GB2188314A - Refractory materials - Google Patents

Abstract

A refractory material suitable for lining furnace walls at elevated temperature by spraying is provided. The main ingredient as a binder is an amorphous calcium aluminate of the formula aluminate of the formula CaO+Al2O3, 3CaO+Al2O3, 3CaO+3Al2O3+CaF2, 11CaO+7Al2O3+CaF2, 12CaO+7Al2O3, 3CaO+3Al2O3+CaSO4 or a crystalline calcium aluminate of the formula 3CaO+3Al2O3+CaSO4. Calcium aluminate having a glass content of not less than 50 wt% and a specific surface area (Blaine value) of not less than 2000 cm<2>/g is preferred in an amount of not less than 50 wt%. Quicksetting after spraying, and low peeling are achieved by such materials.

Description

SPECIFICATION Refractory materials DESCRIPTION The invention relates to refractory materials containing binders. Refractories for lining furnace walls are frequently applied by spraying, and such spraying is often performed for repairing furnace walls. Refractories used for this purpose include binders composed singly or in combination of phosphates, alumina cements and/or crystalline calcium aluminates in powder form.

The refractories are usually applied when the furnace is at about room temperature. When sprayed onto furnace walls at elevated temperatures, the refractories often peel owing to the abrupt evaporation of water, and leave flaws or defects uncovered by refractory lining. Thus it is necessary to cool the furnace for a long standstill time, for instance for three days or more before the walls are ready to be sprayed. These refractories become deficient in refractoriness and low in adhesive power on standing for a long time after application to a furnace wall.

The invention provides a refractory material comprising an amorphous calcium aluminate or a crystalline calcium aluminate of the formula 3CaO+3AI203+CaSO4 as a binder in admixture with other ingredient of kinds known in themselves. This has proved high in quick setting force, and to form a refractory firmly adhering on the furnace wall without peeling or desquamation even if used under high temperature conditions. The resulting refractory rapidly exhibits a high strength, and is effective even when the content of the binder is relatively small. A small rebounding amount has been achieved which affords a high operation efficiency.

The refractory material preferably comprises as a main ingredient an amorphous calcium aluminate of the formula CaO+AI203, CaO+2AI203, 3CaO+A1203, 3CaO+3AI203+CaF2, 11 CaO +7A1203+CaF2, 1 2CaO+7AI203 or 3CaO+3AI203+CaSO4. Mixtures of two or more thereof may be used.

The amorphous calcium aluminates may be prepared by melting the corresponding material at a temperature not lower than 1000"C, preferably not lower than 1400"C, and then quenching the molten mass to a temperature of lower than the crystallization temperature by contacting the molten mass with a cooling medium, such as compressed air or water to form an amorphous calcium aluminate. The degree of amorphousness may be raised by perfect melting of the starting material and by using a high quenching speed.Rapid quenching can be effected by contacting the molten mass with a large quantity of a cooling medium in a quenching time of not more than 5 seconds, preferably not more than one second, to a temperature of not higher than 500"C, preferably not higher than 1 00 C. A small quantity, for example 5 wt% or lower, of a melting point depression agent such as a halide (one example being CaF2), or a boron compound such as borax, may be added to form a perfectly melted mass or accelerate the formation of an amorphous mass. It is preferred that the quantity of melting point depression agent added is as small as possible.

In preparation of an amorphous calcium aluminate having the composition of 3CaO+3AI203+CaSO4, it is desirous that the starting material be melted at a temperature of from 1300 to 1400"C because the calcium aluminate decomposes at 1500 to 1 600 C. Quenching is preferably at an extremely high speed, for example, by blowing a large quantity of a cooling medium into the system to increase the glass content.

It is desirable that the glass content of the amorphous calcium aluminate be not less than 50 wt%, preferably not less than 90 wt%, and more preferably at 100 wt%. Glass content can be determined by comparing the heights of peaks on X-ray diffractiometry charts of perfectly crystallized calcium aluminate (prepared by cooling a molten mass at a cooling rate of not more than 10 degrees C per minute until the temperature of the mass reaches 500"C) with a target amorphous calcium aluminate which has been quenched rapidly. Calculation is in accordance with the following equation: Glass content equals weight of crystalline mass minus weight of quenched mass divided by weight of crystalline mass multiplied by 100.

Crystalline calcium aluminate having the formula 3CaO+3AI203+CaSO4 may be prepared by melting the material at a temperature not lower than 1000"C, preferably at a temperature of from 1300 to 1400"C, followed by cooling. The degree of crystallization may be checked by the X-ray diffractiometry.

In preparation of these calcium aluminates, a CaO source such as quick lime, an A1203 source such as bauxite or alumina, and a CaSO4 source such as anhydrous calcium sulfate are preferably mixed together in a molar ratio as indicated by the desired composition. The mixture is melted by heating in an electric furnace, kiln or open-hearth furnace, and followed by quenching or slowly cooling.

The calcium aluminate is generally used in powdered form. The specific surface area (Braine value) of the calcium aluminate powder should be generally not less than 2000 cm2/g, preferably from 4000 to 7000 cm2/g.

A calcium aluminate powder having an excessively fine particle size causes the problems due to dust, whereas a calcium aluminate powder having too coarse particle size is undesirable because its reaction affinity is depressed, and the initial strength and adhesiveness of the refractory material are lowered.

It is preferred that the amount of calcium aluminate in the refractory material is as large as possible, preferably not less than 50 wt%, more preferably not less than 70 wt%.

Examples of other ingredients used in refractory materials are alumina cements, various phosphates such as aluminium phosphate, alkali silicates such as sodium silicate, various cements such as Portland cements, or a mixture of two or more thereof. These materials may be mixed with the calcium aluminate in preparation of the refractory.

The content of binder in the refractory material may be from 2 to 40 wt%, preferably from 5 to 20 wt%, to provide satisfactory refractoriness. If the content is less than 2 wt%, the binding force becomes deficient, and if more than 40 wt% the refractoriness is lowered.

Further ingredients include alumina base materials, silica base materials, alumina-silica base materials, magnesia base materials, zircona base materials, silicon carbide base materials or a mixture of two or more thereof. The specific kind of the refractory material and its particle size may be selected in consideration of spraying operation to be used, and the strength required for the refractory at an expected use temperature.

The refractory material mixture may be pneumatically conveyed to a sprayer, mixed with water immediately before spraying, and the admixture composed of the binder, the refractory material and water is sprayed onto a furnace wall or a surface to be coated with a refractory lining. If an excessively large amount of water is added, the strength after spraying may be decreased, adhesiveness may be reduced due to evaporation of water with attendant peeling-off or falling-down of the sprayed coating. If the quantity of added water is too small, the quantity of the rebounding material becomes too great. Accordingly, it is preferable that the amount of water added should be as small as possible, preferably from 5 to 20 wt%.

A water reducing agent, such as a lingin sulfonate system compound, may be added to reduce the quantity of water added at this step. Particulariy preferred for this purpose is a plasticizer, such as a condensation polymerization product of naphthalenesulfonic acid and formaldehyde. Other additives, which are usually added in such compositions for similar applications, may also be used. Examples are a retarder, such as an organic acid including an oxycarbonic acid, and an alkali carbonate such as sodium carbonate or sodium bicarbonate.

It is also effective to admix a fine powder of a siliceous or aluminous material, such as a powder of silica flour and alumina powder, with the aim of increasing the strength of the resultant lining at high temperature or reducing the amount of rebounding in the spraying. Conventional binders for refractory materials, such as alumina cements, phosphates and silicates, may also be used in the refractory materials of the invention. The refractory material may be cast and/ortamped or used in spray coating. They may optionally be mixed with a retarder, if necessary during processing.

In the following Examples and Comparative Examples, C means CaO, A means A1203, and S means SO3, and A-CA means an amorphous calcium aluminate.

Example 1 From a mixture of a quick lime (Purity: 98%) and an alumina (Purity: 97%), a CA composition represented by C12A7 was melted in an electric furnace at 1700"C, and the molten mixture was then blown by compressed air of 5 kg/cm2 to cool rapidly to a temperature below 500"C within one second to obtain an A-CA. The thus obtained A-CA was pulverized to have a Blaine value of 5700 cm2/g, and 7 parts by weight of the pulverized ACAwas mixed with 3 parts byweightofan alumina powder,7 parts by weight of a silica flour and 100 parts by weight of a magnesia clinker pulverized to have a particle size of below 5 mm.

The mixture was pneumatically conveyed by a sprayer available from Alvia Co., under the Trade Designation of "Model 260" by a distance of 50 m, and blown with water at a point 5 m before the nozzle in a rate such that 100 parts by weight of the mixture was added with 12 parts by weight of water. And the admixture blown with water was sprayed onto a surface of a wall fabricated with refractory bricks and having an area of 30 m2 and maintained at an average temperature of 327"C.

The refractory composition of this Example was solidified instantaneously after it was sprayed onto the wall with no substantial peeling-off or desquemation and with extremely small rebounding amount of 8 wt%.

The wall was brought to the condition ready for use in 2 hours.

Comparative Example 1 A composition was prepared similarly as in Example 1 except that the tradename High Alumina Cement available from DENKI KAGAKU KOGYO K.K. was used in place of the A-CA and added with 2 parts by weight of a commercially available calcium hydroxide as a quick setting agent. The mixture was sprayed onto a surface of the same wall as that coated with the admixture of Example 1 generally following the operations practiced in Example 1.

The result was that the composition of this Comparative Example could not be used for practical application, since peeling-off or exfoliation of the sprayed coating was often caused when the temperature qf the wall was higherthan 100 C.

Example 2 Similar procedures were repeated as in Example 1, except that molten masses were prepared from the mixtures mixed to form compositions represented, respectiveiy, by CA2, C3A, C3A3+CaF2 and C11A7+CaF2 by heating to 1 800 C, and that respective molten masses were cooled below 500"C within one second.

The results are shown iri Table 1.

TABLE 1 Composition CA2 C C3A3+CaF2 C11A7+CaF2 Rebound(wt%) 10 13 10 8 Time Readyfor Operation 3 hr. 4hr. 3 hr. 2 hr.

Example 3 Using the same materials as used in Example 1, prepared were the compositions having the glass content as shown in Table 2 by varying the pressure of compressed air and by varying the cooling rate. Other conditions and procedures were the same as in Example 1.

The results are shown in Table 2.

TABLE 2 Pressure of Compressed Air (kg/cm2) 4 2.5 1.5 0 Glass Content (%) 80 50 30 0 Rebound(wt%) 10 15 35 40 Time ready for Operation (hr.) 3 4 24* * Note: Some peeling was observed.

Example 4 Similar procedures were repeated as in Example 1, except that the A-CA prepared in Example 1 were changed as set forth in Table 3. The results are shown in Table 3.

TABLE 3 BlaineValue(cm2/g) 1500 2000 4000 Rebound(wt%) 30 20 16 TimeReadyforOperation(hr.) 20* 5 4 *Note: Some peeling was observed.

Example 5 Similar procedures were repeated as in Example 1, except that the mixed amount of A-CA prepared in Example 1 were change to 4.6, 12 and 27 parts by weight, respectively, as set forth in Table 4. The results are shown in Table 4.

TABLE 4 Mixed Amount (part by weight) 4.6 12 27 Rebound(wt%) 15 6 5 Time Ready for Operation (hr.) 3 1.5 1.5 Example 6 An alumina cement was melted at 1800"C in an electric furnace, and the molten mass was blown by compressed air of 5 kg/cm2 to cool rapidly to 500"C within one second, whereby an A-CA was prepared. The A-CA was pulverized to have a Blaine value of 5500 cm2/g, and 7 parts by weight of the pulverized A-CA, 3 parts by weight of an alumina powder, 7 parts by weight of a silica flour, and 100 parts by weight of a magnesia clinker pulverized to have a particle size of below 5 mm were mixed together. During the operations, a portion of molten mass was picked up and subjected to an X-ray diffractiometry to ascertain that the molten mass contained 90 wt% of the amorphous CA.

The mixture was pneumatically conveyed by the "Model 260" sprayer available from Aliva Co. by a distance of 50 m and blown with water at a point 5 m before the nozzle in a rate such that 100 parts by weight of the mixture was added with 12 parts by weight of water. And the admixture blown with water was sprayed onto a surface of a wall fabricated with refractory bricks and having an area oF 15 m2 and maintained at an average temperature of 315 C.

The refractory composition of this Example was solidified instantaneously after if was sprayed onto the wall with no substantial peel-off or desquemation and with extremely small rebounding amount of 9 wt%.

The wall was brought to the condition ready for use as a refractory material within 2 hours.

Example 7 Similar procedures as in Example 6 were repeated except that the A-CA prepared in Example 6 was pulverized to have the rebounding values as set forth in Table 5.

The results are shown in Table 5.

TABLE 5 Blaine Value (cm2/g) 1500 2000 4000 Rebound (wt%) 35 23 20 Time Readyforoperation (hr.) 20* 5 4 *Note: Some peeling was observed.

Example 8 Similar procedures were repeated as in Example 6, except that the mixed amount of A-CA prepared in Example 6 were changed to 4.6, 12 and 27 parts by weight, respectively, as set forth in Table 6. The results are shown in Table 6.

TABLE 6 Mixed Amount of A-CA (part by weight) 4.6 12 27 Rebound(wt%) 17 8 6 Time Ready for Operation (hr.) 3 1.5 1.5 Example 9 Mixtures were prepared by using quick lime as a CaO source, bauxite as an Al203 source, and an hydrous calcium sulfate as a CaSO4source. Each of the used starting materials had been pulverized before being mixed together.

The results of quantitative analyses of respective raw or starting materials are shown in Table 7.

26.3 kg of a quick lime, 52.9 g of bauxite and 20.8 kg of anhydrous calcium sulfate were mixed and pulverized to obtain a mixture, from which pellets were prepared by moulding under pressure. The thus prepared pellets were calcined in a siliconite electric furnace at 1300"C for an hour to obtain a crystalline clinker represented by C4A3S and having the composition as set forth in Table 8.

The calcined dinner was pulverized into fine powder having a Blaine value of 4500 cm2/g. 8 wt% of the calcined and pulverized clinker was mixed with 4 wt% of an alumina powder produced by Sumitomo Chemical Co., Ltd., 8 wt% of a silica flour which was an alloyed iron dust and 80 wt% of a magnesia clinker pulverized to have a particle size of below 5 mm.

The mixture was pneumatically conveyed by the "Model 260" sprayer available from Aliva Co. by a distance of 50 m and blown with water at a point 5 m before the nozzle in a rate such that 100 parts by weight of the mixture was added with 12 parts by weight of water. And the admixture blown with water was sprayed onto a surface of a wall fabricated with refractory bricks and having an area of 30 m2 and maintained at an average temperature of 327"C.

The refractory composition of this example was solidified instantaneously after it was sprayed onto the wall with no substantial peeling-off or desquemation and with extremely small rebounding amount of 7 wt%.

The wall was brought to the condition ready for use in about 2.5 hours after being spray-coated with the refractory composition.

TABLE 7 Quantitative Analysis (wt%) Ingredient Raw Material Ig-Loss Al2O3 CaO SO3Fe2O3 SiO2 MgO TiO2 TOTAL Quick Lime - 0.5 94.7 0.2 0.4 1.9 0.8 - 98.5 Bauxite 0.3 86.2 0.3 0 3.9 5.5 0 3.2 99.4 Anhydrous Calcium Sufate 1.7 0.3 39.4 58.1 0.3 0.1 0 0 99.9 TABLE 8 Quantitative Analysis of Calcined C4A3S (wt%) Ig-loss In-Sol Al2O3 CaO SO3 Fe2O3 SiO2 MgO TiO2 TOTAL F-CaO 0.1 0.5 45.3 33.3 12.1 2.2 3.4 0.2 1.7 98.7 0 Example 10 Similarly as described in Example 9, the materials set forth in Table 9 were mixed to prepare the compositions (Runs Nos. 2 to 9) as set forth in Table 9. Each of the thus prepared compositions was sprayed onto a surface of brick wall maintained at an average temperature of 330 C and having a surface area of 10 m2.

The results are shown in Table 9.

Comparative Example 2 Similar procedures were repeated as in Example 10, except that a binder for a refractory material solely made of an alumina cement was used, as seen from Run No. 1 in Table 9.

The results of quantitative analysis of the calcium aluminate used in the preceding Examples are set forth in the following Table 10.

TABLE 9 Refractory Material Binder (wt%) (wt%) Standstill Time Ready Run Alumina Alumina Silica Magnesia Rebound for Re-operation No. C4A3S Cement Powder Flour Clinker (wt%) (hr) Remarks 1 0 8 4 8 80 25 -* Comp. Ex.

2 4 4 ,, ,, " 15 12 Example 3 6 2 ,, " " 13 3 4 2 0 " 9 85 18 18 5 5 ,, ,, 8 83 10 10 6 10 ,, ,, ,, 78 6 2 7 20 " 3 7 70 5 1.5 8 30 ,, 6 61 6 61 ,, 9 40 ,, ,, 5 52 4 *Note: Peeling was so significant as not to be applied for practical use. The average surface temperature of the retractory bricks was lowered to 150"C and then the spraying operation was repeated. However, the furnace had to be stand still for 2 days until it became in the condition for reuse.

The chemical analyses of the calcium aluminates used in the above Examples are shown in Table 10.

TABLE 10 lg-loss In-Sol Al203 CaO Fe2O3 SiO2 MgO TiO2 CaF2 CA 0.1 0.3 63.9 33.7 0.9 0.8 0.2 0.1 0 CA2 0.1 0.5 76.3 20.8 0.7 1.0 0.2 0.3 0 C3A 0.1 0.2 36.9 60.7 0.8 0.9 0.4 0.1 0 C12A7 0.1 0.3 50.2 47.3 0.6 0.9 0.3 0.2 0 C3A3CaF2 0.1 0.3 54.2 29.7 0.6 0.8 0.2 0.3 12.7 C11A7CaF2 0.1 0.2 50.1 42.6 0.7 0.8 0.2 0.3 5.8

Claims (11)

1. A refractory material comprising an amorphous calcium aluminate or a crystalline calcium aluminate of the formula 3CaO+3AI203+CaSO4 as a binder in admixture with other ingredients, of kinds known in themselves.

2. A refractory material according to claim 1 containing amorphous calcium aluminate of the formula CaO+Al2O3, CaO+2Al2O3. 3CaO+A1203, 3CaO+3AI203+CaF2, 11CaO+7AI203+CaF2, 12CaO+7AI203, or 3CaO+3AI203+CaSO4.

3. A refractory material according to claim 1 or claim 2 containing amorphous calcium aluminate having a glass content of not less than 50 wt%.

4. A refractory material according to any preceding claim containing calcium aluminate having a specific surface area (Blaine value) of not less than 2000 cm2/g.

5. A refractory material according to any preceding claim containing calcium aluminate in an amount of not less than 50 wt%.

6. A refractory material according to any preceding claim containing an alumina cement, phosphate, alkali silicate, Portland cement or a mixture of two or more thereof.

7. A refractory according to any preceding claim containing an alumina base material, silica base material, alumina-silica base material, magnesia base material, zirconia base material, or silicone carbide base materials or a mixture of two or more thereof.

8. A refractory according to any preceding claim containing binder in an amount of from 2 to 40 wt%.

9. A refractory according to any preceding claim containing from 5 to 20 wt% of water.

10. A refractory according to any preceding claim containing a water reducing agent, retarder, fine powder of a siliceous material, fine powder of an alumina material, and or a mixture of two or more thereof.

11. A refractory material substantially as herein described in any of the Examples as distinct from the Comparative Examples.