EP3917880A1 - Method for producing activated carbon - Google Patents

Method for producing activated carbon

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Publication number
EP3917880A1
EP3917880A1 EP20704064.3A EP20704064A EP3917880A1 EP 3917880 A1 EP3917880 A1 EP 3917880A1 EP 20704064 A EP20704064 A EP 20704064A EP 3917880 A1 EP3917880 A1 EP 3917880A1
Authority
EP
European Patent Office
Prior art keywords
alkali metal
hydroxide
weight
lignin
chloride
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP20704064.3A
Other languages
German (de)
French (fr)
Inventor
Nikolai PONOMAREV
Mika SILLANPÄÄ
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Lappeenrannan Lahden Teknillinen Yliopisto LUT
Original Assignee
Lappeenrannan Lahden Teknillinen Yliopisto LUT
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lappeenrannan Lahden Teknillinen Yliopisto LUT filed Critical Lappeenrannan Lahden Teknillinen Yliopisto LUT
Publication of EP3917880A1 publication Critical patent/EP3917880A1/en
Pending legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/30—Active carbon
    • C01B32/312—Preparation
    • C01B32/342—Preparation characterised by non-gaseous activating agents
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/30—Active carbon
    • C01B32/312—Preparation

Definitions

  • the present invention relates to methods for producing activated carbon, in particular to methods comprising pyrolyzing carbonaceous materials with a mixture of alkali metal hydroxide and alkali metal chloride wherein the alkali metal is selected from sodium and potassium.
  • Activated carbon is carbon produced from carbonaceous source materials such as bamboo, coconut husk, willow peat, wood, coir, lignite, coal, petroleum pitch, cellulose and lignin.
  • Activated carbon is typically produced by physical activation or by chemical activation.
  • Drawbacks of physical activation are preliminary carbonization and a high temperature of activation.
  • preliminary pyrolysis usually is not needed.
  • hazardous properties of some chemicals and high chemical consumption make this method challenging.
  • CN106185921 A disclosed a method for preparing porous carbon material using NaCI as a template and kraft lignin as a starting material.
  • the lignin was mixed with NaCI with a lignin to NaCI ratio of 1 :10 by weight.
  • the mixture was then dried at 60- 100 °C and preliminary carbonized at temperature 400-600 °C.
  • the obtained composite was washed using distilled water and dried.
  • the material was mixed with KOH with a mass ratio of 1 :4. Then, the mixture was pyrolyzed at 850 °C and after cooling the obtained carbide was washed using 1 -12 mol/L of HCI.
  • an inorganic part comprising alkali metal carbonate (M 2 CO3) and alkali metal chloride (MCI), and
  • step c) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, alkali metal hydroxide (MOH), and alkali metal chloride (MCI),
  • step f recycling the admixture comprising the alkali metal hydroxide (MOH) and the alkali metal chloride (MCI) to step a).
  • MOH alkali metal hydroxide
  • MCI alkali metal chloride
  • MOH alkali metal hydroxide
  • MCI alkali metal chloride
  • figure 1 shows an exemplary non-limiting embodiment for producing activated carbon, and recycling sodium hydroxide and sodium chloride using the method of the present invention
  • figure 2 shows an exemplary non-limiting embodiment for producing activated carbon, and recycling potassium hydroxide and potassium chloride using the method of the present invention
  • figure 3A shows N2 adsorption-desorption isotherms of activated carbon prepared according to the method of the present invention and according to prior art
  • figure 3B shows NLDFT pore size distribution and SEM images of activated carbons prepared using the method of the present invention and according to prior art
  • figure 4A shows XRD spectra of samples prepared using NaCI, NaOH or
  • figure 4B shows SEM micrographs of samples prepared using NaCI, NaOH or NaOH/NaCI before leaching
  • figure 5A shows FTIR spectra of carbons prepared using NaCI, NaOH and
  • figure 5B shows TGA/DTG curves of carbons prepared using NaCI, NaOH and NaOH/NaCI
  • figure 6A shows N2 adsorption-desorption isotherms of prepared carbons using various NaOH/NaCI ratio
  • figure 6B shows BET and yields prepared carbons using various NaOH/NaCI ratio
  • figure 7A shows the FTIR spectra of raw lignin (a), dried mixtures of lignin-NaCI and lignin-NaOH; guaiacyl propane unit (b); reactions of destruction (c); condensation (d) of lignin in alkaline media
  • figure 7B shows SEM images of lignin-NaOH (e) and SEM images of lignin-NaCI (f) mixtures.
  • the present invention concerns a method for producing activated carbon using a mixture of NaOH and NaCI as the activation agent.
  • the method comprises the following steps:
  • an inorganic part comprising sodium carbonate and sodium chloride, and c) separating the activated carbon and the inorganic part.
  • lignin is used as an exemplary non limiting carbonaceous material.
  • lignin is admixed 1 with a mixture of aqueous sodium hydroxide and sodium chloride to form an admixture as a slurry.
  • the slurry is extruded 2 and transferred to a pyrolysis furnace.
  • the method comprises one or more of: swelling, extruding and drying the admixture of step a) prior to step b).
  • Pyrolysis 3 is performed preferably under inert or at least low O2 content atmosphere.
  • the pyrolysis produces activated carbon together with sodium carbonate and carbon dioxide.
  • the inorganic part comprising the sodium carbonate produced, and the remaining sodium chloride is removed by leaching 4, and the isolated activated carbon is dried 5.
  • the method of the present invention is also suitable for recycling the sodium hydroxide and the sodium chloride used as the activating agent in the process.
  • the recycling comprises
  • step c) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, sodium hydroxide, and sodium chloride,
  • the recycling comprises treating 6 the inorganic part comprising sodium carbonate and sodium chloride with calcium hydroxide to form an admixture comprising precipitated calcium carbonate.
  • the precipitate is then separated 7, and the remaining mixture of sodium hydroxide and sodium chloride is recycled to the admixing 1.
  • the activating agent comprises sodium hydroxide and sodium chloride.
  • the NaOH/NaCI ratio is typically from 1/9 by weight to 3/2 by weight, preferably from 1/4 by weight to 1/1 by weight, most preferably 2/3 by weight.
  • the NaOH/NaCI ratio is 2/3 by weight and the ratio (mixture of NaOH and NaCI)/(carbonaceous material) is 2/3 by weight.
  • a particular carbonaceous material is lignin, preferably hydrolytic lignin.
  • the carbonaceous material such as lignin is mixed with a solution of NaOH and NaCI in deionised water.
  • the obtained admixture is preferably allowed to swell e.g. at room temperature.
  • An exemplary swelling time is 1 h.
  • After swelling the admixture is preferably extruded and also dried.
  • An exemplary drying temperature is 130 °C, and an exemplary drying time is 16 h.
  • the dried material is grinded to produce granules.
  • a typical size of granules is 1 -2 mm.
  • the method comprises pyrolysis of the admixture comprising the carbonaceous material, sodium hydroxide and sodium chloride, preferably pyrolysis of a granulized admixture obtained as described above.
  • the pyrolysis is performed in a tubular furnace.
  • the pyrolysis is performed preferably in inert atmosphere at temperature from 600 °C to 1000 °C, preferably from 700 °C to 900 °C, more preferably from 850 °C to 900 °C.
  • Typical time for pyrolysis is 30-120 min.
  • a particular temperature is 868 °C
  • a particular time for pyrolysis is 47 min when lignin is the carbonaceous material.
  • the inorganic part is leached from the activated carbon preferably with water or with aqueous acid, such as aqueous mineral acid.
  • aqueous mineral acid is hydrochloric acid, such as 0.1 M HCI.
  • the activated carbon is isolated from the solution using a Biichner funnel and washing several times with water until neutral pH.
  • the activated carbon isolated is preferably dried. An exemplary drying is at 130 °C for 16 h.
  • the present invention concerns a method for producing activated carbon using a mixture of KOH and KCI as the activation agent.
  • the method comprises the following steps:
  • lignin is used as an exemplary non limiting carbonaceous material.
  • lignin is admixed 1 with a mixture of aqueous potassium hydroxide and potassium chloride to form an admixture as a slurry.
  • the slurry is extruded 2 and transferred to a pyrolysis furnace.
  • the method comprises one or more of: swelling, extruding and drying the admixture of step a) prior to step b).
  • Pyrolysis 3 is performed preferably under inert or at least low O2 content atmosphere.
  • the pyrolysis produces activated carbon together with potassium carbonate and carbon dioxide.
  • the inorganic part comprising the potassium carbonate produced, and the remaining potassium chloride is removed by leaching 4, and the isolated activated carbon is dried 5.
  • the method is also suitable for recycling the potassium hydroxide and the potassium chloride used as the activating agent in the process. According to this embodiment, the recycling comprises
  • step c) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, potassium hydroxide, and potassium chloride,
  • the recycling comprises treating 6 the inorganic part comprising potassium carbonate and potassium chloride with calcium hydroxide to form an admixture comprising precipitated calcium carbonate.
  • the precipitate is then separated 7, and the remaining mixture of potassium hydroxide and potassium chloride is recycled to the admixing 1.
  • the present invention concerns a method for producing activated carbon using a mixture of KOH and NaCI as the activation agent.
  • the method comprises the following steps:
  • an inorganic part comprising potassium carbonate and sodium chloride, and c) separating the activated carbon and the inorganic part.
  • the method is also suitable for recycling the potassium hydroxide and the sodium chloride used as the activating agent in the process.
  • the recycling comprises
  • step c) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, potassium hydroxide, and sodium chloride,
  • the present invention concerns a method for producing activated carbon using a mixture of NaOH and KCI as the activation agent.
  • the method comprises the following steps:
  • an inorganic part comprising sodium carbonate and potassium chloride, and c) separating the activated carbon and the inorganic part.
  • the method is also suitable for recycling the sodium hydroxide and the potassium chloride used as the activating agent in the process.
  • the recycling comprises
  • step c) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, sodium hydroxide, and potassium chloride,
  • the calcium carbonate formed is isolated and decomposed thermally 8 to produce calcium oxide and carbon dioxide.
  • Treatment of calcium oxide with water 9 produces calcium hydroxide, which is then recycled to the treating step 6.
  • the method thus further comprises
  • chemical reactions related recycling calcium hydroxide comprises the following chemical reactions:
  • Exemplary carbonaceous materials suitable for the method are bamboo, coconut husk, willow peat, wood, coir, lignite, coal, petroleum pitch, cellulose and lignin.
  • a particular carbonaceous material is lignin such as hydrolytic lignin and kraft lignin.
  • An exemplary kraft lignin is BioPiva 100 of UPM.
  • Still another exemplary kraft lignin is precipitated kraft lignin from black liquor.
  • Another particular carbonaceous material is cellulose, preferably microcrystalline cellulose.
  • An exemplary microcrystalline cellulose is AaltoCellTM. Preparation of microcrystalline cellulose has been disclosed in EP 2576629.
  • Still another particular carbonaceous material is sewage sludge of pulp and/or paper.
  • the carbonaceous material is may be powdered, ground of milled prior to use in the method. Also, mixtures of two or more carbonaceous materials can be used.
  • the activating agent comprises alkali metal hydroxide (MOH) and alkali metal chloride (MCI).
  • MOH/MCI ratio is typically from 1/9 by weight to 3/2 by weight, preferably from 1/4 by weight to 1/1 by weight, most preferably 2/3 by weight.
  • biomass of the carbonaceous material is preferably from 1/1 by weight to 0.5/1 by weight, more preferably 0.6/1 by weight.
  • the alkali metal is sodium and NaOH/NaCI ratio is 2/3 by weight and the ratio (mixture of NaOH and NaCI)/(carbonaceous material) is 2/3 by weight.
  • a particular carbonaceous material is lignin, preferably hydrolytic lignin.
  • the alkali metal is potassium and KOH/KCI ratio is 2/3 by weight and the ratio (mixture of KOH and KCI)/(carbonaceous material) is 2/3 by weight.
  • a particular carbonaceous material is lignin, preferably hydrolytic lignin.
  • the alkali metal hydroxide is potassium hydroxide and the alkali metal chloride is sodium chloride KOH/NaCI ratio is 2/3 by weight and the ratio (mixture of KOH and NaCI)/(carbonaceous material) is 2/3 by weight.
  • a particular carbonaceous material is lignin, preferably hydrolytic lignin.
  • the alkali metal hydroxide is sodium hydroxide and the alkali metal chloride is potassium chloride NaOH/KCI ratio is 2/3 by weight and the ratio (mixture of NaOH and KCI)/(carbonaceous material) is 2/3 by weight.
  • a particular carbonaceous material is lignin, preferably hydrolytic lignin.
  • the carbonaceous material such as lignin is mixed with a solution of alkali metal hydroxide (MOH) and alkali metal chloride (MCI) wherein the alkali metal is sodium or potassium in deionised water.
  • MOH alkali metal hydroxide
  • MCI alkali metal chloride
  • the obtained admixture is preferably allowed to swell e.g. at room temperature.
  • An exemplary swelling time is 1 h.
  • After swelling the admixture is preferably extruded and also dried.
  • An exemplary drying temperature is 130 °C, and an exemplary drying time is 16 h.
  • the raw materials for the preparation of activated carbon were hydrolytic lignin, microcrystalline cellulose (MCC)-AaltoCellTM and precipitated kraft lignin from black liquor.
  • the industrial acid hydrolytic lignin from coniferous wood was used (200 pm with 8.5 % moisture content).
  • Microcrystalline cellulose was prepared according to EP 2576629.
  • the kraft lignin was obtained from the black liquor by the following procedure. 500 ml_ of black liquor was mixed with 250 ml_ of 10% w/w HCI and vigorously stirred. The obtained precipitate was isolated from the solution using a Buchner funnel and washed several times with water. The obtained kraft lignin was dried at 130 °C for 16 h.
  • the functional groups of the material were studied using Fourier transform infrared spectroscopy (FTIR) on a Bruker Vertex 70.
  • FTIR Fourier transform infrared spectroscopy
  • the crystalline structure of the material was evaluated using X-ray powder diffraction (XRD) on a high-resolution PANalytical diffractometer. The scans were recorded employing Co Ka radiation at a voltage of 40 kV from 10 ° to 100 ° 20 angles.
  • the crystalline size was calculated using XRD data applying Williamson-Flall (W-FI) method.
  • W-FI Williamson-Flall
  • the structure of the material was studied employing scanning electron microscopy (SEM) on a Hitachi S-4800. Thermogravimetric analyses (TGA) was performed on NETZSCFI TG thermal analyser at a heating rate 10 °C/min from 23 °C to 1000 °C. The yield was calculated using values of raw material and final product masses, respectively.
  • the process variables such as target temperature A (°C), residence time B (min) and lignin to NaOH/NaCI ratio C (g/g) were considered as factors. Three levels of factors, including zero level, were ascribed for each parameter. The minimum, zero and maximum levels are temperature 600, 700 and 800 °C; time 30, 60 and 90 min; lignin to NaOH/NaCI ratio 0.5, 1 .0 and 1 .5 g/g.
  • the BET surface area Y1 (m 2 /g) and yield Y2 (%) were selected as responses for process optimization. To evaluate the reproducibility and validity of experimental findings three repetitions were performed on a zero level. The design of experiments is demonstrated in Table 1 . Experimental data were analysed by RSM using Design Expert 1 1 software and ANOVA analysis was performed to evaluate the adequacy of the models.
  • a. g/g mass ratio (NaOH/NaCI)/lignin
  • the regression coefficients of BET model are increased with increasing of factor values.
  • the yield is decreasing while factors are increasing that can be concluded from the negative values of regression coefficients.
  • the factor of lignin to NaOH/NaCI ratio (C) is the most essential factor since the regression coefficients values are the highest.
  • the influence of temperature (A) on responses is also significant, while time (B) demonstrates the weakest effect.
  • the maximum cannot be achieved for both responses because with increasing of one response another is decreasing. More specifically, the maximum BET surface area appears at 800 °C and 1.5 g/g, while the maximum yield is observed at the minimum temperature 600 °C and 0.5 g/g of ratio.
  • increasing of factors has a positive effect on BET surface whereas the influence on yield is negative.
  • the increasing of ratio, temperature and time lead to the enlarged surface area since the formation of porous texture is occurred in the same manner. Meanwhile, the yield is decreased due to the transformation of micropores to meso- and macropores with the further void formation.
  • the optimal values of factors can be predicted.
  • desirability function has been employed.
  • Table 4 shows predicted and obtained responses at optimal factors.
  • Activated carbons prepared at optimal factors in three repetitions to evaluate reproducibility and difference of predicted and obtained values are negligible indicating additionally validity of the model.
  • the BET surface area of the obtained activated carbons is relatively the same while lignin to activation agent ratio is remarkably lower. Moreover, the NaOH consumption is five times lower compared to the previously reported methods. The discrepancies of yield can be explained by reduced consumption of N2 flow using in experiments.
  • Figure 3A shows adsorption isotherms of carbons prepared after use of NaOH, NaCI and NaOH/NaCI.
  • Adsorption isotherms are described according to the lUPAC report (Tanss, M. et al. (2015) Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (lUPAC Technical Report), Pure and Applied Chemistry, 87(9-10), pp. 1051-1069. doi: 10.1515/pac-2014- 1 1 17).
  • Isotherm curvature of carbon prepared from the mixture of lignin-NaCI exhibits to composite type I, II assigned to micro- macroporous materials.
  • Activated carbons produced of hydrolytic lignin using NaOH or KOH activation are associated with microporous materials.
  • the isotherm of carbon prepared using a mixture of NaOH and NaCI is also coincided with combined isotherm type I, II, since enhanced N2 uptake is observed at pressures below 0.1 p/p° associated with micropore filling, while ascended curvature of the plateau is inherent for macropores.
  • the adsorption hysteresis of carbons prepared using NaOH is attributed to H4 type and accompanied with isotherms I or composite type I, II indicating micropore filling.
  • the hysteresis loop of carbon prepared using only NaCI is associated with isotherm type II of macropores materials. Adsorption capacities of carbons prepared using only NaCI or NaOH are similar whereas N2 uptake of carbons prepared using NaOH/NaCI mixture is more pronounced.
  • the texture properties of carbons prepared using various mixtures are summarized in Table 6.
  • the samples prepared using only NaOH or NaCI has a relatively similar total volume of pores and BET surface area, while carbon prepared using a mixture of NaOH/NaCI demonstrated significantly higher values.
  • the BET surface area and micropore volume of carbon prepared using NaOH/NaCI are larger compared to NaOH or NaCI activated carbons.
  • the total pore volume of carbon prepared using NaCI is caused by micro and macropore formation, while in case of NaOH micro porosity mainly deals with its origin.
  • Table 6 Texture properties and yield of carbons prepared using a various amount of NaOH and/or NaCI.
  • the product of assumed chemical reaction (1 ) is sodium carbonate that can be detected by analytical methods.
  • the characteristic peaks of Na 2 C0 3 are found in XRD spectra of samples prepared using NaOH (Fig. 4A).
  • the corresponding peaks of Na 2 C0 3 are indexed to reference number ICSD98-008-0985 of standard XRD pattern of sodium carbonate.
  • the SEM micrograph of the sample prepared using NaOH before leaching shows different structure compare to NaCI contained samples.
  • the formed crystals shapes are inherent for sodium carbonate.
  • the peaks at 1423, 1426 cm -1 and 832, 876 cm -1 attributed to carbonates are found on FTIR spectra (Fig.
  • Adsorption isotherms of carbons prepared using different NaOFI to NaCI ratio are presented in Figure 6A. Starting from 1/9 ratio to 3/7 ratio adsorption isotherms exhibit Type I of microporous materials. With increasing of NaOFI presence from 4/6 to 6/4 isotherms demonstrated ascended curvature and pronounced hysteresis H4 that more intrinsic for composite isotherm I, II indicating the development of the porous structure. In particular, the slope of isotherm pointing to macropores formation, while hysteresis H4 type is attributed to micropores. The micropore volume and total volume is significantly increased starting from 4/6 ratio lead to enlarged BET surface area (Table 8). Carbons prepared using NaOFI to NaCI ratio from 1/9 to 3/7 are assigned to microporous materials, while samples prepared using 4/6 to 6/4 ratio are also significantly meso- macroporous.
  • a Micropore volume is calculated by DR equation.
  • b Mesopore volume is calculated as a difference between Vtotai and Vmicro.
  • the strong peaks located at 3341 cm -1 and 2934 cm -1 are assigned to stretching vibrations of OH- and C-H in -CH3 or -CH2-, respectively.
  • the significant changes are observed for the following peaks: 1706 cm -1 - aromatic C-H n stretching; 1270 cm -1 - C-O-C stretching of aryl- alkyl ether bond; 1210 cm 1 -C-0 stretching of guaiacyl ring; 1154 cm -1 and 1110 cm -1 - stretching vibrations in guaiacyl propane unit.
  • the mentioned changes are assigned to linkages, which take part in the reaction of destruction (Fig. 7A, c) and condensation (Fig. 7A, d).
  • the peaks at 1420 cm -1 , 884 cm -1 and 776 cm -1 could be indexed to formed carbonate by captured CO2 from the air.
  • FTIR peaks of lignin are not observed for pyrolyzed carbons (Fig.4A) whatever NaOFI or NaCI was used indicating essential degradation of lignin and formation of carbon like structure.
  • Table 10 summarizes the properties of activated carbons prepared by the method of the present invention, namely hydrolytic lignin, kraft lignin and microcrystalline cellulose and optimized process parameters.
  • the method of the present invention has the following advantages
  • the carbonaceous material is directly mixed with MOFI/MCI, wherein M is Na or K, and pyrolyzed in one single step. This led to reduction of operation steps, energy and water consumption.
  • the optimized carbonaceous material such as lignin to NaOFI/NaCI ratio is 1 :0.6 (1 :0.24 of only NaOFI) w/w.
  • the material is micro- macroporous. There is the accessibility of micropores through the macropores.
  • the material can be easily granulated/extruded without additional binder because of chemical interaction between lignin and NaOH.

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Abstract

The present invention relates to methods for producing activated carbon, in particular to methods comprising treating carbonaceous materials, in particular lignin with the mixture of alkali metal hydroxide and alkali metal chloride, where the alkali metal is selected from sodium and potassium. The invention relates also to a method for recycling the alkali metal hydroxide and the alkali metal chloride in the method.

Description

METHOD FOR PRODUCING ACTIVATED CARBON
FIELD
The present invention relates to methods for producing activated carbon, in particular to methods comprising pyrolyzing carbonaceous materials with a mixture of alkali metal hydroxide and alkali metal chloride wherein the alkali metal is selected from sodium and potassium.
BACKGROUND
Activated carbon is carbon produced from carbonaceous source materials such as bamboo, coconut husk, willow peat, wood, coir, lignite, coal, petroleum pitch, cellulose and lignin.
Activated carbon is typically produced by physical activation or by chemical activation. Drawbacks of physical activation are preliminary carbonization and a high temperature of activation. In the case of chemical activation, preliminary pyrolysis usually is not needed. However, hazardous properties of some chemicals and high chemical consumption make this method challenging.
One of the newest approaches includes the use of templated carbon. For example, CN106185921 A disclosed a method for preparing porous carbon material using NaCI as a template and kraft lignin as a starting material. The lignin was mixed with NaCI with a lignin to NaCI ratio of 1 :10 by weight. The mixture was then dried at 60- 100 °C and preliminary carbonized at temperature 400-600 °C. The obtained composite was washed using distilled water and dried. Afterwards, the material was mixed with KOH with a mass ratio of 1 :4. Then, the mixture was pyrolyzed at 850 °C and after cooling the obtained carbide was washed using 1 -12 mol/L of HCI.
However, there is still need for further methods for producing activated carbon.
SUMMARY
The following presents a simplified summary in order to provide a basic understanding of some aspects of various embodiments of the invention. The summary is not an extensive overview of the invention. It is neither intended to identify key nor critical elements of the invention, nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.
It was observed that when a mixture of alkali metal hydroxide and alkali metal chloride wherein the alkali metal is selected from sodium and potassium, was used as an activation agent in a method for producing activated carbon, activated carbon with large surface area was obtained. Furthermore, the cost of the activated carbon could be reduced since the consumption of the chemicals can be reduced.
In accordance with the invention, there is provided a new method for producing activated carbon, the method comprising the following steps:
a) admixing carbonaceous material with alkali metal hydroxide (MOH) and alkali metal chloride (MCI), wherein the alkali metal (M) is selected from sodium (Na) and potassium (K) to form an admixture,
b) pyrolyzing the admixture to yield a mixture comprising
• activated carbon, and
• an inorganic part comprising alkali metal carbonate (M2CO3) and alkali metal chloride (MCI), and
c) separating activated carbon and the inorganic part.
In accordance with the invention, there is also provided a new method for regenerating alkali metal hydroxide (MOH) and alkali metal chloride (MCI), wherein the alkali metal (M) is selected from sodium (Na) and potassium (K) in the method for producing activated carbon, the method comprising
d) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, alkali metal hydroxide (MOH), and alkali metal chloride (MCI),
e) removing the calcium carbonate from the admixture, and
f) recycling the admixture comprising the alkali metal hydroxide (MOH) and the alkali metal chloride (MCI) to step a).
In accordance with the invention, there is also provided a new use for a mixture of alkali metal hydroxide (MOH) and alkali metal chloride (MCI), wherein the alkali metal (M) is selected from sodium (Na) and potassium (K) as an activation agent in a method for producing activated carbon.
A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.
Various exemplifying and non-limiting embodiments of the invention and to methods of operation, together with additional objects and advantages thereof, are best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying figures.
The verbs“to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.
BRIEF DESCRIPTION OF FIGURES The exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below with reference to the accompanying figures, in which figure 1 shows an exemplary non-limiting embodiment for producing activated carbon, and recycling sodium hydroxide and sodium chloride using the method of the present invention, figure 2 shows an exemplary non-limiting embodiment for producing activated carbon, and recycling potassium hydroxide and potassium chloride using the method of the present invention, figure 3A shows N2 adsorption-desorption isotherms of activated carbon prepared according to the method of the present invention and according to prior art, figure 3B shows NLDFT pore size distribution and SEM images of activated carbons prepared using the method of the present invention and according to prior art, figure 4A shows XRD spectra of samples prepared using NaCI, NaOH or
NaOH/NaCI before leaching, figure 4B shows SEM micrographs of samples prepared using NaCI, NaOH or NaOH/NaCI before leaching, figure 5A shows FTIR spectra of carbons prepared using NaCI, NaOH and
NaOH/NaCI, figure 5B shows TGA/DTG curves of carbons prepared using NaCI, NaOH and NaOH/NaCI, figure 6A shows N2 adsorption-desorption isotherms of prepared carbons using various NaOH/NaCI ratio, figure 6B shows BET and yields prepared carbons using various NaOH/NaCI ratio, figure 7A shows the FTIR spectra of raw lignin (a), dried mixtures of lignin-NaCI and lignin-NaOH; guaiacyl propane unit (b); reactions of destruction (c); condensation (d) of lignin in alkaline media, and figure 7B shows SEM images of lignin-NaOH (e) and SEM images of lignin-NaCI (f) mixtures.
DESCRIPTION
According to one embodiment the present invention concerns a method for producing activated carbon using a mixture of NaOH and NaCI as the activation agent. The method comprises the following steps:
a) admixing carbonaceous material with sodium hydroxide and sodium chloride to form an admixture,
b) pyrolyzing the admixture to yield
• activated carbon, and
· an inorganic part comprising sodium carbonate and sodium chloride, and c) separating the activated carbon and the inorganic part.
The method is shown in figure 1. In the figure, lignin is used as an exemplary non limiting carbonaceous material. According to the method, lignin is admixed 1 with a mixture of aqueous sodium hydroxide and sodium chloride to form an admixture as a slurry. The slurry is extruded 2 and transferred to a pyrolysis furnace.
Thus, according to a particular embodiment shown in the figure, the method comprises one or more of: swelling, extruding and drying the admixture of step a) prior to step b).
Pyrolysis 3 is performed preferably under inert or at least low O2 content atmosphere. The pyrolysis produces activated carbon together with sodium carbonate and carbon dioxide. The inorganic part comprising the sodium carbonate produced, and the remaining sodium chloride is removed by leaching 4, and the isolated activated carbon is dried 5.
The method of the present invention is also suitable for recycling the sodium hydroxide and the sodium chloride used as the activating agent in the process. According to this embodiment, the recycling comprises
d) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, sodium hydroxide, and sodium chloride,
e) removing the calcium carbonate from the admixture, and
f) recycling the admixture comprising sodium hydroxide and sodium chloride to step a).
As shown in figure 1 , the recycling comprises treating 6 the inorganic part comprising sodium carbonate and sodium chloride with calcium hydroxide to form an admixture comprising precipitated calcium carbonate. The precipitate is then separated 7, and the remaining mixture of sodium hydroxide and sodium chloride is recycled to the admixing 1.
Without binding to any theory, chemical reactions related to the above described processes can be described by
• formation of sodium carbonate and carbon dioxide during pyrolysis:
6NaOH + 2C = 2Na + 2Na2C03 + 3H2 (a)
4NaOH + C = 4Na + C02 + 2H20 (b)
• regeneration of sodium hydroxide by using calcium hydroxide Ca(OH)2 + Na2C03 = CaCOs + 2NaOH (c)
According to the method of the present invention, the activating agent comprises sodium hydroxide and sodium chloride. The NaOH/NaCI ratio is typically from 1/9 by weight to 3/2 by weight, preferably from 1/4 by weight to 1/1 by weight, most preferably 2/3 by weight. The ratio of the mixture of sodium hydroxide and sodium chloride and the carbonaceous material, i.e.
(mass of the mixture of NaOH and NaCI)
(mass of the carbonaceous material)
is preferably from 1/1 by weight to 0.5/1 by weight, more preferably 0.6/1 by weight. According to a particular embodiment, the NaOH/NaCI ratio is 2/3 by weight and the ratio (mixture of NaOH and NaCI)/(carbonaceous material) is 2/3 by weight. A particular carbonaceous material is lignin, preferably hydrolytic lignin.
According to an exemplary embodiment, the carbonaceous material, such as lignin is mixed with a solution of NaOH and NaCI in deionised water. The obtained admixture is preferably allowed to swell e.g. at room temperature. An exemplary swelling time is 1 h. After swelling the admixture is preferably extruded and also dried. An exemplary drying temperature is 130 °C, and an exemplary drying time is 16 h.
According to a preferable embodiment the dried material is grinded to produce granules. A typical size of granules is 1 -2 mm.
According to one embodiment the method comprises pyrolysis of the admixture comprising the carbonaceous material, sodium hydroxide and sodium chloride, preferably pyrolysis of a granulized admixture obtained as described above. According to an exemplary embodiment, the pyrolysis is performed in a tubular furnace. The pyrolysis is performed preferably in inert atmosphere at temperature from 600 °C to 1000 °C, preferably from 700 °C to 900 °C, more preferably from 850 °C to 900 °C. Typical time for pyrolysis is 30-120 min. A particular temperature is 868 °C, and a particular time for pyrolysis is 47 min when lignin is the carbonaceous material.
After the pyrolysis, the inorganic part is leached from the activated carbon preferably with water or with aqueous acid, such as aqueous mineral acid. Exemplary aqueous mineral acid is hydrochloric acid, such as 0.1 M HCI. According an exemplary embodiment the activated carbon is isolated from the solution using a Biichner funnel and washing several times with water until neutral pH. The activated carbon isolated is preferably dried. An exemplary drying is at 130 °C for 16 h.
According to another embodiment the present invention concerns a method for producing activated carbon using a mixture of KOH and KCI as the activation agent. The method comprises the following steps:
a) admixing carbonaceous material with potassium hydroxide and potassium chloride to form an admixture,
b) pyrolyzing the admixture to yield
• activated carbon, and
• an inorganic part comprising potassium carbonate and potassium chloride, and
c) separating the activated carbon and the inorganic part.
The method is shown in figure 2. In the figure, lignin is used as an exemplary non limiting carbonaceous material. According to the method, lignin is admixed 1 with a mixture of aqueous potassium hydroxide and potassium chloride to form an admixture as a slurry. The slurry is extruded 2 and transferred to a pyrolysis furnace.
Thus, according to a particular embodiment shown in the figure, the method comprises one or more of: swelling, extruding and drying the admixture of step a) prior to step b).
Pyrolysis 3 is performed preferably under inert or at least low O2 content atmosphere. The pyrolysis produces activated carbon together with potassium carbonate and carbon dioxide. The inorganic part comprising the potassium carbonate produced, and the remaining potassium chloride is removed by leaching 4, and the isolated activated carbon is dried 5. The method is also suitable for recycling the potassium hydroxide and the potassium chloride used as the activating agent in the process. According to this embodiment, the recycling comprises
d) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, potassium hydroxide, and potassium chloride,
e) removing the calcium carbonate from the admixture, and
f) recycling the admixture comprising potassium hydroxide and potassium chloride to step a).
As shown in figure 2, the recycling comprises treating 6 the inorganic part comprising potassium carbonate and potassium chloride with calcium hydroxide to form an admixture comprising precipitated calcium carbonate. The precipitate is then separated 7, and the remaining mixture of potassium hydroxide and potassium chloride is recycled to the admixing 1.
Without binding to any theory, chemical reactions related to the above described processes can be described by
• formation of potassium carbonate and carbon dioxide during pyrolysis:
6KOH + 2C = 2K + 2K2CO3 + 3H2 (a)
4KOH + C = 4K + C02 + 2H20 (b)
• regeneration of potassium hydroxide by using calcium hydroxide
Ca(OH)2 + K2CO3 = CaCOs + 2KOH (c) According to still another embodiment the present invention concerns a method for producing activated carbon using a mixture of KOH and NaCI as the activation agent. The method comprises the following steps:
a) admixing carbonaceous material with potassium hydroxide and sodium chloride to form an admixture,
b) pyrolyzing the admixture to yield
• activated carbon, and
• an inorganic part comprising potassium carbonate and sodium chloride, and c) separating the activated carbon and the inorganic part.
The method is also suitable for recycling the potassium hydroxide and the sodium chloride used as the activating agent in the process. According to this embodiment, the recycling comprises
d) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, potassium hydroxide, and sodium chloride,
e) removing the calcium carbonate from the admixture, and
f) recycling the admixture comprising potassium hydroxide and sodium chloride to step a).
According to still another embodiment the present invention concerns a method for producing activated carbon using a mixture of NaOH and KCI as the activation agent. The method comprises the following steps:
a) admixing carbonaceous material with sodium hydroxide and potassium chloride to form an admixture,
b) pyrolyzing the admixture to yield
• activated carbon, and
• an inorganic part comprising sodium carbonate and potassium chloride, and c) separating the activated carbon and the inorganic part.
The method is also suitable for recycling the sodium hydroxide and the potassium chloride used as the activating agent in the process. According to this embodiment, the recycling comprises
d) treating the inorganic part of step c) with calcium hydroxide to produce an admixture comprising calcium carbonate, sodium hydroxide, and potassium chloride,
e) removing the calcium carbonate from the admixture, and
f) recycling the admixture comprising sodium hydroxide and potassium chloride to step a).
According to a preferable embodiment, the calcium carbonate formed is isolated and decomposed thermally 8 to produce calcium oxide and carbon dioxide. Treatment of calcium oxide with water 9 produces calcium hydroxide, which is then recycled to the treating step 6. According to this embodiment, the method thus further comprises
g) converting calcium carbonate to calcium oxide,
h) treating the calcium oxide with water to produce calcium hydroxide, and i) recycling the calcium hydroxide to step d)
Without binding to any theory, chemical reactions related recycling calcium hydroxide comprises the following chemical reactions:
CaCC>3 = CaO + CO2 (d)
CaO + H2O = Ca(OH)2 (e)
Exemplary carbonaceous materials suitable for the method are bamboo, coconut husk, willow peat, wood, coir, lignite, coal, petroleum pitch, cellulose and lignin. A particular carbonaceous material is lignin such as hydrolytic lignin and kraft lignin. An exemplary kraft lignin is BioPiva 100 of UPM. Still another exemplary kraft lignin is precipitated kraft lignin from black liquor.
Another particular carbonaceous material is cellulose, preferably microcrystalline cellulose. An exemplary microcrystalline cellulose is AaltoCell™. Preparation of microcrystalline cellulose has been disclosed in EP 2576629.
Still another particular carbonaceous material is sewage sludge of pulp and/or paper.
The carbonaceous material is may be powdered, ground of milled prior to use in the method. Also, mixtures of two or more carbonaceous materials can be used.
According to the method of the present invention, the activating agent comprises alkali metal hydroxide (MOH) and alkali metal chloride (MCI). The MOH/MCI ratio is typically from 1/9 by weight to 3/2 by weight, preferably from 1/4 by weight to 1/1 by weight, most preferably 2/3 by weight. The ratio of the mixture of alkali metal hydroxide and alkali metal chloride and the carbonaceous material, i.e.
(mass of the mixture of MOH and MCI)
(mass of the carbonaceous material) is preferably from 1/1 by weight to 0.5/1 by weight, more preferably 0.6/1 by weight.
According to a particular embodiment, the alkali metal is sodium and NaOH/NaCI ratio is 2/3 by weight and the ratio (mixture of NaOH and NaCI)/(carbonaceous material) is 2/3 by weight. A particular carbonaceous material is lignin, preferably hydrolytic lignin.
According to another particular embodiment, the alkali metal is potassium and KOH/KCI ratio is 2/3 by weight and the ratio (mixture of KOH and KCI)/(carbonaceous material) is 2/3 by weight. A particular carbonaceous material is lignin, preferably hydrolytic lignin.
According to still another particular embodiment, the alkali metal hydroxide is potassium hydroxide and the alkali metal chloride is sodium chloride KOH/NaCI ratio is 2/3 by weight and the ratio (mixture of KOH and NaCI)/(carbonaceous material) is 2/3 by weight. A particular carbonaceous material is lignin, preferably hydrolytic lignin.
According to still another particular embodiment, the alkali metal hydroxide is sodium hydroxide and the alkali metal chloride is potassium chloride NaOH/KCI ratio is 2/3 by weight and the ratio (mixture of NaOH and KCI)/(carbonaceous material) is 2/3 by weight. A particular carbonaceous material is lignin, preferably hydrolytic lignin.
According to an exemplary embodiment, the carbonaceous material, such as lignin is mixed with a solution of alkali metal hydroxide (MOH) and alkali metal chloride (MCI) wherein the alkali metal is sodium or potassium in deionised water. The obtained admixture is preferably allowed to swell e.g. at room temperature. An exemplary swelling time is 1 h. After swelling the admixture is preferably extruded and also dried. An exemplary drying temperature is 130 °C, and an exemplary drying time is 16 h.
Experimental
Materials The raw materials for the preparation of activated carbon were hydrolytic lignin, microcrystalline cellulose (MCC)-AaltoCell™ and precipitated kraft lignin from black liquor.
The industrial acid hydrolytic lignin from coniferous wood was used (200 pm with 8.5 % moisture content). Microcrystalline cellulose was prepared according to EP 2576629. The kraft lignin was obtained from the black liquor by the following procedure. 500 ml_ of black liquor was mixed with 250 ml_ of 10% w/w HCI and vigorously stirred. The obtained precipitate was isolated from the solution using a Buchner funnel and washed several times with water. The obtained kraft lignin was dried at 130 °C for 16 h.
All other chemicals were supplied by Sigma-Aldrich, were of analytical grade and used as received. The nitrogen for inert atmosphere and porosity analysis was of 99.999% purity.
Characterizations
The functional groups of the material were studied using Fourier transform infrared spectroscopy (FTIR) on a Bruker Vertex 70. The crystalline structure of the material was evaluated using X-ray powder diffraction (XRD) on a high-resolution PANalytical diffractometer. The scans were recorded employing Co Ka radiation at a voltage of 40 kV from 10° to 100° 20 angles. The crystalline size was calculated using XRD data applying Williamson-Flall (W-FI) method. The surface area, pore size distribution, micro and total pore volume and were derived from the data of ish adsorption-desorption isotherm obtained by Tristar® II Plus. The surface area was calculated using Brunauer, Emmett and Teller (BET) applying recommended the procedure for microporous materials. Pore size distribution and micropore volume were evaluated using non-local density functional theory (NLDFT) applying the“N2 @ 77 on Carbon Slit Pores” kernel provided by the software of Tristar® II Plus. The micropore volume was also calculated using the Dubinin-Radushkevich equation. The total volume of pores was calculated at p/po = 0.99. The structure of the material was studied employing scanning electron microscopy (SEM) on a Hitachi S-4800. Thermogravimetric analyses (TGA) was performed on NETZSCFI TG thermal analyser at a heating rate 10 °C/min from 23 °C to 1000 °C. The yield was calculated using values of raw material and final product masses, respectively.
Design of experiments
The process variables such as target temperature A (°C), residence time B (min) and lignin to NaOH/NaCI ratio C (g/g) were considered as factors. Three levels of factors, including zero level, were ascribed for each parameter. The minimum, zero and maximum levels are temperature 600, 700 and 800 °C; time 30, 60 and 90 min; lignin to NaOH/NaCI ratio 0.5, 1 .0 and 1 .5 g/g. The BET surface area Y1 (m2/g) and yield Y2 (%) were selected as responses for process optimization. To evaluate the reproducibility and validity of experimental findings three repetitions were performed on a zero level. The design of experiments is demonstrated in Table 1 . Experimental data were analysed by RSM using Design Expert 1 1 software and ANOVA analysis was performed to evaluate the adequacy of the models.
Table 1 . Factors and responses of experimental design.
a. g/g = mass ratio (NaOH/NaCI)/lignin
Preparation of activated carbon. General procedure. NaOH/NaCI
In a typical experiment using optimized parameters, certain amount of lignin was mixed with a solution containing certain amount of NaOH certain amount of NaCI and certain volume of deionized water. The obtained mixture was let to swell for an hour at room temperature, then was extruded and dried at 130 °C for 16 h. Received dried material was grinded and obtained granules with size 1-2 mm was collected. Afterwards, the material was pyrolyzed in a tubular furnace under an inert atmosphere at 868 °C for 47 min. The temperature, time and lignin to NaOH/NaCI ratio were also varied according to experimental design (Table 1 ). The flow of ish and heating rate were constant and equal 0.5 L/min and 5 °C/min, respectively. After pyrolysis, the inorganic part was leached from the material using 0.1 mol/L of HCI. The material was isolated from the solution using Buchner funnel and washed several times until neutral pH. Then the product was dried at 130 °C for 16 hours. KOH/KCI
The process conditions when KOH and KCI were used instead of NaOH and NaCI are given in Table 2. The KOH and KCI were used simultaneously and KOH/KCI-to- lignin was 3.000 g/g. The process time was 60 min.
Table 2.
Results and discussion
Process optimization
The process optimization of activated carbon production using NaOH/NaCI mixture was performed according to experimental design (Table 1 ). The regression coefficients of each response and values of statistical validity are presented in Table 3.
Table 3. Regression coefficients in coded units and statistical validations of models.
a Temperature; b Time; c Ratio The standard deviation of three repetitions on a zero level (Table 1 ) is negligible for both responses indicating remarkable reproducibility. R-squared of responses approaches unity since the difference between predicted and experimental values are negligible. The model F-value of 84.63 and 135.43 for BET surface area and yield, respectively implies the models are significant since there is only 0.01 % chance can be occurred because of noise. The model p-value below 0.05 indicates model terms are significant, whereas for BET and yield models the p-values are far below 0.05. In addition, according to abovementioned statistical validations, the experimental data of BET surface area in a good agreement with a quadratic model while yield does fit a linear model. The regression coefficients of BET model are increased with increasing of factor values. By contrast, the yield is decreasing while factors are increasing that can be concluded from the negative values of regression coefficients. The factor of lignin to NaOH/NaCI ratio (C) is the most essential factor since the regression coefficients values are the highest. The influence of temperature (A) on responses is also significant, while time (B) demonstrates the weakest effect. The maximum cannot be achieved for both responses because with increasing of one response another is decreasing. More specifically, the maximum BET surface area appears at 800 °C and 1.5 g/g, while the maximum yield is observed at the minimum temperature 600 °C and 0.5 g/g of ratio. Thus, increasing of factors has a positive effect on BET surface whereas the influence on yield is negative. The increasing of ratio, temperature and time lead to the enlarged surface area since the formation of porous texture is occurred in the same manner. Meanwhile, the yield is decreased due to the transformation of micropores to meso- and macropores with the further void formation.
According to obtained mathematical models for BET surface area and yield, the optimal values of factors can be predicted. To maximize both responses simultaneously, desirability function has been employed. Table 4 shows predicted and obtained responses at optimal factors. Activated carbons prepared at optimal factors in three repetitions to evaluate reproducibility and difference of predicted and obtained values are negligible indicating additionally validity of the model.
Table 4. The values of responses at optimal factors using desirability function.
The large value of BET surface area can be achieved at moderate yield using optimal conditions defined by RSM design. To demonstrate the features of the method, the comparison with previously reported chemically activated carbon using lignin was made (Table 5). Table 5. The comparison of the method of the present invention and methods
a. Fierro, V., Torne-Fernandez, V. and Celzard, A. (2007)‘Methodical study of the chemical activation of Kraft lignin with KOH and NaOH’, Microporous and Mesoporous Materials, 101 (3), pp. 419-431. doi: 10.1016/j.micromeso.2006.12.004.
b. Hayashi, J. et al. (2000)‘Preparation of activated carbon from lignin by chemical activation’,
Carbon, 38(13), pp. 1873-1878. doi: 10.1016/S0008-6223(00)00027-0.
The BET surface area of the obtained activated carbons is relatively the same while lignin to activation agent ratio is remarkably lower. Moreover, the NaOH consumption is five times lower compared to the previously reported methods. The discrepancies of yield can be explained by reduced consumption of N2 flow using in experiments.
Carbon texture
Figure 3A shows adsorption isotherms of carbons prepared after use of NaOH, NaCI and NaOH/NaCI. Adsorption isotherms are described according to the lUPAC report (Thommes, M. et al. (2015) Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (lUPAC Technical Report), Pure and Applied Chemistry, 87(9-10), pp. 1051-1069. doi: 10.1515/pac-2014- 1 1 17). Isotherm curvature of carbon prepared from the mixture of lignin-NaCI exhibits to composite type I, II assigned to micro- macroporous materials. Carbon prepared using lignin-NaOH demonstrated isotherm shape of microporous materials with characteristic plateau starting at a point below 0.1 p/p°. Activated carbons produced of hydrolytic lignin using NaOH or KOH activation are associated with microporous materials. The isotherm of carbon prepared using a mixture of NaOH and NaCI is also coincided with combined isotherm type I, II, since enhanced N2 uptake is observed at pressures below 0.1 p/p° associated with micropore filling, while ascended curvature of the plateau is inherent for macropores. The adsorption hysteresis of carbons prepared using NaOH is attributed to H4 type and accompanied with isotherms I or composite type I, II indicating micropore filling. The hysteresis loop of carbon prepared using only NaCI is associated with isotherm type II of macropores materials. Adsorption capacities of carbons prepared using only NaCI or NaOH are similar whereas N2 uptake of carbons prepared using NaOH/NaCI mixture is more pronounced. The texture properties of carbons prepared using various mixtures are summarized in Table 6. The samples prepared using only NaOH or NaCI has a relatively similar total volume of pores and BET surface area, while carbon prepared using a mixture of NaOH/NaCI demonstrated significantly higher values. The BET surface area and micropore volume of carbon prepared using NaOH/NaCI are larger compared to NaOH or NaCI activated carbons. Aforementioned micro- and macro porosity of carbons lead to the largest total pore volume of carbons prepared using NaOH/NaCI. The total pore volume of carbon prepared using NaCI is caused by micro and macropore formation, while in case of NaOH micro porosity mainly deals with its origin. Table 6. Texture properties and yield of carbons prepared using a various amount of NaOH and/or NaCI.
The inertness of NaCI leads to the largest carbon yield, whereas NaOH activated carbon demonstrated reduced yield assumed by the possible chemical interaction between NaOH and carbon. In the case of composite NaOH/NaCI mixture, the lowest yield is also caused by NaCI catalytic effect of thermal degradation. In addition, the NaOH/NaCI composite mixture has a remarkable effect on the porous texture of prepared carbon. The results allow suggesting the different origin of porous structure depending on the used chemical.
The texture properties of carbons prepared using potassium hydroxide and potassium chloride are summarized in Table 7. Table 7. Texture properties and yield of carbons prepared using a various amount of KOH and/or KCI.
When KOH and KCI was used instead of NaOH and NaCI, properties of activated carbon is different. In particular, the surface are is significantly larger.
The origin of pores The role of NaCI
To clarify origin of pores, the pore size was evaluated using NLDFT pore size distribution (Fig. 3B), while the NaCI crystal size was calculated by W-H method using XRD data (Fig. 4A). Both sizes, of pores and crystals, obtained by NLDFT and XRD methods, were also visually compared with sizes observed on SEM micrographs (Fig. 3B and Fig. 4B). The NLDFT pore size distribution (Fig. 3B) exhibits the most probable peak at 63.2 nm that size is indexed to macropores according to IUPAC classification. As seen from Figure 4 and in Table 8, NaCI crystal size derived from information on corresponding XRD peaks and observed crystals on SEM micrograph have similar size of macropores observed in Figure 3B, regardless using of NaOH. Therefore, it can be assumed the template role of NaCI in the process of pore formation. Table 8. The crystal size of NaCI and pore size of carbons prepared using NaCI or NaOH/NaCI.
a Calculated by W-H method using the following diffraction peaks: 1 1 1 , 200, 220, 222, 400, 420 (Fig. 3A).
b Observed NaCI crystals on SEM micrograph (Fig. 3B).
c The most significant peak of macropores obtained by NLDFT pore size distribution (Fig. 2B). d Observed macropore size on SEM micrograph (Fig. 2B). This range is obtained visually from the SEM images.
The role of NaOH Among the macropores, a considerable amount of micropores and some mesopores of carbons prepared using NaOH/NaCI are defined by NLDFT pore size distribution in a good agreement with SEM micrographs (Fig 3B). NaOH activated carbon exhibits micropores while the lack of NaCI leads to the absence of macropores, which are not observed on SEM images (Fig. 3B NaOH). The most probable pores of NaOH activated carbons whatever use of NaCI, defined by NLDFT analyses are micropores. In a high-resolution SEM micrograph (Fig. 3B) the micropores of size around 2 nm can be also found. Micropores could be formed by the following chemical reactions:
6NaOH + 2C = 2Na + 2Na2C03 + 3H2 (1 )
4NaOH + C = 4Na + C02 + 2H20 (2)
The product of assumed chemical reaction (1 ) is sodium carbonate that can be detected by analytical methods. The characteristic peaks of Na2C03 are found in XRD spectra of samples prepared using NaOH (Fig. 4A). The corresponding peaks of Na2C03 are indexed to reference number ICSD98-008-0985 of standard XRD pattern of sodium carbonate. The SEM micrograph of the sample prepared using NaOH before leaching shows different structure compare to NaCI contained samples. The formed crystals shapes are inherent for sodium carbonate. The peaks at 1423, 1426 cm-1 and 832, 876 cm-1 attributed to carbonates are found on FTIR spectra (Fig. 5A) and thermal decomposition of Na2C03 starting at 854 °C is observed as a strong peak on the DTG curve (Fig. 5B). In support of micropores origin, chemical reactions (1 ) and (2) could be assumed since the formation of Na2C03 is confirmed by abovementioned analytical methods. The formation of mesopores 7-10 nm (Fig. 3A) can be also explained by assumed chemical reactions since NaCI crystals corresponded to mesopores size are not observed on SEM and not derived from XRD data. Consequently, the template origin of mesopores is questionable meanwhile chemical formation is more probable.
Effect of NaOH/NaCI ratio
Adsorption isotherms of carbons prepared using different NaOFI to NaCI ratio are presented in Figure 6A. Starting from 1/9 ratio to 3/7 ratio adsorption isotherms exhibit Type I of microporous materials. With increasing of NaOFI presence from 4/6 to 6/4 isotherms demonstrated ascended curvature and pronounced hysteresis H4 that more intrinsic for composite isotherm I, II indicating the development of the porous structure. In particular, the slope of isotherm pointing to macropores formation, while hysteresis H4 type is attributed to micropores. The micropore volume and total volume is significantly increased starting from 4/6 ratio lead to enlarged BET surface area (Table 8). Carbons prepared using NaOFI to NaCI ratio from 1/9 to 3/7 are assigned to microporous materials, while samples prepared using 4/6 to 6/4 ratio are also significantly meso- macroporous.
The effect of NaOFI to NaCI ratio versus BET surface area and yield is presented in Figure 6B. With increasing of NaOFI content BET surface is increased while yield is decreased since the carbon was consumed for voids formation via presumed reactions (1 ) or (2). According to the results presented on Figure 6B and Table 6, the largest BET surface and pore volume at moderate yield are attributed to 4/6 NaOFI to NaCI ratio. Moreover, the meso- and macropores are more intrinsic starting from 4/6 ratio that is beneficial for accessibility of micropores.
Carbons prepared using only NaCI (Fig. 3B) are ascribed to macropores materials, while an even insignificant amount of NaOFI was added (ratio 1/9, Fig. 6A), micropores appeared. Table 9. Structure properties of prepared carbons using various NaOH/NaCI ratio.
a Micropore volume is calculated by DR equation.
b Mesopore volume is calculated as a difference between Vtotai and Vmicro.
c Total volume of pores is calculated at p/p°=0.99.
The interaction between lignin and NaOH
The chemical reactions of lignin in alkali media could sufficiently effect on the porous structure of obtained carbon. Reactions of destruction and condensation are peculiar to lignin in presence of NaOH. Condensation is more inherent for coniferous lignin compared to deciduous lignin. In the present case, the origin of used lignin is coniferous wood that can be confirmed by characteristic FTIR peak at 1266 cm-1 (Fig.7A, a) indexed to guaiacylpropane unit (Fig. 7A, b). In particular, reactions of destruction are caused by split off a-O-4 or b-O-4 bonds (Fig. 7A, c) and condensation can be occurred in a-5 position (Fig. 6A, d). The strong peaks located at 3341 cm-1 and 2934 cm-1 are assigned to stretching vibrations of OH- and C-H in -CH3 or -CH2-, respectively. The significant changes are observed for the following peaks: 1706 cm-1 - aromatic C-Hn stretching; 1270 cm-1 - C-O-C stretching of aryl- alkyl ether bond; 1210 cm 1 -C-0 stretching of guaiacyl ring; 1154 cm-1 and 1110 cm-1 - stretching vibrations in guaiacyl propane unit. The mentioned changes are assigned to linkages, which take part in the reaction of destruction (Fig. 7A, c) and condensation (Fig. 7A, d). The peaks at 1420 cm-1, 884 cm-1 and 776 cm-1 could be indexed to formed carbonate by captured CO2 from the air.
SEM micrographs exhibit the textural changes of lignin-NaOH (Fig. 7B, e) and lignin- NaCI (Fig. 7A, d) mixtures before pyrolysis. One can see the sample of lignin-NaOH has smooth surface whereas roughness and porous structure is observed for lignin- NaCI. The pointed differences are explained by possible interaction between NaOH and lignin via assumed aforementioned interactions (Fig.6A,c,d).
The thermal degradation (see TGA/DTG, Figure 4B) at around 210-220 °C and 310- 335 °C is assigned to cleavage of a-O-4 or b-O-4 linkage. Noteworthy, for the sample, contained NaCI peak at 210-220 °C is not observed while for NaOFI contained samples there are two peaks at 210-220 °C and 310-335 °C indicating possible interaction (Fig.7A,c,d). In support of inference, the cleavage the aryl-aryl bonds at 444-455 °C is only observed for NaOFI-contained samples pointing influence of NaOFI on lignin structure. At 388 °C side chain of -O-CFI3 begins splitting off as a methane for NaCI contained samples.
The FTIR peaks of lignin are not observed for pyrolyzed carbons (Fig.4A) whatever NaOFI or NaCI was used indicating essential degradation of lignin and formation of carbon like structure.
Table 10 summarizes the properties of activated carbons prepared by the method of the present invention, namely hydrolytic lignin, kraft lignin and microcrystalline cellulose and optimized process parameters.
Table 10. Properties of activated carbons prepared according to the method of the present invention.
Accordingly, the method of the present invention has the following advantages
• The carbonaceous material is directly mixed with MOFI/MCI, wherein M is Na or K, and pyrolyzed in one single step. This led to reduction of operation steps, energy and water consumption.
• Significantly lower chemical consumption. In the method, the optimized carbonaceous material such as lignin to NaOFI/NaCI ratio is 1 :0.6 (1 :0.24 of only NaOFI) w/w.
• Regeneration of chemicals is possible. • The material is micro- macroporous. There is the accessibility of micropores through the macropores.
• The material can be easily granulated/extruded without additional binder because of chemical interaction between lignin and NaOH.
· NaOH is more prevalent and cost-efficient compared to KOH.
The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims.

Claims

What is claimed is
A method for producing activated carbon characterized in that the method comprises following steps:
a) admixing carbonaceous material with a mixture of alkali metal hydroxide (MOH) and alkali metal chloride (MCI), wherein the alkali metal (M) is selected from sodium (Na) and potassium (K) to form an admixture, b) pyrolyzing the admixture to yield
• the activated carbon and
• an inorganic part comprising alkali metal carbonate and alkali metal chloride,
and
c) separating the activated carbon and the inorganic part.
2. The method according to claim 1 comprising one or more of: swelling, extruding and drying the admixture of step a) prior to step b).
3. The method according to claim 1 or 2 wherein the carbonaceous material is selected from one or more of bamboo, coconut husk, willow peat, wood, coir, lignite, coal, petroleum pitch, cellulose, lignin, sewage sludge of pulp and/or paper.
4. The method according to claim 1 or 2, wherein the carbonaceous material comprises lignin.
5. The method according to claim 4, wherein the lignin is selected from hydrolytic lignin and kraft lignin, preferably hydrolytic lignin.
6. The method according to claim 1 or 2, wherein the carbonaceous material comprises cellulose, preferably microcrystalline cellulose.
7. The method according to claim 1 or 2, wherein the carbonaceous material comprises sewage sludge of pulp and/or paper.
8. The method according to any of claims 1 -7 wherein MOH/MCI ratio is from 1/9 by weight to 3/2 by weight, preferably from 1/4 by weight to 1/1 by weight, most preferably 2/3 by weight.
9. The method according to any of claims 1 -8, wherein ratio (mass of the mixture of MOH and MCI)
(mass of the carbonaceous material)
is from 1/1 by weight to 0.5/1 by weight, preferably 0.6/1 by weight.
10. The method according to any of claims 1-9, wherein the pyrolyzing is at temperature from 600 °C to 1000 °C, preferably from 700 °C to 900 °C, more preferably from 850 °C to 900 °C.
11. A method for recycling alkali metal hydroxide (MOH) and alkali metal chloride (MCI) in the method according to any of claims 1 -10, the method comprising d) treating the inorganic part with calcium hydroxide to produce an admixture comprising calcium carbonate, alkali metal hydroxide, and alkali metal chloride,
e) removing the calcium carbonate from the admixture, and
f) recycling the alkali metal hydroxide and the alkali metal chloride to step a).
12. A method for recycling the calcium hydroxide in the method according to claim 11 , the method comprising
g) thermally decomposing the calcium carbonate of step e) to produce calcium oxide,
h) treating the calcium oxide with water to produce calcium hydroxide, and i) recycling the calcium hydroxide to step d).
13. The method according to any of claims 1-12 wherein the alkali metal (M) is sodium.
14. The method according to any of claims 1-12 wherein the alkali metal (M) is potassium.
15. The method according to any of claims 1 -12 wherein the alkali metal chloride (MCI) is potassium chloride (KCI) and the alkali metal hydroxide (MOH) is sodium hydroxide (NaOH).
16. The method according to any of claims 1 -12 wherein the alkali metal chloride (MCI) is sodium chloride (NaCI) and the alkali metal hydroxide (MOH) is potassium hydroxide (KOH).
17. Use of a mixture of alkali metal hydroxide (MOH) and alkali metal chloride (MCI), wherein the alkali metal is selected from sodium (Na) and potassium (K) as an activation agent in a method for producing activated carbon.
18. The use according to claim 17, wherein MOH/MCI ratio is from 1/9 by weight to 3/2 by weight, preferably from 1/4 by weight to 1/1 by weight, most preferably
2/3 by weight.
19. The use according to claim 17 or 18 wherein the alkali metal (M) is sodium (Na).
20. The use according to claim 17 or 18 wherein the alkali metal (M) is potassium (K).
21.The use according to claim 17 or 18 the alkali metal chloride (MCI) is potassium chloride (KCI) and the alkali metal hydroxide (MOH) is sodium hydroxide (NaOH).
22. The use according to claim 17 or 18 the alkali metal chloride (MCI) is sodium chloride (NaCI) and the alkali metal hydroxide (MOH) is potassium hydroxide (KOH).
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