Magnetic Head Pulleys. These magnetic head separators replace the standard head pulley on portable crushers or grinders. As the product stream passes over the pulley, ferrous materials are attracted to and remain on the belt until the belt passes away from the pulley. The collected materials are deposited in a pile below the belt while the primary materials on the pulley are thrown forward from the pulley in a normal trajectory. Magnetic head pulleys especially are useful in pulling bits of iron from a stream of greenwaste grindings.

Eddy-Current Separators. These separators use the repelling power of eddy-current fields to separate aluminum and other nonferrous materials from plastics, glass, and other mixed containers. These are particularly effective on container sort lines and are usually placed after the ferrous separation occurs. In combination with ferrous magnets, they provide an extremely effective method for removing metals from the container line.

"I'm not sure that the equipment is going to evolve much differently [from what] it is right now," states Harold Bolstad, sales manager with Dings Company in Milwaukee, WI. "People are fine-tuning their line to meet their needs, have a clean product to sell to [their] customers, and get a higher price for it. They're doing it a couple of ways. First, they're taking the equipment that they have and making it work better, getting a better understanding of the equipment to make it work. The second thing is they're looking at new or additional equipment to help get a clean product and then at what is the cost and the payback."

Bolstad is seeing more experimentation in existing processing lines as it relates to the placement of the magnets in the production process. "Customers today are more concerned about the applications," he says. "They ask a lot more questions about 'Do I have the magnets positioned correctly?' 'What if I do this?' 'What happens if I do that?' They will find a way to tweak their system to get better performance. I see the guy [who] is actually operating the system get involved after [design and construction] because he's seeing that the material going through is not as clean as he wants it, so he goes back upstream and says, 'How can I change things to make them better?'"

Vibratory Finger Screens

Vibratory finger screens help even out the flow of materials across the separation line and separate commodities by size. Vibratory screens work best either at the beginning of the separation process, where the mixed materials might still be in some type of bag, or toward the end of the line, after bulk of the paper has been removed. "One thing that we don't like to do with the vibratory equipment is large volumes of paper since it absorbs the energy," states Ron Zorn of General Kinematics in Barrington, IL. "The finger screen is usually used where a lot of this paper has been removed. Most MRFs use a variety of finger screens. They want to get it sized down to a particular size for their operations and then it hits the picking lines."

Over the years, the use of finger screens has become more sophisticated. "In the past, you could put out a finger screen and they just ran everything across it," notes Zorn. "Now people are starting to pin it down a little bit tighter to the material." One of the biggest challenges in the placement of vibratory finger screens is the issue of controlling fugitive vibrations from the equipment. "We're looking for better ways of balancing vibratory equipment so the forces that this equipment is producing are not transmitted down into the structure. A lot of these facilities are being built close to other businesses, so if I turn on my piece of equipment and want to shake something, I can't shake my neighbor's coffee cup."

There is more attention focused on the design of vibratory screens to improve efficiency. "We're collecting more data from the jobs that we have out," Zorn continues. "The finger design is changing slightly. We're collecting data where we know what opening is working better for certain products. We're getting into control systems, adding more electronics to the equipment so we can monitor strokes. If we get into a condition where there are heavy loads, the electronics can pick this up, change frequencies in the motor, and speed the unit up so we can maintain a constant stroke. You have to keep updating the equipment and keep looking at a better way of doing it."

Other innovations include vibratory screens called "racetracks," which allow the materials to vibrate around in a circle. "If you can't pick your product the first time around, it will come back around to you again," says Zorn. "We're also working on how we form our troughs. We've designed some of our troughs with V-designs so people can push things over to the center. If they are not picking the cardboard or their job isn't to pick the cans, they can just push it over in the center 'V,' and that way product isn't going past every single person down the pick line."

Air Classification

Air classification provides an ability to stratify and separate a light constituent from a heavy constituent by using blasts of air. "If someone wants to separate out cans and plastics, one way of doing this is with an air classifier," says Zorn. Air classification originally was designed to remove stones from wood products, but its use has been significantly expanded over time.

"An air classifier is a vibratory piece of equipment," explains Zorn. "As it approaches a section of the unit, we have a fluidizing plate - we call it boiling up the product where we bring the lights up to the top and the heavies stay at the bottom. Immediately after the fluidizing section, the material hits an air knife that is a thin slot in the unit that we're blowing air through. The air going through there will throw the light product over the gap onto another carrying surface, and the heavy products drop straight out the bottom."

Grapples

Material handlers are used to move materials from one location to another within the processing operations. Often they will be used to transfer the materials from the tipping area to the infeed conveyors. "We build grapples to fit the application," states John Anderson, vice president of Builtrite Material Handlers in Two Harbors, MN. "We've got a fingered-type grapple if we're working in sorting lines so that we can do some sorting prior to putting it on the infeed conveyors. We size it to the customer and the amount of tonnage that they are putting through on a daily basis."

The type of materials that the grapple will handle determines the type of claw arrangement. The claw may consist of two opposing jaws that can handle large pieces of material, such as construction and demolition waste. An orange-peel grapple will have four claws located adjacent to each other that will permit the claw to pick up larger quantities of commingled recyclables or perform preseparations of large materials in order to spread materials for separation.

Determining the type of grapple that's appropriate for a particular location might require a site visit, says Anderson. "If it's an existing facility, we work with the parameters on what they want to reach," he relates. The locations or targets where materials will be picked up and placed are determined. "We determine the length of the boom, and once we figure the boom and tonnage, we know what size crane we are going to put in there, then do our layout and go from there."

Screening Technology

Screening technology involves any mechanical process that separates material by size. It is in screening technology that the biggest advances in separation technology have occurred in the last three to five years. These advances have helped increase the volume throughput of facilities, while significantly lowering the operating costs for these facilities.

The type of separation that occurs with screening is a factor of the size of the openings in the screens, the speed of the screen, and the movement of the materials through the screen. The efficiency of the screen is directly affected by each of these constraints. Use the wrong-size opening and the target materials might pass on by. Have the screen operating too fast and the materials don't have adequate residence within the screen to be effective. If the materials, especially fibers, move in such a way that they layer across the screen, the recovery rate might be reduced.

Early screening involved running the materials through a rotating screen or trommel that tumbled the materials while allowing smaller fractions to separate from the larger fraction. These tumbling screens resulted in increased breakage of glass within the material mix, reduced glass recovery, and the spreading of broken glass throughout the paper fractions. But current screens focus on removing various grades of fibers, especially OCC and newsprint, thereby allowing the mixed paper and containers to pass through to other sorting lines.

"Most screening has three different things that can happen," explains John Willis, vice president of sales and marketing for CP Manufacturing in National City, CA. "In our systems, we use elliptical disks. The disks are designed so you get movement on the materials that's going over them. The disks can grab the material that you want to separate and take it over the top. In the case of an OCC screen, we want a large-enough opening that everything other than OCC will fall through the screen. The disks have to be very heavy and sized such that they get a lot of action on the OCC without having it jam between the disks. The spacing in the openings depends on the type or size you want to go over the top."

Optimizing the screening involves a combination of the angle of deck, speed and design of the disks, and the opening between the disks. "We clock each disk 90º [to each other] so that as the disk rotates we get a side-to-side bouncing action," says Willis. "We have a tendency to bounce the fiber off the cardboard so they can go down the openings. The disk has a serrated leading edge that grabs the cardboard and throws it forward."

While the same type of disk technology works for other fibers, intensive research resulted in using rubber disks to separate newspaper, rather than the steel disks used for the OCC. "The coefficient of friction for rubber works better on papers such as newspaper or mixed paper, which is lighter," says Willis. "On cardboard you can use the serrated disk, grab that cardboard, and just bang it forward. But [with] paper, you have to really take it almost a sheet at a time. We use a number of configurations of that disk, depending on what we want to accomplish. We also utilize a bed of air on top of our screens to aid the paper movement forward and we have a patent on that. We're coming out with a new screen called our V-screen. It will double the amount of material that can be run at one time, and it will do a better job of separation while breaking less glass."

Into the Future

For the recycling processing industry, the rapid growth of the 1990s in which new MRFs were being designed and built almost daily to meet a growing demand has leveled out. As collection programs move toward single stream in order to maximize efficiency and minimize costs, the processing industry is moving toward consolidations as well.

"Single stream has been a huge breakthrough," says Sean Austin, sales manager for Bulk Handling Systems in Eugene, OR. "I highly doubt that you're going to see anything that dramatic as far as cost savings. Everything is based on economy and payback, and you have to get to the point where their investment is going to give them a payback that they need. Fifteen to 20 years ago, it was highly advanced to run your recyclables at the sort belt. Now people are starting to retrofit mechanical sorting devices and screens into their systems to reduce labor."

For many of the small MRFs, the growing volume might be more than they can handle and the cost of retrofitting these new technologies might not be feasible because it shrinks the margin between what they cost and what MRFs make on the sale of the end product. With greater volume comes the opportunity to lower operating costs, so the prediction is to see the big MRFs gain a competitive edge at the expense of smaller MRFs, especially in the large metropolitan areas. "When you look at the cost per ton to process material, the cost goes down significantly if you can run high tonnage through the MRF," states Willis. "In large metropolitan areas where you can combine smaller MRFs that are running single shifts and slow lines, you can put it into one large MRF where you're running multiple shifts and high volumes. Your cost per ton is going to be lower, assuming the equipment is state of the art and low maintenance."

Lynn Merrill is director of public services for the City of San Bernardino, CA.

MSW - May/June 2003