EP3941879A1 - Verfahrenstechnik für halogensalze mit zwei identischen elektroden - Google Patents
Verfahrenstechnik für halogensalze mit zwei identischen elektrodenInfo
- Publication number
- EP3941879A1 EP3941879A1 EP20712909.9A EP20712909A EP3941879A1 EP 3941879 A1 EP3941879 A1 EP 3941879A1 EP 20712909 A EP20712909 A EP 20712909A EP 3941879 A1 EP3941879 A1 EP 3941879A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- electrode
- molten salt
- cathode
- anode
- electrodes
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F5/00—Compounds of magnesium
- C01F5/02—Magnesia
- C01F5/04—Magnesia by oxidation of metallic magnesium
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
- C25C3/04—Electrolytic production, recovery or refining of metals by electrolysis of melts of magnesium
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
- C01D3/00—Halides of sodium, potassium or alkali metals in general
- C01D3/14—Purification
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F5/00—Compounds of magnesium
- C01F5/26—Magnesium halides
- C01F5/30—Chlorides
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
- C25C7/005—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells of cells for the electrolysis of melts
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
- C25C7/02—Electrodes; Connections thereof
Definitions
- the present invention relates to methods and devices for reducing impurities in molten salts, which are used as thermal energy storage in solar thermal power plants, a molten salt being purified in an electrochemical process by applying a voltage between two electrodes.
- the voltage is varied so that different electrodes act as cathodes or anodes in different phases.
- Molten salt has been used for some years in solar thermal power plants as thermal energy storage and enables the efficient large-scale transmission and storage of thermal energy.
- So-called solar salt for example, is used as the heat transfer and storage medium in prior art solar thermal power plants, which is a mixture of two nitrate salts consisting of 40% by weight of potassium nitrate and 60% by weight of sodium nitrate.
- the temperature at which such salt melts can be used is limited by the melting temperature of the salt and its decomposition temperature.
- the maximum operating temperature of such nitrate salts is around 560 ° C. At higher temperatures, decomposition occurs.
- Alternative molten salts that can be used at higher temperatures could improve the efficiency of downstream processes in solar power plants (e.g.
- Salt melts with a higher decomposition temperature for example halogen salt melts, in particular chloride salt melts, the latter having decomposition temperatures of more than 800 ° C., come into consideration as an alternative to nitrate-based solar salt.
- halogen salt melts in particular chloride salt melts, the latter having decomposition temperatures of more than 800 ° C.
- mixtures of MgC / KCl / NaCl show good properties.
- the problem here, however, is that such molten salts are often highly corrosive to metals due to impurities.
- the salts Upon contact with air or moisture, the salts form hydrates, whose decomposition during the heating and melting process results in the formation of corrosive oxygen and / or hydrogen-based impurities, in particular hydroxy compounds, dissolved hydrogen ions (H + ), dissolved oxygen or dissolved water in the Melting salt leads. Furthermore, air can also come into contact with the molten salt during use, in which case corrosive oxygen and / or hydrogen-based impurities can be formed. In order to be able to use high-temperature ursalzschmelzen as a thermal transfer and storage medium, processes are therefore required that can reduce the corrosiveness of the salt melts by purifying the salt melt or removing oxygen- and / or hydrogen-based impurities.
- a method for purifying high-temperature molten salts containing oxygen and / or hydrogen-based impurities is disclosed in the as yet unpublished US patent application 16/003229, the content of which is hereby incorporated by reference. It describes how to purify a molten salt using electrolysis.
- the molten salt is brought into contact with two electrodes, between which an electrical voltage is applied.
- One of the electrodes acts as a cathode, at which hydroxide-based impurities are converted into oxides and hydrogen by reduction, with the oxides precipitating out of the molten salt due to their higher melting point.
- the other electrode functions as the anode, whereby the electrode material is dissolved by oxidation and becomes part of the molten salt.
- Cathode material materials that are electrochemically inert at the voltages applied, such as tungsten, silver, gold, platinum, palladium or nickel alloys.
- Alkali metals, alkaline earth metals, transition metals or semi-metals with a low reduction potential are used as anode material.
- the present invention is therefore based on the object of providing a method for the electrolytic purification of molten salts that avoids the disadvantages of the prior art.
- this object is achieved by a method for purifying salt melts, comprising the following steps:
- the voltage is varied so that during at least a first phase the at least one first electrode acts as a cathode and the at least one second electrode acts as an anode, and during at least a second phase the at least one first electrode acts as an anode and the at least one At least one second electrode acts as a cathode.
- the method differs from the prior art in particular in that the applied voltage is varied so that different electrodes act as cathodes or anodes at different times. Surprisingly, it has been shown that this can reduce or slow down passivation of the cathode.
- an electrode that acts as an anode is gradually dissolved by oxidation of the anode material, the cations formed being transferred to the molten salt.
- electrolysis products that were deposited on the electrode while the electrode previously acted as a cathode can be released.
- Step ii) is to be understood to mean that the electrodes are not spatially in contact with one another, that is, do not touch one another.
- the applied voltage is varied so that the at least one first electrode and the at least one second electrode alternately act as an anode, the respective other electrode acting as a cathode.
- a first voltage and a second voltage are preferably applied alternately, the second voltage corresponding to the first voltage with the opposite sign.
- the voltages can also have opposite signs and different amounts.
- the voltage change can be carried out continuously or discontinuously. So that the function of the two electrodes as cathode or anode can change, the sign of the voltage must be reversed, so that the direction of an electrical current flowing between the electrodes is also reversed.
- the voltage is with a period length in a range from 0.1 to 10 seconds, in particular in a range from 1 to 5 seconds, that is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, in particular 1, 2, 3, 4 or 5 seconds, varies.
- a positive voltage is applied for a period of 4 seconds.
- a voltage with a negative sign is applied - again for a period of 4 seconds. This change is repeated periodically.
- the voltage can also be varied with a shorter or longer period length. However, in this case too long a period length should be avoided.
- the passivation of an electrode acting as a cathode continues to such an extent that the electrode can then no longer function efficiently as an anode or the passivation is no longer completely reversible. Therefore, shorter period lengths are preferred. If the change is too fast, there is a risk of overload.
- a pause can be provided between the voltage changes, during which no voltage is applied in order to avoid an overload when changing ver.
- This pause depends on the applied voltage and the duration of the period. Suitable pause times are 0.1 to 3 seconds, in particular 0.5 to 2 seconds, preferably 1 second.
- An exemplary voltage change is shown in FIG. 11. This shows the two phases of alternating voltage, the phase duration being denoted by t. Pauses are denoted by p.
- the measured electrical current between the electrodes decreases, which can be attributed on the one hand to a falling concentration of impurities and on the other hand to an increasing passivation of the electrode acting as a cathode.
- the voltage can therefore also be varied as a function of a measured current intensity between the electrodes, the sign of the voltage applied between the electrodes being changed after the current intensity has fallen below a certain threshold value.
- the applied voltage and the frequency with which the voltage is varied can also be changed during the course of the process.
- a significantly higher voltage can be applied than is required for the electrolysis in order to additionally remove electrolysis products adhering to an electrode.
- this can stimulate the formation of gases on an electrode, which can lead to the flaking off of deposits on the electrode.
- the electrode can be heated by a high voltage, which can also lead to the removal of deposits, in particular due to different thermal expansion of the deposit and the electrode, a partial melting of the deposit or a partial dissolution of the deposit in the molten salt.
- the method according to the invention can include further measures to counteract the passivation of an electrode.
- Deposits on an electrode can also be removed mechanically, in particular by scraping, but grinding. Ultrasound can also be used to remove deposits.
- Heating elements can be introduced into an electrode so that the electrode can be heated up, and deposits can flake off.
- Another possibility is purging with inert gas or stirring the molten salt, whereby pressure is also exerted mechanically on deposits. is practiced.
- the molten salt can also flow past the electrode and thus reduce the formation of deposits on the electrode.
- a halogen salt melt is used as the salt melt.
- a halogen salt is understood to mean any salt which contains at least one ion of fluorine, chlorine, bromine and / or iodine.
- a chloride salt is preferably used. Chloride salts are generally cheaper than other halogen salts, so that the method according to the invention can be carried out cost-effectively.
- suitable chloride salts can also be easily handled in large quantities, since they are not toxic and have no other negative effects on human safety and health. In principle, however, any other halogen salt can also be used.
- the molten salt used contains cations to balance the charge.
- those cations are used in a halogen salt which form a high-temperature stable salt with the halogen anions.
- a salt is regarded as being stable at high temperatures if its decomposition temperature is above 700 ° C., in particular above 1000 ° C., so that it can be used as a high-temperature heat storage and transfer medium.
- liquid salts are preferred which have a low vapor pressure at the maximum operating temperature.
- Cations of the elements Mg, Ca, Na, K, Li, Sr, Ba, Zn, Al, Sn, Fe, Cr, Mn, Ni and / or mixtures thereof are preferably used in a halogen salt.
- Cations of the elements Mg, Ca, Ba, Na and / or K are particularly preferably used in a halogen salt.
- the molten salt can in particular contain MgC, CaC, NaCl, BaC and / or KCl or consist of one of the compounds mentioned or a mixture thereof.
- a mixture of two or more salts can also be used as the molten salt.
- the mixture of the molten salt can thus have lower melting temperatures than individual salts and the temperature range can thereby be expanded or the minimum temperature can be lowered. In this way, the melting temperature, the Cypruska capacity and the vapor pressure can be adjusted through a targeted combination of salts with the appropriate properties.
- the molten salt can first be exposed to a vacuum.
- the molten salt or the salt can be exposed to a vacuum at a temperature in a range from 20 ° C. to 300 ° C.
- the temperature is preferably in a range from 80 to 250.degree. C., particularly preferably 100 to 200.degree.
- the salt can be dehydrated by the vacuum treatment, whereby the content of impurities can be reduced by reduced hydrolysis during heating.
- the process according to the invention is preferably carried out under an inert gas atmosphere.
- an inert gas atmosphere therefore denotes an atmosphere that is largely free of water and / or oxygen or preferably free of water and oxygen.
- suitable inert gases are nitrogen or argon.
- At least two electrodes are used, each of which acts as a cathode or anode at different times.
- more than two electrodes can also be used, with each electrode being able to act as an anode or cathode in phases due to the variable application of voltages between the electrodes.
- You can do this more than two electrodes are divided into groups, which act alternately as cathode or anode, or the individual electrodes are also used individually in phases as an anode and in phases as a cathode.
- 3, 4, 5, 6, 7, 8, 9, 10 or more electrodes are used. The number of electrodes depends on the space required and the amount of molten salt or impurities.
- each electrode that was used as a cathode in phases is used as an anode in a different phase, with the passivation of the electrode being avoided or reduced in particular by dissolving the anode material.
- an electrode that acts as a cathode electrons are provided, where in particular an oxygen- and / or hydrogen-based contamination is removed from the molten salt by reduction.
- the required electrons are provided by an oxidation reaction on an electrode that acts as an anode, in particular the anode material itself is oxidized and the cations formed pass into the molten salt.
- the at least two electrodes are both used in phases as anode and must therefore comprise a material that is suitable as an anode in the method according to the invention. So that the anode can provide electrons by oxidation, each of the electrodes must comprise a material that has a suitable reduction potential so that by applying a suitable voltage, an oxygen- and / or hydrogen-based impurity at the cathode can be reduced and the anode can be oxidized at the same time.
- the at least one first electrode and / or the at least one second electrode comprise a material with a reduction potential that is not greater than the reduction potential of an oxygen- and / or hydrogen-based contamination.
- the reduction potential is preferably not greater than the reduction potential of an in The element used as the anion of the molten salt, since otherwise an undesired oxidation of the molten salt could take place at the anode.
- the reduction potential is preferably not less than the reduction potential of an element used as a cation in the molten salt, since otherwise an undesired reaction of the electrode with the molten salt could take place.
- the at least one first electrode and / or the at least one second electrode preferably comprise a material with a normal potential (reduction potential) in a range from -0.1 to -3.1 V, preferably in a range from -0.4 to -2.95 V. Particularly preferred one or both electrodes are made of such a material.
- the normal potential describes the electrical potential difference between the electrode and a standard hydrogen electrode (2 H + + 2 e -> H2) under standard conditions.
- the at least one first electrode and / or the at least one second electrode preferably comprise a material which is at a temperature of 500 ° C in a molten salt compared to a tungsten electrode immersed in the molten salt (W 2+ + 2 e -> W) Reduction potential in a range from -0.6 to -1.6 V, preferably in a range from -0.8 to -1.5 V, has.
- one or both electrodes can consist of such a material.
- the at least one first electrode and the at least one second electrode can comprise different materials or consist of the same material.
- the same material is preferably used for both electrodes. Both electrodes are then equally suitable as anode and cathode.
- the at least one first electrode and / or the at least one second electrode comprises an alkali metal, an alkaline earth metal, a transition metal and / or a semimetal.
- Suitable alkali tals are in particular lithium, sodium, potassium or mixtures thereof.
- Ge suitable alkaline earth metals are in particular magnesium, calcium, strontium, barium or mixtures thereof.
- Suitable transition metals are in particular cobalt, nickel, iron, zinc or mixtures thereof.
- Suitable semimetals are in particular boron, silicon or mixtures thereof.
- the reactive alkali metals and alkaline earth metals described can also be located in a matrix structure made of an inert material (for example steel) in order to improve the mechanical stability or to minimize passivation.
- Electrolyte reaction 1 2 AOHB -> 2 AOH + + 2B
- Electrolyte reaction 2 A 2+ + 2B -> AB 2
- Electrolyte reaction 1 2 MgOHCI -> 2 MgOH + + 2 CI
- magnesium is used as the electrode material. Magnesium is oxidized at the anode, whereby the electrode is partially dissolved. The Mg 2+ ions formed are transferred to the molten salt.
- MgOHCI is a typical oxygen-based impurity in MgC melts. In the melt it is in the form of MgOH + and Cl ions. When a voltage is applied, MgOH + reacts at the cathode, forming hydrogen to form MgO. Due to its high melting point and its poor solubility in the molten salt, MgO does not pass into the molten salt, but remains on the cathode or is deposited as a precipitate.
- the electrode used as cathode is passivated during the course of the process. According to the present invention, however, this electrode is used as an anode at a later point in time, with the material of the electrode being dissolved. It is assumed that deposited precipitates (MgO) are released at the electrode, without the present invention being restricted to this theory.
- an anode material is dissolved, with cations being formed which remain in the molten salt.
- An element whose cations are already part of the molten salt used is therefore preferably used as the electrode material.
- MgC is used as the salt, it is particularly preferred to use electrodes made of magnesium.
- An inserted electrode is used up in particular during the process according to the invention. A used electrode can therefore be regularly replaced by a new one.
- Another possibility is the constant supply of electrode material in the form of a wire, strip or a foil.
- the electrode preferably has a large surface and can also be used, for example, in the form of a sheet metal, a mesh, an open-pored foam or a perforated sheet.
- the method according to the invention particularly preferably comprises the following steps:
- molten salt has at least one oxygen and / or at least one hydrogen-based impurity, in particular at least one oxygen-based impurity
- the voltage is varied so that during at least a first phase the at least one first electrode acts as a cathode and the at least one second electrode acts as an anode, and during at least a second phase the at least one first electrode acts as an anode and the at least one second electrode acts as a cathode,
- the molten salt has chloride salts of Mg, Na and K.
- the molten salt particularly preferably comprises MgC, NaCl and KCl and is preferably composed of these three salts.
- a Mg anode is particularly preferably used as the anode.
- the oxygen-based contamination is in particular oxides or hydroxides of Mg, Na and / or K, in particular oxides or hydroxides of Mg.
- FIG. 1 illustrates the method according to the invention by way of example for MgC as molten salt and electrodes which consist of magnesium.
- MgC as molten salt and electrodes which consist of magnesium.
- FIG. 1 illustrates the method according to the invention during the electrochemical purification of a molten salt between an Mg cathode and an Mg anode is switched back and forth (by reversing an applied voltage) to avoid the passivation of the cathode by the MgO formed.
- FIG. 1 illustrates the method according to the invention by way of example for MgC as molten salt and electrodes which consist of magnesium.
- MgO is generated on the surface of the cathode (left Mg electrode), which leads to passivation of the electrode. Therefore, before irreversible passivation occurs (for example 3.5 seconds after the first phase), the function of the two electrodes is changed in a second phase, as shown on the right-hand side of FIG.
- the left Mg electrode with MgO deposits on the surface is used as the anode
- the right Mg electrode is used as the cathode. Due to the reaction of Mg to Mg 2+ on the anode, the MgO deposited on the surface falls off and the electrode surface is renewed.
- impurities such as MgOH + can be removed while avoiding passivation of the cathode.
- the formation of MgO on the cathode, the fall of MgO and the renewal of the electrode surface can be confirmed by microstructural analysis methods such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD).
- SEM scanning electron microscopy
- EDX energy dispersive X-ray spectroscopy
- XRD X-ray diffractometry
- the process according to the invention is preferably carried out at a temperature which is well below the decomposition temperature of the molten salt used. In a preferred embodiment, the process according to the invention is carried out at a temperature in a range from 300 to 800.degree.
- the process is carried out at a temperature in a range from 390 to 650 ° C., preferably at a temperature in a range from 450 to 600 ° C., particularly preferably in a range from 480 to 550 ° C., in particular at about 500 ° C carried out.
- the preferred salts NaCl, KCl, MgC and / or CaC are in solid form below a temperature of 350 ° C., depending on the mixture.
- the system has a NaCl-KCl-MgCh mixture with a minimum melting temperature of approx. 380 ° C.
- the process is carried out at a temperature which is higher than the melting temperature of the salt used.
- one or more of the electrodes used can be in liquid or solid form.
- alkali metals such as Li, Na and / or K are used as the electrode material
- one or more electrodes can be in liquid form. Because of the density differences between the molten salt and the electrode, a liquid electrode can float on the molten salt, for example.
- Alkali metals are preferably used in combination with alkali metal salt melts, which prevents foreign ions from being transferred into the salt melt as a result of the dissolution of the anode and disadvantageously changing its properties.
- the electrode is preferably in solid form.
- alkaline earth metals are preferably used in solid form.
- magnesium is used as the electrode material, the process is preferably carried out at a temperature of 650 ° C or less done.
- the use of a fixed electrode has the advantage that the material of the electrode cannot mix with the molten salt and then settle as a precipitate in lines, valves and pumps that transport the molten salt and are operated below the melting temperature of the electrode material can.
- the concentration of oxygen and / or hydrogen-based impurities in the molten salt can be determined before, during and after the process, for example with the aid of cyclic voltammetry.
- a corresponding method is described in the unpublished US patent application 16/003229 be, to which reference is made here in full.
- a detailed description of cyclovoltammetric measurements to determine impurities in molten salt was also by W. Ding et al. (Electrochemical Measurement of Corrosive Impurities in Molten Chlorides for Thermal Energy Storage, Journal of Energy Storage. 2018; 15: 408-414), to which reference is also made in full.
- the method according to the invention can be used in particular in connection with heat storage systems based on high-temperature molten salts.
- a high-temperature salt melt can, for example, be purified with the aid of the method according to the invention before it is used.
- the method according to the invention can also be used as a heat store or during the use of a high-temperature molten salt
- a molten salt in a storage container can be purified using the method according to the invention or can be continuously obtained in as pure a state as possible.
- the concentration of impurities can also be continuously monitored, for example by means of cyclic voltammetry, and if necessary a purification can be carried out.
- part of the molten salt can be transported from the Vorratsbe into a separate device for purifying molten salt and transported back to the storage container after the purification.
- the object on which the invention is based is achieved by a device for purifying salt melts using the method according to the invention, comprising at least one device for cyclic voltammetric measurements and at least one device for electrochemical purification, the device for electrochemical purification having an anode and comprises a cathode, characterized in that the anode and the cathode consist of the same material.
- the device for electrochemical purification has at least two electrodes made of the same material. Both electrodes are thus suitable as a cathode and an anode. According to the invention, the material must have a suitable reduction potential so that by applying a suitable voltage, an oxygen- and / or hydrogen-based contamination on the cathode can be reduced and the anode can be oxidized at the same time.
- the device for cyclic voltammetric measurements preferably comprises a reference electrode, a working electrode and a counter electrode.
- the electrodes can in particular consist of a material, such as tungsten, which is electrochemically inert under the conditions of the purification of molten salts according to the invention.
- the device according to the invention for purifying molten salts comprises two devices for cyclic voltammetric measurements.
- the device according to the invention can in particular be connected to a Vorratsbefflel ter for a high-temperature molten salt.
- the molten salt can be transported from the storage container into the device according to the invention, purified there and then transported back into the storage container.
- the device according to the invention can accordingly be used to keep a high-temperature molten salt in a storage container as free as possible of oxygen- and / or hydrogen-based impurities during the period of use.
- the device according to the invention can furthermore comprise a heat exchanger.
- a heat exchanger is particularly advantageous when the purification of molten salts in the device according to the invention is to be carried out at a different temperature than the storage temperature of the molten salt in the storage container.
- heat exchanger is a countercurrent heat exchanger. The heat exchanger improves the efficiency of the process if the purification of the molten salt is to be carried out at a temperature other than the storage temperature.
- the device according to the invention preferably further comprises a temperature control unit in order to control the temperature of the electrochemical purification and thus its efficiency.
- the device according to the invention comprises a cold trap with a wall temperature close to the liquidus temperature of the molten salt in order to enable further processing through the precipitation of impurities. This is particularly the case when the solubility of the impurity is reduced at low temperatures. This means that contaminants that are dissolved at a high temperature can be deposited in the cold trap in a targeted manner.
- a device according to the invention is shown by way of example in FIG.
- the device comprises a device for electrochemical purification 1 with two electrodes 2, which consist of the same material and both can be used alternately as anode or cathode.
- the device according to the invention comprises two devices for cyclic voltammetric measurements 3, 4, each of which comprises a reference electrode, a working electrode and a counter electrode.
- the electrodes are immersed in a high-temperature molten salt which is located in a container 6.
- a gas phase 5 is located above the high temperature molten salt.
- the container 6 is connected to a storage container 8 by lines 9.
- a line 9 leads from the storage container 8 through a heat exchanger 7 into the container 6.
- Another line 9 leads from the container 6 through the heat exchanger 7 back to the storage container 8.
- Fig. 2 the flow direction of a high temperature salt melt is shown by arrows.
- the device for electrochemical cleaning 1 lies in the direction of flow between the two devices for cyclic voltammetric measurements 4 and 3, so that with the device for cyclic voltammetric measurements 4 the concentration of impurities before the electrochemical purification and with the device for cyclic voltammetric measurements 3 the concentration Impurities can be determined after the electrochemical purification.
- the following examples relate to the purification of chloride salt melts and were carried out using an autoclave device as shown in FIG. 3 Darge is.
- the device comprises a tubular furnace 22, control devices, which in particular comprise a temperature control unit, a metal container 24 and a sample crucible 23 which is inert to the salts used.
- the sample space is connected to an argon container 20 and a vacuum pump 21 in order to control the atmosphere in the sample space can.
- a tungsten electrode 14 as a reference electrode
- a tungsten electrode 15 as a working electrode for cyclic voltammetric measurements
- a tungsten electrode 16 as a counter electrode for cyclic voltammetric measurements
- an electrode 17 for determining the corrosivity of the molten salt by means of potentiodynamic polarization measurements
- an electrode 18 and one Electrode 19 as a cathode and anode for electrolytic cleaning.
- the autoclave is made of the alloy 1.4876 (Incoloy® 800H).
- the electrode 17 also consists of Incoloy® 800H in order to be able to determine the corrosiveness of the molten salt with respect to this alloy.
- Incoloy® 800H is an iron alloy containing 30.52% by weight of nickel, 20.47% by weight of chromium, 0.58% by weight of manganese, and 0.50% by weight of silicon and contains 0.07 weight percent carbon.
- a mixture with 20 mol% NaCl, 20 mol% KCl and 60 mol% MgC was used as the molten salt.
- 140 g of the salt mixture were evacuated in the sample crucible 23 at room temperature and then heated to 200 ° C. under an argon atmosphere. The temperature was kept at 200 ° C. for one hour under an argon atmosphere in order to dehydrate the salt and thus reduce side reactions to hydroxides. The mixture was then heated to 500 ° C., the salt mixture changing into the liquid phase. 1. Comparative example - prior art method
- a tungsten electrode was used as the cathode 18 for the electrolysis and a magnesium electrode as the anode 19 in accordance with the method from the prior art.
- the electrolysis was carried out for 60 minutes under the voltage of 0.5-0.7V.
- the decrease in current intensity is shown as a function of time in FIG. The current strength drops below the short-circuit current strength of 105 mA within 4 minutes.
- the short-circuit amperage corresponds to the spontaneous current flow when the unused magnesium anode is connected to the tungsten cathode without a voltage being applied.
- the deposition of MgO on the cathode can be observed optically.
- the tungsten cathode is shown before the electrolysis (above) and after the electrolysis (below).
- EDS energy dispersive X-ray spectroscopy
- a magnesium electrode was used for the electrolysis both for the electrode 18 and for the electrode 19, the electrodes 18 and 19 being used alternately as anode and cathode.
- the molten salt was prepared as described above. After the molten salt was heated to 500 ° C., the content of impurities was determined by means of cyclic voltammetry. The measurement was carried out as for the comparative example. The corresponding cyclic voltammogram is shown in FIG. There is a clear signal at around -0.5 V, which is caused by the reduction of MgOH + to MgO and H2.
- the peak current was approximately 50 mA, which corresponds to a peak current density of 313 mA / cm 2 for the contact area between the working electrode and molten salt of 0.16 cm 2 . Since the peak current density is proportional to the MgOH + concentration, it can be concluded that the concentration of MgOH + in the melt was 11938 ⁇ 2379 ppm O.
- the electrolysis was carried out for 120 minutes.
- a voltage of 0.8 V was applied between the two magnesium electrodes 18 and 19, the direction or sign of the voltage being changed every 3 seconds.
- 9 shows the course of the measured current intensity as a function of time. It can be seen that the current remains at a high value of more than 200 mA, especially within the first 15 minutes.
- the sharp drop from about 600 mA to about 200 mA within the first 15 minutes can be traced back to an initially sharp drop in the concentration of MgOH + .
- the jumps in the current strength can be attributed to the removal of deposits on the electrodes (MgO falling off).
- the corrosiveness of the original molten salt and of the molten salt purified according to the invention were determined by means of potentiodynamic polarization measurements.
- 10 shows polarization curves for Incoloy® 800H in the original molten salt and the molten salt purified according to the invention at 500 ° C.
- the working electrode used was electrode 17, which had a contact area of 7.6 cm 2 with the molten salt.
- the tungsten electrodes 14 and 16 were used as the counter electrode and reference electrode.
- the potential feed rate was 1 mV / s.
- the corrosion current was determined from the potentiodynamic polarization curve, which was 10 mA for the Incoloy® 800H for the original molten salt (corrosion current density 1.32 mA / cm 2 ), which, according to Faraday's laws, was a corrosion rate of 15 mm / Year.
- the corrosion current of the salt melt purified according to the invention was le diglich 2.8 mA (corrosion current density 0.42 mA / cm 2 ), which corresponds to a corrosion rate of 4.2 mm / year.
- the purification according to the invention made it possible to reduce the corrosiveness of the molten salt to 28% of the original value.
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DE102019107393.6A DE102019107393A1 (de) | 2019-03-22 | 2019-03-22 | Verfahrenstechnik für Halogensalze mit zwei identischen Elektroden |
PCT/EP2020/057414 WO2020193305A1 (de) | 2019-03-22 | 2020-03-18 | Verfahrenstechnik für halogensalze mit zwei identischen elektroden |
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EP20712909.9A Pending EP3941879A1 (de) | 2019-03-22 | 2020-03-18 | Verfahrenstechnik für halogensalze mit zwei identischen elektroden |
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US (1) | US20220186392A1 (de) |
EP (1) | EP3941879A1 (de) |
DE (1) | DE102019107393A1 (de) |
MA (1) | MA55358A (de) |
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CN113603059B (zh) * | 2021-07-12 | 2022-11-29 | 中国科学院上海应用物理研究所 | 一种熔盐及熔盐的电化学净化方法、电化学装置 |
DE102021131250A1 (de) | 2021-11-29 | 2023-06-01 | Deutsches Zentrum für Luft- und Raumfahrt e.V. | Kontinuierliches Reinigungssystem für Halogensalze |
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US4088550A (en) * | 1977-05-25 | 1978-05-09 | Diamond Shamrock Corporation | Periodic removal of cathodic deposits by intermittent reversal of the polarity of the cathodes |
JPH1121690A (ja) * | 1997-07-07 | 1999-01-26 | Mitsubishi Materials Corp | 金属ウランの精製方法 |
US6676824B2 (en) * | 2001-07-18 | 2004-01-13 | Hatch Associates Ltd. | Process for purification of molten salt electrolytes |
GB0422129D0 (en) * | 2004-10-06 | 2004-11-03 | Qinetiq Ltd | Electro-reduction process |
CN106283112A (zh) * | 2015-05-11 | 2017-01-04 | 中国科学院上海应用物理研究所 | 熔盐的电化学净化方法 |
GB2552526A (en) * | 2016-07-28 | 2018-01-31 | Siemens Ag | Electrochemical method of ammonia generation |
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2020
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- 2020-03-18 EP EP20712909.9A patent/EP3941879A1/de active Pending
- 2020-03-18 WO PCT/EP2020/057414 patent/WO2020193305A1/de active Application Filing
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MA55358A (fr) | 2022-01-26 |
DE102019107393A1 (de) | 2020-09-24 |
US20220186392A1 (en) | 2022-06-16 |
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