Humankind is well aware of the safety hazards
caused by both airborne dust and surface dusty conditions.
The cost of dust pollution to industry and mining
in terms of accelerated maintenance, plant replacement through the
premature wearing in critical areas and lost production is not that well
understood and often overlooked.
The costs can be astronomical while the greatest
and most serious is the loss of person hours due to illnesses associated
with or caused by dust.
The indirect cost to mankind of atmospheric dust
pollution is incalculable and this cost is only marginally understood even
by scientists, engineers and environmentalists.
Savings in any of these areas due to dust control
measures can be substantial and the long-term economics, extremely sound.
All too often, dust control becomes the last item
of importance on the design agenda and one that is all too frequently
dropped in favour of a later "on-site arrangement if we find the
need"
Dust Control or Dust Extraction
The prime objectives to be achieved by any system
devised to control dust, fall into two categories.
- Systems controlling the dust content of the
atmosphere surrounding or emitted from plant or machinery and
maintaining this dust content within reasonable and acceptable limits.
- When this is not practicable, the system should
control the enclosed dusty atmosphere in such a manner that any air
leakage into the enclosure is induced inwards and not permitted to
escape bearing dust outwards.
Scrubbing or Filtration of Dust-Laden Air
Once dust-laden air has been captured at the
source by a ducted extraction system and conveyed to a central point, the
air must be scrubbed or filtered before it can be released as an
inoffensive and de-toxified gas.
This can be achieved in various ways.
- Cartridge filters
- Bag filter systems
- wet scrubbing plants
- cyclone settlement
Dry dust handling
Without doubt, the most convenient and economical
method of dry dust disposal is to return this, untreated, to the product
stream.
An element of thought and design is required in
the selection of the location for this re-introduction point which should
be out of the zone of extraction to prevent re-introduction to the
extraction system.
Tests have shown only some 0.05% of total
available dust is collected from the material stream at a given point and
re-introduction of dust does not result in the next point being
"twice as dusty"
Pelletisation of dust is also an option if the
dust is to be transported.
Dust Slurry Handling
All slurries emanating from dust scrubbers can be
pumped to a central settling sump system where settlement of solids can be
allowed to occur before water can be returned to each scrubber.
Dust Suppression Using Water Spray System
- Boosted pressure spray systems - These systems
utilise high-pressure pumps to supply a constant pressure to a
designed spray system. These systems should be used when potable
water supplies are very erratic.
- Sonic fog spray systems - This type of
system utilises high energy compressed air to break water down into
sub-micronic sized water droplet fog in purpose designed nozzles.
There are many problems with these systems resulting in excessive wear
and high operating costs.
- Moisture addition - Often water spray
systems can be effectively used to allow a percentage of dust allaying
before dust extraction systems are encountered in order to
'pre-condition" the material being handled or processed.
The nature of this type of system is therefore merely to increase the
moisture content of the material.
Spray systems of this nature would be located where the material is
"opened out" or aerated as at a conveyor head pulley or as
material is tipped from a truck.
Control of Wind Blown Material on Open Stockpiles
There are two main problems associated with
stockpile dust suppression.
- The first is that dry material is added
constantly and this has to be catered for.
- The second problem becomes the stabilisation of
the remainder of the dump against wind action.
The initial problem can largely be solved by
tipping the feed conveyor trajectory through an annular ring of sprays of
the flat cone, large droplet type located off the head chute.
Control of the system can be interconnected with
the running of the belt in a loaded condition.
This system is prone to wind problems and a high
percentage of the spray can be lost through wind action or evaporation.
This can be largely eliminated using wind screens around the spray rings.
The stockpile moisture content should always be
monitored and spray water only added as a known proportion to the load
rate of the stockpile.
In this way, a reasonable assessment of the moisture content can be made.
Ideally, spray water at the annular ring must only
be added while material is flowing. The control to effect this
operation can be from the motor control centre (MCC) where a
"loaded" motor running condition would activate the solenoid
control valve to open the sprays.
The main body of the stockpile can be "set
down" from time to time when this becomes excessively dry or when
wind is anticipated.
Sprays of the agricultural type are recommended.
These rain gun units are mounted on protected stand pipes around the
stockpile and are usually activated manually to operate until the surface
is sufficiently wet to prevent wind-blown dust generation before being
turned off.
Water Curtain Applications
Water curtain drenching is sometimes used when
trucks road haul ore some distance over haulages which are unsheltered or
exposed to wind.
In such a case, the truck would drive under the
spray race and hesitate while a pre-determined volume of water is
discharged through the sprays onto the payload. The truck would then
proceed on its journey.
Unusual Achievements
We have successfully removed fish scales from
marine diamond deposits with specialized dust control equipment. The
fish scales were not actually a dust problem but they did interfere with
the optics used to separate diamonds. This is similar to the
problem where we had to defluff diamond concentrates from
underground mining operations, where a slurry explosive is pre-packed in
plastic bags. The slivers of plastic fluoresced in the same way that
diamonds fluoresce and needed to be removed from the process.
