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Cold Crystallization

Introduction:

There are two methods for crystallization of high brine: one is thermal crystallization, which is suitable for substances whose temperature has little effect on solubility. The “sunshine dry salt” in coastal areas is the method used. The other one is cooling the hot saturated solution method. This method is applicable to substances with increased temperature and increased solubility.For example, in the salt lakes of the northern region, there is no crystal on the lake surface when the temperature is high in summer.When the temperature is lowered in winter, the mirabilite and other substances are separated from the salt lake.
Detailed Introduction

There are two methods for crystallization of high brine: one is thermal crystallization, which is suitable for substances whose temperature has little effect on solubility. The “sunshine dry salt” in coastal areas is the method used. The other one is cooling the hot saturated solution method. This method is applicable to substances with increased temperature and increased solubility.For example, in the salt lakes of the northern region, there is no crystal on the lake surface when the temperature is high in summer.When the temperature is lowered in winter, the mirabilite and other substances are separated from the salt lake.


Evaporation Concentration Cooling Crystallization

1)Introduction

Heating the solvent will cause the solution to change from unsaturated to saturated.If temperature falls,the solute whose solubility changes greatly with temperature will precipitate as crystals, which is called cooling crystallization. For example, when there is more KNO3 than NaCl in the mixture of NaCl and KNO3, this method can be adopted. KNO3 will be separated first, and NaCl later.


2)Characteristis of Cooling Crystallization

After cooling crystallization, the mass of the solute becomes smaller.

The quality of the solvent is constant. The mass of the solution is reduced.

The mass fraction of olute becomes smaller.

The state of the solution is saturated.


3)Theory

1. The principle of cooling crystallization is that the solubility of the substance is reduced when the temperature falls.Then the solution is saturated, and the excess insoluble solute will precipitate. The principle of evaporation crystallization is that the solubility is constant when the temperature is constant or the temperature has no great change before and after evaporation, and the excess insoluble solute will precipitate when the the solubility solution is saturated caused by the reduction of water.

2. To separate or purify two soluble substances after mixing, the one which is likely to reach saturation, the crystallization method of which will be used.For example,when sodium chloride contains a small amount of sodium carbonate, the evaporation crystallization will be used.This is the crystallization method of sodium chloride. On the contrary, cooling crystallization will be applied.

3. The solubility curve showing a significant upward trend means that the solubility of the substance increases greatly with temperature. Cooling crystallization will be applied for this kind of substance.When the solubility curve is slightly flat,the dissolutionof the substance does not change much with temperature, and evaporation crystallization will be used.


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