AFG

Fluidised bed opposed jet mill for manufacturing powders with a steep particle size distribution and sharp top size limitation in the range < 5 µm to 200 µm.

Grinding nozzles arranged around the periphery of the grinding chamber
Classifying wheel(s) arranged horizontally in the classifier top section
Cool and contamination-free grinding
Cleaning made easy by the hinge-back and removable classifier top section and the inspection deck in the mill housing

As ALPINE launched the AFG fluidised bed opposed jet mill onto the market in 1981, it succeeded in revolutionising jet milling. Since then, over 1,000 machines have been sold for a wide diversity of applications.

In jet milling, comminution is exclusively the result of interparticle collision in the gas jets. And because there are no machine components in the grinding zone, neither machine wear nor product contamination occurs. This is why jet milling is often used when contamination-free products are required.

Jet milling is suitable for any material hardness value: from Mohs' hardness 1 (talc) to Mohs' hardness 10 (diamonds).

Principle of operation

During expansion, the energy contained in the compressed gas in the form of heat is converted to kinetic energy. The speed of sound is initially a natural limit for the exit velocity. But by using Laval nozzles, the exit velocity can be increased to above the speed of sound. Laval nozzles are characterised by their hourglass shape, which widens downstream of the narrowest cross-sectional point, the nozzle throat.

The length of the divergent part of the nozzle is adapted to suit the operating pressure. Compressed air of 20°C and 6 bar overpressure is frequently used as the grinding gas, and delivers nozzle exit speeds of around 500 m/s. As a result of drawing in gas and product from the fluidised bed, the speed of the gas jets sinks extremely rapidly after exiting the nozzles. Comminution is a result of interparticle collision in the jets of air and also in the core area, i.e. the point where the opposing jets intersect.

Jet mills are impact mills which are used to achieve maximum fineness values at maximum product purity. Such particle sizes can only be obtained in connection with an air classifying step. Spiral jet mills have a static air classifier integrated into the mill housing, whereas fluidised bed opposed jet mills are equipped with a dynamic deflector-wheel classifier.

The fineness is set as a function of the classifying wheel speed. All-important is a high product loading of the nozzle jets in order to achieve a high concentration of particles and thus high impact probabilities. The patented Megajet® nozzles were developed with this in mind. They consist of four small nozzles which as a result of their close proximity generate an underpressure at their common centre, and thus draw particles from the fluidised bed direct into the centre of the nozzle jet. The product level in the machine is controlled by means of load cells or by monitoring the current loading of the classifier drive.

Features

Single-/multi-wheel classifier head
Monobloc ceramic classifying wheels ensure minimum wear when processing abrasive products
Lining: PU, ceramic or special-grade steel to suit the product
Stainless steel or mild steel, Ni alloy
Horizontal or 3D nozzle arrangement, Megajet nozzles
Selective milling, semi-batch mode, automatic quality control for high quality
Load cells for product level control
Material feed via a feed metering system into the classifier head or direct into the milling chamber, or injector feed
Explosion-pressure-shock-proof
Pharma design up to machine size 630/1 AFG, steep particle size distribution, high bioavailability for active substances, minimum off-spec batches, low manpower requirement
Hot-gas mode for mineral products permits cost-effective manufacture of large amounts of ultrafine mineral powders
Circuit-gas systems for pyrophoric products provide maximum safety
If frequent colour change is necessary, the TFG for coloured toner reduces the costs and ensures optimum throughput rates

Applications

Even difficult products can be processed efficiently with the AFG jet mills.

Heat-sensitive materials such as toner, resin, wax, fat, ion exchangers, plant protectors, dyestuffs and pigments
Hard and abrasive materials such as silicon carbide, zircon sand, corundum, glass frits, aluminium oxide, metallic compounds
Highly pure materials where the requirement is contamination-free processing such as fluorescent powders, silica gel, silica, special metals, ceramic raw materials, pharmaceuticals
High-performance magnetic materials based on rare earth metals such as neodymium-iron-boron and samarium-cobalt. Mineral raw materials such as kaolin, graphite, mica, talc
Selectively ground composite materials such as metal alloys