Method for removing foreign substances and decomposition products from liquid salt melts

Abstract

A process for purifying a heat transfer medium in the form of a molten salt in thermal power plants, characterized in that impurities are removed from the molten salt by means of precipitating salts, wherein a) the precipitating salts have the general formula MV(NO3)W, wherein: i) M is selected from metals of the alkaline earth and / or lanthanide group, preferably alkaline earth group, more preferably calcium, barium and strontium; ii) V = 1; andiii) is at least W=2, but preferably 2, 3 or 4, and the precipitating salts themselves are soluble in the molten salt;b) the cations of the precipitating salts precipitate the impurities, wherein the impurities have an anion in the form of an oxyanion of the general formula AxOyz, as salts insoluble in the molten salt; andiv) A is selected from the substances chromium, nickel, molybdenum, iron, carbon, zirconium, aluminum, yttrium, hydrogen or silicon; andv) X=1 or X=2;andv) Y is selected according to the oxidation state of substance A, such that Z has a value of -2 or -1.;

Description

The present invention relates to a method for regenerating or reducing the formation of decomposition products in nitrate salt melts with the aid of a regeneration gas and to a method for removing foreign substances in nitrate salt melts with the aid of precipitation reactions.In thermal thermal power plants, in particular in solar power plants, salt melts, among other things, are used as the heat storage and heat transfer medium. Nitrate salts (alkali metal nitrates, alkaline earth metal nitrates or even mixtures thereof) are already used in such installations either for heat transfer and / or for heat storage. In this case, the salt is brought to a temperature above the melting temperature by concentrating the solar radiation, in order to use it as a melt or fluid. The thermal energy may then be transported by the fluid to other locations of the system and immediately utilized and / or stored. Operating temperatures at nitrate salts of 280-560° C. are customary for such power plants. Since these salt melts are highly corrosive, installations made of austenitic materials (0-35% chromium, 0-35% nickel, 0-10% molybdenum, remainder iron), or Ni-based alloys (0-35% iron, 0-35% chromium, 0-15% molybdenum, remainder nickel), and ceramic materials (carbides, and oxides zirconium, aluminum, yttrium, silicon, or mixtures of the metals) are usually used. However, even with these materials, corrosion and decomposition processes cannot be completely excluded, in particular not during long-term operation (about 20 years). These corrosion processes lead, among other things, to the formation and accumulation of highly toxic compounds dissolved in the liquid salt. Accordingly, the permanent purification of these fluids during operation is necessary to ensure the long-term stability of the nitrate salt, the reactors and storage materials used, as well as to reduce the potential amounts of highly toxic material in the event of leaks or planned disposal of the salts. Chromates in particular are to be mentioned as impurities, since these are highly toxic, environmentally hazardous and carcinogens. A further problem is the thermal decomposition of the nitrate anions, since a high nitrite content in the liquid salt ensures enhanced corrosion of the structural materials used by increased oxide formation.Since there is currently no possibility in the prior art for removing these or other foreign substances and / or decomposition products during ongoing operation, only the cumbersome, very expensive and hazardous disposal of the salt remains as a special waste. The accumulation of the foreign substances and / or decomposition products, in particular chromates, in the nitrate salt melts used during the lifetime of a salt store thus represents great safety risks for humans and the environment in the case of leaks and also in the case of disposal and carries considerable additional costs due to increased safety measures and disposal costs.There is thus a need for a method for purifying a heat carrier, preferably nitrate salt melts, in thermal power plants, such that the formation of decomposition products can be reduced and / or regenerated and / or foreign substances can be removed.Surprisingly, it has been found that foreign substances can be removed with the aid of precipitation salts. Removing foreign matter, especially chromates, lowers operation and disposal costs because they can be removed continuously and thus accumulation can be prevented. This significantly reduces the amount of environmentally harmful waste and thus also disposal costs. In a first embodiment, the object underlying the present invention is achieved by a method for purifying a heat carrier in the form of a salt melt in thermal power plants, characterized in that foreign substances are removed from the salt melt with the aid of precipitation salts, wherein a) the precipitation salts have the general empirical formula M V( NO 3)W wherein: i) M is selected from metals of the alkaline earth group and / or lanthanide group, preferably alkaline earth group, most preferably calcium, barium and strontium; ii) V=1; and iii) is at least W=2, but preferably 2, 3 or 4, and the precipitation salts themselves are soluble in the salt melt, b) the cations of the precipitation salts precipitate the impurities, the impurities having an anion in the form of an oxyanion of the general empirical formula A x O yz as salts insoluble in the salt melt, wherein: iv) A is selected from the substances chromium, nickel, molybdenum, iron, carbon, zirconium, aluminum, yttrium, hydrogen or silicon; and v) X=1 or X=2; and v) Y, depending on the oxidation state of the substance A, is selected, so that a value of -2 or -1 results for Z.The heat carriers in the form of the salt melt, preferably nitrate salt melts, are used according to the invention in thermal power plants, in particular in solar power plants, as a receiving, transport and / or storage medium for the thermal energy. The thermal energy is transferred from the sun indirectly or directly to the melt. This can lead to decomposition of the nitrate anions, which thereby decompose in an equilibrium reaction to nitrite and oxygen (I):As a result of the reaction, the further decomposition or reduction of nitrite anions to nitrogen monooxide, nitrogen dioxide and oxide ions is possible (II):Both the oxygen formed and the oxides formed lead to increased corrosion processes within the plant. These processes are characterized, inter alia, in that they attack materials, such as metals from the alloy, of the plant and this results in accumulation of impurities in the salt melt and corrosion in the power plants.In order to minimize the corrosion processes as well, austenitic material is used in the prior art in thermal power plants which use salt melts as working medium. Usually, these consist of iron-based alloys with additions of γ-mixed crystals comprising 0-35% chromium, 0-35% nickel and 0-10% molybdenum or nickel-based alloys with additions of γ-mixed crystals comprising 0-35% iron, 0-35% chromium and 0-15% molybdenum. In addition, ceramic materials can be embedded into the austenitic material or used separately if necessary, which are usually based on carbides and oxides of zirconium, aluminum, yttrium, silicon or mixtures of the metals.Nevertheless, corrosion of the material used occurs and foreign substances accumulate in the salt melt during the running time of the thermal thermal power plant. Possible impurities are all oxides of the metal alloy used or carbonates, and hydroxides. These foreign substances, in particular chromates, represent an essential problem for humans and the environment for the reasons already mentioned, since there is no possibility in the prior art to remove them during operation.Thus, there is a need for regeneration or reduction of the formation of the decomposition products and for continuous removal of foreign substances from molten salts. Surprisingly, it has been found that in thermal power plants, in particular in solar power plants which use salt melts as working medium, foreign substances can be removed with the aid of precipitation salts.Preferred embodiments of the method and of a device are described further below, wherein all features can be combined with one another in any desired manner and do not restrict the method of the present invention.The method according to the invention provides that foreign substances are removed with the aid of precipitation salts for purifying a heat carrier in the form of a salt melt.In the sense of the present invention, the terms heat carrier and working medium are synonymous and are thus synonyms or interchangeable. Furthermore, in a preferred embodiment, the salt melt used as heat carrier is at least one nitrate salt or mixtures of a plurality of nitrate salts which comprise at least one or more alkali metal nitrates. This may be a mixture of 2, 3, 4 or more alkali metal nitrates. In a further preferred embodiment, the salt melt can be a melt of sodium nitrate and / or potassium nitrate. Accordingly, according to the invention, any desired mixture of sodium nitrate and potassium nitrate or more alkali metal nitrates as salt melt is also possible. An example of a preferred and commercially widely used salt melt is solar salt, a mixture of 60% by weight sodium nitrate and 40% by weight potassium nitrate.Depending on the composition of the salt melt, which preferably consists of nitrate salts, the latter has a specific melting temperature, from which it can be used as working medium in a solar power plant as fluid. Accordingly, the present invention describes a process which is carried out between 200-1000° C., preferably between 250-800° C. and most preferably 280-560° C. At lower temperatures, no complete molten salt would be present and operation of the thermal power plant would not be possible. Higher temperatures would result in high corrosion reaction rates and / or decomposition of the molten salt.In a preferred embodiment, the process is characterized in that decomposition products of the salt melt are additionally reacted back to the starting salt by introduction, directly before or after heat absorption of the heat carrier, of a regeneration gas and / or their formation is mitigated, wherein the regeneration gas a) comprises nitrous gases, oxygen and optionally nitrogen and b) has a content of nitrous gases of at least 0.0001% by volume, preferably 0.02-0.05% by volume, based on the total volume of the regeneration gas.Regeneration gas in the sense of the preferred embodiment is a mixture of nitrous gases, oxygen and optionally nitrogen. The nitrous gases constitute a proportion of less than 0.1% by volume, preferably less than and / or equal to 0.05% by volume, based on the total volume of the regeneration gas mixture. Higher proportions of nitrous gases would be possible but expensive, so that the proportion of 0.05% by volume is preferably not exceeded. This also takes into account the toxicity of the nitrous gases. The proportion of at least 0.0001% by volume, preferably between 0.02% by volume and 0.05% by volume, of the nitrous gases, based on the total volume of the regeneration gas, keeps the toxicity low, with at the same time adequate regeneration action. The remaining portion of the regeneration gas consists of any mixture of oxygen (>20 vol.%) and nitrogen or only of oxygen. Nitrous gases in the context of the present invention are NO, NO 2 and N 2 O and / or any desired mixtures thereof. Furthermore, in this embodiment, the regeneration gas is introduced directly before or after the heat absorption of the heat carrier.Surprisingly, it has been found that in this preferred embodiment, by introducing the gas directly before or after the absorption of heat by the heat carrier, the decomposition products formed in reactions (I) and (II) are almost completely converted back to the desired nitrate anion and / or the formation of the decomposition products can be prevented. The supersaturation by the regeneration gas obtained by this specific introduction, directly before or after the heat absorption of the heat carrier in the form of the salt melt, prevents the decomposition processes represented in reactions (I) and (II). This reduces further corrosion processes in the plant and thus enables more effective and longer operation of the thermal power plant. Thus, in a preferred embodiment, according to the invention, the decomposition products comprise, in particular, substances which can be formed from nitrate anions and / or can be nitrites, oxygen, free oxides and / or nitrogen oxides.In a further embodiment of the present invention, foreign substances are all products which are formed from corrosion reactions of the salt melt with the material used in the thermal power plant and can occur in solution in the salt melt. In particular, according to the invention, these are substances made of γ-mixed crystals of iron, which are used as alloys in austenitic materials, and substances made of ceramic materials. Thus, according to the invention, impurities comprise in particular the oxyanions CrO 42-, Cr 2 O 72-, MoO 42-, ZrO 32-, SiO 32-, SiO 44-, CO 32- and OH -, preferably CrO 42-, Cr2O7NER29_, CO3NER31_and OH-. These accumulate in the salt melt during operation and thus influence the performance and lead to shorter operating times of the thermal power plant.Surprisingly, it has been found that these foreign substances can be removed with the aid of precipitation salts. The removal of chromates in particular is of particular importance here, since thermal power plants often have to take expensive safety precautions in the event of leaks and the complete molten salt has to be disposed of as a special waste at the end of the running time. By removing the contaminants, in particular during the ongoing operation of the thermal power installations, the running time and efficiency of thermal power installations is increased and the waste produced is substantially reduced.Thus, in the method of the present invention, a precipitated salt having the general empirical formula M V( NO 3)W reacts with the impurity having an anion in the form of an oxyanion having the general empirical formula A x O yz to form the insoluble salt.According to the invention, M comprises metals of the alkaline earth and / or lanthanide group, preferably alkaline earth group, most preferably calcium, barium and strontium, wherein V=1 and W corresponds to the oxidation state of the metal and is at least W=2, but preferably 2, 3 or 4. Thus, in particular said oxidation state of the metal M is selected from +II, +III and +IV.Furthermore, in the sense of the present invention, the oxyanion A x O yz, A comprises an element made of the austenitic and / or ceramic material used, which element can pass into the molten salt on account of corrosion processes. A thus comprises substances selected from chromium, nickel, molybdenum, iron, carbides, zirconium, aluminum, yttrium or silicon, wherein X=1 or X=2. Furthermore, it is possible according to the invention to use hydrides and / or oxides of the substances A in the austenitic and / or ceramic material, such that A also comprises hydrogen and thus one of the oxyanions A is x O is yz OH is -, i.e. hydroxide. For the purposes of the present invention, Y is selected depending on the coordination sphere of the substance A or on the valence electron rule familiar to the skilled worker and thus comprises values between 1 and 10, preferably values between 1 and 7. Depending on the oxidation state of the substance A, Y is selected such that a value of -2 or -1 results for Z, thus the oxyanion comprises A x O yz preferably the anions selected from CrO 42-, Cr2O7 2-, MoO4 2-, ZrO3 2-, SiO4 4-, SiO3 2- and CO3 2-, most preferably CrO4 2-, Cr2 O7 2-, CO3 2- and OH -.The general precipitation reaction to precipitate impurities according to the method of the invention, whereby the insoluble salt of the general formula M V( A x O y)u ≅ is formed, can thus be described as follows:where U is the equivalents of precipitated oxyanion.Thus, precipitation reactions are preferred according to the invention in which the precipitation salt M V( A x O y)u ≅ from M v( OH) U, M V( CrO 4)U, M V( Cr 2 O 7)U, MV(MoO4)U, MV(ZrO3)U, M V( SiO 4)U, M V( SiO 3)U and / or M V( CO 3)U. M here comprises metals of the alkaline earth group and / or lanthanide group, preferably alkaline earth group, most preferably calcium, barium and / or strontium.Precipitation reactions in which M V( OH) U, M V( CrO 4)U, M V( Cr 2 O 7)U, and / or M V( CO 3)U, wherein M is selected from calcium, barium and / or strontium, are particularly preferred according to the invention. Accordingly, a preferred subject of the present invention is a process which is characterized in that the precipitation salts comprise inorganic nitrate salts, in particular alkaline earth metal nitrates and / or lanthanide nitrates, most preferably Ca(NO 3)2, Sr(NO 3)2 and / or Ba(NO 3)2.Thus, according to the invention, precipitation salts are inorganic nitrate salts which are not part of the salt melt as heat medium, preferably alkaline earth metal nitrates and / or transition metal nitrates. In particular, lanthanide nitrates, Ca(NO 3)2, Sr(NO 3)2 and / or Ba(NO 3)2 or mixtures thereof should be mentioned here according to the invention. In a particularly preferred embodiment, Sr(NO 3)2 can be used as precipitation salt. Other strontium salts would be conceivable, but would either promote corrosion reactions in the plant (for example SrCl 2) or have poor solubility (for example SrO) due to the anions.Due to the continuous removal of the impurities, for example the chromate, during operation with Sr(NO 3)2 due to precipitation as insoluble strontium chromate, in particular SrCrO 4, time-consuming purifications or expensive disposal of the entire tank contents as hazardous / toxic waste waste at the end of the plant run time are no longer necessary. With other foreign substances, precipitation and separation take place accordingly.Depending on the insoluble salt formed from the precipitation reaction (III) described, M V( A x O y)U ≅, this can be used further. According to the invention, Sr(CrO 4)2 is particularly preferably formed, which can be reused as corrosion protection in alloys. Other insoluble salts of the form M V( A x O y)U ≅, such as lanthanide salts and alkaline earth metal salts, in particular Ba(CrO 4)2 or Ca(CrO 4)2 can likewise be used further as described in the prior art. In particular, Ba(CrO 4)2 can be used further in pyrotechnics as pigment, oxidizing agent or as green dye. Furthermore, Ca(CrO 4)2 can be used in particular as pigment, anticorrosive agent or as depolarizer in batteries. Other uses are known to those skilled in the art and are also possible.In a further preferred embodiment, according to the invention, the molar ratio of foreign substances to precipitation salt is from 1:1 to 1:10. The selection of the ratio depends on the desired reaction time. During the operation of the present invention, ratios of 1:1 to 1:2 can be used in particular, since foreign substances are removed here in a cost-effective manner, which protects the resources, and the precipitation or reaction rate is of low importance. Furthermore, in an additional embodiment, in particular a ratio of 1:5 to 1:10 can be used during the final disposal of the salt melt. This achieves faster precipitation times, since here too long a precipitation time would be uneconomical. Amounts greater than 10-fold excess of precipitation salt no longer influence the reaction rate and are thus unnecessary.In a further embodiment, according to the invention, the concentration of the foreign substances and / or decomposition products is checked via electrochemical and / or optical or spectroscopic sensors both before and / or after the reactor. As a result, the exact amount of precipitation salt and / or regeneration gas required to be introduced can be determined and added to the salt melt. This makes it possible to ensure efficient long-term use of the thermal power plant in a resource-saving manner.FIG. 1 shows an exemplary purification section with: 1 salt line with valve V 1 for separating the precipitation reactor from the thermal power plant system 2 sensor (temperature) for monitoring the salt temperature 3 sensor (electrochemically or optically) for monitoring the salt / gas composition 4 heat source / heating, controlled via the salt temperature for temperature guidance 5 discharge line with valve V 4 for removing precipitated solids 6 container with ceramic inlay 7 propeller stirrer for increasing the residence time of the introduced gas and added precipitant, and better mixing of the individual components in the precipitation reactor 8 inorganic salt 9 heat storage medium 10 heat transfer medium 11 feed for precipitant 12 sensor (pressure) for monitoring overpressure in the container 13 ceramic filter for retaining solid 14 recirculation line into the salt circuit with valve V 3 for separating the reactor from the thermal power plant system 15 salt line with valve V 5 for separating the packed column from the system 16 column body with ceramic inlay 17 ceramic packed bed 18 feed line for regeneration gases 19 regeneration gases (NO x, O 2) 20 recirculation line into the salt circuit with valve V 6 for separating the column from the system 21 gas outletFIG. 2 shows the change over time of the salt composition of the salt melt when treated with a regeneration gas.For the method according to the invention for purifying the salt melt, in one embodiment a purification section can be constructed from a combination of a batch precipitation reactor with a continuously running packed column. Other reactors or reactor combinations which would be suitable for the process are known to the person skilled in the art and can likewise be used for the process according to the invention. The precipitation reactor can be provided from a heatable (steel) container 6 with a chemically inert ceramic inlay. The reactors 6 and 13 may be made of any ceramic and / or austenitic materials known in the art for such a thermal power plant. Particular mention should be made here of materials which use chromium as mixed metal in the alloy. Other alloys which contain γ-mixed metals of iron and whose oxyanions form sparingly soluble salts with the precipitation salts mentioned are likewise possible.Furthermore, it is possible according to the invention that a heater 4 with temperature sensors 2 is present in the plant. The sensors can in principle be installed anywhere, preferably they are located at the inlets and / or outlets of the reactor. The heater 4 with sensors 2 ensures both constant temperature in the salt melt and controlled temperature control for optimized precipitation of the impurities as insoluble salts. The temperature for the operation of such a plant, thus also for the process according to the invention, is between 200° C. and 1000° C., preferably between 250° C. and 800° C., most preferably between 280° C. and 580° C. At lower temperatures, no complete molten salt would be present and operation of the thermal power plant would not be possible. Higher temperatures would result in high corrosion reaction rates and / or decomposition of the molten salt.According to the invention, the concentration of the foreign substances and / or decomposition products is preferably detected or checked via electrochemical and / or optical sensors 3, so that the amount of precipitation salt and / or regeneration gas can be determined. This can take place in particular before and / or after the reactor, i.e. at the inlet and outlet valves V 1 / V 5 and / or V 2 / V 6, but does not exclude, according to the invention, another detection location past which the salt melt flows. If a purification is required, the batch reactor can be connected via the valves V1 and / or V2 at the inlet and outlet and the necessary precipitation reagent, for example Sr(NO 3)2, can be added to the salt via a feed 11. A stirrer, for example a propeller stirrer 7 here, enables a distinct reduction in the residence time and better mixing of the individual components in the reactor. A pressure sensor 12 can be used to monitor the pressure in the reactor in order to avoid overpressure.In the embodiment described here, the ceramic grids 13 prevent the precipitate formed and / or the undissolved precipitant from being carried over into the further salt system. Depending on requirements, the solid formed at the bottom of the reactor can be discharged in a controlled manner via the valve V 4 with the aid of the outlet 5. Depending on the insoluble salt formed, this can be used further.In the embodiment described, the continuously running packed column can consist of a heatable (steel) container 16 with a chemically inert ceramic inlay. Furthermore, it is also possible here according to the invention that a heater 4 with temperature sensors 2 is present in the plant. The sensors can in principle be installed anywhere, preferably they are located at the inlets and / or outlets of the reactor. Between two ceramic grids 13 within the column is a ceramic packing 17 which provides for an increase in the reactive surface area of the salt and thus a shortening of the reaction time during the regeneration of the decomposition products. If regeneration is required, the column can be connected via the valves V5 and / or V6 at the inlet and outlet and the necessary regeneration gas, for example a mixture of nitrous gas(s) and oxygen, can be flowed through the column counter to the flow direction from the bottom to the top to the salt via a gas inlet 18.At the output, electrochemical and / or optical sensors 3 determine the gas concentration and composition. Detection can also take place where elsewhere in the plant according to the method of the invention, but is particularly expedient at the inlet or outlet, since the effect of the regeneration gas can be directly controlled. According to the present invention, the gas composition required for regenerating the salt at the gas inlet is regulated on the basis of the measurement results. In the case of maintenance of the column, the latter can be switched off from the salt system.The concept of the present invention is explained in more detail with the exemplary embodiments which follow and does not restrict the subject matter of the invention to these.Example 1:25 g of solar salt (60% by weight of NaNO 3, 40 % by weight of KNO 3) were mixed together with K 2 CrO 4(0.05 mol %) and melted in a 25 cm long quartz glass tube at 280° C. Subsequently, about 10-fold excess (0.5 mol %) of the precipitation reagent Sr(NO 3)2 was added and mixed well. Directly on salt contact, a dark yellow solid precipitated out of the salt melt. After a waiting time of 10 minutes, the salt mixture was filtered through a glass fibre filter and the filtrate and the filter cake were investigated for chromate residues by means of ion chromatography (IC). The filtrate had largely decolored by the reaction and showed no detectable amounts of chromate in the IC. The filter cake consisted largely of the dark yellow SrCrO 4.Example 2:Example 1 was repeated, but with a chromate to Sr(NO 3)2 ratio of 1:1. ion chromatographic measurements gave only a slight reduction in the chromate content in the filtrate.In this case too, the filter cake consisted for the most part of the dark yellow SrCrO 4. It has thus been found that a stoichiometric reaction with a waiting time of 10 minutes is not sufficient, but an excess of precipitation reagent is required in favor of a shorter reaction time.Example 3: Example 3:Example Regeneration with NO x / O 2:In order to simulate the aging of salts experimentally, the decomposition products, i.e. nitrite (as sodium nitrite) and oxide species (as sodium peroxide), were added to a salt melt made of solar salt (60% by weight of NaNO 3, 40 % by weight of KNO 3). This salt melt was heated to the temperature to be examined and a regeneration gas (rich in oxygen and rich in NO x- ) was passed over the melt.A solar salt mixture of 89% nitrate, 10% nitrite and 1% oxide was prepared. The oxide content corresponded to that of a very strongly aged salt melt. A gas mixture of 80% O 2, 200 ppm NO and remainder N 2 was passed over the salt at 600° C. FIG. 2 illustrates that the nitrite content dropped directly from 10% to about 5% by the addition of the regeneration gas. The oxide content likewise fell off within 160 h, suggesting a degradation of 94% of the oxide content.

Claims

A process for purifying a heat carrier in the form of a salt melt in thermal power plants, wherein foreign substances are removed from the salt melt with the aid of precipitation salts, wherein a) the precipitation salts have the general empirical formula M V( NO 3)W wherein: i) M is selected from metals of the alkaline earth group and / or lanthanide group, preferably alkaline earth group, most preferably calcium, barium and strontium; ii) V=1; and iii) is at least W=2, but preferably 2, 3 or 4, and the precipitation salts themselves are soluble in the salt melt, b) the cations of the precipitation salts precipitate the impurities, the impurities having an anion in the form of an oxyanion of the general empirical formula A x O yz as salts insoluble in the salt melt, wherein: iv) A is selected from the substances chromium, nickel, molybdenum, iron, carbon, zirconium, aluminum, yttrium, hydrogen or silicon; and v) X=1 or X=2; and v) Y, depending on the oxidation state of the substance A, is selected, so that a value of -2 or -1 results for Z.Process according to Claim 1, characterized in that decomposition products of the salt melt are additionally reacted back to the starting salt by introduction, directly before or after heat absorption of the heat carrier, of a regeneration gas and / or their formation is mitigated, wherein the regeneration gas a) comprises nitrous gases, oxygen and optionally nitrogen and b) has a content of nitrous gases of at least 0.0001% by volume, preferably 0.02-0.05% by volume, based on the total volume of the regeneration gas.Method according to claim 1 or 2, characterised in that the salt melt comprises nitrate salts, preferably alkali nitrate salts, most preferably sodium and / or potassium nitrate.Process according to at least one or more of Claims 1 to 3, characterized in that it is carried out at from 200°C to 800°C, preferably from 250°C to 800°C, most preferably from 280°C to 580°C.Method according to at least one or more of Claims 1 to 4, characterized in that the decomposition products are formed from nitrate anions and / or comprise nitrites, oxygen, free oxides and / or nitrogen oxides.Process according to at least one or more of Claims 1 to 5, characterized in that the impurities are salts which are used as anion CrO 42-, Cr 2 O 72-, MoO 42-, ZrO 32-, SiO 44-, SiO 32- and CO 32-, most preferably CrO 42-, Cr 2 O7NER26_, co_ner27__ner28_und OH -.Process according to at least one or more of Claims 1 to 6, characterized in that the precipitation salts comprise inorganic nitrate salts, preferably alkaline earth metal nitrates and / or lanthanide nitrates, most preferably Ca(NO 3)2, Sr(NO 3)2 and / or Ba(NO 3)2.Method according to at least one or more of Claims 1 to 7, characterized in that the molar ratio of the foreign substances to precipitation salts is from 1:1 to 1:10.Method according to at least one or more of Claims 1 to 10, characterized in that it is carried out during operation of the thermal power plant, but can also be carried out at the end of the period of use of the molten salt as heat carrier.Method according to at least one or more of Claims 1 to 11, characterized in that the concentration of the foreign substances from the salt melt is checked via electrochemical and / or optical sensors before and / or after the reactor, as a result of which the amount of precipitation salt to be used is determined.Process according to at least one or more of Claims 1 to 12, characterized in that the insoluble salts formed in b), preferably strontium chromate, are used further as corrosion protection in alloys.

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