METHOD FOR MULTI-STAGE CLASSIFICATION AND RECOVERY OF RARE EARTH ELEMENTS

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Patent Application No. 202411758122.7, filed on Dec. 3, 2024, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to separation and purification of rare earth elements (REEs) in the hydrometallurgical process of ionic rare earths, falling within the technical field of separation and recovery of REE(III) ions.

BACKGROUND

Most of the existing extraction and separation technologies are ion exchange resin and solvent extraction, while industrial methods mostly adopt solvent extraction: multi-stage extraction with different organic solvents (such as amines, esters, ethers, and organophosphorus compounds). Generally speaking, these extractants are flammable, toxic, expensive and easy to cause environmental pollution. Therefore, in response to the issue of effectively separating light rare earth elements (LREEs), middle rare earth elements (MREEs) and heavy rare earth elements (HREEs), developing a cost-effective and pollution-free process method for enriching and separating the three types of rare earths is of great significance for improving the recovery rate of REE(III) ions and simplifying the rare earth extraction process.

SUMMARY

An objective of the present disclosure is to solve the defects existing in the related art to provide a method for multi-stage classification and recovery of REEs, which is not only cheap, environmentally friendly, pollution-free, but also can recycle humic acid (HA), iron (hydr) oxides and inorganic phosphate to classify and recover full suite of REEs (15 types) in solution. By the solution, it can obtain a solution relatively rich in LREE, a solution relatively rich in MREE and a solution relatively rich in HREE.

The present disclosure can realize the classification recovery of REEs in mining and smelting process or specific and efficient recovery of trace elements in natural water.

The present disclosure adopts the following technical solutions.

A method for multi-stage classification and recovery of REEs includes the following six steps.

In step (1), an iron oxide I with a mass concentration of 0.1-5 g/L is added into an MREE separation tank, inorganic phosphate with a concentration of 20-100 μmol/L is added, the pH of a suspension is adjusted to 4-6 using a certain amount of inorganic acid, and the suspension is stirred for more than 8 hours. After the iron oxide I fully reacts with the phosphate, a full suite of rare earth leaching solution is introduced into the MREE separation tank, the mixed liquid is stirred for more than 24 hours, and the mixed liquid is performed solid-liquid separation by suction filtration. Phosphated iron oxide I is recovered for secondary recovery of MREEs, and a filtrate A is collected for later use.

In step (2), an iron oxide II with a mass concentration of 0.1-5 g/L is put into an HREE separation tank. The pH of a solution is adjusted to 7-8 using an inorganic alkali solution, the filtrate A in step (1) is introduced into an HREE separation tank to obtain a mixed filtrate, the mixed filtrate is stirred for more than 24 hours, and the mixed filtrate is performed solid-liquid separation by suction filtration. The iron oxide II is recovered for secondary recovery of HREEs, and a filtrate B is collected for later use.

In step (3), an HA with a mass concentration of 0.1-5 g/L is firstly put into an LREE separation tank, the pH of a solution is adjusted to 7-8 using an inorganic acid, the filtrate B in step (2) is introduced into the LREE separation tank, the mixed liquid is fully stirred for more than 24 hours to obtain a mixed filtrate, and the mixed filtrate is performed solid-liquid separation. The HA is recovered for secondary recovery of LREEs, and a filtrate C is allowed to flow into the HREE separation tank to realize the recovery of HREEs.

In step (4), secondary recovery of MREEs: recovered iron oxide I is placed in an MREE collecting tank, the pH of a suspension is adjusted to 3-5 by adding an inorganic acid, and the suspension is fully stirred for more than 24 hours. After REE(III) ions adsorbed on the iron oxide I are fully desorbed into the solution, solid-liquid separation is performed on the suspension, a solution rich in MREEs is collected, and purified MREE is obtained after drying. The iron oxide I is recycled for 1-3 times.

In step (5), secondary recovery of HREEs: iron oxide II is placed in an HREE recovery tank, the pH of a suspension is adjusted to 3-5 by adding an inorganic acid, and the suspension is fully stirred for more than 24 hours. After REE(III) ions in the filtrate C and adsorbed on the iron oxide II are fully desorbed into the solution, solid-liquid separation is performed on the suspension by suction filtration, a filtrate rich in HREEs is collected, and purified HREE is obtained after drying. The iron oxide II is recycled for 1-3 times.

In step (6), secondary recovery of LREEs: the recovered HA is placed in an LREE collecting tank I, the pH of a suspension is adjusted to 3-5 by adding an inorganic acid, and the suspension is fully stirred for more than 24 hours. After REE(III) ions adsorbed on the HA are completely desorbed into the solution, solid-liquid separation is performed on the suspension by suction filtration, a solution rich in LREEs is collected, and purified and separated LREE after drying is obtained. The HA is recycled for 1-3 times.

Further, the iron oxide I is ferrihydrite, and the synthesis method is as follows: slowly dropping a potassium hydroxide (KOH) solution with a concentration of 1 mol/L into a ferric nitrate (Fe(NO₃)₃) solution with a concentration of 0.5 mol/L, fully stirring at the same time until the pH of the solution reaches 7-8, quickly centrifuging, repeatedly immersing the obtained solid substance with deionized water until a conductivity is lower than 20 μS/cm, and obtaining a pure phase ferrihydrite after freeze-drying. A specific surface area of the synthesized ferrihydrite is about 300 m²/g, and pH at a point of zero charge (pH_zpc)=8.

Further, the iron oxide II is goethite, and the synthesis method is as follows: slowly adding a KOH solution with a concentration of 2.5 mol/L dropwise to a Fe(NO₃)₃ solution with a concentration of 0.2 mol/L, and fully stirring at the same time until the pH of the solution reaches 12; and heating the fully stirred suspension in a 60° C. water bath for aging for 72 hours, and performing high-speed centrifugation; and repeatedly rinsing the solid with deionized water until a conductivity is lower than 20 μS/cm, and obtaining a pure phase goethite after freeze-drying. A specific surface area of the synthesized goethite is about 60 m²/g, and a pH_zpc=9.

Further, in step (1), the pH of the suspension is adjusted to 5-5.5. Weak acidic pH condition is beneficial to the adsorption of inorganic phosphate by ferrihydrite, and improves the ternary complexation between REE(III) ions and phosphate, which is the key to selective adsorption of MREE(III) ions. When pH<3, ferrihydrite may be dissolved to some extent, resulting in adverse effects. When 3<pH<5, the surface of ferrihydrite carries an excessive positive charge, which has a strong repulsive effect on rare earth cations and is not conducive to the adsorption of REE(III) ions. When pH>5.5, the adsorption of phosphate by ferrihydrite weakens, but the direct adsorption of REE(III) ions is enhanced, which is not conducive to the formation of ternary complexation between phosphate and REE(III) ions, and reduces the selectivity to MREE(III) ions, resulting in adverse effects.

Further, in step (2), the pH of the suspension is adjusted to 7-7.5. Under a medium alkaline pH condition, the surface of goethite is negatively charged, which is conducive to the adsorption of positively charged rare earth cations. However, when pH>7.5, the REE(III) ions are prone to form REE hydroxides (REE(OH)₃) precipitates, which is not conducive to the selective adsorption of HREE(III) ions by goethite and can no longer desorb and recover REE(III) ions.

Further, in step (3), the pH of the solution is adjusted to 7-7.5. Under the medium alkaline pH condition, the HA is beneficial to adsorb rare earth cations. However, when pH>7.5, the REE(III) ions are prone to form REE(OH)₃ precipitates, which is not conducive to the selective adsorption of LREE(III) ions by HA, and can no longer desorb and recover REE(III) ions.

Further, in step (4), the pH of the suspension is adjusted to 3-3.5. Under an acidic pH condition, it is beneficial to the desorption of REE(III) ions from the surface of hydropyrite and realize the recovery of REE(III) ions. When pH<3, ferrihydrite may dissolve to a certain extent, thereby generating free Fe³⁺, forming impurities and reducing the purity of REE(III) ions.

Further, in step (5), the pH of the suspension is adjusted to 3-3.5. Under the acidic pH condition, it is beneficial to the desorption of REE(III) ions from the surface of goethite and realize the recovery of REE(III) ions. When pH<3, ferrihydrite may be dissolved to a certain extent, thereby generating free Fe³⁺, forming impurities and reducing the purity of REE(III) ions.

Further, in step (6), the pH of the suspension is adjusted to 3-4. Under the acidic pH condition, it is beneficial to the desorption of REE(III) ions from the surface of goethite and realize the recovery of REE(III) ions. When pH<3, HA may partially dissociate or precipitate, resulting in HA being unable to be recycled.

The present disclosure has the following advantageous effects.

1. The present disclosure can effectively separate and purify full suite of REE(III) ions into a solution rich in LREE, a solution rich in MREE and a solution rich in HREE.

2. The use of organic solvent extractant can be avoided, and the environmental pollution can be reduced. The workload can be reduced for subsequent single-element purification process engineering.

3. The adsorption and desorption fillers used, including the iron oxide I and humus, can be recycled to reduce costs. In the present disclosure, the extractant environment-friendly and pollution-free, and is a natural mineral and natural humus with low price. Note: adsorption reaction means that metal ions or acid ions are fixed on the surface of the adsorption material through electrostatic attraction (physical adsorption) or complexation reaction (chemical adsorption). It is a common low-cost and economical recovery and classification method without additional energy. The combination of different adsorption materials and different solution environments (including pH, ionic strength, etc.) can efficiently and selectively separate target ions.

4. Molecular-level mechanism: (1) under acidic conditions, the iron hydroxyl groups on the surface of ferrihydrite (including isolated iron hydroxyl FeOH, twin iron hydroxyl Fe₂OH, and tri-coordinated iron hydroxyl Fe₃OH) protonate to form positively charged FeOH₂ surface groups, which help to adsorb free phosphate (H₂PO₄ is the main form of inorganic phosphate under acidic conditions). At this time, the inner ring adsorption of phosphate on the surface of ferrihydrite occurs, forming a protonated bidentate binuclear configuration (Fe₂O₂PO₂H). When REE(III) ions are added to the solution, the REE(III) ions interact with phosphate adsorbed on the surface of ferrihydrite, and a stable ternary complex with phosphate as a bridge is generated. Under acidic conditions, the ternary complex generally exhibits a monodentate ternary complex configuration (Fe₂O₂POOREE⁺), while under neutral conditions, the ternary complex changes to a bidentate ternary complex configuration (Fe₂O₂PO₂REE⁺). This ternary complexation mechanism not only significantly enhances the adsorption capacity of REE(III) ions, but also exhibits selectivity for MREE(III) ions during macroscopic adsorption due to the lower adsorption energy of monodentate ternary complex configuration for MREEs.

(2) Goethite has a needle-like morphology, and its main exposed crystal planes are (101) and (210) crystal planes, among which the (210) crystal plane shows higher reactivity. Both crystal planes are rich in isolated iron hydroxyl groups (FeOH), which easily form bidentate coordination configurations (Fe₂O₂REE) with REE(III) ions. Due to the smaller ionic radius, HREE(III) ions are more prone to hydrolysis, and are more prone to complexation with isolated iron hydroxyl groups. Therefore, goethite exhibits significant selective adsorption of HREE(III) ions.

(3) The surface of HA is usually rich in various functional groups including carboxyl (—COOH), hydroxyl (—OH) and phenolic hydroxyl groups. The complex structure and diversified functional groups can provide a variety of adsorption sites for the formation of REE(III) ions and form a variety of coordination configurations. However, the radius of LREE(III) ions is relatively large, the formed complex has small steric hindrance and can form a stable multidentate complex with multiple coordination groups of HA, thus HA has selective adsorption of LREE(III) ions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of separation and collection of LREEs, MREEs and HREEs according to the present disclosure.

FIG. 2 is a diagram of preferential selective adsorption of LREEs by humus.

FIG. 3 is a diagram of selective adsorption of MREE(III) ions by phosphate-loaded ferrihydrite.

FIG. 4 is a diagram of selective adsorption of HREE(III) ions by goethite.

FIG. 5 shows separation effect of LREE(III) ions, MREE(III) ions and HREE(III) ions after solutions are implemented.

DETAILED DESCRIPTION

For clearer objectives, technical solutions and advantages of the present disclosure, the technical solutions of the present disclosure will be described clearly and completely in the following. Obviously, all the described examples are only some, rather than all examples of the present disclosure. Based on the examples in the present disclosure, all other examples obtained by those ordinary skilled in the art without creative efforts belong to the protection scope of the present disclosure.

In the present disclosure, the preferential adsorption behavior of humus to LREE(III) ions, the preferential adsorption behavior of goethite to HREE(III) ions and the preferential adsorption behavior of phosphated ferrihydrite (phosphate-loaded ferrihydrite) to MREE(III) ions are discovered and fully utilized, and a suitable adsorption sequence is constructed to realize an economical, environmentally friendly and efficient separation and purification solution of LREE, MREE and HREE. The adsorption rates of the three types of adsorption systems are as follows.

(1) As shown in FIG. 2, under neutral pH conditions (pH=7), humus has a higher adsorption rate for LREEs (80%-100%), while the adsorption rate for HREEs is very low (only less than 20%), and humus can separate LREE(III) ions from medium and HREE(III) ions. After a simple filtration operation, the humus adsorbing LREE(III) ions can desorb the LREE(III) ions by adjusting the pH (acidity, such as pH=3), and finally a large amount of LREE(III) ions can be released into the LREE collecting tank.

(2) As shown in FIG. 3, phosphate-loaded ferrihydrite has a strong selective adsorption effect on MREs. The adsorption rate of phosphate-loaded ferrihydrite for MREE(III) ions reaches 60%, while the adsorption rate of phosphate-loaded ferrihydrite for LREE(III) ions is only 20%-40%, and the adsorption rate of phosphate-loaded ferrihydrite for HREE(III) ions is about 30%.

(3) As shown in FIG. 4, from LREEs to HREEs, the adsorption rate of goethite for the three types of rare earths increases nearly linearly, leading to a greater enrichment of HREE(III) ions. Therefore, after the separation of MREE(III) ions and LREE(III) ions, the selective enrichment of goethite to HREE is more obvious.

Example 1

The laboratory simulates the purification and separation operation of full suite of rare earth ion standard solution, with a commercial brand of the standard solution being Accustandard (the content of each type of rare earth ion is 100 parts per billion (ppb)).

As shown in FIG. 1 and FIG. 5, in step (1), firstly, ferrihydrite with a water and soil mass concentration of 1 g/L is put into an MREE separation tank. Certainly, the ferrihydrite with a mass concentration of 0.1-5 g/L is also suitable, and sodium dihydrogen phosphate with a concentration of 60 μmol/L is added into the separation tank. Certainly, the sodium dihydrogen phosphate with a concentration of 20-100 μmol/L is also suitable. The pH of the suspension is adjusted to 5 using hydrochloric acid or nitric acid with a concentration of 0.1 mol/L. Certainly, the adjusted suspension with pH=4-6 is also suitable, and the suspension is fully stirred for 8 hours or more. The rare earth full suite of solution (a concentration of each type of adsorbed ion is 100 ppb) is introduced into the MREE separation tank, the mixture liquid is fully stirred for 24 hours, and the mixture liquid is subjected to solid-liquid separation by suction filtration; and the phosphated iron oxide I (surface adsorbed REE(III) ions) is recovered for secondary recovery of MREEs, and a filtrate A is collected for later use. The concentration of REE(III) ions is determined by inductively coupled plasma-mass spectrometry (ICP-MS).

In step (2), an iron oxide II with a mass concentration of 1 g/L is put into an HREE separation tank, and certainly, an iron oxide II with a mass concentration of 0.1-5 g/L is also suitable. The pH of the solution is adjusted to 7.5 using sodium hydroxide with a concentration of 0.1-1 mol/L, and certainly, the adjusted solution with pH=7-8 is also suitable. The filtrate A in step (1) is introduced into an HREE separation tank and the mixed liquid is fully stirred for 24 hours to obtain a mixed filtrate; the mixed filtrate is subjected to solid-liquid separation by suction filtration; and the iron oxide II is recovered for secondary recovery of HREEs, and a filtrate B is collected for later use.

In step (3), humus with a mass concentration of 1 g/L is firstly put into an LREE separation tank, certainly, humus with a mass concentration of 0.1-5 g/L is also suitable, and the pH of the suspension is adjusted to 7-8 using hydrochloric acid or nitric acid with a concentration of 0.1-1 mol/L. The filtrate B in step (2) is introduced into the LREE separation tank, the mixed liquid is fully stirred for 24 hours to obtain a mixed filtrate, and the mixed filtrate is subjected to solid-liquid separation by suction filtration; and the humus (including rare earth) is recovered for secondary recovery of LREEs, and a filtrate C flows into an HREE recovery tank to realize the recovery of the HREEs. As shown in Table 1, it is a rare earth ion concentration table of adsorbent and filtrate.

In step (4), secondary recovery of MREEs: the recovered iron oxide I is placed in an MREE collection tank, and hydrochloric acid or nitric acid with a concentration of 0.1-1 mol/L is added to adjust the pH of the suspension to 3, certainly, the adjusted suspension with a pH range of 3-5 is also suitable, and the mixture is stirred fully for 24 hours; and after the REE(III) ions are completely desorbed into the solution, the suspension is subjected to solid-liquid separation by suction filtration, a solution rich in MREEs is collected, and purified MREE is obtained after simple evaporation and drying. The iron oxide I is recycled for 1-3 times. Table 2 shows the concentration of adsorbed MREE(III) ions and the concentration of recovered MREE(III) ions as follows:

In step (5), secondary recovery of HREEs: an iron oxide II is placed in the HREE recovery tank, and hydrochloric acid or nitric acid with a concentration of 0.1 mol/L is added to adjust the pH of the suspension to 3, certainly, the adjusted suspension with a pH range of 3-5 is also suitable, and the mixture is stirred fully for 24 hours; and after the REE(III) ions adsorbed on iron oxide II and in the filtrate C are desorbed into the solution, the suspension is subjected to solid-liquid separation by suction filtration, the filtrate rich in HREEs is collected, and purified HREE is obtained after simple evaporation and drying. The iron oxide II is recycled for 1-3 times. The concentration of adsorbed HREE(III) ions and the concentration of recovered HREE(III) ions are shown in Table 3:

In step (6), secondary recovery of LREEs: the recovered humus is placed in an LREE collecting tank I, and hydrochloric acid or nitric acid with a concentration of 0.1 mol/L is added to adjust the pH of the suspension to 3, certainly, the adjusted suspension with a pH range of 3-5 is also suitable, and the mixture is stirred fully for 24 hours; and after the REE(III) ions are completely desorbed into the solution, the suspension is subjected to solid-liquid separation by suction filtration, the solution rich in LREEs is collected, and purified and separated LREE is obtained after simple evaporation and drying. The humus is recycled for 1-3 times. The concentration of adsorbed LREE(III) ions and concentration of recovered LREE(III) ions are shown in Table 4:

Finally, it is to be noted that the above examples are only used to describe technical solutions of the present disclosure, and are not intended to limit to the present disclosure. Although the present disclosure has been described in detail with reference to the foregoing examples, those skilled in the art will understand that the technical solutions disclosed in the above examples can still be modified, or some of the technical features thereof can be replaced by equivalents. However, these modifications and substitutions do not make the essence of the corresponding technical solutions depart from the spirit and scope of the technical solutions of various examples of the present disclosure.