Method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate

Abstract

The invention relates to a method for preparing high-purity graphite by roasting and purifying waste graphite through sodium carbonate-sodium borate, and belongs to the technical field of preparation of high-purity graphite through purification of waste graphite. The preparation method comprises the following steps: taking waste graphite containing silicon carbide as a raw material, crushing the raw material, adding 10-30% of an additive of sodium carbonate-sodium borate mixed salt, controlling the mass ratio of sodium carbonate to sodium borate in the mixed salt to be (2.8-3.6): 1, and then roasting at 500-800 DEG C to obtain a roasted product; and then carrying out water immersion treatment, and drying to obtain the product. High-purity graphite with the ash content reduced by 0.02% or below can be obtained by purifying the waste graphite thermal field, the ash removal rate of the waste graphite thermal field is 99% or above, and the purification method has the advantages that the process is simple, the purification efficiency is high, energy consumption is low, products are easy to collect, industrialization is easy to achieve and the like. The method is of great significance to preparation of high-purity graphite through waste graphite thermal field purification.

Description

Technical Field

[0001] The present invention relates to a method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate, and particularly to a method for preparing high-purity graphite by roasting waste graphite thermal field for photovoltaic crystal pulling with sodium carbonate-sodium borate, belonging to the technical field of purifying waste graphite to prepare high-purity graphite. Background Art

[0002] Graphite and its derivative materials have the characteristics of high chemical inertness, good corrosion resistance, high strength, good electrical conductivity, etc., and are widely used in the fields of chemical industry, portable electronics, aerospace, metallurgy, etc. With the large increase in the application of graphite, a large amount of waste graphite is generated in industry. Photovoltaic power generation is a technology that directly converts light energy into electrical energy using the photovoltaic effect at the semiconductor interface. Due to the excellent properties of graphite, there is currently no other material that can replace it. These carbon graphite material products undoubtedly become the manufacturing basis of key equipment in the upstream of the solar photovoltaic power generation industry chain. For the contribution of carbon graphite material products to the solar photovoltaic power generation industry, since the current market of the solar photovoltaic power generation industry is still dominated by policies, in order to reduce costs and achieve widespread acceptance in the market, as an important basic material for polysilicon, if the graphite material does not meet the requirements, then the dream of the solar photovoltaic power generation industry to improve the photoelectric conversion efficiency and reduce costs will not be realized. It is understood that the photoelectric conversion efficiency of solar cells depends to a large extent on the purity of the silicon wafers. To improve the photoelectric conversion efficiency of solar cells, on the one hand, it is necessary to continuously improve the purification process of silicon and increase the purity of the wafers. On the other hand, it is necessary to improve the purification environment of the crystallization silicon-graphite thermal field to be purified. If the graphite purity is not high and there are too many impurities, the crystallization silicon pulling environment will be polluted and the quality of the wafers cannot be guaranteed, then the efficiency of solar cells cannot be improved.

[0003] Graphite products are the industrial foundation for the development of the solar photovoltaic power generation industry, and the two coexist and develop prosperously. The application of carbon in the field of solar photovoltaic power generation mainly focuses on the manufacture of polysilicon raw materials, the wire drawing of single crystal silicon rods, and the casting of polysilicon ingots. Due to the excellent density, hardness, and compressive strength of graphite materials, and the advantages of high temperature resistance, high pressure resistance, corrosion resistance, good electrical conductivity, and stable performance, in the upstream of the solar photovoltaic power generation industry, graphite materials can be made into graphite crucible heating containers for purifying polysilicon, pulling silicon rods, and manufacturing. Therefore, it is very necessary to purify and recycle the waste graphite generated in the photovoltaic industry.

[0004] A high-purity graphite powder and its purification process (Application No.: CN201811405254.6) reports a graphite purification process. In this process, graphite powder raw materials are added into a graphite boat, and the graphite boat is installed in a purification device. The device is evacuated to a pressure of -93 to -100 KPa and the pressure is maintained for 0.5 - 2 h. After the pressure rise rate is qualified, the temperature is raised to 1200 - 1800 °C, and a certain amount of purification gas is introduced. After maintaining the pressure, the device is evacuated to a pressure of -93 to -100 KPa. Pulse purification is carried out during the heating process until a certain temperature in the range of 2000 - 3000 °C is reached and maintained for 1 - 3 h, then the pulse purification is stopped. The device is evacuated and cooled to 1200 - 1500 °C, and then argon is introduced to lower the temperature to room temperature, obtaining high-purity graphite powder. This process can timely remove volatile impurities through a pulsed process at a relatively low temperature, solve the problem of deeply removing key impurities such as B, Al, V, etc. in graphite powder raw materials, and thus obtain high-purity graphite powder with a purity of 99.999% - 99.9999%, providing a practical path for the industrial production of high-purity graphite powder.

[0005] A method for purifying graphite by chlorination roasting (Application No.: CN202010028266.2) reports a method for purifying graphite by chlorination roasting. In this method, graphite raw materials react for 2 - 16 h in an atmosphere of reducing gas and chlorinated gas at a temperature of 400 °C to 1200 °C, and the impurities in the graphite form gaseous metal complexes. Then, gas-solid separation is carried out to obtain purified graphite with a purity greater than 99.5%. The chlorinated gas is a gas containing halogen elements. This patent first discovers that when the pickled product reacts with reducing gas and chlorinated gas at medium and low temperatures of 400 °C to 1200 °C, the oxide impurities therein will form metal complexes with boiling points lower than 1000 °C, such as generating CaFeCl 4 , NaAlCl 4 , KMgCl 3 . These metal complexes are discharged in gaseous form with the reducing gas and chlorinated gas, which can effectively solve the problems of low purity of the final product in low-temperature chemical purification, high feeding requirements, high cost, and complex equipment in high-temperature roasting purification. The purity of the purified product is greater than 99.5%.

[0006] After retrieval, it is found that existing graphite purification methods all have the drawbacks of high cost, low efficiency, and large pollution, and have good effects on conventional impurity phases. However, they are basically ineffective for waste graphite containing silicon carbide (especially the waste graphite heat field used in photovoltaic crystal pulling). Therefore, there are few reports on the technology that high-purity graphite can be obtained by roasting with sodium carbonate and sodium borate followed by water washing. Summary of the Invention

[0007] The present invention first attempts to roast the waste graphite after mixing it with a mixed salt of sodium carbonate and sodium borate. Considering that it is difficult to effectively remove silicon carbide in the waste graphite thermal field by conventional means, sodium carbonate and sodium borate will decompose silicon carbide to form soluble sodium salts under an air atmosphere and at an appropriate temperature, thereby removing most of the silicon carbide. However, due to the low content of silicon carbide in the waste graphite and poor reaction kinetic conditions, the addition amount of the additive is appropriately increased to promote the reaction of silicon carbide in the waste graphite with sodium carbonate and boric acid, thereby improving the purification efficiency. In addition, this method can effectively avoid the generation of toxic gases, which has positive significance for production safety and environmental protection. The specific operation is simple and has high safety, and the valuable elements can be recovered after the roasted product is washed with water, which has positive significance for reducing industrial production costs and energy consumption.

[0008] The technical problem to be solved by the present invention is that there are silicon carbide impurities in the waste graphite thermal field for photovoltaic crystal pulling. Due to its excellent acid and alkali resistance and high temperature resistance, it is difficult to remove by conventional purification means.

[0009] In view of the problems and deficiencies existing in the above-mentioned prior art, the present invention provides a method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate. The graphite obtained by this method has a very low ash content and can fully meet the needs of industrial production. The specific steps are as follows:

[0010] (1) First, crush the waste graphite to particles with a particle size less than 0.088 mm accounting for 85-95%, and then mix the waste graphite and the additive evenly according to a mass ratio of 70-90%:30-10% to obtain a mixed material; 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) Put the mixed material prepared in step (1) into a muffle furnace for roasting, the holding temperature is 500-800 °C, preferably 600-800 °C, and take out the roasted product after roasting.

[0012] (3) Perform water immersion treatment on the roasted product, and dry it after the water immersion treatment to obtain the product.

[0013] The waste graphite includes the waste graphite thermal field for photovoltaic crystal pulling.

[0014] Preferably, the ash content of the waste graphite is 4-9.5%, and the fixed carbon content is above 90%.

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

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

[0017] In the present invention, in the mixed salt, the mass ratio of sodium carbonate to sodium borate is preferably 3 to 3.5:1. The purpose of controlling sodium carbonate and sodium borate in the present invention is to obtain the optimal roasting effect with the least amount of additives. If the amount of sodium carbonate is too large, the sintering situation will be more serious, making the structure of the roasted product too dense, thus affecting the roasting effect and making it difficult to remove it during the water washing process; excessive sodium carbonate will also increase the pressure in the subsequent water washing step, making it difficult to recycle and causing waste of resources. If the amount of sodium carbonate is too small, there will be problems such as poor reaction effect and incomplete impurity removal.

[0018] The particle size of sodium borocarbonate 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 roasting temperature is 500 - 800 °C; the heat preservation time is 60 - 120 min, and the roasting is carried out in a semi-closed or closed system. The semi-closed system includes a muffle furnace. The initial atmosphere of roasting is an air atmosphere. Because the muffle furnace makes the whole roasting system form a semi-closed system, during roasting, the air hardly forms a rapid flow in the furnace. As the roasting progresses, a weak reducing system will be formed in the furnace cavity, thereby ensuring that there is no significant burning loss of the raw materials.

[0020] In actual application, when roasting in an air atmosphere, within the selectable range of the present invention, a higher temperature will affect the recovery rate of high-purity graphite.

[0021] Preferably, the refined product is placed in an acid solution according to a solid-liquid ratio of 1:6 - 10; stirring leaching is carried out at room temperature to 70 °C. After leaching, filtration is carried out, and the solid obtained by filtration is dried to obtain the product. The water leaching time can be 30 - 150 min; during cleaning, generally clean 3 - 5 times until the pH of the washing liquid is 7 - 8; the drying temperature is 80 - 90 °C, and the time is 12 - 36 hours.

[0022] The yield of the high-purity graphite is above 90%. In the present invention, after the waste graphite is treated, the ash removal rate is greater than or equal to 99.5%. The ash content in the product obtained by the present invention is less than or equal to 0.02%, and after optimization, the ash content in the obtained product is less than or equal to 0.012%.

[0023] The beneficial effects of the present invention are:

[0024] (1) This method is easy to operate and does not require too many and overly cumbersome devices.

[0025] (2) The additives used in the present invention are sodium carbonate and sodium borate. After roasting, the product contains soluble sodium salts in addition to graphite. The solution obtained after water leaching treatment can be completely recovered.

[0026] (3) The graphite product treated by this method can retain a high degree of graphitization of the raw material, approaching commercial graphite, and can be directly used as the raw material for downstream products after simple treatment. Specific Embodiments

[0027] The present invention will be further described below in conjunction with specific embodiments;

[0028] In the comparative examples and examples of the invention, the particle sizes of the sodium borocarbonate and sodium borate used are both less than 0.2 mm.

[0029] Example 1

[0030] The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate - sodium borate is as follows:

[0031] (1) First, crush the waste graphite until 86% of the particles are less than 0.088 mm, and then mix it evenly with a sodium carbonate - sodium borate mixed salt in a mass ratio of 90%:10% to obtain a mixed material (that is, the molar ratio of the sodium carbonate - sodium borate mixed salt used to the silicon carbide in the waste graphite is 1.125:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3:1; the content of silicon carbide in the waste graphite is 7.1 wt%.

[0032]

[0033] (2) Put the mixed material prepared in step (1) into a graphite crucible, and put the graphite crucible into a muffle furnace for roasting. The holding temperature is 600 °C, and the holding time is 60 min. After taking it out, a roasted product is obtained.

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

[0035] (4) Put the acid - leached product prepared in step (3) into a drying oven at 85 °C for drying for 24 h to finally obtain a high - purity graphite product.

[0036] The ash content in the high - purity graphite prepared in this example is 0.08%, the impurity removal rate is greater than 99%. The yield of high - purity graphite is 92%. In the obtained high - purity graphite, the fixed carbon content is about 99.92%.

[0037] Example 2

[0038] The method for preparing high-purity graphite from waste graphite by roasting with sodium carbonate-sodium borate is as follows:

[0039] (1) First, crush the waste graphite until the particles smaller than 0.088 mm account for 89%, and then mix it evenly with the sodium carbonate-sodium borate mixed salt in a mass ratio of waste graphite to the mixed salt of 80%:20% to obtain a mixed material (that is, the molar ratio of the amount of the sodium carbonate-sodium borate mixed salt to silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3.5:1; the content of silicon carbide in the waste graphite is 6.7 wt%;

[0040]

[0041] (2) Put the mixed material prepared in step (1) into a graphite crucible, and put the graphite crucible into a muffle furnace for roasting. The holding temperature is 700 °C and the holding time is 90 min. After taking it out, a roasted product is obtained;

[0042] (3) Put the roasted product prepared in step (2) into a container, add deionized water for leaching treatment according to a solid-liquid ratio of 1:7, control the temperature at 50 °C, stir for 90 min, and filter the stirred product to obtain a water-leached product.

[0043] (4) Put the acid-leached product prepared in step (3) into an oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0044] The ash content in the high-purity graphite prepared in this example is 0.03%, the impurity removal rate is greater than 99%. The yield of high-purity graphite is 90%. In the obtained high-purity graphite, the fixed carbon content is about 99.97%.

[0045] Example 3

[0046] The method for preparing high-purity graphite from waste graphite by roasting with sodium carbonate-sodium borate is as follows:

[0047] (1) First, crush the waste graphite until the particles smaller than 0.088 mm account for 89%, and then mix it evenly with the sodium carbonate-sodium borate mixed salt in a mass ratio of waste graphite to the mixed salt of 80%:20% to obtain a mixed material (that is, the molar ratio of the amount of the sodium carbonate-sodium borate mixed salt to silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3.2:1; the content of silicon carbide in the waste graphite is 6.9 wt%;

[0048]

[0049] (2) Put the mixed material prepared in step (1) into a graphite crucible, and place the graphite crucible in a muffle furnace for roasting. The heat preservation temperature is 500 °C, and the heat preservation time is 120 min. After taking it out, a roasted product is obtained;

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

[0051] (4) Put the acid-leached product prepared in step (3) into a drying oven at 85 °C for drying for 24 h, and finally obtain a high-purity graphite product.

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

[0053] Example 4

[0054] The method for preparing high-purity graphite from waste graphite by roasting with sodium carbonate-sodium borate is as follows:

[0055] (1) First, crush the waste graphite so that the particles smaller than 0.088 mm account for 89%, and then mix it evenly according to the mass ratio of waste graphite to sodium carbonate-sodium borate mixed salt of 80%:20% to obtain a mixed material (that is, the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate: sodium borate = 3:1; the content of silicon carbide in the waste graphite is 7.1 wt%;

[0056]

[0057] (2) Put the mixed material prepared in step (1) into a graphite crucible, and place the graphite crucible in a muffle furnace for roasting. The heat preservation temperature is 800 °C, and the heat preservation time is 120 min. After taking it out, a roasted product is obtained;

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

[0059] (4) Put the acid-leached product prepared in step (3) into a drying oven at 85 °C for drying for 24 h, and finally obtain a high-purity graphite product.

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

[0061] Example 5

[0062] The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate is as follows:

[0063] (1) First, crush the waste graphite until 89% of the particles are smaller than 0.088 mm, and then mix it evenly with the sodium carbonate-sodium borate mixed salt at a mass ratio of waste graphite to sodium carbonate-sodium borate mixed salt of 70%:30% to obtain a mixed material (i.e., the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to silicon carbide in the waste graphite is 1.15:1). In the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3.5:1, and the content of silicon carbide in the waste graphite is 7 wt%.

[0064]

[0065] (2) Put the mixed material prepared in step (1) into a graphite crucible, place the graphite crucible in a muffle furnace for roasting, keep the temperature at 600 °C for 90 min, and take it out to obtain a roasted product.

[0066] (3) Put 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 °C, stir for 90 min, filter the stirred product to obtain a water-leached product.

[0067] (4) Place the acid-leached product prepared in step (3) in a drying oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

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

[0069] Comparative Example 1

[0070] The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate is as follows:

[0071] (1) First, crush the waste graphite until particles smaller than 0.088 mm account for 89%, and then mix it evenly with the sodium carbonate-sodium borate mixed salt at a mass ratio of 70%:30% to obtain a mixed material (i.e., the molar ratio of the amount of the sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3:1; the silicon carbide content in the waste graphite is 7 wt%.

[0072]

[0073] (2) Put the mixed material prepared in step (1) into a graphite crucible, and place the graphite crucible in a muffle furnace for roasting. The holding temperature is 300 °C and the holding time is 90 min. After taking it out, a roasted product is obtained.

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

[0075] (4) Place the acid-leached product prepared in step (3) in a drying oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0076] The ash content in the high-purity graphite prepared in this comparative example is 3.7%, and the impurity removal rate is greater than 40%. In the obtained product, the fixed carbon content is about 96.3%, and high-purity graphite cannot be obtained.

[0077] Comparative Example 2

[0078] The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate is as follows:

[0079] (1) First, crush the waste graphite until particles smaller than 0.088 mm account for 90%, and then mix it evenly with the sodium carbonate-sodium borate mixed salt at a mass ratio of 95%:5% to obtain a mixed material (i.e., the molar ratio of the amount of the sodium carbonate-sodium borate mixed salt to the silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3:1; the silicon carbide content in the waste graphite is 6.7 wt%.

[0080]

[0081] (2) Put the mixed material prepared in step (1) into a graphite crucible, and place the graphite crucible in a muffle furnace for roasting. The holding temperature is 700 °C and the holding time is 120 min. After taking it out, a roasted product is obtained.

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

[0083] (4) Put the acid-leached product prepared in step (3) into an oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0084] The ash content in the high-purity graphite prepared in this comparative example is 1.8%, and the impurity removal rate is greater than 70%. In the obtained graphite, the fixed carbon content is about 98.2%, and high-purity graphite cannot be obtained.

[0085] Comparative Example 3

[0086] The method for preparing high-purity graphite from waste graphite by roasting with sodium carbonate-sodium borate is as follows:

[0087] (1) First, crush the waste graphite until 89% of the particles are smaller than 0.088 mm, and then mix evenly according to a mass ratio of waste graphite to sodium carbonate-sodium borate mixed salt of 80%:20% to obtain a mixed material (that is, the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 3.5:1; the content of silicon carbide in the waste graphite is 6.7 wt%.

[0088]

[0089] (2) Put the mixed material prepared in step (1) into a graphite crucible, put the graphite crucible into a muffle furnace for roasting, keep the temperature at 900 °C for 60 min, and take it out to obtain a calcined product;

[0090] (3) Put the calcined product prepared in step (2) into a container, add deionized water for leaching treatment according to a solid-liquid ratio of 1:6, control the temperature at 60 °C, stir for 90 min, filter the stirred product to obtain a water-leached product.

[0091] (4) Put the acid-leached product prepared in step (3) into an oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0092] The ash content in the high-purity graphite prepared in this comparative example is 0.07%, and the impurity removal rate is greater than 99%. The yield of high-purity graphite is 52%. In the obtained high-purity graphite, the fixed carbon content is about 99.93%. When the roasting temperature is relatively high, although graphite with relatively high purity can be obtained, the loss rate of the sample is too high, and there is a serious sintering phenomenon, resulting in too low recovery rate, so it is not suitable to be used.

[0093] Comparative Example 4

[0094] The method for preparing high-purity graphite from waste graphite by sodium carbonate roasting is as follows:

[0095] (1) First, crush the waste graphite until particles smaller than 0.088 mm account for 89%, and then mix evenly with sodium carbonate according to the mass ratio of waste graphite to sodium carbonate of 80%:20% to obtain a mixed material (i.e., the molar ratio of the dosage of sodium carbonate to silicon carbide in the waste graphite is about 2.5:1); the content of silicon carbide in the waste graphite is 6.7 wt%.

[0096]

[0097] (2) Put the mixed material prepared in step (1) into a graphite crucible, and put the graphite crucible into a muffle furnace for roasting. The heat preservation temperature is 700 °C, and the heat preservation time is 90 min. After taking it out, a roasted product is obtained;

[0098] (3) Put the roasted product prepared in step (2) into a container, add deionized water according to the solid-liquid ratio of 1:7 for leaching treatment, control the temperature at 50 °C, stir for 90 min, filter the stirred product, and obtain a water-leached product.

[0099] (4) Put the acid-leached product prepared in step (3) into a drying oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0100] The ash content in the high-purity graphite prepared in this comparative example is 1.3%, and the impurity removal rate is greater than 97%. High-purity graphite was not obtained in the reaction because using only sodium carbonate not only increased the required reaction temperature but also affected the reaction kinetic conditions, changing the original liquid-solid reaction to a solid-solid reaction and affecting the reaction effect.

[0101] Comparative Example 5

[0102] The method for preparing high-purity graphite from waste graphite by sodium borate roasting is as follows:

[0103] (1) First, crush the waste graphite until particles smaller than 0.088 mm account for 89%, and then mix evenly with sodium borate according to the mass ratio of waste graphite to sodium borate of 80%:20% to obtain a mixed material (i.e., the molar ratio of the dosage of sodium borate to silicon carbide in the waste graphite is about 2:1); the content of silicon carbide in the waste graphite is 6.7 wt%.

[0104]

[0105] (2) Put the mixed material prepared in step (1) into a graphite crucible, and place the graphite crucible in a muffle furnace for roasting. The holding temperature is 700 °C and the holding time is 90 min. After taking it out, a roasted product is obtained;

[0106] (3) Put the roasted product prepared in step (2) into a container, add deionized water for leaching treatment according to a solid-liquid ratio of 1:7, control the temperature at 50 °C, stir for 90 min, and filter the stirred product to obtain a water-leached product.

[0107] (4) Place the acid-leached product prepared in step (3) in a drying oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0108] The ash content in the high-purity graphite prepared in this comparative example is 2.8%, and the impurity removal rate is greater than 97%. High-purity graphite was not obtained in the reaction. The reason is that using sodium borate alone does not have the ability to decompose silicon carbide. Sodium borate mainly acts as a flux to strengthen the reaction kinetics during the roasting process. It is difficult to remove impurities when used alone as an additive.

[0109] Comparative Example 6

[0110] The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate is as follows:

[0111] (1) First, crush the waste graphite so that the particles smaller than 0.088 mm account for 89%, and then mix it evenly according to the mass ratio of waste graphite to sodium carbonate-sodium borate mixed salt of 70%:30% to obtain a mixed material (that is, the molar ratio of the amount of sodium carbonate-sodium borate mixed salt to silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:sodium borate = 2.5:1; the content of silicon carbide in the waste graphite is 7 wt%;

[0112]

[0113] (2) Put the mixed material prepared in step (1) into a graphite crucible, and place the graphite crucible in a muffle furnace for roasting. The holding temperature is 600 °C and the holding time is 90 min. After taking it out, a roasted product is obtained;

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

[0115] (4) Place the acid-leached product prepared in step (3) in a drying oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0116] The ash content in the prepared high-purity graphite is 0.53%, and high-purity graphite cannot be obtained. The reason is that too little sodium carbonate added makes it difficult for the reaction to proceed fully, so the impurity removal effect is poor.

[0117] Comparative Example 7

[0118] The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-borate is as follows:

[0119] (1) First, crush the waste graphite until 89% of the particles are smaller than 0.088 mm, and then mix it evenly with a sodium carbonate-borate mixed salt in a mass ratio of waste graphite to sodium carbonate-borate mixed salt of 70%:30% to obtain a mixed material (that is, the molar ratio of the amount of the sodium carbonate-borate mixed salt to the silicon carbide in the waste graphite is 1.15:1); in the mixed salt, by mass ratio, sodium carbonate:borate = 4:1; the content of silicon carbide in the waste graphite is 7 wt%.

[0120]

[0121] (2) Put the mixed material prepared in step (1) into a graphite crucible, and put the graphite crucible into a muffle furnace for roasting. The holding temperature is 600 °C, and the holding time is 90 min. After taking it out, a roasted product is obtained.

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

[0123] (4) Put the acid-leached product prepared in step (3) into a drying oven at 85 °C for drying for 24 h to finally obtain a high-purity graphite product.

[0124] The ash content in the prepared high-purity graphite is 0.53%, and high-purity graphite cannot be obtained. The reason is that too little sodium carbonate added makes it difficult for the reaction to proceed fully, so the impurity removal effect is poor.

Claims

1. A method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate, characterized in that The steps include: (1) First, the waste graphite is crushed to a particle size of less than 0.088 mm, and the particles account for 85-95%. Then, the waste graphite and the additive are mixed in a mass ratio of 70-90%:30-10% to obtain a mixed material; 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) placing the mixed material prepared in step (1) into a muffle furnace for roasting at a holding temperature of 500 to 800° C., preferably 600 to 800° C., and taking out to obtain a roasted product; (3) The roasted product is subjected to water immersion treatment, and then dried to obtain the product.

2. The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate according to claim 1, characterized in that: The waste graphite includes waste graphite thermal field for photovoltaic crystal pulling.

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

4. The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate according to claim 1, characterized in that: The content of silicon carbide in the waste graphite is 3wt%~7.5wt%; and the molar ratio of the mixed salt of sodium carbonate and sodium borate to the silicon carbide in the waste graphite is 2.5~1:1, and more preferably 2~1:

1.

5. The method for preparing high-purity graphite by roasting waste graphite with 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 preferably 3-3.5:

1.

6. The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate according to claim 1, characterized in that: The particle size of sodium borocarbonate and sodium borate is less than 0.2 mm.

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

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

9. The method for preparing high-purity graphite by roasting waste graphite with sodium carbonate-sodium borate according to claim 1, characterized in that: The refined product is placed in an acid solution at a solid-liquid ratio of 1:6-10; stirred and leached at room temperature-70°C, filtered after leaching, and the filtered solid is dried to obtain the product. The immersion time is 30-150 minutes; when washing, generally wash 3-5 times until the pH of the washing liquid 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 by roasting waste graphite with sodium carbonate-sodium borate according to claim 1, characterized in that: The yield of the high-purity graphite is above 90%, and the ash content of the obtained product is less than or equal to 0.02%.

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