Method for producing glasses, glass ceramics and the use of same

Abstract

A method for producing bubble-free glasses is provided, in which a glass mixture that is arsenic-free, antimony-free and tin-free with the exception of any unavoidable raw material impurities and at least one sulfate compound as a refining agent are used. The glass mixture and refining agent are melted and primarily refined in a first region of a melting tank, an average melting temperature (T1) is set at T1>1580 C. and an average melt residence time (t1) is set at t1>2 hours. A secondary refinement is carried out in a second region, an average melting temperature (T2) is set at T2>1660 C. and an average melt residence time (t2) is set at t2>1 hour, and the proportion of the SO.sub.3 resulting from decomposition of the sulfate is reduced to less than 0.002 wt. %.

Claims

1. A method for producing glasses, comprising: using a glass batch that is free of arsenic, antimony, and tin except for unavoidable raw-material impurities; using at least one sulfate compound as refining agent; melting and primarily refining the glass batch and the refining agent in a first region of a furnace melting tank, wherein in the first region, an average melt temperature T.sub.1 is set at 1580 C.<T.sub.11660 C. and an average residence time of the melt t.sub.1 is set at t.sub.1>2 hours; secondarily refining the glass batch and the refining agent in a second region of the melting tank, wherein in the second region, an average melt temperature T.sub.2 is set at 1660 C.<T.sub.21720 C., and an average residence time t.sub.2 of the melt is set at t.sub.2>1 hour, and wherein the proportion of the SO.sub.3 forming due to the decomposition of the sulfate compound is decreased to less than 0.002 wt. % at the latest during the conducting of the secondary refining; and high-temperature refining the glass batch and the refining agent after the secondary refining, wherein an average melt temperature T.sub.3 during the high-temperature refining stage is 1750 C.T.sub.32000 C.

2. The method according to claim 1, wherein the at least one sulfate compound comprises at least one alkali sulfate and/or at least one alkaline-earth sulfate.

3. The method according to claim 2, wherein the at least one sulfate compound further comprises Na.sub.2SO.sub.4.

4. The method according to claim 1, wherein the at least one sulfate compound comprises Na.sub.2SO.sub.4.

5. The method according to claim 1, wherein the at least one sulfate compound comprises BaSO.sub.4 and/or CaSO.sub.4.

6. The method according to claim 1, wherein the at least one sulfate compound is added to the batch in an amount that corresponds to 0.05 to 1 wt. % of SO.sub.3.

7. The method according to claim 1, wherein the step of high-temperature refining is conducted at temperatures of at least 1750 C.

8. The method according to claim 1, wherein the step of high-temperature refining is conducted over a time period of at least 12 min.

9. The method according to claim 1, wherein, in the first region, melting is conducted in an oxidizing manner.

10. The method according to claim 1, further comprising adding nitrate to the glass batch in a concentration of 0 to 3 wt. %.

11. The method according to claim 1, wherein the glasses produced are a transparent, colorless glasses.

12. The method according to claim 1, further comprising adding coloring components so that the glasses produced are transparent, colored glasses.

13. The method according to claim 1, further comprising exposing the glass batch to a thermally treatment sufficient to convert the glass batch into a glass ceramic.

Description

DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained below in more detail on the basis of the figures. Herein:

(2) FIG. 1 schematically shows a furnace melting tank with downstream high-temperature aggregate, and

(3) FIG. 2 shows a gas flow/temperature diagram.

DETAILED DESCRIPTION

(4) A furnace melting tank 1 with a filling wall 2, a bottom wall 3 and an outlet 4 is shown in FIG. 1. The preferred type of furnace tank is a conventional furnace tank that can be heated by fossil fuel with or without supplemental electrical heating.

(5) The furnace melting tank is divided into a first region 10 and a second region 20. The batch is placed in the first region 10, so that initially a raw melt having a porous batch carpet 12 is formed there. Underneath the batch carpet 12 is found a molten batch, in which non-molten particles, particularly the difficult-to-melt components, are still present in part.

(6) Under the batch carpet 12 is formed a counterclockwise principal flow vortex 13, which sweeps past underneath the batch carpet and continually takes up material and converts it into the melt.

(7) This principal flow vortex 13 extends approximately into the central region of the melting furnace 1, whereby partial flows 14 detach from the principal flow vortex 13, and flow into the second region 20. The regions 10 and 20 can be optionally separated by a built-in component, e.g., a wall 5, by which the hot glass melt is forcibly guided to the surface of the melting furnace.

(8) The two regions are separated by the so-called source point 15, which is also designated the hot spot. This is a region with a high local temperature of the melt.

(9) A primary refining is carried out in the first region 10. The temperature T.sub.1 in this region 10 lies above 1580 C. In the second region 20, the temperature T.sub.2 is clearly higher, i.e., over 1660 C. The secondary refining is conducted in this second region.

(10) The average residence time t.sub.1 in the region 10 is more than two hours. The average residence time can be set correspondingly by different parameters, such as, e.g., by the geometric dimensions, particularly the length of the furnace tank.

(11) This is also true for the average residence time t.sub.2 in the second region 20, where the average residence time t.sub.2 shall be at least one hour. By maintaining the temperature and residence time, it is assured that at the outlet 4, the concentration of SO.sub.3 in the melt is <0.002 wt. % and the bubble concentration is <1000/kg.

(12) The outlet 4 is connected to a high-temperature aggregate 6, where the high-temperature refining takes place. The high-temperature refining is conducted at temperatures >1750 C. Since the SO.sub.3 proportion is <0.002 wt. %, the undesired reboil effect cannot occur due to this low SO.sub.3 content, so that a bubble-free glass (<2 bubbles/kg, preferably <1 bubble/kg) can be produced at the end of the high-temperature aggregate 6.

(13) The evolved gas flows (Evolved Gas Analysis measurements, abbreviated as EGA measurements) are plotted in FIG. 2 as a function of temperature for the two regions 10 and 20 of an LAS glass composition. For the measurement, 50 g of batch are heated from room temperature to 1680 C. at 8 K/min, and the evolved gases are analyzed as a function of temperature by means of a mass spectrometer. The diagram in FIG. 2 distinguishes between the gas flows of SO.sub.2 and O.sub.2. From about 1000 C., the evolution of SO.sub.2 and O.sub.2 begins in the porous batch carpet based on the decomposition of barium sulfate. Gases found between the batch particles, such as air, are removed thereby from the batch carpet (strong fluctuations in the curve course). With increasing temperature, the porous batch carpet transitions into a glass melt and the evolution of SO.sub.2 and O.sub.2 again decreases. From about 1600 C., the evolution of SO.sub.2 decreases to nearly zero. The slower decrease of the O.sub.2 from 1600 C. may be caused by impurities.

(14) A pronounced primary refining occurs. Only a small amount of SO.sub.3 or even no SO.sub.3 is available in the secondary refining, so that it is assured that at the end of the furnace tank, thus after the secondary refining region, the SO.sub.3 component is <0.002 wt. %.

(15) The temperatures of the just described gas flows (EGA measurements) cannot be directly converted to furnace tank ratios, since the heating rates and surface-to-volume ratios differ between the laboratory measurements and the furnace tank. The measurements indicate the temperature regions of the evolution of refining gas under laboratory conditions. The actual temperatures of the gas evolution were determined in the small furnace tank test and are shifted to higher temperatures in comparison to the EGA measurements.

(16) The invention will be explained in more detail on the basis of examples:

(17) Compositions 1 to 3 in the following table are glass compositions according to the invention.

(18) TABLE-US-00003 TABLE 1 Glass compositions from the following Examples Composition 4 Composition 5 (comparative (comparative Oxides in wt. % Composition 1 Composition 2 Composition 3 example) example) Al.sub.2O.sub.3 22.2 21.5 21.6 22.2 22.1 BaO 2.02 0.76 2.02 1.96 Fe.sub.2O.sub.3 0.024 0.016 0.02 0.024 0.016 K.sub.2O 0.13 0.07 Li.sub.2O 3.80 3.83 3.59 3.80 3.81 MgO 0.59 1.02 0.86 0.59 0.57 CaO 0.12 SrO 0.49 Na.sub.2O 0.56 0.44 0.56 0.56 0.57 SiO.sub.2 65.0 67.3 66.0 65.0 65.2 SO.sub.3 synthesis 0.52 0.47 0.26 TiO.sub.2 2.26 1.99 2.39 2.26 1.98 ZnO 1.76 1.75 1.79 1.76 1.82 ZrO.sub.2 1.76 1.81 1.74 1.76 1.82 SnO.sub.2 0.2 Light transmission 88.7 88.4 88.5 88.6 86.9 Y in % Chromaticity C 3.2 3.8 3.8 3.3 4.5

(19) SO.sub.3 synthesis means: Quantity of SO.sub.3 in wt. %; the quantity of BaSO.sub.4 that is added to the batch is calculated from the quantity of SO.sub.3. All other data are analytically determined values in the glass.

EXAMPLE I

(20) In the laboratory, a 1.4-kg batch of LAS glass composition 1 (without addition of As.sub.2O.sub.3, SnO.sub.2 and Sb.sub.2O.sub.3) was prepared with conventional raw materials (quartz powder, Al.sub.2O.sub.3, Al hydroxide, Ba nitrate, Na nitrate, Li carbonate, Ba carbonate, MgO, TiO.sub.2, zirconium silicate, ZnO) and 0.53 wt. % SO.sub.3 refining agent as Ba sulfate.

(21) The batch was melted without remnants in the air-fuel operating gas furnace at temperatures of T.sub.1=1620 C. with t.sub.1=3 h and subsequently stirred in an MF coil in the silica glass crucible and kept for t.sub.2=3 h at T.sub.2=1680 C. in order to carry out a secondary refining. After the end of the melting time, the glass was cast and cooled at 20 K/h. Glass produced in this way still contained approximately 600 bubbles/kg of glass. The analyzed SO.sub.3 content was 0.00075 wt. %.

(22) After evaluating the glass in the cold state, the glass, which was nearly free of refining agent and SO.sub.3-poor, was subjected to a high-temperature refining. For this purpose, cylindrical cores were produced from the just described melt suitable for the crucible of the high-temperature refining. A 55-mm high core was heated in a 140-mL Ir crucible again to 1600 C., kept at 1600 C. for 30 min. for uniform thorough melting, and then heated at 975 K/h to 1925 C. and kept for 12 min at the high temperature. Subsequently, the hot glass was cooled to 1500 C. in approximately 8 min, kept for 10 min, and then thermally annealed to room temperature in the cooling furnace.

(23) The glass was completely free of bubbles, all bubbles were removed, and there was no new bubble formation.

(24) The glass was converted into a glass ceramic by thermal treatment. The glass ceramic with a layer thickness of 4 mm had a light transmission Y according to the CIE color system of 88.7% and a chromaticity C* in the CIE-LAB color system of 3.2.

(25) If the temperatures in the 140-mL high-temperature crucible lie below 1925 C., residence times of at least 15 min are necessary in order to arrive at a bubble-free glass from the glass which is free of refining agent and SO.sub.3-poor having bubble numbers of <1000 bubbles/kg. At 2125 C., short residence times are sufficient in order to obtain a bubble-free glass.

EXAMPLE 2

(26) Another LAS glass batch (composition 2 with raw materials comparable to those of Example 1) with 0.47 wt. % SO.sub.3 as Na sulfate was melted at T.sub.1=1580 C. with t.sub.1=3 h and subsequently melted at T.sub.2=1660 C. only with t.sub.2=2 h. The number of bubbles was approximately 950 bubbles/kg. The SO.sub.3 content was approximately 0.0010 wt. %.

(27) The higher the bubble number is prior to introduction into the high-temperature crucible (initial bubble number), the higher the refining temperature and/or the longer the residence time must be in the high-temperature refining crucible. At 1950 C. and after 15 min residence time, the glass was free of bubbles.

EXAMPLE 3

Comparative Example with SO3>0.0020 Wt. %

(28) An 8.6-kg LAS glass batch (composition 1) was melted down with 0.53 wt. % SO.sub.3 as BaSO.sub.4 in the gas furnace for 3 h at 1550 C. and subsequently further melted at 1600 C. for 1 h. The SO.sub.3 content remaining in the glass amounted to 0.0022 wt. %. A subsequent high-temperature refining at 1925 C. with 15 min holding time did not lead to a bubble-free glass. The glass contained small bubbles (max. 100 m diameter), particularly on the walls of the crucible and on the 3-phase interface between glass, crucible, and atmosphere. Based on the partial pressure of the SO.sub.2 (pSO.sub.2) or of the SO.sub.3 concentration, so-called new bubble formation occurred.

EXAMPLE 4

Comparative Example with High Initial Bubble Numbers

(29) An LAS glass batch behaves similarly (composition 1) with 0.53 wt. % SO.sub.3 as Ba sulfate, which was melted down in the gas furnace for only 3 h at 1620 C. (without the 2nd temperature step); the glass contained approximately 2000 bubbles/kg and the SO.sub.3 content was between 0.0022 and 0.0025 wt. % SO.sub.3. After the subsequent high-temperature refining at 1925 C. for a time period of 15 min, the glass was not bubble-free, but rather it contained very small bubbles, many bubbles on the crucible wall and on the 3-phase interface.

(30) Based on these examples, it can be clearly seen that a bubble-free glass can only be produced by maintaining the claimed parameters. The number of bubbles in the comparative examples shows that, in particular, high SO.sub.3 contents lead to a new bubble formation in the high-temperature refining aggregate due to poorly selected primary and secondary refining temperatures. Nevertheless, the addition of sulfate in the batch cannot be omitted, since if it were, the bubble numbers of <1000 that are necessary after the primary and secondary refining would not be obtained.

EXAMPLE 5

Comparative Example without Sulfate

(31) A batch of LAS glass composition 4 without addition of a sulfate (without addition of refining agent) showed more than 5000 very small bubbles/kg and melting remnants in the laboratory furnace after 3 h at 1600 C. with subsequent heating to 1660 C. at 300 K/h and a holding time of 2 h. A subsequent high-temperature refining at 1925 C. with 15 min residence time did not lead to a bubble-free glass. The high initial bubble numbers could not be completely eliminated. In addition, batch remnants in the high-temperature crucible are permanent sources of small bubbles, and in fact, are based on the changing chemistry of the glass and the gas solubility.

EXAMPLE 6

(32) The batch of Example 5 (composition 1), of course containing 0.53 wt. % SO.sub.3 as Ba sulfate led to a maximum of only 600 bubbles/kg after the same temperature-time treatment, i.e., to clearly smaller bubble numbers after the primary and secondary refining. The melt was completely free of remnants and the SO.sub.3 content was 0.0012 wt. %. A subsequent high-temperature refining at 1900 C. with 12 min residence time led to bubble-free glass. All initial bubbles could be reduced and a new bubble formation was not observed.

EXAMPLE 7

(33) An LAS glass composition (composition 3) was melted in a small furnace tank. Commercially available technical raw materials were used (quartz powder, Al.sub.2O.sub.3, Al hydroxide, Na nitrate, K carbonate, Li carbonate, Ca carbonate, Sr carbonate, Ba carbonate, MgO, TiO.sub.2, zirconium silicate, ZnO, Ba sulfate) with a total content of Fe.sub.2O.sub.3 of 0.0200 wt. % The batch contained 0.26 wt. % SO.sub.3, added as Ba sulfate. No coloring oxides were added to the batch. 0.56 wt. % Na.sub.2O was added as Na nitrate. After average melting temperatures of 1580 C. to 1600 C. for the primary refining, the melting temperature for the secondary refining was increased to over 1660 C. (average residence times >3 h in each case). Samplings after the furnace tank showed that the glass was melted free of remnants. The number of bubbles was between 200 and a max. 800 bubbles/kg depending on the melting parameters. The content of SO.sub.3 in each case was under 0.0012 wt. %.

(34) The subsequent high-temperature refining at temperatures between 1760 C. and approx. 1900 C. with average residence times of 12 to 15 min led to glass with bubble numbers of <1 bubble/kg.

(35) The thus-produced, transparent colorless LAS glass was converted into a glass ceramic by ceramicizing, and light transmission Y and chromaticity C* (color) were measured. The glass ceramic with a layer thickness of 4 mm had a light transmission Y according to the CIE color system of 88.5% and a chromaticity C* in the CIE-LAB color system of 3.8.

EXAMPLE 8

Comparative Example with SnO2 with Respect to Obtainable Light Transmission

(36) An LAS glass composition 5 was melted and refined in the gas furnace in the laboratory in the same way as described in Example 1 and also as in Example 2.

(37) The bubble numbers after the primary and secondary refining were between 200 and 600 bubbles/kg, and ZrO.sub.2-containing melting remnants occurred increasingly on the surface.

(38) The thus-produced, colorless LAS glass was converted into a glass ceramic by thermal post-treatment and its transmission and color were measured. The glass ceramic with a layer thickness of 4 mm had a light transmission Y of 86.9% in the CIE color system and a chromaticity C* in the CIE-LAB color system of 4.5 with a 4-mm layer thickness. Due to the addition of SnO.sub.2, the transmission clearly decreases and the chromaticity increases, in comparison to the SnO-free, sulfate-refined glass ceramic.

LIST OF REFERENCE NUMBERS

(39) 1 Furnace melting tank 2 Filling wall 3 Bottom wall 4 Outlet 5 Wall 6 High-temperature crucible 10 First region 12 Batch carpet 13 Principal flow vortex 14 Partial flow 15 Source point 20 Second region