International Journal of Mineral Processing, 4 (1977) 111-129@ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

SUBSIDENCE OF SUSPENSIONS OF PHOSPHATIC SLIME AND ITSMAJOR CONSTITUENTS

D.R. NAGARAJ, L. McALLISTER and P. SOMASUNDARAN

Henry Krumb School of Mines, Columbia University, New York, N.Y. 10027 (US.A.

(Received December 22, 1976)

ABSTRACT

Nagaraj, D.R., McAllister, L. and Somasundaran, P., 1977. Subsidence of suspensions ofphosphatic slime and its major constituents. Int. J. Miner. Process., 4: 111-129.

The dewatering of stable phosphatic clay suspensions resulting from the processing ofphosphate rock is a problem of much practical and theoretical significance. In order toinvestigate this aspect, the sedimentation behavior of suspensions of major components ofphosphatic slimes, or morphologically similar meterials, both separately and in appropriatecombinations, was studied. Systems which behaved very similarly to phosphatic slimes weretermed "model systems". Study of these model systems revealed that the mineralsmontmorillonite and attapulgite, with their special properties - morphology, swelling, ionexchange capacity, surface charge, and viscosity - were responsible for the behavior ofphosphatic slimes.

Mechanisms by which coarse particles and micro air bubbles enhance the dewatering ofthe phosphatic slimes are reviewed. In both cases, creation of paths for water seepage as aresult of their movement is considered to be mainly responsible for the enhanced sub$i-dence of the slurry.

INTRODU~ON

The dewatering of colloidal phosphatic slimes is a problem of utmosttheoretical interest and practical importance. The reasons for the remarkablestability * of these suspensions** are still largely unknown. The practical im-

portance lies in the need for the disposal of such slimes produced during theprocessing of Florida phosphate rock in an environmentally acceptable form,and in the need for recovering the water for recirculation. Currently, millionsof tons of phosphatic slime are stored in vast acreages of ponds that remain asan environmental hazard (Boyle, 1969; Morgan, 1974)._~ral investigations (Davenport et al., 1953; Bureau of Mines, 1975) have

*The distinction between "stability" and "colloidal stability" must be noted; the formerimplies the slow response of the system to change with time, and the latter implies an ex-tremely slow settling of the system, leaving a relatively turbid supernatant.* * Suspension rather than dispersion appears to be the appropriate word according to

Van Olphen (1963, p.5).

112

been carried out in the past, both towards achieving water separation in themost effective manner, and towards understanding the mechanism of settlingof phosphatic slimes. An earlier investigation (Harris et al., 1975) discussed thecharacteristic features accompanying the settling of phosphatic slimes. A three-dimensional network model was suggested. Gelling of the slurry occurredwithin a few seconds after mixing and then fissures and tears formed graduallydue to the movement of coarser particles as well as micro air bubbles throughthe structure. Water seeped through these paths and when it met resistance toi~ continued transport, it formed lenses of water. Further seepage occurredwhen channels opened up between tears, with water finally exiting at theslurry Isupernatant interface in the form of microvolcanoes. The continuousremoval of water finally contracted the channels and consequently, furthersubsidence became difficult. On the whole, the sediment settled slowly as abulky compressible mass, leaving a clear supernatant.

If the movement of either the relatively coarse particles or micro air bubblesthrough the slurry structure makes it unstable, then the addition of coarseparticles to it or the generation of microbubbles in it make it more unstable,and, consequently, could enhance i~ subsidence. The addition of coarseparticles (>50 /lm) to the phosphatic slimes did reduce their stability signifi-cantly; the subsidence rate increased almost 50 times (Somasundaran et al.,1973). Similar observations have also been reported by others (Timberlake,1969; Bureau of Mines, 1972).

The subsidence behavior of the phosphatic slimes has been explained in thepast on the basis of electrostatic repulsive forces among the constituentminerals of the slime. However, not a single proven mechanism exis~, whichis complete in i~elf and which can explain both the remarkable stability ofthese suspensions and their characteristic subsidence behavior. For example,electrostatic repulsion alone cannot explain the typical pH effect on the sub-sidence behavior of the suspensions (see Fig.l). Similarly, the electrostatic re-

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Fig. I. Height of the slurry/supernatant interface versus subsidence time at various pH values(or 1.8% phosphatic slimes; ionic strength adjusted, when possible, to 2.10-3M with KNO3o

113

pulsive forces can neither explain the characteristic physical features - be-lieved to have a significant influence on the stability - observed during thesubsidence of phosphatic slimes, nor the considerable variations in the sub-sidence behavior among different slime samples, which might have comparablezeta potentials. It is to be noted in this regard that the previous investigationsconcerned with the mechanistic aspects of phosphatic slime behavior had ac-tually based their conclusions on the results obtained using industrial slimes ~fwidely varying and, most importantly, often unknown mineralogical compo-sition. A model-systems approach was, therefore, used in this study in anattempt to elucidate the mechanisms involved. These systems were made up ofphosphatic slime constituents of which montmorillonite, attapulgite, kaoliniteand quartz are significant (Boyle, 1969).

The colloidal behavior of the phosphatic clay suspensions is believed to bedue to montmorillonite and attapulgite. As the latter mineral is fibrous innature, it was decided to incorporate in the model systems similar acicularminerals, such as chrysotile and amphibole, in order to determine the possiblerole of a fibrous mineral in the behavior of the suspensions. Thus, the syst.emsinvestigated in the present work were essentially composed of kaolin ormontmorillonite and a fibrous mineral. A system should simulate the followingcharacteristics to be considered to be representative of the phosphatic slimes:(1) an initial period of gelling; (2) slow overall subsidence with a continuouslyvarying settling rate typical of a network structure; (3) absence of significantsegregation of mineral constituents during sedimentation; (4) a clear super-natant and a sharp slurry/supernatant interface; (5) a bulky sediment; (6)presence of tears and channels in the sedimenting structure; and (7) water exitas microvolcanoes. These characteristics have been identified for the phosphaticslime and its subsidence.

The present study using model systems revealed the role of each constituentin determining the stability or the instability of phosphatic slimes. The studyshowed that, in addition to the electrokinetic properties, morphology, swellingproperties, and the ion exchange capacities of the mineral constituents are re-sponsible for the subsidence characteristics of the phosphatic slimes. It alsoshowed that the supernatant clarity is determined mostly by the electrokineticproperties.

EFFECT OF COARSE PARTICLES

Earlier tests (Somasundaran et al., 1973, 1975) to demonstrate the effec~of coarse particles on the stability of phosphatic slimes were carried out withslimes sand, which is the settled material during storage of the as-receivedslimes, and particles of different sizes, shapes, densities, and surface properties.Various particles used include fluorite powder (AR grade); coarse quartztailings from the phosphate flotation circuit of the International Minerals andChemicals Co.; -149 + iO5-,um size clear glass beads and silicone-coated glassbeads; fluorspar, alumina, apatite, chalcopyrite, cassiterite (Bolivian), graphite

114

and molybdenite of selected sizes; additional tes~ were done during the presentstudy using fibrous minerals, chrysotile, attapulgite, and amphibole,

Fig.2 shows the effect of addition of sand and other coarse particles. It alsoshows the effect of removal of the sand in improving the reproducibility ofsubsidence of phosphatic slimes. Various stages typical of a subsidence curveare indicated in Fig.3. As can be seen from these figures, there is an initial zero

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Fig. 2. Height oC the slurry/supernatant interCace versus subsidence time Cor phosphaticslimes with and without coarse additives; vertical bars indicate range oC data Cor replicates.(aCter Somasundaran et al., 1973).

115

subsidence rate period, the duration of which depends on the amount of coarseadditives present. During the subsequent stage - identified as the lenticularstage - the movement of coarse particles leaves tears behind them and, thus,

initiate the formation of water channels, the extent of which is decided by thenumber of coarse particles. These are assisted by tiny air bubbles tearing thenetwork. Water seeps through these tears until it meets resistance, thereupon itforms a lens of water. Further movement of water is possible only if channelsopen up between tears. This stage has been identified as the reticular stage,followed by the vermicular stage, where aggregates of flocs are more stronglylinked by bridging into a continuous mass interspersed by filaments of water.

The effect of weight of additives is illustrated in Fig.4, and that of the ad-100 '-- ~~~'--.. ","""- -

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Fig. 4. Height of the slurry/supernatant interface versus the subsidence time as a function ofthe weight of coarse additives for 2.6% phosphatic slimes, -53+44 JAm quartz particlesadded. Points are omitted for clarity (after Somasundaran et aI., 1973).

100

ditive density and surface properties is shown.in Fig.5. In general, the settlingrate increases with the addition of the coarse particle up to a certain amountabove which the excess of it simply breaks through the slurry and settles atthe bottom. However, slurries of higher solids content can support higheramounts of additives as a result of strengthening of the network due to the in-crease in concentration of the suspension. Similarly, the minimum amount ofan additive required to show any effect depends on the initial concentrationof the suspension. As regards the size of the additive, the -37 .urn particlesare found to be generally less effective than the others, and for particlesgreater than 37 .urn, their effect is independent of size, at least up to 105 .urn.

Fig. 5 shows that an increase in density of the additive is without any bene-ficial effect on the subsidence. This clearly showed that the effect of coarse

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116

100 -GRAPHITE, QUARTZ, SILICONEFLUORITE,APATITE GLASS COATEDALUMINA, BEADS GLASSCHALCOPYRITE, ' B!AOSMOLYBDENITE AND!CASSITERITE

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Fig. 5. Height of the slurry/supernatant interface versus subsidence time as a function of thedensity of coarse additives for the 2.6%, -37 #lorn phosphatic slimes (after Somasundaranet al., 1975).

particles is not primarily due to an increase of the effective weight of the net-work. Cassiterite, possibly owing to its very high density, was observed tobreak through the slurry even before the gelling process was complete and didnot produce any significant enhancement in the subsidence rate. It can be seenfrom Fig.5 that various hydrophilic as well as partially hydrophilic mineralshad approximately the same effect on the subsidence of phosphatic slimes,while the totally hydrophobic silicone-coated glass beads~ which are alsosmooth and spherical, did not enhance the subsidence. It is to be noted thatthe uncoated glass beads were effective in enhancing the subsidence. Thesefindings suggest polar interactions of additives with the particles of the slimenetwork. The conclusion reached in the past that the particle shape was not apredominant factor determining the subsidence was found, as will be seen later,in the present study to be invalid for cases involving coarse particles of ex-treme shape. If the coarse particle additives are of a type that strongly interactwithin the network of the suspension the resulting mixture might becomemore stable than the original suspension. This stabilizing effect could be dueto any of the various special properties that the additive might possess; forexample, unique morphology, high viscosity in aqueous suspensions, etc.

Fig.6a. Height of the slurry/supernatant interface versus subsidence time as a function ofchrysotile to phosphatic slimes weight ratio for the 1.8% slimes.Fig.6b. Height of the slurry/supernatant interface versus subsidence time as a function ofattapulgite to phosphatic slimes weight ratio for 1.8% slimes.Fig.6c. Height of the slurry/supernatant interface versus subsidence time as a function ofamphibole to phosphatic slime solids weight ratio for 1.8% slimes.

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118

When coarse particles of fibrous minerals such as chrysotile, attapulgite, oramphibole, were added to the phosphatic slime, the subsidence suffered but de-creasingly in the above order (Fig.6a, b, c). The suspension after the additionof chrysotile appeared very bulky and viscous. Large amounts of attapulgitealso had a similar effect.

EFFECT OF AIR BUBBLES

The experiments consisted of applying suction above a suspension of phos-phatic slimes contained in a lOO-ml graduated cylinder using an aspirator con-tinuously during sedimentation of the phosphatic clay suspension or periodi-cally (ten minutes of suction every twenty minutes) or using a vacuum pumpperiodically (2.5 sec of suction every thirty minutes).

The results of the effect of generating bubbles in the slurry are given inFig. 7. It can be seen that generation of bubbles does enhance the subsidence

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Fig. 7. Effect of air bubbles (generated in the slurry by applying partial vacuum above it) onthe subsidence of phosphatic slimes.

considerably. Like the micro bubbles that are ordinarily present in the slime,these bubbles are also found to act by altering the physical features of theslurry such that water seepage becomes possible through the channels and thenumerous tears produced by the movement of bubbles.

It can be seen in Fig.7 that applying suction has no effect on the initial tento twenty minutes of the lenticular region of the subsidence curve. Also,suction had only a small ~ffect on the second flat region of the curve, namely,the asymptotic region. However, when suction was applied during the latterregion (Fig. 7, Curve 4), the solids settled to a smaller volume (25-26% of theoriginal volume) in a shorter period than when the suction was not applied(Fig.7, Curve 1). Once the solids had settled to a volume of 25-26%, mild

119

suction above the suspension even for long periods produced no measurableeffect. A stronger suction merely dispersed the sediment.

It can also be seen in Fig.7 that the effects produced by suction in pulsesand that applied continuously' yielded comparable results. The curves in Fig. 7have the same general shape, and, when suction is applied, the curves are al-most parallel in the reticular and the vermicular regions, irrespective of theintensity or manner of application of suction. These regions are least affectedby suction. It might be recalled that a similar result was obtained for the coarseparticle addition up to one gram (Fig.4). On the basis of the above observations~it appears that generation of tiny bubbles by some means within the sedimentitself might offer, either by itself or in combination with other techniques, anattractive way to enhance the subsidence.

SUBSIDENCE OF MODEL SYSTEMS

The following minerals were studied in order to arrive at model systems:(1) Clay minerals - montmorillonite (Wyoming), Al2Si4O1o (OH)2'xH2O; at-

tapulgite (Quincy, Florida), (OH2)4 (OH)2MgsSigO2o'4H2O; and kaolin (Corn-wall, England). These three minerals are the major clay constituents of thephosphate slimes.

(2) Asbestos minerals - amphibole (Rajasthan, India), which closely re-sembles tremolite; and chrysotile (C.E. Refractories Combustion Eng., Inc.)an asbestiform serpentine. These two minerals were chosen solely on thebasis of their prominent fibrous nature, and were included in order to de-termine the role of such a mineral feature.

(3) Quartz (Hot Springs, Arkansas) which is also a constituent of phosphaticslime, but which has neither the properties of the clay minerals nor a fibrousmorphology .

All the minerals except chrysotile were purchased from Wards NaturalScience Establishment. The phosphatic slime used in this study was from thesame source as mentioned in the previous sections, but had 1.75% solids. Allthe minerals except" montmorillonite and kaolin (which were received as finepowders) were ground in a planetary mill for ten to thirty minutes to obtainapproximately 100%-400 mesh fraction. The purity of the minerals was notdetermined. As prior studies with clay mineral systems showed little change inbehavior after cleaning, the minerals used were not treated in any way beforethe experiments were run.

The experiments essentially involved preparation of colloidal suspensions atthe desired pH and ionic strength, observation of the sedimentation processafter mixing, measurement of the clarity of the supernatant at given heightsand times, and determination of the isoelectric point and the point of zerocharge of the minerals used.

The suspensions of minerals and mineral combinations were prepared by in-tensely stirring appropriate amountc; of clay minerals for twenty minutes in therequired volume of water, introducing the additive and thoroughly mixing the

120

combination. 100-ml samples of the suspension were then adjusted to thedesired pH and ionic strength values (the latter using 1M KNO3), and aftertumbling the suspensions at 17 rpm in 100-ml graduated cylinders for twentyminutes, subsidence tests were carried out by recording the descent of theslurry /supernatant interface as a function of time. To measure the clarity ofthe supernatant, samples were drawn from levels at 80% of the total sampleheight and transmittance was measured using a spectrophotometer.

In order to arrive at the model systems, which must behave very similarly tothe phosphatic slimes, the settling behavior of the following mineral systemswas studied as a function of time and compared with that of the phosphaticslime:

(1) Investigation of single mineral systems - montmorillonite, kaolin, at-tapulgite, quartz, chrysotile, and amphibole, as a function of pH.

(2) Investigation of binary systems - binary mixtures of the above minerals.

For brevity, only the relevant binary mixtures, such as those of montmorillo-nite and kaolin with other minerals, will be presented here. The experimentswere conducted as a function of both the minerals combination ratio and pH.

(3) Investigation of ternary systems - again only the relevant ternary

mixtures of montmorillonite plus kaolin or attapulgite with other mineralshave been presented; experiments were carried out as a function of both theminerals combination ratio and pH.

The selection of combination ratios for the binary and the ternary systemswas based on the average mineralogical composition of phosphatic slimes re-ported in literature (Gary et al., 1963; Cox, 1968; Boyle, 1969; Lamont et al.,1975).

The subsidence of phosphatic slime

The subsidence behavior as a function of pH was shown in Fig.l. In a pre-vious study (Somasundaran et al., 1975), the isoelectric point of the suspension(which for such a natural mineral mixture must be used with caution) wasfound to be 2.6. The rate of subsidence is a measure of the stability of thesuspension. The minimum stability indicated at pH 1.4 is in agreement withwhat would be expected near the isoelectric point under high ionic strengthconditions associated with that pH. An increase in pH above the isoelectricpoint of 2.6 should augment the stability of the dispersion, which it does for.a rise in pH to 4.2 and 6.2. Upon further increase to 8.2 (natural pH), theslurry, however, begins to subside at a comparatively faster rate. This be-havior is unexpected on the basis of the isoelectric point obtained; but it canpossibly be accounted for if the attainment of a natural pH of 8.2 is inter-preted to suggest an apparent point of zero charge close to that pH, whichcertainly is questionable for clay systems. Thus, the observations contradictthe general contention expressed in the literature that the electrostatic re-pulsion between the strongly negatively charged clay-type particles solely dic-tate the stability of phosphatic slime. Evidently, there are several other factors,

121

such as morphology and electrokinetic, swelling and ion exchange propertiesof the constituent minerals, responsible for the subsidence behavior of phos-phatic slime.

As mentioned above, the relatively unstable behavior at pH 1.4 could partlybe due to the high ionic strength associated with such a pH value. In addition,however, the evolution of gas bubbles as a result of the reaction of the acidwith carbonates or other acid-avid substances present in the suspension canalso be expected to aid the sedimentation process, as noted in the previoussection. It might also be mentioned that the slime was observed to behavedifferently in the basic and the acid (or neutral) regions. It will be seen laterthat this difference is striking among the model systems also. It is also to benoted from Fig.! that the natural pH of the phosphatic slimes is 8.2, which ismuch different from the measured isoelectric point. This discrepancy appearsto stem from the fact that the particles observed for electrophoretic measure-ments, though possibly the most stable in the suspension, cannot be consideredto meaningfully represent the entire suspension, which contains a variety ofminerals, and also from the experimental artifacts introduced by the ion ex-change properties of the clay minerals in the slime.

Single mineral systems

Fig.8 shows the characteristics of the single minerals, with only the settlingrates indicated. A typical subsidence curve of phosphatic slime is also given forcomparison. None of the single minerals systems showed all the phosphaticslime subsidence characteristics, namely, slow initial subsidence, developmentof tears and fissures in the sediment, a sharp slurry Isupernatant interface and

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TIME, MIN.

Fig.B. Height of the slurry/supernatant interface versus settling time for single mineralsystems; all at 2.5% solids. The numbers in parentheses indicate the pH values of sus-

pensions.

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1°1

201

122

a clear supernatant. Kaolin settled rapidly leaving a turbid supernatant, andthe rate at which this supernatant cleared varied with pH, the low pH valuesbeing favorable. However, even at lowest pH values tested, the supernatant wascomparatively cloudy. If morphology of the minerals is brought into con-sideration, it can clearly be seen that all the minerals with an acicular mor-phology - attapulgite, chrysotile, and amphibole - had settling rates inter-mediate to those of kaolin and montmorillonite. Kaolin and quartz showedsimilar settling characteristics. The very slow settling rate of montmorillonitecan be attributed to its special properties, such as swelling, and its ability toform a rigid network that can trap a large amount of water. The slightly in-creased settling rate at lower pH values might possibly be due to the release ofsodium ions which may interact with the negatively charged montmorilloniteto destabilize the suspension (O'melia, 1972). Kaolin has a simpler structure,and the swelling and ion-exchange properties are relatively minimal, if notabsent; their settling characteristics are, consequently, almost similar to thoseof very fine inert particles such as that of quartz. In alkaline media, all thesuspensions settled slowly, leaving behind a turbid supernatant. The clayminerals expose negative faces at all pH values, and if the pH is sufficientlyhigh, even the edges will be negatively charged. As a consequence, the particleattractive forces are at a minimum, and the particles tend to be dispersed. Therelatively coarse particles tend to settle, but the fines remain suspended in thesupernatant. In a similar manner, electrostatic repulsion among negativelycharged particles can be considered to produce a slow settling suspension and aturbid supernatant in systems of amphibole and, to a lesser extent, in chryso-tile and quartz.

BINARY SYSTEMS

The addition of a second mineral to a single clay mineral suspension pro-duced some remarkable changes. Fig.9 shows the effect of the addition ofchrysotile - an acicular mineral- to a 2.5% suspension of kaolin - one of theclay miner'dJ constituents of phosphatic slime. The approach of the behaviorof the kaolin-chrysotile suspension towards that of the phosphatic slimewith increasing chrysotile/kaolin weight ratio is clearly seen in Fig.9. As theamount of chrysotile increased, the settling rate decreased, clarity of thesupernatant increased, and the suspension appeared bulky, resembling a gel-type structure. The sedimenting structure, now with the fissures and channelsmaking their debut, also appeared similar to that of the phosphatic slime. How-ever, the addition of attapulgite to a suspension of kaolin did not have as mucheffect as chrysotile did, as shown in Fig.l0. At natural pH of the kaolin-at-tapulgite binary (1:1), the two minerals appeared to sediment separately leavinga cloudy supernatant; this suggests lack of interaction between particles and,therefore, the absence of a stable three-dimensional network. At lower pHvalues, the addition of attapulgite did, however, hinder the settling of kaolin,as could have been expected from the single mineral characteristics shown in

123

L, 2.5% Kooll~ in 0Chrysohle, 01

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TIME, MIN.

Fig. 9. Height of the slurry/supernatant interface versus subsidence time at various pHvalues for 2. 5 to 5% kaolin-chrysotile suspensions at chrysotile/kaolin weight ratios of0.04, 0.2, 0.3,0.4,0.6,0.8 and 1.0; ionic strength 2.10-3M.

TIME. MIN.Fig. 10. Height of the slurry/supernatant interface versus settling time for binary systemscontaining kaolin (K) and the minerals: chrysotile (Ch), attapulgite (At), amphibole (Am),and quartz (Q). Points are omitted for clarity. Curve 1 - K:Ch = 1:1 (pH 8.8); curve 2-K: At = 1:4 (pH 8.3); curve 3 - K:Am = 1:1 (pH 6.7 and 8.5); curve 4 - K:At = 1:1(pH 7.8); curve 5 - K:Q = 1:2 (pH 6.4); curve 6 - phosphatic slimes alone (pH 8.2).

Fig. 8. At lower pH values, possibly, the edges of the clay minerals already havea positive charge and hence, an edge-to-face aggregation is possible.

Mixtures of kaolin and quartz settled rapidly at all pH values, and at naturalpH, the two minerals settled separately. The supernatants were invariablycloudy. The kaolin~mphibole system displayed no special features; at pHvalues very much lower than the natural pH ('\.,8) the mixture settled fairlyrapidly, leaving a clear supernatant.

124

A very interesting binary was that constituted by montmorillonite and at-tapulgite. Two settling curves for this binary are given in Fig.II, along withthose for a few other representative binaries and two for phosphatic slimesfor comparison. At the natural pH of the montmorillonite-attapulgite binary

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TIME. MIN

Fig.I1. Height of the slurry/supernatant interface versus settling time for binary systemscontaining montmorillonite (M) and the minerals: attapulgite (At), amphibole (Am), andchrysotile (Ch). Points are omitted for clarity. Curves 1 and 4 - At:M of 4:1 and 6:1respectively; curves 2, 3, and 5 - Am:M of 4:1, 5:1 and 6:1 respectively; curve 6 - Ch:Mof 4: 1; all of the above at pH'\, 5; curves 7 and 8 - phosphatic slimes at pH 6.2 and 8.2

respectively.

('\19), the latter mineral settled rapidly, leaving montmoril~onite still in sus-pension; this observation was judged on the basis of the color difference be-tween the minerals.. At lower pH values, they appeared to forma stable homo-geneous suspension, which still settled very slowly. Increasing the amount ofattapulgite should in general increase the settling rate of the mixture unlessthere is a very specific interaction between the particles which hinders settling.Fig.12 shows the settling characteristics of the montmorillonite-attapulgitesystem at varying weight ratios of attapulgite to montmorillonite. A minimumratio of four appears to be necessary to enhance the settling rate of the binaryto a level comparable with that of the phosphatic slime. Furthermore, at thisratio, the binary suspension resembled the phosphatic slime. Even though thetears and channels, characteristic of the phosphatic slime, were less prominentin the binary mixture, the latter suspension appeared to be as viscous as theformer.

Even though the montmorillonite-amphibole and the montmorillonite-chrysotile binaries had similar settling rates as the montmorillonite-attapulgitesystem, they did not possess the major phosphatic slime features. The mont-morillonite-amphibole suspension was less viscous, settled leaving a slightlyturbid supernatant, and did not display tears or channels. The montmorillo-

125

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nite-quartz system behaved similarly to the montmorillonite-amphibolesystem. The suspensions of montmorillonite-chrysotile which were viscous,developed some of the phosphatic slime features during settling, and under allconditions produced clear supernatants.

In summary, even though none of the binaries completely resembled thephosphatic slime, it was found possible to arrive at a montmorillonite-atta-pulgite binary which resembled the phosphatic slime more closely than theother binaries. It was also evident that the addition of the other two majorconstituents of the phosphatic slime, namely kaolin and quartz, to the mont-morillonite-attapulgite binary can yield systems capable of simulating thebehavior of phosphatic slime.

Ternary systems

Fig.13 shows the representative settling rate characteristics of two of thethree types of ternary systems that were investigated. In both types the totalsolids content was limited to 6%, and various combinations of montmorillo-nite-attapulgite-kaolin (Type 1) and montmorillonite-attapulgite-quartz(Type II) were tested. The ternary system kaolin-attapulgite -quartz (Type III)produced no relevant information. This suggests that montmorillonite has avery important role in determining the subsidence behavior of phosphaticslime. It was found that Type I systems behaved very similarly to phosphaticslime; hence, the systems of this type shown in Fig.13 can be considered to bemodel systems on which further studies might be conducted.

In the previous section it was found that a ratio' of attapulgite to mont-morillonite of four was required; the ternary systems were selected partly onthis basis. It can be seen in Fig.II that systems containing attapulgite in excess

I

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Fig. 12. Height of the slurry/supernatant interface versus settling time for 1-5% mont-

126

Fig. 13. Height of the slurry/supernatant interface versus settling time for some importantternary systems, considered here as model systems, of montmorillonite (M), attapulgite (At),kaolin (K), and quartz (Q). Points are omitted for clarity. Curves 1,2 and 4 - M:K: At at1:3:2 (pH 4-8), 1:1:3 (pH f\J 6) and 1:1:6 (pH 'V 6) respectively; curves 3 and 5 - M:Q:Atat 2:2:2 (pH 3-8) and 1:1:4 (pH f\J4) respectively. The pH values (or the ranges) indicatedin parentheses are those at which the suspensions behaved very similarly to phosphaticslimes. Compare this figure with Fig.1.

of three times that of montmorillonite have subsidence characteristics ve~close to those of phosphatic slime. In fact, these systems exhibited most of thephysical features characteristic of the phosphatic slime. Even these systems attheir natural pH failed, however, to totally simulate the behavior of the phos-phatic slime; the minerals had a tendency in this case to settle separately. Ateven slightly lower pH values, the behavior was much the desired one. Possibly,lowering the pH makes the edges of one or more of the clay minerals to havea charge either close to zero or positive.

Electrophoretic measurements were carried out for all the clay minerals andch~sotile using a zeta meter, but, as expected, did not give any indication ofthe changes in charge on the edges as a function of pH. An additional problemis posed by the nature of the minerals itself; since natural clay minerals in-variably contain impurity minerals of different densities, zeta potential oflighter particles measured with the zeta meter cannot always be considered tobe representative of the entire mineral system. The presence of such impurityminerals might, however, be unimportant as far as the overall subsidence ofthe minerals mixture is concerned.

In the Type I ternaries, two sets of combinations were tried. In the first set,the ratio of montmorillonite to kaolin was kept constant at one, while theamount ofattapulgite was varied. Attapulgite had a tendency to settle prefer-entially when the systems were at their natural pH. Possibly, the needle-likemorphology of attapulgite is partly responsible for this since, at natural pH, the

127

particles might not show the necessary interaction to form a stable three-dimensional network and as a result, attapulgite might be cutting through thefragile network of montmorillonite and kaolin. At lower pH values, the systemswith attapulgite to montmorillonite ratios of three or higher showed the de-sired settling behavior.

In the second series of tests, the solids content was kept constant at six percent, and several weight combinations of montmorillonite, attapulgite andkaolin were tested. These systems, at their natural pH, behaved similarly to theothers mentioned above at their natural pH. At lower pH values, they gave thedesired results when the attapulgite content was comparatively high.

Among the Type II ternaries, the system with a montmorillonite/attapulgitelquartz at 1:4:1 showed the desired results at lower than the natural pH of 9.At natural pH, the minerals again had a tendency to settle separately.

The kaolin-attapulgite-quartz systems were found to be not capable ofproducing suspensions with relevant features. However, when 0.5 unit of mont-morillonite was introduced into a mixture of 1:1:0.5 of the above threeminerals, the resulting combination was strikingly similar to the phosphaticslime in every respect. This observation again shows the important role thatmontmorillonite has in determining the subsidence behavior of the phosphaticslime; attapulgite also appears to have an equally important role. It is not sur-prising that these two minerals almost entirely dictate the behavjor of phos-phatic clays because they both have very special properties. Montmorillonitebelongs to the group of "smectites" characterized by their unique swellingproperty (Van Olphen, 1963). Depending on the conditions, montmorillonitecan immobilize a volume of water several times its original volume. The vis-cosity of the resulting suspension is also high. The ion exchange capacity ofmontmorillonite is also spectacular. Values as high as 100-150 mequiv. per100 g have been reported in the literature. The particles are flaky and, due totheir swelling properties, are able to form gel structures, where cross-links arepossibly formed between the positive edges and negative faces. This cross-linking compensates for the double-layer repulsion between the unit layers ofthe montmorillonite structure. The double-layer repulsion is in turn influencedby the exchange of ions into and out of the inter-layer regions.

On the other hand attapulgite has entirely different but unique properties.It crystallizes in long needles whose length could be about a micron (based onthe electron micrographs taken during the course of this study) and widthcould be about a hundredth of its length. They have been observed to remainin bundles (Haden and Schwint, 1967); similar bundles of attapulgite have alsobeen observed in phosphatic slime (Gary et al., 1963). Attapulgite particles, inaddition, possess an unusually high surface area, and are associated with veryhigh viscosities (which could be as high as 40,000 cp); this latter propertyprevents sedimentation of the particles under certain conditions (Haden andSchwint, 1967). Attaptilgite, unlike montmorillonite, does not swell, and ithas a relatively low ion exchange capacity. Therefore, the unusually largesurface area and the acicular morphology of attapulgite can be considered to

..

128

dictate i~ properties. Nevertheless, attapulgite is also one of the most im-portant gel-forming clays. The bundles of needles disperse with ease in aqueoussuspensions, leading to the formation of a random lattice which can entrapliquid to increase the viscosity of the system (Haden and Schwint, 1967).

The clarity of the supernatant of the mineral systems studied here can beaccounted for by the favorable particle interaction. The electrochemicalcharacteristics of the minerals are suggested to be solely responsible for thedegree of clarity of the supernatant. Any other properties of minerals seem tohave little influence on the clarity. For all the systems investigated here, thesupernatant was clear only at pH values close to or below the natural pH. AtpH values higher than natural pH, the supernatants were invariably cloudy,which cleared only over several hours. Most of the minerals have negativesurfaces at pH values higher than their respective natural pH values. Thus, in amixture of minerals, as one of the minerals acquires a positive charge due to adecrease in pH, heterocoagulation can result due to i~ interaction with thenegative surfaces of other particles, even though this interaction may not in-fluence the sedimentation significantly.

In addition to the model systems developed here, several other systemscould be developed, and made to simulate the behavior of the phosphaticslime by altering the proportions of the minerals and the conditions underwhich they settle. In fact, as much as there are some differences in the be-havior of the phosphatic slimes from various sources, different model systemsmight serve different slime samples. There are, however, some definite featureswhich are characteristic of most phosphatic slimes, and these features havebeen emphasized in this work.

SUMMAR Y AND CONCLUSIONS

The mechanism of sedimentation of colloidal phosphatic slimes has been in-vestigated. The macroscopic mechanism of subsidence or dewatering has beendiscussed by considering the effects of movement of coarse particles and airbubbles through the slurry. The movement of air bubbles through the slurryhad purely physical effects; it altered the physical conditions of the slurry bycreating tears, fissures, and long channels for easy seepage of water. The in-fluence of the movement of coarse particles depended upon the manner inwhich additives interacted with the phosphatic slime particles. If the mineralinteracted strongly with the network, as in the case of chrysotile addition, thephosphatic slime suspensions were more stabilized; while the addition ofcoarse quartz, which does not appear to interact strongly with phosphaticclays, enhanced the subsidence rate of the latter. Totally hydrophobic silicone-coated glass beads did not-have any effect, as they had broken through theslurry even before it gelled.

The microscopic mechanism, responsible for the stability, or lack of it, ofphosphatic slime was investigated by formulating model systems, made fromthe major constituents of the phosphatic slime, which simulated both the be-

129

~»

havior and the appearance of the slime. Of the various combinations ofkaolinite, montmorillonite, attapulgite, quartz, chrysotile, and amphiboletested, the montmorillonite-attapulgite-kaolin ternary and the montmoril-lonite-attapulgite-kaolin-quartz quarternary were found to behave verysimilarly to the phosphatic slime.

During this process of formulation of the model system, the role of eachmineral in determining the behavior of phosphatic slimes was revealed. Theminerals montmorillonite and attapulgite, with their characteristic shape andproperties of swelling, surface charge and viscosity, were mainly responsible forthe unique subsidence behavior of phosphatic clay suspensions.

.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the helpful discussion with Prof. C.C.Harris. Financial support of National Science Foundation (ENG-71-02405)and technical help of T.S. Wong and Y.C. Wang are also gratefullyacknowledged.

REFERENCES

I

Boyle, J.R., 1969. Waste disposal costs of a Florida phosphate operation. Bureau of Mines,IC 8404.

Bureau of Mines Research, 1972. U.S. Dep. of the Interior, Washington, D.C., p. 24.Bureau of Mines, 1975. The Florida phosphate slimes problem - a review and biblio~phy.

Bureau of Mines, IC 8668.Cox, J.L., 1968. Phosphate wastes. In: Proc. Symp. Mineral Wastes Utilization, 1968, liT

Res. Inst., Chicago, m., pp. 50-58.Davenport, J.R, Kieffer, G. W. and Brown, RH., 1953. Disposal of Phosphatic Tailing,

Report No. 661, Tennessee Valley Authority, Division of Chem. Dev., Res. Branch,Wilson Dam, Alabama, pp. 70-96.

Gary, J.J., Feld, II... and Davis, E.G., 1963. Chemical and physical beneficiation of Floridaphosphate slimes. Bureau of Mines, RI6163.

Haden Jr., W.L. and Schwint, lA, 1967. Attapulgite - its properties and applications.1& EC, 59 (9): 59-69.

Harris, C.C., Somasundaran, P. and Jensen, R.R., 1975. Sedimentation of compressiblematerials: analysis of batch sedimentation curve. Powder Technol., 11: 75-84.

Lamont, McLendon, J. T., Clements, I... W., Jr, and Feld, lL., 1975. Characterization studiesof Florida phosphate slimes. Bureau of Mines, R.l 8089.

Morgan, J.J., 1974. Those Nasty Phosphatic Clay Ponds. Environm. Sci. Technol., 8: 312-313.

O'Melia, C.R., 1972. Coagulation and flocculation. In: W.J. Weber (Editor), Physico-Chemical Processes for Water Quality Control. Wiley-Interscience, New York, N.Y.,pp.61-109.

Somasundaran, P., Smith Jr., E.I... and Harris, C.C., 1973. Effect of coarser particles on thesettling characteristics of phosphatic slimes. In: Proc. 1st Int. Conf. in Particle Tech-nology, IITRI, Chicago, Ill., pp. 144-150.

Somasundaran, P., Smith Jr., RL. and Harris, C.C., 1975. Dewatering of phosphate slimesusing coarse additives.. In: Proc. 11th Int. Mineral Processing Congr., Cagliari, Italy, Pap.No. 49.

Timberlake, R.C., 1969. Building land with phosphate wastes. Min. Eng., 21 (12): 38-40.Van alphen, H., 1963. An Introduction to Clay Colloid Chemistry. Interscience, New York,

N.Y.