Most operators want to have a clean plant, but what does that mean? Steve Davis explains how to design systems to reduce dust emissions and spillage from the very beginning.
In a mining environment we start to release fine particles that become dust at the primary crusher, and from there on along a conveyor system through stockpiles, other crushing operations, screening and into storage for loading to transport systems. Most of these stages will be dry materials handling. In many operations there will be a processing stage that further reduces particle size and produces product and waste streams. Processes include dry and wet materials handling.
Larger particles that don’t become airborne or flush away are easier to manage. However, design of systems should consider management and containment and avoidance of spills. Chutes should be designed to load the receiving belt correctly, sized to contain carry over on a shut down and have blockage detectors that are sensibly located. Storage level control to manage fill, methods to prevent fly rock. Don’t overfill belts and do consider the large rocks that could migrate to the edges. Provide methods and access to clean up after the inevitable spill.
Our focus should be on eliminating and controlling levels of fine particles in the materials handling streams. Fine particles are released and generated in material streams from any action that has potential to fragment larger particles into smaller. These actions can be deliberate forms of size reduction using impact, attrition or compression in the design, or they can be incidental from material handling designs that cause high impacts and other forces from materials interaction with wear liners.
When fine particles are generated, they will be released from the main material stream anywhere that circumstances permit. This is typically separation through gravity, air or water flow. Unintended separation causes many problems in materials handling apart from environmental and health issues. Airborne and waterborne fine particles accumulate on structures and mechanical equipment. This accumulation can overload structures, accelerate corrosion, create dust explosions, prevent visual condition monitoring, penetrate bearing and other seals, bury idler rolls, cause belt tracking issues, build up on pulleys and damage belts. The cost of damage and cleanup is high and ongoing and reduces plant availability and productivity. For many bulk materials there will be a cumulative loss of material that could be lost profit. Even if the product can be reclaimed there is additional cost to do so.
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Assuming we want minimum fines release in our plants, there are control methods available.
First establish some data. Start with how dangerous are the dust particles that will be released during material transport. This will set the basis for control, noting that current advice is that all ultrafine particles have potential to cause lung and or skin irritation or worse. Smaller particles, particularly the -10µm and -2.5µm are noted as being worse, especially if silica or organic (coal etc.)
Are they toxic (arsenic or lead concentrates for example)? Are they corrosive when wet or dry, soluble, (salt or potash)? What is the dust explosion level, will a static charge accumulate, (sulphur dust)? What is the dust extinction moisture level, noting that this varies with the overall particle size distribution of the material being handled? Is the Transportable Moisture Limit (TML) applicable? Is the dust hydrophobic? What is the dust lift off velocity? If any of these or other specifics apply, they should determine what dust control methods can and can’t be used. The type of material could affect the selection of processing equipment, e.g. which size reduction process releases the least amount of fines?
We have many options for control of generation and management of fines. It seems reasonable to assume that we would focus on methods that generate less fines as this results in fewer problems. It is unlikely that any bulk materials installation will be free of fines and dust.
Material handling system design impacts the amount of dust generated. Limiting dust generation and release is achieved by avoiding direct impact of material on any surface, minimising material free fall and minimizing any turbulence in the material flow stream.
Design conveyor systems to keep the material on or in the belt. Look at enclosed systems such as pipe conveyors. Use a real surcharge angle – many systems designed using textbook angles are near flooded when the actual angle turns out to be much lower. Belt edge clearance considers many aspects of conveying and should not be reduced.
Design and install flow-controlled chutes and spouts that offer smooth flow and keep the flow stream together to keep impact and turbulence low. We can select liner materials and methods such as modular rotable parts to ensure that liner integrity is easily maintained.
Keep drop heights as low as practical to avoid stream separation and dust release.
Balance the discharge from chutes with the velocity of the receiving conveyor to avoid turbulence, reducing belt cover wear at the same time.
Design chutes and bins to control the airflow and manage dust that is released and into dust collection systems.
Select skirt and impact components to match the material and the flow vectors to contain bulk materials and dust and avoid run back. Install them correctly at the supplier recommended spacing on the belt. Wide spaced skirts can cause many problems. Provide good access so that these components can be adjusted and maintained safely.
Select belt cleaning systems that match the bulk materials and the belt cover, remove as much as possible to avoid carry back and discharge into the chute without disrupting flow. Install them using supplier guidance, if not they may not function. Provide good access so that these components can be adjusted and maintained safely. Look for self-adjusting units, and units designed for maintainability. In Australia we have a wide selection of specialist suppliers who have global experience in many bulk materials that are happy to discuss and guide selection.
Maintaining the condition of materials handling systems will be easier if the design provides safe and considered methods. A well designed and maintained system will generate less spillage and dust and be more reliable.
We can minimise dust generation and release, but there will almost certainly be some dust and spills. There are several methods to containemissions.
Water is the most common method used. It is also a method that is abused badly in my experience. Water is an expensive commodity, but even more expensive if an excess of water displaces saleable product in transport. In many operations water is added before and after transfer chutes through a simple spray bar with little thought of control or quantity. Large volumes of water are also added in dump hoppers and other locations. The intent seems to be that drenching the bulk material will keep the dust down. I rarely see any black belt shut off or assessment of water quantity required, often just a bar with a manual valve. Often this causes more problems than solved. Cleaners are overwhelmed by wet slurry causing carry back, there can be run back and spillage from inclined belts, surcharge angles reduce from design, stockpiles have water issues. Water does suppress dust emission but could be applied more effectively.
The use of overbelt and in chute foam sprays, water plus surfactant plus air, has been proved effective for some applications. Selection of surfactant will be specific to requirements. Sodium lauryl sulphate (SLS) ‘soap’ is common. The bubbles consist of a thin film supported by air, when they contact a surface and break the thin film spreads over the local material adhering to small particles. Water consumption will be lower, potentially only 10 per cent of water only, and the effect lasts longer due to foam binding dust to larger particles. Capital investment is higher but effectiveness and operating costs are much better.
Spray systems are also intended to knock airborne dust down. They are only partially effective if incorrectly designed. Water droplets are generally much larger than dust particles and they tend to ‘push’ dust particles out of the way rather than coalescing and knocking them down. This leads to the use of fog or mist sprays, which are effective in suppressing airborne dust in dump hoppers and chutes. Systems are proven and readily available in Australia.
Effect of drop size on collision with fine particles
If wet suppression is not suitable for the materials handling system, we have fan driven dust collection systems. There are three aspects to consider; what will be done with the collected dust, the collector itself and the system used to carry dust laden air from the source to the collector.
There is one aspect of dust collection that has always puzzled me. The concept of having integrated bag filter and fan systems in a chute. These collect dust relatively well, but then reverse pulse the bags to discharge the dust back into the chute from which it was collected. The dust is likely part collected over and over and otherwise dumped back into the system to be released or collected elsewhere. My feeling is that if dust is collected, there should be a disposal plan. The minimum I can see would be to wet the dust into a paste and return to the flow. As a paste it may be able to pass through the balance of the system without emitting too much dust. Alternates fordry collection are to bag and dispose or recycle. Wet collectors only have thedispose or recycle option in most cases.
There are several types of dust collector available. There could be a pre-collector or cyclone for coarse separation with most types. The most common is the dry bag house system. Dust laden air is pulled into a fabric or paper filter where it is held and either discharged through shaking or reverse pulse air flow. When dust is released, it falls into a discharge hopper for recycle or disposal.
Wet scrubbers are used where bag filters are unsuitable, e.g. for explosive or flammable dust. The dust laden air is drawn through a liquid system and the dust separates from the air into the liquid. The liquid is circulated through settling or classification equipment to remove the dust as a slurry for disposalor recycle.
The fundamental basis for a dust collector to collect dust is for the dust to get from the source to the collector. On many sites I have visited, this is one aspect that stands out as being poorly implemented. There must be sufficient airflow velocity in the system from the extract point to and through the collector to entrain and keep dust particles entrained. If not, the dust will not get to the collector. This means sensibly located extract plenums, not just a convenient spigot, and a carefully designed low friction loss ducting system. If there are several plenums on the same collector, balancing facilities are necessary. Systems with multiple long and convoluted duct runs have too much resistance and are difficult to balance. When the cumulative flows and friction losses are added, then size a suitable collector and fan to provide flow and collect dust. The ducting must be accessible for cleaning. Many systems appear to select the collector and fan first and then randomly run ducts to convenient locations along structure that provides little access. This is one area where I have found it difficult to obtain a complete system design from any single supplier.