FIELD OF THE INVENTION
The present invention relates to a floatation process and apparatus for
beneficiating coal fines in coal preparation plant. More particularly, the invention
relates to an apparatus and a method for ultrasonic treatment of coal to increase
yield of clean coal in a floatation process.
BACKGROUND OF THE INVENTION
In coal preparation, application of ultrasound is known for deashing,
desulphurization and deshaling during flotation along with reagents. Prior art also
teaches application of extra chemicals in addition to flotation reagents, in order
to increase yield of clean coal.
Froth Floatation is a physico chemical separation process, in which fine coal
(0.5mm) particles are selectively separated from the ash minerals. Separation of
hydrophobic particles from the coal slurry/pulp is achieved by using differences
in their affinity towards air bubbles.
The coal is a heterogeneous mixture of coal and small particles of shale/clay.
Some of these contaminant particles are loosely attached and held in place by
ionic or cohesive force over the surface of coal. These particles only need to be
sufficiently displaced to break the ionic and cohesive forces to be removed.
US4537599, entitled "Process for removing sulphur and ash from coal", teaches a
process for treating coal slurry with ultrasonic energy before being subjected to
centrifuge and flotation for removing particles of 2-micron size. After separating
the 2 micron size particles, the slurry is again repulped and subjected to
ultrasonic energy and ozone treatments to release further surface contaminants

with specific pH of 6-9. The difficulties in this process is that the coal slurry is
subjected to more number of times to ultrasonic treatment inducing application
of chemicals (ozone) at specific pH. Due to multiple ultrasonic treatment, the
coal slurry get thermally destabilized.
JP 62216655 entitled "Flotation method for particulate coal", discloses a coal
slurry treatment process, in which the particulate coal slurry is subjected to
ultrasonic wave at high pressure and followed by steps of flotation along
flocculation for removing the ash particles, so that the yield is increased. The
difficulties in this process to get higher yields are that the ultrasonic waves need
to be applied at high pressures and particulate coal slurry warrants treatment in
flotation along with flocculation.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an ultrasonic floatation
process for beneficiating coal fines to increase floatation performance in terms of
yield of clean coal.
Another object of the invention is to propose an ultrasonic floatation process for
beneficiating coal fines to increase floatation performance in terms of yield of
clean coal, in which the coal slurry is subjected to ultrasonic treatment
independently or consecutively during pre-treatment and conditioning.
A still another object of the invention is to propose an ultrasonic floatation
process for beneficiating coal fines to increase floatation performance in terms of
yield of clean coal, in which the ultrasound generates a phenomenon of transient
cavitation and surface activation leading to increase in hydrophobicity.

A further object of the invention is to propose an apparatus for implementing an
ultrasonic floatation process to increase floatation performance in terms of yield
of clean coal.
SUMMARY OF THE INVENTION
Ultrasonic is known to be a three-dimensional pressure wave with a frequency
greater than 20 kilo hertz formed of successive compression and rarefaction
cycles. However, the present inventors recognized that an alternative behavior of
the rarefaction and compression cycles, generates a cavitation process, which
interalia creates a significant effect on any solid phase within the liquid. Mixing
the slurry with ultrasonic treatment provides a thorough mixing of coal-water
slurry for wetting the fine coal particles. The coated clay/ash on the coal surface
is dislodged from the surface in ultrasonic pre-treatment process due to the
phenomena of transient cavitation, surface erosion. By removing the surface
contaminant, the surface activation occurs which leads to an increase in the
hydrophobicity of coal particles.
The effects of ultrasonic pretreatment in heterogeneous systems are:-
• Better dispersion of coal pulp,
• Dislodging of clay/shale/ash particles and surface films,
• Removing surface contaminants i.e. Surface cleaning,
• Generating fresh surface, and
• Increasing the hydrophobicitgy.
During ultrasonic conditioning, the ultrasound makes it possible to obtain fine,
homogeneous, and pure chemical emulsions. Ultrasonic conditioning improves
the effectiveness of a reagent due to more uniform distribution in the suspension

and the activity of the reagents is also increased. The solid/liquid interactions are
weaker cohesion forces; solid/liquid interfaces are more amenable to the
formation of cavitation. The unsettled conditions caused at a solid/liquid interface
can modify the surface properties of minerals, leading to increase in the
adsorption rates of collectors over coal surface.
In the flotation step, the collectors are used to enhance the hydrophobicity of
coal particles in coal flotation. Collectors are hydrocarbon chemicals having polar
and non polar groups. The polar group of the collector is attracted to the coal
particles, and non polar group of the collector oriented towards the bulk solution.
A mono layer of the collector is adsorbed on the coal surface by the polar group,
there by imparting hydrophobicity to the coal particles. If an air interface is
provided in the form of an air phase and if the bond strength between the polar
group and the coal surface is strong enough, the particle get lifted to the surface
by the buoyancy of the air bubble. Flotation collector (hetero polar, non polar
compounds, oils, fatty acids, amines etc) are poorly soluble in water and their
usages in excess amounts are not only wasteful, but also impair the process
inefficiency.
Ultrasound can be efficiently used to emulsify insoluble collectors. Interaction of
immiscible liquid phases in an ultrasonic field gives rise to an effect known as
emulsification. Ultrasound makes it possible to obtain fine, homogeneous, and
chemically pure emulsions. Ultrasonic emulsification necessiciates cavitation. The
regularities of origination and development of cavitation determine the course of
emulsification and its dependence on the intensity and frequency of oscillations,
the presence of dissolved gases, as well as on temperature, pressure, density,
viscosity, and surface tension of interacting liquids.

The rate of emulsification can be estimated by

where V - Emulsion volume
C - Emulsion concentration
S - Interface area
a constant depends on the characteristics of acoustic field, physicochemical
properties of phases and experimental conditions.
Collector molecules are adsorbed on the drops of a nonpolar agent, their polar
groups being oriented to a polar aqueous phase. During the contact of such
drops with minerals particles, the polar groups of the adsorbed collector
molecules interact with the crystal lattice ions of minerals and thus promote
spreading of non polar flotation agent drops over the surface of minerals
particles. Emulsification is often conducted in the presence of stabilizers
(surfactants) that are expected to reduce interfacial surface tension. Surfactant
molecules are adsorbed on the emulsion drops thus preventing their
coalescence.
The cavitation mechanism makes energy available at the interface of a solid
particle suspended in liquid, and this energy is used to implant chemical reagents
on the surface of the particle, thereby improving flotation. The rate of
emulsification rises with the intensity of ultrasound. The frequency rises the
dispersed particles become smaller, and the ultimate concentration of emulsion
increases with the intensity of vibrations, and the rate of emulsification
decreases with growing frequency. The emulsion quality depends on the time of

ultrasonic irradiation. A short term irradiation gives rise to coarse, unstable
emulsions, but a long irradiation may also reduce the stability of emulsions by
inducing their coagulation.
The effects of ultrasonic conditioning in a coal slurry pulp:-
• Efficient dispersion of coal pulp
• Blending of liquids
• Promotion of interfacial diffusion
• Reagents penetration into minerals pores and cracks
• Stimulation of emulsion
Due this mechanism, with the ultrasonic pre-treatment the yield of clean coal can
be enriched compared to without ultrasonic treatment.
Accordingly, the present invention teaches an ultrasonic flotation process for
beneficiating coal fines in coal preparation plants. In the ultrasonic flotation
process, the coal slurry is subjected to ultrasonic treatment during mixing (pre-
treatment) and during conditioning (ultrasonic conditioning). The coal slurry can
be subjected to pretreatment and ultrasonic conditioning independently or
subsequently. During the process of pretreatment of coal slurry, the loosely
bonded ash/shale particles get dislodged over the coal surface, such that the
hydrophobicity increases. During the ultrasonic treatment of the coal slurry,
uniform adsorption of reagents takes place over the coal surface, to make the
kinetics faster. Application of ultrasonic treatment during mixing (pre-treatment)
and during conditioning (ultrasonic conditioning) increases the flotation
performance in terms of yield. According to the invention, the yield of clean coal
has been increased in the existing units with the inventive ultrasonic treatment,
compared to flotation process without ultrasonic treatment at same ash level.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a prior art apparatus for carrying-out a coal-flotation
process.
Figure 2 - shows an apparatus for implementing an ultrasonic floatation
process of coal, according to the invention.
Figure 3 - Graphically shows collector's consumption trend in an ultrasonic
floatation process of coal according to the invention.
Figure 4 - Graphically exhibits frother's consumption trend in the ultrasonic
flotation process of the invention
Figure 5 - shows a bar chart indicating a comparison in size-wise ash analysis
of coal in a beneficiation process with ultrasonic treatment and
without ultrasonic treatment.
Figure 6 - a photographic view of petrography study of clean coal yield from
the inventive process.
DETAIL DESCRIPTION OF THE INVENTION
Figure 1 shows a prior art apparatus for coal beneficiation process. The
apparatus comprises a mixing unit (1) wherein coal is mixed with water to form
coal slurry with desired density of the pulp; a conditioning unit (2), in which the
coal slurry is further mixed with frother and collector (reagents) to respectively
increase hydrophobicity of coal and reduce the surface tension of the coal
particles; a floatation unit (3) for selective separation of coal particles by air

bubbles, the separated particles reporting to upper part with non-recovered
particles remaining at lower part of the floatation unit (3); a dewatering unit (4)
in which the water is removed from the clean coal forth; and a thickener unit (5)
for recovering the water which can be recycled to coal preparation, with the
tailings being discarded.
Figure 2 shows an apparatus for ultrasonic treatment of coal in a floatation
process comprising a mixing unit (6); an ultrasonic bath (7) for conditioning of
the coal slurry and having at least one transducer with generator for ultrasonic
treatment during pretreatment and ultrasonic conditioning of the slurry; a
flotation unit (8); a dewatering unit (9), and a thickener unit (10).
The coal sample collected from an Indian mine. Diesel oil (DO) and a known
frother were used as collector and frother respectively. A flotation cell of 5-lit
capacity was used for flotation test. The Pulp density was 10%, the mixing time
was 3 min, conditioning time was 3 min. The froth samples were collected at
different time intervals. After the final froth sample was collected, the froth
samples were dried, weighed and analyzed for their ash content.
An ultrasonic transducer with generator having frequency of 25 kHz and output
power 2000 Watts was used to convert the electrical energy into sound energy.
The ultrasonic transducer is placed in the flotation cell, which connects the
generator. The ultrasonic treatment can be applied to the coal slurry as per the
requirement.
RESULTS:
To investigate the effect of the ultrasonic treatment on flotation performance,
the yield of clean coal was compared before and after the ultrasonic treatment.

Flotation experiments were carried out with and without ultrasonic treatment by
varying the collector concentration from lkg/t to 3kg/t with increment of 0.5kg/t
by keeping the frother concentration at 0.5kg/t. From Figure 3, it can be seen
that the yield increases with increasing of the collector concentration from lkg/t
to 2 kg/t, and a further increase in the collector concentration, decreases the
yield. Similar trends were observed with and without ultrasonic treatment
assisted flotation process. Maximum yield of 67%, was observed at 2 kg/t before
ultrasonic treatment, whereas the yield increased to 73% with ultrasonic
treatment. This is due to increase in hydrophobicity of coal which leads to an
increase in selectivity used clean coal fines.
With further rise in collector concentration, the yield decreased from 67% to
60% without ultrasonic treatment, whereas the yield reduced from 73% to 67%
with ultrasonic treatment at 3 kg/t of collector concentration. This is due to the
fact that after the starvation level further rise in collector concentration shows
adverse effect on coal flotation. Beyond 2 kg/t collector concentrate, it may
adsorb on the mineral particles thereby reducing the selectivity of coal. Further,
the collector can adsorb in multilayer on coal particles, which reduces the
proportion of hydrocarbon radicals oriented into the bulk solution thereby
decreasing the yield. The experiment reveals that for both with and without
ultrasonic treatment, the starvation limit of the collector for the flotation process
are 2 kg/t.
For studying the effect of ultrasonic treatment on coal flotation, experiments
were carried out with and without ultrasonic treatment for the range of frother
concentration from 0.05 kg/t to 0.25 kg/t with increment of 0.05 kg/t at constant
collector concentration 2 kg/t. The additions of the frothers reduce the surface
tension at the liquid-vapor interface, which results in the production of a finer
bubble size distribution and to stabilize the froth.

In figure 4 Yield was increased for both experiments for example, with and
without ultrasonic treatment. Without ultrasonic treatment, the maximum yield of
73% was obtained at 0.2 kg/t. The yield was increased from 47% to 73% with
the increasing of the frother concentration from 0.05 kg/t to 2 kg/t. With
Ultrasonic treatment, the yield was increased with the increasing of the frother
concentration from 0.05 kg/t to 0.15 kg/t and maximum yield of 77% was
obtained at 0.15 kg/t.
With further rise in frother concentration, the yield was decreased from 73% to
71% at 0.25kg/t for without ultrasonic treatment and with ultrasonic treatment
the yield was decreased from 77% to 74% at 0.25 kg/t. The decrease in yield
with further increasing the frother concentration is attributed to hydrogen
bonding between the frother and hydrated ash mineral matter which results in a
greater recovery of entrainable ash bearing particles.
CHARACTERIZATION
Size wise ash analysis
The representative coal sample weight of 500gr, coal flotation feed (-0.5 mm)
was screened into different particle size fractions. The weight and ash
distribution of each fraction of feed sample is compared with and without
ultrasonic treatment is shown in figure 5.
Figure 5 shows a Comparison of size wise ash analysis of coal with and without
ultrasonic treatment. Without ultrasonic treatment the size wise analysis shows
that the sample contains high percentage (60%) of coarse fraction (>150 micron
size) coal with ash percentage of 26%. The ash% of flotation feed is 26%. The
ash distributed around 26% for each fraction.

With ultrasonic treatment, the size wise ash analysis shows that the sample
contains high percentage (60%) of coarse fraction (>150 micron size) coal with
ash percentage of 26%. After ultrasonic treatment there is not much variations
in size but there is significant changes in ash% of size fraction. The ash
percentage of 500 microns decreased to 25% from 29%. This might be due to
the removal of ultra fine ash particles from coarse surface due to ultrasonic
treatment. There is a bit variation in the ash% for intermediate fractions from
250 microns to 75 microns. The ash percentage of 53 microns particles ash
percentage significantly raised from 30% to 35%. There is not much variation in
the ash percentage of ultrafine (-53 microns) coal particles.
Petrography
To investigate the effect of ultrasonic treatment on coal microstructure,
petrographic studies have been carried out on the concentrate. In the
petrography analysis, it was observed that micro cracks found over the coal
surface which helps bettor adsorption of reagents over the surface, resulting to
increase in flotation performance.
Petrographic studies of clean coal with ultrasonic treatment shows that, there
was micro cracks observed over the coal surface. These micro cracks helps for
better absorption of reagents, it consequently leads to better floatability.

WE CLAIM
1. A method for ultrasonic treatment of coal to increase yield of clean coal in
a floatation process, comprising the steps of:
- mixing coal fines with water to produce coal slurry with desired
density in a mixing unit;
- ultrasonic conditioning of the slurry in an ultrasonic bath having at
least one transducer with a generator by adding collector and
frother at different intervals in varying concentration;
- selective separation of coal particles from the conditioned slurry
transferred from the ultrasonic bath by injecting air bubbles in a
floatation unit to produce clean coal froth;
- recovering water from the clean coal froth, and
- recovering the water for further recycling including discarting of the
tailings,
wherein the transducer with generator having a frequency of at least
25 kHz and output power of 2000 watts, wherein the density of the
coal slurry is at least 10% with mixing time of 03 minutes, wherein the
ultrasonic conditioning of the slurry is conducted for at least 03
minutes, wherein the collector concentration during conditioning is
variable between lkg/ton to 3kgs/ton with a single increment of
0.5kg/ton, and wherein the frother concentration is kept constant at
0.5 kg/ton.

2. The process as claimed in claim 1, wherein the yield of clean coal is
maximum at 73% when collector concentration is 2kg/ton, and wherein
when the collector concentration is increased beyond 2kg/ton, the yield of
clean coal is reduced.
3. The process as claimed in claim 1, wherein the frother concentration when
optionally increased from 0.05kg/ton to 0.15 kg/ton with constant
collector concentration of 2kg/ton, the maximum yield of clean coal is
noted at 77% with frother concentration of 0.15 kg/ton, which is reduced
to 71% when frother concentration is increased to 0.25 kg/ton.
4. The process as claimed in any of the preceding claims, wherein the ash-
percentage of size fraction of coal particles are substantially decreased at
various percentages corresponding to size of the particles.

ABSTRACT

The invention relates to a method for ultrasonic treatment of coal to increase yield of clean coal in a floatation process, comprising the steps of: mixing coal fines with water to produce coal slurry with desired density in a mixing unit;ultrasonic conditioning of the slurry in an ultrasonic bath having at least one
transducer with a generator by adding collector and frother at different intervals in varying concentration; selective separation of coal particles from the conditioned slurry transferred from the ultrasonic bath by injecting air bubbles in a floatation unit to produce clean coal froth; recovering water from the clean coal froth, and recovering the water for further recycling including discarting of the
tailings, wherein the transducer with generator having a frequency of at least 25 kHz and output power of 2000 watts, wherein the density of the coal slurry is at least 10% with mixing time of 03 minutes, wherein the ultrasonic conditioning of the slurry is conducted for at least 03 minutes, wherein the collector concentration during conditioning is variable between 1kg/ton to 3kgs/ton with a single increment of 0.5kg/ton, and wherein the frother concentration is kept constant at 0.5 kg/ton.