Crystallization

Crystallization . Solution separation technique in which the conditions are adjusted in such a way that only one of the solutes can crystallize while the others remain in the solution. This operation is frequently used in industry for the purification of substances that are generally obtained together with impurities.

Summary

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  • 1 Crystallization process
    • 1 Purity of the product
    • 2 Balance and returns
    • 3 Solubility of equilibrium in crystallization
  • 2 Growth and properties of crystals
    • 1 Nucleation
  • 3 Crystallization rate
  • 4 Effect of impurities
  • 5 Effect of temperature on solubility
  • 6 Fractional crystallization
  • 7 Sources

Crystallization process

In this process, a solid substance with a very small amount of impurities dissolves in a minimal volume of solvent (hot if the solubility of the substance to be purified increases with temperature ). Then the solution is allowed to cool very slowly, so that the crystals that separate are from the pure substance, and it is filtered.

Filtrate, containing all impurities, is usually discarded. For fractional crystallization to be an appropriate separation method, the substance to be purified must be much more soluble than impurities under the crystallization conditions, and the amount of Impurities should be relatively small. Impurities are commonly present in low concentrations and they return to the solution even when the solution cools. If extreme purity of the compound is needed, the filtered crystals can be subject to recrystallization and, naturally, each crystallization results in a loss of the desired solute that remains in the mother liquor along with the impurities. The ideal solvent for crystallization of a particular compound is one that:

  • It does not react with the compound.
  • Boil at a temperature below the melting point of the compound.
  • Dissolves large amounts of the compound when hot.
  • Dissolves a small amount of compound when cold.
  • It is moderately volatile and the crystals can be dried quickly.
  • It is non-toxic, non-flammable and inexpensive. Impurities should be insoluble in the solvent so that they can be separated by filtration .

Product purity

A crystal itself is very pure. However, when the crystal harvest is separated from the final magma, especially if it is a crystalline aggregate, the mass of solids retains a considerable amount of mother liquor. Therefore, if the product is dried directly, a contamination occurs that depends on the quantity and degree of impurity of the mother liquor retained by the crystals.

Balance and returns

In many industrial crystallization processes the crystals and mother liquor remain in contact long enough to reach equilibrium, so that the mother liquor is saturated at the final process temperature. The crystallization yield can be calculated from the concentration of the original solution and the solubility at the final temperature. If appreciable evaporation occurs during the process, it must be taken into account.

When the growth rate of the crystals is low, it takes a relatively long time to reach equilibrium, especially when the solution is viscous or when the crystals deposit at the bottom of the crystallizer, so that the surface of crystals exposed to the supersaturated solution is small. In these cases, the final mother liquor may contain considerable supersaturation and the actual yield will be less than that calculated from the solubility curve.

When the crystals are anhydrous, the calculation of the yield is simple, since the solid phase does not contain solvent. If the crystals contain crystallization water, it must be taken into account, since this water is not available for the solute that remains in the solution. Solubility data s are generally expressed in parts by mass of anhydrous material, percent parts by mass of total solvent, or in percentage by mass of anhydrous solute. In these data the crystallization water is not taken into account.

The key to calculating hydrated solute yields is to express all masses and concentrations as a function of hydrated salt and free water. Since the latter remains in the liquid phase during crystallization, concentrations and amounts based on free water can be subtracted to obtain a correct result.

Balance solubility in crystallization

The equilibrium in the crystallization of any system can be defined in terms of its solubility or saturation and supersaturation curve. The supersaturation curve differs from the solubility curve in that its position is not only a property of the system but also depends on other factors such as the cooling range, the degree of agitation and the presence of foreign particles. However under certain conditions, the supersaturation curve for a given system is definable, reproducible, and represents the maximum supersaturation that the system can tolerate, at which point nucleation occurs spontaneously.

The solubility curve describes the balance between the solute and the solvent and represents the conditions under which the solute crystallizes and the mother liquor coexists in thermodynamic equilibrium. The saturation and supersaturation curves divide the concentration-temperature field into three zones:

  • The unsaturated region, to the right of the saturation curve.
  • The stable target region, between the two curves.
  • The oversaturated or labile region, to the left of the supersaturation curve. A typical crystallization equilibrium diagram is shown in the figure below:

According to Mier’s original theory, in the unsaturated region, the solute crystals will dissolve, crystal growth will occur in the meta stable zone, and nucleation will occur instantaneously in the labile zone. Subsequent investigations found how other factors affect nucleation in addition to supersaturation.

Growth and properties of crystals

Nucleation

The nucleation phenomenon is essentially the same for crystallization from solution, crystallization from molten product, condensation of mist droplets in supercooled vapor, and generation of bubbles in superheated liquid in all cases, nucleation s occurs as a consequence of rapid local fluctuations on a molecular scale in a homogeneous phase that is in a state of metastable equilibrium. Crystalline nuclei can be formed from molecules, atoms, or ions. In aqueous solutions they can be hydrated. Due to their rapid movements, these particles are called kinetic units.

For a small volume on the order of 100 oA, kinetic theory states that individual kinetic units vary greatly in location, time, velocity, energy, and concentration. The apparently stationary values ​​of the intensive properties, density, concentration and energy, corresponding to a macroscopic mass of solution, are actually time-averaged values ​​of fluctuations too fast and small to be measured on a macroscopic scale.

Due to fluctuations, an individual kinetic unit frequently penetrates the force field of another, or the two particles momentarily unite, normally they separate immediately, but, if held together, they can be successively joined to other particles. Combinations of this type are called aggregates. The unit of particles, one by one, to an aggregate constitutes a chain reaction that can be considered as a series of reversible chemical reactions according to the following scheme: where A1 is the elemental kinetic unit, and the subscript represents the number of units that make up the aggregate.

When m is small, an aggregate does not behave like a particle that forms a new phase with a defined identity and limit. With increasing m, the aggregate can already be recognized and is called an embryo. The vast majority of embryos have a very short life, breaking to re-form aggregates or individual units. However, depending on the supersaturation, some embryos grow to a size sufficient to reach thermodynamic equilibrium with the solution.

In this case the embryo is called the nucleus. The value of m for a nucleus is in the range of a few units to several hundreds. The value of m for nuclei of liquid water is of the order of 80. The nuclei are in unstable equilibrium: if they lose units they dissolve and if they gain units they transform into a crystal, the sequence of stages in the formation of a crystal is therefore Added => embryo => nucleus => crystal.

Crystallization speed

The growth rate of a crystal is known as the crystallization rate. Crystallization can only occur from supersaturated solutions. Growth occurs first with the formation of the nucleus, and then with its gradual growth. At concentrations above supersaturation, nucleation is thought of as spontaneous, and rapid.

In the metastable region, nucleation is caused by mechanical shock, or by friction, and secondary nucleation can result from the breaking of already formed crystals. It has been observed that the crystallization rate adjusts to the following equation: The values ​​of the exponent m are in the range of 2 to 9, but it has not been correlated as a quantitative value that can be estimated. This speed is average by counting the number of crystals formed in certain periods of time.

This speed depends on its instantaneous surface and the linear speed of the solution, which passes into the solution, as well as the supersaturation. The values ​​of the exponent n are in the order of 1.5, but again there is no correlation in the design of the crystallizers that can estimate it. Crystal growth is a layer-by-layer process, and since it can only occur on the face of the crystal, it is necessary to transport material to the face from the solution.

The diffusion resistance to the movement of the molecules (or ions) towards the growing face of the crystal and the resistance to the integration of these molecules to the face must be considered. Different faces can have different growth rates and these can have different growth rates and these can be selectively altered by adding or removing impurities.

Effect of impurities

The chemical environment, eg the presence of relatively low concentrations of substances outside the species to be crystallized, be it impurities, etc., plays an important role in optimizing the crystallization systems. Their role is very important for various reasons.
First, all materials are impure or contain traces of impurities added during processing. Random variation of impurities is an undesirable effect. Its effect on the species to be crystallized must be well known, if satisfactory control is desired over the crystallization system.

The second, and most important, it is possible to influence the output and control of the crystallization system, or change the properties of the crystals by adding small amounts of carefully chosen additives. This, by adding certain types and amounts of additives it is possible to control the size of the crystals, the size distribution of the crystal, the habit of the crystal and its purity.
The chemical environment can be appropriately used to vary:

  • Significantly altering the crystallization kinetics and hence the crystal size distribution.
  • Have better control of the crystallizer.
  • Improve product quality and / or performance by producing a certain type of glass.
  • Produce very pure crystals of certain materials in which impurities are unacceptable.

Effect of temperature on solubility

Dissolve in a certain amount of a solvent at a specified temperature. Temperature affects the solubility of most substances. Most, but not all, ionic compounds, the solubility of the solid increases with temperature. However, there is no clear correlation between the sign of Dissolution and the variation in solubility with temperature. For example, the dissolution process of CaCl2 is exothermic and that of NH4NO3 is endothermic. But the solubility of both compounds increases with temperature. In general, the effect of temperature on solubility should be determined experimentally.

Fractional crystallization

The dependence of the solubility of a solid on temperature varies considerably. For example, the solubility of NaNO3 increases very rapidly with temperature, while that of NaBr hardly changes. This large variation provides a way to obtain pure substances from mixtures. Fractional crystallization is the separation of a mixture of substances into their pure components based on their different solubilities. Suppose you have a 90 g sample of KNO3 contaminated with 10 g of NaCl.

To purify the former, the mixture is dissolved in 100 mL of water at 60 ° C and then the solution is gradually cooled to 0 ° C. At this temperature the solubilities of KNO3 and NaCl are 12.1 g / 100 g of H2O and 34.2 g / 100 g of H2O respectively. Thus, (90-12) g or 78 g of KNO3 is separated from the solution, but all the NaCl will remain dissolved. In this way, about 90% of the original amount of KNO3 can be obtained in pure form. The KNO3 crystals can be separated from the solution by filtration.

Many of the solid, inorganic and organic compounds used in the laboratory are purified by fractional crystallization. The method works best if the compound to be purified has a steeply curved curve, that is, if it is much more soluble at high temperatures than at low temperatures. Otherwise, a large part of the compound will remain dissolved as the solution cools. Fractional crystallization also works if the amount of impurities in the solution is relatively small.

 

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