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Colloids and Surfaces A: Physicochemical and Engineering Aspects 181 (2001) 159 – 169 Hydrophobic flocculation of sphalerite fines in aqueous suspensions induced by ethyl and amyl xanthates Shaoxian Song *, Alejandro Lopez-Valdivieso, Juan Luis Reyes-Bahena, Hugo Israel Bermejo-Perez Instituto de Metalurgia, Uni6ersidad Autonoma de San Luis Potosi, A6. Sierra Leona 550, Lomas 2a Seccio ´n, C.P.78210, San Luis Potosi, SLP, Mexico Received 30 April 2000; accepted 2 October 2000 Abstract The hydrophobic flocculation of sphalerite fines in aqueous suspensions induced by ethyl and amyl xanthates has been investigated using laser diffraction, electrophoretic light scattering and contact angle measurements. The investigation includes studying the effects of various parameters, namely hydrocarbon chain length of xanthate ions, xanthate concentration, pH, original particle size and stirring strength, and approaching the mechanisms of the hydrophobic flocculation. The experimental results have demonstrated that the hydrophobic flocculation arises as a result of the adsorption of ethyl and amyl xanthate ions on sphalerite, which imparts hydrophobicity to the particles, and thereby hydrophobic interaction between the particles. It closely correlates with sphalerite particle hydrophobicity and original particles sizes. The more hydrophobic and the smaller the particles, the stronger is the hydrophobic flocculation. There are critical ethyl and amyl xanthate concentrations at which the hydrophobic flocculation start to increase sharply, which well accords with the contact angle and negative z potential of sphalerite. The hydrophobic flocculation increased with increasing xanthate concentration despite an simultaneous increase in the negative z potential of sphalerite, meaning that hydrophobic interaction between the particles increased much more strongly than electric double layer repulsion from the adsorption of the xanthate ions. Also, it has been found that there is a direct relationship between particle hydrophobicity and stirring strength for the hydrophobic flocculation. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrophobic flocculation; Floc size; z Potential; Contact angle; Adsorption; Sphalerite fines; Xanthate ions www.elsevier.nl/locate/colsurfa 1. Introduction The flotation response of minerals falls substan- tially when the particles are present in a fine size range. For instance, sulfide minerals flotation were found to have the lower size limit of 6 mm for galena, 8 mm for sphalerite, 9 mm for pyrrhotite, 15 mm for chalcopyrite and 20 mm for pyrite through investigations in laboratory as well as in operating mills [1]. During the last four decades, considerable research work have been * Corresponding author. Fax: +52-4-8254326. E-mail address: [email protected] (S. Song). 0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0927-7757(00)00789-5

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Page 1: Hydrophobic flocculation of sphalerite fines in aqueous suspensions induced by ethyl and amyl xanthates

Colloids and Surfaces

A: Physicochemical and Engineering Aspects 181 (2001) 159–169

Hydrophobic flocculation of sphalerite fines in aqueoussuspensions induced by ethyl and amyl xanthates

Shaoxian Song *, Alejandro Lopez-Valdivieso, Juan Luis Reyes-Bahena,Hugo Israel Bermejo-Perez

Instituto de Metalurgia, Uni6ersidad Autonoma de San Luis Potosi, A6. Sierra Leona 550, Lomas 2a Seccion, C.P.78210,San Luis Potosi, SLP, Mexico

Received 30 April 2000; accepted 2 October 2000

Abstract

The hydrophobic flocculation of sphalerite fines in aqueous suspensions induced by ethyl and amyl xanthates hasbeen investigated using laser diffraction, electrophoretic light scattering and contact angle measurements. Theinvestigation includes studying the effects of various parameters, namely hydrocarbon chain length of xanthate ions,xanthate concentration, pH, original particle size and stirring strength, and approaching the mechanisms of thehydrophobic flocculation. The experimental results have demonstrated that the hydrophobic flocculation arises as aresult of the adsorption of ethyl and amyl xanthate ions on sphalerite, which imparts hydrophobicity to the particles,and thereby hydrophobic interaction between the particles. It closely correlates with sphalerite particle hydrophobicityand original particles sizes. The more hydrophobic and the smaller the particles, the stronger is the hydrophobicflocculation. There are critical ethyl and amyl xanthate concentrations at which the hydrophobic flocculation start toincrease sharply, which well accords with the contact angle and negative z potential of sphalerite. The hydrophobicflocculation increased with increasing xanthate concentration despite an simultaneous increase in the negative z

potential of sphalerite, meaning that hydrophobic interaction between the particles increased much more stronglythan electric double layer repulsion from the adsorption of the xanthate ions. Also, it has been found that there isa direct relationship between particle hydrophobicity and stirring strength for the hydrophobic flocculation. © 2001Elsevier Science B.V. All rights reserved.

Keywords: Hydrophobic flocculation; Floc size; z Potential; Contact angle; Adsorption; Sphalerite fines; Xanthate ions

www.elsevier.nl/locate/colsurfa

1. Introduction

The flotation response of minerals falls substan-tially when the particles are present in a fine size

range. For instance, sulfide minerals flotationwere found to have the lower size limit of 6 mmfor galena, 8 mm for sphalerite, 9 mm forpyrrhotite, 15 mm for chalcopyrite and 20 mm forpyrite through investigations in laboratory as wellas in operating mills [1]. During the last fourdecades, considerable research work have been

* Corresponding author. Fax: +52-4-8254326.E-mail address: [email protected] (S. Song).

0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0927 -7757 (00 )00789 -5

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S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 181 (2001) 159–169160

carried out to improve the poor flotation responseof mineral fines, in order to treat finely dissemi-nated ores to meet the increasing need for miner-als. See the excellent reviews in this regard madeby Fuerstenau [2] and Sivamohan [3]. Variousschemes including floc flotation (carrier flotation,emulsion flotation and shear flocculation flota-tion, etc.) as well as vacuum flotation and elec-trolytic flotation have been proposed. Flocflotation is based on the size enlargement of de-sired mineral fines through selective aggregation,followed by conventional flotation of these finesas flocs. Hydrophobic flocculation is mostly se-lected for this purpose, because hydrophobic flocsare more compact and have greater strength incomparison to electrolytic coagulation and poly-mer flocculation [4].

Hydrophobic flocculation arises as a result ofhydrophobic interaction between particles inaqueous suspensions [5–8]. Except for naturallyhydrophobic particles, it is usually induced bysurfactants upon their adsorption on the particlesand sufficient input of kinetic energy to the sys-tem, and is enhanced by adding non-polar oil.Numerous investigations have been performed onthe hydrophobic flocculation of mineral fines dur-ing the last four decades, in order to delineate thefundamentals of the floc flotation process. Soma-sundaran et al. [9] observed that the aggregationof colloidal alumina in dilute solution of sodiumdodecyl sulfonate did not decrease, but increasedas increasing the surface charges. It was specu-lated that the adsorbed surfactant might bridgebetween the particles thereby lowering the freeenergy of the system. Warren and Koh [10,11]found that scheelite fines with a high surfacecharge aggregated in an aqueous suspension dueto sodium oleate addition and suspension condi-tioning at a high shear rate. This phenomenonwas termed shear flocculation to emphasize therole of shear fields that are attributed to providingparticles kinetic energy to overcome the energybarrier due to electrostatic repulsion [10]. Also,they suggested that hydrophobic interaction andhydrocarbon chain association play very impor-tant roles in the shear flocculation [4,11]. Chiaand Somasundaran [12] have studied the carrierflotation of anatase fines on coarse calcite with

sodium oleate as collector. They found that thelayers of adsorbed oleate ions and the surfacecharge at the solid/water interface of both fineand coarse particle determined whether the fineparticles coat the coarse particles. Lu and Song[8,13] reported that the hydrophobic flocculationof rhodochrosite and hematite fines induced bysodium oleate closely correlated to the particlehydrophobicity; the more hydrophobic the parti-cles, the stronger is the flocculation. The kineticstudy on the hydrophobic flocculation of quartzfines with dodecylamine revealed that the flocswere disrupted as fragments in inertial sub-rangeof turbulence, in comparison to surface erosion inviscous sub-range of turbulence for electrolyticcoagulates [4], indicating that hydrophobic flocsare capable of withstanding greater floc-ruptureforces from turbulent flows.

There are also many reports on the hydropho-bic flocculation of other oxide minerals fines in-duced by surfactants, namely, apatite fines bysodium oleate [14], wolframite fines by sodiumoleate [15], hematite fines by sodium dodecyl sul-fate (SDS) [16] and rutite fines by benzyl arsenicacid [17], etc. However, there are few literatureson the hydrophobic flocculation of sulfide mineralfines. This work aims at the study of the hydro-phobic flocculation of sphalerite fines induced byethyl and amyl xanthates in aqueous suspensions,in order to gain a better understanding of theprocess as applied to the floc flotation of sphaler-ite fines.

Sphalerite is the most common zinc mineral,and its flotation with alkyl xanthates as collectorshas been practiced worldwide since early in 20thcentury. Numerous reviews are available in thisregard, including the recent ones by Fuerstenau[18] and Finkelstein [19]. Although new collectorshave been developed for sphalerite flotation [20],alkyl xanthates still represent the most commoncollector used today in the industry. It is believedthat the flotation mechanism is due to chemisorp-tion of alkyl xanthate ions on sphalerite surfaces,rendering them hydrophobic [18]. It has beenfound that sphalerite flotation is markedly influ-enced by pH. With amyl xanthates as collector,sphalerite flotation is effective only in a pH rangefrom weakly acidic to weakly alkaline condition,

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depending on the xanthate concentration [21].Furthermore, the xanthate concentration requiredfor flotation depends closely on the chain lengthof the collector; being this concentration higher,the shorter is the chain length [21].

In this work, the hydrophobic flocculation ofsphalerite fines and the adsorption of ethyl andamyl xanthate ions on sphalerite were studiedusing laser diffraction and electrophoretic lightscattering measurements, versatile techniques instudying colloidal particles. The particle hydro-phobicity was determined through contact anglemeasurements. The objectives are to characterizethe behavior of the hydrophobic flocculation ofsphalerite fines as affected by several parameters,namely xanthates concentration, hydrocarbonchain length of xanthates, pH, original particlesize and stirring strength, and to study the mecha-nisms of the hydrophobic flocculation.

2. Materials and experimental methods

2.1. Materials

The sample of sphalerite used in this study wasreceived from the Naica mine, in Chihuahua,Mexico. The mineral was crushed by a handhammer, and then purified by hand sorting. Sev-eral pieces of sphalerite were cut to obtain rectan-gles with 2-cm length and 1-cm width for contactangle measurements. Fines of sphalerite were ob-tained through grinding in a vibrating cup mill.By gravity sedimentation, two particle size frac-tions were produced, i.e. −2 and +2 to 5 mm,for aggregation and electrokinetic studies. Fig. 1illustrates the size distribution of the two samplesin suspensions treated with sodium hexam-etaphosphate and ultrasonic for particle disper-sion. The final sphalerite samples were found toassay 93.8 and 92.3% ZnS for the −2 and +2 to5 mm, respectively.

Potassium ethyl and amyl xanthates were sup-plied by the Industria Quımica de Mexico, andwere purified in the following procedure. Xan-thate powders were first dissolved in acetone,followed by filtration; next, the filtrate was mixedwith ethyl ether or amyl ether to precipitate potas-

sium ethyl or amyl xanthate. This procedure wasrepeated twice to obtain pure potassium ethyl andamyl xanthates. Hydrochloric acid and sodiumhydroxide analytical reagents were obtained fromJ.T. Barker that were used to adjust pH ofaqueous suspensions. The water used was distilledfirst, and then treated by passing through resinbeds and a 0.2-mm filter. The residue conductivityof the water was less than 1 mS cm−1.

2.2. Experimental methods

2.2.1. Hydrophobic flocculationHydrophobic flocculation of sphalerite fines in

aqueous suspensions was performed in a condi-tioning tank of 10-cm internal diameter with four1-cm width baffles. The mixer head was connectedto a Servodyne mixer controller and a mixingshaft equipped with a 6-cm width and 2-cm heightS-shape impeller. Mineral suspensions were pre-pared with 1-g sphalerite and 100-ml water, andwere adjusted for pH with either hydrochloricacid or sodium hydroxide solution. Then the sus-pensions were conditioned at a given stirringspeed for a given duration in the absence andpresence of ethyl or amyl xanthate before theywere transferred to the particle size analyzer todetermine floc size distributions.

Fig. 1. Cumulative undersize distributions of the sphaleritesamples.

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2.2.2. Floc size determinationA Shimadzu SALD-1100 laser diffraction parti-

cle size analyzer was used to determine the sizedistribution of dispersed and flocculated suspen-sions. This instrument utilizes a 3-mW laser diodeas the light source, showing excellent stabilityunder output and oscillation. The calculations ofparticle size distributions are based on the Fraun-hofer diffraction theory for large size range andthe Mie scattering theory for small size range.Different from sedimentation method [22,23], thisinstrument reports optical diameters instead ofStokes equivalent diameters. In the case of floccu-lated suspensions, the optical diameters of flocsreported in this study are much larger than thecorresponding Stokes equivalent diameters be-cause of the high porosity of flocs. In order toprotect flocs from breaking in the measurementoperation, ultrasonic was not applied to thesuspensions.

2.2.3. Aggregation degreeThe dispersion and aggregation of the sphaler-

ite suspensions were also evaluated and reportedas aggregation degree (E), given by [24]

E=N0−N

N, (1)

where N0 and N are the particle number concen-tration in suspensions before and after aggrega-tion, respectively. The higher the aggregationdegree, the more powerful is the aggregation. Inthe present study, volumetric mean diameters be-fore and after aggregation were used to calculateaggregation degrees. Considering one floc, N=1,and N0 can be given by

N0=Pf

Vf

V0

(2)

where Pf means floc porosity, Vf and V0 are thevolume of the floc and the original particle, re-spectively. In the event that both the originalparticle and floc are spheres, Eq. (1) can be writ-ten as

E=Pf

d f3

d03−1, (3)

where df and d0 are the volumetric mean diameterof floc and original particle, respectively. Unlikeloose polymer flocs, hydrophobic flocs have amore compact structure, resulting in a floc poros-ity of about 0.5 [25].

2.2.4. z Potential measurementz Potentials of sphalerite particles were deter-

mined using a Coulter Delsa 440SX instrument,which is based on the electrophoretic light scatter-ing technique. A detailed description of the instru-ment can be found elsewhere [26,27]. In thisstudy, −2 mm sphalerite suspensions at 0.1%solids were first conditioned for 5 min with pHregulating reagents in the presence and absence ofthe xanthates. The suspensions were then trans-ferred to the rectangular capillary cell of theinstrument. Measurements were performed using16–19 V cm−1 electric field and 500 Hz frequencywith 100 s duration (using a 2-s on and 1-s offsequence alternating the electric field polarity) atthe upper and lower stationary layers of the cell.Four-angle electrophoretic light scattering was,simultaneously, utilized for z potentials measure-ments. The average value of z potentials from thefour angles was reported as final results. Thetemperature throughout measurements was keptat 25.090.1°C.

2.2.5. Contact angle measurementA Rame-Hart NRL-100-00 goniometer was

used for contact angles measurements. The sur-face of the sphalerite piece used for the measure-ments was polished using first a fine sandpaperand then a diamond paste. The piece was condi-tioned in an xanthate solution at a given pH for10 min. Solid/water/gas interface was obtained byattaching a small air bubble to the surface im-mersed in water, from which the contact anglewas determined.

3. Experimental results

3.1. Effect of alkyl xanthate concentration

The hydrophobic flocculation of sphalerite finesinduced by ethyl and amyl xanthates was per-

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Fig. 2. Volumetric mean diameters of sphalerite flocs as afunction of alkyl xanthates concentration at pH 6.

tion. Fig. 3 shows the percentage of −10 mm flocs(it is close to the lower size limit of 8 mm forsphalerite) in the flocculated sphalerite suspensionas a function of either ethyl or amyl xanthateconcentrations. It can be seen that the percentagesof −10 mm particles in the flocculated suspen-sions decreased as increasing the xanthate concen-trations. This percentage is lower for amylxanthate, indicating that hydrophobic flocculationinduced by amyl xanthate is more effective thanthat by ethyl xanthate. At high concentrations ofamyl xanthate, less than 2% of the flocs are as−10 mm, while with ethyl xanthate the percentageis very high, 23%. Clearly, the hydrophobic floc-culation induced by amyl xanthate is very power-ful to aggregate sphalerite fines in aqueoussuspensions.

3.2. Effect of pH

Fig. 4 depicts the volumetric mean diameter ofsphalerite flocs as a function of pH in the absenceand presence of either 10−2 mol l−1 ethyl xan-thate or 5×10−3 mol l−1 amyl xanthate. With-out xanthate, very weak aggregation (i.e.electrolytic coagulation) occurred at pH below 7.However, owing to the presence of the xanthates,the aggregation was strongly enhanced. As is seen,

formed on the −2 mm sphalerite. Fig. 2 illustratesthe volumetric mean diameter of sphalerite flocsas a function of either ethyl or amyl xanthateconcentrations at pH 6. In the figure, the opensquare on the y-axis indicates the volumetricmean floc diameter in the absence of xanthates. Itis 1.8 mm, a value close to the original volumetricmean diameter of the particles as can be seen inFig. 1. As is noted in Fig. 2, the volumetric meanfloc diameters increased with increasing ethyl andamyl xanthate concentration, indicating that thesphalerite fines aggregate in the presence of thexanthates. This increase was slight in the lowxanthate concentration, and then was sharp aftera critical xanthate concentration. The xanthatewith a longer hydrocarbon chain-length loweredthe critical concentration and increased the flocdiameter. With amyl xanthate, flocs of 27 mm inmean size were obtained while with the sameconcentration of ethyl xanthate the flocs wereabout 11 mm. The critical xanthate concentrationfor the sharp increase of the floc size well coincidewith that for the rapid flotation response of spha-lerite with both amyl and ethyl xanthate, as re-ported by Fuerstenau et al. [21].

As already stated, there is a lower size limit forsulfide mineral flotation. Hence, the percentage offine particles (under lower size limit) in flocculatedsuspensions is an important factor for floc flota-

Fig. 3. Percentages of −10 mm flocs in flocculated suspensionof sphalerite fines as a function of alkyl xanthates concentra-tion at pH 6.

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Fig. 4. Volumetric mean diameter of sphalerite as a function ofpH without and with hydrophobic-flocculation induced byalkyl xanthates.

Fig. 5. Aggregation degree of flocculated sphalerite suspen-sions induced by amyl xanthate as a function of pH on the twosize fraction of sphalerites.

hydrophobic flocs formed from the −2 mm spha-lerite contained 60-fold more original particlesthan those from the +2 to 5 mm sphalerite. Inaddition, the E curve for each size fraction showsa maximum within the pH range of 3–9, inagreement with the results given in Fig. 4 for themaximum values of floc size.

3.4. Effect of stirring strength

Stirring strength in a mixing system is mainlydetermined by stirring speed as long as the im-peller, reactor size, slurry volume and solid con-

amyl xanthate produced larger flocs as comparedwith those from ethyl xanthate, indicating againthat amyl xanthate is more effective than ethylxanthate in inducing flocculation of sphaleritefines. At pH 6, the flocs of 27 and 12 mm involumetric mean diameter are obtained for amylxanthate and ethyl xanthate, respectively. In addi-tion, it can be observed that large flocs are at-tained within the pH range of 3–9 when amylxanthate is used. This pH range is in very goodagreement with that of maximum flotation re-sponse for sphalerite with amyl xanthate, as re-ported by Fuerstenau et al. [21].

3.3. Effect of original particle size

The effect of original particle size on hydropho-bic flocculation was studied using the −2 mm and+2 to 5 mm sphalerite samples. Fig. 5 shows theaggregation degree (E) of sphalerite fines as afunction of pH in the presence of 10−3 mol l−1

amyl xanthate for both samples. To estimate E, afloc porosity of 0.5 was considered for all thecases. As is noted, much larger E values wereobtained with the −2 mm original sphalerite par-ticles in comparison to those for the +2 to 5 mmoriginal particles. The E from the −2 mm spha-lerite are about 60-fold larger than those from the+2 to 5 mm sphalerite. This indicates that the

Fig. 6. Volumetric mean diameters of sphalerite flocs inducedby amyl xanthate as a function of stirring speed.

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Fig. 7. z Potential of sphalerite as a function of pH in theabsence and the presence of alkyl xanthates.

al. [30]. With xanthate ions the IEP shifted to alower pH and the z potential values become morenegative, but up to pH 11. The increase in thenegative z potential is indicative of chemisorptionas has been shown elsewhere [31,32], so that xan-thate ions chemisorbed on sphalerite leading to theformation of zinc xanthate on the surface, inagreement with the adsorption mechanism pro-posed by Fuerstenau [33]. The increment in thenegative z potential D�z � has been related directlyto adsorption density of the chemisorbed ions atthe solid/water interface [34]. As can be seen in Fig.7, this D�z �, caused by xanthate ions on the sphaler-ite, was just up to pH 11, indicating that these ionswere not present at the sphalerite/water interfaceabove this pH value. On the other hand, the largestincrement in D�z � occurs around pH 6 for 10−3 moll−1 amyl xanthate, meaning that adsorption den-sity of this xanthate on the sphalerite was thehighest under this condition, resulting in a maxi-mum value of floc diameter as can be seen in Fig.4. The adsorption density depends on the hydro-carbon chain length and the concentration of thesurfactants. The longer the hydrocarbon chain andthe higher the concentration, the higher is theadsorption density. The adsorption density of ethylxanthate at 5×10−3 mol l−1 on the sphalerite wasabout equivalent to that of amyl xanthate at 10−4

mol l−1 as their z potential curves are similar.

4.2. Particle hydrophobicity effect

Xanthate ions adsorb on sphalerite with theirnon-polar group (hydrocarbon chains) orientatedtoward the bulk suspension, thereby impartinghydrophobicity to the particles. Fig. 8 presents thecontact angles (which is usually used to representhydrophobicity) of the sphalerite as a function ofpH in the absence and presence of 5×10−3 moll−1 amyl xanthate. Indeed, the sphalerite contactangle greatly increased due to amyl xanthate addi-tion. This increase varied markedly with pH. Themaximum contact angle increase occurred at pH 6,being about 36°. At pH above 11, the increasetended to be zero. Clearly, sphalerite contact an-gles closely correlated with the adsorption densityof amyl xanthate ions. The higher the adsorptiondensity, the greater is the contact angle.

centration are constants [28]. Fig. 6 illustrates theeffect of the stirring speed on the volumetric meandiameter of sphalerite flocs induced by amyl xan-thate at pH 6. As is noted, the volumetric mean flocdiameter increased with increasing the stirringspeed until reaching a peak about 900 rpm, fol-lowed by a decline. These results indicate that thestirring strength activates the enlargement of theflocs, which are observed to withstand large floc-rupture forces. The decrease in size of the flocsabove 900 rpm may be due to an increase in flocrupture and floc density. This is being investigatedfurther in our laboratory as to delineate thisphenomenon with experimental evidence and esti-mation of the energy needed to break the flocs.

4. Discussions

4.1. Adsorption of xanthates on sphalerite

The adsorption of both ethyl and amyl xanthateions on sphalerite was studied through electroki-netics. The z potentials of the sphalerite weremeasured as a function of pH in the absence andpresence of ethyl and amyl xanthates. The resultsare given in Fig. 7. As is noted, the sphaleritepresented an isoelectric point (IEP) at pH 3.3, inagreement with that reported by Healy andMoignard [29] and more recently by Laskowski et

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Fig. 8. Contact angle of sphalerite as a function of pH in theabsence and the presence of amyl xanthate.

erful is the hydrophobic flocculation. This conclu-sion also applied to the hydrophobic flocculationof other mineral fines [4,8]. In addition, the parti-cle hydrophobicity effect can be found to be moremarked at great contact angle than at small one.For instance, the volumetric mean floc diameterincreased 4 mm when the contact angle increasedfrom 40 to 48°, in comparison to the increase ofabout 6 mm from 55 and 60°.

As already stated, hydrophobic flocculationarises as a result of hydrophobic interaction be-tween particles. The stronger the hydrophobicattractive force between the particles, the morepowerful is the hydrophobic flocculation. Hence,from the foregoing experimental results, it is clearthat the hydrophobic interaction closely correlateswith particle contact angles. During last twodecades, there were numerous literatures on hy-drophobic interaction [35–37]. Although directforce measurements have found that an attractivehydrophobic interaction exists between hydropho-bic surfaces in aqueous solutions [38–40], theorigin of hydrophobic interaction is still unclear.Rabinovich and Yoon [41] reported that hydro-phobic force increases with surface hydrophobic-ity, based on the force measurements betweensilanated plate and glass sphere. In reverse, themeasurements of the attraction between three dif-ferent kinds of hydrophobic surfaces showed thatthe surface contact angle does not correlate withthe magnitude of hydrophobic forces [42]. Theresults presented in this paper support the formerfinding.

4.3. Surface charges effect

The z potential and contact angle of the spha-lerite were determined as a function of amyl xan-thate concentration at pH 6. The results areshown in Fig. 10. It can be seen that as increasingamyl xanthate concentration, there was a slightincrease of negative z potentials and contact anglein the low concentration range. Following that,the increase became sharp after a critical amylxanthate concentration taking place at about 5×10−5 mol l−1.

If Fig. 10 is superimposed on Fig. 2, it is clearthat the contact angle responded to amyl xanthate

The effect of sphalerite contact angles on thesize distributions of flocculated sphalerite suspen-sions induced by amyl xanthate is shown in Fig.9, where the sphalerite contact angles were ad-justed through varying amyl xanthate concentra-tions. As is seen, the sphalerite floc sizes increasedwith increasing the contact angle. At 60° contactangle, the flocs were two-fold larger than those at40° contact angle. The results suggest that thehydrophobic flocculation is dominated by thesphalerite hydrophobicity, meaning that the morehydrophobic the particle surfaces, the more pow-

Fig. 9. Effect of sphalerite contact angles on the cumulativeundersize distribution of hydrophobic flocs.

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Fig. 10. z Potential and contact angles of sphalerite as afunction of amyl xanthate concentration at pH 6.

not support this conclusion. Amyl xanthate (hav-ing the chain with five carbons) induced a power-ful hydrophobic flocculation for the sphaleritefines. Even ethyl xanthate (with two carbons) alsoinduced the sphalerite fines to be hydrophobicflocculated at a high concentration. From theresults as shown in Fig. 7, it is clear that themechanism by which ethyl xanthate induced amuch weaker hydrophobic flocculation than amylxanthate might be mainly due to its much loweradsorption density on the particles, instead of itsshorter hydrocarbon chain.

As suggested by Lu and Song [13], the hydro-carbon chain length influences surfactant-inducedhydrophobic flocculation through the hydrocar-bon chain association between the particles. Sup-posed that the surfactant molecule length and theparticle separation are represented by l and h,respectively, this interaction only functions in theseparation range of lBhB2l. Commonly, thisseparation range is far from the location of theenergy barrier. Hence, the hydrocarbon chain as-sociation usually does not affect the height andlocation of the energy barrier, but increases thedepth of primary energy valley. In other words,the association between surfactant adsorbing lay-ers on particles may not be helpful to the forma-tion of hydrophobic flocs, but increases thefloc-strength to protect the flocs from rupture.Consequently, hydrocarbon chain length may notstrongly influence surfactant-induced hydrophobicflocculation.

4.5. Stirring strength

The relationship between stirring strength andparticle hydrophobicity in the hydrophobic floc-culation of sphalerite fines induced by amyl xan-thate was given in Fig. 11 where the line indicatesconditions at which the flocs have the volumetricmean diameter (d50) of 15 mm. The contact angleswere adjusted by varying amyl xanthate concen-tration. In this Figure, to the right of the line,flocs with a d50 larger than 15 mm are formed.Conversely, to the left of the line, flocs with a d50

smaller than 15 mm are formed. In the zone ofd50B15 mm, the weaker hydrophobic flocculationoccurs at the point farther from the line; in re-

concentration in the same manner as the floc size,indicating that the hydrophobic flocculationclosely correlates with the particle hydrophobic-ity. However, no correlation between the floccula-tion and the z potential is found. According to theDLVO theory [43], the electric double layer inter-action between particles arisen from particle sur-face charges (which is usually determined by the z

potential) plays an important role for the aggrega-tive stability of fine particle suspensions. The in-crease in negative z potential should give rise tothe increase of the energy barrier, preventing theparticles from aggregation. In the case of amylxanthate adsorbing on sphalerite, increasing thesurface charges did not depress the flocculation,but enhanced it, which is contrary to the DLVOtheory. It may be due to that the amyl xanthateadsorption on sphalerite increased the hydropho-bic attraction much more strongly than the elec-tric double layer repulsion.

4.4. Hydrocarbon chain length effect

The hydrocarbon chain length of adsorbed sur-factants was considered to be an important factorfor hydrophobic flocculation induced by the sur-factants. Zollars and Ali [44] suggested that ahydrocarbon chain with 14 carbons is required toachieve a good hydrophobic flocculation. How-ever, the observation from the present study does

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S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 181 (2001) 159–169168

verse, in the zone of d50\15 mm, the more power-ful hydrophobic flocculation occurs at the pointfarther from the line. It can be noted that thegreater the contact angle, the lower is the stirringstrength required for the hydrophobic floccula-tion. As already stated, a sufficient stirringstrength is needed to overcome the energy barrierfor hydrophobic flocculation. The results given inFig. 11 suggest that increasing particle hydropho-bicity results in the decrease of the energy barrier,which well corresponds with the results given inFig. 9.

cles, the more powerful is the hydrophobicflocculation. The experimental results suggestthat the hydrophobic interaction might be pro-portional to the particle contact angle.

2. A surfactant with a short hydrocarbon chaincan induce a powerful hydrophobic floccula-tion, indicating that hydrocarbon chain lengthmight be not a dominant factor for a surfac-tant-induced hydrophobic flocculation. Also,this flocculation can not be depressed by in-creasing surface charges from surfactant ad-sorption, because the adsorption mightincrease the hydrophobic attraction muchmore strongly than the electric double layerrepulsion.

3. The stirring strength activates the hydrophobicflocculation before it reaches maximum at ahigh value of stirring strength. It has beenfound that there is a direct relationship be-tween the particle contact angle and the stir-ring strength for the hydrophobic flocculation,showing that a lower magnitude of stirringstrength is needed for the flocculation whenthe particles have a greater contact angle.

4. The hydrophobic flocculation is also influ-enced by the size of original particles. Thefiner the original particles, the more powerfulis the hydrophobic flocculation.

Acknowledgements

The financial supports to this work from theConsejo Nacional de Ciencia y Tecnologıa(CONACyT) of Mexico under the grantc485100-5-25840A and the Fondo de Apoyo a laInvestigacion (FAI) of the Universidad Au-tonoma de San Luis Potosi (C99-FAI-11-7.64) aregratefully acknowledged.

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1. It has been demonstrated from the presentstudy that hydrophobic flocculation is effectiveon sphalerite fines in aqueous suspensionsthrough the addition of either ethyl or amylxanthate. It has been found that this hydro-phobic flocculation arises as a result of thechemisorption of the xanthate ions on sphaler-ite, which imparts hydrophobicity to the parti-cles, and thereby the hydrophobic interactionbetween the particles. It strongly correlateswith the degree of particle hydrophobicity,namely that the more hydrophobic the parti-

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