Method for purifying and classifying low-quality boardlike phosphogypsum to prepare building materials

Abstract

This invention discloses a method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials, comprising the following steps: 1) feeding low-quality, compacted phosphogypsum into a ceramic ball mill for grinding to obtain phosphogypsum slurry; 2) feeding the phosphogypsum slurry into an ultrasonic cleaner for ultrasonic vibration cleaning; 3) obtaining a foam product as high-silica tailings and an underflow product as high-purity phosphogypsum slurry; 4) feeding the high-silica tailings into a filter for filtration, and then adding fly ash and an alkali activator to the filtered high-silica tailings to obtain a phosphogypsum-based polymer; 5) filtering the high-purity phosphogypsum slurry through a filter, drying it in an oven, and finally calcining it in a calcining furnace to obtain high-quality β-hemihydrate gypsum powder. This invention is simple and feasible, and particularly effective in purifying low-quality phosphogypsum with many impurities and severe compaction, without generating secondary solid waste. It also provides a reasonable and reliable full-scale application technology route for the resource utilization of low-quality, compacted phosphogypsum.

Description

Technical Field

[0001] This invention relates to the field of harmless and high-value treatment of phosphogypsum, and in particular to a method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials. Background Technology

[0002] The instability and diversity of various impurities in phosphogypsum, especially in low-quality, compacted phosphogypsum, result in high impurity content, difficulty in breaking up the compacted material, and difficulty in removing harmful impurities such as phosphorus and fluorine. This leads to poor environmental performance, low strength of the gypsum powder, and unstable water-to-gypsum ratio. Due to the influence of impurities in phosphogypsum, the consumption of building materials made from phosphogypsum is relatively low. In addition, the low added value and high logistics costs of this product limit the sales radius of building materials made from phosphogypsum, making market promotion difficult and hindering the expansion of production scale.

[0003] Current research on the purification and application of phosphogypsum is increasing, mainly focusing on methods such as flotation, calcination, and acid leaching. Among these, flotation is the preferred method for phosphogypsum resource utilization due to its low cost and simple operation. However, this method has limited purification capabilities and lacks highly efficient flotation reagents, and the resulting flotation tailings still face the problem of stockpiling. Calcination significantly improves the whiteness of phosphogypsum, but the processing cost is too high, and environmental requirements are difficult to meet, hindering industrialization. Acid leaching can produce high-purity, high-whiteness phosphogypsum, but the process is complex and costly, also making application difficult. Given the current situation of large reserves, low value, and poor quality of phosphogypsum, we should explore and find technologies that are low-cost, simple, feasible, and environmentally friendly, with the goal of achieving full-scale application of phosphogypsum and ultimately realizing its high-value utilization and zero emissions.

[0004] In the prior art, patent CN113695085 discloses a combined collector for desilication and decarbonization of phosphogypsum, its solution preparation method, and its application. Using benzyl quaternary ammonium salt as the main desilication collector and non-polar oil as the auxiliary collector, the introduction of benzyl strengthens the hydrogen bonding between the collector and silicate minerals compared to common alkyl straight-chain primary, secondary, and tertiary amines and quaternary ammonium salts. Although this method has stronger collecting power for silica-containing impurities and good desilication effect even in strongly acidic flotation environments, the invention involves a large number of reagents, is complex to formulate, and has high costs, making industrial application difficult.

[0005] Patent CN112808736B discloses a method for purifying and rendering harmless phosphogypsum. Flotation adsorbs or binds some impurities and harmful substances mixed in the phosphogypsum into the foam, thus separating and removing some impurities from the phosphogypsum. The resulting flotation slurry, after solid-liquid separation, yields refined phosphogypsum that meets the Class I phosphogypsum standard. The flotation residue is also purified to obtain harmless flotation residue, which can be managed as Class I residue. However, this method cannot achieve full-scale application of phosphogypsum. Given the large volume of phosphogypsum, a large amount of flotation residue is still generated and cannot be effectively utilized. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials. The method provided by this invention involves dissociating, purifying, and classifying phosphogypsum to prepare building materials. This not only yields high-quality β-phosphogypsum powder products but also allows the conversion of high-silica tailings into phosphogypsum-based polymers, ultimately achieving the high-value and comprehensive application of phosphogypsum.

[0007] The solution of the present invention is:

[0008] A method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials includes the following steps:

[0009] 1) Add water, pH adjuster and phosphorus / fluorine curing agent to low-quality hardened phosphogypsum, make slurry and pour it into a ceramic ball mill for grinding and dissociation to obtain phosphogypsum slurry;

[0010] 2) The phosphogypsum slurry is fed into an ultrasonic cleaner for ultrasonic vibration cleaning;

[0011] 3) The phosphogypsum slurry that has been cleaned in 2) is fed into a flotation machine, flotation reagents are added, and after stirring and aeration, reverse flotation is performed to obtain a foam product as high silica tailings and a bottom product as high-purity phosphogypsum slurry.

[0012] 4) The high-silica tailings are fed into a filter for filtration. The filtered high-silica tailings are mixed with fly ash and an alkali activator to obtain a phosphogypsum-based polymer.

[0013] 5) The high-purity phosphogypsum slurry is filtered, dried in an oven, and finally calcined in a calcining furnace to obtain high-quality β-hemihydrate gypsum powder. The recycled water obtained from filtration throughout the process is used for phosphogypsum slurry preparation, achieving water recycling. Low-quality, hardened phosphogypsum is ultimately processed using this method to obtain phosphogypsum-based polymers and β-hemihydrate gypsum powder suitable for building materials.

[0014] As a preferred technical solution, the solid-liquid ratio of the phosphogypsum slurry in step 1) is 1.3 to 3:2, the grinding time is 10 to 15 minutes, the pH adjuster is lime, and the amount of lime added is 0.2 to 0.3% of the mass of the low-quality calcined phosphogypsum; the amount of phosphorus / fluorine curing agent added is 0.06 to 0.08% of the mass of the low-quality calcined phosphogypsum.

[0015] As a preferred technical solution, the pH adjuster is industrial-grade lime; the phosphorus / fluorine curing agent is one or more of polyaluminum chloride (PAC), polyferric sulfate (PFS), and calcium chloride.

[0016] As a preferred technical solution, the ultrasonic frequency in step 2) is 40KHz, and the ultrasonic oscillation cleaning time is 10-15min.

[0017] As a preferred technical solution, the flotation machine in 3) is a hanging tank aerated flotation machine, the flotation process is a single flotation, the flotation reagent is a pH adjuster and a collector, the pH adjuster is lime, which adjusts the pH of the pulp to 6.5-7.5; the collector is one or more mixed amines selected from ethylenediamine, n-propylamine, and pentylamine, and is acidified with acetic acid at a weight ratio of 0.5-1.0 with the collector, and the dosage is 200-400 g / t.

[0018] As a preferred technical solution, the filter in 4) is a vacuum filter, the concentration of the filtered high-silica tailings is 15-20%, and the composition ratio of the phosphogypsum-based polymer is high-silica tailings: fly ash: alkali activator = 2:6-8:1-2; the alkali activator is one or more of water glass, sodium hydroxide, and calcium hydroxide, with a modulus of 1.3-1.5 and a dosage of 10-15%; the mixture is evenly poured into a mold and compacted, then placed in a curing box and cured at 80℃ for 1 day and at 25℃ for 2 days.

[0019] As a preferred technical solution, the high-purity phosphogypsum in step 5) is dried in an electric thermostatic drying oven at a temperature of 60-70°C, and then calcined in a muffle furnace at a temperature of 160-180°C for 2-3 hours.

[0020] As a preferred technical solution, the filtered water in steps 4) and 5) is returned to the phosphogypsum pulping process, thus achieving water recycling.

[0021] A method for purifying and classifying low-quality calcined phosphogypsum to prepare building materials, employing the above-mentioned technical solution, includes the following steps: 1) Adding low-quality calcined phosphogypsum to water, pH adjuster, and phosphorus / fluorine curing agent, forming a slurry, and then grinding and dissociating it in a ceramic ball mill to obtain phosphogypsum slurry; 2) Sending the phosphogypsum slurry to an ultrasonic cleaner for ultrasonic vibration cleaning; 3) Sending the cleaned phosphogypsum slurry from step 2) to a flotation machine, adding flotation reagent, stirring and aerating, and then performing reverse flotation to obtain a foam product as high-silica tailings and a bottom stream product as high-purity phosphogypsum slurry; 4) Sending the high-silica tailings to a filter for filtration, and then adding fly ash and an alkali activator to the filtered high-silica tailings to obtain a phosphogypsum-based polymer; 5) Filtering the high-purity phosphogypsum slurry through a filter, drying it in an oven, and finally calcining it in a calcining furnace to obtain high-quality β-hemihydrate gypsum powder.

[0022] The entire process mainly consists of four parts: ceramic grinding, ultrasonic cleaning, flotation separation, and filtration and calcination / polymer preparation. The recycled water obtained from filtration is used for phosphogypsum slurry making, realizing the recycling of recycled water. The process ultimately yields phosphogypsum-based polymers and β-hemihydrate gypsum powder (meeting the requirements of the national standard GB / T9776-2022 for building gypsum) that can be used in building materials, without generating secondary solid waste. This solves the environmental pollution and utilization problems caused by phosphogypsum stockpiling. The process realizes the high-value and full-scale application of phosphogypsum.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) The grinding and phosphorus fixation technology of the present invention can dissociate the useful component calcium sulfate in phosphogypsum from unreacted apatite, quartz, iron and aluminum oxides, organic matter and other impurities, thereby improving the monomer dissociation degree of the calcium sulfate component; at the same time, the soluble phosphorus and fluorine in phosphogypsum are fully exposed and dissolved, and firmly bonded with the curing agent to achieve efficient curing of phosphorus and fluorine.

[0025] (2) The ultrasonic oscillation cleaning process of the present invention can oscillate and remove impurities such as organic matter and fine particles adhering to the useful component calcium sulfate, providing favorable conditions for the flotation process stage.

[0026] (3) The flotation reagent system of the present invention is carried out under neutral conditions, without acid or alkali corrosion, and the acidity or alkalinity of the product does not need to be adjusted in the later stage; the collector is a compound mixed amine, which has high selectivity for quartz and organic matter in phosphogypsum, and has a high recovery rate while ensuring the quality of phosphogypsum product.

[0027] (4) The phosphogypsum-based polymer of the present invention can be used in mine restoration, roadbed materials and other fields, solving the problem that the secondary tailings generated after phosphogypsum flotation can only be stockpiled, and providing a technical route for the full-scale application of phosphogypsum.

[0028] The processing method provided by this invention is simple to operate, has a reasonable cost, a short process flow, and strong feasibility for industrial application.

[0029] The purification and application technology provided by this invention involves lightly grinding low-quality, compacted phosphogypsum into a slurry, adding a pH adjuster and a phosphorus-fluorine curing agent to eliminate the compaction phenomenon, solidify the soluble phosphorus and fluorine in the phosphogypsum, and initially dissociate the phosphogypsum from the bonded impurities. The ground phosphogypsum is then vibrated and cleaned using an ultrasonic cleaner to further remove impurities from its surface. Finally, neutral flotation removes quartz and organic matter from the phosphogypsum, resulting in a phosphogypsum that meets the national standard GB / T23456-2018 Grade 1 phosphogypsum. The purified phosphogypsum is then filtered, dried, and calcined to obtain high-purity, high-whiteness, high-quality β-hemihydrate gypsum, meeting the requirements of national standard GB / T9776-2022 building gypsum. The flotation foam tailings are filtered, alkali-activated, and cured to produce a phosphogypsum-based polymer, which can be used for building materials and other applications. The filtrate generated throughout the process can be returned to the system for recycling, and no secondary solid waste is generated. The method of this invention is simple and feasible, and it has a good purification effect, especially for low-quality phosphogypsum with many impurities and severe caking. It also provides a reasonable and reliable technical route for the resource utilization of low-quality caking phosphogypsum. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the process flow of the present invention;

[0031] Figure 2 and Figure 3 The images shown are the XRD and SEM images of the phosphogypsum raw material in Example 3.

[0032] Figure 4 and Figure 5 The images shown are XRD and SEM images of high-purity phosphogypsum after grinding and flotation in Example 3.

[0033] Figure 6 This is a SEM image of β-hemihydrate gypsum from Example 3. Detailed Implementation

[0034] This invention provides a method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials.

[0035] This invention provides a method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials, comprising the following steps:

[0036] 1) Pour low-quality, hardened phosphogypsum into a mill, add water, pH adjuster, and phosphorus and fluorine curing agents, and grind to obtain dissociated phosphogypsum slurry;

[0037] 2) The dissociated phosphogypsum slurry is fed into an ultrasonic cleaner for ultrasonic vibration cleaning to obtain a secondary dissociated phosphogypsum slurry.

[0038] 3) Pour the secondary dissociated phosphogypsum slurry into the flotation machine, add flotation reagents, stir and aerate, and then perform reverse flotation to obtain a foam product as high-silica tailings and a bottom product as high-purity phosphogypsum slurry.

[0039] 4) The high-silica tailings are filtered, then fly ash and alkali activator are added, stirred evenly and cured to obtain phosphogypsum-based polymer.

[0040] 5) The high-purity phosphogypsum slurry is filtered, dried, and calcined to obtain high-quality β-hemihydrate gypsum powder;

[0041] 6) There is no time order restriction between steps 4) and 5).

[0042] Unless otherwise specified, all raw materials used are commercially available products well known to those skilled in the art.

[0043] This invention involves grinding phosphogypsum with water, a pH adjuster, and a phosphorus / fluorine curing agent in a mill. The resulting dissociated phosphogypsum has a pH between 6 and 8, and the soluble phosphorus and fluorine in the slurry are solidified into insoluble substances. The raw material for this invention is low-quality calcined gypsum discharged from phosphorus chemical enterprises. For other phosphogypsum of higher quality, the purification and separation effect is even better. The mill used in this invention must be a ceramic ball mill lined with polyurethane, which is more corrosion-resistant and will not cause secondary pollution to the phosphogypsum compared to ordinary ball mills. The grinding concentration is preferably 40-60%, more preferably 45-55%. There are no special requirements for the pH adjuster and the phosphorus / fluorine curing agent.

[0044] In this invention, the ground phosphogypsum is fed into an ultrasonic cleaner for ultrasonic vibration cleaning to obtain a secondary dissociated phosphogypsum slurry. There are no special requirements for the type of ultrasonic cleaner; the phosphogypsum is ultrasonically cleaned under stirring conditions, and there are no special requirements for the specific implementation of the stirring method.

[0045] In this invention, the secondary dissociated phosphogypsum slurry is poured into a flotation machine, flotation reagents are added, and after stirring and aeration, reverse flotation is performed to obtain a frothy product as high-silica tailings and a bottom stream product as high-purity phosphogypsum slurry. There are no special requirements for the model of the flotation machine; the pulp temperature is room temperature, and the pulp concentration is preferably 30-40%, more preferably 30-35%; the flotation reagents are added in the following order: pH adjuster, collector, with an addition interval of 3-5 minutes and an action time of 3-5 minutes; the flotation pH value is 6.5-7.5, more preferably 7.

[0046] In this invention, the high-silica tailings are filtered, then fly ash and an alkali activator are added, stirred evenly, and cured to obtain a phosphogypsum-based polymer. The filtration is performed using a vacuum filter, the filter cake is washed and filtered, and the resulting filtrate is returned to step 1). There are no special requirements for the type of vacuum filter. The phosphogypsum-based polymer is poured into a test block mold and compacted, then cured in a curing chamber. The curing temperature is 70–90°C on day 1, and 20–30°C on days 2 and 3, for a total of 3 days. There are no special requirements for the model of the test block mold and the curing chamber. Finally, the phosphogypsum-based polymer cured for 3 days is subjected to a compressive strength test.

[0047] In this invention, the high-purity phosphogypsum slurry is filtered, dried, and calcined to obtain high-quality β-hemihydrate gypsum powder. The filtration is performed using a vacuum filter, and the filter cake is washed and filtered. The resulting filtrate is returned to step 1). There are no special requirements for the type of vacuum filter. The drying and calcination are performed in a constant-temperature forced-air drying oven and a muffle furnace, respectively. The drying temperature is 60–70°C, and the drying time is 4–6 hours. The calcination temperature is 160–180°C, and the calcination time is 2–3 hours. There are no special requirements for the model of the drying oven and the muffle furnace.

[0048] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0049] Example 1

[0050] The results of X-ray fluorescence spectroscopy and chemical multi-element analysis of dry phosphogypsum from a certain enterprise are shown in Tables 1 and 2 below.

[0051] Table 1. X-ray fluorescence spectral analysis of phosphogypsum

[0052]

[0053] Table 2. Multi-element chemical analysis of phosphogypsum

[0054]

[0055]

[0056] (1) Pour 500g of phosphogypsum into a ceramic ball mill, add water to make the grinding concentration reach 45%, then add 1g of lime, 0.1g of polyaluminum chloride (PAC), 0.1g of polyferric sulfate (PFS) and 0.1g of calcium chloride. Grind for 10 minutes to obtain dissociated phosphogypsum slurry.

[0057] (2) Pour the dissociated phosphogypsum slurry into a beaker container, place it in an ultrasonic cleaner and vibrate it at a working frequency of 40KHz. Stir the slurry at the same time and clean for 10 minutes.

[0058] (3) Pour the secondary dissociated phosphogypsum slurry into the flotation machine, with a flotation concentration of 35%. Add lime to adjust the pH to 7, and add a mixed amine flotation reagent of ethylenediamine: n-propylamine: acetic acid = 1:1:1 at a dosage of 200 g / t. Aerate and float for 3 min to obtain foam product (high silica tailings slurry) and underflow product (high purity phosphogypsum).

[0059] (4) The flotation tailings and concentrate were filtered, the tailings were kept for later use, and the concentrate was dried again. The resulting high-purity phosphogypsum had a dry weight of 404.4g, a yield of 80.88%, and a recovery rate of 89.13%, which met the first-level requirements of the national standard GB / T 23456-2018. Its main chemical composition and indicators are shown in Table 3.

[0060] Table 3 Main Technical Indicators of High-Purity Phosphogypsum

[0061]

[0062] (5) High-purity phosphogypsum was placed in a muffle furnace and calcined at a temperature of 160℃ for 2 hours to obtain high-quality β-hemihydrate gypsum powder, which meets the requirements of national standard GB / T9776-2022 building gypsum.

[0063] (6) Mix high silica tailings: fly ash: alkali activator in a ratio of 2:8:1.5. The alkali activator is water glass, sodium hydroxide and calcium hydroxide. The modulus is adjusted to 1.3 and the dosage is 13%. Mix them evenly, pour them into a mold and compact them. Place them in a curing box and cure at 80℃ for 1 day and at 25℃ for 2 days. The compressive strength was measured to be 7.1 MPa.

[0064] Example 2

[0065] The results of X-ray fluorescence spectroscopy and multi-element chemical analysis of dry-basis phosphogypsum from a certain enterprise are shown in Tables 4 and 5 below.

[0066] Table 4. X-ray fluorescence spectral analysis of phosphogypsum

[0067]

[0068] Table 5. Multi-element chemical analysis of phosphogypsum

[0069]

[0070] (1) Pour 500g of phosphogypsum into a ceramic ball mill, add water to make the grinding concentration reach 50%, then add 1.5g of lime, 0.15g of polyaluminum chloride (PAC), 0.15g of polyferric sulfate (PFS) and 0.15g of calcium chloride. Grind for 15 minutes to obtain a dissociated phosphogypsum slurry.

[0071] (2) Pour the dissociated phosphogypsum slurry into a beaker container, place it in an ultrasonic cleaner and vibrate it at a working frequency of 40KHz. Stir the slurry at the same time and clean for 15 minutes.

[0072] (3) Pour the secondary dissociated phosphogypsum slurry into the flotation machine, with a flotation concentration of 35%. Add lime to adjust the pH to 7, and add a mixed amine flotation reagent of ethylenediamine:pentylamine:acetic acid = 1:1:1 at a dosage of 300 g / t. Aerate and float for 3 min to obtain foam product (high silica tailings slurry) and underflow product (high purity phosphogypsum).

[0073] (4) The flotation tailings and concentrate were filtered, the tailings were kept for later use, and the concentrate was dried again. The resulting high-purity phosphogypsum had a dry weight of 350.1g, a yield of 70.02%, and a recovery rate of 81.35%, which met the first-level requirements of the national standard GB / T 23456-2018. Its main chemical composition and indicators are shown in Table 6.

[0074] Table 6 Main Technical Indicators of High-Purity Phosphogypsum

[0075]

[0076] (5) High-purity phosphogypsum was placed in a muffle furnace and calcined at a temperature of 170°C for 2 hours to obtain high-quality β-hemihydrate gypsum powder, which meets the requirements of national standard GB / T9776-2022 building gypsum.

[0077] (6) Mix high silica tailings: fly ash: alkali activator in a ratio of 2:7:1. The alkali activator is water glass and sodium hydroxide. The modulus is adjusted to 1.3 and the dosage is 10%. Mix them evenly, pour them into a mold and compact them. Place them in a curing box and cure at 80℃ for 1 day and at 25℃ for 2 days. The compressive strength was measured to be 6.6 MPa.

[0078] Example 3

[0079] The phosphogypsum from Example 1 and Example 2 was mixed in a 1:1 ratio and used as the raw material for this example. The experimental method steps are as follows:

[0080] (1) Pour 500g of phosphogypsum into a ceramic ball mill, add water to make the grinding concentration reach 50%, then add 1.2g of lime, 0.12g of polyaluminum chloride (PAC), 0.12g of polyferric sulfate (PFS) and 0.12g of calcium chloride. Grind for 12 minutes to obtain dissociated phosphogypsum slurry.

[0081] (2) Pour the dissociated phosphogypsum slurry into a beaker container, place it in an ultrasonic cleaner and vibrate it at a working frequency of 40KHz. Stir the slurry at the same time and clean for 12 minutes.

[0082] (3) Pour the secondary dissociated phosphogypsum slurry into the flotation machine, with a flotation concentration of 25%. Add lime to adjust the pH to 7, and add a mixed amine flotation reagent of n-propylamine: pentamamine: acetic acid = 1:1:1 at a dosage of 250 g / t. Aerate and float for 3 min to obtain foam product (high silica tailings slurry) and underflow product (high purity phosphogypsum).

[0083] (4) The flotation tailings and concentrate are filtered. The tailings are kept for later use, and the concentrate is dried again. The resulting high-purity phosphogypsum is placed in a muffle furnace for roasting at a set temperature of 170℃ for 2 hours to obtain high-quality β-hemihydrate gypsum powder, which meets the requirements of the national standard GB / T9776-2022 for building gypsum. The indicators are shown in Table 7.

[0084] Table 7 Main Technical Indicators of β-Hemihydrate Gypsum Powder

[0085]

[0086] (6) Mix high silica tailings: fly ash: alkali activator in a ratio of 2:6:1. The alkali activator is water glass and sodium hydroxide with a modulus of 1.3 and a dosage of 11%. Mix them evenly, pour them into a mold, compact them, and place them in a curing box. Cure at 80°C for 1 day and at 25°C for 2 days. The compressive strength was measured to be 6.2 MPa.

[0087] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials, characterized in that, Includes the following steps: 1) Add water, pH adjuster and phosphorus / fluorine curing agent to low-quality calcined phosphogypsum, make slurry, and then pour it into a ceramic ball mill for grinding and dissociation to obtain phosphogypsum slurry; the grinding time is 10-15 min, and the amount of phosphorus / fluorine curing agent added is 0.06-0.08% of the mass of low-quality calcined phosphogypsum; the phosphorus / fluorine curing agent is one or more of polyaluminum chloride (PAC), polyferric sulfate (PFS), and calcium chloride. 2) The phosphogypsum slurry is fed into an ultrasonic cleaner for ultrasonic vibration cleaning; 3) The phosphogypsum slurry that has been cleaned in 2) is fed into a flotation machine, flotation reagents are added, and after stirring and aeration, reverse flotation is performed to obtain a foam product as high silica tailings and a bottom product as high-purity phosphogypsum slurry. The flotation reagents are a pH adjuster and a collector. The pH adjuster adjusts the pulp pH to 6.5-7.

5. The collector is a mixture of various amines including ethylenediamine, n-propylamine, and pentylamine, and is acidified by stirring with acetic acid at a weight ratio of 0.5-1.0 to the collector. 4) The high-silica tailings are fed into a filter for filtration. The filtered high-silica tailings are then mixed with fly ash and an alkali activator to obtain a phosphogypsum-based polymer. The composition ratio of the phosphogypsum-based polymer is high-silica tailings: fly ash: alkali activator = 2:6~8:1~2. The alkali activator is one or more of water glass, sodium hydroxide, and calcium hydroxide, with a modulus of 1.3~1.

5. 5) The high-purity phosphogypsum slurry is filtered by a filter, dried in an oven, and finally calcined in a calcining furnace to obtain high-quality β-hemihydrate gypsum powder.

2. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 1, characterized in that: The solid-liquid ratio of the phosphogypsum slurry in 1) is 1.3 to 3:2, the pH adjuster is lime, and the amount of lime added is 0.2 to 0.3% of the mass of the low-quality hardened phosphogypsum.

3. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 2, characterized in that: The pH adjuster is industrial-grade lime.

4. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 1, characterized in that: In step 2), the ultrasonic frequency is 40 kHz, and the ultrasonic oscillation cleaning time is 10 to 15 minutes.

5. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 1, characterized in that: The flotation machine in 3) is a hanging tank aerated flotation machine, the flotation process is a single flotation, the pH adjuster is lime, and the collector dosage is 200-400g / t.

6. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 1, characterized in that: The filter mentioned in 4) is a vacuum filter; the phosphogypsum-based polymer is poured into a mold and compacted, then placed in a curing box and cured at 80°C for 1 day and at 25°C for 2 days.

7. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 1, characterized in that: The high-purity phosphogypsum in step 5) is dried in an electric thermostatic drying oven at a temperature of 60-70°C, and then calcined in a muffle furnace at a temperature of 160-180°C for 2-3 hours.

8. The method for purifying and classifying low-quality, compacted phosphogypsum to prepare building materials as described in claim 1, characterized in that: The filtered water in steps 4) and 5) is returned to the phosphogypsum pulping process.

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