By Curt Wells and Paul O’Connor
Aluminum scrap recycling plays a vital role in the operation of the U.S. aluminum industry. Scrap has grown from 25 percent of the total U.S. aluminum supply in 1980 to 42 percent in 2016, providing significant economic and environmental benefits to the industry. Along with these benefits, however, come safety challenges due to hazards in the scrap aluminum supply that can harm remelting plant workers, equipment, and furnaces.
The aluminum industry has made tremendous progress in secondary aluminum plant safety, reducing the total recordable injury rate 55 percent from 2005 to 2016. The relationships between aluminum secondary remelters and the recyclers and traders that supply them with scrap have been essential to that success because scrap management—what a scrap facility would call source control—is at the forefront of remelting facility safety.
Tracking explosions to their source
Molten aluminum explosions are the aluminum industry’s greatest safety concern, and preventing such explosions is its No. 1 safety priority. Such explosions can injure or kill workers, damage or destroy buildings and equipment, and put companies out of business. The most serious such explosion in recent memory, in a Chinese aluminum casting plant in August 2007, resulted in 16 deaths and 59 injuries. U.S. aluminum facilities have experienced at least one major molten aluminum incident per year in the past three years. Luckily, these three incidents did not result in any fatalities, but they did cause critical injuries and millions of dollars in property damages. Not all explosions are major ones, but even smaller ones have enough force to injure or kill someone nearby.
Since 1980, the Aluminum Association’s (Arlington, Va.) molten metal incident reporting program has captured global incident data annually from 200 companies and nearly 300 facilities across the globe. We’ve used the data to identify trends that allow us to prioritize our prevention efforts. The 2017 report, based on incidents in 2016, provides details on 195 molten metal incidents worldwide, dividing them into three levels of severity, with Force 1 the lowest-force incidents and Force 3 the highest-force incidents. The report also categorizes each incident by whether it occurs during melting, casting, transfer, or some other part of the operation.
Across the last 37 years of reporting, we’ve determined that the second-highest proportion of all molten aluminum explosions—and the highest proportion of Force 3 incidents, the most serious—occur in the melting area. We did additional analysis of those melting incidents, looking at their frequency and severity by input material: scrap, sow, alloy, ingot, scrap-sow combinations, or other. Scrap input material resulted in the highest proportion of total incidents and of Force 3 incidents. In other words, the largest single hazard category in aluminum remelting operations is melting incidents involving scrap charge material. That information has led the Aluminum Association to make a primary focus of its industry safety improvement efforts preventing scrap-related molten metal incidents.
The explosive effects of contaminants
Molten aluminum can explode when contaminants enter the melting furnace. The two most common explosive contaminants in aluminum scrap are trapped water and chemical oxidizers. When water gets trapped in or under molten aluminum, it can transform into steam in a fraction of a second and expand approximately 1,700 times—think of a marble instantly turning into a soccer ball. This rapid expansion, plus an exponential rise in temperature, provide enough explosive energy to throw molten metal up to several hundred feet in any direction.
In chemical oxidizer explosions, the contaminant causes a chemical reaction that transforms elemental aluminum into aluminum oxide, generating tremendous amounts of heat and energy that can cause considerable damage to worker health and plant equipment. To give you an example of the amount of energy potentially involved, the Aluminum Association has calculated that a chemical reaction that causes the oxidation of one pound of aluminum is equivalent to detonating three pounds of TNT.
Chemicals other than oxidizers can be contaminants in the aluminum scrap stream as well. In the guidance documents mentioned below, we have compiled a list of the most significant types of chemical contamination in aluminum scrap along with a brief description of the health-related hazards they present when charged into molten aluminum. The most hazardous (in addition to oxidizers) are nitrates, sulfates, fertilizers, and potentially any unknown powders or residues. Incidents involving these materials have high risks of severe burns, other physical injury, severe property damage, and ultimately death. Even a minor amount of powder residue that remains after a piece of equipment has been washed may be enough to trigger an explosion.
A less common category of potentially explosive contaminants is volatile substances commonly found in oils, solvents, and other non-aqueous liquids. Volatile substances readily vaporize at ordinary temperatures; when they are immersed in molten metal, the forceful vaporization that occurs can eject aluminum out of a furnace. The result can be burns and other physical injury, exposure to toxic fumes, severe property damage, fire, and death. Closed containers in aluminum scrap are the source of many of these contaminants. Crimped tubing, aerosol cans, ammunition cartridges, and similar objects may conceal hazardous amounts of water, chemical oxidizers, or volatile substances that can cause a molten metal explosion when introduced into the furnace.
The evolving nature of automobile safety has introduced an entirely new set of remelter safety hazards. Air bags have proliferated in new vehicles, with many new models containing 10 air bags or more. Air bag detonators and seat belt pre-tensioners often are made of aluminum and can end up in the aluminum stream that automobile shredding operations generate. These components often contain the chemical sodium azide, which, when detonated, produces the gas required to inflate most air bags. This chemical can present a hazard in remelting operations, however, and even if the remelter is able to segregate these items from other scrap prior to melting, they potentially need to be handled as hazardous waste for disposal.
One final contaminant worth mentioning is hypodermic needles. For many years, the Aluminum Association tracked the causes of its members’ scrap rejections. In 2001, the tracking program’s final year, nearly half of the 134 rejections the program recorded were due to needles. Needles or syringes can contain liquids unsuitable for introduction into the melting furnace, and they present a hazard in terms of biological infection.
The Aluminum Association believes that all scrap-related melting incidents are preventable. We have found that root causes of hazardous contamination in the aluminum scrap stream include untrained or improperly trained workers, incomplete purchase orders or specifications, lack of adherence to specifications, improperly tagged items, incomplete inspections, hidden conditions, carelessness, and intentional deception for profit. We and our member companies have developed and published several guidance documents on scrap management safety that help our members address these root causes. Our “Guidelines for Scrap Receiving and Inspection Based on Safety and Health Considerations” is written for secondary aluminum remelters, but it provides relevant information for scrap suppliers as well in terms of what your aluminum customers’ material quality expectations are. Our “Guidelines for Handling Molten Aluminum,” the foundational document secondary aluminum remelters use to prevent molten metal explosions across the entirety of casthouse operations, contains specific guidance on scrap management as well. These two documents set out best practices for specifications and purchase orders, receiving and inspection, and contamination and mitigation. You can find these guidance documents, along with other safety information from the Aluminum Association, at www.aluminum.org/bookstore.
Specifications and purchase orders. Well-crafted purchase orders provide the first and best opportunity for remelters to eliminate unsafe materials. Communicating expectations in two areas is key to preventing both molten metal explosions and, more broadly, disruptions throughout the recycling process. First, scrap buyers must explicitly define and describe acceptable scrap and periodically review the standards with scrap suppliers. Second, scrap buyers must describe and clearly define unacceptable scrap due to the specific hazards those items present. Purchase orders should specify in writing that aluminum, aluminum scrap, alloying materials, and fluxes be as free as possible from (1) fertilizers and other materials containing nitrates, sulfates, or other oxidizing agents; (2) any form of water; (3) volatile substances and hydrocarbons; (4) closed containers such as crimped tubing, aerosol cans, automobile air bag inflation cartridges, ammunition cartridges, and fire extinguishers; (5) oxidized metal; and (6) radioactive material.
Adherence to material specification is essential and can greatly reduce the chances of an explosion due to contaminated scrap. This is why certain forms of aluminum scrap—borings, turnings, saw chips, fines, or pit cleanings—that are likely to contain water and oil are not accepted unless specified.
Most aluminum companies have publicly available scrap specifications that scrap sellers can consult to determine what material is suitable for melting. For example, the website of Logan Aluminum (Russellville, Ky.), www.loganrawmaterials.com, contains remelt material specifications, supplier performance expectations, and ratings and reports on supplier performance.
Receiving and inspecting. Remelters’ scrap receiving facilities should implement an extensive screening process on incoming loads to ensure that suppliers have met the established specifications and the material is contaminant-free. One step in that process is the use of radiation detectors positioned to scan truck and railcar loads of material as they enter the property. If they detect radiation, the facility must be ready to implement a strict set of scrap rejection and/or quarantine procedures.
The next step is a visual inspection of the load for the obvious presence of any hazards, such as beverage containers, lighters, aerosol cans or other types of sealed containers, munitions, air bag canisters, crimped tubing, or cold packs that contain ammonium nitrate, which can cause molten metal explosions. Certain personnel are authorized to reject any scrap containing contamination or other hazards.
In some cases, the scrap receiving facility might make use of third-party laboratory tests to pinpoint and measure certain kinds of contamination. These might include testing powder samples for levels of nitrates, sulfates, other oxidizers, and other hazardous materials; measuring the water, volatiles, and oil content of scrap samples by weighing the sample before and after heat drying; and measuring the ratio of aluminum fines to dried scrap in a sample.
Even if a scrap load appears free from contamination, some facilities may prohibit entry if the delivery vehicle interior fails inspection due to the potential for unknown contaminants to transfer from the interior surfaces of the vehicle to the scrap as it is being unloaded.
Contamination and mitigation. When a receiving facility identifies scrap that’s contaminated or is suspected of contamination, workers should mark it with brightly colored warning tags stating “Do Not Charge” or something similar, isolate the material, and contact the scrap supplier to arrange logistics for removal of the load.
The list of contaminants that will cause rejection may differ somewhat between remelting facilities and depend on the circumstances as well as the ability of each plant to handle various degrees of contamination safely. The Aluminum Association has identified aerosol cans, ammunition, closed containers, flammable and radioactive substances, needles, lead, and powder residue as the most common—nearly universally rejected—contaminants.
Secondary aluminum producers and scrap suppliers rely on each other to support safe and successful operations. But we must base that reliance on a clear understanding of the hazards of introducing contaminants into a remelting furnace and strong action across the value chain to prevent those hazards from becoming incidents. Clear guidance, thorough communication, and strong verification and validation all play a role in scrap suppliers and remelters working together to improve the safety of the entire industry.
Curt Wells is senior director of regulatory affairs and Paul O’Connor is a policy intern at the Aluminum Association (Arlington, Va.).