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Characteristics of Graphite Electrode

Application characteristics of graphite electrode production and processing

Electrode introduction

Graphite electrode mainly uses petroleum coke and needle coke as raw materials and coal pitch as a binding agent. It is made by calcination, batching, kneading, pressing, roasting, graphitization and machining. It releases electric energy in the form of electric arc in electric arc furnace The conductor that heats and melts the furnace charge can be divided into ordinary power graphite electrode, high power graphite electrode and ultra high power graphite electrode according to its quality index.

The main raw material for the production of graphite electrodes is petroleum coke, ordinary power graphite electrodes can be added with a small amount of pitch coke, and the sulfur content of petroleum coke and pitch coke cannot exceed 0.5%. Needle coke is also needed when producing high-power or ultra-high-power graphite electrodes. The main raw material for the production of aluminum anodes is petroleum coke, and the sulfur content should be controlled not more than 1.5% to 2%. Petroleum coke and asphalt coke should meet the relevant national quality standards.

Graphite electrode advantages

(1) The increasing complexity of mold geometry and the diversification of product applications have led to higher and higher requirements for the discharge accuracy of spark machines. The advantage of graphite electrode is that it is easier to process, the discharge rate of discharge is high, and the graphite loss is small. Therefore, some customers of group-based spark machine abandoned the copper electrode and switched to graphite electrode. In addition, some electrodes with special shapes cannot be manufactured with copper, but graphite is easier to form, and the copper electrode is heavier, which is not suitable for processing large electrodes. These factors have caused some group-based spark machine customers to use graphite electrodes.

(2) Graphite electrodes are easier to process, and the processing speed is significantly faster than copper electrodes. For example, the milling process is used to process graphite, which is 2 to 3 times faster than other metal processing and does not require additional manual processing, and the copper electrode requires manual grinding. Similarly, if a high-speed graphite machining center is used to manufacture the electrode, the speed will be faster and the efficiency will be higher, and there will be no dust problem. In these processes, choosing a tool and graphite with appropriate hardness can reduce the wear of the tool and the damage of the copper electrode. If you specifically compare the milling time of graphite and copper electrodes, graphite electrodes are 67% faster than copper electrodes. In the general case of electrical discharge machining, the processing using graphite electrodes is 58% faster than that using copper electrodes. In this way, the processing time is greatly reduced, and the manufacturing cost is also reduced.

(3) The design of the graphite electrode is different from the traditional copper electrode. Many mold factories usually have different reserves for roughing and finishing of copper electrodes, while graphite electrodes use almost the same reserve, which reduces the number of CAD / CAM and machine processing. This is the only reason. It is enough to greatly improve the accuracy of the mold cavity.

Graphite electrode processing

(1) Processing form of graphite material
There are three main forms of graphite electrode processing: pressure vibration method, numerical control automatic forming method and mechanical processing method.

(1) Pressure vibration method: The pressure vibration method requires a special machine tool, and the electrode master mold has the opposite shape to the electrode. Graphite materials and forming tools are relatively placed during processing, leaving a certain gap. The machining fluid, which is a mixture of water and abrasives, is injected into its solution. While passing through the machining fluid, the graphite and the forming tool are ultrasonically vibrated. Under the action of the abrasive impact force, the graphite is peeled off to a desired shape. The abrasive is SiC, B4C or diamond. The coarser the abrasive, the faster the processing speed. When the machine tool is working, it adopts sealed type and water curtain type for processing. It is suitable for EDM for mass production.

(2) Numerical control automatic forming method: The numerical control automatic forming method requires a special graphite electrode forming machine, which adopts a numerical control method and has an automatic tool exchange device, which can accommodate multiple tools. In order to improve the processing environment, the processing graphite electrode is surrounded by a water curtain to prevent the graphite dust from being scattered around. In addition, the wet machining method can also be used to prevent the tool from being worn and the graphite dust to be scattered by the water-soluble cutting fluid. The machine tool has a high-performance filtering device, which can separate the cutting fluid and graphite chips, which can ensure continuous processing for a long time. However, its price is relatively high, and it is not widely used in China.

(3) Mechanical processing method: The mechanical processing method is the main method adopted by domestic general mold production units.
Graphite materials can be processed by turning, milling, drilling and grinding. In addition, graphite materials are prone to fly ash during mechanical processing, which has an adverse effect on processing equipment and operators.

(2) Processing procedures of graphite materials

1) Graphite immersed in oil. Before machining, the graphite material should be immersed in the engine oil for tens of hours to allow the oil to penetrate into the graphite material, so that the dust will not fly during the machining and a smoother surface can be obtained.
2) Paint the line. After rough processing of the electrode surface, spray a thin layer of white paint on the scribing surface and then scribing, so that the lines are clear and easy to machine.
3) Perform mechanical processing.

Graphite electrode production characteristics

(1) Long production cycle. The production cycle of ordinary power graphite electrodes is about 45 days, the production cycle of ultra-high power graphite electrodes is more than 70 days, and the production cycle of graphite electrode joints that require multiple impregnations is longer.

(2) Higher energy consumption. The production of 1t ordinary power graphite electrode requires about 6000kW · h of electrical energy, thousands of cubic meters of gas or natural gas, and about 1t of metallurgical coke particles and metallurgical coke powder.

(3) There are many production processes. The production process includes raw material calcination, crushing and grinding, batching, kneading, shaping, roasting, impregnation, graphitization and mechanical processing. Its production requires many special mechanical equipment and kilns with special structure. The construction investment is large and the investment recovery period is long.

(4) A certain amount of dust and harmful gases are generated in the production process, and it is necessary to adopt perfect ventilation and dust reduction and environmental protection measures to eliminate harmful gases.

(5) The carbonaceous raw materials needed for production, such as petroleum coke and coal pitch, are by-products produced and processed by refinery companies and coal chemical companies. The quality and stability of the raw materials are difficult to be fully guaranteed, especially for the production of high-power and ultra-high-power graphite electrodes Needle coke, modified electrode asphalt and low-quinoline insoluble content special impregnant asphalt are urgently needed to be paid attention and actively cooperated by China's petroleum and coal chemical processing enterprises.

Graphite electrode application

(1) Used in electric arc steel making furnace

Electric furnace steelmaking is a major user of graphite electrodes. China's electric furnace steel output accounts for about 18% of crude steel output, and graphite electrodes for steelmaking account for 70% to 80% of the total graphite electrode usage. Electric furnace steelmaking uses graphite electrodes to introduce current into the furnace, and uses high-temperature heat sources generated by the arc between the electrode end and the charge to carry out smelting.

(2) Used in mine thermal electric furnace

The ore-heated electric furnace is mainly used for the production of industrial silicon and yellow phosphorus. Its characteristic is that the lower part of the conductive electrode is buried in the charge, forming an arc in the charge layer, and using the heat energy emitted by the charge of the charge to heat the charge, which requires current Graphite electrodes are required for high-density ore-fired electric furnaces. For example, each 1t of silicon requires about 100kg of graphite electrode, and every 1t of yellow phosphorus requires about 40kg of graphite electrode.

(3) For resistance furnace

Graphitization furnaces for producing graphite products, melting furnaces for melting glass, and electric furnaces for producing silicon carbide are all resistance furnaces. The materials contained in the furnace are both heating resistors and heated objects. Generally, conductive graphite electrodes are embedded in the resistance furnace end In the furnace wall of the Ministry, the graphite electrode used here is not continuously consumed.

(4) Used to prepare special-shaped graphite products

Graphite electrode blanks are also used to process various shaped graphite products such as crucibles, molds, boats and heating elements. For example, in the quartz glass industry, every 1t of fused tube requires 10t of graphite electrode blank; for every 1t of quartz brick, 100kg of graphite electrode blank is consumed.

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