The process of solidifying atoms or molecules into a highly organized form known as a crystal is known as crystallization. A supersaturated solution nucleates the solute in the process of crystallization, which is a technique for turning a solution into a solid under the control of chemical equilibrium. Although crystallization can happen naturally, it also has numerous industrial uses as a separating and purifying step in the chemical, pharmaceutical, and food sectors.
What is Crystallization?
Crystallization, also known as crystallisation, is the process of organizing atoms or molecules into a well-defined, rigid crystal lattice to minimize their energy state. Crystallization is a natural process that occurs when solids harden from liquids or precipitate from liquids or gases.
This procedure can be carried out by causing a physical change, such as a temperature shift, or a chemical change, such as acidity.
Crystallization is a separation technique that is frequently used in the chemical and pharmaceutical industries. The principle of crystallization is based on a compound’s limited solubility in a solvent at a given temperature, pressure, and so on. A transition in these conditions to a lower solubility state will result in the production of a crystalline solid. Although crystallization has been used for thousands of years to produce salt and sugar, many events that occur during crystallization remain unknown.
Crystallization requires two events to occur. First, on the microscopic scale, atoms or molecules cluster together in a process known as nucleation. If the clusters become stable and large enough, crystal formation may ensue.
Nucleation is the initial stage or phase in the crystallization process. The initial atom to form a crystal forms the nucleus of the nucleation, and other atoms form around that nucleus. More unit cells assemble around the nucleus during this process, and a tiny crystal seed forms. Nucleation is the most important step in the crystallization process since it dictates the structure of the entire crystal. As nucleation occurs in a supersaturated solvent and supercooled liquid, seed crystal and nucleus defects can cause catastrophic rearrangements. A supercooled liquid is any liquid that is on the verge of solidifying and requires the formation of an initial nucleus.
- The crystallization process will continue to circle around this nucleus.
- When atoms or molecules in a cooling liquid lose their ability to bounce off each other, the nucleus develops. Instead, they begin to interact, generating solid crystal formations.
- Although pure elements normally form a crystal structure, larger molecules may be difficult to crystalize at normal temperatures and pressures.
- In a supersaturated solution, the solvent containing the desired crystal is completely saturated.
- When the temperature or acidity of the solution lowers, the solubility of the atoms or molecules reduces, and the solvent can hold fewer of them. They “fall out” of the solution and clash as a result. This can also result in nucleation and crystallization.
Other molecules and atoms surrounding the nucleus branch off from the established symmetry, contributing to the seed crystal. This operation might occur quickly or slowly depending on the circumstances. Water may freeze in seconds, while rock crystals like quartz and diamonds take millennia to form. The basic configuration set up around the nucleus determines the complete crystal structure. Variations in crystal formation account for differences in crystals ranging from the uniqueness of a snowflake to the purity of a diamond.
Crystals can only accept a limited number of geometric shapes specified by the molecules’ bonds and interactions. Various shapes result from different bond angles of atoms based on the original nucleus. The pattern will diverge from the predicted one due to impurities in the solution or substance. As observed in snowflakes, even tiny defects in the nucleus can result in completely separate and unique shapes.
Types of Crystallization
The manner in which supersaturation is produced distinguishes crystallization processes/techniques. The following are the most common types of crystallization:
- Evaporative crystallization
- Crystallization from solution or melt cooling
- Precipitation or reactive crystallization
The method chosen is determined by the qualities of the substance to be crystallized, the feed, and the system’s thermodynamics.
The evaporation of the solvent causes crystallization in evaporative crystallization. As a result, this technique produces vapor and a crystal suspension in mother liquor. The extra heat of evaporation is, in theory, captured in the vapor stream. The equilibrium concentration of product will still be present in the mother liquor. This remaining amount of product can be harvested by recycling the mother liquor to the feed. The contaminants will limit the ability to recycle the mother liquor. At some point, the concentration of contaminants will get so high that it will affect crystallization and/or product purity. If this is the case, the mother liquor stream cannot be recycled any longer and must be expelled via a bleed or purge stream.
Cooling crystallization is appealing when the product’s solubility grows dramatically with rising temperature. Cooling crystallization is usually more energy efficient than evaporative crystallization in these instances. In a cooling crystallization process, the feed is cooled in a heat exchanger, which can be either inside or outside the crystallizer.
- The crystallizers’ wall can be utilized as an internal heat exchanger, but the heat exchanger can also be integrated into the crystallizer as cooled tubes or plates. When a liquid is chilled to a temperature below its equilibrium solubility, crystallization can occur.
- The surface of the heat exchanger has the lowest temperature in the system. As a result, cooling must be done carefully to avoid nucleation on the cold surface of the heat exchanger, which will result in encrustation.
- Typically, the temperature difference between the coolant and the crystalizing solution is reduced, the liquid velocity along the surface of the heat exchanger is increased to level out the temperature difference over the length of the heat exchanger, or a scraper is used to keep the surface of the heat exchanger free of solids.
- Alternative cooling methods that do not require a heat exchanger include flash cooling, which includes (partial) evaporation of the solvent, and direct cooling, which involves the insertion of a cooling element.
Melt crystallization can be thought of as a subset of cooling crystallization. The absence of solvents in cooling crystallization from solution means that most melt crystallization techniques operate near to the melting point of the pure product. An impure melt is used as the input for a melt crystallization process. Cooling this melt below the equilibrium temperature results in the creation of a solid phase that is purer than the feed, while the impurities prefer to remain in the impure mother liquid.
The merging of two streams causes supersaturation in precipitation. Precipitation occurs in three ways:
- reactive crystallization,
- pH shift crystallization, and
- anti-solvent (or extractive) crystallization.
Carbonate and phosphate are two well-known examples of ions that alter state with pH. Typically, an interaction between an acid and a base occurs during pH shift crystallization. This type of reaction is often highly quick and can be quite exothermic, particularly when the reactants are present in quite high concentrations. The anti-solvent is commonly combined with a (concentrated) solution in an anti-solvent crystallization. The anti-solvent is normally well-mixable with the solvent, the crystalizing product has a low(er) solubility in the anti-solvent, and the anti-solvent should be recoverable for economic and environmental reasons.
- Precipitation can come down quickly or slowly. When one of the underlying stages in crystallization, such as nucleation, is fast, the precipitation is called fast. A quick conversion stage does not always indicate a short residence duration in all precipitation processes. Longer(er) residence periods are necessary, for example, when a coarser product or a different polymorph is required than in the main conversion/precipitation process.
- Ripening or ageing of the suspension can increase the size of a precipitate. Ripening often occurs at low supersaturation, which prolongs residence time. Precipitation is also recognized for the formation of metastable intermediates near the inlets of the feed streams. These intermediates can be amorphous or (pseudo)polymorphs of the final, thermodynamically stable product.
- The shift from the metastable phase to the thermodynamically stable product necessitates a liquid-mediated recrystallization process that occurs at a low supersaturation and so requires relatively long residence durations. One advantage of such slow re-crystallization is that it is usually followed by a significant increase in product purity.
Fractional crystallization is a word used to describe a process in which multiple crystallization stages are performed to increase the purity of the product and/or the process yield. Metal (metal refining/zone melting), oil & gas (e.g. oil dewaxing), food (e.g. palm oil fractionation and freeze concentration), and chemistry/chemical industries (e.g. paraffin wax de-oiling or ultra-purification of chemicals) are all examples of applications.
- Crystals generated in the first stage of a fractional crystallization process are removed from the mother liquid using devices such as filters, centrifuges, or wash columns and remolten to be utilized as feed in a second crystallization stage.
- Alternatively, the crystals are resuspended in a liquid with a greater product concentration at a higher working temperature rather than remolten. If necessary/beneficial, the crystals can be allowed to develop after being re-suspended. The choice between re-melting and re-suspending is determined by the crystal quality.
- When the crystals already have the desired internal purity, shape, and size, re-suspension is the preferable method. If one or more of these parameters deviates significantly from the end product specifications, re-melting/re-crystallization is usually the best solution. Due to the improved purity of the feed in the second stage, this second crystallization stage will function at a higher temperature than the first stage.
- If there is still a sufficient amount of product in the mother liquor, it can be used as feed for an additional crystallization step. Because the mother liquor contains a lower concentration of product than the feed of the first crystallization stage, this stage will be operated at a lower temperature than the first crystallization stage.
- The solids and liquids in the outlined process move in opposite directions: the crystals rise in temperature while the mother liquor falls. As a result, this tiered crystallization process is often referred to as a counter-current cascade.
In theory, fractional crystallization can relate to both crystallization from solution and crystallization from the melt. In practice, it is more commonly associated with melt crystallization, which explains why the phrase fractional crystallization is sometimes used interchangeably with melt crystallization. Because of their low melting temperatures, melt crystallization is particularly appealing for the separation and ultra-purification of organic compounds.
Fractional/melt crystallization is a separation technology that can be an appealing alternative to distillation for organic mixture separation. Because melt crystallization does not require the use of solvents, process streams may be kept minimal and no procedures or equipment are required for solvent recovery and recycling.
Steps in Crystallization
While the process used to crystallize a product might vary depending on a number of conditions, there are six common steps for crystallization to take place. Solubility curves are used by scientists to construct a framework for developing the required crystallization process. The solubility curves plot temperature vs. solubility to determine the crystallization process parameters.
Step 1: Select the Proper Solvent
Because crystallization is often accomplished by lowering the product’s solubility in a saturated starting solution, an adequate solvent is critical. Some factors to consider while selecting a solvent are:
- Safety and environmental impact
- Fees for disposal
Step 2: Dissolve the product in the solvent by gradually increasing the temperature until all of the product solids have disappeared.
Aside from the solvent, temperature is a crucial element in determining whether or not crystallization will occur. There is a temperature at which the greatest amount of product can be dissolved in a solvent. When this temperature is attained, the solution is saturated, and contaminants in the hot solution can be filtered out.
Step 3: Reduce Solubility by Cooling, Antisolvent Addition, Evaporation, or Reaction (Precipitation) Crystallization Methods
In general, crystallization happens by lowering the solubility of a solute in a solution through one or more of these four processes. When the solubility of a solution decreases, it becomes supersaturated. Crystal nucleation and growth are propelled by supersaturation. It is an important crystallization step since it determines crystal product characteristics like size distribution and phase. The method of crystallization chosen is determined by the crystallization equipment available, the crystallization objectives, and the solute’s solubility and stability in the chosen solvent.
Step 4: Crystallize the Product
As solubility decreases, a point is reached at which crystals nucleate and develop. The metastable limit is the point at which crystal nucleation occurs. The discrepancy between the actual concentration and the solubility concentration at a particular temperature is referred to as supersaturation. When the product crystallizes, very pure crystals develop while impurities remain in solution. Controlling supersaturation, improving batch consistency, and optimizing the product created can all be accomplished by introducing a seeding strategy into the process.
Step 5: Allow the System to Reach Equilibrium in Order to Maximize Solid Product Yield
To achieve a high product yield, one or more crystallization processes (cooling, antisolvent, evaporative, or reactive crystallization) are used. Controlling the degree of supersaturation and understanding the particle mechanism crystals go through are required to create effective crystallization methods.
Step 6: Purify the product and dry it.
The solid, refined particles are the desired product in the majority of crystallization operations. Filtration is required to remove the crystals from the mother liquor. The following are the prerequisites for an efficient filtration procedure in order to obtain the product.
Filtration supplies such as filter paper, a porous plate, and a clean flask are required.
Finally, the pure crystal product is dried either by air or by vacuum. The method employed will be determined by parameters such as the solvent type and the API’s thermal and mechanical stability.
Application of Crystallization
Almost all processes in the production of small-molecule medicines rely on crystallization. It is critical in both manufacturing and medicinal development. Because the qualities of a solid material (polymorphism) can have a significant impact on the process or product’s compliance and effect (dissolution rate, for example), monitoring and managing the separation of solids for various applications via crystallization is critical.
Crystallization of Alum
The crystallization method is used to separate alum crystals from a contaminated sample. Alum can be found in a variety of minerals, including potash. Potash, also known as potassium sulphate, is found in minerals such as kalinite, alunite, and leucite.
When honey is stored over time, the sugar molecules tend to form sugar crystals due to the crystallization process. By storing honey in a cold environment, the rate of formation of sugar crystals can be accelerated.
Purification of Silicon
Sand contains a lot of silicon, which is abundant on the earth’s surface. It is utilized in a number of applications, including the manufacture of solar cells, electronic components, alloys, and so on. The extraction of silicon from sand begins by heating silica in an arc furnace at 1800 degrees Celsius.
The resulting silicon contains a number of impurities and is known as metallurgical grade silicon. The metallurgical grade silicon is ground into a fine powder and treated with hydrogen chloride to extract pure silicon. The mixture is heated further until it is completely evaporated. The procedure is carried out in a vacuum atmosphere. Electronic-grade silicon crystals are subsequently deposited on the electrically heated polysilicon rods. The electronic grade silicon crystals placed on the rod with a purity of more than 99.9% are then violently extracted.
One of the best illustrations of the crystallization process in action is salt harvesting. Initially, the water in seas and salty lakes evaporates due to sun radiation. The salt that is left behind when water is transformed into water vapours is impure and has tiny crystals. It is cleansed and refined further to ensure safe consumption and purification.
The method of desalinating water is similar to that of extracting salt from saline water. The polluted water sample is heated to a high temperature, causing the water to evaporate, leaving behind salt and other contaminants. The water vapours created as a result of evaporation are then treated to a condensation process, yielding clean water. Crystallization can be seen in the process of removing contaminants from clean water.
Photographic film was used to capture photos before digital cameras were introduced. The process of recording images on photographic film necessitated the execution of several chemical processes. The photographic film is made up of several layers, including a scratch-resistant layer, an emulsion layer, an adhesive layer, an antihalation layer, and a film base. The scratch-resistant coating shields the film from damage. The adhesive layer is the firm layer of flexible plastic that provides support to the other layers and binds them to the top of the film foundation. The emulsion layer is a light-sensitive layer composed of gelatin and silver halide grains.
These silver crystals are formed by the crystallization process and help to trap light energy, which is ultimately responsible for picture capture.
There are three types of allotropic carbon: amorphous carbon, graphite, and diamond. The amount of pressure and the pace of metamorphism are the primary differences between the three fundamental forms of carbon. The variation in pressure difference causes carbon to crystallize, which improves its structural strength.
Crystallization is commonly used to purify components such as iodine or sulfur. The iodine is placed in a crucible and covered with a funnel for this purpose. A cotton ball is used to obstruct the end of the funnel. After that, the iodine is heated by placing the apparatus on a lit bunsen burner. Sublimation of the iodine results in the production of purple vapors. The vapors cannot exit the setup and become trapped in the flask. These vapors are then transformed to solid state, allowing the crystals created to adhere to the interior of the curved part of the funnel via the deposition process. The crystals generated throughout the process are completely pure.
The majority of sweeteners are derived from syrups using the crystallization technique. To obtain table sugar or sucrose, for example, sugar cane is boiled at a very high temperature. The majority of the water content in the sugar cane juice evaporates, leaving behind a thick syrup. The syrup is then cooked at a relatively low temperature in a partial vacuum atmosphere. Some of the seedings are artificially added to the syrup and are responsible for hastening the process of sugar crystal formation. These crystals are subsequently separated using a centrifugal machine.
- Abu Bakar MR, Nagy ZK, Saleemi AN, Rielly CD. The impact of direct nucleation control on crystal size distribution in pharmaceutical crystallization processes. Crystal Growth and Design. 2009;9(3):1378-1384
- Tavare, N. S. (1995). Industrial Crystallization. Plenum Press, New York