classifier and its circuit

After initial liberation of a mineral constituent from its ore by crushing, grinding and screening, separation of minerals by size are normally attempted by a classifying process. In mineral processing plant operations, classification and separation of mixtures of fine and coarse particles and also of lighter and heavier particles may be performed in a wet or dry state. The majority of separations are carried out in a liquid environment because of an increased efficiency. The basic technique employed is to allow particles to settle under gravity in a liquid medium (usually water). The higher terminal velocity of irregular shaped, coarser, heavier particles allows these particles to reach the bottom of the vessel at a faster rate compared to particles that are smaller and lighter. Removing the settled particles while the others are still settling offers a simple means of a separation. For very small particles, like clay or silt, whose size approaches colloidal dimensions, long times are required to settle and the small difference in settling rates of these fine particles leads to low separation efficiency.

To accelerate the settling rate of these fine particles, centrifugal forces are employed such as

in cyclones or hydrocyclones.

Spiral classifier

The shape of the spiral classifier tanks is usually rectangular (Fig 1). The feed is introduced at a position about halfway along the length of the settling tank. The tank slopes range from 14° to 18°. The slope is adjusted such that the top end is higher than the height of the overflow weir. The spirals impede the downward slurry movement resulting in some build up. The sides are therefore raised. Classifiers with raised sides are generally called high or H-type classifiers. In contrast, classifiers with low sides and shallow tanks are known as S-type classifiers. The S type classifiers have almost gone out of use. The maximum lengths of H type classifiers are about 14 m with widths of 0.5 to 7 m and spirals up to 2400 mm in diameter. The speed of rotation of the spirals varies inversely with size. Thus classifiers with a 300 mm spiral diameter revolve at about 8-20 rpm while the 2000 mm diameter spirals rotate at about 2-5 rpm to give a sand conveying speed of 2-3.5 m/s. The raking capacity of the large classifiers is approximately 200 t/h while smaller classifiers have raking capacities as low as 1.5 t/h. To some extent the capacities depend on the number and design of the helix in the spiral. The helix could be single, double or even triple pitch. The pitch is related to the diameter of the spirals. It is generally of the order of 0.5 to 0.75 times the diameter of the shaft. The number of helix may be single (simplex) or two side by side (duplex) depending on the dimensions of the tank.

Fig1.spiral classifier

The feed size of particles to spiral classifiers is in the region of 150 microns and coarser. The overflow particle size distribution depends both on the height of the weir and a baffle placed before the weir. The baffle is placed within the tank and located at a distance of approximately 38 mm (maximum about 380 mm) from the weir. The flow rate of the overflow stream ranges from 1 t/h to around 40-45 t/h. Increasing the feed flowrate increases the overflow rate, decreases the residence time and increases the fraction of coarse particle sizes in the overflow stream. A slow feed rate, well spread out along the width, is preferred for finer feeds to eliminate or reduce the presence of coarser sizes in the overflow stream.

Rake classifier

The feed size of particles to spiral classifiers is in the region of 150 microns and coarser. The overflow particle size distribution depends both on the height of the weir and a baffle placed before the weir. The baffle is placed within the tank and located at a distance of approximately 38 mm (maximum about 380 mm) from the weir. The flow rate of the overflow stream ranges from 1 t/h to around 40-45 t/h. Increasing the feed flowrate increases the overflow rate, decreases the residence time and increases the fraction of coarse particle sizes in the overflow stream. A slow feed rate, well spread out along the width, is preferred for finer feeds to eliminate or reduce the presence of coarser sizes in the overflow stream. The structure of rake classifier is shown as fig2.

Fig2.rake classifier

Cone classifier

The cone classifier is the simplest of all of the classifiers, however its use in industry is relatively limited. The classifier vessel is conical in shape. The feed enters the vessel (Fig.3) through a centrally located inlet pipe. Initially the bottom spigot is closed. When the slurry reaches a certain height, the spigot is opened. The settled particles then discharge through the spigot. The finer particles travel with the water to the periphery and overflow into a launder.

Fig3.cone classifier system

Hydrocyclone

Rapid settling and classification is achieved by increasing the force acting on the particles by replacing the gravitational force by centrifugal forces. Several types of equipment based on this principle are used for the purpose, like the hydrocyclone and basket centrifuges. The hydrocyclone is the simplest and is the only one discussed here. The hydrocyclone has no moving parts and is the easiest to operate. Fig. 14 is a sketch of a typical hydrocyclone. The feed entry is either tangential to the centre line of entry or forms an involuted entry. The cross-section of the entry pipe is usually circular, oval or rectangular; each of which provide a different velocity profile inside the feed chamber and the cyclone cone. The top of the feed chamber is closed with a plate through which a pipe known as a vortex finder passes. The bottom of the vortex finder protrudes below the feed chamber.

Below the feed chamber the body of a cyclone is shaped like an inverted cone, which converges to a smaller cone, which serves as the outlet of the coarser size fractions in the feed. The feed chamber and the cones are lined inside with rubber or synthetic linings due to abrasive nature of most metallurgical slurries. The lining material is hard rubber, neoprene or urethane. In some cases, the protective lining is sprayed inside forming a hard monolithic bond with the base metal. The apex is sometimes fitted with a concentric, hardwearing synthetic rubber inner sleeve, which can be squeezed hydraulically or pneumatically to alter the diameter of the opening.

Fig4. hydrocyclone

Operation of Mechanical Classifiers

The feed to the mechanical classifier with a rectangular cross-section is spread along the width and is usually directed towards the top end. On entry, the solids in the slurry commence to settle, the coarser and denser particles settling at a faster rate than the others. Particles settling to the bottom form a layer (region J in Fig. 5), which is least disturbed by the blades of the rakes or spirals and possibly serves to protect the base of the tank. Region 4 is the zone of moving sands dragged into the underflow by the raking mechanism. Above the bottom layers is the zone marked 5 in Fig. 5 where hindered settling occurs. A continuously changing concentration gradient is set up in this layer, the upper portion being least concentrated and the lower end having the maximum concentration of particles. The mechanical rakes or spirals continuously stir this zone, breaking up agglomerated particles and generally accelerating the separation process. The layer marked zone 2 is where maximum agitation takes place, the lighter and smaller particles are separated here where they join with the overflow stream and are carried over to the overflow launder. The heavier particles settle by gravity to zone 3 forming the thick bottom layer. The surface of the top layer 1 is at the same level as the weir allowing the light particles to flow over to the overflow launder.

Fig5. Slurry movement and zones of particle separations in an operating classifier

Hydrocyclone Circuits

Almost all crushing and grinding circuits include hydrocyclones in close circuit to yield a product of the required size distribution. Hydrocyclones are generally installed at an elevated position above the grinding unit so that the coarse underflow product can flow by gravity back to the grinding unit for further size reduction. The configurations adopted in practice are varied. Three typical set ups are illustrated in Fig. 6.

Fig6. Hydrocyclones in closed circuits with grinding mills

For a better control of the product size, hydrocyclones are connected in series (Fig. 7), while for greater throughput cyclones are connected in parallel.

Fig7. Hydrocyclones connected in series, two stage classification

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