FIELD OF INVENTION
The present invention relates to a beneficiation process of coal tailings employing
coal reverse flotation technique.
BACKGROUND OF THE INVENTION
Beneficiation of coal is done to reduce the ash content present in coal in form of
different minerals and maximise the carbon content at a feasible cost. A typical
coal preparation plant consists of two circuits i.e. a coarse circuit and a fines
circuit. In the coarse circuit, particles of size +0.5 mm are processed through a
cyclone, jigs etc. whereas in the fines circuit, finer size particles i.e. -0.5 mm are
treated using flotation, spirals, hydrocyclone etc. During coal beneficiation along
with the clean coal certain by-productsare also produced which contain the
unwanted mineral matter removed from the feed coal. Among these, the by-
product generated during the beneficiation of fine coal are known as coal
tailings. Coal tailings are of the size -0.5 mm and ash content of 40-50%.
Beacuase of their high ash content and low carbon content, the tailings are
generally dumped in tailings ponds without any further end use. Since a large
amount of coal tailings are generated each year, their proper disposal has
become a severe problem for coal washery operators due to issues like land
shortage and environmental concerns. Therefore in order to provide a solution to
this problem further processing of tailings to extract the left over carbon content
is proposed in this invention. The process involves the beneficiation of coal
tailings by coal reverse flotation process using quaternary amines as collector for
floating mineral matter &polymeric carbohydrates as depressants for the coal
particles.This route is completely opposite to conventional coal flotation process
where the coal being naturally hydrophobic in nature is floated by using oily
collectors. However during the new process firstly the coal is depressed into the

feed slurry by using certain polymeric carbohydrates. After this theminerals
found in coal such as quartz, clay minerals (like kaolinite, illiteetc.), feldspar,
carbonates (such as calcite, siderite and dolomite,) and sulphideminerals such as
pyrite etc. and other less commonly found minerals like phosphates, hydroxides
and salts like gypsum, sylvite, and halite, are made a little hydrophobic& lesser
hydrophilic by use of quaternary amines as collector. Finally these minerals
attach themselves to the rising air bubbles & report to the froth phase leaving
the coal in the slurry. Hence in this way separation between coal particles and
mineral matter takes place. The pH for the overall process is kept normal i.e.
around 7 to maximise the recovery.So no need to add pH modifiers during the
process.
The process of reverse flotation is a well known technology used for the
processing of different ores like iron-ores, diasporic–bauxite ores, phosphate
rocks, kaolin mineralsetc. In iron ore processing, the reverse flotation of silica
and silicateshas been tested successfully by both cationic and anionic reagents
toobtain purified ferriferous concentrates. Much work regarding the reverse
flotation of diasporic ores has been reported which involve the effectiveflotation
and collection of alumina–silicate gangue minerals such aspyrophyllite, illite,
kaolinite and chlorite using cationic reagents like
alkyl amine and carboxy hydro-oxidoxime. For phosphate rocks reverse flotation
is considered to be a better route as carbonate minerals can be floated by
anionic collectors atacidic pH while siliceous gangues at neutral pH using cationic
amine collectors.
The coal reverse flotation technique is directly opposite to conventional flotation.
In this method the gangue is floated by use of suitable reagents while the
valuables remain in the suspension and collected as product/concentrate. In case
of coal tailings because of their high ash content (i.e. 40-50%).

In case of coal the concept was developed for the selective removal ofpyritic
sulphur from US coals by several investigators in which potassium amyl xanthate
was
used as collector to float pyrite. The first patent for ash removal of coal through
reverse flotation was done by G P Eveson (United Kingdom Patent 863805-
A). After this the method was tested for South African coals where a mixture of
washed coal and quartz were tested. Meanwhile some work was also carried by
US researchers using reverse flotation as a cleaning methodfor the preparation
of coal–water slurries. The forward flotation reagentsremove the inorganic
impurities from coal but tend to increasethe yield stress and viscosity of the
slurry due to aggregate formationas these reagents make the coal particles more
hydrophobic. Hencethe reverse flotation method appears to be more noteworthy
as theclean coal product is made more hydrophilic. Recently the process has
been widely studied for hard to float low rank coals like brown coals. The
Chinese patents i.e. CN102773168 & CN102773168B allows the use of a
soluble salt (sodium chloride), an inhibiting agent (causticizing corn starch), a
collecting agent (dodecyl amine-hydrochloride) and finally a foaming agent
(methyl isobutyl carbinol) to beneficiate the brown coal. In order to meet the
critical quality parameters of clean coal, the method was developed in the
patents CN104399595 & CN105127002 to reduce the ash of coal slimes.
Reverse flotation has also been explored for purifiying aphanitic graphite
flotation concentrate in the patents CN105692606 & CN105692606B.
Although the above mentioned patents show application of reverse flotation for
various coal types but nonetheless shows its potential for the beneficiation of
degraded poor quality coal by-products like the coal tailings. In addition to this
most of the prior invention either involve two stage beneficiation or more than
two types of chemicals used in reverse flotation.

OBJECTS OF THE INVENTION:
In view of the foregoing limitations inherent in the prior-art, the object of the
invention is to propose a benefication process of coal tailings, of size less than
0.5 mm with ash-content of 40 to 50% employing a single stage reverse flotation
technique.
Another object of the invention is to propose a beneficiation process of coal
tailings of size less than 0.5 mm with ash-content of 40% to 50% employing a
Single stage reverse flotation technique, in which Coal tailings in its standard
form as obtained from coal preparation plants without any prior processing is
used.
A still another object of the invention is to propose a beneficiation process of
coal tailings of size less than 0.5 mm with ash-content of 40% to 50% employing
a Single stage reverse flotation technique, in which as quaternary amines as
collector for floating mineral matter and polymeric carbohydrates as depressants
for the coal particles are used.
SUMMARY OF THE INVENTION
Accordingly, there is provided a process beneficiation of coal tailings of size less
than 0.5 mm with ash-content of 40 to 50% employing a single stage reverse
flotation technique. The present invention involves only single stage flotation
using only two types of chemicals i.e. depressant for coal & collector for mineral
matter. Apart from these two types of chemicals, no need to use other chemicals
like frother, pH modifiers, activators etc. in the present method. So the simplicity
of the process makes it very user friendly as well as very cost effective especially
reducing the cost of whole other class of chemicals. Further the invention does
not involve any prior processing of the feed sample like size reduction, ultrafine
grinding, slime removal etc. as it is suitable to coal tailings of size -0.5 mm i.e.
coal tailings in its standard form as obtained from coal preparation plants can be

direclty beneficiated using this process. Finally the significant ash reduction
achieved in the process i.e. from 40-50% feed ash to a product of ash 22-25%
with a yield of more than 32% makes it a very novel method for beneficiation of
coal tailings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 shows a general Coal Reverse Flotation process.
FIG. 2 shows the mechanical flotation cell used in the current invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
DISCLOSURE
As per the invention, a process for beneficiation of coal tailings of size less than
0.5 mm comprises a reverse flotation process where in selected reagents is
added to coal-water slurry inside a flotation cell followed by vigorous mixing.
Then air is flown through the slurry to form air bubbles and carry the high ash
mineral matter to the froth phase where it is collected in a separate tray. The
remaining slurry in the flotation cell is also transferred to another tray and
subjected to filtration and drying. The coal slurry thus obtained is filtered and
dried and weighed to calculate the yield after which ash analysis is done to
determine product ash. After this the weight of froth phase is also measured and
ash analysis done. The flotation test is performed in a standard laboratory
mechanical flotation machine which has a cell of capacity 2.5 liters. The amount
of feed material (i.e. -0.5 mm coal tailings) taken depend on the pulp density of
the slurry to be prepared and in this case it is between 1:4 to 1:5 ratio on weight
by weight basis. After adding the feed coal tailing sample to the water in the cell,
proper mixing of the coal-water slurry is done by keeping the rotating speed
from 650 to 850 rpm. This speed is tried to be kept constant throughout the
process. The mixing is done for sometime after which a calculated amount of

the depressant which in this invention is a polymeric carbohydrates such as
starch or dextrin. The dosage of the depressant is varied in between from 1.25
kg/t to 1.5 kg/t. After the addition of depressant the mixing is again done for
atleast 10-20 minutes. Finally a quaternary amine such as Dodecyltrimethy
lammonium bromide (DTAB), Cetyl trimethy lammonium bromide (CTAB) and
Dodecyltrimethylammonium chloride (DTAC) was used. The dosage of the
collector is varied in between from 3.0 kg/t to 4.0 kg/t. Again a mixing for
sometime is done so that a uniform mixing of the reagents is done throughout
the slurry. No need of using a frother as the quaternary amines have very high
frothing nature and produce a good froth. Then air at a fixed rate of 3 lpm is
flown through the slurry which resulted in the formation of air bubbles and finally
a stable froth. The high ash coal particles because of their so-called artificially
made hydrophobic nature get attached to the air bubbles and come to the froth
phase where they are recovered in another tray using a scraper. The froth is
collected for about 1- 1.5 minutes after which the air flow is cut off and agitator
is switched off. The only two parameters which are varied during the reverse
flotation of coal tailings are the collector dosage & depressant dosage. Whereas
all the other parameters i.e. pulp density (amount of feed and water), agitator
speed, airflow, conditioning time and collection time are almost kept constant.
The coal tailing used in the present invention is a by-product of the fines circuit
of medium coking coal preparation plant. Its proximate analysis& washability
analysis are given in Table 1& 2 respectively showing a fixed carbon content of
not more than 35%.
Table 1: Proximate analysis of the coal tailings sample



The depressant used for the invention are polymeric carbohydrates like dextrin
(low molecular weight) and starch (high molecular weight). This is the most
important step in coal reverse flotation as the natural hydrophobicity of coal
makes coal to float on its own. Since the concept of coal reverse flotation is
opposite to the nature, hence strongreagents are needed to depress the coal. In
this regard, the depressantsplay a critical role by rendering the coal hydrophilic
in nature and thusenhancing the selectivity of the process.These organic
polymeric depressant, having good watersolubility, are less hazardous than
inorganic depressants& based on adsorption density and thermochemical
measurements, it has been suggested that dextrin/starch adsorptiononto the
coal surface occurs by hydrophobic bonding. It is found that the non-polar part
of the dextrose monomer forms hydrophobic bonding with the coal surface
whereas the polar part is directed away from the surface making the coal surface
hydrophilic as directed by the experimental adsorption free energy calculation.
The increase in the non-polar sites of coal by demineralization increases the
dextrin adsorption and this adsorption on coal system is independent of pH.
Reverse flotation involves the flotation of ash forming minerals which means that
the collectors used should be such that they make the mineral matter

hydrophobic in nature. The surface properties of the minerals determine the
selection of collectors. The presence of low pzc value indicates that silica surface
is negatively charged over the entirepH range which creates an excellent
favourable condition for the adsorption of cationic agents like amines on the
silica surface. In fact long chain amine salts have found to be successful in the
flotation ofquartz. Similarly in the case of alumina too amines have found to be
effective when the alumina surface is negatively charged which means that
alumina flotation occurs by cationic collectors above its pzci.e. in the pH higher
than 9.4 but highly alkaline solutions have to be neglected as the concentration
of cationic species starts to decrease rapidly. So therefore at a normal pH
flotation of most of the mineral matter present in coal can take place.
The adsorption mechanism of quaternary amines at neutral pH values is well
explained by a theory which states that the amine cations adsorb only on the
negatively charged oxide or silicate surfaces by means of electrostatic and
hydrophobic forces from a solution below the critical micelle concentration.When
the adsorption density of the surfactant ions becomes equal tothe criticalmicelle
concentration, the hydrocarbon chains of the aminesbegin to formtwo-
dimensional surface aggregates called hemi-micellesat the silica–solution
interface. As the formationof the hemi-micelles progresses at the interface, the
zetapotentialstarts to reverse charge and both theamine adsorption densityand
flotation recovery sharply increase.
As per the dosages of depressants & collectors, a good number of experiments
experiments were performed using different combinations of depressants &
collectors. The results for these dosages i.e. for depressant dosage above 1.00
kg/t & upto 1.5 kg/t & collector dosagefrom 3.00 kg/t to 4.00 kg/t were
encouraging & shown in Tables 3 & 4 respectively for dextrin and starch.

The analysis of the results show that dextrin is a better depressant for coal than
starch whereas among the collectors DTAC emerges out to be the best. Among
the different permutations & combinations with the depressants & collectors, the
best result is obtained when both dextrin and DTAC used together. This
combination gives a product of ash as low as upto 22% with yield as high as
upto 36%.

In case of starch, the results are better with DTAC than with both DTAB & CTAB.
The overall results obtained with DTAB are somewhat comparable with those of
DTAC. However the use of CTAB as collector should be neglected because of
high product ash as well as low yield which my be attributed to the low
selectivity & high frothing nature of CTAB. This invention brings with it a lot
many advantages. Firstly it carves the way forward for an appropriate use of the
solid wastes generated in the coal preparation plants. This helps in proper
utilization of coal tailings, which are currently discarded along with good profits
for the plant. It will resolves the issues of land disposal of coal tailings and
further pollution of water, air etc. due to dumping of tailings in the nearby areas
of the plant. Finally it will help in conserving our coal reserves for the future as
the wastes/by-products from coal preparation plants can be looked upon as
substantial sources of good quality coal.

WE CLAIM :
1. A beneficiation process for coal tailings of size less than 0.5 mm and
having an ash content of at least 40-50%, the process comprising:
forming a slurry by mixing the coal tailings with water;
adding a polymeric carbohydrate depressant in a ratio of 1.25 to 1.50 kg
per tonne of coal and mixing to form a first mixture;
adding a quaternary amine collector to the first mixture and further mixing
to form a second mixture;
blowing air to the mixture to allow the quaternary amine collector forming
a froth and beneficiated slurry, the slurry being settled down, wherein the
froth consisting of high ash mineral matter, and wherein the beneficiated
slurry constituting clean coal.
2. A beneficiation process as claimed in claim 1, wherein a ratio between the
coal tailings and water for forming the slurry is 1:4 to 1:5 (by wt/wt).
3. The process as claimed in claim 1, wherein the mixing speed of the coal
tailings and water is 650-850 rpm.
4. The process as claimed in claim 1, wherein the depressant is dextrin (TM).
5. The process as claimed in claim 1, wherein the depressant is starch (TM).
6. The process as claimed in claim 1, wherein the quaternary amine collector
is Dodecyltrimethy Ammonium Chloride (DTAC) (TM).
7. The process as claimed in claim 1, wherein froth formation is done at 3
lpm.

8. The process as claimed in claim 1, wherein mixing of the slurry and the
depressant is for 10-20 minutes.
9. The process as claimed in claim 1, wherein a collection time of the froth
is 1-1.5 minutes.
10. The process as claimed in claim 1, wherein the beneficiated slurry has
22-23% of ash with a yield of 35-36% (all by wt%).