Features, Materials Handling, OH&S News

Managing dust

Dust

Kinder mechanical engineer Jack Cain discusses processes to control the exposure of dust in quarrying operations. 

The dust that comes from mineral production, crushing or even from handling aggregates requires management. Luckily there are a number of measures that business can take to reduce dust level exposure to workers and the environment.

According to Kinder mechanical engineer Jack Cain, the formation of this dust is not entirely unwelcome. 

“In some cases, the dust is the intended product,” he said.

“So efforts to negate it are rarely required, nor are they entirely plausible.

“Therefore, the more auspicious approach is to control the spread of this by-product by removing any possible avenues of emission into the environment and exposure to workers.” 

Dust tends to gather in transfer points. If it is discharged into open air, can end up in stockpiles rather than in sealed chutes or hoppers. 

And if the product is exposed to environmental factors such as wind, it can disperse the top layers.

“In any areas where vibration is common, where this jolting movement can bounce the material around, it can cause unsettled particles to wander off the conveyor,” Cain said. 

“The movement of the belt starting, stopping or even just when travelling at high speeds can be all that’s needed to disturb the top layers of material.”

Dust fineness is measured based on particulate matter (PM) size. 

Fine particles (PM2.5) are smaller than 2.5 μm (1 μm = 0.001mm), coarse particles (PM10) are sized between 2.5 μm and 10 μm and “larger than PM10” is for all particles with a diameter of 10 μm or greater.

The occupational exposure limits of respirable dust and respirable crystalline silica is 3.0mg/m3 and 0.05mg/m3, respectively across an eight-hour shift. This includes 10mg/m3 of inhalable dust. 

It is important exposure to dust is maintained within stated exposure standards to mitigate any risks. 

Seeking a solution

Despite the numerous risks associated with dust particles, there are certainly ways reduce exposure.  

“Thankfully, when it comes to mitigating exposure to dust, modern conveyor solutions provide quite the arsenal,” Cain said. 

“Take the transfer, for example, likely the dustiest place on any conveyor.”

According to Cain, skirting is one of the most effective methods of keeping material from spilling out of transfer the moment it touches the belt. 

“Ideally you would want skirts that provide some sort of contact seal, but when the diameter of dust is measured in microns, even a 1mm gap is enough for millions of particles to escape,” he said. 

“So, a proper seal needs this belt contact to be effective, and contact means wear so a skirt needs a way to combat wear,” Cain explained. 

“A self-adjusting skirt, that maintains full effectiveness for its entire lifecycle without the need for direct maintenance is incredibly cost-effective. 

“These self-adjusting skirts are typically quite soft to allow for them to conform to the belt unhindered.”

Cain explained that this softness leaves the skirts vulnerable to the harshness of the material stream, which can cause damage as it flows by.

“So, typically a hard internal skirt that stops just short of the belt is used to keep the material at bay, while the soft skirt picks up the slack and completes the contact seal in relative safety,” he said. 

Cain recommends contactless external skirts “if the wear and tear of contact systems rubs you the wrong way”.

“While they do tend to hit a higher price point in the short term, these marvellous devices tend to make up for it with their almost non-existent maintenance costs,” he pointed out.

“Using their unique geometry to manipulate the power of airflow, a continuous suction effect is created to keep dust in. 

“These contactless skirts still require the support of an internal hard skirt to make sure they don’t clog up with the larger product, but their overall effectiveness is impressive. 

“Of course, with all this skirting it’s imperative that your belt profile is well supported to maintain consistency and prevent gaps, even more so with contactless skirting which requires a very particular spacing for the proper suction flow to be possible,” Cain explained. 

According to Cain, how you support your belt will depend on the conditions of your transfer point.

“Systems that experience low impact from a small fall height or light products can get away with static belt support systems, which provide little in the way of impact support but help maintain that perfect, flat surface across the transfer,” he said. 

“Environments that require at least some impact support should invest in an impact bed, a large structure with flexible rubber rails that alleviate some of the impact force from the belt. 

“For situations where severe impacts are common, a dynamic bed may be required,” he continued.

“These utilise a trough panel suspended by spring elements to support the belt, while the static outer rails maintain the edge profile. 

“These systems all tend to utilise low frictions rails (typically < 0.3 static coefficient of friction) on the wings to create a consistent surface for the belt to move across.

“However, this is still an increase in friction over most rollers. 

“While these rails typically quite resistant to abrasion, they will eventually require replacement as well,” Cain said. 

Looking at the whole picture

Mitigating dust cannot be solved solely at the transfer and needs to be managed across the rest of the conveyor. 

“Compared to the transfer point, the run of the belt typically has far less structure to encase the product and protect it from the open elements,” Cain said.

This requires special equipment adapted to such constraints. 

“Belt covers are physical barriers that attach to the stringers and almost completely enclose the belt, preventing the influence of wind and trapping dust that is disturbed by vibration and belt movement,” Cain said. 

“Belt covers typically are made from rigid materials such as steel or reinforced plastics, which are effective barriers, simple to install, and can be modified with features like inspection and maintenance windows.

“Alternatively, more flexible materials like mesh cloth can be used, which maintain airflow and a degree of visibility while preventing dust migration.”

Water sprays can also be used as an effective dust-suppression tool, though they are not without their drawbacks. 

“The resultant water saturation is undesirable for a lot of products and the wastewater must be dealt with, limiting their desirability for numerous applications,” Cain said.

Likewise, extraction fans can be effective when run across the belt length, but filters must be replaced to maintain effectiveness. 

“Unlike the belt run, discharge points can usually be encased in structure with dust curtains to prevent any dust spreading, but when material is being discharged into open space, an extendable loading chute is more appropriate as it can be used to guide the material down to the stockpile without excessive dust being thrown during the fall,” Cain said. 

Prevention is key

“Controlling and containing dust is simple and affordable with an abundance of modern products,” Cain said.

Cain recommends operators evaluate their conveyor and determine any locations that are a source of dust, so a solution can be applied. 

“Operators who would prefer evaluations confirmed or would prefer professional input on the severity of issues can seek conveyor inspections from the numerous available conveyor solutions providers,” Cain concluded. •

Visit kinder.com.au for more information.

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