Jet milling is the term commonly applied to fluid energy milling. It is a process utilizing the potential energy of a compressible fluid and converting it to kinetic energy within the mill grinding chamber. This occurs when compressed gas is injected through specially designed nozzles. As the gas exits the nozzle it rapidly expands and creates a high-velocity stream within the mill. Raw feed material enters the same chamber through a venturi eductor. As the feed becomes entrained in this high-velocity stream the particles collide. It is this inter-particle collision that is the basis for the comminution of the material.
Size classification is one of the important unit operations used in industries and/or applications involved with processing differently sized solid particles. It is a solid-solid separation process separating particles into two or more fractions. It is commonly used in combination with grinding and comminution of solid materials.
For dry particles, classification is predominately performed on screens or sieves and air classifiers. Screens and sieves are predominately used on particles larger than 200 mesh (70 microns) and very seldom on particles smaller than 325 mesh (44 microns). Air classifiers are used predominately on fine powders but can be used on coarse particles up to 1000 microns in diameter.
Air classifiers employ various airflow techniques including elutriation, free vortex, and forced vortex, either separately or in combination, to achieve the desired classification. Elutriation is a gravity-based particle classification that rinses coarse particles in a flowing stream of air or other gas. It is a relatively crude method that separates the feed’s fine and coarse particles by introducing the feed into the flowing air. The airflow carries the fines to a fines discharge outlet, while the heavier coarse particles fall with gravity against the airflow to a coarse outlet. Increasing or decreasing the airflow adjusts the cut size.
The flow in a free vortex classifier moves in a decaying spiral pattern toward a discharge outlet point, as found in a cyclone. A tangential inlet for the feed and air establishes a circular airflow that forces coarse particles to the chamber’s periphery where they flow to a coarse outlet. The fines remain with the airflow and are carried to a central outlet where they are discharged with the air. The cut size is controlled by adjusting the airflow and physical dimensions of the cyclone.
A forced vortex classifier is more complex and achieves a more precise classification than either the elutriation or free vortex units. In this unit, the airflow is pulled from the chamber’s outer circumference and a classifying wheel sets the airflow, with entrained feed particles, into a rotational motion (forced vortex) around the wheel. The centrifugal force of the wheel pushes the coarse particles out of the open vanes, but the fines pass thought the vanes and are carried to the outlet with the air discharge. The cut size is controlled by varying the wheel speed and airflow through the classifier. These two variables can be adjusted independently and thus allows an infinitely adjustable classification.
A jet or fluid energy mill can have a static (MICRO-JET, JET-O-MIZER) or dynamic (ROTO-JET) classifier.
Static classification occurs as the gas and product recirculate within the mill. This recirculation creates a centrifugal force that directs the larger particles to the outer periphery and away from the mill exhaust. The finest particles, less influenced by this centrifugal force, are too small to overcome axial forces created by the exhaust and are carried out of the mill with the spent gas. It is this process that creates a stratification of particles throughout the milling chamber. In a jet mill with static classification, the air volume is particularly important. Each model has a designed nominal air volume at which the mill is most efficient in classification. Too much or too little airflow will have an adverse effect on classification.
Dynamic classification is provided by a variable speed classifying rotor. The speed of the rotor prevents specific size particles from passing through the rotor and rejecting these particles back to the fluidized zone for additional grinding.
In a jet mill with static classification (MICRO-JET, JET-O-MIZER), the particle size is controlled by a combination of gas pressure and feed rate. With a constant feed rate of granular solids, increased inlet gas pressure results in higher velocities and greater kinetic energy, therefore a finer grind. Lower inlet gas pressures result in less energy and larger particle size. With a constant gas inlet pressure, faster feed rates result in higher mill loading, which reduces residence time, therefore producing a larger particle size. As you lower the feed rate the particle size decreases.
By varying these two parameters you may produce the desired particle size. In a jet mill with dynamic classification (ROTO-JET), particle size is controlled by a combination of inlet gas pressure and classifier rotor speed. Higher inlet gas pressures in combination with the highest classifier speeds result in the finest average particle size and the finest top size. Lower inlet gas pressures in combination with lower classifier speeds result in larger average particle size and a larger top size. The variable speed classifier allows for a wider range of adjustments to tailor the particle distributions to your exact specifications.
Jet milling is typically used to grind a dry powder to a range of 0.5 to 45 microns. Customers generally require smaller particle sizes to increase the surface area of their product. Maximizing surface area provides some of the following benefits:
For particle sizes above 45 microns, a mechanical mill is more energy-efficient and should be used except when heat and contamination are of concern.
A raw feed to a jet mill is usually, but not necessarily, the product of a mechanical mill such as a hammer, air classifying (ACM), or roller mill. Raw feed sizes vary from granular or flake to the low micron range. When selecting a desired feed to a jet mill, remember a finer raw feed will yield a finer product or a faster rate. Similarly, the narrower the distribution of the raw feed, the better the distribution of the jet-milled product.
Raw feed sizes also vary with the actual size mill being used. For example, a Model 00 JET-O-MIZER has a venturi throat size of 1/8″ and therefore all raw feed should be in the range of table sugar or smaller. While the Model 0808 JET-O-MIZER has a venturi throat size of 1-1/2″ and can accept a larger feed size, it is still recommended that the feedstock be in powdered form in order to obtain a similar fineness in the final powder.
A pancake mill is a horizontally designed jet mill, while a loop mill is a vertically oriented jet mill. Both are equipped with static classification. The basic differences are highlighted below:
As indicated above, a pancake mill and loop mill are both jet mills that employ static classification. These mills are also similar in that they have a tangential grinding nozzle pattern. Here are the basic differences of the fluid bed design:
IMPROVED CLASSIFICATION/ SPECIFIC TOP SIZE
If you cannot achieve the required distribution with a pancake mill or loop mill, then a fluid bed design (e.g. ROTO-JET) should be used. With its advanced, dynamic classification it will produce a narrower distribution with the finest top size. Also, any material that has a high aspect ratio (flakes or needles) and is difficult to classify would be better suited for a fluidized bed jet mill.
An example of such an application is toner powder. Toner manufacturers typically require a very specific top size (15-20 microns) while generating minimal fines (less than 4 microns).
Since a fluid bed jet mill has opposed grinding nozzles, “wall effects” that occur in the pancake mill and loop mill are eliminated. The grinding occurs in the center of the milling chamber, preventing direct impingement with mill surfaces.
HARD TO GRIND MATERIALS
Any material that is difficult to fracture, such as polymers, alloys, or some ceramics, typically requires more residence time in the grinding stream than a pancake mill or a loop mill can provide. In these cases, the fluid bed jet mill excels because the classifier wheel prevents the particles from exiting prematurely and rejects them back into the grinding zone.
VARYING OR POOR QUALITY RAW FEED
In a pancake mill and loop mill the raw feed distribution is critical, because the particle size is controlled by a combination of feed rate and gas pressure. In these static mills, if any aspect of the feed distribution changes (mean or top size) then the final product will be affected unless the controlling parameters are changed. A fluid bed jet mill, due to the classifying rotor, will consistently produce in-spec product regardless of fluctuations in the raw feed particle sizes.
Typically, jet milling systems are designed for continuous operation. However, pancake and loop lab units can be operated in a batch mode.
CONTINUOUS OPERATING SYSTEM
BATCH OPERATING SYSTEM
A cyclone (e.g. JET-O-CLONE) is a static solids collection device that can provide primary solids collection. It does not eliminate the baghouse requirement. Fluid Energy cyclones are the most efficient in the industry with capture efficiencies as high as 95%-98%. Listed below are applications where a cyclone is typically utilized.
STERILE PHARMACEUTICAL APPLICATIONS
A JET-O-CLONE cyclone is used in sterile pharmaceutical applications where the customer prefers that the product not contact the filter media due to the potential of fibers contaminating the product. In these cases, the cyclone is a sanitary collection point that can be steam sterilized or placed in an autoclave.
PROCESSING A VARIETY OF MATERIALS IN ONE SYSTEM
A JET-O-CLONE cyclone is a quick-disconnect (where possible), easy-to-clean component as compared to a baghouse, which can be very labor-intensive to disassemble and clean. When a customer is processing many materials in the same system the cyclone is used as the primary collection point. Any cyclone exhaust that is conveyed to the baghouse is considered waste. During changeovers, a customer experiences much less downtime by cleaning the cyclone and not the baghouse.
In applications involving mill exhaust temperatures above 300° F, an EVAPORATIVE COOLER is used to cool down the process stream prior to entering the baghouse. Because the EVAPORATIVE COOLER atomizes water into the exhaust it is recommended that a JET-O-CLONE cyclone be used to remove the solids from the process stream prior to the cooler.
ALLEVIATING/PREVENTING BAGHOUSE PROBLEMS
In applications where powders tend to “cake” and “blind” filter bags, causing excessively high-pressure drop across the baghouse, a JET-O-CLONE cyclone is used to reduce the solids loading within the baghouse. In these applications, the cyclone will alleviate the problem or prevent the need for excessively high filter cloth area in the baghouse
All of our equipment is available in carbon steel; type 304 or 316 series stainless; or exotic alloys except for the following mills, which are stocked in type 316 stainless steel:
A complete set of liners for the MICRO-JET typically includes a top liner, bottom liner, manifold liner ring, outlet extension, and venturi. On some of the production scale, MICRO-JETS the bottom liner and manifold liner ring are an integral piece. In abrasive applications, these liners do not wear at the same rate. The bottom liner is replaced most frequently, followed by the manifold liner ring and venturi.
The following is a list of liner materials commonly used:
Note: Fluid Energy supplies all liners as solid components constructed in the materials listed above. We do not epoxy tiles to the base plate unless requested by the customer. Tiles are not as durable as a solid liner when exposed to the harsh environment inside the milling chamber.
Only the Model 0405 and 0808 JET-O-MIZERS are available in replaceable liner design. These liners can be cast in NiHard, carbon steel with hard facing, or stainless steel with hard facing. All the JET-O-MIZERS can be provided with a spray or weld applied hard facing on the internal surfaces subject to wear. Hard facing can be carbon-based or stainless based on applications where corrosion is of concern.
As discussed previously in this section, due to the opposed orientation of the grinding nozzle, the ROTO-JET mill does not experience the “wall effects” typical in the MICRO-JET or JET-O-MIZER mills. It therefore does not require liners on the internal chamber, although in a high purity application where no amount of metal contamination is tolerable a polymeric coating may be applied.
In the ROTO-JET design, it is the classifying rotor that experiences some wear, although minimal compared to the MICRO-JET and JET-O-MIZER. In these applications the classifying rotor would be constructed in the following materials:
Centrifugal, reciprocating, and rotary screw compressors have all been used in jet milling operations, with the latter being the most common. It is recommended that the compressor deliver a minimum of 100 psig to a maximum of 140 psig to the mill. The higher the grinding pressure, the finer the grind.
The quality of the air is at the discretion of the customer. Oil-free or food-grade lubricated compressors may be necessary for pharmaceutical or specialty chemical or pigment application. Most installations have a compressor package, which consists of a compressor with an aftercooler, refrigerated or desiccant air drier, coalescing filter and a high-pressure receiver. With a refrigerated dryer, a typical dewpoint should be in the 35°-39° F range. Air dryers are not essential but are recommended for preventing condensation and eventual product build-up within the system.
Compressed air is the most common source of energy. In explosive or pyrophoric applications, nitrogen or argon may be utilized. Heated air or superheated steam may be used were tolerated by the product. Higher enthalpy of superheated steam results in higher nozzle velocities and a finer grind or improved rates.
If a jet milling application requires an inert atmosphere, the system gas can be designed for a once-through or a continuous closed-loop recycling operation. The question of which type of design should be used will depend primarily on usage. In a recycling system, a HEPA filter is required before reintroducing the gas back into the compressor. A sealed exhaust blower and low-pressure receiver are also required for this operation. Automated controls with oxygen and pressure sensors can also be provided.
The following methods have been used either separately or in conjunction to handle explosive materials:
Because of the low velocities and fine particle concentrations, the baghouse is the component most susceptible to dust explosions. A relief vent would be sized based on the Kst value of the material and included on the baghouse. Venting is the most common method employed for handling explosive powders. Explosion vents are ducted to a safe area outside the building. It is recommended that the baghouse be positioned as close to an exterior wall as possible. Ducting should be a straight run and be as short as possible.
As discussed above, the system can be operated with an inert atmosphere, eliminating the possibility of an explosion.
Equipment can be provided to sense a pressure rise within the baghouse and release a chemical reagent into the baghouse, suppressing the explosion.
When the above options are not feasible, a baghouse may be designed for explosion containment. For several pharmaceutical applications, we have supplied baghouses that have been designed for 10 bar containment. Since the entire system cannot be designed for this pressure, explosion isolation valves are included at the inlet and exhaust connections of the baghouse. As with the suppression equipment, a sensor is placed in the system to detect a pressure rise. Once detected, the valves are automatically shut, isolating the explosion.