Field of Invention
The present invention relates to a method of flotation for improving clean coal
yield by a column flotation with an external sparger.
Background of the invention
In mineral processing plants froth flotation is widely used for beneficiation of
fines. It is physic - chemical separation process in which clean coal (less ash)
particles selectively separated by air bubbles due to hydrophobicity of coal.
Currently in India coal washeries, have been using conventional flotation cells for
beneficiation of raw coal fines by froth flotation process. Flotation reagents
collector and frother respectively added in conditioning tank prior to flotation.
Collector is hydrocarbon oil, adsorbs on the surface of the coal particles and
enhances the hydrophobicity of coal. Frother is a hetero polar chemical, used to
reduce the surface tension, consequently fine and stable bubble produce.
Conventional flotation cell is a mechanical mixing tank equipped with rotar -
stator arrangement. The rotor-stator mechanism has two functions one is mixing
i.e. to make homogenous coal slurry and second, it provides high shear rate, so
that the pressure of the air decreases to critical pressure and bubbles are
generated. These air bubbles and clean coal particles are hydrophobic, due to
hydrophobic interaction the clean coal particles are collected by air bubbles. The
air bubbles containing clean coal particles will float to the top of the cell via a
forth layer, where the particles are removed by automatic scrapper. The high ash
particles will remain at the bottom of the cell & discarded as tailings. High rotar
speeds are desirable for fine bubble generation whilst it creates turbulence,

which is not suitable for flotation. Hence there is no independent control on
bubble size and it not able to provide very fine bubbles. High turbulence prevails
in flotation cell but quiescent regime is desirable for flotation process. To address
the inherent disadvantage of conventional flotation cell a new flotation process.
To address the inherent disadvantage of conventional flotation cell a new
flotation column with external sparger was designed and demonstrated at bench
scale level.
Flotation Machine is a device for separating multi-component and heterogeneous
materials in a multiphase system including two main stages
Particle - bubble aggregate formation (Collection)
Mass transport (Separation)
The first stage involves aggregate selective formation where bubbles attach to
various particle species at different rates. The second stage corresponds to
aggregate separation, where bubble - particle aggregates are separated from
the pulp by buoyancy to form a froth bed and then to the forth overflow to yield
a concentrate product.
Hence, there exists a necessity to establish a better method of flotation for
improving clean coal yielding.
Objects of the invention.
Therefore, it is an object of the invention to propose a method of flotation for
improving clean coal yield by a column flotation with an external sparger, which
is able to eliminate the disadvantages of the prior art.

Another object of the invention is to propose a method of flotation for improving
clean coal yield by a column flotation with an external sparger, which is capable
of creating higher number of collision between bubble and particles during
counter current motion. Yet another object of the invention is to propose a
method of flotation for improving clean coal yield by a column flotation with an
external sparger, which results high residence times of bubbles and particles for
collecting bubbles.
Summary of the invention
A method of flotation was developed for improving clean coal yield by a flotation
column with an external sparger. The developed flotation column has state of art
facility to generate fine air bubbles by sparger which was arranged near bottom
of the flotation column. For generation of air bubbles compresses air was used
along with part of tailings to the sparger. By recirculating part of tailings to the
flotation column through sparger, so that tailings get more residence time for
flotation process and there was no requirement for fresh water for bubble
generation. Due to the countercurrent contact between rising bubbles and down
flowing coal particles, the clean coal particles selectively separated by air bubbles
due to hydrophobic interaction. By this method of flotation column, the clean
coal yield was increased by 10 units from 44 to 54% at 11% clean coal ash from
the feed ash of 27%.
Brief description of the accompanying drawings:
Fig.l : Shows a line diagram of column floating process according to the
invention.
Fig.2 : Shows a schematic diagram of column floating process according to the
invention.

Fig.3a : Shows effect of air flow rate on clean coal yield in accordance with the
invention.
Fig.3b : Shows effect of air flow rate on separation efficiency according to the
invention.
Fig.4a : Shows effect of collector dosage on clean coal yield as per the invention.
Fig.4b : Shows effect of collector dosage on separation efficiency in accordance
with the invention.
Fig.5a : Shows effect of frother dosage on clean coal yield according to the
invention.
Fig.5b : Shows effect of frother dosage on separation efficiency as per invention.
Fig.6 : Shows comparison of flotation column and mechanical flotation cell
for clean coal ash % against clean coal yield %.
Fig.7 : Shows the product clean coal ash % in flotation column as per
invention.
Fig.8 : Shows the product clean coal ash % in mechanical flotation cell.
Detailed Description of a preferred embodiment of the invention.
For effective treatment of fines, flotation columns are more suitable than
conventional mechanical cells. Line diagram of column flotation process is shown
in Fig 1. The flotation column can be divided into three regions one is sparger
region: where bubble generation takes place and located at bottom of the
column. Second is pulp/collection zone, where collection of clean coal particles
happens and forth zone where the collected clean coal particles transferred to
top of the column. The basic principal involved in a flotation column is the
counter current flow of air bubbles and coal particles and the flow behaviour
similar to a plug flow reactor. The bubbles are generated by external sparger at
the bottom of the column. Bubbles move upward in counter direction to
downward flow of slurry. The attachment of hydrophobic coal particles to the air

bubbles takes place in the lower section of the column between the fee point and
air inlet known as flotation zone. The froth from flotation zone moves to cleaning
zone (between interface and top of the column). The cleaning zone is a mobile
packed bubble bed that is contacted counter currently with wash water from the
top of the column to remove the entrained gangue particles from the froth and
send back to the flotation zone.
The advantages of column flotation compared to conventional cells are : 1) Due
to counter current motion, the probability of bubble-particle collision is higher,
2) High residence times of bubbles and particles for collecting bubbles 3) For
better product without sacrificing recovery, Reduction in the number of stages of
operation, Column has ability to handle a finer feed, Column has simple in design
for construction without any moving parts, It occupies less floor space
requirements.
SPARGERS
The spargers for column flotation can be broadly divided into two types: internal
and external spargers. Internal spargers deliver air into the column through a
material installed within the bottom of column, such materials as sintered glass,
porous rubber or filter cloth, or a single or-milti-nozzle sparger. With external
spargers, air and liquid (either surfactant-containing water or slurry) are brought
into contact outside the column before the mixture is introduced into the column.
Generally, the spargers used for column flotation should be able to provide
bubbles of the desired size and be capable of varying bubble size in order to
meet processing requirements.
In the conventional flotation columns, the spargers (bubble generation devices)
were made up of porous material such as perforated rubber, filter cloth. Sinter
tube and single/multi nuzzled orifices.

The disadvantages of conventional flotation columns are clogging effect:
blockage of pores due to particles and/or precipitates, Efficiency decreases:
tearing and general deterioration with use, Maintance: the need to shut down to
change the sparger, Bubble size: No control on bubble size after processing for
long times due pores blockage.
The spargers should be robust, energy efficient, easily maintaines and/or
replaced, friendly operated, and exhibit little tendency to become plugged and
worn out. The design of sparger plays an important role in deciding the efficiency
of the flotation cell. The sparger design producing smaller bubbles is more
desirable since the interfacial area generated is higher as compared to the bigger
bubbles. However too small bubbles are also not desirable, optimum size range
of bubbles are required. As the bubble size decreases for a given volumetric flow
of the gas, the number of bubbles increases and this leads to increased number
of particle attachment with bubbles. In this regard, four important aspects are as
follows: size of the bubbles, number of bubbles, gas hold up and residence time
of bubbles.
Bubble generation in external designed sparger
The air and water are injected at the air and water inlets of the sparger and
moves Co-currently. The pressure of air is slightly more than the fluid, so that air
can be readily dispersed in to water/slurry. Once a slug of air enters into the
static micro bubble generator, 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 to the shear force developed by moving the aerated water/slurry
against stationary blades and contact between air and water increased by the
intensive mixing provided by the change in the direction of movement.
The sparger is composed of two elements, periodically repeated in an axial
direction and placed in a circular tube. The second element is an identical copy

of the first element with 90° rotation in tangential direction. Each static element
consists of multiple 'X' shapes cross- bars and the angle between these opposite
cross-bars, is 90° . The standard element consists of 3 cross-bars ('X' shaped
pairs of crossed plates over the width of the channel). Flow is induced by
applying a pressure difference. The fluid through the tube, we experience the
crossed bars acting as intermingled combs moving in opposite direction from
one wall to another.
To generate micro bubbles water/slurry is pumped at a relatively high speed
through the inline micro bubble generator while a controlled amount of air is
introduced into the line just before the in line generator. Micro bubble generator
consist of multiples of small blades are placed in such a way that the fluid rapidly
changes its direction while passing through. This creates cavities in the fluid and
at the same time breaks the large bubble into smaller ones, thereby creating
micro bubbles. This technique is capable of producing micro bubble suspensions
greater than 50% air by volume.
Although in this study , only four shear elements are used and can be used up to
16 elements. In line micro bubble generators that have been used are of 3/8
inch and ½ inch in diameter and of varying lengths from 3 inches to 10 inches.
The preferred on at the present time is a ½ inch in diameter by about 10 inches
long with 16 elements. The generator preferentially breaks up big bubbles since
the bubbles as they get smaller have a tendency to be less subject to the shear
stresses.
The in line static elements are of uniform thickness but may be varied in
thickness over their width and length and arranged in other patterns and
arrangements to achieve similar multiple direction changing of the liquid and
shear forces applied to the bubbles to cause them become micro bubbles. Thus
the liquid is exposed to splitting and shear forces causing rapid changes in
direction. However, it is believed to be due primarily to the shear forces created
in the liquid by the rapid reversal of the direction of motion and by the boundary

layer along the surface of the shear elements. The liquid flow is controlled by
means of a variable speed pump. The fluid velocity is increased until the exiting
liquid becomes milky white, which is an indication that micro bubbles have been
produced.
A static mixed consists of a series of stationary mixing elements inserted end-to-
end in a pipe. 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. The
only power required for static mixers is the external pumping power that propels
the fluids through the mixer.
• High mixing between the gas and the water,
• Small diameter bubbles,
• High interfacial area,
• High mass transfer coefficient,
• Plug flow behavior.
The in-line micro bubble generator has a number of special advantages. It uses
less water, there is a low pressure drop through the system, there is no tendency
to plug up, it has no moving parts and it can operate with no external need for
sources of compressed air.
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. While the venture section works satisfactorily in
introducing air or other gas, such can also be done without a venturi using a
pressurized source of the gas.

The Kinetics of Bubble-Particle interaction
The efficiency of the flotation process depends, amongst other factors, on
contact between bubbles and particles, which facilitates selective adherence of
floatable particles to these bubbles. For an efficient extraction process, the
flotation cell should be designed to achieve good mixing of suspending solids and
dispersing air.
In flotation process , the basic mechanism is a successful collision and
attachment between a particle and a bubble.
The probability P of a particle being collected by an air bubble is the product of
these three probabilities:
P=PcPa(1-Pd) (1)
where,
Pc Probability of collision
Pa Probability of adhesion
Pd Probability of detachment
For fine particles, (Pd) can be negligibly small because of the low inertia. The
probability of attachment (Pa) depends mostly on the surface characteristics of
the mineral, the degree of collector adsorption on the mineral surface, and the
induction time required for attaching the hydrophobic particle to the bubble.
The power relationship between Pc and the Dp/Db ration changes with bubble
size (Yoon et al). The relationship may be represented in a generalized from,

Where A and n are the parameters that vary with Reynolds numbers. The values
of A and n are given Table 1 for three different flow regimes.


The probability of collision or the particle-bubble collision rate depends on the
sizes of the particles and bubbles, and the hydrodynamics of the flotation pulp.
The all probabilities are listed below in brief.
Flotation rate
Once P is known from the detailed knowledge of its subprocesses, the first order
flotation rate constant k can be obtained using the following relationship Yoon et
al.(1989; Yoon and Mao,1996):

Where Dp is the particle diameter in the pulp, P is the overall probability of
particle capture by bubbles, and Vg is the superficial gas rate which is the flow
rate divided by the cross-sectional area of the flotation cell. Since Pc varies as
Db-2 at the bubble sizes commonly employed in flotation, k will also vary as Db-2
at given Pa and Vg, providing an explanation for the difficulty in recovering fine
particles. A common solution to this problem is to increase Vg as suggested by
the above mentioned equation. This expression can be used for estimating the
rate constant for a given set of values of bubble size, particle size and particle
hydrophobicity (or induction time).

Flotation Process:
With reference to Fig.2, the various steps of process are,
1. A representative sample of raw coal size of less than 0.5mm is taken and
wet with water at predetermined pulp density. For coal flotation the usual pulp
density varies from 7-15%. After wetting the coal, coal slurry was fed to
conditioning tank (3) where the flotation reagents were added.
2. In the conditioning tank (3), first coal slurry is conditioned with collector,
to enhance hydrophobicity of coal and then frother was added to coal slurry to
generate stable fine bubbles.
3. After preparing the feed, the coal slurry was fed to flotation column (1) by
using feed slurry pump (8). The feed flow rate can be measured by using
magnetic flow meter (9) and can be controlled by using ball valve (7).
4. Fine air bubbles are generated by using external sparger (2) which was
locate near bottom of the flotation column (1). External sparger requires
water/tailings and compressed air to generate air bubbles.
5. Compressor (10) supplies air and air flow rate can be controlled and
measured by needle valve (11) and rotameter (12) respectively. The pressure of
compressed air can be measured by pressure guage (22).
6. For buble generation the tailings were recirculates by using slurry pump
(14) to the sparger. The tailing flow rate can be measured and controlled by
magnetic flow meter (13) and ball valve (15) respectively from the tailings
collecting tank (5).

7. Froth washing system (4) was arranged at the top of the flotation column
(1) for cleaning/washing the entrained ash particles into the forth. Ash particles
are removed along with wash water due to hydrophilicity of ash particles. The
wash water pumped to the flotation column (1) by using centrifugal pump (19)
from the wash water tank (21). The flow rate of wash water can be controlled
and measured by ball valve (20) and rotameter (18) respectively. The washed
forth was collected in froth tank (6).
8. Feed entering to the flotation column (1) above the middle of the column.
Fine air bubbles generated by sparger (2) entering in to the flotation column
from the bottom of the column. Due to buoyancy air bubbles rises to top of the
column whilst coal slurry drops to bottom of the column due to gravity. There
was counter current contact between air bubbles and coal particles, hence clean
coal particles were collected by air bubbles due to hydrophobicity.
Results & Discussions:
A representative minus 0.5mm size semi bituminous flotation feed sample was
taken in this investigation. Ash analysis was carried out according to ASTM D
3174-73 standard shows that the sample contains 27% ash. The representative
sample of the coal flotation feed (-0.5mm) was screened into different particle
size fractions. Total weight and mineral matter (ash) distribution in the particle
size fractions were analysed. The size wise analysis shows that the feed sample
contains 17% of coarse size (>500 microns), 30% of intermediate size (250
microns) and 22% of ultrafine size (<53 microns). The ash content of coarse size
fraction, intermediate size and ultrafine size was 29, 24 and 33% respectively.
The overall ash percentage of coal sample is 27% . High speed diesel oil (HSD)
and commercial frother was used as collector and frother respectively.

Effect of air flow rate:
In the flotation process separation occurs coal particles affinity towards air
bubbles. The bubble generation directly related to air flow rate, which in turn
enhances flotation rate (ka Vg). For effective separation flotation columns
should be operated in bubbly flow i.e. laminar flow. With increasing air flow
rate, then the flow regime shifts to churn bubble flow due to turbulent
conditions. The flotation column experiments were carried out with 3, 4, 5, 6 Ipm
by keeping the wash water rate at 31pm, collector dosage lkg/t and frother
dosage 0. lkg/t and results were shown in fig 3. The yield of clean coal at 11%
ash was 45,47,40 and 34% at 3,4,5 and 6 Ipm respectively. Flotation cell
experiments were carried out by varying air flow rate 3, 4, 4.5 and 5 Ipm other
variables collector dosage and frother dosage kept constant 1 kg/t and 0.1 kg/t
respectively. From the experiment results it was found that the clean coal yield
at 11% ash was 32,37,36 and 34% at flow rate of 3,4,4.5 and 5 Ipm
respectively. At fixed air flow rate, the amount of shear generated high in static
mixer of flotation column than rotar - stator arrangement of mechanical flotation
cell. Due to fine bubble generation the residence time f air bubbles more; it
result the chance of colliding the coal particles with air bubbles increases in
multifold. Hence at each air flow rate the clean coal yield is high in flotation
column compared to flotation cell this is due to fine bubble generation in
flotation column. The separation efficiency of the column was 29,33.27,29.7 and
25at 3,4,5 and 6 Ipm. The separation efficiency of cell was 29,33,30 and 25 at
3,4,4.5 and 5 1pm. The maximum yield was 47 and 37% for flotation and cell
respectively at 11% clean coal ash.

Effect of Collector dosage :
Coal is hydrophobic in nature, and to enhance its hydrophobicity collectors have
been using in coal flotation. Collectors are selectively adsorb on the surface of
coal consequently improves the hydrophobicity. Collectors can be divided in to
two classes, non-ionic and ionic (anionic and cationic) collectors. The mechanism
of non ionic collector is, to adsorb/spread over the coal surface due to high
hydrophobic interaction between non-ionic collector and coal surface. The
adsorbed collector increases the contact angle of coal consequently enhance the
hydrophobicity of coal. The non-ionic collector has two branches one is polar and
other is non-polar. The polar group reacts with polar sites over the mineral
surface and non-polar group orients towards bulk solution. Thus non-polar
branch enhances the hydrophobicity of mineral surface. When air interface is
provided in the form of an air phase and if the bond strength between the
monolayer of the collector and the coal surface is strong enough the particle will
be lifted by the buoyancy of the air bubble.
In this investigation high speed diesel oil was used as collector to increase
hydrophobicity of coal. Diesel oil was added to the coal slurry prior to the
flotation, in the conditioning tank. Diesel oil comes under the non-ionic collector,
it adsorbs or spreads as mono layer over the coal surface due to strong
hydrophobic between diesel oil and coal surface, consequently increases the
hydrophobicity.
Column flotation experiments were carried out by varying the collector dosage
from 1 to 2.5 kg/t with increasing 0.5kg/t of diesel oil per experiment keeping
other variables constant (wash water rate 31pm, air flow rate 41pm and frother
dosage 0.1 kg/t). From the fig 4, it was observed that clean coal yield was
47,48,54 and 46% for 1,1.5,2 and 2.5 kg/t respectively. InDenver flotation cell
the flotation experiments were carried out by changing the collector dosage from
0.5 to 2 kg/t with increment level of 0.5kg/t. From the experiments the clean

coal yield was 35,37.3, 44 and 28% at 0.5,1,1.5 and 2 kg/t respectively. The
maximum yield was 54 and 44% for column and cell respectively at 11% clean
coal ash. With increasing collector concentration number of coal particles get
adsorbed by collector increases thereby the clean coal yield increases. Till the
starvation limit of collector the clean coal yield increases with increasing the
collector dosage. The starvation limit was 2 and 1.5 kg/t for column and cell
respectively. After starvation level further rise in collector concentration shows
adverse effect on flotation of clean coal both in flotation column as well as cell.
After the starvation limit of collector dosage, it adsorbs in multilayer over the
coal surface. Multilayer collector has strong hydrophobic interactions, and due to
these interactions among the coal particles, inter particle attraction will increase
and tends to form coal agglomerations. The agglomerations of coal particles do
not lift by air bubbles due to high density, so the yield of clean coal decreases.
The separation efficiency of flotation column was 33.2, 34.5, 39.2 and 33.3 for
1, 1.5, 2 and 2.5kg/t respectively. For the mechanical flotation cell, the
separation efficiency was 24, 30, 36 and 22 for 0.5, 1, 1.5 and 2kg/t
respectively.
Effect of Frother dosage:
One of the prerequisites for a successful flotation operation is the stability of the
bubble-particle aggregate. A stable bubble is produced by using a frother, the
function of which is to decrease the surface tension at the air-liquid interface. A
frother has a number of functions in flotation - first, it reduces the surface
tension of the air-liquid interface in order that a stable bubble is produced in the
system; secondly, it influences the kinetics of bubble-particles adhesion; thirdly,
it thins the liquid layer by interacting with collector molecules and finally, it

stabilizes the bubble-particle aggregate (Schulman and Laja, 1954; Leja, 1956-
57).
Column flotation experiments were carried out by varying the frother dosage
from the 0.05 kg/t to 0.2 kg/t with increasing 0.05 kg/t of frother per experiment
keeping other variables constant (wash water rate 31pm, air flow rate 41pm and
collector dosage 2 kg/t). From the fig 5, the clean coal yield was 32, 44, 34 and
32% at 0.05, 0.1, 0.15 and 0.2kg/t respectively for mechanical flotation cell. The
clean coal yield was 38, 54, 41 and 35% for 0.05, 0.1, 0.15 and 0.2 kg/t
respectively for column flotation. The clean coal yield was increased to 54 from
38% by increasing frother dosage from 0.05kg/t to 0.1 kg/t. The increase in the
clean coal yield due to the addition of frother, increase in bubble formation,
reduce breakage of bubble, capture of particles reaching to the top of the
column. With further increase in frother dosage from 0.1 kg/t to 0.2 kg/t, the
clean coal yield decreased from 54% to 35% . The decrease in yield with further
increasing the frother concentration is attributed to hydrogen bonding between
frother and hydrated ash mineral matter which results in a greater recovery of
entrainable ash bearing particles.
The separation efficiency of flotation column was 33 39.0, 35 and 30% for 0.05,
0.1, 0.15 and 0.2 kg/t respectively. The separation efficiency of mechanical cell
was 25, 36, 27 and 25% for 0.0, 0.1, .15 and 0.2 kg/t respectively. The increase
in separation efficiency is due to increase in number of air bubbles and thereby
increases the selectivity of coal. With further increase in the frother
concentration the separation efficiency decreases due to collection of ash
particles.
From the fig 6, the maximum yield observed at 54% and separation efficiency is
39.2%. The optimum conditions for column flotation

• Wash water flow rate - 31pm
• Air flow rate-41pm
• Collector Dosage-2kg/t
• Frother Dosage-O.lkg/t
The maximum yield of clean coal was 44% and separation efficiency 36%. The
optimum conditions for mechanical flotation cell:
• Air flow rate-31pm
• Collector dosage-1.5 kg/t
• Frother dosage-0.1 kg/t
From the comparative studies, it was found that flotation column able to produce
11% ash clean coal with 54% yield and 39.2% separation efficiency. Whereas
the mechanical flotation cell, the clean coal yield is 44% with separation
efficiency is 36%. Henceforth flotation column is superior to mechanical flotation
cell for producing clean coal with higher yield.
From the fig 7, the average clean coal ash of flotation column id 9.7%. Minimum
and maximum ash is 6.4% and 11.4% respectively.
From the fig 8, the average clean coal ash of mechanical flotation cell is 10.8%.
The minimum and maximum ash percentage is 9.7% and 12.7% respectively.
By this new flotation column, one can achieve less ash clean coal (9.7%)
compared to mechanical flotation cell, the average clean coal ash is 10.8%.

We Claim:
1. A method of flotation for improving clean coal yield by a flotation column
with an external sparger, the said method comprising;
collecting a sample of raw coal;
disposing the sample in water at predetermined pulp density;
feeding the coal slurry to conditioning tank (3);
adding flotation reagents to the said coal slurry in the tank (3);
conditioning the said coal slurry with collector to enhance hydrophobicity
of coal and then adding frother to the coal slurry to generate stable fine bubbles;
feeding the pretreated coal slurry to a flotation column (1) by a feed
slurry pump (8) measuring and controlling the feed flow rate by a magnetic flow
meter (9) and by a ball valve (7);
supplying compressed air by compressor (10) and water from the tailing
tank (5) to an external sparger (2) located near the bottom of the flotation
column (1) to generate fine air bubbles;
controlling the compressed air flow from the compressor (10) by a needle
valve (11) and air flow by rotameter (12);
recirculating the tailings (5) by slurry pump (14) into the sparger (2) to
generate bubble;
measuring the tailings flow rate by magnetic flow meter (13) and
controlling the said air flow by ball valve (15) from the tailings collecting tank(5);
arranging at the top of flotation column (1), washing system to wash the
entrained ash particles into the froth and to remove ash particles because of
hydrophilicity of ash particles;

pumping the wash water to the flotation column (1) by disposing a
centrifugal pump (19) from the wash water tank (21) allowing washed froth to
be collected in froth tank (6);
controlling the flow rate of wash water by ball valve (20) and measuring
flow by rotameter (18);
wherein the feed enters the flotation column (1) above the middle of the
column and fine air bubbles generated by sparger (2) enters into the flotation
column from the bottom of the column (1) when air bubbles rise to top of the
column while coal slurry drops to bottom of the column due to gravity resulting a
counter current contact between air bubbles and coal particles causing clean coal
particles being collected by air bubbles due to hydrophobicity and drops in the
tank (5).
2. The method as claimed in claim 1, wherein the maximum yield is 54% and
separation efficiency is 392% when optimum condition for column flotation is
established with (i) wash waterflow rate of 3 liter/min. (ii) air flow rate of 4
liter/min. (iii) collector dosage of 2 kg/ton and (iv) frother dosage of 0.1 kg/ton,
wherein average clean coal ash produced in flotation column is 9.7% when
minimum and maximum ash is 6.4% and 11.4% respectively.
3. The method as claimed in claim 1 and 2, wherein the increase of collector
dosage increases the clean coal yield till the starvation limit of 2 kg/t.
4. The method as claimed in claim 1 to 3, wherein the addition of frother
causes stable bubble resulting an improved flotation operation.

5. The method as claimed in claim 1 to 4, wherein the clean coal yield is
increased to 54% from 38% by increasing frother dosage from 0.05 kg/t to 0.1
kg/t.
6. The method as claimed in claim 1 to 5, wherein at each air flow rate clean
coal yield is comparitively more in flotation column when it is 10% more at 11%
clean coal ash and air flow rate of 3 Ipm.

ABSTRACT

The invention relates to a method of flotation for improving clean coal yield by a
column flotation with an external sparger. A sample of raw coal of size less than
0.5mm is wet with water and the coal slurry is fed to a conditioning tank (3)
where flotation reagents are added. The slurry is conditioned with collector and
they frother is added to it to generate fine bubbles. This slurry is fed to flotation
column (1) in a measured and controlled manner. Fine air bubbles are generated
in external burger (2) when compressor (10) supplies air. For bubble generation
the tailings are recirculated by slurry pump (4) to the sparger (2). Frother
washing system is arranged at the top of flotation column for cleaning ash
particles. The washed froth is collected in the froth tank (6). Feed enters the
flotation column above the middle of the column. Fine air bubbles generated by
sparger (2) enters the flotation column (1) from the bottom of the column. Due
to buoyancy air bubbles rises to top of the column whilst coal slurry drops to
bottom of the column due to gravity. The counter current contact between air
bubbles and coal particles, clean coal particles are collected air bubbles due to
hydrophobicity and drops in the tank (5).