Applied Mineralogy Unit, Institute of Geological Sciences...

Clay Minerals (1983) 18, 33-47

L A B O R A T O R Y S E P A R A T I O N O F C L A Y S

B Y H Y D R O C Y C L O N I N G

J. A. B A I N AND D . J. M O R G A N

Applied Mineralogy Unit, Institute of Geological Sciences, 64/78 Gray's Inn Road, London WCIX 8NG, UK

(Received 14 October 1982; revised 12 November 1982)

A B S T R A C T: Although seldom appreciated, the hydrocyclone offers several advantages over the more commonly used sedimentation/decantation method for clay separation. Large amounts of relatively concentrated clay suspensions may be separated in a matter of minutes. In addition, no dispersing agents are required and the shear conditions encountered by the suspension during passage through the hydrocyclone lead to particle disaggregation both in the silt and sub-micron size range. Formulae relating design factors of the hydrocyclone to separation performance exist but are of limited value as they are unable to predict the maximum particle size of the clay product or percentage recovery of any particular size of particle. Such data, determined from practical trials, are given for kaolinite, smectite and mixed- assemblage clay suspensions. Design limitations prevent the effective separation of clay products with 'cut-off' sizes finer than 5 #m.

The hydrocyclone, as a vehicle for sol id- l iquid separations, has a range of uses which vary from de-watering and de-sliming to particle-size fractionation. Its fast throughput of slurry and capaci ty for dealing with suspensions of high solids content are exploited in various mineral dressing operat ions where removal of clay and other fines is a necessary preliminary to t reatment of cleaned or washed sands. Where the clay itself is the principal product of a mineral processing plant, the equipment offers a convenient means of effecting a c lay/s i l t / sand split which yields a clay-rich fraction in a suitable state for fine-particle treatment.

Its simplicity of operat ion and lack of moving parts (other than a slurry pump) may be exploited in small-scale or low-technology processing where, if necessary, power can be supplied by a petrol engine and the equipment made portable (Fig. l a ; see also Kiff, 1977). In prospecting operat ions, the slurry pump can even be coupled to the secondary drive of a Landrover or similar vehicle (Fig. lb).

For precise particle size separat ion there are a number of constraints on its operat ion but as a tool for clay investigations in the labora tory several advantages are offered which seldom appear to be appreciated. Also, although basical ly simple in design, a number of interrelated parameters affect the performance of the cyclone and these are not easy to assess or manipulate. This paper provides an appraisal of the technique for separating clays in compar ison with other sedimentation methods and describes the results of a number of separat ions obtained from small l abora tory hydrocyclones.

~) 1983 The Mineralogical Society

34 J. A. Bain and D. J. Morgan

FIG. 1. (a) Portable cyclone rig (petrol engine). (b) Portable cyclone rig (Landrover drive).

D E S C R I P T I O N OF T H E H Y D R O C Y C L O N E

The hydrocyclone is shown in diagrammatic form in Fig. 2. It consists of an inverted cone, open at the bottom but extended upwards by a short cylindrical section closed at the top. The latter is penetrated by a hollow tube, the vortex finder, which passes down through the cylindrical section at its centre. The cylindrical body also carries a circular or rectangular orifice through which feed suspension is introduced--tangentially---into the vessel. The resulting centrifugal force imposed on the suspension causes the coarser particles, together with a small proportion of unclassified feed, to travel towards the wall of the vessel and describe a spiral descending path to the apex, where they are discharged (the underflow product).

The presence of the vortex finder causes a second, inner, vortex to form; this contains an air core, rotating in the same sense, but moving in the opposite direction to the outer one. Finer material is retained within this inner vortex and is discharged, together with the bulk of the water, through the vortex finder outlet (the overflow product).

Separations described here were obtained with two types of laboratory hydrocyclone.

Clay products by hydroeycloning

�9 OVERFLOW

I FEED �9

rmder

~ ~ ...... ~ c'/clone body 0 ~ included angle)

l apex ONDERFLOW

FIG. 2. S c h e m a t i c d i a g r a m o f c y c l o n e ; i = inlet, v = v o r t e x f inder .

35

A small glass model, produced by Liquid-Solid Separations Ltd., consisted of a 3 cm diameter pyrex body with a standard inlet, to which a series of vortex finder and apex nozzles could be attached by means of springs (Fig. 3a). For controlled separations, feed suspension was supplied by a ] h.p. Mono slurry pump operating through a by-pass valve which allowed the use of a range of pressures from 0-5 to 2.5 kg/cm 2 and ofthroughputs from 4 to 12 litres per minute. Feed density was usually in the range 5-10% solids by weight. For routine purposes, smaller quantities of suspension, with total volumes down to 250 ml, could be dealt with using a 1.5 cm diameter cyclone in conjunction with a small centrifugal pump (Fig. 3b).

For pilot-plant tests, use was made of a 4.5 cm diameter cyclone manufactured by Richard Mozley Ltd. from moulded polyurethane and supplied with push-fit sections for all orifices. This was serviced by a variable-speed pump drawing slurry from a conical sump to which the products could be returned in order to use the system as a test-rig in closed- circuit operation (Fig. 3c). A typical throughput at a pressure of 3.5 kg/cm 2 was 50 litres of suspension per minute.

S E P A R A T I O N T H E O R Y

As in all particle settling operations, the rate at which a specific size of mineral grain travels through a liquid medium is related both to the difference in density between them and to the viscosity of the liquid (ignoring the particle 'shape' factor). For a given size of hydrocyclone, the pressure at which the slurry is pumped through the vessel obviously governs the centrifugal force exerted on a particle in suspension within it and, therefore, the speed with which it travels to the walls in order to be expelled with the underflow discharge. This is the main operating control.

In practice, a number of design factors affect the performance of the cyclone. Length, diameter and conical angle of the vessel itself, together with inlet, vortex finder and apex nozzle diameters, are all important. Bradley (1965) has collated formulae derived


(a) (c)

(b)

FIG. 3. (a) 3 cm diameter cyclone under closed-circuit operating conditions. (b) 1.5 cm diameter cyclone in operation. (c) Sampling of overflow and underftow products from 4.5 cm

diameter cyclone.

Clay produets by hydrocyeloning 37

empirically by various workers which relate the parameters of the cyclone to an 'equilibrium' particle size ds0----defined as the size at which half the particles report to the overflow and half to the underflow.

The formula given below is based on that derived by Dahlstrom (1949), assuming a mean specific gravity of 2.65 for solid particles suspended in water of density 1.00.

47(iv) ~ dso - FO.5 3

ds0 = diameter of particle (#m)

i = inlet diameter (cm)

v = vortex finder diameter (cm)

F = flow rate (litres/min)

From the formula it is seen that dso may be decreased by diminishing the diameters of inlet and/or vortex finder but, by increasing back pressure within the vessel, this also tends to restrict the flow. The greatest effect is produced by changes in flow rate, reductions in ds0 being obtained by increasing throughput (Naylor, 1958). The limiting factor is feed pressure, which increases roughly in proportion to the square of the flow rate. A feed pressure of about 3.5 kg/cm 2 is generally accepted as a working maximum.

Finer separation sizes are also achieved by using smaller cyclones, but, as the minimum size of particle which can be removed varies with the square root of the cyclone diameter, there is a diminishing return in passing to successively smaller vessels. The shorter residence times in small cyclones may also have a marked effect on the separation efficiency as there is a greater potential for short circuiting of particles and inter-particle interference. This effect becomes increasingly important at particle-size separations < 10 gm, and particularly below 5 gm where the technique is of most interest for clay fractionation.

An expression for the diameter of the apex does not appear in the formula. In general, if this is large enough to allow free discharge of coarse material it has only a minor influence on ds0. During correct operation of the hydrocyclone the underflow leaves the apex in an umbrella-shaped discharge incorporating a hollow core through which the air column passes. I f the feed rate is insufficient (i.e. pressure drop falls below an operating minimum) or the slurry is too concentrated, the underflow forms a ropey discharge which can force some of the coarser particles to divert to the overflow. Short-circuiting of coarser particles can also occur if the apex diameter is too small. The relative volumes of water and suspended solids reporting respectively in the underflow and overflow products is a function of the comparative cross-sectional area of the apex and vortex finder orifices, allowing for the presence of the central air core.

The operation of the Dahlstrom formula is illustrated in Table 1, using data from measurements of flow rates with a small 3 cm diameter cyclone fitted with a fixed 0.4 cm inlet and 0.25 cm apex nozzle; changes were made in vortex finder diameter and feed pressure. The relationship between flow (in litres/min) and pumping pressure (in kg/cm 2) was found to be of the form F = kvffi-where the constant k was approximately 8.1, 7.0 and 5-6 for vortex finders of diameters 0.85, 0.60 and 0-42 cm.

Calculated equilibrium particle sizes (dso) are given for each set of conditions, with the exception of the lowest feed pressure operating with the 0.85 cm vortex finder (where the


TABLE 1. Calculated equilibrium particle sizes (dso) for different operating conditions; 3.0 cm cyclone with 0.40 cm inlet and 0.25 cm apex nozzle. Flow

rates and volume discharges were measured with water only.

Calculated dso (~tm) for feed pressures (kg/cm) of

Vortex diameter Volume ratio (cm) o/f:u/f 0.75 1.25 1.75 2.25

0.42 2.4:1 5.9 5.3 4.9 * 0.60 8:1 6.7 6.0 5.5 5.1 0.85 17:1 t 7.0 6.5 6.1

* Vortex finder retaining springs failed. I" Unsatisfactory apex discharge.

apex discharge was unsatisfactory) and the highest feed pressure with the 0.42 cm vortex finder (where the retaining springs were forced apart). The ds0 values vary as expected, although, at the small particle sizes involved, the scope for flexibility is limited. A significant feature of the results is the major change in volume ratios of overflow and under tow discharges brought about by variations in vortex finder diameter.

Although such theoretical calculations are valuable for predicting the effects of changes in operational parameters, they are of limited use in assessing the true working performance of the hydrocyclone. This becomes particularly apparent where the efficiency or sharpness of the separation needs to be known or where there is a requirement for a product with a specific 'cut-off ' size, e.g. an overflow product with a limiting upper particle diameter. In addition, the various formulae proposed do not take into account the particle-size distribution of the feed itself--which undoubtedly has a quite distinct effect on the separation characteristics.

An operational analysis of a hydrocyclone must therefore be based on actual separation trials from which measurements of throughput, volume splits and solids concentration can be made and products collected for determination of particle-size distribution. The examples which follow have been chosen for their illustrative value in demonstrating the potential of the technique and its limitations or disadvantages.

S E P A R A T I O N T E S T S

Fractionation of coarser clays

Fig. 4 reproduces curves for the size analysis of products recovered in a 'china clay' separation of material from a deeply-weathered granite profile in Sind Province, Pakistan. In this case, full recovery of kaolinite was inhibited by the severely indurated and cemented nature of the raw material and effective dispersion of the clay in water had to be aided by attrition scrubbing. Separation of kaolinite was attempted solely by hydrocyclone treatment of a <250 pm (60 mesh) slurry after removal of the bulk of the quartz sand by wet sieving (to prevent blockage of the hydrocyclone orifices).

The 4.5 cm diameter cyclone was used with settings chosen to provide an equilibrium particle diameter (ds0) of 10 pm. A suspension containing 10% solids by weight was passed through the vessel, producing overflow and under tow products bearing 4% and

Clay products by hydrocycloning 39

# �9 ~ / .90 I s / ~ / 180

, ~ ,70

i . 6 o .

@ "30 ~

J | J

,20

,10

2 5 10 20 50 100 200 particle size, microns

FIG. 4. Particle size distributions of Pakistan china clay hydrocyclone products. Data on <250 (60 mesh) suspension. (~) original sample, @ recalculated from products (see text), #m

@ overflow product, @underflow product. Operating parameters: feed pressure 3.5 kg/cm 2, inlet diameter (equivalent) 0.95 cm, vortex finder diameter 1-40 cm, apex nozzle diameter

0.50 cm.

34% of solids respectively. To improve recovery of fines, the underflow was diluted to the original consistency and re-treated, thereby increasing the proportion of feed collected as overflow material from 45 to 60%. Curves 2 and 3 in Fig. 4 indicate the particle size distributions of the combined overflow products and the final undertow, respectively.

It can be seen that the overflow product contains 7% of particles coarser than the ds0 diameter of 10 #m, some ranging up to about 20/~m in size. In practical terms, this can be viewed as a 'cut-off' diameter of 20/~m with 93% of the material finer than 10/~m and 64% finer than 2/~m.

A comparison of the relative weights of the two products indicated that 94% of the (10 /~ m particles present in the feed were recovered in the overflow and only 6% lost in the undertow, the bulk of this loss being accounted for by ultrafine <2/~m particles (see curve 3). This is unseparated material carried down in suspension with the water discharging through the apex.

Reference to curves 1 and 1A in Fig. 4 illustrates a further point of interest about the use of the hydrocyclone for treatment of this particular china clay. Curve 1 represents the particle-size distribution of the feed as measured prior to the separation; curve 1A is the equivalent size distribution derived by combining the data obtained for the two separated products. The difference in position is due to enhanced dispersion of the clay caused by passage through the cyclone and will be referred to in more detail later.


FIG. 5. Transmission electron micrograph of overflow product from Pakistan china clay separation. Scale bar = 1/~m.

In terms of product quality, the overflow fraction was subsequently found to contain 94% of kaolinite, as estimated by thermogravimetry. Fig. 5 shows the euhedral nature of the fully dispersed kaolinite, which is present in both platy and rolled forms.

Fractionation of finer clays

Two further examples are quoted to illustrate the separation characteristics of finer- grained clays of disparate particle size, texture and composition. The first is an indurated Jurassic mudstone from 43 m depth in a borehole at Winstone, Gloucestershire, containing smectite, illite, kaolinite, calcite, quartz and pyrite. The second is an impure Lower Cretaceous fuller's earth from Baulking, Oxfordshire, consisting of Ca-smectite with appreciable amounts of quartz. Suspensions were prepared by prolonged shaking of the samples in distilled water followed by wet sieving at 63 pm (240 mesh) to remove any sand grains. They were found to contain 43% and 84% of <2 pm particles, respectively.

Both were treated in the same way, feeding a 5-5% solids suspension to a 3.0 cm cyclone fitted with a fixed inlet of 0.40 cm diameter and apex nozzle of 0.26 cm diameter. Two trial separations were made in each case, one with a 0.42 cm diameter vortex finder and the other with a 0.85 cm diameter vortex finder. The operating variables and recovery figures are shown in Tables 2 and 3, while the particle size distributions of the feeds and products are illustrated in Figs 6 and 7.

Comparison of curves 2 and 4 in Fig. 6 show that, although the particle-size distribution of the clay fractions separated from the Jurassic mudstone are similar, the use of the larger-diameter vortex finder has given a sharper cut-off at the upper l imit--9 pm compared to 15 /~m. The smaller vortex finder, theoretically offering a finer split in terms of average particle diameter, operates by forcing more water and fine suspended matter into the underflow and produces an underflow product showing a greater proportion of fines (cf. curves 3 and 5). The recovery of <2 pm particles in the overflow is thus poorer,

Clay products by hydrocycloning

TABLE 2. Trial separations on indurated Jurassic mudstone using a 3 cm cyclone with 0.4 cm inlet and 0.26 cm apex nozzle. Details are for products obtained with a 0.85 cm vortex finder (at 2-5 kg/

cm 2 pressure) and a 0.42 cm vortex finder (at 1.0 kg/cm 2 pressure).

41

Solids weight % recovery Vortex finder Volume ratio ratio % solids % solids of <2 #m diameter (cm) o/f: u/f o/f: u/f in o/f in u/f particles

0.42 2.5 : 1 1 : 1 4 10 70 0.85 14.6:1 1.7:1 4 30 88

( • original sample

( ~ original sample recalculated from ( ~ and (~)

0.42 cm vortex finder, pressure 1.0 kg/cm 2

( ~ overflow (49%)

( ~ underflow (51%)


( ~ overflow (63%)

( ~ underflow (37%)

( ~ overflow from (~) (55%)

( ~ underflow from ( ~ (8%)

/}" I ,t'11 | if ," ,J I I

"/,~" / j s ' j

., / /

/ / / /

G / / / / / i

| /

/ /

|

r

50

SO

70

g

so

20

2 5 10 20 particle size, microns

FIG. 6. Particle-size distributions of products from cyclone separation of indurated Jurassic mudstone. Figures in parentheses refer to wt% recovery of solids in product.

a f igure o f 7 0 % be ing o b t a i n e d c o m p a r e d wi th 8 8 % w h e n e m p l o y i n g the la rge v o r t e x

f inder (Tab le 2).

In the la t te r case , r ecyc l ing the over f low to u p g r a d e the p r o d u c t in t e r m s o f c lay c o n t e n t

leads on ly to a m a r g i n a l i m p r o v e m e n t in b o t h cu t -o f f d i a m e t e r a n d a m o u n t o f < 2 /~m


TABLE 3. Trial separations on impure Lower Cretaceous fuller's earth using a 3 cm cyclone with 0.4

cm inlet and 0-26 cm apex nozzle. Details are for products obtained with a 0-85 cm vortex finder (at 2-5 kg/cm ~ pressure) and a 0.42 cm vortex finder (at 1.0 kg/cm ~ pressure).

Solids weight % recovery Vortex finder Volume ratio ratio % solids % solids of <2 pm diameter (cm) o / f :u / f o / f :u / f in o / f in u/f particles

0.42 2.5:1 0-1:1 4 10 55 0.85 15-7:1 1.6:1 4 36 74

( • original sample


Q overflow (48%)

Q underflow (52%)


Q overflow (62%)

(~) underflow (38%)

( ~ overflow from ( ~ (53%)

0 underflow from (~) (9%)

|

2

j f

J 11 J

i " f , , , ~

. . . . / / /

j / j l

k

i 5 10 2O

particle size, microns

f / j

a

FIG. 7. Particle-size distributions of products from cyclone separation of impure Lower Cretaceous fuller's earth. Figures in parentheses refer to wt% recovery of solids in product.

50

100

90

8O

70

6O

L

150

particles (cf. curves 4 and 6), and reduces the overall recovery of <2 pm material from 88 to 81%.

Curve 1A represents a calculated composition for the feed derived from the separate particle-size distributions of overflow and underflow and, when compared with the actual feed (curve 1), indicates an additional disaggregation of particles up to 20 pm in diameter produced simply by passage through the cyclone.

For the fuller's earth separation (Fig. 7), it can be seen that the overflow products obtained from the two vortex finders are identical (curves 2 and 4), both providing a clay- enriched fraction containing 95% particles finer than 2 pm. The smaller vortex finder, however, compels a greater proportion of the fines to discharge in the underflow (cf. curves 3 and 5) and the recovery of <2 pm particles in the overflow is only 55% compared to 74% when the larger vortex finder is fitted. When the cyclone is employed for upgrading clays already rich in <2 #m material the use of a larger vortex finder is obviously advisable

.-fi E .=


TABLE 4. 'Calculated' and 'observed' ds0 values for indurated Jurassic mudstone (Fig. 6) and impure Lower Cretaceous fuller's earth (Fig. 7). (Values used for flow rates were those

actually measured.)

43

Sample d~o

Vortex finder diameter (pressure)

0.42 cm (1.0-1.4 kg/cm :) 0-85 cm (2-5 kg/cm 2)

Both samples: calculated Indurated mudstone: observed Impure fuUer's earth: observed

5.6 5.9 3.2 4.7 1.4 1.7

to prevent an excessive loss of fines in the underflow 'reject'. Repassing the first overflow product reduces the cut-off size of the clay fraction from 7 to 5 /~m (cf. curves 4 and 6) and increases the amount of < 2 / t m material present from 94 to 97%. Overall recovery of <2/~m fines is, as a result, reduced from 74 to 64%.

Calculated and observed d~o values

The two last sets of results also illustrate a feature which is difficult to cater for in deriving theoretical formulae for performance. As shown in Tables 2 and 3, the operating conditions were virtually the same for both samples, comparable results being obtained for ratios of solids and suspension volumes reporting in the products. The products them- selves, however, do show the effects of varying separation efficiency, both in the nominal particle size at which fractionation takes place and in recovery figures. This is a function of particle-size distribution in the feed itself and must be borne in mind when attempting to forecast probable performance of a hydrocyclone. The effect is also seen more clearly when re-passing the products, the finer or coarser fractions obtained from the original treatment of the feed then yielding finer or coarser separations respectively in the re-treatment (cf. curves 4 and 6 in Figs 6 and 7).

The calculated ds0 values for the operating conditions'under which the two clays were treated are shown in Table 4. The two figures of 5.6 #m and 5.9 #m (for the smaller and larger vortex finders) must obviously be the same for both samples. In practice, the actual figures are lower, with the finer fuller's earth yielding a lower equilibrium particle diameter than the mudstone. The lower dso value obtained with the smaller vortex finder is essentially a function of the increased quantities of fines diverted to the underflow.

The use of calculated ds0 values for samples containing large amounts of material much finer than the minimum cut-off size for the cyclone is thus of limited practical use.

S H E A R I N G E F F E C T O F T H E H Y D R O C Y C L O N E

Although mechanical agitation in water and subsequent passage through a screen is an adequate preliminary to hydrocyclone separation, the resulting suspensions may not necessarily be fully dispersed. Where incomplete dispersion is suspected it is good practice to allow the suspension to recirculate within the hydrocyclone system in order to take advantage of the shearing forces within the vessel. The effect of these forces in breaking down aggregates in the medium- to fine-silt size range has already been noted in the discussion of Figs 4 and,6. However, for smectite suspensions, exposure to the high-shear


TABLE 5. Analytical data on pure Lower Cretaceous fuller's earth and hydrocyclone overflow product.

Exchangeable cations (mEq/100 g):~ Surface area* Smectite~" Liquid

(m2/g) (%) Ca 2+ Mg 2+ K + Na + limit

Original sample 780 97.5 65.2 39.4 2.5 2.5 209 Overflow product 791 98.9 66.3 39.2 3.2 5.9 292

* Obtained by ethylene glycol monoethyl ether adsorption under vacuum (Carter et al., 1965). ~" Assuming a surface area of 800 m2/g for 100% smectite. :~ Extracted by ammonium acetate at pH 7.

environment of the hydrocyclone may also result in appreciable disruption of interparticle associations in the sub-micron size range.

Table 5 gives various data obtained from a virtually pure Ca-smectite from Baulking, Oxfordshire, and its hydrocyclone overflow product. This smectite showed no X-ray detectable impurities and >99% particles <2 #m by Andreasen particle-size analysis. As can be seen from Table 5, passage through the hydrocyclone resulted in only a marginal increase in smectite content and no significant change in the exchangeable cation assemblage, but a marked rise in liquid limit from 209 to 292. That this enhancement in liquid limit was due to breakdown of sub-micron sized aggregates was confirmed by transmission electron microscopy. Comparison of micrographs of original sample and overflow product (Fig. 8a,b) shows that the original aggregates and particle clusters, predominantly 0.5-0.8 #m in size, have broken down to much thinner, smaller particles. In both original sample and overflow product, the lath-like morphology predominates (of. Newmann, 1976). An indication of the intensity of the shearing forces experienced by this sample on hydrocycloning may be obtained from noting that liquid limits equivalent

(a) (b)

FIG. 8. Transmission electron micrographs of pure Lower Cretaceous fuller's earth (a) before and (b) after cyclone separation. Scale bar = 1/~m.

Clay products by hydrocycloning 45

to that given by the overflow product were achieved only after subjecting a second portion of the sample to the shear conditions of a domestic liquidizer operating at 13 000 r.p.m. for 30 min.

D I S C U S S I O N

At its present state of development, the hydrocyclone suffers from the major disadvantage of being unable to provide clay products as fine as those capable of being produced by the sedimentation/decantation technique with or without the aid of the centrifuge. The separation size varies to some extent with the size distribution of the feed suspension and, within limitations, the finer the feed the finer will be the resulting clay product. Products with a cut-off size of 10 #m are easily obtained but good separations may become increasingly difficult below this figure. Fineness of separation may be improved by recycling the first clay separate but the sharpness of separation then decreases and overall recovery of fine material is diminished. In the examples described here, a 'cut-off' size finer than 5 ~tm could not be achieved.

The hydrocyclone does, however, have some important advantages over the sedimentation/decantation technique. Perhaps the major one is that large amounts of relatively concentrated clay suspensions may be separated in a matter of minutes. Suspensions used in the present investigation contained 5-10% by weight of solids. For sedimentation/decantation separations the recommended maximum concentration is 1-2% solids by weight, as above this figure hindered settling may result. Clay recovery from suspension is rendered easier and more efficient when high suspension densities are used. Although the bulk of the water in the cyclone separation accompanies the clay in the overflow product, this is also the case with a decantation or elutriation method. Indeed, using these latter procedures very large volumes of dilute suspension are collected for subsequent clay recovery, as repeated treatments are necessary for efficient separation of a given size of particle.

In the sedimentation/decantation technique a dispersing agent is normally added to stabilize the suspension and, once separation is complete, the salt of a divalent cation is added to flocculate the clay. The clay product is then washed with water to remove excess salt and finally treated with a polar organic liquid, e.g. ethanol, and dried. This procedure is time-consuming and impracticable for preparation of large amounts of clay separate. It is also out of the question if clay products containing the same exchangeable cation assemblage as in the original sample are required.

No dispersing agents are necessary for separations made with the hydrocyclone as the shear forces encountered by the clay suspension maintain, and often promote, dispersion. This effect is enhanced if the suspension is allowed to circulate within the system for some minutes prior to separation. The shear forces also minimize thixotropic effects, which is useful when dealing with suspensions containing Na-smectite, palygorskite or sepiolite. A single disadvantage is that gypsum and similar evaporite minerals may be solubilized and increase the amount of dissolved salts in the water. This may be inconvenient if direct oven-drying or freeze-drying is used for eliminating the water by evaporation or sublimation, thus leaving the precipitated salt in the clay, but is seldom of importance if the water is removed by settling or filtering the clay product.

In dealing with small amounts of suspension, the slurry remaining untreated in the cyclone and pump when the latter is switched off may be considered an unacceptable loss

4 6 J. A . Ba in and D. J. M o r g a n

o f m a t e r i a l ( u n l e s s m o r e w a t e r is a d d e d to c a r r y o v e r t h e r e s i d u e ) b u t t h e l a r g e a m o u n t o f

c l a y w h i c h c a n be s e p a r a t e d e a s i l y fo r s u b s e q u e n t t e c h n i c a l t e s t i n g p u r p o s e s is o f

c o n s i d e r a b l e bene f i t t o l a b o r a t o r y e v a l u a t i o n s t u d i e s .

A C K N O W L E D G M E N T S

We thank Mr P. R. Kiff, Dr R. C. Mackenzie and Mr M. J. Stentiford for comments. The paper is published by permission of the Director, Institute of Geological Sciences (NERC).

R E F E R E N C E S

BRADI.I.:Y D. (1965) The Hydrocyclone, pp. 84-88. Pergamon Press, Oxford. CAR'rER D.L., HEiI.MAr~ M.D. & GONZALEZ C.L. (1965) Ethylene glycol monoethyl ether for determining

surface area of silicate minerals. Soil Sci. I~1, 356-360. DAHI.srROM D.A. (1949) Cyclone operating factors and capacities on coal and refuse studies. Mining

Trans. AIME 184, 331-344. KtFF P.R. (1977) Evaluation of a hydrocyclone for on-site sediment separation. J. Sedim. Petrol. 47,

1365-1374. NAYt.OR T.R. (1958) The hydrocyclone in the refining of china-clay. Mine & Quarry Engineering 24,

510--513 NEUMANN B.S. (1976) Lath-shaped montmorillonites in Surrey. Clay Miner. I I, 3-12.

R E S U M F.: Les hydrocyclones, rarement apprecies, olTrent plusieurs avantages p~.r rapport aux m&hodes plus connues de la sbparation des argiles utilisant des m,Jthodes de s~dimentation ou decantation. De grandes quantites de suspensions d'argiles relativement concentr6es peuvent ainsi &re s~parees en I'espace de quelques minutes. En plus, aucun agent dispersant n'est requis et les conditions de contrainte appliqubes ~i la suspension Iors de son passage dans rhydro- cyclone provoquent la desagregation des particules a la fois dans le domaine des limons et de celles inf~rieures au microm6tre. I1 existe des Iormules reliant la g6ometrie de l 'hydrocyclone aux performances de separation mais elles sont de peu de valeur car elles ne peuvent predire la taille maximale des particules de rargile recueillie ou le pourcentage de recuperation d'un type de taille de particule. Ces donn,3es, d6termin6es par des essais, sont fournies pour des suspensions de kaolinite, de smectite ou des m,31anges d'argiles. Des limites ont 6t6 pr6vues pour 6viter la separation d'argiles ayant des tailles inferieures ,5 5 #m.

K U R Z R E F E R A T : Obwohl selten angewendet, hat der tlydrozyklon einige Vorteile gegen0ber der gebriiuchlichen Sedimentationsmethode fiir die Abtrennung der Tonfraktion. GroBe Mengen relativ konzentrierter Tonsuspension k6nnen damit in wenigen Minuten abgetrennt werden. Weiterhin werden keine Dispergierungsmittel ben/3tigt, und die b c i d e r passage durch den Ilydrozyklon auftretended ScherkrS.fte zerst6ren Aggregate in der Schluff und in der Tonfraktion. Die fiir dic Auslegung eines Hydrozyklons bekannten Formeln habcn nur begrenzten Wert, da sic keine Voraussage der maximaten Teilchengr6[~,e des abgetrennten Tons oder der Ausbeute der einzelnen Teilchengr/313en Fraktionen zulassen. Daten hierf0r wurden in praktischen Versuchen bestimmt und werden for Kaolinit, Smectit und f/Jr Misch- suspensionen angegebcn. Konstruktive Einschr~inkungen verhindern die effektive Trennung yon Tonfraktionen mit Ausschlul3grenzen unter 5 ,um.

R E S U M E N: La utilizacion de hidrociclones ofrece varias ventajas sobre los m~todos de sedimentacion/decantaci6n mas corrientemente usados en la separaci6n de arcillas. Una de estas ventajas es la de permitir la separaci6n en unos pocos minutos, de grandes cantidades de suspensiones de arcilla relativamente concentradas. Pot otra parte, no se requiere el uso de agentes dispersantes, pues los esfuerzos cortantes a los que esta sometida la suspension a su paso a traves del hidrociclon produce la desagregaci6n de las particulas, tanto en el rango de la fracci6n limo como en el de la fraccion por debajo de la micra. Las formulas existentes que


relacionan los factores de disefio del hidrocicl6n con su comportamiento en la separaci6n, son de valor limitado ya que no permiten predecir el tamafio m~ximo de particula del producto arcilloso, ni el porcentaje de recuperaci6n de un determinado tarnafio de particula. Tales datos, determinados experimentalmente, se dan para suspension6s de caolinita, esmectita y minerales interestratificados. Limitaciones de disefio impiden la separacion efectiva de fracciones de arcilla con tamafios inferiores a 5/tin.

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