Abstract

Background: Industries such as thermal power plants use coal as a source of energy and release the combustion products into the environment. The generation of these wastes is inevitable and thus needed to be reused. In India, coals with high ash content usually between 25 to 45% are used. The refractory bricks that were used earlier in steel industries were mainly based on silica, magnesia, chrome, graphite. In modern days, several other materials were introduced for the manufacturing of refractory bricks such as mullite, chrome-magnesite, zircon, fused cast, and corundum. The materials selection for refractory brick manufacturing depends on various factors such as the type of furnace and working conditions.

Objectives: The current work aims to focus on the fly-ash subjected to spark plasma sintering process with a maximum temperature of 1500 °C and pressure 60 MPa for 15 minutes and to characterize to observe the properties with respect to their microstructure.

Methods: Fly-ash collected from Rourkela Steel Plant was sintered using spark plasma sintering machine at the Indian Institute of Technology, Kharagpur. The powder placed in a die was subjected to a heating rate of 600-630 K/min, up to a maximum temperature of 1500˚C. The process took 15 minutes to complete. During the process, the pressure applied was ranging between 50 to 60 Mpa. 5-10 Volts DC supply was given to the machine with a pulse frequency of 30-40 KHz. The sintered product was then hammered out of the die and the small pieces of the sintered product were polished for better characterization. The bricks collected from Hindalco Industries were also hammered into pieces and polished for characterization and comparison.

Results: The particles of fly-ash as observed in SEM analysis were spherical in shape with few irregularly shaped particles. The sintered fly-ash sample revealed grey and white coloured patches distributed around a black background. These were identified to be the intermetallic compounds that were formed due to the dissociation of compounds present in fly-ash. High- temperature microscopy analysis of the sintered sample revealed the initial deformation temperature (IDT) of the fly-ash brick and the refractory brick which were found to be 1298 °C and 1543 °C, respectively. The maximum hardness value observed for the sintered fly-ash sample was 450 Hv (4.413 GPa) which is due to the formation of nano-grains as given in the microstructure. The reason behind such poor hardness value might be the absence of any binder. For the refractory brick, the maximum hardness observed was 3400 Hv (33.34 GPa). Wear depth for the sintered fly-ash was found to be 451 μm whereas for the refractory brick sample it was 18 μm.

Conclusion: The fly-ash powder subjected to spark plasma sintering resulted in the breaking up of cenospheres present in the fly ash due to the formation of intermetallic compounds, such as Cristobalite, syn (SiO2), Aluminium Titanium (Al2Ti), Magnesium Silicon (Mg2Si), Maghemite (Fe2O3), Chromium Titanium (Cr2Ti) and Magnesium Titanium (Mg2Ti), which were responsible for the hardness achieved in the sample. A large difference in the maximum hardness values of sintered fly-ash and refractory brick was observed due to the hard nitride phases present in the refractory brick.

Keywords: Fly-ash, refractory bricks, Spark Plasma Sintering (SPS), coal, power plants, thermal.

Graphical Abstract