Presentation on theme: "TOPIC: CLASSIFICATION AND CLASSIFIERS Presented by P Aditya 2013UGMM061 K R RAJU 2013UGMM087 Shubham Goyal 2013UGMM083."— Presentation transcript:
TOPIC: CLASSIFICATION AND CLASSIFIERS Presented by P Aditya 2013UGMM061 K R RAJU 2013UGMM087 Shubham Goyal 2013UGMM083
TOPIC: CLASSIFICATION AND CLASSIFIERS CLASSIFICATION In classification, particles of various sizes, shapes and specific gravities are separated by being allowed to settle in a fluid. The coarser, heavier, and rounder grains settle faster than the finer, lighter, and more angular grains. Water or air is used as fluid, and the size at which a separation is made ranges currently from 20 to 300 Classification may be regarded as based principally on Stokes’s law of sedimentation.
Settling Velocity in a Classifier for fine particles settle at a 1. Velocity directly proportional to the square of their diameter 2. Velocity directly proportional to their apparent specific gravity. 3. Inversely proportional to the viscosity of the fluid. Correction factor F 1 =(1- γ 2/3 )(1- γ)(1-2.5 γ) Where γ is the fraction of the volume of the suspension occupied by the solid.
Shape correction Factor Another factor is required to take care of the irregularity in the shape of the particles. It can be expressed as the ratio of the size of equal-settling particles, one of which is irregular in shape, and the other spherical. F 2 = 1 for spheres F 2 = 1.19 for cubes F 2 = 1.28 for very thin disks Furthermore, the viscosity of the liquid and the acceleration of gravity are viewed as constituting a third factor F 3, the velocity (in centimeters per second) can be expressed as υ = F (Δ – Δ’) r 2 In which F= f 1.f 2.f 3
Desirable Classification Conditions Depends on the use that is to be made of the classifier products If the classifier products are to be subsequently concentrated by tabling, it is advantageous emphasize the effect of differences in specific gravity of the minerals. On the other hand, the classifier is operated merely as a sizing device to assist a grinding mill. Classifiers classes 1.Sorting classifiers using a relatively dense aqueous suspension as the fluid medium. 2.Sizing classifiers using a relatively dilute aqueous suspension as the fluid medium. 3.Sizing classifiers using air as the fluid medium.
Sorting Classifiers In sorting classifiers, the settling conditions are more or less hindered. The separation achieved is sorting. Example Evans Classifier, Anaconda Classifier and the Richards hindered- settling classifier. Hydrotator classifier, Richards pulsator classifier
Sizing Classifiers They do not require additional water besides that present in the suspension which is being treated. They utilize free-settling conditions to effect sizing as much as possible unaffected by specific gravity and shape. Sizing classifiers are sub-divided into settling cones which have no moving parts and mechanical classifiers. Settling cones are conical sheet-metal shells with the apex at the bottom and a peripheral overflow launder. In the Allen Cone the discharge is automatic and involves, of course, some moving parts. Allen Sand cone
Mechanical Classifiers These have gained importance with the increasing use of flotation as a dressing method. Esperanza Classifier consists of along sloping trough on the bottom of which the coarser and heavier grains settle and the finer and lighter grains overflow a terminal weir. Dorr classifier Dorr bowl classifier Dorr multizone classifier Akins Classifier Dorrco sand washer Dorr classifier Dorr Bowl classifier
Performance of classifiers Capacity of a classifiers is directly proportional to 1. The cross- sectional area of the sorting column, 2. The rising velocity of the fluid in the sorting column 3. The solid content in the classifier intake. C = aAυγΔ, Where C is the tons of solid per hour, A the cross-sectional area in sq. feet, υ the velocity in feet per minute, γ the percentage of solids by volume, Δ the specific gravity of the solids & a the constant (62.5*60)/2,000 =1.875
Efficiency of classifiers E =100c (f-t)/f(c-t) Where E is the efficiency as a percentage, C, f, and t are the content of minus x-mesh material in the overflow, feed and underflow. Cost The cost of classification is strikingly small, except when the size at which the classification is carried out is so fine that relatively large machines and extremely dilute pulps must be used. The cost for supplies, electric current, repairs and labor chargeable to classification are very small. Much of the cost is overhead and capital.
CLASSIFICATION AS A MEANS OF CONCENTRATION Separation of fine Valuable Mineral from Coarse Waste Deflocculation of the raw clay, classification into a sedimented residue and suspended washed clay, flocculation of the washed clay, thickening and filtration are usually all the dressing that is required. In this case, classification results in concentration. Free settling is used. Separation of coarse Valuable Mineral from Fine waste Opposite of the preceding separation Classifier sediment is the concentrate Classifier overflow is the tailing Where there is no difference in sp. Gravity between mineral and gangue Examples: Iron ore beneficiation, Phosphate rock beneficiation and gravel washing.
Scrubbing Classifiers There is usually some tenuous bonding of dissimilar grains requiring rupture, a more or less gentle mixing, turning over, or tumbling of the ore is desirable. It can be accomplished in a combination of mill without grinding medium and a classifier. Log washers usually consists of one or more logs carrying metal blades at the surface and placed at a slope of 1 to 1 1/2 inch per feet. The log rotates and thus conveys the ore lumps to the upper end of the trough while breaking them up and turning them over.
Importance of Rheolaveur Coal Washer. The process is hardly appreciated by those not specialising in coal washing. According to Berthelot, the amount treated in 1936 by this process, the world over, amounted to 115 million tons. Roughly then, 1/3rd of all the cleaned coal was cleaned by that process. Concentration by Launder stratification. A means of concentration is in the sluice box of gold mining fame and the rheolaveur coal washer. Both consists essentially of a trough in which a suspension of coarse solids is flowing.
Concentration by sizing and sorting. Attention has already been drawn to the fundamental difference between screen sizing and classifier sizing to with, that specific gravity of the solid is not involved in screen sizing but involved in classifier sizing. This difference can be made the bases for a method of concentration.
Asbestos Concentration. Pneumatic classification is utilized to separate the relatively fluffy asbestos fibre from the granular gangue associated with it. Centrifugal Classification: To save fine particles of heavy minerals in the days prior to the wide spread use of flotation, They were designed many so called centrifugal concentrators. The spread in specific gravity between mineral and gangue is large, centrifugal classifier can not be expected to do more than yield a partial enrichment in sediment.
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