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-ta r-based needle coke was examined in comparison with that of boric acid. Bot h 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 i ron(II) sulfate was very effective to suppress the puffing regardless of re peated impregnation of pitch and baking. The bulk density of the graphitize d rod was found also to be increased by both inhibitors. Rapid heating at g raphitization enhanced puffing of the rod in any case, however this inhibit or always suppressed the puffing. In contrast to the iron(III) oxide, addit ion of boric acid together with iron(II) sulfate was not effective to provi de additional inhibition; suggesting their similar roles in the puffing inh ibition. Iron(II) sulfate impregnated into the pore of needle coke is conve rted into iron(III) oxide during the heat-treatment which plugs the pore pr eventing it from being fully filled with the binder and impregnated pitches during repeated impregnation and baking. Iron(III) oxide was reduced to ir on at an early stage of graphitization, which melts, migrates within the ca rbon 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 Sc i 1975;10(11)=1976-1990). Such conversions of iron(II) sulfate open and ind uce the porosity for puffing causing sulfur- and nitrogen-containing gases to be liberated without provoking the puffing. No remaining iron in the gra phitized rod may support the role of the present puffing inhibitor. Additio n of iron(III) oxide at the kneading stage may decrease the puffing due to the coke derived from impregnation pitch, suppressing the puffing at repeat ed impregnation together with iron(II) sulfate in the pores of needle coke. An additional advantage of increased bulk density of the graphitized rod w as emphasized by both inhibitors, catalytic carbonization and graphitizatio n performed by the inhibitors contributing such advantages. (C) 2000 Elsevi er Science Ltd. All rights reserved.