Paper, metal, plastics can generate combustible dust in recycling facilities

In the 15 years before an explosion occurred that would destroy the A.L. Solutions metal recycling plant, there had been two fatal fires and explosions at the facility. The West Virginia plant, which processed scrap titanium and zirconium metal, continued to ignore safety standards and use inappropriate control measures. Then, in December 2010, a spark of heat in a defective blender ignited milled zirconium particulates, producing a flash fire that lifted some of the powder into the air and creating a burning metal dust cloud. The cloud, in turn, set fire to other dust in the facility, and a secondary explosion blew though the production plant. Three workers were killed and a contract employee injured.

The hazard of combustible dust explosion may be most associated with the wood and food industries, which have seen high-profile incidents in recent years, but the hazard presents a major risk in the recycling industry as well. Materials handled in recycling facilities are highly combustible and people are often unaware of the hazards they present during processing.

A dust explosion occurs when an explosive dust cloud (consisting of an adequately mixed fuel and oxidant) is formed and ignited by a sufficiently energetic ignition source in a confined or partially confined environment, says Paul Amyotte, professor of chemical engineering at Dalhousie University in Halifax.

In recycling, dusts or powders generated during the processing of paper, metal, plastics and rubber (rubber crumb) constitute the fuel. Ignition sources include welding, static electricity and equipment power systems. The confined space may be a building section or piece of equipment.

Processes commonly used in recycling — shredding, cutting and shaving — can create large amounts of combustible dusts. With some materials, handling and conveying can generate fine particles that may be hazardous. In the case of paper and cardboard, for example, the hazard is greatest at the beginning of the processing, says Graeme Norval, associate chair and undergraduate co-ordinator in the department of chemical engineering and applied chemistry at the University of Toronto.

“When they bring in the dry paper and they’re cutting up the bales, they have to shred that down into fine particle size. That’s where you can get dust coming off and airborne matter,” he says. “Once they put it all into water, and they do the dispersion to get the inks off, it’s all wet, there’s no dust. So, dust is not a problem in the whole plant, just in parts of the plant when the material is dry.”

In metal recycling, small particles are created through a range of processes, which, in addition to milling, include scrap chopping, cutting, handling, sawing and filing. Some of these processes, especially baling, compacting and shredding, produce significant amounts of dusts.

Among the most hazardous metal dusts and powders are aluminum, magnesium, tantalum, titanium and zirconium. While metal dust is a known hazard, Norval says, people generally underestimate the likelihood of explosions caused by metal processing.

“When people think about metals in their day-to-day life, they think they aren’t reactive (the quality of reacting spontaneously, rapidly and with heat). But aluminum or iron dusts are actually quite reactive. If you shred that dust finely and get it dispersed in the air and with a large enough amount of heat, it can take off,” he says.

The hazard severity of plastic dusts varies according to the type of plastic and additives. Fibreglass, for example, tends to smoulder rather than flare up and burn, Norval says. Moreover, companies often use processes that avoid dust creation.

“Most places, when they recycle the polymers don’t get into the same processes. They can melt it down and process it as a hot liquid rather than having to shred it,” he says.

However, many dust explosions involve plastic materials. For example, the processing of polyethylene, the plastic used to make grocery bags, presents dust explosion hazards, Amyotte says.

“So polymers like that, plastics, resins, they’re quite hazardous. If you’re shredding plastics, if you’re reducing the particles and generating particles of material down in the explosive range, maybe 100 or 200 microns (1,000 microns = one millimetre), you have to be careful.”

The presence of combustible plastic in many appliances has made their recycling more hazardous. According to a 2005 study on dust explosions in industrial wastes, the recycling of refrigerators is particularly dangerous, in part due to the use of combustible insulation materials, such as polyurethane. This use, the researchers say “produces explosive dust clouds during the crushing operation and transportation of shredded materials.”

Moreover, the foaming agent used for the polyurethane is cyclopentane, a flammable gas. Together, the substances create an explosive atmosphere around dust particles, so any impact, friction or collision caused by crushing or compacting machines will ignite the dust cloud.

“These findings show that the dusts produced in the recycling process can lead to dust explosion easily,” the study concludes.

Because of the number of different kinds of dust that may be present in a facility, companies should have their dusts analyzed, says Simon Fridlyand, president of S.A.F.E. Engineering in Toronto.

“In the recycling industry, you may be handling all sorts of types of products, so you may have different types of dust. One dust is different from another as far as the explosive characteristics are concerned, so it is difficult to establish the different characteristics of the dust unless you test,” he says.

Testing determines the major parameters such as Kst value (maximum rate of pressure rise); MIE (minimum ignition energy); and Pmax (maximum pressure in constant-volume explosion).

Dust explosions are prevented — and their impact limited — primarily through a good housekeeping program. Employees must be cleaning regularly, Fridlyand says, especially to remove dust accumulated on elevated surfaces and hard-to-see building structures.

“Where you have ledges or a joist in the air, that’s where the dust buildup takes place and where there may be a thick layer of dust,” he says. “If you have even a minor explosion, it could cause a second and even a third. It’s like a cascade. And that could be devastating, destroying entire buildings and complexes. It is very important to make sure there is no dust accumulation.”

Another basic prevention strategy is a dust collection system, which extracts dust in the area where it is generated, he adds, such as at shredding machines. The system must be designed to mitigate a potential dust explosion inside the system itself (through venting, for example).

Collection systems and other prevention and protection equipment must be designed and positioned by professional engineers. In Ontario, Fridlyand says, under the PSR (pre-start health and safety review) program, all new and modified equipment needs to be registered to ensure its compliance with current and applicable standards.

In recycling facilities, too, it’s important to separate hazardous materials and processes. As the NFPA 484: Standard for Combustible Metals advises, combustible metals should be separated from other combustible material, such as wood pallets and corrugated cardboard, that could provide extra fuel in case of a fire.

Other protection methods include:

• grounding and bonding (to remove electrostatic ignition sources)

• hot-work permitting

• explosion relief (such as explosion relief walls to prevent building collapse)

• explosion relief venting

• automatic dust explosion suppression systems

• mechanical isolation valves.

Training fundamentals

Managers and workers need to be trained in the fundamentals of a dust explosion, Amyotte says. By understanding the explosion pentagon — fuel, oxidant, ignition source, confinement and mixing or suspension — they can identify the conditions they need to try to eliminate.

Thus, everyone understands proper housekeeping will eliminate the fuel. They will also be looking for ways to reduce ignition sources and prevent actions that cause dust to be raised and suspended in an open area. Of course, managers should also ensure workers understand the hazardous properties of the specific dusts being handled, usually through the use of safety data sheets.

“I go back to education,” Amyotte says. “If vents are being used in your facility, operators should know why they’re there and what problem they’re dealing with. Managers should know what the frequency of inspection needs to be and make sure those inspections are done. All these different levels of responsibility mean you need different knowledge. But, everyone should have the fundamental training in processing a combustible dust.”

The A.L. Solutions explosion in 2010 caused the shutdown of the plant. Following its investigation, the United States Chemical Safety Board recommended the company comply with NFPA 484, develop training materials addressing combustible dust hazards, train employees and conduct regular refresher training.

Norval says fires and explosions in recycling facilities happen more often than people think. He advises managers to never rely on any particular control measure, such as dust analysis, or on workers always following procedures.

“If we plan for the worst and make sure the worst can never happen, then we’re making sure things don’t happen,” he says. “That’s prevention.”

Linda Johnson is a freelance writer based in Toronto. She can be reached at [email protected].

This article originally appeared in the October/November 2015 issue of COS.