dispersion caolin interactions

min = minimum 1 = run No. 2 = run No.

literature Cited

(1) Buzyna, G., M.S. thesis, Institute of Gas Technology, Chicago,

(2) Buzyna, G.. Macriss, R. A , , Ellington, R. T., 48th National

(3) Davis, J . .4., Ph.D. thesis. University of Kansas, Lawrence,

(4) DeVaney. I V . E., Dalton, B. J, Meeks, J . C., J . Chem. Eng.

(5) Fedofitenko. A . , Ruhemann. iM.. Tech. Phys. U.S.S.R. 4,

Ill., 1962.

Meeting, A.I.Ch.E., Denver, Colo., August 1962.

Kan.. 1963.

Data 8, 473 (1963).

36-43 ( 1 917) . - \ - . - , (6) Gonikberg, M. G., Fastowsky, I V . C., Acta Physicochim. C.R.S.S.

(7) Hill, A. E.. J . A m . Chem. SOC. 45. 1143 11923). 12, 67-72 (1940).

(8) Itterbeek, A. V., Verbeke, O., Cijogenics’S, 79-80 (1961).

(9). Johnson. V. J., Stewart, R. B., “Compendium of the Proper- ties of Materials at Low Temperature (Phase 11);’ National Bureau of Standards, Boulder. December 1961.

(10) Kharakhorin, F. F., Znthener. Fit. Zhur. TPkh. Fit. 10, 1533-40 (1940).

(11) K;amer. G. M.. Miller, J . G., J . Phys. C h m 61, 785-8 (193,) .

(12) Mullhaupt, J . T., Di Paolo. F. S.. ‘.Solubility of Helium in Liquid Xenon,” ACS \.\’inter Symposium. IVashington Uni- versity. St. Louis. Mo., Dec. 27, 1960.

(13) Rodewald, h-. C., Ph.D. thesis, University of Kansas, Law- rence. Kan., 1963.

(1 4) Strobridge. T. R.. “Thermodynamic Properties of Nitrogen from 64 to 300 O K . between 0.1 and 200 Atmospheres.” National Bureau of Standards, Boulder, Tech. Note 129 (1962).

(1 5) \\‘eissberger, “Technique of Organic Chemistry.’‘ 2nd ed.. Part 1, p. 320, Interscience, New York. 1949.

RECEIVED for review June 27. 1963 ACCEPTED October 17, 1963

PARTICLE INTERACTIONS IN AQUEOUS

KAOLINITE DISPERSIONS

A L A N S . M I C H A E L S A N D J U S T I N C . B O L G E R 1

Department of Chemical Engineering, ’Massachusetts Institute of Trchnology, Cambridge. .Mass.

The pseudoplastic characteristics of acidic aqueous kaolinite dispersions are attributed to the formation of “card-house” flocs by coulombic interaction between cationic edges and anionic basal surfaces. As pH i s elevated with NaOH, edge charges are neutralized with consequent increase in floc density, and reduction in floc size, suspension yield value, and viscosity. Polyphosphates deflocculate kaolinite under acidic conditions by adsorption on the cationic edge surfaces. Dilute deflocculated slurries exhibit Newtonian viscosities which closely obey Einstein’s equation corrected for particle anisometry and electroviscous effects. Acid decomposition of kaolinite appears to liberate colloidal alumina and silica which adsorb on the basal and edge surfaces, weaken particle interactions, reduce the pH at which deflocculation occurs, and increase the polyphosphate required for deflocculation.

THE rheological and sedimentation characteristics of suspensions of particulate solids in liquid media depend upon the

particle size, shape, size distribution and concentration, and particle-particle interactions within the suspending fluid. In the case of suspensions of colloidal or near-colloidal particles, evidence is strong that particle-surface interactions control suspension properties. Inasmuch as surface interactions are to a major degree governed by the composition of the suspending medium, it follows that the structure and properties of suspensoid systems are similarly governed by the same var- iables.

Aqueous kaolinite slurries exhibit rheologic and sedimentation behavior which is profoundly dependent upon the pH, ionic strength, and ionic composition of the aqueous phase. .4t lo\v pH’s (below ca. 6.0). and in the absence of extraneous electrolytes. kaolinite slurries are rather highly flocculated, exhibit marked pseudoplastic behavior, and possess a finite yield value even at low solids concentrations. .4s the p H (in the absence of added salts) is raised to values above ca. 7.0 with alkali metal hydroxides. deflocculation of the particles ensues abruptly (as evidenced bl- microscopic examination and sedimentation behavior). suspension viscosity drops dramati-

I Prescnr addres<,. Amicon Corp.. Cambrldge, Ma%

cally, and pseudoplasticity and yield value virtually disappear. These changes in properties with p H have, to a degree, been successfully correlated with changes in electrophoretic mobility (zeta potential) of the kaolinite particles (9, 70). Since this 1 to 1 aluminosilicate is electrophoretically negative even in acid media, and since the zeta potential becomes in- creasingly more negative kvith increasing p H (via addition of monovalent bases as NaOH). it might be expected that the classical Verivey-Overbeek (76) theory of double-layer interactions should apply, and the properties of kaolinite suspensions be adequately explained in these terms.

Certain anomalous properties of kaolinite suspensions. however, suggest that straightforkvard double-layer interaction theory is not completely applicable to these systems. For example, addition of ’.indifferent” electrolytes such as sodium chloride to an acidic kaolinite suspension causes a decrease in suspension viscosity kvhich is not consistent with the corresponding change in electrophoretic mobility. Also, addition of alkaline earth hydroxides-e.g.. calcium hydroxide-to an acidic slurry causes only a slight increase in zeta potential but a marked viscosity reduction. Lastly. addition of trace quantities of certain polyanionic electrolytes-e.g., alkali polyphosphates, alkali lignosulfonates. etc.-causes marked deflocculation and reduction in suspension viscosity at pH levels

14 l & E C F U N D A M E N T A L S

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

where the clay is normally well flocculated (3, 75); indeed, deflocculation can be accomplished under strongly acidic (pH,) conditions with traces of polyphosphoric acid (6).

These anomalies have, in large part. been resolved by the observation that kaolinite adsorbs or exchanges anions in acidic media, that this anion-exchange capacity vanishes a t p H above 6.5 where marked rheo1o;;ic changes are observed to take place (a)! and that kaolinite platelets adsorb negatively charged gold particles on their edges ( 7 7 ) . From these adsorption data, it was deduced ( 7 J ) that xnder acid conditions, the basal surfaces of clay platelets (kaolinites as well as montmorillonites) are negatively charged tvhile the platelet edges are positively charged. T h e t\vo double layers on edges and faces have a different character. the face double layer being of the constant charge type, since the charge is due to lattice substitutions of aluminum for silicon: \chile the edge double layer is of the constant potential type. being created by the adsorption of potential-determining ions---i.e., by protonation of hydroxy ions coordinated Lvith aluminum a t the fracture surfaces. The amphoteric nature of aluminum thus accounts for the development of positive (anion-binding) sites under acidic conditions, and for neutralization and charge reversal of these vites as the p H is elevated.

T h e presence of edge and basal-surface charges of opposite sign on kaolinite particles in acidic suspensions permits the formation of particle floscs \vhich differ in an important way from those expected in classical colloidal dispersions. Electro- static edge-to-face attractions lead to the buildup of "card- house" type flocs \vhich are very voluminous and entrap a large quantity of interstitial fluid. From sedimentation and rheologic measurements on kaolinite slurries, the authors (4. ?)have found these card-house flocs to contain only 10% by volume of solid. Slurries in this state of aggregation exhibit pronounced yield values and highly psmeudoplastic behavior under shear, even at low (ca. 2 7 , by volume) solids concentration. The rather abrupt increase in (negative) zeta potential. and concurrent rapid decrease in viscosity of kaolinite slurries on elevation of pH. are satisfactorily accounted for in terms of the neutralization and charge reversal of the positive edge sites, with consequent collapse of the particles.

By the same token. the observation (7.2) that "indifferent" electrolytes such as sodiuin chloride cause deflocculation of the clay can be explained by the ability of these solutes to compress the electrical double layer adjacent to both positively and negatively charged surface rtgions, thereby reducing electrostatic edge-face attractions. ItVhen present at sufficiently high concentrations. such electrolytes can, of course. sufficiently compress the double layers that van der IVaals attractions will predominate. and flocculation \vi11 again ensue. Under these conditions. hoLvever. the most favorable interaction is benveen platelet faces. and dense, card-pack flocs will result; the observed kinetics of sedimrntalion of "salt-flocced" suspensions (-I) and thrir streaming-birefringent behavior (8) confirm this postulate. \'an Olphen (72> 73) has suggested similar mecha- nisms to explain the anomalous pseudoplastic behavior of bentonite gels.

M'hen unsymmetrical electrolytes-e.g,, the alkali lignosulfonates and polyphosphaies or polyphosphoric acid-are added to card-house flocced kaolinite slurries, preferential adsorption or '.charge shielding" by the polyanions a t the positive edges of the flakes results in either eliminaiion o charge or sign reversal at these surfaces, Jvith consequent card- house collapse. T h e potent dispersive action of these com- pounds on kaolinite slurries is adequately accounted for in these terms. Michaels (3) . for example, has shown that a correla-

tion exists among polyphosphate adsorption. positive edge- charge density. and deflocculation of kaolinite suspensions

Recently. the authors developed a structural model for flocculated kaolinite suspensions which has provided a means for gaining insight into the fine structure of such suspensions from macroscopic measurements of their sedimentation and viscous behavior (I. 5). It seemed desirable to use this model and experimental method to re-examine the foregoing hypothesis of the effects of aqueous environment on kaolin suspension structure. and to establish \+hether the chemical and physical interpretations of kaolinite particle interactions are mutually consistent.

Experimental Procedure

Hydrite PD-10. an acid-bleached. well crystallized kaolinite of narroxy particle size distribution, manufactured by the Georgia Kaolin Co.. \yas used throughout. This clay had a mean (equivalent spherical) particle size of 0.55 micron Lvith 90% by weight in the particle size range 0 . 2 to 2.0 microns ( 7 ) . Its cation exchange capacity (as determined by the standard ammonium acetate method) \vas approximately 3 . 0 meq. per 100 grams. Electron photomicrographs ( 7 ) indicate that the axial ratio of the nearly hexagonal kaolin platelets is about 10 to 1 .

Slurries were prepared by mixing weighed quantities of the dry clay. water, and reagents in a IVaring Blendor for 5 minutes. Exact kaolin volume concentrations could be calculated from the specific gravity of these suspensions, measured with a 25- ml. pycnometer. p H was measured lvith a Becknian glasb electrode p H meter using glass and calomel electrodes which were inserted into the suspension. All pH data Lvere obtained a t the same kaolin concentration. Q~ = 0 . 0 3 2 , to minimize the suspension effect. Apparent viscosities and yield values \vere measured using standard RVT and R\-F Brookfield drive heads, coupled to a concentric rotor and stator unit having a uniform gap ividth of 0.15 cm. and a wetted rotor area of 3 7 2 sq. cm. The R\:F head \vas used over the shear stress range from 0 to 61 dynes per sq. cm. and the RVT head over the range from 0 to 6.1 dynes per sq. cm. T h e speed range of the drive units (0 to 20 r . p ,m, ) corresponded to a shear rate range from 0 to 42 set.-' Yield stresses were computed from the final reading to \vhich the torque indicator would descend after the rotor had been run at the lowest speed and then turned off. Typically. these yield stresses were reproducible lvithin 5 to 10%.

A salt \vash procedure was used to purify the acid-bleached kaolin. Six hundred grams of clay were blended into 2 or 3 liters of distilled Water and then diluted to 11-liter total volume in a 5-gallon ja r , T o this lvere added 580 grams of NaCl and sufficient HCI to lower the p H to 3.0. T h e ja r was agitated periodically for 24 hours. T h e clay was then washed lvith distilled water until the CI concentration was less than 0.001 .Y. Dilute HCI and NaCl solutions \vere used to adjust the slurries to the desired final p H and to a C1 concentration of 0.001.V.

HCI NaOH Mcq / 100 gmo Kaolin

Figure 1. Titration curves

$K = 0.032

V O L . 3 NO. 1 F E B R U A R Y 1 9 6 4 15

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

Results and Discussion

Titration Curves. Figure 1 sho\vs the variation of slurr! p H Lvith added N a O H or HCI, both for an acid-bleached cia)- suspension and for a suspension of "salt-washed" clay--i.e.. clay that had been extracted with acidified 1 .V NaCl solution and exhaimtively washed Lvith distilled water (residual chloride

Acid bleaching of kaolinite is conducted under such highly acidic conditions that virtually all foreign exchangeable cations on the clay are replaced with hydrogen. However, the acid environment may also cause breakdown of the clay \vith the liberation of aluminum ion and (probably) formation of colloidal hydrous silica. The aluminum so released from the lattice can become affixed to the cation exchange sites, or eventually deposited as colloidal alumina on the particle surfaces during the final Ivater-Lvashing step. In other Lvords, an acid-bleached clay is, for all practical purposes, an aluminum- hydrogen kaolinite. The salt-xvashing procedure is believed to remove the bulk of any exchangeable aluminum or colloidal alumina. yielding a predominantly sodium hydrogen kaolinite.

The data of Figure 1 support the hypothesis thaL the acid- bleached clay is contaminated Xvith colloidal (or exchangeable) alumina, and;or colloidal silica. Both curves exhibit a \veil- defined inflection point at a pH of ca. 7.5. which presumably represents the point of virtually complete titration of the positively charged edge sites. The acid-bleached clay, ho\sever, requires roughly 2.0 meq. per 100 grams more alkali to achieve this state than does the salt-washed clay. and exhibits an addi- rional inflection at p H ca. 5.5-roughly corresponding to the isoelectric point of hydrous alumina. Inasmuch as the alumina extracted from the clay in the salt-kvashing process \vas found. by separate titration, to total about 0.5 mmole of AI per 100 grams, it is deduced that roughl!- one equivalent of hydroxyl ion is consumed by reaction Ivith 0.5 mole each of nonlattice alumina and silica present in the clay. A second inflection in the titration curve for the salt-washed clay is observed in the vicinity of p H 5; this may be due to the onset of titration-i.e., protonation-of the basal-plane hydroxyls on the alumina faces of the clay crystals.

Effect of pH on Viscosity. Figure 2 shows the shear stress-shear rate curves obtained for a 3.2 volume Yc acid- bleached kaolinite slcrry as the p H is varied by addition of N a O H . In the absence of added caustic. the slurry has an appreciable yield value, a high viscosity, and a very rapid decrease in viscosity with increasing shear rate. As the p H is raised, there is a rapid decreasr in yield value..viscosity, and pseudoplasticity until, a t pH's above about 7,0, the suspensions become SeLvtonian and virtually insensitive to

The rheological data of Figure 2 can be quantitatively analyzed by techniques previously developed by the authors (5) in terms relating to suspension structure. In essence. it is assumed that a flocculated suspension may be regarded as a dispersion of virtually indestructible "flocs" which attractively interact \vith one another to form floc clusters or "aggregates" whose size depends upon the shear rate. At high laminar- shear rates, the aggregate (but not floc) structure is virtually destroyed, and the shear energy is dissipated in viscous flo\v around flocs. and in inelastic collisions benveen flocs. Vnder these conditions. the shear stress is linearly related to the shear rate:

= 1 0 - 3 ~ j .

PH.

T = A ~ @ . F ? + p*G(I + 2.5 @ F ) ( 1 )

where qr. is the floc volume fraction, and rll is a constant char- acterizing the force of adhesion benveen flocs.

IO

20 40 G S h e a r Rate sec- I

Figure 2. Shear diagrams

Unfreated kaolin, 4.y = 0.032

For a rather large group of kaolinite slurries, it \vas found

( 2 )

whence. by extrapolation of the straight-line sections \vhich the curves in Figure 2 approach at high shear rates. values of pF can be estimated. I t has further been found (5) that the yield value of card-house flocculated kaolinite suspensions can be correlated by the relation :

(3 )

Equation 3 permits an independent estimate of Q,; from the measured yield values of the various suspensions.

The constants -1, and A3 in Equa t iox 2 and 3 are directly proportional to one another. and both contain the parameter ( A , ' d i 2 ) ; where A , = interfloc adhesive force and d,. = floc diameter. Since '41 and A 8 have been found to be virtually independent of suspension composition (provided card-house flocculation conditions obtain). it is deduced that treatments ivhich reduce interparticle bonding forces (and thereby interfloc bonding forces) reduce (d,*i proportionately. In other words. weak binding forces favor. as expected. the formation of small flocs.

Calculations of qF for the four suspensions of Figure 2. from Equations 2 and 3. are presented in Table I . The agreement bet\veen the t\vo values is generally good (within 10yc). In the case of the raiv slurry (pH 4.151, there is further good agreement \vith a value of oF. determined independently from sediment density measurements (4 ) . As p H increases. the floc- volume concentration decreases (at constant kaolinite concentration). reaching a value equal to the clay volume concentration a t pH's of 7 or higher. A more meaningful measure of

(5) that :

A I = 120 dynes per sq. cm.

T I . = & i j ( C $ F - @ F o ) 3 200($F - $ , ~ o ) ~

Table I. Floc-Volume Concentrations and Floc-Volume Coefficients for Acid-Bleached Kaolinite Slurries as Functions

of pH (61, = 0.032:

T I

4 15 0 232 - 0 0 240 2 3 0 238 0 236 - 4 5 35 4 0 0 183 1 4 0 212 0 19' 6 0 6 10 1 6 0 116 0 25 0 12- 0 121 3 8 7 to 10 0 0 0 0 1 0

16 I & E C F U N D A M E N T A L S

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

floc structurc is the "floc volume coefficient." C,.,, defined as the ratio of €he volume of flocs to the volume of solid in suspension. 'I'his quantity decreases from 7.4 to essentially unity as the pH i h raised.

.A large floc volume coefficient indicates an extremely open- structured floc containing a large amount of trapped fluid. 'I'he exiitenct. of such '>tructures in a kaolinite suspension is mo\t readily accounted Ibr in terms of a card-house array. An "ideal card-house." however-. wherein each platelet is assumed to have an axial ratio of 10 to 1 and to be in contact Isith six others. ivould have a volurnr coefficient of only about 4. Tha t acidic kaolinite slurriea exhibit coefficient5 of over 7 suggests that even more tenuous floc structures must be present. This observation can be explained by noting that, if the interparticle forces Ivhich produce flocs are large, random edge-face interactions benseen individual particles can )-ield mechanically >table clusters in ishich ):he mean particle coordination number is less than 6.

'l'he fact that elevating suspension pH increases floc density, Lvhile at the same time 'decreasing the floc interactions responsible for non-Sewtonian behavior, at first hand seems para- doxical. These trends are not inconsistent, ho\vever. A Lveakening of the interparticle forces responsible foi flocculation will render unstable to shear loiv-density flocs formed by random particle interactions: such flocs \vi11 either break down or deform under the applied loads: eventually reforming into denser structures xvhich. by virtue of their higher volume density of interparticle contacts, will be mechanically stable. The den5est flocs will. therefore. be formed \\.hen particle attractions are Lseakest At a pH just slightly below that at which deflocculation o8:curs ---i.e., 6.10- the floc volume coefficient attains a value of 3.8. Ishich is very nearly equal to that expected of a "close-packed card-house" floc of 10 to 1 axial ratio disks.

.An increase in mean apparent floc density with increasing p H might also be explained by gradual deflocculation of the clay. Lvith consequent reduction in the volume fraction of residual flocculated solid. This is evidently not the case Lvith these kaolinite suspensions. inasmuch ab slurries at pH's below 7.0 sediment relatively rapidly. yielding a clear supernatant. If partial defloccu1,atiori had taken place in these suspensions, the supernatant fluid \sould have been turbid.

At pH's of 7

or higher. the viscous behavior, sedimentation characteristics. and microscopic appearance of the suspensions confirm that the clay is virtually completely deflocculated. The Einstein constant. A-> defined by the relationship :

Properties of Deflocculated Suspensions.

is found to be approxiniately 10.0 for these suspensions. This is somexshat larger than the value of 8.5 predicted from Kuhn and Kuhn's relationship (2, for oblate spheroids \vith a 1 to 10 axial ratio. Since the kaolinite particles possess an appreciable zeta potential under these conditions, ca. - 100 niv. based on Street and Buchanan's ( 7 0 ) measurements? an electrokinetic contribution t o the viscosity can be expected. .4ccording to

Sinoluchowski (77) :

L4ssuming an electrolyte concentration of 10-3M: the expected increment to the Einstein constant from "electroviscosity" is roughly 107:> Lvhence it appears that the viscous behavior of dilute. deflocculated k,iolinite suspensions is predictable from the classical theory of colloidal dispersions.

I I 0 Solt Woshed [ C l j i OOOlM

Solt Woshed [G I1 0.0002M x Untreoted 1 Day Old

~ ! ' l " ' l " " " ' ' 6 8 IO pH o f S lu r r y

Figure 3. Relative viscosity vs. pH +.K = 0.032

Effect of Salt-Washing on Viscosity-pH Dependency. T h e variation of the viscosity of 3.2 volume yc suspensions of salt-washed kaolinite \cith pH is compared lvith that of a similar suspension of raw kaolinite in Figure 3. T h e ordinate of this plot is the relative viscosity of the respective slurries d t a common shear rate of 42 sec.-'-~that is, the ratio of the apparent viscosity of the slurry to the viscosity of the suspending medium (water). A few points are also plotted for a salt- \sashed clay slurry in \rhich the chloride ion concentration has been reduced from lo-" to 2 X 1@-4.\'. All three suprnsions show a marked viscosity reduction u i t h increaqing pH. until a limiting p H is reached above \\.hich all suspensions exhibit thr same viscosity. The limiting value for the i'a\v clay suspension is about 7.1, and that for the salr-\%ashed clay about 7.8. 'I'hrir values correspond almost perfectly \sith the points of inflection on the titration curves. The salt-\sashed clay slurry (10-3A4 SaC1) sho\vs a much higher viscosity a t a given pH than thr raw clay. and the lo\\ NaC1. salt-Lvashed slurry a still higher value.

1 he reduction in viscosit? ivith increasing pH for all three slurries can be attributed to gradual titration and neutralization of the positive edge sites, Lvith consequent ivrakening and densification of the floc structure and attendant weakrning of the interfloc attractions \vhich arc basically responsible for high apparent viscosity. Hoxvever, the marked differences in viscosity betxveen raw and salt-Lvashed clay (at a yiven pH)% the viscosity variation Lsith chloride ion concentration. and the difference in critical deflocculation pH for raw and salt-\vauhed clay require special conyideration.

Presumably. salt-Ivashed kaolinite is "clean" kaolinite in the sense that impurities such as colloidal alumina and Yilica have been removed from the mineral surface. and that the exchange sites are occupied by Sa'. H+. and C:l- (at positivei). charged edges). Since the counter, ions in this sy.tem are all monovalent. and present in lo\\, concentrations. local elcctrokiiietic potentials \vi11 be high and electrostatic edge-to-face attractions \vi11 be strong. This \t ill result in the formation of durable. open-structured flocs which \\.ill interact \trongly \vith onr another and yield high viscosity suspensions. 'l.hP prirnar) effect of raising the pH \vi11 be to titratc edge site.; via thr reactions :

f 0 )

(7)

r _

--XlO€I?'- + 0 1 3 - + - - . ~ I O I I . H ~ ~ ~

- A I O H . H ~ O + O H - - + --hl(OH)2- + 1i:O

V O L . 3 NO. 1 F E B R U A R Y 1 9 6 4 17

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

/

M I S C I S per Liter Supernatant

Figure 4. Shear stress vs. chloride concentration

The p H a t \vh,ich deflocculation ensues (ca. 7.8) thus represents the point a t which the positive charge density on the particle edges drops to some critical (small) value. T o a first ap- proximation: it may be assumed that p H 7.8 is the isoelectric point of edge alumina.

The presence of a slight excess of electrolyte (in the form of sodium chloride) in this system will operate to compress the electrical double layer and shield the surface charge sites a t both positively and negatively charged areas. This \vi11 reduce the edge-face interactions, thereby reducing suspension viscosity. As the p H is increased in the presence of sodium chloride, the zero-point-of-charge of edge alumina will occur at about the same p H as in the absence of salt; but, in all likeli- hood, the critical p H a t which deflocculation occurs \vi11 be some\vhat depressed with salt present because of the charge- shielding activity of the chloride ion. A s soon. however, as the p H is raised to a level where positive edge charges are effec- tively neutralized, an “indifferent electrolyte” such as sodium chloride \\.ill merely compress the electrical double layer about the particles, thereby tending to promote flocculation rather than to discourage it. This is clearly illustrated in Figure 4 where the shear stress (at constant shear rate) of a 6.5 volume To raw kaolinite slurry a t pH 10 has been measured as a function of sodium chloride concentration. At this pH, in the absence of salt, the clay is completely defiocculated; and the suspensions are Newtonian and of low viscosity. 4 s the sodium chloride concentration is raised, both viscosity and non-New- tonian behavior rapidly increase, indicating reflocculation of the clay. Under these conditions. particle attractions are probably due to dispersion or dipole-interaction forces rather than to coulombic forces. and the most stable flocculated state \vi11 be one in lvhich the interparticle contact area is maximized. For platelike particles, this \vi11 mean formation of relatively dense, card-pack flocs. Evidence supporting such differences in the structure of kaolinite flocs with varia- tions in p H and electrolyte concentration is provided by sediment-volume measurements on slurry columns of varying height. The authors have sho\vn (I) that the limiting density of sedi- ments accumulating under gravity is directly related to the individual floc density. Comparative values of floc-volume coefficients for acidic. salt-free slurries, and alkaline, salt- containing slurries as determined by sedimentation, are shown in Table 11. The substantially smaller floc-volume coefficient observed ivith alkaline, salt-flocced slurries relative to acidic, salt-free slurries strongly suggests the presence of denser, card- pack flocs in the former.

Table II. Floc-Volume Concentration and Floc-Volume Coefficients for Various Kaolinite Slurries as Determined by

Sedimentation Measurements ( 4 ) ( @ h = 0 032)

CFh = Slurry 4F q F / @ h

Acid-bleached, pH 4 0 232 7 3

Acid-bleached, pH 9, 0 06,V NaCl 0 138 4 3

.4cid-bleached, pH 6 0 208 6 5 Salt-washed. pH 4.5 (10-3.V iYaC1) 0 273 8 5

The relatively low viscosity of acid-bleached slurries, as compared with salt-washed kaolinite systems a t the same pH. can be explained as follo\vs. At lo^. p H , colloidal alumina present in the dispersion is protonated to form complex aluminum cations Lvhich occupy. in part a t least. the basal negative charge sites, thereby reducing the charge density (or surface potential) at the basal surfaces. In addition, colloidal silica which is undoubtedly present on clay with alumina as a clay decomposition product. \vi11 exist (even a t low pH) partly as silicate anions; these ions \vi11 probably adsorb on the positively charged edges. thereby reducing the edge density. Thus, the combined action of cationic alumina and anionic silica \vi11 be to shield both basal plane and edge surface charges. and thus to reduce electrostatic interparticle attractions and suspension viscosity.

As the p H is raised, the following sequence of events is likely to take place:

NEUTRALIZATION A N D DISPLACEMENT O F ALUMISA HELD ELECTROSTATICALLY OS THE BASAL SURFACES.

Z-[A1(OH)s(H?O)l] + N a + O H - + Z-Na+ 4- A1(OH)3(HUO):, ( 8 )

This reaction \vi11 occur in the vicinity of the isoelectric point of hydrous alumina (ca. 5.5). The consequence of this reaction \vi11 be an increase in the effective basal-plane charge density and (if the clay edges are still cationic) an increase in edge-face attractions. Street (9) has reported an increase in kaolin suspension viscosity Lvith increasing pH (using S a O H as titrant) jvhich he attributes to the formation of “chain” aggregates. Absence of this effect \vith ”clean” kaolinite pro- vides support for the alternate hypothesis offered here.

SILICA PRESENT INTO SILICATE ASION, COSVERSION O F A LARGER FRACTION O F THE COLLOIDAL

Si(OH), + O H - + Si(0H)lO- + H?O (9)

This reaction \vi11 provide additional anions for adsorption on the positive edge sites, thereby weakening edge-face interactions.

EVENTUAL DISCHARGE (OR CHARGE R E V E R S A L ) OF EDGE

(10) (11)

ALUMINA BY DEPROTONATION (AT CA. PH 7.8).

-AIOHyT + OH--, -AIOH + H?O -AlOH + OH--+ - N O - f H?O

This reaction will. of course. eliminate cationic edge sites and thereby destroy edge-face attractions.

The experimental facts that raw clay slurries have lo\ver viscosities a t any given p H than salt-washed clay slurries, and that raw clay slurries deflocculate at a lower pH. suggest that “charge shielding” of basal and edge surfaces by alumina cations and silicate anions. respectively. is responsible for the observed differences. If it is postulated that deflocculation of the raw clay at p H 7.1 occurs because all residual positive edge sites are occupied by silicate anion, and that edge alumina is essentially neutralized a t p H 7.8, then the quantity of caustic required to elevate the slurry pH from 7.1 to 7.8 should be

18 l & E C F U N D A M E N T A L S

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

essentially ionically equivalent to the silicate adsorbed. From Figure 1, this is found to be approximately 0.4 meq. per 100 grams, in reasonable agreement with the estimate of 0.5 mole of Si per 100 grams determined from the titration curves directly. I t thus appears that the colloidal decomposition pro- ducts of kaolinite can function as partial clay deflocculants; a t pH’s below 6.0, cationic alumina is the active constituent, while a t higher pH’s anionic silica plays the primary role.

Figure 5 shows the variation of kaolin suspension viscosity (at a shear rate of 42 sec.-l) with concentration of added sodium hexametaphosphate, a t a constant p H of 5.2. il’hereas the raw kaolinite slurry require:; ca. 1.3 mmoles of phosphorus per 100 grams for complete d.eflocculation, the salt-washed kaolin requires only ca. 0.8 mmole per 100 grams. T h e viscosities of the fully deflocculated slurries are identical and equal to those of caustic-deflocculated slurries (Figure 2) , indicating that the chemistry of the deflocculation process does not, in any important way, influence the flow behavior of the primary particles (at least, in dilute slurries).

Deflocculation in thir system undoubtedly results from selec- tive adsorption of the polyphosphate anion on the cationic edges of the clay particle, as has been previously postulated (3, 75). T h e additional 0.5 mole of phosphorus per 100 grams required to deflocculate raw (relative to Talt-washed) kaolinite can most readily be explaked by assuming that polyphosphate combines Lvith colloidal alumina in preference to protonated edge alumina. Quantitative support for this postulate is found in the observation that, while partial deflocculation of salt- washed clay (as evidenced by a turbid supernatant) is observed a t polyphosphate concentrations as low as 0.2 mmole of phosphorus per 100 grams. similar partial deflocculation of raw clay is not observed until the polyphosphate concentration reaches about 0.4 mmole of phosphorus per 100 grams. In- deed, the formation of a 1 to 1 molar complex betLveen alumina and polyphosphate is indicated in this system. inasmuch as the raw clay contains approximately 0.5 mmole per 100 grams of ”extractable” alumina. This observation is in substantial agreement with earlier speculation on formation of alumino- phosphate complexes in kaolin dispersions (3) , based on meas-

Deflocculation by Sodium Hexametaphosphate.

I I 1 I I 1 0 01 0 2 0 4 0 8 16 3 2

M mols Phosphorus / IOOgms

Figure 5. Relative viscosity vs. polyphosphate concentration

@ K = 0.032

urements performed on a kaolinite of different origin and properties.

In many respects, the response of acidic kaolinite to an unsymmetrical electrolyte such as sodium hexametaphosphate resembles the response of an isoelectric polyampholvte (such as gelatin) to similar electrolytes. i i i t h polyampholvres, addition of unsymmetrical electrolytes reduces gelation tendency and increases swelling, and shifts the isoelectric point to lower pH’s if the microanion is of higher valence than the microcation (7). Reduced gelation and enhanced swelling are consequences of lessened polycation-anion interactions betLveen (or ivithin) the polyampholyte molecules, and are analogous to the deflocculation phenomena observed with kaolin ; both have their origins in preferential shielding of charges of one sign present on the dispersed ampholytic phase relative to the other. I t would appear that the activity of many other deflocculants for kaolin (sodium metasilicate, sodium lignosulfonate, sodium poly- acrylate, etc.) can be satisfactorily accounted for in these terms, without the necessity of implicating special chemical or chemi- sorptive reactions betiveen the components of the clay and the solute.

Conclusions

Changes in the structural parameters of flocculated kaolinite suspensions, deduced from p H and viscosity data. agree with previous predictions of the effects of electrolyte (72, 74) and polyanion (3, 75) additions. T h e high viscosity and pseudoplasticity of acidic kaolinite supensions are a consequence of strong electrostatic interactions bet\veen particles due to the coexistence of negatively charged basal surfaces and positively charged, protonated alumina edges. Elevation of p H by addition of caustic soda causes gradual LLeakening of interparticle attractions via neutralization of positive edge charges. This is accompanied by a gradual increase in packing density of individual clay particles within flocs. The zero-point-of- charge of the edge surface occurs at approximately p H 7.8, corresponding to complete deflocculation of the clay.

Acid hydrolysis of kaolinite appears to result in the liberation of colloidal alumina and silica, which are adsorbed electrostatically on the basal planes and edges, respectively, thereby reducing particle attractions and suspension viscosity. Kaolin so contaminated deflocculates a t a lower p H (7 .2 ) , but requires more caustic for deflocculation. Deflocculated kaolinite slurries are essentially Neivtonian, and exhibit an Einstein constant somewhat larger than that expected from hydro- dynamic considerations alone; a n electrokinetic contribution to viscosity is indicated. Viscosity of the deflocculated slurries is independent of the nature of the deflocculating agent used.

Sodium hexametaphosphate deflocculates kaolinite slurries by adsorption of the polyphosphate ion on the positively charged particle edges. Alumina-contaminated clay requires more polyphosphate for complete deflocculation than “clean clay” because of preferential reaction of the phosphate anion with the alumina. Shielding of positive edge charges by polyvalent anions (under acidic conditions) appears to be a general characteristic of common kaolin deflocculants.

Acknowledgment

T h e advice and criticism of H. H. Murray. t V . Bundy, and R . Conley of T h e Georgia Kaolin Co., H. van Olphen of the Shell Development Co.. and F. Fowkes of the Sprague Electric Co. are gratefully acknoxvledged.

V O L . 3 NO. 1 F E B R U A R Y 1 9 6 4 19

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

Nomenclature

A , A , AS dF = floc diameter, cm.

= floc-floc attractive force, dynes = Bingham yield value coefficient, dynes/sq. cm. = true yield value coefficient, dynes/sq. cm.

vol. flocs vel. clav in flocs

C F K = floc volume coefficient = --

G = shear rate. s e c . 3 K = Einstein coefficient AK = increment in Einstein coefficient due to electroviscous

effect R = particle radius, cm. Z - = anionic site on kaolin surface e = dielectric constant of suspension medium 7 = apparent viscosity: dyne sec. 'sq. cm. qg? = apparent viscosity at G = 42 set.-' q B = Bingham viscosity, 7 - q8 a t large G X = specific conductivity of suspension medium, ohms 'cm. p, = viscosity of water. dyne sec./'sq. cm. E = electrokinetic potential, mv. T = shear stress, dynes, sq. cm. T~ = true yield value. T +. T~ as G +. 0 pp = floc volume concentration oFO = minimum floc volume concentration exhibiting finite

Q~ = kaolin volume concentration yield value

literature Cited (1) Georgia Kaolin Co., Elizabeth, N. J . , Bull. TSBH-10 (1956). (2) Kuhn, it'., Kuhn, H., Helo. Chim. Acta 28, 97 (1945). (3) Michaels, A. S., Ind. En,c. Chem. 50, 951 (1958). (4) Michaels, A. S.. Bolger, J. C., IND. ENG. CHEM. FUNDAMENTALS

1, 24 (1962). (5) Ibid., p. 153. (6) Michaels, A. S., Tausch, F.. Ind. Enp. Chem. 52, 857 (1960). (7) Overbeek, J. Th. G., Bungenberg de Jong. H. G., in H. R .

Kruyt, "Colloid Science, Vol. 11," p. 219, Elsevier, Amsterdam, 1949.

(8) Schofield; R. K.. Samson, H. R.; Discussions Faraday Soc. 18, 135 (1954).

(9) Strect, N.. .lustraizan J . Chem. 9, 467 (1956). (10) Street. X., Buchanan. A. S.. Ibid.. p. 450. (11) Thiessen. P. A , , 2'. Eit-ktrochm 48, 675 (1942). (12) Van Olphrn, H.. Discussions Faradq Soc. 11, 82 (1951). (13) Van Olphen. H., Proc. Fourth Natl. Conf. Clays and Clay

Minerals. Tat]. Res. Council Publ. 456, 204-24 (1956). (14) Van Olphen H.; PYOC. T r a z ~ . Chim. Pays Bas 69, 1308 (1950). (15) Van \Vazer. , J .> Besmertuk, J.: J . Phqs. Colioid Chfm. 54, 89

(1950), (16) Vyrwey. E. J. \V. , Overbeek, J . Th. G. , "Theory of the

Stability of Lyophobic Colloids." Elsevier, Amsterdam. 1948. (17) Von Smoluchowski, M., Kolioid-Z. 18, 194 (1916).

RECEIVED for review November 19. 1962 ACCEPTED August 27. 1963

Division of Colloid and Surfacr Chemistry, 142nd Meeting, XCS, Atlantic City, N. J.. September 1962.

RATE OF ISOMERIZATION OF CYCLOPROPANE

IN A FLOW REACTOR B . R . D A V I S A N D D . S . S C O T T

Drpartment of Chemical Enginpering. CTn iver s i t~ of British Columbia, Vancouaer. B C , Canada

Experimental studies planned to investigate combined diffusional and chemical effects in porous solids or packed beds might be simplified by the use of a simple-order homogeneous gas phase reaction. The isomerization of cyclopropane to propylene appears to satisfy these requirements potentially, and to retain the simplicity of a binary system. However, the reaction behavior at fast reaction rates must be known if the reaciion i s to be used in a diffusion-controlled experiment. In the present work, the existing kinetic data for this isomerization, obtained in batch reactors at temperatures from 460" to 550" C., were extended to 620" C. by use of flow reactors constructed of borosilicate glass. The extent of secondary decomposition of propylene at these temperatures was investigated, as well as the effect of large surface-volume ratios. Kinetic behavior was measured at all conversion levels by both integral and differential methods. At constant pressure, and temperatures to 620" C., this isomerization behaves as an ideal homogeneous first-order reaction in contact with borosilicate glass equipment.

HE mechanism of isomerization of cyclopropane has been 'carefully investigated by a number of \vorkers ( 7 : 3. 5-7) and found to be a homogeneous first-order gas phase reaction when carried out in borosilicate glass apparatus. Below 1- a tm. total pressure a dependence of the first-order rate constant on the pressure exists. Studies of the probable mechanism. both experimental and theoretical in nature. have indicated (after some controversy) that this isomerization is a molecular rearrangement dependent on a hydrogen shift in the cyclopropane molecule. and that the observed rate is not due to any extent to free radical processes.

In many laboratory studies of catalyst or reactor behavior. particularly those in \yhich a diffusional step is significant, it is convenient to use a simple-order reaction of known behavior. For example. diffusional processes in the pore channels of a

catalytic solid could be easily studied as a single phenomenon by using an ideal homogeneous reaction. Further. an isomerization reaction retains the simplicity of a binary system. HoLvever. the experimental study of diffusion-limited reactions requires relativelj- fast rates of chemical change. The majority of the kinetic studies of cyclopropane isomerization already mentioned have been done in static systems a t temperatures from 460' 10 550' C. Such batch techniques are usually not useful for the measurement of the rates of rapid reactions, and: therefore. results obtained at the higher temperatures by this method have been obtained at subatmospheric preszures to reduce reaction rates to a manageable level. In general. the entire range of conversion levels at different temperatures has not been fully investigated.

Fast reaction rates can best be studied in a flow reactor.

20 I & E C F U N D A M E N T A L S

Dow

nloa

ded

by U

NIV

AU

TO

DE

L E

STA

DO

DE

HID

AL

GO

UA

EH

on

July

6, 2

009

Publ

ishe

d on

May

1, 2

002

on h

ttp://

pubs

.acs

.org

| do

i: 10

.102

1/i1

6000

9a00

3

dispersion caolin interactions

Documents