Liquid metal convection apparatus, seawater desalination system, and seawater desalination method

Abstract

To provide a liquid metal convection device, a seawater desalination system and a seawater desalination method for separating and recovering valuable resources from seawater.SOLUTION: A liquid metal convection device 1 configures a liquid metal convection loop, and accommodates liquid tin as a coolant to generate convective circulation as shown by arrows. The liquid metal convection device 1 comprises: high-temperature piping 11; low-temperature piping 12; a seawater contact chamber 13; an alkali metal separation recovery unit 14 provided at a first place of the low-temperature piping 12; a heavy metal separation recovery unit 15 provided at a second place of the low-temperature piping 12; and a gas component high-temperature desorption unit 16 provided at a third place of the high-temperature piping 11. At both ends of the seawater contact chamber 13, the high-temperature piping 11 and the low-temperature piping 12 are connected in a loop shape. At least on one side of a lower part, the high-temperature piping 11 directly receives sunlight to collect the sunlight and be heated by a parabolic dish type collector 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a liquid metal convection apparatus, a seawater desalination system, and a seawater desalination method for separating and recovering valuable resources from seawater. [Background technology]

[0002] To alleviate water shortages, there is a method of desalination that desalinates seawater to produce fresh water for drinking, industrial use, etc.

[0003] Conventional seawater desalination methods include the reverse osmosis method, which uses a permeable membrane to separate minerals from seawater under high pressure to obtain fresh water, and evaporation methods such as the multi-stage flash method and the multi-effect method, which use thermal energy to evaporate seawater and then cool the resulting steam to obtain fresh water. However, in the multi-stage flash method, the deposition of sparingly soluble salts called scale on the surface of the heat transfer tubes that transmit heat to the seawater hinders heat transfer and reduces the efficiency of freshwater production.

[0004] The conventional seawater desalination methods described above consume large amounts of energy to apply high pressure to seawater or to impart thermal energy to it. As a result, environmental problems such as air pollution due to the use of large amounts of fossil fuels and noise from accidents during fuel transportation occur. Therefore, as a way to solve the environmental problem of large amounts of energy consumption in seawater desalination technology, there are methods that utilize sustainable energy sources such as nuclear reactor energy and solar heat. Methods for collecting this solar heat include parabolic trough type collectors, linear Fresnel type collectors (see Patent Document 1), tower type collectors, and parabolic dish type collectors. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] CN109534428A [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, in the above-mentioned conventional seawater desalination method, seawater from which moisture has been removed is produced as high-concentration salt water. When this high-concentration salt water is discharged into the ocean, there is a problem that it affects abiotic chemical composition, abiotic dissolved oxygen concentration, biological diversity, and biological metabolic rate. On the other hand, the acceleration of industrialization has increased the demand for mineral resources, and new supply sources are being sought. Seawater contains abundant natural minerals as valuable resources; in particular, there is a demand for recovering alkali metals such as magnesium and lithium, and rare metals such as strontium and molybdenum from seawater depending on future technological development and increases in market prices. [Means for Solving the Problems]

[0007] In order to solve the above-mentioned problems, the liquid metal convection device according to the present invention comprises a seawater contact chamber whose upper part is open to the atmosphere, and at both ends of the seawater contact chamber Loop-shaped connected high temperature that can be heated piping and insulated low-temperature piping , comprising death, said seawater contact chamber ,high temperature and said piping and low-temperature piping form Consists of tin or tin alloy as a heat / mass transfer medium. a liquid metal convection loop for accommodating liquid metal and causing the liquid metal to convect, and further low temperature comprises an alkali metal separation and recovery unit for separating and recovering alkali metals dissolved in the liquid metal at a first location of the piping.

[0008] Further, the seawater desalination system according to the present invention comprises the above-mentioned liquid metal convection device, and high temperature a heating means for heating at least a part of the lower side of the piping.

[0009] Further, the seawater desalination method according to the present invention comprises: Made of tin or tin alloy a step of accommodating liquid metal in a loop shape; a step of partially heating the liquid metal to cause natural convection of the liquid metal; a seawater contacting step of bringing seawater into direct contact with the naturally convected liquid metal to evaporate fresh water, and dissolving alkali metals, heavy metals and gas components of seawater into the liquid metal; and separating the alkali metals and heavy metals dissolved in the liquid metal andat least one gaseous component is separation valuable resources to be recovered separation a recovery step. [Advantageous Effects of Invention]

[0010] According to the present invention, valuable resources contained in seawater can be recovered while suppressing discharge of high-concentration salt water simultaneously with seawater desalination. [Brief Description of Drawings]

[0011] [Figure 1] It is a figure showing an embodiment of the seawater desalination system according to the present invention. [Figure 2] It is a cross-sectional view of the liquid metal convection device of FIG. 1, wherein (A) shows a cross-section of a pipe, and (B) shows a cross-section of a seawater contact chamber. [Figure 3] It is a graph showing Gibbs free energy for formation reaction formulas of alkali metal chlorides and oxides in liquid tin, wherein (A) shows Gibbs free energy of formation per chlorine atom for the chloride formation reaction formula, and (B) shows Gibbs free energy of formation per oxygen atom for the oxide formation reaction formula. [Figure 4] It is a schematic diagram for explaining the operation of the liquid metal convection device of FIG. 1. [Figure 5] It is a flowchart for explaining the design procedure of the liquid metal convection device of FIG. 1. [Figure 6] It is a graph showing characteristics of the liquid metal convection device of FIG. 1, wherein (A) shows heating length / wall surface temperature characteristics, and (B) shows heating length / loop aspect ratio characteristics. [Figure 7] It is a table showing the dissolution amount and concentration of valuable metals in liquid tin obtained by the seawater desalination system of FIG. 1. [Figure 8] It is a graph showing, as mass increase, results obtained from an oxidation experiment targeting stationary liquid tin in an atmospheric environment performed without bringing seawater into contact, regarding the surface oxidation characteristics of liquid tin in the seawater contact chamber of FIG. 1. [Figure 9] It is a table showing elemental components of artificial seawater used in the seawater dropping experiment of FIG. 8. [Figure 10] Figure 1 shows the surface condition of tin after artificial seawater is dropped onto liquid tin, with (A) being a scanning electron microscope (SEM) image and (B) being a graph of the analysis results from an energy-dispersive X-ray spectrometer (EDX) attached to the scanning electron microscope. [Figure 11] Figure 1 shows scanning electron microscope (SEM) images and EDX elemental mapping results illustrating the surface state of liquid tin after artificial seawater was added to it. [Figure 12] Figure 1 shows the surface condition of the tin after artificial seawater was dropped onto the liquid tin, and the graph shows the results of X-ray spectroscopy (XRD) analysis. [Figure 13] This is a diagram illustrating the valuable metals that should be recovered. [Modes for carrying out the invention]

[0012] Figure 1 shows an embodiment of the seawater desalination system according to the present invention.

[0013] In Figure 1, the liquid metal convection device 1 constitutes a liquid metal convection loop formed from 316L austenitic steel, etc. Heat / mass transfer medium As such, a liquid metal, such as liquid tin, is contained and circulated by convection as indicated by the arrow. The liquid metal convection device 1 consists of a high-temperature pipe 11, a low-temperature pipe 12, a seawater contact chamber 13, an alkali metal separation and recovery unit 14 located at a first location in the low-temperature pipe 12, a heavy metal separation and recovery unit 15 located at a second location in the low-temperature pipe 12, and a gas component high-temperature desorption unit 16 located at a third location in the high-temperature pipe 11. In this case, the high-temperature pipe 11 and the low-temperature pipe 12 are connected in a loop to both ends of the seawater contact chamber 13. The high-temperature pipe 11 and the low-temperature pipe 12 may be an integrated or separate structure, and as shown in Figure 2(A), for example, they are annular with an outer diameter of 254 mm and a wall thickness of 20 mm, while the seawater contact chamber 13, as shown in Figure 2(B), is rectangular with a width of 100 cm and a height of 5 cm or more, and its upper side is open to the atmosphere. Heat / mass transfer mediumPossible liquid metals for this purpose include sodium, lead, and tin. However, sodium is undesirable because it can cause dangerous reactions when in direct contact with seawater. Furthermore, considering that freshwater may also be used as drinking water, toxic lead is undesirable. Therefore, in the embodiments of this invention, tin or an alloy thereof is used. The density ρ of tin is, ρ = 7374.7 - 675 × 10 -3 T(kg / m 3 ) However, T is absolute temperature (K). It is expressed as follows, and is highly temperature-dependent. Heat / mass transfer medium It functions adequately as such. However, if other than tin is suitable... Heat / mass transfer medium If there is a metal, then that metal Heat / mass transfer medium It can be used as such.

[0014] The high-temperature pipe 11 is heated by direct sunlight on at least one side of its lower section, for example, the left side in Figure 1, and by collecting solar heat through a parabolic dish-type collector 2 with a diameter of, for example, 15 m. In this case, the output Q of the parabolic dish-type collector 2 and the length of the heated section of the high-temperature pipe 11 heated by direct sunlight are defined as the heating length X. Furthermore, the temperature distribution of the heated section of the high-temperature pipe 11 is sloped as much as possible to promote natural convection of the liquid tin. The parabolic dish-type collector 2 receives, for example, direct solar radiation q of 778 W / m². 2 The light-gathering area A is (15 m / 2) 2 If we let it be π, then the output Q of the parabolic dish type collector 2 is: Q = A × q = 137 × 10 3 W (1) This is the result.

[0015] As described later, oxidation of liquid tin by oxygen occurs at temperatures above 500°C, and its melting point is 232°C. Therefore, the temperatures of the high-temperature pipe 11 and low-temperature pipe 12 flanking the seawater contact chamber 13 in the liquid metal convection apparatus 1 are set to, for example, 450°C and 300°C. In this case, the physical properties of the liquid tin in the high-temperature pipe 11 are as follows: density is 6886 kg / m³. 3The viscosity coefficient is 1.21 mPa·s, the specific heat is 0.237 kJ / (kg·K), and the thermal conductivity is 34.9 W / (m·K). The physical properties of liquid tin in the low-temperature pipe 12 are as follows: density is 6987 kg / cm³. 3 The viscosity coefficient is 1.55 mPa·s, the specific heat is 0.241 kJ / (kg·K), and the thermal conductivity is 32.0 W / (m·K). The average of these values ​​is used in the design of the liquid metal convection apparatus 1. Therefore, the Prandtl number Pr of the seawater contact chamber 13 is 0.012 at 337 °C and 0.0094 at 537 °C, so Pr = 0.01.

[0016] Seawater SW is supplied to the seawater contact chamber 13 from the seawater tank 3 by the seawater supply pump 3a, and the seawater SW is sprayed in the seawater contact chamber 13. On the other hand, the freshwater W generated in the seawater contact chamber 13 is supplied to the freshwater tank 4. At this time, heat exchange occurs between the seawater SW and the freshwater W by the heat exchanger 5, which improves the desalination efficiency of the seawater desalination system.

[0017] The alkali metal separation and recovery unit 14 precipitates at predetermined positions from the upstream to the downstream side of the liquid tin, in accordance with the order of thermodynamic stability of alkali metal chlorides and oxides, that is, the order of the Gibbs free energies in the formation reaction equations of the chlorides and oxides, from lowest to highest. Therefore, alkali metal chlorides and oxides dissolved in liquid tin can be separated and recovered. As a means of separation and recovery, multiple valved cartridges, similar to those used in the heavy metal separation and recovery unit 15 described later, can be provided.

[0018] Figure 3 is a graph showing the Gibbs free energies of the formation reaction equations for alkali metal chlorides and oxides in liquid tin, where (A) shows the Gibbs free formation energy per chlorine atom in the chloride formation reaction equation, and (B) shows the Gibbs free formation energy per oxygen atom in the oxide formation reaction equation.

[0019] As shown in Figure 3(A), in the chloride, sodium chloride (NaCl) and lithium chloride (LiCl) are thermodynamically stable at high temperatures (450 °C) and low temperatures (300 °C). In particular, the result for sodium chloride (NaCl) is consistent with the precipitation on the surface of liquid tin described later. The reaction equation for the formation of tin chloride (1 / 2SnCl2) at 300 °C is: SnO + 2NaCl → SnCl2+ Na2O + dG (= 367 kJ / mol) Therefore, it is thermodynamically unstable. Consequently, the order of thermodynamic stability is LiCl, NaCl, 1 / 2MgCl2, 1 / 2SnCl2, and HCl. When chlorine is saturated, LiCl, NaCl, 1 / 2MgCl2, 1 / 2SnCl2, and HCl precipitate in that order at predetermined positions within the alkali metal separation and recovery unit 14, allowing for separation and recovery.

[0020] As shown in Figure 3(B), in oxides, magnesium oxide (MgO), lithium oxide (Li2O), and sodium oxide (Na2O) are thermodynamically stable at high temperatures (450 °C) and low temperatures (300 °C). For example, the reaction equation for the formation of magnesium oxide (MgO) at 300 °C is: MgCl2+ SnO → MgO + SnCl2+ dG (= -5.40 kJ / mol) The reaction equations for the formation of tin oxide (SnO(1)) and tin oxide (1 / 2SnO2(1)) at 300 °C are: Sn + H2O → SnO + H2+ dG (= 3.45 kJ / mol) 1 / 2Sn + H2O → 1 / 2SnO2+ H2+ dG (= -0.10 kJ / mol) Therefore, it is thermodynamically unstable. In contrast, the reaction equation for the formation of tin oxide SnO(2) and tin oxide 1 / 2SnO2(2) at 300 °C is, Sn + 1 / 2O2→ SnO + dG (= -226 kJ / mol) Sn + O2→ SnO2+ dG (= -230 kJ / mol) Therefore, it is thermodynamically stable. Consequently, when oxygen is saturated, MgO, Li2O, Na2O, 1 / 2LiO2, 1 / 2SnO2, and SnO precipitate in that order at predetermined positions within the alkali metal separation and recovery unit 14, allowing for separation and recovery.

[0021] Returning to Figure 1, the heavy metal separation and recovery unit 15 can precipitate heavy metal elements by saturating them in liquid tin and cooling them down to lower their temperature, since the solubility of heavy metal elements dissolved in liquid tin is a function of temperature inherent to each heavy metal element. In this case, the temperature decreases from upstream to downstream of the liquid tin, so the heavy metal elements precipitate at predetermined positions according to their solubility. For example, to separate and recover three types of heavy metal elements, cartridges 15-1, 15-2, and 15-3, each having a mesh, are provided. Valves 15-1a, 15-2a, and 15-3a are provided in each cartridge 15-1, 15-2, and 15-3, allowing different heavy metal elements to be recovered from each cartridge 15-1, 15-2, and 15-3.

[0022] The high-temperature gas component desorption unit 16 desorbs gas components contained in liquid tin, such as hydrogen, nitrogen, and oxygen, under high-temperature and reduced-pressure conditions.

[0023] Figure 4 is a schematic diagram illustrating the operation of the liquid metal convection apparatus 1 shown in Figure 1.

[0024] As shown in FIG. 4, solar heat Q from an output Q of a parabolic dish collector 2 irradiates a high-temperature pipe 11 containing liquid tin, and simultaneously seawater SW is sprayed into the seawater contact chamber 13 in a high-temperature state. As a result, fresh water W in the seawater SW evaporates due to the heat Q, and at the same time alkali metals A1, A2, A3, heavy metals M1, M2, M3, and gases K1, K2, K3 in the seawater SW dissolve into the liquid tin. Next, the liquid tin moves by natural convection to an adiabatic low-temperature pipe 12 as indicated by the arrow. In an alkali metal separation and recovery unit 14 provided in the low-temperature pipe 12, the chlorides and oxides of the alkali metals A1, A2, A3 are precipitated at predetermined positions in accordance with the Gibbs free energy of the generation reaction formula of said chlorides and oxides, and are separated and recovered. Further, in a heavy metal separation and recovery unit 15 provided in the low-temperature pipe 12, the heavy metals M1, M2, M3 are precipitated at predetermined positions in accordance with their solubility, and are separated and recovered. Furthermore, when the liquid tin moves back to the high-temperature pipe 11 by natural convection, the gases K1, K2, K3 are desorbed under high-temperature conditions by increasing temperature and reducing pressure in a gas component high-temperature desorption unit 16 provided in the high-temperature pipe 11.

[0025] Next, the design procedure of the liquid metal convection device 1 in FIG. 1 will be described with reference to FIG. 5.

[0026] First, in step 501, since the output Q of the parabolic dish collector 2 is equal to the heating amount of tin, the flow rate dm / dt of tin is given by dm / dt=Q / [C p ×(T h -T c )] (2) where Q is the output of the parabolic dish collector 2 in formula (1) C p is the average specific heat of tin, 0.239 kJ / (kg·K) T h is the temperature of the high-temperature pipe 11, 450 °C T c is the temperature of the low-temperature pipe 12, 300 °C expressed by the above formula. Therefore, dm / dt=3.82 kg / s.

[0027] Next, in step 502, the wall temperature T of the pipes 11 and 12 of the liquid metal convection device 1w The heat flux q and heat transfer h are given by the following equations. T w =q / h +T out =(Q / πDX) / h+T out (3) However, D has an outer diameter of 254 mm for pipes 11 and 12. X is the output Q of the parabolic dish-type collector 2 and the heating length by direct sunlight, for example, 10 m. T out This is the wall temperature at the end of the heating length X when the heating length X is made very large. Therefore, the wall temperature T w This is represented as shown in the heating length / wall temperature characteristics in Figure 6(A). Therefore, in order to not exceed 500°C, which is the limiting factor for material coexistence, that is, T w If the temperature is <500℃, the corrosion resistance of tin by oxygen is ensured, so for example, let X = 10.0m.

[0028] Static pressure difference P caused by the temperature difference from the equilibrium during steady-state operation of a natural convection loop. h , friction loss Δp c and local loss ΣΔp l It can be expressed by the following equation. P h -Δp c -ΣΔp l =0 (4) In this case, the friction loss Δp c The coefficient of friction of the pipe flowing through the liquid metal convection apparatus 1 can be derived from this coefficient, and its value is a function of the heating length X and the loop height H. Furthermore, the local loss ΣΔp l This is caused by the bends in the liquid metal convection apparatus 1, changes in the path cross-sectional area, and the packed bed formed by the alkali metal separation and recovery unit 14 and the heavy metal separation and recovery unit 15. In particular, the pressure loss in the packed bed is large, and the packing material and the length of the packed bed H R (Metal recovery distance H in Figure 1) R It is determined by ).

[0029] Therefore, in step 503, the height of the packed bed H R Let's assume it's 2.0 mm, for example.

[0030] Next, in step 504, add H to equation (4). R Substitute the values ​​to obtain functions of X and H, and then calculate the aspect ratio X / H.

[0031] In step 505, since X has already been determined, the height H is calculated. In this case, if the aspect ratio X / H is unnatural, repeat steps 503 and 504. If the aspect ratio X / H is not extremely elongated vertically or horizontally, proceed to step 506.

[0032] In step 506, from equation (4), the loop height H of the liquid metal convection apparatus 1 is 4.25 m, so the tin inventory I is 1160 L, given the height H, heating length L, and diameter D.

[0033] Finally, in step 507, the amount of freshwater produced M w The following equation is obtained from the fact that the output Q of the parabolic dish-type collector 2 balances the heat of vaporization of seawater. M w ×[C w (100-20)+C v ]=Q (5) However, C w The specific heat of seawater C v sensible heat of vaporization 100 is the temperature of evaporation (°C). 20 is the ambient temperature (°C). Therefore, the daily water production volume M w This would be 1527 L / d.

[0034] Figure 7 shows the daily amount and concentration of dissolved materials in liquid tin obtained by the seawater desalination system shown in Figure 1. In Figure 7, the amounts of dissolved materials in liquid tin are 18.8 kg of sodium and 2.29 kg of magnesium, with concentrations of 0.234 wt% and 0.0284 wt%, respectively. Note that the alkali metal separation and recovery unit 14 utilizes the fact that the solubility of alkali metals in liquid tin is high at high temperatures and low at low temperatures, so the actual amounts of recovered sodium and magnesium are less than 18.8 kg and 2.29 kg.

[0035] In the seawater contact chamber 13 shown in Figure 1, when seawater comes into direct contact with the liquid tin, the liquid tin oxidizes upon contact with the atmosphere. The operational life of the liquid tin depends on its surface oxidation characteristics.

[0036] Figure 8 shows the seawater contact chamber in Figure 1. 13 This graph shows the mass increase in the results obtained from oxidation experiments conducted on static liquid tin under atmospheric conditions without contact with seawater, illustrating the surface oxidation characteristics of liquid tin.

[0037] In Figure 8, the dropping experiment was performed using the following procedure. 1) Place approximately 12 g of tin in a crucible and heat it until it melts and the surface becomes flat, then let it cool. 2) Place the crucible containing tin into the dripping experiment apparatus and heat it up using the mantle heater. 3) Once the temperature of the liquid tin obtained by the thermocouple stabilizes at around 300 °C, 400 °C, and 500 °C, add a total of 10 mL of artificial seawater having the elemental composition shown in Figure 9, for example, at a rate of 2 mL / 15 minutes. At this time, the dropping rate should be kept low to prevent the artificial seawater from penetrating into the liquid tin and causing a steam explosion. 4) In the dropping experiment or oxidation experiment, the test time should be, for example, 100 hours, 200 hours, 300 hours (or more precise time intervals at 500°C), after which the mixture should be allowed to cool naturally and the crucible removed once it reaches room temperature. 5) Measure the mass increase.

[0038] Figure 8, which shows the amount of the oxide layer of liquid tin in a stationary field at 300°C, 400°C, and 500°C, indicates that at 300°C, the liquid tin is not oxidized, and even at 400°C, the oxide layer of liquid tin is hardly oxidized. However, at 500°C, the liquid tin is greatly oxidized, and the oxide layer of liquid tin becomes larger. When the oxide layer of liquid tin is large, seawater cannot directly contact the liquid tin, and as a result, valuable metals in seawater cannot penetrate the oxide layer and dissolve in the liquid tin. Moreover, when recovering valuable metals, the tin oxide layer other than the valuable metals is also recovered. Therefore, the recovery rate of valuable metals decreases. From this point of view, the temperature of the liquid tin, i.e., the wall temperature T, wThe temperature should be less than 500°C, for example, a maximum of 450°C.

[0039] Next, we will explain the chemical reaction between liquid tin and artificial seawater in the seawater contact chamber 13.

[0040] Figure 10 shows the surface state of the tin after artificial seawater was dropped onto the liquid tin in Figure 1. (A) is a scanning electron microscope (SEM) image, and (B) is a graph of the analysis results obtained by an energy-dispersive X-ray analyzer (EDX) attached to the scanning electron microscope. As shown in Figure 10, sodium chloride (NaCl) and tin oxide (SnO) are observed on the tin surface. This is thought to be because sodium ions and chloride ions present in seawater dissolved in the liquid tin and were deposited as crystals on the tin surface during the cooling process, and also because the tin was oxidized and deposited as tin oxide on the liquid tin surface.

[0041] Figure 11 also shows scanning electron microscope (SEM) images and EDX elemental mapping results illustrating the surface state of the tin after artificial seawater was dropped onto the liquid tin shown in Figure 1. As shown in Figure 11, oxygen (O), chlorine (Cl), sodium (Na), and magnesium (Mg) were observed on the tin surface. From these findings, it is considered that sodium chloride (NaCl) was deposited on the tin surface.

[0042] Figure 12 is a graph showing the results of X-ray spectroscopy (XRD) analysis of the surface state of tin after dropping freshwater (comparative example) and artificial seawater onto the liquid tin shown in Figure 1. As shown in Figure 12(A), peak values ​​of Sn and SnO were observed in the case of freshwater, and in the case of seawater (Figure 12(B)), in addition to these, a peak value of NaCl was observed.

[0043] Thus, sodium, magnesium, and lithium can be expected to be recovered in the alkali metal separation and recovery unit 14 and the heavy metal separation and recovery unit 15. For example, if the liquid tin inventory is 1160 L, the output Q of the parabolic dish-type collector 2 is 137 kW, and 1527 L / d of freshwater is produced, the amount of sodium recovered is 18.8 kg / d, the amount of magnesium recovered is 2.29 kg / d, and if the lithium concentration in seawater is 0.1 to 0.2 mg / L, the amount of lithium recovered can be expected to be 157 to 314 mg / d. As for the method of selecting the valuable metals to be recovered, it is sufficient to consider the concentration of valuable metals contained in seawater and the market price per unit mass. For example, as shown in Figure 13, sodium (Na), magnesium (Mg), lead (Pb), potassium (K), bromine (Br), and sulfur (S) are promising, and lithium (Li) is also promising depending on the market price. Furthermore, in the high-temperature gas component desorption unit 16, hydrogen is a promising element to be recovered.

[0044] In the above-described embodiment, the liquid metal convection device 1 forms a rectangular loop, but other loop shapes, such as a circular loop or an elliptical loop, may also be used. Furthermore, the heating means may be a solar heat collector other than a parabolic dish type collector, or it may be reactor energy or electrical energy that does not utilize solar heat.

[0045] Furthermore, the present invention may be applied to any modification of the embodiments described above that falls within the scope of obviousness. [Explanation of symbols]

[0046] 1: Liquid metal convection device 11: High-temperature piping 12: Cryogenic piping 13: Seawater contact chamber 14: Alkali metal separation and recovery unit 15: Heavy metal separation and recovery unit 16: High-temperature desorption unit for gas components 2: Parabolic dish type collector 3: Seawater tank 3a: Seawater supply pump 4: Freshwater tank 5: Heat exchanger

Claims

1. A seawater contact chamber with its top open to the atmosphere, The seawater contact chamber is connected in a loop to both ends to a high-temperature pipe that can be heated and a low-temperature pipe that is insulated. It is equipped with, The seawater contact chamber, the high-temperature piping, and the low-temperature piping constitute a liquid metal convection loop for containing and circulating a liquid metal, which is tin or a tin alloy, as a heat / mass transfer medium. Furthermore, the liquid metal convection apparatus comprises an alkali metal separation and recovery unit at a first location in the low-temperature piping for separating and recovering alkali metals dissolved in the liquid metal.

2. Furthermore, the liquid metal convection apparatus according to claim 1, further comprising a heavy metal separation and recovery unit for separating and recovering heavy metals dissolved in the liquid metal at a second location on the high-temperature piping side of the first location in the low-temperature piping.

3. Furthermore, the liquid metal convection apparatus according to claim 1 further comprises a high-temperature gas component desorption unit for separating and recovering gas components dissolved in the liquid metal at a third location in the high-temperature piping.

4. The liquid metal convection apparatus according to claim 1, wherein the alkali metal separation and recovery unit separates and recovers the chloride or oxide deposited at a predetermined position in the low-temperature piping according to the Gibbs free formation energy of the reaction equation for the formation of the alkali metal chloride or oxide.

5. The liquid metal convection apparatus according to claim 2, wherein the heavy metal separation and recovery unit separates and recovers the heavy metal deposited at a predetermined position in the low-temperature piping according to the solubility of the heavy metal.

6. The liquid metal convection apparatus according to claim 1, wherein seawater is sprayed onto the liquid metal in the seawater contact chamber so that the seawater comes into direct contact with the liquid metal.

7. A liquid metal convection apparatus according to claim 1, A heating means for heating at least a portion of the lower side of the high-temperature piping, A seawater desalination system equipped with the following features.

8. The seawater desalination system according to claim 7, wherein the heating means is a collector for collecting solar heat.

9. moreover, A seawater supply means for supplying seawater to the seawater contact chamber, A freshwater tank for recovering freshwater from the aforementioned seawater contact chamber and A seawater desalination system according to claim 7, comprising the following:

10. Furthermore, the seawater desalination system according to claim 9 further comprises a heat exchanger for performing heat exchange between the seawater from the seawater supply means and the freshwater to the freshwater tank.

11. A step of containing a liquid metal consisting of tin or a tin alloy in a loop shape, A step of partially heating the liquid metal to cause natural convection of the liquid metal, A seawater contact step involves directly contacting the naturally circulating liquid metal with seawater to evaporate the freshwater, and dissolving the alkali metals, heavy metals, and gaseous components of the seawater into the liquid metal. A valuable resource separation and recovery step of separating and recovering at least one of the alkali metal, heavy metal, and gaseous component dissolved in the liquid metal. A method for desalination seawater that includes the following features.

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