Coke (approximately 75-80% in content)
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Petroleum coke is the most important raw material, and it is formed in a wide range of structures, from highly anisotropic needle coke to nearly isotropic fluid coke. The highly anisotropic needle coke, due to its structure, is indispensable for the manufacture of high-performance electrodes used in electric arc furnaces, where a very high degree of electrical, mechanical and thermal load-bearing capacity is required. Petroleum coke is almost exclusively produced by the delayed coking process, which is a mild slow carbonising procedure of crude oil distillation residues.
Needle coke is the commonly used term for a special type of coke with extremely high graphitizability resulting from a strong preferred parallel orientation of it&#;s turbostratic layer structure and a particular physical shape of the grains.
Binders (approximately 20-25% in content)
Binding agents are used to agglomerate the solid particles to each other. Their high wetting ability thus transforms the mix into a plastic state for subsequent molding or extrusion.
Coal tar pitch is an organic compound and has a distinct aromatic structure. Due to its high proportion of substituted and condensed benzene rings, it already has the distinctly preformed hexagonal lattice structure of graphite, thus facilitating the formation of well-ordered graphitic domains during graphitisation. Pitch proves to be the most advantageous binder. It is the distillation residue of coal tar.
The milled coke is mixed with coal tar pitch and some additives to form a uniform paste. This is brought into the extrusion cylinder. In a first step the air has to be removed by prepressing. Than the actual extrusion step follows where the mixture is extruded to form an electrode of the desired diameter and length. To enable the mixing and especially the extrusion process (see picture on the right) the mixture has to be viscous. This is achieved by keeping it at elevated temperature of approx. 120°C (depending on the pitch) during the whole green production process. This basic form with cylindrical shape is known as "green electrode".
Two types of baking furnaces are in use:
Car bottom furnace
Here the extruded rods are placed in cylindrical stainless steel canisters (saggers). To avoid the deformation of the electrodes during the heating process, the saggers are also filled with a protecting covering of sand. The saggers are loaded on railcar platforms (car bottoms) and rolled into natural gas - fired kilns.
Ring furnace
Here the electrodes are placed in a stone covert cavity in the bottom of the production hall. This cavity is part of a ring system of more than 10 chambers. The chambers are connected together with a hot air circulation system to save energy. The voids between the electrodes are also filled with sand to avoid deformation. During the baking process, where the pitch is carbonized, the temperature has to be controlled carefully because at the temperatures up to 800°C a rapid gas build up can cause cracking of the electrode.
In this phase electrodes have a density around 1,55 &#; 1,60 kg/dm3.
The baked electrodes are impregnated with a special pitch (liquid pitch at 200°C) to give them the higher density, mechanical strength, and electrical conductivity they will need to withstand the severe operating conditions inside the furnaces.
A second baking cycle, or "rebake," is required to carbonize the pitch impregnation and to drive off any remaining volatiles. Rebake temperature reach almost 750°C. In this phase the electrodes can reach density around 1,67 &#; 1,74 kg/dm3.
The final step in graphite manufacture is a conversion of baked carbon to graphite, called graphitizing. During the graphitizing process, the more or less pre-ordered carbon (turbostratic carbon) is converted into a three-dimensionally ordered graphite structure.
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The electrodes are packed in electric furnaces surrounded by carbon particles to form a solid mass. An electric current is passed through the furnace, raising the temperature to approximately °C. This process is usually achieved using either an ACHESON FURNACE or a LENGTHWISE FURNACE (LWG).
With the Acheson furnace the electrodes are graphitized using a batch process, while in a LWG furnace the entire column is graphitized at the same time.
The graphite electrodes (after cooling) are machined to exact dimensions and tolerances. This stage may also include machining and fitting the ends (sockets) of the electrodes with a threaded graphite pin (nipple) joining system.
From Mixing to Machining the production of graphite electrodes takes about 60 days as shown in the picture here below:
Author: Eng. Matteo Sporchia
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Graphite electrode is made of high quality petroleum coke, asphalt coke and other raw materials through crushing, calcining, grinding, medium crushing, screening, batching, kneading, shaping, roasting, graphitization and mechanical processing. Arc furnace and electric furnace are used as conductive materials. Graphite electrodes are widely used in industrial metallurgy and are used in almost every industry.
People often say that several dipping and baking, refers to the production process of many times after dipping bitumen coke and other raw materials, and repeatedly roasted graphitized graphite electrode. Two dipping three baking, refers to the second dipping process, three times roasted graphite electrode. Three dipping and four baking refers to three dipping processes and four roasting of graphite electrode. Impregnation is an important process to strengthen the performance of graphite electrode, which greatly improves the conductivity, flexural resistance and compressive resistance of graphite electrode.
Graphite electrode batching process, calcined coke after crushing, can be different sizes of carbon particles materials, in order to obtain high quality graphite products, we need to different particle size of carbon materials and different kinds of raw materials appropriate mixture. In the production of carbon and graphite products, the correct formulation and accurate batching operation will have a great impact on the pressing and calcining of graphite chemical order. The present production formula is based on specific conditions, after a long period of production practice gradually summarized, improved.
When determining a recipe, attention should be paid to each product in the use of different materials, the aggregate particles and the binder dosage, also, the corresponding adjustment theory tells us that the accumulation of such big balls to take the same situation, one of the largest space filling rate is 74.05%, so in the middle of the ball forming two types of gap and a tetrahedral gap and an octahedral gap and fill in basic sphere of tetrahedral gap sphere diameter is 0.225, and octahedral gap filling large particles sphere of sphere diameter can be equal to 0.441.
Graphite electrode using experimental method to choose the proportion of various granularity, first made experiments, two kinds of particles with a larger particles and another small particles, with big grain weight of 100 grams, and then put it separately and 0-100 grams of the second particle mixing, and then take the maximum density of 100 g of the mixture, then mixed with 0-100 grams of the third kind of particles, then select the third largest mixture density, mixed with the fourth granularity, do a series of experiments, until get the right mix, below is 1.5, 1.0 and 1.0, less ash coke two mixture density of 1.3 mm.
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