GLASS MELTING TANK FOR FULLY ELECTRIC OPERATION

Description

[0001] The invention relates to a glass melting furnace for fully electric operation, comprising a process area for a glass bath, which has a rectangular cross-section characterized by a width and a depth in the horizontal plane and a height up to which the glass bath is provided, an electrode arrangement projecting into the process area and an outlet opening provided in a lower part of the process area.

[0002] Continuously operated glass melting furnaces have long been known in the art. While fossil fuels, such as gas, were traditionally used for heating, current interest in many applications is shifting towards fully electric heating. Such a fully electric glass melting furnace is operated exclusively by direct electrical resistance heating, provided by an electrode array. The electrodes are immersed in the molten glass bath, utilizing its electrical conductivity. Thus, an electric current flows through the molten glass via the electrodes to achieve electrical heating. Examples of electrode materials for such an array include molybdenum, tungsten, tin oxide, and platinum.

[0003] Prior art has already proposed fully electrically operated glass melting furnaces in which the entire processing process, from raw material to glass ready for further processing, is carried out. For example, it is known to introduce the mixture from which the most homogeneous liquid gas, particularly with a specific viscosity, is to be produced into the glass melting furnace from above, where it forms the first layer of the entire glass bath. The electrodes, which extend into the process area for the glass bath, particularly from above, heat the mixture located beneath this layer, enabling the conversion to glass to take place in this upper part of the process area.Since, under ideal process conditions, any remaining gas bubbles rise more easily than the molten glass sinks to the outlet, a refining process takes place, which also involves homogenization through various agitation processes. The finished, liquid glass can be drawn off through an outlet located in the lower part. For typical tonnage ranges, round glass melting furnaces are commonly used.

[0004] Development efforts are attempting to create fully electric glass melting furnaces with rectangular tanks, offering advantages in terms of design, flexibility, and processable tonnage. For example, a high-capacity, fully electric, rectangular glass melting furnace was proposed, featuring a large width and depth but a low height of approximately one meter. However, significant operational problems have emerged, including an unstable process and the production of low-quality molten glass.

[0005] US4693740 A discloses an electrically heated glass furnace with a length of 1.2 m, a glass melt depth of 0.75 m (ratio 1.6), and an outlet opening at the bottom. The electrodes for electrical heating extend from the bottom of the furnace to the upper part of the glass melt. Burners are optionally provided in a separate refining area.

[0006] CN112830661 A describes a glass furnace. The homogenization zone is 2550-2555 mm long, and the glass melt has a depth of 960 mm or more. There is an outlet for the molten glass at the bottom of the homogenization zone. The furnace is heated by burners and electrodes.

[0007] The invention is therefore based on the objective of providing an improved, rectangular and fully electric glass melting furnace, particularly suitable for the production of glass with different quality requirements, which is stable and operational.

[0008] To solve this problem, according to claim 1, a glass melting furnace for fully electric operation is provided, which has: a process area for a glass bath, which has a rectangular cross-section characterized by a width and a depth in the horizontal direction and a height up to which the glass bath is provided, an upper electrode arrangement projecting into the process area from above and acting in an upper part of the process area with at least one pair of electrodes opposite each other in the depth direction, and an outlet opening provided in a lower part of the process area, wherein the ratio of the depth of the process area to the height of the process area is in the range of 1.2 : 1 to 3 : 1.

[0009] The invention is based on the understanding that the ratios of dimensions, particularly depth to height, are essential for the stability of the process and the resulting end product, namely the molten glass that is drawn off through the outlet. The depth direction corresponds to that in which the (oppositely polarized) electrodes of a pair in the electrode arrangement face each other, particularly when positioned close to the wall of the glass melting tank. Operating the electrodes for heating results in certain currents and movements within the glass bath, especially through convection.Ideally, these elements should promote the melting process and the conversion of the raw materials—that is, the mixture fed from above—into molten glass, ideally, as studies have shown, by having individual raw material particles repeatedly pass through the upper, heated portion of the process area. In the lower portion of the process area, however, a calming of the glass bath is desired, particularly to allow any remaining gas bubbles to rise for refining purposes and to enable the extraction of a homogenized molten glass as the product.

[0010] It has been shown that both the depth and height of the glass bath, and thus the process area, and their relationship play a crucial role, with both parameter ranges potentially having technical limitations. For example, increasing depth can also increase the distance between the electrodes, which can lead to a deepening of the resulting flow pattern, particularly that formed by convection, and to a change in the temperature distribution. Additional flow zones may also emerge. Simultaneously, the height of the glass bath determines the retention of the lower portion and its effect (purification, homogenization / mixing), as defined in claim 1.

[0011] Through analyses of various process configurations using observations and analytical approaches, the invention has now succeeded in determining a depth-to-height ratio of the process area, i.e., the glass bath, that results in high-quality product outcomes. Optimization analyses, particularly using simulations, revealed that quality parameters can be optimized when the depth-to-height ratio of the process area for the glass bath is between 1.3:1 and 1.4:1, and especially 1.375:1. In this process, quality parameters related to residence time, bubble rise, sand dissolution, and mixing were optimized. These parameters are characteristic of both the process itself, particularly its stability, and the product, i.e., the glass melt to be drawn off.There are various ways to define key performance indicators (KPIs), for example, by describing whether gas bubbles rise instead of sinking and whether sand grains or other solid particles in the mixture are completely dissolved. This can be demonstrated, for instance, by tracking bubbles or grains in a simulation. Regarding mixing, observation of schlieren formation can be used, for example.

[0012] It has been shown that in the aforementioned range of 3:1 to 1.2:1, with sufficient quality and process stability maintained, depending on the application, but especially in the range of 1.4:1 to 1.3:1, a physical effect can be observed: the temperature in the glass melting tank, particularly in the lower part of the process range, can be lowered, and the flow becomes calmer, especially in the lower part, with positive effects on refining. This directly results in a multitude of advantages.

[0013] This reduces corrosion of the glass melting furnace, leading to a significant increase in the furnace's service life, for example, to eight years or more. The flexibility of the melting capacity, i.e., with regard to throughput / tonnage, can be improved from 70–100% of the maximum capacity, as is typical for all-electric glass melting furnaces, to 45–100%. A particular advantage is that the cullet yield, previously known for all-electric glass melting furnaces, especially for container glass, can be increased to over 80% while maintaining stable operation.

[0014] The described considerations were primarily carried out for soda-lime glass, but can, of course, be transferred to other glasses, possibly with slight modifications to the depth-to-height ratio, so that the invention can also be advantageously applied to, for example, borosilicate glasses and other glasses. This also applies, in comparison to known glass melting furnaces, with regard to the sizes and size ratios mentioned here and discussed further below.

[0015] In addition to the advantages gained through the targeted selection of dimensional ratios, the use of rectangular furnaces offers the further advantage of this approach. Since there is initially no limitation on the width of the process area for the glass bath, and thus also on the width of the glass melting furnace itself, the glass melting furnace can be easily enlarged while maintaining largely consistent high quality, as will be discussed in more detail below, and the stability of the process is still controlled. Particularly in light of these considerations, the glass melting furnace according to the invention can be designed for high tonnages, preferably for a tonnage of at least 50 tons per day (tpd). Extremely high throughputs are therefore possible with the glass melting furnaces according to the invention, for example, up to 400 tpd and more with high glass quality.When combined with the given flexibility regarding throughput, for example, varying throughputs in ranges of 45 to 100 tpd and / or 180 to 400 tpd can be achieved with a single glass melting furnace.

[0016] Within the scope of the present invention, a balancing of interests can ultimately be undertaken. While it can be assumed that the closer one gets to the ideal range, particularly a depth-to-height ratio of 1.3:1 to 1.4:1, especially 1.375:1, the higher the quality of the glass product, which can then, for example, exceed product quality requirements, depth and height, which will be discussed in more detail below, are often technically limited. This means that additional tonnage would have to be provided across the width, where extremely high values ​​are not always practical. Therefore, depending on the specific quality requirements for the glass, excellent and particularly advantageous compromises can be achieved.

[0017] For example, excellent results in terms of quality requirements are achieved in the production of tableware, especially opal glass, when a process depth-to-glass bath height ratio of 2.1:1 to 2.3:1 is selected. For the production of container glass, such as bottles for the beverage industry, a process depth-to-glass bath height ratio of 2.3:1 to 2.6:1 has proven advantageous and more than meets the quality requirements.

[0018] While the quality requirements for the production of cosmetic glass, such as bottles and the like, are already very well met with a process bath depth-to-height ratio of 2.3:1 to 2.0:1, further improvements in product quality, high process stability, high throughput flexibility, and a high possible cullet content in the batch are achieved with process bath depth-to-height ratios below 2:1, for example, in the range of 2.0:1 to 1.5:1. A further improvement in quality is possible if the ratio is reduced below 1.5:1, although the method of supplying electrical power to the melting unit then needs to be optimized.Choosing too shallow a process depth can lead to short distances between the electrodes of at least one electrode pair, resulting in insufficient resistance and excessively high, undesirable currents.

[0019] In summary, it can therefore be said that, particularly depending on the quality requirements, the ratio of depth to height in the glass melting furnace according to the invention can be at most 2.6 : 1, in particular at most 2.0 : 1, preferably at most 1.5 : 1.

[0020] Generally speaking, with regard to the supply of the mixture, i.e., the raw materials, in the glass melting furnace according to the invention, it can also be provided that the mixture is fed in from above, for example via at least one conveyor belt. After the molten glass, i.e., the product, is drawn off through the at least one outlet opening, it can be further processed in various ways, as is generally known, for example to achieve certain shapes.

[0021] In a suitable embodiment, the process area can be designed to end 50 to 150 mm, particularly 100 mm, below the upper edge of the glass melting tank. The height of the glass melting tank itself around the process area is often referred to as the palisade height. For technical reasons, a certain vertical distance should exist between the height of the process area for the glass bath (i.e., the glass bath height) and this palisade height, preferably 100 mm.

[0022] Regarding the height of the process area, it has proven advantageous to select a maximum height of three meters and / or a minimum height of 1.9 meters, preferably between 1.9 and 3.0 meters. For example, if the palisade height of the glass melting furnace is three meters, the glass bath height can be 2.9 meters; with the commonly used palisade height of two meters, the glass bath height of the process area can be 1.9 meters.

[0023] Regarding the depth of the process area (and thus the interior of the glass melting furnace), it can be specified that it is a maximum of six meters and / or a minimum of three meters. Depths exceeding six meters can lead to undesirable effects on heat distribution during heating by the upper electron array, and, in particular, to undesirable flow patterns, such as eddies and / or excessively large openings in the edge regions. Therefore, the depth is expediently chosen to be a maximum of six meters. The depth can be limited to three meters at the bottom, particularly to avoid excessively close proximity of electrodes of an opposing electrode pair, and thus also to prevent excessively high currents and the like.In individual cases, especially when small ratios between depth and height are desired, it may also be advantageous to choose smaller depths, provided the electrode arrangement and its operation are appropriately adapted, if, for example, smaller heights are also required.

[0024] Regarding width, there are initially no restrictions from the perspective of the stability of the driving process, meaning that ultimately any width can be chosen. In practical configurations, the width can range from 3 to 30 meters, although greater widths are also conceivable, for example, up to 40 meters or more. Widths of more than 6 meters, and especially more than 10 meters, are particularly useful for achieving practical tonnage capacities.

[0025] In a simpler, less preferred embodiment, the ratio of the process area's width to its depth can be between 1:1 and 3:1, whereas in the prior art this ratio was typically determined by the resistance characteristics in the glass bath and the glass melting tank, as well as by optimizing the electrical power input (electrode positions). However, such a limitation can be circumvented, particularly by using multiple pairs of electrodes. Generally speaking, various types of such electrode pairs can also be employed, for example, double or quadruple electrodes. The use of different electrode pairs allows for the provision of correspondingly assigned sections or quadrants in the width direction.

[0026] A particularly advantageous embodiment of the present invention provides that the glass melting tank comprises at least one, and in particular modular, width section defined by at least one pair of electrodes of the upper electrode arrangement, containing the at least one pair of electrodes. In this way, rectangular tanks such as the glass melting tank according to the invention can be easily enlarged while maintaining largely consistent high quality, and, moreover, the process stability remains controllable by adhering to the appropriate ratios of the depth of the process area to the height of the process area. For example, further width sections can be added in the width direction, whereby the ratio of the depth of the process area to the height of the process area, i.e., of the glass bath, is maintained, and the resistance characteristics do not change significantly due to the identical design of the width sections with respect to the electrodes.For example, at least three, and in particular at least five, successive width sections can be encompassed by the glass melting furnace. This allows, for instance, the effective working range of high-throughput glass melting furnaces of, say, 50 tpd to be increased to over 400 tpd while maintaining the same high glass quality and process stability. Particularly when using a large number of width sections, it may be necessary to provide more than one outlet for venting the molten glass.

[0027] As is generally known in the prior art, the electrodes of the upper electrode arrangement can also be swivelled out of the process area, i.e. the glass bath, by means of a swiveling device within the scope of the present invention.

[0028] In a further advantageous embodiment of the present invention, the glass melting tank may, in addition to the upper electrode arrangement, also have a lower electrode arrangement located at the bottom and / or laterally, acting on the lower portion of the process area. Thus, in this embodiment of the present invention, heating can also be effected, at least temporarily and / or partially, via at least one further lower electrode arrangement, for example, comprising bottom and / or horizontal electrodes. The combination and power distribution of the various electrode arrangements used for heating can be dependent on various parameters, such as the viscosity of the glass, and can also be adapted to the height of the glass bath, i.e., the process area.For example, additional lower electrode arrangements can also be used during commissioning, for example to prepare an initial glass melt from below, while during continuous operation, when the batch top layer has been formed, heating can be carried out mainly or even exclusively via the upper electrode arrangement.

[0029] Further advantages and details will become apparent from the exemplary embodiments described below and from the drawings. These show: Fig. 1 a basic cross-sectional view of a glass melting tank according to the invention, Fig. 2 a basic top view of a glass melting tank according to the invention, Fig. 3 a schematic diagram to illustrate flow conditions in the glass melting tank, Fig. 4 a schematic diagram of a glass melting tank having three width sections, and Fig. 5 a schematic diagram of a glass melting tank having five width sections.

[0030] Fig. 1 Figure 1 shows a general cross-sectional view of a glass melting furnace 1 according to the invention, in which a process area 4 for a glass bath is formed within a base 2 and lateral walls 3, the height 5 of which is somewhat lower, for example 50 to 150 mm lower, than the palisade height 6 of the wall 3. Preferably, the height 5 of the process area 4 is 100 mm lower than the palisade height 6. The process area 4 also has a depth 7, which is Fig. 1 The electrodes extend in the transverse direction. In the depth direction, electrodes 8 of an electrode pair from an upper electrode arrangement 9 project from above into the process area 5, which, during operation, is the glass bath. The electrodes 8 can also be pivoted out of the process area 4 or the glass bath by means of a pivoting device. The electrodes 8 are positioned opposite each other along the depth 7 in the area of ​​the walls 3 and, with corresponding opposite polarity, serve to generate an electric current through the glass bath and thus heat it. They act on an upper portion of the process area 4. Through an outlet opening 10, molten glass or liquid glass can be drawn off from the lower portion of the process area 4 into a corresponding guide channel 11.

[0031] Fig. 2 Figure 1 shows a top view of the glass melting tank 1, in which the depth 7 and the width 12 of the process area 4 can also be seen. In the present example, four electrode pairs, thus eight electrodes 8, of the upper electrode arrangement 9 are used. As in Fig. 1 As indicated in the lower part of the process area 4, additional electrodes 13 of a lower electrode arrangement 14 can optionally be provided, which can protrude, for example, from the floor 2 or from the lateral walls 3.

[0032] In glass melting furnace 1, the ratio of the depth 7 of process area 4 to the height 5 of process area 4 is in the interval from 1.2 : 1 to 3 : 1. Here, a ratio below 2.0 : 1, in particular approximately 1.5 : 1, is shown.

[0033] Such a depth-to-height ratio of 7 to 5 results in a stable process flow and high-quality glass production. In particular, as shown in Fig. 3 As schematically explained, the flow generated by the operation of the electrodes 8, which arises primarily from convection, is located mainly in the upper portion 15 of the process area 4, while a calmer flow occurs in the lower portion 16, which has a positive effect on refining. The temperature in the glass melting tank 1 can also be lowered, particularly in the lower portion 16 of the process area 4. This leads to a reduction in corrosion, an increase in the flexibility of the throughput (melting capacity), and an increase in the possible cullet fraction.

[0034] The height 5 is preferably in the range of 1.9 to 3.0 meters, particularly 2.8 to 3.0 meters. The depth is preferably in the range of 3 to 6 meters, for example 4.5 to 6.0 meters. It has been shown that with increasing quality requirements, lower ratios of depth 7 to height 5 become more advantageous, so that, for example, for tableware and container glass, glass qualities exceeding the requirements are obtained even with ratios of 2.0 : 1 to 2.5 : 1 or 2.6 : 1, while for some applications, such as cosmetic glass, the ratio can be chosen closer to an optimum in the range of 1.3 : 1 to 1.4 : 1, for example between 1.5 : 1 and 2.0 : 1 or even below 1.5 : 1.

[0035] The width 12, on the other hand, is relatively freely scalable, so that despite technical limitations in height 5 and depth 7, glass melting furnaces 1 with high tonnages, for example up to 400 tpd or even more, can be achieved. This is particularly advantageous for configurations such as those found in Fig. 4 and Fig. 5 shown, it is possible in which the glass melting tank 1 is divided into modular width sections 17, each of which is assigned an electrode pair 8. By adding further width sections 17, for example of three in Fig. 3 to five in Fig. 5The tonnage can be increased while maintaining at least essentially the same glass quality and process stability. For example, if a tonnage of 240 tpd is achieved with a depth of 6.0 meters and a width of 14.3 meters using three width sections of 17, a tonnage of 400 tpd can be achieved using five width sections of 17 (and an area of ​​6 by 23.4 meters).

[0036] The following are some examples of specific dimensions for glass melting furnaces 1 according to the invention. For example, for a glass melting furnace 1 designed for a tonnage of 80 tpd, a depth 7 of 3.8 meters and a width 12 of 7.6 meters can be used with a process area 4 height 5 of 2.5 meters to obtain a depth-to-width ratio of 1.52. A design with a depth 7 of 3.45 meters and a width 12 of 8.4 meters is also conceivable, resulting in a depth-to-height ratio of 1.38. For a glass melting furnace 1 for a tonnage of 90 tpd, for example, a depth 7 of 4.0 meters with a height 5 of 1.9 meters and a width 12 of 8.0 meters can lead to a depth-to-height ratio of 2.1. With a height of 5 of 2.5 meters, a depth of 7 of 4.0 meters and a width of 12 of 9.0 meters, the depth-to-height ratio is 1.74.A depth-to-height ratio of 1.38 results with a height of 5 of 2.5 meters, a depth of 7 of 3.45 meters and a width of 12 of 9.3 meters.

[0037] Even for larger tonnages, concrete examples of possible implementations exist. For instance, with a depth 7 of 5.75 meters and a height 5 of 2.5 meters, i.e., a depth-to-height ratio of 2.3:1, a width 12 of 14.9 meters can be provided to achieve tonnages of 240 tpd. If three width sections 17 were used previously, and five width sections 17 are used instead, a tonnage of 400 tpd is achieved, for example. Similarly, with depths 7 of 3.5 meters and heights 5 of 2.5 meters, i.e., a ratio of 1.4:1, high tonnages can be achieved modularly using width sections 17, for example, 185 tpd with three width sections 17 and 400 tpd with nine width sections 17, even if this results in widths 12 of around 40 meters.

[0038] It should be noted that, particularly in the case of a large number of width sections 17, for example nine width sections 17, several outlet openings 10 may also be provided.

Claims

1. Glass-melting tank (1) for fully electrical operation, having - a process region (4) for a glass bath, which process region has a rectangular cross-section, characterized by a width (12) and a depth (7), in a horizontal plane and has a height (5) up to which the glass bath is provided, - an upper electrode arrangement (9) with at least one pair of electrodes (8) situated opposite one another in the depth direction, which upper electrode arrangement projects in particular from above into the process region (4) and acts in an upper part (15) of the process region (4), and - an outlet opening (10) which is provided in a lower part (16) of the process region (4), serving for fining and homogenization, and is used for withdrawal of a homogenized glass melt, - wherein the ratio of the depth (7) of the process region (4) to the height (5) of the process region (4) is in the range of 1.2: 1 to 3: 1.

2. Glass-melting tank (1) according to Claim 1, characterized in that said glass-melting tank is designed for a tonnage of at least 50 tonnes / day and / or for a use for soda-lime glass.

3. Glass-melting tank (1) according to Claim 1 or 2, characterized in that the ratio of the depth (7) to the height (5) is at most 2.6: 1, in particular at most 2: 1, in particular at most 1.5: 1.

4. Glass-melting tank (1) according to one of the preceding claims, characterized in that the process region (4) ends 50 to 150 mm, in particular 100 mm, below the upper edge of the glass-melting tank (1) and / or the height (5) is at most 3 metres and / or at least 1.9 metres.

5. Glass-melting tank (1) according to one of the preceding claims, characterized in that the depth (7) is at most 6 metres and / or at least 3 metres.

6. Glass-melting tank (1) according to one of the preceding claims, characterized in that the width (12) is in a range of 3 to 30 metres.

7. Glass-melting tank (1) according to one of the preceding claims, characterized in that said glass-melting tank comprises at least one in particular modular width section (17) defined by at least one pair of electrodes (8) of the upper electrode arrangement (9) and containing the at least one pair.

8. Glass-melting tank (1) according to Claim 7, characterized in that said glass-melting tank comprises at least three, in particular at least five, width sections (17) following one another in the width direction.

9. Glass-melting tank (1) according to one of the preceding claims, characterized in that the electrodes (8) of the upper electrode arrangement (9) are able to be pivoted out of the process region (4) by means of a pivoting device.

10. Glass-melting tank (1) according to one of the preceding claims, characterized in that said glass-melting tank further has a bottom-side and / or lateral lower electrode arrangement (14) acting on the lower part (16) of the process region (4).