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	Photo: Don Cecil

Separation of individual types of waste from an incoming waste mass requires a specialized set of equipment for each stage of the process and for each type of waste.

By Daniel P. Duffy

Most of this machinery is adapted from mining operations and has new uses in material recovery facilities (MRFs) where automated recycling operations take place. Separators are necessary for those operations that involve both large quantities and multiple types of waste. Limited material flows of one or two types of recyclable materials can be managed by hand sorting and separation.

For more ambitious recycling operations there are sorters and separators that allow for high productivity and accurate separation of like materials. These sorters and separators use the following techniques based on the material’s physical characteristics: separation by electromagnetic characteristics (magnetic and eddy-current separators), separation by size and shape (disc screeners and rotating trommels), separation by weight (air classifiers and air knives), and separation by color (optical sensors). Recent improvements in control systems and refinements in the separating mechanisms have made these machines more accurate and productive. Generally, MRFs manage mixed wastestreams from multiple locations, and these streams can vary in content and quantity from day to day or even hour to hour. Certain MRFs emphasize certain types of waste, with the emphasis depending on the current local markets for recycled waste materials and the availability of these materials in the wastestream.

Separation by Magnetic Characteristics
Magnetic separators—Simple and effective, magnetic separators simply pluck ferrous metals from the wastestream by means of magnetic attraction. The wastestream passes either under or over an electromagnet, with belt-configured magnetic separators being the most popular for recycling applications. In belt separators, either the belt itself is magnetized or there are magnets installed directly under the moving belt. The belt moves along on rollers until it comes to the last roller and turns under it for its return cycle. Nonferrous waste drops off the end of the belt into a collection bin as the belt turns under the last roller. Meanwhile, the ferrous metal continues to stick to the belt until it is carried to a scraper blade that pries it loose so it can fall into its own receptacle. A different configuration combines magnetism with a perforated rotating drum (trommel). The drum rotates at an angle as waste is fed into it. The ferrous metals cling to its inner surface as nonferrous materials fall through the perforations. The weight of additional ferrous metal forces the accumulated ferrous metals to work their way out the bottom of the drum, falling down a chute to a collection bin.

Eddy-current separators—Separating nonferrous metals (mostly aluminum cans and foil) from the wastestream is a bit trickier. Eddy-current separators take advantage of the fact that even nonferrous metals are electrically conductive to some degree. By rapidly spinning magnetic rotors with alternating polarity immediately adjacent to the waste-stream, alternating electrical current fields are generated within the nonferrous metals. These streams create their own electromagnetic currents, in this case with polarities opposite those of the rotors. The nonferrous metals are repelled from the rotors (in contrast with ferrous metals being attracted to magnets) and jump into nearby collection bins.

Master Magnets Ltd., a British company, manufactures a low-cost eddy-current separator specifically designed for extracting aluminum beverage cans from the wastestream. A compact unit, it is designed to drop into a process line, allowing for easy installation and quick adjustment. Master Magnets also produces a line of magnetic-drum separators. With their larger operating surfaces and powerful electromagnets, the electro-drum separators can handle higher throughputs and larger objects, making them suitable for such large-scale applications as car-shredding plants. The company’s permanent-magnet drums are less powerful and are used in smaller applications (such as removal of metal contaminants from grain). Master Magnets makes four lines of magnetic drums. Its Fragmentiser electro drums have an alternate pole arrangement that allows only ferrous metals to be carried over the drum, thus preventing nonferrous materials from getting trapped. The slag-processing separators are strong enough to extract heavy ferrous slag. The totally enclosed drum separator is designed to prevent fine products from being contaminated or giving off dust. Lastly, its wet drum separators are designed to remove dense media wastestreams and can be configured to operate in concurrent, counter-flow, and countercurrent modes to ensure that materials are recovered from wastestreams with differing liquid contents.

The Dings Magnetic Group of Milwaukee manufactures eddy-current separators, suspended overhead-magnet separators, and small and large magnetic-drum separators. Its eddy-current separators come in two-pulley and four-pulley configurations with a triple-shell design for strength and durability. Optional equipment includes a control panel, a splitter assembly, or custom designs. Its self-cleaning overhead magnets can separate large quantities of ferrous metals and can come as either electro- or permanent magnets. Its belt is made of vulcanized rubber with 1-inch cleats for continuous metal removal. The severe-duty, self-cleaning magnet can be used to recycle rebar from crushed concrete. The company’s drum separators utilize ceramic magnets and can be configured for top feed, up and over feed, and down and under feed. The electro scrap drum is specifically designed for scrap-metal recycling and cleans the extracted steel by tumbling it during the removal process.

Industrial Magnets Inc. (IMI) provides individual magnets made from 50-megagauss-oersteds rare earth materials, as well as drum separators, magnetic pulleys, and suspended magnetic separators. The drum separators are top-fed and continuously self-cleaning. They can be supplied either as the drum alone or as a complete assembly. In addition to providing magnetic belt separators, IMI manufactures a key component of these systems, the magnetic pulley. These pulleys can be configured to a wide variety of belts, and they provide automatic and continuous removal of ferrous metals. IMI’s suspended magnetic separators come in manual-cleaning, easy-cleaning, or self-cleaning designs. Its belts come in standard sizes ranging from 18 inches to 66 inches wide.

The Helsinki-based Metso Corp., normally a supplier of mining and ore-processing equipment, has entered the metals-recycling market. It manufactures the Lindemann nonferrous metal separators, further developing the dry-sorting processes for nonferrous metals. This dry-sorting process is effluent-free and efficiently separates nonferrous metals from all other nonmetallic waste materials. These efficiencies are maintained by optimizing the magnetic flux of an eccentrically arranged magnet system with a variable pole-reversal frequency. The direction and degree of separation of the nonferrous metals are adjustable due to the degree of eccentricity applied to the self-cleaning magnetic rotors located under the processing belt. Operator efficiency is enhanced by the separator’s multifunctional control system. This equipment is suitable for automobile-shredder waste, electronic scrap, plastic waste, nonferrous scrap remelting, and waste-glass processing. It can be used as stand-alone equipment or inserted into a single-stream recycling process.

Separation by Size and Shape
Disc screens—Disc screens are useful for separating large but lightweight objects (such as cardboard or aluminum foil) from small and heavy objects (aluminum cans and pretty much everything else). Waste enters a disc screener by way of an inflow bin fed by a variable-speed conveyor belt. The bin’s surge capacity and the belt’s differing speeds allow for smooth control of the wastestream. Disc screens usually come with adjustable openings to manage different kinds of waste. However, they are most useful for secondary separation. The screener itself consists of a bed lined with multiple discs that intermesh in arranged rows and decks. The discs can be round, oval, or star-shaped with various dimensions and diameters.

Rotating trommels—These are perforated drums with the axis usually set at an angle to allow for gravity feed and discharge. Originally used in mining operations for separating ore from slag, trommels have found another use in mechanical sorting of recycled waste. Their walls are perforated to allow fines (soil, grit, broken-glass shards, organic wastes, and other residue) and other small objects to escape through the sides. Larger objects suitable for recycling are retained and further separated by a series of parallel vanes fixed to the drums’ interior walls to promote gravity separation. Some trommels utilize flights to carry material back up to the top for another pass for thorough separation. Like disc separators, trommels are more useful as a last stage of material separation.

West Salem Machinery of West Salem, OR, provides disc screens that manage high-volume coarse screening applications. Of the material fed into the screen, small items fall down through the disc openings while larger materials are carried up by the rotating discs to a receiving conveyor. It can handle paper, old corrugated cardboard (OCC), shredded tires, and plastics, and it has processing capacities of up to 600 tons per hour. An additional agitating action is imparted to the rotating discs to improve screening efficiencies. Maintenance is made easy by self-cleaning discs, removable side sheets that allow for easy access, and a central lubrication station.

Machinex Industries Inc. of Plessisville, QC, offers a complete line of single-stream recycling machinery, including the Mach series of separators. The Mach OCC 2614, 2617, and 2717 sort corrugated cardboard with processing rates of 35 tons per hour. It is equipped with two decks of inclined screening surface. The Mach Two utilizes adjustable twin decks and friction-type disc fingers to process materials at a rate of 20 tons per hour. The Mach One is specifically designed for single-stream recycling. It has adjustable spacing between the discs and variable operating speeds for sorting flexibility. In addition to the disc screens, Machinex provides a durable trommel installed on a solid one-piece structural steel base.

Photo: Don Cecil

Separators are necessary for operations involving both large quantities and multiple types of waste.

Based in National City, CA, CP Manufacturing Inc. is a major manufacturer (having built more than 300 MRFs worldwide) of a wide range of recycling equipment and automated systems. Its OCC disc screen can handle up to 30 tons per hour with variable-speed drive motors. The unit’s discs can be designed to accommodate any in-feed of corrugated cardboard and paper. Two screening sections house two decks of elliptical discs that are set at 90-degree angles to each other in order to maximize the wavelike movement that sorts the OCC. Its internal support system allows it to be easily retrofitted into most existing recycling processes.

McCloskey International Ltd. of Peterborough, ON, manufactures a complete line of trommels ranging from its 50-horsepower McCloskey 407 to the 175-horsepower McCloskey 733, the largest portable trommel in the world. Each comes with a 180-degree radial stacker and a remote-controlled stockpiling conveyor for ease of discharge. All McCloskey trommels come powered by Caterpillar diesel engines, with rubber-wheeled drum drivers, built-in drum-cleaning brushes, and an exchange system that allows for easy modular replacement of worn-out drums.

In Avon, OH, Doppstadt US manufactures a wide range of municipal solid waste recycling equipment, including a line of rotating trommel screens. The screens come equipped with patented load-sensing devices that ensure maximum utilization of the entire screening area, leading to dramatic increases in efficiency. Portable models come equipped with their own self-propelled drive system. At the other end of the operating spectrum is the model SM 627 trommel. Driven by a 77-horsepower Perkins diesel motor, its two side conveyors allow it to produce three types of separated materials. Self-cleaning capability is provided by hydraulically tensioned cleaning brushes. It is maintenance-friendly thanks to the installation of a central lubrication point, large protection doors, and a double-pinion direct drive. Operational flexibility is provided by division of the trommel into two screening sections, with the screens consisting of holes or replaceable mesh panels. Material feed is from the side, with three conveyor-belt discharges for fine, medium, and oversized particles. A fourth fraction can be separated by use of a stone grid.

Separation by Weight
Air Classifiers—Like trommels, air classifiers got their start in the mining industry, specifically coal mining. An air classifier resembles a large chimney stack with a powerful blower installed in the top that sucks a high-velocity airstream up the stack. At the midpoint of the stack, a controlled wastestream is fed in, and lightweight materials are pulled up and out of the waste mass. Immediately below the waste feed is a rotary airlock that regulates airflow. Heavy and large objects fall to the bottom of the stack for collection and secondary separation. The lighter materials exit the stack and enter a cyclone separator that further sorts them by size and density. Some air separators utilize pulsed air blasts instead of steady streams, featuring offset stack channels passing the waste in and out of the air flows to allow for separation by density.

Air knives—Air itself can be used as a “knife” to separate light materials from the wastestream. In this case, the air knife consists of multiple layers of high-velocity sheet flows operating in parallel. The purpose of the separate airflows is to prevent swirling that would remix the separated materials. This makes it useful for separating materials that vary only slightly in mass, such as different grades of paper (newsprint, glossy magazine stock, or office paper).

The Cincinnati, OH–based Exair Corp. manufactures a wide range of air blower systems, including its Super Air Knife. This equipment reduces noise levels to 69 decibels and lowers air consumption while providing a 40:1 amplification ratio resulting in pressures as high as 80 psig. Its uniform airflow (there are no interruptions or dead spots) is suited for environmental separation applications.

General Kinematics of Crystal Lake, IL, manufactures the De-Stoner air classifier. It can separate and classify a wide range of recyclable materials (MSW and other commingled materials, auto-shredder residue, construction and demolition waste, biomass fuel, and refuse-derived fuel). High-velocity, low-pressure airstreams work in tandem to fluidize and stratify commingled materials according to differences in terminal particle velocity. It’s a completely dry system without the need for water sprays, but it is capable of handling moist materials.

Prodeva Recycling Systems Inc., located in Jackson Center, OH, makes a more extreme version of the air classifier, its Model 500 Flattener Blower. This unit is designed to separate aluminum from steel and bimetal cans, then crush and blow the aluminum cans into trailers, rolloffs, or any type of holding container. It can handle over 2,000 pounds per hour from a continuous feed, and it is equipped with a spring-loaded crusher plate, replaceable drum cleats, and a high-power blower.

Schmidt & Co., based in Germany, is a worldwide manufacturer of dispersed air clarifiers for a variety of industries (cement, stones and earth, ceramics, quartz, ore processing, coal grinding, environmental technology, and recycling systems). It provides three models: compact classifiers, cyclone recirculating air classifiers, and crosscurrent classifiers. These classifiers have a throughput capacity of 300 tons per hour, come equipped with dust separators, and are provided with full-service engineering. The company also manufactures centrifugal air classifiers utilizing either central air admission or tangential air admission.

Separation by Color
Glass-color separators—Developed by the chemical industry to allow for the automated sorting of chemicals by color, the technique of light spectrophotometry (LSP) can also be applied to recycling processes. Specifically, LSP can be used to sort and separate different colors of glass (clear, amber, brown, or green) from ceramic cullet. The different wavelengths reflected to the optical sensor from the glass let the sensor “know” what color the glass is. The actual separation of the different colors of glass is more difficult and has lower removal efficiencies. A recent development in the field of optical sensing is the near-infrared (NIR) sensor. These sensors can differentiate among densities for sorting different types of plastic, or among colors for sorting glass. The sensors are mated to air blowers that are programmed to shoot a stream of air that propels the glass fragment to its appropriate bin.

Nashville, TN–based Magnetic Separation Systems Inc. (MSS) has developed the Glass ColorSort system, utilizing NIR sensors to process 20 tons per hour when separating ceramic cullet from glass and 5 tons per hour when sorting glass by color. So sensitive is its color optical sensor that it can differentiate between all shades of green, amber, yellow, and blue glass, including “Georgia Green” and “Half-White.” It even sorts by sizes ranging from three-eighths-inch to 2 inches. Its hardware improvements (high-speed optical detectors and digital signal processors) are matched by advanced software that utilizes special algorithms that compensate for contamination without the need for intermittent operations and air- or water-cleaning utilities.

Binder+Co, an Austrian company, manufactures the Clarity Plus optical sorting system. It is the first sorter to use a camera system to remove ceramic, stone, and porcelain (CSP) impurities from the glass-fragment stream. The same system also removes non-clear (brown, green, etc.) glass fragments, allowing for the direct removal of clear or white glass only. Inside the Clarity Plus unit, pieces of broken glass are exposed to white radiation. A maximum number of seven independent cameras, which can detect up to 16 million colors, analyze the obtained information from the light reflected off the glass. They also directly control the valves distributed over two blowing-out strips so the strips operate at the right time, blowing out cullet according to the colors defined in the unit’s software menu or defined as impurities. The Clarity Plus is modular, which makes it easy to install into existing recycling facilities.

Though used to sort different types of plastics, the MultiSort IR by National Recovery Technologies Inc. of Nashville, TN, uses proprietary infrared sensing technology and matching high-speed algorithms to process large quantities of polymers based on each of their unique reflective “signatures.” The result is an easy-to-use mass sorting system that can handle 10,000 pounds per hour. Its simple stainless steel design, easy-to-use graphic control panel, and ability to have diagnostics/adjustments performed via modem connection greatly reduce operating and maintenance costs.

Daniel P. Duffy, P.E., is an environmental engineer employed by URS Corp. in Akron, OH.

MSW - May/June 2007