Carbon-coated regenerated calcined sand based on thermal regeneration and its preparation process and application

Abstract

The application discloses carbon-coated regenerated baking sand based on thermal regeneration and a preparation process and application thereof, and belongs to the technical field of casting waste resource utilization.The process comprises the following steps: after waste phenolic resin coated sand is crushed, screened, and magnetically and air selected, the surface of the waste phenolic resin coated sand is pre-coated by using an additive; then the waste phenolic resin coated sand is subjected to segmented heating carbonization baking under an inert atmosphere, so that the resin is pyrolyzed and carbonized in situ to form a carbon film; and then the waste phenolic resin coated sand is subjected to mechanical rubbing and shaping and air selection grading, so that the regenerated baking sand coated with a dense carbon film is prepared.The resin pollutants on the surface of the waste sand are converted into a functional carbon coating with hydrophobic, oleophilic and low-expansion characteristics through the synergistic process of interface modification, secondary reinforcement, gradient carbonization and mechanical shaping.The obtained product has high strength and excellent collapsibility in the application of casting coated sand, has excellent mechanical properties and water resistance and chemical corrosion resistance in building materials, and realizes high-value regeneration and utilization of the casting waste sand.

Description

Technical Field

[0001] This invention relates to the field of foundry waste resource utilization technology, and in particular to carbon-coated regenerated roasted sand based on thermal regeneration, its preparation process and application. Background Technology

[0002] Phenolic resin-coated sand and resin self-hardening sand are widely used in casting production in the automotive, construction machinery, and aerospace industries due to their good formability, high dimensional accuracy, and fast curing speed. This has resulted in a massive amount of waste resin self-hardening sand. my country's annual waste sand discharge exceeds 30 million tons, of which coated sand accounts for over 30%. Direct landfilling of this waste not only occupies land resources but also pollutes soil and groundwater, while wasting high-quality quartz sand resources.

[0003] Existing waste sand recycling technologies are mainly divided into two categories: mechanical recycling and thermal recycling. Mechanical recycling removes the resin film on the surface of sand particles through rubbing and collision, resulting in low recycling efficiency, high residual resin content, and poor recycled sand performance, making it suitable only for low-end casting production. Thermal recycling is currently the mainstream high-end recycling technology in the industry. Through air oxidation roasting at 700-900℃, the resin on the surface of the sand particles is completely burned and decomposed, removing organic matter and obtaining recycled sand with properties close to that of new sand. However, it has the following core drawbacks: High energy consumption and carbon emissions: Complete combustion of resin requires a large amount of excess air for combustion, and high-temperature roasting consumes a lot of energy. At the same time, the organic carbon in the resin is completely converted into CO2 emissions. Each ton of waste sand regeneration produces about 50-80 kg of CO2, resulting in large carbon emissions, which does not meet the dual carbon target requirements. Recycled sand has inherent defects in its performance: after calcination, the surface of the sand particles is a bare quartz mineral surface, which is highly hydrophilic and has poor affinity with organic binders such as phenolic resin. When recoating, more resin needs to be added to achieve the target strength, which increases the cost of downstream applications. Low added value of products: Traditional thermal recycled sand is positioned as a "low-end aggregate to replace new sand", which cannot achieve performance superiority and is difficult to enter high-value-added application scenarios such as high-end casting and 3D printing.

[0004] In response to the above problems, some research in the industry has attempted to optimize the thermal regeneration process, but none of these studies have broken free from the inherent mindset of "completely removing organic matter from the resin," thus failing to fundamentally solve the aforementioned defects. Therefore, developing a new regeneration process that is low in energy consumption and carbon emissions, while also imparting functional properties to the regenerated sand, has become a pressing technical challenge for the industry. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a carbon-coated regenerated calcined sand based on thermal regeneration, its preparation process, and its application.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention first proposes a preparation process for carbon-coated regenerated calcined sand based on thermal regeneration, specifically including the following steps: S1. Pretreatment: The waste foundry phenolic resin coated sand or resin self-hardening sand is crushed and screened to 40-100 mesh, and then subjected to magnetic separation and air separation to obtain crushed sand particles with uniform particle size and no magnetic impurities. The crushed sand particles are heated to 80-100℃ using a high-speed heated sand mixer. Then, silane coupling agent KH-550 diluted with ethanol to a volume concentration of 20% is sprayed through atomization and mixed for 30 seconds. Next, a mixture of high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is sprayed and mixed until uniform. The heating is turned off and the material is discharged after being cooled to <50℃ by blowing cold air to prevent agglomeration, thus obtaining pretreated sand. The coupling agent between the sand particles (inorganic) and the resin (organic) transforms the interface that was originally bound by physical adsorption into a chemical bond interface, which greatly improves the bonding strength between the carbon film formed by subsequent carbonization and the sand particles, making it less prone to peeling off.

[0007] Curing accelerators (p-toluenesulfonic acid or hexamethylenetetramine) initiate a partial cross-linking reaction of the resin under heating conditions, causing the resin film to transform from a liquid state to a gel state or semi-solid state, fixing it to the surface of the sand particles and preventing adhesion or detachment during subsequent transportation. Without pre-curing, the resin film may aggregate into droplets on the sand particle surface due to excessive fluidity during subsequent calcination, resulting in a discontinuous carbon film. The pre-cured gel film maintains uniform coverage during subsequent carbonization, forming a complete carbon shell after carbonization. The presence of nanoparticles alleviates thermal stress caused by the difference in thermal expansion coefficients between the carbon layer and the sand particles, preventing cracking of the carbon layer.

[0008] S2. In-situ carbonization roasting: The pretreated sand is placed in a sealed atmosphere roasting furnace, and an inert protective gas is introduced into the furnace to maintain a slight positive pressure of 10-50 Pa. The oxygen volume fraction in the furnace is ≤0.5%. Drying section: Heat to 200℃ at a heating rate of 5-8℃ / min; Solvent desorption and condensation reaction of the residual resin system further solidify and crosslink. The heating rate should not be too fast, otherwise the rapid evaporation of the solvent may break through the resin film, forming micropores and damaging the integrity of the film.

[0009] Pre-carbonization section: Heating to 400℃ at a heating rate of 3-5℃ / min; At 200-300℃, phenolic resin enters a viscous flow state, further flowing and spreading on the surface of sand particles, forming a uniform coating by relying on surface tension; at 300-400℃, the resin begins to undergo pyrolysis, side chains break, releasing small molecule gases, and at the same time, the molecular structure begins to aromatize, forming a carbonaceous mesophase.

[0010] The heating rate needs to be moderately slow during this stage, allowing sufficient time for the resin to flow and spread while preventing concentrated gas escape that could cause blistering or cracking of the carbon layer. The nano-reinforcing phase acts as a "physical cross-linking point" during this stage, limiting excessive resin flow and maintaining uniform film thickness.

[0011] At around 573℃, quartz undergoes an α-β phase transformation, resulting in a sudden change in volume. However, because the sand grains are encapsulated by an organic / carbon layer, this expansion is constrained, which helps to form a carbon layer under compressive stress, thereby improving the bonding strength.

[0012] Main carbonization section: Heating to 650℃ at a heating rate of 2-4℃ / min; At 400-550℃, the resin main chain begins to break, the aromatic ring structure gradually forms, and a large amount of gas is released; at 550-650℃, the carbon layer gradually changes from amorphous organic matter to amorphous carbon structure (glassy carbon), and heteroatoms such as hydrogen and oxygen are further removed, and the carbon content is greatly increased; the nano-reinforcing phase, especially nano-SiC, remains stable at this temperature and is embedded in the carbon matrix to form a nano-composite carbon structure.

[0013] This is the core stage of carbon layer structure formation. The heating rate is the slowest (2-4℃ / min) to allow the gas to be released slowly, preventing the carbon layer from being broken down; at the same time, it allows the carbon layer sufficient time to rearrange its structure and form densification.

[0014] Insulation section: Insulate for 30-60 minutes to allow the phenolic resin organic binder on the surface of the sand particles to undergo in-situ pyrolysis and carbonization, forming a uniformly coated carbon film on the surface of the sand particles, thus obtaining carbonized calcined sand. The carbon layer in the insulation section is further aromatized and densified, and the remaining small amount of hydrogen is removed; local solid-phase reaction may occur at the interface between the carbon layer and the sand particles to form Si-OC or Si-C chemical bonds, thereby achieving chemical anchoring. Prolonged heat treatment at high temperatures allows the quartz sand grains to undergo annealing, fully releasing lattice stress and reducing subsequent thermal expansion.

[0015] S3. Mechanical rubbing and shaping: The carbonized roasted sand is fed into a hot rubbing and regeneration machine for mechanical rubbing and shaping to remove the loose and poorly bonded excess carbon layer on the surface of the sand particles, so that the carbon film is uniform and dense, and the sand material after rubbing and shaping is obtained. The surface of carbonized sand grains may contain an outer loose layer (regions where pyrolysis was incomplete and gas escaped) and an inner dense layer. Mechanical abrasion utilizes the shearing and collision effects between the sand grains to peel off the loosely bonded layer. At the same time, the abrasion process smooths out the micro-protrusions on the carbon film surface, making the overall surface smoother and more uniform.

[0016] S4. Air classification: The sand after grinding is air-classified to remove fine carbon powder, ash and fine powder impurities, and obtain carbon-coated recycled roasted sand finished product.

[0017] Preferably, in step S1, the mass ratio of crushed sand particles, silane coupling agent KH-550, high residual carbon phenolic resin liquid, curing accelerator, and nano-reinforcing phase is 100:0.1-0.2:0.3-0.5:0.02:0.05; wherein the curing accelerator is p-toluenesulfonic acid or hexamethylenetetramine, and the nano-reinforcing phase is nano-silica or nano-silicon carbide.

[0018] Preferably, in step S3, the rotation speed of the hot scrubbing regeneration machine is 800-1200 r / min, and the scrubbing time is 2-5 min.

[0019] The present invention also proposes a carbon-coated regenerated calcined sand prepared by the aforementioned preparation method, characterized in that the surface of the sand particles is coated with a dense amorphous carbon film with a thickness of 50-500 nm, the product has a loss on ignition of 0.3-1.2%, a thermal expansion rate of 0.50-0.60% from room temperature to 1000℃, and the surface is hydrophobic.

[0020] The present invention also proposes the application of the aforementioned carbon-coated recycled calcined sand in building materials, using the carbon-coated recycled calcined sand as aggregate to prepare cement-based or resin-based building materials.

[0021] Preferably, the building material is one of the following: waterproof mortar, acid and alkali resistant anti-corrosion concrete, epoxy flooring material, and 3D printing mortar.

[0022] In building materials, the carbon coating is hydrophobic, which reduces the water absorption rate of the mortar; as an acid and alkali resistant concrete, the carbon coating is chemically inert, which protects the sand particles from acid and alkali corrosion; as an epoxy flooring, the carbon coating has good compatibility with epoxy resin and strong interfacial bonding.

[0023] This invention also proposes the application of the aforementioned carbon-coated recycled calcined sand in the preparation of coated sand, characterized by comprising the following steps: Preheat the carbon-coated recycled calcined sand to 140-160℃, add thermoplastic phenolic resin, mix for 60-90s, add curing agent hexamethylenetetramine, continue mixing for 30-60s, then add lubricant calcium stearate, mix for 20-40s, unload, cool, crush and screen to obtain the finished coated sand.

[0024] When carbon-coated recycled sand is used as aggregate to prepare coated sand, the amount of resin used may be reduced by 10-20% due to the oleophilic nature of the carbon coating itself, and the bonding strength is higher.

[0025] Preferably, the mass ratio of the carbon-coated recycled calcined sand, thermoplastic phenolic resin, hexamethylenetetramine, and calcium stearate is 10:1.0-2.0:0.10-0.15:0.05-0.08.

[0026] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention breaks through the conventional mindset of treating organic matter as "impurities" that must be completely removed in traditional waste sand recycling processes, and innovatively proposes a functionalized recycling path of "in-situ carbonization + secondary reinforcement." Existing thermal recycling technologies completely burn away the resin film on the surface of waste sand through oxidative roasting, which not only generates carbon dioxide waste gas but also consumes a large amount of energy. In contrast, this invention uses controlled gradient heating carbonization under an inert atmosphere to transform the phenolic resin on the surface of waste sand into a usable carbon film. At the same time, a silane coupling agent, high residual carbon phenolic resin, and nano-reinforcing phase are introduced in the pretreatment stage for secondary reinforcement, achieving a leap from "passive impurity removal" to "active construction." This process design avoids carbon emissions from resin combustion and endows the carbon coating with additional mechanical properties and thermal stability through the nano-reinforcing phase, thus unifying the environmental and economic benefits of waste sand recycling.

[0027] 2. This invention, through a precise four-stage gradient heating carbonization process and mechanical abrasion shaping, yields a composite-structured recycled sand that combines low thermal expansion and a complete carbon coating. The overall performance of the product is significantly superior to existing recycled sand. Specifically: the slow heating in the drying and pre-carbonization stages ensures uniform spreading and pre-curing of the resin film, preventing blistering or cracking of the carbon layer; precise control in the main carbonization and insulation stages allows for full aromatization and densification of the carbon layer, forming chemical bonds with the sand grain surface; mechanical abrasion selectively removes the loose layer, retaining a dense amorphous carbon film of 50-500 nm. The resulting product has a thermal expansion rate of only 0.50-0.60% from room temperature to 1000℃ (lower than 1.15% for virgin sand), a hydrophobic surface, and a controllable loss on ignition of 0.3-1.2%. It combines the low expansion characteristics of quartz sand with the functionalized surface of a carbon coating, achieving a synergistic performance of "1+1>2".

[0028] 3. The carbon-coated recycled calcined sand prepared by this invention has broad applicability and significant added-value effects, simultaneously meeting the high-end application needs of both the casting and building materials industries. In the casting field, the oleophilic nature of the carbon coating makes its compatibility with phenolic resin significantly better than that of bare sand particles. When used as coated sand aggregate, it can reduce resin usage by 10-20%. At the same time, the lubricating properties of the carbon film improve core-shooting fluidity, making it particularly suitable for applications such as aluminum alloy castings where collapsibility is required. In the building materials field, the hydrophobicity and chemical inertness of the carbon coating allow the product to be directly used as a functional aggregate for waterproof mortar, acid and alkali resistant concrete, or for use in 3D printing mortar to improve extrusion performance. This cross-industry applicability significantly increases the added value of recycled waste sand, opening up new avenues for the resource utilization of foundry waste. Attached Figure Description

[0029] Figure 1 This is a flow chart of the preparation process of carbon-coated recycled calcined sand proposed in this invention. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0031] Example 1: A preparation process for carbon-coated regenerated calcined sand based on thermal regeneration, such as... Figure 1 As shown, the specific steps include: S1. Pretreatment: The waste foundry phenolic resin coated sand or resin self-hardening sand is crushed and screened to 40-100 mesh, and then subjected to magnetic separation and air separation to obtain crushed sand particles with uniform particle size and no magnetic impurities. The crushed sand particles are heated to 80°C using a high-speed heated sand mixer. Then, silane coupling agent KH-550 diluted with ethanol to a volume concentration of 20% is sprayed through atomization and mixed for 30 seconds. Next, a mixture of high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is sprayed and mixed until uniform. The heating is turned off and the material is discharged after being cooled to <50°C by blowing cold air to prevent agglomeration, thus obtaining pretreated sand. S2. In-situ carbonization roasting: The pretreated sand is placed in a sealed atmosphere roasting furnace, and an inert protective gas is introduced into the furnace to maintain a slight positive pressure of 10-50 Pa. The oxygen volume fraction in the furnace is ≤0.5%. Drying section: Heat to 200℃ at a heating rate of 8℃ / min; Pre-carbonization section: Heating to 400℃ at a heating rate of 3℃ / min; Main carbonization section: Heating to 650℃ at a heating rate of 4℃ / min; Insulation section: Insulation for 30 minutes allows the phenolic resin organic binder on the surface of the sand particles to undergo in-situ pyrolysis and carbonization, forming a uniformly coated carbon film on the surface of the sand particles, thus obtaining carbonized calcined sand. S3. Mechanical rubbing and shaping: The carbonized roasted sand is fed into a hot rubbing and regeneration machine for mechanical rubbing and shaping to remove the loose and poorly bonded excess carbon layer on the surface of the sand particles, so that the carbon film is uniform and dense, and the sand material after rubbing and shaping is obtained. S4. Air classification: The sand after grinding is air-classified to remove fine carbon powder, ash and fine powder impurities, and obtain carbon-coated recycled roasted sand finished product.

[0032] In S1, the mass ratio of crushed sand particles, silane coupling agent KH-550, high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is 100:0.2:0.3:0.02:0.05; wherein the curing accelerator is p-hexamethylenetetramine and the nano-reinforcing phase is nano-silica.

[0033] In S3, the rotation speed of the hot scrubbing regeneration machine is 1200 r / min, and the scrubbing time is 2 min.

[0034] Example 2: A preparation process for carbon-coated regenerated calcined sand based on thermal regeneration, specifically including the following steps: S1. Pretreatment: The waste foundry phenolic resin coated sand or resin self-hardening sand is crushed and screened to 40-100 mesh, and then subjected to magnetic separation and air separation to obtain crushed sand particles with uniform particle size and no magnetic impurities. The crushed sand particles are heated to 90°C using a high-speed heated sand mixer. Then, silane coupling agent KH-550 diluted with ethanol to a volume concentration of 20% is sprayed through atomization and mixed for 30 seconds. Next, a mixture of high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is sprayed and mixed until uniform. The heating is turned off and the material is discharged after being cooled to <50°C by blowing cold air to prevent agglomeration, thus obtaining pretreated sand. S2. In-situ carbonization roasting: The pretreated sand is placed in a sealed atmosphere roasting furnace, and an inert protective gas is introduced into the furnace to maintain a slight positive pressure of 10-50 Pa. The oxygen volume fraction in the furnace is ≤0.5%. Drying section: Heat to 200℃ at a heating rate of 6℃ / min; Pre-carbonization section: Heating to 400℃ at a heating rate of 4℃ / min; Main carbonization section: Heating to 650℃ at a heating rate of 3℃ / min; Insulation section: Insulation for 45 minutes allows the phenolic resin organic binder on the surface of the sand particles to undergo in-situ pyrolysis and carbonization, forming a uniformly coated carbon film on the surface of the sand particles, thus obtaining carbonized calcined sand. S3. Mechanical rubbing and shaping: The carbonized roasted sand is fed into a hot rubbing and regeneration machine for mechanical rubbing and shaping to remove the loose and poorly bonded excess carbon layer on the surface of the sand particles, so that the carbon film is uniform and dense, and the sand material after rubbing and shaping is obtained. S4. Air classification: The sand after grinding is air-classified to remove fine carbon powder, ash and fine powder impurities, and obtain carbon-coated recycled roasted sand finished product.

[0035] In S1, the mass ratio of crushed sand particles, silane coupling agent KH-550, high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is 100:0.15:0.4:0.02:0.05; wherein the curing accelerator is p-toluenesulfonic acid and the nano-reinforcing phase is nano-silicon carbide.

[0036] In S3, the rotation speed of the hot scrubbing regeneration machine is 1000 r / min, and the scrubbing time is 4 min.

[0037] Example 3: A preparation process for carbon-coated regenerated calcined sand based on thermal regeneration, specifically including the following steps: S1. Pretreatment: The waste foundry phenolic resin coated sand or resin self-hardening sand is crushed and screened to 40-100 mesh, and then subjected to magnetic separation and air separation to obtain crushed sand particles with uniform particle size and no magnetic impurities. The crushed sand particles are heated to 100°C using a high-speed heated sand mixer. Then, silane coupling agent KH-550 diluted with ethanol to a volume concentration of 20% is sprayed through atomization and mixed for 30 seconds. Next, a mixture of high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is sprayed and mixed until uniform. The heating is turned off and the material is discharged after being cooled to <50°C by blowing cold air to prevent agglomeration, thus obtaining pretreated sand. S2. In-situ carbonization roasting: The pretreated sand is placed in a sealed atmosphere roasting furnace, and an inert protective gas is introduced into the furnace to maintain a slight positive pressure of 10-50 Pa. The oxygen volume fraction in the furnace is ≤0.5%. Drying section: Heat to 200℃ at a heating rate of 5℃ / min; Pre-carbonization section: Heating to 400℃ at a rate of 5℃ / min; Main carbonization section: Heating to 650℃ at a heating rate of 2℃ / min; Insulation section: Insulation for 60 minutes allows the phenolic resin organic binder on the surface of the sand particles to undergo in-situ pyrolysis and carbonization, forming a uniformly coated carbon film on the surface of the sand particles, thus obtaining carbonized calcined sand. S3. Mechanical rubbing and shaping: The carbonized roasted sand is fed into a hot rubbing and regeneration machine for mechanical rubbing and shaping to remove the loose and poorly bonded excess carbon layer on the surface of the sand particles, so that the carbon film is uniform and dense, and the sand material after rubbing and shaping is obtained. S4. Air classification: The sand after grinding is air-classified to remove fine carbon powder, ash and fine powder impurities, and obtain carbon-coated recycled roasted sand finished product.

[0038] In S1, the mass ratio of crushed sand particles, silane coupling agent KH-550, high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is 100:0.1:0.5:0.02:0.05; wherein the curing accelerator is p-toluenesulfonic acid and the nano-reinforcing phase is nano-silicon carbide.

[0039] In S3, the rotation speed of the hot scrubbing regeneration machine is 800 r / min, and the scrubbing time is 5 min.

[0040] The following comparison model was also set: Comparative Example 1: The sand to be roasted was placed in a conventional roasting furnace, air was introduced, and the temperature was raised to 900℃ at 10℃ / min and held for 30min to completely oxidize and burn off the resin on the surface of the sand particles, thus obtaining roasted sand.

[0041] Comparative Example 2: Based on Example 2, the difference is that the silane coupling agent modification, high residual carbon resin pre-coating, and nano-reinforcing phase addition steps are not performed. The pretreated sand is obtained directly after crushing, screening, magnetic separation, and air separation. The rest is the same as Example 2.

[0042] Comparative Example 3: Based on Example 2, the difference is that there is no segmented heating. The temperature is directly increased from room temperature to 650°C in one step at a rate of 5°C / min, and held for 45 minutes to obtain carbonized calcined sand. The rest is the same as in Example 2.

[0043] Comparative Example 4: Based on Example 2, the difference is that there is no mechanical grinding and shaping process, and the carbonized roasted sand directly enters the next stage. The rest is the same as Example 2.

[0044] Comparative Example 5: Based on Example 2, the difference is that there is no oxygen control requirement in the oxidizing atmosphere, but otherwise it is the same as Example 2.

[0045] Performance Testing: A portion of carbon-coated recycled calcined sand was used for performance testing of coated sand for foundry applications. The carbon-coated recycled calcined sand was preheated to 150℃, thermoplastic phenolic resin was added, and after mixing for 90 seconds, the curing agent hexamethylenetetramine was added, and mixing continued for 60 seconds. Then, the lubricant calcium stearate was added, and after mixing for 40 seconds, the material was unloaded, cooled, crushed, and sieved to obtain the finished coated sand. The mass ratio of the carbon-coated recycled calcined sand, thermoplastic phenolic resin, hexamethylenetetramine, and calcium stearate was 10:1.0:0.15:0.05. According to JB / T 8583-2008 "Coated Sand for Foundry", the high-temperature flexural strength at 230℃, gas generation at 850℃, curing rate, and high-temperature collapse properties were tested. The results are shown in Table 1. Take 55 parts of carbon-coated recycled calcined sand, 40 parts of P·O 42.5 silicate cement, 5 parts of fly ash, 0.3 parts of polycarboxylate superplasticizer, and 18 parts of water. Test the compressive / flexural strength, impermeability grade, water absorption rate, and acid and alkali corrosion resistance according to GB / T 17671-2021 "Test Method for Strength of Cement Mortar". The results are shown in Table 2. Table 1. Test results of various properties of coated sand Table 2. Test results of various properties of cement mortar Data Analysis: As can be seen from the data in Table 1, the carbon-coated recycled calcined sand prepared in the embodiments of the present invention exhibits excellent comprehensive performance in the application of casting coated sand. The high-temperature flexural strength at 230℃ of Examples 1-3 reached 7.25, 7.82, and 8.16 MPa, respectively, which are significantly higher than those of the comparative groups (5.02-6.58 MPa). This advantage stems from the fact that the present invention forms a chemical bond between the sand particles and the carbon coating through the "molecular bridge" effect of the silane coupling agent KH-550, while the introduction of the nano-reinforcing phase (SiO2 or SiC) effectively inhibits the crack propagation of the carbon layer, thus greatly improving the interfacial bonding strength.

[0046] Regarding the gas evolution at 850℃, the examples showed a relatively low level of 12.2-12.8 mL / g, while Comparative Example 4 had a high level of 16.7 mL / g, and Comparative Example 5 had a low level of 10.5 mL / g but the worst strength. This indicates that the present invention fully transforms the resin into a stable carbon layer through a gradient carbonization process, avoiding the high-temperature decomposition of residual organic matter; the lack of mechanical abrasion (Comparative Example 4) leads to loose carbon layer residue and increased gas evolution; although the oxidizing atmosphere (Comparative Example 5) had a low gas evolution, the carbon coating was burned off, resulting in a sharp drop in strength.

[0047] Curing rate is a key indicator of core-making efficiency. The curing time of the example was only 38-45 seconds, much faster than the 52-70 seconds of the comparative example. This is because the oleophilic surface of the carbon coating has excellent compatibility with the phenolic resin, promoting the wetting and curing reaction of the resin on the sand grain surface.

[0048] The most significant advantage lies in the residual flexural strength at 800℃ (high-temperature collapse resistance). The residual strength in the example is only 0.28-0.35 MPa, while Comparative Example 1 reaches 1.68 MPa and Comparative Example 5 reaches 1.85 MPa. This means that the sand core made using the sand of this invention is extremely easy to collapse and clean after casting, making it particularly suitable for non-ferrous alloy castings such as aluminum alloys. This characteristic stems from the partial oxidative decomposition of the carbon coating at high temperatures, while the nano-reinforcing phase regulates the pyrolysis behavior of the carbon layer.

[0049] When the carbon-coated recycled calcined sand of this invention is used in cement-based materials, its mechanical properties also exhibit significant advantages. In Example 28, the compressive strength reached 51.2-53.8 MPa and the flexural strength 9.6-10.2 MPa, significantly superior to the comparative examples' 43.2-48.1 MPa and 7.5-8.7 MPa. This improvement is attributed to the interfacial compatibility between the carbon coating and cement hydration products, as well as the micro-aggregate effect of the nano-reinforcing phase.

[0050] In terms of durability, the embodiments achieved a water resistance rating of P10-P12, with a 24-hour water absorption rate of only 1.08-1.25%, far superior to the comparative examples' 1.92-4.15%. The hydrophobic properties of the carbon coating (contact angle >90°) effectively blocked the capillary water absorption channels, giving the material excellent waterproof performance. Regarding acid and alkali corrosion resistance, the embodiments showed corrosion coefficients of 0.92-0.95 in 5% H2SO4 and 0.94-0.96 in 5% NaOH, significantly higher than the comparative examples' 0.68-0.86. The chemical inertness of the carbon coating protected the quartz sand matrix from acid and alkali corrosion.

[0051] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A process for preparing carbon-coated regenerated calcined sand based on thermal regeneration, characterized in that, Specifically, the following steps are included: S1. Pretreatment: The waste foundry phenolic resin coated sand or resin self-hardening sand is crushed and screened to 40-100 mesh, and then subjected to magnetic separation and air separation to obtain crushed sand particles with uniform particle size and no magnetic impurities. The crushed sand particles are heated to 80-100℃ using a high-speed heated sand mixer. Then, silane coupling agent KH-550 diluted with ethanol to a volume concentration of 20% is sprayed through atomization and mixed for 30 seconds. Next, a mixture of high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is sprayed and mixed until uniform. The heating is turned off and the material is discharged after being cooled to <50℃ by blowing cold air to prevent agglomeration, thus obtaining pretreated sand. S2. In-situ carbonization roasting: The pretreated sand is placed in a sealed atmosphere roasting furnace, and an inert protective gas is introduced into the furnace to maintain a slight positive pressure of 10-50 Pa. The oxygen volume fraction in the furnace is ≤0.5%. Drying section: Heat to 200℃ at a heating rate of 5-8℃ / min; Pre-carbonization section: Heating to 400℃ at a heating rate of 3-5℃ / min; Main carbonization section: Heating to 650℃ at a heating rate of 2-4℃ / min; Insulation section: Insulate for 30-60 minutes to allow the phenolic resin organic binder on the surface of the sand particles to undergo in-situ pyrolysis and carbonization, forming a uniformly coated carbon film on the surface of the sand particles, thus obtaining carbonized calcined sand. S3. Mechanical rubbing and shaping: The carbonized roasted sand is fed into a hot rubbing and regeneration machine for mechanical rubbing and shaping to remove the loose and poorly bonded excess carbon layer on the surface of the sand particles, so that the carbon film is uniform and dense, and the sand material after rubbing and shaping is obtained. S4. Air classification: The sand after grinding is air-classified to remove fine carbon powder, ash and fine powder impurities, and obtain carbon-coated recycled roasted sand finished product.

2. The preparation process of carbon-coated regenerated calcined sand based on thermal regeneration according to claim 1, characterized in that, In S1, the mass ratio of crushed sand particles, silane coupling agent KH-550, high residual carbon phenolic resin liquid, curing accelerator and nano-reinforcing phase is 100:0.1-0.2:0.3-0.5:0.02:0.05; wherein the curing accelerator is p-toluenesulfonic acid or hexamethylenetetramine, and the nano-reinforcing phase is nano-silica or nano-silicon carbide.

3. The preparation process of carbon-coated regenerated calcined sand based on thermal regeneration according to claim 1, characterized in that, In S3, the rotation speed of the hot scrubbing regeneration machine is 800-1200 r / min, and the scrubbing time is 2-5 min.

4. A carbon-coated regenerated calcined sand prepared according to the preparation method described in claims 1-3, characterized in that, The surface of the sand grains is coated with a dense amorphous carbon film with a thickness of 50-500nm. The product has a loss on ignition of 0.3-1.2%, a thermal expansion rate of 0.50-0.60% from room temperature to 1000℃, and a hydrophobic surface.

5. The application of the carbon-coated recycled calcined sand prepared by the process described in any one of claims 1-3 in building materials, characterized in that, The carbon-coated recycled calcined sand is used as aggregate to prepare cement-based or resin-based building materials.

6. The application of the carbon-coated recycled calcined sand according to claim 5 in building materials, characterized in that, The building material is one of the following: waterproof mortar, acid and alkali resistant anti-corrosion concrete, epoxy flooring material, and 3D printing mortar.

7. The application of the carbon-coated recycled calcined sand prepared by the process described in any one of claims 1-3 in the preparation of coated sand, characterized in that, Includes the following steps: Preheat the carbon-coated recycled calcined sand to 140-160℃, add thermoplastic phenolic resin, mix for 60-90s, add curing agent hexamethylenetetramine, continue mixing for 30-60s, then add lubricant calcium stearate, mix for 20-40s, unload, cool, crush and screen to obtain the finished coated sand.

8. The application of the carbon-coated recycled calcined sand according to claim 7 in the preparation of coated sand, characterized in that, The mass ratio of the carbon-coated recycled calcined sand, thermoplastic phenolic resin, hexamethylenetetramine, and calcium stearate is 10:1.0-2.0:0.10-0.15:0.05-0.08.

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