Minerals Engineering 50–51 (2013) 30–37

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Minerals Engineering

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Interactions of clay minerals in copper–gold flotation:Part 1 – Rheological properties of clay mineralsuspensions in the presence of flotation reagents

0892-6875/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.mineng.2013.06.003

⇑ Corresponding author at: School of Chemical Engineering, University ofQueensland, St. Lucia, Brisbane, QLD 4072, Australia. Tel.: +61 7 3365 7156; fax:+61 7 3365 3888.

E-mail address: yongjun.peng@uq.edu.au (Y. Peng).

Nestor Cruz a, Yongjun Peng a,b,⇑, Saeed Farrokhpay a, Dee Bradshaw a

a Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly, Brisbane, QLD 4068, Australiab School of Chemical Engineering, University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 31 December 2012Accepted 4 June 2013Available online 2 July 2013

Keywords:KaoliniteBentoniteRheologypH modifierCollectorFrother

Clay minerals are often associated with copper, gold and other valuable minerals and place a widespreadproblem in mineral flotation. This paper is one of a series addressing the problem caused by clay mineralsin copper–gold flotation. This study seeks to understand the rheological behaviour of kaolinite and ben-tonite suspensions at natural pH and how the pH modifier, collector and frother normally used in copper–gold flotation affect the rheological properties. It was found that kaolinite and bentonite suspensions mayfollow Newtonian flows or non-Newtonian flows with pseudoplastic characteristics depending on thesolid concentration. Bentonite has a stronger effect on the viscosity of suspensions than kaolinite. ThepH modifier, collector and frother have a potential to alter the rheological behaviour of kaolinite and ben-tonite suspensions and the effect of the pH modifier is more pronounced.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Continuous depletion of mineral resources has resulted in in-creased processing of low grade ores which require fine grindingto liberate minerals, and complex ores containing problematic gan-gue minerals (Ndlovu et al., 2011; Schubert, 2008). A number ofstudies have been performed to understand mineral interactionswhen low grade and complex ores are processed. It has been foundthat in many cases non-Newtonian nature of the slurries becomesa common flow property during the processing of these ores whilethe slurries of normal ores (or the high grade ores) exhibit theNewtonian nature (Akroyd and Nguyen, 2003; Boger, 2000; Boger,2009; Das et al., 2011; Ferrini et al., 1979; Klein et al., 1995; Kleinand Pawlik, 2005; Ndlovu et al., 2011; Nosrati et al., 2011, 2012;Prestidge, 1997a,b; Shi and Napier-Munn, 1996).

Mueller et al. (2010) indicated that the rheology of slurries is acomplex function of the physical properties and processes that oc-cur at the scale of the suspended particles. The most important fac-tors determining the rheology are particle volume fractions,particle shapes, interactions between particles, the spatial arrange-ment of particles and the nature of the bulk flow field. Other fac-tors such as the size and shape distribution of the particles and

inter-particle forces can be important in some suspensions (Muel-ler et al., 2010). Regarding the volume fraction, at low concentra-tions the suspension may be Newtonian with the viscosityindependent of the shear rate. An increase in the concentrationmakes the suspension more strongly non-Newtonian with amarked rise in viscosity as the shear rate decreases suggestingthe possible appearance of a yield stress (He et al., 2004).

It has been also found that elongated particles or particles withlarge aspect ratios produce an anisotropic behaviour as a preferredparticle orientation results under the action of the shear field (Turi-an et al., 1997). The flow around a non-spherical particle is differ-ent from that around a spherical particle, and the particlecontribution to the suspension viscosity is also different. For in-stance, at the same volume fraction the degree of the interactionbetween non-spherical particles will be greater than that betweenspherical particles (Mueller et al., 2010).

According to some researchers the narrow size distribution ofparticles in the suspension produces yield stress and steady shearviscosity values higher than those of particles with the broader sizedistribution at the same volume fraction (He et al., 2004; Yanget al., 2001). It has been also reported that in silica-based suspen-sions the viscosity increases with the addition of fine particles(Olhero and Ferreira, 2004). In suspensions of colloidal particles(less than 1 lm) the interface separating the phases is very largeand this makes the stability and rheology be strongly affected bynon-hydrodynamic electrical double-layers forces and by the waythese forces interact with the prevailing chemistry and state of

Fig. 1. The mode of particle interactions and the Bingham yield stress of kaolinitesuspensions as a function of pH (Ndlovu et al., 2011).

N. Cruz et al. / Minerals Engineering 50–51 (2013) 30–37 31

shear (Turian et al., 1997). The stability of this type of suspensionsis dependent on the state of aggregation of the particles and theirrheology is chemistry and shear sensitive (Turian et al., 1997).

Rheological challenges such as reduced flotation rates and com-plex tailings treatment and pumping during the processing of lowgrade and complex ores have been reported (Ndlovu et al., 2011).Changes in rheology affect the hydrodynamics within flotationcells and therefore affect various sub-processes necessary for effi-cient flotation, such as gas dispersion, particle suspension, bub-ble–particle collision, attachment and detachment (Bakker et al.,2009; Shabalala et al., 2011).

Clay minerals such as kaolinite, bentonite and smectite, whichare often associated with copper, gold and other valuable minerals,place a widespread problem in mineral flotation. There have beennumerous observations of these clay minerals having a deleteriouseffect on mineral flotation grade and recovery, with simultaneousobservations of high flotation pulp viscosity. Rigorous explanationson how clay minerals affect flotation are not available. Currently,the only way to treat clay ores is dilution by either blending themat a small proportion with normal ores or processing them at lowsolid concentrations (Peng and Zhao, 2011). The unique feature ofclay minerals is their rheological behaviour which may be associ-ated with their negative impact on the flotation.

Clay minerals are anisotropic phyllosilicates with colloidal sizesand have a layer structure with one dimension in the nanometrerange. They are comprised of one layer alumina octahedral (O)sheet and one or two silica tetrahedral (T) sheets. The thicknessof the 1:1 (TO) layer is about 0.7 nm, and that of the 2:1 (TOT) layeris about 1 nm. They have several types of surfaces, external basal(planar) and edge surfaces as well as internal (interlayer) surfaces.It has been reported that most clay minerals have a basal perma-nent negative charge caused by isomorphous substitution, andthe charge on the edges is either positive or negative dependingon the pH (Schoonheydt and Johnston, 2006). However, recentwork reported that the two basal plane surfaces of kaolinite canhave positive or negative charge depending on pH (Gupta and Mill-er, 2010). In that study a colloidal force measurement showed thatthe silica tetrahedral face is negatively charged at pH > 4 and thealumina octahedral face is negative at pH > 8 and positive atpH < 6. The anisotropic structure and charge properties lead tothe aggregation of clay minerals increasing the viscosity of suspen-sions. There are three main modes of aggregation, edge–face (EF),face–face (FF) and edge–edge (EE) (Gungor, 2000; Johnston andPremachandra, 2001).

The EF contact (or the house of cards network) forms when theedges are positively charged, or in a slightly alkaline mediumabove the critical salt concentration due to the electrostatic attrac-tion between edges and faces (Lagaly, 2006). This network is char-acterized by non-Newtonian flow of the suspensions, and thedevelopment of yield stress in acidic medium. In the case of aNa+-montmorillonite suspension, with increasing pH the house ofcard network breaks down due to the reduction of the positivecharge on edges. The FF contact (or the band type network) mayform three-dimensional structures when the free energy of the sys-tem is at its lowest level (Rand and Melton, 1977). This networkhas showed some elasticity in contrast to the more rigid house ofcards structure (Lagaly, 2006). The edge–edge contact may occurunder conditions of low ionic strength at the pH value of the iso-electric point of the edge surface of the kaolinite particle (Randand Melton, 1977). When studying the aggregation of kaoliniteparticles, explanation of modes of particle interactions can differif the basal surfaces charges are considered negative or if theychange with pH. By assuming that the silica and alumina basalplanes have a fixed negative charge, the maximum yield stressfor kaolinite suspensions is expected at the isoelectric point ofpH < 3, but this yield stress value occurs for suspensions at pH

5–5.5. New findings about the dependence of the basal surfacecharges on pH (Gupta and Miller, 2010) give a better explanationfor the correlation between the maximum shear-yield stress andthe charges in the clay particles. It is reported that a silica face-alu-mina face interaction is the main mode of aggregation at low pH,and this type of association increases the staking of kaolinite layerspromoting the edge–face and face–face associations with increas-ing pH. At pH 5–5.5 the maximum shear-yield stress value occursand a further increase in pH decreases face–face and edge–faceassociations. When pH is high, all the surfaces of the kaolinite par-ticles become negative and therefore dispersed in the suspension(Gupta et al., 2011). Fig. 1 outlines some proposed modes of aggre-gation of kaolinite particles and rheological properties as a func-tion of pH.

Aggregation of kaolinite suspensions can be also related to sed-imentation behaviour. For dispersed particles the sediment thick-ness increases with time to a maximum, and during settling offlocculated particles the sediment thickness decreases with time.These responses depend on electrolyte concentrations and pH(Nasser and James, 2006). Some other factors affecting the sedi-mentation of a suspension are the density of the particles and med-ium, the gravity, the buoyancy, the network force by floc–floccontact, the drag acting at the interface between the wall of a con-tainer and the suspension, and the pressure difference between thetop and the bottom of the suspension (Nakaishi et al., 2012).

Clay minerals can interact with water, inorganic and organiccompounds which in turn modify the rheology of the suspensions.Clay minerals with a 2:1 layer structure are more reactive thanthose with a 1:1 structure. Swelling clay minerals with a 2:1 layerstructure, contain inorganic exchange cations such as Na+, Mg2+

and Ca2+ that are strongly hydrated in the presence of water, andwhen they are treated with some inorganic chemicals, differentsurfaces and rheological properties are developed (Paineau et al.,2011; Yildiz and Calimli, 2002). Kaolinite, a non-swelling clay min-eral with a 1:1 structure, have low chemical reactivity and its anionexchange capacity is typically higher than their cation exchangecapacity (Kau et al., 1998). The influence of organic compoundson clay mineral suspensions is more complex than inorganic com-pounds (Gungor, 2000). Organic compounds may be adsorbed onthe clay lattice by ion–dipole forces, van der Waals forces, orhydrogen bonding. They may also complex with counter ions ofthe clay, or if they are ionised, they may undergo cation or anion

32 N. Cruz et al. / Minerals Engineering 50–51 (2013) 30–37

exchange with the original counter ions (Swartzen-Allen and Mat-ijevic, 1974; Luckham and Rossi, 1999). On the surface of the par-ticles organic compounds have an effect on the electrical doublelayer interactions and on the Van der Waals interactions (Yalçinet al., 2002). Consequently, they alter the rheological behaviourof clay–water suspensions.

The efficiency of copper–gold flotation may be restrained by thechanged pulp rheology in the presence of clay minerals. During flo-tation, pH modifiers (NaOH, lime or sodium carbonate) are nor-mally used to adjust flotation pH in the alkaline range, whileintroducing cations and anions. Meanwhile, surfactants, collectorsand frothers are added to modify surface hydrophobicity, reducebubble sizes and form appropriate froth. The pH modifiers, collec-tors and frothers may interact with clay minerals and further alterthe pulp rheology. In this study, the rheological properties of kao-linite, a non-swelling clay mineral with a 1:1 structure, and ben-tonite, a swelling clay mineral with a 2:1 structure wereinvestigated in the context of copper–gold flotation in the absenceand presence of pH modifiers and collector and frother surfactants.Following this work, detailed fundamental studies will be carriedout to investigate how these flotation reagents affect the rheolog-ical properties of clay suspensions.

2. Experimental

2.1. Materials

Kaolinite and sodium bentonite were purchased from SibelcoGroup, Australia. Quantitative XRD analysis shows that this kaolin-ite sample contains 85 wt.% purity with 4 wt.% quartz and 11 wt.%muscovite. The bentonite sample contains 63 wt.% montmorillon-ite, 25 wt.% albite and 12 wt.% quartz. Particle size distributionsof kaolinite and bentonite samples were measured by MastersizerMicroplus from Malvern Instruments using a very dilute suspen-sion (about 0.1 wt.% solids) conditioned with dispersants followedby 20 s of ultrasonic dispersion. Results are show in Fig. 2. The par-ticle size distribution of these two samples are very similar with70 wt.% particles smaller than 10 lm.

Three pH modifiers, sodium hydroxide (NaOH), lime (CaO) andsodium carbonate (Na2CO3) with AR grade were used to adjust thepH of clay suspensions at 10. The natural pH of clay mineral sus-pensions varied from about 7.5 to about 8.5. These pH values arewithin the pH range normally used in copper–gold flotation. Potas-sium Amyl Xanthate (PAX), the strongest xanthate collector andCytec Interfroth 6500 (an alkyl aryl ester frother) were also tested.They have been widely used in copper–gold flotation. Brisbane tapwater was used throughout the study. However, the three pH mod-ifiers provide different ions and may suggest the effect of waterquality on the rheology of clay suspensions.

Fig. 2. Particle size distributions of the bentonite and kaolinite used in this study.

2.2. Techniques

2.2.1. Preparation of clay mineral suspensionsThe concentrations of bentonite suspensions tested in this

study were 2, 5 and 10 wt.% equivalent to 0.8, 2.1 and 4.3 vol.%,respectively. The concentrations of kaolinite suspensions were10, 20 and 30 wt.% equivalent to 4.3, 9.1, and 14.6 vol.%, respec-tively. To prepare the suspension, kaolinite or bentonite was addedto tap water while stirring. The samples were stirred continuouslyfor 30 and 60 min for kaolinite and bentonite suspensions, respec-tively. This was because kaolinite particles were much easier todisperse than bentonite particles. The suspension with naturalpH or adjusted at pH 10 using a NaOH, lime or Na2CO3 solution,or conditioned with the collector or frother were subjected to rhe-ology measurements.

2.2.2. Rheology measurementsRheology measurements were conducted within an hour after

preparation of the clay suspension. An Ares rheometer with a Cou-ette geometry (bob and cup) from TA Instruments was used for therheology measurements at the ambient temperature around 22 �C.Each rheology measurement required a sample of 15 dm3 whichwas taken with a 20 dm3 syringe while the suspension was mixed.The rheometer automatically set the distance between the base ofthe cup and the tip of the bob which was 8.5 mm in this study. Theshear rate for the rheology measurement was between 0.1 and350 s�1 and it took around 5 min to cover this range during eachmeasurement. To analyze rheology data, both Bingham yield stressand apparent viscosity at a shear rate of 100 s�1 were calculated.These two values gave very similar trends. In this study only appar-ent viscosity at 100 s�1 was chosen and this could be the averageshear rate value in a flotation cell (Ralston et al., 2007). However,in this paper, rheograms were mainly presented to show rheolog-ical properties and also the type of fluids.

2.2.3. Sedimentation experimentsSome sedimentation experiments were conducted for 5 and

20 wt.% suspensions of bentonite and kaolinite, respectively. Threebentonite suspensions were tested at natural pH, and pH 10 ad-justed by lime and sodium carbonate as pH modifiers. Similar testswere conducted for the kaolinite suspensions. The bentonite orkaolinite suspensions were transferred to 1000 ml cylinders. Theheight of the suspension in each calibrated cylinder was 34 cm.Suspensions were mixed by inverting the cylinders 10 times. Thena photo was taken at specific time intervals to meaure the height ofmudlines and record any changes in the suspensions.

3. Results and discussion

3.1. Kaolinite and bentonite interactions with water

The rheological behaviour of kaolinite and bentonite in thepresence of water was investigated first. Fig. 3 shows the rheo-grams of kaolinite and bentonite suspensions at different concen-trations and relationships between shear stress and shear rateare indicated. At a low kaolinite or bentonite concentration (e.g.,10 wt.% kaolinite, and 2 and 5 wt.% bentonite), the suspension be-haves as a Newtonian fluid with the shear stress versus shear ratecurve being linear and passing through the origin. The constantslope that is independent of the shear rate is known as the viscos-ity. However, at a higher kaolinite or bentonite concentration (e.g.,20 and 30 wt.% kaolinite, and 10 wt.% bentonite), the suspensionbecomes non-Newtonian with pseudoplastic characteristics. Theviscosity of the pseudoplastic suspension described between theshear stress and shear rate decreases with an increase in the shear

Fig. 3. Rheograms of kaolinite and bentonite suspensions at differentconcentrations.

Fig. 4. Apparent viscosity of kaolinite and bentonite suspensions as a function oftheir concentrations at a shear rate of 100 s�1.

N. Cruz et al. / Minerals Engineering 50–51 (2013) 30–37 33

rate. Under shear, the large particle aggregates are separated intosmaller units and, at very high shear, into individual particles. Sucha process rheologically manifests itself as shear-thinningbehaviour.

Fig. 3 also indicates that mineral slurries with less than 10 wt.%kaolinite or 5 wt.% bentonite may not change the Newtonian nat-ure of normal fluids. However, in mineral flotation, the interactionsbetween clay minerals and other minerals may take place. It hasbeen reported that aluminium and iron oxide particles are precip-itated in the presence of clay minerals alternating the stabilisationand re-stabilisation of suspensions depending on pH and the massratio between the oxide and the clay mineral (Lagaly, 1989; Lagalyet al., 2006). As a result, there are posibilities that a low concentra-tion of clay minerals may promote the change of the fluid type inthe presence of other minerals. Studies on the interaction of clayminerals with other minerals in mineral flotation will be reportedin a separate paper.

Fig. 4 shows the apparent viscosity of kaolinite and bentonitesuspensions as a function of their concentrations at a shear rateof 100 s�1. In line with the reports in literature, bentonite has astronger effect on viscosity than kaolinite. For instance, at a10 wt.% (4.3 vol.%) concentration, a bentonite suspension producesan apparent viscosity nearly seven times higher than a kaolinitesuspension. Compared to kaolinite suspensions, bentonite suspen-sions display a significant viscosity at low concentrations due tothe high swelling and flocculation of the fine clay particles produc-ing a viscous gel-like structure (Goh et al., 2010).

Fig. 5. Rheograms of 10 wt.% kaolinite suspensions in the absence and presence ofpH modifiers.

3.2. Kaolinite and bentonite suspensions in the presence of pHmodifiers

The natural pH of kaolinite suspensions was about 7.5. Toexamine the effect of pH modifiers on the rheology of kaolinite sus-pensions, NaOH, lime and Na2CO3 were used to adjust the pH to 10.Rheograms of 10, 20 and 30 wt.% kaolinite suspensions in the ab-sence and presence of pH modifiers are shown in Figs. 5–7,respectively.

Fig. 5 shows that at 10 wt.% kaolinite, pH modifiers have littleeffect on the rheological behaviour of the suspension with Newto-nian characteristics. In the absence and presence of NaOH, limeand Na2CO3, the lines of shear stress versus shear rate are closelylocated. In contrast, lime and Na2CO3 have a significant effect onthe rheological behaviour of kaolinte suspensions at 20 and30 wt.% kaolinite as shown in Figs 6 and 7. While lime increasesthe viscosity at the same shear rate compared to the baseline inthe absence of any pH modifier, Na2CO3 decreases the viscosity.The effect of lime and Na2CO3 on the rheological behaviour of

kaolinite suspensions is more pronounced at 30% kaolinite. AgainNaOH has little effect at both 20 and 30 wt.% kaolinite.

It is interesting to find that lime exacerbates the aggregation ofkaolinite particles corresponding to the increased viscosity of kao-linite suspensions at high kaolinite concentrations. This is in linewith Lagaly (2006) who observed that multivalent cations includ-ing Ca2+ were more strongly attracted to the clay particles thanmonovalent cations including Na+ with a more important effecton the coagulation of clay particles. Abdi and Wild (1993) and Wildet al. (1993) also reported that lime attacked the clay plates at theiredges enveloping the surface of the plates and modifying the dou-ble layer at the surface of the clay plates resulting in the formationof gels. This study also indicates that Na2CO3 may disperse kaolin-ite suspensions at a high kaolinite concentration resulting in re-duced viscosity. In fact, Na2CO3 has been used as a dispersant indifferent industries. It has been reported that multivalent anionssuch as CO2�

3 may add a negative charge to clay particles but nocharge is added by monovalent anions. Only part of the negativecharge associated with a multivalent anion is used by attractionto the exposed cation, while the increase in dispersion potentialof a clay–water system through the adsorption of negativelycharged ions on the clay particle is roughly proportional to the un-used valency of the anions (Rolfe et al., 1960).

Similarly, the pH of bentonite suspensions was adjusted to 10.0by NaOH, lime and Na2CO3 and rheology measurements were per-formed. Fig. 8 shows rheograms of 2 wt.% bentonite suspensions inthe absence and presence of pH modifiers. In all cases, bentonitesuspensions exhibit similar Newtonian fluids, and therefore NaOH,lime and Na2CO3 do not affect the rheological behaviour of 2 wt.%bentonite suspensions. Fig. 9 shows rheograms of 5 wt.% bentonitesuspensions in the absence and presence of pH modifiers. In theabsence of pH modifiers, the 5 wt.% bentonite suspension behaves

Fig. 6. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence ofpH modifiers.

Fig. 7. Rheograms of 30 wt.% kaolinite suspensions in the presence of pH modifiers.

Fig. 8. Rheograms of 2 wt.% bentonite suspensions in the absence and presence ofpH modifiers.

Fig. 9. Rheograms of 5 wt.% bentonite suspensions in the absence and presence ofpH modifiers.

34 N. Cruz et al. / Minerals Engineering 50–51 (2013) 30–37

like a Newtonian fluid. However, in the presence of NaOH, lime andNa2CO3, the suspension becomes non-Newtonian and pseudoplas-tic. Fig. 10 shows rheograms of 10 wt.% bentonite suspensions inthe absence and presence of pH modifiers. In the absence of pHmodifiers, the 10 wt.% bentonite suspension behaves like a non-Newtonian fluid. The addition of pH modifiers increases shearstress at the same shear rate corresponding to higher viscosity.

Unlike in kaolinite suspensions, Na2CO3 acts as an aggregatorinstead of a dispersant in 5 and 10 wt.% bentonite suspensions,producing higher viscosity. NaOH also affects the rheology behav-iour of bentonite suspensions, but does not in the case of kaolinitesuspensions. The effect of lime on bentonite suspensions is not thestrongest either. These phenomena may be associated with theactivation of bentonite in the presence of different pH modifiers.Evangeline et al. (2009) and Raji and Sheela (2009) investigatedthe activation of bentonite treated by a number of sodium com-pounds. They found that Na2CO3 strongly activated bentoniteincreasing its swelling capacity and plasticity, followed by NaOH.The activation of bentonite by sodium compounds has been attrib-uted by an ion exchange reaction where the ions in the bentoniteare replaced by alkali ions. Due to the strong affinity of monovalentcations to water, the activated bentonite is of high swelling capac-ity (Yildiz and Calimli, 2002). In contrast, lime may de-activate so-dium bentonite and then reduce or eliminate its swelling potential.This is due to the substitution of the clay cations by calcium andsubsequent formation of calcium silicate and aluminate hydrates(Abdi and Wild, 1993). However, lime increases the viscosity ofbentonite suspensions as well and therefore the aggregation ofbentonite particles by lime as explained in kaolinite cases stillplays a role in determining the rheological behaviour of bentonitesuspensions.

In this study, tap water was used to minimise the effect of saltson the rheology of kaolinite and bentonite suspensions. However,the significant effect of pH modifiers releasing different ions sug-gests the importance of water quality in determining the rheologyof clay suspensions.

To further confirm the interaction of pH modifiers with kaolin-ite and bentonite suspensions, sedimentation tests were con-ducted. Fig. 11 shows the settling of 20 wt.% kaolinitesuspensions at natural pH (7.8) and pH 10 adjusted by lime and so-dium carbonate as pH modifiers. The settling rate of the suspensionwith sodium carbonate was the fastest in the first 6 h of the test,and the suspension with lime showed the slowest settling rate.The kaolinite suspension at natural pH showed a settling rate be-tween those two suspensions at pH 10.

The settling behaviour of kaolinite suspensions corresponds toflocculated or aggregated particles (Nasser and James, 2006). In aprevious study Cryo-SEM images from sedimentation tests of a4 wt.% kaolinite at pH 8 showed that at the earliest possible stage

Fig. 10. Rheograms of 10 wt.% bentonite suspensions in the absence and presenceof pH.

Fig. 11. Sediment thickness of 20 wt.% kaolinite suspensions at natural pH, and pH10 adjusted by lime and sodium carbonate as pH modifiers as a function of settlingtime.

Fig. 12. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence ofPAX.

Fig. 13. Rheograms of 20 wt.% kaolinite suspensions in the absence and presence ofInterfroth 6500.

Fig. 14. Rheograms of 8 wt.% bentonite suspensions in the absence and presence ofPAX.

N. Cruz et al. / Minerals Engineering 50–51 (2013) 30–37 35

of dispersion EE and EF inter-particle associations already existed,but those association were open, loose and easily disrupted. Thosestructures do not follow the electrostatic DLVO theory since thekaolinite basal plane and edge sites are negatively charged at pH8, but van der Waal forces in the near potential minimum can over-come the charge barrier and induce coagulation (Zbik et al., 2008).It is suggested that in the settling of kaolinite suspensions initiallythere is a predominance of EE structures and as particles settlesliding across each other, particle orientation changes from EE toFF and despite the formation of this structures, settling withoutflocculants is very slow (Zbik et al., 2008).

In this study the sedimentation results for the first 6 h havesome correlation with the rheology measurements for the samesuspensions, however, this relationship is not as expected. Rheol-ogy tests suggest that by adjusting the pH to 10 with lime, aggre-gation will occur and increase the apparent viscosity. On the otherhand, sodium carbonate acts as a dispersant in the kaolinite sus-pension. Probably the first thought about this is that settling veloc-ity is faster in the case of the suspension with aggregated particles.However, aggregates of kaolinite particles in these tests can havemany voids or spaces occupied by water, and settling of this struc-ture is much slower if compared to the case where flocculants areadded (Zbik et al., 2008). Further, Ca2+ cations have a greater con-tribution to the formation of aggregates, as discussed previously,resulting in many modes of interactions between faces and edgesand producing structures similar to house of cards aggregates withsufficient voids inside. This structure gives a higher apparent vis-cosity value and at the same time its settling may be inhibited. So-dium carbonate disperses kaolinite suspensions by reducing thenet-work structures corresponding to the faster settling.

Similar sedimentation tests were conducted on 5 wt.% benton-ite suspensions. However, no sediment formation occurred evenafter 48 h probably due to the swelling property of bentonite par-ticles. This is in agreement with Akther et al. (2008) studying sed-imentation characteristics of two commercial bentonite samples inaqueous solutions at 3% w/v concentration at natural pH and pH 12by a light transmittance technique.

3.3. Kaolinite and bentonite suspensions in the presence of thecollector and frother

In addition to pH modifiers, collectors and frothers are alsoessential in mineral flotation. In this study, PAX and Interfroth6500 were selected to represent collectors and frothers normallyused in copper–gold flotation and their effects on the rheologicalbehaviour of kaolinite and bentonite suspensions were investi-gated at natural pH without the addition of any pH modifier.

Fig. 12 shows rheograms of 20 wt.% kaolinite suspensions in theabsence and presence of PAX. The addition of 50 g/t PAX to the kao-linite suspension possessed the pseudoplastic characteristics andslightly shifted the shear stress versus shear rate curve upwardwith higher shear stress and viscosity. Additions of 100 and150 g/t PAX affected the rheology of the suspension similarly andfurther increased the shear stress and viscosity. A similar trendwas observed when Interfroth 6500 was added to 20 wt.% kaolinitesuspensions. As shown in Fig. 13, the addition of 30 g/t Interfroth6500 slightly increased the rheological properties of the suspen-sion, while additions of 60 and 100 g/t Interfroth 6500 further in-creased these properties.

The effect of PAX and Interfroth 6500 on the rheological behav-iour of 8 wt.% bentonite suspensions was also investigated. The re-sults are shown in Figs. 14 and 15. PAX and Interfroth 6500 have a

Fig. 15. Rheograms of 8 wt.% bentonite suspensions in the absence and presence ofInterfroth 6500.

36 N. Cruz et al. / Minerals Engineering 50–51 (2013) 30–37

similar effect increasing the rheological properties slightly regard-less of the dosage.

In general, PAX and Interfroth 6500 did not modify the rheologyof kaolinite and bentonite suspensions as much as the pH modifier.Schott (1968) found that surfactants such as polyethylene glycol,alkyl ethoxylated surfactants and nonylphenol ethoxylated surfac-tants reduced the viscosity of clay mineral suspensions by breakingup the flocculated clay particles into thinner flakes, while at higherconcentrations, the viscosity increased due to the appearance ofmicelles acting as crosslinks between the particles. Yalçin et al.(2002) investigated the influence of the addition of anionic surfac-tants, sodium dodecyl sulphate and ammonium lauryl sulphate onthe flow properties of bentonite–water systems. They found thatboth surfactants increased the viscosity of bentonite suspensionsdue to interactions between the tails of the surfactants and theedges of the clay particles, which resulted in the formation of moreresistant structures against flowing. This study reveals that bothPAX and Interfroth 6500 tend to cause the aggregation of kaoliniteand bentonite particles, which has very important implications incopper–gold flotation. Firstly, the increased viscosity of kaoliniteand bentonite suspensions after the addition of PAX and Interfroth6500 may affect the flotation performance. Secondly, clay mineralsmay consume PAX and Interfroth 6500 so that enough collectorand frother amounts for the flotation process may not be available.Detailed studies are underway to investigate the adsorption, con-formation and micellisation of these surfactants on clay minerals.

4. Conclusions

Kaolinite and bentonite suspensions follow Newtonian fluids atlow solid concentrations (10 wt.% kaolinite and 2 and 5 wt.% ben-tonite in this study), and non-Newtonian fluids with pseudoplasticcharacteristics at high solid concentrations (20 and 30 wt.% kaolin-ite and 10 wt.% bentonite in this study). Bentonite has a strongereffect on the viscosity of suspensions than kaolinite.

The pH modifier has little effect on the rheological properties ofkaolinite and bentonite suspensions at low solid concentrations(10 wt.% kaolinite and 2 wt.% bentonite in this study) but can sig-nificantly alter the rheological properties of suspensions at high so-lid concentrations (20 and 30 wt.% kaolinite and 5 and 10 wt.%bentonite in this study). While lime exacerbates the aggregationgreatly in kaolinite suspensions, Na2CO3 disperses kaolinite sus-pensions. In contrast, Na2CO3 acts as an aggregator instead of a dis-persant in bentonite suspensions enhancing the aggregation evenmore than lime. NaOH also induces aggregation in bentonite sus-pensions, but has little effect in kaolinite suspensions.

Collector PAX and frother Interfroth 6500 have a similar effecton increasing the rheological properties of kaolinite and bentotinesuspensions at high solid concentrations (20 wt.% kaolinite and

8 wt.% bentonite in this study). However, this effect is less pro-nounced compared to that by the pH modifiers.

Acknowledgments

The authors gratefully acknowledge the discussion of this studywith technical experts from Newmont Mining Corporation andNewcrest Mining Limited supporting the succeeding research pro-ject together with the Australian Research Council. The first authoralso thanks the scholarship provided by the University ofQueensland.

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