EP0275774B1 - Process for preparing chromium-aluminium agglomerates for adding chromium to a melt of aluminium - Google Patents

Abstract

The present invention relates to a process for the production of chromium-aluminum balls for adding chromium into molten aluminum baths. In order to obtain balls containing x% of chromium and y% of aluminum, where x and y are gravimetric contents corresponding to the following relationships: 70</=x</=80 20</=y</=30 x+y=100 an alloy of chromium and aluminum containing gravimetric chromium and aluminum contents approximating to x by an excess and to y by a deficit respectively is prepared by melting and this alloy is then finely ground into a crude powder; the chromium and aluminum contents of the alloy or of the crude powder are determined and, if required, an additional amount of finely divided aluminum is added so as to obtain a powder containing x% of chromium and y% of aluminum, the additional amount of finely divided aluminum corresponding to less than 10% by weight of the crude powder; a compacting is then carried out. Balls essentially consisting of alloy particles having the same melting point and little risk of floating on the surface of the molten aluminum bath, which favors the dissolution of the balls in the latter, are produced.

Description

The present invention relates to a process for producing chrome-aluminum balls for adding chromium to molten aluminum baths.

$\text{20 ≦ y ≦ 30}$

$\text{x+y = 100,}$

par réalisation d'une poudre contenant x% de chrome et y% d'aluminium puis compactage de cette poudre en boulets.More specifically, it relates to the production of such balls, containing x% chromium and y% aluminum where x and y are weight contents corresponding to the following relationships:

$\text{70 ≦ x ≦ 80}$

$\text{20 ≦ y ≦ 30}$

$\text{x + y = 100,}$

by making a powder containing x% chromium and y% aluminum and then compacting this powder into balls.

According to a known method, disclosed by U.S. Patent ^No. 3,592,637, is carried balls designed for the addition of chromium in the molten aluminum bath by compaction of a powder obtained by mixing an amount on the order of 10% to 90% of a finely divided material formed from chromium or a chromium alloy, and approximately 10 to 90% of finely divided aluminum, the presence of which in this form is presented, in this American patent, as essential to ensure the subsequent dissolution of the balls in the molten aluminum bath.

However, it appears that the balls obtained by this known process are not entirely satisfactory; indeed, each ball is formed of two heterogeneous components, namely on the one hand aluminum grains and on the other hand grains of chromium possibly alloyed, which exhibit different behaviors; in fact, the generally smaller particle size of the aluminum powder entering for a large part in the composition of the balls makes that part of this powder separates from the rest of the balls to float on the surface of the aluminum bath, to which this powder mixes poorly; in addition, the aluminum grains and the possibly alloyed chromium grains melt at different temperatures, so that their dissolution in the aluminum bath takes place under poor conditions.

U.S. Patent ^No. 4,564,393 discloses another method is essentially distinguished from the method described by U.S. Patent ^No. 3,592,637 in that a flux is added to the mixture of aluminum powder and chromium powder, optionally alloyed , before compacting into balls; the balls made by this process exhibit heterogeneity similar to that of the balls manufactured according to the teachings of U.S. Patent ^No. 3,592,637, with the same effect, and furthermore have the disadvantage of being more expensive because of the addition of the flow.

The object of the present invention is to remedy these drawbacks, by proposing a process making it possible to economically produce balls having a precise chromium and aluminum content, with a homogeneity conducive to dissolution under the best conditions when incorporated into a molten aluminum bath.

• a) réaliser par fusion un alliage de Cr et Al contenant des teneurs en poids de Cr et d'Al approchant respectivement x par excès et y par défaut,
• b) broyer finement ledit alliage en une poudre brute et analyser les teneurs respectives dudit alliage ou de ladite poudre brute en poids de Cr et d'Al,
• c) si la teneur de l'alliage ou de la poudre brute en poids d'Al est inférieure à y, apporter à la poudre brute un complément d'Al finement divisé pour obtenir ladite poudre contenant x% de Cr et y% d'Al, ledit complément d'Al finement divisé correspondant à moins de 10% du poids de poudre brute.

To this end, the present invention proposes to produce the powder containing x% chromium and y% aluminum, where x and y are defined as indicated above, by the succession of steps consisting in:

• a) by melting an alloy of Cr and Al containing contents by weight of Cr and Al approaching x by excess and y by default respectively,
• b) finely grinding said alloy into a crude powder and analyzing the respective contents of said alloy or of said crude powder by weight of Cr and Al,
• c) if the content of the alloy or of the crude powder by weight of Al is less than y, supply the crude powder with a finely divided complement of Al to obtain said powder containing x% of Cr and y% of Al, said finely divided Al complement corresponding to less than 10% of the weight of crude powder.

The balls resulting from the compacting of the powder thus obtained have all or at least most of their aluminum content in the form of chromium-aluminum alloy grains, having a homogeneous content of these two components and consequently melting at temperatures identical, which we note that they are lower than those of aluminum or chromium considered separately, so that the dissolution of the balls takes place under particularly favorable conditions; the possible addition of finely divided aluminum only intervenes as an adjustment of the aluminum content, in proportions which can be reduced to less than 5% of the weight of crude powder and for example less than 2% or of the order of approximately 2% of the weight of raw powder, thanks to a careful production of the chromium-aluminum alloy, which makes the disadvantages inherent in the presence of aluminum in the form of metallic powder practically negligible, namely the heterogeneity of melting temperature and the risk of flotation of this powder.

On the contrary, the possible addition of aluminum powder contributes to the compactness of the balls, that is to say, helps to avoid excessive friability of the latter.

For this purpose, it is also possible to add to the powder, before compacting, a binder which is advantageously a carbon-based binder such as bakelite, which binder is added in sufficiently small proportions so as not to have any influence on the behavior of the balls when 'they are immersed in a bath of molten aluminum; naturally, the compaction process is chosen by the skilled person according to the absence of binder or the presence of binder, as well as the nature of the binder.

Other characteristics and advantages of the invention will emerge from the description below, relating to a non-limiting mode of implementation.

In this example, it is desired to produce balls containing 75% of chromium and 25% of aluminum and, for this purpose, one begins by producing by fusion, for example by aluminothermy, an alloy of chromium and aluminum having contents in weight of chromium and aluminum approaching respectively 75% by excess and 25% by default.

The alloy thus produced is then finely ground into a crude powder having a particle size advantageously between approximately 0.250 mm and approximately 0.053 mm, and this crude powder or else the alloy itself is analyzed in order to determine the content of the crude powder and of the alloy by weight of chromium and by weight of aluminum.

If these contents correspond respectively to the 75% and 25% sought, no addition of aluminum powder is necessary.

If, on the other hand, the content of the raw powder or of the alloy by weight of aluminum is less than the 25% sought, an aluminum contribution is made in finely divided form to adjust to the required 25% the aluminum content, and to the required 75% the chromium content; experience shows that, by a good mastery of the aluminothermic technique used to make the alloy of chromium and aluminum, the contribution of aluminum in the form of finely divided aluminum powder can be reduced to 2% about or less than 2%, by weight, of the weight of chromium-aluminum alloy powder; the finely divided aluminum thus optionally added to the raw powder advantageously has a particle size less than 420 μm, with a proportion of aluminum powder with a particle size less than 53 μm not exceeding 15% of the total weight of aluminum thus optionally added .

In both cases, a powder is thus obtained containing as precisely as possible 25% chromium and 75% aluminum.

This powder advantageously has a density of the order of 2.5 ± 0.2.

Then, preferably, bakelite is added to this powder, at a rate of approximately 2 ° / 00 by weight of bakelite relative to the weight of the powder containing 25% chromium and 75% aluminum, then the compacting under a pressure and a temperature easily determinable by a person skilled in the art, capable of imparting to the balls obtained by this compacting a density advantageously of the order of 5.6 ± 0.2 and of causing the setting of the bakelite; the balls are then ready for use.

The addition of a binder is preferred insofar as it makes it possible to avoid dusting of the cannonballs; experience has shown that the presence of a small quantity of aluminum in powder form, in the non-alloyed state, makes it possible to considerably reduce the quantity of binder required, and to make the presence of this binder without consequence on the aluminum bath; this quantity of aluminum in powder form, in the non-alloyed state, is for example of the order of 2%, by weight, of the weight of powder of chromium-aluminum alloy.

Alternatively, one can also dispense with any binder by practicing appropriate compaction.

Naturally, the implementation of the method according to the invention is not limited to the production of balls containing 75% chromium and 25% aluminum, and this method can be applied with the same advantages for chromium contents. ranging from 70 to 80%, with the% aluminum complement.

In each case, care is taken to avoid the drawbacks associated with the presence of too large a quantity of unalloyed aluminum in powder form by adjusting the chromium and aluminum contents of the alloy as precisely as possible to reduce as low as possible below 10% the complement of unalloyed aluminum added in powder form to this alloy; preferably, the grain sizes and densities indicated above, which are currently considered preferable, are also respected in each case, although other values can also be chosen without departing from the scope of the present invention.

Claims (12)

1. A process for the production of balls containing x% of Cr and y% of Al where x and y are gravimetric contents corresponding to the following relationships:
   
   $\text{70 ≦ x ≦ 80}$

   

   
   $\text{20 ≦ y ≦ 30}$

   

   
   $\text{x+y = 100}$

   

   
   
   for adding Cr into molten aluminium baths, by producing a powder containing x% of Cr and y% of Al and then compacting this powder into balls, characterized in that said powder is produced by a succession of stages consisting in:

   a) producing, by melting, an alloy of Cr and Al containing gravimetric Cr and Al contents approximating x by an excess and y by a deficit respectively,

   b) finely grinding said alloy into a crude powder and analyzing the gravimetric Cr and Al contents respectively in said alloy or in said crude powder, and

   c) if the gravimetric Al content of the alloy or of the crude powder is less than y, adding to the crude powder an additional amount of finely divided Al in order to obtain said powder containing x% of Cr and y% of Al, said additional amount of finely divided Al corresponding to less than 10% by weight of the crude powder.
2. The process as claimed in claim 1, characterized in that said alloy of Cr and Al is produced in stage a) by the thermite process.
3. The process as claimed in any of claims 1 and 2, characterized in that said alloy of Cr and Al is produced in stage a), with gravimetric Cr and Al contents sufficiently approximating to x and y respectively so that said additional amount of finely divided Al corresponds to less than 5% by weight of the crude powder.
4. The process as claimed in claim 3, characterized in that said alloy of Cr and Al is produced in stage a), with gravimetric Cr and Al contents sufficiently approximating to x and y respectively so that said additional amount of finely divided Al corresponds to approximately 2% by weight of the crude powder.
5. The process as claimed in any of claims 1 to 4, characterized in that the crude powder has a particle size of between 0.250 mm and 0.053 mm.
6. The process as claimed in any of claims 1 to 5, characterized in that the additional amount of finely divided aluminium has a particle size less that 420 µm.
7. The process as claimed in claim 6, wherein not more than 15% by weight of the additional amount of finely divided aluminium has a particle size less than 53 µm.
8. The process as claimed in any of claims 1 to 7, characterized in that the powder containing x% of Cr and y% of Al has a density of the order of 2.5 + 0.2 and a ball has a density of the order of 5.6 + 0.2.
9. The process as claimed in any of claims 1 to 8, characterized in that a binder is added to the powder before compacting.
10. The process as claimed in claim 9, characterized in that the binder is a carbon-containing binder.
11. The process as claimed in claim 10, characterized in that the binder is bakelite.
12. The process as claimed in any of claims 9 to 11, characterized in that the binder is added at a rate of approximately 0.2% by weight of the powder containing x% of Cr and y% of Al, before compacting.