Moates, G.H. and Kennedy, J.K. (1967) Continuous-zone melting. In Fractional Solidification, M. Zief and W.R. Wilcox (eds.), Dekker, New York, 342-373. 

The original Cyanamid process used a single phase fusion melt of an acrylonitrile copolymer (molecular weight 30,000 60,000) and water and extruded, at about 50 bar, through a spinneret (hole size 60-160 pm) directly into a steam pressurized solidification zone, such that the subsequent release of water from the extrudate avoided any deformation of the extruded filaments and their condition remained such that they could be stretched up to some 25 times. The fiber was cooled with water and passed to the atmosphere by way of a pressure lock. 

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