Tuesday 30th Nov, 2021

Dust – do we need it?

Sean Kinder, the Field Applications Engineer at Kinder Australia, explains how engineering controls can drastically reduce dust emissions.

Sean Kinder, the Field Applications Engineer at Kinder Australia, explains how engineering controls can drastically reduce dust emissions.

Dust is the uncontrollable fugitive emission from a process which escapes into the environment. It can occur in almost all bulk material processing industries.

Construction industries require fine aggregates and sands from quarries for road base and concrete. To extract gold, copper, lead, zinc, and nickel from their parent ores in a mine, the processing plant must grind the material to an ultra-fine particulate where it can be separated. Crushing and refining of bauxite into alumina powders is the manufacturing technique of smelting aluminium.

Unfortunately, given its physical properties, dust can cause respiratory and cardiovascular disorders. According to Safe Work Australia to reduce the risks of silicosis an agreed respirable exposure of crystalline silica will be reduced to a time weighted average of 0.05 milligrams per cubic metre. Coal mines also must contend with the hazardous dust particles where prolonged exposure can lead to coal workers pneumoconiosis or chronic obstructive pulmonary disease.

To manage risks, organisations and individuals should perform a risk assessment which may involve utilising the hierarchy of control (elimination, substitution, engineering controls, administrative controls, and personal protective equipment). Elimination and substitution are borderline impossible given that most bulk material handling process facilities require the dust to obtain their final product. Therefore, engineering and administration controls must be utilised with personal protective equipment technologies.

Related stories:

Dust has been observed to occur mostly when presented with an unnatural excitation of its surrounding environment due to turbulence. In a processing plant for example, dust forms in crushing and screening chambers and poorly designed conveyor transfer chutes. It’s observed that if the output air pressure is not equal to the introduced air pressure, and lighter than the processed asset, material in the form of dust will fill all voids in the system and escape via any means possible (holes, gaps). Being a lighter material, fines when excited, take longer to settle than the bulk handled. When designs don’t take into consideration the settling time of the finer particles, they remain in the atmosphere resulting in emission. In some cases, the air pressure can be so great that it will expel from system at a higher volume than the design capacity of the system.

Engineering controls are the first line of defence against dust. To minimise dust emissions from conveyor transfer points, a skirting system is installed in the loading zones to seal accordingly. Skirting systems are generally made up of a hard-internal primary skirt (which holds the larger asset), protecting it from damaging the secondary skirting. Secondary skirting (made of rubber or polyurethane) seals the transfer point from emitting dust into the environment. Unfortunately, the skirting systems are often a rushed component of the overall conveyor design, often being inserted almost during the commissioning phase of projects which prove a lack understanding. Generally, an area of cost cutting and poorly designed, transfer points and skirting systems are one of the first locations to be reviewed during the first months of a new operation. On a conveyor system, they would be considered one of the most frequently complained about locations due to their confined working conditions and dusty environments (which require continuous cleaning by plant operators).

There is a variety of different types of skirtings that can be installed. Selection should always be considered early on during the design stages of the project plan as a number of factors need to be taken into consideration, to ensure that an engineered control is being provided. Some skirts apply a vertical seal, whereas other lay-in and offer self-adjustment. Mounting and clamping systems also vary depend on the allowable heights provided by the surrounding steelwork. Skirting should never be fouled by ‘bolting or screwing through’. By doing this, once the skirting wears away gaps will form and the skirting will not be able to be adjusted leaving a permanent dust emission point, until the skirting is fully replaced. Depending on production requirements, this might be left in this position for several weeks.

An important subject which is often misplaced is that of freeboard, the distance between the outer edge of the chute and the edge of the conveyor belt. This is the space where the secondary dust skirting is installed. Often, in the event of higher loading on to the conveyor belt and increasing throughput, freeboard is sacrificed. This is very problematic for a secondary skirting system. If the conveyor belt mistracks, which is common in transfer points too due to many factors, it will creep under the skirting, popping the skirting material and create gaps. This allows dust to escape and results in emission. An added dilemma is when the conveyor belt tracks back to a central position (or even the other side if conditions are worse), the conveyor belt is known to cut through and prematurely fail the popped skirt. Freeboard should always be considered in conveyor transfer point design.

Conveyor belt sag between idlers in transfer points is another common observed dust emission point on conveyor systems. Sagging is the result of material loading on to the flexible conveyor belt. The weight of the bulk material enables conveyor belt to dip in unsupported areas. Although sag is generally only few millimetres, it is enough for a gap to form between the secondary skirting and the conveyor belt resulting in emission. Sag can be eliminated by utilising a conveyor belt support system or impact bed (if the application includes large primary or secondary lump bulk materials).

Compromising the skirting system and conveyor belt performance itself, some conveyors are designed and fabricated without the understanding of a designated transition and transfer point. The transition point is defined as the area in which the conveyor belt changes its profile from zero degrees (horizontal when it wraps around a pulley) to the desired troughing angle. In confined locations, some conveyors have seen to have ultra-short, borderline non-existent transition points. This results in the bulk material impacting and loading on to the conveyor belt, while the conveyor belt is forming from a zero to 35-degree (or steeper) trough profile.

Attempting to apply a secondary skirting system to an application that is changing its profile seldom functions as intended and generally always result indust emissions.

Another common way of controlling dust is with the addition of water. By adhering to the dust particles, clumping them together and increasing the weight, water quickly supresses the dust slowing is projection velocity and causing it to lose momentum allowing it to fall into the material flow. This technique when implemented well by professionals can dramatically reduce the dust emissions in many applications by carefully and methodically. Attempting to add water via a hose without taking into consideration many other parameters can lead to costly issues including oversaturation of product (material loses specification) or plant blockage due to screens blinding or chutes hanging up.

Important in production of materials, dust will be impossible to eliminate within the bulk materials handling industry. It does need management though. Utilising engineering controls and taking progressive and innovative steps, dust emissions and safety standards can be improved.