What is the difference between precipitation and crystallization?
At NiTech, we are always careful to differentiate between precipitation and crystallization. In chemistry, precipitation is the formation of a solid phase. So, all crystallizations are a sub-set of precipitation processes. It is common to call rapid solid formation precipitation and slow, control formation of a crystalline solid phase crystallization. The difference between the two can cause some confusion for two reasons:
- both form a solid phase; and
- at first glance, the processes appear to be the same – i.e. removal of a solute from a solvent by causing it to form a solid.
Although the phase of the product is the same (i.e. a solid), the characteristic of the product and the mechanism by which it is formed is different between the two processes.
Precipitation occurs when a solute in a solvent becomes supersaturated to such an extent that it nucleates and spontaneously comes out of solution.
Precipitation is rapid and forms a large quantity of small particles, as rate of formation of new solids greatly exceeds the rate of crystal growth. Also, due to the speed of precipitation, other components become entrapped in the precipitate, resulting in a lower purity material; these can include the solvent as well as other components present in the solvent system at the time.
In the case of CBD, this can mean THC, pesticides, solvent and other cannabinoids becoming entrapped in the CBD crystals, requiring resource-intensive recrystallization operations to achieve the required purity.
In addition to a greater number of impurities, the small particle size results in:
- the material becoming more compacted and harder to filter effectively as the flow-channels for fluid to drain are narrower;
- losses during filtration and washing are increased because there is more solid-liquid surface area – negatively impacting on the end-to-end yield;
- increased time to dry as there is more wet surface area to dry; and
- caking of solids on the side of the reactor/stirrer.
For these reasons, although quick and easy, precipitation is not an effective or desirable means of isolating a compound from a solution due to poor process performance.
Crystallization occurs when the solute dissolved in a solvent is supersaturated (but not precipitating spontaneously). An external stimulus, e.g. the presence of seeds, is used to initiate nucleation in a controlled manner. Crystallization is the controlled growth of crystals, often from seed crystals. The seed crystals act as de facto catalysts, which lower the energy barrier of crystallization: it is easier for solute to crystallise on an existing crystal surface than to form its own new solid phase, at relatively levels of low supersaturation.
Controlled crystallization in a COBC is gradual and forms large, uniform particles with high purity and yields and makes downstream processing simpler and more effective.
Crystallization is the desired method for isolating a compound from a system.