Jet mill

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In jet mills ( also called air jet mills ), particles are ground in a gas stream without using mechanical tools such as B. use high-speed rotors. The particles are comminuted using the energy introduced by the grinding gas.

Mostly compressed air is used as grinding gas, rare gases such as. B. argon . Nitrogen can be used for grinding under inert conditions . The use of superheated steam is also possible and is used for special applications. The principle of jet grinding is generally used wherever extremely fine grinding of the ground material is required. A jet mill crushes dry and very hard materials to finenesses in the range of 0.1-100 micrometers . The usual working range is in the area below 10 micrometers.

functionality

The grinding gas is accelerated so much by special nozzles that the respective particles are carried away and meet again. The product particles are mainly shredded autogenously and therefore without any significant wear on the grinding tools, which is why this grinding principle is particularly suitable for the efficient grinding of very hard materials. After the mill, the fine grinding material is separated from the actual grinding gas in a cyclone separator or a dust filter connected downstream of the mill .

Areas of application

Jet mills are used, for example, in the industrial production of powders such as aluminum oxide , Al 2 O 3 , as a basis for modern ceramics. But this type of mill is also used in numerous other applications. With a jet mill you get a controlled particle distribution with minimal contamination . Depending on the design, the grinding chamber can be designed so that the interaction with the wall is as low as possible. For this reason, such a mill construction is also particularly suitable for the production of medicines, such as antibiotics . Other typical areas of application are, in addition to active pharmaceutical ingredients, such as sulfonamines or salicylic acid , the production of pesticides , mineral flours, talc or silicon for semiconductor technology . The usual grinding quantities of such mills are between 50 grams and over 1000 kg of ground material per hour.

Types

Jet mills are used in a wide variety of designs and can be differentiated according to the location of the particle acceleration (jet pipe, injector, free jet) and the location of the particle impact (in the free jet, in the focus of opposing beams, on a baffle plate). Most types of jet mills have an integrated air classifier, which can be static or dynamic, or they are operated in a circuit with an external air classifier . Examples of well-known designs are:

  • the Majac opposed jet mill (acceleration in the injector, shredding in the opposed jet, semi-integrated air separator),
  • the Finnpulva SP opposed jet mill (acceleration in the jet pipe, shredding in the opposite jet, external air separator),
  • the NPK I-mill (acceleration in the injector, crushing on the baffle plate),
  • the spiral jet mill
  • the oval tube jet mill (acceleration in a free jet, shredding in a single jet, integrated static air classifier) ​​and
  • the fluidized bed opposed jet mill (acceleration in the free jet, shredding in the single and in the opposite jet, integrated dynamic air classifier).

The spiral jet mill and the fluidized bed opposed jet mill are most frequently used.

Web links

Individual evidence

  1. ^ S. Ebel, HJ Roth (Ed.): Lexicon of Pharmacy. Georg Thieme Verlag, 1987, ISBN 3-13-672201-9 , p. 405.
  2. ↑ Dr. Roland Nied, Bonstetten