Method for recycling metallic sodium by using density gradient liquid

Abstract

The present invention relates to the field of recycling of sodium slag in the electrolysis process of metallic sodium, and discloses a method for recycling metallic sodium by using density gradient liquid. The method includes: mixing liquid sodium slag with density gradient liquid, and then performing high-gravity stratification to obtain metallic sodium, high-calcium sodium slag and density gradient liquid, wherein the density of the density gradient liquid is 0.97 - 1.55 g / mL. The method provided by the present invention has the advantages of high separation efficiency of sodium and calcium. The separated metallic sodium can be used as a product, and the separated density gradient liquid has the advantages of being recyclable repeatedly, etc., so that the low-value hazardous solid waste sodium slag generated in the process of preparing metallic sodium by electrolyzing sodium chloride is secondarily developed, and the metallic sodium mixed therein is effectively recovered, realizing the reduction of sodium slag and the high-value utilization of resources.

Description

Technical Field

[0001] The present invention relates to the field of recycling of sodium slag in the process of electrolyzing metallic sodium, and particularly relates to a method for recycling metallic sodium by using a density gradient liquid in a high gravity field. Background Art

[0002] As a common alkali metal, metallic sodium has become an important metal widely used in the development of China's national economy due to its rich resources, good electrical conductivity, low density, and active chemical properties. Since metallic sodium was obtained by electrolyzing sodium hydroxide by Professor Davy in the UK in 1807, the preparation process of metallic sodium has been continuously improved and industrial production has been realized. In 1921, American scientist Downs (J.C. Downs) invented a process for electrolyzing molten salts using sodium chloride as the raw material, graphite as the anode, and iron or tungsten as the cathode to obtain metallic sodium and chlorine. Due to its high current efficiency and low raw material cost, it gradually replaced the original sodium hydroxide electrolysis method and became the current mainstream metallic sodium preparation process. As the world's largest producer of metallic sodium, China has obtained huge applications in the fields of nuclear power, pharmaceutical chemistry, etc. In order to reduce the electrolysis temperature of metallic sodium, binary eutectics of calcium chloride and sodium chloride, as well as ternary eutectics of sodium chloride, calcium chloride, and barium chloride have been developed, enabling electrolysis at 580 - 600 °C. During the electrolysis process, while metallic sodium is generated at the cathode, a small amount of metallic calcium is also reduced. The metallic sodium containing calcium is refined in a refining tank to obtain metallic sodium with a purity of 99.5%, and at the same time, sodium slag containing metallic calcium, sodium calcium oxide, and a small amount of molten salt inclusions is obtained at the bottom. It is analyzed that the sodium slag contains more than 50 - 70% of metallic sodium components. Since metallic sodium is extremely prone to chemical reactions in air or water, sodium slag has become a hazardous waste residue in the process of electrolytic preparation of metallic sodium. Developing an efficient separation technology for sodium slag has important economic and environmental value for promoting the secondary utilization of sodium slag resources and reducing the disposal cost of sodium slag.

[0003] The existing methods for treating sodium slag are as follows:

[0004] (1) Preparation of sodium alkoxides and sodium hydroxide by decomposition with low molecular weight alcohols and water.

[0005] This method uses methanol, ethanol, or water as raw materials to react with sodium slag to produce by-products such as sodium methoxide, sodium ethoxide, and sodium hydroxide. Due to the presence of calcium and other impurities in the sodium slag, the purity of the above by-products cannot meet the requirements of some manufacturers, and their applications are limited. For example, patent CN208562194U discloses an industrial metallic sodium slag recycling and resource utilization system, in which sodium slag and methanol react in a methanol tank to obtain a sodium methoxide product.

[0006] (2) Vacuum distillation method.

[0007] Since the boiling point of metallic sodium is 883 °C, which is lower than that of metallic calcium (1440 °C), vacuum distillation is used to reduce the boiling point of metallic sodium, thereby heating the sodium slag at a lower temperature and converting metallic sodium into sodium vapor, which is separated from the sodium slag to obtain high-purity metallic sodium. The main problem with this process is the high energy consumption during the evaporation process. Since metallic sodium is flammable and explosive, higher requirements are imposed on equipment and operation techniques. For example, CN103667708B, CN102634671A, and CN203668481U disclose that sodium vapor is obtained by heating the metallic sodium waste slag in a heating furnace, thereby separating sodium from other impurities, and then the sodium vapor is condensed into liquid sodium and sent to a sodium casting machine to cast sodium ingots.

[0008] (3) Melt extrusion method.

[0009] The characteristic of the mechanical extrusion method is to heat the sodium slag above the melting point of metallic sodium, and then separate metallic sodium from the sodium slag by mechanical extrusion. For example, CN2846436Y, CN203487209U, and CN2846437Y disclose a method for separating molten alkali metal - metallic sodium from infusible impurities by mechanical extrusion and a recovery device including an oil cylinder and a hydraulic system. According to analysis, the problem with this patent is that it is difficult to extrude metallic sodium by the extrusion method when the content of metallic sodium is less than 40%, resulting in an extraction rate of metallic sodium of only 40 - 50%. In addition, the equipment needs to be treated for oxygen and water isolation during the extrusion process, and the treatment of secondary sodium slag containing a high content of metallic sodium also expects new processes for improvement. Chinese patent application CN109371250A also discloses a preparation process for extracting metallic sodium from sodium slag by the molten replacement method, with the steps as follows: 1) Placing the sodium slag in a sodium slag melting tank and heating it under the protection of nitrogen to obtain molten sodium slag; 2) Adding the molten sodium slag into a reaction kettle, and at the same time adding sodium chloride for a replacement reaction to form metallic sodium, calcium chloride, and calcium oxide, obtaining metallic sodium with a purity of 96 - 98%; 3) Then entering a refiner, and after cooling and sedimentation treatment, obtaining 99.7% metallic sodium, which is transferred to a refined sodium storage tank; the residue generated during cooling and sedimentation in the refiner is added to the sodium slag melting tank and then transferred to the reaction kettle to reoccur the replacement reaction; 4) The obtained 99.7% metallic sodium is made into sodium ingots by a sodium casting machine. Chinese patent CN107574318B also reports heating molten salt to form a molten salt liquid, where the molten salt is a substance that does not react with metallic sodium; heating the sodium slag above the melting point of sodium to form a solid-liquid mixture, and then adding the solid-liquid mixture into the molten salt liquid, and obtaining a pure metallic sodium melt on the upper layer of the molten salt liquid.

[0010] Chinese Patent Application CN105821217A discloses a method of repeatedly heating and cooling using a liquid dispersion medium and combining mechanical stirring, and then separating solid sodium metal by cooling, so that the recovery rate of sodium in sodium slag reaches more than 80-95%, significantly improving the recovery efficiency of sodium slag. The main problems of this process are: first, the repeated heating and cooling treatments increase the pressure and energy consumption of equipment heating and heat dissipation; second, there are safety hazards with the use of a large amount of flammable alcohol ether organic solvents. In addition, the effective recovery of metal calcium components in sodium slag is not fully considered, and the secondary sodium slag containing metal calcium is still a flammable hazardous waste.

[0011] In view of the above problems, there is an urgent need to invent a new sodium slag separation process to maximize the recovery of sodium metal from sodium slag and reduce the accumulation of sodium slag. Summary of the Invention

[0012] The purpose of the present invention is to overcome the above problems existing in the prior art and provide a method for recovering sodium metal using a density gradient liquid in a supergravity field, which has a simple process and is energy-saving and efficient.

[0013] The present invention provides a method for recovering sodium metal using a density gradient liquid, which method comprises:

[0014] Mixing liquid sodium slag with a density gradient liquid, and then performing supergravity stratification to obtain sodium metal, high-calcium sodium slag and a density gradient liquid, wherein the density of the density gradient liquid is 0.97-1.55 g / mL.

[0015] Preferably, the density gradient liquid is selected from at least one of heat-conducting silicone oil, liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone, phenyl ethyl ether, ethyl silicone oil, benzyl silicone oil, dimethyl phenyl ether, 2-methylnaphthalene, N,N-dimethylformamide and m-toluidine.

[0016] Preferably, the method further comprises: before mixing the liquid sodium slag with the density gradient liquid, pretreating a part of the liquid sodium slag with oxides on the surface; the pretreatment uses an activation liquid containing alcohol.

[0017] Preferably, the activation liquid containing alcohol further contains a blending solvent, and the blending solvent is selected from at least one of liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone and phenyl ethyl ether.

[0018] The method provided by the present invention has the advantages of high separation efficiency of sodium and calcium. The separated metallic sodium can be used as a product, and the separated density gradient liquid has the advantages of being recyclable repeatedly, etc. Thus, the low-value hazardous waste sodium slag generated from the electrolysis of sodium chloride to prepare metallic sodium is re-developed, and the metallic sodium mixed therein is effectively recovered, realizing the reduction of sodium slag and the high-value utilization of resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a flowchart of a specific embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0021] The present invention provides a method for recovering metallic sodium using a density gradient liquid. As Figure 1 shown, the method includes:

[0022] Mix the liquid sodium slag with the density gradient liquid, and then perform high-gravity stratification to obtain metallic sodium, high-calcium sodium slag, and density gradient liquid, wherein the density of the density gradient liquid is 0.97 - 1.55 g / mL.

[0023] The method provided by the present invention mixes the liquid sodium slag with the density gradient liquid, and then performs high-gravity or centrifugal stratification to obtain metallic sodium, density gradient liquid, and high-calcium sodium slag. This method has a high separation efficiency for sodium and calcium. The separated metallic sodium can be used as a product, and the separated density gradient liquid can be recycled repeatedly. Thus, the low-value hazardous waste sodium slag generated from the electrolysis of sodium chloride to prepare metallic sodium is re-developed, and the metallic sodium mixed therein is effectively recovered, realizing the reduction of sodium slag and the high-value utilization of resources.

[0024] The method provided by the present invention is applicable to the recovery of most industrial sodium slags, which can be directly obtained industrial sodium slags, preferably the hazardous waste sodium slag generated from the electrolysis of sodium chloride to prepare metallic sodium, or can also be sodium slags treated by existing technologies.

[0025] According to a preferred embodiment of the present invention, the liquid sodium slag contains Na and Ca; preferably, based on the total amount of the sodium slag, the content of Na is 60-90% by weight, and the content of Ca is 10-40% by weight. In the method provided by the present invention, in addition to Na and Ca, the sodium slag does not exclude the presence of trace amounts (for example, not more than 2%) of other metal elements, including but not limited to barium, magnesium, and potassium. In addition, the present invention does not particularly limit the forms of Na and Ca in the sodium slag, and sodium slags containing Na and Ca in various forms are applicable to the method provided by the present invention.

[0026] In order to further optimize the separation and recovery effect, it is preferred that the density gradient liquid comprehensively considers density, melting point, boiling point, and chemical stability.

[0027] Preferably, the density of the density gradient liquid is 1.0-1.3 g / mL.

[0028] Preferably, the melting point of the density gradient liquid ≤ 97 °C, preferably -20 to 85 °C.

[0029] Preferably, the boiling point of the density gradient liquid is 98-300 °C, more preferably 105-250 °C.

[0030] In the present invention, unless otherwise specified, the density refers to the density under standard conditions of standard atmospheric pressure, the boiling point refers to the boiling point measured under standard atmospheric pressure conditions, and the melting point refers to the melting point measured under standard atmospheric pressure when the vapor pressures of the solid and liquid phases are the same and the solid and liquid phases coexist in equilibrium.

[0031] According to a preferred embodiment of the present invention, the electrochemical stability window of the density gradient liquid is -3.5 to -1.0 V (Vs. standard hydrogen electrode), preferably -3.0 to -1.8 V (Vs. standard hydrogen electrode). In the present invention, the electrochemical stability window refers to the electrochemical window in which the sodium slag is relatively stable in the density gradient liquid, and can be measured by the electrochemical cyclic voltammetry method as the potential window where there is no electrochemical reaction on the cyclic voltammogram.

[0032] Taking into comprehensive consideration the density, melting point, boiling point, and chemical stability of the density gradient liquid, preferably, the density gradient liquid is selected from at least one of thermal conductive silicone oil, liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone, phenyl ethyl ether, ethyl silicone oil, benzyl silicone oil, dimethyl phenyl ether, 2-methylnaphthalene, N,N-dimethylformamide, and m-toluidine, and more preferably at least one of 2-methylnaphthalene, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, and 1,4-butyrolactone. The specific types of the density gradient liquid can all be obtained through commercial purchase.

[0033] The present invention provides a method for recovering metallic sodium using a density gradient liquid, as Figure 1 shown, the method includes:

[0034] Mixing liquid sodium slag with a density gradient liquid, and then performing high-gravity stratification to obtain metallic sodium, high-calcium sodium slag, and a density gradient liquid, wherein the density gradient liquid is selected from at least one of thermal conductive silicone oil, liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone, phenyl ethyl ether, ethyl silicone oil, benzyl silicone oil, dimethyl phenyl ether, 2-methylnaphthalene, N,N-dimethylformamide, and m-toluidine.

[0035] According to the present invention, preferably, the weight ratio of the amount of the density gradient liquid to the liquid sodium slag is 0.1 - 15:1, and more preferably 0.2 - 5:1. By adopting the preferred implementation manner of the present invention, effective recovery of metallic sodium can be achieved even with a smaller amount of the density gradient liquid.

[0036] The present invention has no particular limitation on the conditions for the mixing. The liquid sodium slag and the density gradient liquid can be directly mixed and then heated, or the liquid sodium slag and the density gradient liquid can be directly mixed. The temperature of the liquid sodium slag is 100 - 240°C.

[0037] In order to better achieve the high-gravity stratification effect of the present invention, the inventors of the present invention found in the research process that selecting an appropriate centrifugal separation factor is more conducive to effectively separating metallic sodium, high-calcium sodium slag, and the density gradient liquid. Preferably, in step (1), the separation factor Fr of the high-gravity stratification is 500 - 5500, and more preferably 1500 - 4500, such as 1500, 2000, 2500, 3000, 3500, 4000, 4500, or any value between any two of them.

[0038] According to the present invention, preferably, the time for the high-gravity stratification is 1 - 500 min, more preferably 2 - 30 min, such as 2 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, or any value between any two of them.

[0039] Preferably, the high-gravity stratification is carried out under heat preservation conditions.

[0040] The present invention has no particular limitation on the temperature of the high-gravity stratification. It can be carried out under heat preservation at the temperature after the liquid sodium slag is mixed with the density gradient liquid. It can also be carried out by heating (for example, the temperature is 97 - 300 °C, preferably 100 - 200 °C) after the liquid sodium slag is mixed with the density gradient liquid and then under heat preservation.

[0041] According to the present invention, oxides may exist in the sodium slag. For the sodium slag with oxides on part of its surface, the present invention may further include a pretreatment process for the sodium slag with oxidized surface. Preferably, the method further includes: before mixing the liquid sodium slag with the density gradient liquid, pretreating part of the liquid sodium slag with oxides on its surface. The purpose of the pretreatment is to dissolve the sodium oxide and a small part of metallic sodium on its surface, provide a fresh sodium slag surface for the subsequent density gradient separation, and accelerate the melting and fusion process between sodium slag particles.

[0042] According to a preferred embodiment of the present invention, the pretreatment uses an activation liquid containing alcohol. The alcohol can be at least one of monohydric alcohols, dihydric alcohols, and polyhydric alcohols. Preferably, the alcohol is an alcohol with C1 - C10.

[0043] According to a preferred embodiment of the present invention, the alcohol is selected from at least one of ethanol, propanol, isopropanol, benzyl alcohol, isobutanol, butanol, 2-methyl-2-propanol, pentanol, ethylene glycol, and glycerol, and more preferably at least one of ethanol, propanol, isopropanol, benzyl alcohol, and isobutanol.

[0044] According to the present invention, preferably, the activation liquid containing alcohol further contains a blending solvent, and the blending solvent is selected from at least one of liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone, and phenyl ethyl ether, and more preferably at least one of diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, and 1,4-butyrolactone.

[0045] In the above pretreatment method, alcohols are used as solvents for sodium oxide and play a role in regulating the dissolution rate and mass transfer during the dissolution process. Substances such as liquid paraffin and esters can accelerate the dissolution process and blend the solvents.

[0046] According to the present invention, preferably, in the activation liquid containing alcohol, the volume ratio of alcohol to the blending solvent is 1:0.5 - 100, and more preferably 1:2 - 50.

[0047] The present invention has a relatively wide selection range for the dosage of the activation liquid containing alcohol. Under preferred circumstances, the weight ratio of the dosage of the activation liquid containing alcohol to the liquid sodium slag is 0.1 - 15:1, and more preferably 0.2 - 5:1.

[0048] The conditions of the pretreatment according to the present invention can be appropriately selected according to the storage time of the sodium slag or the surface smoothness. For fresh sodium slag, density gradient liquid can be directly used for separation; for sodium slag that has been stored for a relatively long time or whose surface is significantly oxidized and dull, an activation liquid containing alcohol can be used to remove the surface oxides. Preferably, the conditions of the pretreatment include: the activation temperature is 10 - 320 °C, preferably 20 - 190 °C; the activation time is 1 - 120 min, preferably 2 - 50 min.

[0049] According to a preferred embodiment of the present invention, the method further includes recycling the density gradient liquid obtained by high-gravity stratification. Specifically, the density gradient liquid obtained by high-gravity stratification can be used for the treatment of the next batch of sodium slag.

[0050] According to the present invention, the high-calcium sodium slag obtained by high-gravity stratification can be further recycled, and the present invention has no particular limitation on the recycling method. Preferably, potential safety hazards can be eliminated through a roasting - leaching process to obtain sodium hydroxide solution and calcium hydroxide products.

[0051] Example 1

[0052] 1 kg of liquid sodium slag from Inner Mongolia Ruixin Chemical Industry Co., Ltd. at a temperature of 140 °C (analysis: 85 wt% metallic sodium, 15 wt% metallic calcium) was mixed with 0.5 L of 2-methylnaphthalene (density: 1.01 g / cm 3 , melting point: 34 °C, boiling point 241 °C, electrochemical window stable and conforming to the active potential of metallic sodium). The mixture was placed in a high-gravity centrifuge under heat preservation conditions, with a centrifugal factor of 2600 and a centrifugation time of 10 min, to obtain 0.58 kg of preliminarily separated metallic sodium (purity 99.3%), 0.42 kg of high-calcium sodium slag (64 wt% sodium, 35 wt% calcium), and 0.45 L of 2-methylbenzene, and 2-methylbenzene can be reused for the recovery of the next batch of sodium slag.

[0053] Liquid sodium can be recovered using a conventional negative pressure absorption method, and the high-calcium sodium slag can be treated by controlled roasting - leaching to obtain sodium hydroxide solution and calcium hydroxide by-products.

[0054] Example 2

[0055] Mix 1 kg of liquid sodium slag from Inner Mongolia Ruixin Chemical Co., Ltd. at a temperature of 140°C (analysis: 85 wt% metallic sodium, 15 wt% metallic calcium) with 0.45 L of 2-methylnaphthalene separated in Example 1. Place this mixture in a high-gravity centrifuge under heat preservation conditions, with a centrifugal factor of 3000 and a centrifugation time of 5 min, to obtain 0.57 kg of preliminarily separated metallic sodium (purity 99.3%), 0.43 kg of high-calcium sodium slag (64 wt% sodium, 35 wt% calcium), and 0.42 L of 2-methylnaphthalene. The 2-methylbenzene can be reused for the recovery of the next batch of sodium slag.

[0056] The metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium slag can be treated by controlled roasting-leaching to obtain sodium hydroxide solution and calcium hydroxide by-product.

[0057] Example 3

[0058] Mix 10 kg of liquid sodium slag from Inner Mongolia Ruixin Chemical Co., Ltd. at 120°C (86 wt% metallic sodium, 14 wt% metallic calcium) with 10 L of ethylene carbonate (density: 1.32 g / cm 3 , melting point: 35°C, boiling point 248°C, and the electrochemical window is stably in line with the active potential of metallic sodium) and then carry out centrifugal separation under high-gravity conditions with a controlled centrifugal factor of 3000 and a centrifugation time of 10 min under constant temperature conditions. Obtain 6.2 kg of preliminarily separated metallic sodium (purity 99.45%), 3.8 kg of high-calcium sodium slag (63 wt% sodium, 37 wt% calcium), and 9.6 L of ethylene carbonate.

[0059] The separated metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium slag can be returned to the electrolytic cell for reuse.

[0060] Example 4

[0061] Mix 10 kg of the same liquid sodium slag as in Example 3 with the ethylene carbonate separated in Example 3. Place this mixture in a high-gravity centrifuge under heat preservation conditions, control the centrifugal factor to be 3500, and the centrifugation time to be 15 min, to obtain 6.3 kg of preliminarily separated metallic sodium (purity 99.5%), 3.7 kg of high-calcium sodium slag (62 wt% sodium, 38 wt% calcium), and 9.2 L of propylene carbonate. This ethylene carbonate can be reused for the recovery of the next batch of sodium slag.

[0062] Example 5

[0063] First, react 5 kg of sodium residue (87 wt% metallic sodium, ~12.5 wt% metallic calcium, and ~0.5 wt% sodium oxide) with 0.5 L of an activating solution containing alcohol (0.1 L of propanol and 0.4 L of propylene carbonate) at 100 °C for 2 min. Then, transfer the purified sodium residue and 1.5 L of propylene carbonate to a high-gravity centrifuge for isothermal separation at 120 °C. Control the centrifugal factor at 3500 and the centrifugation time at 15 min to obtain 3.4 kg of metallic sodium (purity 99.7%) and 1.6 kg of high-calcium sodium residue (59 wt% sodium, 41 wt% calcium), as well as approximately 0.96 L of propylene carbonate, which can be used for the separation of the next batch of sodium residue.

[0064] The separated pure metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium residue can be recovered through controlled calcination treatment.

[0065] Example 6

[0066] First, react 5 kg of liquid sodium residue (87 wt% metallic sodium, 12.5 wt% metallic calcium, and ~0.5 wt% sodium oxide) as in Example 5 with 1 L of an activating solution containing alcohol (0.2 L of ethanol and 0.3 L of vinylene carbonate) at 30 °C for 4 min, and then separate to obtain the surface-purified sodium residue and the alcohol-based activator. Transfer the purified sodium residue and 2 L of vinylene carbonate to a high-gravity centrifuge for isothermal separation at 130 °C. Control the centrifugal factor at 3500 and the centrifugation time at 20 min to obtain 3.5 kg of metallic sodium (purity 99.8%) and 1.5 kg of high-calcium sodium residue (57 wt% sodium, 43 wt% calcium), as well as approximately 0.96 L of vinylene carbonate, which can be used for the separation of the next batch of sodium residue.

[0067] The separated pure metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium residue can be recovered through controlled calcination treatment.

[0068] Example 7

[0069] Mix 2 kg of 160 °C liquid sodium residue (86 wt% metallic sodium, 14 wt% metallic calcium) with 1 L of ethyl methyl carbonate, and then centrifuge in a high-gravity centrifuge under heat preservation conditions. Control the centrifugal factor at 4200 and the centrifugation time at 10 min to obtain 1.3 kg of preliminarily separated metallic sodium (purity 99.5%), 0.7 kg of high-calcium sodium residue (61 wt% sodium, 39 wt% calcium), and 0.96 L of ethyl methyl carbonate. This ethyl methyl carbonate can be used as a density gradient liquid for the recovery of the next batch of sodium residue.

[0070] The metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium residue is subjected to calcination treatment.

[0071] Example 8

[0072] Mix 2 kg of sodium residue (87 wt% metallic sodium, ~12.5 wt% metallic calcium, ~0.5 wt% sodium oxide) with 1 L of density gradient liquid (liquid paraffin, 1,4-butyrolactone, and dimethyl carbonate, with a volume ratio of 1:3:6). Centrifuge this mixture in a high-gravity centrifuge at 150 °C for 12 min, controlling the centrifugal factor to be 4000, to obtain 1.29 kg of separated metallic sodium (purity 99.3%), 0.71 kg of high-calcium sodium residue (62 wt% sodium, 38 wt% calcium), and 0.96 L of density gradient liquid. This density gradient liquid can be reused for the recovery of the next batch of sodium residue.

[0073] This pure metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium residue is returned to the sodium chloride electrolysis cell after chlorination roasting for use.

[0074] Example 9 <M

[0075] First, react 2 kg of sodium residue (87 wt% metallic sodium, ~12.5 wt% metallic calcium, approximately ~0.5 wt% sodium oxide) with 0.5 L of an activating liquid containing alcohol (containing 0.1 L of isopropyl alcohol, 0.1 L of 1,4-butyrolactone, and 0.3 L of dimethyl carbonate) at room temperature for 3 min and then separate to obtain surface-purified sodium residue and the activating liquid containing alcohol.

[0076] Mix this purified sodium residue with 1 L of density gradient liquid (liquid paraffin, 1,4-butyrolactone, and dimethyl carbonate, with a volume ratio of 1:3:6). Centrifuge this mixture in a high-gravity centrifuge at 150 °C for 12 min, controlling the centrifugal factor to be 4000, to obtain 1.33 kg of separated metallic sodium (purity 99.6%), 0.67 kg of high-calcium sodium residue (62 wt% sodium, 38 wt% calcium), and 0.96 L of density gradient liquid. This density gradient liquid can be reused for the recovery of the next batch of sodium residue.

[0077] This pure metallic sodium can be recovered using the negative pressure absorption method, and the high-calcium sodium residue is returned to the sodium chloride electrolysis cell after chlorination roasting for use.

[0078] From the above examples and results, it can be seen that the method provided by the present invention mixes liquid sodium residue with density gradient liquid and then performs high-gravity stratification to obtain metallic sodium, density gradient liquid, and high-calcium sodium residue. This method has a high separation efficiency for sodium and calcium. The separated metallic sodium can be used as a product, and the separated density gradient liquid can be recycled repeatedly, thereby enabling the secondary development of the low-value hazardous waste sodium residue generated in the electrolysis of sodium chloride to prepare metallic sodium, effectively recovering the metallic sodium mixed therein, and achieving the reduction of sodium residue and the high-value utilization of resources.

[0079] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for recovering metallic sodium using a density gradient liquid, the method comprising: Mixing liquid sodium slag with the density gradient liquid, and then performing high-gravity stratification to obtain metallic sodium, high-calcium sodium slag, and the density gradient liquid, wherein the density of the density gradient liquid is 0.97 - 1.55 g / mL; Wherein, the method further comprises: before mixing the liquid sodium slag with the density gradient liquid, pretreating a part of the liquid sodium slag with oxides on its surface; the pretreatment uses an activating liquid containing alcohol.

2. The method according to claim 1, wherein, The melting point of the density gradient liquid ≤ 97 °C.

3. The method according to claim 2, wherein The melting point of the density gradient liquid is -20 to 85 °C.

4. The method according to claim 1, wherein, The boiling point of the density gradient liquid is 98 - 300 °C.

5. The method according to claim 4, wherein, The boiling point of the density gradient liquid is 105 - 250 °C.

6. The method according to claim 1, wherein, The electrochemical stability window of the density gradient liquid is -3.5 to -1.0 V (Vs. standard hydrogen electrode).

7. The method according to claim 6, wherein, The electrochemical stability window of the density gradient liquid is -3.0 to -1.8 V (Vs. standard hydrogen electrode).

8. The method according to claim 1, wherein The density gradient liquid is selected from at least one of thermally conductive silicone oil, liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone, phenyl ethyl ether, ethyl silicone oil, benzyl silicone oil, dimethylphenyl ether, 2-methylnaphthalene, N,N-dimethylformamide, and m-toluidine.

9. The method according to claim 8, wherein, The density gradient liquid is selected from at least one of 2-methylnaphthalene, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, and 1,4-butyrolactone.

10. The method according to claim 1, wherein The liquid sodium slag contains Na and Ca.

11. The method according to claim 10, wherein Based on the total amount of the liquid sodium slag, the content of Na is 60 - 90 wt%, and the content of Ca is 10 - 40 wt%.

12. The method according to claim 1, wherein The weight ratio of the amount of the density gradient liquid used to the liquid sodium slag is 0.1 - 15:

1.

13. The method according to claim 12, wherein, The weight ratio of the amount of the density gradient liquid used to the liquid sodium slag is 0.2 - 5:

1.

14. The method according to claim 1, wherein, The conditions for the high-gravity stratification include: the separation factor is 500 - 5500; the time is 1 - 500 min.

15. The method according to claim 14, wherein, The conditions for the high-gravity stratification include: the separation factor is 1500 - 4500; the time is 2 - 30 min.

16. The method according to any one of claims 1-15, wherein, The alcohol in the activating liquid containing alcohol is selected from at least one of ethanol, propanol, isopropanol, benzyl alcohol, isobutanol, butanol, 2-methyl-2-propanol, pentanol, ethylene glycol, and glycerol.

17. The method according to claim 1, wherein The activating liquid containing alcohol further contains a blending solvent, and the blending solvent is selected from at least one of liquid paraffin, diethyl malonate, propylene carbonate, ethylene carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, vinylene carbonate, diphenyl carbonate, 1,4-butyrolactone, and phenyl ethyl ether.

18. The method according to claim 17, wherein, In the activating liquid containing alcohol, the volume ratio of alcohol to the blending solvent is 1:0.5 - 100.

19. The method according to any one of claims 1-15, wherein, The weight ratio of the amount of the activating liquid containing alcohol used to the liquid sodium slag is 0.1 - 15:

1.

20. The method according to claim 19, wherein, The weight ratio of the amount of the activating liquid containing alcohol used to the liquid sodium slag is 0.2 - 5:

1.

21. The method according to any one of claims 1 - 15, wherein, The conditions of the pretreatment include: the activation temperature is 10 - 320 °C; the activation time is 1 - 120 min.

22. The method according to claim 21, wherein, The conditions of the pretreatment include: the activation temperature is 20 - 190 °C; the activation time is 2 - 50 min.

23. The method according to any one of claims 1-15, wherein, This method further includes recycling the density gradient liquid obtained by high gravity layering.

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