Puffing behavior during the graphitization of coal-tar-based needle coke impregnated with iron(II) sulfate and boric acid

The puffing inhibiting ability of iron(II) sulfate impregnated into coal-tar-based needle coke was examined in comparison with that of boric acid. Both inhibitors exhibited similar ability to suppress the puffing of coal-tar-based needle coke by impregnating from their aqueous solutions, drying and heat-treating at 1223 K before kneading with the binder pitch. Addition of iron(III) oxide at the kneading stage to the needle coke impregnated with iron(II) sulfate was very effective to suppress the puffing regardless of repeated impregnation of pitch and baking. The bulk density of the graphitized rod was found also to be increased by both inhibitors. Rapid heating at graphitization enhanced puffing of the rod in any case, however this inhibitor always suppressed the puffing. In contrast to the iron(III) oxide, addition of boric acid together with iron(II) sulfate was not effective to provide additional inhibition, suggesting their similar roles in the puffing inhibition. Iron(II) sulfate impregnated into the pore of needle coke is converted into iron(III) oxide during the heat-treatment which plugs the pore preventing it from being fully filled with the binder and impregnated pitches during repeated impregnation and baking. Iron(III) oxide was reduced to iron at an early stage of graphitization, which melts, migrates within the carbon and reacts with carbon to form Fe₃C, and decompose to Fe and graphite, finally vaporizing out of the coke grain (Matthews and Jenkins, J Mater Sci 1975;10(11):1976-1990). Such conversions of iron(II) sulfate open and induce the porosity for puffing causing sulfur- and nitrogen-containing gases to be liberated without provoking the puffing. No remaining iron in the graphitized rod may support the role of the present puffing inhibitor. Addition of iron(III) oxide at the kneading stage may decrease the puffing due to the coke derived from impregnation pitch, suppressing the puffing at repeated impregnation together with iron(II) sulfate in the pores of needle coke. An additional advantage of increased bulk density of the graphitized rod was emphasized by both inhibitors, catalytic carbonization and graphitization performed by the inhibitors contributing such advantages.