A method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system

Abstract

The present invention belongs to the field of rare earth hydrometallurgy and relates to a method for decomposing a mixed rare earth concentrate using a gradient water wash solution and alkali. Specifically, a method for staged water washing of alkali cakes in a mixed rare earth concentrate alkali decomposition system is disclosed. Through slurry washing and high-gravity filtration, a small amount of water is used to wash unreacted sodium hydroxide from the alkali cake, reducing wastewater treatment costs. Simultaneously, the common ion effect is utilized to maximize the retention of NaF and Na3PO4 in the slurry alkali cake, achieving the goal of pre-separating NaOH from F and P resources. Ultrasonic technology improves water washing efficiency while reducing water consumption. The present invention also rationally utilizes sodium fluoride, sodium phosphate, sodium carbonate, and a small amount of rare earth elements in the ultrasonic water wash solution to produce glass, achieving high-quality resource utilization.

Description

Technical Field

[0001] The invention belongs to the field of rare earth hydrometallurgy, relates to a method for decomposing mixed rare earth concentrate with gradient water washing liquid alkali, and specifically discloses a method for segmented water washing of alkali cakes in a mixed rare earth concentrate liquid alkali decomposition system. Background Art

[0002] Rare earths are important strategic resources in my country, and the mixed rare earth reserves in Bayan Obo rank first in my country. It is one of the important rare earth deposits in my country. It is mainly composed of fluorocarbon cerium ore and monazite. Because the two have close interbedded relationships, serious intergrowth, and huge differences in physical and chemical properties, they are recognized as "difficult to smelt" minerals.

[0003] Currently, 10% of the Bayan Obo mixed rare earth concentrate is extracted using a liquid alkali decomposition process. While this process is a relatively clean smelting process, offering advantages such as ease of operation and the absence of significant acidic waste gas generation, the alkaline wastewater produced during production is difficult to filter, requiring large volumes of water for washing. This results in significant alkaline wastewater discharge and high treatment costs. Consequently, optimizing the water washing process in the liquid alkali decomposition process remains a daunting task.

[0004] Application number CN 201611134251.4 describes a process for the comprehensive recovery of fluorine and phosphorus from mixed rare earth concentrate by caustic soda decomposition. The process involves hot filtration of the decomposed material at temperatures above 80°C to produce a concentrated caustic solution and an caustic cake. The concentrated caustic solution is then cooled to 30°C to 70°C and filtered to produce sodium phosphate and NaOH products. The caustic cake is then washed, concentrated, and filtered to produce sodium fluoride, while maintaining a solid-liquid ratio of 1:1 to 3. While this process achieves fluorine and phosphorus recovery and recycling of the caustic soda, the purity of the sodium fluoride and phosphate products is relatively low due to the use of cooling crystallization precipitation.

[0005] Application number CN 202010550727.2 describes a method and application for separating fluoride and phosphorus from mixed rare earth alkaline wastewater. This invention primarily involves using a nanofiltration membrane to separate mixed rare earth alkaline wastewater containing fluoride and phosphate ions. While the separation of fluoride and phosphate ions is possible, the NaOH and NaF remain unseparated, and the method may not be suitable for alkaline wastewater containing high concentrations of fluoride (>1.6 g / L) and phosphate (>1.6 g / L).

[0006] Therefore, how to develop a liquid alkali decomposition process for mixed rare earth concentrate with simple operation, low wastewater volume, low treatment cost and high product purity is a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0007] In view of this, the present invention provides a method for segmented water washing of alkali cake in a mixed rare earth concentrate liquid alkali decomposition system to address the shortcomings of the traditional liquid alkali decomposition process of mixed rare earth concentrate, such as difficulty in water washing and filtering of alkali cake, large amount of wastewater and high cost of subsequent water treatment, and low purity of sodium fluoride, sodium phosphate and sodium hydroxide products.

[0008] Specifically, the present invention proposes a new idea for segmented water washing of alkali cakes. First, the solid-liquid ratio is controlled to make the alkali cake form close to a slurry. Then, the slurried alkali liquor and the slurried alkali residue are separated by supergravity filtration. The purpose of slurrying is to make the sodium hydroxide in the slurried alkali liquor reach a saturated state as much as possible, which is beneficial to reduce water consumption and reduce the dissolution of NaF and Na3PO4. The main purpose of introducing ultrasonic water washing is to utilize the cavitation effect of ultrasonic waves in the liquid to continuously peel off sodium fluoride, sodium phosphate and sodium carbonate on the surface of the slurried alkali residue, thereby reducing the number of water washing times and achieving the purpose of reducing water consumption.

[0009] In order to achieve the above object, the present invention provides the following technical solutions:

[0010] A method for washing alkali cake in sections using a mixed rare earth concentrate liquid alkali decomposition system specifically comprises the following steps:

[0011] (1) Slurry washing and ultra-gravity filtration: water is added to the alkali cake and stirred, and after slurry washing for a certain period of time, a slurry material is obtained; the slurry material is filtered and washed with water twice according to the above steps, and after the slurry washing is completed, a slurry alkali solution and a slurry alkali cake are obtained; wherein the slurry alkali solution can be directly returned to the decomposition process of the mixed rare earth concentrate for continued use;

[0012] The main purpose of this operation step is to use a small amount of water to wash the unreacted sodium hydroxide in the alkali cake, and make the slurry alkali solution directly usable in the process of liquid alkali decomposition of Bayan Obo mixed concentrate, thereby reducing the cost of wastewater treatment. At the same time, by utilizing the common ion effect, NaF and Na3PO4 are retained in the slurry alkali cake as much as possible, thereby achieving the purpose of pre-separation of NaOH from F and P resources.

[0013] (2) low-temperature roasting: placing the slurry alkali cake obtained in step (1) in a muffle furnace and roasting at low temperature to obtain a roasted ore;

[0014] The main purpose of this operation step is, on the one hand, to further react the unremoved sodium hydroxide with the undecomposed mixed rare earth concentrate to increase the mineral decomposition rate, and on the other hand, to convert rare earth hydroxide (RE(OH)3) into easily soluble water-washable rare earth oxide (RE2O3) (Formula 1, Formula 2), thereby providing a prerequisite for subsequent reduction in water washing amount, and at the same time converting the residual NaOH into one of the raw materials for glass production (Na2CO3) (Formula 3).

[0015] RE(OH)3→REO(OH)+H2O (Formula 1)

[0016] 2REO(OH)→RE2O3+H2O (Formula 2)

[0017] 2NaOH+CO2→Na2CO3+H2O (Formula 3)

[0018] (3) Crushing and grinding: placing the roasted ore obtained in step (2) in a ball mill for ball milling to obtain fine-grained roasted ore;

[0019] The main purpose of this operation step is to expose some NaF, Na3PO4, and Na2CO3 wrapped in the roasted ore as much as possible, providing good conditions for subsequent acid dissolution and ultrasonic water washing.

[0020] (4) Ultrasonic water washing and filtering: adding water to the fine-grained roasted ore obtained in step (3) and placing the ore in an ultrasonic cleaning apparatus for ultrasonic water washing and filtering to obtain an ultrasonic water washing liquid and an ultrasonic water washing slag; wherein the sodium phosphate, sodium fluoride, and sodium carbonate in the ultrasonic water washing liquid can be recycled;

[0021] The main purpose of this operation step is to use the cavitation effect, acceleration effect and straight flow effect of ultrasound in the liquid to enhance the stripping of sodium fluoride, sodium phosphate and sodium carbonate on the surface of the mineral, so as to allow NaF, Na3PO4 and Na2CO3 to enter the ultrasonic water washing solution for recycling as much as possible, thereby reducing the amount of alkaline wastewater, reducing treatment costs and increasing industrial output value.

[0022] (5) Acid dissolution: The ultrasonic water-washed slag obtained in step (4) is acid-dissolved to obtain a RECl3 solution and an acid-dissolved slag, which can be returned to the liquid alkali decomposition process of the mixed rare earth concentrate to continue decomposition;

[0023] The main purpose of this operation step is to convert RE2O3 into RECl3 (Formula 4), and RECl3 solution is a common raw material for rare earth extraction and is used in the production of rare earth products.

[0024] RE2O3+6HCl→2RECl3+3H2O (Formula 4)

[0025] (6) Evaporation and crystallization: The ultrasonic water washing liquid obtained in step (4) is evaporated and crystallized to obtain a mixed sodium salt (NaF, Na3PO4, Na2CO3) containing trace amounts of rare earth and F and P. The mixed sodium salt can be used as a glass additive;

[0026] The main purpose of this operation step is to obtain a mixed sodium salt containing trace rare earths and NaF, Na3PO4, and Na2CO3 by evaporation and crystallization, thereby obtaining the raw material for preparing glass; on the other hand, to recycle the evaporated water resources to achieve zero discharge of wastewater. The mixed sodium salt after evaporation is analyzed by scanning electron microscopy and energy spectrum ( Figure 2 ).

[0027] (7) Preparation of glass: Add B2O3, Al2O3, ZnO, and Na2CO3 to the mixed sodium salt (NaF, Na3PO4, and Na2CO3) containing trace amounts of rare earths and F and P obtained in step (6) to prepare borate glass.

[0028] The main purpose of this operation step is to avoid the need to separate sodium salts such as NaF, Na3PO4, and Na2CO3 separately, but to directly use the mixed sodium salts to prepare glass, thereby achieving high-value application of resources.

[0029] In summary, the purpose of the present invention is to saturate and enrich the sodium hydroxide in the slurry alkali solution as much as possible through slurry water washing and supergravity filtration, thereby reducing water consumption and the dissolution of sodium fluoride and sodium phosphate, so as to achieve the separation of most of the NaOH from the sodium fluoride and sodium phosphate before the low-temperature roasting process, and avoid the conversion of most of the NaOH into Na2CO3 due to the low-temperature roasting process, thereby affecting the resource recycling effect of NaOH; low-temperature roasting is to continue to decompose the unreacted mixed rare earth concentrate with the remaining sodium hydroxide for secondary decomposition, and at the same time convert the rare earth hydroxide into rare earth oxide that is better filtered, thereby providing more favorable washing conditions for the back-end ultrasonic water washing; ultrasonic water washing utilizes the cavitation effect of ultrasonic waves in the liquid to enhance the stripping effect of sodium fluoride, sodium phosphate and sodium phosphate on the surface of the rare earth hydroxide, thereby achieving the purpose of reducing water consumption.

[0030] Optionally, in step (1), the first washing condition is a solid-liquid ratio of 1:1, a washing time of 10 min to 90 min, and a washing temperature of room temperature; the second washing condition is a solid-liquid ratio of 1:0.5, a washing time of 10 min to 90 min, and a washing temperature of room temperature.

[0031] Furthermore, the filtration method in step (1) is centrifugal filtration.

[0032] Optionally, the calcination temperature in step (2) is 250° C. to 650° C., and the calcination time is 0.5 h to 2.0 h; preferably, the calcination temperature is 300° C. to 400° C., and the calcination time is 0.5 h to 1.0 h.

[0033] Optionally, in step (3), the particle size of the fine-grained roasted ore is ≤50 μm (≥90%).

[0034] Optionally, in step (4), the ultrasonic temperature is 40°C to 100°C, the ultrasonic time is 5min to 30min, the mass ratio of fine-grained roasted ore to water is 1:4 to 1:10, and the number of water washings is 2 to 5 times; preferably, the ultrasonic temperature is 80°C to 90°C, the ultrasonic time is 5min to 10min, the mass ratio of fine-grained roasted ore to water is 1:7 to 1:10, and the number of water washings is 3 to 4 times.

[0035] Optionally, in step (7), B2O3, Al2O3, ZnO, and Na2CO3 are added to a mixed sodium salt containing F and P (NaF, Na3PO4, and Na2CO3) to prepare borate glass, and the dissolving temperature is 800-1000°C, and the glass is kept warm for 1-3 hours, then placed in an annealing furnace, kept warm at 430-480°C for 1-2 hours, and cooled to room temperature with the furnace; preferably, the dissolving temperature is 950-1000°C, and the glass is kept warm for 2 hours, then placed in an annealing furnace, kept warm at 450-470°C for 2 hours, and cooled to room temperature with the furnace.

[0036] It can be seen from the above technical solution that compared with the prior art, the method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system provided by the present invention has the following excellent effects:

[0037] 1) The present invention uses a small amount of water to wash the unreacted sodium hydroxide in the alkali cake, and the slurry alkali solution can be directly used in the process of liquid alkali decomposition of the Bayan Obo mixed concentrate, thereby reducing the cost of wastewater treatment. At the same time, the common ion effect is utilized to allow NaF and Na3PO4 to remain in the slurry alkali cake as much as possible, thereby achieving the purpose of pre-separation of NaOH from F and P resources.

[0038] 2) In the present invention, ultrasonic means are used to improve the water washing efficiency, greatly reducing the amount of water used, and the ultrasonic water washing liquid after washing can be evaporated and crystallized to obtain a mixed sodium salt containing sodium fluoride, sodium phosphate, and sodium carbonate. The sodium fluoride, sodium phosphate, and sodium carbonate do not need to be separated and can be directly mixed with materials such as B2O3, Al2O3, ZnO, and Na2CO3 to prepare borate glass. This process can not only recycle water resources and avoid water resource waste, but also rationally utilize sodium fluoride, sodium phosphate, sodium carbonate, and a small amount of rare earth elements in the ultrasonic water washing liquid, which is economical and environmentally friendly.

[0039] 3) The process route disclosed in the present invention is simple, the process objectives are clear, and the equipment is mature, which can provide convenience for the industrial transformation and upgrading of related enterprises.

[0040] 4) The present invention utilizes a slurry washing and high-gravity filtration process to wash unreacted sodium hydroxide from the alkali cake with a small amount of water, thereby reducing wastewater treatment costs. Simultaneously, the common ion effect is utilized to maximize the retention of NaF and Na3PO4 in the slurry alkali cake, achieving the goal of pre-separating NaOH from F and P resources. Ultrasonic treatment improves washing efficiency while reducing water consumption. The present invention also rationally utilizes sodium fluoride, sodium phosphate, sodium carbonate, and a small amount of rare earth elements in the ultrasonic wash solution to produce glass, achieving high-quality resource utilization.

[0041] 5) The present invention proposes a new concept of rare earth mineral conversion (low-temperature roasting) and double water washing, which not only effectively solves the problem of large water consumption in the traditional liquid alkali decomposition and washing process of mixed rare earth concentrates, but also realizes the high-quality application of associated resources in rare earth minerals and the efficient utilization of sodium hydroxide. The present invention also has simple processes, low costs, and high rare earth leaching rates, opening up new ideas for subsequent clean production of rare earth hydrometallurgy. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0043] Figure 1 This is a process flow chart of the method for segmented water washing of alkali cake in the mixed rare earth concentrate liquid alkali decomposition system of the present invention.

[0044] Figure 2 Surface morphology and surface scanning analysis diagram of mixed sodium salt. DETAILED DESCRIPTION

[0045] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention and the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0046] The embodiment of the invention discloses a method for washing alkali cake in sections in a mixed rare earth concentrate liquid alkali decomposition system.

[0047] For a better understanding of the present invention, the present invention is further specifically described below through the following examples, but it should not be understood as limiting the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above invention content are also considered to fall within the scope of protection of the present invention.

[0048] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0049] It should be noted that the alkali cakes used in all examples of the present invention are products obtained by decomposing rare earth concentrate with liquid alkali, and their main component is rare earth hydroxide (rare earth grade is 28% to 35%).

[0050] Example 1

[0051] 1) adding water to the alkali cake to perform a first slurry washing with water at room temperature under the conditions of a solid-liquid ratio of 1:1 and a washing time of 10 minutes, and then filtering under high gravity; then performing a second slurry washing with water at room temperature under the conditions of a solid-liquid ratio of 1:0.5 and a washing time of 10 minutes, and then filtering under high gravity to obtain slurry alkali liquor and slurry alkali cake;

[0052] 2) placing the slurried alkali cake in a muffle furnace and roasting at 350° C. for 1.0 h to obtain a roasted ore;

[0053] 3) The roasted ore is placed in a ball mill and ball-milled to obtain fine-grained ball-milled ore with a particle size of 50 μm;

[0054] 4) adding water to the fine-grained roasted ore obtained in step (3) and placing the mixture in an ultrasonic cleaner, washing the mixture three times under the conditions of a solid-liquid ratio of 1:10, an ultrasonic temperature of 80° C., and an ultrasonic time of 10 min to obtain ultrasonically washed slag and an ultrasonic washing liquid;

[0055] 5) Evaporating and crystallizing the ultrasonic water washing liquid to obtain a mixed sodium salt containing F and P (a sodium salt mixture of NaF, Na3PO4, and Na2CO3), adding B2O3, Al2O3, ZnO, and Na2CO3 in proportion, and heating at 1000°C for 2 hours, placing in an annealing furnace, heating at 450°C for 2 hours, and cooling to room temperature with the furnace to prepare borate glass;

[0056] 6) Add 8 mol / L hydrochloric acid with a solid-liquid ratio of 1:4 to the ultrasonic water-washed slag obtained in step (3), stir at a speed of 400 r / min, and react at a temperature of 80° C. for 1 h to obtain a rare earth chloride solution and acid-soluble slag.

[0057] Finally, the sodium hydroxide yield is more than 70%, and the rare earth recovery rate is more than 85%.

[0058] Example 2

[0059] 1) adding water to the alkali cake to perform a first slurry washing, wherein the solid-liquid ratio is 1:1, the washing time is 10 minutes, the slurry washing is performed at room temperature, and then the slurry is filtered under high gravity; then performing a second slurry washing, wherein the solid-liquid ratio is 1:0.5, the washing time is 10 minutes, the slurry washing is performed at room temperature, and then the slurry is filtered under high gravity to obtain slurry alkali solution and slurry alkali cake;

[0060] 2) placing the slurried alkali cake in a muffle furnace and roasting at 550° C. for 2.0 h to obtain a roasted ore;

[0061] 3) placing the roasted ore in a ball mill and milling it to obtain fine-grained ball-milled ore with a particle size of ≤50 μm (≥90%);

[0062] 4) adding water to the fine-grained roasted ore obtained in step (4) and placing the mixture in an ultrasonic cleaner, washing the mixture three times under the conditions of a solid-liquid ratio of 1:10, an ultrasonic temperature of 80° C., and an ultrasonic time of 10 min to obtain ultrasonically washed slag and an ultrasonic washing liquid;

[0063] 5) Evaporating and crystallizing the ultrasonic water washing liquid to obtain a mixed sodium salt containing F and P (a sodium salt mixture of NaF, Na3PO4, and Na2CO3), adding B2O3, Al2O3, ZnO, and Na2CO3 in proportion, and heating at 1000°C for 2 hours, placing in an annealing furnace, heating at 450°C for 2 hours, and cooling to room temperature with the furnace to prepare borate glass;

[0064] 6) Add 9 mol / L hydrochloric acid with a solid-liquid ratio of 1:5 to the ultrasonically washed slag, stir at a speed of 600 r / min, and react at a temperature of 90° C. for 1 h to obtain a rare earth chloride solution and acid-soluble slag.

[0065] Finally, the sodium hydroxide yield is more than 80%, and the rare earth recovery rate is more than 90%.

[0066] In addition, in order to further illustrate the excellence of the characteristic elements in the operation process of this case, the inventor conducted the following creative experiments on the technology of the present invention. The specific experimental contents are as follows:

[0067] (1) Pulping and washing

[0068] The traditional liquid caustic soda decomposition process primarily uses NaOH to convert rare earth minerals such as bastnaesite and monazite into acid-soluble RE(OH)3. During this process, F and P are converted into sodium fluoride and sodium phosphate. The presence of sodium salts such as sodium fluoride, sodium phosphate, and sodium hydroxide is not only detrimental to subsequent acid leaching but also affects the purity of the rare earth products, so they need to be removed through water washing. As previously described, the purpose of slurry water washing is to use a small amount of water to wash the unreacted sodium hydroxide from the caustic soda cake, allowing the slurry to be directly used in the caustic soda decomposition process of the Bayan Obo mixed concentrate, reducing wastewater treatment costs. At the same time, the common ion effect is utilized to retain as much NaF and Na3PO4 as possible in the slurry caustic soda cake, achieving the goal of pre-separating NaOH from the F and P resources.

[0069] Practice has shown that the selection of slurry washing process parameters directly affects the washing rates of sodium fluoride, sodium phosphate, and sodium hydroxide in alkaline hydrolysis ore, making it a key step in this invention. Furthermore, the more sodium hydroxide dissolves during this process, the less sodium fluoride and sodium phosphate dissolve, resulting in a higher sodium hydroxide utilization rate. Therefore, measuring the washing rates of sodium hydroxide, sodium fluoride, and sodium phosphate is a simple and effective method for selecting slurry washing process parameters.

[0070] Table 1 shows the relationship between washing rate and washing temperature under the conditions of a solid-liquid ratio of 1:1 and a slurry washing time of 10 minutes. It can be seen that as the washing temperature increases, the washing rates of sodium hydroxide, sodium fluoride, and sodium phosphate increase slightly. During this process, the washing rate of sodium hydroxide is higher, while the washing rates of sodium fluoride and sodium phosphate are lower.

[0071] Table 2 shows the relationship between washing rate and washing time, assuming a solid-to-liquid ratio of 1:1 and a single wash at room temperature. As washing time increases, the washing rates of sodium hydroxide, sodium fluoride, and sodium phosphate increase less significantly, indicating that washing time has little effect on the washing rates of the three.

[0072] Table 3 shows the relationship between the water washing rate and the solid-to-liquid ratio, with a 10-minute water washing time and a single wash at room temperature. It can be seen that the water washing rates of the three materials gradually increase with increasing solid-to-liquid ratio. Within the range of 1:0.5 to 1:1, the water washing rate of sodium fluoride is <4%, and that of sodium phosphate is <0.3%. However, the higher water washing rate of sodium hydroxide is consistent with the purpose of this process.

[0073] As shown in Table 4, the first washing process selected a solid-liquid ratio of 1:1 and a duration of 10 minutes, and the second to seventh washing processes selected a solid-liquid ratio of 1:0.5. This process was carried out at room temperature. The relationship between the washing rate and the number of washings is shown. It can be seen that with the increase in the number of washings, the washing rates of the three increase. In the first washing process, the washing rate of sodium fluoride is only 4.38%, the washing rate of sodium phosphate is only 0.28%, and the washing rate of sodium hydroxide reaches 40.71%; in the third washing process, the washing rate of sodium fluoride is only 24.64%, the washing rate of sodium phosphate is only 2.00%, and the washing rate of sodium hydroxide reaches 85.23%; in the fourth washing process, the washing rate of sodium fluoride is as high as 41.55%. The high dissolution of sodium fluoride in the solution is not conducive to the recovery of sodium hydroxide.

[0074] Therefore, based on the above considerations, the first washing condition is selected as room temperature washing with a solid-liquid ratio of 1:1 and a washing time of 10 minutes; the second to third washing conditions are room temperature washing with a solid-liquid ratio of 1:0.5 and a washing time of 10 minutes; the slurry alkali solution obtained at this time has a high sodium hydroxide content, and relatively low sodium fluoride and sodium phosphate contents.

[0075] Table 1 Relationship between washing rate and washing temperature

[0076]

[0077]

[0078] Table 2 Relationship between washing rate and washing time

[0079] Time (min) 10 20 30 40 50 60 Sodium hydroxide washing rate / % 61.43 61.90 62.38 62.86 62.86 62.86 Sodium fluoride washing rate / % 24.15 25.85 27.42 28.5 28.86 28.86 Sodium phosphate washing rate / % 0.50 0.89 1.04 1.16 1.20 1.22

[0080] Table 3 Relationship between water washing rate and solid-liquid ratio

[0081] Solid-liquid ratio 1:0.5 1:0.6 1:0.7 1:0.8 1:0.9 1:1 1:2 Sodium hydroxide washing rate / % 10.95 19.05 30.24 30.95 36.67 40.71 60.48 Sodium fluoride washing rate / % 2.05 2.90 3.14 3.26 3.5 3.62 18.96 Sodium phosphate washing rate / % 0.02 0.09 0.15 0.17 0.2 0.28 0.48 Solid-liquid ratio 1:3 1:4 1:5 1:6 1:7 1:8 1:9 Sodium hydroxide washing rate / % 61.43 62.38 64.29 68.57 69.05 71.43 73.33 Sodium fluoride washing rate / % 24.15 35.75 42.39 48.31 52.54 56.28 56.64 Sodium phosphate washing rate / % 0.50 2.10 8.98 14.67 26.02 29.31 34.65

[0082] Table 4 Relationship between washing rate and washing times

[0083] Number of times 1 2 3 4 5 6 7 Sodium hydroxide washing rate / % 40.71 73.33 85.23 91.18 94.51 96.89 98.32 Sodium fluoride washing rate / % 4.83 12.56 24.64 41.55 51.33 59.3 65.94 Sodium phosphate washing rate / % 0.28 1.02 2.00 2.95 3.09 14.13 21.96

[0084] (2) Low temperature roasting

[0085] Low-temperature calcination is the most important step in this invention. It converts rare earth hydroxide [RE(OH)3] into acid-soluble rare earth oxide (RE2O3), which not only improves the decomposition efficiency of the rare earth mineral but also enhances the efficiency of washing with sodium fluoride and sodium phosphate. Therefore, minimizing the calcination temperature and time while maintaining the decomposition rate of the rare earth mineral is a key consideration in this step and a key point of this invention. To this end, we further explored the selection of low-temperature calcination conditions.

[0086] As shown in Table 5, when the oxidation roasting time is 2.0h, the relationship between the roasting temperature and the rare earth leaching rate is shown. It can be seen that when the roasting temperature is greater than 250℃, the rare earth leaching rate has a significant increase, and with the gradual increase of the roasting temperature, the rare earth leaching rate shows an increasing trend, but when the roasting temperature is above 450℃, the rare earth leaching rate tends to be stable.

[0087] Table 6 shows the relationship between rare earth leaching rate and oxidation roasting time when the roasting temperature is 450℃. When the roasting temperature is greater than 0.5h, the rare earth leaching rate is significantly improved, and with the extension of roasting time, the rare earth leaching rate shows a trend of gradual increase. However, when the roasting time exceeds 2.0h, the rare earth leaching rate tends to be stable and the growth rate is small.

[0088] Therefore, based on the above considerations, the calcination temperature is selected to be 250℃~650℃ and the calcination time is selected to be 0.5h~2.0h.

[0089] Table 5 Relationship between roasting temperature and rare earth leaching rate

[0090] Calcination temperature / ℃ 250 350 450 550 650 Rare earth leaching rate / % 88.56 89.29 91.08 91.49 91.68

[0091] Table 6 Relationship between roasting time and rare earth leaching rate

[0092] Calcination time / h 0.5 1.0 1.5 2.0 2.5 3.0 Rare earth leaching rate / % 87.62 89.18 90.32 91.49 91.57 91.61

[0093] (3) Ultrasonic water washing

[0094] In addition, the ultrasonic water washing process is a key process of the present invention. Since the presence of sodium fluoride, sodium phosphate, and sodium hydroxide in the fine-grained roasted ore is not conducive to subsequent acid leaching, they need to be washed and removed. Therefore, the cavitation effect, acceleration effect, and straight flow effect of ultrasound in the liquid are used to peel off the sodium fluoride, sodium phosphate, and sodium hydroxide on the surface of the fine-grained roasted ore to achieve the cleaning purpose. This process is not only crucial for improving the rare earth recovery rate, but the water consumption will directly determine the production cost. How to minimize the water consumption of the water washing process while ensuring the decomposition rate of rare earth minerals will be one of the key factors to consider in this process, and it is also the top priority of the present invention. To this end, we have made further research on the selection of ultrasonic water washing parameters.

[0095] Table 7 shows the relationship between water washing rate and ultrasonic temperature under conditions of a solid-to-liquid ratio of 1:4 and a 10-minute ultrasonic wash. It can be seen that the water washing rates of sodium fluoride and sodium phosphate increase with increasing ultrasonic temperature, and stabilize at 80°C.

[0096] Table 8 shows the relationship between water washing rate and solid-to-liquid ratio for a single water wash at an ultrasonic temperature of 80°C for 10 minutes. The solid-to-liquid ratio significantly affects the water washing rate of sodium fluoride and sodium phosphate. With increasing solid-to-liquid ratio, the water washing rate of sodium fluoride and sodium phosphate increases significantly, reaching a maximum at 1:10.

[0097] Table 9 shows the relationship between water washing rate and ultrasonic time for a single wash under the conditions of a solid-liquid ratio of 1:10, an ultrasonic temperature of 80°C, and an ultrasonic time of 10 minutes. It can be seen that as the ultrasonic time increases (5-10 minutes), the water washing rate of sodium fluoride and sodium phosphate increases briefly. This decreases as the ultrasonic time increases (10-30 minutes), reaching its maximum at 10 minutes.

[0098] Table 10 shows the relationship between the washing rate and the number of washes, using five washes with a solid-to-liquid ratio of 1:10, an ultrasonic temperature of 80°C, and an ultrasonic time of 10 minutes. It can be seen that the washing rate of sodium fluoride reached 96.06% in the second wash, while the washing rate of sodium phosphate was even higher, reaching 92.12% in the third wash.

[0099] Therefore, considering the above, the ultrasonic washing conditions are selected as solid-liquid ratio 1:10, ultrasonic temperature 80℃, ultrasonic time 10min, and room temperature water washing 2 to 4 times; at this time, the washing rate of sodium fluoride and sodium phosphate is higher.

[0100] Table 7 Relationship between water washing rate and ultrasonic temperature

[0101] temperature normal temperature 30℃ 40℃ 50℃ 60℃ 70℃ 80℃ 90℃ Sodium fluoride washing rate / % 43.09 51.06 52.74 61.56 65.16 65.64 68.96 69.18 Sodium phosphate washing rate / % 39.48 43.48 51.12 52.56 54.17 54.63 56.62 56.8

[0102] Table 8 Relationship between water washing rate and solid-liquid ratio

[0103]

[0104] Table 9 Relationship between water washing rate and ultrasonic time

[0105]

[0106]

[0107] Table 10 Relationship between washing rate and washing times

[0108] Number of times 1 2 3 4 5 Sodium fluoride washing rate / % 81.82 96.06 98.81 99.64 99.91 Sodium phosphate washing rate / % 70.99 82.24 92.12 99.45 99.76

[0109] The above description of the disclosed embodiments will enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system, characterized in that: The method specifically comprises the following steps: (1) Slurry washing and ultra-gravity filtration: add water to the alkali cake and stir, and slurry wash for a certain period of time to obtain a slurry material; filter the slurry material and wash it twice according to the above steps, and after the slurry washing is completed, obtain slurry alkali solution and slurry alkali cake; in the step (1), the first washing condition is a solid-liquid ratio of 1:1, the washing time is 10 min to 90 min, and the washing temperature is room temperature; The second washing conditions are a solid-liquid ratio of 1:0.5, a washing time of 10 min to 90 min, and a washing temperature of room temperature. The alkali cake used is the product obtained by decomposing rare earth concentrate with liquid alkali. Its main component is rare earth hydroxide, and the rare earth grade is 28% to 35%. (2) Low-temperature roasting: roasting the slurry alkali cake obtained in step (1) at low temperature to obtain roasted ore; the roasting temperature is 250°C to 650°C; (3) Crushing and grinding: ball milling the roasted ore obtained in step (2) to obtain fine-grained roasted ore; (4) Ultrasonic water washing and filtration: adding water to the fine-grained roasted ore obtained in step (3) and ultrasonically washing and filtering to obtain an ultrasonic water washing liquid and an ultrasonic water washing slag; (5) Acid dissolution: The ultrasonic water-washed residue obtained in step (4) is subjected to acid dissolution to obtain a RECl3 solution and an acid-dissolved residue; (6) Evaporation and crystallization: The ultrasonic water washing solution obtained in step (4) is evaporated and crystallized to obtain a mixed sodium salt containing trace amounts of rare earth and F and P, wherein the mixed sodium salt of F and P includes NaF, Na3PO4, and Na2CO3; (7) Preparation of glass: Adding B2O3, Al2O3, ZnO, and Na2CO3 to the mixed sodium salt containing trace amounts of rare earth and F and P obtained in step (6) to prepare borate glass.

2. The method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system according to claim 1, characterized in that: In the step (2), the roasting time is 0.5h~2.0h.

3. The method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system according to claim 1, characterized in that: In the step (3), the particle size of the fine-grained roasted ore is ≤50 μm.

4. The method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system according to claim 1, characterized in that: In the step (4), the ultrasonic temperature is 40°C to 100°C, the ultrasonic time is 5 min to 30 min, the mass ratio of the fine-grained roasted ore to water is 1:4 to 1:10, and the number of water washings is 2 to 5 times.

5. The method for washing alkali cake in a mixed rare earth concentrate liquid alkali decomposition system according to claim 1, characterized in that: In the step (7), B2O3, Al2O3, ZnO and Na2CO3 are added to the mixed sodium salt containing F and P to prepare borate glass, the melting temperature is 800-1000°C, the glass is kept warm for 1-3 hours and then placed in an annealing furnace, kept warm at 430-480°C for 1-2 hours, and then cooled to room temperature along with the furnace.

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