Method for simultaneous removal of arsenic and phosphorus impurities in bayer process sodium aluminate solution

Abstract

The present application relates to sewage treatment technical field, specifically, it is a kind of synchronous removal method of arsenic phosphorus impurities in bayer sodium aluminate solution.The method is added to bayer sodium aluminate solution containing fluorine reagent, and the premix solution is obtained, and the reaction liquid with the caustic soda concentration in the target concentration range is obtained by the premix solution;Reaction liquid is directional cooling crystallization, and sodium fluoroarsenate crystal and sodium fluoro phosphate crystal are obtained, and the removal of arsenic and phosphorus impurities is completed after solid-liquid separation;In the reaction liquid, the target concentration range of caustic soda concentration is 180-200g / L.The method realizes the synchronous removal of arsenic and phosphorus by specific complexation-directional crystallization, and the removal rate of arsenic and phosphorus is high, and the content of sodium aluminate has no influence, and energy consumption is extremely low, operating cost is low, and economic benefit is remarkable.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and more specifically, to a method for the simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solutions produced by the Bayer process. Background Technology

[0002] The Bayer process is currently the mainstream technology for alumina production worldwide, accounting for more than 90% of global alumina capacity. During the Bayer process, arsenic and phosphorus impurities associated with bauxite are released into the sodium aluminate solution as arsenate and phosphate ions during high-temperature and high-pressure leaching, and then continuously accumulate in the circulating mother liquor.

[0003] The continuous accumulation of arsenic and phosphorus impurities poses a series of serious threats to alumina production: First, it worsens the seed crystal decomposition process, reducing the decomposition rate of aluminum hydroxide and the particle size of the product, leading to finer products and decreased strength; second, it affects the purity of alumina products, limiting the production of high-end alumina products; third, it accelerates equipment corrosion and shortens equipment lifespan; and fourth, some impurities are discharged with red mud, causing environmental pollution. Therefore, it is essential to efficiently and deeply remove arsenic and phosphorus impurities from sodium aluminate solutions.

[0004] Currently, the technologies for arsenic and phosphorus removal from sodium aluminate solutions using the Bayer process in the alumina industry mainly suffer from the following shortcomings: (1) Lime precipitation method: This is currently the most widely used impurity removal technology. It removes impurities by adding lime milk to the solution to generate calcium arsenate and calcium phosphate precipitates. However, this method has extremely poor selectivity. Lime will react with sodium aluminate solution in large quantities to generate hydrated calcium aluminate, resulting in an alumina loss rate of more than 5%. At the same time, it generates a large amount of calcium silicate slag that is difficult to dispose of, causing serious secondary pollution.

[0005] (2) Adsorption method: Adsorption materials such as activated carbon, ion exchange resin, and zeolite are used to remove impurities. However, in the Bayer process, under the solution system with high alkalinity, high aluminum and high ionic strength, the adsorption capacity of the adsorption materials is low, the selectivity is poor, and regeneration is difficult and the operating cost is high, so it cannot be implemented on a large scale for industrial application.

[0006] (3) Solvent extraction method: Organic extractants are used to separate arsenic and phosphorus, but the extractants are easily degraded and difficult to separate phases under high caustic alkali environment, resulting in low extraction efficiency. In addition, organic solvents are easy to volatilize and cause environmental pollution, so industrial applications are very rare.

[0007] (4) Coprecipitation method: Coprecipitation is achieved by using the crystallization process of impurities such as sodium oxalate and sodium sulfate. However, this method has low impurity removal efficiency, usually less than 50%, and cannot achieve deep removal. It is also easy to cause loss of the main product.

[0008] To address the aforementioned issues, the industry has yet to develop a simultaneous arsenic and phosphorus removal technology that balances high impurity removal efficiency, high selectivity, low alumina loss, low operating costs, and compatibility with existing Bayer processes. Therefore, developing novel impurity removal processes that meet green chemical engineering requirements is of significant industrial value for the high-quality development of the alumina industry. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for the simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution in the Bayer process.

[0010] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: This invention provides a method for the simultaneous removal of arsenic and phosphorus impurities from a Bayer process sodium aluminate solution. A fluorine-containing reagent is added to the Bayer process sodium aluminate solution to obtain a premix. A reaction solution with a caustic alkali concentration within a target range is obtained from the premix. The reaction solution is then subjected to directional cooling crystallization to obtain sodium fluoroarsenate crystals (nonadecanohydrate) and sodium fluorophosphate crystals (nonadecanohydrate). The removal of arsenic and phosphorus impurities is completed after solid-liquid separation. The target concentration range of the caustic alkali in the reaction solution is 180-200 g / L.

[0011] Based on the above technical solution, the present invention can be further improved as follows.

[0012] Furthermore, the temperature of the reaction solution is 60℃-85℃, and the final crystallization temperature of the directional cooling crystallization is 20℃-45℃.

[0013] Furthermore, the directional cooling crystallization process involves uniformly cooling the temperature to the final crystallization temperature at a cooling rate of 0.5-1℃ / min and maintaining this temperature for 2-4 hours.

[0014] Furthermore, the reaction solution is kept stirred during the directional cooling crystallization.

[0015] Furthermore, in the fluorine-containing reagent, the total molar ratio of fluorine to arsenic and phosphorus is 2-5:1.

[0016] Furthermore, the fluorine-containing reagent is sodium fluoride, potassium fluoride, aluminum fluoride, or ammonium fluoride.

[0017] Furthermore, in the Bayer process sodium aluminate solution, the phosphorus content, calculated as P2O5, is 0.1-3 g / L, and the arsenic content, calculated as As2O3, is 0.05-2 g / L.

[0018] Furthermore, the Bayer process sodium aluminate solution includes any one of the following from the Bayer process alumina production process: crude leaching solution, seed mother liquor, evaporation circulating mother liquor, or washing solution; in the Bayer process sodium aluminate solution, the aluminum concentration is 80-140 g / L for alumina, and the caustic alkali ratio (ak) is 2.6-3.0.

[0019] Furthermore, the reaction solution is obtained by determining the amount of exogenous caustic alkali to be added based on the initial concentration of caustic alkali in the premixed solution, and then adding the exogenous caustic alkali to the premixed solution.

[0020] Furthermore, the solid-liquid separation method is any one of vacuum filtration, plate and frame filtration, sedimentation separation, and centrifugal separation.

[0021] The beneficial effects of this invention are as follows: (1) The method for simultaneous removal of arsenic and phosphorus impurities in sodium aluminate solution by Bayer process of the present invention achieves simultaneous removal of arsenic and phosphorus through specific complexation-directional crystallization, with high arsenic and phosphorus removal rates and no effect on sodium aluminate content, which is far superior to the alumina loss rate of more than 5% in the traditional lime method. (2) The method for simultaneous removal of arsenic and phosphorus impurities in sodium aluminate solution by Bayer process of the present invention has mild reaction conditions, simple steps, no need to add complex high pressure and high temperature equipment, can be directly connected to the existing Bayer process production process, and the modification cost is extremely low. (3) The method for simultaneous removal of arsenic and phosphorus impurities in sodium aluminate solution by Bayer process of the present invention can realize intermittent or continuous operation, flexibly adapt to alumina production lines of different scales, and is suitable for large-scale industrial application. (4) The method for simultaneous removal of arsenic and phosphorus impurities in sodium aluminate solution by Bayer process of the present invention does not involve the addition of toxic and harmful reagents during the process and does not introduce new impurities such as heavy metals; the separated crystalline filter cake can be recovered by simple water washing to recover fluorine, phosphorus and arsenic resources, and the washing water can be directly returned to the washing process of Bayer process for recycling, without secondary pollution, which meets the requirements of green chemical industry and dual carbon development. (5) The method for simultaneous removal of arsenic and phosphorus impurities in sodium aluminate solution by Bayer process of the present invention has extremely low energy consumption, low operating cost and significant economic benefits. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process according to the present invention. Figure 2 The X-ray diffraction pattern of sodium fluorophosphate in the filter cake of Example 1 of the present invention; Figure 3 The X-ray diffraction pattern of sodium fluoroarsenate in the filter cake of Example 1 of the present invention. Detailed Implementation

[0023] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0024] The present invention discloses a method for the simultaneous removal of arsenic and phosphorus impurities from a Bayer process sodium aluminate solution. A fluorine-containing reagent is added to the Bayer process sodium aluminate solution to obtain a premix. A reaction solution with a caustic alkali concentration (Na₂O₃) within a target concentration range is obtained through the premix. The reaction solution is then subjected to directional cooling crystallization to obtain sodium fluoroarsenate crystals and sodium fluorophosphate crystals (nonadecanohydrate and nonadecanohydrate). After solid-liquid separation, the removal of arsenic and phosphorus impurities is completed. The target concentration range of the caustic alkali in the reaction solution is 180-200 g / L.

[0025] The present invention provides a method for the simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution in the Bayer process. This method provides a directional complexation-crystallization method for removing arsenic and phosphorus. Through specific complexation reaction and low-temperature crystallization control, arsenic and phosphorus are precipitated as highly stable sodium fluoroarsenate fluorophosphate fluorophosphate fluorophosphate crystals, achieving deep and simultaneous removal of arsenic and phosphorus, while significantly reducing alumina loss. This method is compatible with existing Bayer process production processes.

[0026] Specifically, the core reaction principle of the synchronous removal method of the present invention is as follows: In a sodium aluminate solution system with high caustic alkali, fluoride ions undergo specific nucleophilic substitution reactions with arsenate and phosphate ions to generate stable fluoroarsenate and fluorophosphate ions. Under low-temperature conditions, these form sodium fluoroarsenate fluorophosphate (Na7(AsO4)2F˙19H2O) and sodium fluorophosphate fluorophosphate (Na₂O) with low solubility. 6.5 (PO4)2F˙19.5H2O) crystals are formed and precipitated from sodium aluminate solution. The aluminate ions and free sodium hydroxide in the solution do not react with the above reaction and will not precipitate during the crystallization process. Therefore, highly selective removal of arsenic and phosphorus can be achieved, while keeping alumina loss to an extremely low level.

[0027] The method can remove arsenic and phosphorus at a rate of over 90%, with a maximum of 95%; it has no effect on sodium aluminate, and the alumina loss can be stably controlled below 0.5%, which is far superior to the alumina loss rate of over 5% in the traditional lime method.

[0028] Preferably, the temperature of the reaction solution is 60℃-85℃, and the final crystallization temperature of directional cooling crystallization is 20℃-45℃. When the cooling crystallization temperature is too high, it will lead to a decrease in the removal rate of arsenic and phosphorus. When the cooling crystallization temperature is too low, it will lead to the precipitation of aluminum hydroxide in the mother liquor, thereby causing the loss of aluminum oxide.

[0029] The method of this invention requires only conventional cooling during the crystallization process, has extremely low energy consumption, and its overall operating cost is only about 30% of that of the traditional lime method, resulting in significant economic benefits.

[0030] Preferably, the directional cooling crystallization process involves uniformly cooling to the final crystallization temperature at a cooling rate of 0.5-1℃ / min and maintaining this temperature for 2-4 hours. Cooling at a uniform rate and maintaining the temperature for a certain period of time can effectively improve the crystallization effect, not only enabling the precipitated crystals to form a solid that is easy to separate from the liquid, but also maintaining the removal rate at a good level.

[0031] Preferably, the reaction solution is kept stirred during directional cooling crystallization; stirring can effectively improve the crystallization efficiency.

[0032] Preferably, in the fluorine-containing reagent, the molar ratio of fluorine to the total amount of arsenic and phosphorus is 2-5:1. Since typical Bayer process sodium aluminate solutions contain a small amount of sodium fluoride, the method of this invention only requires the addition of a small amount of fluorine-containing reagent.

[0033] Preferably, the fluorine-containing reagent is sodium fluoride, potassium fluoride, aluminum fluoride, or ammonium fluoride; further, it can be sodium fluoride or potassium fluoride. Sodium fluoride and potassium fluoride are inexpensive, which can effectively reduce processing costs.

[0034] Preferably, in the Bayer process sodium aluminate solution, the phosphorus content, calculated as P2O5, is 0.1-3 g / L, and the arsenic content, calculated as As2O3, is 0.05-2 g / L.

[0035] Preferably, the Bayer process sodium aluminate solution includes any one of the following: crude leaching solution, seed mother liquor, evaporation circulating mother liquor, or washing solution from the Bayer process alumina production; in the Bayer process sodium aluminate solution, the concentration of alumina is 80-140 g / L, and the caustic alkali ratio ak is 2.6-3.0.

[0036] It should be noted that the "concentration of alumina" mentioned above does not refer to the actual alumina present in the Bayer process sodium aluminate solution. Aluminum in the Bayer process sodium aluminate solution actually exists in the form of sodium hydroxyaluminate. However, in the industry, when detecting metal components in a solution, the standard way to express the results is by using their oxides. Therefore, this patent uniformly uses the expression "concentration of alumina" to represent aluminum actually present in the form of sodium hydroxyaluminate. The same applies to P2O5 and As2O3 mentioned above.

[0037] Preferably, the reaction solution with a caustic alkali concentration within the target concentration range is obtained by determining the amount of exogenous caustic alkali to be added to the premixed solution based on the initial concentration of caustic alkali in the premixed solution.

[0038] When the initial concentration of caustic alkali in the premixed solution is already within the target concentration range, no external caustic alkali needs to be added, and cooling crystallization can be performed directly. The initial concentration of caustic alkali in the premixed solution can be obtained through testing, or the known concentration of caustic alkali can be obtained according to the specific type of solution to be treated before adding the fluorine-containing reagent.

[0039] Preferably, the exogenous caustic alkali is solid sodium hydroxide or high-concentration circulating mother liquor from the Bayer process, wherein the concentration of caustic alkali in the high-concentration circulating mother liquor from the Bayer process is greater than or equal to 240 g / L. By using high-concentration circulating mother liquor from the Bayer process as the exogenous caustic alkali, there is no need to add a large amount of solid sodium hydroxide, and the high-concentration circulating mother liquor from the Bayer process can be reused.

[0040] Preferably, the solid-liquid separation method is any one of vacuum filtration, plate and frame filtration, sedimentation separation, and centrifugal separation.

[0041] The filter cake obtained by the synchronous removal method of this invention can be washed with pure water to recover fluorine, arsenic, and phosphorus resources. The wash water generated is returned to the washing process of Bayer process alumina production for recycling. The resulting purified sodium aluminate solution can be directly returned to the seed decomposition process or leaching process of Bayer process alumina production.

[0042] like Figure 1 As shown, in one embodiment of the present invention, the specific steps of the synchronous removal method are as follows: (1) Pretreatment of ingredients: Add the Bayer process sodium aluminate solution containing arsenic and phosphorus impurities to the premixing tank, and add sodium fluoride and stir until the sodium fluoride is completely dissolved to obtain a uniform premix.

[0043] (2) Caustic alkali concentration control: Detect the concentration of caustic alkali in the premixed solution. When the concentration is not within the target concentration range, add caustic alkali source to obtain the reaction solution.

[0044] (3) Directional cooling crystallization: The reaction solution obtained in step (2) is introduced into the cooling crystallization tank, and the temperature is lowered at a uniform rate to stimulate the crystallization reaction, so that the arsenic and phosphorus in the solution are precipitated in the form of sodium fluoroarsenate nonadecahydrate and sodium fluorophosphate nonadecahydrate crystals, respectively, to obtain a crystallization slurry.

[0045] (4) Solid-liquid separation: The crystallization slurry obtained in step (3) is introduced into a crystallization filter press for solid-liquid separation to obtain a purified sodium aluminate solution after the removal of arsenic and phosphorus, and a filter cake containing sodium fluoroarsenate nonadecahydrate and sodium fluorophosphate nonadecahydrate.

[0046] A washing filter press is used to wash and filter the filter cake, resulting in a filter cake containing fluorine, arsenic, and phosphorus, as well as a filter cake washing liquid. The filter cake containing fluorine, arsenic, and phosphorus can be recycled, and the filter cake washing liquid can be recycled into the alumina washing process.

[0047] The purified sodium chlorate solution can be introduced into the alumina production system.

[0048] The effects of the present invention are illustrated below through specific embodiments and comparative examples. In the following embodiments and comparative examples, the sodium fluoride and sodium hydroxide used are industrial-grade reagents with purities exceeding industry standards; the Bayer process sodium aluminate used is sourced from a large-scale alumina production line in China; the concentrations of caustic alkali (Na2Ok) and Al2O3 in the solution are determined using the alumina industry standard acid-base titration method; the contents of arsenic, phosphorus, and fluorine are determined using inductively coupled plasma optical emission spectrometry (ICP-OES); the crystalline phase is determined using X-ray diffraction (XRD); and the crystal morphology is determined using scanning electron microscopy (SEM).

[0049] Example 1 This embodiment uses the simultaneous removal method of the present invention to remove arsenic and phosphorus impurities from Bayer process sodium aluminate solution. The specific steps are as follows: (1) Pretreatment of ingredients: Take 1L of mother liquor from the Bayer process and test its basic indicators. The test results are: Al2O3 concentration 85g / L, Na2Ok concentration 140g / L, caustic ratio ak=2.7, P2O5 concentration 1.2g / L, As2O3 concentration 0.5g / L. Based on the molar ratio of fluorine to arsenic and phosphorus total amount of 2:1, add sodium fluoride solid at 60℃ and stir until completely dissolved to obtain a homogeneous premixed solution.

[0050] (2) Caustic alkali concentration control: Add solid sodium hydroxide to the above premixed solution, stir until completely dissolved, adjust and stabilize the Na2Ok concentration of the solution to 180 g / L, and obtain the reaction solution to be crystallized.

[0051] (3) Directional cooling crystallization: The reaction solution to be crystallized is cooled to 30°C at a rate of 1°C / min, and crystallized at 30°C for 4 hours. The stirring speed is controlled at 159 r / min. White crystals continue to precipitate in the solution to obtain crystallization slurry.

[0052] (4) Solid-liquid separation: The above crystallized slurry is vacuum filtered to obtain purified sodium aluminate solution and white filter cake.

[0053] The purified sodium aluminate solution obtained in this embodiment was tested, and the results were as follows: the concentration of P2O5 was 0.08 g / L, the phosphorus removal rate was 93.99%, the concentration of As2O3 was 0.05 g / L, the arsenic removal rate was 90%, the concentration of Al2O3 was 84.7 g / L, and the alumina loss rate was 0.35%.

[0054] XRD analysis of the filter cake obtained in this embodiment confirmed that the precipitated crystals were sodium fluoroarsenate nonadecanohydrate (Na7(AsO4)2F˙19H2O) and sodium fluorophosphate nonadecanohydrate (Na6.5 (PO4)2F˙19.5H2O) No other impurities are formed.

[0055] The X-ray diffraction (XRD) pattern of sodium fluorophosphate in the filter cake is as follows: Figure 2 As shown, the X-ray diffraction (XRD) pattern of sodium fluoroarsenate is as follows: Figure 3 As shown.

[0056] Example 2 This embodiment uses the simultaneous removal method of the present invention to remove arsenic and phosphorus impurities from Bayer process sodium aluminate solution. The specific steps are as follows: (1) Pretreatment of ingredients: Take 1L of the mother liquor from the Bayer process evaporation cycle and test its basic indicators. The test results are: Al2O3 concentration 117.6g / L, Na2Ok concentration 189.1g / L, caustic ratio ak=2.65, P2O5 concentration 1.8g / L, As2O3 concentration 0.8g / L. Based on the molar ratio of fluorine to arsenic and phosphorus total amount of 2:1, add sodium fluoride solid at 80℃ and stir until completely dissolved to obtain a homogeneous premixed solution.

[0057] (2) Caustic alkali concentration control: Add solid sodium hydroxide to the above premixed solution, stir until completely dissolved, adjust and stabilize the Na2Ok concentration of the solution to 200 g / L, and obtain the reaction solution to be crystallized.

[0058] (3) Directional cooling crystallization: The reaction solution to be crystallized is cooled to 30°C at a uniform rate of 1°C / min, and crystallized at 30°C for 4 hours. The stirring speed is controlled at 159 r / min. White crystals continue to precipitate in the solution to obtain crystallization slurry.

[0059] (4) Solid-liquid separation: The above crystallized slurry is vacuum filtered to obtain purified sodium aluminate solution and white filter cake.

[0060] The purified sodium aluminate solution obtained in this embodiment was tested, and the results were as follows: the concentration of P2O5 was 0.06 g / L, the phosphorus removal rate was 96.67%, the concentration of As2O3 was 0.05 g / L, the arsenic removal rate was 93.75%, the concentration of Al2O3 was 117.1 g / L, and the alumina loss rate was 0.43%.

[0061] XRD analysis of the filter cake obtained in this embodiment confirmed that the precipitated crystals were sodium fluoroarsenate nonadecanohydrate (Na7(AsO4)2F˙19H2O) and sodium fluorophosphate nonadecanohydrate (Na 6.5 (PO4)2F˙19.5H2O) No other impurities are formed.

[0062] Example 3 This embodiment uses the simultaneous removal method of the present invention to remove arsenic and phosphorus impurities from Bayer process sodium aluminate solution. The specific steps are as follows: (1) Pretreatment of ingredients: Take 1L of the mother liquor from the Bayer process evaporation cycle and test its basic indicators. The test results are: Al2O3 concentration 117.6g / L, Na2Ok concentration 189.1g / L, caustic ratio ak=2.65, P2O5 concentration 1.8g / L, As2O3 concentration 0.8g / L. Based on the molar ratio of fluorine to arsenic and phosphorus total of 3:1, add sodium fluoride solid at 80℃ and stir until completely dissolved to obtain a homogeneous premixed solution.

[0063] (2) Caustic alkali concentration control: Add solid sodium hydroxide to the above premixed solution, stir until completely dissolved, adjust and stabilize the Na2Ok concentration of the solution to 240 g / L, and obtain the reaction solution to be crystallized.

[0064] (3) Directional cooling crystallization: The reaction solution to be crystallized is cooled to 30°C at a uniform rate of 1°C / min, and crystallized at 30°C for 4 hours. The stirring speed is controlled at 159 r / min. White crystals continue to precipitate in the solution to obtain crystallization slurry.

[0065] (4) Solid-liquid separation: The above crystallized slurry is vacuum filtered to obtain purified sodium aluminate solution and white filter cake.

[0066] The purified sodium aluminate solution obtained in this embodiment was tested, and the results were as follows: the concentration of P2O5 was 0.05 g / L, the phosphorus removal rate was 97.22%, the concentration of As2O3 was 0.04 g / L, the arsenic removal rate was 95%, the concentration of Al2O3 was 117.3 g / L, and the alumina loss rate was 0.25%.

[0067] XRD analysis of the filter cake obtained in this embodiment confirmed that the precipitated crystals were sodium fluoroarsenate nonadecanohydrate (Na7(AsO4)2F˙19H2O) and sodium fluorophosphate nonadecanohydrate (Na 6.5 (PO4)2F˙19.5H2O) No other impurities are formed.

[0068] Example 4 This embodiment uses the simultaneous removal method of the present invention to remove arsenic and phosphorus impurities from Bayer process sodium aluminate solution. The specific steps are as follows: (1) Pretreatment of ingredients: Take 1L of the mother liquor from the Bayer process evaporation cycle and test its basic indicators. The test results are: Al2O3 concentration 117.6g / L, Na2Ok concentration 189.1g / L, caustic ratio ak=2.65, P2O5 concentration 1.8g / L, As2O3 concentration 0.8g / L. Based on the molar ratio of fluorine to arsenic and phosphorus total amount of 4:1, add sodium fluoride solid at 80℃ and stir until completely dissolved to obtain a homogeneous premixed solution.

[0069] (2) Caustic alkali concentration control: Add solid sodium hydroxide to the above premixed solution, stir until completely dissolved, adjust and stabilize the Na2Ok concentration of the solution to 300 g / L, and obtain the reaction solution to be crystallized.

[0070] (3) Directional cooling crystallization: The reaction solution to be crystallized is cooled to 30°C at a uniform rate of 1°C / min, and crystallized at 30°C for 4 hours. The stirring speed is controlled at 159 r / min. White crystals continue to precipitate in the solution to obtain crystallization slurry.

[0071] (4) Solid-liquid separation: The above crystallized slurry is vacuum filtered to obtain purified sodium aluminate solution and white filter cake.

[0072] The purified sodium aluminate solution obtained in this embodiment was tested, and the results were as follows: the concentration of P2O5 was 0.05 g / L, the phosphorus removal rate was 97.22%, the concentration of As2O3 was 0.04 g / L, the arsenic removal rate was 95%, the concentration of Al2O3 was 117.4 g / L, and the alumina loss rate was 0.17%.

[0073] XRD analysis of the filter cake obtained in this embodiment confirmed that the precipitated crystals included sodium fluoroarsenate nonadecanohydrate (Na7(AsO4)2F˙19H2O) and sodium fluorophosphate nonadecanohydrate (Na 6.5 In addition to (PO4)2F˙19.5H2O, sodium carbonate monohydrate (Na2CO3˙H2O) and sodium oxalate (Na2C2O4) are also formed. The content of sodium fluoroarsenate nonadecahydrate and sodium fluorophosphate nonadecahydrate in the obtained filter cake accounts for 60%.

[0074] Comparative Example 1 This comparative example does not include sodium fluoride to verify the necessity of adding sodium fluoride.

[0075] The only difference between this comparative example and Example 2 is that sodium fluoride is not added in step (1), while the other steps and process parameters are completely consistent with Example 2.

[0076] The purified sodium aluminate solution obtained in this comparative example was tested, and the results were as follows: the concentration of P2O5 was 1.75 g / L, the phosphorus removal rate was 2.78%, the concentration of As2O3 was 0.78 g / L, the arsenic removal rate was 2.5%, there was no obvious impurity removal effect, and no target crystals were precipitated.

[0077] It is evident that crystallization is difficult to occur without the addition of sodium fluoride.

[0078] Comparative Example 2 This comparative example does not involve alkali concentration adjustment to verify the effect of Na2Ok concentration in the seed mother liquor on the impurity removal effect.

[0079] The only difference between this comparative example and Example 1 is that sodium hydroxide is not added in step (2), while the other steps and process parameters are completely consistent with Example 1.

[0080] The purified sodium aluminate solution obtained in this comparative example was tested, and the results were as follows: the concentration of P2O5 was 0.45 g / L, the phosphorus removal rate was 62.45%, the concentration of As2O3 was 0.38 g / L, the arsenic removal rate was 52.5%, the impurity removal effect was lower than that of Example 1, and the precipitated filter cake contained 30% aluminum hydroxide.

[0081] It can be seen that when the concentration of caustic alkali is not between 180-200 g / L, the concentration of caustic alkali has a significant impact on the crystallization effect.

[0082] Comparative Example 3 This comparative example does not involve alkali concentration adjustment to verify the effect of Na2Ok concentration in the seed mother liquor on the impurity removal effect.

[0083] The only difference between this comparative example and Example 2 is that sodium hydroxide is not added in step (2), while the other steps and process parameters are completely consistent with Example 2.

[0084] The purified sodium aluminate solution obtained in this comparative example was tested, and the results were as follows: P2O5 concentration was 0.065 g / L, phosphorus removal rate was 96.38%, As2O3 concentration was 0.052 g / L, arsenic removal rate was 93.5%, Al2O3 concentration was 117.0 g / L, and alumina loss rate was 0.51%. The impurity removal effect was basically the same as in Example 2, with a slight increase in alumina loss.

[0085] It can be seen that when the concentration of caustic alkali is between 180-200 g / L, the specific adjustment of its concentration has almost no effect on the impurity removal effect.

[0086] Comparative Example 4 This comparative example does not undergo cooling treatment to verify the necessity of cooling crystallization.

[0087] The only difference between this comparative example and Example 2 is that: in step (3), no cooling treatment is performed, and the solution is kept at the original temperature of 80°C. The remaining steps and process parameters are completely consistent with Example 2.

[0088] The purified sodium aluminate solution obtained in this comparative example was tested, and the results were as follows: the concentration of P2O5 was 1.80 g / L, the phosphorus removal rate was 0%, the concentration of As2O3 was 0.8 g / L, the arsenic removal rate was 0%, no crystals were precipitated, and there was no impurity removal effect.

[0089] Comparative Example 5 This comparative study changed the final cooling temperature to verify the relationship between crystallization temperature and impurity removal effect.

[0090] The only difference between this comparative example and Example 2 is that the temperature is reduced to 40°C in step (3). All other steps and process parameters are completely consistent with Example 2.

[0091] The purified sodium aluminate solution obtained in this comparative example was tested, and the results were as follows: the concentration of P2O5 was 0.26 g / L, the phosphorus removal rate was 85.8%, the concentration of As2O3 was 0.16 g / L, the arsenic removal rate was 80%, and white crystals were precipitated. The impurity removal effect was slightly lower than that of Example 2.

[0092] It is evident that when the final crystallization temperature is high, the overall impurity removal effect decreases.

[0093] Comparative Example 6 In this comparative example, the sample was filtered directly after cooling without any heat preservation, in order to verify the crystallization time and the impurity removal effect.

[0094] The only difference between this comparative example and Example 2 is that the filter is directly filtered after cooling to 40°C in step (3). The other steps and process parameters are completely consistent with Example 2.

[0095] The purified sodium aluminate solution obtained in this comparative example was tested, and the results were as follows: the concentration of P2O5 was 0.57 g / L, the phosphorus removal rate was 68.33%, the concentration of As2O3 was 0.48 g / L, the arsenic removal rate was 40%, the white crystalline precipitate was fine and difficult to filter, and the purified sodium aluminate solution was turbid. The impurity removal effect was far lower than that of Example 2.

[0096] It is evident that without heat preservation and crystallization after increasing the final cooling temperature, the impurity removal effect is further reduced.

[0097] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process, characterized in that, A fluorine-containing reagent is added to the Bayer process sodium aluminate solution to obtain a premixed solution. A reaction solution with a caustic alkali concentration within the target concentration range is obtained through the premixed solution. The reaction solution is subjected to directional cooling crystallization to obtain sodium fluoroarsenate crystals and sodium fluorophosphate crystals. After solid-liquid separation, the removal of arsenic and phosphorus impurities is completed. The target concentration range of the caustic alkali in the reaction solution is 180-200 g / L.

2. The method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process according to claim 1, characterized in that, The temperature of the reaction solution is 60℃-85℃, and the final crystallization temperature of the directional cooling crystallization is 20℃-45℃.

3. The method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process according to claim 2, characterized in that, The directional cooling crystallization process involves uniformly cooling the temperature to the final crystallization temperature at a cooling rate of 0.5-1℃ / min and maintaining this temperature for 2-4 hours.

4. The method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process according to claim 3, characterized in that, The reaction solution is kept stirred during the directional cooling crystallization.

5. A method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process, according to any one of claims 1-4, characterized in that... In the fluorine-containing reagent, the molar ratio of fluorine to the total amount of arsenic and phosphorus is 2-5:

1.

6. The method for simultaneous removal of arsenic and phosphorus impurities from Bayer process sodium aluminate solution according to claim 5, characterized in that, The fluorine-containing reagent is sodium fluoride, potassium fluoride, aluminum fluoride, or ammonium fluoride.

7. The method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process according to claim 5, characterized in that, The sodium aluminate solution prepared using the Bayer process contains 0.1-3 g / L of phosphorus (P2O5) and 0.05-2 g / L of arsenic (As2O3).

8. The method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process according to claim 7, characterized in that, The Bayer process sodium aluminate solution includes any one of the following from the Bayer process alumina production: crude leaching solution, seed mother liquor, evaporation circulating mother liquor, or washing solution; in the Bayer process sodium aluminate solution, the aluminum concentration is 80-140 g / L and the caustic alkali ratio (ak) is 2.6-3.

0.

9. A method for simultaneous removal of arsenic and phosphorus impurities from sodium aluminate solution using the Bayer process, according to any one of claims 1-4, characterized in that, The reaction solution is obtained by determining the amount of exogenous caustic alkali to be added based on the initial concentration of caustic alkali in the premixed solution, and then adding the exogenous caustic alkali to the premixed solution.

10. A method for simultaneous removal of arsenic and phosphorus impurities from a Bayer process sodium aluminate solution according to any one of claims 1-4, characterized in that, The solid-liquid separation method is any one of vacuum filtration, plate and frame filtration, sedimentation separation, and centrifugal separation.

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