High purity fused grain in the system alumina-chromia-magnesia

Abstract: High purity fused grain and refractory compositions that contain a high purity fused grain made up of 35 to 85 wt.% chromia, 5 to 55 wt. % alumina, and 5 to 45 wt.% magnesia. Burned refractory brick having such a chemistry was found to unexpectedly possess superior resistance to AOD and electric furnace slags.

Description: BACKGROUND OF THE INVENTION This invention relates to high purity refractories and, more particularly, it concerns an improved high purity fused grain made of alumina, chromia and magnesia oxides. Commercial high purity refractories based upon alumina, chromia, and magnesia, are divided into two groups. One group is based upon picrochromite (MgO · Cr₂O₃ in amounts of about 80% Cr₂O₃ and 20% MgO) and the other group is based upon alumina-chromia in various combinations. Grains used to make these refractories are either sintered or fused. Fused grains are preferred due to their low porosity as it is very difficult to sinter high purity inert refractory oxides. Refractories based upon picrochromite are used in high temperature basic environments such as in critical wear areas of some steelmaking furnaces. Refractories based on alumina-chromia and chromia-alumina are used in high temperature more acidic environments such as coal gasifiers. Many steelmaking or refining vessels have acidic slags initially. Later during the production cycle slag conditions turn basic. A classic example of this is the Argon Oxygen Decarburization (AOD) vessel. A desirable refractory for such applications would contain combinations of all three oxides -- magnesia, alumina, and chromia. Such a refractory should have improved resistance to changing slag conditions compared to either end member (MgO-Cr₂O₃ or Al₂O₃-Cr₂O₃). Such a refractory should also have improved thermal shock resistance compared to chrome-alumina compositions. A refractory based upon Al₂O₃-Cr₂O₃-MgO should be a cost effective means for resisting a variety of slags within a given operation. The three component system should allow a decrease in the total chromic oxide level without negatively impacting slag resistance. Lowering the Cr₂O₃ content of the refractory could significantly lower cost as chromic oxide is by far the most costly ingredient in this system. U.S. Patent No. 3,615,775 discloses compositions made from 80 to 95 wt. % alumina and the balance magnesia and chromia. This mixture when fired contained coarse alumina grains having some chromia in solid solution bonded by a MgO-Al₂O₃-Cr₂O₃ matrix. U.S. Patent No. 4,490,474 discloses a fused cast refractory consisting of a uniform mixture of 65 to 98 wt.% Cr₂O₃, 0.75 to 4 wt. % MgO, and 1 to 34.25 wt. % Al₂O₃. In both of these references, the magnesia content is so low that the beneficial aspects of a refractory containing higher levels of magnesia could not be realized. GB-A-1 496 356 discloses a metal casting mould comprising a synthethic spinel which contains 10 to 35 wt.% Cr₂O₃, 26 to 65 wt.% MgO, 10 to 30 wt.% Al₂O₃ and 5 to 30 wt.% FeO. The synthetic refractory spinels may be obtained by calcining mixtures of suitable precursors for the oxide constituents of the spinel or from refractory bricks as used magnesium-chrome refractory bricks. The mould is formed by hardening a refractory slurry containing the refractory spinel in finely divided form which has been placed in contact with a disposable pattern, the pattern being a replica of the part to be cast. Refractories based on the system Al₂O₃-CrO₃-MgO are also known from EP-A-0 210 457. These refractories contain 31 to 76 wt.% Al₂O₃, 3 to 20 wt.% Cr₂O₃ and 19.5 to 53 wt.% MgO. The compounds may be used as additives in refractory magnesia bricks to improve their thermal shock resistance. The magnesia bricks are useful at high temperatures in combination with temperature cycling, mechanical stress, and basic environments, such as occur in lime or cement kilns. The compositions of the refractories known from these references are outside the ranges of the present invention. There is no disclosure or suggestion, that these refractories are resistant to both acidic and basic metallurgical slags. In light of the foregoing, there is a need for an improved high purity refractory which is resistant to both acidic and basic metallurgical slags. SUMMARY OF THE INVENTION In accordance with the present invention, it has been unexpectedly discovered that low porosity (less that 15%) refractory brick can be made which essentially consists only of three oxides -- alumina, chromia, and magnesia. The ranges of these oxides occur between 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia. Prior art indicated that compositions within this range would have higher porosity because the oxides, particularly chromia, were thought to be inert and difficult to sinter. Shapes made from various high purity grains and having an overall chemical composition within the range 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia were found to have exceptional resistance to both acid and basic metallurgical slags -- such as those found in an AOD vessel or a steel ladle. Accordingly, a principal object of the present invention is to provide a refractory consisting primarily of alumina, chromia, and magnesia oxides, and which can resist a range of metallurgical slags ranging from acidic to basic in their composition. Another and more specific object of the invention is the provision of a high purity fused grain consisting essentially of 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia. Preferably, said grain contains no more than 2 wt.%, more preferably less than 1 wt.% total impurities such as silica, iron oxide, and limes. Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow taken in conjunction with the accompanying drawings and tables. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a ternary diagram of the mix compositions in the Al₂O₃-Cr₂O₃-MgO system; Fig. 2 is a ternary diagram of the melting isotherms of the Al₂O₃-Cr₂O₃-MgO system; and Fig. 3 is a ternary diagram of the isothermal section at 1700° ± 15° C of the Al₂O₃-Cr₂O₃-MgO system. DETAILED DESCRIPTION OF THE INVENTION To determine if some unique compositions existed with the Al₂O₃-Cr₂O₃-MgO system, three areas within this system were explored as shown in Fig. 1. Within Area I, a mix A was made which had an analysis of 70 wt.% alumina, 15 wt. % chromia, and 15 wt. % magnesia. This mix is represented by a dot within the center of area I. Likewise, to investigate the magnesia-rich portion of the ternary, a mix B was made which corresponded to the center of Area II (70 wt. % magnesia, 15 wt.% alumina, and 15 wt.% chromia). To explore the chromia-rich portion of this system, four contrasting mixtures were investigated. These mixtures are labelled C, D, E, F, and are found within area III. To obtain the desired positions within the ternary, mixes (Table I) were formulated using various fused and sintered grains having contrasting levels of alumina, chromia and magnesia. Various levels of fine alumina, chromia and magnesia were blended with the coarse fused grains in order to obtain the desired screen analysis as shown in Table II. The mixes were blended with a temporary binder, such as, 3.3 wt. % lignosulfonate with 0.4 wt. % motor oil, dry pressed into a brick shape, and fired at an elevated temperature, for examples, 1543°C (2810° F) for 10 hours. Some mixes contained a very minor water addition to achieve a pressable consistency. The physical properties of the brick were then measured (Table III). It is well known that high purity refractory oxides such as the three components in this system are difficult to sinter and produce a shape with low porosity. This occurs because these pure materials, especially chromia, are chemically inert. In spite of this, it was quite unexpected that the two high chromia mixes (Mix E and F) had low apparent porosities near 12.5%. These two compositions also had relatively high hot strength. This further indicates that the shapes had undergone sintering. Mixes B and D had unexpectedly low Modulus of Elasticity (MOE) values. These low MOE values may indicate that these compositions would have excellent thermal shock resistance. It is quite unusual to find fused grain shapes with low MOE values since it is generally accepted that shapes made predominantly from fused grains have poor thermal shock resistance. To determine the corrosion resistance of the compositions within the system Al₂O₃-Cr₂O₃-MgO, a series of slag tests were conducted (Table IV). In an AOD slag test Mix A with 15% chromia showed over twice as much erosion as the standard composition. The standard used in these tests was a composition called Guidon ®. This standard composition consists essentially of fused magnesia-chrome ore and it has by chemical analysis about 59 wt.% magnesia, 20 wt.% chromia, and 7 wt.% alumina. Mix E, however, was remarkable in its ability to resist the corrosive effects of the AOD slag. It was virtually unaffected in this test. Mixes B and E did not show any distinctive advantage in a combination ladle slag and degasser slag test. This test was developed to simulate a degasser environment in which early in the operation a basic slag can be drawn into the degasser and later the slag in contact with the refractories is enriched in iron oxide. In an electric furnace slag test Mixes E and F and to a lesser degree Mixes C and D showed an advantage over the standard. Mixes E and F were remarkable in their ability to resist this slag. However, Mix F in spite of its low erosion loss appeared to be prone to bloating during the test. The tendency of some of these mixes to bloat may be due to fluctuating changes in the valence states of iron oxide. The results of these slag tests clearly show that unique slag-resistant compositions exist within Area III of Fig. 1. Compositions within this area are very refractory as shown by Fig. 2 which indicates compositions within this area having melting points between about 1950-2050° C (3542-3722° F). Fig. 3 indicates that there are two main phases which are stable at high temperatures in Area III. These phases are a solid solution series between magnesia aluminate and picrochromate and a magnesia phase which contains a solid solution of chromia and alumina. As such, the present invention is directed to the refractory compositions within Area III of Fig. 1 as well as those compositions which lie 5 wt.% on the outside of area III towards all three end members of the ternary diagram, i.e. a high purity fused refractory grain consisting essentially of 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia. The present invention is also directed to a refractory shape which contains more than 5 wt.% of said high purity fused grain. In another aspect the present invention is directed to an improved refractory lining for an argon oxygen decarburization "AOD" vessel as well as an improved refractory lining for an electric arc furnace which contains more than 5 wt.% of a fused grain made from 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia. Table VI shows the chemical analysis of compositions A, B, C, D, E, and F shown in Figure 1 and described in Tables I-IV. Thus it will be appreciated that as a result of the present invention, low porosity, high purity refractory brick essentially comprised of chromia, alumina, and magnesia can be manufactured and said refractories are shown to have exceptional resistance to acid as well as basic slags. Thus, in a preferred embodiment the present invention also relates to a high purity fused grain in which the grain contains no more than 2 wt.%, preferably less than 1 wt.% total impurities such as silica, iron oxide and lime, as well as to a refractory shape containing said grain.

Claims: A high purity fused refractory grain consisting essentially of 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia. The high purity fused grain according to claim 1, in which the grain contains no more than 2 wt.%, preferably less than 1 wt.% total impurities such as silica, iron oxide, and lime. A refractory shape which contains more than 5 wt.% of the high purity fused grain consisting essentially of 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt. % magnesia. The refractory shape of Claim 3, wherein the grain contains no more than 2 wt.%, preferably less than 1 wt.% total impurities such as silica, iron oxide, and lime. An improved refractory lining for an Argon Oxygen Decarburization (AOD) vessel which contains more than 5 wt.% of a fused grain made from 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia. An improved refractory lining for an electric arc furnace which contains more than 5 wt.% of a fused grain made from 35 to 85 wt.% chromia, 5 to 55 wt.% alumina, and 5 to 45 wt.% magnesia.

Inventors: PAVLICA STANLEY R, RENKEY ALBERT L, PAVLICA, STANLEY R., RENKEY, ALBERT L.
Applicants: INDRESCO INC, INDRESCO INC.
Application year: 1992
Main classification: C04B35
IPC codes: C04B35