Method for preparing sodium sulfate from high-salinity wastewater

Abstract

The invention provides a method for preparing sodium sulfate from high-salinity wastewater. The method comprises the following steps: adding lanthanum oxide into the high-salinity wastewater, then adding calcium hydroxide, and finally adding sodium hydroxide to obtain sodium sulfate. According to the method, lanthanum oxide is added into high-salinity wastewater, specific coordinate bonds are formed by surface hydroxyl groups and phosphate radicals, the phosphate radicals are selectively adsorbed, the purpose of removing the phosphate radicals is achieved, calcium hydroxide is added, calcium hydroxide is more easily combined with fluorine to generate calcium fluoride with a smaller solubility product, fluorine ions are removed, and the purpose of removing the phosphate radicals is achieved. Finally, sodium hydroxide is added, sodium sulfate in the solution is induced to be supersaturated and separated out, sodium sulfate separation is achieved, and by-products such as calcium fluoride and sodium sulfate generated through the method can achieve resource recycling.

Description

Technical Field

[0001] The present invention relates to the field of resource utilization, and particularly to a method for preparing sodium sulfate from high-salt wastewater. Background Art

[0002] With the rapid development of the rare earth industry, a large amount of high-salt wastewater is generated during the manufacturing and purification processes of rare earth products due to the extensive use of chemical agents. This wastewater contains high concentrations of sulfate (SO4 2⁻ > 20 g / L), fluoride (F - > 10 g / L), and a relatively low phosphorus content (P2O5 < 5 g / L), with an annual discharge exceeding 200 million tons.

[0003] Currently, the main method for treating wastewater is the calcium precipitation-evaporation crystallization method, which is widely used especially in treating high-salt wastewater containing pollutants such as sulfate and fluoride ions. Calcium precipitation utilizes calcium ions (Ca 2+ ) to chemically react with anions such as sulfate ions and fluoride ions in the wastewater to form insoluble calcium salt precipitates. Evaporation crystallization is to evaporate the wastewater by heating or other means. As the water evaporates continuously, the salt concentration in the wastewater gradually increases. When it reaches the saturation state, the salt substances crystallize out, realizing the separation of salts and water. However, during the calcium precipitation process, phosphorus, fluoride, and sulfur will form complex coprecipitates, and the purity of sodium sulfate is less than 85%, which cannot meet the resource utilization standard of industrial by-product salts; evaporation crystallization also faces the unfavorable situation of high energy consumption and difficult to form effective benefits. Summary of the Invention

[0004] Aiming at the problems in the prior art that phosphorus, fluoride, and sulfur form complex coprecipitates, the purity of sodium sulfate is less than 85%, and the economic benefit of the evaporation crystallization process is poor, the present invention provides a method for preparing sodium sulfate from high-salt wastewater.

[0005] The technical method of the present invention is as follows: A method for preparing sodium sulfate from high-salt wastewater, comprising: Adding lanthanum oxide to the high-salt wastewater to adsorb phosphate ions in the high-salt wastewater, and filtering to obtain a first filtrate and a first filter residue; Adding calcium hydroxide to the first filtrate, and the addition amount of calcium hydroxide can make fluoride ions in the first filtrate form calcium fluoride, and filtering to obtain a second filtrate and a second filter residue; Adding sodium hydroxide to the second filtrate, and the addition amount of sodium hydroxide can make sulfate ions in the second filtrate precipitate sodium sulfate, and filtering to obtain a third filtrate and a third filter residue, and the third filter residue is sodium sulfate; Wherein, the high-salt wastewater contains phosphate ions, fluoride ions, sulfate ions, and sodium ions. The alkalinity of the high-salt wastewater is lower than 0.5 mol / L.

[0006] Specifically, the step of adding calcium hydroxide includes a first stage, a second stage, and a third stage; in the first stage, calcium hydroxide is added to reduce the fluoride ion concentration to 10 g / L; in the second stage, calcium hydroxide is added to reduce the fluoride ion concentration to 5 g / L; in the third stage, calcium hydroxide is added to make the fluoride ion concentration less than 1.0 g / L. Here, in the first stage, the second stage, and the third stage, the temperature range for each stage is 50 - 60 °C, and the time range is 0.5 - 2 h.

[0007] Specifically, the form of sodium hydroxide is solid, and the purity of sodium hydroxide is greater than or equal to 99%.

[0008] Further, the method further includes: dephosphorizing the first filter residue to obtain trisodium phosphate. Here, the dephosphorization treatment uses sodium hydroxide, and the concentration of sodium hydroxide is 1.5 - 3 mol / L.

[0009] Further, the method further includes: removing the sulfate ions and calcium carbonate adsorbed on the surface of the second filter residue to obtain calcium fluoride with a purity greater than 98%. Here, the removal uses hydrochloric acid, and the concentration of hydrochloric acid is 1.5 - 2.5 mol / L.

[0010] Further, the method further includes: washing the third filter residue with a sodium hydroxide solution to obtain sodium sulfate with a purity greater than 98%; wherein, the concentration of the sodium hydroxide solution is 5 - 6 mol / L.

[0011] The beneficial effects of the present invention are as follows: First, the present invention first adds lanthanum oxide to the high-salt wastewater, uses the surface hydroxyl groups to form specific coordination bonds with phosphate ions, selectively adsorbs phosphate ions to achieve the purpose of removing phosphate ions, then adds calcium hydroxide. Since calcium hydroxide is more likely to combine with fluorine to form calcium fluoride with a smaller solubility product, fluoride ions are removed. Finally, by adding sodium hydroxide, the supersaturation of sodium sulfate in the solution is induced to precipitate, realizing the separation of sodium sulfate. The by-products such as calcium fluoride and sodium sulfate generated by the present invention can be recycled.

[0012] Second, the present invention dephosphorizes the filter residue after adding lanthanum oxide to the high-salt wastewater to obtain trisodium phosphate, realizing resource recycling.

[0013] Third, the present invention washes sodium sulfate to remove residual impurities, and finally the purity of sodium sulfate reaches 98%, significantly improving the product quality.

[0014] Fourth, the present invention removes the sulfate ions and calcium carbonate adsorbed on the surface of the second filter residue to obtain calcium fluoride with a purity greater than 98%, significantly improving the product quality. Description of the Drawings

[0015] Figure 1Flow chart of a method for preparing sodium sulfate using high-salt wastewater. Detailed implementation mode

[0016] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0017] The present invention provides a process for targeted separation of phosphorus-fluorine-sulfur and high-value purification of sodium sulfate from high-salt wastewater. The method uses a synergistic technology of selective adsorption-gradient precipitation-supersaturated crystallization-alkali solution regeneration (also known as targeted separation of phosphorus-fluorine-sulfur-sodium sulfate regeneration) to achieve efficient separate recovery and resource utilization of phosphorus, fluorine, and sulfur.

[0018] The present invention provides a method for preparing sodium sulfate using high-salt wastewater, as Figure 1 shown, including: S1. Add lanthanum oxide to the high-salt wastewater to adsorb and precipitate phosphate ions in the high-salt wastewater, and then filter to obtain a first filtrate and a first filter residue.

[0019] In one embodiment, due to the large amount of sodium fluoride contained in the rare earth alkali-converted wastewater, when sodium fluoride is purified by the MVR technology, wash water with low alkalinity, high fluorine, and high sulfur will be generated, and this kind of wash water belongs to high-salt wastewater.

[0020] Here, the high-salt wastewater contains phosphate ions, fluoride ions, sulfate ions, and sodium ions. Here, the concentration of sulfate ions is greater than 20 g / L, the concentration of fluoride ions is greater than 10 g / L, and the concentration of P2O5 is less than 5 g / L. Here, the concentration of phosphate ions is calculated by converting the amount of P2O5. Specifically, the high-salt wastewater mainly includes sodium fluoride, trisodium phosphate, sodium sulfate, and sodium hydroxide. The alkalinity of the high-salt wastewater is lower than 0.5 mol / L.

[0021] Specifically, in a low-alkalinity state (for example, lower than 0.5 mol / L), lanthanum oxide powder uses surface hydroxyl groups to form specific coordination bonds with phosphate ions (PO4 3- ), and selectively adsorbs phosphorus. This is because the charge density matching degree of PO4 3- and La 3+ is higher, and the two are more likely to adsorb. In addition, there is a certain chemical bond reaction and partial physical adsorption in the adsorption of phosphate ions by lanthanum oxide, and its adsorption ability is greater than that of single chemical adsorption. Here, the addition amount of lanthanum oxide powder is determined according to the content of phosphorus pentoxide in the actual waste alkali water. For example, adding in a molar ratio of lanthanum ion: phosphate ion of 1:1, in the subsequent phosphorus removal treatment, lanthanum oxide powder can be repeatedly obtained and reused in step S1.

[0022] In one embodiment, the method further includes: performing dephosphorization treatment on the first filter residue to obtain trisodium phosphate. Specifically, sodium hydroxide is added to the first filter residue for dephosphorization treatment, and the dephosphorization rate is ≥98%. The phosphorus-rich alkaline solution can crystallize out trisodium phosphate. For example, the liquid-solid ratio of the first filter residue to sodium hydroxide is 4-5:1. The preferred liquid-solid ratio is 3:1. Here, the concentration of sodium hydroxide is 1.5-3 mol / L. The present invention realizes the recycling of resources by recovering trisodium phosphate. At the same time, after the lanthanum oxide adsorption becomes supersaturated, it can be eluted with a high-alkalinity alkaline solution to break the La-O-PO3 bond, generate new surface hydroxyl groups, restore the adsorption capacity, and be recycled again.

[0023] S2. Add calcium hydroxide to the first filtrate. The added amount of calcium hydroxide can cause the fluoride ions in the first filtrate to form calcium fluoride, and then filter to obtain a second filtrate and a second filter residue.

[0024] In the present invention, calcium hydroxide is added. Since calcium hydroxide is more likely to combine with fluorine to form calcium fluoride with a smaller solubility product, the formation of calcium sulfate is reduced, and ultimately more calcium fluoride is ensured to be generated. This is because the solubility product of CaF2 is about 6 orders of magnitude lower than that of CaSO4. At the same calcium ion concentration, F - preferably combines with Ca 2+ to form a precipitate.

[0025] In the present invention, the added amount of calcium hydroxide is slightly lower than the consumption required for complete reaction of dissolving fluorine. This is because according to the molar ratio of calcium fluoride, the final fluorine residue in the solution is 0.5-1 g / L. Experiments have found that there is still fluorine residue even when calcium hydroxide is in excess or sufficient. To avoid waste caused by excessive addition of calcium hydroxide, its added amount is slightly lower than the consumption required for complete reaction of fluorine.

[0026] Specifically, the step of adding calcium hydroxide is to add calcium hydroxide in three stages. In the first stage, calcium hydroxide is added to reduce the fluoride ion concentration to 10 g / L; in the second stage, calcium hydroxide is added to reduce the fluoride ion concentration to 5 g / L; in the third stage, calcium hydroxide is added to make the fluoride ion concentration lower than 1.0 g / L (preferably 0.8-1.0 g / L). Here, according to the reaction equation of sodium fluoride and calcium hydroxide, the theoretical input amount in each stage is calculated to reduce it to the preset range in each stage. Here, the staged treatment mainly has two functions: First, the staged addition is equivalent to forming crystal seeds in the early stage, which helps to make the crystal form of the subsequently generated calcium fluoride better and the structure more stable; second, maintaining the fluorine in an excessive state in the early stage can avoid the reaction of sulfate radicals with calcium to generate unnecessary impurities.

[0027] Specifically, in the first stage, the second stage, and the third stage, the temperature range in each stage is 50-60 °C, and the time range is 0.5-2 h.

[0028] In one embodiment, the method further includes: removing sulfate ions and calcium carbonate adsorbed on the surface of the second filter residue to obtain calcium fluoride with a purity greater than 98%. Here, hydrochloric acid is used for removal, and the concentration of hydrochloric acid is 1.5 - 2.5 mol / L. The second filter residue is crude calcium fluoride (CaF2 purity 94%). The ratio of the second filter residue to hydrochloric acid is 2 - 4:1. The removal time is 0.5 - 2 h.

[0029] S3. Add sodium hydroxide to the second filtrate. The addition amount of sodium hydroxide can cause sulfate ions in the second filtrate to precipitate sodium sulfate. After filtration, a third filtrate and a third filter residue are obtained, and the third filter residue is sodium sulfate.

[0030] Specifically, in the present invention, by adding sodium hydroxide to the second filtrate (alkaline water after phosphorus and fluorine removal), the alkalinity is increased, inducing supersaturation precipitation of sodium sulfate, and the crystallization rate reaches more than 84%. The obtained third filtrate (high-concentration alkaline solution) is directly recycled to the alkali conversion process. Here, sodium hydroxide exists in solid form, namely flake alkali, and its purity is greater than or equal to 99%. This is because the alkalinity required for complete precipitation of sulfate ions is much greater than 10 mol / L. When the alkalinity reaches 10 mol / L, continuous addition of flake alkali will cause a large amount of solid alkali to be insoluble, resulting in too high a content of sodium hydroxide in sodium sulfate during subsequent filtration and causing the product quality to fail to meet the standard. Therefore, the alkalinity is increased to about 10 mol / L. At this time, about 84% of sulfate ions precipitate, and the remaining sulfate ions enter the production cycle with the high-concentration alkaline solution.

[0031] Specifically, during the addition of sodium hydroxide, the stirring time is 15 - 20 min, and it is cooled to 40 - 50 °C.

[0032] Furthermore, the third filter residue is washed with a 5 - 6 mol / L sodium hydroxide solution, and the fluoride ion residue is reduced to ≤0.05%, and the purity of the third filter residue is increased to ≥98%. If the alkalinity is too high (greater than 6 mol / L), there will be too much residual sodium hydroxide in sodium sulfate and the purity will be low; if the alkalinity is too low (less than 5 mol / L), a large amount of sodium sulfate will dissolve. Here, the fluorine content in the washed solution is relatively high, and it can be mixed with the second filtrate for defluorination again. Here, the washing temperature is 40 - 50 °C.

[0033] In summary, through the method of the present invention, the removal rates of fluorine and phosphorus reach more than 95% (up to 99% at most), the recovery rate of sodium sulfate reaches 84% (the purity can reach 98% after washing), and there is no wastewater discharge, and the effect is excellent.

[0034] In the present invention, unless otherwise specified, all preparation raw materials are commercially available products well-known to those skilled in the art.

[0035] The present invention is described in detail below through examples and experimental examples. However, these are only examples and do not limit the present invention in any form.

[0036] Example 1 A method for preparing sodium sulfate from high-salt wastewater, comprising: Step 1: Provide high-salt wastewater Take 10 L of high-salt wastewater, and detect that the alkalinity of the high-salt wastewater is 0.31 mol / L, wherein the F - content is 22 g / L, the content of P2O5 is 1.8 g / L (calculated by converting the amount of phosphate ions to P2O5), and the SO4 2- content is 38 g / L. The high-salt wastewater includes sodium fluoride, trisodium phosphate, sodium sulfate, and sodium hydroxide.

[0037] Step 2: Selective adsorption of phosphorus and closed-loop regeneration of the adsorbent Selective adsorption of phosphorus: Take 5 L of the high-salt wastewater in Step 1, add lanthanum oxide powder to the high-salt wastewater, stir at 50 - 60 °C for 15 min, and form specific coordination bonds with PO4 using surface hydroxyl groups 3- to selectively adsorb phosphorus, and filter to obtain a first filtrate and a first filter residue. Here, the addition amount of lanthanum oxide powder: the high-salt wastewater is 20.42 g:5 L. After the reaction is completed, measure that the content of P2O5 in the wastewater is less than 0.0010 g / L.

[0038] Closed-loop regeneration of the adsorbent: Dephosphorize the first filter residue with 2 mol / L NaOH solution, and the dephosphorization rate ≥ 98%. The phosphorus-rich alkaline solution can crystallize out trisodium phosphate with a purity greater than 98% to achieve recycling.

[0039] Step 3: Gradient precipitation of fluorine and pickling purification Gradient precipitation of fluorine: Calculate the amount of calcium hydroxide added in each stage according to the reaction of fluoride ions and calcium hydroxide. Add calcium hydroxide to the first filtrate in three stages. According to the molar ratio Ca 2+ :F⁻ = 1:2, preset the residual F⁻ = 0.7 g / L, keep the temperature at 60 - 65 °C, and add a total of 207.6 g of Ca(OH)2.

[0040] In the first stage, add 60% calcium hydroxide at 20 g / min and react at 60 - 65 °C for 15 min to reduce the fluoride ion concentration to 10 g / L; in the second stage, add 30% calcium hydroxide at 15 g / min and react at 60 - 65 °C for 15 min to reduce the fluoride ion concentration to 5 g / L; in the third stage, add 10% calcium hydroxide at 10 g / min and react at 60 - 65 °C for 15 min to make the fluoride ion concentration 0.7 g / L, obtaining a second filtrate and 232 g of a second filter residue. Inspection shows that the fluorine content in the second filtrate is 0.74 g / L.

[0041] Pickling and purification: 232 g of the second filter residue (crude calcium fluoride CaF2 purity 94.05%, SO4 2- content 4.31%) was washed with 2 mol / L hydrochloric acid, with a liquid-solid ratio of 2:1 and a washing time of 30 min to remove adsorbed SO4 2- and CaCO3 impurities, and the purity was increased to >98.3%. After drying, 214.45 g with a content of 98.60% and SO4 2- <0.1% reached the industrial calcium fluoride standard.

[0042] Step 4: Supersaturated precipitation of sodium sulfate and caustic washing Supersaturated precipitation of sodium sulfate: To the second filtrate (alkaline water initial NaOH concentration 1.34 mol / L after phosphorus and fluoride removal, F - content 0.74 g / L, SO4 2- content 36 g / L), 1732 g of caustic soda (solid sodium hydroxide, purity 99%) was added, stirred for 15 - 20 min to completely melt the caustic soda; cooled to 40 - 50 °C, the alkalinity was gradually increased to 10 mol / L to induce supersaturated precipitation of the third filter residue (sodium sulfate), and the crystallization rate reached 86.06%, resulting in 252 g of sodium sulfate (sodium sulfate content 91.92%, F - content 1.29%). The third filtrate (high-concentration caustic solution F - content 0.1 g / L, alkalinity 10.25 mol / L, SO4 2- content 5.02 g / L) was directly recycled to the alkali conversion process.

[0043] Caustic washing: 252 g of the third filter residue was washed with 1 L of 5.02 mol / L NaOH solution at 40 °C, and after drying, it was 224.4 g of refined sodium sulfate with a content of 98.62% and F-<0.1%. The alkalinity of the washing solution was detected to be 5.42 mol / L, F - content 3.8 g / L, SO4 2- content 7.02 g / L, which can be added to the fluoride removal step to reduce the subsequent addition amount of caustic soda.

[0044] Example 2 A method for preparing sodium sulfate from high-salt wastewater, comprising: Step 1: Provide high-salt wastewater Take 10 L of high-salt wastewater, and detect the alkalinity of the high-salt wastewater to be 0.26 mol / L, where F - content is 21 g / L, the concentration of P2O5 is 2.4 g / L, and SO4 2- content is 42 g / L. The high-salt wastewater includes sodium fluoride, trisodium phosphate, sodium sulfate, and sodium hydroxide.

[0045] Step 2: Selective Adsorption of Phosphorus and Closed-loop Regeneration of Adsorbent Selective adsorption of phosphorus: Take 5 L of the high-salt wastewater from Step 1, add lanthanum oxide powder to the high-salt wastewater, stir at 50 - 60 °C for 15 min, and form specific coordination bonds with PO4 using surface hydroxyl groups to selectively adsorb phosphorus. Then filter to obtain the first filtrate and the first filter residue. Here, the addition amount of lanthanum oxide powder is 27.2 g:5 L of high-salt wastewater. After the reaction is completed, measure the content of P2O5 in the wastewater to be less than 0.0010 g / L. 3- Closed-loop regeneration of adsorbent: Dephosphorize the first filter residue with 2 mol / L NaOH solution, with a dephosphorization rate ≥ 98%. The phosphorus-rich alkaline solution can crystallize out trisodium phosphate with a purity greater than 98%, realizing recycling.

[0046]

[0047] Step 3: Gradient Precipitation of Fluoride and Pickling Purification Gradient precipitation of fluoride: Calculate the amount of calcium hydroxide added in each stage according to the reaction between fluoride ions and calcium hydroxide. Add calcium hydroxide to the first filtrate in three stages. According to the molar ratio Ca 2+ :F⁻ = 1:2, preset the residual F - content to be 0.7 g / L, keep the temperature at 60 - 65 °C, and add a total of 197.6 g of Ca(OH)2.

[0048] In the first stage, add 60% calcium hydroxide at 20 g / min and react at 60 - 65 °C for 15 min to reduce the fluoride ion concentration to 10 g / L; in the second stage, add 30% calcium hydroxide at 15 g / min and react at 60 - 65 °C for 15 min to reduce the fluoride ion concentration to 5 g / L; in the third stage, add 10% calcium hydroxide at 10 g / min and react at 60 - 65 °C for 15 min to make the fluoride ion concentration 0.7 g / L, obtaining the second filtrate and 221 g of the second filter residue. Inspection shows that the fluoride content in the second filtrate is 0.68 g / L.

[0049] Pickling purification: Wash 221 g of the second filter residue (coarse calcium fluoride CaF2 purity 93.80%, SO4 2- content 4.51%) with 2 mol / L hydrochloric acid. Among them, the liquid-solid ratio is 2:1, and the washing time is 30 min to remove the surface-adsorbed SO4 2- and CaCO3 impurities, and the purity is increased to > 98.3%. After drying, the content is 202.45 g with a purity of 98.50% and SO4 2- < 0.1%, meeting the industrial calcium fluoride standard.

[0050] Step 4: Supersaturated Precipitation of Sodium Sulfate and Alkaline Solution Washing ​Sodium sulfate supersaturated precipitation: For the second filtrate (the initial NaOH concentration of the alkaline water after phosphorus and fluoride removal is 1.3 mol / L, F - content is 0.68 g / L, SO4 2- content is 38 g / L), add 1732 g of flake caustic soda (solid sodium hydroxide, purity 99%), stir for 15 - 20 min to completely melt the flake caustic soda; cool to 40 - 50 °C, gradually increase the alkalinity to 10 mol / L to induce the supersaturated precipitation of the third filter residue (sodium sulfate), with a crystallization rate of 84.63%, resulting in 258 g of sodium sulfate (sodium sulfate content is 93%, F - content is 1.09%). The third filtrate (high-concentration alkaline solution F - content is 0.14 g / L, alkalinity is 10.15 mol / L, SO4 2- content is 5.84 g / L) is directly recycled to the alkali conversion process.

[0051] Alkaline solution washing: Wash 258 g of the third filter residue with 1 L of 5.02 mol / L NaOH solution at 40 °C, and after drying, it is 222 g of refined sodium sulfate with a content of 98.5%, F - <0.1%. The alkalinity of the washing solution is detected to be 5.32 mol / l, F - content is 3.2 g / L, SO4 2- content is 8.62 g / L, which can be added to the fluoride removal step to reduce the subsequent addition amount of flake caustic soda.

[0052] Comparative Example 1 Step 1: Take 10 L of high-salt wastewater and detect its main impurity content: F - content is 22 g / L, SO4 2- content is 38 g / L, P2O5 content is 1.8 g / L, and alkalinity is 0.31 mol / L.

[0053] Step 2: Take 5 L of the high-salt wastewater from Step 1, add lanthanum oxide powder to the high-salt wastewater, stir at 50 - 60 °C for 15 min, and use the surface hydroxyl groups to form specific coordination bonds with PO4 3- to selectively adsorb phosphorus, filter to obtain the first filtrate and the first filter residue. Here, the addition amount of lanthanum oxide powder: for 5 L of high-salt wastewater is 20.42 g:5 L. After the reaction is completed, measure the P2O5 content in the wastewater to be less than 0.0010 g / L.

[0054] Step 3: According to the reaction of fluoride ions and calcium hydroxide, according to the molar ratio Ca 2+ :F⁻ = 1:2, preset the residual F -The content is 0.7 g / L. Keep the temperature at 60 - 65 °C. A total of 207.6 g of Ca(OH)₂ is added. Calcium hydroxide is added rapidly without gradients. After addition, react for 30 min. Filter by suction to separate, obtaining 246 g of the second filter residue (crude calcium fluoride) and the second filtrate.

[0055] Step 4: Wash 246 g of the second filter residue (crude calcium fluoride with a CaF₂ purity of 80.42% and a SO₄ 2- content of 12.6%) with 2 mol / L hydrochloric acid. Among them, the liquid - solid ratio is 2:1, and the washing time is 30 min to remove the adsorbed SO₄ 2- and CaCO₃ impurities, obtaining 192 g of calcium fluoride with a content of 96.52% and a SO₄ 2- content of 2.4%.

[0056] Step 5: Add 1732 g of flake soda (solid sodium hydroxide with a purity of 99%) to the second filtrate (the initial NaOH concentration of the alkali water after phosphorus and fluorine removal is 1.32 mol / L, the F - content is 3.6 g / L, and the SO₄ 2- content is 30 g / L), stir for 15 - 20 min to completely melt the flake soda; cool to 40 - 50 °C, gradually increase the alkalinity to 10 mol / L to induce the supersaturated precipitation of the third filter residue (sodium sulfate), with a crystallization rate of 79.93%, obtaining 198.14 g of sodium sulfate (the sodium sulfate content is 90.14% and the F - content is 1.73%) and the third filtrate (high - concentration alkali solution with an F - content of 0.12 g / L, an alkalinity of 10.15 mol / L, and a SO₄ 2- content of 6.02 g / L), which is directly recycled to the alkali conversion process.

[0057] Step 6: Put all the crude sodium sulfate in Step 5 into 1 L of pure alkali solution with a concentration of 5.02 mol / l and wash at 40 °C, then filter by suction to separate, obtaining the alkali washing solution and high - purity sodium sulfate; the detected alkalinity of the washing solution is 5.22 mol / L, F - : 3.4 g / L, SO₄ 2- : 6.42 g / L, which can be added to the defluorination step to reduce the subsequent addition amount of flake soda; Step 7: The high - purity sodium sulfate obtained in Step 6 is 167 g after drying, with a content of 98.42% and F - <0.1%.

[0058] It can be seen that, compared with Example 1, during the precipitation process of fluorine, calcium fluoride containing a large amount of SO4 2- impurities is produced, which not only interferes with the subsequent extraction of sodium sulfate but also is not conducive to the purification of calcium fluoride.

[0059] Although embodiments of the present invention have been shown and described, those of ordinary skill in the art will appreciate that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing sodium sulfate using high-salt wastewater, characterized in that, Comprising: Adding lanthanum oxide to high-salt wastewater to adsorb phosphate ions in the high-salt wastewater, and filtering to obtain a first filtrate and a first filter residue; Adding calcium hydroxide to the first filtrate, and the addition amount of calcium hydroxide can make fluoride ions in the first filtrate form calcium fluoride, and filtering to obtain a second filtrate and a second filter residue; Adding sodium hydroxide to the second filtrate, and the addition amount of sodium hydroxide can precipitate sodium sulfate from the sulfate ions in the second filtrate, and filtering to obtain a third filtrate and a third filter residue, and the third filter residue is sodium sulfate; Wherein, the high-salt wastewater contains phosphate ions, fluoride ions, sulfate ions and sodium ions.

2. The method according to claim 1, characterized in that The step of adding calcium hydroxide includes a first stage, a second stage and a third stage; Adding calcium hydroxide in the first stage to reduce the fluoride ion concentration to 10 g / L; Adding calcium hydroxide in the second stage to reduce the fluoride ion concentration to 5 g / L; Adding calcium hydroxide in the third stage to make the concentration of fluoride ions lower than 1.0 g / L.

3. The method according to claim 2, wherein The temperature range of the first stage is 50 - 60 °C, and the time range is 0.5 - 2 h; The temperature range of the second stage is 50 - 60 °C, and the time range is 0.5 - 2 h; The temperature range of the third stage is 50 - 60 °C, and the time range is 0.5 - 2 h.

4. The method according to claim 1, wherein The form of the sodium hydroxide is solid, and the purity of the sodium hydroxide is greater than or equal to 99%.

5. The method according to claim 1, wherein The method further includes: Performing dephosphorization treatment on the first filter residue to obtain trisodium phosphate.

6. The method according to claim 5, wherein The dephosphorization treatment uses sodium hydroxide, and the concentration of the sodium hydroxide is 1.5 - 3 mol / L.

7. The method according to claim 1, characterized in that, The method further includes: Removing sulfate ions and calcium carbonate adsorbed on the surface of the second filter residue to obtain calcium fluoride with a purity greater than 98%.

8. The method according to claim 7, wherein The removal uses hydrochloric acid, and the concentration of the hydrochloric acid is 1.5 - 2.5 mol / L.

9. The method according to claim 1, characterized in that The method further includes: Washing the third filter residue with a sodium hydroxide solution to obtain sodium sulfate with a purity greater than 98%; wherein, the concentration of the sodium hydroxide solution is 5 - 6 mol / L.

10. The method according to claim 1, characterized in that, The alkalinity of the high-salt wastewater is lower than 0.5 mol / L.

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