BULKtalk, Dust suppression, Technical articles

BULKtalk: Dealing with dust

: Dust in bulk materials handling is a safety, environmental and cost problem. It is a problem that should be managed systematically to the best of our technical ability. Steve Davis explains.

Dust in bulk materials handling is a safety, environmental and cost problem. It is a problem that should be managed systematically to the best of our technical ability. Steve Davis explains.

Dust can kill directly or indirectly. Dust may not be visible. Breathing in dust can result in a range of occupational illnesses and diseases depending on the size and composition and concentration of dust particles, exposure time and its effect on the body, of dust particles. 

Particles below 100 microns can be breathed in. These are visible to the naked eye and are caught in the nose, throat, and upper respiratory tract. Below 10 microns, respirable dust contains dust particles are so small they are invisible and reach deep into the lungs. Dust below 2.5 microns is considered a severe risk.

Different types of dust particles have different health effects. E.g., respirable crystalline silica dust scars the lungs, and inhalable lead dust can damage the central nervous system. Several diseases are the result of long-term exposure to dust, and it may take years or decades before the disease develops. All dusts must be considered dangerous until data proves otherwise. All bulk materials must be considered dusty, either inherent in the material, through degradation in handling or from fugitive dust from spillage.

Health impact from dusts include blood toxicity, allergic reactions, bacterial and fungal infection, lung scarring and fibrosis, pulmonary diseases (pneumoconiosis and chronic obstructive pulmonary diseases such as bronchitis and emphysema). 

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We have a duty of care to prevent exposure to dust. Safe Work Australia has a document “Workplace exposure standards for airborne contaminants” that provides guidance. There are many other guidelines available.

Poor dust management of explosive and combustible dusts results in injuries and fatalities (and consequential damage costs) globally. Dust explosions and fires are reported often around the world. An internet search shows some significant explosions have been recorded back to the 1800’s. The most recent being in August 2020, when ammonium nitrate stored at the Port of Beirut in Lebanon exploded, causing over 218 deaths, 7000 injuries, and US$15 billion in property damage, and leaving an estimated 300,000 people homeless. 

I worked in a steelworks where dust buildup over years was enough to bring the roof down from combined corrosive effect and structural overload. I have been in many storage areas where the dust buildup was cause for concern, and others where design and housecleaning has limited this problem. Dust build up that gets wet can cause localised corrosion.

Collapse of structures risks injury, death and cost of downtime and repair. A small explosion or a seismic event can bring the dust down and create a massive explosion.

Environmental impact is often a case of measuring and complying with legislation and specific site rules. Exceed the limits and likely a fine is paid. Accurate measurement of residual dust levels on a large site is not simple. It often takes a visual emission to raise a problem. Other aspects of environment can be expensive and protracted, such as when the local community complains of coal dust on washing or similar. 

Cross contamination of products on stockpiles is another potential issue, either from discolouring or impact on the quality of the product. Dust shadows on stockpiles and open conveyors are highly visible from satellite maps.

Once an issue has been raised in the community, it may be difficult and expensive to mitigate in an acceptable manner and less efficient than designing an integrated system to manage dust from initiation.

What guidance is available to engineers and designers to design bulk materials handling plant to provide good dust management? 

Realistically, I can find little practical guidance. Electrical standards give guidance on zoning to limit the chance of ignition and other Standards on test processes and levels. None appear to consider dust in a dynamic environment, where bulk material is in motion on conveyors and the like and in storage where we fill and empty. There is some guidance on air flow velocities in dust collection ducting to prevent settling and excess wear. Guidance on any form of suppression, water, fog, foam is supplier specific or perhaps from previous experience. Available guidance assumes generally assumes dust is present and does not consider reducing dust generation. 

As designers and engineers, we still have a duty to minimise risk. The first item on the design list is to define the dust. As most materials handled are well known, we should be able to readily understand whether there are probable risks from toxicity, fire, or explosion. Any organic, or silica containing, or inherently toxic material is a definite risk, sodium salt probably not. Unless there is no proven risk from dust, we should dig deeper. We should also understand whether dust has value and should be returned to the process or be disposed. In some materials, high dust content reduces its value. 

The next issue should be to determine which additional data is required and how to obtain this. Where will we obtain data to assess dust content, toxicity, flammability and explosivity for design purposes? Material safety data sheets (MSDS) rarely have sufficient detail and are full of ‘not determined’ statements for these and other aspects. What else is needed; dust lift off and settling velocities? Web search? Possibly, but probably not the same as your material. 

When searching for nut coke information, the only reference found was for a US petroleum coke from 1970s, a wholly different material. Does the supplier have data? Not often in my experience. Specialised laboratories can test for all required information, but time and cost are considerations.

A recent plant modification occasionally handled a material that was known to be virtually dust free through many years of operation. A dust specialist was involved and directed that all electrical equipment should be zoned against explosions. We accepted this to keep the project moving, knowing that the specialist was unaware of how to test for explosivity or that five components are necessary to create explosion. We were unable to get data from the material supplier or end user, and the time (year waiting period) and cost to test for the data that could define the risk was too long. 

What environmental restrictions have been imposed on the site by State and Local Authority environmental departments? These may be physical, such as full enclosures, or limiting local and perimeter dust levels. Off-site disposal of dust may be prohibited or only to high-cost landfill.

With information on dust properties and requirements, we can move on to design considerations. Start at the top, for example in a mining operation is it possible to change the blast pattern to reduce dust generation? Can water sprays alone contain dust from a 400-tonne haul truck dump? Further along, can conveyor speeds be reduced to limit dust lift off? Is transfer chute design dust generating / containing? We took belt cuts throughout a sulphur system and proved approximately 0.5 percent increase in fine particles and dust at each non flow controlled transfer point. Are conveyor covers as effective at containing dust and facilitating clean up as a full gallery? Is the belt cleaning system effective and maintainable? Can a boom stacker be designed to follow the top of the stockpile to reduce drop height? Are telescopic chutes or DSH hoppers best for dust control for loading out? Is dust management integral to the design process, or an afterthought? The ANZ region includes many innovative suppliers of dust management equipment. 

Dust suppression is a good system where the dust is amenable, and the best option is to keep the dust in the product stream. Choice of suppression medium and addition rate are important. Location where suppressant is added is more important still. Belt cleaning must accommodate the added suppressant. Poorly designed and operated systems may result in site spillage and clean up. Spillage dries out to form a major fugitive dust source.

Dust suppression water sprays can be anything from a perforated plastic drink bottle or a pipe on the end of a hose hanging over a conveyor to more sophisticated devices, but I cannot find any methodology to determine how much water to add where to get the best results. Should water be sprayed over the conveyor, in the transfer chute or elsewhere in the system? Should the suppressant be added before stockpiling or via sprays onto the surface? Can water addition be managed to suit requirements via online moisture monitoring or blackbelt monitor? 

There are many industry “rules of thumb” regarding the percentage of water in the bulk to prevent dust emission, but few try to match water added to achieve this. Many suppressant systems do not function well because design does not consider maintenance. Randomly adding water is expensive and can have downstream handling, process, and shipping consequences. 

Water containing suppressants and foaming agents can be much more effective than plain water. Water usage may be cut by 90 per cent and the suppression effect lasts longer. The only guidance is from experience and suppliers, and there may be some development cost. Fogging systems use much less water than sprays and are more effective on finer dusts. 

Dust collection systems seem obvious solutions for dust management. They can be used to collect and return dust to the transport system or to remove dust for retreatment or disposal. Dust collector selection should be determined to suit the dust. Explosive dusts and baghouses must be compatible, as these are a common source of explosions when incorrectly configured. Scrubbers offer a wet product that may be a better option for some plants and toxic materials. Electrostatic and cyclone collectors are less common for bulk materials.

The main problems I observe with dust collection systems is collector size, lack of maintenance access to the system and the random routes of ducting that are generally determined by available support on existing structures to facilitate installation, with little regard to functionality such as flow resistance and maintenance. Ducting often resembles a large plumbing system with inappropriate bend radii and junctions, flanged connections, etc., even water valves, that provide significant pressure drops and low velocity zones where dust can settle. I recently saw a duct run with four 90-degree bends to match structure when a single 45-degree bend would have been appropriate. Another problem exists with systems that attempt to draw from many widely separated locations resulting in a spider web of lengthy, often horizontal, ductwork. These systems often have multiple blockages and no-flows because the air flow cannot be balanced. Dust collectors can only function if they have adequate capacity and dust laden air reaches the collector. Although two or more smaller systems will cost more, they are more likely to be functional.

A final design point relates to dew point. In a dust collection system, the air pressure is lower than atmospheric, and the dew point is lower than ambient. Will condensed water run to low point discharge, or will it mix with dust to form paste which coats and eventually blocks the inside of the ducting? 

Dust has more recognition as a bulk material problem than ever, and this focus will not disappear. We can manage dust well, but this does mean adapting better methods so that investment is in functional and effective systems. 

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