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Mineral processing
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Mineral processing is the process of separating commercially valuable minerals from their ores in the field of extractive metallurgy.[1] Depending on the processes used in each instance, it is often referred to as ore dressing or ore milling.
Beneficiation is any process that improves (benefits) the economic value of the ore by removing the gangue minerals, which results in a higher grade product (ore concentrate) and a waste stream (tailings). There are many different types of beneficiation, with each step furthering the concentration of the original ore. Key is the concept of recovery, the mass (or equivalently molar) fraction of the valuable mineral (or metal) extracted from the ore and carried across to the concentrate.
History
[edit]
Before the advent of heavy machinery, raw ore was broken up using hammers wielded by hand, a process called "spalling". Eventually, mechanical means were found to achieve this. For instance, stamp mills were being used in central Asia in the vicinity of Samarkand as early as 973. There is evidence the process was in use in Persia in the early medieval period. 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.[2] 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.
Iron beneficiation has been evident since as early as 800 BC in China with the use of bloomery.[3] A bloomery is the original form of smelting and allowed people to make fires hot enough to melt oxides into a liquid that separates from the iron. Although the bloomery was promptly phased out by the invention of the blast furnace, it was still heavily relied on in Africa and Europe until the early part of the second millennium. The blast furnace was the next step in smelting iron which produced pig iron.[4] The first blast furnaces in Europe appeared in the early 1200s around Sweden and Belgium, and not until the late 1400s in England. The pig iron poured from a blast furnace is high in carbon making it hard and brittle, making it hard to work with. In 1856 the Bessemer process was invented that turns the brittle pig iron into steel, a more malleable metal.[4] Since then, many different technologies have been invented to replace the Bessemer process such as the electric arc furnace, basic oxygen steelmaking, and direct reduced iron (DRI).[5]
For sulfide ores, a different process is taken for beneficiation. The ore needs to have the sulfur removed before smelting can begin. Roasting is the primary method of separating, where wood was placed on heaps of ore and set on fire to help with oxidation.[6][7]
- 2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2
The earliest practices of roasting were done outside, allowing large clouds of sulfur dioxide to blow over the land causing serious harm to surrounding ecosystems, both aquatic and terrestrial. The clouds of sulfur dioxide combined with local deforestation for wood needed for roasting compounded damages to the environment,[6] as seen in Sudbury, Ontario and the Inco Superstack.[7]
The simplest method of separating ore from the 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 'bundles' 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.
Types of separation
[edit]Disaggregation
[edit]Beneficiation can begin within the mine itself. Most mines will have a crusher within the mine itself where separation of ore and gangue minerals occurs and as a side effect becomes easier to transport. After the crusher the ore will go through a grinder or a mill to get the ore into fine particles. Dense media separation (DMS) is used to further separate the desired ore from rocks and gangue minerals. This will stratify the crushed aggregate by density making separation easier. Where the DMS occurs in the process can be important, the grinders or mills will process much less waste rock if the DMS occurs beforehand. This will lower wear on the equipment as well as operating costs since there is a lower volume being put through.[8]
Physical separation
[edit]After the milling stage the ore can be further separated from the rock. One way this can be achieved is by using the physical properties of the ore to separate it from the rest of the rock. Prior to any physical separation process, sizing of ore particles is important for effective separation. This is done by using either Industrial Screens or Classifiers.[9]
These processes are gravity separation, flotation, and magnetic separation. Gravity separation uses centrifugal forces and specific gravity of ores and gangue to separate them.[10] Magnetic separation is used to separate magnetic gangue from the desired ore, or conversely to remove a magnetic target ore from nonmagnetic gangue.[11] DMS is also considered a physical separation.
Chemical separation
[edit]Some ore physical properties can not be relied on for separation, therefore chemical processes are used to separate the ores from the rock. Froth flotation, leaching, and electrowinning are the most common types of chemical separation. Froth flotation uses hydrophobic and hydrophilic properties to separate the ore from the gangue. Hydrophobic particles will rise to the top of the solution to be skimmed off.[12][13] Changes to pH in the solution can influence what particles will be hydrophilic. Leaching works by dissolving the desired ore into solution from the rock.[14] Electrowinning is not a primary method of separation, but is required to get the ore out of solution after leaching.
Unit operations
[edit]Mineral processing can involve four general types of unit operation: 1) Comminution – particle size reduction; 2) Sizing – separation of particle sizes by screening or classification; 3) Concentration by taking advantage of physical and surface chemical properties; and 4) Dewatering – solid/liquid separation.
In all of these processes, the most important considerations are the economics of the processes, which 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. This, however, becomes difficult to do with fine particles since they prevent certain concentration processes from occurring.
Comminution
[edit]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[15] 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 equipment in crushing are jaw crushers, gyratory crushers and cone crushers whereas rod mills and ball mills, usually 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.
Sizing
[edit]
Sizing is the general term for separation of particles according to their size. 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,[16] bar screens, wedge wire screens, radial sieves, banana 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 include 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.
Concentration
[edit]There are a number of ways to increase the concentration of the wanted minerals: in any particular case, the method chosen will depend on the relative physical and surface chemical properties of the mineral and the gangue. Concentration is defined as the number of moles of a solute in a volume of the solution. In case of mineral processing, concentration means the increase of the percentage of the valuable mineral in the concentrate.
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, buoyant forces), one of which is resistance to motion (drag force) by a viscous medium such as heavy media, water or, less commonly, air.
Gravity separation is one of the oldest technique in mineral processing but has seen a decline in its use since the introduction of methods like flotation, classification, magnetic separation and leaching. Gravity separation dates back to at least 3000 BC when Egyptians used the technique for separation of gold.
It is necessary to determine the suitability of a gravity concentration process before it is employed for concentration of an ore. The concentration criterion is commonly used for this purpose, designated in the following equation (where represents specific gravity):
- for CC > 2.5, suitable for separation of particles above 75 microns in size
- for 1.75 < CC < 2.5, suitable for separation of particles above 150 microns in size
- for 1.50 < CC < 1.75, suitable for separation of particles above 1.7 mm in size
- for 1.25 < CC < 1.50, suitable for separation of particles above 6.35 mm in size
- for CC < 1.25, not suitable for any size
Although concentration criteria is a useful rule of thumb when predicting amenability to gravity concentration, factors such as particle shape and relative concentration of heavy and light particles can dramatically affect separation efficiency in practice.
Classification
[edit]There are several methods that make use of the weight or density differences of particles:[17]
- 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[18]
- 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
- Multi-gravity separators including Knelson, Mozley (multi or enhanced) gravity separator, Salter cyclones (multi-gravity separator) and the Kelsey jig)
- Inline pressure jigs
- Reichert cones
- Sluices
- Elutriators
- Optima classifier- a water -only density separator. It is an engineered system that consists of feed preparation, beneficiation/separation and dewatering modules to ensure a high-quality product with the correct moisture content. Typical products consist of ash below 16% and a calorific value of at least 27-million joules per kilogram.
These processes can be classified as either density separation or gravity (weight) separation. In dense media separation a media is created with a density in between the density of the ore and gangue particles. When subjected to this media particles either float or sink depending on their density relative to the media. In this way the separation takes place purely on density differences and does not, in principle, relay on any other factors such as particle weight or shape. In practice, particle size and shape can affect separation efficiency. Dense medium separation can be performed using a variety of mediums. These include, organic liquids, aqueous solutions or suspensions of very fine particles in water or air. The organic liquids are typically not used due to their toxicity, difficulties in handling and relative cost. Industrially, the most common dense media is a suspension of fine magnetite and/or ferrosilicon particles. An aqueous solution as a dense medium is used in coal processing in the form of a belknap wash and suspensions in air are used in water-deficient areas, like areas of China, where sand is used to separate coal from the gangue minerals.
Gravity separation is also called relative gravity separation as it separates particles due to their relative response to a driving force. This is controlled by factors such as particle weight, size and shape. 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 very fine particles to occur (in the range of 5 to 50 micron) by increasing the driving force of separation in order to increase the rate at which particles separate. In general, single G process are only capable of processing particles that are greater than approximately 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
[edit]
Froth flotation is an important concentration process. This process can be used to separate any two different particles and operated by the surface chemistry of the particles. In flotation, bubbles are introduced into a pulp and the bubbles rise through the pulp.[19] In the process, hydrophobic particles become bound to the surface of the bubbles. The driving force for this attachment is the change in the surface free energy when the attachment occurs. These bubbles rise through the slurry and are collected from the surface. To enable these particles to attach, careful consideration of the chemistry of the pulp needs to be made. These considerations include the pH, Eh and the presence of flotation reagents. The pH is important as it changes the charge of the particles surface and the pH affects the chemisorption of collectors on the surface of the particles.
The addition of flotation reagents also affects the operation of these processes. The most important chemical that is added is the collector. This chemical binds to the surface of the particles as it is a surfactant. The main considerations in this chemical is the nature of the head group and the size of the hydrocarbon chain. The hydrocarbon tail needs to be short to maximize the selectivity of the desired mineral and the headgroup dictates which minerals it attaches to.
The frothers are another important chemical addition to the pulp or slurry as they enable stable bubbles to be formed. This is important because if the bubbles coalesce, minerals will fall off their surface. The bubbles however should not be too stable as this prevents easy transportation and dewatering of the concentrate formed. The mechanism of these frothers is not completely known and further research into their mechanisms is being performed.
Depressants and activators are used to selectively separate one mineral from another. Depressants inhibit the flotation of one mineral or minerals while activators enable the flotation of others. Examples of these include CN−, used to depress all sulfides but galena and this depressant is believed to operate by changing the solubility of chemisorbed and physisorbed collectors on sulfides. This theory originates from Russia. An example of an activator is Cu2+ ions, used for the flotation of sphalerite.
There are a number of cells able to be used for the flotation of minerals. these include flotation columns and mechanical flotation cells. The flotation columns are used for finer minerals and typically have a higher grade and lower recovery of minerals than mechanical flotation cells. The cells in use at the moment can exceed 300 m3. This is done as they are cheaper per unit volume than smaller cells, but they are not able to be controlled as easily as smaller cells.
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.
Staged flotation reactors (SFRs) split the flotation process into three defined stages per cell. They are becoming increasingly more common in use as they require much less energy, air and installation space.
Electrostatic separation
[edit]There are two main types of electrostatic separators. These work in similar ways, but the forces applied to the particles are different and these forces are gravity and electrostatic attraction. The two types are electrodynamic separators (or high tension rollers) or electrostatic separators. In high tension rollers, particles are charged by a corona discharge. This charges the particles that subsequently travel on a drum. The conducting particles lose their charge to the drum and are removed from the drum with centripetal acceleration. Electrostatic plate separators work by passing a stream of particles past a charged anode. The conductors lose electrons to the plate and are pulled away from the other particles due to the induced attraction to the anode. These separators are used for particles between 75 and 250 micron and for efficient separation to occur, the particles need to be dry, have a close size distribution and uniform in shape. Of these considerations, one of the most important is the water content of the particles. This is important as a layer of moisture on the particles will render the non-conductors as conductors as the layer of the water is conductive.
Electrostatic plate separators are usually used for streams that have small conductors and coarse non-conductors. The high tension rollers are usually used for streams that have coarse conductors and fine non-conductors.
These separators are commonly used for separating mineral sands, an example of one of these mineral processing plants is the CRL processing plant at Pinkenba in Brisbane Queensland. In this plant, zircon, rutile and ilmenite are separated from the silica gangue. In this plant, the separation is performed in a number of stages with roughers, cleaners, scavengers and recleaners.
Magnetic separation
[edit]Magnetic separation is a process in which magnetically susceptible material is extracted from a mixture using a magnetic force. This separation technique can be useful in mining iron as it is attracted to a magnet. In mines where wolframite was mixed with cassiterite, such as South Crofty and East Pool mine in Cornwall or with bismuth such as at the Shepherd and Murphy mine in Moina, Tasmania, magnetic separation was used to separate the ores. At these mines a device called a Wetherill's Magnetic Separator (invented by John Price Wetherill, 1844–1906)[1] was used. In this machine the raw ore, after calcination was fed onto a moving belt which passed underneath two pairs of electromagnets under which further belts ran at right angles to the feed belt. The first pair of electromagnets was weakly magnetised and served to draw off any iron ore present. The second pair were strongly magnetised and attracted the wolframite, which is weakly magnetic. These machines were capable of treating 10 tons of ore a day. This process of separating magnetic substances from the non-magnetic substances in a mixture with the help of a magnet is called magnetic separation..
This process operates by moving particles in a magnetic field. The force experienced in the magnetic field is given by the equation f=m/k.H.dh/dx. with k=magnetic susceptibility, H-magnetic field strength, and dh/dx being the magnetic field gradient. As seen in this equation, the separation can be driven in two ways, either through a gradient in a magnetic field or the strength of a magnetic field. The different driving forces are used in the different concentrators. These can be either with water or without. Like the spirals, washwater aids in the separation of the particles while increases the entrainment of the gangue in the concentrate.
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.
Dewatering
[edit]Dewatering is an important process in mineral processing. The purpose of dewatering is to remove water absorbed by the particles which increases the pulp density. This is done for a number of reasons, specifically, to enable ore handling and concentrates to be transported easily, allow further processing to occur and to dispose of the gangue. The water extracted from the ore by dewatering is recirculated for plant operations after being sent to a water treatment plant. The main processes that are used in dewatering include dewatering screens, sedimentation, filtering, and thermal drying. These processes increase in difficulty and cost as the particle size decreases.
Dewatering screens operate by passing particles over a screen. The particles pass over the screen while the water passes through the apertures in the screen. This process is only viable for coarse ores that have a close size distribution as the apertures can allow small particles to pass through.
Sedimentation operates by passing water into a large thickener or clarifier. In these devices, the particles settle out of the slurry under the effects of gravity, or centripetal forces. These are limited by the surface chemistry of the particles and the size of the particles. To aid in the sedimentation process, flocculants and coagulants are added to reduce the repulsive forces between the particles. This repulsive force is due to the double layer formed on the surface of the particles. The flocculants work by binding multiple particles together while the coagulants work by reducing the thickness of the charged layer on the outside of the particle. After thickening, slurry is often stored in ponds or impoundments. Alternatively, it can pumped into a belt press or membrane filter press to recycle process water and create stackable, dry filter cake, or "tailings".[20]
Thermal drying is usually used for fine particles and to remove low water content in the particles. Some common processes include rotary dryers, fluidized beds, spray driers, hearth dryers and rotary tray dryers. This process is usually expensive to operate due to the fuel requirement of the dryers.
Other processes
[edit]Many mechanical plants also incorporate hydrometallurgical or pyrometallurgical processes as part of an extractive metallurgical operation. Geometallurgy is a branch of extractive metallurgy that combines mineral processing with the geologic sciences. This includes the study of oil agglomeration[21][22][23][24]
A number of auxiliary materials handling operations are also considered a branch of mineral processing such as storage (as in bin design), conveying, sampling, weighing, slurry transport, and pneumatic transport.
The efficiency and efficacy of many processing techniques are influenced by upstream activities such as mining method and blending.[25]
Case examples
[edit]In the case of gold, after adsorbing onto carbon, it is put into a sodium hydroxide and cyanide solution. In the solution the gold is pulled out of the carbon and into the solution. The gold ions are removed from solution at steel wool cathodes from electrowinning. The gold then goes off to be smelted.[14]
Lithium is hard to separate from gangue due to similarities in the minerals. In order to separate the lithium both physical and chemical separation techniques are used. First froth flotation is used. Due to similarities in mineralogy there is not complete separation after flotation. The gangue that is found with lithium after the flotation are often iron bearing. The float concentrate goes through magnetic separation to remove the magnetic gangue from the nonmagnetic lithium.[26]
Conferences
[edit]European Metallurgical Conference (EMC)
[edit]EMC, the European Metallurgical Conference has developed to the most important networking business event dedicated to the non-ferrous metals industry in Europe. From the start of the conference sequence in 2001 at Friedrichshafen it was host to some of most relevant metallurgists from all countries of the world. The conference is held every two years by invitation of GDMB Society of Metallurgists and Miners and is particularly directed to metal producers, plant manufactures, equipment suppliers and service providers as well as members of universities and consultants.
See also
[edit]Notes
[edit]- ^ "mineral processing | metallurgy | Britannica". www.britannica.com. Retrieved 2022-04-02.
- ^ Adam Robert Lucas (2005), "Industrial Milling in the Ancient and Medieval Worlds: A Survey of the Evidence for an Industrial Revolution in Medieval Europe", Technology and Culture 46 (1): 1-30 [10-1 & 27]
- ^ Wagner, Donald B. (1999). "The Earliest Use of Iron in China". Metals in Antiquity: 1–9 – via Oxford: Archaeopress.
- ^ a b Wagner, Donald B. (2008). "Science and Civilization in China Volume 5-11". Ferrous Metallurgy – via Cambridge University Press.
- ^ "Secondary Steelmaking: Principles and Applications". CRC Press. Retrieved 2020-04-08.
- ^ a b Greenwood, Norman N. (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
- ^ a b "Photo History of Sudbury's Stressed Environment". users.vianet.ca. Retrieved 2020-04-08.
- ^ Haldar, S.K. (2017). Platinum-Nickel-Chromium Deposits. Elsevier Inc. ISBN 978-0-12-802041-8.
- ^ Wills, Barry A.; Finch, James E. (2016). Wills' mineral processing technology: an introduction to the practical aspects of ore treatment and mineral recovery (Eighth ed.). Amsterdam Boston, Mass Heidelberg: Elsevier. ISBN 978-0-08-097053-0.
- ^ Falconer, Andrew (2003). "Gravity Separation: Old Technique/New Methods" (PDF). Physical Separation in Science and Engineering. 12: 31–48. doi:10.1080/1478647031000104293.
- ^ Yu, Jianwen (2017). "Beneficiation of an iron ore fines by magnetization roasting and magnetic separation". International Journal of Mineral Processing. 168: 102–108. Bibcode:2017IJMP..168..102Y. doi:10.1016/j.minpro.2017.09.012.
- ^ "Introduction to Mineral Processing: Froth Flotation". Retrieved September 2, 2017.
- ^ Ramachandra Rao, S. (2006). "Physical and Physico-Chemical Processes". Resource Recovery and Recycling from Metallurgical Wastes. Waste Management Series. Vol. 7. pp. 35–69. doi:10.1016/S0713-2743(06)80088-7. ISBN 978-0-08-045131-2 – via Enslevier.
- ^ a b Vinal, J.; Juan, E.; Ruiz, M.; Ferrando, E.; Cruells, M.; Roca, A.; Casado, J. (2006). "Leaching of gold and palladium with aqueous ozone in dilute chloride media". Hydrometallurgy. 81 (2): 142–151. Bibcode:2006HydMe..81..142V. doi:10.1016/j.hydromet.2005.12.004 – via Elsevier Science Direct.
- ^ 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. Archived from the original on 2010-10-29. Retrieved 2010-08-07.
- ^ 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 ft (120 cm) minus in diameter while recovering all the 2-inch (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.
- ^ Lowrie, Raymond L; Society for Mining, Metallurgy and Exploration (2002), SME mining reference handbook, Society for Mining, Metallurgy, and Exploration, ISBN 978-0-87335-175-1 - chapter 17 - Classification section by Paul D. Chamberlin
- ^ "Mill Machines: The Wilfley table". Copper Country Explorer. Archived from the original on 2014-08-26. Retrieved 2010-08-07.
- ^ Hoque, Mohammad Mainul; Peng, Zhengbiao; Evans, Geoffrey; Doroodchi, Elham (2023). "Influence of bubble surface loading on particle-laden bubble rising dynamics in a fluid flow system". Minerals Engineering. 195 108043. Bibcode:2023MiEng.19508043H. doi:10.1016/j.mineng.2023.108043.
- ^ "Tons Per Hour Product Guide 2016" (PDF). Archived from the original (PDF) on 2019-02-09. Retrieved 2016-03-05.
- ^ Preparation of salty coals with oil-agglomeration technology / V. S. Biletskyi, A. Kheloufi, P. V. Sergeev // 9th International conference on coal science (ICCS'97), 7–12 September 1997, Essen, Germany. V. 1. P.535-538.
- ^ Investigation of laws of selective flocculation of coals with syn-thetic latexes / P. V. Sergeev, V. S. Biletskyy // ICCS'97. 7–12 September 1997, Essen, Germany. V. 1. p. 503-506.
- ^ C.-W. Fan, R. Markuszewski, and T. D. Wheelock, "Oil Agglomeration of Coal in Salt Solutions: Effects of Hydrophobicity and Other Parameters on Coal Recovery" Archived 2017-04-28 at the Wayback Machine
- ^ Beletskyi V., Shendrik T. Ennobling of salty coals by means of oil agglomeration Technical and Geoinformational Systems in Mining. Proceedings of the School of underground mining, Dnipropetrovsk/Yalta, 2–8 October 2011. / CRC Press Taylor & Francis Group, London, UK. A Balkema Book. 2011. p. 135-140.
- ^ Whitacre, J., Iorio, A., Schellenberg, S. "Coal Blending: Business Value, Analysis, and Optimization"
- ^ tadesse, Bogale; Makuei, Fidele; Albijanic, Boris; Dyer, Laurence (2019). "The beneficiation of lithium minerals from hard rock ores: A review". Minerals Engineering. 131: 170–184. Bibcode:2019MiEng.131..170T. doi:10.1016/j.mineng.2018.11.023. S2CID 105940721.
References
[edit]- 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
- Wills, B.A., Finch, J. (2015): Wills' Mineral Processing Technology, An Introduction to the Practical Aspects of Ore Treatment and Mineral Recovery; 8th Edition, 512 pp, ISBN 978-0-08-097053-0
Sources
[edit]- Various articles in J. Day & R. F. Tylecote, Metals in the Industrial Revolution (Institute of Metals, London 1991).
Further reading
[edit]- Carter, C. Barry; M. Grant Norton (2007). "Beneficiation". Ceramic materials: science and engineering. Berlin: Springer. p. 347. ISBN 978-0-387-46270-7. OCLC 77012326. Retrieved 15 May 2009.
- Smith, Mick R.; L. Collis; P. G. Fookes (2001). "Beneficiation". Aggregates: Sand, Gravel and Crushed Rock Aggregates for Construction Purposes. London: Geological Society. pp. 124–129. ISBN 1-86239-079-7. OCLC 46600322. Retrieved 15 May 2009.
- Baartjes, N. L. (2007). "Diamonder Beneficiation: Structural Impediments And How To Unblock These In South Africa" (PDF). Diamonds Source to Use 2007. The Southern African Institute of Mining and Metallurgy. Archived from the original (PDF) on 2 October 2011. Retrieved 15 May 2009.
- Suptelya, V. V.; S. A. Martynov; V. P. Butuzov; V. A. Khvan (July 1977). "Beneficiation of synthetic diamonds by ultrasound". Journal of Mining Science. 13 (4): 439–441. Bibcode:1977JMinS..13..439S. doi:10.1007/BF02498516. S2CID 129150818.
Mineral processing
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Definition and Objectives
Mineral processing, also known as ore dressing or beneficiation, is the practice and technology of extracting maximum value from raw ores or rocks by separating valuable minerals from waste materials, known as gangue, through a series of physical and chemical processes.[1] These processes aim to transform heterogeneous run-of-mine ore into homogeneous products suitable for further treatment or direct sale, without altering the fundamental physical and chemical identities of the minerals involved.[6] The primary objectives of mineral processing include the liberation of valuable minerals from the surrounding host rock or gangue, typically achieved by reducing particle size to free the desired components; concentration to increase the proportion of valuable minerals in the product; and the production of marketable outputs such as high-value concentrates and residual tailings for disposal or reuse.[1] Liberation ensures that valuable minerals are sufficiently exposed for separation, while concentration enhances the economic viability by upgrading the ore to a higher value state, often through methods that minimize waste and optimize resource utilization.[7] Ultimately, these objectives focus on maximizing recovery efficiency, producing transportable and dry concentrates, and managing environmental impacts by handling tailings responsibly.[1] Mineral processing, or beneficiation, differs from extractive metallurgy in that it primarily employs mechanical and physicochemical techniques to prepare and concentrate ores, whereas extractive metallurgy encompasses subsequent chemical processes, such as smelting and refining, to extract and purify metals from those concentrates.[8] This distinction positions mineral processing as the initial stage in the broader extractive metallurgy framework, focused on physical upgrading rather than elemental extraction via high-temperature reactions.[9] Key performance metrics in mineral processing include the recovery rate, which measures the efficiency of extracting valuable minerals and is expressed as the percentage of the valuable component in the feed that reports to the concentrate, often ranging from 80% to 90% depending on ore type and process conditions; and the grade, defined as the concentration or purity of the valuable mineral in the product, typically quantified as a percentage by weight, such as 50% copper in a concentrate.[1] These metrics are critical for evaluating process effectiveness, as higher grades and recovery rates directly influence economic returns while balancing trade-offs in separation efficiency.[10]Scope and Importance
Mineral processing encompasses the preparation and separation of valuable minerals from primary sources, such as ores extracted through mining operations including surface and underground methods, as well as secondary sources like recycled materials, tailings, and industrial byproducts such as slag and concrete. This scope extends to a diverse array of materials, including base and precious metals (e.g., copper, gold, iron ore), industrial minerals (e.g., sand, gravel, kaolin, phosphate for construction and ceramics), and coal for energy applications, involving techniques like comminution, classification, and concentration to upgrade raw feeds into marketable products.[1] The importance of mineral processing lies in its role as a foundational step in supplying essential raw materials to key industries, including electronics (requiring rare earth elements and metals like copper for semiconductors and wiring), construction (relying on aggregates and industrial minerals for building materials), and the global energy transition (where critical minerals such as lithium, nickel, cobalt, and graphite are vital for battery production in electric vehicles and renewable energy storage). These materials enable the shift toward low-carbon technologies, with demand for battery minerals projected to surge due to clean energy deployments. For instance, lithium, nickel, cobalt, manganese, and graphite are crucial for enhancing battery performance, longevity, and energy density, while rare earth elements support permanent magnets in wind turbines and electric motors.[11][12] As of 2025, global mineral production volumes underscore the scale of these operations, with iron ore reaching approximately 2.5 billion metric tons, copper at 23 million metric tons, aluminum smelter output at 72 million metric tons, and critical minerals like lithium surging to 240,000 metric tons amid battery demand growth of nearly 30% in the prior year. Processing capacities are heavily concentrated, particularly in China, which accounts for about 69% of global rare earth mine production and 92% of its processing, alongside 80% of cobalt processing and over 50% of output for 18 other minerals, highlighting supply chain vulnerabilities in the energy transition. Mineral processing interconnects with downstream processes like hydrometallurgy by providing concentrated ores or intermediates as feedstocks for aqueous extraction and purification of metals such as copper, gold, and zinc.[13][14][15][16]Historical Development
Early Methods
Mineral processing originated in ancient civilizations, where rudimentary techniques were employed to extract valuable minerals from ores. In ancient Egypt, as early as the predynastic period (c. 3000 BCE), gold was recovered from alluvial deposits using simple panning methods, involving the swirling of sediment in water to separate heavier gold particles. Hand sorting was also common, with workers manually picking out visible nuggets or high-grade ore from placer deposits along the Nile River and Eastern Desert.[17] The Romans advanced these practices during their expansion into Europe and Africa, applying sluicing—channels that directed water flow to wash away lighter gangue materials—for gold extraction, notably through hydraulic mining at sites like Las Médulas in Spain (1st century CE). Similar gravity-based washing methods were used for tin mining in Cornwall under Roman influence, where ore was hand-sorted and washed in streams to concentrate cassiterite.[18] During the medieval period, mechanical innovations began to replace purely manual labor, enhancing efficiency in ore preparation. Stamp mills, first documented in Persia around the 11th century and widespread in Europe by the 13th century, used heavy wooden stamps powered by animal or water to crush ore into finer particles for subsequent processing.[19] This mechanized crushing was crucial for breaking down hard rock ores of gold and silver, allowing better liberation of minerals.[19] Amalgamation, involving the use of mercury to bind with gold or silver particles, had been known since Roman times but gained prominence for silver recovery in the mid-16th century with the development of the patio process in Mexico, later adopted in European mines. Mercury was mixed with the pulverized material, forming an amalgam that could be separated and retorted to distill the mercury.[20] The 18th and 19th centuries marked a transition toward more powered and systematic approaches, driven by the Industrial Revolution. Water wheels, refined in the mid-18th century by engineer John Smeaton, provided reliable mechanical power for grinding mills, replacing inconsistent animal-driven systems and enabling continuous ore comminution in larger operations. Smeaton's overshot designs increased efficiency to around 60-70%, powering arrastra or edge-runner mills that ground ore into slurries for separation.[21] Early jigging for gravity separation emerged as a key innovation, with mechanical jigs using pulsating water to stratify ore particles by density; by the late 19th century, devices like the Baum jig automated this process, concentrating heavy minerals such as tin and tungsten from tailings.[22] These advancements laid the groundwork for modern unit operations in mineral processing. A pivotal figure in documenting these early techniques was Georgius Agricola, a 16th-century German scholar whose 1556 work De Re Metallica provided the first comprehensive treatise on mining and processing.[23] Agricola detailed methods like ore washing, roasting, and amalgamation, drawing from Saxon mining practices, and illustrated early machinery such as water-powered bellows and stamps.[23] His text emphasized systematic approaches to ventilation, ore dressing, and assaying, influencing subsequent European developments.20th Century Innovations
The invention of froth flotation in 1905 marked a pivotal advancement in mineral processing, enabling the selective separation of valuable minerals from complex ores on an industrial scale. Francis Elmore and his associates developed the process, building on earlier bulk oil methods, with the first commercial application at the Broken Hill mines in Australia treating lead-zinc sulfide ores. This innovation involved introducing air bubbles to create a froth that carried hydrophobic sulfide particles, such as galena and sphalerite, to the surface while hydrophilic gangue sank, drastically improving recovery rates from low-grade deposits. Early patents, including British No. 7803 filed in April 1905 by Minerals Separation Ltd. engineers E.L. Sulman, H.F.K. Picard, and John Ballot, formalized the froth flotation technique using minimal oil (0-20 lb/ton) and agitation to generate stable bubbles, distinguishing it from prior oil-heavy approaches.[24][25] In the early 1900s, the development of ball mills and rod mills revolutionized comminution, providing more efficient and controlled size reduction for ore preparation compared to stamp mills. Ball mills, invented by Brückner in Germany in 1876 but widely adopted in mineral processing by the 1900s, utilized rotating drums filled with steel balls to grind ores into fine powders, achieving uniform particle sizes essential for downstream separation. Rod mills, emerging around the same period to address excessive fines production in ball mills, employed long steel rods as grinding media in cylindrical mills, producing coarser products suitable for initial grinding stages in circuits handling sulfide and oxide ores. These innovations enabled continuous, high-throughput operations, with ball mills becoming standard in circuits for gold, copper, and iron ore processing by the 1920s.[26][27] Another significant innovation was the cyanide leaching process for gold and silver recovery, patented in 1887 by John Stewart MacArthur, which allowed efficient extraction from low-grade and refractory ores using dilute aqueous sodium cyanide solutions to dissolve the metals for subsequent precipitation.[28] Magnetic separation advanced with the introduction of drum separators in the early 20th century (patented 1919), enhancing the recovery of ferromagnetic minerals like magnetite from low-grade iron ores. These devices featured rotating drums with embedded electromagnets that attracted and lifted magnetic particles from a pulp stream, allowing continuous separation and reducing manual sorting needs in wet processing plants.[29] Complementing this, electrostatic separators gained prominence in the 1930s through Johnson's selective process, which exploited differences in mineral conductivity to separate non-magnetic materials, such as rutile from ilmenite in beach sands, using high-voltage fields to charge and deflect particles.[30] Following World War II, automation in mineral processing began with the integration of conveyor systems and basic control instrumentation, facilitating safer and more efficient material handling in large-scale operations. Belt conveyors, improved for durability with synthetic materials, enabled continuous transport of ores from crushers to mills, as seen in open-pit iron and coal mines where they replaced discontinuous haulage methods. Early instrumentation, including pneumatic controls and simple sensors for flow and level monitoring, allowed rudimentary automation of processes like grinding circuits, optimizing throughput and reducing labor in post-war expansion of copper and phosphate plants.[31][32]Fundamentals
Mineral Liberation
Mineral liberation refers to the process of reducing ore particle size through comminution to expose and separate individual grains of valuable minerals from the surrounding gangue material, thereby enabling effective downstream separation. This step is fundamental in mineral processing, as it transforms locked mineral particles into discrete, free grains that can be targeted by separation techniques.[33][34] The size at which minerals are adequately liberated depends on several key factors, including the ore's mineral texture, the distribution of grain sizes within the ore matrix, and the nature of associations between valuable minerals and gangue. For instance, ores with fine-grained textures or strong intergrowths, such as those involving silicates and sulfides, require finer grinding to achieve sufficient exposure, whereas coarser-grained deposits may liberate at larger particle sizes. These factors influence the efficiency of liberation, as overly complex textures can lead to incomplete separation even at reduced sizes.[35][36] To quantify the degree of liberation, models such as the Gaudin-Meloy model are employed, which build on geometric assumptions of mineral grain arrangement and random breakage to predict the proportion of free mineral grains in a particle population. Originally developed by Gaudin in 1939 based on cubic grain structures and extended by Meloy in subsequent works to incorporate detachment and grain size distributions, the model uses techniques like point counting on polished sections—where a grid is superimposed to estimate the area or volume fraction of liberated minerals—or modern image analysis for automated assessment of 2D and 3D liberation spectra. These methods allow for the calculation of liberation indices, such as the percentage of fully liberated particles, providing a basis for process optimization.[37][38][39] Achieving optimal liberation is critical, as it balances high mineral recovery rates—potentially increasing yields by 7–12% in targeted operations—with minimized energy expenditure, given that excessive grinding can elevate costs without proportional benefits. Inadequate liberation results in locked particles that reduce separation efficiency, while over-liberation produces fines that complicate handling and increase slimes formation. Thus, liberation analysis guides the selection of appropriate grind sizes to enhance overall process economics. Liberated particles form the basis for subsequent separation based on their inherent properties.[40][33]Separation Principles
Mineral processing separation principles rely on exploiting inherent differences in the physical, chemical, and surface properties of liberated mineral particles to achieve selective partitioning into concentrate and tailings streams.[41] These properties become exploitable after mineral liberation, which exposes individual grains for differential treatment.[42] The choice of property depends on the ore's composition and the desired separation sharpness, with processes designed to amplify subtle differences through controlled environments like fluid media or applied fields. Physical properties form the basis for many mechanical separation methods, primarily density, magnetic susceptibility, and electrical conductivity. Density differences drive gravity-based separations, where heavier valuable minerals settle faster than lighter gangue in a fluid medium, as seen in applications for tungsten ores with densities exceeding 7 g/cm³ compared to siliceous gangue around 2.65 g/cm³.[43] Magnetic susceptibility quantifies a mineral's response to an applied magnetic field, enabling separation of ferromagnetic or paramagnetic species like magnetite (susceptibility ~10^{-3} m³/kg) from diamagnetic silicates. Electrical conductivity distinguishes conductive sulfides, such as chalcopyrite, from insulating quartz in electrostatic processes, where charged particles are deflected differently in an electric field.[41] Chemical properties, particularly solubility and reactivity, underpin hydrometallurgical separations like selective leaching, where target minerals dissolve preferentially in aqueous solutions. Solubility variations allow extraction of metals like gold via cyanidation, exploiting its high solubility in cyanide complexes while gangue remains undissolved.[44] Reactivity differences facilitate processes such as acid leaching of oxide ores, where reactive copper minerals dissolve in sulfuric acid at rates up to 90% extraction, contrasting with inert silicates.[45] These methods often involve coordination chemistry to enhance dissolution kinetics through complex formation.[46] Surface properties govern interfacial behaviors critical for processes involving liquid-solid or solid-air interactions, including hydrophobicity/hydrophilicity and zeta potential. Hydrophobicity, induced by collector adsorption on mineral surfaces, enables froth flotation by promoting attachment to air bubbles; for instance, sulfide minerals treated with xanthates achieve contact angles >50°, rendering them oleophilic while hydrophilic gangue wets and sinks.[47] Zeta potential, measuring the electric potential at the slipping plane of a particle in suspension, influences coagulation and dispersion; negative zeta potentials around -30 mV for quartz at neutral pH promote stability, but dispersant addition shifts it to enhance selective flocculation of clays in iron ore processing.[48] A general measure of separation performance is the efficiency , derived from mass balance principles as , where is the fractional recovery of the valuable mineral in the concentrate and is the fractional recovery of the gangue in the same stream. This equation arises from the overall material balance across the separator: the feed mass splits into concentrate and tailings , with valuable assay conserved as , yielding ; similarly for gangue, where denotes gangue assay. The product form captures both the capture of valuables and rejection of impurities, with ideal for perfect separation.[49][50]Ore Preparation
Comminution
Comminution is the initial stage in mineral processing where run-of-mine ore is reduced in size through mechanical means to facilitate subsequent liberation and separation of valuable minerals.[1] This process typically involves sequential crushing and grinding operations, consuming a significant portion of the overall energy in mineral processing plants, often up to 50% or more.[51] The primary goal is to break down the ore from large fragments, typically 1 meter or greater, to finer particles suitable for downstream processing, while minimizing energy use and equipment wear.[52] The comminution process is divided into distinct stages: primary, secondary, and tertiary. Primary crushing reduces large ore chunks to a manageable size, commonly using jaw crushers that apply compressive forces to handle feed sizes up to 1.5 meters and produce output around 100-300 mm.[1] Secondary crushing further refines the material, employing cone crushers that operate on similar compressive principles but with higher speeds and finer settings to achieve sizes of 10-50 mm, suitable for harder ores.[51] Tertiary grinding then achieves the fine particle sizes needed for liberation, typically 10 microns to 1 mm, using equipment such as semi-autogenous grinding (SAG) mills, which combine ore and steel balls for impact and abrasion, or ball mills that rely on cascading media for finer reduction.[1] Comminution mechanisms include compression, impact, and attrition, each suited to different ore properties. Compression, dominant in jaw and cone crushers, applies slow, sustained force ideal for hard, abrasive ores like quartzite, as it minimizes fines generation.[53] Impact, used in hammer or vertical shaft impactors, delivers high-velocity blows effective for softer, friable ores such as limestone, producing more uniform particles but risking over-crushing in brittle materials.[54] Attrition, prevalent in ball and SAG mills, involves rubbing and shearing between particles or media, best for achieving fine sizes in both hard and soft ores but consuming more energy for tougher materials due to surface wear.[53] Energy requirements for comminution are estimated using Bond's Work Index equation, a standard empirical model developed by Fred C. Bond in 1952. The equation calculates the net energy in kilowatt-hours per short ton as: where is the ore's work index (a measure of grindability in kWh/short ton), is the 80% passing size of the product in microns, and is the 80% passing size of the feed in microns.[55] This model accounts for the non-linear increase in energy needed as particle size decreases, with typical values ranging from 10-20 kWh/short ton for common ores like copper sulfides.[56] Key challenges in comminution include overgrinding, which generates excessive fines that complicate downstream separation and increase energy waste, and liner wear, accelerated by abrasive ores leading to frequent maintenance and downtime.[57] Overgrinding is particularly problematic in grinding mills where insufficient control allows particles to recirculate excessively.[58] Liner wear in SAG and ball mills can reduce equipment life by up to 50% in high-abrasion environments.[59] Mitigation strategies for overgrinding involve optimized circuit design with intermediate classification using hydrocyclones to remove fines early and staged grinding to target specific size ranges.[1] For liner wear, selecting durable materials like high-chrome alloys or rubber-polyurethane composites extends life by 20-30%, while advanced designs with optimized lifter profiles improve energy transfer and reduce abrasion exposure.[59] Real-time monitoring and AI-driven adjustments further prevent excessive wear by maintaining optimal mill loads and speeds.[58]Sizing and Classification
Sizing and classification are essential stages in mineral processing that follow comminution, where the ore is reduced to liberate valuable minerals, and involve separating particles based on their size to prepare material for subsequent separation processes.[60] Sizing refers to the process of segregating particles into discrete size fractions, typically using mechanical screens, while classification sorts particles in a fluid medium, often exploiting differences in settling velocities or centrifugal effects to achieve finer separations.[61] These operations ensure that downstream processes receive material with optimal particle size distributions, enhancing overall efficiency and recovery rates in mineral extraction.[62] Screening is a primary sizing method that utilizes perforated surfaces to separate coarse from fine particles, with vibratory screens employing mechanical vibration to fluidize the feed and promote passage of undersized material through apertures.[63] Vibratory screens, often inclined at 15-25 degrees, operate at frequencies of 800-3600 vibrations per minute, achieving high throughput for particles up to 300 mm, though efficiency decreases for finer sizes below 6 mm due to blinding and moisture effects.[60] Grizzly screens, featuring parallel bars spaced 25-150 mm apart, are rugged devices used for initial scalping of very coarse ore (over 100 mm) ahead of crushers, relying on gravity and minimal vibration to reject oversize rocks while allowing fines to pass.[63] The performance of screening equipment is evaluated using efficiency curves, such as the Tromp curve, which plots the probability of a particle reporting to the oversize stream against its size, providing a measure of sharpness of separation; an ideal curve approaches a step function at the cut size, with real curves showing reduced efficiency indicated by the imperfection factor.[62] Classification extends sizing into finer ranges by suspending particles in a liquid or air medium, where separation occurs based on differential settling velocities under gravity or enhanced forces.[64] Hydrocyclones, the most common classifiers, generate centrifugal forces up to 1000 times gravity through tangential feed injection into a conical vessel, causing larger, faster-settling particles to spiral downward to the underflow while finer particles exit via the vortex finder in the overflow; this method is effective for particles below 300 µm, with cut sizes typically 10-100 µm depending on cyclone geometry and operating pressure.[61] Rake classifiers use reciprocating rakes to drag settled coarse solids up an inclined trough against a countercurrent flow of water, suitable for sizes 0.5-10 mm, while spiral classifiers employ a continuously rotating spiral conveyor in a settling tank to lift oversize material, offering gentler handling for friable ores in the 0.15-1 mm range.[64] The underlying principle contrasts hindered settling in dilute pulps, where interparticle interactions slow finer particles less than coarser ones, with centrifugal classification accelerating this differential based on size-to-mass ratios.[5] Particle size analysis is critical for monitoring and optimizing sizing and classification, employing techniques that quantify distributions to ensure process control.[65] Sieve analysis, a traditional dry or wet method, stacks standardized wire-mesh sieves (e.g., the Tyler series, which uses square openings decreasing geometrically from 125 mm to 20 µm in √2 ratios) and shakes the sample to determine cumulative mass percentages retained on each sieve, providing robust data for coarse fractions above 50 µm but limited by agglomeration in fines.[65] Laser diffraction, a modern optical technique, measures the angular distribution of laser light scattered by particles in suspension, inferring size distributions from Mie or Fraunhofer theory across a wide range (0.1-3000 µm) with high reproducibility, though it assumes spherical shapes and requires dispersion validation for irregular mineral particles.[66] These analyses adhere to standards like ISO 9276 for general procedures and Tyler's for sieve specifications, enabling precise characterization of grind fineness.[67] In closed-circuit grinding, classification plays a pivotal role by returning oversize particles from the classifier to the mill for further size reduction, maintaining a consistent product fineness (e.g., 80% passing 75 µm for flotation feed) and increasing circuit capacity by 20-50% compared to open circuits through better control of circulating load.[68] Hydrocyclones are preferred in such setups for their compact design and ability to handle high tonnages, with the classifier's cut size dictating the mill's power draw and overall energy efficiency.[1]Separation Processes
Gravity Concentration
Gravity concentration is a physical separation process that exploits differences in the specific gravity of minerals to separate valuable heavy minerals from lighter gangue materials, typically in a liquid medium such as water. This method relies on the application of gravitational forces, often enhanced by mechanical or fluid dynamics, to achieve stratification and hindered settling of particles. It is particularly effective for coarse to fine particles where density contrasts are significant, and it serves as a primary or preconcentration step in mineral processing flowsheets.[69] The fundamental principles include hindered settling, where particles in a suspension move relative to each other under gravity, with heavier particles settling faster due to the increased effective density of the medium formed by the particles themselves. Stratification occurs in devices like jigs, shaking tables, and spirals, promoting the layering of denser particles at the bottom or along the flow path. In jigs, pulsated water flow creates alternating upward and downward currents through a bed of ore, facilitating the separation by allowing heavy minerals to penetrate to the lower layers while lighter ones rise. Shaking tables use a reciprocating motion combined with a thin water film to drive heavy particles along the riffled deck longitudinally, while lighter particles are washed transversely. Spirals employ a helical trough where centrifugal and gravitational forces stratify particles radially, with heavies concentrating near the inner wall. Particle sizing is a prerequisite for optimal performance, as uniform feed sizes enhance separation efficiency.[69][1] Key equipment includes jigs, which utilize mechanical or pneumatic pulsation of water to achieve hindered settling and stratification for particles ranging from 75 μm to 200 mm, commonly applied to ores like gold and coal. Dense media separation (DMS) involves suspending dense materials such as ferrosilicon in water to create a medium with adjustable specific gravity, typically above 2.6 t/m³; heavy minerals sink while lighter gangue floats, with ferrosilicon's spherical or angular particles ensuring medium stability and low viscosity for efficient separation. The effectiveness of gravity concentration is governed by the concentration criterion, defined as (density of mineral - density of fluid) / (density of gangue - density of fluid), where values greater than 2.5 enable easy separation down to 75 μm particle sizes.[1][70][71] Applications are prominent in processing alluvial deposits, where jigs and spirals recover heavy minerals like gold from placer sands, often achieving plant recoveries ranging from 20% to 90% depending on particle size and equipment. In coal washing, DMS with ferrosilicon media efficiently removes ash and sulfur, upgrading low-grade coals to meet quality specifications. For tungsten ores, gravity methods such as shaking tables and jigs serve as the primary concentration technique, effectively recovering scheelite or wolframite from gangue due to their high density (around 7.0 specific gravity).[72]Froth Flotation
Froth flotation is a physicochemical separation process that exploits differences in the surface wettability of minerals to concentrate valuable components from ore slurries. In this method, hydrophobic particles attach to air bubbles introduced into the pulp, rising to form a froth layer that is skimmed off as concentrate, while hydrophilic particles remain in the cell.[49] The process is particularly effective for fine particles liberated during grinding, typically in the range of 10–150 micrometers.[49] The process begins with conditioning, where the ore slurry is mixed with chemical reagents to modify mineral surfaces. Collectors, such as xanthates (e.g., sodium ethyl xanthate), adsorb onto target mineral surfaces through chemisorption, rendering them hydrophobic and promoting bubble attachment.[73] Frothers, like pine oil or methyl isobutyl carbinol (MIBC), are added to stabilize the froth by reducing bubble surface tension and facilitating stable bubble formation.[49] Depressants, such as sodium cyanide or lime, are used to prevent unwanted minerals from floating by making their surfaces hydrophilic, enhancing selectivity.[73] Following conditioning, the slurry enters flotation cells where air is sparged through the pulp, creating bubbles that collide with and adhere to the conditioned particles.[49] Flotation kinetics describe the rate at which particles are recovered and are commonly modeled as a first-order process, where the recovery at time follows , with as the ultimate recovery and as the rate constant.[74] The rate constant can be expressed as , where represents selectivity (influenced by attachment efficiency) and is the probability of particle-bubble collision, which depends on bubble size, aeration rate, and particle characteristics.[74] Two primary types of froth flotation are direct and reverse. In direct flotation, the valuable hydrophobic minerals float to form the concentrate, while gangue remains in the tailings; this is standard for most sulfide ore processing.[49] Reverse flotation inverts this by floating the hydrophilic gangue away, leaving the valuable minerals in the cell, often applied when gangue is easier to render hydrophobic.[49] Flotation occurs in mechanical cells, which use impellers for agitation and air dispersion, or column cells, which employ countercurrent flow with spargers for finer bubbles and reduced entrainment via wash water addition; columns typically offer higher selectivity for fine particles.[49] Froth flotation is widely applied to sulfide ores, such as those containing copper, lead, and zinc, where it achieves high recoveries of 85–95% for liberated particles.[49] For copper sulfides like chalcopyrite, xanthates selectively float the mineral in alkaline conditions.[73] In lead-zinc ores, sequential circuits first recover lead as galena using collectors and depressants like zinc sulfate to suppress sphalerite, followed by zinc flotation after activation.[73] Typical circuits include rougher stages for initial concentrate production, cleaner stages for upgrading purity, and scavenger stages to recover additional valuables from rougher tailings, often arranged in multi-stage banks to optimize overall recovery and grade.[49]Magnetic Separation
Magnetic separation exploits differences in the magnetic susceptibilities of minerals to achieve physical separation using applied magnetic fields, primarily targeting ferromagnetic and paramagnetic species while leaving diamagnetic materials unaffected. This technique is integral to mineral processing, enabling the concentration of valuable ores such as iron oxides and titanium-bearing minerals from low-grade feeds. The method's selectivity stems from the ability to generate non-uniform magnetic fields that exert differential forces on particles, facilitating efficient recovery without chemical reagents.[75] The core principle governing particle separation is the magnetic force acting on a susceptible particle in a non-uniform field, expressed as where represents the volume magnetic susceptibility, the particle volume, the magnetic flux density, the field gradient, and the permeability of free space (4 × 10⁻⁷ H/m). This force competes with gravitational, hydrodynamic, and interparticle forces; for effective separation, it must dominate to deflect magnetic particles toward collection zones. Higher susceptibility and field gradients enhance separation of weakly magnetic minerals, with typical susceptibilities for magnetite exceeding 10⁻¹ (SI units) and ilmenite around 10⁻⁴ to 10⁻³.[76] Separators are categorized by magnetic field intensity: low-intensity magnetic separators (LIMS) and high-intensity magnetic separators (HIMS). LIMS, typically drum-type units operating at 0.1–0.3 T, are designed for highly magnetic ferromagnetic minerals like magnetite, using permanent or electromagnetic drums where feed passes over or through the rotating drum to attract and remove magnetic particles. These are widely applied in wet form for iron ore beneficiation, processing slurries at high throughput (up to 500 t/h per unit) to recover over 95% of magnetite from taconite ores, minimizing silica content in concentrates.[75] HIMS, including rare earth roll separators, achieve fields of 0.8–2 T using neodymium-iron-boron magnets and target paramagnetic minerals such as ilmenite (FeTiO₃). Rare earth rolls feature a series of magnetic rolls with thin belts conveying dry feed tangentially, where paramagnetic particles adhere to the roll and are deflected into a separate chute upon field release. This dry process is standard in heavy mineral sands beneficiation, upgrading ilmenite from 40–50% TiO₂ to 55–60% by removing non-magnetic gangue like quartz and zircon, with recoveries exceeding 90% for particles in the 0.1–1 mm range.[75] Wet magnetic separation predominates in iron ore circuits, employing slurries (20–40% solids) in LIMS or wet HIMS to handle fines below 100 μm, as water reduces viscosity and enhances particle suspension, improving selectivity for hematite-ilmenite mixes in banded iron formations. Dry separation, via rare earth or induced rolls, suits arid environments or pre-concentration of heavy mineral sands, avoiding dewatering costs but requiring feed moisture below 2% to prevent dust and adhesion issues; for instance, dry processing recovers 80–85% ilmenite from beach sands at rates up to 50 t/h. Proper particle sizing from prior comminution ensures liberation, as oversized particles reduce efficiency.[75] Induced roll magnetic separators, a subtype of dry HIMS, use electromagnetic induction to magnetize the roll surface, creating intense local gradients up to 0.5 T/m for fine paramagnetic recovery. Demagnetization in these units occurs post-separation in a non-magnetic discharge zone, where the decaying field releases captured particles, averting agglomeration and enabling clean splits; residual magnetism is minimized via coil design to below 5% of peak field. Matrix design optimizes separation by arranging conductive or ferromagnetic elements along the roll to amplify gradients, though in induced rolls, this manifests as layered coil windings and roll linings (e.g., stainless steel) that sustain fields without saturation, boosting throughput by 20–30% in ilmenite circuits.[75]Electrostatic Separation
Electrostatic separation is a dry beneficiation technique employed in mineral processing to separate particles based on their electrical conductivity and surface charge properties within a high-voltage electric field. This method exploits the differential behavior of conducting and non-conducting minerals, where charged particles are deflected or pinned according to their ability to retain or dissipate charge. It is particularly valuable for processing dry, free-flowing feeds and serves as an alternative to wet methods in arid environments or for moisture-sensitive materials.[77][78] The core principles involve three primary charging mechanisms in high-voltage fields: corona charging, induction charging, and contact charging. In corona charging, particles are ionized by bombardment from a corona discharge generated by a high-voltage electrode, allowing conductors to rapidly discharge while insulators retain charge for deflection. Induction charging occurs when uncharged particles contact a charged surface, inducing opposite polarity charges that cause conductors to neutralize quickly upon grounding. Contact charging, or triboelectrification, arises from electron transfer during particle-particle or particle-wall collisions, influenced by material work functions and effective for dielectric minerals. These mechanisms enable selective charging, with separation achieved by trajectory differences in the electric field.[77] Key equipment includes high-tension roller separators and plate separators, often with upstream conditioning to optimize particle charging. High-tension rollers feature a grounded rotating roll where feed particles are introduced near a high-voltage electrode; conductors lose charge and are thrown off centrifugally, while non-conductors adhere briefly before mechanical removal. Plate separators use parallel electrodes to create uniform or gradient fields, directing charged particles to collectors. Conditioning may involve milling to increase surface area and promote tribocharging, though specific implementations vary by application.[79][80] In mineral sands processing, electrostatic separation effectively isolates conducting minerals such as rutile and ilmenite from non-conductors like zircon and quartz. For instance, high-tension roll separators applied to Indian beach sands in the 125–500 μm size fraction achieve rutile grades of 96.6% and recoveries of 98.9% under optimized conditions of 102°C temperature, 1.75 tph feed rate, and 132 rpm roll speed. Beyond minerals, the technique extends to recycling, where it separates metals from plastics and sorts plastic mixtures based on triboelectric charge differences; equipment like electrostatic drum separators recovers 2–12% residual copper from plastic-metal mixes at throughputs up to several tons per hour.[78][80][81] Efficiency depends on factors like particle size and environmental conditions, with optimal performance for sizes between 0.1 and 2 mm, where finer fractions (e.g., 40–800 μm) enhance separation resolution without excessive dust. Low humidity is critical to maintain charge integrity, as moisture dissipates charges and reduces selectivity; feeds are typically dried to below 1% moisture content. These controls ensure high-purity products, though multi-stage setups may be needed for complex ores.[79][77]Automated Ore Sorting
Automated ore sorting represents a sensor-based technology that enables the high-throughput, particle-by-particle separation of valuable minerals from waste rock in mineral processing operations. This method relies on detecting physical properties of individual ore particles to make real-time decisions for rejection or acceptance, typically applied after coarse crushing to sizes ranging from 10 to 300 mm. By integrating advanced sensors and ejection mechanisms, it facilitates pre-concentration early in the flowsheet, enhancing overall efficiency.[82] Key sensors in automated ore sorting include X-ray transmission (XRT), which measures atomic density by detecting X-ray attenuation to identify dense mineral inclusions like sulfides or diamonds; near-infrared (NIR) spectrometry, which analyzes molecular bonds through spectral reflection and absorption for mineral discrimination; and color cameras, which capture surface color and texture for grade detection in applications such as industrial minerals. These sensors operate non-invasively, scanning particles at speeds up to 3 m/s to support bulk throughputs exceeding 1000 tons per hour. XRT is particularly versatile for bulk ores due to its penetration capability, while NIR excels in surface-sensitive sorting of carbonates or silicates.[82][83] Sorting systems commonly employ belt sorters, where ore particles are conveyed on a vibrating feeder belt under sensor arrays for sequential scanning, or free-fall chutes, allowing particles to drop through detection zones for double-sided analysis. Pneumatic ejection, using high-pressure air jets from nozzle arrays, is the standard mechanism for diverting rejected particles into separate streams, enabling precise separation without mechanical contact. For instance, TOMRA's COM series belt sorters utilize XRT for particle sizes of 8–120 mm, while their PRO series free-fall systems incorporate NIR for broader size ranges up to 300 mm. These configurations ensure scalability for industrial-scale operations.[82][83] Algorithms for real-time decision-making process sensor data through machine learning models, such as convolutional neural networks, to recognize features like mineral composition or texture patterns and classify particles as accept or reject. These systems perform binary decisions in milliseconds, often calibrated via offline training on sample datasets to achieve detection accuracies above 95%. Deep learning enhancements, like TOMRA's OBTAIN and CONTAIN technologies, integrate multi-sensor inputs for improved precision in complex ores.[82][84][85] The primary benefits of automated ore sorting include pre-concentration that upgrades feed grades by 20–50%, significantly reducing the mass flow to downstream comminution and reducing energy consumption by up to 50% in milling circuits. It also minimizes water usage and tailings volume, promoting sustainability. Applications are prominent in diamond processing, where XRT sorting at mines like Karowe achieves 96–98% recovery rates for particles of 60–125 mm at 250 tons per hour, and in base metal ores such as copper, tin, and tungsten, exemplified by 90% recovery in tin operations at San Rafael mine.[82][86]Dewatering and Product Handling
Dewatering Techniques
Dewatering techniques in mineral processing involve mechanical methods to remove water from slurries, producing concentrated solids suitable for transport, storage, or further processing. These methods primarily include thickening, filtration, and centrifugation, which target the separation of fine particles from liquids in concentrates and tailings streams. Thickening relies on gravity sedimentation enhanced by flocculants, while filtration and centrifugation use pressure or centrifugal forces to form solid cakes with reduced moisture content.[1] Thickening occurs in large tanks where slurries settle under gravity, allowing solids to form a dense underflow while clarified water overflows for recycle. Flocculants, such as anionic polymers, are added to aggregate fine particles, accelerating settling and increasing underflow solids concentration to 40-60%. The process is governed by sedimentation theory, where the settling velocity of individual particles follows Stokes' law: , with as settling velocity, and as particle and fluid densities, as gravitational acceleration, as particle diameter, and as fluid viscosity. This equation applies to spherical particles in laminar flow conditions typical of mineral slurries, though hindered settling in concentrated suspensions modifies the effective velocity.[87][88][1] Filtration removes water by passing slurry through a porous medium under vacuum or pressure, forming a filter cake of compacted solids. Vacuum filters, such as rotary drum types operating at 0.5-0.8 bar, suit coarser materials, while pressure filters like vertical plate units at 6-16 bar handle finer concentrates. Cake formation involves initial deposition on the filter medium, followed by buildup where the cake's resistance dominates flow. The specific cake resistance , a measure of permeability, is modeled using Darcy's law adapted for constant pressure filtration: the flow rate decreases as , where is filtration time, is mass of cake per unit area, is viscosity, is medium resistance, is filter area, and is pressure drop. Higher values, often 10^9 to 10^11 m/kg for mineral cakes, indicate poorer dewaterability due to fine particles or poor flocculation.[1][89][89] Centrifugation employs high-speed rotation to generate forces up to thousands of times gravity, separating solids from liquids in continuous or batch modes. Decanter centrifuges, with perforated baskets or scrolls, are common for mineral concentrates, producing cakes via centrifugal sedimentation and expression. This method is effective for sludges with 5-35% solids, achieving rapid dewatering without additional chemicals in some cases.[1][90] These techniques collectively enable moisture reduction in mineral concentrates to below 10%, essential for economic transport and smelting; for instance, pressure filtration yields 7-11% moisture in copper concentrates and 6-13% in zinc concentrates.[91][1]Drying Techniques
Following mechanical dewatering, thermal drying removes residual moisture from concentrates to produce a stable, transportable product, typically achieving 1-5% moisture content. Common methods include rotary dryers, which tumble material in a heated rotating drum using hot gas streams (often from combustion or waste heat) at temperatures of 100-600°C, suitable for large volumes of coarse concentrates like iron ore pellets. Flash dryers suspend fine particles in a high-velocity hot gas (300-1000°C) for rapid evaporation in seconds, ideal for ultrafine or heat-sensitive materials such as zinc or phosphate concentrates. Spray dryers atomize slurry into a drying chamber with co-current hot air (150-400°C), forming hollow spheres for easy handling, though less common in minerals due to energy intensity. These processes are energy-intensive, consuming 20-50 kWh per tonne, and require emission controls for dust and volatiles, with innovations focusing on heat recovery and solar-assisted drying for sustainability.[92][93]Tailings Management
Tailings management in mineral processing involves the safe handling, storage, and long-term disposal of waste materials generated after ore beneficiation, aiming to mitigate environmental, health, and safety risks while maximizing resource recovery. These tailings consist of finely ground rock, water, and residual chemicals, often stored in engineered facilities such as dams or impoundments. Effective management requires integrating geotechnical engineering, environmental monitoring, and regulatory compliance to prevent failures and contamination.[94] Tailings are categorized by their consistency and deposition method, which influence water management and stability. Conventional slurry tailings, with 15–35% solids content, are pumped as a fluid mixture and deposited in impoundments where water drains naturally, but they pose higher risks due to liquidity. Thickened tailings, achieving 50–65% solids via high-rate thickeners, reduce water content and beach slope for better consolidation. Paste tailings, with 65–80% solids, exhibit yield stress for subaqueable deposition without segregation, suitable for underground backfill. Dry-stacked or filtered tailings, exceeding 80% solids through pressure filtration, enable stacking without impoundments, minimizing water release and footprint. These densified forms enhance water recovery and reduce seepage compared to slurries.[94][95][96] Tailings storage facilities are constructed using various dam raising methods to accommodate ongoing deposition. Upstream construction builds raises on the tailings beach itself, using minimal borrowed materials for cost efficiency, but relies on tailings consolidation for stability, increasing liquefaction risks during seismic events. Downstream construction raises the dam on the original foundation away from the tailings, providing greater structural independence and resistance to failure, though it requires more earthfill and higher costs. Centerline methods combine elements of both, balancing material use and stability by raising along the dam crest. Selection depends on site geology, seismicity, and economics, with downstream preferred for high-risk areas.[97][98][99] Significant risks in tailings management stem from geotechnical instability, leading to dam failures that release toxic slurries, causing fatalities, ecosystem damage, and contamination. The 2019 Brumadinho dam collapse in Brazil, an upstream facility at an iron ore mine, exemplifies these hazards; liquefaction of the tailings mass due to internal erosion and poor drainage triggered a catastrophic flow, killing 270 people and polluting the Paraopeba River with heavy metals over 300 km. Such incidents highlight vulnerabilities in older dams, where static and dynamic loading, combined with inadequate monitoring, exacerbate failure probabilities, with global reports of 3-6 catastrophic failures annually, corresponding to an estimated annual probability of approximately 1 in 3,000 to 1 in 6,000 for individual facilities based on around 18,000 worldwide tailings storage facilities. Recent incidents, such as the 2024 Chinchorro failure in Chile and 2025 events in Indonesia, underscore ongoing challenges. Mitigation involves regular stability assessments, seepage control, and emergency preparedness.[100][101][102][103][104] Reprocessing tailings offers potential to recover residual metals using modern technologies, transforming waste into resources amid rising mineral demand. Techniques such as flotation, gravity separation, and hydrometallurgical leaching can extract valuables like gold, copper, and rare earths from legacy deposits, with recovery rates up to 70% in some cases, depending on original ore grade and tailings age. Sensor-based sorting and bioleaching further enable selective recovery, reducing environmental liabilities while supporting circular economy principles. Economic viability improves with metal prices and technological advances, as demonstrated in U.S. operations reprocessing historic tailings for critical minerals.[105][106][107] Regulatory frameworks have evolved to address these challenges, with the Global Industry Standard on Tailings Management (GISTM), launched in 2020 by the International Council on Mining and Metals (ICMM) and United Nations Environment Programme (UNEP), providing a comprehensive benchmark. The GISTM mandates a lifecycle approach, including site-specific risk assessments, independent audits every three years, and public disclosure of facility data, applying to all tailings types with a goal of zero harm. It requires operators to maintain an interdisciplinary knowledge base for design, operation, and closure. As of 2025, 67% of the 836 tailings facilities managed by ICMM members are in full conformance with the GISTM, influencing over 1,800 facilities worldwide and driving adoption of safer practices post-disasters like Brumadinho.[108][109][110][103]Chemical and Auxiliary Processes
Chemical Separation Methods
Chemical separation methods in mineral processing encompass hydrometallurgical techniques that exploit chemical reactions to selectively dissolve and extract valuable minerals from ores, often applied to low-grade or complex deposits where physical methods alone are insufficient. These processes, such as leaching, rely on the differential solubility of minerals in aqueous solutions under controlled chemical conditions, enabling the separation of metals like gold, copper, and uranium. Leaching typically involves the percolation or agitation of reagents through prepared ore, followed by downstream recovery of dissolved metals via precipitation, solvent extraction, or ion exchange.[111] Heap leaching represents a cost-effective chemical separation approach for low-grade ores, particularly in gold extraction using cyanide as the primary reagent. Crushed ore is stacked on impermeable liners to form heaps, through which a dilute sodium cyanide solution (0.05–0.5% NaCN) is irrigated, dissolving gold via the reaction:This process achieves gold recovery rates of 60–90% over periods of 30–90 days, with the pregnant leach solution collected at the base for further processing.[112] For copper and uranium, acid leaching with sulfuric acid is widely employed; in uranium processing, 10–100 kg of H₂SO₄ per tonne of ore oxidizes tetravalent uranium to the soluble hexavalent state, forming uranyl sulfate complexes and yielding 85–95% recovery in 4–24 hours at 40–60°C.[113] Similarly, sulfuric acid leaches copper from oxide minerals via:
with recoveries up to 80% in heap configurations for low-grade deposits.[114] The kinetics of these leaching reactions are frequently governed by diffusion limitations, modeled by the shrinking core approach, which describes a reactive core diminishing inward as the mineral dissolves, surrounded by an inert ash layer. For diffusion-controlled leaching through the ash layer, the model follows:
where is the conversion fraction of the solid reactant B, is the rate constant incorporating diffusion coefficients and particle radius, and is time; this equation applies to scenarios like chalcopyrite or uraninite leaching where reagent diffusion through product layers limits the rate.[115] The model highlights how particle size, porosity, and temperature influence overall efficiency, with activation energies typically 20–50 kJ/mol for mineral systems.[116] Key reagents in chemical separation include complexing agents that stabilize dissolved metals—such as cyanide for gold or ammonia for copper—and oxidants like oxygen, hydrogen peroxide, or sodium chlorate to maintain appropriate redox potentials (Eh). For instance, in uranium leaching, manganese dioxide serves as an oxidant to elevate Eh above 400 mV, ensuring uranium solubility.[113] pH and Eh are critically controlled using additives like lime (for pH 10–11 in cyanidation to minimize toxic HCN formation) or sulfuric acid (pH 1–2 for base metals), guided by Pourbaix diagrams to position the target mineral in its leachable domain while suppressing impurities.[117] These parameters optimize selectivity and yield, with pH deviations reducing extraction by up to 50% in sensitive systems.[118] Integration of chemical methods with physical preprocessing, such as grinding to liberate minerals, enhances accessibility and kinetics in leaching operations. Hybrid approaches, like combining chemical acid leaching with bioleaching, further improve efficiency for refractory ores by using microbial oxidants to precondition sulfides before chemical dissolution.[119]