Method for stepwise and efficiently purifying key impurity sodium oxalate in sodium aluminate solution by utilizing ultrasonic waves

Abstract

The invention provides a method for stepwise and efficiently purifying a key impurity sodium oxalate in a sodium aluminate solution by utilizing ultrasonic waves, and particularly relates to the technical field of aluminum oxide production. The method provided by the invention comprises the following steps: adding alkali containing Na into a sodium aluminate solution to adjust the concentration of Na2O in the sodium aluminate solution to be 180-260g / L, after the sodium aluminate solution is cooled, adding a seed crystal, controlling the concentration of the seed crystal to be 5-40g / L, and carrying out ultrasonic gradient purification; the step-by-step ultrasonic purification comprises the step of sequentially performing strong ultrasonic assisted crystallization and weak ultrasonic assisted crystallization; the power of the strong ultrasound is 0.1-0.6 W / L of the sodium aluminate solution; the power of the weak ultrasound is 0.01-0.05 W / L of the sodium aluminate solution; the time of the ultrasonic step purification is 10 minutes to 15 hours. The method provided by the invention has the advantages of low caustic alkali loss rate, high sodium oxalate purification rate, reduction of the purification cost, high purification efficiency, simple method and easy realization of industrialization, and the obtained filter residue can be recycled as a seed crystal.

Description

Technical Field

[0001] The invention belongs to the technical field of alumina production, and in particular relates to a method for efficiently purifying sodium oxalate, a key impurity in a sodium aluminate solution, by utilizing ultrasonic steps. Background Art

[0002] In the industrial preparation of alumina, due to the problem of excessively high content of organic matter such as humus in bauxite resources, when the Bayer cycle is running, the macromolecular organic matter in the sodium aluminate solution gradually turns into sodium oxalate and accumulates in a cycle. A large amount of precipitated sodium oxalate accumulates on the surface of aluminum hydroxide, hindering the agglomeration of aluminum hydroxide and the growth of crystals, reducing the activity of decomposition seeds, resulting in reduced decomposition rate, fine particle size of aluminum hydroxide products, excessive sodium content in alumina products, and other problems, resulting in reduced product quality. Therefore, how to effectively solve the removal of sodium oxalate and achieve efficient utilization of bauxite has become a difficult problem that plagues the high-quality development of alumina companies.

[0003] Common methods for removing sodium oxalate include solution combustion, adsorption, seed washing, chemical precipitation and crystallization. The solution combustion method has high energy consumption and production application costs; the adsorption method can only remove certain specific organic matter and reduce the critical concentration of sodium oxalate, but has no removal effect on sodium oxalate itself; the chemical precipitation method usually causes the loss of alumina and is therefore less used. Although the crystallization method has the advantages of low energy consumption, low equipment investment and low production and application costs, it still has problems such as high energy consumption for evaporation and concentration, serious loss of aluminum and alkali, low crystallization efficiency, and difficulty in filtering fine needle-shaped sodium oxalate crystals that need to be solved urgently. Summary of the invention

[0004] In view of this, the object of the present invention is to provide a method for efficiently purifying sodium oxalate, a key impurity in sodium aluminate solution, by using ultrasonic step-by-step. The method provided by the present invention has a low caustic soda loss rate and a high sodium oxalate purification rate. The obtained filter residue can be recycled as a seed crystal, thereby reducing the purification cost. The method is simple and easy to realize industrialization.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] The present invention provides a method for efficiently purifying sodium oxalate, a key impurity in a sodium aluminate solution, by using ultrasonic step-by-step methods, comprising the following steps:

[0007] Adding a sodium-containing alkali to the sodium aluminate solution to adjust the concentration of Na2O in the sodium aluminate solution to 180-260 g / L, adding seed crystals after the sodium aluminate solution is cooled, controlling the concentration of the seed crystals to 5-40 g / L, and performing ultrasonic step purification;

[0008] The ultrasonic step-by-step purification includes sequentially performing strong ultrasonic assisted crystallization and weak ultrasonic assisted crystallization;

[0009] When the strong ultrasound is used to assist crystallization, the power of the strong ultrasound is 0.1 to 0.6 W / L. 铝酸钠溶液 ;

[0010] When the weak ultrasound is used to assist crystallization, the power of the weak ultrasound is 0.01 to 0.05 W / L. 铝酸钠溶液 ;

[0011] The time of the ultrasonic step purification is 10 minutes to 15 hours.

[0012] Preferably, the ultrasonic step purification comprises: firstly performing strong ultrasonic assisted crystallization, and then performing weak ultrasonic assisted crystallization when the concentration of sodium oxalate in the sodium aluminate solution is ≤1 g / L.

[0013] Preferably, the frequencies of the strong ultrasound and the weak ultrasound are independently 20 to 40 kHz.

[0014] Preferably, the seed crystals include sodium oxalate and / or aluminum hydroxide.

[0015] Preferably, after the ultrasonic step purification, the method further comprises: performing solid-liquid separation on the obtained purified liquid to obtain a filter cake and a purified liquid.

[0016] Preferably, the purified liquid is used for preparing alumina.

[0017] Preferably, the filter cake is used directly as a seed and / or as a Na-containing alkali for purifying the sodium aluminate solution after lime causticization;

[0018] The filter cake includes sodium oxalate, sodium carbonate and sodium hydroxide.

[0019] Preferably, the Na-containing alkali comprises sodium hydroxide and / or sodium carbonate.

[0020] Preferably, the temperature after cooling is 30-70°C.

[0021] Preferably, the concentration of sodium oxalate in the sodium aluminate solution is >1 g / L.

[0022] The invention provides a method for efficiently purifying sodium oxalate, a key impurity in a sodium aluminate solution, by using ultrasonic step-by-step purification. The method comprises the following steps: adding an alkali containing Na to the sodium aluminate solution to adjust the concentration of Na2O in the sodium aluminate solution to 180-260 g / L, adding crystal seeds after the sodium aluminate solution is cooled, controlling the concentration of the crystal seeds to 5-40 g / L, and performing ultrasonic step-by-step purification; the ultrasonic step-by-step purification comprises sequentially performing strong ultrasonic assisted crystallization and weak ultrasonic assisted crystallization; during the strong ultrasonic assisted crystallization, the power of the strong ultrasound is 0.1-0.6 W / L 铝酸钠溶液 When the weak ultrasound is used to assist crystallization, the power of the weak ultrasound is 0.01 to 0.05 W / L. 铝酸钠溶液; The time of ultrasonic step purification is 10min to 15h. The present invention adjusts the concentration of alkali containing Na to increase the caustic ratio (the molar ratio of sodium oxide and aluminum oxide in the sodium aluminate solution), so that the equilibrium concentration of sodium oxalate in the sodium aluminate solution is reduced, which is beneficial to the crystallization of sodium oxalate; ultrasonic step purification of sodium oxalate is used to greatly improve the crystallization reaction environment, significantly reduce the crystallization metastable zone, effectively strengthen the crystallization process, and purify the sodium aluminate solution; by controlling the concentration of Na2O, the concentration of crystal seeds, the power of strong ultrasound, the power of weak ultrasound, the ultrasound time and the cooling temperature, the synergistic effect is achieved to increase the crystallization rate of sodium oxalate, reduce energy consumption, reduce the loss of aluminum and alkali, and make impurities easy to separate.

[0023] The invention timely adjusts the ultrasonic time of each stage according to the content of sodium oxalate, so as to control the properties such as the morphology and particle size of sodium oxalate, and avoids the problems of too fine sodium oxalate crystal particles and difficulty in filtering caused by long-term high-power ultrasonic irradiation. The obtained by-product filter cake can be reused in the circulation process, and no "three wastes" are generated, which is green and environmentally friendly. The provided method is simple to operate, has low energy consumption, and is suitable for industrial production.

[0024] The results of the examples show that the method provided by the present invention has a sodium oxalate purification rate of 63-82% and a caustic soda loss rate caused by crystallization of 10-17%, which is superior to the single ultrasonic method and the traditional crystallization method under the same conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0026] Figure 1 This is a schematic diagram of the process of efficiently purifying sodium oxalate, a key impurity in a sodium aluminate solution, by using ultrasonic step-by-step methods according to the present invention;

[0027] Figure 2 The SEM images and mass percentages of each element of the sodium oxalate obtained in Comparative Example 1 and Example 1 are shown;

[0028] Figure 3 The XRD diagrams of sodium oxalate obtained in Comparative Example 1 and Example 1 are shown. DETAILED DESCRIPTION

[0029] The present invention provides a method for efficiently purifying sodium oxalate, a key impurity in a sodium aluminate solution, by using ultrasonic step-by-step methods, comprising the following steps:

[0030] Adding a sodium-containing alkali to the sodium aluminate solution to adjust the concentration of Na2O in the sodium aluminate solution to 180-260 g / L, adding seed crystals after the sodium aluminate solution is cooled, controlling the concentration of the seed crystals to 5-40 g / L, and performing ultrasonic step purification;

[0031] The ultrasonic step-by-step purification includes sequentially performing strong ultrasonic assisted crystallization and weak ultrasonic assisted crystallization;

[0032] When the strong ultrasound is used to assist crystallization, the power of the strong ultrasound is 0.1 to 0.6 W / L. 铝酸钠溶液 ;

[0033] When the weak ultrasound is used to assist crystallization, the power of the weak ultrasound is 0.01 to 0.05 W / L. 铝酸钠溶液 ;

[0034] The time of the ultrasonic step purification is 10 minutes to 15 hours.

[0035] In the present invention, unless otherwise specified, the raw materials and equipment used are commercially available products well known in the art.

[0036] In the present invention, the sodium aluminate solution is preferably derived from the product of the aluminum ore dissolution stage, and its main components are preferably sodium hydroxide, sodium aluminate, and sodium oxalate.

[0037] In the present invention, the concentration of sodium oxalate in the sodium aluminate solution is preferably >1 g / L; in a specific embodiment, the concentration of sodium oxalate in the sodium aluminate solution may be 1.5 g / L, 2 g / L, 3 g / L, 4 g / L, 5 g / L, 7 g / L, 9 g / L or 10 g / L.

[0038] In the present invention, the alkali containing Na preferably includes sodium hydroxide and / or sodium carbonate. In the present invention, the concentration of Na2O in the sodium aluminate solution is 180-260 g / L. In a specific embodiment, the concentration of Na2O in the sodium aluminate solution can be 180 g / L, 190 g / L, 200 g / L, 210 g / L, 220 g / L, 230 g / L, 240 g / L, 250 g / L or 260 g / L. Based on the common ion effect, the present invention increases the concentration of Na2O in the sodium aluminate solution to the above range, increases the caustic ratio, and reduces the concentration of oxalate ions under equilibrium conditions, thereby reducing the content of sodium oxalate in the solution. When the amount of Na-containing alkali added is too small, the crystallization rate of sodium oxalate will be slowed down and the ultrasonic time will be prolonged. When the amount of Na-containing alkali added is too large, the viscosity, tension and other solution properties of the sodium aluminate solution will be greatly changed, which will significantly weaken the effect of ultrasound and increase energy consumption. At the same time, excessive Na2O concentration will also cause aluminum, caustic soda (sodium hydroxide and sodium carbonate), and sodium oxalate in the sodium aluminate solution to crystallize together during the crystallization process, resulting in great aluminum and caustic soda losses.

[0039] In the present invention, the temperature of the sodium aluminate solution is preferably ≥ 80° C., and the temperature of the sodium aluminate solution after cooling is preferably 30-70° C. In specific implementation operations, the temperature after cooling can be 30° C., 40° C., 50° C., 60° C. or 70° C. Since the sodium aluminate solution is directly produced from the factory and has a high temperature, the solubility of sodium oxalate is high at this time and it is not easy to crystallize. When the temperature is lowered, the precipitation of sodium oxalate is increased, which is convenient for crystallization.

[0040] In the present invention, the seed crystal preferably includes sodium oxalate and / or aluminum hydroxide. In the present invention, the concentration of the seed crystal is 5 to 40 g / L. In the specific implementation operation, the concentration of the seed crystal can be 5 g / L, 10 g / L, 20 g / L, 30 g / L or 40 g / L. Adding seed crystals can shorten the induction period of the crystallization process, break the activation energy barrier that hinders the formation of crystalline products, and significantly increase the nucleation rate of the reaction system. The addition of seed crystals will also reduce the supersaturation of the reaction system to a certain extent, thereby affecting the metastable zone width of the crystallization reaction. Since the seed crystals are highly active in the sodium aluminate solution, adding seed crystals induces the crystallization of sodium oxalate, thereby reducing the equilibrium concentration of sodium oxalate. However, too high a seed crystal concentration will also cause the viscosity, tension and other solution properties of the sodium aluminate solution to change significantly, weakening the effect of ultrasound, and too high a seed crystal concentration will not significantly increase the crystallization rate, but increase the cost.

[0041] In the present invention, the ultrasonic step-by-step purification of sodium oxalate includes sequentially performing strong ultrasonic assisted crystallization and weak ultrasonic assisted crystallization. In the present invention, during the strong ultrasonic assisted crystallization, the power of the strong ultrasound is 0.1 to 0.6 W / L. 铝酸钠溶液 In the specific implementation operation, the power of the strong ultrasound can be 0.1W / L 铝酸钠溶液 , 0.2W / L 铝酸钠溶液 , 0.3W / L 铝酸钠溶液 、0.4W / L 铝酸钠溶液 , 0.5W / L 铝酸钠溶液 or 0.6W / L 铝酸钠溶液 The frequency of the strong ultrasound is preferably 20 to 40 kHz. In the specific implementation, the frequency of the strong ultrasound can be 20 kHz, 30 kHz or 40 kHz. In the present invention, when the weak ultrasound assists crystallization, the power of the weak ultrasound is 0.01 to 0.05 W / L 铝酸钠溶液 In the specific implementation operation, the power of the weak ultrasound can be 0.01W / L 铝酸钠溶液 , 0.02W / L 铝酸钠溶液 , 0.03W / L 铝酸钠溶液 , 0.04W / L 铝酸钠溶液 or 0.05W / L 铝酸钠溶液The frequency of the weak ultrasound is preferably 20 to 40 kHz. In a specific implementation, the frequency of the weak ultrasound can be 20 kHz, 30 kHz or 40 kHz.

[0042] In the present invention, the ultrasonic step-by-step purification of sodium oxalate preferably includes: first performing strong ultrasound-assisted crystallization, and then performing weak ultrasound-assisted crystallization when the concentration of sodium oxalate in the sodium aluminate solution is ≤1g / L. The present invention utilizes two stages of ultrasonic power with different intensities for ultrasound, which can effectively promote the crystallization and precipitation of sodium oxalate molecules and thoroughly purify the sodium aluminate solution. A single long-term strong ultrasound will cause the sodium oxalate to be too fine and difficult to filter, and long-term high-power ultrasound will increase the cost; a single long-term weak ultrasound will have a poor purification effect. The present invention adjusts the ultrasonic power in time according to the sodium oxalate content, which is convenient for controlling the properties such as the morphology and particle size of sodium oxalate, and avoiding long-term high-power ultrasonic irradiation resulting in the sodium oxalate crystals being too fine and difficult to filter.

[0043] In the present invention, the time for ultrasonic step-by-step purification of sodium oxalate is 10 minutes to 15 hours. In the specific implementation operation, the time for ultrasonic step-by-step purification of sodium oxalate can be 1 hour, 5 hours, 10 hours or 15 hours.

[0044] In the present invention, after the ultrasonic step-by-step purification of sodium oxalate, the method preferably further comprises: performing solid-liquid separation on the obtained purified liquid to obtain a filter cake and a purified liquid.

[0045] The present invention has no special requirements on the method of solid-liquid separation, and can adopt technical means well known in the art, such as filtration, centrifugation, etc.

[0046] In the present invention, the purified liquid is preferably used for preparing alumina.

[0047] In the present invention, the filter cake preferably comprises sodium oxalate, sodium carbonate and sodium hydroxide, and the filter cake is preferably used directly as a seed crystal and / or as a Na-containing alkali for purifying the sodium aluminate solution after being causticized with lime.

[0048] In the present invention, the step of lime causticizing the filter cake preferably comprises: mixing the filter cake, calcium oxide and water, separating the solid from the liquid, and drying to obtain sodium hydroxide.

[0049] The present invention has no special requirements on the drying method, and can adopt technical means well known in the art, such as drying.

[0050] The method provided in the present application solves the problems existing in the current crystallization process such as high energy consumption of evaporation and concentration, serious loss of aluminum and alkali, low crystallization efficiency, and difficulty in filtering fine needle-shaped sodium oxalate crystals through ultrasonic external field enhancement, common ion effect and seed addition strategy. The method is simple to operate, has lower energy consumption and does not produce "three wastes". It can efficiently purify sodium oxalate, a key impurity in sodium aluminate solution, reduce the loss of caustic soda caused by sodium oxalate filter cake, and promote the circulation of caustic soda in alumina production.

[0051] The principle of the present invention can be analyzed from the following three aspects:

[0052] (1) Power ultrasonic technology, which is known as "industrial MSG" by the American industry, is a high-tech technology based on physics, mechanical vibration, materials and other disciplines. When ultrasound acts on a solution, it will produce ① a perturbation effect, that is, the action of ultrasound causes the molecules and ions in the solution to deviate from their original state, increase the probability of collision, and have a certain impact on the dynamic balance of the solution; ② a turbulent effect, that is, the disturbance caused by the cavitation bubbles generated by ultrasound to the solution after they break, accelerating the irregular movement and collision of particles in the solution; ③ an interface effect, that is, the internal changes of the solution caused by the gathering of bubbles in the solution can destroy the outer layer of mineral particles, form cavities, accelerate the migration and transformation of substances, and increase the metal leaching rate; ④ an energy-gathering effect, that is, a high-pressure impact flow that can break the intermolecular force is generated in the solution, changing the metastable state generated by hydrogen bonds, reducing the connection of ions in the system, and accelerating the mass transfer process. In particular, the cavitation effect generated by ultrasound forms a local high-temperature and high-pressure environment in the reaction system, which accelerates the reaction rate. The solution is strongly agitated, significantly reducing the diffusion resistance of the solute, and improving the mass transfer efficiency. Therefore, the physical and chemical effects of ultrasound can change the physical and chemical properties of the treated medium, accelerate and strengthen the reaction process. High-power ultrasound can improve chemical reaction conditions, accelerate chemical reaction rate, increase sodium oxalate crystallization rate, and reduce caustic soda loss in the process of step-by-step efficient purification of key impurity sodium oxalate in sodium aluminate solution, thereby achieving efficient purification. At the same time, the fine needle-shaped sodium oxalate obtained without ultrasonic crystallization is difficult to filter, which seriously reduces the circulation efficiency in alumina production. The application of step-by-step ultrasound helps sodium oxalate crystals transform from fine needles to flakes and blocks, which can significantly improve the above harmful behaviors.

[0053] (2) Based on the common ion effect, the concentration product constant of sodium oxalate Ksp = [Na + ] 2 ·[C2O4 2- ], which is constant at the same temperature. The higher the caustic ratio, the lower the equilibrium concentration of sodium oxalate in the sodium aluminate solution. When the Na2O concentration of the sodium aluminate solution is increased, the concentration of oxalate ions under equilibrium conditions will decrease, and the sodium oxalate content in the solution will be less.

[0054] (3) Adding seed crystals can shorten the induction period of the crystallization process, break the activation energy barrier that hinders the formation of crystalline products, and significantly increase the nucleation rate of the reaction system. The addition of seed crystals will also reduce the supersaturation of the reaction system to a certain extent, thereby affecting the width of the metastable zone of the crystallization reaction. Since seed crystals are highly active in sodium aluminate solution, they can induce the crystallization of sodium oxalate, thereby reducing the equilibrium concentration of sodium oxalate.

[0055] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described in conjunction with specific embodiments below. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Any modification, equivalent replacement, improvement, etc. made to the implementation methods of the present invention based on the technical essence and general principles of the present invention without creative work should be within the scope of protection of the present invention.

[0056] The process of efficiently purifying the key impurity sodium oxalate in sodium aluminate solution by ultrasonic cascade, such as Figure 1 shown.

[0057] Example 1

[0058] Sodium hydroxide or sodium carbonate is added to a sodium aluminate solution with a sodium oxalate impurity content of 2.31 g / L to adjust the concentration of Na2O in the sodium aluminate solution to 210 g / L. When the sodium aluminate solution is cooled to 40°C, seed crystals are added to control the seed crystal concentration to 20 g / L. The sodium oxalate is purified by ultrasonic step-wise purification, and strong ultrasonic field-assisted crystallization is first performed at a frequency of 28 kHz and a power of 0.5 W / L. 铝酸钠溶液 When the sodium oxalate concentration in the sodium aluminate solution is ≤1g / L, weak ultrasonic field assisted crystallization is performed with a frequency of 28kHz and a power of 0.03W / L. 铝酸钠溶液 The ultrasonic step purification time is 5 hours. After solid-liquid separation, filter cake and purified liquid are obtained. The filter cake is circulated in the crystallization process as a seed, or sodium hydroxide is produced by lime causticization and circulated in the crystallization process as an alkali; the purified liquid obtained is returned to the dissolution mother liquor tank for the preparation of alumina.

[0059] Comparative Example 1

[0060] Add sodium hydroxide or sodium carbonate to a sodium aluminate solution with a sodium oxalate impurity content of 2.31 g / L to adjust the concentration of Na2O in the sodium aluminate solution to 210 g / L. When the sodium aluminate solution is cooled to 40°C, add seed crystals to control the seed concentration to 20 g / L, continue stirring for 5 hours, and obtain filter cake and purified liquid through solid-liquid separation.

[0061] Comparative Example 2

[0062] Sodium hydroxide or sodium carbonate was added to the sodium aluminate solution with a sodium oxalate impurity content of 2.31 g / L to adjust the concentration of Na2O in the sodium aluminate solution to 210 g / L. When the sodium aluminate solution was cooled to 40°C, seed crystals were added to control the seed concentration to 20 g / L. Strong ultrasonic field assisted crystallization was performed with a frequency of 28 kHz and a power of 0.5 W / L. 铝酸钠溶液 , the ultrasonic time is 5h, and after solid-liquid separation, filter cake and purified liquid are obtained.

[0063] Comparative Example 3

[0064] Sodium hydroxide or sodium carbonate was added to the sodium aluminate solution with a sodium oxalate impurity content of 2.31 g / L to adjust the concentration of Na2O in the sodium aluminate solution to 210 g / L. When the sodium aluminate solution was cooled to 40°C, seed crystals were added to control the seed concentration to 20 g / L. Weak ultrasonic field assisted crystallization was performed with a frequency of 28 kHz and a power of 0.03 W / L. 铝酸钠溶液 , the ultrasonic time is 5h, and after solid-liquid separation, filter cake and purified liquid are obtained.

[0065] The sodium oxalate purification rate and the caustic soda loss rate caused by crystallization in the filtrate obtained in Example 1 and Comparative Examples 1 to 3 were calculated using Formula 1 and Formula 2 to obtain Table 1.

[0066] Sodium oxalate purification rate = (sodium oxalate content in sodium aluminate solution - sodium oxalate content in purified solution) / sodium oxalate content in sodium aluminate solution × 100% Formula 1;

[0067] Caustic soda loss rate = mass of sodium carbonate and sodium hydroxide in filter cake / mass of filter cake × 100% Formula 2.

[0068] Table 1 Comparison of sodium oxalate purification rate and caustic soda loss rate in Example 1 and Comparative Examples 1 to 3

[0069] Sodium oxalate purification rate Caustic soda loss rate due to crystallization Example 1 74.7％ 11.1％ Comparative Example 1 61.9％ 28.5％ Comparative Example 2 68.1％ 14.7％ Comparative Example 3 64.4％ 17.8％

[0070] It can be seen from Table 1 that compared with conventional conditions and ultrasonic continuous purification methods, the efficiency of ultrasonic step-by-step purification of sodium oxalate is significantly improved, and the caustic soda loss rate is lower.

[0071] The crystallization habit of the crystal is mainly determined by the chemical composition and structure of the crystal, and is also closely related to external conditions (such as temperature, pressure, concentration, viscosity, impurities). The SEM images and mass percentages of each element of the sodium oxalate obtained in Comparative Example 1 and Example 1 were compared and analyzed. Figure 2 As shown. The sodium oxalate crystallized without ultrasound is a fine needle-shaped crystal, indicating that its axial growth rate is greater than its radial growth rate, which may be related to the presence of organic matter such as sodium humate in the sodium aluminate solution. These organic matter will reduce the surface activity of sodium oxalate crystals and inhibit their growth.

[0072] After applying step-by-step ultrasonic crystallization, the fine needle-like sodium oxalate morphology becomes less and less, and the flake or block morphology becomes the most important manifestation. The mass percentage of each element shows that the precipitated sodium oxalate crystals contain multiple elements such as Na, O, C, Al, K and Cl. With the application of step-by-step ultrasonic crystallization, the C content in the precipitated product increased significantly and the Al and K contents decreased significantly, indicating that the step-by-step ultrasonic crystallization significantly increased the sodium oxalate content in the product, while significantly reducing the loss of multiple valuable elements such as Al and K.

[0073] XRD was used to analyze the phase structure changes of sodium oxalate during the stepwise ultrasonic crystallization and the conventional crystallization. Figure 3 As shown. After no ultrasonic crystallization and step-by-step ultrasonic crystallization, three substances, sodium hydroxide, sodium carbonate and sodium oxalate, appeared in the precipitated products, corresponding to JCPDS: 81-1160, 72-0628 and 20-1149, respectively. After applying step-by-step ultrasonic crystallization, the growth direction of the precipitated product changed from preferential orientation growth along the (400) crystal plane to growth along the (-111) crystal plane. Therefore, after applying step-by-step ultrasonic crystallization, the preferential orientation growth mechanism of the crystal can be broken through by inputting ultrasonic energy, ensuring the uniform growth of the crystal in three-dimensional space.

[0074] Example 2

[0075] The time of ultrasonic step purification was controlled to be 10 h, and other conditions were the same as those in Example 1.

[0076] Comparative Example 4

[0077] The ultrasonic time was 10 h, and other conditions were the same as those in Comparative Example 1.

[0078] The sodium oxalate purification rate in the filtrate obtained in Example 2 and Comparative Example 4 and the caustic soda loss rate caused by crystallization were calculated to obtain Table 2.

[0079] Table 2 Comparison of sodium oxalate purification rate and caustic soda loss rate in Example 2 and Comparative Example 4

[0080] Sodium oxalate purification rate Caustic soda loss rate due to crystallization Example 2 78.8％ 10.9％ Comparative Example 4 71.4％ 22.2％

[0081] It can be seen from Table 2 that, compared with conventional conditions, the efficiency of ultrasonic step-by-step purification of sodium oxalate is significantly improved, and the caustic soda loss rate is lower.

[0082] Example 3

[0083] When the cooling temperature is controlled to 60°C, seed crystals are added. Other conditions are the same as those in Example 1.

[0084] Comparative Example 5

[0085] When the cooling temperature is controlled to 60°C, seed crystals are added. Other conditions are the same as those in Comparative Example 1.

[0086] The sodium oxalate purification rate in the filtrate obtained in Example 3 and Comparative Example 5 and the caustic soda loss rate caused by crystallization were calculated to obtain Table 3.

[0087] Table 3 Comparison of sodium oxalate purification rate and caustic soda loss rate in Example 3 and Comparative Example 5

[0088] Sodium oxalate purification rate Caustic soda loss rate due to crystallization Example 3 77.6％ 16.3％ Comparative Example 5 65.4％ 27.4％

[0089] It can be seen from Table 3 that, compared with conventional conditions, the efficiency of ultrasonic step-by-step purification of sodium oxalate is significantly improved, and the caustic soda loss rate is lower.

[0090] Example 4

[0091] The Na2O concentration in the sodium aluminate solution was controlled to be 240 g / L, and the other conditions were the same as those in Example 1.

[0092] Comparative Example 6

[0093] The Na2O concentration in the sodium aluminate solution was controlled to be 240 g / L, and other conditions were the same as those in Comparative Example 1.

[0094] The sodium oxalate purification rate in the filtrate obtained in Example 4 and Comparative Example 6 and the caustic soda loss rate caused by crystallization were calculated to obtain Table 4.

[0095] Table 4 Comparison of sodium oxalate purification rate and caustic soda loss rate in Example 4 and Comparative Example 6

[0096] Sodium oxalate purification rate Caustic soda loss rate due to crystallization Example 4 63.8％ 12.3％ Comparative Example 6 51.2％ 26.7％

[0097] It can be seen from Table 4 that, compared with conventional conditions, the efficiency of ultrasonic step-by-step purification of sodium oxalate is significantly improved, and the caustic soda loss rate is lower.

[0098] Example 5

[0099] The seed concentration was controlled to be 30 g / L, and other conditions were the same as those in Example 1.

[0100] Comparative Example 7

[0101] The seed concentration was controlled to be 30 g / L, and other conditions were the same as those in Comparative Example 1.

[0102] The sodium oxalate purification rate in the filtrate obtained in Example 5 and Comparative Example 7 and the caustic soda loss rate caused by crystallization were calculated to obtain Table 5.

[0103] Table 5 Comparison of sodium oxalate purification rate and caustic soda loss rate in Example 5 and Comparative Example 7

[0104] Sodium oxalate purification rate Caustic soda loss rate due to crystallization Example 5 81.7％ 15.1％ Comparative Example 7 73.4％ 22.6％

[0105] It can be seen from Table 5 that, compared with conventional conditions, the efficiency of ultrasonic step-by-step purification of sodium oxalate is significantly improved, and the caustic soda loss rate is lower.

[0106] Example 6

[0107] The power of strong ultrasonic field assisted crystallization is controlled to 0.1W / L 铝酸钠溶液 The power of weak ultrasonic field assisted crystallization is 0.01W / L 铝酸钠溶液 , other conditions are the same as in Example 1.

[0108] The sodium oxalate purification rate in the filtrate obtained in Example 6 and the caustic soda loss rate caused by crystallization were calculated to obtain Table 6.

[0109] Table 6 Comparison of sodium oxalate purification rate and caustic soda loss rate in Example 6

[0110] Sodium oxalate purification rate Caustic soda loss rate due to crystallization Example 6 69.8％ 13.6％ Comparative Example 1 61.9％ 28.5％

[0111] Through the above experiments, it can be seen that the use of ultrasonic step-by-step purification of sodium oxalate can significantly promote the crystallization effect, reduce the caustic soda content caused by the filter cake, and promote the caustic soda circulation in alumina production; the caustic soda and purified liquid after sintering are more suitable for the alumina production system. This application makes the purification cost of sodium oxalate, a key impurity in sodium aluminate solution, lower, the purification efficiency is higher, and it is easy to achieve industrialization.

[0112] The present invention solves the problems existing in the current crystallization process such as high energy consumption of evaporation and concentration, serious loss of aluminum and caustic soda, low crystallization efficiency, difficulty in filtering fine needle-shaped sodium oxalate crystals, resulting in low circulation efficiency of alumina production, etc. through ultrasonic external field enhancement, common ion effect and seed addition strategy, and provides a feasible purification method for sodium oxalate, a key impurity in sodium aluminate solution.

[0113] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for efficiently purifying sodium oxalate, a key impurity in sodium aluminate solution, using ultrasonic steps, characterized in that: The following steps are involved: Adding a sodium-containing alkali to the sodium aluminate solution to adjust the concentration of Na2O in the sodium aluminate solution to 180-260 g / L, adding seed crystals after the sodium aluminate solution is cooled, controlling the concentration of the seed crystals to 5-40 g / L, and performing ultrasonic step purification; The ultrasonic step-by-step purification includes sequentially performing strong ultrasonic assisted crystallization and weak ultrasonic assisted crystallization; When the strong ultrasound is used to assist crystallization, the power of the strong ultrasound is 0.1 to 0.6 W / L. 铝酸钠溶液 ; When the weak ultrasound is used to assist crystallization, the power of the weak ultrasound is 0.01 to 0.05 W / L. 铝酸钠溶液 ; The time of the ultrasonic step purification is 10 minutes to 15 hours.

2. The method according to claim 1, characterized in that The ultrasonic step-by-step purification comprises: firstly performing strong ultrasonic assisted crystallization, and then performing weak ultrasonic assisted crystallization when the concentration of sodium oxalate in the sodium aluminate solution is ≤1 g / L.

3. The method according to claim 1 or 2, characterized in that: The frequencies of the strong ultrasound and the weak ultrasound are independently 20 to 40 kHz.

4. The method according to claim 1, characterized in that: The seed crystals include sodium oxalate and / or aluminum hydroxide.

5. The method according to claim 1, characterized in that The ultrasonic step purification further comprises: performing solid-liquid separation on the obtained purified liquid to obtain a filter cake and a purified liquid.

6. The method according to claim 5, characterized in that The purified liquid is used for preparing aluminum oxide.

7. The method according to claim 5, characterized in that The filter cake is directly used as a seed crystal and / or as a Na-containing alkali for purifying the sodium aluminate solution after being causticized with lime; The filter cake includes sodium oxalate, sodium carbonate and sodium hydroxide.

8. The method according to claim 1, characterized in that The Na-containing alkali includes sodium hydroxide and / or sodium carbonate.

9. The method according to claim 1, characterized in that: The temperature after cooling is 30-70°C.

10. The method according to claim 1, characterized in that The concentration of sodium oxalate in the sodium aluminate solution is >1 g / L.

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