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Not so precious metals

By Linda Johnson
| www.cos-mag.com

A laboratory researcher at the University of Wisconsin-Madison was removing a few grams of titanium metal powder from a bag with a spatula when the powder suddenly caught fire. In an effort to put out the fire, the researcher doused the flames with water — which caused the fire to intensify. He then grabbed an ABC-rated fire extinguisher (which is an improper extinguishing agent in this situation) and was able to extinguish the fire due to the small amount of burning metal.

A later investigation concluded the fire was likely caused by static charge or chemical contamination of the spatula.

Titanium, along with magnesium, is one of the most common causes of burning metal, or Class D, fires. Combustible metals — which also include aluminum, lithium, potassium, zirconium and sodium — are used in many warehouses and factories, as well as laboratories. Not flammable in large pieces, these metals can ignite when a manufacturing process, such as sanding or grinding, produces “fines” (filings or shavings).

In the above case, the researcher, fortunately, was wearing his personal protective equipment (PPE) and did not sustain any injuries. It’s important employers provide all workers handling combustible metals with the appropriate PPE.

John McMahon, safety officer with Nova Scotia Community College in Halifax, says selection of PPE depends on the initial hazard assessment, when materials used, particle sizes and explosivity (KST test) are identified. Primarily, PPE must be flame resistant and provide insulation against the flash of the fire.

Basic body protection consists of coveralls, trousers and shirts, often made of cotton treated with a flame-resistant chemical, McMahon says. With other materials, such as Nomex, the fire-resistance property is inherent in the fibre.

Above all, he adds, synthetic materials should be avoided because they ignite easily, may melt into the skin and tend to accumulate static electricity so can become an ignition source.

Clothing should be designed not to collect dust, he adds. The weave should be tight, so material is smooth, with no cuffs, pockets or similar features. Dust attached to clothing can itself ignite if a person is exposed to a flash fire.

“If you did have an ignition source coming at you and you’re all wrapped up in dust, you’re going to be part of the ball of flame,” he says.

Protecting hands, feet

Several heat-resistant fabrics are used to make gloves. The polymers Kevlar, Nomex and PBI, which are used in firefighting suits, do not melt at high temperatures, says Patricia Dolez, a researcher at CTT Group Saint-Hyacinthe, Que. These fabrics are inherently fire resistant and, she adds, last longer and perform better than materials that are manufactured and then treated.

Polymers are increasingly replacing leather, she says. While it is effective, leather becomes hard and reduces dexterity when wet and with age.

Woven or knit gloves, made of Kevlar, Nomex or PBI, can be made stronger when dipped into a solution of neoprene or butyl rubber (both polymers) or silicone, Dolez says.

“You lose a bit of dexterity, but you gain some resistance to heat.”

Where the risk of heat is very high, gloves or mitts made of aluminized material may be selected, she says. The radiant heat is reflected off the aluminized surface.

Safety shoes should always be worn. Generally, high top (at least 150 millimetres high) leather safety shoes, without exposed metal, are recommended, says Guy Colonna, division manager with the Quincy, Mass.-based National Fire Protection Association (NFPA). NFPA 484 also states that trousers should cover the tops of footwear.

Socks and all other undergarments should be cotton, not synthetic, he adds. And any metal fasteners, snaps or zippers that may touch the skin should be avoided.

Shielding face, eyes

While good protective clothing for the body is available, there is not much for the head area, Colonna says. Currently, most of the materials used to make face shields and hard hats are not resistant to the extreme heat of a flash fire caused by combustible metals.

However, he adds, workers should wear eye protection. A safety shield will provide some initial protection. To better protect the eyes against the intense light that comes from a flash fire, workers — particularly those handling metals like lithium — should use welders’ goggles, which reduce the brilliance of the light.

“The face shield is ultimately going to melt. I can wear that kind of equipment; it will give me some protection. But if I am engulfed in an actual fireball, that type is equipment isn’t going to protect me,” he says.

PPE should be selected, too, with the goal of reducing static. Moving dust generates static, so workers required to move dust (such as cleaning out a dust collector) or who perform tasks likely to create an explosive atmosphere, should wear clothing made of static dissipative material, Colonna says.

“You don’t want your clothing to also be able to generate static. Then the static charge will be jumping off your clothing and may become an ignition source for the combustible dust atmosphere,” he says.

The protective ability of clothing and gloves can be greatly compromised by the presence of other materials, such as solvents, McMahon says. Moreover, these contaminants increase the chance the protective items themselves may ignite.

“You might be working with magnesium, for example, which is used in the aircraft industry. Then you spend part of the day in the paint shop, where there are petroleum solvents, greases or oils. So you’re actually picking up accelerants on your clothes. That can be a true concern,” he says.

In addition to removing surface contaminants, it is essential to inspect gloves carefully as they age, Dolez says. About 60 per cent to 70 per cent of performance can be lost before the deterioration becomes visible.

Heat accelerates aging, and oxygen and UV rays degrade polymers, she says. Aging causes loss of strength, which could cause material to break. Thus, it’s important to consider replacing gloves before the manufacturer’s suggested date.

Extinguishing metal fires

Along with proper PPE, it is very important employees understand what type of extinguishing agents to use — and not use — for metal fires. The type of metal determines the type of extinguishing agent. Generally, however, the best way to extinguish a Class D fire is by using a “dry powder” extinguishing agent, usually a graphite powder or sodium chloride mix, says Eugene Marotta, president of Ottawa-based Fire Prevention Canada. The agent must be absolutely dry — water reacts with burning metals and may spread the fire. Foams, carbon dioxide, nitrogen and halocarbon cleaning agents should also not be used.

The Class D extinguisher, which has a five-pointed yellow star on the label, is designed so the powder can be applied at low velocity.

“You apply a type D extinguisher at low velocity with a circular motion on top of the fire like a blanket, as opposed to a spraying action. It’s a gentle shower of chemical dust landing on top of the fire, so it smothers it,” says Marotta. “The fire doesn’t necessarily go out immediately. Metal fires are very hot. You have to be patient and wait until it cools down before you investigate whether it’s out.”

Employers are responsible for ensuring workers are trained on proper procedures in case they have a fire, says Ed Gadbois, fire training officer at Lakeland College in Vermillion, Alta. Workers should know what extinguisher to use, how to use it and where it is.

Extinguishers should be located within 75 feet of the area, he says. While other fire extinguishers have a reach-of-stream of about 12 feet to 15 feet, the dry powder extinguisher used for burning metals has a reach-of-stream of about 3 feet to 4 feet. Combustible metal fires can also be extinguished by sand or dirt provided in a large bucket close to the work area, adds Gadbois.

In the event of a fire, workers should make sure someone else knows.

“So shout ‘fire’ and activate an alarm. Even though you’re getting the extinguisher to put it out, make sure somebody is there to help in case it gets away from you,” says Gadbois.

Linda Johnson is a freelance writer based in Toronto. She can be reached at lindajohnson@sympatico.ca.

This article originally appeared in the February/March 2015 issue of COS.

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