Author Archives: admin

Loop Flash Dryer System

Loop Dryer Process Layout

Fluid Energy loop flash dryers are used to dry and calcine granular materials.  The overall drying system requires additional equipment beyond the dryer unit.  In the picture at the right, is a dryer system using a Fluid Energy ThermaJet.  To transport the granular material into the dryer and remove the moisture, additional feeding, separation, and storage equipment is necessary in addition to the dryer unit.

In this article we will review the main components of this type of drying system.  In considering whether to purchase a loop dryer system, it is a good to know the basic process requirements.  Below is a schematic of a ThermaJet process showing the main pieces of equipment.

Main Supporting Equipment for Drying System:

  1. Air Blower. Used to blow air into the ThermaJet dryer for drying.
  2. Air Heater. Used to heat air between 200F and 1300F before entering loop dryer to effectively dry the material.
  3. Material Feeder. Hopper with screw feeder meters material into ThermaJet for drying. Feeder has controls to meter material at a consistent, fixed rate. The material is fed through a venturi funnel into the dryer.
  4. Receiver Bin/Rotary Airlock Valve. A cyclone separator (e.g. Jet-O-Clone Collector) is used to remove the majority of product from the gas stream exiting the dryer. This is the main product stream of fine powder from the mill. Product is collected in a Receiver Bin and fed via gravity from Receiver Bin through rotary valve into packaging container.
    Reverse Pulse Collector/Rotary Airlock. Dust collector contains cartridges/filters to filter out remaining finer powder in gas stream. This is the secondary stream of product from the dryer. When pulsing cartridges to clean them, finer material falls to bottom of collector and is gravity fed through rotary valve to packaging. Other equipment has been used instead of or in addition to a pulse collector: Wet scrubbers, electrostatic precipitators, HEPA filters. Some of the factors which influence the selection of the product collection equipment may be the number of products handled in a system, frequency of product changes, desire to segregate fine material from finished product, explosive nature of the product, toxic nature of the product.
  5. Exhaust fan. Ensures proper air flow from the pulse collector.

Work Process

Below is a work process diagram outlining the basic steps in a drying process, and how the equipment is used to produce a dried product.  There are two main process flows into the loop dryer, granular solids to be dried and heated air.  The blended air and solid streams are then separated into the exhaust air and dried powder product.  The steps shown below may have additional sub steps depending on the applications and specifications of the final product.

Quality Control & Analytical Equipment

During the grinding process sampling of the material is necessary to ensure the material is dried to the target specification. Processing personnel are supported by a quality control laboratory to verify material is in spec. The key measurement in the process is the final moisture level of the product. In addition, particle size, pH, and color of the final product may be measured:

  • Moisture: Moisture level of initial and final powder is measured in oven or dedicated moisture analyzer.
  • Particle Size and Size Distribution:
    – Laser diffraction measurement of particle size and size distribution
    – Grind gage to test for oversize particles within the powder
    – Sieve screens for coarse measurement of particle size distribution
    – pH: Slurried powder is checked for changes in pH.
    – Color: Spectrophotometer used to measure shifts in color versus unmilled standard.

Additional measurements may be made in addition to those listed above depending on the requirements of the final material.

There are numerous variations on the basic system outlined here. If you are looking to dry ground powders and have questions on what equipment is required for your application, please contact Fluid Energy Processing and Equipment at this link with any questions.

References:
1. R.H. Perry and D.W. Green, Chemical Engineering Handbook, 8th Ed., 2007, p. 21-61 to 21-62.

New Application Methodology

Fluid Energy manufactures three (3) complete lines of jet mills (Jet-O-Mizer, MicroJet, RotoJet) and a line of flash driers (ThermaJet). This equipment is used in the production of fine powders in industries ranging from food/pharmaceuticals to specialty chemicals, plastic fillers, additives and even propellants.

Such an expansive range of applications requires flexible testing protocols to establish feasibility through optimization. Whether dealing with a current customer looking to improve an existing material or a new customer with an unknown material, the Fluid Energy Test Center, in coordination with our technical department, work to efficiently and effectively determine the optimal milling and/or drying parameters.


Customers present their applications in several ways. Requests can be as simple as “grind as fine as possible” or “bone dry” to specifying a very select particle size distributions, rates and/or moisture levels.

Our initial step involves reviewing the request and comparing it our previous experiences. In addition to scanning our databases for previous history, Fluid Energy engineers and technicians apply their 75+ years of combined experience in the jet milling and flash drying field towards a solution.

The history allows us to use a proven method to quickly assess products that have been tested previously, while the practical experience allows us to “think outside the box” when needed.

The foundation of our new application methodology is our Test Center. Located in Telford, PA, the Fluid Energy Test Center operates lab and pilot size Jet-O-Mizer, MicroJets, and RotoJets. We also have lab and pilot size ThermaJet systems for flash drying trials. In addition to the specific Fluid Energy equipment, the Test Center is stocked with support equipment used to pre-condition feeds that are too large or too wet for our equipment, multiple collection devices, multiple heating sources and even inert gases capabilities for the safe processing of volatile materials.

Some trials last ½ day, while others run the better part of week. We often start our testing one mill or system but end up sliding over to an alternate piece of equipment. There is a lot of trial and error with jet milling and flash drying testing. Our experience and data base streamline that approach. Here are a few applications that demonstrate the problems and eventual solutions developed in our Test Center:

Suspension Drying Application

The Fluid Energy sales department was contacted by a customer that had a wet suspension with the consistency of pudding and needed a dry powder less than 3% moisture. Mechanical dewatering could only get the material to the “pudding -like” phase and it was heat sensitive. After reviewing the test data sheet supplied by the customer, we felt we could get enough heat into the product, even with the temperature limitations, but the feed was not in a form that could be metered consistently into a ThermaJet flash dryer.

Pre-conditioning the feed was our top priority. Back mixing is a method we use to mix previous dried material in with wet feed to make a “feed-able” consistency. Since we only had drums of wet material, the Test Center devised a method of manually air drying a portion of the wet feed, then de-lumping it so it could be used as seed material for the back mixer. Once enough dry back was created, the wet feed was metered into the back mixer using a positive displacement pump and mixed with the seed material generating the necessary consistency for entry into our pilot size ThermaJet.
With the back-mix loop functional, the operators performed a series of trials eventually dialing in the optimum conditions. The data and test method were incorporated into the design of a production system.

Moisture Sensitive Micronizing

With the reduction in particle size created by fluid energy jet mills, the physical characteristics of the material can change. Some materials change color, some become explosive, while many experience a dramatic shift in bulk density. We must be aware of the potential for these types of changes and design material testing to accommodate the property changes safely.
An application that required a testing redesign occurred when we were asked to micronize an extremely moisture sensitive material. The feed samples arrived in granular form, which was not sensitive to ambient moisture conditions. From previous experience we anticipated moisture pick up was going to be an issue, but we were surprised at the level and speed at which it occurred. Initial testing used compressed air passed through a desiccant air dryer. After a few adjustments, the jet mill was able to meet the particle size specification; however, the sample solidified within minutes of processing.
The decision was made to move from compressed air to compressed nitrogen. Using nitrogen, the milling worked well but the samples failed to stay in a deagglomerated form. Apparently, the stray air entering the mill inlet with the feed was providing enough ambient moisture for the superfine powder to absorb. The solution required us to mill, collect and package the samples under a nitrogen atmosphere. As system was designed in the test center that allow all facets of the process to be conducted under nitrogen and the critical material handling phases occur inside a nitrogen flooded glove box. This arrangement has been used for other application ranging oxygen sensitive to pyrophoric feed stocks.

Multi-Component Milling

When more the one component of a blend needs to be jet milled, we have two choices: (1) mill all components separately then blend or (2) blend first then mill. Fluid Energy was approached by a customer with a proven product but aging process equipment. In fact, the equipment being used to produce their specific blend of micronized materials was being recommissioned for other use and would no longer be available. The old blend process was not a jet milling process, so the customer’s request left the Test Center with an empty canvas to work with.
As with many applications, processing costs were being squeezed, so the goal was to match the current specification and performance with the least number of steps. The operators performed a series of trials on each component to determine individual grind-abilities. Then they combined raw materials with similar grind rates and ran combination milling trials to dial in the minimum steps needed to create the blend. Since the customer had several blend recipes and particle size requirements, multiple trials were required. Each step was documented and reviewed. One by one the Test Center was able to dial in the conditions, creating a replacement process with good economics.

Polymer Drying

The Test Center received a request to process a large batch of wet material that was a base material coated with a polymer. A review of the data sheets showed a slight level of heat sensitivity but not enough to cause concern. The test protocol called for a series of trials where the wet feed rate and temperatures would be adjusted to find the optimum throughput conditions.
After the first run was completed, the samples were analyzed for moisture, color, and consistency. During the visual inspection of the sample, flakes were found in the dried material. The final product specification called for a powder, so the flakes were unacceptable. Fearing melting was the cause, all operating temperatures were lowered but the flakes remained. The system was broken down and inspected.
The flakes were traced to the two rotary valves used to meter material in and out of the system. Some of polymer was being peeled off the base material by the blades of the rotary valve and extruded into a flake as the valves turned. Once the operators recognized the problem; the rotary valves were removed, and the system was balanced using an alternate method. By eliminating the pinch point problem caused by the rotary valves, the flake problem was eliminated allowing the optimization trial to continue. All subsequent systems drying this polymer blend follow the same “pinch less” design.

These are just a few examples of troubleshooting unconventional milling and drying processes. If your application could use this type of optimization, use this link to contact Fluid Energy Processing and Equipment.

1. David Cook, David B. Hastie, Tom Hicks and Peter W. Wypych. A Novel Approach to Rotary Valve Venting. Centre for Bulk Solids and Particulate Technologies, University of Wollongong, Northfields Avenue, Wollongong NSW 2522, Australia

Fluid Energy Processing Divison

Fluid Energy Equipment division specializes in ultra-fine grinding and flash drying of powders. The Processing division of Fluid Energy dries and mills products with the full product line of equipment that Fluid Energy designs and builds. All three types of mills and the loop dryer are available for use at the Hatfield processing facility.

In addition to the equipment designed and built by Fluid Energy, the Hatfield facility has additional capabilities to process granular materials at different scales for milling and drying. These facilities can be used for interim production while new equipment is being assembled, or the processing division can process materials on an ongoing basis as a contract manufacturing or toll arrangement.

In addition to processing in Fluid Energy designed mills and dryers, there are several complimentary powder processing operations at the Hatfield facility. These include the following:

1. Screening and Particle size classification
2. Coarse Grinding
3. Blending/Coating
4. Repackaging
5. Food Grade Processing

Screening and particle size classification can be used to separate out material that does not fall within the necessary particle size specification. Coarse grinding is typically more energy efficient than fine jet-milling. Materials for fine grinding can be “pre-ground” in a coarse grinder to reduce the amount of fine grinding required.

Blending/Coating can combine two or more materials with different properties to create the final product. Materials can be repackaged in smaller or larger containers as required by customers. Separate areas are maintained for processing of Food Grade materials to minimize contamination with technical grade materials.

Characterization of the initial and final powders is done at the Hatfield facility to confirm the desired processing is realized.

Screening and Particle size classification. Vibratory screen with ultrasonic capability is available to create cut sizing of product. Coarse material and/or fine material above or below a specified size can be extracted from the bulk material. A Roto-sizer is also available for this type of classification. This device can separate material down to sizes of 3-microns.

 

Coarse Grinding. Coarse grinding equipment is available at the Hatfield facility. This equipment can be used as the only step in communition of the granular material. Or, if sizes below 45 microns are required, this can be an initial step before finer grinding in one of the jet mills. Pin mill and hammer mill equipment are available.

Blending/Coating. Ribbon and paddle mixers are available for blending of different powders. These have also been used to blend a liquid product on top of a powder to create a coated material. Similarly, a coated product has been made by blending two powders where one of the powders is a lower melting temperature material. Using hot air to melt the lower melting point material creates a coating on the second. Jet mills run at low pressure can also be used for more intense blending with minimal grinding of the particles.

Repackaging. If material needs to be repackaged from small containers or bags to larger ones, this can be handled at the Hatfield facility. Also, if the material needs to be moved from larger containers to smaller ones, the reverse process can be done.

Food Grade Milling. Fluid Energy has a food grade area within the site that meets the FDA 21CFR 117 regulations. This area can be used for milling and blending of food grade materials. The equipment is in a separate building away from the technical grade processing. This ensures no contamination of the food grade products.

Quality Control Laboratory. The Hatfield site can characterize the powder before and after processing in various ways. Typically, communition processing requires measurement of particle size changes in the granular material. The quality control lab can measure various aspects of particle size along with additional powder characteristics which include the following:

  • Particle Size and Size Distribution
  • Laser diffraction measurement of particle size and size distribution
  • Grind gage to test for oversize particles within a powder sample
  • Sieve screens for coarse measurement of particle size distribution
  • pH: Slurried powder can be checked for shifts in pH.
  • Moisture: Moisture level of final powder is measured in oven or dedicated moisture analyzer
  • Color: Spectrophotometer used to measure shifts in color against unmilled standard

In addition, other specialized testing (e.g. Gas Chromatograph) have been developed to meet customer’s needs as required.

Contact us here to discuss powder processing needs where Fluid Energy Processing can help.

Spiral Jet, Pancake Mill Case Studies

Fluid Energy Mill Overview

Jet milling or fluid-energy grinding via a spiral jet mill is a common method of comminution or pulverizing granular material into a fine powder with a narrow size distribution.

The spiral jet mill, also called a pancake mill, is shown in the drawing to the left. This design was first described by NH Andrews in 1936 (US2032827)1. Several nozzles are placed in the outer perimeter of the mill through which the grinding medium, gas or steam, enters.

The solid particles are introduced into the grinding chamber through a venturi feed injector. The outlet is placed in the center of the mill chamber. Grinding and static classification both occur within the cylindrical chamber. The vortex formed by the jets causes coarse particles within the mill to be transferred to the outer zone, as fines exit through the central outlet.

The Micro-Jet pancake mill is a horizontally oriented jet mill for most applications but the orientation can be adjusted for space and processing considerations. Spiral jet mills, like the Fluid Energy Micro-Jet, are notable for their robust design and compactness. Their direct air operation avoids the need for separate drive units. Capable of grinding dry powders to 0.5-45-micron averages, this mill generates a maximum number of “fines” resulting in high surface areas in the final product.2 As with all Fluid Energy jet mills, the grinding process occurs with no heat buildup when using ambient gases.

A few case studies of the Micro-Jet spiral jet grinder are described below.

Pigment and Abrasives Grinding

The Model 42 Micro-Jet was made to grind a chemical intermediate to a 6-micron average, at a rate of 2 metric tons an hour.

The initial raw feed size was 325 mesh (~45 microns) material. This model also features replaceable silicon carbide liners for all product contact surfaces to provide maximum abrasion resistance. The mill utilizes 4,000 cfm of air compressed to 100 psi.

In a high-temperature configuration, the Model 42 Micro-Jet utilizes super-heated steam as the grinding fluid. Steam requirements range from 4,000-9,000 lbs/hr at 150 psig. The steam is typically super-heated from 550ºF (290ºC) to 1,000ºF (540ºC). These units have been designed for Titanium Dioxide (TiO2) applications producing a particle size of 0.5 microns at a rate of 4000 lbs/hr. The same unit has also been used for grinding red and yellow iron oxide pigments.

Polymer Grinding

The Model 24 Micro-Jet grinds fluoropolymers to a particle size range of 10-15 microns at a rate of 200-400 lbs/hr. The system is equipped with a feed impact chamber for reducing larger particles prior to entry into the mill.

The unit is constructed in type 304 stainless steel with replaceable nozzles and liners. Fluid Energy has ground many types of fluoropolymers including virgin and reprocessed grades.

Sanitary Micro-Jet and Pharmaceutical Production System

The Model 8 sanitary Micro-Jet meets USDA and pharmaceutical guidelines for cleaning and sterilization. This unit is equipped with FDA approved O-rings and gaskets.

All sanitary designs have mirror polish and provide easy access to product and gas contact areas for cleaning and sterilization. There are no threaded connections in product contact areas. Sanitary models can be made in sizes from Model 4 up to Model 24.

The Model 8 Micro-Jet System was designed to grind penicillin to less than a 10-micron average at a rate of 50 lbs/hr. This system is equipped with a cyclone for primary sterile collection and a baghouse for secondary product collection.

A HEPA filter with an exhaust fan was also included to provide maximum particulate capture of the system exhaust. All product contact areas are constructed in type 316 stainless steel with pharmaceutical polish and sanitary O-Rings. The system utilizes 100 scfm of air compressed to 100 psig.

Individual Micro-Jet systems may be tailored to optimize both the desired particle size and the production rate. The airflow rate, air pressure, and grinding pattern are easily adjusted by means of interchangeable grinding nozzles and liners. Abrasive, sticky, and contamination-sensitive products can all be processed by means of specialized Micro-Jet liners. Specialized liners include:

• Alumina
• Tungsten Carbide
• Silicon Carbide
• PTFE
• Urethane
• Polyethylene

These case studies are just a few examples of granular solids that can be jet-milled in a spiral or pancake mill. A list of additional product applications can be found here on the Micro-Jet equipment web page.

References:
1. Andrews, NH, 1936, Method of And Apparatus for Providing Material in Finely Divided Form, US2550390.
2. R.H. Perry and D.W. Green, Chemical Engineering Handbook, 8th Ed., 2007, p. 21-61.

Frequently Asked Questions: Jet Milling and Particle Classfication

Loop Mill (JET-O-MIZER) Pancake Mill (MICRO-JET) Fluid Bed Mill (ROTO-JET)

Below are several frequently asked questions on jet milling and particle size classification. If your question is not answered here, feel free to contact us using this link.

1.What is jet milling?
2. What is the size classification?
3. What are the different types of classifiers?
4. How does a jet mill classify particles?
5. How is the output particle size controlled?
6. When is jet milling used?
7. What are typical raw feed sizes and/or size distributions?
8. What is the basic difference between a pancake mill (e.g. MICRO-JET) and a loop mill (e.g. JET-O-MIZER)?
9. How does the fluid bed jet mill (e.g. ROTO-JET) differ from the pancake mill and loop mill?
10. What applications are best suited for a fluid bed mill design?
11. Are jet mills for continuous or batch operation?
12. What components are required for a jet milling system?
13. When is the cyclone included in a jet milling system?
14. What are the typical materials of construction?
15. What types of liners are available for a Fluid Energy MICRO-JET?
16. What types of liners are available for a Fluid Energy JET-O-MIZER?
17. What types of liners are available for a Fluid Energy ROTO-JET?
18. What are the typical compressor requirements for a jet milling system?
19. What compressible fluids are used in jet milling systems?
20. Can the jet-milling system be designed for closed-loop gas operation?
21. What system design features are employed for explosive applications?
22. What are the typical raw materials used in jet-mills? What is the particle size of the feed and the final product?

 

1. What is jet milling?

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.

 

2. What is size classification?

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.

 

3. What are the different types of classifiers?

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.

 

4. How does a jet mill classify particles?

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.

 

5. How is the output particle size controlled?

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.

 

6. When is jet milling used?

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:

• Improved reaction rates for catalysts and explosives
• Improved absorption in the body for pharmaceuticals
• Improved suspension of specialty chemicals
• Improved color characteristics for pigments
• Smoother textures for pigments or foods
• Improved strength qualities of products

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.

 

7. What are typical raw feed sizes and/or size distributions?

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.

 

8. What is the basic difference between a pancake mill (e.g. MICRO-JET) and a loop mill (e.g. JET-O-MIZER)?

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:

• While both mills are capable of grinding to 1-10 microns, a pancake mill (e.g. MICRO-JET) will grind a few microns finer (assuming the same processing conditions). As such, a pancake mill should be used if you are looking to obtain maximum grinding and more surface area.
• A pancake mill due to its simple geometry is easy to line with a variety of materials such as ceramics or special alloys for abrasive products; or PTFE or UHMW polyethylene for sticky products.
• A pancake mill typically produces a particle size distribution with a high concentration of fine particles.
• A loop mill (e.g. JET-O-MIZER) is simple to feed with a wide variety of materials. It does not experience the “blow-back” problems that could occur when feeding very light bulk density materials into a pancake mill.
• Because the loop mill is easier to feed with all materials, there is no required ramp up of pressure when starting the mill. In effect, the operating pressures can be set before feed is introduced into the mill.
• The JET-O-MIZER models have the widest range of throughput, from 1-gram batches to rates as high as 3 tons per hour.
• A loop mill typically produces a particle size distribution with a normal bell-shaped curve and less fines.

 

9. How does the fluid bed jet mill (e.g. ROTO-JET) differ from the pancake mill and loop mill?

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:
• The fluid bed mill employs a variable speed classifier wheel.
• The fluid bed mill grinding nozzles are directly opposed rather than tangential.
• The fluid bed system can be totally automated through a PC or PLC.

 

10. What applications are best suited for a fluid bed mill 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).

ABRASIVE MATERIALS

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.

 

11. Are jet mills for continuous or batch operation?

Typically, jet milling systems are designed for continuous operation. However, pancake and loop lab units can be operated in a batch mode.

 

12. What components are required for a jet milling system?

CONTINUOUS OPERATING SYSTEM

• Volumetric or gravimetric screw feeder
• MICRO-JET, JET-O-MIZER or ROTO-JET mill
• Compressor package
• JET-O-CLONE cyclone (optional)
• PLC/PC based controls (primarily ROTO-JET — optional)
• Baghouse
• HEPA (optional)
• Exhaust fan (optional)

BATCH OPERATING SYSTEM

• Volumetric or vibratory feeder
• MICRO-JET or JET-O-MIZER
• Compressor package or compressed gas cylinders
• Filter collection bag *, **
• JET-O-CLONE cyclone (optional—extends batch time)
• HEPA filter (optional)
* Must not be used with materials that have explosion potential
** Recommend placement in a fume hood

 

13. When is a cyclone included in a jet milling 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.

HIGH-TEMPERATURE APPLICATION

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

 

14. What are the typical materials of construction?

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:
• Model 00 Jet-O-Mizer (also available in hardened 630 SS)
• Model 0101 Jet-O-Mizer (also available in hardened 630 SS)
• Model 0202 Jet-O-Mizer (also available in hardened 630 SS)
• Model 4 Micro-Jet (Sanitary or Standard designs)
• Model 8 Micro-Jet (Sanitary or Standard designs)

 

15. What types of liners are available for a Fluid Energy MICRO-JET?

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:

• Series 300 stainless steel for corrosive materials
• Hardened Series 630 stainless steel for mildly abrasive materials
• Hardened steel for abrasive materials
• Alumina used for light-colored abrasive materials
• Tungsten carbide for dark-colored abrasive materials
• PTFE for sticky materials
• UHMW polyethylene for sticky materials
• Silicon carbide for abrasive applications (production-sized MICRO-JETS)

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.

 

16. What types of liners are available for a Fluid Energy JET-O-MIZER?

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.

 

17. What types of liners are available for a Fluid Energy ROTO-JET?

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:

• Hastelloy
• Stellite
• Ceramic-steel duplex
• Tool steel

 

18. What are the typical compressor requirements for a jet milling system?

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.

 

19. What compressible fluids are used in jet milling systems?

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.

 

20. Can the jet-milling system be designed for closed-loop gas operation?

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.

 

21. What system design features are employed for explosive applications?

The following methods have been used either separately or in conjunction to handle explosive materials:

RELIEF VENTING

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.

INERT ATMOSPHERE

As discussed above, the system can be operated with an inert atmosphere, eliminating the possibility of an explosion.

SUPPRESSION

Equipment can be provided to sense a pressure rise within the baghouse and release a chemical reagent into the baghouse, suppressing the explosion.

ISOLATION/CONTAINMENT

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.