Car-bottom roasting furnaces are widely used in graphite electrode processing plants. These furnaces are derived from the trolley-type heat treatment furnaces used in the casting industry and have been modified according to the specific requirements of roasting graphite products. Based on production needs, several identical furnaces form a furnace group, accompanied by a set of transportation equipment and a loading/unloading station. A typical car-bottom furnace has the following primary dimensions. Each car-bottom furnace can load over 100 large standard graphite electrodes with a diameter of 600 mm and a length of 3000 mm per batch, with a maximum load capacity of up to 170 tons. A processing plant with an annual output of 20,000 tons of large standard ultra-high-power graphite electrodes (including secondary roasting) requires the simultaneous operation of around 15 such large car-bottom furnaces.
The furnace chamber of a car-bottom furnace is a fixed structure, and the “car bottom,” equipped with a walking mechanism, is moved in or out of the chamber along the tracks on the furnace floor by a transfer cart. Each side of the furnace chamber is equipped with 4-6 fuel nozzles, while the furnace roof is equipped with 3-4 high-temperature fans and cooling water nozzles. The rear wall of the furnace has an exhaust gas outlet, and the furnace door is made of a welded steel frame with cast iron strips embedded around it. The furnace door is operated by an electric or hydraulic lifting mechanism.
Raw billets are first transported to the loading/unloading station and placed into steel cylinders of corresponding specifications. The billets are surrounded by packing material, and the steel cylinders are placed vertically on the “car bottom.” The transfer cart then moves the “car bottom” into the car-bottom furnace, the furnace door is closed, and fuel is introduced for heating. Several high-temperature fans on the furnace roof ensure continuous convection of the high-temperature gas to reduce temperature differences within the furnace. The roasting temperature is controlled according to a specified heating curve. For the first roasting of raw billets, the maximum temperature reaches 850°C, and the roasting cycle lasts 288-336 hours. For secondary roasting, the maximum temperature is about 800°C, with a roasting cycle of only 72-96 hours.
After the roasting and heating cycle is complete, the products are cooled within the furnace (with appropriate water spraying) to around 250°C. The furnace door is then opened, and the “car bottom” is pulled out by the transfer cart and sent to the unloading station where the products are removed from the steel cylinders. Exhaust gases in the furnace are drawn out by an exhaust fan and typically treated by high-temperature combustion to meet emission standards. The energy consumption of car-bottom roasting furnaces is slightly high, with energy consumption for the first roasting ranging from 4.6 to 5.8 GJ/t, and for secondary roasting from 2.5 to 3.8 GJ/t.