THE EFFECT OF FLOCCULANTS AND THEIR DEGRADATION PRODUCTS ON MOLYBDENITE FLOTATION S.Castro' and J.S. Laskowski I Department of Metalhurgical Engineering.Universityof Concepcion,chile INB Keevil Imstitute of Mining Engineering University of British Columbia,Vancowner, Canada ABSTRACT The quality of recycled process water is an important issue in the flotation of Cu-Mo ores Processing of Cu-Mo ores includes two steps: a bulk flotation where molybdenite is recovered together with Cu and Fe sulfides; and a subsequent selective flotation step where molybdenite is separated from depressed copper sulfides. Flocculants are usually employed in the middling thickeners in the copper plant, and in the Cu-Mo bulk concentrate ahead of the molybdenite plant. However, the floatability of molybdenite, similarly to other naturally hydrophobic minerals, is highly sensitive to the effect of both natural and synthetic polymers. In this work flotation tests demonstrate that conventional high-molecular weight anionic polyacrylamides(PAM) are strong molybdenite depressants. Low-molecular weight shear degraded polyacrylamides in spite of losing flocculation efficiency maintain depressing ability for molybdenite. Also a non-ionic flocculant, polyethylene oxide(PEO), has been studied in this project. Our results indicate that PEO is an efficient flocculant for molybdenite suspensions in a wide pH range. However, similarly to polyacrylamides, commercial PEO can also depress molybdenite flotation. Keywords: flocculation, polyacrylamide; polyethylene oxide, molybdenite flotation; shear degraded flocculants. INTRODUCTION Modern processing plants must have closed water circuits in which process water is recycled back after removal of solids in the solid/liquid separation unit operations. With the present trend towards higher flocculant levels in the thickening (e.g. paste technology) and filtration dewatering the likelihood of flocculants build-up in recycle streams over time is very large. The possible presence of residual flocculants or degraded derivatives in water which is recycled back to a flotation process raises the question about the effect of flocculants on flotation of Cu-Mo ores, and in particular on flotation of molybdenite which is an inherently hydrophobic mineral. These effects were studied in coal flotation, and since coal along with graphite, molybdenite and talc belongs to the same group of inherently hydrophobic solids there is a lot of to learn from these results and to apply directly to the case of molybdenite flotation In many flotation processes, for example, in rejection of pyrite from coal, rejection of talo and graphite in the flotation of sulfide ores, polymeric agents are commonly applied as depressants. At one point, dextrin was implemented at the Utah concentrator as molybdenite depressant in the selective flotation of Cu sulfides from molybdenite(Shirely, 1979)
THE EFFECT OF FLOCCULANTS FLOCCULANTS FLOCCULANTS FLOCCULANTS AND THEIR DEGRADATION DEGRADATION DEGRADATION DEGRADATION PRODUCTS PRODUCTS PRODUCTS PRODUCTS ON MOLYBDENITE MOLYBDENITE MOLYBDENITE MOLYBDENITE FLOTATION FLOTATION FLOTATION FLOTATION S.Castro S.Castro S.Castro S.Castro1 and J.S. Laskowski Laskowski Laskowski Laskowski2 1 Department of Metallurgical Engineering, University of Concepcion, Chile 2 NB Keevil Institute of Mining Engineering, University of British Columbia, Vancouver, Canada ABSTRACT ABSTRACT ABSTRACT ABSTRACT The quality of recycled process water is an important issue in the flotation of Cu-Mo ores. Processing of Cu-Mo ores includes two steps: a bulk flotation where molybdenite is recovered together with Cu and Fe sulfides; and a subsequent selective flotation step where molybdenite is separated from depressed copper sulfides. Flocculants are usually employed in the middling thickeners in the copper plant, and in the Cu-Mo bulk concentrate ahead of the molybdenite plant. However, the floatability of molybdenite, similarly to other naturally hydrophobic minerals, is highly sensitive to the effect of both natural and synthetic polymers. In this work flotation tests demonstrate that conventional high-molecular weight anionic polyacrylamides (PAM) are strong molybdenite depressants. Low-molecular weight shear degraded polyacrylamides in spite of losing flocculation efficiency maintain depressing ability for molybdenite. Also a non-ionic flocculant, polyethylene oxide (PEO), has been studied in this project. Our results indicate that PEO is an efficient flocculant for molybdenite suspensions in a wide pH range. However, similarly to polyacrylamides, commercial PEO can also depress molybdenite flotation. Keywords Keywords Keywords Keywords: flocculation, polyacrylamide; polyethylene oxide, molybdenite flotation; shear degraded flocculants. INTRODUCTION INTRODUCTION INTRODUCTION INTRODUCTION Modern processing plants must have closed water circuits in which process water is recycled back after removal of solids in the solid/liquid separation unit operations. With the present trend towards higher flocculant levels in the thickening (e.g. paste technology) and filtration dewatering the likelihood of flocculants build-up in recycle streams over time is very large. The possible presence of residual flocculants or degraded derivatives in water which is recycled back to a flotation process raises the question about the effect of flocculants on flotation of Cu-Mo ores, and in particular on flotation of molybdenite which is an inherently hydrophobic mineral. These effects were studied in coal flotation, and since coal along with graphite, molybdenite and talc belongs to the same group of inherently hydrophobic solids there is a lot of to learn from these results and to apply directly to the case of molybdenite flotation. In many flotation processes, for example, in rejection of pyrite from coal, rejection of talc and graphite in the flotation of sulfide ores, polymeric agents are commonly applied as depressants. At one point, dextrin was implemented at the Utah concentrator as molybdenite depressant in the selective flotation of Cu sulfides from molybdenite (Shirely, 1979)
In the Cu-Mo sulfide ore processing plants the stage of bulk flotation of copper sulfidesandmolybdeniteis followed bythe second stage (moly plant)which includes selectiveflotationof molybdenite and depression of copper sulfides. It has been a common practice that the Cu-Mobulk concentrate is stored in a thickener in which also flocculants can be applied to increasesolids content inthe feed tothemolyplant.In doingso itis often assumed that theuseof lowmolecularweight flocculants minimizesflocculationandsodoesnotaffectmuchflotationHowever, Shirley (1979) pointed out that in plant practice “most of commonly used flocculantsare excellent depressants for molybdenite even if it has been collected with an oil.Therefore,flocculants should not be used on middling thickeners in the copper circuit or the molybdeniteplantfeed thickenerunless absolutely necessaryThe objective of this paper is to discuss the effect of anionic polyacrylamide flocculantsand their degradation products - which are polymer segments of low-molecular weight - on thefloatability ofmolybdenite.Polyethylene oxide (PEO),a non-ionic flocculant and its ability todepressmolybdeniteflotationhasalsobeenstudiedUSEOFPOLYMERSINMINERALPROCESSINGVarious polymers-low molecular weight dispersants/depressants and high molecularweight flocculants - are utilized in mineral processing circuits. Since they must be water soluble,these polymers are highly hydrophilic macromolecules.Common examples in the former groupare dextrins,lowmolecular weight polyacrylates (e.g.Cataflot,Dispex,etc.),polystyrenesulfonate (Pss1o used in coal-water slurries),and in the latter group polyacrylamides are thebest known.Also starch is in thegroup of high molecular weight polymers used as flocculants(by the way starch in combination with lime was the first flocculant patented in 1928 for theclarification of a coal's mine effluents (Kitchener, 1978).It is necessary to be pointed out thatfrom the chemical point of view both dextrin and starch are the samepolysaccharides whichdiffer only by molecular weight (Figure 1).Polyacrylamides, the most common commercial flocculants, are to some extent anionic(expressed as degree of anionicity) as shown in Figure 1 and so they can also be treated as co-polymers of poly?acrylamide and polyacrylic acid. High molecular weight PAMs with a degreeof anionicity in the range 20 to 30 are claimed to be the most efficient in thickening tailings (Xuand Cymerman, 1999).OH(CH2-CH-)m(-CH2-CH-)n11CONH2COONaCe-HOHFigure 1. α-D-Glucose, structural unit of dextrin and starch, and the monomer of polyacrylamide.The main function of flocculants used in solid/liquid unit operations is to produce largeand strong flocs. It is generally accepted that polymers used as flocculants aggregate suspensionsof fine particles by a bridging mechanism. The bridging is considered to be a consequence of the
In the Cu-Mo sulfide ore processing plants the stage of bulk flotation of copper sulfides and molybdenite is followed by the second stage (moly plant) which includes selective flotation of molybdenite and depression of copper sulfides. It has been a common practice that the Cu-Mo bulk concentrate is stored in a thickener in which also flocculants can be applied to increase solids content in the feed to the moly plant. In doing so it is often assumed that the use of low molecular weight flocculants minimizes flocculation and so does not affect much flotation. However, Shirley (1979) pointed out that in plant practice “most of commonly used flocculants are excellent depressants for molybdenite even if it has been collected with an oil. Therefore, flocculants should not be used on middling thickeners in the copper circuit or the molybdenite plant feed thickener unless absolutely necessary”. The objective of this paper is to discuss the effect of anionic polyacrylamide flocculants and their degradation products - which are polymer segments of low-molecular weight - on the floatability of molybdenite. Polyethylene oxide (PEO), a non-ionic flocculant and its ability to depress molybdenite flotation has also been studied. USE OF POLYMERS POLYMERS POLYMERS POLYMERS IN MINERAL MINERAL MINERAL MINERAL PROCESSING PROCESSING PROCESSING PROCESSING Various polymers - low molecular weight dispersants/depressants and high molecular weight flocculants - are utilized in mineral processing circuits. Since they must be water soluble, these polymers are highly hydrophilic macromolecules. Common examples in the former group are dextrins, low molecular weight polyacrylates (e.g. Cataflot, Dispex, etc.), polystyrene sulfonate (PSS10 used in coal-water slurries), and in the latter group polyacrylamides are the best known. Also starch is in the group of high molecular weight polymers used as flocculants (by the way starch in combination with lime was the first flocculant patented in 1928 for the clarification of a coal’s mine effluents (Kitchener, 1978). It is necessary to be pointed out that from the chemical point of view both dextrin and starch are the same polysaccharides which differ only by molecular weight (Figure 1). Polyacrylamides, the most common commercial flocculants, are to some extent anionic (expressed as degree of anionicity) as shown in Figure 1 and so they can also be treated as copolymers of poly?acrylamide and polyacrylic acid. High molecular weight PAMs with a degree of anionicity in the range 20 to 30 are claimed to be the most efficient in thickening tailings (Xu and Cymerman, 1999). (CH2-CH-)m(-CH2-CH-)n ׀ ׀ CONH2 COO-Na Figure 1. α-D-Glucose, structural unit of dextrin and starch; and the monomer of polyacrylamide. The main function of flocculants used in solid/liquid unit operations is to produce large and strong flocs. It is generally accepted that polymers used as flocculants aggregate suspensions of fine particles by a bridging mechanism. The bridging is considered to be a consequence of the
adsorption of the segments of the flocculant macromolecules onto the surfaces of more than oneparticle.The optimum flocculation occurs at flocculant dosages corresponding to a particlecoverage that is significantly less than complete. Incomplete surface coverage ensures that thereis sufficient unoccupied surface available on each particle for the adsorption of segments of theflocculant chains during collision of the particles.Thus,at low polymer coverage, the adsorbedpolymer can destabilize the suspension by bridging flocculation, but since these macromoleculesare hydrophilic at a high coverage (high polymer dosages)the polymer adsorbed layers causerepulsion. These results in stabilization of the suspension, the phenomenon referred to as stericstabilization.There are many direct contact angle measurements which show that hydrophobic solidsbecome less hydrophobic and loose floatability in aqueous solutions of water-soluble polymers.Klassen in his monograph on coal flotation (Klassen, 1963) listed many polysaccharides asdepressants for coal flotation. The use of dextrin to depress coal and float pyrite with xanthatewas patented byDOEfor desulfurizing flotation of fine coal (Miller and Deurbrouck,1982)Pradipand Fuerstenau(1987)testedtheeffectofvariouspolymers onthewettabilityofanthracite andshowedthattheanthracite becomeslesshydrophobic intheir presence.Similarresults were reported byMoudgil (1983).Wie and Fuerstenau (1974)reported strongdepressingeffect of dextrin on the wettability of molybdenite in acidic solutions, such a depression hasrecentlybeen confirmed byBeaussart etal (2012).Polymeric substances known as humic acids often appear in process water (obtained fromlakes or rivers).These are poorlydefined anionic polymers with phenolic and carboxylic groups,which were shown to affect strongly wettability of graphite (Wong and Laskowski, 1984) andalso wettability of molybdenite (Laskowski and Yu,1994).These effects wereparticularlysignificantinacidic solutionswherehumic acidsbecomelesssolubleandprecipitate.Pawliketal.(1997)confirmed thatveryhydrophobicbituminous coal canbecometotally hydrophilicatrelatively low concentrations of humic acids. One of the important gangue minerals in SouthAfrican sulfideoresthatcontainplatinum istalc.Sinceit isnaturallyhydrophobicit tendstofloat well and it is common to depress it using guar gum (or other polysaccharides such as starchorcarboxymethyl cellulose).EEFFECTOFFLOCCULANTSONCOALFLOTATIONThis important aspect of coal flotation has been extensively discussed in the book byPikkat-Ordynsky and Ostry (1972).In their tests they used slightly anionic polyacrylamide(PAM)with molecular weight of 3x10and non-ionic polyethylene oxide (PEO)with molecularweight of 7 x 106. Flotation tests were carried out in a 1.5 L lab flotation cell, at 150 g solids/Lpulpdensity
adsorption of the segments of the flocculant macromolecules onto the surfaces of more than one particle. The optimum flocculation occurs at flocculant dosages corresponding to a particle coverage that is significantly less than complete. Incomplete surface coverage ensures that there is sufficient unoccupied surface available on each particle for the adsorption of segments of the flocculant chains during collision of the particles. Thus, at low polymer coverage, the adsorbed polymer can destabilize the suspension by bridging flocculation, but since these macromolecules are hydrophilic at a high coverage (high polymer dosages) the polymer adsorbed layers cause repulsion. These results in stabilization of the suspension, the phenomenon referred to as steric stabilization. There are many direct contact angle measurements which show that hydrophobic solids become less hydrophobic and loose floatability in aqueous solutions of water-soluble polymers. Klassen in his monograph on coal flotation (Klassen, 1963) listed many polysaccharides as depressants for coal flotation. The use of dextrin to depress coal and float pyrite with xanthate was patented by DOE for desulfurizing flotation of fine coal (Miller and Deurbrouck, 1982). Pradip and Fuerstenau (1987) tested the effect of various polymers on the wettability of anthracite and showed that the anthracite becomes less hydrophobic in their presence. Similar results were reported by Moudgil (1983). Wie and Fuerstenau (1974) reported strong depressing effect of dextrin on the wettability of molybdenite in acidic solutions; such a depression has recently been confirmed by Beaussart et al (2012). Polymeric substances known as humic acids often appear in process water (obtained from lakes or rivers). These are poorly defined anionic polymers with phenolic and carboxylic groups, which were shown to affect strongly wettability of graphite (Wong and Laskowski, 1984) and also wettability of molybdenite (Laskowski and Yu, 1994). These effects were particularly significant in acidic solutions where humic acids become less soluble and precipitate. Pawlik et al. (1997) confirmed that very hydrophobic bituminous coal can become totally hydrophilic at relatively low concentrations of humic acids. One of the important gangue minerals in South African sulfide ores that contain platinum is talc. Since it is naturally hydrophobic it tends to float well and it is common to depress it using guar gum (or other polysaccharides such as starch or carboxymethyl cellulose). EEFFECT EEFFECT EEFFECT EEFFECT OF FLOCCULANTS FLOCCULANTS FLOCCULANTS FLOCCULANTS ON COAL FLOTATION FLOTATION FLOTATION FLOTATION This important aspect of coal flotation has been extensively discussed in the book by Pikkat-Ordynsky and Ostry (1972). In their tests they used slightly anionic polyacrylamide (PAM) with molecular weight of 3x106 and non-ionic polyethylene oxide (PEO) with molecular weight of 7 x 106 . Flotation tests were carried out in a 1.5 L lab flotation cell, at 150 g solids/L pulp density
NNENONTHSa%*SDNI20.51.5COLLECTOR DOSAGE,kg/tFigure 2. Effect of PAM on flotation of bituminous coal with oily collector. Dosage of PAM:curve 1, 0, curve 2, 5 g/m; curve 3, 10 g/m2; curve 4, 80 g/m2; curve 5, 150 g/m (Pikkat-Ordynskyand Ostry,1972)Both flocculants were found to strongly depress coal flotation:the higher theflocculantdosage, the smaller the yield of the concentrate, higher its ash content, and lower the ash contentintheflotationtailings.Theseeffectsbegintobevisibleatadosageof1g/mofPAM and weremorepronounced for the flotation feeds with a highyield of very fineparticles.Figure 2 showsthe results of their flotation tests on the effect of PAM on flotation of bituminous coal. As seen,at150g/m3ofPAM,depressionistotal.Theloweryieldsofcleancoal wereexplainedbytheadsorption of hydrophilic macromolecules onto coal particles that makes these particleshydrophilic and higher ash contents of the concentrate results from non-selective flocculation ofcoal particles with gangue.Hey's results (Hey, 1985) are perfectly in line with these conclusions;Figure 3 shows that kinetics of coal flotation slows down with increasing dosage of PAM. Inthesetests,ahighvolatilematterbitusfloatedusing2-ethylhexanolatapulpdensityof120gsolids/L,anditwasfoundthatallthetestedflocculantssloweddownthestudiedflotation process in a similar way (Fig.3)
0 0.5 1 1.5 COLLECTOR DOSAGE, kg/t 0 20 40 60 80 100 ASH IN TAILINGS, % 1 3 4 5 12 16 20 24 ASH IN CONCENTRATE, % 5 4 3 2 1 0 20 40 60 80 100 CONCENTRATE YIELD, % 5 1-4 3 1,2 4 Figure 2. Effect of PAM on flotation of bituminous coal with oily collector. Dosage of PAM: curve 1, 0; curve 2, 5 g/m3 ; curve 3, 10 g/m3 ; curve 4, 80 g/m3 ; curve 5, 150 g/m3 (PikkatOrdynsky and Ostry, 1972). Both flocculants were found to strongly depress coal flotation: the higher the flocculant dosage, the smaller the yield of the concentrate, higher its ash content, and lower the ash content in the flotation tailings. These effects begin to be visible at a dosage of 1 g/m3 of PAM and were more pronounced for the flotation feeds with a high yield of very fine particles. Figure 2 shows the results of their flotation tests on the effect of PAM on flotation of bituminous coal. As seen, at 150 g/m3 of PAM, depression is total. The lower yields of clean coal were explained by the adsorption of hydrophilic macromolecules onto coal particles that makes these particles hydrophilic and higher ash contents of the concentrate results from non-selective flocculation of coal particles with gangue. Hey’s results (Hey, 1985) are perfectly in line with these conclusions; Figure 3 shows that kinetics of coal flotation slows down with increasing dosage of PAM. In these tests, a high volatile matter bituminous coal was floated using 2-ethylhexanol at a pulp density of 120 g solids/L, and it was found that all the tested flocculants slowed down the studied flotation process in a similar way (Fig. 3)
Tpsl1Feststoffgehalt:120g/1anoFlototiensmittel 2-ot?OmgFHGesamtkonzentroteEmgFM8mgFM12mgFMOmgFM560t1sEFigure 3. Effect of flocculant concentration (from 0 to 20 mg/L) on the kinetics of coal flotation.n stands for the selectivity coefficient calculated from concentrate yield and ash contents oftheflotation products (Hey, 1985)POLYMERSINFLOTATIONOFMOLYBDENITEMolybdenitecrystallochemicalstructureFigure 4 shows the crystallochemical structure of graphite and molybdenite, twoanisotropic inherently hydrophobic minerals.In molybdenite, sheets of molybdenum atoms aresandwiched between two sheets of sulfur atoms. The sulfur and molybdenum atoms within thelayers are strongly covalently bonded, but the successive layers of sulfur atoms are held togetherby weak van der Waals bonds. These bonds provide excellent cleavage characteristics parallel tothe base of the hexagonal crystals, producing a hydrophobic surface (sulfur does not formhydrogenbondswithwater)
Figure 3. Effect of flocculant concentration (from 0 to 20 mg/L) on the kinetics of coal flotation. η stands for the selectivity coefficient calculated from concentrate yield and ash contents of the flotation products (Hey, 1985). POLYMERS POLYMERS POLYMERS POLYMERS IN FLOTATION FLOTATION FLOTATION FLOTATION OF MOLYBDENITE MOLYBDENITE MOLYBDENITE MOLYBDENITE Molybdenite Molybdenite Molybdenite Molybdenite crystallochemical crystallochemical crystallochemical crystallochemicalstructure structure structure structure Figure 4 shows the crystallochemical structure of graphite and molybdenite, two anisotropic inherently hydrophobic minerals. In molybdenite, sheets of molybdenum atoms are sandwiched between two sheets of sulfur atoms. The sulfur and molybdenum atoms within the layers are strongly covalently bonded, but the successive layers of sulfur atoms are held together by weak van der Waals bonds. These bonds provide excellent cleavage characteristics parallel to the base of the hexagonal crystals, producing a hydrophobic surface (sulfur does not form hydrogen bonds with water)
oSoMooCFigure4.Crystallochemicalstructureofgraphite(left)andmolybdenite(right)As a result of thistype of structurethesurfacechargeondifferent sides of themolybdenite crystal (faces and edges), and in general surface properties of these sides, aredifferent.The use of electrokinetic technigues tozeta-potential of the particles ofmeasureanisotropic minerals, the particles which carry different charges on different sides, is not basedonanysoundscience(Laskowski,2012).Thiswasdoctnentedforotheranisotropicmineralse.g.for kaolinite (Johnson et al. 2000)and for talc (Burdukova et al.2007).The electrokineticmeasurements for such minerals may lead to entirely wrong conclusions. Molybdenite alsobelongs to a group of anisotropic minerals and thus all the measurements that have been reportedin literaturemust betreated with caution.Since molybdenite is recovered by flotation, its surface properties have been widelystudied. In such cases, wettability studies and the use of contact angle techniques face a severeproblem associated with sample preparation. For example, as reported by Arbiter et al (1975),forcleavedmolybdenite surfacesthemeasuredcontactanglesexceeded8Odegrees.Howeverforthe specimens polished parallel to its cleavage plane the contact angle was only 70 degrees.Chander andFuerstenau(1972)confirmedlarge differences incontact anglesmeasured onhydrophobic facesand hydrophilic edges of molybdenite.These differenceswerealso reportedby Lopez-Valdivieso etal. (2006).As Figure 5 shows, the contact angles measured on molybdenite faces were in the rangeof 60 degrees, while the edges were completely hydrophilic. These measurements also confirmArbiter et al's results (1975) in that the contact angle values practically do not depend on pHThe zeta-potential measurements, for example the results provided by Chander et al. (1975),show practically constant values at pH>6 (around -40 mV) and linear decrease at pH <6 downto -25mV at a pH of 3.But since this is an anisotropic mineral which particles possess twodifferent sides it is impossibleto figure out what are the zeta-potential valuesfor these differentsides
Figure 4. Crystallochemical structure of graphite (left) and molybdenite (right). As a result of this type of structure the surface charge on different sides of the molybdenite crystal (faces and edges), and in general surface properties of these sides, are different. The use of electrokinetic techniques to measure zeta-potential of the particles of anisotropic minerals, the particles which carry different charges on different sides, is not based on any sound science (Laskowski, 2012). This was documented for other anisotropic minerals, e.g. for kaolinite (Johnson et al. 2000) and for talc (Burdukova et al. 2007). The electrokinetic measurements for such minerals may lead to entirely wrong conclusions. Molybdenite also belongs to a group of anisotropic minerals and thus all the measurements that have been reported in literature must be treated with caution. Since molybdenite is recovered by flotation, its surface properties have been widely studied. In such cases, wettability studies and the use of contact angle techniques face a severe problem associated with sample preparation. For example, as reported by Arbiter et al (1975), for cleaved molybdenite surfaces the measured contact angles exceeded 80 degrees. However for the specimens polished parallel to its cleavage plane the contact angle was only 70 degrees. Chander and Fuerstenau (1972) confirmed large differences in contact angles measured on hydrophobic faces and hydrophilic edges of molybdenite. These differences were also reported by Lopez-Valdivieso et al. (2006). As Figure 5 shows, the contact angles measured on molybdenite faces were in the range of 60 degrees, while the edges were completely hydrophilic. These measurements also confirm Arbiter et al.’s results (1975) in that the contact angle values practically do not depend on pH. The zeta-potential measurements, for example the results provided by Chander et al. (1975), show practically constant values at pH>6 (around – 40 mV) and linear decrease at pH < 6 down to -25 mV at a pH of 3. But since this is an anisotropic mineral which particles possess two different sides it is impossible to figure out what are the zeta-potential values for these different sides
70605040DutCaC3020h0.15MofCaC10Contact angle, (°):01068101214pHFigure 5. Effect of pH on contact angle measured on faces and edges of MoS2 crystal (Lopez-Valdivieso etal.,2006).In addition to the conventional contact angle measurements, Wie and Fuerstenau (1974)also measured the contact angle between a free iso-octane droplet in water (across water phase)At pH 4, the contact angle was determined to be about 150 degrees, in alkaline solutions itdecreased to 1o0 degrees, and so these measurements turned out to be much more sensitive to pH(to electrical charge at solid/liquid interface). In the further analysis of these data, Chander et al.(2007) showed an excellent correlation between molybdenite oil flotation recovery and oil/watercontact angle of molybdenite (Figure 6)(in oil flotation an oil phase is substituted for thegaseous phase)100RA18%1.6480(OSO0T)270MOLYBDENIIE1.0OILFLOTATION600.8OCONTACTANGLE,e0.650104682pHFigure 6. The effect of pH on the oil flotation of fine molybdenite without the addition of asurfactant. Also plotted is the effect of pH on the oil/water contact angle of molybdenite,expressed in terms of the flotation dewetting relation (1-cos)(Chander, et al., 2007)In the thermodynamic criterion of flotation Gnor=r(cos-1), but for correlationpurposes, the parameter was needed that increases with increasing flotation recovery, and that iswhyinFigure6theresultswereplottedintermsofthequantity(1-cos0).Figure6showsa
pH 4 6 8 10 12 14 Contact angle, (º) Contact angle, (º) Contact angle, (º) Contact angle, (º) -10 0 10 20 30 40 50 60 70 Edge without CaCl 2 Face without CaCl 2 Face with 0.001 M of CaCl 2 Face with 0.15 M of CaCl 2 Figure 5. Effect of pH on contact angle measured on faces and edges of MoS2 crystal (LópezValdivieso et al., 2006). In addition to the conventional contact angle measurements, Wie and Fuerstenau (1974) also measured the contact angle between a free iso-octane droplet in water (across water phase). At pH 4, the contact angle was determined to be about 150 degrees, in alkaline solutions it decreased to 100 degrees, and so these measurements turned out to be much more sensitive to pH (to electrical charge at solid/liquid interface). In the further analysis of these data, Chander et al. (2007) showed an excellent correlation between molybdenite oil flotation recovery and oil/water contact angle of molybdenite (Figure 6) (in oil flotation an oil phase is substituted for the gaseous phase). Figure 6. The effect of pH on the oil flotation of fine molybdenite without the addition of a surfactant. Also plotted is the effect of pH on the oil/water contact angle of molybdenite, expressed in terms of the flotation dewetting relation (1 cos ) − θ (Chander, et al., 2007). In the thermodynamic criterion of flotation (cos 1) Gflot LV ∆ = − γ θ , but for correlation purposes, the parameter was needed that increases with increasing flotation recovery, and that is why in Figure 6 the results were plotted in terms of the quantity (1 cos ) − θ . Figure 6 shows a
verygood correlation between oil flotation of molybdeniteand thewettability ofmolybdenitesurface byoil. Since these contact angle measurements were carried out on freshly cleaved facesof a molybdenite crystal, these data seem to indicate that the zero-point of charge of molybdenitefaces is situated around pH 4-5.EffectofdextrinonmolybdeniteflotationTable 1 shows Wie and Fuerstenau's results (1974) on the effect of dextrin on thewettabilityofmolybdenitesurfaceTable 1. Contact angles of air bubbles on molybdenite as a function of dextrin at pH 3.8 and pH9.4 in 2x10-3 MKClDextrin concentration (mg/L)Contact angle at pH 3.8Contact angle at pH9.408683670.263451.578.300Asthesemeasurementsdemonstrate,theeffectofdextrinisquitevisibleevenat1mgofdextrin/L(similareffectof dextrin oncoal wettability was shownbyMilleretal.(1984).Sincethe basic structural unit of dextrin and starch, α-D-glucose (Fig. 1), is the same it is obvious thatstarchwill affectthewettability ofmolybdenite surface inthe sameway.EffectofpolyacrylamidesonmolybdeniteflotationThe effect of commercial flocculants on the floatability of molybdenite has been studiedbyLaskowski andCastro(1999).Figure7showstheresultsofourtestsontheeffectofpolyacrylamide flocculants (Nalco 9809) on the floatability of different size fractions ofmolybdenite.1001204060Mean particle size,mmFigure 7.Effect of particle size and Nalco9809flocculant onmolybdeniteflotation.Flotation agents:isopropyl xanthate28g/t,MIBC36g/t, pH 11.Numbers on the curves stand for dosage of Nalco 9809 flocculant in g/t (Laskowskiand Castro, 1999)
very good correlation between oil flotation of molybdenite and the wettability of molybdenite surface by oil. Since these contact angle measurements were carried out on freshly cleaved faces of a molybdenite crystal, these data seem to indicate that the zero-point of charge of molybdenite faces is situated around pH 4-5. Effect of dextrin dextrin dextrin dextrin on molybdenite molybdenite molybdenite molybdenite flotation flotation flotation flotation Table 1 shows Wie and Fuerstenau’s results (1974) on the effect of dextrin on the wettability of molybdenite surface. Table 1. Contact angles of air bubbles on molybdenite as a function of dextrin at pH 3.8 and pH 9.4 in 2x10-3 M KCl. Dextrin Dextrin Dextrin Dextrin concentration concentration concentration concentration (mg/L) Contact Contact Contact Contact angle at pH 3.8 Contact Contact Contact Contact angle at pH 9.4 0 86 83 0.2 67 63 1.57 42 45 8.3 0 0 As these measurements demonstrate, the effect of dextrin is quite visible even at 1 mg of dextrin/L (similar effect of dextrin on coal wettability was shown by Miller et al. (1984). Since the basic structural unit of dextrin and starch, α-D-glucose (Fig. 1), is the same it is obvious that starch will affect the wettability of molybdenite surface in the same way. Effect of polyacrylamides polyacrylamides polyacrylamides polyacrylamides on molybdenite molybdenite molybdenite molybdenite flotation flotation flotation flotation The effect of commercial flocculants on the floatability of molybdenite has been studied by Laskowski and Castro (1999). Figure 7 shows the results of our tests on the effect of polyacrylamide flocculants (Nalco 9809) on the floatability of different size fractions of molybdenite. Figure 7. Effect of particle size and Nalco 9809 flocculant on molybdenite flotation. Flotation agents: isopropyl xanthate 28 g/t, MIBC 36 g/t, pH 11. Numbers on the curves stand for dosage of Nalco 9809 flocculant in g/t (Laskowski and Castro, 1999). 20 40 60 Me an p article size, m m 40 60 80 100 Mo recovery, % 0 2 5 1 0 3 0
In these tests a commercial molybdenite flotation concentrate was classified in aCyclosizer and the averageparticle size of these size fractions was measured using aQantachrome MicroscanParticle Size Analyzer.All the sizefractions were purifiedbytreatingwith diethyl ether to remove any organic contamination, with sulfuric acid to remove oxidelayers,and with sodium cyanideto leach out surfacemetallic impurities.Theprocedure wasfollowedbywashingwithdistilledwater.Theflotationtestswerecarriedoutina1.5LAgitairflotation cell using for each experiment 15 g of molybdenite and 485 g of quartz (-60 +400mesh).Two polyacrylamide flocculants, Magnafloc 139 and Nalco 9809, were tested.The results clearly demonstrate the depressing effect a polyacrylamide-type flocculanthas on the flotation of molybdenite.These results also indicate that this polymer has a morepronounced effect on the flotation of fine particles.This may also indicate that PAMmacromoleculespreferentiallyadsorbontotheedgesofmolybdeniteparticles.Efect of polyacrylamide degradation products on molybdenite flotationDissolutionof high molecularweight flocculants is a slowprocess.Atthe beginning,stage (a), the solid powder swells to large gel-lumps (Owen at al. 2002). At stage (b) the gellumps are no longer visible, although the polymer chains may be far from fully dispersed. Stage(c)representstheoptimalagingtimeforaflocculation,dispersionofthepolymerchainsiseithercomplete or at the maximum level expected forthe solvent.Another change that the polymeric flocculants may be subjected to in the circuits ofmineral processing pants result from shear forces (either from pumping to unit operations orfrom the high degree of turbulence experienced during mixing).Shear degradation ofpolyacrylamides is well documented (Scott et al., 1996). Sheared flocculant solutions showreducedviscosity,andreducedperformanceasflocculants(HendersonandWheatley,1987)In this work aqueous solutions of polyacrylamide-type flocculant (A-110 provided byCytec-Chile)were subjected to shear degradation under a range of high-speed stirring conditions(Castro and Laskowski, 2004) (according to Ferrera at al. (2009) A-110 flocculant has molecularweight of 3.9 x 106 and its degree of anionicity is 18.1%). Since flocculation depends onmolecularweight oftheflocculant, settling testswereusedtoevaluatetheeffectof shearing,ie.the effect of flocculant molecular weight on flocculation.Figure 8 shows the settling rate of thefine quartz suspension (a -325/+400 mesh purified quartz), at a constant dosage of A-110flocculant of 5 g/t as a function of stirring time. As Figure 8 demonstrates, the tested flocculantwas losing its flocculationpoweras a result of stirring
In these tests a commercial molybdenite flotation concentrate was classified in a Cyclosizer and the average particle size of these size fractions was measured using a Qantachrome Microscan Particle Size Analyzer. All the size fractions were purified by treating with diethyl ether to remove any organic contamination, with sulfuric acid to remove oxide layers, and with sodium cyanide to leach out surface metallic impurities. The procedure was followed by washing with distilled water. The flotation tests were carried out in a 1.5 L Agitair flotation cell using for each experiment 15 g of molybdenite and 485 g of quartz (-60 +400 mesh). Two polyacrylamide flocculants, Magnafloc 139 and Nalco 9809, were tested. The results clearly demonstrate the depressing effect a polyacrylamide-type flocculant has on the flotation of molybdenite. These results also indicate that this polymer has a more pronounced effect on the flotation of fine particles. This may also indicate that PAM macromolecules preferentially adsorb onto the edges of molybdenite particles. Effect of polyacrylamide polyacrylamide polyacrylamide polyacrylamide degradation degradation degradation degradation products products products products on molybdenite molybdenite molybdenite molybdenite flotation flotation flotation flotation Dissolution of high molecular weight flocculants is a slow process. At the beginning, stage (a), the solid powder swells to large gel-lumps (Owen at al. 2002). At stage (b) the gel lumps are no longer visible, although the polymer chains may be far from fully dispersed. Stage (c) represents the optimal aging time for a flocculation, dispersion of the polymer chains is either complete or at the maximum level expected for the solvent. Another change that the polymeric flocculants may be subjected to in the circuits of mineral processing pants result from shear forces (either from pumping to unit operations or from the high degree of turbulence experienced during mixing). Shear degradation of polyacrylamides is well documented (Scott et al., 1996). Sheared flocculant solutions show reduced viscosity, and reduced performance as flocculants (Henderson and Wheatley, 1987). In this work aqueous solutions of polyacrylamide-type flocculant (A-110 provided by Cytec-Chile) were subjected to shear degradation under a range of high-speed stirring conditions (Castro and Laskowski, 2004) (according to Ferrera at al. (2009) A-110 flocculant has molecular weight of 3.9 x 106 and its degree of anionicity is 18.1%). Since flocculation depends on molecular weight of the flocculant, settling tests were used to evaluate the effect of shearing, i.e., the effect of flocculant molecular weight on flocculation. Figure 8 shows the settling rate of the fine quartz suspension (a -325/+400 mesh purified quartz), at a constant dosage of A-110 flocculant of 5 g/t as a function of stirring time. As Figure 8 demonstrates, the tested flocculant was losing its flocculation power as a result of stirring
0.403oSetlingrate400800200600100012001400160018009Surring tme, sFigure 8. Effect of stirring (shear degradation time) on settling rate of a quartz suspension (-325+ 400 mesh, 20 % solids) at A-110 (polyacrylamide Cytec flocculant) dosage of 5 g/t and naturalpHof 6.5 (the dashed line denotes the settling rate without flocculant)(CastroandLaskowski,2004)Figure 9 demonstrates that the tested non-shear degraded flocculant depresses stronglymolybdenite flotation; depressing effect of the flocculant was quite dramatic starting at 1-2 ppmflocculant concentration and was total at 5 ppm. These micro-flotation tests (in a Partridge andSmith cell (1971) were carried out with a fine molybdenite (-65/+ 200 mesh, -210/+74 μm)prepared by crushing a massive high purity natural crystal of molybdenite.As Figure 8 showsthe flocculant macromolecules are being degraded and broken to shorter chains; in its originalform the flocculant turned out to be a strong depressant for molybdenite floatability. In the nextstage the effect of flocculant shearing on its performance in the molybdenite flotation processwas tested at 30 ppm of MIBC and as Figure 10 reveals, the flocculant degraded by shearing isstill depressingmolybdenite flotation, particularly at lowpH.o8060Recorrory15n10Polymerconcentration,ppmFigure 9.Effect of A-110flocculant concentration on the flotation of molybdeniteat 30Ppmof MIBC.Polyacrylamide and natural pH of6.5 (Castro and Laskowski,2004)
Stirring time, s Stirring time, s Stirring time, s Stirring time, s 0 200 400 600 800 1000 1200 1400 1600 1800 Settling rate, cm Settling rate, cm Settling rate, cm Settling rate, cm -1 0.0 0.1 0.2 0.3 0.4 0.5 Figure 8. Effect of stirring (shear degradation time) on settling rate of a quartz suspension (-325 + 400 mesh, 20 % solids) at A-110 (polyacrylamide Cytec flocculant) dosage of 5 g/t and natural pH of 6.5 (the dashed line denotes the settling rate without flocculant) (Castro and Laskowski, 2004). Figure 9 demonstrates that the tested non-shear degraded flocculant depresses strongly molybdenite flotation; depressing effect of the flocculant was quite dramatic starting at 1-2 ppm flocculant concentration and was total at 5 ppm. These micro-flotation tests (in a Partridge and Smith cell (1971) were carried out with a fine molybdenite (-65/+ 200 mesh, -210/+74 μm) prepared by crushing a massive high purity natural crystal of molybdenite. As Figure 8 shows the flocculant macromolecules are being degraded and broken to shorter chains; in its original form the flocculant turned out to be a strong depressant for molybdenite floatability. In the next stage the effect of flocculant shearing on its performance in the molybdenite flotation process was tested at 30 ppm of MIBC and as Figure 10 reveals, the flocculant degraded by shearing is still depressing molybdenite flotation, particularly at low pH. Polymer concentration, ppm Polymer concentration, ppm Polymer concentration, ppm Polymer concentration, ppm 0 5 10 15 20 Recovery, % Recovery, % Recovery, % Recovery, % 0 20 40 60 80 100 Figure 9. Effect of A-110 flocculant concentration on the flotation of molybdenite at 30 ppm of MIBC. Polyacrylamide and natural pH of 6.5 (Castro and Laskowski, 2004)