Observations on induced aeration in agitated slurries

The Chemical

Engineering

Journal, 54 (1994)

199

199-205

Short Communication Observations on induced aeration in agitated slurries C. Aldrich and J.S.J. van Deventer Department of Metallurgical Engineering, University of SteL?enbosch, Private Bag X5018, Stelknbosch 7600 (South Afiical

and observed a decrease in the rate of gas induction with an increase in solids loading. They also observed a decrease in the rate of induced aeration with an increase in particle size. In this communication the results of these limited studies are extended, and it will be shown that the effect of slurries on the rate of induced aeration can be explained in terms of the apparent density and viscosity of the slung.

meceived February 13, 1992; in final form November 23,

1993)

2. Experimental Abstract Despite its importance as a fundamental operation, research on induced aeration in slurries in agitated vessels has received little attention up to the present. In experiments conducted with a hollow pipe and shrouded Rushton turbine impellers and shuries composed of ore as well as synthetic particulates, it is shown that the onset of gas induction is not affected by solids concentrations of less than approximately 15% by mass. Beyond this critical point induced aeration decreases with an increase in the solids concentration, particle size and particle density of the slung. These effects can be interpreted in terms of the effect of the solids on the apparent properties of the fluid, i.e. the viscosity and density.

1. Introduction Since it is a convenient way of contacting gases and liquids in chemical engineering operations such as the treatment of waste water and fermentation processes, induced aeration in agitated vessels has been studied in a variety of contactor designs and aeration conditions. Despite its importance as a fundamental operation, research has been focused mainly on gas induction in homogeneous liquids, and little attention has been paid to shuries. Sawant et al. [l] investigated the effect of an aqueous suspension of fine dolomite particles (smaller than 60 pm) on the rate of gas induction in a Denvertype flotation cell and concluded that solids loadings of up to 20% had no appreciable effect on aeration. In a similar investigation, Arbiter et al. [2] experimented with glass beads and haematite ore of various sizes in a Fagergren and a Denver flotation machine and solids mass loadings of up to 50%,

procedures

The apparatus consisted of a variable-speed motor connected to a motor shaft, the speed of which could be varied from 0 to 1400 rev mini. Three different impellers were employed, namely a sixblade and a 12-blade Rushton turbine impeller, as well as a pipe impeller, the geometrical details of which are illustrated in Fig. 1. These impellers were mounted individually on a hollow tapering shaft, which could be sealed at the bottom by mounting the turbine impellers. The turbine impellers were enclosed in draft tubes or stators to allow for better induction of gas. These stators shown in Fig. 2 were each provided with 16 evenly spaced apertures situated below the liquid surface after immersion of the impeller assembly and facilitated the flow of induced gas into the bulk liquid. As a result of the low pressure field generated by the rotation of the impeller blades in the liquid, gas entered the aerator through a sealed unit on top of the stator, which TOP

SIDE

F’ig. 1. Design of turbine and pipe impellers (all dimensions in millimetres).

0923-0467/94/$07.00 0 1994 Elsevier Science S.A. All rights reserved SSDI 0923-0467(93)00201-T

C. Aldrich,

200

J.S.J. van Deventer

/ Induced

aeration

in agitated

slumies

that the slurry particles were fully suspended in the vessel prior to the onset of aeration, it was difficult to conduct comparative experiments on an extended scale. Since the actual mechanism is poorly understood, no fundamental analysis of gas induction is available in the literature, and this communication presents similarly semiquantitative relationships between rheological parameters and gas induction in slurries, rather than a mathematical description of induced aeration.

S80 I

I 3. Critical aeration

Fig. 2. Geometries

of stators

(ah dimensions

in milhrnetres).

contained the top of the hollow shaft and was connected to the stator by a valve. When the pipe impeller was used, the valve was closed and gas entered through the hollow shaft and the tips of the impeller arms, from where it was dispersed into the bulk of the liquid. The stator and rotor were submerged in slurries contained by a cylindrical Perspex vessel with a capacity of 6 1, the inside of which was lined with four evenly spaced baffles, each projecting 19 mm into the vessel. The immersion depth of the impeller was accurately controlled by the small jack on which the vessel was supported. The onset of gas induction was recorded when the 6rst bubbles appeared on the bulk fluid surface. Seven different kinds of slurry were investigated, as indicated in Table 1. Because the composition of the slurries was chosen to ensure

TABLE

1. Properties

of liquids and shuries

The critical point of gas induction, i.e. the point at which minute quantities of gas are induced into the fluid (manifested by the fhst few bubbles to emerge on the bulk fluid surface), was not affected significantly by solids loadings below approximately 15%, as shown in Pigs. 3-5, where the onset of gas induction in aqueous slurries is compared with water. Above loads of about 20-259/o, the solids loading appears to have a more pronounced effect on the onset of gas induction, as is illustrated by the 50% quartz, 25% calcium fluoride and 38% resin suspensions in Pigs. 3-5. This is to be expected since, at these solids concentrations, inter-particle effects probably contribute considerably to the apparent viscosity of the fluid [3, 41, which is known to have an effect on the onset of induced aeration [ 1, 5, 61. Figures 3 and 4 also show that the onset of aeration is inhibited by an increase in viscosity in

used

Slurrya &lass%) Water 25 Mass% aqueous 63 Mass% aqueous 65 Mass% aqueous Resin

PP (kg me3)

Shapeb (-)

&P)

998 1190 1304 1315 11;

13 38

Baddeleyite Polystyrene Nylon CaFZ Siliceous

20

ore’

(pm)

brine solution sucrose solution sucrose solution 23; 50 15 10 9 25

Qum

dP

“AU solids are suspended in water, unless Indicated otherwise. ‘C, cylindrical; I, irregular; S, spherical. ‘Suspended in an aqueous sucrose solution with a density of 1315

1 2.2 60 178

710

1270

S

-

63 142 800 1790 90

2650 3730 1020 1020 3180

I I S C I

-

106

2170

I

kg mm3 and viscosity

of 178 cP, denoted

by S.

C. Aldrich,

CRITICAL 0.8

FROUDE

-WATER

J.S.J. van Deventer

NUMBER

/ Induced

aeration

in agitated slurries

201

(NC2 D */gh)

q 20%

SIL&S

x

+ 15%

BADD

n 50%

11%

RESIN

-

65%

QUARTZ

0 23%

SUCROSE

1 -4

0.7

x 10%

PS

0.6-_

...I

RESIN

I 0 .I...

o-5_

i gj

n

1

I

I

330

IMPELLER

IMMERSION

Critical aeration in slurries agitated with the T6 impeller: -, ore; PS, polystyrene; BADD, baddeleyite.

0.8

FROUDE

-WATER 0 10%

-- 65% + 20%

PS

0.6_1-.-.-----.-..I.--

NUMBER SUCROSE SIL&S

[mm1

DEPTH

Fig. 3.

CRITICAL

160

140

120

100

IN, * D

water; - - -

, 65% aqueous sucrose solution; SK&S, siliceous

*/gh)

x 23%

RESIN

0 11%

RESIN

A 50%

QUARTZ

X 25%

CaF2

. . . . . . . . . ..~.._.._.......

____ ....

__.._,:

-i

i 90

100

IMPELLER

110

IMMERSION

120

130

DEPTH

Fig. 4. Critical aeration in slurries agitated with the T12 impeller: -, siliceous ore; PS, polystyrene.

absence of particles. Density has been shown not to affect the onset of aeration significantly [ 6 1. Although an increase in the particle size appears to have a marginally inhibitive effect on the onset of gas induction with the pipe impeller (it does not seem to have an effect as far as the turbine impellers are concerned), too few data are available to draw firm conclusions with regard to this and the effect of other particle parameters.

140

[mm] water; ---,

65% aqueous sucrose solution; SK&S,

4. The rate of induced aeration Previous research [ 1, 5, 7 ] has established that the only physical properties of homogeneous fluids affecting induced aeration are the density and the viscosity of the fluid. On the basis of these observations, it can be expected that the apparent density and viscosity of multiphase media affect the rate of induced aeration similarly.

202

C. Aldrich, J.S.J. van Deventer / Induced aeration in agitated slurries

CRITICAL

FROUDE

NUMBER

(Nc2 D 2/gh)

0.9 -WATER 0.8

q

13%

RESIN

x 10%

PS

m 10%

NYLON

A 50%

QUARTZ

+ 38%

RESIN

....

-.

0.7

_.._.....__......__...,...........~.....................,............

0.6

_~~~.~.~~~~~...~~~....~~.....~....~.~.....~.....~.....~..~.~....~....~

A

.+.

_._

30

50

70

90

IMPELLER Fig.5. Critical

110

IMMERSION

130

DEPTH

aeration in slurries agitated with the pipe impeller:

4.1. Solids l!imdin.g The presence of solids in the medium affects the apparent viscosity of the medium and, if the density of the solids differs from that of the fluid, the apparent density of the slurry is affected as well. It is well known that an increase in the solids loading of slurries results in an increase in the apparent viscosity of the fluid [3] and, depending on the density of the particles, also increases the apparent

lOOO*Ae

I...

150

[mm] -,

water; PS, polystyrene.

density of the fluid. The apparent viscosity of the fluid is furthermore dependent on the density of the particles and the size of the particles, as well as on the shapes of the particles and the prevailing shear rates in the flow [3]. The adverse effect of an increase in the solids loading on induced aeration above the critical point, however, was clearly observed in experiments with both the pipe and the turbine impellers, as indicated in Figs. 6-10, where

[-I

-WATER

40

30

20

10

.- 25% --

63%

0

9%

0

50%

BRINE SUCROSE

;:-’

NYLON

;, Y/ , :---:---, ,, QUARTZ

n 10%

POLYSTYRENE

x

RESIN

23%

I

,w

L

,x.

0

n . ‘.

0

0

0

0

0

00 __--

0

o

0

00

o

_--

___---

__--

_p___--------

_--7

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Fr-Frc [-] Fig. 6. Induced aeration in slurries using the T6S80 impeller-stator

assembly.

0.9

1

C. Aldrich,

/ Induced

aeration in agitated slurries

203

i-1

1 OOO*Ae 60

J.S.J. van Dtwenter

r-

1 -

WATER

50 -

25%

BRINE

63%

SUCROSE

q 15% BADDELEYITE 011%

RESIN

+ 25%

CaF2

Q 23%

RESIN

0.2

0.1

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Fr-Frc [-I Fig. 7. induced aeration in slurries using the T6SlOO impeller-&&or

1 OOO*Ae

assembly.

[-I

35 -WATER 30

.- 25% -

25

63% q

20 15

25%

x 50%

BRINE SUCROSE

CaF2 Z!

QUARTZ

c

10 5 0

0.2

0.4

0.6

0.8

1

1.2

Fr-Frc f-1 Fig. 8. Induced aeration in slurries using the T12S80 impeller-stator assembly.

all the curves representing slurries lie below the curves representing water (i.e. show lower rates of gas induction). For comparative purposes the rates of gas induction in a 25 mass% aqueous brine solution with a viscosity of 2.2 cP and a density of 1190 kg mW3, as well as an aqueous sucrose solution with a viscosity of 60 CP and a density of 1304 kg mW3 have been included in Figs. 6-10. Owing to the small difference in their viscosities (see Table l),

the difference in the rates of induced aeration in the brine solution and the water can be attributed almost entirely to the difference in the densities (approximately 20%) of the fluids. No direct comparison between the water and the aqueous sucrose solution is possible, however, since both the density and the viscosity of the sucrose solution differ significantly from those of water. None the less inclusion of the sucrose data in Figs. 6-10 serves as a useful reference to the behaviour of the slurries

C. Aldrich, J.S.J. van Dcmenter / Induced

204

1 OOO*Ae

aeration in agitated slutis

[-I

-WATER -25% q

BRINE

63%

SUCROSE

23%

RESIN

X 25%

CaF2

0.2

0.4

0.6

0.8

Fr-Frc Fig. 9. Induced

aeration

in slurries

1

1.2

1.4

L-1

using the T12SlOO

impeller-stator

assembly.

L-1

lOOO*Ae 35,

I -WATER

30

SUCROSE RESIN

0

9%

0

50%

QUARTZ

1 +

10%

POLYSTYRENE

25 2.

-~ 63% ha 38%

0

NYLON

0.2

1

0.4

/

0.6

0.8 Fr-Frc

Fig. 10. Induced

aeration

in slurrles

1

1.2

1.4

[-I

using the pipe impeller.

investigated. The decrease in the rates of gas induction with an increase in the solids loading in the shnry can probably be ascribed to the increase in the apparent density of the fluid in the presence of higher solids concentrations, as well as to the increase in the apparent viscosity of the fluid. The effect is especially noticeable at slurry concentrations above approximately 20%, probably owing to the effect of interactions between the particles that become significant above this concentration level.

This increases both the apparent viscosity of the medium as well as the buoyancy of the gas in the liquid and impedes prolonged entrainment of the gas in the liquid, in turn hindering dispersion of the gas into the bulk liquid. A further contributing factor is the lower fraction of liquid available to aeration owing to the space occupied by the particles. As is shown in Figs. 6 and 10, the rate of gas induction in the polystyrene and nylon slurries which have essentially the same

C. Aldrich, J.S.J. van Deventer

/ Induced

aeration in agitated slurries

205

densities as water is lower than would have been expected if only the dilution effect (approximately 10%) of the solids were to be taken into account.

by the concentration of solids present, as well as by the relative densities of the solids with regard to the liquid phase.

4.2. Particle size If the effects of the 25% calcium fluoride and the 23% resin slurries on the rate of induced aeration are compared in Figs. 7-9, it can be seen that the resin has a more inhibitive effect on the rate of gas induction than the calcium fluoride, despite its slightly lower solids concentration and its lower particle density. The same trend is observed when the nylon and polystyrene slurries are compared in Pig. 10. The nylon particles (1790 pm) are considerably larger than the polystyrene particles (800 pm) and are therefore associated with a commensurately lower rate of gas induction. (These observations are not definitive, since the different sphericities of the nylon and polystyrene particles cannot be entirely discounted.) Prom these observations it can be concluded that a larger particle size has an adverse effect on the rate of induced aeration in slurries.

References

4.3. Particle density Too few data were obtained to establish the effect of particle density on induced aeration but, from the limited data available, it would appear as if higher particle densities result in decreased rates of gas induction. This effect is best illustrated in Pig. 7, where the 15% baddeleyite slurry had a significantly stronger suppressive effect on induced aeration than the 23% resin slurry, for example. The baddeleyite had both a lower solids content and particle size than the resin, but a much higher particle density. 6. Conclusions The induced aeration of slurries decreases with an increase in (a) the solids content, (b) the particle size and (c) the particle density of the slurry. These effects can be interpreted in terms of the effects of the solids on the apparent properties of the fluids, namely viscosity and density. The apparent viscosity of the medium is a complex function of the physical properties of the constituents of the medium, as well as the prevailing flow conditions in the medium, and can be expected to be increased by an increase in the solids content of the slurry. An increase in the apparent density of the fluid medium leads to a decrease in the rate of gas induction and the apparent density of the slurry is directly affected

1 S.B. Sawant, J.B. Joshi, V.G. Pangarkar and R.D. Mhaskar, Chem. Eng. J., 21 (1981) 11. 2 N. Arbiter, C.C. Harris and R.F. Yap, Int. J. Min. Proc., 14 (1976) 257. 3 B. Clarke, Trans. Inst. Chem. Eng., 45 (1967) T251. 4 C. Castillo and H.C. Williams, Chem. Eng. Commun., 3 (1979) 529. 5 J.B. Joshi, A.B. Pandit and M.M. Sharma, Chem. Eng. Sci., 37(6) (1982) 813. 6 J.B. Joshi, Chem. Eng. Commun., 5 (1980) 109. 7 C. Koen and B. Pingaud, 2nd Eur. Cwqf on Mixing, March 30-April I, 1977, Paper F5.

Appendix

A: Nomenclature

Ae d

QlNd 3, aeration number ( - ) impeller diameter (m) particle size of slurry solids (m) F’roude number (- ) NC2d 2/gh, critical F’roude number ( - ) gravitational acceleration (m s-“) impeller immersion depth (m) impeller speed (rad s- ‘) impeller speed at onset of aeration (rad

d, Fr Prc 9 h N NC

S-1)

PIPE

pipe impeller ( - ) rate of induced aeration (m3 s- ‘) stator with inner diameter of 74 mm (outer diameter 80 mm) ( - ) stator with inner diameter of 94 mm (outer diameter 100 mm) ( - ) six-bladed turbine impeller ( - ) T6 impeller combined with stator with inner diameter of 74 mm (outer diameter

Q S80 SlOO T6 T6S80

80 mm) (-) T6 impeller combined with stator with inner diameter of 94 mm (outer diameter 100 mm) (-) 12-bladed turbine impeller ( -) T6 impeller combined with stator with inner diameter of 74 mm (outer diameter

T6SlOO

T12 T12S80

T12SlOO

Greek CL PP

4

80 mm) (-) T12 impeller combined with stator with inner diameter of 94 mm (outer diameter 100 mm) (-)

letters viscosity (cP) particle density (kg rnp3) solids mass fraction in shrrry ( - )