GB2098198A

GB2098198A - Zircon-containing refractories

Info

Publication number: GB2098198A
Application number: GB8114108A
Authority: GB
Original assignee: Flogates Ltd
Current assignee: Flogates Ltd
Priority date: 1981-05-08
Filing date: 1981-05-08
Publication date: 1982-11-17
Also published as: ES512000A0; GB2098198B; ES8306077A1; AU8285082A; ZA822817B; IN156857B

Abstract

Alumina-magnesia-chromic oxide refractory compositions yield remarkably stronger and less expansion- susceptible mouldings upon firing, when modified so they contain at least 1% by weight of zircon flour, and firing temperatures can be lowered or firing times reduced. An exemplary composition is a blend of particulate alumina, 35 to 97%, with 1 to 10% zircon flour, particulate magnesia and particulate chromic oxide, the latter two components being 1 to 10%; the percentages are by weight of the blend. Molten metal erosion resistance of the fired zircon-containing refractory is such that the compositions lend themselves to the production of orificed, metal teeming components.

Description

SPECIFICATION Zircon-containing refractories The present invention relates to zircon-containing refractories.

Fired refractory bodies are commonly required in the metallurgical industry to be capable of resisting the degrading efects of molten metal and associated slags. Exemplary refractory bodies are the valve plates of sliding gate valves for controlling metal teeming, teeming nozzles and wear-resisting liners for such plates and nozzles.

One refractory in common usage is mullite-bonded alumina of high density. A major drawback of mullite-bonded alumina is its relatively poor resistance to chemical attack by steels having high manganese contents. Manganese compounds in the steel preferentially attack the mullitebonding phase and result in rapid wear in the orifice area of orificed bodies and, where these are valve plates, the wear can lead to loss of teeming rate control.

Investigation of three component alumina-based systems have shown that alumina-chromic oxide-magnesia formulations have the potential for giving improved chemical attack resistance particularly with erosive steels such as those mentioned above. At the same time these systems maintain the good thermal shock properties normally associated with mullite-bonded material.

Good thermal shock resistance is often needed in metal teeming operations where teeming is discontinuous. Discontinuous teeming is practiced during ingot teeming, for instance.

Such three-component formulations exhibit shortcomings, however. One shortcoming is their poor volume stability on firing. In this regard one finds there is a tendency to high thermal expansion.

We have been experimenting with alumina-chromic oxide-magnesia mixes and have discovered that unexpected improvements in their properties can be gained by modest additions of zircon, i.e. zirconium silicate. Significant improvements in strength, improved chemical attack resistance and dramatically reduced thermal expansion on firing, have been noted.

According to the present invention, therefore, there is provided a refractory composition for use in producing pressed and fired refractory bodies, which is a blend of four separate particulate components, namely alumina, chromic oxide, magnesia and zircon, the major component of the blend by weight being alumina, and 1 to 10% by weight of the blend being the zircon component.

Before pressing to shape, the composition will be blended with a binder such as starch, for instance in the weight ratio of 100 parts composition to 1 part binder. One exemplary formulation according to the invention comprises 90% alumina, 4% magnesia, 3% chromic oxide and 3% zircon, the percentages being by weight of the blend. The alumina content can range from 35 to 97%, and the composition can include a further, optional ingredient which is alumina-containing grog in an amount up to 35% by weight of the blend.

The invention comprehends a pressed and fired refractory body made from a composition as defined in the last paragraph.

The invention further comprehends a method of making a refractory body comprising procuring a blend of alumina particles, chromic oxide particles, magnesia particles and zircon particles in which the latter are present in an amount of 1 to 10% by weight of the blend and the alumina particles are the major component by weight of the blend, combining the blend with a green binder, shaping the binder-containing blend under pressure to form a green moulding of the required body, and firing the body at a temperature of 1 550 C or higher.

Tests on refractory bodies according to the invention exhibit a percentage porosity between 18 and 20 and a bulk density between 3.10 and 3.15 after firing in the range 1550 to 1 650 C.

The same bodies had a cold crushing strength in the range 1 20 to 1 70 mega Newtons per square metre after firing in the range 1 550 to 1 650 C and a permanent volume change after firing in the said range between - 0.10 to + 0.03%.

By way of comparison, a composition otherwise substantially the same but omitting the zircon had a cold crushing strength between 70 and 95 NNM-2, and a significantly worse volume stability, after firing within the same temperature range. The zircon-free composition had a percentage volume change (an increase) between 0.7 and 1.5% after firing within the same range.

The invention will now be described by way of example only.

Compositions according to the invention, for use in manufacturing pressed and fired refractory shapes, comprise blends of four or optionally five components each in particulate form. The components are alumina, magnesia, chromic oxide and zircon. The optional component if employed is an alumina-containing grog which can be selected from sintered or fused mullite, bauxite and alumina-silicate grogs containing 45 to 90% Al2O3 by weight of the grog.

Before pressing, compositions according to the invention are compounded with a green binder. One suitable binder is starch, compounded in the weight ratio of one part to one hundred parts of the composition.

Other binders can be substituted, as the addressee will recognise. Suitable binders can be inorganic or organic: e.g. alkali or alkali earth metal borates, phosphates and silicates, lignin sulphonates and dextrin.

We have found that the presence of 1 to 10%, preferably 3 to 6%, of zircon by weight of the blend enables us to prepare fired bodies having properties which are significantly improved over those of bodies made from comparable formulations from which, however, the zircon is missing.

It is believed that best results are achieved by using components of high purity.

The alumina component may comprise sintered, fused or calcined particles and should have an Al203 content of 98% or more by weight of this component, for preference.

The magnesia component may comprise fused or calcined particles and should have an MgO content of 97% or more by weight of this component. Preferably, the magnesia used has been calcined, at a temperature ranging from 800 to 1400"C.

The chromic oxide component should preferably comprise not less than 96% Cr203 by weight of this component.

Preferably, the zircon component has a ZrSiO4 content of 98% or more by weight of this component.

The magnesia and chromic oxide components should each amount to not less then 1% by weight of the blend, and each may range up to 10% by weight thereof. For instance, 3 to 6% by weight of the blend can be magnesia and 2 to 4% can be chromic oxide.

The alumina component preponderates over any of the other components in compositions according to the invention. It can amount to 35 to 97%, e.g. 70 to 92%, by weight of the blend. If it is desired to keep the alumina component low in this range, the composition can include the optional, fifth component in an amount up to 35% by weight of the blend. As noted above, this component contains Al203 in an amount in the range 45 to 90% by weight thereof.

Restricting the amount of the alumina component present, and including appreciable amounts of the fifth component, can reduce the cost of the composition. the fifth component should also help stabilise the expansion characteristics of the composition when fired.

As is known in the art, the composition should be prepared from graded particulate materials.

The zircon particle size should be 45 microns or smaller and both the magnesia and chromic oxide should have mean particle sizes within the range 1 to 5 microns.

20 to 50% by weight of the blend, e.g. 40 to 50%, can be sintered or fused alumina having a particle size in the range 0.5 to 2 mm 10 to 30% on the same basis, e.g. 1 8 to 24 or 25%, can be sintered or fused alumina having a particle size smaller than 0.5 mm.

5 to 20% on the same basis, e.g. 1 2 to 18% can be sintered, fused or calcined alumina having a particle size or 45 microns or smaller.

The fifth component, when used, can have a particle size in the range 0.5 to 2 mm.

The following Table summarises compositions according to the invention. Percentages in the Range columns are by weight of the blend.

Preferred Material Size % Range % Range Alumina (Sintered or fused; min. 98 wt % Al2O3 0.5-2 mm 20-50 40-50 Alumina (Sintered or fused; min. 98 wt % Al2O3) < 0.5mm 10-30 18-24 Alumina (Sintered, fused or calcined, min. 98 wt % Al203) < 45 it 5-20 12-18 Magnesia (Fused or calcined; min. 97 wt % MgO) 1-5,u 1-10 3-6 Chromic oxide (Min.

96 wt % Cr203) 1-5ju 1-10 2-4 Zircon flour (Min.

98wt%ZrSiO4) < 451l 1-10 3-6 Alumina-silicate grog (Sintered or fused Mullite, Bauxite, Grogs, of 45-90 wt % Al2O3) 0.5-2 mm 0-35 0-22 Compositions according to the invention are mixed with a suitable binder before pressing. The pressing operation can be conventional and will not be described here. After pressing, the resulting green shapes are fired at elevated temperatures e.g. in the range 1550 to 1650 C.

Suitable firing temperatures and times will depend inter alia on pressings dimensions and will be ascertainable by the addressee.

It will be noted that firing temperatures below 160"C are possible. By contrast, temperatures greater than 1600"C are necessary for comparable compositions from which zircon is omitted, if excessive firing periods are to be avoided.

Fired bodies made from the present compositions have exhibited a percentage porosity between 18 and 20 and a bulk density between 3.10 and 3.15 after firing in the range 1550 to 1650 C. The bodies have been found to possess a cold crushing strength in the range 120 to 170 mega Newtons per square metre after firing in the range 1550 to 1650 C and a permanent volume change after firing in the said range between - 0.10 to + 0.03%.

With the attainment of such properties, the invention is particularly suitable for use in manufacturing valve plates for sliding gate valves, as well as teeming nozzles or parts thereof for instance to be associated with such valves.

Specific examples of the invention are now given.

Example 1 A blend was made of the following ingredients: Tabular Alumina ) 0.5-2 mm 45% < 0.5 mm 20% < 45 y 10% Calcined Alumina < 45 1 15% Magnesia 4% Chromic oxide 3% Zircon flour < 45 it 3% The percentages were by weight of the blend.

The Tabular alumina and the calcined alumina were obtained from Alcoa, the latter component being designated by makers' reference Al 2. The magnesia was Steetley Minerals Limited's Lycal and the chromic oxide was Accrox F from British Chrome and Chemicals Limited.

110 parts by weight of the blend was compounded with 1 part by weight of starch, and sample pressings were then made and fired at 1550 C or 1650 C. Porosity, bulk density, permanent volume change on firing and cold crushing strength determinations were performed, the results being reported in Table 2 hereunder.

Example 2 Example 1 was repeated with changes to the blend, as follows. The chromic oxide content was increased to 6% by weight of the blend and the calcined alumina was reduced to 12% by weight. The results of measurements on fired samples, as under Example 1 are reported in Table 2.

Comparative Example.

In this comparative example, which is outside the scope of this invention, the zircon component was omitted. The calcined alumina content was increased to 18% by weight of the blend. Again, the results of measurements on fired samples, as under Example 1, are reported in Table 2.

Table 2 Example Bulk Permanent Volume Cold crushing and firing % Density Change on firing Strength temp C Porosity g/cc % MNM-2 Example 1 1550"C 18.7 3.12 +0.02 128.4 Example 2 1550"C 19.1 3.13 It 0.03 139.5 Comparative Example 1550 C 19.2 3.07 + 0.73 71.5 Example 1 1650"C 18.5 3.14 -0.03 154.6 Example 2 1650"C 18.7 3.14 -0.10 164.6 Comparative Example 1650on 20.3 3.04 + 1.41 94.8 The remarkably better volume stability and cold crushing strengths the inventive compositions (Examples 1 and 2) have over the Comparative Example is clearly demonstrated by the above tabulation.

Erosion or chemical attack resistance of Example 1 and the Comparative Example have been assessed qualitatively by reference to standard commercial mullite bonded alumina valve plates.

Example 1 fared better, and the Comparative Example worse, then the standard mullite-bonded alumina. An orifice in the latter, initially 50 mm diameter, eroded to approximately 54 mm after molten steel had been teemed through the orifice for a given length of time. Under the same teeming conditions, and with orifices initially 50 mm in diameter, in Example 1 the orifice enlarged to approximately 51 mm; in the Comparative Example, the enlargement was markedly greater, the orifice finally measuring 56 mm. A remarkable increase in attack resistance was therefore attained by this example of the invention.

Our present investigations indicate that controlled, yet modest, additions of finely-divided zircon are accompanied by very significant practical improvements in fired refractories based on alumina, magnesia and chromic oxide. Without wishing to be bound by theory, it is thought that the zircon has a controlling effect on expansion and that it affects spinel formation. It may, for instance, modify the spinel chemistry. Whatever the mechanism involved, the zircon appears responsible for reducing reaction, i.e. firing, temperatures or conversely for accelerating spinel formation. The greatly increased cold crushing strengths reported in Table 2 for Examples 1 and 2 over the Comparative Example are indicative of the advanced spinel formation that is thought to take place when zircon is present.

The formultions according to the invention can be modified by routine additions of further ingredients. Such ingredients and the reasons for their addition will be known to the addressee.

One exemplary additive is a plasticizer which is often a clay such as bentonite.

Other compositions have been prepared and, after mixing with binder as in Example 1, samples have been pressed, fired at 1550"C or 1650"C and their properties measured as before. Details for the examples according to the invention Examples 3 to 7) and for three further comparative examples (A, B and C), which are outside the scope of this invention, are presented in Table 3 below. Table 3

EXAMPLES COMPARATIVE EXAMPLES 3 4 5 6 7 A B C Tabular Alumina 0.5 mm - 2 mm 45 45 45 45 45 45 45 45 Tabular Alumina < 0.5 mm 20 20 20 20 20 20 20 20 20 Tabular Alumina > 45 10 10 10 10 10 10 10 10 Calcined Alumina Al2 > 45 17 10 14 14 9 17 16 14 Magnesia (Lycal) 1.5 3 5 4 4 5 6 8 Chromic Oxide (Accrox F) 3 3 3 4 9 3 3 3 Zircon Flour > 45 4.5 9 3 3 3 - - Starch (Green Binder) +1 +1 +1 +1 +1 +1 +1 +1 Fired 1550 C/2 Hr. Soak Apparent Porosity % 19.5 18.1 19.6 18.8 20.0 19.7 23.2 24.8 Bulk Density g/cc 3.10 3.14 3.10 3.13 3.09 3.05 2.89 2.83 P.V.C. on Firing % -1.35 -0.16 -0.04 -0.05 +0.36 +1.63 +3.05 +5.92 C.C.S. MN/M 127.0 161.3 132.5 144.5 120.5 68.7 65.5 65.0 Fired 1650 C/2 Hr. Soak Apparent Porosity % 19.6 17.2 19.4 18.5 19.4 21.1 24.7 26.3 Bulk Density g/cc 3.09 3.18 3.13 3.14 3.12 2.97 2.84 2.79 P.V.C. on Firing % -0.02 -0.30 -0.08 -0.23 +0.20 +4.06 +7.92 C.C.S. MN/M 152.0 176.0 162.0 168.0 155.5 85.5 80.2 76.7

Claims

1. A refractory composition for use in producing pressed and fired refractory bodies, which is a blend of four separate particulate components, namely alumina, chromic oxide, magnesia and zircon, the major component of the blend by weight being alumina, and 1 to 10% by weight of the blend being the zircon component.

2. A composition according to claim 1, when mixed with a green binder ready for pressing.

3. A composition according to claim 1 or claim 2, wherein the zircon component has a ZrSiO4 content of 98% or more by weight of this component.

4. A composition according to claim 1, 2 or 3, wherein the alumina component comprises sintered, fused or calcined particles and has an Al2O3 content of 98% or more by weight of this component.

5. A composition according to claim 1, 2, 3 or 4, wherein the magnesia component comprises fused or calcined particles and has an MgO content of 97% or more by weight of this component.

6. A composition according to any of claims 1 to 5, wherein the magnesia and chromic oxide components each comprise at least 1 % by weight of the blend.

7. A composition according to claim 6, wherein the magnesia and chromic oxide components are present in an amount of 1 to 10% by weight of the blend.

8. A composition according to claim 6 or claim 7, wherein the alumina component is present in an amount of 35 to 97% by weight of the blend, and the composition further includes an optional fifth component comprising alumina-containing grog in an amount of O to 35% by weight of the blend.

9. A composition according to claim 8, wherein the fifth component is selected from sintered or fused mullite, bauxite or alumina-silicate grogs.

10. A composition according to claim 8 or claim 9, wherein 45 to 90% of the fifth component by weight is Al203.

11. A composition according to any of claims 1 to 10, which comprises 70 to 92% alumina particles, 3 to 6% magnesia particles, 2 to 4% chromic oxide particles and 3 to 6% zircon particles, and the composition optionally further including 0 to 22% alumina-containing grog, the percentages all being by weight of the blend.

1 2. A composition according to any of claims 1 to 11, wherein the magnesia particles have a mean particle size in the range of 1 to 5 zircons.

1 3. A composition according to any of claims 1 to 12, wherein the chromic oxide particles have a mean particle size in the range 1 to 5 microns.

1 4. A composition according to any of claims 1 to 13, wherein the zircon particles have a mean particle size of less than 45 microns.

1 5. A composition according to any of claims 1 to 14, wherein 20 to 50% by weight of the blend is alumina particles having a particle size in the range 0.5 to 2 mm, 10 to 30% on the same basis is alumina particles having particle sizes smaller than 0.5 mm and 5 to 20% on the same basis is alumina particles having particles sizes of 45 microns or smaller.

16. A composition according to any of claims 1 to 15, which comprises 45% tabular alumina particles of 0.5 to 2 mm particle size, 20% tabular alumina particles smaller than 0.5 mm, 10% tabular alumina particles of 45 microns particle size or smaller and 15% calcined alumina particles of 45 microns particle size or smaller, 4% magnesia particles, 3% chromic oxide and 3% zircon flour, the percentages being by weight of the blend.

1 7. A refractory composition substantially as herein described by way of example.

1 8. A pressed and fired refractory body made from the composition claimed in any one of claims 1 to 17.

1 9. A refractory body according to claim 18, having a percentage porosity between 1 8 and 20 and a bulk density between 3.10 and 3.1 5 after firing in the range 1 550 to 1650"C.

20. A refractory body according to claim 18 or claim 19, having a cold crushing strength in the range 1 20 to 1 70 mega Newtons per square metre after firing in the range 1 550 to 1650"C and a permanent volume change after firing in the said range between - 0.10 to + 0.03%.

21. A pressed and fired refractory body substantially as herein described by way of example.

22. A method of making a refractory body comprising procuring a blend of alumina particles, chromic oxide particles, magnesia particles and zircon particles in which the latter are present in an amount of 1 to 10% by weight of the blend and the alumina particles are the major component by weight of the blend, combining the blend with a green binder, shaping the binder-containing blend under pressure to form a green moulding of the required body, and firing the body at a temperature of 1 550 C or higher.

23. A method of making a refractory body substantially as herein described by way of example.