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What advantages does a jet mill have?
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.
How does jet milling compare with other methods of grinding such as ball mill or a hammer mill?
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) cut-off 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 a jet mill the temperature of the air leaving the jets is cooled to about 200 degrees F due to the Joules Thompson effect and the product leaves no warmer than the air used for the grinding.
Oxidizers and highly explosive materials are often ground. When grinding abrasive products like alumina, silica and Ti02 the Jet Mill is lined with ceramics having a mhos hardness of 9.5 (compared to a diamond 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 here 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 precedence over size distribution.
What particle size range does a jet mill produce?
A jet 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 of harder 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 edges by running at elevated feed rates with low grinding pressure, a process that changes how the material compacts.
What materials can be ground in a jet mill?
Any powder that is crystalline or friable can be ground in a jet mill. For example diamonds can easily be ground but an amorphous glass is difficult to fine grind. Most materials may also be flash dried and ground at the same time using heated air or superheated steam. If the material is not a slurry and can be fed to the mill injector it can be dried and ground in a mill. The most common materials ground in a mill are Titanium Dioxide, Iron Oxide and similar pigments, abrasive materials such as Aluminium 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.
What are the characteristics of a jet milled products?
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 important to have a very narrow particle size distribution for maximum productivity. The disks used on computer hard drives are polished with precisely milled abrasives. Abrasives in toothpaste are another example they must b aggressive but not enough to remove enamel.
Partical size is crtiacalin titanium dioxide pigment because the greatest reflectory of light (producing the brightest white) occurs when the particle size of the pigment approaches a precise relationship to the wavelength of the visible light. Controlling particle size then is used to control product colour 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 jet 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. Filler s and extenders such as clay and silica products are precisely milled for use in fine paper and plastic products. Coating such as waxes and various polymers are jet milled to achieve special high gloss finishes on paper. Jet milled pesticides herbicides and other plant related products cover much more surface area using less of the active ingredient thus reducing costs.
How much contamination will a Jet Mill add to my product?
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 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 simple made from materials that may easily be removed from the diamond in subsequent processing. In those rare cases when working with materials tat 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.
Can jet mill only be used for particle size reduction?
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 materials may be fed to the jet mill at the same time resulting in a perfect homogeneous blend of 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 atomising spray nozzles either directly in to the grinding chamber in to 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.