Classifiers
Classifiers – Kinds Gravity Settling Settling tank, Elutriator, Spitzkasten, Double cone classifier, Hydraulic Jig, Riffled table Centrifugal separators Hydro‐cyclones y y Electrostatic separators Floatation Floatation
Gravity Settling Tank Tank‐ large volume – medium (H ( 2O) low velocity ) Suspension containing particles – introduced into tank very fines carried away with the liquid small particles – travel farther from inlet before reaching bottom high Ut – collect near inlet If in solid form introduced down a chute, the position at which the particles are deposited will be determined by the rate at which they lose the horizontal component of their velocities
Surface velocity classifier Surface velocity classifier
Spitzkasten Commercial adaptation of surface velocity classifier Commercial adaptation of surface velocity classifier Wooden Pyramidal boxes / riveted steel sheet cones Conical vessels are arranged in the order of increasing C/s area for area for Amount of liquid to be handled includes all the water used for classifying in previous vessels partly because it is desired to reduce in stages the tl b it i d i d t d i t th surface velocity of the fluid flowing from one vessel to next Coarsest collected near feed point, finest at the exit of last compartment Generally for sizing operations for dp < 4 mesh particles i.e., 5 mm works with high concentrations : 2‐15 kg/s Used popularly for ore dressing Limitations /Disadvantages Fractions not sharply separated Size of material collected in each unit depends on : rate of feeding of suspension upward velocity of the liquid in the vessel diameter of vessel
Relatively large quantity of water required Fractions withdrawn from spigots contain large amount of water to prevent plugging
Free Settling Classifier separating particles based on their size shape and separating particles based on their size, shape and density, using a stream of gas or liquid flowing in a direction usually opposite to the direction of y pp sedimentation Fluid at controlled velocity fed from bottom Smaller or lighter particles rise to the top (overflow) because their terminal falling velocities are lower than the velocity of the rising fluid. Solids with higher Ut settle down against the upward current Relatively smaller solids (lower Ut) rise up along with the current Series of such elutriators with relatively larger C/s can be used to separate the solids into multiple fractions Predominately used for particles with size (>1μm)
Elutriation or Sizing by Free settling Elutriation or Sizing by Free settling
Double cone classifier Double cone classifier Fixed external cone‐with source of fluid (hydraulic water), spigot at tip, peripheral overflow launder around open end Concentric inner truncated cone –supported so that the height of this cone relative to outer cone – adjusted by a hand wheel Feed flows down‐within inner cone‐out of the opening against a baffle
Analysing column
Hydraulic or other fluid and feed mixed @ vicinity of baffle and rise in space between two cones Adjustment of height of inner cone and control of hydraulic water – determines fluid velocity in space between cones (i.e. analyzing column) Capacity of double‐cone classifier = 1.8 A*Y**s A – Minimum C/s area of annular space (in Sq. ft.) Y‐ Volume concentration, i.e., vol. of solid/ vol. of total ‐Velocity in the analyzing column (fps) s – Density of solids (lb/cu. Ft.)
Gayco Pneumatic Classifier
Dry solid feed enters through hollow shaft‐falls upon rotating feeder plate Circulating fan causes air to circulate upward and outward through the inner cylinder – carrying fines into outer cone Circulating air returns through slots in the inner cylindrical Circulating air returns through slots in the inner cylindrical section Oversize settles down through the circulating air into bottom of inner cone‐separately removed Fines settle out through outer cone –removed separately at bottom For 100 – 400 mesh particles
http://sturtevantinc.com/demo/Whirlwind.html
Tabling
Shaking table ‐ Panning of gold – gold miner‐ to separate more dense gold from less dense rock/gangue dense rock/gangue Separation of two materials of different densities by passing dilute pulp over a table or deck d k Inclination – 2 to 5 degrees from horizontal Reciprocating horizontal motion or shake – Reciprocating horizontal motion or shake 150 to 37 per minute – slow forward motion and a fast return‐ causes material to move forward Surface of table – covered with canvas, linoleum, cement‐provided with grooves or cleats or form riffles‐parallel to direction of p table motion or at an angle Cleats – 0.25”‐0.5” wide and (1/16)” to (1/8)” hi h (1/8)” high, spaced 0.5 to 2” apart d 0 5 t 2” t Net movement of particles – resultant of two forces applied to them‐ pp tilt of table and velocity and thickness of water flowing over it
Tabling
Jigging
Special form of hindered settling Stratification of particles into layers– giving opportunity to settle or fall for short periods of time and then removing the stratified layers i.e., separation based on initial falling velocities of particles When resistance force to friction effects are not developed yet, i.e., drag force ~0 Acceleration of particle ‘a’ = (1‐(/ s))g Initial velocity depends on (i) gravity, (ii) density Initial velocity depends on (i) gravity (ii) density of particle and of the fluid Independent of size and shape of particle Thus sorting of two materials based on density Thus sorting of two materials based on density Relative acceleration of two materials aA/aB = {(A‐ ) B)/ (B‐ ) A}
Fixed screen diaphragm jig
Jigging‐Basis
Curve 1:slate particle of same shape and size as coal particle in curve 2 Curve 2: large coal particle Curve 3: medium slate particle Curve 4: smallest slate particle Curve 5: small coal particle distance a particle falls in time ‘t’ is the integral p g of (vdt) within limits of 0 to t, i.e., the area under curve If the time of sorting or settling allowed to extend If the time of sorting or settling allowed to extend to t2 – area under curves 2 qnd 4 are nearly equal‐impossible to separate the two particles‐ equal jigging i.e., they settle equal distances in equal jigging – i e they settle equal distances in the allowed settling period
Jigging‐ based on the initial velocity of the falling particle
Electrostatic classification Rolls
Pinpoint electrode Based on difference of behaviour of particles in electric field (i) Electric charge on particle, (ii) difference between dielectric constants of particle and surrounding medium Conductive particle coming in contact with charged surface or screen rolls sometimes replaced by inclined chutes
Conductance separators A‐ Initial Material B‐less conductive C‐more conductive
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