The dewatering of polyclay suspensions

I89

Powder Technology, 7 (1973) X89-204 0 Elsewer Sequoia SA , Lausanne - Pnnted m The Netherlands

The Dewatering of Polyclay Suspensions C. C. DELL

and W. T. H. KELEGHAN

Departmrnrof Mrnrng and Mmerat Sconces, The Unmxwy,

Leeds LSZ 9JT (Gr Bmarn!

(Recewed March 15, 1972, m revised form July 31,1972)

Summary Tlte behaviour of claysflocculated ,vith long-chain pofyelectroiytes (termed poIycZay systems) Ivas studied in the laboratory under both dynamic and equilibrium conditions_ The ultimate degree of delvatering of a giuen ciay Ivas shoxn to depend upon one variable only, the pressure transmitted through solid-solid contact. or “solids pressure”. The rate at lvhich this ultimate condition IS approached, however, depends upon manv variables. One vartable, stirring, lvas stud.+< ,IvJety and lvas slwbvn to hat,e beneficial effecrs both in providing an easy escape route for lvlter and in removing frtcttonal support by the ~rJs of the vessel. A three-regime physical model fir the thickening of polyclav systems is propose 1.

1. INTRt~DUCTION

The ex’remes of thickening behaviour may be typified by granular systems and polyclay systems The term “rolyclay system” is used for brevity to describe a sutpension of clay particles which have been treated with a synthetic, iong-chain, polyeIectrolyte flocculant. In a granular system, the solids may be considered to be either completely in suspension or completely settled. In contrast, the llocs in a poiyclay system settle rapidly to form a loose sediment and further dewatering then occurs through the gradual collapse of the fioc structure The extent of this collapse is determmed by the compressive forces transmitted mechanically by the overlying solids or “solids pressure”. In simple terms, an analogy to the difference between granular and polyclay systems is provided by the difference between the packing properties of hailstones and snowflakes. The former do not compress substantially after falling, whereas the delicate

structure formed by the latter IS progressively destroyed as the weight of overlying material increases For sedimentation purposes, an ideal suspension is defined as one m which an element of sohds settles at a velocity V which depends solely upon the IocaI concentration C. The granular suspenstons encountered industrially show some deviation from ideality. but it is nevertheless possible to predict the performance of conventronal thickeners reasonably accurately by assuming Ideal behavtour. The conventional thickener (Fig. 1) IS a comparattvely shallow vessel of large cross-sectrona! area. It is not possible m such a umt to thicken flocculated coal washer-y tarlmgs (a polyclay system) to the point at whtch further dewatering becomes unnecessary. In recent years an alternative form of thickener_ known as a deep cone, has been developed for tathngs disposal_ In essence such a thickener is simply a deep, steep-sided conical tank (Fig_ 2) Although t” deep cone has proved capable of producmg satrsfactory underflows economically_ the prediction of its performance in the settlement of polyclay materrais is fraught with difiiculty. In such a system, settling velocity does not depend only upon local sohds concentration_ It is also affected by such factors as solids pressure, flocculant dosage, stirring environment and wall support. There would appear to be three possible solutions to the problem of predicting thickening behaviour in deep cones These are: (1) The acquisition of operating experience from measurements made on existmg full-scale cones (2) The constructton of a continuous, pilot-scale cone to which a simple scale-up factor could be applied. (3) The derivation of a complete model of thickening behaviour from laboratory tests A rehable design method based upon simple

190

C C DELL,

feed well

Fig

1 A

Feed

contmuous

thickener

’ P

2.3 Dynamzc sedimezztatiozz experiments In these experiments, samples were taken durmg settling, by means of a simple coring technique, in order to build up a picture of the distribution of flocculated solids withm the sedimentation vessel at a particular time. A number of such runs under identical Initial conditions enabled a set of solids dzstribution curves to be constructed_ Sets of curves were derived for both the stirred and unstzrred conditions

1

Pad--E-~ L---j--~----5”pply

All-

3

EXPERIMENTAL

3.1 Equzlibrizmz

laboratory tests would clearly be preferable to the lirst two expedients. The question of relating batch settling experiments to the performance of deep cone thickeners will be discussed later in thrs paper.

2. EXPERIMENTAL

KELEGKi?:

2.2 Equilibrizzzn sedinzezztation experzmezzts In these tests, polyclay systems were allowed to settle until equrhbrium was reached, either with or without slow stirring After equilibrium had been attained, samples were taken in order to discover the distribution of solids within the sediment. AU sedimentation experiments were earned out in a perspex settling coluznn (50 znm diam.). Either a single vertical aluminium rod (13 mm dram.) or combinations of thinner silver steel rods (4 mm diam.) were used for stzrnng purposes.

Inlet

\

T H

2.1 Studzes of tire equilibrium dewaterizzg of polyclu~ sediments under applied suction Polyclay samples were subjected to varying degrees of suction until equilibrium was reached (6 hours) and their residual moisture contents were measured The filtration process was carried out on a sintered glass funnel using only a very thin cake to ensure that the compression effect from the weight of the solids in the cake itself was negligible compared with that due to the applied suction

I

conventional

W

METHODS

(see also Append=

1)

The experimental work considered herein1 can be classified under three general headmgs.

RESULTS

dewaterirx

under applied

suction

Lf residual moisture content, expressed as the volume of sediment containing unit weight of solids, or dilution, D, is plotted against applied suction S on log-log scales, it is found that a substantially linear relationship exists over two orders of magnitude for each combination of solid material, polyelectrolyte and flocculant dosage used (Figs. 3-6). It should be noted that nezther the type of flocculant nor its dosage have any appreciable effect upon the slope. or position of the lines, but that the type of clay does have a profound effect. The minor effect that flocculant type and dosage have upon compressibility is at first sight surprising,

DEWATERINC

OF POLYCLAY

5

SUSPENSIONS

103

2 Applmd Strctoon

Ftg

3 EITect of flocculant type upon

5

Fg.

103

191

the filtratmn

2

1+

5 (

Id

2

1

behawour

of

5

Wheldale

10‘

shale.

2

5

Appllrds~ctlrm( WlJ)

4. Effect of flocculant dosage upon the filtratmn behawour of Wheldale shale.

since it is a well-established fact that high dosages are conducive to easy dewatering It must be remembered, however, that these tests all refer to equilibrium conditions and that the results are independent of the rate of water expulsion, which would obviously be important in the operation of a practical thickener_ High dosages,_ if they result in

the for-matron of larger floes, might well lead to an easier Quid escape route without altering the strength of the ffoc structure. The implicatron of this work IS that in deep cone thickeners a certain theoretical minimum amount of solids is required to bring the dilution of the underflow down to a certain value. By making two

C C DELL,

Ftg

5. Effect of flo~~uiant

Fig

6 Comparison

type upon

W. T_ H

KELEGHAN

the filtratmn behaviour of DevonshIre ball clay

of the fiitration behawour

of Wheldale ShaleandDevonshlreball

simple assumptions it is possible to derive fkom the corresponding filtration relationship an expression which relates the height h of a column of sediment at equilibrium with the dilution at its base. For example, if the suction relationship for a particular material is given by

Cay.

D=KISX then D = K2h-Xt(1--r) where K2 is a constant defined by the density of the material and K, and x, its suction test character-

DEWATERING

Fig

10

Fig

7

OF POLYCLAY

Companson

-3

8

Companson

193

SUSPENSIONS

of equihbrium dewatenng results for 200 BSS Wheldale shale

I

I

5

I

of equdlbnum

I

I 2

103

lpplwd

kesrare

( N/m2 ]

dewatering results for IO0 BSS Howden Clougb clay

istics (see Appendix 2). The implicit assumptions in such a derivation are, firstly, that floe breakdown in sedimentation is caused solely by solids pressure and, secondly, that at equilibrium the overlying solids exhibit their maximum theoretical solids pressure. 32 Equilibrium sedimentation results Seen equilibrium sedimentation

data,

in

which the dilution at the base of the column is plotted against the maximum theoretical solids pressure, are illustrated in Figs. 7 and 8. The corresponding filtration line 1s shown for comparison.. It will be seen that the pulps which have been stirred produce points which lie on or slightly above the filtration line, while those which have not been stirred give points well above the line. When the assumptions of the previous subsec-

C. C DELL,

194 Solldr

Snmplc

-200

0

Dorogr stwng

-3

I

10

10

10

Ftg. 10

Dtlutmn/sohds

5 kgltonnc Slnglr . 112’

danmeter

I

I

pressure distributron

wthm

I

I

I

2

5

10Z

pressure distnbutton

rod

I

5

I

2

10)

S0,,.3* PIr*r”rL (*In?1

9. Ddution/sobds

-31

Shale

365 P

Conllgurnlloll

2

lo2

Fig

B S 5. Wheldnlr

Polyllot

Pollrlrclrolllr

W. -I- H. KELEGHAN

a stlrred equtbbrrum sednnent of Wheldale shale.

I

I

2

5

Soltdr

Prrsrsrr

I Wnzl

withtn H stirred equibbnum

tlon were examined it was found that the floe breakdown is, to all intents and purposes, caused solely by compression_ Direct impact of the slow-moviqg, stir-r-fugdevice and the effect of the shear forces play comparatively minor roles. However, the second assumption, namely that at equilibrium overlying solids exert their maximum,

I

lo3

I 2

sednnent of Howden Clough clay

theoretical solids pressure, is not necessarily true as wall support and inter-particle bridging occur in the absence of stirring. The model of an equilibrium sediment, outlined in the previous subsection and based upon filtration Ata, wi’; only bc valid if the sediment is formed in the presence of slow stirring and if the sphere of influence of the stirring device is

DEWATERING

OF

POLYCLAY

SUSPENSIONS

sufficient to prevent bridging throughout the settling vessel. Further evidence of the dominant role of sohds pressure rn sedinl<.;_+ formatron is given by Figs 9 and 10. If the mecha-usm of sediment formation IS one of pure compression and the maximum theoretical solids pressure i~ exert& (as has been postulated for stirred sedrmentation using a smgle, large stirring rod), the solids distribution data for all sediments of a given material free from segregation effects should be coincidental with the filtration line for that material and its projection to pressure values lower than those obtained experimentally. Although there are slight differences in slope, the proximity of the two sets of data in Figs. 9 and 10 represents an encouraging agreement between theory and experiment. The solids distributions within the stirred sediments are similar to the distributions which would have been predicted from simple liltration tests. In contrast, the unstirred equilibrium sedimentation results of Figs. 7 and 8 suggest that, beyond a

195

certain pomf the dilution generated at the column base becomes insensitive to the total weight of sohds in the column. For example, in unstirred sedimentation tests using --2OO B.S.S. Wheldale shale, it appears that dilutions lower than approxrmately 1.85 x 10e3 m3/kg are not formed (Fig 7) When the amount of solids in the column is greater than that necessary to generate thus mrnimum dilution value, a zone of almost constant dilution is formed in the lower part of the equilibrium sediment (see Fig. 1 I, a dilution profile representing the sediment from which the circled pomt in Fig 7 was derived) This phenomenon is attributed to arching effects similar to those commonly observed in work with dry powders2_ The polyclay structure begins to break down when subjected to compression until a point is reached at which any further increase in the pressure exerted by the overlying solids is transmitted directly to the walls of the settiing column ln this way a zone of constant dilution is propagated upwards from the base of the vesseL

196

C

Unstn-ed

expenment

WI

s.twred

experiment

(9 5

//

8.:,1 L 1c

u

5

0

S

FIN

13. SohIs

descent

paths

for

experiments

U

and

S (note

s

dtfferences

C.

in ordmate

scale)

DELL,

W

T.

H.

KELEGHAN

DEWATERING

OF POLYCLAY

197

SUSPENSIONS

3.3 Dynamic sedimentation results A pair of specimen solids distribution curves are shown in Fig. 12 While concentrations remain low, the downward velocity of the mud-line is virtually independent of the presence of stirring. This does not, however, indicate an identical situation within stirred and unstirred suspensions during the early stages of settlement The more pronounced curvature of the ‘*stirred” curves shows that the propagation of higher concentrations (lower dilutions) in the lower regions of the column begins almost immediately. As concentrations throughout the column begin to rise, the effect of stirring upon the rate of settling becomes increasingly apparent. The advantage of mild stirring is illustrated by the solids descent paths of Fig. 13 and the concentration profiles of Fig. 14. The solids descent paths show the settling behaviour of individual solids elements from various parts of the initial suspension (a solids element being defined by its numerical value on the abscissa of the solids distribution curves) and demonstrate that gentle agitation aids sedimentatlon rates in all parts of the suspension. The concentration profiles show the distributions of concentration throughout the settling column at selected moments and illustrate directly the higher concentrations generated in a stirred environment. It is believed on the basis of the work outlined herein that the main function of slow stirring in thickening is the promotion of escape channels for

the returnmg fluld This view is supported by experimental evidence. Observation of the conditions at the column walls revealed that well-channelled sediment structures were formed in both stIrred and uns:,rred test,, the effect being more marked in the stirred case. Powever, it was the behaviour in the region of the rod itself which was of greater Interest. From the commencement of stirred sedimentation tests, a sigmticant upward flow of liquid through a relatively loose, uncompacted zone in the “wake” of the large stirring rod was apparent. A simple experiment using coloured dye demonstrated the importance of this effect. Dye, Injected into the lowest layers of the suspension, appeared above the mud-line at what was effectively the rear of the stirring rod within G very short time.

4 A MODEL

OF BATCH

SETTLING

Coe and ClevengeP believed thaf at the crItIca settling point in a batch settling test, some degree of particle-particle contact existed in all layers of the suspension, with the particles at the suspension boundary just touching those immediately below them They postulated that further dewatering could only occur through compression of the sohds. However, in view of the present work, it is not accepted that all the solids are necessarily in com-

E

DEWATERING

OF POLYCLAY

199

SUSPENSIONS

cx Fig

17 Concentration

prolile for a

sedtment

Concentrotlon of sohds

at eqmhbrmm

For a polyclay system, an equilibrium concentration profile of the form shown in Fig 17 is implied by the theoretical model of Section 3.1. C, IS the concentration at which compresslon effects (2-e. solids pressure) first become important. The acceptance that pressure caused by physical contact between floes is the cause of floe breakdown suggests that, in the region where there is no solids pressure, the floes should behave as if they are rigid particles. This being so, the concentration profiles obtained experimentally should be a hybrid of the two theoretical forms The point at which compression effects become zi~gnilicant should be indicated by a point of inflexion in the curve (C,= CX) Experimental profiles, specimens of which are shown 111Fig. 14, appear to confirm the postulates of the precedmg sections. They sve credence to each of the settling regimes proposed. Profiles U5 and S.5 illustrate clearly the existence of an upper zone of constant composition. The upper layers of the suspensions had remained effectively at their feed concentration. Inspection of all the profiles obtained expertmentally revealed that a point of inflexion of the typz predicted became apparent whenever the concentration rose appreciably above 400 kg/m3 (e.g. U60, S60). At all concentrations greater than this, the curves had a shape consistent with dewatermg by compression. Comings et aL4 and Scott’ have

produced curves :vhlch appear to confirm the existence of such an inflexlon point In the retarded settling zone. as there is no contact there can be no floe breakdown and the floes behave as single particles. The existence of this zone beyond the critical point of Coe and Clevenger may explain why Fltch6 was unable to find evidence of mechanical support in their so-called “compression regime”. 5

CHANNELL!NG CLAYS

IN THE SEDIMENTATION

OF POLY-

It is a widely accepted view, supported by the present work, that the upward flow of water from a settling polyclay system is far from uniform It occurs m upward movmg streams of channels rather than uniformly past each particle as might be expected with an idcal suspension The escape of liqmd from a particular level in the retarded settling regime will depend upon the hydrostatic pressure exerted by the solids above rhar level It is possible to visual& a situation in which the thickening material behaves as a Bingham plastic under conditions of low shear (no floe deformation) and dlsplaced liquid escapes & channels through the material in whlzh rhe flow is Newtonian As very few particles are free to move into the channels, the pressure within them is lower than in the main body of the suspension and the flow of hquid mto them IS maintained.

C. C

According to the Hagen-Poiseuille law7, the volume flow through a capillary is proportional to the fourth power of its radius. It might be expected, therefore, that for a given average porosity, the greatest flow of liquid from a settling suspension will occur when the packing of floes is uneven, and the least will occur when it is uniform_ A wellchannelled structure would be expected to dewater more rapidly than an unchannelled one It has been shown experimentally that slow stlrring promotes a well-channelled structure and aids dewatermg rates.

6 THE ROLE OF STIRRING IN POLYCLAY TAT1 ON

SEDIMEN-

Investlgatlon of the effect of slow stirring in batch tests has shown that its presence facilitates the escape of liquid from the retarded settling and compression regimes. By so doing, stirring enhances settling rates and puts more material into compression in a given time, thereby producing higher concentrations at the base of a batch test cylmder As an example, consider the concentration profiles for runs U6O and S60 (Fig 14) The arca bounded by a concentration profile and a horizontal line drawn at the mud-line value for the particular run is a direct measurement of the total amount of solids in the settIing column A horizontal line drawn to cut a profile indicates the amounts of material above and below a certain concentration Examining profile U60, it will be seen that approximately ha!f the solids have reached the compression regime and that the concentration at the base of the column has risen to 525 kg/m3. However, profile S60 reveals that in this cake nearly all the solids have entered the compression regime and that the concentration at the base is considerably higher, namely 670 kg/m3_ Mild stirring will play a similar role in an operatmg deep cone thickener. By promoting rhe upward flow of liquid, the presence of stirring will produce greater compressive forces in the cone under any given operating conditions In terms of thickener operation, the use of a stirring device will improve performan= in one of two ways Either (1) it will enable the throughput rate of the thickener at a given underflow thickness to be increased, or (2) it wiIl improve underflow thickness at a given throughput rate.

DELL,

W T

H

KELEGHAN

A number of points must be made regarding the use of slow stirring in practice. If the stirring rods are too narrow, an unthickened zone of palyclay may tend to remain as a relatively undisturbed mass with the stirring rods cutting through it like a knife through butter On the other hand, with comparatively thick material, the entire mass could re rolve as a single body, sliding along the walls of the cc-e. To be effective, the stirring device must deform the thickening system, and it is therefore probably advisable to use a combination of moving and stationary rods Ideally, the stirring arrangement should be designed in such a way that all parts of the cone dre stirred regularly and steps should be taken to ensure that there 1s not a large discrepancy between rod velocities at the periphery and centre of the cone

7 THICKENING

IN DEEP CONES

7.1 Deep cone operation Traditional thickening theory suggests that depth should play no part, but it has been shown m practice that deep cone thickeners are of value in the treatment of polyclay suspensions’. Two distmct thickening zones may be observed in an operating deep cone, namely a dilute zone and a mud zone. It is probable that the mud zone encompasses both the retarded settling and compression zones observed m batch tests, although there is insufficient evidence to say whether there is a distinct boundary between them in an operating thickener. However, It can be said with certainty that the compression zone must exist if a particular concentration is exceeded (see Section 4). As thickener height is increased, the underflow concentration which is theoretically possible is also increased. This must be so since the maximum solids pressure which can be exerted in sedimentation is directly proportional to the amount of overlying solids However, the depth of solids alone does not determine the solids pressure produced in operation. The compressive forces generated are also a function of the residence time of the material. If the residence time of material m a given cone is increased (by reducing feed and underflow rates) and mud zone height is maintained, t%e compression regime will extend upwards Solids will therefore spend longer in compression and will be subject to greater mechanical forces This will lead to the production of thicker underflows. Conversely, a

DEWATERING

OF POLYCLAY

201

SUSPENSIONS

decrease in retention time wiIl produce a shallower layer of compression within the mud zone and underflow thickness will be reduced. 7.2 The prediction of tkrckener behaviour from laboratorJ7tests The assumption of ideal behaviour, namely that settling veIocity and particle flux depend solely upon local solids concentratton’, is implicit III the traditional design methods for continuous thickeners It is not possible to apply such a simplified concept to deep cone thickening Factors other than concentration influence settling velocities at the higher concentrations present within a deep cone; such factors include solids pressure, residence time, particle bridging, thickener depth, the presence of slow stirring and stirrer speed. To predict thickening behaviour, it is necessary to he able to build up a compiete picture of condittonr. within the vessel It is concervable that the mteractions (if any) between indivtdual variables couId be examined in a factorially-designed batch setthng experiment. However, such an examination would be of little value since it IS not possibIe to generate the high concentrations present in deep cone underflows in the laboratory_ This point will be emphasised by the figures cttcd in Section 7.3. There IS, therefore, no simple means of predicting the throughput rate of an operatmg deep cone from small-scale batch tests. Although the dynamic settling condition has many variables, it has been shown that the final state of the sohds in an equilibrium filtration test depends only upon the pressure appIied. It is thus possible to set the limit of performance of a deep cone by means of a laboratory test. Unfortunately, the equilibrium condition IS not approached in operating thickeners. 7.3 The ~imitatiortsof deep cones It has been shown that the performance of a deep cone thickener depends upon the compressive forces actmg within the mud zone Both the presence of slow stirring and an increase in residence time facilitate compression and lead to the production of thrcker underflows However, there is a limit to the performance of a thickener which is set by the size of the vesseI. The logarithmic relationship between dilution and applied pressure is such that, in a thickened sediment, a very larse increase in overlying sohds is required to produce a comparatively small decrease in dilution. An increase in overlying

solids is synonymous wtth an increase in thickener size The hypothetical calculatrons which follow show that the production of extremely thick material from a deep cone is unthinkable simply because of the scale of vessel which v,ould be necessary. Consider the de-watermg of flocculated samples of Howden Clough clay in filtration tests (Fig 8) These experiments gave an equation of the form

By suitable manipulation, this equation gives a relationship between sediment height h and dilution D at the base of such a sediment which is of the form D = 9.95 x 1O-4 11-O 15-/ Substituting the correct value for D in this second equation, it is found that the mimmum height for the formation of material containing 65% soltds by weight (deep cone underflow consistency) IS 1 54 m. Likewise. the minimum height for the formatton of material containmg 800/bsolids (filter-cake consistency) is 16 6 m (54 ft.), a tenfold Increase. The height which would have to be provided In practice to produce an underflow of SO0? solids would be even greater than the quoted value (possibly by a factor of 3 or 4), because experience shows that deep cones operate at a point considerably short of equihbrmm. Further difficulties would undoubtedly arise from the discharge of such stiff material as wall support and arching would increase progressively with increasing underflow concentration It would appear. therefore, that the greatest promtse lies 1~1better control, so that cones may operate for a greater proportion of the time under conditions which at present they reach cnly occasionally.

8 CONCLUSIONS

The followmg aspects of sedimentation behaviour have been shown to be characteristic of polyclay systems. (1) The ultimate degree of dewatermg of a polyclay system IS governed by a linear logarithmic relationship relating pressure applied to residual moisture content, expressed either as dilution or concentration (2) A polyclay suspension, after Initial free settlement, forms a structure which is channelled.

C C DELL.

202 These channels assist the escape of displaced hqurd. (3) The escape of fhnd is further assisted by slow strrrmg which creates a loose, broken zone in the wake of the stirring rod. (4) The presence of gentle stirring removes static fraction at the walls of the setthng vessel. At hrgh concentrations, 111the absence of stirring, particle bridging occurs Floes collapse until the particle arrangement 1s such that any further increase in the pressure exerted from above is transmitted directly to the vessel walls Beyond this point, no further reduction in dilution occurs (5) The rare at which the floe structure collapses is a function of many variables, m contrast to the .rlrunate degree of collapse which depends only upon the pressure apphed. To predict plant performance from laboratory tests would therefore involve either simulating operating conditrons rn all respects in the laboratory or constructmg a complete physical model of the rystem Neither of these problems have been solved. (It will be remembered that, tn a granular system, settling rate is rektted to concentratron alone ) (6) Three regimes of settling may be observed _ (i) An Uppi_ free-settling zone of constant compositron. (II) A retarded settling zone of variable composition. (iii) A compression zone of variable compositron in which floe breakdown occurs For a given solid, the boundary between the second and thud regimes occurs at a substantrally constant concentration

ACKNOWLEDGEMENTS

REFERENCES 1 W T H Keleghan, The effect of sturmg upon the dew.mzrmg propertiesof ftocculnted clay suspensions. Ph D Thcsrs. Umv of Leeds. 1971 2 G A Turn-r and M E Fayed, Sedimeritatlon of line powders m d~r. Pouder Technol, 4 (1970/71) 241-249

T H. KELEGHAN

3 H S Coe and G H Clevenger, Methods for determinmg the capacltles oi shme-settlmg tanks. Trans Am. Ins? Mrnrng ,‘tter Engrv, 55 (1916) 356 4 E W Commgs, C E Pruisse and C. De Bord, Contmuous setthng and thlchening, Ind Eng Chcm, 46 (1954) 1164-I 172 5 K J Scott, Contmuous thvzkenmg of flocculated suspensions. Ind Eng Chenz Ftrndonrentals.9 (1970) 432427 6 E B Fitch. Sedlmentatlon process fundamentals,

_ Trnrls Am

Ins? ,%l~nEngrs. 223 (1962j 129 7 J M Kay. An Introductro,r to Flurd Mechonzcs and Hrar Transfer, Cambndge Umv. Press, Cambndge, 1963, Chapter 6 8 G H Arrowsmtth, Flltratmn and dewatermg m the coai Industry, F‘r~rmtron Scparorron. 8 (1971) 413417 9 G J Kynch, A theory of sed~mentatmn, l-runs Faradq Sot , 4s (1952) 166-176

APPENDLX (A)

Solrd

1 EXPERIMENTAL

DETAILS

nzaterrais

Crushed shale from Wheldale Colliery, Castleford, Yorkshire_ (ii) Dried ball clay from Devonshire (iii) Clay sample from Howden Clough, Morley. Yorkshire (1)

(B)

Polyelectrolytes

Polyflok 365P, manufactured by Yorkshire Chemrcals Ltd., Kirkstall Road, Leeds LS3 ILL. (11) Magnafloc R156, manufactured by Allied Collards Ltd , Low Moor, Bradford, Yorkshire_ (in) Flocbel FCl80, manufactured by Float-Ore Ltd , Apex Works, Willowbank, Uxbridge, Middlesex These three chemicals are all aniontc polymers normally produced by the hydrolysis of polyacrylamide or polyacrylonitrile. (1)

(C)

The

(i)

The authors thank Professor P. A. Young. Department of Mining and Mineral Scienc. s, Leeds University, for helpful discussion A&nowledgement is also made to the National Coal Board for a grant in aid of this research, but the views expressed are those of the authors and not necessarily those of the Board

W

preparation

Choice

of

of

suspensions

coaguiant

Calcium chloride was chosen as coagulant for this research programme. Preliminary batch settling tests indicated that 40 cm3 of OS o/0calcium chloride solution per 50 g of dry solids in suspension produced clear supematant liquid_ Choice of polyelectrolyte On the basis of the consistency of its performance m a factorially-designed batch settling experiment, Polyfloc 365P was chosen for use m this work except where another llocculant was necessary for purposes of comparison. (ii)

(iii)

Flocculation

The preparation of a flocculated suspension of

C C. DELL, W. T. H KELEGHAN

204 Substituting

-

1

=

s, v

for D in eqn. (I),

for P from eqn. (3):

(&&T/)1-x

K,p-==

= KDh’l(1-X)

which simplifies to give

v=

p‘

(3)

S,K,

Substituting for V in eqn (2), dP

Substituting

=

g-8

h = KB

1CC- &J P” _ S&A

&ir’/”

--xl

Expressed in terms oi ddution, becomes

this relationship

dh

I

dP F = KCP1-=

Therefore p = n’,JI’”

v=

-X)

K, is a constant de&xxi by the density of the material in question ard by its suction test characteristics. KF=

KA

(

S,E,

9sl(sS-ss,)(1-x)

X,(1-*)

>