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Air-Jet Milling System

Air-Jet Milling Process Layout

Fluid Energy mills such as pancake, loop, and fluid bed mills, are used to break down granular material to fine powders.  All jet mills are continuously operating devices so an overall milling system requires additional equipment beyond just the mill.  In the picture at the right, is a jet milling system using a Fluid Energy Jet-O-Mizer.  Equipment is needed to transport the coarse or granular material into the mill in a consistent fashion.  After the mill grinds the material into a finer powder, additional separation and storage equipment is necessary beyond the mill.

In this article we will review the main components of a milling system.  When considering whether to purchase a milling system, it is a good to know the basic process requirements.  Below is a schematic of a Jet-O-Mizer process showing the main pieces of equipment.

 

 

Main Supporting Equipment for Jet-Milling System:

  1. Compressor. Air compressor raises air pressure from ambient to 100-125 psi. Air pressure of 100-125 psi is required to effectively grind powder. (Note: Fluid Energy does not supply air compressors or Air Dryers & Filters (Items 1 and 2). We can provide technical support for purchase by the customer.)
  2. Air Dryer & Filter. These components remove moisture and unwanted particulates for the feed air. There are many dryer and filter options available.
  3. Material Feeder. Hopper with screw feeder meters material into Jet-o-Mizer for grinding. Feeder has controls to meter material at a consistent, fixed rate. The material is fed through a venturi funnel using a vacuum set up with a pusher nozzle at the mill inlet.
  4. Jet-O-Clone Collector/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 jet mill. This is the main product stream of fine powder from the mill. Product is collected in Receiver Bin and fed via gravity from Receiver Bin through rotary valve into packaging container. Cyclones are not always needed and can be eliminated making the Reverse Pulse Collector (#5) the primary milled product collector.
  5. Reverse Pulse Collector/Rotary Airlock. Dust collector contains cartridges/filters to separate out remaining finer powder in gas stream. This is the secondary stream of product from the mill. 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.
  6. Exhaust fan (not shown). Ensures proper air flow from the pulse collector.

The schematic above details a Jet-O-Mizer system, but all Fluid Energy milling system follow the same basic layout. MicroJets, like the Jet-O-Mizer, use a venturi induction feed method and use an exhaust for flow assistance, if needed. The RotoJet classifying jet mill, aka fluid bed mill, operates under a vacuum so the venturi injection system is not always needed but a high intensity exhaust fan is an absolute necessity. Even with the physical differences, all three (3) type of Fluid Energy jet mills (MicroJet, Jet-O-Mizer, RotoJet) need consistent feed and product separation equipment. The size and orientation of the support equipment will vary with the space available, size of feed and finished material containers and other restrictions like area electrical classifications. Systems can be made to fit into glove boxes, bunkers, open warehouse or even outside.

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.