Comminution mechanism

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  • I was unable to understand how varying the exponents imply proportionality wrt crack length for Bonds theory and for other theories
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  • there is a critical value for the crack length at any particular level of stress at which the increased stress level at the crack tip is sufficient to break the atomic bond at that point. Such rupture of the bond will increase the crack length, thus increasing the stress concentration and causing a rapid propagation of the crack through the matrix, thus causing fracture
  • Comminution mechanism

    1. 1. Comminution MechanismsBy: Reyhane MazahernasabFeb2013
    2. 2. Content Introduction Comminution theory Mechanics of particle fracture Comminution mechanisms2
    3. 3. Introduction• Why comminution?………..To create particles in a certain size andshape…….…To increase the surface area available fornext process………..To liberate valuable minerals held withinparticles[2]3
    4. 4. Introduction• Grinding and crushing usually account for more than 30to 50% of the total power used in the concentrationprocess, but this can rise as high as 70% for hard finelydispersed and intergrown ores.[1] but about 5 % of allelectricity generated is used in size reduction[2]4
    5. 5. Introduction• Where this lost energy is consumed ?1. Deforming the particle to its elastic limit2. Compacting particles after fracture3. Overcoming friction between particles4. Elastically deforming milling surfaces5. Deformation of fractured particlesThis energy is dissipated as heat.[4]5
    6. 6. Introduction• Aim:• The general goal of the project Mechanisms ofcomminution is to enhance understanding of particlebreakage, which shall lead to improved comminutionsystems and more efficient utilization of energy for sizereduction and mineral liberation.[3]6
    7. 7. Comminution Theory• In the crystalline lattice of minerals, these inter-atomic bonds areeffective only over small distances, and can be broken if extended bya tensile stress. [5]7
    8. 8. Comminution Theory• The relationship between energy and breakage may beexpressed in the equation:dE= -K.dx/dxn• Rittinger: the new surface area produced proportionalto the energy consumed [6]n=1 E=K(1/x2 – 1/x1)8
    9. 9. Comminution TheoryKick: the same relative reduction in volume is obtainedfor constant energy input per unit mass irrespective of theoriginal size.n=2 E=K.ln(x1/x2)Kicks law is reasonably accurate in the crushing rangeabove about 1 cm in diameter [6]9
    10. 10. Comminution Theory• Bond: the work input is proportional to the new cracktip length produced in particle breakage.n=1.5 E= 2K(1/√x2 -√x1)• Avilable in the range of conventional rod-mill and ball-mill grinding.[6]10
    11. 11. Comminution Theory[8]11
    12. 12. Mechanics of particle fractureFlaws are stress concentrators [9]• Even when rocks are uniformly loaded, the internal stresses are notevenly distributed[5]12
    13. 13. Mechanics of particle fracture• Griffith showed that materials fail by crack propagationwhen this is energetically feasible.[5]• For crack to propagate:Strain energy > surface energy createdRequires appropriate crack propagation mechanism[2]13
    14. 14. Mechanics of particle fractureFlaws are stress concentrators14
    15. 15. Mechanics of particle fractureVirtually no stress isrequired to bring aboutbond breakage, stress isrequired to provide theenergy necessary forcrack propagation andthe consequentproduction of newsurface. [7]15
    16. 16. Mechanics of particle fracture• It should be noted that although it is not necessary toprovide enough energy to strain all bonds to the point ofbreaking, more energy is required than that which is justsufficient to provide the free energy of the new surfaces.Because bonds away from the eventual fracture surfacesalso become strained, hence absorb energy.[7]16
    17. 17. Mechanics of particle fracture• Rumbf: for smaller particles having fewer flaws, the appliedstress at which fracture occurs is greater. Irrespective of the distribution and density of flaws, agreater stress is required to fracture a smaller particle:strain energy is proportional to volume so the amount ofenergy available at a given stress condition decreases asthe particle size decreases.[7]17
    18. 18. Mechanics of particle fracture• The manner in which a particle fractures depends on (i)the nature of the particle; and (ii) the manner in which thefracture force is applied.[13]• Grain boundary fracture: The fracture toughness forgrain boundary cracking is lower than that for randomplane intragranular cracking, because atoms are arrangedirregularly in the grain boundary region.[12]18AB, showing regions of coincidence and non-coincidencebetween atoms in the neighbouring grains
    19. 19. Mechanics of particle fracture19• Interfacial fracture: Cracking along these interfaces willoccur preferentially whenever they are present. Likesedimentary rocks and conglomerates.• Interphase fracture: interphase fracture is defined ascracking along the boundary between two differentcrystalline phases. [12]Bonding across the boundary between the differentphases is stronger than that for interfacial boundariesbut not as strong as that across grain boundaries in thepure, single-phase mineral. [12]
    20. 20. Comminution mechanisms• Shatter (impact):• This mechanism of fracture is induced by rapidapplication of compressive stress.• high speed 10 – 2000 m.s-1 [10]• A broad spectrum of product sizes is produced and thisprocess is unselective20
    21. 21. Comminution mechanisms• shattering process consists of a series of steps in which theparent particle is fractured and this is followed immediatelyby the sequential fracturing of successive generations ofdaughter fragments until all of the energy available forfracture is dissipated.• Examples: industrial autogenous, rod and ball mills. [11]21
    22. 22. Comminution mechanisms22
    23. 23. Comminution mechanisms• Cleavage: Strain is applied as compression stress• Occurs when the energy applied is just sufficient to loadcomparatively few regions of the particle to the fracturepoint and only a few particles result. [7]23
    24. 24. Comminution mechanisms• When the original solid has some preferred surfacesalong which fracture is likely to occur, cleavage results.• The size distribution of the product particles is relativelynarrow [11]• low speed 0,01 – 10 m.s-1• Examples: jaw crushers, toggle crushers. [10]24
    25. 25. Comminution mechanisms25
    26. 26. Comminution mechanisms• Attrition: Strain between two or more solidsurfaces as a result of shearing action[10]• Attrition occurs when the particle is large and the stressesare not large enough to cause fracture.26
    27. 27. Comminution mechanisms• parent particle hardly changes size but the attritionprocess generates a significant number of particles thatare much smaller than the parent size.• Examples: occurs in autogenous mills where largeparticles are present to act as media.[11], shearing actionbetween ring sieve and rotor in rotor beater mills, crossbeater mills, ultra-centrifugal mills, etc. [10]27
    28. 28. Comminution mechanisms28
    29. 29. Comminution mechanisms29
    30. 30. Conclusion• The manner in which the particle fractures depends on thenature of the particle and on the manner in which theforce on the particle is applied.• The greatest problem is that most of the energy input to acrushing or grinding machine is absorbed by the machine,and only a small fraction of the total energy is availablefor breaking the material.• With knowing fracture mechanism of a specific ore wecan choose comminution machine correctly and also wecan design machines with higher efficiency.30
    31. 31. References• [1] Progress in mineral processing technology, Halim Demirel and SalihErsayin, Hacettepe university,Ankara, 1994• [2] an E-book chapter 10• [3] http://www.ltu.se/centres/camm• [4] http://www.chemeng.ed.ac.uk• [5] Mineral Processing Technology, Recovery, by Barry A. Wills, TimNapier-Munn., Elsevier Science & Technology Books, October 2006• [6] mineral crushing and grinding circuits, A.J. Lynch, Julius KruttshnittMineral Research Centre, department of mining and metallurgicalEngineering,university of Queensland, Australia, 1989• [7] introduction to mineral processing, Errol G. Kelly, David J Spottswood198931
    32. 32. References• [8] http://tresen.vscht.cz/kot/english/files/2012-03-particle-sizing-comminution• [9] www.scs.illinois.edu/~chem584/.../chem584.mechanicalfailure• [10] Size reduction within the context of sample preparation, Helmut Pitsch,Retsch Application Support• [11] Modeling and Simulation of Mineral Processing Systems, R.P. KingDepartment of Metallurgical Engineering University of Utah, USA, 2001• [12] Fracture toughness and surface energies of minerals: theoreticalestimates for oxides, sulphides, silicates and halides D. Tromans , J.A.Meech, September 2002• [13] Chemical Metallurgy, Chiranjib Kumar Gupta, 200332

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