Quaternary nitric acid heat storage molten salt with low melting point and wide liquid range and preparation method thereof

Abstract

The invention discloses quaternary nitric acid heat storage molten salt with low melting point and wide liquid range and a preparation method thereof, and the quaternary nitric acid heat storage molten salt comprises the following components in percentage by mass: 5-9 wt% of sodium nitrate, 21-26 wt% of calcium nitrate, 48-55 wt% of potassium nitrate and 5-20 wt% of lithium nitrate. The quaternary nitric acid heat storage molten salt has a lower melting point, and the initial melting point is about 76 DEG C; the thermal stability is excellent, the temperature can be stably maintained at about 560 DEG C, and reliable application under the high-temperature working condition is guaranteed; the liquid range is 110-560 DEG C, the wide liquid range characteristic is achieved, and diversified industrial operation temperature scenes can be adapted; the specific heat capacity is up to 1.61 J g <-1 > K <-1 >, and the relatively high specific heat capacity can be excellent in heat storage and release links.

Description

Technical Field

[0001] The invention relates to the technical field of heat storage molten salts, and in particular to a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range and a preparation method thereof. Background Art

[0002] The current energy shortage problem is becoming increasingly serious, and the safe and stable storage and utilization of renewable energy is particularly important. Energy storage technology plays a huge role in solving problems such as low energy utilization caused by energy fluctuations and discontinuities. The combination of solar thermal power plants and molten salt heat storage systems effectively improves the thermal efficiency, stability and reliability of the system.

[0003] At present, the more mature commercial molten salt systems include Solar Salt (60% NaNO3 + 40% KNO3), Hitec (7% NaNO3 + 53% KNO3 + 40NaNO2) and Hitec L (45% KNO3 + 48% Ca (NO3) 2 + 7% KNO3). The working temperatures of these three mixed molten salts are: 220 ~ 600 ℃, 142 ~ 535 ℃ and 120 ~ 500 ℃. However, these three have many shortcomings when the heat storage system is running. For example, the high melting points of the three will not only cause blockage of the molten salt pipeline, but also increase the operating cost of the system; Hitec is very easy to decompose and produce toxic gases due to the instability of sodium nitrite in high temperature environment; Hitec XL has a large content of calcium nitrate, and its high viscosity will hinder the normal flow of liquid in the pipe.

[0004] With the development of the molten salt heat storage market, extensive research has been carried out on the design and development of multi-element low-melting-point molten salt heat storage materials. At present, the development of binary and ternary molten salts is relatively mature. Patent CN 106590546 A discloses a ternary mixed molten salt heat storage material, which is composed of KNO3, NaNO3 and LiNO3, and the measured melting point is 110-140°C; Patent CN107312502 A discloses a ternary molten salt system KNO3-Mg(NO3)2-NaNO3 based on KNO3-NaNO3, and Mg(NO3)2 is mixed in through a reasonable treatment method, and finally a formula with a melting point of 160°C is screened out, and its melting point is still too high; Patent CN 116218482 A discloses a low-melting-point ternary mixed molten salt heat storage material, which is composed of KNO3-Ca(NO3)2-NaNO3, and its melting point is about 125°C.

[0005] Regarding the study of quaternary system molten salt, patent CN 103074040 A discloses two low melting point mixed molten salt thermal storage materials, wherein the first formula is composed of KNO3, NaNO2, Ca(NO3)2, and NaNO3, with a melting point of about 130°C; the second formula is composed of KNO3, LiNO3, Ca(NO3)2, and NaNO3, and the components of the mixed molten salt reaching the lowest eutectic point are 50wt% KNO3, 20wt% LiNO3, 20wt% Ca(NO3)2, and 10wt% NaNO3, with a starting point (melting point) temperature of 82.2°C and an end point (eutectic point) temperature of 123.2°C, and the problem of high melting point still exists. The lowering of the melting point of the thermal storage material can reduce the heating temperature of the energy storage system, which reduces the power consumption for heating on the one hand, and reduces the heat loss at low temperature on the other hand. It is preliminarily estimated that the thermal storage efficiency increases by 0.6% to 0.8% for every 1°C reduction in the melting point. Therefore, designing and developing new mixed molten salts with excellent performance, low melting point, high specific heat and high decomposition temperature is of great significance to improving the economy and safety of the molten salt system. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range, which has a lower melting point and avoids the problems of high melting point of the multi-component heat storage molten salt in the prior art, which leads to easy blockage of the molten salt pipeline and high operating cost of the system; at the same time, the quaternary nitric acid heat storage solution of the present invention has strong stability and is not easy to decompose; the preparation process of the molten salt is simple, and it is convenient for industrial production.

[0007] In order to solve the above technical problems, the present invention provides a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range, which comprises, by mass percentage, 5-9wt% of sodium nitrate, 21-26wt% of calcium nitrate, 48-55wt% of potassium nitrate, and 5-20wt% of lithium nitrate.

[0008] Furthermore, by mass percentage, it includes 7-8wt% of sodium nitrate, 21-24wt% of calcium nitrate, 48-52wt% of potassium nitrate, and 15-19wt% of lithium nitrate.

[0009] Furthermore, by mass percentage, it includes 7.83 wt % of sodium nitrate, 23.32 wt % of calcium nitrate, 50.85 wt % of potassium nitrate, and 18 wt % of lithium nitrate.

[0010] Based on the prior art, multi-component molten salts are used to obtain components with lower melting points by trial and error, but it is difficult to obtain the components with the lowest melting point, and the steps are cumbersome and inefficient. The components of the quaternary nitric acid heat storage molten salt of the present invention are calculated by the optimized thermodynamic model established by the 6 sub-binary systems and 4 sub-ternary systems corresponding to the quaternary system, and verified by experiments. The established quaternary mixed salt phase diagram model has self-consistent characteristics and can be reduced to corresponding ternary and binary phase diagrams. Among them, the 6 sub-binary systems are NaNO3-Ca(NO3)2, KNO3-Ca(NO3)2, LiNO3-Ca(NO3)2, NaNO3-KNO3, NaNO3-LiNO3, KNO3-LiNO3; the 4 ternary systems are NaNO3-KNO3-LiNO3, NaNO3-KNO3-Ca(NO3)2, NaNO3-LiNO3-Ca(NO3)2, KNO3-LiNO3-Ca(NO3)2.

[0011] The thermodynamic model calculation method comprises the following steps:

[0012] (1) A thermodynamic database was built based on the thermodynamic parameters of pure single-component salts (NaNO3, KNO3, LiNO3, Ca(NO3)2), and the liquid phase Gibbs free energy expression of all sub-binary systems involved in the quaternary system was constructed:

[0013] G=n A 0 G A +n B 0 G B -TΔS config +(n AB Δg AB / twenty one)

[0014] Where n A and n B are the mole fractions of A and B, respectively, n AB is the mole fraction of the (AB) pair, 0 G A and 0 G B is the Gibbs free energy of components A and B, ΔS config is the configurational entropy, which is described by randomly distributed component pairs (AA), (BB) and (AB), Δg AB are model parameters, where and is the model parameter to be determined, T is the temperature, unit K. In each binary system, the parameter Δg in formula (1) AB Expand to an empirical polynomial:

[0015]

[0016] Formula (2) is used to perform thermodynamic optimization on the binary system, where and are the model parameters to be determined.

[0017] (2) The six binary sub-systems of NaNO3-Ca(NO3)2, KNO3-Ca(NO3)2, LiNO3-Ca(NO3)2, NaNO3-KNO3, NaNO3-LiNO3, and KNO3-LiNO3 were mixed in mass ratios of 1:9, 2:8, ..., 9:1 by the melt high-temperature mixing method, and then DSC tests were performed to record the liquidus point (termination point) temperature of each system.

[0018] (3) Optimize the thermodynamic parameters of the binary system in step (1) based on the liquidus point data obtained from the binary system experiment in step (2).

[0019] (4) Similarly, based on the thermodynamic parameters of the six binary systems, four ternary system thermodynamic databases were constructed, the ternary phase diagram was drawn, the eutectic composition and temperature of the ternary system were calculated, and the thermodynamic parameters of the ternary system were optimized through experiments.

[0020] (5) Based on the thermodynamic parameters of binary and ternary systems, a self-consistent thermodynamic database of quaternary systems is constructed, and the quaternary phase diagram is drawn to deduce the ratio and temperature of the eutectic point of the quaternary system. Among them, the quaternary system can be reversed to the ternary system and the binary system, that is, when any component of the quaternary system is set to zero, it can be completely reduced to the ternary system and remain strictly equal to the ternary system phase diagram.

[0021] The method for constructing a theoretical model of a quaternary molten salt phase diagram of the present invention can accurately guide the preparation of mixed salts with a wide liquid range and low melting points, and effectively improve the configuration accuracy and efficiency of multi-component mixed molten salts.

[0022] The second aspect of the present invention provides a method for preparing the quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range as described in the first aspect, comprising the following steps:

[0023] S1. Grind and dry sodium nitrate, calcium nitrate, potassium nitrate and lithium nitrate respectively for later use;

[0024] S2. Weigh sodium nitrate, calcium nitrate, potassium nitrate and lithium nitrate according to the mass percentage of each component, and mix them evenly to obtain a mixed powder;

[0025] S3, melting the mixed powder at high temperature under vacuum state;

[0026] S4, grinding into powder after cooling, and melting again at high temperature under vacuum state;

[0027] S5. Repeat step S4 1-3 times to obtain a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range.

[0028] Furthermore, in S1, the drying is specifically as follows: placing the ground raw materials in a muffle furnace, heating the temperature to 140-160° C. at a heating rate of 10-20° C. / min, and drying at the temperature for 20-24 hours.

[0029] Furthermore, in S2, the mixing specifically includes placing the raw materials in an alumina crucible and mixing them by mechanical stirring.

[0030] Furthermore, in S3 and S4, the high temperature melting temperature is 350±10°C.

[0031] Furthermore, in S3, the high temperature melting time is 10-12 hours.

[0032] Furthermore, in S4, the high temperature melting time is 20-24 hours.

[0033] Furthermore, in S1 and S4, the particle size of the powder after grinding is 50-200 mesh.

[0034] Beneficial effects of the present invention:

[0035] The quaternary nitric acid heat storage molten salt of the present invention has the characteristics of low melting point, wide liquid range, and high specific heat capacity, and can meet the needs of industrial applications, specifically:

[0036] The quaternary nitric acid heat storage molten salt of the present invention has a lower melting point, with an initial melting point of about 76°C. Compared with Solar Salt, Hitec and Hitec XL, the melting points are significantly reduced by 144°C, 66°C and 44°C respectively, showing the advantage of low melting point.

[0037] The quaternary nitric acid heat storage molten salt of the present invention has excellent thermal stability and can be stably maintained at about 560° C., ensuring reliable application under high temperature conditions.

[0038] The corresponding liquid temperature zone (liquid range) of the quaternary nitrate molten salt of the present invention during normal use spans 110°C to 560°C, has the characteristics of a wide liquid range, and can be adapted to a variety of industrial operating temperature scenarios.

[0039] The specific heat capacity of the quaternary nitrate molten salt of the present invention is as high as 1.61 J g -1 K -1 , the higher specific heat capacity can perform well in heat storage and release.

[0040] This patent uses the method of thermodynamic phase diagram to build a self-consistent thermodynamic phase diagram model of quaternary nitrates. The eutectic composition of the low-melting-point quaternary nitrate molten salt system is screened out, avoiding the blindness and tediousness of the traditional "trial and error method", improving the accuracy of predicting eutectic composition and temperature, and providing new ideas and methods for finding multi-component low-melting-point molten salt systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solution of the present invention, the drawings required for use in the embodiments are briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0042] Figure 1 is the temperature-DSC curve of the quaternary nitrate molten salt sample of Example 1 of the present invention;

[0043] Figure 2 This is the temperature-DSC curve of the quaternary nitrate molten salt sample of Example 2 of the present invention;

[0044] Figure 3 is the specific heat capacity curve of the quaternary nitrate molten salt sample of Example 2 of the present invention;

[0045] Figure 4 is the temperature-TG curve of the quaternary nitrate molten salt sample of Example 2 of the present invention;

[0046] Figure 5 This is the temperature-DSC curve of the quaternary nitrate molten salt sample of Example 3 of the present invention;

[0047] Figure 6 This is the temperature-DSC curve of the quaternary nitrate molten salt sample of Comparative Example 1 of the present invention;

[0048] Figure 7 This is the temperature-DSC curve of the quaternary nitrate molten salt sample of Comparative Example 2 of the present invention;

[0049] Figure 8 It is the ternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3 when LiNO3=0wt%;

[0050] Fig. 9 It is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=5wt%;

[0051] Fig.10 It is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=10wt%;

[0052] Fig.11 It is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=15wt%;

[0053] Fig.12 It is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=18wt%;

[0054] Fig.13 It is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=20wt%;

[0055] Fig.14 It is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=25wt%;

[0056] Fig.15 This is the quaternary thermodynamic phase diagram of KNO3-Ca(NO3)2-NaNO3-LiNO3 when LiNO3=30wt%. DETAILED DESCRIPTION

[0057] The technical solution of the present invention will be described clearly and completely below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] Example 1

[0059] This embodiment relates to a method for preparing a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range, comprising the following steps:

[0060] Step 1, using a 0.1 mg electronic analytical balance to weigh 15 wt% lithium nitrate, 50.30 wt% potassium nitrate, 26.06 wt% calcium nitrate and 8.63 wt% sodium nitrate after drying, and mechanically stirring and mixing them in an alumina crucible;

[0061] Step 2: Place the evenly mixed sample in a muffle furnace under vacuum at 350°C for 12 hours to melt and mix the sample. After the sample is cooled, place it in a glove box and grind it into powder. Place it in a muffle furnace under vacuum at 350°C for 24 hours.

[0062] Step 3: After the sample is cooled, it is placed in a glove box and ground into powder to obtain the quaternary nitrate molten salt heat storage material.

[0063] The quaternary nitrate molten salt sample material obtained in this embodiment was subjected to thermal physical property tests, such as Figure 1 As shown, the eutectic point (termination point) is 112.05°C.

[0064] Example 2

[0065] This embodiment relates to a method for preparing a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range, comprising the following steps:

[0066] Step 1, using a 0.1 mg electronic analytical balance to weigh 18 wt% lithium nitrate, 50.85 wt% potassium nitrate, 23.32 wt% calcium nitrate and 7.83 wt% sodium nitrate after drying, and mechanically stirring and mixing them in an alumina crucible;

[0067] Step 2: Place the evenly mixed sample in a muffle furnace under vacuum at 350°C for 12 hours to melt and mix the sample. After the sample is cooled, place it in a glove box and grind it into powder. Place it in a muffle furnace under vacuum at 350°C for 24 hours.

[0068] Step 3: After the sample is cooled, it is placed in a glove box and ground into powder to obtain the quaternary nitrate molten salt heat storage material.

[0069] The quaternary nitrate molten salt sample material obtained in this embodiment was subjected to thermal physical property tests, such as Figure 2 As shown in Figure 1, the eutectic point (termination point) is 110.70°C. Figure 3 As shown, the average specific heat capacity of the molten salt is 1.61 J g -1 K -1 .like Figure 4 As shown in the figure, the TG curve of the molten salt from room temperature to 500℃ is a horizontal straight line. The molten salt begins to volatilize a small amount from 541.32℃, and the mass loss is 1.5% at this time, which shows that the molten salt has high stability. When the temperature rises to 560℃, the mass loss reaches 3%, which is the decomposition temperature of the salt. When the temperature rises to 586.5℃, the mass loss is 7%, reaching the limit use temperature.

[0070] Example 3

[0071] This embodiment relates to a method for preparing a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range, comprising the following steps:

[0072] Step 1, using a 0.1 mg electronic analytical balance to weigh 20 wt% lithium nitrate, 49.43 wt% potassium nitrate, 22.51 wt% calcium nitrate and 8.06 wt% sodium nitrate, and mechanically stirring and mixing them in an alumina crucible;

[0073] Step 2: Place the evenly mixed sample in a muffle furnace under vacuum at 350°C for 12 hours to melt and mix the sample. After the sample is cooled, place it in a glove box and grind it into powder. Place it in a muffle furnace under vacuum at 350°C for 24 hours.

[0074] Step 3: After the sample is cooled, it is placed in a glove box and ground into powder to obtain the quaternary nitrate molten salt heat storage material.

[0075] The quaternary nitrate molten salt sample material obtained in the embodiment was subjected to thermal physical property test, such as Figure 5 As shown, the eutectic point (termination point) is 112.55°C.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 is that the ratio of the raw materials is: 6.56wt% lithium nitrate, 45.32wt% potassium nitrate, 37.68wt% calcium nitrate and 10.44wt% sodium nitrate. The quaternary nitrate molten salt sample material obtained in the comparative example was tested for thermal properties, and the starting temperature was 81.7℃ and the peak point was 91.5℃. Figure 6 It can be seen that the molten salt has not reached the eutectic point. As the temperature increases, the heat flow curve changes greatly and has poor stability.

[0078] Comparative Example 2

[0079] The difference between this comparative example and Example 1 is that the ratio of the raw materials is: 20wt% lithium nitrate, 50wt% potassium nitrate, 20wt% calcium nitrate and 10wt% sodium nitrate. The quaternary nitrate molten salt sample material obtained in the comparative example was subjected to thermal property tests, as shown in FIG. Figure 7 As shown, the eutectic point (termination point) temperature is 120.3°C.

[0080] The comparison of the proportions and thermal properties test results of the quaternary nitric acid heat storage molten salts of Examples 1-3 and Comparative Examples 1-2 is shown in Table 1.

[0081] Table 1

[0082]

[0083] As can be seen from Table 1, the quaternary nitric acid heat storage molten salt of the present invention has a lower melting point, especially in Example 2, the quaternary nitric acid heat storage molten salt has a lower melting point, and the starting melting point is about 76° C. The melting point in Comparative Example 2 is relatively high. Although the starting point and peak point temperatures in Comparative Example 1 are not high, the molten salt does not reach the eutectic point. As the temperature increases, the heat flow curve changes greatly and the stability is poor.

[0084] The following examples provide a method for predicting the proportion of a quaternary nitric acid heat storage molten salt and a eutectic point, comprising the following steps:

[0085] Step 1: Build a thermodynamic database based on the thermodynamic parameters of pure single-component salts (NaNO3, KNO3, LiNO3, Ca(NO3)2), and combine the experimental data of binary systems to evaluate and optimize the liquid phase Gibbs free energy of all sub-binary systems involved in the quaternary system.

[0086] The formula for the liquid phase Gibbs free energy is as follows:

[0087] G=n A 0 G A +n B 0 G B -TΔS config +(n AB Δg AB / twenty three)

[0088] Where n A and n B are the mole fractions of A and B, respectively, n AB is the mole fraction of the (AB) pair, 0 G A and 0 G B is the Gibbs free energy of components A and B, Δg AB is the model parameter, ΔS config is the configurational entropy, which is described by randomly distributed pairs of components (AA), (BB) and (AB):

[0089]

[0090] Where n m and X m are the mole number and mole fraction of the component (m = A or B) in the cationic sublattice, respectively. m is the equivalent mole fraction of the component. mn and X mn are the number of moles and mole fraction of nm, respectively. m , Y m and X mn The expression is as follows:

[0091] X m =n m / (n A +n B ) (5)

[0092] Y m =Z m n m / (Z A n A +ZB n B )=Z m X m / (Z A X A +Z B X B ) (6)

[0093] X mn =n mn / (n AA +n BB +n AB ) (7)

[0094] In the formula, Z A and Z B are the coordination numbers of cations A and B respectively. They are related to n m and n mn The relationship is as follows:

[0095] Z A n A =2n AA +n AB (8)

[0096] Z B n B =2n BB +n AB (9)

[0097] In each binary system, the parameter Δg in equation (3) AB It can be expanded into an empirical polynomial:

[0098]

[0099] Formula (10) is used to perform thermodynamic optimization on the binary system, where and are the model parameters to be determined.

[0100] Among them, the experimental data were obtained specifically as follows: the six sub-binary systems of NaNO3-Ca(NO3)2, KNO3-Ca(NO3)2, LiNO3-Ca(NO3)2, NaNO3-KNO3, NaNO3-LiNO3, and KNO3-LiNO3 were mixed in mass ratios of 1:9, 2:8...9:1 by the melt high-temperature mixing method, and then DSC tests were performed to record the liquidus point (termination point) temperature of each system.

[0101] The thermodynamic binary interaction parameters of the binary system are listed in Table 2.

[0102] Table 2

[0103]

[0104]

[0105] Step 2: Extrapolate from the binary system to the ternary system, and use the thermodynamic data of the binary system combined with the experimental data of the ternary system to evaluate the thermodynamic model parameters of the ternary system.

[0106] For the NaNO3-KNO3-LiNO3 ternary system, the composition and temperature of the eutectic point were extrapolated using the binary parameters of NaNO3-KNO3, NaNO3-LiNO3 and KNO3-LiNO3, respectively, and the composition and temperature of the eutectic point after optimization were 9.92wt%NaNO3-59.68wt%KNO3-30.40wt%LiNO3 and 125.97℃, respectively.

[0107] The thermodynamic parameter optimization results of the NaNO3-KNO3-LiNO3 ternary system are as follows:

[0108] Δg LiNa =(-1799.30+3.79T)-769.86X LiLi -1145.33X NaNa +176.10816Y K

[0109] For the NaNO3-KNO3-Ca(NO3)2 ternary system, the composition and temperature of the eutectic point were extrapolated using the binary parameters of NaNO3-KNO3, NaNO3-Ca(NO3)2 and KNO3-Ca(NO3)2, which were 7.35wt%NaNO3-51.61wt%KNO3-41.04wt%Ca(NO3)2 and 140.60°C, respectively. The composition and temperature of the eutectic point obtained after optimization were 12.92wt%NaNO3-48.00wt%KNO3-39.08wt%Ca(NO3)2 and 130.72°C, respectively.

[0110] The thermodynamic parameter optimization results of the NaNO3-KNO3-Ca(NO3)2 ternary system are as follows:

[0111]

[0112] For the NaNO3-LiNO3-Ca(NO3)2 ternary system, the composition and temperature of the eutectic point were extrapolated using the binary parameters of NaNO3-LiNO3, NaNO3-Ca(NO3)2 and LiNO3-Ca(NO3)2, which were 45.10wt%NaNO3-32.37wt%LiNO3-22.53wt%Ca(NO3)2 and 184.68°C, respectively. The composition and temperature of the eutectic point obtained after optimization were 43.86wt%NaNO3-30.87wt%LiNO3-25.27wt%Ca(NO3)2 and 182.34°C, respectively.

[0113] The thermodynamic parameter optimization results of the NaNO3-LiNO3-Ca(NO3)2 ternary system are as follows:

[0114]

[0115] For the KNO3-LiNO3-Ca(NO3)2 ternary system, the composition and temperature of the eutectic point were extrapolated using binary parameters of KNO3-LiNO3, KNO3-Ca(NO3)2 and LiNO3-Ca(NO3)2, which were 57.19wt%KNO3-20.72wt%LiNO3-22.09wt%Ca(NO3)2 and 118.92°C, respectively. The composition and temperature of the eutectic point obtained after optimization were 56.08wt%KNO3-20.46wt%LiNO3-23.46wt%Ca(NO3)2 and 115.45°C, respectively.

[0116] The thermodynamic parameter optimization results of the KNO3-LiNO3-Ca(NO3)2 ternary system are as follows:

[0117]

[0118] Step 3: Combine the thermodynamic parameters of the binary system with the thermodynamic parameters of the ternary system to establish a quaternary system thermodynamic database. Using the quaternary system thermodynamic database, add different proportions of LiNO3 to the KNO3-Ca(NO3)2-NaNO3 ternary phase diagram to draw a quaternary phase diagram, where LiNO3 is added in increments of 1wt% by mass, such as Figure 8-15 , based on the quaternary phase diagram drawn with different proportions of LiNO3, the components with the predicted lowest eutectic point were screened out.

[0119] like Figure 8 As shown, when LiNO3=0wt%, the lowest eutectic point composition is 0wt% lithium nitrate, 48.00wt% potassium nitrate, 39.08wt% calcium nitrate and 12.92wt% sodium nitrate, and its eutectic temperature is 130.72°C.

[0120] like Fig. 9 As shown, when LiNO3=5wt%, the lowest eutectic point composition is 5wt% lithium nitrate, 48.57wt% potassium nitrate, 34.81wt% calcium nitrate and 11.62wt% sodium nitrate, and its eutectic temperature is 124.84°C.

[0121] like Fig.10 As shown, when LiNO3=10wt%, the lowest eutectic point composition is 10wt% lithium nitrate, 49.39wt% potassium nitrate, 30.51wt% calcium nitrate and 10.10wt% sodium nitrate, and its eutectic temperature is 119.15°C.

[0122] like Fig.11 As shown, when LiNO3=15wt%, the lowest eutectic point composition is 15wt% lithium nitrate, 50.30wt% potassium nitrate, 26.06wt% calcium nitrate and 8.63wt% sodium nitrate, and its eutectic temperature is 113.05℃, which is only 1℃ different from the measured eutectic temperature of 112.05℃ in Example 1, and the predicted results are basically consistent.

[0123] like Fig.12 As shown, when LiNO3=18wt%, the lowest eutectic point composition is 18wt% lithium nitrate, 50.85wt% potassium nitrate, 23.32wt% calcium nitrate and 7.83wt% sodium nitrate, and its eutectic temperature is 109.03℃, which is only 1.67℃ different from the measured eutectic temperature of 110.70℃ in Example 2, and the predicted results are basically consistent.

[0124] like Fig.13 As shown, when LiNO3=20wt%, the lowest eutectic point composition is 20wt% lithium nitrate, 49.43wt% potassium nitrate, 22.51wt% calcium nitrate and 8.06wt% sodium nitrate, and its eutectic temperature is 110.69°C, which is only 1.86°C different from the measured eutectic temperature of 112.55°C in Example 3, and the predicted results are basically consistent.

[0125] like Fig.14 As shown, when LiNO3=25wt%, the lowest eutectic point composition is 25wt% lithium nitrate, 49.43wt% potassium nitrate, 16.53wt% calcium nitrate and 9.04wt% sodium nitrate, and its eutectic temperature is 117.54°C.

[0126] like Fig.15 As shown, when LiNO3=30wt%, the lowest eutectic point composition is 30wt% lithium nitrate, 52.19wt% potassium nitrate, 7.76wt% calcium nitrate and 10.05wt% sodium nitrate, and its eutectic temperature is 124.01°C.

[0127] When the proportion of each component of the NaNO3-KNO3-LiNO3-Ca(NO3)2 quaternary system is 5-9wt% sodium nitrate, 21-26wt% calcium nitrate, 48-55wt% potassium nitrate and 10-20wt% lithium nitrate, the eutectic temperature is 110-120℃. The molten salt material prepared by the lowest eutectic point shows excellent performance, with a melting point (initial melting point) of about 76℃ and can be stably maintained at about 560℃, ensuring reliable application under high temperature conditions, and a specific heat capacity of up to 1.61J·g -1 ·K -1 .

[0128] The present invention has been described in detail above in conjunction with specific implementations and exemplary examples, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art understand that, without departing from the spirit and scope of the present invention, a variety of equivalent substitutions, modifications or improvements may be made to the technical solution of the present invention and its implementation methods, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be subject to the attached claims.

Claims

1. A quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range, characterized in that: Calculated by mass percentage, the invention comprises 5-9wt% of sodium nitrate, 21-26wt% of calcium nitrate, 48-55wt% of potassium nitrate and 5-20wt% of lithium nitrate.

2. The quaternary nitrate heat storage molten salt with a low melting point and a wide liquid range as claimed in claim 1, characterized in that: Calculated by mass percentage, the invention comprises 7-8wt% of sodium nitrate, 21-24wt% of calcium nitrate, 48-52wt% of potassium nitrate and 15-19wt% of lithium nitrate.

3. The low melting point and wide liquid range quaternary nitrate heat storage molten salt according to claim 1, characterized in that: Calculated by mass percentage, it includes 7.83wt% of sodium nitrate, 23.32wt% of calcium nitrate, 50.85wt% of potassium nitrate, and 18wt% of lithium nitrate.

4. A method for preparing the quaternary nitric acid heat storage molten salt with low melting point and wide liquid range as described in any one of claims 1 to 3, characterized in that: The steps include: S1. Grind and dry sodium nitrate, calcium nitrate, potassium nitrate and lithium nitrate respectively for later use; S2. Weigh sodium nitrate, calcium nitrate, potassium nitrate and lithium nitrate according to the mass percentage of each component, and mix them evenly to obtain a mixed powder; S3, melting the mixed powder at high temperature under vacuum state; S4, after cooling, grinding into powder, and melting again at high temperature under vacuum state; S5. Repeat step S4 1-3 times to obtain a quaternary nitric acid heat storage molten salt with a low melting point and a wide liquid range.

5. The method for preparing the quaternary nitric acid heat storage molten salt with low melting point and wide liquid range as claimed in claim 4, characterized in that: In S1, the drying is specifically as follows: placing the ground raw materials in a muffle furnace, heating the temperature to 140-160° C. at a heating rate of 10-20° C. / min, and drying at the temperature for 20-24 hours.

6. The method for preparing the quaternary nitric acid heat storage molten salt with low melting point and wide liquid range as claimed in claim 4, characterized in that: In S2, the mixing specifically includes placing the raw materials in an alumina crucible and mixing them by mechanical stirring.

7. The method for preparing the low melting point and wide liquid range quaternary nitric acid heat storage molten salt according to claim 4, characterized in that: In S3 and S4, the high temperature melting temperature is 350±10°C.

8. The method for preparing the low melting point and wide liquid range quaternary nitric acid heat storage molten salt according to claim 4, characterized in that: In S3, the high temperature melting time is 10-12 hours.

9. The method for preparing the low melting point and wide liquid range quaternary nitric acid heat storage molten salt according to claim 4, characterized in that: In S4, the high temperature melting time is 20-24 hours.

10. The method for preparing the quaternary nitric acid heat storage molten salt with low melting point and wide liquid range as claimed in claim 4, characterized in that: In S1 and S4, the particle size of the powder after grinding is 50-200 mesh.

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