How to Adjust Oxo Acidity

Abstract

The present invention relates to a method for adjusting the oxo-acidity of a molten metal hydroxide salt, the method comprising the steps of: 2 O, O 2- , O.H. - providing an oxo-acidity control component; and contacting said oxo-acidity control component with said molten salt of a metal hydroxide to adjust the oxo-acidity of said molten salt of a metal hydroxide. The method allows better utilization of the available temperature range for the molten salt of a metal hydroxide by reducing the corrosiveness of the metal hydroxide.

Description

[Technical field]

[0001] The present invention relates to a method for adjusting the oxo-acidity of a molten metal hydroxide salt, which allows better utilization of the available temperature range of the molten salt of the metal hydroxide by reducing the corrosiveness of the metal hydroxide. [Background technology]

[0002] Although molten salts are generally highly corrosive, their physical and chemical properties make them attractive for certain applications: in particular, molten hydroxide salts may be useful as neutron moderators in nuclear fission processes, and can be used over a wider temperature range than molten salts such as chlorides, nitrates, and carbonates, which are useful for energy storage, etc.

[0003] Despite the corrosive nature of molten metal hydroxide salts, their use as a neutron moderator in nuclear fission processes has been described. For example, WO 2020 / 157247 uses single crystal corundum as a corrosion-resistant material in contact with molten hydroxide moderator salts in molten salt fission reactors (MSRs). However, single crystal corundum is expensive, limiting its use as a construction material in large-scale systems.

[0004] Molten salts may contain water and other components that contribute to the defining property of the molten salt, known as its "oxo-acidity." In hydroxide-containing molten salts, the hydroxide ion is an amphoteric species and can accept a proton to form HO, but also donate a proton to form the superoxide ion O. 2- The water contained in the molten salt reacts as shown in Equation 1 and Equation 2.

[0005] [ka]

[0006] [ka]

[0007] In this specification, pH=-log 10 [H + ], pOH=-log 10 [OH - ], we define oxo acidity as pH2O=-log 10 [HO], oxobasicity pO 2- =-log 10 [O 2- ] is defined as

[0008] Oxo acidity can contribute to predicting the stability of certain species in molten salts, as described by BL Tremilon in Chemistry in Non-Aqueous Solvents, Springer Netherlands, Dordrecht, 1974, doi:10.1007 / 978-94-010-2123-4 and Acid-Base Effects in Molten Electrolytes: Molten Salt Chemistry, 1987: pp. 279-303. For example, the representative material alumina is slightly soluble in acidic and neutral melts and very soluble in basic melts. In acidic melts, AlO + In the basic melt, it dissolves as AlO2 - However, Tremilon points out that the combination of oxidizing species and base stabilizes the system, explaining why species that are more easily oxidized are more stable in basic media. Conversely, oxidizing species are generally less stable in acidic systems where bases tend to combine with acidic species, resulting in the favoring of reduced species. However, for many metal alloys there exists an oxo-acidity range within which the alloy can exist in a stable equilibrium under oxo-acidic / oxo-basic conditions. Thus there exists a range of water concentrations in molten hydroxide that is stable enough to be used as a containment material.

[0009] WO2018 / 229265 also discloses an MSR with molten metal hydroxide as moderator salt. The molten moderator salt may contain a redox element with a reduction potential greater than the reduction potential of the material in contact with the molten moderator salt, or may be a chemical species that controls the oxo-acidity of the molten moderator salt, such as water. WO2018 / 229265 suggests bubbling water gas through the molten moderator salt, or using an inert cover gas consisting of a chemical species that controls the redox potential and / or oxo-acidity of the melt, with exemplary chemical species being H2O, H2 and HF. However, WO2018 / 229265 does not disclose how to actually control the oxo-acidity.

[0010] The object of the present invention is to provide a process which makes it possible to utilize the large temperature range between the melting point and the boiling point of molten hydroxide salts in industrial applications and further to provide a process which makes it possible to use molten hydroxides on a large scale. Summary of the Invention

[0011] The present invention relates to a method for adjusting the oxo-acidity of a molten metal hydroxide salt, the method comprising the steps of: 2- , and O.H. - providing an oxo-acidity control component; and contacting the oxo-acidity control component with a molten salt of a metal hydroxide to adjust the oxo-acidity of the molten salt of the metal hydroxide. 2- , and O.H. -The concentration of one or more of the metal hydroxides may be adjusted to correspond to a corresponding target concentration, for example to provide oxo-neutral conditions. By adjusting the oxo-acidity, corrosion in the molten metal hydroxide salt can be mitigated, and therefore the method can also be considered as a method of adjusting oxo-acidity to mitigate corrosion in the molten metal hydroxide salt, or a method of mitigating corrosion in the molten metal hydroxide salt. In an embodiment of the method, the oxo-acidity is adjusted to oxo-neutral conditions, for example for a particular lining material, in the step of contacting the oxo-acidity control component with the molten salt of the metal hydroxide to adjust the oxo-acidity of the molten salt of the metal hydroxide. The metal hydroxide may be any metal hydroxide as desired, but the metal hydroxide is preferably an alkali metal hydroxide, for example, sodium hydroxide, potassium hydroxide, lithium hydroxide, or a mixture thereof, or the metal hydroxide may be an earth alkali metal hydroxide, for example, calcium or magnesium hydroxide. Similarly, the metal hydroxide may be a hydroxide of a different metal.

[0012] In one example, a method for adjusting the oxo-acidity of a molten metal hydroxide salt includes providing a vessel having an inner surface made from a material of interest, the vessel containing a molten salt of the metal hydroxide, the vessel having a heat source and / or heat sink configured to create a temperature gradient in the range of 0.1° C. / cm to 10° C. / cm in the molten salt of the metal hydroxide, and 2- , and O.H. - the oxo-acidity control component; and contacting the oxo-acidity control component with the molten salt of the metal hydroxide to adjust the oxo-acidity of the molten salt of the metal hydroxide.

[0013] By adjusting the oxo-acidity of the molten salt of the metal hydroxide, the method allows adjusting the oxo-acidity to an optimal value for the material in contact with the molten salt of the metal hydroxide, thereby minimizing the corrosion of the material that would otherwise be caused by the molten hydroxide salt, and thus the method of the present disclosure may be used in any context in which a molten hydroxide salt is useful or appropriate. For example, the method may be used in a molten salt fission reactor (MSR) where the hydroxide salt serves as a moderator for the nuclear fission process, in an energy or heat storage vessel where the hydroxide salt provides a medium for energy or heat storage, or in a scrubber unit operating with molten hydroxide, e.g., pure molten hydroxide. HO and / or O 2- It is particularly preferable to estimate the target concentration of OH even if the molten salt is a molten salt of a metal hydroxide. - Estimating the target concentration of is related as shown in Equation 1 and Equation 2.

[0014] The molten salt of the metal hydroxide can be contacted with the oxo acidity control component in any manner desired, for example, the oxo acidity control component can be in gas, liquid or solid form and can be directly contacted with the molten salt of the metal hydroxide.

[0015] The method may use a process gas comprising an inert carrier gas and an oxo-acidity control component. For example, the oxo-acidity control component may be provided in a process gas comprising an inert carrier gas, and the method may further comprise contacting the process gas comprising the oxo-acidity control component with a molten salt of a metal hydroxide to adjust the oxo-acidity of the molten salt of the metal hydroxide. In this document, an inert gas is any gas that does not react with the molten salt of the metal hydroxide or with a material that comes into contact with the molten salt of the metal hydroxide. Exemplary inert gases are nitrogen (N2) and noble gases, such as helium, neon, argon, and mixtures or combinations thereof. By providing the oxo-acidity control component in the process gas, the amount of oxo-acidity control component that comes into contact with the molten salt of the metal hydroxide can be easily controlled, thereby adjusting and maintaining the oxo-acidity of the molten salt of the metal hydroxide within a window of oxo-acidity that is most suitable for protecting the lining of the vessel in which the molten salt of the metal hydroxide is located.

[0016] In this specification, the oxo acidity control component is an OH acidity control component in a molten salt, particularly a molten salt of a metal hydroxide. - , O 2- , and HO. OH may be any chemical entity, such as an element, molecule, or ion, that can affect the concentration of at least one of OH - , O 2- The effect on the concentration of at least one of OH, OH, and HO may be direct or indirect, and the effect may include, for example, increasing or decreasing the concentration according to Equation 1 and Equation 2. In particular, OH - , O 2- , and HO are all considered oxo acidity control components in the context of the present method; similarly, OH - Or O 2-and molecules containing appropriate counterions are also considered oxo acidity control components. Water, H2O, especially in the form of steam, is a preferred oxo acidity control component. Water, H2O, may also exist as a hydrate in salts or crystals, and salts containing hydrates may also be used as oxo acidity control components. When a salt contains a hydrate, the number of water molecules in the salt is usually written as "xH2O" along with the stoichiometric composition of the salt, and the value of x may be taken to determine the amount of salt to adjust the oxo acidity. Other oxo acidity control components are metal oxide salts, such as oxide salts of the same metal as the metal in a molten salt of a metal hydroxide. OH - , O 2- , and / or molecules capable of binding HO are also considered oxo acidity control moieties herein.

[0017] When a treatment gas is used, the treatment gas is brought into contact with a molten salt of a metal hydroxide. This allows the oxo-acidity control component to also come into contact with the molten salt of a metal hydroxide, and the oxo-acidity of the molten salt of a metal hydroxide can be adjusted. In general, the amount of the oxo-acidity control component brought into contact with the molten salt of a metal hydroxide depends on the concentration of the oxo-acidity control component in the treatment gas, the pressure of the treatment gas, and the amount of treatment gas brought into contact with the molten salt of a metal hydroxide, for example, an amount expressed in unit volume per unit time, for example, m 3 The amount of oxo acidity control component relevant to a particular embodiment of the method is determined by the amount of OH in the molten salt of the metal hydroxide. - , O 2- , and an estimate of at least one target concentration (s) of HO and OH present in the molten salt of the selected oxo acidity control component and metal hydroxide. - , O 2- , and HO.

[0018] The oxo acidity control component can also be added to the molten salt of the metal hydroxide without using a process gas. For example, a solid metal oxide such as lithium or sodium oxide can be added to the molten salt in the form of solid pellets to reduce the OH in the molten salt of the metal hydroxide. - , O 2-In another example, a molten salt of potassium hydroxide hexahydrate can be titrated into a molten salt of a metal hydroxide to produce a molten salt of OH. - , O 2- , H2O. It is also possible to contact an oxide, such as Li2O or Na2O, with a molten salt of a metal hydroxide in which the metal oxide is in a molten state.

[0019] The molten salt of metal hydroxide may be in any type of vessel, pipe or tube when in contact with the process gas. The material in contact with the molten salt of metal hydroxide is referred to herein as the "lining material". The lining material is thus exposed to the molten salt of metal hydroxide. Generally, a material has an "oxo-acidity window" where it is optimally resistant to corrosion. If the oxo-acidity is too high or too low, the lining material cannot be practically used because the corrosion rate by the molten salt of metal hydroxide is too high. The oxo-acidity corresponding to the oxo-acidity window is sometimes referred to as the "oxo-neutral" condition. The vessel has an interior surface made of the lining material. The vessel may be made of any material, such as a metal, metal alloy, ceramic material, or combination thereof, and this material is referred to herein as the vessel material. The interior surface may be the surface of the vessel material such that the lining material is the vessel material, or the vessel material may be coated with a further material to provide the lining material. For example, the container material may be a metal alloy, such as a nickel-based alloy, a nickel-based superalloy, Hastelloy, nickel. As used herein, a nickel-based alloy is an alloy having at least 50% w / w nickel.

[0020] When the metal hydroxide molten salt is in a container, the metal hydroxide molten salt may be stationary or the metal hydroxide molten salt may circulate in the container by natural convection, forced convection or forced circulation. In general, forced circulation involves stirring the metal hydroxide molten salt. Stirring may be accomplished by any method. Natural convection is considered herein to include movement of the metal hydroxide molten salt due to temperature and / or component concentration gradients in the metal hydroxide molten salt without active manipulation to affect convection. The metal hydroxide molten salt is generally considered to be stationary herein if no active measures are taken to create temperature and / or concentration gradients. In contrast, forced convection is considered to include movement of the metal hydroxide molten salt caused by actively introducing temperature and / or concentration gradients, particularly temperature. For example, localized heating of a volume of the metal hydroxide molten salt causes local expansion of the metal hydroxide molten salt near the heat source, resulting in movement of the metal hydroxide molten salt. Similarly, localized cooling of a volume of molten metal hydroxide salt will cause localized contraction of the metal hydroxide molten salt near the heat sink, resulting in movement of the metal hydroxide molten salt. Due to forced convection and forced circulation, the oxo-acidity of the metal hydroxide molten salt is generally uniform, i.e., the oxo-acidity varies within 30% of the average oxo-acidity across the volume of the metal hydroxide molten salt. Forced circulation is herein expressed in terms of volume displacement over time, measured in units per hour (or h -1 For example, the volume displacement is 0.1h -1 From 100 hours -1 Within the range of, for example, 1h -1 From 20 hours -1 In this way, forced circulation and forced convection are advantageous in avoiding situations where local variations in the molten salt of the metal hydroxide create regions where the oxo acidity is outside the oxo acidity window.

[0021] The molten salt of the metal hydroxide is preferably placed in a container, typically with a cover gas above the molten salt of the metal hydroxide, for example the container can have a lid covering the molten salt of the metal hydroxide to provide a closed system, but the lid can also have an opening to control the composition and pressure of the cover gas. The cover gas can be maintained at a pressure above ambient pressure, for example in the range of 1 bar to 10 bar. The cover gas can be an inert gas or a process gas comprising an oxo acidity control component. The cover gas can comprise water vapor as the oxo acidity control component, for example at a partial pressure in the range of 0.01 bar to 2 bar, for example 0.02 bar to 0.5 bar. When the cover gas comprises an oxo acidity control component, the cover gas can be bubbled through the molten salt of the metal hydroxide and recycled to the cover gas, in particular the content of the oxo acidity control component (for example expressed in partial pressure) can be replenished in the cover gas. For example the oxo acidity control component can be added directly to the cover gas, which can be bubbled through the molten salt of the metal hydroxide. By using a process gas containing an oxo-acidity control component as a cover gas and bubbling the cover gas through a molten salt of a metal hydroxide to recirculate the process gas back into the cover gas, an environment in which the oxo-acidity of the molten salt of a metal hydroxide can be easily controlled can be created.

[0022] As an example, a gas is bubbled through a molten salt of a metal hydroxide. The gas may be an inert gas, i.e., an inert gas without an oxo acidity control component, a process gas containing an oxo acidity control component, or a gaseous oxo acidity control component. When the gas contains an oxo acidity control component, the amount of gas bubbled through the molten salt of the metal hydroxide takes into account the intended amount of oxo acidity control component to be contacted with the molten salt of the metal hydroxide, and the amount of gas bubbled through the molten salt of the metal hydroxide may be expressed in terms of the volume of inert gas relative to the volume of the molten salt of the metal hydroxide per unit time, and the units are per hour (or h -1 The volume of inert gas bubbled through the volume of the metal hydroxide molten salt can be 0.1 h -1 From 10 hours -1 range, e.g. 0.5h-1 From 2 hours -1 When gas is bubbled through a volume of a molten salt of a metal hydroxide, the volume of gas bubbled through the volume of the molten salt of a metal hydroxide can be in the range of 2h -1 If the temperature exceeds 100° C., the bubbles may cause forced circulation of the molten salt of the metal hydroxide.

[0023] The oxo acidity control component may be present in the metal hydroxide before the salt is melted, and therefore the oxo acidity control component will also be present in the metal hydroxide salt once molten. However, due to the high temperatures typically used to melt the salt and the potential for reaction of the oxo acidity control component with other components, the content of the oxo acidity control component is not constant over time. For example, the oxo acidity control component may evaporate from the molten salt.

[0024] This method uses OH in a molten salt of a metal hydroxide. - , O 2- , and estimating a target concentration of at least one of HO, O 2- , and O.H. - The target concentration of at least one of the following can be estimated at any temperature at which the metal hydroxide is molten. Generally, at least one temperature is sufficient to provide a useful estimate of the target concentration. However, the concentrations of HO, O 2- , and O.H. -The at least one target concentration of is preferably estimated at at least three different temperatures in the range between the melting point and boiling point of the metal hydroxide salt or between the melting point of the metal hydroxide salt and 1000° C. The temperatures, e.g. the at least three temperatures, are preferably selected to be within the intended temperature operating range of the setup. The at least three temperatures are different, preferably the different temperatures being at least 10° C. apart from each other, but the temperatures are preferably distributed over the temperature range in which the metal hydroxide salt is molten, e.g. the temperatures can be selected at points at least 50° C., at least 100° C. or at least 200° C. apart from each other. For example, the temperatures can include a first temperature, e.g. a “low point temperature”, in the range from the melting point of the metal hydroxide salt to the melting point of the metal hydroxide salt + 100° C., a second temperature, e.g. a “mid point temperature”, in the range of ±50° C. from the midpoint between the melting point and boiling point of the metal hydroxide salt, and a third temperature, e.g. a “high point temperature”, in the range from 100° C. below the boiling point of the metal hydroxide salt to the boiling point of the metal hydroxide salt. HO, O 2- , and O.H. - is estimated at at least three different temperatures within the melting and boiling points of the metal hydroxide salt, particularly where the temperatures are at least 50° C. or at least 100° C. apart from one another, the inventors have surprisingly found that the estimates of the target concentrations are useful over the entire temperature range of the corresponding molten salt of the metal hydroxide, thereby providing a simple approach to utilize the entire temperature range of the metal hydroxide salt.

[0025] In general, the target concentration represents the oxo-acidity window of the material, e.g. the lining material of a vessel containing a molten salt of a metal hydroxide, where the resistance to corrosion is optimal so that corrosion is minimized; the target concentration may be a point or a range, typically expressed in moles / L or moles / kg. The target concentration generally depends on the lining material, e.g. the chemical composition of the lining material, the temperature range of use. The theoretical analysis of Ni, Cr, and Fe (the main components of typical high-nickel alloys) highlights a common oxo-acidity window in which they are stable in NaOH at 800 °C. In these cases, the H2O concentration, expressed as p(H2O) = -Log[H2O], should be included between 2.5 and 5.6, e.g. between 2.5 and 3.1. Figure 2 shows the theoretical potential oxo-acidity diagrams of Ni, Fe, and Cr metals, with the common theoretical areas of stability highlighted as shaded areas. The thick outline shows the stability window of a molten salt of sodium hydroxide at 800 °C. However, the theoretical potential oxo-acidity diagram in Figure 2 only applies to pure Ni, Fe and Cr metals. The inventors have now found that for alloys containing about 90% w / w Ni, the oxo-acidity window for HO is in the range of 0.1 to 40 mmol HO per Kg of molten metal hydroxide, preferably in the range of 1 to 15 mmol HO per Kg of molten metal hydroxide. Thus, a target concentration can be defined for a particular lining material. The target concentration is OH - , O 2- , and H2O, or OH - , O 2- , and H2O. - , O 2- , and HO contribute to oxo acidity, and OH - , O 2-By estimating the target concentrations of one, two or all three of H2O, H2O, and H2O, and contacting the molten salt of the metal hydroxide with a process gas containing the oxo-acidity control component, the oxo-acidity of the molten salt of the metal hydroxide can be adjusted, in particular controlled, to be within the oxo-acidity range of the lining material according to Henry's law. In general, when the oxo-acidity control component is provided in gas form, it is assumed that the amount of the oxo-acidity control component that dissolves in the molten salt of the metal hydroxide is proportional to the partial pressure of the oxo-acidity control component in contact with, e.g., on, the molten salt of the metal hydroxide (see FIG. 3). This minimizes corrosion of the lining material. In a specific example, the molten salt of the metal hydroxide is placed in a vessel having an inner surface made of a lining material and is filled with OH. - , O 2- At least one target concentration of H2O is defined for the lining material. The inventors have found an empirical correlation between the partial pressure of water vapor in the process gas and the water concentration in the sodium hydroxide molten salt, as shown in Figure 3.

[0026] Without application of an oxo-acidity control component to the molten salt of metal hydroxide, there may be only limited variations in oxo-acidity throughout the volume of the molten salt of metal hydroxide, especially when the molten salt of metal hydroxide is stationary in the vessel or when the molten salt of metal hydroxide is circulating by natural convection. In general, when the molten salt of metal hydroxide is stationary in the vessel, there may be local variations in the oxo-acidity of the molten salt of metal hydroxide, but the oxo-acidity of the molten salt of metal hydroxide more than 20 cm, e.g. more than 50 cm or more than 100 cm from the vessel wall is considered to be limited in the effect of the molten salt of metal hydroxide on the vessel wall. However, the risk of corrosion is particularly pronounced at the interface between the molten salt of metal hydroxide and any material in contact with the molten salt of metal hydroxide, e.g. lining material. In a specific example, the molten salt of the metal hydroxide is placed in a vessel having an inner surface made of a lining material, and the oxo-acidity control component is contacted with the molten salt of the metal hydroxide located at a distance ranging from 0 cm to 100 cm, for example 0 cm to 50 cm or 0 cm to 20 cm from the lining material. In particular, the molten salt of the metal hydroxide located within 100 cm, or within 50 cm or within 20 cm can be contacted with the oxo-acidity control component over a distance from the wall of the vessel, for example the inner surface made of the lining material. In this way, the molten salt of the metal hydroxide may be stationary, for example in the vessel, and the molten salt of the metal hydroxide located more than 100 cm, for example more than 50 cm or more than 20 cm away from the lining material may not be contacted with the oxo-acidity control component, since the molten salt of the metal hydroxide will cause limited corrosion of the material of the inner wall of the vessel, for example the lining material, over this distance from the inner wall of the vessel. For example, the oxo acidity control component, e.g., contained in an inert carrier gas or in gaseous form, can be bubbled through the molten salt of the metal hydroxide at or over a distance ranging from 0 cm to 100 cm, e.g., 0 cm to 50 cm, or 0 cm to 20 cm from the wall of a lining material, e.g., a vessel containing the molten salt of the metal hydroxide.

[0027] The vessel may have any size and shape desired. For example, vessels, particularly storage vessels, have a central volume defined by a distance from the vessel wall. Thus, the vessel has a central volume that is at least 20 cm, at least 50 cm, or at least 100 cm away from the vessel wall. Molten salts of metal hydroxides within the central volume are generally not believed to contribute to corrosion of the vessel's inner walls. An exemplary vessel volume is 1 m 3 10m from 3 In the context of the present method, the vessel may be a pipe or conduit, for example a pipe or conduit for adding a salt of a metal hydroxide, for example in a molten state, to a storage vessel.

[0028] Industrial applications of molten salts of metal hydroxides, particularly when the molten salts of metal hydroxides are used for energy storage, may include being able to add heat to or remove heat from the molten salt of metal hydroxides in order to take advantage of the large temperature range between the melting point and boiling point of the molten salt of metal hydroxides. Thus, in one example, the molten salt of metal hydroxides is disposed in a container, the container including a heat source and / or a heat sink configured to create a temperature gradient in the molten salt of metal hydroxides in the range of 0.1° C. / cm to 100° C. / cm, e.g., 0.1° C. / cm to 10° C. / cm, over a distance in the range of, e.g., 5 cm to 50 cm. Other relevant temperature gradients are in the range of 0.1° C. / cm to 5° C. / cm, 0.15° C. / cm to 2° C. / cm, or 1° C. to 5° C. / cm. The temperature gradient may be defined as the temperature difference and the distance between the points at which the temperature is measured. Typically, the temperature difference is recorded from a reference point, e.g., representing the molten salt of metal hydroxides, and from further points, as required, representing the heat source and / or the heat sink. The temperature gradient may be expressed in terms of a distance, such as the distance from a heat sink to a point in the molten salt of the metal hydroxide, or the distance from a heat source to a point in the molten salt of the metal hydroxide, and the distance may range from 1 cm to 100 cm, for example from 10 cm to 50 cm. Thus, in one example, a temperature gradient recorded, for example, from a heat sink to a point in the molten salt of the metal hydroxide, or from a heat source to a point in the molten salt of the metal hydroxide, may range from 1° C. over 10 cm to 10° C. over 10 cm, or from 10° C. to 100° C. over 50 cm. In general, heat may be added to or removed from the molten salt of metal hydroxide to produce forced convection in the molten salt of metal hydroxide, and is particularly relevant for contacting the molten salt of metal hydroxide over a distance from a lining material, e.g., the wall of a vessel containing the molten salt of metal hydroxide, when the molten salt of metal hydroxide is thus in contact with a heat source or heat sink, and forced circulation is employed to provide a uniform oxo-acidity of the molten salt of metal hydroxide in the range of 0 cm to 100 cm or 0 cm to 50 cm. Thus, the method of the present invention is particularly advantageous for large-scale use of molten metal hydroxide salt for energy storage, since it can protect the inner wall of the vessel, e.g., the lining material.In a specific embodiment, the method is for adjusting the oxo-acidity of a molten metal hydroxide salt in an energy storage system having a container in which the molten metal hydroxide salt is disposed, comprising: H2O, O. 2- , and O.H. - The at least one target concentration of is estimated from theoretical calculations, prior knowledge of the particular material, e.g., pure metal. The molten salt of metal hydroxide is circulated within the vessel by forced convection obtained from a heat sink and / or heat source, configured to create a temperature gradient in the range of 0.1° C. / cm to 10° C. / cm over a distance in the range of 5 cm to 20 cm from the heat sink and / or heat source, as appropriate, to a point in the molten salt of metal hydroxide. For example, the temperature gradient can be at least 20° C. over a distance of 20 cm.

[0029] The use of metal hydroxide molten salts as moderators in MSRs involves similar heat addition and removal. For example, nuclear fission reactions generate heat, which is removed from the MSR to convert the generated heat into electricity. When metal hydroxide molten salts are used in MSRs, heat is usually removed from the metal hydroxide molten salt with the aid of a heat exchanger, which thus creates forced convection in the metal hydroxide molten salt, and the oxo acidity control component can be added anywhere in the MSR where the metal hydroxide molten salt is located. For example, the oxo acidity control component can be water vapor contained in an inert cover gas, where the cover gas represents the process gas, and the cover gas can optionally be bubbled through the metal hydroxide molten salt in a recycle loop and constitute the point of addition of water vapor.

[0030] The method of the present invention is also advantageous in other applications of molten salts of metal hydroxides. A gas stream purification operated by contacting a polluted gas with a molten salt of a metal hydroxide can be operated in a scrubber unit having a vessel with a lining material, the lining material being protected by the methodology disclosed herein. A target concentration of an oxo-acidity control component can be co-fed by bubbling into the polluted gas stream in the vessel of the scrubber unit, simultaneously determining the purification of the gas stream and the oxo-acidity adjustment of the molten salt of the metal hydroxide.

[0031] The molten salt of the metal hydroxide can be contacted with a process gas containing an oxo-acidity control component. In a specific example, the oxo-acidity control component is added to the process gas by sublimating the oxo-acidity control component from the solid state. For example, a metal oxide such as sodium oxide or lithium oxide can be sublimated to produce a partial pressure of a gas phase molecular metal oxide that can be mixed with the process gas and used to control the oxo-acidity of the molten salt of the metal hydroxide.

[0032] In yet another example, the oxo acidity control component is added to the process gas as a liquid by spraying or mist generation. For example, water may be sprayed into the process gas to achieve the desired concentration of OH in the molten salt of the metal hydroxide. - , O 2- , and / or H2O can be provided in the droplet concentration in the process gas.

[0033] OH in molten metal hydroxides - , O 2-The target concentrations of , and HO can be estimated using any procedure as desired. For pure metals, the target concentrations are available from the scientific literature (see FIG. 2). However, the inventors have discovered that when a metal contains other components, such as alloyed metals, non-alloyed metals, and / or non-metallic components, such as carbon, nitrogen, oxygen, boron, and / or silicon, even if the metal is up to about 99% pure, the presence of the other components affects the electrochemical properties compared to the same metal in pure form without the other components, thereby making the metal containing the components differently, for example, more amenable to corrosion from molten hydroxides, than the corresponding pure metal. The inventors have devised a method to generate accurate data correlating the steady-state concentration of oxo-acidity control components in a molten salt of metal hydroxide with the hydroxide's corrosive attack on the lining material. In particular, different metal materials have different polarization characteristics as defined by the open circuit potential, breakdown potential, and passivation potential of the material. By detecting these electrochemical parameters, the corrosion factors of the material in the investigated environment can be identified. The method is similar to that used in aqueous corrosion studies and has been adapted with modifications to study corrosion in molten salts of metal hydroxides. The employed setup is particularly advantageous as it allows the target concentration of the material of interest to be analyzed on a bench scale. Experimental results for an exemplary nickel alloy containing about 90% w / w nickel are shown in FIG. 4. In this example, a three-electrode arrangement can be used in which three electrodes are in contact with the molten salt of metal hydroxide. This electrode arrangement includes the lining material of interest as the working electrode, a reference electrode, and a counter electrode made of pure nickel or other suitable metal such as a nickel-based superalloy suspected to have good resistance to molten hydroxide corrosion. In one embodiment, a β-sodium alumina reference is used as the reference electrode. The potentials reported in this disclosure are referenced to this reference electrode. An exemplary setup includes a high-temperature electrochemical cell consisting of a container, e.g., a metal container, in which a crucible made of an inert material, e.g., graphite, is placed, the crucible containing the molten salt of metal hydroxide. The container has a lid to maintain control of the atmosphere of the experiment, e.g., the atmosphere above the molten salt of metal hydroxide.The lid further has openings to allow the penetration of the electrodes, and gas inlets and gas outlets for adding and removing gases, e.g., the process gas to be analyzed. All openings can be closed and / or opened as appropriate depending on the experimental setup. Gas inlets can also be used for the addition of non-gaseous components to the molten salt of metal hydroxide. An exemplary electrochemical cell is shown in FIG. 1. The cell can measure potential versus current as potentiodynamic polarization and can include sensors, computers, etc. for controlling and measuring electrical parameters. It may include a multichannel potentiostat / galvanostat controlled by a computer, such as PARSTAT (Princeton Applied Research, Hampshire, the UK). The potentiostat / galvanostat can be set to automatically target a desired potential between the working and reference electrodes by passing an appropriate current between the working and counter electrodes. Polarization of the working electrode can be accomplished potentiodynamically such that the potential is continuously changed. This change occurs at a sweep rate of 20 mV / s or 50 mV / s. Before the polarization diagram is experimentally established, the corrosion potential of the working electrode relative to the reference electrode can be determined under open circuit conditions, i.e. under conditions of zero applied current. The open circuit potential reaches an approximately constant value after a few minutes to a few hours. The working electrode can then be anodically polarized up to the transmission potential, starting from a potential 100 mV negative from the open circuit potential. Due to the stochastic nature of the corrosion phenomenon, the polarization test can be repeated at least three times under the test conditions adopted for each material to be studied. Furthermore, the scale / corrosion products formed during the polarization test on the samples can be metallographically examined with post-analysis, for example by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS), to evaluate whether the material underwent microstructural changes upon polarization. In the exemplary setup, the test conditions investigated may differ in the target concentration of the oxidizing acidity control components in the process gas.As an example, argon is used as a carrier gas in either dry or wet form, and wet argon gas as an exemplary process gas can be generated by contacting argon with water in a thermostatic water bath at a temperature ranging from, for example, 30° C. to 90° C.

[0034] In another aspect, the present invention relates to a method for determining the oxo acidity window of a material, the method comprising: Selecting the target material and metal hydroxide; Providing a crucible of inert material; placing a metal hydroxide in a crucible of an inert material and heating the metal hydroxide to obtain a molten salt of the metal hydroxide; providing a working electrode made from a material of interest, a reference electrode, and a counter electrode made from an inert metal; inserting the working electrode, the reference electrode, and the counter electrode into a molten salt of a metal hydroxide; applying a gas above the molten salt of a metal hydroxide and doping the gas with an oxo acidity control component; passing a current between the working and counter electrodes and recording the polarization of the working electrode; Determining the oxo acidity window of the material of interest from the polarization of the working electrode The process includes the steps of:

[0035] In one embodiment, a method for determining an oxo acidity window of a material is provided, the method comprising: Selecting the target material and metal hydroxide; Providing a crucible made of an inert material; placing a metal hydroxide in a crucible of an inert material and heating the metal hydroxide to obtain a molten salt of the metal hydroxide; Inserting a corrosion test coupon made of the material of interest into a molten salt of metal hydroxide; adding an oxo acidity control component to the treated gas and contacting the treated gas with a molten salt of a metal hydroxide; Determining the oxo-acidity window of materials from weight loss of corrosion test coupons. The process includes the steps of:

[0036] The two methods for determining the oxo-acidity window of a material are as follows: In the first aspect, i.e., in the method for adjusting the oxo-acidity of a molten salt, H2O, O 2- , and O.H. - and the target material may be a lining material of a vessel for containing a molten salt of a metal hydroxide. The metal hydroxide may be any metal hydroxide, for example an alkali metal or earth alkali metal hydroxide, and the oxo acidity control component may be as defined above. The inert material may be any material suspected to have good resistance to molten hydroxide corrosion, for example graphite.

[0037] These methods are suitable for estimating the oxo-acidity window. In a first embodiment, the oxo-acidity window of the material of interest is determined from the polarization of the working electrode. In a second embodiment, this is done by measuring the corrosion rate of the material by measuring the weight loss of a corrosion test coupon in the oxo-acidity range. The metal hydroxide may be any metal hydroxide, for example an alkali metal or earth alkali metal hydroxide.

[0038] Thus, the corrosion rate is determined by measuring the difference in weight before and after exposure of a corrosion test coupon of a selected material to a molten salt of a metal hydroxide in contact with a process gas containing an oxo-oxide control component, e.g., at a controlled partial pressure. For example, the corrosion rate can be expressed in units of length per time, e.g., mm / year (mm / y), relative to the thickness of the corrosion test coupon. The oxo-acidity window of a material is determined as the oxo-acidity, e.g., the range of oxo-acidity, that results in the lowest corrosion rate. In this specification, a corrosion rate of 0.1 mm / y is generally considered acceptable for materials used in MSRs, energy or heat storage vessels, or scrubber units operating on molten hydroxide.

[0039] The counter electrode may be made from a metal suspected of having good resistance to molten hydroxide corrosion, such as nickel, e.g. pure nickel, or a metal such as a nickel-based superalloy suspected of having good resistance to molten hydroxide corrosion. The reference electrode may be alumina-based, e.g. a β-sodium alumina reference electrode.

[0040] Therefore, by estimating the oxo-acidity of the molten salt of the metal hydroxide and contacting the process gas containing the oxo-acidity control component with the molten salt of the metal hydroxide at a controlled partial pressure, the oxo-acidity can be maintained within the oxo-acidity window, i.e., oxo-neutral conditions can be provided for the lining material, and corrosion of the lining material from the molten salt of the metal hydroxide can be minimized. For example, the oxo-acidity control component can be water vapor, and water vapor can be added to the process gas to provide a partial pressure of water in the process gas. The molten salt of the metal hydroxide is at a temperature much higher than the boiling point of water even when the pressure is increased, and the water added to the process gas becomes vaporous regardless of the conditions of the molten salt of the metal hydroxide. The water vapor concentration in the process gas can be expressed as a volume percentage, and the water vapor concentration in the process gas can be freely selected. For example, the water vapor concentration in the process gas can be in the range of 5% V / V to 95% V / V. Correspondingly, the concentration of the inert carrier in the process gas can range from 95% V / V to 5% V / V. However, typically water vapor is described by its partial pressure in the process gas. The partial pressure of water vapor ranges, for example, from 0.01 bar to 2 bar, for example, from 0.02 bar to 0.5 bar. The partial pressure of water vapor and the amount of process gas suitable for an embodiment of the present method are determined by the concentration of OH in the molten salt of metal hydroxide. - , O 2- , and at least one target concentration estimate(s) of HO.

[0041] In a specific example, the oxo acidity control component is water vapor, and the water vapor is added to the treatment gas to provide a partial pressure of water in the treatment gas. For example, the water vapor can be added to the treatment gas by contacting the treatment gas with water. Any method can be used to contact the treatment gas with water, and one example is a method of bubbling the treatment gas through a water bath, for example, a constant temperature water bath. The treatment gas bubbled through the water bath can be an inert carrier gas that does not contain moisture, or the treatment gas can already contain, for example, a trace amount of water vapor, particularly an amount of moisture below a target concentration. After bubbling the treatment gas through the water bath, the treatment gas containing water vapor is contacted with a molten salt of a metal hydroxide. The partial pressure of water vapor in the treatment gas can be controlled by at least one of controlling the temperature of the water bath, controlling the residence time of the treatment gas in the water bath, and controlling the pressure of the treatment gas in the water bath. In general, for the water in the water bath to be liquid, the water bath will be at a temperature below the boiling point of water. The optimum temperature range for obtaining a suitable partial pressure of water in the process gas is from 25°C to 90°C, for example from 30°C to 50°C.

[0042] Any embodiment of the invention may be used with any aspect of the invention, and the advantages for a particular embodiment apply equally when an embodiment is used with a particular aspect. [Brief description of the drawings]

[0043] The invention will now be explained in more detail with the aid of examples and with reference to the schematic drawings. [Figure 1] FIG. 1 is a diagram showing an electrochemical cell for estimating a target concentration of at least one of OH − , O 2 − , and H 2 O in a molten salt of a metal hydroxide in this specification. [Diagram 2] FIG. 2 shows the potential oxo acidity diagram for Ni, Fe, and Cr. [Diagram 3] FIG. 3 shows an empirical correlation between the partial pressure of water in the process gas and the steady-state concentration of H2O in a molten salt of sodium hydroxide. [Figure 4]FIG. 4 shows potential difference data measured on Ni alloys in molten NaOH at 600° C. [Diagram 5] FIG. 5 is a graph showing the corrosion rate of Ni alloys in molten NaOH.

[0044] The present invention is not limited to the embodiment or embodiments illustrated in the drawings, and therefore, when reference signs are used after features recited in the appended claims, it should be understood that such signs are used only to improve the clarity of the claims and in no way limit the scope of the claims.

[0045] The term "comprises" as used in the present specification and claims means "consisting at least in part of." When interpreting descriptions containing the term "comprises" in the present specification and claims, there may be features other than the feature(s) preceded by this term in each description. Related terms such as "comprises" and "includes" are to be interpreted in the same manner. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] The present invention relates to a method for adjusting the oxo-acidity of a molten metal hydroxide salt, which method will now be illustrated in the following non-limiting examples.

[0047] Example 1 An experiment was set up to investigate the correlation between the partial pressure of water in the process gas and the steady-state concentration of H2O in the molten salt of sodium hydroxide. Specifically, NaOH was added to a graphite crucible in a pure nickel container. The container had a lid with an opening for adding gas and an opening for removing gas so that the composition of the gas at the top of the crucible could be controlled, and the container also had openings for a thermometer and a gas analysis probe. The container was placed in a mineral wool container, and heated by passing an electric current through a heating wire in the mineral wool, heating the crucible to 600 °C to melt the sodium hydroxide. After the sodium hydroxide melted, the amount of water vapor in the gas at the top of the crucible was gradually increased, and the amount of water in the molten sodium hydroxide was measured after increasing the amount of water vapor in the gas. The results showed a linear correlation between the amount of water vapor in the gas and the amount of water in the molten sodium hydroxide, as shown in Figure 3.

[0048] Example 2 Two high nickel commercial alloys, containing 70% w / w or more nickel, were analyzed to determine the target concentrations. One alloy contains approximately 90% w / w nickel with iron, manganese, silicon, copper, and carbon. The iron, manganese, silicon, copper, and carbon are considered to be trace amounts, but are present in sufficient quantities to require the alloy to be analyzed to determine the target concentrations. The other alloy contains 70% w / w or more nickel, 10% w / w or more chromium, 5% w / w or more iron, and other elements. The H2O, O 2- , O.H. - The target concentration of cannot be predicted from the target concentrations of the individual components in their pure form.

[0049] Samples of the two alloys were analyzed in graphite standard crucibles using sodium hydroxide as an exemplary metal hydroxide salt. The analyses were carried out at temperatures ranging from the melting point of sodium hydroxide to 900°C. Water vapor was used as an oxo-acidity control component, and the water content in the molten salt of sodium hydroxide was determined from the correlation shown in Figure 3.

[0050] Specifically, the two alloys were analyzed in an electrochemical cell 1 as shown in Figure 1. The alloy containing 90% w / w or more of nickel was supplied by Q-metal as a wire with a diameter of 1 mm, and the alloy containing 70% w / w or more of nickel was supplied by Merck as a wire with a diameter of 1 mm. The electrochemical cell 1 contained a vessel 2 made of pure nickel, which contained a graphite crucible 20. Pellets of NaOH were added to the crucible 20, and the vessel 2 with the crucible 20 was placed in a vessel of mineral wool as an insulating material 23, and heated by applying an electric current to a heating wire 231 made of copper to melt the NaOH and provide a molten salt of metal hydroxide 3. NaOH was obtained from Honeywell at 600°C with a nominal purity of 98% or more.

[0051] The vessel 2 had a lid 21 attached to a cell support 24, which had openings 22 for the working electrode 11, reference electrode 12, counter electrode 13, and thermocouple 14, as well as a gas inlet 41 and a gas outlet 42. It is understood that the openings 22 may be used for any item or device that will be in suitable contact with the molten salt of metal hydroxide 3. The gas inlet 41 and gas outlet 42 contained stainless steel pipes and pumps for adding / removing the process gas to be analyzed.

[0052] The working electrode 11 was made from one of the alloys to be analyzed, while the reference electrode 12 and counter electrode 13 were made from pure nickel. The reference electrode 12 was contained in a membrane 121 of β-alumina 121. The electrodes 11, 12, 13 were connected to a PARSTAT multichannel potentiostat / galvanostat (not shown) controlled by a computer (not shown). The potentiostat / galvanostat was set to maintain a potential between the working and reference electrodes by passing a direct current between the working electrode 11 and the counter electrode 13, and the potential was continuously varied to analyze the polarization of the working electrode 11. Specifically, the potential was varied at a sweep rate of 20 mV / s or 50 mV / s.

[0053] Before the polarization diagram was experimentally established, the corrosion potential of the working electrode 11 was determined against the reference electrode 12 under open circuit conditions, i.e. with zero applied current. An almost constant value of the open circuit potential was usually reached after a few minutes. The working electrode 11 was then anodically polarized starting from a potential 100 mV minus the open circuit potential up to the permeability potential. Since the corrosion phenomenon is stochastic, the polarization test was repeated three times for each material of the working electrode 11.

[0054] Furthermore, the formation of scale / corrosion products during polarization tests of the tested alloys was metallographically investigated using post-analysis by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM / EDS) to assess whether any microstructural changes had occurred in the materials after polarization.

[0055] Argon was used as a carrier gas, and an exemplary process gas was wet argon, generated by bubbling argon through a water bath (not shown) at temperatures of 36° C., 50° C. or 90° C. Wet argon was introduced into vessel 2 via gas inlet 41. Excess gas was removed from vessel 2 via gas outlet 42 to maintain the pressure at ambient pressure.

[0056] The results of this practical example demonstrate the methodology for finding the optimal oxo-acidity window for a given material, but the results are not exhaustive. Multiple test conditions can be evaluated to accurately evaluate the oxo-acidity window. Furthermore, while this example uses a single molten salt temperature, multiple temperatures can be appropriately evaluated to define a suitable oxo-acidity window in a practical commercial setup that considers temperature transients.

[0057] The results for a nickel alloy containing about 90% w / w nickel are shown in Figure 4. It shows the change in the determined current vs. potential. Different electrochemical responses were obtained for the same type of sample when different partial pressures of water (ppH2O) in the process gas used to adjust the steady-state concentration of water in the molten salt were used. By comparing the different profiles, we defined the target oxo-acidity conditions to be used in the oxo-acidity control of the full-scale setup. Figure 4 shows a clear corrosion reduction for ppH2O = 5.5968% in a cover gas with a total pressure of 1 atm. There are two major features in the figure that indicate the improvement due to the adjustment of the water pressure. First, the peak in the potential region from 0.4 to 1.2 V is assigned to the corrosion potential region. The higher the potential, the more corrosion resistant the material is at the conditions of use. The black line corresponds to the case where no water was added to the molten salt, which showed the worst results with a corrosion potential peak at 0.6 V. This salt was highly oxobasic and easily corroded the sample. It is noteworthy that an excess of ppHO in the process gas was also poorly effective in mitigating the corrosion of the samples, although to a lesser extent. The blue and green lines correspond to the oxo-acidic conditions of the salt, determining a corrosion potential around 0.77 V. Finally, the red line shows the corrosion mitigation that can be achieved by carefully controlling the targeted oxo-acidity. Under these conditions, we believe that the molten salt is in an oxo-neutral state, i.e. intermediate between oxo-basic and oxo-acidic for the chosen lining material. In this oxo-acidity window, the peak of the corrosion potential increases significantly, reaching a value of 1.1 V.

[0058] Additionally, another region of the plot shows the mitigation of corrosion achieved by appropriate water target concentrations. In the potential region from 1.2V to 2V, the formation of a protective passive layer is observed. The lower the current, the stronger the protective passivation. If the target concentration of the lining material is appropriate, the chemistry of the material surface is stabilized, allowing the formation of a stable metal oxide that protects the uncorroded material layer below the surface. In this region of the plot, it is observed that the first ppH2O (red line) is the best at protecting against corrosion, while the oxo-basic region (black line) is the worst at promoting the formation of a stable oxide layer on the material, and the second (green) and third (blue) ppH2O concentrations determine oxo-acidic conditions and have similar partial protective effects.

[0059] Example 3 An experiment was set up to analyze the 90% nickel alloy used in Example 2. Samples of the alloy were analyzed in alumina crucibles using sodium hydroxide as an exemplary metal hydroxide salt. The analysis was carried out at temperatures ranging from the melting point of sodium hydroxide to 900°C. Water vapor was used as the oxo acidity control component and the amount of water in the molten salt of sodium hydroxide was determined from the correlation in Figure 3.

[0060] Specifically, pellets of NaOH were added to an alumina crucible, which was then placed in a container of mineral wool for insulation, and heated by passing an electric current through a copper heating wire wrapped around the crucible to melt the NaOH and provide a molten salt of metal hydroxide. The NaOH was obtained from Honeywell with a nominal purity of 98% or higher at 600°C.

[0061] The alloys were supplied by Q-metal as corrosion coupons with a thickness of 3 mm, length of 20 mm and width of 7 mm. The coupons were cleaned and dried before being weighed and inserted into molten NaOH. The coupons were removed from the molten NaOH after one week and the molten NaOH residue was removed from the coupon surface. The weight loss of each coupon was recorded and expressed relative to the coupon surface area (i.e. length x width) in mg / cm. 2The corrosion rate was calculated from the exposure time to molten NaOH and expressed in mm / year (mm / y) relative to the thickness of the corrosion test specimen. The results are shown in Figure 5. Figure 5 shows the weight change and corrosion rate at different oxo-acidity levels, determined at a corrosion rate of ±0.1 mm / y. Different corrosion rates were obtained for the same type of specimen when the partial pressure of water (ppH2O) in the process gas used to adjust the steady-state concentration of water in the molten salt was different. Figure 5 shows the results obtained from the weight change and the results of inductively coupled plasma optical emission spectroscopy (ICP-OES). The minimum corrosion rate obtained from the weight change calculation was 0 mm / y, with a p[H2O] of 2.27.

[0062] Reference Sign List 1. Electrochemical cell 2 containers 20 Crucible 21 Lid 22 Opening 23 Insulation 231 Heating wire 24 Cell Support 3 Molten salts of metal hydroxides 11 Working electrode 12 Reference electrode 121 Membrane 13. Opposite 14 Thermocouple 41 Gas inlet 42 Gas outlet

Claims

1. 1. A method for adjusting the oxo-acidity of a molten metal hydroxide salt, comprising: H in molten salts of metal hydroxides ２ O, O ２－ , O.H. － estimating at least one target concentration of; Providing an oxo acidity control component; and contacting the oxo-acidity control component with the molten salt of the metal hydroxide to adjust the oxo-acidity of the molten salt of the metal hydroxide; A method comprising the steps of:

2. 2. The method for adjusting the oxo acidity of a molten salt according to claim 1, wherein the oxo acidity control component is supplied in a process gas comprising an inert carrier gas, and the method further comprises contacting the process gas comprising the oxo acidity control component with the molten salt of a metal hydroxide to adjust the oxo acidity of the molten salt of a metal hydroxide.

3. 3. The method for adjusting the oxo-acidity of a molten salt according to claim 2, wherein the oxo-acidity control component is water vapor, and the water vapor is added to the process gas to adjust the partial pressure of water in the process gas.

4. The water vapor is added to the process gas by bubbling the process gas through a water bath, and the partial pressure of the water vapor in the process gas is controlling the temperature of the water bath; controlling the residence time of the treatment gas in the water bath; and controlling the pressure of the treatment gas in the water bath; 4. The method for adjusting the oxo acidity of a molten salt according to claim 3, wherein the oxo acidity is controlled by at least one of the following:

5. 3. The method for adjusting the oxo-acidity of a molten salt according to claim 2, wherein the oxo-acidity control component is sublimated from a solid state and added to the treatment gas.

6. 3. The method for adjusting the oxo-acidity of a molten salt according to claim 2, wherein the oxo-acidity control component is added to the treatment gas as a liquid by spraying or misting.

7. The oxo acidity control component is H ２ O, H ２ 3. The method for adjusting the oxo acidity of a molten salt according to claim 2, wherein the oxo acid is selected from the group consisting of HF and HF.

8. The molten salt of metal hydroxide is placed in a vessel having an inner surface made of a lining material, and OH － , O ２－ , and H ２ 2. The method for adjusting the oxo-acidity of a molten salt according to claim 1, wherein at least one target concentration of O is defined in the lining material.

9. 9. The method for adjusting the oxo acidity of a molten salt according to claim 8, wherein the molten salt of the metal hydroxide is stationary or circulated within the vessel by forced convection or forced circulation.

10. 9. The method for adjusting the oxo-acidity of a molten salt according to claim 8, wherein the oxo-acidity control component is contacted with the molten salt of a metal hydroxide located at a distance in the range of 0 cm to 100 cm from the lining material.

11. 9. The method for adjusting the oxo-acidity of a molten salt according to claim 8, wherein the vessel includes a heat source and / or a heat sink configured to generate a temperature gradient in the range of 0.1° C. / cm to 10° C. / cm in the molten salt of the metal hydroxide.

12. 12. The method for adjusting the oxo acidity of a molten salt according to claim 1, wherein a cover gas above the molten salt of metal hydroxide is maintained at a pressure equal to or greater than ambient pressure.

13. The method for adjusting the oxo acidity of a molten salt according to claim 12, wherein the cover gas is the process gas.

14. 12. The method for adjusting the oxo acidity of a molten salt according to claim 2, wherein the process gas is bubbled through the molten salt of a metal hydroxide.

15. H ２ O, O ２－ , O.H. － The method for adjusting the oxo acidity of a molten salt according to any one of claims 1 to 11, wherein the at least one target concentration is estimated at at least three different temperatures within the range between the melting point and boiling point of the metal hydroxide salt.

16. 1. A method for determining the oxo acidity window of a material, comprising: Selecting the target material and metal hydroxide; providing a crucible of inert material; placing said metal hydroxide in a crucible of said inert material and heating said metal hydroxide to provide a molten salt of said metal hydroxide; providing a working electrode made from the material of interest, a reference electrode, and a counter electrode made from an inert metal; inserting the working electrode, the reference electrode, and the counter electrode into the molten salt of metal hydroxide; pouring a gas over said molten salt of metal hydroxide and adding an oxo acidity control component to said gas; applying a current between the working electrode and the counter electrode and recording the polarization of the working electrode; determining an oxo-acidity window of the material of interest from the polarization of the working electrode; A method comprising the steps of:

17. The oxo acidity control component is H ２ O, H ２ 17. The method for adjusting the oxo acidity of a molten salt according to claim 16, wherein the oxo acid is selected from the group consisting of HF and HF.

18. 1. A method for determining the oxo acidity window of a material, comprising: Selecting the target material and metal hydroxide; providing a crucible of inert material; placing the metal hydroxide in a crucible made of the inert material and heating the metal hydroxide to obtain a molten salt of the metal hydroxide; Inserting a corrosion test coupon made of the target material into the molten salt of metal hydroxide; adding an oxo acidity control component to the treat gas and contacting the treat gas with the molten salt of the metal hydroxide; determining the oxo-acidity window of the material from the weight loss of the corrosion test coupon; A method comprising the steps of:

19. The oxo acidity control component is H ２ O, H ２ 19. The method for adjusting the oxo acidity of a molten salt according to claim 18, wherein the oxo acidity is selected from the group consisting of HF and HF.

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