NO143152B - USE OF ALUMINUM OXYDE-CONTAINED ILL-FIXED CONCRETE AS CONTACT MATERIAL WITH MELTED ALUMINUM OR ALLOYS THEREOF - Google Patents

Description

The invention relates to the use of refractory concrete for contact

with masses of molten aluminum and its alloys, both as construction material for melting furnaces and as auxiliary equipment for such furnaces, such as plinths, troughs, re-bottling ladles or casting ladles,

which are often in contact for a long time with molten aluminium.

The refractory materials that have been used in metal foundries contain silicon dioxide, usually in the form of various silicates.

The relative proportion of silicon dioxide varies depending on the type of material. It amounts to approx. 60% by weight in the products that be-

are drawn as clay-like or as silicon aluminum-containing products (35% aluminum oxide), and 35% in aluminum-containing products (60%

aluminum oxide) on the basis of sillimanite, bauxite, mullite or other products. The aluminum oxide as such contains silicon dioxide in an amount of approx. 1%. The magnesium products or chromium/magnesium products often contain silicon dioxide in an amount of up to 10%.

This silicon dioxide is prone to react with

reducing metals, such as aluminium, according to the reaction equation below:

This reaction by reduction of silicon dioxide or of silicates with reducing metal releases a lot of energy. It can theoretically continue until one of the two starting materials has been completely consumed. In practice, the turnover rate is sufficiently slow that light reducing metals and their alloys can be melted in furnaces built from materials containing silicon dioxide. However, various phenomena are encountered which quickly adversely affect the service life of the ovens or the associated auxiliary equipment. A slow impregnation of the refractory material occurs. This is transferred to a black and very durable mass which consists of agglomerates of corundum and aluminium. This mass is conductive to heat and electricity, and this can have serious consequences for an electro-

oven and cause significant energy losses. Swelling and cracks form especially if the refractory material has a low aluminum oxide content. The particles of the refractory material loosen from the furnace and are found in the aluminum castings in the form of hard inclusions. Oxides, which are popularly referred to as "oxide fungi", form on the bottom of the oven and especially on the oven walls which

comes into contact with the bathroom. These oxide fungi increase in size until they block a large part of the furnace. The oxide mushrooms are very hard and in practice cannot be detached from the road.

In British Patent Document No. 1135147 and in the article'of

Von W. Helling and E. Kistermann with the title "Salzimpragnier-verfahren fiir Zustellung von Aluminium-Schmelz - und Warmhalteofen" in the journal "Aluminium", 33 (1957), issue 8, pp. 514-520, these are significant difficulties for the aluminum industry described, and it is proposed to remedy these difficulties by applying a glaze to the inner walls of the oven consisting of a mixture of 80% sodium chloride and 20% cryolite. This mixture has a melting point of 795°C. As the mixture is highly flowable, it will easily penetrate the surface pores of the chamotte-based refractory material. The glaze usually has a visible thickness of 1-2.

mm, but it penetrates up to 6 mm into the refractory material.

The high sodium content in this glaze flux and the relatively low melting point that results from it make it not refractory and not resistant. According to the authors, it is necessary to repeat the impregnation from time to time. A technical application manual from la Société Servimétal ancien départment, fonderie soudure Otalu, 87 Rue Pierre Joigneaux, 92 Bois-Colombes, France entitled "Les procédés de glacage des garnissages réfractaires des fours de fusion et de coulée utilisés das les; fonderies d 'aluminium' is more detailed. After it has been established that molten aluminum has attacked the furnace, two glazing methods are proposed in the brochure using a mixture consisting of 80% NaCl and 20% AlF^ . 3NaF. The deep impregnation method corresponding to the one described in the aforementioned German article is abandoned in favor of a method known as "surface impregnation" which is more advantageous especially because it is simpler, faster, consumes less glaze flux and requires lower working temperatures . This improved method consists in that, after the oven has been dried and fired, the surface temperature of the refractory lining is regulated to 750-780°C, after which the bottom of the oven is sprinkled with 6-10 kg of flux per cm 2 using a sprayer powered by compressed air, the aluminum is filled in the oven, preferably ordinary commercial aluminium, approx. 1 hour after melting has begun, the charge being calculated so that it should not amount to more than a third of the furnace's normal capacity, 2-3 kg of flux per m 2 of the furnace bottom is sprinkled while the charge is half-melted, another flux is sprinkled at the end of melting while the bath has a temperature of* 730-750°C, on the slag which is thereby made dry and powdery and thus makes it easier to remove this.when skimming after 4-5 minutes of degassing reaction and refining reaction as is usual, the kiln is completely emptied and the kiln bottom carefully scraped, a new glaze is applied as described above, with the exception that another filling of metal is calculated so that it makes up half of the kiln's normal capacity, a third glaze as described above is applied, with the exception of certain changes in the composition of the glaze, and the glaze is renewed 5 hours later. The oven is thus completed once and for all, and it is applied

a maintenance glaze once per month. This complex work operation and the costs of this, the lack of mobility and the materials for these original glazes which have since been repeated, have prevented the further development of these methods, all the more so as the addition of a relatively volatile flux will invariably contaminate the treated aluminium; The aluminum smelters in reality continue to work as before and factor in the production costs the necessary investments for frequent replacement of the furnaces, the rapid reduction of the furnaces' capacity, the lack of movement of the furnaces when cleaning after each charge and the costs which. due to the loss of aluminium, and all the time with the fear that a

"hard spot" in the molten aluminum could damage an expensive tool island below the furnace. These methods have not been successful because they have not been able to solve the following problem. If an impregnation is carried out with a flux with a low melting point, the impregnation can be carried out more easily, but the flux evaporates quickly. If an impregnation is carried out with a refractory flux, it is necessary to make this liquid and easy to apply, to heat the furnace to an elevated temperature at which it will not be resistant or only poorly resistant and damaged.

In the article entitled "Oxydation des alliages fondus, réaction avec les réfractaires" by Michel Drouzy and Michel Richard in the journal Fonderie 332, March 1974, pp. 121-128,

it is noted that J.W. Fruehling and J.D. Hanawalt has shown that a fluorine-containing atmosphere protects a refractory furnace intended for processing liquid magnesium (Protective Atmosphere for Melting Magnesium Alloys, Modern Casting 56, August 1969, pp. 159-164), and it is noted that this fluorine-containing atmosphere as can well be used to protect a refractory material against an aluminum bath.

The protection obtained is temporary (a few days at most)

due to the amount of fluorine that the refractory material can absorb in its pores, at the same time that the fluorine-containing compound introduced into the furnace is consumed. For a furnace or crucible which is not used under a restricted atmosphere, the permanent renewal of the atmosphere will practically prevent the achievement of correct protection. In addition, the release of large quantities of fluorine-containing gas is too dangerous for a fluorine-containing atmosphere to be commonly used in the foundries.

These difficulties cannot be overcome by introducing fluorine-containing compounds in the same mass as the refractory material as it is known that fluorine-containing compounds are agents which facilitate melting and which reduce the refractoriness of the products to which they are added. However, attempts have already been made to add fluorine-containing derivatives to building materials which are not intended for contact with molten aluminium, but this affects the refractoriness essentially under two types of conditions which will be discussed in more detail. ' On the one hand, attempts have been made to mix some fluorine into refractories bound with a phosphate (see, inter alia, the article by Herbert D. Sheets, Jack J. Bulloff and Wiston H . Duckworth at the Batelle Memorial Institute entitled "Phosphate bonding of refractory compositions" published in Brick & Clay Record, July 1958, or the article in Berichte der Deutschen Keramischen Gesellschaft, Volume 37 (1960), Issue 8, pp. 362-267, by Von H. Betchel and G. Ploss entitled "Uber das Abbinden von Keramischer Rohstoffen mit monoaluminium-phosphat-Losung (Feuerfestbinder 32)". Thus, an attempt is made to improve concrete adhesion by adding fluorine. It is hoped that the formation of fluorophosphates will be beneficial in achieving this effect. In order to counteract the unfavorable impact on refractoriness, expensive refractory components have been used, such as aluminum oxide flakes and Zr02 etc. according to the first article,

while, according to the second article, it is warned that a high pyroscopic resistance cannot usually be achieved in practice if the presence of fluorine can be tolerated for the purpose of forming a glassy phase of fluorophosphate, in order to accelerate the bond. On the other hand, attempts have been made to produce glassy, soda-containing products, presented under the term refractory concretes, according to the book entitled "Hitzebestandiger Beton" by Nekrassow (Bauverlag GMBH Wiesbaden Berlin 1961) j especially in the third and fourth chapters, by utilizing soluble glass and sodium fluorosilicate. The high content of sodium in these glasses lowers their fire resistance. A chamotte with a fire resistance of up to 1580°G will only be able to withstand 900°C if a soluble glass and fluorosilicate are added to it, which are strong melting agents. To achieve a refractoriness of 1000°C, it is necessary to add a chromite (see table p. 239).

In none of these publications is the special case treated

with a furnace for molten aluminum.

In US patent no. 3261699, refractory stones intended for aluminum electrolysis furnaces are described.

The inventive idea of this patent document is to make

the furnace of the same ingredients as in the electrolysis bath, more specifically of cryolite and A^O^. With this solution, the bathroom is not contaminated even if the oven is attacked during the electrolysis. To avoid

this disadvantage, it is expressly stated in column 5 of the patent document, lines 30-32, that if additives are used, they should

these are used in relatively small amounts, i.e. less than 1%. This is correct provided that in the patent document synthetic aluminum oxide is to be used, i.e. aluminum oxide produced by the Bayer process, and the cryolite is also synthetic cryolite. It is thus a very expensive product that is synthetically produced.

In French patent document 1545678, a highly refractory, aluminum oxide-containing cement is described which contains approx. 60% clinker, approx. 40% A1203, approx. 2% AlNa^Fg and 0.18-2.5% sodium citrate. The cryolite, AlNa^g, helps form ceramic bonding

in use, as its "melting" effect promotes the formation of liquid phases in which reactions between aggregates and binders take place. The cement is not proposed to be used for

inhibit the action of molten aluminum.

According to the invention, it has been shown that it is possible to use a refractory material which is relatively inexpensive because it is made from naturally occurring materials, and which is sufficiently resistant to molten aluminum even if it contains silicon dioxide in a significant amount.

The present invention makes it possible to produce

an object, such as a furnace or auxiliary equipment, which is intended for contact with molten aluminum and which is not substantially attacked chemically or physically by the aluminum bath and which is capable of withstanding elevated temperatures and is relatively inexpensive because it is not based on the manufacture of special refractory products or synthetic refractory products, such as aluminum oxide flakes, ZtO^ or synthetic aluminum oxide.

The invention thus relates to the use of a refractory concrete with a slump temperature under load of over 1000°C and containing 40-60% by weight aluminum, expressed as Al20.j, 4-14% by weight calcium, expressed as CaO, in the form of calcium aluminate cement, 20 -60% by weight of silicon, expressed as SiO 2 , and 0.1-10% by weight, preferably 0.2-2% by weight, of fluorine which forms part of the concrete's structure, to inhibit the impact of molten aluminum on a concrete intended to come into contact with the molten aluminum.

The very low proportion of fluorine in the pulp has been shown to.

be sufficient to prevent chemical or physico-chemical attack of aluminum and insufficient to reduce the refractoriness unacceptably. The incorporation of fluorine, even if it is present in relatively small quantities, has a radical effect on the wettability of the molten aluminum and the refractory material. The refractory material does not get wet and is no longer attacked by the aluminium.

The production of a material with such properties is surprisingly considered in connection with what is stated in US patent document no. 3261699, not only because it is expressly stated therein that it is not necessary to use silicon dioxide in significant quantities like all the other additives, but especially because it could not be foreseen that the addition of the silicon dioxide, which is a relatively easily reducible material, can nevertheless be tolerated in a material for contact with molten aluminium, when fluorine is added to the material. In the aforementioned US patent, the addition of fluorine to the aluminum oxide is actually justified by the composition of the electrolyte, but not by the need to protect the rest of the material consisting of aluminum oxide produced by the Bayer process, which is a non-reducible material. The effect and purpose of the fluorine, on the other hand, are completely different in the material according to the invention. It prevents attack on SiG^ by the molten aluminium.

The shrinkage temperature (Tg j.) under load is a characteristic of the refractory material's refractoriness and mechanical strength. It is determined in accordance with ISO standard R 1893

(F), October 1970. The test essentially consists of a cylindrical test piece with a diameter of 50 mm and a certain height of

the material to be examined is placed in an oven between the pistons of a device used which makes it possible to apply a constant load of 2 kg/cm 2 to the test piece, and to record the temperature when the original height of the test piece is deformed by 0.5% at a heating rate in the oven of 10°C per minute up to 500°C, followed by a heating rate of

5°C per minute at over 500°C.

As it has been suggested that the fluorine forms part of the material's structure, it will be understood that the fluorine is not present in the form of an adsorbed gas in the pores of an agglomerate or in the form of a coating applied to the surface and in the pores of the agglomerate. The fluorine is present and distributed in the mass in solid form combined

with the agglomerate, or in the form of a solid fluorine compound associated with the agglomerate. Most often, the distribution of the fluorine in the mass is uniform. An analysis of removed test pieces at locations at different distances from the surface intended for contact with the molten aluminum shows the presence of the fluorine. This will practically only be de-sorbed above the working temperatures.

Apart from the fact that it is surprising that such a material satisfies the requirement for fire resistance and is not wetted by molten aluminum at such low fluorine contents and at such high silicon contents

hold, the production presented unexpected difficulties. It has actually been shown that the fluorine compounds and the aluminum oxide which are known as retarders for the binding of ordinary cements and concretes, exert a different effect together with the refractory agglomerates with which they are associated, forming a material according to the invention. The presence of the fluorine induces a false bond. It is therefore necessary to use special work methods.

The first of these is that it is necessary to stir the agglomerate, the fluorine compound, the hydraulic binder and the water for a sufficient time after these have been brought together, in order to avoid this false bonding. A stirring time of approx. 10 minutes is usually sufficient. It is also recommended to choose a fluoride content for the final material that is as low as possible within the effective range. It is likewise beneficial to add a bond-retarding agent.

The first step of this production method consists in mixing an agglomerate and the hydraulic binder.

Products that have a high content of aluminum oxide can be used as agglomerate, e.g. an aluminum oxide content of

over 45% by weight or even over 55% by weight.

An agglomerate comprising kyanite, sillimanite, bauxite, diaspore, corundum, andalusite, gibbsite or synthetic mullite can thus be used. Agglomerates containing special refractory products can also be used, such as products of magnesium oxide, chromite, chromium/magnesium oxide, forsterite, dolomite, coal-containing products based on graphite or coke, silicon carbide products, ZrO^ products, zirconium silicate or nitride products. However, according to the invention it is possible to avoid

use the above-mentioned expensive products while obtaining satisfactory results when using more common ones. agglomerates.

These agglomerates are particularly those which have an aluminum oxide content of 35-45% by weight, often referred to as clay-like products, and those which contain aluminum oxide in an amount of 10-35% by weight, referred to as silicon-clay-like products, the rest, with the exception of smaller amounts of contaminants are made up of silicon dioxide.

The granulometry of the agglomerate is normal. It can e.g.

have the following particle size distribution:

Calcium aluminate cement is used as a binder. In particular, a molten cement sold under the trademark Lafarge<®>

and containing 40% aluminum oxide, a fused cement sold under the trade name "Secar 162" and containing 40% aluminum oxide, a fused cement sold under the trade name "Secar 250" and containing 70% aluminum oxide or a fused cement sold under the trade name "Supersecar" and contains 80% aluminum oxide, is used.

The binder usually makes up 10-35% by weight of the agglomerate.

The agglomerate and the binder are mixed for at least one minute, usually for 1-10 minutes.

The second step of the described production method consists of

in to the mass of agglomerate and binder to add 0.1-10%,

expressed as fluorine, of one or more fluorine-containing products and mixing again for a few minutes.

Among the fluorine compounds that can be used are alkali metal fluorides, alkaline earth metal fluorides and the fluorosilicates.

It is advantageous to use a mixture of two fluorine compounds, one of which is more volatile or more soluble in water than the other. An example of such a mixture is the system consisting of 0.8-1.2% sodium fluorosilicate and 0.2-0.8% calcium fluoride, which gives satisfactory results.

The third work step consists in stirring the mixture of the agglomerate, the binder and the fluorine product in water in an amount of 2-20% of the mixture's weight.

It is preferred to add a binding retarder

to the mixed water. The binding retarder usually makes up 0.2-2% of the mixture of agglomerate, binder and fluorine product.

Among the retarders that can be used, they are preferred

so-called "clogging agents" that make the grain surface impermeable to agents that reduce solubility.

These water-soluble or surface-active "clogging agents" are in particular glycolic acid, glycolaldehyde, tartaric acid, glyceric acid, glycerol, pyruvic acid, glyceraldehyde, dihydroxyacetaone, maleic acid, succinic acid, malonic acid, tartaric acid,

erythrol, dihydroxytartaric acid, α- and 3-ketoglutaric acids, "arabitol, gluconic acid, galactonic acid, sorbitol, citric acid, salicylic acid, the diphenols (resorcinol or hydroquinone), benzoquinone, gallus-

acid, dioxane, organic materials fom flocculate in the presence of Ca<++>, for example digallic acid, casein, proteins such as albumin,

gum, pepsin, melamine resin, lignosulfonates, fatty acids incl

12-18 carbon atoms, oleic acid, naphthenic acids, benzoic acid, penta-_ chlorophenol, dialkylene glycols, mono- and polyethanolamines,

carbohydrates, glucose, sucrose, starch, cellulose and other sugars etc.

It is surprising that the presence of fluorine, which is a known retarder used for casting concrete, makes it necessary according to the invention to add another retarder to avoid a false bond, and especially since aluminum oxide as such is also known to have retarding properties against bonding in mortars and normal building concrete.

After adding the stirring water, the kneading is continued

for at least 8 minutes, preferably at least 10 minutes.

A material is thereby obtained which is sufficient to

pour into the mold and let harden, then slowly heat the material, e.g. to 100-120°C, after which the temperature is increased to approx. 700°C to form the refractory material for the use according to the invention.

The refractory material can be used in the form of a pourable material or in the form of hard-stamped pieces or stones for the construction of furnaces and other equipment for molten aluminium.

Examples 1-10

Place on the refractory material to be examined

two discs with a height of 5 mm and a diameter of 15 mm of each of three alloys chosen for their graded aggressiveness from weak to very strong. The refractory pieces and the discs on them are then placed in an electric muffle furnace for 48 hours at 800°C. This temperature is in reality an average of the temperatures used in smelting furnaces or holding furnaces for aluminium, and the times chosen are sufficient to provide sufficient sensitivity during the experiment.

The reaction number is determined using an index which is the sum of three reaction indices for each of the alloys and which is defined as follows: 0: No reaction and no visible mark on the refractory

material.

1: Faint mark on the refractory material, but the disc's volume is retained and the disc can be easily removed by hand.

2: The disc" is anchored to the refractory material and its volume has decreased by at least 50% or the final ring has a largest dimension that does not exceed 22 mm, or "mushroom" begins to form on the refractory material.

3: The disc is anchored to the refractory material, and its volume has decreased by more than 50% or the formed ring has a largest dimension of more than 22 mm in diameter, or there is strong 'mushroom formation' on the refractory material.

The experiments (2 slices of three alloys) are always carried out in duplicate.

Taking into account some variation in the results (it is usually allowed that the reaction should not take place until after an initial period of arbitrary duration) the maximum index obtained is calculated for each alloy.

The alloys have the following composition:

Alloy with weak aggressiveness A: Zn 0%

Aggressive alloy B: Zn 0.22%

Very aggressive alloy G: Zn 2%

In the three alloys, the other elements besides aluminum are: iron 0.33%, silicon 8.7%, copper 3.1% and magnesium 0.22%.

In the table I below, the type of fluorine product used is indicated in the second column, the content of fluorine product in the third column and the obtained index in the fourth column. The experiment marked with an asterisk is a comparison experiment that was carried out with a refractory material that did not contain fluorine. The agglomerate is a mixture of chamotte and the binder, which makes up 30% by weight of the agglomerate, is a calcium aluminate with 40% Al203.

Example 11

In a concrete mixer, 30 kg chamotte with a grain size of 2-4 mm, 22.5 kg chamotte with a grain size of less than 0.1 mm and 22.5 kg "Secar" are kneaded for 2 minutes.

0.5 kg of CaF2 and 1 kg of Na,,SiFg are added, and the kneading is continued for 2 minutes.

A solution of a retarder is prepared by adding 10 1 water per kg retarder.

This solution is poured into the concrete mixer and kneaded for 9 minutes.

The mass is removed from the concrete mixer and poured between a form and a counter form which are kept at a distance of 40 mm from each other.

Curing was allowed to take place for 24 hours, avoiding any drying out. To achieve this, moist bags are used.

The counter mold is removed and the cast piece is allowed to dry in air for 48 hours.

The hemispherical piece of the material according to the invention is carefully heated for 24 hours to 115°C, avoiding any contact between the piece and a flame.

The temperature is gradually increased to 700°C at a rate of 30°C per hour. The temperature of 700°C is maintained for 6 hours.

A test piece of this type then withstands repeated attacks of molten aluminum for over 4 months.

Example 12

Example 11 is repeated, but with the change indicated in table II.

Example 13

Example 11 is repeated, but with the change indicated in Table II and by applying the material by ramming to the mold in a stamped state.

Claims (3)

1. Use of a refractory concrete with a slump temperature under load of over 1000°C and containing 40-60% by weight aluminium, expressed as A12C>3, 4-14% by weight calcium, expressed as CaO, in the form of calcium aluminate cement, 20- 60% by weight of silicon, expressed as SiO2, and 0.1-10% by weight, preferably 0.2-2% by weight, of fluorine which forms part of the structure of the concrete, to inhibit the effect of molten aluminum on a concrete intended to come into contact with the molten aluminum. 2. Use according to claim 1 where the fluorine is present as CaF2 or as a mixture of CaF2 and a fluorosilicate. 3. Use according to claim 2, where CaF2 is present in an amount of 0.8-1.2% by weight and the fluorosilicate in an amount of 0.2-0.8% by weight.