MOLTEN METAL PROCESSING APPARATUS

Abstract

The present invention relates to a composite material comprising wollastonite fibres embedded within a ceramic matrix. The wollastonite fibres are bonded to the ceramic matrix by a glassy bonding phase comprising a glass component comprising at least 80 wt % of oxides of calcium, silicon and aluminium. The material is used in the processing of molten metal, e.g. as a pump, degasser, flux injector or scrap submergence device.

Claims

1. A molten metal processing apparatus comprising acomposite material comprising wollastonite fibres embedded within a ceramic matrix, wherein the wollastonite fibres are bonded to the ceramic matrix by a glassy bonding phase comprising a glass component comprising at least 80 wt % of oxides of calcium, silicon and aluminium.

2. The apparatus according to claim 1, wherein glassy bonding phase further comprises mullite.

3. The apparatus according to claim 1, wherein the mullite is in a form of crystallite particles embedded into the glass component.

4. The apparatus according to claim 1, wherein the wollastonite fibres are bondedly embedded into the glassy bonding phase.

5. The apparatus according to claim 1, wherein the comprises: (a) 0.5 to 20 wt % wollastonite fibres; (b) 0.5 to 40 wt % glassy bonding phase; (c) 0 to 50 wt % ceramic matrix; (d) 0 to 50 wt % carbon material; and (e) 0 to 15 wt % other additives, wherein a sum of components (a) to (e) is greater than 90 wt % of the composite material.

6. The apparatus of claim 5, wherein the carbon material comprises graphite and/or amorphous carbon.

7. The apparatus of claim 5, wherein the ceramic matrix comprises a beta silicon carbide and/or alpha silicon carbide.

8. The apparatus according to claim 1, wherein the glass component comprises greater than 90 wt % of oxides of calcium, aluminium and silicon.

9. The apparatus according to claim 1, wherein a content of oxides of aluminium and silica in the glass component is at least 60 wt %.

10. The apparatus according to claim 1, wherein there is a gradient of calcium concentration across the glass component of the glassy bonding phase.

11. The apparatus according to claim 1, wherein a concentration of calcium is higher in the glass component proximal to the wollastonite fibres relative to the glass component distal to the wollastonite fibres.

12. The apparatus according to claim 1, wherein the glassy bonding phase bonds the wollastonite fibre, ceramic matrix and/or graphite, if present, together.

13. The apparatus according to claim 1, wherein the ceramic matrix comprises one or more of silica; alumina; carbides of Si, Ti, W, Ta, Nb, Zr, Hf, V, Cr, Mo; silicon nitride; magnesia; zirconia; boron nitride; and aluminium nitride.

14. The apparatus according to claim 1, comprising: (a) 0.5 to 10 wt % wollastonite fibres; (b) 0.5 to 30 wt % glassy bonding phase; (c) 20 to 50 wt % silicon carbide; (d) 5.0 to 50 wt % graphite; and (e) 1 to 15 wt % other additives, wherein a sum of components (a) to (e) is greater than 90 wt % of the composite material.

15. (canceled)

16. The apparatus according to claim 15, wherein the apparatus is selected from the group consisting of a pump, a degasser, a flux injector and a scrap submergence device.

17. The apparatus according to claim 16, wherein the apparatus is a degasser and a shaft and/or a rotor of the degasser comprises the composite material.

18. The apparatus according to claim 17, wherein the apparatus comprises a shaft and a rotor, and wherein the shaft and the rotor comprise a one piece construction.

19. The apparatus according to claim 18, wherein the shaft comprises a diameter that gradually increases in size proximal to the rotor.

20. The apparatus according to claim 19, wherein an interface angle between the shaft and the rotor is at least 100°.

21. A process for producing the apparatus of claim 1 comprising: (a) providing a precursor composite mixture comprising wollastonite fibres, a ceramic matrix and a glassy bonding phase or precursors thereof; (b) depositing the mixture into a mould; and (c) sintering the mixture at a temperature of at least 800° C. for sufficient time to partially transform the wollastonite fibres into the glassy bonding phase.

22-27. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0074] FIG. 1a is a schematic diagram of the process of producing an apparatus of the present disclosure using clay as a green binder.

[0075] FIG. 1b is a schematic diagram of the process of producing an apparatus of the present disclosure using carbonaceous resin as a green binder.

[0076] FIG. 2 is are XRD spectra of a composite material of the present disclosure (top spectra) compared to a conventional composite material (bottom spectra).

[0077] FIG. 3 is a SEM image of wollastonite fibres used in the composite material of the present disclosure.

[0078] FIG. 4 is a SEM image of the composite material after being sintered at 1260° C.

[0079] FIG. 5 is another a SEM image of the composite material after being sintered at 1260° C.

[0080] FIG. 6a is a photograph of a one piece shaft rotor of the present disclosure after pressing.

[0081] FIG. 6b is a photograph of a one piece shaft rotor of the present disclosure after sintering and machining.

[0082] FIG. 7 is a SEM image of a composite material derived from a resin bonded formulation.

DETAIL DESCRIPTION OF A PREFERRED EMBODIMENT

[0083] As illustrated in FIG. 1 a, the apparatus of the present disclosure may be produced through mixing graphite, a ceramic (e.g. SiC), a source of alumina (e.g. clay) and additives (e.g. FeSi and/or silicon metal as an oxidation inhibitor for the graphite).

[0084] The mixture is dried at 120° C. to reduce the moisture level down from about 15 to 30 wt % to less than 1.0 wt % or less than 0.5 wt %. The mixture is then crushed to achieve an average particle size distribution of 10 μm to 2 mm. Wollastonite fibre and further SiC then added and mixed, with the homogeneous mixture aged at least 8 hrs before drying and filling the moulds of the apparatus. The moulds are then iso-pressed at 400 bar for 25-60 seconds after which the green ceramic is dried at 120° C. before being sintered at 1260° C. for sufficient time for the wollastonite to partially react with the clay to form a SiO.sub.2—CaO—Al.sub.2O.sub.3 glassy bonding phase. Preferably, the green ceramic is sintered for sufficient time at the sintering temperature to transform some of the clay material into mullite.

[0085] It will be understood that a combination of sintering temperature and sintering time is able to produce the glassy bond and the mullite. For the formation of the glassy bond, the sintering temperature is preferably at least 800° C. or at least 900° C. or at least 1000° C. or at least 1100° C. or at least 1200° C. For the formation of the mullite phase the sintering temperature is preferably at least 1000° C. or at least 1100° C. or at least 1200° C. or at least 1300° C.

[0086] The moulded apparatus may then be machined to the required shape and surface finish. An optional sealant may be applied, such as the vacuum impregnation of the apparatus with a borax-boric acid solution and subsequent firing at 980° C., followed by additional machining if required.

[0087] In a variation, the inorganic binder (e.g. clay) may be partially replaced with a carbonaceous resin binder. Within this embodiment, the glassy bonding phase forms by reaction of fibre with alumina/clay present in the mix. An alternative source of alumina may be required from which the glassy bond phase could be derived from. In this variant, the glassy bonding phase plus a beta form of silicon carbide act as both a binder and a graphite oxidation inhibitor.

EXAMPLES

[0088] Raw Materials:

[0089] SiC Particle Size—75-400 μm, Purity—4=95%

[0090] Graphitep13 Particle Size—200-600 μm, Purity—>92%

[0091] Clay Particle Size—5-40 μm, loss on Ignition—10-13%

[0092] clay composition: alumina 28-35%, Silica 50-58%, Iron oxide 2-3%, titanium oxide 1-3% and alkali and alkaline metals 2-3%.

[0093] FeSi Particle size—50-180 μm, Silicon—71.5-80%

[0094] Borax—Particle size<75 μm, % H2O at 110° C.-<1=0.3%

[0095] Si metal Particle size—40-100 μm, Purity—min 96%

[0096] Binder Resin—Novolac™ with about 80% solids

[0097] Viscosity at 75° C.-13-17 Poise, Solid content at 160° C., 2hrs—79.5-82%

[0098] Transverse Bending Strength (TBS), Porosity and Oxidation Resistance (Graphite)

[0099] Composite mixtures (standard clay bonded mix) were formed comprising:

[0100] 36 parts by weight silicon carbide

[0101] 29 parts by weight graphite

[0102] 21.5 parts by weight clay

[0103] 5 parts by weight silicon metal

[0104] 2 parts by weight FeSi

[0105] 0 (C-1), 2.5 (E-1), 5.0 (E-2) and 7.5 parts (E-3) wollastonite fibre

[0106] The mixture was moulded into test samples (200 m×150 mm×150 mm) and sintered at 1260° C. for 60 minutes. The test samples were then tested for density, porosity, flexural strength and oxidation resistance, with the results presented in Table 1.

TABLE-US-00001 TABLE 1 E-1 E-2 E-3 2.5 wt % 5.0 wt % 7.5 wt % C-1 Wollastonite Wollastonite Wollastonite Density, gm/cc  2.19 ± 0  2.19 ± 0 2.17 ± 0    2.16 ± 0 Porosity, %   14 ± 0   12 ± 0 13 ± 0   13 ± 0 TBS, MPa 17.44 ± 1 22.67 ± 1 21 ± 1 21.22 ± 2 weight loss % 5.8 4.8 4.9 4.5 750° C., 1 hr

[0107] The results indicate that the additional wollastonite fibre results in a significant increase in flexural strength, with example E-1 having about 30% greater flexural strength compared to the test sample produced without wollastonite fibre. Furthermore, the additional of wollastonite fibre also increased oxidation resistance. Improved oxidation resistance (as indicated with a lower % weight loss) is thought to be as a result of an increase in the glassy bonding phase from the dissociation of the wollastonite fibres around 800° C. and higher, which results in a reduced porosity of the composite.

[0108] FIG. 2 provides a comparison of the sample E-1 (top XRD spectra) with the comparative example C-1 (bottom XRD spectra). The wollastonite crystalline structure is clearly visible along with increased levels of mullite, which the disassociated wollastonite may have also contributed to. The graphite phase was not characterised in the XRD analysis. It is estimated that the proportion of wollastonite fibres that were transformed into the glassy bonding phase or anorthite phase (CaSi.sub.2Al.sub.2O.sub.8) is approximately 78 wt %.

[0109] FIG. 3 is an SEM image of wollastonite fibres as received, with the fibres being of various length in the range of about 2 μm to 50 μm. FIG. 4 is an SEM of the composite material after it has been sintered at 1260° C. The EDS analysis of particle (A) indicates that it comprises Ca—Si—Al—Oand trace amount of Na and K indicative of the glassy bonding phase. However, the elongated shape of particle A is suggestive that the core of the particle comprises wollastonite and the mechanical properties thereof. It is postulated that due to the gradual and incomplete disassociation of the wollastonite fibres during the sintering process that there will be a gradient of calcium concentration stemming from the interface of the wollastonite fibre; the glassy bonding phase immediately adjacent the fibre that the calcium is mitigating into; and the glass phase further removed from the wollastonite fibre, which may still comprise a composition similar to that of the clay from which is was derived. EDS analysis confirms the concentration gradient calcium extending from the fibre/glassy bonding phase boundary. The differences in glass composition means that the glassy bonding phase comprises a portion which has a lower melting point conducive to lowering the composite material's porosity and portion which has a higher melting point conducive to increasing the erosive resistance of the composite material.

[0110] EDS analysis also confirmed higher concentration of Al at the grain boundaries in association with Si and Ca, which is consistent with the formation of anorthite during the sintering process.

[0111] The glass component of the glassy bonding phase had a composition of approximately 20 wt % CaO, 37 wt % Al.sub.2O.sub.3 and 43 wt % SiO.sub.2 immediately adjacent the wollastonite fibre.

[0112] Whilst, on a practical level, it may be difficult to analyse regions proximal and distal of the wollastonite fibre, a variation of glass composition in terms of calcium level will also be consistent with this glassy bonding phase formation mechanism.

[0113] As illustrated in the SEM image of FIG. 5, the composite material composites a silicon carbide phase 10 and a graphite phase 30 which is bonded together with a glassy bonding phase 20. EDS spectra confirmed that silicon carbide phase 10 comprised silicon and carbon; the graphite phase 30 comprised essentially carbon and the glassy bonding phase 20 comprised calcium, aluminium, silicon, iron and oxygen. Wollastonite fibres detected by the XRD spectra (FIG. 2) would be expected to be embedded in the glassy bonding phase 20. The image confirms that the glassy bonding phase is securely bonding to the silicon carbide and graphite phase with no substantial presence of voids in the structure.

[0114] Hot Flexural Strength

[0115] C-2: standard clay bonded mix with 0% wollastonite.

[0116] C-3: standard clay bonded mix with 0% wollastonite with vacuum impregnation with a borax-boric acid solution and subsequently fired at 980° C.

[0117] E-4: standard clay bonded mix with 2.5 wt % wollastonite.

[0118] E-5 standard clay bonded mix with 2.5 wt % wollastonite with vacuum impregnation in accordance with C-3.

[0119] The results (Table 2) demonstrate that the addition of wollastonite decreases porosity by 19% prior to vacuum impregnation; and that hot flexural strength (or Hot Modulus of Rupture (HMOR)) substantially increases at 800° C., the maximum operating temperature of the apparatus in contact with molten aluminium. The wollastonite examples have a decreased hot flexural strength at 1200° C. due to the softening of the glassy bonding phase at these elevated temperatures.

TABLE-US-00002 TABLE 2 C-2 C-3 E-4 E-5 Density, gm/cc 2.19 ± 0 2.22 ± 0 2.19 ± 0 2.22 ± 0 Porosity, % .sup. 16 ± 0 .sup. 14 ± 0 .sup. 13 ± 1 .sup. 13 ± 1 TBS, MPa   8 ± 1   9 ± 1 .sup. 10 ± 1 .sup. 11 ± 1 @RT HMOR, MPa 3.68 ± 1 5.85 ± 0 5.24 ± 1 8.04 ± 1 @800° C. HMOR, MPa 4.50 ± 2 4.44 ± 0 4.05 ± 1 2.57 ± 0 @1200° C. Thermal NA 35.6 NA 32 conductivity, W/mK at RT % Weight loss   5.3 ± 0.5   3.6 ± 0.2   4.9 ± 0.4   2.7 ± 0.1 @ 750/1 Hr.

[0120] FIG. 6a is a photo of a one piece shaft/rotor apparatus for the use in molten aluminium refining. FIG. 6b is a photo of the same apparatus after machining has taken place, resulting in a graduated shaft diameter between line BC. The interface angle between the shaft and rotor (angle ABC) is approximately 150° resulting in a mechanically robust shaft/rotor free of any connection joint (i.e. integral). The intersection joint ABC is defined by a radius of 25 mm.

[0121] The material of construction is such that the usually high wear rotor component is sufficiently high wearing that a two piece design is not required to enable the rotor to be regularly replaced due to performance deterioration resulting from erosion of the rotor.

Additional Examples

[0122] The following examples provide a variety of formulations used to produce composite materials comprising a glassy bonding phase derived from wollastonite. The relevant functionality of the derived composites extends beyond the reported TBS and oxidative resistance. Changes to the proportion of the components influence other functionality including thermal shock, hardness and HMOR.

[0123] Resin Bonded Formulations

TABLE-US-00003 TABLE 3 R 1 R 2 R 3 R 4 R 5 R 6 R 7 R 8 R9 Wollastonite, wt % 1.0 2.5 3.5 5.0 10 10 10 10 7.0 SiC, wt % 30 39 39 40 42 20 10 57 38 Graphite, wt % 40 35 30 25 20 39 43 5 40 Si wt % 9.5 10 10 6 11 11 11 2 0 FeSi wt % 3 3 3 2.5 2.5 2.5 2.5 2.5 0.5 Borax wt % 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 Clay, wt % 3 1 1 8 1 10 10 10 5 Alumina, wt % 10 6 10 10 10 4 10 10 6 Binder Resin, wt % 10 10 10 10 10 10 10 10 10 Density, g/cc 2.06 2.08 2.01 2.15 2.13 1.86 1.86 1.97 1.90 Porosity, v/v % 14 ± 0 15 ± 1 20 ± 1 16 ± 0 18 ± 1 23 ± 1  23 ± 1  32 ± 0  24 ± 1 TBS, MPa 12 ± 0 13 ± 1 12 ± 1 13 ± 1 18 ± 0 6 ± 2 4 ± 0 2 ± 1  6 ± 0 Oxidation at  8 ± 2  6 ± 0  7 ± 1  7 ± 1  5 ± 1 7 ± 2 9 ± 2 6 ± 0 14 ± 1 750° C. for 1 hr

[0124] With reference to Table 3, examples R1 to R5 cover a range of wollastonite contents, with oxidation resistance values being generally reflective of the graphite content in the formulation. The TBS value remained relatively stable except for Example R5, which exhibited the highest transverse bending strength due to the specific balance of components. SiC content was found to have a positive correlation to strength, however example R8 where SiC highest, strength is reduced, due to less bonding phase resulting high porosity. Likewise, insufficient SiC may also adversely affect the TBS value, as indicated in Examples R6 & R7.

[0125] With Reference to FIG. 7 (Example R6), the resin bonded formulations when processed in accordance to the process as outline in FIG. 1b, results in a composite comprising silicon carbide particles (spectrum 35) embedded in graphite (spectrum 36 & 37) and the presence of a glassy bonding phase (spectrum 38). The glassy bonding phase contained the oxide forms of silicon, calcium and aluminium.

[0126] Examples R2 and R4 have the same strength, although the Si metal content in the formulation is lower in R4. This is attributed from higher wollastonite and clay content which forms an increased glassy bonding phase. The additional glassy bonding phase compensates for the reduced levels of beta SiC derivable from the lower silicon metal content in the starting formation. The reduced Si metal content in R4 has also resulted in a relatively higher oxidation rate taking into account the lower graphite level in Example R4.

[0127] Higher formation of glassy bonding phase and the presence of beta SiC (derived from Si metal and carbonised resin) in Example R7 results in improved oxidative resistance relative to the composite derived from the formulation in Example R9.

[0128] Clay Bonded Formulations

TABLE-US-00004 TABLE 4 C-A C-B C-C C-D C-E C-F Wollastonite, % 0.5 10 1.0 10 1.0 10 SiC, % 15 15 25 25 50 40 Graphite, % 50 50 29 20 5 5 Si Metal 5 5 5 5 5 5 FeSi % 5 5 5 10 4 10 Clay % 24.5 15 35 30 35 30 Density gm/cc 2.07 2.05 2.20 2.16 2.35 2.19 Porosity % 15 ± 0 13 ± 1 13 ± 0 17 ± 2 16 ± 0 19 ± 1 TBS, MPa 15 ± 1 15 ± 1 17 ± 0 20 ± 1 24 ± 0 25 ± 4 Oxidation at 10 ± 2  9 ± 2  7 ± 1  5 ± 0  2 ± 1  2 ± 0 750° C. for 1 hr

[0129] Table 4 illustrates a range of formulations and TBS and oxidative resistance of the derived composite materials formed in accordance to the process used for sample E-1. The wollastonite content of the formulations is at the boundaries of the preferred ranges, with the higher wollastonite content generally correlating to higher TBS values.