Process for preparing silicate from plant ash comprising crystalline silica using multivalent anion-containing salts

Abstract

The invention relates to a method for producing silicates from plant ash comprising crystalline silica. The method comprises reacting the plant ash with a base in the presence of an additive which is a salt comprising a multivalent anion. The invention also relates to a silicate obtainable by said method and to a method for preparing precipitated silica from said silicate. The invention also relates to a reaction mixture which can be used in said method.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to European Application Nos. 22306640.8, 22306641.6, and 22306642.4, all filed on October 28, 2022, the entire contents of which are incorporated herein by reference for all purposes. Technical field

[0003] The present invention relates to a method for producing silicates, preferably alkali metal silicates, from plant ash containing crystalline silica. The method comprises reacting the plant ash with an alkali in the presence of an additive which is a salt containing a polyvalent anion. The present invention also relates to a silicate obtainable by said method and a method for preparing precipitated silica from said silicate. The present invention also relates to a reaction mixture to be used in said method. Background art

[0004] Silicon dioxide (SiO 2 ), also known as silica, is a common silicon compound in nature. Naturally occurring silicon dioxide exists in amorphous and crystalline forms such as cristobalite, tridymite, and quartz, the latter being the main component of sand.

[0005] Quartz sand is often used in the production of silicates (especially sodium silicate), which can be obtained, for example, by hydrothermally treating quartz sand with a strong base such as sodium hydroxide or potassium hydroxide, or by melting quartz sand with sodium carbonate at a high temperature of about 1400 °C - 1500 °C.

[0006] Sodium silicate is commonly used as a raw material for the preparation of precipitated silica, which is a form of synthetic silica in amorphous form.

[0007] Both silicates and precipitated silica are materials with a wide range of uses and have various applications in the most diverse technical fields, from construction to the detergent, tire, adhesive, food, and pharmaceutical industries, and their global demand is constantly increasing.

[0008] However, the main drawback of the above - mentioned methods is that the sand used as raw material is not a renewable resource on the human time scale, as its replenishment is achieved through the processes of rock erosion and weathering over geological periods.

[0009] In addition, the aforementioned conventional method for manufacturing silica by sand melting requires high energy consumption due to the need to heat the reactants to a high temperature.

[0010] Therefore, there is clearly still a need to find a method for producing silicates and silica that is not only more environmentally sustainable but also cost-effective.

[0011] One possible renewable source that can be envisaged is the ash derived from the combustion of plants, and in particular the plant ash derived from the combustion of plants rich in silica and / or their parts (such as rice husks).

[0012] Rice husks are agricultural residues of the rice milling industry and are abundant in rice-producing countries. After combustion, approximately 20% of the weight of the rice husks is converted into ash, which contains up to 97 wt.% of silica in addition to trace amounts of carbon impurities and various metals. A portion of the silica contained in the ash is in crystalline form, and the main crystalline phase is typically cristobalite.

[0013] Similarly, other plant ashes particularly rich in silica are those derived from the combustion of trees and / or sugarcane, especially the combustion of tree wood and / or sugarcane bagasse. Also in this case, the ash can contain up to 97 wt.% of silica in addition to carbon and metal impurities, and a portion of this silica contained in the ash is in crystalline form, where the main crystalline phase is typically quartz.

[0014] Given the large amount of silica contained in these ashes and their inherently renewable nature, many efforts have been made to try to extract silica from them, as this could represent an economically viable option for obtaining silicates and precipitated silica. This can also solve the problem of the proper disposal of rice husks (as mentioned earlier, rice husks are waste from the milling industry).

[0015] US2020 / 0399134 describes a method for producing sodium silicate solution from the ash of combusted organic matter (such as rice husk ash), which involves washing the ash and reacting the rinsed ash in the presence of sodium hydroxide to obtain a sodium silicate solution.

[0016] In contrast, WO 2017 / 063901 discloses the preparation of silicates by reacting rice husk ash with a silicate precursor. This document also describes the preparation of precipitated silica from the silicates thus produced.

[0017] For example, the preparation of silica from rice husk ash is also described in WO 2004 / 073600, which discloses a method for preparing precipitated silica by adding an acid to a silicate solution obtained by caustic digestion of biomass ash (such as rice husk ash).

[0018] One of the key aspects of the method using plant ash as a starting material (such as rice husk ash, wood ash, and / or bagasse) is that, as previously mentioned, the ash contains silica with a crystalline part. Different from the amorphous form that is relatively easily eroded and dissolved when the ash reacts under alkaline conditions (such as alkaline digestion / caustic digestion), the crystalline part is more difficult to dissolve and these processes often result in a low conversion rate of silica or require elevated temperatures. Another problem related to the use of plant ash involves the fact that since the ash is a complex material, it is usually difficult to obtain silicate that is easily separated from the remaining impurities because these impurities often remain suspended.

[0019] Therefore, there is still a need to develop an environmentally friendly, cost-effective, and high-yield novel method for the production of silicate and precipitated silica. Summary of the Invention

[0020] The present invention relates to a method for producing silicate, preferably alkali metal silicate, from plant ash, wherein the method comprises the step (a) of reacting:

[0021] (i) Plant ash obtained from the combustion of silica-containing plant parts and / or plants, wherein the plant ash contains crystalline silica;

[0022] with

[0023] (ii) An alkali, preferably an alkali metal hydroxide;

[0024] wherein the reaction of step (a) is carried out in a reaction mixture comprising:

[0025] A dispersion medium, preferably an aqueous dispersion medium, and an additive, wherein the additive is a salt containing a polyvalent anion.

[0026] According to an embodiment of the present invention, the silicate is obtained in liquid form as a silicate solution, preferably an aqueous silicate solution, and the method further comprises the step (b) of separating the silicate solution obtained after step (a) from the impurities derived from the ash, which impurities contain carbon products and metals.

[0027] According to another embodiment of the present invention, the silicate is obtained in solid form as a solid silicate, and the method further comprises the step (c) of drying the silicate solution obtained after step (b) after the step (b) to obtain the silicate in solid form.

[0028] Furthermore, the present invention relates to a silicate, preferably an alkali metal silicate, which can be obtained in liquid form as a silicate solution by a method comprising step (b), or in solid form as a solid silicate by a method comprising step (c).

[0029] Furthermore, the present invention relates to a method for preparing precipitated silica, the method comprising the following steps:

[0030] (I) Producing a silicate solution, which is achieved by a method comprising step (b) or by a method comprising producing a silicate in solid form (by a method comprising step (c)) and redispersing the silicate in solid form in a dispersion medium, preferably an aqueous dispersion medium, and

[0031] (II) Reacting the silicate solution thus produced and optionally further silicate solutions other than the silicate solution thus produced, NaOH and / or a minor silica source with an acidifying agent to effect precipitation of silica.

[0032] Furthermore, the present invention provides a novel reaction mixture for producing silicates, preferably alkali metal silicates, from wood ash and / or for preparing precipitated silica by the method defined above, the reaction mixture comprising:

[0033] (i) Wood ash obtained from the combustion of silica-containing plant parts and / or plants, wherein the wood ash contains crystalline silica;

[0034] (ii) A base, preferably an alkali metal hydroxide;

[0035] (iii) A dispersion medium, preferably an aqueous dispersion medium, and

[0036] (iv) An additive, wherein the additive is a salt comprising a polyvalent anion.

[0037] The present invention solves the aforementioned problems of the prior art by providing a method for producing silicates and precipitated silica from wood ash, which method allows for improved dissolution of the silica contained in the ash, whether in amorphous or crystalline form, and in particular of the crystalline part of said silica.

[0038] Crystalline silica is more difficult to dissolve than the amorphous form, and during the normal alkaline digestion of the ash with the base, the crystalline silica only dissolves partially or not at all unless higher temperatures and / or longer reaction times are employed. To solve this problem, the inventors conducted numerous experiments and unexpectedly found that the presence of an additive (which is a salt comprising a polyvalent anion) in the reaction mixture of the present invention not only allows for improved dissolution of the amorphous part of the silica contained in the wood ash, but also allows for improved dissolution of the crystalline part of the silica contained in the wood ash.

[0039] Furthermore, the present invention is particularly advantageous because it allows to obtain a silicate solution that can be effectively separated from impurities derived from the ash, thereby obtaining the desired yield and purity. In addition, the present invention allows to reduce the crystalline silica content in the waste (carbon cake) obtained at the end of the process, thereby reducing the potentially hazardous waste (if dried).

[0040] In addition, the process for producing silicate and the process for producing precipitated silica according to the present invention are not only environmentally friendly because they use rice husk ash (which is of renewable origin), but also economically advantageous because lower temperatures can be used without affecting the overall yield. Detailed Description

[0041] Before describing the problem of the present invention in detail, the following should be considered:

[0042] It should be understood that the present invention is not limited to the specific embodiments described, as such embodiments can vary (naturally). It should also be understood that the terms used herein are not intended to be restrictive, as the scope of the present invention will be limited only by the appended claims.

[0043] As used herein, the singular forms "a / an" and "the" include both singular and plural referents, unless the context clearly indicates otherwise. For example, "additive" means one additive or more than one additive, or "salt" means one salt or more than one salt.

[0044] The terms "comprising", "comprises" and "consisting of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, unrecited members, elements or method steps. It will be understood that the terms "comprising", "comprises" and "consisting of" as used herein include the terms "consisting of", "consists" and "consists of".

[0045] Throughout this application, the term "about" is used to indicate that a value includes the standard deviation of the error for the equipment or method used to determine that value.

[0046] The terms "rice husk" and "rice hull" are used interchangeably herein. The same applies to the terms "rice husk ash" and "rice hull ash", which are also abbreviated as RHA.

[0047] As used herein, the term "average" refers to number average, unless otherwise specified.

[0048] As used herein, the terms "% by weight", "wt.-%", "wt.%", "weight percentage", or "percentage by weight" are used interchangeably. The same applies to the terms "% by volume", "vol.-%", "vol.%", "volume percentage" or "percentage by volume", or "% by mole", "mol-%", "mol.%", "mole percentage" or "percentage by mole".

[0049] A numerical range recited by endpoints includes all integers and (where appropriate) fractions contained within that range (e.g., 1 to 5 can include 1, 2, 3, 4 when referring to, for example, the number of elements, and can also include 1.5, 2, 2.75, and 3.80 when referring to, for example, a measured value). The recited endpoints also include the endpoint values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0050] All reference documents cited in this specification are hereby incorporated by reference in their entirety. In particular, the disclosures of all references specifically mentioned herein are incorporated by reference.

[0051] Unless otherwise defined, all terms used in the disclosure of the present invention, including technical and scientific terms, have the meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. By further guidance, definitions of terms are included to better appreciate the disclosure of the present invention.

[0052] In the following paragraphs, different alternatives, embodiments, and variations of the present invention are defined in more detail. When ranges of values of the same parameter are separated, each alternative and embodiment so defined can be combined with any other alternative and embodiment, and this applies to each variation, unless clearly stated to the contrary or clearly incompatible. In particular, any feature designated as being preferred or advantageous can be combined with any other one or more features designated as being preferred or advantageous.

[0053] Furthermore, in one or more embodiments, the specific features, structures, or characteristics described in this specification can be combined in any suitable manner, as will be apparent to those skilled in the art from this disclosure. Additionally, although some embodiments described herein include some features (in addition to other features included in other embodiments), combinations of features of different embodiments are intended to be within the scope of the present invention and form different embodiments, as will be understood by those in the art.

[0054] The present invention relates to a method for producing silicate from plant ash, wherein the method comprises the step (a) of reacting the following:

[0055] (i) Plant ash obtained from the combustion of silica-containing plant parts and / or plants, wherein the plant ash contains crystalline silica;

[0056] with

[0057] (ii) an alkali;

[0058] wherein the reaction of step (a) is carried out in a reaction mixture comprising a dispersion medium and an additive, wherein the additive is a salt containing a polyvalent anion.

[0059] According to the present invention, the silicate is preferably an alkali metal silicate and the alkali is preferably an alkali metal hydroxide.

[0060] The dispersion medium can be any suitable medium for dispersing the plant ash, the alkali and / or the additive. Preferably, the dispersion medium is an aqueous dispersion medium, more preferably water.

[0061] The polyvalent anion typically contains several oxygen atoms. In some embodiments, the polyvalent anion is an oxyanion, optionally having one or more protons attached thereto. In some embodiments, the polyvalent anion does not contain any heteroatoms other than oxygen. In some other embodiments, the polyvalent anion (which may be an oxyanion or an oxyanion having one or more protons attached thereto) contains one or more heteroatoms other than oxygen; the at least one heteroatom is typically an element of Group 13, 14, 15 or 16 of the Mendeleev periodic table based on the new IUPAC system.

[0062] The polyvalent anion is optionally selected from the group consisting of borate anions, carbonate anions, phosphate anions and sulfate anions.

[0063] The polyvalent anion can be selected from the group consisting of borate [(BO 3 ) 3- , carbonate [(CO 3 ) 2- , carboxylate, phosphate [(PO 4 ) 3- , hydrogen phosphate [(HPO 4 ) 2- , pyrophosphate [(P 2 O 7 ) 4- , acid dihydrogen phosphate [(HP 2 O 7 ) 3- and (H 2 P 2O 7 ) 2- , triphosphate [(P 3 O 10 ) 5- , hydrogen triphosphate [(HP 3 O 10 ) 4- , (H 2 P 3 O 10 ) 3- and (H 3 P 3 O 10 ) 2- , phosphite [(HPO 3 ) 2- , pyrophosphite [(H 2 P 2 O 5 ) 2- , sulfite [(SO 3 ) 2- , sulfate [(SO 4 ) 2- , peroxymonosulfate [(SO 5 ) 2- , thiosulfate [(S 2 O 3 ) 2- , dithionite [(S 2 O 4 ) 2- , metabisulfite [(S 2 O 5 ) 2- , dithionate [(S 2 O 6 ) 2- , persulfate [(S 2 O 7 ) 2- , peroxydisulfate [(S 2 O 8 ) 2- , trithionate [(S 3 O 6 ) 2- , tetrathionate [(S 4 O 6 ) 2- , pentathionate [(S 5 O 6 ) 2- and having the general formula S n [(SO 3 ) 2- 2 ​Higher polysulfate anions, where n is an integer in the range from 4 to 18.

[0064] In some first embodiments, the polyvalent anion (which may be an oxyanion or an oxyanion having one or more protons attached thereto) contains one or more heteroatoms other than oxygen, where the at least one heteroatom is an element of Group 13, 14, or 15 of the Mendeleev periodic table of the elements based on the new IUPAC system.

[0065] The polyvalent anion is optionally selected from the group consisting of: carboxylate, phosphate [(PO 4 ) 3- , hydrogen phosphate [(HPO 4 ) 2- , pyrophosphate [(P 2 O 7 ) 4- , acid pyrophosphate [(HP 2 O 7 ) 3- and (H 2 P 2 O 7 ) 2- , triphosphate [(P 3 O 10 ) 5- , acid triphosphate [(HP 3 O 10 ) 4- , (H 2 P 3 O 10 ) 3- and (H 3 P 3 O 10 ) 2- , phosphite [(HPO 3 ) 2- , pyrophosphite [(H 2 P 2 O 5 ) 2- , carbonate [(CO 3 ) 2- , borate [(BO 3 ) 3- and combinations thereof.

[0066] The first carboxylate polyvalent anion according to the invention is oxalate (C 2 O 4 2- ).

[0067] In addition to oxalate, the carboxylate polyvalent anions according to the invention typically have the general formula

[0068] R**(-COO - ) N

[0069] wherein N is an integer greater than or equal to 2, preferably from 2 to 10, more preferably from 2 to 5, still more preferably 2 or 3, and R** is a C 1 -C 30 N-valent hydrocarbon group (wherein N is defined as above), which may optionally be interrupted by one or more heteroatoms and / or substituted by one or more functional groups other than -COO - groups.

[0070] R** may also be substituted by one or more acidic carboxylic acid groups (-COOH); examples of carboxylate polyvalent anions substituted by one or more acidic carboxylic acid groups are monoacid dicarboxylates derived from tricarboxylic acids (having one -COOH group and two -COO - groups), monoacid tricarboxylates derived from tetracarboxylic acids (having one -COOH group and three -COO - groups), diacid dicarboxylates derived from tetracarboxylic acids (having two -COOH groups and two -COO - groups) and combinations thereof. Despite the above description, R* generally does not contain any acidic carboxylic acid groups (-COOH).

[0071] The N-valent hydrocarbon group may be straight-chain, branched or cyclic. It may be saturated or unsaturated. It may be aliphatic or aromatic.

[0072] According to the present invention, R** may be a C 1 -C 30 N-valent saturated (e.g., C 1 -C 30 N-valent alkanediyl) or unsaturated (e.g., C 1 -C 30 N-valent alkenediyl or alkynediyl) hydrocarbon group.

[0073] In addition, R** is preferably a C 1 -C 20 N-valent hydrocarbon group, more preferably a C 1 -C 15 N-valent hydrocarbon group, still more preferably a C 1 -C 10 N-valent hydrocarbon group, even more preferably a C 1 -C 10 N-valent hydrocarbon group, most preferably a C 1 -C 3 N-valent hydrocarbon group. The one or more heteroatoms may be selected from the group consisting of O, N, S and combinations thereof.

[0074] The one or more except -COO - Other functional groups can be selected from -OH, -NH 2 , -X (wherein X is a halogen atom (including -F, -Cl, -Br, -I and combinations thereof)), -C(=O)NH 2 、-NO 2 , =NH, =O and a combination thereof.

[0075] The carboxylate polyvalent anion may be of the general formula - OOC-R'-COO - dicarboxylate, where R' is C 1 -C 30 A divalent hydrocarbon radical which may be optionally interrupted by one or more heteroatoms and / or by one or more - Other functional groups are substituted.

[0076] Preferably, the group R' corresponds to a group R** as defined above, wherein N=2.

[0077] The one or more heteroatoms may be selected from the group consisting of O, N, S, and combinations thereof.

[0078] The one or more except -COO - Other functional groups can be selected from -OH, -NH 2 、-NO 2 , =NH, =O and a combination thereof.

[0079] Preferably, the dicarboxylate is selected from:

[0080] - aliphatic saturated dicarboxylate, preferably selected from oxalate (C 2 O 4 2- ), malonate (C 3 H 2 O 4 2- ), succinate (C 4 H 4 O 4 2- ), glutarate (C 5 H 6 O 4 2- ), adipate (C 6 H 8 O 4 2- ), sebacate (C 10 H 16 O 4 2-aliphatic saturated dicarboxylate radicals of a group consisting of it and its combinations, more preferably oxalate radicals;

[0081] - aliphatic unsaturated dicarboxylate radicals, preferably selected from the group consisting of maleate radicals (C 4 H 2 O 4 2- ), maleate radicals (C 4 H 2 O 4 2- , (2Z)-but-2-enedioate radicals), fumarate radicals (C 4 H 2 O 4 2- , (2E)-but-2-enedioate radicals), itaconate radicals (C 5 H 4 O 4 2- ), glutaconate radicals (C 5 H 4 O 4 2- , (E)-pent-2-enedioate radicals) and their combinations; aliphatic unsaturated dicarboxylate radicals of a group consisting of

[0082] - substituted aliphatic dicarboxylate radicals, preferably aliphatic hydroxy dicarboxylate radicals such as glycerate radicals (C 3 H 2 O 5 2- ), malate radicals (C 4 H 4 O 5 2- ), tartrate radicals (C 4 H 4 O 6 2- ) or their combinations, aliphatic amino dicarboxylate radicals such as aspartate radicals (C 4 H 5 NO 4 2- ), aliphatic keto dicarboxylate radicals such as pyruvate radicals (C 3 O 5 2- ) and / or 2-oxoglutarate radicals (C 5 H 4 O 5 2- ) and their combinations, more preferably malate radicals; and

[0083] - aromatic dicarboxylate radicals, preferably selected from the group consisting of benzene-1,3-dicarboxylate radicals (C 7 H 4 O 42- )), benzene-1,4-dicarboxylate (C 7 H 4 O 4 2- ), phthalate (C 8 H 4 O 4 2- ), isophthalate (C 8 H 4 O 4 2- ), terephthalate (C 8 H 4 O 4 2- ) and combinations thereof.

[0084] According to the present invention, the carboxylate polyvalent anion can be a tricarboxylate having the following general formula:

[0085]

[0086] wherein R* is a C 1 -C 30 trivalent hydrocarbon group, which may optionally be interrupted by one or more heteroatoms and / or substituted by one or more functional groups other than -COO - .

[0087] Preferably, the group R* corresponds to the group R** as defined above, where N = 3.

[0088] The one or more heteroatoms may be selected from the group consisting of O, N, S and combinations thereof.

[0089] The one or more functional groups other than -COO - may be optionally selected from the group consisting of -OH, -NH 2 , -NO 2 , =NH, =O and combinations thereof.

[0090] Preferably, the tricarboxylate is selected from:

[0091] - unsubstituted tricarboxylates, including unsubstituted aromatic tricarboxylates [such as trimesate (C 9 H 3 O 6 3- )] and unsubstituted aliphatic tricarboxylates, preferably unsubstituted aliphatic tricarboxylates, more preferably propane-1,2,3-tricarboxylate (C 6 H 5 O 6 3- ) and / or aconitate (C6 H 3 O 6 3- )); and

[0092] - a substituted tricarboxylate, preferably an aliphatic hydroxytricarboxylate, more preferably an aliphatic hydroxytricarboxylate selected from the group consisting of citrate (C 6 H 5 O 7 3- ), isocitrate (C 6 H 5 O 7 3- ), oxalosuccinate (C 6 H 3 O 7 3- ) and combinations thereof, even more preferably citrate.

[0093] According to an embodiment of the present invention, the carboxylate polyvalent anion may be a polycarboxylate having the general formula as described above, wherein N is greater than 3.

[0094] The polycarboxylate may be selected from the group consisting of tetracarboxylates (N = 4) such as ethylenediaminetetraacetate (C 10 H 12 N 2 O 8 4- ), pentacarboxylates (N = 5) such as diethylenetriaminepentaacetate (C 14 H 18 N 3 O 10 5- ), polycarboxylate polymers (preferably aminopolyacetates) and combinations thereof.

[0095] According to a preferred embodiment of the present invention, the polyvalent anion is selected from dicarboxylates, tricarboxylates and combinations thereof. More preferably, the polyvalent anion is selected from aliphatic saturated dicarboxylates (preferably oxalate), aliphatic hydroxy dicarboxylates (preferably malate), aliphatic hydroxy tricarboxylates (preferably citrate) and combinations thereof.

[0096] According to another preferred embodiment of the present invention, the polyvalent anion is different from the silicate anion; more preferably, it is different from any silicon-containing anion. However, in a specific embodiment of the present invention, the polyvalent anion is a silicate anion. The silicate anion may be selected from the group consisting of orthosilicate [(SiO 4 ) 4- , metasilicate [(SiO 3 ) 2- , polymeric metasilicate {[(SiO 3 ) 2- ​n , where n is an integer greater than 1}, metasilicate [(Si 2 O 7 ) 6- , hexafluorosilicate [(SiF 6 ) 2- , hexahydroxysilicate {[Si(OH) 6 2-}, and combinations thereof. For the avoidance of doubt, in this particular embodiment, the additive of interest (which is then a salt containing a silicate anion) is not produced by reacting wood ash with an alkali in the reaction mixture, although its chemical properties and those of the silicate produced by reacting wood ash with an alkali in the reaction mixture may be the same.

[0097] In some other embodiments, the polyvalent anion (which may be an oxyanion or an oxyanion with one or more protons attached thereto) contains one or more heteroatoms other than oxygen, where the at least one heteroatom is an element of Group 16 of the Mendeleev periodic table based on the new IUPAC system.

[0098] Then, the polyvalent anion can be an oxyanion selected from the group consisting of sulfite anions, selenite anions, and tellurite anions.

[0099] According to a preferred embodiment of the present invention, the polyvalent anion is a sulfite anion.

[0100] The sulfite anion can be selected from the group consisting of: sulfite [(SO 3 ) 2- , sulfate [(SO 4 ) 2- , peroxymonosulfate [(SO 5 ) 2- , thiosulfate [(S 2 O 3 ) 2- , dithionite [(S 2 O 4 ) 2- , metabisulfite [(S 2 O 5 ) 2- , dithionate [(S 2 O 6 ) 2- , disulfate [(S 2 O 7 ) 2- , peroxydisulfate [(S 2 O 8 ) 2- , trithionate [(S 3 ​O 6 ) 2- , tetrathionate [(S 4 O 6 ) 2- , pentathionate [(S 5 O 6 ) 2- , and higher polythionate anions having the general formula S n [(SO 3 ) 2- 2 , where n is an integer in the range from 4 to 18.

[0101] Preferably, the thiooxyanion is sulfate [(SO 4 ) 2- .

[0102] The polyvalent anion can also be a selenooxyanion.

[0103] In particular, the selenooxyanion can be selenite [(SeO 3 ) 2- and / or selenate [(SeO 4 ) 2- .

[0104] The polyvalent anion can also be a tellurooxyanion.

[0105] In particular, the tellurooxyanion can be tellurite [(TeO 3 ) 2- , metatellurate [(TeO 4 ) 2- , orthotellurate [(TeO 6 ) 6- or a combination thereof.

[0106] The salt preferably contains an alkali metal cation, an ammonium cation, or a quaternary ammonium cation having the formula NR 4 + (where R = C 1 -C 20 , preferably C 1 -C 10 , more preferably C 1 -C 5 alkyl groups), more preferably an alkali metal cation.

[0107] Preferably, the salt contains a cation selected from the group consisting of Na + , K + , Cs + , Li + , Rb + and combinations thereof; more preferably, the salt is Na + , K​+ or a combination thereof.

[0108] Furthermore, preferably, the salt is selected from the group consisting of sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, and combinations thereof. Thus, particularly preferably, the salt is sodium citrate (Na 3 C 6 H 5 O 7 ), potassium citrate (K 3 C 6 H 5 O 7 ), or a combination thereof. Also preferably, the salt is sodium sulfate (Na 2 SO 4 ), potassium sulfate (K 2 SO 4 ), or a combination thereof.

[0109] According to the present invention, the reaction mixture may contain more than one additive as defined above. In other words, the reaction mixture according to the present invention may contain more than one salt containing a polyvalent anion as defined above, for example, the reaction mixture according to the present invention may contain more than one salt containing a thiooxo anion as defined above.

[0110] According to the present invention, the amount of the additive in the reaction mixture should be significantly higher than the catalytic amount and should be adjusted according to the level of crystalline silica that is not dissolved by alkaline digestion (i.e., the reaction of plant ash (i) with alkali (ii) and the dispersion medium (iii)).

[0111] Preferably, the amount of the additive in the reaction mixture is at least 1 g / L, at least 5 g / L, at least 10 g / L, or at least 15 g / L, more preferably at least 20 g / L. It may be at least 30 g / L, at least 50 g / L, or even at least 100 g / L. The maximum amount of the additive is not particularly limited, and it should be understood that this amount should advantageously not exceed its solubility limit in the dispersion medium; in fact, the amount of the additive in the reaction mixture is usually at most 500 g / L and may be at most 300 g / L, at most 150 g / L, or at most 100 g / L.

[0112] Preferably, the amount of plant ash in the reaction mixture is at least 10 wt.%, at least 15 wt.%, at least 20 wt.%, more preferably at least 22 wt.% based on the total weight of the reaction mixture. The maximum amount of the additive is not particularly limited, and it should be understood that this amount should advantageously not exceed its solubility limit in the dispersion medium; in fact, the amount of plant ash in the reaction mixture is at most 50 wt.% based on the total weight of the reaction mixture and may be at most 40 wt.%, at most 35 wt.%, or at most 30 wt.%.

[0113] Based on the weight of plant ash in the reaction mixture, the molar amount of the additive can be at least 0.01 mmol / g, at least 0.02 mmol / g, or at least 0.05 mmol / g. Preferably, it is at least 0.10 mmol / g, at least 0.20 mmol / g, or at least 0.30 mmol / g. It can be at least 0.50 mmol / g or even at least 0.70 mmol / g. As noted above, there is no particular limitation on the maximum amount of the additive; that being said, based on the weight of plant ash in the reaction mixture, the molar amount of the additive is generally at most 10 mmol / g and can be at most 4.0 mmol / g, at most 2.0 mmol / g, at most 1.5 mmol / g, or at most 1.0 mmol / g.

[0114] In addition, it is preferred that the amount of base in the reaction mixture is advantageously characterized by a weight ratio (R p ) [SiO 2 :[Na 2 O] that is well known to those skilled in the art, producers of silicates and / or precipitated silica. 2 :[Na 2 O] is preferably from 2 to 10, more preferably from 3 to 8, even more preferably from 3 to 6, and most preferably from 3 to 4.

[0115] According to the present invention, the reaction mixture can be formed by bringing together plant ash, a base, a dispersion medium, and an additive and / or a precursor of the additive. In fact, it has been found that the additive, which is a salt containing a polyvalent anion, can advantageously be added to the reaction mixture as the salt already formed and / or can be formed in situ by adding a precursor of the salt, in particular an acid containing a polyvalent anion, to the reaction mixture.

[0116] The acid containing a polyvalent anion is the acid equivalent of the above-mentioned polyvalent anion according to any one of the embodiments of the present invention.

[0117] The acid containing a polyvalent anion can be selected from the group consisting of boric acid (BH 3 O 3 ), carbonic acid (CH 2 O 3 ), carboxylic acids, phosphoric acid (H 3 PO 4 ), pyrophosphoric acid (H 4 P 2 O 7 ), triphosphoric acid (H 5 P 3 O 10 ), phosphorous acid (H 3 PO 3 ), pyrophosphorous acid (H 4P 2 O 5 )), sulfuric acid (H 2 SO 4 ), sulfurous acid (H 2 SO 3 ), peroxymonosulfuric acid (H 2 SO 5 ), thiosulfuric acid (H 2 S 2 O 3 ), dithionous acid (H 2 S 2 O 4 ), pyrosulfurous acid (H 2 S 2 O 5 ), dithionic acid (H 2 S 2 O 6 ), pyrosulfuric acid (H 2 S 2 O 7 ), peroxydisulfuric acid (H 2 S 2 O 8 ), trithionic acid (H 2 S 3 O 6 ), tetrathionic acid (H 2 S 4 O 6 ), pentathionic acid (H 2 S 5 O 6 ), and higher polythionic acids having the general formula S n [(SO 3 H) 2 (where n is an integer in the range from 4 to 18) and combinations thereof.

[0118] Optionally, the acid containing a polyvalent anion is selected from carboxylic acids, phosphoric acid (H 3 PO 4 ), pyrophosphoric acid (H 4 P 2 O 7 ), triphosphoric acid (H 5 P 3 O 10 ), phosphorous acid (H 3 PO 3 ), pyrophosphorous acid (H 4 P 2 O 5 ), carbonic acid (CH 2 O 3 ), boric acid (B H 3 O 3) and combinations thereof. Preferably, the acid containing a polyvalent anion is a carboxylic acid selected from dicarboxylic acids, tricarboxylic acids, and combinations thereof.

[0119] The dicarboxylic acid can be selected from the group consisting of: oxalic acid (C 2 H 2 O 4 ), malonic acid (C 3 H 4 O 4 ), succinic acid (C 4 H 6 O 4 ), glutaric acid (C 5 H 8 O 4 ), adipic acid (C 6 H 10 O 4 ), sebacic acid (C 10 H 18 O 4 ), fumaric acid (C 4 H 4 O 4 ), maleic acid (C 4 H 4 O 4 , trans-butenedioic acid), maleic acid (C 4 H 4 O 4 , cis-butenedioic acid), itaconic acid (C 5 H 6 O 4 ), pentenedioic acid (C 5 H 6 O 4 , (E)-pent-2-enedioic acid), glycolic acid (C 3 H 4 O 5 ), malic acid (C 4 H 6 O 5 ), tartaric acid (C 4 H 6 O 6 ), aspartic acid (C 4 H 7 NO 4 ), pyruvic acid (C 3 H 2 O 5 ), oxaloacetic acid (C 4 H 4 O 5 ), terephthalic acid (C 8 H 6 O 4) and combinations thereof. Preferably, it is oxalic acid and / or maleic acid.

[0120] The tricarboxylic acid may be selected from the group consisting of malonic acid (C 6 H 8 O 6 ), aconitic acid (C 6 H 6 O 6 ), trimellitic acid (C 9 H 6 O 6 ), citric acid (C 6 H 8 O 7 ), isocitric acid (C 6 H 8 O 7 ), oxalosuccinic acid (C 6 H 6 O 7 ) and combinations thereof; preferably, it is citric acid.

[0121] Possibly, the acid containing the polyvalent anion is an equivalent acid of the thiooxyanion mentioned above. Preferably, the acid of the thiooxyanion is selected from the group consisting of: sulfuric acid (H 2 SO 4 ), sulfurous acid (H 2 SO 3 ), peroxymonosulfuric acid (H 2 SO 5 ), thiosulfuric acid (H 2 S 2 O 3 ), dithionous acid (H 2 S 2 O 4 ), pyrosulfurous acid (H 2 S 2 O 5 ), dithionic acid (H 2 S 2 O 6 ), pyrosulfuric acid (H 2 S 2 O 7 ), peroxydisulfuric acid (H 2 S 2 O 8 ), trithionic acid (H 2 S 3 O 6 ), tetrathionic acid (H 2 S 4 O 6 ), pentathionic acid (H 2 S 5 O 6 ), having the general formula Sn [(SO 3 H) 2 where n is an integer in the range from 4 to 18) and combinations thereof. More preferably, the acid is sulfuric acid.

[0122] In a specific embodiment (corresponding to the specific embodiments described above for the polyvalent anion), the acid containing the polyvalent anion is at least one silicic acid.

[0123] Particularly preferred precursors of the additive are acids containing polyvalent anions selected from the group consisting of sulfuric acid, citric acid, and combinations thereof.

[0124] Another possible precursor of the additive is an acid salt containing a monovalent anion, such as sodium hydrogen oxalate (NaHC 2 O 4 ) or sodium dihydrogen phosphate (NaH 2 PO 4 ) or sodium hydrogen sulfate (NaHSO 4 ); as is well known to those skilled in the art, such acid salts can react with a base (such as NaOH) to obtain one or more salts containing polyvalent anions, such as sodium oxalate, or Na 2 HPO 4 and Na 3 PO 4 at least one of.

[0125] Yet another possible precursor of the additive is an acid anhydride or oxide of an acid containing a polyvalent anion, such as on the one hand citric anhydride, maleic anhydride, or succinic anhydride, and on the other hand carbon dioxide or sulfur trioxide; as is well known to those skilled in the art, when the dispersion medium is an aqueous dispersion medium, such acid anhydrides or oxides can be converted into the corresponding diacids (such as succinic acid or carbonic acid respectively), and the diacids themselves can react with a base (such as NaOH) to obtain salts containing polyvalent anions, such as sodium succinate or sodium carbonate.

[0126] When using an acid containing a polyvalent anion, an acid salt containing a monovalent anion, an acid anhydride of an acid containing a polyvalent anion, an oxide of an acid containing a polyvalent anion, or a combination thereof as a precursor of the additive, it should be understood that a sufficient amount of base must be present in the reaction medium to not only react with the plant ash to produce silicate, but also to react with the acid, acid salt, acid anhydride, oxide, or combination thereof and convert it into the additive, i.e., the corresponding salt of the acid, acid salt, acid anhydride, oxide, or combination thereof.

[0127] Preferably, the additive and / or the precursor of the additive (especially an acid containing a polyvalent anion, such as an acid containing a thiooxyanion) can be added to the reaction mixture during step (a), and / or to the mixture obtained after a pre-dissolution step (a') carried out before step (a), so as to form the reaction mixture according to the present invention. The pre-dissolution step (a') includes reacting plant ash (i) in the presence of an alkali (ii) and a dispersion medium (iii) and allowing at least partial dissolution of the plant ash. Since plant ash is a complex material which contains not only crystalline silica but also impurities derived from the combustion of plants and / or plant parts, it is believed that the pre-dissolution step (a') can contribute to obtaining improved dissolution in the presence of the additive and / or the precursor of the additive (especially an acid containing a polyvalent anion) during the subsequent step (a).

[0128] According to an embodiment of the present invention, the additive and / or the precursor of the additive (especially an acid containing a polyvalent anion) can be added directly to the ash before step (a') or step (a), and / or added to the plant before combustion.

[0129] According to the present invention, preferably, the alkali (ii) is an alkali metal hydroxide selected from the group consisting of NaOH, KOH, LiOH, CsOH, RbOH, NH 3(水溶液) and combinations thereof, and / or an alkali having the following general formula NR 4 + OH - wherein R = C 1 -C 20 , preferably C 1 -C 10 , more preferably C 1 -C 5 hydrocarbyl group). More preferably, the alkali (ii) is NaOH, KOH or a combination thereof.

[0130] In addition, preferably, the silicate produced by the method of the present invention is an alkali metal silicate. More preferably, the silicate is sodium silicate, potassium silicate or a combination thereof.

[0131] According to an embodiment, step (a) of the method of the present invention is carried out at a reaction temperature of from 120 °C to 250 °C, preferably from 120 °C to 230 °C, more preferably from 165 °C to 205 °C, and most preferably from 170 °C to 190 °C.

[0132] According to another embodiment, step (a) of the method of the present invention is carried out at a reaction temperature of from 120 °C to 250 °C, preferably from 120 °C to 230 °C, more preferably from 200 °C to 230 °C, and even more preferably from 205 °C to 220 °C.

[0133] Not wishing to be bound by any particular theory or mechanism, it has been found that the method of the present invention can surprisingly and advantageously be carried out without the need for the relatively high temperatures typically employed in the methods of the prior art, and in particular, can be carried out at temperatures from 120 °C to 250 °C. It is believed that the presence of the additive in combination with the other components of the reaction mixture employed in the method of the present invention permits improved dissolution of the silica contained in the plant ash, and in particular, the crystalline portion of said silica, which is more difficult to dissolve by alkaline digestion than the amorphous portion, i.e., by simply reacting the ash (i) with an alkali (ii) and a dispersion medium (iii).

[0134] Furthermore, step (a) of the method according to the present invention should be carried out for a duration sufficient to effect dissolution of the crystalline silica.

[0135] Preferably, step (a) is carried out for at least 10 minutes, more preferably more than 30 minutes, even more preferably more than 60 minutes, most preferably more than 90 minutes, but most preferably less than 240 minutes. More preferably, the reaction time is the amount of time during which the reaction temperature of step (a) as mentioned above is maintained.

[0136] According to an embodiment of the present invention, the amount of crystalline silica is at least 0.1 wt.%, preferably at least 1 wt.%, more preferably at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 25 wt.%, at least 30 wt.%, at least 40 wt.%, at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, at least 80 wt.%, or at least 90 wt.%, based on the total content of silica (SiO 2 ) contained in the ash.

[0137] In an embodiment of the present invention, the amount of crystalline silica is preferably from 5 wt.% to 50 wt.%, based on the total content of silica (SiO 2 ) contained in the ash.

[0138] The crystalline silica according to the present invention may comprise a crystalline form selected from the group consisting of quartz, cristobalite, tridymite, and combinations thereof.

[0139] According to the present invention, the silica-containing plant is preferably an angiosperm, more preferably a monocotyledon or a dicotyledon, and still more preferably a plant belonging to a family selected from the group consisting of Poaceae, Equisetaceae, Cyperaceae, Cucurbitaceae, Cannabaceae, Arecaceae, Brassicaceae, and combinations thereof.

[0140] The silica-containing plant may also be a tree. Preferably, the tree is selected from the group consisting of pine, oak, birch, elm, and combinations thereof.

[0141] Preferably, plants belonging to the Poaceae family are selected from the group consisting of rice, wheat, sugarcane, bamboo, oats, barley, rye, sorghum, triticale, reed canarygrass, reed, corn, miscanthus, and combinations thereof.

[0142] Preferably, the plant belonging to the Equisetaceae family is field horsetail.

[0143] Preferably, the plant belonging to the Cyperaceae family is sedge.

[0144] Preferably, plants belonging to the Cucurbitaceae family are selected from the group consisting of melons, watermelons, pumpkins, cucumbers, and combinations thereof.

[0145] Preferably, the plant belonging to the Cannabaceae family is cannabis.

[0146] Preferably, the plant belonging to the Arecaceae family is a palm tree.

[0147] Preferably, the plant belonging to the Brassicaceae family is rapeseed.

[0148] According to the examples, the silica-containing plants are plants selected from the group consisting of rice, wheat, rapeseed, barley, bamboo, field horsetail, sedge, watermelon, and combinations thereof.

[0149] According to the present invention, plant ash is obtained from the burning of silica-containing plant parts and / or plants.

[0150] Preferably, the parts of the silica-containing plant (i.e., the silica-containing plant parts) are selected from the group consisting of roots, stems, leaves, flowers, fruits, husks, stalks, stems, woods, and combinations thereof.

[0151] According to the present invention, the parts of the silica-containing plant can also be derived from the processing of plants, such as straw (e.g., cereal straw), bagasse (e.g., sugarcane bagasse), oil (e.g., palm oil), sawdust (e.g., tree sawdust), and / or pellets (e.g., wood pellets).

[0152] The plant parts can be selected from the group consisting of rice husks, rice straw, wheat husks, wheat straw, barley straw, barley husks, sugarcane bagasse, sugarcane leaves, bamboo stems, bamboo leaves, corn cobs, palm tree oil, miscanthus stems, miscanthus leaves, sedge leaves, watermelon fruits, tree woods, and combinations thereof.

[0153] In an example of the present invention, the silica-containing plant is rice, and preferably, the part of the silica-containing plant is the husk.

[0154] According to this example, once burned, rice husk ash will contain a relatively high amount of silica, which is preferably at least 5 wt.%, more preferably at least 10 wt.%, and most preferably at least 15 wt.% based on the total weight of the ash.

[0155] According to the present invention, the combustion of the silicon dioxide-containing plant part and / or the plant is carried out by conventional techniques by burning the silicon dioxide-containing plant part and / or the plant.

[0156] According to an embodiment, the silicon dioxide-containing plant part and / or the plant can be subjected to at least one pretreatment. Preferably, the at least one pretreatment is washing with water, more preferably washing with acidified water.

[0157] Preferably, before combustion, a compound selected from the group consisting of Na 2 CO 3 、K 2 CO 3 、NaOH, KOH, and combinations thereof, and / or any other similar compounds known to those skilled in the art can be added to the silicon dioxide-containing plant part and / or the plant to reduce the amount of crystalline silicon dioxide produced thereby. Preferably, if the pretreatment is carried out, the compound is added to the silicon dioxide-containing plant part and / or the plant after the pretreatment.

[0158] Preferably, the combustion of the silicon dioxide-containing plant part and / or the plant is carried out at a temperature from 300 °C to 1500 °C, preferably from 500 °C to 1000 °C. According to an embodiment, the combustion of the silicon dioxide-containing plant part and / or the plant is carried out at a temperature of at least 700 °C, preferably at a temperature from 700 °C to 1000 °C, and the silicon dioxide-containing plant part and / or the plant preferably contains a compound selected from the group consisting of Na 2 CO 3 、K 2 CO 3 、NaOH, KOH, and combinations thereof, as described above.

[0159] According to another embodiment, the combustion of the silicon dioxide-containing plant part and / or the plant is carried out at a temperature below 700 °C, preferably at a temperature from 500 °C to below 700 °C.

[0160] According to an embodiment, the plant ash is subjected to at least one pretreatment before being used in step (a) and / or (a') of the method of the present invention. Preferably, the at least one pretreatment is or includes washing with water and / or washing with acidified water; more preferably, the at least one pretreatment is or includes washing with acidified water.

[0161] In addition, according to an embodiment, a compound selected from the group consisting of Na 2 CO 3 、K 2 CO 3Compounds of the group consisting of NaOH, KOH and combinations thereof and / or other similar compounds known to those skilled in the art are added to the plant ash before it is used in step (a) and / or (a'). Preferably, if said pretreatment is carried out, the compound is added to the plant ash after said pretreatment.

[0162] According to a first preferred embodiment of the invention, at least a part of the crystalline silica is cristobalite. In this first embodiment, in addition to cristobalite, the crystalline silica may also contain quartz and / or tridymite. In this first embodiment, most of the crystalline silica is preferably cristobalite. In this first embodiment, based on the total content of crystalline silica, preferably at least 50 wt.%, at least 60 wt.%, at least 70 wt.% or at least 80 wt.%, more preferably at least 90 wt.%, even more preferably at least 99 wt.%, and most preferably about 100 wt.% of the crystalline silica is cristobalite. In this first embodiment, most preferably, the crystalline silica consists essentially of cristobalite. In this first embodiment, most preferably, the crystalline silica contains only trace amounts of quartz and / or tridymite. In this first embodiment, preferably, the amount of cristobalite is at least 0.1 wt.%, preferably at least 1 wt.%, more preferably at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 25 wt.%, at least 30 wt.%, at least 40 wt.%, at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, at least 80 wt.% or at least 90 wt.% based on the total content of silicon dioxide (SiO 2 ) contained in the ash. In this first embodiment, the amount of cristobalite contained in the plant ash may range from 5 wt.% to 50 wt.% based on the total content of silicon dioxide (SiO 2 ) contained in the ash. In this first embodiment of the invention, wherein at least a part of the crystalline silica is cristobalite, and preferably, wherein most of the crystalline silica is cristobalite, step (a) of the method of the invention is preferably carried out at a reaction temperature of from 165 °C to 205 °C, more preferably from 170 °C to 190 °C. Still in this first embodiment, wherein at least a part of the crystalline silica is cristobalite, and preferably, wherein most of the crystalline silica is cristobalite, the most preferred silica-containing plant is rice, and the preferred rice parts are rice husks and rice straws, and the most preferred rice part is rice husks (because once rice husks are burned, their ash will contain a relatively high amount of silicon dioxide, which is preferably at least 5 wt.%, more preferably at least 10 wt.%, and most preferably at least 15 wt.% based on the total weight of the ash).

[0163] According to another preferred embodiment of the present invention, at least a part of the crystalline silica is quartz. In this other embodiment, most of the crystalline silica is preferably quartz, and the remaining part, based on the total content of crystalline silica up to 100 wt.%, includes cristobalite and / or tridymite. In this other embodiment, based on the total content of crystalline silica, preferably at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, or at least 80 wt.% of the crystalline silica, more preferably at least 90 wt.%, even more preferably at least 99 wt.%, and most preferably about 100 wt.% is quartz. In this other embodiment, most preferably, the crystalline silica consists essentially of quartz. In this other embodiment, most preferably, the crystalline silica contains only trace amounts of cristobalite and / or tridymite. In this other embodiment, preferably, the amount of quartz is at least 0.1 wt.%, preferably at least 1 wt.%, more preferably at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 25 wt.%, at least 30 wt.%, at least 40 wt.%, at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, at least 80 wt.%, or at least 90 wt.% based on the total content of silicon dioxide (SiO 2 ) contained in the ash. In this other embodiment of the present invention, the amount of quartz contained in the plant ash can range from 5 wt.% to 50 wt.% based on the total content of silicon dioxide (SiO 2 ) contained in the ash. In this other embodiment of the present invention, where at least a part of the crystalline silica is quartz, and preferably, where most of the crystalline silica is quartz, step (a) of the method of the present invention is preferably carried out at a reaction temperature of from 200 °C to 230 °C, more preferably from 205 °C to 220 °C. Still in this other embodiment, where at least a part of the crystalline silica is quartz, and preferably, where most of the crystalline silica is quartz, preferably, the silicon dioxide-containing plant is selected from the group consisting of trees, sugarcane, and combinations thereof; more specifically:

[0164] - Preferably, the tree is selected from the group consisting of pine trees, oak trees, birch trees, elm trees, and combinations thereof;

[0165] - Preferably, the part of the silicon dioxide-containing plant (i.e., the silicon dioxide-containing plant part) is wood, such that the plant part is tree wood;

[0166] - The part of the silicon dioxide-containing plant can also be derived from the processing of the plant, such as bagasse (e.g., sugarcane bagasse), sawdust (e.g., tree sawdust), and / or pellets;

[0167] - More preferably, the silicon dioxide-containing plant part is tree wood and / or sugarcane bagasse;

[0168] - Once burned, the ash of the tree wood and / or the bagasse ash advantageously contains a relatively high amount of silica, which is preferably at least 5 wt.%, more preferably at least 10 wt.%, and most preferably at least 15 wt.% based on the total weight of the ash.

[0169] According to the present invention, the silicate is preferably in the form of a silicate solution, more preferably an aqueous silicate solution, and the method of the present invention may further include a step (b) of separating the silicate solution obtained after step (a) from the impurities derived from the ash, which impurities include carbon products and metals.

[0170] The metals are, for example, metals (and / or metalloids) selected from the group consisting of metals of the boron group (especially B and / or Al), alkali metals (especially K), alkaline earth metals (especially Ca and / or Mg), transition metals (especially at least one of Fe, Mn, Ni, Ti, and Zn), and combinations thereof.

[0171] According to an embodiment, the impurities derived from the ash may include additional chemical elements selected from the group consisting of elements of the nitrogen group (especially P and / or As), halogens (especially Cl), and combinations thereof.

[0172] According to the present invention, the impurities may be present in the silicate solution as insoluble solids and / or in a solvated form.

[0173] Any conventional separation technique (such as centrifugation and / or filtration) may be used during step (b) to separate the silicate solution obtained after step (a) from the impurities derived from the ash, which impurities include carbon products and metals.

[0174] According to another embodiment, the silicate is in the form of a solid silicate, and the method of the present invention further includes a step (c) of drying the silicate solution obtained after step (b) to obtain the silicate in solid form.

[0175] Preferably, the drying of the silicate solution obtained after step (b) is carried out by a conventional drying technique.

[0176] The conventional drying techniques are preferably selected from the group consisting of turbo drying, rotary flash drying, atomization, spray drying, and combinations thereof.

[0177] In addition, preferably, the conventional drying technique is carried out in the absence of oxygen and / or in a short time to avoid oxidation and degradation of the product.

[0178] The present invention further relates to a silicate, preferably an alkali metal silicate, which can be obtained in liquid form as a silicate solution by the method including step (b) described above. Preferably, the silicate solution is an aqueous solution of silicate.

[0179] A remarkable feature of the silicate of the present invention, especially the alkali metal silicate, is that it generally contains additives. The silicate of the present invention may contain the same amount, substantially the same amount or a lower amount of additives as the amount of additives introduced into the reaction mixture, which especially depends on the nature of the additives and the operating conditions of steps (a) and (b).

[0180] Preferably, the silicate solution is a solution characterized in that the silicate content is at least 5 wt.%, more preferably at least 10 wt.%, even more preferably at least 15 wt.%, most preferably at least 20 wt.%, and even most preferably at least 22 wt.% by weight based on the total weight of the solution.

[0181] In addition, preferably, the silicate solution is a solution characterized in that the ratio [SiO 2 :[Na 2 O] is from 2 to 10, more preferably from 3 to 8, even more preferably from 3 to 6, and most preferably from 3 to 4.

[0182] Without wishing to be bound by a particular theory, it has been found that when the silicate solution is used for the production of precipitated silica, the ratio allows for an advantageous and effective silica synthesis because it results in a good yield, a defined structure and limited salt formation.

[0183] As pointed out above, it is preferred that the silicate produced by the method of the present invention is an alkali metal silicate. Nevertheless, those skilled in the art should understand that the method of the present invention can also be easily used to produce silicates other than alkali metal silicates. Possibly, an alkali metal silicate is first produced according to step (a) as described above, or according to steps (a) and (b), or according to steps (a), (b) and (c), and then the alkali metal silicate is reacted with a hydroxide of a metal other than an alkali metal hydroxide. Possibly, the reaction involves an ion exchange between the alkali metal silicate and the hydroxide of a metal other than an alkali metal hydroxide. The hydroxide of a metal other than an alkali metal hydroxide can in particular be an alkaline earth metal hydroxide, such as magnesium hydroxide, calcium hydroxide, strontium hydroxide and / or barium hydroxide. Well-known examples thereof taught in encyclopedias (such as Wikipedia Encyclopedia https: / / en.wikipedia.org / wiki / Sodium_silicate) and educational publications (such as https: / / melscience.com / BE-en / chemistry / experiments / chemgarden-v3_calcium-silicate / ) are the ion exchange reaction of sodium silicate with calcium hydroxide to obtain calcium silicate or sodium calcium mixed silicate. Similarly, sodium aluminosilicate can be obtained by reacting aluminum hydroxide with an alkali metal silicate, as taught, for example, in https: / / www.fao.org / fileadmin / user_upload / jecfa_additives / docs / monograph17 / additive-391-m17.pdf.

[0184] The present invention further relates to a silicate, preferably an alkali metal silicate, which can be obtained in solid form as a solid silicate by the method including step (c) described above.

[0185] Preferably, the solid silicate is characterized by a ratio of [SiO 2 :[Na 2 O] of from 2 to 10, more preferably from 3 to 8, even more preferably from 3 to 6, and most preferably from 3 to 4.

[0186] In addition, it is preferred that the solid silicate is characterized by a humidity of at most 10%.

[0187] More preferably, the solid silicate is characterized by a particle size measurement value of at least 100 μm.

[0188] The present invention also relates to a method for preparing precipitated silica, the method comprising the following steps:

[0189] (I) Produce a silicate solution, which is achieved by the method including step (b) above or by a method including producing a silicate in solid form (by the method including step (c) above) and redispersing the silicate obtained in solid form in a dispersion medium; and

[0190] (II) React the silicate solution so produced and optionally an additional silicate solution other than the silicate solution so produced, NaOH, and / or a minor silica source with an acidifying agent to effect precipitation of silica.

[0191] Preferably, the dispersion medium is an aqueous dispersion medium, more preferably water, and the silicate solution to be used in step (II) obtained after step (I) is an aqueous silicate solution.

[0192] According to an embodiment of the present invention, step (II) includes adding a silicate solution other than the silicate solution produced according to step (I), i.e., the silicate solution so produced, NaOH, and / or a minor silica source to the silicate solution produced according to step (I) to adjust the [SiO 2 :[Na 2 O] ratio, and thus adjust the [SiO 2 :[Na 2 O] ratio of the finally precipitated silica.

[0193] According to the present invention, the silicate solution other than the silicate solution so produced is a silicate solution produced by any conventional method other than the method of the present invention.

[0194] According to the present invention, the minor silica source is silica produced by any conventional method other than the method of the present invention and / or naturally occurring silica. Preferably, the minor silica source is amorphous silica.

[0195] The acidifying agent may be an inorganic acid (preferably, an inorganic acid selected from the group consisting of sulfuric acid (H 2 SO 4 ), hydrochloric acid (HCl), nitric acid (HNO 3 ), phosphoric acid (H 3 PO 4 ) and combinations thereof, and / or carbonic acid), an organic acid (preferably, an organic acid selected from the group consisting of acetic acid, formic acid and combinations thereof) or a combination thereof.

[0196] In a preferred embodiment of the present invention, the acidifying agent is sulfuric acid, and the method includes removing sulfate anions (SO 4 2-An additional step (III) of separating the salt in solid or liquid form. This salt containing sulfate anions (SO 4 2- ) is preferably an alkali metal sulfate; more preferably, it is sodium sulfate (Na 2 SO 4 ), potassium sulfate (K 2 SO 4 ) or a combination thereof. In a more preferred embodiment, the salt containing sulfate anions obtained after step (III) is recycled by adding it to the reaction mixture in step (a) of the method of the present invention.

[0197] Without wishing to be bound by a particular theory or mechanism, it has been found that at the end of the method for preparing precipitated silica, when sulfuric acid is used as the acidifying agent, a salt containing sulfate anions is obtained, which can advantageously be used as an additive according to the present invention by adding it to the reaction mixture used in step (a) of the method of the present invention, so as to advantageously obtain a recycling method.

[0198] Furthermore, the present invention relates to a reaction mixture for producing silicates, preferably alkali metal silicates, from plant ash by a method comprising steps (a) (possibly steps (a) and (b), possibly steps (a), (b) and (c)) as defined above, and / or for preparing precipitated silica by a method comprising steps (I) and (II) (possibly steps (I), (II) and (III)) as defined above.

[0199] The reaction mixture comprises:

[0200] (i) Plant ash obtained by burning plant parts containing silica, said plant ash containing crystalline silica;

[0201] (ii) An alkali;

[0202] (iii) A dispersion medium; and

[0203] (iv) An additive which is a salt containing polyvalent anions.

[0204] Preferably, the plant ash (i), alkali (ii), dispersion medium (iii) and additive (iv) are as described above in any one of the embodiments of the present invention.

[0205] If the disclosure content of any patent, patent application and publication incorporated by reference into this application conflicts with the description of this application to such an extent that it may lead to unclear terms, then this description should prevail.

[0206] The present invention will now be illustrated by the following examples, which are not intended to be limiting.

[0207] Example

[0208] Materials and Methods

[0209] All starting materials used in the examples were commercially available. For Examples 1 - 4, rice husk ash (RHA) with the following characteristics was used:

[0210] - Silicon dioxide content measured by the Assay purity method: 83.1% of the total sample;

[0211] - Cristobalite content measured by XRD: 40% of the total SiO 2 content;

[0212] - Carbon content measured by carbon / sulfur analysis: 13.5% of the total SiO 2 content.

[0213] Determination of cristobalite and carbon content

[0214] The determination of the cristobalite fraction content in each sample was carried out according to the following procedure, which is divided into two parts: the first part is dedicated to creating the calibration curve, while the second part is dedicated to preparing the samples, performing the analysis, and calculating the cristobalite content by XRD.

[0215] XRD analyzer

[0216] The diffractometer used in the experiment was the X’PertPro from Malvern - Panalytical, with the following configuration:

[0217] Copper X - ray tube, reflection, Bragg - Brentano configuration, Bragg - Brentano HD mirror, 1 / 8° divergence slit, 4 mm mask, 0.02 rad pre - soller slit, 1 / 4° pre - anti - scatter slit, 0.02 rad post - soller slit, 1 / 4° post - anti - scatter slit, X - accelerator detector, sample holder rotating 1 time per second.

[0218] Calibration curve

[0219] The standard samples for the calibration curve were prepared with the following raw materials:

[0220] - Cristobalite was provided by Solvay (n°22MAU302), with a purity of 66.6 wt% + / - 2.5 wt%. It also contained tridymite and amorphous silica

[0221] - Amorphous silica was provided by Solvay (n°19STD078)

[0222] - α-Al 2 O 3 (purity > 99%)

[0223] Nine samples with different amounts of cristobalite and a constant amount of α-Al 2 O 3 (20 wt%) were prepared as shown in Table 1 below. The following steps were followed to prepare each sample:

[0224] - Mix for 30 minutes in a Turbula shaker;

[0225] - Manually grind the powder with an agate mortar and pestle;

[0226] - Mix for 30 minutes in a Turbula shaker.

[0227] Table 1

[0228]

[0229] For each of the nine samples, three XRD sample holders with a diameter of 16 mm were prepared, which were back-loading sample holders.

[0230] Each XRD sample holder was analyzed from 2θ = 33° to 40° at 200 seconds / step with a step size of 0.017°.

[0231] The area of the peak of Al 2 O 3 at 35.14° and the combined area of the peaks of cristobalite at 36.08° and 38.38° were measured using HighScore Plus 4.8 software.

[0232] The calibration curve area ratio between the cristobalite peak and the Al 2 O 3 peak was established as a function of the weight percentage of cristobalite in the sample.

[0233] Preparation and quantification of samples

[0234] For each measurement, two preparations were carried out according to the same procedure described below to ensure quantitative representativeness.

[0235] Weigh 1.2 g of the sample in a glass bottle and add 0.3 g of α-Al 2 O 3 . The following steps were followed to ensure uniform mixing:

[0236] - Mix for 30 minutes in a Turbula shaker;

[0237] - Manually grind the powder with an agate mortar and pestle;

[0238] - Mix in a Turbula shaker for 30 minutes.

[0239] For each preparation, load a 16 mm diameter back-loading XRD sample holder and analyze according to the following analysis method; 2θ = 33° to 40° at 200 seconds / step, step size 0.017°, copper X-ray tube, reflection, Bragg-Brentano configuration, Bragg-Brentano HD mirror, 1 / 8° divergence slit, 4 mm mask, 0.02 rad pre-Soller slit, 1 / 4° pre-anti-scatter slit, 0.02 rad post-Soller slit, 1 / 4° post-anti-scatter slit, X-accelerator detector, sample holder rotates 1 time per second. Measure the peak area using HighScore Plus 4.8 software.

[0240] The combined area of 2 cristobalite peaks (at 36.08° and 38.38°) and the peak area of Al at 35.14° 2 O 3 The ratio of the peak area is used to determine the cristobalite content by using a calibration curve.

[0241] Carbon / sulfur analysis

[0242] Analyze a 200 mg sample in a Horiba EMIA 320-V2. Iron balls and tin balls are used as combustion accelerators. Use a CS26 - 3.19% calibration sensor.

[0243] Determination of purity analysis

[0244] Ignite 1 g of the sample in a peeled platinum dish at 1000 °C for 1 hour, cool in a desiccator and weigh. Moisten the resulting solid with water and add 10 mL of hydrofluoric acid in small increments. Then evaporate the mixture to dryness on a steam bath and then cool. Add 10 mL of hydrofluoric acid and 0.5 mL of sulfuric acid and then evaporate to dryness. Then slowly raise the temperature until all the acid has evaporated. Then ignite the sample at 1000 °C, cool in a desiccator, and then weigh. The ratio between the difference in the final weight on one hand and the weight of the initially ignited portion on the other hand and the weight of the original sample on the other hand represents the weight percentage of SiO 2 2.

[0245] Potentiometry

[0246] Use a Titrando 808 to determine the weight ratio (Rp) [% weight (SiO 2 ) / % weight (Na 2O). The device is equipped with a reference electrode Ag / AgCl in 3M KCl and a tungsten working electrode. Each Rp is measured repeatedly, and the Rp value is the average between two measurements.

[0247] Weigh 0.5 g of the sample and make up to 30 mL with demineralized water. The titrant solution is 0.1 N HCl solution. The volume V1 (mL) is determined as the equivalence of the titration. After the equivalence, add 0.5 mL of the titrant solution.

[0248] Then, add 50 mL of KF solution (50 g / l KF / ethanol (50 / 50) solution in water) and react for 3 minutes. Then, add 15 mL of 0.1 N HCl solution. Titrate the excess HCl with 1N NaOH solution, and the volume V2 (mL) is the equivalence point of the titration.

[0249] Then calculate Rp according to the following formula:

[0250] Rp = (0.31 * V1) / (1.5(15 * 1 - V2 * 1) + (0.5 * 0.1))

[0251] Example 1 - Comparative example without any additives

[0252] Introduce the following reagents into a stirred PARR combustion bomb: 50 mL of 104 g / L NaOH solution and 16.4 g of RHA. Then introduce the PARR combustion bomb into an oven. Raise the temperature at a rate of 1 °C / min to 180 °C. Once this temperature is reached, leave the mixture at 180 °C for 1 hour and then cool until it reaches room temperature. Centrifuge the obtained solution at 4500 tr / min for 35 minutes to separate the residual solid and the solution. Then take out the supernatant for analysis of Rp by potentiometric technique. The results are shown under the "Results" heading in Table 3 below.

[0253] Example 2 - Using potassium citrate (formed in situ) as an additive according to the present invention

[0254] Introduce the following reagents into a stirred PARR combustion bomb: 50 mL of 104 g / L NaOH, 2.7 g of citric acid, 2.35 g of KOH and 16.4 g of RHA. Then introduce the PARR combustion bomb into an oven. Raise the temperature at a rate of 1 °C / min to 180 °C. Once this temperature is reached, leave the mixture at 180 °C for 1 hour and then cool until it reaches room temperature. Centrifuge the obtained solution at 4500 tr / min for 35 minutes to separate the residual solid and the solution. Then take out the supernatant for analysis of Rp by potentiometric technique. The results are shown under the "Results" heading in Table 3 below.

[0255] Example 3 - Using Sodium Citrate as an Additive According to the Invention

[0256] The following reagents were introduced into a stirred PARR bomb: 11.99 g of 30% NaOH by mass, 1.54 g of sodium citrate (Na 3 C 6 H 5 O 7 ), 11.57 g of RHA, and 24.92 g of deionized water. The PARR bomb was then introduced into an oven. The temperature was increased to 180 °C at a rate of 1 °C / min. Once this temperature was reached, the mixture was left at 180 °C for 1 hour and then cooled until room temperature was reached. The resulting solution was centrifuged at 4500 tr / min for 35 minutes to separate the residual solid and the solution. The supernatant was then removed for analysis of Rp by potentiometric techniques. The results are shown under the "Results" heading in Table 3 below.

[0257] Example 4 - Comparative Example with KCl as an Additive

[0258] The following reagents were introduced into a stirred PARR bomb: 50 mL of 104 g / L NaOH, 0.955 g of KCl, and 16.4 g of RHA. The PARR bomb was then introduced into an oven. The temperature was increased to 180 °C at a rate of 1 °C / min. Once this temperature was reached, the mixture was left at 180 °C for 1 hour and then cooled until room temperature was reached. The resulting solution was centrifuged at 4500 tr / min for 35 minutes to separate the residual solid and the solution. The supernatant was then removed for analysis of Rp by potentiometric techniques. The results are shown under the "Results" heading in Table 3 below.

[0259] Examples 5 and 5a - Using K 2 SO 4 as an Additive According to the Invention

[0260] Example 5a is a repetition of Example 5.

[0261] The following reagents were introduced into a stirred PARR bomb: 50 mL of 104 g / L NaOH, 1.115 g of K 2 SO 4 and 16.4 g of RHA. The PARR bomb was then introduced into an oven. The temperature was increased to 180 °C at a rate of 1 °C / min. Once this temperature was reached, the mixture was left at 180 °C for 1 hour and then cooled until room temperature was reached. The resulting solution was centrifuged at 4500 tr / min for 35 minutes to separate the residual solid and the solution. The supernatant was then removed for analysis of Rp by potentiometric techniques. The results are shown under the "Results" heading in Table 3 below.

[0262] Example 6 - Using Na 2 SO 4 as an additive according to the present invention

[0263] Introduce the following reagents into a stirred PARR bomb: 50 mL of 104 g / L NaOH, 0.910 g of Na 2 SO 4 and 16.4 g of RHA. Then introduce the PARR bomb into an oven. Increase the temperature at a rate of 1 °C / min to 180 °C. Once this temperature is reached, leave the mixture at 180 °C for 1 hour and then cool until room temperature is reached. Centrifuge the resulting solution at 4500 tr / min for 35 minutes to separate the residual solid and the solution. Then take the supernatant for analysis of Rp by potentiometric techniques. The results are shown under the "Results" heading in Table 3 below.

[0264] Examples 7 and 7a to 7q - Using various salts containing polyvalent anions as additives

[0265] Example 7. Example 7 is a "blank" experiment without an additive. Otherwise, all operating conditions of Example 7 are the same as those of Examples 7a to 7q involving additives; these operating conditions will be described in detail below.

[0266] Examples 7a to 7q. Introduce the following reagents into a stirred PARR bomb: 50 mL of 104 g / L NaOH, 10.00 mmol of the additive specified in Table 2 below and 16.4 g of RHA. Then introduce the PARR bomb into an oven. Increase the temperature at a rate of 1 °C / min to 180 °C. Once this temperature is reached, leave the mixture at 180 °C for 1 hour and then cool until room temperature is reached. Centrifuge the resulting solution at 4500 tr / min for 35 minutes to separate the residual solid and the solution. Then take the supernatant for analysis of Rp by potentiometric techniques.

[0267] The amount of the additive can be adjusted downwards to 5 mmol and, conversely, upwards to 15 mmol or 20 mmol, depending on the specific nature of the additive.

[0268] Table 2 - List of additives according to Examples 7 to 7q

[0269]

[0270]

[0271] Results

[0272] The results obtained with each experiment carried out according to Examples 1 - 6 are shown in Table 3 below.

[0273] "Rp target value" means the theoretical Rp (SiO 2 / Na 2 O) by weight based on the amount of NaOH wt.% and SiO 2 wt.% (from RHA) introduced.

[0274] "Rp" is the ratio measured by potentiometric technique.

[0275] Table 3

[0276]

[0277] Example 1 (for comparison purposes) shows that at a reaction temperature of 180 °C, crystalline silica cannot dissolve without any additives. Example 4 (also for comparison purposes) confirms that not all salts are suitable additives for improving the dissolution of crystalline silica: using a salt containing a monovalent anion (such as KCl) as an additive does not result in effective dissolution of crystalline silica.

[0278] In contrast, using salts containing polyvalent anions according to the present invention such as potassium citrate, sodium citrate, K 2 SO 4 or Na 2 SO 4 (as shown in Examples 2, 3, 5, 5a and 6) at the same reaction temperature of 180 °C results in a much higher Rp and a significant increase in the dissolution yield of SiO 2 up to a maximum of ≥ 79%. This clearly shows that the method according to the present invention using salts containing polyvalent anions greatly improves the dissolution level of crystalline silica even at lower temperatures such as 180 °C.

Claims

1. A method for producing alkali metal silicate from plant ash, wherein the method comprises the step (a) of reacting the following: (i) Plant ash obtained from the combustion of silica-containing plant parts and / or plants, wherein the plant ash contains crystalline silica; with (ii) An alkali, preferably an alkali metal hydroxide; wherein the reaction of step (a) is carried out in a reaction mixture comprising a dispersion medium, preferably an aqueous dispersion medium, and an additive, wherein the additive is a salt containing a polyvalent anion, and the salt is selected from the group consisting of sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, and combinations thereof.

2. The method according to claim 1, wherein, the step (a) is carried out at a reaction temperature of from 120 °C to 250 °C, preferably from 120 °C to 130 °C, for at least 10 minutes, preferably more than 30 minutes.

3. The method according to claim 1 or 2, wherein, The amount of crystalline silica is at least 0.1 wt.%, preferably at least 1 wt.%, more preferably at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 25 wt.%, at least 30 wt.%, at least 40 wt.%, at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, at least 80 wt.%, or at least 90 wt.% based on the total content contained in the ash. 2 ​ 4. The method according to claim 3, wherein, The amount of crystalline silica is at least 30 wt.% based on the total content of SiO contained in the ash. 2 ​ 5. The method according to any one of the preceding claims, wherein, the crystalline silica comprises a crystalline form selected from the group consisting of quartz, cristobalite, tridymite, and combinations thereof.

6. The method according to claim 5, wherein, at least a part of the crystalline silica is cristobalite.

7. The method according to claim 6, wherein, The amount of cristobalite is at least 0.1 wt.%, preferably at least 1 wt.%, more preferably at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 25 wt.%, at least 30 wt.%, at least 40 wt.%, at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, at least 80 wt.%, or at least 90 wt.%, based on the total SiO content contained in the ash, and preferably, most of the crystalline silica is cristobalite. 2 ​ 8. The method according to claim 7, wherein, The amount of cristobalite is at least 20 wt.% based on the total SiO 2 content contained in the ash.

9. The method according to any one of claims 6 to 8, wherein, the silica-containing plant is an angiosperm, preferably a monocotyledon or a dicotyledon, more preferably a plant belonging to a family selected from the group consisting of Poaceae, Equisetaceae, Cyperaceae, Cucurbitaceae, Cannabaceae, Arecaceae, Brassicaceae, and combinations thereof, still more preferably a plant selected from the group consisting of rice, wheat, rapeseed, barley, bamboo, wild horsetail, sedge, watermelon, and combinations thereof.

10. The method according to claim 9, wherein, the plant ash is rice husk ash.

11. The method according to any one of claims 6 to 10, wherein, step (a) is carried out at a reaction temperature of from 165 °C to 205 °C, preferably from 170 °C to 190 °C.

12. The method according to claim 5, wherein, at least a part of the crystalline silica is quartz.

13. The method according to claim 12, wherein, The amount of quartz is at least 0.1 wt.%, preferably at least 1 wt.%, more preferably at least 5 wt.%, at least 10 wt.%, at least 20 wt.%, at least 25 wt.%, at least 30 wt.%, at least 40 wt.%, at least 50 wt.%, at least 60 wt.%, at least 70 wt.%, at least 80 wt.%, or at least 90 wt.%, based on the total content of SiO contained in the ash, and preferably, most of the crystalline silica is quartz. 2 ​ 14. The method according to claim 13, wherein, The amount of quartz is at least 20 wt.% based on the total content of SiO 2 contained in the ash.

15. The method according to any one of claims 12 to 14, wherein, the silica-containing plant is selected from the group consisting of trees, sugarcane, and combinations thereof, and wherein the silica-containing plant parts are derived from the processing of trees and / or sugarcane.

16. The method according to claim 15, wherein, the plant ash is tree sawdust, wood pellets, bagasse, or a combination thereof.

17. The method according to any one of claims 12 to 16, wherein, step (a) is carried out at a reaction temperature of from 200 °C to 230 °C, preferably from 205 °C to 220 °C.

18. The method according to any one of the preceding claims, wherein, The salt is sodium citrate (Na 3 C 6 H 5 O 7 ), potassium citrate (K 3 C 6 H 5 O 7 ), or a combination thereof.

19. The method according to any one of claims 1 to 17, wherein, The salt is sodium sulfate (Na 2 SO 4 ), potassium sulfate (K 2 SO 4 ), or a combination thereof.

20. The method according to any one of the preceding claims, wherein, the reaction mixture is formed by bringing together the plant ash, the base, the dispersion medium, and the additive and / or the precursor of the additive.

21. The method according to claim 20, wherein, the precursor of the additive is an acid containing polyvalent anions selected from the group consisting of sulfuric acid, citric acid, and combinations thereof.

22. The method according to any one of the preceding claims, wherein, the alkali metal silicate is in the form of a silicate solution, preferably an aqueous silicate solution, and wherein the method further comprises a step (b) of separating the silicate solution from the impurities derived from the ash, said impurities comprising carbonaceous products and metals.

23. The method according to claim 22, further comprising a step (c) of drying the silicate solution obtained after step (b) to obtain a silicate in solid form.

24. An alkali metal silicate obtainable by the method according to claim 22 in liquid form as a silicate solution or by the method according to claim 23 in solid form as a solid silicate.

25. The alkali metal silicate according to claim 24, which is a silicate solution in liquid form and contains an additive, wherein the additive is sodium citrate (Na 3 C 6 H 5 O 7 ), potassium citrate (K 3 C 6 H 5 O 7 ), or a combination thereof.

26. A method for preparing precipitated silica, the method comprising the following steps: (I) producing an alkali metal silicate solution by the method according to claim 22 or by a method comprising producing a silicate in solid form by the method according to claim 23 and redispersing the silicate in solid form in a dispersion medium, preferably an aqueous dispersion medium, and (II) reacting the alkali metal silicate solution so produced and optionally an additional silicate solution other than the alkali metal silicate solution so produced, NaOH, and / or a minor silica source with an acidifying agent to effect precipitation of silica.

27. The method according to claim 26, wherein, the acidifying agent is sulfuric acid, a salt containing sulfate anions is formed simultaneously with the precipitated silica, and the method comprises a step (III) of separating the salt containing sulfate anions in solid or liquid form.

28. The method according to claim 27, further comprising recycling the salt containing sulfate anions obtained after the precipitation reaction to step (a).

29. The method according to claim 27 or 28, wherein, The additive is sodium sulfate (Na 2 SO 4 ), potassium sulfate (K 2 SO 4 ), or a combination thereof.

30. A reaction mixture for producing an alkali metal silicate from plant ash by the method according to any one of claims 1 to 23 and / or for preparing precipitated silica by the method according to any one of claims 26 to 29, comprising: (i) plant ash obtained by burning plant parts containing silica and / or plants, wherein the plant ash contains crystalline silica; (ii) a base, preferably an alkali metal hydroxide; (iii) a dispersion medium, preferably an aqueous dispersion medium, and (iv) an additive, wherein the additive is a salt containing polyvalent anions selected from the group consisting of sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, and combinations thereof.

31. The reaction mixture according to claim 30, wherein, The salt is sodium citrate (Na 3 C 6 H 5 O 7 ), potassium citrate (K 3 C 6 H 5 O 7 ), or a combination thereof.

32. The reaction mixture according to claim 30, wherein, The salt is sodium sulfate (Na 2 SO 4 ), potassium sulfate (K 2 SO 4 ) or a combination thereof.

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