With emission control standards becoming more stringent, many shredder plant operations are taking a more proactive approach to air emissions. The metal recycling industry has matured and grown during the past several decades. With growth comes increased regulatory pressures and public scrutiny. It is important to review past practices, implementing new procedures and technology as needed to handle future emission challenges.
The recycling of end-of-life vehicles (ELVs) and other obsolete consumer products is a well-established industry with many economic and environmental benefits. Each year, as many as 30 million ELVs are processed worldwide through an infrastructure that typically concludes when the bulk of the vehicle ends up in a metal shredding facility. It is estimated that there are some 700 such facilities worldwide.
The environmental benefits of recycling metals vs. production from raw materials are well-documented. The energy required to recycle aluminum is 1/20th of that required for production of virgin metal, according to Alcoa Recycling. Along with these energy savings comes a decrease in greenhouse gas emissions. Even though the net environmental benefits of recycling ELVs are clear, it is important to recognize that the recycling process is not without environmental concerns.
Concerns include vibration, noise, stormwater run-off, waste disposal and air emissions. Each of these concerns must be addressed through detailed study and application of appropriate technology to minimize impact. The intent of this article is to focus on air emission problems and solutions around the scrap yard.
As material that will become shredder feedstock is delivered to the scrap facility it is off-loaded and sorted into the appropriate areas based on scrap type. This process has the potential to generate dust as the incoming scrap is moved from pile to pile. This is particularly true when the incoming scrap has a high dirt content.
The shredder feedstock is loaded onto the infeed conveyor where it is conveyed to a feed ramp. Feedstock is then fed into the shredding chamber using a roller feeding device. As the feedstock enters the chamber it is struck by rapidly rotating hammers, which rip and deform the material until the particle size has been reduced sufficiently to fall through sizing grids around the circumference of the chamber. During the shredder process, any dirt and dust contained within the feed material can be liberated and carried into the air. In addition, as the material is pulverized, additional dust is created.
As the shredded material exits the shredder it is conveyed through various arrangements of downstream equipment. There are many process flow alternatives designed to produce a clean shredded ferrous product. Options include magnetic separation only, air sifting (Z-box) followed by magnetic separation and magnetic separation followed by air sifting. Regardless of configuration, the potential exists for dust emissions. It is important to note that emissions are an inherent part of most process equipment, even when there is not a typical source, such as a stack or chimney.
The shredded ferrous material is ultimately conveyed and stored in large stockpiles. There is potential for significant dust generation as the shredded steel falls from the end of the stacking conveyor. Large drop distances are also potential problem areas at various other material transfer points within the plant.
Auto shredder residue (ASR) is the mix of materials remaining after the ferrous fraction has been removed. This material is typically sized (with trommels and screens) before being conveyed to eddy current separators and air-ejection sensor sorters for nonferrous metal recovery. The remaining material is then stored, waiting for transport either to an off-site specialized sorting facility or to the landfill. The light dusty nature of ASR creates great potential for fugitive dust in this area of the plant.
Water and Foam
The injection of water and foam are commonly used dust-suppression strategies. This technology is utilized in many process industries, including coal mining, aggregate crushing and metals recycling.
Water injection systems (WIS) are designed to assist in dust emission control by injecting water into the shredder box during operation. The injected water turns into steam when it contacts the hot shredded fragments inside the shredder chamber. Particulate dust reduction then takes place as airborne steam droplets capture dust particles.
In addition to dust suppression, the system also reduces the frequency and magnitude of explosions within the shredding chamber. The steam generated by the WIS lowers the oxygen concentration inside the mill.
It is important that the shredder is fed in a consistent manner to provide plenty of material to fill the shredding chamber.
Foam injection is similar to water injection in that the foam is injected into the shredding chamber with the purpose of capturing emissions at the source. The foam injection system often consists of a mixing/pumping unit and piping system. The mixing unit mixes a custom chemical with water and air to produce the foam, which is then pumped to strategic locations within the shredder.
Foam-injection systems are typically run continuously without any feedback from the shredding equipment. This is in contrast to WIS, which typically varies water flow based on system feedback. The foam systems have a consistent amount of foam flowing into the shredder regardless of the shredder load. For this reason, the foam systems tend to perform better in shredders lacking consistent feed.
Both WIS and foam injection systems have the potential for wet and “sticky” material if they are not correctly configured to specific operating conditions.
Dry shredder emission control involves extensive dust-collection equipment with vacuum points at and around the shredder. Vacuum air from the shredder is conveyed through a network of ducts and air emission-control devices. Typical control equipment includes cyclones, scrubbers and bag houses filters. These systems must be designed to handle very large dust volumes and shredder explosions.
In Europe, the prevailing technique involves suction from the top section of the shredder. This method provides for effective evacuation of the shredder chamber. However, sometimes it can be too aggressive, leading to excessive material suction, which can lead to clogged duct work and loss of metals. In addition, there is a greater risk of secondary explosions within downstream emission control equipment, as fumes that rise to the top section of the shredder are carried to control vessels and ignited by sparks.
Other methods of dust extraction around the shredder have also been successfully implemented. Two common locations for secondary dust extraction include the shredder discharge hood and the area above the shredder inlet. Suction from these locations provides some of the benefits of suction from the shredding chamber with less risk of clogging and secondary explosions. Several shredders are operating currently with a combination of suction locations.
The dust-laden air stream is typically introduced to a high-efficiency cyclone as the primary control device. The cyclone is designed to remove a large portion of the particulate through centrifugal force as the air spirals through the cyclone cone. The captured particulates are discharged through a rotary air lock while the exiting air continues on to additional control devices such as filters or wet scrubbers.
The air stream typically continues on to some form of wet scrubber. Wet scrubbers today are usually a form of venturi design and offer high collection efficiency (99 percent or greater into the submicron range) and the added benefit of safety against hot embers, which can cause fires within bag houses. The wet scrubbing process comes with operational drawbacks, namely high power consumption (often more than 500 horsepower) and process water disposal.
Shredder Emission Control Trends
Regulatory and public scrutiny of shredder operations varies widely. Many existing shredder permits cover only particulate matter (PM) concerns without addressing hazardous air pollutants (HAPs). Leading environmental consultants today are seeing more requests by authorities to identify and handle any HAP emissions for new permits.
Evaluation of potential contaminants such as metals (lead), volatile organic compounds (VOCs, such as benzene), poly-nuclear aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are required. For the most part, agencies have been satisfied with promises to implement strict institutional controls (material acceptance plans, vehicle inspection and pre-shred processing) to remove chemicals of concern from the shredder feed.
It is clear that the outcomes of any stack tests are somewhat arbitrary and highly dependent on the condition of material being fed into the shredder. Automobiles that do not have fluids drained will emit VOCs and PAHs, batteries will produce lead, and transformers or old capacitors will exhibit PCBs emissions. An effective pre-shred screening procedure is the best defense to HAPs.
There is an increasing trend within the shredding industry to be proactive to environmental concerns. This is evident in the increase of indoor facilities and installation of advanced shredder suction systems on some shredders in California. These air systems are designed to address VOCs in addition to PM.
As the shredding industry evolves and environmental regulations expand, the use of advanced control equipment will become more common.
The author is vice president of technology with Metso Minerals Industries Inc. in its San Antonio office. He can be contacted at firstname.lastname@example.org.