Processing technology with a proven worldwide track record for the carbon industry

Carbon materials are used in many sectors of industry. Along with raw material selection, the production technology has a crucial influence on the properties of the carbon which is embedded in a pitch or synthetic resin binder matrix. Mixing in the high-viscosity binder presents a major challenge. That is the reason why EIRICH mixing systems are used worldwide in these applications.

The characteristics profile of the carbon materials which are part of the ceramics industry is largely determined by the microstructure and the properties of the individual constituents contained in the microstructure. In contrast to other ceramic materials, melting or sintering is not possible with carbon. An additional binder matrix is needed for microstructure bonding. Pitch or synthetic resin with a high carbon content are used for that purpose. Typical binders include coal tar pitch and to a lesser extent petroleum pitch and polymers such as phenolic resins. The solid raw materials are graphite, coke and carbon black. All of them have a graphitic crystalline structure, but the formation and size of the individual crystals differ.

Stringent demands are placed on material processing. High uniformity and maximum density are essential. This requires intensive, homogeneous mixing of the binders and
solid raw materials. Mixing takes place at temperatures up to 300°C. When producing large parts, the mixture is formed using extrusion or vibro compaction while the material is still hot. With smaller parts, the mixture is ground again and formed to a green body on hydraulic or isostatic presses. Through tempering in the absence of oxygen at 600°C - 1200°C, the binder is converted to carbon. Volatiles are released, creating a coke matrix. This is followed by graphitization, a high-temperature treatment process which takes place at 1800°C - 3000°C.

In volume terms, the most important products for the carbon materials industry are anodes and cathodes for aluminum and graphite electrodes used in steel production. There are also a number of special applications, for example carbon blocks for blast furnaces. Relatively large-grain graphite materials are used for these applications and for electrodes. Special fine-grain materials are needed for carbon brushes, bearing and sealing materials and for high-temperature applications.

The aluminum industry is the world's largest consumer of electrodes. In the electrolytic extraction of aluminum, both electrodes are made of graphite. The function of the cathode is to lead current. However due to the electrochemical processes which take place, the graphite in the anode is consumed and must be continuously renewed or replaced. So-called Söderberg anodes are used at older plants. The unbaked electrode mass is filled into steel shells. The high temperature of the molten bath causes graphitization.

The second major application for carbon materials is the production of steel from scrap or sponge iron in electric arc furnaces. An arc is struck between the graphite electrodes and the charge, melting the metal. The electrodes are exposed to extreme temperature peaks and gradients. The reduction of oxides on the graphite, dissolution of carbon in the molten steel and oxidation of the graphite in contact with air results in continuous consumption of the electrodes in this application as well.

Around 40 years ago, the carbon materials industry discovered the advantages of EIRICH processing technology, initially as a replacement for mixers running at low speeds and as mixer-coolers installed downstream of kneaders. Producers quickly recognized the opportunities offered by the new technology. The EMC© process (Eirich Mixing Cascade), with a hot-mixer and a remixer-cooler with throughput rates of 15 - 60 t/h backed up by 20 references worldwide, is available for anode fabrication. Due to the excellent cost/benefit ratio, EIRICH technology became the industry standard. More than half of the anode manufacturers worldwide use at least one EIRICH mixer, predominantly in continuous operating, for hot mixing as well as for subsequent cooling to pressing temperature which can be pre-selected with ± 3 K precision. Several hundred machines are currently in operation at well-known producers around the world which enabled them to substantially enhance the properties of the materials. The fact that a single EMC© mixer line is able to support production of 600,000 tons p.a. of aluminum demonstrates the performance capabilities of EIRICH processing technology.

EIRICH has also simplified the production of cathodes for the aluminum industry. As is the case for graphite electrodes, a special electrical heating technique is available which heats up the coke up to mixing temperature. Typical throughput rates for batch processing of graphite electrodes and cathodes are 4 -12 t/h. The EIRICH product portfolio for these applications ranges from single machines to complete plants including raw material dosing and weighing as well as mixing and filling of the correct quantities into the press. Of course, the whole process is operated fully automatically.

Inductive mixers with capacities between 5 liters and 400 liters which can be used to produce special products have been available for several years. Due to the inherent system design, they are capable of handling any consistency including extremely viscoplastic consistencies. Inductive heating of the mixing pan directly generates heat in the walls of the mixing pan. Fast heat transfer between the material which is being intensively mixed and the rotating walls of the mixing pan produces a high surface specific energy transfer. Dynamic regulation inherent for inductive heating systems makes it possible to select the exact temperature with high reproducibility. The chemical-thermal reaction in the mixture, called defuming, can be started quickly and stopped at exactly the right time when the mass is dry enough for further processing or forming. Heating capacities of 5 kW - 250 kW make it possible to process mixtures containing materials with high melting points such as solid resins rather than resin solutions. Solid resins introduce less volatiles, reducing process times and lowering porosity. This provides a basis for development of new materials, and that includes other industries which use synthetic resin bonding as well.

Producers who have chosen EIRICH mixing systems confirm that investment costs are up to 50% lower and operating and maintenance costs are up to 30% less. This is the reason why EIRICH has recently acquired a number of new orders, e.g. for Aluminium Bahrain (ALBA) PL6, Bahrain. The order includes an EMC© system for anode paste for the aluminum industry with a hot-mixer and a remixer-cooler. The system has a throughput rate of 55 t/h. The equipment is being supplied by the French engineering group Fives Solios.

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Contact: Stefan Vucic, e-mail: stefan.vucic@eirich.de