Crystallization

Crystallization refers to the formation of solid crystals from a homogeneous solution. It is essentially a solid-liquid separation technique and a very important one at that.

Example of Crystallization

1. Water freezing

2. Removing sucrose from beet solutions

3. Removing KCl from an aqueous solution

Crystals are grown in many shapes, which are dependent upon downstream processing or final product requirements. Crystal shapes can include cubic, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic, and trigonal. In order for crystallization to take place a solution must be "supersaturated". Supersaturation refers to a state in which the liquid (solvent) contains more dissolved solids (solute) than can ordinarily be accomodated at that temperature.

As with any separation method, equilibrium plays an important role. Below is a general solubility curve for a solid that forms hydrate (a compound that has one or more water molecules attached) as it cools.

In Figure 1, X may be any solid that can form hydrates such as Na₂S₂O₃. The number of hydrate molecules shown in Figure 1 is strictly arbitrary and will vary for each substance.

So how do you grow crystals? Let's consider an example that is fairly easy to envision. Take a pot of boiling water and add table salt while stirring to make a water-salt solution. Continue adding salt until no more salt will dissolve in the solution (this is a saturated solution). Now add one final teaspoon of salt. The salt that will not dissolve will help the first step in crystallization begin. This first step is called "nucleation" or primary nucleation. The salt resting at the bottom of the pot will provide a site for nucleation to occur.

Primary nucleation is the first step in crystallization. Simply defined, it's the growth of a new crystal

On an industrial scale, a large supersaturation driving force is necessary to initiate primary nucleation. The initiation of primary nucleation via this driving force is not fully understood which makes it difficult to model (experiments are the best guide). Usually, the instantaneous formation of many nuclei can be observed "crashing out" of the solution. You can think of the supersaturation driving force as being created by a combination of high solute concentration and rapid cooling. In the salt example, cooling will be gradual so we need to provide a "seed" for the crystals to grow on. In continuous crystallization, once primary nucleation has begun, the crystal size distribution begins to take shape. Think about our salty water, as you look at Figure 2 describing the progression of crystallization.

Figure 2: Progression of Crystallization

The second chief mechanism in crystallization is called secondary nucleation. In this phase of crystallization, crystal growth is initiated with contact. The contact can be between the solution and other crystals, a mixer blade, a pipe, a vessel wall, etc. This phase of crystallization occurs at lower supersaturation (than primary nucleation) where crystal growth is optimal.

Secondary nucleation requires "seeds" or existing crystals to perpetuate crystal growth. In our salt example, we bypassed primary nucleation by "seeding" the solution with a final teaspoon of salt. Secondary nucleation can be thought of as the workhorse of crystallization.

Again, no complete theory is available to model secondary nucleation and it's behavior can only be anticipated by experimentation. Mathematic relationships do exist to correlate experimental data. However, correlating experimental data to model crystallization is time consuming and often considered extreme for batch operations, but can easily be justified for continuous processes where larger capital expenditures are necessary. For batch operations, only preliminary data measurements are truly necessary.

We've discussed how crystallization occurs once supersaturation is reached, but how do we reach supersaturation? We have already covered one such method in our salt crystallization example. Since the solubility of salt in water decreases with decreasing temperature, as the solution cools, its saturation increases until it reaches supersaturation and crystallization begins (Figure 3). Cooling is one of the four most common methods of achieving supersaturation. It should be noted that cooling will only help reach supersaturation in systems where solubility and temperature are directly related. Although this is nearly always the case, there are exceptions. In Figure 3, you'll note that Ce₂(SO₄)₃ actually becomes less soluble in water at higher temperatures.

Figure 3: Solubilities of Several Solids

The four most common methods of reaching supersaturation in industrial processes are:

1. Cooling (with some exceptions)

2. Solvent Evaporation

3. Drowning

4. Chemical Reaction

In an industrial setting, the solute-solvent mixture is commonly referred to as the "mother liquor".

Drowning describes the addition of a nonsolvent to the solution which decreases the solubility of the solid. A chemical reaction can be used to alter the dissolved solid to decrease its solubility in the solvent, thus working toward supersaturation. Each method of achieving supersaturation has its own benefits. For cooling and evaporative crystallization, supersaturation can be generated near a heat transfer surface and usually at moderate rates. Drowning or reactive crystallization allows for localized, rapid crystallization where the mixing mechanism can exert significant influence on the product characteristics.

Equipment Used in Crystallization

1. Tank Crystallizers

This is probably the oldest and most basic method of crystallization. In fact, the "pot of salt water" is a good example of tank crystallization. Hot, saturated solutions are allowed to cool in open tanks. After crystallization, the mother liquor is drained and the crystals are collected. Controlling nucleation and the size of the crystals is difficult. The crystallization is essentially just "allowed to happen". Heat transfer coils and agitation can be used. Labor costs are high, thus this type of crystallization is typically used only in the fine chemical or pharmaceutical industries where the product value and preservation can justify the high operating costs.

2. Scraped Surface Crystallizers

An example may be the Swenson-Walker crystallizer consisting of a trough about 2 feet wide with a semi-circular bottom. The outside is jacketed with cooling coils and an agitator blade gently passes close to the trough wall removing crystals that grow on the vessel wall.

3. Forced Circulating Liquid Evaporator-Crystallizer

Just as the name implies, these crystallizers combine crystallization and evaporation, thus the driving forces toward supersaturation. The circulating liquid is forced through the tubeside of a steam heater. The heated liquid flows into the vapor space of the crystallization vessel. Here, flash evaporation occurs, reducing the amount of solvent in the solution (increasing solute concentration), thus driving the mother liquor towards supersaturation. The supersaturated liquor flows down through a tube, then up through a fluidized area of crystals and liquor where crystallization takes place via secondary nucleation. Larger product crystals are withdrawn while the liquor is recycled, mixed with the feed, and reheated.

4. Circulating Magma Vacuum Crystallizer

In this type of crystallizer, the crystal/solution mixture (magma) is circulated out of the vessel body. The magma is heated gently and mixed back into the vessel. A vacuum in the vapor space causes boiling at the surface of the liquid. The evaporation causes crystallization and the crystals are drawn off near the bottom of the vessel body.