HK1066248B - Extraction of metals - Google Patents

Description

Metal extraction

no marking

Technical Field

The present invention relates to a process for producing metals from a metal material, such as a metal oxide.

Background

It is known that a process for producing metal from a metal-containing ore comprises the steps of: 1) selecting ores; 2) reducing the concentrate at elevated temperature and in the presence of a suitable reducing agent and producing a crude metal; 3) and refining the crude metal.

The present invention relates to an alternative method for producing metals from metal-containing materials based on electrochemical cell applications.

Prior Art

A. A paper entitled "electrochemical reduction of titanium" is published in 1993, Metallurgical transformations 24B, 6 months, page 449-445 (authors: TH Okabe, M Nakamura, T Oishi and K Ono)

The Okabe et al paper discloses an electrochemical method for removing oxygen dissolved in titanium.

The paper reports experimental work in an electrolytic cell containing a titanium cathode and a graphite anode with dissolved oxygen up to 1400 ppm. Cathode and anode immersion in molten CaCl₂In an electrolyte bath. A potential of between 0 and 6V is applied between the anode and the cathode. Using CaCl₂Calcium is generated and the reaction of calcium is promoted by reducing the activity of CaO, an electrolysis byproduct. Due to the use of a potential across the anode and cathode, CaCl₂The calcium potential in (b) increases at the titanium cathode surface. This results in calcium or CaCl being produced as a result of the electrolysis₂Cathode deoxidation by calcium of high activity. The oxygen ions generated, mainly in the deoxygenated product of the electrolyte, react at the graphite anode to form CO or CO₂And removing from the system.

B. A paper entitled "electrochemical deoxidation of yttrium-oxygen solid solutions" is published at pages 150-.

The Okabe et al paper discloses an electrochemical method for removing oxygen dissolved in yttrium.

The paper describes experimental work on solid yttrium containing dissolved oxygen. Yttrium is placed in a titanium cage cathode and then immersed in the meltMelting CaCl₂In an electrolyte bath. CaCl₂The electrolyte bath was contained in a titanium crucible and a constant voltage of between 3.2 and 3.8V was applied between the cathode and the graphite anode immersed in the electrolyte. The electrolysis was carried out at 1223K (950 ℃ C.) for a given time.

C. International application PCT/GB99/01781 (patent publication WO99/64638) (Fray et al).

The international application of Fray et al discloses two possible applications of a "discovery" in the field of metallurgical electrochemistry.

One application is the direct production of metals from metal oxides.

Another application is the removal of impurities that are "dissolved" in solid metals. The same basic approach is considered suitable for both applications.

The "discovery" is that electrochemically ionizing oxygen contained in solid metal is carried out so that oxygen is dissolved in the electrolyte (refer to page 5, lines 14-16). The international application discloses that when a suitable negative voltage is applied in an electrochemical cell using an oxygen-containing metal as the cathode, a reaction occurs whereby oxygen is ionized and can then be dissolved into the electrolyte in the electrolytic cell.

The international application discloses an electrolytic cell comprising a body of a metallic substance, such as a metal oxide with dissolved impurities, as the cathode of the cell. The cathode is immersed in a molten bath of a suitable electrolyte. A predetermined voltage lower than the decomposition potential of the electrolyte is applied between the cathode and a suitable anode (a separate graphite anode or an electrolyte crucible). The potential is selected to be of a value that ionizes and diffuses selected impurities (e.g., O, S, C or N) through the metal body and into the electrolyte in which it is dissolved.

The international application lists a number of metals which are believed to be suitable for use in the above process. These metals are titanium (Ti), silicon (Si), germanium (Ge), zirconium (Zr), hafnium (Hf), samarium (Sm), uranium (U), aluminum (Al), magnesium (Mg), neodymium (Nd), molybdenum (Mo), chromium (Cr), niobium (Nb), or any of their alloys.

Reference is made to pages 9-14 of the international application in which all examples refer toAnd "purging" and/or reduction to titanium, titanium oxide and certain titanium/aluminum alloys, Ti6Al 4V. Example 12 relates to the preparation of a coating from TiO₂And Al₂O₃The mixture of (a) and (b). The applied voltage in the different examples varied from as low as 1.75V (see example 2) to 3.3V (see example 3). Most experiments were performed at a voltage of 3.0V. The processing time is different. The crucible used is made of aluminum, graphite or titanium, so that the anode is a crucible or a separate graphite rod. The only electrolyte used in all examples was CaCl₂。

Summary of The Invention

The applicant carried out the experimental work at the mineral technology centre of the new cassel laboratory in order to repeat the tests carried out in the above-mentioned prior art documents.

Experimental work led to the following findings and inventions.

1. Can melt CaCl₂In the middle electrolysis, titanium with very low oxygen concentration is directly prepared from titanium oxide.

However, there is a need for improved electrolytic cells for reducing titanium oxide in electrolytic cells, and the international application of Fray et al is silent as to how to build electrolytic cells for the reduction of good electrical insulators, such as titanium oxide, among others. According to the test apparatus disclosed in the international application of Fray et al, titanium oxide reduction cannot be achieved within the parameters required therefor.

Accordingly, a first aspect of the invention is based on the recognition that: the cathode is of the type in contact with TiO₂And CaCl₂Resulting in severe effects on the titanium oxide reduction process in the electrical contact of the electrolyte. Although the mechanism is only rudimentary, it is likely that the correct choice of materials and types of electrical contacts is an important aspect of the design of the cell for reducing metal oxides and the choice of electrolyte for this purpose.

Thus, a first aspect of the invention is a process for producing a metal or alloy from a metalliferous material by removing an impurity (I) selected from O, S or N from a solid metalliferous material by electrolysis in an electrolytic cell using a molten halide salt or a mixture of halide salts as an electrolyte, wherein the cation of the salt is selected from Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y, the process comprising carrying out the electrolysis under the following conditions:

(a) the potential applied between the anode and the cathode of the electrolytic cell is selected to avoid, to a certain extent, a permanent decomposition of the electrolyte, thereby avoiding a substantial deposition of electrolyte cations on the cathode; and

(b) the body is part of a cathode forming an electrolytic cell, the cathode comprising a conductor for electrical connection of the cathode to an electrical potential, the conductor having a high resistance to chemical attack by an electrolyte at elevated temperatures, and the conductor being at least partially immersed in the electrolyte; and

(c) o, S or N are removed from the cathode and enter the solution and/or chemically react with the electrolyte cations.

The metal-containing material may comprise an oxide, sulphide, carbide or nitride of said metal.

Preferably, the metalliferous material is a titanium-containing material.

Preferably, the impurity is oxygen.

Preferably, the titanium-containing material is titanium oxide.

Preferably, the anode is made of graphite.

Preferably, the electrolyte is CaCl₂。

2. Carbon was detected in the reduced metal pellets produced in the experiment.

Although the source of carbon was the carbon anode used in the experiments, the mechanism of carbon incorporation into the reduced metal was not fully understood. The absolute content of carbon in certain points of the metal pellets is too high to be negligible.

Accordingly, a second aspect of the invention provides a process for producing a metal or alloy from a metal material by removing impurities (I) selected from O, S or N from a solid metal material by electrolysis in an electrolytic cell having as electrolyte a molten halide salt or a mixture of halide salts, wherein the cations of said salts are selected from Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y, which process comprises carrying out the electrolysis under the following conditions:

(a) selecting the potential applied between the anode and cathode of the cell so as to avoid, to some extent, permanent decomposition of the electrolyte, thereby avoiding substantial deposition of electrolyte cations on the cathode and substantially preventing migration of anode material towards and into the cathode;

(b) the body forms part of the cathode of the cell; and

(c) o, S or N are removed from the cathode and enter the solution and/or chemically react with the electrolyte cations.

Preferably, the cathode comprises a conductor having a high resistance to chemical corrosion of the electrolyte at high temperatures and serving to connect the cathode and an electrical potential, and the conductor is at least partially immersed in the electrolyte.

The metal material may comprise an oxide, sulfide, carbide or nitride of said metal.

Preferably, the metalliferous material is a titanium-containing material.

Preferably, the impurity is oxygen.

Preferably, the titanium-containing material is titanium oxide.

Preferably, the anode is made of graphite.

Preferably, the electrolyte is CaCl₂。

3. When the above-mentioned method of the present invention is used, it is confirmed that TiO is involved in the reaction₂Ball-contacted Al₂O₃May be reduced and alloyed with the reduced titanium.

4. Using the above process, it was found that CaCl can be melted₂From SiO by electrolysis in medium₂And reduced to silicon. But nevertheless with TiO₂Reduction phase, SiO is observed₂The degree of chlorine evolution during reduction is high.

5. Attempts have also been made to use Al in the above-mentioned manner₂O₃And reducing the pellets to obtain Al.

It was found that the reduction of Al occurred only in the vicinity of the position between the pellet and the electrical lead connecting the cathode and the power supply. The portion of the pellet remote from the cathode lead is not reduced at all.

This finding again indicates that the conductivity of the cathode is a factor in the reduction process.

Accordingly, it is a further aspect of the present invention to provide a cathode for use in the above method, wherein the cathode comprises a body of metallic material distributed around one or more electrical conductors which are substantially inert in the electrolyte at elevated temperatures and a plurality of reduction zones are provided on the cathode.

The mechanism by which the electrolytic process described by the application proposed in the international application of Fray et al and in the article of Okabe et al removes oxygen from titanium, titanium oxide, yttrium and aluminium-titanium alloys has not been understood to date. The mechanism disclosed in the article by Okabe et al is not correct as proposed by the international application of Fray et al. Both mechanisms are believed to be speculative in the context of the other metals and oxides involved. Furthermore, there is evidence that the type of electrolyte affects the process parameters, but its performance and role in current mechanisms is ambiguous and only qualitative.

Test data of the invention

A. Reduction of titanium oxide

I. First test

The purpose of the first test was to confirm (or conversely) passage through molten CaCl₂The direct electrochemical reduction of titanium oxide to produce titanium metal.

In particular, the purpose of the first test is to validate (or conversely) the device described in the international application of Fray et al. Therefore, the test conditions were kept as close as possible to those in the examples of the international application.

According to the international application of Fray et al, the basic principle of the method is based on the application of an appropriate negative potential to the cell to ionize the oxygen in the oxide and subsequently dissolve it in the electrolyte.

Test method and apparatus

The test apparatus is shown in FIG. 1.

Referring to fig. 1, an electrochemical cell includes a graphite crucible with a graphite lid. The crucible serves as the anode of the electrolytic cell. Stainless steel rods were used to ensure electrical contact between the d/c power supply and the crucible. The cathode of the electrolytic cell is composed of Kanthal heat-resistant steel wire or platinum wire connected with one end of a power supply and TiO hung at the other end of the metal wire₂Small balls. Alumina tubes were used as the insulator around the cathode.

A type B thermocouple contained within an alumina sheath was immersed within the electrolyte adjacent to the pellet.

Two types of pellets may be used. One is a slip casting type and the other is a die pressing type. Both types of beads consist of analytical grade TiO₂And (5) preparing powder. Both types of pellets were sintered in air at 850 ℃. One molded pellet and one slip cast pellet were used in the test.

The test was carried out at 950 ℃. The voltage applied between the crucible wall and Kanthal heat-resistant wire (Kanthal) or platinum wire reaches 3V.

The power supply maintains a constant voltage throughout the test. LabVIEW (TM) data acquisition software was used to record the voltage and resulting cell current.

Test results

Referring to fig. 2 and 3, the constant voltage (3V) used in the experiment produced an initial current of about 1.2A. During the first 2 hours, a continuous drop in current was observed. Then a gradual increase in current to 1A was again observed.

At the end of the test, the cell was removed from the furnace and quenched in water. Dissolving solid CaCl in water₂And two pellets were recovered.

Scanning electron micrographs of the cross section of the two beads are shown in figures 4 and 5.

Analysis with an Electron Probe Microanalyzer (EPMA) confirmed the presence of virtually pure metallic titanium in both pellets. Analysis also showed regions of partially reduced titanium. EPMA results are shown in FIGS. 6 and 7.

Carbon was detected at various locations within the pellet and its content varied up to 18 wt%.

B. Reduction of silicon

I. Test method and apparatus

The test apparatus was essentially the same as the apparatus for titanium oxide reduction. The cathode is composed of a platinum-rhodium wire and SiO suspended at the end of the wire₂And (4) forming small balls.

The test was carried out at 950 ℃.

Test results

The voltage used in the experiment was 3V, which produced an initial current of about 1.5A, shown in fig. 8. The current was then observed to gradually drop to 0.65A.

To overcome the resistance and overvoltage, the operating potential was chosen to be 3V. However, despite the working potential being lower than CaCl₂A theoretical decomposition potential of 3.25V at 950 ℃, but chlorine evolution was also observed at 3V.

The test was terminated after 4 hours. By mixing CaCl₂Dissolved in water to separate the partially reduced beads. The surface and the inside of the specimen were analyzed by a scanning electron microscope.

Analysis of the bead surface indicated the presence of some oxygen, indicating only partial reduction in these regions.

However, the oxygen concentration in these regions is much lower than SiO₂The oxygen concentration in (c) is as shown in FIG. 9.

The structure of the partially reduced region of the pellet is shown in fig. 10. Regions of different phases, e.g. SiO₂And 2 CaO. SiO₂See fig. 11-13.

Pure unreduced SiO is present in the center of the pellet₂。

Pure Si was identified near the platinum wire as shown in fig. 14-17.

Many modifications may be made to the above-described invention without departing from the spirit and scope thereof.

Claims (7)

1. A process for producing a metal or alloy from a metalliferous material by removing an impurity selected from O, S or N from a solid metalliferous material by electrolysis in an electrolytic cell including an anode, a cathode and a molten halide salt or mixture of halide salts as an electrolyte, in which the cation of the salt is selected from the group consisting of Ca, Ba, Li, Na, K, Mg, Sr, Cs and Y, which process includes carrying out the electrolysis under the following conditions:

(a) selecting the potential to be applied between the anode and the cathode of the electrolytic cell to avoid permanent decomposition of the electrolyte;

(b) the cathode comprises a body of metallic material and one or more conductors for electrically connecting the cathode to an electrical potential, the body of metallic material being distributed around the conductors, the conductors having a high resistance to chemical attack by an electrolyte at high temperatures and a plurality of reduction zones, and the conductors being at least partially immersed in the electrolyte; and

(c) o, S or N are removed from the cathode and enter the solution and/or chemically react with the electrolyte cations.

2. The method defined in claim 1 wherein the metal material comprises an oxide, sulfide, carbide or nitride of said metal.

3. The method defined in claim 1 or claim 2 wherein the metalliferous material is a titanium-containing material.

4. The method defined in claim 3 wherein the titanium-containing material is titanium oxide.

5. The process as described in claim 1 or 2, wherein the impurity is oxygen.

6. A method as claimed in claim 1 or 2, wherein the anode is made of graphite.

7. A cathode for use in the method of claim 1, comprising a body of metalliferous material distributed around one or more electrical conductors that are substantially inert in the electrolyte at the elevated temperature and that provide a plurality of reduction zones at the cathode.