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On the separation of fine particles by centrifugation
International Journal of Mineral Processing, 14 (1985) 265--273 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
265
ON THE SEPARATION OF FINE PARTICLES BY CENTRIFUGATION
K.N. HAN and W.C. SAY
Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 5 7701, U.S.A. (Received January 7 , 1 9 8 3 ; revised and accepted July 30, 1984)
ABSTRACT Han, K.N. and Say, W.C., 1985. On the separation of fine particles by centrifugation. Int. J. Miner. Process., 14 : 265--273. An investigation of recovering finely divided mineral particles by centrifugal forces has been carried out. A theoretical analysis of the separation efficiency for various heavy minerals with density ranging between 5 and 19.3 g/cm 3 against silica particles has been made initially. The size range considered in this study was 1--5 microns. The separation efficiency of these heavy minerals from silica was found to show typically a recovery over 95% with 80% purity. A system containing cassiterite and silica particles with 1--5 microns has been chosen as a model system in experiments. The experimental results were in good agreement with the theoretical predictions. Better separations were observed at higher pH's than pH 4 due to better dispersion of these minerals at the higher pH range.
INTRODUCTION The recovery of valuable minerals in the fine size region is needed to conserve dwindling raw material resources. Mineral industries are losing many millions of dollars each year due to the loss of these minerals by not being able to recover fine particles. There have been numerous attempts to recover such fine minerals by various techniques including flotation and flocculation (Collins and Read, 1971; Trahar and Warren, 1976; Fuerstenau, 1980). However, only a few of these techniques have proven to show a moderate success. A major inherent problem of treating fine particles is the fact that first, the slurry concentration is usually low resulting in poor collision frequency for flotation or flocculation to be effective and second, the motion of fine particles is so slow due to small size that any conventionaltechnique usually fails t o c o n c e n t r a t e t h e s e p a r t i c l e s in a r e l a t i v e l y s h o r t t i m e s c a l e . The current investigation used controlled centrifugal forces to overcome t h e s e p r o b l e m s . I t is w e l l k n o w n t h a t s o m e c e n t r i f u g e s d e s i g n e d f o r t h e industrial application can accelerate the motion of particles by up to 60,000
0301-7516/85]$03.30
© 1985 Elsevier Science Publishers B.V.
266 fold over that obtained with gravity force alone. In this study, an attempt was made first to examine whether an effective separation o f heavy minerals from light gangue minerals was possible using centrifugal forces. A theoretical analysis of the separation efficiency for various heavy minerals against silica particles was made initially. A series of experiments was then carried out to confirm the findings from such a theoretical analysis. Cassiterite and silica fines with 1--5 microns have been used in this study as a model system. The effect of centrifugal force, pH of the solution and time o f centrifugation on the separation efficiency was studied. THEORETICAL CONSIDERATION An equation describing the settling rate o f particles in the radial direction under centrifugal forces can be easily derived and has the form given by eq. 1 (Coulson and Richardson, 1978): dr --
d ~ ( p s - P) ¢°~r (1)
=
dt
18~
where: dr/dt d Ps, P ¢o p
= velocity of particles along the r-direction
= = = =
diameter o f particles density of particles and of fluid angular velocity viscosity of fluid
I
II
I
I
I"2
-,
Fig. 1. S c h e m a t i c diagram o f the centrifuge basket.
267 Equation 1 assumes that the intensity of centrifugal force is sufficiently strong enough to maintain the profile of the liquid/air interface practically vertical as indicated in Fig. 1 and that the motion of particles under the influence o f the centrifugal force is horizontal and in the radial direction. The equation also assumes that the acceleration term for particles is negligible under these conditions. It should also be noted that the particle Reynolds number is far less than 0.1 under the conditions studied throughout this investigation. Upon integration of Eq. 1: t=
18#
r2 In - d2co2(Ps-- P) rl
(2)
Equation 2 represents the time, t required for particles with diameter d moving from r, to r2 at an angular velocity co (see Figure 1). Equation 2 can be rearranged to yield: d = kta,~(ps -
p)
In
(3)
Equation 3 is useful for the determination o f the size of the particles reporting to Region II (Fig. 1) for given t and ,0. Particles whose diameter is greater than d will report to Region II. Therefore, the weight percent of particles reporting to Region II can be evaluated, once the size distribution of particles is known. By a way of example, let us take an ore consisting of a heavy and a light mineral. The separation of the heavy from the light by centrifugation is to be effected. The recovery percent of heavy and light minerals and the percent purity can be evaluated by the following equations. max Wh = ~ Mi,h i
(4)
max Wt,h = ~ Mi,h i=o
(5)
max
w,= ~
Mi,,
(6)
i max
Wt,1 = ~ i=o
Mi,l
(7)
268
%G
Wh+Wl X 100
=
=
(S)
X 100, for heavy minerals
% R e = \WtJ
X 100, for light minerals
(9) (10)
where Wh, W1
= weight of heavy and light minerals, respectively, having size greater than i Mi,h, Mi,l = weight of heavy and light minerals, respectively, having size i Mt,h, Mt,1 = total weight o f heavy and light minerals respectively %G = percent grade of heavy minerals %Re = percent recovery. Calculations were made to examine recoveries and purities of various minerals reporting Region II at various angular velocities. In this analysis, the size distribution of heavy as well as light minerals is assumed to follow a standard normal distribution with the mean particle diameter of 3 microns and variance of 1. Density values considered in this analysis are 2.6, 5.0, 7.0, 9.0, and 19.3 g/cm 3. Figures 2, 3, and 4 represent the weight percentage o f particles travelling for a distance of 0.95 cm as a function o f time given by eq. 9 or 10. The angular velocities considered are 17.2, 39.0 and 209.4 rad/s. As can be seen in Fig. 2 for example, the recovery o f particles after 200 seconds centrifugation at the angular velocity o f 17.2 rad/s is nearly 100% I00
,
hi C.9
-
I--
/ 80
/
2:
,., o
_, .
/
//
/
~s0
II
¢O
'
/
/ /
~
//
//
/
.
.
.
.
.
.
~ _ _ ~
/ "
/ /
/
/
.
/
DENSITY ,,
/
/
19.3 o/©m 3
------
/ /
.....
//
9.0 7.0 ~.o
-----
2.6
o"',Oo,. ///// e,.
III
/
:
"
"
I
I 0
0
/I// I I00
I 200
i 300
I 400
I 500
TIME, S
Fig. 2. The weight percent of particles settled in centrifuge at angular velocity of 17.2 rad/s for various densities.
269
for density of 19.3 g/cm 3, 85% for 9.0, 73% for 7.0, 45% for 5.0 and 0% for 2.6. It should also be noted that under these conditions, the purity is 100% for all particles heavier than 2.6 g/cm 3 of density, assuming each system consisting of a heavy mineral and a gangue with 2.6 g/cm 3 density. This means that it is possible that minerals with the standard normal distribution and with the density greater than 5.0 g/cm 3 can be easily separated from silica by centrifugation at an angular velocity of 17.2 rad/s. The time required to achieve this separation can be reduced from 200 s to about 45 s at an angular velocity o f 39.0 tad/s, and to less t h a n 2 s at 209.4 rad/s (see Figs. 3 and 4).
~ :~___~_
,oo
,,=, I..: ,',// IJ
u. 8 0
~." /
/
: III
l\,'//
/
I~ [,71 6°1~,7/
OE,,~IT, _
/
......
/
,"
~, I;ili >-Iii:'/
/
~=4°1+Ii I " I::II: "~
/
/
/
/
19'3 l / c m l
.....
....
,.o T.o
------
~.o ~.~
,.,,,,
~9.or,,,,,.
fir
0
[
I
0
i
I00
i
200
I
300 TIME,
I
400
500
S
Fig. 3. The weight percent of particles settled in centrifuge at angular velocity of 39.0 rad/s for various densities. I00
/
~
----~- ~
--
~ec
,,,
Z
/
(n':' 60 "' _o
{
/ /
OE.S,T¥ ........ -------
/
19.3 g/cm 3 9.0 7.0
I-
~,0i.
-
-
/
~- 2c i
0 0
~o
--
--,
2.6
.
,
i 2
I
i 6
4 TIME,
I 8
I0
S
F i g . 4. T h e w e i g h t p e r c e n t o f particles settled in c e n t r i f u g e at angular v e l o c i t y o f 2 0 9 . 4 rad/s f o r various densities.
270
It is also seen in these figures that the recovery increases as the time of centrifugation is increased at the expense of purity. For example, the recovery of all heavy minerals is better t h a n 95% at 500 seconds, while the purity is not better than 50% at 17.2 rad/s (Fig. 2). Figure 5 shows that the purity o f heavy minerals given by eq. 8 can be projected as a function of centrifugation time. In a simple way of illustrating the purity of heavy minerals after separation in centrifugal force, two minerals with densities of 7 and 2.6 g/cm 3 were chosen. An equal a m o u n t of these two minerals with the same size distribution was to be introduced into a centrifuge. Ignoring the interaction between particles, it was found that 100% purity of heavy minerals could be obtained within 200 s at an angular velocity 17.2 rad/s in one second at 209.4 rad/s. TIME, ! I ~ ~ ~
I00
S
2 I
3 I
4 I
5 [
\\\ 90 O x ~
60
~
"
CJ~ r o d / @
3 1T.2 .~
40 . . . .
w" Q ~
209.4
2O
0
I
I00
L
I
I
200
300
4.00
TIME,
I
500
S
Fig. 5. The projected percent grade of heavy minerals with density 7 g/cm 3 separated from silica at angular velocities 17.2 and 209.4 rad/s. EXPERIMENTAL
A centrifuge, Damon/IEC Model CH was used in this investigation. Although the centrifuge is capable of running continuously with the aid o f perforated basket, batch tests were conducted in this study using a solid basket without holes. The batch system is easier to operate and y e t satisfactorily shows the effect of important variables on the recovery and the selectivity of heavy minerals. The centrifuge has seven knobs with which centrifugal force can be adjusted. The lowest speed can be obtained on position 1 (164 rpm = 17.2 rad/s) and the highest on position 7 (3400 rpm = 362.3 tad/s). Cassiterite as heavy and silica as light mineral were used as a model sys-
271 tem. Minerals were ground separately in a ceramic ball mill and particles with size range 1--5 microns were collected by repetitive sedimentation and decantation. The pH o f the slurry solution was adjusted with reagent grade HC1 and NaOH. The centrifuged particles after a set time were collected and analyzed by weight when a single mineral was used and by an X-ray diffractometer when a mixture o f cassiterite and silica was used. The settling distance, (r2 - rl) was fixed at 0.95 cm throughout the experiments (Fig. 1) and temperature was at 20 + 2°C. The capacity of centrifuge is about 320 ml. The centrifuge was loaded initially with about 300 ml of pure water and set at a constant speed until running at steady state when about 20 ml o f a slurry containing 0.2% by weight of either a single mineral or a mixture of cassiterite and silica was introduced into the center of the basket. Samples were withdrawn by a 70ml syringe at a depth of 0.95 cm from the air/liquid interface. The time required for sampling was within 10 s. When the speed of the centrifuge was too high to carry out sampling without disturbance, the speed of the centrifuge was reduced to the lowest before the sample was withdrawn. Because the X-ray analysis required at least 0.125 g o f sample, the above experiment was usually repeated until a desirable a m o u n t of samples was collected. Electrophoresis measurements were made using a Zeta meter. RESULTS AND DISCUSSION A slurry containing a single mineral, either cassiterite or silica was introduced into the centrifuge basket running fully at a set speed of 39 rad/s. I00
I
L
I
l
I
C A S S I T E R I T ~ ~ 80
,,, > o o ,d
H
y/j ///
,.
:--
.
o
,.o ,.o
•
///
• U3 :
-
,0
39.0rad/s
_
~
40
0 0
t
I
I
40
80
120 TIME,
Fig.
I 200
S
6 . The percent r e c o v e r y
angular velocity 39.0 rad/s.
I 160
of
cassiterite
and
s i l i c a o f 1 - - 5 microns in d i a m e t e r
at
272
Samples reported to Region II were collected at a set time and the a m o u n t of particles settled as a function o f time was measured. The results are plotted in Fig. 6. The centrifugation was carried out at three different pH values, namely pH 4, 7, and 9. From the tests carried out with single mineral particles, it can be seen that the recovery of cassiterite as well as silica is best at pH 4 and worst at pH 9. This is because as the pH of the solution approaches near the point of zero charge of these minerals (PZC of SiO2 = pH 3; PZC of SnO2 = pH 4), flocculation of these particles occurs resulting in large flocs which helps settling of these particles faster but the purity would have been impaired if these minerals were to be recovered from a mixture. On the other hand, at pH 9 complete dispersion of both mineral particles was observed. Another series of experiments were carried out where a slurry containing the same a m o u n t o f cassiterite and silica was introduced into the centrifuge. The particles centrifuged at a set angular velocity were collected at a set time and analyzed using an X-ray diffractometer. The time o f centrifugation was adjusted to recover better than 95% recovery o f cassiterite. The grade expressed in the weight percent o f cassiterite reported into Region II was determined and the results are plotted in Fig. 7. The results are reasonably in good agreement with those obtained in the tests with single mineral systems (Fig. 6). Because of the interaction between mineral particles, the reproducibility was not as good as the single mineral system, especially at pH 4. Nevertheless, there is a consistent trend seen in these results and the results obtained earlier. It can be concluded that the efficient separation o f ultrafine and heavy ,oo
~
~.
s?+ ,,, 8 0 IE
-----~
,
...... I2I
I.-
~-LX
°
< u. 0
pH
W 40 o < o: (D
- - -
z~---
9.0
. . . .
[] . . . .
6.5
O - -
4.0
ae 2 0
0 0
I I00
I ZOO
I 300
T IME, S
Fig. 7. The percent grade of cassiterite obtained at angular velocity of 39.0 rad/s from a mixture of cassiteriteand silicaparticlesof 1--5 microns.
273
minerals is possible by the use of centrifugal forces. The separation efficiency of cassiterite from silica particles with size range 1--5 microns was found to show typically a recovery over 95% with 80% purity. ACKNOWLEDGEMENT
This research has been partially supported b y the Faculty Grant Committee of South Dakota School of Mines and Technology.
REFERENCES Collins, D.N. and Read, A.D., 1971. Treatment of slimes. Miner. Sci. Eng., 3: 19--31. Coulson, J.M. and Richardson, J.F., 1978. Chemical Engineering, Vol. II. Pergamon Press, New York, N.Y. Fuerstenau, D.W., 1980. Fine particle flotation, In: P. Somasundaran (Editor), Fine Particles Processing. AIME, New York, N.Y., pp. 669--705. Trahar, W.J. and Warren, L.J., 1976. The flotability of very fine particles - - a review. Int. J. Miner. Process., 3: 103--131.
265
ON THE SEPARATION OF FINE PARTICLES BY CENTRIFUGATION
K.N. HAN and W.C. SAY
Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 5 7701, U.S.A. (Received January 7 , 1 9 8 3 ; revised and accepted July 30, 1984)
ABSTRACT Han, K.N. and Say, W.C., 1985. On the separation of fine particles by centrifugation. Int. J. Miner. Process., 14 : 265--273. An investigation of recovering finely divided mineral particles by centrifugal forces has been carried out. A theoretical analysis of the separation efficiency for various heavy minerals with density ranging between 5 and 19.3 g/cm 3 against silica particles has been made initially. The size range considered in this study was 1--5 microns. The separation efficiency of these heavy minerals from silica was found to show typically a recovery over 95% with 80% purity. A system containing cassiterite and silica particles with 1--5 microns has been chosen as a model system in experiments. The experimental results were in good agreement with the theoretical predictions. Better separations were observed at higher pH's than pH 4 due to better dispersion of these minerals at the higher pH range.
INTRODUCTION The recovery of valuable minerals in the fine size region is needed to conserve dwindling raw material resources. Mineral industries are losing many millions of dollars each year due to the loss of these minerals by not being able to recover fine particles. There have been numerous attempts to recover such fine minerals by various techniques including flotation and flocculation (Collins and Read, 1971; Trahar and Warren, 1976; Fuerstenau, 1980). However, only a few of these techniques have proven to show a moderate success. A major inherent problem of treating fine particles is the fact that first, the slurry concentration is usually low resulting in poor collision frequency for flotation or flocculation to be effective and second, the motion of fine particles is so slow due to small size that any conventionaltechnique usually fails t o c o n c e n t r a t e t h e s e p a r t i c l e s in a r e l a t i v e l y s h o r t t i m e s c a l e . The current investigation used controlled centrifugal forces to overcome t h e s e p r o b l e m s . I t is w e l l k n o w n t h a t s o m e c e n t r i f u g e s d e s i g n e d f o r t h e industrial application can accelerate the motion of particles by up to 60,000
0301-7516/85]$03.30
© 1985 Elsevier Science Publishers B.V.
266 fold over that obtained with gravity force alone. In this study, an attempt was made first to examine whether an effective separation o f heavy minerals from light gangue minerals was possible using centrifugal forces. A theoretical analysis of the separation efficiency for various heavy minerals against silica particles was made initially. A series of experiments was then carried out to confirm the findings from such a theoretical analysis. Cassiterite and silica fines with 1--5 microns have been used in this study as a model system. The effect of centrifugal force, pH of the solution and time o f centrifugation on the separation efficiency was studied. THEORETICAL CONSIDERATION An equation describing the settling rate o f particles in the radial direction under centrifugal forces can be easily derived and has the form given by eq. 1 (Coulson and Richardson, 1978): dr --
d ~ ( p s - P) ¢°~r (1)
=
dt
18~
where: dr/dt d Ps, P ¢o p
= velocity of particles along the r-direction
= = = =
diameter o f particles density of particles and of fluid angular velocity viscosity of fluid
I
II
I
I
I"2
-,
Fig. 1. S c h e m a t i c diagram o f the centrifuge basket.
267 Equation 1 assumes that the intensity of centrifugal force is sufficiently strong enough to maintain the profile of the liquid/air interface practically vertical as indicated in Fig. 1 and that the motion of particles under the influence o f the centrifugal force is horizontal and in the radial direction. The equation also assumes that the acceleration term for particles is negligible under these conditions. It should also be noted that the particle Reynolds number is far less than 0.1 under the conditions studied throughout this investigation. Upon integration of Eq. 1: t=
18#
r2 In - d2co2(Ps-- P) rl
(2)
Equation 2 represents the time, t required for particles with diameter d moving from r, to r2 at an angular velocity co (see Figure 1). Equation 2 can be rearranged to yield: d = kta,~(ps -
p)
In
(3)
Equation 3 is useful for the determination o f the size of the particles reporting to Region II (Fig. 1) for given t and ,0. Particles whose diameter is greater than d will report to Region II. Therefore, the weight percent of particles reporting to Region II can be evaluated, once the size distribution of particles is known. By a way of example, let us take an ore consisting of a heavy and a light mineral. The separation of the heavy from the light by centrifugation is to be effected. The recovery percent of heavy and light minerals and the percent purity can be evaluated by the following equations. max Wh = ~ Mi,h i
(4)
max Wt,h = ~ Mi,h i=o
(5)
max
w,= ~
Mi,,
(6)
i max
Wt,1 = ~ i=o
Mi,l
(7)
268
%G
Wh+Wl X 100
=
=
(S)
X 100, for heavy minerals
% R e = \WtJ
X 100, for light minerals
(9) (10)
where Wh, W1
= weight of heavy and light minerals, respectively, having size greater than i Mi,h, Mi,l = weight of heavy and light minerals, respectively, having size i Mt,h, Mt,1 = total weight o f heavy and light minerals respectively %G = percent grade of heavy minerals %Re = percent recovery. Calculations were made to examine recoveries and purities of various minerals reporting Region II at various angular velocities. In this analysis, the size distribution of heavy as well as light minerals is assumed to follow a standard normal distribution with the mean particle diameter of 3 microns and variance of 1. Density values considered in this analysis are 2.6, 5.0, 7.0, 9.0, and 19.3 g/cm 3. Figures 2, 3, and 4 represent the weight percentage o f particles travelling for a distance of 0.95 cm as a function o f time given by eq. 9 or 10. The angular velocities considered are 17.2, 39.0 and 209.4 rad/s. As can be seen in Fig. 2 for example, the recovery o f particles after 200 seconds centrifugation at the angular velocity o f 17.2 rad/s is nearly 100% I00
,
hi C.9
-
I--
/ 80
/
2:
,., o
_, .
/
//
/
~s0
II
¢O
'
/
/ /
~
//
//
/
.
.
.
.
.
.
~ _ _ ~
/ "
/ /
/
/
.
/
DENSITY ,,
/
/
19.3 o/©m 3
------
/ /
.....
//
9.0 7.0 ~.o
-----
2.6
o"',Oo,. ///// e,.
III
/
:
"
"
I
I 0
0
/I// I I00
I 200
i 300
I 400
I 500
TIME, S
Fig. 2. The weight percent of particles settled in centrifuge at angular velocity of 17.2 rad/s for various densities.
269
for density of 19.3 g/cm 3, 85% for 9.0, 73% for 7.0, 45% for 5.0 and 0% for 2.6. It should also be noted that under these conditions, the purity is 100% for all particles heavier than 2.6 g/cm 3 of density, assuming each system consisting of a heavy mineral and a gangue with 2.6 g/cm 3 density. This means that it is possible that minerals with the standard normal distribution and with the density greater than 5.0 g/cm 3 can be easily separated from silica by centrifugation at an angular velocity of 17.2 rad/s. The time required to achieve this separation can be reduced from 200 s to about 45 s at an angular velocity o f 39.0 tad/s, and to less t h a n 2 s at 209.4 rad/s (see Figs. 3 and 4).
~ :~___~_
,oo
,,=, I..: ,',// IJ
u. 8 0
~." /
/
: III
l\,'//
/
I~ [,71 6°1~,7/
OE,,~IT, _
/
......
/
,"
~, I;ili >-Iii:'/
/
~=4°1+Ii I " I::II: "~
/
/
/
/
19'3 l / c m l
.....
....
,.o T.o
------
~.o ~.~
,.,,,,
~9.or,,,,,.
fir
0
[
I
0
i
I00
i
200
I
300 TIME,
I
400
500
S
Fig. 3. The weight percent of particles settled in centrifuge at angular velocity of 39.0 rad/s for various densities. I00
/
~
----~- ~
--
~ec
,,,
Z
/
(n':' 60 "' _o
{
/ /
OE.S,T¥ ........ -------
/
19.3 g/cm 3 9.0 7.0
I-
~,0i.
-
-
/
~- 2c i
0 0
~o
--
--,
2.6
.
,
i 2
I
i 6
4 TIME,
I 8
I0
S
F i g . 4. T h e w e i g h t p e r c e n t o f particles settled in c e n t r i f u g e at angular v e l o c i t y o f 2 0 9 . 4 rad/s f o r various densities.
270
It is also seen in these figures that the recovery increases as the time of centrifugation is increased at the expense of purity. For example, the recovery of all heavy minerals is better t h a n 95% at 500 seconds, while the purity is not better than 50% at 17.2 rad/s (Fig. 2). Figure 5 shows that the purity o f heavy minerals given by eq. 8 can be projected as a function of centrifugation time. In a simple way of illustrating the purity of heavy minerals after separation in centrifugal force, two minerals with densities of 7 and 2.6 g/cm 3 were chosen. An equal a m o u n t of these two minerals with the same size distribution was to be introduced into a centrifuge. Ignoring the interaction between particles, it was found that 100% purity of heavy minerals could be obtained within 200 s at an angular velocity 17.2 rad/s in one second at 209.4 rad/s. TIME, ! I ~ ~ ~
I00
S
2 I
3 I
4 I
5 [
\\\ 90 O x ~
60
~
"
CJ~ r o d / @
3 1T.2 .~
40 . . . .
w" Q ~
209.4
2O
0
I
I00
L
I
I
200
300
4.00
TIME,
I
500
S
Fig. 5. The projected percent grade of heavy minerals with density 7 g/cm 3 separated from silica at angular velocities 17.2 and 209.4 rad/s. EXPERIMENTAL
A centrifuge, Damon/IEC Model CH was used in this investigation. Although the centrifuge is capable of running continuously with the aid o f perforated basket, batch tests were conducted in this study using a solid basket without holes. The batch system is easier to operate and y e t satisfactorily shows the effect of important variables on the recovery and the selectivity of heavy minerals. The centrifuge has seven knobs with which centrifugal force can be adjusted. The lowest speed can be obtained on position 1 (164 rpm = 17.2 rad/s) and the highest on position 7 (3400 rpm = 362.3 tad/s). Cassiterite as heavy and silica as light mineral were used as a model sys-
271 tem. Minerals were ground separately in a ceramic ball mill and particles with size range 1--5 microns were collected by repetitive sedimentation and decantation. The pH o f the slurry solution was adjusted with reagent grade HC1 and NaOH. The centrifuged particles after a set time were collected and analyzed by weight when a single mineral was used and by an X-ray diffractometer when a mixture o f cassiterite and silica was used. The settling distance, (r2 - rl) was fixed at 0.95 cm throughout the experiments (Fig. 1) and temperature was at 20 + 2°C. The capacity of centrifuge is about 320 ml. The centrifuge was loaded initially with about 300 ml of pure water and set at a constant speed until running at steady state when about 20 ml o f a slurry containing 0.2% by weight of either a single mineral or a mixture of cassiterite and silica was introduced into the center of the basket. Samples were withdrawn by a 70ml syringe at a depth of 0.95 cm from the air/liquid interface. The time required for sampling was within 10 s. When the speed of the centrifuge was too high to carry out sampling without disturbance, the speed of the centrifuge was reduced to the lowest before the sample was withdrawn. Because the X-ray analysis required at least 0.125 g o f sample, the above experiment was usually repeated until a desirable a m o u n t of samples was collected. Electrophoresis measurements were made using a Zeta meter. RESULTS AND DISCUSSION A slurry containing a single mineral, either cassiterite or silica was introduced into the centrifuge basket running fully at a set speed of 39 rad/s. I00
I
L
I
l
I
C A S S I T E R I T ~ ~ 80
,,, > o o ,d
H
y/j ///
,.
:--
.
o
,.o ,.o
•
///
• U3 :
-
,0
39.0rad/s
_
~
40
0 0
t
I
I
40
80
120 TIME,
Fig.
I 200
S
6 . The percent r e c o v e r y
angular velocity 39.0 rad/s.
I 160
of
cassiterite
and
s i l i c a o f 1 - - 5 microns in d i a m e t e r
at
272
Samples reported to Region II were collected at a set time and the a m o u n t of particles settled as a function o f time was measured. The results are plotted in Fig. 6. The centrifugation was carried out at three different pH values, namely pH 4, 7, and 9. From the tests carried out with single mineral particles, it can be seen that the recovery of cassiterite as well as silica is best at pH 4 and worst at pH 9. This is because as the pH of the solution approaches near the point of zero charge of these minerals (PZC of SiO2 = pH 3; PZC of SnO2 = pH 4), flocculation of these particles occurs resulting in large flocs which helps settling of these particles faster but the purity would have been impaired if these minerals were to be recovered from a mixture. On the other hand, at pH 9 complete dispersion of both mineral particles was observed. Another series of experiments were carried out where a slurry containing the same a m o u n t o f cassiterite and silica was introduced into the centrifuge. The particles centrifuged at a set angular velocity were collected at a set time and analyzed using an X-ray diffractometer. The time o f centrifugation was adjusted to recover better than 95% recovery o f cassiterite. The grade expressed in the weight percent o f cassiterite reported into Region II was determined and the results are plotted in Fig. 7. The results are reasonably in good agreement with those obtained in the tests with single mineral systems (Fig. 6). Because of the interaction between mineral particles, the reproducibility was not as good as the single mineral system, especially at pH 4. Nevertheless, there is a consistent trend seen in these results and the results obtained earlier. It can be concluded that the efficient separation o f ultrafine and heavy ,oo
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Fig. 7. The percent grade of cassiterite obtained at angular velocity of 39.0 rad/s from a mixture of cassiteriteand silicaparticlesof 1--5 microns.
273
minerals is possible by the use of centrifugal forces. The separation efficiency of cassiterite from silica particles with size range 1--5 microns was found to show typically a recovery over 95% with 80% purity. ACKNOWLEDGEMENT
This research has been partially supported b y the Faculty Grant Committee of South Dakota School of Mines and Technology.
REFERENCES Collins, D.N. and Read, A.D., 1971. Treatment of slimes. Miner. Sci. Eng., 3: 19--31. Coulson, J.M. and Richardson, J.F., 1978. Chemical Engineering, Vol. II. Pergamon Press, New York, N.Y. Fuerstenau, D.W., 1980. Fine particle flotation, In: P. Somasundaran (Editor), Fine Particles Processing. AIME, New York, N.Y., pp. 669--705. Trahar, W.J. and Warren, L.J., 1976. The flotability of very fine particles - - a review. Int. J. Miner. Process., 3: 103--131.