Glass melting device and glass manufacturing method

Abstract

This glass melting device comprises: a melting tank that stores molten glass; and an electrode that energizes and heats the molten glass. The molten glass has an electric resistivity of 0.2 Ωm or more at 1600°C. The melting tank has a first section that surrounds the molten glass and a second section that is disposed below the first section and surrounds the molten glass with an inner cross-sectional area smaller than that of the first section, a step section being formed between the first section and the second section. The electrode is disposed so as to protrude, from a hole provided in the step section, toward the inner side of the first section. The step section includes a first furnace material that surrounds at least the hole and a second furnace material that constitutes a corner section of the inner side of the step section. The first furnace material has a higher electric resistivity than the molten glass, and the second furnace material has higher corrosion resistance than the first furnace material.

Description

Glass melting apparatus and glass manufacturing method

[0001] This application claims priority to Japanese Patent Application No. 2024-065887, filed April 16, 2024, the contents of which are incorporated herein by reference.

[0002] Patent Document 1 describes a fully electric melting furnace for melting glass, which includes a melting tank having an upper pool 2 and a lower pool 3 having a smaller internal cross-sectional area than the upper pool 2, with the upper pool 2 and the lower pool 3 connected via transition bricks 6. The transition bricks 6 are provided with a plurality of electrodes 7, which electrically heat the molten glass in the melting tank. A melting tank with this type of structure is also called a double-decker type (two-story type).

[0003] Patent Document 2 describes a melting furnace for producing molten glass by electrical heating, the furnace having a plurality of electrodes at its bottom. The bottom of the melting furnace includes first bricks surrounding the electrodes and second bricks arranged between the first bricks, the first bricks having better corrosion resistance than the second bricks, and the second bricks having higher electrical resistivity than the first bricks.

[0004] Chinese Patent No. 1217867 International Publication No. 2019 / 004434

[0005] Electric melting furnaces are also used to produce glass with higher electrical resistivity than typical soda-lime glass, such as alkali-free glass. When electrically heating molten glass with high electrical resistivity, current can flow into the furnace materials surrounding the electrodes under certain conditions. This can result in insufficient electrical heating and a deterioration in the quality of the resulting glass. To avoid this phenomenon, it is possible to use furnace materials with high electrical resistivity, such as dense zircon sintered bricks, for the furnace materials surrounding the electrodes. However, such furnace materials with low electrical resistivity have relatively low corrosion resistance at high temperatures. Therefore, continued operation at high temperatures to produce high-quality glass is likely to erode melting tanks made of dense zircon sintered bricks, shortening the equipment's lifespan. In particular, double-decker melting tanks are prone to erosion at high temperatures due to the shape of the tank, which can locally increase the molten glass velocity. Therefore, there is a need for a glass melting apparatus that can operate at high temperatures to ensure glass quality while maintaining a long lifespan.

[0006] In view of the above, one aspect of the present disclosure provides a technology that can maintain the life of a glass melting apparatus without reducing the quality of the glass produced.

[0007] One aspect of the present disclosure is a glass melting apparatus including a melting tank that accommodates molten glass and electrodes that electrically heat the molten glass, wherein the molten glass has an electrical resistivity of 0.2 Ωm or more at 1600°C, the melting tank having a first portion surrounding the molten glass and a second portion that is disposed below the first portion and has an inner cross-sectional area smaller than that of the first portion and surrounds the molten glass, a step portion is formed between the first portion and the second portion, the electrode is disposed so as to protrude into the first portion from a hole formed in the step portion, the step portion includes at least a first furnace material that surrounds the hole and a second furnace material that forms an inner corner of the step portion, the first furnace material has a higher electrical resistivity than the molten glass, and the second furnace material has a higher corrosion resistance than the first furnace material.

[0008] According to one aspect of the present disclosure, the life of the glass melting equipment is maintained without reducing the quality of the glass produced.

[0009] 9 is a cross-sectional view of a glass melting apparatus according to an embodiment of the present disclosure; FIG. 9 is a cross-sectional view of a glass melting apparatus according to a reference example; FIG. 10 is a top view of the melting tank shown in FIG. 1; FIG. 11 is an enlarged view of a portion of the melting tank shown in FIG. 1 near a step portion; FIG. 12 is a top view of a shelf-like member; FIG. 9 is a cross-sectional view of a melting tank according to a modified example, and corresponds to FIG. 4; FIG. 9 is a cross-sectional view of a melting tank according to another modified example, and corresponds to FIG. 4; FIG. 9 is a cross-sectional view of a melting tank according to yet another modified example, and corresponds to FIG. 4; FIG. 9 is a perspective view of a melting tank according to a modified example, having a rectangular cross-sectional shape; FIG. 9 is a cross-sectional view taken along line A-A of FIG.

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding components are denoted by the same reference numerals, and descriptions thereof may be omitted.

[0011] First, the basic structure of a glass melting apparatus according to one embodiment will be described with reference to Fig. 1. The glass melting apparatus 10 is an apparatus for melting glass raw material G1 to produce molten glass G2. The produced molten glass G2 is subjected to subsequent processes such as forming and annealing to produce glass, and the obtained glass can be further subjected to processes such as cutting and polishing to produce a desired glass article.

[0012] As shown in Fig. 1, the glass melting apparatus 10 includes a melting tank 20 that accommodates molten glass G2 and electrodes 30 that electrically heat the molten glass G2. The glass melting apparatus 10 according to this embodiment is preferably a fully electric melting furnace that melts glass frit G1 only by electrically heating the molten glass G2. The fully electric melting furnace is an apparatus that has only a plurality of electrodes 30 as a heat source for melting the glass frit G1. However, the glass melting apparatus 10 may also include auxiliary heating means other than the electrodes 30.

[0013] The top of the melting tank 20 may be open, allowing frit G1 to be introduced from above. The frit G1 is prepared by mixing multiple types of materials. The frit G1 may contain a fining agent. The frit G1 may also contain glass cullet obtained from discarded glass in order to recycle glass. The frit G1 may be powdered raw material or granulated raw material obtained by granulating powdered raw material. The frit G1 is determined depending on the desired glass composition. The introduced frit G1 forms a layer on the liquid surface LS of the molten glass G2. The frit G1 gradually melts due to heat transferred from the molten glass G2. Therefore, as shown in FIG. 1 , during operation of the glass melting apparatus 10, the melting tank 20 may contain frit G1 and molten glass G2. The molten glass G2 gradually moves downward and is removed from the outlet 24.

[0014] The outlet 24 may be connected to a conduit 25 as shown in Fig. 1. The molten glass G2 flowing out of the outlet 24 moves through the conduit 25 and is subjected to a subsequent process. The shape of the conduit 25 is not particularly limited, and may extend linearly or may be bent as shown in Fig. 1 depending on the position of a downstream device.

[0015] The layer of frit G1 formed in the upper part of the melting tank 20 covers the liquid surface LS of the molten glass G2. This layer can suppress the escape of heat and / or volatile components from the molten glass G2. In the cold-top system, the layer of frit G1 may cover preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more of the area of ​​the liquid surface LS of the molten glass G2. The layer of frit G1 may also substantially cover the entire liquid surface LS of the molten glass G2. The maximum surface temperature of the layer of frit G1 is preferably 500°C or less, more preferably 350°C or less. Although FIG. 1 shows the use of the glass melting apparatus 10 in a cold-top system, the glass melting apparatus 10 may also be used in other systems, such as a hot-top system.

[0016] The molten glass G2 may have an electrical resistivity (specific resistance) of 0.2 Ωm or more at 1600° C. An example of the molten glass G2 having such a high electrical resistivity is alkali-free glass. The alkali-free glass is a glass containing Na ２ O.K. ２ The term "glass substantially free of alkali metal oxides" refers to glass that is substantially free of alkali metal oxides such as 0.1% by mass or less in total. The electrical resistivity of the molten glass G2 at 1600°C may be 2.5 Ωm or less.

[0017] The alkali-free glass is, for example, SiO ２ 54% to 73%, Al ２ O ３ 10% to 23%, B ２ O ３ The content of B is 0.1% to 12%, MgO is 0% to 12%, CaO is 0% to 15%, SrO is 0% to 16%, BaO is 0% to 15%, and the total content of MgO, CaO, SrO, and BaO is 8% to 26%. ２ O ３ , MgO, CaO, SrO, and BaO are not essential components but are optional components. From the viewpoint of achieving both a high strain point and high solubility, the alkali-free glass should contain, in mass % on an oxide basis, SiO ２ 58 to 66%, Al ２ O ３ 15-22%, B ２ O ３ It is preferable that the composition contains 5 to 12% of SiO, 0 to 8% of MgO, 0 to 9% of CaO, 3 to 12.5% ​​of SrO, 0 to 2% of BaO, and a total of 9 to 18% of MgO, CaO, SrO, and BaO. Furthermore, when it is desired to obtain a particularly high strain point, it is preferable that the composition contains, in mass % on an oxide basis, SiO ２ 54 to 73%, Al ２ O ３ 10.5 to 22.5%, B ２ O ３From the viewpoint of obtaining low dielectric loss or low transmission loss, the alkali-free glass preferably contains, in mass % on an oxide basis, 0.1 to 5.5% of SiO, 0 to 10% of MgO, 0 to 9% of CaO, 0 to 16% of SrO, 0 to 2.5% of BaO, and a total of 8 to 26% of MgO, CaO, SrO, and BaO. ２ 48 to 59%, Al ２ O ３ 5 to 20%, B ２ O ３ It is preferable that the content of MgO is 16 to 25%, MgO is 0 to 6%, CaO is 0 to 8%, SrO is 0 to 15%, BaO is 0 to 20%, and the total content of MgO, CaO, SrO and BaO is 10 to 20%.

[0018] The melting tank 20 according to this embodiment is a so-called double-decker melting tank. The melting tank 20 has a first portion P1 and a second portion P2 disposed below the first portion P1. The first portion P1 has a first internal cross-sectional area, and the second portion P2 has a second internal cross-sectional area, which is smaller than the first internal cross-sectional area. In other words, the internal space of the melting tank 20 is narrower at the bottom than at the top. In this specification, the internal cross-sectional area refers to the area of ​​a cross section of the internal space of the melting tank 20 taken along a plane perpendicular to the vertical direction (horizontal direction). The side of the internal space of the melting tank 20 is referred to as the inside, and the side facing the outside of the melting tank 20 is referred to as the outside.

[0019] FIG. 3 shows a top view of the glass melting apparatus 10 of FIG. 1 . As shown in FIG. 3 , the first portion P1 located at the top of the melting tank 20 has a rectangular cylindrical shape with a polygonal cross section. An axis extending in the vertical direction and passing through the center of the polygon of the first portion P1 may be referred to as the central axis CA. The second portion P2 may have a bottomed shape with a rectangular cylindrical cross section. While the centers of the first portion P1 and the second portion P2 may be offset from each other when viewed from above, as shown in FIG. 3 , it is preferable that the center of the second portion P2 coincides with the center of the first portion P1, i.e., passes through the central axis CA. The direction from the central axis CA toward the outside, or the direction from the outside toward the central axis CA, may be referred to as the inward / outward direction.

[0020] The cross-sectional shape of the first portion P1 is hexagonal in the embodiment shown in Fig. 3, but may be a polygon other than a hexagon, for example, a triangle to a pentagon, a heptagon to a dodecagon, or a shape having ten or more triangles. The polygon may be either an even-numbered polygon or an odd-numbered polygon. From the viewpoint of the symmetry of the temperature distribution of the molten glass G2, the polygon is preferably a regular polygon or a polygon with chamfered corners (a polygon having alternating long and short sides). By chamfering the corners of the regular polygon, it is possible to reduce areas that cannot be electrically heated (dead space).

[0021] As described above, the difference in the internal cross-sectional area between the first portion P1 and the second portion P2 creates a step between the first portion P1 and the second portion P2. That is, a step ST is formed between the first portion P1 and the second portion P2. In this specification, the step ST is a portion extending horizontally, more specifically, from the central axis CA of the melting tank 20 toward the outside of the melting tank 20. The step ST has an inner end face exposed to the inside of the melting tank 20 and an outer end face exposed to the outside of the melting tank 20. When viewed from above, the step ST is an annular portion protruding inward from the melting tank 20, and the corner STe of the step ST is also annular ( FIG. 3 ). Although the corner STe is shown in FIG. 1 as a sharp, right-angled portion in cross section, the corner STe may be rounded or chamfered. Note that the first portion P1, the second portion P2, and the step portion ST are convenient divisions for explaining the shape of the melting tank 20, and do not necessarily represent components (parts) that are combined to manufacture the melting tank 20.

[0022] 1, the step ST extends in a substantially horizontal direction, but the step ST may be inclined so that the step ST approaches the lower side of the melting tank 20 as it extends inward. In this specification, the term "substantially horizontal" includes not only the strictly horizontal direction but also a direction inclined by ±5° from the horizontal direction.

[0023] The step portion ST may be supported from below by a support member (not shown) provided on the outside of the second portion P2, in which case the load of both the step portion ST and the first portion P1 disposed thereon is supported.

[0024] A plurality of electrodes 30 for electrical heating are provided on the step ST. More specifically, the electrodes 30 are rod-shaped and disposed in holes H formed in the step ST, protruding from the holes H into the inside of the melting tank 20. The electrodes 30 may protrude directly upward from the step ST, or may protrude obliquely upward from the step ST as shown in FIG. 1. When the electrodes 30 are inclined, the inclination may be 5 to 60 degrees relative to the vertical direction.

[0025] In the embodiment shown in FIG. 3 , the electrodes 30 are provided near each vertex of the polygon of the first portion P1. By providing the electrodes 30 near the vertices of the polygon, it is possible to reduce areas that cannot be heated by electrical current (dead space). The cross-sectional shape of the first portion P1 may be a shape with uneven side lengths, such as a rectangle. In such a case, in order to obtain an appropriate distance for the electrical current path 31, the electrodes 30 may be provided between the vertices on the long sides in addition to positions other than near the vertices, as in the embodiment shown in FIG. 9 (the embodiment of FIG. 9 will be described later). The material of the electrodes 30 is not particularly limited, but may be, for example, molybdenum (Mo).

[0026] The electrodes 30 may each be held in an electrode holder 40. The electrode holder 40 is provided in a hole H in the step portion ST and may have the function of holding the outer periphery of the electrode 30 as well as the function of cooling the electrode 30 and the furnace material surrounding the electrode 30. The cooling function of the electrode holder 40 can prevent the molten glass G2 from leaking out of the melting tank 20 through the hole H. A coolant such as water is supplied to the electrode holder 40, and the coolant dissipates heat from the electrode holder 40 to the outside. The electrode holder 40 may also hold the lower end of the electrode 30.

[0027] When the glass melting apparatus 10 is used to produce glass with high electrical resistivity, such as the aforementioned alkali-free glass, electrical leakage to the melting tank 20 (a phenomenon in which current flows through the wall material of the melting tank 20) ​​is likely to occur. To prevent this, the first furnace material br1, which has a higher electrical resistivity than the molten glass G2, can be used as the material for the step ST. It is preferable that at least the portion surrounding the electrode 30 be made of the first furnace material br1. FIG. 2 shows a cross-sectional view of a glass melting apparatus 110 according to a reference example. The conduit 25 is not shown in the glass melting apparatus 110 of FIG. 2. In the glass melting apparatus 110 according to the reference example shown in FIG. 2, the entire step ST is made of the first furnace material br1, which has a higher electrical resistivity than the molten glass G2. The configuration of the reference example, in which the entire step ST is made of a single type of furnace material, is preferable from the viewpoints of reducing the complexity of manufacturing the melting tank 20 and more reliably preventing electrical leakage over a wide area. In this specification, the term "wall member" refers to a member that constitutes the dissolution tank 20, and includes a first side wall member 21, a second side wall member 22, a shelf-like member 23, and a bottom wall member 26, which will be described later.

[0028] However, the first furnace material br1, which has a higher electrical resistivity than the molten glass G2, often has low corrosion resistance at high temperatures. Furthermore, in a double-decker melting tank with a step ST, the flow velocity of the molten glass G2 is fast near the step ST, particularly near the inner corner STe of the step ST. Therefore, in a double-decker melting tank, the step ST, especially the corner STe, is susceptible to the force of the flow of the molten glass G2 and is susceptible to erosion. Therefore, in a melting tank 120 ( FIG. 2 ) of the glass melting apparatus 110 according to the reference example, in which the entire step ST is made of furnace material br1 and the corner STe of the step ST is also made of furnace material br1, corrosion is likely to progress from the corner STe of the step ST. In other words, the corrosion resistance of the corner STe may be the rate-limiting factor for the life of the melting tank 20. On the other hand, if the operating conditions such as the temperature and flow of the molten glass G2 are changed in order to prioritize maintaining the life of the melting tank 20, the quality of the molten glass, and ultimately the quality of the glass obtained from the molten glass, may not be sufficient.

[0029] Referring again to FIG. 1 , in a glass melting apparatus 10 according to one embodiment of the present disclosure, a step ST in a melting tank 20 includes a first furnace material br1 having a higher electrical resistivity than molten glass G2 and a second furnace material br2 different from the first furnace material br1. The second furnace material br2 is made of a material having higher corrosion resistance against molten glass G2 than the first furnace material br1. As shown in FIG. 1 , the first furnace material br1 constitutes at least an area surrounding the electrode 30, and the second furnace material br2 constitutes an inner corner STe of the step ST. In this embodiment, the second furnace material br2, which has high corrosion resistance, constitutes the corner STe, which is susceptible to the force of the flow of molten glass G2 and prone to deterioration. This improves the corrosion resistance of the corner STe. Therefore, the life of the melting tank 20 can be maintained without changing operating conditions such as temperature.

[0030] The electrical resistivity of the first furnace material br1 may be larger than that of the molten glass G2, and may be, for example, preferably 5 Ωm or more, more preferably 10 Ωm or more at 1600°C. The electrical resistivity of the first furnace material br1 may be 100 Ωm or less at 1600°C. A specific example of the first furnace material br1 is dense zircon fired brick. Dense zircon fired brick is mainly made of zircon (ZrSi ４ ). The zircon content of the dense zircon fired bricks is, for example, 95% by mass or more. In this way, the first furnace material br1 contains a material with a higher electrical resistivity than the molten glass G2, which prevents unintended current from flowing through the furnace material of the melting tank 20 during electrical heating. Therefore, the molten glass G2 can be heated to a desired temperature, ensuring high quality of the resulting glass.

[0031] The second furnace material br2 is a material that has higher corrosion resistance against molten glass G2 at high temperatures, for example, 1500°C or higher, than the first furnace material br1. The second furnace material may be electroformed brick, and specific examples thereof include zirconia-based electroformed brick, alumina-based electroformed brick, alumina-zirconia-based electroformed brick, and AZS (Al-Zr-Si) electroformed brick. Among these, zirconia electroformed brick is preferred because of its high corrosion resistance against molten glass G2, and high-zirconia electroformed brick is more preferred. The high-zirconia electroformed brick has a zirconia content of, for example, 90% by mass or more.

[0032] The first part P1 and the second part P2 connected via the step ST may include a third furnace material br3. The third furnace material br3, like the second furnace material br2, may be made of a material having higher corrosion resistance to molten glass G2 than the first furnace material br1, preferably electrocast bricks, more preferably high-zirconia electrocast bricks. The third furnace material br3 and the second furnace material br2 may be made of different materials, or may be made of the same material.

[0033] The conduit 25 may be made of a fourth furnace material br4. The fourth furnace material br4 may be made of a material different from the first furnace material br1, the second furnace material br2, and the third furnace material br3, or may be made of the same material as any of the first furnace material br1, the second furnace material br2, and the third furnace material br3. For example, the fourth furnace material br4 may be made of a furnace material having a higher electrical resistivity than the molten glass G2, preferably dense zircon fired brick, more specifically, mainly made of zircon (ZrSi ４ ) may be a brick containing.

[0034] The specific configuration of the melting tank 20 will be described below. As shown in FIG. 1 , the melting tank 20 is composed of multiple connected components. The melting tank 20 includes a first sidewall member 21 that surrounds the internal space of the first portion P1 of the melting tank 20, a second sidewall member 22 that surrounds the internal space of the second portion P2 of the melting tank 20, and a shelf-like member 23 that connects the lower end of the first sidewall member 21 to the upper end of the second sidewall member 22. The first sidewall member 21 has a first internal cross-sectional area, and the second sidewall member 22 has a second internal cross-sectional area that is smaller than the first internal cross-sectional area. Furthermore, a bottom wall member 26 is connected to the bottom of the second sidewall member 22. The first sidewall member 21, the second sidewall member 22, and the shelf-like member 23 may be surrounded by a thermal insulation member (e.g., thermal bricks) on the outside.

[0035] Figure 4 is an enlarged view of the shelf-like member 23 and its surrounding area of ​​the dissolution tank 20 shown in Figure 1, with the electrodes 30 and electrode holders 40 omitted. The first sidewall member 21, the second sidewall member 22, and the shelf-like member 23 may each include multiple small members connected laterally. It is also preferable that the first sidewall member 21 and the second sidewall member 22 are each undivided members when viewed vertically. The lines between members in Figure 4 indicate the positions where the members abut and connect to each other.

[0036] In the embodiment shown in FIG. 4 , the step ST is formed of a shelf-like member 23. The shelf-like member 23 is a member extending in the horizontal direction, and an inner end surface 231 of the shelf-like member 23 is exposed inside the melting tank 20, and the inner end of the shelf-like member 23 forms a corner STe of the step ST. The shelf-like member 23 includes a first furnace material br1 surrounding at least the hole H and a second furnace material br2 adjacent to the first furnace material br1 and including the corner STe of the step ST. The first furnace material br1 and the second furnace material br2 may have the same thickness, or, as shown in FIG. 4 , the thickness of the second furnace material br2 may be smaller than the thickness of the first furnace material br1. That is, a step is formed on the underside 234 of the shelf-like member 23 at the boundary between the first furnace material br1 and the second furnace material br2. As a result, the outer edge of the lower surface of the second furnace member br2 comes into contact with the inner surface of the first furnace member br1, effectively preventing leakage of molten glass.

[0037] The second furnace material br2 may be provided to a position a distance d2 away from the inner end surface 231 of the shelf-like member 23. The distance d2 may be appropriately set depending on the type of molten glass, the type of furnace material (more specifically, the electrical resistivity of the first furnace material br1, the electrical resistivity of the second furnace material br2, etc.), the arrangement of the electrodes, etc. From the viewpoint of improving corrosion resistance at high temperatures, the distance d2 may be preferably 50 mm or more.

[0038] The boundary line B between the first furnace material br1 and the second furnace material br2 does not necessarily have to be a straight line as shown in Fig. 4, but may be curved (an example will be described later in detail). In addition, the direction of the boundary line B may be parallel to the vertical direction as shown in Fig. 4, but may also be inclined inward or outward at a predetermined angle, for example, 30° or less, from the vertical direction.

[0039] The distance d2 may be set so that the boundary between the first furnace material br1 and the second furnace material br2 contacts the second side wall member 22. That is, the lower end of the boundary line B as seen in a cross section cut in the thickness direction (vertical direction) of the shelf-like member 23 may be on the upper end surface 223 of the second side wall member 22. This reduces the possibility that the molten glass G2 will leak from the inside to the outside of the melting tank 20 through the boundary between the first furnace material br1 and the second furnace material br2.

[0040] In this embodiment, the first furnace material br1 may be provided in an area surrounding at least the electrode 30, i.e., the area surrounding the hole H. However, it is preferable that the first furnace material br1 is provided in an area on the upper surface of the first furnace material br1 (the upper surface 233 of the shelf-like member 23) that extends from the outer end surface 232 to a position at least a distance d1 inward from the hole H.

[0041] As shown in Fig. 4, the shelf-like member 23 preferably comprises the second refractory material br2 and the first refractory material br1. When viewed from above, the second refractory material br2 is preferably arranged in a ring shape at the inner end of the shelf-like member 23, and the remaining portion is preferably composed of the first refractory material br1 (Fig. 3). In Fig. 3, the second refractory material br2 is shown in gray (matt finish).

[0042] The distance d1, i.e., the distance from the inner edge of the hole H on the upper surface 233 of the shelf-like member 23 to the second furnace material br2 (FIG. 4), can be appropriately set depending on the type of molten glass, the type of furnace material, the arrangement of the electrodes, etc. For example, if a current flows from one electrode 30 to an adjacent electrode 30 via the second furnace material br2 and takes a tentative path along which the current would most easily flow, it is preferable to set the distance d1 so that the current is prevented from flowing along the tentative path. Here, a preferred distance d1 will be explained with reference to FIG. 5. FIG. 5 shows a top view of the shelf-like member 23 and the multiple electrodes 30. For ease of explanation, FIG. 5 illustrates only two of the multiple electrodes 30, and the inclination direction of the electrodes is different from that in FIG. 3. As shown in FIG. 5, the distance between the electrodes (the shortest distance when the electrodes are inclined) is D1, the distance of the current path in the second furnace material br2 is D2, and the electrical resistivity of the molten glass is ρ glass , the electrical resistivity of the first furnace material br1 is ρ ＤＺ , the electrical resistivity of the second furnace material br2 is ρ ＨＺ In this case, D1×ρ glass For the value of d1×ρ ＤＺ ×2 + D2 × ρ ＨＺ It is preferable that the value of is sufficiently large. More specifically, the formula (d1×ρ ＤＺ ×2 + D2 × ρ ＨＺ ) ≧ k × (D1 × ρ glass ) may be preferably 5 times or more, more preferably 7 times or more, and even more preferably 10 times or more. By setting k in the above range, it is possible to prevent current from flowing into the second furnace material br2. Furthermore, taking into consideration the scale of the melting tank 20, etc., k may be preferably 500 times or less. Specifically, the distance d1 may be 20 mm or more and 400 mm or less.

[0043] The arrangement of the electrodes 30 in this embodiment is not particularly limited, but it is preferable that the current path 31 formed by electrically connecting two electrodes 30 with wiring 32 be along a side of the polygon of the first portion P1 or the first side wall member 21, as shown in Fig. 3. A transformer 33, for example, may be provided in the middle of the wiring 32. The transformer 33 applies an AC voltage to the multiple electrodes 30. The phase of the AC voltage is adjusted so that a current flows through each current path 31.

[0044] 1, the electrode 30 is inclined in a direction away from the wall of the first portion P1 as it goes upward. By positioning the upper end of the electrode 30 farther from the first sidewall member 21 of the melting furnace 20 than the lower end of the electrode 30, electrical leakage can be further suppressed. This prevents erosion of the melting furnace 20 due to electrical leakage and enables sufficient electrical heating of the molten glass.

[0045] FIG. 6 shows a modified melting tank 20. FIG. 6 corresponds to FIG. 4. The basic structure of the melting tank 20 shown in FIG. 6 is similar to that shown in FIG. 4, but the arrangement of the shelf-like member 23 and the second sidewall member 22 is different. In the example shown in FIG. 6, the inner end surface 231 of the shelf-like member 23 is not exposed to the interior of the melting tank 20, but faces and is connected to the outer surface 222 of the second sidewall member 22. The upper surface 233 of the shelf-like member 23 and the upper end surface 223 of the second sidewall member 22 are flush with each other. Thus, in the example shown in FIG. 6, the corner portion STe of the step portion ST is formed by the upper end portion of the second sidewall member 22.

[0046] In the example shown in FIG. 6 , the entire second sidewall member 22 is formed from the second furnace material br2. However, as long as the corner portion STe is formed from the second furnace material br2, it is also possible to form at least the upper end portion of the second sidewall member 22 from the second furnace material br2, rather than the entire second sidewall member 22, and to form the remaining portion of the second sidewall member 22 from another furnace material. For example, the region from the upper end surface 223 to a position spaced downward by a distance equal to the thickness of the shelf-like member 23 may be formed from the second furnace material br2. However, forming the entire second sidewall member 22 from the second furnace material br2 is preferable because it improves the corrosion resistance of the wall in the lower portion (second portion P2) of the melting tank 20 where the flow rate of the molten glass G2 is relatively high. Furthermore, it is possible to eliminate a material boundary between the corner portion STe of the step portion ST and the bottom wall member 26, thereby reducing the possibility of the molten glass G2 leaking from the inside to the outside.

[0047] FIG. 7 shows another modified melting tank 20. FIG. 7 corresponds to FIG. 4. The basic structure of the melting tank 20 shown in FIG. 7 is similar to that shown in FIG. 4, except for the arrangement of the first and second furnace materials br1 and br2 included in the shelf-like member 23. In the example shown in FIG. 7, the boundary line B between the first and second furnace materials br1 and br2 in a cross section taken along the thickness direction (vertical direction) of the shelf-like member 23 is not linear but curved. More specifically, as shown in FIG. 7, the boundary line B extends downward from the upper surface 233 of the shelf-like member 23, but then bends at a substantially right angle midway to extend outward, and then bends again at a substantially right angle to extend downward. Since the boundary line B between the first furnace material br1 and the second furnace material br2 has such a shape, the length of the path of the boundary line B from the upper surface 233 to the lower surface 234 of the shelf-like member 23 is increased, and the possibility of the molten glass G2 leaking from the inside to the outside at the boundary between the first furnace material br1 and the second furnace material br2 can be reduced.

[0048] In the example shown in Fig. 7, when viewed from the inside-outside direction, the convex portion pt1 of the first refractory material br1 is engaged with the concave portion rs2 of the second refractory material br2, and the concave portion rs1 of the first refractory material br1 is engaged with the convex portion pt2 of the second refractory material br2. This engagement makes it difficult for the first refractory material br1 and the second refractory material br2 to shift vertically, resulting in a melting furnace 20 that is less susceptible to damage from stimuli such as vibrations, compared to the configuration shown in Fig. 4. In the example shown in Fig. 7, the convex portion pt1 of the first refractory material br1 is formed on the upper side, and the convex portion pt2 of the second refractory material br2 is formed on the lower side. However, the first refractory material br1 may have the concave portion rs1 on the upper side and the convex portion pt1 on the lower side, and the second refractory material br2 may have the convex portion pt2 on the upper side and the concave portion rs2 on the lower side, and the first refractory material br1 and the second refractory material br2 may be engaged with each other.

[0049] Figure 8 shows another modified melting tank 20. Similar to the melting tank 20 shown in Figure 6, the inner end surface 231 of the shelf-like member 23 is not exposed and is connected to the second sidewall member 22. Therefore, the corner STe of the step ST is formed by the upper end of the second sidewall member 22. Furthermore, the shelf-like member 23 is entirely made of the first furnace material br1, and the second sidewall member 22 is entirely made of the second furnace material br2.

[0050] The melting furnace 20 shown in Figure 8 differs from the example shown in Figure 6 in that, when viewed in a cross section cut in the thickness direction of the shelf-like member 23, the boundary line between the shelf-like member 23 and the second sidewall member 22, i.e., the boundary line B between the first furnace material br1 and the second furnace material br2, is curved rather than straight. As shown in Figure 8, the boundary line B extends downward from the upper surface 233, bends at a nearly right angle midway, extends outward, and then bends at a nearly right angle again, extending downward. The curved boundary line B between the shelf-like member 23 and the second sidewall member 22 increases the length of the path of the boundary line B when viewed in the cross section in the thickness direction, thereby reducing the possibility of molten glass G2 leaking from the inside to the outside between the shelf-like member 23 and the second sidewall member 22. In other words, in the example shown in Figure 8, the convex portion pt1 of the first furnace material br1 (shelf-like member 23) and the concave portion rs2 of the second furnace material br2 (second sidewall member 22) engage with each other. Therefore, the first furnace material br1 and the second furnace material br2 are less likely to shift in the vertical direction, and the structure is less likely to be damaged by stimuli such as vibrations.

[0051] The embodiment described above is based on a melting tank 20 having a regular polygonal cross section. However, as an example of a glass melting apparatus having a cross section other than that, Fig. 9 shows a perspective view of a glass melting apparatus 10 including a melting tank 20 having a rectangular cross section. Fig. 10 also shows a partial cross section taken along line A-A in Fig. 9.

[0052] Similar to the melting tank 20 having a regular polygonal cross section described above, the melting tank 20 shown in Figure 9 also includes a first portion P1, a second portion P2 located below the first portion P1 and having a smaller internal cross-sectional area than the first portion P1, and a step portion ST formed between the first portion P1 and the second portion P2. Multiple electrodes 30 are disposed protruding into the first portion P1 through holes formed in the step portion ST. The step portion ST includes a first furnace material br1 surrounding at least the hole and having a higher electrical resistivity than molten glass, and a second furnace material br2 forming an inner corner portion STe of the step portion ST and having a higher corrosion resistance than the first furnace material. In Figure 9, the second furnace material br2 is shown in gray (matt finish) as in Figure 3.

[0053] In the embodiment shown in FIG. 9 , a plurality of electrodes 30 are arranged along each long side of the rectangle. This allows a current path 31 of an appropriate length to be formed along the long sides. In this embodiment, the electrodes 30 are not arranged along the short sides. Therefore, as shown in FIG. 9 , the first furnace material br1 for preventing electricity from flowing to the wall member does not need to be provided along the short sides of the step portion ST. The portion along the short sides of the step portion ST can be formed from the second furnace material br2. In this way, the first furnace material br1 and the second furnace material br2 can be appropriately set depending on the arrangement of the electrodes 30. In this embodiment, the third furnace material br3 may also be made of the same material as the second furnace material br2.

[0054] Furthermore, one embodiment of the present disclosure may be a glass manufacturing method including producing molten glass G2 using the above-described glass melting apparatus 10, shaping the obtained molten glass G2, and cooling it to obtain glass. The obtained glass can be further processed into a desired shape to obtain a glass article. The glass article may be, for example, a glass plate for a display such as a flat panel display, a glass plate for electronic devices other than displays, or a laboratory instrument. However, the shape of the glass article is not limited to a plate shape.

[0055] Although the present disclosure has been described above based on the embodiments, the present disclosure is not limited to these embodiments. Furthermore, the above embodiments can be subject to various changes, modifications, substitutions, additions, deletions, and combinations within the scope of the claims, and these also fall within the technical scope of the present disclosure.

[0056] REFERENCE SIGNS LIST 10 Glass melting apparatus 20 Melting tank 21 First side wall member 22 Second side wall member 23 Shelf-like member 26 Bottom wall member 30 Electrode 40 Electrode holder 222 Outer surface of second side wall member 223 Upper end surface of second side wall member 231 Inner end surface of shelf-like member 232 Outer end surface of shelf-like member 233 Upper surface of shelf-like member 234 Lower surface of shelf-like member br1 First furnace material br2 Second furnace material G1 Glass frit G2 Molten glass H Hole P1 First portion P2 Second portion ST Step portion STe Corner portion of step portion

Claims

1. A glass melting apparatus comprising a melting tank for accommodating molten glass and electrodes for electrically heating the molten glass, wherein the molten glass has an electrical resistivity of 0.2 Ωm or more at 1600°C, the melting tank has a first portion surrounding the molten glass and a second portion located below the first portion and surrounding the molten glass with an internal cross-sectional area smaller than that of the first portion, a step is formed between the first and second portions, the electrodes are positioned so as to protrude into the first portion from holes formed in the step, and the step includes at least a first furnace material surrounding the holes and a second furnace material constituting an internal corner of the step, the first furnace material has an electrical resistivity higher than that of the molten glass, and the second furnace material has higher corrosion resistance than the first furnace material.

2. The glass melting apparatus according to claim 1, wherein the first furnace material is dense zircon fired bricks, and the second furnace material is electroformed bricks.

3. The glass melting apparatus according to claim 2, wherein the electroformed bricks are electroformed zirconia bricks.

4. The glass melting apparatus according to claim 1, wherein the melting tank has a first sidewall member constituting the first portion, a second sidewall member constituting the second portion, and a shelf-like member connecting the lower end of the first sidewall member and the upper end of the second sidewall member, and the inner end of the shelf-like member forms the corner of the step.

5. The glass melting apparatus according to claim 1, wherein the melting tank has a first sidewall member constituting the first portion, a second sidewall member constituting the second portion, and a shelf-like member connecting the lower end of the first sidewall member and the upper end of the second sidewall member, and the upper end of the second sidewall member forms the corner of the step.

6. The glass melting apparatus of claim 5, wherein said second sidewall member is constructed from said second furnace material.

7. A glass melting apparatus as claimed in claim 1, wherein the first furnace material and the second furnace material are adjacent to each other, and when viewed in a cross section along the vertical direction of the melting tank, the boundary between the first furnace material and the second furnace material is curved.

8. The glass melting apparatus according to claim 1, wherein the molten glass is alkali-free glass.

9. A method for producing glass, comprising: producing the molten glass using the glass melting apparatus according to any one of claims 1 to 8; and shaping and cooling the molten glass to obtain glass.

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