Modified activated carbon as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polycrystalline silicon production, in particular to modified activated carbon and a preparation method and application thereof.The preparation method includes the steps that activated carbon serves as a raw material, nitric acid oxidation, phosphoric acid impregnation and thiourea group loading are sequentially conducted, and the modified activated carbon with the surface rich in carboxylic acid groups and thiourea chelation sites is prepared; a composite functional group system with electrostatic attraction, coordination chelation and pore capture capabilities is constructed on the surface of activated carbon through a multi-step modification process, so that the problems of poor selectivity, low adsorption capacity and limited wet adsorption in a traditional method are solved, and the metal impurities in the cold hydrogenated slurry high-boiling residues are efficiently and greenly removed.

Description

Technical Field

[0001] The present invention relates to the technical field of polysilicon production, and particularly to a modified activated carbon, a preparation method thereof, and an application thereof. Background Art

[0002] In the fields of organosilicon materials and polysilicon production, the slurry high-boiling substances (mainly containing chlorosilane compounds) generated in the cold hydrogenation process often contain metal impurities such as aluminum and titanium (existing in the forms of Al 3+ and Ti 4+ plasma). The existence of these impurities will affect the subsequent recycling of chlorosilane and product quality. Therefore, the removal of metal impurities is a key link.

[0003] In the prior art, the removal of metals from the cold hydrogenation slurry high-boiling substances mainly relies on chemical complexation method or distillation method; the chemical complexation method forms a complex by adding a specific complexing agent (such as organic carboxylic acid, amine compound) with metal ions, and then realizes the removal through phase separation. However, this method has problems such as complex process route, large consumption of complexing agent, harsh reaction conditions, etc. Moreover, the generated complex waste liquid contains a large amount of organic matter and heavy metals, with high treatment cost and great environmental risk; the distillation method separates chlorosilane and metal impurities through high-temperature rectification. However, the chlorosilane high-boiling substance system has high viscosity and high boiling point, resulting in extremely high energy consumption during the distillation process, and metal impurities are likely to deposit on the surface of the equipment and exacerbate the equipment corrosion at high temperature, significantly increasing the maintenance cost.

[0004] Activated carbon is widely used in the fields of industrial wastewater treatment, gas purification, etc. due to its rich pore structure and certain surface adsorption capacity; however, traditional activated carbon has significant defects in the adsorption of high-valent metal ions such as Al 3+ and Ti 4+ : on the one hand, its surface functional groups are mainly hydroxyl groups, carbonyl groups, etc., with weak coordination ability to high-valent metal ions and poor adsorption selectivity; on the other hand, it lacks specific chelating sites, resulting in low adsorption capacity and being difficult to meet the removal requirements of trace but highly harmful metal impurities in the chlorosilane system; in addition, chlorosilane compounds have the characteristic of being easily hydrolyzed by water (such as generating HCl and siloxane polymers), which makes the traditional wet adsorption process unable to be directly applied - the introduction of moisture will not only cause the hydrolysis of chlorosilane and result in material loss, but also generate corrosive waste liquid, further exacerbating the process complexity.

[0005] The information disclosed in this background art section is only intended to deepen the understanding of the overall background art of the present invention, and should not be regarded as an admission or any form of implication that this information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a modified activated carbon, its preparation method and application. Through a multi-step modification process, a composite functional group system with electrostatic attraction, coordination chelation and pore capture capabilities is constructed on the surface of the activated carbon to solve the problems of poor selectivity, low adsorption capacity and limited wet adsorption in traditional methods, so as to achieve efficient and green removal of metal impurities in the high-boiling components of the cold hydrogenation slurry.

[0007] The first object of the present invention is to provide a preparation method of modified activated carbon, including: using activated carbon as raw material, successively undergoing nitric acid oxidation, phosphoric acid impregnation and loading thiourea groups to obtain modified activated carbon with carboxylic acid groups and thiourea chelation sites rich on the surface.

[0008] More specifically, the activated carbon used is coconut shell activated carbon.

[0009] During the nitric acid oxidation process, the carbon skeleton on the surface of the activated carbon is oxidized by nitric acid to introduce oxygen-containing functional groups. The specific reaction is:

[0010] C + 4HNO3 → -COOH + NOx↑ + 2H2O;

[0011] In the above process, some phenolic hydroxyl groups (-OH), carbonyl groups (C=O), etc. are generated, but mainly carboxylic acid groups, which improve the surface polarity and coordination ability.

[0012] Phosphoric acid molecules react with oxygen-containing functional groups such as hydroxyl groups (-OH) and carboxylic acid groups (-COOH) on the surface and inside of the activated carbon, promoting dehydration and partial decomposition of the carbon skeleton, and gradually expanding the originally isolated micropores through etching and connection of the carbon structure to form mesopores with larger pore diameters.

[0013] In the above preparation method, carboxylic acid groups are first generated on the outer surface of the activated carbon particles and at the openings of the mesopores during nitric acid oxidation (because the oxidant first contacts these areas), forming an "outer layer negative charge enrichment area". When Al 3+ , Ti 4+ in the chlorosilane high-boiling components flow through the activated carbon particles with the fluid, the -COO - of the outer layer carboxylic acid groups quickly captures metal ions through electrostatic attraction to form a preliminary adsorption; after thiourea molecules diffuse into the mesopores through the ethanol solution, they are uniformly loaded on the inner wall of the mesopores and deep in some connected micropores through hydrogen bonding (-NH2 binds to -COOH) and pore adsorption to form inner layer specific chelation sites. The pre-adsorbed metal ions diffuse inward along the mesopore channels under the drive of the concentration gradient and undergo a strong coordination reaction with the sulfur (S) and nitrogen (N) atoms of the thiourea groups to form stable chelate structures (such as five-membered ring and six-membered ring complexes).

[0014] The three-step process complements each other. Nitric acid oxidation pre-introduces oxygen-containing functional groups such as carboxylic acid groups on the surface of activated carbon micropores, which not only provides anchor points for the subsequent covalent fixation of thiourea groups to ensure the uniform distribution of chelation sites, but also serves as primary adsorption sites for metal ions to achieve outer-layer pre-capture; subsequent phosphoric acid activation uses these functional groups as reaction guiding points to directionally etch the carbon skeleton through esterification and dehydration, connecting and expanding some micropores into mesopores to form a hierarchical pore network of "mesopore high-efficiency diffusion channels - micropore strong adsorption sites", enabling metal ions to rapidly mass transfer to the inner-layer thiourea chelation sites through mesopores. At the same time, the outer-layer carboxylic acid groups pre-enrich ions through electrostatic interaction, ultimately achieving a double improvement of "optimized mass transfer path - synergistic adsorption sites", significantly enhancing the capture efficiency and capacity of the material for target pollutants.

[0015] As a preferred embodiment of the present invention, nitric acid oxidation is carried out using nitric acid with a concentration of 7-15%, treating at 70-100 °C for 1-5 h;

[0016] Preferably, the nitric acid concentration is 10%, treating at 80 °C for 2 h;

[0017] More specifically, activated carbon and nitric acid solution are added to the reaction kettle according to a solid-liquid ratio of 1:5-1:10 (g / mL), stirred and heated to the set temperature, after constant-temperature reaction, cooled to room temperature, dehydrated by a dehydrator and washed with deionized water until the pH of the filtrate is neutral to avoid the influence of residual acid on subsequent steps, and then vacuum dried at 80 °C for 12 h;

[0018] Too low concentration or temperature will lead to insufficient generation of carboxylic acid groups, with weak electrostatic attraction and coordination ability; too high concentration or temperature may cause over-oxidation, damaging the pore structure of activated carbon and reducing the specific surface area.

[0019] As a preferred embodiment of the present invention, phosphoric acid impregnation is carried out using phosphoric acid with a concentration of 4-8%, activating at 110-130 °C for 2-5 h, then placing it into a tubular furnace and heating to 380-420 °C, keeping warm, washing and drying after cooling to room temperature to obtain nitric acid-phosphoric acid double-modified activated carbon;

[0020] Preferably, the phosphoric acid concentration is 5%, activating at 120 °C for 3 h, and the temperature of the tubular furnace is 400 °C;

[0021] More specifically, the oxidized activated carbon is immersed in phosphoric acid solution, ultrasonically activated at 120 °C for 3 h and then placed into a tubular furnace, heated to 400 °C at a rate of 5 °C / min, kept warm for 1 h, and phosphoric acid reacts with activated carbon at high temperature to form a microporous structure. The reaction equation is:

[0022] C + H3PO4 → C-O-PO3H2 + CO↑ + H2O↑;

[0023] After cooling to room temperature, wash with boiling water until there is no acid residue, and dry at 110 °C for 6 hours to obtain nitric acid-phosphoric acid double-modified activated carbon;

[0024] The phosphorus-oxygen functional group C-O-PO3H2 formed by the combination of phosphoric acid and the surface of activated carbon can enhance the surface polarity of activated carbon, and cooperate with the carboxylic acid groups (-COOH) introduced by nitric acid oxidation to provide more active sites for the subsequent loading of thiourea groups; CO and H2O are the gas products released during the reaction process, and microporous structures are formed inside the activated carbon through gas overflow, improving the specific surface area and porosity of the material;

[0025] Low concentration of phosphoric acid or low temperature leads to insufficient activation and insignificant pore expansion, affecting the diffusion of metal ions. High concentration of phosphoric acid or high temperature may cause excessive carbonization of activated carbon and collapse of the pore structure.

[0026] As a preferred embodiment of the present invention, the loading of thiourea groups is carried out by impregnating with a thiourea ethanol solution with a concentration of 4-8%, and then washing and drying;

[0027] Preferably, the concentration of the thiourea ethanol solution is 5%;

[0028] More specifically, prepare a thiourea ethanol solution with a concentration of 5% (ethanol volume fraction ≥ 95%), immerse the activated carbon after activation in the thiourea ethanol solution according to a solid-liquid ratio of 1:8-1:12 (g / mL), perform ultrasonic oscillation impregnation at 80 °C for 6 h (ensure that thiourea fully diffuses into the pores), and control the pH between 7-8, then use deionized water to wash away the residual thiourea ethanol solution, and then vacuum dry at 60-80 °C for 4-6 h;

[0029] A low concentration of thiourea ethanol solution will result in insufficient chelating sites and low adsorption capacity. A high concentration of thiourea ethanol solution is likely to cause the aggregation of thiourea at the pore mouth, blocking the channels;

[0030] After thiourea (H2NCSNH2) is dissolved in ethanol, the amino group (-NH2) in its molecule combines with the carboxylic acid group (-COOH) generated by nitric acid oxidation on the surface of activated carbon through a hydrogen bond (-NH2…O=C-OH). At the same time, due to the sufficient space provided by the mesopores formed by phosphoric acid activation, the thiourea molecules are evenly dispersed and adsorbed on the inner wall of the pores to achieve preliminary loading;

[0031] During the drying process (60-80 °C), the amino group (-NH2) of thiourea and the carboxylic acid group (-COOH) undergo a dehydration condensation reaction to form an amide bond (-CONH-), covalently grafting the thiourea group onto the surface of activated carbon, while retaining the sulfur atom (S) and amino nitrogen atom (N) in the thiocarbonyl group (-CS-) as chelating sites. This reaction stably anchors the thiourea group, preventing it from falling off during the adsorption process and ensuring the adsorption of metal ions (such as Al 3+ 、Ti 4+) coordination ability.

[0032] As a preferred embodiment of the present invention, the particle size of the activated carbon is 0.5 - 1 mm; if the particles are too large (>1 mm), it will increase the internal diffusion resistance, and it is difficult for metal ions to reach the chelation sites deep in the pores; if the particles are too small (<0.5 mm), it is easy to cause an increase in the pressure drop of the adsorption bed layer, affecting the operation efficiency of industrial devices;

[0033] Before nitric acid oxidation, it is placed in deionized water and ultrasonically cleaned 2 - 3 times (15 - 30 minutes each time) to remove surface dust and impurity ions remaining in the production process; the cleaned activated carbon is placed in a vacuum drying oven and dried at 110 - 125 °C for 6 - 8 hours until the difference in weight between two consecutive weighings is <0.1% (i.e., in a constant weight state), ensuring that the water content is <0.5%.

[0034] The second object of the present invention is to provide a modified activated carbon prepared by the above - mentioned preparation method of modified activated carbon; the modified activated carbon prepared by the above - mentioned preparation method uses activated carbon with a particle size of 0.5 - 1 mm as the raw material, introduces carboxylic acid groups through nitric acid oxidation to provide electrostatic attraction and preliminary coordination, impregnates and activates with phosphoric acid to optimize the pore structure and promote mass transfer, and loads thiourea groups to form specific chelation sites. The three work together to construct a "gradient adsorption structure of outer - layer carboxylic acid pre - adsorption - inner - layer thiourea strong chelation", which has both the specificity of chemical adsorption and the advantage of physical pore mass transfer, perfectly adapts to the dry adsorption environment of chlorosilane high - boilers, effectively solves the adsorption defects of traditional activated carbon, and at the same time overcomes the problems of complex existing metal removal processes, strong corrosiveness, and difficult waste liquid treatment, realizing the efficient removal of metal impurities and the greening of the process.

[0035] The third object of the present invention is the application of the modified activated carbon in the removal of metals from chlorosilane high - boilers, and the application includes a device for removing metals from chlorosilane high - boilers;

[0036] The device for removing metals from chlorosilane high - boilers includes:

[0037] An adsorption column body filled with modified activated carbon inside;

[0038] More specifically, the outer shell of the adsorption column body is made of corrosion - resistant Hastelloy, and a gas distributor (such as a perforated sieve plate) and a fluid homogenization device are provided inside to ensure that the chlorosilane high - boilers pass through the modified activated carbon bed layer evenly; 3 - stage series - connected adsorption units are provided inside the adsorption column body, and each unit is filled with modified activated carbon (loading density 0.4 g / cm 3 ); every 4 adsorption units are taken as a group, and there are 3 groups in total. The 3 groups cycle in the adsorption, desorption, and standby states.

[0039] As a preferred embodiment of the present invention, the device for removing metals from chlorosilane high - boilers further includes:

[0040] The temperature control unit is used to control the adsorption temperature of the adsorption column body. More specifically, the temperature control unit adopts a jacketed adsorption column design, and heat transfer oil is introduced into the jacket;

[0041] The gas protection unit is used to maintain the oxygen content in the adsorption column body less than 10ppm and desorption, more specifically, to maintain the oxygen content in the column less than 10ppm, to prevent the entry of external oxygen to cause oxidation of chlorosilane or failure of functional groups on the surface of activated carbon; when the modified activated carbon is saturated with adsorption, the gas protection unit switches to the desorption mode, and introduces high-temperature nitrogen to purge the activated carbon in the column, taking the adsorbed metal chloride out with the desorption gas, while preventing oxygen from contacting the high-temperature activated carbon to cause oxidation loss;

[0042] An automated feeding unit is used to continuously feed the high-boiling chlorosilane. More specifically, the automated feeding unit includes a centrifuge, and the slurry is pre-treated by centrifugation to remove solid particles; and also includes a metering pump or a magnetic pump and a flow meter, etc., to achieve continuous feeding of the high-boiling product;

[0043] The three units work together to create a suitable temperature, inert gas environment and stable feeding conditions for the main body of the adsorption column, significantly improving the removal effect and operating stability of the entire device on metal impurities in high-boiling chlorosilanes.

[0044] As a preferred embodiment of the present invention, a method for using a chlorosilane high-boiling metal removal device comprises:

[0045] After the solid particles of chlorosilane high boiling products are removed by the automated feeding unit, liquid high boiling products are obtained; removing solid particles can prevent these particles from blocking the pore structure of modified activated carbon. Modified activated carbon has a rich pore structure and is a key place for metal ion adsorption. If solid particles enter and block these pores, it will seriously affect the diffusion of metal ions into the activated carbon, thereby reducing the adsorption efficiency and adsorption capacity; at the same time, reducing solid impurities also helps to protect subsequent equipment and pipelines, and reduce the risk of equipment wear and blockage;

[0046] The gas protection unit maintains the oxygen content in the adsorption column body at <10ppm; the chlorosilane component in the high-boiling chlorosilane is prone to oxidation reaction in an aerobic environment, and the generated oxidation products may block the adsorption sites of the activated carbon, reducing the adsorption performance of the activated carbon for metal ions; in addition, the functional groups on the surface of the activated carbon (such as thiourea groups, etc.) may also be oxidized and destroyed in the presence of oxygen, resulting in a decrease in its chelating ability with metal ions. Maintaining a low-oxygen environment can effectively avoid these problems and ensure the efficient adsorption process;

[0047] Liquid high boiling substances pass through the main body of the adsorption column, and metal ions are selectively chelated with the functional groups on the surface of the modified activated carbon; the carboxylic acid groups on the outer layer of the modified activated carbon can quickly adsorb a large number of metal ions, play a role in preliminary enrichment and concentration reduction, and create favorable conditions for the deep chelation of the inner thiourea groups. The thiourea groups have strong selectivity and chelating ability for specific metal ions, which can ensure the efficient removal of target metal ions while reducing the impact on other components;

[0048] The adsorption saturated modified activated carbon is subjected to high-temperature nitrogen purging by a gas protection unit to desorb metals to obtain desorbed gas, and the modified activated carbon is regenerated and recycled; more specifically, the nitrogen temperature is 300-350°C, and the high-temperature nitrogen purging desorption process can restore the adsorption performance of the modified activated carbon and achieve recycling, which not only reduces production costs, but also reduces waste generation, meeting the requirements of green chemistry and sustainable development; at the same time, the desorption operation is carried out under nitrogen protection to avoid oxidation reaction of the activated carbon due to contact with oxygen at high temperature, thereby extending the service life of the activated carbon.

[0049] The metal chloride is recovered after the desorbed gas is condensed; the desorbed gas flowing out of the adsorption column contains high-temperature gaseous metal chloride (such as AlCl3, TiCl4, etc.), and the desorbed gas is introduced into the condenser. The condenser reduces the temperature of the desorbed gas to below the condensation temperature of the metal chloride through the cooling medium, so that the metal chloride is converted from gas to liquid, thereby achieving separation from nitrogen. The condensed liquid metal chloride is collected in a recovery tank for further processing and utilization;

[0050] This adsorption and desorption method builds an efficient and green treatment system through multi-unit collaboration and process innovation: first, the automated feeding unit centrifugal filtration removes solid particles to ensure that the pores of the modified activated carbon are unobstructed and reduce equipment wear; second, the gas protection unit uses high-purity nitrogen to maintain the oxygen content in the adsorption column less than 10ppm, avoiding chlorosilane oxidation and the failure of the activated carbon functional groups, and creating a stable inert environment; at the same time, the modified activated carbon achieves rapid enrichment and deep removal of metal ions through the gradient structure of "outer carboxylic acid pre-adsorption + inner thiourea chelation"; after adsorption saturation, high-temperature nitrogen at 300-350℃ is purged and desorbed in an inert atmosphere, and the regenerated activated carbon is recycled, which not only protects the structure but also reduces costs; finally, the desorbed gas is condensed to recover metal chlorides to achieve resource recycling. This method is closely linked and breaks through traditional bottlenecks. While efficiently removing metal impurities, it achieves a closed loop of "adsorption-regeneration-recycling", which is both economical and environmentally friendly, and significantly improves the value of industrial applications.

[0051] As a preferred embodiment of the present invention, when the liquid high-boiling substances pass through the main body of the adsorption column, the adsorption temperature is controlled by the temperature control unit to be 60 - 80 °C; the adsorption temperature of 60 - 80 °C is deeply adapted to the "gradient functional group adsorption" mechanism of the modified activated carbon: it not only avoids the increase in the viscosity of high-boiling substances and the rise in mass transfer resistance caused by low temperature, but also prevents the decomposition of thiourea groups or the cracking of chlorosilanes caused by high temperature; within this temperature range, the negative charge density of carboxylic acid groups and the coordination bond energy of thiourea groups reach the best balance, improving the synergistic efficiency of outer-layer electrostatic capture and inner-layer chelation reaction, reducing the diffusion path resistance of metal ions in the pores, and significantly optimizing the removal rate and adsorption capacity.

[0052] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) Using activated carbon as the raw material, through three steps of modification including nitric acid oxidation, phosphoric acid impregnation, and loading thiourea groups, carboxylic acid groups are introduced to provide electrostatic attraction and preliminary coordination, phosphoric acid impregnation optimizes the pore structure to promote mass transfer, and loading thiourea forms specific chelation sites, constructing a "gradient adsorption structure of outer-layer carboxylic acid pre-adsorption - inner-layer thiourea strong chelation", which combines the advantages of chemical adsorption specificity and physical pore mass transfer, and can effectively solve the adsorption defects of traditional activated carbon;

[0053] 2) The metal removal device for chlorosilane high-boiling substances integrates the main body of the adsorption column, the temperature control unit, the gas protection unit, and the automatic feeding unit, and each unit works synergistically: the temperature control unit maintains the optimal adsorption temperature of 60 - 80 °C, the gas protection unit ensures a low-oxygen environment to avoid material oxidation and the failure of functional groups, and the automatic feeding unit removes solid particles and realizes continuous feeding, jointly creating stable and efficient operating conditions for the adsorption column and significantly improving the overall performance of the device;

[0054] 3) Adopting a "pretreatment - adsorption - desorption - recovery" closed-loop process, first remove solid particles through the automatic feeding unit to protect the pores of activated carbon, and use the gradient adsorption structure of modified activated carbon to efficiently remove metal ions in a low-oxygen environment; after adsorption saturation, high-temperature nitrogen purging is used to regenerate the activated carbon for recycling, and at the same time, metal chlorides are recovered by condensation; this method takes into account both efficiency and environmental protection, realizes resource recycling while reducing costs, and breaks through the limitations of traditional processes. Description of the Drawings

[0055] Figure 1 is a schematic structural diagram of the metal removal device for chlorosilane high-boiling substances of the present invention;

[0056] Reference numerals in the drawings: 1, main body of the adsorption column; 2, modified activated carbon; 3, temperature control unit; 4, gas protection unit; 5, automatic feeding unit; 6, condenser. Detailed Embodiments

[0057] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention will be given with reference to the drawings of the specification.

[0058] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0059] Secondly, the so-called "embodiment" refers to specific features, structures, or characteristics that may be included in at least one implementation manner of the present invention. The phrase "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it an individual or alternative embodiment that is mutually exclusive with other embodiments.

[0060] This embodiment provides a method for preparing modified activated carbon:

[0061] Example 1:

[0062] S1 Select coconut shell activated carbon with a particle size of 0.8 - 1.0 mm, place it in deionized water and ultrasonically clean it 3 times, with each ultrasonic time being 20 min, to remove surface dust and impurity ions; the cleaned activated carbon is placed in a vacuum drying oven and dried at 120 °C for 7 h until the difference in weight between two consecutive weighings is < 0.1%, ensuring that the water content is < 0.5%.

[0063] S2 Add the pretreated activated carbon and a 10% nitric acid solution to the reaction kettle according to a solid-liquid ratio of 1:8 (g / mL), stir and heat to 80 °C, and react at a constant temperature for 2 h; after the reaction is completed, cool to room temperature, dehydrate through a dehydrator, and then wash with deionized water until the pH of the filtrate is neutral to avoid the influence of residual acid on subsequent steps; finally, vacuum-dry the material at 80 °C for 12 h to complete the nitric acid oxidation step and introduce a large number of carboxylic acid groups on the surface of the activated carbon;

[0064] S3 Immerse the oxidized activated carbon in a 5% phosphoric acid solution and perform ultrasonic activation at 120 °C for 3 h to allow the phosphoric acid to fully penetrate into the pores of the activated carbon. Subsequently, transfer the material to a tubular furnace, heat it to 400 °C at a rate of 5 °C / min, hold for 1 h, cool to room temperature after the reaction is completed, wash with boiling water until there is no acid residue, and then dry at 110 °C for 6 hours to obtain nitric acid-phosphoric acid double-modified activated carbon;

[0065] S4 Prepare a 5% thiourea ethanol solution (ethanol volume fraction is 95%), immerse the above double-modified activated carbon in the thiourea ethanol solution according to a solid-liquid ratio of 1:10 (g / mL), perform ultrasonic oscillation impregnation at 80 °C for 6 h, and at the same time control the pH of the solution to be between 7 and 8. After the impregnation is completed, use deionized water to wash away the residual thiourea ethanol solution, and then vacuum-dry the activated carbon at 70 °C for 5 h to form modified activated carbon 2.

[0066] Example 2:

[0067] Different from Example 1, coconut shell activated carbon with a particle size of 0.6 - 0.8 mm was used in step S1;

[0068] Example 3:

[0069] Different from Example 1, nut shell activated carbon with a particle size of 0.8 - 1.0 mm was used in step S1;

[0070] Comparative Example 1:

[0071] Different from Example 1, no phosphoric acid impregnation was carried out;

[0072] S1 Select coconut shell activated carbon with a particle size of 0.8 - 1.0 mm, place it in deionized water and ultrasonically clean it 3 times, with each ultrasonic time being 20 min, to remove surface dust and impurity ions; the cleaned activated carbon is placed in a vacuum drying oven and dried at 120 °C for 7 h until the difference in weight between two consecutive weighings is < 0.1%, ensuring that the water content is < 0.5%.

[0073] S2 Add the pretreated activated carbon and a 10% nitric acid solution according to a solid - liquid ratio of 1:8 (g / mL) to the reaction kettle, stir and heat up to 80 °C, and keep the temperature constant for 2 h; after the reaction is completed, cool it to room temperature, dehydrate it through a dehydrator, and then wash it with deionized water until the pH of the filtrate is neutral to avoid the influence of residual acid on subsequent steps; finally, vacuum - dry the material at 80 °C for 12 h to complete the nitric acid oxidation step and introduce a large number of carboxylic acid groups on the surface of the activated carbon;

[0074] S3 Prepare a 5% thiourea ethanol solution (ethanol volume fraction is 95%), immerse the above - mentioned nitric acid - oxidized activated carbon according to a solid - liquid ratio of 1:10 (g / mL) in the thiourea ethanol solution, carry out ultrasonic oscillation impregnation at 80 °C for 6 h, and at the same time control the solution pH to be between 7 - 8. After the impregnation is completed, use deionized water to wash away the residual thiourea ethanol solution, and then vacuum - dry the activated carbon at 70 °C for 5 h to form modified activated carbon 2.

[0075] Comparative Example 2:

[0076] Different from Example 1, no nitric acid oxidation was carried out;

[0077] S1 Select coconut shell activated carbon with a particle size of 0.8 - 1.0 mm, place it in deionized water and ultrasonically clean it 3 times, with each ultrasonic time being 20 min, to remove surface dust and impurity ions; the cleaned activated carbon is placed in a vacuum drying oven and dried at 120 °C for 7 h until the difference in weight between two consecutive weighings is < 0.1%, ensuring that the water content is < 0.5%.

[0078] S2 Immerse the coconut shell activated carbon in a 5% phosphoric acid solution, perform ultrasonic activation at 120 °C for 3 h to allow the phosphoric acid to fully penetrate into the pores of the activated carbon. Subsequently, transfer the material to a tubular furnace, heat it to 400 °C at a rate of 5 °C / min, hold for 1 h. After the reaction ends and cools to room temperature, wash it with boiling water until there is no acid residue, and then dry it at 110 °C for 6 hours to obtain phosphoric acid-modified activated carbon 2;

[0079] S3 Prepare a 5% thiourea ethanol solution (ethanol volume fraction is 95%). Immerse the above-mentioned phosphoric acid-modified activated carbon 2 in the thiourea ethanol solution at a solid-liquid ratio of 1:10 (g / mL), perform ultrasonic oscillation impregnation at 80 °C for 6 h, and at the same time control the solution pH between 7 and 8. After the impregnation ends, use deionized water to wash away the residual thiourea ethanol solution. Subsequently, dry the activated carbon under vacuum at 70 °C for 5 h to form modified activated carbon 2.

[0080] Comparative Example 3:

[0081] The difference from Example 1 is that the thiourea group is not loaded;

[0082] S1 Select coconut shell activated carbon with a particle size of 0.8 - 1.0 mm, place it in deionized water and ultrasonically clean it 3 times, with each ultrasonic time being 20 min to remove surface dust and impurity ions; the cleaned activated carbon is placed in a vacuum drying oven and dried at 120 °C for 7 h until the difference in weight between two consecutive weighings < 0.1%, ensuring that the water content < 0.5%.

[0083] S2 Add the pretreated activated carbon and a 10% nitric acid solution to the reaction kettle at a solid-liquid ratio of 1:8 (g / mL), stir and heat to 80 °C, and carry out a constant-temperature reaction for 2 h; after the reaction ends, cool to room temperature, dehydrate through a dehydrator, and then wash it with deionized water until the pH of the filtrate is neutral to avoid the influence of residual acid on subsequent steps; finally, dry the material under vacuum at 80 °C for 12 h to complete the nitric acid oxidation step and introduce a large number of carboxylic acid groups on the surface of the activated carbon;

[0084] S3 Immerse the oxidized activated carbon in a 5% phosphoric acid solution, perform ultrasonic activation at 120 °C for 3 h to allow the phosphoric acid to fully penetrate into the pores of the activated carbon. Subsequently, transfer the material to a tubular furnace, heat it to 400 °C at a rate of 5 °C / min, hold for 1 h. After the reaction ends and cools to room temperature, wash it with boiling water until there is no acid residue, and then dry it at 110 °C for 6 hours to obtain nitric acid-phosphoric acid double-modified activated carbon.

[0085] Assemble the modified activated carbon 2 prepared in the above Examples 1-3 and Comparative Examples 1-3 into the adsorption column main body 1. The outer shell of the adsorption column main body 1 is made of corrosion-resistant Hastelloy, and a gas distributor (such as a porous sieve plate) and a fluid homogenization device are provided inside to ensure that the high-boiling chlorosilane uniformly passes through the bed layer of the modified activated carbon 2; three series-connected adsorption units are provided in the adsorption column main body 1, and each unit is filled with the modified activated carbon 2 (packing density 0.4g / cm 3 ); Every 4 adsorption units form a group, and there are 3 groups in total. The 3 groups cycle among the adsorption, desorption and standby states respectively.

[0086] Apply the adsorption column main body 1 to the metal removal device for high-boiling chlorosilane. The metal removal device for high-boiling chlorosilane further includes:

[0087] The temperature control unit 3 adopts a jacketed adsorption column design. Dimethyl silicone oil is introduced into the jacket as heat transfer oil, and an electric heating rod and a PID temperature controller are equipped. The temperature control accuracy is ±1°C, and the adsorption temperature of the adsorption column main body 1 is accurately controlled at 60-80°C;

[0088] Gas protection unit 4: It consists of a high-purity nitrogen cylinder group (purity ≥ 99.999%), a mass flow meter, a pressure sensor, etc., to maintain the oxygen content in the adsorption column main body 1 < 10 ppm with high-purity nitrogen; and provide nitrogen purge desorption at a temperature of 300°C;

[0089] Automated feeding unit 5: It includes a horizontal screw centrifuge for centrifugally pretreating the slurry to separate solid particles with a particle size ≥ 5 μm; a high-precision metering pump is used in combination with an electromagnetic flow meter to achieve continuous and stable feeding of the high-boiling chlorosilane.

[0090] The usage method of the above metal removal device for high-boiling chlorosilane is as follows:

[0091] Start the temperature control unit 3. Under the action of the electric heating system and the PID temperature controller, the dimethyl silicone oil in the jacket stabilizes the temperature of the adsorption column main body 1 at 75°C;

[0092] Before starting the adsorption column, turn on the gas protection unit 4. The high-purity nitrogen cylinder group (purity ≥ 99.999%, pressure: 15 MPa) passes through the mass flow meter and purges the adsorption column at a flow rate of 25 L / min for 45 min to reduce the oxygen content in the column to 8 ppm; during the adsorption process, nitrogen is continuously introduced, and the flow rate is adjusted to 10 L / h to maintain a slightly positive pressure environment of 8 kPa in the column to prevent external oxygen from entering;

[0093] The liquid high-boiling substances pass through the adsorption column main body 1 from bottom to top and sequentially pass through three series-connected adsorption units;

[0094] When the metal ion concentration at the outlet of the adsorption column reaches 5 ppm (penetration threshold), it is determined that the modified activated carbon 2 is saturated with adsorption and switched to the desorption mode:

[0095] High-temperature purging: The gas protection unit 4 heats nitrogen to 300 °C through an electric heating device and introduces it into the adsorption column at a flow rate of 70 L / h for 2.5 h of purging. The high-temperature nitrogen causes the chelation bonds between metal ions and the surface functional groups of activated carbon to break, and metal chlorides are discharged with the desorbed gas.

[0096] Cooling for standby: After purging, heating is stopped, and normal-temperature nitrogen (flow rate 15 L / h) is continuously introduced to cool the activated carbon for 1.5 h. When the temperature drops below 40 °C, the modified activated carbon 2 resumes its adsorption performance and is transferred to the standby state, waiting for a new round of adsorption tasks;

[0097] The desorbed gas flowing out of the adsorption column enters the shell-and-tube condenser 6. Using industrial circulating water as the cooling medium, the temperature of the desorbed gas is reduced to 35 °C, causing gaseous metal chlorides (AlCl3, TiCl4) to condense into a liquid state. The condensed metal chlorides are collected through a pipeline into a stainless-steel recovery tank and can be further purified and utilized through processes such as distillation and crystallization.

[0098] The modified activated carbon prepared in the above Examples 1-3 and Comparative Examples 1-3 was treated with the above device and method for chlorosilane high-boiling substances with the same initial metal concentration, and the data in Table 1 were obtained:

[0099] Table 1

[0100]

[0101] Examples 1-3 all adopted the modification processes of nitric acid oxidation, phosphoric acid impregnation, and loading thiourea groups. Using activated carbon with different particle sizes or materials as raw materials, they all showed excellent metal removal capabilities;

[0102] In Comparative Example 1, due to the absence of phosphoric acid impregnation, the metal concentration after adsorption was significantly higher than that in the examples, and the recovery purity was lower. This was because the lack of the phosphoric acid impregnation step prevented some micropores of the activated carbon from being connected and expanded into mesopores, making it difficult to form a hierarchical pore network of "mesopore efficient diffusion channels - micropore strong adsorption sites". Metal ions were restricted in diffusion and could not fully chelate with the inner thiourea groups, resulting in poor adsorption effects. Moreover, it was difficult for metal chlorides to completely detach during the desorption process, affecting the recovery purity;

[0103] In Comparative Example 2, due to the absence of nitric acid oxidation, the activated carbon surface lacked sufficient oxygen-containing functional groups such as carboxylic acid groups. This led to the inability to form effective electrostatic pre-adsorption on the outer layer, and metal ions could not be preliminarily enriched, increasing the adsorption burden on the inner thiourea groups. The adsorption efficiency and capacity decreased, and at the same time, the desorption effect and the recovery purity of metal chlorides were also affected;

[0104] In Comparative Example 3, since the thiourea group was not loaded and only the carboxylic acid group introduced by nitric acid oxidation was relied on for adsorption, due to the relatively weak adsorption force and poor selectivity of the carboxylic acid group for metal ions, deep removal of metal ions could not be achieved, resulting in a relatively high metal concentration after adsorption, and the recovery purity of metal chlorides was also affected during the desorption process.

[0105] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A preparation method of modified activated carbon, characterized in that, Comprising: Using activated carbon as raw material, successively through nitric acid oxidation, phosphoric acid impregnation and loading of thiourea groups, a modified activated carbon with carboxylic acid groups and thiourea chelating sites rich on the surface is prepared.

2. The preparation method of the modified activated carbon according to claim 1, characterized in that, The nitric acid oxidation is carried out by using nitric acid with a concentration of 7 - 15% and treating at 70 - 100 °C for 1 - 5 h.

3. The preparation method of the modified activated carbon according to claim 1, characterized in that, The phosphoric acid impregnation is carried out by using phosphoric acid with a concentration of 4 - 8%, activating at 110 - 130 °C for 2 - 5 h, then placing it in a tubular furnace, heating to 380 - 420 °C, keeping warm, washing after cooling to room temperature, and drying to obtain nitric acid - phosphoric acid double - modified activated carbon.

4. The preparation method of the modified activated carbon according to claim 1, characterized in that, The loading of thiourea groups is carried out by impregnating with a thiourea ethanol solution with a concentration of 4 - 8%, and then washing and drying.

5. The preparation method of the modified activated carbon according to claim 1, characterized in that, The particle size of the activated carbon is 0.5 - 1 mm. The modified activated carbon prepared by the preparation method of the modified activated carbon according to any one of claims 1 - 5.

7. Use of the modified activated carbon as described in claim 6 in the removal of metals from high-boiling chlorosilanes, characterized in that, The application includes a metal removal device for chlorosilane high - boilers; The metal removal device for chlorosilane high - boilers includes: An adsorption column main body, which is filled with the modified activated carbon inside.

8. Use of the modified activated carbon according to claim 7 in the removal of metals from high-boiling chlorosilanes, characterized in that, The metal removal device for chlorosilane high - boilers further includes: A temperature control unit for controlling the adsorption temperature of the adsorption column main body; A gas protection unit for maintaining the oxygen content in the adsorption column main body < 10 ppm and for desorption; An automatic feeding unit for continuously feeding chlorosilane high - boilers.

9. Use of the modified activated carbon as described in claim 8 in the removal of metals from high-boiling chlorosilanes, characterized in that, The using method of the metal removal device for chlorosilane high - boilers includes: After removing solid particles from the chlorosilane high - boilers by the automatic feeding unit, a liquid high - boiler is obtained; The liquid high - boiler passes through the adsorption column main body, and metal ions selectively chelate with the surface functional groups of the modified activated carbon; The modified activated carbon saturated in adsorption is subjected to high - temperature nitrogen purging desorption of metals by the gas protection unit to obtain a desorbed gas, and the modified activated carbon is recycled after regeneration; The desorbed gas is condensed to recover metal chlorides.

10. Use of the modified activated carbon as described in claim 9 in a metal removal device for high-boiling chlorosilanes, characterized in that, When the liquid high - boiler passes through the adsorption column main body, the adsorption temperature is controlled by the temperature control unit to be 60 - 80 °C.

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