TITLE
A method of producing carbon foam from high ash coals by converting into swelled coal
using solvent extraction method
FIELD OF THE INVENTION
The present invention relates to coal based carbon foam and more particularly to low ash coal
derived from solvent extraction treatment of high ash coal that provide porous and structurally
strong carbon materials. Products utilizing the coal based porous carbon foam are also
described.
BACKGROUND OF THE INVENTION
Carbon foams are materials of very high carbon content that have appreciable void volume.
Typically, swelling coals are heated at rates such that the coal swells and produces a coke or a
carbon foam. Cokes and carbon foams have a high degree of porosity and are typically of low
density. Carbon foams have potential utility in a variety of applications because of their unique
properties such as temperature resistance, strength, and low density. For example, carbon
foams are typically fire resistant and may exhibit significant strength, even at extreme
temperatures, which makes these materials suitable for use as light weight thermal barriers,
wall panels, and as baffles for high intensity flames. These materials may also function as water
contamination removal.
Carbon foams have been produced from various materials including, but not limited to. pitches,
mesophase carbon materials, foamed synthetic plastics, coals, and hydrogenatcd coal produce.
Coal is a relatively inexpensive and relatively non-toxic starting material to produce carbon
products. Such products have included metallurgical cokes, activated carbons, and carbon
foams. Specifically, swelling bituminous coals are typically used in me production of
metallurgical cokes since these coals typically "swell" upon heating to produce a porous carbon
mass. This porous carbon mass is referred to as a coke. As described above, the void volume
within coke is not arranged in cells having the regular size, shape, distribution, and orientation
of the cells within carbon foam. As a result, coke docs not have the combination of beneficial
properties exhibited by carbon foams. Therefore, coke has limited, if any, utility in the
applications to which carbon foams may be directed (United States Patent 7,767,183 B2).

The number of processes have been used to control the swelling behaviour of the coal. For
example, carbon foams have been produced from swelling bituminous coals using processes
where the coal is foamed under high process atmospheric pressures. In other processes, the coal
is oxidized or devolatized prior to foaming. In still other process, the coal is foamed in a high-
strength mold which, apparently, mechanically limits the maximum carbon foam volume.
Many processes have been developed to produce carbon foam. Such processes often involve a
template foaming, use of blowing agents and high-pressure conditions to produce carbon foam
(for example US patent no. 7,544,222 B2). Those processes are complicated and involve high
pressure vessel. They have used costly and sophisticated feed materials. Hence, the cost of
production goes up significantly.
United Slates Patent 7,767,183 B2 provides a method to produce carbon foam from swelling
coals that do not require the use of high process pressures, oxidized coal, devolatized coal, or
high-strength, foam expansion confining molds. The three steps heating of swelling coals is
done to produce the graphitized carbon foam.
United States Patent 8,048,528 B2 provides a method in which carbon foam is produced having
a density between 0.3 and 0.4 g/cc by the controlled heating of coal particulate preferably up
to ¼ inch in diameter in a "mold" and under a non-oxidizing atmosphere. These coal-based
cellular or porous products have ash contents greater than about 3%, with ash contents in the
range about 7 % to 15 % being most typical.
Carbon foam is made in two steps. First step is the foaming process and second step is
carbonization. There have been many examples in which they have applied high pressure for
foaming and slow heating rate (2 to 3 °C/min). This requires high pressure vessel and increased
process time to get the desired product, (for example, US patent no. 7,824,645 B2). I o improve
these deficiencies, the high swelling coal is treated at high heating rate between 10 to 20 °C/min
to increase the porosity and to reduce the treatment time.
Though there are different approaches taken to produce carbon foams however, there is need
where coals with high ash content can be used to produce carbon foams.

OBJECTS OF THE INVENTION
The object of the invention is to increase the swelling of high ash coal by reducing the ash
content of the coal through solvent extraction method.
Still another object of the invention is to use high swelling coal produced by solvent extraction
method in carbon foam production.
Another object of the invention is to heat the coal at faster rate during the self-foaming of the
coal.
Still another object of the invention is to develop new uses of high ash coals.
SUMMARY OF THE INVENTION
The present invention relates to a process in which high ash coal (with an ash content of 17
weight% or more) is treated using solvent extraction method to increase the swelling property
of the coal and to decrease the ash content of the coal. The process includes mixing the coal
and mixed solvent (NMP + EDA) in desired ratio; heating the mixture to 200 C; separating the
low ash and high swelling coal; heating the high swelling coal in a boat in inert atmosphere;
and carbonizing the foam to produce carbon foam. The developed carbon foam can be used for
water purification including reducing TDS and TSS.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 Process flow diagram for production of medium and low ash swelled coal
Figure 2: Process flow sheet of converting coal into foam
Figure 3: Foam produced from 5% ash coal
Figure 4: Foam produced from <1% ash coal
Figure 5: Foam produced from nitric acid treated coal tar pitch

DETAILED DESCRIPTION
The process of the present invention provides for a direct route to produce self-supporting,
three-dimensional carbon materials, from swelling bituminous coals. In the present invention.
coal of less than or equal to 250-micron particle size (-60 mesh) is taken in a round bottom
flask. N-methyl pyrrolidone (NMP) is taken as solvent and ethylenediamine (EDA) is taken as
co solvent for the extraction of clean coal. Solvent mixture is taken as ten times of the coal.
Solvent to co solvent ratio is maintained at 17:1. The coal and solvent mixture is heated in a
rotary vacuum evaporator under normal atmospheric pressure, at 170-180 °C for 1 hour. After
the extraction is complete, solution is filtered in 500 SS wire mesh. The filtrate and residue is
obtained. The residue part that is obtained as the cake from the filter mesh is called residue
coal. This filtrate is filtered for the second time to further reduce the ash percentage of the coal
The process is explained in figure 1. According to figure 1, coal and solvents are mixed in step
I and extraction is carried out. The solvents dissolve the hydrocarbon of the coal and leave out
the mineral matter (ash particles). The slurry containing dissolved coal and suspended mineral
matter passes through two stage filtrations to separate out the medium ash swelled coal and
low ash swelled coal. The first filtration (2) is carried out with 500 SS wire mesh (25 micron),
which separates residue and dissolved coal with solvent. The dissolved coal and solvent passes
through another filter (3) of approximately 5-micron size, this separates the coarse dissolved
coal (called medium ash swelled coal - MASC) and fine dissolved coal (low ash swelled coal
- LASC). The medium ash swelled coal (MASC) remains on the top of the filter paper and low
ash swelled coal (LASC) passes through filler along with solvent. This LASC and solvent goes
to solvent separation system (4), which separates the solvent and LASC.
Two different grades of swelled coal arc obtained from the two-step filtration. The medium ash
swelled coal (MASC) is obtained on the top of the second filtration system The solvent is
recovered from the second stage filtrate in vacuum evaporator. This is done under vacuum.
Pressure inside the system is maintained at 700 mm Hg by a vacuum pump. Temperature is
kept at 120-140 °C. This process is carried out until a dry cake of clean coal is obtained in the
flask. Separated solvent is collected in a receiver flask, placed after the condenser. The clean
coal is then removed from the flask and washed with distilled water to remove the residual
solvent. Then it is dried at 110 °C for 3 hours to get the dry clean coal of very low ash content
(called low ash swelled coal - LASC),

Coal swelling is measured by free swelling index (FSI). The test is done according to AS ASTM
D720 standard. The feed coal is having typical free swelling index around 5. The medium ash
swelled coal (termed "MASC") and low ash swelled coal (termed LASC") obtained after
solvent extraction process is having FSI 6.5 and 8 respectively. The feed coal is having the ash
content about 17%. The medium ash swelled coat has around 5% ash content while the low ash
swelled coal has less than 1 % ash content.
The swelled coal (low ash or medium coal) is placed in a container or boat and heated to a first
temperature in a tube furnace. This heating is performed in a non-oxidizing atmosphere, such
as nitrogen. The first temperature is 50-100 "C more than the initial plastic temperature of the
coal. The initial plastic temperature of the coal is that temperature at which the coal particles
begins to soften and becomes sufficiently plastic to adhere to each other. The initial plastic
temperature of the coal may vary depending on the coal and process conditions. The typical
plastic temperature range of coal is between 350-550 "C. First temperature may vary for coal
to coal. The first temperature is taken between 500 -650 OC, typically between 575 to 635 °C
The carbon structure also depends on heating rale. The heating rate play an important role in
deciding the pore size, shape and interconnectivity of carbon foam. The heating is takes place
between 5-20 °C/min, more specifically between 10-15 °C/min. The high heating rate gives
uniform pore diameter as volatile matter escape very fast from the coal sample. The heating
time also decreases because of high heating rate, leading to energy saving. The swelled coal is
maintained at the first temperature for about 1 hour. The product is called green foam. This
green foam is heated to second temperature between 900 - 1200 °C, more specifically about
1000 °C and maintained for about 2 hours at that temperature. The heating is done at slow rate
about 3 °C /min. The foam is cooled to room temperature. The obtained foam can be cut in
desired shape and size. The foam has low density.
According to figure 2, the high ash coal is treated using solvent extraction method [101] to
reduce the ash content and to increase the swelling of the coal, the solvent extraction step
[101] contains mixing of coal and solvents to form the slurry, heating of the slurry, filtering of
the slurry in two steps and recovery of the solvent. The two-stage filtration system in solvent
extraction process gives two products namely low and medium ash swelled coal. The swelled
coal obtained in previous step [101] is taken to the self-foaming [102] step. The coal is placed
in boat or contained in the tube furnace and heated at the rate of about 15 oC min 10 600 °C
temperature and kept for 1 hour at that temperature to remove most of the volatile matter. The
product obtained in self-foaming step is called green foam. This green foam is heated slowly

to 1000 oC and carbonized [103] at that temperature for 2 hours. The product obtained is called
carbonized foam, which can be cut into different size as per requirement.
Example 1
Carbon foam was produced in following manner, A 100-g sample of comminuted (less than
250-micron size) sub-bituminous coal was mixed in NMP and EDA (NMP: EDA was 17:1) in
1:10 ratio (coal to solvent). As per 1:10 coal to solvent ratio, solvent is taken as 1000 mL. Since
NMP and EDA is in 17:1 ratio, NMP is taken as 945 mL and EDA is taken as 55 mL, The sub-
bituminous coal (called feed coal) has 17% ash and 56.7% fixed carbon. The coal and solvent
slurry was heated to 180 °C in round bottom flask for 1 hour. After the completion of extraction.
the slurry is filtered out using 500 BSS wire mesh. The filtration separates filtrate and residue.
The residue is washed with water, dried and weighed, The filtrate is passes through 5-micron
filter paper. This filtration separates medium ash coal as top product and low ash coal with
solvent as filtrate respectively. The top product (called medium ash swelled coal - MASC) is
washed with distilled water, dried, weighed and sent for analysis. The MASC has 5% ash. The
yield of MASC is between 35-40% The filtrate was further subjected to rotary vacuum
evaporator at 120-140 °C under 700 mm Hg vacuum to recover the remaining solvent and low
ash swelled coal (LASC), The yield of LASC is between 10-15% and its ash content is less
than 1%. The two-stage filtration gave two products having different swelling property, The
first product MASC and second product LASC was having 6.5 and 8 FSl respectively. The
proximate analysis of these coals is presented in table 1.
Table 1: Proximate analysis of coals

Both the products showed good swelling index but their ash content was different The ash
content plays an important role in increasing and decreasing the density of the foam, pore size
and its structure. The two coals (MASC and LASC) were taken separately in the boat and

placed in tube furnace and heated from room temperature to 600 UC at a rate of 15 °C/min in
inert (N2 gas) environment al atmospheric pressure. The heating rate was kept high so that most
of the volatile matter escape in short time. The coal sample was held at 600 °C for 1 hour after
which the sample was cooled to room temperature. The sample was converted to green foam.
The green foam is carbonized in the next step by heating to 1000 °C This green foam is again
heated from room temperature to 1000 °C at 3 °C/min in inert (N2 gas) environment at
atmospheric pressure. The green foam is maintained at 1000 °C for 2 hours. The inert gas flow
was maintained at 2 liter per minute (1pm), The product is called carbonized foam. The
carbonized foam produced from MASC and LASC were subjected to subsequent analysis. The
SEM image of carbon foam produced from MASC and LASC is presented in Fig 3 and 4*
Different properties are shown in table 2.
Table 2: Different properties of carbon foam produced from swelled coals

Figure 3 shows that foam produced from MASC exhibits pores of 350-500 micron diameters
(i.e. macrpores) with windows of 15-25-micron diameter. Pore distribution is uniform and
pores are not circular. Structure is interconnected throughout with carbon walls of 40-45-
micron thickness. Figure 4 shows that foam produced from LASC exhibits well inier connected
structure. Pore distribution is uniform but there is wide range of pore sizes present in the foam,
Pores of 215-micron and 50-micron diameter are observed. Cell walls are not smooth and
cracks are present. Ash particles are visible on the surface of the walls. Wall thickness is
varying from 10-16 micron. 1 'he MASC had good compressive strength, i.e. 8.3 MPa, The bulk
density was 0.21 and 0.22 g/cc for MASC and LASC respectively.
The carbonized foam of MASC was used for contamination (TDS and TSS) removal from the
waste water. The waste water was taken in the beaker and the sample of carbonized MASC
was put into the water and stirred for 5 hours. The initial and final weight of the sample was

noted down along the TDS and TSS concentration of water before and after treatment. Initially.
the waste water had TDS 4.31 (g/L) and TSS11.1 (ppm) concentration. After the treatment with
carbon foam the concentration reduced to TDS 3.85 (g/L) and TSS 6.4 (ppm). The results are
presented in table 3.
Table 3: TDS and TSS removal results by medium ash swelled coal

The analysis shows that carbon foam produced from swelled coals are of superior properties.
Therefore, this process may provide a novel and inexpensive route through which high ash coal
can be converted to a good quality carbon foam.
p]xample 2
In another example, nitric acid treated coal tar pitch (NATCTP) is selected for producing
carbon foam. The NATCTP was taken in the boat and placed in tube furnace and heated from
room temperature lo 600 °C at a rate of 15 °C/min in inert (N2 gas) environment at atmospheric
pressure. The heating rate was kept high so that most of the volatile matter escape in short time.
The NATCTP sample was held at 600 °C for 1 hour after which the sample was cooled to room
temperature. The sample was converted to green foam. The green foam was carbonized in the
next step by heating to 1000 °C. This green foam is again heated from room temperature to
1000 °C at 3°C/min in inert (N2 gas) environment at atmospheric pressure. The green foam is
maintained at 1000 °C for 2 hours. The inert gas flow was maintained at 2 liter per minute
(lpm). The product is called carbonized foam. The carbonized foam produced from NATCTP
was subjected to subsequent analysis. The SEM image of carbon foam produced from
NATCTP is presented in Fig 5. Different properties are shown in table 4.

Table 4: Properties of carbon foam produced from nitric acid treated coal tar pitch

Figure 5 describes the self-foam prepared from NATCTPA. Pores are inter-connected with
non-uniform pore size. Cell walls are not smooth but cracks are not observed. Pore diameters
are in the range of 50-236 microns. A special type of pore geometry is observed for this foam.
A big pore is formed on the surface & then small pores are formed on the walls of that big pore.
Ligaments between two consecutive pores are 25-30 microns thick. The bulk density of carbon
foam is 0.12 g/cc and compressive strength is 7.1 MPa. The density is lower than the carbon
foam produced from coal samples. The compressive strength is comparable with carbon foam
produced from coal.
In the description section, carbon foam and carbonized foam has been used interchangeably

CLAIMS ^ j^-.
1. A method for producing carbonized foam from coal, the process comprising;
heating a mixture of coal and Solvent;
separating residue and filtrate using a wire mesh at a first filtration step;
subjecting the filtrate to a second filtration step to obtain residue in the form of a
medium ash swelled coal and second stage filtrate;
separating the solvent from second stage filtrate to obtain a low ash swelled coal;
heating the low ash and the medium ash swelled coal separately to a first temperature
of about 500 to 650 °C, preferably from 575 to 635 °C, in an inert environment and
holding the coal at the first temperature for a predetermined time to obtain a green
carbon foam;
heating the green foam to a second temperature of about 950 to 1050 °C in an inert
environment and holding the green foam at the second temperature for a predetermined
time to obtain carbonized foam.
2. The method as claimed in claim 1, wherein the coal is high ash coal with ash content
of 17 weight percent or more,
3. The method as claimed in claim 1, wherein coal is comminuted with particle size of
less than 250 microns.
4. The method as claimed in claim 1, wherein solvent comprises a mixture of N-Methyl-
2-pyrrolidone (NMP) and Ethylenediamine (EDA) in a ratio of 17:1.
5. The method as claimed in claim 1, wherein coal and solvent arc mixed in ratio of 1:10
and heated at a temperature range between 170-180 °C for approximately 1 hour.
6. The method as claimed in claim 3, wherein the coal and solvent mixture is heated at a
rate of 15°C/min,

7. The method as claimed in claim 1, wherein residue and filtrate are separated by using
a 500 BSS wire mesh at first filtration stage.
8. The method as claimed in claim 1, wherein the medium ash swelled coal is separated
from filtrate at the second stage filtration step using a 5-micron filter paper,
9. The method as claimed in claim 1, wherein the solvent is separated from the second
stage filtrate using a vacuum evaporator by maintaining pressure at 700 mm and
keeping the temperature at 120-140 °C.
10. The method as claimed in claim 1, wherein the medium ash swelled coal and low as
swelled coal are held at the first temperature to achieve the uniform temperature
throughout the coal.
11. The method as claimed in claim 1, wherein first temperature is approximately 50 tolOO
°C more than the initial plastic temperature of the coal.

12. The method as claimed in claim 1, wherein the medium ash swelled coal and low as
swelled coat are healed to first temperature range at a rate 5 to 20 °C/min, preferably in
the range oflO to 15 "C7min„
13. The method as claimed in claim 1, wherein the green foam is heated to second
temperature range at a rate of 3 °C/min.
14. The method as claimed in claim 1, wherein the medium ash swelled coal has ash content
of approximately around 5 weight percentage.
15. The method as claimed in claim 1, wherein the low ash swelled coal has ash content of
less than 1 weight %.
16. The method as claimed in claim 1, wherein the medium ash swelled coal and the low
ash swelled coal have free swelling index (FSI) 6.3-6.7 and 7.8-8.2 respectively.
17. The carbonized foam as produced by any of the preceding claims.

18. The carbonized foam as per any of the preceding claims, wherein the carbonized foam
produced from medium ash swelled coal has bulk density of 0,20-0.25 g/cc and
compressive strength of 7.5 - 9.1 MPa.
19. The carbonized foam as per any of the preceding claims, wherein the carbonized foam
produced from low ash swelled coal has bulk density of 0.18-0.24 g/cc and compressive
strength of 6.7-7.4 MPa.