Analysis of carbon fines obtained from the off-gas dust of calcium carbide furnace

The off-gas dust of calcium carbide furnace is a hazardous waste of large quantities in China. It, however, contains valuable materials and should be recovered not only for environmental and safety reasons but also for economic reasons. This work examines carbon fines separated from the off-gas dust of a calcium carbide furnace by acid leaching using HCl and HF solutions. The carbon fines are characterized using various instrumental techniques. It is found that the carbon fines mainly consist of hollow spheres of 0.5–3.5 µm in size with multilayer graphitic shells, a BET surface area of 124.5 m²/g and a mean pore size of 4.0 nm. Some of the carbon fines enclosed smaller carbon spheres that are either hollow or enclosed with inorganic species, such as magnetic Fe₃C, which can be separated magnetically. The carbon fines are good sorbent as demonstrated by quick adsorption of Congo Red in an aqueous solution to capacities of 216 or 296 mg/g with or without the magnetic separation, respectively. A mechanism for the formation of carbon spheres is proposed.