FIELD OF THE INVENTION
The present invention relates to a system to produce less than 1 mm size fine
particle for uniform dispersion of immiscible fluid having continuous and
discontinuous phases.
BACKGROUND OF THE INVENTION
In mineral processing and pulp industry, selective separation of desired and
undesired component is achieved through air bubbles. For example, in coal
flotation process, pure coal particles are carried away by air bubbles in a vertical
counter-current flow of a feed constituting coal mineral fed downward and air
sparged through spargers at bottom of the flotation column. As the bubbles
move upward, the coal particles are attached to the air bubbles due to its
hydrophobic nature. In this type of processes, bubble size, numbers of bubbles,
gas hold up as well as hydrodynamics are key parameters to determine the
efficiency of the attachment. The parameters namely, bubble size, numbers of
bubbles, and gas hold up are the determining factors for interface available to
have the reactions, mass transfer and gas particle interaction for attachment. In
general, the key factor for the process is how good the sparger or dispersion
system to have better interface. A schematic representing a process utilizing the
internal sparger system is shown in Figure l. Figure 1 depicts a process where a
feed constituting of desired and undesired particles is fed from top and air
bubbles are generated via a sparger system at bottom. As the bubbles move
upward, it comes in contact with the feed material, the particles having affinity
to the bubbles are attached to the bubbles and then carried away by bubbles on
the top and recovered. In this type of process, the sparger is the critical element
of the separation system.

There are two broad categories of spargers known in the art:
a) Internal spargers where air or gas is passed directly into a column
b) External spargers where gas and liquid are contacted outside of the
column and then the mixture is directed into the bottom of the column.
Figure 2 shows the embodiment with internal and external spargers. The internal
spargers are categorized into two streams: porous spargers (such as sintered
glass) and single and multi nozzle spargers (such as punched plate) [Finch and
Dobby, 89]. The internal spargers are prone to clogging after certain hours of
operation in the industrial environment of the abrasive slurries and the corrosive
waters, and hence it requires lot of maintains and plant shutdown to change new
one. To overcome the above issue, external spargers based on static elements
are presently implemented in the industry. A schematic representing a process
utilizing the external sparger system is shown in Figure 3. The continuous phase
is contacted to discontinuous phase in the external sparger with static elements.
Basically, the static elements works in two ways: first it provides a higher strain
rate near static elements due to higher velocity gradient, and secondly, they
provide transverse flow in order to enhance stretching of the discontinuous
phase which leads to higher interfacial area. Once, the interfacial forces becomes
large enough due to increase in the interface area such that the interface area
cannot withstand the interfacial forces, the small bubbles or droplets break-up.
The advantages of external spargers over internal spargers are multifold: less
chance of clogging or plugging with solids, online sparger maintenance, and a
degree of control over bubble size (by manipulating number of static elements,
and by manipulating gas and fluid stream flow). Recently number of static
external spargers are used in the laboratory, pilot, and in industrial columns. To
name the spargers used by Yoon et al., 1988 in flotation column for mineral

beneficiation and sparger used by Libby et al., 1982 for solvent extraction. The
static spargers used by Yoon et al., 1988 are shown in Figure 4.
BUBBLE GENERATION MECHANISM IN STATIC MIXERS USED AS EXTERNAL
SPARGERS
A static mixer consists of a series of stationary mixing elements inserted end-to-
end in a pipe (for example see Figure 2 (b) showing Kenics mixer). Each element
is a specially designed rigid structure that divides the flow and recombines it in a
geometric sequence. Mixing and contacting take place as the fluids are sheared
and directed radially across the pipe or duct . In other words, the presence of
elements provides transverse or secondary flow. The only power required for
static mixers is the external pumping power that propels the fluids through the
mixer.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a device to produce less than
1 mm size fine particles for uniform dispersion of immiscible fluids having
continuous and discontinuous phases.
A further object of invention is to propose a device to produce less than 1 mm
size fine particles for uniform dispersion of immiscible fluids having continuous
and discontinuous phases, in which the spargers are operated where
gravitational forces working upward or downward direction of main flow.

A still further object of the invention is to propose a device to produce less than
1 mm size fine particles for uniform dispersion of immiscible fluids having
continuous and discontinuous phases, which is enabled to eliminate accumulation
of air bubbles near lateral walls of the sparger due to buoyancy force.
SUMMARY OF THE INVENTION
According to the invention, the system compress at least two types of external
static cross spargers. The spargers consist of especially designed 'X' Shaped
baffles in its one unit; the second element is just rotated by 90° to the axial
direction to get the mirroring effect of the flow profile. As flow move forward,
this baffles act as barrier to the flow providing higher strain rate as well as the
baffles are oriented in such a way that it provides transverse or secondary flow
leading to extra strain rate. This allows breaking bubbles into finer bubbles and
increasing the gas hold-up. In flotation process generally bubble size below 1
mm is required to carry out hydrophobic minerals at the top and it is found that
the spargers developed in this study are able to provide the range of bubble size
and gas hold up suitable for flotation process.
The system comprises a first device, static three cross sparger which each
employs an element consisting of three cross X-shaped baffles each at 90° with
respect to each other followed by 90 degree rotated copy of this element and
these two elements form a periodic unit. A second device constituting a static six
cross sparger employing an element consisting of six cross X-shaped baffles. On
the basis of requirement and operation, a number of periodic modules can be
added in the system. The dispersion phenomenon increases the interfacial area
and hence mass and heat transfer increases. The system is enabled to achieve

dipersion of one phase in another phase for faster heat and mass transfer as well
as recovery of desired products.
The system incorporates two types of micro-bubble generators/spargers with
static elements along axial direction of the conduit are used. The static cross
spargers are micro bubble generators consisting of gas inlet and liquid/slurry
inlet and outlet for the stream.
The two elements of the cross spargers are the periodic elements which can be
added on in plurality to make a complete sparger.
The crossbar element have length to diameter ratio (L/D) in the range of 0.5 to
1.5.
The system comprises means to bypass materials from the bottom of the bubble
column to the spargers to increase the residence time of bubbles as well as to
avoid extra liquid to be used to be injected to carry air in spargers.
The static cross spargers are placed outside the floatation column.
The static cross spargers are placed in horizontal direction once the residence
time of the bubble is small enough such that the bubbles does not reach the
lateral wall in that time. This can be achieved via high inlet liquid velocity.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is a schematic diagram of a flotation process for mineral processing
according to prior art where air bubbles are generated at bottom using internal
spargers to capture hydrophobic particles.
Figure 2 is the prior art spargers in flotation column (a) internal conventional
porous sparger, (b) static sparger.
Figure 3 is the schematic diagram of the invented process where air bubbles are
generated at bottom via external spargers, air and continuous phase being
contacted in the external sparger having a series of static elements.
Figure 4 shows external spargers of prior art disclosed by Yoon et al., 1988 to
generate micro bubbles.
Figure 5 shows the inventive system having an external sparger with (a) Static
three cross sparger with three plates, (b) the basic first element with 3 plates,
(c) Static six cross sparger with 6 plates and (d) basic first element with 6 plates.
Figure 6 shows the Grace Curve (Critical Capillary number Vs Viscosity ratio)
which exhibits criterion for bubble breakup in shear flow and elongation flow.
Figure 7 shows schematics about procedure to calculate gas hold up in a
flotation column using pressure transducers (PT).
Figure 8 shows bubble size distribution without frother for static cross spargers.

Figure 9 shows bubble size distribution with frother for static cross spargers.
Figure 10 shows the gas holdup for static cross spargers with and without
forther.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, the system comprises two static cross
spargers based on cross-bar 'X' shaped baffles. Figure 5 shows the proposed two
sparger of the system used as external micro bubble generators. The basic
building element of the static three cross sparger consists of three plates/bafflers
covering whole cross-section. The static element consists of two 'X' shaped
cross-bars (Figure 4b shows the three plates which makes two 'X' cross-bars
series) and the angle between these opposite cross-bars is 90°. The second
element is identical to the first element (Figure 4 (a)) but rotated by 90° with
respect to the axial direction. These two element are the periodic elements of
the sparger. The second static six cross sparger consists of six static element of
'X' shaped cross-bars which makes three 'X' cross-bars series in the cross
section. The angle between these opposite cross-bars is 90°. The second
element is the copy of first element but rotated by 90° with respect to the axial
direction. In these spargers only four elements are shown, however in reality 10-
16 elements can be used depending on the process.
According to the invention the spargers of the system are configured with static
elements, air and water are injected at the air and water inlets of the static mixer
and moves co currently. Once a slug of air with water passes the static elements,
it is broken into small pieces to form micro bubbles by shearing action of the
static blades of the generator. The bubbles are formed due the shear force

developed by the moving the aerated waster/slurry against stationary blades and
contact between air and water increased by the intensive mixing provided by the
changing the direction of movement. Bubble deformation or drop defoermation is
mainly governed by the Capillary number which is ratio of the shear stress
exerted on the bubble or drop by the flow field and the interfacial shear stress :

From the dispersed phase point of view, this number can be seen as the ratio of
the "destructive" viscous forces responsible for the rupture of the secondary
phase over the "protective" surface tension forces opposing the deformation. The
inertial forces do not dominate due to the laminar flow regime. If the Capillary
number is small, the interfacial stress withstands the shear stress and no
breakup occurs. Above a critical value, CaCrit, viscous shear stress overrules the
interfacial stress and the bubble/drop fragments in smaller bubbles/drops.
Experimentally, Cacrit, is determined by slowly increasing the shear rate and
evaluating whether or not the deformed shape is stable on the long run. Taylor
(1934) and Grace (1982) showed that the Cacrit depends both on viscosity ratio
(p=ηc/ηd) and the flow type. Figure 6 show Grace's experimental results for

Newtonian liquids in both simple shear and 2-D elongational (plane hyperbolic)
flow for a wide range of the viscosity ratio p between dispersed and continuous
phase. At lower viscosity ratios p<0.1 (p=dispersed/continuous), in a simple
shear field, very high stresses must be generated in order to break a bubble or a
drop. With viscosity ratios above 4.0 steady simple shear flows cannot generate
stresses high enough to lead to break-up. With extensional shear flows however,
the capillary numbers remain within reasonable values for a very wide range of
viscosity ratios. The viscosity of the dispersed phase is a major parameter in the
dispersion phenomenon. Note that in real flow condition, there is always mix up
of shear and elongational flows and hence the curve for breakup in industrial or
real flow case are somewhere between the two curves in Grace plot as shown in
Figure 6. In the turbulent flowregime, it has been shown that the width of the
bubbles/drops distribution increase with the viscosity ratio.
The behavior of the size reduction in the mixer can be explained by a kinetic
phenomenon. With the dispersed phase passing through the obstacles formed by
the element, the drops break in a repetitive process until the kinetic equilibrium
is reached between breakup and coalescence. The point where this equilibrium is
reached is a function of the dissipated energy and the liquid or gas system
involved. The mixing length to establish the equilibrium can be directly compared
to the mixing time in tanks.
Flows in a device comprise shear as well as elongational flow. Elongational flow
is better for breakup, however complete elongational flow is impossible to realize
for a flow in a device.

Note that a surfactant or frothing agent is present in the water to assist the
bubbles in their formation and give them sufficient stability and assistance in
preventing coalescence of the bubbles in industrial flotation column.
EXPERIMENTAL PROCEDURE
In this invention, the spargers constitute static three cross sparger and static six
cross sparger (see Figure 5) which are tested in a lab scale bubble column with
diameter of 10 cm and height 2m. At the bottom of the bubble column, the
external spargers are arranged for generating air bubbles in to the column. The
sparger has air inlet and water inlet, in which water act as a continuous fluid and
air as dispersed fluid elements. The water flow rate is controlled by the
centrifugal pump and the air inlet is measured by the air rota meter. For
calculating the pressure inside the column, pressure transducer is arranged in
the bubble column which can be used to calculate the gas hold up (see Dobby
and Finch (1989)) as follows :

Where - Gas holdup in the bubble column
P1 - Density of the water
ΔP - Pressure difference between two points
g - Gravitational acceleration.
- Height difference between two points.
The pressure difference (ΔP) is measured using pressure transducers. Initially
the bubble column was filled with water and the pressure transducer reading
was noted. After reaching the steady state value in the pressure transducer, air

flow was started. The steady state pressure transducer reading was noted with
air flow rate. The air flow rate range was varied from 3-15 1pm (litre per
minute). The water flow rate range was pumped with the pressure of 3-10
kg/cm2 and the respective flow rate was measured with measuring cylinder for a
stipulated time. Figure 7 shows the schematics of a device with pressure
transducers to get the gas hold up.
EXPERIMENTAL RESULTS
Bubble size distribution without frother and with frother
Figure 8 and 9 shows the quantitative bubble size distribution without and with
frother. It is evident qualitatively that frothers play a major role in reducing the
bubble size. With frothers, very fine and uniform bubbles less than 0.5 mm size
are observed.
Gas holdup Studies
Gas holdups have been calculated for static three cross sparger and static six
cross sparger with and without frother. The pressure transducer measurements
were taken to calculate gas holdups as described in equation (2). The gas
holdups for all the tested spargers are found to be higher with frother than
without frother as shown in Table 1. The reason is obvious that a frother reduces
the surface tension of the water significantly. Without adding the frother the gas
holdups are 5.9% and 6.1% for static three cross sparger and static six cross
sparger, respectively. By adding a commercial frother (2ml frother in 40 liter of
water), the experiments are repeated by keeping the similar conditions as done
without frother. The gas holdups are found to be 19.9% and 19.7% for, static

three cross sparger andf static six cross sparger, respectively. Relative
comparison of a sparger performance is shown in a histogram as shown in Figure
10. As evident from above analysis keeping in mind industrial practice, the static
cross spargers are suitable for flotation columns. Overall, it can be concluded
that the new designs provide better gas holdups than the commercially available
spargerrs.

References:
Finch and Dobby, Column flotation, year 1989, Pergamon Press
Yoon, R. H. Adel, G. T. and Luttrell, G. H. year 1988, U.S. patent application
number 5761008, A process and apparatus for separating fine particles by
microbubble flotation together with a process and apparatus for generation of
microbubbles.
Libby, D. R. Magnolia, Chen, S. 1, year 1982, U. S. patent application number
4314974, solvent extraction method using static mixers.

Taylor, G.I., "The formation of emulsions in definable fields of flow, "Proc. R.
Soc. London, Ser. A 146 (1934), pp 501-523.
Grace, H. P., "Dispersion phenomena in high viscosity immiscible fluid systems
and application of static mixers as dispersion devices in such systems," Chem.
Eng. Commun.14 (1982),225-277.

WE CLAIM :
1. A bubble coloumn device to generate ultrafine and uniformly dispersable
particles of inmiscible fluid having continuous and discontinuous phases,
comprising at least two external Sparger devices, wherein the first sparger
device consisting of at least one static three cross sparger having a static
element comprising three plates to form at least two x-cross bars
opposedly disposed at an angle of 90° with respect to the axial direction,
and wherein the second sparger device consists of at least one static six
cross sparger having six plates to form at least three X-cross bars; a
centrifugal pump to control flow of water through the water inlet of the
spargers, and acting as a continuous fluid; a source to supply air inside
the bubble column device to act as dispersed fluid elements; an air rota
meter to measure the air inlet into the column device through the air
inlets of the spargers; and a pressure transducer to calculate the gas
hold-up inside the bubble coloumn, wherein the spargers are disposed
vertically at the bottom of the bubble column to operate against the
gravity forces acting upward or downward direction of the main flow, and
wherein the second sparger device is rotatably disposed at 90° with
respect to the axial direction.
2. The device as claimed in claim 1, wherein the static elements have a
length to diameter ratio of 0.5 : 1.5.
3. The device as claimed in claim 1, comprising means to bypass materials
from the bottom of the bubble column to the spargers to increase the
residence time of the bubbles.

4. The device as claimed in claim 1, wherein the spargers operate as micro-
bubble generators and comprise outlets for the liquid/slurry stream.
5. The device as claimed in claim 1, wherein uniform bubbles of less than 0.5
mm size is generated.

ABSTRACT

The invention relates to a bubble column device to generate ultrafine and
uniformly dispersable particles of inmiscible fluid having continuous and
discontinuous phases, comprising at least two external Sparger devices, wherein
the first sparger device consisting of at least one static three cross sparger
having a static element comprising three plates to form at least two x-cross bars
opposedly disposed at an angle of 90° with respect to the axial direction, and
wherein the second sparger device consists of at least one static six cross
sparger having six plates to form at least three X-cross bars; a centrifugal pump
to control flow of water through the water inlet of the spargers, and acting as a
continuous fluid; a source to supply air inside the bubble column device to act as
dispersed fluid elements; an air rotameter to measure the air inlet into the
column device through the air inlets of the spargers; and a pressure transducer
to calculate the gas hold-up inside the bubble column, wherein the spargers are
disposed vertically at the bottom of the bubble column to operate against the
gravity forces acting upward or downward direction of the main flow, and
wherein the second sparger device is rotatably disposed at 90° with respect to
the axial direction.