Sizing Methods Screening Classification

Transcription

1 Sizing Methods There are two methods of industrial sizing. i. Screening ii. Classification Screening is generally carried out on relatively coarse material, as the efficiency decreases rapidly with fineness. Screening is generally limited to materials above about 250 microns in size, finer sizing normally being undertaken by classification. Classification: Classification is defined as a method of separating mixtures of mineral particles into two or more products according to their settling velocities in water, in air or in other fluids as given in below figure. Industrial classification may be carried out in different types of classifiers and these classifiers are; hydraulic classifiers, mechanical classifiers and cyclones. Basically they all work according to the principle that the particles are suspended in water which has a slight upward movement relative to the particles. Particles below a certain size and density are carried away with the water-flow, whereas the coarser and heavier particles will settle.

2 Size control Introduction Size control With size control we understand the process of separating solids into two or more products on basis of their size. This can be done dry or wet. As mentioned earlier neither crushers nor grinding mills are too precise in their size reduction job and a lot of size fractions are misplaced. By using optimum size control the result can be improved both regarding capacity, size and particle shape. Size control by duties To prevent undersize in the feed from blocking the next size reduction stage (scalping) Size control To prevent oversize from moving into the next size reduction or operation stage (circuit sizing) To prepare a sized product (product sizing) Size control by methods In mineral processing practices we have two methods dominating size control processes: Screening using a geometrical pattern for size control. Bars Wire Circle Square Rectangle Rectangle Classification using particle motion for size control. BASICS IN MINERAL PROCESSING 1

3 Size control Screens Performance of screens will fall back on three main parameters: Motion Inclination Screening media Screen motions Size control Circular motion Inclined Straight line throw Inclined Horizontal Elliptical motion Straight line motion Horizontal Screening by stratification By building up a material bed on a screen deck the material will stratify when the motion of the screen will reduce the internal friction in the material. This means that the finer particles can pass between the larger ones giving a sharp separation. Screening by free fall If we use the double inclination used for stratification (from up to degrees) we are in free fall, meaning that no particle layer can build up on the screen deck. The particles will now be sized directly via the screening media, giving a higher capacity, (or a more compact installation), but also less sharpness in separation. Optimal use when a large amount of fines shall be removed fastly. Stratification Separation 2 BASICS IN MINERAL PROCESSING

4 Size control Screen types There are many types of screens, but they can be reduced to the four types shown below. Of these types approx. 80 % used worldwide are of type single inclination, stratification screens. The other are of type double, triple or multiple inclination, where screening by stratification and free fall are combined for different applications. Single inclination Stratification screen Circular (15 deg.) Linear 0 5 (deg.) Still the leader in selective screening Data sheet, see 4:6 Double inclination Free fall Compact - high capacity paid for by lower selectivity Typical in circuit screening Data sheet, see 4:7 Size control Triple inclination Combine capacity and selectivity Typical control screen for advanced product fractions Data sheets see 4:8 Multiple inclination ( banana screen ) Effective Thin-layer screen Popular in coal and metallic mining BASICS IN MINERAL PROCESSING 3

5 Size control Selection of screening media Selection of the correct size and type of screen is important. Equally important is the selection of the screening media. This refers not only to a correct aperture related to the cut size, but also to the wear in operation of these screens. Below a short selection guide to screening media can be found. Rubber or polyurethane? Feed size Select >35 mm dry Rubber 60 sh Size control <0-50 mm wet <40 mm dry/moist Look out for: Because Absorbes impact Resistant to sliding abrasion Polyurethane Very good against sliding abrasion Accurate separation Rubber 40 sh (soft) Very flexible Prevents blinding Oil in rubber applications Hot water or acids in PU-applications What thickness? General rule for min. thickness Max feed size 4 = Panel thickness What happens if we go...? THINNER THICKER + + Capacity Accuracy Service life + Blinding/Pegging + Tendency N.B.: Thickness should not exceed required product size What type of panel Bolt down panels, pretensioned for easy installation and guaranteed screening performance. Tension mats with hooks fits all screens designed with cambered decks and tensioning rails. Wire mesh panels offer superior open area and are quickly available. Self supporting panels, for screens of open frame design for tough applications. 4 Modular systems provide flexibility in wear material/hole configuration combinations. BASICS IN MINERAL PROCESSING

6 What hole size? (Inclined deck) Size control General guideline for wire mesh: Required product size plus 5 10% General guideline for rubber panels: Required product size plus 25 30% General guideline for PU panels: Required product size plus 15 20% Size control What type of hole? The standard choice For improved service life (coarse screening) For improved capacity For improved accuracy and dewatering Particle size Mesh or Micron? mesh* micron mesh micron mesh micron 2½ ½ *Taylor serie (US) Mesh number = the number of wires per inch or the number of square apertures per inch micron BASICS IN MINERAL PROCESSING 5

7 Physical Concentration Methods 1. Separation dependent on optical and radioactive properties of minerals, i.e. hand pickling, optical sorting, radioactive sorting, etc. 2. Separation dependent on specific gravity (density) difference of minerals, i.e. heavy-media separation, gravity concentration by use of tables, jigs, cones, etc. 3. Separation utilizing the different surface properties (i.e. surface chemistry) of the minerals, i.e. froth flotation, etc. 4. Separation dependent on magnetic properties of the minerals, i.e. low and high, dry and wet magnetic separation, etc. 5. Separation dependent on electrical conductivity properties of the minerals, i.e. electrostatic separation, etc.

8 1- Gravity separation A- Gravity separation involves feeding grounded ore into a pulsating body of water, which serves to settle out the heavy material while floating away the light material. If the heavy material is what you wishes to keep, then it is taken from the bottom. If the light material is what you wishes to keep, then it is taken from the top.

9 B- Cyclone : The tangential inlet shape of the cyclone forces feed to travel in a rapid circular path. The circular motion of the slurry creates the centrifugal force necessary for particle settling.

10 2- Electrostatic separator An electrostatic beneficiator works because different minerals have different electrostatic affinities -- will absorb different amounts of charge depending upon their composition, and hence are deflected different amounts by an electric field. After grains are sieved by size, they are placed through a beneficiator. After a few passes through beneficiators, we have separated different minerals fairly well. (There's no change in physical or chemical identity; there's only separation of minerals.)

11 3- Magnetic separator Magnetic separation is most commonly used to separate natural magnetic iron ore (magnetite) from a variety of lessmagnetic or nonmagnetic material. Today, magnetic separation techniques are used to beneficiate over 90 percent of all domestic iron ore.

12 4- Floatation Froth flotation is a highly versatile method for physically separating particles based on differences in the ability of air bubbles to selectively adhere to specific mineral surfaces in a mineral/water slurry. The particles with attached air bubbles are then carried to the surface and removed, while the particles that remain completely wetted stay in the liquid phase. Froth flotation can be adapted to a broad range of mineral separations, as it is possible to use chemical treatments to selectively alter mineral surfaces so that they have the necessary properties for the separation. It is currently in use for many diverse applications, with a few examples being: separating sulfide minerals from silica gangue (and from other sulfide minerals); separating potassium chloride (sylvite) from sodium chloride (halite); separating coal from ash-forming minerals; removing silicate minerals from iron ores; separating phosphate minerals from silicates; and even nonmineral applications such as de-inking recycled newsprint. It is particularly useful for processing fine-grained ores that are not amenable to conventional gravity concentration.

13 Fig : The flotation system includes many interrelated components, and changes in one area will produce compensating effects in other areas. Flotation Reagents Collectors. These are used primarily to make solids hydrophobic and promote adhesion to air bubbles or oil droplets. Common examples are fatty acids, sulfonates and amines. Frothers. Frothers promote the formation of a metastable froth phase that facilitates the removal of particles carried by air bubbles to the top of the flotation cell. Examples of frothers are pine oil, long-chain alcohols.

14 Auxiliary Reagents. These reagents include depressants, which are used to prevent solids from becoming hydrophobic, and activators, which promote the adsorption of reagents onto selected solids. FLOTATION PRQCESS Flotation is a method of separating an ore species froth another, based on its hydrophobia surface characteristics either natural or induced when present as a suspension in water with air bubble. Due to the affinity of the desired mineral to adhere to air bubbles it is floated out ' of the ore slurry. Schematics of various sub processes controlling flotation system is given in fig.

15 Fig. : Various sub-processes controlling flotation system The successful industrial practice of flotation involves knowledge and optimisation of four important components of flotation process namely, 1- Mineraiogical characteristics of the ore (mineral association, liberation size, presence of slime particles and soluble species contributed by the ore). 2. Surface colloid and reagent chemistry which determines selectivity of separation (colllectors, frothers, activators, depressant, modifier, dispersants etc.)

16 3. Process engineering (feed preparation that is size reduction, cell design, control system etc.. 4. Operating parameters such as aeration rate, temperature, Eh/ ph, ionic strength and flotation circuit configuration. Overall separation efficiency in flotation is dependent on 1. Surface chemistry factors such as particle bubble attachment, mineral reagent interactions, reagent chemistry etc. These factors are related to equilibrium considerations contributing selectivity to separation. 2. Hydrodynamics factors which contribute to the kinetics of flotation such as agitation, air flow rate, dispersion and cell design etc. control recovery of minerals. Important physico-chemical variables in flotation are : (a) Rote of mineral/ water interface. (b) Surface charge on the minerals. (c) Effect of hydrocarbon chain length of the collector. (d) Effect of neutral molecules. (e) Rote of polar functional group of the collector. (f) Role of solution chemistry of the collector.

17 (g) Role of inorganic ions (activator and depressant). (h) Effect of temperature (i) Ore properties i.e. grade, minerology, degree of oxidation, liberation of minerals. Flotation cell