Comminution Mechanisms

By: Reyhane Mazahernasab

Feb2013

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ContentIntroduction

Comminution theory

Mechanics of particle fracture

Comminution mechanisms

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Introduction

• Why comminution?

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To create particles in a certain size and shape

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To increase the surface area available for next process

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To liberate valuable minerals held within particles[2]

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Introduction

• Grinding and crushing usually account for more than 30 to 50% of the total power used in the concentration process, but this can rise as high as 70% for hard finely dispersed and intergrown ores.[1] but about 5 % of all electricity generated is used in size reduction[2]

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Introduction

• Where this 'lost' energy is consumed ?

1. Deforming the particle to its elastic limit

2. Compacting particles after fracture

3. Overcoming friction between particles

4. Elastically deforming milling surfaces

5. Deformation of fractured particles

This energy is dissipated as heat.[4]

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Introduction• Aim:• The general goal of the project Mechanisms of

comminution is to enhance understanding of particle breakage, which shall lead to improved comminution systems and more efficient utilization of energy for size reduction and mineral liberation.[3]

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Comminution Theory• In the crystalline lattice of minerals, these inter-atomic bonds are

effective only over small distances, and can be broken if extended by a tensile stress. [5]

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Comminution Theory• The relationship between energy and breakage may be

expressed in the equation:

dE= -K.dx/dxn

• Rittinger: the new surface area produced proportional to the energy consumed [6]

n=1 E=K(1/x2 – 1/x1)

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Comminution TheoryKick: the same relative reduction in volume is obtained

for constant energy input per unit mass irrespective of the original size.

n=2 E=K.ln(x1/x2)

Kick's law is reasonably accurate in the crushing range above about 1 cm in diameter [6]

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Comminution Theory• Bond: the work input is proportional to the new crack

tip 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]

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Comminution Theory

[8]

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Mechanics of particle fracture Flaws are stress concentrators [9]• Even when rocks are uniformly loaded, the internal stresses are not

evenly distributed[5]

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Mechanics of particle fracture • Griffith showed that materials fail by crack propagation

when this is energetically feasible.[5]

• For crack to propagate:

Strain energy > surface energy created

Requires appropriate crack propagation mechanism[2]

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Mechanics of particle fracture Flaws are stress concentrators

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Mechanics of particle fracture Virtually no stress is required to bring about bond breakage, stress is required to provide the energy necessary for crack propagation and the consequent production of new surface. [7]

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Mechanics of particle fracture

• It should be noted that although it is not necessary to provide enough energy to strain all bonds to the point of breaking, more energy is required than that which is just sufficient to provide the free energy of the new surfaces. Because bonds away from the eventual fracture surfaces also become strained, hence absorb energy.[7]

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Mechanics of particle fracture

• Rumbf: for smaller particles having fewer flaws, the applied

stress at which fracture occurs is greater. Irrespective of the distribution and density of flaws, a

greater stress is required to fracture a smaller particle: strain energy is proportional to volume so the amount of energy available at a given stress condition decreases as the particle size decreases.[7]

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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 the fracture force is applied.[13]

• Grain boundary fracture: The fracture toughness for grain boundary cracking is lower than that for random plane intragranular cracking, because atoms are arranged irregularly in the grain boundary region.

[12]

AB, showing regions of coincidence and non-coincidence between atoms in the neighbouring grains

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Mechanics of particle fracture • Interfacial fracture: Cracking along these interfaces will

occur preferentially whenever they are present. Like sedimentary rocks and conglomerates.

• Interphase fracture: interphase fracture is defined as cracking along the boundary between two different crystalline phases. [12]

Bonding across the boundary between the different phases is stronger than that for interfacial boundaries but not as strong as that across grain boundaries in the pure, single-phase mineral. [12]

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Comminution mechanisms• Shatter (impact): • This mechanism of fracture is induced by rapid

application of compressive stress.• high speed 10 – 2000 m.s-1 [10]• A broad spectrum of product sizes is produced and this

process is unselective

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Comminution mechanisms

• shattering process consists of a series of steps in which the parent particle is fractured and this is followed immediately by the sequential fracturing of successive generations of daughter fragments until all of the energy available for fracture is dissipated.

• Examples: industrial autogenous, rod and ball mills. [11]

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Comminution mechanisms

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Comminution mechanisms

• Cleavage: Strain is applied as compression stress

• Occurs when the energy applied is just sufficient to load comparatively few regions of the particle to the fracture point and only a few particles result. [7]

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Comminution mechanisms

• When the original solid has some preferred surfaces along which fracture is likely to occur, cleavage results.

• The size distribution of the product particles is relatively narrow [11]

• low speed 0,01 – 10 m.s-1

• Examples: jaw crushers, toggle crushers. [10]

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Comminution mechanisms

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Comminution mechanisms

• Attrition: Strain between two or more solid surfaces as a result of shearing action[10]

• Attrition occurs when the particle is large and the stresses are not large enough to cause fracture.

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Comminution mechanisms• parent particle hardly changes size but the attrition

process generates a significant number of particles that are much smaller than the parent size.

• Examples: occurs in autogenous mills where large particles are present to act as media.[11], shearing action between ring sieve and rotor in rotor beater mills, cross beater mills, ultra-centrifugal mills, etc. [10]

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Comminution mechanisms

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Comminution mechanisms

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Conclusion• The manner in which the particle fractures depends on

the nature of the particle and on the manner in which the force on the particle is applied.

• The greatest problem is that most of the energy input to a crushing or grinding machine is absorbed by the machine, and only a small fraction of the total energy is available for breaking the material.

• With knowing fracture mechanism of a specific ore we can choose comminution machine correctly and also we can design machines with higher efficiency.

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References

• [1] Progress in mineral processing technology, Halim Demirel and Salih Ersayin, 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, Tim

Napier-Munn., Elsevier Science & Technology Books, October 2006• [6] mineral crushing and grinding circuits, A.J. Lynch, Julius Kruttshnitt

Mineral Research Centre, department of mining and metallurgical Engineering,university of Queensland, Australia, 1989

• [7] introduction to mineral processing, Errol G. Kelly, David J Spottswood 1989

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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. King

Department of Metallurgical Engineering University of Utah, USA, 2001• [12] Fracture toughness and surface energies of minerals: theoretical

estimates for oxides, sulphides, silicates and halides D. Tromans , J.A. Meech, September 2002

• [13] Chemical Metallurgy, Chiranjib Kumar Gupta, 2003