Graphite is a high-energy crystalline carbon material, because of its unique structure and conductivity, thermal conductivity, lubricity, high temperature, chemical stability and other characteristics, it has a high value in the high performance material, are widely used in metallurgy, mechanical In the fields of environmental protection, chemical industry, fire resistance, electronics, medicine, military industry and aerospace, it has become an indispensable non-metallic material for modern industry and the development of high, new and cutting-edge technologies. It is increasingly important in the development of the national economy. Industry experts predict: "The 20th century is the century of silicon, and the 21st century will be the century of carbon." China's natural graphite has good geological conditions, wide distribution, abundant resources and good quality. Its reserves and output rank first in the world and it is one of the dominant minerals in China. Natural graphite can be classified into two types: crystalline graphite (scale) and cryptocrystalline graphite (earth) depending on the degree of crystallization. The characteristics of crystalline graphite ore are that the grade is not high, the fixed carbon content is generally not more than 10%, and the local special enrichment section can reach 20% or more, but the graphite ore has good selectivity and the flotation concentrate grade can reach More than 85% is one of the best floatable ore in nature. The grade of cryptocrystalline graphite is higher, the fixed carbon content is generally 60% to 80%, and the highest is up to 95%, but the ore is less selectable. With the continuous development of technology, ordinary high-carbon graphite products can no longer meet the requirements of various industries, so it is necessary to further improve the purity of graphite. However, China's graphite processing technology is relatively low, and its products are mainly based on raw materials and primary products. The high impurity content of the products limits its application range. In this way, on the one hand, domestic graphite products are cheap in the international market, resulting in a large outflow of graphite resources; on the other hand, high-purity ultra-fine graphite products required by the domestic market are more dependent on imports. Therefore, it is of practical significance to study the preparation process of high-purity graphite. To study the method of purifying graphite, it is necessary to first find out the composition of impurities present in the graphite ore. Although the natural graphite in various places contains different impurity components, the approximate composition is similar. These impurities are mainly silicate minerals such as potassium, sodium, magnesium , calcium, aluminum, etc. The purification process of graphite is to take effective measures to remove this part of impurities. At present, the methods for purifying graphite at home and abroad mainly include flotation method, alkali acid method, hydrofluoric acid method, chlorination roasting method, high temperature method and the like. Among them, the alkali acid method, the hydrofluoric acid method and the chlorination roasting method belong to the chemical purification method, and the high temperature purification method belongs to the physical purification method.
First, the main method of graphite purification
(1) Flotation method Flotation method is a commonly used method for purifying minerals. Since the graphite surface is not easily wetted by water, it has good floatability and is easy to separate from impurity minerals. The graphite is subjected to beneficiation by a flotation method. The flotation of graphite ore is generally carried out by positive flotation followed by reverse flotation of the flotation concentrate. A higher grade graphite concentrate can be obtained by flotation. Flotation graphite concentrate grades can usually reach 80% to 90%, using multi-stage grinding, the purity can reach 98%. Conventional crystalline graphite flotation collector as kerosene, diesel oil, heavy oil, sulfonate, sulfate, phenols, and carboxylic acid ester, a foaming agent commonly used for the oil # 2, # 4 oil, pine oil, an ether Alcohol and butyl ether oil, etc., the adjusting agent is lime and sodium carbonate, and the inhibitors are water glass and lime. The common collector for flotation of cryptocrystalline graphite is coal tar. Commonly used foaming agents are eucalyptus oil and pine oil. The commonly used regulator is sodium carbonate. The commonly used inhibitors are water glass and sodium fluorosilicate. The graphite concentrate purified by flotation can only reach a certain range, because some impurities are impregnated into the graphite scales in a very fine granular form, and even if it is finely ground, the monomer cannot be completely dissociated, so it is difficult to completely remove it by physical beneficiation method. This part of impurities is generally only used as the first step in the purification of graphite. The method of further purifying graphite is usually a chemical method or a high temperature method.
(2) Alkali acid method The alkali acid method is the main method for chemical purification of graphite, and it is also a relatively mature process. The method includes a system such as NaOH-HCl, NaOH-H2SO4, NaOH-HCl-HNO3. Among them, the NaOH-HCl method is the most common. The principle of purifying graphite by alkali acid method is to uniformly homogenize NaOH and graphite according to a certain ratio, and impurities such as silicate, aluminosilicate, quartz and the like in the graphite at a high temperature of 500-700 ° C and sodium hydroxide A chemical reaction occurs to form soluble sodium silicate or acid-soluble sodium aluminosilicate, which is then removed by washing with water to achieve the purpose of desiliconization; another part of impurities such as metal oxides remain in the graphite after alkali fusion. The product after desiliconization is leached with an acid to convert the metal oxide therein into a soluble metal compound, and impurities such as carbonate in the graphite and the acid-soluble compound formed during the alkali leaching react with the acid to enter the liquid. The phase is separated from the graphite by filtration and washing. Graphite is chemically inert and has good stability. It is insoluble in organic solvents and inorganic solvents, and does not react with lye; it has no reaction with many acids except for strong oxidizing acids such as nitric acid and concentrated sulfuric acid, especially It is resistant to hydrofluoric acid; it does not react with water and steam at temperatures below 6000 °C. Therefore, the properties of graphite remain unchanged during the purification process.
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