Dust Control Design
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'.
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.
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 it, 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 that 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.
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 is returned to each scrubber.
- 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.
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 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 sprays 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.
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 de-fluff 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.