Hydrothermal process for the preparation of silicate and precipitated silica from spent foundry sand

Abstract

The invention relates to a process for the preparation of silicate, preferably an alkali metal silicate, from a spent foundry sand using a hydrothermal route comprising the step of reacting said spent foundry sand with a base, preferably an alkali metal hydroxide, in an aqueous reaction medium, wherein the process is free of any thermal pre-treatment step as applied in prior art processes. The invention also relates to a process for the preparation of precipitated silica using said silicate.

Description

[0001] HYDROTHERMAL PROCESS FOR THE PREPARATION OF SILICATE AND PRECIPITATED SILICA FROM SPENT FOUNDRY SAND

[0002] TECHNICAL FIELD

[0003] The present invention relates to a hydrothermal process for the preparation of silicate, preferably alkali metal silicate, from spent foundry sand (SFS). The present invention also concerns a process for the preparation of precipitated silica from this silicate.

[0004] TECHNICAL BACKGROUND

[0005] Silicon dioxide (SiO?), also known as silica, is a silicon compound that is commonly found in nature. Naturally-occurring silica exists both in amorphous and crystalline forms such as cristobalite, tridymite, and quartz, the latter being the major constituent of sand. Natural quartz sand, such as Fontainebleau sand, is frequently employed for the preparation of silicate, in particular sodium silicate, which can be obtained, for example, via a furnace route, namely by fusion of said sand with alkali metal carbonates, hydrogen carbonates or hydroxides, at high temperatures of around 1500 °C, or by hydrothermal treatment of said sand with strong bases, such as sodium hydroxide, as described, for example, in FR1112807A, EP0164073A2, EP0033109A1, or WO2017 / 063901.

[0006] Silicates and, particularly, sodium silicates are commonly employed as raw materials for the precipitation of precipitated silica, which is a form of synthetic silica in amorphous form. Both silicates and precipitated silica are highly versatile materials with a variety of applications in the most diverse technological fields, from constructions to detergents, tire, adhesives, food, and pharmaceutical industries, and their global demand is constantly increasing.

[0007] However, the above-mentioned processes for producing silicates and precipitated silica have the major disadvantages that natural quartz sand, used as raw material, is not a renewable resource over human timescales as its replenishment happens through rocks erosion or weathering processes over geological time.

[0008] It appears thus clear that there is the need to identify and valorize new sustainable circular source of SiCL that can be employed to produce silicates and precipitated silica in a more environmentally friendly way while at the same time assuring good quality and yields of the final product. A possible alternative for this purpose can be envisaged in spent foundry sand (SFS), a SiCh-rich waste which is produced in large amounts by the foundry industries. Spent foundry sand (SFSs) or waste foundry sand (WFSs) is in fact the most common waste from foundries which use new, virgin natural quartz sand to make casting molds to design metal parts of various sizes. Although such sand can be reused multiple times within the foundry, heat and mechanical abrasion eventually render the sand unsuitable for use in casting molds, and a portion of the sand is continuously removed and replaced with new virgin sand.

[0009] SFS thus represents a large amount of the waste produced by the foundry industries and its management and disposal (mainly in landfill) not only is an increasing environmental issue but also involves high costs.

[0010] In this context, the recovery and reuse of SFS plays a fundamental role as its recycling can save energy, costs, as well as reduce the need to mine virgin materials and exploit natural resources. Hence, from a circular economy perspective, there is a rising need towards the development of new environmentally friendly approaches for the conversion of SFS into value added products, such as silicates and precipitated silica.

[0011] However, one of the criticalities related to the use of spent foundry sand as starting material for producing silicate and precipitated silica is connected to the high variability of its chemical nature, morphology, and elemental composition which can be substantially different depending on the type of SFS, type of foundry processes, and on how the waste is managed locally by the various foundries. Moreover, different impurities may be present which, if not properly handled, can be detrimental for the production of silicates of high quality (i.e., having high Rp values) and with satisfactory yields.

[0012] IN201941016001 discloses a process for obtaining sodium silicate though a catalytic hydrothermal reaction route using spent foundry sand.

[0013] This process involves a hydrothermal treatment of spent foundry sand using a solution of concentrated caustic soda over a period of 6 to 36 hours over a temperature range between 150 °C and 350 °C in a closed pressure vessel. According to this document, before being subjected to said hydrothermal treatment, the spent foundry sand is subjected to a thermal pre-treatment at a temperature of about 400 °C to 600 °C for about 5 to 6 hours in order to remove the residual carbon present in the sand, which yields light brown coarse sand. The color removed sample is then treated with a solution of acidic catalyst to remove any volatile soluble impurities and to modify the surface of the spent foundry sand. The sample is then further washed to remove any acidic residues.

[0014] A similar approach is disclosed in IN202041002565 and IN201941026558 which all use similar thermal pre-treatments of the SFS before hydrothermal treatments.

[0015] However, all these documents disclose a thermal pre-treatment of the SFS which is necessary to purify the sand from inorganic carbon and / or organic compounds contained in the sand to be able to obtain a silicate having the desired properties for further applications, such as for the production of precipitated silica. Such a thermal pre-treatment, however, increases the CO2 production of the method and the overall process steps and time.

[0016] In this context, it remains the need to develop a novel process for producing circular silicate and precipitate silica from a spent foundry sand which is easy, environmentally friendly, cost-efficient, and at the same time assures that the desired properties of the final product are obtained.

[0017] SUMMARY OF THE INVENTION

[0018] The present invention relates to a process for the preparation of a silicate from a spent foundry sand via a hydrothermal route, wherein said silicate can in turn be used for the preparation of silica, in particular, precipitated silica.

[0019] A first objective of the present invention is to provide a process for the hydrothermal preparation of a silicate from a spent foundry sand, said process comprising:

[0020] - optionally, a physical pre-treatment step (a”) of purifying the spent foundry sand,

[0021] - a step (a) of reacting, in a closed vessel, the spent foundry sand and, optionally in addition a sand other than a spent foundry sand, with a base, in an aqueous reaction medium, by heating up to a temperature of above 100 °C, said process being characterized in that it is free of step (a’), before step (a), of thermally pre-treating the spent foundry sand in an atmosphere comprising oxygen by heating up to a temperature of between 200 °C and 600 °C for a period of time of 20 minutes to 12 hours, and said process being preferably free of step (a’), before step (a), of any thermally pre-treating of the spent foundry sand. According to the present invention, the reaction of step (a) is thus carried out in a reaction mixture comprising the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction medium.

[0022] In a preferred embodiment, the aqueous reaction medium comprises an additive, wherein said additive is a salt comprising a multivalent anion, preferably a salt comprising a multivalent anion selected from the group consisting of: sulfur oxyanions, dicarboxylate anions, tricarboxylates, and combinations thereof, more preferably selected from the group consisting of_ citrate, sulfate, or a combination thereof, even more preferably selected from the group consisting of: sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, or a combination thereof.

[0023] Preferably, said additive is formed in situ, namely by adding to the aqueous reaction medium of step (a) a precursor of the additive which is possibly an acid comprising a multivalent anion selected from the group consisting of citric acid, sulfuric acid, or a combination thereof.

[0024] Preferably, the aqueous reaction medium of step (a) preferably further comprises a plant ash, a secondary silica source, preferably an amorphous silica source, or a combination thereof.

[0025] According to the present invention, after said step (a), an aqueous liquid mixture comprising silicate and possibly further containing solid by-products is obtained.

[0026] A further objective of the present invention is therefore to provide a process for the preparation of a solution of silicate, wherein the process of the invention further comprises a step (b) of separating said solid by-products from the aqueous liquid mixture to obtain a solution of silicate, which is an aqueous solution of silicate.

[0027] According to an embodiment of the present invention, the process further comprises a step (c) of adding to the solution of silicate at least one of a secondary silica source, an alkali metal hydroxide, preferably NaOH, a silicate other than the silicate obtained in step (b), or a mixture thereof.

[0028] According to an embodiment, the solution of silicate obtained after step (b), or (c), can be dried (step (d)) so as to obtain a solid silicate.

[0029] According to an embodiment, the process of the invention comprises, before step (a), a physical pre-treatment step (a”) of purifying the spent foundry sand. Advantageously, this enables the removal of impurities possibly contained in the spent foundry sand. Said impurities are preferably impurities selected from the group consisting of: metals, carbon-rich impurities, clays, metallic silicates, and other insoluble impurities, or a combination thereof.

[0030] According to the present invention, said pre-treatment step (a”) is not a thermal pre-treatment (a’). As a matter of fact, the process according to the invention is free of the thermal pre-treatment (a’) as defined in claim 1 before step (a) (as an essential feature of the invention), and is preferably free of any thermal pre-treatment.

[0031] The silicate obtained by the process according to the present invention (steps (b), (c), or (d)) is preferably an alkali metal silicate and can be advantageously used as raw material for the preparation of silica, in particular precipitated silica.

[0032] Said silicate is indeed preferably characterized by a SiCh / MxO weight ratio (Rp) of at least 2.0, preferably of at least 2.5, more preferably of up to 2.6, or, according to an embodiment of the present invention, where the reaction mixture of step (a) further comprises a plant ash, an amorphous silica source, or a combination thereof, can be above 2.6 and preferably up to 3, more preferably between 3 and 3.6, even more preferably up to 4, wherein M is a metal, preferably wherein M is an alkali metal and x is 2.

[0033] Said silicate is also preferably characterized by an amount of SiCh (i.e., SiCh final) of up to 100% of the amount of silica contained in the sand used in step (a) of the process (i.e., SiCh initial). Preferably, SiCh final is between 70% and 100% of SiCh initial. This corresponds to the yield of the process according to the present invention (i.e., the yield of SiCh recovered from the sand).

[0034] A further objective of the invention is therefore the use of said silicate for the preparation of silica, in particular, precipitated silica.

[0035] Accordingly, an objective of the invention is also the provision of a process for the preparation of precipitated silica, wherein said process comprises the steps of:

[0036] (I) preparing a silicate by the process according to the present invention,

[0037] (II) reacting the silicate obtained in step (I) and, optionally in addition a silicate other than said silicate obtained in step (I), with at least one acidifying agent in a liquid medium, preferably an aqueous liquid medium, to obtain a suspension of precipitated silica,

[0038] (III) separating the precipitated silica from the liquid medium to provide wet precipitated silica, and (IV) drying said wet precipitated silica to obtain precipitated silica.

[0039] Preferably, according to an embodiment, said process for the preparation of precipitated silica comprises, after step (I) and before step (II) a step (I’) of adding to the silicate obtained in step (I) at least one of a secondary silica source, an alkali metal hydroxide, preferably NaOH, a silicate other than the silicate obtained in step (I), or a mixture thereof, so as to obtain a new silicate.

[0040] Accordingly, step (II) comprises reacting the new silicate obtained in step (I’) and, optionally in addition a silicate other than the new silicate obtained in step (I’), with at the one acidifying agent in a liquid medium, preferably an aqueous liquid medium, to obtain a suspension of precipitated silica.

[0041] The present invention also concerns a reaction mixture for the preparation of a silicate from a spent foundry sand and / or for the preparation of a precipitated silica by the process as defined above, comprising:

[0042] (i) a spent foundry sand which has not been subjected to a thermal pretreatment (a’) having total carbon content of at least 1 wt%, more preferably of at least 2 wt%, even more preferably of at least 3 wt%, most preferably of at least 4 wt% based on the total weight of the spent foundry sand;

[0043] (ii) a base;

[0044] (iii) an aqueous liquid medium, preferably water;

[0045] (iv) optionally a sand other than a spent foundry sand, preferably a natural sand;

[0046] (v) optionally an additive that is a salt comprising a multivalent anion, preferably a salt comprising a multivalent anion selected from the group consisting of: sulfur oxyanions, dicarboxylate anions, tricarboxylates, and combinations thereof, more preferably a salt comprising a multivalent anion selected from the group consisting of: sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, or a combination thereof; and

[0047] (vi) optionally a plant ash, an amorphous silica source, or a combination thereof.

[0048] The present invention solves the aforementioned problems of the prior art by providing a process for the preparation of silicate from a spent foundry sand via a direct hydrothermal route without the need of thermally pre-treating said sand. The process of the present invention advantageously allows obtaining silicate and precipitated silica having the desired quality and yield as those obtainable by the conventional approach starting from natural quartz sand (i.e., pure sand, such as Fontainebleau sand).

[0049] The process of the present invention is thus particularly advantageous not only from an environmental but also from an economic point of view as it allows to recycle a waste material such as the spent foundry sand with an overall reduction of the CO2 emissions and, at the same time, produce high quality silicate and precipitated silica, without affecting the overall yield of the final products.

[0050] DETAILED DESCRIPTION OF THE INVENTION

[0051] Before the issues of the invention are described in detail, the following should be considered:

[0052] It is to be understood that this invention is not limited to particular embodiments described, since such embodiments may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[0053] As used herein, the singular forms “a”, “an”, and “the” include both singular and plural referents unless the context clearly dictates otherwise. By way of example, “a compound” means on compound or more than one compound.

[0054] The terms “comprising”, “comprises”, and “comprised of’ as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms “comprising”, “comprises” and “comprised of’ as used herein comprise the terms “consisting of’, “consists”, and “consists of’.

[0055] Throughout this application, the term “about” is used to indicate that a value includes the standard deviation of error for the device or method being employed to determine the value.

[0056] As used herein, the terms “% by weight”, “wt.-%”, “wt%”, “weight percentage”, or “percentage by weight”, are used interchangeably.

[0057] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75, and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point 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.

[0058] For the avoidance of doubt, “X is free of Y” is used herein under its usual, commonly accepted meaning that X is completely free of Y.

[0059] For the purposes of the present invention, the term “liquid medium” is also referred to, interchangeably, as “dissolving medium” or “dispersing medium”.

[0060] The term “closed vessel” is used as a synonym of “oven”, “pressure reactor”, or of any other vessel or container which allows the hydrothermal process according to the present invention to be carried out under autogenous pressure.

[0061] According to the present invention, the expression “aqueous reaction medium” indicates a “liquid reaction medium” comprising, or essentially consisting of, or consisting of water, preferably deionized water.

[0062] According to the embodiment where the “aqueous reaction medium” indicates a “liquid reaction medium” comprising water, preferably deionized water, said liquid reaction medium can further comprise one or more liquids / solvents other than said water, preferably other than said deionized water, such as, C1-C4 alcohols preferably selected from the group consisting of: ethanol, isopropanol, n-butanol, iso-butanol, butanol, or combinations thereof.

[0063] Similar considerations apply to the expression “aqueous liquid mixture” and “aqueous liquid medium”.

[0064] The term “strong base” is used herein under its usual, commonly accepted meaning, namely a base that is completely dissociated in the aqueous reaction medium of the present invention, especially in water, when water is said aqueous reaction medium. According to the present invention, an aqueous base with an association constant pKb measured at 25 °C less than about 0, corresponding to pKameasured at 25 °C greater than about 14, is considered a strong base.

[0065] In other words, when in water at 25°C, the base may behave as a strong base (that it is to say it has a pKb < 0) or as a weak base having a pKb < 5.0 (such as NH3 or Na4P2O?), preferably < 4.0 (such as ISfeCCh), more preferably < 2.0 (such as NasPCU or ISfeGeCh). In accordance with the present invention, a base that behaves as a strong base when in water at 25°C is further preferred over a base that behaves as a weak base when in water at 25°C. The term “spent foundry sand” abbreviated SFS is used herein to refer to the waste sand deriving from foundry processes, such as the sand used for casting metal parts. The expressions “spent foundry sand” and “waste foundry sand” abbreviated WFS are used herein interchangeably.

[0066] The terms “natural quartz sand” or “natural sand” are used herein to refer to “pure sand” or “high purity sand”. An example of such sand is Fontainebleau sand.

[0067] The terms “natural quartz sand”, “natural sand”, “pure sand” or “high purity sand” may be used interchangeably and are used to distinguish such sand from a spent foundry sand (SFS).

[0068] As used herein, the term “room temperature” is used as a synonym of “ambient temperature” and refers to a temperature comprised between 15 °C and 25 °C, e.g., 20 °C.

[0069] The expression SiCh / MxO indicates the SiCh / MxO weight ratio: [%weight (SiCh) / %weight (MXO)], also defined as “Rp” or “Rp value”, which is a parameter which describes the quality of the silicate (and the corresponding silica obtained therefrom). M is a metal, preferably selected from the group consisting of alkaline metals and, accordingly, x is preferably 2. For the avoidance of any doubts, alkaline metals correspond to the group 1 elements according to IUPAC nomenclature. Where the silicate is a sodium silicate, the Rp value is represented by the SiCh / lSfeO weight ratio [%weight (SiCh) / %weight (Na2O)].

[0070] All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

[0071] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0072] In the following passages, different alternatives, embodiments, and variants of the invention are defined in more detail. Each alternative and embodiment so defined may be combined with any other alternative and embodiment, and this for each variant unless clearly indicated to the contrary or clearly incompatible when the value range of a same parameter is disjoined. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0073] Furthermore, the particular features, structures, or characteristics described in the present description may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are mean to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

[0074] The present invention refers to a process for the preparation of a silicate from a spent foundry sand comprising a step (a) of reacting, in a closed vessel, the spent foundry sand and, optionally in addition a sand other than a spent foundry sand, with a base, in an aqueous reaction medium, by heating up to a temperature of above 100 °C.

[0075] According to the present invention, the reaction of step (a) is thus carried out in a reaction mixture comprising the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction medium.

[0076] Preferably the aqueous reaction medium is water, more preferably deionized water.

[0077] Moreover, the process according to the present invention is characterized in that it is free of any step (a’), before step (a), of thermally pre-treating the spent foundry sand. For the purposes of the present invention, said step (a’) is also called “thermal pre-treatment step (a’)”.

[0078] According to the present invention said step (a’) is a step of thermally pretreating the spent foundry sand in an atmosphere comprising oxygen so as to burn carbon and / or organic compounds which can be comprised in the spent foundry sand. Said step (a’) is carried out by heating up to a temperature of between 200 °C and 600 °C, possibly between 300 °C and 600 °C, for a period of time of 20 minutes to 12 hours, possibly 20 minutes to 60 minutes.

[0079] According to a particularly preferred embodiment, said step (a’) is any thermally pre-treating of the spent foundry sand.

[0080] The base used in step (a) in preferably used in an amount of up to 25 % by weight, preferably of up to 20 % by weight, more preferably of up to 15% by weight, based on the total weight of the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction medium. Preferably said amount is of at least 9 % by weight, more preferably comprised between 9 % by weight and 13 % by weight.

[0081] According to the present invention, the total weight of the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction medium corresponds to the initial weight of said ingredients (i.e., the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction medium), namely to the weight of said ingredients (which are mixed together to form the reaction mixture) before the hydrothermal reaction of step (a) begins.

[0082] According to a preferred embodiment, said base is preferably a strong base.

[0083] According to a particularly preferred embodiment, said base is an alkali metal base, preferably an alkali metal strong base, even more preferably an alkali metal hydroxide.

[0084] Said alkali metal hydroxide is preferably selected from the group consisting of NaOH, KOH, LiOH, CsOH, RbOH, and combinations thereof, preferably selected from the group consisting of sodium hydroxide (NaOH) and potassium hydroxide (KOH), most preferably sodium hydroxide (NaOH).

[0085] According to a particularly preferred embodiment of the invention, when the base is an alkali metal base as described above, said silicate is an alkali metal silicate; the alkali metal silicate is typically of formula A2O.nSiO2 wherein A is the alkali metal and n is a real number ranging from 1.0 to 5.0, preferably from 2.0 to 4.5 and more preferably from 3.0 to 4.0. More preferably, said silicate is selected from the group consisting of sodium silicate and potassium silicate, most preferably sodium silicate; the sodium silicate is typically of formula Na2O.nSiO2 wherein n is a real number ranging from 1.0 to 5.0, preferably from 2.0 to 4.5 and more preferably from 3.0 to 4.0.

[0086] According to a particularly preferred embodiment of the invention, the silicate is sodium silicate and the base is sodium hydroxide.

[0087] Preferably, the reaction of step (a), i.e., the hydrothermal dissolution of the spent foundry sand, the sand other than a spent foundry sand if any, in the presence of the base, is carried out at a temperature of at least 160 °C, preferably at least 190 °C, more preferably of at least 200 °C, even more preferably of at most 250 °C, still more preferably of at most 300 °C.

[0088] According to a particularly preferred embodiment, the temperature of step (a) is betweenl60 °C and 250 °C, more preferably between 190 and 230 °C, most preferably between 200 °C and 210 °C. The timing of the reaction of step (a), i.e., the residence time in the closed vessel, may vary. For the purposes of the present invention, the residence time is the amount of time for which the reaction of step (a) is carried out. Preferably, step (a) should be carried out for a duration sufficient to achieve dissolution of the crystalline silica (quartz) contained in the sand. According to an embodiment, said residence time may range from at least 30 minutes to less than 24 hours, preferably between 30 minutes and 10 hours, more preferably between 30 minutes and 6 hours, even more preferably between 30 minutes and 5 hours. A timing between 2 and 5 hours has been found to be the most suitable.

[0089] The closed vessel in which the reaction of step (a) is carried out, allows performing the hydrothermal dissolution under autogenous pressure which is the pressure formed during the heating up of the reaction mixture comprising the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction medium. In other words, the step (a) of the process according to the present invention is carried out under pressure, which is an autogenous pressure.

[0090] According to a particularly preferred embodiment of the invention the aqueous reaction medium of step (a) can comprise an additive which is a salt comprising a multivalent anion. Said additive may also be formed by adding to the aqueous reaction medium of step (a) a precursor of the additive which is, preferably, an acid comprising a multivalent anion.

[0091] According to the present invention, the multivalent anion usually comprises several oxygen atoms. In some embodiments, the multivalent anion is an oxyanion, optionally with one or more proton(s) attached thereto.

[0092] The oxyanion may contain one or more heteroatom(s) other than oxygen; the at least one heteroatom is typically an element of group 13, 14, 15 or 16 of Mendeleev’s Periodic Table of the Elements, based on new IUPAC system. In particular, the oxyanion can be selected from the group consisting of boron oxyanions, carbon oxyanions, phosphorus oxyanions, sulfur oxyanions, selenium oxyanions, tellurium oxyanions and mixtures thereof.

[0093] The multivalent anion can be an inorganic oxyanion. Examples of inorganic oxyanions are: borate [(BCE)3'], carbonate [(CCE)2’], carboxylates, phosphate [(PCE)3’], hydrogenophosphate [(UPCE)2], pyrophosphate [(P2O?)4'], acid diphosphates [(EEECE)3' and (H2P2O7)2], triphosphate [(PsCEo)5'], acid triphosphates [(HPsOio)4', (EEPsOio)3' and (EEPsOio)2’], phosphite [(HPO3)2], pyrophosphite [(H2P2O5)2], sulfite [(SCE)2'], sulfate [(SCE)2'], peroxomonosulfate [(SOs)2’], thiosulfate [(S2O3)2'], dithionite [(S2O4)2'], metabisulfite [(S20s)2’], dithionate [(S20e)2'], disulfate [(S2O?)2'], peroxy di sulfate [(S20s)2'], trithionate [(SsOe)2'], tetrathionate [(S40e)2'], pentathionate [(SsOe)2’] and higher polythionates of general formula Sn[(SO3)2']2 wherein n is an integer in the range of from 4 to 18, selenite [(SeCE)2'] and selenate [(SeCU)2'], tellurite [(TeCh)2'], metatellurate [(TeCU)2'], orthotellurate [(TeOe)6'] and mixtures thereof.

[0094] Alternatively, the multivalent anion can be an organic oxyanion. Organic oxyanions are typically carboxylates.

[0095] A first carboxylate multivalent anion according to the invention is oxalate (C2O42).

[0096] Except oxalate, carboxylates multivalent anions according to the invention have typically the general formula

[0097] R**(-COO )N wherein N is an integer greater than or equal to 2, preferably from 2 to 5, more preferably 2 or 3, and R** is a C1-C10 N-valent hydrocarbon group with N as defined above, which can optionally be interrupted by one or more heteroatom(s) and / or substituted by one or more functional group(s) other than -COO'. Said one or more functional group(s) other than -COO' can be selected from the group consisting of -COOH, -OH, -NH2, -X wherein X is a halogen atom, - C(=O)NH2.-NO2, =NH and =0.

[0098] Often, R** is C1-C10 N-valent hydrocarbon group with N as defined above, period (i.e. it is uninterrupted and unsubstituted).

[0099] The N-valent hydrocarbon group is preferably a C1-C3 N-valent hydrocarbon group. It can be linear, ramified or cyclic. It can be saturated or unsaturated. It can be aliphatic or aromatic.

[0100] Examples of dicarboxylates are: (i) unsubstituted dicarboxylates such as: oxalate, malonate, succinate, glutarate, adipate, sebacate, and itaconate (CsH4O42' ); (ii) substituted aliphatic dicarboxylates including hydroxydicarboxylates like tartronate, malate, and tartarate (C4H40e2'), aminodicarboxylates like aspartate, and ketodicarboxylates like mesoxalate and 2-oxoglutarate; and (iii) unsubstituted aromatic dicarboxylates, such as o-phthalate, isophthalate, and terephthalate.

[0101] Examples of tricarboxylates are: (i) unsubstituted aliphatic tricarboxylates, such as propane-1, 2, 3 -tri carb oxy late and aconitate; (ii) substituted aliphatic tricarboxylates such as citrate, isocitrate, and oxalosuccinate; and (iii) unsubstituted aromatic tricarboxylates such as trimesate.

[0102] Examples of higher carboxylates are ethylenediaminetetraacetate, diethylenetriamine pentaacetate, and polycarboxylate polymers such as aminopolyacetates.

[0103] In general, the multivalent anion differs from a silicate anion, and even from any silicon-containing anion. Yet, in a special embodiment of the present invention, the multivalent anion is a silicate anion such as metasilicate [(SiOs)2’], polymeric metasilicate { [(SiC>3)2’]n, wherein n is an integer greater than 1 and pyrosilicate [(Si2O?)6']. For the avoidance of doubt, in this special embodiment, the additive, which is then a salt comprising a silicate anion, is not produced by reacting the spent foundry sand, the sand other than a spent foundry sand if any, with the base in the aqueous reaction medium, although its chemical natural and the chemical nature of the silicate produced by reacting the spent foundry sand, the sand other than a spent foundry sand if any, with the base in the aqueous reaction medium can be identical.

[0104] According to a first preferred embodiment of the invention, the multivalent anion is a sulfur oxyanion, with sulfate [( SO4)2"] being especially preferred.

[0105] According to another preferred embodiment of the invention, the multivalent anion is a dicarboxylate and / or a tri carb oxy late. More preferably, it is selected from the group consisting of aliphatic unsubstituted dicarboxylates, aliphatic hydroxydicarboxylates, aliphatic unsubstituted tricarboxylates, aliphatic hydroxytricarboxylates, and combinations thereof.

[0106] The salt preferably comprises an alkali metal cation, an ammonium cation, or a quaternary ammonium cation of formula NR.4+(where R = C1-C20, preferably a C1-C5 hydrocarbon group), more preferably an alkali metal cation, still more preferably Na+, K+, Cs+, Li+, Rb+, or a combination thereof, and the most preferably, it is Na+, K+or a combination thereof.

[0107] It is especially preferred that said salt is selected from the group consisting of sodium citrate, potassium citrate, sodium sulfate, potassium sulfate and combinations thereof. Thus, it is notably especially preferred that said salt is sodium citrate (NasCeHsO?), potassium citrate (K3C6H5O7), or a combination thereof. It is also especially preferred that said salt is sodium sulfate (ISfeSCU), potassium sulfate (K2SO4), or a combination thereof.

[0108] According to the invention, the amount of the additive within the reaction mixture should be significantly above a catalytic amount and should be adapted depending on the level of quartz (crystalline silica) which is not dissolved in the hydrothermal reaction of the spent foundry sand, the sand other than a spent foundry sand if any, with the base in the aqueous reaction medium according to step (a) of the process of the present invention.

[0109] Preferably, the amount of additive within the reaction mixture is of at least 1 g / L, at least 5 g / L, at least lOg / L or at least 15 g / L, more preferably at least 20 g / L. It can be of 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, it being understood that this amount should advantageously not exceed its solubility limit in the aqueous reaction medium; in practice, the amount of additive within the aqueous reaction medium is generally of at most 500 g / L and can be of at most 300 g / L, at most 150 g / L or at most 100 g / L.

[0110] The additive, which is a salt comprising a multivalent anion, can be advantageously added to the reaction mixture as a salt which is already formed and / or can be formed in situ by adding, to the aqueous reaction medium, a precursor of the salt, especially an acid comprising a multivalent anion.

[0111] Said acid comprising a multivalent anion is the acid equivalent of the multivalent anion mentioned above according to any one of the embodiments of the present invention.

[0112] An especially preferred precursor of the additive is an acid comprising a multivalent anion selected from the group consisting of sulfuric acid, citric acid and combinations thereof.

[0113] Another possible precursor of the additive is an acid salt comprising a monovalent anion, e.g. sodium hydrogenoxalate (NaPK^CU) or sodium dihydrogenophosphate (NaLLPCU) or sodium hydrogenosulfate (NaHSCU).

[0114] Still another possible precursor of the additive is an anhydride or an oxide of an acid comprising a multivalent anion, e.g. respectively citric anhydride, maleic anhydride or succinic anhydride on the one hand, and carbon dioxide or sulfur trioxide on the other hand.

[0115] When an acid comprising a multivalent anion, an acid salt comprising a monovalent anion, an anhydride of an acid comprising a multivalent anion, an oxide of an acid comprising a multivalent anion, or a combination thereof is used as a precursor of the additive, it is understood that a sufficient amount of the base must be present in the aqueous reaction medium, not only for reacting with the sand to produce the silicate but also for reacting with said acid, acid salt, anhydride, oxide or combination thereof and converting it into the additive, namely the corresponding salt of said acid, acid salt, anhydride, oxide or combination thereof.

[0116] According to an embodiment of the invention, the additive and / or a precursor of the additive (especially, an acid comprising a multivalent anion) can be added to the aqueous reaction medium of step (a) and / or directly to the spent foundry sand and, optionally in addition the sand other than a spent foundry sand, before step (a).

[0117] According to an embodiment of the present invention, the aqueous reaction medium of step (a) can further comprise a plant ash, an amorphous silica source, or a combination thereof, so as to increase the Rp of the silicate thus produced, preferably up to values above 2.6 and preferably, up to 3, more preferably comprised between 3 and 3.6, even more preferably up to 4.

[0118] Preferably said plant ash, amorphous silica source, or a combination thereof, is added to the aqueous reaction medium of step (a) in an amount of up to 30% by weight, preferably 25% by weight based on the total weight of the spent foundry sand, the sand other than a spent foundry sand if any, the base, and the aqueous reaction.

[0119] The plant ash comprises crystalline silica and can also comprise a portion of amorphous silica. The plant ash is generally obtained from a combustion of a silica-containing plant part and / or plant. Preferably, the part of a silica- containing plant (i.e., silica-containing plant part) is selected from the group consisting of a root, a stem, a leaf, a flower, a fruit, a husk, a culm, a stalk, wood and combinations thereof. The part of the silica-containing plant can also derive from a processing of the plant, such as straw (e.g. cereals straw), bagasse (e.g. sugar cane bagasse), oil (e.g. palm oil), sawdust (e.g. tree sawdust), and / or pellet (e.g. wood pellet).

[0120] Said plant part can be selected from the group consisting of rice husk, rice straw, wheat husk, wheat straw, barley straw, barley husk, sugar cane bagasse, sugar cane leaves, bamboo stem, bamboo leaves, corncob, palm tree oil, miscanthus stalk, miscanthus leaves, sedge leave, watermelon fruit, tree wood and combinations thereof.

[0121] Preferably, the silica-containing plant is rice and, preferably, the part of said silica-containing plant is a husk.

[0122] Rice husk ash (RHA) is particularly preferred as it contains a relatively high amount of silica which is preferably of at least 5 wt.%, more preferably at least 10 wt.%, most preferably at least 15 wt.% based on the total weight of the ash.

[0123] Without wishing to be bound to a specific theory or mechanism, it has been found that the use of the additive or a precursor thereof as described above in the process according to the present invention is particularly advantageous as it allows obtaining an improved dissolution of the crystalline silica (quartz) contained in the sand, which is normally difficult to dissolve without employing elevated temperatures and / or long reaction times.

[0124] It has indeed been found that the addition of said additive or a precursor thereof contributes increasing the dissolution yield of the SiCh contained in the sand (i.e., %SiO2 initial) during the dissolution step (a) and thus the amount of SiCh contained in the silicate thus produced (i.e., %SiO2 final). In other words, this allows increasing the yield of SiCh recovered from the sand.

[0125] Moreover, it has been found, that in the above embodiment wherein the aqueous reaction mixture can further comprise a plant ash, an amorphous silica source, or a combination thereof, the addition of the additive or a precursor thereof is particularly advantageous and desirable. This is because the additive allows improving not only the dissolution of the amorphous silica (possibly contained in the plant ash and / or deriving from the amorphous silica source) but also helps the dissolution of the crystalline silica (which is in addition to that of the sand) possibly deriving from the plant ash. As already mentioned, compared to the amorphous form, crystalline silica is more difficult to dissolve and, in normal hydrothermal processes, it is difficult to achieve efficient dissolution and thus high yields of SiCh recovery.

[0126] In conclusion of step (a) of the process according to the present invention, an aqueous liquid mixture comprising silicate and, possibly, solid by-products is obtained.

[0127] Preferably, said aqueous liquid mixture is water, even more preferably deionized water.

[0128] According to an embodiment of the present invention, if solid by-products are present, the aqueous liquid mixture obtained in step (a) can be also defined as a solid-liquid mixture comprising aqueous liquid silicate (in the form of a silicate solution which is an aqueous silicate solution) and solid by-products.

[0129] Accordingly, the process according to the present invention further comprises, after step (a), a step (b) of separating said solid by-products from the aqueous liquid mixture to obtain a solution of silicate, which is an aqueous solution of silicate.

[0130] Said solid by-products are also defined for the purposes of the present invention as “residual solids”. The content of such solid by-products in the aqueous liquid mixture obtained in step (a), if any, may be between 0.1% and 35 % by weight, preferably between 5% and 30% by weight based on the total weight of the aqueous liquid mixture.

[0131] Said solid by-products can typically comprise carbon-rich impurities (i.e., carbon-rich solids), metal impurities, such as aluminum or iron impurities, unreacted spent foundry sand and / or any other insoluble materials typically originally contained in the spent foundry sand.

[0132] Said carbon-rich solids are generally by-products deriving from the carbon originally present in the SFS and / or from the heating of carbon-containing compounds possibly contained in the SFS (such as, for example, the chemical binders contained in chemically bonded sand as disclosed below) which occurs in the closed vessel during the hydrothermal dissolution of the spent foundry sand in step (a).

[0133] Aluminum or iron impurities are generally due to the type of the SFS (green sand and / or chemically bonded sand as disclosed below) and / or to the foundry processes, where said SFS was employed (such as, for example, ferrous metal casting).

[0134] The solid by-products are thus separated from the liquid silicate according to step (b) which is carried out in order to eliminate the above-mentioned possible residual impurities and to recovery the silicate in the form of a solution of silicate, which is an aqueous solution of silicate.

[0135] The separation of the residual solid by-products from the aqueous liquid phase of the solid-liquid mixture, which liquid phase comprises the aqueous liquid silicate (in the form of an aqueous solution of silicate), can be carried out using any separation technique and equipment that are commonly used by a skilled person to separate a solid phase from a liquid phase.

[0136] As suitable techniques well known to the skilled person to achieve such a solid / liquid separation, are: decantation, filtration, centrifugation, and combinations thereof.

[0137] Decantation is advantageously achieved using a blanket settling tank, a circular settling tank, a rectangular settling tank, a lamellar settling tank or a combination thereof. Filtration is advantageously achieved using a vacuum-drum filter, a rotary filter, a filter press, belt filter or a combination thereof. It is also possible to use a carbon bed to eliminate, for example, aluminum or iron present in the SFS. Centrifugation is advantageously achieved using a two-phase decanter centrifuge, a three-phase decanter centrifuge, a hollow bowl centrifuge, a disk stack centrifuge, a hermetic bowl centrifuge, a multi-chamber centrifuge (i.e. a centrifuge with an inner centrifuge chamber and exterior mounted) or a combination thereof.

[0138] According to a preferred embodiment, the separation step (b) is carried out by filtration, more preferably carried out with a vacuum filter.

[0139] In conclusion of step (b), i.e., when the silicate has been separated from the residual solid by-products, a recovered (aqueous) solution of silicate is obtained.

[0140] According to an embodiment, it may be desirable having a process which is free of any post-treatment steps of the aqueous liquid mixture comprising silicate obtained after step (a) so that an aqueous solution of silicate is directly obtained after step (a). Preferably, said post-treatments steps are post-treatments, such as a solid / liquid separation, aimed at removing residual impurities, such as solid by-products as mentioned above, possibly contained in the aqueous liquid mixture comprising silicate obtained after step (a).

[0141] More preferably, according to said embodiment, the process of the invention is free of said step (b) as disclosed above.

[0142] Without wishing to be bound to a specific theory, it has been found that such an embodiment wherein the process is free of any post-treatments, may be desirable depending, for example, on the initial composition of the spent foundry sand and / or the particular uses and applications of the final silicate and, possibly, of the precipitated silica deriving therefrom.

[0143] Preferably, following step (a) or (b), the process according to the present invention can comprise a step (c) of adding to the (aqueous) solution of silicate at least one of a secondary silica source, an alkali metal hydroxide, preferably NaOH, a silicate other than the silicate obtained in step (a) or (b), or a mixture thereof.

[0144] According to the invention, after step (c) a new aqueous solution of silicate is obtained.

[0145] Without wishing to be bound to a specific theory, it has been found that this may be advantageous to adjust the characteristic of the silicate, in particular, to achieve the desired Rp values. According to the invention, the silicate other than the silicate obtained in step (a) or (b), is a silicate produced by any conventional process other than the process of the present invention.

[0146] Preferably said silicate other than the silicate obtained in step (a) or (b) is an alkali metal silicate, more preferably sodium or potassium silicate, even more preferably sodium silicate.

[0147] According to the invention, said secondary silica source is a silica produced by any conventional process other than the process of the present invention and / or naturally occurring silica. Preferably, said secondary silica source is amorphous silica.

[0148] According to an embodiment, it may be desirable, either following step (b) or directly following step (a) (i.e., according to the embodiment wherein the process of the invention is free of any post-treatment steps, including step (b)), or following step (c), to dry the silicate solution so obtained to obtain a solid silicate (according to step (d)). Said solid silicate, is a solid silicate in amorphous form.

[0149] The dying step (d) can be carried out using any known means. Preferably, the drying step (d) is carried out by atomization. To this end, use may be made of any type of suitable atomizer, in particular a rotary, nozzle, oven, furnace, liquid pressure or two-fluid atomizer.

[0150] Preferably the silicate obtained according to the process of the present invention (i.e, the silicate obtained after step (a), (b) and / or (c), as well as the solid silicate obtained after step (d)) has a SiCh / MxO ratio (Rp) of at least 2, preferably of at least 2.5, preferably of up to 2.6.

[0151] According to a particularly preferred embodiment of the invention, wherein the aqueous reaction medium of step (a) further comprises a plant ash, an amorphous silica source, or a combination thereof, and / or wherein the process according to the present invention further comprises step (c) as defined above, the Rp of said silicate can be increased up to values above 2.6 and preferably of up to 3, more preferably comprised between 3 and 3.6, even more preferably up to 4.M and x are as defined above.

[0152] Preferably, the amount of SiCh (i.e., %SiO2 final) contained in the solution of silicate obtained after step (a), (b) and / or (c), as well as the solid silicate obtained after step (d) of the process of the invention, is of up to 100% of the amount of silica initially contained in the sand used in step (a) of the process (i.e., %SiO2 initial). Preferably, %SiO2 final is between 70% and 100% of %SiO2 initial. According to an embodiment, the spent foundry sand used in the process of the invention is selected from the group consisting of: green sand (GS), chemically bonded sand (CBS), or a combination thereof. Typically, green sand is a type of SFS which, in addition to SiCh, further comprises clay, in particular bentonite, coal, and water. Chemically bonded sand, instead, is a type of SFS which, in addition to SiCh, typically comprises chemical binders.

[0153] Preferably, the spent foundry sand used in the process according to the present invention comprises from 85 wt% to 99 wt% of SiCh (%SiO2 initial), generally between 92 wt% and 97 wt%. The SiCh in the spent foundry sand is typically only crystalline silica.

[0154] Typically, green sand comprises from 85 wt% to 95 wt% SiCh, while chemically bonded sand comprises from 93 wt% to 99 wt% SiCh. For comparison, the silica content in pure sand, such as Fontainebleau sand is generally between 97.0 wt% and 100.0 wt%, often between 98.0 wt% and 100.0 wt%, sometimes between 99.0 wt% and 100.0 wt%. According to the present invention, the SiCh content in the spent foundry sand is measured by X-Ray fluorescence.

[0155] The carbon content in a spent foundry sand is generally between 0.1 wt% and 5 wt%. For comparison, the carbon content in high purity sand is 0 wt%.

[0156] According to the present invention, the carbon content in the spent foundry sand is measured by carbon / sulfur analysis.

[0157] According to an embodiment of the present invention, green sand has preferably the following composition: 85-95 wt% SiCh, 8-10 wt% bentonite, 4-7 wt% coal, and 3-4 wt% water.

[0158] Green sand typically comprises metal impurities such as iron, likely deriving from foundry processes such as ferrous metal casting, and aluminum, likely deriving from the clays, particularly from the bentonite which is an aluminosilicate clay. In fact, clays correspond to metallic silicates and thus could bring a many different metal impurities to the final silicate depending to the nature of the clay, such as metals, earth-alkaline metals, and the like.

[0159] According to an embodiment of the present invention, green sand comprises from 3 wt% to 5.5 wt% AI2O3 (Al impurity) measured by X-ray fluorescence and / or from 0.5 wt% to 3 wt% Fe20s (Fe impurity) measured by X- ray fluorescence. According to an embodiment of the present invention, chemically bonded sand has preferably the following composition: 93-99 wt% SiCh and 1-3 wt% chemical binder.

[0160] Typically, the chemical binders of the chemically bonded SFS are organic or inorganic binders. The organic binders are generally selected from the group consisting of: alkyd, phenolic acid, phenolic urethane, phenolic ester, furan acid, furan resin, phenolic resin, or a combination thereof. Chemically bonded sand typically comprises organic impurities (mainly carbon-rich impurities) deriving from the chemical binder as well as metal impurities (such as aluminum and iron impurities) in this case likely deriving from the metal casting of the foundry processes.

[0161] According to an embodiment of the present invention, chemically bonded sand comprises from 0.5 w% to 1.8 wt% AI2O3 (Al impurity) and / or from 0.1 wt% to 1.5 wt% Fe20s (Fe impurity).

[0162] There might be situations where a purification of the SFS is particularly advantageous, in particular, when the SFS contains a high amount of metal impurities.

[0163] Accordingly, in a particularly preferred embodiment, the process of the invention comprises, before step (a), a physical pre-treatment step (a”) of purifying the spent foundry sand. A purified spent foundry sand can thus be obtained.

[0164] Preferably said physical pre-treatment step (a”) is advantageously a step of purifying the spent foundry sand from carbon impurities and / or metal impurities and / or from particles having a size less than 100 pm so to provide a purified spent foundry sand mainly comprising or consisting of quartz (crystalline silica).

[0165] Said physical pre-treatment (a”) is preferably selected from the group consisting of: sieving, mechanical attrition, density selection, granulometry selection, magnetic removal of magnetic elements, or a combination thereof.

[0166] Particularly preferred are the physical pre-treatments selected from the group consisting of: sieving, mechanical attrition, or a combination thereof.

[0167] The sieving is generally carried out to select the preferred silica-rich phase (i.e., quartz) of the SFS, which is instead generally composed of different phases depending on the type of the SFS as disclosed above (such as: clays, quartz, carbon, and / or chemical binders forming polymeric resin). Preferably, said sieving is carried out to select particles having a size bigger than 60 pm, preferably a size bigger than 100 pm. These particles of the SFS are the preferred selected particles to be used in the melting process of the invention as they mainly correspond to quartz (crystalline silica) particles.

[0168] Said sieving is also advantageous to purify the SFS (prior to step (a)) from clays particles and the corresponding metal impurities (in particular aluminum). Said clays particles generally have a size comprised between 6 pm and 100 pm, preferably between 20 pm and 100 pm.

[0169] Said sieving is also advantageous to remove from the SFS (prior to step (a)), particles having a size below 10 pm, preferably below 1 pm, (i.e. fine particles) in order to prevent exposure to harmful products, such as STOT RE products (i.e., Specific Target Organ Toxicity- Repeated Exposure, referring to adverse, non-lethal, organ-specific health effects of a toxic substance that manifest after multiple exposures).

[0170] Preferably, the mechanical attrition is carried out when the SFS is composed of different phases with a strong link between them.

[0171] According to a particularly preferred embodiment, the mechanical attrition is carried out when the SFS is a green sand (GS).

[0172] Advantageously mechanical attrition removes clays, polymeric resins, and / or carbon at the quartz surface without modifying (particularly, without reducing) the quartz particle size.

[0173] Still in situations where the SFS contains a high amount of metal impurities, it might be particularly advantageous to reduce this amount of impurities by operating the step (a) of the invented process not to with a single silica source, namely the SFS, but with a mixture of the spent foundry sand and a sand other than a SFS, said sand other than a SFS having a purity (SiCh content) greater than the one of the SFS.

[0174] Without wishing to be bound to a specific theory, it has been found that using a purified SFS or a sand, having a higher purity than the SFS, that is a mixture of a SFS and of a sand other than a SFS in the process of the present can be particularly advantageous when the final silicate (and the silica produced therefrom) are intended for specific uses such as, for example, for use as polymer reinforcement in a polymer composition. In such case the physical pre-treatment (a”) of the spent foundry sand is desirable especially to remove carbon impurities (particularly, inorganic carbon impurities) possibly present in the initial spent foundry sand that could act as undesired “side filler” in the polymer composition. There are still other situations where it may be advantageous to apply the step (a) of the invented process not to a single silica source, namely the SFS, but to several ones. A mere reason could be that several silica sources could be “at hand” and that the process can work with a variety of sources. Such other possible silica sources include sands other than SFS that have a purity (SiCh content) lower than or equal to the one of the SFS, such as dirty sands, possibly polluted / contaminated sands.

[0175] This being said, the sand other than a spent foundry sand, when present, is advantageously natural sand. An example of such sand is Fontainebleau sand. The purity (SiCh content) of the sand other than a spent foundry sand, possibly natural sand, is preferably of at least 95.0 wt%, more preferably at least 97.0 wt%, still more preferably at least 99.0 wt%. between 99 wt% and 100 wt%.

[0176] The weight of the sand other than a spent foundry sand can range from 0 up to 85% by weight, preferably from 0 up to 75%, possibly from 0 up to 60%, 50%, from 0 to 40%, from 0 to 30%, from 0 to 20%, from 0 to 10%, from 0 to 5.0%, from 0 to 2.0% or from 0 to 1.0% by weight, based on the total weight of sand (that is to say based on the combined weight of the spent foundry sand and of the sand other than a spent foundry sand); it can also be zero (that is to say that no sand other than a spent foundry sand is then used). Besides, the weight of the sand other than a spent foundry sand can be of at least, at least 0.1%, at least 0.2%, at least 0.5%, at least 1.0%, at least 2.0%, at least 5.0%, at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60% or at least 70% by weight based on the total weight of sand.

[0177] The sand other than a spent foundry sand, if any, is advantageously put together, preferably mixed, with the spent foundry sand before the step (a). Putting together, preferably mixing, the spent foundry sand and the sand other than a spent foundry can be done in a dedicated container before their introduction in the vessel. Alternatively, this can be done in the vessel itself before reaction has started; when introducing both sands in the vessel, the aqueous reaction medium may already be present therein so as to facilitate the dispersion and homogenization of both sands. The base may be introduced in the vessel after the aqueous reaction medium, the spent foundry sand and the sand other than a spent foundry sand have been introduced in it.

[0178] According to an embodiment, when the SFS is a chemically bonded sand (CBS), the process of the invention can be free of the physical pre-treatment step (a”) of purifying the spent foundry sand. The present invention also relates to the use of the silicate as obtained or obtainable according to the process of the invention for the preparation of silica, in particular precipitated silica. Said silicate is preferably the silicate obtained after step (a), (b), (c), and / or (d) of the process of the present invention.

[0179] The invention relates also to a process for the preparation of a precipitated silica comprising the steps of:

[0180] (I) preparing a silicate by the process according to the present invention as disclosed according to any of the above embodiments,

[0181] (II) reacting the silicate obtained in step (I) and, optionally in addition a silicate other than said silicate obtained in step (I), with at least one acidifying agent in a liquid medium, preferably an aqueous liquid medium, to obtain a suspension of precipitated silica,

[0182] (III) separating the precipitated silica from the liquid medium, preferably an aqueous liquid medium, to provide a wet precipitated silica, and

[0183] (IV) drying said wet precipitate silica to obtain precipitated silica.

[0184] Said silicate is preferably the silicate obtained after step (a), (b), (c), and / or (d) of the process of the present invention and is as disclosed according to any one of the above embodiments.

[0185] The silicate used in the process for the preparation of precipitated silica may consist of the silicate obtained or obtainable by the process according to the present invention.

[0186] Alternatively, the silicate used in the process for the preparation of precipitated silica may represent only a portion of the overall silicate employed in step (II).

[0187] According to this embodiment, step (II) comprises reacting the silicate obtained in step (I) (i.e., the silicate obtained by the process according to the present invention) and, in addition thereto, a different silicate. Said different silicate is preferably an alkali metal silicate, more preferably sodium or potassium silicate, even more preferably sodium silicate.

[0188] According to the invention said silicate other than the silicate obtained in step (I), is preferably a silicate produced by any conventional process other than the process of the present invention.

[0189] According to another embodiment, the process of the present invention is a process for the preparation of precipitated silica comprising: (I) preparing a silicate by the process according to the present invention as disclosed according to any of the above embodiments,

[0190] (I’) adding to the silicate obtained in step (I) at least one of a secondary silica source, an alkali metal hydroxide, preferably NaOH, a silicate other than the silicate obtained in step (I), or a mixture thereof, so as to obtain a new silicate,

[0191] (II) reacting the new silicate obtained in step (I’) and, optionally in addition a silicate other than the new silicate obtained in step (I’), with at least one acidifying agent in a liquid medium, preferably an aqueous liquid medium, to obtain a suspension of precipitated silica,

[0192] (III) separating the precipitated silica from the liquid medium to provide wet precipitated silica, and

[0193] (IV) drying said wet precipitated silica to obtain precipitated silica

[0194] Said silicate of step (I) is preferably the silicate obtained after step (a), (b), (c), and / or (d) of the process of the present invention and is as disclosed according to any one of the above embodiments.

[0195] According to the present invention, the silicate other than the silicate obtained in step (I) is as defined above.

[0196] According to the invention, said secondary silica source is a silica produced by any conventional process other than the process of the present invention and / or naturally occurring silica. Preferably, said secondary silica source is amorphous silica.

[0197] Preferably, the new silicate obtained in step (F) is an alkali metal silicate, more preferably sodium or potassium silicate, even more preferably sodium silicate.

[0198] According to the present invention, the silicate other than the new silicate obtained in step (F) is preferably an alkali metal silicate, more preferably sodium or potassium silicate, even more preferably sodium silicate. According to the invention said silicate other than the new silicate obtained in step (F), is preferably a silicate produced by any conventional process other than the process of the present invention.

[0199] The choice of the acidifying agent is made in a way well known in the art.

[0200] The acidifying agent is preferably selected from the group consisting of: a mineral acid, preferably selected from the group consisting of: sulfuric acid (H2SO4), hydrochloric acid (HC1), nitric acid (HNO3), phosphoric acid (H3PO4), and combinations thereof, and an organic acid, preferably selected from the group consisting of: acetic acid, formic acid, carbonic acid, and combinations thereof.

[0201] The acidifying agent can be dilute or concentrated; its normality can be between 0.4 and 36N, for example, between 0.6 and 2.5N. In particular, in the case where the acidifying agent is sulfuric acid, its concentration can be between 40 and 180 g / 1, for example between 60 and 130 g / 1.

[0202] Preferably, step (II) is carried out at a temperature of at least 40 °C, preferably of at least 60 °C, even more preferably of at least 75°C, most preferably of at least 80°C.

[0203] The process for the preparation of precipitated silica can be carried out according to conditions well-known in the art, preferably by the processes disclosed in US11241370, WO23118281, US10011495, FR2985993, US5547502, US9938154, US2009214449, US10259715, US9369215, US11208331, or US11279623, all incorporated by reference herein.

[0204] Particularly, the preparation of precipitated silica by reaction of a silicate, preferably sodium silicate, with an acidifying agent, can be carried out according to any known method of preparation, in particular by addition of an acidifying agent to a feedstock of silicate, preferably sodium silicate, or else by simultaneous addition, total or partial, of acidifying agent and of silicate, preferably sodium silicate, to a feedstock of water, or of silicate, or of acidifying agent.

[0205] In the process according to the invention, on conclusion of the step of the reaction of precipitation between silicate and acidifying agent (step (II)), a suspension of precipitated silica is obtained.

[0206] The precipitated silica is subsequently separated (step (III)).

[0207] The separation carried out in the preparation process according to the invention, usually comprises a filtration, followed by a washing operation, if necessary. The filtration is carried out according to any suitable method, for example using a filter press, a belt filter or a vacuum filter.

[0208] The silica suspension thus recovered (filtration cake) is subsequently dried.

[0209] Drying (step (IV)) can be carried out according to any means known in the art as described above for step (d). In a preferred embodiment of the invention, the acidifying agent is sulfuric acid and the process comprises a further step (IF) of separating the salt containing a sulfate anion (SO42') obtained after the precipitation reaction of silica, in solid or liquid form. This salt containing a sulfate anion (SCU2'), which is co-produced with the precipitated silica and separated therefrom during step (II’), is preferably an alkali metal sulfate; more preferably, it is sodium sulfate (Na2SO4), potassium sulfate (K2SO4), or a combination thereof. In a more preferred embodiment, said salt containing a sulfate anion obtained after step (III) is recycled by adding it into the aqueous reaction mixture of step (a) of the process of the invention.

[0210] Without wishing to be bound to a specific theory or mechanism, it has been found that, at the end of the process for preparing a precipitated silica, when sulfuric acid is employed as the acidifying agent, a salt containing a sulfate anion is obtained, which can be advantageously employed as the additive according to the invention, by adding it into the aqueous reaction mixture used in step (a) and / or to the spent foundry sand and optionally to the sand other than a spent foundry sand of the process of the invention so to advantageously obtain a circular process.

[0211] In an embodiment of the present invention where the silicate used in step (II) disclosed above is a silicate which still comprises carbon impurities, the above process for the preparation of a precipitated silica is a process for the preparation of a precipitated silica-carbon composite, which may be notably used as polymer reinforcement.

[0212] Accordingly, step (II) of reacting said silicate with at least one acidifying agent in a liquid medium, preferably an aqueous liquid medium, as disclosed above results in a suspension of precipitated silica-carbon composite. Step (II) as disclosed above is thus a step of separating the precipitated silica-carbon composite from the liquid medium, preferably an aqueous liquid medium, to provide a wet precipitated silica-carbon composite, and step (IV) as disclosed above is a step of drying said wet precipitate silica-carbon composite to obtain precipitated silica-carbon composition.

[0213] Furthermore, the invention relates to a reaction mixture. Said reaction mixture can be used and is suitable for producing a silicate, preferably an alkali metal silicate, from a spent foundry sand by the process as defined above comprising step (a) and being free of any thermal pre-treatment step (a’), and / or for preparing a precipitated silica by the process as defined above. The reaction mixture comprises:

[0214] (i) spent foundry sand, which has not been subjected to a thermal pretreatment (a’) as disclosed above;

[0215] (ii) a base;

[0216] (iii) an aqueous liquid medium;

[0217] (iv) optionally a sand other than a spent foundry sand;

[0218] (v) optionally an additive that is a salt comprising a multivalent anion; and

[0219] (vi) optionally a plant ash, an amorphous silica source, or a combination thereof.

[0220] Preferably, the spent foundry sand (i), the base (ii), the aqueous liquid medium (iii), the sand other than a spent foundry sand (iv), the additive (v), and, the plant ash and / or an amorphous silica source (vi), are as described above according to any one of the embodiments of the present invention.

[0221] Preferably, spent foundry sand (i) is a spent foundry sand which has not been subjected to any thermal pre-treatment (a’) according to any of the embodiments according to the present invention as described above. Besides, the spent foundry sand (i) has a total carbon content of at least 1 wt%, more preferably of at least 2 wt%, even more preferably of at least 3 wt%, most preferably of at least 4 wt% based on the total weight of the spent foundry sand, said total carbon content being measured by carbon / sulfur analysis.

[0222] The present invention will now be illustrated by the following examples, which are not intended to be limiting.

[0223] EXAMPLES

[0224] Materials and methods

[0225] All starting materials used in the examples are commercially available.

[0226] Potentiometry method to determine Rp

[0227] A Titrando 808 was used in order to determine the weight ratio (Rp) [%weight (SiCh) / %weight (Na2O)]. The device was equipped with a reference electrode Ag / AgCl in KC1 3M and a working electrode in tungsten. Each Rp was measured in duplicate and the Rp values presented were the average between the two measurements. 0.5 g of sample was weighed and completed with 30 mL of demineralized water. The titration solution was a 0. IN HC1 solution. The volume Vi (mL) was determined as the equivalence of the titration. After the equivalence, 0.5 mL of titration solution was added.

[0228] Afterward, 50 mL of KF solution (50 g / 1 of KF in a water / ethanol (50 / 50) solution) was added and allowed to react for 3 minutes. Then, 15 mL of 0. IN HC1 solution were added. The excess of HC1 was titrated by a NaOH IN solution and the volume V2(mL) was the equivalent point of the titration.

[0229] The Rp was then calculated following the formula below:

[0230] Rp = (0.31 *VI) / (1.5(15* 1-V2* 1) +(0.5*0.1))

[0231] X-Ray Fluorescence

[0232] The elemental analysis was done by XRF (X-Ray Fluorescence). Glass disks were prepared using commercial fusion instrument (LeNeo from Malvern Panalytical, X-300 from Katanax or other equivalent system).

[0233] For the preparation of the glass disk, the sample was mixed with lithium metaborate, lithium tetraborate and non-wetting agent (LiBr). The fusion was carried out in platinum crucibles at 1050 °C for 20 minutes before pouring into the platinum mould to obtain a glass disk.

[0234] The measurement was realized thanks to a XRF instrument (Epsilon 3XL from Malvern-Panalytical).

[0235] Standard geological samples (ACE, ANG, BEN, or other equivalent) and 3 blanks were used to determine the calibration of the desired elements.

[0236] Different measurement conditions allowed an optimization of the signal for the following elements:

[0237] - for Al, Mg, Na, Si: 5kV, 6pA, under Helium sweep.

[0238] - for P: 10 kV, 30pA under Helium scan.

[0239] - for Ca, K, Ti: 12 kV, 25 pA, under Helium scan.

[0240] - for Fe and Mn: 20 kV, 15 pA in air.

[0241] Laser granulometry to determine the Particle Size Distribution (PSD) measurement

[0242] The particle sizes of the powders were determined by means of a laser granulometer (Malvern Mastersizer 3000). The Fraunhofer optical model was used for the measurement. This measurement allows to determine the following parameter (based on the volume distribution): - Dio: particle size below which 10 % of the distribution lies (from cumulative curve);

[0243] - D50 [median]: particle size at 50% of the cumulative curve;

[0244] - D90: particle size below which 90 % of the distribution lies (from cumulative curve)..

[0245] Carbon sulfur analysis (C / S method) to determine the carbon content

[0246] A 200 mg sample was analyzed in Horiba EMIA 320-V2. Lecocel®, iron and tin balls were used as combustion accelerators. CS26 - 3.19 % was used to calibrate the sensor.

[0247] Example 0 - Sand characterization

[0248] Different samples of spent foundry sand and high purity sand (for comparison) have been analyzed as following.

[0249] Example 0.1 - Spent Foundry Sand 1 (SFS1)

[0250] Spent foundry sand 1 (SFS1) presents the following characteristics:

[0251] Particle size distribution by laser granulometry

[0252] The granulometric distribution of SFS1 presents the following main values:

[0253] Carbon / Sulfur analysis

[0254] SFS1 presents a carbon content of 4.3 wt.% according to the carbon / sulfur analysis.

[0255] X-Ray Fluorescence (elemental analysis)

[0256] Example 0.2 - Spent Foundry Sand 2 (SFS2)

[0257] Spent foundry sand 2 (SFS2) presents the following characteristics:

[0258] Particle size distribution by laser granulometry

[0259] The granulometric distribution of SFS2 presents the following main values:

[0260] Carbon / Sulfur analysis

[0261] SFS2 presents a carbon content of 2.4 wt.% according to the carbon / sulfur analysis.

[0262] X-Ray Fluorescence (elemental analysis)

[0263] Example 0.3 - Spent Foundry Sand 3 (SFS3)

[0264] Spent foundry sand 3 (SFS3) presents the following characteristics: Particle size distribution by laser granulometry

[0265] The granulometric distribution of SFS3 presents the following main values:

[0266] Carbon / Sulfur analysis

[0267] SFS1 presents a carbon content of 3.9 wt.% according to the carbon / sulfur analysis.

[0268] X-Ray Fluorescence (elemental analysis)

[0269] Example 0.4 - Spent Foundry Sand 4 (SFS4)

[0270] Spent foundry sand 4 (SFS4) corresponds to SFS3 of Example 0.3 after a purification step, described as follows:

[0271] Purification of the sand SFS3:

[0272] Purification of the first spent foundry sand (SFS3) was realized by granulometry selection (sieving), using a sieve having a cut-off threshold = 100 pm.

[0273] The purified spent foundry sand (SFS4) thus only contains particles having a particle size bigger than 100 pm which are the selected particles for the purpose of the present invention (to be employed for the production of silicate as disclosed in the following examples) as they are rich in silica. Spent foundry sand 4 (SFS4) presents the following characteristics:

[0274] Particle size distribution by laser granulometry.

[0275] The granulometric distribution of SFS4 presents the following main values:

[0276] Carbon / Sulfur analysis:

[0277] SFS4 presents a carbon content of 3.2 wt.% according to the carbon / sulfur analysis.

[0278] X-Ray Fluorescence (elemental analysis)

[0279] Example 0.5 - Spent Foundry Sand 5 (SFS5)

[0280] Spent foundry sand 5 (SFS5) presents the following characteristics:

[0281] Particle size distribution by laser granulometry

[0282] The granulometric distribution of SFS5 presents the following main values: Carbon / Sulfur analysis

[0283] SFS5 presents a carbon content of 1.1 wt.% according to the carbon / sulfur analysis.

[0284] X-Ray Fluorescence (elemental analysis)

[0285] Example 0.6 - High Purity Sand (comparative)

[0286] The high purity sand presents the following characteristics:

[0287] Particle size distribution by laser granulometry

[0288] The granulometric distribution of said high purity sand presents the following main values:

[0289] Carbon / Sulfur analysis

[0290] The high purity sand presents a carbon content of 0 wt.% according to the carbon / sulfur analysis.

[0291] X-Ray Fluorescence (elemental analysis)

[0292] Example 0.7 - Spent Foundry Sand 6 (SFS6)

[0293] Spent foundry sand 6 (SFS6) presents the following characteristics:

[0294] Particle size distribution by laser granulometry

[0295] The granulometric distribution of SFS7 presents the following main values:

[0296] Carbon / Sulfur analysis

[0297] SFS6 presents a carbon content of 4.5 wt.% according to the carbon / sulfur analysis.

[0298] X-Ray Fluorescence (elemental analysis) Example 1 - Silicate preparation

[0299] The spent foundry sand described in Example 0 was been employed to produce silicate according to the process of the present invention as disclosed in the following Examples 1.1 to 1.12.

[0300] Example 1.1

[0301] In a PARR combustion bomb, the following reagents were introduced: 19.76 g of NaOH solution at 30 wt%, 17.1 g of spent foundry sand 1 (SFS1) and 13.14 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 1 hour at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 1” (SI).

[0302] Example 1.2

[0303] In a PARR combustion bomb, the following reagents were introduced: 16.4 g of NaOH solution at 30 wt%, 15.8 g of spent foundry sand 1 (SFS1) and 17.8 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 2 hours at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 2” (S2).

[0304] Example 1.3

[0305] In a PARR combustion bomb, the following reagents were introduced: 19.76 g of NaOH solution at 30 wt%, 17.1 g of spent foundry sand 1 (SFS1) and 13.14 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 3 hours at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 3” (S3).

[0306] Example 1.4

[0307] In a PARR combustion bomb, the following reagents were introduced: 18.8 g of NaOH solution at 30 wt%, 19 g of spent foundry sand 1 (SFS1) and 12.2 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 3 hours at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 4” (S4).

[0308] Example 1.5

[0309] In a PARR combustion bomb, the following reagents were introduced: 18.8 g of NaOH solution at 30 wt%, 19 g of spent foundry sand 1 (SFS1) and 12.2 g of water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 215°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 5 hours at 215°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 5” (S5).

[0310] Example 1.6

[0311] In a PARR combustion bomb, the following reagents were introduced: 21.1 g of NaOH solution at 30 wt%, 14.4 g of spent foundry sand 2 (SFS2) and 14.5 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 1 hour at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 6” (S6).

[0312] Example 1.7

[0313] In a PARR combustion bomb, the following reagents were introduced: 21.1 g of NaOH solution at 30 wt%, 14.4 g of spent foundry sand 2 (SFS2) and 14.5 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 3 hours at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 7” (S7).

[0314] Example 1.8

[0315] In a PARR combustion bomb, the following reagents were introduced: 17 g of NaOH solution at 30 wt%, 16 g of spent foundry sand 2 (SFS2) and 17 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 215°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 5 hours at 215°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 8” (S8).

[0316] Example 1.9

[0317] In a PARR combustion bomb, the following reagents are introduced: 15.6 g of NaOH solution at 50 wt%, 12.5 g of spent foundry sand 4 (SFS4), 12.9 g of rice husk ashes and 9 g of deionized water. The PARR combustion bomb is then set under stirring and introduced in an oven. The temperature is raised up to 215°C with a ramp of l°C / min. Once the temperature is reached, the mixture is left for 5 hours at 215°C and then cooled down until room temperature is reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained is centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 9” (S9).

[0318] Example 1.10

[0319] In a PARR combustion bomb, the following reagents are introduced: 15.6 g of NaOH solution at 50 wt%, 12.5 g of spent foundry sand 5 (SFS5), 12.9 g of rice husk ashes and 9 g of deionized water. The PARR combustion bomb is then set under stirring and introduced in an oven. The temperature is raised up to 215°C with a ramp of l°C / min. Once the temperature is reached, the mixture is left for 5 hours at 215°C and then cooled down until room temperature is reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained is centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 10” (S10).

[0320] Example 1.11 (comparative)

[0321] In a PARR combustion bomb, the following reagents are introduced: 18.8 g of NaOH solution at 30 wt%, 14.6 g of high purity sand and 16.6 g of deionized water. The PARR combustion bomb is then set under stirring and introduced in an oven. The temperature is raised up to 215°C with a ramp of l°C / min. Once the temperature is reached, the mixture is left for 5 hours at 215°C and then cooled down until room temperature is reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained is centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 11” (Si l).

[0322] Example 1.12

[0323] In a PARR combustion bomb, the following reagents are introduced: 17 g of NaOH solution at 30 wt%, 16 g of spent foundry sand 4 (SFS4) and 17 g of deionized water. The PARR combustion bomb is then set under stirring and introduced in an oven. The temperature is raised up to 215°C with a ramp of l°C / min. Once the temperature is reached, the mixture is left for 5 hours at 215°C and then cooled down until room temperature is reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained is centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 12” (S12).

[0324] Results

[0325] For each of the liquid sodium silicates obtained according to Examples 1.1 to 1.12 above, the experimental Rp (Rp exp or Rp) was measured by infrared spectroscopy technique (IR-ATR) and compared to the target one (Rp target).

[0326] The results are shown in Table 1 below.

[0327] Rp target is the theoretical ratio Rp = %wt SiO2 / %wt Na2O (concentrations in weight) based on the quantity of SiO2(from the Spent Foundry Sands), Na2O (from the NaOH) and water introduced. The Rp targeted is so called Rp(theo). Parallely, the Rp actually obtained (i.e., the Rp measured) is the Rp(exp) or Rp.

[0328] Table 1

[0329] If the targeted Rp(theo) is too high (typically above 3.6), that is the silica content is really much more important than the NaOH amount, the dissolution power is not strong enough to allow the effective dissolution of the silica contained in the raw material (i.e., in the spent foundry sand used in the hydrothermal process of the invention). There is indeed a threshold value above which the dissolution is not thermodynamically possible. Indeed, in the case of sand, quartz dissolution yield is thermodynamically limited to about 75%. Thus about 25% of the initial SiCh content (i.e., of the SiCh contained in the spent foundry sand used in the process of the present invention) will never be dissolved and thus never recovered in the liquid silicate.

[0330] Moreover, without wishing to be bound to a specific theory, it has been found that a too high Rp could also lead to the formation of crystalline silica in the liquid sodium silicate. Such a reaction has to be avoided because crystalline silica would disturb the further silica precipitation process.

[0331] Example 2 - Preparation of precipitated silica

[0332] Example 2.1

[0333] In a 2 L glass reactor, 969 g of water and 28.5 g of sodium silicate are introduced. The suspension so obtained is heated to 72 °C and kept at this temperature for 5 minutes before being re-heated to 82 °C. Simultaneously to this temperature profile, 80 g / L of sulfuric acid is introduced under stirring (350 rpm) at a flow rate of 5.9 g / min until the pH reaches 8.7. A sodium silicate solution with a ratio %wtSiO2 / %wtNa2O (Rp) equal to 3.5 and a concentration of 230 g / L and sulfuric acid with a concentration equal to 80 g / L are simultaneously introduced into the reactor over a period of 58 minutes, wherein the sodium silicate solution is introduced at a flow rate of 12 g / min and the sulfuric acid is introduced at a flow rate regulated in such a way as to maintain the pH of the reaction medium at a value of 8.7. At the end of the 58 minutes, the sodium silicate flow rate is stopped. The flow rate of the 80 g / L sulfuric acid is put at 5.4 g / min that allows to maintain the pH of the solution above 5.2.

[0334] The slurry thus obtained is filtered and washed through a filter (20 wt% dry cake extract). The resulting cake is then mechanically broken down and the resulting slurry dried using a spray dryer.

[0335] Example 2.2

[0336] In a 2 L glass reactor, 969 g of water and 28.5 g of sodium silicate are introduced. The suspension so obtained is heated to 72 °C and kept at this temperature for 5 minutes before being re-heated to 82 °C. Simultaneously to this temperature profile, 80 g / L of sulfuric acid is introduced under stirring (350 rpm) at a flow rate of 5.9 g / min until the pH reaches 9.1. A sodium silicate solution with a ratio %wtSiO2 / %wtNa2O (Rp) equal to 2.5 and a concentration of 230 g / L and sulfuric acid with a concentration equal to 80 g / L are simultaneously introduced into the reactor over a period of 60 minutes, wherein the sodium silicate solution is introduced at a flow rate of 12 g / min and the sulfuric acid is introduced at a flow rate regulated in such a way as to maintain the pH of the reaction medium at a value of 9.1. At the end of the 60 minutes, the sodium silicate flow rate is stopped. The flow rate of the 80 g / L sulfuric acid is put at 5.4 g / min that allows to maintain the pH of the solution above 5.2.

[0337] The slurry thus obtained is filtered and washed through a filter (20 wt% dry cake extract). The resulting cake is then mechanically broken down and the resulting slurry dried using a spray dryer.

[0338] Example 3 - Attrition mechanical regeneration

[0339] Example 3.1 - Purification of the Spent Foundry Sand 6 (SFS6)

[0340] Purification of the spent foundry sand (SFS6 - Example 0.7) was realized by a regeneration process, which is a mechanical attrition treatment comprising the following steps.

[0341] A preliminary sieving correlated with a screener was used to remove block sand with a size more than 3 mm. Then, the spent foundry sand was dried at a temperature of more than 100 °C by using a fluidized bed to obtain a moisture of less than 1%. The dried sand was then introduced into a hopper before passing into a first attrition chamber through a screw. The sand flow rate was 6 kg / min.

[0342] The attrition process involved two millstones rotating in the opposite direction in a rotating drum. The pressure of the two millstones on the rotating drum was set using the pressure of a hydraulic cylinder allowing the breaking of the shell around the sand particles.

[0343] After this step in a first attrition chamber, the sand was evacuated in a second attrition chamber wherein the same conditions as in the first chamber were used. In total, four steps in attrition chambers were performed for the present experiment.

[0344] To evacuate the fine particles so obtained and mainly composed by coal and clay, an aspirator was installed above the attrition chamber with a flow rate of 5000 m3 / h. By this methodology it was possible to purify 360 kg spent foundry sand per hour.

[0345] At the end of the mechanical attrition process, a regenerated spent foundry sand was obtained. Particle size distribution by laser granulometry

[0346] The granulometric distribution of the SFS6 presents the following main values before and after the mechanical attrition treatment:

[0347] Carbon / Sulfur analysis

[0348] SFS6 presents the following carbon content according to the carbon / sulfur analysis before and after the mechanical attrition treatment:

[0349] X-Ray Fluorescence (elemental analysis)

[0350] The profile of the impurities before and after the mechanical attrition treatment was measured by X-ray fluorescence and is reported below: Example 4 - Silicate preparation

[0351] Example 4.1

[0352] In a PARR combustion bomb, the following reagents were introduced: 19.76 g of NaOH solution at 30 wt%, 17.1 g of regenerated spent foundry sand 6 (SFS6 - according to Example 3.1) and 13.14 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200 °C with a ramp of 1 °C / min. Once the temperature was reached, the mixture was left for 1 hour at 200 °C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The sumatant (liquid phase) is a solution of sodium silicate, also called “Silicate 4.1” (S4.1).

[0353] Example 4.2

[0354] In a PARR combustion bomb, the following reagents were introduced: 14.97g of NaOH solution at 30 %wt, 12 g of spent foundry sand 6 (SFS6 - according to Example 0.7, namely a spent foundry sand not subjected to a purification step) and 23.03 g of deionized water. The PARR combustion bomb was then set under stirring and introduced in an oven. The temperature was raised up to 200°C with a ramp of l°C / min. Once the temperature was reached, the mixture was left for 2 hours at 200°C and then cooled down until room temperature was reached (there was no ramp for the temperature decrease, the cooling was by inertia). The solution obtained was centrifuged at 4500 tr / min for 35 min to separate the residual solid and the solution. The surnatant (liquid phase) was so called “Silicate 4.2” (S4.2).

[0355] Results

[0356] For each of the liquid sodium silicates obtained according to Examples 4.1 and 4.2 above, the experimental Rp (Rp exp or Rp) was measured by potentiometric method as described below: Table 2

[0357] The profile of the impurities was measured by X-ray fluorescence and is reported in Table 3 below:

[0358] Table 3

[0359] Example 5 - Preparation of precipitated silica

[0360] Example 5.1

[0361] This example describes the preparation of a precipitated silica from a silicate with a regenerated spent foundry sand (silicate produced according to Example 4.1 - “Silicate S.l”).

[0362] In a 25 L stainless steel reactor is introduced 5.943 kg of water, 7.738 kg of a sodium silicate solution (SiCh / Na?!) ratio = 1.6; SiCh concentration = 13.11 wt%) and 166 g of ISfeSCU (solid). The same sodium silicate solution is used throughout the process. After starting the agitation (250rpm), the tank feed thus constituted is heated to 79 °C. 7.7 wt% sulfuric acid solution is introduced into the reactor at a flowrate of 525 g / min during 20 min. Next, a 96 wt% sulfuric acid solution is simultaneously introduced over 14.24 min period at a flowrate of 6.8 g / min to reach pH 8. At the same time, the temperature is increased from room temperature up to reach 92°C.

[0363] Sodium silicate at a flowrate of 93 g / min and a 96 wt% sulfuric acid solution are then introduced simultaneously over a period of 20.1 min. The flowrate of the 96 wt% sulfuric acid solution is regulated so that the pH of the reaction medium is maintained at a value of 8.00.

[0364] At the end of this simultaneous addition, the pH of the reaction medium is brought to a value of 4.1 with 96 wt% sulfuric acid, and the reaction mixture is matured for 2 minutes. A slurry is obtained.

[0365] The reaction slurry is filtered and washed on a filter press to obtain a cake. The cake obtained is disintegrated by a mechanical action and by a chemical action with alumina additive 0.3%wt. The resulting slurry is dried by means of a nozzle spray dryer to obtain a precipitated silica SI in accordance with the invention having the following characteristics:

[0366] Table 4

Claims

C L A I M S1. A process for the hydrothermal preparation of a silicate from a spent foundry sand comprising:- optionally, a physical pre-treatment step (a”) of purifying the spent foundry sand,- a step (a) of reacting, in a closed vessel, the spent foundry sand and, optionally in addition a sand other than a spent foundry sand, with a base, in an aqueous reaction medium, at a temperature of above 100 °C, said process being characterized in that it is free of step (a’), before step (a), of thermally pre-treating the spent foundry sand in an atmosphere comprising oxygen by heating up to a temperature of between 200 °C and 600 °C for a period of time of 20 minutes to 12 hours.

2. The process according to claim 1, which is free of step (a’), before step (a), of any thermally pre-treating of the spent foundry sand.

3. The process according to claim 1 or 2, wherein the base is used in an amount of up to 25% by weight, preferably of up to 20% by weight, more preferably of up to 15% by weight, even more preferably between 9 % by weight and 13% by weight, based on the total weight of the spent foundry sand, the sand other than a spent foundry sand, if any, the base, and the aqueous reaction medium.

4. The process according to claim 1, 2 or 3, wherein the silicate is an alkali metal silicate, preferably sodium silicate, and wherein the base is an alkali metal base, preferably an alkali metal hydroxide, more preferably sodium hydroxide (NaOH).

5. The process according to any one of the preceding claims, wherein step (a) is carried out by heating up to a temperature of at least 160 °C, preferably of at least 200 °C, more preferably of at most 300 °C, even more preferably between 160 °C and 250 °C, still more preferably between 190°C and 230 °C, most preferably between 200 °C and 215 °C, preferably wherein step (a) is carried out for a period of time of at least 30 minutes, preferably of less than 24 hours, more preferably between 30 minutes and 10 hours, even more preferably between 30 minutes and 6 hours, still more preferably between 30 minutes and 5 hours, most preferably between 2 and 5 hours.

6. The process according to any of the preceding claims, wherein the reaction medium of step (a) comprises an additive which is a salt comprising a multivalent anion, preferably a salt comprising a multivalent anion selected from the group consisting of: sulfur oxyanions, dicarboxylate anions, tricarboxylates, and combinations thereof, more preferably a salt comprising a multivalent anion selected from the group consisting of: sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, or a combination thereof; and wherein the aqueous reaction medium of step (a) preferably further comprises a plant ash, a secondary silica source, preferably an amorphous silica source, or a combination thereof.

7. The process according to any of the preceding claims, wherein, after step(a), an aqueous liquid mixture comprising silicate and solid by-products is obtained and wherein the process further comprises a step (b) of separating said solid by-products from the aqueous liquid mixture to obtain a solution of silicate, which is an aqueous solution of silicate.

8. The process according to any of the preceding claims further comprising a step (c) of adding to the solution of silicate at least one of a secondary silica source, an alkali metal hydroxide, preferably NaOH, a silicate other than the silicate obtained in step (b), or a mixture thereof.

9. The process according to any one of the preceding claims further comprising a step (d) of drying the solution of silicate so as to obtain a solid silicate.

10. The process according to any one of the preceding claims comprising, before step (a), the physical pre-treatment step (a”) of purifying the spent foundry sand, wherein said physical pre-treatment is preferably selected from the group consisting of: sieving, mechanical attrition, densityselection, granulometry selection, magnetic removal of magnetic elements, or a combination thereof.

11. The process according to any of the preceding claims, wherein the solution of silicate or the solid silicate obtained in any of steps (b), (c), or (d), is characterized by a SiCh / MxO weight ratio of at least 2, preferably of at least 2.5, preferably of up to 2.6 or up to 3, preferably between 3 and 3.6, more preferably up to 4, wherein M is a metal, preferably wherein M is an alkali metal and x is 2, and preferably wherein said silicate is characterized by an amount of SiCh of up to 100% of the amount of silica contained in the sand used in step (a), preferably by an amount of between 70 % and 100%.

12. A process for the hydrothermal preparation of a silicate from a spent foundry sand comprising:- a physical pre-treatment step (a”) of purifying the spent foundry sand, selected from the group consisting of: sieving, mechanical attrition, density selection, granulometry selection, magnetic removal of magnetic elements, or a combination thereof,- a step (a) of reacting, in a closed vessel, the spent foundry sand and, optionally in addition a sand other than a spent foundry sand, with a base, in an aqueous reaction medium, at a temperature of above 100 °C, said process being characterized in that it is free of step (a’), before step (a), of thermally pre-treating the spent foundry sand in an atmosphere comprising oxygen by heating up to a temperature of between 200 °C and 600 °C for a period of time of 20 minutes to 12 hours.

13. The process according to claim 12, which is free of step (a’), before step (a), of any thermally pre-treating of the spent foundry sand.

14. Use of the silicate obtained or obtainable by the process according to any of the preceding claims for the preparation of silica, in particular, precipitated silica.

15. A process for the preparation of precipitated silica comprising the steps of:(I) preparing a silicate by the process according to any one of claims 1 to 11 or by the process according to any one of claims 12 to 13,(II) reacting the silicate obtained in step (I) and, optionally in addition a silicate other than said silicate obtained in step (I), with at least one acidifying agent in a liquid medium, preferably an aqueous liquid medium, to obtain a suspension of precipitated silica,(III) separating the precipitated silica from the liquid medium to provide wet precipitated silica, and(IV) drying said wet precipitated silica to obtain precipitated silica.

16. A process for the preparation of precipitated silica comprising the steps of:(I) preparing a silicate by the process according to any one of claims 1 to 11 or by the process according to any one of claims 12 to 13,(I’) adding to the silicate obtained in step (I) at least one of a secondary silica source, an alkali metal hydroxide, preferably NaOH, a silicate other than the silicate obtained in step (I), or a mixture thereof, so as to obtain a new silicate,(II) reacting the new silicate obtained in step (I’) and, optionally in addition a silicate other than the new silicate obtained in step (I’), with at least one acidifying agent in a liquid medium, preferably an aqueous liquid medium, to obtain a suspension of precipitated silica,(III) separating the precipitated silica from the liquid medium to provide wet precipitated silica, and(IV) drying said wet precipitated silica to obtain precipitated silica.

17. A reaction mixture suitable for the preparation of a silicate from a spent foundry sand by the process as defined in any one of claims 1 to 11 or by the process as defined in any one of claims 12 to 13, and / or suitable for the preparation of a precipitated silica by the process as defined in claim 15 or 16, comprising:(i) a spent foundry sand which has not been subjected to a thermal pretreatment (a’) as specified in any one of claims 1 to 2 or in any one of claims 12 to 13, having total carbon content of at least 1 wt%, more preferably of at least 2 wt%, even more preferably of at least 3 wt%, most preferably of at least 4 wt% based on the total weight of the spent foundry sand;(ii) a base;(iii) an aqueous liquid medium, preferably water;(iv) optionally a sand other than a spent foundry sand, preferably a natural sand;(v) optionally an additive that is a salt comprising a multivalent anion, preferably a salt comprising a multivalent anion selected from the group consisting of: sulfur oxyanions, dicarboxylate anions, tricarboxylates, and combinations thereof, more preferably a salt comprising a multivalent anion selected from the group consisting of: sodium citrate, potassium citrate, sodium sulfate, potassium sulfate, or a combination thereof; and(vi) optionally a plant ash, an amorphous silica source, or a combination thereof.

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