Suppress, Collect & Contain Silica Dust at the Conveyor Belt
BY AsphaltPro Staff
In June 2023, the Mine Safety and Health Administration (MSHA) proposed a rule to lower exposure limits for respirable crystalline silica to a permissible exposure limit (PEL) of 50 µg/m3 for a full-shift exposure, calculated as an 8-hour time-weighted average. “The amendment basically cuts by half the allowance of workers’ exposure to respirable silica dust,” said Jerad Heitzler, lead instructor for Martin Engineering, Neponset, Illinois. Although some in the mining industry might anticipate slow adoption of the amendment, Heitzler said the new rule “has all the teeth it needs to be enforced.”
“If you’re in the mining industry, I think it’s advantageous to get ahead of this and have an idea of if your facility may be exposing workers beyond the permissible level,” Heitzler said. The rule states the companies exceeding the new exposure limits must take immediate corrective action such as changes to job scheduling and job tasks or the use of respirators.
However, as Heitzler reads the rule, MSHA is eventually expecting mines to comply with the rule primarily via engineered controls that remove or reduce the hazard at the source, such as suppression, collection or containment.
During a recent Martin Engineering webinar, Heitzler cited a study of German coal mines, which found that 85% of all dust came from conveyor belts. “That’s only one study, but what we need to take away from it is that conveyor belts contribute quite a bit [of dust],” he said. “That may not be the case at your facility, but you can’t underestimate how dirty and dusty conveyor belt transfer points can be.”
Here, Heitzler walks us through some best practices for the use of engineered controls at the conveyor belt.
Respirable dust (smaller than 10 µm) comprises particles so small that they are invisible under normal lighting conditions. “Airborne dust gets caught up in the mucous membranes of the nose and the throat, nose hair, etc., but respirable dust is so small that it bypasses those natural filters,” Heitzler said.
Suppression
The first category of engineered controls is suppression, which uses either water or water with surfactant to suppress the dust. “Suppression manipulates the weight of the dust, helping the particles fall out of suspension,” Heitzler said. “It also manipulates cohesiveness, so the dust particles are more likely to attach to one another. If you get enough cohesion of that material, that respirable dust particle becomes bigger and takes less air speed for it to fall out of suspense.”
However, there are several concerns with suppression. First of all, the risk of freezing in colder climates. And second, respirable silica dust may be so fine that water alone may not be enough; surfactants may be required to change the water surface tension and droplet size. Nozzle configuration is also important here. “Nozzle configuration really causes that water droplet to be smaller, which lets it connect or attach to that dust to increase its cohesiveness and weight,” Heitzler said.
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He also cautions on the use of high volume and high pressure suppression. “It’s beneficial to use more nozzles at lower rates than one nozzle at a higher rate,” Heitzler said. According to the Industrial Minerals Dust Control Handbook, which Heitzler highly recommends, one should only start with 0.5% moisture-to-product ratio and the spray nozzle should be at the beginning of the process. Fog nozzles, on the other hand, need to be at the exit.
The other consideration to make with suppression tactics are the potential unintended consequences. “If you keep adding moisture at different points to try to control the dust, you’ve radically changed your product by the end of those processes,” Heitzler said. It can also lead to screens being blinded off, pluggage build up on chutes, and the amount of carry-back resulting in excess cleanup.
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Collection
The second method of engineered controls is collection. Heitzler highlights two types of collection systems: central ventilation/dust collection and locally ventralized/insertable dust collection.
The former are widely used in a variety of industries, Heitzler said, but are expensive and require frequent maintenance. He prefers the latter. “Locally ventralized units are machine mounted units that vacuum the dust, in this instance, off of a transfer point and then conglomerate that dust into a filter that is simply changed out.”
According to studies Martin Engineering has performed on insertable units, the company has found they usually cost 50% less than a central unit and save 35% of the energy. “I’m not sure what the savings on maintenance would be, but I imagine they’re significant,” Heitzler said.
If using a central unit, Heitzler said pick up points should be at least six feet away from the end of a transfer point and air speeds inside of a transfer point should be less than 500 feet per minute. “If they’re more than 500 feet per minute, a dust collection system is going to be ineffective.”
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Containment
“The most effective way to control dust at a conveyor point transfer point is containment,” Heitzler said. “In many cases, we’ve completely eliminated a facility’s reliance on collection or suppression simply by manipulating the air speed inside of a transfer point to eliminate the need for suppression or collection.”
According to Heitzler, dust isn’t a problem unless it’s exposed to air speeds that can carry it around. “By slowing down these air speeds to under 200 feet per minute, the dust falls out of that air flow.”
Heitzler provided one example of a facility using rubber to block excess air from escaping at the transfer point. “The less air that you have to deal with exiting the transfer point, the slower the air speeds will be, the less dust you’ll have,” he said.
In another example, the facility used a passive dust collection system of bags.
“The idea of this is the air will hit a curtain as it exits, and the path of least resistance for that air will be up into [a] bag,” Heitzler said. “Some of that air will vent through that bag, but the respirable dust will be captured by that bag and fall down back onto the cargo.”
Skirt board exit curtains are an important part of an effective containment strategy. “The idea of these curtains is that the curtains don’t interrupt the cargo, but the curtains do disrupt the air. It helps slow the air speed to under 200 feet per minute,” he said, adding that the traditional arrangement of spacing curtains about a belt width apart tends to be the least expensive and most effective, though “you might need multiple sets depending on what the overall airspeeds are.”
Heitzler said containment is more effective when the vertical fall of the material is minimized. “If you’re dropping cargo 50 feet from one belt to another, you might need some suppression or collection,” he said. “If you’re dropping 20 feet or less, you should take a strong look at containment because it’s incredibly effective at those drops.”
What about PPE? In the rule, MSHA states respirator use must be non-routine and that workers wearing respirators to reduce their exposure must undergo medical evaluations. “The way I understand it is that respirators can be used until you get those engineered controls [established],” Heitzler said. The problem with respirators, he continues, is that workers are protected so long as they wear the respirator, however, once they remove the respirator, that dust remains on their hair, clothes, etc. “Then they inhale it all day long until they wash it off, so it could still be problematic.”
Whether you find suppression, collection or containment to be the best strategy for your facility, Heitzler stresses the importance of minimizing dust—and not only because of the new MSHA standards.
“We underestimate how important [dust containment] is to attracting and retaining workers,” he said. “If we bring a new operator to the mine and on his first day he walks in and is exposed to that dust and has to wear a respirator, guess what he’s doing when he comes home at the end of the day? He’s searching for a new job.”
“Not only is dust control critical to protecting your workers,” he continued, “but I think as we move forward in the mining industry, we’ve got to make these facilities cleaner and safer in order to retain workers.”