WO2019083471A2

WO2019083471A2 - A production method for scandium metal or al-sc alloys using electrolysis method from scandium salt mixtures prepared by adding of cacl2 and/or mgcl2 compounds to scf2 compound obtained from scandium compound in the form of (nh4)2nascf6

Info

Publication number: WO2019083471A2
Application number: PCT/TR2018/050005
Authority: WO
Inventors: Ali Safder IPLIKCIOGLU; Serif KAYA
Original assignee: Minertek Mineral Teknolojileri Madencilik Sanayi Ve Ticaret Anonim Sirketi
Priority date: 2017-03-21
Filing date: 2018-01-04
Publication date: 2019-05-02
Also published as: TR201704220A2; WO2019083471A3

Abstract

The invention relates to a production method for scandium metal in pure or alloy form using molten salt electrolysis or electrolysis method from scandium salt mixtures prepared by adding CaCI2 and/or MgCI2 salts to ScF3 compound, in order to lower its melting temperature, which is obtained by reacting the initial scandium compound, having the form of (NH4)NaScF6, with HCI in aqueous medium and removing (NH4) and Na with CI from its structure

Description

A Production Method For Scandium Metal or Al-Sc Alloys Using Electrolysis Method From Scandium Salt Mixtures Prepared By Adding Of CaCh and/or MgC Compounds To ScF₃ Compound Obtained From Scandium Compound In The Form of (NH₄)2NaScF₆ Technical Field

The invention relates to a production method for scandium metal in pure or alloy form, using molten salt electrolysis or electrolysis method from ScF₃-CaCI₂, ScF₃-MgCl2 or ScF₃-CaCI₂- MgCI₂ salt mixtures prepared by adding CaCI₂ and/or MgCI₂ salts to ScF₃ compound, in order to lower its melting temperature, which is obtained by reacting the initial scandium compound having the form (NH₄)₂NaScF₆, with HCI in aqueous medium and removing (NH₄) and Na with CI from its structure.

Prior Art

Scandium is one of the transition metals, which belong to the 3B group of the periodic table, and known as one of the rare earth elements, which is exceptionally enriched in nature. Scarcity of the ores having sufficient grade for feasible processing, constitute a great obstacle against production and use of this metal in various industries. Until today, trace amounts of it have been identified among more than 100 minerals within the content of uranium, tin, iron, tungsten, tantalum, zirconium, titanium and other rare earth element ores, which are processed feasibly; and it has been produced as a byproduct. In addition, substantial amounts of scandium element present in lateritic nickel-cobalt ores have lately brought the possibility of producing scandium as a byproduct of processing these resources, and various studies have been set off. An example study is provided by patent application no. TR201308682 titled 'High pressure acid leaching of refractory ores containing nickel, cobalt and scandium and recovering scandium from metal loaded leach solutions and purification precipitates', claimed by 'Meta Nikel Kobalt Madencilik Sanayi ve Ticaret Anonim Sirketi'. Another example is the patent application titled 'Recovering scandium and its derivatives from leach solutions loaded with metals, which are obtained by leaching lateritic ores containing nickel, cobalt and scandium, and secondary sources containing scandium', claimed by 'Meta Nikel Kobalt Madencilik Sanayi ve Ticaret Anonim Sirketi'. The aforementioned applications are related to scandium recovery from lateritic nickel-cobalt ores.

Another example is given by the US application no. US31 1 1467. The mentioned application requires pure NaF, ScF₃ and Sc₂0₃ salts in order to conduct the electrolysis process. First, NaF and ScF₃ salts are mixed and melted at 800 Ό, and then Sc ₂0₃ salt is dissolved inside this melt. Production of aluminum-scandium alloys is one of the most important areas where scandium element is used, where a 0.2-0.8% scandium is added into aluminum for applications which require high strength, corrosion resistance and weldability. These alloys are in general obtained by adding pure scandium metal or a master alloy containing 1 -20% scandium into molten aluminum metal.

One of the methods used for obtaining pure scandium metal is based on metallothermic reduction of pure Sc₂0₃ compound with gaseous calcium metal at high temperature. However, since the compound is a highly stable compound during the reduction, the reduction process may not fully take place and a certain amount of scandium metal is lost in the form of CaSc₂0₄. Another method for obtaining pure scandium metal is converting the Sc₂0₃ compound into relatively less stable ScF₃ form using HF gas at high temperature and metallothermic reduction with calcium metal at high temperature. However, HF gas used in converting the Sc₂0₃ compound into ScF₃ form is an extremely hazardous and corrosive compound, and imposes technical difficulties during the process. As a result of the reaction, CaF₂ compound is produced along with the scandium metal, leading to the problem of separating scandium from this compound. Furthermore, there are additional operational costs associated with the vacuum distillation method used in this process in order to overcome the impurity problem due to the tantalum pots in which the reduction process takes place during the applied process and due to the calcium metal used in the reaction. In order to overcome these problems, it has been proposed to perform the reduction process of pure Sc₂0₃ or ScF₃ compound with calcium in the presence of aluminum metal, thereby recovering scandium aluminum alloy instead of pure scandium metal. The reduction process has been claimed to become easier and more efficient by dissolving the scandium metal, which has been reduced to metallic form, in molten aluminum metal. However; impurity problems during the reduction process at high temperatures due to tantalum pots and calcium metal, generation of AUCa phase in processes where Sc₂0₃ compound is used as the initial compound, and requirement of using HF gas in processes where ScF₃ compound is used as the initial compound still constitute the technical problems which should be overcome during these processes. It has been proposed that it is possible to obtain scandium metal alternatively by using molten salt electrolysis method in order to overcome the problems encountered during metallothermic reduction methods conducted at high temperatures with calcium metal [1 ]. The molten salt method is based on dissolving the metal, which is desired to be recovered, inside an appropriate salt mixture at high temperature and separate it into its ions, and meanwhile, applying electric current to the molten salt mixture, in order to achieve the reduction of the desired element and selectively collect it at the cathode. Salts that may be used in this process include fluoride, chloride, bromide and iodide, among which fluoride, chloride and mixtures thereof have been the most preferred [2]. Among mixtures of molten salts containing chloride and fluoride, fluoride containing salts are the most preferred ones due to having higher stability at high temperatures, absence of humidity absorption problem (not being hydroscopic) in contrast to chlorides, and having high current efficiencies [3].

Therefore, aforementioned disadvantages and lack of adequate solutions in the background art have made it necessary to make a development in the technical field related to scandium metal production. Purpose of the Invention

The present invention relates to a production method for scandium metal and Al-Sc alloys by electrolysis of scandium salt mixtures obtained by adding of CaCI₂ and/or MgCI₂ compounds to ScF₃ compound obtained from scandium compound in the form of (NH₄)2NaScF₆, while it meets the aforementioned requirements, overcomes all disadvantages and provides further advantages.

The primary objective of the invention is production of pure scandium metal, directly obtaining aluminum-scandium alloys containing 0.2-0.8% scandium and obtaining aluminum-scandium master alloys containing 1 -20% scandium.

An objection of the invention is to obtain scandium metal by using molten salt method, in order to overcome the technical problems associated with metallothermic reduction method. It is also obtaining fluoride-chloride containing salts in order to achieve this.

An objection of the invention is to obtain ScF₃-CaCI₂, ScF₃-MgCl2 or ScF₃-CaCl2-MgCl2 salt mixtures, in order to be used in electrolysis method, by adding CaCI₂ and/or MgCI₂ to ScF₃ compound, which is obtained from initial (NH₄)2NaScF₆ raw material by reacting it with HCI in aqueous medium and removing (NH₄) and Na with CI from its structure, without using calcination process.

Another objection of the invention is to decrease the melting temperature of the ScF₃ which has a high melting point (1552 Ό), by adding of CaCI ₂ and/or MgCI₂ to it. This is due to the requirement of having the salts used in molten salt electrolysis, to be in molten phase during the process. Therefore, the melting temperature of the obtained salt mixture is preferred to be relatively low. Therefore, one unit of ScF₃ by weight, together with 15 units of CaCI₂ and/or MgCI₂ salt mixture by weight is heated and they are found to be in molten state at a relatively low -800 Ό temperature. A similar objection of the invention is to prevent mixing a multitude of high purity salts.

In order to achieve the aforementioned objections, a production method for pure scandium metal, wherein, it comprises; a) obtaining ScF₃ compound from the scandium compound having the form of (NH₄)2NaScF₆, b) obtaining a scandium salt mixture by adding CaCI₂ and/or MgCI₂ to the obtained ScF₃ compound in order to lower the melting point, c) electrolysis of the obtained scandium salt mixture by using molten salt electrolysis method, process steps. Structural and characteristic features of the invention with all its advantages shall become apparent with the detailed description given below and the appended drawings, therefore, assessment should be based on these drawings and the detailed description.

Brief Description of Drawings

Figure 1 : A view of the cell system intended to be used to obtain pure scandium metal Figure 2: A view of the cell system intended to be used to obtain aluminum-scandium alloy

Figure 3: XRD graph of the ScF₃ obtained by reacting (NH₄)2NaScF₆ compound with 4 M

HCI at 95 °C for 2 hours

Figure 4: XRD graph of the Al-Sc alloy obtained by electrolysis

Figure 5: Optical microscope image of the Al-Sc alloy obtained by electrolysis (x190 magnification)

Figure 6: Optical microscope image of the Al-Sc alloy obtained by electrolysis (x1300 magnification) and micro-hardness measurement of Al and AI₃Sc phases

Drawings should not be scaled and the details that are not essential for understanding the present invention may have been omitted. Moreover, at least substantially identical elements or elements with at least substantially identical functions are denoted with the same numbers. Detailed Description of the Invention

In this detailed description; the preferred embodiments of the production method for scandium metal and Al-Sc alloys by electrolysis of scandium salt mixtures obtained by adding CaCI₂ and/or MgCI₂ to the ScF₃ compound which is obtained from the scandium compound in the form of (NH₄)2NaScF₆, are given herein to merely help understanding the subject better and they are not intended to limit the invention.

The invention relates to production method for scandium metal in pure or alloy form. The scandium production method of the invention comprises three steps. The first step is obtaining ScF₃ by reacting the scandium compound having the form of (NH₄)2NaScF₆ with HCI in aqueous medium in order to remove (NH₄) and Na with CI from its structure. In the second step, ScF₃- CaCI₂, ScF₃-MgCl2 or ScF₃-CaCI₂-MgCl2 scandium salt mixtures are obtained by adding CaCI₂ and/or MgCI₂ to ScF₃ compound in order to decrease the melting temperature of the ScF₃ which has a high melting point (1552 Ό). The reason for adding CaCI₂ and/or MgCI₂ to ScF₃ (1552 Ό) is the requirement to have the salts used in molten salt electrolysis to be in molten state. Therefore, the melting temperature of the obtained salt mixture is preferred to be relatively low. Therefore, one unit of ScF₃ by weight, together with 15 units of CaCI₂ and/or MgCI₂ salt mixture by weight is heated and they are found to be in molten state at a relatively low temperature of -800 ^<C. In the third step, ScF ₃-CaCI₂, ScF₃-MgCI₂ or ScF₃-CaCI₂-MgCI₂ fluoride-chloride salt mixtures are used in molten salt electrolysis, in order to obtain pure scandium metal. The scandium compound of the invention, having the form of (NH₄)₂NaScF₆, is reacted with 2-6 M HCI solution (preferably 4 M) at a temperature between 25-95 Ό (preferably at 95 for a duration between 1 -6 hours (preferably 4 hours).

Figure 1 shows a cell system which is used to obtain pure scandium metal. ScF₃-CaCI₂, ScF₃- MgCI₂ or ScF₃-CaCI₂-MgCI₂ scandium salt mixture obtained by adding CaCI₂ and/or MgCI₂ to ScF₃, which is obtained in the first step, is fed to the electrolysis cell, in order to obtain pure scandium metal.

The salts used in molten salt electrolysis are required to be in molten phase. Therefore, the melting temperature of the required salt mixture is preferred to be relatively low. The fluorinated- chlorinated salt mixture is observed to be in molten state at a relatively low temperature of -800 Therefore, electrolysis temperature during elec trolysis should be higher than 800 Ό where the salt mixture will be in a molten state.

Over the abovementioned temperatures; Na, Ca, Mg, Sc, F and CI elements are in their ionic states, and in general in Na⁺, Ca⁺², Mg⁺², Sc⁺³, F^~ and CI^" forms. Therefore, when an electric current is passed through the molten mixture at a certain potential, positively charged Na⁺, Ca⁺², Mg⁺², Sc⁺³ ions will be attracted by the cathode, while negatively charged F^~ and CI^" ions will be attracted by the anode. Depending on the relative stability of these ions with respect to each other, the applied potential will result in electrochemical oxidation and reduction reactions at the anode and cathode regions. Depending on the composition of the ScF₃-CaCI₂, ScF₃-MgCl2 or ScF₃-CaCl2-MgCl2 salt mixture, the preferred temperature should be 10-15 Ό above the melting temperature of the salt mixture.

The circuit is powered on to maintain 2-8 volts of potential difference. The current density applied to the circuit is maintained at a constant level between 0.5-1 .0 A/cm². Due to the relatively lower applied potential, Na⁺, Ca²⁺ and Mg²⁺ ions are prevented to react, while only Sc³⁺ ions are reduced at the cathode and collected on the electrolysis cell in a metallic form. Therefore, limiting the potential applied to the circuit, below 3 volts, will prevent the Na, Ca and Mg contamination problem. Whereas, due to the oxidation reactions at the anode, F^~ and CI^" ions will react with the graphite anode and leave the cell in the form of various fluoro-chloro carbon gases. These gases leaving the anode are environmentally undesired and hence may be collected later and disposed of. Scope of this study does not cover disposal of these gases. If it is preferred, more stable anode materials which does not react with fluorine-chlorine may be used instead of the graphite anode, resulting only fluorine-chlorine gas output at the anode, in order to provide easier disposal of these gases. These output gases may be converted into more stable and less hazardous fluoride-chloride compounds using a gas collector unit. When fluorine-chlorine gas outlet is not preferred in the circuit after the reduction reactions, Sc₂0₃ compound may be added to the system no more than 10% by weight in the total salt bath and this compound may be dissolved into Sc⁺³ and O ² ions inside the molten salt phase. Since O ² ions are oxidized at lower potentials in comparison to F^~ and CI^" ions, F^" and CI^" ions may be prevented to react by adjusting the potential difference applied to the circuit. Thereby, carbon monoxide and carbon dioxide gases, which are environmentally easier to dispose of, are emitted as a result of the carbon and oxygen reactions at the graphite anode. Alternatively, instead of graphite, by using more stable anode materials which does not react with oxygen, oxygen gas, which does not possess any environmental risk, may be emitted at the anode. When oxidation and reduction reactions end at the anode and the cathode, Sc⁺³ ions in the molten mixture, are separated from the salt mixture and pure scandium metal aggregates are obtained at the bottom of the cell.

If it is desired to obtain an aluminum-scandium alloy containing 0.2-0.8% scandium or an aluminum-scandium master alloy containing 1 -20% scandium; then pure aluminum metal is added to the bottom of the cell before the reaction as shown in Figure 2. Subsequently, ScF₃- CaCI₂, ScF₃-MgCl2 or ScF₃-CaCl2-MgCl2 salt mixture obtained from the calcination process is added on top of the Al metal.

Thereafter, both phases are liquefied by increasing the temperature of the cell over a value (preferably 800-1 100 Ό) where the aluminum and the fluorinated-chlorinated salt mixture are in their molten states. When melting is completed, liquid aluminum is collected at the bottom of the cell due to the density difference, while fluorinated-chlorinated salt mixture is collected on top of the liquid aluminum.

A current shall be applied to the circuit in order to create a potential difference of 2-8 volts. Applying relatively lower potentials preferably not exceeding 3 volts prevent Na⁺, Ca²⁺, Mg²⁺ ions to react, ensuring only Sc³⁺ ions to be reduced at the cathode and to be dissolved in the liquid aluminum collected at the bottom of the electrolysis cell, in molten state. Due to oxidation reactions occurring in the anode, F and CI^" ions will react with the graphite anode and will leave the cell in the form of various fluoro-chloro carbon gases. Since these gases emitted at the anode, are environmentally undesired, these gases may be collected and disposed of later. If it is preferred, more stable anode materials which does not react with fluorine and chlorine may be used instead of the graphite anode, resulting in only fluorine and chlorine gas output at the anode, in order to provide easier disposal of these gases.

These output gases may be converted into more stable and less hazardous fluoride-chloride compounds using a gas collector unit. When fluorine-chlorine gas outlet is not preferred in the circuit after the reduction reactions, Sc₂0₃ or Al₂0₃ compounds may be added to the system no more than 10% by weight in the salt mixture and scandium oxide may be dissolved into Sc⁺³ and O ² ions, or the aluminum oxide may be dissolved into Al⁺³ and O ² ions, inside the molten salt phase. Since O ² ions are oxidized with lower potentials in comparison to F and CI^" ions, F and CI^" ions may be prevented to react by adjusting the potential difference applied to the circuit. Thereby, carbon monoxide and carbon dioxide gases, which are environmentally easier to dispose of, are emitted as a result of the carbon and oxygen reactions at the graphite anode. Alternatively, by using more stable anode materials which does not react with oxygen, instead of graphite, oxygen, which does not possess any environmental risk, may be emitted at the anode. Applying a potential difference of 2-8 volts, Sc³⁺ and Al⁺³ ions are reduced simultaneously and dissolved in the liquid aluminum collected at the bottom of the electrolysis cell in molten state. Thereby applying 2-8 volts potential difference to the cell and if the process is maintained until the desired scandium concentration; an aluminum-scandium alloy containing 0.2-0.8% scandium or, with longer electrolysis durations, an aluminum-scandium master alloy with 1 -20% scandium may be obtained. After the electrolysis process, again the aluminum- scandium alloy collected at the bottom of the cell in liquid state is separated from the molten fluorinated-chlorinated salt phase and an aluminum-scandium alloy is obtained. REFERENCES

Xiao Y. Yan and D.J. Fray, Molten salt electrolysis for sustainable metals extraction and materials processing - A review, in Electrolysis: Theory, Types and Applications, Shing Kuai and Ji Meng, Editors. 2010, Nova Science: New York. p. 255-302.

Zhu, H., Rare Earth Metal Production by Molten Salt Electrolysis, in Encyclopedia of Applied Electrochemistry, G. Kreysa, K.-i. Ota, and R. Savinell, Editors. 2014, Springer New York. p. 1765-1772.

Yuriy Shtefanyuk, et al. Production of Al-Sc alloy by electrolysis of cryolite-scandium oxide melts, in TMS (The Minerals, Metals & Materials Society). 2015. Florida: John Wiley & Sons, Inc.

Claims

1. A production method for pure scandium metal, wherein, it comprises; a) obtaining ScF₃ compound from the scandium compound having the form of (NH₄)₂NaScF₆, b) obtaining a scandium salt mixture by adding CaCI₂ and/or MgCI₂ to the obtained

ScF₃ compound in order to lower the melting point, c) electrolysis of the obtained scandium salt mixture by using molten salt electrolysis method, process steps.

2. The production method according to claim 1 , wherein; the ScF₃ compound mentioned in process step a is obtained by,

• reacting the initial scandium compound having the form of (NH₄)2NaScF₆ with HCI in aqueous medium and removing (NH₄) and Na with CI from its structure process step.

3. The production method according to claim 1 , wherein; the scandium compound having the form of (NH₄)2NaScF₆, is reacted with HCI at a temperature between 25-95 Ό for a duration between 1 -6 hours.

4. The production method according to claim 1 , wherein; ScF₃-CaCI₂ scandium salt mixture is obtained by adding CaCI₂ to ScF₃ mentioned in process step b.

5. The production method according to claim 1 , wherein; ScF₃- MgCI₂ scandium salt mixture is obtained by adding MgCI₂ to ScF₃ mentioned in process step b.

6. The production method according to claim 1 , wherein; ScF₃-CaCI₂-MgCI₂ scandium salt mixture is obtained by adding CaCI₂- MgCI₂ to ScF₃ mentioned in process step b.

7. The production method according to claim 1 , wherein; the electrolysis process mentioned in step c comprises;

• heating the scandium salt mixture until molten state at a temperature between 800- • applying an electrical current to the obtained molten salt bath through anode and cathode in order to maintain 2-8 volts potential difference,

• obtaining pure scandium metal aggregates at the bottom of the electrolysis cell by separating the Sc⁺³ ions in the melt from the salt mixture by means of this electrical current, process steps.

8. The production method according to claim 7, wherein; when it is desired to obtain an aluminum-scandium alloy containing 0.2-0.8% scandium or an aluminum-scandium master alloy containing 1 -20% scandium, it comprises; · adding aluminum to the bottom of the electrolysis cell,

• adding scandium salt mixture on top of the aluminum metal,

• heating the aluminum metal and the fluorinated-chlorinated scandium salt mixture until molten at a temperature between 800-1 100 Ό,

• applying an electrical current to the obtained molten salt bath through the anode and the cathode in order to maintain 2-8 volts potential difference,

• obtaining aluminum-scandium alloy by separating the liquefied aluminum-scandium alloy from the molten fluorinated-chlorinated salt mixture by means of this electrical current, process steps.

9. The production method according to claim 8, wherein; the mentioned scandium salt mixture is ScF₃-CaCI₂, ScF₃-MgCI₂ or ScF₃-CaCl2-MgCI₂.

10. The production method according to claim 8, wherein; when mentioned anode material is graphite; fluoro-chloro carbon gas or carbon monoxide-carbon dioxide gases are emitted.

11. The production method according to claim 10, wherein; when carbon monoxide-carbon dioxide gas emission is preferred; it comprises,

• adding Sc₂0₃ or Al₂0₃ compound no more than 10% by weight to the salt mixture and dissolving scandium oxide into Sc⁺³ and O ² ions, or dissolving the aluminum oxide into Al⁺³ and O ² ions, inside the molten salt phase, adjusting the potential difference applied to the circuit without exceeding 6 volts, in order to oxidize only O ² ions, process steps.

12. The production method according to claim 8, wherein ; when the anode material mentioned in step d is a stable anode material which does not react with F^~, F₂ gas is emitted; and when it is a stable anode material which does not react with O , 0₂ gas is emitted.

13. The production method according to claim 12, wherein; when emission of O₂ gas is preferred; it comprises, adding Sc₂0₃ or AI2O3 compound no more than 10% by weight, to the salt mixture and dissolving scandium oxide into Sc⁺³ and O ² ions, or dissolving the aluminum oxide into Al⁺³ and O ² ions, inside the molten salt phase, adjusting the potential difference applied to the circuit without exceeding 6 volts, in order to oxidize only O ² ions, process steps.