GLASS MELTING

Abstract

The invention relates to a glass melting process comprising melting glass cullet in a submerged combustion melter comprising at least one submerged burner, under oxidizing conditions, wherein the glass cullet comprises increased levels of contaminants.

Claims

1. Glass melting process comprising melting glass cullet in a submerged combustion melter comprising at least one submerged burner, under oxidizing conditions, wherein the glass cullet comprises more than 0.5 wt % or more than 0.8 wt % or more than 1.0 wt %, or more than 1.2 wt % of organic contamination and/or more than 2.5 wt % or more than 3 wt %, or more than 3.5 wt % or more than 4.0 wt % or more than 4.5 wt % or more than 5 wt % boron expressed as B2O3, and/or high melting contaminants of more than 20 ppm, more than 75 ppm, more than 100 ppm more than 150 ppm or more than 200 ppm, more than 250 ppm or more than 300 ppm.

2. The process of claim 1 wherein the boron content in the final raw material mix is less than 20% by weight, preferably between 10 and 15% by weight.

3. The process of claim 1 wherein the concentration of high melting contaminants, such as ceramic contaminations, is preferably kept below 2, preferably below 1 wt % in the final raw material mix.

4. The process of claim 1 wherein the organics concentration in the cullet are preferably below 5 wt %, preferably below 3 wt %.

5. The process of claim 1, wherein the melter comprises at least one submerged burner, and the said 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%, preferably at least 10%, more preferably at least 15% higher than the level the melt would be at if no burners are firing.

6. The process of claim 5, wherein the submerged burners are controlled such that no significant foam layer is generated over the top of the melt level.

7. The process of claim 1, wherein the melting chamber walls comprise double steel walls separated by circulating cooling liquid, preferably water.

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

9. The process of claim 1, wherein heat is recovered from the hot fumes to preheat the raw materials.

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 The process of claim 1, wherein part at least of the melt is withdrawn continuously or batchwise from the melter.

11. 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 an substantially toroidal flow pattern.

12. The process of claim 1, wherein the melting step comprises melting the solid batch 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.

13. The process of claim 12 wherein towards the melter bottom, the flow vectors change orientation showing outward and then upward components.

14. The process of claim 1 further comprising downstream internal or external fiberizing for forming mineral wool fibers.

15. The process of claim 1 further comprising downstream forming of continuous fibers.

Description

[0051] 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:

[0052] FIGS. 1a and 1b are schematic representations of a toroidal flow pattern;

[0053] FIG. 2 shows schematically a vertical section through a melter followed by a downstream forming device; and

[0054] FIG. 3 is a schematic representation of a burner layout.

[0055] With reference to FIGS. 1a and 1b, 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 in the melt, ensures good stirring of the melt, and absorption of raw material including glass cullet into the melt.

[0056] It has been found that the burner arrangement and control to obtain the above described toroidal melt flow pattern may ensure appropriate mixing in the melt as well as the required turbulence to sufficiently increase the melt volume (or reduce the melt density) to reach the objective of the present invention. Foam formation is particularly reduced, as the gas bubbles reaching the top of the melt are reabsorbed and mixed within the melt as a result of the toroidal flow pattern.

[0057] 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 surface 18 of the melt being entrained into the fumes. A raw material feeder 10 is arranged at the upper chamber 5 and is designed to load fresh raw material including man-made mineral fibers into the melter 1 at a point 11 located above the melt surface 18 and close to the side wall of the melter. The feeder 10 comprises a horizontal feeding means, for example a feed screw, which transports the raw material mix to a hopper fastened to the melter, the bottom of which may be opened and closed by a vertical piston. In the alternative, an underlevel feeder may charge raw material directly into the melt, under the level of 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 ground as required. In the alternative, a syphon-type outlet may be used which concomitantly continuously controls the level of the melt in the melter.

[0058] The temperature within the melt may be between 1350.degree. C. and 1450.degree. C., preferably about 1400.degree. 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.

[0059] The melter 1 may be mounted on dampers adapted to absorb vibrational movements.

[0060] 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 (eg oxygen, industrial oxygen have an oxygen content 95% by weight or oxygen enriched air) used in the burners. The fumes are preferably filtered prior to release to the environment, optionally using dilution with ambient air to reduce their temperature prior to filtering.

[0061] The melt may then be discharged continuously or batch wise into a downstream processing and/or forming equipment 20 known per se for desired applications.

[0062] The melt obtained is of high quality. The above described production process is less energy demanding then known processes, because of 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 turbulence and stirring leads 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 more easy and less costly to maintain and repair and which further allow for recycling of the energy withdrawn from the cooling fluid. In addition, the high turbulence allows for use of highly contaminated cullet, such as cullet comprising contaminations by organic compounds and/or by boron which are significantly beyond those tolerated in known processes.

[0063] It has been found that despite the relatively high level of contamination of the cullet used at relatively high rate, hence constituting a highly contaminated melt, the melt produced is advantageously used for forming mineral wool by external or internal centrifugation. The obtained mineral wool fibers are of high quality and appropriate length. This shows that the high contamination level has not affected the rupturing of the fibers as a result of the presence of otherwise undesirable contaminants. Similarly, it has been found that the highly contaminated melt may advantageously be used for forming continuous fibers, such as fibers for reinforcement. Here again, the fibers have shown desirable strength patterns which are not significantly affected by the presence of otherwise undesirable contaminants.