EP0343378A1 - Process for producing metallic titanium, zirconium, chromium, samarium or neodymium from their oxides - Google Patents

Abstract

Process for producing metallic titanium (Ti), zirconium (Zr), chromium (Cr), samarium (Sm) and neodymium (Nd) by reducing their oxides. In a first variant, a melt of the particular metal oxide and an alkaline earth metal halide is first prepared and the same alkaline earth metal which, in its compound as an alkaline earth metal halide is present in the melt, is then added thereto. In a second variant, a melt of the alkaline earth metal and the corresponding alkaline earth metal halide is first prepared and the metal oxide which is to be reduced is then added to the melt. In both cases, the mixture is maintained in the molten state until the reduced metal is formed and the latter is finally separated from the other reduction products.

Description

The invention relates to a method according to the preamble of claim 1 or 2.

According to the current state of the art, five process steps are necessary for the industrial production of titanium from TiO₂:
- Production of titanium chloride from titanium oxide after the reaction
TiO₂ + 2Cl₂ + 2C = TiCl₄ + 2 CO.
- Reduction of titanium tetrachloride with magnesium or sodium to titanium sponge after the reaction
TiCl4 + 2 Mg = 2 MgCl₂ + Ti
TiCl4Y + 4 Na = 4 NaCl + Ti
- Vacuum distillation of the titanium sponge thus produced to remove Mg and MgCl₂ or Na and NaCl
- Compacting the titanium sponge into electrodes
- Two times remelting of titanium sponge in a vacuum arc furnace or in an electron beam furnace to a titanium block.

This manufacturing process is technically and economically very complex. There has been no lack of efforts to produce titanium by direct reduction of TiO₂. Because of the high heat of formation of the titanium oxide up to 225.8 kcal / mol and the high solubility of the oxygen in the titanium mixed crystal, there is still no industrially usable process.

A method for direct reduction is known for example from DE-AS 20 34 385. The metal oxides to be reduced, the slag components and the metal acting as a reducing agent are mixed as a powder, and the powder mixture is partially melted in a water-cooled induction crucible. Since the required inductive coupling can only take place on the metallic component of the mixture, the process only starts up slowly. In addition, the reducing agent (Ca) must be added in excess.

Because of the long process time and the excess of reducing metal, a disproportionate amount of this is lost through evaporation. The process also suffers from a high energy requirement. Nevertheless, it has not proven possible to exceed the experimental laboratory quantities from 100 g to 150 g and to extend the process to a production scale.

Due to the high affinity of alkaline earth metals such as calcium, magnesium, barium and others to oxygen, an extensive reduction of titanium oxide with these metals is fundamentally possible. Because of the high vapor pressure of these elements, the reduction must be carried out at a relatively low temperature of approx. 1000 ° C.

On a laboratory scale, titanium could be produced with 0.1% to 0.2% oxygen by reduction with calcium (Ulrich Zwicker: Book "Titan und Titanlegierungen", page 28, Springer Verlag, Berlin, 1974). The problem, however, is the difficulty in separating the reactive metal from the alkaline earth oxide formed.

In addition, the metallic titanium in this reaction is present in the solid phase in small particles, so that after the separation, a vacuum distillation to remove excess alkaline earth metal requires a compaction of the titanium particles with subsequent, possibly two, remelting. For this reason, the process could not assert itself in terms of production technology.

Titanium oxide (TiO₂) melts at 1825 ° C. Reduction of the titanium oxide melt at a temperature above 1825 ° C, e.g. with calcium, is not possible because calcium already evaporates at a temperature of 1484 ° C and is no longer available for the reduction.

The invention is therefore based on the object of specifying a method of the type described in the introduction in which alkaline earth metals can be used economically for direct reduction of the oxides in order to be able to produce the metals and metal alloys concerned on an industrial scale.

According to the invention, this object is achieved by the measures specified in the characterizing part of claim 1 or 2.

The slag melt formed from said metal oxides and the alkaline earth metal halide has, depending on the mixing ratio, a substantially lower melting point than the pure metal oxides.

Since alkaline earth metals in a corresponding halide melt are freely soluble over the entire temperature and concentration range, the vapor pressures of the dissolved alkaline earth metals become in accordance with Raoult's law
P _x = N _x x P _x * where P _x = vapor pressure of the solute x
N _x = molar pressure of the solute x
P _{x *} = vapor pressure of the pure substance x
reduced.

Surprisingly, it has been shown in experiments that the alkaline earth metal in a titanium / alkaline earth halide melt does not escape in vapor form even at temperatures above 1500 ° C. and is largely retained in titanium oxide for reduction.

In addition to calcium, alkaline earth metals such as magnesium or barium can also be used for the purpose of the invention. It is advisable to replace the fluorspar (CaF₂) contained in the slag, which is particularly suitable for calcium, in whole or in part by the corresponding fluoride, such as MgF₂ or BaF₂. If the slag does not contain magnesium fluoride or barium fluoride, magnesium or barium must be added together with calcium. Since all alkaline earth metals are soluble in the liquid calcium, the evaporation losses of these elements are significantly smaller.

In the method according to the invention, the reduction takes place, for example, for the production of titanium in a liquid slag phase by setting a slag temperature which is above the melting point of titanium, ie at 1675 ° C. The titanium remains liquid. Due to the difference in density, the liquid titanium is inevitably separated from the Slag phase. The liquid titanium can then be poured into blocks.

• a) Schmelzen in einem wassergekühlten Kupfertiegel nach dem Elektroschlackeumschmelzverfahren, vor­zugsweise mit einer nichtverzehrbaren Elektrode,
• b) Schmelzen in einem wassergekühlten Kupfertiegel mittels Lichtbogen mit einer nichtverzehrbaren Elektrode, und
• c) Schmelzen in einem wassergekühlten Kupfertiegel mit Hilfe von Plasmabrennern.

Due to the high reactivity of some metals such as titanium, it is advisable to melt and reduce the oxide-halide slag in a ceramic-free crucible, such as a water-cooled copper trough. The following methods can be used for this:

• a) melting in a water-cooled copper crucible using the electroslag remelting process, preferably with an inedible electrode,
• b) melting in a water-cooled copper crucible by means of an arc with a non-consumable electrode, and
• c) Melting in a water-cooled copper crucible with the help of plasma torches.

The slag temperature can be set as desired in all three of the furnaces mentioned above by appropriate energy supply. With a sensible design of the melting furnace, the reduction of the oxides in the slag and the removal of the metal melt can be carried out continuously.

The conditions in the characterizing part of claim 1 or 2, according to which the separation is to take place at a temperature above the melting point of the metal to be obtained, can be accomplished in two ways. For metals such as Ti, Zr and Cr, whose melting point is above the reaction temperature, heating is necessary. In the case of metals such as Nd and Sm, the melting point of which is below the reaction temperature, heating is unnecessary.

It is particularly advantageous if the process is carried out under an increased protective gas pressure (above 1 bar). This has the advantage that the decomposition of e.g. Fluorspar (CaF₂) is suppressed at high temperatures.

What has been described above for the direct reduction of titanium oxide also applies analogously to the direct reduction of the other oxides to the metals. In the following, the invention is explained using four examples.

Example 1:

In a water-cooled copper crucible, 1000 g of TiO₂ / CaF₂ mixture with a composition of 50% TiO₂ and 50% CaF₂ were melted by means of an arc using a non-consuming electrode. 500 g of calcium were added to the molten TiO₂ / CaF₂ slag. The temperature of the slag melt was 1600 ° C. The added calcium immediately dissolved without noticeable evaporation. The system was switched off after two minutes. After cooling, the solidified slag was crushed. Numerous metallic particles with a size between 0.5 mm and 5 mm were found. Subsequent investigations of these metallic particles with the microsensor showed that it is pure titanium.

Example 2:

In an ESR system, 50 kg of powder mixture with 50% TiO₂ and 50% CaF₂ were melted with a non-consuming graphite electrode under an argon protective gas and heated to 1700 ° C. Then 30 kg of calcium granules were added to the slag. The system was switched off after a holding period of 30 minutes. The liquid titanium (approx. 13 kg) had collected on the bottom of the copper mold and could be removed after cooling. The oxygen content of the titanium thus produced was approximately 1100 ppm.

Example 3:

In a plasma furnace with a water-cooled copper crucible, 100 kg of ZrO₂ / CaF₂ mixture with 50% ZrO₂ were melted at a pressure of 1000 mbar. The plasma gas was argon. At a slag temperature of 1900 ° C, the slag melt was reduced with approx. 60 kg calcium. The system was switched off after a holding period of 30 minutes. Here too, the zirconium had collected on the bottom of the copper trough and could be removed after cooling.

Example 4:

In a plasma furnace with a water-cooled copper crucible, 100 kg of Cr₂O₃ / CaF₂ mixture with 50% Cr₂O₃ are melted at a pressure of 1000 mbar under an Ar atmosphere. The plasma gas was argon. At a slag temperature of approx. 1900 ° C, the chromium oxide in the slag was reduced with 50 kg calcium. The system was switched off after a holding period of 30 minutes. The metallic chrome thus formed had collected on the bottom of the copper trough and could be removed after cooling.

Claims (10)

1. A process for producing metals and metal alloys whose oxygen affinity is lower than that in the alkaline earth metals by direct reduction of their oxides in a mixture of these oxides with at least one alkaline earth metal halide and the alkaline earth metal in question and subsequent separation of the metal formed or the metal alloy, characterized in that a) in a first step, produces a melt from the metal oxide in question and the alkaline earth metal halide, b) in a second step, the same alkaline earth metal is added to the melt, which is contained in its compound as alkaline earth metal halide in the melt, and which keeps the metal or the metal alloy in the molten state and finally c) in a third step, the metal or metal alloy thus formed is separated from the other reaction products at a temperature which is above the melting point of the metal or metal alloy to be obtained 2. Process for producing metals and metal alloys whose oxygen affinity is lower is as that of the alkaline earth metals, by direct reduction of their oxides in a mixture of these oxides with at least one alkaline earth metal halide and the relevant alkaline earth metal and subsequent separation of the metal or metal alloy formed, characterized in that a) in a first step, produces a melt from the alkaline earth metal in question and its halide, b) in a second step, the at least one metal oxide to be reduced is metered into the melt and keeps the melt liquid until the corresponding metal or metal alloy is formed, and finally c) in a third step, the metal or metal alloy thus formed is separated from the other reaction products at a temperature which is above the melting point of the metal or metal alloy to be obtained. 3. The method according to claim 1, characterized in that the metal oxide and Alkaline earth metal halide mixed in a weight ratio of 30:70 to 70:30. 4. The method according to claim 1, characterized in that one keeps the melt of the metal oxide and the alkaline earth metal halide during the addition of the alkaline earth metal at a temperature between 1500 ° C and 2000 ° C. 5. The method according to claim 4, characterized in that the melt is kept at a temperature between 1675 ° C and 1900 ° C. 6. The method of claim 1 or 2, characterized in that is used as the alkaline earth metal halide is a fluoride. 7. The method according to claim 6, characterized in that calcium fluoride is used as the alkaline earth metal halide. 8. The method of claim 1 or 2, characterized in that the pressure above the melt is at least 1 bar. 9. The method of claim 1 or 2, characterized in that it is carried out in an atmosphere of with the metals and metal alloys, non-reacting gases. 10. Application of the method according to claim 1 or 2 to the direct reduction of metals from the group Ti, Zr, Cr, Sm and Nd.