FIELD OF INVENTION
The present invention relates to development of a process flow sheet for a novel
beneficiation process to produce low ash clean coal from high ash Indian coals.
BACKGROUND OF THE INVENTION
Coal and coal products will play an increasingly important role in fulfilling the energy
needs of our society. Most of the coals are associated with mineral matter, which
makes it unsuitable for efficient utilization, such as carbonization, gasification,
combustion, or liquefaction. Hence, demineralization of coal has tremendous
applications in the field of metallurgical industries, thermal power plants and other
industries, which require coals with low ash content. Accordingly, physical as well as
chemical coal cleaning (beneficiation) processes have been explored. In general,
physical coal cleaning processes involve pulverizing the coal to release the
impurities, wherein the fineness of the coal generally governs the degree to which
the impurities are released. However, because the costs of preparing the coal rise
exponentially with the amount of fine to be treated, there is an economic optimum
in size reduction. Moreover, grinding coal even to extremely fine sizes may not be
effective in removing all the impurities. Based on the physical properties that effect
the separation of the coal from the impurities, physical coal cleaning method are
generally divided into four categories: gravity, flotation, magnetic and electrical.
In contrast to physical cola cleaning, chemical coal cleaning techniques are in a very
early stage of development. As far as Indian scenario is concerned, most of the
coals present in India are of drift origin and low grade having high mineral matter
contents varying from 5% to greater than 50%. Moreover, the mineral matter is

finely disseminated inside the coal matrix and is at times firmly bound. Again, since
washability characteristics of Indian coal is not goold, it is difficult to remove the
mineral matter from coal by conventional physical cola washing techniques based on
specific gravity difference such as heavy media and froth flotation, are of limited use
for applications in coal beneficiation to produce low ash coals for metallurgical coke
making and power generation.
Chemical leaching of coal is a technology to produce ultra clean coal where the ash
content of clean coal is as low as ~1.0% basing on mineralogical composition of the
feed coal. There are potential use of this ultra clean coal both as a fuel and nonfuel.
An effort was made to decrease theash content of high ash Indian coals up to ~4.0-
5.0% in clean coal through laboratory as well as in bench scale studies. In the
present invention, an effort has been made to develop a process flow sheet of a
pilot plant for treating high ash Indian coals through a series of alkali and acid
treatment steps under different operating conditions to produce low ash coal. The
proposed pilot plant is envisaged to operate on a batch process. Each batch shall
process 500 kg of raw coal. In the case of the chemical leaching processes for
removing ash from coal, the inorganic substances constituting the ash content of
coal are reacted with chemical agents and separated from the coal for removal.
Various chemicals are used for the chemical beneficiation process. Some of these
chemicals will have a tendency to dissolve certain inorganic constituents
preferentially to others and the actual chemical to be used may depend upon the
inorganic content of the carbonaceous material which is fed to the process.
Following the end of World War II, it was learned that German coal processing
plants treated coal with aqueous sodium hydroxide at elevated temperatures and
pressures, and thereafter extracted the coal with aqueous hydrochloric acid. The
process reduced the sulphur and ash content of the coal. (Crawford, BIOS Final
Report No. 522, Item No. 30, Feb. 19,1946, British Intelligence Objectives Sub-
Committee, London (A.T.I.- 118668, Central Documents Office, Wright-Patterson

Airforce Bdse, Dayton, Ohio). Subsequently, the U.S. Bureau: of Mines evaluated a
similar process for treating coal, involving leaching with aqueous sodium hydroxide
at 225°C, both with and without a final stage extraction with aqueous hydrochloric
acid. In a report made by Reggel et al in 1972,it was concluded that the sequence of
sodium hydroxide treatment and hydrochloric acid extraction removed most of the
mineral matter originally present in thecoal. Am. Chem. Soc. Div. of Fuel Chem.
Preprints, 17(1): 44-48. Battelle Memorial Institute had developed a similar
process,which is decsribed in Stambaugh et al U.S. Pat No. 4,055,400 of 1977. An
aqueous alkaline slurry of coal is heated at an elevated temperature and pressure to
leach out sulphur and mineral matter. The Battelle process may optionally include
last stage extraction with aqueous acid to reduce the final ash content. (Stambaugh
et al, Hydrocarbon Processing , 54 (7): 115-116 (1975)). AN alternative process has
undergone extensive development at Iowa State University, Ames, Iowa. The
"Ames" process uses oxidative desulphurization in aqueous slurry of sodium
carbonate. Typical conditions are 0.2M Na2CO3 at an oxygen partial pressure of
about 4 atmosphere and temperatures of 120-140 deg. C for 1-2 hrs. This
development was reviewed in detail by Dr. T.D. Wheelock in 1981. (Chem. Eng.
Commun., 12; 137-159). In one representative test, using temperatures of 120-140
deg. C, the total sulfur content of the coal was reduced 70% and the pyritic sulphur
content was reduced 78%. (Wheelock (1981), above cited, at pages 148-149).
Such processes are known as processes for removing ash from coal or coke
(Japanese Patent Publication No. 466/1942), a process for removing sulfur and ash
from coals (Japanese Patent Publication No. 23711/1971) and a coal deashing
process (Japanese Patent Disclosure No. 133487/1980). The processes (1) and (2)
with use of an acid or alkali are practiced usually with the application of pressure
and heat to dissolve the metallic components for the removal of ash. When
practiced under moderate conditions, these processes were almost unable to
achieve any ash removing effect and are therefore not suitable as deashing
processes. The process (3) wherein oxidation is followed by an acid or alkali

treatment is the same as the processes (1) and (2) in principle and is such that the
FeS2 components which are difficult to dissolve are first oxidized and thereafter
dissolved. With the process (4) wherein hydrofluoric acid or hydrogen fluoride gas is
used for treatment, coal is treated with hydrogen fluoride gas since SiO2 is not easily
soluble in acids or alkalis to separate Si in the form of gaseous SiF4 to achieve a
deashing effect. However, the use of hydrofluoric acid or hydrogen fluoride gas,
which is highly toxic and corrosive, involves many difficulties. Thus an actually
effective and useful process for removing ash from coal still remains to be
developed although the deashing of coal is avery important technique for the
effective use of coal.
The patent literature is replete with chemical coal beneficiation processes. For
example, U.S. Patent No. 4,424,062 discloses a process for chemically removing ash
coal by immersing ash containing coal in an aqueous solution containing
hydrochloric acid or citric acid in combination with acidic ammonium fluoride. U.S.
Patent No. 3,993,455 discloses a process for removing mineral matter from coal by
the treatment of the coal with aqueous alkali such as sodium hydroxide, followed by
acidification with strong acid. Similarly, U.S. Patent No. 4,055,400 discloses a
method of extracting sulphur and ash from coal by mixing the coal with an aqueous
alkaline solution, such as ammonium carbonate.
U.S. Patent No. 4,071,328 discloses a method of removing sulphur from coal by first
hydrogenating the coal and the hydrogenated coal is subsequently contacted with
an aqueous inorganic acid solution. U.S. Patent No. 4,127,390 discloses a process
for reducing the sulphur content of coal by treatment with an aqueous sodium
chloride solution. U.S. Patent No. 4,134,737 discloses a process for the production of
beneficiated coal wherein the coal is digested in caustic, then treated in mineral acid
and then treated in nitric acid.

U.S. Patent No. 4,083,940 discloses a process for cleaning coal by contacting the
coal with an aqueous leaching solution containing nitric and hydrofluoric acid. U.S.
Patent No. 4,169,710 discloses comminuting and cleaning coal of sulphur and ash by
contacting the coal with a hydrogen halide, such as HF (aqueous and/or anhydrous).
U.S. Patent No. 4,408,999 discloses beneficiating coal by subjecting the coal to
electromagnetic radiation in the presence of a strong inorganic acid, such as
hydrofluoric acid. In turn, U.S.Patent No. 4,305,726 discloses a chemical method of
treating coal to remove ash and sulphur comprising treating the coal with
hydrochloric and hypochlorous acid in the presence of ferric and ferrous sulphate,
while U.S. Patent No. 4,328,002 discloses a method of treating coal to remove ash
and sulphur involving preconditioning coal particles in the presence of an aqueous
solution of an oxidant, such as H2O2 or HF, washing the so-treated coal, treating the
washed coal with further oxidant and then passivating the coal with for example, an
ammonium salt and then neutralizing with alkali metal hydroxide.
U.S Patent No. 4,516,980 discloses a process for producing low-ash, low sulphur
coal by a two-stage alkaline treatment using sodium carbonate or bicarbonate as the
reagent. The alkaline treated coal is then extracted with aqueous mineral acid; and
U.S.Patent No. 3,998,604 discloses a coal demineralization process whereby ground
coal is treated with aqueous acid, such as HCI, H2SO4 or H2CO3 and then subjected
to froth flotation in the presence of a gas selected from CI2SO2 or CO2.
All the above patents however mostly deal with laboratory scale process
development of the chemical benefication process. In the present invention, an
effort has been made to develop a process flow sheet of a pilot plant for treating
high ash Indian coals through a series of alkali and acid treatment steps under
different operating conditions to produce low ash coal (4-5%). The proposed pilot
plant is envisaged to operate on a batch process. Each batch shall process 500 kg of
raw coal.

OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a develop process flow
sheet for beneficiation process to produce low ash dean coal which eliminates the
disadvantages of prior art.
Another object of the present invention is to propose a develop process flow sheet
for beneficiation process to produce low ash clean coal which is economic and
commercially viable.
A further object of the present invention is to propose a develop process flow sheet
for beneficiation process to produce low ash clean coal which consume less power.
A still further object of the present invention is to propose a develop process flow
sheet for beneficiation process to produce low ash clean coal which is eco-friendly.
SUMMARY OF THE INVENTION
Leaching or solid extraction is done to dissolve mineral matter in coal using a
solvent. The acidic and/or basic components present in mineral matter react with
the solvents, gets dissolved and then removed. The present invention describes a
pilot scale process flow sheet for treating coals crushed to -30/-72 BS mesh size or
fine clean coal obtained from flotation circuit coal for removal of ash-forming
minerals which includes series of alkali and acid treatment steps under various
operating conditions. The steps include treatment of the feed coal in an aqueous
alkaline solution at an elevated temperature under atmospheric and elevated
pressures followed by reaction/extraction with an aqueous acidic solution at
atmospheric temperature and pressures. This is a process to produce low ash (~4.0-

5.0% ash) clean coal from high ash Indian coals with 75-85% yield. The proposed
pilot plant is envisaged to operate on a batch process. Each batch shall process
500kg of raw coal. The present invention is a step towards making chemical
leaching process commercially feasible for various applications.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Fig.1 shows process flow sheet of Chemical Leaching Pilot Plant
Fig.2 shows block flow diagram of the Pilot Plant for Chemical Leaching of Coal
Fig.3 shows proposed layout for development of Pilot Scale Facilities for
Chemical Leaching of Coal
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
Process Units
The process flow sheet of the chemical leaching pilot plant is shown in Fig.1. The
total pilot plant facility is divided into 5 zones, each zone serving the specific
purpose of facilitating the various requirements for the up-gradation of the bench
scale to pilot plant scale unit. The block flow diagram which shows the five zones is
depicted inFig.2. The proposed layout for the development of the chemical leaching
pilot plant facility is shown in Fig.3.

Zone 1:
Zone 1 consists of Raw material storage area. The covered shed shall consist of area
for storing raw coal, finished product and a room for chemical storage.
The raw coal received from the mine is stored in one part of the covered shed. The
storage facility shall be available for storing upto 50 t of coal. The final product shall
be stored in the second part of the covered shed.
A separate room shall be made available for storing the chemicals. The acids shall
be stored in cans or drums. The alkali shall be stored in racks as it comes in bags.
A limekiln is envisaged to generate fresh lime powder from the limestone. The
limestone raw material is envisaged to be in an open area.The fresh lime from the
kiln is envisaged to be stored in covered containers. Alternatively, fresh lime form
external sources can also be purchased and used.
Zone 2:
Zone 2 shall consist of two major sections:
- Coal preparation
- Feed preparation
a) Coal preparation section:
The coal preparation section shall consist of coal conveyors, ball mill to pulverize
the coal to -72 mesh (0.025 mm), screens to separate out -72 mesh powders,
powder storage bunkers.
The raw coal of size 0-25 mm from the storage area is dozed into ground
hoppers and lifted to an elevation of 15 m with the help of bucket elevators or
suitable equipment and fed to a continuous ball mill. If the input coal size is

larger than 25 mm, then a primary crusher to reduce the size to 0-25 mm may
be required to be provided. The primary crushing can be provided at the ground
level.
The ball mill shall crush the coal to -72 mesh and the output from the ball mill is
screened to separate the -72 mesh material using suitable screens. The under
size material shall be stored into a storage bunker while the over size material is
re-circulated back to the ball mill.
The total capacity of the coal preparation unit shall be 2-3 tph. The fine coal shall
be stored in a bunker with a capacity of 2 t. The bunker shall be provided with
load cells to continuously monitor the weight of the powder while filing and also
during discharge. The bunker shall also be provided with suitable vibratory
equipment to facilitate the smooth flow of the fine material from the bunker,
during discharges. The fine powders from the bunker shall be discharged
through a rotary feeder arrangement.
If the moisture content of the coal is high, either the inherent moisture or the
external moisture (i.e >5%) then a suitable rotary dryer shall be provided to dry
the coal before it is screened. This will be essential to avoid sticking and choking
of the screens and bunkers.
The coal preparation unit is envisaged to operate once a day to generate 2 T
materials which shall suffice a 4 batch operation of 500 kg each per day.
b) Feed preparation section:
The feed preparation section shall consist of the following units:
- Coal slurry preparation -2m3 capacity
- Acid preparation -2m3 capacity
- Alkali preparation -3m3 capacity

Slaked Iime preparation -2m3 capacity
All the feed preparation units shall be single walled circular tanks fitted with stirring
and agitation equipment of suitable capacity. While the unit for cola slurry
preparation shall be capable of withstanding abrasion of the coal powders and the
other units for the acid, alkali and the lime units shall be of acid, alkali and corrosion
resistance. All pipe lines, feed pumps, delivery pumps; valves (both gate valves and
the control valves) shall be suitable for acid, alkali and corrosion resistance.
The feed preparation tanks for the coal slurry, alkali and the slaked lime shall have
suitable inlets for feeding the fine powders, the flow controlled by a gate valve. Acid
preparation tank shall have suitable inlet for feeding the concentrated acids into the
tank. Each of the tank shall have dilution water inlet, the flow controlled through a
control valve. Each of the tanks shall be provided with level sensors for control
purposes. Each of the tanks shall also be provided with sample collection points at
suitable locations which shall be operated manually.
1) Coal slurry preparation unit:
The coal slurry preparation tank shall be a circular tank of 2m3 capacity. The tank
shall be fitted with suitable stirring mechanism to stir the material while the slurry is
being prepared. The slurry prepared may be in the form of dough or cake or thick
slurry and hence heavy capacity stirrers are envisaged.
The coal slurry preparation tank shall have inlet pipe for feeding the coal powder
into the tank. The feed shall be controlled through a gate valve, which shall also act
as an isolation valve for sealing purposes. The process water shall be fed to the tank
and the feed line shall be provided with a control valve. The tank shall be provided
with level indicators for control purposes.
The coal slurry prepared in the tank may be like dough or cake or thick slurry and
requires forced discharge form the tank. A compressed air line shall be provided to

pressurize the tank to facilitate discharging the slurry out of the tank. The outlet
shall be provided with a gate valve. This valve shall also act as an isolator of the
tank apart from controlling the flow.
The output from the coal slurry tank shall be transported to the reactor zone.
The coal powder from the storage bunker is fed to the slurry preparation tank in
batches of 500 kg coal powder. The feeding may be controlled by both the load cell
data at the coal powder storage bunker and also the slurry tank level. Once the coal
powder is taken into the tank, process water is fed to the tank is controlled manner
to make the slurry of required composition. The flow of water is controlled through
the flow control valve based on the tank level signal. The mixture is continuously
stirred while the material is being added.
Once the slurry is made, the outlet gate valve is opened and the outflow of the
slurry form the tank is aided with the compressed air to ensure complete emptying
of the tank.
2) Acid preparation unit:
The tank used for the acid preparation shall be provided with inlet for the feed, inlet
for the process eater, stirrer, and outlet. Each of the feed lines for the acid, water
and the outlet may be provided with control valves to regulate the feed inlet and
also the pipe lines may be provided with flow sensors. The tank shall be provided
with level sensors. The acid inlet and the tank outlet shall be provided with pumps
to feed and evacuate the material.
The tank, valves, pumps, flow and level sensors shall be acid resistant. The
concentrated acid from the storage tanks in the storage area is pumped to the acid
preparation tank in the required quantities for preparation of diluted acids with

various dilutions. Process water is used to prepare the necessary diluted acids. The
contents in the tank are stirred while the diluted acids are prepared.
The final diluted acid is then pumped with the help of pumps to the reactor zone
and the flow is controlled through the control valves in the outlet pipe line.
3)Alkali preparation unit:
The stirred tank used for the alkali preparation shall be provided with a hopper for
the feed alkali, inlet pipes for the process water and recycled alkali, outlet for the
alkali solution. The hopper shall be provided with load cells for measuring the
amount of alkali entering into the system so that requisite alkali concentration can
be maintained inside the tank. Each of the feed lines for water and the outlet may
be provided with control valves to regulate the feed inlet and also the pipe lines may
be provided with flow sensors. The tank shall be provided with level sensors. The
tank outlet shall be provided with pumps to feed and evacuate the material.
The tank, valves, pumps flow and level sensors shall be alkali resistant. The
concentrated alkali from the storage area is charged through the hopper to the alkali
preparation tank in the required quantities for preparation of diluted alkali with
various dilutions. Process water is used to prepare the necessary dilute alkali. The
contents in the tank are stirred while the diluted alkali is being prepared.
The final diluted alkali is then pumped with the help of pumps to the reactor zone
and the flow is controlled through the control valves in the outlet pipe line. In
addition to the fresh alkali, recycled alkali is also added to the reactor.
4) Slaked lime preparation unit:
The tank used for the slaked lime preparation shall be provided with hopper for the
feed, inlet for the process water, stirrer, and outlet. The hopper shall be provided
with load cells for measuring the amount of lime entering into the system so that

requisite lime concentration can be maintained inside the tank. Each of the feed
lines for water and the outlet may be provided with control valves to regulate the
feed inlet and also the pipe lines may be provided with flow sensors. The tank shall
be provided with level sensors. The tank outlet shall be provided with pump to feed
and evacuate the material.
The tank, valves, pumps, flow and level sensors shall be corrosion resistant.
The lime from the storage area is charged through the hopper to the slaked lime
preparation tank in the required quantities for preparation of lime with various
dilutions. Process water is used to prepare the necessary slaked lime. The contents
in the tank are stirred while the lime is being prepared.
The final slaked lime is then added to the recycled alkali to precipitate out calcium
silicates. The precipitate is prepared inside a thickener and the regenerated alkali is
recycled back and charged to the reactors along with fresh alkalis.
Methodology to be followed for chemical leaching of minerals in the pilot
plant
Process Description for Chemical Leachino Pilot Plant
Coal Sample Preparation:
The feed can be fine clean coals generated in coal washeries through froth flotation
process or a middling product. The feed sample can be taken directly as obtained
after froth flotation treatment if the size requirement in -30 BS mesh or can be
crushed to -72 BS mesh size (-0.025mm) for improving the kinetics of the process.
The coal samples will be analysed for their ash content before processing. The
major minerals normally found in coals are silicates or shales, quartz and/or
sandstone, pyrites and carbonates such as siderites and ankerites. It has been found
that even at 373 deg. K using dilute to moderately concentrated NaOH solution,

kaolin is converted into a crystalline sodium derivative i.e. Na2O.Al2O3.l.8SiO2. The
solubility of this sodium-aluminium-silicate derivative is not very high in alkali
solutions but it is fairly soluble in dilute alkali followed by washing with mineral acid.
Chemicals:
Commercially available sodium hydroxide (NaOH) pellets in combination with
hydrochloric acid (HO) are to be used in the present method. Dilute aqueous NaOH
solution of 10-50% concentration and 10-20% HCI will be prepared and used for the
chemical leaching process described below:
The treatment process:
Feed coal will be collected form the coal storage sheds and will be fed into a hopper
using a bucket elevator. The coal will then enter into a ball mill from the hopper in
which it will be crushed to desired size range by allowing sufficient residence time.
The crushed coal will then be screened to obtain -30 BS mesh and/or -72 BS me4sh
size. Oversize coal at the top of the screen will be recycled back to ball mill. The
product from the screen will be stored in a storage hopper of 2 t capacity. The cola
powder from the storage hopper will be fed into the slurry preparation tank. In the
slurry preparation tank, 500kg of feed coal will be mixed with 1000-3000 litres of
water using a agitator. The coal slurry will then be pumped to the first reactor
(Reactor 1). In other tank, alkali solution of a particular concentration will be
prepared (500-2500 kg NaOH in 2000-4000 litres of water) so as to maintain
aqueous alkali concentration of 10-50% inside Reactor 1. The cola slurry as well as
the prepared alkali solution will be fed into the first reactor of capacity 7.0 Nm3. The
first reactor is a jacketed reactor having an agitator in which temperature up to
~100°C can be achieved at atmospheric pressure. In all the above preparation
tanks, the agitator speed has to be maintained at around at around 200 rpm to
facilitate proper mixing.

In Reactor 1, the temperature will be maintained at 85-90°C by using saturated
steam inside the jacket. The leaching reaction of cola with aqueous NaOH solution
will be carried out inside the first reactor for a period of nearly 2 to 5 hr based on
requirement in reduction of mineral matter percentage in coal and for achieving
optimum leaching sequence in the series of reactors.The stirrer speeds inside the
reactors can be varied using a variac for homogenous mixing of the coal slurry with
the NaoH solution. A condenser will be mounted at the top of the reactor 1 for
continuous reflux. After completion of reaction in the first reactor, the reaction
mixture can be sent to the second reactor directly or through the rotary drum filter
for filtration under vacuum and washing.
The filter cake that is formed on the filter cloth of the rotary drum filter is
continuously washed with wash water so as to remove the silicates and other
products those are formed as a result of reaction of the mineral matter constituents
with NaOH. Water washing is done by spraying water from the top of the rotary
drum filter. The filter cake that comes out of the rotary drum filter will be sent for
preparation of coal slurry. Filtrate coming out of the rotary drum filter will be
pumped to a separate storage tank.
The coal slurry prepared from filter cake will then be pumped to pressure reactor
(Reactor 2) or acid reactor 3. A pressure of 10kg/cm2 is maintained inside the
second reactor by circulating compressed air into the reactor. At the highest
pressure, a temperature of nearly 180°C can be reached by injecting steam through
the jacket. Inside Reactor 2, 150-170°C temperature will be maintained. Here, the
stirrer speed inside the reactor is maintained at around 200 rpm. A
condenser/cooling coil arrangement is present at the top of the reactor 2 for
continuous reflux. After completion of reaction time, the product from reactor 2 will
be again send back to filtration unit for filtration. Filtration will be carried out in
similar way.

The slurry prepared using the filter cake can also be fed into Reactor 3 which is a
reactor for treating coal slurry with 5-20% of HCI at room temperature and
atmospheric pressure. This reactor will be used for washing/treating alkali treated
coal with acid so that most of the dissolved silicates and other reaction products
gets washed away. Here also, the stirrer speed inside the reactor is maintained at
around 200 rpm. Then the acid treated coal will be sent to the filtration unit where
filtration will be carried out. Product after filtration from pressure Reactor 2 or acid
reactor 3 basing on sequence maintained may then be sent to Reactor 4 if required
which is an acid reactor similar to Reactor 3. There will be facility for maintaining
different leaching sequence i.e. alkali-acid-alkali-acid as well as alkali-alkali-acid-acid
using the four reactors.
After completion of reaction in the final reactor, the slurry will be sent to filtration
unit 2 (rotary drum filter 2). Filtration will be carried out in the same way, as was
done using the filtration unit 1. The coal filter cake obtained after the final treatment
will be stored inside a tank and after drying it will be sent for various physical,
chemical, rheological, petrographical and other special analysis. The alkali and acid
filtrates will be sent to two different storage tanks.
The filtrate coming out from the filter after alkali treatment will be neutralized with
lime to precipitate out the silicates and other undesirable constituents. The rest of
the filtrate which contains mostly pure NaOH will be sent to a triple effect
evaporator to obtain requisite concentration of NaOH. The concentrated NaOH
solution (coming out of the evaporator) will be pumped back to the recycle NaOH
storage tank from which it will be again charged into the reactor along with fresh
alkali solutions. Similarly, the filtrate coming out after the acid treatment will also be
recycled back to a recycle acid storage tank. After particular number of tests when
the acids get contaminated with impurities, the spent acid will discharged into a
storage tank in the product disposal section from which it will be sent to a tanker
and will be disposed off in a safe place.

Requirement of services:
Approximately 500 kW peak power requirement is estimated for the total plant
operation. No emergency power facility is proposed since the operations are batch
processes.
Approximately 30,000 litres or 25 cubic m of water requirement is estimated per day
for the steam generation, process as well as wash water. A water storage tank of
suitable capacity shall be provided to facilitate uninterrupted supply of water to the
various units. The water shall be soft having not more than 10 ppm hardness. If the
water available is hard, the same shall be provided with suitable softening
equipment.
Steam shall be used as the heating medium for the reactors. Approximately, 800
kgs/hr of superheated steam at 250°C and a pressure of 15kg/cm2 is estimated. The
steam shall be used to heat the material in the reactors upto 185°C.
Compressed air is envisaged to pressurize the pressure reactor at 10 kg pressure.
Compressed air is also envisaged for pneumatically activated control valves. A
compressed air buffer storage tank with 5000 litres capacity is envisaged to facilitate
uninterrupted supply of air to the plant units. Suitable dryer arrangements shall be
provided to supply dry compressed air to the control valves.
Modes of Operation:
Three modes of operations are envisaged for the leaching process. Any of the
operating modes can be finally adopted and the process optimized for achieving the
maximum reduction of ash content in the coal. The details of the three different
modes of operation are given below:
Final Product
The coal cake obtained after the final treatment and filtration in the rotary vacuum
filter 2 is collected separately. The coal cake is expected to have a moisture content

of about 20%. The cake shall be air dried to reduce the moisture and then bagged
in 1 tonne jumbo bags. The bags are then stored in product storage area for further
transportation to the user. The coal cake samples from each batch are collected and
analyzed for various physical, chemical, Theological, petrographical and other special
analysis.

WE CLAIM
1. A process flow sheet for a novel beneficiation process to produce low ash clean
coal from high ash coal comprises:-
- coal preparation;
- feed preparation which comprises coal slurry, alkali, acid and slaked lime
preparation;
- a plurality series of alkali and acid treatment in reactors;
- filtration of coal slurry;
- treatment in a thickness and alkali concentration using multi-effect
evaporator;
- storing the dried coal in a bag.

2. The process as claimed in claim 1, wherein coal preparation step comprising 500
kg coal crushed to -30 or -72 BS mesh size or fine coal obtained from floatation
of circuit coal.
3. The process as claimed in claim 1, wherein coal slurry is prepared in the dough
or cake or thick slurry by mixing prepared coal with water.
4. The process as claimed in claim 1, wherein acid preparation comprising an
addition of water in concentrate acid for preparation of diluted acid with various
concentration.
5. The process as claimed in claim 1, wherein alkali preparation comprises an
addition of water so that requisite alkali concentrate can be maintained.
6. The process as claimed in claim 1, wherein slaked lime preparation comprises an
addition of water so that requisite lime concentration can be maintained.

7. The process as claimed in claim 1, wherein the alkali is sodium hydroxide
(NaOH).
8. The process as claimed in claim 1, wherein the acid is hydrochloric acid or
sulphuric acid.
9. The process as claimed in claim 1, wherein the alkali concentration is 10-50%
and the acid concentration is 10-20%.
10. A process as claimed in claim 1, wherein the treatment steps in reactor
comprises a treatment of the feed coal in an aqueous alkaline solution at an
elevated temperature under atmospheric pressure and elevated pressure
followed by reaction/extraction with an aqueous acidic solution at atmospheric
temperature and pressures.
11. The process as claimed in claim 10, wherein alkaline temperature up to 80-85
deg. C under atmospheric pressure is done in the first reactor 1, alkaline
treatment up to 180-185 deg. C under elevated pressure up to 10 Bar is done in
second reactor 2 and acidic treatment under atmospheric temperature and
pressure is done in reactor 3 and reactor 4.
12. The process as claimed in claim 10, wherein super heated steam at 250°C shall
be used to heat the material in the reactor up to 185°C.
13. The process as claimed in claim 10, wherein the leaching reaction of coal with
aqueous NaOH solution will be carried out inside the first reactor for a period of
nearly 2 to 5 hours.

14. The process as claimed in claim 10, wherein the stirrer speed inside the reactor
is maintained at around 200 rpm.
15. The process as claimed in claim l,wherein soft water having not more than 10
ppm hardness is used.
16. The process as claimed in claim 1, wherein pressure in reactor is created by the
compressed air supplied at pressure of 10 kg/cm2.
17. The process as claimed in claim 1, wherein coal filter cake obtained after final
treatment contains 20% moisture which shall be air dried to reduce moisture.
18. The process as claimed in claim 13, wherein the product coal ash can be
reduced upto 4-5% by weight and yield is 75-85% after leaching.
19. The process as claimed in claim 1, wherein different leaching sequence i.e. alkali
-acid-alkali-acid as well as alkali-alkali-acid-acid can be used in four reactors.

Leaching or solid extraction is done to dissolve mineral matter in coal using a
solvent. The acidic and/or basic components present in mineral matter react with
the solvents, gets dissolved and then removed. The present invention describes a
pilot scale process flow sheet for treating coals crushed to -30/-72 BS mesh size or
fine clean coal obtained from flotation circuit coal for removal of ash-forming
minerals which includes series of alkali and acid treatment steps under various
operating conditions. The steps include treatment of the feed coal in an aqueous
alkaline solution at an elevated temperature under atmospheric and elevated
pressures followed by reaction/extraction with an aqueous acidic solution at
atmospheric temperature and pressures. This is a process to produce low ash (~4.0-5.0% ash) clean coal from high ash Indian coals with 75-85% yield. The proposed
pilot plant is envisaged to operate on a batch process. Each batch shall process
500kg of raw coal. The present invention is a step towards making chemical
leaching process commercially feasible for various applications.