Method for recovering magnesium hydroxide from phosphorus tailings by stepwise sedimentation separation

Abstract

The application discloses a method for recovering magnesium hydroxide from phosphorus tailings by step-by-step precipitation separation, which comprises the following steps: mixing phosphorus tailings with a sulfuric acid solution and reacting; adding an oxidizing agent to oxidize Fe 2+ into Fe 3+ ; adding an aluminum-containing precipitant, adjusting the pH with an alkaline adjusting agent, and separating the solid from the liquid after the reaction; adding ammonium persulfate, adjusting the pH of the system with an alkaline adjusting agent, and making manganese ions precipitate in the form of manganese dioxide after the reaction; separating the solid from the liquid; and adding an alkaline precipitant, and making magnesium ions precipitate in the form of magnesium hydroxide by adjusting the pH. The application can efficiently remove impurity ions such as phosphate, iron and manganese from the phosphorus tailings acid leaching solution, and can prepare high-purity magnesium hydroxide products. The method simplifies the process flow in the prior art, reduces reagent consumption and operation cost, efficiently removes phosphate and iron ions under low-pH conditions, avoids the generation of ammonium magnesium phosphate in the subsequent process, selectively removes manganese ions, and prevents the problem of co-precipitation when magnesium hydroxide is precipitated.

Description

Technical Field

[0001] This invention belongs to the field of resource recycling technology, and in particular relates to a method for the step-by-step precipitation separation and recovery of magnesium hydroxide from phosphorus tailings. Background Technology

[0002] The phosphate chemical industry poses a severe environmental challenge due to the massive amounts of phosphate tailings generated during phosphate mining and beneficiation. These tailings are typically transported to tailings ponds in slurry form. Over the years, this storage not only occupies vast amounts of land and incurs enormous costs for pond construction and maintenance, but also poses potential safety risks such as dam instability and landslides. Furthermore, the trace heavy metals and residual flotation reagents contained in phosphate tailings can cause continuous and irreversible complex pollution to surrounding water bodies, soil, and the atmosphere through leachate erosion and windblown dust, highlighting the growing environmental hazards and pressure for remediation.

[0003] Faced with this severe situation, the resource utilization of phosphate tailings, transforming them from "waste" into "secondary resources," has become an inevitable choice and urgent need to promote the green, low-carbon, and circular sustainable development of the phosphate chemical industry. Phosphate tailings have a complex chemical composition, containing not only incompletely recovered phosphorus but also high levels of magnesium, a major valuable component with extremely high recovery value. If they can be efficiently recovered and converted into high-value-added magnesium-based chemicals, both environmental and economic benefits will be achieved simultaneously.

[0004] Acid leaching-precipitation is the mainstream technical route for recovering magnesium resources from phosphate tailings. This process typically uses inorganic acids such as sulfuric acid to leach the phosphate tailings, allowing magnesium to enter the solution phase in ionic form. Subsequently, magnesium hydroxide or magnesium carbonate is obtained through alkaline precipitation. However, this technical route faces the challenge of deep purification due to the complexity of the acid leaching solution system in practical industrial applications. Phosphate tailings acid leaching solution is a typical multi-component, multi-impurity system, containing a high concentration of phosphate (PO4) ions. 3- ), with divalent iron (Fe) 2+ ) and trivalent iron (Fe) 3+ Iron ions existing in the form of ) and divalent manganese (Mn) 2+ Mg ions are key impurities affecting the quality of magnesium recovery products. Specifically, these impurities can trigger a series of cascading technical bottlenecks during subsequent precipitation: First, when ammonia is used as a precipitant to adjust the pH to approximately 6.0, phosphate ions in the solution will react with magnesium ions (Mg²⁺ ions). 2+ ) and ammonium ions (NH4) +The reaction produces magnesium ammonium phosphate (MgNH4PO4·6H2O, struvite), a precipitate with extremely low solubility. This reaction competes with the precipitation process of the target product, magnesium hydroxide, resulting in the loss of a large amount of magnesium in the non-target product and significantly reducing the magnesium recovery rate. Secondly, iron and manganese are colorimetric ions. If they are not thoroughly removed before precipitation, they will be directly incorporated into the crystal lattice of the final product, causing the magnesium hydroxide product to appear yellowish-brown or grayish-black, severely degrading its physical appearance and whiteness, making it unable to meet the stringent whiteness requirements of high-end applications such as flame retardant materials and advanced rubber fillers. Particularly problematic is that the pH value at which manganese hydroxide (Mn(OH)2) begins to precipitate (approximately 8.5) is very close to the pH range at which magnesium hydroxide (Mg(OH)2) precipitates in large quantities (approximately 10.5). Traditional hydrolysis methods easily cause co-precipitation of manganese hydroxide and magnesium hydroxide, forming a difficult-to-separate colloidal copolymer of hydroxides. This not only reduces product purity but is also the core obstacle preventing the product from meeting whiteness standards.

[0005] While existing technologies have attempted to address these issues, such as stepwise precipitation, solvent extraction, or ion exchange, these methods often suffer from lengthy processes, high operating costs, large reagent consumption, or the introduction of new impurities. For example, while organic complexing agents can achieve selective impurity removal, they are costly and may cause secondary pollution; simple hydroxide precipitation methods struggle to achieve efficient separation of manganese and magnesium due to overlapping pH windows. Therefore, the industry urgently needs to develop an innovative integrated process that can efficiently, economically, and deeply remove key impurities such as phosphate, iron, and manganese from phosphate tailings acid hydrolysis solutions, while simultaneously ensuring high magnesium recovery and high product purity. Summary of the Invention

[0006] To address the aforementioned technical challenges, deeply remove phosphate, iron, and manganese ions, and improve the whiteness and purity of magnesium hydroxide products, this invention provides a step-by-step precipitation separation and recovery method for magnesium hydroxide from phosphate tailings. This method simplifies the existing process flow, reduces reagent consumption and operating costs, and can efficiently remove phosphate and iron ions under low pH conditions, avoiding the formation of magnesium ammonium phosphate in subsequent processes. It also achieves selective removal of manganese ions, preventing co-precipitation problems during magnesium hydroxide precipitation. The resulting magnesium hydroxide can be used in high-value-added fields.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for recovering magnesium hydroxide from phosphate tailings by step precipitation separation, comprising the following steps: (1) Acid hydrolysis: Phosphate tailings are mixed with sulfuric acid solution to obtain acid hydrolysis solution.

[0008] Preferably, the liquid-to-solid mass ratio of the sulfuric acid solution to the phosphate tailings is 2-5:1, more preferably 3:1; the concentration of the sulfuric acid solution is 10%-30%, more preferably 20%.

[0009] Preferably, the temperature of the mixing reaction is 25~100℃, more preferably 60℃; the time of the mixing reaction is 0.5~5 hours.

[0010] In the technical solution of the present invention, magnesium is efficiently leached out through acid hydrolysis, while calcium and other elements are fixed in the acid hydrolysis residue in the form of calcium sulfate, thereby greatly simplifying the subsequent process; the acid hydrolysis solution is mainly composed of magnesium ions, and also contains impurities such as phosphate, iron, and manganese.

[0011] (2) Phosphorus and iron removal: An oxidant is added to the acid hydrolysate to remove Fe from the acid hydrolysate. 2+ Oxidized to Fe 3+ Add an aluminum-containing precipitant, adjust the pH with an alkaline regulator, and after the reaction, separate the solid and liquid to obtain a primary purified liquid. Preferably, the oxidant is selected from at least one of hydrogen peroxide, sodium hypochlorite, potassium permanganate, and ozone; the aluminum-containing precipitant is selected from at least one of aluminum sulfate, aluminum chloride, and aluminum nitrate; and the alkalinity regulator is selected from at least one of ammonia, sodium hydroxide, and potassium hydroxide.

[0012] Preferably, the molar ratio of Al in the aluminum-containing precipitant to phosphate (PO4) in the acid hydrolysate is 1.0~1.8:1, more preferably 1.3~1.8:1.

[0013] Preferably, the pH is adjusted to 3.0-5.0, and the reaction time after pH adjustment is 0.5-3 hours.

[0014] In the technical solution of this invention, the addition of an oxidant can reduce Fe 2+ Oxidized to Fe 3+ Phosphate and iron ions co-precipitate as aluminum phosphate (AlPO4) and iron phosphate (FePO4), respectively. After solid-liquid separation, a primary purified solution is obtained. Insufficient aluminum in the aluminum-containing precipitant results in incomplete phosphorus removal, affecting product purity; excessive aluminum increases the required alkaline adjuster, raising costs. Too low a pH leads to incomplete phosphate precipitation; while too high a pH results in the formation of magnesium ammonium phosphate, causing a decrease in Mg recovery.

[0015] (3) Manganese removal: Ammonium persulfate is added to the primary purification solution, and the pH of the system is adjusted by an alkaline regulator. After the reaction, manganese ions are precipitated in the form of manganese dioxide. Solid-liquid separation is performed to obtain a deep purification solution.

[0016] Preferably, the S2O8 in the ammonium persulfate 2- Mn in the primary purification solution2+ The molar ratio is 1.0~4:1.

[0017] Preferably, the pH is adjusted to 7.0-8.0; the reaction time after pH adjustment is 0.5-3 hours.

[0018] In the technical solution of this invention, ammonium persulfate oxidizes divalent manganese to tetravalent manganese, generating manganese dioxide, and itself becomes sulfate, without introducing new impurities. Too little persulfate results in incomplete removal of Mn ions, affecting whiteness and purity; too much persulfate leads to excessive cost. Under the above conditions, deep manganese removal is possible, and a deeply purified solution is obtained after solid-liquid separation.

[0019] In the technical solution of this invention, the aluminum ions remaining in step (2) may generate aluminum hydroxide in situ. As a highly efficient flocculant, it can adsorb suspended fine MnO2 precipitate particles, causing the fine particles to aggregate into larger flocs, greatly improving the sedimentation rate and filtration efficiency. In addition to assisting in capturing fine MnO2 particles, a small amount of aluminum hydroxide can also adsorb trace impurities (such as trace amounts of Fe) that have not been completely precipitated in the primary purification liquid. 3+ PO4 3- The solution is further purified to remove residual or trace amounts of silicon, heavy metal impurities, etc. introduced during the acidification process, ensuring the subsequent preparation of high-purity magnesium hydroxide (purity ≥99.2%) and preventing trace impurities from affecting the quality of the final product.

[0020] (4) Precipitate magnesium hydroxide: Add an alkaline precipitant to the deep purification solution and adjust the pH to precipitate magnesium ions in the form of magnesium hydroxide.

[0021] Preferably, the alkaline precipitant is selected from at least one of ammonia, sodium hydroxide, and potassium hydroxide.

[0022] Preferably, the pH is adjusted to 10.0~11.0.

[0023] Preferably, the reaction time after pH adjustment is 0.5 to 4 hours.

[0024] In some specific embodiments, the precipitate obtained in step (4) is filtered, washed, and dried to obtain high-purity magnesium hydroxide; the washing is done with deionized water; the drying temperature is 100~110℃; and the drying time is 1.5~6 hours.

[0025] In the technical solution of this invention, after step (4), the aforementioned potentially residual aluminum ions are converted into soluble aluminate (AlO2) under this strongly alkaline condition. - This prevents aluminum hydroxide precipitate from entering the product, thus ensuring the high purity of the final product.

[0026] The beneficial effects of this invention are as follows: (1) The method provided by the present invention completely avoids the formation of magnesium ammonium phosphate. The final magnesium hydroxide product has a purity of over 99.2% and a whiteness of over 94. The content of key impurities is extremely low. The product indicators can directly meet the stringent requirements of flame retardants, high-end fillers and other fields.

[0027] (2) By precisely controlling key steps, namely, pre-deep phosphorus removal at low pH to avoid magnesium loss and selective manganese removal under near-neutral conditions to avoid co-precipitation, this invention achieves efficient recovery of magnesium, with a comprehensive recovery rate of over 92%, significantly enhancing the comprehensive utilization value of phosphorus tailings, a waste resource.

[0028] (3) This invention utilizes the synergistic effect of substances within the system to simplify and optimize the process: the generated aluminum hydroxide is used to capture elemental sulfur through flocculation, eliminating additional steps; the amphoteric nature of aluminum hydroxide allows for automatic dissolution and separation during the final sedimentation stage, achieving "self-cleaning". This design makes the process more compact and the operation simpler.

[0029] (4) The method provided by the present invention uses common industrial chemicals, does not require expensive reagents or special equipment, has a simple process and a wide parameter window, is highly adaptable, and has the dual advantages of low cost and high reliability. At the same time, the whole process is free of secondary pollution, is environmentally friendly, and has outstanding prospects for industrial application and economic benefits. Attached Figure Description

[0030] Figure 1 This is a flowchart illustrating the step-by-step precipitation separation and recovery of magnesium hydroxide from phosphorus tailings in an embodiment of the present invention. Detailed Implementation

[0031] The following embodiments are merely some, not all, of the embodiments of the present invention. Therefore, the detailed descriptions of the embodiments provided below are not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0032] In this invention, unless otherwise specified, all equipment and raw materials are commercially available or commonly used in the industry. The methods described in the following embodiments are conventional methods in the art, unless otherwise specified.

[0033] The phosphorus tailings in the following examples are from Dayukou, Hubei Province, and were crushed and ground before acid hydrolysis.

[0034] Example 1 (1) Acid hydrolysis: 20% sulfuric acid and phosphorus tailings were reacted at 60℃ for 2 hours at a liquid-solid mass ratio of 3:1. After solid-liquid separation, acid hydrolysis solution was obtained. The composition of acid hydrolysis solution was determined to be Mg: 20000 ppm, P: 12000 ppm, Mn: 600 ppm, Fe: 500 ppm.

[0035] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4) = 1.5:1. The pH was adjusted to 4.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The filtrate contained less than 100 ppm of phosphorus and less than 10 ppm of total Fe.

[0036] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Ammonium persulfate was added to the filtrate at a ratio of 1.8:1. The pH was adjusted to 7.5 with an alkaline regulator under stirring. The reaction was carried out for 1 hour and then filtered. The Mn content in the filtrate was less than 10 ppm.

[0037] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0038] Example 2 (1) Acid hydrolysis: 15% sulfuric acid and phosphorus tailings were reacted at 25°C for 4 hours at a liquid-solid mass ratio of 4:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 18000 ppm, P: 10000 ppm, Mn: 500 ppm, Fe: 400 ppm.

[0039] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4) = 1.5:1. The pH was adjusted to 4.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The filtrate contained less than 100 ppm of phosphorus and less than 10 ppm of total Fe.

[0040] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Add ammonium persulfate at a ratio of 1.8:1, adjust the pH to 7.5 with an alkaline regulator under stirring, react for 1 hour, and filter; the Mn content in the filtrate is <10 ppm.

[0041] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0042] Example 3 (1) Acid hydrolysis: 25% sulfuric acid and phosphorus tailings were reacted at 100℃ for 1 hour at a liquid-solid mass ratio of 2:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 19000 ppm, P: 11500 ppm, Mn: 550 ppm, Fe: 450 ppm.

[0043] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4) = 1.3:1. The pH was adjusted to 4.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The filtrate contained less than 100 ppm of phosphorus and less than 10 ppm of total Fe.

[0044] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Add ammonium persulfate at a ratio of 1.8:1, adjust the pH to 7.5 with an alkaline regulator under stirring, react for 1 hour, and filter; the Mn content in the filtrate is <10 ppm.

[0045] (4) Precipitation of magnesium: pH is adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 is precipitated after 1 hour of reaction. The precipitate is filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0046] Example 4 (1) Acid hydrolysis: 20% sulfuric acid and phosphorus tailings were reacted at 80℃ for 3 hours at a liquid-solid mass ratio of 5:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 22500 ppm, P: 12500 ppm, Mn: 650 ppm, Fe: 520 ppm.

[0047] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4)=1.3:1. Under stirring, the pH was adjusted to 4.5 with 25% ammonia water and the reaction was carried out for 1 hour. The mixture was then filtered. The filtrate contained less than 100 ppm of phosphorus and less than 10 ppm of total Fe.

[0048] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+Add ammonium persulfate at a ratio of 1.8:1, adjust the pH to 7.5 with an alkaline regulator under stirring, react for 1 hour, and filter; the Mn content in the filtrate is <10 ppm.

[0049] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0050] Example 5 (1) Acid hydrolysis: 20% sulfuric acid and phosphorus tailings were reacted at 60℃ for 2 hours at a liquid-solid mass ratio of 3:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 20000 ppm, P: 12000 ppm, Mn: 600 ppm, Fe: 500 ppm.

[0051] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4) = 1.3:1. The pH was adjusted to 3.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The filtrate contained less than 100 ppm of phosphorus and less than 10 ppm of total Fe.

[0052] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Add ammonium persulfate at a ratio of 1.8:1, adjust the pH to 7.5 with an alkaline regulator under stirring, react for 1 hour, and filter; the Mn content in the filtrate is <10 ppm.

[0053] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0054] Example 6 (1) Acid hydrolysis: 20% sulfuric acid and phosphorus tailings were reacted at 60℃ for 2 hours at a liquid-solid mass ratio of 3:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 20000 ppm, P: 12000 ppm, Mn: 600 ppm, Fe: 500 ppm.

[0055] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4) = 1.8:1. The pH was adjusted to 4.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The filtrate contained less than 100 ppm of phosphorus and less than 10 ppm of total Fe.

[0056] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Ammonium persulfate was added at a ratio of 4:1, and the pH was adjusted to 7.5 with an alkaline adjuster under stirring. The reaction was carried out for 1 hour, and then filtered. The Mn content in the filtrate was <10 ppm.

[0057] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0058] Comparative Example 1 (1) Acid hydrolysis: 20% sulfuric acid and phosphorus tailings were reacted at 60℃ for 2 hours at a liquid-solid mass ratio of 3:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 20000 ppm, P: 12000 ppm, Mn: 600 ppm, Fe: 500 ppm.

[0059] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+ After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4)=0.9:1. The pH was adjusted to 4.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The P element in the filtrate reached 1200 ppm and the total Fe <10 ppm.

[0060] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Ammonium persulfate was added at a ratio of 1.8:1, and the pH was adjusted to 7.5 with an alkaline adjuster under stirring. The reaction was carried out for 1 hour, followed by filtration. The filtrate contained Mn. 2+ <10 ppm.

[0061] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0062] Comparative Example 2 (1) Acid hydrolysis: 20% sulfuric acid and phosphorus tailings were reacted at 60℃ for 2 hours at a liquid-solid mass ratio of 3:1 to obtain acid hydrolysis solution. The composition of the acid hydrolysis solution was determined to be Mg: 20000 ppm, P: 12000 ppm, Mn: 600 ppm, Fe: 500 ppm.

[0063] (2) Precipitation of phosphorus: Take 1L of the above acid hydrolysis solution, add 5 mL of 30% hydrogen peroxide to remove Fe 2+After oxidation, aluminum sulfate was added at a ratio of n(Al):n(PO4) = 1.5:1. The pH was adjusted to 4.5 with 25% ammonia solution under stirring. The reaction was carried out for 1 hour and then filtered. The filtrate contained P < 100 ppm and total Fe < 10 ppm.

[0064] (3) Manganese removal: according to n(S2O8) 2- ): n(Mn 2+ Ammonium persulfate was added at a ratio of 0.9:1, and the pH was adjusted to 7.5 with an alkaline regulator under stirring. The reaction was carried out for 1 hour and then filtered. The Mn content in the filtrate was 85 ppm.

[0065] (4) Precipitation of magnesium: The pH of the filtrate was adjusted to 10.5 with 25% ammonia water, and Mg(OH)2 was precipitated after reacting for 1 hour. The precipitate was filtered, washed with deionized water, and dried at 105℃ for 6 hours to obtain magnesium hydroxide product.

[0066] Performance test results The magnesium hydroxide recovered from Examples 1-5 and Comparative Examples 1-2 was tested for whiteness, and its purity and recovery rate were calculated. The test results are shown in Tables 1 and 2.

[0067] Whiteness testing method: The blue light whiteness (R457) of the sample is measured using a whiteness meter, in accordance with GB / T 23774—2009 "Determination of Whiteness" or GB / T 5950—2008 "Determination of Whiteness of Pigments for Coatings".

[0068] The purity of magnesium hydroxide was determined by titration, in accordance with HG / T 3607-2024 "Industrial Magnesium Hydroxide". The recovery rate of magnesium hydroxide was calculated by the impurity deduction method.

[0069] Magnesium recovery rate is calculated using the following formula:

[0070] In the formula: m 产品 For magnesium hydroxide product quality (g); w (Mg) 产品 The mass fraction of magnesium in the product (%) V 酸解液 The volume of the acid hydrolysate is in L. C (Mg) 酸解液 The concentration of magnesium in the acid hydrolysis solution is expressed in mg / L.

[0071] Table 1

[0072] Table 2

[0073] The reaction temperature and reaction time in Tables 1 and 2 are the temperature and time in step (1) acid hydrolysis.

[0074] As can be seen from the table above, the present invention can stably prepare high-purity magnesium hydroxide under a wide range of acidolysis conditions, with product purity ≥99.2%, whiteness ≥93, magnesium recovery rate ≥92%, and strong process adaptability and good stability.

[0075] In the phosphorus precipitation step, a n(Al):n(PO4) ratio of 1.3:1 to 1.8:1 achieves deep phosphorus removal (P < 100 ppm), avoids the formation of magnesium ammonium phosphate, and ensures high product purity and high magnesium recovery rate. Insufficient aluminum ions result in incomplete phosphorus removal, affecting product purity. In the manganese removal step, n(S2O8) 2- ): n(Mn 2+ Deep manganese removal (Mn < 10 ppm) can be achieved within a pH range of 1.8:1 to 4:1, ensuring high product whiteness; however, if the persulfate content is too low, Mn ions cannot be completely removed, affecting whiteness and purity. Phosphorus precipitation achieves good impurity removal within a pH range of 3.5–4.5, with a wide process control window, making it easy to scale up industrially.

[0076] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle 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 recovering magnesium hydroxide from phosphate tailings by step precipitation separation, characterized in that, Includes the following steps: (1) Acid hydrolysis: Phosphate tailings are mixed with sulfuric acid solution and reacted to obtain acid hydrolysis solution; (2) Phosphorus and iron removal: An oxidant is added to the acid hydrolysate to remove Fe from the acid hydrolysate. 2+ Oxidized to Fe 3+ Add an aluminum-containing precipitant, adjust the pH with an alkaline regulator, and after the reaction, separate the solid and liquid to obtain a primary purified liquid. (3) Manganese removal: Ammonium persulfate is added to the primary purification solution, and the pH of the system is adjusted by an alkaline regulator. After the reaction, manganese ions are precipitated in the form of manganese dioxide. Solid-liquid separation is performed to obtain a deep purification solution. (4) Precipitate magnesium hydroxide: Add an alkaline precipitant to the deep purification solution and adjust the pH to precipitate magnesium ions in the form of magnesium hydroxide.

2. The method according to claim 1, characterized in that, In step (1), the liquid-solid mass ratio of the sulfuric acid solution to the phosphate tailings is 2 to 5:1, more preferably 3:1; the concentration of the sulfuric acid solution is 10% to 30%, more preferably 20%.

3. The method according to claim 1, characterized in that, In step (1), the temperature of the mixing reaction is 25~100℃, more preferably 60℃; the time of the mixing reaction is 0.5~5 hours.

4. The method according to claim 1, characterized in that, In step (2), the oxidant is selected from at least one of hydrogen peroxide, sodium hypochlorite, potassium permanganate and ozone; the aluminum-containing precipitant is selected from at least one of aluminum sulfate, aluminum chloride and aluminum nitrate; and the alkalinity regulator is selected from at least one of ammonia, sodium hydroxide and potassium hydroxide.

5. The method according to claim 1, characterized in that, In step (2), the molar ratio of Al in the aluminum-containing precipitant to phosphate (PO4) in the acid hydrolysate is 1.0~1.8:1, more preferably 1.3~1.8:1; Preferably, in step (2), the pH is adjusted to 3.0~5.0, and the reaction time after pH adjustment is 0.5 hours~3 hours.

6. The method according to claim 1, characterized in that, In step (3), the S2O8 in the ammonium persulfate 2- Mn in the primary purification solution 2+ The molar ratio is 1.0~4:

1.

7. The method according to claim 1, characterized in that, In step (3), the pH is adjusted to 7.0~8.0; the reaction time after pH adjustment is 0.5~3 hours.

8. The method according to claim 1, characterized in that, In step (4), the alkaline precipitant is selected from at least one of ammonia, sodium hydroxide and potassium hydroxide.

9. The method according to claim 1, characterized in that, In step (4), the pH is adjusted to 10.0~11.0; Preferably, in step (4), the reaction time after pH adjustment is 0.5 to 4 hours.

10. The method according to claim 1, characterized in that, The precipitate obtained in step (4) is filtered, washed, and dried to obtain high-purity magnesium hydroxide; the washing is done with deionized water; the drying temperature is 100~110℃; and the drying time is 1.5~6 hours.

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