The grinding air is injected tangentially via Laval nozzles in the nozzle ring into the machine. This causes a spiral jet of air to form in the grinding zone, from which the mill derives its name. A high pressure forms in the mill as a result of the spiral flow of air that can rise to 1 bar overpressure in operation without product. The integrated injector is charged with compressed air which ensures that the product is conveyed into the machine against the overpressure present in the machine. This, however, is associated with a considerable compressed air consumption, which can be as much as 30% of the total grinding air requirement.
The feed product circulates close to the nozzle ring and is thus intercepted repeatedly by the air jets exiting the nozzles. Comminution is the result of inter-particle collision caused by the particles flowing at different speeds in the nozzle jet. Comminuted material is conveyed along with the air to the discharge, the spiral flow subjects the particles to a classification: only fine particles are discharged, coarse particles remain in the mill.
Alpine® AS Spiral Jet Mills are characterized by a special geometry of the mill housing in the area of the discharge, which contributes towards a finer classification effect and a sharp top cut.
The product also has an effect on the air flow within the machine: the more product there is in the machine, the more the spiral flow is braked and the lower is the centrifugal force and the coarser the end product. The relationship of the product to the classifying air flow rate is therefore the most significant parameter for setting the fineness of the spiral jet mill. In principle, different nozzle angles can also be used to achieve the required fineness values, whereby the complete nozzle ring must be exchanged in this case.
In the case of jet milling, one takes advantage of the fact that as compressed gas exits a nozzle, it is accelerated to extremely high speeds. In expanding, 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 characterized 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 fluidized bed, the speed of the gas jets sinks extremely rapidly after exiting the nozzles.
Comminution is a result of inter-particle 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. 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 fluidized 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 under-pressure at their common center, and thus draw particles from the fluidized bed direct into the center 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.
The wet material is fed through customized feeding systems into the grinding and drying section of the dryer's main body. The grinding rotor disperses the wet material into very fine particles and the fine particles are fluidized in the grinding chamber by temperature-controlled, hot gas coming in from the air heater.
The hot air (or inert gas) can be heated up to 650 °C and as the wet product is dispersed, it is reduced in size in the bottom section of the dryer. The system is kept under a negative pressure by the exhaust fan and the surface area of the product is increased enormously so that water (or other solvents) is evaporated instantaneously.
The dry and fine particles are conveyed with the gas stream to the top section of the dryer where a separator classifies the particles by size. Then the particles pass the classifier at the set cut point and are conveyed with the exhaust air to a powder air separating system like the cyclone, cyclone filter or cyclone scrubbers.
The Micron Dryer flash dryer retains a fluidized bed of product in the drying chamber to ensure a low level of adhesion of wet material on the inside wall of the drying chamber. Moreover, process parameters like classifier speed and outlet temperature can be adapted to control moisture content and particle size of the end product.
Ultra fine regular end product - Due to its specially designed dispersion rotor, our flash dryers are able to produce ultra-fine powders in one step from suspensions, slurries, pastes and dough, filter cakes or wet powders.
High evaporation capacity - Due to the combination of high ?T with a good dispersion of the wet product in the air, our flash dryers can evaporate large quantities of liquids.
Compact design - Compared to other direct drying technologies, the flash dryer requires up to 50% less space.
Multi-purpose - Filter cake or slurry, in the same machine. Therefore with the Drymeister-H unlimited mechanical dewatering is possible previous to the final drying stage.
Easy access - Either for cleaning, inspection or maintenance inside the dryer, our flash dryers offers the best accessibility from the market, regardless which size.
Various systems available - Open, closed- and recycle system concepts.