REFRACTORY PRODUCT AND ITS USE

Abstract

A shaped and fired coarse ceramic refractory product and a process for the preparation of the product and use the product, the shaped and fired coarse ceramic refractory product comprises (a) a chemical composition comprising a content of Al.sub.2O.sub.3: at least 40 wt.-%: Y.sub.2O.sub.3: 2.0-57 wt.-%; ZrO.sub.2: below 42.0 wt.-%; and (b) a bond matrix comprising at least an yttria-containing crystalline mixed oxide with cubic modification cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

Claims

1. A shaped and fired coarse ceramic refractory product comprising (a) a chemical composition comprising a content of Al.sub.2O.sub.3: at least 40 wt.-%; Y.sub.2O.sub.3: 2.0-57 wt.-%; ZrO.sub.2: below 42.0 wt.-%; and (b) a bond matrix comprising at least an yttria-containing crystalline mixed oxide with cubic modification cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

2. Refractory product according to claim 1, wherein the yttria-containing crystalline mixed oxide with cubic crystal structure of the ternary system is yttrium-aluminium garnet (Y.sub.3Al.sub.5O.sub.12).

3. Refractory product according to claim 1, wherein the yttria-containing crystalline mixed oxide with cubic crystal structure is yttria fully stabilized zirconia or a mixture of yttria fully stabilized zirconia and Y.sub.3Al.sub.5O.sub.12.

4. Refractory product according to claim 1, any of claims 1 to 3, wherein the sum of the oxides SiO.sub.2, TiO.sub.2, Fe.sub.2O.sub.3, alkalis, and HfO.sub.2 further contained in the product is at most 2.2 wt.-%, preferably at most 1.7 wt.-%.

5. Refractory product according to claim 1, wherein the chemical composition comprises at least 60 wt. % of Al.sub.2O.sub.3.

6. Refractory product according to claim 1, wherein the chemical composition comprises between 2.0 to 25 wt. % Y.sub.2O.sub.3.

7. A process for producing a shaped and fired coarse ceramic refractory comprising a chemical composition comprising a content of Al.sub.2O.sub.3: at least 40 wt.-%; Y.sub.2O.sub.3: 2.0-57 wt.-%; ZrO.sub.2: below 42.0 wt.-%, the process comprising the step of: providing a bond matrix comprising at least an yttria-containing crystalline mixed oxide with cubic modification of cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

8. The process according to claim 7, wherein the Y.sub.2O.sub.3 for providing the yttria-containing crystalline mixed oxide has a degree of purity >98.5% of Y.sub.2O.sub.3/the total sum of rare earth oxides.

9. The process according to claim 7, wherein a raw material mixture comprises granular high purity fused corundum and/or sintered corundum with an Al.sub.2O.sub.3-content of >98.5 wt. %, and with grain sizes of between 0-1 mm and/or between 1-5 mm.

10. The process according to claim 7, wherein the bond matrix is formed by firing the raw material mixture after the shaping of the raw material mixture.

11. The process according to claim 7, wherein the average grain size of the Y.sub.2O.sub.3 is less than 63 m.

12. A process comprising the step of using the product according to claim 1 as a refractory material exposed to a reducing atmosphere.

13. A vessel for the production of hydrogen and carbon monoxide rich gases comprising a product according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGS. 1a to 3b show a microstructure of embodiments of the refractory product.

[0013] FIG. 4 shows weight loss after long-term exposure to reducing hot gases containing steam of embodiments of the refractory product compared with a conventional high fired and high purity corundum brick.

DETAILED DESCRIPTION

[0014] The invention is based on the surprising recognition that the resistance of an Al.sub.2O.sub.3-containing coarse ceramic refractory product to reducing hot gases containing steam can be improved to a significant measurable effect when the bond matrix of the fired product comprises at least an yttria-containing crystalline mixed oxide with cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2. It was found that the bond matrix of the refractory product according to the present invention both exerts an effective protective function for the refractory product against alumina evaporation and advantageously contributes to a high thermal resistance and stability of the refractory product.

[0015] According to the present invention, a fired coarse ceramic refractory product is provided which comprises the following characteristics: [0016] The product has a chemical composition according to which the following oxides are present in the following amounts, respectively: [0017] Al.sub.2O.sub.3: at least 40.0 wt.-%; Y.sub.2O.sub.3: 2.0-57.0 wt.-%; ZrO.sub.2: below 42.0 wt.-%; [0018] the bond matrix of the product comprises at least an yttria-containing crystalline mixed oxide with cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

[0019] For the purposes of the present invention, the yttria-containing mixed oxide with cubic crystal structure is refractory yttrium-aluminium garnet (Y.sub.3Al.sub.5O.sub.12, YAG) or refractory yttria fully stabilized Zirconia (YFSZ) or a mixture of both. YAG chemically consisting of 42.9 wt. % Al.sub.2O.sub.3 and 57.1 wt.-% Y.sub.2O.sub.3 is the Al.sub.2O.sub.3-richest compound in the binary system Y.sub.2O.sub.3Al.sub.2O.sub.3. YAG has a high melting temperature of about 1900 C. and a similar coefficient of thermal expansion to Al.sub.2O.sub.3. YFSZ contains about 8 mol. % (about 13 wt. %) Y.sub.2O.sub.3 and melts at about 2680 C.

[0020] YAG and YFSZ as at least partial component of the bond matrix is detectable on the finished refractory product by means of X-ray powder diffraction analysis, e.g. using Rietveld-method, and/or microscopic examination of, for example, thin sections, if necessary also including the determined chemical composition of the product, e.g. measured by DIN EN ISO 12677.

[0021] In accordance with the invention, it was found out that the presence of further oxides, which may be present in the refractory product according to the invention in addition to the oxides Al.sub.2O.sub.3, Y.sub.2O.sub.3 and ZrO.sub.2, e.g. as raw material impurities, may have a negative influence on the resistance to steam-containing reducing hot gases and/or on the hot properties. These include, for example, SiO.sub.2, TiO.sub.2, Fe.sub.2O.sub.3 and alkalis, the content of which in total should not exceed 1.5 wt.-%, preferably should not exceed 1.0 w.-%, in the finished product. In addition, it should be taken into account that HfO.sub.2 occurs in ZrO.sub.2 synthesized from zircon sands, e.g. by electrofusion, with a content of up to 2 wt.-%. In this respect, in a preferred embodiment, it is provided that the refractory product according to the invention comprises a chemical composition according to which the sum of the oxides SiO.sub.2, TiO.sub.2, Fe.sub.2O.sub.3, alkalis, and HfO.sub.2 is at most 2.2 wt.-%, preferably at most 1.7 wt.-%, e.g. determined by DIN EN ISO 12677.

[0022] Accordingly, as raw material for providing the scaffolding grain of the product according to the invention, granular high purity fused corundum and/or sintered corundum with an Al.sub.2O.sub.3-content of >98.5 wt. %, preferably >99 wt. %, most preferably >99.5 wt. %, and with grain sizes customary for coarse ceramic products, e.g. 0-1 mm and/or 1-5 mm, are preferably used. Grain sizes are determined by sieve analyses, e.g. according to DIN 66165. According to certain non-restricted embodiments, other Al.sub.2O.sub.3-containing granular raw materials with aforementioned grain sizes e.g. sintered and/or fused spinel and/or sintered and/or fused YAG and/or sintered and/or fused materials of the system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2, may also be provided for the scaffolding grain of the refractory product, at least in part.

[0023] The present invitation also encompasses a process for the production of the refractory product including the step of providing a bond matrix comprising at least an yttria-containing crystalline mixed oxide with cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

[0024] According to certain non-limiting embodiments, YAG can be provided in the bond matrix of the refractory product, for example, via a reaction of Al.sub.2O.sub.3 and Y.sub.2O.sub.3 in an appropriate mixing ratio in-situ during firing of the refractory product at temperatures in the range of 1500 C. to 1800 C., preferably 1600 C. to 1760 C. The reaction to YAG proceeds via the intermediate formation of Y.sub.4Al.sub.2O.sub.9 (YAM, monoclinic) and YAlO.sub.3 (YAP, orthorhombic). In the context of the invention, in-situ formed YAG has been detected already when fired at lower temperature, e.g. 1400 C. However, it has been found that a sufficient high heat resistance and stability is provided when the product according to the invention is fired at the aforementioned higher firing temperature.

[0025] Yttria raw material is available on the market as a synthetic product with a high degree of purity whereby, in sense of the present invention, purity means the ratio of Y.sub.2O.sub.3-content/sum total rare earth oxides (Y.sub.2O.sub.3/TREO). Y.sub.2O.sub.3 has a cubic structure and melts at about 2410 C. According to the invention, yttrium oxide powder is used, for example, with a degree of purity of >98.5%, preferably 99%, most preferably 99.9%, whereby the sum content of non-rare earth impurities, such as Fe, Al, Ca, SO.sub.4.sup.2 and Cl.sup., is below 0.15 wt. %. The average grain size of the yttrium oxide powder used is less than 63 m, preferably less than 25 m, and more preferably less than 10 m, e.g. measured by laser diffraction (d.sub.50-value).

[0026] As Al.sub.2O.sub.3 raw material for YAG-formation, preference is given to using high purity fused alumina (fused corundum) and/or sintered alumina (sintered corundum) and/or ground calcined alumina powder available on the market, which are characterized by an Al.sub.2O.sub.3-content of >99 wt. %, in particular >99.5 wt. %. Whereby, in the context of the present invention, it has been found out that the Al.sub.2O.sub.3 raw material may be provided as powder with grain size less than 0.5 mm, preferably less than 0.1 mm, and/or grain with grain sizes customary for coarse ceramic products, e.g. 0-1 mm and/or 1-5 mm, whereby gain sizes are determined by sieve analyses, e.g. according to DIN 66165. It has been surprisingly found out that, during firing of the shaped refractory product, yttria even reacts with coarser corundum grains at their surface by formation of YAG as part of the bond matrix.

[0027] In accordance with certain non-limiting embodiments, YFSZ can be obtained in the bond matrix of the refractory product, via a reaction of, preferably commercially available, monoclinic and/or tetragonal ZrO.sub.2 and Y.sub.2O.sub.3 powder in an appropriate mixture in-situ during firing of the refractory product at temperatures in the range of 1500 C. to 1800 C., preferably 1650 C. to 1760 C., the ZrO.sub.2 being provided preferably in powder form with grain size less than 0.5 mm, preferably less than 0.1 mm, determined e.g. as sieve passage.

[0028] According to certain non-limiting embodiments, a mixture of YAG and YFSZ can be provided in the bond matrix of the refractory product, via a reaction of Al.sub.2O.sub.3, Y.sub.2O.sub.3 and ZrO.sub.2 in an appropriate mixing ratio in-situ during firing of the refractory product in the temperature range of 1500 C. to 1800 C., preferably 1650 C. to 1760 C., using raw materials as described above. In this context, it was recognised that YAG-formation occurs only after YFSZ formation is complete.

[0029] In the context of the present invention, it was found that separately produced (synthetic) YAG, e.g. by solid-state reaction, or separately synthetically produced YFSZ, e.g. commercially available electro-fused, or a mixture of both can be provided at least proportionally in the bond matrix of the refractory product when used as a raw material preferably in powder form with grain size less than 0.5 mm, preferably less than 0.1 mm, determined e.g. as sieve passage.

[0030] According to certain non-limiting embodiments, YAG can be prepared separately, for example, by mixing yttria and alumina powder of the types described above in a suitable ratio, e.g. stoichiometric, with the addition of a suitable organic pressing and/or binding agent such as lignin sulfonates or wax emulsion, optionally in combination with water, so to form a pressable composition. This is followed by a shaping process, e.g. uniaxial pressing or extrusion, followed by firing of the shaped and dried body at a temperature above 1400 C., preferably above 1450 C. The dimensions of the fired shaped body are to be selected in such a way that the desired maximum grain size can be obtained after crushing and sieving and optionally milling.

[0031] According to another aspect of the present invention, separately produced YAG may be provided at least in part as Al.sub.2O.sub.3-containing scaffolding grain when used as raw material with grain sizes common for coarse ceramic products, e.g. 0.1-1 mm and/or 1-5 mm, determined e.g. by sieve analysis.

[0032] According to the invention, the Al.sub.2O.sub.3-containing scaffolding grain having the aforementioned grain sizes is mixed with a mixture of the YAG and/or YFSZ forming components, respectively with powdery YAG and/or YFSZ, and with a temporary binder or binder mixture, for example an organic binder such as lignin sulfonates and dextrin, and with water if required. The temporary binder and water can be added in the necessary proportions, in particular in such proportions that the prepared batch (mix) has a moist crumbly consistency. The coarse ceramic mix is shaped to give bricks. For shaping, various well-known processes may be used whereby; in particular, the complexity of desired geometric shape and number of pieces to be shaped has to be taken into account (Handbook of Refractory Materials, op. cit.). Where applicable, the shaped product may still be dried before firing, for example in a dryer. The brick is subsequently fired at temperatures in the range of 1500 C. to 1800 C., preferably 1650 C. to 1760 C. The firing may preferably be carried out for a duration in the range of 4 to 12 hours at the aforementioned firing temperatures.

[0033] The product according to the invention is preferably used in furnaces, reformers such as secondary steam reformers, reactors or vessels with reducing atmospheres. In this respect, it is also an object of the invention to use the product according to the invention as lining material for gasification plants, in particular as lining material for a vessel for producing hydrogen and carbon monoxide rich syngas at temperatures above 1000 C.

[0034] It is further an object of the invention to provide a vessel for the production of hydrogen and carbon monoxide rich syngas at temperatures above 1000 C. that is at least partially lined with the product according to the invention, e.g. as a wall brick and/or as the partition between the combustion zone and the catalytic zone.

[0035] In summary, the features of the present invention are:

[0036] 1. A shaped and fired coarse ceramic refractory product comprising [0037] (a) a chemical composition comprising a content of Al.sub.2O.sub.3: at least 40 wt.-%; Y.sub.2O.sub.3: 2.0-57 wt.-%; ZrO.sub.2: below 42.0 wt.-%; and [0038] (b) a bond matrix comprising at least an yttria-containing crystalline mixed oxide with cubic modification cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

[0039] 2. Refractory product according to feature 1, wherein the yttria-containing crystalline mixed oxide with cubic crystal structure of the ternary system is yttrium-aluminium garnet (Y.sub.3Al.sub.5O.sub.12).

[0040] 3. Refractory product according to feature 1, wherein the yttria-containing crystalline mixed oxide with cubic crystal structure is yttria fully stabilized zirconia or a mixture of yttria fully stabilised zirconia and Y.sub.3Al.sub.5O.sub.12.

[0041] 4. Refractory product according to any of features 1 to 3, wherein the sum of the oxides SiO.sub.2, TiO.sub.2, Fe.sub.2O.sub.3, alkalis, and HfO.sub.2 further contained in the product is at most 2.2 wt.-%, preferably at most 1.7 wt.-%.

[0042] 5. Refractory product according to any of features 1 to 4, wherein the chemical composition comprises at least 60 wt. % of Al.sub.2O.sub.3.

[0043] 6. Refractory product according to any of features 1 to 5, wherein the chemical composition comprises between 2.0 to 25 wt. % Y.sub.2O.sub.3.

[0044] 7. A process for producing a shaped and fired coarse ceramic refractory comprising a chemical composition comprising a content of Al.sub.2O.sub.3: at least 40 wt.-%; Y.sub.2O.sub.3: 2.0-57 wt.-%; ZrO.sub.2: below 42.0 wt.-%, the process comprising the step of: [0045] providing a bond matrix comprising at least an yttria-containing crystalline mixed oxide with cubic modification of cubic modification of the ternary system Al.sub.2O.sub.3Y.sub.2O.sub.3ZrO.sub.2.

[0046] 8. The process according to feature 7, wherein the Y.sub.2O.sub.3 for providing the yttria-containing crystalline mixed oxide has a degree of purity >98.5%, such as >99% or such as 99.9% of Y.sub.2O.sub.3/the total sum of rare earth oxides.

[0047] 9. The process according to feature 7 or 8, wherein a raw material mixture comprises granular high purity fused corundum and/or sintered corundum with an Al.sub.2O.sub.3-content of >98.5 wt. %, preferably >99 wt. %, most preferably >99.5 wt. %, and with grain sizes of between 0-1 mm and/or between 1-5 mm.

[0048] 10. The process according to any of features 7 to 9, wherein the bond matrix is formed by firing the raw material mixture after the shaping of the raw material mixture.

[0049] 11. The process according to any of features 7 to 10, wherein the average grain size of the Y.sub.2O.sub.3 is less than 63 m, such as less than 25 m or such as less than 10 m.

[0050] 12. Use of the product according to any of features 1 to 6 as a refractory material exposed to a reducing atmosphere.

[0051] 13. A vessel for the production of hydrogen and carbon monoxide rich gases comprising a product according to any of claims 1 to 6.

[0052] The following examples and drawings are provided for the purpose of illustration and are not intended to restrict the scope of protection of the present invention.

EXAMPLES

[0053] The contents of powdery and granular raw materials used for the production of the examples and a conventional comparative alumina brick (reference) are listed in Table 1. Yttria powder used had an average grain size (d.sub.50) of approx. 5 m and a purity (Y.sub.2O.sub.3/TREO) of 99.99%. Homogenous mixing of the components with additional approx. 0.8 wt. % lignin sulfonate and approx. 2.2 wt. % of water was done in an intensive mixer. The moist crumbly mixtures obtained in this way were pressed at a forming pressure of about 80 MPa to give bricks having a volume of about 4300 cm.sup.3 (brick height approx. 15 cm). After drying at approx. 110 C. to constant weight, the dried bricks were fired at a temperature of 1720 C. for 6 hours.

[0054] Certain properties of the fired bricks are shown in Table 1. Content of YAG and YFSZ was determined by X-ray powder diffraction analysis including chemical analysis. Bulk density and apparent porosity were measured according to DIN EN 993-1, cold crushing strength according to DIN EN 993-5 and, as a measure of thermal resistance and stability, creep in compression according to DIN EN 993-9.

TABLE-US-00001 TABLE 1 Example Item 1 2 3 4 5 6 Reference Mix (wt. %) Fused alumina 1-3 mm 45 45 45 45 45 42.5 45 Fused alumina 0-1 mm 40 40 40 40 40 27.5 40 Calcined alumina 0-0.1 mm 6.4 3.3 15 Yttria 0-0.1 mm 8.6 11.8 15 5.8 2.1 Monoclinic ZrO.sub.2 0-0.1 mm 9.2 12.9 Y-FSZ 0-0.1 mm 30 Properties YAG (wt. %) 15.1 20.7 26.3 7.7 Y-FSZ (wt. %) 10.6 15 30 Al.sub.2O.sub.3 (wt. %) 90.9 87.7 84.5 84.5 84.5 69.9 99.5 Y.sub.2O.sub.3 (wt. %) 8.6 11.8 15.0 5.8 2.1 3.9 ZrO.sub.2 (wt. %) 8.9 12.4 25.1 SiO.sub.2, TiO.sub.2, Fe.sub.2O.sub.3, 0.4 0.3 0.3 0.7 0.8 0.9 0.4 alkalis, HfO.sub.2 (wt. %) Bulk density (g/cm.sup.3) 3.26 3.25 3.24 3.30 3.28 3.40 3.24 Apparent porosity (%) 18.8 19.9 20.6 19.9 20.5 22.4 18.0 Cold crushing strength 50 70 72 61 50 37 112 (MPa) Creep at 1550 C., load 0.2 MPa; Z.sub.5-25 (%) 0.19 0.19 0.09 0.10 0.21 0.15 0.21

[0055] It has been found that the fired bricks according to the invention display excellent creep behaviour (Z.sub.5-25-value, deformation between 25 h and 5 h test period) which is at least equal to a conventional high-fired and high-purity corundum brick (reference).

[0056] FIGS. 1a and 1b show the microstructure of inventive example 1, FIGS. 2a and 2b of inventive example 4, and FIGS. 3a and 3b of inventive example 6. It can be seen the scaffolding grain (100), bond matrix (101) containing YAG (102) or YFSZ (103) or both as well as open pores (104).

[0057] In order to quantitatively evaluate the improved resistance to reducing hot gases containing steam, hydrogen and carbon oxides, bricks according to the invention produced as described in example 1 to 6 were subjected to a comparative practical trial in the combustion chamber of a vessel for industrial production of syngas using the catalytic process for about 7 months. The maximum application temperature was at about 1200 C. A conventional high-purity corundum brick produced as described above (reference) acted as the comparative material. The weight of all bricks to be tested was measured before and after the trial and the respective percent weight loss was determined. Based on this, the comparative percent weight loss (CWL) was calculated according to the following equation:

CWL (%)=(weight loss of example (%).Math.100%)/(weight loss of reference (%)).

[0058] The results are listed in Table 2 and are also illustrated in the graph of FIG. 4.

TABLE-US-00002 TABLE 2 Example Item 1 2 3 4 5 6 Reference Comparative 60.2 52.5 40.1 59.4 74.6 48.7 100 weight loss CWL (%)

[0059] The results shown in table 2 reflect the clear superiority of the product according to the invention over high-purity corundum bricks in terms of resistance to hot gases containing steam, hydrogen and carbon oxides. Regarding examples 4 to 6, a trial-related destabilization of the cubic zirconia phase could not be detected.

[0060] It has also been unexpectedly discovered that the thermal conductivity of the product according to the invention is noticeably lower than that of commercially available corundum bricks that, among other things, has a positive effect on energy saving.