What can be
done to make
water work in
the suppression
of coal dust?
The imperative
is to first
consider the
effects of coal
dust control on
the calorific
value of coal
in addition to
dislodging the
buoyancy of
coal dust
particles. No
one also wants
to achieve coal
dust
suppression at
the expense of
its use as a
source of
energy.
Critical to the
discussion is
the hydrophobic
nature of coal
surfaces.
Surface
chemistry is
the most
important
phenomenon to
understand and
comprehend.
Characteristics
of the coal
dust
suppressants
that have been
used, newer
developments in
coal dust
suppression and
their
mechanisms of
action will be
discussed in
the article.
History is
important to
assess why
initial dust
suppressants
did not achieve
the expected
results whilst
understanding
the efficacy of
superior
performing coal
dust
suppressants
and their
evolution over
time.
Water
water
everywhere…
Traditionally,
the water spray
is the most
commonly used
coal dust
suppressant as
raising
moisture
content in coal
dust is meant
to agglomerate
coal dust
particles
through the
capillary force
of water
molecules.
However, the
drawbacks with
water include
short term
effectiveness
due to
unpredictable
rapid water
evaporation
rates in summer
seasons that
are dependent
on atmospheric
conditions. The
rationale for
using water was
that wetting of
coal adequately
at the face,
would minimize
dust created
during
transport at
the transfer
points. Water
has high
surface tension
and the same
surface charge
profile as coal
surfaces, which
is mainly
hydrophobic.
Although coal
mineral matter
is hydrophilic
it comes as
inferior to the
hydrophobic
coal macerals
which render
domination of
hydrophobic
sites over
hydrophilic
sites. The
hydrophilicity
of water can
only achieve a
reasonable but
not consistent
and long-long
lasting which
would result in
the use of more
water as
opposed to
minimizing
water usage
owing to
possible
interfacial
chemistry
changes to
water.
Principally the
need was to
make water work
at the coal
face which led
to the
development of
products that
could achieve
amphiphilicity
at the coal
face.
The
dust
suppression
evolution
The
evolution of
coal dust
suppressants
revolved around
reducing the
surface tension
of water
without loss of
the wetting
capabilities of
water itself
preferable at a
small cost. In
order to
achieve that
there was the
need to harness
surface
chemistry of a
product that
when in aqueous
solution, the
concentration
would be higher
at the surface
than it would
be in the bulk
of the liquid.
The solution to
making water
work was
answered
through the
application of
surfactants. By
definition,
they are
organic
compounds
consisting of a
polar head
group and a
hydrophobic
tail.
Generally, the
head groups can
be hydrophilic,
lipophobic or
oleophobic
whereas the
tail groups can
be hydrophobic,
lipophilic and
oleophilic.
When added to
water the
surfactant can
be used to
increase the
coal dust
suppression to
the water spray
system. The
hydrophilic
head groups
make the
surfactants
soluble in
polar solvents
such as water
and they
determine the
main
characteristics
of the
surfactants. On
the contrary,
the hydrophobic
tails are
usually made up
of hydrophobic
chains which
render
insolubility in
polar solvents
such as water.
The discussion
will further
divide them
based on
hydrophilic
group which
will be
described in
the mechanism
of action
section.
Other
methodologies
It
would be biased
to strictly
focus on just
the use of
surfactants as
such is a broad
topic which
requires a
balanced
approach. Other
methods of
making water
work has been
attempted and
used.
Magnetized
water, atomized
water sprays
electrostatically
charged fogs,
high-pressure
sprays and
sprays that use
foams or
wetting agents
have been
utilized. The
challenges
faced in using
these
alternatives
included excess
water leading
to belt
slippage and
reduction of
conveyor
cleaning
systems, wet
and sticky
fines which
possibly
accumulated
within chutes
and around
transfer
points.
Inherently
water is not an
effective
binder hence
coal dust
particles tend
to dislodge and
become loose
leading to
their buoyancy
and release
back as
inhalable and
respirable coal
dust. In fog
spray
applications,
an ultra-fine
mist of water
agglomerates
the very
particles based
on the
principle that
droplets of
similar sizes
as coal dust
particles are
more
effectively
combined in the
process. In
water spray
systems, air
currents
produced blow
away much of
the coal dust
cloud with coal
dust
suppression
achieved
through slowing
the air
movement within
the loading
zone.
The
challenges to
the alternative
methods above
included
variations in
dust
concentration
and particle
size based on
the nature of
the coal mining
operations. The
basis of the
fog spray, for
instance, was
solely focused
on physical
properties such
as particle
size which is
not always
effective given
the coal dust
particles do
not necessarily
become dislodge
inequitable
size
distributions.
The extent to
which dust
suppression was
implemented was
also dependent
on the
potential of
coal dust being
a health
hazard, public
nuisance, the
existing
regulations and
emission rates.
The undesirable
effect of
excess moisture
content is poor
wettability as
a result of
repeated use of
water to
achieve coal
dust
suppression can
negatively
alter materials
performance
with a
reduction in
its calorific
value.
Surfactant
technology
The
mechanism of
action for
surfactants in
coal dust
control is
entirely
dependent on
adsorption that
occurs on the
interface
between the
surface-active
agent and the
coal surface.
The coal
surface is
primarily
hydrophobic in
nature with
secondary
hydrophilic
sites hence the
amphiphilicity
of surfactants
enables a dual
mechanism
through
lowering the
surface tension
of water whilst
converting the
coal surface to
hydrophilicity
via adsorption
onto the
hydrophobic
sites. The
surfactant
solution
succumbs to
physical
changes as a
result of the
decrease in
surface tension
with increased
concentration
of surfactant
until a point
is reached
where it
remains
relatively
constant as the
surfactant
molecules form
a unimolecular
layer on the
surface.
Intricately,
adsorption
takes places
after addition
of surfactant
into the water,
the hydrophilic
head groups of
surfactants in
the polar
environment of
the water
whereas the
hydrophobic
tails interact
with the
hydrophobic
coal surface.
Dipole forces
are responsible
for hydrophobic
interactions at
the coal face
which converts
them
hydrophobic
sites into
hydrophilic
sites hence
improving coal
wetting
performance.
The
dust control
revolution
So
what separates
the different
products
marketed for
coal dust
suppression? In
particular, how
can we manage
rapidly
generated,
fast-moving
dynamic dust?
As we focus on
practical
solutions to
coal dust
suppression,
Global Road
Technology
(GRT) offers
the Australian
made products
GRT: ActivateUG
(underground)
and GRT:
Activate which
can make water
work. The
choice of coal
dust
suppressant
should factor
in effects on
coal calorific
values, the
percentage used
per ton and
ability to
super-activate
water. The
key-differentiating
factor is
SPEED. GRT:
Activate works
faster than any
other
technology on
the market.
Each
application and
mine differ –
GRT solutions
and delivery
hardware are
tailor-made to
suit any coal
dust
suppression
need to the
satisfaction of
the
client.
REFERENCES
- Chang, P., Zhao, Z., Xu, G., Ghosh, A., Huang, J., and Yang, T. 2020. Evaluation of the coal dust suppression efficiency of different surfactants: A factorial experiment. Colloids and Surfaces A. 595. 1-8.
- Chen, J., Hu, X., Fang, Y., Jin, G., and Xia, Y. 2019. What dominates the interfacial properties of extended surfactants: Amphipathicity or surfactant shape? Journal of Colloid and Interface Science. 547. 190-198.
- Hendriks, C.A.M., and Claus, H. 1963. Effect of Dust Suppression Measures on the Prevalence of Coal Workers Pneumoconiosis in the Dutch Coal Mines. Brit. J. Industr. Med. 20. 288-297.
- Petavratzi, E., Kingman,S., and Lowndes, I. 2005. Particulates from mining operations: A review of sources, effects and regulations. Minerals Engineering. 18. 1183-1199.
- Xu, G., Chen, Y., Eksteen, J., and Xu, J. 2018. Surfactant-aided coal dust suppression: A review of evaluation methods and influencing factors. Science of the Total Environment. 639. 1060-1076.