Mineral processing

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A set of Cornish stamps

Before the advent of heavy machinery the raw ore was broken up using hammers wielded by hand, a process called "spalling". Before long, mechanical means were found to achieve this. For instance, stamp mills were used in Samarkand as early as 973. They were also in use in medieval Persia. By the 11th century, stamp mills were in widespread use throughout the medieval Islamic world, from Islamic Spain and North Africa in the west to Central Asia in the east.[1] A later example was the Cornish stamps, consisting of a series of iron hammers mounted in a vertical frame, raised by cams on the shaft of a waterwheel and falling onto the ore under gravity.

The simplest method of separating ore from gangue consists of picking out the individual crystals of each. This is a very tedious process, particularly when the individual particles are small. Another comparatively simple method relies on the various minerals having different densities, causing them to collect in different places: metallic minerals (being heavier) will drop out of suspension more quickly than lighter ones, which will be carried further by a stream of water. The process of panning and sifting for gold uses both of these methods. Various devices known as 'buddles' were used to take advantage of this property.[when?] Later, more advanced machines were used such as the Frue vanner, invented in 1874.

Other equipment used historically includes the hutch, a trough used with some ore-dressing machines and the keeve or kieve, a large tub used for differential settlement.

Unit operations[edit]

Mineral processing can involve four general types of unit operation: comminution – particle size reduction; sizing – separation of particle sizes by screening or classification; concentration by taking advantage of physical and surface chemical properties; and dewatering – solid/liquid separation. In all of these processes, the most important considerations are the economics of the processes and this is dictated by the grade and recovery of the final product. To do this, the mineralogy of the ore needs to be considered as this dictates the amount of liberation required and the processes that can occur. The smaller the particles processes, the greater the theoretical grade and recovery of the final product, but this however is difficult to do with fine particles as they prevent certain concentration processes from occurring.


Comminution is particle size reduction of materials. Comminution may be carried out on either dry materials or slurries. Crushing and grinding are the two primary comminution processes. Crushing is normally carried out on "run-of-mine"[2] ore, while grinding (normally carried out after crushing) may be conducted on dry or slurried material. In comminution, the size reduction of particles is done by three types of forces-compression, impact and attrition. Compression and impact forces are extensively used in crushing operations while attrition is the dominant force in grinding. The primarily used equipments in crushing are-jaw crushers, gyratory crushers and cone crushers whereas rod mills and ball mills, closed circuited with a classifier unit, are generally employed for grinding purposes in a mineral processing plant. Crushing is a dry process whereas grinding is generally performed wet and hence is more energy intensive.


Sizer 2000 for screening coarse to small particles


The simplest sizing process is screening, or passing the particles to be sized through a screen or number of screens. Screening equipment can include grizzlies,[3] bar screens,wedge wire screens, radial sieves, banna screens, multi-deck screens, vibratory screen, fine screens, flip flop screens and wire mesh screens. Screens can be static (typically the case for very coarse material), or they can incorporate mechanisms to shake or vibrate the screen. Some considerations in this process includes the screen material, the aperture size, shape and orientation, the amount of near sized particles, the addition of water, the amplitude and frequency of the vibrations, the angle of inclination, the presence of harmful materials, like steel and wood, and the size distribution of the particles.

Classification refers to sizing operations that exploit the differences in settling velocities exhibited by particles of different size. Classification equipment may include ore sorters, gas cyclones, hydrocyclones, rotating trommels, rake classifiers or fluidized classifiers.

An important factor in both comminution and sizing operations is the determination of the particle size distribution of the materials being processed, commonly referred to as particle size analysis. Many techniques for analyzing particle size are used, and the techniques include both off-line analyses which require that a sample of the material be taken for analysis and on-line techniques that allow for analysis of the material as it flows through the process.



Gravity concentration[edit]

Gravity separation is the separation of two or more minerals of different specific gravity by their relative movement in response to the force of gravity and one or more other forces (such as centrifugal forces, magnetic forces), one of which is resistance to motion (drag force) by a viscous medium such as heavy media or water.


It is necessary to determine the suitability of a gravity concentration process before it is employed for concentration of an ore. A criteria called as concentration criterion is commonly used for this purpose. It is defined as-

Concentration Criterion (CC)= (SG of heavy mineral-SG of fluid)÷(SG of light mineral-SG of fluid), where SG=specific gravity

  • for CC < 1.25, not suitable for any size

There are several methods that make use of the density differences of particles-

  • Heavy media or dense media separation (these include, baths, drums, larcodems, dyana whirlpool separators, and dense medium cyclones)
  • Shaking tables, such as the Wilfley table[4]
  • Spiral separators
  • Reflux Classifier
  • Jig concentrators are continuous processing gravity concentration devices using a pulsating fluidized bed.(RMS-Ross Corp. Circular Jig Plants)
  • Centrifugal bowl concentrators, such as the Knelson concentrator and Falcon Concentrator
  • Multi gravity separators (Falcon Concentrator, Knelson, Mozley (Enhanced Gravity Separator), Salter Cyclones (Multi-Gravity Separator) and the Kelsey Jig)
  • Inline pressure Jigs
  • Reichert Cones
  • Sluices

These processes can be classified as either dense medium separation or gravity separation. The difference between the two that gravity separation does not use a dense medium to operate, only water or air. Dense medium separation can be performed with a variety of mediums. These include, organic liquids, aqueous solutions, suspensions in water and suspensions in air. Of these, most industrial processes use suspensions in water. The organic liquids are not used due to their toxicity and difficulties in handling. The aqueous solution as a dense medium is used in coal processing in the form of a belknap wash and the suspension in air is used in water-deficient areas, like china, where sand is used to separate coal from the gangue minerals. The dense medium separation is also classified as absolute gravity separation as the sinks and the floats travel in different directions. The gravity separation is also called relative gravity separation as they separate particles due to their differences in the magnitude of the particle response to a driving force.

These processes can also be classified into multi-G and single G processes. The difference is the magnitude of the driving force for the separation. Multi-G processes allow the separation of fine particles to occur and these particles can be in the range of 10 to 50 micron. The single G process are only capable of processing particles that are greater than 80 micron in diameter.

Of the gravity separation processes, the spiral concentrators and circular jigs are two of the most economical due to their simplicity and use of space. They operate by flowing film separation and can either use washwater or be washwater-less. The washwater spirals separate particles more easily but can have issues with entrainment of gangue with the concentrate produced..

Froth flotation cells used to concentrate copper and nickel sulfide minerals, Falconbridge, Ontario.

Froth flotation[edit]






This process was invented in the 19th century in Australia. It was used to recover a sphalerite concentrate from tailings, produced using gravity concentration. Further improvements have come from Australia in the form of the Jameson Cell, developed at the University of Newcastle, Australia. This operated by the use of a plunging jet that generates fine bubbles. These fine bubbles have a higher kinetic energy and as such they can be used for the flotation of fine grained minerals, such as those produced by the isamill.

Electrostatic separation[edit]




Magnetic separation[edit]



Automated Ore Sorting[edit]

Modern, automated sorting applies optical sensors (visible spectrum, near infrared, X-ray, ultraviolet), that can be coupled with electrical conductivity and magnetic susceptibility sensors, to control the mechanical separation of ore into two or more categories on an individual rock by rock basis. Also new sensors have been developed which exploit material properties such as electrical conductivity, magnetization, molecular structure and thermal conductivity. Sensor based sorting has found application in the processing of nickel, gold, copper, coal and diamonds.






Other processes[edit]



See also[edit]


  2. ^ Run-of-mine: The raw mined material as it is delivered prior to treatment of any sort. "Dictionary of Mining, Mineral, and Related Terms". Hacettepe University - Department of Mining Engineering. Retrieved 2010-08-07. 
  3. ^ Grizzly: a grid of iron bars that allows ore of the correct size to travel down the ore pass to the bottom of the mine, ready for hoisting to the surface. An active, articulating "grizzly" that is able to roll, scrub, clean and discharge oversize rock and boulder of up to 4 foot (1220 mm minus)diameter, while recovering all the 2 inch minus (51 mm minus) slurry material for further screening, separation and recovery of target metals/minerals is the DEROCKER system (RMS-Ross Corporation) "Geevor Tin Mine: Grizzly men". Geevor Tin Mine Museum. Retrieved 2010-08-07. 
  4. ^ "Mill Machines: The Wilfley Table". Copper Country Explorer. Retrieved 2010-08-07. 
  • Dobby, G.S., and Finch, J.A., 1991, Column Flotation: A Selected Review, Part II, 4(7-11) 911-923
  • Finch, J.A., 1995, Column Flotation: A Selected Review-Part IV: Novel Flotation Devices, Minerals Engineering, 8(6), 587-602
  • Miettinen, T, Ralston, J., and Fornasiero, D., The Limits of Fine Particle Flotation, Minerals Engineering, 23, 420-437 (2010)
  • Nguyen, A.V., Ralston, J., Schulze, H.S., 1988, On modelling of bubble–particle attachment probability in flotation, Int. J. Min. Proc., 53(4) 225-249
  • Probstein, R. F. (2003) Physicochemical Hydrodynamics: An introduction, Hoboken, New Jersey, John Wiley & Sons, Inc., 141-142.
  • Ralston, J. Fornasiero, D., Hayes, R., 1999, Bubble Particle Attachment and Detachment in Flotation, Int. J. Min. Proc., 56(1-4) 133-164


  • Various articles in J. Day & R. F. Tylecote, Metals in the Industrial Revolution (Institute of Metals, London 1991).