GB2283486A - Magnesia based refractory compositions - Google Patents

Description

2283486 1.1 TITLE Magnesia 'based shaped and unshaped refractory

Compositions This invention relates to shaped and unshaped refractory compositions based on magnesia.

Because of their high melting temperature and basic character, refractory materials based on magnesium oxide are used predominantly for the lining of industrial furnaces in the iron and steel industry and the cement and lime industry. The main disadvantage of this material is due to the high coefficient of thermal expansion resulting in a low resistance to abrupt fluctuations in temperature.

The maximum permissible variation in temperature can be expressed by the following equation:

(1-u) - K where (P.-L. Coble & W.D.

a - E CY -- E -a -- g K b,') = temperature difference (K) tensile strength (N - mm-2) modulus of elasticity (N - mm-2) coefficient of thermal expansion (K-1) Poisson's ratio shape factor Kingery, J- Am- Ceram Soc- 38 (1955) 11 pp. 33 - 37).

Although it has been possible through the manufacture of magnesia bricks with a suitable grain structure to reduce the modulus of elasticity of pure magnesia bricks, this has led to an improvement in the thermal shock resistance (TSR) to only a limited extent. The crucial step in enhancing the TSR of magnesia products is the addition of TSR-improving components. As a result of the difference in the thermal coefficient of expansion between MgO and the second component, textural stresses occur in the material, which cause a diminution in the modulus of elasticity. In addition the coefficient of thermal expansion (CTE) of such systems can be reduced. The TSR can be enhanced in this way. Chromium ore has proved to be an effective additive. However, magnesia products containing chromium ore have the disadvantage that chromium ore reacts during use of the material at high temperatures with CaO-containing components such as lime, cement clinker and alkali compounds to form water-soluble chromates (CaCr6+0 4 and K,Xr6+04) which contain sexivalent, toxic and environmentally harmful chromium.

Said above-mentioned disadvantage of the addition of chromium ore is overcome by the magnesia (periclase)spinel bricks (Refratecluiik Bericht No. 28). The MgO - - 3 A1203 spinel contained in the refractory material to improve the TSR instead of chromium ore can be produced during the firing or else be introduced into the brick as a pre-synthesised fused and/or sintered spinel (P. Bartha, Zement-Kalk-Gips 35 (1982) pp. 500 507); W. Zednicek, Radex-Rundschau, 1983, pp. 210 - 244.

The TSR was able to be enhanced considerably in the case of MgA'204containing magnesia materials. The service life of the lining material e. g. in the transition zone of the cement rotary kilns is thereby prolonged to a significant extent. Relevant tests on the periclase products have shown that the MgO - Al,-)O spinel, 3 is attacked at temperatures above 1200 IC by CaO and at temperatures above 1400 OC by lime-rich compounds with a C/S mass ratio > 1-87 with the new formation of mostly I^"- 1+4" rl, I + 11;1 r, n. &I n rr-AN 19P n.

1. a k, W CL 7AlnO-3(C,)A A1203(C A) 7) 3CaO 3 (M Nishio; H. Iwaho, Shinagawa Technical Report 34 (1991) No. 34, pp. 75 - 90; P. Bartha, Zement- Ka lk- Gips 38 (1985), pp. 96 - 99).

To improve the thermal shock resistance of fired refractory shaped bodies of magnesia, in particular highgrade sintered magnesia with low iron content, it is known. (DE-PS 22 49 814) to add 1 to 5 wt % Zr0.1) in a grain size up to 5 mm, wherein the proportion below 1 mm 2 3 $ CL 4 must come to not more than 50 wt % but preferably 0 wt %. An improvement in the TSR is accordingly only attained if the Zr02 is present in coarse- grained form. A further disadvantage consists in the fact that the coarse- grained Zr02 first has to be manufactured separately from the available raw material. The manufacture is expensive, since the Zr02 has to be mixed with a binder and granulated or briquetted.

In DE-OS 37 39 900 a method is described for manufacturing a refractory composition for the brick lining of furnaces or high-temperature zones of furnaces for the manufacture of fired dolomite, fired lime or fired magnesite. Such compositions of sintered or fused magnesia with a purity of at least 97 wt % MgO and an addition of 10 - 25 wt % of a zirconium compound with a ZrO.) content of not less than 98 wt % (baddeleyite) exhibit excellent volume stability, chemical resistance to the charge and structural stability. The TSR of a fired brick attained a value of > 30 air quenchings.

The refractoriness of magnesia products (unfired or fired bricks and compounds) can be enhanced by a ZrO,, additive of 0-5 to 6 wt %, i.e. the formation of the lowmelting reaction products is avoided (DE-OS 25 0-1 556). The proportions of the trivalent oxide R203 and Fe,,03 in the product must not fall below the value of 3 wt % and 2 wt %. Previously described bricks can however contain additions of chromium ore or magnes ia- chromium sinter in amounts up to 50 wt %. A further method for manufacturing fired magnesia bricks, to which up to 5. 5 wt % Zr02 's added, is described in DE-PS 26 46 430. An A1203 carrier such as fused corundum or calcined clay of 2 to 10 wt % was added to magnesia-zirconium oxide granulations, wherein spinels can also form (ef. DE-OS 25 07 556). In both methods the surplus CaO was made harmless by the Zr02- From DE-PS-36 14 604 it is known that zirconium oxide in small proportions (0. 5 - 4 wt %) improves the deposit behaviour of a magnesia-spinel brick (10 - 50 spinel and 50 - 90 % magnesia). The aim here is to achieve a promotion of deposit formation also at high temperatures, in particular during overheating in the sinter zone of rotary kilns in the cement industry. The zirconium oxide is present in a grain size range of < 0. 1 mm. In this way the depth of penetration of the bricks is reduced and the actual bricks have to be protected against chemothermal attacks of the cement clinker and the thermal insulation towards the outside has to be improved.

Particular spinel-forming mixtures as a TSRimproving additive in the case of refractory magnesia bricks are known from DE-PS 36 17 904. Such 6 compositions consist of 0.5 - 10 wt % Zr02, 22.5 - 39.8 wt % MgO and A1203 made up to 100%. BY an addition of the above-mentioned raw material the non-reactivity of the periclase-spinel bricks to CaO and lime-rich materials with a C/S ratio > 1.87 was enhanced. It was shown that the formation of lime-containing decomposition products was delayed to a high degree or suppressed. In addition, no Y-C2S formation was observed with magnesiaspinel bricks if the spinel-forming mixtures were used instead of pure MA-spinel. In this way a tendency of the deposit to disintegrate in the contact during the C2S conversion from the V into the 0 form was largely prevented, and the deposit remained intact on the during the cooling. This phenomenon is said to be advantageous for the life of the furnace- As described, it is known to solve the problem of poor thermal shock resistance of refractory compositions based on magnesia either by additions of various components such as chromium ore or spinel in proportions of 10 to 30 wt % or by an addition of coarse-grained zirconium oxide- By the addition of various components such as chromium ore or spinel, a diminution of the mechanical strength and a change in the chemical composition occur. This has a disadvantageous effect on the life and the 7 - possible uses of refractory products of this kind. The use of coarse- grained zirconium oxide serves above all to make the surplus CaO harmless. In addition, the coarsegrained zirconium oxide has to be manufactured separately in order to obtain granules of the desired size.

This invention seeks to provide shaped and unshaped refractory compositions, consisting of magnesia raw material and zirconium oxide, which possess without further additions a high thermal shock resistance, a good high-temperature strength and a high chemical resistance to CaO and basic decomposition products with a C/S ratio > 1.87. According to this invention there is provided a refractory composition comprising magnesia raw material and zirconium oxide, characterised in that the magnesia raw material is a sintered and/or fused magnesia with an Mg content of at least 96 wt % and the zirconium oxide exhibits a purity of at least 95 wt % together with a grain size of less than 0. 040 mm and the quantitative proportion of the zirconium oxide is 1.0 to 2.5 wt %.

By the use of high-purity starting materials and a zirconium oxide in a small grain size and in a low amount of 1-0 to 2.5 wt %, refractory compositions have been obtained which possess a very high thermal shock resistance and a high hot strength.

8 - It is critical that the zirconium oxide is added in a grain size of less than 0.040 mm. The effects thereby achieved are surprising in view of the fact that in DE-PS 22 49 814 it is explicitly pointed out that test series have shown that additions of finely dispersed zirconium oxide do not improve the thermal shock resistance. The further developments which have taken place in the meantime relate in the main to spinel-forming compositions.

It is of advantage furthermore that the zirconium oxide does not have to be prepared additionally, since the Zr02 powder present in the original state already exhibits the desired grain size.

By reason of the highpurity starting materials the spinel content is sufficiently small that interfering attacks by CaO and lime-rich materials are largely prevented. During the heating and cooling, there are generated by the zirconium oxide particles finely dispersed according to the invention in the magnesia material, as a result of the low thermal expansion of the zirconium oxide particles, stress fields which lead to a significant improvement in the thermal shock resistance, -p without the mechanical strength, measured as modulus of elasticity, and the chemical character of the ground material being altered. The retention of the chemical 8 2 composition of the ground material magnesia guarantees tolerance of high temperatures, expressed in the hot modulus of rupture, and the chemical resistance to CaO and lime-rich materials. This unexpected effect occurs however only within the stated amounts to be used. A higher amount of zirconium oxide than 2.5 wt % already leads to a diminution in the strength values.

There can be manufactured with the refractory composition according to this invention bricks which e.g. as lining material for industrial furnaces guarantee a considerably longer service life than the bricks known to date.

According to a further embodiment of the invention the zirconium oxide addition can take place in unstabilised form (baddeleyite) and/or in stabilised form. Particularly good properties are obtained if the proportion of zirconium oxide comes to 1.5 to 2.5 wt %. The invention also provides that the magnesia raw material used possesses the following grain fractions:

1.00 to 5.00 mm 30 to 45 wt ',o 0.09 to 1.00 mm 15 to 30 wt % < 0.09 mm 30 to 40 wt O/o C4 Lhe shaped refra k-.ory products according to this invention are manufactured from mixtures of the starting materials to which there is added as a temporary binder i 11 t 1 waste sulphite liquor together with a suitable quantity of water. This binder ensures that the blanks have sufficient green strength after the demoulding and does not introduce any interfering components into the initial mixture.

According to the invention refractory compositions are compacted and shaped in a suitable mould by pressing. The mouldings are after the shaping dried at 100 to 200 OC and subsequently fired at 1700 to 1800 OC.

The unshaped refractory compositions according to the invention are in the case of shipment as solids provided with a temporary binder such as synthetic resin or with chemical binders on a sulphate or phosphate base.

The shaped and unshaped refractory compositions according to the invention are preferably used for the brick lining of furnaces or of hightemperatures regions and transition zones of kilns for cement manufacture or valve plates in the steel industry and in the supplying of the regenerative chambers of glass melting furnaces.

This invention is further described and explained with reference to the following examples and the accompanying drawing showing a graph of the residual dynamic modulus of e-lasticity plotted against the number of quenchings in air.

-1 - 1.

ExamDle 1 A mixture of 98.17 parts by wt sintered magnesia with an MgO content of 98.6 wt % and 1.83 parts by wt zirconium oxide additive (monoclinic, baddeleyite) with a ZrOn+Hf02 content of 99.6 wt % was manufactured, wherein the sintered magnesia had the following grain size distribution:

1.00 to 3.15 mm 40 wt % 0.09 to 1.00 mm 30 wt % < 0.09 mm 30 wt % The zirconium oxide additive had a grain size of 016 mm. The starting materials were mixed carefully, moistened with the necessary aqueous waste sulphite liquor and then compressed to bricks with a pressing pressure of 120 N - mm-2. After the demoulding the bricks were dried at 110 to 1200C and finally subjected to firing at 1700 'C. The properties of the bricks are given in the table below. Example 2 The composition is similar to that in Example 1, but baddeleyite was replaced by an addition of volume stabilised zirconium oxide with a ZrO-- HM, content of 95 w 1-1 % and a CaO content of 4 - 4 w-.', 110, referred to the zirconium oxide addition. The grain spectrum of the zirconium addition lies in the range < 0.040 mm. The properties of the bricks are given in the table below. ExamDle 3 A mixture of 98.17 wt % of the same magnesia and 1.83 wt % of the identical zirconium oxide as in Example 1 was manufactured.

The mixture has the grain size distribution: 1.00 to 5.00 mm 0.09 to 1.00 mm < 0.09 mm wt % 25 wt % 35 wt % There was added as binder to the mixture 4 wt % of sodium polyphosphate, referred to the total mass of the refractory composition- The following table gives the properties of the samples from the mixture after firing at 1700 OC.

The following table lists the property values determined on the products according to Examples 1 to 3 and, as a comparison, the properties of two thermal shock resistant products used in practice to date, which are referred to as Type 1 and Type 2.

r 1 -3 Composition and Known prior art According to the invention properties Type 1 Type 2 Example 1 Example 2 Example 3 Sintered magnesia wt % 90.5 87 98.17 98.17 98.17 A1,0, wt 1k 9.5 13 Zirconium oxide addition - - 1.83 1.83 1.83 wt % Bulk density (9.CM-3] 2.90 2.89 2.94 2.93 2.92 Hot modulus of rupture at 1.3 1.8 6.1 10.3 5.5 1400 C (MPal.

Edyno at 25 "C [GPal 13.6 20.4 81.2 73.1 70.1 Edyn71Edyni) 1M 46.2 58.6 78.1 77.8 75.0 EdynGO/EdynO 37.2 33.5 47.3 not not determined determined dynamic modulus of elasticity, calculated from the measurement of the resonance frequency obtained by a bending vibration excitation on samples with the dimensions 150 x 25 x 25 mm3 residual dynamic modulus of elasticity after 7 or 60 temperature changes 1200 OC <-> room temperature; high values signify good TSR.

As further illustration of the outstanding properties of the compositions according to the invention, there is shown below for Examples 1 to 2 and C for Comparison Examples Type 1 and Type 2 a graph of the residual dynamic modulus of elasticity plotted against the number of quenchings in air.

'I

Claims (11)

1. A refractory composition comprising magnesia raw material and zirconium oxide, characterised in that the magnesia raw material is a sintered and/or fused magnesia with an Mg content of at least 96 wt % and the zirconium oxide exhibits a purity of at least 95 wt % together with a grain size of less than 0.040 mm and the quantitative proportion of the zirconium oxide is 1.0 to 2.5 wt %.

2- A refractory composition according to Claim 1, characterised in that the zirconium oxide is used in unstable (baddeleyite) form.

3. A refractory composition according to Claim 1, characterised in that the zirconium oxide is used in stabilised form.

4. A refractory composition according to any one of Claims 1 to 3, characterised in that the addition of zirconium oxide is 1.

5 to 2.5 wt r, CoMpoS,4 I. A refractory iL4i-on according to any one of Claims 1 to 4, characterised in that the magnesia granulation has the following fractions:

i 1 1.00 to 5.00 ram 0.09 to 1.00 mm < 0.09 mm to 45 wt % 15 to 30 wt % 30 to 40 wt %.

6. A refractory composition according to any one of Claims 1 to 5, characterised in that the latter contain as a temporary binder aqueous waste sulphite liquor.

7. Shaped refractory compositions according to any one of claims 1 to 6, manufactured by pressing into a mould, subsequent drying at a temperature of 100 to 120 OC and then firing at a temperature of 1700 to 1800 'C.

8. Unshaped refractory compositions according to any one of claims 1 to 5, manufactured as solids with a temporary binder on a synthetic resin base or with chemical binders on a sulphate or phosphate base.

9. Use of a refractory composition according to Claim 7 for the brick lining of furnaces or of high temperature regions and transition zones of kilns for cement manufacture and lime manufacture or as gate-type valve joints in the steel industry and in the supplying of the regenerative chambers of glass melting furnaces.

1.

10. Refractory compositions substantially as described herein and as exemplified.

11. A refractory material when manufactured from a composition according to any preceding claims.