REFRACTORY FILTER

Abstract

A refractory filter suitable for filtering molten metal, such as steel, and a method and powdered composition for producing said filter. The filter comprises refractory material, said refractory material comprising: 60-90 wt % alumina; 8-30 wt % zirconia; and 3-20 wt % magnesia. The powdered composition comprises: 60-90 wt % alumina; 8-30 wt % zirconia; and 3-20 wt % magnesia, wherein the powdered composition comprises less than 12.5 wt % reactive alumina, calcined alumina or a mixture thereof, and wherein the remainder of the alumina is tabular alumina. The method comprises: providing a powdered composition in accordance with the invention; forming a filter precursor from the powdered composition and a liquid component; and firing the filter precursor to form a refractory filter.

Claims

1. A refractory filter for filtering molten steel, comprising refractory material, said refractory material comprising: 60-90 wt % alumina; 8-30 wt % zirconia; and 3-20 wt % magnesia.

2. The refractory filter of claim 1, wherein the refractory material is substantially silica-free.

3. The refractory filter of claim 1, wherein the refractory filter has a compressive strength of at least 4 MPa.

4. The refractory filter of claim 1, wherein the refractory filter has at least one first surface forming a side face of the filter and two opposed second surfaces forming the through-flow faces of the filter, the second surfaces having an area of no greater than 500 cm.sup.2.

5. The refractory filter of claim 1, wherein the filter is framed.

6. The refractory filter of claim 1, wherein the refractory filter is either a foam filter, a cellular filter, or a pressed filter.

7. The refractory filter of claim 1, wherein the refractory material further comprises up to 5 wt % titania.

8. The refractory filter of claim 1, wherein the refractory material is made using less than 12.5 wt % reactive or calcined alumina.

9. A powdered composition for making a refractory filter comprising 60-90 wt % alumina; 8-30 wt % zirconia; and 3-20 wt % magnesia, wherein the powdered composition comprises less than 12.5 wt % reactive alumina, calcined alumina or a mixture thereof, and wherein the remainder of the alumina is tabular alumina.

10. The powdered composition of claim 9, wherein the powdered composition comprises from 0 to 10 wt % reactive alumina, calcined alumina or a mixture thereof.

11. The powdered composition of claim 9, wherein the powdered composition comprises at least 60 wt % tabular alumina.

12. The powdered composition of claim 9, wherein the tabular alumina has a D50 particle size of less than 500 μm.

13. The powdered composition of claim 9, wherein the tabular alumina comprises a mixture of finer grade tabular alumina having a D50 particle size of 20 to 50 μm and coarser grade tabular alumina having a D50 particle size of 100 to 500 μm.

14. The powdered composition of claim 13, wherein the ratio of finer grade tabular alumina to coarser grade tabular alumina is from 40:60 to 60:40.

15. The powdered composition of claim 9, wherein the reactive alumina, when present, has a D50 particle size of less than 10 μm.

16. The powdered composition of claim 9, wherein the magnesia has a D50 particle size of less than 30 μm.

17. The powdered composition of claim 9, wherein the zirconia has a D50 particle size of less than 1 μm.

18. The powdered composition of claim 9, wherein the powdered composition comprises less than 1 wt % silica.

19. The powdered composition of claim 18, wherein the powdered composition is substantially free of silica.

20. The powdered component of claim 9, wherein the magnesia is at least partially replaced by ceria.

21. The powder composition of claim 9, further comprising up to 5 wt % titania.

22. Use of a powdered composition according to claim 9 to form a refractory filter.

23. A method for the production of a refractory filter, comprising: providing a powdered composition according to claim 9; forming a filter precursor from the powdered composition and a liquid component; and firing the filter precursor to form a refractory filter.

24. The method of claim 23, wherein the filter precursor is dried prior to firing.

25. The method of claim 23, wherein forming the filter precursor comprises 3D printing.

26. The method of claim 23, wherein forming the filter precursor comprises: combining the powdered composition and the liquid component to form a slurry, and impregnating a reticulated foam substrate with the slurry to form the filter precursor.

27. The method of claim 26, wherein the reticulated foam substrate is impregnated with the slurry by spraying, roller impregnation, dipping, centrifuging, or any combination thereof.

28. The method of claim 23, wherein the filter precursor is fired at a temperature of greater than 1500° C.

29. The method of claim 23, wherein the filter precursor is fired for at least 30 minutes.

30. The method of claim 23, wherein the filter precursor is fired in an oxidizing atmosphere.

Description

[0080] Embodiments of the invention will now be described by way of example and with reference to the accompanying FIGURES in which:

[0081] FIG. 1 is a graph showing the friability of refractory filters, as measured by the level of broken particles of filter material following vibration.

EXAMPLE 1

[0082] Preparation of a Refractory Filter

[0083] A reticulated polyurethane foam piece was impregnated with a slurry using a combination of rollers and spraying until a desired weight was achieved. The slurry comprised approximately 90% powdered composition and 10% rheology modifiers (anti-foamer, dispersants, humectant, binder and viscosity modifiers). Water was added so as to give the required slurry viscosity.

[0084] The impregnated foam piece was then dried in an oven set at 150° C. before being fired. The firing was conducted in a tunnel (continuous) kiln set at a temperature of 1620° C.

[0085] Cold Crush Strength

[0086] The cold crush strength test is used to assess the compression strength of a filter at room temperature. Cold crush strength was determined using a test method as specified by the German Foundry Association (BDG (Bundesverband der Deutschen Giesserei-lndustrie) Directive P100, September 2012 Edition), in accordance with DIN EN 993-5: Methods of test for dense shaped refractory products—Part 5: Determination of cold crushing strength. Briefly, a refractory filter (100×100×25 mm, 10 ppi, unframed), prepared as described above, was positioned on a support of 25 mm diameter. Using a ram of the same diameter, the filter was put under load at a speed of 20 mm/min until breakage occurred. The resultant maximum force was used to determine the cold crush strength.

[0087] Metal Pouring Test

[0088] Molten stainless steel at a temperature of 1610-1620° C. was poured through a refractory filter (100×100×25 mm, 10 ppi, unframed), prepared as described above. The filter was held in a two-sided support and positioned 700 mm beneath a bottom-pour ladle with a 30 mm nozzle. The filter was deemed to pass this test if it stayed intact and did not rupture when a minimum of 30 kg molten stainless steel was poured through the filter.

[0089] Results

[0090] Filters (unframed) having the dimensions 100×100×25 mm were prepared from 8 ppi reticulated polyurethane foam pieces using the method described above. Filters were made using different powder compositions according to the recipes in Table 1 below.

[0091] The compression strength of the filters and their ability to withstand a molten steel pouring test was tested as described above. The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Alumina (wt %) Zirconia Tabular Reactive (wt %) Compression (D50: (D50: D50: D50: Magnesia Other Molten strength Comp. 40 μm) 2.5 μm) 0.4 μm 15 μm (wt %) (wt %) steel test (MPa) A 70 0 30 0 0 0 Fail 4.5 B 70 29  0 0 0 .sup. 1.sup.b Fail 6.3 C 70 20  10 0 0 0 Fail 2.3 D 72 5 20 0 3 0 Pass 3.6 (30 kg) E 70 5 20 0 5 0 Pass 4.6 (150 kg) F 75 0 20 0 5 0 Pass 4.5 (50 kg) G 67.5 5 20 0 7.5 0 Pass 4.4 (30 kg) H 62.5 5 20 0 12.5 0 Pass 4.0 (50 kg) O 65 5 16 0 14 0 Pass 4.6 (50 kg) J 65 5 10 0 20 0 Pass 3.1 (30 kg) K 82 5 8 0 5 0 Pass 6.3 (50 kg) L 74 5 16 0 5 0 Pass 5.4 (50 kg) M 55 5 20 0 20 0 Pass 4.4 (50 kg) N 66 5 24 0 5 0 Pass 4.9 (50 kg) O 65 5 25 0 5 0 Pass 4.1 (30 kg) P 60 0 30 0 10 0 Pass 4.9 (50 kg) Q 70  5.sup.a 20 0 5 0 Pass 4.7 (100 kg) R 72.5 0 20 0 7.5 0 Pass 5.4 (50 kg) S 62 5 20 8 5 0 Pass 4.5 (30 kg) T 60 9 10 15 6 0 Fail 3.6 U 65 10  10 10 5 0 Pass 4.8 (30 kg) V 62.5  12.5 0 20 5 0 Fail 2.1 W 67 15  15 0 3 0 Fail 2.3 X 70 5 20 0 1.7  .sup. 3.3.sup.c Pass 2.6 (50 kg) Y 70 5 20 0 1.7  .sup. 3.3.sup.d Fail 5.9 .sup.aD50: 0.4 μm; .sup.bSilica; .sup.cCeria; .sup.dYttria

[0092] Filters made using Compositions A-C, which comprised no magnesia, did not pass the molten steel test and ruptured upon impact. Compositions D-J, which comprised between 3-20 wt % magnesia and 67.5-77 wt % alumina, passed the molten steel test. Compositions K-P, which comprised 8-30 wt % zirconia (D50 0.4 μm), also passed the molten steel test.

[0093] Composition E, which comprised 5 wt % magnesia, 20 wt % zirconia (D50 0.4 μm), 70 wt % tabular alumina and 5 wt % reactive alumina, was found to give a strong filter that was able to withstand up to 150 kg molten steel. Composition Q, which comprised reactive alumina having a smaller D50 particle size (0.4 μm vs 2.5 μm for composition E), also showed good strength in the metal pouring test.

[0094] Compositions R-W showed that filters comprising higher levels of reactive alumina (e.g. 12.5 wt % or above) and/or higher levels of zirconia with a D50 particle size of 15 μm (e.g. 15 wt % or above) were weaker and did not pass the molten steel test, although filters comprising a mixture of zirconia having smaller and larger particle sizes (e.g. compositions S and U) did pass the molten steel test.

[0095] Composition X, in which the magnesia was partially substituted by ceria, passed the molten steel pouring test, whereas Composition Y, in which the magnesia was partially substituted by yttria, failed.

EXAMPLE 2

[0096] Powdered composition E was selected for further testing.

[0097] Friability Test

[0098] The friability of a filter prepared from powdered composition E (referred to as “Filter E”) was compared to three commercially available framed and un-framed zirconia-based filters of the same dimensions (75×75×25 mm, prepared from a 10 ppi reticulated polyurethane foam), having zirconia levels of >90%. 117 of each type of filter were packed into a box, standing on edge in three layers. The box was vibrated on a table for 20 minutes. Following vibration, the crumbs resulting from breakage of the filters were weighed.

[0099] It was observed that Filter E had significantly lower friability than the commercially available filters (comparative examples X, Y, Z) (FIG. 1).

[0100] Comparison of the structure of Filter E with a standard zirconia filter by SEM analysis indicated that sintering of the refractory material is more complete in Filter E. This is thought to be the reason why the filter of the invention has lower friability than standard zirconia filters.

[0101] Deformation Test

[0102] A refractory filter of circular cross-section (150 mm diameter, 30 mm depth) was prepared from a 10 ppi reticulated polyurethane foam impregnated with a slurry formed from powdered composition E (Filter E′). The deformation of Filter E′ was compared with that of a commercially available filter having the same dimensions but a zirconia level greater than 90%. The filters were supported across a 110 mm span. A 170 g weight was placed on top of each filter, in the middle of the upper surface. The filters was exposed to a temperature of 1620° C. for 2.5 hours.

[0103] Following the test procedure, the deformation (i.e. sagging) of Filter E′ was measured as being 3 mm, whereas for the commercially available filter the deformation was 5 mm.

EXAMPLE 3

[0104] A further composition (Composition Z) based on composition E was formulated, with half of the 40 μm grade tabular alumina being replaced with a coarser grade of tabular alumina having a D50 particle size of 200 μm. The water demand of composition Z was found to be 15% less than composition E and composition Z showed even less shrinkage after firing (around 4.5% shrinkage compared with 6% shrinkage for composition E).

[0105] Filters made with composition Z (dimensions: 75×75×25 mm) were tested using the cold crush strength and metal pouring tests described in Example 1. The filters were found to have a higher crush strength than filters made using composition E, and were easily able to withstand 100 kg molten steel poured at 1640° C. without any sign of rupturing.

EXAMPLE 4

[0106] Composition Z was tested with addition of small quantities of titanium dioxide. Addition of 0.5 wt % titania to composition Z was found to increase shrinkage by an extra 1.5%, bringing the total shrinkage to 6% (in line with conventional zirconia filters). Addition of 2 wt % titania was found to increase shrinkage by an extra 4%.

[0107] The metal capacity of filters with composition Z and comprising 0.5 wt % titania was drastically improved compared with filters made using composition E. Circular filters having a diameter of 150 mm were able to withstand 600 kg molten steel without rupturing. The cold crush strength and friability performance of the filter was also found to be improved.

[0108] A filter made using a composition comprising 10 wt % zirconia, 5 wt % magnesia and 1 wt % titania, with the remainder being made up of a 50:50 mixture of 40 μm and 200 μm tabular alumina, also performed well and the slurry was found to be easier to pump.