Coal gangue-based multi-phase composite prb material and preparation method and application thereof

Abstract

The application discloses a preparation method of a coal gangue-based multi-phase composite PRB material, and specifically comprises the following steps: crushing, screening and grinding the coal gangue, and then performing thermal activation; mixing the activated coal gangue with an alkali-activating liquid, and performing alkali activation and partial crystallization under heating conditions, so that part of active silicon-aluminum components in the coal gangue forms a zeolite crystal phase, another part of the active silicon-aluminum components forms a silico-aluminate gel precursor, and part of unreacted minerals are reserved; directly granulating the obtained reaction materials without separating the zeolite crystals, and then performing maintenance, washing and drying to obtain the coal gangue-based multi-phase composite PRB material. The material is filled in a permeable reaction wall on a groundwater pollution plume migration path, and through ion exchange, surface adsorption, complexation, precipitation fixation and microbial adhesion, one or more pollutants in groundwater, such as cadmium, arsenic, mercury, zinc and the like, fluorides and ammonia nitrogen, are removed. The application realizes the resource utilization of the coal gangue, and takes into account the reaction activity, particle stability and hydraulic permeability of the PRB material.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization and groundwater pollution remediation technology, specifically involving a coal gangue-based multiphase composite PRB material and its preparation method, as well as the application of this material in the remediation of groundwater containing heavy metals such as cadmium, arsenic, mercury, and zinc, fluorides, and ammonia nitrogen. Background Technology

[0002] Coal gangue is a solid waste generated during coal mining and washing. Its main chemical components include silicon dioxide, aluminum oxide, and certain amounts of iron oxide, calcium oxide, magnesium oxide, and residual carbon. Major minerals may include kaolinite, quartz, illite, chlorite, calcite, and iron-bearing minerals. Long-term stockpiling of coal gangue occupies land and may cause environmental problems such as dust, spontaneous combustion, acid leaching, and the migration of harmful elements. Converting coal gangue into environmental remediation materials is an important way to achieve solid waste reduction and resource utilization.

[0003] Heavy metal pollutants such as cadmium, arsenic, mercury, and zinc, once introduced into groundwater, are characterized by their wide migration range, long duration, and difficulty in natural attenuation. Excessive levels of fluoride and ammonia nitrogen also degrade groundwater quality. Commonly used methods such as pumping, chemical precipitation, and membrane separation require continuous pumping, chemical dosing, or equipment maintenance. Permeable reactive bar (PRB) technology involves placing a reactive media layer along the migration path of contaminated groundwater. Groundwater passes through the reactive media under the influence of a natural hydraulic gradient, and pollutants are removed through adsorption, ion exchange, complexation, precipitation, or biological processes. This technology offers advantages such as in-situ remediation, minimal surface disturbance, and lower operation and maintenance requirements.

[0004] Permeable reactive barrier (PRB) materials are typically made from zero-valent iron, activated carbon, zeolite, limestone, or phosphate minerals. These materials possess redox, adsorption, ion exchange, or precipitation / fixation functions, respectively. However, the applicability of any single material to different forms of pollutants is limited, and some materials suffer from high costs, easy powder loss, increased head loss after filling, or caking over long-term operation. Using industrial solid waste to prepare PRB materials can reduce raw material costs, but it is still necessary to consider pollutant removal performance, particle strength, water stability, permeability, and environmental safety.

[0005] Existing coal gangue water treatment materials mainly include coal gangue percolation packing material after direct crushing and grading, sintered coal gangue ceramsite, coal gangue-based zeolite, and coal gangue-based polymers. When using coal gangue particles directly, the ion exchange capacity and reactivity of the material are limited by the mineral composition of the raw materials; sintered ceramsite and high-purity coal gangue-based zeolite usually require higher temperatures or longer crystallization times; high-purity zeolite powder also needs to be separated, washed, dried, and re-shaped. Ordinary coal gangue-based polymers are mostly designed for density and high strength, but their pore connectivity is not entirely consistent with the hydraulic permeability requirements of PRB (porous rebar).

[0006] Therefore, there is a need for a method to prepare granular PRB material using coal gangue as the main raw material, so that the silicon and aluminum components in the coal gangue can be moderately activated and partially crystallized under low energy consumption conditions, and the gel components formed by the incomplete crystallization can be used to achieve particle self-cementation, while retaining the mineral skeleton and interconnected pores, thereby forming a multiphase composite material that can treat a variety of groundwater pollutants and is suitable for filling permeable reactive walls. Summary of the Invention

[0007] This invention provides a coal gangue-based multiphase composite PRB material, its preparation method, and its application, in order to solve the problems of insufficient reactivity of existing coal gangue water treatment materials, numerous steps in the preparation and secondary molding of high-purity zeolite, insufficient permeability of ordinary cementitious materials, and the inconvenience of direct filling of powder materials.

[0008] To achieve the above objectives, the present invention provides a method for preparing a coal gangue-based multiphase composite PRB material, comprising the following steps: S1. Raw material pretreatment: The coal gangue is crushed, screened and ground to obtain coal gangue powder; After being ground, coal gangue is passed through a 60-300 mesh sieve. For raw materials with high sulfide or harmful element content, one or more of magnetic separation, gravity separation or particle size separation is used to remove high-risk components during crushing, screening or grinding. S2. Thermal activation: Coal gangue powder is kept at 500-750℃ for 1-4 hours and then cooled to obtain activated coal gangue. Thermal activation causes the aluminum silicate minerals such as kaolinite in the coal gangue to undergo dehydroxylation and form amorphous silica-alumina components with high reactivity, while retaining quartz and some unreacted minerals as the skeleton components of subsequent particles. S3, Alkali activation and partial crystallization: Activated coal gangue is mixed with alkali activation solution and stirred at 60-100℃ for 1-4 hours, followed by crystallization at 80-140℃ for 12-24 hours to obtain multiphase reaction material; The alkaline activation solution is one or more of sodium hydroxide solution, sodium carbonate solution, and water glass. The amount of alkaline component added to the alkaline activation solution is 5-20% of the mass of activated coal gangue, and the liquid-solid ratio is 0.5-3.0 mL / g. During partial crystallization, some of the active silica-alumina components in the activated coal gangue form type A zeolite, type X zeolite, type P zeolite, analcime, sodalite, or other zeolite crystal phases; another part of the active silica-alumina components remain as a precursor of aluminosilicate gel that can continue to condense; quartz and other unreacted minerals are retained in the reactants; during alkali activation and partial crystallization, 1-10% by weight of zeolite seed crystals can be added to the activated coal gangue, and the zeolite seed crystals are selected from type A zeolite, type X zeolite, type P zeolite, analcime, sodalite, and natural zeolite.

[0009] S4. Granulation: After partial crystallization, without separating and purifying the zeolite crystals in the multiphase reactants, wet granules with a particle size of 2-15 mm are directly produced using one of the following methods: disc granulation, extrusion granulation, rolling granulation, or compression molding. During the granulation process, one or more of the following can be added: coal gangue particles, quartz sand, bentonite, biochar, sodium bicarbonate, or plant-derived pore-forming agents. The total amount added is 5-20% of the mass of the activated coal gangue.

[0010] S5. Curing and post-treatment: The wet particles are cured at 40-90℃ for 7-14 days to allow the incompletely crystallized aluminosilicate gel precursor to continue to condense and form a cemented structure. After curing, the particles are washed with water, or a combination of water washing and carbonization is used to reduce the free alkali in the particles. The particles are washed until the pH of the extract is 6-7, and then dried at 60-105℃ to obtain the coal gangue-based multiphase composite PRB material.

[0011] This invention also provides a coal gangue-based multiphase composite PRB material prepared by the above method. This material is a particulate material with interconnected pores, comprising a zeolite crystalline phase, an aluminosilicate gel phase, and a residual mineral framework. The zeolite crystalline phase is dispersed in the aluminosilicate gel phase, which binds the zeolite crystalline phase and the residual mineral framework together. The material may also contain one or more of the following: iron-containing phase, calcium-containing phase, magnesium-containing phase, and carbonaceous components, formed from primary or activated coal gangue.

[0012] When the coal gangue-based multiphase composite PRB material is subjected to leaching tests, the leaching concentrations of arsenic, cadmium, mercury, lead, chromium, and other harmful elements are not higher than 0.010 mg / L, 0.005 mg / L, 0.001 mg / L, 0.010 mg / L, 0.050 mg / L, and 1.00 mg / L, respectively, and the pH is 6.5-8.5, which meets the emission standards.

[0013] This invention also provides the application of the above-mentioned coal gangue-based multiphase composite PRB material in groundwater pollution remediation, wherein the groundwater pollutants are one or more of heavy metals such as cadmium, arsenic, mercury, and zinc, fluorides, and ammonia nitrogen. The material can be filled alone or mixed with quartz sand, natural zeolite, and biochar and filled into a permeable reactive barrier along the migration path of the groundwater pollution plume; a flow-guiding layer, a reactive layer, and a support layer can be sequentially arranged along the groundwater flow direction, with the reactive layer filled with the coal gangue-based multiphase composite PRB material (…). Figure 1 ).

[0014] The material removes cadmium, zinc, and ammonium ions primarily through ion exchange generated by the pores of the zeolite crystal phase and the negatively charged sites of the silica-alumina framework, as well as complexation and adsorption by hydroxyl groups on the particle surface. It removes arsenate, arsenite, and fluoride ions primarily through ligand exchange, electrostatic adsorption, and precipitation fixation on the iron-containing, aluminum-containing, and calcium-containing phases. It removes mercury ions primarily through adsorption and complexation on the iron-containing components, carbonaceous components, and silica-alumina gel surface. The outer surface of the particles and the interconnected pores also provide an attachment interface for indigenous microorganisms in groundwater, promoting the biotransformation of ammonia nitrogen.

[0015] Compared with existing technologies, this invention uses the silica-alumina components, iron- and calcium-containing components, and residual minerals in coal gangue as a common reaction basis. Through thermal activation and controlled partial crystallization, a multiphase structure of "zeolite crystal phase - aluminosilicate gel phase - residual mineral framework" is formed. The zeolite crystal phase provides ion exchange and microporous adsorption sites, the gel phase provides cementation strength and surface complexation sites, the residual minerals maintain the particle framework and connect water flow channels, and the iron-, calcium-, and carbonaceous components further undertake anion adsorption, precipitation fixation, and microbial attachment functions. The partially crystallized reactants can be directly granulated without separation and purification, reducing the solid-liquid separation, repeated washing, and secondary gelling during the preparation of high-purity zeolite. The resulting material can synergistically remove groundwater pollutants such as cadmium, arsenic, mercury, zinc, fluoride, and ammonia nitrogen through multiple mechanisms while maintaining hydraulic permeability. It is suitable for constructing low-cost, replaceable, and long-term operating permeable groundwater reactive barriers, and provides a material basis for the large-scale resource utilization of coal gangue and in-situ remediation of groundwater with complex pollution. Attached Figure Description

[0016] Figure 1 This is a schematic diagram illustrating the filling and function of the coal gangue-based multiphase composite PRB material of the present invention in a groundwater permeable reactive barrier. Detailed Implementation

[0017] The present invention will be further described below with reference to embodiments and application examples. Operations not specifically described in the embodiments are performed under conventional conditions in the art; the raw materials and reagents used can be obtained through conventional means unless otherwise specified. The main chemical composition of the coal gangue described in the following examples is as follows: SiO2 45%-65%, Al2O3 20%-40%, Fe2O3 2%-10%, CaO 0.5%-8%, MgO 0.2%-4%, and loss on ignition 5%-20%; the main minerals include kaolinite, quartz and illite, and it also contains chlorite, calcite and iron-bearing minerals. Example 1: Preparation of coal gangue-based multiphase composite PRB materials 1. Take coal gangue raw material, crush it and screen out the material with a particle size greater than 0.25mm, remove some iron-rich magnetic impurities by magnetic separation, then grind it and pass it through a 100-mesh sieve to obtain coal gangue powder. 2. Weigh 100 parts by weight of coal gangue powder, place it in a muffle furnace, raise the temperature to 600°C at a heating rate of 5°C / min, hold for 2 hours, and let it cool naturally to room temperature to obtain activated coal gangue. 3. Weigh 100 parts by weight of activated coal gangue, add 8 parts by weight of sodium hydroxide, 10 parts by weight of water glass and water (liquid-solid ratio mL:g is 0.55:1), and stir to mix evenly; stir the mixture at 80℃ for 2 hours, then transfer it to a closed reaction vessel and crystallize at 100℃ for 14 hours to obtain a multiphase reaction material containing zeolite crystal phase, aluminosilicate gel precursor and residual minerals; 4. After crystallization, no solid-liquid separation is performed. The moisture content of the multiphase reaction material is adjusted to 35%, and granulation is carried out using a disc granulator to obtain shaped particles with a particle size of 2-6 mm.

[0018] 5. The molded particles were cured at 60℃ for 7 days to allow the gel structure inside the particles to continue to condense. After curing, the particles were washed with water until the pH of the extract was 6.8, and then dried at 60℃ until the mass was constant to obtain the coal gangue-based multiphase composite PRB material, denoted as CG-PRB-1.

[0019] Example 2: Preparation of coal gangue-based multiphase composite PRB materials 1. The method is the same as step 1 in Example 1; 2. The method is the same as step 2 of Example 1, except that the coal gangue powder is kept at 550℃ for 3 hours; 3. Weigh 100 parts by mass of activated coal gangue, add 8 parts by mass of sodium carbonate, 12 parts by mass of water glass, 1 part by mass of P-type zeolite seed crystals and water (liquid-solid ratio mL:g is 0.6:1), stir and react at 70℃ for 3 h, and then crystallize at 90℃ for 12 h to obtain multiphase reaction material. 4. The method is the same as steps 4 and 5 in Example 1, except that after crystallization, the material is directly extruded and granulated to produce wet particles with a particle size of 3-8 mm. After curing at 70°C for 12 days, the material is washed and dried to obtain coal gangue-based multiphase composite PRB material, denoted as CG-PRB-2.

[0020] Example 3: Preparation of coal gangue-based multiphase composite PRB material with coarse-grained skeleton 1. The methods in steps 1-3 are the same as those in steps 1-3 of Example 1, to prepare multiphase reaction materials; 2. The method is the same as steps 4 and 5 of Example 1, except that 10% of activated coal gangue particles and 3% of activated coal gangue sodium bicarbonate are added before granulation. After mixing evenly, the mixture is rolled and granulated to obtain shaped particles with a particle size of 4-10 mm. The shaped particles are cured at 80°C for 10 days, washed with water and dried to obtain coal gangue-based multiphase composite PRB material, denoted as CG-PRB-3.

[0021] Comparative Example 1 The crystallization step (crystallization at 100°C for 14 hours) in step 3 of Example 1 was removed, and the remaining steps were the same as in Example 1, resulting in alkali-activated coal gangue particles that did not form zeolite crystal phase, denoted as CG-C1.

[0022] Comparative Example 2 The method is the same as in Example 1, except that the thermal activation of the coal gangue powder in step 2 is not performed. Instead, the coal gangue powder is directly activated by alkali, crystallized, granulated, cured, washed and dried to obtain the comparative material, which is denoted as CG-C2.

[0023] Comparative Example 3 The coal gangue was directly crushed, screened, and washed with water. Coal gangue particles with a diameter of 2-6 mm were used as a comparative material and denoted as CG-C3.

[0024] Example 4: Experiments on the treatment of cadmium- and zinc-containing groundwater by the materials prepared in Examples 1-3 and Comparative Examples 1-3. Formulating Cd-containing 2+ 1.0 mg / L and Zn 2+ Simulated groundwater at a concentration of 10.0 mg / L was used, and the pH was adjusted to 6.7. 1.00 g of the materials prepared in Examples 1-3 and Comparative Examples 1-3 were weighed and added to 100 mL of simulated groundwater. After shaking at 25℃ and 150 r / min for 24 h, the mixture was filtered. The concentrations of cadmium and zinc in the water samples were determined by atomic absorption spectrophotometry or corresponding standard methods. The results are shown in Table 1. Table 1 Results of Cadmium and Zinc Removal Tests ; As shown in Table 1, the coal gangue-based multiphase composite PRB materials prepared in Examples 1-3 showed significantly improved cadmium and zinc removal rates and ion exchange and surface fixation capabilities compared to Comparative Examples 1-3.

[0025] Example 5: Experiment on the treatment of arsenic and fluoride-containing groundwater by the material prepared in Example 1. Formulating As 5+ 0.1 mg / L and F -Simulated groundwater at a concentration of 2.0 mg / L was adjusted to pH 6.6. 1.00 g of CG-PRB-1 was added to 100 mL of the simulated groundwater, and the mixture was shaken for 24 h at 25℃ and 150 rpm. After the reaction, the mixture was filtered, and the concentrations of arsenic and fluoride were determined using atomic fluorescence spectrometry and fluoride ion-selective electrode method, respectively. The arsenic removal rate was 97.7%, and the fluoride removal rate was 89.4%.

[0026] Example 6: Experiment on the treatment of mercury-containing groundwater by the material prepared in Example 1. Preparation of Hg 2+ Simulated groundwater with an initial concentration of 0.01 mg / L was adjusted to pH 7.2. 1.00 g of CG-PRB-1 was weighed and added to 100 mL of the simulated groundwater, and the mixture was shaken at 25 °C for 24 h. After the reaction, the mercury concentration in the water sample was determined by atomic fluorescence spectrometry, and the mercury removal rate was 93.0%.

[0027] Example 7: Experiment on the treatment of ammonia nitrogen-containing groundwater by the material prepared in Example 2. Simulated groundwater with an initial ammonia nitrogen concentration of 5.0 mg / L was prepared, and the pH was adjusted to 6.8. CG-PRB-2 was packed into a column with an inner diameter of 5.0 cm and a height of 60 cm, with a material packing height of 40 cm. The water was continuously fed in at a flow rate of 0.5 mL / min, and the ammonia nitrogen concentrations in the influent and effluent were determined using Nessler's reagent spectrophotometry. After 30 days of operation, the average ammonia nitrogen removal rate was 91.7%.

[0028] Example 8: Determination of the continuous treatment effect of coal gangue-based multiphase composite PRB material on complex polluted groundwater.

[0029] An acrylic glass column with an inner diameter of 5.0 cm and an effective length of 60 cm was used. The column was sequentially filled with a quartz sand guiding layer, a CG-PRB-1 reaction layer, and a quartz sand support layer; the reaction layer was 10 cm thick and had a packing density of 0.95 g / cm³. 3 Simulated groundwater containing Cd 2+ 0.1 mg / L, As 5+ 0.05 mg / L, Hg 2+ 0.01 mg / L, Zn 2+ 5.0 mg / L, F - The concentrations of the influent were 2.0 mg / L and the ammonia nitrogen concentration was 5.0 mg / L. The pH of the influent was 6.8, and the flow rate was 0.25 mL / min.

[0030] The system operated continuously for 30 days, collecting influent and effluent samples every 24 hours. The concentrations of each pollutant were determined using appropriate standard methods, and the head difference between the column inlet and outlet was recorded. During the stable operation phase, the average removal rates of cadmium, arsenic, mercury, zinc, fluoride, and ammonia nitrogen were 96.4%, 86.3%, 91.1%, 84.8%, 89.2%, and 96.6%, respectively.

[0031] Example 9: Environmental Safety Test The coal gangue-based multiphase composite PRB materials prepared in Examples 1-3 were subjected to leaching tests, and the concentrations of arsenic, cadmium, mercury, lead, chromium, fluoride and other harmful elements in the leachate were determined. The results are shown in Table 2. Table 2 Leaching test results of coal gangue-based multiphase composite PRB materials

[0032] The results in Table 2 show that the leaching concentrations of each harmful element in the coal gangue-based multiphase composite PRB materials prepared in Examples 1-3 all meet the emission limits.

Claims

1. A method for preparing a coal gangue-based multiphase composite PRB material, characterized in that, The steps are as follows: (1) The coal gangue is crushed, screened and ground, and then screened again to obtain coal gangue powder; (2) Calcine the coal gangue powder at 500-750℃ for 1-4 h, and then cool it to obtain activated coal gangue; (3) Mix activated coal gangue with alkaline activation solution and stir at 60-100℃ for 1-4 h, then crystallize at 80-140℃ for 12-24 h, so that some of the active silica-alumina components in the activated coal gangue form zeolite crystal phase, and another part of the active silica-alumina components form aluminosilicate gel precursor, while retaining some unreacted minerals to obtain multiphase reaction material; (4) Adjust the moisture content of the multiphase reactant and granulate it directly to obtain shaped particles; cure the shaped particles at 40-90℃ for 7-14 days to allow the aluminosilicate gel precursor to condense and form an aluminosilicate gel phase; wash and dry the particles after curing to obtain coal gangue-based multiphase composite PRB material.

2. The preparation method of the coal gangue-based multiphase composite PRB material according to claim 1, characterized in that: In step (1), the coal gangue is ground and then passed through a 60-200 mesh sieve. During the crushing, screening or grinding process, components enriched with sulfides and harmful elements are removed by one or more methods such as magnetic separation, gravity separation or particle size separation.

3. The preparation method of the coal gangue-based multiphase composite PRB material according to claim 1, characterized in that: In step (3), the alkaline activation solution is one or more of sodium hydroxide solution, sodium carbonate solution, and water glass; the amount of alkaline component added in the alkaline activation solution is 5-20% of the mass of activated coal gangue.

4. The preparation method of the coal gangue-based multiphase composite PRB material according to claim 1, characterized in that: Step (3) Add zeolite seed crystals. The zeolite seed crystals are selected from type A zeolite, type X zeolite, type P zeolite, analcime, sodalite, and natural zeolite. The amount of zeolite seed crystals added is 1-10% of the mass of activated coal gangue.

5. The method for preparing coal gangue-based multiphase composite PRB material according to claim 1, characterized in that: In step (4), one of the following methods is used: disc granulation, extrusion granulation, rolling granulation, and compression molding; the particle size of the wet particles is 2-15 mm; one or more of the following are added during the granulation process: coal gangue particles, quartz sand, bentonite, biochar, sodium bicarbonate, and plant-derived pore-forming agent, and the total amount added is 5-20% of the mass of activated coal gangue.

6. The preparation method of the coal gangue-based multiphase composite PRB material according to claim 1, characterized in that: The washing in step (4) is done by water washing, or by a combination of water washing and carbonization treatment, so that the pH of the particle extract after washing is 6-7 and the drying temperature is 60-105℃.

7. The coal gangue-based multiphase composite PRB material prepared by the preparation method of any one of claims 1-6.

8. The application of the coal gangue-based multiphase composite PRB material as described in claim 7 in the remediation of groundwater pollution.

9. The application according to claim 8, characterized in that, Coal gangue-based multiphase composite PRB material is filled alone or mixed with one or more of quartz sand, natural zeolite, and biochar; a flow guiding layer, a reaction layer, and a support layer are set sequentially along the groundwater flow direction, with the reaction layer filled with coal gangue-based multiphase composite PRB material.

10. The application according to claim 8, characterized in that: Groundwater pollutants include one or more of the following: cadmium, arsenic, mercury, zinc, fluoride, and ammonia nitrogen.

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