the pulp rheology of some australian gold ores

8/12/2019 The Pulp Rheology of Some Australian Gold Ores

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THE PULP RHEOLOGY OF SOME AUSTRALIAN GOLD ORES

BSTR CT

Janine L. Beazley and James H. Kyle

Western Australian School of MinesKalgoorlie Western Australia.

The effects of pH, pH modifier (lime or

caustic soda) and cyanide on the pulprheology of a number of ore types rangingfrom highly siliceous to highly kaolinitichas been investigated. All experimentswere performed in fresh water at solidsconcentrations of 25 and 45 solids and inthe pH range 6 to 11. The kaolinitic oresexhibited greater changes in rheology withpH and calcium ion concentration than didthe siliceous ores.

INTRODUCTION

In recent years there has beenincreasing interest in the subject of pulprheology. Viscous pulps can cause mixingproblems in the Carbon-in-Pulp process forthe extraction of gold which, in turn, maylead to low recoveries. This is becausegood mixing is essential for the effectiveleaching and loading of gold ontoactivated carbon. Viscous pulps canseriously hinder this process. Inaddition, low recoveries caused bypumping, grinding and screeningdifficulties have been attributed to pulpviscosity (Snow et al., 1987; Osan et al.,1988).

In general terms the viscosity of aslurry depends on the following propertiesof the ore:

(i) Mineralogy

(ii) Particle size(iii) Solids concentration in the pulpiv) Chemical environment (e g pH,

ionic strength)

This paper discusses the rheology ofseveral different gold ore pulps andcompares the effects that the use ofcaustic soda, lime and cyanide have uponthe pulp viscosity as a function of bothpH and solids concentration.

Rheology

The flow behaviour of a fluid is

governed by its viscosity. Fluid flowproperties may be classified as Newtonianor Non-Newtonian. For a Newtonian fluidthe shear stress (force/unit area) isdirectly proportional to the rate of shear(velocity gradient), the proportionalityconstant being the viscosity:

where s = shear stress (Pa)u = viscosity (Pals)

dv/dy = rate of shear (s-

However, for most mineral slurries therelationship between shear stress andshear rate is more complex. Fluids of thistype are classified as Non-Newtonian.

A variety of equations and models havebeen employed to describe flow behaviourof non-Newtonian fluids. These include theMooney equation (Dinsdale Moore, 1962)and the power law (Wilkinson, 1960).Although these models are useful forillustrating flow behaviour, they are notusually used in describing plantperformance. The simpler term 'lapparentviscosityM (ua) defined by equation (2)has found wider use (Osan et a1 1988)

Non-Newtonian fluids can be dividedinto two groups: time-dependent and tirne-independent.

There are three main types of time-independent fluids - pseudoplastic,dilatant and Bingham plastic. The generalrelationship between shear stress andshear rate for these fluids is shown inFigure 1.


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PULP RHEOLOGY OF SOME USTR LI N GOLD ORE S

EXPERIMENTAL

Pretreatment of Ores

Shear ate

FIGURE 1 Types of Time-independentFluid Behaviour

Many Non-Newtonian fluids obey a PowerLaw (equation 3) over certain ranges ofshear rate:

with the apparent viscosity ua (asdefined in equatio n 2) being gi ven by:

For such a fluid, a plot of log sversus log (dv/dy) will give a straightline of slope n. For pseudoplastic fluidsnl.

For a pseudoplastic, the apparentviscosity decreases with increasing shearrate whereas for a dilatant fluid theopposite is true. Bingham plastics,however, require the application of acertain minimum shear stress before anyflow will occur. This minimum shear stressis called the yield stress (Y).

For time-dependent non-Newtonianfluids, two types of behaviour are evident

thixotropic and rheopectic. In theformer, the apparent viscosity decreaseswith increasing time of shear at aconstant shear rate. This type ofbehaviour may be due to a breakdown in thestructure of the fluid with time or withincreasing rate of shear. In rheopecticbehaviour, a gradual formation ofstruct ure in the fluid is establis hed withtime. This leads to an increase in shearstress at a constant shear rate.

In this paper we will present theresults of a rheological study of fourdifferent ore types. All ore slurriesexhibited non-Newtonian behaviour and somerequired a yield stre ss for flow to occur.These latter ores are described as yieldp s e u d ~ p l a s t i c ~ ~ Osa n et al., 198 8)

The ores examined were from CentralNorseman Gold Mines (siliceous), KidstonGold Mine (siliceous/muscovitic) andPad din gto n and Hannan's Sou th Gold Min es(kaolinitic). The chemical and c rystalcomposi tion of these ores as determined byX-ray Fluore scence and Diffra ction areshown in Tables 1 and 2.

For the testwork, a bulk sample of eachore was crushed to


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84 WORLD GOLD 89

Bohlin Visco 88BV Viscometer20 1 f

All data were obtained using the BohlinVisco 88BV Viscometer. The availablemeasuring systems are summarised in Table3. In this study, only the narrow and wide

gap configurations were used employing astationary cup and a rotating bob. Thepulp was suspended using a vibromixerprior to measurements being taken.

Table 3

MEASURING SYSTEMS FOR BOHLIN VISCO 88BV

Spind le CUP Height Measur ingDiameter Diameter of bob S ys tem

W m )

14 15 4 21 0 Narr ow gap25 27 5 37 5 Narrow gap3 33 0 45 0 N ar r ow gap

14 33 0 21 0 Wide gap25 33 0 37 5 Wide gap

The torque on the bob was recorded atvarious speeds (20 to 100 rpm) and theshear stress and shear rates calculated asfollows:

Shear stress =

where T = Torque (Nm)

b =radius of the bob (m)

= height of the bob (m)

Shear rate = 2 R ~ ~ wRC2-Rb2)

where Rc = radius of the cup (m)w = angular speed (s-l)

The apparent viscosity was thencalculated from equation (2). Thixotropicbehaviour, if it occurred, was establishedby observing the change in torque withtime at constant bob speed.

RESULTS AND DISCUSSION

Rheology of Norseman Ore

The rheograms for Norseman ore pulps at

solids concentrations of 25 and 45 areshown in Figure 2. The pulp exhibitsdilatant behaviour in that the apparentviscosity increases with increasing rateof shear. However, the apparentviscosities for this pulp arecomparatively low, particularly at lowshear rates where apparent viscosities aresimilar to that of water (1 centipoise,CPS)

SHEAR RATE 1 se t

FIGURE 2 Rheograms of Norseman Ore Pulpsat 25 and 45 solids.

The viscosity of the pulp at 45 solids

was significantly higher than at 25 , butonly at high shear rates with thedifference increasing with increasingshear rate.

The effects of pH, lime, caustic sodaand cyanide addition were all minimal.This behaviour is presumably due to thelow clay mineral content of the ore,although the precise surface chemistry ofthis behaviour is unknown and is atpresent the subject of investigation(Meagher et al., 1988).

Kidston Ore

The rheograms for Kidston ore pulps at25 and 45 solids are shown in Figures 3aand 3b. At 25 solids this pulp also showsdilatant behaviour, but at 45 solids therheology is more complex. At low shearrates it behaves as a yield pseudoplasticbut at higher rates of shear the rheogramshows the characteristics of a dilatantfluid.

SHEAR RATE 1 s e t

FIGURE 3a Rheograms for Kidston Ore Pulpsat 25 Solids.


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PULP RHEOLOGY OF SOME USTR LI N GOLD ORE S 85

SHEAR RATE l/sec)

FIGURE 3b Rheograms for Kidston Ore Pulpsat 45 Solids.

This change in behaviour isdemonstrated by a Power Law plot, log svs. log dv/dy (Figur e 4) which sh ows adistinctive change i slope at a shea rr at e of ab ou t 300 s Below t hi s s he arrate n l whichcorresponds to dilatant behaviour.

SHEAR RATE 1 SEC 1

FIGURE 4 Power Law Plot for Kidston OrePulp at 45 Solids and NaturalOre pH (6.35).

The appare nt viscosity of this ore,unlike Norseman, did increase withincrea sing pH. This effect was morenoticeable at the hiaher solidsconcen tratio n of 45 : There was als o ameasurable difference between th e effectof caustic soda and lime on the pulpviscosity. Both these effects were mostsignificant at high pH and high percentsol ds.

At 45 solids and pH 11, there was alsoa measurable yield stress which was notevident at lower pH or at 25 solids.

The differences between this ore andNorseman may be related to the presence ofa signi ficant amount of muscovite in theore.

Paddington Ore

At 25 solids (Figu re 5a), therheograms for this ore, like bothsiliceous ores above, showed dilatantbehaviour but with a more noticeable pHdependence. At 45 solids however, therheology chan ges (Figur e 5b).

SHEAR RATE 1 se t

FIGURE 5a Rheograms for Paddington OrePulps at 25 Solids.

0 oo an in u

SHEAR RATE 1 set

FIGURE 5b Rheograms for Paddington OrePulps at 45 Solids.

At pH 9 and below, the behaviour issimilar to Kidston ore - at low shearrates behaving like a yield pseudoplasticbut changing to dilatant behaviour athigher shear rates. The changeover shearra e is, however, mucp lower - about 75s instead of 300 s- . The use of causticsoda produced slightly lower viscositiesthan lime at th is pH.


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WORLD GOLD 89

At pH 11, the rheology is similar butwith greatly increased yield stress andapparent viscosity. In addition, there isnow a marked difference in apparentviscosity between the pulps employing limeor caustic soda as the pH modifier, with

lime now producing the lower viscositypulp, and a small but noticeable decreasecaused by the addition of cyanide to the

pulp

The rheology of this ore pulp isgreatly influenced by the kaolinitepresent in the ore.

Hannanrs South

The Hannan's South ore contained morekaolinite than the Paddington ore andhence was expected to show similar pulprheology but with higher yield and shearstresses. However, this was not the case,particularly at higher concentrations ofsolids and higher pH.

At 25 solids (Figure 6a) the rheogramsindicate yield pseudoplastic behaviour atlow shear rates changing o dilatant at ashear rate of about 75 s-I. The apparentviscosities and shape of the rheogram arevery similar to those for Paddington oreat 45 solids (pH 9 and below). Theapparent viscosities increase slightlywith pH but are independent of themodifier used.

SHEAR RATE1 se t

FIGURE6a Rheograms for Hannan's South

Ore Pulps at 25 Solids.

The rheograms at 45 solids and pH 9(Figure 6b) illustrate yield pseudoplastic

behaviour. There is no evidence ofdilatant behaviour at the highest measuredshear rates. The yield stresses and theapparent viscosities are higher than thoseobtained for Paddington ore (45 solids).

SHEAR RATE1 /se t

FIGURE 6b Rheograms for Hannanfs SouthOre Pulps at 45 Solids.

Initial investigations of the rheogramsfor Hannan's South ore pulps at pH 11indicate that the apparent viscosity isextremely high in comparison to the otherores studied in this work. In addition theapparent viscosity changed with timeindicating thixotropic behaviour. Nofurther data on this system is availableas yet.

Surface hemistry and Pulp Rheology

Until recently (Meagher et al., 1988),there has been little attempt to explainmineral pulp rheology in terms of surfacechemistry. However, some generalobservations can be made.

The mineral particles will nearly allcarry a negative surface charge at thepH's studied. The role of dissolvedcounterions in the pulp will be to balancethis surface charge in the diffuse double-layer surrounding the mineral particles.If these counterions become specificallyadsorbed to a mineral surface, then theymay negate the surface charge to such anextent that coagulation may occur in thepulp, resulting in an increase in theapparent pulp viscosity.

The ion most likely to specificallyadsorb on a ineral surface in the resentsystem is CaT+ or at pH ~ 1 0 , a(0H)'. Thelatter ion is known to specifically adsorbonto quartz surfaces at pH 11 and above(Fuerstenau et al., 1985). It is therefore

possible that specific adsorption of thision onto clay minerals (kaolinite,muscovite) will occur to a significantextent at pH 9 and above in mineralsystems. This would result in somecoagulation of these minerals in the pulpresulting in a change in pulp rheology andincreased apparent viscosities. This was


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PULP RHEOLOGY OF SOME USTR LI N GOLD ORES

observed for all ores containing either REFERENCBBmuscovite or kaolinite, with the onenotab le excep tion of Paddin gton ore, but Dinsdale, A. and Moore, F., 1962,only at 45% solids. llV isc osi ty nd it's Mea sur eme nt, ** hap man

and Hall, London.

The rheograms presented for the fourgold ores studied show extremely variablepulp rheology the only real similaritybeing that they all exhibited Non-Newtonian behaviour.

For the siliceo us ore (Norseman), thepulps generally exhibited dilatantbehaviour with minimal effect of pH, lime,caustic or cyanide addition on apparentviscosity.

The presence of muscovite in theKidston ore pulps caused an unusualrheogram in which a cha nge frompseudoplastic to dilatant behaviour

occurred at a cer tain shear rate. Thisbehaviour was also observed for thekaolin itic ores (Paddi ngton and Hannan'sSouth) under certain conditions. Suchbehaviour may be an indication of thehigher shear rates causing flocculation inthe pulps, although why this might occuris unknown at this stage of theinvestigation.

The presence of kaolinite in the oresobviously has a major effect on theapparent pulp viscosity. The highest yieldstresses and shear stresses were found inthe Hannan's South ore pulps, which alsocontained the most kaolinite. ThePaddington ore which also containedkaolin ite but in lesser amoun ts, producedsimila r effec ts to the Hannan's South orebut showed smaller yield stresses andshear stresses.

The use of caustic soda instead of limeas a pH modifier generally lowered theapparent viscosity with one notableexception Paddington ore pulps at 45%solids in which case the opposite wastrue.

The ore pulp with the highestconcentration of kaolinite, Hannan'sSouth, exhibited thixotropic behaviour atpH 11 at solids concentrations of both 25%and 45%. This phenomenon has not yet beenfully investigated.

CKNOWLEDQEMENTB

Fuers tenau , M.C., Mille r, J.D., and Kuhn,

M.C., 1985, che mis try of Flota tion, SME-AIME, New York.

Meagh er, L. et al., 19 88, The eff ect ofdissolved ions on the rheology ofconcentrated quar tz suspension sIM thInt ern ati ona l Con gre ss on Ftheology,Ausralian Society of Ftheology,Sydney,Australia.

Osa n, M.A., Hor sle y, R.A. and Bar ham ,S . 1988, The FtheologicalCharacteristics of Several WesternAustralian Gold Ore Slurries and theEffectiveness of Viscosity Modi fier sIwRandol Perth International GoldConference, Perth, Western Australia,Randol Int ernati onal Ltd., Golden,Colorado, U.S.A.

Snow, R.J., et al., 19 87, Inc rea sin gSlurry Concentrations in the Carbon-in-Pulp Pro cess I1* 2 th Slur ry TechnologyAssociation Conference, New Orleans,U.S.A.

Wilkin son, W.L., 1960, Non-NewtonianFluids,'* Per gam on Pres s, London.

The research described in this paperwas funde d by ICI (Aust.) Ltd. in th e formof a Postgraduate Research Scholarship toMrs. Jani ne Beazley.

the pulp rheology of some australian gold ores

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