Frequently Asked
Questions
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| Pilot-scale Micron Master® mill crated for shipment |
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What advantages does a jet mill have? |
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The main advantage of a jet mill over other kinds of mills is that you can grind a friable or crystalline material to 1 – 10 microns average particle size and classify in a very narrow particle size range at the same time. There are no moving parts to wear out or generate heat in a jet mill and no screens to plug or be punctured. There is no attritional heat because of the cooling effect of the jets. |
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| How does jet milling compare with other methods of grinding such as a ball mill or a hammer mill? |
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Well designed, high speed mechanical mills can grind some friable materials into the low micron size range but wear and product contamination is a serious problem as is attritional heat. Practically, the high speed mechanical mill (hammer etc. mill) cutoff is 200 mesh with a typical mid range of 80 mesh. Abrasive products will seriously erode most hammer mills adding metallic contamination to the product. Materials that degrade with heat or have low melt temperatures are a problem in hammer mills because of the heat generated in the mill.
In 1936 the first commercially practical jet mill
was introduced. Called the
Micronizer, it was the predecessor to our current Micron-Master® design. Up to that time, dry grinding in the sub-sieve range of 625 to 2500 theoretical mesh size range was impractical. In a jet mill the temperature of the air leaving the jets is cooled to about –200 degs F due to the Joules Thompson effect and the product leaves no warmer than the air used for the grinding. (Friction from collisions and contact with the grinding chamber is offset by the cooling effect of the expanding air).
Oxidizers and highly explosive materials are often ground in the Micron-Master®. When grinding abrasive products like
alumina, silica and TiO2 the Micron-Master® is lined with ceramics having a mhos hardness of 9.5 (compared to diamond at 10) which allow virtually zero contamination of the product.
A ball mill with a classifier will produce a fine product but the particle size distribution curve is very wide and when the correct average size is reached the number of fines is usually too high. Ball mills can be lined with ceramics to reduce contamination from abrasive products but there is constant wear of the media, which may contaminate the product. Ball mills are often used in high volume, mine mouth operations where cost/ton has precedence over size distribution.
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What particle size range does a jet mill produce? |
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Typically the Micron-Master® mill will grind friable or crystalline materials down to the 1 to 10 micron average particle size range. Some products, such as some molybdenum compounds, paint pigments and similar products, can be reduced to particles as small as 200 nanometers. Work in the nano size range can also be the deagglomeration of nano size particles.
Particles larger than 10 microns are generally hard-to-fracture polymers such as toner compounds or hard waxes, and some organic materials, but if a larger size is wanted, many products can be ground larger than 10 microns by reducing power to the mill or by increasing the rate of feed. Some products are simply
polished to remove sharp edges by running at elevated feed rates with low grinding pressure, a process that changes how the material compacts.
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| What materials can be ground in a jet mill? |
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Any powder that is crystalline or friable can be ground in a jet mill. For example, diamonds can easily be ground in the Micron-Master® mill but an amorphous glass is difficult to fine grind. Moist materials may also be flash dried and ground at the same time using heated air or superheated steam. Generally, if the material is not a slurry and can be fed to the mill injector, it can be dried and ground in a Micron-Master® mill. The most common materials ground in the Micron-Master® mill are Titanium Dioxide, Iron Oxide and similar pigments; abrasive materials such as Aluminum Oxide, Silica,
Wollastonite, Cerium Oxide and Diamond; chemicals such as Barium
Titanate, Azodicarbonamide, Sodium Benzoate, TCC and TCP; many cosmetic products and many pharmaceutical products… to name a few. |
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| What are the characteristics of a jet milled product? |
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One of the most important characteristics of a jet milled product is the huge increase in surface area. When reduced to 5 microns, a 30 mesh product has 1,643,000 times more particles and the surface area is 118 times greater. This allows faster reaction times for chemicals, faster burn rates in solid fuel rockets (air to air missiles) more powerful explosives, stronger plastics and adhesives, and better pharmaceuticals.
The particle size is also important since very fine abrasives are used to polish lenses of all types so there are no scratches. If the polish is too coarse the lens will have scratches and if it is too fine processing time will increase. In the polishing industry, it is very important to have a very narrow particle size distribution for maximum productivity. The disks used on computer hard drives are polished with very precisely milled abrasives. Abrasives in toothpaste are another example, they must be aggressive but not enough to remove enamel.
Particle size is critical in titanium dioxide pigment because the greatest reflectivity of light (producing the brightest white) occurs when the particle size of the pigment approaches a precise relationship to the wavelength of visible light. Controlling particle size, then,
is used to control product color and brilliance.
Cosmetics exhibit a silky smoothness when ground in the jet mill, which is a highly desirable property for facial cosmetics.
Pharmaceuticals for asthma patients need to pass deep into the lungs and medicines comprised of very fine particles travel deeper. The Micron-Master® mill will grind pharmaceutical powders fine enough to pass through a hypodermic needle. An increase in potency can be achieved with an increase in surface area so a lower dosage of the drug is required to do the same job.
Fillers and extenders such as clay and silica products are precisely milled for use in fine paper and plastic products. Coatings such as waxes and various polymers are jet milled to achieve special high gloss finishes on paper.
Jet milled pesticides, herbacides and other plant related products will cover much more surface area using less of the active ingredient thus reducing cost. |
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| How do you adjust particle size in a jet mill? |
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Particle size is adjusted primarily by a change in feed rate. When the rate is reduced, finer particles result because there is more energy available per particle to accelerate the particles and the entire fluid mass. Collisions become more violent and pressure gradients become larger.
Some products require exceptionally high levels of applied energy to grind and classify. An increase in pressure will increase the flow of the compressed gas and may improve the grind
characteristics.
An increase in temperature may also be used to modify the profile of the product.. The velocity of the air jets increases about one ft/sec for every degree F of increase in temperature. Using the heat of compression of the gas compressor will often improve the quality of the grind. Some products are best ground using super heated steam supplied through supersonic nozzles into the grinding chamber. Others require air heated beyond what a compressor can supply. Units have been installed operating up to 1,200 deg F.
The profile of the mill may also be modified to adjust the output particle size. Some particles break very quickly and the mill must be designed to process the material through with very low resonance time. Others require many collisions and therefore high resonance time for proper classification. Particles that have strong molecular bonding may require very high energy collisions and special designs facilitate this activity. Also the smaller the particle the lower the energy of each collision for a given velocity. To achieve greater size reduction the particle velocity must be increased. |
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| Is
compressed air the only gas used to power a jet mill? |
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No. Commercially, compressed air is by far the most commonly used gas but super heated steam (no moisture is present in the super heat state) is used in very large installations grinding primarily titanium dioxide pigment. Nitrogen is commonly used when a material must be ground in an inert atmosphere to protect from oxidation and possible fire or even explosion of the product. Argon has also been used for this purpose if the less expensive nitrogen was unsuitable. Light gases such as helium have been experimented with because of the possibility of even higher velocity impacts. |
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| How will you determine what size mill should be used for my application? |
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We manufacture Micron-Master® mills in 12 sizes, from a grams-per-minute sized 1-in.unit up to the
tons-per-hour 42-in. model. The size number represents the actual inside diameter of the grinding chamber, and we most commonly determine the proper size based on data generated from tests performed on the client’s material. The client provides the desired production feed rate and particle size and we extrapolate or directly correlate the mill size based on the test data. The mills scale up proportional to gas flow. And, since we have an extensive custom grinding and test facility with over 12000 SCFM of oil-free air available for testing, we often are able to test on the exact required Micron-Master® mill. |
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| How much contamination will a Micron-Master® mill add to my product? |
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Correctly specified the mill will add no contamination to the product, or amounts so small as to be undetectable and of no significance. When grinding abrasive materials such as
alumina, silica, iron oxides etc. the mill is lined with tungsten carbide or silicon carbide ceramics with MOH hardness of 9.6 (with diamond being a 10). These linings have been developed over the past 35 years and were one of the first uses of this type of ceramic lining.
When grinding diamond powder the mill lining becomes a consumable and the lining is simply made from materials that may easily be removed from the diamond in subsequent processing. In those rare cases when working with materials that allow no contamination whatsoever, as is sometimes the case in the electronics field, the liners are made of the same material as is being ground. Today’s ceramic linings are fully dense and very hard, with long wear life and allow little if any contamination of the product being ground. |
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| What other equipment is necessary to operate a jet mill? |
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The equipment absolutely necessary to operate a jet mill are:
- A source of fluid energy such as a compressor , steam generator with
superheater, or some other compressed gas generation system - evaporation of liquid nitrogen, for example.
- A means to meter the material to the pulverizer at a constant rate of feed such as a volumetric feeder (screw feeder or vibratory feeder) or loss-in-weight feeder, rotary valve.
- A means to separate the ground product from the spent air such as a simple free hanging filter bag, cyclone collector with bag house secondary, a reverse-pulse cleaned bag house, a scrubber or an electrostatic precipitator.
Other equipment could be used to enhance the operation such as remote control systems, potent compound containment systems, aspiration air filtration systems and many material handling alternatives.
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Can jet mills only be used for particle size reduction? |
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A jet mill has many applications beyond size reduction.
One of the important secondary uses for jet mills is to blend powders. Two or more streams of material may be fed to the jet mill at the same time resulting in a perfect homogeneous blend at the output. One product may also be coated by another as well as blended. In some instances liquid additives have been injected under pressure through one or more atomizing spray nozzles either directly into the grinding chamber, into the main grinding air inlet or into the vortex at the mill exit.
Another use of the jet mill is for polishing the sharp edges on particles to make them flow or compress better. Generally very low pressures are required for this process.
Flash drying of products containing water or solvents may be accomplished by powering the mill with hot air or superheated steam. |
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What is the smallest amount of material that can be processed? |
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This depends upon what size mill is being used. Our smallest Micron-Master® mill is the 1-in. model. It is being used to grind samples as small as one gram in biological and pharmaceutical laboratories. |
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Can I lease a jet mill? |
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Yes there are a limited number of mills available for lease. |
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How can I be sure jet milling will work for me before purchasing a Micron-Master® mill? |
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In our testing and custom grinding facility, we can handle samples from 50 grams to 50 tons. We have over 12,000 SCFM of oil free air available with mills from the 2-in. through the 24-in. in many configurations ready to test your products. Upon inspection of the MSDS and the material we do, however, reserve the right to refuse a test if the product is not compatible with our operation. |
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What environmental controls are necessary? |
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Certainly the primary collection system must be designed to protect your people and the environment. A secondary collection system is prudent to guarantee a minimum of ultra fine dust particles escaping to the environment. If you choose to launder dust collector bags in house, local municipality and state guidelines on water emissions must be met. |
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Can jet milling systems be automated? |
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Yes, but our mills are so simple to operate that very few people bother. But, the control system in a loss-in-weight feeder may be used to trigger a shut down of the mill in case there is no feed, and a pressure sensor could be installed to shut down the mill if there is not enough air. These are the key parameters to monitor. |
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How can I estimate required air compressor horsepower? |
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The amount of air that each mill size uses is published elsewhere on this web site. Roughly, you can estimate that each compressor horsepower will deliver about 4 SCFM of air. A 300 HP oil free screw, twin element compressor will deliver 1200 SCFM of air. |
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| Can the mill operate at lower pressures?
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Yes. Often lower pressures are used to allow polishing of particles rather than grinding. In some instances, customers have wanted a broader distribution curve, which can be accomplished using lower pressures. |
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How
much heat is generated in the grinding process? |
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There is always heat generated by friction of high speed particles rubbing over mill surfaces and from the millions of collisions taking place in the mill. However, because of the Joule-Thompson Effect of an air temperature drop when throttling, there is no net temperature increase when the mill is operating. The product temperature will
approach the temperature of the compressed air supplied to the mill. |
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