Editor’s note: This article first appeared in the January/February 2016 issue of MSW Management.
Recycling is not an easy thing to do, and its difficulties increase with scale. For large communities and cities, industrial scale recycling operations are required. This further requires the adoption of industrial methods utilizing specialized machinery that can extract specific materials from the wastestream with sufficient capacity to manage incoming waste loads. Technically, the process of recycling MSW presents a series of challenges and problems.
The first is the nature of the material itself. MSW collected at the curbside and hauled by collection trucks to the landfill or material recovery facility (MRF) is extremely heterogeneous with waste materials of all types and characteristics (density, water content, chemical composition, electromagnetic potential, and sizes), mixed and compacted together into a mass that makes separation and extraction inherently difficult.FREE Infographic on Landfill Management: 6 Tips for Excellence in Landfill Operations. Covering publicity, education, engineering, long-term planning, and landfill gas waste-to-energy. Download it now!
And not every article of waste is composed of a single unique material. A significant proportion of the wastestream consists of composite materials that combine two or more types. This can include something as simple as glass jars with screwed on metal lids and pasted on paper labels, or complex composite packaging like children’s drink boxes that use a layer of aluminum foil sandwiched between layers of plastic and paper. It is easy to imagine how difficult (if not practically impossible) it is to economically recycle the individual components of composite packaging materials.
External factors can indirectly limit the amount of waste being recycled. Though they are definitely “doing well by doing good” by promoting sustainability and preserving natural resources, it is no secret that private recyclers (like any other business) are in business to make money. Even publicly owned and operated recycling agencies strive to show a positive annual budget balance. So, if there is a lack of market demand for a recycled material, this lack of demand will reduce the amount generated by recycling due to the iron law of supply and demand.
A lack of locally available recycling facilities can create another external bottleneck to higher recycling rates. Whether expanding existing facilities or building new ones, the recycler requires capital investment. And investors need an adequate return on their investment. If economic conditions depress demand (and demand growth) for recycled materials, investors will find other uses for their money.
The material recovery facility is the one type of facility that meets all of these needs: efficient separation of materials by means of specialized equipment, the ability to manage large tonnages of incoming waste, providing quality products that meet market demands, and ensuring efficient employment of capital investment. Traditionally, MRFs were simple affairs where materials already presorted by homeowners and businesses were received and manually organized and stockpiled for shipment to market. The type of labor-intensive MRF is usually referred to as a multi-stream, or “clean” MRF.
The second type of MRF accepts a mixed wastestream direct from waste collection and hauling trucks. The mixed waste mass is a process by machinery designed for efficient, sequential removal and segregation of each type of waste material. This type of capital-intensive MRF is called a single-stream, or “dirty” MRF. It is this type of facility that processes most of our recycled waste and utilizes electromagnetic and eddy current separators for metal recovery. These separators represent both the most technically advanced and simple to operate machines used at a MRF, and are the perfect examples of technological solutions applied to the problems of recycling . . . and the most valuable.
And, given the profitability and consistent market demand for ferrous and non-ferrous metals of all kinds, scrap metal remains the crown jewel of recycling.
According to EPA (2012), the wastestream consists (on average) of the following material categories. The total amount of MSW generated by the US (2012) came to about 251 million tons (prior to recycling). Recycling and composting removed 86.6 million tons of waste (34.5% of the total) from the wastestream, leaving 164.4 million tons to be landfilled (53.8%) or incinerated to generate electricity (11.7%). Recycling over one third of the wastestream over all is a significant accomplishment, one obtained by achieving the recycling rates for each type of material.
Combining these two data sets gives the recycling rate for each type of material. “Other” would include items like car batteries and old tires that are often banned from landfilling, resulting in very high recycling rates. “Yard Trimming” recycling rates represent extensive composting operations that are performed at other locations than MRFs. “Paper and Paperboard” have always had high recycling rates for bulk newsprint, office paper, and old cardboard.
That leaves “Metals” as the material with the second-highest MRF-related recycling rates at over one third. And there are several good reasons for this relatively high recycling rate. The recycling and sale of scrap metals tends to be profitable on a per ton basis. From 2003 to 2008, the average price of ferrous and non-ferrous scrap metals nearly doubled. The market crash of 2008 set back these price advances. Prime scrap steel fell from a peak of $700 per ton, to a low of $120. Scrap aluminum fell from $0.60 per pound, to $0.20. The price of copper, a material so much in demand that criminals risked electrocution by ripping copper wire out of live substations, fell from $3, to $1 per pound. But with the economic recovery came the return of higher scrap metals prices, and the value of scrap metal markets have nearly recovered to pre-recession levels. This increased value of scrap metal refocuses interest in efficient scrap metal removal at MRFs.
Scrap Metal and the Recycling Process
Scrap metal removal is just one stage of the recycling process performed by a MRF. Waste shows up at a local MRF in the back of a waste collection truck. Regional MRFs may receive waste from large open top trucks, baled loads of waste, or even from railroad cars. Upon arrival, the comingled wasted is dumped onto the MRFs tipping floor. From there, the waste is usually pushed by wheeled dozers or front end loaders onto conveyor belts or hoppers. Conveyor belts then carry the waste from receiving to sorting. The belts themselves come in various types (slider-belt, chain-driven, roller-type, steel-belt, or rubber-belt) and often come with sides that flip over the top of the mounded waste to prevent loss during transport.
The sorting stage could be as simple as a manual sorting platform. This labor-intensive operation relies on humans to pull out and extract certain types of waste as it passes by their station, and place it into a receiving hopper. Though many single-stream MRFs include a manual sorting stage, most rely more on sorting machinery for automated waste extraction. Often, the comingled waste passes through shredders and pulverizes to reduce the object and particle sizes to facilitate removal. In some of the newer plants, organics (paper, paperboard, food waste, yard trimmings, etc.) is subject to saturated steam pulping which renders this material into a pulp suitable for feeding a gas producing digester.
The workflow sequence and separation stages vary from MRF to MRF. But typically, the following steps occur. Non-recyclable material is removed as much as possible up front for disposal in the landfill. Machines called disc separators can remove large cardboard items (sheets stock, boxes, etc.), newspaper, and mixed paper. The extracted materials are sent to hoppers for bailing and stockpiling. The remaining waste gets fed into a sorter line where large containers (mostly plastic) are removed. Electromagnetic separators extract ferrous metals and eddy current separators remove non-ferrous metals. This leaves the wastestream’s residual glass, which is removed and sorted by color.
The above is a relatively simple MRF operation. More complicated and sophisticated MRFs rely on a wider assortment of separation and sorting machines-and less on labor. These machines utilize various methods that match the waste materials physical characteristics. Magnetic and eddy current separators extract metals based on their electromagnetic characteristics. Disc screens and rotating trommels separate containers and paper or paperboard based on size and shape. Air classifiers and air knives sort paper by weight. Optical sensors sort and separate plastic and glass by color.
Once separated and sorted, the recycled materials enter the last stage of the recycling process. Here, the extracted products are then sent through compactors, can flatteners, and baling systems to increase their density prior to stockpiling. From stockpiles, the materials are loaded on to trucks or railcars that ship them to market.
Scrap Metal Separation by Electromagnetic Characteristics
As mentioned above, the potentially most lucrative portion of the recycled wastestream is its metals component, both ferrous and non-ferrous. But how is this done? More importantly, how is this done in the most cost-effective and profitable manner?
Electromagnetic separators are simple in design and operation. They extract ferrous metals from the wastestream by means of magnetic attraction. Configurations can vary from overhead to under the conveyor belt. Overhead electromagnets pulls waste up out of the wastestream as it travels underneath on the conveyor belt.
Belt separators incorporate electromagnets installed directly under the belt. As the conveyor belt moves to its last roller and turns under itself, all of the waste except the ferrous metal falls off at the turn under point into a waiting receptacle or another conveyor belt. The ferrous metal, on the other hand, sticks to the underside of the belt due to magnetic attraction. It then continues to a scraper blade, which pries off the ferrous metal so it can fall into a designated hopper.
A different version of a magnetic separator combines electromagnets with a rotating trommel. Essentially a rotating drum set at an angle, a trommel usually has holes perforating in its surface to allow the escape of small particles and objects. This configuration lines the drum surface with electromagnets. As waste passes through the trommel most of the waste continues out the bottom while ferrous metals cling to the interior wall. Eventually the weight of the accumulated ferrous metals overcomes the force of magnetic attraction ad the ferrous metals work their way out of the drum into a collection chute and storage bin.
The removal of non-ferrous metals requires a more complicated process. Lacking direct magnetic attraction characteristics, non-ferrous metals like aluminum need to have a current induced in them. This is accomplished by a rapidly rotating rotor mounting magnets with alternating polarity. The rotating magnetic fields create alternating currents within the on-ferrous metals. In turn, these alternating currents create electromagnetic fields of their own. These opposing magnetic fields repel each other, causing the non-ferrous metal to jump off the conveyor belt into adjacent collection bins.
Major Suppliers of Magnetic and Eddy Current Separators
Bulk Handling Systems LLC (BHS) is noted for its complete MSW recycling systems. For example, BHS has developed the first MSW recovery system in the US to keep 70% of waste out of the landfill, including the recovery of 90-99% of high value commodities like PET plastics-on the first pass. Included with these high value materials are non-ferrous metals like aluminum cans that are extracted by eddy current separators. In addition to high recovery rates and productivity, their compact-sized eddy current separators are built for durability and minimal maintenance.