Eutectic points

Ciyst lliz tion. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure soHd phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where soHd crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is stiU soluble in the remaining Cg aromatics Hquid solution,... 

The lead—copper phase diagram (1) is shown in Figure 9. Copper is an alloying element as well as an impurity in lead. The lead—copper system has a eutectic point at 0.06% copper and 326°C. In lead refining, the copper content can thus be reduced to about 0.08% merely by cooling. Further refining requites chemical treatment. The solubiUty of copper in lead decreases to about 0.005% at 0°C. 

Silver readily forms alloys with lead. Lead is often used as a base metal solvent for silver recovery processes. The lead—silver system is a simple eutectic having the eutectic point at 2.5 wt % silver and 304°C. The soHd solubihty of silver in lead is 0.10 wt % at 304°C, dropping to less than 0.02 wt % at 20°C. 

Highway. Rock salt, solar salt, and in some cases in Europe, evaporated salt are used to maintain traffic safety and mobiUty during snow and ice conditions in snowbelt regions throughout the world. Sodium chloride melts ice at temperatures down to its eutectic point of —21.12°C. Most snowstorms occur when the temperature is near 0°C, where salt is very effective. More than 40% of dry salt produced in the United States is used for highway deicing. 

Pure ethylene glycol freezes at —I2,7°C, Exact composition and temperature for eutectic point are unknown, since solutions in this region turn to viscous, glassy mass that makes it difficult to determine the true freezing point. For the concentrations lower than eutectic, ice forms on freezing, while on the concentrated, solid glycol separates from the solution. 

Both liquid and vapor phases are totally miscible. Conventional vapor/liqiiid eqiiilihriiim. Neither phase is pure. Separation factors are moderate and decrease as purity increases. Ultrahigh purity is difficult to achieve. No theoretical limit on recovery. Liquid phases are totally miscible solid phases are not. Eutectic system. Sohd phase is pure, except at eutectic point. Partition coefficients are very high (theoretically, they can be infinite). Ultrahigh purity is easy to achieve. Recovery is hmited by eutectic composition. 

Most alloy systems are more complicated than the lead-tin system, and show intermediate phases compounds which form between components, like CuAlj, or AljNi, or FojC. Their melting points are, usually, lowered by alloying also, so that eutectics can form between CuAlj and A1 (for example), or between AljNi and Al. The eutectic point is always the apex of the more or less shallow V formed by the liquidus lines. 

The three phase diagrams, or parts of diagrams, shown in Fig. A1.28, all have a eutectic point. Mark the point with an arrow and list the eutectic temperature and composition in wt% (the co-ordinates of the point). 

At 183°C. The liquid composition has reached the eutectic point (the bottom of the V). This is the lowest temperature at which liquid is stable. At this temperature all the remaining liquid transforms to two solid phases a tin-rich a phase, composition Xpb= 1.45% and a lead-rich (3 phase, composition Xpi, = 71%. This reaction ... 

DEF. A eutectic reaction is a three-phase reaction, by which, on cooling, a liquid transforms into two solid phases at the same time. It is a phase reaction, of course, but a special one. If the bottom of a liquid-phase field closes with a V, the bottom of the V is a eutectic point. 

At the eutectic point the three phases are in equilibrium. The compositions of the two new phases are given by the ends of the line through the eutectic point. 

Check, using the phase rule, that three phases can coexist only at a point (the eutectic point) in the lead-tin system at constant pressure. If you have trouble, revise the phase rule on p. 327. 

The aluminium casting alloys are mostly based on the Al-Si system (phase diagram Fig. A1.31). It is a classic eutectic system, with a eutectic point at about 11% Si and... 

The iron-carbon system has a eutectic find it and mark it on the diagram (Fig. A1.37). At the eutectic point the phase reaction, on cooling, is... 

A hyper-eutectic alloy containing, say, 50% Sb starts to freeze when the temperature reaches the liquidus line (point a in Fig. 20.39). At this temperature pure pro-eutectic Sb nucleates as the temperature continues to fall, more antimony is deposited from the melt, and the composition of the liquid phase moves down the liquidus line to the eutectic point. When this is reached, the remainder of the melt solidifies. The microstructure of alloys of eutectic composition varies somewhat with alloy system, but generally consists of an aggregate of small particles, often platelets, of one of the phases comprising the eutectic in a continuous matrix of the other phase. Finally, the microstructure of the hypereutectic 50% Sb alloy already mentioned... 

With any solution, there will be one concentration which remains liquid until it reaches a freezing point, and then will freeze solid. This is the eutectic mixture, and its freezing point is the eutectic point of the solute (see Figure 12.2). At all other concentrations, as... 

The general theory was worked out by Roozeboom (Zeitschr. physik. Chem., 1899) from the standpoint of the theory of thermodynamic potential. The equations (2a, h), (3a, h) of the preceding section apply equally well to the present case, and details need not be given here. The liquid solidifies at a constant temperature when it has the same composition as the solid deposited— the so-called eutectic point. 

These materials are essentially combustion improvers and tend to have fairly simple formulations (e.g., 3% copper chloride, 7% manganese chloride, 90% ammonium chloride). They are designed to change the crystalline structure within the clinker crystal lattice and raise the clinker eutectic point, thus minimizing the formation of noncombustible clinker, residual ash, and other deposits. Feed rates are approxiimately 0.5 to 2.0 lb per bone-dry ton. 

E8.12 The melting point of 1,4-dichlorobenzene is 326.4 K and that of naphthalene is 353.4 K. The eutectic point occurs at a temperature of 303.4 K and a mole fraction of naphthalene in the liquid phase of 0.394. Assume ideal liquid solutions, no solid solubility, and ArusCp.m = 0 and calculate AfusHm for 1,4-dichlorobenzene. 

Euler s theorem 209. 612 europium hydroxide, heat capacity 584-5 eutectic point in solid 4- liquid phase equilibria 421... 

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