Caustic leached rice husk ash as absorbent material

Abstract

The invention relates to caustic leached RHA absorbent materials obtained as a residue during alkaline silica production in hydrothermal process, and use of residue as CO2 absorbent material.

Description

[0001] CAUSTIC LEACHED RICE HUSK ASH AS ABSORBENT MATERIAL

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The invention relates to caustic leached Rice Husk Ash (RHA) obtained as waste product from sodium silicate (waterglass) material production process and preparation process thereof. The invention also relates to the use of that caustic leached Rice Husk Ash (RHA) residue as acidic gas (ab)sorbent material.

[0004] BACKGROUND OF THE INVENTION

[0005] The present invention relates to the field of caustic leached RHA residue / waste obtained during production of sodium silicate, wherein RHA is used as a silica source. The inventive caustic leached RHA residue is found to be very efficient as a low cost acid gas sorbent material, especially as a CO2 sorbent material.

[0006] In recent years, there has been a growing interest in the use of treated silica / silicate materials as CO2 sorbent materials. These are mainly high porous silica materials and / or amine treated silicas to absorb the gaseous materials (especially CO2) being released while manufacturing goods.

[0007] In the art, RHA is known as a sustainable source for producing sodium silicate (waterglass) and / or activated carbon. The processes for production thereof by use of RHA is also known in the art. However, in the process of making commercial pure activated carbon from RHA (to be used as a sorbent material), the removal of silicate residue from the pores of the carbon particles is a two stage washing or rinsing process. Consequently after leaching out the silica an acid wash step is needed in order to rid all the residual sodium silicate from (blocking) the pores of activated carbon as already disclosed in US 6,114,280 (A) prior art.

[0008] In the art, it is also known highly porous activated carbon to be used as CO2 sorbent. Since CO2 is an acid gas, a sodium silicate or even a surface treated sodium silicate can also be used to absorb CO2 through acid base neutralization. Therefore, a sodium silicate / activated carbon composite could be used advantageously as CO2 sorbent, with the advantage of its mesoporous structure. It is known to produce carbons with developed mesoporosity that are obtained from rice husk (RH) as being disclosed in the article from P.M. Yeletsky et all, in Microporous and Mesoporous Materials, 121 (2009) 34-40 doi:10.1016 / j.micromeso.2008.12.025, ISSN: 1387-181 1 , titled as “Synthesis of mesoporous carbons by leaching out natural silica templates of rice husk”.

[0009] There are many other prior arts disclosing either silica material itself or activated carbon as absorbent material with a focus on improved porosity which is certainly necessary for absorption applications.

[0010] KR 26503098 (B1) patent discloses improved porosity using the liquified carbonized rice husk (no separation), to imply the porosity improvement is for the non-woven fabric filter itself, using the splashing implementation. CN 10749745 (B) patent discloses a bi-model pore size distribution porous carbon composite, using sodium chloride as template, in high efficiency catalyst application. This is to be considered as an improvement of making the porous carbon (similar like above mentioned US 6, 1 14, 280 prior art).

[0011] None of the prior art discloses use of a caustic leached RHA residue (waste of alkali silicate production) as a sustainable and low cost sorbent material to capture CO2 in industrial processes with improved CO2 absorption capacities. The use of this specific caustic leached RHA residue (silicate composite powder), which is a waste in RHA hydrothermal silicate process; that has improved CO2 absorption capability when compared to known porous silica sorbents.

[0012] In the present invention, the residue / waste (caustic leached RHA residue) of RHA while alkali silicate production is being used as CO2 adsorbent as being cost efficient and still to be considered as sustainable source for CO2 capturing. The present invention therefore supports the CO2 capture cycle during alkaline silicate production itself without a need of using additional CO2 capture material. The present invention is not intended for the silicate and / or activated carbon material itself or the manufacturing thereof, but rather the residue material (that can be still called as silicate and activated carbon composite) being released during production of those composites and use of it as CO2 capture material.

[0013] It is therefore an object of the present invention to provide a new acidic gas sorbent material for absorbing production gases; especially CO2 gas; such as during production of silicate material itself or any other production sites as CO2 capture material.

[0014] BRIEF SUMMARY OF THE INVENTION

[0015] After thorough investigation, the inventors of the present invention have surprisingly found that the caustic leached RHA composite material, which is the waste of alkaline silicate (sodium silicate / water glass) production remained after washing and filtering the silicate product, as defined in claim 1 , solves the above technical problem as it provides a sustainable and low cost CO2 absorbent / capture material, which also supports the CO2 capture cycle during metal silicate production, without a need of additional CO2 capturing material.

[0016] Therefore, the inventors of the present invention further supports the use of above mentioned caustic leached RHA waste composite material (formed of silicate and activated carbon) as a CO2 capturing material.

[0017] DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a sorbent / absorbent material obtained from caustic leached Rice Husk Ash (RHA) during alkaline silicate production process, wherein the absorbent material is separated from alkaline silicate solution and obtained as residue of caustic leached Rice Husk Ash (RHA). Therefore, the first aspect of the invention is an absorbent composite material obtained from caustic leached rice husk ash (RHA), from alkaline silicate production process, wherein the absorbent composite material is separated from alkaline silicate solution and obtained as a residue of caustic leached Rice Husk Ash (RHA).

[0019] In some aspects of the invention the composite absorbent material is obtained from hydrothermal process of alkaline silicate production wherein rice husk ash (RHA) is used as the silicate source and; wherein the absorbent material is obtained as the residue of caustic leached RHA and separated from alkaline silicate solution by filtration followed by drying to obtain a solid or powder composite.

[0020] In some aspects of the absorbent composite material according to the invention is formed of a composite material comprises silicate (caustic & SiO2) and activated carbon.

[0021] In some aspects of the invention, the residue of mentioned caustic leached RHA can be named as sodium silicate / activated carbon composite.

[0022] In some aspects of the absorbent composite material according to the invention, the material has a pore volume of less than 0.5 cm3 / g, preferably less than 0.2 cm3 / g but not less than 0.05 cm3 / g, most preferably in the range of 0.1 cm3 / g to 0.2 cm3 / g.

[0023] In some aspects of the absorbent composite material according to the invention, the material has a carbon content of (C wt %) more than 11 %, preferably more than 15 %, more preferably more than 20 %.

[0024] In some aspects of the absorbent composite material according to the invention, the material has a carbon content of (C wt %) less than 90 wt%, preferably less than 70 wt%, more preferably less than 70 wt% by weight of the absorbent composite material.

[0025] Carbon content is determined by carbon / sulfur analyzer (LECO, model SC832).

[0026] In some aspects of the absorbent composite material according to the invention, comprises a silica content of (SiO2 wt %) more than 10 %, preferably more than 15 %, more preferably more than 20 %.

[0027] In some aspects of the absorbent composite material according to the invention, the material has a (SiO2 wt %) of less than 50 wt%, preferably less than 40 wt%, by weight of the absorbent composite material.

[0028] Silica content of the ignited particles is determined by the acid digestion weight loss, by HF and H2SO4, after 1000°C for 1 hour in the muffle furnace, divided by the dried particle weight.

[0029] In some aspects of the absorbent material according to the invention, the material has a BET SA of higher than 100 m2 / g, preferably higher than 150 m2 / g, more preferably higher than 200 m2 / g.

[0030] BET SA is measured by Micromeritics Gemini VII N2 surface area analyzer, using the BET nitrogen adsorption method of Brunaur et al., J. Am. Chem. Soc., 60, 309 (1938). Advantageously, a new CO2 absorption material is obtained from the caustic leached RHA residue (silicate composite powder), which is later compared to known CO2 absorption materials within the present invention, such as; known porous silica material itself, or the RHA raw material itself. It has been demonstrated with the data given in the experimental part that proves increased CO2 capturing capacities with the inventive absorbent material.

[0031] Commercial alkali metal silicates (waterglass) are prepared by following two general known methods which are either the hydrothermal method (liquid) or the furnace method (solid waterglass), which are already known from the art.

[0032] The silica source (sand / quartz, RHA) used is typically a small particle sized sand or an ash. The alkali metal may be sodium, potassium or lithium, with sodium preferred.

[0033] In the hydrothermal method, the hydrothermal reaction of silica source with aqueous caustic, (e.g.: sodium hydroxide or potassium hydroxide) takes place at temperatures between 150 °C and 250 °C. In this method, excess sand or silica source, caustic and water are agitated in a pressure reactor and the unreacted sand or silica source is then separated from the formed sodium silicate solution by filtration. The resulting molar ratio is generally between 2.5 and 2.8 (SiO2 / Na2O).

[0034] In the furnace method, silica source and a caustic base (such as; sodium carbonate or sodium hydroxide) are reacting together at high temperatures. In this method, the solid silicate waterglass is produced by heating stoichiometric amounts of silica source and sodium carbonate (Na2COs) up to 1100 °C to about 1400 °C in a gas-fired furnace or electric or plasma furnace. The formed waterglass is cooled, crushed and dissolved in water to form the liquid sodium silicate solution with a molar ratio of between 3.0 and 3.8 molar ratio (SiO2 / Na2O). The solubilized silicate solution is then filtered by filtration to remove the non-soluble heavy metal complexes and unreacted sand.

[0035] In both methods, the mixture is filtered to remove unreacted silica source and any other insoluble species or residues. Insoluble heavy metal complexes and / or salts are removed from alkali metal silicate mixture by filtration, centrifugation, or vibrating screening, preferably by filtration, such as pressure leaf filtration. Pressure leaf filtration is the preferable filtration method.

[0036] Within this invention, it has been proved that filtered residue from a hydrothermal process can be used further as CO2 absorbent, otherwise it would be considered as trash to be destroyed later.

[0037] A second aspect of the invention is a method for preparing said caustic leached RHA composite residue.

[0038] In some aspects of the present invention hydrothermal method is used for waterglass production, wherein RHA is used as the silica source and leached RHA residue / leftover is obtained as absorbent material.

[0039] The process for alkali metal silicate production from rice husk ash is a caustic leaching process, which is already known from the prior art; as mentioned above.

[0040] In some aspects of the present invention, any known hydrothermal caustic leaching process for alkali metal silicate production can be applied to obtain caustic leached RHA residue and claimed as an absorbent material within the present invention.

[0041] In the hydrothermal method, the hydrothermal reaction of silica source (RHA) with aqueous caustic, (e.g.: sodium hydroxide or potassium hydroxide) takes place at temperatures between 150°C and 250°C and under pressure in a pressure reactor. In this method, excess silica source, caustic and water are agitated in a pressure reactor and the unreacted silica source and metal impurities are then separated from the formed sodium silicate solution by filtration.

[0042] The production of caustic silicate solution from RHA is a caustic leached process as explained below. And, in some aspects of the present invention, the alkali metal silicate is made through hydrothermal process at temperatures between 150°C to 250°C, preferably between 150°C to 200°C.

[0043] In some aspects of the present invention, the alkali metal silicate is sodium, potassium or lithium silicate, preferably sodium silicate.

[0044] In the process of alkali metal silicate production; rice husk ash is leached with alkali metal hydroxide, wherein the alkali metal hydroxide is selected form sodium, potassium or lithium hydroxide, preferably sodium hydroxide, under heat and pressure; i.e. hydrothermal caustic digestion in pressure reactor. The purpose of applying heat and / or pressure is to enhance the caustic leaching of silica out of the rice husk ash. Therefore, a higher silica content (molar ratio) sodium silicate can be obtained. The resulting molar ratio is generally between 2.5 and 2.8 (SiO2 / Na2O).

[0045] In some aspects of the present invention, the RHA silica source is heated with caustic solution (such as; sodium hydroxide (NaOH)), which reacts with RHA silica source to create sodium silicate solution as shown with the chemical equation below; where "n" represents the silica / alkali molar ratio.

[0046] In some aspects of the present invention, the caustic leaching process is followed with filtration and washing steps to obtain solid or powder composite absorbent material. After filtration and washing, most of the alkali metal silicate (e.g: sodium silicate) is collected as the desired product and called as waterglass to be used later for silica production. The waste / residue of this caustic leaching process is the caustic leached RHA which has been obtained after filtration and washing steps, the absorbent material as claimed within the present invention. This absorbent material can also be named as powder composite, silicate residue or activated carbon composite. In some aspects of the present invention, the process for preparing a caustic leached RHA residue composite comprises the following steps; a) providing rice husk ash or mixture of rice husk ash with sand as the silicon dioxide source, b) mixing said silicon dioxide source with an alkali base in water at temperatures between 150°C to 250°C for caustic digestion, and c) removing alkali silicate material from the caustic leached RHA residue by filtration d) drying and obtaining caustic leached RHA residue.

[0047] According to the process of the present invention, the alkaline agent can be sodium hydroxide, potassium hydroxide or lithium hydroxide, preferably is sodium hydroxide.

[0048] According to the process of the present invention, it is preferred that the temperature is between 150°C to 200°C for caustic digestion.

[0049] A further aspect of the invention is the use of caustic leached RHA residue composite as acidic gas sorbent.

[0050] In a preferred embodiment of the invention the absorbent material is suitable as CO2 sorbent or capture material.

[0051] In some aspects of the present invention, in the method for obtaining caustic leached RHA residue after separating the alkali silicate particles from the caustic silicate solution, the caustic leached RHA residue may comprises small amount of sodium silicate particles with pores.

[0052] In some aspects of the present invention, above mentioned absorbent composite is formed of porous silicate material and activated carbon, and suitable as CO2 sorbent to be used as a sustainable material.

[0053] A third aspect of the invention is the use of caustic leached RHA as gas absorbent composite material, especially as CO2 absorbent material.

[0054] BRIEF DESCRIPTION OF THE FIGURES

[0055] For the purpose of better illustrating the advantages and properties of the claimed caustic leached RHA residue composite, object of the present invention, graphs are attached as a non-limiting examples.

[0056] Figure 1 (a) and (b) graphs showing pore size distribution of all inventive and comparative examples, by using N2 adsorption / desorption method.

[0057] Fig 1a) corresponds to Comparative Example 1 and Inventive Example 1 ;

[0058] Fig 1 b) corresponds to Inventive Example 1 compared to Comparative Examples 1 to 3.

[0059] Figure 2 is a graph showing CO2 adsorption isotherm data of Inventive Example 1 and Comparative Examples 2 (untreated porous silica-Sipernat® 50 commercial grade) and 3 (amine treated Sipernat® 50 silica) Explanations:

[0060] As used herein, the term “about” encompasses the range of experimental error that occurs in any measurement.

[0061] The invention may be better understood by reference to the following examples in which the parts and percentages are by weight unless otherwise indicated.

[0062] EXPERIMENTAL PART

[0063] The invention is further illustrated in detail hereinafter with reference to examples and comparative examples, without any intention to limit the scope of the present invention.

[0064] Abbreviations

[0065] RHA : Rice Husk / Hull Ash

[0066] LOI : Loss on ignition

[0067] SA : Surface Area

[0068] Method of Measurements

[0069] Silica content (SiO2%)

[0070] Silica content of the ignited particles was measured by the acid digestion weight loss, by HF and H2SO4, after 1000°C for 1 hour in the muffle furnace, divided by the dried particle weight.

[0071] Carbon content (C content)

[0072] Carbon content is determined by carbon / sulfur analyzer (LECO, model SC832).

[0073] BET SA

[0074] BET SA is measured by Micromeritics Gemini VII N2 surface area analyzer, using the BET nitrogen adsorption method of Brunaur et al., J. Am. Chem. Soc., 60, 309 (1938), and such technique is well known to those skilled in the art.

[0075] Pore volume

[0076] The pore volumes are obtained directly from the adsorption isotherm at P / Po of 0.90 to 0.95, using the TriStar II Nitrogen adsorption / desorption analyzer from Micromeritics. The pore size distribution were calculated using BJH (Barrett Joyner-Halenda) method from desorption isotherm. Such technique is well known to those skilled in the art.

[0077] CO2 Isotherm Adsorption

[0078] To measure CO2 Adsorption performance, the CO2 Isotherm Adsorption data is obtained directly from the adsorption isotherm with a total absolute gas saturation pressure of 760 mmHg using the TriStar II Nitrogen adsorption / desorption analyzer from Micromeritics. For the isotherm adsorption, CO2 was used as the absorptive gas instead of nitrogen and the temperature was fixed at 25°C. Such a technique is well known to those skilled in the art.

[0079] EXAMPLES

[0080] Comparative Example 1 is a raw RHA material itself, before caustic leaching hydrothermal process.

[0081] Inventive Example 1 is a caustic leached RHA residue, obtained according to the present invention and used as a CO2 sorbent, sodium silicate / activated carbon composite

[0082] Comparative Example 2 is an untreated porous silica. Commercially available silica grade is used directly. Eg; SIPERNAT® 50 is used as a commercial grade from Evonik Industries AG.

[0083] Control Example 3 is an amine treated silica. Commercially available silica grade is treated Eg; SIPERNAT® 50 is treated for this examples to be used as a control examples, as amine treated silicas are already known as CO2 absorbent materials. of Inventive Example 1 - Caustic Leached RHA Residue Composite

[0084] In a method for producing alkaline silicate composite particles which are separated, by filtration, from a caustic silicate solution produced from caustic digestion of rice husk ash (RHA). The ash (RHA), being obtained from thermal pyrolysis of rice husk and contains silica particles. The caustic digested RHA is then being separated from the caustic silicate solution after filtration and dried to obtain caustic leached residue composite.

[0085] In the hydrothermal method of the present invention, the hydrothermal reaction of RHA and / or combination of RHA with sand (quartz) with aqueous caustic, (e.g.: sodium hydroxide or potassium hydroxide) takes place at temperatures between 150°C and 250°C. In this method, RHA or RHA and sand mixture, caustic, and water are agitated in a pressure reactor and the unreacted RHA or the mixture with sand is then separated from the formed sodium silicate solution by filtration. The resulting residue is then dried and comprises mainly caustic leached RHA residue (sodium silicate / activated carbon composite) to be used as absorbent material, including CO2 capture material. 3- Amine Treated Silica

[0086] The amine surface treatment of precipitated silica was prepared by the dropwise addition of 27g of 3- aminopropyltri-ethoxysilane (Dynasylan® AMEO) into 20g of precipitated Silica (SIPERNAT® 50), under low sheer mixing. The resulting material was then oven-cured at 90°C for 6hrs.

[0087] RESULTS AND DISCUSSION

[0088] In Table 1 and Figures 1(a, b) & 2; the chemical and physical properties of the examples have been presented. Table 1: Chemical and physical properties of Examples

[0089] *based on wt. percentages of the total amount of examplified absorbent materials

[0090] As shown in the Table 1 above, the raw RHA material (silicate source material in comparative example 1) has low surface area, and also quite low pore volume, those reflect that the raw material is not very porous, accordingly CO2 absorption data cannot be obtained or nearly no CO2 has been absorbed. Even if it has high amount of SiO2 content, it does not enable to absorb CO2

[0091] It is already known in the art that highly porous materials can be used as gas sorbent alone or after a treatment. Therefore, the inventive caustic leached RHA composite (formed of SiO2 and carbon content) is compared with one untreated porous silica (comparative example 2) and amine treated porous silica (comparative example 3) for CO2 absorption.

[0092] Considering comparative example 2 and control example 3; normally it is expected to have a large volume for a silica, that will undergo for amine treatment and later to be used as CO2 capturing material. If we consider the non-treated silica material in comparative example 2, it is obvious that it has a very large pore volume, which has been treated with amine to obtain control example 3. These amine treated silicas are very well known in the art and have been used for CO2 capturing. Amine is used for CO2 capturing regardless of pore size of the treated silica then. CO2 is connected / reacted with amine and stick on the surface of treated silica material.

[0093] But if the silica material itself is used for CO2 capturing, it will not be as successful as an amine treated material. Due to high pore size distribution, CO2 will not be as readily captured.

[0094] Within this invention, it has been proved that lower than 0.5 cm3 / g, preferably lower than 0.2 cm3 / g and above 0.05 cm3 / g pore volume will enable a higher CO2 absorption when compared to silica materials. This composite material comprises higher amount of carbon material and does not require an additional step of treatment which would require additional sources. Simply within present invention, it has been proved that a waste material or a silicate production residue has been used for CO2 absorption which could be considered as sustainable CO2 absorption composite material.

[0095] Within this invention it has been also proved that inventive caustic leached RHA residue composite material can absorb CO2 better than raw RHA material itself or even untreated silica and comparable with the amine treated silica materials as well.

[0096] After leaching the RHA raw material with sodium hydroxide during the hydrothermal process, followed with washing and filtration for collecting the sodium silicate product, the left over RHA residue composite provides a high surface area and porous material which comprises still enough amount of SiO2 and carbon content (inventive example 1 compared to comparative example 1). Despite having lower BET SA (when compared to untreated silica), it is providing higher CO2 absorption capacity, due to having higher carbon content. Therefore, there likely to be a desired balance among C content, SiO2 content, pore volume and BET SA, to be able to absorb maximum amount of CO2 for inventive composite type of materials.

[0097] Porosity difference between the RHA raw material and the left over residue can be compared by their pore size distribution by using N2 adsorption / desorption method as shown in Figure 1 a as well. When compared to the untreated highly porous silica example (comparative example 2), the pore volume of the current inventive material (inventive example 1) which has lower surface area and pore volume, it is obvious that a balance between pore volume and surface area needs to be present. This is also very evident from the pore size distribution comparison in Fig 1 b showing all examples in one table.

[0098] As can be seen from figure 2, CO2absorption of the inventive example increases from 0.1 to 1 mmol / g under pressure increasing from Oto 760 mmHg, which is more similar to the untreated silica material. Further, while the CO2 absorption may not be as much as the amine treated silica material (reach up to 1 .4 mmol / g) under the same conditions; the inventive example has close to 1 mmol / g CO2 absorption, which makes it quite competitive. Not wish to be bound by the theory, this increase in CO2 absorption can be supported with the data being provided in Table 1 , as BET SA and C content of the caustic leached RHA composite increased compared to non-treated RHA silicate source.

Claims

CLAIMS1 . An absorbent composite material comprising a carbon content of (C wt %) more than 11 wt % and less than 90 wt % measured by carbon / sulfur analyzer, a silica content of (SiO2 %) more than 10 wt % and less than 50 wt % measured by the acid digestion weight loss, wherein the absorbent composite material is a residue of caustic leached Rice Husk Ash (RHA).

2. The absorbent composite material according to any claim 1 comprises a carbon content of (C wt %) more than 15 %, and less than 70 wt% of the absorbent composite material.

3. The absorbent composite material according to claim 1 or 2 comprises a silica content of (SiO2 wt %) more than 15 %, and less than 40 wt% by weight of the absorbent composite material.

4. The absorbent composite material according to any one of the one of the preceding claims has pore volume of less than 0.5 cm3 / g and above 0.05 cm3 / g, preferably less than 0.2 cm3 / g and above 0.05 cm3 / g, measured by BJH (Barrett Joyner-Halenda) method from desorption isotherm.

5. The absorbent composite material according to any one of the preceding claims has BET SA of higher than 100 m2 / g, preferably higher than 150 m2 / g, more preferably higher than 200 m2 / g, measured by BET nitrogen adsorption method.

6. The absorbent composite material according to claim 1 , wherein the absorbent composite material is obtained from hydrothermal process of alkaline silicate production wherein rice husk ash (RHA) is used as the alkaline silicate source and wherein the absorbent material is obtained as the residue of caustic leached RHA and separated from alkaline silicate solution by filtration followed by drying.

7. The absorbent composite material according to claim 6, wherein the absorbent composite material is obtained by following the steps; a) providing rice husk ash or mixture of rice husk ash with sand as the silicon dioxide source, b)mixing said silicon dioxide source with an alkali base (caustic) in water at temperatures between 150 to 250 °C for caustic digestion, and c) removing alkali silicate material from the caustic leached RHA residue by filtration d) drying and obtaining caustic leached RHA residue.

8. Use of residue of caustic leached rice husk ash (RHA) obtained from alkaline silicate production process, as gas absorption material according to claim 6 or 7.

9. Use of residue of caustic leached rice husk ash (RHA) obtained from alkaline silicate production process, as gas absorption material according to claim 6 or 7, wherein the caustic leached rice husk ash is separated from alkaline silicate solution and obtained as a residue of caustic leached Rice Husk Ash (RHA).

10. Use of residue of caustic leached rice husk ash (RHA) according to claims 6 or 7, as an absorbent material to absorb CO2.

11. Use of residue of caustic leached rice husk ash (RHA) according to claim 10, wherein the absorbent material has a CO2 absorption capacity of 0.1 to 1.0 mmol / g under pressure from 0 to 760 mmHg.