A method for synergistic solidification of heavy metal contaminated soil using coal gangue and fly ash

Abstract

This invention discloses a method for synergistic solidification of heavy metal-contaminated soil using coal gangue and fly ash. This invention relates to the field of soil pollution control and remediation technology, addressing two major challenges: secondary pollution from self-contaminated heavy metals in remediation materials and antagonistic solidification of anionic and cationic heavy metals. A multi-component sulfur-based modifier is prepared by mixing sodium sulfide nonahydrate and thiourea at a mass ratio of 2 to 4:1, and then pre-passivated by high-energy ball milling with coal gangue and fly ash. This reduces the TCLP leaching amount of self-contaminated heavy metals in the solid waste to less than 5% of the original amount. The pre-passivated solid waste is then combined with an alkaline activator and a hydrated... A dual-target solidification material was prepared using lime, lightly calcined magnesium oxide, and gypsum dihydrate. The solidification material was mixed with heavy metal contaminated soil at a dosage of 10% to 25% of the dry weight of the contaminated soil. The mixture was cured at 18℃ to 22℃ for 7 to 28 days, during which C-(A)-S-H gel, Mg / Al-LDHs mineral phase, and AFt ettringite were generated in situ. The solidification rate of cationic heavy metals was not less than 99%, the solidification rate of anionic heavy metals was not less than 90%, and the unconfined compressive strength after 28 days was not less than 350 kPa.

Description

Technical Field

[0001] This invention relates to the field of soil pollution control and remediation technology, specifically a method for synergistically solidifying heavy metal-contaminated soil using coal gangue and fly ash. Background Technology

[0002] Heavy metal contamination from industrial legacy sites is one of the major challenges in current environmental remediation. In actual contaminated sites, Pb... 2+ Cd 2+ Zn 2+ Cu 2+ Cationic heavy metals often coexist with heavy metals such as As(V) and Cr(VI) which exist in the form of oxygen-containing anions, forming a complex multi-element compound pollution system.

[0003] Solidification / stabilization technology is one of the most engineered mainstream technologies for the remediation of heavy metal contaminated sites. Using industrial solid wastes such as coal gangue and fly ash as silica-alumina precursors, and NaOH solution or water glass solution as alkaline activators, the following series of reactions can occur under alkaline conditions: OH- in the alkaline activator... - It attacks the Si-O-Si and Si-O-Al bonds of amorphous SiO2 and Al2O3 in coal gangue and fly ash, causing them to depolymerize and form soluble silicate [Si(OH)4] and aluminate [Al(OH)4]. - The monomers are further polymerized to form a three-dimensional network geopolymer gel with Si-O-Al-O as the backbone, which has the ability to adsorb and physically encapsulate heavy metal ions. When the chemical analysis of coal gangue and fly ash raw materials shows the presence of calcium ions, they can also be hydrated to generate C-(A)-SH gel, which further solidifies cationic heavy metals through lattice substitution and physical encapsulation. This technical route is called "polymer solidification technology for contaminated soil based on fly ash and coal gangue". Its typical process is as follows: coal gangue is crushed and ground and mixed with fly ash at a certain mass ratio to prepare a silica-alumina precursor. NaOH solution or water glass alkaline activation solution is prepared. The alkaline activation slurry is fully mixed with the target contaminated soil at 10% to 20% of the dry weight of the contaminated soil and cured for 7 to 28 days under normal temperature and high humidity conditions to form solidified soil.

[0004] However, the aforementioned existing technologies have two fundamental technical flaws: Firstly, during the mineralization and combustion processes of coal gangue and fly ash, a certain amount of soluble heavy metals such as Pb, Cd, As, and Cr are encapsulated in their glassy networks and residual lattices. Current technologies directly apply solid waste to the soil without any pretreatment. The highly alkaline environment (pH 11 to 13) generated during the alkali activation process promotes the release of large amounts of heavy metals carried by the solid waste into the pore liquid. This creates a "leaching-solidification" competition effect with the original target pollutants in the soil, resulting in a significant depletion of effective solidifying agent resources on the pollutants carried by the solid waste. In severe cases, this can lead to an increase rather than a decrease in the overall leaching toxicity of heavy metals at the site after the remediation project is completed, making it impossible to achieve the remediation goals.

[0005] Secondly, the strongly alkaline environment (pH 11 to 13) maintained by the alkali-activated curing system is extremely favorable for Pb. 2+ Cd 2+ Isocational heavy metals precipitate as hydroxides and are tightly encapsulated by C-(A)-SH gel, but for AsO4... 3- CrO4 2- For anionic heavy metals, a high pH value significantly increases their thermodynamic solubility, exacerbating their leaching and migration risks.

[0006] The existing technology system does not include any functional mineral phase construction methods specifically for anionic heavy metals, resulting in the TCLP leaching concentration of As(V) and Cr(VI) exceeding the national standard limit by several times or even dozens of times, and the contradiction between anionic and cation solidification antagonism has been difficult to resolve for a long time. Summary of the Invention

[0007] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for synergistic solidification of heavy metal contaminated soil using coal gangue and fly ash, so as to solve the two major technical problems of secondary pollution risk caused by heavy metals carried by remediation materials and antagonistic solidification of anionic and cationic heavy metals.

[0008] The technical solution adopted in this invention is as follows: A method for synergistic solidification of heavy metal contaminated soil using coal gangue and fly ash includes a pre-passivation mechanochemical activation step, a dual-target solidification material preparation step, and a construction and maintenance step.

[0009] Pre-passivation mechanochemical activation step: Prepare a multi-component sulfur-based modifier by mixing Na2S·9H2O (sodium sulfide nonahydrate) and thiourea at a mass ratio of 2:1 to 4:1, wherein S... 2- The total concentration is 0.5 mol / L to 2.0 mol / L, and it should be prepared immediately before use. Coal gangue, fly ash, and the aforementioned multi-component sulfur-based modifier are added together to a planetary high-energy ball mill and ball-milled at a ball-to-material mass ratio of 10:1 and a rotation speed of 300 r / min to 400 r / min for 2 to 4 hours to obtain pre-passivated and activated solid waste powder. During the ball milling process, mechanical force crushes the solid waste particles to a particle size d.90 No larger than 45 μm, with a specific surface area increased to no less than 400 m². 2 / kg, and releases the heavy metal cations Me sealed in the solid waste lattice. 2+ ;Released Me 2+ With S 2- An in-situ precipitation reaction occurs, producing a solubility product constant Ksp of 10. -26 Up to 10 -36 Metal sulfide (MeS) precipitation (including PbS, CdS, ZnS); the -NH2 and -C=S functional groups in thiourea provide organic chelation coordination and fixation for residual heavy metals that are not completely precipitated during ball milling, forming a dual fixation guarantee; finally, the amount of self-carried heavy metals leached from the pre-passivated activated solid waste powder by TCLP is reduced to less than 5% of that before ball milling.

[0010] Preparation steps of dual-target solidification material: The alkali activator, quicklime Ca(OH)2, lightly calcined magnesium oxide MgO, gypsum dihydrate CaSO4·2H2O and pre-passivated activated solid waste powder are uniformly mixed to prepare dual-target solidification material. The alkali activator was prepared by mixing a NaOH solution with a concentration of 8 mol / L to 12 mol / L and a water glass solution with a modulus adjusted to 1.2 to 2.0 at a volume ratio of 1:1 to 2:1, followed by sealing and standing for 24 hours to mature. The liquid-solid mass ratio was controlled at 0.35 to 0.45. The amount of quicklime used was 2% to 8% (mass fraction) of the total mass of coal gangue and fly ash, used to supplement calcium ions in the system and promote the formation of C-(A)-SH gel. The amount of lightly calcined magnesium oxide (active MgO content not less than 80%, calcination temperature 750℃ to 900℃) added made the Mg / Al molar ratio in the system reach 2:1 to 3:1, used to drive the in-situ synthesis of Mg / Al-LDHs mineral phases. The amount of gypsum dihydrate used was 3% to 8% (mass fraction) of the total mass of coal gangue and fly ash, providing SO42-. 2- Ions drive the in-situ generation of AFt ettringite; when the aluminum content of coal gangue and fly ash is insufficient, resulting in a Si / Al molar ratio of less than 2.0 in the system, sodium aluminate NaAlO2 (purity not less than 95%) is added to supplement the aluminum source, so that the Si / Al molar ratio of the system is adjusted to 2.0 to 3.5.

[0011] Construction and maintenance steps: Mix the dual-target solidification material with the target heavy metal contaminated soil at a solidification agent dosage of 10% to 25% (mass fraction) of the dry weight of the contaminated soil, and maintain it for 7 to 28 days at a temperature of 18℃ to 22℃ and a relative humidity of not less than 95%.

[0012] Method for determining the dosage of multi-component sulfur-based modifier: First, TCLP leaching tests were performed on coal gangue and fly ash respectively according to HJ 832 standard to obtain the results. Leaching concentration of heavy metals in coal gangue (mg / L) and leaching concentration in fly ash (mg / L); Calculate the TCLP leaching molar amount of each heavy metal in a unit mass of solid waste: ; In the formula, (mol / kg) is the first The TCLP leaching molar amount of a certain heavy metal in a unit mass of coal gangue. (mol / kg) is the first The TCLP leaching molar amount of several heavy metals in a unit mass of fly ash. (L) represents the total volume of the TCLP leachate. According to the HJ 557 standard, the volume of the leachate is 20 times the sample mass (unit: mL / g, converted to L). (g / mol) is the first Molar mass of a heavy metal element (kg) represents the mass of the coal gangue sample taken for TCLP testing. (kg) represents the mass of the fly ash sample taken for TCLP testing, and 1000 is the conversion factor from mg to g.

[0013] The system needs to be S 2- Total molar amount of heavy metals fixed by precipitation (mol) is calculated using the following formula: ; In the formula, (kg) represents the total actual mass of coal gangue to be used. (kg) represents the total mass of fly ash actually intended for use. For need to be S 2- The number of target heavy metals that are precipitated and fixed. This represents the summation over all target heavy metal types.

[0014] Theoretical requirement of Na2S·9H2O (kg) is calculated using the following formula: ; In the formula, This is an excess factor, ranging from 1.2 to 1.5, with a typical value of 1.3, used to compensate for S during the ball milling process. 2- Partial oxidation loss in the air, The molar mass of sodium sulfide nonahydrate is taken as 240.18 g / mol. For S in the system 2- The proportion derived from Na₂S·9H₂O is 0.70 to 0.80, with the remaining 20% ​​to 30% provided by thiourea. The mass fraction (decimal form) of Na2S in industrial Na2S·9H2O is not less than 0.60, and 1000 is the conversion factor between g and kg. Thiourea dosage. (kg) from It is calculated by dividing by the mass ratio of Na2S·9H2O to thiourea (2:1 to 4:1).

[0015] Total amount of multi-sulfur-based modifier It should meet the following requirements: ; In the formula, This represents the percentage of the total mass of sulfur-based modifiers to the total mass of coal gangue and fly ash.

[0016] Method for determining the amount of lightly calcined magnesium oxide: Based on the XRF chemical analysis data of coal gangue and fly ash, combined with their respective alkali activation activity coefficients (0.75 to 0.80 after calcination and activation, and 0.60 to 0.65 for uncalcined), calculate the initial total molar amount of active Al in the system. (mol) and the active Mg provided by the solid waste itself 2+ molar amount (mol); The proportion of active Al allocated to LDH synthesis is determined according to the severity of anionic heavy metal pollution. (When the concentration of anionic heavy metals is high, use 0.45 to 0.50; when the concentration is relatively low, use 0.30 to 0.35). Therefore, the molar amount of active Al allocated to LDHs synthesis is... According to the target Mg / Al molar ratio (Take 2 to 3) Calculate the required Mg 2+ Total molar amount ; deducting the active Mg provided by the solid waste itself 2+ Afterwards, the amount of lightly calcined magnesium oxide that needs to be added... (kg) is calculated using the following formula: ; In the formula, The molar mass of MgO is taken as 40.30 g / mol. The mass fraction (decimal form) of active MgO in the lightly calcined MgO product shall not be less than 0.80. This is the conversion factor between g and kg.

[0017] Dual-target synergistic solidification mechanism: After the dual-target solidification material is mixed with contaminated soil, it undergoes three mineral phase evolution stages in sequence during the normal temperature and high humidity curing process.

[0018] During the rapid dissolution and initial condensation stage of curing (0-24 h): OH in the alkali activator -The amorphous SiO2 and Al2O3 on the surface of pre-passivated solid waste particles are rapidly attacked, causing a depolymerization reaction and releasing large amounts of [Si(OH)4] and [Al(OH)4]. - When monomers enter the pore liquid, the pH of the system rapidly rises to 11 to 13, increasing the free Pb in the soil. 2+ Cd 2+ When cationic heavy metals begin to precipitate as hydroxides, MgO undergoes simultaneous hydration upon light calcination (MgO + H2O → Mg(OH)2). 2+ It begins to accumulate in the pore fluid, Ca(OH)2 dissolves rapidly, and Ca... 2+ It enters the solution and participates in subsequent C-(A)-SH gel nucleation.

[0019] During the in-situ reconstruction phase of the three major mineral phases ([Si(OH)4] and [Al(OH)4]) from 1 to 7 days of curing,... - Monomers undergo condensation polymerization, gradually forming a three-dimensional Si-O-Al-O network framework (geopolymer gel), Ca 2+ It participates in the formation of C-(A)-SH gel with silicon-aluminum monomers, which physically encapsulates and solidifies cationic heavy metals through lattice substitution, enriching Mg in the pore fluid. 2+ With Al 3+ Under alkaline conditions (pH 9 to 12), co-precipitation occurs, forming an in-situ Mg / Al-LDHs mineral phase, and AsO4 in the soil pore fluid. 3- and CrO4 2- It begins to be captured by interlayer ion exchange of LDHs, Ca 2+ [Al(OH)4] - and SO4 from gypsum dissolution 2- All three components participate in the nucleation and crystal growth of AFt ettringite.

[0020] During the mineral phase densification and solidified body strength development stage (7 to 28 days of curing): C-(A)-SH gel continues to densify, the physical barrier against cationic heavy metals thickens, the Mg / Al-LDHs grain size increases, the interlayer ion exchange tends to saturate, and AsO4... 3- and CrO4 2- A thermodynamically stable intercalation equilibrium is reached between LDHs layers, and AFt needle-like crystals continue to grow, intertwining with C-(A)-SH gel and geopolymer framework to form a dense three-dimensional mineral framework network.

[0021] The beneficial effects that this invention can achieve are as follows: 1. After the present invention is subjected to mechanical and chemical pre-passivation treatment assisted by a multi-sulfur-based modifier, the amount of heavy metals carried by coal gangue and fly ash leached by TCLP is reduced to less than 5% of that before ball milling, and the repair material itself no longer constitutes a secondary pollution source.

[0022] 2. This invention utilizes in-situ generated Mg / Al-LDHs mineral phase interlayer exclusive ion exchange, achieving an As(V) solidification rate of over 95% and a Cr(VI) solidification rate of over 90%, with the TCLP leaching concentration meeting the limits of GB 5085.3 standard.

[0023] 3. This invention utilizes lattice substitution and physical encapsulation of C-(A)-SH gel to reduce Pb content. 2+ Cd 2+ Zn 2+ The curing rate reaches over 99%, and the TCLP leaching concentration meets national standards.

[0024] 4. The 28-day unconfined compressive strength (UCS) of the solidified soil of the present invention is not less than 350 kPa, which meets the technical requirements for the mechanical stability of the solidified body in contaminated site remediation projects.

[0025] 5. Under simulated acid rain erosion conditions (pH range of 4 to 9), the cumulative leaching amount of heavy metals in the cured product is reduced by more than 70% compared with traditional cement-based curing methods, and the entire life cycle meets strict environmental regulatory standards.

[0026] 6. The core aggregate of this invention comes from bulk industrial solid waste, which reduces material costs by 40% to 60% compared to traditional cement-based solidification solutions, significantly reduces carbon emissions, and synergistically achieves large-scale resource utilization of coal gangue and fly ash. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the overall process flow of the method of the present invention. Figure 2 A schematic diagram of the pre-passivation mechanochemical activation mechanism; Figure 3 A schematic diagram illustrating the principle of the synergistic solidification mechanism of in-situ reconstruction of dual-target mineral phases. Figure 4 This is a structural diagram of the complete system device for the engineering implementation of the present invention. Detailed Implementation

[0028] The following is in conjunction with the appendix Figure 1 - Appendix Figure 4 The specific embodiments of the present invention will be further described below. It should be noted that the description of these embodiments is for the purpose of helping to understand the present invention, but does not constitute a limitation of the present invention.

[0029] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0030] The technical requirements for raw materials are as follows: The chemical composition requirements for coal gangue are: SiO2 content of 35% to 60% (mass fraction), Al2O3 content of 15% to 35% (mass fraction), and the sum of SiO2 and Al2O3 not less than 60% (mass fraction). The raw ore is first coarsely crushed to a particle size of less than 5 mm using a jaw crusher. Depending on the required activity, it can be calcined and activated in a muffle furnace at 550℃ to 750℃ for 2 to 3 hours to break down the layered structure of residual kaolinite and other clay minerals, significantly increasing the proportion of amorphous SiO2 and Al2O3 and alkali-activated activity. After calcination, it is naturally cooled to room temperature for later use. Before use, TCLP leaching tests are performed according to HJ 832 standard, and the leaching concentrations of heavy metals such as Pb, Cd, As, Cr, and Zn are recorded as the basis for calculating the accurate dosage of the sulfur-based modifier.

[0031] Fly ash conforming to GB / T 1596, Grade I or II, should be preferred. The sum of SiO2 and Al2O3 should not be less than 65% (mass fraction), and the CaO content should be 3% to 15% (mass fraction). High-calcium fly ash should be preferred to facilitate the formation of C-(A)-SH gel. The residue on a 45 μm square-hole sieve should not exceed 25%, and the specific surface area should not be less than 300 m². 2 / kg; TCLP leaching test was performed and recorded.

[0032] The working solution of the multi-sulfur-based modifier is prepared by dissolving Na2S·9H2O (industrial grade, effective Na2S content not less than 60%) and thiourea (industrial grade, purity not less than 98%) separately in deionized water at a mass ratio of 2:1 to 4:1, and then mixing them together. 2- The total concentration is 0.5 mol / L to 2.0 mol / L; prepare immediately before use to avoid prolonged exposure to air and potential damage from sulfur dioxide. 2- Oxidation loss; the total dosage is controlled within the range of 3% to 6% (mass fraction) of the total mass of coal gangue and fly ash. The specific dosage is determined by a precise quantitative algorithm based on the solid waste TCLP leaching test data.

[0033] The alkali activator is prepared by mixing an 8 mol / L to 12 mol / L solution of industrial-grade NaOH (purity not less than 96%) with an industrial liquid water glass solution with an initial modulus of 2.8 to 3.3, which is then adjusted to a target modulus of 1.2 to 2.0 by NaOH, at a volume ratio of 1:1 to 2:1, and then sealing and allowing it to stand for 24 hours to mature. The liquid-solid mass ratio is controlled at 0.35 to 0.45.

[0034] Dual-target mineral phase control additives include: lightly calcined magnesia (obtained from natural magnesite calcined at 750℃ to 900℃, with an active MgO content of not less than 80%, a purity of not less than 85%, and a specific surface area of ​​not less than 15 m²). 2 / g); quicklime (industrial grade hydrated lime, effective Ca(OH)2 content not less than 90%, fineness less than 75 μm); gypsum dihydrate (industrial by-product gypsum, SO3 content not less than 35%, fineness less than 75 μm); sodium aluminate NaAlO2 (industrial grade, purity not less than 95%, added when the Si / Al molar ratio of the system is less than 2.0 to supplement the aluminum source).

[0035] The specific implementation process of the present invention is described below with reference to a specific embodiment: The soil at an abandoned coal chemical plant site is affected by long-term industrial pollution and exhibits typical characteristics of complex heavy metal pollution with anions and cations: the soil pH is 5.8 (weakly acidic), the water content is 22%, and the main pollutants are Pb (cationic) 1200 mg / kg, Cd (cationic) 85 mg / kg, As(V) (anionic) 320 mg / kg, and Cr(VI) (anionic) 180 mg / kg.

[0036] The physicochemical properties of the selected raw materials are as follows: coal gangue contains 48.2% (mass fraction) SiO2, 22.5% (mass fraction) Al2O3, 3.1% (mass fraction) CaO, and 1.8% (mass fraction) MgO. The TCLP leaching concentration of Pb is 2.1 mg / L, and the TCLP leaching concentration of As is 0.9 mg / L. Fly ash contains 51.6% (mass fraction) SiO2, 28.1% (mass fraction) Al2O3, 8.3% (mass fraction) CaO, and 1.2% (mass fraction) MgO. The TCLP leaching concentration of Pb is 0.8 mg / L, and the TCLP leaching concentration of As is 0.3 mg / L.

[0037] Based on one ton of dry contaminated soil, the dual-target solidification material formula is as follows: 100 kg of pre-passivated coal gangue (activated by ball milling), 80 kg of pre-passivated fly ash, 7.2 kg of Na2S·9H2O (added simultaneously with solid waste during the ball milling stage), 2.4 kg of thiourea (the mass ratio of Na2S·9H2O to thiourea is 3:1), 45 L of 10 mol / L NaOH solution, 30 L of water glass solution adjusted to Ms=1.6, 18 kg of lightly calcined MgO (active MgO content not less than 85%), 14 kg of quicklime Ca(OH)2, 9 kg of gypsum dihydrate CaSO4·2H2O, 3 kg of sodium aluminate NaAlO2 (fine-tuning the Si / Al molar ratio to 3.0), and the total amount of solidifying agent is 18% (mass fraction) of the dry mass of the contaminated soil. The design value of the Mg / Al molar ratio of the system is 2.5:1, and the system is designed to target and coordinate the coexisting As(V) and Cr(VI) elements in the site.

[0038] Pre-passivation treatment: Weigh 100 kg of coal gangue and 80 kg of fly ash according to the above ratio, and prepare a multi-component sulfur-based modifier working solution (S) with Na2S·9H2O and thiourea. 2- The total concentration of 1.2 mol / L was added to a planetary high-energy ball mill (ball-to-material mass ratio 10:1, rotation speed 350 r / min) and milled for 3 h to obtain pre-passivated and activated solid waste powder with a specific surface area of ​​430 m². 2 / kg, d 90 The concentration was 38 μm. Testing showed that the TCLP leaching concentration of Pb carried by the solid waste decreased from 2.1 mg / L to 0.09 mg / L after pre-passivation treatment (down to 4.3% before ball milling), meeting the requirements for secondary pollution control.

[0039] Preparation of dual-target solidification material: Pre-passivated activated solid waste powder, alkaline activator (NaOH solution and molding water glass solution mixed at a volume ratio of 1.5:1, sealed and allowed to stand for 24 h for curing), quicklime Ca(OH)2, lightly calcined MgO, gypsum dihydrate CaSO4·2H2O, and sodium aluminate NaAlO2 are uniformly mixed in a high-speed mixer (60 r / min, 5 min) to prepare the finished dual-target solidification slurry. The slurry has a shelf life of no more than 4 h and must be prepared and used immediately.

[0040] Construction and curing: The prepared dual-target solidification slurry is mixed with the target contaminated soil at a solidification agent dosage of 18% (dry soil mass fraction). The grouting is carried out by in-situ deep mixing pile machine with a grouting pressure of 0.3 MPa and the mixture is stirred to 0.5 m below the bottom of the contaminated layer. The mixture is then cured for 28 days at a temperature of 20±2℃ and a relative humidity of not less than 95%.

[0041] The curing effect test results (after 28 days of curing) are as follows: the unconfined compressive strength (UCS) after 28 days is 423 kPa, which meets the requirement of not less than 350 kPa; the Pb TCLP leaching concentration is 0.8 mg / L (curing rate 99.3%), which meets the requirement of GB 5085.3 standard limit of not more than 5 mg / L; the Cd TCLP leaching concentration is 0.05 mg / L, which meets the requirement of not more than 1 mg / L; the As(V) TCLP leaching concentration is 0.3 mg / L (curing rate 94.2%), which meets the requirement of not more than 5 mg / L; and the Cr(VI) TCLP leaching concentration is 0.8 mg / L (curing rate 91.5%), which meets the requirement of not more than 5 mg / L. XRD mineral phase identification simultaneously detected LDHs characteristic peaks (2θ=11.7° and 23.3°), AFt characteristic peaks (2θ=9.1°), and C-(A)-SH diffuse peaks. The simultaneous verification of the three phases confirmed the effective operation of the dual-target synergistic solidification mechanism.

[0042] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications without departing from the spirit and scope of the present invention. Therefore, any modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for synergistically solidifying heavy metal-contaminated soil using coal gangue and fly ash, characterized in that, Includes the following steps: A multi-component sulfur-based modifier was prepared by mixing Na₂S·9H₂O and thiourea at a mass ratio of 2:1 to 4:

1. Coal gangue, fly ash, and the multi-component sulfur-based modifier were then added to a planetary high-energy ball mill and ball-milled at a ball-to-material mass ratio of 10:1 and a rotation speed of 300 to 400 r / min for 2 to 4 hours to obtain pre-passivated and activated solid waste powder. During the ball milling process, mechanical force released the heavy metal cations Me sealed in the solid waste lattice. 2+ The Me 2+ With S 2- The in-situ formation solubility product constant Ksp is 10. -26 Up to 10 -36 The metal sulfide MeS precipitate, and the -NH2 and -C=S functional groups in thiourea chelate and coordinate the residual heavy metals to fix them, so that the TCLP leaching amount of the self-carried heavy metals in the pre-passivated activated solid waste powder is reduced to less than 5% before ball milling. An alkaline activator, slaked lime (Ca(OH)2), lightly calcined magnesium oxide (MgO), and gypsum dihydrate (CaSO4·2H2O) are uniformly mixed with the pre-passivated activated solid waste powder to prepare a dual-target solidification material. The lightly calcined magnesium oxide (MgO) contains no less than 80% active MgO, and the amount added makes the Mg / Al molar ratio in the system reach 2:1 to 3:

1. The amount of slaked lime used is 2% to 8% (mass fraction) of the total mass of coal gangue and fly ash, and the amount of gypsum dihydrate used is 3% to 8% (mass fraction) of the total mass of coal gangue and fly ash. The dual-target solidification material was uniformly mixed with the target heavy metal contaminated soil at a solidification agent dosage of 10% to 25% (mass fraction) of the dry weight of the contaminated soil. The mixture was then cured for 7 to 28 days at a temperature of 18°C ​​to 22°C and a relative humidity of not less than 95%. During the curing process, C-(A)-SH gel, Mg / Al-LDHs mineral phase, and AFt ettringite were simultaneously generated in situ. The cationic heavy metals were solidified through lattice substitution and physical encapsulation by the C-(A)-SH gel, and AsO4 was captured through interlayer ion exchange of the Mg / Al-LDHs mineral phase. 3- and CrO4 2- By using AFt ettringite to assist in curing and improve the mechanical strength of the cured body, the synergistic curing of anionic and cationic multi-element heavy metals can be achieved.

2. The method according to claim 1, characterized in that, The mass ratio of Na2S·9H2O to thiourea is 3:

1.

3. The method according to claim 1, characterized in that, The total amount of the multi-component sulfur-based modifier is determined as follows: TCLP leaching tests are performed on coal gangue and fly ash according to HJ 832 standard to obtain leaching concentration data for each target heavy metal; based on the test data, the TCLP leaching molar amount of each target heavy metal per unit mass of solid waste is calculated; then, using the total mass of the actual coal gangue and the total mass of fly ash as weights, the summation is applied to all target heavy metal types to obtain the amount of heavy metal that needs to be leached by S in the system. 2- Total molar amount of heavy metals fixed by precipitation; based on the consumption of 1 mole of S per mole of heavy metal cations. 2- Based on the stoichiometric relationship, an excess coefficient of 1.2 to 1.5 is introduced to calculate the theoretical requirement of Na2S·9H2O; the amount of thiourea is determined by dividing the theoretical requirement of Na2S·9H2O by the mass ratio of Na2S·9H2O to thiourea; the total amount of multi-component sulfur-based modifier is 3% to 6% (mass fraction) of the total mass of coal gangue and fly ash.

4. The method according to claim 1, characterized in that, The specific surface area of ​​the pre-passivated activated solid waste powder is not less than 400 m². 2 / kg, particle size d 90 No larger than 45 μm.

5. The method according to claim 1, characterized in that, The lightly calcined magnesium oxide (MgO) is calcined at a temperature of 750℃ to 900℃, has a purity of not less than 85%, and a specific surface area of ​​not less than 15 m². 2 / g; The amount of lightly calcined magnesium oxide (MgO) added is determined as follows: based on the complete chemical analysis data of coal gangue and fly ash and their respective alkali activation activity coefficients, the initial total molar amount of active Al and the amount of active Mg provided by the solid waste itself are calculated. 2+ molar quantity; The proportion of active Al allocated to LDH synthesis (ranging from 0.30 to 0.50) is determined based on the severity of anionic heavy metal pollution, and the required Mg is calculated in conjunction with the target Mg / Al molar ratio. 2+ Total molar amount, minus the active Mg provided by the solid waste itself. 2+ After determining the molar amount, the required amount of lightly calcined magnesium oxide can be calculated.

6. The method according to claim 1, characterized in that, The Mg / Al molar ratio is determined according to the following principles: when As(V) is the main anionic heavy metal in the polluted soil, the Mg / Al molar ratio is 3:1; when Cr(VI) is the main anionic heavy metal in the polluted soil, the Mg / Al molar ratio is 2:1 to 2.5:

1.

7. The method according to claim 1, characterized in that, Also includes: When the calculated Si / Al molar ratio of the system obtained from the complete chemical analysis of coal gangue and fly ash raw materials is less than 2.0, sodium aluminate (NaAlO2) with a purity of not less than 95% is added to the alkaline activator to supplement the aluminum source, so that the Si / Al molar ratio of the system is adjusted to 2.0 to 3.

5.

8. The method according to claim 1, characterized in that, The alkali activator is prepared by mixing a NaOH solution with a concentration of 8 mol / L to 12 mol / L and a water glass solution with a modulus adjusted to 1.2 to 2.0 at a volume ratio of 1:1 to 2:1, followed by sealing and allowing it to stand for 24 hours to mature. The liquid-solid mass ratio is 0.35 to 0.

45.

9. The method according to claim 1, characterized in that, The dual-target solidification material is mixed with contaminated soil and solidified in situ. A deep mixing pile machine is used to inject and mix the dual-target solidification material into the contaminated soil. The injection pressure is 0.2 MPa to 0.5 MPa, and the mixing depth is 0.5 m below the bottom of the contaminated layer.

10. The method according to claim 1, characterized in that, The coal gangue has a SiO2 content of 35% to 60% (mass fraction) and an Al2O3 content of 15% to 35% (mass fraction), and the sum of SiO2 and Al2O3 is not less than 60% (mass fraction). The fly ash has a SiO2 content of 40% to 60% (mass fraction), an Al2O3 content of 20% to 35% (mass fraction), a CaO content of 3% to 15% (mass fraction), a sieve residue of no more than 25% on a 45 μm square mesh sieve, and a specific surface area of ​​no less than 300 m². 2 / kg.

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