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 mohs 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.
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.
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.
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 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.
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.
The equipment absolutely necessary to operate a jet mill are:
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.