AU639072B2 - Improved chromia-magnesia refractory body - Google Patents

Description

i i COMMONWEALTH OF AUSTRAL 3 9 7 PATENTS ACT 1952 FORM Case: C-2280c Class: Int. Class Application Number: Lodged: Complete specification: Lodged: Accepted: Published: Priority: Related Art: 0@

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Name of Applicant: Address of Applicant: Actual Inventor/s: A e* dr Address for Service:

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NORTON COMPANY 1 New Bond Street, Box Number 15008, Worcester, MA 01615-0008, United States of America.

ANTHONY K. BUTKUS; and SCOTT D. MARTIN.

E.F. WELLINGTON CO., Patent and Trade Mark Attorneys, 312 St. Kilda Road, Melbourne, 3004, Victoria.

Complete Specification for the invention entitled: "IMPROVED CHROMIA-MAGNESIA REFRACTORY BODY" The following statement is a full description of this invention including the best method of performing it known to us.

1 TECHNICAL FIELD This invention relates to chromia-magnesia refractory products which exhibit improved resistance to silica. More specifically, the invention is concerned with refractory articles of chromia bonded co-fused chromia and magnesia.

TECHNICAL BACKGROUND Co-fused chromia and magnesia refractory bodies bonded with an in situ formed microcrystalline chromia or chromia-magnesia bond are known from U.S. Patent No. 4,435,514 (Hartline), which discloses 20-40% fused chromia-magnesia fines in its mixtures see the table at the foot of column 3 of Hartline.

The bodies are disclosed to have excellent resistance to erosion by molten silica rich slags, particularly Western U.S. coal slag. While these bodies are superior to prior refractory bodies in regard to resistance to siliceous materials, it has been found that under certain circumstances of extended exposure to extremely high siliceous content materials, they still exhibit an amount of deterioration which is considered unsatisfactory. Thus, the resistance to slag has been found to decrease significantly when the percentage of fines reaches about 20%, hence there is a need for chromia-magnesia refractory bodies having an even greater resistance to siliceous materials such as slags than can be obtained by Hartline.

Goo Accordingly, it is an object of the invention to produce co-fused chromia and magnesia bodies having less deterioration in the presence of molten siliceous materials than the co-fused bodies of Hartline.

SUMMARY OF THE INVENTION It has now been discovered that improved silica resistance can be obtained from co-fused chromia and magnesia o. bodies by substantially avoiding the presence of co-fused chromia and magnesia particles which are finer than about 150 microns, that is, avoiding the presence of more than about of co-fused chromia and magnesia particles which are finer ii-n about 150 microns.

1- U -ur~lr II 1 -1 i i Thus, in accordance with the present invention, there is provided in a refractory product comprising grains of cofused chromia and magnesia bonded with an in situ formed microcrystalline bond of chromia, in which the in situ formed microcrystalline bond of chromia is formed from chromia sufficiently fine to readily react with the grains of co-fused chromia and magnesia at the firing temperature to which they are subjected but not so fine as to be reactive therewith at ambient temperature, the improvement wherein less than 10% of said grains of co-fused chromia and magnesia can pass through a sieve opening of 150 microns.

Said refractory product can be derived from a raw batch refractory mix comprising grains of co-fused chromia and magnesia and a finely particulate source of chromia, in the absence of more than 10 weight of co-fused grains that pass through a sieve opening of 150 microns, which mix constitutes another aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The refractory bodies of this invention comprise co-fused chromia and magnesia grain particles bonded with an in situ S formed microcrystalline chromia bond.

The co-fused chromia and magnesia grain particles used herein are generally prepared in any manner known in the art.

One suitable such method entails fusion of a mixture of chromia and magnesia, casting into an ingot, and then crushing the ingot to form the grains. Generally, chromia will be. present in the grain as about 70 to 90 weight with the magnesia present in an amount of about 30 to 10%, though the grain may also contain impurities in amounts up to about preferably only up to about The co-fused grain will normally be used in a mixture of particle 'sizes to improve the packing density thereof. The maximum size of the grain particles is not critical however there shouldbe a sufficient mixture of particle sizes so that a dense packed mixture can be formed in which the porosity is less than about 30%, preferably less than about 16%. Generally all of S^ the particles will be fine enough to pass through a 3.5 mesh screen (sieve openins of 5.6 mm). To produce the refractory O 2 bodies having improved resistance to siliceous slags, substantially all particles capable of passing through a 100 mesh screen (sieve openings of 150 microns) are removed. The co-fused particles which pass a 100 mesh screen can be readily removed from a mixture of such particles with larger ones by screening or other suitable technique. While the avoidance of all -100 mesh grain particles is preferred to produce the most slag resistant refractory products, a substantial improvement in slag resistance over that obtained by U.S. Patent 4,435,514 occurs when the weight percent of -100 mesh particles is less than about 40% of the total mixture, preferably less than about of the total mixture, more preferably less than about and still more preferably less than about 1%.

Preferably, the co-fused particles will be used in a mixed particle size distribution in which about 20 to 45 weight will pass 3.5 mesh but not pass 6 mesh (sieve openings 3.35 mm), about 30 to 55 weight will pass 6 mesh but not pass mesh (sieve openings 328 microns), about 15 to 30 weight will pass 45 mesh but not pass 100 mesh (sieve openings 150 microns) only up to about 5 weight that will pass 100 mesh.

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The microc-ystalline chromia bond is produced from very '...fine particles of chromia or a source of chromia. The chromia or a chromia source preferably should have a particle size of about 5 micron or finer, most preferably about 1 micron or The chromia needs to be sufficiently fine to readily react with the grains at the firing temperature to which they Swill be subjected, but not so fine as to be reactive therewith ambient temperature.

In addition to the co-fused grains and the chromia bond, 3'...the refractory body may further contain fine alumina particles in an amount of up to about 5 weight preferably up to about 2%.

To prepare the refractory bodies having improved resis- 2/26/90 3 tance to siliceous slags, the co-fused-: chromia and magnesia grain particles, the very fine chromia particles, and the optional alumina particles are simply mixed together, formed into the desired shape, and fired at an elevated temperature.

Oftentimes the forming will be accomplished by the addition of water, surfactants, temporary binders such as Swift's colloid (animal glue) or a 25% dextrin-water solution, and similar such forming additives which will be removed during the subsequent firing of the green body. Generally, the mixture will contain about 50 to 93 parts by weight of the co-fused grain, about to 7 parts by weight of the very fine chromia particles, up to about 5 parts by weight alumina, and up to about 10 parts by weight forming additives per 100 parts by weight total of said grains. Preferably, the mixture contains about 60 to 80 parts by weight of the co-fused grain, about 40 to 20 parts by weight of the very fine chromia particles, up to about 2 parts by weight alumina, and about 1 to 8 parts by weight forming additives. Most 'referably, the mixture will contain about 65 to 75 parts by weight of the co-fused grain, correspondingly about 35 to 25 parts by 2A weight of the very fine chromia particles, up to about 1 part by ":'eight alumina, and about 2 to 6 parts by weight of forming additives.

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While it is not an absolute necessity that the grain and S'..bond materials be highly pure, the quality of the final product is directly related to the degree of impurity. Silica is a particularly undesirable impurity. Thus the final product should *pbe about 92 weight or more chromia and magnesia, preferably at least about The processing or fabrication method used to form the refractory product of the present invention is basically standard procedure well known in the art and, therefore, forms no part of the present invention per se. The green refractory products :c*an be shaped or placed by all of the usual techniques such as so pressure molding, casting, tamping, ramming, and the like. The green product is then fired or heat treated in the conventional C-2280C 2/26/90 4

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i Imanner, generally at temperatures up to about 1600"C.

The resultant refractory bodies generally have densities of about 3.7 to about 4.2 g/cc with minimal void spaces. The final product preferably exhibits a theoretical density of about 84% or higher, with no more than about 13% open pores and 3% closed pores. Preferably, the average pore diameter will be less than about 9 microns and no more than about 10% by volume of the pores have diameters above about 20 microns. More preferably, the mean pore diameter is about 5 microns or less.

*-10 In the following non-limiting examples, all parts and percents are by weight unless otherwise specified.

EXAMPLE I A co-fused chromia and magnesia refractory body was prepared by cold pressing the following composition: Ingredient Parts Chromia-magnesia 23 Chromia-magnesia Chromia-magnesia (-45+80) 17 Alumina 1 0* Chromia (-800) 29 Swift's colloid (animal glue) 4 The alumina was grade A15SG, a "super ground" alumina of Aluminum Company of America. The chromia was extremely fine chromia dust having a particle size finer than 800 mesh.

The above composition was mixed, cold pressed into shape, and then fired at about 1750QC. for 4 hours. The properties of the refractory body thus formed were: Property English Units Metric Units Density 242.2 #/ft 3 3.88 g/cc Modulus of Elasticity 13x10 6 psi 90 GPa C-2280C 2/26/90 5 1- RT Modulus of Rupture 2,900 psi 20 GPa 2642*F. Modulus of Rupture 2,407 psi 1450*C. Modulus of Rupture 16.6 GPa Cold crushing strength 12,000 psi 82.7 MPa Standard wet chemical analyses were performed on the refractory body and the composition was: Cr 2 3 83.55%, MgO 13.37%, CaO 0.11%, A1 2 0 3 1.61%, SiO 2 0.87%, and Fe 2 0 3 0.49%.

To evaluate the resistance to slag of the refractory body, it was subjected to a 6 hour drip slag test at 1550°C. using an artificial western type slag. The test was as detailed in A.S.T.M. C768-85 except that the artificial black mesa slag was used in powder form (rather than a rod) and it "drips" into a reaction crucible where it becomes viscous and then flows onto the sample and (ii) a low oxygen partial pressure of 4 atmospheres was used to increase the severity of the test.

The results showed a silica penetration to a depth of 4.76 mm and no observed surface erosion.

*0 EXAMPLE II A refractory body is produced by vibrocasting a complex shape from the following composition: n Ingredient Parts Chromia-magnesia 18 Chromia-magnesia Chromia-magnesia (-45+80) 12 Chromia-magnesia (-80+100) Chromia (-800) Darvan 811-D deflocculant 0.15 Water 4.3 C-2280C 2/26/90 6 The pH of the mixture is adjusted to 1D-11 before vibrocasting. The deflocculant is a conventional sodium acrylate copolymer surfactant from R.T, Vanderbilt Co. The resultant mixture is thixotropic and flows under vibration.

The results of the slag drip test as in Example I are essentially the same.

COMPARATIVE EXAMPLE A The procedure of Example I was repeated to produce a refractory body, except that the chromia-magnesia particle distri- M]0 bution included particles finer than 100 mesh as taught by U.S.

a Patent No. 4,435,514 (Hartline). The specific composition used

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was:

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Ingredient Parts Chromia-magnesia 17.5 Chromia-magnesia 22.4 Chromia-magnesia (-45+80) 14.0 Chromia-magnesia (-80+100) 16.1 Calcined magnesia (-100) 5.1 **Chromia (-800) 24.9 ooo9 Swift's colloid The resultant refractory body was then subjected to the slag drip test of Example I and the results showed substdntial S surface erosion by the slag and slag penetration to a depth of about 12.7 mm.

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The presence of magnesia in the in situ formed bond is detrimental to the slag resistance of the body, even when no -100 mesh co-fused chromia-magnesia particles are present.

The matter contained in each of the following claims is to be read as part of the general description of the present invention.

C-2280C 2/26/90 7

Claims (14)

1. In a refractory product comprising grains of co-fused chromia and magnesia bonded with an in situ formed micro- crystalline bond of chromia, in which the in situ formed micro- crystalline bond of chromia is formed from chromia sufficiently fine to readily react with the grains of co-fused chromia and magnesia at the firing temperature to which they are subjected but not so fine as to be reactive therewith at ambient temperature, the improvement wherein less than 10% of said grains of co-fused chromia and magnesia can pass through a sieve opening of 150 microns.

2. The refractory product of Claim 1 comprising from to 93 parts by weight co-fused grains and from 7 to 50 parts by weight bond. 15 3, The refractory product of Claim 1 or 2 wherein said co-fused grains contain 70 to 90% by weight chromia and about 30 to 10% by weight magnesia.

4. The refractory product of Claim 1 or 2 wherein said grains are present in an amount of from 60 to 80% by 20 weight and said bond is present in an amount of from to 20% by weight.

5. The refractory product of Claim 1 or 2 wherein said S grains are present in an amount of from 65 to 75% by weight and said bond is present in an amount of from S to 25% by weight.

6. The refractory product of any one of Claims 1 to wherein said grains have a particle size distribution such that the grains pack to produce a maximum of 30% pores by volume and wherein the average size of said pores is 8 midrons or less. I

7. The refractory product of any one of Claims 1 to 6 Swherein -said grains have a particle size distribution of Si 20 to 45 weight passing 3.5 mesh but not 6 mesh, low to 55 weight passing 6 mesh but not 45 mesh, to 30 weight passing 45 mesh but not 100 mesh, and less than 5 weight passing 100 mesh.

8. The refractory product of any one of Claims 1 to 7 further containing alumina in an amount of up to

9. The refractory product of any one of Claims 1 to 7 further containing alumina in an amount of up to 2%.

10. The refractory product of any one of Claims 1 to 7 further containing alumina in an amount of up to 1%. :;ii 11. A raw batch refractory mix comprising grains of co- fused chromia and magnesia and a finely particulate source of chromia, in the absence of more than 10 weight of co-fused grains that pass through a sieve opening of 150 microns, said finely particulate source of chromia being sufficiently fine to readily react with the grains of co-fused chromia and magnesia at the firing temperature to which they are to be subjected but not so fine as to be reactive therewith at ambient temperature. o *o S 12, The raw batch of Claim 11 wherein said grains are present in an amount of from 50 to 93 parts by weight, said fine chromia source is present in an amount of from 7 to 50 parts by weight, and alumina is present in an amount up to 5 parts by weight magnesia.

13. The raw batch of Claim 11 or 12 wherein said grains consist essentially of 70 to 90% by weight chromia and 30 to by weight magnesia. 14". The raw batch of any one of Claims 11 to 13 wherein said grains have a particle size distribution of 20 to 45 weight passing 3.5 mesh but not 6 mesh, 30 to 55 weight passing 6 mesh but not 45 mesh, 15 to 30 weight passing 45 mesh but not i 100 mesh, and less than 5 weight passing 100 mesh. 9 A/RR/1919 1 ii The raw batch of any one of Claims 11 to 14 wherein said grains are present in an amount of from 60 to by weight and said fine chromia source is present in an amount of from 40 to 20% by weight. 16, The raw batch of any one of Claims 11 to 14 wherein said grains are present in an amount of from 65 to by weight and said fine chromia source is present in an amount of from 35 to 25% by weight.

17. The raw batch of any one of Claims 11 to 16 wherein the fine chromia source has an average particle size of 1 micron or less.

18. The raw batch of any one of Claim 11 to 17 further containing alumina in an amount of up to 2%.

19. The raw batch of any one of Claim 11 to 17 further containing alumina in an amount of up to 1%.

20. The raw batch of any one of Claims 11 to 19 further comprising up to 10 parts by weight of a forming S additive per 100 parts by weight total of said grains, said fine chromia source, and any of said alumina. 21, A refractory product according to Claim 1, substantially as hereinbefore described in Example 1. I o 22. A raw batch r.efactory mix according to Claim 11, substantially as hereinbefore described in Example 1. DATED this 27th day of April, 1993 *00 NORTON COMPANY, By its Patent Attorneys, E. F. WELLINGTON CO., By-: -,P SC IN S BRUCE S. WELLINGTON