Red mud biochar loaded filter aid and preparation method and application thereof

Abstract

The application discloses a red mud biochar loaded filter aid and a preparation method and application thereof, and belongs to the technical field of red mud dewatering treatment. The preparation method of the red mud biochar loaded filter aid provided by the application comprises the following steps: mixing dry and crushed red mud and biomass raw materials to prepare pulp, and then sequentially performing spray drying and co-pyrolysis reaction to obtain a loaded precursor; mixing the loaded precursor with a modifier, wherein the modifier comprises a silane coupling agent; and calcining to obtain the red mud biochar loaded filter aid. The red mud biochar loaded filter aid realizes double resource utilization of industrial solid waste and agricultural waste, can significantly reduce the filtration resistance during red mud dewatering, improves the filtration rate, has good structural stability and compression resistance in the filtration process, and can be separated and regenerated from the filter cake after use, and has stable recycling performance.

Description

Technical Field

[0001] This invention relates to the field of red mud dewatering technology, and in particular to a red mud biochar-supported filter aid, its preparation method, and its application. Background Technology

[0002] Red mud is a large-scale industrial solid waste generated during alumina production. Its fine particles and strong hydrophilicity make dewatering extremely difficult, becoming a key bottleneck restricting the treatment, disposal, and resource utilization of red mud. Currently, red mud dewatering mainly adopts a combination of physical filtration and chemical conditioning, in which the performance of the filter aid directly affects the dewatering effect and treatment cost.

[0003] Existing filter aids have the following main shortcomings: conventional inorganic filter aids have a simple pore structure, making it difficult to effectively break down the hydrophilicity of red mud particles, resulting in slow dewatering rates and high moisture content in the filter cake; single biochar filter aids have low mechanical strength, making them prone to breakage during filtration and affecting dewatering performance; existing supported filter aids have high carrier costs, low solid waste resource utilization rates, and active components are prone to detachment, causing secondary pollution. Summary of the Invention

[0004] In view of this, the present invention provides a red mud biochar-supported filter aid, its preparation method, and its application. The present invention obtains the filter aid by co-pyrolyzing red mud and biomass and further modifying it. When used for red mud dewatering, it can significantly improve filtration efficiency and reduce the moisture content of the filter cake, while simultaneously achieving dual resource utilization of red mud and biomass. It has the advantages of low processing cost and environmental friendliness.

[0005] In a first aspect, the present invention provides a method for preparing a red mud biochar-supported filter aid, comprising the following steps: The dried and pulverized red mud was mixed with biomass raw materials to form a slurry, which was then spray-dried and co-pyrolyzed in sequence to obtain a supported precursor; the mass ratio of the red mud to the biomass raw materials was (3~5):1. The supported precursor is mixed with a modifier, the modifier including a silane coupling agent; the mass of the modifier is 2-4% of the mass of the supported precursor; calcination yields the red mud biochar supported filter aid.

[0006] Preferably, the mass ratio of the red mud to the biomass raw material is (3~5):1; the inlet air temperature of the spray dryer is 180~220℃, and the outlet air temperature is 80~100℃.

[0007] Preferably, the co-pyrolysis reaction specifically includes: first heating to 250~300℃ for pyrolysis for 1.5~2.5h, and then heating to 500~600℃ for pyrolysis for 2~3h; the calcination temperature is 300~350℃, and the calcination time is 1~2h.

[0008] Preferably, the silane coupling agent comprises at least one of γ-glycidoxypropyltrimethoxysilane (KH-560), γ-aminopropyltriethoxysilane (KH-550), vinyltrimethoxysilane, and methacryloxypropyltrimethoxysilane.

[0009] Preferably, the modifier further includes graphene, and the mass ratio of the silane coupling agent to graphene is 1:(0.1~5).

[0010] Preferably, when mixing the supported precursor with the modifier, a solvent is first added to the supported precursor, and then it is mixed with the modifier; the solvent is a mixed solution of water and ethanol, and the volume ratio of water to ethanol is (1~3):1.

[0011] Secondly, the present invention provides a red mud biochar-supported filter aid prepared by the above preparation method.

[0012] Thirdly, the present invention provides the application of the above-mentioned red mud biochar-supported filter aid in red mud dewatering treatment.

[0013] Fourthly, the present invention provides a method for dewatering red mud, comprising the following steps: The above-mentioned red mud biochar-supported filter aid was added to the red mud slurry, mixed evenly, and then filtered and dehydrated.

[0014] Preferably, the amount of the red mud biochar-supported filter aid added is 5-10% of the dry weight of the red mud in the red mud slurry.

[0015] Preferably, the red mud slurry further includes a flocculant and a coagulant aid, wherein the amount of flocculant added is 0.1-0.3% of the dry weight of the red mud in the red mud slurry; and the amount of coagulant added is 1-3% of the dry weight of the red mud in the red mud slurry.

[0016] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) The present invention uses red mud and biomass raw materials to prepare a supported precursor by co-pyrolysis. By utilizing the synergistic effect of red mud mineral components and biomass pyrolysis products during co-pyrolysis, biochar is uniformly loaded on the surface of red mud to form a composite material with a multi-level pore structure, which significantly improves the specific surface area and adsorption performance of the material, and provides a good structural basis for subsequent modification and filtration application.

[0017] (2) In this invention, silane coupling agent is used as a modifier to modify the co-pyrolysis products. The silane coupling agent introduces active functional groups on the surface of the material through chemical bonding, thereby improving the surface hydrophobicity of the material and its compatibility with polymeric flocculants.

[0018] (3) When the red mud biochar-supported filter aid prepared in this invention is used for red mud dewatering, it can form a structurally stable porous filter cake layer on the surface of the filter cloth, significantly reducing filtration resistance and increasing filtration rate. At the same time, it has good compatibility with chemical agents such as flocculants and coagulants. Through the synergistic effect of physical filtration and chemical conditioning, it can effectively reduce the moisture content of the filter cake and achieve efficient deep dewatering of red mud.

[0019] (4) The present invention uses red mud and biomass as raw materials to prepare filter aids, realizing the dual resource utilization of industrial solid waste and agricultural waste, reducing the raw material cost of filter aids; after use, the filter aids can be separated and regenerated from the filter cake, and the recycling performance is stable, further reducing the comprehensive treatment cost of red mud dewatering, and has good economic and environmental benefits. Detailed Implementation

[0020] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0021] This invention addresses the technical problems of existing red mud dewatering filter aids, such as poor filtration effect, incomplete dewatering, and low solid waste resource utilization rate. It proposes a new approach to prepare supported filter aids using red mud and biomass as raw materials through a combination of co-pyrolysis and modification.

[0022] This invention provides a method for preparing a red mud biochar-supported filter aid, comprising the following steps: The dried and pulverized red mud was mixed with biomass raw materials to make a pulp, and then spray-dried and co-pyrolyzed in sequence to obtain a supported precursor. The supported precursor is mixed with a modifier, including a silane coupling agent; calcination yields a red mud biochar supported filter aid.

[0023] In this invention, the red mud needs to be dried and pulverized before use. Fresh red mud has a high moisture content, typically 60% to 80%, which can affect the uniformity of mixing with biomass and the subsequent co-pyrolysis effect if used directly. Therefore, the red mud is sent to a dryer for drying at a temperature of 80-100℃, such as 80℃, 85℃, 90℃, 95℃, or 100℃, until the moisture content is ≤15%. In this invention, unless otherwise specified, the moisture content refers to the mass fraction of water in the total mass of the material. If the drying temperature is too low, the drying time will be prolonged and the efficiency will be reduced; if the drying temperature is too high, it may cause changes in the mineral structure of the red mud, affecting its performance as a carrier. After drying, the red mud is cooled to room temperature and then sent to a pulverizing device to be pulverized to a particle size ≤100 mesh, such as 70-80 mesh, 80-100 mesh, 90-100 mesh, or 100 mesh. Crushing the red mud into this particle size range is beneficial to improving the mixing uniformity of red mud and biomass, increasing the reaction contact area, and promoting the synergistic effect in the co-pyrolysis process.

[0024] Biomass raw materials also need to be dried and pulverized before use. This invention does not have specific limitations on the type of biomass raw material, as long as it can form biochar through pyrolysis. In some preferred embodiments, the biomass raw material includes one or more of corn stalks, wheat stalks, sawdust, and rice husks. These biomass raw materials are widely available and inexpensive, and after pyrolysis, they can form biochar with a porous structure, which is beneficial for synergistically improving filtration performance with red mud carriers. After removing impurities, the biomass raw material is pulverized to a particle size of 2-5 mm, for example, 2 mm, 3 mm, 4 mm, or 5 mm. Pulverizing to this particle size range facilitates uniform mixing with red mud during the mixing and pulping process, and also facilitates subsequent spray drying and molding. The pulverized biomass is then fed into a dryer and dried at 70-90°C until the moisture content is ≤10%, for example, 70°C, 75°C, 80°C, 85°C, or 90°C. The selection of the drying temperature must balance drying efficiency and avoid biomass pyrolysis; within this temperature range, moisture can be effectively removed without damaging the biomass structure.

[0025] This invention involves mixing dried and pulverized red mud with biomass raw materials to form a pulp. The purpose of mixing and pulping is to ensure that the red mud and biomass are in full contact and evenly dispersed, laying the foundation for subsequent spray drying and co-pyrolysis reactions.

[0026] In some preferred embodiments, the mass ratio of red mud to biomass feedstock is (3~5):1, for example, 3:1, 3.5:1, 4:1, 4.5:1, or 5:1. Within this mass ratio range, red mud and biochar can form a good synergistic effect during co-pyrolysis. If the proportion of red mud is too high, the biomass content is relatively insufficient, resulting in a smaller amount of biochar and making it difficult to fully utilize the porous adsorption and filtration effects of biochar; if the proportion of biomass is too high, the role of red mud as a carrier is weakened, the mechanical strength of the obtained precursor may decrease, and the resource utilization rate of red mud will decrease.

[0027] In this invention, an appropriate amount of deionized water is added when mixing red mud and biomass raw materials to adjust the slurry concentration to 30-50 wt%, for example, 30 wt%, 35 wt%, 40 wt%, 45 wt%, or 50 wt%. The slurry concentration needs to be controlled within a suitable range: if the concentration is too low, the water content will be too high, increasing the energy consumption of subsequent spray drying; if the concentration is too high, the slurry viscosity will be too high, making it difficult to mix the red mud and biomass evenly.

[0028] In this invention, the slurry after mixing red mud and biomass raw materials is preferably stirred at a constant temperature of 50-70℃ for 40-60 minutes, for example, at 50℃, 55℃, 60℃, 65℃, or 70℃ for 40 minutes, 45 minutes, 50 minutes, 55 minutes, or 60 minutes, respectively. Heating and stirring promotes further dispersion of red mud particles in the slurry and ensures thorough wetting of the biomass, which is beneficial for uniform mixing. Too short a stirring time results in insufficient mixing; too long a stirring time reduces efficiency and increases energy consumption.

[0029] This invention involves feeding a homogenized slurry obtained through mixing and pulping into a spray dryer for drying and shaping. The purpose of spray drying is to rapidly dry the liquid slurry into mixed particles with a certain particle size and shape, providing a structural basis for subsequent co-pyrolysis reactions.

[0030] In some preferred embodiments, the inlet air temperature for spray drying is 180~220℃, for example, 180℃, 190℃, 200℃, 210℃, or 220℃; the outlet air temperature is 80~100℃, for example, 80℃, 85℃, 90℃, 95℃, or 100℃. The inlet air temperature is a key parameter in spray drying: if the inlet air temperature is too low, the drying rate is slow, and the slurry may stick to the walls or be insufficiently dried; if the inlet air temperature is too high, it may lead to premature carbonization of the biomass surface or changes in the mineral structure of the red mud. The outlet air temperature reflects the temperature of the drying exhaust gas and is related to the final moisture content of the product. Within this range of inlet and outlet air temperatures, mixed particles with suitable moisture content and uniform particle size distribution can be obtained. The particle size of the mixed particles obtained after spray drying can be 0.5~2mm, which is beneficial for the uniformity of the subsequent co-pyrolysis reaction.

[0031] Through spray drying, red mud and biomass are uniformly mixed at the particle size. At the same time, the particles have a certain porosity, which is conducive to the heat transfer and the release of volatile products during co-pyrolysis.

[0032] In this invention, the mixed particles obtained by spray drying are fed into a co-pyrolysis reactor for co-pyrolysis under inert gas protection. Co-pyrolysis is one of the core steps of this invention, which uses high-temperature treatment to pyrolyze biomass into biochar, while simultaneously achieving uniform loading of biochar on the surface of red mud.

[0033] In some preferred embodiments, the co-pyrolysis reaction specifically includes staged heating pyrolysis: first heating to 250~300℃ for pyrolysis for 1.5~2.5h, and then heating to 500~600℃ for pyrolysis for 2~3h.

[0034] The first stage of pyrolysis occurs at temperatures between 250 and 300°C, such as 250°C, 260°C, 270°C, 280°C, 290°C, or 300°C; the pyrolysis time is 1.5 to 2.5 hours, such as 1.5 hours, 1.8 hours, 2.0 hours, 2.2 hours, or 2.5 hours. The main function of this stage is to remove volatile impurities and residual moisture from the mixed particles, while simultaneously initiating the initial pyrolysis of the biomass. Within this temperature range, hemicellulose and some cellulose in the biomass begin to decompose, but the porous structure of the biochar is not yet fully formed. This first stage of pyrolysis prevents the rapid escape of volatile components during subsequent high-temperature pyrolysis, which could damage the porous structure.

[0035] The second stage of pyrolysis occurs at temperatures between 500 and 600°C, such as 500°C, 520°C, 550°C, 580°C, or 600°C; the pyrolysis time is 2 to 3 hours, such as 2 hours, 2.2 hours, 2.5 hours, 2.8 hours, or 3 hours. This stage is crucial for biochar formation and loading. Within this temperature range, cellulose and lignin in the biomass undergo complete pyrolysis, forming biochar with a rich porous structure. Simultaneously, the mineral components in the red mud interact with the biochar surface, achieving uniform loading of biochar on the red mud surface. If the pyrolysis temperature is too low, the biomass pyrolysis will be incomplete, resulting in low biochar yield and underdeveloped pore structure; if the pyrolysis temperature is too high, it may lead to excessive graphitization of the biochar, causing pore structure collapse and a decrease in specific surface area.

[0036] This invention employs the aforementioned staged heating method, which allows for the gradual release of volatile components, avoiding rapid escape and damage to the pore structure. Simultaneously, the preliminary carbon skeleton formed in the first stage of pyrolysis provides a structural basis for the second stage of high-temperature pyrolysis, facilitating the formation of a more developed pore structure. The biochar specific surface area in the supported precursor obtained by this invention can reach 300 m². 2 / g or higher, with a porosity of over 60%.

[0037] The co-pyrolysis reaction must be carried out under the protection of an inert gas to prevent the biomass from burning in the air. The inert gas can be one or more of nitrogen, argon, and helium, with nitrogen being preferred as the protective gas due to its wide availability and low cost.

[0038] After the co-pyrolysis reaction was completed, the product was cooled to room temperature to obtain a red mud biochar supported precursor. In this precursor, biochar was uniformly loaded on the surface of red mud, forming a composite material with a multi-level porous structure.

[0039] This invention improves the surface hydrophobicity, mechanical strength, and compatibility with chemical agents of the filter aid by introducing a modifier to modify the surface of the precursor. The modifier includes a silane coupling agent. Silane coupling agents are a class of organosilicon compounds with bifunctional groups; one end of the compound can react with hydroxyl groups on the surface of inorganic materials to form a chemical bond, while the other end has an organic functional group, which can improve the compatibility of the material with organic compounds.

[0040] In some preferred embodiments, the mass of the modifier is 2-4% of the mass of the supported precursor, for example, 2%, 2.5%, 3%, 3.5%, or 4%. The amount of modifier used needs to be controlled within an appropriate range: if the amount is too low, the modification effect will be insignificant; if the amount is too high, it may cause the modifier to agglomerate on the material surface, which will affect the filter aid performance.

[0041] In this invention, the silane coupling agent includes at least one of γ-glycidoxypropyltrimethoxysilane (KH-560), γ-aminopropyltriethoxysilane (KH-550), vinyltrimethoxysilane, and methacryloxypropyltrimethoxysilane. Different types of silane coupling agents have different organic functional groups and can be selected according to actual needs. KH-560 contains epoxy groups and can react with a variety of polymers; KH-550 contains amino groups and has better compatibility with anionic flocculants; vinyl and methacryloxy silanes contain unsaturated bonds and can participate in free radical reactions. In some preferred embodiments, the silane coupling agent is KH-560, whose epoxy groups can undergo ring-opening crosslinking reactions during subsequent calcination to form a more stable network structure.

[0042] In some preferred embodiments, the modifier further includes graphene, and the mass ratio of the silane coupling agent to graphene is 1:(0.1~5), more preferably 1:(0.15~4). The silane coupling agent can form an organic modification layer on the material surface, improving surface hydrophobicity and compatibility with flocculants; graphene can fill the pore structure, enhancing the mechanical strength of the material and optimizing the pore structure. The two work synergistically to improve the structural stability and compressive strength of the filter aid during the filtration process, avoiding a decline in filtration performance due to breakage.

[0043] In this invention, when mixing the supported precursor and the modifier, a solvent is first added to the supported precursor, and then mixed with the modifier. The solvent is preferably a mixture of water and ethanol, with a preferred volume ratio of water to ethanol of (1~3):1. The amount of solvent used is 20%~30% of the precursor mass. Stirring ensures the precursor is completely wetted and evenly dispersed. Then, the modifier is added, and the mixture is stirred at a constant temperature of 60~80℃ for 30~40 min, for example, at 60℃, 65℃, 70℃, 75℃, or 80℃ for 30 min, 32 min, 35 min, 38 min, or 40 min, respectively. The hydrolysis of silane coupling agents requires water, but pure water has limited solubility for silane coupling agents and a high surface tension, making it difficult to fully wet the porous structure of the precursor. Ethanol, as a co-solvent, can reduce the surface tension of the solution, improve the wettability of the precursor particles, and simultaneously promote the uniform dispersion of the silane coupling agent in the solution. Furthermore, graphene is a hydrophobic carbon material that easily aggregates in pure water. The addition of ethanol improves the dispersibility of graphene, allowing it to adhere uniformly to the material surface and pores. The water and ethanol mixture at the above volume ratio ensures the complete hydrolysis of the silane coupling agent while achieving uniform dispersion and effective loading of the modifier on the material surface. Heating and stirring promote the adsorption and reaction of the modifier on the material surface. Within the above temperature range, the silane coupling agent hydrolyzes under the action of water provided by the solvent and undergoes a condensation reaction with hydroxyl groups on the material surface, forming stable chemical bonds; graphene can then be uniformly dispersed and adhered to the material surface and pores. After stirring, the mixture is dried at a constant temperature of 70–90°C to constant weight to completely remove the solvent, yielding the modified mixture.

[0044] This invention involves calcining the modified and mixed materials. The purpose of calcination is to further solidify the modified layer, promote the chemical bonding between the silane coupling agent and the material surface, and simultaneously enable the modified layer to form a more stable bond with the substrate.

[0045] In some preferred embodiments, the calcination temperature is 300-350°C, and the calcination time is 1-2 hours, for example, calcination at 300°C, 310°C, 320°C, 330°C, 340°C, or 350°C for 1 hour, 1.2 hours, 1.5 hours, 1.8 hours, or 2 hours. The calcination treatment at 300-350°C removes unreacted physically adsorbed residues and promotes the ring-opening cross-linking reaction of the grafted epoxy groups, forming a stable three-dimensional network structure, while simultaneously enabling a tighter composite between graphene and the modified layer. If the calcination temperature is too low, curing will be insufficient, and the modified layer will not bond firmly; if the calcination temperature is too high, the organic components may be over-carbonized, losing their modifying function.

[0046] After calcination, the product is cooled to room temperature and pulverized to a particle size ≤100 mesh, for example, to 70-80 mesh, 80-100 mesh, 90-100 mesh, or 100 mesh, to obtain a red mud biochar-supported filter aid. Pulverizing to this particle size range facilitates the uniform dispersion of the filter aid in the red mud slurry and also makes it easier to form a uniform filter cake layer during the filtration process.

[0047] This invention also provides a red mud biochar supported filter aid prepared by the above preparation method, which has the following structural features: red mud serves as the carrier skeleton, and biochar is uniformly loaded on the surface and pores of the red mud to form a composite material with a multi-level pore structure; a silane coupling agent is grafted onto the material surface through chemical bonding, and after high-temperature calcination, a stable silicon-oxygen inorganic layer or inorganic hybrid layer is formed; graphene is filled in the pore structure to further optimize the pore structure and mechanical properties of the material.

[0048] This filter aid combines adsorption and filtration functions. Its porous structure provides physical retention and adsorption, while the surface modification layer improves the compatibility of the filter aid with chemical agents. Furthermore, the filter aid has enhanced mechanical strength, enabling it to maintain structural integrity and resist breakage during the filtration process.

[0049] This invention also provides the application of the above-mentioned red mud biochar-supported filter aid in red mud dewatering treatment.

[0050] The present invention also provides a method for dehydrating red mud, comprising the following steps: The above-mentioned red mud biochar-supported filter aid was added to the red mud slurry, mixed evenly, and then filtered and dehydrated.

[0051] This invention does not impose special limitations on the red mud slurry. It can be the red mud underflow obtained after sedimentation and separation during alumina production, or the red mud slurry discharged after multi-stage washing, or the red mud slurry transported to the stockpile via pipeline. In industrial practice, the solids content of red mud slurry is usually 20%~35%, it is highly alkaline, has fine particles, strong hydrophilicity, and poor filtration performance, requiring solid-liquid separation treatment.

[0052] In some preferred embodiments, the amount of the red mud biochar-supported filter aid added is 5-10% of the dry weight of the red mud in the red mud slurry, for example, 5%, 6%, 7%, 8%, 9%, or 10%. The amount of filter aid needs to be adjusted according to the characteristics of the red mud and the dewatering requirements: if the amount is too low, the filter aid effect will be insignificant; if the amount is too high, the filter cake will thicken, the filtration resistance will increase, and the filter aid will be wasted.

[0053] In some preferred embodiments, the red mud slurry further includes flocculants and coagulants. The addition of flocculants and coagulants can synergistically work with filter aids to further optimize the filtration performance of red mud particles.

[0054] The flocculant can be polyacrylamide, and its addition amount is 0.1% to 0.3% of the dry weight of the red mud in the red mud slurry, for example, 0.1%, 0.15%, 0.2%, 0.25%, or 0.3%. Polyacrylamide causes fine red mud particles to agglomerate into larger flocs through bridging, which is beneficial for filtration. If the addition amount is too low, the flocculation effect will be insignificant; if the addition amount is too high, it may cause the slurry viscosity to be too high, which will increase the filtration resistance.

[0055] The coagulant can be selected from polyaluminum chloride and / or polyferric sulfate, and its addition amount is 1-3% of the dry weight of the red mud in the red mud slurry, for example, 1%, 1.5%, 2%, 2.5% or 3%. The coagulant disrupts the surface charge stability of the red mud particles through charge neutralization, promoting particle agglomeration. If the addition amount is too low, the effect will be insignificant; if the addition amount is too high, it may introduce too many impurities.

[0056] After adding flocculant and coagulant aid to the red mud slurry, let it stand for 10-20 minutes to allow the agents to fully exert their effects. Then add red mud biochar-supported filter aid and continue stirring for 20-30 minutes to ensure that the red mud biochar-supported filter aid is evenly dispersed in the red mud slurry.

[0057] Filtration and dewatering can be achieved using solid-liquid separation equipment such as plate and frame filters. The filtration pressure can be 0.3~0.5 MPa, for example, 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, or 0.5 MPa; the filtration temperature can be 25~40℃, for example, 25℃, 30℃, 35℃, or 40℃; and the filtration time can be 30~60 min, for example, 30 min, 40 min, 45 min, 50 min, or 60 min. Under these filtration conditions, efficient filtration and dewatering of red mud can be achieved.

[0058] During the filtration process, the filter aid forms a porous filter cake layer on the surface of the filter cloth, which plays a role in interception and adsorption, reducing filter cloth clogging. At the same time, the filter aid works synergistically with flocculants and coagulants to promote the aggregation of red mud particles and significantly improve the dewatering rate. After filtration, filter cake and filtrate are obtained. The moisture content of the filter cake can be reduced to below 35%, which can be directly used for stockpiling, transportation, or subsequent resource utilization. The filtrate can be used for red mud pretreatment or filter aid preparation, realizing water resource recycling.

[0059] The method of the present invention may further include processing the filter cake obtained after filtration, recovering the undiminished filter aid therein, and regenerating and recycling it. This design significantly reduces the cost of using filter aids and improves the economics of the process.

[0060] Specifically, the filtered filter cake is fed into a crushing device for crushing, and then into a screening machine for separation based on the particle size difference or other physical property differences between the filter aid and the dewatered red mud, to obtain the unused filter aid and dewatered red mud.

[0061] The separated filter aid is fed into a regeneration reactor, where a regenerant is added for regeneration. In some preferred embodiments, the regenerant is a mixture of dilute hydrochloric acid and ammonium chloride. The dilute hydrochloric acid dissolves alkaline substances adsorbed on the surface of the filter aid, while ammonium chloride provides ammonium ions that exchange with the surface adsorption sites, restoring the filter aid's activity. The concentration and amount of the regenerant can be adjusted according to the degree of contamination of the filter aid. For example, an aqueous solution of ammonium chloride and hydrochloric acid can be used, wherein the mass fraction of ammonium chloride is 3-5%, the mass fraction of hydrogen chloride is 1-3%, and the amount of regenerant is 20-30% of the mass of the waste filter aid.

[0062] The regeneration temperature can be 50-60℃, and the regeneration time can be 30-40 minutes. Under these conditions, impurities adsorbed on the surface of the filter aid can be effectively removed, restoring its filter aid activity. After drying and activation, the regenerated filter aid can be recycled for red mud dewatering. Experiments have shown that the filter aid of this invention can be recycled more than 20 times, with an activity recovery rate of over 90% after regeneration, and its filter aid activity remains above 85% after 20 cycles.

[0063] The separated dehydrated red mud can be used to prepare high-value-added products such as building aggregates and soil conditioners, realizing the resource utilization of solid waste.

[0064] The red mud biochar-supported filter aid prepared in this invention has significant advantages over existing filter aids when used for red mud dewatering. Using fresh red mud with a moisture content of approximately 70% as raw material for dewatering, the filter aid of this invention, in conjunction with flocculants and coagulants, achieves a filtration rate of up to 2.5 mL / (min·cm). 2 The moisture content of the filter cake can be reduced to below 35%. Furthermore, the filter aid of this invention can be recycled, further reducing overall treatment costs.

[0065] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0066] Example 1 This embodiment provides a method for preparing a red mud biochar-supported filter aid.

[0067] (1) Raw material pretreatment ① Red mud pretreatment: Take fresh red mud and dry it at 90℃ until the moisture content is 12%, then crush it to 80-100 mesh to obtain homogenized red mud powder.

[0068] ② Biomass pretreatment: Corn stalks and sawdust are mixed at a mass ratio of 3:1, impurities are removed, and the mixture is crushed to 3-4 mm and dried at 80℃ until the moisture content is 8% to obtain pretreated biomass raw materials.

[0069] ③ Mixing and blending: Mix red mud powder and biomass raw materials at a mass ratio of 4:1, add deionized water to adjust the slurry concentration to 40wt%, and stir at a constant temperature of 60℃ for 50 minutes to obtain a homogenized mixed slurry.

[0070] (2) Preparation of supported precursors by co-pyrolysis The mixed slurry was spray-dried, with the inlet air temperature controlled at 200℃ and the outlet air temperature at 90℃, forming mixed particles with a particle size of 0.5-2mm. The mixed particles were then placed in a tube furnace and, under nitrogen protection, pyrolyzed at 300℃ for 2 hours, followed by pyrolysis at 550℃ for 2.5 hours. After cooling, a red mud biochar-supported precursor was obtained. The specific surface area of ​​the obtained precursor was measured to be 360 ​​m². 2 / g, with a porosity of 66%.

[0071] (3) Modification molding Add 25% by mass of a mixed solvent, which is a mixture of deionized water and ethanol in a volume ratio of 2:1, to the precursor and stir until the precursor is completely wetted and evenly dispersed. Then add 3% by mass of a modifier, which is a mixture of silane coupling agent KH-560 and graphene in a mass ratio of 5:1. Stir at 70°C for 35 min, and then dry at 80°C to constant weight to completely remove the mixed solvent, obtaining a modified solid material. Calcine the modified solid material at 320°C for 1.2 h, cool, and then pulverize to 80-100 mesh to obtain a red mud biochar supported filter aid.

[0072] Example 2 This embodiment provides a method for preparing a red mud biochar-supported filter aid.

[0073] (1) Raw material pretreatment ① Red mud pretreatment: Take fresh red mud and dry it at 80℃ until the moisture content is 15%, then crush it to 70-80 mesh to obtain homogenized red mud powder.

[0074] ② Biomass pretreatment: After removing impurities from the corn stalks, crush them to 2-3 mm and dry them at 70℃ until the moisture content is 10% to obtain pretreated biomass raw materials.

[0075] ③ Mixing and blending: Mix red mud powder and biomass raw materials at a mass ratio of 3:1, add deionized water to adjust the slurry concentration to 30wt%, and stir at a constant temperature of 50℃ for 60min to obtain a homogenized mixed slurry.

[0076] (2) Preparation of supported precursors by co-pyrolysis The mixed slurry was spray-dried, with the inlet air temperature controlled at 200℃ and the outlet air temperature at 90℃, forming mixed particles with a particle size of 0.5-2mm. The mixed particles were then placed in a tube furnace and, under nitrogen protection, pyrolyzed at 250℃ for 2.5 hours, followed by pyrolysis at 500℃ for 3 hours. After cooling, a red mud biochar-supported precursor was obtained. The specific surface area of ​​the obtained precursor was measured to be 300 m². 2 / g, with a porosity of 60%.

[0077] (3) Modification molding Add 25% by mass of a mixed solvent, which is a mixture of deionized water and ethanol in a volume ratio of 2:1, to the precursor and stir until the precursor is completely wetted and evenly dispersed. Then add 2% by mass of a modifier, which is a mixture of silane coupling agent KH-560 and graphene in a mass ratio of 5:1. Stir at 60°C for 40 min, and then dry at 80°C to constant weight to completely remove the mixed solvent, obtaining a modified solid material. Calcine the modified solid material at 300°C for 1.5 h, cool, and then pulverize to 70-80 mesh to obtain a red mud biochar supported filter aid.

[0078] Example 3 This embodiment provides a method for preparing a red mud biochar-supported filter aid.

[0079] (1) Raw material pretreatment ① Red mud pretreatment: Take fresh red mud and dry it at 100℃ until the moisture content is 10%, then crush it to 90-100 mesh to obtain homogenized red mud powder.

[0080] ② Biomass pretreatment: After removing impurities, the sawdust is crushed to 4-5mm and dried at 90℃ until the moisture content is 7% to obtain pretreated biomass raw materials.

[0081] ③ Mixing and blending: Mix red mud powder and biomass raw materials at a mass ratio of 5:1, add deionized water to adjust the slurry concentration to 50wt%, and stir at a constant temperature of 70℃ for 40 minutes to obtain a homogenized mixed slurry.

[0082] (2) Preparation of supported precursors by co-pyrolysis The mixed slurry was spray-dried, with the inlet air temperature controlled at 200℃ and the outlet air temperature at 90℃, forming mixed particles with a particle size of 0.5-2mm. The mixed particles were then placed in a tube furnace and, under nitrogen protection, pyrolyzed at 300℃ for 1.5 hours, followed by pyrolysis at 600℃ for 2 hours. After cooling, a red mud biochar-supported precursor was obtained. The specific surface area of ​​the obtained precursor was measured to be 380 m². 2 / g, with a porosity of 68%.

[0083] (3) Modification molding Add 25% by mass of a mixed solvent, which is a mixture of deionized water and ethanol in a volume ratio of 2:1, to the precursor and stir until the precursor is completely wetted and evenly dispersed. Then add 4% by mass of a modifier, which is a mixture of silane coupling agent KH-560 and graphene in a mass ratio of 5:1. Stir at 65°C for 35 min, and then dry at 80°C to constant weight to completely remove the mixed solvent, obtaining a modified solid material. Calcine the modified solid material at 350°C for 1 h, cool, and then pulverize to 90-100 mesh to obtain a red mud biochar supported filter aid.

[0084] Example 4 The difference between this embodiment and Example 1 is that the biomass raw materials in step (1) are different. In this embodiment, corn stalks and rice husks are compounded at a mass ratio of 2:1. The modifier in step (3) is silane coupling agent KH-560, which does not contain graphene. The remaining steps and conditions are the same as in Example 1.

[0085] Example 5 The difference between this embodiment and Embodiment 1 is that in step (2), the temperature is first raised to 280℃ and kept at a constant temperature for 2 hours, and then raised to 520℃ and kept at a constant temperature for 2.5 hours. The remaining steps and conditions are the same as in Embodiment 1.

[0086] Example 6 The difference between this embodiment and Example 1 is that in step (1), the red mud powder and biomass raw materials are mixed at a mass ratio of 4.5:1; and in step (3), the modifier is composed of silane coupling agent KH-560 and graphene at a mass ratio of 1:4. The remaining steps and conditions are the same as in Example 1.

[0087] Comparative Example 1 Compared with Example 1, the difference in this comparative example is that step (3) of modification and molding is not performed, that is, the red mud biochar supported precursor obtained in step (2) is directly used as a filter aid.

[0088] Comparative Example 2 The difference between this comparative example and Example 1 is that this comparative example does not involve adding deionized water to form a slurry and spray drying. Instead, the homogenized red mud powder from step (1) and the pretreated biomass raw material are directly mixed uniformly in a mixer. The mixture is then placed in a tube furnace and, under nitrogen protection, heated to 300°C for 2 hours and then heated to 550°C for 2.5 hours. After cooling, a red mud biochar supported precursor is obtained. The remaining steps and conditions are the same as in Example 1.

[0089] Comparative Example 3 This comparative example uses diatomaceous earth as a filter aid.

[0090] Comparative Example 4 This comparative example uses biochar as a filter aid. The preparation method of biochar is as follows: After removing impurities, corn stalks are pulverized to 2-3 mm and dried at 70℃ to a moisture content of 10% to obtain pretreated biomass raw materials. Then, the corn stalks are pyrolyzed at 550℃ for 2.5 h under nitrogen protection to prepare biochar, which is then pulverized to 80-100 mesh to obtain a single biochar filter aid.

[0091] Comparative Example 5 This comparative example uses activated carbon-supported filter aid as the filter aid.

[0092] The preparation method of activated carbon-supported filter aid is as follows: Commercially available powdered activated carbon with a particle size of 80-100 mesh is used as a carrier. A modifier accounting for 3% of its mass is added to the activated carbon. The modifier is a mixture of silane coupling agent KH-560 and graphene in a mass ratio of 5:1. Then, a mixed solvent accounting for 25% of the activated carbon mass is added. The mixed solvent is a mixture of deionized water and ethanol in a volume ratio of 2:1. The mixture is stirred until it is evenly dispersed. The mixture is stirred at 70°C for 35 min, and then dried at 80°C to constant weight to completely remove the mixed solvent. The mixture is then calcined at 320°C for 1.2 h, cooled, and pulverized to 80-100 mesh to obtain the activated carbon-supported filter aid.

[0093] Comparative Example 6 The difference between this comparative example and Example 1 is that conventional drying is used instead of spray drying in this comparative example, while the remaining steps and conditions are the same as in Example 1. Step (2) of this comparative example is as follows: The mixed slurry was spread evenly on an enamel tray and dried in an oven at 105℃ until the moisture content was ≤10%. The dried material was then mechanically crushed and sieved to obtain mixed particles with a particle size of 0.5-2mm. The mixed particles were placed in a tube furnace and, under nitrogen protection, first heated to 300℃ and kept at that temperature for 2 hours, then heated to 550℃ and kept at that temperature for 2.5 hours. After cooling, a red mud biochar supported precursor was obtained. The specific surface area of ​​the obtained precursor was measured to be 221 m². 2 / g, with a porosity of 48.2%.

[0094] Application Examples 1-13 (1) Add flocculant and coagulant to red mud slurry with a solid content of 30%, let it stand for 20 minutes; then add the filter aid of the example or comparative example, and stir for 30 minutes.

[0095] (2) Filtration and dewatering: The conditioned slurry is fed into a plate and frame filter press and filtered at 25±3℃ for 45 minutes to obtain filter cake and filtrate.

[0096] (3) Filter aid recovery and regeneration: The filter cake is crushed and sieved to separate the unused filter aid; 25% of its mass of regenerator is added to the separated filter aid, wherein the regenerator is an aqueous solution of ammonium chloride and hydrochloric acid, wherein the mass fraction of ammonium chloride is 4% and the mass fraction of hydrogen chloride is 2%; the filter aid is stirred and regenerated at 55°C for 35 min, and after drying and activation, the regenerated filter aid is obtained.

[0097] See Table 1 for specific parameter settings.

[0098] Table 1 Specific parameter settings for application examples 1-13

[0099] Note: In Table 1, the amounts of filter aid, flocculant, and coagulant added refer to their percentages relative to the dry mass of red mud in the red mud slurry.

[0100] The test results of the above application examples are shown in Table 2.

[0101] Table 2 Test results of application examples 1-13

[0102] Note: In Table 2, the calculation method for the filter aid activity recovery rate is as follows: Filter aid activity recovery rate = (Initial moisture content of the red mud slurry to be dewatered - Measured filter cake moisture content of the regenerated filter aid) / (Initial moisture content of the red mud slurry to be dewatered - Reference filter cake moisture content of the fresh filter aid) × 100%. The regenerated filter aid and the fresh filter aid were tested under completely identical test conditions for red mud dewatering experiments.

[0103] As shown in Table 2, the red mud biochar-supported filter aids of Examples 1-6 of this invention exhibit excellent filtration and regeneration performance. The diatomaceous earth filter aid of Comparative Example 3 suffers from severe filter cloth clogging and high filtration energy consumption during application. The biochar filter aid of Comparative Example 4 is easily broken, affecting dewatering efficiency and exhibiting poor regeneration performance. The activated carbon-supported filter aid of Comparative Example 5 has a high cost and poor compatibility with flocculants and coagulants. The red mud biochar-supported filter aids of Examples 1-6 of this invention demonstrate superior dewatering performance and economic efficiency compared to existing filter aids, and can achieve "waste-to-waste treatment."

[0104] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a red mud biochar-supported filter aid, characterized in that, Includes the following steps: The dried and pulverized red mud was mixed with biomass raw materials to form a slurry, which was then spray-dried and co-pyrolyzed in sequence to obtain a supported precursor; the mass ratio of the red mud to the biomass raw materials was (3~5):

1. The supported precursor is mixed with a modifier, the modifier including a silane coupling agent; the mass of the modifier is 2-4% of the mass of the supported precursor; calcination yields the red mud biochar supported filter aid.

2. The preparation method according to claim 1, characterized in that, The inlet air temperature of the spray dryer is 180~220℃, and the outlet air temperature is 80~100℃.

3. The preparation method according to claim 1, characterized in that, The co-pyrolysis reaction specifically includes: first heating to 250~300℃ for pyrolysis for 1.5~2.5h, and then heating to 500~600℃ for pyrolysis for 2~3h; The roasting temperature is 300~350℃, and the roasting time is 1~2h.

4. The preparation method according to claim 1, characterized in that, The silane coupling agent includes at least one of γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, vinyltrimethoxysilane, and methacryloxypropyltrimethoxysilane; The modifier also includes graphene, and the mass ratio of silane coupling agent to graphene is 1:(0.1~5).

5. The preparation method according to claim 1, characterized in that, When the supported precursor is mixed with the modifier, a solvent is first added to the supported precursor, and then it is mixed with the modifier; the solvent is a mixed solution of water and ethanol, and the volume ratio of water to ethanol is (1~3):

1.

6. The red mud biochar supported filter aid prepared by the preparation method according to any one of claims 1 to 5.

7. The application of the red mud biochar-supported filter aid as described in claim 6 in the dewatering treatment of red mud.

8. A method for dewatering red mud, characterized in that, Includes the following steps: The red mud biochar-supported filter aid described in claim 6 is added to the red mud slurry, mixed evenly, and then filtered and dehydrated.

9. The red mud dewatering method as described in claim 8, characterized in that, The amount of the red mud biochar-supported filter aid added is 5-10% of the dry weight of the red mud in the red mud slurry.

10. The red mud dewatering method as described in claim 8, characterized in that, The red mud slurry also includes flocculants and coagulants. The amount of flocculants added is 0.1 to 0.3% of the dry weight of red mud in the red mud slurry; the amount of coagulants added is 1 to 3% of the dry weight of red mud in the red mud slurry.

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