A method for direct leaching of calcium, phosphorus and synchronous extraction of associated rare earth elements from phosphate ore by phosphoric acid

Abstract

The application discloses a method for directly leaching calcium and phosphorus in phosphate ore by phosphoric acid and simultaneously extracting associated rare earth elements. After the phosphate ore is directly leached by the phosphoric acid, solid-liquid separation is performed to obtain strong-acid phosphoric acid leaching solution containing phosphorus, calcium and rare earth elements, total rare earth elements are directly extracted under strong-acid conditions by using p-nonyl phenyl dihydrogen phosphate / TBP synergistic organic phase, the loaded organic phase is recycled after reverse extraction of the rare earth elements by sulfuric acid and reverse extraction of co-extracted iron by oxalic acid, and mixed rare earth oxides are prepared through oxalate precipitation and roasting of the reverse extraction solution of the rare earth elements. Sulfuric acid is added into the raffinate to precipitate calcium in the form of dihydrate gypsum, and convert calcium dihydrogen phosphate into phosphoric acid, and the obtained phosphoric acid solution is returned to leaching after concentration adjustment. The method places the rare earth recovery before the formation of gypsum, can reduce the loss of the rare earth elements entering the gypsum, avoids the introduction of exogenous impurity ions in the process, and can realize the recycling of the phosphoric acid.

Description

Technical Field

[0001] This invention belongs to the field of comprehensive utilization of phosphate rock resources and hydrometallurgical technology. Specifically, it relates to a method for recycling phosphate rock by directly leaching it with phosphoric acid, extracting and recovering total rare earth elements in a strongly acidic phosphate system, and regenerating phosphoric acid by calcium precipitation with sulfuric acid after rare earth recovery. Background Technology

[0002] Rare earth elements (REEs) in phosphate rock are typically low in content and mainly enter the apatite lattice via isomorphism, exhibiting a high degree of symbiosis with Ca and P at the mineral scale. This makes it difficult to achieve separate enrichment of REEs through conventional beneficiation. Therefore, the recovery of associated REEs usually needs to be carried out in conjunction with the hydrometallurgical processing of phosphate rock.

[0003] Traditional wet-process phosphoric acid production often involves the direct decomposition of phosphate rock with sulfuric acid, generating a large amount of calcium sulfate during the dissolution of the phosphate rock. Because rare earth elements and calcium have a strong mineralogical symbiotic relationship, some rare earth elements are easily carried, adsorbed, or incorporated into the crystal structure of gypsum during the leaching stage, thus reducing the rare earth recovery rate and increasing the difficulty of subsequently recovering rare earth elements from phosphogypsum.

[0004] Direct leaching of phosphate rock with phosphoric acid first decomposes the apatite crystal lattice, allowing Ca, P, and associated rare earth elements to enter the liquid phase, without the need to introduce sulfate ions during the leaching stage. The resulting leachate is characterized by high acidity, high phosphate concentration, high calcium content, and relatively low rare earth element concentration. If a large amount of alkaline reagent is used for neutralization before extraction, it can easily cause a local pH increase and induce the redeposition of rare earth phosphates; if calcium is precipitated with sulfuric acid before rare earth recovery, the problem of rare earth loss along with calcium sulfate may occur again.

[0005] Commonly used acidic phosphorus-containing extractants such as P204, P507, and Cyanex 272 show a significant decrease in extraction capacity under high acidity conditions. Therefore, there is an urgent need to establish a combined process that can be directly linked to phosphoric acid leaching, directly recover rare earth elements in a strongly acidic phosphate system, and arrange the calcium precipitation step after rare earth recovery. This would decouple the timing of rare earth recovery and gypsum formation, while simultaneously converting calcium-containing phosphate into recyclable phosphoric acid. Summary of the Invention

[0006] The purpose of this invention is to provide a method for directly leaching calcium and phosphorus from phosphate rock with phosphoric acid and simultaneously extracting associated rare earth elements. This method is based on a core process of "phosphoric acid leaching—direct extraction of total rare earth elements with strong acid—stepwise back-extraction—calcium precipitation with sulfuric acid to regenerate phosphoric acid." The rare earth recovery step is placed before the formation of calcium sulfate to reduce the risk of rare earth loss with gypsum and to achieve the recycling of the phosphoric acid leaching agent.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: 1) Phosphoric acid leaching: The phosphate rock is dried and finely ground, and leached with phosphoric acid as the leaching agent under suitable liquid-to-solid ratio, temperature, and stirring conditions. After the reaction, the solid and liquid are separated to obtain a strongly acidic phosphoric acid leaching solution and a silica-rich leaching residue. The preferred conditions are: -200 mesh particle mass fraction not less than 85%, phosphoric acid concentration 4.5–7.5 mol / L, liquid-to-solid ratio 6:1–10:1 mL / g, temperature 45–65 ℃, leaching time 4–8 h, and stirring speed 200–400 r / min.

[0008] 2) Total rare earth extraction: Phosphoric acid leachate is used directly as the extraction feed. The feed is in a strongly acidic range with pH < 1. Multi-stage countercurrent extraction is performed on the organic phase of sulfonated kerosene containing p-nonylphenyl dihydrogen phosphate and TBP. This allows the trivalent rare earth elements such as La, Ce, Pr, Nd, Sm, Gd, Dy, and Y to enter the organic phase as a whole, achieving separation from Ca, Mg, and Al.

[0009] p-Nonylphenyl dihydrogen phosphate exhibits high effective acidity under strong acid conditions. TBP, as a synergistic component, can improve the stability of rare earth extractants in the organic phase and, at an appropriate volume fraction, enhance rare earth extraction and reduce aluminum co-extraction. The organic phase contains 5–15 vol% p-nonylphenyl dihydrogen phosphate and 1–10 vol% TBP, with the balance being sulfonated kerosene. Preferably, the organic phase consists of 10 vol% p-nonylphenyl dihydrogen phosphate, 5 vol% TBP, and 85 vol% sulfonated kerosene. 3–6 stages of countercurrent extraction are used, with an aqueous / organic phase volume ratio (A / O) of 1:1–3:1, an extraction temperature of 20–35 °C, and a single-stage contact time of 5–20 min; preferably, five stages of countercurrent extraction are used, with an A / O ratio of 2:1, a temperature of 27±1 °C, and a single-stage contact time of 10 min.

[0010] 3) Rare earth back-extraction: A multi-stage countercurrent back-extraction process is used to transfer the rare earth-supported organic phase into the aqueous phase using sulfuric acid solution, yielding a rare earth sulfate back-extraction solution. The sulfuric acid solution concentration is 0.5–2.0 mol / L, with 3–6 stages of countercurrent back-extraction and an A / O ratio of 1:2–1:6. Preferably, 1 mol / L H₂SO₄, five stages of countercurrent back-extraction, and an A / O ratio of 1:4 are used.

[0011] 4) Co-extraction for iron removal and organic phase regeneration: The organic phase after rare earth back-extraction is used for co-extraction of iron using oxalic acid solution with a concentration of 0.5–1.5 mol / L. Three to six stages of countercurrent back-extraction are employed, with an A / O ratio of 1:1–1:4. Preferably, 1 mol / L H₂C₂O₄, five stages of countercurrent back-extraction, and an A / O ratio of 1:2 are used. The organic phase after water washing is returned to the extraction process for recycling.

[0012] 5) Rare earth product preparation: Sodium oxalate and other oxalate precipitants are added to the rare earth back-extraction solution to precipitate the rare earth in the form of oxalate; the precipitate is then separated into solid and liquid, washed, dried and calcined to obtain a mixed rare earth oxide enrichment.

[0013] 6) Calcium precipitation and phosphoric acid regeneration in the raffinate: The raffinate after rare earth extraction mainly consists of calcium dihydrogen phosphate and free phosphoric acid. Based on the Ca content in the raffinate... 2+ Add sulfuric acid according to the content measurement, so that Ca 2+ With SO4 2- Calcium sulfate crystals are formed, releasing phosphoric acid in the process. SO42- control is preferred. 2- / Ca 2+ The molar ratio is 1.0 to 1.2:1. Sulfuric acid is slowly added at 60 to 80 °C and the reaction is carried out for 0.5 to 2 h, followed by aging for 0.5 to 2 h to promote the growth of gypsum dihydrate crystals and filtration.

[0014] The calcium precipitation and re-acidification process can be represented by the following formula: Ca(H2PO4)2 + H2SO4 + 2H2O → CaSO4·2H2O↓ + 2H3PO4. The CaSO4·2H2O obtained by filtration can be used as a gypsum by-product, and the filtrate is the regenerated phosphoric acid solution. After the filtrate is concentrated by evaporation or by adding phosphoric acid to adjust it to the required concentration, it is returned to the phosphoric acid leaching step, and the gypsum washing solution can be incorporated into the regenerated phosphoric acid solution.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) Phosphate ore is first leached directly with phosphoric acid. No calcium sulfate is generated during the decomposition of apatite and the release of rare earth elements, thus avoiding early losses caused by the synchronous migration of rare earth elements and gypsum during the traditional direct decomposition of sulfuric acid.

[0016] (2) The total rare earth elements are directly extracted in a strongly acidic phosphate solution using a nonylphenyl dihydrogen phosphate / TBP system, which reduces the need for large-scale neutralization operations, thereby reducing the risk of rare earth phosphate reprecipitation and achieving the separation of rare earth elements from Ca, Mg and Al.

[0017] (3) By back-extracting rare earth with sulfuric acid and co-extracting iron with oxalic acid, the preparation of rare earth products and the regeneration of organic phase are connected, and the organic phase can be recycled back to the extraction process.

[0018] (4) After rare earth has been basically removed from the aqueous phase, sulfuric acid is added to the raffinate to precipitate calcium, which can convert Ca(H2PO4)2 into CaSO4·2H2O and regenerate H3PO4, realizing the process organization of "first recovering rare earth, then generating gypsum, and then recycling phosphoric acid", thereby improving the comprehensive utilization of phosphorus, calcium and rare earth. Detailed Implementation

[0019] The present invention will be further described below with reference to the embodiments. The following embodiments are used to illustrate the process route, operating conditions and representative experimental results of the present invention, and are not intended to limit the scope of protection of the present invention.

[0020] The raw material used in the examples was flotation phosphate concentrate from a mine in Guizhou. Mineralogical analysis showed that apatite was the main mineral, accounting for about 86.23 wt%, quartz about 6.07 wt%, and dolomite about 5.08 wt%. Rare earth elements mainly entered the apatite lattice in isomorphous form and were highly associated with Ca and P.

[0021] Example 1: Direct Leaching of Phosphoric Acid The phosphate rock was dried and finely ground to achieve a particle size fraction of over 85% (-200 mesh). The phosphate rock was then mixed with 6.0 mol / L phosphoric acid at a liquid-to-solid ratio of 8:1 mL / g and leached at a constant temperature of 55 ℃ with stirring at 300 r / min for 6 h. After leaching, solid-liquid separation was performed, and the leaching residue was washed and dried. The filtrate was used as the feed solution for subsequent rare earth extraction.

[0022] Under the above conditions, the leaching rates of rare earth elements, Ca, and P reached 96.49%, 98.42%, and 99.17%, respectively. XRD analysis showed that the characteristic peaks of apatite basically disappeared after leaching, and SiO2 became the main residual phase in the leaching residue, indicating that phosphoric acid can effectively destroy the apatite lattice and allow associated rare earth elements to enter the liquid phase in synergy with Ca and P.

[0023] When the response surface methodology was used to further optimize the phosphoric acid concentration, liquid-to-solid ratio, temperature, and time, the model prediction conditions were: phosphoric acid concentration 6.78 mol / L, liquid-to-solid ratio 8.77 mL / g, temperature 56.49 ℃, and leaching time 6.76 h. The predicted rare earth leaching rate was 98.04%. The relative error between the verification experiment and the model prediction was within ±2%.

[0024] Example 2: Extraction of total rare earth elements from strongly acidic phosphoric acid leachate by p-nonylphenyl dihydrogen phosphate / TBP The phosphoric acid leachate obtained in Example 1 was used as the extraction feed. The feed was in a strongly acidic range with pH < 1, and no large-scale alkali neutralization was used. The organic phase consisted of 10 vol% p-nonylphenyl dihydrogen phosphate, 5 vol% TBP, and 85 vol% sulfonated kerosene. Five-stage countercurrent extraction was carried out at 27±1 °C for 10 min under single-stage contact conditions, with an A / O ratio of 2:1.

[0025] Under the above conditions, the total rare earth extraction rate can reach over 99%; Fe is completely co-extracted into the organic phase, while the co-extraction of Al and Ca is relatively low, and Mg is basically retained in the aqueous phase. This indicates that the p-nonylphenyl dihydrogen phosphate / TBP system can achieve the separation of total rare earth elements from the main Al, Ca, and Mg impurities in a strongly acidic phosphate system.

[0026] Example 3: Rare earth back-extraction, iron back-extraction and organic phase regeneration The rare earth-supported organic phase obtained in Example 2 was subjected to five-stage countercurrent back-extraction using 1 mol / L H2SO4, with an A / O ratio of 1:4, a temperature of 27±1 ℃, and a single-stage contact time of 10 min. The back-extraction rate, based on total rare earth content, reached over 97%; while the back-extraction rate of co-extracted Fe in this step was less than 5%, thus achieving the separation of rare earth and iron.

[0027] The organic phase after rare earth back-extraction was further subjected to five-stage countercurrent iron back-extraction using 1 mol / L H2C2O4, with an A / O ratio of 1:2, a temperature of 27±1 ℃, and a single-stage contact time of 10 min, achieving an iron back-extraction rate of 98.3%. The organic phase after iron back-extraction was washed with deionized water and returned to the extraction process of Example 2 for recycling.

[0028] Example 4: Preparation of mixed rare earth oxides Sodium oxalate was added to the rare earth back-extraction solution obtained in Example 3 to precipitate the rare earth elements as oxalate. The precipitate was then filtered, washed, dried, and calcined to obtain a mixed rare earth oxide concentrate.

[0029] Example 5: Regeneration and reuse of calcium precipitation and phosphoric acid in the raffinate The raffinate obtained after extraction in Example 2 was used as the original calcium precipitation solution. The Ca content was determined by ICP. 2+ To calculate the concentration of sulfuric acid added, heat the raffinate to 70 °C while stirring, and slowly add sulfuric acid to make SO42-. 2- / Ca 2+ The molar ratio was 1.05:1; after the acid was added, the reaction continued for 1 hour, and then aged for another 1 hour to allow the calcium sulfate crystals to grow fully.

[0030] After the reaction, the mixture was filtered to obtain gypsum residue mainly composed of CaSO4·2H2O and regenerated phosphoric acid filtrate. The gypsum was washed with a small amount of water, and the washings were combined with the regenerated phosphoric acid filtrate. The filtrate was then concentrated by evaporation or by adding phosphoric acid to adjust the phosphoric acid concentration to 6.0 mol / L before being returned to Example 1 as a leaching agent.

[0031] Since the calcium precipitation step is set after the total rare earth extraction, the concentration of rare earth in the aqueous phase entering the gypsum crystallization step has been significantly reduced, which reduces the chance of rare earth co-precipitation or entrainment with calcium sulfate from the process sequence. At the same time, the calcium dihydrogen phosphate in the original extract is converted into phosphoric acid, realizing the separation of calcium as a gypsum by-product and the regeneration of phosphoric acid leaching agent.

[0032] Comparative Example 1: Strong Acid Suitability of Commonly Used Acidic Phosphorus-Containing Extractants Under the same strongly acidic phosphate conditions, P204, P507, Cyanex 272, and p-nonylphenyl dihydrogen phosphate were compared as main extractants. As the acidity of the feed solution increased, the total rare earth extraction capacity of each extractant decreased to varying degrees. When the feed solution pH was 1.8, the total rare earth extraction rates of P204 and p-nonylphenyl dihydrogen phosphate were 97.79% and 100%, respectively, while the total rare earth extraction rate of P507 was also close to 80%. When the pH of the feed solution decreased from 1.8 to 0.1, the extraction rates of yttrium by P204 and P507 dropped sharply to below 10%. Specifically, at pH=0.5, the extraction rate of total rare earth elements by P507 dropped to 0%, while the extraction rate of total rare earth elements by Cyanex272 remained between 0% and 6%. In contrast, the extraction rate of yttrium by p-nonylphenyl dihydrogen phosphate decreased slowly. At pH 0.5 and 0.1, the extraction rates of yttrium by p-nonylphenyl dihydrogen phosphate were 76.05% and 45.62%, respectively, indicating that it is more suitable for the subsequent rare earth recovery of strongly acidic phosphoric acid leachates.

[0033] The above embodiments demonstrate that this invention organically integrates phosphoric acid leaching, direct extraction of total rare earth elements with strong acid, stepwise back-extraction, preparation of mixed rare earth oxides, and precipitation of calcium with sulfuric acid and regeneration of phosphoric acid in the raffinate, forming a complete comprehensive utilization process for phosphate rock. In practical industrial applications, the number of extraction stages, phase ratio, amount of back-extraction agent, and precipitation conditions can be adjusted within the scope defined in the claims, based on the raw material composition, free acidity of the leachate, Ca / P ratio, and total rare earth concentration.

[0034] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for directly leaching calcium and phosphorus from phosphate rock with phosphoric acid and simultaneously extracting associated rare earth elements, characterized in that, Includes the following steps: 1) The phosphate rock is dried and finely ground, and then leached with phosphoric acid as the leaching agent. Solid-liquid separation is performed to obtain a strongly acidic phosphoric acid leaching solution containing phosphorus, calcium and rare earth elements and a silicon-rich leaching residue. 2) The strongly acidic phosphoric acid leachate obtained in step 1) is directly used as the extraction feed. The feed is in the strongly acidic range of pH < 1. Multi-stage countercurrent extraction is performed on the sulfonated kerosene organic phase containing p-nonylphenyl dihydrogen phosphate and TBP to obtain a rare earth-loaded organic phase and a raffinate with calcium dihydrogen phosphate as the main calcium component. 3) The rare earth-loaded organic phase obtained in step 2) is back-extracted using sulfuric acid solution to obtain a rare earth-containing back-extraction solution and a rare earth-depleted organic phase; 4) The co-extracted iron in the rare earth-removed organic phase obtained in step 3) is back-extracted using oxalic acid solution. The resulting organic phase is then washed and regenerated with water and returned to step 2) for recycling. 5) Add oxalate precipitant to the rare earth-containing back-extraction solution obtained in step 3), and wash, dry and calcine the obtained rare earth oxalate precipitate to obtain mixed rare earth oxides; 6) Add sulfuric acid to the raffinate obtained in step 2) to convert the calcium in it into calcium sulfate crystals. Separate the solid and liquid to obtain calcium sulfate byproduct and regenerated phosphoric acid solution. The regenerated phosphoric acid solution is returned to step 1) as a leaching agent after the concentration is adjusted.

2. The method according to claim 1, characterized in that, In step 1), the mass fraction of -200 mesh phosphate rock is not less than 85%, the phosphoric acid concentration is 4.5-7.5 mol / L, the liquid-solid ratio is 6:1-10:1 mL / g, the leaching temperature is 45-65 ℃, the leaching time is 4-8 h, and the stirring speed is 200-400 r / min.

3. The method according to claim 2, characterized in that, The conditions for step 1) are: phosphoric acid concentration 6.0 mol / L, liquid-solid ratio 8:1 mL / g, leaching temperature 55 ℃, leaching time 6 h, and stirring speed 300 r / min.

4. The method according to claim 1, characterized in that, In step 2), the organic phase contains 5–15 vol% p-nonylphenyl dihydrogen phosphate and 1–10 vol% TBP, with the balance being sulfonated kerosene.

5. The method according to claim 1, characterized in that, Step 2) Use 3 to 6 stages of countercurrent extraction, with an aqueous phase to organic phase volume ratio of 1:1 to 3:1, an extraction temperature of 20 to 35 ℃, and a single-stage contact time of 5 to 20 min.

6. The method according to claim 1, characterized in that, In step 3), the concentration of sulfuric acid solution is 0.5–2.0 mol / L, and countercurrent extraction is performed using 3–6 stages with an A / O ratio of 1:2–1:

6.

7. The method according to claim 1, characterized in that, In step 4), the concentration of oxalic acid solution is 0.5–1.5 mol / L, and countercurrent extraction is performed using 3–6 stages with an A / O ratio of 1:1–1:

4.

8. The method according to claim 1, characterized in that, The amount of sulfuric acid added in step 6) is based on SO4 2- With Ca 2+ The molar ratio is controlled at 1.0 to 1.2:

1. Sulfuric acid is added at 60 to 80 °C and reacted for 0.5 to 2 h, followed by aging for 0.5 to 2 h to allow calcium to crystallize out as CaSO4·2H2O. The resulting phosphoric acid solution is then concentrated by evaporation or mixed with supplemental phosphoric acid to adjust to the concentration required in step 1) before being recycled.