Method for producing reduced graphene oxide

Abstract

The present invention relates to a method for producing reduced graphene oxide from Kish graphite, the method 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 oxidant while gas generated during the expansion step is still at least partially present so that the expanded Kish graphite is simultaneously oxidized, exfoliated, and reduced to reduced graphene oxide.

Description

[0001] The present invention relates to a method for producing reduced graphene oxide from expanded Kish graphite. In particular, the reduced graphene oxide is used, for example, as a coating or as a cooling agent in the metal industry, including the steel, aluminum, stainless steel, copper, iron, copper alloys, titanium, cobalt, metal composites, and nickel industries.

[0002] Kish graphite is a by-product produced during the steelmaking process, particularly during the blast furnace or ironmaking process. Kish graphite is typically formed on the free surface of molten iron during cooling. It originates from molten iron at temperatures of 1300°C to 1500°C, cooled at rates of 0.40°C / minute to 25°C / hour during transport in torpedo cars, or at higher rates during ladle transfer. Mills generate large quantities of Kish graphite annually.

[0003] Since Kish graphite contains a large amount of carbon (typically greater than 50 wt%), it is a good candidate for producing graphene-based materials. In general, graphene-based materials include: graphene, graphene oxide, and reduced graphene oxide.

[0004] Reduced graphene oxide (rGO) is produced by reducing the oxygen content of graphene oxide (GO). Reduced graphene oxide consists of one or more graphene sheets containing fewer oxygen functional groups than graphene oxide. Due to its attractive 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 chemical methods using reducing agents such as hydrazine, ascorbic acid, urea, or NaOH, or by thermal reduction at high temperatures in an inert atmosphere. However, rGO with a low oxygen content (i.e., less than 10 wt%) is very difficult to obtain. Some oxygen groups, such as epoxy groups, are very difficult to reduce using these methods. In addition, the resulting rGO contains many defects and therefore exhibits very low electrical conductivity.

[0006] It is also known to obtain reduced graphene oxide from treated Kish graphite according to the Hummers process, which comprises the following steps:

[0007] - oxidation of Kish graphite with sodium nitrate (NaNO3), sulfuric acid (H2SO4) and sodium permanganate or potassium permanganate (KMnO4), and

[0008] - reducing the graphene oxide to obtain reduced graphene oxide.

[0009] Patent application WO2018178845 discloses a method for producing reduced graphene oxide from Kish graphite, the method comprising:

[0010] A. Provide Kish graphite,

[0011] B. The pretreatment step of the Kish graphite comprises the following sub-steps in the following order:

[0012] i. A screening step in which the Kish graphite is size-classified as follows:

[0013] a) Kish graphite with a size below 50 μm,

[0014] b) Kish graphite with a size greater than or equal to 50 μm, excluding the fraction a) of Kish graphite with a size less than 50 μm,

[0015] ii. a flotation step of the fraction b) of Kish graphite having a size greater than or equal to 50 μm,

[0016] iii. an acid leaching step, wherein acid is added so that the weight ratio (acid amount) / (Kish graphite amount) is from 0.25 to 1.0,

[0017] iv. optionally, washing and drying the Kish graphite,

[0018] 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

[0019] D. Reducing graphene oxide to reduced graphene oxide.

[0020] However, when sodium nitrate (NaNO3) is used for the oxidation step, toxic gases are generated, resulting in a polluting process. In addition, the oxidation time is very long (about 3 hours).

[0021] Patent application WO2019220228 discloses a method for producing reduced graphene oxide from Kish graphite, the method comprising:

[0022] A. Provide Kish graphite,

[0023] B. A pretreatment step of the Kish graphite comprising the following sub-steps in the following order:

[0024] i. A screening step in which the Kish graphite is graded by size as follows:

[0025] a) Kish graphite with a size below 50 μm,

[0026] b) Kish graphite with a size greater than or equal to 50 μm, excluding the fraction a) of Kish graphite with a size less than 50 μm,

[0027] ii. a flotation step of the fraction b) of Kish graphite having a size greater than or equal to 50 μm,

[0028] iii. an acid leaching step, wherein acid is added so that the weight ratio (acid amount) / (Kish graphite amount) is from 0.25 to 1.0,

[0029] C. performing an oxidation step of the pretreated Kish graphite with an acid, ammonium nitrate (NH4NO3) and an oxidizing agent and exfoliating the obtained graphite oxide into graphene oxide,

[0030] D. Reducing graphene oxide to reduced graphene oxide.

[0031] However, although the process using NH4NO3 is less polluting than the process using NaNO3, there is a need to further provide an even less polluting process and to reduce energy consumption.

[0032] Additionally, although the oxidation time using NH 4 NO 3 is shorter (i.e., 1 hour and 30 minutes) than the oxidation time of the method using NaNO 3 (i.e., 3 hours), there is still a need to reduce the process duration and thus to increase the productivity of synthesizing reduced graphene oxide.

[0033] Therefore, the object of the present invention is to provide an industrial process for obtaining reduced graphene oxide of good quality in the shortest possible time. In addition, the object of the present invention is to provide a process for producing reduced graphene oxide from kish graphite that is less polluting than the prior art processes.

[0034] This is achieved by providing a method for producing reduced graphene oxide from Kish graphite, the method comprising:

[0035] - Provide Kish graphite,

[0036] - intercalation of Kish graphite with persulfate and acid at room temperature to obtain intercalated Kish graphite,

[0037] - expanding the intercalated Kish graphite at room temperature to obtain expanded Kish graphite,

[0038] - While the gas generated during the expansion step is still at least partially present, mixing the expanded Kish graphite with at least an acid and an oxidizing agent such that the expanded Kish graphite is simultaneously oxidized, exfoliated and reduced to reduced graphene oxide.

[0039] The method according to the invention may also have the optional features listed below, considered individually or in combination:

[0040] - the gas produced during the expansion step contains O2,

[0041] - not cleaning the expanded Kish graphite before mixing it with at least the acid and the oxidizing agent,

[0042] - Expansion is carried out in a closed container,

[0043] - Expansion is carried out in open containers,

[0044] - mixing the expanded Kish graphite with at least an acid and an oxidizing agent less than eight hours after commencing the expansion step,

[0045] - mixing the expanded Kish graphite with at least an acid and an oxidizing agent less than one hour after starting the expansion step,

[0046] - 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 by volume of the gases generated during the expansion step is still present in the expanded Kish graphite,

[0047] - 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 by volume of the gases generated during the expansion step is still present in the expanded Kish graphite,

[0048] - first mix the expanded Kish graphite with the acid, then gradually add the oxidant,

[0049] - the addition of the oxidant lasts from 30 seconds to 180 seconds,

[0050] - the method according to the invention comprises the additional step of mixing the reduced graphene oxide with H2O2 in order to eliminate residual oxidizing agents,

[0051] - the method according to the invention comprises the additional step of mixing the reduced graphene oxide with HCl, H2SO4, HNO3 or a mixture thereof in order to remove by-products formed during the oxidation of the expanded Kish graphite,

[0052] The method according to the invention comprises the additional step of rinsing the reduced graphene oxide with water.

[0053] The method according to the present invention allows for the rapid production of reduced graphene oxide. In particular, the retention of gases produced during the expansion step modifies the kinetics of the oxidation step and allows oxidation, exfoliation, and reduction to proceed simultaneously. Thus, after mixing the expanded Kish graphite with the acid and oxidant, there are no separate exfoliation and reduction steps. Furthermore, because the method specifically involves intercalation at room temperature, expansion at room temperature, and oxidation in the absence of salt, it is easily implemented on an industrial scale and is less polluting than prior art methods.

[0054] Other features and advantages of the present invention will be described in more detail in the following description, which is provided for purposes of illustration only and is not intended to be limiting in any way.

[0055] The following terms are defined:

[0056] - Graphite means an allotropic form of the element carbon, consisting of graphene layers stacked parallel to each other in a three-dimensional, crystalline, long-range order.

[0057] Graphite oxide means chemically modified graphite which is prepared by extensive oxidative modification of the basal planes.

[0058] Graphene oxide means one or more layers of graphene containing oxygen functional groups including keto, carboxyl, epoxy and hydroxyl groups. It can take the form of several morphological variations such as sheets and worm-like structures.

[0059] Reduced graphene oxide is a form of graphene oxide with reduced oxygen content. It can take the form of several morphological variations such as sheets, wrinkle-like structures and worm-like structures.

[0060] - Pristine graphene means that the graphene is in its original condition, ie ideal and without any defects.

[0061] Room temperature means that the chemical reaction is carried out without regulating the temperature by cooling or heating. In other words, no attempt is made to control the temperature of the reaction. Room temperature is preferably 0°C to 45°C at atmospheric pressure, more preferably 1°C to 30°C, and even more preferably 15°C to 25°C.

[0062] In the first step (step A) of the method according to the invention, raw Kish graphite is provided.

[0063] Kish graphite is preferably a residue from the steelmaking process. It is collected at the surface of the molten iron after it has been tapped from the blast furnace. As the iron cools during tapping and transport to the steelmaking plant, it becomes supersaturated and carbon comes out of solution as graphite flakes that float to the surface of the iron. The graphite flakes can be skimmed from the molten iron fed to the basic oxygen furnace. The graphite flakes consist of a mixture of graphite (precipitated from the supersaturated iron), lime-rich slag (from the desulfurization operation), and some iron (skimmed along with the graphite and slag). A large amount of the iron is recovered for recycling, and the remaining Kish is ready for other applications.

[0064] According to one variant of the method according to the invention, the Kish graphite is preferably pretreated (step B) in order to increase its purity to above 90%.

[0065] The pretreatment of Kish graphite preferably comprises the following sequence of sub-steps:

[0066] i. A screening step in which the Kish graphite is graded by size as follows:

[0067] a) Kish graphite with a size below 50 μm,

[0068] b) Kish graphite with a size greater than or equal to 50 μm,

[0069] The fraction a) of Kish graphite with a size below 50 μm is removed,

[0070] ii. a flotation step of the fraction b) of Kish graphite having a size greater than or equal to 50 μm, iii. an acid leaching step wherein acid is added in a weight ratio of 0.25 and 1.0 relative to the Kish graphite.

[0071] iv. Optionally, washing and drying the Kish graphite.

[0072] In step Bi), the screening step can be performed using a screening machine.

[0073] After sieving, a fraction a) of Kish graphite with a size below 50 μm is removed. In practice, without wishing to be bound by any theory, it is believed that Kish graphite with a size below 50 μm contains very small amounts of graphite, typically less than 10%. Preferably, a fraction a) of Kish graphite with a size below 55 μm is removed. More preferably, a fraction a) of Kish graphite with a size below 60 μm is removed.

[0074] In steps B.i) and B.ii), the size of fraction b) of Kish graphite is preferably less than or equal to 300 μm, more preferably less than or equal to 275 μm, and even more preferably less than or equal to 250 μm. Therefore, any fraction of Kish graphite having a size greater than 300 μm, 275 μm or 250 μm is removed before step B.ii).

[0075] Preferably, flotation step B.ii) is performed using an aqueous flotation reagent solution. For example, the flotation reagent is a frother selected from the group consisting of methyl isobutyl carbinol (MIBC), pine oil, polyethylene glycol, xylenol, S-benzyl-S'-n-butyl trithiocarbonate, S,S'-dimethyl trithiocarbonate, and S-ethyl-S'-methyl trithiocarbonate. Advantageously, the flotation step is performed using a flotation apparatus.

[0076] In step B.iii), the weight ratio of acid to Kish graphite is from 0.25 to 1.0, advantageously from 0.25 to 0.9, and more preferably from 0.25 to 0.8. Below 0.25, there is a risk that the Kish graphite may not be sufficiently purified. Above 0.8, there is a risk that a large amount of chemical waste may be generated.

[0077] Preferably, the acid is selected from hydrochloric acid, phosphoric acid, sulfuric acid, nitric acid, and mixtures thereof.

[0078] Preferably, the pretreatment of the pristine Kish graphite consists of the above-described sequence of sub-steps Bi to B.iv.

[0079] The pretreated Kish graphite obtained after step B) of the process according to the invention has a high purity, i.e. at least 90%. In addition, the crystallinity is improved compared to conventional processes, thereby allowing higher thermal and electrical conductivity and thus higher quality.

[0080] After the Kish graphite has been provided and optionally pretreated, it is intercalated with persulfate and acid at room temperature to obtain intercalated Kish graphite (step C).

[0081] Without wishing to be bound by any theory, it is believed that persulfate acts as an oxidant, oxidizing the edges of the Kish graphite layers. Because persulfate is a significant oxygen donor, it further expands the gap between the two graphene layers, allowing the acid to more easily enter between the graphene layers. Simultaneously, a certain amount of persulfate may be dragged between the graphene layers by the acid. It is believed that the persulfate dragged between the graphene layers will decompose and release O₂ and SO₃, causing transient pressure between the graphene layers and, therefore, the exponential expansion of graphite at room temperature.

[0082] Preferably, the weight ratio of persulfate relative to Kish graphite is from 1 to 8, more preferably from 1 to 6, and advantageously from 1 to 5. This further improves intercalation.

[0083] Preferably, the weight ratio of acid to Kish graphite is from 2 to 8, more preferably from 4 to 8. In practice, if the acid-to-Kish graphite ratio is below 2, there is a risk that only a portion of the Kish graphite will expand. If the acid-to-Kish graphite ratio is above 8, there is a risk that the expansion will occur very slowly and the volume expansion will be reduced. It is believed that the excess acid prevents the persulfate from being entrained in the interlayers of the Kish graphite. Consequently, this prevents the release of oxygen from the decomposition of the persulfate and, therefore, the exponential expansion of the Kish graphite.

[0084] Preferably, the persulfate is selected from the group consisting of peroxydisulfate anions S2O8 2- More preferably, the persulfate is selected from the group consisting of sodium persulfate (Na2S2O8), ammonium persulfate ((NH4)2S2O8) and potassium persulfate (K2S2O8), and mixtures thereof.

[0085] Preferably, the acid is a strong acid. More preferably, the acid is selected from H2SO4, HCl, HNO3, H3PO4, C2H2Cl2O2 (dichloroacetic acid), HSO2OH (alkylsulfonic acid), and mixtures thereof.

[0086] Preferably, the Kish graphite is mixed with the acid first, followed by the addition of the persulfate. The mixing of the Kish graphite and the acid can be performed by mechanical agitation or stirring to further improve the homogeneity of the mixture. This mixing preferably lasts for 5 to 20 minutes. The persulfate is preferably added gradually.

[0087] Preferably, after the addition of the acid and persulfate, the mixture is mechanically stirred to make it homogenous. Stirring is preferably continued for 2 minutes to 10 minutes, more preferably for 3 minutes to 7 minutes.

[0088] Step C) preferably lasts from 2 minutes to 30 minutes, more preferably from 2 minutes to 10 minutes.

[0089] After the Kish graphite has been intercalated, it is expanded (step D).

[0090] Preferably, the expansion is carried out naturally by allowing the mixture of Kish graphite, persulfate and acid to stand at room temperature.

[0091] During expansion, gases are produced. The gases in question include O2 and SO3 produced in the case of sulfuric acid and ammonium persulfate by the reaction between the acid and the persulfate as shown below:

[0092] (NH4)2S2O8+2H2SO4→H2S2O8+2NH4HSO4

[0093] When the temperature reaches 65℃, 2H2S2O8→2H2SO4+2SO3+O2.

[0094] In one variant of the invention, the expansion is carried out in a closed container so that the gas generated during expansion can be more easily retained.

[0095] In another variation, the expansion is carried out in an open vessel. It is believed that the gas is sufficiently intercalated between the layers of graphite to avoid its rapid release from the mixture.

[0096] At the end of step D), the expanded Kish graphite is preferably not cleaned. Such cleaning would facilitate the release of the gases generated during the expansion step.

[0097] 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 time of addition of these reactants is adapted to limit the release of excess gas generated during the expansion step. Preferably, the expanded Kish graphite is mixed with the reactants of the next step less than eight hours after the start of the expansion step, more preferably less than one hour after the start of the expansion step, and 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 time of addition of the reactants of the next step is not particularly limited. The closure of the container makes it possible to keep the gas sufficiently between the graphene layers and to start the next step at any given time. That is, the expanded Kish graphite is preferably mixed with the reactants of the next step less than eight hours after the start of the expansion step, more preferably less than one hour after the start of the expansion step, to limit the gas pressure in the closed container.

[0098] According to one variant of the invention, the intercalation step C) and the expansion step D) are carried out simultaneously.

[0099] After the Kish graph has been expanded, it is mixed with at least an acid and an oxidizing agent at room temperature to begin oxidation of the expanded Kish graphite to graphite oxide (step E).

[0100] It has been unexpectedly observed that starting the oxidation step while the gases generated during the expansion of the intercalated Kish graphite have not yet been fully released changes the kinetics of the reaction and allows oxidation, reduction, and exfoliation to proceed simultaneously. This reduction in the number of process steps and processing time is a significant improvement over prior art methods.

[0101] Without wishing to be bound by any theory, the inventors understand that when an oxidant is added, the presence of gases trapped between the graphene layers, and in particular O 2 , accelerates the temperature increase, thereby causing the graphite oxide to exfoliate and reduce.

[0102] In the present context, "at least partially present" means that a portion of the gas generated during the expansion step remains trapped between the graphene layers and cannot be released into the atmosphere. Preferably, this represents at least 5% by volume of the maximum volumetric amount of gas generated during the expansion step. This maximum volumetric amount assumes 100% reaction of the persulfate with the acid to form H2S2O8, which decomposes 100% according to the above chemical equation, assuming O2 and SO3 are ideal gases and calculated by adding the volume of O2 and the volume of SO3. More preferably, the portion of gas generated during the expansion step and remaining trapped between the graphene layers represents 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% by volume of the maximum volumetric amount of gas generated during the expansion step. The more gas remains trapped between the graphene layers of the expanded Kish graphite, the more it stimulates the simultaneous oxidation, exfoliation, and reduction in step E).

[0103] During expansion in a closed container at room temperature, by connecting the container to a gas injector or to a mass spectrometer, it is possible to measure the volume of gas released and thus maintain the percentage of the volume of gas trapped between the graphene layers relative to the maximum amount of gas generated during the expansion step. The latter is preferred for better accuracy.

[0104] Preferably, the acid is selected from H2SO4, HCl, HNO3, H3PO4, C2H2Cl2O2 (dichloroacetic acid), HSO2OH (alkylsulfonic acid), and mixtures thereof. Preferably, the weight ratio of the (concentrated) acid relative to the Kish graphite is 25 to 75.

[0105] Preferably, the oxidant is selected from potassium permanganate (KMnO4), H2O2, O3, H2S2O8, H2SO5, KNO3, NaClO, and mixtures thereof. More preferably, the oxidant is potassium permanganate. Preferably, the weight ratio of the oxidant to the Kish graphite is 2 to 10.

[0106] According to one embodiment of the present invention, a salt is further added to the mixture of Kish graphite, acid, and oxidant. Preferably, the salt is selected from NaNO3, NH4NO3, KNO3, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2, Al(NO3)3, and mixtures thereof. Preferably, the weight ratio of the salt relative to the Kish graphite is from 0.2 to 2. In other words, it appears that oxidizing the expanded Kish graphite in the absence of any salt allows for shorter oxidation times. The ability to remove salt from the oxidation step significantly limits contamination. Therefore, the expanded Kish graphite is preferably mixed with the acid and oxidant without any salt. In other words, the mixture of step E) preferably consists of expanded Kish graphite, acid, and oxidant.

[0107] Preferably, the expanded Kish graphite is mixed with the acid first, and then the oxidant is added. Preferably, the oxidant is added gradually to avoid excessive temperature increases. The addition preferably lasts from 30 seconds to 180 seconds.

[0108] After the acid and oxidant have been added to the expanded Kish graphite, the mixture is preferably stirred until oxidation, exfoliation and reduction have occurred. Stirring is preferably continued for 5 to 50 minutes, more preferably for 20 to 40 minutes.

[0109] Step E) preferably lasts from 5 minutes to 60 minutes, more preferably from 15 minutes to 45 minutes. Such durations are a great improvement over prior art methods.

[0110] When the reaction is complete, step E) is ended.

[0111] In a preferred embodiment, the reduced graphene oxide obtained is treated to eliminate the remaining oxidant (step F). In the case of potassium permanganate, H2O2 is preferably used as shown below:

[0112] 2KMnO4+H2O2+3H2SO4→2MnSO4+O2+K2SO4+4H2O.

[0113] Then, an acid such as HCl, H2SO4, HNO3, and mixtures thereof may be added to eliminate by-products such as Mn2O7 and MnO2 formed during the oxidation / stripping / reduction steps.

[0114] After the reduced graphene oxide has been obtained, it is optionally rinsed with non-deionized or deionized water to reach a neutral pH (step G).

[0115] After the reduced graphene oxide has been obtained, it is optionally dried (step H). Drying can be carried out in particular with air, by sublimation drying, vacuum drying or freeze drying. Freeze drying is preferred because it further facilitates the separation of the rGO flakes.

[0116] By applying the method according to the invention, reduced graphene oxide (rGO) is obtained comprising one or several layers of graphene with 10 to 25 wt.-% oxygen.

[0117] According to one variation of the present invention, rGO is further reduced to microwave-reduced graphene oxide (MW-rGO) (step J).

[0118] Preferably, in step J), the catalyst is selected from: pristine graphene, graphene nanosheets, graphite, or graphite nanosheets. More preferably, the catalyst is pristine graphene. Without wishing to be bound by any theory, it is believed that due to the properties, form, and characteristics of pristine graphene, pristine graphene can better absorb electromagnetic fields in the form of microwaves. In fact, pristine graphene is a single layer of graphite composed of carbon bonded together in a hexagonal honeycomb lattice. It is an allotrope of carbon with a planar structure of atoms bonded in sp2, which attracts microwaves and can easily absorb them.

[0119] Preferably, the weight ratio of rGO to catalyst is from 50 to 1000. Advantageously, the weight ratio of rGO to catalyst is from 75 to 125. Due to such a ratio, the reduction of rGO to MW-rGO is further improved, resulting in MW-rGO with even fewer oxygen groups.

[0120] Preferably, the microwave frequency is 300 MHz to 100 GHz, preferably 1 GHz to 5 GHz, and for example 2.45 GHz.

[0121] Preferably, step J) is performed using a microwave frequency heating device, preferably a microwave oven.

[0122] Advantageously, the power of the microwave frequency heating device is from 100W to 100KW, more preferably from 100W to 2000W.

[0123] Preferably, the microwaving is performed for a period of at least 2 seconds, more preferably 2 seconds to 3600 seconds. This further improves the reduction of graphene oxide.

[0124] Thus, microwave reduced graphene oxide (MW-rGO) comprising one or several layers of graphene with less than 10 wt. %, more preferably less than 7 wt. % oxygen can be obtained.

[0125] Preferably, the reduced graphene oxide is deposited on a metal substrate to improve some properties of the metal substrate, such as corrosion resistance.

[0126] In another preferred embodiment, reduced graphene oxide is used as a cooling agent. In practice, the reduced graphene oxide can be added to a cooling fluid. Preferably, the cooling fluid can be selected from the group consisting of 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.

[0127] For example, the metal substrate is selected from the group consisting of: aluminum, steel, stainless steel, copper, iron, copper alloys, titanium, cobalt, metal composites, and nickel.

[0128] The present invention will now be described in further detail based on examples which are provided for reference only. The examples are not limitative. Example:

[0129] Kish graphite is obtained from steelmaking. Kish graphite is then sieved to classify by size as follows:

[0130] a) Kish graphite with a size below <63 μm, and

[0131] b) Kish graphite with a size greater than or equal to 63 μm.

[0132] The fraction a) of Kish graphite with a size below 63 μm was removed.

[0133] A flotation step was performed on fraction b) of Kish graphite with a size greater than or equal to 63 μm. The flotation step was performed using a Humboldt Wedag flotation cell with MIBC as a frother. The following conditions were applied: cell volume (l): 2, rotor speed (rpm): 2000, solids concentration (%): 5-10, frother type: MIBC, frother addition (g / T): 40, conditioning time (s): 10, and water conditions: natural pH, room temperature.

[0134] The Kish graphite was then leached with aqueous hydrochloric acid at a weight ratio of 0.5 acid / Kish graphite. It was then washed with deionized water and dried in air at 90°C. The purity of the pretreated Kish graphite was 95%.

[0135] Next, 10 g of Kish graphite was added to 30 mL of 98% H₂SO₄ in an open container. The reaction mixture was stirred continuously at room temperature for 15 minutes to obtain a homogeneous mixture. Then, still at room temperature, 30 g of ammonium persulfate ((NH₄)₂S₂O₄) was gradually added to the mixture for intercalation. The mixture was then homogenized by stirring for 5 minutes.

[0136] The mixture was then left in an open container at room temperature for 30 minutes, during which time expansion occurred.

[0137] Then, still at room temperature, 250 mL of H2SO4 98% was added to the mixture containing the expanded Kish graphite and then 0.5 g.s. -1 35 g of KMnO4 was added gradually to start the oxidation process, exfoliation and reduction simultaneously.

[0138] At the end of the stepwise addition of KMnO4, still at room temperature, the mixture was mechanically agitated for 30 minutes to complete the oxidation / stripping / reduction steps.

[0139] Then, 50 mL of H2O2 35% was added to eliminate the remaining KMnO4. Subsequently, 100 mL of HCl 36% was added to eliminate the Mn2O7 and MnO2 formed during the oxidation / stripping / reduction steps.

[0140] Finally, the mixture was neutralized with deionized water to pH 7 and freeze-dried to obtain rGO in powder form.

[0141] 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.

[0142] These analyses determined that the product obtained at the end of the process according to the invention is a reduced graphene oxide having 16% by weight of O and a weight ratio C / O of 4.95, very similar to the reduced graphene oxide obtained by the processes of the prior art.

[0143] Analysis by transmission electron microscopy also revealed that the reduced graphene oxide took the form of nanosheets, i.e., nanoscale objects with one nanometer-scale external dimension and two other significantly larger external dimensions. Specifically, the thickness was approximately two graphene layers, and the width and length were in the micrometer range. These results indicate that, compared to the prior art, the exfoliation process according to the present invention was superior and the lattice distortion was lower.

Claims

1. A method for producing reduced graphene oxide from Kish graphite, comprising: - Provide 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, - While the gas generated during the expansion step is still at least partially present, mixing the expanded Kish graphite with at least an acid and an oxidizing agent such that the expanded Kish graphite is simultaneously oxidized, exfoliated and reduced to reduced graphene oxide.

2. The method of claim 1, wherein the gas generated during the expansion step comprises O2.

3. The method according to claim 1 or 2, wherein the expanded Kish graphite is not cleaned before being mixed with at least an acid and an oxidizing agent.

4. The method according to claim 1 or 2, wherein the expansion is carried out in a closed container.

5. The method according to claim 1 or 2, wherein the expansion is carried out in an open container.

6. The method of claim 1 or 2, wherein the expanded Kish graphite is mixed with the at least acid and oxidizing agent less than eight hours after initiating the expansion step.

7. The method of claim 1 or 2, wherein the expanded Kish graphite is mixed with the at least acid and oxidizing agent less than one hour after starting the expansion step.

8. The method according to claim 1 or 2, wherein at least 5% by volume of the maximum amount by volume of the gas generated during the expansion step is still present in the expanded Kish graphite when the expanded Kish graphite is mixed with the at least acid and oxidizing agent.

9. The method according to claim 1 or 2, wherein at least 30% by volume of the maximum amount by volume of the gas generated during the expansion step is still present in the expanded Kish graphite when the expanded Kish graphite is mixed with the at least acid and oxidizing agent.

10. The method according to claim 1 or 2, wherein the expanded Kish graphite is first mixed with the acid and then the oxidizing agent is added stepwise. The method of claim 10 , wherein the addition of the oxidizing agent lasts from 30 seconds to 180 seconds.

12. The method according to claim 1 or 2, comprising the further step of mixing the reduced graphene oxide with H2O2 to eliminate remaining said oxidant.

13. The method of claim 1 or 2, comprising the further step of mixing the reduced graphene oxide with HCl, H2SO4, HNO3, or a mixture thereof to remove by-products formed during oxidation of the expanded Kish graphite.

14. The method according to claim 1 or 2, comprising the further step of rinsing the reduced graphene oxide with water.