Unit operations Mineral processing

1 unit operations

1.1 comminution
1.2 sizing
1.3 concentration
1.4 gravity concentration
1.5 froth flotation
1.6 electrostatic separation
1.7 magnetic separation
1.8 automated ore sorting
1.9 dewatering

unit operations

mineral processing can involve 4 general types of unit operation: comminution – particle size reduction; sizing – separation of particle sizes screening or classification; concentration taking advantage of physical , surface chemical properties; , dewatering – solid/liquid separation. in of these processes, important considerations economics of processes , dictated grade , recovery of final product. this, mineralogy of ore needs considered dictates amount of liberation required , processes can occur. smaller particles processes, greater theoretical grade , recovery of final product, difficult fine particles prevent concentration processes occurring.

comminution

comminution particle size reduction of materials. comminution may carried out on either dry materials or slurries. crushing , grinding 2 primary comminution processes. crushing carried out on run-of-mine ore, while grinding (normally carried out after crushing) may conducted on dry or slurried material. in comminution, size reduction of particles done 3 types of forces: compression, impact , attrition. compression , impact forces extensively used in crushing operations while attrition dominant force in grinding. used equipment in crushing are-jaw crushers, gyratory crushers , cone crushers whereas rod mills , ball mills, closed circuited classifier unit, employed grinding purposes in mineral processing plant. crushing dry process whereas grinding performed wet , hence more energy intensive.

sizing

screening ore through sieve, fixed nitrogen research laboratory, 1930



sizer 2000 screening coarse small particles

sizing general term separation of particles according size.

the simplest sizing process screening, or passing particles sized through screen or number of screens. screening equipment can include grizzlies, bar screens, wedge wire screens, radial sieves, banana screens, multi-deck screens, vibratory screen, fine screens, flip flop screens , wire mesh screens. screens can static (typically case coarse material), or can incorporate mechanisms shake or vibrate screen. considerations in process include screen material, aperture size, shape , orientation, amount of near sized particles, addition of water, amplitude , frequency of vibrations, angle of inclination, presence of harmful materials, steel , wood, , size distribution of particles.

classification refers sizing operations exploit differences in settling velocities exhibited 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 , sizing operations determination of particle size distribution of materials being processed, commonly referred particle size analysis. many techniques analyzing particle size used, , techniques include both off-line analyses require sample of material taken analysis , on-line techniques allow analysis of material flows through process.

concentration

there number of ways increase concentration of wanted minerals: in particular case method chosen depend on relative physical , surface chemical properties of mineral , gangue. concentration defined number of moles of solute in volume of solution. in case of mineral processing concentration means increase of percentage of valuable mineral in concentrate.

gravity concentration

gravity separation separation of 2 or more minerals of different specific gravity relative movement in response force of gravity , 1 or more other forces (such centrifugal forces, magnetic forces, buoyant forces), 1 of resistance motion (drag force) viscous medium such heavy media, water or, less commonly, air.

gravity separation 1 of oldest technique in mineral processing has seen decline in use since introduction of methods flotation, classification, magnetic separation , leaching. gravity separation dates @ least 3000 bc when egyptians used technique separation of gold.

it necessary determine suitability of gravity concentration process before employed concentration of ore. concentration criterion commonly used purpose, designated



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represents specific gravity):

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{\displaystyle cc={\frac {sg(heavy\ mineral)-sg(fluid)}{sg(light\ mineral)-sg(fluid)}}}




for cc > 2.5, suitable separation of particles above 75 micron in size
for 1.75 < cc < 2.5, suitable separation of particles above 150 micron in size
for 1.50 < cc < 1.75, suitable separation of particles above 1.7 mm in size
for 1.25 < cc < 1.50, suitable separation of particles above 6.35 mm in size
for cc < 1.25, not suitable size

although concentration criteria useful rule of thumb when predicting amenability gravity concentration, factors such particle shape , relative concentration of heavy , light particles can dramatically affect separation efficiency in practice.

classification

there several methods make use of weight or density differences of particles:

heavy media or dense media separation (these include sepro condor dms, baths, drums, larcodems, dyana whirlpool separators, , dense medium cyclones)
shaking tables, such wilfley table
spiral separators
reflux classifier
jig concentrators continuous processing gravity concentration devices using pulsating fluidized bed.(rms-ross corp. circular jig plants)
centrifugal bowl concentrators, such knelson concentrator , falcon concentrator
multi gravity separators (falcon concentrator, knelson, mozley (multi or enhanced gravity separator manufactured gravity mining ltd, england. www.gravitymining.com), salter cyclones (multi-gravity separator) , kelsey jig)
inline pressure jigs
reichert cones
sluices
elutriators

these processes can classified either density separation or gravity (weight) separation.

in dense media separation media created density in between density of ore , gangue particles. when subjected media particles either float or sink depending on density relative media. in way separation takes place purely on density differences , not, in principle, relay on other factors such particle weight or shape. in practice, particle size , shape can affect separation efficiency. dense medium separation can performed using variety of mediums. these include, organic liquids, aqueous solutions or suspensions of fine particles in water or air. organic liquids typically not used due toxicity, difficulties in handling , relative cost. industrially, common dense media suspension of fine magnetite and/or ferrosilicon particles. aqueous solution dense medium used in coal processing in form of belknap wash , suspensions in air used in water-deficient areas, areas of china, sand used separate coal gangue minerals.

gravity separation called relative gravity separation separates particles due relative response driving force. controlled factors such particle weight, size , shape. these processes can classified multi-g , single g processes. difference magnitude of driving force separation. multi-g processes allow separation of fine particles occur (in range of 5 50 micron) increasing driving force of separation in order increase rate @ particles separate. in general, single g process capable of processing particles greater approximately 80 micron in diameter.

of gravity separation processes, spiral concentrators , circular jigs 2 of economical due simplicity , use of space. operate flowing film separation , can either use washwater or washwater-less. washwater spirals separate particles more can have issues entrainment of gangue concentrate produced.

froth flotation

froth flotation cells used concentrate copper , nickel sulfide minerals, falconbridge, ontario.

froth flotation important concentration process. process can used separate 2 different particles , operated surface chemistry of particles. in flotation, bubbles introduced pulp , bubbles rise through pulp. in process, hydrophobic particles become bound surface of bubbles. driving force attachment change in surface free energy when attachment occurs. these bubbles rise through slurry , collected surface. enable these particles attach, careful consideration of chemistry of pulp needs made. these considerations include ph, eh , presence of flotation reagents. ph important changes charge of particles surface , eh affects chemisorption of collectors on surface of particles.

the addition of flotation reagents affects operation of these processes. important chemical added collector. chemical binds surface of particles surfactant. main considerations in chemical nature of head group , size of hydrocarbon chain. hydrocarbon tail needs short maximize selectivity of desired mineral , headgroup dictates minerals attaches to.

the frothers important chemical addition pulp enables stable bubbles formed. important if bubble coalesce, minerals fall off surface. bubbles should not stable prevents easy transportation , dewatering of concentrate formed. mechanism of these frothers not known , further research mechanisms being performed.

depressants , activators used selectively separate 1 mineral another. depressants inhibit flotation of 1 mineral or minerals while activators enable flotation of others. examples of these include cn, used depress sulfides galena , depressant believed operate changing solubility of chemisorbed , physisorbed collectors on sulfides. theory originates russia. example of activator cu ions, used flotation of sphalerite.

there number of cells able used flotation of minerals. these include flotation columns , mechanical flotation cells. flotation columns used finer minerals , typically have higher grade , lower recovery of minerals mechanical flotation cells. cells in use @ moment can exceed 300 m. done cheaper per unit volume smaller cells, not able controlled smaller cells.

this process invented in 19th century in australia. used recover sphalerite concentrate tailings, produced using gravity concentration. further improvements have come australia in form of jameson cell, developed @ university of newcastle, australia. operated use of plunging jet generates fine bubbles. these fine bubbles have higher kinetic energy , such can used flotation of fine grained minerals, such produced isamill.

electrostatic separation

there 2 main types of electrostatic separators. these work in similar ways, forces applied particles different , these forces gravity , electrostatic attraction. 2 types electrodynamic separators (or high tension rollers) or electrostatic separators. in high tension rollers, particles charged corona discharge. charges particles subsequently travel on drum. conducting particles lose charge drum , removed drum centripetal acceleration. electrostatic plate separators work passing stream of particles past charged anode. conductors lose electrons plate , pulled away other particles due induced attraction anode. these separators used particles between 75 , 250 micron , efficient separation occur, particles need dry, have close size distribution , uniform in shape. of these considerations, 1 of important water content of particles. important layer of moisture on particles render non-conductors conductors layer of water conductive.

electrostatic plate separators used streams have small conductors , coarse non-conductors. high tension rollers used streams have coarse conductors , fine non-conductors.

these separators commonly used separating mineral sands, example of 1 of these mineral processing plants crl processing plant @ pinkenba in brisbane queensland. in plant, zircon, rutile , ilmenite separated silica gangue. in plant, separation performed in number of stages roughers, cleaners, scavengers , recleaners.

magnetic separation

magnetic separation process in magnetically susceptible material extracted mixture using magnetic force. separation technique can useful in mining iron attracted magnet. in mines wolframite mixed cassiterite, such south crofty , east pool mine in cornwall or bismuth such @ shepherd , murphy mine in moina, tasmania, magnetic separation used separate ores. @ these mines device called wetherill s magnetic separator (invented john price wetherill, 1844–1906)[1] used. in machine raw ore, after calcination fed onto moving belt passed underneath 2 pairs of electromagnets under further belts ran @ right angles feed belt. first pair of electromagnets weakly magnetised , served draw off iron ore present. second pair magnetised , attracted wolframite, weakly magnetic. these machines capable of treating 10 tons of ore day.this process of separating magnetic substances non-magnetic substances in mixture of magnet called magnetic separation..

this process operates moving particles in magnetic field. force experienced in magnetic field given equation f=m/k.h.dh/dx. k=magnetic susceptibility, h-magnetic field strength, , dh/dx being magnetic field gradient. seen in equation, separation can driven in 2 ways, either through gradient in magnetic field or strength of magnetic field. different driving forces used in different concentrators. these can either water or without. spirals, washwater aids in separation of particles while increases entrainment of gangue in concentrate.

automated ore sorting

modern, automated sorting applies optical sensors (visible spectrum, near infrared, x-ray, ultraviolet), can coupled electrical conductivity , magnetic susceptibility sensors, control mechanical separation of ore 2 or more categories on individual rock rock basis. new sensors have been developed exploit material properties such electrical conductivity, magnetization, molecular structure , thermal conductivity. sensor based sorting has found application in processing of nickel, gold, copper, coal , diamonds.

dewatering

dewatering important process in mineral processing. purpose of dewatering remove water absorbed particles increases pulp density. done number of reasons, specifically, enable ore handling , concentrates transported easily, allow further processing occur , dispose of gangue. water extracted ore dewatering recirculated plant operations after being sent water treatment plant. main processes used in dewatering include dewatering screens such sepro-sizetec screens, sedimentation, filtering, , thermal drying. these processes increase in difficulty , cost particle size decreases.

dewatering screens operate passing particles on screen. particles pass on screen while water passes through apertures in screen. process viable coarse ores have close size distribution apertures can allow small particles pass through.

sedimentation operates passing water large thickener or clarifier. in these devices, particles settle out of slurry under effects of gravity, or centripetal forces. these limited surface chemistry of particles , size of particles. aid in sedimentation process, flocculants , coagulants added reduce repulsive forces between particles. repulsive force due double layer formed on surface of particles. flocculants work binding multiple particles while coagulants work reducing thickness of charged layer on outside of particle. after thickening, slurry stored in ponds or impoundments. alternatively, can pumped belt press or membrane filter press recycle process water , create stackable, dry filter cake, or tailings .

thermal drying used fine particles , remove low water content in particles. common processes include rotary dryers, fluidised beds, spray driers, hearth dryers , rotary tray dryers. process expensive operate due fuel requirement of dryers.