ENGINEERING PROCESS FOR HALOGEN SALTS, USING TWO IDENTICAL ELECTRODES

Abstract

The invention relates to a process and devices for reducing impurities in molten salts, a molten salt being purified in an electrochemical process by applying a voltage between two electrodes. According to the invention, the voltage is varied so that in different phases different electrodes act as cathode or anode.

Claims

1. A process for purifying salt melts, comprising the following steps: i) providing a salt melt, wherein said salt melt contains at least one oxygen-based and/or at least one hydrogen-based contaminant; ii) contacting the salt melt electrically and physically with at least one first electrode and at least one second electrode, wherein said electrodes are not in mutual contact within the salt melt; iii) variably applying a voltage between said at least one first electrode and said at least one second electrode, so that an electric current flows between the electrodes, wherein said oxygen- and/or hydrogen-based contaminant is at least partially removed by an electrochemical reaction on at least one of the electrodes; characterized in that the voltage is varied in such a way that said at least one first electrode acts as the cathode and said at least one second electrode acts as the anode during at least one first phase, and said at least one first electrode acts as the anode and said at least one second electrode acts as the cathode during at least one second phase.

2. The process according to claim 1, characterized in that the voltage is varied in such a way that said at least one first electrode and said at least one second electrode alternately act as the anode, the other electrode respectively acting as the cathode.

3. The process according to claim 2, characterized in that the voltage is varied with a period length within a range of from 0.1 to 10 seconds, especially within a range of from 1 to 4 seconds.

4. The process according to claim 1, characterized in that said salt melt includes a halogen salt, optionally a chloride salt.

5. The process according to claim 1, characterized in that said salt melt includes a cation selected from the group consisting of Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, and Ni.

6. The process according to claim 1, characterized in that said at least one first electrode and/or said at least one second electrode include a material having a reduction potential that is not higher than a reduction potential of an oxygen- and/or hydrogen-based contaminant.

7. The process according to claim 1, characterized in that said at least one first electrode and said at least one second electrode are made of the same material, or from different materials.

8. The process according to claim 1, characterized in that said at least one first electrode and/or said at least one second electrode include an alkali metal, optionally lithium, sodium, or potassium, an alkaline earth metal, optionally magnesium, calcium, strontium, or barium, a transition metal, especially cobalt, nickel, iron, or zinc, or a metalloid, optionally boron, or silicon.

9. The process according to claim 1, characterized in that said process is performed at a temperature within a range of from 300 to 800° C.

10. A device for purifying salt melts using a process according to claim 1, comprising at least one device for cyclic voltammetric measurements, and at least one device for electrochemical purification, wherein said device for electrochemical purification includes an anode and a cathode, characterized in that said anode and cathode are made of the same material.

11. The process according to claim 2, characterized in that said salt melt includes a halogen salt, optionally a chloride salt.

12. The process according to claim 3, characterized in that said salt melt includes a halogen salt, optionally a chloride salt.

13. The process according to claim 2, characterized in that said salt melt includes a cation selected from the group consisting of Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, and Ni.

14. The process according to claim 3, characterized in that said salt melt includes a cation selected from the group consisting of Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, and Ni.

15. The process according to claim 4, characterized in that said salt melt includes a cation selected from the group consisting of Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, and Ni.

16. The process according to claim 11, characterized in that said salt melt includes a cation selected from the group consisting of Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, and Ni.

17. The process according to claim 12, characterized in that said salt melt includes a cation selected from the group consisting of Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, and Ni.

Description

EXAMPLES

[0062] The following Examples relate to the purification of chloride salt melts and were carried out by means of an autoclave device as shown in FIG. 3. The device includes a tube furnace 22, control devices comprising, in particular, a temperature control unit, a metallic container 24, and a sample crucible 23, which is inert towards the salts employed. The sample compartment is connected to an argon container 20 and a vacuum pump 21, in order to be able to control the atmosphere in the sample compartment. Six electrodes are connected to the sample crucible: a tungsten electrode 14 as the reference electrode, a tungsten electrode 15 as the working electrode for cyclic voltammetric measurements, a tungsten electrode 16 as a counter-electrode for cyclic voltammetric measurements, an electrode 17 for determining the corrosiveness of the salt melt by means of potentiodynamic polarization measurements, and an electrode 18 and an electrode 19 as the cathode and anode for electrolytic purification. The autoclave is made of the alloy 1.4876 (Incoloy® 800H). The electrode 17 is also made of Incoloy® 800H in order to be able to determine the corrosiveness of the salt melt towards this alloy. Incoloy® 800H is an iron alloy containing 30.52% by weight nickel, 20.47% by weight chromium, 0.58% by weight manganese, 0.50% by weight silicon, and 0.07% by weight carbon.

[0063] A mixture comprising 20 mole % NaCl, 20 mole % KCl and 60 mole % MgCl.sub.2 was employed as the salt melt. At room temperature, 140 g of the salt mixture was evacuated in the sample crucible 23 at room temperature, and then heated at 200° C. under an argon atmosphere. The temperature was maintained at 200° C. for one hour under an argon atmosphere, in order to dehydrate the salt and thus reduce side reactions to form hydroxides. Subsequently, the mixture was heated at 500° C., wherein the salt mixture underwent a transition to the liquid phase.

1. Comparative Example—Process According to the Prior Art

[0064] In a comparative experiment, a tungsten electrode was employed as the cathode 18, and a magnesium electrode as the anode 19, for the electrolysis in accordance with the process from the prior art. The electrolysis was performed for 60 minutes under a voltage of 0.5-0.7 V. In the course of the electrolysis, a fast decrease of the measured current was seen, which can be attributed to the formation of MgO on the tungsten cathode and thus to the passivation of the cathode. The decrease of current is shown in FIG. 4 as a function of time. Already within 4 minutes, the current decreases below the short-circuit current of 105 mA. The short-circuit current corresponds to the spontaneous flow of current when the unused magnesium anode is connected to the tungsten cathode without applying a voltage. The deposition of MgO at the cathode can be observed optically. In FIG. 5, the tungsten cathode is shown before the electrolysis (top), and after the electrolysis (bottom). By using energy-dispersive X-ray spectroscopy (EDS), it could be confirmed that the deposits at the tungsten cathode are MgO. In FIG. 6, corresponding EDS spectra are shown.

[0065] By using cyclic voltammetry, it was determined that only 15% of the MgOHCl contaminants could be removed after 60 minutes of electrolysis. FIG. 7 shows corresponding cyclic voltammetric measuring curves before the electrolysis and after the electrolysis. The measurements were performed with a potential feed rate of 200 mV/s. The contact area between the tungsten electrode 15 (working electrode) and the salt melt was 0.16 cm.sup.2.

2. The Process According to the Invention

[0066] In an Example according to the invention, a magnesium electrode was employed for the electrolysis for both electrode 18 and electrode 19, wherein electrodes 18 and 19 were alternately employed as the anode and cathode. The salt melt was prepared as described above. After the salt melt had been heated to 500° C., the content of contaminants was determined by cyclic voltammetry. The measurement was performed in the same way as for the Comparative Experiment. The corresponding cyclic voltammogram is shown in FIG. 8. A clear signal is found at about −0.5 V, which is caused by the reduction of MgOH.sup.+ to MgO and H.sub.2. Before the electrolysis, the peak current was about 50 mA, which corresponds to a peak current density of 313 mA/cm.sup.2, when the contact area between the working electrode and salt melt is 0.16 cm.sup.2. Since the peak current density is proportional to the MgOH.sup.+ concentration, it can be concluded that the concentration of MgOH.sup.+ in the melt was at 11938±2379 ppm O.

[0067] The electrolysis was performed for 120 minutes. A voltage with an absolute value of 0.8 V was applied between the two magnesium electrodes 18 and 19, wherein the direction or the sign of the voltage was swapped every 3 seconds. FIG. 9 shows the course of the measured current as a function of time. It is found that the current remains at a high value of more than 200 mA, especially within the first 15 minutes. The sharp drop of about 600 mA to about 200 mA within the first 15 minutes can be attributed to an initially strongly decreasing concentration of MgOH.sup.+. The leaps in the current can be attributed to the removal of deposits on the electrodes (falling off of MgO),

[0068] After the electrolysis, the content of MgOH.sup.+ was again determined by cyclic voltammetry, the cyclic voltammogram being shown in FIG. 8. The peak current for the reduction of MgOH.sup.+ dropped to 25 mA, the concentration of MgOH.sup.+ was thus reduced to 5969±1188 ppm O, which is half the original value. The corrosiveness of the original salt melt and of the salt melt purified according to the invention were determined by means of potentiodynamic polarization measurements. FIG. 10 shows polarization curves for Incoloy® 800H in the original salt melt and in the salt melt purified according to the invention at 500° C. Electrode 17, which had a contact area to the salt melt of 7.6 cm.sup.2, was employed as the working electrode. The tungsten electrodes 14 and 16 were employed as the counter-electrode and reference electrode, respectively. The potential feed rate was 1 mV/s. Using the Tafel equation, the corrosion current was determined from the potentiodynamic polarization curve, which corrosion current was 10 mA for Incoloy® 800H for the original salt melt (corrosion current density: 1.32 mA/cm.sup.2), which corresponds to a corrosion rate of 15 mm/year according to Faraday's law. The corrosion current of the salt melt purified according to the invention was only 2.8 mA (corrosion current density: 0.42 mA/cm.sup.2), which corresponds to a corrosion rate of 4.2 mm/year. Thus, the corrosiveness of the salt melt could be lowered to 28% of the original value by the purification according to the invention.