EP0102892A1 - Process for manufacturing high purity metals and alloys - Google Patents

Abstract

A process for the production of high purity metals or metallic alloys comprises the steps of: (a) producing a metal or metallic alloy, the non-metallic inclusions of which are, preferably, easily reducible oxides of the base metal; (b) milling the metal or metallic alloy thus obtained and agglomerating the milled metal or metallic alloy with an agglomerating agent and a reducing agent, so as to form balls; and (c) subjecting the balls to a reducing treatment under regulated conditions of reduced pressure and elevated temperature, at which the reducing agent acts on the non-metallic inclusions while substantial sublimation of the alloying metal or metals is avoided. The invention is particularly applicable to the production of high purity chromium.

Description

The present invention relates to a method for manufacturing metals or metal alloys of high purity, in particular metallic chromium.

Modern industries increasingly demand high purity metals and metal alloys for the manufacture of mechanical parts.

This is the case in particular for the noble parts of aeronautical turbo-engines which require super-refractory alloys, called "super-alloys" of very high quality, because they are the most heavily stressed parts, both from the point of view thermal than mechanical. Such parts include stationary and movable fins of turbines, turbine disks, combustion chambers, nozzles, etc.

In order to obtain satisfactory characteristics, the superalloys concerned require extremely careful and sophisticated processing from very high quality raw materials, both in chemical purity and in regularity. This is in particular the case for pure metallic chromium which is used as an alloying element providing the superalloys with resistance to hot oxidation.

There are currently two techniques for producing pure metallic chromium, namely the electrolytic technique and the alumino-thermal technique.

The electrolytic technique makes it possible to obtain a metallic chromium of very good chemical purity which however contains too many gases which are very harmful for superalloys, in particular oxygen, hydrogen and nitrogen.

To improve the quality of the metallic chromium obtained by electrolysis, a reduction degassing is then carried out under vacuum, so that the content of chromium oxygen falls from 2000 to 5000 ppm for the raw electrolysis product to 300 - 500 ppm for the metallic chromium obtained after treatment. This treatment also makes it possible to appreciably lower the hydrogen and nitrogen contents as well as the contents in certain volatile metals such as lead or in certain. metalloids like sulfur.

It is generally this electrolytic chromium degassed under vacuum which, because of its high purity and its low oxygen content, is preferred for the production of noble parts of aeronautical turbomachines.

The aluminothermic technique consists of reducing chemically pure chromium oxide (99.5 to 99.7% Cr203) at very high temperature, ie above 2000 ° C, using powdered aluminum. Even if by a suitable choice of the raw materials used and by an evolved and very careful reaction technology we manage to obtain very satisfactory chemical purities on the whole and in some cases better than by electrolysis, the igneous elaboration used inevitably leads to the presence in the pure metallic chromium obtained, returned to ambient temperature, of non-metallic oxidized inclusions of alumina and chromium oxide. The oxygen content which is the consequence thereof and which varies inversely with the content of residual reducing agent in metallic chromium, in this case the aluminum content, is then prohibitive for the noblest aeronautical uses which, moreover, cannot tolerate very low residual aluminum contents.

The process of the invention makes it possible to manufacture different metals, in particular chromium, and different alloys, with high purity.

The process of the invention is based essentially on a primary production of a metal or of a metal alloy preferably containing non-metallic inclusions oxidized from the base metal, easily reducible, which is then ground and agglomerated before being subjected to a vacuum reducing treatment.

The primary production of the metal or of the metal bond is preferably obtained by an unbalanced aluminothermic reaction allowing the content of difficultly reducible aluminous inclusions to be reduced to a minimum, but this preparation can also be obtained by other techniques, for example by silicothermal , by reduction in an electric oven, etc., provided that the characteristics of the non-metallic inclusions make it possible to reproduce the subsequent stages of grinding and reducing treatment under vacuum.

• a) élaborer un métal ou un alliage métallique dont les inclusions non métalliques sont préférentiellement des oxydes du métal de base facilement réductibles,
• b) broyer le métal ou l'alliage métallique ainsi obtenu et agglomérer le métal ou l'alliage métallique broyé avec un agglomérant et un agent réducteur pour former des boulets, et
• c) soumettre les boulets à un traitement réducteur sous vide dans des conditbns réglées de pression et de température pour que l'agent réducteur réagisse sur les inclusions non métalliques réductibles et qu'il n'y ait pas de sublimation substantielle du métal ou des métaux d'alliage, le cas échéant.

According to the essential characteristic of the method of the invention, it comprises the steps which consist in:

• a) developing a metal or a metal alloy whose non-metallic inclusions are preferably easily reducible base metal oxides,
• b) grinding the metal or the metal alloy thus obtained and agglomerating the ground metal or metal alloy with a binder and a reducing agent to form balls, and
• c) subjecting the balls to a reducing treatment under vacuum under controlled conditions of pressure and temperature so that the reducing agent reacts on the reducible non-metallic inclusions and that there is no substantial sublimation of the metal or of the metals alloy, if applicable.

As indicated above, the metals or metal alloys which can be obtained with high purity by the process of the invention are those likely to include reducible non-metallic inclusions which can be practically eliminated at the end of the grinding and reduction steps under vacuum, such as the own oxides of the base metal.

Among the metals which can be produced with the process of the invention, mention may in particular be made of chromium, titanium, vanadium, molybdenum, manganese, niobium and tungsten. Likewise, the alloys envisaged in the context of the invention are alloys comprising at least one of the preceding metals, and / or boron, these alloys also comprising ferro-alloys in general.

In accordance with the preferred embodiment of the invention, step a) comprises an aluminothermic reaction between at least one metal oxide and divided aluminum, the reaction being unbalanced by an aluminum defect relative to the usual amount, for producing a metal or a metal alloy containing reducible non-metallic inclusions consisting mainly of inclusions of the base metal oxide, the occurrence of inclusions of alumina Ai ₂ 0 ₃ being minimized.

This defect in aluminum, which can represent from 0.5 to 8%, preferably from 2 to 5%, by weight of the usual amount, is essential in order to minimize the inclusions of alumina which are the most difficult to reduce.

This aluminothermic reaction unbalanced by a voluntary and significant defect in aluminum compared to the usual quantity goes completely against the usual aluminothermic processes where one always uses higher quantities of aluminum and closer to stoichiometry of the reaction, so to obtain the maximum yield, for which a product is obtained whose non-metallic inclusions consist for the most part of inclusions of alumina which are difficult to reduce.

As indicated previously, the preferred metal for carrying out the process of the invention is chromium.

The metallic chromium will advantageously be prepared by an unbalanced aluminothermic reaction, of the type indicated above, between chromium oxide, optionally an additive such as potassium dichromate, and divided aluminum. The use of additives of this kind is well known in the field of aluminothermy to provide additional oxygen and to heat the aluminothermic reaction.

The grinding step (b) is advantageously carried out using an impact mill, for example a hammer mill.

In the preferred embodiment of the invention, the grinding of the metal or of the metal alloy is a so-called "purifying" grinding. Which makes it possible to produce a certain flow of sweeping air to partially entrain the released non-metallic inclusions. during grinding. This purification associated with grinding is not compulsory, but of course preferred, because it allows a first physical separation of the non-metallic inclusions before the reducing treatment of step (c). However, it should be noted that non-metallic inclusions released during the purifying grinding seem to be preferably the inclusions of base metal oxide, for example the inclusions of Cr ₂ 0 ₃ in the caa of the production of metallic chromium.

The purifying grinding is advantageously supplemented by elimination by sieving or any other selective separation of the finest particles from the ground product in which almost all of the non-metallic inclusions released by the grinding are found concentrated.

The ground product thus obtained, from which part of the non-metallic inclusions will have been eliminated, is then agglomerated with an agglomerant and a reducing agent to form balls. The binder will advantageously consist of an organic compound or a mixture of organic compounds capable of leaving, during the heating of step (c), a carbon skeleton which completes the reducing action of the reducing agent. Thus, the binder can be constituted by a mixture of bakelite and furfuraldehyde and the reducing agent will advantageously be constituted by carbon black.

During step (b), the balls are shaped in a conventional compacting press and then baked at a set temperature, for example between 200 and 230 ° C to avoid any oxidation of the metal or metals constituting the balls, while obtaining satisfactory cohesion.

In accordance with the preferred embodiment of the invention, the reducing treatment is carried out in a vacuum oven and is optionally supplemented by sweeping using a non-oxidizing or reducing gas which is not soluble in the metal or the alloy.

The metal product obtained after reduction treatment under vacuum is cooled in a neutral atmosphere and can then be used in the manufacture of metal parts.

Other characteristics and advantages of the invention will emerge from the detailed description which follows and which refers to the production of metallic chromium of high purity by an unbalanced aluminothermic reaction. This description is only a non-limiting example and cannot in any way limit the scope of the invention.

I type a

Chromium oxide (Cr ₂ O ₃ ), potassium dichromate (Cr ₂ O ₇ K ₂ ) and divided aluminum are introduced into an aluminothermic crucible, lined with a refractory material. and potassium dichromate are advantageously commercial products having a particle size between 0 and 15 μ, while the divided aluminum consists of grains less than 1 mm.

Chromium oxide and potassium dichromate are present in the proportions of the classic aluminothermic reaction, while aluminum is present with a defect compared to the proportion of the classic aluminothermic reaction. As indicated above, this defect in aluminum can represent from 0.5 to 8%, preferably from 2 to 5% by weight of the usual amount.

The three constituents are mixed thoroughly and the reaction is initiated in the crucible as appropriate. The reaction temperature quickly reaches a value of about 2200 ° C., and at the end of the reaction, the metal is collected at the bottom of the crucible, and a supernatant slag.

The analysis of the metallic chromium obtained shows that by deliberately choosing to remain below the usual proportions, with the consequence of not achieving the optimum extracting yield, the residual aluminum content in the metallic chromium drops to very low levels below 0.01% (100 ppm). This analysis also shows that the content of non-metallic inclusions rises very quickly to reach high levels of 0.40 to 0.80%, or even more, but that these non-metallic inclusions are almost entirely composed of chromium oxide. not reduced (Cr ₂ 0 ₃ ) with very little Al ₂ O ₃ .

This way of proceeding contrasts sharply with the conventional way where one always seeks, for economic reasons, the maximum yield by using higher amounts of aluminum, while remaining compatible with the maximum admissible residual aluminum content. Thus, if a conventional aluminothermic reaction is carried out between chromium oxide and aluminum in the usual amount, one obtains for a residual aluminum content in metallic chromium of 0.1% maximum (maximum content usually accepted for aeronautical uses) of contents of inclusions from 1500 to 2500 ppm in almost all or at least for the most part, in the form of alumina Al ₂ O ₃ .

Without wishing to limit the invention to a particular theory, we can however attempt to explain the mechanism of the phenomenon observed. Contrary to a previously accepted opinion, inclusions

. non-metallic alumina Al ₂ O ₃ present in normal solidified pure metallic chromium, do not consist of reaction slag (chromium corundum, i.e. aluminous slag from reduction by aluminum) poorly decanted and having been trapped during the passage of the metal from the liquid state to the solid state. On the contrary, it is a secondary alumina formed either at the time of solidification, or even in the metal in the solid state slightly below the temperature of the solidus, therefore at a still very high temperature where the diffusion phenomena and the reactivity of the components is still very high. This secondary alumina would in fact result from the reaction, the equilibrium conditions having shifted with the drop in temperature where the state is by nature out of equilibrium, between chromium oxide or oxygen dissolved in chromium metal and excess residual aluminum also present in chromium metal.

Thus, in conventional aluminothermic reactions, where the amounts of aluminum are greater and approach the stoichiometric amount of the reaction, the residual aluminum is in an amount large enough to be able to reduce all of the chromium oxide or dissolved oxygen and there is even an unused excess in chromium. The non-metallic inclusions which remain trapped in the solid metal are then almost entirely formed of alumina A1 ₂ 0 ₃ .

If, on the contrary, the operation is carried out, in accordance with the process of the invention, with an aluminum defect with respect to the previous usual quantity, the residual aluminum present is in insufficient quantity to be able to reduce all of the chromium oxide or of the dissolved oxygen. All or almost all aluminum present is oxidized by the oxygen present (phase equilibrium in slow cooling) and the excess of chromium oxide or unreduced dissolved oxygen precipitates in the form of non-metallic inclusions Cr ₂ 0 ₃ . There are then all the less aluminous inclusions A1 ₂ 0 ₃ and all the more Cr ₂ O ₃ as the reduction has been carried out with an increasingly marked aluminum defect (unbalanced reaction by default).

The unbalanced aluminothermic reaction of step a) obviously gives a slightly less good yield than in conventional aluminothermic processes.

With the process of the invention, the elemental chromium is nevertheless reduced and the final product obtained is a metallic chromium of high purity, identical to normal aluminothermic metallic chromium of good quality, except that it contains a very high oxygen content ( 2000 to 3000 ppm or more), but in the almost exclusive form of non-metallic Cr ₂ O ₃ inclusions (0.40 to 0.80% or more) with the presence of very few aluminous inclusions A1 ₂ 0 ₃ (100 to 400 ppm corresponding to 50 to 200 ppm of oxygen bound to aluminum).

Metallic chromium is therefore obtained with non-metallic inclusions mainly constituted by Cr ₂ O ₃ inclusions which are easy to remove and secondarily by alumina inclusions, which are more difficult to remove, but in small quantities.

Step b

The metallic chromium from step a) is ground in an impact mill advantageously constituted by a high energy mill of the hammer type (mobile hammers / counter hammers) until a fine powder is obtained which passes entirely through a sieve with 200 µ mesh opening. The high impact energy of the grinder causes the grains to burst, which releases, at least in large part, the non-metallic inclusions Al ₂ 0 ₃ and Cr ₂ 0 ₃ contained in the metal, the inclusions Cr ₂ 0 ₃ appearing preferentially released. .

In the present example, the grinding is a purifying grinding which produces ventilation, that is to say a certain flow of purge air. This ventilation can be produced directly by the crusher itself or indirectly by an additional device, such as a blower. This sweeping air makes it possible to ventilate the product during grinding, which on the one hand prevents the product from overheating, and therefore possibly its oxidation and nitriding by ambient air, and on the other hand causes the most fractions. fine and lighter in the sweeping air stream, that is to say preferably the non-metallic inclusions released, the density of which is lower.

The air flow can be adjusted voluntarily to accentuate, if desired, the purifying effect. Likewise, this purifying effect can be supplemented by elimination by sieving or any other selective separation of the finest particles from the ground product in which almost all of the non-metallic inclusions released by grinding are found concentrated.

The purified chromium powder thus obtained is then intimately mixed with a reducing agent and a binder. The latter is advantageously constituted by a mixture of bakelite and furfuraldehyde. Furfuraldehyde aims to facilitate agglomeration when cold, the bakelite dissolved in the furfuraldehyde forming cold glue, as well as the subsequent polymerization of the hot bakelite. Of course, other thermosetting agglomerants and other solvents can be used.

The reducer, for its part, is advantageously constituted by carbon black which complements the carbon of the bakelite. The respective amounts of these products are variable but are generally adjusted, with a slight excess, to the residual oxygen content of the ground product. For example, the reducing / binder mixture may consist of 0.1% bakelite, 0.3% of furfuraldehyde and 0.05 to 0.2% of carbon black, these percentages being based on the weight of the crushed product.

The mixture obtained is shaped into balls or pellets by means of a conventional compacting press, such as a ball press with tangential wheels or a tableting press. After agglomeration, the mixture is baked at the appropriate temperature (200 to 230 ° C approximately) to remove the volatile furfuraldehyde and polymerize the bakelite which forms a binder and gives resistance to the balls or pellets.

It should however be noted that the oven temperature must be limited to the minimum necessary in order to avoid any oxidation of the product.

Step c

The balls or pellets obtained in the previous step are then subjected to a reducing treatment under vacuum at 1100 ° -1400 ° C under high vacuum of the order of 10 ^-4 mm of mercury.

At the start of the vacuum heating cycle, the bakelite decomposes at around 600 ° C, leaving a carbon skeleton which is added to the carbon black introduced as a reducing agent in the mixture. Once arrived at the processing temperature, this carbon reacts on the oxygen of the _Cr203 remaining in the product but practically not on the oxygen of the alumina Al ₂ O ₃ because to reduce the alumina it would be necessary to operate at a higher temperature. high and reach deeper voids.

In this regard, it should be noted that, already at temperatures of 1200 to 1300 ° C under voids of 10-4 mm of mercury, the chromium sublimes and a non-negligible part vaporizes. This explains why it is not economically possible to push further the reduction of residual alumina and justifies the "unbalanced" development of step a) in order to limit the residual alumina to its strictest minimum.

The vacuum is reduced in the treatment oven to 10 mm of mercury by sweeping controlled by a non-oxidizing or reducing gas, such as hydrogen, which has the particularity of being practically not soluble in solid chromium.

Due to the relatively low voids and relatively low temperatures imposed by chromium sublimation, processing can take several hours to achieve an almost complete reaction.

Once the reaction is complete and after cooling in a neutral atmosphere, a product is obtained containing at most 300 to 400 ppm of total oxygen in the form of 200 to 300 ppm of alumina approximately containing 100 to 150 ppm of oxygen and about 500 ppm maximum of unreduced chromium oxide containing about 150 ppm of oxygen. It is therefore a chromium of high purity which makes it possible to develop superalloys which can be used in particular in the manufacture of the noble parts of aeronautical turbo-engines.

It should be noted that the use of a conventional starting product without imbalance in step a) would necessarily lead, insofar as it is desired to lower the oxygen content to the required level of about 300 ppm, treatment allowing the reduction of Al ₂ O ₃ by carbon which, in addition to the preceding drawbacks, would bring a rise in the residual aluminum content of the finished product to levels not acceptable by the users preparing the superalloys.

Of course, the invention is not limited to the preferred embodiment described above and one can envisage other alternative embodiments without departing from the scope of the invention. Thus, step a) can be carried out other than by aluminothermy, for example by silicothermal or else by reduction in an electric furnace, to obtain a metal or alloy comprising oxidized non-metallic inclusions of the base metal.

By way of non-limiting examples, the manufacture of ferro-chromium or chromium metal by reduction with silicon metal or silico-chromium, as well as the manufacture of ferro-tungsten or ferro-molybdenum, by way of non-limiting examples. by reduction with one; high-grade ferro-silicon or metal silicon.

For reduction in an electric oven, mention may be made, by way of nonlimiting example, of the manufacture of ferro-vanadium in an electric oven, followed by an alumino-thermy.

Claims (15)

1. A method of manufacturing a metal or a metal alloy of high purity, characterized in that it comprises the steps consisting in: (a) developing a metal or a metal alloy, the non-metallic inclusions of which are preferably easily reducible base metal oxides, (b) grinding the metal or the metal alloy thus obtained and agglomerating the ground metal or metal alloy with a binder and a reducing agent to form balls, and (c) subjecting the balls to a reducing treatment under vacuum under controlled conditions of pressure and temperature so that the reducing agent reacts on the non-metallic inclusions and that there is no substantial sublimation of the metal or alloy metals, if applicable. 2. Method according to claim 1, characterized in that the metal is chosen from chromium, titanium, vanadium, molybdenum, manganese, niobium and tungsten and that the alloy comprises at least one of previous metals and / or boron, the alloy also comprising ferro-alloys in general. 3. Method according to one of claims 1 and 2, characterized in that step (a) comprises an aluminothermic reaction between at least one metal oxide and divided aluminum, the reaction being unbalanced by an aluminum defect with respect to the usual amount, to produce a metal or a metal alloy containing reducible non-metallic inclusions consisting mainly of inclusions of the basic metal oxide, the appearance of inclusions of alumina Al ₂ 0 ₃ being reduced to minimum. 4. Method according to claim 3, characterized in that the defect in aluminum represents from 0.5 to 8%, preferably from 2 to 5%, by weight of the usual amount. 5. Method according to one of claims 3 and 4, for the preparation of metallic chromium of high purity, characterized in that the aluminothermic reaction is carried out between chromium oxide and optionally an additive, such as potassium dichromate, and divided aluminum. 6. Method according to one of claims 1 to 5, characterized in that the grinding of metal or of the metal alloy is carried out in an impact mill, for example in a hammer mill. 7. Method according to one of claims 1 to 6, characterized in that the grinding of the metal or of the metal alloy is a purifying grinding producing a certain flow of sweeping air to partially entrain non-metallic inclusions released during grinding. 8. Method according to claim 7, characterized in that the purifying grinding is followed by a selective separation of the released non-metallic inclusions. 9. Method according to one of claims ₁ to 8, characterized in that the binder is a bakelite-furfuraldehyde mixture. 10. Method according to one of claims 1 to 9, characterized in that the reducing agent is carbon black. 11. Method according to one of claims 1 to ₁ 0, characterized in that the balls are shaped in a compacting press and then steamed at a set temperature to avoid any oxidation of the metal or metals constituting the balls, while obtaining satisfactory cohesion. 12. Method according to claim 11, characterized in that the oven temperature is between 200 and 230 ° C. 13. Method according to one of claims 1 to 12, characterized in that the reducing treatment is carried out in a vacuum oven and that a sweep is used by means of a non-oxidizing or non-reducing reducing gas. soluble in metal or alloy. 14. The method of claim 13, for the manufacture of metallic chromium of high purity, characterized in that one subjects chrome balls containing chromium oxide and alumina to a reducing treatment at a temperature from 1100 to 1400 ° C and under a high vacuum and that, during treatment, the pressure is reduced to a lower vacuum level by sweeping with a non-oxidizing or reducing gas, such as hydrogen. 15. Method according to one of claims ₁ to 14, characterized in that the metal product obtained after reducing treatment under vacuum is cooled in a neutral atmosphere.