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Comminution 

 

Summary

This review looks at the basic concepts of comminution, including principles and different theories of comminution. It also tells the recent trends in the comminution industry and provides links to different simulation packages available, university and research centers and important books and other references.

Introduction

Most of the minerals are finely disseminated and intimately associated with the gangue so; they must be initially liberated before separation can be done. Comminution is the process in which the particle size of the ore is progressively reduced until the clean particles of mineral can be separated by such methods as are available. Comminution in its earliest stages can also be carried out in order to make the freshly excavated material easier to handle by scrapers, conveyors, and ore carriers, and in the case of quarry products to produce material of controlled particle size.

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. Comminution in the mineral processing plant takes place in a sequence of crushing and grinding processes. Crushing reduces the particle size of run-of-mine ore to such a level that grinding mill can further grind it until the mineral and gangue is substantially produced as separate particles.

Crushing is accomplished by compression of the ore against rigid surfaces, or by impact against surfaces in a constrained motion path. Grinding is accomplished by abrasion and impact of the ore by the free motion of unconnected media such as rods, balls, or pebbles.

Crushing is usually a dry process, and is performed in several stages, reduction ratios being small, ranging from three to six in each stage. The reduction ratio of a crushing stage can be defined as the ratio of maximum particle size entering to maximum particle size leaving the crusher, although other definitions are sometimes used. There are a number of crushers available as jaw, gyratory, cone, roll, and impact crushers.

Grinding is usually performed wet to provide a slurry feed to the concentration process, although dry grinding has limited applications. There is an overlapping size area where it is possible to crush or grind the ore. From a number of case studies, it appears that at the fine end of crushing operations equivalent reduction can be achieved for roughly half the energy and costs required by grinding mills (Flavel, 1978).

Tumbling mills for size reduction with either steel rods (rod mills) or balls (ball mills), or sized ore (AG & SAG mills) as the grinding media used depending upon the size and energy considerations.

Stirred mills represent the broad category of mills, which use a stirrer to provide motion to the steel, ceramic, or rock media. Both vertical and horizontal configurations exist, and since they can operate with smaller media sizes, they are far more suitable for fine grinding applications than ball mills. Stirred mills are claimed to be more energy efficient (by up to 50%) than conventional ball mills (Stief et al., 1987).

Principles of Comminution

The increase in stress at a site is proportional to the square root of the crack length perpendicular to the stress direction (Inglis, 1913). Therefore, 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. When fracture does occur, some of the stored energy is transformed into free surface energy, which is the potential energy of atoms at the newly produced surfaces.

Mainly breakage achieved by crushing, impact, and attrition, and all three modes of fracture (compressive a, tensile b, and shear c) can be discerned depending on the rock mechanics and the type of loading.

When an irregular particle is broken by compression, or crushing, the products fall into two distinct size ranges coarse particles resulting from the induced tensile failure, and fines from compressive failure near the points of loading, or by shear at projections. Minimizing the area of loading can reduce the amount of fines produced and this is often done in compressive crushing machines by using corrugated crushing surfaces (Partridge, 1978). In impact breaking, due to the rapid loading, a particle experiences a higher average stress while undergoing strain than is necessary to achieve simple fracture, and tends to break apart rapidly, mainly by tensile failure. The products are often very similar in size and shape. Attrition (shear failure) produces much fine material, and may be undesirable depending on the comminution stage and industry sector. Attrition occurs mainly in practice due to particle- particle interaction (inter-particle comminution), which may occur if a crusher is fed too fast, contacting particles thus increasing the degree of compressive stress and hence shear failure.

Theory of Comminution

Comminution theory is concerned with the relationship between energy input and the particle size made from a given feed size. The greatest problem is that the machine itself absorbs most of the energy input to a crushing or grinding machine, and only a small fraction of the total energy is available for breaking the material. It is to be expected that there is a relationship between the energy required breaking the material and the new surface produced in the process, but this relationship can only be explained if the energy consumed in creating new surface can be separately measured. Another factor is that a material, which is plastic, will consume energy in changing shape, a shape that it will retain without creating significant new surface. All the theories of comminution assume that the material is brittle, so that no energy is adsorbed in processes such as elongation or contraction, which is not finally utilized in breakage.

The oldest theory is that of Von Rittinger (1867), which states that the energy consumed in the size reduction is proportional to the area of new surface produced.

Where E is the energy input, D1 is the initial particle size, D2 is the final particle size, and K is a constant.

Kick (1885) stated that the work required is proportional to the reduction in volume of the particles concerned. Where f is the diameter of the feed particles and p the diameter of the product particles, the reduction ratio R is f/p. According to Kick's law, the energy required for comminution is proportional to log R/log 2.

Bond (1952) 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. In particles of similar shape, the surface area of unit volume of material is inversely proportional to the diameter. The crack length in unit volume is considered to be proportional to one side of that area and therefore inversely proportional to the square root of the diameter.

For practical calculations the size in microns which 80% passes is selected as the criterion of particle size. The diameter in microns which 80% of the product passes is designated as P, the size which 80% of the feed passes is designated as F, and the work input in kilowatt hours per short ton is W. Bond's third theory equation is

Where Wi is the work index. The work index is the comminution parameter, which expresses the resistance of the material to crushing and grinding; numerically it is the kilowatt-hours per short ton required to reduce the material from theoretically infinite feed size to 80% passing 100 microns.

Hukki (1975) suggests that the relationship between energy and particle size is a composite form of the three laws. The probability of breakage in comminution is high for large particles, and rapidly diminishes for fine sizes. On the basis of Hukki's evaluation, Morrell (2004) has proposed a modification to Bond's

where Mi is the material index related to the breakage property of the ore and K is a constant chosen to balance the units of the equation. The application of the new energy-size relation has been shown to be valid across the size range covered by most modern grinding circuits, i.e. 0.1-100 mm.

Grindability

Ore grindability refers to the ease with which materials can be comminuted, and data from grindability tests are used to evaluate crushing and grinding efficiency. Most widely used parameter to measure ore grindability is the Bond work index Wi that is described in previous paragraph. Berry and Bruce (1966) developed a comparative method of determining the grindability of an ore. The method requires the use of a reference ore of known grindability. The reference ore is ground for certain time and the power consumption recorded. An identical weight of the test ore is then ground for a length of time such that the power consumed is identical with that of the reference ore. If r is the reference ore and t the ore under test, then from Bond's Equation,

While Bond is the best-known grindability test for rod and ball mills, in recent years the SPI (SAG Power Index) test has become popular for SAG mills. The SPI test is a batch test, conducted in a 30.5 cm diameter by 10.2 cm long grinding mill charged with 5kg of steel balls. Two kilograms of sample are crushed to 100% minus 1.9 cm and 80% minus 1.3 cm and placed in the mill. The test is run with several screening iterations until the sample is reduced to 80% minus 1.7 mm. The time required to reach a P80 of 1.7 mm is then converted to an SAG power index Wsag via the use of a proprietary transformation (Starkey and Dobby, 1996):

The parameters K and n are empirical factors whilst fsag incorporates a series of calculations (unpublished), which estimate the influence of factors such as pebble crusher recycle load, ball load, and feed size distribution. The test is essentially an indicator of an ore's breakage response to SAG abrasion events.

Recent Developments in Comminution

A relatively new comminution device, the high pressure grinding rolls (HPGR) adapted in cement industry and got application in diamond crushing, utilizes compression breakage of a particle bed, in which energy efficient inter-particle breakage occurs (Schrnert, 1988). The reduction ratio obtained in a single pass through the HPGR is substantially higher than that obtained in conventional rolls crushers. Some evidence has also been reported for downstream benefits such as reduced grinding strength and improved leachability due to micro-cracking (Knecht, 1994). The HPGR offers a realistic potential to markedly reduce the comminution energy requirements needed by tumbling mills. Reports have suggested the HPGR to be between 20 and 50% more efficient than conventional crushers and mills (Esna-Ashari and Kellerwessel, 1988).

In addition of HPGR, some other mills are also developed such as Poittemill, HPROMILL, Sala Agitated mill (SAM), IsaMill, ANI-Metprotech Stirred Vertical mill, ALPINE ATR Mill, MaxxMill, KD Tower Mill, Vibration mills such as Eccentric vibrating mill (ESM), VibroKinetic Energy (VKE) Mill, Centrifugal mills such as ZRM centrifugal tube mill, Aachen centrifugal mill, Jet mills, Hicom mills etc. (Wang and Eric, 2003)

Wang and Forssberg (2001) summarized the product size-specific energy input relations obtained by various recently developed mills such as MaxxMill, Drais mill, ESM, SAM and HPRM as well in the comminution of limestone. Clearly, these new or developed mills have shown a superior performance for size reduction and energy saving compared to the conventional ball mill.

Modeling and Simulation Software

JKSimMet is a general-purpose computer software package for the analysis of comminution and classification circuits in mineral processing operations. JKSimMet integrates all tasks associated with data analysis, optimization, design and simulation, including the storage and manipulation of models, data and results, within one package. Mass balancing and model fitting of complete circuits are standard features. It is fully interactive and operates with high-resolution color graphics. These graphics facilitate the display of detailed plant flowsheets and accompanying information.

Metso Minerals Co. has developed a crushing plant simulator named BRUNO. This simulator has been utilized to facilitate the comminution equipment selection process. The program was a DOS based mass balance program that kept track of the tonnage rate of each size fraction in various circuit flows.

Sandvik Rock Processing AB, Sweden introduced a simulator PlantDesigner for optimization and design of crushing and screening plant circuits. This PlantDesigner software is a PC program in Windows 98/ME or Windows 2000/XP for design of flowsheets, simulation of processes and calculation of mass balances for crushing and screening plants. The programming language is a structured subset of C with object-oriented extensions.

The USIM PAC simulator for design/optimization of mineral processing plant has been developed for 16 years by BRGM, France. The latest version available is the USIM PAC 3.0, which incorporates the modern developments. This is a user-friendly steady-state simulator that allows processing engineers and researchers to model plant operations with available experimental data and determine optimal plant configuration that meets production targets.

The University of Utah, USA has developed a simulator MODSIM. The simulator offers the versatility to the user to modify and adapt the models of the unit operations that are used. The underlying theme for the models that are used in the MODSIM is the population balance method. In addition to the application in engineering, the MODSIM has been used as an academic tool to enhance the educational experience of students of mineral processing.

DEM Solutions software is used to simulate, analyze, and visualize particle flows so that you can get a feel for what is happening in a situation involving particle kinematics, momentum, and heat and mass transfer.

The Australian CSIRO reports on the use of the Discrete Element Method to research the functioning of mills. They have used a discrete element method to simulate the charge motion in a centrifugal mill with various loadings.

SGS Group software CEET® is an internet-based software tool that enables accurate design and forecasting of plant throughputs, operating costs and grind quality. Capital investment and production planning decisions can thus be based on well-defined representative data sets representing the resource model.

Books & Magazines

The most important book for understanding the basic of the comminution is Mineral Processing Technology by B. A. Wills. The Other important book will be Advances in Comminution by S. K. Kawatra.

In addition of this different conferences on comminution, crushing and milling publish their proceedings that is the most useful source for getting the recent developments in this field. The proceedings includes:

University & Research

The Julius Kruttschnitt Mineral Research Center in Queensland, Australia, is one of the leaders in comminution research.

Other university and research center includes:

  • CSIRO Australia
  • Centre of Mineral Research at Cape Town
  • Mineral Research Centre Malaysia
  • Mineral Exploration Research Centre, Canada
  • CAMIRO, Canada
  • University of British Columbia, Canada
  • University of Utah, USA

There are many more universities and organizations involve in the development of advance comminution methods and simulation software.

References

Berry, T.F. and Bruce, R.M. (1966), A simple method of determining the grindability of ores, Can. Min. J. (Jul.), 63.

Bond, F.C. (1952), The third theory of comminution, Trans. AIMF, 193, 484.

Esna-Ashari, M. and Kellerwessel, H. (1988), Inter- particle crushing of gold ore improves leaching, Randol Gold Forum 1988, Scottsdale, USA, 141-146.

Flavel, M.D. (1978), Control of crushing circuits will reduce capital and operating costs, Min. Mag., Mar., 207.

Hukki, R.T. (1975), The principles of comminution: An analytical summary, Engng. Min. J., 176(May), 106.

Inglis, C.E. (1913), Stresses in a plate due to the presence of cracks and sharp comers, Proc. Inst. Nav. Arch.

Kick, F. (1885), Des Gesetz der Proportionalem wider-stand und Seine Anwendung, Felix, Leipzig.

Knecht, J. (1994), High-pressure grinding rolls - a tool to optimize treatment of refractory and oxide gold ores, Fifth Mill Operators Conf., AusIMM, Roxby Downs, Melbourne (Oct.), 51-59.

Partridge, A.C. (1978), Principles of comminution, Mine and Quarry, 7(Jul./Aug.), 70.

Schnert, K. (1988), A first survey of grinding with high-compression roller mills, Int. J. Min. Proc., 22, 401-412.

Starkey, J. and Dobby, G. (1996), Application of the minnovex SAG power index at five canadian SAG plants, SAG1989 Conf., Vancouver, Canada, 345-360.

Stief, D.E., Lawruk, W.A., and Wilson, L.J. (1987), Tower mill and its application to fine grinding, Min. Metall. Proc., 4(1), Feb., 45-50.

Von Rittinger, P.R. (1867), Lehrbuch der Aufbereitungs Kunde, Ernst and Korn, Berlin.

Wang, Y. M. and Forssberg (2003), International overview and outlook on comminution technology, Freningen Mineralteknisk Forskning / Swedish Mineral Processing Research Association

Wang, Y. M. and Forssberg, E. (2001), New milling technology, Technical report, MinFo, Stockholm.

Wills, B.A. (2006), Mineral Processing Technology, Elsevier Science & Technology Books, 110-117

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