Graphite purification method
2018-11-03 Source： Click：1552
Graphite is a high energy crystalline carbon material. Graphite has a unique structure and electrical conductivity, thermal conductivity, lubricity, high temperature resistance and chemical stability, and has high application value in high performance materials. Widely used in metallurgy, machinery, environmental protection, chemical, refractory materials, electronics, medicine, military, aerospace and other fields. As an essential non-metallic material for the development of modern industry and high-tech, it plays an increasingly important role in the development of the national economy.
China's natural graphite has superior geological conditions, wide distribution, abundant resources and good quality. Its reserves and output rank first in the world. It is one of the main minerals in China. Natural graphite can be classified into crystalline graphite (graphite) and invisible graphite (earth) according to its crystallinity. Crystalline graphite ore is one of the best flotation ore in the natural world where the carbon content is generally less than 10%, the local special richness is up to 20%, and the washable and flotation concentrate grade is up to 85%. The content of invisible graphite is high, the fixed carbon content is generally 60%-80%, and the highest is up to 95%, but the ore washability is poor.
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. Because graphite surface is not easily wetted by water, it has good floatability and is easy to separate from impurity minerals. In China, flotation method is basically used for graphite. Performing ore dressing.
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%.
The commonly used collectors for flotation crystal graphite are kerosene, diesel oil, heavy oil, sulfonate, sulfate, phenol and carboxylate. The common foaming agents are 2# oil, 4# oil, pine oil, 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.
The process of purifying graphite by alkaline acid method can be divided into two processes of alkali fusion and acid hydrolysis. The main chemical reactions of the alkali fusion process are as follows:
At a suitable temperature, Na2O·mSiO2 can form a low m value water-soluble sodium silicate, and the reactants can be washed with water for purification.
The alkali substance reacts with hydrochloric acid and reacts as follows:
The alkali acid method can obtain a graphite product having a fixed carbon content of 99% or more. This method is widely used in industry, and has been transferred from the manual operation of the soil method to the relatively advanced process of continuous washing using a melting furnace and a V-groove. The melting process can be carried out in a rotating tube furnace or in a cast iron pan under manual agitation, but with poor safety. The melting temperature is 500 to 800 ° C, and the reaction is about 1 h. The amount of alkali depends on the nature of the ore, and is generally 400 to 450 kg/t. The acid amount is 450 to 500 kg/t, and pickling is carried out at normal temperature.
A disadvantage of the alkaline acid process is that high temperature calcination is required and energy consumption is large. The reaction time is long and the equipment is seriously corroded. In addition, from the current literature, the purity of high-purity graphite is less than 99.9%.
(3) Hydrofluoric acid method
Any silicate can be dissolved by hydrofluoric acid, a property that makes hydrofluoric acid a special agent for the treatment of poorly soluble minerals in graphite. Since 1979, the purification methods of gaseous hydrogen fluoride, liquid hydrofluoric acid system and ammonium fluoride salt system have been developed at home and abroad. Among them, liquid hydrofluoric acid method is the most widely used, which uses impurities in graphite and hydrofluoric acid to form It is soluble in fluoride and volatiles to achieve purification. The main chemical reactions are as follows:
However, hydrofluoric acid reacts with CaO, MgO, Fe2O3 and the like to precipitate. The response is as follows:
In order to solve the above precipitation problem, a small amount of fluorosilicic acid, dilute hydrochloric acid, nitric acid or sulfuric acid may be added to the hydrofluoric acid to remove interference of impurity elements such as Ca, Mg, and Fe. When fluorosilicic acid is present, its reaction is as follows:
When purifying by hydrofluoric acid method, the graphite and a certain proportion of hydrofluoric acid are added together to the reactor with the agitator after preheating. After sufficient wetting, the time is stirred, and the temperature of the reactor is controlled by the thermostat. After the specified time is reached. The excess acid solution is removed in time, 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 hydrofluoric acid method is a relatively good purification program. It has been industrialized in the 1990s, and countries such as Europe and the United States are more common than in China. Because the method is highly corrosive to equipment and highly toxic, more than a decade ago, people used two steps of dilute acid and fluoride to remove impurities in graphite. Japanese, French and other national patents have described the use of ammonium hydrogen fluoride or ammonium fluoride to react with graphite powder containing 93% carbon, which can increase the fixed carbon content of graphite to 99.95%. In view of the enormous toxicity of hydrofluoric acid, the production process must have strict safety protection and wastewater treatment systems.
(4) Chlorination roasting method
The chlorination roasting method is to add a certain amount of reducing agent to the graphite powder, calcine it at a certain temperature and a specific atmosphere, and then pass chlorine gas to carry out a chemical reaction, so that the valuable metal in the material is converted into a gas phase or a condensed phase having a lower melting point. The chloride and the complex escape and separate from the remaining components for the purpose of purifying the graphite.
The impurities in graphite are heated at high temperature and can be decomposed into simple oxides such as SiO2, Al2O3, Fe2O3, CaO, MgO, etc. under the action of reducing agents. These oxides have higher melting points, see Table 1, and their chlorine The melting point of the metal complex formed by the compound or other trivalent metal chlorides (such as CaFeCl4, NaAlCl4, KMgCl3, etc.) is lower, as shown in Table 2. The vaporization of these chlorides escapes, increasing the purity of the graphite.
The metal complex discharged in a gaseous state quickly becomes a condensed phase due to a decrease in temperature, and this characteristic can be used to carry out the treatment of the escaped exhaust gas.
Table 1 Melting boiling point of main oxide impurities
Table 2 Melting boiling point of some chloride impurities
Test procedure: mixing graphite sample and a certain proportion of reducing agent coke into corundum tube, setting porcelain sieve plate and porcelain ball in the lower part of corundum tube to block the falling of graphite column, and sealing both ends of corundum tube without leaking . The corundum tube is placed in a furnace for heating, and firstly, nitrogen gas is introduced to drive out the air in the tube to prevent oxidation of the graphite at a high temperature. When the set temperature is reached, the nitrogen gas is turned off and chlorine gas is introduced. The volatile chloride or complex formed by the chlorination reaction passes through the condensate bottle, is filtered, and is discharged into the atmosphere. After a certain period of time in the chlorination reaction, the chlorine gas is turned off, and then nitrogen gas is introduced to drive out the residual chlorine gas and the chloride gas.
The chlorination roasting method has the advantages of energy saving, high purification efficiency (>98%), and high recovery rate. The toxicity, serious corrosiveness and serious environmental pollution of chlorine gas limit the promotion and application of the chlorination roasting process to a certain extent. Of course, this process is difficult to produce graphite of ultimate purity, and the process system is not stable enough, which also affects the application of the chlorination method in actual production. This method needs further improvement and improvement.
(5) High temperature purification method
Graphite is one of the most boiling substances in nature, with a melting point of 3850±50°C and a boiling point of 4500°C. The boiling point of silicate minerals is below 2750°C (the boiling point of quartz). The boiling point of graphite is much higher than the impurities contained. The boiling point of the silicate. This property is the theoretical basis for the purification of graphite by high temperature methods.
The graphite powder is directly charged into the graphite crucible, and heated to 2300 to 3000 ° C in a purification furnace which is supplied with an inert gas and a Freon protective gas for a certain period of time, and impurities in the graphite may overflow, thereby purifying the graphite. The high-temperature method generally uses high-carbon graphite containing more than 99% of carbon purified by flotation or chemical method as raw material, and the graphite can be purified to 99.99%. If the process conditions are further improved, the quality of the crucible can be improved, and the purity can reach 99.995%. the above.
The high-temperature method can produce more than 99.99% of ultra-high-purity graphite, but the fixed carbon of the raw material is required to be more than 99%, and the equipment is expensive, the investment is huge, the production scale is limited, the electric furnace heating technology is strict, and the air needs to be isolated, otherwise graphite When heated to 450 ° C in hot air, it begins to be oxidized. The higher the temperature, the greater the loss of graphite. Only in special industries (such as national defense, aerospace, etc.) where the quality of graphite is very high, high-purity graphite is produced in small batches by high-temperature method.
Second, the advantages and disadvantages of graphite purification methods
The flotation method is one of the lowest energy consumption and reagent consumption and the lowest cost in the conventional purification method of minerals, which is the biggest advantage of the flotation method for purifying graphite. However, the use of flotation method to purify graphite can only achieve a limited improvement in the grade of graphite. For flaky graphite, the use of multi-stage grinding can not only dissociate its completely monomer, but also is not conducive to the protection of large scales of graphite. Therefore, it is neither economical nor scientific to further improve the graphite grade by flotation. To obtain high carbon graphite with a carbon content of 99% or more, it is necessary to chemically purify the graphite.
(1) Alkali acid purification method. The graphite containing carbon purified by alkaline acid method can reach more than 99%, and has the characteristics of less one-time investment, higher product grade and strong process adaptability. Moreover, it has the advantages of conventional equipment and versatility (except for graphite, many non-metallic minerals can be purified by alkaline acid method). Alkaline acid method is the most widely used method in China today; its disadvantage is energy consumption. Long reaction time, large loss of graphite and serious pollution of wastewater.
(b) Hydrofluoric acid 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 the hydrofluoric acid is highly toxic and highly corrosive. Strict safety protection measures must be taken during the production process. The strict requirements for the equipment also lead to an increase in cost. In addition, the wastewater produced by the hydrofluoric acid method is very toxic and corrosive. Strong, it needs to be strictly treated before it can be discharged, and environmental protection investment also greatly reduces the cost of the hydrofluoric acid method.
(3) Chlorination roasting method. The low calcination temperature and the small chlorine consumption of the chlorination roasting method greatly reduce the production cost of graphite, and the carbon content of the graphite product is comparable to that after the treatment with the hydrofluoric acid method. The recovery rate of the calcination method is high. However, because chlorine is toxic and corrosive, it requires high equipment operation and needs to be tightly sealed. The exhaust gas must be properly disposed of, so it limits its promotion and application to a certain extent.
(4) The biggest advantage of the high temperature method is that the carbon content of the product is extremely high, up to 99.995%. The disadvantage is that the high temperature furnace must be specially designed and constructed, the equipment is expensive, and the one-time investment is large. In addition, the energy consumption is large and the electricity cost is high. Increased production costs. Moreover, the harsh production conditions also make the application range 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, and industrialization cannot be realized.
Comparative analysis shows that several methods of graphite purification have their own advantages, and they all have certain defects. The alkali acid method is easy to operate, the production cost is low, and the requirements for production conditions are also low, but the graphite produced has a low fixed carbon content, which cannot be 99.9% from the current point of view. The hydrofluoric acid method has a good effect of removing impurities, and the fixed carbon content of the product is high, but the hydrofluoric acid is highly toxic and highly corrosive, and the safety protection measures and production conditions are strict, and the wastewater is not easy to handle. Chlorination roasting is also required to be tightly sealed due to the toxic and corrosive nature of chlorine. The high-temperature method can produce very high-grade high-purity graphite, but it cannot be promoted because of its own limitations, and it is only used in a small range.