Crystallisation, velocity

The crystal growth rate has been found in many eases to be extremely rapid, more rapid than can be accounted for on the diffusion hypothesis thus Tammann (foe. cit.) found for benzophe-none a maximum crystallisation velocity of 2 4 mm. per minute (Walton and Judd, J. Phys. Ohem. xvni, 722,1914). Much higher values, e.g. 6840 mm. per minute for water and 60,000 mm. per minute for phosphorus (Gernez, O.R, xcv. 1278, 1882) have been recorded. In some cases the rate was found independent of the speed of rotation of the stirrer and occasionally the reaction velocity followed a bimolecular law instead of the simple unimolecular expression which holds true for solution. 

A few cases have been recorded (Dreyer, Zeit Phys, Ghem, XLVlii. 487, 1904) where the effect of the addition agent on the crystallisation velocity is to be attributed to an alteration in the diffusion constant of the solute in the solution, thus the velocity of crystallisation of formanilide is increased and not decreased by the addition of methyl or ethyl alcohol to the fused salt. 

Since the temperature at which the spontaneous formation of crystal nuclei has its maximum value is, in general, below that at which the velocity of crystallisation is a maximum, it is possible, by rapid cooling, to pass through the temperatures of maximum crystallisation velocity and maximum formation of nuclei, and to obtain the liquid at a temperature at which the velocity of crystallisation (and also of crystal nuclei formation) becomes negligible. Since the viscosity increases with fall of temperature, the liquid passes into a glassy mass, which will remain (practically) permanent even in contact with the crystalline sohd, Some substances can be supercooled to the glassy state more readily than others. 

Pressure appears to decrease the maximum crystallisation velocity (Hassel-blatt, Z. anorgan. Chem, 1921, irp, 325). 

Marc and Freundlich have put forward the view that the velocity of crystallisation of supercooled liquids is influenced by adsorption phenomena, the dissolved substance being adsorbed on the surface of the crystals. In this connection, Freundlich and Oppenheimer have shown that the crystallisation velocity of supercooled water is frequently, if not always, increased by colloidal substances the particles of which are non-spherical, whereas particles which are spherical, and also the truly dissolved substances, lower the velocity. 

Padoa, Aecad. Lincei, Atti, 1904, 13, 329 Freundlich, Z. physikaL Chem., 1911 > 245 For a study of the crystallisation velocity of isomorphous mixtu res,... 

Precipitation is effected in hot solutions, provided the solubility and the stability of the precipitate permit. Either one or both of the solutions should be heated to just below the boiling point or other more favourable temperature. At the higher temperature (a) the solubility is increased with a consequent reduction in the degree of supersaturation, (b) coagulation is assisted and sol formation decreased, and (c) the velocity of crystallisation is increased, thus leading to better-formed crystals. 

Although Carnelley once thought he had been able to superheat ice ( hot ice ), it is almost certain that no solid can be maintained alone at a temperature higher than its melting-point. Tammann (Zeitschr. physik. Chem., 68, 257, 1910) finds, however, that a crystalline solid may, under certain circumstances, be superheated in the presence of its melt. This occurs when the supply of -heat to the crystal is sufficiently great in comparison with the linear velocity of crystallisation of the supercooled liquid (cf. Findlay Phase Hide). 

The velocity of linear crystallisation has boon determined in the case of fused solids by Tainmann and his co-workers loo. cit), in Buporsaturabed solutions by Mare Zeit. Phys. Ghem, LXi. 385,1908, Lxvn. 4t70,1909, Lxvm. 104,1909, Lxxin. ()8o, 1910, lxxv. 710 1911, Lxxix. 71, 1912), Lo Blanc (ibid. Lxxvn. 614, 1911), and nthers. 

Increase in temperature causes a slight decrease in refractive index. The heat of dissolution 4 of the octahedral form is - 7530 calories at 18° C. The velocity of crystallisation from supersaturated solutions corresponds with 5 - dcjdt = fre4, where c is the concentration the temperature coefficient for the interval 0 to 25° C. is zero. 

The velocities of crystallisation of supercooled water, as determined in a tube 1 metre in length and 0-7 cm. in diameter, and expressed as cms. per minute, are given in the accompanying table.2... 

The maximum velocity of crystallisation evidently lies below —9-07° C., but, owing to spontaneous solidification of the water, it was not found possible to make determinations at lower temperatures. 

Different from the dissolution of amorphous polymers is that of semi-crystalline ones. Dissolution of these polymers is much more difficult than that in the glassy state, as the enthalpy of melting has to be supplied by the solvent. Many solvents, which are able to dissolve tactic but glassy polymers, are unable to dissolve the same polymer in the crystalline state. Asmussen et al. (1965) have found that the velocity of dissolution of crystalline polymers as a function of temperature closely resembles the velocity of crystallisation versus temperature curves. Polymers formed at the highest rate of growth also dissolve at the highest rate. 

In unstrained (quasi-isotropic) crystallisation processes the crystallisation starts from a number of point-nuclei, and progresses in all directions at an equal linear velocity (v). In the case of isothermal crystallisation the radius of the crystallised regions increases by an equal amount per unit of time (v = constant). The rate of growth is very much dependent, however, on the temperature of crystallisation. 

In this connection, most attention has been paid to the study of the velocity of crystallisation of a supercooled liquid, the first experiments in this direction having been made by Gernez on the velocity of crystallisation of phosphorus and sulphur. Since that time, the velocity of crystallisation of other supercooled liquids has been investigated such as acetic acid and phenol by B. Moore supercooled water by Tumlirz and by Walton and Judd and a number of organic... 

In the case of certain substances the velocity of crystallisation does not remain constant within this range of temperature, but may increase or decrease. This behaviour is attributed to differences in the molecular complexity of the liquid and solid substances (Tammann, Gedenkboek aangehodm aan J.M.mn Bemmehny 1910, p. 297. 

Crystallisation or scale formation on heat exchanger surfaces is influenced by a number of system variables (see Chapter 8) including temperature, pH supersaturation conditions and the flow velocity. In general these are fixed parameters and may not be changed without affecting the requirements of the process. 

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