Process method for recovering sodium salt and molybdenum salt from waste liquid

Abstract

The invention relates to a process method for recovering sodium salt and molybdenum salt from waste liquid, which comprises the following steps: 1, recovering the sodium salt: (1.1) introducing carbon dioxide gas into a container filled with the waste liquid, carbonizing and crystallizing sodium carbonate in the waste liquid to obtain sodium bicarbonate crystals and mother liquor I, and then executing the step (1.2); (1.2) after separating the crude sodium bicarbonate crystal, judging whether the content of molybdenum ions in the mother liquor I reaches a set value, if so, executing the step (2), and if not, refluxing the mother liquor I into the container and executing the step (1.1); and 2, recovering molybdenum salt in the mother liquor 1. As sodium salt is extracted in a carbonization crystallization mode, water in waste liquid is recycled in the process of separating sodium carbonate, and sodium molybdate is concentrated in a system along with continuous separation of sodium bicarbonate, the primary crystallization mother liquor (namely mother liquor I) contains higher sodium molybdate concentration, and sodium molybdate is convenient to separate.

Description

Technical Field

[0001] The invention relates to the technical field of petrochemical industry, and in particular to a process for recovering sodium salt and molybdenum salt from waste liquid. Background Art

[0002] The PO / SM device uses ammonium molybdate as an epoxidation catalyst. Since the catalyst will dissolve into the wastewater, the wastewater is neutralized with sodium hydroxide and then discharged into the waste alkali incineration device. After incineration, waste containing sodium carbonate and sodium molybdate is generated.

[0003] The main components of PO / SM waste alkali are sodium carbonate with a concentration of 80-99%wt, sodium molybdate with a concentration of 0-1%wt, and other impurities such as sodium chromate. For the recovery of sodium salt and molybdenum salt, as disclosed in the Chinese invention patent application with patent application number CN202410218010.6 (publication number CN118084058A), "A method for recovering sodium carbonate and molybdenum from incineration slag after incineration", (1) dissolving the incineration slag after incineration with water, and then filtering to remove insoluble matter; (2) evaporating and crystallizing the unsaturated solution of sodium carbonate, sodium molybdate and soluble impurities produced in step 1, and separating the crystal slurry to obtain sodium carbonate crystals; (3) taking out part of the mother liquor from the evaporation and crystallization of sodium carbonate in step 2 and adjusting the pH value to acidic with acid solution; (4) separating molybdate and impurity elements from the acidic mother liquor treated in step 3; (5) stripping and / or analyzing the molybdate-rich part separated in step 4 to obtain a molybdate solution; (6) evaporating and crystallizing the molybdate solution obtained in step 5 to obtain high-purity molybdate crystals.

[0004] The patent adopts the method of evaporation crystallization to recycle sodium carbonate in step (2), but the crystallization process is formed by the collision of molecules, atoms or ions in the liquid phase. The higher the concentration of the solute in the solution, the easier it is to form crystals. Specifically, in the above patent, in sodium carbonate and sodium molybdate solution, carbonate ions and molybdate ions can form crystals with sodium ions. Evaporation crystallization requires increasing the concentration of carbonate and sodium ions in the solvent at the same time. After the two ions are oversaturated, sodium carbonate crystals are produced. This process increases the sodium ion concentration, and therefore also increases the chance of sodium ions and molybdate collision crystallization, resulting in sodium carbonate crystals containing sodium molybdate impurities. Molybdenum is a heavy metal. If you want to obtain high-purity carbonate, you need to control the heavy metal content. For example, GB1887-2007 requires a mass fraction of ≤0.0005%wt for the heavy metal content in sodium bicarbonate.

[0005] The patent specification

[0008] paragraph discloses that the incineration slag after incineration described in the present invention may be composed of 30%-99% by mass of sodium carbonate, 0.1%-50% by mass of sodium molybdate, and 0.1-20% by mass of impurity elements.

[0006] It can be seen that there may be a very low content of molybdenum ions in the waste liquid. In this case (such as the recovery of sodium salt and molybdenum salt in the PO / SM waste alkali mentioned above), the conventional evaporation and crystallization separation method is difficult to control the distribution coefficient of molybdenum in the crystalline phase and the liquid phase, resulting in excessive molybdenum in the crystallized sodium salt. This will result in a very low concentration of sodium molybdate in the system, that is, due to the excessive amount of sodium molybdate carried in the evaporated crystallized sodium carbonate, the concentration of sodium molybdate in the liquid phase continues to decrease, and the concentration effect cannot be achieved, which ultimately leads to the inability to separate sodium molybdate in the subsequent separation steps. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a process for recovering sodium salt and molybdenum salt from waste liquid with high recovery rate and high recovery purity in view of the current status of the prior art.

[0008] The technical solution adopted by the present invention to solve the above technical problems is: a process for recovering sodium salt and molybdenum salt from waste liquid, comprising the following steps:

[0009] 1. Recovering sodium salt comprises the following steps:

[0010] (1.1) introducing carbon dioxide gas into a container containing waste liquid, so that the sodium carbonate in the waste liquid is carbonized and crystallized to obtain crude sodium bicarbonate crystals and mother liquor 1, and then performing step (1.2);

[0011] (1.2) After separating the crude sodium bicarbonate crystals, determine whether the platinum ion content in the mother liquor 1 reaches the set value. If yes, perform step 2. If not, reflux to the container and perform step (1.1);

[0012] 2. Recover the molybdenum salt in the mother liquor 1.

[0013] During the crystallization process, adsorption and deposition will occur on the crystal surface, causing the crystallized sodium bicarbonate crystals to contain impurities. Therefore, the step 1 also includes the following steps:

[0014] (1.3) Rinse the crude sodium bicarbonate crystals separated in step (1.2) with a saturated sodium bicarbonate solution. The present invention uses a saturated solution containing sodium bicarbonate to rinse the crystals, which minimizes the amount of water introduced into the system and is beneficial to the concentration and separation of sodium molybdate in the solution. If water is directly used to wash the crystals, the crystals will dissolve in the rinse water, causing the crystal product to be lost. At the same time, low-concentration rinse water mixed into the crystallization separation system will reduce the overall salt concentration of the system, which is not conducive to the concentration and separation of sodium molybdate. The present invention uses a saturated sodium bicarbonate solution to rinse the crystals, which minimizes the introduction of water.

[0015] The single crystallized sodium bicarbonate product can meet the requirements of industrial products. If the purity requirement is high, further refining is required. Therefore, in the step 1, the following steps are also included:

[0016] (1.4) adding the aqueous solution into step (1.3) to prepare the washed sodium bicarbonate crystals into a slurry;

[0017] (1.5) heating the slurry generated in step (1.4) to generate sodium carbonate and carbon dioxide;

[0018] (1.6) The sodium carbonate generated in step (1.5) is subjected to carbonization crystallization, and after separation, refined sodium bicarbonate crystals and mother liquor II are obtained, and mother liquor II is returned to step (1.1) and / or added to the aqueous solution of step (1.4). The present invention adopts the method of preparing a slurry with crude sodium bicarbonate and water and then heating it with a low-temperature heat source, so that the sodium bicarbonate is decomposed into sodium carbonate and carbon dioxide with greater solubility. After secondary carbonization crystallization of sodium carbonate, a high-purity sodium bicarbonate product can be obtained. At the same time, the present invention adopts wet heat decomposition, and the heating heat source adopts a low-grade heat source to minimize energy consumption. Part of the mother liquor with sodium molybdate can be used to dissolve the sodium bicarbonate crystals and recover the sodium molybdate to the maximum extent.

[0019] In the above scheme, the container is a pressure vessel, and the carbon dioxide gas comes from a chemical plant containing carbon dioxide in the exhaust gas. If the crystal growth rate is too fast during the crystallization process, the crystal will be wrapped in the mother liquor rich in impurities, which will lead to the production of crystals containing impurities. Controlling the carbonate ion concentration in the liquid phase is the most effective means to regulate the crystallization rate. In the step (1.1), the carbon dioxide concentration is 10-100%, and the pressure is 0-10 bar (g). Since the carbon dioxide concentration in various exhaust gases varies widely and is unstable, it is difficult to control the carbonization rate by directly performing carbonization crystallization. This patent controls the carbon dioxide gas phase partial pressure in the carbonization process by controlling the pressure of the exhaust gas, and then controls the carbonate concentration in the liquid phase to regulate the crystallization rate, thereby reducing the possibility of sodium bicarbonate crystals encapsulating sodium molybdate.

[0020] Preferably, the step 2 comprises the following steps:

[0021] (2.1) Recover the molybdenum salt in the mother liquor 1 to obtain a crude molybdenum-containing compound and a mother liquor 3, and return the mother liquor 3 to step (1.1) to recover the sodium salt to the maximum extent.

[0022] In the above scheme, in the step (2.1), the method for recovering the crude molybdenum-containing compound is at least one of the following methods: resin adsorption, evaporative crystallization, and acidification crystallization under acidic or alkaline conditions.

[0023] However, the existing methods for separating sodium salts such as evaporation cannot well control the sodium molybdate carried in the separated sodium carbonate, which may lead to the inability to concentrate the sodium molybdate, or even the inability to separate due to the excessive amount of sodium molybdate carried in the sodium carbonate. In the process of separating sodium carbonate in the present application, the water in the waste liquid is recycled, so as the sodium bicarbonate is continuously separated, the sodium molybdate will be concentrated in the system. Since the technology adopted in the sodium bicarbonate separation process of the present patent can effectively reduce the amount of sodium molybdate carried by the sodium bicarbonate crystals, the primary crystallization mother liquor (i.e., mother liquor 1) contains a higher sodium molybdate concentration, and the sodium molybdate can be separated. Therefore, the sodium molybdate can be separated by evaporation crystallization, resin adsorption and other methods.

[0024] Since molybdic acid is insoluble in water, the mother liquor after separation of sodium bicarbonate can be directly acidified, the pH value can be adjusted to 0-7, and molybdic acid crystals can be separated. Therefore, the step 2 also includes the following steps:

[0025] (2.2) The crude molybdenum-containing compound is dissolved in an acidic solution for acidification and crystallization to generate molybdic acid crystals and mother liquor IV.

[0026] In order to further improve the purity of molybdic acid, the step 2 further comprises the following steps:

[0027] (2.3) dissolving the molybdic acid crystals produced in step (2.2) in an alkaline solution, and then adding an acidic solution again for acidification and crystallization to obtain refined molybdic acid.

[0028] Preferably, the step 2 further comprises the following steps:

[0029] (2.4) reacting the molybdic acid crystals produced in step (2.2) or the refined molybdic acid produced in step (2.3) with an alkaline substance to obtain the desired molybdate salt substance, such as sodium molybdate, potassium molybdate, ammonium molybdate, etc., which can be regenerated using sodium hydroxide, potassium hydroxide, ammonia water, etc.

[0030] Compared with the prior art, the invention has the following advantages: for waste liquid containing sodium carbonate and sodium molybdate, the invention adopts a method of pressing carbon dioxide into the waste liquid to form sodium bicarbonate crystals, the reaction selectivity is high, the purity of the crystallized sodium salt is high, and as the crystallization proceeds, the sodium ion concentration in the solution is continuously reduced, so that the crystallization of sodium ions and molybdate ions on the surface of the sodium bicarbonate crystals is effectively avoided, and the purity of the sodium bicarbonate is improved; in addition, the sodium bicarbonate crystals are obtained by chemical reaction of a low-cost carbon source with sodium carbonate, rather than by a high-energy consumption method such as heating evaporation, which is energy-saving and environmentally friendly.

[0031] Because the sodium salt is extracted by carbonization crystallization, the water in the waste liquid is recycled during the separation of sodium carbonate. As the sodium bicarbonate is continuously separated, sodium molybdate will be concentrated in the system. Therefore, the primary crystallization mother liquor (i.e., mother liquor 1) contains a higher concentration of sodium molybdate, which is convenient for separating sodium molybdate. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The process flow chart of an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present embodiment is further described in detail below in conjunction with the accompanying drawings.

[0034] like Figure 1 As shown, the process for recovering sodium salt and molybdenum salt from waste liquid in this preferred embodiment comprises the following steps:

[0035] 1. Recovering sodium salt comprises the following steps:

[0036] (1.1) introducing carbon dioxide gas into a container containing waste liquid, so that the sodium carbonate in the waste liquid is carbonized and crystallized to obtain sodium bicarbonate crystals and mother liquor 1, and then performing step (1.2);

[0037] The container is an existing pressure vessel capable of regulating pressure, and the carbon dioxide gas comes from a chemical device containing carbon dioxide in the exhaust gas. If the crystal growth rate is too fast during the crystallization process, the crystal will be wrapped in the mother liquor rich in impurities, which will lead to the production of crystals containing impurities. Controlling the carbonate ion concentration in the liquid phase is the most effective means to regulate the crystallization rate. Preferably, the carbon dioxide concentration is 10-100% and the pressure is 0-10 bar(g); carbonization crystallization can be carried out at room temperature.

[0038] (1.2) After separating the crude sodium bicarbonate crystals, determine whether the platinum ion content in the mother liquor 1 reaches the set value (platinum ion content 0-100 g / 100 g solution). If so, perform step 2; if not, return to the container and perform step (1.1); the platinum ion content can be determined by an existing test method, such as testing with an atomic absorption spectrometer.

[0039] (1.3) washing the crude sodium bicarbonate crystals separated in step (1.2) with a saturated sodium bicarbonate solution;

[0040] (1.4) adding the aqueous solution to step (1.3) to prepare the washed sodium bicarbonate crystals into a slurry, preferably, the sodium bicarbonate concentration in the slurry is ≮5%wt;

[0041] (1.5) heating the slurry generated in step (1.4) at a temperature of ≮50° C. to generate sodium carbonate and carbon dioxide;

[0042] (1.6) The sodium carbonate generated in step (1.5) is subjected to carbonization crystallization to obtain refined sodium bicarbonate crystals and mother liquor 2 after separation, and the mother liquor 2 is returned to step (1.1) and / or added to the aqueous solution in step (1.4).

[0043] Second, recovering the molybdenum salt in the mother liquor 1 comprises the following steps:

[0044] (2.1) Recover the molybdenum salt in the mother liquor one to obtain a crude molybdenum-containing compound and a mother liquor three, and return the mother liquor three to step (1.1) to recover the sodium salt to the maximum extent; the crude molybdenum-containing compound is recovered by at least one of the following methods: resin adsorption, evaporative crystallization, and acidification crystallization under acidic or alkaline conditions. These methods are prior arts and will not be described in detail here.

[0045] (2.2) The crude molybdenum-containing compound is dissolved in an acidic solution (such as nitric acid or sulfuric acid) for acidification and crystallization to generate molybdic acid crystals and mother liquor IV. The acidification and crystallization is carried out under acidic conditions.

[0046] (2.3) dissolving the molybdic acid crystals produced in step (2.2) in an alkaline solution (such as sodium hydroxide), and then adding an acidic solution again for acidification and crystallization to obtain refined molybdic acid.

[0047] (2.4) reacting the molybdic acid crystals produced in step (2.2) or the refined molybdic acid produced in step (2.3) with an alkaline substance to obtain the desired molybdate salt substance, such as sodium molybdate, potassium molybdate, ammonium molybdate, etc., which can be regenerated using sodium hydroxide, potassium hydroxide, ammonia water, etc.

[0048] The process method of this embodiment is described as follows:

[0049] 1. The wastewater of this embodiment comes from the PO / SM device. The PO / SM device uses ammonium molybdate as an epoxidation catalyst. Since the catalyst will dissolve into the wastewater, the wastewater is neutralized with sodium hydroxide and discharged into the waste alkali incineration device. After incineration, waste containing sodium carbonate and sodium molybdate is generated. After the waste is dissolved in water and then filtered and purified, it is the wastewater of this embodiment. Of course, the process method of this embodiment can also treat other wastewater containing sodium carbonate and sodium molybdate.

[0050] 2. A sodium carbonate, sodium bicarbonate and sodium molybdate solution system is formed in the waste liquid. Due to the different ratios of the three materials, mixed sodium salts will be produced during crystallization. When preparing the solution, the concentrations of sodium carbonate, sodium bicarbonate and sodium molybdate are controlled so that only sodium bicarbonate is crystallized. Preferably, the sodium carbonate concentration is ≯50 kg / 100 kg water, the sodium bicarbonate concentration is ≯30 kg / 100 kg water, and the sodium molybdate concentration is ≯25 kg / 100 kg water.

[0051] 3. The existing carbonization crystallization process can only use a high-concentration carbon dioxide gas source, which has a small gas source selection range and high cost. This embodiment controls the carbon dioxide gas phase partial pressure in the carbonization process by controlling the pressure of the waste gas, and then controls the carbonate concentration in the liquid phase, thereby regulating the crystallization rate and reducing the possibility of sodium bicarbonate crystals encapsulating sodium molybdate. Therefore, this embodiment can use low-concentration carbon dioxide-containing waste gas, thereby increasing the gas source selection range and saving costs. The chemical plant containing carbon dioxide gas in the waste gas can be furnace flue gas, coal-to-hydrogen, natural gas-to-hydrogen, heavy oil-to-hydrogen, synthesis gas and other devices.

[0052] 4. This embodiment uses carbonization crystallization to recover sodium salt. During the cyclic operation of steps (1.1) and (1.2), sodium salt is continuously recovered and the concentration of molybdenum salt is continuously increased.

[0053] 5. If the sodium bicarbonate crystals do not have high purity requirements when used, the sodium bicarbonate crystals separated in step (1.1) can be used. If sodium bicarbonate crystals with higher purity are required, steps (1.3), (1.4), (1.5), and (1.6) are also required.

[0054] 6. In step (1.3), the crude sodium bicarbonate crystals are rinsed with a saturated sodium bicarbonate solution, which can minimize the amount of water introduced into the system and facilitate the concentration and separation of sodium molybdate in the solution. If water is directly used to wash the crystals, the crystals will dissolve in the rinse water, causing the crystal product to be lost. At the same time, low-concentration rinse water mixed into the crystallization separation system will reduce the overall salt concentration of the system, which is not conducive to the concentration and separation of sodium molybdate. In this embodiment, a saturated sodium bicarbonate solution is used to rinse the crystals, which minimizes the introduction of water.

[0055] 7. In step (1.6), the carbonization crystallization method is the same as step (1.1), and part of the mother liquor containing sodium molybdate can be added to the aqueous solution of step (1.4) to dissolve the sodium bicarbonate crystals and recover the sodium molybdate to the maximum extent.

[0056] 8. If the product in step (2.4) is not produced, this embodiment does not need to use a process or medium with explosion hazard, which greatly reduces the safety risk of this technology.

[0057] 9. When the molybdenum salt is recovered, since the water in the waste liquid is recycled in the process of separating sodium carbonate, as the sodium bicarbonate is continuously separated, sodium molybdate will be concentrated in the system. Due to the sodium bicarbonate separation process, the technology adopted in this patent can effectively reduce the amount of sodium molybdate carried by the sodium bicarbonate crystals, so the primary crystallization mother liquor (i.e., mother liquor 1) contains a higher sodium molybdate concentration, and the sodium molybdate can be separated. However, the existing methods for separating sodium salts such as evaporation cannot well control the sodium molybdate entrained in the separated sodium carbonate, which may lead to the inability to concentrate sodium molybdate, or even the inability to separate due to the large amount of sodium molybdate carried in the sodium carbonate. However, after separating the sodium bicarbonate in this embodiment, a sodium molybdate solution with a higher concentration can be obtained, so the sodium molybdate can be separated by evaporation crystallization, resin adsorption, etc.

[0058] The present embodiment uses resin adsorption to separate sodium molybdate in an acidic environment, which has the following advantages over the resin adsorption to recover sodium molybdate in an acidic environment in the patent cited in the background art:

[0059] Since the carbonization crystallization method is used in the separation and purification of sodium salt in this embodiment, sodium bicarbonate and sodium molybdate are present in the mother liquor. Since the solubility of sodium bicarbonate is much lower than that of sodium carbonate, the carbonate content in the mother liquor is much lower than that in the mother liquor of the evaporative crystallization method. Therefore, the amount of acid used in acidification and neutralization is smaller, and the content of sodium salts such as sodium sulfate, sodium nitrate, and sodium chloride produced is less.

[0060] In summary, the present embodiment adopts carbon dioxide, water and sodium carbonate to react to generate sodium bicarbonate crystals with low energy consumption, high selectivity, high product purity, and less wastewater. The acidification method is used to generate molybdic acid crystals, which has high selectivity, can obtain high-purity products, and has low cost. Ammonia water, sodium hydroxide and molybdic acid can be used to react to generate high-purity sodium molybdate or ammonium molybdate, with rich product lines, and acidic resin is used to recover molybdenum in wastewater, reducing molybdenum loss.

[0061] The sodium salt and molybdenum salt recovered by the process method of this embodiment have the following test results:

[0062] (1) Sodium salt recovery rate: ≮99%

[0063] (2) Sodium bicarbonate quality: food grade premium

[0064] (3) Molybdenum recovery rate: ≮99%

[0065] (4) Sodium molybdate products: superior products

[0066] (5) Ammonium molybdate products: superior products

[0067] (6) Molybdenum content in discharged wastewater: ≯1 mg / L.

Claims

1. A process for recovering sodium salt and molybdenum salt from waste liquid, comprising the following steps:

1. Recovering sodium salt comprises the following steps: (1.1) introducing carbon dioxide gas into a container containing waste liquid, so that the sodium carbonate in the waste liquid is carbonized and crystallized to obtain sodium bicarbonate crystals and mother liquor 1, and then performing step (1.2); (1.2) After separating the crude sodium bicarbonate crystals, determine whether the platinum ion content in the mother liquor 1 reaches the set value. If yes, perform step 2. If not, reflux to the container and perform step (1.1); 2. Recover the molybdenum salt in the mother liquor 1.

2. The process according to claim 1, characterized in that: In the step 1, the following steps are also included: (1.3) Rinse the crude sodium bicarbonate crystals separated in step (1.2) with a saturated sodium bicarbonate solution.

3. According to claim 2, it is characterized in that: In the step 1, the following steps are also included: (1.4) adding the aqueous solution into step (1.3) to prepare the washed sodium bicarbonate crystals into a slurry; (1.5) heating the slurry generated in step (1.4) to generate sodium carbonate and carbon dioxide; (1.6) The sodium carbonate generated in step (1.5) is subjected to carbonization crystallization to obtain refined sodium bicarbonate crystals and mother liquor 2 after separation, and the mother liquor 2 is returned to step (1.1) and / or added to the aqueous solution in step (1.4).

4. The process according to claim 1, characterized in that: The container is a pressure container, the carbon dioxide gas comes from a chemical plant whose waste gas contains carbon dioxide, and in the step (1.1), the carbon dioxide concentration is 10-100% and the pressure is 0-10 bar(g).

5. The process according to any one of claims 1 to 4, characterized in that: The step 2 comprises the following steps: (2.1) Recover the molybdenum salt in the mother liquor 1 to obtain a crude molybdenum-containing compound and a mother liquor 3, and return the mother liquor 3 to step (1.1).

6. The process according to claim 5, characterized in that: In the step (2.1), the method for recovering the crude molybdenum-containing compound is at least one of the following methods: resin adsorption, evaporative crystallization, and acidification crystallization under acidic or alkaline conditions.

7. The process according to claim 5, characterized in that: The step 2 also includes the following steps: (2.2) The crude molybdenum-containing compound is dissolved in an acidic solution for acidification and crystallization to generate molybdic acid crystals and mother liquor IV.

8. The process according to claim 7, characterized in that: The step 2 also includes the following steps: (2.3) dissolving the molybdic acid crystals produced in step (2.2) in an alkaline solution, and then adding an acidic solution again for acidification and crystallization to obtain refined molybdic acid.

9. The process according to claim 8, characterized in that: The step 2 also includes the following steps: (2.4) reacting the molybdic acid crystals produced in step (2.2) or the refined molybdic acid produced in step (2.3) with an alkaline substance to obtain the desired molybdate substance.

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