Classification © Classification is a method of separating solid particles into fractions based upondaccording to their terminal falling/settling velocities. © Suppose, for example, that the solid particles to be separated are fed in suspension into a tank (containing water) of large cross-sectional area. When the feed stream enters the tank, the horizontal velocity component decreases and the particles start to settle, The faster-settling particles will reach the bottom of the tank before the slower-settling particles. Thus, the faster-settling particles will accumulate near the inleVentrance, while the slower-settling particles which are carried farther (because of relatively slow-settling rates) and will concentrate nearer the exit/outlet, © Another way to separate the particles would be to place two particles having different settling velocities (rates) in a rising stream of water. If the velocity of the water is so adjusted that it lies between the terminal settling velocities of the two particles then the slower-settling particle will be carried upward by the water and the faster-settling particle will simultaneously move downward against the water stream and settle out to the bottom, thus achieving a separation, © A device that separates the solids into two fractions is called a classifier. The product streams that are obtained from any classifier are : (i) a partially drained fraction containing the coarse material, called the sand and (ii) a fine fraction together with the remaining liquid medium, called the overflow. © In the classification operation, the coarse solids that are settled at the bottom of a pool of fluid pulp are removed by gravity, mechanical means or induced pressure, while the solids which do not settle are taken out as an overflow from the pool © All wet classifiers work upon the difference in rate of settling/settling rate between coarse and fine particles. The settling rate of a particle depends upon its size and density and the particle will settle under the conditions of free settling or hindered settling depending upon the concentration of solids. © When the particle is at sufficient distance from the vessel walls and from other particles, so that its fall is not affected by them, the process is called free settling. In practice, the concentrations of suspensions (high concentrations of solids to liquid) used in the industry is usually high so that the particles are very close together and thus the collision between the particles is practically continuous. © When the motion of the particle is impeded or affected by other particles (as they being very close to each other), the process is called hindered settling. Gravity Settling Tank * It is the simplest type of classifier. It consists of a large tank with provisions for a suitable inlet and outlet. * A slurry feed enters the tank through an inlet connection. As soon as the slurry feed enters the tank, its linear velocity decreases 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 inleVentrance, 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 the occurrence of considerable overlapping of size, no sharp separation is possible with this classifier, Fluid in, Fluid out wide range of Fee price sizes particies (fines) A B c A = Coarse particles B = Intermediate particles C = Small particles Fig. 4.1 : Gravity settling tank Cone Classifier * A cone classifier is simply a cone (conical vessel), installed point down, with a ischarge launder around the top (of the cone). ‘* The feed is introduced in the form of a suspension through a fed inlet provided at the centre at the top. The coarse fraction (the partially drained fraction containing the coarse material) collects at the point of the cone (ie., at the apex) and is withdrawn, periodically or continuously. The fine fraction along with the remaining portion of the liquid is removed from the launder as an overflow. The separation achieved with this unit is only an approximate one. Cone cla They are used in ore-dressing plants sifiers are used for relatively crude work because of low cost of installation. Foot Overflow product (fine fraction) i Fig. 4.2: Cone classifier Coarse materialDouble-cone Classifier This classifier uses hydraulic water for classification (a stream of additional water supplied to a classifier is called hydraulic water). ‘The double-cone clas is shown in Fig. 4.3. It consists of a conical vessel incorporating a second hollow cone in it. The inner cone is slightly larger in angle, arranged apex downwards and is movable in a vertical direction. The bottom portion of the inner cone is cut away and its position (height) relative to the outer cone is regulated by a screw adjustment (not shown). The feed to be separated is fed in the form of a suspension to the centre 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 from the inner cone and a rising stream of water are mixed below the inner cone. Then they flow through an annular space between the two cones. Classification occurs in the annular space, the small/fine particles are carried away in the overflow, whereas the large particles/coarse particles settle against the hydraulic water to the bottom and are removed periodically. Fees ‘water and fine:solid overiow (overflow fines) Movableladjustable cone Fixed cone Coarse sallds/panicles Fig, 4.3: Double-cone classifier MECHANICAL CLASSIFIERS Rake Classifier ‘The rake classifier such as the Dorr classifier consists of a rec ingular tank with a sloping/inclined bottom, The tank is provided with movable rakes (reciprocating rakes), The feed in the form of suspension (slurry) is introduced continuously near the middle of the tank. The lower end of the tank has a weir overflow (discharge weir) from which the fines that are not settled leave with the overflow liquid,Feed Coarse solid (sand product) Overflow product (fine solid + quid) “~ Coarse material Fig. 4.4 : Rake classifier ©The heavy material (coarser particles) sink to the bottom of the tank. The rakes serap the settled solids and move them upwards along the bottom of the tank towards the top/upper end of the tank from where they are discharged. The reciprocating rakes keep the slury 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 (called the sand)]. Spiral Classifier * Iris a mechanical classifier. The spiral classifier such as the Akins classifier consists of a semicylindrical trough (a trough which is semicircular in cross-section) inclined to the horizontal, The trough is provided with a slow-rotating spiral conveyor and a liquid overflow at the lower end. The spiral conveyor moves the solids which settle to the bottom upward towards the top of the trough. © Slurry is fed continuously near the middle of the trough. The slurry feed rate is so adjusted that fines do not have time to settle and are carried out with the overflow liquid. Heavy particles have time to settle, they settle to the bottom of the trough and the spiral conveyor moves the settled solids upward along the floor of the trough towards the top of the trough from where they are discharged. ‘© Rake and spiral classifiers are used along with ball mills in closed-circuit grinding. Feed Overiiow coarse solids product Fig. 4.5 : Spiral classifier