The effect of solids on the behaviour of the downcomer of a jameson cell

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 04 Issue: 11 | Nov-2015, Available @ http://www.ijret.org 252
THE EFFECT OF SOLIDS ON THE BEHAVIOUR OF THE
DOWNCOMER OF A JAMESON CELL
Ramiro Escudero, Eloy Tapia, Ricardo Morales
Instituto de Investigaciones en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo,
Morelia 58000, Mexico. Tel/Fax: (+52) 443 322 3500 ex. 4018
*Corresponding autor: ramiro1963@gmail.com
Abstract
The effect of solids on the behaviour of the downcomer of a Jameson cell was studied in terms of the hydrophobic/hydrophilic
character of the solids. Hydrophobic (carbon), and hydrophilic (silica sand) solids were used, separately. The experiments were
carried out under controlled conditions of gas flow rate, pulp flow rate, and pulp consistency. The observed operating variables
were the extension of the downcomer operating regions (pulp jet, mixing, and collection) and gas hold-up. It was observed that
gas bubbles are smaller and more uniform in size when the pulp is comprised of silica (hydrophilic particles), as compared with
pulps consisted of carbon (hydrophobic particles).
When measuring a profile of gas holdup in the separation cell, experimental results show that a more homogeneous radial holdup
distribution is achieved in the case of a slurry with silica sand rather than the pulp made of carbon.
Key words: Jameson cell, downcomer, separation cell, superficial phase velocity, gas hold-up, hydrophobic solids,
hydrophilic solids.
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INTRODUCTION
Inverted flotation columns, or Jameson cells, were
developed initially to separate, by froth flotation, fine
particles, which normally are difficult to float in
conventional flotation systems (mechanical cells and
flotation columns) [1]. The effectiveness of flotation process
in the Jameson cell has been shown even in non mineral
separation processes, as for instances recovering organics in
solvent extraction applications. [2, 3, 4]
The Jameson cell comprises two bodies: the downcomer and
the separation cell. The down-comer constitutes the inverted
bubbling column, and in this part of the cell, both the slurry
and the gas bubbles are in close contact forming
hydrophobic solids-bubble aggregates. The slurry and the
aggregates are discharged down into the separation cell.
Because of its diameter, the separation cell observes a
smaller superficial phase velocities when compared to the
downcomer. This allows the aggregates to float and be
collected at the surface of the cell as concentrate.
Depending on the physicochemical characteristics of the
slurry-gas dispersion and operating parameters such as the
superficial gas and slurry velocities in the downcomer, the
resulting extension of the operating zones (free slurry jet,
mixing, and collection), bubble size, and gas hold-up in the
downcomer can be controlled; therefore, the product from
the downcomer, which depends from the operating
conditions, affects the behaviour of materials in the
separation cell, determining the characteristics of axial and
radial circulation and mixing, as reported by Koh and
Schwarz [5], mentioning that the local value of the turbulent
dissipation rate has a direct influence on the local particle-
bubble detachment rate.
Regarding the effect of operating conditions (nozzle
diameter, downcomer diameter, jet velocity, and jet length),
on the Jameson cell performance, Tasdemir and co-workers
[6, 7], concluded that downcomer diameter observes a very
little effect on gas entrainment rate while increasing values
of other factors had an increasing effect on it.
The effect of solids content (hydrophobic and hydrophilic
solids) fed to a Jameson cell is analysed in terms of the
downcomer behaviour.
EXPERIMENTAL
The experiments were carried out in a laboratory Jameson
cell (downcomer: 5 cm in diameter, 180 cm in length;
separation cell: 50 cm in diameter, 100 cm in length), made
of transparent acrylic plexiglass.
The slurry and the air were fed separately into the
downcomer through inlets located at its top. The flowrates
of the feed (1.7 to 6.1 L/min), and tailings were controlled
by means a peristaltic pumps (Masterflex: 0 – 20 L/min).
The fed gas (0.17 to 2.11 L/min) was fixed and controlled
using an automatic air mass flow meter. Both the superficial
gas and liquid flowrate (Jg, and Jl, respectively) are referred
to the downcomer cross-sectional area.
Tap water was mixed with carbon or silica particles (D80 = -
30 µm), separately, to form low density slurries (1% and 2%
solids (w/w)). The slurries were fed into the Jameson cell
after conditioning in a tank with a certain amount of
surfactant (MIBC).
Gas hold-up was estimated in the downcomer through
conductivity measurements (Tacussel électronique; CDRV

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62) of both, the slurry only and the slurry-gas dispersion, by
applying the Maxwell’s conductivity model: [4]
ε = [1 – (κd / κl)] / [1 + 0.5(κd / κl)]
(1)
being κd the conductivity of the dispersion (in this work is
the conductivity of the gas-slurry) and κl the continuum
conductivity (in this work is the slurry only conductivity);
and єg is the volumetric fraction of the non conducting
dispersed phase (or the dispersed gas bubbles) named gas
hold-up. Therefore, once the slurry only conductivity and
the gas-slurry conductivity are known, the gas hold-up can
be estimated.
Maxwell’s conductivity model has proven to be an accurate
and reliable method to estimate the volumetric fraction of
non conducting phase dispersed in a conducting continuum.
[8, 9]
The conductivity of the slurry (continuous phase), with no
air, was continuously measured through a flow cell installed
in the top point of the downcomer, in this way the property
κl in Maxwell´s model (eq. (1)) is known; on the other hand,
the electrical conductivity of the slurry-gas dispersion (κd) is
constantly measured by using a conductivity flow cell
located at the outlet of the downcomer [10]. With the proper
knowledge of both κl, and κd, the gas holdup can be
estimated (eq. (1)).
Radial and axial profiles of gas hold-up and superficial gas
velocity, in the separation cell, are estimated by using a
laboratory home-made probe. [11] In this case, the slurry
only conductivity and the conductivity of the gas-slurry
dispersion are measured by using the “gas separation
method.” [12]. This method consists in measuring the gas-
slurry dispersion conductivity, κd, with an open conductivity
flow cell, while the slurry only conductivity, κl, is measured
in a siphon conductivity flow cell, where the gas bubbles are
excluded. Therefore equation (1) can be applied to estimate
the gas holdup in the point of measurement. The probe is
presented in Figure 1, whereas the experimental apparatus is
shown in Figure 2.
open conductivity
flow cell
siphon conductivity
flow cell
open
conductivity
flow cell
siphon
conductivity
flow cell
one way
valve
pressure
transducer
2 m
Figure 1. Gas holdup probe to measure the superficial gas
velocity [8], the conductivity of the gas-slurry dispersion
(open conductivity flow cell), and the slurry only
conductivity (siphon conductivity flow cell [9].
Air
Flowrate
Conductivity Flow Cell
Downcomer
Lounder
Feed
Separation
Cell
Conductivity
Flow Cell
Baffle
Conditioning tank
Concentrate
Tailings
Figure 2. Schematic representation of the experimental
setup.
RESULTS AND DISCUSSION
It has been experimentally observed that the hydrophobic
degree of a dispersed phase in a gas-liquid dispersion has an
effect on bubbles coalescence conduct, and therefore on the
gas hold-up behaviour of the dispersion; this demeanour, in
turn, will affect the circulation-mixing patterns in such
dispersions. [13]
The addition of hydrophobic or hydrophilic solids in an
aqueous phase in amounts to produce low consistency
slurries is visualised in Figures 3 and 4 in terms of the gas
hold-up as a function of the superficial gas velocity in the
downcomer.
Figure 3 shows the stability range of the operating
conditions (Jg and Jl), on the existence of collection zone in
the down-comer as a proper performance of the flotation
process. There is observed that the collecting zone in the
downcomer increases with the superficial slurry velocity. It
is important to visualise the operation of the downcomer
where apparently the gas and slurry flows in co-current;
however, there is a counter-current flow between these
phases, the slurry moving downwards while the gas bubbles
move upwards in the slurry. Under this scene, larger bubbles
will be retained for longer periods of time as compared with
small bubbles which are easily transported downwards by
the stream.

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Figure 3. Effect of the superficial gas (Jg), and slurry
velocities (Jl), on the gas hold-up in the downcomer. Slurry
with 1 % (w/w) of carbon particles, and without frother.
The large retention time of the gas bubbles in the
downcomer will create large gas holdup values. It has been
pointed out that hydrophobic solids may induce bubbles
coalescence resulting in the formation of larger bubbles.
[14] In carbon slurries – gas systems, the carbon particles
present a hydrophobic behaviour and, therefore, formation
of large bubbles is likely to occur because of bubbles
coalescence induced by the hydrophobic nature of the
carbon particles, as it was visually observed in these
experimental systems.
The effect of hydrophilic solids on the downcomer
performance is presented in Figure 4 where silica slurries-
gas systems are illustrated. Silica is a well known
hydrophilic material, this characteristic is due to the
formation of silanol groups on its surface when it is
immersed in aqueous media. It can be observed that
operating superficial slurry velocity (in the downcomer) is
restricted to a narrower range. This behaviour is caused by
the production of small bubbles whose coalescence is
prevented by the presence of the hydrophilic particles of
silica.
0 1 2 3 4 5 6 7 8 9
Jg, cm/s
0
10
20
30
40
Eg,%v/v
Jl = 5.6 cm/s
7.9
10.6
13.4
15
17.8
19.6
21.3
Figure 4. Gas Holdup behaviour in the downcomer as a
function of both the superficial gas and liquid velocities.
Silica-water slurry without frother (surface tension of the
liquid = 72 dyn/cm), 1 % (w/w) solids.
In light of the above, small bubbles present a small
resistance to the slurry flow to maintain the packed bed of
bubbles into the slurry stream in the downcomer collection
zone, being pushed downwards by the gas that is fed into the
downcomer, decreasing the extension of the collection zone,
and then being replaced by the jet zone. This effect is
compensated by increasing the superficial slurry velocity in
the downcomer in order to sustain a proper collection zone
operation.
The effect of increasing the slurry consistency by adding
carbon or silica up to 2% is presented in Figures 5 and 6.
These flotation systems are performed with surfactant
additions.
Figure 5. Gas hold-up in the downcomer for two solids
content (2 %, w/w) in the slurry. Surface tension of the
liquid equal to 65 dyn/cm. (90 ppm of frother MIBC).
Figure 5 shows that, under the same slurry velocities, there
is not an appreciable difference in the behaviour of the gas
hold-up in the down-comer when the slurry consistency is
changed from 1 to 2% solids; however, the gas hold-up
values are lower as compared with the gas hold-up in the
equivalent gas-carbon slurry system without surfactant
additions (see Figure 3), which indicates a decrease in
bubble size as a result of the surfactant presence.
Even with the surfactant additions, the range of stability of
the collection zone in terms of the superficial gas velocity
maintained up to 5 cm/s. When the superficial gas velocity
exceeds 5 cm/s, the increase in the slurry hydrophobic solids
content appears to promote bubbles coalescence, since the
gas hold-up in the down-comer increases markedly.
Figure 6 shows the gas hold-up behaviour in the down-
comer under superficial silica slurry velocities from 5 to 21
cm/s; this figure compares the flotation system, under
surfactant additions, containing 1% solids with that of 2%
solids.

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Figure 6. Changes on gas hold-up in the downcomer with
the superficial gas and slurry velocities. The slurry is
constituted by silica particles (1 and 2%, w/w), and 20 ppm
of surfactant.
The experimental measurements indicate that under
surfactant additions the stability of the collection zone is
maintained up to a gas hold-up value of 20% in the range of
superficial slurry velocity between 5 cm/s and 21 cm/s.
It can be seen that for each fixed value of superficial slurry
velocity exist a particular maximum superficial gas velocity
at which the collection zone is found; and, over that
maximum value the collection zone is replaced by the jet
zone. Under the last circumstances the flotation process
becomes erratic and the operation is unstable.
These results show that in the downcomer stable operating
zone, there is a linear relationship among the gas hold-up
and the superficial gas velocity for every fixed condition of
superficial slurry velocity, this may indicate that bubbles are
more homogeneous in size and more evenly distributed in
the down-comer, preventing bubbles coalescence because of
the hydrophilic nature of the silica particles (and because the
surfactant additions).
By using the experimental device depicted in figure 1, the
gas hold-up and the superficial gas velocity were estimated
in the separation cell, in order to observe the effect of the
downcomer behaviour in the distribution of the gas
dispersion in this separation cell. Figures 7 and 8 show the
gas hold-up distribution in the separation cell as a function
of the superficial gas velocity in the downcomer for a gas-
carbon slurry flotation system, and the gas-silica slurry
system, respectively.
0 1 2 3 4 5 6 7 8
Jg down-comer, cm/s
0.5
1
1.5
2
Egradial,%v/v
r = 0
r/2
r
r = 0
r / 2
r
Figure 7. Radial distribution of the gas hold-up in the
separation cell. Slurry made with carbon particles (2 %,
w/w), and surface tension of the liquid = 65 dyn/cm.
Figure 7 shows a radial distribution of the gas content in the
separation cell for a carbon-water slurry. It is noticed that
the radial gas hold-up differences in the separation cell are
maintained when the superficial gas velocity in the
downcomer does not exceed 5 cm/s, being lower in the
center of the cell (r = 0), indicating that the larger gas
bubbles float in that region; however, when the superficial
gas velocity in the downcomer exceeds the 5 cm/s, the gas
hold-up becomes lower close to the wall of the separation
cell as a result of a high circulation and mixing produced by
the operating conditions.
Radial differences in superficial gas velocity and gas hold-
up will promote circulation and mixing in the separation
cell, which will induce turbulences affecting the
metallurgical performance of the process.
Figure 8 presents the experimental gas hold-up values in the
radial direction in the separation cell when the flotation
system consisted of gas-silica slurry. It can be observed that
there are small radial differences in gas hold-up all the way
through the operating conditions of the downcomer.
0 1 2 3 4 5 6 7
Jg down-comer, cm/s
0
0.5
1
1.5
2
2.5
3
3.5
Egradial,%v/v
r = 0
r / 2
r
r = 0
r / 2
r
Figure 8. Radial distribution of the gas holdup in the
separation cell. Slurry made with silica particles (2 %,
w/w), surface tension = 71 dyn/cm (20 ppm of MIBC).
The radial gas distribution pattern in the silica slurry (2%,
w/w solids) is maintained through the entire range of
superficial gas velocity in the downcomer. The gas hold-up
tends to decrease slightly as the measurement point is farther
from the downcomer; this indicates that the gas bubbles are
homogeneous in diameter, and the downcomer operation is
mainly affected by the type of solids in the slurry.
These observations suggest that gas bubbles are
exceptionally uniform in size and distribution in the radial
direction of the separation cell, which may be explained by
the hydrophilic nature of silica that prevents bubbles to
collide and eventually to coalesce.
CONCLUSIONS
The experimental results from this work show that the
operation of the downcomer in the Jameson cell is affected
by the superficial gas velocity, the superficial slurry
velocity, and the hydrophobic – hydrophilic nature of the
solids in the slurry.

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The presence of a collection zone in the downcomer
depends on the balance between the superficial gas velocity
and the superficial slurry velocity. Unbalanced gas-slurry
operations of the downcomer produce the unsteadiness of
the collection zone in such a way that the entire downcomer
is occupied by a free slurry jet. Under these circumstances
the separation process of hydrophobic particles by the gas
bubbles in the downcomer does not exist, and the behaviour
of the device as an inverted flotation column does not
succeed.
It was observed that the presence of hydrophobic solids in
the slurry induces bubbles coalescence in the downcomer,
producing a given distribution of bubbles that is reflected in
a remarkable turbulent behaviour in the separation cell. On
the contrary, slurries consisted of hydrophilic solids prevent
the bubbles coalescence, and produce more homogeneous
size of bubbles as well as a stable packed bubbles fluidized
bed in the down-comer.
Invariably, the process produces differences of gas content
in the radial direction of the separation cell. However, these
differences are larger when slurries consisted of
hydrophobic solids. Therefore, the circulation, and mixing,
in the separation cell is enhanced by the production of large
bubbles in the downcomer, particularly, under high gas and
slurry flow rates.
REFERENCES
[1] Dawson, W. J., Yannoulis, G. F., Atkinson, B. W.,
Jameson, G. J. 1996. Applications of the Jameson cell
in the Australian coal industry. In Column’96.
Proceedings of the International Symposium on Column
Flotation, The Met. Soc. Of CIM, pp. 233-246.
[2] Clayton, R. L. 1994. Recovering organic from raffinate
using Jameson cells. A paper presented at Arizon AIME
Conference, Tucson, AZ, December 1994, pp. 1-12.
[3] Dawson, W. J., Jackson, B. R. 1995. Evolution of
Jameson cells for solvent extraction applications. A
paper presented at Copper Hydrometallurgy Forum,
Brisbane, Australia, September, 1995, pp. 1-10.
[4] Maxwell, J. C. 1892. A Treatise of Electricity and
Magnetism. Vol. 1 Oxford Univ. Press, Part II, pp. 435-
449.
[5] Koh, P.T.L., and Schwarz, M.P. 2009. CFD Models of
microcell and Jameson flotation cells. Proceedings of
the Seventh International Conference on CFD Minerals
and Process Industries CSIRO, Melbourne, Australia,
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[6] Tasdemir, T., Tasdemir, A., and Oteyaka, B. 2011. Gas
entrainment rate and flow characterization in
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Problems of Mineral Processing. Vol 47, pp 61-78.
[7] Tasdemir, T., Tasdemir, A., and Gecgel, Y. 2011.
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[8] Tavera, F. J., Gomez, C. O., Finch, J. A. 1996. Novel
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[9] Tavera, F. J., Escudero, R., Gomez, C. O., Finch, J. A.
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[10]Tavera, F. J., Gomez, C. O., Finch, J. A. 1998.
Conductivity flow cells for measurements on
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[11]Tavera, F. J., Escudero, R. 2010. On-line superficial
bubble surface flux measurements in mineral flotation
systems. Proceedings of the Symposium on Chem.
Engng. And Environmental Tech., Aguascalientes,
México, pp 170-176.
[12]Tavera, F. J. 1996. Ph. D. Thesis, McGill University,
Montreal, Canada.
[13]Spyridopoulos, M., Simons, S., Neethilng, S., Cillers, S.
2004. Effect of humic substances and particles on
bubble coalescence and foam stability in relation to
dissolved air flotation process. Physicochemical
Problems of Mineral Processing, 38, pp. 37-52.
[14]Tse, K. L., Martin, T., McFarlane, C. M., Nienow, A.
W. 2003. Small bubble formation via a coalescence
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Science, 58 (2003), pp. 275-286.

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