When to use evaporation to crystallize, and when to use evaporation to concentrate cooling and crystallization?
Almost the same meaning is to remove the solvent so that the solute is supersaturated and separated from the solvent.
1. Evaporative crystallization is mainly used to extract the solute from the aqueous solution of a single solute, such as sodium chloride from the solution of sodium chloride.
2. Evaporative crystallization is mainly used for the solubility of the solute that is not greatly affected by temperature. For example, sodium chloride contains a small amount of potassium chloride (but it must be filtered if the water cannot be evaporated).
3. Heating, evaporation, concentration and crystallization are mainly used for the purification of solutes whose solubility is greatly affected by temperature. For example, potassium chloride contains a small amount of sodium chloride in evaporation, concentration and crystallization to separate a solute with low solubility in a multi-solute solution.
Evaporation and crystallization are directly heated in the evaporating dish to evaporate the solution until a large number of crystals (or crystal film appear) are stopped, and the remaining solvent is evaporated to dryness with the waste heat of the evaporating dish.
The cooling crystallization first needs to be heated and concentrated to obtain a hot saturated solution, and then filtered while hot to remove insoluble impurities, then cooled to crystallize and filtered. The obtained crystals may also contain other impurities. If further purification is required, recrystallization is performed.
The principle of cooling a saturated solution and cooling crystallization is the same. The solution is saturated and the solute is precipitated by cooling. This method is generally used for solutes whose solubility changes greatly with temperature. The only difference is that the starting point of cooling is different.
Evaporative solvent crystallization promotes the solution to saturation and precipitation of solutes through the continuous reduction of solvent. This method is mainly used for solutes with small solubility changes with temperature.
Evaporation crystallization: the solubility is unchanged, reducing the solvent, solute precipitation.
Saturation of cooling heat: As the temperature decreases, the solubility decreases and solutes precipitate.