Method for recovering by-products from sodium chromate alkaline liquor by stepwise acidification and method for preparing chromium salts

Abstract

This invention relates to the field of chromium salt preparation technology, and discloses a method for recovering byproducts from the acidification of sodium chromate alkaline solution and a method for preparing chromium salts. The method includes the following steps: adding a first acidifying agent to the sodium chromate alkaline solution for primary acidification, followed by crystallization and filtration to obtain a primary filtrate and a sodium carbonate filter cake; wherein the final pH of the primary acidification reaction is 11-13; adding a second acidifying agent to the primary filtrate for acidification, followed by crystallization and filtration to recover aluminum hydroxide and sodium salts. This invention employs a stepwise acidification process, which preferentially yields sodium carbonate, followed by aluminum hydroxide and the remaining sodium salts. It achieves the stepwise separation of acidification byproducts such as sodium carbonate, aluminum hydroxide, and other sodium salts, avoiding the problem of difficult utilization caused by the simultaneous precipitation and separation of byproducts.

Description

Technical Field

[0001] This invention relates to the field of chromium salt preparation technology, specifically to a method for recovering byproducts by graded acidification of sodium chromate with alkaline solution and a method for preparing chromium salts. Background Technology

[0002] The main method for producing chromium salts is the oxidative roasting method of chromite ore. This method involves oxidizing and roasting a mixture of chromite ore, soda ash, filler, or slag at 1100~1200℃ to convert the chromium in the chromite ore into water-soluble sodium chromate. The roasted clinker is then leached to obtain an alkaline sodium chromate solution, which, after impurity removal, can be used to produce chromium salts.

[0003] The main impurities in sodium chromate alkaline solution are sodium aluminate, sodium hydroxide, and sodium carbonate. The traditional method for impurity removal involves directly adding an acidifying agent (sulfuric acid or sodium bisulfate) to the sodium chromate alkaline solution to adjust the pH to around 10. At this point, sodium hydroxide and sodium carbonate are converted into sodium sulfate, a low-value-added soluble substance, while sodium aluminate is converted into aluminum hydroxide precipitate. Alkali is then added back to the liquid phase, and the pH is adjusted to 13 for vanadium removal. The vanadium-removed filtrate is further treated with sulfuric acid to adjust the pH to 3-4, thus acidifying the sodium chromate into sodium dichromate. This process is simple and practical, but it has limitations. High solution viscosity affects mass and heat transfer, a large amount of inexpensive sodium sulfate is produced as a byproduct, the value of the alkaline solution is not fully utilized, and significant carbon emissions are generated.

[0004] Currently, there are also methods that use carbon dioxide as an acidifying agent for impurity removal. For example, patent application CN201110153029.X discloses a method for continuously neutralizing alkaline solutions to generate gibbsite. This method uses carbon dioxide as a neutralizing agent to acidify the pH of the alkaline solution to 9.58~9.62 in a single step. Patent application CN201210482808.9 employs a carbon fractionation method for preparing sodium chromate, which uses industrial waste gas to reduce the pH to 8.5~9.5 in a single step.

[0005] However, the above-mentioned method using carbon dioxide as an acidifying agent has the following problems: When the concentration of alkaline water containing sodium chromate exceeds a certain range (for example, when the concentration of sodium chromate, calculated as Na2Cr2O7•2H2O, is greater than 180 g / L), sodium carbonate generated by the carbonation of sodium hydroxide will precipitate simultaneously with aluminum hydroxide generated by the carbonation of sodium aluminate, causing the sodium carbonate and aluminum hydroxide precipitates to require further separation, making it impossible to directly recover sodium carbonate and aluminum hydroxide. Furthermore, due to the electrical properties of aluminum hydroxide, the viscosity of the liquid phase will increase sharply, making solid-liquid separation difficult and affecting CO2 mass transfer, resulting in a lower local pH, promoting the formation of sodium aluminate, and making subsequent aluminum sludge dissolution difficult, thus affecting the resource utilization of aluminum sludge. Conversely, when the concentration of alkaline water containing sodium chromate is low, only aluminum hydroxide precipitates, while sodium carbonate and sodium bicarbonate are difficult to precipitate. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of difficulty in separating and recovering by-products in existing technologies, and to provide a method for recovering by-products by graded acidification of sodium chromate alkaline solution and a method for preparing chromium salts. This method first adopts a primary acidification. By controlling the pH of the reaction endpoint of the primary acidification, sodium carbonate can be obtained preferentially. Then, further acidification can be carried out to obtain aluminum hydroxide and the remaining sodium salts. Thus, each by-product can be obtained stepwise.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a method for recovering byproducts by graded acidification of sodium chromate alkaline solution, wherein the sodium chromate alkaline solution contains NaCrO4, Na2CO3, NaAlO2, and NaOH, and the method includes the following steps: (1) Add a first acidifying agent to the sodium chromate alkaline solution for primary acidification, crystallize and filter to obtain a primary filtrate and a sodium carbonate filter cake; wherein the pH of the reaction endpoint of the primary acidification is 11~13; (2) Add a second acidifying agent to the first filtrate for acidification, crystallize and filter to recover aluminum hydroxide and sodium salt.

[0008] Preferably, the first acidifying agent is at least one of sodium bicarbonate, carbon dioxide, and industrial exhaust gas containing carbon dioxide; and / or, the final pH of the primary acidification reaction is 12.2 to 12.8.

[0009] Preferably, the second acidifying agent contains at least one of carbon dioxide and sodium bicarbonate, and step (2) includes the following sub-steps: (21) Add a second acidifying agent to the primary filtrate for secondary acidification, crystallize, and then filter to obtain a secondary filtrate and an aluminum hydroxide filter cake. (22) Add a second acidifying agent to the secondary filtrate for tertiary acidification, crystallize and filter to obtain sodium bicarbonate and tertiary filtrate.

[0010] Preferably, the endpoint pH of the secondary acidification reaction is 9.5-11; more preferably 10-10.5; and / or, the endpoint pH of the tertiary acidification reaction is 7-9.5.

[0011] Preferably, the method further includes: returning at least a portion of the sodium carbonate obtained in step (1) to the chromite roasting section for reuse.

[0012] Preferably, the method further includes: returning the sodium bicarbonate obtained in step (22) as at least a portion of the first acidifier and / or the second acidifier to step (1) and / or step (2) for reuse.

[0013] Preferably, the second acidifying agent is sulfuric acid and / or sodium bisulfate, and step (2) is as follows: Acidification is carried out by adding a second acidifying agent at least once to the primary filtrate. The final pH of the acidification reaction is 7-9.5. After crystallization, the solution is filtered to recover aluminum hydroxide and sodium sulfate.

[0014] Preferably, when the first acidifying agent and / or the second acidifying agent contains CO2 gas, the partial pressure of CO2 is 0.01 MPa to 1 MPa, more preferably 0.05 to 0.15 MPa.

[0015] Preferably, the method further includes, in step (1), concentrating the sodium chromate alkaline solution before primary acidification, and / or, concentrating the sodium chromate alkaline solution after any primary acidification and before crystallization.

[0016] Preferably, the concentration of the sodium chromate alkaline solution, calculated as Na2Cr2O7·2H2O, is 200~1400 g / L, more preferably 600~1200 g / L.

[0017] Secondly, the present invention provides a method for preparing chromium salts, comprising a method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to the first aspect.

[0018] The technical solution of the present invention has at least the following beneficial effects: This invention targets a multi-component alkaline water system containing sodium chromate (sodium chromate, sodium carbonate, sodium hydroxide, and sodium aluminate). First, a primary acidifying agent is used for initial acidification. By controlling the pH at the endpoint of the primary acidification reaction, sodium carbonate can be preferentially obtained. Subsequent acidification yields aluminum hydroxide and the remaining sodium salts. This allows for the stepwise acquisition of various byproducts, avoiding the problems of inseparability between sodium carbonate, aluminum hydroxide, and other sodium salts, and the difficulty in utilizing byproducts.

[0019] Furthermore, this invention employs a specific acidifying agent and precisely adjusts the pH of the liquid phase through the precise addition of the acidifying agent, gradually converting NaOH, NaAlO2, and Na2CO3 in alkaline water into sodium carbonate, aluminum hydroxide, and sodium bicarbonate, which are then sequentially crystallized and recovered, thereby obtaining high-value-added byproducts of sodium carbonate, aluminum hydroxide, and sodium bicarbonate. This invention can recover 40%–45% of the Na ions in sodium chromate solution in the form of sodium carbonate or sodium bicarbonate. Combining this invention with the sodium dichromate preparation process can significantly reduce the production of cheap byproduct sodium sulfate generated by sulfuric acid acidification. The byproduct sodium carbonate can be returned to the chromite roasting section as raw material, simultaneously reducing sodium carbonate consumption and carbon dioxide emissions, achieving low-carbon and high-efficiency production in the chromite industry. Attached Figure Description

[0020] Figure 1 This is a flowchart of the method for recovering byproducts by graded acidification of sodium chromate alkaline solution provided in Embodiment 1 of the present invention; Figure 2 E-pH diagram of the alkaline solution system in the method for recovering byproducts by graded acidification of sodium chromate alkaline solution provided by the present invention. Detailed Implementation

[0021] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0022] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0023] In a first aspect, the present invention provides a method for recovering byproducts by stepwise acidification of sodium chromate alkaline solution, wherein the sodium chromate alkaline solution contains NaCrO4, Na2CO3, NaAlO2 and NaOH, and the method includes the following steps: (1) Add a first acidifying agent to the sodium chromate alkaline solution for primary acidification, crystallize and filter to obtain a primary filtrate and a sodium carbonate filter cake; wherein the pH of the reaction endpoint of the primary acidification is 11~13; (2) Add a second acidifying agent to the first filtrate for acidification, crystallize and filter to recover aluminum hydroxide and sodium salt.

[0024] In some embodiments, the first acidifying agent is at least one of sodium bicarbonate, carbon dioxide, and carbon dioxide-containing industrial exhaust gas; and / or, the final pH of the primary acidification reaction is 12.2 to 12.8.

[0025] In some embodiments, the second acidifying agent contains at least one of carbon dioxide and sodium bicarbonate, and step (2) includes the following sub-steps: (21) Add a second acidifying agent to the primary filtrate for secondary acidification, crystallize, and then filter to obtain a secondary filtrate and an aluminum hydroxide filter cake. (22) Add a second acidifying agent to the secondary filtrate for tertiary acidification, crystallize and filter to obtain sodium bicarbonate and tertiary filtrate.

[0026] In some embodiments, the endpoint pH of the secondary acidification reaction is 9.5-11; preferably 10-10.5; and the endpoint pH of the tertiary acidification reaction is 7-9.5.

[0027] Therefore, in this preferred embodiment, different byproducts can be obtained by stepwise acidification and by controlling the pH of the reaction endpoint of each acidification step: first-stage acidification yields sodium carbonate, second-stage acidification yields aluminum hydroxide, and third-stage acidification yields sodium bicarbonate.

[0028] In some embodiments, the method further includes returning at least a portion of the sodium carbonate obtained in step (1) to the chromite roasting section for reuse. Thus, the sodium carbonate recovered in step (1) can be used as a raw material for the chromite roasting furnace.

[0029] In some embodiments, the method further includes: returning the sodium bicarbonate obtained in step (22) as at least a portion of the first acidifying agent and / or the second acidifying agent to steps (1) and / or (2) for reuse. That is, the sodium bicarbonate obtained in step (22) can be returned as at least a portion of the first acidifying agent to step (1), or as at least a portion of the second acidifying agent to step (2), or as at least a portion of both the first and second acidifying agents to steps (1) and (2). Thus, the sodium bicarbonate recovered from the three-stage acidification in step (22) can be used as the acidifying agent for the first-stage acidification, or it can be reused as the acidifying agent for the second-stage acidification.

[0030] Through research, the inventors discovered that the CO2 acidification process is a complex gas-liquid-solid heterogeneous reaction process, with each reaction formula shown in equations (1-1) to (1-6). Using HSC Chemistry software, the inventors calculated the Gibbs free energy, enthalpy change, and entropy change of each reaction in the acidification process, and compared the magnitude of the Gibbs free energy changes of reactions (1) to (6) to illustrate the feasibility of the alkaline liquid stepwise separation in the above-mentioned scheme adopted in this invention, and further verified the order of stepwise separation of each component of the alkaline liquid.

[0031] CO2 + H2O = H2CO3 (1-1) CO2 + 2NaOH = Na2CO3 + H2O (1-2) 2NaAlO2+CO2+3H2O =2Al (OH)3+Na2CO3 (1-3) Na2CrO4+2CO2+H2O =Na2Cr2O7+2Na HCO3 (1-4) Al(OH)3+NaHCO3=NaAlCO3(OH)2+H2O (1-5) Na2CO3+CO2+H2O =2NaHCO3 (1-6) By comparing the magnitudes of the free energy changes of each reaction, it was found that the ease with which CO2 reacts with the main components in the alkaline solution is: NaOH > NaAlO2 > Na2CO3 > Na2CrO4, indicating that each carbonization reaction has a clear priority during the carbonization process. Therefore, in the stepwise acidification separation process of this invention, NaOH, NaAlO2, and Na2CO3 in alkaline water can be sequentially converted into sodium carbonate, aluminum hydroxide, and sodium bicarbonate, which have relatively low solubility, and then separated from them, using CO2.

[0032] Furthermore, the inventors used HSC Chemistry software to calculate the potential E-pH phase diagram of the system, comparing the existing forms of each substance at different pH values ​​to further illustrate the feasibility of the alkaline liquid stepwise separation method used in this invention. See also Figure 2 As shown by the E-pH diagram of the alkaline solution system, when the solution is in a strongly alkaline environment with a pH of 11 to 14, the aluminum in the solution is mainly AlO2. - Aluminum exists in ionic form. When the solution pH is less than 11 and greater than 3, aluminum mainly exists as the water-insoluble Al(OH)3. Therefore, in this invention, during the primary acidification process to convert NaOH into sodium carbonate, the solution pH should be maintained above 11. At this point, CO2 preferentially reacts with free NaOH to form Na2CO3, while aluminum remains as AlO2. - Sodium carbonate exists in ionic form. Since the solubility of Na₂CO₃ is significantly lower than that of Na₂CrO₄ and NaAlO₂, and considering the existing Na₂CO₃ in alkaline water, Na₂CO₃ is more easily precipitated and recovered from the liquid phase. Therefore, primary acidification can effectively recover sodium carbonate. According to the E-pH diagram, during secondary acidification, adjusting the pH to below 11 converts NaAlO₂ into aluminum hydroxide precipitate for initial precipitation. During tertiary acidification, the sodium carbonate not precipitated in primary acidification and the newly generated sodium carbonate during the aluminum removal process in secondary acidification are further acidified, converting the relatively less soluble sodium carbonate into even less soluble sodium bicarbonate, thus causing sodium bicarbonate to precipitate. The remaining saturated sodium bicarbonate in the liquid phase can be further recovered through concentration and crystallization.

[0033] In some embodiments, the second acidifying agent is sulfuric acid and / or sodium bisulfate. Step (2) involves adding the second acidifying agent to the primary filtrate at least once for acidification. The final pH of the acidification reaction is 7-9.5. After crystallization, the solution is filtered to recover aluminum hydroxide and sodium sulfate. After completing the recovery of sodium carbonate in step (1), step (2) in this invention can also be performed by conventionally adding sulfuric acid and / or sodium bisulfate. In specific implementation, the second acidifying agent can be added once or multiple times, such as in two or three separate additions, depending on the viscosity of the liquid.

[0034] In some embodiments, when the first acidifying agent and / or the second acidifying agent contains CO2 gas, the partial pressure of CO2 is 0.01 MPa to 1 MPa, preferably 0.05 to 0.15 MPa.

[0035] In feasible methods, the temperature for acidification, crystallization, and filtration can be 25°C to 130°C.

[0036] In some embodiments, the method further includes, in step (1), concentrating the sodium chromate alkaline solution before the first-stage acidification, and / or, after any first-stage acidification and before crystallization, concentrating the sodium chromate alkaline solution. This is because converting the relatively highly soluble component into a relatively less soluble component during each stage of acidification promotes the saturation precipitation of the less soluble component, and the method of increasing the concentration of alkaline water through concentration is more conducive to improving the recovery rate of sodium carbonate and sodium bicarbonate.

[0037] In some embodiments, the concentration of the sodium chromate alkaline solution, calculated as Na2Cr2O7·2H2O, is 200~1400 g / L, preferably 600~1200 g / L.

[0038] Secondly, the present invention provides a method for preparing chromium salts, comprising a method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to the first aspect.

[0039] Combining the method of recovering byproducts by alkaline acidification of sodium chromate solution of the present invention with the sodium dichromate preparation process can significantly reduce the production of cheap sodium sulfate byproducts generated by sulfuric acid acidification. The byproduct sodium carbonate can be returned to the chromite roasting section as raw material, thereby reducing sodium carbonate consumption and carbon dioxide emissions, and realizing low-carbon and high-efficiency production in the chromite industry.

[0040] The present invention will be further described below through embodiments, but the scope of protection of the present invention is not limited thereto.

[0041] Example 1 This embodiment illustrates a method for recovering byproducts through the stepwise acidification of sodium chromate alkaline solution. See also... Figure 1 The method is as follows: Sodium chromate alkaline solution (sodium chromate in Na2Cr2O) 7•The solution (containing 330 g / L Na₂H₂O, 20 g / L Na₂CO₃, 50 g / L NaAlO₂, and 50 g / L NaOH) was concentrated to a concentration of 608.8 g / L (Na₂Cr₂O₇•₂H₂O). This solution was then fed into a primary acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. After the pH of the reaction solution reached 12.5, crystallization was maintained at 85°C for 4 hours. The viscosity of the solution after primary acidification was 316 Pa·s. Filtering the reaction product yielded a sodium carbonate filter cake that was almost free of aluminum hydroxide. The filtrate was then fed into a secondary acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. After the pH of the reaction solution reached 10, crystallization was maintained at 85°C for 5 hours. Filtering the reaction product yielded an aluminum hydroxide filter cake containing a small amount of sodium carbonate impurities. The aluminum removal rate of the secondary acidification was close to 99%. The filtrate was then fed into a tertiary acidification reactor. Carbon dioxide was introduced into a three-stage acidification reactor at a partial pressure of 0.03 MPa. Once the pH of the reaction solution reached 9, crystallization was carried out at 85°C for 4 hours. After three-stage acidification, the material was filtered to obtain a sodium bicarbonate filter cake with an aluminum hydroxide content of less than 0.5%. The filtrate was concentrated and crystallized to increase its concentration to Na₂Cr₂O. 7• The concentration of 2H2O is 923 g / L, thus allowing for further recovery of sodium bicarbonate.

[0042] Sodium carbonate recovery rate after primary acidification = molar amount of sodium in recovered sodium carbonate ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution).

[0043] Sodium carbonate that has undergone primary acidification refers to sodium carbonate converted from sodium carbonate and sodium hydroxide.

[0044] The recovery rate of the sodium carbonate-sodium bicarbonate mixture after secondary acidification = (molar amount of sodium in the recovered sodium carbonate and sodium bicarbonate mixture) ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution).

[0045] Sodium bicarbonate recovery rate after tertiary acidification = Sodium in recovered sodium bicarbonate ÷ (Molar amount of sodium in sodium hydroxide in concentrated alkaline solution + Molar amount of sodium in sodium carbonate in concentrated alkaline solution + Molar amount of sodium in sodium aluminate in concentrated alkaline solution).

[0046] Sodium bicarbonate recovery rate after concentration and crystallization = sodium in recovered sodium bicarbonate ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution).

[0047] The total recovery rate of sodium-containing compounds = primary recovery rate + secondary recovery rate + tertiary recovery rate + concentration and crystallization recovery rate. The calculation results for the above recovery rates are shown in Table 1.

[0048] Table 1

[0049] Example 2 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... An alkaline sodium chromate solution (sodium chromate as Na2Cr2O7•2H2O = 200 g / L, Na2CO3 = 12 g / L, NaAlO2 = 20.3 g / L, NaOH = 35.6 g / L) was fed into a primary acidification reactor. Carbon dioxide was introduced, and the partial pressure of CO2 was 0.11 MPa. After the pH of the reaction solution reached 12.7, it was concentrated to a concentration of 903.5 g / L as Na2Cr2O7•2H2O. The solution was then kept at 100℃ for 3.5 h to allow crystallization.

[0050] The sodium carbonate recovery rate after primary acidification is calculated as follows: (Molar amount of sodium in the recovered sodium carbonate ÷ (Molar amount of sodium in the concentrated alkaline solution + Molar amount of sodium in the concentrated alkaline solution + Molar amount of sodium in the concentrated alkaline solution) = 71.6%. The sodium carbonate contains virtually no aluminum hydroxide.

[0051] Example 3 This embodiment illustrates a method for recovering byproducts through the stepwise acidification of sodium chromate alkaline solution. The method specifically involves: A low-concentration alkaline solution of sodium chromate (330 g / L Na₂Cr₂O₇•₂H₂O, 20 g / L Na₂CO₃, 50 g / L NaAlO₂, and 50 g / L NaOH) was directly fed into a primary acidification reactor. Carbon dioxide was introduced at a partial pressure of 0.03 MPa. After the pH of the reaction solution reached 12.5, crystallization was maintained at 85°C for 4 hours, with no solid precipitation. The solution was then fed into a secondary acidification reactor, where CO₂ was introduced at a partial pressure of 0.03 MPa to bring the pH of the reaction solution to 10. Crystallization was then maintained at 85°C for 5 hours. The resulting material was filtered to obtain an aluminum hydroxide filter cake, with the aluminum removal rate of the secondary acidification approaching 99%. The filtrate was then fed into a tertiary acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. After the pH of the reaction solution reached 9, crystallization was maintained at 85°C for 4 hours. After the reaction, the material was filtered to obtain sodium bicarbonate. The filtrate was concentrated and crystallized to increase the concentration to 911 g / L (based on Na₂Cr₂O₇•₂H₂O), thereby further recovering sodium bicarbonate.

[0052] Sodium carbonate recovery after secondary acidification = Molar amount of sodium in the recovered sodium carbonate and sodium bicarbonate mixture ÷ (Molar amount of sodium in sodium hydroxide in alkaline solution + Molar amount of sodium in sodium carbonate in alkaline solution + Molar amount of sodium in sodium aluminate in alkaline solution) = 2.35%.

[0053] The recovery rate of sodium bicarbonate under tertiary acidification is calculated as follows: = (Molar amount of sodium in the recovered sodium carbonate and sodium bicarbonate mixture) ÷ (Molar amount of sodium in sodium hydroxide in the alkaline solution + Molar amount of sodium in sodium carbonate in the alkaline solution) (Amount + Molar amount of sodium in sodium aluminate in alkaline solution) = 25.4%.

[0054] Sodium bicarbonate recovery rate after concentration and crystallization = sodium in recovered sodium bicarbonate ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 50.23%.

[0055] The total recovery rate of sodium-containing compounds = primary recovery rate + secondary recovery rate + tertiary recovery rate + concentration and crystallization recovery rate = 77.98%. The calculation results for the above recovery rates are shown in Table 2.

[0056] Table 2

[0057] Example 4 This embodiment illustrates a method for recovering byproducts through the stepwise acidification of sodium chromate alkaline solution. The method specifically involves: A sodium chromate alkaline solution (sodium chromate as Na₂Cr₂O₇•₂H₂O: 330 g / L, Na₂CO₃: 20 g / L, NaAlO₂: 50 g / L, NaOH: 50 g / L) was concentrated to a concentration of 608.8 g / L (Na₂Cr₂O₇•₂H₂O). This solution was then fed into a primary acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. Once the pH of the reaction solution reached 12.5, crystallization was maintained at 85°C for 4 hours. The resulting material was filtered to obtain a sodium carbonate filter cake that contained almost no aluminum hydroxide. The filtrate was then fed into a secondary acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. Once the pH of the reaction solution reached 10.2, crystallization was maintained at 85°C for 5 hours. The resulting material was filtered to obtain an aluminum hydroxide filter cake containing a small amount of sodium carbonate impurities. The aluminum removal rate of the secondary acidification was close to 99%. The filtrate was fed into a three-stage acidification reactor. Carbon dioxide was introduced into the reactor at a partial pressure of 0.03 MPa. After the pH of the reaction solution reached 9.3, crystallization was carried out at 85°C for 4 hours. After three-stage acidification, the material was filtered to obtain a sodium bicarbonate filter cake with an aluminum hydroxide content of less than 0.5%. The filtrate was concentrated and crystallized to increase the concentration to 900 g / L (Na₂Cr₂O₇•₂H₂O), thereby further recovering sodium bicarbonate.

[0058] Sodium carbonate recovery rate after primary acidification = molar amount of sodium in recovered sodium carbonate ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 55.3%.

[0059] Sodium carbonate that has undergone primary acidification refers to sodium carbonate converted from sodium carbonate and sodium hydroxide.

[0060] The recovery rate of the sodium carbonate-sodium bicarbonate mixture after secondary acidification = (molar amount of sodium in the recovered sodium carbonate and sodium bicarbonate mixture) ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 6.77%.

[0061] Sodium bicarbonate recovery rate after tertiary acidification = Sodium in recovered sodium bicarbonate ÷ (Molar amount of sodium in sodium hydroxide in concentrated alkaline solution + Molar amount of sodium in sodium carbonate in concentrated alkaline solution + Molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 14.5%.

[0062] Sodium bicarbonate recovery rate after concentration and crystallization = sodium in recovered sodium bicarbonate ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 4.25%.

[0063] The total recovery rate of sodium-containing compounds = primary recovery rate + secondary recovery rate + tertiary recovery rate + concentration and crystallization recovery rate = 80.82%. The calculation results for the above recovery rates are shown in Table 3.

[0064] Table 3

[0065] Example 5 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... The sodium chromate alkaline solution (sodium chromate as Na2Cr2O7•2H2O was 330 g / L, Na2CO3 was 20 g / L, NaAlO2 was 50 g / L, and NaOH was 50 g / L) was concentrated to 608.8 g / L as Na2Cr2O7•2H2O and sent to the first-stage acidification reactor. 52.5 g of sodium bicarbonate (which was recovered by the method in Example 1) was added to each liter of alkaline water. Carbon dioxide was continuously introduced at a partial pressure of 0.05 MPa until the pH of the reaction solution reached 12.1. Then, the solution was kept at 60°C for 5 hours to crystallize.

[0066] The sodium carbonate recovery rate after primary acidification is calculated as follows: (molar amount of sodium in the recovered sodium carbonate - molar amount of sodium in the added sodium bicarbonate) ÷ (molar amount of sodium in the concentrated alkaline solution + molar amount of sodium in the concentrated alkaline solution + molar amount of sodium in the concentrated alkaline solution) = 57.1%, wherein the mass content of aluminum hydroxide in the sodium carbonate is less than 0.5%.

[0067] Example 6 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... The sodium chromate alkaline solution (sodium chromate as Na2Cr2O7•2H2O was 330 g / L, Na2CO3 was 20 g / L, NaAlO2 was 50 g / L, and NaOH was 50 g / L) was concentrated to 550 g / L as Na2Cr2O7•2H2O and sent to the first-stage acidification reactor. 88 g of sodium bicarbonate (which was recovered by the method in Example 1) was added to each liter of alkaline water. Carbon dioxide was continuously introduced at a partial pressure of 0.05 MPa until the pH of the reaction solution reached 12.5. Then, the solution was kept at 60°C for 5 hours to crystallize.

[0068] The sodium carbonate recovery rate after primary acidification is calculated as follows: (molar amount of sodium in the recovered sodium carbonate - molar amount of sodium in the added sodium bicarbonate) ÷ (molar amount of sodium in the concentrated alkaline solution + molar amount of sodium in the concentrated alkaline solution + molar amount of sodium in the concentrated alkaline solution) = 51.3%, wherein the mass content of aluminum hydroxide in the sodium carbonate is less than 0.5%.

[0069] Example 7 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... A sodium chromate alkaline solution (sodium chromate as Na2Cr2O7•2H2O 330 g / L, Na2CO3 20 g / L, NaAlO2 50 g / L, NaOH 50 g / L) was directly fed into the primary acidification reactor. 52.5 g of sodium bicarbonate (obtained through the method in Example 1) was added per liter of alkaline water. Carbon dioxide was continuously introduced at a partial pressure of 0.05 MPa until the pH of the reaction solution reached 12. Crystallization was then maintained at 60°C for 5 hours. No solid precipitated from the reaction solution. The solution was then fed into the secondary acidification reactor, where CO2 was introduced at a partial pressure of 0.03 MPa to bring the pH of the reaction solution to 10. Crystallization was then maintained at 85°C for 5 hours. The reactants were filtered to obtain aluminum hydroxide filter cake. The aluminum removal rate of the secondary acidification was close to 99%. The filtrate was fed into a three-stage acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. Once the pH of the reaction solution reached 9, crystallization was carried out at 85°C for 4 hours. The resulting material was filtered to obtain sodium bicarbonate. The filtrate was then concentrated and crystallized to a concentration of 885 g / L (Na₂Cr₂O₇•₂H₂O), allowing for further recovery of sodium bicarbonate.

[0070] Total recovery rate of sodium-containing compounds = (sodium in recovered sodium bicarbonate - molar amount of sodium in added sodium bicarbonate) ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 79.56%.

[0071] Example 8 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... An alkaline sodium chromate solution (sodium chromate is 200 g / L Na2Cr2O7•2H2O, Na2CO3 is 12 g / L, NaAlO2 is 20.3 g / L, and NaOH is 35.6 g / L) was fed into a primary acidification reactor. Sodium bicarbonate was added until the pH of the reaction solution reached 12.4. The solution was then concentrated to a concentration of 827.9 g / L Na2Cr2O7•2H2O. The solution was then kept at 60°C for 5 hours to allow crystallization.

[0072] The sodium carbonate recovery rate after primary acidification is calculated as follows: (molar amount of sodium in the recovered sodium carbonate - molar amount of sodium in the added sodium bicarbonate) ÷ (molar amount of sodium in the concentrated alkaline solution + molar amount of sodium in the concentrated alkaline solution + molar amount of sodium in the concentrated alkaline solution) = 68.9%.

[0073] Example 9 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... An alkaline sodium chromate solution (sodium chromate as Na₂Cr₂O₇•₂H₂O 330 g / L, Na₂CO₃ 20 g / L, NaAlO₂ 50 g / L, NaOH 50 g / L) was fed into a primary acidification reactor. Sodium bicarbonate (obtained through the method in Example 1) was added until the pH of the reaction solution reached 12.5. Crystallization was maintained at 75°C for 4 hours. No solid precipitated from the reaction solution. The solution was then fed into a secondary acidification reactor, where sodium bicarbonate was added to bring the pH to 11. The reaction solution was filtered to obtain an aluminum hydroxide filter cake. The filtrate was fed into a tertiary acidification reactor, where CO₂ was introduced to bring the pH to 9. The reaction solution was concentrated and crystallized to a concentration of 906 g / L (Na₂Cr₂O₇•₂H₂O), and the remaining sodium bicarbonate was recovered.

[0074] Total recovery rate of sodium-containing compounds = (sodium in recovered sodium bicarbonate - molar amount of sodium in added sodium bicarbonate) ÷ (molar amount of sodium in sodium hydroxide in concentrated alkaline solution + molar amount of sodium in sodium carbonate in concentrated alkaline solution + molar amount of sodium in sodium aluminate in concentrated alkaline solution) = 78.38%.

[0075] Example 10 This embodiment illustrates a method for recovering byproducts through the stepwise acidification of sodium chromate alkaline solution. The method specifically involves: A sodium chromate alkaline solution (sodium chromate as Na₂Cr₂O₇•₂H₂O: 330 g / L, Na₂CO₃: 20 g / L, NaAlO₂: 50 g / L, NaOH: 50 g / L) was concentrated to 608.8 g / L (Na₂Cr₂O₇•₂H₂O) and fed into a primary acidification reactor. 105 g of sodium bicarbonate (obtained through the method in Example 1) was added per liter of alkaline water. The pH of the reaction solution reached 12.5, and crystallization was maintained at 75°C for 4 hours. The reaction solution was filtered to obtain sodium carbonate, and the filtrate was fed into a secondary acidification reactor. CO₂ was introduced into the secondary acidification reactor at a partial pressure of 0.05 MPa to bring the pH of the reaction solution to 11. Crystallization was then maintained at 95°C for 3.5 hours. The reactants were filtered to obtain aluminum hydroxide filter cake. The aluminum removal rate of the secondary acidification was close to 99%. The filtrate was fed into a three-stage acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.08 MPa. Once the pH of the reaction solution reached 9, crystallization was carried out at 83°C for 4 hours. The reactants were then filtered to obtain sodium bicarbonate. The filtrate was concentrated and crystallized to increase its concentration to 880 g / L (Na₂Cr₂O₇•₂H₂O), thereby further recovering sodium bicarbonate.

[0076] The sodium carbonate recovery rate after primary acidification is calculated as follows: (molar amount of sodium in the recovered sodium carbonate - molar amount of sodium in the added sodium bicarbonate) ÷ (molar amount of sodium in the sodium hydroxide in the alkaline solution + molar amount of sodium in the sodium carbonate in the alkaline solution + molar amount of sodium in the sodium aluminate in the concentrated alkaline solution) = 55.7%, wherein the mass content of aluminum hydroxide in the sodium carbonate is less than 0.5%.

[0077] The sodium carbonate recovery rate after secondary acidification = the molar amount of sodium in the recovered sodium carbonate and sodium bicarbonate mixture ÷ (the molar amount of sodium in the alkaline solution + the molar amount of sodium in the sodium carbonate in the alkaline solution + the molar amount of sodium in the sodium aluminate in the alkaline solution) = 6.53%.

[0078] Sodium bicarbonate recovery rate after tertiary acidification = molar amount of sodium in the recovered sodium bicarbonate ÷ (molar amount of sodium in sodium hydroxide in alkaline solution + molar amount of sodium in sodium carbonate in alkaline solution + molar amount of sodium in sodium aluminate in alkaline solution) = 16.3%.

[0079] The sodium bicarbonate recovery rate after concentration and crystallization = sodium in the recovered sodium bicarbonate ÷ (molar amount of sodium in sodium hydroxide in alkaline solution + molar amount of sodium in sodium carbonate in alkaline solution + molar amount of sodium in sodium aluminate in alkaline solution) = 7.1%. The calculation results of the above recovery rate are shown in Table 4.

[0080] Table 4

[0081] Example 11 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... An alkaline sodium chromate solution (sodium chromate as Na₂Cr₂O₇•₂H₂O = 200 g / L, Na₂CO₃ = 12 g / L, NaAlO₂ = 30.3 g / L, NaOH = 30.3 g / L) was fed into a primary acidification reactor. Carbon dioxide was introduced, and the CO₂ partial pressure was 0.04 MPa. After the pH of the reaction solution reached 12.7, the solution was concentrated to a sodium chromate concentration of 747.3 g / L (Na₂Cr₂O₇•₂H₂O). Crystallization was carried out at 75°C for 4 hours. The filtrate was fed into a secondary acidification reactor, where sodium bisulfate was added to bring the pH of the reaction solution to 11. The reaction solution was filtered to obtain an aluminum hydroxide filter cake. The filtrate was fed into a tertiary acidification reactor, where sodium bisulfate was added to bring the pH of the reaction solution to 7.2. Aluminum hydroxide precipitated in stages, and the recovery rate of aluminum hydroxide reached 100% in both the secondary and tertiary acidification processes.

[0082] The sodium carbonate recovery rate after primary acidification is calculated as follows: (Molar amount of sodium in the recovered sodium carbonate ÷ (Molar amount of sodium in sodium hydroxide in the concentrated alkaline solution + Molar amount of sodium in sodium carbonate in the concentrated alkaline solution + Molar amount of sodium in sodium aluminate in the concentrated alkaline solution) = 62.8%. The sodium carbonate contains virtually no aluminum hydroxide.

[0083] Example 12 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... A sodium chromate alkaline solution (sodium chromate as Na₂Cr₂O₇•₂H₂O: 330 g / L, Na₂CO₃: 20 g / L, NaAlO₂: 50 g / L, NaOH: 50 g / L) was concentrated to a concentration of 608.8 g / L (Na₂Cr₂O₇•₂H₂O). This solution was then fed into a primary acidification reactor, where carbon dioxide was introduced at a partial pressure of 0.03 MPa. Once the pH of the reaction solution reached 12.5, crystallization was maintained at 85°C for 4 hours. The resulting material was filtered to obtain a sodium carbonate filter cake containing almost no aluminum hydroxide. The filtrate was then fed into a secondary acidification reactor, where sodium bisulfate was added to bring the pH of the reaction solution to 11. The reaction solution was filtered again to obtain an aluminum hydroxide filter cake. This filtrate was then fed into a tertiary acidification reactor, where sodium bisulfate was added to bring the pH of the reaction solution to 7. Aluminum hydroxide precipitated in stages, and the recovery rate of aluminum hydroxide reached 100% in both the secondary and tertiary acidification processes.

[0084] The sodium carbonate recovery rate after primary acidification is calculated as follows: (Molar amount of sodium in the recovered sodium carbonate ÷ (Molar amount of sodium in the concentrated alkaline solution + Molar amount of sodium in the concentrated alkaline solution + Molar amount of sodium in the concentrated alkaline solution) = 56.19%. The sodium carbonate contains virtually no aluminum hydroxide.

[0085] Example 13 This embodiment uses the method of Example 1 to recover byproducts from sodium chromate through a stepwise acidification process with an alkaline solution. The difference lies in... A sodium chromate alkaline solution (sodium chromate as Na₂Cr₂O₇•₂H₂O 330 g / L, Na₂CO₃ 20 g / L, NaAlO₂ 50 g / L, NaOH 50 g / L) was directly fed into a primary acidification reactor. Carbon dioxide was introduced, and the CO₂ partial pressure was 0.03 MPa. After the pH of the reaction solution reached 12.5, crystallization was carried out at 85℃ for 4 hours. No solid precipitated from the reaction solution. The solution was then fed into a secondary acidification reactor, where sodium bisulfate was added to bring the pH of the reaction solution to 11. The reaction solution was filtered to obtain an aluminum hydroxide filter cake. The filtrate was fed into a tertiary acidification reactor, where sodium bisulfate was added to bring the pH of the reaction solution to 7. The reaction solution was concentrated and crystallized to a concentration of 903 g / L (Na₂Cr₂O₇•₂H₂O). After filtration, sodium sulfate was obtained as a byproduct.

[0086] Comparative Example 1 The concentrated alkaline sodium chromate solution (sodium chromate calculated as Na2Cr2O7•2H2O is 608.8 g / L, Na2CO3 is 36.5 g / L, NaAlO2 is 92.3 g / L, and NaOH is 92.3 g / L) was fed into an acidification reactor for acidification. The pH value at the end of the acidification reaction was 9. The reaction solution was then subjected to solid-liquid separation. The viscosity of the reaction solution was 1685 Pa·S, and the filter cake was a mixture of aluminum mud, sodium bicarbonate, and sodium carbonate.

[0087] Comparative Example 2 A low-concentration alkaline solution of sodium chromate (sodium chromate is 330 g / L as Na2Cr2O7•2H2O, Na2CO3 is 20 g / L, NaAlO2 is 50 g / L, and NaOH is 50 g / L) was directly fed into the acidification reactor for acidification. The pH value at the end of the acidification reaction was 9. The reaction solution was then subjected to solid-liquid separation, and the filter cake was a mixture of aluminum mud, sodium bicarbonate, and sodium carbonate.

[0088] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for recovering byproducts by graded acidification of sodium chromate alkaline solution, characterized in that, The sodium chromate alkaline solution contains NaCrO4, Na2CO3, NaAlO2, and NaOH. The method includes the following steps: (1) Add a first acidifying agent to the sodium chromate alkaline solution for primary acidification, crystallize and filter to obtain a primary filtrate and a sodium carbonate filter cake; wherein the pH of the reaction endpoint of the primary acidification is 11~13; (2) Add a second acidifying agent to the first filtrate for acidification, crystallize and filter to recover aluminum hydroxide and sodium salt.

2. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 1, characterized in that, The first acidifying agent is at least one of carbon dioxide, carbon dioxide-containing industrial exhaust gas, and sodium bicarbonate; and / or, the final pH of the primary acidification reaction is 12.2 to 12.

8.

3. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 1, characterized in that, The second acidifying agent contains at least one of carbon dioxide and sodium bicarbonate, and step (2) includes the following sub-steps: (21) Add a second acidifying agent to the primary filtrate for secondary acidification, crystallize, and then filter to obtain a secondary filtrate and an aluminum hydroxide filter cake. (22) Add a second acidifying agent to the secondary filtrate for tertiary acidification, crystallize and filter to obtain sodium bicarbonate and tertiary filtrate.

4. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 3, characterized in that, The endpoint pH of the secondary acidification reaction is 9.5-11; preferably 10-10.5; and / or, The final pH value of the tertiary acidification reaction is 7-9.

5.

5. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 3 or 4, characterized in that, The method further includes: The sodium carbonate obtained in step (1) is returned to the chromite roasting section for reuse, at least a portion of which is used as raw material; and / or, The sodium bicarbonate obtained in step (22) is returned to step (1) and / or step (2) for reuse as at least a portion of the first acidifier and / or the second acidifier.

6. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 1, characterized in that, The second acidifying agent is sulfuric acid and / or sodium bisulfate, and step (2) is as follows: Acidification is carried out by adding a second acidifying agent at least once to the primary filtrate. The final pH of the acidification reaction is 7-9.

5. After crystallization, the solution is filtered to recover aluminum hydroxide and sodium sulfate.

7. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 1, characterized in that, When the first acidifying agent and / or the second acidifying agent contains CO2 gas, the partial pressure of CO2 is 0.01 MPa to 1 MPa, preferably 0.05 to 0.15 MPa.

8. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to claim 1, characterized in that, The method further includes concentrating the sodium chromate alkaline solution before primary acidification, and / or concentrating the sodium chromate alkaline solution after any primary acidification and before crystallization.

9. The method for recovering byproducts by graded acidification of sodium chromate alkaline solution according to any one of claims 1-9, characterized in that, The concentration of the sodium chromate alkaline solution, calculated as Na2Cr2O7·2H2O, is 200~1400 g / L, preferably 600~1200 g / L.

10. A method for preparing a chromium salt, characterized in that, A method for recovering byproducts by using sodium chromate alkaline solution in a stepwise acidification process according to any one of claims 1-9.

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