GB2107358A

GB2107358A - Process for reducing metal oxide to metal powder

Info

Publication number: GB2107358A
Application number: GB08228826A
Authority: GB
Inventors: Michael Graham Down
Original assignee: Westinghouse Electric Corp
Current assignee: Westinghouse Electric Corp
Priority date: 1981-10-08
Filing date: 1982-10-08
Publication date: 1983-04-27
Also published as: JPS5877506A; GB2107358B; FR2514369B1; FR2514369A1

Abstract

The oxide is reacted with molten metal, preferably at a temperature not exceeding 600 DEG C and with agitation. When reaction is complete the excess reducing agent may be removed by distillation. Reduction of TiO2 with Li is particularly exemplified, but Li-Mg or Li-Ca may also be used. The powder product has a particle size of 0.1-0.5 micron.

Description

SPECIFICATION Process for reducing metal oxide to metal powder This invention relates to metal oxide reduction to metal powder through the use of a liquid metal reducing agent. It especially relates to the reduction of the oxides of reactive metals, such as titanium, hafnium and zirconium.

In the past the difficult to reduce oxides of the metals titanium, zirconium, thorium, uranium, vanadium, etc. have been reduced by a liquid metal reducing agent as described in U.S. Patent Nos.

1,573,083; 1,704,257; 1,814,719; 2,446,062; 2,537,068; 2,653,869; and 2,707,679. While other reducing agents have been utilized liquid calcium has been typically preferred because of its oxide's very negative free energy of formation.

However the use of liquid calcium as a reducing agent has been limited to temperatures above 8500 C, the melting point of pure calcium. The actual reduction temperatures taught by previous investigators has ranged from approximately 9000C to 13500C. These high reduction temperatures in combination with localized high temperatures generated by the heat released by the reduction reaction itself have tended to cause agglomeration of the powders produced. These powder agglomerates are deleterious in that they can entrap calcium oxide, calcium and other impurities which are difficult to remove by the leaching techniques used to clean the metal powders.

Accordingly, the present invention resides in a process for reducing a metal oxide to a substantially unoxidized metal powder which comprises forming a reaction mixture of said metal oxide in a quantity of a liquid metal reducing agent in excess of that stoichiometrically required to fully reduce said metal oxide; and reacting said metal oxide with said liquid metal reducing agent until said metal oxide has been substantially reduced to said unoxidized metal powder except for minor impurity levels of oxygen.

During the reduction reaction, it has proved convenient to agitate or stir the reaction mixture of metal oxide and reducing agent to reduce the number and/or size of metal powder agglomerates formed.

Desirably, the metal oxide is reduced at a temperature below approximately 6000 C. The liquid metal reducing agent utilized is selected from the group of lithium, lithium-sodium, lithiummagnesium, and lithium-calcium. Preferably the reaction mixture is stirred during reduction.

After the reduction of the metal oxide by an excess quantity of the liquid metal reducing agent has taken place, the excess liquid metal then remaining can be distilled off. Preferably, the liquid metal reducing agent is lithium.

In an especially convenient embodiment of the invention, a mixture of titanium oxide and liquid lithium, in a quantity in excess of that stoichiometrically required to fully reduce the titanium oxide, is formed. The reduction reaction takes place at a temperature between approximately 350 and 6000C under an inert atmosphere until the titanium oxide has been substantially reduced to the unoxidized metal. The excess lithium is then vacuum distilled from the mixture leaving titanium powder and lithium oxide powder. During the reduction and distillation steps of the process the mixture is stirred to limit powder particle agglomeration. After distillation the lithium oxide and any remaining lithium metal are leached from the titanium powder and the resulting titanium powder washed and dried.

The resulting metal powder has a basic particle size of approximately 0.1 to approximately 0.5 micron which may be used in the manufacture of powder metal parts or as a gettering material.

Lithium is the preferred liquid metal reducing agent since it has a low melting temperature of approximately 1 800C which allows low reduction temperatures between 3500C and 6000C to be used in reducing the oxides of the transition metals. All transition metals except for zirconium, hafnium, thorium, lanthanum, uranium, scandium and yttrium can be reduced by this liquid metal reduction process using lithium. A lithium-magnesium solution containing up to approximately 50% magnesium may substitute for pure lithium. Solutions containing approximately 50% lithium and 50% magnesium have a maximum melting temperature of approximately 270 to 3500C. The aforementioned exceptions are due to the unfavorable thermodynamics of those reduction reactions.Lithium is also preferred in that it can easily be vacuum distilled at 6000 C. For the more stable oxides such as those already mentioned, that is, zirconium, hafnium, thorium, lanthanum, uranium, etc., a solution of lithium and calcium may be used. Liquid metal reductant solutions of lithium and calcium can contain up to approximately 75 wt.% calcium and have a melting temperature of approximately 2300C at the maximum calcium content. These low melting temperatures make liquid reduction of the transition metal oxides possible at a much lower temperature than previously used in prior art liquid calcium reduction operations. A reduction temperature between 6000C and the melting temperature of the liquid reductant is possible.While pure magnesium which has a melting temperature of 6500C or pure calcium which has a melting temperature of 8500C may be used in this invention they are less preferred than the use of pure lithium or lithium magnesium or lithium calcium solutions. The higher temperatures required in using a magnesium or calcium reduction process lead to a greater amount of aggiomeration of the metal powder particles produced. This can be alleviated to some extent however by stirring or agitation of the mixture during the reduction process so as to keep the titanium metal particles produced suspended in solution. But the use of these pure high temperature melting metals is clearly less preferred.The reduction of the metal oxides by the liquid metal reductant is thermodynamically achieved by using an amount of reductant in excess of that stoichiometrically required by the reduction chemical equation. While an amount of reductant as low as 200% of that stoichiometrically required can be used in accordance with this invention it is preferable that the liquid reductant be present and at least 1000% of that stoichiometrically required so that not only will the reaction go to completion but the fluidity of bath is enhanced and allows easy stirring of the mixture and contact between the suspended metal particles will be limited. It is also important that the chemistry of both the oxide being reduced and the liquid metal reductant be tightly controlled and should be as pure as possible.This is especially critical with respect to the nitrogen contents in that nitrogen will not be removed by the liquid metal reductant and in fact in many cases will be picked up by the metal which one is trying to produce. In addition the use of metal chlorides should be avoided since these can contaminate the metal powder and be deleterious to its use as a metal powder for powder metallurgical applications. As already mentioned the reaction mixture containing the metal oxide and liquid metal reducing agent should be agitated during the reduction reaction to alleviate the formation of powder agglomerates.

This agitation or stirring can take the form of mechanicai means, that is, a stirring implement actually being position inside the bath to stir the bath or a vibratory mechanism to vibrate the chamber containing the bath. It can also take the form of a magnetic pump pumping the reaction mixture through the reaction mixture container or a magnetic stirrer stirring the bath without actual contact with the bath. In addition baffles or other impediments may be positioned within the pot containing the bath so as to assist in breaking up agglomerates as they impact against the baffles or impediments.

The invention will now be illustrated with reference to the following Example: Example 250 grams of high purity lithium was placed inside a stainless steel container. The lithium typically should have a purity of less than 200 ppm nitrogen content. Pigment grade titanium dioxide in the amount of approximately 40 grams was then added to the stainless steel container. In these proportions the lithium was present in approximately 1 800% of that stoichiometrically required to reduce the titanium dioxide to titanium metal. The stainless steel container containing these materials was then heated and as the temperature exceeded 1 800C the lithium ingot melted and a suspension of titanium dioxide particles was formed within the mixture as the mixture is stirred. At approximately 3500C the first noticeable signs of reaction were observed.The reaction mixture was then heated to 5000C and held there for a few minutes to complete the reduction of the titanium dioxide which proceeds according to the following equation:

This reduction was performed inside of a glove box holding an argon atmosphere at a pressure slightly in excess of atmospheric pressure. The mixture now containing lithium metal, titanium metal and lithium oxide was then transferred to a vacuum distillation apparatus where the mixture was heated to approximately 6000C under a 10-4 torr vacuum. The mixture was agitated during distillation.

Substantially all of the lithium metal was distilled from the titanium metal and lithium oxide using this distillation procedure and the lithium metal was recovered on a cold finger extending into the distillation chamber. Titanium powder and lithium oxide powder were ieft in the stainless steel reaction chamber. The mixture of titanium and lithium oxide powders was then washed with an aqueous medium so as to dissolve the lithium oxide particles and any lithium metal remaining in the mixture.

Many solvents including water or mild acids are of course possible, but it should be mentioned that solvents containing chlorides, for example, hydrochloric acid, should be avoided so as to prevent chloride contamination of the titanium powder. After leaching as described above, the titanium powder was washed in deionized water and then dried. Examples of the powder produced are shown in Figures 1 and 2 which are scanning electron micrographs of the titanium powder. As can be seen, the titanium powder was in the form of agglomerates and in looking at Figure 2, in which the powder is shown at a higher degree of magnification than in Figure 1, it can be seen that the powder has a basic particle size of approximately 0.1-0.5 microns. The agglomerates observed in Figure 1 were made up of these basic particles.

The lithium hydroxide produced by leaching was treated by CO2+CI2 according to the following equations to produce lithium chloride. Electrolysis of the lithium chloride was then performed to recover lithium and chlorine.

2LiOH+CO2

Li2CO3+H20 (1) Li2CO3+Cl2

2LiCI +CO2+wO2 (2) electrolysis 2LiCI

2L1 +Cl2 (3) Net reaction 2LiOH

2Li +H20+02 The lithium metal recovered by the above process and through distillation was then recycled and used to reduce additional oxide material.

Claims

1. A process for reducing a metal oxide to a substantially unoxidized metal powder which comprises forming a reaction mixture of said metal oxide in a quantity of a liquid metal reducing agent in excess of that stoichiometrically required to fully reduce said metal oxide; and reacting said metal oxide with said liquid metal reducing agent until said metal oxide has been substantially reduced to said unoxidized metal powder except for minor impurity levels of oxygen.

2. A process according to claim 1, wherein the metal oxide is reacted with the liquid metal reducing agent at a temperature below approximately 6000 C.

3. A process according to claim 1 or 2, wherein excess metal reducing agent is distilled from the substantially reduced metal.

4. A process according to claim 1,2 or 3, wherein the liquid metal reducing agent is selected from calcium, magnesium, lithium, sodium or their solutions with each other.

5. A process according to claim 4, wherein the liquid metal reducing agent is selected from iithium-magnesium solutions, lithium-sodium solutions and lithium-calcium solutions.

6. A process according to any of claims 1 to 5, wherein the liquid metal reducing agent is a lithium-calcium solution and the metal oxide is an oxide of a transition metal.

7. A process according to claim 6, wherein the metal oxide is an oxide of a group IV transition metal.

8. A process according to claim 1, 2, 3 or 4, wherein the metal oxide is an oxide of titanium and the liquid metal reducing agent is lithium.

9. A process according to any of claims 1 to 8, wherein the reaction mixture is agitated or stirred during the reaction step.

1 0. A process according to any of the preceding claims, wherein the metal powder is separated from the reaction mixture and cleaned.

11. A process according to claim 1, which comprises forming a reaction mixture of a titanium oxide powder in the required stoichiometric excess of liquid lithium; reacting the titanium oxide with the liquid lithium at a temperature between approximately 350 and 6000C under an inert atmosphere until said titanium oxide has been substantially reduced to the unoxidized metal; vacuum distilling excess liquid lithium from the substantially reduced titanium; stirring said reaction mixture during said reacting and distilling steps; leaching lithium oxide and any remaining lithium metal out of said titanium; washing the resulting titanium metal powder in water; and drying said powder.

1 2. A process according to claim 11, which further comprises reclaiming and recycling the lithium metal separated out during distillation and leaching.

13. A process for reducing a metal oxide to a substantially unoxidized metal powder as claimed in claim 1 and substantially as described herein with particular reference to the foregoing Example.

1 4. Metal powder when made by a process as claimed in any of the preceding claims.