A method for preparing a high-purity metal sodium salt

Abstract

This invention provides a method for preparing high-purity metallic sodium salt, belonging to the field of salt production technology. The preparation method includes adding calcium hydroxide and sodium carbonate to gypsum-type brine, separating the resulting precipitate to obtain primary refined brine; the primary refined brine is then subjected to a nanofiltration system to obtain low-nitrate refined brine and high-nitrate refined brine; a portion of the low-nitrate refined brine is evaporated and crystallized, and the resulting saturated salt slurry is dehydrated and washed using the low-nitrate refined brine; the washed salt slurry is then dehydrated, dried, and impurity-removed to obtain high-purity metallic sodium salt. The sodium salt prepared by this invention contains SO42-. 2‑ The content remains consistently below 0.01%.

Description

Technical Field

[0001] This invention relates to the field of salt production technology, specifically to a method for preparing high-purity metallic sodium salt. Background Technology

[0002] The main industrial method for producing metallic sodium is the electrolysis of molten sodium chloride. This process yields metallic sodium and chlorine gas as a byproduct. The principle is that during the electrolysis of molten sodium chloride, the sodium chloride melts at high temperatures into an ionic conductor (containing no water), in which sodium ions (Na+) are released. + ) moves towards the cathode, chloride ions (Cl) - The sodium chloride moves towards the anode. Under the action of direct current, a reduction reaction occurs at the cathode to produce metallic sodium, and an oxidation reaction occurs at the anode to produce chlorine gas. In the molten electrolysis method, sodium chloride raw material with a purity of at least ≥99% is required. Impurities in the sodium chloride raw material will affect the electrolysis efficiency and the service life of the electrolysis equipment during the electrolysis process. Among these, sulfate (SO4) ions... 2- The sulfate content is a core indicator affecting electrolysis efficiency and service life. Excessive sulfate content will cause more slag accumulation, equipment blockage, reduced current efficiency, increased power consumption, and aggravated corrosion of the electrolytic cell.

[0003] Currently, high-end customers are increasingly demanding strict requirements on the sulfate content of metallic sodium salts, with some requiring it to be below 0.02%, and high-end products even requiring it to be below 0.01%. Existing technologies control the sulfate content of metallic sodium salts by sieving. For example, Chinese patent application CN106241836A, entitled "An Energy-Saving Metallic Sodium Salt and Its Preparation Method and Application," discloses the following steps: (1) Glauber's salt or gypsum-based brine is purified, evaporated and concentrated, centrifuged and dehydrated, and dried; (2) the product obtained in step (1) is sieved through a 20-30 mesh sieve, retaining the salt that can pass through the sieve; (3) the product after sieving in step (2) is sieved through a 60-140 mesh sieve, retaining the salt that cannot pass through the sieve, thus obtaining the energy-saving metallic sodium salt of this invention. The metallic sodium salt SO4 obtained after sieving in the above-mentioned prior art... 2- The content is between 0.009% and 0.018%, SO4 2- The content fluctuates greatly, making it difficult to meet the SO4 requirements. 2- High-end demand with a stable content of less than 0.01%. Summary of the Invention

[0004] In view of this, the present invention provides a method for preparing high-purity metallic sodium salts, thereby achieving a high SO4 content in the prepared metallic sodium salts. 2- The aim is to maintain the content stably below 0.01%.

[0005] To achieve the above objectives, the present invention provides a method for preparing high-purity metallic sodium salts, comprising the following steps:

[0006] 1) Add calcium hydroxide and sodium carbonate to the gypsum-based brine, and separate the resulting precipitate to obtain primary refined brine; 2) The primary refined brine is processed through a nanofiltration system to obtain low-nitrate refined brine and high-nitrate refined brine; 3) Part of the low-nitrate brine is evaporated and crystallized, and the saturated salt slurry obtained at the bottom is washed after dehydration. The washing process uses the remaining low-nitrate brine. 4) After washing, the salt slurry is dehydrated, dried, and impurities are removed to obtain high-purity metallic sodium salt.

[0007] The preparation method provided by this invention combines a nanofiltration system with washing the saturated salt slurry after evaporation and crystallization using low-nitrate refined brine. Specifically, it utilizes the selective sieving principle of nanofiltration, which allows monovalent ions to pass through and divalent ions to be retained, to reduce the sulfate content in the primary refined brine from the source. Combined with countercurrent washing with low-nitrate refined brine, it fully replaces and washes away the high-nitrate mother liquor and sulfate impurities adhering to the surface of the salt particles, thereby significantly reducing the sulfate residue in the salt and ensuring that the sulfate content in the finished high-purity metallic sodium salt is less than 0.01%.

[0008] Optionally, the gypsum-type brine comprises the following components in terms of mass concentration: NaCl: 292–307 g / L, CaSO4: 3.9–4.2 g / L, MgSO4: 0.6–1.0 g / L, and Na2SO4: 1.1–1.5 g / L.

[0009] Optionally, the primary refined brine comprises the following components at the following mass concentrations: NaCl: 292–305 g / L, Na₂SO₄: 6.6–7.1 g / L, Ca… 2+ ≤20 mg / L, Mg 2+ ≤10 mg / L.

[0010] Optionally, the low-nitrate brine comprises the following components in mass concentrations: Na2SO4: ≤0.5 g / L, NaCl: ≥290 g / L.

[0011] Optionally, the high-nitrate brine includes the following components in mass concentration: Na2SO4: ≥55 g / L, NaCl: ≤20 g / L.

[0012] Optionally, the evaporation crystallization is a double-effect evaporation system, which includes a first-effect evaporation chamber and a second-effect evaporation chamber.

[0013] The dual-effect evaporation system provided by this invention controls the sulfate concentration in the first and second effect evaporation chambers to cause sodium chloride crystals to precipitate.

[0014] Optionally, the first-effect evaporation chamber and the second-effect evaporation chamber are connected in series.

[0015] Optionally, the saturated salt slurry undergoes solid-liquid separation before washing.

[0016] This invention separates saturated salt slurry into solid and liquid components using a hydrocyclone before washing, significantly saving washing liquid and improving washing efficiency.

[0017] Optionally, the concentrated mother liquor obtained from the evaporation and crystallization is discharged externally.

[0018] The dual-effect evaporation system provided by this invention enriches sodium sulfate in the mother liquor and discharges it from the system, thus systematically inhibiting the sulfate content.

[0019] This invention also provides a method for preparing high-purity sodium metallic salts, wherein the high-purity sodium metallic salt contains SO4. 2- The mass content is less than 0.01%.

[0020] The high-purity sodium metal salt obtained by this invention has a stable sulfate content of less than 0.01%.

[0021] The above-described technical solution of the present invention has at least the following beneficial effects: The preparation method provided by this invention combines nanofiltration, countercurrent washing and other steps to form a new process that continuously reduces sulfate content throughout the entire process, resulting in a high-purity metallic sodium salt with a sulfate content that is consistently less than 0.01%. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0023] Example 1 A method for preparing a high-purity metallic sodium salt, comprising the following steps: 1) The raw material is gypsum-based brine, which contains: NaCl 305.25 g / L, CaSO4 3.97 g / L, MgSO4 0.69 g / L, Na2SO4 1.21 g / L (other components are present in low amounts and can be ignored). 2) Lime (calcium hydroxide) is added to the gypsum-based brine to form magnesium hydroxide precipitate and remove magnesium ions. Soda ash (sodium carbonate) is added to form calcium carbonate precipitate and remove calcium ions. The brine is then filtered to remove sludge, yielding primary refined brine. The Na₂SO₄ content is 6.77 g / L, NaCl is 297.45 g / L, and Ca... 2+ 18.61 mg / L, Mg 2+ 9.47 mg / L; 3) The primary refined brine enters the nanofiltration system to obtain permeate (low-nitrate refined brine) and retentate (high-nitrate refined brine). The permeate is collected, and the low-nitrate refined brine is found to contain 295.18 g / L NaCl and 0.48 g / L Na2SO4; the high-nitrate refined brine contains 2.04 g / L NaCl and 55.53 g / L Na2SO4.

[0024] 4) A portion of the low-nitrate refined brine is sent to the first-effect and second-effect evaporation chambers for independent evaporation and crystallization. During the production process, concentrated mother liquor is discharged periodically and in measured quantities (the flow rate of the discharged concentrated mother liquor is controlled at 15m). 3 / h~25m 3 ( / h), effectively avoiding the accumulation of sulfate impurities in the evaporation system and achieving efficient crystallization of sodium chloride.

[0025] 5) Saturated salt slurry is continuously discharged from the first and second effect salt legs. After solid-liquid concentration by a hydrocyclone, the slurry enters the salt washing unit. The remaining low-nitrate brine is used as the washing liquid throughout the process for countercurrent precision washing of the salt crystals (the flow rate of the washing liquid is controlled at 30 m³ / s). 3 / h~35 m 3 The volume ratio of low-nitrate brine used for evaporation and crystallization to that used for washing is 4:1; this effectively replaces the high sulfate mother liquor adsorbed on the surface of the salt crystals, further reducing the sulfate residue in the finished product.

[0026] 6) After washing and purification, the clean salt slurry is centrifuged for dehydration, fluidized bed drying, and vibrating sieve for impurity removal. It is then metered, packaged, and stored to finally produce high-purity metallic sodium salt product.

[0027] The main indicators (mass percentage) of the finished sodium metallic salt prepared in Example 1 were as follows: NaCl 99.85%, SO42- 2- 0.008%, moisture 0.040%, water-insoluble matter 0.0045%, total calcium and magnesium ions 0.0017%.

[0028] Example 2 1) The raw material is gypsum-based brine, which contains: NaCl 306.32 g / L, CaSO4 4.2 g / L, MgSO4 0.71 g / L, Na2SO4 1.20 g / L (other components are present in low amounts and can be ignored). 2) Lime (calcium hydroxide) is added to the gypsum-based brine to form magnesium hydroxide precipitate and remove magnesium ions. Soda ash (sodium carbonate) is added to form calcium carbonate precipitate and remove calcium ions. The brine is then filtered to remove sludge, yielding primary refined brine. The Na₂SO₄ content is 6.81 g / L, NaCl is 298.44 g / L, and Ca... 2+ 18.83 mg / L, Mg 2+ 9.52 mg / L; 3) The primary refined brine enters the nanofiltration system, yielding permeate (low-nitrate refined brine) and retentate (high-nitrate refined brine). The permeate is collected, and the low-nitrate refined brine is found to contain 296.38 g / L NaCl and 0.50 g / L Na₂SO₄; the high-nitrate refined brine contains 2.06 g / L NaCl and 57.05 g / L Na₂SO₄. 4) A portion of the low-nitrate refined brine is sent to the first-effect and second-effect evaporation chambers for independent evaporation and crystallization. During the production process, concentrated mother liquor is discharged periodically and in measured quantities (the flow rate of the discharged concentrated mother liquor is controlled at 15m). 3 / h~25m 3 ( / h), effectively avoiding the accumulation of sulfate impurities in the evaporation system and achieving efficient crystallization of sodium chloride.

[0029] 5) Saturated salt slurry is continuously discharged from the first and second effect salt legs. After solid-liquid concentration by a hydrocyclone, the slurry enters the salt washing unit. Low-nitrate brine is used as the washing liquid throughout the process for countercurrent precision washing of the salt crystals (the flow rate of the washing liquid is controlled at 30 m³ / s). 3 / h--35 m 3 The volume ratio of low-nitrate brine used for evaporation and crystallization to that used for washing is 4:1, which effectively replaces the high sulfate mother liquor adsorbed on the surface of the salt crystals, further reducing the sulfate residue in the finished product.

[0030] 6) After washing and purification, the clean salt slurry is centrifuged for dehydration, fluidized bed drying, and vibrating sieve for impurity removal. It is then metered, packaged, and stored to finally produce high-purity metallic sodium salt product.

[0031] The main indicators (mass percentage) of the finished sodium metal salt prepared in Example 2 were as follows: NaCl 99.87%, SO42- 2- 0.009%, moisture 0.042%, water-insoluble matter 0.0044%, total calcium and magnesium ions 0.0017%.

[0032] Comparative Example 1 The difference from Example 1 is that low-nitrate brine is not used for countercurrent washing.

[0033] The specific steps are as follows: 1) The raw material is gypsum-type brine, which includes the following components in terms of mass concentration: NaCl 305.25 g / L, CaSO4 3.97 g / L, MgSO4 0.69 g / L, and Na2SO4 1.21 g / L; 2) Lime (calcium hydroxide) is added to the gypsum-based brine to form magnesium hydroxide precipitate and remove magnesium ions. Soda ash (sodium carbonate) is added to form calcium carbonate precipitate and remove calcium ions. The brine is then filtered to remove sludge, yielding primary refined brine. The Na₂SO₄ content is 6.77 g / L, NaCl is 297.45 g / L, and Ca... 2+18.61 mg / L, Mg 2+ 9.47 mg / L; 3) The primary refined brine enters the nanofiltration system to obtain permeate (low-nitrate refined brine) and retentate (high-nitrate refined brine). The permeate is collected, and the low-nitrate refined brine is found to contain 295.18 g / L NaCl and 0.48 g / L Na2SO4; the high-nitrate refined brine contains 2.04 g / L NaCl and 55.53 g / L Na2SO4.

[0034] 4) All low-nitrate refined brine is fed into the first-effect and second-effect evaporation chambers for independent evaporation and crystallization. During the production process, concentrated mother liquor is discharged periodically and in measured quantities (the flow rate of the discharged concentrated mother liquor is controlled at 15m). 3 / h~25m 3 ( / h), effectively avoiding the accumulation of sulfate impurities in the evaporation system and achieving efficient crystallization of sodium chloride.

[0035] 5) The salt legs of the first and second effects continuously discharge saturated salt slurry, which is then concentrated by a hydrocyclone before entering the salt washing machine.

[0036] 6) After washing and purification, the clean salt slurry is centrifuged for dehydration, fluidized bed drying, and vibrating sieve for impurity removal. It is then metered, packaged, and stored to finally obtain the finished sodium metal salt product.

[0037] The main indicators (mass percentage) of the finished sodium metallic salt prepared in Comparative Example 1 were as follows: NaCl 99.81%, SO42- 2- 0.016%, moisture 0.041%, water-insoluble matter 0.005%, total calcium and magnesium ions 0.0023%.

[0038] As can be seen from the comparison between Example 1 and Comparative Example 1, the low-nitrate brine countercurrent washing method in this invention... The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-purity metallic sodium salt, characterized in that, Includes the following steps: 1) Add calcium hydroxide and sodium carbonate to the gypsum-based brine, and separate the resulting precipitate to obtain primary refined brine; 2) The primary refined brine is processed through a nanofiltration system to obtain low-nitrate refined brine and high-nitrate refined brine; 3) Part of the low-nitrate brine is evaporated and crystallized, and the saturated salt slurry obtained at the bottom is washed, using the remaining low-nitrate brine for washing; 4) After washing, the salt slurry is dehydrated, dried, and impurities are removed to obtain high-purity metallic sodium salt.

2. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The gypsum-type brine comprises the following components in terms of mass concentration: NaCl: 292–307 g / L, CaSO4: 3.9–4.2 g / L, MgSO4: 0.6–1.0 g / L, and Na2SO4: 1.1–1.5 g / L.

3. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The primary refined brine comprises the following components in the following mass concentrations: NaCl: 292–305 g / L, Na₂SO₄: 6.6–7.1 g / L, Ca… 2+ ≤20 mg / L, Mg 2+ ≤10 mg / L.

4. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The low-nitrate brine comprises the following components in mass concentrations: Na2SO4: ≤0.5 g / L, NaCl: ≥290 g / L.

5. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The high-nitrate brine comprises the following components by mass concentration: Na2SO4: ≥55 g / L, NaCl: ≤20 g / L.

6. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The evaporation crystallization is a double-effect evaporation system, which includes a first-effect evaporation chamber and a second-effect evaporation chamber.

7. The method for preparing high-purity metallic sodium salt according to claim 6, characterized in that, The first-effect evaporation chamber and the second-effect evaporation chamber are connected in series.

8. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The saturated salt slurry undergoes solid-liquid separation before washing.

9. The method for preparing high-purity metallic sodium salt according to claim 1, characterized in that, The concentrated mother liquor obtained from evaporation and crystallization is discharged externally.

10. A high-purity sodium metal salt prepared by the method according to any one of claims 1-9, characterized in that, The high-purity sodium metal salt SO4 2- The mass content is less than 0.01%.