SUMMARYThis review looks at the basic concepts of comminution, including principles anddifferent theories of comminution. It also tells the recent trends in thecomminution industry and provides links to different simulation packagesavailable, university and research centers and important books and otherreferences.INTRODUCTIONMost of the minerals are finely disseminated and intimately associated with thegangue so; they must be initially liberated before separation can be done.Comminution is the process in which the particle size of the ore is progressivelyreduced until the clean particles of mineral can be separated by such methodsas are available. Comminution in its earliest stages can also be carried out inorder to make the freshly excavated material easier to handle by scrapers,conveyors, and ore carriers, and in the case of quarry products to producematerial of controlled particle size.Blasting can be described as the first stage of comminution carried out in themine site in order to remove ores from their natural beds. Comminution in themineral processing plant takes place in a sequence of crushing and grindingprocesses. Crushing reduces the particle size of run-of-mine ore to such a levelthat grinding mill can further grind it until the mineral and gangue is substantiallyproduced as separate particles.Crushing is accomplished by compression of the ore against rigid surfaces, orby impact against surfaces in a constrained motion path. Grinding isaccomplished by abrasion and impact of the ore by the free motion ofunconnected media such as rods, balls, or pebbles.Crushing is usually a dry process, and is performed in several stages, reductionratios being small, ranging from three to six in each stage. The reduction ratio ofa crushing stage can be defined as the ratio of maximum particle size enteringto maximum particle size leaving the crusher, although other definitions aresometimes 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 concentrationprocess, although dry grinding has limited applications. There is an overlappingsize area where it is possible to crush or grind the ore. From a number of casestudies, it appears that at the fine end of crushing operations equivalentreduction can be achieved for roughly half the energy and costs required bygrinding mills (Flavel, 1978).Tumbling mills for size reduction with either steel rods (rod mills) or balls (ballmills), or sized ore (AG & SAG mills) as the grinding media used dependingupon the size and energy considerations.
Stirred mills represent the broad category of mills, which use a stirrer to providemotion to the steel, ceramic, or rock media. Both vertical and horizontalconfigurations exist, and since they can operate with smaller media sizes, theyare far more suitable for fine grinding applications than ball mills. Stirred millsare claimed to be more energy efficient (by up to 50%) than conventional ballmills (Stief et al., 1987).Principles of ComminutionThe increase in stress at a site is proportional to the square root of the cracklength perpendicular to the stress direction (Inglis, 1913). Therefore, there is acritical value for the crack length at any particular level of stress at which theincreased stress level at the crack tip is sufficient to break the atomic bond atthat point. Such rupture of the bond will increase the crack length, thusincreasing the stress concentration and causing a rapid propagation of thecrack through the matrix, thus causing fracture. When fracture does occur,some of the stored energy is transformed into free surface energy, which is thepotential energy of atoms at the newly produced surfaces.Mainly breakage achieved by crushing, impact, and attrition, and all threemodes of fracture (compressive a, tensile b, and shear c) can be discerneddepending on the rock mechanics and the type of loading.When an irregular particle is broken by compression, or crushing, the productsfall into two distinct size ranges coarse particles resulting from the inducedtensile failure, and fines from compressive failure near the points of loading, orby shear at projections. Minimizing the area of loading can reduce the amountof fines produced and this is often done in compressive crushing machines byusing corrugated crushing surfaces (Partridge, 1978). In impact breaking, due tothe rapid loading, a particle experiences a higher average stress whileundergoing strain than is necessary to achieve simple fracture, and tends tobreak apart rapidly, mainly by tensile failure. The products are often very similarin size and shape. Attrition (shear failure) produces much fine material, and maybe 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, contactingparticles thus increasing the degree of compressive stress and hence shearfailure.
Theory of ComminutionComminution theory is concerned with the relationship between energy inputand the particle size made from a given feed size. The greatest problem is thatthe machine itself absorbs most of the energy input to a crushing or grindingmachine, and only a small fraction of the total energy is available for breakingthe material. It is to be expected that there is a relationship between the energyrequired breaking the material and the new surface produced in the process,but this relationship can only be explained if the energy consumed in creatingnew surface can be separately measured. Another factor is that a material,which is plastic, will consume energy in changing shape, a shape that it willretain without creating significant new surface. All the theories of comminutionassume that the material is brittle, so that no energy is adsorbed in processessuch as elongation or contraction, which is not finally utilized in breakage.The oldest theory is that of Von Rittinger (1867), which states that the energyconsumed in the size reduction is proportional to the area of new surfaceproduced.Where E is the energy input, D1 is the initial particle size, D2 is the final particlesize, and K is a constant.Kick (1885) stated that the work required is proportional to the reduction involume of the particles concerned. Where f is the diameter of the feed particlesand p the diameter of the product particles, the reduction ratio R is f/p.According to Kicks law, the energy required for comminution is proportional tolog R/log 2.Bond (1952) developed an equation which is based on the theory that the workinput is proportional to the new crack tip length produced in particle breakage,and equals the work represented by the product minus that represented by thefeed. In particles of similar shape, the surface area of unit volume of material isinversely proportional to the diameter. The crack length in unit volume isconsidered to be proportional to one side of that area and therefore inverselyproportional to the square root of the diameter.For practical calculations the size in microns which 80% passes is selected asthe criterion of particle size. The diameter in microns which 80% of the productpasses is designated as P, the size which 80% of the feed passes is designatedas F, and the work input in kilowatt hours per short ton is W. Bonds third theoryequation 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 materialfrom theoretically infinite feed size to 80% passing 100 microns.Hukki (1975) suggests that the relationship between energy and particle size isa composite form of the three laws. The probability of breakage in comminutionis high for large particles, and rapidly diminishes for fine sizes. On the basis ofHukkis evaluation, Morrell (2004) has proposed a modification to Bonds 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 ofthe new energy-size relation has been shown to be valid across the size rangecovered by most modern grinding circuits, i.e. 0.1-100 mm.GrindabilityOre grindability refers to the ease with which materials can be comminuted, anddata from grindability tests are used to evaluate crushing and grindingefficiency. Most widely used parameter to measure ore grindability is the Bondwork index Wi that is described in previous paragraph. Berry and Bruce (1966)developed a comparative method of determining the grindability of an ore. Themethod requires the use of a reference ore of known grindability. The referenceore is ground for certain time and the power consumption recorded. An identicalweight of the test ore is then ground for a length of time such that the powerconsumed is identical with that of the reference ore. If r is the reference ore andt the ore under test, then from Bonds Equation,While Bond is the best-known grindability test for rod and ball mills, in recentyears the SPI (SAG Power Index) test has become popular for SAG mills. TheSPI test is a batch test, conducted in a 30.5 cm diameter by 10.2 cm longgrinding mill charged with 5kg of steel balls. Two kilograms of sample arecrushed 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 to80% minus 1.7 mm. The time required to reach a P80 of 1.7 mm is thenconverted 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 ores breakageresponse to SAG abrasion events.Recent Developments in ComminutionA relatively new comminution device, the high pressure grinding rolls (HPGR)adapted in cement industry and got application in diamond crushing, utilizescompression breakage of a particle bed, in which energy efficient inter-particlebreakage occurs (Schrnert, 1988). The reduction ratio obtained in a single passthrough the HPGR is substantially higher than that obtained in conventional rollscrushers. Some evidence has also been reported for downstream benefits suchas reduced grinding strength and improved leachability due to micro-cracking(Knecht, 1994). The HPGR offers a realistic potential to markedly reduce thecomminution energy requirements needed by tumbling mills. Reports havesuggested the HPGR to be between 20 and 50% more efficient thanconventional 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 Verticalmill, ALPINE ATR Mill, MaxxMill, KD Tower Mill, Vibration mills such asEccentric vibrating mill (ESM), VibroKinetic Energy (VKE) Mill, Centrifugal millssuch as ZRM centrifugal tube mill, Aachen centrifugal mill, Jet mills, Hicom millsetc. (Wang and Eric, 2003)Wang and Forssberg (2001) summarized the product size-specific energy inputrelations 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 forsize reduction and energy saving compared to the conventional ball mill.Modeling and Simulation SoftwareJKSimMet is a general-purpose computer software package for the analysis ofcomminution 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, dataand results, within one package. Mass balancing and model fitting of completecircuits are standard features. It is fully interactive and operates with high-resolution color graphics. These graphics facilitate the display of detailed plantflowsheets and accompanying information.Metso Minerals Co. has developed a crushing plant simulator named BRUNO.This simulator has been utilized to facilitate the comminution equipmentselection process. The program was a DOS based mass balance program thatkept 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. ThisPlantDesigner� software is a PC program in Windows 98/ME or Windows2000/XP for design of flowsheets, simulation of processes and calculation of
mass balances for crushing and screening plants. The programming languageis a structured subset of C with object-oriented extensions.The USIM PAC simulator for design/optimization of mineral processing planthas been developed for 16 years by BRGM, France. The latest versionavailable is the USIM PAC 3.0, which incorporates the modern developments.This is a user-friendly steady-state simulator that allows processing engineersand researchers to model plant operations with available experimental data anddetermine optimal plant configuration that meets production targets.The University of Utah, USA has developed a simulator MODSIM. Thesimulator offers the versatility to the user to modify and adapt the models of theunit operations that are used. The underlying theme for the models that areused in the MODSIM is the population balance method. In addition to theapplication in engineering, the MODSIM has been used as an academic tool toenhance the educational experience of students of mineral processing.DEM Solutions software is used to simulate, analyze, and visualize particleflows so that you can get a feel for what is happening in a situation involvingparticle kinematics, momentum, and heat and mass transfer.The Australian CSIRO reports on the use of the Discrete Element Method toresearch the functioning of mills. They have used a discrete element method tosimulate the charge motion in a centrifugal mill with various loadings.SGS Group software CEET® is an internet-based software tool that enablesaccurate design and forecasting of plant throughputs, operating costs and grindquality. Capital investment and production planning decisions can thus bebased on well-defined representative data sets representing the resourcemodel.Books & MagazinesThe 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 mostuseful source for getting the recent developments in this field. The proceedings includes:IMPC proceedingsSAG conferenceComminution conference