Etringite crystal material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of solid waste recycling, and provides an ettringite crystal material as well as a preparation method and application thereof. According to the invention, industrial solid wastes such as ardealite, phosphorus tailings and red mud are used as main raw materials, and the ettringite crystal material is obtained through drying pretreatment, specific proportion mixing, addition of an auxiliary agent to promote an ion replacement reaction and aging solidification, so that multi-ion synergistic migration and crystal structure oriented growth are realized; harmless, recycling and ecological integrated utilization of the three wastes is realized, and a new sustainable solution can be provided for large-scale treatment and ecological restoration of industrial solid wastes.

Description

Technical Field

[0001] This invention relates to the field of solid waste recycling technology, specifically to an ettringite crystal material, its preparation method, and its application. Background Technology

[0002] Phosphogypsum is a byproduct of the wet process for producing phosphoric acid. Its main component is calcium sulfate dihydrate (CaSO4·2H2O), which contains soluble phosphorus, fluorides, heavy metals, and other harmful elements. Its high water content and acidic residue lead to long-term leachate discharge from the stockpile, causing groundwater pollution and soil acidification.

[0003] Phosphate tailings are tailings from the flotation and separation of phosphate rock. They are mainly silica sand particles (SiO2 content is usually >70%), containing residual phosphates, heavy metals and fine-grained sticky substances. After being stockpiled, they are prone to causing soil erosion and instability of tailings dams.

[0004] Red mud is a highly alkaline waste residue (pH usually >12) generated during the production of alumina. It is mainly composed of Fe2O3, Al2O3, SiO2 and titanium compounds. It has high alkalinity, and its long-term storage can cause soil alkalization, groundwater pollution and heavy metal migration.

[0005] The common methods for dealing with the aforementioned "three wastes" include stockpiling, sealing off sites, and chemical modification, but these pose significant long-term safety hazards and lack a stable long-term treatment path. Summary of the Invention

[0006] Therefore, it is necessary to provide an ettringite crystal material, its preparation method, and its application.

[0007] Currently, synergistic research schemes for multi-source solid waste are becoming a research hotspot. However, existing research still faces the following technical bottlenecks: ① The system design lacks directional control of ion reactions, leading to unstable formation of ettringite and uneven solidification structure; ② Insufficient precision in controlling reaction parameters (pH, Ca / Al ratio, water-to-solid ratio, etc.) results in large fluctuations in material properties; ③ The long-term stability of harmful elements after synergistic solidification has not been systematically verified, and there is still a potential leaching risk; ④ The unreasonable matching of chemical activities among multi-component wastes limits the sustainability of synergistic mineralization and the high-value utilization of materials.

[0008] The present invention adopts the following technical solution: This invention provides a method for preparing ettringite crystal materials, using phosphogypsum, phosphorus tailings, red mud, and additives as raw materials. The additives are used to adjust the pH of the system and provide a Ca source or active silica-alumina components. The method includes the following steps: To obtain phosphogypsum, pretreatment is performed: aging is carried out to reduce the free water content to ≤12%; Phosphorus tailings were obtained, and pretreatment was performed: coarse particles were removed by sieve screening through an 80-100 mesh screen, and the material was dried to a moisture content of ≤5%. Red mud was obtained and pretreated by acid washing and dealkalization to adjust the pH to 8.5-9.5. The pretreated phosphogypsum, phosphogypsum tailings and red mud are mixed together, and additives are added to obtain a homogeneous mixture. Water is added to the mixed material, the water-to-solid ratio is controlled at 0.3-0.4, and the mixture is stirred. An ion exchange reaction is carried out at room temperature to obtain a reaction slurry. The reaction slurry is aged and solidified to obtain ettringite crystal material.

[0009] The reaction mechanism of ettringite crystals is as follows: CaSO4·2H2O+Al 3+ +Ca 2+ +SO4 2− +OH - →Ca6Al2(SO4)3(OH) 12 ·26H2O.

[0010] In some embodiments, the phosphogypsum is selected from the by-product of wet phosphoric acid production, with a CaSO4·2H2O content ≥90%, pH 4.0~5.0, and contains a small amount of soluble phosphorus, fluorine and heavy metal ions; the phosphate tailings are selected from phosphate ore beneficiation tailings, with a SiO2 content ≥70% and a particle size distribution mainly between 75~180 μm; and / or the red mud is selected from Bayer process alumina waste residue, containing Al2O3 20~30%, Fe2O3 30~40%, pH 10~12, and is strongly alkaline.

[0011] Preferably, the pretreatment process parameters are as follows: phosphogypsum aging for 7-10 days (free water ≤12%); red mud pH adjusted to 8.5-9.5 after acid washing; and phosphate tailings drying ≤5%.

[0012] Preferably, the mass ratio of phosphogypsum, phosphogypsum tailings, and red mud is (40~60):(20~30):(15~25). More preferably, the mass ratio of phosphogypsum, phosphogypsum tailings, and red mud is 50:25:20.

[0013] In some embodiments, the additive comprises a modifier and a stabilizer. The modifier is selected from quicklime or polyferric phosphate, and the stabilizer is selected from sodium silicate or sodium aluminate. The amount of additive added is 5-15% of the total mass of phosphogypsum, phosphogypsum tailings, and red mud, preferably 7-12%. The mass ratio of modifier to stabilizer is preferably (5-10):(2-5), more preferably 7:3.

[0014] Preferably, the uniformity of the dry mix is ​​confirmed by visual inspection and sieve residue test (<5%). The purpose of this step is to ensure that the reactive species are in full contact at the microscale, creating a uniform reaction environment for the subsequent ion exchange reaction.

[0015] Preferably, the conditions for the ion exchange reaction are: constant temperature 25±5℃, reaction time 2~4 hours.

[0016] Preferably, the aging and curing conditions are: 90% humidity, 20~25℃ temperature, aging for 7~10 days, followed by drying and curing.

[0017] This invention provides an ettringite crystal material prepared by the above method. The ettringite crystal material solidifies environmentally harmful elements through both chemical bonding and lattice containment.

[0018] The present invention also provides the use of the above-mentioned ettringite crystal material in the preparation of mine backfill, roadbed filler and soil remediation agent.

[0019] The present invention also provides an ecological restoration material comprising the ettringite crystal material prepared by the above method.

[0020] Compared with the prior art, the core technical advantages of this invention are: This invention utilizes a composite system primarily composed of phosphogypsum (providing sulfate and calcium sources), phosphorus tailings (providing a silica-alumina phosphate and SiO2 framework to enhance ion exchange), and red mud (providing alkalinity and aluminum-iron oxides). Through specific process conditions and additive combinations to regulate the ion reaction rate and crystal formation environment, the prepared ettringite crystal material exhibits stable lattice containment, capable of immobilizing heavy metals and fluoride-phosphorus ions at the molecular level, achieving long-lasting harmlessness. The proportions of the three wastes and their functional complementarity are crucial to the solidification reaction; the absence of any component hinders ettringite formation or reduces its stability.

[0021] This invention utilizes an ion replacement-synergistic solidification mechanism centered on the mineralization reaction of ettringite, by regulating the Ca content in the system. 2+ Al 3+ With SO4 2- The ratio of ions and reaction conditions enable phosphogypsum, red mud, and phosphate tailings, along with specific additives, to achieve an acid-base complementary reaction under low energy consumption. During the reaction, Ca... 2+ With Al 3+ Stable ettringite (Ca6Al2(SO4)3(OH) is generated through ion replacement. 12 ·26H2O) crystals, whose porous tunnel structure can effectively contain and fix F - PO4 3- Pb 2+ Cr 3+ Harmful ions are neutralized, achieving dual solidification through chemical bonding and lattice containment. This mechanism transforms potential pollutants in solid waste into stable components within the crystal structure, fundamentally realizing the harmless and mineralized fixation of multi-source solid waste.

[0022] This invention is the first to construct a multi-component reaction system consisting of "phosphogypsum (Ca, S source) - red mud (Al, Fe source) - phosphogypsum tailings (Si source)," achieving synergistic mineralization through elemental complementarity. In this system, the SiO2 framework of phosphogypsum tailings regulates the pore structure and crystal growth direction; red mud provides aluminum and iron ions to promote ettringite nucleation; phosphogypsum provides sulfate and calcium sources to maintain a stable ionic balance; and modifiers and stabilizers regulate the ionic reaction rate and crystal formation environment. The synergistic effect of these components significantly improves the density and stability of the material, with a solidification efficiency more than 60% higher than that of a single solid waste system.

[0023] This invention employs a four-stage preparation process: pretreatment, dry mixing and homogenization, ion replacement reaction, and aging and curing. The entire process is completed at room temperature and pressure, without the need for calcination or high-pressure curing. Compared with traditional cement-based curing systems, the energy consumption of this invention's preparation process is reduced by 80%, and carbon emissions are reduced by approximately 90%, while maintaining the high strength and low leaching performance of ettringite crystal materials, achieving green, low-carbon, and high-value utilization. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0025] Figure 1 The image is a scanning electron microscope (SEM) image (magnification 5000×) of the material obtained in Example 1.

[0026] Figure 2 The image shows the X-ray diffraction (XRD) pattern of the material obtained in Example 2.

[0027] Figure 3 The graph shows the statistical line graph of the leaching concentration of harmful elements in the application test of the material obtained in Example 1 under different ash-sand ratios. Detailed Implementation

[0028] The technical concept of this invention lies in providing a method and application for the synergistic preparation of ettringite crystal materials from three wastes (phosphogypsum, phosphate tailings, and red mud). The raw materials are derived from industrial solid waste. Through pretreatment, mixing and homogenization in a specific ratio, and ettringite ion replacement reaction, needle-like or plate-like ettringite crystals are generated, whose lattice can effectively contain PO4. 3- F - and Pb 2+ Cr 3+Harmful ions are removed to achieve solidification within the crystal structure. Sodium silicate or sodium aluminate, as an additive, provides a soluble silicon or aluminum source during this process, promoting crystal framework growth and pore filling, thus enhancing the material's density. The mineralization and solidification process involves aging in a constant temperature chamber (90% humidity, 20-25℃) for 7-10 days to promote full growth of the ettringite crystals. Subsequently, it is dried in an oven at 60±10℃ for 24-48 hours, and then demolded to obtain the environmental remediation material synergistically modified from waste materials.

[0029] The present invention will be further described in detail below with reference to specific embodiments, so that those skilled in the art can more clearly understand the present invention. The following embodiments are only used to illustrate the present invention, and are not intended to limit the scope of the present invention. Based on the specific embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention. In the embodiments of the present invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art; in the embodiments of the present invention, unless specifically specified, the technical means used are conventional means well known to those skilled in the art.

[0030] Key raw material sources and pretreatment instructions: Phosphogypsum: a byproduct of wet phosphoric acid production, with a CaSO4·2H2O content ≥90%, pH 4.0~5.0, and containing small amounts of soluble phosphorus, fluorine, and heavy metal ions. Preferably, the pretreatment process is as follows: aging at room temperature (20~25℃) for 7~10 days to reduce the free water content to ≤12% to minimize the dilution effect on the subsequent reaction system.

[0031] Phosphate tailings: tailings from phosphate ore beneficiation, with SiO2 content ≥70% and particle size distribution mainly between 75~180 μm. Preferably, the pretreatment process is as follows: after removing coarse particles through an 80~100 mesh sieve, the material is dried to a moisture content ≤5% to ensure uniform particle distribution in the mixed system.

[0032] Red mud: Bayer process alumina waste residue, with Al2O3 content of 20-30%, Fe2O3 content of 30-40%, pH 10-12, exhibiting strong alkalinity. Preferably, the pretreatment process involves acid washing and de-alkali removal with a 0.5-1.0 mol / L dilute sulfuric acid solution, stirring for 30-45 minutes, followed by filtration, and adjusting the pH to 8.5-9.5. This significantly reduces the alkalinity of the system and activates the reactivity of the aluminum components.

[0033] Quicklime: CaO content ≥90%.

[0034] Polyferric phosphate: An inorganic polymeric iron salt containing phosphate groups. Appearance: Dark reddish-brown viscous liquid or brownish-yellow powder. Total iron (calculated as Fe) content: ≥10.5%, available P2O5 content: ≥3.0%~6.0%, basicity: 8.0%~15.0%, pH (1% aqueous solution): 1.5~2.5, density (20℃): ≥1.40 g / cm³ (liquid). It provides instantaneous neutralization of strong acids and bases and acts as a pH buffer. Polyferric phosphate combines the rapid reactivity of strongly acidic phosphate and polyferric iron, capable of pulling the strongly alkaline red mud (pH 12+) to the optimal ettringite formation range of 8.8~9.2 within 10~30 minutes. This is faster and more precise than quicklime and polyferric sulfate, avoiding localized over-alkalinity that could delay ettringite formation or lead to its conversion into monosulfide hydration products. It provides a "dual-core" active component (Fe). 3+ +PO4 3- ), Fe 3 + Al replacement part 3+ Iron enters the ettringite lattice, forming iron-substituted ettringite (Ca6[Al2O3]2O3). 2-x Fe x ]2(SO4)3(OH) 12 ·26H2O), significantly improves the early strength of crystals (strength can be increased by more than 30% after 3 days). PO4 3- Prioritizes residual soluble phosphorus, fluorine, and heavy metals (Pb) in the system. 2+ Cd 2+ Zn 2+ In situ, extremely insoluble phosphate precipitates (FePO4, Ca5(PO4)3F, etc.) are generated, and free phosphorus and fluorine are solidified in a secondary manner, resulting in a leaching concentration that is 50% to 70% lower than that of a single polyferric or quicklime system.

[0035] Polyferric phosphate can be prepared in the laboratory or industrially using the following methods: Ferrous sulfate (FeSO4·7H2O) is dissolved in water, and an appropriate amount of sulfuric acid is added to adjust the pH to 1.0-2.0 to form a ferrous sulfate solution. Then, phosphoric acid or a phosphate (such as sodium dihydrogen phosphate or sodium pyrophosphate, controlling the P / Fe molar ratio to 0.05-0.2) is added as a phosphorus source. The solution is heated to 40-60℃, and an oxidizing agent (hydrogen peroxide H2O2 or sodium chlorate, etc., controlling the reaction between the oxidizing agent and Fe) is slowly added. 2 + (Molar ratio of 0.5~1.0), stirring and oxidizing for 2~4 hours, so that Fe... 2+ Oxidized to Fe 3+A hydrolysis-polymerization reaction occurs; the temperature is controlled not to exceed 70℃ during the reaction to avoid over-polymerization; after the reaction is completed, continue stirring and aging for 1-2 hours, then cool to room temperature to obtain a dark reddish-brown viscous liquid polyferric phosphate. The obtained product has a total iron content ≥10.5%, a P2O5 content of 3.0%~6.0%, a basicity of 8.0%~15.0%, and a pH (1% solution) of 1.5~2.5, meeting the usage requirements.

[0036] Sodium silicate (Na2SiO3): Modulus 2.5~3.0.

[0037] The following example illustrates this.

[0038] Example 1 This embodiment provides a method for the synergistic preparation of ettringite crystal materials using three types of waste, including the following steps: S1, Obtain the reaction raw materials and perform pretreatment.

[0039] The raw materials are obtained as follows, by weight: 50 parts phosphogypsum, 25 parts phosphogypsum tailings, 20 parts red mud, 7 parts quicklime, and 3 parts sodium silicate.

[0040] The phosphogypsum was left to age naturally at room temperature (20~25℃) for 8 days until the free water content dropped to below 12% (measured by drying method).

[0041] The tailings were screened through an 80-mesh sieve to remove impurity particles larger than 0.18 mm and dried until the moisture content was ≤5%.

[0042] The red mud was treated with dilute sulfuric acid (concentration 0.5 mol / L) to remove alkali. After stirring for 30 minutes, it was filtered, and the pH was adjusted to 9.0±0.5 (monitored with a pH meter). It was then dried for later use.

[0043] S2, prepare the mixture.

[0044] The pretreated phosphogypsum, phosphogypsum tailings and red mud were placed in a mechanical mixer and dry-mixed at 300 rpm for 5 minutes. Then quicklime and sodium silicate were added and dry-mixed for another 3 minutes to ensure uniform distribution (no lumps visible) to obtain the mixture.

[0045] S3, ion displacement reaction.

[0046] Slowly add water (20-25℃) to the mixture while stirring (500 rpm), controlling the water-to-solid ratio at 0.3 to form a homogeneous slurry. Place the slurry in a constant temperature water bath (25℃) and stir for 3 hours to promote the formation of ettringite, thus obtaining the reaction slurry.

[0047] Specific reaction mechanism: Phosphogypsum provides Ca 2+ and SO4 2-Red mud provides Al 3+ Sodium silicate enhances the silicon source, generating Ca6Al2(SO4)3(OH). 12 • 26H2O crystals, which solidify heavy metals (such as Pb and Cr) and fluorine-phosphorus compounds.

[0048] S4, aging and curing.

[0049] The reaction slurry is poured into a mold (100mm×100mm×100mm) and aged for 9 days at 90% humidity and 20~25℃. Then it is dried in an oven at 60℃ for 24 hours, demolded, and the calcite crystal material is obtained, which can also be used as an environmental remediation material.

[0050] The materials obtained in this embodiment were subjected to structural characterization and performance testing.

[0051] 1. Structural characterization.

[0052] Scanning electron microscopy (SEM) procedure: Take a fresh cross-section of the aged and dried material (approximately 5 mm × 5 mm × 3 mm), fix it to the sample stage with conductive adhesive, and sputter gold onto the surface for 30–60 s. Observe under accelerating voltage of 5–15 kV and working distance of 8–10 mm, with a magnification of 500–10000x.

[0053] The results are as follows Figure 1 As shown, needle-like and platy ettringite crystals are interwoven and distributed within the matrix, forming a dense three-dimensional network structure. This structure significantly improves the overall strength and impermeability of the material, while simultaneously achieving lattice containment and physical sealing of harmful ions, verifying the effectiveness of the ettringite ion replacement reaction mechanism of this invention.

[0054] 2. Performance Testing: (1) Strength.

[0055] According to GB / T 17671-1999 standard, 40 mm × 40 mm × 160 mm specimens were prepared and cured for 28 days in a curing chamber with a temperature of 20±2℃ and a humidity of ≥95%. A universal testing machine was used to apply a load at a rate of 2.4 kN / s, and the failure load was recorded. The compressive strength (unit: MPa) and flexural strength (unit: MPa) were calculated.

[0056] The results showed that the ettringite crystal material had a 28-day compressive strength of 28.5 MPa and a flexural strength of 5.2 MPa.

[0057] (2) Leaching experiment.

[0058] Referring to the HJ 557-2010 standard, 5 g of sample was crushed to ≤5 mm, added to deionized water (liquid-to-solid ratio 10:1), and immersed in a rotary shaker (30 r / min) for 18 h. After filtration, the concentrations of fluoride ions, soluble phosphorus, and heavy metals (Pb, Cr, Cd) were determined by inductively coupled plasma mass spectrometry (ICP-MS) and compared with the Class III limits of GB / T 14848-2017.

[0059] The results showed that the pH of the ettringite crystal material solution was 7.5, the fluoride ion concentration was ≤4.8 mg / L, the soluble phosphorus concentration was ≤0.25 mg / L, and the leaching of heavy metals (Pb, Cr, Cd) were all lower than the Class III limit of the groundwater quality standard (GB / T 14848-2017).

[0060] 3. Soil remediation simulation experiment: Soil remediation simulation experiment procedure: Topsoil (0-20 cm) from acidic polluted farmland surrounding a phosphate chemical enterprise in Hubei Province was collected, air-dried, and sieved through a 5 mm sieve. The initial pH was 4.32-4.68. A control group (CK) and an experimental group (T1: 5% material addition, T2: 10% material addition) were set up. Each pot contained 2.00 kg of dry soil, and the remediation agent (environmental remediation material from Example 1) was mixed thoroughly with the soil. The soil was cultured for 30 days in an artificial climate chamber (25℃ during the day, 18℃ at night, 12 h / d light, 70% humidity), maintaining a field water holding capacity of 60%-70%. After culture, soil samples (0-15 cm) were collected, air-dried, and sieved. pH, available phosphorus, perfluorinated compounds, heavy metals, and other indicators were measured.

[0061] Test results: In group T2, the pH rose to 6.8, the heavy metal fixation rate was ≥95%, and the vegetation coverage increased by 22%.

[0062] The ettringite crystal material obtained in Example 1 was used as a cementing material. Mortar specimens were prepared by mixing the cementing material with standard sand at different cement-sand ratios (mass ratio of cementing material to standard sand was 1:8, 1:12, 1:15, and 1:20, respectively). After standard curing, a leaching toxicity test for harmful elements was conducted (referring to standard HJ 557-2010) to determine the Pb content in the leachate. 2+ Cr 3+ Cd 2+ F - The concentrations of harmful elements were measured, and line graphs showing changes over time or under different conditions were plotted to conduct long-term leaching toxicity tracking experiments.

[0063] The results are as follows Figure 3 As shown in the line graph of harmful element leaching concentration in the material, it can be seen that: Pb 2+ Cr 3+ Cd 2+ and F -The leaching amount of plasma tends to stabilize over time, and eventually falls below the Class III limit of the Groundwater Quality Standard (GB / T 14848-2017), indicating that the material of this invention has long-term chemical stability and environmental safety.

[0064] Example 2 This embodiment provides a method for the synergistic preparation of ettringite crystal materials using three wastes. The method steps are basically the same as those in Embodiment 1, with the only difference being: (1) In step S1, the raw materials, by weight, are: 45 parts phosphogypsum, 28 parts phosphogypsum tailings, 22 parts red mud, 8 parts polyferric phosphate, and 4 parts sodium silicate. The pretreatment process differs as follows: The phosphogypsum is naturally aged at room temperature (20~25℃) for 7 days to reduce the free water content to below 12% (determined by drying). The red mud is treated with 0.8 mol / L sulfuric acid for dealkalization, stirred for 45 minutes, filtered, and the pH is adjusted to 8.5 before drying for later use.

[0065] (2) In step S2, the water-to-solid ratio of the mixture is controlled to be 0.35.

[0066] (3) In step S3, the reaction is stirred for 2.5 hours.

[0067] (4) In step S4, the product is aged for 8 days and dried at 50°C.

[0068] The materials obtained in this embodiment were subjected to structural characterization and performance testing.

[0069] 1. Structural characterization.

[0070] X-ray diffraction (XRD) analysis procedure: The aged and dried bulk material is pulverized, passed through a 200-mesh sieve, and the powder sample is placed and flattened in a zero-background sample holder. Cu target Kα radiation (λ=0.15418 nm) is used, with a tube voltage of 40 kV, a tube current of 40 mA, a scanning range of 2θ=5°~70°, a step size of 0.02°, and a dwell time of 0.3 s per step. Characteristic diffraction peaks of ettringite are identified by comparison with standard PDF cards.

[0071] The results are as follows Figure 2 As shown: The system exhibits distinct characteristic diffraction peaks of ettringite (2θ≈9.1°, 15.8°, 22.9°), accompanied by small amounts of characteristic peaks of gypsum and hematite. This indicates that ettringite crystals are the dominant mineral phase in the system, demonstrating a complete reaction, significant synergistic mineralization effect of the three wastes, and high structural stability of the product.

[0072] 2. Performance testing.

[0073] (1) The strength test method is described in Example 1. The results showed that the compressive strength after 28 days was 30.2 MPa and the flexural strength was 5.8 MPa.

[0074] (2) The leaching test method is the same as in Example 1. The results showed that the pH of the ettringite crystal material solution was 7.2, the fluoride ion concentration was ≤3.5 mg / L, the soluble phosphorus concentration was ≤0.20 mg / L, and the leaching amount of heavy metals (Pb, Cr, Cd) all met the standards.

[0075] 3. Roadbed filling simulation experiment.

[0076] The testing method and steps are as follows: Prepare specimens with a diameter of 150 mm and a height of 120 mm, and conduct compaction degree (≥95%) and California bearing ratio (CBR) tests according to JTJ 034-2000 standard. Use SEM to analyze porosity (≤15%) and calculate carbon emissions (85% reduction compared to conventional cement).

[0077] The results showed that the compaction degree was 95%, the bearing capacity ratio (CBR) was ≥8%, and carbon emissions were reduced by 85% compared to traditional cement. Microscopic analysis (SEM) showed that ettringite needle-like crystals were densely distributed, and the porosity was ≤15%.

[0078] Example 3 This embodiment provides a method for the synergistic preparation of ettringite crystal materials using three wastes. The method steps are basically the same as those in Embodiment 1, with the only difference being: (1) In step S1, the raw materials, by weight, are: 55 parts phosphogypsum, 22 parts phosphogypsum tailings, 18 parts red mud, 6 parts quicklime, and 4 parts sodium silicate. The red mud is treated with 0.8 mol / L sulfuric acid for dealkalization, stirred for 45 min, filtered, and the pH is adjusted to 9.5. It is then dried for later use. The phosphogypsum tailings are sieved through a 100-mesh sieve to remove impurities and dried until the moisture content is ≤5%.

[0079] (2) In step S2, the water-to-solid ratio of the mixture is controlled to be 0.32.

[0080] (3) In step S3, the reaction is stirred for 4 hours.

[0081] (4) In step S4, the product is aged for 10 days and dried at a temperature of 48 hours.

[0082] The materials obtained in this embodiment were subjected to structural characterization and performance testing.

[0083] 1. Structural characterization results show that XRD analysis confirms the high peak strength of ettringite, proving the effectiveness of the curing mechanism.

[0084] 2. Performance testing.

[0085] (1) Strength method steps: Specimen preparation: After crushing the ettringite material, mix it with standard mortar (material:standard sand = 1:3), add water and stir (water-to-solid ratio 0.5). Cast into prism specimens of 40 mm × 40 mm × 160 mm, compact them on a vibrating table and smooth them.

[0086] Curing conditions: The specimens were cured for 28 days in a standard curing chamber with a temperature of 20±1°C and a humidity of ≥90%.

[0087] Strength test: Using a universal testing machine, the compressive strength (40 mm × 40 mm of the specimen under pressure) and bending strength (three-point bending method, span 100 mm) were recorded at a uniform loading rate of 2400 N / s.

[0088] Three specimens were tested in each group, and the average value was taken as the final result.

[0089] The results showed that the compressive strength after 28 days was 26.8 MPa and the flexural strength was 4.9 MPa.

[0090] (2) Leaching experiment: Sample preparation: The ettringite material was crushed and passed through a 100-mesh sieve (particle size ≤ 0.15 mm), and 10.0 g of dry basis sample was taken.

[0091] Leaching procedure: Add deionized water (liquid-to-solid ratio 10:1) to a 500 mL stoppered conical flask. Shake horizontally at 110±10 rpm for 18 hours in a constant temperature shaker (25±2°C).

[0092] Analysis and detection: The leachate was filtered (using a 0.45 μm filter membrane), and fluoride ions (F) were determined by ion chromatography (HJ 84-2016). - ) and soluble phosphorus (PO4) 3- Heavy metals (Pb, Cr, Cd) were determined using inductively coupled plasma mass spectrometry (HJ 766-2015).

[0093] Compare with the Class III limit (F) of the "Groundwater Quality Standard" (GB / T 14848-2017). - ≤1.0 mg / L, PO4 3- ≤0.3mg / L).

[0094] The results show: The pH of the ettringite crystal material solution is 7.8, fluoride ion concentration is ≤5.0 mg / L, soluble phosphorus concentration is ≤0.30 mg / L, and the fixation rate of heavy metals (Pb, Cr, Cd) is ≥98%.

[0095] 3. Soil remediation simulation experiment: Soil preparation: Take typical acidic contaminated soil (pH 4.5, total lead 136 mg / kg), air dry and pass through a 5 mm sieve.

[0096] Experimental design: A control group (CK) and a treatment group (T1: 5% material addition, T2: 10% material addition) were set up, with 4 replicates for each group, and each pot containing 2.0 kg of soil.

[0097] Culture conditions: Cultured in an artificial climate chamber (25°C during the day / 18°C at night) for 30 days, maintaining soil moisture at 60–70%.

[0098] Indicator testing: Soil samples were taken after cultivation to measure pH (GB / T 7859-1995) and heavy metal fixation rate (HJ 491-2019).

[0099] Example results: After adding 10% of the material from Example, the soil pH increased from 4.5 to 6.8, and the heavy metal fixation rate was ≥95%.

[0100] Comparative Example 1 This comparative example provides a method for the synergistic preparation of remediation materials from two types of waste. The method steps are basically the same as those in Example 1, except that in step S1, the raw materials, by weight, are: 60 parts phosphogypsum, 30 parts phosphorus tailings, 8 parts polyferric phosphate, and 4 parts sodium silicate. Red mud is not included.

[0101] The material obtained in this experimental example was subjected to the same tests as in Example 1. The method of the present invention could not form ettringite crystals, and the material stability was poor. (1) The compressive strength after 28 days is only 15.6 MPa.

[0102] (2) The leaching of soluble phosphorus is >1.2 mg / L, and the leaching of heavy metals exceeds the standard.

[0103] (3) The vegetation coverage rate increased by only 5%.

[0104] Comparative Example 2 This comparative example provides a method for the synergistic preparation of remediation materials from two types of waste. The method steps are basically the same as those in Example 2, except that in step S1, the raw materials, by weight, are: 50 parts phosphogypsum, 25 parts red mud, 7 parts quicklime, and 3 parts sodium silicate. Phosphate tailings are not included.

[0105] The material obtained in this experimental example was subjected to the same tests as in Example 2, and it was found that the material prepared by this method had poor performance. (1) The compressive strength after 28 days is only 22.1 MPa.

[0106] (2) Porosity > 25%, leachable fluoride ions > 8 mg / L.

[0107] Insufficient mechanical strength limits its application effectiveness.

[0108] Comparative Example 3 This comparative example provides a method for the synergistic preparation of remediation materials from two types of waste. The method steps are basically the same as those in Example 1, except that the raw materials are directly mixed without aging, sieving, or acid washing.

[0109] Strength test: In accordance with GB / T 17671-1999 standard, the material was made into a 40 mm × 40 mm × 160 mm prism. After standard curing for 28 days, the compressive strength was determined using a press at a loading rate of 2400 N / s.

[0110] Leaching experiment: In accordance with the HJ 557-2010 standard, 10 g of sample was crushed and passed through a 100-mesh sieve, deionized water was added (liquid-solid ratio 10:1), and the mixture was shaken for 18 hours. After filtration, the concentrations of fluoride ions and soluble phosphorus were determined by ion chromatography.

[0111] Porosity test: The mercury porosimetry method was used to determine the ratio of total pore volume to apparent volume of a dry sample.

[0112] The results showed that the 28-day compressive strength was only 12.3 MPa. The fluoride ion leaching concentration was >10 mg / L, exceeding the Class III limit (≤1.0 mg / L) of the "Groundwater Quality Standard" (GB / T 14848-2017). The porosity was >30%, indicating a loose material structure.

[0113] Analysis of the causes: Lack of pretreatment led to uneven ionic reactions, hindered the formation of ettringite crystals, and resulted in poor solidification of harmful elements.

[0114] Comparative Example 4 This comparative example provides a method for the synergistic preparation of remediation materials from two types of waste. The method steps are basically the same as those in Example 1, except that only a mixture of three types of waste is used, without the addition of quicklime or sodium silicate.

[0115] Tests showed that the 28-day compressive strength was 18.5 MPa, the leached soluble phosphorus was >1.5 mg / L, the material structure was loose, and the XRD pattern showed that the characteristic peak of ettringite was weak, accompanied by a large amount of gypsum and hematite impurities.

[0116] This indicates that the lack of additives caused pH control to fail, resulting in insufficient nucleation of ettringite and the inability to achieve a synergistic solidification mechanism.

[0117] Not limited to the above experimental examples, the method of preparing ettringite crystal materials by synergistic treatment of three wastes in this invention forms a stable mineral phase dominated by ettringite through the dual effects of ion replacement and mineralization solidification, achieving chemical complementarity and synergistic solidification among the three wastes: (1) The alkaline components of red mud neutralize the acidity of phosphogypsum, stabilizing the system pH between 7 and 9; Al 3+Fe 3+ With Ca 2+ SO4 2- Under hydration conditions, ettringite crystals are formed, achieving lattice encapsulation and fixation of harmful ions; phosphorus tailings provide SiO2 framework support and pore regulation, improving the material's density and compressive strength; modifiers and stabilizers work together to control the reaction rate, avoiding structural inhomogeneity caused by rapid hardening.

[0118] (2) The present invention optimizes the raw material ratio, pretreatment process, material dry mixing and homogenization, ion replacement reaction and aging treatment reaction conditions. The prepared ettringite material system ensures mechanical properties and achieves long-term stable solidification of harmful ions. It has both engineering filling performance and ecological restoration function, providing a reliable way for the high-value utilization of waste.

[0119] (3) Through long-term leaching and cyclic wet-drying experiments, the ion solidification rate of the material can be stably maintained at over 95%, and the comprehensive utilization rate of waste reaches over 80%, realizing the simultaneous transformation of multi-source solid waste into harmless and resource-based waste.

[0120] (4) This invention utilizes the principle of ion synergistic reaction to promote the directional growth and interweaving of ettringite in a porous matrix by controlling pH (7.0~9.0) and Ca / Al molar ratio (3.0~3.5), forming a dense and stable mineral network structure. The 28-day compressive strength of the crystal material is 26.8~30.2 MPa and the flexural strength is 5.0~5.8 MPa, which fully meets the bearing requirements of mine backfilling, roadbed filling and foundation improvement projects.

[0121] (5) Compared with the traditional red mud-phosphogypsum system (compressive strength 15~22 MPa), the mechanical properties of the ettringite crystal material of the present invention are improved by about 40%~80%. At the same time, the ettringite crystal in the reaction system has good crystal stability. After high temperature (60℃) aging and water immersion for 24 hours, there is no obvious decomposition of the structure, indicating that the material has excellent long-term durability and environmental adaptability.

[0122] (6) Taking a typical medium-sized phosphate chemical and alumina enterprise as an example, based on an annual solid waste production of 1 million tons, this technology can dispose of more than 800,000 tons of solid waste, saving about 50 mu of land for stockpiling and reducing secondary pollution prevention and control investment by about 10 million yuan per year. The materials produced can be directly used in mine backfilling, road base courses, and land remediation projects, with a potential market value of about 200-300 million yuan per year, demonstrating significant comprehensive economic and social benefits. This invention promotes the transformation of solid waste disposal towards resource utilization, low carbon emissions, and sustainability by achieving integrated utilization of three wastes (waste gas, wastewater, and solid waste), high-value utilization, and ecological restoration, which is in line with the national "Solid Waste Pollution Prevention and Control Law" and the requirements for green mine construction.

[0123] It should be noted that the above embodiments are only for further elaboration and explanation of the technical solution of the present invention, and are not intended to further limit the technical solution of the present invention. The method of the present invention is only a preferred embodiment and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing ettringite crystal material, characterized in that, Using phosphogypsum, phosphogypsum tailings, red mud, and additives as raw materials, wherein the additives are used to adjust the pH of the system and provide a Ca source or active silica-alumina components, the process includes the following steps: To obtain phosphogypsum, pretreatment is performed: aging is carried out to reduce the free water content to ≤12%; Phosphorus tailings were obtained, and pretreatment was performed: coarse particles were removed by sieve screening through an 80-100 mesh screen, and the material was dried to a moisture content of ≤5%. Red mud was obtained and pretreated by acid washing and dealkalization to adjust the pH to 8.5-9.

5. The pretreated phosphogypsum, phosphogypsum tailings and red mud are mixed together, and additives are added to obtain a homogeneous mixture. Water is added to the mixed material, the water-to-solid ratio is controlled at 0.3-0.4, and the mixture is stirred. An ion exchange reaction is carried out at room temperature to obtain a reaction slurry. The reaction slurry is aged and solidified to obtain ettringite crystal material.

2. The method for preparing the ettringite crystal material according to claim 1, characterized in that, The phosphogypsum is selected from the by-product of wet phosphoric acid production, with a CaSO4·2H2O content ≥90%, pH 4.0~5.0, and contains a small amount of soluble phosphorus, fluorine and heavy metal ions; The phosphorus tailings are selected from phosphate rock beneficiation tailings, with an SiO2 content ≥70% and a particle size distribution mainly between 75 and 180 μm; and / or The red mud was selected from Bayer process alumina waste residue, containing 20-30% Al2O3, 30-40% Fe2O3, and pH 10-12, and is strongly alkaline.

3. The method for preparing the ettringite crystal material according to claim 2, characterized in that, The mass ratio of phosphogypsum, phosphogypsum tailings and red mud is (40~60):(20~30):(15~25).

4. The method for preparing the ettringite crystal material according to claim 3, characterized in that, The additive includes a modifier and a stabilizer. The modifier is selected from quicklime or polyferric phosphate, and the stabilizer is selected from sodium silicate or sodium aluminate. The amount of additive added is 5-15% of the total mass of phosphogypsum, phosphogypsum tailings, and red mud.

5. The method for preparing the ettringite crystal material according to any one of claims 1 to 4, characterized in that, The conditions for the ion replacement reaction are: constant temperature 25±5℃, reaction time 2~4 hours.

6. The method for preparing the ettringite crystal material according to claim 5, characterized in that, The aging and curing conditions are: 90% humidity, 20~25℃ temperature, and aging for 7~10 days.

7. The ettringite crystal material prepared according to any one of claims 1 to 6.

8. The ettringite crystal material according to claim 7, characterized in that, The ettringite crystal material solidifies environmentally harmful elements through both chemical bonding and lattice containment.

9. The use of the ettringite crystal material according to claim 7 in the preparation of mine backfill, roadbed filler, and soil remediation agent.

10. An ecological restoration material, characterized in that, The ettringite crystal material comprising any one of claims 1 to 6.

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