PROCESS FOR THE PREPARATION OF A SILICA MELT

Abstract

Fly ash and/or rice husk ash is molten in a submerged combustion melter, possibly together with fluxing agent and/or further vitrifiable material, and vitrified upon cooling.

Claims

1. Process for the preparation of a silica melt comprising at least 35 wt % silica, preferably at least 40 wt % silica, more preferably at least 45 wt % silica or at least 50 wt % silica, wherein fine silica powder is fed below bubbling melt level in a submerged combustion melter comprising at least one submerged burner arranged in the bottom of the melter.

2. The process of claim 1 wherein the fine silica powder is fly ash and/or rice husk ash.

3. The process of claim 1 wherein the at least one submerged burner is controlled such as to maintain the melt in a turbulent state such that the volume of the turbulent melt is at least 8% higher than the level the melt would have if no burners are firing.

4. The process according to any of claim 1, wherein it is operated such that no significant foam layer is generated over the top of the melt level.

5. The process according to any of claim 1, wherein further a fluxing agent is introduced into the melt, in combination with the fine silica powder.

6. The process of claim 5, wherein the fluxing agent is selected from sodium oxide, potassium oxide, lithium oxide, lead oxide, zinc oxide, calcium oxide, barium oxide, magnesium oxide, strontium oxide and boron oxide, and combinations thereof.

7. The process of claim 6 wherein the fluxing agent is added in an amount ranging between 0.5 and 25 wt % of the composition.

8. The process of claim 1 comprising feeding additional vitrifiable raw material into the melter.

9. The process of claim 8 wherein the additional vitrifiable raw material is fed above the melt level in the melter.

10. The process of claim 8, wherein the vitrifiable raw material is fed below the bubbling melt level.

11. The process of claim 1, wherein at least a portion of the melt is withdrawn from the melter and allowed to vitrify upon cooling to produce a vitrified product.

12. The process of claim 11 wherein the vitrified product is further treated as appropriate for the preparation of concrete compositions, construction elements, for road constructions, or for use as vitrified raw material in glass melting processes.

13. The process of claim 1 wherein the melting chamber walls are cooled, comprise double steel walls separated by circulating cooling liquid, the energy withdrawn by the cooling liquid being recycled, and the inner melter walls are not lined with refractory material.

14. The process of claim 1 wherein heat is recovered from the hot fumes and/or from the cooling liquid.

15. The process of claim 1 wherein part at least of the melt is withdrawn continuously or batchwise from the melter.

16. The process of claim 1 wherein the submerged combustion is performed such that a substantially toroidal melt flow pattern is generated in the melt, having a substantially vertical central axis of revolution, comprising major centrally inwardly convergent flows at the melt surface; the melt moves downwardly at proximity of the vertical central axis of revolution and is recirculated in an ascending movement back to the melt surface, thus defining a substantially toroidal flow pattern.

17. The process of claim 1 wherein the melting step comprises melting the fine silica powder material, in a submerged combustion melter by subjecting the melt to a flow pattern which when simulated by computational fluid dynamic analysis shows a substantially toroidal melt flow pattern in the melt, comprising major centrally inwardly convergent flow vectors at the melt surface, with the central axis of revolution of the toroid being substantially vertical.

18. The process of claim 13 wherein towards the melter bottom, the flow vectors change orientation showing outward and then upward components.

19. The process of claim 1 wherein submerged combustion burners are arranged at the melter bottom in a substantially annular burner zone, preferably on a burner circle.

20. The process of claim 1 wherein the burners are arranged with a distance between adjacent burners of about 250-1250 mm.

21. The process of claim 1 wherein each burner axis and/or a speed vector of the melt moving upwards over or adjacent to the submerged burners is slightly inclined from the vertical, for example by an angle which is 1, 2, 3 or 5 and/or which is 30 towards the center of the melter.

22. The process of claim 1 wherein each central burner axis is inclined by a swirl angle with respect to a vertical plane passing through a central vertical axis of melter and the burner center, the swirl angle being 1, 2, 3, 5 and/or 30, 20, 15 or 10.

23. A submerged combustion melter (1) comprising a melting chamber (3), a melt outlet (9) and a chimney for evacuation of flue gases, burners (21,22,23,24,25,26) arranged under the melt level in the bottom of the melter, and a feeder (10) for powdery or fine material arranged below the melt level and/or between the melt level and the bubbling melt level, the burners being arranged and controlled such as to maintain at normal operating conditions a sufficient turbulence within the melt such that the melt volume is increased by at least 8% as compared to the volume the melt would have at the same temperature, in the absence of any burner firing.

Description

[0047] An embodiment of a melter suitable for use in accordance with the present invention is described below, with reference to the appended drawings of which:

[0048] FIGS. 1a and 1b are schematic representations of a toroidal flow pattern;

[0049] FIG. 2 shows a vertical section through a melter; and

[0050] FIG. 3 is a schematic representation of a burner layout.

[0051] With reference to FIGS. 1a and 1 b, a toroidal flow pattern is preferably established in which melt follows an ascending direction close to submerged burners 21, 22, 23, 24, 25, 26 which are arranged on a circular burner line 27, flows inwardly towards the center of the circular burner line at the melt surface, and flows downwards in the proximity of the said center. The toroidal flow generates agitation and turbulence in the melt, ensures good stirring of the melt, and absorption of raw material and gas bubbles into the melt.

[0052] The illustrated melter 1 comprises: a cylindrical melting chamber 3 having an internal diameter of about 2.0 m which contains the melt; an upper chamber 5; and a chimney for evacuation of the fumes. The upper chamber 5 is equipped with baffles 7 that prevent any melt projections thrown from the melt surface 18 being entrained into the fumes. A raw material feeder 10 is arranged in the melting chamber wall, below the bubbling melt level and is designed to load fresh powdery ash and fluxing agent into the melter 1. A powdery or fine raw material feeder may be arranged below the melt level and/or between melt level and bubbling level of melt. The feeder 10 comprises a horizontal feeding means, for example a feed screw or a piston, which transports the fly ash and/or the rice husk ash possibly admixed with fluxing agent and/or other raw materials for preparation of a glass melt, directly into the melt. The bottom of the melting chamber comprises six submerged burners 21, 22, 23, 24, 25, 26 arranged on a circular burner line 27 concentric with the melter axis and having a diameter of about 1.4 m. The melt may be withdrawn from the melting chamber 3 through a controllable outlet opening 9 located in the melting chamber side wall, close to the melter bottom, substantially opposite the feeding device 10. The melt withdrawn from the melter may then be allowed to cool and solidify and possibly ground as required for downstream use. Such downstream use may include cullet preparation for later use in glass manufacturing. It may also include actual use of the melt for glass formation, including fiberization as is known per se. Other uses include grinding of the vitrified material for use in cement and/or concrete compositions, construction materials etc.

[0053] The temperature within the melt may be between 1200 C. and 1600 C., depending on the composition of the melt, desired viscosity and other parameters. Preferably, the melter wall is a double steel wall cooled by a cooling liquid, preferably water. Cooling water connections provided at the external melter wall allow a flow sufficient to withdraw energy from the inside wall such that melt can solidify on the internal wall and the cooling liquid, here water, does not boil.

[0054] The submerged burners comprise concentric tube burners operated at gas flows of 100 to 200 m/s, preferably 110 to 160 m/s and generate combustion of fuel gas and oxygen containing gas within the melt. The combustion and combustion gases generate agitation within the melt before they escape into the upper chamber and then through the chimney. These hot gases may be used to preheat the raw material and/or the fuel gas and/or oxidant gas (e.g. oxygen, industrial oxygen have an oxygen content 95% by weight or oxygen enriched air) used in the burners. The fumes are preferably filtered or otherwise treated prior to release to the environment, optionally using dilution with ambient air to reduce their temperature prior to filtering.

[0055] It has been determined that in a melter as described and controlled as per the invention requirements, the melt level is increased by 30-50% as compared to the level the melt would have at the same temperature when no burners are firing. The melt level with no burners firing has been calculated on the basis of the melt composition and has been verified by letting the melt harden in the melter. The level of the turbulent aerated melt has been determined in normal operating mode, by a laser pointer averaging the measured values over a 5 minutes time period. Similar devices would be appropriate to. Interestingly, the melt flow pattern as desired does not generate any significant foam over the melt level. It is understood that the gas bubbles are reabsorbed into the melt by the relevant flows, rather than to be allowed to accumulate over the top of the melt.

[0056] The above described production process is energy efficient due to the choice of submerged combustion melters that allow for improved energy transfer to the melt, shorter residence times and thus less heat loss, and because the high stirring and turbulence lead to a more homogenous melt at reduced melt viscosity, which in turn may allow for operation at reduced temperatures. Furthermore, submerged combustion may advantageously be performed in water-cooled melters which are easier and less costly to maintain and repair and which further allow for recycling of the energy withdrawn from the cooling fluid. Furthermore, the underlevel feeding of the powdery ash material reduces the risk of contamination of the fumes, and eases the incorporation of the powdery ash material into the melt with concomitant energy transfer to the freshly charged material.

[0057] As a first example, the vitrified product obtained comprises 73 wt % SiO.sub.2, 22 wt % B.sub.2O.sub.3, 1.5 wt % Na.sub.2O and K.sub.2O, and trace amounts of other oxides, adding up to 100 wt %. Such vitrified product may be used as such or may be further combined with raw materials to produce other glass compositions.

[0058] As an alternative example, the use of CaO, MgO, and Na.sub.2O and/or K.sub.2O as fluxing agents may lead to a composition as follows: 69 wt % SiO.sub.2, 8 wt % CaO, 2 wt % MgO, 15 wt % Na.sub.2O+K.sub.2O, and trace amounts of other oxides to add up to 100 wt %.

[0059] As a further example, fly ash, Al.sub.2O.sub.3, B.sub.2O.sub.3, CaO, MgO and Na.sub.2O and K.sub.2O may be mixed in suitable proportions to produce a C-glass composition at the outlet of the submerged combustion melter equipped with bottom burners as described above. A typical C-glass composition comprises 64-68 wt % SIO.sub.2, 3-5 wt % Al.sub.2O.sub.3, 4-6 wt % B.sub.2O.sub.3, 11-15 wt % CaO, 2-4 MgO, 7-10 wt % Na.sub.2O+K.sub.2O and trace amounts of other oxides to add up to 100%.

[0060] Similarly, rice husk ash, Al.sub.2O.sub.3, B.sub.2O.sub.3, CaO, MgO and Na.sub.2O and K.sub.2O may be mixed in suitable proportions to produce a E-glass composition at the outlet of the submerged combustion melter equipped with bottom burners as described above. A typical E-glass composition comprises 52-62 wt % SIO.sub.2, 12-16 wt % Al.sub.2O.sub.3, 0-10 wt % B.sub.2O.sub.3, 16-25 wt % CaO, 0-5 MgO, 0-2 wt % Na.sub.2O+K.sub.2O and trace amounts of other oxides to add up to 100%.