Nanoscale Analysis on Spark Plasma Sintered Fly-Ash Bricks and their Comparative Study with SiN-Zr Refractory Bricks
Authors: Sahoo D.K. , Kishor M.S.V.R. , Sahoo D.P. , Sarkar S. , Behera A. 
Published: March 2020

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.