Artificial graphite

1.Raw materials of artificial graphite include petroleum coke, needle coke and coal tar pitch.Petroleum coke is a combustible solid product obtained from coking of petroleum residue and petroleum asphalt. It is black and porous, and the main element is carbon,ash content is very low, generally below 0.5%. Petroleum coke belongs to the category of easily graphitized carbon. Petroleum coke is used in chemical industry, metallurgy, etc.It is widely used in the industry and is the main raw material for producing artificial graphite products and carbon products for electrolytic aluminum.

Petroleum coke can be divided into green coke and calcined coke according to heat treatment temperature. The former is petroleum coke obtained from delayed coking,it contains a large amount of volatile matter and has low mechanical strength. Calcined coke is obtained by calcining green coke. Most refineries in China only produce coke,the calcination operation is mostly carried out in the carbon plant.

Petroleum coke can be divided into high sulfur coke (more than 1.5% sulfur), medium sulfur coke (0.5% – 1.5% sulfur) and low sulfur coke (sulfur content less than 0.5%). Generally, low sulfur coke is used for the production of artificial graphite products.

Needle coke is a high-quality coke with obvious fibrous texture, low coefficient of thermal expansion and easy graphitization.

When the coke block breaks, it can be divided into long and thin strip particles according to the texture (the ratio of length to width is generally above 1.75).

Anisotropic fibrous structure can be observed below, so it is called needle coke.

The anisotropy of needle coke’s physical and mechanical properties is very obvious, and it has good conductivity and heat conduction parallel to the particle’s long axis.

The properties and thermal expansion coefficient are low. During extrusion, the long axis of most particles is arranged according to the extrusion direction. Therefore, needle coke is the key raw material for making graphite, which has low resistivity, small thermal expansion coefficient and good thermal shock resistance.

 Needle coke is divided into oil based needle coke produced from petroleum residue and coal based needle coke produced from refined coal tar pitch

Coal tar pitch is one of the main products of coal tar deep processing. It is a mixture of various hydrocarbons, black at room temperature.

Semi solid or solid with high viscosity, without fixed melting point, softens after heating, and then melts, with a density of 1.25  1.35g/cm3.

According to its softening point, it can be divided into low temperature asphalt, medium temperature asphalt and high temperature asphalt. The yield of medium temperature pitch is 54 56% of coal tar.

The composition of asphalt is extremely complex, which is related to the nature of coal tar and the content of heteroatoms, and is also affected by the coking process system and the addition of coal tar.

 Influence of working conditions. There are many indicators to characterize the characteristics of coal tar pitch, such as asphalt softening point, toluene insoluble (TI), quinoline insoluble material (QI), coking value, coal tar pitch rheology, etc.

 Coal tar pitch is used as binder and impregnating agent in the carbon industry, and its performance affects the production process and product quality of carbon products.

 Quantity has a great impact. Binder asphalt is generally used with moderate softening point, high coking value β Medium temperature or medium temperature modified asphalt with high resin,medium temperature asphalt with low softening point, low QI and good rheological property shall be used as impregnant.

 Calcination: the carbonaceous raw materials are heat treated at high temperature to discharge the moisture and volatile matter contained, and the physical and chemical properties of the raw materials are improved accordingly.

The production process of performance is called calcination. Generally, carbon raw materials are calcined with gas and its own volatile as the heat source, with the highest temperature of 1250 – 1350 .

Calcination has made profound changes in the structure and physical and chemical properties of carbonaceous raw materials, mainly reflected in the improvement of the density, mechanical strength and conductivity of coke have improved the chemical stability and oxidation resistance of coke, laying a foundation for subsequent processes.

The calcination equipment mainly includes tank calciner, rotary kiln and electric calciner. Calcination quality control index is the true density of petroleum coke.Not less than 2.07g/cm3, resistivity not more than 550 μ Ω. m. The true density of needle coke is not less than 2.12g/cm3, and the resistivity is not greater than five hundred μ Ω.m.

Crushing and batching of raw materials:

Before batching, the large pieces of calcined petroleum coke and needle coke shall be subject to intermediate crushing, grinding and screening.Medium crushing is usually used to crush materials of about 50mm through jaw crusher, hammer crusher, roller crusher, etc

Prepare 0.5-20mm particle size materials required for further crushing to ingredients.

Grinding is to grind the carbon raw materials to 0.15mm by hanging rod type ring roller mill (Raymond mill), ball mill and other equipment ,or the process of small powder particles with particle size below 0.075mm.

 Screening is to divide the materials with a wide size range after crushing into size ranges through a series of sieves with uniform openings.For the narrow process of several particle size fractions, the current electrode production usually requires 4-5 particle size fractions and 1-2 powder size fractions.

The batching is to calculate, weigh and focus the aggregate, powder and binder of various particle sizes according to the formula requirements

Production process. The scientific suitability of the formula and the stability of the batching operation are the most important factors that affect the product quality and performance

 One of the factors.

 The formula needs to determine five aspects:

 ① Select the type of raw materials;

 ② Determine the proportion of different kinds of raw materials;

 ③ Determine the particle size composition of solid raw materials;

 ④ Determine the amount of binder;

 ⑤ Determine the type and amount of additives.

 Kneading: under a certain temperature, stir and mix a certain amount of carbon particles and powders with a certain amount of binder,

 The process of kneading plastic paste is called kneading.

 Kneading process: dry mixing (20-35 min) wet mixing (40-55 min)

 The function of kneading:

 ① During dry mixing, various raw materials shall be mixed evenly, and solid carbonaceous materials with different particle sizes shall be mixed and filled evenly

 Filling to improve the compactness of the mixture;

 ② After coal tar pitch is added, the dry material and asphalt are mixed evenly, and the liquid asphalt is evenly coated and wetted on the particle surface to form a

 The asphalt tack coat is used to bond all materials together to form a homogeneous plastic paste, which is conducive to molding;

 ③ Some coal tar pitches infiltrate into the internal voids of carbonaceous materials, further improving the density and cohesiveness of the paste.

 Molding: the molding of carbon materials refers to the plasticity of the kneaded carbon paste under the external force exerted by the molding equipment

 Deformation, and finally forming green billets (or products) with certain shape, size, density and strength.

 Operation of extrusion

 ① Cooling materials: disc cooling materials, cylinder cooling materials, kneading cooling materials and other methods to discharge volatile matter and reduce to appropriate temperature (90-120 ) increase the cohesive force to make the paste even, which is conducive to forming for 20-30 minutes

 ② Loading: press to lift the baffle plate —- blanking in 2-3 times —- 4-10MPa compaction

 ③ Pre pressing: pressure 20-25MPa, time 3-5 min, vacuum pumping at the same time

 ④ Extrusion: lower baffle plate of the press — 5-15MPa extrusion — shearing — turn into the cooling water tank

 Technical parameters of extrusion: compression ratio, temperature of press material chamber and nozzle, cooling material temperature, pre pressing pressure time, extrusion pressure

 Force, extrusion speed, cooling water temperature

 Inspection of green billet: bulk density, appearance knocking and analysis

 Roasting: It refers to the process that the green carbon products are put into a specially designed heating furnace for high temperature heat treatment under the protection of filling materials, so that

 The process of carbonization of coal tar pitch in green body. The pitch coke formed after carbonization of coal tar pitch consolidates the carbonaceous aggregate and powder particles

 Together, the calcined carbon products have higher mechanical strength, lower resistivity, better thermal stability and chemical stability.

 Baking is one of the main processes in the production of carbon products, and also an important part of the three heat treatment processes in the production of graphite electrodes

 First, the roasting production cycle is long (22-30 days for the first roasting and 5-20 days for the second roasting), and the energy consumption is high. Green roasting

 The quality of the finished product has a certain impact on the quality and production costs.

 The coal tar pitch in the green billet is coked during the roasting process, and about 10% of the volatile matter is discharged, while 2-3% of the volume is produced

 Shrinkage, mass loss 8-10%. The physical and chemical properties of carbon billet have also changed significantly, due to the increase of porosity

 1.70g/cm3 is reduced to 1.60g/cm3, and the resistivity is 10000 μ Ω. m drops to 40-50 μ Ω. m, the mechanical strength of the baked billet is also

 Greatly improved.

 The secondary roasting is a process in which the pitch immersed in the pores of the calcined product is carbonized after the calcined product is impregnated.

 The production of graphite with high volume density requires secondary baking, and the joint blank also requires tertiary leaching and tertiary baking or secondary leaching and tertiary baking.

 Main types of baking furnace: continuous operation – ring furnace (with cover and without cover), intermittent operation of tunnel kiln – inverted flame kiln

 Car bottom type baking furnace, box type baking furnace

 Roasting curve and maximum temperature: primary roasting — 320, 360, 422, 480 hours, 1250  secondary roasting — 125

 Inspection of calcined products at 700-800  for 240 and 280 hours: appearance knocking, resistivity, bulk density, compressive strength

 Anatomy of the Ministry Structure

 Impregnation is to put carbon materials in a pressure vessel and immerse liquid impregnant asphalt under certain temperature and pressure

 The process of penetrating into the electrode pores of the product. The purpose is to reduce the porosity of products, increase the bulk density and mechanical strength of products,

 Improve the conductivity and thermal conductivity of products.

 The technological process and relevant technical parameters of impregnation are as follows: baked billet – surface cleaning – preheating (260-380 , 6-10 Hour) – loading into the impregnating tank – vacuumizing (8-9KPa, 40-50min) – asphalt injection (180-200 ) – pressurizing

 (1.2-1.5MPa, 3-4 hours) — asphalt return — cooling (inside or outside the tank)

 Inspection of impregnates: impregnation weight gain rate G=(W2-W1)/W1 × Weight gain rate of 100% primary impregnation ≥ 14% secondary impregnation

 Weight gain rate ≥ 9% Weight gain rate of the third impregnation ≥ 5%

 Graphitization refers to heating the carbon products to 2300  or above in the protective medium of the high temperature electric furnace, so as to make the amorphous and disordered layer structure

 High temperature heat treatment process in which carbon is transformed into three-dimensional ordered graphite crystalline structure.

 2 Purpose and function of graphitization

 ① Improve the conductivity and thermal conductivity of carbon materials (the resistivity is reduced by 4-5 times, and the thermal conductivity is increased by about 10 times);

 ② Improve thermal vibration resistance and chemical stability of carbon materials (linear expansion coefficient reduced by 50-80%);

 ③ Make carbon materials have lubricity and anti-wear property;

 ④ Discharge impurities and improve the purity of carbon materials (the ash content of products is reduced from 0.5-0.8% to about 0.3%).

 3 Realization of graphitization process

 The graphitization of carbon materials is carried out at a high temperature of 2300-3000 , so it can only be realized by electric heating in industry

 Now, that is, the current directly passes through the heated calcined product. At this time, the calcined product loaded into the furnace is not only a conductor that generates high temperature through the current,

 It is also an object heated to high temperature.

 At present, the furnace types widely used are Acheson graphitization furnace and LWG furnace. The former has large output

 The temperature difference is large and the power consumption is high. The latter has short heating time, low power consumption, uniform resistivity but is not easy to install joints.

 The control of graphitization process is to determine the electric power curve corresponding to the temperature rise through temperature measurement

 Electric time is 50-80 hours for Acheson furnace and 9-15 hours for LWG furnace.

 The power consumption of graphitization is large, generally 3200-4800KWh, and the process cost accounts for 20-35% of the total production cost

 Inspection of graphitized products: appearance knock and resistivity test

 Machining: the purpose of carbon graphite material machining is to achieve the required size, shape

 Precision, etc., to make graphite products that meet the use requirements.