Method for preparing high value-added products by comprehensive utilization of phosphorus tailings

Abstract

The application provides a method for preparing high value-added products by comprehensive utilization of phosphor tailings, which comprises the following steps: firstly, high-temperature forging and decomposition of the phosphor tailings to obtain calcined powder; then, water digestion and carbonization by introducing appropriate CO2 to obtain first filter residue containing calcium carbonate and first filtrate; adding hydrochloric acid to the first filter residue for acid leaching and filtering separation to obtain second filter residue containing SiO2 and second filtrate containing CaCl2; adding a certain amount of NH3H2O to the second filtrate containing CaCl2 and introducing a certain amount of CO2 for carbonization, and then filtering separation to obtain light calcium carbonate and third filtrate containing NH4 + ; adding an alkaline solution to the first filtrate for filtering separation to obtain magnesium hydroxide and fourth filtrate with alkalinity. The method is simple, and the obtained light calcium carbonate and magnesium hydroxide meet the preparation standards of the industry. The fourth filtrate is reacted with the third filtrate, magnesium-containing and phosphorus-containing components are supplemented into the reaction system, struvite is generated by crystallization, waste liquid generation and discharge are avoided, and the value-added utilization of industrial by-products is easy to realize.

Description

Technical Field

[0001] This invention relates to the field of comprehensive utilization technology of phosphate rock flotation tailings, and more specifically, to a method for the full-scale comprehensive utilization of phosphate tailings to prepare high value-added products. Background Technology

[0002] The main chemical components of phosphate rock flotation tailings (hereinafter referred to as phosphate tailings) are CaO, MgO, P2O5, SiO2, and CO2 in the form of carbonates. The main mineral component of phosphate tailings is dolomite [CaMg(CO3)2], along with fluorapatite, small amounts of quartz (SiO2), and calcite. Calcium and magnesium content is relatively high in phosphate tailings. Currently, the main methods for treating phosphate tailings include carbonation, acid hydrolysis, and ammonia leaching.

[0003] Chinese Patent CN115321573A discloses a method for recovering and processing phosphate tailings to prepare light calcium carbonate, magnesium oxide, and phosphate concentrate. First, the phosphate tailings are pretreated, calcined, and digested to obtain a digested slurry. Calcium and magnesium are separated through phase transfer. Then, the filtrate and filter residue are dissolved and carbonized separately to obtain light calcium carbonate and phosphate concentrate solid products. The filtrate containing magnesium bicarbonate is pyrolyzed and the pH is adjusted to obtain magnesium carbonate, which is then calcined to obtain magnesium oxide. This method requires a complex process and is difficult to implement industrially. Chinese Patent CN119219048A discloses a method for leaching calcium from phosphate tailings and a method for mineralizing and sealing carbon dioxide to co-produce light calcium carbonate. The phosphate tailings are ball-milled and calcined to obtain calcined ore powder. A protonated organic amine solution is then added for leaching to obtain a calcium-containing leachate. Finally, carbon dioxide is introduced into the calcium-containing leachate for carbonation, and solid-liquid separation yields light calcium carbonate. While this method has advantages such as simple process and high purity of light calcium carbonate product, it also has disadvantages such as low calcium ion recovery rate, high energy consumption of mill, large consumption of organic solvents and strong acid reagents, and high requirements for production equipment. Chinese Patent CN112694115A discloses a method for producing high-quality calcium carbonate and magnesium hydroxide from phosphorus tailings. First, hydrochloric acid and phosphorus tailings in a certain ratio are mixed and reacted thoroughly in a decomposition tank to obtain a first filtrate containing calcium chloride and magnesium chloride. Ammonium bicarbonate is then added to the filtrate for crystallization, and solid-liquid separation yields calcium carbonate with more impurities and a second filtrate containing calcium chloride and magnesium chloride. Ammonium bicarbonate is added again for a second crystallization, and solid-liquid separation yields light calcium carbonate and a third filtrate containing magnesium chloride. Ammonia gas is then introduced for solid-liquid separation to obtain magnesium hydroxide and evaporated ammonium chloride crystals. However, the calcium carbonate obtained by this method contains magnesium carbonate, and it has drawbacks such as multiple leaching processes, complex procedures, and low economic efficiency. In view of the limitations of the above analysis, there is an urgent need to find a new process and method with higher economic efficiency and low carbon emissions to separate calcium and magnesium from phosphorus tailings and produce high-purity, high-value-added calcium and magnesium products. Summary of the Invention

[0004] To overcome the above deficiencies, this invention provides a method for the comprehensive utilization of phosphorus tailings to prepare high value-added products, aiming to solve the problems mentioned in the background.

[0005] This invention is implemented as follows: The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a method for the full-scale utilization of phosphate tailings to prepare high value-added products, including the following steps: Step 1: Pre-treat the phosphate tailings by drying, grinding, and calcining to obtain calcined powder; Step 2: Digest the calcined powder with pure water to obtain a digested slurry. Pass CO2 into the digested slurry until no precipitate is produced to precipitate calcium. Then, perform the first solid-liquid separation to separate calcium and magnesium, and obtain the first filtrate and the first filter residue. Step 3: After adding a certain amount of hydrochloric acid to the first filter residue, a second solid-liquid separation is performed to obtain a second filtrate containing CaCl2 and a second filter residue containing impurities such as SiO2. Step 4: Add NH3·H2O to the second filtrate to adjust to a suitable alkaline pH, introduce CO2 to carbonize until calcium carbonate precipitates, and then perform a third solid-liquid separation to obtain calcium carbonate filter residue and the third filtrate. After washing and drying the calcium carbonate filter residue, light calcium carbonate product is obtained. Step 5: Add a fixed proportion of alkaline solution to the first filtrate, adjust the pH at a certain temperature until magnesium hydroxide precipitate appears, and then perform a fourth solid-liquid separation to obtain magnesium hydroxide filter residue and a fourth filtrate. After washing and drying the magnesium hydroxide filter residue, high-purity magnesium hydroxide can be obtained. Step Six: After recovering sodium carbonate, the fourth filtrate reacts with the third filtrate under appropriate conditions to generate struvite, which is used for the adsorption of high-concentration ammonia nitrogen wastewater.

[0006] A further improvement is that, in step one above, the phosphate tailings are dried and crushed and then calcined in a muffle furnace to obtain calcined powder. The calcination conditions are a calcination temperature of 750~1050℃ and a calcination time of 1~2h. Through calcination, the carbonates in the phosphate tailings are decomposed into metal oxides and carbon dioxide, of which the carbon dioxide can be used for subsequent carbonization reactions.

[0007] Furthermore, in step two above, the calcined powder is digested with water by adding 10 to 30 times its mass of water to the calcined powder, stirring at room temperature for 60 minutes, and keeping the stirring speed at 300 rpm. At this time, calcium and magnesium ions can be fully dissolved into the solution.

[0008] A further improvement is that, in step two above, the digested slurry is carbonized by introducing CO2 at room temperature for 40-80 minutes, and the CO2 flow rate is 100-250 ml / min. Under these conditions, the calcium-magnesium separation effect is optimal.

[0009] Furthermore, in step two above, the first filter residue obtained by separation is placed in an oven and dried at 100±5℃ for 2 hours to obtain carbonized residue, and the first filtrate is collected and stored in a cool and dry place.

[0010] A further improvement is that, in step three above, the concentration of hydrochloric acid as the leaching agent is 1~3 mol / L, the leaching temperature is 30~90℃, the leaching time is 30~180min, and the leaching liquid-to-solid ratio is 4~10ml / g. Under these conditions, calcium ions can be fully leached out.

[0011] A further improvement is made in step four above, where NH3·H2O is added to the second filtrate to adjust the pH to 8.5-9.5. Under these conditions, the cost is lower and it is easier to remove the Fe impurity. 3+ .

[0012] A further improvement is that in step four above, CO2 is introduced for carbonization, the carbonization temperature is 25~70℃, the carbonization time is 30~180 min, the final pH of carbonization is 9.0~11.0, and the CO2 flow rate is 40~100 ml / min. Under these conditions, the high yield and high quality of light calcium carbonate can be guaranteed, resulting in good additional economic benefits.

[0013] Furthermore, in step four above, the separated calcium carbonate filter residue is placed in an oven and dried at 100±5℃ for 2 hours to obtain light calcium carbonate.

[0014] A further improvement is that, in step five above, the first filtrate is added to an alkaline solution to precipitate magnesium hydroxide. The precipitation endpoint pH is 13.0~14.0, the precipitation temperature is 25~70℃, the growth time is 1~3h, the aging time is 0.5~1h, and the aging temperature is 80℃. Under these conditions, the magnesium hydroxide particles are fine and uniform, resulting in higher product quality.

[0015] Furthermore, in step five above, the alkaline solution is one of sodium hydroxide solution, ammonia water, or lime milk.

[0016] Furthermore, in step five above, the separated magnesium hydroxide filter residue is placed in an oven and dried at 100±5℃ for 2 hours to obtain high-purity magnesium hydroxide.

[0017] A further improvement is that, in step six above, after recovering sodium carbonate, the fourth filtrate reacts with the third filtrate to generate struvite, which is used for ammonia nitrogen adsorption in wastewater. The final pH of the reaction is 6.5~7.5, the reaction time is 0.2~0.5h, and the ratio of the third filtrate to the fourth filtrate is 1.2~2.

[0018] A further improvement is that, in step five above, when precipitating the magnesium hydroxide, magnesium hydroxide seed crystals accounting for 0.5~5.0 wt% of the theoretical magnesium hydroxide yield are added to the system.

[0019] A further improvement is that, in step six above, when generating the struvite, struvite seed crystals accounting for 1-5 wt% of the theoretical struvite yield are added to the system; by supplementing the reaction system with magnesium- and phosphorus-containing components, the molar ratio of n(Mg):n(N):n(P) in the mixed solution is adjusted to 1.05-1.15 : 1.0 : 1.0; the supplemented magnesium-containing component comes from the magnesium hydroxide filter residue obtained in step five, the first filtrate obtained in step two, or a combination of the two; the phosphorus-containing component can be phosphoric acid or sodium phosphate.

[0020] The beneficial effects of this invention are: 1. The filter residue obtained by the existing carbonation method is magnesium-containing calcium carbonate, which has low product purity. The present invention cleverly utilizes the difference in calcium and magnesium ion concentration in phosphate tailings to achieve selective separation of calcium ions as calcium carbonate precipitation and magnesium ions as soluble magnesium bicarbonate under a certain pH. This step completes the calcium-magnesium separation with a simple method, solving the core problem of incomplete calcium-magnesium separation and making it easier to produce high-purity light calcium carbonate products.

[0021] 2. The process flow design of this invention is more integrated and simplified. By adopting the strategy of primary carbonization separation and secondary carbonization purification, it avoids complex phase transfer and multiple crystallization steps. At the same time, the whole process mainly uses bulk chemical raw materials such as water, carbon dioxide, hydrochloric acid, and ammonia, resulting in low reagent costs. Key steps such as digestion and primary carbonization are carried out at room temperature, which significantly reduces process energy consumption and reduces the stringent requirements on production equipment.

[0022] 3. The process of this invention is simple, with low processing cost and high calcium-magnesium separation efficiency. Through leaching and carbonization, high-purity light calcium carbonate, high-purity magnesium hydroxide, and struvite can be produced. The products have high added value and are easy to industrialize, realizing the effective comprehensive utilization of phosphorus tailings. This invention breaks through the traditional approach of only recovering calcium and magnesium elements. It innovatively uses the fourth and third filtrates as raw materials and synthesizes struvite (magnesium ammonium phosphate) products with high added value by supplementing magnesium and phosphorus-containing components into the reaction system. This approach not only completely recovers phosphorus elements from phosphorus tailings but also realizes a closed-loop cycle of ammonium ions in the process, transforming potential ammonia nitrogen pollutants into useful resources and avoiding the generation and discharge of high-concentration ammonia nitrogen waste liquid and phosphorus-containing waste liquid. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the process flow of the present invention; Figure 2 This is the XRD pattern of the phosphate tailings used in the examples; Figure 3 The XRD pattern of the light calcium carbonate prepared in the example is shown. Figure 4 The XRD pattern of the layered magnesium hydroxide prepared in the example is shown. Figure 5 Here is the XRD pattern of the magnesium ammonium phosphate prepared in the example; Figure 6 Here is a SEM image of the magnesium ammonium phosphate prepared in the example; Figure 7 This is a SEM image of the light calcium carbonate prepared in the example. Detailed Implementation

[0025] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0026] In the following examples, the phosphorus tailings came from a phosphorus chemical enterprise. Their chemical composition, after elemental analysis by XRF, is shown in the table below:

[0027] Example 1 A method for fully utilizing phosphate tailings to produce high-value-added products, such as Figure 1 As shown, it includes the following steps: (1) The phosphorus tailings are pretreated, dried and ground, and then calcined in a muffle furnace at 900℃ for 1 hour to form calcined powder. Through calcination, the dolomite in the phosphorus tailings is converted into calcium oxide and magnesium oxide. The released CO2 is collected, purified and used for subsequent carbonization reaction. The calcination reaction formula is as follows: CaCO3→CaO+CO2↑ MgCO3→MgO+CO2↑ (2) Add 10 times the mass of water to the calcined powder and stir at room temperature for 60 min to digest, obtaining a digested slurry; pass CO2 through the digested slurry at room temperature for carbonation for 40 min at a CO2 flow rate of 100 ml / min, producing calcium carbonate precipitate, and separate calcium and magnesium by solid-liquid separation, obtaining a first filtrate containing magnesium ions and carbonized residue; the digestion reaction equation is as follows: CaO + H₂O → Ca(OH)₂ MgO + H₂O → Mg(OH)₂ The carbonization reaction equation is as follows: Mg(OH)₂ + 2CO₂ → Mg(HCO₃)₂ Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (3) Hydrochloric acid was added to the carbonized slag as a leaching agent for acid leaching. The concentration of hydrochloric acid was 1 mol / L, the leaching temperature was 60℃, the leaching time was 60 min, and the leaching liquid-to-solid ratio was 8 ml / g. Calcium carbonate in the carbonized slag dissolved to form calcium chloride. The solid-liquid separation yielded impurities (SiO2, etc.) and a calcium-containing solution. The acid leaching reaction equation is as follows: CaCO3 + 2HCl → CaCl2 + H2O + CO2↑ (4) NH3·H2O was added to the second filtrate to adjust the pH to 8.5 to remove iron ions; then CO2 was introduced for carbonation at a temperature of 50℃ for 60 min, with a final pH of 9.0 and a CO2 flow rate of 40 ml / min; calcium carbonate was further purified through secondary carbonation, and the reaction equation is as follows: CaCl₂ + CO₂ + 2OH⁻ - →CaCO3 + H2O + 2Cl - (5) The first filtrate was added to an alkaline solution to precipitate magnesium hydroxide. The precipitation endpoint pH was 13.0, the precipitation temperature was 50℃, the growth time was 2h, the aging time was 0.5h, and the aging temperature was 80℃. Solid-liquid separation yielded magnesium hydroxide precipitate. The reaction equation is as follows: Mg(HCO3)2 + 4OH - →Mg(OH)₂↓+2H₂O+CO₃ 2- (6) After recovering sodium carbonate, the fourth filtrate reacts with the third filtrate to generate struvite for ammonia nitrogen adsorption. The final pH of the reaction is 6.5, the reaction time is 0.5 h, and the ratio of the third filtrate to the fourth filtrate is 2.

[0028] It should be noted that recovery rate is a key indicator for measuring process efficiency. It reflects the proportion of calcium / magnesium elements effectively extracted and converted into the target product from the initial phosphate tailings raw material. Specifically: Calcium recovery rate = (Mass of calcium in the obtained light calcium carbonate product / Mass of calcium in the raw phosphate tailings) × 100%

[0029] Magnesium recovery rate = (mass of magnesium in the obtained magnesium hydroxide product / mass of magnesium in the raw phosphorus tailings) × 100%.

[0030]

[0031] In this experimental example, the recovery rate of calcium was 90.37% and the recovery rate of magnesium was 84.32% as measured by the above method; the CaCO3 content in the calcium carbonate product was 95.67%, which meets the GB / T 19281-2014 standard for industrial calcium carbonate; the Mg(OH)2 content in the magnesium hydroxide product was 85.42%, which meets the HG / T 3607-2024 standard for industrial magnesium hydroxide.

[0032] Example 2 A method for fully utilizing phosphate tailings to produce high-value-added products, such as Figure 1 As shown, it includes the following steps: (1) The phosphorus tailings are pretreated, dried and ground, and then calcined in a muffle furnace at 950°C for 1 hour to form calcined powder. Through calcination, the dolomite in the tailings is converted into calcium oxide and magnesium oxide. The released CO2 is collected, purified and used for subsequent carbonization reaction. (2) Add 20 times the mass of water to the calcined powder and stir at room temperature for 60 min to digest it and obtain digested slurry. Pass CO2 into the digested slurry at room temperature for carbonation. The carbonation time is 60 min and the CO2 flow rate is 150 ml / min. Calcium carbonate precipitate is produced. Solid-liquid separation is used to separate calcium and magnesium and obtain the first filtrate containing magnesium ions and carbonized slag.

[0033] (3) Hydrochloric acid was added to the carbonized slag as a leaching agent for acid leaching. The concentration of the leaching agent hydrochloric acid was 2 mol / L, the leaching temperature was 70℃, the leaching time was 90 min, and the leaching liquid-solid ratio was 8 ml / g. Calcium carbonate in the carbonized slag dissolved to generate calcium chloride. The solid-liquid separation yielded impurities (SiO2, etc.) and a calcium-containing solution.

[0034] (4) NH3·H2O was added to the second filtrate to adjust the pH to 8.5 to remove iron ions; CO2 was then introduced for carbonization at a temperature of 60℃ for 90 min, with the final pH of 10.0 and a CO2 flow rate of 40 ml / min; calcium carbonate was further purified through secondary carbonization.

[0035] (5) The first filtrate was added to an alkaline solution to precipitate magnesium hydroxide. The precipitation endpoint pH was 13.5, the precipitation temperature was 70℃, the growth time was 3h, the aging time was 1h, and the aging temperature was 80℃. The solid-liquid separation yielded magnesium hydroxide precipitate.

[0036] (6) After recovering sodium carbonate, the fourth filtrate reacts with the third filtrate to generate struvite for ammonia nitrogen adsorption. The final pH of the reaction is 6.5, the reaction time is 0.5 h, and the ratio of the third filtrate to the fourth filtrate is 1.5.

[0037] In this experimental example, using the above method, the recovery rate of calcium was 95.36% and the recovery rate of magnesium was 80.78%. The CaCO3 content in the calcium carbonate product was 98.36%, which meets the GB / T 19281-2014 standard for industrial calcium carbonate. The Mg(OH)2 content in the magnesium hydroxide product was 90.54%, which meets the HG / T 3607-2024 standard for industrial magnesium hydroxide.

[0038] Example 3 A method for fully utilizing phosphate tailings to produce high-value-added products, such as Figure 1 As shown, it includes the following steps: (1) The phosphorus tailings are pretreated, dried and ground, and then calcined in a muffle furnace at 850°C for 1 hour to form calcined powder. Through calcination, the dolomite in the tailings is converted into calcium oxide and magnesium oxide. The released CO2 is collected, purified and used for subsequent carbonization reaction. (2) Add 15 times the mass of water to the calcined powder and stir at room temperature for 60 min to digest it and obtain digested slurry. Pass CO2 into the digested slurry at room temperature for carbonation. The carbonation time is 60 min and the CO2 flow rate is 200 ml / min. Calcium carbonate precipitate is produced. Solid-liquid separation is used to separate calcium and magnesium and obtain the first filtrate containing magnesium ions and carbonized slag.

[0039] (3) Hydrochloric acid was added to the carbonized slag as a leaching agent for acid leaching. The concentration of the leaching agent hydrochloric acid was 3 mol / L, the leaching temperature was 60℃, the leaching time was 75 min, and the leaching liquid-solid ratio was 10 ml / g. Calcium carbonate in the carbonized slag dissolved to generate calcium chloride. The solid-liquid separation yielded impurities (SiO2, etc.) and a calcium-containing solution.

[0040] (4) NH3·H2O was added to the second filtrate to adjust the pH to 8.5 to remove iron ions; CO2 was then introduced for carbonation at a temperature of 70℃ for 90 min. The final pH of the carbonation was 11.0 and the CO2 flow rate was 40 ml / min. Calcium carbonate was further purified through secondary carbonation.

[0041] (5) The first filtrate was added to an alkaline solution to precipitate magnesium hydroxide. The precipitation endpoint pH was 13.5, the precipitation temperature was 70℃, the growth time was 1h, the aging time was 1h, the aging temperature was 80℃, and the magnesium hydroxide precipitate was obtained by solid-liquid separation.

[0042] (6) After recovering sodium carbonate, the fourth filtrate reacts with the third filtrate to generate struvite for ammonia nitrogen adsorption. The final pH of the reaction is 7.0, the reaction time is 0.2 h, and the ratio of the third filtrate to the fourth filtrate is 2.

[0043] In this experimental example, using the above method, the recovery rate of calcium was 85.32% and the recovery rate of magnesium was 75.44%. The CaCO3 content in the calcium carbonate product was 95.39%, which meets the GB / T 19281-2014 standard for industrial calcium carbonate. The Mg(OH)2 content in the magnesium hydroxide product was 85.09%, which meets the HG / T 3607-2024 standard for industrial magnesium hydroxide.

[0044] The calcium recovery rate, magnesium recovery rate, CaCO3 content in calcium carbonate product, and Mg(OH)2 content in magnesium hydroxide product measured in Examples 1-3 are shown in Table 1: Table 1

[0045] Based on the comparative analysis of the experimental data provided in Embodiments 1 to 3 of this invention, it can be clearly identified that the optimized combination of process conditions can achieve better overall results within the stated process parameter range. This preferred condition shows significant advantages in the two core indicators of product purity and element recovery rate. The better process conditions are as follows: Optimal conditions for calcination: calcination temperature 950℃, calcination time 1 h; as shown in Example 2, under these conditions, the highest calcium recovery rate (95.36%) and the highest purity of light calcium carbonate product (98.36%) were achieved. The higher temperature is conducive to the more complete decomposition of carbonate minerals such as dolomite, generating more active oxides, which lays the foundation for subsequent efficient digestion and separation.

[0046] The preferred conditions for the first carbonization (calcium-magnesium separation) are: CO2 flow rate 150 mL / min and carbonization time 60 min (see Example 2). These conditions are between those of Example 1 (lower flow rate and time) and Example 3 (higher flow rate), which can ensure sufficient reaction while avoiding unnecessary energy consumption and time costs. This is the balance point for achieving efficient calcium-magnesium separation and directly contributes to a high calcium recovery rate.

[0047] The preferred conditions for the magnesium hydroxide precipitation step were: precipitation endpoint pH 13.5, precipitation temperature 70°C, growth time 3 h, and aging time 1 h (see Example 2). Under these optimized conditions, Example 2 obtained the highest magnesium hydroxide product purity (90.54%). The higher pH and temperature, combined with sufficient growth and aging time, facilitated the more complete precipitation and growth of magnesium hydroxide crystals, thereby reducing impurity encapsulation and obtaining a product with more uniform particle size and higher purity.

[0048] In summary, the combination of process conditions used in Example 2 (calcination: 950℃ / 1h; first carbonization: CO2 150mL / min / 60 min; magnesium hydroxide precipitation: pH 13.5, 70℃, 3h growth, 1h aging) is the best implementation scheme within the scope currently verified by this application. This scheme achieves a good balance between maximizing product value (purity of light calcium carbonate 98.36%, purity of magnesium hydroxide 90.54%) and optimizing resource recovery efficiency (calcium recovery rate 95.36%), providing a clear and efficient process path for the industrialization of this invention.

[0049] Examples 1 to 3 investigated the implementation effects of the method under different process parameters (including but not limited to calcination temperature, solids ratio of digested slurry, carbonization time and flow rate, precipitation pH, etc.). All examples successfully achieved efficient extraction and conversion of calcium and magnesium elements in phosphate tailings, and ultimately obtained high-purity products that meet national or industry standards. Based on the implementation results of the above examples, compared with the prior art, the present invention can operate stably and produce qualified products within the range of key process parameters (such as calcination temperature of 850~950℃, first carbonization time of 40~80 min, etc.). This confirms that the process conditions provided by the present invention are wide and reliable, and have good adaptability to fluctuations in raw materials and operating conditions, providing a solid practical foundation for large-scale industrial production and avoiding the problem of unstable operation caused by overly stringent parameter control.

[0050] According to the data from the examples in Table 1, the recovery rate of calcium ranges from 85.32% to 95.36%, and the recovery rate of magnesium ranges from 75.44% to 84.32%. This indicates that the method can efficiently enrich and recover the main valuable elements in phosphate tailings, resulting in high resource utilization. The CaCO3 content in the prepared light calcium carbonate products is all higher than 95% (95.39%~98.36%), meeting and exceeding the requirements of GB / T 19281-2014 "Industrial Calcium Carbonate" standard. The Mg(OH)2 content in the prepared magnesium hydroxide products is all higher than 85% (85.09%~90.54%), conforming to HG / T 3607-2024 "Industrial Magnesium Hydroxide" standard. This shows that the present invention can produce high-value-added fine chemical products, rather than low-value industrial fillers, greatly improving the economic benefits of phosphate tailings conversion.

[0051] The end-stage design of this invention involves reacting the phosphorus-containing fourth filtrate with the ammonium-containing third filtrate to successfully crystallize struvite (magnesium ammonium phosphate). This process converts the ammonium and phosphate ions generated within the system into struvite products that function as slow-release fertilizers and water treatment adsorbents, thus completely realizing the recovery and utilization of phosphorus and nitrogen elements. This process fundamentally avoids the generation and discharge of high-concentration ammonia nitrogen wastewater and phosphorus-containing wastewater, transforming potential environmental pollution into marketable products. It embodies the advanced environmental protection concept of treating waste with waste and turning waste into treasure, and meets the core requirements of green chemistry and circular economy development.

[0052] In summary, the present invention, through the above embodiments, fully demonstrates that the comprehensive utilization method for phosphorus tailings it provides has the characteristics of reasonable process flow, mild operating conditions, high resource recovery rate, superior product value, and outstanding environmental friendliness. This method successfully transforms bulk industrial solid waste into a variety of high-purity chemical products urgently needed by the market, providing an efficient, economical, and environmentally friendly technical solution for the sustainable development of the phosphorus chemical industry, and has broad prospects for industrial application.

[0053] Example 4 This embodiment optimizes steps (5) and (6) based on embodiment 2. Steps (1) to (4) are exactly the same as in embodiment 2.

[0054] Step (5): Add sodium hydroxide solution to the first filtrate, adjust the pH to 13.5 at 70°C with stirring. When the system becomes slightly turbid, add 2.0 wt% of the pre-prepared nano-sheet magnesium hydroxide seed crystals, and then continue the reaction. The growth time is 3 h, the aging time is 1 h, and the aging temperature is 80°C. Then, perform the fourth solid-liquid separation to obtain magnesium hydroxide filter residue and the fourth filtrate. Part of the obtained magnesium hydroxide filter cake is used for subsequent drying, and the other part is retained as the magnesium source in step (6).

[0055] Without seed crystals, when alkali is added to a filtrate rich in Mg(HCO3)2, magnesium hydroxide molecules spontaneously nucleate everywhere in the solution, forming a large number of tiny, disordered crystal nuclei. This results in small crystal particle size, forming colloidal precipitates that are difficult to filter and wash, time-consuming operation, high product water content, poor bulk density, and easy encapsulation of impurity ions, affecting product purity.

[0056] By adding pre-prepared nanosheet magnesium hydroxide seeds, these seeds provide ready-made, regular attachment surfaces for magnesium hydroxide molecules in the solution. Magnesium ions and hydroxide ions in the solution will preferentially deposit on specific crystal faces of these seeds instead of reforming crystal nuclei themselves. This is called "heterogeneous nucleation," and its energy barrier is much lower than that of "homogeneous nucleation." By using sheet-like seeds, the newly grown magnesium hydroxide can be guided to develop along this morphology, ultimately obtaining larger, more uniform, and denser sheet-like crystals.

[0057] The core reaction in this step is the precipitation of magnesium ions: Mg(HCO3)2 + 4NaOH → Mg(OH)2↓ + 2Na2CO3 + 2H2O (The added seed crystal Mg(OH)2 itself does not participate in the reaction, but serves as a template to guide the growth of newly generated Mg(OH)2 on it.) Through the above treatment, the crystals become larger and more regular, which speeds up the solid-liquid separation process and makes the filter cake denser; it reduces the encapsulation of impurities and improves the washing efficiency, thereby increasing the purity of the final magnesium hydroxide product from 90.54% to 92.5%; the specific flake morphology may give it better performance in applications such as flame retardants.

[0058] Step (6): After mixing the fourth filtrate after recovering sodium carbonate with the third filtrate, since the fourth filtrate mainly comes from the mother liquor of magnesium hydroxide precipitation, its phosphorus content is low and insufficient to completely precipitate the ammonium ions in the third filtrate. Therefore, it is necessary to first add phosphoric acid solution to the mixture so that the phosphorus in the mixture reaches the theoretical stoichiometric value for reaction with ammonium ions. Then, add the wet filter cake of magnesium hydroxide retained in step (5) to the mixture. The molar ratio of n(Mg):n(N):n(P) in the mixture is precisely controlled to be 1.10:1.0:1.0 by the amount of addition. Subsequently, 3.0 wt% of the theoretical guanoite yield is added, and the pH of the reaction endpoint is controlled to be 7.0. The reaction time is 30 min. After the reaction is completed, the solid and liquid are separated and the guanoite product is collected.

[0059] The fourth filtrate, after the precipitation of magnesium hydroxide, has an alkalinity (mainly from sodium carbonate) that can be used to adjust the pH environment for the struvite synthesis reaction, but its phosphate content is limited. The third filtrate, on the other hand, provides the main ammonium ions (NH4+). + To achieve complete recovery of ammonium and phosphorus and avoid ammonia nitrogen pollution, phosphate ions must be added to ensure sufficient reactants. At the same time, a large amount of magnesium ions have been removed. If magnesium is not added, the reaction to synthesize struvite will not be able to proceed completely due to insufficient magnesium source, resulting in low recovery rates of phosphorus and ammonia nitrogen, which will remain in the wastewater and cause pollution. Struvite will spontaneously crystallize to form fine crystals, which settle slowly and have a low yield.

[0060] By returning a portion of the magnesium hydroxide filter cake obtained in step (5) to the system as a magnesium source for the synthesis of struvite, the waste is recycled within the system. By supplementing with phosphoric acid and magnesium hydroxide filter cake (magnesium source) from within the system, and controlling the ratio of n(Mg):n(N):n(P) = 1.10 : 1.0 : 1.0 in the mixed solution to ensure a slight excess of magnesium ions, it is possible to ensure that expensive or environmentally harmful phosphate and ammonium ions are completely precipitated, maximizing the recovery rate. Similar to the precipitation principle of magnesium hydroxide, the addition of struvite seed crystals can induce the formation of large-particle, easily recyclable struvite crystals, significantly improving the settling rate and product yield.

[0061] This step is the formation reaction of struvite. The added magnesium hydroxide first reacts with ammonium ions in the solution to provide a magnesium source. Mg(OH)2 + 2NH4 + → Mg 2+ + 2NH3·H2O Subsequently, magnesium, ammonium, and phosphate ions combine to form struvite precipitate: Mg 2+ + NH4 + + HPO4 2- + 6H2O → MgNH4PO4·6H2O↓ (Under near-neutral pH conditions, phosphate mainly exists as HPO4) 2- (Formal existence)

[0062] This embodiment achieves efficient co-precipitation of phosphorus and ammonia nitrogen through this optimization, increasing the phosphorus precipitation yield to 96.8%, converting almost all phosphorus into valuable products. The concentrations of ammonia nitrogen and total phosphorus in the mother liquor after the reaction are extremely low (<50mg / L), eliminating the two major water pollution factors at the source, and transforming externally added phosphorus resources into high-value-added struvite products, thus converting environmental protection costs into product revenue.

[0063] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for fully utilizing phosphate tailings to prepare high-value-added products, characterized in that, Includes the following steps: Step 1: Pre-treat the phosphate tailings by drying, grinding, and calcining to obtain calcined powder; Step 2: Digest the calcined powder with pure water to obtain a digested slurry. Pass CO2 into the digested slurry until no precipitate is produced to precipitate calcium. Then, perform the first solid-liquid separation to separate calcium and magnesium, and obtain the first filtrate and the first filter residue. Step 3: After adding a certain amount of hydrochloric acid to the first filter residue, a second solid-liquid separation is performed to obtain a second filtrate containing CaCl2 and a second filter residue containing SiO2 impurities. Step 4: Add NH3·H2O to the second filtrate to adjust to a suitable alkaline pH, introduce CO2 to carbonize until calcium carbonate precipitates, and then perform a third solid-liquid separation to obtain calcium carbonate filter residue and the third filtrate. After washing and drying the calcium carbonate filter residue, light calcium carbonate product is obtained. Step 5: Add a fixed proportion of alkaline solution to the first filtrate, adjust the pH at a certain temperature until magnesium hydroxide precipitate appears, and then perform a fourth solid-liquid separation to obtain magnesium hydroxide filter residue and a fourth filtrate. After washing and drying the magnesium hydroxide filter residue, high-purity magnesium hydroxide can be obtained. Step Six: After recovering sodium carbonate, the fourth filtrate reacts with the third filtrate under appropriate conditions to generate struvite, which is used for the adsorption of high-concentration ammonia nitrogen wastewater.

2. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step one above, the phosphate tailings are dried and crushed and then calcined in a muffle furnace to obtain calcined powder. The calcination conditions are a calcination temperature of 750~1050℃ and a calcination time of 1~2h. Through calcination, the carbonates in the phosphate tailings are decomposed into metal oxides and carbon dioxide, of which the carbon dioxide can be used for subsequent carbonization reactions.

3. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step two above, the calcined powder is digested with water by adding 10 to 30 times its weight of pure water to the calcined powder, stirring at room temperature for 60 minutes, and maintaining a fixed stirring speed of 300 rpm; the digested slurry is carbonized by introducing CO2 at room temperature for 40 to 80 minutes, with a CO2 flow rate of 100 to 250 ml / min; the first filter residue obtained is placed in an oven and dried at 100±5℃ for 2 hours to obtain carbonized residue, and the first filtrate is collected and stored in a cool and dry place.

4. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step three above, the concentration of hydrochloric acid as the leaching agent is 1~3 mol / L, the leaching temperature is 30~90℃, the leaching time is 30~180min, and the leaching liquid-to-solid ratio is 4~10ml / g.

5. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step four above, NH3·H2O is added to the second filtrate to adjust the pH to 8.5~9.

5.

6. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step four above, CO2 is introduced for carbonation at a temperature of 25-70℃ for 30-180 min, with a final pH of 9.0-11.0 and a CO2 flow rate of 40-100 ml / min. The separated calcium carbonate filter residue is placed in an oven and dried at 100±5℃ for 2 h to obtain light calcium carbonate.

7. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step five above, the first filtrate is added to an alkaline solution to precipitate magnesium hydroxide. The precipitation endpoint pH is 13.0~14.0, the precipitation temperature is 25~70℃, the growth time is 1~3h, the aging time is 0.5~1h, and the aging temperature is 80℃. The alkaline solution is one of sodium hydroxide solution, ammonia water, or lime milk. The separated magnesium hydroxide filter residue is placed in an oven and dried at 100±5℃ for 2h to obtain high-purity magnesium hydroxide.

8. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step six above, after recovering sodium carbonate, the fourth filtrate reacts with the third filtrate to generate struvite, which is used for ammonia nitrogen adsorption in wastewater. The final pH of the reaction is 6.5~7.5, the reaction time is 0.2~0.5h, and the ratio of the third filtrate to the fourth filtrate is 1.2~2.

9. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step five above, when precipitating the magnesium hydroxide, magnesium hydroxide seed crystals accounting for 0.5~5.0 wt% of the theoretical magnesium hydroxide yield are added to the system.

10. The method for preparing high-value-added products from phosphate tailings using full-scale utilization as described in claim 1, characterized in that, In step six above, when generating the struvite, struvite seed crystals accounting for 1-5 wt% of the theoretical struvite yield are added to the system; by supplementing the reaction system with magnesium- and phosphorus-containing components, the molar ratio of n(Mg):n(N):n(P) in the mixed solution is adjusted to 1.05-1.15 : 1.0 : 1.0; the supplemented magnesium-containing component comes from the magnesium hydroxide filter residue obtained in step five, the first filtrate obtained in step two, or a combination thereof; the phosphorus-containing component can be phosphoric acid or sodium phosphate.

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