Method for producing float glass from mineral materials including anhydrous sodium hydroxide

Abstract

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Description

Title: PROCESS FOR MANUFACTURING FLOAT GLASS FROM MINERAL MATERIALS INCLUDING ANHYDROUS SODIUM HYDROXIDE

[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 (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 being melted. 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.). The elimination of bubbles in the glass generally requires the addition of a refining agent such as sodium sulfate, the release of sulfur oxide of which carries the residual bubbles of carbon dioxide and water towards the surface of the glass.Sulfur oxide, however, is a particularly corrosive gas. Adding water to glass has already been proposed as a refining agent. In a conventional soda-lime-silica glass manufacturing process, CO2 emissions are generally around 20% of the total mass of raw materials used. Furthermore, carbon dioxide is a greenhouse gas, and 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.

[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, the energy cost of providing said raw materials or the overall energy expenditure enabling the fusion of the mixture.

[0004] As previously stated, glass is usually prepared by melting in a furnace raw materials comprising a significant portion of carbonates.

[0005] During melting, carbonates release carbon dioxide, the bubbles of which contribute to the stirring of the melting mass. 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.

[0006] Sodium hydroxide has been proposed as an alternative to sodium carbonate and has the effect of reducing the CO2 generated during glass manufacturing, for example in publications US3753743A or BE761040A. Sodium hydroxide is used in the form of sodium hydroxide solution, with a sodium hydroxide concentration of between 30 and 75% by weight.

[0007] However, as described in the remainder of this description, it appeared that using a sodium hydroxide solution at such a concentration has the consequence of greatly deteriorating the flow of the vitrifiable mixture because it becomes very pasty due to the large quantity of water added and of greatly increasing the energy expenditure necessary for the manufacture of the glass. Also, the chemical homogeneity of the glass appeared insufficient with such a solution.

[0008] Patent applications FR3086942 and WO2022 / 229568 also describe the use of sodium hydroxide for the formation of a sodium-calcium silico glass.

[0009] The object of the present invention is to propose a mixture intended for the production of sodium-calcium silicate glass, in particular for the manufacture of float glass, making it possible to solve the preceding problems and in particular to obtain a homogeneous distribution of sodium oxide and silicon oxide in the final glass, while maintaining good flow. of the vitrifiable mixture and reducing the energy consumption required for said production

[0010] Another object of the present invention is to propose a melting method making it possible to preserve the elements of the melting furnace, in particular by limiting the emission of dust into it during melting.

[0011] To this end, the experiments carried out by the applicant company have shown the advantages of using sodium hydroxide in anhydrous form as a source of sodium in the initial raw material mixture, in particular if its density and preferably its morphology are specifically adapted to such use. Anhydrous form means any source of sodium hydroxide exhibiting a mass loss of less than 5% after heat treatment at 150°C for 1 h.

[0012] More specifically, the invention relates in particular 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 60 and 80%, preferably between 70 and 75%, - Na2O: between 5 and 20%, preferably between 10 and 20%, - CaO: between 5 and 20%, preferably between 5 and 15%, - MgO: between 0 and 10%, preferably between 0.1 and 7%, - AI2O3: between 0 and 10%, preferably between 0.1 and 3%, - K2O: between 0 and 10%, preferably between 0 and 2%, - Iron oxide: 0 to 15%, preferably between 0 and 10%, - B2O3: between 0 and 1% excluded, preferably less than 0.5%, - other oxide(s): between 0 and 10% cumulative, preferably between 0 and 5%, cumulatively the remainder being made up of unavoidable impurities.

[0013] The said mixture comprises: - at least one source of silicon chosen in particular from silica, in particular in the form of sand, a glass cullet, in particular a mixture of silica and glass cullet, - at least one source of sodium in the form of sodium hydroxide, - at least one source of calcium, preferably chosen from limestone, quicklime or slaked lime or a mixed oxide of calcium with at least one element chosen from the group consisting of Si, Mg, in particular a calcium silicate, said source of calcium being able to be at least partly cullet, - optionally at least one source of magnesium preferably chosen from dolomite, possibly calcined, magnesite (MgCOs), brucite (Mg(OH)2) or a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca, in particular a magnesium silicate, - optionally at least one compound chosen from the group consisting of feldspar, in particular of formula (K, NajAISisOs, nepheline or phonolite, calcined or hydrated alumina, - optionally recycled glass cullet, - optionally a slag in which the sodium hydroxide is in anhydrous form.

[0014] Preferably the mixture is made up, for more than 90% by weight or even more than 95% by weight, preferably even more than 99% by weight, of the constituents mentioned above.

[0015] Preferably, the mixture according to the invention is water-free. This means that no water is added to the mixture and that any moisture present comes from the raw materials. Preferably, the mixture contains less than 1% by weight of water, preferably less than 0.5% by weight of water, more preferably less than 0.2% by weight of water, relative to the total weight of the mixture.

[0016] In said mixture according to the invention, the sodium hydroxide is in anhydrous form and preferably has an apparent density of less than 1.2 g / cm 3 , more preferably an apparent density of less than 1.0 g / cm 3 , or even less than 0.8 g / cm 3 .

[0017] In the present description, the term "particle" describes an individualized entity of anhydrous sodium hydroxide.

[0018] Apparent density, or apparent volumetric mass, is a quantity usually used mainly with substances in the form of particles (or grains), in order to account for the mass of material contained in a given volume, including the volume of interstitial air.

[0019] Apparent density (also called bulk density) is measured as follows:

[0020] According to an easily reproducible operating protocol for measuring the apparent density of the granular mixture, the latter is first poured into a container, for example a bucket, of known mass and volume. The container must be at least 1 liter to have sufficient precision and respect an aspect ratio allowing to limit the compaction of the mixture, by checking the formula: where Lmax is the maximum extent of the container in a given direction, by analogy with the Feret diameter of a particle, and V is the volume of said container.

[0021] It is also important to ensure that the mixture is poured gently, without any movement of the bucket or mechanical compression of the mixture, in order to limit settling of the mixture as much as possible. The filled bucket is then weighed to determine the mass of the poured mixture. The apparent density is the ratio between the measured mass of the mixture and the volume of the bucket.

[0022] By the expression "for the preparation of a molten glass of target composition", it is meant that the various raw materials previously cited are present in said mixture in proportions suitable for the final production, after melting according to the techniques of the art, of the glass of the target composition described above.

[0023] This mixture of raw materials is therefore intended to be heated to a temperature and under conditions allowing its fusion to obtain a glass meeting the said target composition.

[0024] According to particular and advantageous embodiments of the present invention which can of course be combined with each other where appropriate: - Said anhydrous sodium hydroxide is in the form of a set of particles having a circularity of less than 0.85, preferably less than 0.80 or even greater than 0.75. - Said anhydrous sodium hydroxide is in the form of a set of particles having an aspect ratio greater than 1.15, preferably greater than 1.20. - Said anhydrous sodium hydroxide is in the form of a set of particles whose average Feret diameter is greater than 5 millimeters, greater than 10 millimeters. - Anhydrous sodium hydroxide comprises a total water proportion of less than 30% by weight, preferably less than 25% by weight. - Sodium hydroxide is the only source of sodium (apart from possible additions of cullet or feldspar, nepheline or phonolite). - Alternatively, the sodium is provided in the form of a mixture of sodium hydroxide and sodium carbonate Na2CO3, the anhydrous sodium hydroxide according to the invention preferably representing more than 20% by weight of said mixture, or even more than 50% by weight of said mixture. - A source of calcium is a mineral calcium silicate, preferably natural. - A source of calcium is calcium oxide. - A source of calcium is calcium hydroxide. - A source of magnesium is a mineral magnesium silicate, preferably natural. - A source of calcium is a mineral calcium silicate, preferably natural, comprising, in weight percentage, more than 30% of SiO2 and more than 10% of CaO, preferably more than 15% 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 source. - A source of magnesium is a mineral magnesium silicate, preferably natural, comprising, in weight percentage, more than 30% of SiO2 and more than 10% of MgO, preferably more than 15% of 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. - A source of magnesium is brucite Mg(OH)2. - The raw materials of said molten bath comprise a source of calcium as previously described and a source of magnesium as previously described. - A source of potassium is potassium hydroxide.

[0025] Silica is usually introduced into the raw material mix in the form of sand.

[0026] If necessary, as indicated above, the mixture of raw materials may also advantageously comprise an AI carrier, a precursor of alumina in the glass, in particular in the group consisting of a feldspar, hydrated or calcined alumina or alternatively a blast furnace slag.

[0027] The mixture of raw materials may also include in small proportions a colorant such as iron oxide, cobalt oxide, chromium oxide.

[0028] According to the invention, as little carbonate as possible, or even no carbonate, is introduced into the mixture of raw materials. Preferably, the sum of the weight of alkali carbonate and alkaline earth carbonate is less than 30%, and preferably less than 10%, and preferably less than 5%, and preferably less than 1% by weight, or is even zero in the mixture of raw materials. According to a possible advantageous embodiment, the mixture of raw materials may be substantially free of any carbonate. It is advantageously capable of releasing only a minimal portion of carbon oxide during its heating and melting into glass due, for example, to the addition of coke to the initial mixture.

[0029] To produce the glass, the Si carrier is introduced into the raw material mixture in the form of sand, the alkali carriers are advantageously introduced into the raw material mixture in the form of hydroxides, in particular in the form of anhydrous NaOH and of a particle size adapted according to the invention. The possible aluminum carrier can be introduced into the raw material mixture in the form of feldspar powder.

[0030] Preferably, the mixture of raw materials is moistened to reduce the release of fine particles, the total humidity of the mixture ultimately being preferably less than 4.5%, more preferably less than 3.5%, more preferably less than 3%.

[0031] Each raw material is introduced into the raw material mixture in such a quantity that the molar percentage of its cation (such as Si, Na, Al, Fe, etc.) relative to the sum of the moles of all cations is the same as in the final glass. As previously stated, the raw materials in the mixture are chosen to lead to a glass whose target composition falls within the framework (the percentage ranges for the different oxides) described previously.

[0032] The mixture of raw materials is heated until a molten glass is obtained, generally in a furnace. The heating is higher or lower in temperature and for a longer or shorter time depending on the quality of the glass required, in particular depending on the degree of tolerance for unmelted particles (called "unmelted") and bubbles. Generally, the maximum heating temperature of molten glass is between 1200 and 1700°C. For the transformation of the mixture of raw materials into glass, glass melting techniques well known to those skilled in the art can be used. This transformation can be carried out in any type of furnace such as an electric electrode furnace, an overhead burner furnace such as a transverse burner furnace or a loop furnace, or a submerged burner furnace.

[0033] For heating and melting glass, the raw material mixture may be introduced into a furnace in a powdered state, which implies that each raw material it contains is in a powdered state. For heating and melting glass, the raw material mixture may be introduced into a furnace in a compounded state comprising cullet and the raw material mixture, the latter being powdered where appropriate.

[0034] The invention also relates to a method of manufacturing a glass having this same target composition, comprising the fusion of a mixture of raw materials according to one of the preceding claims, said mixture constituting a melting bath, said method comprising the following steps: a) the necessary quantities of said raw materials are selected to obtain, after melting, a glass of said target composition, said raw materials being chosen from at least: - at least one source of silicon chosen in particular from silica, in particular in the form of sand, a glass cullet, in particular a mixture of silica and glass cullet, - at least one source of sodium in the form of sodium hydroxide, - at least one source of calcium, preferably chosen from limestone, quicklime or slaked lime or a mixed oxide of calcium with at least one element chosen from the group consisting of Si, Mg, in particular a calcium silicate, said source of calcium being able to be at least partly cullet, - optionally at least one source of magnesium preferably chosen from dolomite, possibly calcined, magnesite (MgCOs), brucite (Mg(OH)2) or a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca, in particular a magnesium silicate, - optionally at least one compound chosen from the group consisting of feldspar, in particular of formula (K, Na)AISi3Os, nepheline or phonolite, calcined or hydrated alumina, - optionally recycled glass cullet, - optionally a slag, b) said mixture of said raw materials is carried out according to said quantities, c) said mixture is melted and cooled under conditions allowing said glass to be obtained, in which said sodium hydroxide in said mixture is in anhydrous form.

[0035] Preferably, the sodium hydroxide has an apparent density of less than 1.2 g / cm 3 , more preferably an apparent density of less than 1.0. Examples

[0036] 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: [Table 1]

[0037] Table 2 below gives the proportions of the different raw materials for the 5 prepared mixtures: [Table 2]

[0038] The mixtures are differentiated by the apparent density and the morphology of the sodium hydroxide source used.

[0039] The geometric characteristics of the different forms of sodium hydroxide used were determined by image analysis using ImageJ® software. The median diameter, the Feret diameter, the circularity and the aspect ratio were thus determined for each of the sodium hydroxides and their values ​​are given in Table 3 below.

[0040] More precisely :

[0041] The circularity of a particle is given by the formula S = 4TT (A / P 2 ), with A area and P the perimeter of said particle.

[0042] The Feret diameter is classically defined as the distance between two parallel tangents to the periphery of the projected surface of the particle in one direction.

[0043] The median diameter dso of each sodium hydroxide sample is also measured by image analysis using ImageJ ® software.

[0044] The equivalent diameter of a particle is defined as the diameter of a circle whose perimeter is equivalent to the perimeter of said particle, as measured on said images.

[0045] The aspect ratio of a particle is the ratio of its greatest length to its shortest length.

[0046] We then define an average circularity, Feret diameter, equivalent diameter and aspect ratio for all the particles in each sodium hydroxide sample, as reported in Table 3 below.

[0047] The proportions of free water and bound water of these different qualities of sodium hydroxide are also reported there.

[0048] Classically, the median diameter is the diameter for which 50% of the particles in number have a diameter less than this diameter and 50% a diameter greater than this diameter.

[0049] [Table 3]

[0050] Total water is the sum of free water plus bound water.

[0051] The percentage of bound water is therefore obtained by subtracting the percentage of free water from the percentage of total water (“remainder” in table 3 above).

[0052] Bound water means water chemically bound to the structure via strong bonds, i.e. typically the hydroxyl group -OH in a general formulation NaOH. The temperature for the removal of bound water (dehydroxylation) is well above 100°C. In the case of dehydroxylation of anhydrous sodium hydroxide, the water released is essentially bound water according to the formula

[0053] By anhydrous form is meant in particular any source of sodium hydroxide showing a mass loss of less than 5%, preferably less than 2%, after heat treatment at 150°C for 1 h.

[0054] Free water, on the contrary, means all water molecules bound by weak interactions. Free water can, for example, correspond to the water in solution in which sodium hydroxide is present as described in application US3753743A, but also to the water present between the sodium hydroxide particles, in particular due to the hygroscopic nature of this compound. Unlike bound water, this water is weakly bound to sodium hydroxide by weak bonds.

[0055] Thus, an “anhydrous” sodium hydroxide comprises less than 5% free water and preferably less than 2% free water, in the sense previously described.

[0056] For the purposes of the present invention, a sodium hydroxide is therefore said to be anhydrous when it comprises less than 5% free water.

[0057] Mixture B is in accordance with the teaching of publication US3753743A. Mixtures C and D are in accordance with the subject of the present invention. Example A is a comparative example in which sodium carbonate is conventionally used as the sodium source.

[0058] The performance and qualities of the raw material mixtures according to examples A to D above, including the quality of the glass after their melting, are measured according to the following different criteria: 1°) Flow measurement by slope angle tests

[0059] The angle of repose is a method for evaluating the flowability of powder, carried out by predicting the flow of vitrifiable mixtures at the outlet of the mixer until they are put into the kiln.

[0060] This technique makes it possible to estimate, in relation to the reference mixture (mixture A), how the other vitrifiable mixtures flow. If the flow is weak or poor, the risks of clogging on the conveyor belts or in the kiln hoppers are greater.

[0061] Regarding the angle of repose, the batch mixture is introduced into a trapdoor funnel. Then the trapdoor is removed to allow the batch mixture to flow onto a sheet of graph paper, resulting in a cone. The height of the cone is measured and its circumference is plotted on the graph paper to then deduce its average diameter. Finally, with the height and diameter of the cone, the angle of repose of the batch mixture is calculated according to the formulas: Angle of repose (degrees) = 180 x Arc Tangent (height of the slope / (1 / 2 x diameter of the cone)) / TT. The interpretation of the measurement results is made on the basis of the classification present in the Engineering Technique - Forming of Solids. Ref J3380 V1.

[0062] The results obtained are grouped in Table 4 below.

[0063] The reference mixture (Mixture A) has very good flow, as do mixtures C and D, with angles of repose between 25 and 30° (see the Technical Engineering classification - Forming solids. Ref J3380 V1). Mixture D has good flow with a slope angle of 32°. On the other hand, with its slope angle greater than 50°, mixture B has poor flow and requires the use of suitable devices to assist discharge in the event of the mixture passing into a hopper, for example. In addition, the risks of mixer clogging or blockage on the conveyor belts are high in the case of mixture B.

[0064] Mixtures B, C, D were characterized and the results obtained are presented in Table 4 below. Mixtures C and D according to the invention, and particularly Example C, can be considered as the one which flows the most freely.

[0065] [Table 4] 2°) Measurement of emissivity

[0066] A mixture of low-emissivity raw materials produces a thermal mirror on the surface of the composition lump, which reduces the melting kinetics and increases the energy consumption of the furnace to achieve the same degree of melting.

[0067] The emissivity of the various mixtures shown in Table 5 below was measured from the reflectance spectra using the law of conservation of energy for opaque materials: absorbance a(À, T) + reflectance p(À, T) = 1. Applying Kirchhoff's law, the spectral emittance can be expressed in terms of total reflectance, E(À, T) = a(À, T) with: where BT 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.

[0068] 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.

[0069] For easy handling, the vitrifiable mixture samples are pressed by applying a force of 0.4 tonnes / cm 2 and wetted with 2% water prior to measurement.

[0070] [Table 5]

[0071] In order to improve the heat transfer between the flame and the glass, the skilled person seeks to maximize the emissivity of the batch as much as possible. Too low an emissivity is a sign of high thermal reflectivity which could result in a screening effect. This will produce an increase in the temperature of the roof and a low temperature of the glass bath. The data in Table 5 show that the use of anhydrous sodium hydroxide according to Examples C and D improves the thermal absorption of the batch compared to the use of sodium carbonate (mixture A). 3°) Energy required for fusion

[0072] A reduction in energy consumption logically leads to a reduction in costs (less energy to be used to melt the raw material mixture) and in the carbon footprint of glass production (less gas to be used, for example).

[0073] The energy required to melt the different raw material mixtures was determined using FactSage 8.0 software. The energy is calculated over the temperature range 25 and 1400°C.

[0074] The results of the evaluation are reported in Table 6 below: [Table 6]

[0075] Table 6 shows that with mixture B (50% free water), the fusion energy of the vitrifiable mixture is drastically increased: +23% compared to the reference (mixture A). On the contrary, with the other mixtures C and D according to the invention, the energy consumption is significantly reduced compared to this same reference by approximately 11%. 4°) Homogeneity of the glass obtained after fusion

[0076] Non-homogeneous glass has "waves" (optical defects), i.e. areas where the refractive index varies locally due to a variation in chemical composition.

[0077] Glass samples were melted in a cylindrical platinum crucible from the mixtures indicated in Table 2. Each sample was heated in air at 1480°C for 2 hours. The cooled glass was 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 15 kV. The analysis consisted of determining the mass percentages of Na2O, CaO and SiO2 over the height of the slide (or of the melted sample) at 50 measurement points, the measurement interval being 500 microns.

[0078] On this basis, it is possible to calculate the mean concentration of Na2O, CaO and SiO2 and the standard deviations. A criterion for homogeneity of Na2O, CaO and SiO2 in the glass is provided by the standard deviation ratio divided by the mean concentration (oNa2O / [Na2O], oCaO / [CaO] and oSiO2 / [SiO2]).

[0079] The results obtained are reported in Table 7 below. [Table 7]

[0080] The homogeneity of sodium oxide and silicon oxide appears particularly improved in the case of mixtures C and D according to the invention, and more particularly in the case of mixture C according to the invention.

[0081] All of the results previously set out and the advantages of the present invention, materialized by the superiority of mixtures C and D according to the invention, are summarized below: - good flow of the raw material mixture, - improved emissivity of the vitrifiable mixture, which implies better heat transfer from the flames to the mixture of raw materials during melting, - a significant decrease in the overall fusion energy of the mixture, - better homogeneity of the glass finally obtained.

Claims

Claims 1. Mixture of raw materials for the manufacture of a glass of the following composition: - SiO2: between 60 and 80%, preferably between 70 and 75%, - Na2O: between 5 and 20%, preferably between 10 and 20%, - CaO: between 5 and 20%, preferably between 5 and 15%, - MgO: between 0 and 10%, preferably between 0.1 and 7%, - AI2O3: between 0 and 10%, preferably between 0.1 and 3%, - K2O: between 0 and 10%, preferably between 0 and 2%, - Iron oxide: 0 to 15%, preferably between 0 and 10%, - B2O3: between 0 and 1% excluded, preferably less than 0.5%, other oxide(s): between 0 and 10% cumulative, preferably between 0 and 5% cumulative, the remainder being made up of unavoidable impurities, said mixture comprising: - at least one source of silicon chosen in particular from silica, in particular in the form of sand, a glass cullet, in particular a mixture of silica and glass cullet, - at least one source of sodium in the form of sodium hydroxide, - at least one source of calcium, preferably chosen from limestone, quicklime or slaked lime or a mixed oxide of calcium with at least one element chosen from the group consisting of Si, Mg, in particular a calcium silicate, said source of calcium being able to be at least partly cullet, - optionally at least one source of magnesium preferably chosen from dolomite, possibly calcined, magnesite or a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca, in particular a magnesium silicate, - optionally at least one compound chosen from the group consisting of feldspar, in particular of formula (K,Na)AISi3Os, nepheline or phonolite, calcined or hydrated alumina, - optionally slag, - optionally recycled glass cullet, characterized in that the sodium hydroxide is in anhydrous form.

2. Mixture according to claim 1, in which the anhydrous sodium hydroxide has an apparent density of less than 1.2 g / cm 3 .

3. Mixture according to claim 1 or 2, in which the anhydrous sodium hydroxide is in the form of a set of particles having a circularity less than or equal to 0.85, preferably less than 0.80, or even less than 0.

75.

4. Mixture according to one of the preceding claims, in which said anhydrous sodium hydroxide is in the form of a set of particles having an aspect ratio greater than 1.15, preferably greater than 1.

20.

5. Mixture according to one of the preceding claims, in which said anhydrous sodium hydroxide is in the form of a set of particles whose average Feret diameter is greater than 5 millimeters, greater than 10 millimeters.

6. Mixture according to one of the preceding claims, in which the anhydrous sodium hydroxide comprises a total water proportion of less than 30% by weight, preferably less than 25% by weight.

7. Mixture according to one of the preceding claims, in which said anhydrous sodium hydroxide is the only source of sodium, apart from any additions of cullet or feldspar, nepheline or phonolite.

8. Mixture according to one of the preceding claims, in which the sodium is provided in the form of a mixture of anhydrous sodium hydroxide and sodium carbonate Na2CO3, the anhydrous sodium hydroxide preferably representing more than 20% by weight of said mixture, or even more than 50% by weight of said mixture.

9. Mixture according to one of the preceding claims, in which a source of calcium is a mineral calcium silicate comprising, in weight percentage, more than 30% of SiO2 and more than 10% of CaO, preferably more than 15% 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 source.

10. Mixture according to one of the preceding claims, in which a source of magnesium is a mineral magnesium silicate comprising, in weight percentage, more than 30% of SiO2 and more than 10% of MgO, preferably more than 15% of 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.

11. Mixture according to one of the preceding claims, in which a source of potassium is a potassium hydroxide.

12. Mixture according to one of the preceding claims, in which a source of calcium is a calcium hydroxide.

13. Mixture according to one of the preceding claims, in which a source of magnesium is a magnesium hydroxide.

14. Mixture according to one of the preceding claims in which the anhydrous sodium hydroxide has an apparent density of less than 1.0 g / cm 3 , preferably less than 0.8 g / cm 3 .

15. A method of manufacturing a glass having a target composition, comprising melting a mixture of raw materials according to one of the preceding claims, said mixture constituting a melt bath, said target composition meeting the following criteria, in weight percentages: - SiO2: between 60 and 80%, preferably between 70 and 75%, - Na2O: between 5 and 20%, preferably between 10 and 20%, - CaO: between 5 and 20%, preferably between 5 and 15%, - MgO: between 0 and 10%, preferably between 0.1 and 7%, - AI2O3: between 0 and 10%, preferably between 0.1 and 3%, - K2O: between 0 and 10%, preferably between 0 and 2%, - Iron oxide: 0 to 15%, preferably between 0 and 10%, - B2O3: between 0 and 1% excluded, preferably less than 0.5%, other oxide(s): between 0 and 10% cumulative, preferably between 0 and 5% cumulative, the remainder consisting of unavoidable impurities, said process being characterized in that it comprises the following steps: a) the necessary quantities of said raw materials are selected to obtain, after melting, a glass of said target composition the raw materials of said molten bath, said raw materials being chosen from at least: - at least one source of silicon chosen in particular from silica, in particular in the form of sand, a glass cullet, in particular a mixture of silica and glass cullet, - at least one source of sodium in the form of sodium hydroxide, - at least one source of calcium, preferably chosen from limestone, quicklime or slaked lime or a mixed oxide of calcium with at least one element chosen from the group consisting of Si, Mg, in particular a calcium silicate, said source of calcium being able to be at least partly cullet, - optionally at least one source of magnesium preferably chosen from dolomite, possibly calcined, magnesite or a mixed oxide of magnesium with at least one element chosen from the group consisting of Si, Ca, in particular a magnesium silicate, - optionally at least one compound chosen from the group consisting of feldspar, in particular of formula (K,Na)AISi3Os, nepheline or phonolite, calcined or hydrated alumina, - optionally slag, - optionally recycled glass cullet, b) said mixture of said raw materials is carried out according to said quantities, c) said mixture is melted and cooled under conditions allowing said glass to be obtained, in which the sodium hydroxide in said mixture is anhydrous.

16. Manufacturing method according to the preceding claim, in which the anhydrous sodium hydroxide has an apparent density of less than 1.2 g / cm 3 , preferably an apparent density of less than 1.0 g / cm 3