With spiral jet mill technology, the size reduction is achieved by collisions between particles of the substance, previously accelerated by high-speed jet streams. Target PSD is generally below 30μm when adopting this technology.
Jet milling does not use mechanical components, but instead uses pressurized gas to create high particle velocity and high-energy impact between particles. After exiting the jet-mill chamber, the process gas is separated from the solid particles with a cyclone filter. Compared with mechanical milling, jet milling reduces metal contamination and, because process temperature is relatively constant, can be used for heat-sensitive products.
Jet milling, however, has relatively low productivity, large equipment size, and high process-gas flow requirements, therefore requiring high process knowledge to optimize its use. Micronization using spiral jet milling has been a poorly understood “black box” process, but process understanding is growing as pharmaceutical companies work together with equipment manufacturers. Parameters include equipment geometry, temperature, and powder characteristics (e.g., humidity, density, impurities, and incoming particle size distribution) as well as the variables of pressure and solid-feed rate (i.e., specific energy). Improved understanding can lead to improvements in productivity, particle size distribution, and gas consumption, for example. Temperature control for cryogenic applications is also being investigated and adopted.
The desired particle size depends on the solid-dosage requirements with typical API particle sizes in the 2–5 micrometer range. The minimum particle size that can be obtained using a jet mill depends on the inner friability of the material. Many APIs are synthesized or precipitated at the nanometer-scale but are agglomerated, and a jet mill can also be used to deagglomerate these APIs down to their original nanometer base-particle size.
Because of the prevalence of potent compounds, micronization is increasingly being performed under isolation. A mill can be enclosed inside a barrier isolator for processing highly potent drugs. In addition, jet mills can be operated in sterile conditions by integrating the mill in an isolator or a restricted access barrier system (RABS) as needed.
Some products are not considered potent, yet still require containment because of environmental, health, and safety (EHS) concerns as a result of the small particles created after the micronization process.
FPS has custom-designed containment systems that provide the ability to jet mill high volumes of product under containment.
High degree of material fineness
Internal classifier and good uniformity of size distribution
Low maintenance due to absence of moving parts
Rapid disassembly and easy cleaning
Cryogenic application: possibility of working at low temperatures
Good productivity due to constant and continuous processing