Method for removing oxalate in sodium aluminate solution

Abstract

The invention provides a method for removing oxalate in a sodium aluminate solution, and relates to the technical field of aluminum oxide production processes. The method for removing oxalate in the sodium aluminate solution is used for treating an intermediate product sodium aluminate solution in a Bayer process production process, and mainly comprises the following steps: S10, adding coarse seeds into the intermediate product sodium aluminate solution for producing aluminum oxide by the Bayer process to perform first-stage decomposition, and cooling to 70-75 DEG C to separate out aluminum hydroxide slurry; s20, cooling the prepared aluminum hydroxide slurry to 40 to 50 DEG C; s30, filtering and separating the cooled aluminum hydroxide slurry to obtain a filter cake and a filtrate, and washing the filter cake with a hot liquid to obtain a washing liquid; s40, carrying out causticization treatment on the obtained washing liquid to prepare calcium oxalate; and roasting the obtained filter cake to prepare aluminum oxide. According to the invention, the operation difficulty and equipment requirements of alumina production can be reduced, and water consumption and power consumption are reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alumina production process, in particular to a method for removing oxalate from sodium aluminate solution. BACKGROUND

[0002] The production of alumina generally adopts the Bayer process. The Bayer process has the characteristics of simple process, high product quality, low production cost and low comprehensive energy consumption. The basic principle of the Bayer process is to dissolve alumina in bauxite at a certain temperature with caustic soda solution to obtain sodium aluminate solution. Then, aluminum hydroxide is added as seed, and the decomposition and crystallization are carried out under the conditions of temperature reduction and stirring to precipitate. The produced aluminum hydroxide is calcined to lose water to become product alumina. The seed mother liquor after decomposition is concentrated by evaporation and used to dissolve a new batch of bauxite.

[0003] In the process of producing alumina by the Bayer process, the sodium aluminate solution as an intermediate product often contains a large amount of oxalate impurities. Excessive oxalate impurities will affect the quality of alumina products. In the prior art, the methods for removing oxalate impurities include evaporation crystallization method, adding seed method, oxalate growth agent method and wet oxidation method. The commonly used method in production at present is to remove oxalate by causticizing treatment. The principle of causticizing treatment is to utilize the characteristics that sodium oxalate is easily soluble in hot water but slightly soluble in caustic soda solution. First, the sodium oxalate adsorbed on the surface of the fine seed is washed to dissolve in the solution. Then, lime milk is added to the washing solution rich in sodium oxalate to perform a causticizing reaction, so that the sodium oxalate is converted into calcium oxalate precipitate. After sedimentation and separation, the calcium oxalate precipitate is discharged from the process, and the caustic soda solution is recovered. The general operation steps of causticizing treatment are as follows: before adding coarse seed to decompose sodium aluminate, fine seed is first added to the sodium aluminate solution to adsorb oxalate impurities; then the fine seed is washed multiple times, and the washing solution is collected and mixed with lime milk to perform a causticizing reaction; after the reaction is completed, sedimentation and separation are performed to obtain calcium oxalate precipitate and recovered caustic soda solution; the calcium oxalate precipitate can be treated as general solid waste, and the recovered caustic soda solution can be returned to the production process for recycling.

[0004] The causticizing treatment method has the advantages of not only effectively removing sodium oxalate but also recycling caustic soda solution, reducing production cost and reducing environmental pollution. However, the existing causticizing treatment method needs to add fine seed, adsorb and filter, and wash multiple times, so the process is relatively complex, the requirements for equipment and operation are high, and it also increases additional water consumption and power consumption, which is not conducive to cost reduction and efficiency improvement. SUMMARY

[0005] The present application provides a method for removing oxalate from sodium aluminate solution, which can simplify operation difficulty and equipment requirement, and reduce water consumption and power consumption.

[0006] The technical scheme adopted by the present application is:

[0007] A method for removing oxalate from sodium aluminate solution, which is used for treating intermediate product sodium aluminate solution in a Bayer process for producing alumina, and comprises the following steps:

[0008] S10. Adding coarse seeds to the intermediate product sodium aluminate solution in the Bayer process for producing alumina to perform a first-stage decomposition, and cooling to 70-75 DEG C to precipitate aluminum hydroxide slurry;

[0009] S20. Cooling the prepared aluminum hydroxide slurry to 40-50 DEG C;

[0010] S30. Filtering and separating the cooled aluminum hydroxide slurry to prepare filter cake and filtrate, and washing the filter cake with hot liquid to obtain washing liquid;

[0011] S40. Performing causticization treatment on the obtained washing liquid to prepare calcium oxalate; and calcining the prepared filter cake to prepare alumina.

[0012] Further, the coarse seeds in the S10 step are aluminum hydroxide crystals with a particle size D50 in the range of 80-120 mu m.

[0013] Further, the proportion of crystals with a particle size of 60 mu m or less in the aluminum hydroxide crystals is 10-20%.

[0014] Further, in the S20 step, the aluminum hydroxide slurry is passed into a heat exchanger for cooling treatment; the heat exchanger comprises a wide-channel heat exchanger or a jacketed heat exchanger.

[0015] Further, in the S20 step, the cooling rate of the aluminum hydroxide slurry is greater than 5 DEG C / min / L.

[0016] Further, in the S30 step, a flat disc filter is used for filtering and separating, and at least one filtering and two washing are performed.

[0017] Further, the washing liquid after the first washing is used as secondary washing water for reverse washing.

[0018] Further, in the S30 step, the hot liquid used for washing is hot water, and the temperature of the hot water is 90-95 DEG C.

[0019] Further, the mixture solution containing calcium oxalate in the step S40 is sent to a pressure filtration process, and the pressure filtration obtains solid calcium oxalate and weak alkali liquor, and the weak alkali liquor is returned to the settling tank used in the previous process of the Bayer process for washing the red mud.

[0020] Further, the step S40 adopts lime milk to perform causticization treatment on the washing liquor to form calcium oxalate, and the addition amount of the lime milk reaches a CaO in the lime milk and Na2C2O4 molecule in the original sodium aluminate solution ratio of 1.2-2 or more.

[0021] The beneficial effects of the present application are:

[0022] 1. The present application slows down the precipitation of oxalate by setting the decomposition temperature at 70-75℃ when adding coarse seeds to the sodium aluminate solution for decomposition, so that most of the aluminum hydroxide can be crystallized first, and then the temperature is lowered through a heat exchanger to accelerate the precipitation of oxalate, so that the crystalline oxalate on the outer layer of the aluminum hydroxide crystals is in the state of adhering to the surface of the inner layer, greatly reducing the amount of oxalate crystals between the inner layer crystals of the aluminum hydroxide, and then the surface oxalate crystals can be easily separated by filtration and washing, reducing the residual amount of oxalate in the aluminum hydroxide crystals, and avoiding the fine grain refinement affecting product quality. Finally, calcium oxalate is recovered by causticization, and aluminum hydroxide crystals are obtained by calcination. The whole process cancels the addition of fine seeds for multi-stage separation and decomposition treatment, as well as the multi-stage washing steps, so that the process complexity, operation difficulty, and water and power consumption are reduced, thereby solving the problems of the prior art, such as the need to add fine seeds, adsorption and filtration, and multiple washing steps, complex process, high requirements for equipment and operation, and increased additional water and power consumption. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0024] Fig. 1 The process flow chart of the present application;

[0025] Fig. 2 The product flow chart of the present application. DETAILED DESCRIPTION

[0026] In the description of the present application, it needs to be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, which are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0027] The following disclosure provides many different embodiments or examples for implementing different structures of the present application. For simplicity of the present application's disclosure, the descriptions of the components and settings of specific examples in the following are described. Of course, they are only examples and the purpose is not to limit the present application.

[0028] The embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0029] The existing Bayer method production generally uses caustic treatment method to remove oxalate in sodium aluminate solution, which has the advantages of effectively removing and recycling caustic lye, reducing production cost and reducing environmental pollution. However, the existing caustic treatment method needs to add fine seeds, adsorption and filtration and multiple washing, the process is relatively complex, the requirements for equipment and operation are higher, and it also increases the additional water consumption and power consumption, which is not conducive to cost reduction and efficiency improvement. Moreover, the fine seeds are taken from the crystallized sodium hydroxide crystals, and the recycling of the fine seeds in the process flow causes the enrichment of the system, and the long-term use also increases the residual amount of oxalate in the alumina product.

[0030] In order to solve the above problems in the prior art, the present application provides a method for removing oxalate in sodium aluminate solution, which is used to treat the intermediate product sodium aluminate solution in the production process of Bayer method, reduce the content of oxalate in sodium aluminate solution and ensure the quality of alumina product. The method for removing oxalate in sodium aluminate solution can reduce the operation difficulty and equipment requirement of alumina production, and reduce water consumption and power consumption. Please refer to Figs. 1-2 , the method for removing oxalate in sodium aluminate solution mainly includes the following steps:

[0031] S10. Adding coarse seeds to the intermediate product sodium aluminate solution in the production of Bayer method alumina to carry out a stage of decomposition, and cooling to 70-75℃ to precipitate aluminum hydroxide slurry. The decomposition temperature of 70-75℃ is higher than the temperature of about 50-60℃ in the general caustic treatment method, and the precipitation of oxalate crystal can be effectively inhibited in this temperature range.

[0032] S20. The prepared aluminum hydroxide slurry is passed into a heat exchanger and rapidly cooled to 40-50°C. During the rapid cooling process, the solubility of sodium oxalate in the 40-50°C interval drops by 80% (compared to 70°C). When the cooling rate is >5°C / min / L, the crystalline particle size is <10 μm, which is conducive to surface adsorption. During this process, the oxalate crystals in the solution will precipitate and adsorb on the surface of the aluminum hydroxide, thereby replacing the slow and small-scale cooling process in the prior art, which requires the addition of fine seeds and coarse seeds for multi-stage decomposition. The fine seeds are used to adsorb oxalate crystals and separate oxalate.

[0033] S30. Filtration and separation are performed to obtain a filter cake and a filtrate, and the filter cake is washed with a hot liquid to obtain a washing liquid. Through the filtration and washing steps, the aluminum hydroxide crystals in the aluminum hydroxide slurry are separated, and the oxalate crystals attached to the surface of the aluminum hydroxide crystals are removed.

[0034] S40. The obtained washing liquid is subjected to causticization treatment to obtain calcium oxalate, and the obtained filter cake is calcined to obtain aluminum oxide. This step is used to recover oxalate and to calcine the aluminum hydroxide crystals at high temperature to obtain the desired aluminum oxide product. Since the oxalate impurities in the aluminum hydroxide crystals are effectively removed, the quality of the obtained aluminum oxide product can be guaranteed.

[0035] In the traditional seed decomposition process, aluminum hydroxide is precipitated at the contact between the primary grains, acting as a binder to bond the primary grains together to form a relatively solid agglomerate. However, the oxalate inclusions between the aluminum hydroxide particles hinder the bonding and filling of aluminum hydroxide, making the connection between the agglomerates loose and the gap large, reducing the efficiency of this bonding effect and reducing the product aluminum hydroxide grain size. In addition, the strength of this oxalate-wrapped aluminum hydroxide monomer is also reduced, and during calcination, the oxalate crystals in the aluminum hydroxide decompose at 250°C, causing the particles to break, further reducing the particle size of the product aluminum oxide and increasing the sodium content in the aluminum oxide, reducing product quality. Therefore, the existing sodium aluminate decomposition generally adds fine seeds to remove oxalate to avoid the inclusion of oxalate impurities during the precipitation of aluminum hydroxide crystals in the sodium aluminate decomposition. In the present invention, the method for removing oxalate from the sodium aluminate solution is to set the decomposition temperature to 70-75°C when the coarse seed is added to the sodium aluminate solution for decomposition, slowing down the precipitation of oxalate, allowing most of the aluminum hydroxide to crystallize first, and then accelerating the precipitation of oxalate by heat exchanger cooling, so that the crystalline oxalate on the outer layer of the aluminum hydroxide crystal is in the state of adhering to the inner layer surface, greatly reducing the amount of oxalate crystals between the inner layer crystals of the aluminum hydroxide, and then through filtration and washing, the surface oxalate crystals can be easily separated, reducing the residual amount of oxalate in the aluminum hydroxide crystal, and avoiding the fine grain size affecting product quality. Finally, calcium oxalate is recovered by causticization, and aluminum hydroxide crystals are obtained by calcination. The entire process eliminates the addition of fine seeds for multi-stage separation and decomposition treatment, as well as the multi-stage washing steps, reducing the complexity of the process, the difficulty of operation, and water and electricity consumption, thereby solving the problem of the need to add fine seeds in the causticization treatment method in the prior art, adsorption and filtration, and multiple washing steps, complex process, high requirements for equipment and operation, and additional water and electricity consumption.

[0036] Further, in the S10 step of the present invention, the coarse seed used is a larger particle size aluminum hydroxide crystal, with a particle size mainly distributed around 20-200 μm. Specifically, the proportion of particles with a particle size of 60 μm or less in the coarse seed is 10%-20%, and the D50 (median diameter of particle size distribution) of the coarse seed particles is 80-120 μm.

[0037] Further, in the step S20 of the present application, the heat exchanger used is arranged at the end of the sodium aluminate decomposition tank, and after the sodium hydroxide slurry is precipitated by the decomposition reaction of sodium aluminate, the oxalate can be precipitated by rapid cooling immediately, thereby improving the production efficiency and saving equipment space. In addition, the heat exchanger used can be a wide channel heat exchanger or a jacketed heat exchanger, etc. The jacketed heat exchanger is composed of two straight pipes with different diameters, which are sleeved together to form a concentric sleeve. Each section of the sleeve is called a pass, and the number of passes can be increased or decreased according to the required heat transfer area. When heat exchange occurs, one fluid flows through the inner pipe, and the other fluid flows through the annular gap, and the heat is transferred through the inner pipe wall. Generally, the hot fluid is introduced from the upper part, and the cold fluid is introduced from the lower part. The two ends of the outer pipe in the sleeve are welded or flanged connected with the inner pipe, and the inner pipe is usually flange connected with the U-shaped elbow pipe, which is convenient for cleaning and increasing or decreasing the heat transfer pipe. The jacketed heat exchanger can quickly reduce the temperature of the sodium hydroxide slurry to the required temperature range for the reaction, improve the production efficiency, and at the same time ensure the quality and stability of the medicine. At the same time, the wide channel heat exchanger is usually composed of multiple plates with special corrugated shapes, and the plates form wide channels between them. The cold and hot fluids flow alternately in the channels and exchange heat through the plates. The unique "S" type fluid channel design makes the fluid flow in a turbulent state in the channel, improving the heat exchange efficiency. At the same time, the wide channel design is not easy to block, and it is suitable for handling fluids containing fibers, particles and high viscosity. The sodium hydroxide slurry has high solid content and large particles. The wide channel heat exchanger can effectively solve the problems of blockage and stagnation in the heat exchanger, ensure good passability, and realize rapid cooling.

[0038] Further, in the step S30 of the present application, the filter equipment used is a flat disc filter. The flat disc filter is mainly composed of a flat disc assembly, a driving device, a feeding device, a washing device, a spiral unloading device, a distribution head, a vacuum system, etc. The flat disc assembly is a circular material disc, which is divided into multiple sectors by a pressing strip according to different specifications, and filter cloth is laid on each sector. When working, the driving device drives the flat disc to rotate, and the filter chamber passes through the cloth area, the filtering area, the washing area and the unloading area in turn. The vacuum system extracts the filtrate through the distribution head, and the solid particles are trapped on the filter cloth to form filter cake. The filter cake can be washed and finally discharged through the spiral unloading device. The flat disc filter can perform efficient solid-liquid separation through the design of vacuum suction and horizontally rotating flat disc, and has good filtering effect and low moisture content of filter cake. In addition, multiple washing areas can be arranged on the flat disc filter, so that the filter cake can be washed multiple times, improving the purity and quality of the product and reducing the water consumption. Specifically, in the step S30, the rapidly cooled aluminum hydroxide slurry is filtered and separated once and washed twice, so as to ensure that the oxalate impurity crystals attached to the surface of the aluminum hydroxide crystals are effectively separated. Preferably, the hot liquid used for the first washing is hot water, and the temperature of the hot water is 90-95℃. After the first washing, the washing liquid is used as the second washing water for reverse washing, so as to reduce the amount of hot water for washing and increase the washing effect of oxalate.

[0039] Further, in the step S40 of the present application, lime milk is used to perform caustic treatment on the washing liquid to form calcium oxalate, the mixture solution containing calcium oxalate is sent to a filter pressing process, and solid calcium oxalate and weak alkali liquid are obtained by filter pressing, the weak alkali liquid is recycled and returned to the settling tank used in the previous process of the Bayer process for washing red mud. Moreover, when the lime milk is added, the caustic treatment is performed with the CaO in the lime milk and the Na2C2O4 molecules in the original sodium aluminate solution in a ratio of 1.2 or above to ensure that the oxalate in the washing liquid is basically completely recovered. Meanwhile, the roasting of the aluminum hydroxide filter cake uses a circulating roaster, which is composed of a roaster and a cyclone and a U-shaped sealing tank directly connected to the roaster. The heat energy required in the roasting process of the circulating roaster is generated by direct combustion of fuel in the roaster, and the high-temperature flue gas generated by the combustion is both a heat carrier and a boiling medium. The aluminum hydroxide is in contact with the hot flue gas in the roaster and is rapidly heated to form alumina through dehydration. The hot flue gas and alumina particles enter the cyclone for gas-solid separation, and the separated alumina is collected as a product, while the hot flue gas is recycled back to the roaster to continue the heating process. In one or more other embodiments, a rotary kiln or a flash roaster or other equipment can also be used.

[0040] Comparative Example 1

[0041] Take 10 m³ of sodium aluminate solution, which is an intermediate product of the Bayer process for producing alumina, and add fine seed crystals of sodium hydroxide with a D50 of 10-20 μm to perform one-stage decomposition. The fine seed crystals are washed by a classification cyclone to obtain a first washing liquid. Coarse seed crystals with a D50 of 90-110 μm are added to the aluminum hydroxide slurry after the treatment of the fine seed crystals to perform two-stage decomposition, and the temperature is lowered to 55°C to precipitate the aluminum hydroxide slurry. After the temperature is lowered, a flat disc filter is used to separate the filter cake and the filtrate, and the filter cake is washed with 90°C hot water for three times to obtain a second washing liquid. The first washing liquid and the second washing liquid are mixed, lime milk (CaO concentration of 120 g / L) is added to the obtained mixed washing liquid, and caustic treatment is performed with the CaO in the lime milk and the Na2C2O4 molecules in the original sodium aluminate solution in a ratio of 1.2 or above to obtain calcium oxalate, which is recovered and dried at 80°C for 2 hours.

[0042] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 5.7 g / L, and the solid mass of the recovered calcium oxalate after drying is 49.7 Kg, and the removal and recovery rate of oxalate is 91.18%.

[0043] Comparative Example 2

[0044] Take the intermediate product sodium aluminate solution 10 m³ of Bayer process for producing alumina, add D50 of 90-110 μm sodium hydroxide crystal coarse seed for one-stage decomposition, and precipitate aluminum hydroxide slurry at 60℃; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, and rapidly cool to 45℃ at a cooling rate of 6℃ / min / L; after cooling, filter separation is carried out using a flat disc filter to obtain filter cake and filtrate, and the filter cake is washed twice using 90℃ hot water to obtain a washing liquid; add lime milk (CaO concentration 120 g / L) to the obtained washing liquid, and perform causticization treatment to achieve a CaO in the lime milk and Na2C2O4 in the original sodium aluminate solution at a molecular ratio of 1.2 or more, to obtain calcium oxalate, which is recovered and dried at 80℃ for 2 h.

[0045] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 6.2 g / L, and the solid mass of the recovered calcium oxalate after drying is 38.8 Kg, and the removal and recovery rate of oxalate is 65.44%, which does not reach the quality requirement of a recovery rate of 80% or more, and has a large difference from the removal and recovery rate of the multi-stage decomposition by adding fine seeds and coarse seeds in Comparative Example 1.

[0046] Comparative Example 3

[0047] Take the intermediate product sodium aluminate solution 10 m³ of Bayer process for producing alumina, add D50 of 90-110 μm sodium hydroxide crystal coarse seed for one-stage decomposition, and precipitate aluminum hydroxide slurry at 70℃; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, and rapidly cool to 55℃ at a cooling rate of 5℃ / min / L; after cooling, filter separation is carried out using a flat disc filter to obtain filter cake and filtrate, and the filter cake is washed twice using 90℃ hot water to obtain a washing liquid; add lime milk (CaO concentration 120 g / L) to the obtained washing liquid, and perform causticization treatment to achieve a CaO in the lime milk and Na2C2O4 in the original sodium aluminate solution at a molecular ratio of 1.2 or more, to obtain calcium oxalate, which is recovered and dried at 80℃ for 2 h.

[0048] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 5.3 g / L, and the solid mass of the recovered calcium oxalate after drying is 34.9 Kg, and the removal and recovery rate of oxalate is 68.86%, which does not reach the quality requirement of a recovery rate of 80% or more, and has a large difference from the removal and recovery rate of the multi-stage decomposition by adding fine seeds and coarse seeds in Comparative Example 1.

[0049] Example 1

[0050] Take the intermediate product sodium aluminate solution 10 m³ of Bayer process for producing alumina, add D50 of 90-110 μm sodium hydroxide crystal coarse seed for one-stage decomposition, and precipitate aluminum hydroxide slurry at 70℃; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, and rapidly cool to 45℃ at a cooling rate of 6℃ / min / L; after cooling, filter separation is carried out using a flat disc filter to obtain filter cake and filtrate, and the filter cake is washed twice using 90℃ hot water to obtain a washing liquid; add lime milk (CaO concentration 120 g / L) to the obtained washing liquid, and perform causticization treatment when the CaO in the lime milk and Na2C2O4 in the original sodium aluminate solution are in a molecular ratio of 1.2 or more to obtain calcium oxalate, which is recovered and dried at 80℃ for 2 h.

[0051] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 5.8 g / L, the solid mass of the recovered calcium oxalate after drying is 50.3 Kg in 10 m³ of the original sodium aluminate solution, the removal and recovery rate of oxalate is 90.69%, which meets the quality requirement of a recovery rate of 80% or more, and is close to the removal and recovery rate of the multi-stage decomposition by adding fine seeds and coarse seeds in Comparative Example 1.

[0052] Example 2

[0053] Take the intermediate product sodium aluminate solution 10 m³ of Bayer process for producing alumina, add D50 of 90-110 μm sodium hydroxide crystal coarse seed for one-stage decomposition, and precipitate aluminum hydroxide slurry at 72℃; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, and rapidly cool to 40℃ at a cooling rate of 6℃ / min / L; after cooling, filter separation is carried out using a flat disc filter to obtain filter cake and filtrate, and the filter cake is washed twice using 95℃ hot water to obtain a washing liquid; add lime milk (CaO concentration 120 g / L) to the obtained washing liquid, and perform causticization treatment when the CaO in the lime milk and Na2C2O4 in the original sodium aluminate solution are in a molecular ratio of 1.2 or more to obtain calcium oxalate, which is recovered and dried at 80℃ for 2 h.

[0054] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 6.6 g / L, the solid mass of the recovered calcium oxalate after drying is 57.1 Kg in 10 m³ of the original sodium aluminate solution, the removal and recovery rate of oxalate is 90.47%, which meets the quality requirement of a recovery rate of 80% or more, and is close to the removal and recovery rate of the multi-stage decomposition by adding fine seeds and coarse seeds in Comparative Example 1.

[0055] Example 3

[0056] Take the intermediate product sodium aluminate solution 10 m³ of Bayer process for producing alumina, add D50 of 90-110 μm sodium hydroxide crystal coarse seed for one-stage decomposition, and precipitate aluminum hydroxide slurry at 72℃; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, and rapidly cool to 50℃ at a cooling rate of 5℃ / min / L; after cooling, filter separation is carried out using a flat disc filter to obtain filter cake and filtrate, and the filter cake is washed twice using 95℃ hot water to obtain a washing liquid; add lime milk (CaO concentration 120 g / L) to the obtained washing liquid, and perform causticization treatment to achieve a CaO in the lime milk and Na2C2O4 in the original sodium aluminate solution at a molecular ratio of 1.5 or more, to obtain calcium oxalate, which is recovered and dried at 80℃ for 2 h.

[0057] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 7.5 g / L, and the solid mass of the recovered calcium oxalate after drying is 60.2 Kg in 10 m³ of the original sodium aluminate solution, and the removal and recovery rate of oxalate is 83.94%, reaching the quality requirement of a recovery rate of 80% or more, which is close to the removal and recovery rate of the multi-stage decomposition by adding fine seeds and coarse seeds in Comparative Example 1.

[0058] Example 4

[0059] Take the intermediate product sodium aluminate solution 10 m³ of Bayer process for producing alumina, add D50 of 90-110 μm sodium hydroxide crystal coarse seed for one-stage decomposition, and precipitate aluminum hydroxide slurry at 72℃; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, and rapidly cool to 50℃ at a cooling rate of 5℃ / min / L; after cooling, filter separation is carried out using a flat disc filter to obtain filter cake and filtrate, and the filter cake is washed twice using 95℃ hot water to obtain a washing liquid; add lime milk (CaO concentration 120 g / L) to the obtained washing liquid, and perform causticization treatment to achieve a CaO in the lime milk and Na2C2O4 in the original sodium aluminate solution at a molecular ratio of 1.5 or more, to obtain calcium oxalate, which is recovered and dried at 80℃ for 2 h.

[0060] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 7.5 g / L, and the solid mass of the recovered calcium oxalate after drying is 60.2 Kg in 10 m³ of the original sodium aluminate solution, and the removal and recovery rate of oxalate is 83.94%, reaching the quality requirement of a recovery rate of 80% or more, which is close to the removal and recovery rate of the multi-stage decomposition by adding fine seeds and coarse seeds in Comparative Example 1.

[0061] Example 5

[0062] Take 10m³ of sodium aluminate solution which is an intermediate product of Bayer process for producing alumina, add coarse seed of sodium hydroxide crystal with D50 of 90-110μm to carry out one-stage decomposition, and reduce temperature to 75℃ to precipitate aluminum hydroxide slurry; pass the prepared aluminum hydroxide slurry into a jacketed heat exchanger, reduce temperature at a speed of 6℃ / min / L to 40℃; after temperature reduction, use a flat disc filter to carry out filtration separation to obtain filter cake and filtrate, and use 90℃ hot water to wash the filter cake twice to obtain washing liquid; add lime milk (CaO concentration of 120g / L) to the obtained washing liquid, and carry out causticization treatment to achieve a CaO to Na2C2O4 molecular ratio of 2 or more in the lime milk and the original sodium aluminate solution, to obtain calcium oxalate, and recover and dry at 80℃ for 2h.

[0063] The mass concentration of sodium oxalate is determined by Dionex ICS-90 ion chromatograph. The results show that the mass concentration of sodium oxalate (Na2C2O4) in the original sodium aluminate solution is 5.8g / L, and the solid mass of the recovered calcium oxalate after drying is 50.5Kg in 10m³ of the original sodium aluminate solution, and the removal and recovery rate of oxalate is 91.05%, reaching the quality requirement of more than 80% recovery rate, and being close to the removal and recovery rate of the multi-stage decomposition by adding fine seed and coarse seed in Comparative Example 1.

[0064] The above shows and describes the basic principles, main features and advantages of the present application. It should be understood by those skilled in the art that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application, and are not intended to limit the present application, and various changes and improvements can be made to the present application without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for removing oxalate from a sodium aluminate solution, for treating an intermediate product sodium aluminate solution in a Bayer process production, characterized in that, The method comprises the following steps: S10. Adding coarse seed into sodium aluminate solution, an intermediate product of Bayer process for producing alumina, to perform a first decomposition, and cooling to 70-75℃ to precipitate aluminum hydroxide slurry; S20. Cooling the prepared aluminum hydroxide slurry to 40-50℃; S30. Filtering and separating the cooled aluminum hydroxide slurry to obtain filter cake and filtrate, and washing the filter cake with hot liquid to obtain washing liquid; S40. Performing causticization treatment on the obtained washing liquid to obtain calcium oxalate; and calcining the obtained filter cake to prepare alumina.

2. The method for removing oxalate from a sodium aluminate solution according to claim 1, characterized in that, The coarse seed in the S10 step is aluminum hydroxide crystal with D50 of particles in the range of 80-120μm.

3. The method for removing oxalate from a sodium aluminate solution according to claim 2, characterized in that, The proportion of crystal with particle size of 60μm or less in the aluminum hydroxide crystal is 10%-20%.

4. The method for removing oxalate from a sodium aluminate solution according to claim 1, wherein In the S20 step, the aluminum hydroxide slurry is cooled in a heat exchanger; the heat exchanger comprises a wide channel heat exchanger or a jacketed heat exchanger.

5. The method of claim 1, wherein the sodium aluminate solution is a spent pickling solution. In the S20 step, the cooling rate of the aluminum hydroxide slurry is greater than 5℃ / min / L.

6. The method of claim 1, wherein the sodium aluminate solution is a spent pickling solution. In the S30 step, a flat disc filter is used for filtering and separating, and at least one filtering and two washing are performed.

7. The method of claim 6, wherein the sodium aluminate solution is removed from the oxalate solution by, The washing liquid after the first washing is used as reverse washing water for the second washing.

8. The method of claim 1, wherein the sodium aluminate solution is a spent pickling solution. In the S30 step, the hot liquid used for washing is hot water with a temperature of 90-95℃.

9. The method for removing oxalate from a sodium aluminate solution according to any one of claims 1 to 8, characterized in that, In the S40 step, the mixture solution containing calcium oxalate is sent to a filter pressing process to obtain solid calcium oxalate and weak alkali liquid, and the weak alkali liquid is returned to the settling tank used in the pre-process of the Bayer process for washing red mud.

10. The method for removing oxalate from a sodium aluminate solution according to any one of claims 1 to 8, characterized in that, In the S40 step, lime milk is used for causticization treatment on the washing liquid to form calcium oxalate, and the addition amount of lime milk is such that the ratio of CaO in the lime milk to Na2C2O4 molecules in the original sodium aluminate solution is 1.2-2 or more.

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