Abstract
M.Tech.
Mechanical alloying/mechanical grinding by means of high energy ball milling has
been considered as a promising materials processing technique to synthesise
materials. In this study it has been employed in an attempt to simulate the wear
products formed during the drilling of hematite ore by WC-Co drill bits, so as to try
and find an explanation for the rapid wear of these drill bits during the drilling of
hematite ore. A PM400/2 high energy ball has been used to mill individual powders
of hematite, WC and a mixture of WC-hematite, hematite-cobalt and WC-Cohematite
powders. In the high energy ball milling of hematite alone particle size
reduction of up to less than 10 nm was observed by both XRD and Mossbauer
Spectroscopy (MS), no phase transformation was observed as a result of milling the
powder up to - 30 hours. The high energy ball milling of WC also showed no new
phases but particle size reduction and XRD peak size broadening. Mechanical
alloying of WC-hematite revealed a more effective grinding compared to hematite
alone. The hematite particles were reduced to nanosized particles as in the previous
systems but the fraction of particles at nanosize level increased drastically from 59 %
in the hematite alone system to 100 % in the WC- hematite system, after 30 hours of
milling as observed from the increase in the abundance of the central supper
paramagnetic doublet in the Mossbauer Spectra. A new phase was observed from both
XRD and MS in the cobalt-hematite system and this phase was identified as CoFe 2O4 .
An increase in the composition of cobalt from 20 %-50 % in the mixture showed an in
increase in the percentage abundance of the CoFe 2O4 phase (38 %-55 % respectively)
with increase in milling time. The formation of this phase might be one of the causes
of the rapid wear of WC-Co drill bits during the drilling of hematite ores. Particle size
reduction to nanosized particles has also been observed in the cobalt-hematite system.
Mechanical alloying of WC-Co-hematite did not reveal any new phases in the
samples milled for 30 hours and analysed by XRD. Peak size broadening was
observed with increase in milling time. The MS results showed an increase in the
intensity of the central doublet with milling time. The results also show decrease in
the magnetic field of the hematite with increase in milling time. This has been
associated to reduction in particle size with increase in milling time