Method for producing reduced graphene oxide

Abstract

The present invention relates to a method for producing reduced graphene oxide from kish graphite, comprising providing kish graphite, intercalating the kish graphite with a persulfate and an acid at room temperature to obtain intercalated kish graphite, expanding the intercalated kish graphite at room temperature to obtain expanded kish graphite, and mixing the expanded kish graphite with at least an acid and an oxidizing agent, at least partially in the presence of gas evolved during the expansion step, so that the expanded kish graphite is simultaneously oxidized, exfoliated, and reduced to reduced graphene oxide.

Description

Technical Field

[0001] The present invention relates to a method for producing reduced graphene oxide from expanded kish graphite. In particular, reduced graphene oxide has uses, for example, as a coating or as a cooling reagent in the metal industry including the steel, aluminum, stainless steel, copper, iron, copper alloy, titanium, cobalt, metal composite, nickel industries.

Background Art

[0002] Kish graphite is a by-product generated during the steelmaking process, particularly the blast furnace process or the ironmaking process. In fact, kish graphite is usually formed on the free surface of molten iron during its cooling. It is derived from molten iron at 1300 to 1500 °C, which is cooled at a cooling rate between 0.40 °C / min and 25 °C / h when transported by torpedo car, or at a higher cooling rate during ladle transport. A huge tonnage of kish graphite is produced annually in steel plants.

[0003] Since kish graphite usually contains a large amount of carbon exceeding 50% by weight, it is a good candidate for manufacturing graphene-based materials. Usually, graphene-based materials include graphene, graphene oxide, and reduced graphene oxide.

[0004] It is known to produce reduced graphene oxide (rGO) by reducing the oxygen content in graphene oxide (GO). Reduced graphene oxide is composed of one or several layers of graphene sheets with fewer oxygen functional groups than graphene oxide. Due to its interesting properties such as high thermal conductivity, high electrical conductivity, hydrophobicity, and high specific surface area, reduced graphene oxide has many applications.

[0005] For example, reduced graphene oxide can be produced by a chemical process using reducing agents such as hydrazine, ascorbic acid, urea, and NaOH, or by thermal reduction at high temperature in an inert atmosphere. However, it is very difficult to obtain rGO with a low oxygen content, i.e., an oxygen content of less than 10 wt%. Some oxygen groups such as epoxy groups are very difficult to reduce in these processes. Furthermore, the obtained rGO contains many defects and thus exhibits a very low electrical conductivity.

[0006] It is also known to obtain reduced graphene oxide from kish graphite treated according to the Hummers method including the following steps. - A step of oxidizing kish graphite with sodium nitrate (NaNO 3 ), sulfuric acid (H 2 SO 4 ) and sodium permanganate or potassium permanganate (KMnO 4 ), - A step of reducing the graphene oxide to obtain reduced graphene oxide.

[0007] Patent application WO2018178845 discloses reduced graphene oxide from kish graphite including the following: A. A step of providing kish graphite, B. A pre-treatment step of the kish graphite including the following consecutive sub-steps, i. A sieving step in which the kish graphite is classified by size as follows, a) Kish graphite having a size of less than 50 μm, b) Kish graphite having a size of 50 μm or more, and a sub-step of removing fraction a) of the kish graphite having a size of less than 50 μm, ii. A flotation step of fraction b) of the kish graphite having a size of 50 μm or more, iii. An acid filtration step of adding an acid such that the weight ratio of (amount of acid) / (amount of kish graphite) is between 0.25 and 1.0, iv. Optionally, a step of washing and drying the kish graphite C. An oxidation step of the pretreated kish graphite obtained after step B) with an acid, sodium nitrate and an oxidizing agent to obtain graphene oxide, and D. A reduction step of reducing the graphene oxide to reduced graphene oxide.

[0008] However, when the oxidation step is carried out with sodium nitrate (NaNO 3 ), toxic gases are generated, leading to pollution problems. Also, the oxidation time is very long (about 3 hours).

[0009] Patent application WO2019220228 discloses a method for producing reduced graphene oxide from kish graphite, including the following. A. A step of providing kish graphite B. A pretreatment step of the kish graphite, including the following successive sub-steps i. A sieving step in which the kish graphite is classified by size as follows a) Kish graphite having a size of less than 50 μm b) A sub-step of removing fraction a) of the kish graphite having a size of less than 50 μm from the kish graphite having a size of 50 μm or more ii. A flotation step of fraction b) of the kish graphite having a size of 50 μm or more iii. An acid filtration step of adding an acid such that the weight ratio of (amount of acid) / (amount of kish graphite) is between 0.25 and 1.0 C. An oxidation step of the pretreated kish graphite with an acid, ammonium nitrate and an oxidizing agent, and exfoliation of the obtained oxidized graphite to graphene oxide D. A reduction step of reducing the graphene oxide to reduced graphene oxide.

[0010] However, the method using NH 4 NO 3 is NaNO3 Although it has less pollution than the method using

[0011] Furthermore, the oxidation time is 3 compared with the oxidation time of the method using 4 NO 3 which is shorter when using

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0013] Therefore, an object of the present invention is to provide an industrial method for obtaining high-quality reduced graphene oxide in as short a time as possible. Furthermore, an object of the present invention is to provide a method with less pollution for the production of reduced graphene oxide from kish graphite compared to the methods of the prior art.

Means for Solving the Problems

[0014] This is achieved by providing a method for producing reduced graphene oxide from kish graphite, which includes the following. - Providing kish graphite, - Intercalating kish graphite with persulfate and an acid at room temperature to obtain intercalated kish graphite, - Expand the intercalated kish graphite at room temperature to obtain expanded kish graphite. - Mix the expanded kish graphite with at least an acid and an oxidizing agent while at least a portion of the gas generated during the expansion step remains, such that the expanded kish graphite is simultaneously oxidized, exfoliated, and reduced to form reduced graphene oxide.

[0015] The method according to the invention can also have any of the following features, considered individually or in combination. - The gas generated during the expansion step contains O 2 and - The expanded kish graphite is not washed before being mixed with at least an acid and an oxidizing agent. - The expansion is carried out in a closed container. - The expansion is carried out in an open container. - The expanded kish graphite is mixed with at least an acid and an oxidizing agent less than 8 hours after the start of the expansion step. - The expanded kish graphite is mixed with at least an acid and an oxidizing agent less than 1 hour after the start of the expansion step. - At least 5% by volume of the maximum amount of the gas generated during the expansion step remains in the expanded kish graphite when the expanded kish graphite is mixed with at least an acid and an oxidizing agent. - At least 30% by volume of the maximum amount of the gas generated during the expansion step remains in the expanded kish graphite when the expanded kish graphite is mixed with at least an acid and an oxidizing agent. - The expanded kish graphite is first mixed with the acid, and then the oxidizing agent is gradually added. - The addition of the oxidizing agent lasts for 30 to 180 seconds. - The method according to the invention includes an additional step of mixing the reduced graphene oxide with H 2 O 2 to remove the remaining oxidizing agent. - The method according to the invention uses HCl, H 2SO 4 、 HNO 3 or a mixture thereof, and further comprises an additional step of removing by-products formed during the oxidation of the expanded kish graphite, - The method according to the present invention includes an additional step of rinsing the reduced graphene oxide with water.

[0016] The method according to the present invention enables the production of reduced graphene oxide in a rapid manner. In particular, the retention of the gas generated during the expansion step changes the kinetics of the oxidation step, enabling oxidation, exfoliation and reduction to occur simultaneously. Thus, after mixing the expanded kish graphite with the acid and the oxidizing agent, there is no separate exfoliation step and reduction step. Also, since this method particularly includes intercalation at room temperature, expansion at room temperature, and oxidation without using salts, it is easy to implement on an industrial scale and is less polluting than the prior art methods.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Other features and advantages of the present invention are described in more detail in the following description, which is provided for purely illustrative purposes and is not intended to be in any way limiting.

[0018] Define the following terms. - Graphite means an allotrope of elemental carbon consisting of crystalline, long-range ordered graphene layers stacked parallel to each other in three dimensions. - Graphite oxide means a chemically modified graphite prepared by extensive oxidation modification of the basal plane. - Graphene oxide means one or several layers of graphene containing oxygen functional groups including ketone groups, carboxyl groups, epoxy groups and hydroxyl groups. It can take the form of some morphological variations such as platelet or worm-like structures. - Reduced graphene oxide is a form in which the oxygen content of graphene oxide has decreased. It can take the form of some morphological variations such as platelet, wrinkled structure, worm-like structure, etc. - pristine graphene means that the graphene is in its original state, i.e., ideal and completely defect-free. - room temperature means performing a chemical reaction without adjusting the temperature by cooling or heating. In other words, no attempt is made to control the temperature of the reaction. Room temperature is preferably between 0 and 45 °C, more preferably between 1 and 30 °C, and even more preferably between 15 and 25 °C at atmospheric pressure.

[0019] In the first step (step A) of the method according to the present invention, raw kish graphite is provided.

[0020] Preferably, the kish graphite is a residue of the steelmaking process. After being tapped from the blast furnace, it accumulates on the surface of the molten iron. Since the iron cools while being tapped and transported to the steel shop, the molten iron becomes supersaturated and carbon comes out of solution as flakes of graphite floating on the surface of the iron. The graphite can be scooped from the molten iron supplied to the basic oxygen furnace. This consists of a mixture of graphite (precipitated from supersaturated iron), lime-rich slag (resulting from the desulfurization operation), and some iron (scooped up with the graphite and slag). The large lumps of iron are recovered for recycling and the remaining kish can be used for other purposes at any time.

[0021] According to a modification of graphite according to the present invention, the kish graphite is preferably pretreated (step B) to increase its purity to more than 90%.

[0022] The pretreatment of the kish graphite preferably includes the following successive sub-steps. i. A sieving step in which the kish graphite is classified by size as follows a. Kish graphite having a size of less than 50 μm b. Kish graphite having a size of 50 μm or more The fraction a) of kish graphite having a size of less than 50 μm is removed. ii. A flotation step using a fraction b) of chrysotile graphite with a size of 50 μm or more iii. An acid filtration step of adding an acid to chrysotile graphite at a weight ratio of 0.25 to 1.0 iv. Optionally, washing and drying of chrysotile graphite

[0023] In step B.i), the screening step can be carried out with a screening machine.

[0024] After screening, the fraction a) of chrysotile graphite having a size of less than 50 μm is removed. In fact, without intending to be bound by any theory, chrysotile graphite having a size of less than 50 μm is typically considered to contain a very small amount of graphite of less than 10%. Preferably, the fraction a) of chrysotile graphite having a size of less than 55 μm is removed. More preferably, the fraction a) of chrysotile graphite having a size of less than 60 μm is removed.

[0025] In steps B.i) and B.ii), the fraction b) of chrysotile graphite is preferably 300 μm or less, more preferably 275 μm or less, and even more preferably 250 μm or less. Therefore, any fraction of chrysotile graphite having a size exceeding 300 μm or 275 μm or 250 μm is removed before step B.ii).

[0026] Preferably, the flotation step B.ii) is carried out using a flotation reagent in an aqueous solution. For example, the flotation reagent is a foaming agent selected from methyl isobutyl carbinol (MIBC), pine oil, polyglycol, xylenol, S-benzyl-S'-n-butyl trithiocarbonate, S,S'-dimethyl trithiocarbonate, and S-ethyl-S'-methyl trithiocarbonate. Advantageously, the flotation step is carried out using a flotation device.

[0027] In step B.ii), the weight ratio of the acid to the kish graphite is between 0.25 and 1.0, preferably between 0.25 and 0.9, and more preferably between 0.25 and 0.8. If it is less than 0.25, there is a risk that the kish graphite will not be sufficiently purified. If it exceeds 0.8, there is a risk of generating a huge amount of chemical waste.

[0028] Preferably, the acid is selected from hydrochloric acid, phosphoric acid, sulfuric acid, nitric acid, and mixtures thereof.

[0029] Preferably, the pretreatment of the raw kish graphite consists of the continuous sub-steps of B.i to B.iv above.

[0030] The pretreated kish graphite obtained after step B) of the method according to the present invention has a high purity, that is, at least 90% purity. Also, the crystallinity is improved compared to the conventional method, enabling higher thermal conductivity and electrical conductivity, and thus higher quality.

[0031] When kish graphite is provided and optionally pretreated, it is intercalated with a persulfate and an acid at room temperature to obtain intercalated kish graphite (step C).

[0032] Without wishing to be bound by any theory, it is believed that the persulfate acts like an oxidizing agent to oxidize the edges of the kish graphite layers. Since the persulfate is an important oxygen donor, the gap between the two graphene layers further expands, making it easier for the acid to enter between the graphene layers. At the same time, a certain amount of persulfate can be drawn in by the acid between the graphene layers. The persulfate drawn into the graphene layers decomposes and releases O 2 and SO 3 , which is thought to cause an instantaneous pressure between the graphene layers, resulting in a rapid expansion of the graphite at room temperature.

[0033] Preferably, the weight ratio of the persulfate to the kish graphite is between 1 and 8, more preferably between 1 and 6, and advantageously between 1 and 5. This further improves the intercalation.

[0034] Preferably, the weight ratio of the acid to the kish graphite is between 2 and 8, more preferably between 4 and 8. In fact, if the ratio of the acid to the kish graphite is less than 2, only a part of the kish graphite may expand. If the ratio of the acid to the kish graphite exceeds 8, the expansion occurs very slowly and the volume expansion may decrease. The excess acid is considered to prevent the persulfate from being drawn into the interlayer of the kish graphite. Therefore, it hinders the release of oxygen due to the decomposition of the persulfate, and thus the rapid expansion of the kish graphite.

[0035] Preferably, the persulfate is selected from those containing the peroxydisulfate anion S 2 O 8 2- More preferably, the persulfate is sodium persulfate (Na 2 S 2 O 8 ), ammonium persulfate ((NH 4 ) 2 S 2 O 8 ), potassium persulfate (K 2 S 2 O 8 ) and mixtures thereof.

[0036] Preferably, the acid is a strong acid. More preferably, the acid is selected from H 2 SO 4 , HCl, HNO 3 , H 3 PO 4 , C 2 H 2 Cl 2 O 2 (dichloroacetic acid), HSO 2 OH (alkylsulfonic acid) and mixtures thereof.

[0037] Preferably, first, kish graphite is mixed with an acid, and then a persulfate is added. The mixing of kish graphite and the acid is carried out by mechanical stirring or agitation, and the uniformity of the mixture can be further improved. It is preferable to continue the mixing for 5 to 20 minutes. The persulfate is preferably added gradually.

[0038] Preferably, after the addition of the acid and the persulfate, the mixture is mechanically stirred to homogenize the mixture. The stirring is preferably continued for 2 to 10 minutes, more preferably 3 to 7 minutes.

[0039] Step C) is preferably continued for 2 to 30 minutes, more preferably 2 to 10 minutes.

[0040] When the kish graphite is intercalated, it is expanded (Step D).

[0041] Preferably, this expansion is naturally carried out by leaving the mixture of kish graphite, persulfate and acid at room temperature.

[0042] During the expansion, gas is generated. The gas in question is O 2 and SO 3 resulting from the reaction between the acid and the persulfate, as illustrated in the case of sulfuric acid and ammonium persulfate. (NH 4 ) 2 S 2 O 8 +2H 2 SO 4 →H 2 S 2 O 8 +2NH 4 HSO 4 When the temperature reaches 65 °C, 2H 2 S 2 O 8 →2H 2 SO 4 +2SO 3 +O 2

[0043] In a modification of the present invention, the expansion is carried out in a closed container so that the gas generated during expansion is more easily retained.

[0044] In another modification, the expansion is carried out in an open container. The gas is thought to intercalate sufficiently between the graphite layers to avoid rapid release of the gas from the mixture.

[0045] At the end of step D), it is preferable not to wash the expanded kish graphite. Such washing would contribute to the release of the gas generated during the expansion step.

[0046] Preferably, step D) is stopped by adding the reactants of the next step to the expanded kish graphite. When the expansion is carried out in an open container, the timing of the addition of these reactants is adapted to limit the release of too much gas occurring during the expansion step. Preferably, the expanded kish graphite is mixed with the reactants less than 8 hours after the start of the expansion step, more preferably less than 1 hour after the start of the expansion step, even more preferably less than 30 minutes after the start of the expansion step. When the expansion is carried out in a closed container, the timing of the addition of the reactants of the next step is not particularly limited. Closing the container makes it possible to sufficiently maintain the gas between the graphene layers and to start the next step at any time. Nevertheless, in order to limit the gas pressure in the closed container, it is preferable to mix the expanded kish graphite with the reactants of the next step less than 8 hours after the start of the expansion step, more preferably less than 1 hour after the start of the expansion step.

[0047] According to one modification of the present invention, the intercalation step C) and the expansion step D) are carried out simultaneously.

[0048] When the kish graphite is expanded, it is mixed at room temperature with at least an acid and an oxidizing agent to initiate the oxidation of the expanded kish graphite to oxidized graphite (step E).

[0049] When the oxidation step is initiated while the gas generated during the expansion of the intercalated kish graphite is not completely released, it was surprisingly observed that the kinetics of the reaction change and oxidation, reduction, and exfoliation can occur simultaneously. The reduction in the number of such process steps and the processing time represents a significant improvement over prior art processes.

[0050] Without wishing to be bound by any theory, the inventors understand that the presence of gas trapped between the graphene layers, particularly O 2 accelerates the temperature rise when an oxidizing agent is added. As a result, the oxidized graphite self-exfoliates and is reduced.

[0051] In the context of the present invention, "at least partially present" means that a portion of the gas generated during the expansion step remains trapped between the graphene layers and is not released into the atmosphere. Preferably, it represents at least 5% by volume of the maximum amount in the volume of the gas generated during the expansion step. Assuming that 100% of the persulfate reacts with the acid to form H 2 S 2 O 8 and that 100% of it decomposes, and assuming that O 2 and SO 3 are ideal gases, this maximum amount in volume is calculated by summing the volume of O 2 and the volume of SO 3 . More preferably, the portion of the gas generated during the expansion step and trapped between the graphene layers corresponds to at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% of the maximum amount in the volume of the gas generated during the expansion step. The more gas is trapped between the graphene layers of the expanded kish graphite, the more they simultaneously activate oxidation, exfoliation, and reduction in step E.

[0052] The percentage of the volume of the released gas and of the gas trapped between the resulting graphene layers at the volume of the maximum amount of gas occurring during the expansion step, at room temperature in a closed container, can be measured by connecting the container to a gas syringe or a mass spectrometer during the expansion. The latter is preferred because it is more accurate.

[0053] Preferably, the acid is H 2 SO 4 , HCl, HNO 3 , H 3 PO 4 , C 2 H 2 Cl 2 O 2 (dichloroacetic acid), HSO 2 OH (alkylsulfonic acid) and mixtures thereof. Preferably, the weight ratio of the acid (concentrate) to the kish graphite is between 25 and 75.

[0054] Preferably, the oxidizing agent is potassium permanganate (KMnO 4 ), H 2 O 2 , O 3 , H 2 S 2 O 8 , H 2 SO 5 , KNO 3 , NaClO and mixtures thereof. More preferably, the oxidizing agent is potassium permanganate. Preferably, the weight ratio of the oxidizing agent to the kish graphite is between 2 and 10.

[0055] According to an embodiment of the present invention, a salt is further added to the mixture of kish graphite, acid and oxidizing agent. Preferably, the salt is NaNO 3 , NH 4 NO 3 , KNO 3 , Ni(NO 3 ) 2 , Cu(NO 3 ) 2 , Zn(NO 3 ) 2 , Al(NO3 ) 3 and selected from among those and their mixtures. Preferably, the weight ratio of the salt to the expanded kish graphite is between 0.2 and 2. That being said, the oxidation of expanded kish graphite without using a salt seems to allow for a shorter oxidation time. The ability to remove the salt from the oxidation step significantly suppresses contamination. Therefore, preferably, the expanded kish graphite is mixed with the acid and the oxidizing agent without using a salt. In other words, the mixture in step E) preferably consists of expanded kish graphite, an acid, and an oxidizing agent.

[0056] Preferably, the expanded kish graphite is first mixed with the acid, and then the oxidizing agent is added. Preferably, in order to avoid an excessive temperature rise, the oxidizing agent is added gradually. The addition preferably continues for 30 to 180 seconds.

[0057] When the acid and the oxidizing agent are added to the expanded kish graphite, the mixture is preferably stirred until oxidation, exfoliation, and reduction occur. The stirring preferably continues for 5 to 50 minutes, more preferably 20 to 40 minutes.

[0058] Step E) continues for 5 minutes to 60 minutes, more preferably 15 to 45 minutes. Such a duration is a major improvement over prior art processes.

[0059] When the reaction is complete, step E) ends.

[0060] In a preferred embodiment, the obtained reduced graphene oxide is treated to remove the remaining oxidizing agent (step F). H 2 O 2 is preferably used as shown below in the case of potassium permanganate. 2KMnO 4 +H 2 O 2 +3H 2 SO 4 →2MnSO 4 +O 2 +K 2 SO 4+4H 2 O

[0061] Next, to remove by-products such as Mn 2 O 7 and MnO 2 formed during the oxidation / detachment / reduction step, acids such as HCl, H 2 SO4, HNO 3 and mixtures thereof can be added.

[0062] Once reduced graphene oxide is obtained, it is optionally rinsed with non-deionized water or deionized water to reach neutral pH (Step G).

[0063] Once reduced graphene oxide is obtained, it is optionally dried (Step H). Drying is carried out, in particular, by air, freeze-drying, vacuum drying, freeze-drying, etc. Freeze-drying is preferred as it further advantageously separates the rGO flakes.

[0064] By applying the method according to the present invention, reduced graphene oxide (rGO) containing one or several layers of graphene having oxygen between 10 and 25% by weight is obtained.

[0065] According to a variant of the present invention, the rGO is further reduced to microwave-reduced graphene oxide (MW-rGO) (Step J).

[0066] Preferably, in Step J, the catalyst is selected from pristine graphene, graphene nanoplatelets, graphite or graphite nanoplatelets. More preferably, the catalyst is pristine graphene. Without wishing to be bound by any theory, due to the nature, morphology and properties of pristine graphene, it is believed that pristine graphene can better absorb the electromagnetic field in the form of microwaves. In fact, pristine graphene is a single layer of graphite composed of carbon bonded in a hexagonal honeycomb lattice. It is an allotrope of carbon in a structure of sp2-bonded atoms where microwaves are attracted and can be easily absorbed.

[0067] Preferably, the weight ratio of rGO to the catalyst is included between 50 and 1000. Advantageously, the weight ratio of rGO to the catalyst is between 75 and 125. Thanks to such a ratio, the reduction of rGO to MW-rGO is further improved, and MW-rGO with even fewer oxygen groups is obtained.

[0068] Preferably, the microwave frequency is between 300 MHz and 100 GHz, preferably between 1 and 5 GHz, for example, 2.45 GHz.

[0069] Preferably, step J) is carried out using a microwave frequency heating device. Preferably, it is a microwave oven.

[0070] Advantageously, the microwave frequency heating device has an output between 100 W and 100 KW, more preferably between 100 and 2000 W.

[0071] Preferably, the heating by microwave is carried out for at least 2 seconds, more preferably between 2 and 3600 seconds. Thereby, the reduction of graphene oxide is further improved.

[0072] In this way, microwave-reduced graphene oxide (MW-rGO) containing one or several layers of graphene with less than 10 wt%, more preferably less than 7 wt% oxygen can be obtained.

[0073] Preferably, the reduced graphene oxide is deposited on a metal substrate to improve some properties such as the corrosion resistance of the metal substrate.

[0074] In another preferred embodiment, reduced graphene oxide is used as the cooling reagent. In fact, reduced graphene oxide can be added to the cooling fluid. Preferably, the cooling fluid can be selected from water, ethylene glycol, ethanol, oil, methanol, silicone, propylene glycol, alkylated aromatic compounds, liquid Ga, liquid In, liquid Sn, potassium formate, and mixtures thereof. In this embodiment, the cooling fluid can be used to cool the metal substrate.

[0075] For example, the metal substrate can be selected from aluminum, steel, stainless steel, copper, iron, copper alloy, titanium, cobalt, metal composites, and nickel.

[0076] The present invention will now be described in more detail based on examples implemented for information purposes only. They are not limiting.

Examples

[0077] Kish graphite was obtained from steelmaking. The kish graphite was then sieved and classified by size as follows. a) Kish graphite having a size of less than 63 μm, and b) Kish graphite having a size of 63 μm or more

[0078] The fraction a) of kish graphite having a size of less than 63 μm was removed.

[0079] A flotation step was performed on the fraction b) of kish graphite having a size of 63 μm or more. The flotation step was carried out using a Humboldt Wedag flotation machine with MIBC as the foaming agent. The following conditions were applied. Cell volume (l): 2, rotor speed (rpm): 2000, solids concentration (%): 5 - 10, foaming agent, type: MIBC, foaming agent, addition (g / T): 40, conditioning time (seconds): 10, water conditions: natural pH, room temperature.

[0080] Next, the chrysotile graphite was filtered with hydrochloric acid in an aqueous solution at a weight ratio of acid / chrysotile graphite of 0.5. Then, it was washed with deionized water and dried in air at 90 °C. The purity of the pretreated chrysotile graphite was 95%.

[0081] Next, 10 g of chrysotile graphite was added to 30 mL of 98% H 2 SO 4 in an open container. The reaction mixture was continuously stirred at room temperature for 15 minutes to obtain a homogeneous mixture. Then, at room temperature, 30 g of ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) was gradually added to the mixture for intercalation. Then, it was stirred for 5 minutes to homogenize the mixture.

[0082] Then, this mixture was left in an open container at room temperature for 30 minutes during which it swelled.

[0083] Then, at room temperature, 250 mL of 98% H 2 SO 4 was added to the mixture containing the swollen chrysotile graphite, followed by the gradual addition of 35 g of KMnO 4 at 0.5 g.s -1 to simultaneously initiate the oxidation process, exfoliation, and reduction.

[0084] At the end of the stepwise addition of KMnO 4 , the mixture was mechanically stirred at room temperature for an additional 30 minutes to complete the oxidation / exfoliation / reduction step.

[0085] Next, 50 mL of 35% H 2 O 2 was added to remove the remaining KMnO 4 . Subsequently, 100 mL of 36% HCl was added to remove the Mn 2 O 7 and MnO 2 formed during the oxidation / exfoliation / reduction step.

[0086] Finally, the mixture was neutralized to pH 7 with deionized water and freeze-dried to obtain rGO in powder form.

[0087] The obtained reduced graphene oxide was analyzed by scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), elemental analysis, and Raman spectroscopy.

[0088] These analyses confirmed that the product obtained at the end of the process according to the present invention is reduced graphene oxide with 16 wt% O and a C / O weight ratio of 4.95, and is very similar to the reduced graphene oxide obtained by the prior art process.

[0089] Also, from the analysis by transmission electron microscopy, it was revealed that this reduced graphene oxide is in the form of nanoplatelets, that is, nano-objects with one outer dimension on the nanoscale and the other two outer dimensions being significantly larger. In particular, the thickness was on the order of two graphene layers, and the width and length were in micrometers. These results suggest that in the method according to the present invention compared to the prior art, exfoliation proceeds well and the lattice strain is low.

Claims

**Claim 1** A method for producing reduced graphene oxide from kish graphite, comprising the following steps, namely: - providing kish graphite; - intercalating kish graphite with a persulfate and an acid at room temperature to obtain intercalated kish graphite; - leaving the intercalated kish graphite with the persulfate and the acid at room temperature to obtain expanded kish graphite; - mixing the expanded kish graphite with at least an acid and an oxidizing agent during a period when at least a part of the gas containing O₂ generated during the expansion step remains, within less than 8 hours after the start of the expansion step, so that the expanded kish graphite is simultaneously oxidized, exfoliated and reduced to form reduced graphene oxide; wherein the acid is selected from H₂SO₄, HCl, HNO₃, H₃PO₄, C₂H₂Cl₂O₂, alkylsulfonic acid and mixtures thereof; and the oxidizing agent is selected from KMnO₄, H₂O₂, O₃, H₂S₂O₈, H₂SO₅, KNO₃, NaClO and mixtures thereof. A method. **Claim 2** The method according to claim 1, wherein the expanded kish graphite is not washed before being mixed with at least an acid and an oxidizing agent. **Claim 3** The method according to claim 1 or 2, wherein the expansion is carried out in a closed container. **Claim 4** The method according to any one of claims 1 to 3, wherein the expansion is carried out in an open container. **Claim 5** The method according to any one of claims 1 to 4, wherein the expanded kish graphite is mixed with at least an acid and an oxidizing agent within less than 1 hour after the start of the expansion step. **Claim 6** The method according to any one of claims 1 to 5, wherein when the expanded kish graphite is mixed with at least an acid and an oxidizing agent, at least 5% by volume of the maximum amount of the gas generated during the expansion step remains in the expanded kish graphite. **Claim 7** The method according to any one of claims 1 to 6, wherein when the expanded kish graphite is mixed with at least an acid and an oxidizing agent, at least 30% by volume of the maximum amount of the gas generated during the expansion step remains in the expanded kish graphite. **Claim 8** The method according to any one of claims 1 to 7, wherein the expanded kish graphite is first mixed with an acid, and then an oxidizing agent is gradually added.

9. The method according to claim 8, wherein the addition of the oxidizing agent continues for 30 to 180 seconds.

10. mixing the reduced graphene oxide with H ２ O ２ and including an additional step of removing the remaining oxidizing agent, the method according to any one of claims 1 to 9.

11. The reduced graphene oxide is mixed with HCl, H ２ SO ４ , HNO ３ or a mixture thereof to include an additional step of removing by-products formed during the oxidation of expanded kish graphite, the method according to any one of claims 1 to 10.

12. The method according to any one of claims 1 to 11, further comprising the additional step of rinsing the reduced graphene oxide with water.