Systems for Powder Classification

Classifiers may be divided into two categories: counterflow equilibrium and crossflow separation.

Counterflow can occur in either a gravitational or centrifugal field. The field force and the drag force act in opposite directions and particles leave the separation zone in one of two directions according to their size. At the "cut" size, particles are acted on by two equal and opposite forces; hence, they stay in equilibrium in the separation zone. In gravitational systems these particles remain in a state of suspension, while in a centrifugal field the equilibrium particles revolve at a fixed radius, which is governed by the rate at which material is withdrawn from the system. They would therefore accumulate to a very high concentration in a continuously operated classifier, if they were not distributed to the coarse and fine fractions by a stochastic mixing process.

In a crossflow classifier, the feed material enters the flow medium at one point in the classification chamber, at an angle to the direction of fluid flow, and is fanned out under the action of field, inertia, and drag forces. Particles of different sizes describe different trajectories and so can be separated according to size.

Counterflow Equilibrium Classifiers in a Gravitational Field: Elutriators. Elutriation is a process of grading particles by means of an upward current of fluid, usually air or water. The grading is carried out in one or a series of containers, the bodies of which are cylindrical and the bases of which are inverted cones. The cut size is changed by altering the volume flow rate and the cross-sectional areas of the elutriation chambers. The flow medium is usually air, although water is used occasionally.

In air elutriators, air containing particles sweep up through the system at a preset flow rate. Particles with a settling velocity lower than the air flow rate are carried out with the air stream, whereas larger particles are retained in the elutriation chamber. The separation is very slow but can be speeded up by the use of zig-zag classifiers, which act as a succession of elutriators in series.

Zig-Zag Classifiers. Several versions of the zig-zag classifiers are available (Fig. 9). These may be categorized as gravitational or centrifugal counterflow classifiers. A feed rating worm (b) feeds the unclassified material (c) into a classifying chamber. Radially arranged blades on the outer face of the classifier rotor (d) speed the inflow of material up to the peripheral velocity of the rotor suspending it, extra air being admitted through (e). The dust-air mixture is then sucked in to the zig-zag-shaped rotor channels where classification takes place (Fig. 10). Fine material is sucked into the classifier center (g), where it leaves via a cyclone. The coarse material (f) is expelled by centrifugal force. At the periphery it is flushed by the incoming air before being discharged. Gravitational instruments operate in the 1 to 100 /'m size range, centrifugal instruments from 0.1 to 6 /'m.

Fig. 9 The Alpine zig-zag centrifugal laboratory classifier

Fig. 10 Mode of action of a zig-zag classifier. The small balls are the fine material; the large balls are the coarse material. Bolts indicate the material guide motion

Cross-flow gravitational classification is performed with the Warmain Cyclosizer (Ref 5), which is a hydraulic cyclone elutriator (Fig. 11). Using inverted cyclones as separators with water as the flow medium, samples of between 25 to 200 g are reduced to five fractions having cut sizes (for quartz) of 44, 33, 15, and 10 /,!m. The cyclones are arranged in series, and during a run the oversize for each cyclone is trapped and subjected to elutriating action for a fixed time period. At the end of a run the trapped materials are extracted by opening the valves at the apex of each cyclone in turn, and after decantation, the solids are recovered by filtration and evaporation.

Fig. 11 Principle of the Warmain Cyclosizer

Counterflow Centrifugal Classifiers. The Bahco Classifier is a centrifugal elutriator (Fig. 12). The sample is introduced into a spiral air current created by a hollow disc rotating at 3500 rpm. Air and dust are drawn through the cavity in a radially inward direction against centrifugal forces. Separation into different size fractions is made by altering the air velocity, which is effected by changing the air inlet gap by the use of spacers. Instrument calibration is necessary. For the sample, 5 to 10 g of powder are required, which can be graded in the size range 5 to 100 /'m.

Fig. 12 Simplified schematic diagram of the Bahco microparticle classifier. 1, electric motor; 2, threaded spindle; 3, symmetrical disk; 4, sifting chamber; 5, container; 6, housing; 7, top edge; 8, radial vanes; 9, feed point; 10, feed hole; 11, rotor; 12, rotary duct; 13, feed slot; 14, fanwheel outlet f ■ 1

Fig. 12 Simplified schematic diagram of the Bahco microparticle classifier. 1, electric motor; 2, threaded spindle; 3, symmetrical disk; 4, sifting chamber; 5, container; 6, housing; 7, top edge; 8, radial vanes; 9, feed point; 10, feed hole; 11, rotor; 12, rotary duct; 13, feed slot; 14, fanwheel outlet

Crossflow Centrifugal Classifiers. The principle of a crossflow centrifugal laboratory classifier is illustrated in Fig. 13. A vaned rotor produces a centrifugal field, while at the same time air is drawn into the center of the rotor. All but about 5% of the air intake, induced by a positive displacement pump downstream of the classifier, enters the classification zone through a very narrow gap formed between the rotor and the stator. This leads to a very high turbulence in the preclassification zone. The material enters the classifier through a venturi-type nozzle with the remaining 5% of air. Between planes 1 and 2 the ratio of centrifugal force to drag force is kept very nearly constant by a diverging radial cross section. This zone is the classification zone. The smaller particles are carried out through the middle and the larger ones move toward the stator, where they undergo disaggregation until they reach the exit. The cut size of these machines ranges from 0.5 to 50 /Jm.

Fig. 13 A cross-flow centrifugal laboratory classifier

Crossflow Elbow Classifier. In the cross-flow air classifier (Fig. 14) the main air is introduced at ai and secondary air at a2. Both streams are bent around a solid wall (b), and the resulting flow follows the bend without leaving the wall or forming vortices. The so-called Coanda effect helps to maintain the flow around the bend for approximately 90°, and this is enhanced by the application of suction.

Fig. 14 Principle of the cross-flow elbow classifier
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