Minerals Engineering 45 (2013) 170-179Contents lists available at SciVerse ScienceDirectMINERALSENGINEERINGMinerals EngineeringELSEVITERjournalhomepage:www.elsevier.com/locate/minengFrom amine molecules adsorption to amine precipitate transport by bubbles:A potash ore flotation mechanism *Janusz S. Laskowski*Norman B.Keevil Institute of MiningEngineering.University of British Columbia,Vancouver,BC, CanadINFOARTICLEABSIRACTArticle history:Recent investigations summarized in this review have been conveniently grouped into (i)those dealingReceived 3 November 2012with the mechanism of action of the reagents applied in the flotation of potash ores, and (ii) those focusedAccepted 11 February 2013on the flotation properties of salt-type minerals and explanation of the remarkable selectivity betweenfloatable sylvite, and non-floatable halite.This paper is confined to thefirst group.It is argued that in dis-cussing the mode of action of long-chain primary amines in the flotation of potash ores account must beThe paper is dedicated to the late Professotaken of the way in which these amines are applied by industry. Because they are water insoluble theyJan Lejaare melted by heating up to 70-90 °C and then they are dispersed in acidified aqueous solution, Onceadded to the flotation pulp,the hot amine dispersion rapidly cools down to a temperature far belowKeywords:the Krafft point.The rapid conversion from a hot emulsion to a cold precipitate is a very severe transfor-SylvitePotash oremation. Since nothing is known about the kinetics of these changes and phase instability only the labFlotationtests in which the adopted reagent preparation procedures closely follow the industrial practice havePotash ore flotationbeen considered in this review.Amines2013 Elsevier Ltd.All rights reservedPrecipitationContents1701.Introduction2.171Early research3.172Krafftpoint4173Electrical charge5.173Use of amines in commercial potash ore flotation1746.Amine precipitate in sylvite flotation1757.Molecular films.1768.The mechanism..9.176Frothersinpotashflotation17810.Summary178Acknowledgements178References1. Introductionnot considered until after Jeanprost (1928) showed that theflota-tionmustbeconducted ina saturated solution of suchminerals.Applying flotation to treat ores containing water-solublesalts-The minerals sylvite (KCl) and halite (NaCl), two major compo-as pointed out by Gaudin in his monograph (Gaudin, 1957)- wasnentsofthemostimportantpotashore-thesylviniteore-canbe separated by flotation that is carried out in a NaCl-KCl saturatedbrine.At20°C,1.450kgof theKCl-NaClsaturated solutioncon-* Based on 2010 Gaudin lecture which was presented at the SME Annual Meeting.tains about 0.300kg of NaCl, 0.150kg of KCl and 1kg of waterDenver, February 28, 2011.(Gaska et al., 1965). Thus the NaCI-KCI saturated brine is approxi-* Fax: +1 604 822 5599.mately6mol/Lsolution of thesetwo salts.TheNaClconcentrationE-mail address: jsi@apsc.ubc.ca0892-6875/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.mineng.2013.02.010
From amine molecules adsorption to amine precipitate transport by bubbles: A potash ore flotation mechanism q Janusz S. Laskowski ⇑ Norman B. Keevil Institute of Mining Engineering, University of British Columbia, Vancouver, BC, Canada article info Article history: Received 3 November 2012 Accepted 11 February 2013 The paper is dedicated to the late Professor Jan Leja Keywords: Sylvite Potash ore Flotation Potash ore flotation Amines Precipitation abstract Recent investigations summarized in this review have been conveniently grouped into (i) those dealing with the mechanism of action of the reagents applied in the flotation of potash ores, and (ii) those focused on the flotation properties of salt-type minerals and explanation of the remarkable selectivity between floatable sylvite, and non-floatable halite. This paper is confined to the first group. It is argued that in discussing the mode of action of long-chain primary amines in the flotation of potash ores account must be taken of the way in which these amines are applied by industry. Because they are water insoluble they are melted by heating up to 70–90 C and then they are dispersed in acidified aqueous solution. Once added to the flotation pulp, the hot amine dispersion rapidly cools down to a temperature far below the Krafft point. The rapid conversion from a hot emulsion to a cold precipitate is a very severe transformation. Since nothing is known about the kinetics of these changes and phase instability only the lab tests in which the adopted reagent preparation procedures closely follow the industrial practice have been considered in this review. 2013 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 2. Early research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 3. Krafft point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 4. Electrical charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 5. Use of amines in commercial potash ore flotation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 6. Amine precipitate in sylvite flotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 7. Molecular films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 8. The mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 9. Frothers in potash flotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 10. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 1. Introduction Applying flotation to treat ores containing water-soluble salts – as pointed out by Gaudin in his monograph (Gaudin, 1957) – was not considered until after Jeanprost (1928) showed that the flotation must be conducted in a saturated solution of such minerals. The minerals sylvite (KCl) and halite (NaCl), two major components of the most important potash ore – the sylvinite ore – can be separated by flotation that is carried out in a NaCl–KCl saturated brine. At 20 C, 1.450 kg of the KCl–NaCl saturated solution contains about 0.300 kg of NaCl, 0.150 kg of KCl and 1 kg of water (Gaska et al., 1965). Thus the NaCl–KCl saturated brine is approximately 6 mol/L solution of these two salts. The NaCl concentration 0892-6875/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.02.010 q Based on 2010 Gaudin lecture which was presented at the SME Annual Meeting, Denver, February 28, 2011. ⇑ Fax: +1 604 822 5599. E-mail address: jsl@apsc.ubc.ca Minerals Engineering 45 (2013) 170–179 Contents lists available at SciVerse ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
171JS. Laskowski/Minerals Engineering 45 (2013) 170-179in seawater is about 0.6 M, there is thus a huge difference in theelectrolyte concentrations between potash ore flotation pulpsand pulps in other flotation operations. Only now is it becomingHaliteapparent that dissimilarity between the potash ore flotation andother flotation systems results mostly from differences in the ionicstrength.This paper reviews recent advances made in understanding thenature of phenomena taking place in the flotation carried out inNacI+ KCl saturated brine, that is in the potash ore flotation pro-cess. The purpose of this paper is to draw a common threadthrough seemingly disparate pieces of evidence, and to reconcilevariousexperimentalresultsandvarioustheoriesputforwardinthe area of potash ore flotation.For the sake of discussion, the papers dealing with various as-pects of potash ore flotation have been grouped into:(i) Those which discuss the mechanism of action of long-chainflotation collectors in saturated brine that leads to a goodSylviteflotation of KCl in such an environment.(ii) Those studying the differences between flotation propertiesof various water soluble salts which make some of themfloatable while the others are not.The latter group will not be considered in this review, the read-Nater is referred to several excellent publications on this topic. RogersandSchulman(1957)andRogers(1957)werethefirsttoconsiderhydration as the phenomenon responsible for the surface proper-Kties of alkali halides. They pointed out that ions like Na, K, andCl-,etc.,arestronglyhydrated andthat thepropertiesof thesur-faces of the minerals containing such ions in water are to a largeCIextent determined by these ions' hydration. This model was fur-ther developed by Miller et al. (Hancer et al., 2001; Cao et al..Dodecylammonium ion2010:Cao et al..2011:Ozdemir et al.,2011).who were able toshow that hydration phenomena at salt crystal surfaces provide aFig.1. Schematic representation of the mechanism of collection of sylvite and lackgood explanation for the flotation properties of floatable sylviteof collection of halite by dodecylammonium ions (D.W. Fuerstenau and M.C.and non-floatable halite (advancing contact angle on KCl was mea-Fuerstenau, 1956).sured to be 7.9 ±0.5° while such an angle measured on NaCl wasO).ThedifferencesinthehydrationbetweenKclandNaClsurfacesis believed to affect the adsorption of flotation collectors on theseTable 1salts.Some important contributions in the area of potash ore flotationYearContributors2. Early research1937JE. Kirby1956D.W.Fuerstenau, M.C. Fuerstenau1957J. Rogers, J.H. SchulmanIt was not until the 1950s that the first detailed papers on the1951-1988R. Bachman,A Singewald, H. Schubertadsorption of aliphatic amines on halides were published by Fuer-1968RJ. Roman, M.C. Fuerstenau, D.C. Seidelstenau and Fuerstenau (1956) - papers that represent the begin-19771982J Leja, V.A. Arsentievning of scientific researchonthe flotation fundamentalsof1985D.A. Cormode1988V.A. Arsentiev, T.V. Dendyuk, S.L Gorlovskipotash ores. In this flotation process, two isomorphous minerals1982 -presentS.N.Titkov- sylvite and halite - are separated by flotation. The best separa-1990 -presentJD. Miller and co-workerstion between these minerals, which differ only in the cation, are1986 - presentJ.S. Laskowski and co-workersobtainedwithcationiccollectors.The only interpretation that makes sense (Gaudin, 1957)is thatthe ammonium ion fits in the place of potassium at the sylvite sur-However,a comparison of some fundamentalface but does not fit in the place of sodium at the halite surface.tionprocesses.correlations found in froth flotation, for example the relationshipFig.1 shows schematically themechanism proposed byFuerstenaubetweencollectorconcentrationandrecovery.forthecommonflo-and Fuerstenau (1956).Some of the benchmarks in the development of the potash oretationsystemsandthepotashoreflotationsystemisstunning.Itisknown that the recovery curve,plotted versus collector concentra-flotation process are listed in Table 1. While the author has triedtion, drops to zero when collector concentration approaches theto tabulate all important developments in this area, only those thatcritical micelle concentration. This is shown in Fig. 2.are consistent with the view presented in this paper will be dis-As Fig. 2 demonstrates, whenever the collector concentrationcussed further. The list begins with Kirby's patent (US Patentapproaches the critical micelle concentration, micelles appear in2,088,325), which introduced straight-chain primary amines intothe solution and flotation drops to zero. This is not surprising sincethe technology of potash ore flotation as a universal collector.micelles are colloidal hydrophilic entities and their accumulationMany ideas - especially in the early period of the process' devel-on the mineral surface must render such a surface hydrophilic.opmentweremoreorlessdirectlytransplantedfromotherflota-
in seawater is about 0.6 M, there is thus a huge difference in the electrolyte concentrations between potash ore flotation pulps and pulps in other flotation operations. Only now is it becoming apparent that dissimilarity between the potash ore flotation and other flotation systems results mostly from differences in the ionic strength. This paper reviews recent advances made in understanding the nature of phenomena taking place in the flotation carried out in NaCl + KCl saturated brine, that is in the potash ore flotation process. The purpose of this paper is to draw a common thread through seemingly disparate pieces of evidence, and to reconcile various experimental results and various theories put forward in the area of potash ore flotation. For the sake of discussion, the papers dealing with various aspects of potash ore flotation have been grouped into: (i) Those which discuss the mechanism of action of long-chain flotation collectors in saturated brine that leads to a good flotation of KCl in such an environment. (ii) Those studying the differences between flotation properties of various water soluble salts which make some of them floatable while the others are not. The latter group will not be considered in this review, the reader is referred to several excellent publications on this topic. Rogers and Schulman (1957) and Rogers (1957) were the first to consider hydration as the phenomenon responsible for the surface properties of alkali halides. They pointed out that ions like Na+ , K+ , and Cl, etc., are strongly hydrated and that the properties of the surfaces of the minerals containing such ions in water are to a large extent determined by these ions’ hydration. This model was further developed by Miller et al. (Hancer et al., 2001; Cao et al., 2010; Cao et al., 2011; Ozdemir et al., 2011), who were able to show that hydration phenomena at salt crystal surfaces provide a good explanation for the flotation properties of floatable sylvite and non-floatable halite (advancing contact angle on KCl was measured to be 7.9 ± 0.5 while such an angle measured on NaCl was 0). The differences in the hydration between KCl and NaCl surfaces is believed to affect the adsorption of flotation collectors on these salts. 2. Early research It was not until the 1950s that the first detailed papers on the adsorption of aliphatic amines on halides were published by Fuerstenau and Fuerstenau (1956) – papers that represent the beginning of scientific research on the flotation fundamentals of potash ores. In this flotation process, two isomorphous minerals – sylvite and halite – are separated by flotation. The best separation between these minerals, which differ only in the cation, are obtained with cationic collectors. The only interpretation that makes sense (Gaudin, 1957) is that the ammonium ion fits in the place of potassium at the sylvite surface but does not fit in the place of sodium at the halite surface. Fig. 1 shows schematically the mechanism proposed by Fuerstenau and Fuerstenau (1956). Some of the benchmarks in the development of the potash ore flotation process are listed in Table 1. While the author has tried to tabulate all important developments in this area, only those that are consistent with the view presented in this paper will be discussed further. The list begins with Kirby’s patent (US Patent 2,088,325), which introduced straight-chain primary amines into the technology of potash ore flotation as a universal collector. Many ideas – especially in the early period of the process’ development were more or less directly transplanted from other flotation processes. However, a comparison of some fundamental correlations found in froth flotation, for example the relationship between collector concentration and recovery, for the common flotation systems and the potash ore flotation system is stunning. It is known that the recovery curve, plotted versus collector concentration, drops to zero when collector concentration approaches the critical micelle concentration. This is shown in Fig. 2. As Fig. 2 demonstrates, whenever the collector concentration approaches the critical micelle concentration, micelles appear in the solution and flotation drops to zero. This is not surprising since micelles are colloidal hydrophilic entities and their accumulation on the mineral surface must render such a surface hydrophilic. Fig. 1. Schematic representation of the mechanism of collection of sylvite and lack of collection of halite by dodecylammonium ions (D.W. Fuerstenau and M.C. Fuerstenau, 1956). Table 1 Some important contributions in the area of potash ore flotation. Year Contributors 1937 J.E. Kirby 1956 D.W. Fuerstenau, M.C. Fuerstenau 1957 J. Rogers, J.H. Schulman 1951–1988 R. Bachman, A. Singewald, H. Schubert 1968 R.J. Roman, M.C. Fuerstenau, D.C. Seidel 1977–1982 J. Leja, V.A. Arsentiev 1985 D.A. Cormode 1988 V.A. Arsentiev, T.V. Dendyuk, S.I. Gorlovski 1982 – present S.N. Titkov 1990 – present J.D. Miller and co-workers 1986 – present J.S. Laskowski and co-workers J.S. Laskowski / Minerals Engineering 45 (2013) 170–179 171
172J.S.Laskowski/MineralsEngineering45(2013)170-179CMC10-1100Distilledowater:Oe(%) 10250j6%Brine:4%BrineC142%Brine:8%BrineC12C10/10-P口-?2-6'5-4U-1Concentration (log C)Fig. 2. Effect of concentration of sodium alkyl-sulfonates on flotation of barite:empty circles, sodium tetradecyl sulfonate (C14): filled circles, sodium dodecylI1--10sulfonate (C12): empty squares, sodium decyl sulfonate (C10) Vertical arrows020406080100indicate the critical micelle concentrations for the three studied alkyl-sulfonates at40 -C.(Dobias, 1986),Right-hand side insert shows a micelleon solid surface.Temperature(C)Fig.5.Effectofelectrolyteconcentration on theKraftpointofdodecylammoniumchloride (Laskowskietal.,2007).100What is surprising is that this relationship - as shown by Romanet al. (1968) - is quite different in the potash flotation (Fig. 3).80Fig.3showsthattheflotationofsylvitecommenceswhentheainosianosoTetradecylsolubility limit of amineis exceeded. Together,Figs.2and 3oo:Dodecylindicate that critical micelle concentration (c.m.c.) should not be60confusedwiththesaturationconcentrationofthehydrated crystals(Cases andVillieras,1992).b40O3. Krafft point20Forionicsurfactants.thesolubilitycurveplottedasafunctionoftemperature reveals two large domains (Fig. 4). At temperaturesOdVbelow the Kraft point (Tk),the solubility curve describes the satu-010610610410-310-ration concentration of a hydrated crystal in equilibrium withmonomers (single surfactant molecules)in solution.At T<Tk,Amine Addition (mol/l)when concentration of the surfactant increases(over solubilityFig.3. Relationship between KCl recovery and amine addition (Roman et al., 1968).limit) the hydrated crystals start appearing in the solution; thisphasehas a lamellar structure, with the polargroups lying alongthe interface with water. At T>Tk.when the surfactant concentra-tion is increased, the monomers associate to form micellar aggre-gates; the concentration at which micelles first occur is referredto as a critical micelle concentration (cmc). In the three distinctSolubilityzones inFig.4distinctspeciesappear:inZoneIonlysinglesurfac-curvelamellarphasetant molecules (monomers): in Zone Il hydrated crystals in equilib-eLyotropicphasesrium with monomers; and in Zone Ill micelles in equilibrium withexagonalphasemonomers. At temperatures lower than the Krafft point, the solu-bility is too low for micellization. The Krafft point is also definednmasthetemperatureatwhich thesolubility curvereachesthe criti-cal micelle concentration;further increases in temperaturesharplyasymmetricmicellesenhancethesolubilityof thesurfactantduetotheformationofHydrated crystalsMicellarsolutionsphericalmicellesmicelles.ZAs this discussion reveals, the hydrated crystals which appear atAtemperature T< Tk should not be confused with micelles present.CMC curvewhen T>Tk.Fig.2demonstrates that flotation drops to zero whenSimplemolecules1thecollectorconcentrationapproachescmc,whileFig.3showsVthatsylviteflotationcommenceswhentheamine(collector)con-TkTemperature (deg C) The term "amine" stands in this publication for alkylamine with the number ofFig. 4. Schematic representation of the solubility of ionic surfactants versustemperature.carbonatomshigherthan12
What is surprising is that this relationship – as shown by Roman et al. (1968) – is quite different in the potash flotation (Fig. 3). Fig. 3 shows that the flotation of sylvite commences when the solubility limit of amine1 is exceeded. Together, Figs. 2 and 3 indicate that critical micelle concentration (c.m.c.) should not be confused with the saturation concentration of the hydrated crystals (Cases and Villieras, 1992). 3. Krafft point For ionic surfactants, the solubility curve plotted as a function of temperature reveals two large domains (Fig. 4). At temperatures below the Krafft point (TK), the solubility curve describes the saturation concentration of a hydrated crystal in equilibrium with monomers (single surfactant molecules) in solution. At T < TK, when concentration of the surfactant increases (over solubility limit) the hydrated crystals start appearing in the solution; this phase has a lamellar structure, with the polar groups lying along the interface with water. At T > TK, when the surfactant concentration is increased, the monomers associate to form micellar aggregates; the concentration at which micelles first occur is referred to as a critical micelle concentration (cmc). In the three distinct zones in Fig. 4 distinct species appear: in Zone I only single surfactant molecules (monomers); in Zone II hydrated crystals in equilibrium with monomers; and in Zone III micelles in equilibrium with monomers. At temperatures lower than the Krafft point, the solubility is too low for micellization. The Krafft point is also defined as the temperature at which the solubility curve reaches the critical micelle concentration; further increases in temperature sharply enhance the solubility of the surfactant due to the formation of micelles. As this discussion reveals, the hydrated crystals which appear at temperature T < TK should not be confused with micelles present when T > TK. Fig. 2 demonstrates that flotation drops to zero when the collector concentration approaches cmc, while Fig. 3 shows that sylvite flotation commences when the amine (collector) conFig. 5. Effect of electrolyte concentration on the Krafft point of dodecylammonium chloride (Laskowski et al., 2007). -6 -5 -4 -3 -2 -1 Concentration (log C) 0 50 100 Recovery (%) C14 C12 C10 CMC Fig. 2. Effect of concentration of sodium alkyl-sulfonates on flotation of barite: empty circles, sodium tetradecyl sulfonate (C14); filled circles, sodium dodecyl sulfonate (C12); empty squares, sodium decyl sulfonate (C10). Vertical arrows indicate the critical micelle concentrations for the three studied alkyl-sulfonates at 40 C. (Dobias, 1986). Right-hand side insert shows a micelle on solid surface. Fig. 3. Relationship between KCl recovery and amine addition (Roman et al., 1968). Fig. 4. Schematic representation of the solubility of ionic surfactants versus temperature. 1 The term ‘‘amine’’ stands in this publication for alkylamine with the number of carbon atoms higher than 12. 172 J.S. Laskowski / Minerals Engineering 45 (2013) 170–179
173JS.Laskowski/Minerals Engineering 45 (2013) 170-179centrationexceedsthesolubilitylimit.The difference between Figs. 2 and 3 suggest the obvious: thecolloidal species which appear in these two systems must havevery different properties, and a further discussion of these phe-nomena requires some knowledge of the Krafft point of long-chainamines.Additionally.theeffectofelectrolyteconcentrationontheamineKrafftpointmustalsobecharacterizedTheavailableinformationontheKrafftpointofaminesisratherlimited. The Krafft point values for dodecyl (C12) and octadecyl(C18)ammoniumchlorideswerereportedtobe26Cand56Crespectively (Brandrup and Immergut, 1975).For dodecyl ammo-nium chloride Dai and Laskowski (1991) provided a value of17 °C. Theoretic estimations (Laskowski, 1994) indicate that theKrafft point for amines should dramatically increase with brineconcentration (the term brine used in this text stands for theNaCl-KCl saturated brine at room temperature). Fig. 5 shows theeffectofbrineconcentrationontheKrafftpointofdodecylamineIt is 80C at 16% brine concentration (approximately1MsolutionFig. 6. NaCl (average size 280 μm) and KCI (average size 35 μm) particles inofNaCl-KCl).IfinconcentratedelectrolytesolutionsC12andC18saturated NaCl-KCI brine (Roman et al., 1968)aminesobservethesamerelationshipbetweenKrafftpointandchain length,then it can be expected that in a 1M solution ofNaCI-KCI the Krafft point for C18 amine exceeds 100C.This leadsto the conclusion that all commercial potash ore flotation plants,aswell as all lab flotation experiments, are carried out at tempera-tures much lower than the Krafft temperature of long-chain pri-5x10~MDDA·HCImary amines. Micelles do not exist in such systems. What will5appear in the pulp when amines precipitate are solid particles, hy-drated crystals.1x10-3M35x10M4.Electrical charge3.5x10-MAnother important contribution brought about in Roman et al.'s(1968) paper was evidence that the particles of sylvite and halitecarry an electrical charge. Postulation of the electrical charge onthe particles suspended in brine was considered a blasphemy at1x104M-1the time, but evidence for it was quite convincing. Roman et al.showed that while the suspensions of fine sylvite particles sus-Total ionic strength=2x10-3Mpended in their own brine were very stable,and the suspensions3of fine halite in their own brine were stable too, they both imme-981071112diately coagulated when mixed together (Fig. 6). The obviouspHexplanation was that these particles carried different electricalcharge.Somewhatlater,Milleretal.(1992)usedDopplerelectro-Fig, 7. Effect of pH and concentrationthe electrophoretic mobility of themphoresis and proved that while sylvite particles in brine carry neg-colloidal precipitate in aqueous solutions of dodecylamine (Laskowski et al., 1988)ative electrical charge, halite particles are charged positively.At that time it was already known (Castro et al. 1986: Laskow-Na'strongly interact with interfacial water molecules and stabilizeski et al., 1988) that the particles of the precipitating amine are alsothe interfacial water layer at the structure-maker Nacl surface.electrically charged. As Fig.7 demonstrates, colloidal amine parti-Consequently,octa-decyl amine (ODA)adsorption by the replace-clesare characterized by the clear iso-electricpoint which is situ-ment of interfacial of water molecules cannot take place. In theated at a pH ofapproximately11.Thus,theamineparticlesarecase of KCl with the larger cation, K',it is found that ODA adsorp-positivelycharged below pH 11 and negatively charged whention is possible by attachment of the positively charged polar headpH>11.group at the structure breaker KCl surface defects (Cao et al., 2010).These parallel but independent observations of electrical chargeHowever,whilethisexplainsthedifferencebetweengoodflotationof both sylvite and the precipitating amine,made it possible to ex-of KCI and poor flotation of NaCl it does not explain the correlationplain (Laskowski, 1994)the sylvite flotation curves published bybetween flotation response and surface charge of colloidal amineSchubert (1967, 1988) (Fig. 8). The analysis reveals that whileparticles shown in Fig. 8.KClfloatswithaminesatpH<10.5,theflotationofNaCltakesplace only when this pH is exceeded. A comparison of Fig. 7 withFig. 8 leads to the conclusion that the negative electrical charge5.Useof amines in commercial potash ore flotationofsylviteparticlesandthepositivechargeoftheamineprecipitateresultsfirstinCoulombicattractionwhichthenleadstopotashoreLong-chain primary amines utilized as a collector are practicallyflotation.Hancer et al. (2001) questioned the surface charge-controlledinsoluble in water. Leja (1983)compared various experimentalcollector adsorption model and showed that the flotation of solubledata on the solubility of long-chain primary amines and concludedthat for all C,>16 amines, solubility in brine converges to levelssalts is dictated bythe ability of respected cations and anionstobelow10-"mole/L.Thepoor solubilityof primary amines was alsoorganizethestructureof interfacial water.Small cationssuchas
centration exceeds the solubility limit. The difference between Figs. 2 and 3 suggest the obvious: the colloidal species which appear in these two systems must have very different properties, and a further discussion of these phenomena requires some knowledge of the Krafft point of long-chain amines. Additionally, the effect of electrolyte concentration on the amine Krafft point must also be characterized. The available information on the Krafft point of amines is rather limited. The Krafft point values for dodecyl (C12) and octadecyl (C18) ammonium chlorides were reported to be 26 C and 56 C, respectively (Brandrup and Immergut, 1975). For dodecyl ammonium chloride Dai and Laskowski (1991) provided a value of 17 C. Theoretic estimations (Laskowski, 1994) indicate that the Krafft point for amines should dramatically increase with brine concentration (the term brine used in this text stands for the NaCl–KCl saturated brine at room temperature). Fig. 5 shows the effect of brine concentration on the Krafft point of dodecylamine. It is 80 C at 16% brine concentration (approximately 1 M solution of NaCl–KCl). If in concentrated electrolyte solutions C12 and C18 amines observe the same relationship between Krafft point and chain length, then it can be expected that in a 1 M solution of NaCl–KCl the Krafft point for C18 amine exceeds 100 C. This leads to the conclusion that all commercial potash ore flotation plants, as well as all lab flotation experiments, are carried out at temperatures much lower than the Krafft temperature of long-chain primary amines. Micelles do not exist in such systems. What will appear in the pulp when amines precipitate are solid particles, hydrated crystals. 4. Electrical charge Another important contribution brought about in Roman et al.’s (1968) paper was evidence that the particles of sylvite and halite carry an electrical charge. Postulation of the electrical charge on the particles suspended in brine was considered a blasphemy at the time, but evidence for it was quite convincing. Roman et al. showed that while the suspensions of fine sylvite particles suspended in their own brine were very stable, and the suspensions of fine halite in their own brine were stable too, they both immediately coagulated when mixed together (Fig. 6). The obvious explanation was that these particles carried different electrical charge. Somewhat later, Miller et al. (1992) used Doppler electrophoresis and proved that while sylvite particles in brine carry negative electrical charge, halite particles are charged positively. At that time it was already known (Castro et al., 1986; Laskowski et al., 1988) that the particles of the precipitating amine are also electrically charged. As Fig. 7 demonstrates, colloidal amine particles are characterized by the clear iso-electric point which is situated at a pH of approximately 11. Thus, the amine particles are positively charged below pH 11 and negatively charged when pH > 11. These parallel but independent observations of electrical charge of both sylvite and the precipitating amine, made it possible to explain (Laskowski, 1994) the sylvite flotation curves published by Schubert (1967, 1988) (Fig. 8). The analysis reveals that while KCl floats with amines at pH < 10.5, the flotation of NaCl takes place only when this pH is exceeded. A comparison of Fig. 7 with Fig. 8 leads to the conclusion that the negative electrical charge of sylvite particles and the positive charge of the amine precipitate results first in Coulombic attraction which then leads to potash ore flotation. Hancer et al. (2001) questioned the surface charge-controlled collector adsorption model and showed that the flotation of soluble salts is dictated by the ability of respected cations and anions to organize the structure of interfacial water. Small cations such as Na+ strongly interact with interfacial water molecules and stabilize the interfacial water layer at the structure-maker NaCl surface. Consequently, octa-decyl amine (ODA) adsorption by the replacement of interfacial of water molecules cannot take place. In the case of KCl with the larger cation, K+ , it is found that ODA adsorption is possible by attachment of the positively charged polar head group at the structure breaker KCl surface defects (Cao et al., 2010). However, while this explains the difference between good flotation of KCl and poor flotation of NaCl it does not explain the correlation between flotation response and surface charge of colloidal amine particles shown in Fig. 8. 5. Use of amines in commercial potash ore flotation Long-chain primary amines utilized as a collector are practically insoluble in water. Leja (1983) compared various experimental data on the solubility of long-chain primary amines and concluded that for all Cn > 16 amines, solubility in brine converges to levels below 108 mole/L. The poor solubility of primary amines was also Fig. 6. NaCl (average size 280 lm) and KCl (average size 35 lm) particles in saturated NaCl–KCl brine (Roman et al., 1968). Fig. 7. Effect of pH and concentration on the electrophoretic mobility of the colloidal precipitate in aqueous solutions of dodecylamine (Laskowski et al., 1988). J.S. Laskowski / Minerals Engineering 45 (2013) 170–179 173
174J.S. Laskowski/Minerals Engineering 45 (2013) 170-179100treatmentwiththecationiccollector.Inaddition-intheflotationof coarse fractions - an extender oil is also utilized. A frother (e.g.0MIBC) is added just prior to the flotation cells (Strathdee et al.,KCI2007).6C126. Amine precipitate in sylvite flotation40Ci6!As demonstrated by Leja (1983)."in quiescent environment no20contact angle or pick-up of sylvite particleswasobserved even100afterdepositionofamine-alcoholpasteonthesurfaceoftheun-104stirred brine. However, after thorough stirring for a few minutes,80KCl particles were picked up and contact angle was developed onNaciKCl discs." The bubble/sylvite attachment was clearly possibleSwhen the collector was manually placed directly on the surfaceof the bubble. When a wire coated with the collector-alcohol pasteCi6 i40was placed in brine on the other hand, there was no bubble/sylvite"particle pick-up,even after hours of immersion. But as soon as theC1220probe was touched first to the captive bubble, and the latter wascontactedwithaKCldisc,contactangleimmediatelydevelopedI0These findings point toward adequate agitation as an important24681012step,withoutwhichthesurfactantsutilizedinpotashoreflotationpHare not ableto perform theirfunction.Since long-chain amines are insoluble in brine, this led to theFig. 8. The effect of pH on flotation of sylvite and halite with C12 and C16 primaryamines (Schubert, 1967).conclusion that the mechanism responsible for flotation in thisparticular case must be different from conventional flotation inwhich collector adsorption renders the treated mineral hydropho-evidenced by Rogers (1957).He observed that the surface tensionbic.Itwaspostulatedthatthecollectorinthepotashoreflotationmeasurements with saturated NaCl-KCI brine to which dodecy-pulpistransportedbybubbles.lammoniumchloridecrystals were added(equivalent ofTheeffectsdescribedbyLejawerequantifiedbyBurdukovaand2.26 × 10-4 mole/L) did not froth even after a week and surfaceLaskowski (2009). Their tests were designed to clarify how the pre-tension dropped only by a few mJ/m2cipitating amine particles function in a potash ore flotation system.The dosage of amines in industrial plants in Saskatchewan is inThe amine dispersion was placed either onto the surface of a bub-the range from 60 to 95 g/t (Strathdee et al., 1982) which corre-ble,whichwasthencontactedwithaKClplateandmeasurementsponds to approximately 10-mole/L, thus exceeding by far theofcontactanglefollowed,or the amine dispersion was placed ontoamine solubility limit. Before use the amines are melted by heatingthe surface of aKCl plate,which was then contacted witha bubblethem up to 70-90 C, and are then neutralized with hydrochloric orto measure the contact angle (Fig. 9). Special procedures had to beaceticacids.Thehot emulsion/dispersion isintroduced into theflo-developed for these tests since, in the former case, any previoustation pulp,which is at room temperature.Since this temperaturecontactof theKClplatewiththesolution/airinterfacewasto beismuchbelowtheKrafftpointoftheutilizedamines.precipitationtotallyprevented.Inthelattercasethebubblewasnottobecon-ensues.Accordingtoallreportedobservations,awhiteprecipitatetaminatedbypossiblesurfactantadsorption(SchreithoferandLas-appears immediately when the hot emulsion of amine is added tokowski,2006;Burdukova and Laskowski,2009:Laskowski,2010).thepotashflotationpulp,andthat it accumulatesonthesurfaceofIn 1995, Wang et al. reported (Wang et al., 1995) that frother.bubbles.depending on the way it is introduced into the pulp, may have aIn commercial plants, after crushing and mechanical deslimingstrong effect on potash oreflotation (this effect wasalso confirmedthe potash ore still requires the subsequent application of "blind-by other researchers, see for example Monte and Oliveira, 2004).ers" to depress residual slimes and conserve valuable collectorThe effect of thepresence of MiBC was also studied in the tests dis-(Arsentiev et al.,1988).Depressants (blinders) include: carboxy-cussed in this paper. Dodecyl amine was heated (70 C) and thenmethyl cellulose, guar gum, and starch, etc. This is followed bydispersed in a hot (70 °C) diluted HCl solution. In separate testsBAFig. 9. (A) DDA colloidal particles on the surface of the KCl, and (B) DDA colloidal particles on the surface of the bubble (Burdukova and Laskowski, 2009)
evidenced by Rogers (1957). He observed that the surface tension measurements with saturated NaCl–KCl brine to which dodecylammonium chloride crystals were added (equivalent of 2.26 104 mole/L) did not froth even after a week and surface tension dropped only by a few mJ/m2 . The dosage of amines in industrial plants in Saskatchewan is in the range from 60 to 95 g/t (Strathdee et al., 1982) which corresponds to approximately 104 mole/L, thus exceeding by far the amine solubility limit. Before use the amines are melted by heating them up to 70–90 C, and are then neutralized with hydrochloric or acetic acids. The hot emulsion/dispersion is introduced into the flotation pulp, which is at room temperature. Since this temperature is much below the Krafft point of the utilized amines, precipitation ensues. According to all reported observations, a white precipitate appears immediately when the hot emulsion of amine is added to the potash flotation pulp, and that it accumulates on the surface of bubbles. In commercial plants, after crushing and mechanical desliming the potash ore still requires the subsequent application of ‘‘blinders’’ to depress residual slimes and conserve valuable collector (Arsentiev et al., 1988). Depressants (blinders) include: carboxymethyl cellulose, guar gum, and starch, etc. This is followed by treatment with the cationic collector. In addition – in the flotation of coarse fractions – an extender oil is also utilized. A frother (e.g. MIBC) is added just prior to the flotation cells (Strathdee et al., 2007). 6. Amine precipitate in sylvite flotation As demonstrated by Leja (1983), ‘‘in quiescent environment no contact angle or pick-up of sylvite particles was observed even after deposition of amine-alcohol paste on the surface of the unstirred brine. However, after thorough stirring for a few minutes, KCl particles were picked up and contact angle was developed on KCl discs.’’ The bubble/sylvite attachment was clearly possible when the collector was manually placed directly on the surface of the bubble. When a wire coated with the collector-alcohol paste was placed in brine on the other hand, there was no bubble/sylvite particle pick-up, even after hours of immersion. But as soon as the probe was touched first to the captive bubble, and the latter was contacted with a KCl disc, contact angle immediately developed. These findings point toward adequate agitation as an important step, without which the surfactants utilized in potash ore flotation are not able to perform their function. Since long-chain amines are insoluble in brine, this led to the conclusion that the mechanism responsible for flotation in this particular case must be different from conventional flotation in which collector adsorption renders the treated mineral hydrophobic. It was postulated that the collector in the potash ore flotation pulp is transported by bubbles. The effects described by Leja were quantified by Burdukova and Laskowski (2009). Their tests were designed to clarify how the precipitating amine particles function in a potash ore flotation system. The amine dispersion was placed either onto the surface of a bubble, which was then contacted with a KCl plate and measurement of contact angle followed, or the amine dispersion was placed onto the surface of a KCl plate, which was then contacted with a bubble to measure the contact angle (Fig. 9). Special procedures had to be developed for these tests since, in the former case, any previous contact of the KCl plate with the solution/air interface was to be totally prevented. In the latter case the bubble was not to be contaminated by possible surfactant adsorption (Schreithofer and Laskowski, 2006; Burdukova and Laskowski, 2009; Laskowski, 2010). In 1995, Wang et al. reported (Wang et al., 1995) that frother, depending on the way it is introduced into the pulp, may have a strong effect on potash ore flotation (this effect was also confirmed by other researchers, see for example Monte and Oliveira, 2004). The effect of the presence of MIBC was also studied in the tests discussed in this paper. Dodecyl amine was heated (70 C) and then dispersed in a hot (70 C) diluted HCl solution. In separate tests Fig. 8. The effect of pH on flotation of sylvite and halite with C12 and C16 primary amines (Schubert, 1967). Fig. 9. (A) DDA colloidal particles on the surface of the KCl, and (B) DDA colloidal particles on the surface of the bubble (Burdukova and Laskowski, 2009). 174 J.S. Laskowski / Minerals Engineering 45 (2013) 170–179