A method for removing impurities and recovering potassium resources from an alumina production system

Abstract

The application discloses a method for removing impurities and recycling potassium resources from an alumina production system, and belongs to the technical field of non-ferrous metallurgy and resource comprehensive utilization. In view of the problem that potassium impurities in the circulating lye in the Bayer alumina production process continuously accumulate, and affect the production stability and alkali consumption, the application introduces sodium sulfate into the alkaline solution of the system, and through accurate control of the reaction temperature, alkali concentration and crystallization time, the potassium, sodium and sulfate ions in the solution are selectively combined, and the potassium sodium sulfate double salt solid is crystallized and separated out, so that the product is obtained after solid-liquid separation. The liquid is returned to the main process of the alumina after being purified by evaporation, the separated impurity salt is treated by causticization, and a reaction system is formed with the circulating mother liquor. The application has the advantages of simple process, effective connection with the existing alumina production line, impurity purification and valuable element extraction of the alumina system, and environmental and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of non-ferrous metal metallurgy and comprehensive resource utilization technology, specifically to a method for removing potassium impurities and recovering resources during alumina production, particularly a method for selectively crystallizing potassium sodium sulfate double salt from an alkaline aluminate solution in the Bayer process alumina system, thereby achieving system purification and recovery of valuable elements. Background Technology

[0002] Potassium, a minor component of bauxite ore, is introduced into the Bayer process alumina system during ore leaching. With increasing cycle times, potassium accumulates in the Bayer process mother liquor, leading to excessive potassium content in the mother liquor for some companies. Increased potassium levels degrade the quality of the produced alumina and cause caustic alkali loss, resulting in higher costs. Therefore, it is essential to control the potassium content in Bayer process alumina.

[0003] Currently, the industry's conventional method for dealing with such impurity accumulation is "salt discharge," which involves periodically discharging a portion of the circulating mother liquor with high impurities, or diluting it before discharge. This method not only results in the loss of alkali and aluminum, but the discharged mother liquor or salt residue also poses a serious threat to the environment, and the treatment costs are high. Another approach is to try to extract valuable elements from the system, but existing technologies are often complex or difficult to integrate effectively with the existing alumina main process, resulting in poor economic efficiency.

[0004] Therefore, there is an urgent need to develop an integrated clean production method that can remove potassium impurities from the alumina production system in situ and efficiently, while realizing resource utilization and not affecting the operation of the main process. Summary of the Invention

[0005] (a) Technical problems to be solved This invention aims to overcome the shortcomings of existing technologies and provide a method for removing potassium impurities and recovering potassium sodium sulfate double salt from an alumina production system. This method can effectively purify the circulating alkaline solution, reduce the impurity content of the system, stabilize the main production process, and simultaneously convert harmful impurities into economically valuable byproducts, achieving a balance between environmental and economic benefits.

[0006] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: a method for removing impurities and recovering potassium resources from an alumina production system, characterized by comprising the following steps: S1: Obtain alkaline circulating mother liquor containing potassium, sodium, and sulfate ions and low-caustic alkali strong filtrate from the alumina production system; S2: Sodium sulfate is introduced into the strong filtrate or water and dissolved by heating at a temperature above 50 degrees Celsius; S3: Mix the solution obtained in step S2 with the recycled mother liquor obtained in step S1 to form a reaction system; S4: Control the crystallization conditions of the reaction system so that potassium ions, sodium ions and sulfate ions in the solution crystallize to precipitate potassium sodium sulfate double salt solid; S5: Separate the solid-liquid mixture obtained in step S4 to obtain the potassium sodium sulfate double salt product; S6: Evaporate and concentrate the mother liquor obtained in step S5 until the concentration of caustic alkali Na2O is above 300 g / L, and separate the solid and liquid to remove impurity salts. S7: Return the mother liquor obtained from purification in step S6 to the main alumina production process; S8: The impurity salt obtained in step S6 is causticized to recover sodium salt, and the solid-liquid separation is performed to obtain causticized liquid. S9: Dissolve a small amount of sodium sulfate in the causticizing solution obtained in step S8 and mix it with the circulating mother liquor obtained in step S1 to form a reaction system instead of the solution in step S2.

[0007] Preferably, in step S1, the concentration of caustic alkali Na2O in the circulating mother liquor is 230-250 g / L, and the concentration of elemental K is 70 g / L; the concentration of caustic alkali Na2O in the strong filtrate is 80 g / L, and the concentration of elemental K is 25 g / L.

[0008] Preferably, the amount of sodium sulfate introduced in step S2 is based on a molar ratio of K2O to Na2SO4 in the solution of 1:0.8-1.

[0009] Preferably, the crystallization conditions in step S4 include: a crystallization temperature of -15℃ to 15℃, a crystallization time of 5 hours or more, a caustic alkali Na2O concentration of 160-260 g / L in the mixed system, and stirring during the crystallization process. For the high caustic alkali Na2O concentration required in the mixed system, the circulating mother liquor needs to be evaporated and concentrated to a suitable caustic alkali concentration before mixing to ensure that the mixed system can reach the required caustic alkali value. According to the present invention, by mixing a high-potassium circulating mother liquor with a low-caustic-alkali strong filtrate in a certain proportion, adjusting the caustic-alkali concentration of the system to 160 to 260 g / L, and inducing potassium-sodium sulfate double salt crystallization under low-temperature conditions, the selective precipitation of potassium is achieved by utilizing the low solubility of the double salt under low-temperature and high-alkali conditions. Simultaneously, by evaporating and concentrating the separated mother liquor to a caustic-alkali concentration of over 300 g / L, the residual sodium salt precipitates out as sodium carbonate and sodium sulfate. After solid-liquid separation, the mother liquor is returned to the main process, avoiding the loss of alkali and aluminum. The precipitated sodium salt impurities are converted into sodium hydroxide solution through causticization treatment and then used in the double salt crystallization step, forming a closed-loop material cycle.

[0010] Preferably, before or after step S4, seed crystals are added to the reaction system, wherein the seed crystals are potassium sodium sulfate double salt crystals.

[0011] Preferably, the impurity salt in step S6 comprises sodium carbonate, sodium sulfate, and a small amount of sodium hydroxide.

[0012] Preferably, in step S8, the mass ratio of carbon alkali to calcium oxide in the causticizing process is 1:1.2-1.3, and the concentration of caustic alkali Na2O in the resulting causticizing solution is 80-100 g / L.

[0013] Preferably, in step S9, the amount of sodium sulfate added is such that the SO4 in the causticizing solution is reduced. 2- The molar ratio of K2O to total K2O in the solution is 0.8-1:1.

[0014] Preferably, the chemical composition of the potassium sodium sulfate double salt is xK2SO4. Na2SO4 or its hydrate, wherein the value of x is 1-2.

[0015] In some embodiments, the potassium sodium sulfate compound salt product can be used as a raw material for compound fertilizers and as a raw material for potassium sulfate or some other compound salts.

[0016] Secondly, this invention provides the application of potassium sodium sulfate double salt in impurity control in alumina production systems, wherein the chemical composition of the potassium sodium sulfate double salt is xK2SO4. Na2SO4 or its hydrate, wherein the value of x is 1 to 2, is a double salt that is crystallized from potassium-containing circulating mother liquor at -15℃ to 15℃ and a caustic alkali concentration of 160-260 g / L, for the targeted removal of potassium impurities.

[0017] According to the present invention, the crystallization process of the potassium sodium sulfate double salt depends on the molar ratio of potassium ions, sodium ions, and sulfate ions in the system. When the molar ratio of K2O to Na2SO4 is controlled at 1:0.8-1, it is beneficial to form a stable double salt structure with an x ​​value in the range of 1 to 2. The solubility of this double salt decreases significantly at low temperatures, and with a crystallization time of more than 5 hours, a solid product with uniform particle size and high purity can be obtained.

[0018] Thirdly, the present invention provides a method for recycling causticizing liquid in the removal of potassium impurities in an alumina production system. The causticizing liquid is obtained by causticizing reaction of sodium salt impurities precipitated in step S6. During the causticizing process, the mass ratio of carbon alkali to calcium oxide is 1:1.2 to 1.3, and the caustic alkali concentration of the causticizing liquid after the reaction is 80-100 g / L. After the causticizing liquid is used to dissolve and replenish sodium sulfate, it replaces the original strong filtrate in the double salt crystallization reaction, thereby realizing the internal recycling of sodium resources.

[0019] According to the present invention, before mixing with the circulating mother liquor, the residual sulfate concentration in the causticizing solution needs to be supplemented with sodium sulfate based on the K2O content in the total solution to reduce the SO4 concentration in the causticizing solution. 2-The molar ratio of K2O to total K2O is maintained at 0.8 to 1:1 to ensure stoichiometric balance during the crystallization of the double salt. This operation avoids the continuous consumption of external strong filtrate and reduces the input of fresh water and chemicals.

[0020] Fourthly, this invention provides a method for controlling the effect of crystallization temperature on the composition of potassium sodium sulfate double salt and potassium removal efficiency. When the crystallization temperature is controlled between 15 and 0 degrees Celsius, the potassium content in the double salt is relatively high, and the potassium removal rate can reach 55% to 60%. When the crystallization temperature drops to 0 to -15 degrees Celsius, the sodium content in the double salt increases, but the residual sulfate concentration in the mother liquor is lower, which is suitable for working conditions with stricter sulfate control requirements.

[0021] Fifthly, the present invention provides the application of seed crystal-induced crystallization technology in the preparation of potassium sodium sulfate double salt, wherein the seed crystal is a pre-prepared potassium sodium sulfate double salt crystal, which is added before or during the cooling process of the reaction system to the target crystallization temperature.

[0022] According to the present invention, the addition of seed crystals can shorten the nucleation induction period and promote crystal growth on the surface of existing crystal nuclei, thereby obtaining a complex salt product with a more concentrated particle size distribution and better sedimentation performance, which is beneficial to the efficient execution of subsequent solid-liquid separation operations. Without seed crystals, the crystallization process easily forms a large number of fine crystal nuclei, leading to increased slurry viscosity and difficulty in filtration.

[0023] In a sixth aspect, the present invention provides a method for controlling the composition of impurity salts in the evaporation and concentration step. The impurity salts are mainly composed of sodium carbonate, sodium sulfate and a small amount of sodium hydroxide. The precipitation conditions are that the concentration of caustic alkali in the mother liquor is evaporated to more than 300 grams per liter, and the salt is discharged after standing at about 40 degrees Celsius.

[0024] According to the present invention, after evaporation and concentration, carbonate and sulfate ions in the mother liquor precipitate due to decreased solubility. After solid-liquid separation, the sulfate concentration in the mother liquor can be reduced to 2.88 to 5 g / L, meeting the sulfate control requirements for alumina production. This step effectively prevents the accumulation of sulfate ions in the system, avoiding adverse effects on the Bayer process leaching and decomposition steps.

[0025] Seventhly, the present invention provides a material balance control strategy for the entire process flow. By monitoring the potassium, sodium, and sulfate content of the circulating mother liquor, strong filtrate, causticizing liquid, and double salt products, the amount of sodium sulfate added, the mixing ratio, and the crystallization parameters are dynamically adjusted to maintain the stability of the concentration of each ion in the system.

[0026] According to the present invention, in continuous operation mode, approximately 120 to 150 kg of sodium sulfate needs to be added for every 1 cubic meter of circulating mother liquor (K concentration 70 g / L), which can produce approximately 180 to 220 kg of potassium sodium sulfate double salt, of which the potassium content is equivalent to approximately 30% to 35% K2O. This double salt can be used directly as a raw material for potash fertilizer or further processed into potassium sulfate products, and has clear market value.

[0027] Compared with the prior art, the present invention provides a method for removing impurities and recovering potassium resources from an alumina production system, which has the following beneficial effects: (1) Source control and purification process: This invention directly treats the impurity solution inside the alumina production system. Harmful potassium impurities are removed from the system in solid form through ion exchange and chemical crystallization, which fundamentally solves the problem of impurity accumulation, reduces alkali consumption and equipment scaling risk, and stabilizes the main process operation.

[0028] (2) Resource recycling, turning waste into treasure: The "waste" that traditionally needs to be discharged and treated is transformed into potassium sodium sulfate double salt products with market value. This product can be used as a raw material for compound fertilizers and potassium sulfate salts or some other compound salts, realizing the transformation from "pollution negative benefits" to "resource positive benefits".

[0029] 3) Strong process adaptability: The process steps of this invention are simple and easy to integrate with existing Bayer process production lines. No large-scale modification of the main process is required, and the investment and operating costs are low.

[0030] (4) Environmentally friendly: It achieves "zero discharge" or "reduction" of waste liquid, avoids pollution of the environment by alkaline and saline wastewater, and conforms to the development direction of green production and circular economy. Attached Figure Description

[0031] Figure 1 This is a flowchart illustrating the working steps of the present invention. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] As described in the background section above, during the Bayer process for alumina production, potassium ions are carried into the system along with bauxite and accumulate in the circulating mother liquor, leading to increased caustic alkali concentration, decreased leaching efficiency, and potential equipment scaling. Traditional potassium removal methods, such as cryogenic denitrification and lime causticization, suffer from drawbacks such as significant alkali loss, difficult-to-handle byproducts, and inability to utilize resources effectively. To address these issues, this invention provides a method for removing potassium impurities from the alumina production system and recovering potassium sodium sulfate double salt. By constructing a closed-loop material circulation system, selective removal and high-value recovery of potassium are achieved without losing alkali and aluminum in the main process.

[0034] In a first aspect, the present invention provides a method for removing potassium impurities and recovering potassium sodium sulfate double salt from an alumina production system, comprising the following steps: S1: Obtain alkaline circulating mother liquor containing potassium, sodium, and sulfate ions and low-caustic alkali strong filtrate from the alumina production system; S2: Introduce sodium sulfate into the strong filtrate solution or water, and heat to dissolve it; S3: Mix the solution obtained in step S2 with the recycled mother liquor obtained in S1 to form a reaction system; S4: Control the crystallization conditions of the reaction system so that potassium ions, sodium ions and sulfate ions in the solution crystallize to precipitate potassium sodium sulfate double salt solid; S5: Separate the solid-liquid mixture obtained in step S4 to obtain the potassium sodium sulfate double salt product; S6: The mother liquor obtained in step S5 is concentrated by high-temperature evaporation to remove impurities and solid-liquid separation to remove impurity salts. S7: Return the mother liquor obtained from purification in step S6 to the main alumina production process; S8: The impurity salt obtained in step S6 is causticized to recover sodium salt, and the solid-liquid separation is performed to obtain causticized liquid. S9: Dissolve a small amount of sodium sulfate in the causticizing solution obtained in step S8 and mix it with the recycled mother liquor obtained or processed in step S1 to form a reaction system instead of step S2.

[0035] According to the present invention, the circulating mother liquor in step S1 is derived from the mother liquor after separation and washing in the Bayer process leaching step, and its typical composition is: caustic alkali Na2O concentration of 230-250 g / L, elemental K concentration of 65-70 g / L, sulfate concentration of 15-25 g / L, and carbonate alkali Na2CO3 concentration of 20 to 30 g / L; the strong filtrate is the filtrate obtained after the sodium aluminate solution is decomposed and filtered with aluminum hydroxide, and its typical composition is: caustic alkali Na2O concentration of 70 to 90 g / L, elemental K concentration of 20 to 30 g / L, and sulfate concentration of 5 to 10 g / L.

[0036] In some embodiments, the sodium sulfate mentioned in step S2 is industrial-grade anhydrous sodium sulfate or sodium sulfate decahydrate, with a purity of not less than 95%. The amount of sodium sulfate introduced is 1 to 0.8 to 1 based on the molar ratio of K2O to Na2SO4 in the solution. For example, when treating 1 cubic meter of circulating mother liquor (containing 70 g K per liter, equivalent to approximately 84.7 g K2O per liter), approximately 120 to 150 kg of Na2SO4 needs to be added. The dissolution process is carried out in a stirring dissolution tank, with the temperature controlled above 50 degrees Celsius, preferably 55 to 65 degrees Celsius, the stirring speed being 60 to 100 rpm, and the dissolution time being 30 to 60 minutes, ensuring that the sodium sulfate is completely dissolved and there are no undissolved particles.

[0037] In some embodiments, the mixing operation in step S3 is carried out in a jacketed cooling crystallization reactor. The sodium sulfate solution obtained in step S2 and the circulating mother liquor obtained in step S1 are pumped into the reactor at a volume ratio of 1:0.8 to 1.2. Before mixing, if the caustic soda concentration of the circulating mother liquor exceeds 260 g / L, it is pre-concentrated to the target range by an evaporator. After mixing, the total volume of the system is 1.8 to 2.2 cubic meters, and the caustic soda Na2O concentration is adjusted to 160 to 260 g / L, preferably 180 to 240 g / L.

[0038] In some embodiments, the crystallization conditions in step S4 are controlled as follows: The reactor jacket cooling system is activated, and the system temperature is reduced from the initial 50-60 degrees Celsius to the target crystallization temperature at a cooling rate of 1-3 degrees Celsius per hour. The target crystallization temperature is 15 to -15 degrees Celsius, preferably 0 to 10 degrees Celsius. Throughout the cooling and isothermal phases, continuous stirring is performed at a stirring speed of 40 to 80 revolutions per minute. The crystallization time is not less than 5 hours, preferably 6 to 10 hours. Seed crystals are added 30 minutes before reaching the target temperature or during the cooling process. The seed crystals are pre-prepared potassium sodium sulfate double salt crystals.

[0039] In some embodiments, the chemical composition of the potassium sodium sulfate double salt is xK2SO4. Na2SO4 or its hydrate, wherein the value of x is 1 to 2. The solubility of this double salt is significantly reduced under low temperature and high alkalinity conditions. For example, at 200 g / L of caustic alkali and a temperature of 5 degrees Celsius, its solubility is less than 5 g / L, while at the same alkalinity and a temperature of 30 degrees Celsius, its solubility can reach more than 20 g / L, thereby achieving low-temperature selective crystallization.

[0040] In some embodiments, the solid-liquid separation in step S5 is performed using a vacuum drum filter or a centrifuge. The filter medium is a polypropylene filter cloth with a pore size of 10 to 20 micrometers.

[0041] In some embodiments, the mother liquor evaporation and concentration in step S6 is carried out in a forced circulation evaporator. The evaporation temperature is 95 to 105 degrees Celsius, and the vacuum degree is 0.08 to 0.095 MPa. The evaporation endpoint is reached when the caustic soda (Na₂O) concentration in the mother liquor reaches 300 to 350 g / L. The concentrate is transferred to a settling tank and allowed to stand at 40 to 50 degrees Celsius for 4 to 8 hours to allow sodium carbonate, sodium sulfate, and a small amount of sodium hydroxide to crystallize out. Solid-liquid separation is performed using a settling centrifuge with a separation factor of 800 to 1200, yielding an impurity salt filter cake and purified mother liquor.

[0042] In some embodiments, the sulfate concentration of the purified mother liquor in step S7 is 2.88 to 5 g / L, and the carbonate concentration is less than 5 g / L, which meets the control indicators for circulating mother liquor in alumina production and can be directly pumped back to the Bayer process leaching process for use as a blending solution.

[0043] In some embodiments, the causticization reaction in step S8 is carried out in a causticization tank with stirring. The impurity salt filter cake obtained in step S6 is mixed with water at a mass ratio of 1:1.5 to 2.5 to prepare a slurry, which is then heated to 80 to 90 degrees Celsius. Quicklime (CaO content not less than 85%) is added at a mass ratio of carbonaceous alkali (calculated as Na2CO3) to calcium oxide of 1:1.2 to 1.3. The reaction time is 2 to 4 hours, and the stirring speed is 50 to 80 rpm. After the reaction is completed, the slurry is settled and filtered to obtain a causticized liquid and residue. The concentration of caustic alkali Na2O in the causticized liquid is 80 to 100 g / L, and the suspended solids content is less than 0.5 g / L.

[0044] In some embodiments, in step S9, the causticizing solution obtained in step S8 is used to replace the original strong filtrate. Before mixing with the circulating mother liquor, sodium sulfate needs to be added according to the residual sulfate concentration in the causticizing solution (calculated at 20 grams per liter) and the K2O content in the total solution to reduce the SO4 content in the causticizing solution. 2- The molar ratio of sodium sulfate to total K2O is maintained at 0.8 to 1:1. The added sodium sulfate is dissolved at a temperature above 50 degrees Celsius, mixed with the causticizing solution, and then blended with the circulating mother liquor according to the target caustic base number.

[0045] Secondly, this invention provides the application of potassium sodium sulfate double salt in impurity control in alumina production systems. This double salt is crystallized from potassium-containing circulating mother liquor at 15 to -15 degrees Celsius and a caustic alkali concentration of 160 to 260 g / L, thereby achieving the targeted removal of potassium impurities. The double salt composition is xK₂SO₄. The x value in Na2SO4 is affected by the crystallization temperature: when it is between 15 and 0 degrees Celsius, x is close to 2, and the potassium content is high; when it is between 0 and -15 degrees Celsius, x is close to 1, the sodium content increases, but the residual sulfate in the mother liquor is lower.

[0046] Thirdly, this invention provides a method for recycling causticizing solution in the removal of potassium impurities in an alumina production system. The causticizing solution is obtained by causticizing sodium salt impurities and is used to dissolve sodium sulfate before participating in the crystallization of double salts, thereby realizing the internal recycling of sodium resources and reducing the consumption of external strong filtrate.

[0047] Fourthly, this invention provides a method for controlling the effect of crystallization temperature on the composition of the complex salt and the potassium removal efficiency. By adjusting the temperature, an optimal match can be achieved between the potassium removal rate and the quality of the complex salt.

[0048] Fifthly, this invention provides an application of seed-induced crystallization technology. The addition of seed crystals can shorten the induction period and improve crystal morphology and sedimentation properties.

[0049] Sixthly, this invention provides a method for controlling the composition of impurity salts in the evaporation and concentration step. Salt removal by allowing the mixture to stand at 40 degrees Celsius with a caustic alkali concentration of 300 g / L or higher can effectively precipitate sodium carbonate and sodium sulfate.

[0050] Seventhly, this invention provides a material balance control strategy. By monitoring the ion concentration of each stream, the amount of salt added, the mixing ratio, and the crystallization parameters are dynamically adjusted.

[0051] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0052] Example 1: 500 ml of circulating mother liquor from the Bayer process production line of Sanmenxia Cayman Aluminium Co., Ltd. was taken. Its composition was: caustic alkali concentration (Na2O) 230-250 g / L, alumina concentration (Al2O3) 120 g / L, and elemental potassium concentration 70 g / L. The concentration of caustic alkali (Na2O) in the strong filtrate was 80 g / L, and the concentration of elemental potassium was about 25 g / L. A small amount of water was mixed with the strong filtrate and placed in a reactor equipped with stirring and temperature control. The mother liquor was heated to 80 °C, and industrial anhydrous sodium sulfate (Na2SO4) was slowly added (calculated based on the total K2O content, with a K2O to Na2SO4 molar ratio of 1:1). After the addition was complete, the mixture was stirred at a constant temperature for 30 min. The mother liquor was mixed with the circulating mother liquor until the caustic alkali concentration reached 180 g / L. The mixture was then slowly cooled to 15 °C with stirring, and crystallized at this temperature for 5 hours. After solid-liquid separation, potassium sodium sulfate double salt was obtained, containing 31.32% elemental potassium (K) and 9.41% elemental sodium (Na). The filtrate composition was determined, showing an elemental potassium concentration of 25 g / L, a sulfate concentration of 17.19 g / L, and a potassium ion removal rate of 55%.

[0053] The separated mother liquor was evaporated and concentrated to a caustic alkali concentration of 300 g / L. After cooling, the salt was discharged, and the mother liquor after solid-liquid separation was returned to the main alumina production process. Its sulfate concentration at around 40 °C reached 2.88-5 g / L. Solid impurity salts were causticized to recover sodium salt. During causticization, the ratio of carbon-alkali to calcium oxide in the impurity salts was 1:1.3. The caustic alkali value of the causticization solution was calculated based on 100 g / L, but due to errors, the caustic alkali value was slightly lower. The sodium sulfate concentration in the resulting causticization solution was 131.46 g / L. After adding a small amount of sodium sulfate to dissolve it, it was mixed with the collected circulating mother liquor, and the potassium-sodium sulfate double salt crystallization process was repeated. The resulting double salt had a potassium-sodium ratio of 31.13% K and 10.36% elemental Na. The K concentration in the mother liquor was 24.54 g / L, and the sulfate concentration was relatively high, around 25 g / L.

[0054] Example 2: The same batch of circulating mother liquor as in Example 1 was used, but it was pre-evaporated and concentrated to 75% of its original volume at high temperature. Subsequent steps were the same as in Example 1, but the concentration of caustic alkali after mixing the circulating mother liquor with the strong filtrate was adjusted to 240 g / L. In this example, the residual K and sulfate concentrations in the mother liquor after separating the potassium sodium sulfate double salt were lower, with a K concentration of 20 g / L (removal rate of 60-70%) and a sulfate concentration of 9.71 g / L. However, in the potassium sodium sulfate double salt, K accounted for only 27.07%, and Na accounted for 13.15%.

[0055] Example 3: The same batch of circulating mother liquor as in Example 1 was used, and the subsequent steps were the same as in Example 2, except that the temperature for mixing the circulating mother liquor with the strong filtrate was adjusted to 10 °C. In this example, the residual sulfate concentration in the mother liquor after separating the potassium sodium sulfate double salt was lower, at 6.13 g / L. However, the residual K concentration in the solution was 30 g / L, with a removal rate of only 50%. Furthermore, elemental K accounted for only 24.33% and Na for 16.64% of the potassium sodium sulfate double salt.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for removing impurities and recovering potassium resources from an alumina production system, characterized in that, Includes the following steps: S1: Obtain alkaline circulating mother liquor containing potassium, sodium, and sulfate ions and low-caustic alkali strong filtrate from the alumina production system; S2: Sodium sulfate is introduced into the strong filtrate or water and dissolved by heating at a temperature above 50 degrees Celsius; S3: Mix the solution obtained in step S2 with the recycled mother liquor obtained in step S1 to form a reaction system; S4: Control the crystallization conditions of the reaction system so that potassium ions, sodium ions and sulfate ions in the solution crystallize to precipitate potassium sodium sulfate double salt solid; S5: Separate the solid-liquid mixture obtained in step S4 to obtain the potassium sodium sulfate double salt product; S6: Evaporate and concentrate the mother liquor obtained in step S5 until the concentration of caustic alkali Na2O is above 300 g / L, and separate the solid and liquid to remove impurity salts. S7: Return the mother liquor obtained from purification in step S6 to the main alumina production process; S8: The impurity salt obtained in step S6 is causticized to recover sodium salt, and the solid-liquid separation is performed to obtain causticized liquid. S9: Dissolve a small amount of sodium sulfate in the causticizing solution obtained in step S8 and mix it with the circulating mother liquor obtained in step S1 to form a reaction system instead of the solution in step S2.

2. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, In step S1, the concentration of caustic alkali Na2O in the circulating mother liquor is 230-250 g / L, and the concentration of elemental K is 70 g / L; the concentration of caustic alkali Na2O in the strong filtrate is 80 g / L, and the concentration of elemental K is 25 g / L.

3. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, The amount of sodium sulfate introduced in step S2 is based on a molar ratio of K2O to Na2SO4 in the solution of 1:0.8-1.

4. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, The crystallization conditions described in step S4 include: a crystallization temperature of -15℃ to 15℃, a crystallization time of 5 hours or more, a caustic alkali Na2O concentration of 160-260 g / L in the mixed system, and stirring during the crystallization process. For the high caustic alkali Na2O concentration required in the mixed system, the circulating mother liquor needs to be evaporated and concentrated to a suitable caustic alkali concentration before mixing to ensure that the mixed system can reach the required caustic alkali value.

5. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, Before or after step S4, seed crystals are added to the reaction system, wherein the seed crystals are potassium sodium sulfate double salt crystals.

6. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, The impurity salt in step S6 consists of sodium carbonate, sodium sulfate, and a small amount of sodium hydroxide.

7. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, In step S8, the mass ratio of carbon alkali to calcium oxide in the causticizing process is 1:1.2-1.3, and the concentration of caustic alkali Na2O in the resulting causticizing solution is 80-100 g / L.

8. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, In step S9, the amount of sodium sulfate added is such that the SO4 in the causticizing solution is reduced. 2- The molar ratio of K2O to total K2O in the solution is 0.8-1:

1.

9. The method for removing impurities and recovering potassium resources from an alumina production system according to claim 1, characterized in that, The chemical composition of the potassium sodium sulfate double salt is xK2SO4. Na2SO4 or its hydrate, wherein the value of x is 1-2.

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