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Properties of graphite electrodes

Release time:2019-11-04 From: Zhengnai Industry in China

Product preheating temperature and warm-up time

The preheating temperature and preheating time of the product have certain influence on the impregnation quality and the impregnation production cost. The insufficient preheating temperature will affect the penetration of the asphalt into the pores of the roasting product. It is known that the optimal preheating temperature (heating medium temperature in the furnace) is 350 °C ± 30 ° C. The calcined electrode is preheated in a steel basket, and the product in the middle of the basket is not as hot as the product at the edge of the basket. Therefore, the product to be installed in the middle of the basket must reach a temperature required by the process, and a longer warm-up time must be passed.
 

 

Vacuum degree and vacuum exhaust time

The purpose of vacuum exhaust is to discharge the gas in the pores of the product opening to facilitate the penetration of the impregnant. The degree of vacuum and the length of the vacuum exhaust time mean the amount of gas discharged, and will directly affect the impregnation weight gain.
 

Preheating temperature of the impregnating agent and heating temperature of the impregnation tank

The preheating temperature and the heating temperature should ensure that the fluidity of the impregnating agent is in a good state, the viscosity is relatively low, and it is easy to penetrate into the pores of the product, but the temperature is too high and not good. Generally, the medium temperature asphalt having a softening point of 60~75 ° C is heated. When it reaches about 180 °C, the viscosity change is already small, and the light components have begun to decompose and volatilize. The preheating temperature of the impregnating agent is maintained at 160~180 ° C, and the immersion temperature is maintained at the same temperature range.
 

Pressure and pressurization time during impregnation

The impregnating agent is immersed in the pores of the product mainly by capillary action and osmosis. The capillary action is affected by the pore pore size and the interaction force between the solid and liquid interfaces, while the osmosis is affected by the permeability coefficient and the viscosity of the impregnant, and with the pressure and pressure. Pressure time related.


 

Impregnation cycle

The impregnation operation is a periodic operation, and a complete operation process including preheating of the roasting product, canning, vacuuming, pressurizing, cooling, and discharging the product is called an impregnation cycle. The production of high-density carbon and graphite products requires several impregnation operations to achieve the desired density, that is, several impregnation cycles.
 
The impregnation operation is a periodic operation, and a complete operation process including preheating of the roasting product, canning, vacuuming, pressurizing, cooling, and discharging the product is called an impregnation cycle. 

Index of HP Graphite elelctrode

Item Unit HP
300-800mm
Electric resistivity Electrode μΩm 5.8-6.6
Nipple 4.2-5.0
Flexural strength Electrode Mpa 10.0-13.0
Nipple 18.0-22.0
Elastic modulus Electrode Gpa 8.0-12.0
Nipple 14.0-16.0
Bulk density Electrode ≥g/cm3 1.62-1.66
Nipple 1.74-1.80
CTE(100-600℃) Electrode x10-6/℃ 1.6-1.9
Nipple 1.1-1.4
Ash   0.3

Purpose of impregnation

Impregnation is the process of forcing liquid material into the pores of a carbon product at a certain temperature and pressure. The purpose of impregnating carbon products is to increase the density of the product, reduce porosity and permeability, increase the mechanical strength of the product, and improve electrical and thermal conductivity.
 

Impregnation of graphite electrodes

Impregnation process and impregnation work
The impregnation of the carbon product must be carried out under a certain pressure and temperature, and the viscosity of the impregnating agent is lowered by heating, and the surface tension is reduced. A certain pressure is applied to catch the impregnating agent into the pores of the carbon product. The solid material is placed in a liquid impregnant which is immersed in the pores of the solid material. This action is chemically referred to as “impregnation”. The ability to impregnate is expressed as “impregnation work” which reflects the ability of a liquid to replace a gas on a solid surface.


 

Pore formation

The raw material of the graphite product is petroleum coke or pitch coke. During the roasting process, due to the decomposition, polycondensation and carbonization of the coal tar pitch, the carbon residue rate of the formed pitch coke is generally only about 50%. The volume of coal tar pitch forming pitch coke is smaller than the volume occupied by coal tar pitch in the green body. Although the green body shrinks slightly during calcination, many different sized pores are left in the calcined product.
 

Effect of Porosity on Physicochemical Properties of Graphite Products

For the graphite electrode, as the porosity of the finished product increases, the density decreases, the electrical resistivity increases, and the mechanical strength decreases. Graphite products with a large porosity have a faster oxidation rate at a certain temperature, a lower corrosion resistance, and a gas or liquid is more easily penetrated.

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