Investigation of strength and reduction reactivity during heat treatment in simulated-experimental blast furnace of carbon-containing pellet prepared by vapor deposition of tar to cold-bonded pellet with large particle size

Optimum conditions for preparing carbon-containing pellets with a high crushing strength and high reactivity using three kinds of cold-bonded pellets (CPs) prepared by different methods are investigated in this study. The carbon-containing pellets, which have a size fraction suitable for blast furnace are prepared by completely filling the pores in CPs with carbonaceous materials derived from coke oven gas (COG) tar by the vapor deposition (VD) method. A flow-type quartz fixed-bed reactor is used for this method, using a combination of tar pyrolysis at 700 °C and VD at 350 °C. The changing pore size distribution, distribution of the carbonaceous material, and crushing strength of the VD samples are then measured for the prepared composites. The carbonaceous materials derived from COG tar completely fills into the mesopores and macropores of the CPs, which is prepared by using Portland cement (PCB) or Carboxymethyl cellulose (CMC), as the binder, and the aging method in stainless-steel container, with increasing VD treatment time. When PCB is treated by the VD method, the C content and crushing strength tends to increase with increasing VD treatment time, and reached 16 wt%-dry and 45 daN, respectively, until 60 min; these values are comparable to the strength of metallurgical-coke with DI1506 = 87.1. Additionally, carbonaceous materials are uniformly deposited in the particles inside. The changes in the reduction behavior and crushing strength of the prepared VD sample under various heat conditions (simulation-values in experimental-blast furnaces in COURSE50 project). The reduction extent of the VD samples prepared from PCB reaches up to 30-40% at 850 °C. These rates increase above 850 °C and attain a value >95% until 950 °C irrespective of the heating and atmospheric conditions. Fe2O3 and Fe3O4 in the VD samples are reduced to FeO and/or α-Fe until 850 °C under any condition used, while FeO is completely reduced to α-Fe by 950 °C. The cold crushing strength of VD samples is maintained up to 850 °C. Although this value drastically decreases at 900 °C, which is the temperature at which α-Fe formation occurred, there is no decrease in strength for the reduction of Fe2O3 to Fe3O4, which has been observed reduction disintegration occurs in a conventional blast furnace.