FIELD OF THE INVENTION
The present invention relates to a coal demineralization set up and method for
treating high ash coal materials to substantially reduce the ash bearing inorganic
impurities. More particularly the demineralization is to be performed through high
pressure reactor connected with a high temperature-pressure filtration system.
BACKGROUND OF THE INVENTION
Coals, in general are inferior in quality and difficult to clean. The quality of coal
in terms of ash content and its composition plays a vital role in controlling both the
production rate and the quality of hot metal in blast furnace. Since the mineral matter
is finely disseminated inside the coal matrix, it is difficult to reduce the ash content
below a certain level by conventional physical beneficiation techniques. In contrast,
chemical leaching process involves addition of chemicals which react with the
mineral matter which are locked inside the coal matrix and allows it to be easily
removed.
The chemical leaching process involves dosing of chemicals which react with
the mineral matters that are locked inside the coal matrix and remove them without
any loss of carbonaceous matter. Chemical leaching experiment for coal mainly
involves alkali (NaOH) leaching in the first step and subsequently acid (HCl) leaching
in the second stage. The schematic diagram of the experimental procedure followed
for chemical treatment of coal is illustrated in FIG. 1.
During the alkali leaching stage, the coal is mixed with sodium hydroxide
solution and the resultant coal slurry is taken to the high pressure reactor for high
temperature demineralization. The treated coal slurry is taken to the filter in order to
separate the filtrate (spent reagent) and coal cake. The coal cake is then washed to
recover as much residual alkali as possible that is retained in the wet coal cake. The
spent alkali contaminated with inorganic impurities is then sent to the alkali
regeneration system and the resultant regenerated alkali solution is recycled back to
the process. Subsequently during the acid leaching stage, the coal cake
(intermediate product) obtained from the alkali leaching stage, is mixed with the
required amount of hydrochloric acid to makeup the desired concentration and for
maintaining coal-liquid ratio. The acid leaching reaction is carried out at atmospheric

pressure and room temperature. After the final treatment with acid, coal slurry is
filtered and the coal cake retained is washed with water to produce low ash product
clean coal.
The alkali leaching step is crucial because it determines the economic
feasibility of the overall chemical leaching process. Sodium hydroxide is an
expensive base and every unit of sodium hydroxide lost during the process
contributes heavily to the total process cost. Thus sodium hydroxide recovery
becomes an important step. During alkali leaching stage, the ionic species such as
Na+ in aqueous alkali solution are in equilibrium with the solid phase of sodium
aluminium silicate (sodalite), which is crystalline in nature. Some proportion of
sodium is present in the form of sodalities Na8 (Al6 Si6 O24)(OH)2.2H2O also known
as caged sodalite. The sodium present in the form of silicate and aluminate in liquid
solution can be recovered through regeneration process as illustrated in FIG.1.
However, it is difficult to regenerate caustic from soadlite and hence it is necessary
to either prevent or minimize its formation.
In the prior art, the alkalized coal slurry is cooled down to a low temperature
before separation of coal and spent alkali solution. It is evident from
PCT/AU1987/000080 (U.S. Patent No. US 4936045) that coal slurry leached at high
temperature around 170°C to 230°C under autogenous hydrothermal pressure is
rapidly cooled down to the temperature below 100°C. Subsequently, the low
temperature coal slurry separated into alkalized coal cake and spent alkali solution.
According to PCT/IN2009/000328 (U.S. Patent No. 8647400), an improved
process flow sheet at pilot plant scale was 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 invention, filtration was
carried out at 70°C-80°C even though the alkali leaching was carried out at 160°C-
180°C.
The above mentioned prior arts suffers from the problems such as additional
step of energy requirement for cooling the coal slurry that leads to more energy loss
and also high viscous coal slurry at low temperature slows down the filtration rate.

OBJECTS OF THE INVENTION
In view of the foregoing limitations inherent in the prior-art, it is an object of
the invention to develop a system and a process for coal demineralization in which
less amount of sodalite is produced while alkali leaching and thereby larger extent of
spent alkali can be recovered.
SUMMARY OF THE INVENTION
In one aspect, the invention provides an improved process for coal
demineralization comprising steps of preparing a coal slurry, feeding the alkalized
coal slurry into a high pressure vessel, heating the coal slurry to temperature range
of 120- 200°C, pressurizing the high pressure vessel using inert gas at 5-10 bar,
maintaining the coal slurry temperature at 120- 200° C for 15- 90 min reaction time
under pressure of 8-10 bar, transferring the hot coal slurry at 120-200°C directly into
a mesh arrangement of a high temperature pressure filter, separating the alkali
leached coal cake from spent alkali solution, washing the alkali leached coal cake by
hot water at 60°C-80oC, acid leaching the alkali leached coal cake by adding aqueous
hydrochloric acid solution with 5 - 10% concentration on weight basis, separating the
acid leached coal cake from spent acid solution and washing the acid leached coal
cake by water.
In another aspect, the invention provides an autoclave system for coal
demineralization comprising a high pressure reactor for alkali leaching of a coal
slurry, and a high temperature pressure filter for separating alkali leached coal cake
from spent alkali solution, the high pressure reactor comprising a high pressure
vessel housed within, a coal slurry inlet via a ball valve configured at the high
pressure vessel for feeding the coal slurry, an agitator configured at the high
pressure vessel to stir the coal slurry, a ceramic heater configured at the high
pressure vessel to heat the coal slurry, an inert gas inlet via a needle valve
configured at the high pressure vessel to inject inert gas to maintain pressure inside,
the high pressure reactor coupled to a filter vessel of the high pressure filter via a
slurry transfer pipe, the filter vessel being housed within the high temperature
pressure filter, a slurry inlet via a ball valve configured at the filter vessel through
which the alkali leached coal slurry is received at a mesh arrangement, the mesh
arrangement configured at inside of the filter vessel to separate the spent alkali

solution and get collected at the filter vessel, and a wash water inlet via a ball valve
and a compressed air inlet via a ball valve configured at the filter vessel to wash and
separate spent alkali from the alkali leached coal cake.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates a schematic of chemical leaching process for coal.
FIG. 2 illustrates an autoclave system for coal demineralization in accordance with
an embodiment of the invention.
FIG. 3a shows disassembled view of a mesh arrangement of the autoclave system
for separation of alkali leached coal cake from spent alkali in accordance with an
embodiment of the invention.
FIGS. 3b and 3c shows top view and bottom view of the mesh arrangement of the
autoclave system in accordance with an embodiment of the invention.
FIG. 4 illustrates various steps of a process for coal demineralization in accordance
with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide an improved process for coal
demineralization, the process comprising steps of preparing a coal slurry by mixing a
coal with aqueous alkali solution having NaOH content of 15-25% by weight in such
a way to maintain the ratio of coal to alkali solution as 1:5 on weight basis, feeding
the alkalized coal slurry into a high pressure vessel, heating the coal slurry to
temperature range of 120- 200°C, pressurizing the high pressure vessel using inert
gas at 5-10 bar in order to suppress the vaporization tendency of aqueous alkali
solution at high temperature condition, maintaining the coal slurry temperature at
120- 200° C for 15- 90 min reaction time under pressure of 8-10 bar created by the
inert gas, transferring the hot coal slurry at 120-200°C directly into a mesh
arrangement of a high temperature pressure filter due to the pressure difference
between the high pressure reactor and the high temperature pressure filter,
separating the alkali leached coal cake from spent alkali solution, washing the alkali
leached coal cake by hot water at 60°C-80oC, acid leaching the alkali leached coal
cake by adding aqueous hydrochloric acid solution with 5 - 10% concentration on

weight basis, separating the acid leached coal cake from spent acid solution, and
washing the acid leached coal cake by water.
Another embodiment of the invention provides an autoclave system (100) for
coal demineralization, the autoclave system (100) comprising: a high pressure
reactor (104) for alkali leaching of a coal slurry, and a high temperature pressure
filter (108) for separating alkali leached coal cake from spent alkali solution, the high
pressure reactor (104) comprising a high pressure vessel (112) housed within, a coal
slurry inlet via a ball valve (BV1) configured at the high pressure vessel (112) for
feeding the coal slurry, an agitator (120) configured at the high pressure vessel (112)
to stir the coal slurry, a ceramic heater (116) configured at the high pressure vessel
(112) to heat the coal slurry, an inert gas inlet via a needle valve (NV1) configured at
the high pressure vessel (112) to inject inert gas to maintain pressure inside, the high
pressure reactor (104) coupled to a filter vessel (136) of the high pressure filter (108)
via a slurry transfer pipe (132), the filter vessel (136) being housed within the high
temperature pressure filter (108), a slurry inlet via a ball valve (BV3) configured at
the filter vessel (136) through which the alkali leached coal slurry is received at a
mesh arrangement (140), the mesh arrangement (140) configured at inside of the
filter vessel (136) to separate the spent alkali solution and get collected at the filter
vessel (136), and a wash water inlet via a ball valve (BV4) and a compressed air
inlet via a ball valve (BV5) configured at the filter vessel (136) to wash and separate
spent alkali from the alkali leached coal cake.
Shown in FIG. 2 is an autoclave system (100), newly designed, for coal
demineralization. The autoclave system (100) comprises a high pressure reactor
(104) and a high temperature pressure filter (108). The reactor (104) is configured for
alkali leaching of a coal slurry and the high temperature pressure filter (108) is
configured to separate spent alkali solution from the alkali leached coal cake using
mesh arrangement.
The high pressure reactor (104) comprises a high pressure vessel (112)
housed within. The high pressure vessel (112) is configured with a coal slurry inlet
through a ball valve (BV1) by which a coal slurry is fed in the high pressure vessel
(112). A ceramic heater (116) is configured to the high pressure vessel (112)
circumferentially, to heat and maintain the coal slurry temperature.

The high pressure vessel (112) is configured with a needle valve (NV1)
through which inert gas is injected to maintain pressure inside the vessel. The vessel
(112) is also provided with an agitator (120) configured to it, to stir the coal slurry.
The agitator (120) is magnetically coupled with a motor and continuous cooling water
supply (not shown). The continuous water supply through magnetic drive jacket (not
shown) is configured to cool the temperature of the agitator (120).
The pressure vessel (112) is also provided with a pressure gauge (PG1) to
measure the pressure inside the vessel (112).
Safety head can be provided to prevent damages to the body and parts of the
autoclave by safe release of pressure in case of accidental or unusual pressure
build-up.
Flush bottom valve (FB1) is connected to bottom of the vessel (112). Its main
function is complete discharge of liquid from the vessel (112) from bottom.
The pressure vessel (112) is further provided with a helical coil (124) which is
connected to a cooling water tank (128) to cool down the temperature of the
pressure vessel (112). Brine can also be used in place of water.
Temperature controller (PID) (not shown) is provided to monitor and maintain
the desired operation temperature inside the reactor. Pressure display is provided
along with option for alarm setup.
The pressure vessel is further provided with a vent valve (NV2) to vent the
inside air out while feeding the coal slurry.
The pressure vessel may be provided with sampling valve.
Before starting on the autoclave system (100), the reactor (104) is made sure
that all the valves are closed and nuts and bolts are tightened. The agitator (120) is
switched on at a speed of 200-500 rpm as desired.
The high pressure reactor (104) is further coupled to a filter vessel (136) of
the high temperature pressure filter (108) by means of slurry transfer pipe (132)
made up of stainless steel. The filter vessel (136) is housed within the high
temperature pressure filter (108). The pipe (132) is connected to a slurry inlet at the
vessel (136) via a ball valve (BV3), through which the hot coal slurry is transferred in

the vessel (136). The transfer is due to the pressure difference created between the
high pressure reactor (104) and the high temperature pressure filter (108).
The high temperature pressure filter (108) comprises a filter vessel (136)
housed within the reactor (104) and a mesh arrangement (140) which is affixed at
the inner side of the filter vessel (136). The mesh arrangement (140) is configured to
receive the alkalized coal slurry through the ball valve (BV3) from the high pressure
reactor (104) to separate the spent alkali solution and get collected at the filter vessel
(136).
Shown in FIG. 3a is the mesh arrangement (140) with its component in
disassembled manner. The mesh arrangement (140) comprises a bottom support
(144), a gasket (148), a perforated plate (152), a mesh (156) and a top clamp (160).
The bottom support (144) is fixed inside of the filter vessel (136) over which the
gasket (148) is fixed. The perforated plate (152) is placed over the gasket (148). The
mesh (156) is placed over the perforated plate (152). The entire arrangement of the
bottom support (144), the gasket (148), the perforated plate (152) and the mesh
(156) is clamped via the top clamp (160), fixed above the mesh.
FIGS. 3b and 3c shows the top and bottom view of the mesh arrangement (140)
with its component in assembled manner.
The bottom support (144), the perforated plate (152), the mesh (156) and the top
clamp (160) are all made up of stainless steel SS316. The size of the opening of the
mesh (156) is 20 micron. Since the perforated plate (152) is fixed inside the filter
vessel (136) and as it has to bear the maximum pressure from the alkali leached
coal slurry, the thickness of the perforated plate (152) is kept more.
The alkali leached coal cake is washed by hot water at 60°C- 80oC. A wash water
inlet through ball valve (BV4) is configured at the filter vessel (136) to supply hot
water. The hot water can also be supplied via the slurry transfer pipe (132) from the
pressure vessel (112). The filter vessel (136) is also provided with a pressure gauge
(PG2) to measure the pressure within.
A compressed air inlet through a ball valve (BV5) is configured at the top of the
filter vessel (136) to deliver pressurized air flow to assist in the separation of the
alkali leached coal cake from spent alkali using hot water.
The filter vessel (136) is further provided with vent valve (BV6).

The filter vessel (136) may be provided with pressure relief valve.
The filter vessel (136) is further provided with a cooling jacket having a cooling
water inlet (164) and a cooling water outlet (168) to cool down the temperature of the
filtrate remaining in the filter vessel (136).
Shown in FIG. 4 is a flow diagram depicting series of steps of a process (400)
for coal demineralization in the autoclave system (100) (shown in FIG. 1), in
accordance with an embodiment of the invention.
At step (404), a coal slurry is prepared by mixing a coal with aqueous alkali
solution of NaOH. The content of the alkali solution is 15-25% by weight in such a
way to maintain the ratio of coal to alkali solution as 1:5 on weight basis.
The average size of the coal is 0.5 -1.00 mm.
The composition of the coal is shown in Table 1


At step (408), the alkalized coal slurry is fed in the high pressure vessel (112) of
the high pressure reactor (104). Before feeding the coal slurry, the agitator (120) is
switched on in order to avoid the settling of the coal particles.
At step (412), the coal slurry is heated to maintain the coal slurry temperature in
the range of 120 - 200°C. This heating is achieved by the ceramic heater (116) fixed
to the high pressure vessel (112).
At step (416), the high pressure vessel (112) is pressurized using inert gas at 5-
10 bar in order to suppress the vaporization tendency of aqueous alkali solution at
high temperature condition. In an embodiment the inert gas can be nitrogen gas and
other inert gases. The inert gas is injected through the needle valve (NV1).
At step (420) the coal slurry temperature is maintained at 120 - 200° C for 15 to
90 min reaction time under pressure of 8-10 bar created by an inert gas.
At step (424) the hot coal slurry is transferred at 120-200°C directly to the mesh
(156) of the mesh arrangement (140) of the high temperature pressure filter (108)
due to the pressure difference created between the high pressure reactor (104) and
the high temperature pressure filter (108).
It should be understood that during alkali leaching stage, the ionic species such
as Na+, Al(OHX,SiO-,OH-,CO-in aqueous alkali solution are in equilibrium with
the solid phase of sodium aluminium silicate (sodalite), which is crystalline in nature.
Some proportion of sodium is present in the form of sodalite
Na3(Al6Si6O,+)(0H): .2H2O also known as caged sodalite. The sodium present in the
form of silicate and aluminate in liquid solution can be recovered through
regeneration process. However, it is difficult to regenerate caustic from soadlite and
hence it is necessary to either prevent or control its formation.
The thermodynamic equilibrium relation reveals that increase in solution
temperature depresses the sodalite formation and enhances more Na in the solution
form as sodium silicate and sodium aluminate. It supports the idea that filtration at
high temperature minimizes the sodalite formation by reducing the equilibrium
constant when increasing the temperature.
This transfer of the hot coal slurry directly without cooling, to the mesh
arrangement helps in lowering the viscosity of the hot coal slurry compared to that of
cooled conventionally. Therefore it becomes easy in separation of the spent alkali

from the coal cake. The transfer of the hot coal slurry directly without cooling also
lowers the formation of the sodalite hence more volume of spent alkali can be
recovered.
After the slurry is transferred, FBV1 is closed and the agitator (120) and ceramic
heater (116) are switched off.
As soon as the coal slurry is transferred to the filter vessel, filtration starts
automatically due to the inbuilt pressure above the mesh arrangement (140).
Simultaneously the tap water is continuously circulated through the cooling jacket of
the filter to cool down the filtrate in the filter vessel (136). The filter vessel (136)
comprises the cooling water inlet (164) and the cooling water outlet (168) for
circulation of the water.
At step (428) the alkali leached coal cake is separated from spent alkali solution
using the mesh arrangement (140).
At step (432) the alkali leached coal cake is washed by hot water at 60°C-80oC.
This hot water is supplied from the water wash inlet by valve (BV4). The compressed
air is provided by the valve (BV5) assisting in the separation of the spent alkali from
the coal cake and also in washing of the coal cake.
After filtration, the system is depressurised by opening the vent valve (BV6). A
flush bottom valve (FBV2) is opened to collect the filtrate solution. After collecting the
filtrate solution, the cake has to be washed in order to remove the residual alkali from
the cake.
At step (436) alkali leached coal cake is acid leached by adding aqueous
hydrochloric acid solution with 5 - 10% concentration on weight basis.
At step (440) the acid leached coal cake is separated from spent acid solution.
Finally, at step (444) the acid leached coal cake is washed by water.
The coal is demineralized from 42 % to 8-14% and the product coal yield is 60-
70% on dry basis.
Advantages
The developed system and process lowers the formation of sodalite and
thereby more amount of spent alkali can be recovered.

WE CLAIM
1. An improved process for coal demineralization, the process comprising
steps of:
preparing a coal slurry by mixing a coal with aqueous alkali solution having
NaOH content of 15-25% by weight in such a way to maintain the ratio of
coal to alkali solution as 1:5 on weight basis;
feeding the alkalized coal slurry into a high pressure vessel;
heating the coal slurry to temperature range of 120- 200°C;
pressurizing the high pressure vessel using inert gas at 5-10 bar in order to
suppress the vaporization tendency of aqueous alkali solution at high
temperature condition;
maintaining the coal slurry temperature at 120- 200° C for 15- 90 min
reaction time under pressure of 8-10 bar created by the inert gas;
transferring the hot coal slurry at 120-200°C directly into a mesh
arrangement of a high temperature pressure filter due to the pressure
difference between the high pressure reactor and the high temperature
pressure filter;
separating the alkali leached coal cake from spent alkali solution;
washing the alkali leached coal cake by hot water at 60°C-80oC;
acid leaching the alkali leached coal cake by adding aqueous hydrochloric
acid solution with 5 - 10% concentration on weight basis;
separating the acid leached coal cake from spent acid solution; and
washing the acid leached coal cake by water.
2. The process as claimed in claim 1, wherein the composition of coal is
Inorganics (all in wt. %)
SiO2: 16.00-24.50, Al2O3: 7.00-11.00, Cr2O3 0.02-0.05, TiO2: 0.50 - 0.10,
MnO: 0.02 -0.04, MgO: 1.00-1.50, P: 0.15-0.22, CaO: 2.00-2.50, Fe2O3:
3.00-6.00, K2O: 0.40-0.60, Na2O: 0.20-0.50, Total 35.00-45.00,
Organics, %( all in wt. %): 55.00-65.00.

3. The process as claimed in claim 1, wherein the average size of the coal is
0.5 -1.00 mm.
4. A process as claimed in claim 1, wherein the coal is demineralized from 42
% to 8-14%.
5. A process as claimed in claim 1, wherein the product coal yield is 60-70%
on dry basis.
6. A process as claimed in claim 1, wherein the inert gas is nitrogen gas.
7. An autoclave system (100) for coal demineralization, the autoclave system
(100) comprising:
a high pressure reactor (104) for alkali leaching of a coal slurry, and a high
temperature pressure filter (108) for separating alkali leached coal cake from
spent alkali solution,
the high pressure reactor (104) comprising a high pressure vessel (112)
housed within,
a coal slurry inlet via a ball valve (BV1) configured at the high pressure
vessel (112) for feeding the coal slurry,
an agitator (120) configured at the high pressure vessel (112) to stir the coal
slurry,
a ceramic heater (116) configured at the high pressure vessel (112) to heat
the coal slurry,
an inert gas inlet via a needle valve (NV1) configured at the high pressure
vessel (112) to inject inert gas to maintain pressure inside,
the high pressure reactor (104) coupled to a filter vessel (136) of the high
pressure filter (108) via a slurry transfer pipe (132), the filter vessel (136)
being housed within the high temperature pressure filter (108),
a slurry inlet via a ball valve (BV3) configured at the filter vessel (136)
through which the alkali leached coal slurry is received at a mesh
arrangement (140), the mesh arrangement (140) configured at inside of the
filter vessel (136) to separate the spent alkali solution and get collected at
the filter vessel (136), and

a wash water inlet via a ball valve (BV4) and a compressed air inlet via a
ball valve (BV5) configured at the filter vessel (136) to wash and separate
spent alkali from the alkali leached coal cake.
8. The autoclave system (100) for coal demineralization as claimed in claim 7,
wherein the mesh arrangement (140) comprises a bottom support (144)
fixed inside of the filter vessel (136), a gasket (148) fixed above the bottom
support (144), a perforated plate (152) placed above the gasket (148), a
mesh (156) of 20 micron placed above the perforated plate (152), and a top
clamp (160) configured above the mesh to clamp the bottom support, the
gasket, the perforated plate and the mesh together.
9. The autoclave system (100) for coal demineralization as claimed in claim 7,
wherein the bottom support (144), the perforated plate (152), the mesh (156)
and the top clamp (160) are made up of stainless steel SS316.
10. The autoclave system (100) for coal demineralization as claimed in claim 7,
wherein the average size of the coal is 0.5-1.00 mm.
11. The autoclave system (100) for coal demineralization as claimed in claim 7,
wherein the composition of the coal is Inorganics (all in wt. %) SiO2: 16.00 -
24.50, Al2O3: 7.00 - 11.00, Cr2O3: 0.02 - 0.05, TiO2: 0.50 - 0.10, MnO: 0.02 -
0.04, MgO: 1.00 - 1.50, P: 0.15 - 0.22, CaO: 2.00 - 2.50, Fe2O3: 3.00 - 6.00,
K2O: 0.40 - 0.60, Na2O: 0.20 - 0.50, Total 35.00 - 45.00, Organics, %( all in
wt. %): 55.00 - 65.00.
12. The autoclave system (100) for coal demineralization as claimed in claim 7,
wherein both the high pressure vessel (112) and the filtration vessel (136)
comprises a pressure gauge (PG1 and PG2) to monitor the pressure within.