The production of silicon carbide is energy-intensive and produces large amounts of by-products. Researchers at Fraunhofer IKTS have developed an environmentally friendly recycling process to turn these back into high-quality silicon carbide.
Due to its tremendous hardness — nearly as hard as diamond — and heat resistance, the industrial material silicon carbide (chemical formula: SiC) is a sought-after commodity in the industry sector. SiC has applications in the chemical industry, as technical ceramics and for refractories, and is also used in semiconductors. It is produced using the Acheson process, in which quartz sand and petrol coke are heated in a cylindrical furnace to around 2500 °C. The resulting carbothermic reduction produces silicon carbide as its end product. The process is not complicated but produces a great deal of CO2: around 2.4 tons of the greenhouse gas are released for 1 ton of SiC from the carbothermic reduction alone. This is in addition to the enormous power consumption of 7.15 MWh/t, which is required to run the furnace for days. This releases another 1.8 tons of CO2 for each ton of SiC.
“The Acheson process is nearly 130 years old and is one of the classic, dirty industrial processes that are no longer in step with our times,” explains Jörg Adler, department head for non-oxide ceramics at Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Dresden. His team and their industry partner ESK-SiC have now developed a method for recycling the resulting waste and by-products. The RECOSiC process reduces industrial pollution and turns the recycled materials into high-quality silicon carbide.
High-tech furnace transforms waste and by-products into SiC
At the edges of the giant open-air Acheson furnaces, a significant portion of the primary raw materials — quartz sand and petrol coke, cannot react fully to form silicon carbide because the temperature is not high enough. Due to the sheer size of the furnaces and the enormous heat generated, installation in the factory building is hardly possible. Contamination from the air is an additional factor for open-air furnaces. Previously, the incompletely formed SiC, along with the waste and by-products, could only be used for inferior applications, or had to be disposed of. This is where the patented new process comes in. “We heat these by-products in a high-tech furnace with an inert atmosphere to temperatures up to 2400 °C. This produces more high-quality SiC. The energy input under the new process is 80 % lower than in the Acheson process, which also reduces CO2 emissions significantly,” explains Adler.
In contrast to the Acheson process, the high-tech furnace produces very little waste and by-products. Impurities are separated or vaporized during the chemical reaction. Under high heat, metals such as iron form lumps, which can be removed mechanically. As a result, the yield is nearly 100 %. Another major advantage: The process produces high-quality SiC with a purity grade of 99 % or even more.