Waste graphite purification method based on ammonium chloride solid chlorinating agent

Abstract

The invention relates to the technical field of waste graphite purification, in particular to a waste graphite purification method based on an ammonium chloride solid chlorinating agent. The method comprises the following steps: S1, raw material pretreatment: screening and drying recycled waste graphite to obtain pretreated waste graphite powder; s2, a solid chlorinating agent is mixed, specifically, the pretreated waste graphite powder and ammonium chloride serving as the solid chlorinating agent are mixed according to a preset proportion, and a mixed material to be roasted is obtained; the mixed material is placed in a roasting device, a two-stage heating roasting procedure is executed under the inert carrier gas atmosphere, the first-stage heating roasting is used for achieving the chlorination reaction, the second-stage heating roasting is used for impurity removal and purification, roasting tail gas is subjected to absorption treatment, and purified graphite is obtained after cooling. Compared with the traditional chlorine chlorination, the method adopting the ammonium chloride solid chlorinating agent and the staged heating chloridizing roasting purification method has the advantage that the operation safety and environmental protection property are obviously improved.

Description

Technical Field

[0001] This application relates to the field of waste graphite purification technology, and in particular to a method for purifying waste graphite based on ammonium chloride solid chlorinating agent. Background Technology

[0002] Graphite is widely used in numerous industries such as lithium batteries and semiconductors, and is a crucial supporting material for future technological revolutions. However, this also presents the challenge of recycling waste graphite. The wide range of sources and complex composition of recycled graphite make its reuse difficult, and the purity of the recycled graphite directly determines the performance and comprehensive utilization value of the products. Waste graphite contains various metallic and non-metallic impurities such as Cu, Fe, Al, Ni, Li, Si, and S, which are embedded in the graphite structure and difficult to remove. Traditional purification techniques, such as flotation, achieve low purity (<95%), while alkaline-acid methods cause severe pollution, and high-temperature purification consumes extremely high energy (3000-4000℃), hindering industrial production.

[0003] Chlorination roasting is widely used in metallurgical processes. It involves adding chlorine or other chlorides to gangue to convert target metal or non-metal compounds into low-boiling-point chlorides, which are then removed by medium-to-high temperature extraction or acid leaching. Due to its high extraction efficiency and adaptability to raw materials, it is widely used in the recovery of metals such as copper, lead, zinc, gold, silver, and nickel. The core of chlorination purification lies in the targeted reaction between chlorine-containing components and impurities. Compared to traditional methods, it reduces energy consumption by more than 80%, and byproducts can be recovered by adding a condenser at the tail end. With the surge in demand for high-purity graphite from the new energy industry (such as lithium-ion battery anode materials), chlorination purification has significant potential in battery recycling and semiconductor raw material preparation. However, using traditional chlorine roasting makes it difficult to control the amount of chlorine added, and chlorine poses serious hazards to human health and the environment, thus limiting its industrial application. Summary of the Invention

[0004] This application provides a method for purifying waste graphite based on ammonium chloride solid chlorinating agent to solve the above problems. The method includes: S1. Raw material pretreatment: The recovered waste graphite is screened and dried to obtain pretreated waste graphite powder. S2. Solid chlorinating agent mixing: The pretreated waste graphite powder is mixed with ammonium chloride as a solid chlorinating agent according to a preset ratio to obtain a mixture to be roasted. S3. Segmented heating chlorination roasting purification: The mixture is placed in a roasting device and a two-stage heating roasting process is performed under an inert carrier gas atmosphere. The first stage of heating roasting is used to realize the chlorination reaction, and the second stage of heating roasting is used to remove impurities and purify the material. The roasting tail gas is absorbed and treated, and purified graphite is obtained after cooling.

[0005] The above technical solution, employing ammonium chloride solid chlorinating agent and a segmented heating chlorination-roasting purification method, significantly improves operational safety and environmental friendliness compared to traditional chlorine chlorination. The solid chlorinating agent decomposes in situ within the reaction system, allowing for more thorough contact between active chlorine species and impurities, thus improving chlorination efficiency, particularly for removing low-boiling-point metallic impurities. Segmented heating control achieves a synergistic effect between low-temperature chlorination and high-temperature deep purification, effectively controlling energy consumption and avoiding the extremely high temperatures required for traditional high-temperature purification, thereby reducing production costs. Furthermore, the use of an inert carrier gas ensures that graphite is not oxidized at high temperatures, maintaining its structural integrity.

[0006] Optionally, the raw material pretreatment includes: passing the recovered waste graphite through a 200-mesh sieve and drying it at 75-105℃ for 24 hours to obtain pretreated waste graphite powder.

[0007] The above technical solution effectively removes coarse particulate impurities by sieving waste graphite through a 200-mesh sieve, while ensuring a suitable specific surface area of ​​the graphite powder. This facilitates the uniform adhesion of the chlorinating agent to the graphite particle surface during mixing. Precisely controlling the drying temperature at 75-105℃ and the drying time at 24 hours maximizes the removal of moisture from the raw materials, preventing the decomposition of moisture during calcination and the generation of steam, which could affect the purity of the chlorination atmosphere and improve the efficiency and stability of the chlorination reaction. This precise pretreatment lays the foundation for subsequent high-purity purification.

[0008] Optionally, the solid chlorinating agent is mixed using a ball mill under the following conditions: 300 rpm and 60 min. After mixing, the mixture is sealed for later use.

[0009] By employing the above technical solution and using a ball mill for high-energy mixing, along with precise control of the rotation speed (300 rpm) and mixing time (60 min), this embodiment ensures extremely high dispersion uniformity of ammonium chloride in waste graphite. This uniform mixing significantly improves the efficiency and thoroughness of the chlorination reaction, allowing the chlorinating agent to fully function in the subsequent staged heating chlorination roasting purification S3, thereby increasing the impurity removal rate. Furthermore, immediate sealing after mixing effectively prevents a decrease in the activity of the chlorinating agent, ensuring process stability and batch consistency.

[0010] Optionally, the initial purity of the recycled waste graphite is less than 90%; the amount of ammonium chloride added is 50% to 130% of the mass of the waste graphite.

[0011] By limiting the initial purity of waste graphite to less than 90% through the above technical solution, the application scenario of this invention is clarified, namely, for recycled raw materials with high impurity content. By controlling the amount of ammonium chloride added within the range of 50% to 130% of the mass of waste graphite, it is possible to ensure sufficient chlorination of impurities and obtain high-purity graphite while avoiding excessive waste of chlorinating agent and the resulting increase in tail gas treatment load. This limitation ensures the technical feasibility and economic rationality of the purification process.

[0012] Optionally, the inert carrier gas atmosphere is one or more of argon, nitrogen, and helium.

[0013] By employing the above technical solution, one or more of argon, nitrogen, and helium are selected as inert carrier gases. This ensures that graphite does not suffer oxidation loss during the entire staged heating chlorination roasting purification process, maintaining the yield and structural integrity of the purified graphite. The efficient flow of the inert carrier gas ensures that volatile impurities can be quickly and thoroughly removed from the reaction system, avoiding secondary contamination and thus improving purification efficiency.

[0014] Optionally, the conditions for the first stage of heating and calcination are: carrier gas flow rate of 200 mL / min, heating rate of 10 °C / min, calcination temperature of 400–800 °C, and holding at that temperature for 180 min.

[0015] By precisely limiting the carrier gas flow rate, heating rate, calcination temperature, and holding time, this embodiment ensures that the first-stage chlorination reaction can proceed with high efficiency and thoroughness. The temperature range of 400–800°C is the temperature range for effective decomposition of the chlorinating agent and complete chlorination of impurities. The 180-minute holding time ensures the thoroughness of the main chlorination reaction, laying the foundation for subsequent deep purification and significantly improving the impurity removal efficiency.

[0016] Optionally, the conditions for the second stage of heating and calcination are: carrier gas flow rate of 200 mL / min, heating rate of 3℃ / min, calcination temperature of 1200~1600℃, and holding temperature for 60~300 min.

[0017] By employing the above technical solution and raising the second-stage calcination temperature to 1200–1600℃, high-boiling-point chloride impurities and non-metallic impurities can be effectively removed, achieving deep purification of graphite and obtaining a high-purity product. Using a relatively low heating rate of 3℃ / min helps reduce energy consumption and ensures uniform heating of the material. The holding time setting of 60–300 min allows the operator to flexibly adjust according to the desired final purity, ensuring thorough purification.

[0018] Optionally, the roasting exhaust gas is absorbed and treated by a dilute sulfuric acid aqueous solution and a sodium hydroxide absorption device.

[0019] The above technical solution employs a dilute sulfuric acid aqueous solution absorption tower and a sodium hydroxide absorption device connected in series to treat the roasting tail gas, achieving targeted removal of both acidic and alkaline pollutants. The dilute sulfuric acid effectively absorbs alkaline ammonia, preventing environmental pollution caused by direct ammonia emissions; while sodium hydroxide effectively neutralizes acidic hydrogen chloride, ensuring the environmental friendliness of the tail gas emissions. This two-stage absorption treatment system improves the efficiency and reliability of tail gas purification, making the entire waste graphite purification process highly environmentally friendly.

[0020] Optionally, the roasting temperature of the first stage of heating and roasting is preferably 600°C.

[0021] By employing the above technical solution, and preferably setting the first stage of heating and roasting at 600°C, this embodiment maximizes chlorination efficiency. This temperature point fully utilizes the thermal decomposition characteristics of ammonium chloride, enabling the active chloride to react efficiently with low-boiling-point metal impurities, thereby achieving a high purification effect with lower energy consumption and reducing the load on subsequent deep purification.

[0022] Optionally, the roasting temperature of the second stage of heating and roasting is preferably 1500℃, and the holding time of the second stage of heating and roasting is preferably 180min.

[0023] Through the above technical solution, by preferably setting the second-stage heating and calcination temperature to 1500℃, this embodiment effectively controls energy consumption and equipment requirements while ensuring the removal of high-boiling-point impurities. Simultaneously, the preferred 180-minute holding time ensures thorough purification, resulting in high-purity purified graphite. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart illustrating a method for purifying waste graphite based on ammonium chloride solid chlorinating agent, as provided in an embodiment of this application. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0027] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0028] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0029] Figure 1 A flowchart illustrating a method for purifying waste graphite based on ammonium chloride solid chlorinating agent, as provided in one embodiment of this application, is shown below. Figure 1 Shown: First, the recycled waste graphite raw materials (such as scrap lithium battery anode materials) are screened in a raw material pretreatment unit to remove coarse impurities such as metal fragments. Then, they are dried to remove residual moisture, preventing safety issues or affecting chlorination efficiency at subsequent high temperatures. Next, in the solid chlorinating agent mixing unit, the dried waste graphite powder is mixed with solid chlorinating agent ammonium chloride (… Mix the materials evenly according to the preset mass ratio. Transfer the mixture to the calcination device and start the first stage of heating and calcination under a continuously supplied inert carrier gas atmosphere (e.g., industrial nitrogen). During this stage, ammonium chloride decomposes to produce active chlorides, which react with low-boiling-point metal oxide impurities in graphite to generate corresponding volatile chlorides, which are discharged with the tail gas. After completing the first stage of heat preservation, continue heating to the second stage purification temperature range. The main function of this stage is to completely vaporize the remaining high-boiling-point chloride impurities and non-metallic impurities (such as...). This process achieves final purification. The exhaust gases generated during the entire roasting process are directed to the exhaust gas absorption device 40 for absorption and treatment, ensuring environmental friendliness.

[0030] In the raw material pretreatment unit, the recovered waste graphite is mechanically sieved and hot-air dried to obtain raw materials with uniform particle size and low moisture content. In the solid chlorinating agent mixing unit, suitable mixing equipment (e.g., a V-type mixer) is used to mix the pretreated graphite powder with... The chlorinating agent is mixed in a predetermined ratio to ensure uniform dispersion on the surface of the graphite particles. The mixture is then loaded into the reaction crucible / tube of the calcination apparatus. The calcination apparatus is equipped with a heating furnace and a temperature control system for precise control of the staged heating temperature profile. In step S3, inert carrier gas is continuously introduced into the reaction system through the inert carrier gas inlet to maintain an inert atmosphere and remove volatile chlorides. The first stage of heating calcination raises the temperature to the chlorination temperature of 400-800°C, mainly to achieve the volatilization of impurities such as Fe, Cu, and Ni. The second stage of heating calcination raises the temperature to the purification temperature of 1200-1600°C to remove high-boiling-point impurities such as Si, which are difficult to volatilize at lower temperatures. The treated tail gas enters the tail gas absorption device for purification through the tail gas outlet. Finally, the cooled purified graphite is collected in the purified graphite collection unit.

[0031] The effect of ammonium chloride purity on the experiment is as follows: A mixture of waste graphite powder (88.6% purity) and ammonium chloride (30-110%) was placed in a tube furnace. Argon gas (99.99% purity) was introduced into the furnace, and the temperature was increased to 500℃ at a rate of 5℃ / min and held for 180 min. Then, the temperature was increased to 1300℃ at a rate of 3℃ / min and held for 120 min. After cooling to room temperature, the sample was removed. The purity of the purified graphite powder is shown in Table 1. As the amount of ammonium chloride added increased, the purity of the graphite showed a trend of first increasing and then decreasing. At 70%, the purity of the waste graphite was 98.49%. Considering all factors, 90% was adopted as the optimal amount of ammonium chloride added in the subsequent examples.

[0032] Table 1. Effect of Ammonium Chloride Addition on the Purity of Waste Graphite Amount added (%) Graphite purity (%) 50 90.33 70 98.49 90 98.05 110 92.50 130 94.44

[0033] In other embodiments, the raw material pretreatment unit can employ other types of mechanical means, such as a high-speed shear crusher or an air jet mill, instead of a vibrating screen for particle size control. The mixing equipment in the solid chlorinating agent mixing unit can also be a vertical sand mill to enhance mixing uniformity. The calcination device can be replaced by a horizontal tube furnace or a high-temperature rotary kiln, provided it can achieve precise temperature control and maintain an inert atmosphere. The inert carrier gas can be a single high-purity argon gas, or a mixture of argon and industrial nitrogen. The tail gas absorption device can employ structures such as a spray tower, packed tower, or bubbling absorption tank to purify acidic or alkaline tail gases.

[0034] In some embodiments, the recycled waste graphite is further defined, specifically including passing the recycled waste graphite through a 200-mesh sieve and drying it at 75~105°C for 24 hours to finally obtain pretreated waste graphite powder.

[0035] This embodiment specifically employs a vibrating screen or drum screen as the screening equipment to ensure that all waste graphite particles can pass through a 200-mesh sieve. The drying equipment uses a drying oven with a precise temperature control system, accurately controlling the temperature between 75 and 105°C. This temperature range effectively removes physically adsorbed water and some bound water from the graphite surface and interior, while avoiding structural changes or oxidation that may occur at excessively high temperatures. The drying time is set at 24 hours. This pretreatment method ensures the particle size uniformity of the waste graphite raw material, improves the efficiency of subsequent solid chlorination agent S2 mixing, and reduces the moisture content of the raw material to an extremely low level, providing a stable prerequisite for subsequent high-temperature roasting.

[0036] In other embodiments, the sieving operation can employ more efficient equipment such as an air classifier or an ultrasonic vibrating screen. The drying temperature range can be adjusted according to the specific source and moisture content of the waste graphite; for example, for materials with low moisture content, drying at 75°C for 12 hours can be used. Optionally, the drying step can be interchanged with the sieving step, or a second sieving can be performed after drying to further ensure particle size uniformity.

[0037] In some embodiments, the solid chlorinating agent mixture S2 is achieved by mixing in a ball mill, and the mixing conditions are precisely defined as a rotation speed of 300 rpm and a mixing time of 60 min. After mixing, the material is sealed for later use.

[0038] In the solid chlorinating agent mixing unit, a planetary ball mill was used as the mixing equipment. The ball mill, through its internal grinding balls and high-speed rotation, provides an efficient mixing environment for the materials. Pretreated waste graphite powder and ammonium chloride were fed into the ball mill, with the speed set at 300 rpm and the mixing time precisely controlled at 60 minutes. This high-energy mixing method maximizes the contact area between the ammonium chloride and graphite powder, creating an optimal reaction interface for the subsequent in-situ chlorination reaction during roasting. After mixing, the material was transferred to a sealed container to prevent environmental moisture from affecting the ammonium chloride.

[0039] In other embodiments, other high-energy mixing equipment, such as high-speed shear mixers or vertical mills, can be selected to achieve similar mixing effects. The rotation speed and mixing time can be adjusted appropriately according to the type of equipment selected and the characteristics of the material. For example, if a V-type mixer is used, the mixing time may be appropriately extended to compensate for insufficient mixing energy.

[0040] In some embodiments, the initial purity of the raw materials and the amount of ammonium chloride added are limited, wherein the initial purity of the recycled waste graphite raw materials is less than 90%, and the amount of ammonium chloride added is 50% to 130% of the mass of the waste graphite.

[0041] The raw materials for recycling waste graphite, especially graphite from waste lithium battery anode materials, usually have low initial purity (less than 90%) and contain a large number of metal oxides and non-metallic impurities, requiring intensive purification.

[0042] This implementation method primarily targets the purification of waste lithium-ion battery graphite anode materials with an initial purity below 90%, emphasizing their recycling value. In the solid chlorinating agent mixing unit, pretreated waste graphite powder (e.g., material with an initial purity of 88%) and ammonium chloride are weighed. The amount of ammonium chloride added is precisely controlled within the range of 50% to 130% of the waste graphite mass, for example, 100%. This ratio ensures that sufficient active chlorinating substances participate in the reaction during the staged heating chlorination roasting purification S3 process, achieving effective removal of high-content impurities.

[0043] In other embodiments, for waste graphite with particularly high impurity content (e.g., initial purity below 80%), the amount of ammonium chloride added can preferably be set in the high end of the range of 110% to 130% to ensure the completeness of the chlorination reaction. For waste graphite with a single type of impurity, it can be considered to add it in the low end of the range of 50% to 80% to optimize costs.

[0044] In some embodiments, the inert carrier gas atmosphere used in the segmented heating chlorination roasting purification S3 is limited to one or more of argon, nitrogen, and helium.

[0045] During high-temperature chlorination roasting, the reaction atmosphere must be strictly controlled to prevent graphite from being oxidized at high temperatures, which would lead to graphite loss and a decrease in purity. This embodiment ensures the stability of graphite during the staged heating chlorination roasting purification of S3 by limiting the composition of the inert carrier gas.

[0046] In the calcination apparatus, an inert carrier gas is continuously introduced into the reaction crucible / tube through the inert carrier gas inlet. In this embodiment, high-purity argon can be used as the sole inert carrier gas, or industrial nitrogen can be used to reduce costs. In certain specific embodiments, high-purity helium can also be used to utilize its high thermal conductivity to help achieve uniform heating of the reaction system. The inert carrier gas not only provides an inert environment but also acts as a scavenger, carrying away the gaseous products generated in the reaction (including hydrogen chloride, ammonia, and volatile metal chlorides) from the reaction zone and directing them to the tail gas outlet.

[0047] In other embodiments, the ratio of inert carrier gas can be adjusted according to cost and purification requirements. For example, low-cost industrial nitrogen can be used as the main carrier gas, with a small amount of high-purity argon added to balance economy and atmosphere purity. The inert carrier gas can also be mixed with other non-oxidizing gases (such as CO), as long as it is ensured that it does not react adversely with graphite at the calcination temperature.

[0048] In some embodiments, the conditions for the first stage of heating and calcination are defined as follows: carrier gas flow rate of 200 mL / min, heating rate of 10 °C / min, calcination temperature of 400–800 °C, and holding at that temperature for 180 min.

[0049] The first stage of heating and roasting is a crucial step in achieving the main chlorination reaction. Its primary objective is to effectively remove low-boiling-point metallic impurities from graphite by utilizing the active chlorinating substances produced by the decomposition of ammonium chloride. This implementation method ensures the efficient execution of the chlorination reaction through precise control of temperature, rate, flow rate, and time.

[0050] In the first stage of the staged heating chlorination roasting purification of S3, the mixture is placed in the reaction crucible / tube of the roasting apparatus. The temperature control system raises the temperature to 400–800 °C, for example, 600 °C, according to the first stage heating rate curve, and maintains this temperature for 180 minutes. Under these conditions, ammonium chloride decomposes to produce… and The reactive substances react with metal oxide impurities such as Fe, Cu, and Ni to form volatile metal chlorides. A carrier gas flow rate of 200 mL / min ensures that the volatile chlorides produced during the reaction are promptly discharged from the furnace, preventing backflow and secondary reactions.

[0051] In other embodiments, the heating rate can be adjusted according to the furnace size and material loading. For example, for large-scale processing, the heating rate can be appropriately reduced to 5°C / min to ensure uniform heating of the material. The carrier gas flow rate can be adjusted according to the reactor volume and tail gas treatment load. For example, in some cases, it can be increased to 300 mL / min to accelerate the discharge of volatile substances. The holding time can be optimized between 120 min and 240 min depending on the impurity content of the raw material.

[0052] In some embodiments, the conditions for the second stage of heating and calcination are defined as follows: carrier gas flow rate of 200 mL / min, heating rate of 3 °C / min, calcination temperature of 1200–1600 °C, and holding at that temperature for 60–300 min.

[0053] The second stage of heating and roasting is a crucial step in achieving deep purification. Its main goal is to remove high-boiling-point impurities (such as those that did not fully volatilize or chlorinate during the first stage of low-temperature chlorination) This embodiment ensures the efficiency and thoroughness of deep purification through high temperature and a slow heating rate.

[0054] In the second stage of the segmented heating chlorination roasting purification of S3, the temperature control system continues to raise the furnace temperature from the first stage holding temperature to 1200–1600℃, for example, 1500℃. The heating rate is set to 3℃ / min, which is lower than that of the first stage, in order to protect the stability of the roasting equipment and the material. The carrier gas flow rate is maintained at 200mL / min to continuously remove residual impurities volatilized at high temperatures. The holding time of 60–300min is crucial to ensure deep purification, for example, it is set to 180min to achieve the goal of ultimate purity.

[0055] In other embodiments, the carrier gas flow rate can be fine-tuned according to the furnace structure and purification requirements. The holding time can be optimized based on the content of high-boiling-point impurities in the raw material; for example, when the impurity content is low, the holding time can be 60 minutes. If higher purity is required, the holding time can be extended to 300 minutes.

[0056] In some embodiments, the roasting exhaust gas is absorbed and treated by a dilute sulfuric acid aqueous solution and a sodium hydroxide absorption device.

[0057] During the staged heating chlorination roasting purification of S3, ammonium chloride decomposes to produce alkaline ammonia, acidic hydrogen chloride, and volatile metal chlorides. These gases must undergo rigorous purification before being emitted. This embodiment employs a series-connected acid-base absorption system to ensure thorough exhaust gas treatment.

[0058] Specifically, the exhaust gas from the roasting unit enters a dilute sulfuric acid aqueous solution absorption tower through the exhaust gas inlet. This absorption tower can employ a packed tower structure to increase the gas-liquid contact area and improve... Absorption efficiency. The exhaust gas then enters a sodium hydroxide absorption unit, which can also employ a spray tower structure to efficiently neutralize residual gases. Gases. By using dilute sulfuric acid aqueous solution and sodium hydroxide solution in series, the main pollutants in the exhaust gas are absorbed and purified.

[0059] In other embodiments, the dilute sulfuric acid aqueous solution absorption tower can be replaced by a bubbling absorption tank or a spray tower structure. The sodium hydroxide absorption device can be replaced by a sodium carbonate or calcium hydroxide slurry absorption device to neutralize the acidic gas. Furthermore, a condensation device can be added before the dilute sulfuric acid aqueous solution absorption tower to recover high-boiling-point metal chlorides, further reducing the load on the absorption device.

[0060] In some embodiments, the roasting temperature of the first stage of heating and roasting is preferably 600°C.

[0061] The temperature of the first stage of heating and calcination is a key parameter affecting the efficiency of the chlorination reaction. In this embodiment, this temperature is preferably limited to 600℃. The working principle is that at 600℃, the decomposition rate of ammonium chloride and the concentration of the generated active chlorides reach an ideal equilibrium point. At this temperature, the reaction rate between the active chlorides and the metal oxide impurities (such as Fe, Cu, Ni, etc.) in graphite is the fastest, and the corresponding volatile chlorides can be fully generated. At the same time, this temperature is relatively low, which helps to protect the refractory materials of the calcination device and reduce energy consumption.

[0062] In the segmented heating chlorination roasting purification S3, the temperature control system precisely sets the first stage holding temperature range of 302 to 600℃. Holding at 600℃ for 180 minutes ensures the completeness of the chlorination reaction. This optimal temperature selection maximizes the chlorination volatilization efficiency of the main impurities, significantly outperforming cases where chlorination is performed at the 400℃ or 800℃ endpoints.

[0063] The specific experimental procedure is as follows: A mixture of waste graphite powder (88.6% purity) and ammonium chloride (90% addition) was placed in a tube furnace. Argon gas (99.99% purity) was introduced into the furnace, and the temperature was increased at a rate of 5℃ / min to 400-800℃. After holding at this temperature for 180 min, the temperature was increased to 1300℃ at a rate of 3℃ / min and held for 120 min. After cooling to room temperature, the sample was removed. The purity of the purified graphite powder is shown in Table 2. The optimal purity of graphite (98.49%) was achieved at a calcination temperature of 600℃. Therefore, 600℃ was used as the optimal chlorination calcination temperature in subsequent examples.

[0064] Table 2. Effect of chlorination roasting time on the purification of waste graphite <![CDATA[Roasting temperature with chlorine ( o °C)]]> Graphite purity (%) 400 97.00 500 97.97 600 98.49 700 97.11 800 98.43

[0065] In other embodiments, if the boiling point of the main impurities in the waste graphite raw material is slightly higher or lower, the first stage roasting temperature can be finely adjusted, for example, by holding at 550°C or 650°C, to adapt to the requirements of different impurity components on the chlorination reaction rate.

[0066] In some embodiments, the roasting temperature of the second stage of heating and roasting is preferably 1500°C, and the holding time of the second stage of heating and roasting is preferably 180 min.

[0067] The purpose of the second-stage heating and roasting is to achieve deep purification and remove high-boiling-point impurities, thus requiring higher temperatures and sufficient holding time. In this embodiment, the roasting temperature is preferably limited to 1500℃, and the holding time is preferably limited to 180 minutes. The working principle is that 1500℃ is one of the minimum effective temperatures required to ensure the complete vaporization of non-metallic oxide impurities such as Si, while avoiding the enormous energy consumption and equipment requirements associated with higher temperatures. The 180-minute holding time ensures that the material has sufficient time at 1500℃ to complete the volatilization of residual impurities, thereby raising the graphite purity to a high level.

[0068] In the segmented heating chlorination roasting purification S3 process, the temperature control system precisely sets the second-stage holding temperature range to 1500℃ and maintains it for 180 minutes. Under an inert carrier gas atmosphere at 1500℃, residual high-boiling-point chloride impurities and non-metallic impurities are completely vaporized and removed via a carrier gas flow rate of 200 mL / min. This optimized combination achieves the goal of obtaining the highest purity within a reasonable energy consumption range.

[0069] The specific experiment is as follows: A mixture of waste graphite powder (88.6% purity) and ammonium chloride (90% addition) was placed in a tube furnace. Argon gas (99.99% purity) was introduced into the furnace, and the temperature was increased at a rate of 5°C / min to 500°C. After holding at 500°C for 180 min, the temperature was increased at a rate of 3°C / min to 1200-1600°C and held for 120 min. After cooling to room temperature, the sample was removed. The purity of the purified graphite powder is shown in Table 3. When the calcination temperature was 1500°C, the purity of graphite was 99.90%. Considering energy consumption, 1500°C was selected as the optimal calcination temperature for impurity removal in subsequent examples.

[0070] Table 3. Effect of purification roasting temperature on the purification of waste graphite <![CDATA[Impurity removal roasting temperature ( o °C)]]> Graphite purity (%) 1200 97.89 1300 98.49 1400 99.85 1500 99.90 1600 99.91

[0071] A mixture of waste graphite powder (88.6% purity) and ammonium chloride (90% addition) was placed in a tube furnace. Argon gas (99.99% purity) was introduced into the furnace, and the temperature was increased at a rate of 5℃ / min to 500℃. After holding at 500℃ for 180 min, the temperature was increased to 1500℃ at a rate of 3℃ / min and held for 90-210 min. After cooling to room temperature, the sample was removed. The purity of the purified graphite powder is shown in Table 4. The optimal purity of graphite (99.96%) was achieved when the calcination time was 180 min.

[0072] Table 4. Effect of purification roasting time on the purification of waste graphite Impurity removal roasting time (min) Graphite purity (%) 90 97.86 120 99.90 150 99.95 180 99.96 210 99.95

[0073] In other embodiments, if the required final graphite purity is slightly lower, the holding time of the second-stage heating and calcination can be appropriately shortened to 120 minutes. If the raw material contains impurities that are extremely difficult to volatilize, the calcination temperature can be increased to 1600°C, and the holding time can be extended to 300 minutes, provided the equipment allows, to achieve the highest purification effect. A vacuum induction furnace can be selected as the calcination apparatus to further accelerate the volatilization rate of impurities at 1500°C.

Claims

1. A method for purifying waste graphite based on ammonium chloride solid chlorinating agent, characterized in that, include: S1. Raw material pretreatment: The recovered waste graphite is screened and dried to obtain pretreated waste graphite powder. S2. Solid chlorinating agent mixing: The pretreated waste graphite powder is mixed with ammonium chloride as a solid chlorinating agent according to a preset ratio to obtain a mixture to be roasted. S3. Segmented heating chlorination roasting purification: The mixture is placed in a roasting device and a two-stage heating roasting process is performed under an inert carrier gas atmosphere. The first stage of heating roasting is used to realize the chlorination reaction, and the second stage of heating roasting is used to remove impurities and purify the material. The roasting tail gas is absorbed and treated, and purified graphite is obtained after cooling.

2. The method according to claim 1, characterized in that, The raw material pretreatment includes: passing the recovered waste graphite through a 200-mesh sieve and drying it at 75-105℃ for 24 hours to obtain pretreated waste graphite powder.

3. The method according to claim 2, characterized in that, The solid chlorinating agent is mixed by ball milling under the following conditions: 300 rpm and 60 min. After mixing, the mixture is sealed and stored for later use.

4. The method according to claim 2, characterized in that, The initial purity of the recycled waste graphite is less than 90%; the amount of ammonium chloride added is 50% to 130% of the mass of the waste graphite.

5. The method according to claim 1, characterized in that, The inert carrier gas atmosphere is one or more of argon, nitrogen, and helium.

6. The method according to claim 1, characterized in that, The conditions for the first stage of heating and calcination are as follows: carrier gas flow rate of 200 mL / min, heating rate of 10℃ / min, calcination temperature of 400~800℃, and holding temperature for 180 min.

7. The method according to claim 1, characterized in that, The conditions for the second stage of heating and calcination are as follows: carrier gas flow rate of 200 mL / min, heating rate of 3℃ / min, calcination temperature of 1200~1600℃, and holding temperature for 60~300 min.

8. The method according to claim 1, characterized in that, The roasting exhaust gas is absorbed and treated by a dilute sulfuric acid aqueous solution and a sodium hydroxide absorption device.

9. The method according to claim 1, characterized in that, The preferred roasting temperature for the first stage of heating and roasting is 600℃.

10. The method according to claim 1, characterized in that, The preferred roasting temperature for the second stage of heating and roasting is 1500℃, and the preferred holding time for the second stage of heating and roasting is 180min.

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