Method for producing float glass from a mixture of mineral materials comprising a calcium silicate

Abstract

The invention relates to a method for producing flat glass, comprising melting a mixture of raw materials constituting a melt bath, the mixture of raw materials comprising a calcium silicate comprising, in percentage by weight, more than 30% of SiO2 and more than 20% of CaO, preferably at least 25% of CaO, CaO and SiO2 together representing more than 60%, or even more than 70% or indeed even more than 80% of the total weight of said silicate, and the silicate being introduced into the mixture in the form of a powder, the median grain diameter of which is less than or equal to 400 micrometres.

Description

PROCESS FOR MANUFACTURING FLOAT GLASS FROM A MIXTURE OF MINERAL MATERIALS COMPRISING A CALCIUM SILICATE

[0001] The invention relates to the field of glass melting, in particular for the manufacture of float glass as used in particular in the construction and automotive sectors.

[0002] Glass is usually prepared by melting in a furnace raw materials comprising silica and at least one silica flux such as sodium carbonate, and at least one alkaline earth (to give the glass resistance to hydrolysis) such as limestone (calcium carbonate) and dolomite (CaMg(CO3)2). During melting, the carbonates release carbon dioxide, the bubbles of which contribute to the stirring of the mass during melting. Furthermore, some carbonates such as dolomite, even before releasing their CO2, break down into finer particles according to the phenomenon known as decrepitation, which can be quite violent and generate dust that clogs and even corrodes the various conduits equipping the furnaces (chimneys, regenerators, etc.).Removing bubbles from glass generally requires the addition of a refining agent such as sodium sulfate, the release of sulfur oxide from which carries residual carbon dioxide and water bubbles to the glass surface. However, sulfur oxide is a particularly corrosive gas. It is desirable to develop glass manufacturing processes that generate as little CO2 as possible for environmental reasons, while still producing good quality glass at an acceptable cost, particularly from the point of view of the energy expenditure required for glass manufacturing.

[0003] In addition to the release of CO2 directly during the melting process of the raw material bath, it is therefore important to consider the glass manufacturing process as a whole, taking into account other factors such as the cost of raw materials, their transport or even the energy cost of providing said raw materials and melting them.

[0004] The invention relates to a mixture of raw materials for the preparation of a molten glass whose target composition corresponds to the formulation below, in weight percentage: SiO2: between 65 and 80%, preferably between 70 and 75%, Na2O: between 8 and 20%, preferably between 10 and 20%, CaO: between 5 and 20%, preferably between 5 and 15%, MgO: between 0 and 10%, preferably between 0 and 7%, Al2O3: between 0 and 10%, preferably between 0.5 and 3%, K2O: between 0 and 10%, preferably between 0 and 2%, Iron oxide: 0 to 15%, preferably between 0 and 10%, other oxide(s): between 0 and 5% cumulatively, preferably less than 2% of B2O3, and so highly preferred less than 1% B2O3.

[0005] the rest being made up of unavoidable impurities.

[0006] This mixture of raw materials is intended to be heated to a temperature and under conditions allowing its melting to obtain a glass meeting the said target composition.

[0007] The originality of the present invention lies in the choice of raw materials. Indeed, contrary to the prejudices generally present in the field, and illustrated by the previously cited publications, it was discovered that it was possible to use silicates as raw material in a flat glass manufacturing process to limit the energy consumption of its manufacturing process while maintaining good homogeneity of the final glass.

[0008] According to the invention, it was in fact found by the applicant company that the granulometry of such mineral raw materials should also be taken into account for the preparation of the initial mixture in order to limit the energy consumption of such a flat glass manufacturing process.

[0009] Thus, according to a first aspect, for soda-lime-silica glasses, among the different oxides constituting the glass, calcium oxide is the one that most impacts the homogeneity of the glass due to the difference between its optical index and that of silica, the main constituent of the glass network. CaO is however necessary in the composition of flat glass: inexpensive, easy to find in nature, it reinforces the hydrolytic resistance and helps to control the viscosity and the liquidus temperature of the molten bath of raw material.

[0010] However, a non-homogeneous glass can easily exhibit "waves", i.e. areas where the refractive index varies locally due to a variation in composition. Sometimes, even if the variation is less than 0.1% by weight of calcium oxide, not detectable by conventional analytical methods, these variations lead to optically observable distortions.

[0011] For soda-lime-silica glass intended for the production of flat glass, the standard limestone has a PSD (PSD for particle size distribution) such that the median diameter D 50 is of the order of or greater than 600 microns, with maximum particle sizes of up to 2 or even 3 mm. In order to improve the homogeneity of the glass, it may be considered to reduce the grain size distribution of the raw material and particularly the PSD of the limestone used in the initial bath of raw materials, as shown by the publication "Influence of limestone grain size on glass homogeneity, Glass Technology, Society of Glass Technology, 2010, 51 (3), pp.116".

[0012] In a conventional glass melting furnace as illustrated in the attached figure, another aspect of the raw materials must also be taken into account if such a reduction in PSD is envisaged.

[0013] Schematically illustrates a furnace 2 for melting raw materials. The raw materials are introduced as a mixture into the furnace 7 in the form of a compact lump 1. The lump 1 initially floats on the molten glass 6. The lower surface of the lump first melts due to the convection of the molten glass under the effect of electrical resistance 4 (electrical boosting) arranged in the lower part of the furnace. A bubbler 8 increases the convection of the glass and thus mechanically induces better homogeneity of the glass. The upper surface of the lump is also heated by radiation thanks to a series of burners 3 positioned at the side walls of the furnace as shown schematically below. After melting of the lump, a foaming phenomenon 5 is observed due to the refining of the glass.

[0014] It has been demonstrated by the experiments carried out by the applicant company that the efficiency of heating by burners 3 is linked to the emissivity of the raw materials used and in particular of the calcium source used. Thus, a raw material with low emissivity produces a thermal mirror on the surface of the tile, which reduces the melting kinetics and increases the energy consumption of the furnace to achieve the same degree of melting.

[0015] Therefore, if we act on the particle size distribution (PSD) of the raw material and particularly the PSD of the calcium source used, a major disadvantage is that the emissivity of the raw material decreases. Thus, a lower emissivity results in less efficient heat transfer. As a result, if the use of a finer-grained limestone allows on the one hand to improve the chemical and optical homogeneity of the glass and in particular the distribution of calcium oxide in the final glass, on the other hand it increases the energy consumption of the furnace since the energy transfer will be less favorable.

[0016] Based on these two antagonistic effects, it is usual to use, as a raw material source of calcium (or CaO in the final glass), limestone whose median diameter is of the order of 600 micrometers, or even greater than 600 micrometers.

[0017] The object of the present invention is thus to propose a glass manufacturing process allowing a homogeneous distribution of calcium oxide in said final glass, while reducing the energy consumption necessary for said manufacturing.

[0018] More specifically, the present invention relates to a method for manufacturing a glass having a target composition, comprising the melting of a mixture of raw materials constituting a melt bath, said target composition meeting the following criteria, in weight percentages:

[0019] SiO2: between 65 and 80%, preferably between 70 and 75%,

[0020] Na2O: between 8 and 20%, preferably between 10 and 20%,

[0021] CaO: between 5 and 20%, preferably between 5 and 15%,

[0022] MgO: between 0 and 10%, preferably between 0 and 7%,

[0023] Al2O3: between 0 and 10%, preferably between 0.5 and 3%,

[0024] K2O: between 0 and 10%, preferably between 0 and 2%,

[0025] Iron oxide: 0 to 15%, preferably between 0 and 10%,

[0026] other oxide(s): between 0 and 5% cumulative, including preferably less than 2% B2O3, and very preferably less than 1% B2O3.

[0027] the remainder being made up of unavoidable impurities,

[0028] said method being characterized in that the mixture of raw materials of the molten bath comprises a calcium silicate comprising, in weight percentage, more than 30% of SiO2 and more than 20% of CaO, preferably at least 25% of CaO, CaO and SiO2 together representing more than 60%, or even more than 70% or even more than 80% of the total weight of said silicate and in that said calcium silicate is introduced into the mixture in the form of a powder whose median grain diameter is less than or equal to 400 micrometers.

[0029] According to preferred but non-limiting embodiments of the scope of the present invention:

[0030] - Said calcium silicate has the following composition, in weight percentages: SiO2: between 30 and 60%, preferably between 40 and 55%, CaO: between 25 and 55%, preferably between 35 and 50%, Fe2O 3 : between 0 and 4%, for example between 0.1 and 0.5%, Al2O 3 : between 0 and 8%, for example between 0.5 and 2%,CO2: between 0 and 20%, in particular between 0 and 15%,less than 5% of other oxides, preferably less than 3% of other oxides.Said calcium silicate is introduced into the mixture in the form of a powder whose median grain diameter is between 20 micrometers and 400 micrometers, preferably between 30 micrometers and 350 micrometers, and very preferably between 40 micrometers and 300 micrometers.Said calcium silicate is introduced into the mixture in the form of a powder whose maximum diameter is less than 1500 micrometers, preferably less than 1250 micrometers and very preferably is less than 1000 micrometers.The mixture of raw materials comprises:said calcium silicate,silica, in particular in the form of sand,at least one source of sodium preferably chosen from sodium hydroxide NaOH, sodium carbonate Na2CO3 or a mixture of sodium hydroxide NaOH and sodium carbonate Na2CO. 3, optionally at least one source of magnesium chosen from a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca or a magnesium hydroxide, optionally feldspar (K,Na)AlS i3 O8, or another source of aluminum such as hydrated or calcined alumina, or phonolite or nepheline or optionally limestone slag CaCO3 ,optionally dolomite,optionally recycled glass cullet.The raw material mixture comprises sodium hydroxide as a sodium source.The raw material mixture comprises a magnesium silicate as a magnesium source, said mineral magnesium silicate preferably comprising, in weight percentage, more than 30% SiO2 and more than 10% MgO, preferably more than 15% MgO, MgO and SiO2 together representing more than 60%, or even more than 70% or even more than 75% of the total weight of said source.Said magnesium source is a magnesium silicate corresponding to the following composition, in weight percentages:SiO2: between 40 and 55%, preferably between 45 and 50%,Al2O3: between 0 and 10%, for example between 1 and 10%,MgO: between 20 and 40%, preferably between 25 and 35%,Fe2O 3 : between 0 and 4%, for example between 1 and 3%, less than 5% of other oxides, preferably less than 3% of other oxides, optionally water, in particular present in said source in the form of hydroxide(s), preferably in an amount of less than 20% and in particular between 5 and 15%. Said magnesium source is a magnesium silicate corresponding to the following composition, in weight percentages: SiO2: between 55 and 70%, preferably between 58 and 65%, Al2O3: between 0 and 10%, for example between 1 and 10%, MgO: between 20 and 40%, preferably between 25 and 35%, Fe2O 3 : between 0 and 4%, for example between 0.5 and 2%, less than 5% of other oxides, preferably less than 3% of other oxides, optionally water, in particular present in said source in the form of hydroxide(s), preferably in an amount of less than 20% and in particular between 5 and 15%. Said magnesium source is a magnesium silicate corresponding to the following composition, in weight percentages: SiO2: between 30 and 50%, preferably between 35 and 45%, Al2O3: between 0 and 10%, for example between 1 and 5%, MgO: between 25 and 45%, preferably between 30 and 40%, Fe2O 3 : between 0 and 10%, for example between 5 and 10%, less than 5% of other oxides, preferably less than 3% of other oxides, optionally water, in particular present in said source in the form of hydroxide(s), preferably in an amount of less than 20% and in particular between 5 and 15%. Said magnesium source is a magnesium hydroxide corresponding to the following composition, in weight percentages: MgO: between 50 and 75%, preferably between 55 and 70%, H2O: between 10 and 35%, preferably between 25 and 35%, SiO 2 : between 0 and 10%, for example between 0.5 and 5%,CaO: between 0 and 5%, for example between 0.5 and 2%,Fe2O 3 : between 0 and 4%, for example between 0.5 and 2%,

[0031] less than 5% of other oxides, preferably less than 3% of other oxides.The raw materials of said melt include said calcium silicate and a source of magnesium such as a magnesium silicate, in particular as described above.Recycled glass cullet is introduced into the melt.The recycled glass cullet represents between 5 and 70% of the total weight of the melt.Said silicate is a natural silicate, that is to say used in its initial geological composition after extraction from its deposit, in particular without chemical alteration aimed at modifying its initial mineral composition, that is to say it does not undergo any chemical transformation.

[0032] - The manufacturing process includes the following steps:

[0033] a) raw materials are selected as described above

[0034] to form the fusion bath,

[0035] b) determining the quantities of said raw materials required to obtain a glass of said target composition,

[0036] (c) the said materials are mixed in the said quantities,

[0037] d) the said mixture is melted and cooled under conditions allowing the said glass to be obtained.

[0038] The invention also relates to a mixture of raw materials as described above and comprising in particular a calcium silicate comprising, in weight percentage, more than 30% of SiO2 and more than 20% of CaO, preferably more than 25% of CaO, CaO and SiO2 together representing more than 60%, or even more than 70% or even more than 80% of the total weight of said silicate and in which said silicate is introduced into the mixture in the form of a powder whose median grain diameter is less than or equal to 400 micrometers.

[0039] The present invention relates in particular to a mixture of raw materials comprising and preferably consisting of:said calcium silicate,silica, in particular in the form of sand,at least one source of sodium preferably chosen from sodium hydroxide NaOH, sodium carbonate Na2CO3or a mixture of sodium hydroxide NaOH and sodium carbonate Na2CO 3, optionally at least one source of magnesium chosen from a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca or a magnesium hydroxide, optionally feldspar (K,Na)AlS i3 O8, or another source of aluminum such as hydrated or calcined alumina, or phonolite or nepheline or optionally limestone slag CaCO3 , optionally dolomite, optionally recycled glass cullet.

[0040] Unless otherwise stated, all particle size values ​​(median diameter D50, maximum diameter D max ) in the present description and the claims are given in mass and are obtained by conventional sieving techniques, in particular for the calcium silicate according to the invention.

[0041] The invention will be better understood by reading the following examples intended to illustrate its advantages, without of course the invention being considered as limited to these, in any of the aspects described. Examples

[0042] In the following examples, different mixtures of raw materials were prepared in order to compare a mixture as currently used for the manufacture of glass for an identical final glass composition, which has substantially the following composition:

[0043] Elements Percentage weight%SiO273.1CaO9.5MgO2.0Al2O31.1Fe2O30.1Na2O13.5K2O0.3 Example 1 (prior art)

[0044] According to a first example, a glass corresponding to the previous composition is synthesized, according to current techniques.

[0045] Table 2 below gives the proportions of the different raw materials and the final composition of the glass thus obtained:

[0046] D50 (µm)kg / ton glassSiO2TiO2Al2O3Fe2O3CaOMgONa2OK2OSO3Sand37655999,40,10,40,10,00,00,00,10,0Feldspar1873970,00,116,80,30,40,15,76,40,0Carbonate soda3651740,00,00,00,00,00,058,20,00,0Dolomite615811,00,00,20,133,019,00,00,00,0Limestone596880,10,00,00,055,60,40,00,00,0Sulfate soda NM80,00,00,00,00,00,043,20,055,9Calcin-20073,40,11,10,19,82,013,60,30,2Final composition glass73,10,11,10,19,52,013,50,30,5

[0047] NM: Not Measured

[0048] The particle size of the different raw materials is obtained by sieving.

[0049] The measurements are carried out on approximately 100g of representative sample (accuracy of 10 -2 g). The test sample is placed on the upper sieve of the column (Standard NF X 11501). The column is stirred for 5 minutes in a ROTAP sieve machine (Retsch AS200 TAP). The residue from the first sieve containing the grains is weighed (accuracy of 10 -2 g). The rejection of each sieve is added in the same way to that of the previous sieves, and the cumulative mass is noted each time.

[0050] The results are expressed as a cumulative % relative to the final total mass MF.

[0051] X1% = M1 x 100 / MF

[0052] X1 = % of cumulative rejection on sieve 1

[0053] M1 = mass of cumulative rejects on sieve 1

[0054] MF = final mass of product.

[0055] This calculation will be carried out for each nominal sieve opening.

[0056] The D50 is calculated from an interpolation of the upper and lower grain sizes closest to 50%.

[0057] In particular the D 50 is classically determined by the following equation:

[0058] D 50 = [T<50%] + (50% - [X<50%]) × ([T<50%] - [T>50%]) / ([X<50%] - [X>50%])

[0059] With :

[0060] T<50% = Sieve opening less than 50% of rejection

[0061] T>50% = Sieve opening greater than 50% of rejection

[0062] X<50% =% of cumulative refusal on T<50%

[0063] X>50% =% of cumulative refusal on T>50%

[0064] We can thus deduce the particle size distribution by weight as reported in tables 2 to 4. Example 2 (comparative)

[0065] In this example, the same raw materials were used as in example 1 but a limestone with a finer grain size, i.e. D, was used.50 equal to 114 micrometers. Example 3:

[0066] In this example the mix of raw materials is this time as described in table 3 below.

[0067] In this initial mixture, another mineral material consisting of a calcium silicate according to the invention was introduced as a reagent to replace the limestone, i.e. with a median diameter D50 equal to 107 micrometers.

[0068] Analysis by classical techniques shows that this natural mineral material corresponds to the composition described in table 3 below:

[0069] kg / tonne glassSiO2TiO2Al2O3Fe2O3CaOMgONa2OK2OSO3Sand53799.40.10.40.10.00.00.00.10.0Feldspar3870.00.116.80.30.40.15.76.40.0Sodium carbonate1740.00.00.00.00.00.00.058.20.00.0Dolomite811.00.00.20.133.019.00.00.00.0Limestone530.10.00.00.055.60.40.00.00.0Calcium silicate4452.20.00.60.444.70.50.10.10.0Sulfate soda80,00,00,00,00,00,043,20,055,9Calcin20073,40,11,10,19,82,013,60,30,2Final glass composition73,10,11,10,19,52,013,50,30.5 Example 4:

[0070] In this example, the same raw materials were used as in Example 3, but a calcium silicate with a larger particle size, i.e. D 50 equal to 826 micrometers.

[0071] Table 4 below shows the granulometric characteristics determined using the method previously described for the different sources of calcium used as raw material in the previous examples:

[0072] “Coarse” limestone “Fine” limestone “Fine” calcium silicate “Coarse” calcium silicate Example 1234D 50 (µm)596114107826Max grain size (µm)20002508004000Grain proportion<100 µm (%)63042<5

[0073] The following measurements are taken:

[0074] 1°) The emissivity of the calcium sources used is measured from the reflectance spectra using the law of conservation of energy for opaque materials: absorbance α(λ, T) + reflectance ρ(λ, T) = 1. By applying Kirchhoff's law, the spectral emittance can be expressed in terms of total reflectance,

[0075] ε(λ, T) = α(λ, T)

[0076] with : ,

[0077] where B T is the spectral irradiance of the black body at temperature T of 2000°C, corresponding to the temperature of the air-gas flame of the burner.

[0078] Room temperature hemispherical reflectance measurements are performed with a 150 mm integrating sphere mounted on a Lambda spectrophotometer between 300 and 2500 nm, using a Spectralon plate as a reflectance reference.

[0079] For easy handling, the samples are pressed by applying a force of 0.4 tonnes / cm 2 and wetted with 10% water prior to measurement.

[0080] 2°) The mixtures of the above examples are melted in a typical soda-lime glass melting furnace based on flame technology shown schematically below.

[0081] Measurement of the energy consumed in the furnace for melting the mixtures:

[0082] During the various melting tests corresponding to the examples above, the gas consumption is adjusted in order to maintain a similar profile temperature inside the furnace and at all points within it. From this, we deduce a gas consumption and therefore energy consumption necessary to reach the same heating point.

[0083] 3°) Homogeneity in CaO in the final glass:

[0084] A sample is melted in a cylindrical platinum crucible from the same raw material mixture as described above. The sample is heated in air at 1480°C for 2 hours. The cooled glass is then cored and cut to obtain a slide comprising the cross-section of the cylinder. This slide was polished, carbon-plated and analyzed using an electron microprobe at 15kV. The analysis consisted of determining the mass percentage of CaO over the entire height of the slide (or the melted sample) at 100 measurement points, the measurement interval was 250 microns.

[0085] It is thus possible on this basis to calculate the average of the CaO concentration and the standard deviation. A criterion of homogeneity of CaO in the glass is provided by the ratio standard deviation divided by the average concentration (σ CaO / [CaO]).

[0086] The results obtained are reported in Table 5 below:

[0087] Example1234Calcium source “Coarse” limestone“Fine” limestone“Fine” calcium silicate“Coarse” calcium silicateEmissivity0.570.200.270.44Glass homogeneity(σ CaO / [CaO]) (%)5.5NM3.5>5Energy consumed by the first burner in the furnaceReference> Ref.-10% / RefNM

[0088] Example 3 according to the invention, in which a source of calcium consisting of a calcium silicate according to the invention is used as raw material for said melt, allows an energy adjustment on the first burner 1 (closest to the charging, i.e. the entry of the raw materials into the furnace) in the sense of a reduction in energy expenditure of approximately 10%. Also, in the sense previously described, the homogeneity of calcium oxide appears to be improved in the case of example 3 according to the invention.

[0089] To assess the optical quality (or homogeneity) of the glass, the "shadowgraph" technique is used, which reveals variations in refractive index and / or local variations in thickness. Depending on the type of defect, it is possible to filter out only defects due to variations in refractive index or chemical homogeneity, as described in publication WO2002012869A1. Periodically, samples are collected from all along the ribbon and the intensity of the defects is measured (0 if nothing, 1 if the defect is barely visible, 2 if it is more visible, etc.), each intensity value being defined using reference samples.

[0090] This determines an evaluation or "rating" which is the average calculated from the intensities of the glasses, as defined using the said reference samples and on a production of glass with a thickness of 9.5 mm.

[0091] The results obtained for the glasses of examples 1 and 3 are reported in table 6 below.

[0092] Shadowgraph rating [daily average]Example 1Example 3Production (9.5mm thickness)1,10,7

[0093] It can be seen that example 3 according to the invention has fewer defects than reference example 1.

Claims

A method of manufacturing a glass having a target composition, comprising melting a mixture of raw materials constituting a melt bath, said target composition meeting the following criteria, in weight percentages: SiO2: between 65 and 80%, preferably between 70 and 75%, Na2O: between 8 and 20%, preferably between 10 and 20%, CaO: between 5 and 20%, preferably between 5 and 15%, MgO: between 0 and 10%, preferably between 0 and 7%, Al2O3: between 0 and 10%, preferably between 0.5 and 3%, K2O: between 0 and 10%, preferably between 0 and 2%, Iron oxide: 0 to 15%, preferably between 0 and 10%, other oxide(s): between 0 and 5% cumulatively, the remainder being impurities unavoidable,said process being characterized in that the mixture of raw materials comprises a calcium silicate comprising, in weight percentage, more than 30% of SiO2 and more than 20% of CaO, preferably at least 25% of CaO, CaO and SiO2 together representing more than 60%,or even more than 70% or even more than 80% of the total weight of said silicate and in that said calcium silicate is introduced into the mixture in the form of a powder whose median grain diameter is less than or equal to 400 micrometers., Manufacturing process according to claim 1, wherein said calcium silicate has the following composition, in weight percentages: SiO2: between 30 and 60%, preferably between 40 and 55%, CaO: between 25 and 55%, preferably between 35 and 50%, Fe2O 3 : between 0 and 4%, for example between 0.1 and 0.5%, Al2O 3 : between 0 and 8%, for example between 0.5 and 2%,CO2: between 0 and 20%, in particular between 0 and 15%,less than 5% of other oxides, preferably less than 3% of other oxides. Method according to one of the preceding claims, in which said calcium silicate is introduced into the mixture in the form of a powder whose median grain diameter is between 20 micrometers and 400 micrometers, preferably between 30 micrometers and 350 micrometers, and very preferably between 40 micrometers and 300 micrometers. Method according to one of the preceding claims, wherein said calcium silicate is introduced into the mixture in the form of a powder whose maximum diameter is less than 1500 micrometers, preferably less than 1250 micrometers and very preferably is less than 1000 micrometers. Method according to one of the preceding claims, in which the mixture of raw materials comprises:said calcium silicate,silica, in particular in the form of sand,at least one source of sodium preferably chosen from sodium hydroxide NaOH, sodium carbonate Na2CO3 or a mixture of sodium hydroxide NaOH and sodium carbonate Na2CO 3, optionally at least one source of magnesium chosen from a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca or a magnesium hydroxide, optionally feldspar (K,Na)AlS i3 O8, or another source of aluminum such as hydrated or calcined alumina, or phonolite or nepheline or optionally limestone slag CaCO3 , optionally dolomite, optionally recycled glass cullet. A method according to any preceding claim, wherein the raw material mixture comprises sodium hydroxide as a sodium source. A method according to any preceding claim, wherein the mixture of raw materials comprises a magnesium silicate as a source of magnesium, said mineral magnesium silicate preferably comprising, in weight percentage, more than 30% SiO2 and more than 10% MgO, preferably more than 15% MgO, MgO and SiO2 together representing more than 60%, or even more than 70% or even more than 75% of the total weight of said source. Method according to the preceding claim, in which said magnesium source is a magnesium silicate corresponding to the following composition, in weight percentages:SiO2: between 40 and 55%, preferably between 45 and 50%,Al2O3: between 0 and 10%, for example between 1 and 10%,MgO: between 20 and 40%, preferably between 25 and 35%,Fe2O 3 : between 0 and 4%, for example between 1 and 3%, less than 5% of other oxides, preferably less than 3% of other oxides, possibly water, in particular present in said source in the form of hydroxide(s), preferably in a quantity of less than 20% and in particular between 5 and 15%. Method according to claim 8 in which said magnesium source is a magnesium silicate corresponding to the following composition, in weight percentages: SiO2: between 55 and 70%, preferably between 58 and 65%, Al2O3: between 0 and 10%, for example between 1 and 10%, MgO: between 20 and 40%, preferably between 25 and 35%, Fe2O 3 : between 0 and 4%, for example between 0.5 and 2%, less than 5% of other oxides, preferably less than 3% of other oxides, possibly water, in particular present in said source in the form of hydroxide(s), preferably in a quantity of less than 20% and in particular between 5 and 15%. Method according to claim 8 in which said magnesium source is a magnesium silicate corresponding to the following composition, in weight percentages: SiO2: between 30 and 50%, preferably between 35 and 45%, Al2O3: between 0 and 10%, for example between 1 and 5%, MgO: between 25 and 45%, preferably between 30 and 40%, Fe2O 3 : between 0 and 10%, for example between 5 and 10%, less than 5% of other oxides, preferably less than 3% of other oxides, possibly water, in particular present in said source in the form of hydroxide(s), preferably in an amount of less than 20% and in particular between 5 and 15%. Method according to one of claims 1 to 6 in which said source of magnesium is a magnesium hydroxide corresponding to the following composition, in weight percentages: MgO: between 50 and 75%, preferably between 55 and 70%, H2O: between 10 and 35%, preferably between 25 and 35%, SiO 2 : between 0 and 10%, for example between 0.5 and 5%,CaO: between 0 and 5%, for example between 0.5 and 2%,Fe2O 3 : between 0 and 4%, for example between 0.5 and 2%, less than 5% of other oxides, preferably less than 3% of other oxides. Method according to one of the preceding claims in which the raw materials of said molten bath comprise said calcium silicate and a source of magnesium such as a magnesium silicate, in particular as described according to one of the preceding claims 8 to 11. Method according to one of the preceding claims, in which recycled glass cullet is introduced into the melting bath. Method according to the preceding claim, in which the recycled glass cullet represents between 5 and 70% of the total weight of the melt. A mixture of raw materials as described in one of the preceding claims comprises a calcium silicate comprising, in weight percentage, more than 30% SiO2 and more than 20% CaO, preferably more than 25% CaO, CaO and SiO2 together representing more than 60%, or even more than 70% or even more than 80% of the total weight of said silicate and wherein said calcium silicate is introduced into the mixture in the form of a powder whose median grain diameter is less than or equal to 400 micrometers. Mixture of raw materials according to the preceding claim comprising:said calcium silicate,silica, in particular in the form of sand,at least one source of sodium preferably chosen from sodium hydroxide NaOH, sodium carbonate Na2CO3or a mixture of sodium hydroxide NaOH and sodium carbonate Na2CO 3,optionally at least one source of magnesium chosen from a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca or a magnesium hydroxide, optionally feldspar (K,Na)AlS i3 O8, or another source of aluminum such as hydrated or calcined alumina, or phonolite or nepheline or optionally limestone slag CaCO3 , optionally dolomite, optionally recycled glass cullet.

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