Reducing the specific graphite consumption is one major issue for electric steel plants and can be achieved in several ways, particularly by protecting the electrode surface from oxidation or at least delaying the start of the oxidation process. For more than 40 years, the most efficient technique applied in electric steel production is the special Graphite protective coating for graphite electrodes. All over the world, Graphite is the only producer of this type of coating which is used in metallurgy (electric steel production) as well as in the production of non-metal and mineral products by electric arc treatment (mineral wool, corund, silicium, etc.). The production of protective coating is a high-tech process made on machines designed especially for this purpose.
On EAFs, where water spray cooling is applied for reducing the specific graphite consumption, a further reduction of 10 to 15% can be achieved by using coated electrodes. On LFs, however, the specific graphite consumption can be reduced by up to 30% by using coated electrodes (depending on the operation conditions of the furnace).
TECHNOLOGY FOR THE PRODUCTION OF PROTECTIVE COATING
The classic coating consists of three layers applied on the electrode surface consecutively. The first two of them consist of aluminium alloys with an aluminum content of more than 75%. The third layer consists of pure aluminium.
The newly coating contains two further layers of metal and graphite. The total thickness of coating is 0,5 to 0,8 mm.
PROPERTIES OF THE PROTECTIVE COATING
• high temperature stability in an oxidizing atmosphere
• corrosion stability against dust, slag, metal splashs, etc.
• ideal adhesion: mechanical and chemical connection on the electrode surface
• thermal shock resistance
• absolute gas impermeability
• excellent electrical conductivity
• abrasion resistance
• no negative influence on steel.
The aluminium layer of the protective coating melts at a relatively low temperature – at about 600°C. It thus remains on the graphite surface as a liquid film with a constant thickness, guaranteeing excellent gas impermeability and resulting in increased resistance of the coating towards changes in temperature and thermal shocks.
20 to 35% of the current run through the protective coating due to its very high electrical conductivity (40 to 80 times higher compared to graphite). Thus, the current carrying capacity of the electrodes can be increased respectively.
Higher current carrying capacity of an electrode with protective coating decreases the probability of butt losses and breakages.
PROPERTIES OF COATED ELECTRODES
Coated electrodes show reduced side oxidation when they are in operation. The protective coating delays the start of the oxidation process unless it oxidizes itself. Afterwards, the oxidation process in the lower part of the electrode column, meanwhile without coating, continues the same way and with the same speed as the one of an uncoated electrode.
CHARACTERISTICS OF THE ELECTRODE CONSUMPTION
The electrode consumption depends on the following four factors varying according to the furnace type:
• consumption due to side oxidation - 30 - 65%
• tip consumption - 30 - 60%
• consumption due to tip losses - 3 - 10%
• consumption due to breakages - 1 - 10%
Electrode producers normally sum up the first three elements and consider this sum as technological consumption. Technological consumption plus consumption due to brea- kages are the gross consumption.
With the other conditions remaining unchanged, consumption due to side oxidation is proportional to the surface subject to oxidation. Protective coating reduces the length of the oxidized surface by 35 to 50%. The oxidation cone is reduced, the tip diameter is increased by 25 to 35 mm. Thus, the tip consumption as well as the consumption due to tip losses is reduced as the walls of the socket at the tip are thicker. As a conse- quence, savings in specific graphite consumption of up to 30% can be achieved.
The effect of the coating can be clearly seen if a non-coated and a coated electrode are assembled in one column. Within the same temperature range, the oxidation process of the coated electrode in the connection starts much later.
ADVANTAGES OF COATED ELECTRODES
The following targets can be achieved by using electrodes with Graphite coating:
• electrode quality is improved (durability and conductivity)
• specific graphite consumption in steel plants is reduced by up to 30%
• graphite costs in electric steel plants are reduced
• efficiency in production is increased (time saving for staff and handling)