A method for preparing a fluorine-free exothermic and heat-insulating riser

Abstract

This invention discloses a method for preparing a fluorine-free heating and insulating riser, relating to the field of casting technology. The method includes first modifying micron-nano composite aluminum powder with a silane coupling agent, then in-situ reducing and loading zero-valent iron-cerium dioxide catalytic sites, followed by sequentially coating with phenolic resin and a potassium nitrate-manganese dioxide composite oxygen source, and finally forming a silica shell through interfacial hydrolysis and condensation to obtain a core-shell structured composite heating agent. Separately, hollow aluminosilicate microspheres are etched to create pores, then vacuum-impregnated and filled with an organic-inorganic composite aerogel, followed by hydrophobic modification and coating with nano-alumina sol to obtain a composite insulating material. The heating agent is then mixed with the insulating material, mullite powder, corundum powder, expandable graphite, waste glass powder, and a binder, and the mixture is vibrated, compacted, and allowed to cure to obtain the finished product. This invention solves the problems of difficult heating and poor insulation in fluorine-free risers by constructing a multi-level core-shell heating structure and a porous, multi-level insulating structure.

Description

Technical Field

[0001] This invention relates to the field of casting technology, specifically to a method for preparing a fluorine-free exothermic and heat-insulating riser. Background Technology

[0002] In metal casting production, the liquid and solidification shrinkage during the cooling process of castings easily leads to defects such as shrinkage cavities and porosity. Risers, as core components that compensate for shrinkage and ensure the quality of castings, are an indispensable part of the casting process. Among them, heat-generating and heat-insulating risers combine heat generation and insulation functions, significantly extending the liquid state holding time of molten metal and improving feeding efficiency, and have become the mainstream application solution in the industry. Traditional heat-generating and heat-insulating risers often add fluorides as fluxing activators. Fluorine not only volatilizes and produces harmful gases, enriching and polluting recycled molding sand, but also easily causes fish-eye defects in castings and affects spheroidization, contradicting the requirements of green casting development. Therefore, fluorine-free heat-generating and heat-insulating risers have become a research hotspot. Existing technologies mostly use fluorine-free salts to replace fluorides, achieving fluorine-free formulations and solving the environmental and casting quality risks associated with fluorine-containing risers. However, existing fluorine-free risers, due to the removal of fluorides, lose their fluxing activation and reaction control functions, and their core heat generation and insulation performance have significant shortcomings, hindering their large-scale promotion. In terms of thermal performance, the increased activation energy of the aluminothermic reaction easily leads to problems such as difficulty in ignition and unstable combustion. Furthermore, the thermal systems are mostly simple physical mixtures, resulting in uncontrollable reaction processes, short durations of high temperatures, and low thermal efficiency. Regarding insulation performance, conventional insulation materials such as expanded perlite are prone to shrinkage and collapse under the high temperatures of casting, causing damage to their pore structure and a sharp increase in thermal conductivity. Simultaneously, poor compatibility between the insulation material and the system interface further degrades insulation performance, making it difficult to achieve long-term thermal insulation. Therefore, specifically improving the thermal and insulation performance of fluorine-free thermal insulation risers is a pressing technical problem that needs to be solved in the field of casting materials. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing a fluorine-free heating and heat-insulating riser to solve the technical problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] A method for preparing a fluorine-free exothermic and heat-insulating riser includes the following steps:

[0006] (1) Micron-sized spherical aluminum powder and nano-sized aluminum powder were ultrasonically dispersed in anhydrous ethanol and then grafted with silane coupling agent KH560 to obtain a surface-modified aluminum powder dispersion.

[0007] (2) Add anhydrous ferric chloride and nano-cerium dioxide to the dispersion in step (1), and carry out an in-situ reduction reaction with anhydrous ethanol solution of sodium borohydride in an ice-water bath under nitrogen protection to obtain an aluminum powder dispersion loaded with active sites.

[0008] (3) Add thermoplastic phenolic resin prepolymer, nano potassium nitrate and nano manganese dioxide sequentially to the dispersion in step (2) for coating treatment, and obtain pre-coated heating agent powder after drying.

[0009] (4) Disperse the pre-coated heating agent powder in hexane, add Span-80 and tetraethyl orthosilicate hydrolysate mixture to carry out interfacial hydrolysis and polycondensation reaction to obtain core-shell structured composite heating agent;

[0010] (5) After calcining, porous hollow aluminum silicate ceramic microspheres are etched with sodium hydroxide solution to obtain hydrophilic surface porous hollow microspheres.

[0011] (6) Hollow microspheres with pores are vacuum impregnated, gelled and aged in a mixed system of acidic silica sol, water-soluble methyl phenolic resin prepolymer and p-toluenesulfonic acid to obtain wet gel-filled microspheres.

[0012] (7) After the wet gel-filled microbeads were replaced with isopropanol solvent, they were hydrophobically modified with a hexane solution of trimethylchlorosilane to obtain hydrophobic aerogel-filled microbeads.

[0013] (8) After coating the hydrophobic aerogel-filled microspheres with nano-alumina sol and heat-treating them, the composite thermal insulation material is obtained by fumigation with ammonia gas to remove acid.

[0014] (9) Dry mix the composite insulation material with mullite powder, fused white corundum powder, fluorine-free reaction regulator, neutral expandable graphite, and low-melting-point borosilicate waste glass powder, then add the core-shell structure composite heating agent from step (4) and mix well. Finally, add alkaline phenolic resin binder, curing agent and silane coupling agent KH560 and mix well to obtain riser blank.

[0015] (10) The riser blank from step (9) is vibrated and compacted, and then allowed to stand at room temperature to solidify, to obtain a fluorine-free heat-generating and heat-insulating riser.

[0016] In the technical solution of this invention, (1) the principle of improving the heating effect of the fluorine-free riser is as follows: First, the micron-nano composite aluminum powder is modified by silane coupling agent to eliminate the reaction obstacle of the inert oxide layer on the surface of the aluminum powder, providing a uniform anchoring point for the catalytic site, while avoiding the risk of hydrogen evolution when the nano aluminum powder comes into contact with water. The reaction contact area of ​​the heating system is increased by the micron-nano composite structure. Then, in an anhydrous and oxygen-free system, sodium borohydride is used for gentle reduction to anchor the nano zero-valent iron-cerium dioxide binary synergistic catalytic site on the surface of the modified aluminum powder in situ, which greatly reduces the activation energy of the aluminothermic reaction and solves the problems of difficult ignition and insufficient heating of the fluorine-free system. Then, the aluminum powder fuel and the phenolic resin are pre-coated to achieve the effect of fluorine-free fuel and fluorine-free heating. The spatial physical isolation of the potassium nitrate-manganese dioxide composite oxygen source prevents premature contact and side reactions between the fuel and oxygen source during storage and mixing. At the same time, the oxygen release rate can be controlled by the thermal decomposition rate of the resin film, regulating the aluminothermic reaction process to prevent early deflagration. Finally, a porous silica dry gel network is generated in situ on the outer layer of a single particle using a reverse microemulsion system, completely isolating it from air and preventing oxidation. During the high-temperature service period of molten metal casting, the gel layer melts and transforms into a dense ceramic shell in situ, further controlling the reaction rate and preventing the shell from blocking the feeding channels. The resulting multi-level core-shell structure enables the stable triggering and controllable continuous execution of the exothermic reaction, significantly extending the duration of high temperature. (2) The principle of improving the insulation effect of fluorine-free risers is as follows: First, through high-temperature calcination and mild etching with dilute alkali, mesoporous channels connecting the inner cavity are constructed on the surface of high-temperature resistant hollow aluminum silicate ceramic microspheres, exposing active hydrophilic silanol groups, while completely preserving the high strength and high softening temperature of the matrix, thus eliminating the high-temperature shrinkage and collapse defects from the root; then, with the help of hydrophilic capillary force and vacuum negative pressure impregnation, silica sol-phenolic resin is completely introduced into the inner cavity of the microspheres, and in-situ solidification is used to form an organic-inorganic interpenetrating network composite wet gel. Through isopropanol solvent replacement and trimethylchlorosilane grafting modification, the capillary shrinkage stress during the normal pressure drying process is eliminated to avoid The aerogel structure collapses, simultaneously achieving overall superhydrophobicity of the material. This prevents water-based binders from seeping in and damaging the insulation structure during riser molding, successfully constructing a multi-level insulation structure consisting of an air cavity, a low thermal conductivity composite aerogel, and a ceramic microsphere shell. Finally, the outermost layer is coated with alcohol-based nano-alumina sol to form a refractory layer to resist molten metal corrosion. Ammonia fumigation completely neutralizes the residual strong acid in the system, eliminating toxic interference to the subsequent alkaline phenolic resin curing system and ensuring process compatibility. Ultimately, the insulation material maintains an intact insulation skeleton and stable low thermal conductivity even at high casting temperatures, effectively delaying the solidification of molten metal inside the riser.

[0017] Preferably, in step (1), the mass ratio of micron-sized spherical aluminum powder to nano-sized aluminum powder is 25:(8-12).

[0018] Preferably, in step (2), the mass ratio of anhydrous ferric chloride to nano-cerium dioxide is 5:(0.5-1.5).

[0019] Preferably, in step (3), the mass ratio of the thermoplastic phenolic resin prepolymer, nano potassium nitrate, and nano manganese dioxide is 1.5:(4-6):(2-3).

[0020] Preferably, in step (5), the calcination temperature is 640-650℃ and the calcination time is 2-3h.

[0021] Preferably, in step (6), the mass ratio of water-soluble methyl phenolic resin prepolymer to p-toluenesulfonic acid is 2.5:(1-2).

[0022] Preferably, in step (8), the mass ratio of hydrophobic aerogel-filled microspheres to nano-alumina sol is 3:(9-10).

[0023] Preferably, in step (9), the mass ratio of the composite insulation material, mullite powder, and fused white corundum powder is 30:(9-10):(4-5).

[0024] Preferably, in step (9), the mass ratio of neutral expandable graphite to low-melting-point borosilicate waste glass powder is 1.2:(2-3).

[0025] The present invention found in experiments that when a highly active core-shell composite exothermic agent is used in combination with highly thermally insulating hydrophobic hollow ceramic microspheres, the extremely high temperature gradient and violent volume expansion generated at the moment of ignition of the exothermic agent will produce extreme mechanical shear stress and thermal shock at the interface between the rigid exothermic phase and the brittle insulating phase. This can easily lead to the large-area compression and rupture of the hollow microsphere skeleton and the breakage of the resin bonding bridge, resulting in microcracks. This can then cause the molten metal to invade along the cracks and completely destroy the long-term insulating structure. To address the problem of insulation failure caused by extreme thermal stress concentration at the interface, this invention introduces neutral expandable graphite and low-melting-point borosilicate waste glass during the riser blank mixing stage. During service, when the riser is in contact with high-temperature molten metal, a small amount of neutral expandable graphite rapidly expands when heated to 300–500°C, forming a loose, porous, worm-like flexible buffer layer in situ. This layer, like a miniature airbag, perfectly absorbs the localized volume expansion stress caused by the intense reaction of the exothermic agent, effectively protecting the fragile ceramic microspheres from crushing. Simultaneously, its neutral nature avoids the toxic interference of residual acid on the alkaline phenolic resin curing system. As the local temperature rises... As the temperature continues to rise to 600-800℃, the low-melting-point waste glass powder begins to melt and form a liquid glass with a certain viscosity. This liquid phase can not only quickly coat the surface of expanded graphite to form an anti-oxidation armor to ensure its continued elastic buffering effect, but also act as a high-temperature liquid adhesive. Under capillary action, it can quickly penetrate and fill any new microcracks caused by thermal shock in situ. Through the synergistic effect of this flexible buffering and stress absorption and the dynamic physical phase transformation of the high-temperature liquid phase self-healing cracks, the problem of cracking failure caused by extreme thermal stress at the interface between the heating phase and the insulation phase is completely solved at a very low cost, achieving efficient heating and long-term insulation performance.

[0026] Preferably, in step (9), the mass ratio of the composite insulation material to the core-shell composite heating agent is 30:(38-42).

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 1. Through a multi-level core-shell structure design, the aluminothermic reaction is stably triggered and continuously carried out, significantly extending the high-temperature service life; 2. A multi-level thermal insulation structure is constructed to ensure the integrity and stability of the thermal insulation skeleton at high temperatures, effectively delaying the solidification of molten metal; 3. By utilizing flexible buffering and high-temperature self-healing mechanisms, interfacial thermal stress cracking is eliminated, achieving a balance between efficient heat generation and long-term heat preservation. Attached Figure Description

[0029] Figure 1 This is a low-magnification SEM image of the surface of the fluorine-free heating and heat-insulating riser prepared in Example 1 of the present invention.

[0030] Figure 2 This is a magnified SEM image of the surface of the fluorine-free heating and heat-insulating riser prepared in Example 1 of the present invention.

[0031] Figure 3 This is a high-magnification SEM image of the surface of the fluorine-free heating and heat-insulating riser prepared in Example 1 of the present invention.

[0032] Figure 4 This is the XPS test spectrum of the surface of the fluorine-free heating and heat-insulating riser prepared in Example 1 of the present invention.

[0033] Figure 5 The XRD pattern of the fluorine-free exothermic and heat-insulating riser prepared in Example 1 of this invention is shown. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] A method for preparing a fluorine-free exothermic and heat-insulating riser includes the following steps:

[0037] (1) Take 25g of micron-sized spherical aluminum powder (150 mesh) and 11g of nano-sized aluminum powder (80nm) and add them to 200mL of anhydrous ethanol. Place them in an ultrasonic cleaner and ultrasonically disperse them for 25min at 200W power and room temperature. Prepare a weakly acidic pre-hydrolyzed solution of silane coupling agent KH560: mix 0.8g of KH560, 5mL of anhydrous ethanol, 0.2mL of deionized water and 1 drop of glacial acetic acid and stir at room temperature for 30min. Add all the prepared pre-hydrolyzed solution to the above aluminum powder dispersion, heat the system to 45℃, and stir at a constant temperature of 300r / min for 60min to obtain a surface-modified aluminum powder dispersion.

[0038] (2) Add 5g of anhydrous ferric chloride to the dispersion obtained in step (1) and stir until completely dissolved. Then add 1.2g of nano-cerium dioxide powder. Transfer the system to a three-necked flask equipped with a condenser and an exhaust device. Purge the system with high-purity nitrogen (50mL / min) for 15min to exhaust the oxygen in the system. Then place the reaction vessel in an ice-water bath and control the temperature at 3℃. While stirring at 200r / min, add 100mL of anhydrous ethanol solution containing 2.5g of sodium borohydride dropwise at a rate of 1mL / min. After the addition is completed, continue stirring under ice-water bath conditions for 75min to obtain an aluminum powder dispersion loaded with active sites.

[0039] (3) Add 1.5g of thermoplastic phenolic resin prepolymer (50% solid content, ethanol solvent) to the dispersion in step (2), heat to 60℃, and stir at 300r / min for 45min; then cool the system to 25℃, slowly add 5.5g of nano potassium nitrate (particle size 100nm) and 2.8g of nano manganese dioxide (particle size 50nm), and mix at a low speed of 150r / min for 30min; after mixing, perform vacuum filtration, and dry the filter cake in a vacuum drying oven at 50℃ for 6h to obtain pre-coated exothermic agent powder.

[0040] (4) Disperse the pre-coated exothermic powder obtained in step (3) into 150 mL of n-hexane, add 0.5 g of surfactant Span-80, and stir at 300 r / min for 10 min; then prepare a hydrolyzed mixture of tetraethyl orthosilicate: mix 4 g of tetraethyl orthosilicate, 1.5 mL of deionized water and 0.5 mL of glacial acetic acid evenly; add the hydrolyzed mixture dropwise to the reaction system, slowly heat to 40 °C and stir for 120 min; after the reaction is completed, filter, wash the filter cake twice with n-hexane, dry in a vacuum drying oven at 60 °C for 12 h, and sieve with a 180 mesh standard sieve to obtain the core-shell structured composite exothermic agent.

[0041] (5) Take 30g of porous hollow aluminum silicate ceramic microspheres (50 mesh), place them in a corundum crucible, put them in a box-type muffle furnace and heat them to 645℃ at a heating rate of 5℃ / min. After constant temperature calcination for 2.5h, let them cool naturally to room temperature. Add the calcined microspheres to 150mL of sodium hydroxide aqueous solution with a concentration of 0.5mol / L, stir and etch for 30min at 60℃ and 200r / min. After completion, immediately vacuum filter and wash repeatedly with deionized water until the washing solution is neutral. Place them in an 80℃ vacuum drying oven and dry for 8h to obtain hydrophilic surface pore-forming hollow microspheres.

[0042] (6) The hollow microspheres obtained in step (5) are added to a mixture consisting of 150 mL of acidic silica sol (pH=3, solid content 30%), 2.5 g of water-soluble methyl phenolic resin prepolymer and 1.8 g of p-toluenesulfonic acid; the system is placed in a vacuum impregnation vessel and kept under negative pressure of -0.09 MPa for 20 min, then restored to normal pressure and stirred at 250 r / min for 30 min, then stopped stirring and allowed to stand at room temperature for 75 min to gel, and then heated to 50 °C and aged for 150 min to obtain wet gel-filled microspheres.

[0043] (7) The wet gel-filled microspheres obtained in step (6) were immersed in 200 mL of isopropanol and allowed to stand for 12 h for solvent replacement; then the microspheres were transferred to a mixed solution containing 15 mL of trimethylchlorosilane and 150 mL of n-hexane and modified by constant temperature reflux at 50 °C for 24 h; after modification, the microspheres were filtered and placed in a 60 °C forced air drying oven to dry at normal pressure for 8 h to obtain hydrophobic aerogel-filled microspheres.

[0044] (8) Disperse 30g of hydrophobic aerogel-filled microbeads into 98g of alcohol-based nano-alumina sol with a solid content of 20%, stir at 200r / min for 40min, vacuum filter, and dry in a forced-air drying oven at 80℃ and normal pressure for 6h; after drying, transfer to a box-type muffle furnace and heat treat at 150℃ for 2h. After natural cooling, place the microbeads in a sealed container, fumigate with ammonia gas for 30min, and sieve with a 120-mesh standard sieve to obtain composite thermal insulation material.

[0045] (9) Take 30g of the composite insulation material obtained in step (8), 9.7g of mullite powder, and 4.8g of fused white corundum powder, and prepare 4.5g of fluorine-free reaction regulator (prepared by uniformly mixing 2.7g of sodium chloride, 0.9g of sodium sulfate, and 0.9g of sodium borate). Add the above materials together with 1.2g of neutral expandable graphite and 2.7g of low-melting-point borosilicate waste glass powder (D50=20μm, softening point 550℃) into a small spherical sand mixer in the laboratory. Dry mix at 40 r / min for 2.5 min; then add 41 g of the core-shell composite exothermic agent obtained in step (4) and continue mixing for 1.2 min; prepare 1.6 g of liquid composite organic ester curing agent (prepared by uniformly mixing 1.07 g of glycerol acetate and 0.53 g of ethylene glycol diacetate), and finally add 5.5 g of pure alkaline phenolic resin binder, 1.6 g of the prepared liquid composite organic ester curing agent, and 0.04 g of silane coupling agent KH560 to the sand mixer, maintain the speed and continue mixing for 3.5 min, and discharge the riser blank.

[0046] (10) Add the riser blank obtained in step (9) into a standard cylindrical riser mold, place it on a vibration table and compact it for 2 minutes under the conditions of 0.5 mm amplitude and 50 Hz frequency, and then let it stand at room temperature for 25 minutes to demold; place the demolded riser blank in a room temperature and 50% relative humidity environment to cure for 24 hours, and finally obtain a fluorine-free heat-generating and heat-insulating riser.

[0047] Example 2

[0048] A method for preparing a fluorine-free exothermic and heat-insulating riser includes the following steps:

[0049] (1) Take 25g of micron-sized spherical aluminum powder (150 mesh) and 9g of nano-sized aluminum powder (80nm) and add them to 200mL of anhydrous ethanol. Place them in an ultrasonic cleaner and ultrasonically disperse them for 25min at 200W power and room temperature. Prepare a weakly acidic pre-hydrolyzed solution of silane coupling agent KH560: mix 0.8g of KH560, 5mL of anhydrous ethanol, 0.2mL of deionized water and 1 drop of glacial acetic acid and stir at room temperature for 30min. Add all the prepared pre-hydrolyzed solution to the above aluminum powder dispersion, heat the system to 45℃, and stir at a constant temperature of 300r / min for 60min to obtain a surface-modified aluminum powder dispersion.

[0050] (2) Add 5g of anhydrous ferric chloride to the dispersion obtained in step (1) and stir until completely dissolved. Then add 0.8g of nano-cerium dioxide powder. Transfer the system to a three-necked flask equipped with a condenser and an exhaust device. Purge the system with high-purity nitrogen (50mL / min) for 15min to exhaust the oxygen in the system. Then place the reaction vessel in an ice-water bath and control the temperature at 3℃. While stirring at 200r / min, add 100mL of anhydrous ethanol solution containing 2.5g of sodium borohydride dropwise at a rate of 1mL / min. After the addition is completed, continue stirring under ice-water bath conditions for 75min to obtain an aluminum powder dispersion loaded with active sites.

[0051] (3) Add 1.5g of thermoplastic phenolic resin prepolymer (50% solid content, ethanol solvent) to the dispersion in step (2), heat to 60℃, and stir at 300r / min for 45min; then cool the system to 25℃, slowly add 4.5g of nano potassium nitrate (particle size 100nm) and 2.3g of nano manganese dioxide (particle size 50nm), and mix at a low speed of 150r / min for 30min; after mixing, perform vacuum filtration, and dry the filter cake in a vacuum drying oven at 50℃ for 6h to obtain pre-coated exothermic agent powder.

[0052] (4) Disperse the pre-coated exothermic powder obtained in step (3) into 150 mL of n-hexane, add 0.5 g of surfactant Span-80, and stir at 300 r / min for 10 min; then prepare a hydrolyzed mixture of tetraethyl orthosilicate: mix 4 g of tetraethyl orthosilicate, 1.5 mL of deionized water and 0.5 mL of glacial acetic acid evenly; add the hydrolyzed mixture dropwise to the reaction system, slowly heat to 40 °C and stir for 120 min; after the reaction is completed, filter, wash the filter cake twice with n-hexane, dry in a vacuum drying oven at 60 °C for 12 h, and sieve with a 180 mesh standard sieve to obtain the core-shell structured composite exothermic agent.

[0053] (5) Take 30g of porous hollow aluminum silicate ceramic microspheres (50 mesh), place them in a corundum crucible, put them in a box-type muffle furnace and heat them to 645℃ at a heating rate of 5℃ / min. After constant temperature calcination for 2.5h, let them cool naturally to room temperature. Add the calcined microspheres to 150mL of sodium hydroxide aqueous solution with a concentration of 0.5mol / L, stir and etch for 30min at 60℃ and 200r / min. After completion, immediately vacuum filter and wash repeatedly with deionized water until the washing solution is neutral. Place them in an 80℃ vacuum drying oven and dry for 8h to obtain hydrophilic surface pore-forming hollow microspheres.

[0054] (6) The hollow microspheres obtained in step (5) are added to a mixture consisting of 150 mL of acidic silica sol (pH=3, solid content 30%), 2.5 g of water-soluble methyl phenolic resin prepolymer and 1.3 g of p-toluenesulfonic acid; the system is placed in a vacuum impregnation vessel and kept under negative pressure of -0.09 MPa for 20 min, then restored to normal pressure and stirred at 250 r / min for 30 min, then stopped stirring and allowed to stand at room temperature for 75 min to gel, and then heated to 50 °C and aged for 150 min to obtain wet gel-filled microspheres.

[0055] (7) The wet gel-filled microspheres obtained in step (6) were immersed in 200 mL of isopropanol and allowed to stand for 12 h for solvent replacement; then the microspheres were transferred to a mixed solution containing 15 mL of trimethylchlorosilane and 150 mL of n-hexane and modified by constant temperature reflux at 50 °C for 24 h; after modification, the microspheres were filtered and placed in a 60 °C forced air drying oven to dry at normal pressure for 8 h to obtain hydrophobic aerogel-filled microspheres.

[0056] (8) Disperse 30g of hydrophobic aerogel-filled microbeads into 92g of alcohol-based nano-alumina sol with a solid content of 20%, stir at 200r / min for 40min, vacuum filter, and dry in a forced-air drying oven at 80℃ and normal pressure for 6h; after drying, transfer to a box-type muffle furnace and heat treat at 150℃ for 2h. After natural cooling, place the microbeads in a sealed container, fumigate with ammonia gas for 30min, and sieve with a 120-mesh standard sieve to obtain composite thermal insulation material.

[0057] (9) Take 30g of the composite insulation material obtained in step (8), 9.3g of mullite powder, and 4.2g of fused white corundum powder, and prepare 4.5g of fluorine-free reaction regulator (prepared by uniformly mixing 2.7g of sodium chloride, 0.9g of sodium sulfate, and 0.9g of sodium borate). Add the above materials together with 1.2g of neutral expandable graphite and 2.4g of low-melting-point borosilicate waste glass powder (D50=20μm, softening point 550℃) into a small spherical sand mixer in the laboratory. Dry mix at 40 r / min for 2.5 min; then add 39 g of the core-shell composite exothermic agent obtained in step (4) and continue mixing for 1.2 min; prepare 1.6 g of liquid composite organic ester curing agent (prepared by uniformly mixing 1.07 g of glycerol acetate and 0.53 g of ethylene glycol diacetate), and finally add 5.5 g of pure alkaline phenolic resin binder, 1.6 g of the prepared liquid composite organic ester curing agent, and 0.04 g of silane coupling agent KH560 to the sand mixer, maintain the speed and continue mixing for 3.5 min, and discharge the riser blank.

[0058] (10) Add the riser blank obtained in step (9) into a standard cylindrical riser mold, place it on a vibration table and compact it for 2 minutes under the conditions of 0.5 mm amplitude and 50 Hz frequency, and then let it stand at room temperature for 25 minutes to demold; place the demolded riser blank in a room temperature and 50% relative humidity environment to cure for 24 hours, and finally obtain a fluorine-free heat-generating and heat-insulating riser.

[0059] Example 3

[0060] A method for preparing a fluorine-free exothermic and heat-insulating riser includes the following steps:

[0061] (1) Take 25g of micron-sized spherical aluminum powder (150 mesh) and 10g of nano-sized aluminum powder (80nm) and add them to 200mL of anhydrous ethanol. Place them in an ultrasonic cleaner and ultrasonically disperse them for 25min at 200W power and room temperature. Prepare a weakly acidic pre-hydrolyzed solution of silane coupling agent KH560: mix 0.8g of KH560, 5mL of anhydrous ethanol, 0.2mL of deionized water and 1 drop of glacial acetic acid and stir at room temperature for 30min. Add all the prepared pre-hydrolyzed solution to the above aluminum powder dispersion, heat the system to 45℃, and stir at a constant temperature of 300r / min for 60min to obtain a surface-modified aluminum powder dispersion.

[0062] (2) Add 5g of anhydrous ferric chloride to the dispersion obtained in step (1) and stir until completely dissolved. Then add 1.0g of nano-cerium dioxide powder. Transfer the system to a three-necked flask equipped with a condenser and an exhaust device. Purge the system with high-purity nitrogen (50mL / min) for 15min to exhaust the oxygen in the system. Then place the reaction vessel in an ice-water bath and control the temperature at 3℃. While stirring at 200r / min, add 100mL of anhydrous ethanol solution containing 2.5g of sodium borohydride dropwise at a rate of 1mL / min. After the addition is completed, continue stirring under ice-water bath conditions for 75min to obtain an aluminum powder dispersion loaded with active sites.

[0063] (3) Add 1.5g of thermoplastic phenolic resin prepolymer (50% solid content, ethanol solvent) to the dispersion in step (2), heat to 60°C, and stir at 300r / min for 45min; then cool the system to 25°C, slowly add 5g of nano potassium nitrate (100nm particle size) and 2.5g of nano manganese dioxide (50nm particle size), and mix at a low speed of 150r / min for 30min; after mixing, perform vacuum filtration, and dry the filter cake in a vacuum drying oven at 50°C for 6h to obtain pre-coated exothermic agent powder.

[0064] (4) Disperse the pre-coated exothermic powder obtained in step (3) into 150 mL of n-hexane, add 0.5 g of surfactant Span-80, and stir at 300 r / min for 10 min; then prepare a hydrolyzed mixture of tetraethyl orthosilicate: mix 4 g of tetraethyl orthosilicate, 1.5 mL of deionized water and 0.5 mL of glacial acetic acid evenly; add the hydrolyzed mixture dropwise to the reaction system, slowly heat to 40 °C and stir for 120 min; after the reaction is completed, filter, wash the filter cake twice with n-hexane, dry in a vacuum drying oven at 60 °C for 12 h, and sieve with a 180 mesh standard sieve to obtain the core-shell structured composite exothermic agent.

[0065] (5) Take 30g of porous hollow aluminum silicate ceramic microspheres (50 mesh), place them in a corundum crucible, put them in a box-type muffle furnace and heat them to 645℃ at a heating rate of 5℃ / min. After constant temperature calcination for 2.5h, let them cool naturally to room temperature. Add the calcined microspheres to 150mL of sodium hydroxide aqueous solution with a concentration of 0.5mol / L, stir and etch for 30min at 60℃ and 200r / min. After completion, immediately vacuum filter and wash repeatedly with deionized water until the washing solution is neutral. Place them in an 80℃ vacuum drying oven and dry for 8h to obtain hydrophilic surface pore-forming hollow microspheres.

[0066] (6) The hollow microspheres obtained in step (5) are added to a mixture consisting of 150 mL of acidic silica sol (pH=3, solid content 30%), 2.5 g of water-soluble methyl phenolic resin prepolymer and 1.5 g of p-toluenesulfonic acid; the system is placed in a vacuum impregnation vessel and kept under negative pressure of -0.09 MPa for 20 min, then restored to normal pressure and stirred at 250 r / min for 30 min, then stopped stirring and allowed to stand at room temperature for 75 min to gel, and then heated to 50 °C and aged for 150 min to obtain wet gel-filled microspheres.

[0067] (7) The wet gel-filled microspheres obtained in step (6) were immersed in 200 mL of isopropanol and allowed to stand for 12 h for solvent replacement; then the microspheres were transferred to a mixed solution containing 15 mL of trimethylchlorosilane and 150 mL of n-hexane and modified by constant temperature reflux at 50 °C for 24 h; after modification, the microspheres were filtered and placed in a 60 °C forced air drying oven to dry at normal pressure for 8 h to obtain hydrophobic aerogel-filled microspheres.

[0068] (8) Disperse 30g of hydrophobic aerogel-filled microbeads into 95g of alcohol-based nano-alumina sol with a solid content of 20%, stir at 200r / min for 40min, vacuum filter, and dry in a forced-air drying oven at 80℃ and normal pressure for 6h; after drying, transfer to a box-type muffle furnace and heat-treat at 150℃ for 2h. After natural cooling, place the microbeads in a sealed container, fumigate with ammonia gas for 30min, and sieve with a 120-mesh standard sieve to obtain composite thermal insulation material.

[0069] (9) Take 30g of the composite insulation material obtained in step (8), 9.5g of mullite powder, and 4.5g of fused white corundum powder, and prepare 4.5g of fluorine-free reaction regulator (prepared by uniformly mixing 2.7g of sodium chloride, 0.9g of sodium sulfate, and 0.9g of sodium borate). Add the above materials together with 1.2g of neutral expandable graphite and 2.5g of low-melting-point borosilicate waste glass powder (D50=20μm, softening point 550℃) into a small spherical sand mixer in the laboratory. Dry mix at 40 r / min for 2.5 min; then add 40 g of the core-shell composite exothermic agent obtained in step (4) and continue mixing for 1.2 min; prepare 1.6 g of liquid composite organic ester curing agent (prepared by uniformly mixing 1.07 g of glycerol acetate and 0.53 g of ethylene glycol diacetate), and finally add 5.5 g of pure alkaline phenolic resin binder, 1.6 g of the prepared liquid composite organic ester curing agent, and 0.04 g of silane coupling agent KH560 to the sand mixer, maintain the speed and continue mixing for 3.5 min, and discharge the riser blank.

[0070] (10) Add the riser blank obtained in step (9) into a standard cylindrical riser mold, place it on a vibration table and compact it for 2 minutes under the conditions of 0.5 mm amplitude and 50 Hz frequency, and then let it stand at room temperature for 25 minutes to demold; place the demolded riser blank in a room temperature and 50% relative humidity environment to cure for 24 hours, and finally obtain a fluorine-free heat-generating and heat-insulating riser.

[0071] Example 4

[0072] A method for preparing a fluorine-free exothermic and heat-insulating riser includes the following steps:

[0073] (1) Take 25g of micron-sized spherical aluminum powder (150 mesh) and 12g of nano-sized aluminum powder (80nm) and add them to 200mL of anhydrous ethanol. Place them in an ultrasonic cleaner and ultrasonically disperse them for 25min at 200W power and room temperature. Prepare a weakly acidic pre-hydrolyzed solution of silane coupling agent KH560: mix 0.8g of KH560, 5mL of anhydrous ethanol, 0.2mL of deionized water and 1 drop of glacial acetic acid and stir at room temperature for 30min. Add all the prepared pre-hydrolyzed solution to the above aluminum powder dispersion, heat the system to 45℃, and stir at a constant temperature of 300r / min for 60min to obtain a surface-modified aluminum powder dispersion.

[0074] (2) Add 5g of anhydrous ferric chloride to the dispersion obtained in step (1) and stir until completely dissolved. Then add 1.5g of nano-cerium dioxide powder. Transfer the system to a three-necked flask equipped with a condenser and an exhaust device. Purge the system with high-purity nitrogen (50mL / min) for 15min to exhaust the oxygen in the system. Then place the reaction vessel in an ice-water bath and control the temperature at 3℃. While stirring at 200r / min, add 100mL of anhydrous ethanol solution containing 2.5g of sodium borohydride dropwise at a rate of 1mL / min. After the addition is completed, continue stirring under ice-water bath conditions for 75min to obtain an aluminum powder dispersion loaded with active sites.

[0075] (3) Add 1.5g of thermoplastic phenolic resin prepolymer (50% solid content, ethanol solvent) to the dispersion in step (2), heat to 60°C, and stir at 300r / min for 45min; then cool the system to 25°C, slowly add 6g of nano potassium nitrate (100nm particle size) and 3g of nano manganese dioxide (50nm particle size), and mix at a low speed of 150r / min for 30min; after mixing, perform vacuum filtration, and dry the filter cake in a vacuum drying oven at 50°C for 6h to obtain pre-coated exothermic agent powder.

[0076] (4) Disperse the pre-coated exothermic powder obtained in step (3) into 150 mL of n-hexane, add 0.5 g of surfactant Span-80, and stir at 300 r / min for 10 min; then prepare a hydrolyzed mixture of tetraethyl orthosilicate: mix 4 g of tetraethyl orthosilicate, 1.5 mL of deionized water and 0.5 mL of glacial acetic acid evenly; add the hydrolyzed mixture dropwise to the reaction system, slowly heat to 40 °C and stir for 120 min; after the reaction is completed, filter, wash the filter cake twice with n-hexane, dry in a vacuum drying oven at 60 °C for 12 h, and sieve with a 180 mesh standard sieve to obtain the core-shell structured composite exothermic agent.

[0077] (5) Take 30g of porous hollow aluminosilicate ceramic microspheres (50 mesh), place them in a corundum crucible, put them in a box-type muffle furnace and heat them to 650℃ at a heating rate of 5℃ / min. After constant temperature calcination for 3h, let them cool naturally to room temperature. Add the calcined microspheres to 150mL of 0.5mol / L sodium hydroxide aqueous solution and stir and etch them at 60℃ and 200r / min for 30min. After completion, immediately vacuum filter and wash repeatedly with deionized water until the washing solution is neutral. Place them in an 80℃ vacuum drying oven and dry for 8h to obtain hydrophilic surface pore-forming hollow microspheres.

[0078] (6) The hollow microspheres obtained in step (5) are added to a mixed system consisting of 150 mL of acidic silica sol (pH=3, solid content 30%), 2.5 g of water-soluble methyl phenolic resin prepolymer and 2 g of p-toluenesulfonic acid; the system is placed in a vacuum impregnation vessel and kept under negative pressure of -0.09 MPa for 20 min, then restored to normal pressure and stirred at 250 r / min for 30 min, then stopped stirring and allowed to stand at room temperature for 75 min to gel, and then heated to 50 °C and aged for 150 min to obtain wet gel-filled microspheres.

[0079] (7) The wet gel-filled microspheres obtained in step (6) were immersed in 200 mL of isopropanol and allowed to stand for 12 h for solvent replacement; then the microspheres were transferred to a mixed solution containing 15 mL of trimethylchlorosilane and 150 mL of n-hexane and modified by constant temperature reflux at 50 °C for 24 h; after modification, the microspheres were filtered and placed in a 60 °C forced air drying oven to dry at normal pressure for 8 h to obtain hydrophobic aerogel-filled microspheres.

[0080] (8) Disperse 30g of hydrophobic aerogel-filled microbeads into 100g of alcohol-based nano-alumina sol with a solid content of 20%, stir at 200r / min for 40min, vacuum filter, and dry in a forced-air drying oven at 80℃ and normal pressure for 6h; after drying, transfer to a box-type muffle furnace and heat-treat at 150℃ for 2h. After natural cooling, place the microbeads in a sealed container, fumigate with ammonia gas for 30min, and sieve with a 120-mesh standard sieve to obtain composite thermal insulation material.

[0081] (9) Take 30g of the composite insulation material obtained in step (8), 10g of mullite powder, and 5g of fused white corundum powder, and prepare 4.5g of fluorine-free reaction regulator (prepared by uniformly mixing 2.7g of sodium chloride, 0.9g of sodium sulfate, and 0.9g of sodium borate). Add the above materials together with 1.2g of neutral expandable graphite and 3g of low-melting-point borosilicate waste glass powder (D50=20μm, softening point 550℃) into a laboratory small spherical sand mixer, and heat at 40°C. Dry mix at r / min for 2.5 min; then add 42 g of the core-shell composite exothermic agent obtained in step (4) and continue mixing for 1.2 min; prepare 1.6 g of liquid composite organic ester curing agent (prepared by uniformly mixing 1.07 g of glycerol acetate and 0.53 g of ethylene glycol diacetate), and finally add 5.5 g of pure alkaline phenolic resin binder, 1.6 g of the prepared liquid composite organic ester curing agent, and 0.04 g of silane coupling agent KH560 to the sand mixer, maintain the speed and continue mixing for 3.5 min, and discharge the riser blank.

[0082] (10) Add the riser blank obtained in step (9) into a standard cylindrical riser mold, place it on a vibration table and compact it for 2 minutes under the conditions of 0.5 mm amplitude and 50 Hz frequency, and then let it stand at room temperature for 25 minutes to demold; place the demolded riser blank in a room temperature and 50% relative humidity environment to cure for 24 hours, and finally obtain a fluorine-free heat-generating and heat-insulating riser.

[0083] Example 5

[0084] A method for preparing a fluorine-free exothermic and heat-insulating riser includes the following steps:

[0085] (1) Take 25g of micron-sized spherical aluminum powder (150 mesh) and 8g of nano-sized aluminum powder (80nm) and add them to 200mL of anhydrous ethanol. Place them in an ultrasonic cleaner and ultrasonically disperse them for 25min at 200W power and room temperature. Prepare a weakly acidic pre-hydrolyzed solution of silane coupling agent KH560: mix 0.8g of KH560, 5mL of anhydrous ethanol, 0.2mL of deionized water and 1 drop of glacial acetic acid and stir at room temperature for 30min. Add all the prepared pre-hydrolyzed solution to the above aluminum powder dispersion, heat the system to 45℃, and stir at a constant temperature of 300r / min for 60min to obtain a surface-modified aluminum powder dispersion.

[0086] (2) Add 5g of anhydrous ferric chloride to the dispersion obtained in step (1) and stir until completely dissolved. Then add 0.5g of nano-cerium dioxide powder. Transfer the system to a three-necked flask equipped with a condenser and an exhaust device. Purge the system with high-purity nitrogen (50mL / min) for 15min to exhaust the oxygen in the system. Then place the reaction vessel in an ice-water bath and control the temperature at 3℃. While stirring at 200r / min, add 100mL of anhydrous ethanol solution containing 2.5g of sodium borohydride dropwise at a rate of 1mL / min. After the addition is completed, continue stirring under ice-water bath conditions for 75min to obtain an aluminum powder dispersion loaded with active sites.

[0087] (3) Add 1.5g of thermoplastic phenolic resin prepolymer (50% solid content, ethanol solvent) to the dispersion in step (2), heat to 60°C, and stir at 300r / min for 45min; then cool the system to 25°C, slowly add 4g of nano potassium nitrate (100nm particle size) and 2g of nano manganese dioxide (50nm particle size), and mix at a low speed of 150r / min for 30min; after mixing, perform vacuum filtration, and dry the filter cake in a vacuum drying oven at 50°C for 6h to obtain pre-coated exothermic agent powder.

[0088] (4) Disperse the pre-coated exothermic powder obtained in step (3) into 150 mL of n-hexane, add 0.5 g of surfactant Span-80, and stir at 300 r / min for 10 min; then prepare a hydrolyzed mixture of tetraethyl orthosilicate: mix 4 g of tetraethyl orthosilicate, 1.5 mL of deionized water and 0.5 mL of glacial acetic acid evenly; add the hydrolyzed mixture dropwise to the reaction system, slowly heat to 40 °C and stir for 120 min; after the reaction is completed, filter, wash the filter cake twice with n-hexane, dry in a vacuum drying oven at 60 °C for 12 h, and sieve with a 180 mesh standard sieve to obtain the core-shell structured composite exothermic agent.

[0089] (5) Take 30g of porous hollow aluminum silicate ceramic microspheres (50 mesh), place them in a corundum crucible, put them in a box-type muffle furnace and heat them to 640℃ at a heating rate of 5℃ / min. After constant temperature calcination for 2h, let them cool naturally to room temperature. Add the calcined microspheres to 150mL of sodium hydroxide aqueous solution with a concentration of 0.5mol / L, stir and etch for 30min at 60℃ and 200r / min. After completion, immediately vacuum filter and wash repeatedly with deionized water until the washing solution is neutral. Place them in an 80℃ vacuum drying oven and dry for 8h to obtain hydrophilic surface pore-forming hollow microspheres.

[0090] (6) The hollow microspheres obtained in step (5) are added to a mixed system consisting of 150 mL of acidic silica sol (pH=3, solid content 30%), 2.5 g of water-soluble methyl phenolic resin prepolymer and 1 g of p-toluenesulfonic acid; the system is placed in a vacuum impregnation vessel and kept under negative pressure of -0.09 MPa for 20 min, then restored to normal pressure and stirred at 250 r / min for 30 min, then stopped stirring and allowed to stand at room temperature for 75 min, then heated to 50 °C and aged for 150 min to obtain wet gel-filled microspheres.

[0091] (7) The wet gel-filled microspheres obtained in step (6) were immersed in 200 mL of isopropanol and allowed to stand for 12 h for solvent replacement; then the microspheres were transferred to a mixed solution containing 15 mL of trimethylchlorosilane and 150 mL of n-hexane and modified by constant temperature reflux at 50 °C for 24 h; after modification, the microspheres were filtered and placed in a 60 °C forced air drying oven to dry at normal pressure for 8 h to obtain hydrophobic aerogel-filled microspheres.

[0092] (8) Disperse 30g of hydrophobic aerogel-filled microbeads into 90g of alcohol-based nano-alumina sol with a solid content of 20%, stir at 200r / min for 40min, vacuum filter, and dry in a forced-air drying oven at 80℃ and normal pressure for 6h; after drying, transfer to a box-type muffle furnace and heat-treat at 150℃ for 2h. After natural cooling, place the microbeads in a sealed container, fumigate with ammonia gas for 30min, and sieve with a 120-mesh standard sieve to obtain composite thermal insulation material.

[0093] (9) Take 30g of the composite insulation material obtained in step (8), 9g of mullite powder, and 4g of fused white corundum powder, and prepare 4.5g of fluorine-free reaction regulator (prepared by uniformly mixing 2.7g of sodium chloride, 0.9g of sodium sulfate, and 0.9g of sodium borate). Add the above materials together with 1.2g of neutral expandable graphite and 2g of low-melting-point borosilicate waste glass powder (D50=20μm, softening point 550℃) into a laboratory small spherical sand mixer, and heat at 40°C. Dry mix at r / min for 2.5 min; then add 38 g of the core-shell composite exothermic agent obtained in step (4) and continue mixing for 1.2 min; prepare 1.6 g of liquid composite organic ester curing agent (prepared by uniformly mixing 1.07 g of glycerol acetate and 0.53 g of ethylene glycol diacetate), and finally add 5.5 g of pure alkaline phenolic resin binder, 1.6 g of the prepared liquid composite organic ester curing agent, and 0.04 g of silane coupling agent KH560 to the sand mixer, maintain the speed and continue mixing for 3.5 min, and discharge the riser blank.

[0094] (10) Add the riser blank obtained in step (9) into a standard cylindrical riser mold, place it on a vibration table and compact it for 2 minutes under the conditions of 0.5 mm amplitude and 50 Hz frequency, and then let it stand at room temperature for 25 minutes to demold; place the demolded riser blank in a room temperature and 50% relative humidity environment to cure for 24 hours, and finally obtain a fluorine-free heat-generating and heat-insulating riser.

[0095] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that the core-shell composite exothermic agent in step 9 is replaced with an equal mass of a mixture of micron-sized spherical aluminum powder and nano-sized aluminum powder (weight ratio 25:11).

[0096] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that the composite insulation material in step 9 is replaced with porous hollow aluminosilicate ceramic microspheres of equal mass.

[0097] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that neutral expandable graphite and low-melting-point borosilicate waste glass powder are not added in step 9.

[0098] Comparative Example 4: The difference between Comparative Example 4 and Example 1 is that neutral expandable graphite is not added in step 9.

[0099] Comparative Example 5: The difference between Comparative Example 5 and Example 1 is that low-melting-point borosilicate waste glass powder is not added in step 9.

[0100] Performance testing:

[0101] 1. Heating Performance Test: The simulated casting thermocouple temperature measurement method was used for testing. A standard sample with an inner diameter of φ50mm and a height of 100mm was prepared from the riser to be tested. An S-type platinum-rhodium thermocouple was pre-embedded in the center of the riser cavity, and the thermocouple was connected to a paperless recorder. QT450-10 ductile iron molten metal at 1450℃ was poured uniformly into the riser cavity. After filling, the top of the riser was immediately sealed with insulation cotton. The temperature change curve inside the riser over time was recorded using the paperless recorder. The highest heating temperature reached by the sample and the duration of temperature maintenance above 1200℃ were read from the curve. Each sample was tested in parallel three times, and the arithmetic mean was taken as the final result. The test results are shown in Table 1.

[0102] 2. Thermal Conductivity Test: The thermal conductivity of the samples was tested using the laser flash method. The riser blank to be tested was prepared into circular samples with a diameter of 12.7 mm and a thickness of 2 mm. The thermal diffusivity and specific heat capacity of the samples were measured using a laser thermal conductivity meter under both room temperature (25℃) and high temperature (1200℃) conditions. The thermal conductivity at the corresponding temperature was calculated using a formula. Each group of samples was tested in parallel five times, and the arithmetic mean was taken as the final result. The lower the thermal conductivity, the better the insulation performance of the riser. The test results are shown in Table 1.

[0103] 3. Metal molten metal solidification time extension test: Using the standard comparative thermometry method, two sets of identical φ50mm×100mm cylindrical specimens were prepared. One set was the riser specimen to be tested, and the other set was a blank specimen with a common quartz sand riser. A type K thermocouple was embedded in the center of the inner cavity of both sets of specimens. QT450-10 ductile iron molten metal at 1450℃ was simultaneously poured into both sets of specimens. The time taken for the molten metal to cool from the pouring temperature to the solidus temperature of ductile iron (1120℃) was recorded using a paperless recorder, denoted as t1 (riser to be tested) and t0 (blank specimen), respectively. The solidification time extension of the specimen was calculated using the formula: Solidification time extension = (t1-t0) / t0×100%. Each set of specimens was tested in parallel three times, and the arithmetic mean was taken as the final result. The test results are shown in Table 1.

[0104] 4. Feeding Efficiency Test: A standard stepped specimen casting method was used to prepare QT450-10 ductile iron stepped specimens with dimensions of 300mm × 100mm × step thickness of 20 / 40 / 60 / 80mm. The riser sample to be tested was installed at the maximum hot spot of the specimen. Molten iron at 1450℃ was used for casting. After casting, the specimen was naturally cooled to room temperature. The specimen and riser were dissected, and the effective feeding volume V1 and the total riser volume V0 were measured using three-dimensional scanning. The feeding efficiency of the specimen was calculated according to the formula: Feeding Efficiency = V1 / V0 × 100%. Each group of specimens was tested in parallel twice, and the arithmetic mean was taken as the final result. The test results are shown in Table 1.

[0105] Table 1:

[0106]

[0107] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a fluorine-free heating and heat-insulating riser, characterized in that, Includes the following steps: (1) Micron-sized spherical aluminum powder and nano-sized aluminum powder were ultrasonically dispersed in anhydrous ethanol and then grafted with silane coupling agent KH560 to obtain a surface-modified aluminum powder dispersion. (2) Add anhydrous ferric chloride and nano-cerium dioxide to the dispersion in step (1), and carry out an in-situ reduction reaction with anhydrous ethanol solution of sodium borohydride in an ice-water bath under nitrogen protection to obtain an aluminum powder dispersion loaded with active sites. (3) Add thermoplastic phenolic resin prepolymer, nano potassium nitrate and nano manganese dioxide sequentially to the dispersion in step (2) for coating treatment, and obtain pre-coated heating agent powder after drying. (4) Disperse the pre-coated heating agent powder in hexane, add Span-80 and tetraethyl orthosilicate hydrolysis mixture to carry out interfacial hydrolysis and polycondensation reaction to obtain core-shell structured composite heating agent; (5) After calcining, porous hollow aluminum silicate ceramic microspheres are etched with sodium hydroxide solution to obtain hydrophilic surface porous hollow microspheres. (6) Hollow microspheres with pores are vacuum impregnated, gelled and aged in a mixed system of acidic silica sol, water-soluble methyl phenolic resin prepolymer and p-toluenesulfonic acid to obtain wet gel-filled microspheres. (7) After the wet gel-filled microbeads were replaced with isopropanol solvent, they were hydrophobically modified with a hexane solution of trimethylchlorosilane to obtain hydrophobic aerogel-filled microbeads. (8) After coating the hydrophobic aerogel-filled microspheres with nano-alumina sol and heat-treating them, the composite thermal insulation material is obtained by fumigation with ammonia gas to remove acid. (9) Dry mix the composite insulation material with mullite powder, fused white corundum powder, fluorine-free reaction regulator, neutral expandable graphite, and low-melting-point borosilicate waste glass powder, then add the core-shell structure composite heating agent from step (4) and mix well. Finally, add alkaline phenolic resin binder, curing agent and silane coupling agent KH560 and mix well to obtain riser blank. (10) The riser blank from step (9) is vibrated and compacted, and then allowed to stand at room temperature to solidify, to obtain a fluorine-free heat-generating and heat-insulating riser.

2. The method for preparing a fluorine-free heating and heat-insulating riser according to claim 1, characterized in that, In step (1), the mass ratio of micron-sized spherical aluminum powder to nano-sized aluminum powder is 25:(8-12).

3. The method for preparing a fluorine-free heating and insulating riser according to claim 1, characterized in that, In step (2), the mass ratio of anhydrous ferric chloride to nano-cerium dioxide is 5:(0.5-1.5).

4. The method for preparing a fluorine-free heating and insulating riser according to claim 1, characterized in that, In step (3), the mass ratio of thermoplastic phenolic resin prepolymer, nano potassium nitrate, and nano manganese dioxide is 1.5:(4-6):(2-3).

5. The method for preparing a fluorine-free heating and insulating riser according to claim 1, characterized in that, In step (5), the roasting temperature is 640-650℃ and the roasting time is 2-3h.

6. The method for preparing a fluorine-free heating and heat-insulating riser according to claim 1, characterized in that, In step (6), the mass ratio of water-soluble methyl phenolic resin prepolymer to p-toluenesulfonic acid is 2.5:(1-2).

7. The method for preparing a fluorine-free heating and heat-insulating riser according to claim 1, characterized in that, In step (8), the mass ratio of hydrophobic aerogel-filled microspheres to nano-alumina sol is 3:(9-10).

8. The method for preparing a fluorine-free heating and insulating riser according to claim 1, characterized in that, In step (9), the mass ratio of the composite insulation material, mullite powder, and fused white corundum powder is 30:(9-10):(4-5).

9. The method for preparing a fluorine-free heating and insulating riser according to claim 1, characterized in that, In step (9), the mass ratio of neutral expandable graphite to low-melting-point borosilicate waste glass powder is 1.2:(2-3).

10. The method for preparing a fluorine-free heating and insulating riser according to claim 1, characterized in that, In step (9), the mass ratio of the composite insulation material to the core-shell composite heating agent is 30:(38-42).

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