Corrosion-resistant graphite material for heat exchange graphite cooler and preparation process thereof

Abstract

The application relates to the technical field of graphite material processing, and discloses a kind of corrosion-resistant graphite material for heat exchange graphite cooler and its preparation process, process includes: graphite matrix is pretreated with sodium carbonate solution and dried, clean the inner wall of pore and give pore wall basicity;Lignin sulfonate calcium powder is added into heated pine tar in batches, and heat preservation stirring is carried out, physical adsorption water and bound water in lignin sulfonate calcium are driven out, and organic coating is carried out simultaneously, to prepare pre-dispersed dewatering slurry;The slurry is mixed with high-softening-point modified coal tar pitch and furfuryl alcohol resin to prepare a multi-component composite impregnant;The pretreated graphite matrix is vacuum pressure impregnated, so that the composite impregnant is filled in the inside of the pore;Finally, temperature is raised to solidify, and the corrosion-resistant graphite material is obtained.The application eliminates the steam hole defects caused by residual moisture vaporization during solidification and temperature rise, and solves the problem of agglomeration and sedimentation of lignin sulfonate calcium in the organic impregnation system.

Description

Technical Field

[0001] This invention relates to the field of graphite material processing technology, and more specifically, to a corrosion-resistant graphite material for heat exchange graphite coolers and its preparation process. Background Technology

[0002] Graphite heat exchange coolers are widely used in heat exchange applications involving highly corrosive chemical media such as chlor-alkali, sulfuric acid, hydrochloric acid, and phosphoric acid. Graphite materials have excellent thermal conductivity and inherently good chemical corrosion resistance. However, after molding, sintering, and graphitization, a large number of interconnected pores (porosity typically 15%–25%) exist inside. Corrosive media can continuously penetrate into the material through these pores, leading to leakage and failure. Industrially, organic resin impregnation is commonly used to seal the graphite pores, imparting the required density and corrosion resistance to the material.

[0003] Existing impregnation processes mainly use single organic resins such as furfuryl alcohol resin or phenolic resin as impregnating agents. Furfuryl alcohol resin exhibits significant volume shrinkage during curing (curing shrinkage rate approximately 8%–15%), resulting in limited uniformity of the cured layer in sealing the pores and a relatively high residual porosity. Furthermore, the cured organic resin and the graphite pore walls are physically interlocked without chemical bonding, leading to limited interfacial adhesion. Under long-term erosion by corrosive media, the impregnated layer easily peels off from the pore walls, re-exposing the pores to the corrosive environment.

[0004] To address these shortcomings, high softening-point modified coal tar pitch was compounded with furfuryl alcohol resin as an impregnating agent. Pine tar was further introduced as a polar bridging component to improve the compatibility and stability of the two agents. Calcium lignosulfonate was introduced to improve the wetting and spreading of the impregnating agent on the inner walls of the graphite channels. Simultaneously, an alkaline pretreatment process was used to clean the inner walls of the graphite channels. However, calcium lignosulfonate is a strongly hydrophilic powder, with a commercial product typically containing 8%–15% water. When directly added to an organic impregnating agent system primarily composed of modified pitch and furfuryl alcohol resin, the significant difference in polarity between the two causes the calcium lignosulfonate particles to agglomerate, settle, and stratify within the organic system, failing to disperse evenly and penetrate the micropores of the graphite. Residual moisture rapidly vaporizes during the curing and heating of the impregnating agent, generating vapor bubbles that are sealed by the cured layer, forming numerous randomly distributed spherical vapor pore defects within the impregnating layer, severely weakening its density. The aforementioned problems restrict the effective application of calcium lignosulfonate in multi-component composite impregnation systems and are key technical obstacles to the current composite impregnation process. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a corrosion-resistant graphite material for heat exchange graphite coolers and its preparation process.

[0006] A process for preparing a corrosion-resistant graphite material for a heat exchange graphite cooler includes the following steps:

[0007] The graphite matrix was immersed in a sodium carbonate solution, removed and dried to obtain a pretreated graphite matrix with sodium carbonate remaining on the inner wall of the pores. Calcium lignosulfonate powder was added in batches to pine tar heated to 120-140℃, and stirred at 120-140℃ for 1-2 hours. The mixture was then cooled to below 60℃ to obtain a pre-dispersed dehydrated slurry, wherein the mass ratio of calcium lignosulfonate to pine tar was (20-40):(60-80). Modified coal tar pitch is heated to 120-130℃ to melt it into a fluid state, and the pre-dispersed dehydrated slurry and furfuryl alcohol resin are added in sequence. The mixture is kept at 120-130℃ and stirred until homogeneous to obtain a composite impregnating agent. After the pretreated graphite matrix is ​​evacuated, the composite impregnating agent is injected and kept at 120-130°C, and then impregnated under pressure of 0.3-0.8 MPa. The impregnated graphite material is heated to 150–200°C at a heating rate not exceeding 2°C / min, and then cured at this temperature for 2–4 hours.

[0008] Preferably, the mass fraction of each component in the composite impregnating agent is: 40%–50% modified coal tar pitch, 30%–40% furfuryl alcohol resin, and 10%–20% pre-dispersed dehydrated slurry, with the sum of the mass fractions of each component being 100%.

[0009] Preferably, the viscosity of the furfuryl alcohol resin at 25°C is 100–500 mPa·s.

[0010] Preferably, the modified coal tar pitch has a softening point of 90–110°C and a carbonization residue rate of 55%–70%.

[0011] Preferably, the modified coal tar pitch is obtained by oxidative heat treatment of coal tar pitch.

[0012] Preferably, the sodium carbonate solution has a mass fraction of 3% to 8%, the soaking time is not less than 2 hours, and the drying is carried out at 110 to 120°C until constant weight is achieved.

[0013] Preferably, the moisture content of the calcium lignosulfonate powder does not exceed 15%.

[0014] Preferably, in the vacuuming step, the vacuum degree is not less than 0.09 MPa and is maintained for more than 30 minutes before the composite impregnating agent is injected; the pressure holding impregnation time is 1 to 3 hours.

[0015] Preferably, after the curing is completed, the temperature is raised to 600-900°C under nitrogen protection with a volume fraction of not less than 99.5% for carbonization treatment, and the holding time is 1-2 hours.

[0016] A corrosion-resistant graphite material for heat exchange graphite coolers prepared by the above-described process.

[0017] The beneficial effects of this invention are as follows: 1. Elimination of vapor pore defects and improved density of the impregnated layer. The calcium lignosulfonate-pine tar pre-dispersion and dehydration process thoroughly removes the physically adsorbed water and bound water inside and outside the calcium lignosulfonate particles in a high-temperature pine tar medium at 120-140°C, eliminating the root cause of vapor bubble formation and sealing by the cured layer during curing heating, so that the impregnated cured layer is free of vapor pore defects, and the porosity of the material is lower than that of the existing process of directly mixing calcium lignosulfonate with high moisture content into the organic impregnation system.

[0018] 2. Eliminating phase separation and agglomeration, ensuring uniform composition of the impregnating agent within each pore. Pine tar organic coating transforms the surface of strongly hydrophilic calcium lignin sulfonate particles into an organophilic one, enabling them to disperse uniformly and stably in the modified pitch-furfuryl alcohol resin organic system without agglomeration or sedimentation. This ensures consistent impregnating agent composition within each graphite pore, resulting in uniform weight gain and pore blockage across graphite matrices from the same batch. This eliminates the agglomeration, sedimentation, and phase separation phenomena caused by polarity differences in calcium lignin sulfonate within the organic impregnation system, a problem present in existing technologies.

[0019] 3. Multi-component carbonaceous superposition (applicable to carbonization conditions), improving pore sealing integrity. The high carbonization residual rate of modified coal tar pitch (approximately 55%–70%) and the additional carbonaceous contribution during the carbonization of calcium lignosulfonate (applicable to situations requiring carbonization treatment) result in a lower carbonization volume shrinkage of the overall cured product compared to a single organic resin impregnation system, leading to higher pore sealing integrity and further reduction in residual porosity.

[0020] 4. Alkali pretreatment improves the wettability of the pore inner wall, enhances impregnation uniformity, and improves interfacial adhesion. Alkali pretreatment removes residual processing impurities from the pore inner wall surface. After drying, the alkaline pore wall, containing trace amounts of sodium carbonate, enhances the wetting affinity of the phenolic hydroxyl-containing pine tar and calcium lignin sulfonate components in the impregnating agent. This allows these components to spread fully on the pore wall surface, thereby improving the uniformity of the impregnated layer distribution on the pore wall and the interfacial adhesion, and enhancing the peel resistance of the impregnated layer under long-term exposure to corrosive media.

[0021] 5. Pine tar plays different roles in the pre-dispersion step and the impregnating agent preparation step, eliminating the need for special additives. In step two, pine tar acts as a dehydration and dispersion medium, and in step three, it also acts as a polar bridging phase. The same raw material plays different roles in the two steps, eliminating the need for additional special compatibilizers or dispersants, thus minimizing the types of raw materials used in the impregnating agent.

[0022] 6. The raw material sources are stable, and the process is ready for direct industrial implementation. High softening point modified coal tar pitch, furfuryl alcohol resin, pine tar, calcium lignosulfonate, and sodium carbonate are all bulk industrial raw materials or industrial by-products, with wide availability and controllable costs; the processing methods used in each step of the preparation process are compatible with existing graphite impregnation production lines. Attached Figure Description

[0023] Figure 1 This is a bar chart comparing the open porosity of two sets of samples before and after impregnation according to the present invention. Figure 2 These are SEM comparison images of the cross-sections of the impregnated and cured layers of control sample A and experimental sample B in this invention; Figure 3 This is a line graph showing the change in the ratio of calcium lignosulfonate precipitate height in the two impregnating agents of the present invention with standing time; Figure 4 This is a comparison diagram of the particle dispersion state of the two impregnating agents of the present invention after standing for 4 hours and taking samples from the upper part under an optical microscope (50x). Figure 5 This is a bar chart comparing the initial and post-corrosion interfacial shear strength of two sets of samples from this invention. Figure 6 These are SEM comparison images of the interface sections between the impregnation layer and the graphite pore wall of samples C and D of the present invention. Figure 7 This is a bar chart comparing the residual porosity and 24-hour leakage of the three impregnation schemes of the present invention; Figure 8 This is a line graph showing the corrosion mass loss rate of samples from the three immersion schemes of the present invention in 30% hydrochloric acid solution as a function of immersion time. Detailed Implementation

[0024] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples.

[0025] Example 1 This embodiment discloses a preparation process for corrosion-resistant graphite material for heat exchange graphite coolers. The process uses a commercially available porous graphite matrix with a porosity of 18% as raw material and includes the following steps: Step 1: Alkaline pretreatment of graphite matrix. The graphite matrix is ​​immersed in a 3% sodium carbonate solution at room temperature for 2 hours. After being removed, it is dried at 110℃ to constant weight to obtain a pretreated graphite matrix with trace amounts of sodium carbonate adhering to the inner wall of the pores and the pore walls being alkaline.

[0026] Step 2: Preparation of calcium lignosulfonate-pine tar pre-dispersed dehydrated slurry. Pine tar was heated to 120°C. While stirring, calcium lignosulfonate powder with a water content of 12% was added to the hot pine tar in batches (each batch should not exceed 1 / 3 of the total amount, and the next batch should be added after the bubbles on the surface of the pine tar tend to stabilize). The mixture was kept at 120°C and stirred for 60 min, and then cooled to below 60°C to obtain anhydrous pre-dispersed slurry. The mass ratio of calcium lignosulfonate to pine tar was 20:80.

[0027] Step 3: Preparation of multi-component composite impregnating agent. Modified coal tar pitch (obtained from coal tar pitch through oxidative heat treatment) with a softening point of 90℃ and a carbonization residue rate of 55% is heated to 120℃ to melt it into a fluid state. Under stirring, the pre-dispersed dehydrated slurry obtained in Step 2 and furfuryl alcohol resin with a viscosity of 100 mPa·s at 25℃ are added sequentially. The system temperature is maintained at 120℃ throughout the process, and the mixture is stirred and mixed for 30 min until all components are uniformly coexisting to obtain the composite impregnating agent. The mass fraction of each component is: modified coal tar pitch 40%, furfuryl alcohol resin 40%, pre-dispersed dehydrated slurry 20%, and the sum of the three is 100%.

[0028] Step 4: Vacuum pressure impregnation. Place the pretreated graphite matrix obtained in Step 1 into an impregnation container, evacuate to a vacuum level of not less than 0.09 MPa and maintain it for 30 min to remove gas from the pores; then inject the composite impregnating agent obtained in Step 3. Maintain the temperature of the impregnation container and the impregnating agent at 120℃ throughout the process by external heating, and impregnate at 0.3 MPa for 1 h to allow the impregnating agent to fully penetrate into the graphite pores; after the pressure holding is completed, release the pressure, remove the impregnated graphite material, and remove excess impregnating agent from the material surface.

[0029] Step 5: Curing treatment. After cooling the impregnated graphite material obtained in Step 4 to 60°C, heat it to 150°C at a heating rate of 2°C / min, and keep it at 150°C for 2 hours to cure completely, so that the composite impregnating agent in the pores is completely cured and formed, and corrosion-resistant graphite material is obtained.

[0030] Example 2 This embodiment discloses a preparation process for corrosion-resistant graphite material for heat exchange graphite coolers. The process uses a commercially available porous graphite matrix with a porosity of 20% as raw material and includes the following steps: Step 1: Alkali pretreatment of graphite matrix. The graphite matrix is ​​immersed in a 5% sodium carbonate solution at room temperature for 2 hours. After immersion, it is dried at 115℃ to constant weight to obtain the pretreated graphite matrix.

[0031] Step 2: Preparation of calcium lignosulfonate-pine tar pre-dispersed dehydrated slurry. Pine tar was heated to 130℃. Under stirring, calcium lignosulfonate powder with a water content of 10% was added to the hot pine tar in batches (each batch should not exceed 1 / 3 of the total amount, and the next batch should be added after the bubbles on the surface of the pine tar tend to stabilize). The mixture was kept at 130℃ and stirred for 90 min, and then cooled to below 60℃ to obtain anhydrous pre-dispersed slurry. The mass ratio of calcium lignosulfonate to pine tar was 30:70.

[0032] Step 3: Preparation of multi-component composite impregnating agent. Modified coal tar pitch (obtained from coal tar pitch through oxidative heat treatment) with a softening point of 100℃ and a carbonization residue rate of 62% is heated to 125℃ to melt it into a fluid state. Under stirring, the pre-dispersed dehydrated slurry obtained in Step 2 and furfuryl alcohol resin with a viscosity of 300 mPa·s at 25℃ are added sequentially. The system temperature is maintained at 125℃ throughout the process, and the mixture is stirred for 30 min until all components coexist uniformly to obtain the composite impregnating agent. The mass fraction of each component is: modified coal tar pitch 45%, furfuryl alcohol resin 40%, and pre-dispersed dehydrated slurry 15%, with the sum of the three being 100%.

[0033] Step 4: Vacuum pressure impregnation. Place the pretreated graphite matrix obtained in Step 1 into an impregnation container, evacuate to a vacuum level of not less than 0.09 MPa and maintain it for 35 min; then inject the composite impregnating agent obtained in Step 3. Maintain the temperature of the impregnation container and the impregnating agent at 125℃ throughout the process by external heating, and impregnate at a pressure of 0.5 MPa for 2 h; after the pressure holding is completed, release the pressure, take out the impregnated graphite material, and remove the excess impregnating agent from the surface of the material.

[0034] Step 5: Curing treatment. After cooling the impregnated graphite material obtained in Step 4 to 70°C, heat it to 175°C at a heating rate of 1°C / min, and keep it at 175°C for 3 hours to obtain corrosion-resistant graphite material.

[0035] Example 3 This embodiment discloses a preparation process (including carbonization treatment, suitable for operating conditions with temperatures greater than 400°C) of corrosion-resistant graphite material for heat exchange graphite coolers. The process uses a commercially available porous graphite matrix with a porosity of 22% as raw material and includes the following steps: Step 1: Alkali pretreatment of graphite matrix. The graphite matrix is ​​immersed in an 8% sodium carbonate solution at room temperature for 3 hours. After immersion, it is dried at 120℃ to constant weight to obtain the pretreated graphite matrix.

[0036] Step 2: Preparation of calcium lignosulfonate-pine tar pre-dispersed dehydrated slurry. Pine tar was heated to 140℃. Under stirring, calcium lignosulfonate powder with a water content of 15% was added to the hot pine tar in batches (each batch should not exceed 1 / 3 of the total amount, and the next batch should be added after the bubbles on the surface of the pine tar tend to stabilize). The mixture was kept at 140℃ and stirred for 120 minutes, and then cooled to below 60℃ to obtain anhydrous pre-dispersed slurry. The mass ratio of calcium lignosulfonate to pine tar was 40:60.

[0037] Step 3: Preparation of multi-component composite impregnating agent. Modified coal tar pitch (obtained from coal tar pitch through oxidative heat treatment) with a softening point of 110℃ and a carbonization residue rate of 70% is heated to 130℃ to melt it into a fluid state. Under stirring, the pre-dispersed dehydrated slurry obtained in Step 2 and furfuryl alcohol resin with a viscosity of 500 mPa·s at 25℃ are added sequentially. The system temperature is maintained at 130℃ throughout the process, and the mixture is stirred and mixed for 60 min until all components coexist uniformly to obtain the composite impregnating agent. The mass fraction of each component is: modified coal tar pitch 50%, furfuryl alcohol resin 30%, pre-dispersed dehydrated slurry 20%, and the sum of the three is 100%.

[0038] Step 4: Vacuum pressure impregnation. Place the pretreated graphite matrix obtained in Step 1 into an impregnation container, evacuate to a vacuum level of not less than 0.09 MPa and maintain it for 45 min; then inject the composite impregnating agent obtained in Step 3. Maintain the temperature of the impregnation container and the impregnating agent at 130℃ throughout the process by external heating, and impregnate at a pressure of 0.8 MPa for 3 h; after the pressure holding is completed, release the pressure, take out the impregnated graphite material, and remove the excess impregnating agent from the surface of the material.

[0039] Step 5: Curing and Carbonization Treatment. After cooling the impregnated graphite material obtained in Step 4 to 60°C, the temperature is increased to 200°C at a heating rate of 2°C / min. The material is then held at 200°C for 4 hours to ensure complete curing of the composite impregnating agent within the pores. After curing, nitrogen gas with a volume fraction of not less than 99.5% is introduced into the furnace to replace the air inside the furnace and is continuously introduced. The temperature is further increased to 800°C at a heating rate of 2°C / min. The material is then held at 800°C for 1.5 hours to carbonize, converting the organic curing product into carbonaceous residue, thus obtaining corrosion-resistant graphite material.

[0040] Example 4 This embodiment discloses a preparation process for corrosion-resistant graphite material for heat exchange graphite coolers. The process uses a commercially available porous graphite matrix with a porosity of 19% as raw material and includes the following steps: Step 1: Alkali pretreatment of graphite matrix. The graphite matrix is ​​immersed in a 6% sodium carbonate solution at room temperature for 4 hours. After immersion, it is dried at 115℃ to constant weight to obtain the pretreated graphite matrix.

[0041] Step 2: Preparation of calcium lignosulfonate-pine tar pre-dispersed dehydrated slurry. Pine tar was heated to 135℃. Under stirring, calcium lignosulfonate powder with a water content of 8% was added to the hot pine tar in batches (each batch should not exceed 1 / 3 of the total amount, and the next batch should be added after the bubbles on the surface of the pine tar tend to stabilize). The mixture was kept at 135℃ and stirred for 80 min, and then cooled to below 60℃ to obtain anhydrous pre-dispersed slurry. The mass ratio of calcium lignosulfonate to pine tar was 35:65.

[0042] Step 3: Preparation of multi-component composite impregnating agent. Modified coal tar pitch (obtained from coal tar pitch through oxidative heat treatment) with a softening point of 105℃ and a carbonization residue rate of 65% is heated to 128℃ to melt it into a fluid state. Under stirring, the pre-dispersed dehydrated slurry obtained in Step 2 and furfuryl alcohol resin with a viscosity of 250 mPa·s at 25℃ are added sequentially. The system temperature is maintained at 128℃ throughout the process, and the mixture is stirred for 45 min until all components coexist uniformly to obtain the composite impregnating agent. The mass fraction of each component is: modified coal tar pitch 48%, furfuryl alcohol resin 35%, and pre-dispersed dehydrated slurry 17%, with the sum of the three being 100%.

[0043] Step 4: Vacuum pressure impregnation. Place the pretreated graphite matrix obtained in Step 1 into an impregnation container, evacuate to a vacuum level of not less than 0.09 MPa and maintain it for 40 min; then inject the composite impregnating agent obtained in Step 3. Throughout the process, maintain the temperature of the impregnation container and the impregnating agent at 128℃ through external heating, and impregnate under pressure of 0.6 MPa for 2.5 h; after the pressure holding is completed, release the pressure, take out the impregnated graphite material, and remove the excess impregnating agent from the surface of the material.

[0044] Step 5: Curing treatment. After cooling the impregnated graphite material obtained in Step 4 to 75°C, heat it to 185°C at a heating rate of 1.5°C / min, and keep it at 185°C for 3.5 h to obtain corrosion-resistant graphite material.

[0045] Example 5 This embodiment discloses a preparation process for a corrosion-resistant graphite material for a heat exchange graphite cooler, comprising the following steps: Step 1: Alkali Pretreatment of Graphite Matrix The graphite matrix to be impregnated is immersed in a sodium carbonate solution with a mass fraction of 3%–8% for at least 2 hours to allow the solution to fully penetrate the graphite pores and clean the oily residues and particles on the inner wall surface of the pores. After removal, it is dried at 110–120°C to constant weight. After drying, a trace amount of sodium carbonate remains on the inner wall of the pores, making the pore walls alkaline. Preferably, the sodium carbonate solution has a mass fraction of 5%, the immersion time is 2 hours, and the drying temperature is 115°C.

[0046] The alkaline pore walls are more conducive to the wetting and spreading of phenolic hydroxyl components (phenolic components in pine tar, calcium lignosulfonate) in the subsequent impregnation agent, thereby improving the interfacial adhesion between the impregnation layer and the pore walls. Compared with the existing technology, this step does not require the use of special interface treatment agents, and only uses commonly used industrial alkaline solutions to simultaneously complete the two functions of pore cleaning and pore wall wettability improvement.

[0047] Step 2: Preparation of calcium lignosulfonate-pine tar pre-dispersed dehydrated slurry Pine tar is heated to 120–140°C. While stirring, calcium lignosulfonate powder (moisture content ≤15%) is added in batches to the hot pine tar (each batch should not exceed 1 / 3 of the total amount of calcium lignosulfonate; the next batch should be added only after the bubbles on the surface of the pine tar have stabilized and the rate of moisture vaporization has significantly decreased, to avoid concentrated moisture vaporization causing slurry splashing). The mixture is kept at 120–140°C and stirred for 1–2 hours. After stirring, the temperature is lowered to below 60°C to obtain a pre-dispersed dehydrated slurry. The mass ratio of calcium lignosulfonate to pine tar is (20–40):(60–80). Preferably, the pine tar is heated to 130°C, the stirring time is 90 min, and the mass ratio of calcium lignosulfonate to pine tar is 30:70.

[0048] This step is the core step of this process. It simultaneously removes moisture from the calcium lignosulfonate particles and organically modifies the particle surface through the following two functions: (1) Removal of physically adsorbed water and bound water: The high-temperature pine tar medium at 120-140℃ causes the physically adsorbed water and bound water in the calcium lignosulfonate particles to evaporate completely during the heating and stirring process, and the resulting slurry does not contain residual free water. This eliminates the root cause of vapor pore defects caused by moisture vaporization during the curing heating stage when calcium lignosulfonate with high moisture content is directly mixed into the organic impregnation system.

[0049] (2) Organic modification of particle surface: Pine tar contains phenolic polar components. During high-temperature stirring, the phenolic components are adsorbed onto the surface of calcium lignosulfonate particles, changing the particle surface from strong hydrophilicity to organic philicity. The organically coated calcium lignosulfonate particles form a uniform and stable dispersion in pine tar. When introduced into the subsequent composite impregnating agent in the form of pre-dispersed slurry, no agglomeration, sedimentation, or phase separation occurs, ensuring that all components in the impregnating agent coexist uniformly.

[0050] Pine tar contains both hydrophobic terpenoid components and phenolic polar components, belonging to an intermediate polar mixture. As a dehydration and pre-dispersion medium, it forms an effective wetting contact with calcium lignosulfonate particles in this step, making the particle surface fully organic. At the same time, its intermediate polar characteristics enable it to further assume the function of a polar bridging phase in the asphalt-resin system in step three. Pine tar plays different process roles in steps two and three, respectively, as a dehydration and dispersion medium and a polar bridging phase. The same raw material is used in both steps, eliminating the need for repeated feeding.

[0051] Compared with the existing technology of directly adding calcium lignosulfonate into the organic impregnation system, this step eliminates the agglomeration and sedimentation of calcium lignosulfonate particles due to their incompatibility with the organic system caused by hydrophilicity, as well as the root cause of vapor pore defects formed by the vaporization of residual moisture during curing and heating. This allows calcium lignosulfonate to be uniformly dispersed in the multi-component organic impregnating agent.

[0052] Step 3: Preparation of multi-component composite impregnating agent High-softening-point modified coal tar pitch is heated to 120–130°C until it melts into a fluid state. While stirring, the calcium lignosulfonate-pine tar pre-dispersion slurry obtained in step two and furfuryl alcohol resin are added sequentially. The system temperature is maintained at 120–130°C throughout the process, and stirring is carried out for 30–60 minutes until all components are uniformly coexisting, resulting in a multi-component composite impregnating agent. The mass fractions of each component are: modified coal tar pitch 40%–50%, furfuryl alcohol resin 30%–40%, and calcium lignosulfonate-pine tar pre-dispersion slurry 10%–20%, with each component taking values ​​within the above ranges and the sum of the mass fractions of the three components being 100%. Preferably, the mass fractions of each component are: modified coal tar pitch 45%, furfuryl alcohol resin 40%, and pre-dispersion dehydrated slurry 15%; the mixing time is 30 minutes.

[0053] Among them, high softening point modified coal tar pitch is obtained by oxidative heat treatment of coal tar pitch to increase its softening point to 90-110℃, with a carbonization residual rate of approximately 55%-70%. After carbonization, it forms a carbon-integrated bond with the graphite matrix, reducing residual porosity caused by the solidification shrinkage of the organic phase; furfuryl alcohol resin (viscosity of 100-500 at 25℃) The pine tar has good fluidity at room temperature (mPa·s), which can uniformly penetrate into the micropores of graphite. It has a low curing temperature and can ensure the sealing and density of the impregnated and cured layer under normal working conditions where the operating temperature does not exceed 300℃. Pine tar, with its intermediate polarity, enables the modified pitch (non-polar aromatic) and furfuryl alcohol resin (medium polar hydroxyl resin) to coexist uniformly and without phase separation in the composite impregnating agent. At the same time, its phenolic components stabilize the calcium lignosulfonate particles in a uniformly dispersed state. Calcium lignosulfonate exists in the composite impregnating agent in a uniformly dispersed state with organic coating of pine tar, which makes the wetting and spreading of the impregnating agent on the inner wall of the pores more uniform and contributes additional carbon during carbonization.

[0054] Step 4: Vacuum pressure impregnation The pretreated graphite matrix obtained in step one is placed in an impregnation container, and a vacuum is drawn to a level not lower than 0.09 MPa and maintained for at least 30 minutes to remove gas from the graphite pores. Then, the composite impregnating agent obtained in step three is injected into the impregnation container while maintaining the flow dynamics of the modified coal tar pitch component at 120–130°C. Throughout the impregnation process, the temperature of the impregnation container and the impregnating agent is maintained at 120–130°C through external heating to prevent the modified coal tar pitch component from re-curing due to cooling, which could block the pores and hinder the impregnating agent's penetration. Impregnation is carried out at a pressure of 0.3–0.8 MPa for 1–3 hours to allow the impregnating agent to fully penetrate the graphite pores. After the pressure holding period, the pressure is released, the impregnated graphite material is removed, and excess impregnating agent is removed from the material surface. Preferably, the impregnation pressure is 0.5 MPa, and the pressure holding impregnation time is 2 hours.

[0055] This step employs conventional vacuum pressure impregnation operations in the field. Since step one has improved the wettability of the pore walls, step two has eliminated the agglomeration of calcium lignin sulfonate, and step three has prepared a composite impregnating agent with uniform composition, the impregnating agent in this step can fill the interior of each graphite pore more uniformly. The resulting impregnated layer has a consistent composition in each pore, and the impregnation weight gain and pore blockage degree of each graphite matrix in the same batch are uniform.

[0056] Step 5: Curing treatment After cooling the impregnated graphite material obtained in step four to below 80°C, it is heated to 150–200°C at a heating rate not exceeding 2°C / min, and held at this temperature for 2–4 hours to allow the composite impregnating agent within the pores to completely solidify and form a corrosion-resistant graphite material. The curing heating rate should not exceed 2°C / min to avoid uneven heating within the impregnated layer, which could lead to localized thermal stress concentration. Preferably, the heating rate is 1°C / min, the curing temperature is 175°C, and the curing time is 3 hours.

[0057] For applications requiring operation at higher temperatures (>400℃), after the above curing process, a further carbonization treatment can be performed at 600–900℃ under nitrogen (volume fraction ≥99.5%) protection for 1–2 hours (preferably 800℃ for 1.5 hours). This process converts the organic curing products into carbonaceous residues, improving the material's structural stability under high-temperature conditions. Under conventional heat exchange graphite cooler operating conditions (operating temperature ≤300℃), the curing treatment is the final step and carbonization is unnecessary.

[0058] Experimental verification Experiment 1: The effect of pre-dispersion and dehydration process on the porosity of the impregnated and cured layer 1. Experimental Objective The effect of the calcium lignosulfonate-pine tar pre-dispersion dehydration process on eliminating steam pore defects and reducing the residual porosity of the impregnated and cured layer was verified.

[0059] 2. Preparation of experimental samples Two sets of comparative samples were prepared using commercial porous graphite matrix with a porosity of 20% as raw material.

[0060] Control sample A (without pre-dispersion and dehydration): The graphite matrix was pretreated with alkali solution according to step one of Example 2 (5% sodium carbonate solution, soaked for 2 hours, and dried at 115°C to constant weight); pine tar and calcium lignosulfonate powder with a water content of 10% were directly mixed with modified coal tar pitch (62% carbonation residue) with a softening point of 100°C and a viscosity of 300 at 25°C at 125°C without any pre-dispersion and dehydration treatment. A one-time mixing of furfuryl alcohol resin at mPa·s was performed; the mass fractions of each component were: 45% modified coal tar pitch, 40% furfuryl alcohol resin, 10.5% pine tar, and 4.5% calcium lignosulfonate, with the sum of the four being 100% (the amounts of pine tar and calcium lignosulfonate were calculated from 15% of the pre-dispersed dehydrated slurry in Example 2, i.e., pine tar accounted for 70% and calcium lignosulfonate accounted for 30% in the slurry, and the absolute amounts of each component were exactly the same as those of experimental sample B, except that it was not pre-dispersed and dehydrated); the mixture was stirred for 30 min to obtain a water-containing direct-mixed impregnating agent; vacuum pressure impregnation was performed according to step four of Example 2 (impregnation pressure 0.5 MPa, holding time 2 h, and maintaining impregnation temperature at 125℃ throughout); curing was performed according to step five of Example 2 (heating rate 1℃ / min, holding at 175℃ for 3 h).

[0061] Experimental Sample B (Example 2): Prepared according to all the steps of Example 2, including the pre-dispersion and dehydration process in step two.

[0062] 3. Experimental conditions The open porosity was determined using the Archimedes method (referring to GB / T 1966), with deionized water as the immersion medium and a test temperature of (20±2)℃. SEM cross-section observation was performed using a scanning electron microscope. The cross-section was cut with diamond wire and then polished to a surface roughness Ra not exceeding 0.4 μm. The gold sputtering thickness was approximately 10 nm, the accelerating voltage was 15 kV, and imaging was performed in backscattered electron (BSE) mode.

[0063] 4. Experimental Procedure (1) Prepare five control sample A and five experimental sample B (size 40 mm × 40 mm × 30 mm) according to the above preparation conditions.

[0064] (2) The porosity of each sample before and after immersion was determined by Archimedes method, and the mean and standard deviation of the five samples were calculated.

[0065] (3) Take one sample from each group and cut it along the central section to prepare a metallographic section. Observe the morphology of the pores inside the impregnated and cured layer under SEM. Count the number of vapor pores in each field of view (area 1 mm²), take the average value of 5 fields of view, and record the equivalent diameter range of vapor pores.

[0066] 5. Experimental Results Table 1 Comparison of porosity and vapor pore characteristics of samples prepared by the two processes

[0067] Figure 1 This is a bar chart comparing the open porosity of two groups of samples before and after immersion.

[0068] Figure 2 This is a SEM comparison image of the cross-section of the impregnated and cured layer of control sample A and experimental sample B.

[0069] Experiment 2: Comparison of the dispersion stability of calcium lignosulfonate in composite impregnating agents 1. Experimental Objective Verify the effectiveness of the pre-dispersion dehydration process in preventing the agglomeration and sedimentation of calcium lignosulfonate in the composite impregnating agent.

[0070] 2. Preparation of experimental samples Two composite impregnating agents were prepared according to the parameters of Example 2: Impregnating agent 1 (direct mixing, control): Pine tar and calcium lignosulfonate powder with a moisture content of 10% were directly mixed at 125°C with modified coal tar pitch (carbonization residue rate of 62%) with a softening point of 100°C and furfuryl alcohol resin with a viscosity of 300 mPa·s at 25°C without pre-dispersion and dehydration treatment. The mass fractions of each component were: modified coal tar pitch 45%, furfuryl alcohol resin 40%, pine tar 10.5%, and calcium lignosulfonate 4.5%, with the sum of the four being 100% (the dosage of each component was consistent with the parameters of Example 2 in impregnating agent 2, except that pre-dispersion and dehydration treatment was not performed, and the calcium lignosulfonate was directly added with a moisture content of 10%), and stirred for 30 min.

[0071] Impregnating agent 2 (pre-dispersion dehydration, Example 2): Prepare calcium lignosulfonate-pine tar pre-dispersion dehydration slurry according to step two of Example 2 (pine tar is heated to 130°C, the mass ratio of calcium lignosulfonate to pine tar is 30:70, kept warm and stirred for 90 min, and then cooled to below 60°C). Then prepare the composite impregnating agent according to step three (modified coal tar pitch 45%, furfuryl alcohol resin 40%, pre-dispersion dehydration slurry 15%, stirred at 125°C for 30 min).

[0072] 3. Experimental conditions Static settling experiment: 100 mL of each impregnating agent was placed in a transparent heat-resistant cylinder with an inner diameter of 25 mm and kept at a constant temperature of 120℃ in an oil bath. The height of the solid precipitate at the bottom of the cylinder was read (mm) after standing for 0.5 h, 1.0 h, 2.0 h, and 4.0 h, respectively. The precipitate height ratio (the ratio of precipitate height to the total height of the liquid column, %) was calculated. Each impregnating agent was repeated 3 times, and the mean and standard deviation were taken.

[0073] 4. Experimental Procedure (1) Prepare 300 mL of impregnating agent 1 and 300 mL of impregnating agent 2 according to the above preparation conditions, and put 100 mL of each into a graduated cylinder.

[0074] (2) Place the graduated cylinder in a 120℃ constant temperature oil bath and start timing. Read the height of the bottom sediment layer at 0.5 h, 1.0 h, 2.0 h and 4.0 h respectively and calculate the sediment layer height ratio.

[0075] (3) After standing for 4.0 h, take about 5 mg of sample from 5 mm below the liquid surface of each graduated cylinder and place it on the slide of an optical microscope. Observe the dispersion state of the calcium lignosulfonate particles under a 50x objective lens and take pictures to record.

[0076] 5. Experimental Results Table 2 Comparison of calcium lignosulfonate sedimentation behavior in the two impregnating agents

[0077] Figure 3 This is a line graph showing the change in the ratio of calcium lignosulfonate precipitate height in the two impregnating agents with standing time.

[0078] Figure 4 Comparison of particle dispersion under an optical microscope (50x) of samples taken from the upper part of the two impregnating agents after standing for 4 hours.

[0079] Experiment 3: Effect of Alkali Pretreatment on the Interfacial Bond Strength of the Impregnated Layer 1. Experimental Objective The study aimed to verify the effect of sodium carbonate alkaline solution pretreatment on improving the interfacial bonding strength between the impregnated layer and the graphite pore wall, and its stability under long-term action in corrosive media.

[0080] 2. Preparation of experimental samples Two sets of comparative samples were prepared using commercial porous graphite matrix with a porosity of 20% as raw material.

[0081] Sample C (without alkali pretreatment): The graphite matrix was prepared directly according to steps two to five of Example 2 without the alkali pretreatment in step one (all parameters are completely consistent with Example 2).

[0082] Sample D (Example 2): Prepared according to all the steps of Example 2, including the alkaline pretreatment in step one (soaking in 5% sodium carbonate solution for 2 h and drying at 115°C to constant weight).

[0083] 3. Experimental conditions Interfacial shear strength test: The impregnated and cured samples were machined into small cubes of 20 mm × 20 mm × 10 mm (with the impregnated layer intact). The samples were fixed to the shear test fixture with two-component epoxy adhesive. After curing at room temperature for 24 h, a shear force was applied on a tensile testing machine at a loading rate of 1 mm / min. The peak load at which failure occurred at the interface between the impregnated layer and the graphite matrix was recorded and converted into interfacial shear strength (MPa). Five samples were tested in each group, and the mean and standard deviation were taken.

[0084] Post-corrosion strength test: Take another sample from each group, soak it in 30% hydrochloric acid solution at (25±2)℃ for 720 h, remove it, remove the surface liquid and dry it, and determine the interfacial shear strength according to the above method, and calculate the strength retention rate (the ratio of post-corrosion strength to initial strength, %).

[0085] 4. Experimental Procedure (1) Prepare 12 samples of sample C and 12 samples of sample D (size 40 mm × 40 mm × 30 mm) according to the above preparation conditions, and take 10 samples of each group to process into test cubes.

[0086] (2) The initial interfacial shear strength of the uncorroded group (5 pieces each) was determined according to the method described in the experimental conditions.

[0087] (3) The corrosion immersion group (5 pieces each) was immersed for 720 h and then dried. The interfacial shear strength was measured and the strength retention rate was calculated.

[0088] (4) Take another cross-sectional sample from each group, prepare a polished cross-section, and observe the interface bonding state between the impregnation layer and the graphite pore wall under SEM.

[0089] 5. Experimental Results Table 3. Effect of alkaline pretreatment on the interfacial bonding strength of the impregnated layer

[0090] Figure 5 The bar chart shows the comparison of the initial and post-corrosion interfacial shear strength of the two groups of samples.

[0091] Figure 6 The images show a comparison of SEM images of the interface between the impregnated layer and the graphite pore wall in samples C and D.

[0092] Experiment 4: Effect of different impregnation formulations on the corrosion resistance of materials 1. Experimental Objective By comparing three schemes—single organic resin impregnation, two-component compound impregnation, and the multi-component composite impregnation of the present invention—the comprehensive advantages of the composite impregnation formulation of the present invention in terms of residual porosity, permeability, and corrosion resistance quality loss are verified.

[0093] 2. Preparation of experimental samples Using commercial porous graphite matrix with a porosity of 20% as raw material, all three groups of samples were pretreated with alkaline solution in step one of Example 2 (soaked in sodium carbonate solution with a mass fraction of 5% for 2 hours and dried at 115°C to constant weight).

[0094] Impregnation scheme 1 (single furfuryl alcohol resin): furfuryl alcohol resin with a viscosity of 300 mPa·s at 25℃ was used as the single impregnating agent. The resin was impregnated at 125℃ and 0.5 MPa for 2 h (the impregnation temperature was the same as that of schemes 2 and 3 to ensure uniformity of comparison conditions); the temperature was increased to 175℃ at a heating rate of 1℃ / min and cured at that temperature for 3 h.

[0095] Impregnation Scheme 2 (Modified Pitch-Furfuryl Alcohol Resin Two-Component): Modified coal tar pitch with a softening point of 100℃ (carbonization residue rate of 62%) and furfuryl alcohol resin with a viscosity of 300 mPa·s at 25℃ are mixed at a mass ratio of 45:55 (the furfuryl alcohol resin is used to make up the proportion of pine tar and calcium lignin sulfonate so that the absolute amount of modified coal tar pitch is the same as that in Scheme 3), and stirred at 125℃ for 30 min; impregnated under pressure at 0.5 MPa and 125℃ for 2 h; heated to 175℃ at a heating rate of 1℃ / min and cured at that temperature for 3 h.

[0096] Impregnation scheme 3 (multi-component composite formulation of Example 2): prepared according to all the steps of Example 2 (modified coal tar pitch 45%, furfuryl alcohol resin 40%, pre-dispersed dehydrated slurry 15%; 0.5 MPa, 2 h; heating to 175℃ at 1℃ / min, holding for 3 h).

[0097] 3. Experimental conditions (1) Residual porosity: Archimedes method, under the same conditions as Experiment 1.

[0098] (2) Permeability test: Each sample was processed into a disc with a diameter of 50 mm and a thickness of 10 mm. The disc was installed in the permeability test device. A 30% hydrochloric acid solution was introduced into one side with a pressure difference of 0.2 MPa. The leakage liquid on the other side was collected and weighed within 24 hours. The leakage amount (g) in 24 hours was calculated. Three samples were tested in each group and the average value was taken.

[0099] (3) Corrosion mass loss rate: The samples of each scheme were processed into thin plates of 40 mm × 40 mm × 10 mm and continuously immersed in a 30% hydrochloric acid solution at (25±2)℃ for 2160 h; every 360 h, the samples were taken out, rinsed with deionized water, and dried at 80℃ to constant weight. The mass was recorded and the cumulative mass loss rate (%) was calculated. Three samples were tested for each group and the average value was taken.

[0100] 4. Experimental Procedure (1) Prepare 10 samples (40 mm × 40 mm × 30 mm) according to the three impregnation schemes respectively. Take 3 samples from each scheme to process into corrosion test thin plates, 3 samples to process into permeability test discs, and 2 samples to determine residual porosity.

[0101] (2) Determine the residual porosity of each sample according to the experimental conditions (1).

[0102] (3) Measure the 24-hour leakage of each sample according to experimental conditions (2).

[0103] (4) Conduct corrosion immersion experiments according to experimental conditions (3), weigh the samples at 0, 360, 720, 1080, 1440, 1800 and 2160 h respectively, and record the cumulative mass loss rate.

[0104] 5. Experimental Results Table 4 Comparison of main performance indicators of the three impregnation schemes

[0105] Table 5. Cumulative mass loss rate (%) during corrosion immersion for the three methods

[0106] Figure 7 A bar chart comparing the residual porosity and 24-hour leakage of the three impregnation schemes.

[0107] Figure 8 The graph shows the corrosion mass loss rate of samples from three different immersion schemes in 30% hydrochloric acid solution as a function of immersion time.

[0108] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A preparation process for a corrosion-resistant graphite material for a heat exchange graphite cooler, characterized in that, Includes the following steps: The graphite matrix was immersed in a sodium carbonate solution, removed and dried to obtain a pretreated graphite matrix with sodium carbonate remaining on the inner wall of the pores. Calcium lignosulfonate powder was added in batches to pine tar heated to 120-140℃, and stirred at 120-140℃ for 1-2 hours. The mixture was then cooled to below 60℃ to obtain a pre-dispersed dehydrated slurry, wherein the mass ratio of calcium lignosulfonate to pine tar was (20-40):(60-80). Modified coal tar pitch is heated to 120-130℃ to melt it into a fluid state, and the pre-dispersed dehydrated slurry and furfuryl alcohol resin are added in sequence. The mixture is kept at 120-130℃ and stirred until homogeneous to obtain a composite impregnating agent. After the pretreated graphite matrix is ​​evacuated, the composite impregnating agent is injected and kept at 120-130°C, and then impregnated under pressure of 0.3-0.8 MPa. The impregnated graphite material is heated to 150-200℃ at a heating rate not exceeding 2℃ / min, and then cured at this temperature for 2-4 hours.

2. The preparation process according to claim 1, characterized in that, The mass fractions of each component in the composite impregnating agent are: 40%–50% modified coal tar pitch, 30%–40% furfuryl alcohol resin, and 10%–20% pre-dispersed dehydrated slurry, with the sum of the mass fractions of each component being 100%.

3. The preparation process according to claim 1, characterized in that, The viscosity of the furfuryl alcohol resin at 25°C is 100–500 mPa·s.

4. The preparation process according to claim 1, characterized in that, The softening point of the modified coal tar pitch is 90–110℃, and the carbonization residue rate is 55%–70%.

5. The preparation process according to claim 4, characterized in that, The modified coal tar pitch is obtained by oxidative heat treatment of coal tar pitch.

6. The preparation process according to claim 1, characterized in that, The sodium carbonate solution has a mass fraction of 3% to 8%, the soaking time is not less than 2 hours, and the drying is carried out at 110 to 120°C until constant weight is achieved.

7. The preparation process according to claim 1, characterized in that, The moisture content of the calcium lignosulfonate powder does not exceed 15%.

8. The preparation process according to claim 1, characterized in that, In the vacuuming step, the vacuum level is not lower than 0.09 MPa and is maintained for more than 30 minutes before the composite impregnating agent is injected; the pressure holding impregnation time is 1 to 3 hours.

9. The preparation process according to claim 1, characterized in that, After the curing is completed, the temperature is raised to 600-900°C under nitrogen protection with a volume fraction of not less than 99.5% for carbonization treatment, and the holding time is 1-2 hours.

10. A corrosion-resistant graphite material for a heat exchange graphite cooler prepared by the preparation process described in any one of claims 1 to 9.

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