FORMED FIRED REFRACTORY MATERIAL HAVING A HIGH LEVEL OF SPECTRAL EMISSION, METHOD FOR PRODUCTION THEREOF AND METHOD FOR INCREASING THE LEVEL OF SPECTRAL EMISSION OF REFRACTORY SHAPED BODIES

Abstract

A process for producing a refractory material for use in the superstructure of glass melting tanks contains, as main components, SiO.sub.2, SiC and a binder or binder mixture. A particulate substance, which in the spectral range from 1 μm to 5 μm and at temperatures above 1000° C. has a spectral emission capability which is higher than the spectral emission capability of the matrix of the refractory material, is incorporated into the matrix of the refractory material. A method of increasing the spectral emissivity of shaped, fired, refractory materials, is also provided.

Claims

1-20. (canceled)

21. A process for producing a refractory material for use in the superstructure of glass melting tanks, the process comprising the following steps: providing SiO.sub.2, SiC and a binder or binder mixture as main components; and incorporating into a matrix of the refractory material a particulate substance having a spectral emission capability being higher than a spectral emission capability of a matrix of the refractory material in a spectral range from 1 μm to 5 μm and at temperatures above 1000° C.

22. The process according to claim 21, which further comprises: providing silicon carbide contained in the particulate substance; mixing the particulate substance with at least one particulate SiO.sub.2 raw material and a binder or binder mixture to form a pressable composition, shaped to give bricks; and drying and subsequently firing the bricks.

23. The process according to claim 22, which further comprises: mixing in SiC having a particle size of <1.5 mm as the substance containing silicon carbide; providing a content of silicon carbide in the material to be from 0.2% by weight to 20% by weight; and mixing in lignosulfonates, dextrin, calcium hydroxide, phosphates or substances having an equivalent effect with a proportion in the composition of not more than 6% by weight as a binder or binder mixture.

24. The process according to claim 23, wherein the SiC has a particle size of <1 mm, and the content of silicon carbide in the material is from 0.3% by weight to 15% by weight.

25. The process according to claim 22, which further comprises providing the substance containing silicon carbide with an SiO.sub.2 layer.

26. The process according to claim 22, which further comprises providing recycled material or kiln furniture as the substance containing silicon carbide.

27. The process according to claim 22, which further comprises providing amorphous or crystalline SiO.sub.2 or a mixture of amorphous and crystalline SiO.sub.2 having an SiO.sub.2 content of at least 96% by weight and a particle size of 0-6 mm in an amount of at least 78% by weight as the SiO.sub.2 raw material.

28. The process according to claim 27, wherein the particle size is 0-4 mm.

29. The process according to claim 22, which further comprises firing the bricks at a temperature above 1200° C.

30. The process according to claim 22, which further comprises firing the bricks at a temperature in a range from 1300° C. to 1550° C.

31. A method of increasing the spectral emissivity of shaped, fired, refractory materials, the method comprising the following step: embedding in a matrix of the refractory material a substance having a total emissivity being at least 15% higher than an emissivity of a matrix of the refractory material at a temperature range above 1000° C.

32. The method according to claim 31, wherein the refractory material is silica bricks for use in a superstructure and side walls of glass melting tanks.

33. The method according to claim 31, which further comprises providing the refractory material with a silicon dioxide content of at least 78% by weight and a particulate substance containing silicon carbide dispersed in a matrix of the refractory material, providing a quantity of silicon carbide in the material of from 0.2% by weight to 20% by weight and providing not more than 6% by weight of miscellaneous substances, with a total being 100% by weight.

34. The method according to claim 33, wherein the quantity of silicon carbide in the material is from 0.3% by weight to 15% by weight.

35. The method according to claim 33, wherein the matrix has an SiO.sub.2 content of at least 90% by weight.

36. The method according to claim 33, wherein the matrix has an SiO.sub.2 content of at least 94% by weight.

37. The method according to claim 33, which further comprises: mixing the substance containing silicon carbide with at least one particulate SiO.sub.2 raw material and a binder or binder mixture selected from the group consisting of lignosulfonates, dextrin, calcium hydroxide, phosphates and substances having an equivalent effect with an addition of water to form a pressable composition, shaped to give bricks; and drying and subsequently firing the bricks at a temperature above 1200° C.

38. The method according to claim 37, which further comprises firing the bricks in a temperature range of from 1300° C. to 1550° C.

39. The method according to claim 33, which further comprises using SiC as the substance containing silicon carbide.

40. The method according to claim 33, which further comprises using SiC having an SiO.sub.2 surface layer as the substance containing silicon carbide.

41. The method according to claim 33, which further comprises using a recycled material or kiln furniture as the substance containing silicon carbide.

42. The method according to claim 37, which further comprises mixing in amorphous or crystalline SiO.sub.2 or a mixture of amorphous and crystalline SiO.sub.2 in an amount of at least 78% by weight as the SiO.sub.2 raw material.

Description

[0015] The invention is illustrated by way of example with the aid of a drawing. The radiation behavior of a shaped body (1) according to the invention is compared with that of commercial materials for the superstructure of glass melting tanks, a silica brick (2) and a cast AZS material (3). The figures show:

[0016] FIG. 1 the spectral emissivities (measurement temperature 1200° C.),

[0017] FIG. 2 the corresponding temperature-dependent total emissivities.

[0018] The measurement principle used here, as also in the working examples of this document, for determining the spectral emissivities is based on the comparison of the spectral radiative heat flow density of the sample material with that of the black radiator at the same temperature and under identical optogeometric conditions (known as static radiation comparison principle). The spectral emissivity measured at a particular temperature is used to calculate the total emissivity corresponding to this temperature and averaged over the wavelengths.

[0019] The invention is based on the surprising recognition that the heat radiation capability of a fired, shaped refractory body can be improved to a significant measurable extent when a substance having a high emissivity is present dispersed in the matrix of the shaped body, with the substance being compatible with the matrix. In contrast to a coating applied on one side, in the case of the material according to the invention the high-emission material is already a constituent of the microstructure of the material, resulting in the total shaped body having an improved radiation capability and a use-related removal of material due to prevailing corrosive stresses not leading to loss of the improved radiation capability, as in the case of a thinly coated material surface. Furthermore, the material of the invention can be lined with refractory bricks as are conventionally used in the tank superstructure.

[0020] The invention provides for the use of silicon carbide as high-temperature-resistant, nonoxidic high-emission material. Silicon carbide (SiC) is usually produced by a carbothermic reduction and carbonization of high-purity silica sand (SiO.sub.2) by means of petroleum coke at from 2000° C. to 2400° C. by the Acheson process. A characteristic of SiC in high-temperature use at temperatures of up to about 1600° C. is the formation of a passivating layer of silicon dioxide as a result of reaction with atmospheric oxygen from the furnace atmosphere (known as passive oxidation). This process takes place as early as in the production of the material of the invention during a conventional firing. It has surprisingly been found that the protective SiO.sub.2 layer formed around the remaining SiC core, i.e. the high-emission material, is significantly strengthened and protected against corrosion (compatibility) by a high SiO.sub.2 content according to the invention in the matrix of at least 90% by weight, preferably at least 94% by weight. This also applies in particular to the surface or to the microstructure adjoining the surface of the shaped body, which ultimately determines the radiation behavior in later use. The use of SiC having a particle size of less than 1.5 mm, preferably less than 1 mm, has been found to be advantageous.

[0021] After the positive effect of SiO.sub.2 was recognized in the context of the invention, an advantageous aspect of a particular embodiment of the invention is to use SiC particles which already have a protective SiO.sub.2 layer, preferably by use of recycled material such as kiln furniture.

[0022] The high SiO.sub.2 content in the matrix also results in, inter alia, the material of the invention gaining the thermomechanical properties required for high-temperature use, in particular the creep behavior under pressure. This is ensured by a conventional production firing, with the crystalline SiO.sub.2 constituents tridymite and/or crystobalite being largely formed in the matrix from the SiO.sub.2 raw materials used.

[0023] The raw materials basis for formation of the matrix of the material of the invention is amorphous SiO.sub.2 or crystalline SiO.sub.2 or a mixture of the two having a particle size of 0-6 mm, preferably 0-4 mm, as is customary for industrial refractory coarse ceramic materials. For example, transparent fused silica or cloudy fused silica or a mixture of the two is used as amorphous SiO.sub.2; the SiO.sub.2 contents of these are greater than 99% by weight. During firing of the bricks, conversion into crystobalite takes place above a temperature of about 1150° C. As crystalline raw materials, preference is given to using natural quartzites, silica sands and quartz flours consisting mineralogically of fl-quartz and having SiO.sub.2 contents of greater than 96% by weight, either individually or as a mixture. At a high proportion of quartz-rich raw materials, commensurate addition of a mineralizer which, in an economical manner, promotes the required substantial conversion of the quartz into crystobalite and tridymite during firing of the shaped bodies and does not destroy the radiation properties of these by reaction with the high-emission material is necessary. Calcium hydroxide Ca(OH).sub.2, for example, meets these criteria and has been found to be particularly suitable because it additionally acts as binder.

[0024] According to the invention, the silicon carbide-containing high-emission material is mixed with at least one particulate SiO.sub.2 raw material and with a suitable binder or binder mixture, optionally in combination with water, so as to form a pressable composition. As binders, it is possible to use, for example, lignosulfonates (waste sulfite liquor), dextrin, calcium hydroxide and phosphates. The raw materials comprising SiO.sub.2 are assembled in such a way that at least 78% by weight of SiO.sub.2 is present in the dry matter, taking into account the fact that the matrix of the subsequently shaped, dried and fired material comprises at least 90% by weight of SiO.sub.2, preferably at least 94% by weight. The proportion of carbide-containing substance in the mixture is selected so that from 0.2% by weight to 20% by weight, preferably from 0.3% by weight to 15% by weight, based on the fired material, is present.

[0025] The prepared composition is, for example, shaped to give bricks and the bricks are dried. The bricks are subsequently fired under conditions generally customary for SiO.sub.2-rich, refractory materials at sintering temperatures above 1200° C., preferably in the range from 1300° C. to 1550° C. The bricks treated in this way have formed a matrix which is advantageously predominantly crystalline, i.e. comprises crystobalite or tridymite or a mixture of the two, with the quartz content being very low, preferably less than 1% by weight.

[0026] The following working examples are provided for the purpose of illustration and are not intended to restrict the scope of protection of the invention.

[0027] Examples 1 to 3: the particulate raw material components X-ray-amorphous fused silica having a maximum particle size of 4 mm and a typical particle size distribution and various amounts of SiC having a particle size of 0-1 are together mixed homogeneously as 100% by weight with addition of an additional 1% by weight of waste sulfite liquor and 3.5% by weight of water. The proportions of SiC are 0% by weight (example 1), 5% by weight (example 2) and 15% by weight (example 3), with, in the case of addition of 0% by weight and 5% by weight, the proportion needed in each case to make up 15% by weight of SiC being replaced by silica having the appropriate particle size. The mixtures obtained in this way are pressed at a pressing pressure of about 80 MPa to give shaped bodies. After drying at 110° C. to constant weight, the compacts are fired at a sintering temperature of about 1450° C. The proportion of crystalline SiO.sub.2 (crystobalite) determined by X-ray diffraction in the fired shaped bodies is greater than 50% by weight.

[0028] Examples 4 to 6: compared to examples 1 to 3, crystalline SiO.sub.2 having a maximum particle size of 3 mm is used as SiO.sub.2 raw material component and the proportions of SiC having a particle size of 0-1 mm are 0% by weight (example 4), 0.5% by weight (example 5) and 5% by weight (example 6). The proportion required in each case to make up 5% by weight of SiC is replaced by crystalline SiO.sub.2 having the appropriate particle size. In addition, 0.5% by weight of waste sulfite liquor, 4% by weight of water and about 3% by weight of calcium hydroxide are added and mixed until the mixture is homogeneous. The shaped bodies pressed at a pressing pressure of about 80 MPa and subsequently dried to constant weight at 110° C. are fired at a sintering temperature of about 1450° C. The proportion of unconverted quartz in the fired shaped bodies is less than 1% by weight.

[0029] The critical properties determined are shown in the following table. As characterizing parameter for the radiation behavior, the total emissivity averaged over all wavelengths at 1600° C. is reported.

TABLE-US-00001 Example Example Example Example Example Example Feature 1 2 3 4 5 6 SiO.sub.2 raw amorphous amorphous amorphous crystalline crystalline crystalline materials basis Bulk density 1.83 1.87 1.89 1.83 1.84 1.83 (g/cm.sup.3) Open 19.9 19.2 20.7 21.1 20.8 21.1 porosity (%) Cold 22 24 23 47 45 49 compressive strength (MPa) 99.7 94.9 85.3 96.2 95.8 92.3 SiO.sub.2 content (% by weight) SiC content (*) — 4.77 14.30 — 0.38 3.81 (% by weight) Total emissivity 0.51 0.72 0.80 0.50 0.62 0.72 at 1600° C. (o.d.) Increase — +41% +57% — +24% +44% (*) in accordance with DIN EN ISO 21068-1/2

[0030] The radiation properties of the fired shaped bodies which are not according to the invention of examples 1 and 4 correspond to those of conventional silica bricks, with the shaped body of example 4 also being comparable in terms of the further properties to a conventional silica brick material for use in the superstructure of glass melting tanks. It can be readily seen from the examples that the radiation properties are measurably improved very effectively by the incorporation according to the invention of the high-emission material into the shaped body matrix. Even a very small amount of high-emission material in the matrix surprisingly brings about a drastic improvement, as can be seen from the comparison of the total emissivities at 1600° C. of examples 4 and 5.

[0031] All fired shaped bodies according to the invention (examples 2, 3, 5 and 6) display excellent creep behavior under pressure in accordance with EN 993-9 which corresponds to conventional silica bricks, characterized in that, at a test temperature of 1600° C. and a load of 0.2 MPa, the creep is less than 0.2% between hold times of 5 and 25 h.

[0032] A shaped body according to the invention produced as described in example 3 was subjected to a temperature of 1600° C. for 100 hours in an electrically heated furnace. The radiation properties subsequently measured correspond to those of the original shaped body. Furthermore, shaped bodies according to the invention produced as described in examples 2 and 6 were used under realistic conditions in the superstructure of a glass melting tank for soda-lime glass for somewhat more than one month. The subsequently measured radiation properties of the shaped body surface which had been exposed to the hot furnace atmosphere likewise correspond to those of the unused, original material.

[0033] It can readily be seen from the working examples that the invention provides, by simple means, an improvement which is unusual and was in no way foreseeable.