Introduction to graphite processing and purification

1.1 Graphite

Graphite is one of the layered crystalline minerals of carbon. It has many unique physical and chemical properties such as self-lubrication, electrical conductivity and high temperature resistance. Graphite is classified into scaly graphite and cryptocrystalline graphite due to the difference in degree of crystallization. The former is a large-grained crystalline graphite, which is in the form of a sheet or plate. The latter is in the form of a mass or a powder. The cryptocrystalline graphite is also commonly called microcrystalline graphite or earthy graphite. Grade flake graphite is generally low, fixed carbon content (CGD%) is generally not more than 10%, especially enriched local graphite mine area is up to 20% or more, which is good optional **, floating beneficiation grade up More than 85%, graphite has good quality and wide industrial use. It is the most valuable type of graphite at present. Graphite often associated with mineral mica, feldspar, quartz, tremolite, garnet and the small amount of pyrite, calcite, rutile sometimes useful components associated and vanadium. The cryptocrystalline graphite ore is produced by a microcrystalline aggregate, and its crystal form can only be observed under an electron microscope. The ore is mostly dense and has a fixed carbon content of 60% to 80% or even more than 90%. The selectivity is poor. The impurity minerals (quartz, calcite, etc.) in the ore are difficult to separate, so its industrial application is not as extensive as scaly graphite. The market price is also lower. How to overcome these shortcomings and make the use of cryptocrystalline graphite better is a hot issue in the material science community.

1.2 Structure of Graphite Graphite has three other carbon atoms attached to the periphery of each carbon atom, and the arrangement is a honeycomb-shaped hexagon. It is a hexagonal system with a complete layered cleavage. The cleavage surface is mainly composed of molecular bonds, and its molecular attraction is weak, so its natural floatability is very good. Since each carbon atom emits an electron, those electrons can move freely, so graphite is an electrical conductor. There are two main ways of stacking carbon atoms. The layers are combined with van der Waals force: one structure of graphite is repeated in the order of ABAB..., which has the symmetry of hexagonal crystals, called hexagonal graphite. It is called α-graphite and its structure is shown in the figure below. The other structure is repeatedly stacked in the order of ABCABC. This graphite has the symmetry of the trigonal system, called trigonal graphite, also known as β-graphite, and its structure is shown in Figure 1-1. The unit cell parameters of the trigonal graphite are a = 0.3635 nm, and α = 39 ° 3'.

Figure 1-1 The spatial structure of graphite crystals There is no pure graphite in nature. It often contains impurities such as SiO2, A12O3, FeO, CaO, P2O5, and CuO. These impurities often appear as minerals such as quartz, pyrite, and carbonate. In addition, there are water, asphalt , CO2, H2, CH4, N2 and other gas parts.

1.3 Properties of graphite Due to its special structure, graphite has the following special properties: 1) High temperature resistance type: graphite has a melting point of 3850±50°C and a boiling point of 4250°C. Even if it is burned by ultra-high temperature arc, the weight loss is small. The coefficient of thermal expansion is also small. The strength of graphite increases with increasing temperature, and at 2000 ° C, the strength of graphite is doubled. 2) Conductivity and thermal conductivity: The conductivity of graphite is one hundred times higher than that of general non-metallic minerals. Thermal conductivity than steel, iron, lead and other metal materials. The thermal conductivity decreases with increasing temperature, and even at very high temperatures, graphite becomes a thermal insulator. Graphite is electrically conductive because each carbon atom in graphite forms only three covalent bonds with other carbon atoms, and each carbon atom still retains one free electron to transport the charge. 3) Lubricity: The lubricating property of graphite depends on the size of the graphite scale. The larger the scale, the smaller the friction coefficient and the better the lubrication performance. 4) Chemical stability: Graphite has good chemical stability at normal temperature, and is resistant to acid, alkali and organic solvents. 5) Plasticity: Graphite has good toughness and can be pressed into very thin sheets. 6) Thermal shock resistance: When graphite is used at normal temperature, it can withstand severe changes in temperature without damage. When the temperature is abrupt, the volume of graphite does not change much and cracks are not generated.

1.4 Use of graphite 1.4.1 Application of graphite in the electronics industry The electronics industry is the main market for high-tech graphite products. The improved graphite produced by high-tech is used in the cooling system of electronic products to enable rapid heat dissipation of electronic products, such as liquid crystal displays; ultra-pure graphite for lithium- ion batteries and alkaline battery electrodes can improve conductivity, ultra-pure expanded graphite foil for fuel The battery electrode plate can improve the electrical conductivity, reduce the weight, and the like. Experts estimate that the annual total demand for alkaline batteries in the world will increase by 4% to 5%, and the total annual demand for lithium-ion batteries will increase by 6% to 8%. It is expected that the consumption of ultra-pure graphite in the battery industry will increase rapidly. Fuel cells are the most promising consumer of ultra-pure graphite. Global warming is threatening the sustainable development of human society. Most countries are working hard to research and develop clean energy to reduce carbon dioxide emissions. Fuel cells are an efficient and clean energy source. Once the production cost is reduced to a market acceptable range, the demand for fuel cells will be huge. There is reason to believe that the energy market has a large demand for ultra-pure expanded graphite. Roskill, a foreign consulting firm, estimates that in the short term, the demand for ultra-pure expanded graphite for fuel cells in the world is 15,000 tons. Applications in the electronics industry include a wide range of electro-carbon graphite products, including electrodes, brushes, carbon rods, carbon tubes, anode plates, graphite gaskets, and non-inductive conductive coating agents, electrical contact fillers, and A cathode ray tube coating or the like serves as a good conductor of electricity. Graphite provides an opportunity for the development of the electronics industry: the old-fashioned recorder is to make the movable contact and the wire-wound wire contact directly contact, often causing hair to touch, inevitably generating noise, seriously affecting measurement accuracy and reliability. It also shortens the service life. In order to overcome the above shortcomings, a new generation of recorders is coated with a layer of an organic conductive film with graphite added to the wound wire resistance. 1.4.2 Application of Graphite in the Field of Lubrication Graphite is used in the field of lubricants, including dry powder graphite lubricants, water-based lubricants, and oil-based lubricants. With the good film formation of graphite powder **, people have used direct rubbing, roller coating and other methods to make the graphite powder form a dry powder film on the surface of the lubricated workpiece to achieve lubrication of low-speed light-load operation equipment; using graphite powder to fly ** Powder splash lubrication of the enclosed gear reducer can also be achieved. The graphite powder is blended with gasoline or alcohol, and is applied to the lubrication of the overhead crane of the bridge crane, which not only ensures the conductive performance of the roller, but also improves the lubrication condition between the axles, thereby improving the life of the live roller by three times. In the manufacture of seamless steel tubes and the lubrication of steel wire dry drawing production, the multi-component composite agent composed of graphite can prevent the sintering of the contact surface caused by severe conditions such as 1000 ° C high temperature and heavy load, and ensure the normal production. 1.4.3 Graphite is used to make diamonds. Due to the small reserves of natural diamonds and the extremely uneven distribution of the world's reserves, the value of large-grain diamonds is extremely high. Pure large-grained natural diamonds are called diamonds. In addition, it is the hardest crystal currently present on the earth (Mohs hardness 10), which can be used to cut and polish any hard material. Therefore, diamond cutting metal and drilling rock are widely used in mining and machinery industries. In addition, because diamonds have the highest hardness, abrasives used to process diamonds have so far only used diamond. At present, a large number of natural (purity, particle size is not enough as gem -quality diamond), synthetic diamond is used in machinery, cemented carbide, mining, drilling and other fields. The diamonds used in jewelry and cutting tools, drill bits and abrasives far exceed the annual output of natural diamonds in the world. Therefore, the diamond has a large gap, and a large amount of diamond needs to be artificially synthesized. This makes the research on the synthesis of diamond with graphite as raw material. . At present, the process of artificially synthesizing diamond has become mature, and the diamond synthesized by various processes has largely met the demand for diamond in various fields. Synthetic diamond requires high pressure technology, which is determined by the thermodynamic properties of diamond and graphite. At normal temperature and pressure, graphite is more stable than diamond: graphite heat of combustion is 393.51 kJ/mol, and diamond is 395.41 kJ/mol. A rough calculation shows that the pressure required to convert graphite into diamond is about 1.5 × 1010 Pa, so the main technique for synthetic diamond is high pressure technology. In the 1950s, graphite was successfully converted into diamond through experiments. Due to the development of high-pressure technology, the technology of artificially synthesizing diamond by using molten metal (Ni, Cr, Mn, Fe, CO, etc.) as a catalyst at a high pressure of 0.5 to 1×1010 Pa and a high temperature of 923 to 2123 °C was also explored. At present, the methods of synthesizing diamond in the industry mainly include a static pressure method and a shock wave method. In the synthesis of larger granular diamonds, the seeding method is now widely used: it is grown in a few days at high pressure and high temperature (6×1010Pa, 1527°C) to a particle size of several millimeters and weighs several carats. Gem-quality synthetic diamond. Although this synthetic diamond can not completely replace natural diamond, it is expected that in the near future, with the advancement of synthetic diamond technology, synthetic diamond will certainly meet the increasing demand for diamond products in various industries.

1.5 Prospects for the application of graphite The new use of graphite products has been continuously developed, and the development of graphite heat treatment technology has made these new opportunities possible. The ability to purify and modify graphite and its carbon products is key to the future growth of the graphite industry. Recently, scientists in the United States and France have used graphite layers to create a schematic model of electronic circuits and integrated circuits. Some researchers believe that graphite carbon nanotubes and graphite layer circuits have applications. Improvements in new high purity graphite products are being applied in some new areas. For example, highly conductive, graphitized carbon is being tested for airport runways and bridge friction materials, graphite foil, electronics industry and lubricants that can be heated by asphalt. The airport runway experiment has initially proved that the use of heated asphalt can melt 50 mm of snow per hour, which can prevent snow accumulation under almost any snowfall. The use of this bitumen allows the airport runway to be used under blizzard conditions, allowing road bridges to avoid snow and ice and reduce traffic accidents. An example of an expanded application of graphite technology is a new type of purified synthetic graphite, which is a raw material for the brake industry. Graphite can also be used as a friction modifier for braking. Its function is to provide lubrication so that the vehicle can be stopped in a controlled state rather than suddenly stopped while braking. The presence of SiC in ordinary synthetic graphite is a serious problem. As long as there is one, the brake rotor worth $200 can be scratched and rendered ineffective. Purification of graphite at high temperatures removes SiC therefrom. Now, graphite minerals are being explored and researched. It is expected that new-purpose graphite products will be continuously researched and developed, and the application prospect of graphite is limitless.

1.6 Purification method of graphite 1.6.1 Flotation method The purity of natural flaky graphite is generally only 1.5% to 10%. Because of its good floatability, the enrichment of this kind of graphite is mainly flotation. Flotation graphite concentrate grades can reach 95%, usually 79% to 90%. Since silicate minerals are impregnated in graphite scales, further enrichment by mineral processing is difficult, so chemical or thermal methods must be used to further remove impurities from the graphite. The grade of cryptocrystalline graphite is generally high, up to 50-60%. Some mines can produce high-grade cryptocrystalline graphite with a grade above 90%. However, due to the poor selectivity of this kind of graphite ore, the general beneficiation process can only be limited. Its grade, so cryptocrystalline graphite is usually removed by chemical purification to remove impurities in the graphite to obtain higher purity graphite. The treatment of cryptocrystalline graphite by physical beneficiation method is not good, the concentrate grade is not high enough, and the recovery rate of graphite is also very low. Therefore, the high carbon cryptocrystalline graphite products required in modern industry are generally prepared by chemical purification of cryptocrystalline graphite ore. If necessary, thermodynamic methods can also be used to obtain higher purity graphite products. In addition, some flaky graphite with low degree of crystallinity and fine particle size may undergo scale bending and kinking so that the sheet structure is destroyed, which greatly reduces the selectivity of graphite, although such graphite ore meets the requirements of flaky graphite. Structural characteristics, chemical purification is still widely used to improve the grade. 1.6.2 Chemical method of graphite removal The chemical method of graphite purification is to use impurities such as strong acid, strong alkali or other compounds to act on graphite, and convert it into water-soluble substance. After washing and drying, the final product is obtained. Commonly used methods include an alkali acid method, a chlorination roasting method, a hydrofluoric acid method, and the like. 1.6.2.1 Alkali acid method The alkali acid method (high temperature melting method) is the main method of chemical purification, and it is also a relatively mature process. There are many mineral impurities in microcrystalline graphite, and the purpose of chemical treatment should be to remove oxides of these impurities, such as SiO2, Al2O3, Fe2O3, MgO, CaO, and the like. When calcined at a high temperature of about 600 °C, the impurity minerals in the microcrystalline graphite can be decomposed into oxides, and the amount of caustic soda and process parameters can be controlled to remove most of the silicon. For other oxides, hydrochloric acid can be used to form It is removed by soluble salts. From the mineralogical standpoint, graphite quartz and kaolinite removed more easily, soluble in alkali quartz, kaolinite converted to hydrated aluminum silicate, such materials are insoluble in water and soluble with hydrochloric acid characteristics It is beneficial to the deashing of graphite. The sodium silicate Na2O · mSiO2 formed by the reaction of quartz and alkali can form a low modulus water-soluble sodium silicate as long as a certain temperature is controlled, while other salts are soluble in hydrochloric acid, and the reactants can be washed with water. The purpose of the purification. The alkali fusion reaction is as follows: 2NaOH + mSiO2 → Na2O · mSiO2 + H2O (gas) 2 NaOH + mSiO2 + nAl2O3 → Na2O · mSiO2 · n Al2O3 + H2O (gas) 2 NaOH + mSiO2 + n Fe2O3 → Na2O · mSiO2 · n Fe2O3 + H2O (gas 2NaOH+mSiO2+ nFe2O3+nAl2O3→ Na2O·mSiO2·n Fe2O3·n Al2O3+ H2O (gas) Sodium ferric silicate (Na2O·mSiO2·n Fe2O3) and sodium aluminosilicate (Na2O · mSiO2 · n Al2O3) formed during this process And the solid solution of sodium iron aluminosilicate (Na2O · mSiO2 · n Fe2O3 · n Al2O3) has a small solubility in water, but can be easily dissolved in an acid to form a soluble salt and removed. In the acid leaching process, the sodium silicate should not be strictly controlled to form silicic acid, because the Na 2 SiO 3 in the acid solution, the generated H 2 SiO 3 (metasilicate) is gradually placed or changed conditions (such as adding acid or adding electrolyte) Condensation forms a colloidal solution of polysilicic acid (ie, a silicic acid sol) or a silicic acid gel which has a large water content and is transparent and elastic, and is difficult to eradicate. The acid leaching reaction is as follows: Na2O·mSiO2+2(m+1)HCl→mH2SiO3+2NaCl Na2O ·mSiO2·k Fe2O3·n Al2O3+[6(k+n)+2m] HCl→ 1 2 Next I am two Sentence mH2SiO3 +2nAlCl3+2kFeCl3+2NaCl+3(k+n)H2O After acid leaching, the reactants can be washed with water to achieve the purpose of purification. The main factors affecting the purification effect are the ingredients, calcination temperature and time, and water washing strength. In general, impurities in graphite are a series of complex compounds containing aluminum, silicon, iron, calcium, magnesium , and sulfur. Only a small amount of these impurities are water-soluble, and most of them need to pass roasting, acid leaching, and the like. It reacts with various reagents to form soluble substances, so that solid phase hard-to-separate which is bonded, twisted and interspersed with graphite is converted into a water-soluble phase, and then separated from graphite by washing, and finally To achieve the purpose of producing high purity graphite. 1.6.2.2 Chlorination roasting method Chlorination roasting method is to mix fine graphite powder with a certain amount of reducing agent, calcine at high temperature and specific atmosphere, and then pass chlorine gas to carry out chemical reaction to form chloride or complex of gas phase or condensed phase. The material escapes, thereby achieving the purpose of purifying graphite. SiO2+2Cl2→SiCl 4+CO2 2Fe2O3+6Cl2→4FeCl3+3CO2 2Al2O3+6Cl2+3C→4AlCl3+3CO2 The impurities in graphite can be decomposed into oxides at high temperatures, such as SiO2, Fe2O3, Al2O3, MgO, these oxides The melting point is higher (see Table 1-1). Table 1-1 Melting point of main oxides in graphite

The chlorination reaction occurs after chlorine gas is introduced under a certain high temperature and atmosphere to convert the oxide into a chloride having a lower melting point. It can be seen from Table 1-1 that MgCl2 and CaCl2 have higher melting points, but can form metal complexes with boiling points below 1000 °C with other trivalent metal chlorides at high temperatures, for example, CaFCl, KMgCl3. These metal complexes are formed. It starts to be discharged in a gaseous state, but it quickly becomes a condensed phase due to a decrease in temperature, and this characteristic can be utilized for exhaust gas treatment. Thus, at less high temperatures, these chlorides will vaporize and the graphite will be purified when impurities are excluded from the graphite system. The chlorination roasting method has the advantages of energy saving, high purification efficiency (≥98%), high recovery rate, etc., but the shortcomings of exhaust gas, serious pollution, serious corrosion of equipment and high cost of chlorine gas limit the popularization and application of the method. 1.6.2.3 Hydrofluoric acid method Any silicate can be dissolved by hydrofluoric acid. This property makes hydrofluoric acid a special agent for the treatment of insoluble minerals in graphite. Since 1979, gaseous hydrogen fluoride has been developed at home and abroad. The acid process of liquid hydrofluoric acid system and the purification process of ammonium fluoride salt system. This series of methods is collectively referred to as the hydrofluoric acid method. The liquid hydrofluoric acid method utilizes impurities in graphite and HF to form a compound dissolved in water and volatiles, and then rinsed with water to remove impurities. During purification, the sample and a certain proportion of hydrofluoric acid are added to the reactor with a stirrer after preheating. After sufficient wetting, the time is stirred. The temperature of the reactor is controlled by a thermostat, and is removed in time after the specified time. Excess acid, the filtrate is recycled, and the filter cake is washed with hot water until neutral, and then dehydrated and dried to obtain the product. The leaching agent used is hydrofluoric acid, mixed acid and waste acid. Hydrofluoric acid reacts almost all of the impurity minerals in the graphite, and the removal rate of ash exceeds 70%. With the strengthening of the reaction conditions, the deashing effect is improved, the deashing rate reaches 78%, and after the reaction reaches a certain level, the reaction strength is increased, and the deashing rate is not significantly improved. The main reason is that HF ​​forms part of the precipitate such as CaF2 during the reaction. , MgF2, etc., the coverage of the sediment prevents further progress of the reaction. In order to solve the above problem, a small amount of an acid which can dissolve the precipitate of the above fluoride is added to the hydrofluoric acid to form a mixed acid such as dilute hydrochloric acid, nitric acid or sulfuric acid. A compound which can remove an impurity element such as Ca, Mg or Fe in a further step. The above process is represented by a chemical equation: Na2O+2HF→2NaF+H2O K2O+2HF→2KF+ H2O SiO2+4HF→H2SiF6 CaO+2HF→CaF2↓+ H2O Al2O3+6HF→2AlF3+3H2O MgO+2HF→MgF2↓+ H2O Fe2O3+ 6HF→2FeF3↓+3H2O When a mixed acid is present, the following reaction is simultaneously carried out: CaF2 + H2SiF6→CaSi6+2HF MgF2+ H2SiF6→MgSiF6 +2HF 2FeF3 + 3H2SiF6→Fe(SiF6)3 +6HF When hydrochloric acid or dilute nitric acid is present The solubility of poorly soluble fluorides is greatly increased. Hydrofluoric acid and mixed acid can dissolve almost all minerals at normal temperature and pressure, and it is an ideal chemical deliming agent for removing minerals. 1.6.3 High-temperature method An important property of graphite graphite is its high melting point and boiling point. Graphite is one of the most melting substances in nature (sublimation point: 3850±50°C), while the boiling point of silicate minerals is 2750. °C (quartz boiling point) or less, it is theoretically considered that as long as the graphite is heated to 2700 ° C or higher, the low boiling point of the impurities can be utilized, so that they are first vaporized and removed, and after a certain period of time, all the impurities can be removed. This is the theoretical basis for the purification of graphite by high temperature method. The high carbon graphite powder is directly loaded into the furnace for graphitization and purification, or the graphite is charged into the crucible for graphitization and purification. The ash in the natural graphite at high temperature can be vaporized and escaped, and the graphite has high temperature resistance, and the graphite crucible has good electrical and thermal conduction. High-temperature resistance (a method of purification due to the ash of the graphite crucible passing through the high-temperature gasification at 2700 ° C or higher), so high-temperature graphitization purification has become an effective purification method adopted by carbon manufacturers in the future. 1.6.4 Advantages and Disadvantages of Graphite Purification Method Although flotation method is a commonly used method, it is the one with the lowest energy consumption and reagent consumption and the lowest cost in the conventional purification scheme of minerals. This is the largest purification of graphite by beneficiation method. advantage. However, the use of beneficiation method to purify cryptocrystalline ink can only improve the grade of graphite. For flaky graphite, the concentrate after flotation is usually 80-90%, and the highest is about 95%. The commodity position is very difficult. The original silicate minerals and compounds of elements such as K, Na, ca, Mg, Al, etc. are impregnated in graphite scales in a very fine state, and the multi-stage grinding can not dissociate the monomers, and is not conducive to the protection of graphite scales. . Therefore, the use of flotation to further improve the graphite grade is very limited. To obtain high carbon graphite with a carbon content of about 99%, the graphite must be purified chemically. The advantages of the chemical purification method are: chemical purification can make the cryptocrystalline graphite have a carbon content of more than 99%, and has the characteristics of less one-time investment, high product quality and strong adaptability. The acid-base method is the most widely used method in China today. Besides the inherent characteristics of the above-mentioned chemical purification method, it has the advantages of easy realization and versatility. In addition to graphite, many non-metallic minerals can be purified. The advantages of using the alkali acid method are that the production cost is high, the amount of graphite loss is large, and the wastewater is seriously polluted. The biggest advantage of the high temperature method: the product contains extremely high carbon content, which can reach more than 99.995%. The disadvantage is that it must be specially designed and built to high temperature furnace, which is expensive and has huge investment. In addition, the high electricity bill also makes the application of this method extremely limited. Only in the case of national defense, aerospace and other special requirements for the purity of graphite products, this method is considered to be used for small batch production of graphite. The advantage of the chlorination roasting method is that the low calcination temperature and chlorine consumption make the production cost of graphite greatly reduced, and the carbon content of the graphite product is comparable to that of the hydrofluoric acid method, compared with the chlorination roasting method. The recovery rate is higher, followed by the three waste treatment of the chlorination roasting method. The main advantage of the hydrofluoric acid method is high impurity removal efficiency, high product quality, low impact on the performance of graphite products, and low energy consumption. The disadvantage is that hydrofluoric acid is highly toxic and highly corrosive. Strict safety protection measures must be taken during the production process. The requirements for equipment also lead to an increase in cost. In addition, the wastewater produced by the hydrofluoric acid method is highly toxic and corrosive. It needs to be strictly treated before it can be discharged. The investment in environmental protection has greatly reduced the cost of the hydrofluoric acid method. Considering the advantages and disadvantages of various purification methods, the alkaline acid method in the chemical purification method is easier to operate in the laboratory. Therefore, the alkaline acid method is selected herein.

Chapter II Research Ideas on Graphite Purification

2.1 Experimental materials

2.1.1 SEM analysis of graphite ore The experimental raw materials were used in the Datian County graphite ore mine in Fujian. Figure 2-1 is a scanning electron micrograph of the graphite ore. It can be seen from the figure that the graphite crystal is complete, the plate crystal form is obvious, the graphite particle size is relatively uniform, and the particle size is about 2 to 5 μm, which belongs to fine flaky graphite.

Figure 2-1 Scanning electron micrograph of graphite ore 2.1.2 Analysis of chemical composition of graphite ore The chemical composition of graphite ore is shown in Table 2-1. Table 2-1 Chemical composition analysis of graphite ore /%

It can be known from the table that the ore impurities are mainly Si, Al, Fe, and also contain K, Na, Mg, and the like. 2.1.3 Phase analysis The phase analysis is performed by X-ray diffraction analysis, as shown in Figure 2-2. The main impurities contained in the graphite sample are quartz and clay , and thus the phase of impurities such as silicon oxide and aluminum oxide exists in the form of free quartz and partially complex silicate. The quartz peak in the X-ray diffraction pattern is not obvious, but the chemical analysis of the quartz content is relatively high, indicating that the free quartz crystal is poor, the chemical activity is high, and it is easy to chemically react with sodium hydroxide during chemical treatment. The reagent acts to form the corresponding silicate and is removed. Other complex silicates can be converted into soluble silicates and separated from graphite by calcination with alkali. Therefore, the alkali acid method can effectively remove various impurities in the graphite. Improve the purity of graphite.

Figure 2-2 X-ray diffraction analysis of graphite ore 2.2 Apparatus and reagents The instruments used in this experiment are shown in Table 2-2. Table 2-2 Experimental instruments

The reagents used in this experiment are shown in Table 2-3. Table 2-3 Experimental reagents

2.3 Experimental procedure and steps 2.3.1 Experimental procedure

2.3.2 Experimental procedure 1) Alkali melting process: Mix a certain amount of NaOH solution and graphite in proportion, put it into the muffle furnace and react according to the predetermined temperature and time. The cooled product after the reaction is washed in the centrifuge cup to the pH value. 7. 2) Acid leaching process: the alkali-fused graphite is added to a certain amount of hydrochloric acid to be immersed to remove unreacted impurities and precipitates, and then washed by centrifugation until the pH is neutral, and dried in a drying oven at 105-110 ° C. Dry, and finally get the finished product.

2.4 Detection method Volatile determination method: according to GB/T 3521295; ash determination method: according to GB/T 3521295.

Chapter III Experimental Results and Analysis

3.1 The effect of NaOH solution concentration on graphite purity The mass ratio of NaOH to graphite was 20%, the alkali melting temperature was 500 °C, and the time was 90 min. The concentration of NaOH solution was changed to carry out the experiment. The effects of different NaOH solution concentrations on the purification performance are shown in Table 3-1 and Figure 3-1. Table 3-1 Effect of NaOH Solution Concentration on Purification

Figure 3-1 Effect of NaOH solution concentration on purification effect It can be seen from the experimental results that when the concentration of NaOH solution is less than 35%, the purity of graphite increases with the concentration of NaOH solution, but when it exceeds 35%, the purity of graphite decreases. . This is because the concentration of the NaOH solution is too low, which may result in insufficient reaction, which does not achieve the purification effect. However, when the concentration is too high, the NaOH solution is difficult to mix uniformly with the graphite ore, and the purification effect is rather reduced, and the excessive concentration will inevitably cause NaOH. Waste and increase production costs. After considering these two factors comprehensively, it is advisable to determine the concentration of NaOH solution to be 35%. 3.2 Effect of NaOH dosage on graphite purity The concentration of NaOH solution is 35%, the alkali melting temperature is 500 °C, and the time is 90 min. The experiment is carried out by changing the amount of NaOH. The effects of different NaOH dosages on the purification performance are shown in Table 3-2 and Figure 3-2. Table 3-2 Effect of NaOH dosage on purification

Figure 3-2 The relationship between the amount of NaOH and the purification effect is shown in Figure 3-2. The graphite carbon content increases with the increase of NaOH dosage. The best effect is obtained when the mass ratio of NaOH to graphite is 32.16%. Since the impurities are highly dispersed in the graphite and a part of the impurities are encapsulated in the graphite particles, the presence of excess NaOH will facilitate the reaction to proceed while increasing the degree of completion of the reaction. Even if the product does not have water solubility, the acid leaching process can make up for this deficiency due to the different degrees of acid solubility of these substances, and does not affect the purification effect of graphite. As shown in the figure, if the amount of alkali is too small, the amount of the reaction reagent will be insufficient, and the meaning of roasting will eventually be lost. If the amount of addition is too large, even if no side reaction affects the purification effect, the production cost will be increased because of the high price of NaOH. Therefore, the amount of NaOH is preferably 32.16%.

3.3 The effect of alkali melting temperature on the purity of graphite Take the concentration of NaOH solution 35%, the amount of NaOH 32.16%, the alkali melting time is 90 (min), change the alkali melting temperature. The effects of different temperatures on the purification performance are shown in Table 3-3 and Figure 3-3. Table 3-3 Effect of alkali melting temperature on purification

Figure 3-3 Effect of alkali melting temperature on purification effect The calcination temperature directly affects the chemical reaction process of sodium hydroxide and impurities. The temperature does not meet the requirements, the chemical reaction is difficult to carry out or the reaction is incomplete, and the purification effect is not obtained. Too high a temperature not only wastes fuel and reduces equipment life, but also causes partial graphite oxidation, which reduces the recovery rate. The melting point of NaOH is 328 ° C, so the melting temperature is generally not lower than 328 ° C, and the experiment is carried out in the range of 450 to 650 ° C. It can be seen from the results that in this temperature range, the higher the temperature, the lower the ash and volatile content of the final product, and the optimum alkali melting temperature is 550 °C. 3.4 Effect of alkali melting time on graphite purity The concentration of NaOH solution is 35%, the mass ratio of NaOH to graphite is 32.16%, and the alkali melting temperature is 550 °C, which changes the alkali melting time. The effects of different times on the purification effect are shown in Table 3-4 and Figure 3-4. Table 3-4 Effect of alkali melting time on purification

Figure 3-4 Effect of alkali melting time on purification effect It can be seen from the experimental results that as the time increases, the purity of the product increases continuously, and the fixed carbon content reaches a maximum at 90 min, and then the time is extended, and the purity of graphite is lowered. The trend, taking into account energy consumption, efficiency and other factors, determine 90min as the alkali fusion time of subsequent experiments. Effect of concentration 3.5 HCl purity graphite 3.5.1 selection hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid are the most widely used four acids. Phosphoric acid is easy to combine with various metal ions to form insoluble phosphate and hydrogen phosphate. It is not suitable for leaching various mineral impurities in graphite as acid leaching reagent; nitric acid is a strong oxidizing volatile acid with poor stability. See light is easy to decompose, strong corrosive to equipment, decomposition products (including reduction and oxidation products) are highly corrosive, toxic, proliferative phosgene, and explosive at high temperatures. These properties make nitric acid the same as phosphoric acid. Experimental leaching agent. The properties of sulfuric acid (diluted) and hydrochloric acid are more suitable as leaching agents. For metal ions (especially transition metal ions), chloride ions (CIˉ) have the ability to complex with them (generally, these complex ions or The compound has a large solubility) and is widely used as a leaching of various transition metal ions. The transition metal ions involved in the graphite sample of this experiment have iron ions, and under normal conditions, they do not form complex ions with chloride ions. Therefore, the leaching ability of hydrochloric acid and sulfuric acid for iron impurities is similar, but the solubility of sulfate is better than that of chloride. Slightly smaller. Therefore, HCl was selected as the leaching agent in this experiment. 3.5.2 Experimental content Alkali fusion and acid leaching process: the concentration of NaOH solution is 35%, the mass ratio of NaOH to graphite is 32.16%, the alkali melting temperature is 550 °C, and the alkali melting time is 90 min. The amount of HCl was 10 ml, the acid leaching time was 2 h, and the temperature was 70 °C. The effect of changing the HCl concentration on the purification effect is shown in Table 3-5 and Figure 3-5. Table 3-5 Effect of HCl concentration on purification

Figure 3-5 The effect of HCl concentration on the purification effect is shown in Figure 3-5. When the concentration of hydrochloric acid is below 3%, the fixed carbon content increases with the increase of concentration, but when it is above 3%, with the concentration of hydrochloric acid The increase in fixed carbon content has declined. This is because: the concentration of hydrochloric acid directly affects the concentration of hydrogen ions in the acid leaching process, thereby significantly changing the rate of the reaction, but the concentration of the acid solution should be as low as possible under the condition that the reaction with impurities is complete, because Na2SiO3 is in the acid solution. The silicic acid is formed in the middle, and the silicic acid is deposited immediately after it is formed, but many silicic acid molecules are aggregated to form a multi-molecular group, and then deposited slowly. If the concentration of hydrochloric acid is too high, the rate of formation of metasilicate and the reaction tendency increase. At this time, the silicic acid is more likely to precipitate out, and the solubility of impurities such as sodium silicate is relatively reduced, so that after purification The ash content of the graphite product is correspondingly increased (see Table 3-5), and the violent volatilization of hydrochloric acid caused by the increase of the acid concentration will also lead to environmental pollution and the actual amount of deficiency, so the optimal acid concentration is 3%. . 3.6 Effect of HCl dosage on graphite purity Alkali fusion and acid leaching process: the concentration of NaOH solution is 35%, the mass ratio of NaOH to graphite is 32.16%, the alkali melting temperature is 550 °C, and the alkali melting time is 90 min. The acid leaching time was 2 h, the temperature was 70 ° C, and the HCl concentration was 3% of the best result of the last experiment. The effect of changing the amount of HCl on the purification effect is shown in Table 3-6 and Figure 3-6. Table 3-6 Effect of HCl dosage on purification

Figure 3-6 Effect of the amount of HCl on the purification effect The experimental results show that the purity of graphite is optimal when the amount of HCl is 10 ml. At this time, if the amount of hydrochloric acid is increased, the purity of graphite decreases. The pickling is mainly to dissolve some impurities such as iron silicate and neutralize the alkali remaining after washing with water. The amount of acid should not be too large, otherwise the subsequent processing of the product is complicated. Therefore, the amount of HCl used here is 10 ml.

Chapter IV Conclusion

In this paper, the process research on the purification of fine flake graphite is done. According to the analysis of the properties of fine scale flake graphite ore with fixed carbon content of 88.00% in Datian County, Fujian Province: the crystal of graphite is complete, the crystal form of flake is obvious, the particle size is relatively uniform, about 2~5μm, belonging to fine flaky graphite; The composition analysis shows that the ore impurities are mainly oxides of silicon, aluminum and iron; X-ray diffraction analysis shows that the impurity phase is mainly quartz and clay, so the phase of impurities such as silicon oxide and aluminum oxide is free quartz and partially complex silicon. The acid salt form exists in which the free quartz crystal is inferior and the chemical activity is high.在分析了各种关于石墨提纯的方法后，结合石墨原矿的特点认为碱酸法更易于实现本实验的要求：原矿中结晶较差的游离石英在进行化学处理时易与氢氧化钠等化学试剂作用生成相应的硅酸盐而被除去，其他复杂硅酸盐经加碱焙烧也可转化为可溶性硅酸盐而与石墨分离，因此采用碱酸法可以有效脱除石墨中的各种杂质，提高石墨纯度。为确定碱酸法制备高纯石墨的最佳工艺条件，对碱熔过程进行了NaOH溶液的浓度、NaOH用量、碱熔温度、碱熔时间的单因素实验，酸浸过程进行了HCl浓度、HCl用量的单因素实验。通过对石墨产品纯度的检测，确定碱酸法制备高纯石墨的最佳工艺条件为：碱熔温度550℃，时间90min，NaOH用量为石墨质量的32.16%，NaOH浓度35 %;酸浸过程中HCl用量10ml，HCl浓度为3%。采用该工艺条件进行实验，实验最终石墨产品固定碳含量可达到98.15 %。本实验对石墨除杂的后续工作有重要的指导意义，对于一些其他矿产点、或者是其他地区产的隐晶石墨或者鳞片状石墨也会有重要的指导意义。

gas assisted mould & moulding

vacuum assisted injection molding,gas assisted mould & moulding planes,gas assisted mould & moulding kit,gas assisted injection molded products

Global Injection Molds Co., LTD , https://www.globalinjectionmolds.com