Methods for producing silicon carbide whisker-reinforced refractory composition

Abstract

Methods for forming monolithic refractory compositions may include providing a particulate refractory composition including 2 to 90 mass-% alumina, aluminosilicate, or mixtures thereof; 2 to 70 mass-% silicon carbide; 2 to 10 mass-% carbon; 1 to 10 mass-% Si powder; 1 to 3 mass-% microsilica; and up to 5 mass-% ferrosilicon. The methods may further include adding an amount of water to the particulate refractory composition to form a uniform mixture, installing the uniform mixture and allowing it to set, such that the monolithic refractory composition is required, and heat-treating the set mixture at a temperature no higher than 1200 C. under atmospheric conditions to form a monolithic refractory composition. The methods may optionally include heat-treating the obtained monolithic refractory composition to form silicon carbide whiskers within the monolithic refractory composition.

Claims

1. A method of forming a monolithic refractory composition, the method comprising: (a) providing a particulate refractory composition comprising: (i) 2 to 90 mass-% alumina, aluminosilicate or mixtures thereof; (ii) 2 to 70 mass-% silicon carbide; (iii) 2 to 10 mass-% carbon; (iv) 1 to 10 mass-% silicon powder; and (v) 1 to 3 mass-% microsilica; (b) adding an amount of water to the particulate refractory composition to form a uniform mixture; (c) allowing the uniform mixture to set; (d) heat-treating said set mixture at a temperature no higher than 1200 C. under atmospheric conditions to form the monolithic refractory composition; and (e) heat-treating the monolithic refractory composition to form silicon carbide whiskers within the monolithic refractory composition.

2. The method according to claim 1, wherein in step (a), the particulate refractory composition further comprises up to 5 mass-% ferrosilicon.

3. The method according to claim 1, wherein in step (a), the particulate refractory composition further comprises up to 5 mass-% cement.

4. The method according to claim 1, wherein said step (e) is carried out by contacting said monolithic composition obtained at the end of step (d) with a molten metal.

5. The method according to claim 1, wherein said step (e) of heat-treating is carried out at a temperature of up to 1600 C.

6. The method according to claim 1, wherein step (b) comprises applying the uniform mixture to a vessel by casting, ramming, gunning, or spray casting before allowing the uniform mixture to set.

7. The method according to claim 1, wherein the particulate refractory composition of step (a) comprises from 5 to 50 mass-% silicon carbide, on the basis of the total mass of the particulate refractory composition.

8. The method according to claim 1, wherein the particulate refractory composition of step (a) comprises from 1 to 6 mass-% microsilica, on the basis of the total mass of the particulate refractory composition.

9. The method according to claim 1, wherein the particulate refractory composition of step (a) comprises from 2 to 5 mass-% carbon, on the basis of the total mass of the particulate refractory composition.

10. The method according to claim 1, wherein the carbon in said particulate refractory composition of step (a) is present as carbon black, graphite, fusible carbon, or a mixture thereof.

11. The method according to claim 1, wherein the particulate refractory composition of step (a) is substantially free of TiO.sub.2.

12. The method according to claim 1, wherein said monolithic refractory composition forms a refractory lining having a thickness ranging from 7 mm to 10 mm.

13. The method according to claim 1 wherein step (e) of heat-treating is repeated at least once.

14. The method according to claim 13, wherein step (e) of heat-treating is repeated throughout the lifetime of the monolithic refractory composition.

15. The method according to claim 1, wherein the mass ratio of silicon powder to microsilica in the particulate refractory composition of step (a) ranges from 1:1 to 3:1.

16. A monolithic refractory composition formed by the method according claim 1.

17. A method of forming a refractory composition, the method comprising: adding water to a particulate composition to form a mixture, the particulate composition comprising: (i) 2 to 90 mass-% alumina, aluminosilicate, or a mixture thereof; (ii) 2 to 70 mass-% silicon carbide; (iii) 2 to 10 mass-% carbon; (iv) 1.5 to 4 mass-% silicon powder; and (v) 1.3 to 2 mass-% microsilica; depositing the mixture on a surface of a vessel; applying a first heat treatment to the deposited mixture at a first temperature ranging from 800 C. to 1200 C. to form the refractory composition, the refractory composition forming a refractory lining on the surface of the vessel; and applying a second heat treatment to the refractory composition at a second temperature higher than the first temperature, the second temperature ranging from 1200 C. to 1600 C., wherein the second heat treatment forms silicon carbide whiskers in the refractory composition.

18. The method of claim 17, wherein applying the second heat treatment includes contacting an inner surface of the refractory composition with a molten metal.

19. A method of forming a refractory composition, the method comprising: adding water to a particulate composition to form a mixture, the particulate composition comprising: (i) 2 to 90 mass-% alumina, aluminosilicate, or a mixture thereof; (ii) 2 to 70 mass -% silicon carbide; (iii) 2 to 10 mass-% carbon; (iv) 1 to 10 mass-% silicon powder; and (v) 1 to 3 mass-% microsilica; depositing the mixture on a surface of a vessel: applying a first heat treatment to the deposited mixture at a first temperature less than 1200 C. to form the refractory composition, wherein the retractory composition has a thickness ranging from 7 mm to 10 mm; and applying a second heat treatment to the refractory composition by contacting the refractory composition with a molten metal at a second temperature ranging from 1200 C. to 1600 C., wherein the second heat treatment forms silicon carbide whiskers in the refractory composition.

20. The method of claim 19, wherein the particulate composition comprises 1.5 to 4 mass-% silicon powder with respect to the total mass of the particulate composition, and wherein the mixture comprises 4 to 6 mass-% water with respect to the total mass of the mixture.

Description

SHORT DESCRIPTION OF THE FIGURES

(1) The invention will be further illustrated by reference to the following figures:

(2) FIG. 1 represents an SEM picture of a monolithic refractory material below the surface zone, obtained according to the method of the present invention;

(3) FIG. 2 represents an SEM picture of a surface of a monolithic refractory material, obtained according to the method of the present invention;

(4) FIG. 3 represents an SEM picture of a monolithic refractory material obtained with to a method not according to the present invention;

(5) FIG. 4 represents an SEM picture of a monolithic refractory material obtained with to a method not according to the present invention;

(6) FIG. 5 represents an SEM picture of a monolithic refractory material obtained with the method according to the present invention;

(7) FIG. 6 represents an SEM picture of a monolithic refractory material obtained with the method according to the present invention;

(8) FIG. 7 represents an SEM picture of a monolithic refractory material obtained with the method according to the present invention;

(9) FIG. 8 represents an SEM picture of a monolithic refractory material obtained with the method according to the present invention;

(10) FIG. 9 represents a graphic illustration for carrying out a wedge splitting test on monolithic refractory samples;

(11) FIG. 10 represents the results of a wedge splitting test on a sample according to the present invention and a comparative sample.

(12) FIG. 11 represents the result of a corrosion test on a sample according to the present invention using acidic slag;

(13) FIG. 12 represents the result of a corrosion test on a sample not according to the present invention using acidic slag;

(14) FIG. 13 represents the result of a corrosion test on a sample according to the present invention using neutral slag;

(15) FIG. 14 represents the result of a corrosion test on a sample not according to the present invention using neutral slag;

(16) FIG. 15 represents the result of a corrosion test on a sample according to the present invention using basic slag;

(17) FIG. 16 represents the result of a corrosion test on a sample not according to the present invention using basic slag.

(18) It is understood that the following description and references to the figures concern exemplary embodiments of the present invention and shall not be limiting the scope of the claims.

DETAILED DESCRIPTION OF THE INVENTION

(19) The present invention according to the appended claims provides a method for forming monolithic refractory compositions, such as monolithic refractory linings for metallurgical vessels, in several distinct steps.

(20) It has been known to the skilled person in the art that refractory compositions with improved physical properties such as higher modulus of rupture and cold crushing strength may be achieved if said compositions comprise silicon carbide whiskers. However, as shown in the discussed state of the art, there has always existed a perceived difficulty of obtaining refractory compositions comprising silicon carbide whiskers, requiring either addition of pre-formed SiC-whiskers into the refractory composition prior to firing, or addition of specific additives to a refractory composition and firing under controlled atmosphere.

(21) According to the present invention, it has been found that silicon carbide whisker containing refractory compositions may be formed, for example when used as refractory linings in metallurgical vessels, by a new process of heat treating the refractory lining at a high temperature under non-inert atmospheric conditions, such as for example during use of the metallurgical vessel or the like. In order to achieve this, a refractory particulate composition as described herein is provided, and mixed with water to form a refractory paste for installation in the required environment and location. The refractory paste is then installed, allowed to set and heat-treated under normal atmosphere at a temperature of no higher than 1200 C., such as for example between 800 C. and 1200 C., or for example between 800 C. and 1000 C. The said heat treatment step at no higher than 1200 C. allows the refractory composition to set, harden and dry, such that essentially no more water is contained in said composition. The obtained monolithic refractory composition then may form a refractory lining which has refractory properties, but which may not yet provide improved physical properties caused by a presence of silicon carbide whiskers. The advantage of the monolithic refractory composition so obtained is that it may serve as a starting material for providing a reinforced monolitihic refractory composition containing silicon carbide whiskers.

(22) Silicon carbide whiskers are formed within the monolithic refractory composition during a second heat treatment step, during which the installed monolithic refractory composition after the (first) heat treatment undergoes a further (second) heat treatment at a temperature which is higher than the first heat treatment temperature, under non-inert atmospheric conditions, such as under a normal air atmosphere. During said second heat treatment step, which may be carried out at the same time as normal use of the metallurgical vessel, i.e. wherein some or all of the heat for heat treatment is provided by the presence of a molten metal in contact with said monolithic material, silicon carbide whiskers are formed within said monolithic refractory composition. The formation of said silicon carbide whiskers within said monolithic refractory composition (such as a monolithic refractory lining) then leads to improved physical properties such as increased modulus of rupture and increased cold crushing strength of the refractory lining. The said further heat treatment may be effected entirely or in part by the presence of molten metal, such as molten iron, within a metallurgical vessel lined with said refractory composition, causing the monolithic refractory composition to warm up to a second heat treatment temperature, at which temperature silicon carbide whiskers are formed within said refractory composition and an improved monolithic refractory composition is formed during use thereof.

(23) It has been found that, in the case of monolithic refractory linings for metallurgical vessels, and although at the time of the invention there was a pre-conception that silicon carbide whiskers are not normally formed during heat treatment under normal atmospheric conditions, silicon carbide whiskers can be formed in the non-exposed portions of the monolithic refractory composition, or more precisely within said composition in regions ranging from just below the internal facing surface of the monolithic refractory lining which is in direct contact with the molten metal from which heat is transferred to affect said heat treatment, and towards the external edge of said composition, up to a depth at which a threshold temperature is reached at which silicon carbide whiskers are formed. After an initial heat treatment during which silicon carbide whiskers are formed, the said heat-treatment may be repeated once or several times, such as each time when the metallurgical ladle comprising said refractory composition is in use, by heat transfer from a molten metal to the refractory composition. As the composition of the lining is repeatedly treated at high temperature, silicon carbide whiskers are formed.

(24) During normal use of the metallurgical lining, there will be regular wear and abrasion at the internally facing surface of the refractory lining, causing the portions of the lining close to the internal surface to be worn off, and exposing previously non-exposed regions thereof directly to molten metal providing heat for said heat treatment. As more and more of the said lining wears off at the internal side thereof, the refractory lining will be heated up to a temperature at which silicon carbide whiskers are formed up to a region which lies closer and closer towards the external (wall) facing end of the lining, such that silicon carbide whiskers are formed in regions that did not comprise silicon carbide whiskers prior to repeated heat treatment. In this way, as a refractory lining formed according to the present invention wears off over time and during use, the composition of the lining itself changes such that its physical properties are improved over time, and its lifetime therefore is considerably improved.

(25) According to one aspect of the present invention, an initial monolithic refractory composition is formed by (a) providing a particulate refractory composition comprising among others silicon carbide, (b) forming a uniform refractory mixture by addition of water, (c) installing said mixture where it is required (such as along the internal walls of a metallurgical vessel to a thickness required for providing adequate refractory protection), and (d) heat treating said installed mixture at a temperature such that the installed mixture sets and dries out sufficiently such that it can be used as a refractory lining during use of the metallurgical vessel. Silicon carbide whiskers are formed within said metallurgical vessel during a further (second) step of heat treating said lining, which in one embodiment is carried out during normal operation of the metallurgical vessel through heat transfer from a molten metal, such as molten iron, to said refractory lining.

(26) It has been found that besides the presence of silicon carbide in the particulate refractory composition, the presence of both metallic silicon powder and silica encourage the formation of silicon carbide whiskers under the conditions according to the method of the present invention. The present invention refers to methods for making refractory monolithics, which are installed e.g. by a ramming or casting technique and which are re-enforced by the presence of silicon carbide whiskers, formed in-situ during heating up cycles. The compositions of the materials formed by the inventive method are optimised in their metal, non-oxide and carbon contents, in order to allow the growth of the desired non-oxide (silicon carbide) phases (whiskers), initiated after reaching specific temperatures (e.g. about 1300 to 1400 C.). The method according to the invention does not require a strictly defined atmosphere composition. It was found that the in-situ formation of the silicon carbide whiskers and thus, the re-enforcement effect itself could be achieved by heating up in air.

EXAMPLES

(27) In the following, modulus of rupture (MOR) and cold crushing strength (CCS) were measured in accordance with European Standards EN 1402-5 and EN 1402-6 respectively.

(28) A number of particulate refractory compositions according to Examples 1 to 3 and Comparative Examples 1 to 3 were formed for use in the method according to the present invention. The components of these compositions are listed in Table I:

(29) TABLE-US-00001 TABLE I Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3 Brown fused 58.2 57.0 57.3 59.7 57.0 54.9 alumina 0-10 mm Calcined/reactive 12.6 12.3 13.9 13.0 13.0 13.3 alumina SiC 95%-99% 0.35 14.6 14.3 14.4 15.0 14 13.8 mm SiC 95%-99% 1 4.9 4.8 4.8 5.0 5 4.6 mm Microsilica 1.9 1.9 1.4 1.5 3.8 1.4 Calcium aluminate 1.5 1.4 1.4 1.5 1.4 1.4 cement Carbon carriers 3.9 3.8 3.3 3.3 3.8 7.3 Si powder <70 m 1.9 3.8 2.9 0.5 1.9 2.8 Al powder 0.39 0.38 0.38 0.40 0.38 0.37 Deflocculants 0.15 0.15 0.15 0.15 0.15 0.15

(30) The alumina used is particulate brown fused alumina, the particles having a particle size distribution such that all the particles have a particle diameter between 0 and 10 mm. Furthermore, the particle size distribution is chosen so broadly, that substantial amounts of particles are present in each of the particle diameter ranges 0 to 0.2 mm, 1 to 3 mm, 3 to 6 mm and 6 to 10 mm. The carbon carriers used are a mixture of carbon black, graphite and solid hydrocarbon having a carbon residue of at least about 5% by weight after coking (such as e.g. bitumen, asphalt or others). The deflocculants used are a mixture of STPP, anti-caking agents (e.g. sodium naphthalinsulfonate) and citric acid.

(31) The particulate refractory compositions displayed in Table I were mixed with water (4 to 6 mass-%), installed and fired under normal atmosphere at no more than 1200 C., and subsequently fired again under normal atmosphere at 1400 C. or 1600 C., in order to obtain dried blocks of approximately 32 kg finished monolithic refractory material. All blocks had an oxidised zone of material at their surfaces, of a thickness of approximately 7 to 10 mm.

(32) FIGS. 1 and 2 show SEM pictures of samples from the monolithic refractory material obtained according to Example 1 above after firing at 1400 C. under normal atmosphere for 5 hours. FIG. 1 is an SEM picture of an internal portion of the said block, which was not exposed to the atmosphere during firing. FIG. 2 is an SEM picture of an external sample of the said block (taken from within 10 mm of a surface), which was exposed to the atmosphere during firing. It can be clearly seen that whiskers are present both internally and on the surface of the block, while the occurrence of whiskers is more frequent internally.

(33) FIGS. 3 and 4 show SEM pictures of samples from the monolithic refractory material obtained according to Comparative Example 1 after firing at 1400 C. and 1600 C. respectively. FIGS. 5 and 6 show SEM pictures of samples from the monolithic refractory material obtained according to Example 1 after firing at 1400 C. and 1600 C. respectively. FIGS. 7 and 8 show SEM pictures of samples from the monolithic refractory material obtained according to Example 2 after firing at 1400 C. and 1600 C. respectively.

(34) In the case of Comparative Example 1, no whisker formation is detected at either firing at 1400 C. nor 1600 C. (FIGS. 3 and 4). In the case of Examples 1 and 2, some whisker formation can be observed after firing at 1400 C. (FIGS. 5 and 7), and strong or very strong whisker formation is detected after firing at 1600 C. (FIGS. 6 and 8).

(35) Furthermore, the mechanical properties of the finished products formed after firing at 1400 C. and 1600 C. using the particulate refractory compositions according to Examples 1 to 3 and Comparative Examples 1 to 3 were measured. The results are shown in Table II:

(36) TABLE-US-00002 TABLE II Ex. 1 Ex. 2 Ex. 3 Comp. Ex. 1 Comp. Ex. 3 MOR (1400 C.) 12.5 12.3 11.6 9.0 8.5 MOR (1600 C.) 12.6 13.0 13.6 9.6 8.4 CCS (1400 C.) 136.0 150.0 128.0 110.3 91.9 CCS (1600 C.) 126.6 136.1 141.4 98.9 79.3 N.B. The values for MOR and CCS are expressed in N/mm.sup.2

(37) The improved values for modulus of rupture and cold crushing strength are apparent.

(38) The samples obtained using the composition according to Example 1 and Comparative Example 1 were tested by a wedge splitting test. The cubic samples were cast, and fired at 1400 C. for 5 hrs according to the method of the present invention. The test method is represented in FIG. 9. The results of the test are graphically presented in FIG. 10.

(39) The wedge splitting test was initially developed to characterise the tensile-like behaviour of mineral based composites, which are known to be very brittle. The main parameters obtained from the results of that test are splitting tensile strength (directly correlated to the tensile strength) and the fraction energy (post cracking stability). As is seen in FIG. 10, the material formed according to the present invention withstood notable higher horizontal stress before cracking. The recalculated values of tensile strength were 4.13 MPa and 6.04 MPa for samples of Comparative Example 1 and Example 1 respectively.

(40) The wedge splitting test shows that the resistance against crack-initiation of monolithic refractory compositions formed according to the present invention (Example 1) was clearly better than that of the material formed according to the Comparative Example 1, and that the resistance against crack-propagation was comparable in both cases. Thus whisker formation increases the mechanical strength of material without causing increased brittleness.

(41) Corrosion resistance of the materials formed according to Example 1 and Comparative Example 1 was also tested. The corrosion resistance was tested against iron EN-GJL-250 and three different slags: (a) acidic slag of basicity 0.59 (representing acidic cupola slag); (b) normal slag of basicity 0.75 (representing normal cupola slag); and (c) basic slag of basicity 1.54 (representing the case of runners applications). Samples formed according to Example 1 and Comparative Example 1 were dried and exposed to the various slags for 18 hours case (a) and 8 hours for (b) and (c). The cut sections of the samples after test (a) are shown in FIGS. 11 and 12. The cut sections of the samples after test (b) are shown in FIGS. 13 and 14. The cut sections of the samples after test (c) are shown in FIGS. 15 and 16. The measured corrosion values (in mm.sup.2) are shown in Table III.

(42) TABLE-US-00003 TABLE III Ex. 1 Comp. Ex. 1 acidic slag 1032 1041 neutral slag 662 597 basic slag 964 957

(43) In all cases it is shown that the corrosion resistance of the monolithic refractory materials formed using the particulate refractory materials according to Example 1 and Comparative Example 1 are similar.

(44) It has been shown that monolithic refractory materials formed according to the method of the present invention acquire improved mechanical properties once fired at elevated temperatures under atmospheric conditions, while at the same time not suffering any drawbacks such as increase brittleness or reduced resistance to slag corrosion.