Controlling dust in materials recycling

Recycling is now firmly at the heart of the raw material supply chain across Europe where companies specializing in materials management – whether that’s commercial wastes, industrial byproducts, metals, electronics, biomass, card and paper, or plastics – are maximizing the value of their feedstocks, while simultaneously improving production efficiency.

Within these waste streams, fine materials (which manifest as dust) can account for a substantial proportion of the total output, necessitating measures to control and contain spillage and emissions. Difficult and costly to dispose of, dust can also create problems throughout recycling operations.

Fugitive dust emissions create an unhealthy work environment in and around the plant. To make matters worse, certain wastes emit toxic particulates when processed, especially materials with corrosive properties or those containing fragments of adhesives, coatings and other substances considered hazardous.

Nuisance, inhalable and respirable

Although most of this particulate matter (PM) is deemed little more than a nuisance, particles ≤ 100 μm in size are considered “inhalable” and particles smaller than PM 40 (≤ 40 μm) are usually invisible to the naked eye. Many recycling processes create particle emissions of less than PM 10 (≤ 10 μm) which are considered “respirable”. And PM 2.5 (≤ 2.5 μm), known as fine particles, can penetrate deeply into the lungs, reaching the alveoli (the tiny air sacs where oxygen exchange occurs). At this level some particles cross into the bloodstream.

For particles of all sizes, without regular manual cleaning, dust builds up, covering walkways and stairs, engulfing control units, obscuring signage and eventually making access impossible. Overburdened maintenance teams can struggle to keep on top of the cleanup which, in turn, can lead to safety shortcuts as well as the risk to workers’ respiratory health. In some recycling operations, fine airborne materials are combustible, leading to a significant risk of dust fires and explosions.

Additionally, dust is known to foul exposed machine components, causing them to wear quicker and require servicing and replacement sooner. Particulates also clog air intakes of equipment, prompting the need for extra maintenance and downtime. 

With the exception of crushing, grinding and shredding, the most common sources of dust in any recycling plant are conveyor transfer points. As loose material drops onto a conveyor belt, particulates disperse into the air. At the other end of the conveyor, fines that fail to discharge at the head pulley get carried back and are dispersed as dust and spillage along the return path of the system.

Containment is critical

Years of on-site observation and research have proven that the starting point to improve air quality around conveyors is to contain the discharge. This is best achieved by tackling the underlying issues through a comprehensive evaluation of the root causes of dust then optimising the belt transfer design and operation at source.

Poor transfer point design is among the main causes of dust emissions. Air flow induced by the continuous discharge of material pushes air through the enclosure of a transfer point, and the subsequent turbulence forces the airborne dust out through any available opening. Operators generally find that passive dust control through retroactive transfer point design changes are more practical and cost effective than misting systems or HVAC solutions.

Increases in production volumes and conveyor speeds can exacerbate existing dust issues. If the system was not originally designed for greater throughput, then airborne dust, increased material build-ups from spillage and belt mistracking are inevitable. This can result in more frequent stoppages and increased exposure to hazards. 

Chute first, ask questions later

A well-designed transfer chute has several key features that minimise the dispersal of fine material as it drops and settles onto a conveyor belt.

Firstly, the length, angle, size, and shape of a chute dictate precisely how material arrives onto the belt below. Optimising these factors depends on numerous variables, but some designs perform better than others. For example, with centered, sloped and spoon-shaped chute configurations, material is eased onto the belt with less impact and little or no rebounding. This design reduces the risk of belt damage, lowers air turbulence and minimizes load shifting, which is a main cause of mistracking.

The use of wearliners prevents the falling materials from damaging the skirtboard. This also protects the skirting designed to seal the transfer point, increasing the equipment life. Additionally, fitting externally facing wearliners enables easy access to enclosures for faster and safer maintenance. These designs also raise the chute height to allow more room for dust to settle in the stilling zone.

Skirt around the subject

Along the length of the loading zone, the entire enclosure needs to be well-sealed with belt skirting:

Single skirting should be cut to the shape of the belt’s trough angle for a tighter seal. The best designs can be mounted externally for easy and safe adjustment.

Dual skirting is an engineered design incorporating a primary and secondary seal that provides an additional layer of protection from spillage and fugitive dust. Dual skirting is available in urethane and rubber.

Self-adjusting skirting systems use gravity to deliver light downward pressure to minimize the need for maintenance and can be the right solution in some situations.

Cradling the impact

The weight of material landing onto a conveyor can cause the belt to sag between the skirting and idlers. The efficiency and performance of a transfer point can be greatly improved with the installation of cradle support beneath the belt. Impact cradles absorb the force of falling material to prevent damage to the belt and structure. The shock-absorbing bars help seal the environment and mitigate spillage. In addition, fitted under the belt, slider cradles with abrasion-resistant polyurethane bars retain the seal along the entire length of the skirtboard, producing an air-tight settling zone.

Curtain call

After the initial impact, turbulent air can be steadied by extending the transfer chute enclosure along a specified length of conveyor, providing a larger space for larger particulates to drop back onto the belt. While some flow of air is still going to be prevalent, the key is slowing it to under 1 m/s, slow enough for settlement to happen. Adding a tail panel and dust curtains is essential to slowing airflow, but simply adding them at the ends does not necessarily accomplish the proper stilling environment required. 

Understanding the air flow and then strategic curtain placement is the key to increasing the length of travel for the dust particles giving them more time to agglomerate and settle onto the belt. Chutes that are properly sealed and retrofitted with three curtain zones recirculate the airflow and allow dust to settle. Field tests show a drastic reduction in emissions of both nuisance and respirable dust with the correct curtain set-up.

Collective decision

The transfer point features described above can be collectively referred to as “passive dust control”, using engineered design solutions to minimise the creation of airborne dust and direct it back into the material flow. However, in some circumstances, such as when there are length or space restrictions for chutes which prevent an extended settling zone, then dust bags and mechanical air cleaning devices may be necessary. They typically harness the airflow to collect dust deposit it back onto the belt. However, they can require more maintenance and monitoring, so sealing is always preferred to minimise servicing costs.

Discharge debris

Fugitive material at the discharge zone can also be problematic. Carryback from inadequate belt cleaning can lead to spillage and cause airborne material along the entire length of the belt return. Besides the build-up of material beneath the system, conveyors exposed to wind and weather can spread fugitive dust across a wider area.

To prevent carryback the optimum cleaning solution requires technologies that reduce the need for regular maintenance and inspection help to prevent unscheduled downtime. When it comes to the kinds of materials being recycled, leading manufacturers of belt cleaners have devised durable solutions capable of cleaning belts of the most abrasive, corrosive, acidic, or liquid substances without causing any damage to the belt itself.

Conclusion

The diversity of waste feedstocks and the process of crushing, shredding and grinding invariably give rise to large quantities of particulates which can easily become airborne if not tightly controlled. While conveyor transfer points are not the only source of dust, they represent one of the most prevalent generators of particulate emissions.

Dust is one of the fastest-growing industrial health issues, and solutions to control it are available. Beyond health and safety, eliminating airborne dust delivers numerous benefits, including better housekeeping, improved environmental management, less maintenance, process efficiency, greater productivity and whole-life cost reduction.

For a complete guide to Dust Management, refer to Martin Engineering’s Foundations book Section 4 (info.martin-eng.com/foundations-book-download-page-25).

By addressing root causes, following best practices and installing quality components, recycling operators can tackle dust methodically. Once the major dust sources are addressed, eliminating emissions from other parts of the operation becomes easier, with the ultimate goal of a completely clean, efficient and above all safe operation. 

Case study: Energy-from-waste processing in Germany

A major energy-from-waste plant in Germany processes around 300 000 t of waste annually. The facility handles a variety of materials and generates energy in the form of electricity and heat, while also recovery residual materials for reuse. A key part of the operation are its 40 conveyor belts – from 800 mm to 1200 mm in width and typically running at 1.3 m/s – used to transport abrasive materials through the process. Persistent carryback on most of the conveyors led to excessive cleaning demands, posing a problem for productivity and creating potential safety risks.

Martin Engineering’s experts visited the site and recommended an innovative belt cleaning solution – the CleanScrape^® Primary Cleaner and the highly effective SQC2™ Secondary Cleaner. Both types of cleaners are perfectly suited for tough conditions where space is limited. They have long service life and deliver effective belt cleaning without risk to belt or splice. CleanScrape^® Primary Cleaner is especially durable and has a unique design: it is installed diagonally across the discharge pulley forming a three-dimensional curve, and incorporates a matrix of tungsten carbide tips. While applying minimal pressure to the belt, it removes hardened material without damaging the belt.

A total of 30 CleanScrape^® Primary Cleaners and 5 SQC2™ Secondary Cleaners were installed on the affected conveyors. The installation significantly reduced the amount of cleaning required, as well as unscheduled downtime, thereby reducing the cost of operation. Thanks to the optimised belt cleaning performance brought by the CleanScrape^® Primary Cleaner, as well as by the combination of the primary and secondary cleaners and their durability, the plant achieved a higher level of productivity with much less maintenance.