Yoichi Kawano

Suppression of puffing during the graphitization of pitch needle coke by boric acid

Puffing of pitch needle coke was successfully suppressed by both impregnating boric acid and adding ferric oxide. The boric acid was impregnated into calcined coke and preheated above 870 K to convert boric acid to boron trioxide which melts above 850 K to get into the pore. Filled boron trioxide inhibits the binder pitch getting into the pore at the kneading and sublimes to open the pore above 1800 K at the early graphitization stage, through which liberated nitrogen can be released without puffing at the later graphitization. Boron trioxide reacts with carbon to produce CO and boron carbide, the latter substance enhancing the graphitization. The former reaction reduces the carbon yield. Hence there is an optimum amount to inhibit puffing sufficiently with least carbon loss.

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.

Mechanisms for puffing inhibition of coal tar based needle coke with disodium hydrogen phosphate (DHP) and boric acid

Puffing inhibition of coal tar based needle coke was effectively achieved regardless of the repeated impregnating and heating rate at the graphitization by impregnation with disodium hydrogen phosphate (DHP) and boric acid into the needle coke. Inhibitors were found dispersed into pores of the coke by impregnating and heat-treating the inhibitors on the coke. Such inhibitors in the pore prohibited the binder and impregnation pitches to penetrate into the pore at the kneading, baking and repeated impregnation. Such inhibitors vaporized or reacted with carbon in the pore at the early graphitization to leave some space between needle coke wall and binder pitch carbon, through which puffing causing species such as nitrogen and sulfur can liberate from the coke without expansion. The dispersion of inhibitors was observed in the pore of the coke by a computer-aided microanalyzer (CMA). Complete decomposition of DHP at the early graphitization may emphasize its inhibition activity compared to the activity of boric acid which is converted to boron carbide in the pore.