Technical Field

[0001] The present disclosure relates to a zirconia-mullite composite, in particular a zirconia-mullite composite for refractory and high-temperature applications.

Background of the Disclosure

[0002] Alumina-zirconia- silica (AZS) refractories, refractories based on the ternary alumina-zirconia- silica system, are used for a number of applications, for example, for use in glass manufacturing as linings for glass furnaces. AZS materials are advantageous for refractory applications owing to their ability to retain their strength and dimensional integrity at high temperatures. AZS materials are suited particularly to the corrosive environments required for glass manufacturing owing to their high resistance to acidic slag conditions and thermal shock. AZS refractories are made commonly by fuse-casting, where an electric arc is used to fuse the material into a liquid (glass) and then cast into moulds. AZS refractories that are produced in this way typically have a microstructure of laths of Al ₂ O ₃ -ZrO ₂ eutectic, primary Al ₂ O ₃ and ZrO ₂ phases, and typically around 20 vol% residual glass. Consequently, the microstructures of large blocks of AZS refractories typically consist of ~5-20 wt% silica and approximately equal proportions of Al ₂ O ₃ and ZrO ₂ .

[0003] To produce these refractories, high-quality raw materials are required in order to produce a high-purity and high-performance product. As a result, it is expensive to create AZS refractories. Known starting materials include corundum (Al ₂ O ₃ ) and zircon (ZrO ₂ -SiCh), or gibbsite (Al(OH) ₃ ) and zircon; mullite (3AI ₂ O ₃ -2SiO ₂ ) and partially stabilised zirconia (Y- ZrO ₂ or Ce-ZrO ₂ ); high-purity clay (Al ₂ O ₃ .2SiO ₂ .2H ₂ O) and corundum and partially stabilised zirconia; or kyanite, sillimanite, or andalusite (Al ₂ O ₃ -2SiO) ₂ and corundum and partially stabilised zirconia.

[0004] AZS refractories are one of the few materials capable of long-term use in the corrosive acidic-slag-rich environment required for glassmaking. The high glass content and the high manufacturing cost, however, limit their applications in other industries.

[0005] There thus exists a need to provide an AZS refractory that can be made more inexpensively relative to existing refractories while still meeting performance requirements. [0006] The Al ₂ O ₃ -ZrO ₂ -SiO ₂ ternary system also has been investigated in order to create zirconia-mullite (ZrO ₂ -3Al ₂ O ₃ -2SiO ₂ ) composites in order to attempt to take advantage of specific attributes of mullite and zirconia. These composites have been investigated mostly for fracture and wear resistance properties rather than refractory applications, and thus the compositions of existing composites are limited to the binary zirconia-mullite system. Compositional deviation from this binary system risks the presence of residual glass, which decreases the fracture, wear, and corrosion resistance of the composites. Such compositions thus are not particularly suited to refractory applications and in particular are not suited for applications in the glassmaking industry.

[0007] The present invention seeks to provide a zirconia-mullite composite that can be produced relatively inexpensively and that is suitable for refractory applications.

Summary of the Invention

[0008] According to a first aspect, there is provided a method of forming a biphasic zirconia-mullite composite, comprising heating a body of material comprising alumina, zirconia, and silica to a temperature between approximately 1550 C and approximately 1700 C; wherein the body of material has a composition on an alumina-zirconia-silica (AZS) phase diagram within a region bounded by compositions having weight ratios of 98.0:2.0:0.0, 0.0:30.0:70.0, 0.0:67.0:33.0, and 67.0:33.0:0.0 ZrO ₂ :SiO ₂ :Al ₂ O ₃ .

[0009] In some embodiments, the region is bounded by compositions having weight ratios of 73.0:9.0:18.0, 68.0:28.0:4.0, 15.0:52.0:33.0, and 22.0:24.0:54.0 ZrO ₂ :SiO ₂ :Al ₂ O ₃ .

[0010] In some embodiments, the region is bounded by compositions having weight ratios of 67.0:13.0:20.0, 63.0:29.0:8.0, 18.0:50.0:32.0, and 28.0:24.0:48.0 ZrO ₂ :SiO ₂ :Al ₂ O ₃ .

[0011] In a second aspect, there is provided a method of forming a biphasic zirconiamullite composite, comprising heating a body of material comprising alumina, zirconia, and silica to a temperature between approximately 1550 C and approximately 1700 C; wherein the body of material has a composition on an alumina-zirconia-silica (AZS) phase diagram that falls within the light yellow region of FIGURE 1.

[0012] In some embodiments, the body of material has a composition on the AZS phase diagram that falls within the dark yellow region of FIGURE 1. [0013] In some embodiments, the body of material has a composition on the AZS phase diagram that falls within the green region of FIGURE 1.

[0014] In some embodiments, the body of material comprises fly ash.

[0015] In some embodiments, fly ash comprises between approximately 20 and 80 wt% of the body of material.

[0016] In some embodiments, fly ash comprises between approximately 28 and 65 wt% of the body of material.

[0017] In some embodiments, fly ash comprises between approximately 30 and 50 wt% of the body of material.

[0018] In some embodiments, fly ash comprises approximately 35 wt% of the body of material.

[0019] In some embodiments, the body of material comprises bauxite.

[0020] In some embodiments, bauxite comprises between approximately 2.5 to 20 wt% of the body of material.

[0021] In some embodiments, bauxite comprises between approximately 12 and 20 wt% of the body of material.

[0022] In some embodiments, the body of material comprises zircon.

[0023] In some embodiments, zircon comprises between approximately 18 and 65 wt% of the body of material.

[0024] In some embodiments, zircon comprises between approximately 35 and 65 wt% of the body of material.

[0025] In some embodiments, additional zirconia is added as a nucleating agent and template to the body of material prior to heating.

[0026] In some embodiments, additional zirconia is added in an amount between approximately 12 and 39 wt%. [0027] In some embodiments, additional zirconia is added in an amount between approximately 27 and 39 wt%.

[0028] In some embodiments, additional zirconia is added in an amount between approximately 33 and 39 wt%.

[0029] In some embodiments, the additional zirconia is unstabilised zirconia.

[0030] In some embodiments, mullite is added as a nucleating agent and template to the body of material prior to heating.

[0031] In some embodiments, the body of material includes fly ash and bauxite in a ratio that is approximately 74:3 fly ash:bauxite by weight.

[0032] In some embodiments, the body of material includes fly ash and bauxite in a ratio that is approximately 4: 1 fly ash:bauxite by weight.

[0033] In some embodiments, the body of material is heated for a time period greater than or equal to 10 hours.

[0034] In some embodiments, the time period is greater than or equal to 15 hours.

[0035] In some embodiments, the time period is greater than or equal to 20 hours.

[0036] In some embodiments, a flux is added to the body of material prior to heating.

[0037] In some embodiments, the body of material is heated to approximately 1600°C.

[0038] According to a third aspect, there is provided a biphasic zirconia-mullite composite formed by a process according to either the first or second aspects.

[0039] According to a fourth aspect, the biphasic composite comprises an equiaxed zirconia majority phase by volume and a fibrous mullite minority phase by volume, wherein the composite includes between approximately 37 and 73 wt% zirconia.

[0040] In some embodiments, the equiaxed zirconia majority phase comprises spherically shaped zirconia grains.

[0041] In some embodiments, the zirconia grains are sized in a range -3-15 pm. [0042] In some embodiments, the composite includes between approximately 37 and 73 wt% zirconia.

[0043] In some embodiments, the composite includes between approximately 46 and 73 wt% zirconia.

[0044] In some embodiments, the composite includes between approximately 46 and 67 wt% zirconia.

[0045] In some embodiments, the composite includes between approximately 48 and 67 wt% zirconia.

[0046] In some embodiments, the composite includes between approximately 51 and 67 wt% zirconia.

[0047] In some embodiments, the composite includes between approximately 56 and 67 wt% zirconia.

[0048] In some embodiments, the composite is formed from a body of material comprising at least one of the following: fly ash, bauxite, and/or zircon.

[0049] In some embodiments, the mullite phase includes percolated mullite.

[0050] In some embodiments, the composite is formed by a method according to the first or second aspect.

[0051] In a fifth aspect, there is provided the use of a zirconia-mullite composite according to the third or fourth aspect as a refractory material.

[0052] In some embodiments, the refractory material is a lining for a glassmelting furnace or glass tank.

[0053] According to a sixth aspect, there is provided a method of forming a biphasic zirconia-mullite composite, comprising heating a body of material comprising alumina, zirconia, and silica, wherein the body of material comprises at least one source of glass for transient liquid-phase densification and zirconia as a nucleating agent and template; wherein upon heating, the zirconia and/or mullite precipitates from the glass, effectively eliminating the glass, thereby effecting transient liquid-phase densification of the composite. [0054] In some embodiments, the body of material further comprises mullite as a nucleating agent and template.

[0055] In some embodiments, the body of material comprises fly ash and/or zircon as sources of glass.

[0056] Other aspects, features, and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are a part of this disclosure and which illustrate, by way of example, principles of the inventions disclosed.

Brief Description of the Figures

[0057] The present disclosure will become better understood from the following detailed description of various non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

[0058] FIGURE 1 shows a ternary phase diagram of the AZS system showing compositions A to R.

[0059] FIGURE 2 shows an SEM image of composition A at 50X magnification.

[0060] FIGURE 3 shows an SEM image of composition A at 300X magnification.

[0061] FIGURE 4 shows an SEM image of composition B at 50X magnification.

[0062] FIGURE 5 shows an SEM image of composition B at 300X magnification.

[0063] FIGURE 6 shows an SEM image of composition C at 50X magnification.

[0064] FIGURE 7 shows an SEM image of composition C at 300X magnification.

[0065] FIGURE 8 shows an SEM image of composition D at 50X magnification.

[0066] FIGURE 9 shows an SEM image of composition D at 300X magnification.

[0067] FIGURE 10 shows an SEM image of composition E at 50X magnification.

[0068] FIGURE 11 shows an SEM image of composition E at 300X magnification. [0069] FIGURE 12 shows an SEM image of composition F at 50X magnification.

[0070] FIGURE 13 shows an SEM image of composition F at 300X magnification.

[0071] FIGURE 14 shows an SEM image of composition G at 50X magnification.

[0072] FIGURE 15 shows an SEM image of composition G at 300X magnification.

[0073] FIGURE 16 shows an SEM image of composition H at 50X magnification.

[0074] FIGURE 17 shows an SEM image of composition H at 300X magnification.

[0075] FIGURE 18 shows an SEM image of composition I at 50X magnification.

[0076] FIGURE 19 shows an SEM image of composition I at 300X magnification.

[0077] FIGURE 20 shows an SEM image of composition I at 50X magnification.

[0078] FIGURE 21 shows an SEM image of composition J at 300X magnification.

[0079] FIGURE 22 shows an SEM image of composition K at 50X magnification.

[0080] FIGURE 23 shows an SEM image of composition K at 300X magnification.

[0081] FIGURE 24 shows an SEM image of composition L at 50X magnification.

[0082] FIGURE 25 shows an SEM image of composition L at 300X magnification.

[0083] FIGURE 26 shows an SEM image of composition M at 50X magnification.

[0084] FIGURE 27 shows an SEM image of composition M at 300X magnification

[0085] FIGURE 28 shows an SEM image of composition N at 50X magnification.

[0086] FIGURE 29 shows an SEM image of composition N at 300X magnification.

[0087] FIGURE 30 shows an SEM image of composition O at 50X magnification.

[0088] FIGURE 31 shows an SEM image of composition O at 300X magnification.

[0089] FIGURE 32 shows an SEM image of composition P at 50X magnification. [0090] FIGURE 33 shows an SEM image of composition P at 300X magnification.

[0091] FIGURE 34 shows an SEM image of composition Q at 50X magnification.

[0092] FIGURE 35 shows an SEM image of composition Q at 300X magnification.

[0093] FIGURE 36 shows an SEM image of composition R at 50X magnification.

[0094] FIGURE 37 shows an SEM image of composition R at 300X magnification.

[0095] FIGURE 38 shows a ternary phase diagram of the AZS system showing compositions 1 to 37.

[0096] FIGURE 39 shows an SEM image of composition 1 at 100X magnification.

[0097] FIGURE 40 shows an SEM image of composition 1 at 2000X magnification.

[0098] FIGURE 41 shows an SEM image of composition 2 at 100X magnification.

[0100] FIGURE 42 shows an SEM image of composition 2 at 2000X magnification.

[0101] FIGURE 43 shows an SEM image of composition 3 at 100X magnification.

[0102] FIGURE 44 shows an SEM image of composition 3 at 2000X magnification.

[0103] FIGURE 45 shows an SEM image of composition 4 at 100X magnification.

[0104] FIGURE 46 shows an SEM image of composition 4 at 2000X magnification.

[0105] FIGURE 47 shows an SEM image of composition 5 at 100X magnification.

[0106] FIGURE 48 shows an SEM image of composition 5 at 2000X magnification.

[0107] FIGURE 49 shows an SEM image of composition 6 at 100X magnification.

[0108] FIGURE 50 shows an SEM image of composition 6 at 2000X magnification.

[0109] FIGURE 51 shows an SEM image of composition 7 at 100X magnification.

[0110] FIGURE 52 shows an SEM image of composition 7 at 2000X magnification.

[0111] FIGURE 53 shows an SEM image of composition 8 at 100X magnification. [0112] FIGURE 54 shows an SEM image of composition 8 at 2000X magnification.

[0113] FIGURE 55 shows an SEM image of composition 9 at 100X magnification.

[0114] FIGURE 56 shows an SEM image of composition 9 at 2000X magnification.

[0115] FIGURE 57 shows an SEM image of composition 10 at 100X magnification.

[0116] FIGURE 58 shows an SEM image of composition 10 at 2000X magnification.

[0117] FIGURE 59 shows an SEM image of composition 11 at 100X magnification.

[0118] FIGURE 60 shows an SEM image of composition 11 at 2000X magnification.

[0119] FIGURE 61 shows an SEM image of composition 12 at 100X magnification.

[0120] FIGURE 62 shows an SEM image of composition 12 at 2000X magnification

[0121] FIGURE 63 shows an SEM image of composition 13 at 100X magnification.

[0122] FIGURE 64 shows an SEM image of composition 13 at 2000X magnification.

[0123] FIGURE 65 shows an SEM image of composition 14 at 100X magnification.

[0124] FIGURE 66 shows an SEM image of composition 14 at 2000X magnification.

[0125] FIGURE 67 shows an SEM image of composition 15 at 100X magnification.

[0126] FIGURE 68 shows an SEM image of composition 15 at 2000X magnification

[0127] FIGURE 69 shows an SEM image of composition 16 at 100X magnification.

[0128] FIGURE 70 shows an SEM image of composition 16 at 2000X magnification.

[0129] FIGURE 71 shows an SEM image of composition 17 at 100X magnification.

[0130] FIGURE 72 shows an SEM image of composition 17 at 2000X magnification

[0131] FIGURE 73 shows an SEM image of composition 18 at 100X magnification.

[0132] FIGURE 74 shows an SEM image of composition 18 at 2000X magnification. [0133] FIGURE 75 shows an SEM image of composition 19 at 100X magnification.

[0134] FIGURE 76 shows an SEM image of composition 19 at 2000X magnification.

[0135] FIGURE 77 shows an SEM image of composition 20 at 100X magnification.

[0136] FIGURE 78 shows an SEM image of composition 20 at 2000X magnification.

[0137] FIGURE 79 shows an SEM image of composition 21 at 100X magnification.

[0138] FIGURE 80 shows an SEM image of composition 21 at 2000X magnification.

[0139] FIGURE 81 shows an SEM image of composition 22 at 100X magnification.

[0140] FIGURE 82 shows an SEM image of composition 22 at 2000X magnification.

[0141] FIGURE 83 shows an SEM image of composition 23 at 100X magnification.

[0142] FIGURE 84 shows an SEM image of composition 23 at 2000X magnification.

[0143] FIGURE 85 shows an SEM image of composition 24 at 100X magnification.

[0144] FIGURE 86 shows an SEM image of composition 24 at 2000X magnification.

[0145] FIGURE 87 shows an SEM image of composition 25 at 100X magnification.

[0146] FIGURE 88 shows an SEM image of composition 25 at 2000X magnification

[0147] FIGURE 89 shows an SEM image of composition 26 at 100X magnification.

[0148] FIGURE 90 shows an SEM image of composition 26 at 2000X magnification.

[0149] FIGURE 91 shows an SEM image of composition 27 at 100X magnification.

[0150] FIGURE 92 shows an SEM image of composition 27 at 2000X magnification.

[0151] FIGURE 93 shows an SEM image of composition 28 at 100X magnification.

[0152] FIGURE 94 shows an SEM image of composition 28 at 2000X magnification

[0153] FIGURE 95 shows an SEM image of composition 29 at 100X magnification. [0154] FIGURE 96 shows an SEM image of composition 29 at 2000X magnification.

[0155] FIGURE 97 shows an SEM image of composition 30 at 100X magnification.

[0156] FIGURE 98 shows an SEM image of composition 30 at 2000X magnification.

[0157] FIGURE 99 shows an SEM image of composition 31 at 100X magnification.

[0158] FIGURE 100 shows an SEM image of composition 31 at 2000X magnification.

[0159] FIGURE 101 shows an SEM image of composition 32 at 100X magnification.

[0160] FIGURE 102 shows an SEM image of composition 32 at 2000X magnification.

[0161] FIGURE 103 shows an SEM image of composition 33 at 100X magnification.

[0162] FIGURE 104 shows an SEM image of composition 33 at 2000X magnification.

[0163] FIGURE 105 shows an SEM image of composition 34 at 100X magnification.

[0164] FIGURE 106 shows an SEM image of composition 34 at 2000X magnification.

[0165] FIGURE 107 shows an SEM image of composition 35 at 100X magnification.

[0166] FIGURE 108 shows an SEM image of composition 35 at 2000X magnification.

[0167] FIGURE 109 shows an SEM image of composition 36 at 100X magnification.

[0168] FIGURE 110 shows an SEM image of composition 36 at 2000X magnification.

[0169] FIGURE 111 shows an SEM image of composition 37 at 100X magnification.

[0170] FIGURE 112 shows an SEM image of composition 37 at 2000X magnification.

Detailed Description

[0171] The inventors have created a method that allows zirconia-mullite composites to be made from a greater range of compositions in the ternary AZS system than through the application of compositions fabricated conventionally from zirconia + mullite mixtures or zircon + alumina mixtures. Accordingly, the amount of zirconia present in the final composite may be up to ~73 wt%. In contrast, conventional zirconia-mullite composites are limited to contents of ~37 wt% zirconia and conventional AZS refractories are limited to contents of ~46 wt% zirconia. Composites produced by the present method may provide superior refractory properties relative to existing zirconia-mullite composites and conventional AZS refractories, and further the produced composites have little to no residual glass, which facilitates their use across a wide range of refractory applications.

[0172] Conventional zirconia + mullite compositions are fabricated by solid-state sintering. Conventional zircon + alumina composites are fabricated by reaction sintering (decomposition and sintering). The compositions of starting materials for these methods must be tightly controlled and are limited to small ranges of compositions.

[0173] The present invention includes heating a body of material in the AZS ternary system with a silica content greater than the zirconia-mullite binary system to temperatures between approximately 1550 C and 1700 °C. Advantageously, the body of material may be formed from low-purity inexpensive materials such as fly ash (containing alumina and silica), bauxite (also containing alumina and silica), and zircon (containing zirconia and silica). This is counter-intuitive to conventional wisdom as each of these raw materials is known to generate glass (owing to silica content), which is known to be detrimental to the thermal and mechanical properties of produced composites. The present invention allows a wide range of aluminosilicates to be used despite their associated glass generation. This is because the present invention, unlike the conventional fabrication methods of zirconia + mullite and zircon + alumina mixtures, combines reaction sintering with transient liquid-phase sintering. The blended glass recrystallises as zirconia + mullite and thus a residual glass phase is not present in zirconia-mullite composites produced by the present method.

[0174] The term 'body of material' refers to the starting materials used for the sintering process. Typically this refers to a mix of small particles from multiple sources, such as but not limited to fly ash, bauxite, and zircon as described above. The composition of the body of material' on an alumina-zirconia-silica (AZS) phase diagram thus refers to the overall amounts of alumina, zirconia, and silica in the mixture of sources prior to heating.

[0175] These produced zirconia-mullite composites may be suited particularly to refractory applications, such as blocks for lining glassmelting furnaces or glass tanks. The produced composites also may be suitable for use as refractory castables, refractory ramming mixes, refractory crucibles, refractory setters, refractory tubes, refractory plates, refractory grains, refractory aggregates, kiln shelving, kiln posts, heat shields, fibre blankets, fibre boards, fibre shapes, fuse-cast shapes, corrosion-resistant shapes, coatings, whiteware, sanitaryware, dinnerware, wear-resistant tiles, military armour, proppants, corrosion-resistant shapes, glassceramics (crystallised glasses), filters, electrical insulators, electronic substrates, catalysts, and/or, inter alia, infrared transmitting windows.

[0176] For ease of reference throughout this specification, embodiments using fly ash, bauxite, and zircon will be described. However, it will be understood by the person skilled in the art that these are not the only components that may be used to create a composition in the ternary AZS system. Fly ashes are a waste product with variable composition containing components other than the majority silica and alumina. However, the majority of fly ashes have -55 to 67 wt% silica. Similarly, the chemical composition of bauxite also varies. Bauxite is defined by being an alumina-rich mineral, with minority silica and other components. Other aluminosilicate sources such as mullite, kyanite, sillimanite, andalusite, topaz, pyrophyllite, clays (such as kaolinite or illite), pyrophyllite, phyllite-schist, saprolite clay, kyanite-staurolite, anorthosite, syenite, emery rock, and other naturally occurring aluminosilicate raw materials also may be suitable as at least part of the starting body of material for forming the present composites without departing from the scope of the present invention(s).

[0177] In preferred embodiments, fly ash makes up between approximately 20 and 80 wt% of the starting materials (the initial body of material being heated). In particularly preferred embodiments, fly ash makes up between approximately 28 and 65 wt% of the body of material. In particularly preferred embodiments, fly ash makes up between approximately 30 and 50 wt% of the body of material. In even more particularly preferred embodiments, fly ash makes up approximately 35 wt% of the body of material.

[0178] In preferred embodiments, bauxite makes up between approximately 2.5 and 20 wt% of the starting materials. In particularly preferred embodiments, bauxite makes up between approximately 12 and 20 wt% of the starting materials.

[0179] In preferred embodiments, zircon makes up between approximately 18 and 65 wt% starting materials. In particularly preferred embodiments, zircon makes up between approximately 35 and 65 wt% of the starting materials.

[0180] Advantageously, bauxite, fly ash, and zircon are all inexpensive raw materials and are relatively abundant and available for use. The present method allows the creation of zirconia-mullite composites using these inexpensive materials, in contrast to existing methods, which require rigorous control of the composition of the starting materials. FIGURES 1 and 38 show the limited range of compositions of commercial AZS refractories. Bauxite, fly ash, and zircon all have significant amounts of silica, which are disadvantageous for many existing refractory or other engineered materials. Accordingly, existing methods instead require the use of high-purity starting materials in circumscribed compositional amounts to achieve these compositions.

[0181] Without wishing to be bound to theory, it is thought that the following processes occur during heating to produce a high-purity, homogeneously distributed, and dense zirconiamullite composite: firstly, unstabilised zirconia will dissolve in glass (as generated by fly ash, bauxite, and/or another silica-containing source) to create a zirconium aluminosilicate glass. At the same time, zircon will decompose into zirconia precipitates and zirconium silicate glass. As these two glasses are miscible and chemically complex, a rapid diffusion medium is produced, which leads to an oversaturation of zirconia in the glass. This results in further zirconia precipitation. Pre-existing zirconia precipitates act as nucleating agents and templates, thereby assisting in zirconia precipitation.

[0182] The precipitation of zirconia from the glass changes the chemistry and rheology of the liquid (glass) phase, resulting in oversaturation of alumina, thereby facilitating the precipitation of mullite. Pre-existing mullite grains also act as nucleating sites and templates to assist in mullite precipitation. The mullite is pre-existing within the starting materials, rather than being formed from a reaction melt.

[0183] As zirconia continues to precipitate from the glass, there reaches a point of effective exhaustion of excess (overs aturated) zirconia from the glass, which is dictated by the liquid (glassy) solubility limit of zirconia in glass. At this point, the remaining residual glass acts effectively as an aluminosilicate glass. Similarly, as mullite precipitates, the alumina content of the residual glass decreases, resulting in a point of effective exhaustion of excess (oversaturated) alumina, which is dictated by the liquid (glassy) solubility limit of alumina in glass. This results in the eventual generation of a residual glass consisting predominantly of silica.

[0184] This remaining residual glass then is minimised or eliminated from the microstructure owing to the solid solubilities of silica in the zirconia and mullite phases, resulting in a high-purity zirconia-mullite composite. Otherwise stated, the residual silica glass is removed through transient liquid-phase densification. The micro structure formed is effectively biphasic, consisting of equiaxed zirconia and mullite fibres. This is in stark contrast to a number of existing AZS refractory materials, which instead are triphasic composites including a corundum (alumina) phase in addition to the zirconia and mullite. The additional alumina phase in these refractory materials provides negative thermal and mechanical properties compared to biphasic composites without said alumina phase. Additionally, the mullite of the present invention is in the form of fibrous mullite as opposed to equiaxed mullite as typically found. In preferred embodiments, the fibrous mullite also may percolate during the heating process to form a percolated scaffold or structure.

[0185] Otherwise stated, the method produces a zirconia-mullite composite with a microstructure of equiaxed zirconia and mullite fibres, achieved from transient liquid-phase (glass) densification at approximately 1550 C to 1700 C. The produced composite has a microstructure that differs from both conventional AZS refractory materials and conventional mullite-zirconia composites. On one hand, commercial AZS refractory materials typically have large grains of lath shape on a millimetre scale but the produced composite has comparatively small grains of spherical shape of sizes -3-15 pm. On the other hand, conventional mullitezirconia composites typically have grain sizes limited to a few microns. The produced composite thus has a relatively large grain size range compared to that of conventional zirconiamullite composites. Unlike existing zirconia-mullite composites, which have a maximal zirconia content of approximately 37 wt%, and commercial AZS refractories, which have a maximal zirconia content of approximately 46 wt%, the produced composites may have a zirconia content of up to approximately 73 wt%. In preferred embodiments, the zirconia content of the produced composite is between approximately 46 and 73 wt%. In particularly preferred embodiments, the composite includes between approximately 46 and 67 wt% zirconia. In especially particularly preferred embodiments, the composite includes between approximately 48 and 67 wt% zirconia. In even more preferred embodiments, the composite includes between approximately 51 and 67 wt% zirconia. In even more especially preferred embodiments, the composite includes between approximately 56 and 67 wt% zirconia. [0186] It will be understood by a person skilled in the art that the minimal and maximal zirconia content to produce the composite are dependent on, among other things, the relative content of alumina and silica present. Accordingly, compositions with less than 37 wt% still may be suitable as starting materials, for example the range of compositions highlighted in dark yellow in FIGURE 1 include compositions with as little as 15 wt% zirconia. In preferred compositions, shown in green, the zirconia content may be between 18 and 67 wt%. FIGURE 1 also shows a number of example compositions. The zirconia contents of selected compositions are listed below:

[0187] In preferred embodiments, the heating is carried out for a time period of 10 hours or more. In particularly preferred embodiments, the heating is carried out for a time period of 15 hours or more. In particularly preferred embodiments, the heating is carried out for a time period of 20 hours or more.

[0188] Existing AZS refractories are comprised of alumina and zirconia phases with residual glass between the grains of the phases. The stability against corrosion of these refractories relies on the dissolution of alumina from the AZS refractories by the molten glass, which establishes an alumina-rich interface, which retards further alumina dissolution from the AZS refractories. Thus, the corrosion-resistance properties are reliant on the solubility of alumina in the glass.

[0189] In contrast, in the present invention, the microstructure instead is comprised of a zirconia and mullite composite with little to no glass between these grains and with a higher zirconia content. As zirconia is much less soluble in glass than alumina, the susceptibility to corrosion (i.e., solubility) is decreased. This results in a more corrosion-resistant refractory. Further, as the melting point of alumina is -2050 C and that of zirconia is -2715 C, the thermal resistance of the refractory to high-temperature deformation from viscous glass deformation also is increased. [0190] The increased thermal stability of the composites according to the present invention allows their use at higher temperatures than conventional AZS materials can withstand. In particular, the composites may be suited to uses such as in glassmelting tanks, where conventional AZS materials presently are used at their performance limit, which is to say the thermal stability of conventional AZS materials limits the temperature at which a glassmelting tank using said material can be used. A glassmelting tank utilising materials according to the present invention thus could be used at higher temperatures. This also may allow the use of these materials in more demanding applications.

[0191] As used herein, the term 'percolated' refers to a completely or effectively completely continuous and interconnected microstructure, scaffold, or network that extends through the entire body (rather than in isolated regions, as in triaxial porcelains and other mullitic products), is direct-bonded (thereby excluding glass from between mullite grains), and consequently is structurally stable such that it can resist high-temperature deformation up to, in principle, the melting point (or decomposition temperature) of mullite (1850 C). This is unlike conventional mullite products, in which the corrosion and temperature resistances are limited by chemical attack or softening, respectively, of the intergranular glass.

[0192] Another advantage of the produced composites is that they have a reduced porosity compared with conventional zirconia- mullite composites. This is because, in conventional AZS materials, large pores grow as extensive grain growth occurs. However, in the present composites, this is avoided as single-crystal zirconia and mullite growth to dimensions of between ~3 and 15 pm occurs instead, thereby avoiding the formation of large pores. The reduced porosity results in composites that are substantially impermeable, thereby improving the chemical resistance and gas- and liquid-sealing capacities of the refractories.

[0193] The resulting microstructure may be improved by providing an amount of zirconia (which may be unstabilised, partially stabilised, or fully stabilised) to the starting composition, which provides extensive initial nucleation and templating sites for zirconia precipitation during heating. The surfaces of the added zirconia may be thought of as a 'nucleating agent and template.' In preferred embodiments, the additional zirconia is added in an amount between 12 and 39 wt%. In particularly preferred embodiments, additional zirconia is added in an amount between approximately 27 and 39 wt%. In especially preferred embodiments, additional zirconia is added in an amount between approximately 33 and 39 wt%. It will be understood that when partially stabilised, or fully stabilised zirconia is added to the starting composition, these percentages refer to the net zirconia content exclusive of the fraction of stabiliser in the stabilised zirconia. In alternative embodiments, zircon may be used in place of zirconia. In further alternative embodiments, other zirconia-containing minerals may be used, preferably in an amount which results in a zirconia content as described above. Alternatively or additionally, additional mullite may be added to the starting composition to act as nucleation and templating sites for mullite precipitation during heating.

[0194] Adding zirconia and/or mullite to the body of material (the starting materials) as a nucleating agent and template allows for increased precipitation of zirconia and/or mullite respectively, and in turn the micro structure with a reduced porosity due to the single-crystal growth. As described earlier, composites made using the present method undergo a process which eliminates glass from the microstructure through continued precipitation of zirconia and mullite. Adding zirconia and/or mullite specifically as nucleation agents and as templating sites ensures that this process occurs.

[0195] In a particularly preferred embodiment, the method includes the use of fly ash, bauxite, zircon, and zirconia, such as unstabilised zirconia, to produce the initial body of material that is heated to produce the zirconia-mullite composite. These are favoured as raw materials partly owing to their low cost but also owing to the glass provided through the fly ash and zircon. Fly ash also is a waste product, providing an environmentally beneficial recycling stream for incinerators and coal-burning facilities, such as power plants. As the process is flexible in its design and use of compositions, it is possible to use fly ash and bauxite (both of which have variable compositions) as starting materials, which is not possible in existing methods due to the requirement for tight control over the composition of the starting material. Additional zirconia is added as a nucleating agent and to provide templating site. The range of compositions for existing commercial AZS refractories are shown in FIGURE 1 (in grey oval).

[0196] In some embodiments, a flux may be added to soften the glass and/or enhance liquid-phase formation and/or increase diffusion and/or lower the temperature and time required to form the composite. Fluxes may include, for example, oxides and/or salts of alkalies, alkaline earths, transition metals, semimetals, metalloids, and/or lathanides; halogens may also be suitable fluxes. Examples of fluxes that are common to bauxite and fly ash, which may be added are iron oxide, alkalies, and alkaline earths. These are favoured particularly because they may be present in the raw materials already, reducing the chance of generating undesirable reactions during the process. Otherwise stated, intrinsic fluxes also may be present in the starting materials, such as other oxides present in aluminosilicate raw materials.

[0197] FIGURE 1 shows a ternary phase diagram of the alumina-zirconia- silica system. A polygon indicating acceptable compositions for the present invention is shown in light yellow (1) and is bounded below by the zirconia-mullite binary system (the line between point 1 (98:2:0 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ) and point 4 (0:30:70 ZrO ₂ :SiO ₂ :Al ₂ O ₃ )) and bounded above by the join between zircon and high-silica fly ash (the line between point 2 (67 :33 :0 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ) and point 3 (0:67:33 ZrO ₂ :SiO ₂ :Al ₂ O ₃ )). A more preferred region of compositions (2) is shown in dark yellow. This region is a polygon bounded by point 5 (73:9:18 ZrO2:SiO2:A12O3), point 6 (68.0:28.0:4 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ), point 7 (15:52:33 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ), and point 8 (22:24:54 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ). An even more preferred range of compositions (3) is shown in green. This region is a polygon bounded by point 9 (67:13:20 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ), point 10 (63:29:8 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ), point 11 (18:50:32 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ) and point 12 (28:24:48 ZrO ₂ :SiO ₂ :Al ₂ O ₃ ). All listed ratios are weight ratios. The preferred range of compositions (3) provides the best microstructures in terms of bulk density, amounts of zirconia and mullite, microstructural homogeneity, and minimal residual glass.

[0198] FIGURE 1 also shows the preferred compositions of current commercial AZS compositions in the grey oval (4). These fall below the zirconia-mullite binary system (line between points 1 and 4) and typically consist of zirconia, alumina, and glass, without any significant content of mullite. The zirconia content of these AZS refractories is limited to between approximately 30 and 46 wt%.

[0199] The preferred compositions are described as being placed between the zirconiamullite binary system (line between points 1 and 4 in FIGURE 1) and zircon high-silica fly ash mixtures (line between points 2 and 3 in FIGURE 1). Critical to the method is that the silica content is greater than is present in the zirconia-mullite binary system. Otherwise stated, the silica contents in the overall AZS system are greater than those in the zirconia-mullite binary system. This is contrary to existing methods for producing zirconia + mullite composites (i.e., solid-state sintering) that are limited to compositions on the join between zirconia and mullite represented by the line between points 1 and 4 to avoid the production of any residual silica, and existing methods that use zircon + alumina composites (for example reaction sintering) to generate zirconia + mullite composites. All of these compositions are designed to lie on or below the zirconia-mullite binary system in order to avoid the presence of residual glass. [0200] The present method relies on heating compositions of higher silica content in order to provide glass, allow oversaturation of zirconia in the glass (liquid phase), and allow oversaturation of alumina in the glass (liquid phase), thereby facilitating transient liquid phase densification to occur consequently. Any residual silica then is dissolved in the zirconia and/or mullite. Otherwise stated, the transient liquid-phase densification process involves recrystallisation of the glass phase into zirconia and mullite, such that there is little to no glass phase in the final composite.

[0201] FIGURE 1 also shows the locations of example compositions A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q, and R. SEM images at two magnifications (50X and 300X) for each of these samples are shown in FIGURES 2 to 37 and their properties and starting compositions are tabulated in the following section.

EXAMPLES

[0202] A number of example compositions and the features of the resultant microstructures after sintering at 1600 °C for 10 hours is tabulated below. A scale between X and XXXXXX has been used to indicate the relative concentration of each phase, with X representing a low concentration and XXXXXX indicating a very high concentration. — represents the case when the phase was not observed in the produced composite. These examples utilize a body of material, which is to say starting materials comprising fly ash, bauxite, zircon, and zirconia.

[0203] Samples A to I show the effects of increasing proportions of zircon and added zirconia in the starting materials. These produce mullite-zirconia composites with very high relative matrix densities.

[0204] Samples I to R show the effect of increasing proportions of zirconia in the starting materials. These also produce mullite-zirconia composites with very high relative matrix densities.

[0205] Further example compositions, numbered as samples 1 to 37, also were investigated. These were sintered at 1600 °C for 20 hours. The compositions' locations in the AZS system are shown in the ternary phase diagram in FIGURE 38 and corresponding SEM images at two magnifications (100X and 2000X) for each sample are shown in FIGURES 39 to 112.

[0206] Samples 1 to 6 investigated starting compositions with high amounts of zirconia and low amounts of alumina (no bauxite added). These produced composites of zircon and zirconia with only trace amounts of glass, with no mullite present. Also included in this sample set is sample 37, which was 100 wt% zircon (no alumina present). This resulted in a single unaltered zircon phase microstructure with no glass.

[0207] By contrast, sample 7, which also had no bauxite added but contained a fraction of fly ash (containing silica and alumina), did produce a mullite and zirconia biphasic microstructure.

[0208] Samples 8 to 11 show the effect of increasing fly ash content in the starting materials on the resultant microstructures. With increasing fly ash content, the amount of mullite in the micro structure increased and the amount of zirconia decreased.

[0209] Samples 12 to 14 investigated varying the relative compositions of fly ash, zircon, and zirconia in the starting materials. These produced microstructures with mostly zirconia and a small amount of mullite, with very high matrix densities. These samples showed the comparative effect of adding to the starting materials a proportion of zirconia, which acts as a nucleating agent and template. This facilitated the formation of a high zirconia content.

[0210] Samples 15 to 18 used only fly ash and zircon as starting materials, varying the relative amounts of each. This showed that, with increasing fly ash content, more mullite was formed. However for the extreme 65:35 fly ash:zircon by weight starting body, a large amount of zirconia still was present in the final microstructure despite the low amount of zircon.

[0211] Samples 19 to 23 maintained equal proportions of fly ash and bauxite with the remainder's being zirconia. These showed the presence of alumina, with larger amounts being observed for larger proportions of fly ash + bauxite relative to zirconia. Further, the relative matrix density decreased for these compositions with increasing ratio of fly ash + bauxite to zirconia by weight.

[0212] Samples 24 to 28 maintained a constant ratio of 7:3 fly ash:bauxite by weight, with the remainder's being zirconia. These produced microstructures with high-relative-density matrices and increasing mullite contents with greater amounts of fly ash. [0213] Samples 29 to 33 maintained a constant ratio of 4:1 of fly ash:bauxite by weight, with the remainder's being zirconia. These produced microstructures with low to high amounts of mullite and a majority phase of zirconia, with high relative matrix densities.

[0214] Samples 34 to 36 investigated compositions with microsilica added in place of zirconia. While there were still only trace amounts of residual glass in samples 34 and 35, these were expected to have a higher amount of glass relative to the other samples. This indicates the dissolution of silica in mullite and zirconia. Sample 36, being predominantly fly ash, resulted in mullite and a high amount of residual glass.

[0215] For compositions with high amounts of silica and low amounts of zirconia (such as samples 36, 9, 10, and 11), microstructures with relatively low amounts of zirconia and high amounts of glass were observed. This would be detrimental to the mechanical and thermal properties of the produced material.

[0216] For compositions with low amounts of zirconia and greater amounts of alumina, such as sample 23, a relatively low amount of zirconia and lower matrix density, which mean a greater apparent porosity, were observed. This is disadvantageous for use in corrosive environments and for mechanical strength.

[0217] For compositions that have very high amounts of zirconia and low amounts of silica and alumina, such as samples 1 to 5, zirconia precipitation was reduced or did not occur, and little residual glass was present.

[0218] For high-zirconia and high-silica compositions, such as samples 34, 35, and 37, relatively high amounts of glass and high amounts of zircon were observed.

[0219] Samples 6 to 8, 12 to 14, 19, 20, 27, and 33 are representative of preferred embodiments of composites produced by the present invention. More preferred embodiments are exemplified by samples 15 to 18, 24 to 26, and 29 to 32. [0220] In this specification, the word 'approximately' should be understood as meaning ±1 wt% when referring to compositional ranges and ±20 °C when referring to temperatures.

[0221] In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents that operate in a similar manner to accomplish a similar technical purpose.

[0222] In this specification, the word 'comprising' is to be understood in its 'open' sense, that is, in the sense of 'including', and thus not limited to its 'closed' sense, that is the sense of 'consisting only of. A corresponding meaning is to be attributed to the corresponding words 'comprise', 'comprised', and 'comprises' where they appear.

[0223] The reference in this specification to any prior publication (or information derived from it), or to any matter that is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0224] In addition, the foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and/or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.

[0225] Furthermore, whereas the invention(s) have described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention(s). Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realise yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment.