A high-salt rare earth wastewater step-by-step selective precipitation separation method

Abstract

The application relates to the technical field of wastewater treatment and resource recovery, in particular to a high-salt rare earth wastewater step-by-step selective precipitation separation method. The method comprises the following steps: detecting wastewater components and calculating salt strength, correcting metal ion precipitation windows according to the salt strength to determine a step-by-step precipitation sequence; adding a composite trapping agent to separate heavy metals under weakly acidic and weakly reducing conditions; introducing crystal seeds in a rare earth precipitation window and combining multi-parameter feedback control to realize selective precipitation and an end point of the rare earth; adjusting mother liquor to magnesium and calcium precipitation windows in sequence to realize magnesium-calcium staggered separation; and finally, disposing and recycling the precipitated slag and high-salt purified mother liquor. The application can effectively solve the problems of inaccurate precipitation window and multi-metal co-precipitation in a high-salt system, improve the rare earth recovery rate and slag phase purity, and realize safe recycling of the high-salt mother liquor.

Description

Technical Field

[0001] This application relates to the field of wastewater treatment and resource recycling technology, and in particular to a stepwise selective precipitation separation method for high-salt rare earth wastewater. Background Technology

[0002] In the production processes of rare earth smelting, extraction and separation, and ion-type rare earth leaching, large amounts of high-salinity wastewater containing chloride salts, ammonium salts, calcium and magnesium ions, heavy metal ions, and residual rare earth ions are generated. This type of wastewater is characterized by high salinity, high ionic strength, and frequent fluctuations in composition. Existing treatment processes typically employ neutralization precipitation, sulfide removal, flocculation sedimentation, membrane concentration, or evaporation crystallization. In practice, the wastewater is generally first pH-adjusted, and a precipitant is added to cause metal ions to precipitate as hydroxides or carbonates. Subsequently, solid-liquid separation is used to remove the slag phase. Some processes also incorporate subsequent membrane treatment or evaporation steps for further purification of the mother liquor or salt recovery.

[0003] However, in the existing technology, it is difficult to achieve precise stepwise separation of heavy metals, rare earths and calcium and magnesium ions in the face of complex working conditions such as drastic changes in ion activity and overlapping precipitation windows of multiple metal ions in high-salt systems. This often leads to problems such as insufficient purity of rare earth products, mixed discharge of hazardous waste and inability to directly reuse high-salt mother liquor. Summary of the Invention

[0004] This application provides a stepwise selective precipitation separation method for high-salt rare earth wastewater to solve the above-mentioned problems. The method includes: S1. After homogenizing and removing solid impurities from the high-salt rare earth wastewater, the concentrations of rare earth ions, calcium ions, magnesium ions, heavy metal ions, chloride salts, ammonium salts, and conductivity in the high-salt rare earth wastewater are detected, and the salt intensity of the wastewater is determined based on the chloride salt concentration, ammonium salt concentration, and conductivity. Based on the salt intensity of the wastewater, the precipitation windows for heavy metals, rare earths, magnesium, and calcium are corrected to obtain a stepwise precipitation control sequence in which heavy metals, rare earths, magnesium, and calcium precipitate in sequence. S2. According to the heavy metal precipitation window, the high-salt rare earth wastewater is adjusted to an acidic weak reducing state, and sulfur-containing scavenging agent and iron salt composite scavenging agent are added to make the heavy metal ions in the wastewater form sulfide precipitates, iron-based co-precipitates or sulfide-iron-based composite precipitates. After solid-liquid separation, heavy metal precipitate residue and heavy metal removal filtrate are obtained. S3. According to the rare earth precipitation window, adjust the heavy earth removal filtrate to a state where rare earth preferentially precipitates while calcium and magnesium do not co-precipitate in large quantities. Add carbonate precipitant and rare earth carbonate seed crystals or reflux rare earth precipitation seed crystals to the heavy earth removal filtrate, so that rare earth ions preferentially nucleate and grow into rare earth carbonate precipitates on the surface of the seed crystals. During the rare earth precipitation process, collect turbidity change data, conductivity change data, and rare earth residual amount data of the filtrate. Determine the rare earth precipitation endpoint based on the turbidity inflection point, the stabilization of conductivity change rate, and the decrease in rare earth residual amount of the filtrate. Obtain rare earth enriched precipitate residue and rare earth removal mother liquor through solid-liquid separation. S4. According to the magnesium precipitation window, adjust the rare earth mother liquor to a state where magnesium preferentially precipitates while calcium does not precipitate in large quantities, so that magnesium ions form magnesium hydroxide precipitate or basic magnesium salt precipitate, and obtain magnesium precipitate residue and magnesium removal mother liquor by solid-liquid separation. S5. According to the calcium precipitation window, add carbonate precipitant, bicarbonate precipitant or carbonate-containing circulating liquid to the magnesium removal mother liquor to form calcium carbonate precipitate. After solid-liquid separation, obtain calcium precipitate residue and high-salt purification mother liquor. S6. The heavy metal precipitate, rare earth enrichment precipitate, magnesium precipitate, calcium precipitate, and high-salt purification mother liquor are separately diverted for treatment. The heavy metal precipitate is disposed of as hazardous waste. The rare earth enrichment precipitate enters the rare earth recovery process. The magnesium precipitate and calcium precipitate are disposed of or utilized as resources. The high-salt purification mother liquor is reused in rare earth production or salt recovery processes after passing tests for residual rare earth, heavy metals, and hardness.

[0005] By constructing a dynamic correction mechanism with wastewater salinity as the core variable, the above technical solution solves the problem of inaccurate precipitation window caused by changes in ion activity coefficient in high-salt systems due to the traditional fixed pH. Its beneficial effect is that it can automatically adjust the precipitation priority of heavy metals, rare earths, magnesium, and calcium according to actual working conditions, ensuring that heavy metals are preferentially removed in the acidic section without entraining rare earths, and that rare earths preferentially nucleate in the weakly acidic section to avoid co-precipitation of calcium and magnesium. Ultimately, it achieves precise separation of the four target components in time and space, significantly improving the rare earth recovery rate and the purity of each precipitate.

[0006] Optionally, in step S1, the wastewater salt intensity is determined based on the concentrations of chloride ions, ammonium ions, calcium ions, magnesium ions, sodium ions, rare earth ions, and heavy metal ions in the high-salt rare earth wastewater, and the wastewater salt intensity is correlated with the apparent concentration of each metal ion, the effective concentration of the precipitant, and the historical precipitation curve to correct the precipitation initiation conditions of heavy metals, rare earths, magnesium, and calcium. When the increased salt intensity of wastewater leads to a decrease in the effective precipitation capacity of carbonate, the calcium precipitation window is delayed and the instantaneous addition rate of carbonate during the rare earth precipitation stage is reduced in order to maintain the state in which rare earths are preferentially precipitated while calcium and magnesium are not co-precipitated in large quantities.

[0007] By introducing the above technical solution and modeling the correlation between salt intensity, effective concentration of precipitant and historical precipitation curves, the system can quantify the shielding effect of high salt environment on carbonate activity. Thus, when salt intensity increases, it actively adopts a synergistic strategy of "delaying the calcium window" and "reducing the instantaneous acceleration rate". Its beneficial effect is that it effectively inhibits the premature nucleation of calcium carbonate under high salt conditions, controls the calcium entrainment rate of rare earth precipitation section at an extremely low level, and ensures the full growth of rare earth carbonate on the seed crystal surface.

[0008] Optionally, in step S2, the acidic weak reducing state is pH 2.5 to 4.8 and redox potential -150mV to +150mV; The sulfur-containing precipitant is added before or in stages with the iron salt composite precipitant, so that at least one of lead, cadmium, and copper first forms sulfide precipitates. Then, the iron salt composite precipitant adsorbs and co-precipitates at least one of zinc, nickel, and chromium or co-precipitates iron-based substances, thereby separating the heavy metal precipitate from the subsequent rare earth enrichment precipitate.

[0009] By setting specific pH and redox potential (ORP) windows and utilizing the staged addition sequence of sulfur-containing and iron salt composite scavengers, the differences in solubility products of different heavy metal sulfides and the adsorption bridging effect of iron salt hydrolysis products are taken advantage of. This allows easily precipitable metals such as Pb, Cd, and Cu to preferentially form sulfide cores, while difficult-to-precipitate metals such as Zn, Ni, and Cr are encapsulated and co-precipitated by iron-based flocculants. The beneficial effect is that it achieves highly efficient and deep removal of heavy metals (removal rate ≥99%), while avoiding the reduction and dissolution of rare earth ions or the formation of colloids under strong reducing conditions, thus ensuring the physical isolation between heavy metal slag and subsequent rare earth slag.

[0010] Optionally, before solid-liquid separation in step S2, residual sulfides in the heavy reaction solution are detected. When the residual sulfide content exceeds the allowable value for subsequent rare earth seed-induced precipitation, iron salts, oxygen-containing gas, or oxidants are added to the de-gravity reaction solution to convert the free sulfides into iron-sulfur precipitates or low-migration forms before solid-liquid separation, thereby reducing the interference of residual sulfides on the nucleation process of rare earth carbonates.

[0011] By adding a residual sulfide detection and passivation step before solid-liquid separation, and by using iron salt to generate FeS precipitate or by introducing oxygen / oxidant to oxidize sulfur ions into elemental sulfur or sulfate, the poisoning effect of free sulfides on the surface of rare earth seed crystals is eliminated. The beneficial effects are that sulfides prevent sulfides from hindering the heterogeneous nucleation of rare earth carbonates, shorten the induction period of rare earth precipitation, and significantly improve the particle size distribution and filtration performance of rare earth precipitate particles.

[0012] Optionally, in step S3, the rare earth precipitation window is pH 4.8 to 6.8, and satisfies the conditions that the rare earth carbonate reaches a supersaturated state, the calcium carbonate does not reach a rapid nucleation state, and the magnesium hydroxide is not generated in large quantities. The rare earth carbonate seed crystals or reflux rare earth precipitation seed crystals are lanthanum carbonate, cerium carbonate, mixed rare earth carbonates, basic rare earth carbonates, or fine crystals obtained by washing and classifying the previous batch of rare earth enrichment precipitate residue. The seed crystal dosage is 0.5% to 15% of the theoretical precipitation amount of rare earths in the wastewater.

[0013] By strictly controlling the reaction system within the thermodynamic selectivity window of pH 4.8–6.8 and introducing rare earth carbonate seeds with matching chemical composition, the low-energy-barrier nucleation sites provided by the seeds are utilized to promote the epitaxial growth of rare earth ions. At the same time, this pH range is below the threshold for the large-scale precipitation of magnesium hydroxide, and the rapid nucleation of calcium is inhibited by controlling the carbonate concentration. Its beneficial effect is that it enables the preferential precipitation of rare earth before the large-scale co-precipitation of calcium and magnesium, which greatly improves the grade of REO in the rare earth enriched precipitate and reduces the content of calcium and magnesium impurities.

[0014] Optionally, in step S3, the carbonate precipitant is added in a two-stage manner: The first stage is the induction nucleation stage, in which carbonate precipitant is slowly added in a low pH range while maintaining a high stirring intensity, so that rare earth ions form rare earth carbonate microcrystal nuclei on the surface of the seed crystal; the second stage is the crystal growth stage, in which the stirring intensity is reduced and carbonate precipitant is added again after the formation of rare earth carbonate microcrystal nuclei, so that the rare earth carbonate microcrystal nuclei grow into rare earth enriched precipitate particles that can be settled and filtered.

[0015] The above technical solution employs a two-stage addition strategy of "low pH high shear-induced nucleation" and "high pH low shear crystal growth." The first stage utilizes high stirring intensity to enhance mass transfer and ensure uniform coverage of microcrystal nuclei on the seed surface, avoiding local oversaturation that could lead to homogeneous nucleation. The second stage reduces the shear force that breaks down the microcrystal nuclei by lowering the stirring intensity, thus promoting the Ostwald maturation process. The beneficial effect is that it causes rare earth precipitate particles to transform from fine colloids into large-diameter, high-density, easily settling particles, significantly shortening the solid-liquid separation time and reducing the moisture content of the sludge cake.

[0016] Optionally, in step S3, the dosing rate of the carbonate precipitant is adjusted based on the turbidity change data and conductivity change data. When the rate of increase in turbidity reaches the rare earth nucleation judgment condition and the rate of decrease in conductivity does not exceed the calcium-magnesium coprecipitation judgment threshold, the carbonate precipitant addition acceleration rate is maintained or increased. When the rate of decrease in conductivity increases abnormally and the decrease in the residual amount of calcium or magnesium ions in the filtrate exceeds the set ratio, the addition of carbonate precipitant should be reduced or suspended to inhibit the entrainment of calcium and magnesium with rare earth enrichment precipitate residue.

[0017] The above technical solution establishes a dual-parameter feedback regulation mechanism based on the turbidity rise rate (characterizing rare earth nucleation) and conductivity decrease rate (characterizing ion consumption and co-precipitation risk). The reaction kinetics are monitored in real time. Once an abnormal drop in conductivity is detected, indicating that calcium and magnesium have begun to participate in precipitation, the dosing rate is immediately intervened. Its beneficial effect is that it dynamically adapts to fluctuations in influent water quality, ensuring complete rare earth precipitation while minimizing the risk of calcium and magnesium co-precipitation, thus achieving precise and intelligent reagent dosing.

[0018] Optionally, in step S3, the endpoint of rare earth precipitation is determined as follows: When at least two of the following conditions are met simultaneously: the turbidity change rate is less than 10% to 30% of the maximum turbidity change rate in the previous stage within a continuous set time, the absolute value of the conductivity change rate is less than 0.1% to 1.0% / min, and the amount of rare earth residue in the filtrate after filtration is less than the set recovery threshold, the addition of carbonate precipitant is stopped, and the rare earth enrichment precipitate is aged and then separated into solid and liquid phases.

[0019] The above technical solution replaces the traditional fixed time endpoint control by adopting a multi-dimensional combination of criteria, including turbidity change rate platform, conductivity change rate stabilization, and filtrate rare earth residue compliance. The reaction endpoint is cross-validated from three dimensions: kinetic stagnation, thermodynamic equilibrium, and target achievement. Its beneficial effect is that it effectively avoids rare earth loss due to insufficient reaction time or secondary calcium and magnesium entrainment due to excessive addition, ensuring the stability of rare earth recovery rate and the high purity of precipitate.

[0020] Optionally, in step S4, the magnesium precipitation window is pH 9.2 to 11.2, and an alkaline regulator is added under the condition that the added carbonate concentration is lower than the calcium rapid nucleation concentration, so that magnesium ions preferentially form magnesium hydroxide precipitate or basic magnesium salt precipitate. In step S5, the calcium precipitation window is pH 8.5 to 10.5. After the magnesium ion concentration decreases, carbonate precipitant, bicarbonate precipitant, or circulating liquid containing carbonate ions is added to form calcium precipitate mainly composed of calcium carbonate, thereby achieving staggered separation of magnesium precipitate and calcium precipitate.

[0021] Through the above technical solution, by strictly controlling the low carbonate environment in the S4 stage and utilizing only OH... -Magnesium is precipitated, and after the magnesium ion concentration is significantly reduced, calcium carbonate is introduced in stage S5 to precipitate calcium. This utilizes the difference in solubility and nucleation competition mechanism between magnesium hydroxide and calcium carbonate under different ionic environments to achieve staggered separation of magnesium and calcium. The beneficial effects are that high-purity magnesium hydroxide and calcium carbonate precipitates are obtained, avoiding the resource recovery difficulties caused by the mixing of the two, and significantly reducing the hardness of the final mother liquor, eliminating the risk of scaling during the reuse process.

[0022] Optionally, in step S6, the residual rare earth concentration, heavy metal residual concentration, calcium and magnesium hardness, suspended solids, conductivity, chloride concentration and ammonium concentration of the high-salt purification mother liquor are detected. When the high-salt purified mother liquor meets the reuse index, it is reused in rare earth leaching, slurry washing, extraction preparation, tail gas absorption or salt recovery processes. When at least one of the calcium and magnesium hardness, ammonium salt concentration, or chloride salt concentration fails to meet the reuse criteria, the high-salt purified mother liquor is introduced into softening, deammoniation, membrane concentration, electrodialysis, evaporation crystallization, or salt recovery processes.

[0023] By establishing a mother liquor quality testing system covering seven key indicators and diverting the mother liquor to direct reuse or advanced treatment paths based on the test results, the above technical solutions achieve differentiated matching for different reuse processes (such as leaching with high chlorine tolerance and liquid preparation with high hardness sensitivity). The beneficial effects are that it maximizes the recycling rate of water resources, avoids system failures caused by the reuse of unqualified mother liquor, and reduces the treatment load and operating costs of end-of-pipe wastewater. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 A flowchart of a stepwise selective precipitation separation method for high-salt rare earth wastewater is provided in one embodiment of this application; Figure 2 A trend diagram showing the relationship between rare earth recovery rate and calcium and magnesium entrainment amount is provided in one embodiment of this application; Figure 3 This is a comparison image of the SEM morphology of the precipitate provided in an embodiment of this application; Figure 4 A graph showing the changes in turbidity and conductivity over time in stage S3, provided in an embodiment of this application. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0027] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0028] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0029] Example 1 This embodiment provides a stepwise selective precipitation separation method for high-salt rare earth wastewater based on optimal process parameters, verifying the overall feasibility of the technical solution of the present invention and the synergy of various technical effects.

[0030] Step 1: Take 10L of high-salt rare earth wastewater, filter it through a screen to remove solid impurities, and then homogenize it. Initial water quality was tested: pH 4.2, conductivity 185 mS / cm, chloride ion 120 g / L, ammonium nitrogen 15 g / L, total rare earth elements (as REO) 2500 mg / L, calcium ion 8000 mg / L, magnesium ion 6000 mg / L, and total heavy metals (Pb+Cd+Cu+Zn) 450 mg / L. The wastewater salinity was calculated, and the sedimentation window was adjusted according to a preset model to determine the sedimentation order as: heavy metals → rare earth elements → magnesium → calcium.

[0031] Step 2: Heavy Metal Removal. Adjust the pH of the wastewater to 3.5 by adding dilute sulfuric acid, controlling the ORP to -50mV. First, add sodium sulfide solution (10% concentration), at a dosage of 1.2 times the theoretical molar amount of heavy metals, and react for 15 min. Then, add ferrous sulfate solution (20% concentration), at a dosage of 1.0 times the theoretical molar amount of heavy metals, and react for 20 min. The residual sulfide concentration is detected as 0.8 mg / L (higher than the allowable value of 0.2 mg / L). A small amount of ferrous sulfate is added, and aeration is performed for 5 min to reduce the residual sulfide concentration to 0.05 mg / L. The wastewater is then filtered using a plate and frame filter press to obtain heavy metal precipitate and a heavy metal-free filtrate.

[0032] Step 3: Rare Earth Selective Precipitation. Adjust the pH of the filtrate to 5.6 and control the temperature at 40℃. Add a batch of prepared rare earth carbonate fine crystals as seed crystals, at a dosage of 5% of the theoretical rare earth precipitation amount in the wastewater. Start a two-stage addition program: In the first stage, within the pH range of 5.0–5.6, slowly add 1.0 mol / L sodium bicarbonate solution at a rate of 0.5 L / h, with a stirring intensity of 300 r / min, for 30 min until the turbidity rises rapidly; in the second stage, within the pH range of 5.6–6.3, adjust the dropping rate to 1.2 L / h and reduce the stirring intensity to 100 r / min. Monitor turbidity and conductivity in real time during the process. When the turbidity change rate is less than 20% of the peak value and the absolute value of the conductivity change rate is less than 0.5% / min for 5 min, and the rare earth residue in the filtrate is sampled and found to be less than 20 mg / L, stop the addition. After aging for 30 min, filter to obtain rare earth enriched precipitate and rare earth removal mother liquor.

[0033] Step 4: Magnesium precipitation. Adjust the pH of the rare earth removal mother liquor to 10.2. No additional carbonate is added during this process; only sodium hydroxide is used to adjust the alkalinity. The reaction time is 40 minutes. After solid-liquid separation, magnesium precipitate residue and magnesium removal mother liquor are obtained.

[0034] Step 5: Calcium precipitation. Adjust the pH of the magnesium removal mother liquor to 9.5, add sodium carbonate solution (molar ratio Ca:CO3 = 1:1.1), and react for 45 minutes. After solid-liquid separation, obtain calcium precipitate residue and high-salt purification mother liquor.

[0035] Step 6: Product disposal. Heavy metal precipitates are sent to hazardous waste disposal; rare earth enrichment precipitates are washed and then acid-dissolved for recovery; magnesium and calcium precipitates are collected separately for later use; high-salt purification mother liquor that passes testing (rare earth <10mg / L, heavy metals <0.1mg / L, hardness <50mg / L) is reused for front-end solution preparation.

[0036] The results show that the rare earth enrichment precipitate obtained in this embodiment has a REO grade of 65.4%, a calcium content of only 1.2%, a magnesium content of 0.8%, and heavy metal content below the detection limit; the total rare earth recovery rate is 98.7%. The magnesium-based precipitate has an MgO content of 88.5%, and the calcium-based precipitate has a CaCO3 content of 92.1%. The high-salt purification mother liquor is clear and transparent, with no tendency to scale. These results confirm that the present invention successfully achieves efficient separation and resource recovery of multiple components through salt strength correction, segmented precipitation, and intelligent endpoint control.

[0037] Example 2 This embodiment aims to verify the effect of acidic weak reducing state (pH and ORP range) on heavy metal removal efficiency and subsequent rare earth precipitation, and to examine the technical effect under the lower limit of parameter values.

[0038] With all other preparation conditions the same as in Example 1, only the acidic weak reducing state in step S2 was adjusted to pH 2.5 and redox potential -150mV. Under these conditions, a sulfur-containing scavenger and an iron salt composite scavenger were added in the same proportion.

[0039] The results show that, under the lower limit of these parameters, the total heavy metal removal rate still reached 96.5%, slightly lower than in Example 1, but the rare earth loss rate in the filtrate was controlled within 0.5%. After passivation treatment with residual sulfides, the nucleation induction period of the subsequent rare earth precipitation stage was extended by about 5 minutes compared to Example 1, but the final rare earth recovery rate remained at 97.8%, and the heavy metal entrainment in the rare earth slag met the requirements for hazardous waste isolation. These results demonstrate that even at the edge of the pH and ORP ranges, the method of this invention can still maintain effective heavy metal separation and rare earth protection functions, exhibiting good process adaptability.

[0040] Example 3 This embodiment aims to verify the effects of the upper limit of the pH value of the rare earth precipitation window and the lower limit of the seed crystal dosage on the selectivity and precipitation performance of rare earths.

[0041] With all other preparation conditions the same as in Example 1, only the upper limit of the rare earth precipitation window pH in step S3 was adjusted to 6.8, and the amount of seed crystals added was adjusted to 0.5% of the theoretical amount of rare earth precipitation in the wastewater.

[0042] The results showed that under pH 6.8 conditions, the rare earth precipitation reaction rate accelerated, but the residual calcium ion content in the filtrate increased slightly, leading to an increase in the calcium content in the rare earth enriched precipitate to 2.5% (1.2% in Example 1). When the seed crystal dosage was 0.5%, the average particle size (D50) of the precipitate decreased from 110 μm in Example 1 to 75 μm, and the filtration time was correspondingly extended by 30%, but the rare earth recovery rate still reached 97.2%. This result proves that even under the boundary conditions of the parameter range, the present invention can still achieve preferential precipitation and recovery of rare earths. Although the product purity and filtration performance fluctuate slightly, the overall technical solution remains feasible and effective.

[0043] Example 4 This embodiment aims to verify the effectiveness of the staggered peak separation strategy for magnesium-based and calcium-based precipitation, especially to examine the comparative results under conditions where carbonate concentration is not strictly controlled.

[0044] With all other preparation conditions the same as in Example 1, in step S4, while adjusting the pH to 10.2, an excessive amount of sodium carbonate was mistakenly added (to make the carbonate concentration reach the concentration for rapid calcium nucleation), and the "low carbonate" control strategy was not implemented. The remaining steps were the same as in Example 1.

[0045] The results showed that due to the excessively high carbonate concentration in stage S4, some calcium ions precipitated prematurely during the magnesium precipitation stage, resulting in a decrease in MgO content to 65% and a rise in CaCO3 content to 25% in the mixed precipitate, failing to achieve effective separation of magnesium and calcium. In the subsequent stage S5, the calcium ion concentration had significantly decreased, leading to a substantial reduction in the amount of calcium-based precipitate produced. This reverse test result confirms the necessity of the technical characteristic of "adding carbonate concentrations lower than the rapid calcium nucleation concentration." Only by strictly implementing staggered separation can high-purity magnesium and calcium slag be obtained.

[0046] Example 5 This embodiment is a performance and effect verification embodiment, which aims to quantitatively evaluate the significant advantages of the method of the present invention in terms of rare earth recovery rate, slag phase purity and mother liquor reusability by setting a comparative example.

[0047] Comparative Example 1: The traditional one-time neutralization and precipitation method was used. High-salt rare earth wastewater from the same batch was taken, and lime slurry was directly added to adjust the pH to 9.0, causing all metal ions to co-precipitate. After filtration, a mixed residue was obtained, and the filtrate was treated separately.

[0048] Comparative Example 2: A conventional sulfide degravation + fixed pH carbonate precipitation method was used. S2 was the same as in Example 1, but seed crystals were not used in S3, and carbonate was added until the pH reached 6.5, without online feedback or endpoint determination.

[0049] The test items include: rare earth recovery rate, REO grade in rare earth slag, calcium and magnesium entrainment in rare earth slag, heavy metal removal rate, filtration time of precipitated slag (per unit volume), and high-salt mother liquor reuse qualification rate.

[0050] The specific test results are shown in Table 1.

[0051]

[0052] As shown in Table 1, the rare earth recovery rates of Examples 1-3 were consistently above 97%, significantly better than the 85.4% of Comparative Example 1 and 91.5% of Comparative Example 2. Particularly in terms of rare earth slag grade, Example 1 achieved 65.4%, while Comparative Example 1, due to severe calcium-magnesium co-precipitation, had a grade of only 42.1%, with a calcium-magnesium entrainment as high as 18.5%, severely impacting subsequent rare earth refining. Furthermore, the filtration time of Example 1 was only 3.5 min / L, much shorter than the 12.5 min / L of the Comparative Example, indicating that seed induction and two-stage addition significantly improved the physical properties of the precipitate.

[0053] like Figure 2As shown, compared with Comparative Examples 1 and 2, the rare earth recovery rate of Examples 1 to 3 is generally higher, and the calcium and magnesium entrainment is generally lower. This indicates that the cascade separation process can reduce calcium and magnesium co-precipitation while improving the rare earth recovery rate. With the implementation of the cascade separation process of the present invention, the rare earth recovery rate shows an upward trend, while the calcium and magnesium entrainment shows a significant downward trend. The two are obviously negatively correlated, which confirms the technical effect of "rare earth preferential precipitation while calcium and magnesium do not co-precipitate in large quantities".

[0054] like Figure 3 As shown, the rare earth carbonate particles obtained in Example 1 are regular spherical or ellipsoidal in shape, with uniform particle size distribution and dense surface; while the precipitate formed in Comparative Example 2 under seedless conditions is mostly fine flocculent material, loosely packed, which explains why Example 1 has superior filtration performance.

[0055] like Figure 4 As shown, in Example 1, the turbidity curve rises rapidly in the initial stage of addition and shows a clear plateau near the endpoint, while the conductivity curve shows a step-like decline and tends to flatten at the endpoint. The inflection points of the two are highly consistent, which verifies the accuracy and reliability of the "turbidity-conductivity dual-parameter endpoint determination" method proposed in this invention, and avoids the insufficient or excessive precipitation caused by fixed time control in Comparative Example 2.

[0056] Experimental results show that the high-salt purified mother liquor prepared by this invention meets the reuse criteria in Examples 1-3 and can be directly returned to the rare earth production system. However, the mother liquor of Comparative Example 1 cannot be directly reused due to excessive hardness. Therefore, the method provided by this invention can be used to prepare process solutions for preventing and / or solving the problems of low resource recovery rate, mixed slag phases, and difficulty in reusing mother liquor in the treatment of high-salt rare earth wastewater, and has significant industrial application value.

[0057] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for graded selective sedimentation separation of high-salt rare earth wastewater, characterized in that, include: S1. After homogenizing and removing solid impurities from the high-salt rare earth wastewater, the concentrations of rare earth ions, calcium ions, magnesium ions, heavy metal ions, chloride salts, ammonium salts, and conductivity in the high-salt rare earth wastewater are detected, and the salt intensity of the wastewater is determined based on the chloride salt concentration, ammonium salt concentration, and conductivity. Based on the salt intensity of the wastewater, the precipitation windows for heavy metals, rare earths, magnesium, and calcium are corrected to obtain a stepwise precipitation control sequence in which heavy metals, rare earths, magnesium, and calcium precipitate in sequence. S2. According to the heavy metal precipitation window, the high-salt rare earth wastewater is adjusted to an acidic weak reducing state, and sulfur-containing scavenging agent and iron salt composite scavenging agent are added to make the heavy metal ions in the wastewater form sulfide precipitates, iron-based co-precipitates or sulfide-iron-based composite precipitates. After solid-liquid separation, heavy metal precipitate residue and heavy metal removal filtrate are obtained. S3. According to the rare earth precipitation window, adjust the heavy earth removal filtrate to a state where rare earth preferentially precipitates while calcium and magnesium do not co-precipitate in large quantities. Add carbonate precipitant and rare earth carbonate seed crystals or reflux rare earth precipitation seed crystals to the heavy earth removal filtrate, so that rare earth ions preferentially nucleate and grow into rare earth carbonate precipitates on the surface of the seed crystals. During the rare earth precipitation process, collect turbidity change data, conductivity change data, and rare earth residual amount data of the filtrate. Determine the rare earth precipitation endpoint based on the turbidity inflection point, the stabilization of conductivity change rate, and the decrease in rare earth residual amount of the filtrate. Obtain rare earth enriched precipitate residue and rare earth removal mother liquor through solid-liquid separation. S4. According to the magnesium precipitation window, adjust the rare earth mother liquor to a state where magnesium preferentially precipitates while calcium does not precipitate in large quantities, so that magnesium ions form magnesium hydroxide precipitate or basic magnesium salt precipitate, and obtain magnesium precipitate residue and magnesium removal mother liquor by solid-liquid separation. S5. According to the calcium precipitation window, add carbonate precipitant, bicarbonate precipitant or carbonate-containing circulating liquid to the magnesium removal mother liquor to form calcium carbonate precipitate. After solid-liquid separation, obtain calcium precipitate residue and high-salt purification mother liquor. S6. The heavy metal precipitate, rare earth enrichment precipitate, magnesium precipitate, calcium precipitate, and high-salt purification mother liquor are separately diverted for treatment. The heavy metal precipitate is disposed of as hazardous waste. The rare earth enrichment precipitate enters the rare earth recovery process. The magnesium precipitate and calcium precipitate are disposed of or utilized as resources. The high-salt purification mother liquor is reused in rare earth production or salt recovery processes after passing tests for residual rare earth, heavy metals, and hardness.

2. The method according to claim 1, characterized in that, In step S1, the wastewater salt intensity is determined based on the concentrations of chloride ions, ammonium ions, calcium ions, magnesium ions, sodium ions, rare earth ions, and heavy metal ions in the high-salt rare earth wastewater. The wastewater salt intensity is then correlated with the apparent concentration of each metal ion, the effective concentration of the precipitant, and historical precipitation curves to correct the precipitation initiation conditions for heavy metals, rare earths, magnesium, and calcium. When the increased salt intensity of wastewater leads to a decrease in the effective precipitation capacity of carbonate, the calcium precipitation window is delayed and the instantaneous addition rate of carbonate during the rare earth precipitation stage is reduced in order to maintain the state in which rare earths are preferentially precipitated while calcium and magnesium are not co-precipitated in large quantities.

3. The method according to claim 1, characterized in that, In step S2, the acidic weak reducing state is pH 2.5 to 4.8 and redox potential -150mV to +150mV; The sulfur-containing precipitant is added before or in stages with the iron salt composite precipitant, so that at least one of lead, cadmium, and copper first forms sulfide precipitates. Then, the iron salt composite precipitant adsorbs and co-precipitates at least one of zinc, nickel, and chromium or co-precipitates iron-based substances, thereby separating the heavy metal precipitate from the subsequent rare earth enrichment precipitate.

4. The method according to claim 1, characterized in that, Before the solid-liquid separation in step S2, the residual sulfides in the heavy reaction solution are detected. When the residual sulfide content exceeds the allowable value for subsequent rare earth seed-induced precipitation, iron salts, oxygen-containing gas, or oxidants are added to the de-gravity reaction solution to convert the free sulfides into iron-sulfur precipitates or low-migration forms before solid-liquid separation, thereby reducing the interference of residual sulfides on the nucleation process of rare earth carbonates.

5. The method according to claim 1, characterized in that, In step S3, the rare earth precipitation window is pH 4.8 to 6.8, and the conditions are met that the rare earth carbonates reach a supersaturated state, the calcium carbonates do not reach a rapid nucleation state, and the magnesium hydroxides are not generated in large quantities. The rare earth carbonate seed crystals or reflux rare earth precipitation seed crystals are lanthanum carbonate, cerium carbonate, mixed rare earth carbonates, basic rare earth carbonates, or fine crystals obtained by washing and classifying the previous batch of rare earth enrichment precipitate residue. The seed crystal dosage is 0.5% to 15% of the theoretical precipitation amount of rare earths in the wastewater.

6. The method according to claim 1, characterized in that, In step S3, the carbonate precipitant is added in a two-stage manner: The first stage is the induction nucleation stage, in which carbonate precipitant is slowly added in a low pH range while maintaining a high stirring intensity, so that rare earth ions form rare earth carbonate microcrystal nuclei on the surface of the seed crystal; the second stage is the crystal growth stage, in which the stirring intensity is reduced and carbonate precipitant is added again after the formation of rare earth carbonate microcrystal nuclei, so that the rare earth carbonate microcrystal nuclei grow into rare earth enriched precipitate particles that can be settled and filtered.

7. The method according to claim 1, characterized in that, In step S3, the dosing rate of the carbonate precipitant is adjusted based on the turbidity change data and conductivity change data. When the rate of increase in turbidity reaches the rare earth nucleation judgment condition and the rate of decrease in conductivity does not exceed the calcium-magnesium coprecipitation judgment threshold, the carbonate precipitant addition acceleration rate is maintained or increased. When the rate of decrease in conductivity increases abnormally and the decrease in the residual amount of calcium or magnesium ions in the filtrate exceeds the set ratio, the addition of carbonate precipitant should be reduced or suspended to inhibit the entrainment of calcium and magnesium with rare earth enrichment precipitate residue.

8. The method according to claim 1, characterized in that, In step S3, the endpoint of rare earth precipitation is determined as follows: When at least two of the following conditions are met simultaneously: the turbidity change rate is less than 10% to 30% of the maximum turbidity change rate in the previous stage within a continuous set time, the absolute value of the conductivity change rate is less than 0.1% to 1.0% / min, and the amount of rare earth residue in the filtrate after filtration is less than the set recovery threshold, the addition of carbonate precipitant is stopped, and the rare earth enrichment precipitate is aged and then separated into solid and liquid phases.

9. The method according to claim 1, characterized in that, In step S4, the magnesium precipitation window is pH 9.2 to 11.2, and an alkaline regulator is added under the condition that the added carbonate concentration is lower than the calcium rapid nucleation concentration, so that magnesium ions preferentially form magnesium hydroxide precipitate or basic magnesium salt precipitate. In step S5, the calcium precipitation window is pH 8.5 to 10.

5. After the magnesium ion concentration decreases, carbonate precipitant, bicarbonate precipitant, or circulating liquid containing carbonate ions is added to form calcium precipitate mainly composed of calcium carbonate, thereby achieving staggered separation of magnesium precipitate and calcium precipitate.

10. The method according to claim 1, characterized in that, In step S6, the residual rare earth concentration, heavy metal residual concentration, calcium and magnesium hardness, suspended solids, conductivity, chloride concentration and ammonium concentration of the high-salt purification mother liquor are detected. When the high-salt purified mother liquor meets the reuse index, it is reused in rare earth leaching, slurry washing, extraction preparation, tail gas absorption or salt recovery processes. When at least one of the calcium and magnesium hardness, ammonium salt concentration, or chloride salt concentration fails to meet the reuse criteria, the high-salt purified mother liquor is introduced into softening, deammoniation, membrane concentration, electrodialysis, evaporation crystallization, or salt recovery processes.

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