Mineral Processing

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Mineral Processing Jaw crusher gyratory Crusher beneficiaton roll crusher screening separation classifier grinding crushing law dry grinding wet grinding Ned university My-203

Transcript of Mineral Processing

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2. 3 (Ore) 3. 4 WHAT CONSTITUTES MINERAL BENEFICIATION? Ore is an aggregate of minerals and contains valuable and gangue minerals . The mineral beneficiation involves separation of gangue minerals from ore. It has three steps Liberation , Separation, Concentration. 1) Liberation of valuable mineral by size reduction. 2) Separation of coarse and fine particles. 3) Concentration to separate the gangue minerals to increase the metal grade. If the first step is not done correctly, the second step will be incomplete. 4. LIBERATION BY SIZE REDUCTION. Consider a cube ( 10 cm dia )of the ore having MINERAL and GANGUE. Suppose it has cubic grains of 10 mm. Assumptions 1. crushing is conducted to yield grains of same size. 2. crystals in the ore are intimately joined with each other. Till the ore lump is crushed to 10 mm dia, all grains are locked. If the ore lump is crushed to particles of 5 mm dia, some grains are FREE and some are LOCKED. It cannot be assumed that all particles are free since they are of 5 mm dia ( smaller than 10 mm dia). Locked grains 5 =10 x10 mm 5. Even with heavy overgrinding, particles cannot be liberated completely. The crushed particles are obtained in the form of LOCKED MIDDLINGS. MINERALS OF EQUAL ABUNDANCE. The cube is sectioned along a vertical plane. Visualise that lattices with parameters 10mm and 5 mm are superimposed with their axes parallel. It appears that , in the plane of section, alternate rows A and C only have free looking particles. Other alternate rows B and D do not have them. The free particles are 1,3,5,13,15,17. ( They may be locked in other planes ). Free particles of mineral and gangue 6 out of 48 (1/8) ( and not 6 out of 24). Degree of liberation of each constituent- 3 out of 24 (1/8). 6 6. MINERALS OF UNEQUAL ABUNDANCE. The two constituents are not equally abundant. 1. The less abundant mineral is not free at all unless the particles are finer than the grain size. 2. To free the less abundant mineral, the particles must be made much finer than the grain size. 3. The more abundant mineral is always freer than the less abundant mineral. 7 bauxite LIBERATION BY DETACHMENT. If the ore lump is made of mineral grains bonded loosely, fracturing to the grain size results in complete liberation. Eg: Pebble phosphate rock MOSTLY LIBERATION NEEDS SIZE REDUCTION. 7. 8 A SIMPLIFIED FLOW CHART FOR ORE DRESSING 8. Comminution Communiation is one of the primary operations in mineral dressing ;takes place in a sequence of crushing and grinding processes. COMMINUTION - The whole operation of reducing the raw ore to the size required for mechanical separation or metallurgical processing. It is used to - create particles in a certain size and shape, - increase the surface area available for next process - liberate valuable minerals held within particles. Comminution Crushing Grinding When raw materials are processed in solid state, particle size reduction is essential. The ROM ore may be as big as 3 m in size. Minerals should be liberated before concentration. This liberation accomplished by comminution. 9. 11 OBJECTIVES OF COMMINUTION. Reduction of large lumps into smaller sizes. Production of solids of desired size ranges. Breaking apart valuable minerals from gangue ( liberation of valuables ). Convenience of handling the ore and its transportation. Preparation of feed material for different ore processing techniques ( Eg:- granular material for gravity separation, fine particles for froth flotation etc.) . Difference between crushing and grinding- Crushing ( dry) - Size reduction occurs preferentially on large fragments. Grinding (wet) - Size reduction is less selective- all pieces get ground to fine particles. 10. 12 S. No Process Size reduction 1. Explosive shattering Infinite size to 1 m 2. Primary crushing 1m 100 mm 3. Secondary crushing 100 mm- 10 mm 4. Coarse grinding 10 mm- 1 mm 5. Fine grinding 1 mm- 100 microns 5. Very fine grinding 100 microns 10 microns 6. Superfine grinding 10 microns- 1 micron STEPS IN COMMINUTION Size reduction/Comminution Process : extremely energy-intensive . 5 % of all electricity generated is used in size reduction . Efficiency of size reduction : 1 %. Blasting can be described as the first stage of comminution carried out in the mine site in order to remove ores from their natural beds. 11. 13 Energy utilisation in comminution = (Energy theoretically needed for a particular degree of size reduction ) ------------------------------------------------------------------------------------------------ ( Energy actually consumed ) Actual energy consumed = power input to the mill = energy needed to move the working parts with suitable load+ energy needed to overcome friction+ energy needed to grind away the metal from the working face+ energy needed for size reduction. A major part of the energy is lost as HEAT. ENERGY IN COMMINUTION If the particles of the ore has SINGLE MINERAL only FREE PARTICLES. If they have two or more minerals- LOCKED PARTICLES. Normally the ore consists of at least two minerals which are intimately interlocked. DEGREE OF LIBERATION of a certain mineral is the percentage of that mineral occurring as FREE PARTICLES w.r.t the total of the mineral occurring in the FREE and LOCKED forms. DEGREE OF LOCKING - of a mineral is the percentage occurring in LOCKED PARTICLES w.r.t the total occurring in the FREE and LOCKED forms. 12. 14 Very inefficient at creating new surface area (~1-2%) Surface area is equivalent to surface energy Comminution energy is 60-85 % of all energy used A number of energy "laws" have been developed Assumption - energy is a power function of D dE= differential energy required, dD= change in a particle dimension, D = magnitude of a length dimension, K = energy use/weight of material, and n = exponent ENERGY IN COMMINUTION n DK dD dE Rittingers, Kicks and Bonds theories are used to evaluate any crushing process. Rittingers law deals with measurements of surface areas Kicks law deals with volumes of products particles Bonds theory deals with lengths of cracks formed. Kick applies to coarse sizes (> 10 mm), Bond applies down to 100 m, Rittinger applies to sizes < 100 m . 13. 15 It is impossible to estimate accurately the amount of energy required to effect a size reduction of a given material. A number of empirical laws have been proposed. The two earliest laws are due to KICK and VON RITTINGER, and a third law due to BOND has also been proposed. Von Rittingers Law (1867) :- It states that the energy consumed in the size reduction is proportional to the area of new surface produced. E - the energy input, D1- the initial particle size, D2- the final particle size, K - a constant. Kr = Rittinger's Constant and fc = crushing strength of the material, f- feed, p-product. ) D 1 D 1 (fKE fp cr 14. 16 Kicks law (1885) States that the work required is proportional to the reduction in volume of the particles concerned. Energy required for producing a specified reduction is proportional to the log of the reduction ratio. It is based on the assumption that geometrically similar particles would always break in geometrically similar manner, irrespective of the size. p f eck D D logfKE Kk = Kick's Constant and fc = crushing strength of the material 15. 17 Bonds law (1952) Energy required is based on geometry of a crack expansion as it opens up. Bond has developed an equation which is based on the theory that the work input is proportional to the new crack tip length produced in particle breakage, and equals the work represented by the product minus that represented by the feed. The total work input (represented by a given weight of crushed or ground product ) is inversely proportional to the square root of the diameter of the product particles. Kb = Bond's Constant and fc = crushing strength of the material Fracture of the ore lump occurs by the extension of existing cracks. Such cracks lower the activation energy of fracture. ) D 1 D 1 (fKE fp cb 16. 18 LIMITATIONS OF COMMINUTION LAWS. The size and shape which can be determined by these methods is not very perfect. It is determined on the average method . The methods discussed above cant measure the accurate area of the particles. So results may not be accurate. It is very difficult to determine accurate area of the finer particles by these methods. There is no suitable method available for the determining the area of the fine particles. Even if the above conditions are determined, the surface areas of cracks present in big particles would remain unaccounted. If all the above conditions are considered then the area of the cracks which are present in the particles cant be calculated. It is the major drawbacks of the calculating the area of the particle surface. 17. 19 OPERATING VARIABLES Comminution of the ROM ore is affected by 5 variables. 1. Moisture content of solids. Moisture < 3-4 % by wt. no difficulties. Moisture content > 4wt%- Ores become sticky, clog the machines. Excess moisture ( > 50 wt%) Easy to feed and remove the product from the size reduction area. Size reduced minerals are easily transported as slurry. Grinding is normally done in wet condition ( Crushing is a dry process). 2. Reduction ratio ( RR ). Average diameter of the feed / average diameter of the product. In primary crushers , RR = Gape / Set. Gape- Size of the receiving opening ( max size acceptable in the machine). Set - Size of discharging opening ( max. size passing through the discharge end). Normal RR - 3-7 ( for coarse crushing ), - as high as 100 ( for fine grinding). Energy requirement increases with RR. 18. 20 3. Free crushing. Individual particles are crushed freely. The crushed product is removed from the crushing zone quickly. It avoids formation of excessive amounts of fines ( the number of contacts are less, hence less crushing). 4. Choke feeding. It is the reverse of Free crushing. The feed hopper of the