March/April 1991
All metals can be dangerous in their molten form, but aluminum--one of the few materials routinely melted by scrap processors--has particularly serious implications.
By Michael Mattia
Michael Mattia is director of risk management for the institute of Scrap Recycling Industries (Washington, D.C.).
During a tour of the Reynolds Aluminum Recycling facilities in Richmond, Va., Sisco Griffith points to a 4-foot piece of solid metal embedded 30 feet up into the side of a building. "A reminder of an aluminum explosion 20 years ago," explains the scrap buyer for Reynolds.
The embedded metal is an eerie symbol of the potential power of a molten aluminum explosion. Due to its low melting temperature (approximately 1,100 degrees F), aluminum can be easily sweated off of ferrous scrap or directly melted into molds for transportation to processing facilities. The danger in these operations comes from the violent reaction that occurs when molten aluminum combines with a variety of substances.
The Dangers of Water
Water introduced into molten aluminum can cause a dangerous explosion because of either a physical or a chemical reaction. A physical reaction occurs when water reacts to the molten aluminum to form pockets of steam. These steam pockets can quickly expand, causing molten metal to be thrown out of the melting furnace. These steam pockets have thrown molten aluminum up to 30 feet in all directions.
A chemical reaction can occur at high temperatures, when aluminum reacts with water to form aluminum oxide and release hydrogen, says James Wood, Resource Consultants Inc. (Nashville, Tenn.). This chemical reaction is exothermic: It releases both heat and energy. In addition, because the released hydrogen is exposed to temperatures that far exceed its autoignition temperature of 751 degrees F, an explosion is highly probable.
Avoiding Problems
Where does the water come from? In scrap materials, moisture contamination can include:
liquid in used beverage containers, radiators, and engine parts,
liquid trapped in closed containers,
crimped pipe, tubing, or bales of processed scrap, and
condensation from cold metal.
Moisture can also be introduced into molten aluminum when charging large aluminum sows, which can contain embedded moisture that was sealed in during casting. Unless recognized with precautions taken to slowly charge them into the molten metal, these sows could quickly release the moisture, causing an explosion.
Moisture can pose a problem when pouring molten aluminum into molds, too, if the molds contain any moisture. To eliminate such risk, the molds should be preheated immediately prior to the introduction of the aluminum.
An explosion also is possible from molten aluminum contacting water during a bleed-out in the direct chill casting process. According to Seymour G. Epstein, technical director for the Aluminum Association Inc. (Washington, D.C.), this situation can produce an aluminum-water explosion when considerable quantities of molten metal collect below the water's surface and an unknown triggering action takes place.
Epstein notes that these explosions may be prevented by:
maintaining sufficient water depth that allows the metal to sufficiently cool before striking the bottom and
applying a protective organic coating on exposed surfaces.
Extending the Hazard List
The potential for molten aluminum explosions also exists when residues of nitrates are in scrap. Nitrates may explode when shocked, when exposed to heat or flame, or by spontaneous chemical reaction.
According to Wood, inorganic nitrates (noncarbon compounds) pose a greater explosion hazard than organic nitrates (compounds of carbon) because they quickly release oxygen to other materials--particularly aluminum, which is an ever-ready recipient of oxygen--causing them to detonate. As with water, the oxygen molecules in the nitrates react in the molten aluminum to form aluminum oxide and nitrogen. And at molten temperatures, this reaction produces a potent exothermic reaction and explosion.
Inorganic nitrates include sodium nitrate, potassium nitrate, and ammonium nitrate, a principal chemical component in fertilizers, herbicides, insecticides, and explosives. Thus, any scrap that previously contained any of these products could include ammonium nitrate residues, presenting a potentially hazardous outcome. In 1990, in fact, one worker was killed and another was seriously injured when scrap aluminum from a trailer used to haul ammonium nitrate fertilizer was charged into a furnace.
Oxidized metals such as iron, cobalt, manganese, and zinc also present a hazard when introduced into molten aluminum. The reason: The ready release of oxygen from these metals readily binds with the aluminum to form aluminum oxide, which, in turn, can cause an exothermic chemical reaction that results in a powerful explosion.
Other materials with the potential to cause explosions include small chips, fine turnings, and dust of aluminum--all of which are easily ignitable and pyrophoric, a term that means they may ignite spontaneously in the air. In addition, concentrated aluminum dust clouds present an explosion hazard since powdered aluminum reacts violently with strong oxidizers such as chlorine-based chemicals and generates a great amount of heat.
Protecting Employees Through Equipment
Because of the dangers inherent in molten aluminum, all personnel working near molten aluminum should wear appropriate head, eye, and face protection. This includes safety shoes with metatarsal guards to protect the feet from falling objects and clothing made of material that will not continue to bum if contacted by molten aluminum. According to Donald Pierce, manager of operations technology for Reynolds's recycling division, cloth meeting this requirement includes specially treated cotton or wool or nonmeltable synthetic fabric. Untreated synthetic materials or blends offer little or no protection against molten metal. In fact, polyester can continue to bum when contacted by molten metal.
Additional protective apparel should be worn by employees working very near molten aluminum. B.J. Sasser, director of corporate safety for Reynolds Metals Co. (Richmond, Va.), recommended in an October 1990 article in Foundry Management and Technology, that the following added protection should be worn by these employees:
laceless safety pourer's boots that can be easily removed if splashed (laced boots can be worn if covered with spats to prevent the capture of molten metal by the laces),
leggings designed to keep splashes of metal from entering the top of the boot,
heat-resistant and/or flame-retardant gloves,
jackets and aprons made from treated wool, leather, or aluminized fabrics.
When requiring workers to wear multiple layers of protective clothing while working in hot environments, consideration should be given to the potential for employees to develop heat stress. Training, careful selection of protective equipment, and modification of work practices and procedures are essential elements to reduce this hazard.
Chemical Reaction of Aluminum and Water
Molten aluminum reacts with water to create aluminum oxide plus hydrogen. The creation of aluminum oxide is an exothermic reaction. Another danger: The hydrogen will be present at temperatures above it autoignition temperature, likely resulting in an explosion.
Chemical Reaction of Aluminum and Ammonium Nitrate
Molten aluminum reacts with ammonium nitrate to create aluminum oxide, nitrogen, and hydrogen. The hydrogen will be at temperatures above its autoignition temperature, making explosions likely.•
All metals can be dangerous in their molten form, but aluminum--one of the few materials routinely melted by scrap processors--has particularly serious implications.