Differential Settling 1y3p14

DIFFERENTIAL SETTLING (CLASSIFICATION) Differential Settling is the separation of solid particles into several size fractions based upon their settling velocities in a particular medium. Differential Settling is also known as classification. The density of the medium is less than that of either of the two substances to be separated. In differential settling, both light and heavy materials settle through the medium. (Geankoplis, 2005) BASIC TYPES OF CLASSIFICATION (a) Pool Classification In pool classification, a suspension of particles is fed into and through a pool of some kind at such a rate that only a part of them – the coarser and faster-settling part have time to settle out. The remainder overflows as fines (b) Hydraulic Classification In hydraulic classification, the feed suspension is introduced into or above one or more columns or pockets through which water is rising at a controlled velocity. Coarse paritcles subside through the pockets and are removed through spigots. Those that settle slower than the rising velocity in any column or pocket are (at least ideally) prevented from subsiding and must overflow. CLASSIFICATION EQUIPMENT

Surface Velocity Classifier/Gravity Settling Tank

It is the simplest type of classifier. It consists of a large tank with provisions for a suitable inlet and outlet. Mechanism

A slurry feed enters the tank through an inlet connection. As soon as the slurry enters the tank, its linear velocity decreased as a result of the enlargement of cross-sectional area. Solid particles start to settle under the influence of gravity. The faster-settling particles (coarse particles) will be collected at the bottom of the tank near the inlet/entrance, while the slower settling particles (small particles) will be carried farther into the tank before they reach the bottom of the tank. The very fine particles are carried away in the liquid overflow from the tank. Vertical baffles placed at various distances from the inlet within the tank allow for the collection of several fractions (different grades of particles) according to the terminal falling velocities. Because of occurrence of considerable overlapping of size, no sharp separation is possible with this classifier. Spitzkasten

It consists of a series of conical vessels of increasing diameter in the direction of flow.

Mechanism The slurry feed enters the top of the first vessel where the larger, faster-settling, particles are separated. The overflow including unseparated solids, feeds into the top of the second vessel, where another separation occurs since the velocity is lower than that in the first vessel. Each succeeding vessel will separate a different range of particles depending upon settling velocities of the particles and upon the fluid velocity in the vessel. In each vessel, the velocity of the upflowing fluid is independently controlled to give the desired size range of product from that vessel.

Elutriator

Elutriator is a vertical tube through which a fluid es upwardly at a specific velocity while a solid mixture, whose separation is desired, is fed into the top of the column. Mechanism The large particle, which settle at a velocity higher than that of the rising fluid, are collected at the bottom of the column, and the smaller particles are carried out of the top of the column with the fluid. Several columns of different diameter in series may be used to bring about a further separation.

Double Cone Classifier

This classifier uses hydraulic water for classification. It consists of a conical vessel inside of which is a second hollow cone. The inner cone is slightly larger in angle, arranged apex downwards and is movable in vertical direction. The bottom portion cone is cut away and its position (height) relative to the outer cone is regulated by a screw adjustment.

Mechanism The feed to be separated is fed in the form of a suspension to the center of the inner cone. It flows downward through the inner cone and out at a baffle at the bottom of the inner cone. Hydraulic water is fed near the outlet for the coarse material. The solids form the inner cone and a rising stream of water are mixed below the inner cone and then flow through an annulus space between the two cones. Classification action occurs in the annular space, the small/fine particles are carried away in the overflow while the large particles/ coarse particles settle against the hydraulic water to the bottom and are removed periodically Rake Classifier

This classifier consists of a rectangular tank with a sloping/inclined bottom. The tank is provided with movable rakes (reciprocating rakes). Mechanism The feed in the form of a suspension/slurry is introduced continuously near the middle of the tank. The lower end of the tank has a weir overflow liquid. The heavy material (coarser particles) sink to the bottom of the tank. The rake scrap the settled solids upward along the bottom of the tank toward the top of the tank for discharge from a sand-discharge chute. The reciprocating rakes keep the slurry in continuous agitation. The time of raking stroke is so adjusted that fines do not have time to settle and so remain near the surface of the slurry, while the heavy particles have time to settle (they settle, scrapped upward and removed as a dense slurry)

Pneumatic Classifier

It has almost same appearance of a cone classifier, but operates on a different principle. The dry solid feeds through a hollow shaft and falls upon the rotating feeder plates which distributes the particles across the cylindrical classification section. The circulating fan causes air to circulate upward and outward through the inner cylinder, carrying the fines into the outer cone where they settle out and are removed from the bottom. The circulating air returns through slots in the inner cylindrical section. The oversize material settles down through the circulating air into the bottom of the inner cone and is separately removed. Bowl Classifier

It consists of a shallow cylinder with a gentle conical bottom. The feed slurry is fed at the center near the surface, and the liquid flows outward in a radial direction. The solids settle out according to their particle size with the fines overflowing at the bowl wall. The heavy sludge is scraped towards the center for discharge.

Spiral Vane Classifier

It is a device consisting of a cylindrical tank, a central shaft, and spiral or radial arms which sweep the bottom. Feed is pumped in as a slurry. The solids settle and are swept to the center and removed continuously. The fines overflow into a launder.