Deflocculation of Kaolin Suspensions – The Effect of ......Kaolin Sedlec Ia produced by Sedlecký...

© Appl. Rheol. 25 (2015) 24151 | DOI: 10.3933/ApplRheol-25-24151 | 1 |

tween particles are strong, interacting particles do notaggregate, and the prepared suspension is stable andlow viscous. A lot of works deal with the measurementof zeta-potential of mineral suspensions in dependenceon the content of additives and the actual value of pH[3 – 9]. The information obtained from zeta-potentialmeasurement of kaolin suspensions is not easy to inter-pret because of the complex structural character ofkaolinite (silica and alumina faces have different surfacepotentials [10]). The dominant particle interactionsinfluence the internal structure and consequently alsothe rheological behavior of kaolin suspensions [3, 11]. Theviscosity measurements, therefore, can provide funda-mental information on the quality of suspensions.Changes in flow behavior of mineral suspensions causedby additives are usually described in the form of thedependence of yield stress on the content of additivesand the actual value of pH [3, 4, 8, 9]. The yield stress isfound to be inversely proportional to square of zeta-potential and directly proportional to a power of the vol-ume fraction of solid particles in the suspension [3]. Only few works deal with detailed viscosity mea-surements of aqueous kaolin suspensions, see [12] forthe primary suspensions and [13 – 20] for the suspen-sions with additives. Unfortunately, these works are

1 INTRODUCTION

Reducing viscosity by addition of a small amount of suit-able electrolyte is widely used for aqueous kaolin sus-pensions in ceramic, paper and dye industries to facili-tate transport, agitation, and other mechanical opera-tions. This process is called deflocculation, due to theassumed mechanism of the particle interactions in a liq-uefied suspension. Mineral kaolinite is a dominant com-pound of the kaolin clays. Kaolinite particles with theirtwo-face layer structure (the octahedral alumina layercovered by hydroxyl groups and the tetrahedral silicalayer with excreted oxygen [1, 2]) are susceptible to cre-ating of hydrogen bonds and thus exhibit strong elec-trochemical interactions in aqueous media. This is thereason why a small addition of an electrolyte influencesthe finally achieved viscosity and stability of aqueouskaolin suspensions. Adsorption of chemical agents ondifferent parts of kaolin structure (faces or edges) isstrongly affected by pH value of the suspension. The intensity of electrochemical interactions ismanifested via zeta-potential. It gives information onthe difference of potentials between the particles cov-ered by electrical double layer and the surroundingmedium. If zeta-potential is high, repulsion forces be -

Deflocculation of Kaolin Suspensions –The Effect of Various Electrolytes

V. Penkavová1*, M. Guerreiro2, J. Tihon1, J. A.C. Teixeira2

1Department of Multiphase Reactors, Institute of Chemical Process Fundamentals AS CR,Rozvojová 135, 165 02 Prague, Czech Republic

2Centre of Biological Engineering, University of Minho, Campus Gualtar, 4710-057, Braga, Portugal

* Corresponding author: [email protected]

Received: 16.5.2014, Final version: 2.10.2014

Abstract:Viscosity reduction of aqueous kaolin suspensions by conventional additives (deflocculation) is studied, using standard vis-cosity measurements. Apparent viscosity at 100 s-1, and flow behavior index n give complex information about changes of vis-cosity and flow character of deflocculated suspensions. Several widely used deflocculants – electrolytes and polyelectrolytes –are tested in a wide range of concentrations. The optimum concentrations of these deflocculants, which result in minimumapparent viscosity of suspension, are found. Sedimentation stability of deflocculated suspensions is monitored. Inorganic elec-trolytes are found to be more effective in viscosity reduction. On the other hand, low-molecular-weight polyelectrolytes pro-duce more stable final suspensions.

Key words:kaolin suspensions, deflocculants, viscosity measurements, stability of suspensions

| DOI: 10.3933/APPLRHEOL-25-24151 | WWW.APPLIEDRHEOLOGY.ORG

focusing on a limited number of additives and their con-centrations. The observed flow curves exhibit mostlyshear-thinning character, but in some cases shear thick-ening [13, 18] or even anti-thixotropy [19] has beenobserved. Practically all used additives decrease viscos-ity of aqueous kaolin suspensions. Only the additiveswith the presence of Ca2+ ions play an opposite role,increasing suspension viscosity [18]. Settling of kaolinsuspensions is widely studied as a separation processconnected with flocculated kaolin suspensions [9,21 – 26]. A typical observed parameter is the settling timein dependence on type and concentration of the floccu-lant agents. Also deposit and supernatant quality areinspected, e.g. via turbidity measurement [26], and acharacteristic density profile in settling suspension isexpected [21]. Nasser and James [23] studied sedimen-tation of both flocculated and deflocculated kaolin sus-pensions. The character of sedimentation process dif-fers strongly for these two situations. In flocculated sus-pensions particles create aggregates due to dominantVan der Waals interactions and these aggregates settlefast with sharp interface between structured depositand clear supernatant. Deflocculated suspensions con-tain the particles, which are influenced mainly by elec-trostatic repulsions, and therefore they settle slowly.The deposit grows from bottom and the border betweenthe deposit and turbid supernatant is not clear. The aim of this work is to study the effect of addedelectrolytes on the changes in viscosity of aqueouskaolin suspensions. The results of viscosity measure-ments are presented for several widely used defloccu-lants in wide range of concentrations (from 0.1 to 3 %).Also the stability of resulting deflocculated suspensionsis qualitatively classified and the final evaluation ofdeflocculation process is given.

2 EXPERIMENTAL

2.1 MATERIALS

Kaolin “Sedlec Ia” produced by SedleckyKaolin Inc. wasused for preparing aqueous kaolin suspensions. Thiskaolin contains more than 90 % of mineral kaolinite(Al2O3.2SiO2.2H2O) with wide particle size distribution(typically 60 % of particles are smaller than 2 μm, neg-ligible amount of particles is larger than 60 μm). TheBET surface area is 18.57 m2/g. The list of the electrolytesused as deflocculation additives is presented in Table 1.NaOH and Na2CO3 are well-defined chemical com-pounds. The sodium hexametaphosphate (SHMP) is amixture of polymeric metaphosphates with the prevail-ing hexamer (NaPO3)6. The water glass (WG) is 36 %aqueous solution of a sodium silicate with ratio

SiO2:Na2O about 3 : 1. Three samples of NaCMC with dif-ferent molar masses, which differ also by degree of sub-stitution, were used as polyelectrolytes. The primary suspensions were prepared by blend-ing kaolin with distilled water. Resulted suspensionswere let at rest for 3 days and then gently agitated untilreaching homogeneous state. Deflocculated suspen-sions were prepared in a similar way, by blending kaolinwith aqueous electrolyte solutions. The concentrationof additives, cadd, is expressed in weight percentage rel-


Table 1: List of the additives used as deflocculants (SHMP –sodium hexametaphosphate, WG – water glass, NaCMC –sodium salt of carboxymethylcellulose, *DS – Degree of sub-stitution=number of carboxymethyl groups per anhydroglu-cose unit).

Figure 1: Sedimentation test of 10, 20, 30, and 40% aqueouskaolin suspensions. a) Final state after 70 hours. b) Sedimen-tation course (h = total height, d = height of the deposit).

a)

b)

atively to the mass of kaolin. A simple sedimentationtest was used to determine the suitable concentrationsof the tested aqueous kaolin suspensions. The suspen-sions containing 10, 20, 30 and 40 %wt. of kaolin werelet at rest for 70 hours and their settling was monitoredby a high resolution camera. Typical result of the mon-itoring is shown in Figure 1a (photograph of the finalstate) and Figure 1b (course of the sedimentation). Thissettling test proved that the primary (not deflocculat-ed) suspensions containing 30 % or more of kaolin aresufficiently stable against sedimentation. The subse-quent viscometry testing was made with the 30, 35, and40 % primary aqueous kaolin suspensions, modified byadditions of deflocculants. Prior the viscosity measure-ments, the suspensions had been kept at rest and shak-en from time to time. After becoming homogeneous,they were used for the viscosity measurements.

2.2 VISCOMETRY

Brookfield viscometers LVDV-II+Pro Extra and HBDV-IIIUltra were used for viscosity measurements. The cham-ber SC4-13R/RP (Rc = 9.53 mm, depth 64.77 mm) wasused in combination with the spindle SC4-18(R= 8.74 mm, L= 31.72 mm, Leff = 35.53 mm) or SC4-31(R= 5.88 mm, L= 25.15 mm, Leff = 30.68 mm). The vanespindle V72 (R= 10.84 mm, L= 43.38 mm) in a largebeaker (Rc ≈ 35 mm) was used in preliminary test. All vis-cosity measurements were done at 25 °C. The viscosityresults are expressed in the shear stress σ= M/(2πR2Leff)and apparent shear rate D = 2Ω/(1 – κ2) or the corre-sponding apparent viscosity η = σ/D as the primarydata, obtained from the commercial software Rheo-Calc32, Brookfield Engineering Labs. Here, M is thetorque, Rc the container radius, R the spindle radius, Lthe spindle length, Leff the effective spindle length, Ωthe angular rotation speed of spindle, and κ = R/Rc. It iswell known [27, 28, 29] that the flow curve D= D(σ) coin-cides with true viscosity function g· = g· (σ), where g·stands for the shear rate, only for Newtonian flowbehavior or narrow enough gaps, and provides accept-able estimates only for liquids with the flow index n >0.3 assuming R/Rc > 0.8. First, the time necessary for reaching steady visco-metric flow after a step change of rotation speed wasdetermined. This test was done for the 35 7% primarysuspension with the spindle SC4-18. An example of the

shear stress response on a staircase change of apparentshear rate is given in Figure 2. In the test with 60 s delayson each step, the duration 30 s was sufficient for reach-ing the steady state. Therefore, the staircase coursewith the step period of 30 s was used in all the subse-quent viscosity measurements. Repeating the staircaseexperiment twice without changing the sample thenverified that an eventual sedimentation did not affectthe viscosity measurements. The apparent wall slip (AWS) [30] could influenceviscosity measurements of colloidal suspensions andleads to their misinterpretation. This effect can beexpected especially for smooth cylindrical spindleswith narrow gaps (h = Rc – R), i.e. the correspondingapparent viscosity at a given shear stress would behigher than real one. For wide gaps AWS effect is weak-er and for vane spindles completely avoided [31]. There-


Figure 2: Shear stress response σ to staircase course of appar-ent shear rate D measured for 35 % aqueous kaolin suspension.

Figure 3: Steady state flow curves measured for 35 % aqueouskaolin suspension with using three different spindles.

Figure 4: Example of steady state flow curves measured for40% aqueous kaolin suspensions without (black) and withSHMP additive (0.2% grey, 0.5% white symbols). Two inde-pendent samples were measured (series a and b). Data arefitted and extrapolated using power-law model (Equation 1).

fore, two cylindrical spindles (SC4-18, SC4-31) with dif-ferent gap thickness and the vane spindle V72 in abeaker were used to inspect whether the AWS effectplay a role in our viscosity measurements. The resultsfor the 35 % primary suspension are shown in Figure 3.As the almost identical flow curves was obtained for allthe used sensors, the AWS effect is found not to be play-ing an important role in our viscosity measurementswithin the given range, accuracy and sensitivity of pri-mary viscometric data. Steady state flow curves for alltested samples were then measured using the spindleSC4-18. This configuration with the ratio R/Rc = 0.92matches satisfactorily the narrow gap approximation.The reproducibility of viscosity measurements isdemonstrated for 40 % kaolin suspensions with addi-tion SHMP. The viscosity data for two independentsamples (marked as series a and b) given in Figure 4 arealmost identical, confirming good reproducibility. Theflow curves can be fitted very well over the measuredrange of D by the power-law model

(1)

where K stands for the consistency coefficient, and n isthe flow index. As seen in Figure 4, an addition of elec-trolyte strongly affects not only the consistency of sam-ples, but also the character of shear thinning behaviorfrom almost ideal plastic (n= 0.15) to practically New-tonian (n= 0.97). Note that the analogous data can bedrawn for all studied samples, for various kaolin con-centrations and different electrolyte additives. A partof results gives flow indexes less than 0.3. The flowcurves corresponding to them differ from the true vis-cosity functions. In these cases, the apparent shearrates calculated from angular rotation speeds of thespindle are determined with quite large errors, over-whelming 10 % (see the study of Estelle at al. [29] forthe analysis of using Newtonian approximation forpower law liquids). In spite of this inaccuracy the maingoal of this study (i.e. finding of the optimum additiveconcentration for the maximum reduction of suspen-sion viscosity) could be satisfactorily realized.

3 RESULTS AND DISCUSSION

Not only low viscosity but also good homogeneity andstability are needed to classify the suspensions as rea-sonable from the technological viewpoint. In this sec-tion, first the viscosity and stability results are discussedseparately, and then the final evaluation for all useddeflocculants is given.

3.1 APPARENT VISCOSITY

The fluid mechanics of inelastic non-Newtonian liquids,so called Generalized Newtonian Liquids [32, 33], isbased on the notion of viscosity material function. Evenif the viscosity function is represented by the simplepower-law model, the related two-parameter informa-tion can be too complex for a pragmatic use by chemicaland production engineers. For this reason, the resultsof physicochemical studies on deflocculation [14 – 16,18, 20] are commonly presented in terms of the appar-ent viscosity, which is measured at some a priori chosenreference apparent shear rate D. The problem, in com-paring the viscosity results from various sources, lies indifferences of the chosen reference D (e. g. D= 1 s-1 in[14], 58 s-1 in [15], 65 s-1 in [18], 120 s-1 in [16], 50 and 199 s- 1

in [20]). To clarify this point, we present in Figure 5 vis-cosity data from two series of our measurements (40 %suspensions with addition of SHMP and NaOH) forthree different D. For both additives there is a sharpdecrease of apparent viscosity at some characteristicconcentration of additive, cadd, independent of the cho-sen reference D. On the other hand, the level of viscositydecrease given by the ratio between the viscosity ofdeflocculated suspension and that of primary suspen-sion is strongly shear rate dependent. Much higher vis-cosity reduction is observed at low than at high shearrates (e.g. data from Figure 5a provide viscosity reduc-tion 5000 times at D= 1 s-1 and just 100 times atD= 100 s-1). There is also a limitation in arbitrariness forchoosing the reference D: when comparing viscosityfunctions with very different flow behavior indexes andchoosing too high extrapolated D, it would be possibleto operate close to intersections of the flow curves,where the apparent viscosity is the same at all concen-


Figure 5: Variation of apparent viscosity η at various D with concentration of additive cadd for 40 % aqueous kaolin suspensionswith addition of a) SHMP and b) NaOH.

a) b)

trations cadd (see Figure 4). In this study, the viscositydata are specified by the apparent viscosity η atD= 100 s-1, which is marked as η100. For keeping the com-plete experimental information, the information onflow behavior index n accompany the η100 data. As it isobvious from Equation 1, this information enables tocalculate, if needed, the value of consistency coefficientand to obtain the value of apparent viscosity at any oth-er reference D.

Inorganic additivesThe viscosity changes with addition of SHMP and NaOH

were firstly tested for suspensions with three concen-trations of kaolin, i.e. 30, 35, and 40 %wt. As depicted inFigure 6, both these deflocculants exhibit a sharp vis-cosity decrease (Figure 6a and c) connected with thechange in flow behavior from shear thinning to New-tonian one (Figure 6b and d). For all the kaolin concen-trations these changes occur at the same concentrationsof additives, cadd. The resulting viscosities η100 exhibitsimilar values for all the kaolin concentrations and theboth tested deflocculants. For SHMP addition (Figure 6a)the apparent viscosity does not change significantlyafter reaching the minimum. For NaOH (Figure 6c) there


Figure 6: Variation of the apparent viscosity η100 and flow index n with concentration of additive cadd for 30, 35 and 40 % aque-ous kaolin suspensions with addition of a), b) SHMP and c), d) NaOH.

Figure 7: Variation of a) apparent viscosity η100 and b) flow index n with concentration of additives cadd for 40 % aqueous kaolinsuspensions with addition of inorganic electrolytes.

a) b)

c) d)

a) b)

is only narrow range of cadd with reduced apparent vis-cosity, followed by significant viscosity increase andrecovery of shear thinning behavior (Figure 6d) whenNaOH is overdosed. At cadd ≈ 3 % the apparent viscosityrises back to the values of primary suspensions. Due toagreement of these viscosity trends for all the kaolinconcentrations, the subsequent viscosity measure-ments for the other electrolytes were done only with the40 % primary suspension. The results for all the studiedinorganic additives are given in Figure 7a (apparent vis-cosity) and Figure 7b (flow behavior index). All the elec-trolytes reduce the apparent viscosity of suspensions tothe same level of several mPas. This reduction of viscos-ity is connected with reaching of Newtonian behavior.The optimum cadd (necessary for reaching the minimumapparent viscosity) are however different: 0.1 % forNaOH, 0.2 % for Na2CO3, and mixture SHMP/Na2CO3,0.3 % for WG, and 0.4 % for SHMP.

Organic additivesThe viscosity changes of 40 % aqueous kaolin suspen-sion with NaCMC were studied for three different molarmasses of the polymer. The results are again represent-ed by the apparent viscosities η100 (Figure 8a) and theflow indexes n (Figure 8b). The low molecular weightNaCMC-90 works quite well as deflocculant: the mini-mum apparent viscosity is achieved at cadd ≈ 0.8 %. Thedecreasing apparent viscosity is again accompanied byincreasing of flow index but, in this case, the flowbehavior remains slightly shear thinning (n≈ 0.8). Themedium-molecular-weight NaCMC-250 shows similartrends, but the decrease of viscosity and the increase offlow index are much smaller. The viscosity changes dueto addition the high-molecular-weight NaCMC-700 arenot significant at concentrations below 0.4 %. At higherconcentrations, the viscosity increase (not measured)indicates that this high-molecular-weight polymer actsrather as thickener than a deflocculant.

Common trendsThe viscosity data from Figures 7a and 8a are plotted inFigure 9 versus the concentrations of additives recalcu-lated to moles of Na+ per gram of kaolin. This plot showsthat the strongest deflocculating effect corresponds toapproximately same concentrations of Na+ ions, thuspossible dissociation centers, for all the tested defloc-culants (minima around 0.03 mmol/g). Deflocculationof aqueous kaolin suspensions is the process stronglydependent on pH. The measured pH of suspensions aregiven in Figure 10. Values of pH vary with addition ofelectrolytes from neutral (SHMP), over weak alkali (pri-mary kaolin suspensions, NaCMC additives) and medi-um alkali (WG, Na2CO3), to strong alkali (NaOH). Withrespect to the isoelectric point of kaolin suspensionslocated at pH ≈ 3 [5, 6, 8], all tested suspensions shouldexhibit high negative zeta-potential values.

3.2 STABILITY OF SUSPENSIONS

This study was not primarily focused on settling pro -cesses, only qualitative observations of sedimentationwere made. It consisted in (i) visual observations of themovement of the boundary between the settling phaseand the clear supernatant, and (ii) probing the harddeposit after a long-time staying. These observationsresulted in a qualitative classification shown in Table 2.It is distinguishing between five grades of the sedimen-tation stability, from long-time stable suspensions(grade 5) to quickly setting suspensions (grade 1). Gen-erally speaking, the deflocculants should reduce viscos-ity and, at the same time, stabilize the suspensionagainst sedimentation because of preventing floccula-tion. However, the real sedimentation process is com-plicated by many aspects of the dispersed phase, e.g. bypolydispersity of the colloidal systems under consider-ation. As a result, the sedimentation stability getsworse with addition of deflocculants, but there arelarge differences among various deflocculants.


Figure 8: Variation of a) apparent viscosity η100 and b) flow index n with concentration of NaCMC additives cadd for 40 % aque-ous kaolin suspensions.

a) b)

The worst stability is exhibited by WG suspensionswithin the optimum range of cadd, for which the rigiddeposit quickly grows at the bottom. Stability of NaOHsuspensions within the optimum range of cadd is medi-um, but outside of this range is very good – same as sta-bility of primary suspension. All inorganic additives pro-duce rather unstable suspensions. On the other side,the sedimentation stability of suspensions deflocculat-ed by NaCMC polyelectrolytes is good. These suspen-sions remain more stable probably because they pre-serve certain shear thinning behavior and thereforetheir apparent viscosity is higher at low shear rateswhich are typical for the process of sedimentation.

3.3 EVALUATION OF DEFLOCCULANTS

The optimum concentration of electrolyte necessary forreaching the maximum reduction of suspension viscos-ity is smallest in the case of NaOH. Little bit higher opti-mum concentration is obtained for Na2CO3, and themixture SHMP/Na2CO3, followed by WG, then by SHMP,and finally by medium- and low-molecular-weightNaCMC. However, as shown above, one must be carefulabout using the larger than optimum cadd, especially inthe case of NaOH. As the using of WG leads to highlyunstable suspensions, Na2CO3 and SHMP (eventuallytheir mixture) seem to be the best choices. The appli-cation of Na2CO3 as deflocculant can be recommendedas the most environment-friendly. In principle all theused inorganic electrolytes are able to decrease viscos-ity of aqueous kaolin suspension to the similar level(less than 10 mPas) and to change shear thinningbehavior to the Newtonian one. In the case of NaCMC,the lower molar mass results in lower viscosities ofdeflocculated suspension. Anyway, even the low-mol-ecular-weight NaCMC-90 does not reduce viscosity

with such efficiency as the inorganic deflocculants andthe character of flow behavior still remains shear thin-ning. In contrary to inorganic electrolytes, which pro-duce rather unstable suspensions, the using of NaCMCpolyelectrolytes leads to quite stable suspensions. It isa question of a given technological process, whether itwould be better to use Na2CO3 (as the best inorganicdeflocculant) or low-molecular-weight NaCMC (provid-ing more viscous suspensions but with better stabilityagainst sedimentation). A difference should be noted between the presentresults and those, obtained by Rossington et al. [14] intheir experimental study with similar electrolytes. Theefficiency of viscosity decreasing, when Na2CO3 and WGis used, is found different. Reduction of apparent vis-cosity at 1 s-1 of 40 % batch suspension (defined mixtureof kaoline, quartz, alumina, etc.) was about three ordersof magnitude in the case of WG but only one order whenNa2CO3 was used. This small efficiency of Na2CO3 is sur-prising because this is a deflocculant recommended by


Figure 10: Variation of pH with concentration of additive caddfor 40% aqueous kaolin suspensions and all tested defloccu-lants.

Figure 9: Variation of apparent viscosity η100 with concentra-tion of Na+ ions for 40% aqueous kaolin suspensions and alltested deflocculants.

Table 2: The subjective classification of sedimentation stabili-ty of 40 % aqueous kaolin suspensions with additives: Thehighest grade 5 corresponds to stable suspensions and thelowest grade 1 to unstable, quickly settling suspensions. Thegray fields mark the range of optimum concentrations whichled to the lowest viscosities of suspensions.

our kaolin producer and works very well in our case.Probably the behavior of batch suspension can differfrom pure kaolin suspension used in our work. Also dif-ferent compositions and qualities of used kaolin min-erals can play role (e.g. the specific surface area 26.9and 18.6 m2/g is stated for Florida (USA) and Sedlec(Czech Republic) deposits, respectively). Also Vlasak atal. [20] have found slightly higher efficiency of WG thanNa2CO3 (with apparent viscosities at 199 s-1 two timessmaller). As they used kaolin from the same deposit andproducer (kaolin Sedlec Ia), a small discrepancy in theviscosity results suggests that also different batches ofthe same mineral can behave differently.

4 CONCLUSIONS

The efficiency of deflocculation by addition of variouselectrolytes to aqueous kaolin suspensions was testedvia standard viscosity measurements. Variance of twoparameters, apparent viscosity η100 and flow behaviorindex n, with an additive concentration cadd was usedto give complex information about changes of viscosityand flow character of deflocculated suspensions. Theindependence of deflocculation effect on kaolin con-centration was verified. For all three tested kaolin con-centrations (30, 35, and 40 %) the range of additive con-centrations, which led to lowest viscosities, was thesame at the case of two tested additives (SHMP andNaOH). Decreasing of suspension viscosity using opti-mum deflocculant concentration is always connectedwith change of flow character from shear thinningtoward to Newtonian. Addition of any inorganic elec -tro lyte led to suspensions having seriously Newtoniancharacter and the same level of viscosity less than 10mPas, only different amount of additive was need.Using organic polyelectrolyte led to more consistentsuspensions and both apparent viscosity and flowbehavior index depended on molar mass of usedNaCMC. An excess of deflocculant led to recovery ofshear thinning behavior and increase of apparent vis-cosity in the case of inorganic electrolytes. This isextremely significant when NaOH is used. Qualitativeobservations of the stability of prepared suspensionswere also made. The suspensions containing inorganicelectrolytes was found not very stable, in the contraryorganic polyelectrolytes produced more stable suspen-sions. In summary, inorganic electrolytes produce reallylow viscous, but not very stable suspensions, whereasorganic polyelectrolytes produce more viscous, butrather stable suspensions. It depends on concrete tech-nological process what should be preferred.

ACKNOWLEDGEMENTS

The support by Czech Science Foundation GACR throughthe contract P101/12/0585 is gratefully acknowledged.

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[33] Morrison FA: Understanding Rheology, Oxford UniversityPress, New York (2001).


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