CA1125791A

CA1125791A - Compositions suitable for producing ceramic coatings

Info

Publication number: CA1125791A
Application number: CA319,500A
Authority: CA
Inventors: Wilhelm Pirkl; Lorenz Dotsch; Gunther Wieland
Original assignee: Didier Werke AG
Current assignee: Didier Werke AG
Priority date: 1978-01-11
Filing date: 1979-01-11
Publication date: 1982-06-15
Also published as: FR2414484B1; ZA7991B; AT370068B; ATA3979A; IT7947562A0; IT1113709B; DE2800988B1; GB2012258A; DE2800988C2; FR2414484A1; GB2012258B; BE873381A

Abstract

ABSTRACT

COMPOSITIONS SUITABLE FOR PRODUCING
CERAMIC COATINGS

There is disclosed a composition suitable for producing cermic coatings which comprises a granular aggregate of sintered magnesia, chromite or mixtures thereof and, on a dry weight basis 2 to 6% w/w clay, up to 2% w/w magnesium sulphate or sodium silicate or mixtures thereof, 0.5 to 1.5% w/w ferric sulphate or aluminium sulphate or ferrous chloride and 0.2 to 0.6%
w/w of sodium perborate, sodium percarbonate, or sodium perchlorate or mixtures thereof. The composition can be used to produce ceramic coatings on refractory materials by mixing it with water, allowing the mixture to stand and then applying it to a refractory substrate, drying it and then heating it to a high temperature.

Description

~12S~91 1.

"COMPOSITIONS SUITABLE FOR PRODUCING
CERA~lIC COATINGS"

The invention relates to compositions suitable for producing a ceramic coating on the refractory lining of a metallurgical vessel in particular tundishes containing molten steel in plants for continuous casting. The 5. composition is based on a granular aggregate of sintered magnesia, chromite or mixtures thereof and a binding agent consisting of clay, magnesium sulphate, sodium silicate, additives and water.
The durability required of the lining of tundishes 10. in plants for continuous casting is incessantly in-creasing because there is an increasing tendency to use the process of sequence casting. Refractory lining of the tundish with firebricks or high-grade magnesia bricks is well known. However, the brickwork is severely 15. damaged by cleaning and repair works between pourings, and for magnesia brickwork it is also necessary to maintain a high temperature between pourings to avoid the formation of cracks in the brickwork caused by thermal stress. The refractory brick lining of the tundish is 20. to a large extent covered with a protective coating containing magnesia which is applied by spreading or spraying~ This coating, however, must not only be dried, but it must also be carefully preheated to a compara-tively high temperature to prevent spalling of the 25. refractory material in contact with the molten steel.
After use of the tundish the coating is removed and replaced by a new one for the purpose of cleaning and particularly for renewal of the outlet.
DT-AS 1 238 832 provides for the use of a binding 30. agent consisting of two types of sodium silicate for ~

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mixtures of refractory material with a basic or non-acid material as the main constituent. ~y means of this binding agent it is-intended, particularly for materials in the form of a spray in combination with bentonite, 5. to obtain quick setting and high initial strength. The mixtures of material may contain further compatible cold binding agents such as magnesium salts, tar or pitch and flux, and additives for example mill scale, fayalite, aluminium oxide, or finely divided silicon dioxide to 10. obtain a vitrified bond. The use of such mixtures of materials as a coating on the refractory lining of metallurgical vessels, however, requires careful pre-heating and sintering after drying of the solidified layer and it is difficult to obtain a sufficiently 15. satisfactory, uniform and coherent coating, which can be easily removed after use.
The object of the invention is to provide an improved ceramic material which is readily applied by spreading or spraying and forms a uniform coating adhering 20. to the refractory lining and which, after drying, can be preheated quickly in a short time without damage by spalling and cracking and then can be brough into contact with molten steel. In addition it is desirable that the coating can be easily removed as a layer from the 25. refractory lining, so as to enable the cleaning and renewal of the metallurgical vessel to be carried out without damage to the *efractory lining.
According to the present invention a ceramic material for coating the refractory lining of a metallurgical 30. vessel, for example a tundish, containing molten steel in l~S~7~1 3.

plants for continu~us casting,comprises a granular aggregate of sintered magnesia, chromite or mixtures thereof and on a dry weight basis 2 to 6% w/w clay, especially a refractory clay, up to 2~ w/w magnesium 5. sulphate (corresponding to about 4% w/w hydrated magnesium sulphate) or sodium silicate or mixtures there-of, 0.5 to 1.5% w/w ferric sulphate or aluminiu~ sulphate or ferrous chloride and 0.2 to 0.6% w/w of sodium per-borate, sodium percarbonate or sodium perchlorate or 10. mixtures thereof. These per compounds give off oxygen in aqueous media.
In a preferred embodiment the material according to the invention contains up to 2G~o w/w magnesium sulphate, 0.5 - 1.5% w/w ferric sulphate and 0.2 to o.6% w/w sodium 15~ perborate.
The invention also extends to a method of forming a refractory coating on a refractory substrate, e.g. a refractory brick or refractory lining of a metallurgical vessel, which comprises mixing a composition in accord-20. ance with the invention with water (e.g. 1 part waterper 100 parts of dry solids),allowing the mixture to stand and applying the resuiting mixture to the refractory ~ubstrate, drying the coating and then heating it to a high temperature e.g. to about 1000C.
25. ~ne ceramic material according to the invention may be deposited on the refractory lining by spreading or spraying and may then be dried in a known way at low temperatures. The material adheres well to the refractory lining.
After drying the coating may be quickly pre-heated. During hardening, drying and the subsequent ~ast '7 4.

heating and contact with molten steel the coating shows high resistance to cracking and spalling. Also during further contact with the molten steel the coating remains strong and practically free of cracks.
5- The advantageous resistance of the coating to loading by drying and particularly to preheating and thermal shoc'~ from the molten steel is t~ought to depend on the interaction of the clay component with the additives and in particular with ferric sulphate and sodium perborate.
10. Although the reasons for the favourable properties of the material have not so far been elucidated, it has nevertheless been established that compositions of the same type in which the binder only is different, or compositions containing a different compound for 15. production of a gas by decomposition, do not lead to the same useful results during drying or fast preheating.
Thus using sodium polyphosphate instead of magnesium sulphate as a binder on the one hand and using sodium bicarbonate instead of sodium perborate on the other 20. hand, in each case resulted in coatings with cracks and ~~~ flaking off from the base material, and thus these coatings could not be used.
The present invention may be put into practice in various ways and a number of specific examples and 25. comparison examples will be described to illustrate the invention. In the Examples all parts are on a dry weight basis unless otherwise indicated.
Examples l, 2, 3, 4 and 6 are for comparison.
Exam~le l 30. This is a comparison example. An aggregate of 1125'79 5.

granular sintered magnesia and chromite having the following chemical composition was used :

sintered magnesia chromite .
CaO 9.1 0.5 free CaO 1.4 SiO2 3.8 5.5 10. Fe23 3-3 14.9 A120~ 0.6 26.8 Cr203 32.0 MgO 83.2 18.7 15.
1QSS on ignition 0.5 1.2 bulk density30f granules

2-6 mm; g/cm 3.09 specific gravity 3.57 3.93 20.
55 parts of magnesia of particle size less than 3 mm (0-3 mm), and 24 parts of magnesia of particle size less than 0.1 mm (0-0.1 mm) were dry blended with 10 parts by weight of chromite having a particle size of 25. less than 1 mm (0-1 mm) and 4 parts of ground and air sifted clay which consists predominantly of kaolimitic clay minerals ~9% A1203) and 5.5. parts of a 2:1 mixture of magnesium sulphate and sodium silicate.
The dry blended mixture was then mixed with 1 part of 3o. water per 100 parts of dry blended materials. The mixture was allowed to stand to mature for 5 to 10 minutes. The 112S'^~g~
6.

material so obtained was well suited for brushing or spreading.
Using this mixture, coatings of 50 x 50 cm area were deposited on a refractory base material of 5. magnesia brick and then dried and fired. The thickness of the coatings was fixed at 3 cm. The test area was initially dried for 30 minutes at a low temperature and then heated to a surface temperature of about 1000C by -means of a gas burner of the type commonly used for 10. drying and heating of refractory linings in steel making plants.
During application of the coating, drying, firing and after gradual cooling, the coating was examined for formation of cracks and spalling.

15. Examples_2 to 7 - Example l was repeated. The additives (and the dextrin in Example 7) were dry blended with the aggregate and binder before the water was added in each example.
20. Examples 1, 2, 3, 4 and 6 are comparison examples.
The proportions of ingredients and resulting properties of the coating are listed in the Table below.
As can be seen from the Table, examples 5 and 7 accord-ing to the lnvention showed the best actual service 25. properties. The success of this ceramic material accord-ing to the invention can already be confi~med in practice for coatings in contact with molten steel.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition adapted for use in the production of a ceramic coating for a refractory lining of a metallurgical vessel comprising a granular aggregate of sintered magnesia, chromite or mixtures thereof and, on a dry weight basis 2 to 6% w/w clay, up to 2% w/w magnesium sulphate or sodium silicate or mixtures thereof, 0.5 to 1.5% w/w ferric sulphate or aluminium sulphate or ferrous chloride and 0.2 to 0.6% w/w sodium percarbonrate, sodium percarbonate, or sodium perchlorate or mixtures thereof.

2. A composition as claimed in Claim 1 which contains up to 2% w/w magnesium sulphate, 0.5 to 1.5% w/w ferric sulphate and 0.6% w/w sodium perborate.

3. A method of forming a refractory coating on a refractory substrate which comprises mixing a composition as claimed in Claim 1 or 2 with water, allowing the mixture to stand and applying the resulting mixture to the refrac-tory substrate, drying the coating and then heating it to a high temperature.

4. A refractory substrate whenever provided with a refractory coating by a method which comprises mixing a composition as claimed in Claim 1 or 2 with water, allowing the mixture to stand and applying the resulting mixture to the refractory substrate, drying the coating and then heating it to a high temperature.

5. A metallurgical vessel having a refractory lining whenever provided with a refractory coating by a method which comprises mixing a composition as claimed in Claim 1 or 2 with water, allowing the mixture to stand and applying the resulting mixture to the refractory substrate, drying the coating and then heating it to a high temperature.