Refractory composition for producing compact castable and...

Coating processes – Spraying

Reexamination Certificate

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Details

C427S422000, C427S426000, C427S427000, C501S094000

Reexamination Certificate

active

06277446

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a cement-free, castable refractory composition for producing dense cast refractories usable as linings of vessels for molten metals such as ladles, tundishes, troughs, etc., and a method for wet-gunning it.
Refractory materials for linings of vessels for molten metals have lately been changing from bricks to easy-to-apply castables, as the cast refractories have been provided with more and more improved durability. However, casting methods are still disadvantageous in terms of time and labor needed for placement of forms in casting. On the other hand, gunning methods contribute to reduction in time and labor consumption and have versatility in repair, because these methods do not require the placement of forms and can quickly make partial repair. Accordingly, the gunning methods have been finding more and more applications.
Though most widely used now is a dry-gunning method, it produces refractory layers with poor durability and brings about rebounds and dust, resulting in a poor working environment. To overcome the problems of the dry-gunning methods, developments have thus been made recently to provide various gunning methods such as a semi-dry gunning method, a wet-gunning method, etc., and refractory materials therefor.
In the semi-dry gunning method, the gunning refractory material is premixed with part of necessary water by a mixer, pneumatically conveyed to a gunning nozzle by a dry-gunning machine, mixed with the remaining water, or with a solution or a suspension containing a hardening agent in the gunning nozzle (or before reaching the gunning nozzle), and then gunned through the nozzle. The semi-dry gunning methods are exemplified in Japanese Patent Laid-Open No. 61-111973, and Japanese Patent Publication Nos. 2-27308, 6-17273, 5-63437 and 5-21866, etc.
Japanese Patent Laid-Open No. 61-111973 discloses the combination of a hardening accelerator and sodium silicate as a binder, and Japanese Patent Publication Nos. 2-27308, 6-17273 disclose low-cement castables as gunning materials. Also, both of Japanese Patent Publication Nos. 5-63437 and 5-21866, owned by the same applicant, disclose castable refractory materials containing ultrafine powder and dispersing agents, which are not hardened at room temperature. Specifically, in the former reference a refractory clay is used as an indispensable component and hardened by Ca(OH)
2
, sodium silicate or sodium aluminate introduced at a nozzle to improve the resistance to dry-explosive spalling. In the latter reference, a refractory composition to which a moisture retention agent is added is premixed with water in an amount of ⅕-¾ of the normally required amount in a factory to provide a wet mixture, which is then hardened by sodium silicate, sodium aluminate or colloidal silica introduced at a nozzle.
Though improvements were achieved to some extent in these gunning methods in reducing dust generation and rebound loss, these gunning methods still produce refractory layers with poor adhesion, homogeneity and density, since the refractory materials should be mixed instantaneously with water or an aqueous solution in a nozzle, resulting in poor mixing at an unstable water ratio.
On the other hand, the wet-gunning method is a method in which gunning refractories are premixed with all of water required for gunning, or a method in which factory-premixed refractories delivered in a premixed form are gunned. In the wet-gunning methods, it is possible to add a small amount of an aqueous solution of a hardening agent or a hardening-adjusting agent to the gunning refractories in a nozzle. The wet-gunning methods are classified into a pneumatically conveying method by a gunning machine and a pump-conveying method depending on how to convey gunning materials. In the case of the pumping method, compressed air is introduced into the nozzle to spray refractory materials. The present invention belongs to the latter category. Depending on the amount of water added, there are provided gunning materials with various workabilities ranging from a plastic level to a slurry level.
These wet-gunning methods are exemplified in Japanese Patent Publication Nos. 57-7350, 62-21753, 2-33665, 2-1795, etc. Because the gunning materials disclosed in Japanese Patent Publication No. 57-7350 are in the form of a slurry having a water content of 10-20%, they are not expected to be placed into dense refractory layers suitable for linings of vessels for molten metals. In the latter three methods, the gunning refractory materials are delivered in the form of a moist premix with water or a solution, making it possible for them to be produced and stored in factories as in the present invention. These wet-gunning methods contribute to the reduction of labor and the improvement of the working environment because tempering is not needed at the gunning site.
However, these conventional methods do not utilize the dispersion action of ultrafine refractory powder to reduce the amount of water added. They are also still not different in composition from refractory materials of the previous generations, for instance, because of maximum size of 4 mm or less in particles contained in the gunning materials. In addition, they use gunning machines as refractory-conveying means. Accordingly, the gunned refractory layers are insufficient in denseness and thus considerably poor in durability, as compared with those produced from castable refractory materials, particularly dense castables.
Accordingly, an object of the present invention is to provide a cement-free, castable refractory composition capable of producing dense cast refractories having high density, strength and corrosion resistance at a low water content, and a method for gunning such a cement-free, castable refractory composition.
When ultrafine refractory powder is dispersed in water, there are two types of force, an attraction force and a repulsion force, working among the suspended particles, and the combined force of these two forces is applied to the particles. The attraction force is a van der Waals force, and the repulsion force is an electrostatic force between electric double layers on particle surfaces. When the repulsion force exceeds the attraction force by the function of the dispersing agent, the suspended particles are kept in a dispersed state. In this state, water held among the agglomerated particles is set free as free water which contributes to increase the flowability, making the refractory materials flowable at a low water content.
When an electrolyte (coagulant) releasing ions such as H
+
, Mg
2+
, Ca
2+
, Al
3+
, SO
4
2−
, CO
3
2−
, etc. is added in a concentration higher than a certain level in this state, the repulsion force decreases such that the attraction force becomes relatively larger than the repulsion force, resulting in rapid coagulation of the ultrafine refractory powder. What utilizes this mechanism is called dense castable refractory materials such as ultra-low-cement castables, low-cement castables, etc. Particularly in the ultra-low-cement castables containing 3 weight % or less of alumina cement, it may be regarded that the alumina cement functions as a coagulant.
However, it is of course impossible to achieve good workabilities at a low-water content simply by adding both of the dispersing agent and the coagulant to the refractory materials, because both additives having opposite properties act simultaneously. Nevertheless, actual refractory materials are usually provided as products containing both dispersing agent and coagulant. Accordingly, the refractory material should have such a composition as to have a mechanism that in a case where the refractory material tempered with a small amount of water is cast while good workability is available by the function of the dispersing agent, setting takes place by the function of the coagulant after the lapse of a certain period of time.
There have been known methods for that purpose; (i) a method using as a coagulant a material such

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