FIELD OF THE INVENTION
The present invention relates to a method for treating high ash coal to substantially
reduce the ash bearing inorganic impurities along with regeneration and recycling of
spent alkali and acid during chemical demineralization of coal.
BACKGROUND OF THE INVENTION
High ash coal cannot be utilized directly in the industrial applications like steel
making, power generation and synthetic fuel generation due to its lower calorific
value and poor combustibility. The main problem persists in high ash coal be.ng
the finely disseminated minerals in coal matrix with high ash content of 35-45%
compared to that of worldwide figure of 17-20%.
Due to the presence of the near gravity material in high ash coal, the yield in the
physical beneficiation method is limited to 35-40% at 15% product ash level. Fig.
1 shows the operation zone for physical beneficiation method. It clearly shows
that clean coal yield decreases continuously when moving towards low ash zone.
Due to poor liberation characteristics of high ash coal or for that matter any
poorly liberated high ash coal from other sources, when aspiring for low ash coal
using present physical beneficiation process, the product coal yield becomes very
small and subsequently makes the beneficiation process uneconomical. This
disadvantage of physical beneficiation process necessitated the development of
chemical leaching process which can bring down the mineral content of high ash
coal without much loss of carbonaceous matter in the form of rejects.
Chemical leaching process involves addition of chemicals which selectively react
with the mineral matter which are locked inside the coal matrix and get it
dissolved into liquid phase subsequently. Various chemicals are used for the
chemical beneficiation process. Some of these chemicals 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.
The inorganic portion of coal, which contributes for ash content, mainly consists
of impurities like silica, alumina and oxides of iron, calcium, phosphorous,
magnesium, sodium, potassium etc. The majority of coal ash is contributed by
the presence of silica (55-60%) and alumina (20-25%).
According to U.S. Patent No. 8647400, an improved process flow sheet at pilot
plant scale is disclosed for treating high ash coals through a series of alkali and
acid treatment steps under different operating conditions to produce low ash
coal. However, in the said disclosure, process for regeneration and recovery of
spent chemicals is not developed as the part of the benef.ciation flow sheet.
Development of an integral process flow sheet combined with regeneration
process is a necessity in order to have a sustainable process that is economically
viable and environmental friendly.
OBJECT OF THE INVENTION:
In view of the foregoing limitations inherent in the prior-art, the object of the
invention is to develop an integrated process for beneficiating and producing low
ash clean coal by beneficiating a high ash coal along with regeneration and
recovery of spent chemicals.

SUMMARY OF THE INVENTION:
In one aspect, the invention provide an integrated process for beneficiating a
coal comprising steps of preparing an aqueous alkali solution, the aqueous alkali
solution having concentration of 15-25% (w/w), preparing a coal slurry, the coal
slurry being prepared by mixing the coal with the aqueous alkali solution in such
a way to maintain the ratio of coal to aqueous alkali solution as 1:5 on weight
basis, transferring the coal slurry to atleast one alkali reactor, each of the alkali
reactor having a heating/cooling jacket, pressurizing the alkali reactor by means
of compressed air at 5-12 bar pressure for suppressing vaporization tendency of
the aqueous alkali solution at high temperature condition, heating the coal slurry
by injecting steam of 170° to 200° C temperature at 10-15 bar pressure into the
heating/cooling jacket, maintaining the coal slurry temperature at 100° to 180° C
for 40 to 90 min reaction time, cooling the coal slurry to temperature of 70-90
deg. C by circulating cooling water through the heating/cooling jacket of the
alkali reactor, separating the alkali leached coal from the aqueous alkali solution
using a first agitated nutsche filter, the first agitated nutsche filter being
operated at 2-5 bar pressure, regenerating the aqueous alkali solution by
purging CO2, followed by causticization using slaked lime solution, washing the
alkali leached coal by a counter-current method, acid leaching the alkali leached
coal in atleast one acid reactor, wherein the acid reactor is added in an aqueous
acidic solution having concentration of 5 to 10 % (w/w), the coal to acid solution
ratio being maintained as 1:5 on weight basis, the alkali leaching being done for
20-60 min at room temperature and at atmospheric pressure, separating the acid
leached coal from the aqueous acidic solution using a second agitated nutsche
filter, the second agitated nutsche filter being operated at 2-5 bar pressure,
regenerating the aqueous acidic solution by adding concentrated sulfuric acid
having 98% concentration w/w in such a manner that the aqueous acidic

solution to sulfuric acid ratio being 1:0.05 (by weight), and washing the acid
leached coal by counter-current method.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 shows graphical representation between clean coal yield vs ash.
Fig. 2 shows process flow diagram for an integrated process for beneficiating a
coal.
Fig. 3 shows schematic of the integrated process for beneficiating the coal.
Fig. 4 shows the integrated process flow sheet for beneficiating the coal.
Fig. 5 shows schematic representation of an alkali regeneration process.
Fig. 6 shows schematic diagram of an acid recovery process.
Fig. 7 shows schematic diagram of multi-stage beneficiating of the coal.
Fig. 8 shows layout of beneficiating of a coal pilot plant.
DETAILED DESCRIPTION OF THE INVENTION:
Various embodiments of the invention provide an integrated process for
beneficiating a coal, the process comprises a method of preparing an aqueous
alkali solution, the aqueous alkali solution having concentration of 15-25%
(w/w), preparing a coal slurry, the coal slurry being prepared by mixing the coal
with the aqueous alkali solution in such a way to maintain the ratio of coal to
aqueous alkali solution as 1:5 on weight basis, transferring the coal slurry to

atleast one alkali reactor, each of the alkali reactor having a heating/cooling
jacket, pressurizing the alkali reactor by means of compressed air at 5-12 bar
pressure for suppressing vaporization tendency of the aqueous alkali solution at
high temperature condition, heating the coal slurry by injecting steam of 170° to
200° C temperature at 10-15 bar pressure into the heating/cooling jacket,
maintaining the coal slurry temperature at 100- to 180° C for 40 to 90 min
reaction time, cooling the coal slurry to temperature of 70-90 deg. C by
circulating cooling water through the heating/cooling jacket of the alkali reactor,
separating the alkali leached coal from the aqueous alkali solution using a first
agitated nutsche filter, the first agitated nutsche filter being operated at 2-5 bar
pressure, regenerating the aqueous alkali solution by purging CO2, followed by
causticization using slaked lime solution, washing the alkali leached coal by a
counter-current method, acid leaching the alkali leached coal in atleast one acid
reactor, wherein the acid reactor Is added in an aqueous acidic solution having
concentration of 5 to 10 % (w/w), the coal to acid solution ratio being
maintained as 1:5 on weight basis, the alkali leaching being done for 20-60 min
at room temperature and at atmospheric pressure, separating the acid leached
coal from the aqueous acidic solution using a second agitated nutsche filter, the
second agitated nutsche filter being operated at 2-5 bar pressure and
regenerating the aqueous acidic solution by adding concentrated sulfuric acid
having 98% concentration w/w in such a manner that the aqueous acidic
solution to sulfuric acid ratio being 1:0.05 (by weight) and washing the acid
leached coal by counter-current method.
Shown in FIGS. 2, 3 and 4 is an integrated process (100) for beneficiating a high
ash coal (hereinafter "coal").

The coal is a mixture of 55-65% organic matter and Si02 = 18 - 24; Al203 = 8 -
11; Cr203 = 0.02 - 0.05; Ti02 = 0.5 - 1.0; MnO = 0.02 - 0.04; MgO = 1.0-1.5; P
= 0.15 - 0.22; CaO = 2.0 - 2.5; Fe203 = 4.0 - 6.0; K.0 = 0.4 - 0.6; and Na20 =
0.2 - 0.5 (all in wt. %). The average size of the coal lies in the range of 0.5mm
to 1.00 mm.
The process (100) starts with the coal preparation section in which a bucket
elevator lifts the coal from raw coal storage and feeds in to the ball mill inlet
chute.
In an embodiment the capacity of the bucket elevator can be of 1 ton/hr.
The raw coal gets ground with the help of ball mill followed by screening of the
coal using centrifugal screen to provide the required coal size for leaching
process in the beneficiation of the coal.
In an embodiment the capacity of the ball mill can be 1 ton/hr.
In an embodiment 30 mesh centrifugal screen can be used.
Then, the coal sample obtained is stored in a coal bunker facilitated with a load
cell assembly for weight measurement and a rotary feeder at the outlet for
smooth discharge of coal powder.
In an embodiment the capacity of the coal bunker can be of 2 ton.
Next in sequence (as shown in FIG. 4) is the feed preparation area which
comprises of a coal slurry preparation tank (1), an alkali preparation tank (2) and
a slaked lime preparation tank (3). All the feed preparation tanks are provided
with agitator, level switches, control valve for regulating flow rate, flow meter
and flow integrator at inlet line for flow measurement. Further, these tanks are
also provided with recirculation arrangement for enhancing the vertical mixing of
tank content.

In an embodiment the capacity of the coal slurry preparation tank, the alkali
preparation tank and the slaked lime preparation tank can be 2.5 m3, 3 m3 and
2.5 m3 respectively.
At step (102) an aqueous alkali solution is prepared in the alkali preparation tank
(2). The aqueous alkali solution has concentration of 15-25% (w/w).
At step (104), a coal slurry is prepared in the coal slurry preparation tank (1).
The coal slurry is prepared by mixing the coal with the aqueous alkali solution in
the coal slurry preparation tank, in such a manner to maintain the ratio of coal to
aqueous alkali solution as 1:5 on weight basis.
In an embodiment the alkali is sodium hydroxide solution.
The coal slurry is sent to an alkali reactor. The alkali reactor is located at the
production area along an acid reactor.
For efficiently benefiting the coal, more than one alkali reactor and acid
reactor can be used as shown in FIG. 4.
Shown in FIG. 4 are two alkali reactors Reactor-1 (5), Reactor-2 (6) and two acid
reactors Reactor-3 (7) and Reactor-4 (8). The production area further comprises
a first agitated nutsche filter (9) for alkali, a second agitated nutsche filter (10)
for acid, a carbonation tank (11), an open nutsche filter (12) and a causticizer
(13).
At step (106) the coal slurry is transferred to at least one alkali reactor. Each of
the alkali reactors has a heating/cooling jacket. This heating/cooling jacket is
configured to heat/cool the coal slurry.

The first two reactors, Reactor-1 (Rl) and Reactor-2 (R2), are meant for alkali
leaching of the coal.
In an embodiment Reactor-1 (Rl) and Reactor-2 (R2) can have the holdup
capacities of 7 m3 and 8 m3 respectively.
The vessel for alkali leaching reactors Rl and R2 is made up of SS-316 and the
material of construction of heating jacket arrangement is mild steel. The
agitators of Reactor-1 & Reactor-2 are provided with variable frequency drives
(VFD) for controlling and regulating the speed from 0 to 200 rpm. The agitator
speed of the alkali reactors (5,6) is maintained from 100 to 160 rpm.
Superheated steam generated from water tube boiler is the main source of heat
input for reactors (Rl and R2), causticizer (13) and a triple effect evaporator
(18). The triple effect evaporator (18) is configured to concentrate the dilute
alkali solution. Radar type level sensors are used in both reactors Rl and R2 to
measure the level of high temperature the coal slurry. Further, 5-point
thermowell are provided for the measurement of average temperature of reactor
content and additionally two separate Resistance Temperature Detectors (RTDs)
are installed to observe the jacket side temperature. Pressure sensors are
provided at reactors Rl & R2 and heating/cooling jacket side to continuously
monitor the pressure of the system.
At step (108) the alkali reactors Rl and R2 are pressurized by means of
compressed air. The pressure of the compressed air is 5-12 bar. This is for
suppressing vaporization tendency of the aqueous alkali solution at high
temperature condition.

At step (110) the coal slurry is heated by injecting steam of 170° to 200° C
temperature at 10-15 bar pressure into the heating/cooling jacket.
At step (112) the coal slurry is maintained at temperature 100° to 180° C. for 40
to 90 min reaction time.
At step (114) the coal slurry is cooled to temperature of 70° C - 90° C by
circulating the cooling water through the heating/cooling jacket of the alkali
reactor.
At step (116) the alkali leached coal is separated from the aqueous alkali solution
using the first agitated nutsche filter (9). The first agitated nutsche filter (9)
operates at 2-5 bar pressure.
The first agitated nutsche filter (9), provided with polypropylene (PP) filter cloth
of 10 micron pore diameter as a filter medium for separation of the coal and the
alkali solution.
In an embodiment the first agitated nutsche (9) filter has the holdup capacity of
6m3.
It is provided with the facility for compressed air inlet for pressurisation from top
and is also connected with vacuum pump below the filter cloth/medium for
generating vacuum to enhance the filtration rate as well as to reduce the
moisture content of the coal.
Filtration area has to be designed in such a way to maintain the coal cake
thickness at 7-8 cm. The first agitated nutsche filter (9) can be equipped with
forward and backward rotating agitator whose speed can be regulated from 8
rpm to 16 rpm. The first agitated nutsche filter plays an important role in
accomplishing solid-liquid separation, washing of coal and reslurrification of the
coal slurry.

At step (116 A) the aqueous alkali solution is regenerated by purging COa,
followed by causbcizabon using slaked lime solution.
The schematic representation of an alkali regeneration process (500) is shown in
FIG 5. The process for alkali regeneration involves two stages viz carbonabon
(502) and causticization (506). Carbonabon reaction is carried out in the
carbonabon tank (11) having the provision of sparger arrangement for purgmg
CO2 and heating jacket for injecting steam for heating the solution.
In an embodiment the carbonabon tank has capacity of 1.5 m3.
During carbonabon (502), carbon dioxide gas is purged into the spent alkali
solution, thus forming the silica-alumino precipitate. The resultant slurry ,s
ffltered and washed at (504), using the open nutsche filter (12). The Ik* thus
obtained is then causflcised at (506), in the causticizer (13) using slaked l,me
prepared at the lime preparabon tank (3) to recover the causbc soda from the
carbonate solubon. Now the calcium carbonate formed is washed and obta.ned
as a by-product at (508).
Causbcizer is a jacketed vessel of made up of SS 316 tank provided with agitator
assembly.
In an embodiment capacity of the Causbcizer can be of 5m3.
Mixing tanks (14,15) are provided for hot water washing of silica-alumina
precipitate and calcium carbonate precipitate, respectively. Further, addibonal
mixing tank (16) is provided for the carbonic acid washing of calcium carbonate
during the second stage of washing.

At step (118) the alkali leached coal is washed by a counter-current method. In
an embodiment water can be used for counter-current method.
The washing step of the coal plays important role in recovery of aqueous alkali
solution after filtration step and reduces the residual alkali content of coal. In
turn the reduction of residual alkali content (unreacted) in coal cake decreases
the acid consumption during acid leaching step (explained later). Similarly, the
washing of coal after acid leaching stage is very crucial in order to ensure the
quality of low ash coal product.
The critical process parameters that influence the washing stage of coal are
volume of wash water (500-800 liter), number of washing cycles (5) and wash,ng
«me per cycle (15-30 min). The main challenge involved in the reduction of wash
water quantity as the increased wash water consumption would increase the
energy requirement during evaporation process.
A counter current washing circuK is implemented for coal washing is illustrated in
FIG 4, as part of the integrated process. It involves 5 washing stages w,th first
stage of coal using the spent wash water Aon second stage for washing, second
stage of coal utilizes spent wash water from third stage and so on, til, the four
washing steps. Fresh water is used for 5» washing stage (final) of coal. Th
spent wash water obtained from first stage of washing * taken to the tnple
effect evaporator (18) for concentration.
Subsequenny, the water washed coal obtained after alkali leaching is reslurrified
and pumped in to an acid leaching circuK. The aqueous alkali solution is taken to
the alkali regeneration section to remove the silica-alumina impurities present m
it.

At step (120) the alkali leached coal is acid leached in atleast one acid reactor,
by adding an aqueous acidic solution having concentration of 5 to 10 % (w/w),
the coal to acid solution ratio being maintained as 1:5 on weight basis. The
reaction time is 20-60 min at room temperature and at atmospheric pressure.
In an embodiment the acid is hydrochloric acid (HCI).
In an embodiment, for efficient acid leaching, the acid reactors, Reactor-3 (R3)
and Reactor-4 (R4) are used. The Reactor-3 (R3) and Reactor-4 (R4) are made
up of SS-316 coated with Fibre Reinforced Plastic (FRP) lining.
In an embodiment the holdup the capacity of the reactors R3 and R4 can be 7
m3 each.
Similar to the alkali leaching reactors, the agitators of R3 & R4 are provided with
variable frequency drives (VFD) for regulating the speed from 0 to 200 rpm.
Ultrasonic sensors are used in both R3 and R4 to measure the level of acidified
coal slurry inside the vessel. Even though acid reactors are designed for room
temperature reactions, RTDs are installed in both reactors to monitor the
temperature raise during acid addition due to heat of dilution.
In the acid reactors, the coal slurry is dosed with acid solution for maintaining
the desired concentration.
At step (122) acid leached coal is separated from the aqueous HCI acid solution
using a second agitated nutsche filter (10). The second agitated nutsche filter
(10) operates at 2-5 bar pressure.

The schematic diagram of the acid recovery/regeneration process is shown in
FIG. 6. The aqueous HCI acid solution comprises of silica-alumina compounds in
suspension.
At step (122A) the aqueous hydrochloric acid solution is regenerated by adding
concentrated sulfuric acid having 98% concentration w/w in such a manner that
the aqueous acidic solution to sulfuric acid ratio as 1:0.05 (by weight). The
resultant solution is allowed for cooling till the temperature of the solution
reaches 30-33°C. Flocculant is added to the spent acid solution. The silica-
alumina particles are agglomerated and form floes.
Finally, the solution is filtered to separate the silica-aluminium component as a
precipitate and a regenerated acid solution in the second agitated nutsche filter
(10). After filtration, the aqueous HCI acid solution is recycled back to the
process and the precipitate is collected as a by-product.
At step (124) the acid leached coal is washed by counter-current. In an
embodiment water can be used for the counter-current method.
The entire integrated process (100) is designed to operate using a PLC based
semi-automated control system for ensuring smooth and safe operation. The
control system is categorized into two main parts. One is Supervisory Control and
Data Acquisition (SCADA) system and the second one is supervisory system. The
main function of SCADA is to acquire the data, automate the
machinery/equipments and to control the process. On the other hand,
supervisory system does the higher level process calculations, process variable
optimization and automatic report generation.

In an embodiment the automation system uses Rockwell based hardware and
software control modules are used. Sophisticated instruments like radar level
indicator, temperature and pressure transmitters, magnetic flow meters,
pneumatic controllers etc. can be installed to measure and control the chemical
leaching process parameters. All the important measured variables are
continuously (i.e. for every sampling time) scanned and stored through the
server client based data storage system. All the motors in the plant are provided
with Intelligent Motor Control Center (IMCC) which provides all the valuable
information about motor load, energy consumption, ON/OFF status and trip
status to PLC during the operation.
In an embodiment a multi stage process flow scheme can also be developed to
achieve maximum reduction of ash content from coal having poor liberation
characteristics and high ash content. The schematic of multistage process flow
diagram is shown in FIG. 7.
The chemical leaching study at pilot plant has been carried out for different coals
namely flotation tailings and middlings. As a result, the optimum operating
conditions of the process are identified. The optimum operating parameters
identified for demineralisation of tailings/middlings coal are tabulated in Table. 1.
Table 1. Optimum process parameters for demineralisation of tailings/middlings
coal


The layout of chemical leaching pilot plant for coal benefication process is shown in FIG.
8.
It is to be noted that the integrated process (100) is designed for batch process.
The integrated process (100) is designed In a closed circuit where the reagents
can be regenerated and then recycled back to the process.
A utility section for supply of steam, process water, cooling water, compressed
air and electricity is required. The process flow sheet is designed to beneficiate
the run of mines (ROM), middling, tailings and clean coal from captive washeries.


Advantages
Using the process as mentioned above, one can beneficiate the coal with poor
liberation characteristics as well as high ash content. Moreover low ash coal can
be produced without deteriorating the yield.

WE CLAIM:
1. An integrated process for beneficiating a coal, the process comprising
steps of:
preparing an aqueous alkali solution, the aqueous alkali solution having
concentration of 15-25% (w/w);
preparing a coal slurry, the coal slurry being prepared by mixing the coal
with the aqueous alkali solution in such a way to maintain the ratio of coal
to aqueous alkali solution as 1:5 on weight basis;
transferring the coal slurry to atleast one alkali reactor, each of the alkali
reactor having a heating/cooling jacket;
pressurizing the alkali reactor by means of compressed air at 5-12 bar
pressure for suppressing vaporization tendency of the aqueous alkal.
solution at high temperature condition;
heating the coal slurry by injecting steam of 170° to 200° C temperature
at 10-15 bar pressure into the heating/cooling jacket;
maintaining the coal slurry temperature at 100° to 180° C for 40 to 90
min reaction time;
cooling the coal slurry to temperature of 70-90 deg. C by circulating
cooling water through the heating/cooling jacket of the alkali reactor;
separating the alkali leached coal from the aqueous alkali solution using a
first agitated nutsche filter, the first agitated nutsche filter being operated
at 2-5 bar pressure;
regenerating the aqueous alkali solution by purging CO2, followed by
causticization using slaked lime solution;
washing the alkali leached coal by a counter-current method;

acid leaching the alkali leached coal in atleast one acid reactor, wherein
the acid reactor is added in an aqueous acidic solution having
concentration of 5 to 10 % (w/w), the coal to acid solution ratio being
maintained as 1:5 on weight basis,
the alkali leaching being done for 20-60 min at room temperature and at
atmospheric pressure;
separating the acid leached coal from the aqueous acidic solution using a
second agitated nutsche filter, the second agitated nutsche filter being
operated at 2-5 bar pressure;
regenerating the aqueous acidic solution by adding concentrated sulfur.
acid having 98% concentration w/w in such a manner that the aqueous
acidic solution to sulfuric acid ratio being 1:0.05 (by weight); and
washing the acid leached coal by counter-current method.
2 The process as claimed in claim 1, wherein composition of the coal being
mixture of 55-65% organic matter and SiO2 = 18 - 24; MA - 8 -11;
Cr2O3 = 0.02 - 0.05; TiO2 = 0.5 -1.0; MnO=0.02 - 0.04; MgO = 1.0-1.5; P
= 0.15 - 0.22; CaO = 2.0 - 2.5; Fe2O3 = 4.0 - 6.0; K2O = 0.4 - 0.6; and
Na2O = 0.2 - 0.5 (all in wt.%).
3. The process as claimed in daim 1, wherein the average size of the coal is 0.5
mm to 1 mm.
4. The process as claimed in daim 1, wherein the alkali is sodium hydroxide
(NaOH).
5. The process as claimed in claim 1, wherein the acid is hydrochloric acid
(HCI).

6. The process as claimed in claim 1, wherein the agitator speed of each of
the alkali reactor is 100-160 rpm.
7. The process as claimed in claim 1, wherein water is used in the counter
current method while washing the alkali leached coal and the acid leached
coal.

8. The process as claimed in claim 1, wherein each of the acid reactor is
provided with a variable frequency drive configured to control and
regulate the speed from 0 to 200 rpm.
9. The process as claimed in claim 1, wherein each of the alkali reactor is
provided with a variable frequency drive configured to control and
regulate the speed from 0 to 200 rpm.