Froth flotation 2


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Froth flotation 2

  1. 1. FrothFlotation_2 Collection in the FrothLayer ReagentsBy Pambudi Pajar Pratama BEng, MScBy Pambudi Pajar Pratama BEng, MSc
  2. 2. Collection in the Froth Layer Once a particle and bubble have come in contact,the bubble must be large enough for its buoyancy tolift the particle to the surface. This is obviously easier if the particles are low- density(as is the case for coal) than if they are high-density(such as lead sulfide). The particle and bubble must remain attached whilethey move up into the froth layer at the top of the cell. The froth layer must persist long enough to either flowover the discharge lip of the cell by gravity, or to beremoved by mechanical froth scrapers. If the froth is insufficiently stable, the bubbles will breakand drop the hydrophobic particles back into theslurry prematurely. However, the froth should not be so stable as tobecome persistent foam, as a foam is difficult toconvey and pump through the plant.
  3. 3. Collection in the Froth Layer The surface area of the bubbles in the froth is alsoimportant. Since particles are carried into the frothby attachment to bubble surfaces, increasingamounts of bubble surface area allows a morerapid flotation rate of particles. At the same time, increased surface area alsocarries more water into the froth as the filmbetween the bubbles. Since fine particles that are not attached to airbubbles will be unselectively carried into the frothalong with the water (entrainment), excessiveamounts of water in the froth can result insignificant contamination of the product withgangue minerals.
  4. 4. ReagentsThe properties of raw mineral mixtures suspended in plain water are rarelysuitable for froth flotation. Chemicals are needed both to control the relativehydrophobicities of the particles, and to maintain the proper frothcharacteristics. There are therefore many different reagents involved in thefroth flotation process, with the selection of reagents depending on the specificmineral mixtures being treated. (Klimpel, 1995).
  5. 5. Reagents1. Collectorso Collectors are reagents that are used to selectively adsorbonto the surfaces of particles.o They form a monolayer on the particle surface that essentiallymakes a thin film of non-polar hydrophobic hydrocarbons.o The collectors greatly increase the contact angle so thatbubbles will adhere to the surface.o Selection of the correct collector is critical for an effectiveseparation by froth flotation.o Collectors can be generally classed depending on their ioniccharge: they can be nonionic, anionic, or cationic, as shown inFigure 1.o The nonionic collectors are simple hydrocarbon oils, while theanionic and cationic collectors consist of a polar part thatselectively attaches to the mineral surfaces, and a non-polarpart that projects out into the solution and makes the surfacehydrophobic.o Collectors can either chemically bond to the mineral surface(chemisorption), or be held on the surface by physical forces(physical adsorption).
  6. 6. ReagentsFigure 1. Basic collector types, after Glembotskii et al. (1972). In the structures,“R” represents a hydrocarbon chain, different collectors will use differenthydrocarbons for “R”
  7. 7. Reagents Non-Ionizingo Non-polar hydrocarbons that do not dissociate in water. Cationico Based on pentavalent nitrogen cation. Carboxylico Sodium oleate and fatty acids with this polar group occurin vegetable oils. Collector for hematite and other metaloxide minerals. Strong collector, low selectivity. Sulfates and Sulfonateso Less-used than fatty acids. Less collecting power, higherselectivity. Xanthates and Dithiophosphateso Carbon is tetravalent, has four bonds; phosphorus ispentavalent with five bonds. Sulfur atoms chemically bondto sulfide mineral surface.
  8. 8. Reagents1.1. Chemisorptiono In chemisorption, ions or molecules from solution undergo achemical reaction with the surface, becoming irreversiblybonded. This permanently changes the nature of the surface.Chemisorption of collectors is highly selective, as the chemicalbonds are specific to particular atoms.1.2. Physisorptiono In physisorption, ions or molecules from solution becomereversibly associated with the surface, attaching due toelectrostatic attraction or van der Waals bonding. Thephysisorbed substances can be desorbed from the surfaceif conditions such as pH or composition of the solutionchanges. Physisorption is much less selective thanchemisorption, as collectors will adsorb on any surface thathas the correct electrical charge or degree of naturalhydrophobicity.
  9. 9. Reagents1.3. Nonionic Collectorso Hydrocarbon oils, and similar compounds, have an affinity forsurfaces that are already partially hydrophobic. They selectivelyadsorb on these surfaces, and increase their hydrophobicity. Themost commonly-floated naturally-hydrophobic material is coal.Addition of collectors such as #2 fuel oil and kerosene significantlyenhances the hydrophobicity of the coal particles without affectingthe surfaces of the associated ash-forming minerals. This improvesthe recovery of the coal, and increases the selectivity between coalparticles and mineral matter.o Fuel oil and kerosene have the following advantages overspecialized collectors for froth flotation:1) They have low enough viscosity to disperse in the slurry and spreadover the coal particles easily, and2) They are very low-cost compared to other compounds which canbe used as coal collectors.o In addition to coal, it is also possible to float naturally-hydrophobicminerals such as molybdenite, elemental sulfur, and talc withnonionic collectors. Nonionic collectors can also be used as“extenders” for other collectors. If another, more-expensivecollector makes a surface partially hydrophobic, adding a nonpolaroil will often increase the hydrophobicity further at low cost.
  10. 10. Reagents1.4. Anionic Collectorso Hydrocarbon Anionic collectors are weak acids oracid salts that ionize in water, producing a collectorthat has a negatively-charged end that will attachto the mineral surfaces, and a hydrocarbon chainthat extends out into the liquid, as shown in Figure 2.Figure 2. Adsorption of anionic collector onto a solid surface. Theanionic portion is responsible for the attachment of the collectormolecule to the surface, while the hydrophobic part alters the surfacehydrophobicity.
  11. 11. Reagents1.4.1. Anionic Collectors for Sulfide Mineralso The most common collectors for sulfide minerals arethe sulfhydryl collectors, such as the various xanthatesand dithiophosphates.o Xanthates are most commonly used, and havestructures similar to what is shown in Figure 3.o Xanthates are highly selective collectors for sulfideminerals, as they chemically react with the sulfidesurfaces and do not have any affinity for thecommon non-sulfide gangue minerals.o Other highly-selective collectors for use with sulfideminerals, such as dithiophosphates, have somewhatdifferent adsorption behavior and so can be used forsome separations that are difficult using xanthates.
  12. 12. Reagents1.4.1. Anionic Collectors for Sulfide Minerals1.4.2. Anionic Collectors for Oxide Mineralso The collectors available for flotation of oxide minerals arenot as selective as the collectors used for sulfide mineralflotation, as they attach to the surface by electrostaticattraction rather than by chemically bonding to thesurface. As a result, there is some collector adsorption ontothe minerals that are not intended to float.Figure 3. Structure of a typical xanthate collector (ethyl xanthate).The OCSS- group attaches irreversibly to the sulfide mineral surface.Using xanthates with longer hydrocarbon chains tends to increase thedegree of hydrophobicity when they adsorb onto the surface.
  13. 13. Reagents1.4.2. Anionic Collectors for Oxide Mineralso A typical anionic collector for oxide mineral flotationis sodium oleate, the sodium salt of oleic acid, whichhas the structure shown in Figure 4. The anionic groupresponsible for attaching it to the mineral surface isthe carboxyl group, which dissociates in water todevelop a negative charge. The negatively-chargedgroup is then attracted to positively-charged mineralsurfaces.Figure 4. The structure of oleic acid, a very commonly-used anioniccollector.
  14. 14. Reagents1.4.2. Anionic Collectors for Oxide Mineralso Since particles that are immersed in water develop a netcharge due to exchanging ions with the liquid, it is oftenpossible to manipulate the chemistry of the solution so thatone mineral has a strong positive charge while otherminerals have a charge that is either only weakly positive,or negative. In these conditions, the anionic collector willpreferentially adsorb onto the surface with the strongestpositive charge and render them hydrophobic.1.5. Cationic Collectorso Cationic collectors use a positively-charged amine group(shown in Figure 5) to attach to mineral surfaces. Since theamine group has a positive charge, it can attach tonegatively-charged mineral surfaces. Cationic collectorstherefore have essentially the opposite effect from anioniccollectors, which attach to positively-charged surfaces.Cationic collectors are mainly used for flotation of silicatesand certain rare-metal oxides, and for separation ofpotassium chloride (sylvite) from sodium chloride (halite).
  15. 15. Reagents1.5.1. Cationic CollectorsFigure 5. Primary, secondary, and tertiary amine groups that can be usedfor cationic collectors.2. Frotherso Frothers are compounds that act to stabilize air bubbles sothat they will remain well-dispersed in the slurry, and willform a stable froth layer that can be removed before thebubbles burst. The most commonly used frothers arealcohols, particularly MIBC (Methyl Isobutyl Carbinol, or 4-methyl-2-pentanol, a branched-chain aliphatic alcohol) orany of a number of water-soluble polymers based onpropylene oxide (PO) such as polypropylene glycols.
  16. 16. Reagents2. Frotherso The polypropylene glycols in particular are veryversatile, and can be tailored to give a wide range offroth properties. Many other frothers are available,such as cresols and pine oils, but most of these areconsidered obsolete and are not as widely used asthey once were. Some work has also been done usingsalt water (particularly seawater) as the frothingagent, and the process has been used industrially inRussia (Klassen and Mokrousov, 1963; Tyurnikova andNaumov, 1981).2.1. Function of Frothers:Klimpel (1995) found that use of different frothersproduced changes in the flotation rate (K) andrecovery (R) values in coal flotation, and reached thefollowing conclusions:
  17. 17. Reagents2.1. Function of Frothers:o When frother dosage was held constant whilecollector dosage was increased, it was found that theflotation rate went through a maximum and thendecreased. This was observed for all frother types andall particle size fractions. The difference between thefrother families studied was that the collector dosagethat produced the maximum value of K was different.o For all of the frother types, the finest (-88 µm) andcoarsest (+500 µm) particles tended to float moreslowly than the intermediate-size particles.o Changes in flotation rate were due to both changesin the coal particle size, and to frother/ collectordosage. While the contribution of particle size wasgenerally more significant, the reagent dosage effectprovides a useful means for adjusting K in the plant.
  18. 18. Reagents2.1. Function of Frothers:o With aliphatic alcohol frothers, the flotation rate maximumwas much more pronounced than for the Propylene Oxide(PO) and combined Propylene Oxide/Alcohol (PO-AlcoholAdduct) frothers.o Regardless of frother type, increasing the frother dosage toincrease recovery always leads to less selective flotation.o The PO and PO-Alcohol Adduct frothers are more powerfulrecovery agents than alcohol frothers, and thereforeshould be used at lower dosages.o Over-dosing with alcohol frothers leads to a slower flotationrate, because excesses of these frothers tend to destabilizethe froth. This effect does not occur with the PO and PO-Alcohol frothers, and so overdosing with these frothersleads to high recovery with poor selectivity.o PO frothers with molecular weights of 300 to 500 areoptimal for coal recovery.
  19. 19. Reagents2.1. Function of Frothers:o Alcohol frothers tend to be more effective for fine-particlerecovery than for coarse- particle recovery. To recovercoarse particles, the alcohol frother and the hydrocarboncollector dosages should both be high. The alcohol will stillprovide reasonable selectivity at these high dosages.o The high-molecular-weight PO-based frothers are moreeffective for coarse particle flotation than the alcohol orlow-molecular-weight PO frothers, but also have a lowerselectivity. For both good coarse-particle recovery andgood selectivity, the PO frothers should be used at lowdosage, with low collector dosage as well. The PO-AlcoholAdduct frothers are even more effective for coarse-particle recovery, and need to be used at even lowerdosages.o The optimal frother for high recovery with good selectivitywill often be a blend of members of the various frotherclasses examined. It is reported that such frother blendingwill give enough benefit to be worth the effort inapproximately half of all coal flotation operations.
  20. 20. Reagents2.1. Function of Frothers:o None of the frothers in the three categories studied willchange the shape of the grade/recovery curve. Changesin frother type and dosage simply move the flotationresults along the curve. Similarly, changes in hydrocarboncollector dosage also mainly move the performancealong the grade/recovery curve.o For medium and coarse coal size fractions, the totalgangue recovered is linearly related to the total coalrecovered. It is only for the finest particles that the ganguerecovery increases non-linearly with increasing coalrecovery.o When floating coals with a broad particle size range, themajority of the gangue reaching the froth is from the finerparticle size fractions.o As the rate of coal flotation increases, the rate of gangueflotation increases proportionately. This is typical of a frothentrainment process acting on the gangue.
  21. 21. Reagents2.2. Synthetic and Natural Frotherso The original frothers were natural products, such as pine oiland cresylic acid. These are rich in surface-active agentsthat stabilize froth bubbles, and are effective frothers.o As natural products, they are not pure chemicals, butinstead contain a broad range of chemicals other thanthose that are effective frothers. Some of thesecompounds can act as collectors by attaching to mineralsurfaces.o As a result, these frothers are also weak collectors. Whilethis can have the advantage of reducing the amount ofcollector that needs to be added separately, it introducessome problems with process control.o If the frother is also a collector, then it becomes impossibleto alter the frothing characteristics and the collectingcharacteristics of the flotation operation independently.
  22. 22. Reagents2.2. Synthetic and Natural Frotherso Synthetic frothers, such as the alcohol-type andpolypropylene glycol-type frothers, have the advantage thattheir effectiveness as collectors is negligible. It is thereforepossible to increase the frother dosage without also changingthe quantity of collector in the system. This in turn makes theflotation process much easier to control.3. Modifierso Modifiers are chemicals that influence the way thatcollectors attach to mineral surfaces. They may eitherincrease the adsorption of collector onto a given mineral(activators), or prevent collector from adsorbing onto amineral (depressants). It is important to note that justbecause a reagent is a depressant for onemineral/collector combination, it does not necessarilyfollow that it is a depressant for other combinations. Forexample, sodium sulfide is a powerful depressant for sulfideminerals being floated with xanthate, but does not affectflotation when sulfide minerals are floated with thecollector hexadecyl trimethyl ammonium bromide.
  23. 23. Reagents3.1. pH Controlo The simplest modifiers are pH control chemicals. Thesurface chemistry of most minerals is affected by the pH.o For example, in general minerals develop a positivesurface charge under acidic conditions and a negativecharge under alkaline conditions.o Since each mineral changes from negatively-charged topositively-charged at some particular pH, it is possible tomanipulate the attraction of collectors to their surfaces bypH adjustment.o There are also other, more complex effects due to pH thatchange the way that particular collectors adsorb onmineral surfaces.
  24. 24. Reagents 3.1. pH Controlo Sulfhydryl collectors suchas xanthate ions competewith OH- ions to adsorb onmineral surfaces, and soadsorption is a function ofpH. This makes it possiblefor sulfhydryl collectors tobe used to progressivelyseparate specific minerals.The pH where thexanthate ion wins thecompetition with OH- ionsdepends both on theconcentration of xanthatein solution, and on thespecific sulfide mineralpresent, as shown in Figure6.Figure 6. Schematic of the pHresponse curves for sulfhydryl collectoradsorption on different sulfideminerals.
  25. 25. Reagents3.1. pH ControlFigure 6. Schematic of the pH response curves for sulfhydryl collectoradsorption on different sulfide minerals. These curves mark theboundary where the given mineral becomes sufficiently hydrophobicto float. Both xanthates and dithiophosphates exhibit curves of thisform, with different pH values and concentrations for each type ofcollector (Fuerstenau et al., 1985).
  26. 26. Reagents3.1. pH ControlFor example, assume a mixture ofpyrite (FeS2), galena (PbS), andchalcopyrite (CuFeS2). From Figure 6,we see that if the pH and xanthateconcentrations are in region (A), thenxanthate does not adsorb on any ofthe minerals and no minerals float. Ifthe pH and xanthate concentrationsare altered to move into region (B),then only chalcopyrite becomeshydrophobic and floats. In region (C),both chalcopyrite and galena willfloat, and in region (D) all threeminerals will float. It is therefore possiblyto progressively lower the pH to floatfirst chalcopyrite, then galena, andthen pyrite, producing concentratesfor each mineral and leaving behindany non-floatable silicate ganguemineralsFigure 6. Schematic of thepH response curves forsulfhydryl collectoradsorption on differentsulfide minerals.
  27. 27. Reagents3.1. pH ControlFor example, assume a mixture ofpyrite (FeS2), galena (PbS), andchalcopyrite (CuFeS2). From Figure 6,we see that if the pH and xanthateconcentrations are in region (A), thenxanthate does not adsorb on any ofthe minerals and no minerals float. Ifthe pH and xanthate concentrationsare altered to move into region (B),then only chalcopyrite becomeshydrophobic and floats. In region (C),both chalcopyrite and galena willfloat, and in region (D) all threeminerals will float. It is therefore possiblyto progressively lower the pH to floatfirst chalcopyrite, then galena, andthen pyrite, producing concentratesfor each mineral and leaving behindany non-floatable silicate ganguemineralsFigure 6. Schematic of thepH response curves forsulfhydryl collectoradsorption on differentsulfide minerals.
  28. 28. Reagents3.1. pH ControlFigure 6. Schematic of thepH response curves forsulfhydryl collectoradsorption on differentsulfide minerals. Acidso The acids used are generally thosethat that give the greatest pH changeat the lowest cost, with sulfuric acidbeing most popular. A key point tokeep in mind is that the anion of theacid can potentially have effects of itsown, separate from the lowering of thepH. There are therefore some caseswhere acids other than sulfuric acidare useful. Alkaliso Like acids, the most popular alkalis arethose that are cheapest, with thelowest-cost alkali generally being lime(CaO or Ca(OH)2). However, thecalcium ion often interacts withmineral surfaces to change theirflotation behavior. In some cases thecalcium ions have beneficial effects,while in other cases they change theflotation in undesirable ways. It maytherefore be necessary to use sodium-based alkalis such as NaOH orNa2CO3, because the sodium cationgenerally does not have anysignificant effect on the particlesurface chemistries.
  29. 29. Reagents3.2. Activatorso Activators are specific compounds that make itpossible for collectors to adsorb onto surfaces thatthey could not normally attach to.o A classic example of an activator is copper sulfate asan activator for sphalerite (ZnS) flotation with xanthatecollectors (Fuerstenau et al., 1985).o When untreated, xanthate cannot attach to thesphalerite surface because it forms a zinc-xanthatecompound that quickly dissolves:ZnS(s)+ Xanthate-S(s) + ZnXanthate (aq)The surface of the sphalerite can be activated byreacting it with a metal ion that does not form asoluble xanthate, such as soluble copper fromdissolved copper sulfate:ZnS(s) + CuSO4(aq) CuS(s) + ZnSO4(aq)
  30. 30. Reagents3.2. Activatorso This forms a thin film of copper sulfide on the sphaleritesurface, which allows for stable attachment of thexanthate, rendering the sphalerite particle hydrophobicand floatable. Other metals such as silver and lead can alsobe used to activate zinc, but copper is cheaper than silverand less toxic than lead.o It is also possible to adsorb specific ions onto the surfacethat can promote attachment of the collector. Forexample, silica (SiO2) normally has a strongly-negativesurface charge at approximately neutral pH, and thereforehas little affinity for anionic collectors such as oleic acid.o However, calcium ions specifically adsorb onto silicasurfaces, and the negative charge of the calcium ions canactually reverse the surface charge, making it positive.o It is then possible for the anionic collectors toelectrostatically attach to the calcium-activated silicasurface.
  31. 31. Reagents3.3. Depressants o Depressants have the opposite effect of activators, bypreventing collectors from adsorbing onto particularmineral surfaces. Their typical use is to increaseselectivity by preventing one mineral from floating,while allowing another mineral to float unimpeded.3.3.1. Cyanideo Cyanide (CN-) is a particularly useful depressant insufide mineral flotation. Its activity is believed to bedue to its ability to complex with, and in some casesdissolve, a number of metal ions, preventing them fromattaching to the xanthate molecules. In particular, it isa strong depressant for pyrite (FeS2), and can be usedto “deactivate” sphalerite that has been activated bycopper ions in solution (Fuerstenau et al, 1985).
  32. 32. Reagents3.3.1. CyanideFigure 7. Effect of cyanide depressant on flotation of mineralsas a function of pH. It is interesting to note that flotation ofgalena (PbS) is unaffected by the presence of cyanide.
  33. 33. Reagents3.3.2. Limeo Lime is added as either CaO or Ca(OH)2, and when itdissolves it contributes calcium ions that can adsorb ontomineral surfaces. In combination with its strong alkalinenature, this makes it particularly useful in manipulatingsulfide flotation. It is less useful in oxide mineral flotation,because it can activate the flotation of silica by anioniccollectors, causing it to float along with the other oxideminerals.3.3.3. Organic Depressantso A large number of organic compounds are useful asflotation depressants. These tend to be soluble polymers(such as starch) that selectively coat mineral surfaces andprevent collector from attaching. An example of this is inthe “reverse flotation” of silica from iron ore, where the silicatailings are floated using a cationic collector at a pH of 8.5 -11, leaving behind the iron oxide minerals. Starch acts as adepressant for iron oxide in this process, preventing it frombeing floated by the cationic collector.
  34. 34. Reagents Recovery of Gangue Particles in Flotation:Entrainment or Hydrophobicity?o There is always some gangue material that isrecovered in the froth.o For example, in flotation of coal, a portion of the ash-forming minerals and pyrite will be carried into thefroth along with the coal.o It is common to believe that this gangue mineral canbe prevented from floating if only the correctdepressant can be discovered. In the case of coal,over 42 different chemicals have been reported bymany investigators to depress pyrite flotation, and yetnone of them have ever been useful industrially. This isbecause, in most cases, the pyrite was not trulyhydrophobic in the first place (Kawatra and Eisele,1992, 2001).
  35. 35. Reagents Recovery of Gangue Particles in Flotation:Entrainment or Hydrophobicity?o In a froth flotation machine, there are two ways that aparticle can reach the froth layer.o It can be carried into the froth by attachment to an airbubble (true flotation), or it can be suspended in thewater trapped between the bubbles (entrainment).While true flotation is selective between hydrophobicand hydrophilic particles, entrainment is nonselective,and so entrained particles are just as likely to be gangueminerals as they are to be the valuable mineral. Ifparticles are sufficiently coarse, then they settle rapidlyenough that they are not carried into the froth byentrainment.o As they become finer, particles settle more slowly and sohave more time to be entrapped in the froth, and haveless tendency to drain away. Clay particles in particular,which are only a few micrometers in size, are very easilyentrained.
  36. 36. Reagents Recovery of Gangue Particles in Flotation:Entrainment or Hydrophobicity?o For particles that are less than a few micrometers in size, theirrate of recovery into the froth by entrainment is equal to therate of recovery of water into the froth.o For example, if 20% of the water entering a flotation cell iscarried into the froth, then up to 20% of the fine particlesentering the cell will be entrained. The entrainment of coarserparticles will be less than 20%, due to their greater ability todrain from the froth.o In addition to the entrained particles, gangue is carried into thefroth by being physically locked to the floatable particles. Inthe case of coal, much of the pyrite consists either of sub-micron pyrite grains that are never liberated from the coal, orof pyrite particles whose surfaces consist primarily of coal, andtherefore behave as if they were coal particles.o The recovery of entrained particles can only be reduced bylowering the fraction of the water recovered into the froth.
  37. 37. Reagents Recovery of Gangue Particles in Flotation:Entrainment or Hydrophobicity?o The recovery of locked particles can only bechanged significantly by either grinding to a finer sizeto improve liberation, or by rejecting the lockedparticles along with the least-floatable liberatedparticles, which sacrifices recovery of the valuablemineral.o Depressant chemicals are not useful in either of thesecases, as they are only useful for preventing thehydrophobic bubble attachment and true flotation ofparticles. Before deciding to use a depressant, it istherefore important to first determine whether theparticles to be depressed are actually beingrecovered by true flotation in the first place, orwhether there are other causes.
  38. 38. FrothFlotation_3 EquipmentsBy Pambudi Pajar Pratama BEng, MScBy Pambudi Pajar Pratama BEng, MSc