A method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate.

Abstract

This invention relates to a method for preparing high-purity graphite from waste graphite through sodium carbonate-sodium borate roasting purification, belonging to the technical field of waste graphite purification for high-purity graphite preparation. The preparation method involves using silicon carbide-containing waste graphite as raw material. After crushing the raw material, 10-30% of a sodium carbonate-sodium borate mixed salt additive is added, controlling the mass ratio of sodium carbonate to sodium borate in the mixed salt to be 2.8-3.6:1. The mixture is then roasted at 500℃-800℃ to obtain the roasted product. Following this, a water leaching treatment is performed, followed by drying to obtain the final product. This invention can obtain high-purity graphite with an ash content reduction of less than 0.02% through waste graphite thermal purification, achieving a deashing rate of over 99%. This purification method has advantages such as simple process, high purification efficiency, low energy consumption, easy product collection, and easy industrialization. This method is of great significance for the preparation of high-purity graphite from waste graphite thermal purification.

Description

Technical Field

[0001] This invention relates to a method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate, and particularly to a method for preparing high-purity graphite from waste graphite used in photovoltaic crystal pulling by calcining sodium carbonate-sodium borate, belonging to the technical field of waste graphite purification and preparation of high-purity graphite. Background Technology

[0002] Graphite and its derivatives possess characteristics such as high chemical inertness, good corrosion resistance, high strength, and good electrical conductivity, making them widely used in chemical, portable electronics, aerospace, and metallurgical fields. With the significant increase in graphite applications, a large amount of waste graphite is generated in industry. Photovoltaic power generation utilizes the photovoltaic effect at semiconductor interfaces to directly convert light energy into electrical energy. Due to the superior properties of graphite, no other material can currently replace it. These carbon-graphite materials undoubtedly form the manufacturing foundation for key equipment in the upstream of the solar photovoltaic power generation industry chain. To reduce costs and achieve widespread market acceptance, graphite materials, as an important basic material for polycrystalline silicon, are crucial. If graphite materials are not manufactured to meet requirements, the dream of improving photoelectric conversion efficiency and reducing costs in the solar photovoltaic power generation industry will be unattainable. It is understood that the photoelectric conversion efficiency of solar cells largely depends on the purity of the silicon wafer. To improve the photoelectric conversion efficiency of solar cells, on the one hand, it is necessary to continuously improve the silicon purification process to increase wafer purity. On the other hand, it is necessary to improve the purification environment of the silicon-graphite thermal field to be purified. If the graphite purity is low and there are too many impurities, the silicon pulling environment will be contaminated, and the wafer quality cannot be guaranteed, thus the efficiency of the solar cell cannot be improved.

[0003] Graphite products form the industrial foundation for the development of the solar photovoltaic power generation industry; both coexist and thrive. The application of carbon in solar photovoltaic power generation is mainly concentrated in the manufacture of polycrystalline silicon raw materials, the drawing of monocrystalline silicon rods, and the casting of polycrystalline silicon ingots. Due to its excellent density, hardness, and compressive strength, as well as its advantages such as high temperature resistance, high pressure resistance, corrosion resistance, good electrical conductivity, and stable performance, graphite materials can be used in the upstream of the solar photovoltaic power generation industry to make graphite crucible heating containers for purifying polycrystalline silicon, drawing silicon rods, and manufacturing. Therefore, the purification and regeneration of waste graphite generated in the photovoltaic industry is essential.

[0004] A high-purity graphite powder and its purification process (application number: CN201811405254.6) discloses a graphite purification process. The process involves adding graphite powder raw material into a graphite boat, which is installed in a purification device. The device is evacuated to a pressure of -93 to -100 kPa and held at that pressure for 0.5-2 hours. After the pressure rise rate is deemed acceptable, the temperature is increased to 1200-1800℃, a certain amount of purification gas is introduced, and the pressure is held before evacuating to a pressure of -93 to -100 kPa. During the heating period, pulse purification is performed until a certain temperature within the 2000-3000℃ range is reached and held for 1-3 hours. Then, the pulse purification is stopped. The device is then evacuated and cooled to 1200-1500℃, and argon gas is introduced to lower the temperature to room temperature, thus obtaining high-purity graphite powder. This process can remove volatile impurities in a timely manner through a pulsed process at a relatively low temperature, solving the problem of deep removal of key impurities such as B, Al, and V in graphite powder raw materials, thereby obtaining high-purity graphite powder with a purity of 99.999% to 99.9999%, providing a practical and feasible path for the industrial production of high-purity graphite powder.

[0005] A method for chlorinating and calcining graphite for purification (application number: CN202010028266.2) discloses a method for chlorinating and calcining graphite for purification. The method involves reacting graphite raw material at a temperature of 400℃ to 1200℃ in a reducing gas and chlorine gas atmosphere for 2 to 16 hours, during which impurities in the graphite form gaseous metal complexes; then, gas-solid separation is performed to obtain purified graphite with a purity greater than 99.5%; wherein the chlorine gas is a halogen-containing gas. This patent is the first to discover that by reacting the acid-washed product with reducing and chlorinating gases at a low to medium temperature of 400℃ to 1200℃, the oxide impurities will generate metal complexes with boiling points below 1000℃, such as CaFeCl4, NaAlCl4, and KMgCl3. These metal complexes are discharged in gaseous form along with the reducing and chlorinating gases, which can effectively solve the problems of low purity of the final product after low-temperature chemical purification and high requirements, high cost, and complex equipment for high-temperature roasting purification. The purity of the purified product is greater than 99.5%.

[0006] Search results revealed that existing graphite purification methods suffer from drawbacks such as high cost, low efficiency, and significant pollution. While they are effective against conventional impurity phases, they are largely ineffective against waste graphite containing silicon carbide (especially waste graphite hot zones used in photovoltaic crystal pulling). Therefore, there are few reports on technologies that can obtain high-purity graphite by calcining with sodium carbonate and sodium borate followed by water washing. Summary of the Invention

[0007] This invention is the first to attempt to calcine a mixture of waste graphite and sodium carbonate / sodium borate. Considering that silicon carbide in the thermal field of waste graphite is difficult to remove effectively using conventional methods, sodium carbonate and sodium borate decompose silicon carbide in air at appropriate temperatures to generate soluble sodium salts, thus removing most of the silicon carbide. However, due to the low silicon carbide content in waste graphite and poor reaction kinetics, the amount of additives added was appropriately increased to promote the reaction between silicon carbide in waste graphite and sodium carbonate and borate, thereby improving purification efficiency. Furthermore, this method effectively avoids the generation of toxic gases, which is beneficial for production safety and environmental protection. The operation is simple and safe, and the valuable elements can be recovered after washing the calcined product, which is beneficial for reducing industrial production costs and energy consumption.

[0008] The technical problem to be solved by this invention is that silicon carbide impurities exist in the hot zone of waste graphite used in photovoltaic crystal pulling. Due to its excellent acid and alkali resistance and high temperature resistance, it is difficult to remove by conventional purification methods.

[0009] To address the problems and shortcomings of the existing technology, this invention provides a method for preparing high-purity graphite from waste graphite obtained by calcining sodium carbonate-sodium borate. The graphite obtained by this method has extremely low ash content, which can fully meet the needs of industrial production. The specific steps are as follows:

[0010] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, with 85-95% of the particles being smaller. Then, the waste graphite and additives are mixed evenly at a mass ratio of 70-90%:30-10% to obtain a mixture. The additive is a mixed salt of sodium carbonate and sodium borate. The waste graphite contains silicon carbide. In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 2.8-3.6:1.

[0011] (2) The mixture prepared in step (1) is placed in a muffle furnace and roasted at a holding temperature of 500~800℃, preferably 600~800℃. The roasted product is then obtained.

[0012] (3) The roasted product is soaked in water and then dried to obtain the product.

[0013] The waste graphite includes waste graphite hot zones used in photovoltaic crystal pulling.

[0014] Preferably, the waste graphite has an ash content of 4-9.5% and a fixed carbon content of over 90%.

[0015] Preferably, the waste graphite is crushed to a particle size of less than 0.088 mm with a particle content of 85-95% and a particle size of greater than 0.088 mm with a particle content of 5-15%.

[0016] Preferably, the silicon carbide content in the waste graphite is 3 wt% to 7.5 wt%, more preferably 4% to 5.5 wt%; and the molar ratio of the mixed salt of sodium carbonate and sodium borate to the silicon carbide in the waste graphite is 2.5 to 1:1, more preferably 2 to 1:1.

[0017] In this invention, the preferred mass ratio of sodium carbonate to sodium borate in the mixed salt is 3-3.5:1. The purpose of controlling the amount of sodium carbonate and sodium borate in this invention is to achieve the optimal roasting effect with the least amount of additives. Excessive sodium carbonate can lead to more severe sintering, making the roasted product structure too dense and affecting the roasting effect, making it difficult to remove during the washing process; excessive sodium carbonate also increases the pressure of subsequent washing steps, making it difficult to recover and causing resource waste. Conversely, insufficient sodium carbonate can lead to poor reaction results and incomplete impurity removal.

[0018] The particle size of sodium borate and sodium borate is less than 0.2 mm, preferably less than or equal to 0.18 mm, and more preferably 0.175-0.125 mm.

[0019] Preferably, the calcination temperature is 500~800℃; the holding time is 60~120min; and the calcination is carried out in a semi-closed or closed system, the semi-closed system including a muffle furnace. The initial atmosphere for calcination is an air atmosphere. Because the muffle furnace makes the entire calcination system a semi-closed system, the air hardly flows rapidly inside the furnace during calcination. As calcination proceeds, a weak reducing system is formed inside the furnace cavity, thereby ensuring that the raw material does not suffer significant burn-off.

[0020] In practical applications, when using air atmosphere calcination, within the range selectable by this invention, a high temperature will affect the recovery rate of high-purity graphite.

[0021] Preferably, the refined product is placed in an acidic solution at a solid-liquid ratio of 1:6~10; leaching is carried out with stirring at room temperature to 70°C, followed by filtration. The filtered solid is then dried to obtain the final product. The leaching time is 30~150 minutes; during washing, it is generally washed 3~5 times until the pH of the wash solution is 7~8; the drying temperature is 80~90°C, and the drying time is 12~36 hours.

[0022] The yield of high-purity graphite is above 90%. In this invention, waste graphite, after treatment, achieves a deashing rate of ≥99.5%. The ash content of the product obtained by this invention is ≤0.02%, and after optimization, the ash content of the obtained product is ≤0.012%.

[0023] The beneficial effects of this invention are:

[0024] (1) This method is simple to operate and does not require a lot of equipment that is too complicated to operate;

[0025] (2) The additives used in this invention are sodium carbonate and sodium borate. The product after calcination contains soluble sodium salts in addition to graphite. The solution obtained after water immersion treatment can be completely recovered.

[0026] (3) The graphite products processed by this method can retain a high degree of graphitization of the raw materials, which is close to that of commercial graphite. After simple processing, they can be used directly as raw materials for downstream products. Detailed Implementation

[0027] The present invention will be further described below with reference to specific embodiments;

[0028] In the comparative examples and embodiments of the invention, the particle size of sodium borate and sodium borate used is less than 0.2 mm.

[0029] Example 1

[0030] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0031] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 86%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 90%:10% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.125:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3:1; the silicon carbide content in the waste graphite is 7.1 wt%.

[0032]

[0033] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 600℃ for 60 minutes, and then take it out to obtain the calcined product.

[0034] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:7 for leaching treatment, control the temperature at 40℃, stir for 60 minutes, filter the stirred product to obtain the water-leached product.

[0035] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0036] The high-purity graphite prepared in this embodiment has an ash content of 0.08% and an impurity removal rate greater than 99%. The yield of high-purity graphite is 92%. The fixed carbon content in the obtained high-purity graphite is approximately 99.92%.

[0037] Example 2

[0038] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0039] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 80%:20% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3.5:1; the silicon carbide content in the waste graphite is 6.7 wt%.

[0040]

[0041] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 700℃ for 90 minutes, and then take it out to obtain the calcined product.

[0042] (3) Place the roasted product obtained in step (2) into a container, add deionized water at a solid-liquid ratio of 1:7 for leaching treatment, control the temperature at 50℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0043] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0044] The high-purity graphite prepared in this embodiment has an ash content of 0.03% and an impurity removal rate of greater than 99%. The yield of high-purity graphite is 90%. The fixed carbon content in the obtained high-purity graphite is approximately 99.97%.

[0045] Example 3

[0046] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0047] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 80%:20% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3.2:1; the silicon carbide content in the waste graphite is 6.9 wt%.

[0048]

[0049] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 500°C for 120 min, and then take it out to obtain the calcined product.

[0050] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:9 for leaching treatment, control the temperature at 60℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0051] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0052] The high-purity graphite prepared in this embodiment has an ash content of 0.09% and an impurity removal rate greater than 99%. The yield of high-purity graphite is 94%. The fixed carbon content in the obtained high-purity graphite is approximately 99.91%.

[0053] Example 4

[0054] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0055] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 80%:20% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3:1; the silicon carbide content in the waste graphite is 7.1 wt%.

[0056]

[0057] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 800℃ for 120 min, and then take it out to obtain the calcined product.

[0058] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:9 for leaching treatment, control the temperature at 70℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0059] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0060] The high-purity graphite prepared in this embodiment has an ash content of 0.02% and an impurity removal rate of greater than 99%. The yield of high-purity graphite is 88%. The fixed carbon content in the obtained high-purity graphite is approximately 99.98%.

[0061] Example 5

[0062] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0063] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 70%:30% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3.5:1; the silicon carbide content in the waste graphite is 7wt%.

[0064]

[0065] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 600℃ for 90 minutes, and then take it out to obtain the calcined product.

[0066] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:8 for leaching treatment, control the temperature at 60℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0067] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0068] The high-purity graphite prepared in this embodiment has an ash content of 0.06% and an impurity removal rate of greater than 99%. The yield of high-purity graphite is 89%. The fixed carbon content in the obtained high-purity graphite is approximately 99.94%.

[0069] Comparative Example 1

[0070] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0071] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 70%:30% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3:1; the silicon carbide content in the waste graphite is 7wt%.

[0072]

[0073] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 300°C for 90 minutes, and then take it out to obtain the calcined product.

[0074] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:8 for leaching treatment, control the temperature at 70℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0075] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0076] The high-purity graphite prepared in this comparative example had an ash content of 3.7% and an impurity removal rate of over 40%. The resulting product had a fixed carbon content of approximately 96.3%, failing to yield high-purity graphite.

[0077] Comparative Example 2

[0078] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0079] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 90%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 95%:5% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3:1; the silicon carbide content in the waste graphite is 6.7 wt%.

[0080]

[0081] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 700℃ for 120 min, and then take it out to obtain the calcined product.

[0082] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:7 for leaching treatment, control the temperature at 60℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0083] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0084] The high-purity graphite prepared in this comparative example had an ash content of 1.8% and an impurity removal rate of over 70%. The resulting graphite contained approximately 98.2% fixed carbon, thus failing to produce high-purity graphite.

[0085] Comparative Example 3

[0086] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0087] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 80%:20% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3.5:1; the silicon carbide content in the waste graphite is 6.7 wt%.

[0088]

[0089] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 900℃ for 60 minutes, and then take it out to obtain the calcined product.

[0090] (3) Place the roasted product obtained in step (2) into a container, add deionized water at a solid-liquid ratio of 1:6 for leaching treatment, control the temperature at 60℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0091] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0092] The high-purity graphite prepared in this comparative example had an ash content of 0.07% and an impurity removal rate greater than 99%. The yield of high-purity graphite was 52%. The fixed carbon content in the obtained high-purity graphite was approximately 99.93%. Although higher purity graphite can be obtained at higher calcination temperatures, the sample exhibits excessively high burn-off rates and severe sintering, resulting in low recovery rates, and therefore is not suitable for this method.

[0093] Comparative Example 4

[0094] The specific steps of this method for preparing high-purity graphite from waste graphite by calcining sodium carbonate are as follows:

[0095] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate are mixed evenly at a mass ratio of 80%:20% to obtain a mixture (i.e., the molar ratio of sodium carbonate to silicon carbide in the waste graphite is approximately 2.5:1); the silicon carbide content in the waste graphite is 6.7 wt%.

[0096]

[0097] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 700℃ for 90 minutes, and then take it out to obtain the calcined product.

[0098] (3) Place the roasted product obtained in step (2) into a container, add deionized water at a solid-liquid ratio of 1:7 for leaching treatment, control the temperature at 50℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0099] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0100] The high-purity graphite prepared in this comparative example had an ash content of 1.3% and an impurity removal rate of over 97%. The reaction did not yield high-purity graphite because using sodium carbonate alone not only increases the required reaction temperature but also affects the reaction kinetics, turning the original liquid-solid reaction into a solid-solid reaction and thus impacting the reaction outcome.

[0101] Comparative Example 5

[0102] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium borate are as follows:

[0103] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium borate are mixed evenly at a mass ratio of 80%:20% to obtain a mixture (i.e., the molar ratio of sodium borate to silicon carbide in the waste graphite is approximately 2:1); the silicon carbide content in the waste graphite is 6.7 wt%.

[0104]

[0105] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 700℃ for 90 minutes, and then take it out to obtain the calcined product.

[0106] (3) Place the roasted product obtained in step (2) into a container, add deionized water at a solid-liquid ratio of 1:7 for leaching treatment, control the temperature at 50℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0107] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0108] The high-purity graphite prepared in this comparative example had an ash content of 2.8% and an impurity removal rate of over 97%. The reaction did not yield high-purity graphite because sodium borate alone lacks the ability to decompose silicon carbide. Sodium borate primarily acts as a flux during calcination, enhancing reaction kinetics; as a single additive, it is insufficient for removing impurities.

[0109] Comparative Example 6

[0110] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0111] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 70%:30% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 2.5:1; the silicon carbide content in the waste graphite is 7wt%.

[0112]

[0113] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 600℃ for 90 minutes, and then take it out to obtain the calcined product.

[0114] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:8 for leaching treatment, control the temperature at 60℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0115] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0116] The ash content of the prepared high-purity graphite was 0.53%, which failed to produce high-purity graphite. The reason is that the addition of too little sodium carbonate makes it difficult for the reaction to proceed fully, resulting in poor impurity removal.

[0117] Comparative Example 7

[0118] The specific steps of the method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate are as follows:

[0119] (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, accounting for 89%. Then, the waste graphite and sodium carbonate-sodium borate mixed salt are mixed evenly at a mass ratio of 70%:30% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1). In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 4:1; the silicon carbide content in the waste graphite is 7wt%.

[0120]

[0121] (2) Place the mixture obtained in step (1) into a graphite crucible, place the graphite crucible into a muffle furnace for calcination, keep it at 600℃ for 90 minutes, and then take it out to obtain the calcined product.

[0122] (3) Place the roasted product prepared in step (2) into a container, add deionized water at a solid-liquid ratio of 1:8 for leaching treatment, control the temperature at 60℃, stir for 90 minutes, filter the stirred product to obtain the water-leached product.

[0123] (4) The acid-leached product prepared in step (3) is placed in a drying oven at 85°C for 24 hours to dry, and finally a high-purity graphite product is obtained.

[0124] The ash content of the prepared high-purity graphite was 0.53%, which failed to produce high-purity graphite. The reason is that the addition of too little sodium carbonate makes it difficult for the reaction to proceed fully, resulting in poor impurity removal.

Claims

1. A method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate, characterized in that... Includes the following steps: (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, with 85-95% of the particles being smaller. Then, the waste graphite and additives are mixed evenly at a mass ratio of 70-90%:30-10% to obtain a mixture. The additive is a mixed salt of sodium carbonate and sodium borate. The waste graphite contains silicon carbide. In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 2.8-3.6:

1. (2) Place the mixture prepared in step (1) into a muffle furnace for roasting. The holding temperature is 500~800℃. After taking it out, the roasted product is obtained. (3) The roasted product is soaked in water and then dried to obtain the product.

2. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The waste graphite includes waste graphite hot zones used in photovoltaic crystal pulling.

3. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The waste graphite has an ash content of 4-9.5 wt% and a fixed carbon content of over 90%.

4. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The waste graphite contains 3 wt% to 7.5 wt% silicon carbide; and the molar ratio of the mixed salt of sodium carbonate and sodium borate to the silicon carbide in the waste graphite is 2.5 to 1:

1.

5. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: In the mixed salt, the mass ratio of sodium carbonate to sodium borate is 3~3.5:

1.

6. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: Sodium borate and sodium borate have a particle size of less than 0.2 mm.

7. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The roasting time is 60~120min.

8. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The roasting is carried out in a semi-closed or closed system, the semi-closed system including a muffle furnace; the initial atmosphere for roasting is an air atmosphere.

9. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The refined product is placed in an acidic solution at a solid-liquid ratio of 1:6~10; it is stirred and leached at room temperature to 70°C, filtered after leaching, and the filtered solid is dried to obtain the product; the water leaching time is 30~150 minutes; during washing, it is generally washed 3~5 times until the pH of the washing solution is 7~8; the drying temperature is 80~90°C and the time is 12~36 hours.

10. The method for preparing high-purity graphite from waste graphite by calcining sodium carbonate-sodium borate according to claim 1, characterized in that: The yield of high-purity graphite is above 90%; the ash content of the obtained product is less than or equal to 0.02%.

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