Desulfurized red mud-based filling material and preparation method thereof

Abstract

The application relates to a desulfurization red mud-based filling material and a preparation method thereof, and belongs to the technical field of solid waste resource utilization. The method comprises the following steps: mixing and grinding dry low-iron high-alkali red mud and secondary aluminum ash to obtain pre-desulfurization powder; mixing the pre-desulfurization powder with water according to a set liquid-solid ratio to obtain pre-desulfurization slurry; using the pre-desulfurization slurry as a flue gas desulfurizer to perform wet desulfurization on sulfur-containing flue gas, and controlling a desulfurization temperature to simultaneously realize flue gas purification and gel activation of the pre-desulfurization slurry, so that a desulfurization slurry body with gel activity is obtained; and mixing the desulfurization slurry body, fly ash and water to obtain pumpable filling slurry.

Description

Technical Field

[0001] This application relates to the field of solid waste resource utilization technology, and in particular to a desulfurized red mud-based backfill material and its preparation method. Background Technology

[0002] Red mud is a highly alkaline solid waste generated during alumina production, producing approximately 1.0 to 1.8 tons of red mud for every ton of alumina produced. The sheer volume of red mud is enormous, and it is rich in alkaline components such as Na₂O. Long-term stockpiling not only occupies significant land resources but also easily leads to soil and water pollution through rainwater leaching, posing serious environmental risks and land pressure. Secondary aluminum ash is a hazardous waste generated during aluminum smelting and recycled aluminum production. It is rich in aluminum nitride (AlN), metallic aluminum, and small amounts of toxic and harmful impurities. It reacts violently with water, releasing ammonia gas, which can easily cause safety accidents. Its harmless treatment process is complex, costly, and extremely difficult to safely dispose of. Fly ash is a major byproduct of coal-fired power plant power generation, possessing a certain degree of pozzolanic activity, but its resource utilization rate is limited. Currently, the low-cost, large-scale resource-based disposal of aluminum industry solid wastes such as red mud and secondary aluminum ash, as well as fly ash, is limited by technology. The stockpiling of large quantities of solid waste not only wastes resources but also places a heavy burden on the ecological environment. There is an urgent need to develop an efficient resource-based technology capable of large-scale disposal of such solid wastes.

[0003] There is an urgent need for low-cost, high-performance backfill materials for mine goaf filling. Existing red mud-based backfill material technologies have the following bottlenecks: First, they rely on external cementing materials such as cement and slag, which are costly and result in low solid waste disposal capacity; second, they only handle a limited variety of solid wastes and cannot address the combined risks of red mud alkali pollution and secondary aluminum ash. Summary of the Invention

[0004] This application provides a desulfurized red mud-based backfill material and its preparation method to solve the following technical problem: how to develop a low-cost, high-performance backfill material. In a first aspect, embodiments of this application provide a method for preparing a desulfurized red mud-based backfill material, the method comprising: Dry, low-iron, high-alkali red mud is mixed and ground with secondary aluminum ash to obtain pre-desulfurized powder. The pre-desulfurization powder is mixed with water at a set liquid-solid ratio to obtain a pre-desulfurization slurry; Using the pre-desulfurized slurry as a flue gas desulfurizing agent, wet desulfurization is performed on sulfur-containing flue gas, and the desulfurization temperature is controlled to simultaneously achieve flue gas purification and gelation activation of the pre-desulfurized slurry, thereby obtaining a desulfurized slurry with gelation activity. The desulfurization slurry, fly ash, and water are mixed to obtain a pumpable filling slurry.

[0005] Optionally, the dried low-iron, high-alkali red mud meets the following requirements: the mass fraction of Fe2O3 is less than 30%, the mass fraction of Na2O is greater than 5%, and the moisture content is less than 5%.

[0006] Optionally, the mass ratio of the dried low-iron, high-alkali red mud to the secondary aluminum ash is 100:(5~30).

[0007] Optionally, the proportion of particles with a diameter less than 0.125 mm in the pre-desulfurized powder is not less than 90 wt%, and the proportion of particles with a diameter less than 0.125 mm in the fly ash is not less than 90 wt%.

[0008] Optionally, the solid content of the pre-desulfurized slurry is 15wt%~25wt%.

[0009] Optionally, under standard conditions, the volume V of the sulfur-containing flue gas and the dry basis mass m of the pre-desulfurized slurry satisfy: V:m = (2~5):1, where V is in cubic meters and m is in kilograms.

[0010] Optionally, the temperature of the wet desulfurization process is 75℃~95℃, and the mass concentration of SO2 in the sulfur-containing flue gas is 2000 mg / m³. 3 ~5000mg / m 3 .

[0011] Optionally, the solid content of the desulfurization slurry is ≥50wt%, and the ratio of the dry basis mass of the desulfurization slurry to the mass of the fly ash is 5:(1~3).

[0012] Optionally, the solid content of the pumpable filling slurry is 55wt% to 65wt%.

[0013] Secondly, embodiments of this application provide a desulfurized red mud-based backfill material, which is prepared by the method described in the first aspect.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing a desulfurized red mud-based backfill material. The method includes: mixing and grinding dried low-iron, high-alkali red mud with secondary aluminum ash to obtain pre-desulfurized powder; mixing the pre-desulfurized powder with water at a set liquid-solid ratio to obtain a pre-desulfurized slurry; using the pre-desulfurized slurry as a flue gas desulfurizing agent to perform wet desulfurization on sulfur-containing flue gas, and controlling the desulfurization temperature to simultaneously achieve flue gas purification and gelation activation of the pre-desulfurized slurry, thereby obtaining a desulfurized slurry with gelation activity; and mixing the desulfurized slurry, fly ash, and water to obtain a pumpable backfill slurry. In this embodiment, dried low-iron, high-alkali red mud and secondary alumina ash are mixed and ground to form a pre-desulfurization slurry. This pre-desulfurization slurry is then used directly as a flue gas desulfurizing agent for wet desulfurization of sulfur-containing flue gas. During the wet desulfurization process, the strong alkali provided by the dried low-iron, high-alkali red mud and the weak alkali generated by the hydrolysis of secondary alumina ash constitute a synergistic desulfurization system. This system absorbs sulfur dioxide from the sulfur-containing flue gas while consuming the free alkali in the dried low-iron, high-alkali red mud to eliminate alkali pollution from the dried low-iron, high-alkali red mud, and causes the aluminum nitride in the secondary alumina ash to undergo directional hydrolysis and be converted into active tetrahydroxyaluminate. In a strongly alkaline environment, the active tetrahydroxyaluminate reacts with the active silicon and calcium components in the dried low-iron, high-alkali red mud, as well as the sulfuric acid generated during desulfurization. The root undergoes an in-situ hydration reaction, spontaneously constructing a multidimensional cementing network mainly composed of hydrated calcium silicate (CSH), hydrated calcium aluminate (CAH), and ettringite (AFt), thereby giving the desulfurization slurry cementing activity. Subsequently, the desulfurization slurry is compounded with fly ash. The active silica-alumina components in the fly ash undergo a pozzolanic reaction in the alkaline environment of the desulfurization slurry, generating additional hydration products to fill and strengthen the original cementing network. At the same time, the fine particles of fly ash optimize the rheological properties of the pumpable filling slurry. Thus, this application transforms the flue gas desulfurization process into an intrinsic driving force for the cementing activity of the filling material in a complete solid waste system without adding any external cementing materials, achieving low cost and high performance simultaneously through an integrated process. Attached Figure Description

[0015] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic flowchart illustrating a method for preparing a desulfurized red mud-based backfill material, as provided in an embodiment of this application. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0020] Figure 1 This is a schematic flowchart illustrating a method for preparing a desulfurized red mud-based backfill material, as provided in an embodiment of this application.

[0021] Please see Figure 1 In a first aspect, embodiments of this application provide a method for preparing a desulfurized red mud-based backfill material, the method comprising: S1. Dry, low-iron, high-alkali red mud is mixed and ground with secondary aluminum ash to obtain pre-desulfurized powder. S2. The pre-desulfurization powder is mixed with water at a set liquid-solid ratio to obtain a pre-desulfurization slurry; S3. Using the pre-desulfurized slurry as a flue gas desulfurizing agent, wet desulfurization is performed on sulfur-containing flue gas, and the desulfurization temperature is controlled to simultaneously achieve flue gas purification and gelation activation of the pre-desulfurized slurry, thereby obtaining a desulfurized slurry with gelation activity. S4. The desulfurization slurry, fly ash and water are mixed to obtain a pumpable filling slurry.

[0022] Mixed grinding provides safe and highly active pre-desulfurization powder for subsequent reactions through physical encapsulation and ultrafine grinding. The pre-desulfurization powder is dispersed in water to transform into a pre-desulfurization slurry with suitable solid content and flowability, preparing the reaction medium for wet desulfurization. Using the pre-desulfurization slurry as a flue gas desulfurizing agent, wet desulfurization of sulfur-containing flue gas is performed. The alkaline component (sodium oxide from dried low-iron, high-alkali red mud) and the weakly alkaline component (ammonia generated from the hydrolysis of secondary alumina ash) in the pre-desulfurization slurry jointly absorb sulfur dioxide from the sulfur-containing flue gas, generating sulfites and sulfates. Aluminum nitride and metallic aluminum in the secondary alumina ash undergo directional hydrolysis to generate ammonia and active tetrahydroxyaluminate. This active tetrahydroxyaluminate reacts with sulfate generated during desulfurization and active silicon and calcium components from the dried low-iron, high-alkali red mud through hydration reactions, forming in-situ hydrated calcium silicate, hydrated calcium aluminate, and ettringite, among other multi-dimensional cementitious networks, endowing the desulfurization slurry with cementitious activity. Finally, the desulfurized slurry with gelling activity is compounded with fly ash. In the alkaline environment provided by the desulfurized slurry, the active silica and active alumina in the fly ash vitreous undergo a pozzolanic reaction, releasing active silica and aluminum components, which continuously participate in the hydration gelling reaction, extending and densifying the original gelling network, significantly improving the early strength and long-term stability of the backfill material. The resulting pumpable backfill slurry can be directly transported to the goaf of the mine for backfilling, and after hardening, it forms a desulfurized red mud-based backfill material.

[0023] Secondary aluminum ash directional hydrolysis process: AlN + 4H2O → Al(OH)3+ NH3↑ Al(OH)3+ OH - → [Al(OH)4] - The formation reaction of ettringite: 3CaO·Al2O3+ 3CaSO4+ 32H2O → 3CaO·Al2O3·3CaSO4·32H2O (AFt) In some embodiments, the dried low-iron, high-alkali red mud meets the following requirements: the mass fraction of Fe2O3 is less than 30%, the mass fraction of Na2O is greater than 5%, and the moisture content is less than 5%.

[0024] Iron in red mud mainly exists in the form of hematite (Fe2O3), and the content of Fe2O3 directly affects the cementitious activity of red mud. When the Fe2O3 content is too high (≥30%), the relative proportion of active silicon and aluminum components in the red mud decreases, and the iron phase is difficult to form cementitious hydrates during hydration. Instead, it may encapsulate active particles, inhibiting the formation of cementitious products such as calcium silicate hydrate (CSH) and ettringite (AFt). Controlling the Fe2O3 mass fraction to less than 30% ensures that a sufficient proportion of active SiO2 and Al2O3 is retained in the red mud, enabling the red mud to fully participate in the hydration reaction during subsequent wet desulfurization and fly ash co-compounding processes, providing the material basis for cementitious strength.

[0025] Na₂O is the main contributor to the alkalinity of red mud, and its content determines the alkalinity of the red mud. One of the core steps of this application is to use the pre-desulfurized slurry as a flue gas desulfurizing agent for wet desulfurization. An alkaline environment is crucial for absorbing SO₂ and simultaneously activating the pre-desulfurized slurry. When the Na₂O mass fraction is greater than 5%, the red mud can provide sufficient free alkali to achieve two functions: First, during the wet desulfurization process, it forms a strong-alkali-weak-alkali synergistic desulfurization system with NH₃ generated from the hydrolysis of secondary aluminum ash, efficiently absorbing SO₂ in the flue gas; Second, the strongly alkaline environment promotes the directional hydrolysis of AlN and metallic aluminum in the secondary aluminum ash, generating highly active tetrahydroxyaluminate ([Al(OH)₄)₄) in situ. - The active ions react with sulfate ions generated during wet desulfurization and active silicon and calcium components in red mud to form a cementing network of CSH, CAH, and AFt. If the Na2O content is too low (less than or equal to 5%), the alkalinity is insufficient, the efficiency of wet desulfurization decreases, and the aluminum ash hydrolysis is incomplete, making it difficult to activate the cementing activity.

[0026] AlN in secondary aluminum ash undergoes violent hydrolysis upon contact with water, producing ammonia gas. If the moisture content of red mud is too high (≥5%), the presence of moisture during the mixing and grinding process with secondary aluminum ash will trigger a localized hydrolysis reaction, leading to premature ammonia release and posing safety hazards (ammonia irritation, poisoning risk) and premature consumption of the effective component (AlN). Simultaneously, a red mud moisture content ≥5% reduces grinding efficiency, making it difficult to achieve the required fineness (particles smaller than 0.125mm accounting for no less than 90wt%), thus affecting the subsequent reaction activity at the solid-liquid-gas multiphase interface. Therefore, pre-drying the red mud to a moisture content of less than 5% ensures it is in a loose powder state, facilitating uniform mixing and grinding with secondary aluminum ash, while preventing the aluminum ash from reacting with water during grinding, thus ensuring process safety and operability.

[0027] In some embodiments, the mass ratio of the dried low-iron, high-alkali red mud to the secondary aluminum ash is 100:(5~30).

[0028] During wet desulfurization, aluminum nitride and metallic aluminum in secondary aluminum ash hydrolyze to generate ammonia. This ammonia, along with the strong alkali provided by dried low-iron, high-alkali red mud, forms a strong-alkali-weak-alkali synergistic desulfurization system. If the mass ratio of dried low-iron, high-alkali red mud to secondary aluminum ash is less than 100:5, the amount of ammonia generated by hydrolysis is insufficient, the contribution of the weak alkali component to the absorption of sulfur dioxide in the sulfur-containing flue gas weakens, desulfurization efficiency decreases, and the gel activation effect of the pre-desulfurization slurry is insufficient. Secondly, the tetrahydroxyaluminate ions generated in situ by the hydrolysis of secondary aluminum ash are key active components for constructing cementitious networks such as ettringite. When the mass ratio of dried low-iron, high-alkali red mud to secondary aluminum ash is less than 100:5, the concentration of active tetrahydroxyaluminate ions in the system is insufficient, the hydration reaction is limited, the gel activity of the desulfurization slurry is difficult to effectively activate, and ultimately the mechanical properties of the filling material (such as 3-day compressive strength and 7-day compressive strength) cannot meet the mine filling requirements.

[0029] Although the dry, low-iron, high-alkali red mud forms a physical coating on the secondary aluminum ash particles during the mixing and grinding process, which can inhibit the uncontrollable hydrolysis of the secondary aluminum ash to a certain extent, when the mass ratio of dry, low-iron, high-alkali red mud to secondary aluminum ash is higher than 100:30, the physical coating effect is insufficient to completely isolate the secondary aluminum ash particles from the contact with moisture. This may lead to a violent hydrolysis reaction during the preparation of the pre-desulfurization slurry, generating a large amount of ammonia gas in a short period of time, causing process safety risks (such as ammonia gas accumulation and slurry boiling).

[0030] In some embodiments, the proportion of particles with a diameter less than 0.125 mm in the pre-desulfurization powder is not less than 90 wt%.

[0031] After the pre-desulfurization powder is mixed with water to form a pre-desulfurization slurry, the particle size of the solid particles directly affects the diffusion path of sulfur dioxide gas from the gas phase to the liquid phase and the reaction rate on the surface of the solid particles. The finer the particles, the larger the specific surface area, the more sufficient the contact between sulfur dioxide molecules and the solid alkaline components, and the faster the desulfurization reaction rate. If the proportion of particles with a diameter less than 0.125 mm in the pre-desulfurization powder is less than 90% by weight, the alkaline components inside the coarse particles cannot participate in the reaction in time, and the desulfurization efficiency will decrease significantly. Secondly, the active components in the secondary aluminum ash are encapsulated inside the particles. Only by fully grinding the particles to a sufficiently fine size can aluminum nitride and metallic aluminum be exposed on the particle surface, allowing them to fully contact the alkaline aqueous solution and undergo hydrolysis reactions during subsequent slurry preparation and wet desulfurization processes. If the proportion of particles smaller than 0.125 mm in the pre-desulfurization powder is less than 90 wt%, the aluminum nitride hydrolysis will be incomplete. On the one hand, this will lead to insufficient ammonia generation and the inability to effectively establish a weak alkali desulfurization system. On the other hand, the unhydrolyzed secondary aluminum ash remaining in the final filling material may continue to hydrolyze slowly in the humid environment of the mine, bringing long-term safety risks.

[0032] In some embodiments, the solid content of the pre-desulfurized slurry is 15wt% to 25wt%.

[0033] Solid content refers to the percentage of the mass of solid matter (i.e., pre-desulfurization powder obtained by mixing and grinding dried low-iron, high-alkali red mud with secondary aluminum ash) in the pre-desulfurization slurry to the total mass of the pre-desulfurization slurry (the mass of solids plus the mass of water). In the embodiments of this application, the solid content of the pre-desulfurization slurry is 15 wt% to 25 wt%, that is, every 100 parts by mass of pre-desulfurization slurry contains 15 to 25 parts by mass of pre-desulfurization powder, with the remainder being water.

[0034] In some implementations, under standard conditions, the volume V of the sulfur-containing flue gas and the dry basis mass m of the pre-desulfurized slurry satisfy: V:m = (2~5):1, where V is in cubic meters and m is in kilograms.

[0035] Under standard conditions (temperature 0℃, pressure 101.325 kPa), the ratio between the volume (V, in cubic meters) of sulfur-containing flue gas introduced into the wet desulfurization system and the dry basis mass (m, in kilograms) of the pre-desulfurization slurry used concurrently is defined as follows: Each kilogram of dry pre-desulfurization slurry corresponds to a sulfur-containing flue gas volume of 2 to 5 cubic meters. The pre-desulfurization slurry, acting as a flue gas desulfurizing agent, derives its desulfurization capacity from the strong alkaline component (OH- produced by the hydrolysis of Na2O) provided by the dried, low-iron, high-alkali red mud in the pre-desulfurization slurry. - This system utilizes a strong-base-weak-base synergistic desulfurization system formed by the combined action of sulfur dioxide (SO2) and the weakly alkaline component (NH3) generated from the hydrolysis of secondary aluminum ash. When the V:m ratio is between 2:1 and 5:1, the dry basis weight of the slurry per unit volume of flue gas is moderate, allowing sufficient time for sulfur dioxide in the flue gas to react with the alkaline components in the slurry as it passes through the slurry layer. The alkaline components in the red mud continuously dissolve to replenish the consumed alkalinity, and the ammonia generated from the hydrolysis of secondary aluminum ash also participates in the absorption, ensuring that the pre-desulfurization slurry maintains high desulfurization activity throughout the gas-liquid contact process.

[0036] In some embodiments, the temperature of the wet desulfurization process is 75℃~95℃, and the mass concentration of SO2 in the sulfur-containing flue gas is 2000 mg / m³. 3 ~5000mg / m 3 .

[0037] Wet desulfurization involves multiple reactions, including sulfur dioxide absorption, neutralization, and secondary aluminum ash hydrolysis, all of which are affected by temperature. Within the temperature range of 75℃ to 95℃, the aluminum nitride hydrolysis rate in the secondary aluminum ash is moderate: below 75℃, aluminum nitride hydrolysis is too slow, resulting in insufficient ammonia production and weak contribution from the weak alkali component, with desulfurization primarily relying on the strong alkali of the red mud; above 95℃, the hydrolysis reaction is excessive, leading to concentrated ammonia release, with some ammonia escaping the slurry before participating in desulfurization, causing waste and increasing the risk of ammonia escape from the tail gas. Within the temperature range of 75℃ to 95℃, the aluminum nitride hydrolysis rate and the dissolution-escape balance of ammonia in the slurry are optimal, allowing ammonia to exist stably in the slurry as dissolved NH3·H2O, reacting with the OH- released from the red mud. - A complementary alkalinity buffer system is formed, which makes the pH value of the slurry drop slowly during the absorption of sulfur dioxide, thus maintaining a high desulfurization efficiency.

[0038] The hydrolysis of aluminum nitride in secondary aluminum ash is a strongly exothermic reaction, and the hydrolysis rate increases exponentially with increasing temperature. By controlling the desulfurization temperature within the range of 75℃ to 95℃, aluminum nitride is completely converted via the following directional hydrolysis pathway: AlN + 4H₂O → Al(OH)₃ + NH₃↑. The generated Al(OH)₃ is further converted into tetrahydroxyaluminate ([Al(OH)₄)) with gelling activity in a strongly alkaline environment. - Below 75℃, the hydrolysis reaction kinetics are slow. Even after tens of minutes to several hours of desulfurization, some aluminum nitride remains unreacted. This residual aluminum nitride enters the desulfurization slurry and eventually remains in the backfill material. When it encounters water (such as mine seepage), it slowly releases ammonia, posing a long-term environmental safety risk. Above 95℃, the hydrolysis reaction is too vigorous, generating a large amount of ammonia and heat in a short time. This may cause the slurry to boil violently and ammonia to escape in a concentrated manner. This not only results in ammonia loss but may also cause the active tetrahydroxyaluminate to undergo a condensation reaction due to local overheating, forming amorphous aluminum hydroxide precipitates and losing its gelling activity. Within the temperature range of 75℃ to 95℃, the hydrolysis reaction proceeds at a controllable rate. Ammonia is released steadily and immediately participates in sulfur dioxide absorption. The concentration of active tetrahydroxyaluminate gradually increases to a suitable level, providing sufficient active precursors for the subsequent in-situ construction of hydrated calcium silicate, hydrated calcium aluminate, and ettringite gelling network with the sulfate generated from desulfurization and the calcium and silicon components in the red mud.

[0039] In the wet desulfurization process, sulfur dioxide is absorbed by the pre-desulfurization slurry and successively converted into sulfurous acid and sulfuric acid. Sulfate ions react with calcium ions in the pre-desulfurization slurry and active tetrahydroxyaluminate ions generated from the hydrolysis of secondary alumina to form ettringite (3CaO·Al2O3·3CaSO4·32H2O). Eettringite is the main contributing phase to the early strength of the backfill material, and its formation rate and crystal morphology are highly sensitive to temperature. Within the temperature range of 75℃ to 95℃, the ettringite formation rate is moderate, and crystals are uniformly distributed in needle-like or columnar forms on the surface and in the pores of red mud and fly ash particles, forming a dense and continuous cementitious network. Below 75℃, the ettringite formation rate is slow, the amount of early hydration products is small, and the 3-day compressive strength of the backfill material is insufficient. Above 95℃, ettringite formation is too rapid, the crystals are large and unevenly distributed, and the cementitious network has defects. Simultaneously, at high temperatures, ettringite may transform into monosulfide-type hydrated calcium sulfoaluminate (AFm), leading to a decrease in volume stability. On the other hand, the sulfur dioxide concentration in sulfur-containing flue gas is 2000 mg / m³. 3 Up to 5000 mg / m 3 Within a certain range, the concentration of sulfate generated during desulfurization and the concentration of active tetrahydroxyaluminate are in an appropriate molar ratio (SO4). 2- AlO2 - The ratio is approximately 1:1 to 3:1, which is conducive to the large-scale formation of ettringite.

[0040] In some embodiments, the proportion of particles with a diameter less than 0.125 mm in the fly ash is not less than 90 wt%.

[0041] Fly ash is a solid waste generated by coal-fired power plants. The vitreous structure of fly ash contains active silica and alumina. In this application, fly ash is mixed with a desulfurization slurry exhibiting cementitious activity, further enhancing the mechanical properties of the backfill material through pozzolanic reaction. The pozzolanic activity of fly ash is closely related to its specific surface area. When the proportion of particles smaller than 0.125 mm in fly ash is not less than 90 wt%, the specific surface area of ​​the fly ash particles increases significantly, making it easier for the silica and alumina in the vitreous structure to dissolve in the alkaline environment provided by the desulfurization slurry. The desulfurization slurry originates from the wet desulfurization process, and its pH value is typically maintained between 8 and 11, containing abundant calcium ions and active aluminate. In alkaline slurry, the Si-O-Si and Al-O-Al bonds in the vitreous network of finely ground fly ash particles break, releasing active silicate and aluminate ions. These ions undergo a secondary hydration reaction with the original calcium ions, active aluminate ions, and sulfate ions in the desulfurization slurry, generating additional hydrated calcium silicate and ettringite, which fill the pores of the original cementitious network, making the filling material more compact.

[0042] In this application, the particle size requirement for the pre-desulfurization powder is the same as that for fly ash (both require particles smaller than 0.125 mm to account for no less than 90 wt%). When the particle size distributions of the two solid components are similar, during the mixing process of the desulfurization slurry and fly ash, the fly ash particles can be uniformly dispersed in the interparticle spaces of the desulfurization slurry, forming a continuously graded particle packing system. This particle size matching is beneficial for improving the bulk density of the filling slurry and reducing the interparticle void water, thereby achieving good pumpability under a solid content of 55 wt% to 65 wt%.

[0043] In some embodiments, the solid content of the desulfurization slurry is ≥50wt%, and the ratio of the dry basis mass of the desulfurization slurry to the mass of the fly ash is 5:(1~3).

[0044] The desulfurization slurry is an active slurry rich in hydrated calcium silicate, hydrated calcium aluminate, and ettringite cementitious networks, formed after simultaneous activation during wet desulfurization. A solid content of no less than 50 wt% in the desulfurization slurry indicates a high concentration of cementitious products per unit volume. When the dry basis of the desulfurization slurry is mixed with fly ash in a ratio of 5:1 to 5:3, a reasonable mass balance is achieved between the cementitious substances provided by the desulfurization slurry and the active silica-alumina components provided by the fly ash. Within this ratio range, the cementitious products in the desulfurization slurry can fully coat and bind the fly ash particles, while the amount of fly ash added is neither too much, diluting the cementitious system, nor too little, failing to exert a synergistic strengthening effect.

[0045] A certain amount of hydrated calcium silicate and ettringite have already been generated in situ in the desulfurization slurry. These primary cementitious products constitute the early strength framework of the backfill material. When fly ash is added, the active silica and alumina in the fly ash undergo pozzolanic reaction driven by the residual alkalinity of the desulfurization slurry, generating additional hydrated calcium silicate and ettringite. When the dry basis of the desulfurization slurry is 5:1 to 5:3 by mass, a continuous and complementary cementitious network is formed between the primary cementitious products and the secondary hydration products. This ensures that insufficient fly ash will lead to insufficient strength in the later stages, while excessive fly ash will not dilute the primary cementitious system or delay early strength development.

[0046] In some embodiments, the solids content of the pumpable filling slurry is 55wt% to 65wt%.

[0047] The solids content of pumpable backfill slurry refers to the percentage of the mass of solid matter (including dry-basis solids from the desulfurization slurry and fly ash) in the backfill slurry obtained by uniformly mixing desulfurization slurry, fly ash, and water, relative to the total mass of the backfill slurry (mass of solids plus the mass of all water). When the solids content of the pumpable backfill slurry is in the range of 55% to 65% by weight, the solid particles in the slurry are uniformly dispersed in water, forming a suspension system with appropriate yield stress and plastic viscosity. This system can maintain homogeneity during pipeline transportation and is not prone to sedimentation or segregation; it can flow smoothly under pump pressure and fill irregular goaf spaces by self-leveling upon reaching the goaf.

[0048] Secondly, embodiments of this application provide a desulfurized red mud-based backfill material, which is prepared by the method described in the first aspect.

[0049] The desulfurized red mud-based backfill material is realized based on the preparation method of the above-mentioned desulfurized red mud-based backfill material. The specific steps of the preparation method of the desulfurized red mud-based backfill material can be referred to the above embodiments. Since the desulfurized red mud-based backfill material adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0050] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0051] Example 1 Use dried low-iron, high-alkali red mud and secondary aluminum ash with the chemical composition shown in Table 1.

[0052] Table 1. Chemical composition of dried low-iron, high-alkali red mud and secondary alumina ash.

[0053] Dry, low-iron, high-alkali red mud and secondary aluminum ash were mixed at a mass ratio of 100:30 and ground until the proportion of particles with a particle size less than 0.125 mm was ≥90 wt%, thus obtaining pre-desulfurization powder. The pre-desulfurization powder was then mixed with water to prepare a pre-desulfurization slurry with a solid content of 15 wt%. The pre-desulfurization slurry was used as a flue gas desulfurizing agent to perform wet desulfurization on sulfur-containing flue gas. Under standard conditions, the volume V of the sulfur-containing flue gas and the dry basis mass m of the pre-desulfurization slurry satisfied V:m = 5:1 (V is in cubic meters, m is in kilograms), and the SO2 mass concentration in the sulfur-containing flue gas was 2000 mg / m³. 3The desulfurization temperature is 95℃; after desulfurization, a desulfurized slurry is obtained; the desulfurized slurry, fly ash and water are mixed evenly, and the ratio of the dry basis mass of the desulfurized slurry to the mass of fly ash is 5:3, to obtain a pumpable filling slurry with a solid content of 65wt%.

[0054] Example 2 The only difference between this embodiment and Embodiment 1 is that the solid content of the pumpable filling slurry is 55wt%.

[0055] Example 3 The only difference between this embodiment and Embodiment 1 is that the ratio of the dry basis mass of the desulfurization slurry to the mass of fly ash is 5:1.

[0056] Example 4 The only difference between this embodiment and Embodiment 1 is that the SO2 concentration in the sulfur-containing flue gas is 5000 mg / m³. 3 .

[0057] Example 5 The only difference between this embodiment and Embodiment 1 is that the desulfurization temperature is 75℃.

[0058] Example 6 The only difference between this embodiment and Embodiment 1 is that the volume V of the sulfur-containing flue gas and the dry basis mass m of the pre-desulfurized slurry satisfy V:m=2:1.

[0059] Example 7 The only difference between this embodiment and Embodiment 1 is that the solid content of the pre-desulfurized slurry is 25wt%.

[0060] Example 8 The only difference between this embodiment and Embodiment 1 is that the mass ratio of dried low-iron, high-alkali red mud to secondary aluminum ash is 100:5.

[0061] Example 9 The only differences between this embodiment and Embodiment 1 are: the mass ratio of dried low-iron, high-alkali red mud to secondary aluminum ash is 100:15; the solid content of the pre-desulfurization slurry is 20wt%; the volume V of the sulfur-containing flue gas and the dry basis mass m of the pre-desulfurization slurry satisfy V:m=3:1; the desulfurization temperature is 90℃; and the SO2 mass concentration in the sulfur-containing flue gas is 3500mg / m³. 3 The dry weight ratio of the desulfurization slurry to the fly ash weight is 5:2; the solid content of the pumpable filling slurry is 60wt%.

[0062] Comparative Example 1 The difference between this comparative example and Example 1 is that the dried low-iron, high-alkali red mud is not mixed with secondary aluminum ash; instead, the dried low-iron, high-alkali red mud is directly used to prepare a pre-desulfurization slurry for wet desulfurization. All other aspects are the same as in Example 1.

[0063] Comparative Example 2 The difference between this comparative example and Example 1 is that the pre-desulfurized powder is not subjected to wet desulfurization, but is directly mixed with fly ash and water to obtain a pumpable filling slurry. All other aspects are the same as in Example 1.

[0064] Comparative Example 3 The difference between this comparative example and Example 1 is that the desulfurization temperature is 60°C. Everything else is the same as in Example 1.

[0065] Comparative Example 4 The difference between this comparative example and Example 1 is that the SO2 mass concentration in the sulfur-containing flue gas is 6000 mg / m³. 3 Everything else is the same as in Example 1.

[0066] The desulfurization efficiency during the preparation process of Examples 1-9 and Comparative Examples 1-4 was tested, and the compressive strength of the obtained pumpable filling slurry after molding and curing was also tested. The results are shown in Table 2.

[0067] Table 2. Experimental data for each embodiment and comparative example.

[0068] As shown in Table 1, the desulfurized red mud-based backfill materials prepared in each embodiment of this application achieve a desulfurization efficiency of over 97.8% and a 7-day compressive strength of ≥2.0 MPa without the intervention of any external cementing materials, thus meeting the requirements for mine backfilling.

[0069] Comparative Example 1, without the addition of secondary aluminum ash, exhibited a higher desulfurization efficiency (99.41%), even surpassing some embodiments. This is because the dried, low-iron, high-alkali red mud itself possesses strong alkalinity, sufficient for efficient absorption of sulfur dioxide in flue gas. However, due to the lack of active tetrahydroxyaluminate ions provided by the hydrolysis of secondary aluminum ash, a sufficient hydrated calcium silicate and ettringite cementitious network could not be formed in the desulfurization slurry. Consequently, the 3-day compressive strength (1.2 MPa) and 7-day compressive strength (1.4 MPa) of the filling material were significantly lower than those of the embodiments. This comparative example demonstrates that the core role of secondary aluminum ash in this application is not only in participating in desulfurization but also as a key source of cementitious activity. The active tetrahydroxyaluminate ions generated by its hydrolysis are essential components for constructing a multidimensional cementitious network and imparting mechanical properties to the filling material.

[0070] Comparative Example 2 did not undergo flue gas desulfurization activation and had extremely low intensity.

[0071] Comparative Example 3 showed that the desulfurization temperature was too low and the strength did not meet the standard.

[0072] Comparative Example 4 increased the SO2 concentration in sulfur-containing flue gas to 6000 mg / m³. 3The desulfurization efficiency decreased to 94.13%, but the 3-day compressive strength (2.1 MPa) and 7-day compressive strength (2.5 MPa) of the backfill material were close to those of Example 1. This result indicates that when the SO2 concentration is too high, the amount of SO2 absorbed by the pre-desulfurization slurry per unit mass exceeds the reaction capacity between the active tetrahydroxyaluminate ions and calcium ions in the system. Some sulfate ions cannot participate in the ettringite formation reaction in time, leading to a decrease in desulfurization efficiency. However, since the cementitious network skeleton already formed in the desulfurization slurry is not significantly affected, and the residual alkalinity is still sufficient to drive the pozzolanic reaction of fly ash, the mechanical properties of the backfill material are basically maintained. This comparative example illustrates that the method of this application has good process adaptability within the SO2 concentration fluctuation range. However, to balance desulfurization efficiency and backfill performance, the SO2 concentration should be controlled between 2000 and 5000 mg / m³. 3 .

[0073] This application provides a desulfurized red mud-based backfill material and its preparation method, aiming to solve the following technical problems: (1) Existing red mud-based backfill materials rely on external cementing materials such as cement, which leads to high costs and limited solid waste disposal capacity; (2) The problem of the difficulty in coordinating the environmental risks caused by the high alkalinity of red mud and the safety risks of secondary aluminum ash hydrolysis upon contact with water; (3) The problem of insufficient gelling activity and mechanical properties of filling materials under the whole solid waste system that cannot meet the requirements of mine filling.

[0074] Based on this, one or more technical solutions in the embodiments of this application also have at least the following technical effects or advantages: High-value utilization of bulk solid waste: Using low-iron and high-alkali red mud, secondary aluminum ash and fly ash as all raw materials, 100% solid waste replacement is achieved without the need for external cementing materials such as cement and slag, which significantly alleviates the pressure of red mud storage, eliminates the safety hazards of aluminum ash and improves the utilization rate of fly ash.

[0075] Waste treatment with waste, simultaneously purifying flue gas: Utilizing the strong alkalinity of red mud and the weak alkali produced by the hydrolysis of aluminum ash as desulfurizing agents, the desulfurization efficiency is consistently above 97.8% when treating industrial sulfur-containing flue gas, ensuring that the flue gas meets emission standards. There is no need to purchase desulfurizing agents such as limestone / lime, achieving dual environmental benefits of solid waste disposal and flue gas treatment.

[0076] Under conditions of complete solid waste and no cement, the mechanical properties of the filling material meet the standards for filling engineering in mining goaf areas. The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for preparing a desulfurized red mud-based backfill material, characterized in that, The method includes: Dry, low-iron, high-alkali red mud is mixed and ground with secondary aluminum ash to obtain pre-desulfurized powder. The pre-desulfurization powder is mixed with water at a set liquid-solid ratio to obtain a pre-desulfurization slurry; Using the pre-desulfurized slurry as a flue gas desulfurizing agent, wet desulfurization is performed on sulfur-containing flue gas, and the desulfurization temperature is controlled to simultaneously achieve flue gas purification and gelation activation of the pre-desulfurized slurry, thereby obtaining a desulfurized slurry with gelation activity. The desulfurization slurry, fly ash, and water are mixed to obtain a pumpable filling slurry.

2. The method according to claim 1, characterized in that, The dried, low-iron, high-alkali red mud meets the following requirements: Fe2O3 mass fraction less than 30%, Na2O mass fraction greater than 5%, and moisture content less than 5%.

3. The method according to claim 1, characterized in that, The mass ratio of the dried low-iron, high-alkali red mud to the secondary aluminum ash is 100:(5~30).

4. The method according to claim 1, characterized in that, The pre-desulfurized powder contains particles with a diameter less than 0.125 mm, accounting for no less than 90 wt%, and the fly ash contains particles with a diameter less than 0.125 mm, accounting for no less than 90 wt%.

5. The method according to claim 1, characterized in that, The solid content of the pre-desulfurized slurry is 15wt%~25wt%.

6. The method according to claim 1, characterized in that, Under standard conditions, the volume V of the sulfur-containing flue gas and the dry basis mass m of the pre-desulfurized slurry satisfy the following: V:m = (2~5):1, where V is in cubic meters and m is in kilograms.

7. The method according to claim 1, characterized in that, The wet desulfurization temperature is 75℃~95℃, and the SO2 mass concentration in the sulfur-containing flue gas is 2000 mg / m³. 3 ~5000mg / m 3 .

8. The method according to claim 1, characterized in that, The solid content of the desulfurization slurry is ≥50wt%, and the ratio of the dry basis mass of the desulfurization slurry to the mass of the fly ash is 5:(1~3).

9. The method according to claim 1, characterized in that, The solid content of the pumpable filling slurry is 55wt%~65wt%.

10. A desulfurized red mud-based backfill material, characterized in that, The desulfurized red mud-based filling material is prepared by the method described in any one of claims 1 to 9.

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