~4

The effect of different structural modifications such as change In alkyl chainlength. branching of the alkyl chain and addJtlon of functional groups such as eth~and methyl groups on surfactant adsorpUon has been investigated In detaJl.9.14.IS Animportant Var1atJon In surfactant structure Js the position of furrtlonal groups.However. systematk studies on the effect of the position of fwx:tlonal groups onadsorptkJn an lacking. In thJs work. the effect of position of methyl and sulfonategroups on the benzene r1r1g on the adsorption and mJcelUzatlon of alkyl xylenesulfonates has been Jnvestigated uslr1g mlcrocalonmetry. electrokJneUcs and SU~tension. Calonmetry provides accurate thermodynamtc data and these data are he1pl'ulfor elucidating the mechanJsms of mJcellfzatlon and adsorpUon. Calorimetry has -extensfveJy used to measure the enthalples of mJce1lfzation of ~e surfactants 16-18and also the excess enthalpy of mJxJng of surfactant mixtures. 19. 0 EXtension ofcalorfmetnc work to study adsorption at solld/Jlquid Interface has been Ifmfted.21.25The emphasis in most of the work has been on the estabiJshment of the technique tostudy adsorption and studies show the potenUaJ of thJs techntque to understalxl theadsorption mechanJsms.

'~

Z:..;;.t;-~pial~8Iict

a1~..ciIEtdl~ceJI.readwastJIe!

cOlIC

~dete:

aZei

RE$~

Para S ~Meta S

Inl"lfOPPCbeinJsurU~charadsobull<Ulan-~

~~hydr3CetJSUlkh~

Exnertmental Conditt

Method~

Surfac~ T~~I0~ Surface t~nslon was measured usingr1ng tenslomel~r set to lh~ lest temperature.

water-jacketed d

: Ion on alumina. a gram was condition. Then. 5 mJ of surfactant solution of deNaCI was added and the sluny was contr1fuged and the Supernatant analyzed ft

edwcd

Adsorntlon: For adso:0.03 Janol/m3 NaCI for 2 houconcentration In 0.03 lanolInhours. The sluny was then cconcentration. ~

~~urfactant Ana~I~: Th(DUoS UV-VIs spectrophotometer

1eck:113' SI~Intowas11Ieani!the I

~(teau

surfactant concentration wat a wavelength of 254 nm. ana~ :d

I

294

EXPERIMENTALMatrnalg

~Mixed Micelle FonnaU~

It~ j.; ;:c >'. ~ i

~ ~~ "

Surface tension was also used to measure the CMC of the mixtures <i ~ twosurfactants and the results are given in fig. 5. Also. the theoretal CMC values for thedifferent mixtures calculated using the ideal solution and regular soluUon theorIes ~shown in this figure. The system shows a negative deviation from the ideal behavb .which is not nonnally expected for mlxtUTes of S1mJJar surfactants. Regular soIutbitheory fits the data well With an interacUon parameter (W) of -0.32. The deviations fr'-aideality can be attributed to the difficulty tn packing of the molecules in the mixedmicelles. However. prior to applying the regular soluUon theory to model the data. It IInecessary to verify the validity of the thennodynamic assumpUons of the theory asappUed to the system under investigation. Regular solution theory is based on theassumption that the excesss Gibbs free energy of mixing is given by

~

XIX2WRr 3}JmIx:

where Xl and X2 are the mole fractions of the two surfactants In the mixedmIcelle and W is the interaction parameter. Regular solution theory also assumes thatthe a~ entropy and volume of mJxJng are zero and herreI

HmII: = XIX2WRI 4)

Recently. H(enthalpy of mixingthat the assumptJodata adequately.

10 and Scamehom"'u have measuredonJc surfactants using calorimetry a19b even though the theory fits the su

:ea!xi

The excess enthalpy of mixing of the two surfactants In this study was me~using microcalor1metry and is shown In figure 6. The Interaction parameter obtained byfitting the excess enthalpy data was -0.21. This value Is very close to -0.32 obtained (IUD~e surface tension data. Indicating that the assumptions are valid for our system.

,0

rI

1e sulfonate and meta xylene sulfonateFigure 5 CMC or mixtur oCparaxy

.0

1;.:-0uCI%X2:...0

>-Q.-J'"Z--%w

InU)wUxw

~

-o""~~~~30

/0 ~\.\

I,

20

\Ip

I/.10 \

I0'- - , '" ~-~0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 t.O

..oLE fRACTION Of Weta S IN WICElLE

figure 6. Excess enthalpy of mixed micelle fOn11ation of para xylene sulfonate and

meta xylene sulfonate.

CONCWSION"

:/..I.~/~

It is evident from this work that the small changes in the surfactant structure.such as change In the position of sulfonate and methyl groups on alkyl xylene sulfonates

c~have a marked effect on the adsorption characteristics. The Para S adsorbed to agreater extent than the Meta S on alumina. The lower adsorption of Meta S is attributedto its lower hYdrophobicity and Increased stertc hindrance t9 the packing of themoleCules In the hemimicelles. MicrocalOrlnieny is sensitive to such small changes in$tJ'ucture and is an excellent tool to study mechanism of surfactant adsorption. The$tudfes Indicate that at low adsorption densities. enthalpy Js the driving force while athigher concentrations. the adsorption is governed by entropic changes due to~icelle formation. It helped confirm the steric hindrance to the packing of the~lecules as the reason for the dilTerence in the adsorption of the two surfactants.SurpnsJngiy. mixed micellization of the two surfactants is non-ideal and regularsoltJtion theory fits the data well. MJcrocalorJmetry shows that the thermodynamicassumptions of the theory are valid for the surfactant mixtures.

.-..0

ACKNOWLEDGEMENI'S

'gular Mixing

REfERENCES

-J L ~

0.6 0.7 0.& 0.9

>r W.ta S

and meta xylene sulfonate.

I ,,",

f ;~ "'.

::

.',

:.,'

'~:i'

~

;;";1:'~.

;~

2. H. S. Halma and P. Somasundaran. bt wlmproved on Recovety by Surfactant ~ :.~~Polymer JO100d1ng" . D. O. Shah and R S. Schechter. editors. Academic Press. New~;;York. 1977. c";;;;

3. J. D. Swa1en. D. L. AUara. J. D. Andrade. E. A Chandross. S.Garoft". J.lsrae~~,;;T. J. McCarthy. R Murray. R F. Pease. J. F. Ra~ K. J. Wynne and H. Yu, J.A.-.:2-ti!.a. 932 (1~. -~?:

4. M. J. Schwuger, bt"AnionJc Surfactants". Surfactant Science Sertes. E. H. Lucas.eQ;~cReynders. editor. Vol. 11. Marcel Dekker. New York. 1981.

5. M. S. Wr1gi1ton. editor. ~terfadal Processes: EneIgy Conversion and S~".;::'f:Advarx:es In ChemJstty Series. l.8.i. Amel1can Chemkal Society. Washington, D. c;j,::;1980 'c'-

6. J. H,'Fendler and E. J. Fendler. "Catalysis bt Micellar and Macromolecular ~;~Systems". AcademJc ~. New York. 1975. ~~~,

7. P. &xnaswx1aran am D. W. Fuerstenau. J. Pitys. Chern.. 10:. 90 (1966). ;-8. J. F. ~horn. R S. Schechter and W. H. Wade, J. Colk)ld Interface Scl.~. 463

(1982).9. P. Somasundaran. R Mkldleton and K. V. Viswanathan. bt wStructurc/Perf~

Relatk>nshlp bt Swfactants". M. J. Rosen. editor. American Chemical SocietySymposium Sertes. 2.5s1. 270. 1984.

10. L. Matos. J. Raver and G. Serratrtce. J. ColJold Interface ScL, ill, 341 (1989).11. M. Dahanayake and M. J. Rosen. bt"Structure/Perfonnance Relationship in

Surfactants". M. J. Rosen. editor. American Chemical Society Symposium ~~. 49. 1~.

12. M. J, Rosen. z. 7Jlu. B. Gu and D. S. Murphy. Langmulr..i. 1273 (1~1.13. A. Sokolowski. B. Burczyk andJ. Beger, Colloids Surfaces. 32. 373 (1989).14. P. Somasundaran. "Adsorptk>n from Flooding Solutions bt Porous Media", Annual

Report SubmJtted to DOE. NSF and a Consortium of Supporting IndustrialOrganizatk>ns, Columbia UnIversity. New York. 1987.

15. S. J. LewIs. L. A. Verkruyse and S. J. Salter, Society of PetroleumEngineers/Department of Energy Preprtnt, 14910 (1986).

16. G. OkJrsson. J. Pbys. Chml.. 89.. 1473 (1985).17. N. A. Mazer am G. O~n. J. Pi1ys. Chern.. B§;. 4584 (1982).18. K..S. Blrdl, Collold POIym. Sci.. .2§.l. 45 (1983).19. Edward Fu, D.E.S. Thesis. Columbia University, New York. 1987.20. J. F. Rathman andJ. F. Scarrr.hom. Langmulr,...4.. 474 (1988).21. S. Partyka. M. Undheimer. S. ZalnI. E. Keh and B. Brun. langmuir. 2.101 (19861.22. L. A. Non. L. A. Woodbury and T. E. Bun:hfield. Colloids Surfaces. a. 349 (1984).23. R Denoyel. F. Rouquerol andJ. Rouquerol. CoUokis Surfaces. 31, 295 (1989).24. N. M. van Os and G. Haandr11m1an. langmuir. 3,. 1051 (1989).25. S. Partyka. W. Rudzinski. B. Brun and J. H. Clint. langmuir. .5.. 297 (1989).26. B. W. Bany and G. J. F. Russell. J. CoIIokllnterface Scl..i.Q. 174 (1972).27. W. Drost-Hansen. bt "Chemistty and Physa of Interfaces-II". American Chemk:a1

Society. Washington. D.C.. 1971. RaseD;28. P. M. Holland. In "Structurc/Performarx:e Relationship In Surfactants". M. J.

editor. American Chemical SocIety Symposium Series. 25J. 141. 1984.

,

I

Invited lecture by P. Somasundara:1H,nIY Kntmb School of ,\(i/lu, Co/llmbia Uniy,rzity. Hew York, USA

DrrRODUCTION

233

operations the clays are liberated fro~ the matrix. It is mainly

this clay which, when present along with other minerals, appears

to be responsible for the slow-settlinq behavior of sliQes. The

role of clays has been inv..tiqated recently by Naqaraj, McAllis-

ter and Somasundaran [12J who studied the sedimentation behavior

of suspensions of various cocbinations c~ major mineral compo-

nents of phosphatic slio£ ~r morpholoqically similar materials

The systems ~nvestigated were essentially composed of kaoli-

nite c= montmorillonite, quartz and a fibrous mineral (attapul-

gite, chrysotile or amphibole). Table llists various the mine-

ral systems that were studied. It was stipulated that a system

should simulate the following settling characteristics of typi-

cal slow-settling slimes: (a) an initial period of qellinq;

(b) slow overall subsid~nce' with a continu~usly chanqi.r.q sl!ttl-

ing r~te that is typic~ of a network structure: (c) absence of

significant segregation of mineral constituents durinq sed~en-

tation; Cd) a clear supernatant and a sharp slurry/supernata~t

interface; Ce) a bulky sediment; (f) presence of tears and

channels in the sedimenting structure, and (g) water exiting &s

microvolcanoes. A system exhibiting such characteristics can

also be used as a model system' for controlled studies onslow

settling'. The study of Naqaraj et al. [l2J showed that none of

the constituent minerals themselves or their binaries resembled

industrial slime with respect to the above characteristics.

From the various combinations of kaolinite, mont=o:i~lonite

attapulqite, quartz, chrysotile, and amphibole studied, the

montmorillonite-attapulqite-kaolinite ternary and the montmoril-

lonite-attapulqite-kaolinite-quartz quarternary were found to

235

t

TABLE 1

Mineral Systems Studied

SJ.nqle Minerals:

Hant80rillonite

~oliD

Attapulqite

Q'.1artZ

Chrysoule

~~b~~l.

Binary Syst_:

~liD-cbryso~l. 1:1 At~pulqit.-Kont=oril1oait. 4:1 an4 6:1

1ao1iD-Attapulqite 1:4 and 1:1 Amphibo1e-MQn~ri11onit. 4:1, 5:1 aDd 6:1

!ao11n-A8phibo1. 1:1 Chryso~i1e-Montmcri11onite 411

bo1iD-Quartz 1:2

Ternary Syst_:

*Hontmorillonite-Attapulqi te-XAolin

Hon tmorilloni te-Attapul q i te-Qua~z

~lin-Atta~i te-Quartz

Ouaternary System:- --.Non tmorilloni te- At tapulqi te- Kaolin-Quart:

behave very similarly to the industrial slime The study using'

these systems also enabled us to determine the role of each

consti~uent. The clay minerals, montcorillonite and attapulqite,

w~re founc ~o be responsible for the slow sedimentation rate

owinq possibly to their peculiar morpholoqy and swellinq pro-

perties. For example, whereas montmorillonite-attapu19ite-kaoli-

nite-quartz behaved similarly to industrial slime, the ternary

system kaolinite-attapulqite-quartz showed no similarity - suq-

qestinq that montmorillonite does playa major role in determi-

ninq the settlinq behavior of slimes. S~larly, systems without

attapulqite or other fibrous-type mineral were also found to

settle differently from typical slimes.

236

period.. It is easily analysed alonq the lines of the modified

Kozeny model for flow throuqh porous media [14 J. Here settlinq

ends-abruptly with little further decrea.e in heiiht; such ma-

terial is said to be incompres8ible.

Fiq. 1. Typical batch sedimentation curve.

On the other hand, when the system contains particles of

diffe:ent sizes, one can obtain multiple interfaces [lS-11J.

Davies [lSJ has shown that the existence of such Qultiple in-

terfaces can b. seen up to a critical solid concentration of

about 30 to JS vol.'. The appearance of multiple interfaces is

attributed to the faster settling of the larger spheres. Above

the critical concentration, this differentia+ settling is pre-

vented by interparticle distances that are smaller than the

diameter of the smaller particle.

In contrast to the above cases, the aedimentation c~rves of

dilute suspensions of clay-type materials display a reverse

S-shape such as the one shown in Fig. 2, which is in fact so

238

TABLE 2

MAjor Criteria in Dewatering

SoUd concentration of sed~~

Vol~ of Hdl.-D~

Sett1.1nq rate

SlIperDatant clarity

Cake yield strenqth

Cake ~i.t.ur.

~w~~w...~

w'X...u.0...z0L&Jz

Fiq. 2. Reverse S-shape sedimentation curve.

239

ea=b

solid concentration

region. T

hese are

shown

in F

i;. 3.

three different

types of

settling curve.

are obtained,

one fo:

particle. A

t low

shear

rate., the

flocs qroup

into large

clus-

curved that

no reqion

could be

described as

exhibitinq a

con-

stant sett1inq

rate period

[19J. M

oreover, settlin9

continues

in this

ca.. at

an ever-decreasin9

rate over

an extrem

ely lon9

240

tended three

dimensional

network.

On

the basis

of this

codel

ters called

aggregates, and

the aqgregates

in turn

form

an ex-

time

period, suggesting

that the

material

is com

pressible.

The

sedimentation

of such

clay suspensions

has been

studied

by a

number

of investigators

~O

-2J J. A

notew

orthy quantitative

study in

thi, area

is that

of M

ichaels and

Bolqer

[24J w

ho re-

lated the

settlinq rate

of flocculated

particles to

the m

icro-

structure of

the floc.

and the

properties of

tM

sun"oundinq ~

um.

The

basic flow

unit

in the

Michaels-B

olger m

odel is

a -=

all

cluster of

particles called

a floc,

rather than

the individual

concentrations [24J.

suggest that the container diameter has a negligible ef~ect on

the settling of flocculated slurries. The effect of container

diameter on the settlinq of a phosphatic clay suspension exami-

ned by us (26J, however, showed that the diameter has a defi-

nite effect on the settlinq rate (Fiq. 4). An increase in dia-

meter caused a decrease in the settlinq rate for diameters up

to about 8 cm and above that, a slight increase. This observa-

tion can, however, be explained with the help of a phenomenolo-

qical model that we have developed.

F1q. 4. Diaqram illustratinq the effect of container

diameter on the heiqht of phosphate slime-super-

natant interface; initi~ solids concentration,

2.6' -37 pm slime; no additives, height of the

slime column, 17.8 cm, time of settling, 8 h (26] .

243

BATCH SETTLING PROCESSES

Oar model is based on the actual visual observation of ~.

..ttling .ystems and the formulation of the .implest plausible

equations for the settling curve. which di.play an S-.hape with

no region exhibiting a con.tant .ettling rate period and .ettl-

inq continuing at an ever-increa.inq rate over an extremely

lonq time period. rn fact it i. in this respect, i.e., where

constant settlinq rate period is abs.nt, that these .y.t~

fail to be satisfactorily represented by the previous models of

Kynch, Richardson-Zaki, etc.

Fiqure 5 illustrate. the observed sequence of the settlinq.

In the first one or two .econds after one .tops mixinq the

slur:y, the rotational movement of the 8uspension cease. due to

gelling- The gelling process itself appear. to be complete over

a period of several minutes, and then due to the downward move-

ment of coarser particles and the upward movement of micro air

bubble. through the structure, fissures and tears beqin to form.

Water seeps up throuqh these tears and when the water meets

re.istance to its continued transport, lenses of water ~e for-

mad. Further seepaqe occurs when channels open up between

tears, with water finally exitinq at the slurry-supernatant in-

terface in the form of microvolcanoes. At this point one can

clearly see the interface settlinq down quickly and water alonq

wi th entrained particles can be seen spouting through the cen-

ter of the dome. of the volcanoes. Water also seeps along the

walls of the container, foldinq the slurry away from the ~ll

at the interfac.. The observed effect of the container di~eter

on the rate of settling is in fact attributed to this seepa;e

244

S£DJI£KTATlOli P~SSES FlJR 8ATCII ~1TIc.'IS

J5 min 'tl2h

Fiq. 5. Sedimentation processes for batch conditions (26].

2.6' phosphatic slimes, 0.5 q coarse qraphite tracer.

Observations: 0 min, suspension qels almost i=:edia-

tely followinq mixinq and the rotational mov~.nt

terminates; -10 min, risinq air bubbles and descen-dinq particles create vertical tears; ,y 13 min to-35 min, water concentrates into lenses aroucd thetears; - 2 h, channelinq and microvolcanoes L~ance

dewaterinq process; -4 h, water filaments depleted,

channels beqin to close and no further siqnificant

sedimentation.

245

, "~"Ae= particles should kbetter additives. ReSults given in Fig. 7, ~owever, show that an

increase in the density of the A~"'i..~..- It4Va prOduced no enhanceme~in Settling: this suggested that the Obser\.ed effects were not

246

TlME.h

Fiq. 6. Diaqram illustratinq the effect of addition of

quartz flotation tailinqs on the .ettlinq of 2.6'-37 pm phosphatic slime, vertic~l bars indicate

ranqe when larqer than the symbol (26].

due to ~~y increase in the weiqht of the network. It is inte-

restinq to not. here that ca..it.rite, the heaviest oin.ral,

produced no effect. on .et.t.linq. A careful examination of the

.et.tlinq suspensions showed that the heavy cassiterite part.i-

cles had broken through the slurry before the slurry had a

chance to gel and trap them. Apparently, since the particles

had thus uready left the slurry, it could produce no effect

Oc.8ub8equent settl'inq. Hydrophobic 8ilicon-coated ql&.$ b~ads.

247

\ GRAfl..BTE. QUARTZ.

FW)RITE J...A;~,A!l~.AWMINA. ~ -RITE. MOLYBDENITEANO CASSITERITE90

.~I-CONTfD.~w~~WI-~w%...I&-0I-%!:2LA!%

80,

70;I

10

SO

.-CONTROL

-"-:

.,..02h -""

-o-!- -e ~ "-- - -CONTROL .. 8 h - -a

~ :'::':':::...~~~~.~ c - -. --~- t9 ~ ..,--

2t.h.a.-t-~:.. 0 0-- 4-('# I I I .,--~ I I

to 2.0 3.0 '.0 5.0 6.0 10

DENSITY OF COARSE ADOmve. 9 / ~

1.0

Fi9. 7. Dia~am illustratin9 the effect of density of coarseadditives on the .ettlin9 of 2.6\ -37 pm slime ~6J.

which also broke through the slurry too early, similarly failed

to produce any effect on .ettlinq. In contrast, uncoated (and

thus hydrophilic) gla.. beads and quartz particles of the same

.ize did get trapped in the .lurry and were effective in en-

hancing the settling. Thus variou. hydrophilic and partially

hydroph~lic ~erals had approximately the same effect on the

.ubsidence, while totally hydrophobic .ilicone coated glass

beads did not have much effect. This findin9 suqges~ed that

some polar interactions with the aqueous medium is necessary

248

,to hold and trap the coarse particles in the slurry durinq qel-

linq in order that there can be subsequent movements in the

slurry for the creation of tears.

In ~is reqard, it appears ~at addition of certain polymers

to ~e whole slurry or treatment of ~e coarse additives wi~

polymers under certain conditions miqht be beneficial to settl-

inq. The effec~ of polymers co~d possibly be due ~o ~e en-

hanced bridqinq of par~icles ~o clay and resultant better ~rap-

pinq of particles by the slurry. Whe~her such bridqinq occurs

or no~, results of La Mer et ale [28J qiven in Fiq. 8 do show

tha~ addi~ion of polymers can produce siqnificant effects on

settlin9. It is to be noted, however, that while polymers may

increase the initial settlin9 rate, the final solid c~n~.e~t can

even be lower than where there is no such increase, often due

TIME -

Fi9. 8. Sedimentation in the absence (curve I) and in the

presence (curve II) of polymer [28J.

249

to almost permanent entrapment of large quantities of vater

inside the bulky flocs that may form in the presence of poly-

mers. In this respect, the correct choice of polymers and the

proper conditioninq of th. slurry with it, is important.

Recently a polyethylene oxide polymer has been successfully

used by the u.s. Bureau of Mine. for the dewaterinq of Florida

phosphatic clay wast.. both in batch t.ata and in a continuous

test with a trOImDel [29] .

Effect of Air Bubbles-

The experiments conducted to determine the effect of air

bubbles consisted of the application of auction above a suspen-

sion of the slurry. The results obtained are shown in Fig.

Fig. 9. Diagram illustratinq the effect of air bubbles

generated by suction on the settllnq of phosphaticslime [12].

250

Plq. 10. Schematic repres.n~ation of ..tt1inq of sl~es

8howinq princi?a~ reqions [27J.

It can be seen from the results given in Fig. 10 that the gene-

ration of bubbles enhances subsidence significantly. Like the

microbubbles that are ordinarily present in slime, these bub-

bles are also found to act by alterinq ths physical features of

the slurry such that water seepaqe becomes possible throuqh

more channels. It can a~so be seen that suction has no effect

on .ettlinq during the ini~ial ten to twen~y minutes. Also,

once the solids settle to . volume of 25 to 26', mild suction

above the suspension even for lonq periods produces no measur-

able effect. A stronqer suction was found to merely disperse

the sediment. The above observations $uqqest that qeneration

251

parameters than in the reticular stage. A better fit for the

dewaterinq rate has been found to be qiven by the exponential

relation (9) and it represents a moderatinq rate of interaqqre-

qate water expulsion as compared with that durinq the reticular

stalie.

dWadt

~

- -k exp .( w(g)a

This Equation a~so can be rewritten to re~ate ~~e descent of

the interface to settlinq time in the form qiven below:

B(t) . HCt2 - b In(tlt2(10)

Floccular Staqe _~(t3) to - B(-)

When all the water in the macrochannals be~ween aggregates

has been expelled, the end of the vermicular staqe is reached

and furth~r dewatering can take place due to the consolidation

of individual flocs. Water removal at th~s stage, called the

floccular stage, can take place due to two processes: r~ati-

vely fast expulsion of interfloc water in the microchannels

between flocs and relatively slow expulsion of intrafloc water

from within the flocs. The forces involved are qravitational

and opposinq viscous forces. Each of these processes follow

firs~-order kine~ics:

dWf. -kW~- ..

dt(11)

where Wf is now the fraction of inter- and intra-floc water.

2SS

Plq. 11. Illustration of the fit of the phenomenoloqical

model [27 J.

CONCLUDING REMARKS

A detailed description of behavior of slow-settlinq suspen-

sions has been given here along with the mechanism by which

dewaterinq occurs and the mechanisms by which additives help

the dewatering process. The role of certain major factors such

as vessel diameter and the properties of additiv~s has also

been discussed. FurthermOre a phenomenoloqical settlinq model

that satisfactorily represents the observed behavior of slime

has been reviewed. The problem of dewaterinq is very i=portant

as it can be a very costly operation (Table 3). Dependinq on

the technique used, it can cost anywhere up to $50 per dry ton

and this ranqe of expense is indeed ~prohibitively hiqh.

.,~

TABLE 3

COat of Dewatarinq of Sludq.a by Various Hatbods l30J

Obviou81y there is a great need to develo~ new technoloqy and

to achieve this I there is also an .quai need to fully advance

our scientific underatandinq of the.e systems. Much more re-

.earch certainly needs to be done before we can claim sucb .

full understandinq. Detailed and .ystematic experim.nt~ stu-

dies in the areas discussed above. phys~al and chemical alike,

are in dire need of the proper formulation of settlinq model.

and of full understandinq of the ..ttlinq behavior of slow-

settlinq slimes and sludqes. the processinq of which is steadi-

ly qaininq in importance every day.

ACKNOWLEDGEMENTS

The support of the Particulate and Uultipha.~ Processinq

proqram of the National Science Foundation (ENG 78 25213) is

gratefully acknowledged. Authors of Reference. 12, 13, and 27,

published from our laboratory are also acknowledqed

258

REFE~CES

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Wastes, Ann Arbor Sci. Pub., Inc., Ann Arbor, Mich., 1971.

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with Special Ref. to Par~iculate Media, Prentice-Hal~,

EDqelwood Cliffs, 1965.

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1932.

S. Carman P. C. Trans. Inst. Chem. Enqrs., 15,150 (1937)

6. Carman P. C. J. Soc. Chem. Ind., 57, 225 (1938).-7. Mangelsdorf P. C. J. Appl. Phys., 30, 442 (1959).-S. Erqan S. Chem. Eng. Progr., ~ 89 (1962).

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12. Naqaraj D. R., McAllister L. and Somasundaran P. Int. J.Mn1. Proc., 4, 111 (19-;7).-

13. Somasundaran P. and S:esty G. C. Dewaterinq of Slow-se~tlin~

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Techno1oqy, private publication by XIT Research Institute,Chicaqo, 1977.

14. Carman P. C. Flow of Gases Throuqh Porous Media, Academic

Press, New York, 1956.

15. Davies R. and Kaye B. B. Particle Size Analysis Symposium,

Bradford University, ~q1and, 1970.

5, 61 (197l/72).16. Davies R. and Kaye B. H. Powder Tech.

17. Smith T. N. Trans. Ins~. Chaco En~rs., 43, 769 (1965).-18. Davies R. Powder Tech., 2, 43 (1968/69).

-

259

19. Somasundaran P., Smith E. L., Jr. an4 Barris C. C. Effect

. of Coarser Particles on the Settlinq Characteristics of

Phosphatic Slimes, Proc. 1st Int. Cont. on Particle Techno-

logy, Chi~aqo, Auqust 21"-24,1973, I.I.T. Research Insti-

-tute, 144-150.

20. Wadsworth M. E. and Cutler I. B. Kining Enq., !J 830 (1956).

21. La Mer V. K. and Smellie Jr. R. B. J. Colloid Sci., 12,-230 (1957).

22. La Mer V. K. and Smell~. Jr. R. B. J. Colloid Sci., 12-566 (1957).

23. Bramer H. C. and Soak R. D. Ind. EDqnq. Chea. Process

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30

260

ABSTRACT

Properties of slow-settling suspensions have been described

and it was shown that clays when present along with o~her mine-"

rals appear to be responsible for the slow-settling behaviour

of siimes.The mechanism by which additives help the dewatering process

has been discussed. A phenomenological settling model thatsatisfactorily represents the observed phenomena has been

reviewed.

, ,RESUME

On a decrit les proprietes des suspensions a sedimentationlente et on a montre que les &%qiles en presence d' aut res ~e-

raux sont responsables de la sed~entat1on len~e des bones.

On a discute le mecanisme qrace auquel les acditifs aiden~

lea processus de deshydratation. On a passe en revue les

travaux sur le modele phenomenoloqique de sedimenta~ion quidecrit d 'une maniere sufficante le comportement observe des

bones.

ZUSA.~ASSUNG

Die Eigenschaften der lanqsam sedimentierenden Suspensionenwerden erortert und gleichzeitig wird qezeiqt, da! der Ton in

Anwesenheit van anderen Mineralien ein lanqsames Absetzen der

Schlluane verursacht.Diskutiert wird der Mechanismus, infolqe dessen die Beimen-

gen den EntwasserungsprozeB unterstUtzen. Es wird ein Uber-

blick der Arbei ten zum Thema des phanomenoloqischen Modellsdes Absetzvorqanqes geqeben, da& das beobachtete Verbal ten der

Schlimme vollstandig beschreibt.

261

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262