Production of glass from a mixture comprising calcium oxide, and glass furnace

Abstract

The invention relates to a glass production method comprising the production of a glass precursor mixture for a glass furnace, in which water, sand and sodium carbonate are mixed in weight proportions of between 0 and 5%, 40 and 65%, and greater than 0 and at most 25% respectively, and, after at least 10 minutes, calcium oxide is added in a weight proportion of between 1 and 20% of the total. The invention relates to a method for producing glass using a mixture containing, in particular, calcium oxide, and a glass melting furnace, said method and furnace using a burner with a flame directed at the glass batch.

Claims

1. A process for manufacturing glass comprising: preparing a glass precursor mixture for a glassmaking furnace, in which water, sand and sodium carbonate are mixed in mass proportions of between more than 0 and 5%, 40% and 65%, and more than 0 and not more than 25%, respectively, and, adding, after a delay of at least 10 minutes, calcium oxide in a mass proportion of between 1% and 20% of the total, wherein the glass precursor mixture is prepared and the calcium oxide is added without supplying thermal energy.

2. The process as claimed in claim 1, wherein said delay is at least one hour.

3. The process as claimed in claim 1, wherein said delay is between at least 10 minutes and less than one hour for a mixture of water, sand and sodium carbonate with a moisture content of not more than 4.1%.

4. The process as claimed in claim 1, wherein the sodium carbonate has a particle size of less than 5% passing through a 0.075 mm screen, less than 15% passing through a 0.150 mm screen and less than 5% not passing through a 0.600 mm screen.

5. The process as claimed in claim 1, wherein said mixture of water, sand and sodium carbonate has a moisture content of not more than 3% with sodium carbonate of particle size predominantly greater than 0.500 mm and less than 1.000 mm and a moisture content of not more than 2% with sodium carbonate of particle size predominantly less than 0.250 mm.

6. The process as claimed in claim 1, wherein said delay is less than 72 hours and an initial temperature of the raw materials to be mixed is at least 30° C.

7. The process as claimed in claim 1, wherein the calcium oxide has a particle size such that 70% to 90% by mass does not pass through a 0.1 mm screen.

8. The process as claimed in claim 1, wherein the calcium oxide has a particle size such that more than 90% by mass does not pass through a 0.1 mm screen and less than 5% by mass does not pass through a 4 mm screen.

9. The process as claimed in claim 1, wherein the calcium oxide has a mean particle size of between 1 and 5 mm.

10. The process as claimed in claim 1, wherein the precursor mixture is used in a glassmaking furnace less than 1 hour after its preparation for a particle size of 90% or more by mass passing through a 0.1 mm screen.

11. The process as claimed in claim 1, wherein the precursor mixture is used in a glassmaking furnace less than 8 hours after its preparation for a particle size of 70% or more by mass passing through a 2 mm screen.

12. The process as claimed in claim 1, wherein said sand is dry.

13. The process as claimed in claim 1, wherein the water is present in said sand, preferably to 3% to 4% by mass.

14. The process as claimed in claim 1, wherein the calcium oxide is free of deliberate addition of aluminum oxide.

15. The process as claimed in claim 1, wherein cullet is added to the glass precursor mixture, before or after the addition of calcium oxide, in a mass proportion of between 5% and 40% of the total.

16. The process as claimed in claim 1, wherein the glass precursor mixture is prepared in the solid state.

17. The process as claimed in claim 1, wherein the glass precursor mixture is prepared at a temperature between ambient air temperature and the ambient air temperature increased by 20° C.

18. The process as claimed in claim 1, wherein said mixture is fired in an electric furnace.

19. The process as claimed in claim 1, further comprising: introducing the glass precursor mixture into a glassmaking furnace, and melting the mixture with at least one flame burner directed toward the mixture.

20. The process as claimed in claim 19, wherein an oxidant supplied to the burner is oxygen.

21. The process as claimed in claim 19 wherein the water, sand, sodium carbonate and calcium oxide are present in mass proportions of between 0 and 5%, 40% and 65%, 1% and 25%, and 1% and 20%, respectively.

22. An industrial glassmaking furnace for performing the process as claimed in claim 1, comprising a molten glass tank, a combustion heating chamber located above the tank and delimited by breast walls, gables and a crown, a fume evacuation pipe in communication with the heating chamber, a loop burner placed in a direction parallel to the fume evacuation pipe, and a flame burner directed toward the molten glass tank.

23. The furnace as claimed in claim 22, wherein the flame burner is placed in a crown of the furnace.

24. The furnace as claimed in claim 22 wherein an oxidant is oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will become apparent on examining the detailed description below, and the appended drawings, in which:

(2) FIG. 1 is a schematic perspective view of a glassmaking furnace according to one embodiment.

(3) FIG. 2 shows curves of heating as a function of time for limestone and quicklime.

(4) FIG. 3 shows curves of heating as a function of time for three mixtures containing lime.

(5) FIG. 4 shows several curves of heating as a function of time for ten tests as a function of the temperature of the starting materials, of the moisture content, of the delay between premixing and the introduction of the quicklime, and of the particle size of the sodium carbonate.

(6) FIG. 5 shows a selection of the curves of FIG. 4 on a moisture content parameter.

(7) FIG. 6 shows a selection of the curves of FIG. 4 on the temperature parameter.

(8) FIG. 7 shows a selection of the curves of FIG. 4 on the particle size parameter of the sodium carbonate.

(9) The appended drawings may serve not only to complete the invention, but also to contribute toward its definition, where appropriate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) The glassmaking furnace 1 has at least one loop burner and at least one crown burner. The loop burner is oriented substantially horizontally, close to an oxidant inlet. The flame extends substantially horizontally over the bath. The bath is composed at the start of heating of the starting materials to be melted, i.e. of the glass precursor mixture, and then of the molten glass undergoing production, gradually transformed into industrial glass having the desired quality. The crown burner is oriented substantially vertically in a summit wall of the furnace. The flame extends substantially vertically toward the bath.

(11) The glassmaking furnace 1 comprises a molten glass tank 2 for a batch production. The glassmaking furnace 1 comprises a combustion chamber 3 located above the molten glass bath and an upper wall 4 composed of a crown 5 and vertical parts known as the breast walls (length) or gables (width) 6 delimiting the combustion chamber 3. The glassmaking furnace 1 comprises at least one loop burner 7 fed with fuel oil or gas. The glassmaking furnace 1 comprises at least one crown burner 8 fed with fuel oil or gas. The glassmaking furnace 1 comprises an oxidant inlet 9. The oxidant may be air and/or oxygen.

(12) The burner 8 is installed in the crown 5. The burner 8 is a flame burner directed toward the upper surface of the bath, from the top downward. The burner 8 is positioned so that its flame is located outside the zone where the movement of gas generated by the burner 7 is maximal. The burner 8 is positioned substantially at the top of the crown 5. The burner 8 is positioned substantially in the middle of the furnace 1 in the direction of the length.

(13) An aperture or nook 11 for feeding the furnace 1 with raw materials to be melted, notably with precursor mixture, is provided in one of the breast walls. The members for withdrawing the refined glass have not been shown.

(14) The tank 2 and the upper wall 4 are made of refractory materials, reinforced with an outer metallic structure remote from the high-temperature zones. The burner 7 is a flame burner oriented horizontally in the combustion chamber 3. The burner 7 is installed below the oxidant inlet 9.

(15) The glassmaking furnace 1 comprises a fume outlet 10 housed in one of the vertical walls 6 above the molten glass bath. The burner 7 and the fume outlet 10 may be provided on the same small side so that the flame of said burner 7 and the fumes follow a U shaped path in the combustion chamber 4. The U shaped path is referred to as a loop path in the usual jargon. The burner 7 and the fume outlet 10 may be parallel. The burner 7 and the fume outlet 10 emerge in the combustion chamber 3.

(16) Downstream of the fume outlet 10 in the direction of movement of the fumes, the installation may comprise a flue. The flue is a substantially horizontal fume pipe. The flue is in fluid communication with the combustion chamber 3 via the fume outlet 10. The flue is made of refractory materials reinforced with an outer metallic structure which is remote from the high-temperature zones. The flue is free of valves. The flue conducts the fumes to a chimney or a heat recovery device or a regenerator for heating the oxidant.

(17) The combined use of the loop burner 7 and of the crown burner 8 offers a high yield and glazing of the surface of the bath. The glazing is rapid melting of the surface zone of the bath subjected to the action of the flame of the crown burner 8. Rapid melting prevents the release of dusts from said zone. Glazing is obtained more quickly.

(18) Moreover, tests relating to the delay time D between the mixing (the action of mixing) of the water, the sand and the calcium carbonate and the addition of calcium oxide (quicklime) were conducted in relation with the temperature T.sub.sm of the starting materials corresponding to the mean ambient storage temperature and the moisture content H of the sand/soda mixture measured. Alumina, for example in the form of feldspath, feldspathoid and/or calcined alumina is also mixed with the water, sand and sodium carbonate. These tests are reported in FIGS. 4 to 7. The temperature measured is on the y axis and the time on the x axis. The curves were set on the x axis on a common reference at the moment of introduction of the quicklime into the mixture containing a premix beforehand. The delay D goes from 20 minutes for tests 1, 5 and 6 to 60 minutes for test 7.

(19) Here, the water was supplied to a dry sand and mixed for 3 minutes. Next, sodium carbonate and alumina were mixed with the wet sand for 2 minutes. Measurement of the moisture content H and of the temperature T of the premix was performed. The water present before the introduction of the sodium carbonate and the alumina reacts with the sodium carbonate via a hydration reaction of the sodium carbonate, with a rise in temperature of a few degrees. The sodium carbonate reacts with said water at least in the tests of curves 1 to 3. Free water remains in the test of curve 4 since the subsequent addition of calcium oxide brings about a strong and vigorous temperature increase. Substantially no free water remains in the tests of curves 1 to 3 since the subsequent addition of calcium oxide does not bring about any temperature increase. Furthermore, supplying water, as a check, more than one hour after the addition of calcium oxide brings about a strong and vigorous temperature increase.

(20) Later, the calcium oxide was added and mixed. The mixing action was performed in a mixer of concrete mixer type with a volume of 150 liters. The amounts used in each test are: 19 to 20 kg. The nature and origin of the starting materials are the same for tests 1 to 10. Finally, tests 1 to 10 were performed by the same person following the same protocol, with the same concrete mixer at the same rotation speed. The taking of the measurement and the measurement precision correspond to semi-industrial tests that are closer to the reality of a production run than a fundamental research laboratory, one aim being to identify phenomena that take place at the industrial scale. The masses used are 13 kg of sand, 4 kg of sodium carbonate, 2 kg of calcium oxide, 0.24 kg of alumina and water to reach the desired percentage.

(21) The sand has a composition: SiO.sub.2 at least 99%, Al.sub.2O.sub.3 less than 1%, K.sub.2O less than 0.1%, TiO.sub.2 less than 0.03%, Fe.sub.2O.sub.3 less than 0.015%. The other elements are in trace amount. The sand has a particle size D.sub.50 of between 0.20 and 0.25 mm. The sand has a particle size with not more than 3% of screen retainings of 0.355 mm, and not more than 1% of passage through a 0.125 mm screen.

(22) The sodium carbonate has a composition: Na.sub.2CO.sub.3 99.75%, NaCl 0.03% and H.sub.2O less than 0.1%. The other elements are in trace amount. The sodium carbonate has a particle size D.sub.50 of between 0.15 and 0.25 mm. The sodium carbonate has a particle size with not more than 0.5% of screen retainings of 0.600 mm, at least 90% of screen retainings of 0.150 mm and not more than 2% of passage through a 0.075 mm screen.

(23) The calcium oxide has a composition: CaO at least 93%, MgO less than 2%, CO.sub.2 less than 2%, Fe.sub.2O.sub.3 less than 0.1%, S less than 0.06%. The other elements are in trace amount. The calcium oxide has a particle size D.sub.50 of between 0.08 and 0.12 mm. The calcium oxide has a particle size with not more than 1.6% of screen retainings of 5.00 mm, and not more than 55% of passage through a 0.090 mm screen.

(24) The maximum temperature T.sub.max reached within the hour following the addition of calcium oxide is measured. The temperature measurement is performed by inserting a temperature probe into the mixture contained in the mixer, the mixer having been switched off. The first temperature clip observed on all the curves in FIG. 4 corresponds to the step of withdrawing the temperature probe, addition of the calcium oxide, switching on the mixer for 2 minutes, inserting the temperature probe again. The second temperature clip observed in curves 1, 2 and 3 corresponds to an additional step of adding excess water beyond the amounts indicated to check the presence of calcium oxide more than one hour after the introduction of said calcium oxide. This addition of water is reflected by an exothermic reaction of hydration of the calcium oxide, transforming it into calcium hydroxide. The temperature rise observed after said addition of excess water makes it possible to deduce that the calcium oxide remained present beforehand in the mixture.

(25) Furthermore, fine observation of all of the curves before the addition of calcium oxide shows a temperature increase indicating a water-sodium carbonate reaction. The temperature reached rises with the proportion of water, notably by comparison between curves 1, 2, 3, 7 and 4, on the one hand, and 6 and 5, on the other hand.

(26) Before the addition of calcium oxide, a temperature maximum is reached, i.e. very rapidly for curve 4 in about 1 minute after the end of the mixing action, i.e. about 3 minutes after placing the sodium carbonate and the alumina in contact with the sand and the water, i.e. more slowly for the other curves in about 10 minutes after the end of the mixing action. The temperature reduction after the maximum indicates that the water-sodium carbonate reaction has ceased. The end of said reaction indicates that either all the available water has been taken up, or that all the available sodium carbonate has been hydrated and there is free water remaining. Thus, the rapid reaction of curve 4 corresponds to the hydration of the sodium carbonate with excess water.

(27) After the addition of the calcium oxide, the temperature is measured: D=20 minutes T.sub.sm=30° C. H=1%. T.sub.max<T.sub.rm+15° C. D=30 minutes T.sub.sm=30° C. H=2%. T.sub.max<T.sub.rm+15° C. D=30 minutes T.sub.sm=30° C. H=3%. T.sub.max<T.sub.rm+15° C. D=30 minutes T.sub.sm=30° C. H=5%. T.sub.max>100° C. D=20 minutes T.sub.sm=1° C. H=2.7%. T.sub.max<T.sub.rm+15° C. D=20 minutes T.sub.sm=1° C. H=1.8%. T.sub.max<T.sub.rm+15° C. D=60 minutes T.sub.sm=30° C. H=4.1%. T.sub.max<T.sub.rm+15° C. D=25 minutes T.sub.sm=30° C. H=3.44%. T.sub.max<T.sub.rm+15° C. D=30 minutes T.sub.sm=30° C. H=5.1%. T.sub.max>100° C. D=30 minutes T.sub.sm=30° C. H=3.8%. T.sub.max>60° C.

(28) Heating below 10° C. takes place on mixing (the action of mixing) the water-sand-sodium carbonate in tests 2 to 4 and 7. Tests 4, 9 and 10 are unsatisfactory due to excessive heating on introduction of the calcium oxide. Comparison of tests 2 and 6, on the one hand, and 3 and 5, on the other hand, shows that the initial temperature of the raw materials T.sub.rm has little to no influence on the maximum temperature T.sub.max. Comparison of tests 2, 3 and 4, on the one hand, and 5 and 6, on the other hand, shows that the moisture content has little influence below a threshold. The threshold is located between more than 4.1% and less than 5% for D=30 minutes. However, the influence of the duration D has an upper limit set by the ability of the sodium carbonate to absorb the available free water. However, the tests show that the amount of water must be largely inferior to the theoretical maximum threshold.

(29) Moreover, the particle size of the sodium carbonate has an influence on the duration D. To a certain extent, the finer the particle size, the more quickly the water is absorbed but there is a risk of initiating setting to a solid. In the event of setting to a solid, the water remains available for the quicklime, whence heating that it is desired to avoid.

(30) In the case of a high particle size of the sodium carbonate, the Applicant puts forward the hypothesis that the reaction with water is limited, said reaction taking place at the surface of the sodium carbonate grains but sparingly or not at all inside said grains. The particle size of the sand has little influence on account of the virtually nonexistent ability of SiO.sub.2 to become hydrated.

(31) Test No. 2 was performed with a cold concrete mixer, at about 0° C., which slowed down the sodium carbonate hydration reaction. Test No. 2 is not entirely representative in the curve section prior to the addition of calcium oxide. In general, an energy input may be performed in the form of heating the concrete mixer and/or mixing at a higher temperature than the ambient temperature, for example with a flame burner, electric heating, or injection of steam into the mixture, while remaining at a mixing temperature below 47° C.

(32) Thus, the tests with 4.1% moisture content in the mixture prior to the addition of quicklime for a duration D of at least one hour and at 3% moisture content for a duration D of at least 10 minutes with a common sodium carbonate particle size offer satisfactory results. The low influence of the duration D beyond 10 minutes on account of the temperature maximum reached before 10 minutes, is such that a maximum of 4.1% moisture content in the mixture prior to the addition of quicklime for a duration D of 10 minutes is advantageous and would even be versatile relative to measurement imprecisions or industrial tolerances.

(33) Analysis of the left part of the curves provides information. Between time 0 and the moment of withdrawal of the probe for the purpose of supplying CaO—at 20; 30; 60 minutes depending on the tests—the change in temperature reflects the sodium carbonate hydration reaction on contact with the wet sand. Between these moments, a time period is identified in which the local temperature maximum T.sub.Na is found. The local temperature maximum T.sub.Na indicates that the sodium carbonate hydration reaction has substantially terminated: D=20 minutes T.sub.rm=30° C. H=1%. T.sub.Na 5 to 7 minutes. D=30 minutes T.sub.rm=30° C. H=2%. T.sub.Na 13 to 15 minutes. D=30 minutes T.sub.rm=30° C. H=3%. T.sub.Na 5 to 7 minutes. D=30 minutes T.sub.rm=30° C. H=5%. T.sub.Na 1 to 2 minutes. D=20 minutes T.sub.rm=1° C. H=2.7%. T.sub.Na 11 to 13 minutes. D=20 minutes T.sub.sm=1° C. H=1.8%. T.sub.Na about 15 minutes. D=60 minutes T.sub.sm=30° C. H=4.1%. T.sub.Na 17 to 19 minutes. D=25 minutes T.sub.sm=30° C. H=3.44%. T.sub.Na 7 to 9 minutes. D=30 minutes T.sub.sm=30° C. H=5.1%. T.sub.Na>25 minutes. D=30 minutes T.sub.sm=30° C. H=3.8%. T.sub.Na 25 to 27 minutes.

(34) The initial temperature T.sub.rm of the starting materials has an influence on the rate of the water-sodium carbonate reaction. At T.sub.rm=30° C., the reaction is faster than at T.sub.rm=1° C. by comparison between tests 2 and 6; 3 and 5. The speed of the reaction in test No. 4 corroborates a presence of excess water enabling faster hydration of the sodium carbonate. The relative slowness of the reaction in test No. 7 shows a water-sodium carbonate equilibrium. The stability between tests No. 1 and No. 3 shows that a duration D of about 10 minutes is sufficient and robust with raw materials at an initial temperature of 30° C. or more. Such a stability between tests No. 1 and No. 3, and between tests No. 6 and No. 5 shows that, with sodium carbonate in excess relative to water, the reaction speed is sparingly dependent on the water content.

(35) Moreover, during the subsequent addition of excess water in tests 2 and 3 and during the addition of calcium oxide in test 4, the temperature increased very rapidly and a strong evolution of dusts took place simultaneously. The right side of the curve in test 1 starting from 1:49:20 is not representative for reasons intrinsic to test 1. This type of reaction is typical of the hydration of quicklime, which is a highly exothermic reaction. The immediate hydration of the quicklime added to a mixture containing 5% water and the absence of hydration of quicklime added to a mixture containing 2% or 3% water are thus confirmed. In addition, the temperature curves of tests 3 and 7 containing 3% and 4.1% water, respectively, have very similar shapes before and after the addition of quicklime. This strong similarity indicates that the mixture containing 4.1% water does not contain any free water.

(36) Tests 8 and 9 were conducted with sodium carbonate fines passing through a 0.250 mm screen whereas test 10 was conducted with coarse sodium carbonate particles not passing through a 0.500 mm screen and passing through a 1.000 mm screen. The origin and batch of sodium carbonate are the same for tests 1 to 7. Screening was performed.

(37) Tests 8 and 10 were chosen with a moisture content suggesting a satisfactory result, whereas test 9 was chosen with a high moisture content to test the possible influence of the particle size on the maximum moisture content. The curve of test 8 is close to the curve of test 3. Test 8 is interpreted as producing total consumption of the free water by the sodium carbonate in a relatively short time of less than 10 minutes and a temperature increase of less than 15° C. relative to the initial temperature T.sub.rm. The fine particle size does not have any major impact at the moisture content of 3.44%. Test 9 at a high moisture content reveals a much slower sodium carbonate hydration reaction than in test 4. This is explained by the setting to a solid of the precursor mixture accompanied by crusting phenomena liable to slow down the reaction.

(38) Test 10 at a large particle size and 3.8% moisture content gives a curve different from the other tests in the sodium carbonate hydration step. The temperature rises for more than 25 minutes, which indicates continuation of the sodium carbonate hydration reaction. During the withdrawal of the temperature probe for the purpose of introducing the calcium oxide, an uncertainty remains regarding whether or not the temperature maximum has been reached. The slowness of the sodium carbonate hydration results in a reduced available active surface of the sodium carbonate on account of the large particle size of the sodium carbonate.

(39) During the addition of calcium oxide, test 8 shows a temperature increase comparable to that of tests 3 and 7, which is thus satisfactory. The presence of available water to hydrate the calcium oxide is very low. Test 9 shows a temperature increase comparable to that of test 4, which is thus too high. Decreasing the particle size does not afford any advantageous effect in the step of adding calcium oxide and presents risks of setting to a solid. Such risks may be reduced by selecting a moisture content of 2% or less.

(40) During the addition of calcium oxide, test 10 shows a temperature increase of about 30 to 35° C. above the temperature T.sub.rm. This increase leads to a temperature above 60° C. starting from T.sub.rm=30° C. At 60° C., the risk of releasing irritant dusts is high. Increasing the particle size provides a risk of excessive heating on addition of the calcium oxide, in particular if the temperature T.sub.rm is greater than 15° C. Such a risk may be reduced by selecting a moisture content of 3% or less.

(41) The absence of benefit and certain drawbacks of the exclusively fine and exclusively coarse particle sizes are deduced from tests 8 to 10. It is thus preferable to provide a source of sodium carbonate with a particle size centered between 0.250 mm and 0.500 mm. This may include minor fractions of particles, some of which are less than 0.250 mm and others greater than 0.500 mm, as shown by tests 1 to 7. Thus, a sodium carbonate particle size with less than 5% passing through a 0.075 mm screen, less than 15% passing through a 0.150 mm screen and less than 5% not passing through a 0.600 mm screen is suitable for use.

(42) In the case of supplying sodium carbonate with a high particle size, then the moisture content will be limited to 3%. The hydration of the sodium carbonate will be faster than in test 10 and the temperature after introduction of the calcium oxide will remain within a range of +0 to +15° C. relative to the ambient temperature.