FIELD OF THE INVENTION

The present invention relates to an improved process for preparation of
graphene oxide from natural graphite by a dry process. More particularly, the
present invention relates to an improved process for manufacture of grapheme
oxide from pure natural graphite in a modified dual drive planetary ball mill.

BACKGROUND OF THE INVENTION

Graphene is the youngest member of the nanocarbon family. It is a single layer
of sp2-bonded carbon atoms arranged in a honeycomb shaped hexagonal lattice.
This one atom thick carbon material has outstanding physical, mechanical,
thermal, electrical, optical and chemical properties (Geim A.K. and Novoselov
K.S. Nat Nater 2007;6(3); 183-91). It is the hypothetical infinite aromatic sheet
of sp2-bonded carbon that is the 2-D counterpart of naturally occurring 3-D
graphite ( Debye, P, Scherver P, Physics, Z, 1917; 18, 291-301). It is found in
the 7t-stacked hexagonal structure of graphite with an interlayer spacing of 3.34
A0, which is the van der Waals distance for sp2-banded carbon (S. Niyogi et al J.
Am.Chem Soc. 2006; 128, 7720-7722), Graphene holds enormous potential for
transforming the next generation technologies including computer chips, mobile
phones, electronic gadgets, flexible displays, solar cells, structural composite etc.
Graphite is a layered material and can be considered as a stack of individual
grapheme layers. Graphene is produced by reducing expholiated grapheme oxide
(GO) through chemical process. The use of oxidizing conditions in the exfoliation

of graphite produces grapheme oxide (GO). This GO is subjected to reduction
process for production of reduced grapheme oxide (RGO) or grapheme using
various reductants (Wencheng Du et al, J. Mater.Chem.A,2013; 1,10592).
Graphene is produced by different processes which includes micromechanical
cleavage (K. S. Novoselov, et al., Science, 2004; 306, 666), chemical vapour
deposition (CVD), epitaxial growth on SiC substrates (K.S. Kim et al., Nature,
2009; 457, 706; H.X.Zhangand P.X.Feng, Carbon, 2010; 48,359), chemical
reduction of exfoliated graphite oxide (S.Stankovich et al., Carbon, 2007;
45,1558), liquid phase exfoliation of graphite (Daniele Nuvoli et al, J.
Mater.Chem.,2011; 21,3428) and unzipping of carbon nanotubes. However each
of the above methods has its own advantages as well as limitations depending
on its target application. (Novoselov, Geim A.K., Morozov S.V. Jiang D, Zhang Y,
Dubonos Sv, et al., Science 2004; 306 95296).
Reference is made to Hummers et al., with respect to preparation of graphite
oxide. J.Am.Chem.Soc.80, 1339(1958) where in KNnO4 and NaNO3 in conc.
H2SO4 is used for preparation of graphene oxide (GO).
Reference is also made to an improved Hummers method by D.C. Marcano et al.,
(ACS Nano 2010,4 4806) where in NaNO3 is excluded and amount of KMnO4 is
increased in the process with a 9:1 mixture of H2SO4/H3PO4 to produce greater
amount of GO and this GO is more oxidized than that prepared by Hummer's
method.

Tsuoshi Nakajiona (Carbon 1994; 32, 3 469) has investigated graphite oxide
formation by using modified Staudenmair method and an electrochemical
process. Further, US Patent Number 20060241238A1 mentions the use of
microwave or radio frequency for exfoliation of intercalated graphite for
production of grapheme nano platelets.
PCT application WO20110543058A1 mentions the use of wet ball milling for the
exfoliation of graphite flakes in a suitable organic solvent N, N-
dimethylformamide (DMF). The balls in the ball mill are coated with soft polymer
to reduce the damage to the graphite structure during milling process.
US Patent US 707125881 describes a process in which mechanical attrition is
employed to micro or nano scaled graphite crystallites in polymeric carbon for
producing nano scaled graphene plate metal.
Mechanical peeling technique has been used by W. Zhao et al (J Mat. Chem
2010;20, 5817-5819) wherein wet ball milling was introduced or exfoliation of
graphite. Ball milling was adopted in an attempt to exfoliate thick graphite flakes
mechanically into grapheme in liquid media. Here shear forces are applied by the
ball mill. Similar reference is made to work done by Catharina Knieke et al.
(Carbon, 2010;48,3196-3204) where in a vertical laboratory stirred media is used
to delaminate the graphite particles under surfactant solution consisting of
deionised water and sodium dodecyl sulphate SDS.

Although the first discovery of grapheme was carried out by scotch tapping
peeling, however, the process is not found to be economical for mass
production. Further the production of a few layer graphene by chemical vapor
deposition is although promising, however, the end-product exhibits az low
purity mixture of amorphous carbon. The method also does not address
applications of graphene that require macro-scale deployment and high yield.
According to prior art, the most prominent technique for a scalable production of
few layer graphene is the chemical reduction or thermal treatment of graphene
oxide (GO) following the known Hummer's method. Chemical oxidation of
graphite is an established method using concentrated acids (sulfuric acid, nitric
acid, and phosphoric acid) and highly oxidizing agents (potassium permanganate
and potassium perchlorate). However, the oxidation method usually requires
several steps, temperature controls, tedious and long experimental time for the
preparation of GO.
Further, in the multistep processes, the concentrated acids used in oxidization
including the chemicals needed to reduce GO, interalia enhances the total cost
towards production, safety and environmental measures involved in large scale
production. In order to achieve commercial viability, the synthesis process of GO
needs to be simplistic and cost effective.
Therefore, there is a need of a process for mass production of graphene oxide
GO from high pure natural graphite flake / powder where no acids and chemical
treatment is required for preparation of reduced GO or graphene.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved process for
preparation of graphene oxide from natural graphite in a modified dual drive
planetary ball mill.
Another object of the present invention is to propose an improved process for
preparation of graphene oxide from natural graphite in a modified dual drive
planetary ball mill in which the shearing action of the balls in a dual drive vertical
swing planetary ball mill is made use of.
Still another object of the present invention to propose an improved process for
preparation of graphene oxide from natural graphite in a modified dual drive
planetary ball mill in which chemicals and acids are not used.
Yet another object of the present invention is to propose an improved process
for preparation of graphene oxide from natural graphite in a modified dual drive
planetary ball mill which involves a single step operation.
A further object of the present invention is to propose an improved process for
preparation of graphene oxide from natural graphite in a modified dual drive
planetary ball mill which is environmental friendly process with zero waste.
Accordingly the present invention provides an improved process for preparation
of graphene oxide from natural graphite in a modified dual drive planetary ball
mill which comprises the steps of oxidizing high pure natural graphite

powder/flake in a dual drive planetary ball mill, exfoliation of graphite
powder/flake inside the ball mill jar, peeling of layer for production of graphene
oxide in mass scale without using any acids, chemical additives.
The present invention makes use of the high pure natural graphite powder/flake
obtained from raw graphite ore using the process as disclosed in Indian Patent
197530 (A novel route to produce high purity graphite).
In one embodiment of the invention, purity of graphite (>99%) is obtained
through prior art mineral beneficiation process using high pure natural graphite
powder/flake, which is used as the feedstock for graphene oxide production.
In another embodiment of the invention, a vertical swing dual drive planetary
ball mill designed, fabricated and used for preparation of graphene oxide in
mass scale from high pure natural graphite powder/flake.
The inventive process does not use any acid or chemical inside the ball mill to
produce graphene oxide and is a complete dry process. Further, the process as
per the current invention does not yield any bi-product and the conversion of
graphite to graphene oxide in the range of 18-25%.
The process makes use of high pure graphite powder / flake with particle size
ranging from 600 micron to 5 micron, wherein the graphene oxide obtained is
having particle size ranging from 3 nm to 200 nm. The graphene oxide produced
as per the current invention possesses at least 10 stacking layers. Graphene
oxide obtained is having well defined Raman spectra confirming D peak and
around 1350 cm"1 and G peak at and around 1580 cm"1.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows an isometric view of a vertical swing planetary ball mill according
to the invention.
Figure 2 shows the scanning electron microscope pictures of high pure graphite
feed produced by prior art process using raw natural graphite, the pictures
showing the shape and size of feed graphite at 200 micron scale b) shows at 50
micron scale c) shows at 10 micron scale and d) shows at 5 micron scale.
Figure 3 shows the field emission scanning electron microscope of surface
structure of sample 1 graphene oxide material obtained after dry planetary ball
milling at 200 nano micron scale according to the invention.
Figure 4 shows the field emission scanning electron microscope of surface
structure of sample 2 graphene oxide material at 200 nano micron scale obtained
in this invention after dry planetary ball milling according to the invention.
Figure 5 shows the transmission electron microscope pictures of sample 1
graphene oxide obtained according to the invention, which interalia exhibiting
graphene oxide sheet with many folds and a thick graphene layers (>10), b)
exhibit the selected area diffraction pattern that confirms and displays the typical
hexagonal crystalline structure of graphene oxide, c) exhibits graphene oxide
sheet with folds at 50 nm scale d) exhibit the selected area diffraction pattern of
the graphene oxide that condirms and displays the typical hexagonal crystalline
structure of graphene oxide.

Figure 6 shows the transmission electron microscope pictures of sample 2
graphene oxide according to the invention; a) shows graphene oxide sheet with
many folds at 50 nm scale, b) shows the selected area diffraction pattern that
confirms and displays the typical hexagonal crystalline structure of graphene
oxide, c) shows the hexagonal lattice fringe in d planer spacing in layered
structured of graphene oxide.
Figure 7 shows the Raman spectroscopy of sample 1 graphene oxide.
Figure 8 shows the Raman spectroscopy of sample 2 graphene oxiode.
DETAILED DESCRIPTION OF THE INVENTION
The present invention describes an improved process for manufacture of
graphene oxide from natural graphite. According to the present invention
graphene oxide is obtained via ball milling high pure graphite powder/flake in
dual drive planetary ball mill.
As shown in figure 1, a structural frame (1) in which the vertical swing planetary
ball mill is fixed. A rotating arm (2) where two ball mills (3) freely rotate around
their own horizontal axis independently of the gyro rotation through a pulley
arrangement (4) and a motor (6). Another motor (7) is placed at the bottom
portion of the frame (1) which rotates a gyro pulley (5) that rotates the arm (2)
of the mill.

Further, the ball mill of the invention has the following unique design -
parameters:
a) the distance between gyratory shaft and ball mill axis is 290 mm;
b) the radius of ball mill is 100 mm with volume 4.24 liters in the vertical
swing planetary ball mill.
c) The rotation of the gyro speed ranges from 200 RPM to 300 RPM and
the rotation of the ball mill in reverse direction o the rotation of
gyratory motion is 300 RPM to 400 RPm.
The duration of planetary ball milling ranges from 2 hours to 24 hours.
The process of the present invention describes a single stage preparation of
graphene from high pure natural graphite powder/flake in a ball milling without
using any chemicals and additives.
The process of the present invention involves preparation of feed high pure
graphite powder/flake following the prior art process described in Indian Patent
(197530; A novel route to produce high purity graphite) in which high pure
>99.00% fixed carbon graphite powder / flake from 90.00 % fixed carbon
graphite is produced by combination of physical and chemical benefication
process. In the present process natural graphite of 3-15 % fixed carbon samples
were sourced from mines. Graphite ore is ground to its liberation size and the
carbon content is enhanced to 80-95% by flotation process. Then the flotation

product is subjected to chemical leaching using NaOH at different concentration
in pressure reactor. Then it is filtered and washed thoroughly in water. The
product is treated with dilute HCI to remove the alkali element and maintain the
pH. Then the product is again beneficiated using flotation column or flotation cell
to make it high pure upto 99.99% carbon content.
The process of invention involves a vertical swing dual drive planetary ball mill.
The dual drive planetary mill consists of a gyratory shaft and two cylindrical steel
jars, both are rotated simultaneously and separately at high speed by separate
electrical motors. In dual drive planetary mill, the combination of motions of the
rotating gyration arms and the rotation of the individual jar greatly enhances the
forces within the container effecting the grinding. The mechanism of the grinding
in this type f ball mill is that the centrifugal forces acting on the grinding jar wall
initially carry the grinding ball in the direction in which the grinding jar is
rotating. Difference occurred between the speed ratio of the grinding jars walls
and balls, this result in strong frictional forces acting on the sample. As the
rotational movement increases, Coriolis forces act on the balls to displace them
from the grinding jar wall. The balls fly through the grinding jar interior and
impact against the sample on the opposite grinding jar wall. This releases
considerable dynamic impact energy. The combination of the frictional forces and
impact forces causes the high degree of delamination of layered graphite flake /
powder inside the ball mills.

The present invention involves the above described modified dual drive planetary
mill (see Figure 1) that has a gap of 290 mm between the gyratory shaft axis
and the ball mill axis. The two mill jars are of diameter 200 mm and length 135
mm of volume 4.2 liters each. The two independent power inputs to gyratory
rotation and ball mill are from 5 horse power electrical motors.
The ball mills rotate about their own axis's and also around the gyrator axis
simultaneously independently in same direction as gyrator moves or in opposite
direction. In this present process, the rotation of the gyration of the dual drive
planetary mill is around 200 to300 RPM and the rotation of the ball mill is kept in
the ranges of 300 to 400 RPM. The ball mills and fixed axis gyratory rotate at
critical speed ratios ranging from 2.5 to 2.7.
In the method of the present invention, 60 gram weigh of the feed of high pure
graphite powder / flake material is put in each the ball mill. Nearly, 1000 grams
of the weight of the chrome hardened steel balls of different sizes like 10 mm, 6
mm in the ratio of 1:1 is taken as charge.
In the present method, referring to the above dual drive planetary milling, the
duration of the milling operation of high pure graphite is varied from 4 hours to
24 hours and the yield of production of graphene oxide GO linearly varies
accordingly. In referring to the present invention, in dual drive planetary milling
process of high pure graphite, the particle sizes of graphene oxide GO so
obtained varies from 200 nanometer to 3 nanometer.

The graphene oxide GO obtained by de-lamination of high pure graphite in dry
ball milling is tested and analyzed using scanning electron microscopy, field
emission scanning electron microscopy, transmission electron microscopy,
Raman spectrograph instrument. The scanning electron microscope used in test
and analysis is of Model JEOL (JSM-6510, voltage 10-20 KV. The samples are
taken with ultra thin film gold or by an ion spatter JFC-100. The Raman
spectrometer used in test and analysis is of model Reinshaw in Via Reflex 514
nm. The transmission electron microscope model is Tecnai 20 G2 of FEI make.
The filament in use is W/LaB6.
EXAMPLES
The following example is given by way of illustration of the present invention and
therefore should not be construed to limit the scope of the present invention.
Raw graphite powder with low fixed carbon content was resourced from mine
and was processed through physical and chemical method as described in Indian
Patent 197530 to get high pure graphite powder/flake of more than 99.0%
carbon content. These high pure graphite powder / flake were taken as starting
material for preparation of graphene oxide GO. Figure 1 shows the scanning
electron microscope pictures of high pure graphite feed at different scale, a)
shows the shape and size of feed graphite at 200 micron scale b) shows at 50
micron scale c) shows at 10 micron scale and d) shows at 5 micron scale.

Example 1
Sixty grams of high pure graphite flake / powder having fixed carbon content
99.99% was taken into each of the ball mill jar as shown in to figure 1. Around 1
kg of high chromium steel ball of size 10 mm and 6 mm with a ratio of 1:1 were
taken as media in the ball mill jar. The mouth of the ball mill jar was closed and
placed in the dual drive planetary mill. The gyratory axis of the dual drive
planetary mill was subjected to rotate at 200 RPM and the ball mills were
subjected to rotate at 300 RPM. The duration of the milling was 24 hours. After
milling the ball mills were kept at normal temperature for two hours then kept at
4 Deg centigrade for another 6 hours before the mouth was opened.
In this process of production of graphene oxide, there is no use of any acid zand
or chemical additives inside the mill during high energy milling of high pure
graphite powder/flake. There is also absolute no use liquid medial inside the ball
mill jar confirming to dry preparation of graphene oxide. The ground fine
graphite powder was collected from the ball mill and was subjected to tests and
analysis for standard characterization. Referring to figure 3 where it shows the
field emission scanning electron microscope pictures of surface structure of the
dry produced graphene oxide is shown. Referring to figure 5, it shows the
transmission electron microscope pictures of sample 1 graphene oxide obtained
in this invention. The figure 5(a) shows graphene oxide sheet with many folds
and a thick graphene layers (>10). The figure 5 (b) shows the selected area

diffraction pattern that confirms and displays the typical hexagonal crystalline
structure of graphene oxide. The figure 5 (c) shows graphene oxide sheet with
folds at 50 nm scale. The figure 5 (d) shows the selected area diffraction
pattern of the graphene oxide that confirms and displays the typical hexazgonal
crystalline structure of graphene oxide.
Raman spectroscopy was used as powerful, non-destructive tool in order to
characterize carbon materials, particularly for distinguishing their ordered and
disordered crystal structures. The typical features for carbon in Raman spectr are
the G line around 1582cm-l and the D line around 1350cm-l. The G line is
usually assigned to the C sp2 atoms, while the D line is a breathing mode of k-
point phonons of Alg symmetry. Extensive studies have determined that the
positions, intensities and widths of these bands are dependent on the ordering
of the sp2 sites in varying compositions of amorphous and crystalline carbon
compounds. Figure 7 shows the Raman spectra of graphene oxide produced by
dry milling of high pure graphite powder. In this spectrum, the G band is
broadened and shifted slightly to 1566.97 cm"1, whereas the intensity of the D
band at 1331.31cm"1 increases substantially and the 2D line broadened at 2679.9
cm"1. These phenomena could be attributed to the significant decrease of the
size of the in-plane sp2 domains due to oxidation. The Raman mapping of the
2D to G peak ratio illustrates the layers of the graphene oxide films, here it is
1.712 which indicates very thick layers and evident by TEM. For monolayer
graphene, it is 4. Thus it indicates that the ground product is graphene oxide.

Example 2
Nearly sixty grams of high pure graphite flake / powder having fixed carbon
content 99.99% was taken into each of the ball mill jar. Around 1 kg of high
chromium steel ball of size 10 mm and 6 mm with a ratio of 1:1 were taken as
media in the ball mill jar. The mouth of the ball mill jar was closed and placed in
the dual drive planetary mill. The gyrator axis of the dual drive planetary mill was
subjected to rotate at 300 RPM and the ball mills were subjected to rotate at
400 RPM. The duration of the milling was 18 hours. After milling the ball mills
were kept at normal temperature for two hours then kept at 4 Deg centigrade
for another 6 hours before the mouth was opened.
In this process of production of graphene oxide also, there is no use of any acid
or chemical additive inside the mill during high energy milling of high pure
graphite powder/flake. There is also absolute no use liquid media inside the ball
mill jar confirming to dry preparation of graphene oxide.
The ground fine graphite powder was collected from the ball mill and was
subjected to tests and analysis for standard characterization. Referring to figure
4 where it shows the field emission scanning electron microscope pictures of
surface structure of the dry produced graphene oxide is shown. Figure 6 shows
the transmission electron microscope pictures of sample 2 graphene oxide

obtained in this invention. The figure 6(a) shows graphene oxide sheet with
many folds at 50 nm scale. The figure 6 (b) shows the selected area diffraction
pattern that confirms and displays the typical hexagonal crystalline structure of
graphene oxide. The figure 6 (c) displays the typical hexagonal crystalline
structure of graphene oxide.
Figure 8 shows the Raman spectra of graphene oxide produced by dry milling of
high pure graphite powder. In this spectrum, the G band is broadened and
shifted slightly to 1578.29 cm"1, whereas the intensity of the D band at 1347.2
cm"1 increases substantially and the 2D line broadened at 2712.26 cm"1. These
phenomena could be attributed to the significant decrease of the size of the in-
plane sp2 domains due to oxidation. The Raman mapping of the 2D to G peak
ratio illustrates the layers of the graphene oxide films, here it is 1.71 which
indicates very thick layers and evident by TEM. For monolayer graphene, it is 4.
Thus it indicates that the ground product is graphene oxide.
In conclusion, the method of present invention allows to use natural high pure
graphite in a simple form without using any liquid, acid or any chemical additive
confirming to dry production of graphene oxide. The graphene oxide produed by
this method is environmental friendly and does not produce any waste or by-
product. Besides, the process can be scaled up for mass production of
graphene oxide. Accordingly, the method of present invention can avoid the
drawback of tortuous conventional technique.

The advantages of the current invention are :
i) The process uses natural high pure graphite produced raw low fixed
carbon content graphite sourced from mines.
ii) This is simple one step process to produce graphene oxide from high
pure natural graphite powder/flake.
iii) The process does not use any liquid medium.
iv) It does not use any acid or chemical additives.
v) It is environmental friendly and the process does not produce any
waste or bi-product.
vi) The process can be scaled up for mass production of graphene oxide
vii) Maximum yield of graphene oxide can be obtained.

WE CLAIM:
1. An improved process for preparation of graphene oxide from natural
graphite in a modified dual drive planetary ball mill comprising the
steps of:-
feeding natural graphite of more than 99% purity into a dual drive
high energy planetary ball mill in a dry method;
allowing a solid state oxidizing reaction to graphite in the dual drive
high energy planetary ball mill wherein oxidizing of graphite powder
occurs inside the jars of the high enery gall mill; and
peeling of or de-lamination of graphite powder / flake occurred during
the milling process inside the dual drive high energy planetary ball mill.
2. A process as claimed in claim 1, wherein graphite particles size varies
in the range of 600 micron to 5 micron.
3. A process as claimed in claim 1, wherein the graphene oxide has at
least ten stacking layers.

4 . A process as claimed in claim 1, wherein the graphene oxide has well
defined Raman spectra confirming D at and around 1350 cm"1 and G
peak at and around 1580 cm"1.
5. A process as claimed in claim 1, wherein balls of size range 6 mm to
10 mm are used in the ball mill.
6. A process as claimed in claim 1, wherein 20% of the volume of the ball
mill jar is occupied by the feed material and balls as charge.
7. A process as claimed in claim 1, wherein the fixed axis gyro rotates in
the range of 200 to 250 RPM and wherein the ball mills rotates in a
speed range of 300-400 RPM.