METHOD FOR MANUFACTURING GLASS FIBERS FROM UNPROCESSED MINERAL MATERIALS

Abstract

A method for manufacturing glass includes the melting of a mixture of raw materials constituting a melting bath including a source of silicon, a source of sodium, a source of boron and at least one source of calcium selected from a mixed oxide of calcium and at least one element selected from Si, Mg, Al, in particular a calcium silicate and/or a calcium aluminium silicate, and/or at least one source of magnesium selected from a mixed oxide of magnesium and at least one element selected from Si, Ca, in particular a magnesium silicate and optionally a source of aluminium selected from a mixed oxide of aluminium and at least one element selected from the group consisting of Si, Ca, Na, K, wherein the sources of calcium and/or of magnesium and/or of aluminium are natural mineral materials, that is to say are obtained from a natural geological medium and are unprocessed.

Claims

1. A method for manufacturing glass having a target composition, comprising melting a mixture of raw materials constituting a melting bath, said target composition meeting the following criteria, in weight percentages: SiO.sub.2: between 50 and 75%, Na.sub.2O: between 10 and 25%, CaO: between 5 and 15%, MgO: between 1 and 10%, B.sub.2O.sub.3: between 0 and 10%, Al.sub.2O.sub.3: between 0 and 8%, K.sub.2O: between 0 and 5%, Iron oxide: between 0 and 3%, other oxide(s): between 0 and 5% in total by weight, the remainder consisting of unavoidable impurities, said method comprising: a) selecting the raw materials of said melt from, at least: a silicon source, at least one sodium source, at least one boron source, at least one calcium source selected from a natural mineral calcium silicate comprising, in percentage by weight, more than 30% of SiO.sub.2 and more than 10% CaO, CaO and SiO.sub.2 together representing more than 60% of the total weight of said source, and/or at least one magnesium source selected from a natural mineral magnesium silicate comprising, in percentage by weight, more than 30% of SiO.sub.2 and more than 10% MgO, MgO and SiO.sub.2 together representing more than 60% of the weight of said source, optionally at least one aluminum source selected from a mixed aluminum oxide with at least one element selected from the group consisting of Si, Ca, Na, K, wherein said calcium and/or magnesium and/or aluminum sources are natural mineral materials, that is, unprocessed mineral materials originating from a natural geological environment, optionally, limestone CaCO.sub.3 or a calcium hydroxide such as Portlandite Ca(OH).sub.2, optionally dolomite CaMg(CO.sub.3).sub.2, optionally, hydrated alumina (Al(OH).sub.3) or calcinated alumina Al.sub.2O.sub.3, b) determining the composition of said sources of calcium and/or magnesium and/or natural aluminum, c) on the basis of said composition(s) determined according to point b), determining necessary quantities of said raw materials to obtain a glass of said target composition, d) mixing said materials according to said quantities, e) melting said mixture and cooling said mixtures under conditions making it possible to obtain said glass.

2. The method according to claim 1, wherein said calcium source is a natural mineral calcium silicate comprising, in percentage by weight, more than 30% of SiO.sub.2 and more than 15% CaO, CaO and SiO.sub.2 together representing more than 60% of the total weight of said source.

3. The method according to claim 2, wherein said magnesium source is a natural mineral magnesium silicate comprising, in percentage by weight, more than 30% of SiO.sub.2 and more than 15% MgO, MgO and SiO.sub.2 together representing more than 60% of the weight of said source.

4. The method according to claim 2, wherein the raw materials of said molten bath comprise a calcium source which is a natural mineral calcium silicate comprising, in percentage by weight, more than 30% SiO.sub.2 and more than 15% CaO, CaO and SiO.sub.2 together representing more than 70% of the total weight of said source and a magnesium source which is a natural mineral magnesium silicate comprising, in percentage by weight, more than 30% SiO.sub.2 and more than 10% MgO, MgO and SiO.sub.2 together representing more than 70% of the total weight of said source.

5. The method according to claim 1, wherein said magnesium source is a natural mineral material corresponding to the following composition, in percentages by weight: SiO.sub.2: between 40 and 55%, Al.sub.2O.sub.3: between 0 and 10%, MgO: between 20 and 40%, MgO and SiO.sub.2 cumulatively representing at least 70%, Fe.sub.2O.sub.3: between 0 and 4%, less than 5% of other oxides, optionally water.

6. The method according to claim 1, wherein said magnesium source is a natural mineral material corresponding to the following composition, in percentages by weight: SiO.sub.2: between 55 and 70%, Al.sub.2O.sub.3: between 0 and 10%, MgO: between 20 and 40%, MgO and SiO.sub.2 cumulatively representing at least 85%, Fe.sub.2O.sub.3: between 0 and 4%, less than 5% of other oxides, optionally water.

7. The method according to claim 1, wherein said magnesium source is a natural mineral material corresponding to the following composition, in percentages by weight: SiO.sub.2: between 30 and 50%, Al.sub.2O.sub.3: between 0 and 10%, MgO: between 25 and 45%, MgO and SiO.sub.2 cumulatively representing at least 70%, Fe.sub.2O.sub.3: between 0 and 10%, less than 5% of other oxides, optionally water.

8. The method according to claim 1, wherein said calcium source is a natural mineral material corresponding to the following composition, in percentages by weight: SiO.sub.2: between 30 and 55%, CaO: between 35 and 55%, CaO and SiO.sub.2 cumulatively representing at least 80%, Fe.sub.2O.sub.3: between 0 and 4%, Al.sub.2O.sub.3: between 0 and 5%, CO.sub.2: between 0 and 20%, less than 5% of other oxides.

9. The method according to the claim 1, wherein said calcium source is a natural mineral material corresponding to the following composition, in percentages by weight: SiO.sub.2: between 40 and 55%, CaO: between 10 and 30%, CaO and SiO.sub.2 cumulatively representing at least 55%, Al.sub.2O.sub.3: between 10 and 40%, Fe.sub.2O.sub.3: between 0 and 4%, Na.sub.2O: between 0 and 4%, less than 5% of other oxides.

10. A method comprising providing at least one magnesium source as described in claim 5 and at least one calcium source as raw materials.

11. The method according to claim 1, comprising introducing recycled glass cullet and/or recycled mineral fibers into the molten bath.

12. The method according to claim 11, wherein the recycled glass cullet and/or recycled mineral fibers represent between 1 and 50% of the total weight of the molten bath.

13. The method according to claim 1, wherein the glass cullet satisfies the following composition, in percentage by weight: SiO.sub.2: between 65 and 80%, Na.sub.2O: between 5 and 20%, CaO: between 5 and 20%, Al.sub.2O.sub.3: between 0 and 10%, MgO: between 0 and 5%, Fe.sub.2O.sub.3: between 0 and 2%, less than 5% of other oxides.

14. (canceled)

15. A mixture of raw materials comprising: a silicon source, at least one sodium source, at least one boron source, at least one calcium source selected from a natural mineral calcium silicate comprising, in percentage by weight, more than 30% of SiO.sub.2 and more than 10% CaO, CaO and SiO.sub.2 together representing more than 60%, or more than 70% of the total weight of said source, and/or at least one magnesium source selected from a natural mineral magnesium silicate comprising, in percentage by weight, more than 30% of SiO.sub.2 and more than 10% MgO, MgO and SiO.sub.2 together representing more than 60% of the weight of said source, optionally at least one aluminum source selected from a mixed aluminum oxide with at least one element selected from the group consisting of Si, Ca, Na, K, wherein said calcium and/or magnesium and/or aluminum sources are natural mineral materials, that is, unprocessed mineral materials originating from a natural geological environment, optionally, limestone CaCO.sub.3 or a calcium hydroxide, optionally dolomite CaMg(CO.sub.3).sub.2, optionally, hydrated alumina (Al(OH).sub.3) or calcinated alumina Al.sub.2O.sub.3.

16. The method according to claim 1, wherein said target composition meeting the following criteria, comprises, in weight percentages: SiO.sub.2: between 60 and 70%, Na.sub.2O: between 10 and 20%, CaO: between 5 and 10%, MgO: between 2 and 5%, CaO and MgO together representing between 5 and 20% B.sub.2O.sub.3: between 2 and 8%, Al.sub.2O.sub.3: between 1 and 6%, K.sub.2O: between 0.5 and 2%, Na.sub.2O and K.sub.2O together representing between 12 and 20%, Iron oxide: less than 2%, other oxide(s): less than 3% in total.

17. The method according to claim 1, wherein the silicon source includes one or more of silica, a glass cullet and recycled mineral fibers, the at least one sodium source includes sodium hydroxide NaOH, sodium carbonate Na.sub.2CO.sub.3 or a mixture of sodium hydroxide NaOH and sodium carbonate Na.sub.2CO.sub.3 the at least one boron source is selected from a boron oxide or a mixed oxide of boron with at least one element selected from the group consisting of Si, Mg and Ca, the at least one calcium source is a calcium silicate and/or an aluminum and calcium silicate, the at least one magnesium source is a magnesium silicate, the optional at least one aluminum source is an aluminum silicate.

18. The method according to claim 1, wherein said obtained glass is in the form of fibers after fiber drawing.

19. The method according to claim 2, wherein CaO and SiO.sub.2 together representing more than 70% of the total weight of said source.

20. The method according to claim 3, wherein MgO and SiO.sub.2 together representing more than 70% of the weight of said source.

21. The method according to claim 4, wherein CaO and SiO.sub.2 together represent more than 80% of the total weight of said at least one calcium source and MgO and SiO.sub.2 together represent more than 80% of the total weight of said at least one magnesium source.

Description

EXAMPLES

[0115] According to a first series of examples, different mixtures of raw materials are prepared in order to compare a mixture as currently used for the manufacture of glass wool or C-glass to obtain a substantially identical glass composition, which substantially has the following formulation in percentage by weight:

TABLE-US-00001 TABLE 1 Elements Weight % SiO.sub.2 65.6 CaO 8.1 B.sub.2O.sub.3 4.5 Na.sub.2O 16.4 MgO 2.4 Al.sub.2O.sub.3 2.1 Fe.sub.2O.sub.3 <1 (pref <0.5) K.sub.2O 0.7 Other oxides impurities

Example 1 (Prior Art)

[0116] According to a first example, a typical glass composition corresponding to the preceding formulation is synthesized according to the current techniques.

[0117] Table 2 below gives the proportions of the various raw materials and the final composition of the mixture thus obtained:

TABLE-US-00002 TABLE 2 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 572.90 99.60 0.05 0.01 0.03 0.03 Feldspar (K,Na)AlSi.sub.3O.sub.8 120.80 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 237.50 58.10 41.89 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.5 48.5 28.8 Limestone CaCO.sub.3 83.90 0.05 0.02 55.55 0.25 0.06 44.11 Dolomite CaMg(CO.sub.3).sub.2 111.40 0.05 0.05 30.60 21.30 0.16 47.80 Final glass composition (% weight) 65.6 2.10 16.5 0.6 8.1 2.4 4.5 0.06

[0118] The mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is 190 grams.

Example 2

[0119] In this example, the mixture of raw materials is this time as described in Table 3 below.

[0120] Into this initial mixture, to replace the dolomite, another natural mineral material of a magnesium and silicon oxide, directly taken from a quarry called Trimouns, located in Luzenac, France, was introduced as reagent.

[0121] The hydroxides are considered according to the present invention as being part of the chemical composition of the source, unlike free water (that is present in the form of moisture in the natural mineral material). They are indicated in the table below in H.sub.2O equivalent.

[0122] This material is directly introduced, without any chemical processing and after simple grinding aimed at adjusting the particle size thereof, in mixture with the other constituents in proportions adjusted accordingly to obtain a molten bath of oxides with a composition very close to that of the reference example 1.

TABLE-US-00003 TABLE 3 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 526.50 99.60 0.05 0.01 0.03 0.03 Feldspar (K,Na)AlSi.sub.3O.sub.8 120.80 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 237.50 58.1 41.9 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.5 48.5 28.8 Limestone CaCO.sub.3 145.30 0.05 0.02 55.55 0.25 0.06 44.11 Trimouns 75 61.7 0.2 0.02 31.5 0.68 4.84 Final glass composition (% weight) 65.6 2.10 16.5 0.6 8.1 2.40 4.5 0.10

[0123] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is this time 164 grams, i.e. a decrease of 14% relative to the reference example.

Example 3

[0124] In this example, the mixture of raw materials is this time as described in Table 4 below.

[0125] Into this initial mixture, to replace the dolomite, another natural mineral material was introduced as reagent, namely a magnesium silicate from a quarry located in Carino, Spain. The oxide composition of this mineral material is given below.

TABLE-US-00004 TABLE 4 Bath K.sub.2O Name Formula grams SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O Weight % CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Sand SiO.sub.2 547.50 99.60 0.05 0.01 0.03 0.03 Feldspar (K,Na)AlSi.sub.3O.sub.8 120.80 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 237.10 58.10 41.89 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.50 48.55 28.85 Limestone CaCO.sub.3 143.60 0.05 0.02 55.55 0.25 0.06 44.11 Carino 64.8 39.1 2.4 0.04 0.05 1.50 36.5 8.23 12.15 Final glass composition (% weight) 65.6 2.3 16.5 0.64 8.1 2.4 4.5 0.6

[0126] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is this time 163 grams, i.e. a decrease of 14% relative to the reference example.

Example 4

[0127] In this example, the mixture of raw materials is this time as described in Table 5 below.

[0128] More specifically, in this example, the mineral material Carino described in the previous example, as well as another natural mineral material, namely a calcium silicate directly from a quarry located in Hermosillo, Mexico, are used.

TABLE-US-00005 TABLE 5 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 487.2 99.6 0.05 0.01 0.03 0.03 Feldspar (K,Na)AlSi.sub.3O.sub.8 108.6 70.6 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 239.2 58.1 41.89 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.6 21.5 48.55 28.85 Hermosillo 171.6 40.50 0.43 0.06 0.08 46.50 0.60 0.24 11.54 0.12 Carino 62.8 39.14 2.44 0.04 0.05 1.50 36.5 8.23 12.15 Final glass composition 65.6 2.10 16.5 0.6 8.10 2.40 4.50 0.60

[0129] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is this time 120 grams, i.e. a decrease of 37% relative to the reference example.

Example 5

[0130] In this example, the mixture of raw materials is this time as described in Table 6 below.

[0131] More specifically, in this example, the same natural silicates of magnesium and calcium as in the preceding example were used, but sodium hydroxide is used this time as a raw material source of Na.

TABLE-US-00006 TABLE 6 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 487.1 99.60 0.05 0.01 0.03 0.03 Feldspar (K,Na)AlSi.sub.3O.sub.8 108.7 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-hydroxide NaOH 179.3 77.49 22.51 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.6 21.50 48.55 28.85 Hermosillo 171.6 40.50 0.43 0.06 0.08 46.50 0.60 0.24 11.5 0.12 Carino 62.9 39.1 2.4 0.04 0.05 1.50 36.5 8.2 12.1 Final glass composition (wt. %) 65.6 2.10 16.5 0.6 8.10 2.40 4.50 0.60

[0132] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is this time 20 grams, i.e. a decrease of 90% relative to the reference example.

[0133] According to a second series of examples, different mixtures of raw materials are prepared in order to obtain a glass with a composition substantially identical to the formulation described in Table 1, but using recycled glass cullet as raw material.

Example 6 (Comparative)

[0134] According to one reference example, a typical glass composition is synthesized for the production of glass wool or C-glass corresponding to the composition described in Table 7, according to the current techniques and the raw materials commonly used but by introducing 36% of bottle glass cullet therein, the composition of which is given below.

[0135] Table 7 below gives the proportions of the various raw materials and the final composition of the mixture thus obtained:

TABLE-US-00007 TABLE 7 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 292.20 99.60 0.05 0.01 0.03 0.03 Bottle cullet 420.50 72.50 1.50 11.70 0.30 12.30 0.90 0.06 Feldspar (K,Na)AlSi.sub.3O.sub.8 85.00 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 157.30 58.10 41.89 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.50 48.55 28.85 Dolomite CaMg(CO.sub.3).sub.2 95.10 0.05 0.05 30.60 21.30 0.16 47.80 Final glass composition (wt. %) 65.6 2.10 16.5 0.6 8.1 2.4 4.5 0.1

[0136] The mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is 111 grams.

Example 7

[0137] In this example, the mixture of raw materials is this time as described in Table 8 below.

[0138] More specifically, cullet was used in this example, and as a source of magnesium, the natural mineral compound Trimouns described above was used. These materials are directly introduced, without any chemical processing and after simple grinding aimed at adjusting the particle size thereof, in mixture with the cullet and the other constituents in proportions adjusted accordingly to obtain a molten mixture with a composition very close to that of the reference example.

TABLE-US-00008 TABLE 8 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 98.40 99.60 0.05 0.01 0.03 0.03 Bottle cullet 658.00 72.50 1.50 11.70 0.30 12.30 0.90 0.06 Feldspar (K,Na)AlSi.sub.3O.sub.8 64.50 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 112.20 58.10 41.89 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.50 48.55 28.85 Trimouns 57.4 61.67 0.19 0.02 31.52 0.68 4.84 Final glass composition (wt. %) 65.60 2.10 16.57 0.53 8.10 2.40 4.50 0.09

[0139] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is this time 48 grams, i.e. a decrease of 75% relative to the reference example 1 and of 57% relative to comparative example 6.

Example 8

[0140] In this example, the mixture of raw materials is this time as described in Table 9 below.

[0141] More specifically, cullet was used in this example, and as a source of magnesium, the natural mineral compound Carino described above was used. These materials are directly introduced, without any chemical processing and after simple grinding aimed at adjusting the particle size thereof, in mixture with the cullet and the other constituents in proportions adjusted accordingly to obtain a molten mixture with a composition very close to that of the reference example.

TABLE-US-00009 TABLE 9 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 123.10 99.60 0.05 0.01 0.03 0.03 bottle cullet 652.00 72.50 1.50 11.70 0.30 12.30 0.90 0.06 Feldspar (K,Na)AlSi.sub.3O.sub.8 58.50 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-carbonate Na.sub.2CO.sub.3 114.40 58.10 41.89 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.50 48.55 28.85 Carino 49.5 39.1 2.4 0.04 0.05 1.50 36.52 8.23 12.15 Final glass composition (wt. %) 65.6 2.1 16.6 0.5 8.1 2.4 4.5 0.46

[0142] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. The amount of CO.sub.2 released is this time 48 grams, i.e. a decrease of 75% relative to the reference example 1 and of 57% relative to comparative example 6.

Example 9

[0143] In this example, the mixture of raw materials is this time as described in Table 10 below.

[0144] More specifically, cullet was used in this example, and as a source of magnesium, the natural mineral compound Carino described above was used, along with NaOH as a source of sodium. These materials are directly introduced, without any chemical processing and after simple grinding aimed at adjusting the particle size thereof, in mixture with the cullet and the other constituents in proportions adjusted accordingly to obtain a molten mixture with a composition very close to that of the reference example.

TABLE-US-00010 TABLE 10 Bath SiO.sub.2 Al.sub.2O.sub.3 Na.sub.2O K.sub.2O CaO MgO B.sub.2O.sub.3 Fe.sub.2O.sub.3 CO.sub.2 H.sub.2O Name Formula grams Weight % Sand SiO.sub.2 123.10 99.60 0.05 0.01 0.03 0.03 bottle cullet 652.00 72.50 1.50 11.70 0.30 12.30 0.90 0.06 Feldspar (K,Na)AlSi.sub.3O.sub.8 58.50 70.60 17.05 5.65 5.20 0.10 0.01 0.20 Na-hydroxide Na.sub.2CO.sub.3 85.80 77.49 22.51 Borax Na.sub.2B.sub.4O.sub.75H.sub.2O 92.60 21.50 48.55 28.85 Carino 49.5 39.14 2.4 0.04 0.05 1.50 36.5 8.2 12.15 Final glass composition 65.60 2.10 16.6 0.50 8.10 2.40 4.50 0.46

[0145] As in example 1, the mixture of raw materials is introduced hot into a platinum crucible in a flame furnace (air-gas or oxy-gas combustion) at 1450 C. until the mixture is completely melted for a total duration of 3 h15 including 120 min of refining. This melting does not release CO.sub.2.

[0146] The advantages and quality of the glass obtained from the molten baths of glass according to the above examples 1 to 9 are indicated in Table 11 below, wherein various assessment criteria obtained according to the following measurements were reported:

[0147] 1) Yield

[0148] This is the ratio between the amount of glass produced and the amount of raw materials charged. The higher this ratio, the higher the amount of glass that can be produced, and the lower the gas emissions (CO.sub.2, H.sub.2O) are.

[0149] 2) Amount of sand: which is the amount of sand used relative to the reference example 1 (as a percentage of weight saved). In addition to health considerations related to excessive intake of sand (silicosis), decreasing the amount of sand used in favor of other mineral materials such as natural silicates makes it possible to reduce the energy needed to melt the glass, as the most refractory raw material of the melt is generally silica.

[0150] 3) Energy consumed: this is the amount of energy saved relative to the reference example 1 (as a percentage). This measurement corresponds to the energy necessary for the melting of the mixture of raw materials corresponding to each example.

[0151] 4) Refining quality (or bubble rate):

[0152] The number of bubbles per kilogram of molten glass is measured at 1480 C. for 120 minutes. The higher this index is, the better-quality the refining is.

[0153] ***: number of example bubbles/number of example 1 reference bubbles <100%

[0154] ****: number of example bubbles/number of example 1 reference bubbles <50%

[0155] 5) SiO.sub.2 homogeneity:

[0156] The quality index is proportional to the SiO.sub.2 homogeneity (as measured by microprobe/EDS) of the molten glass at 1480 C. for 120 min.

[0157] Homogeneity is measured by a series of measurements of the amount of SiO.sub.2 in different points of the glass and a standard deviation is then determined.

[0158] **: SiO.sub.2 standard deviation (measured by microprobe/EDS)>0.5%

[0159] *** SiO.sub.2 standard deviation (measured by microprobe/EDS between 0.1 and 0.5%

[0160] ****: SiO.sub.2 standard deviation (measured by microprobe/EDS)<0.1%

TABLE-US-00011 TABLE 11 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Yield (%) 82 84 83 86 91 87.5 92.3 91.7 94.2 Amount of sand saved (%) Ref. 8 4 15 15 Ref. 45 58 58 Energy saved (%) Ref. 2.1 2.3 7.7 18.7 Ref. 10.9 10.9 17 Refining Ref. *** *** *** Ref. *** *** SiO.sub.2 homogeneity ** *** *** *** *** ** *** *** ***

[0161] The comparison of the results above shows that examples 2 to 5 and 7 to 9 according to the invention show quality indices generally greater than reference examples 1 to 6.

[0162] Examples 4 and 5 according to the invention wherein the raw materials of said molten bath used in combination are a calcium source consisting of a natural mineral calcium silicate and a magnesium source consisting of a natural mineral magnesium silicate appear particularly advantageous, according to all the criteria reported in the above table 9.