Destiny by Density

Separating materials using liquid baths involves immersing chopped, shredded or granulated commingled scrap material in a water-only or modified water solution so that a targeted material separates within the bath according to its density. Water has a specific gravity of 1, so anything with a specific gravity greater than 1 will sink, and material with a specific gravity less than 1 will float. Although there are water-only media baths being used in the industry, it is more common for a processor to add a salt such as calcium chloride, calcium nitrate and potassium carbonate to increase the specific gravity of the bath so that more types of materials can be sorted. At this point, the bath is referred to as a "heavy" media bath because now the specific gravity of the solution is greater than 1.

Processors can place several baths in a series, each with a different specific gravity to target different materials. Pumps are normally used to circulate the medium and to provide direction to the material being separated so that it can be removed effectively.

"There are only a handful of heavy media separation systems in use in the United States today, and they are mainly used to recover metal from wire chopping operations," says John Groscurth, president of Wire Recycling Services, a wire processing consulting service in Richardson, Texas. Groscurth has been involved in the wire reclaiming market for more than 24 years and has seen hundreds of wire separation plants throughout his career.

"Most of these methods are not new and have been used in the mining industry for years," he says. "One company even uses a sluice setup much like prospectors used in the old days to recover gold."

In a sluice, material and water are directed down a long chute. As the water cascades over the material it carries the lighter particles with it. Friction from the bottom of the chute keeps the larger particles from carrying away.

A shaker or wet table is another method borrowed from the mining industry that uses specific gravity and differential motion to achieve separation. In this type of application, material is fed in a slurry form onto a rectangular or trapezoidal table from one corner. Water is then cascaded across the short end of the table at 90 degrees to the conveying action. At the same time, the table moves back and forth along the long axis – slowly forward, then quickly back – much in the same fashion as someone pulling the tablecloth from under a place setting. This differential action causes the lighter material to separate and be washed down, as the heavier material proceeds along the long length of the table directed by riffles.

Shine Brothers, which claims to be one of the top five processors of chopped wire in the U.S., uses a heavy media system to process this scrap stream. The company first chops the wire and then removes as much of the nonferrous metal as possible with an eddy current system. However, not all the metal can be removed in this first step, so the remaining scrap stream is emersed in a heavy media liquid bath and then sent through a series of hydrocyclones. This second stage is used to remove the remaining metal and to separate targeted plastics like polyvinyl chloride.

"There are two main ways to separate the fluff – electrostatically or with a wet system," says Greg Vaughn, general manager of Shine Brothers. "We use the wet system because we think it is more efficient."

Heavy media baths are also sometimes used to remove PVC contamination from large quantities of plastic flake. In these situations, bubbles from a carbonated bath more readily latch on to the PVC particles, causing them to float while the other types of plastics sink. The PVC then can be siphoned off the top of the bath. In another type of bath, PVC can be made to sink and the polyethylene to float.

In most heavy media separation systems, the salt or additive is soluble. Because of this, any recovered material from the bath must be thoroughly washed.

Another type of system uses a suspended salt, such as calcium carbonate. The material still has to be washed, but when recovering PVC, it is not as critical to wash off 100 percent of the calcium carbonate because it is used as a filler for PVC anyway. This system has been developed by Plastic Recovery Systems, Toledo.

ORT, Oberlander Recycling Technik GmbH, Germany, makes a heavy media plant that also uses a suspended solution. But instead of using a salt, it uses atomized ferro-silicon in water. Since FeSi is magnetic, it can be recovered easily using an internal magnet, demagnetized and returned to the solution. FeSi also allows the specific gravity of the bath to be adjusted between 1 and 3.

"We use ferro-silicon instead of a salt because we can achieve a high specific gravity of 3 in our media," says Wolfgang Haberer of Oberlander. "This enables our system to effectively float and sort out aluminum which has a specific gravity of 2.7. Or we can adjust the bath to 2 so that the aluminum will sink and lighter fractions such as light stones, rubber and plastic will float."

The system has a capacity of 4 to 6 tons per hour and Oberlander officials recommend using two passes for optimum recovery of mixed streams. The first pass separates non-metallic material from nonferrous metals. The second pass recovers lighter aluminum, aluminum alloys and magnesium, with specific gravities between 1.74 and 2.7, from heavier copper, lead and zinc, with specific gravities between 7.14 and 11.34.

In the Oberlander system, the separation takes place in a drum about 4 feet in diameter and 4 feet long that rotates on its horizontal axis so that there is constant mixing of the solution to the proper consistency. The drum also allows for a more compact layout.

Another separation system is called a rising current classifier. In this type of system, granulated, commingled material is fed through the top of a tube that is centered in a large, upright, drum-like canister. The canister is filled with a water-based solution that is pumped in through the bottom. As the heavier particles sink, they are directed to suction outlets on the bottom. Lighter material floats and cascades over a lip into a ringed outer container and then it is pushed out and collected.

Still another wet system is the hydrocyclone. In a hydrocyclone a slurry is introduced under pressure at the tangential entry, causing a swirling motion to the flow. A primary and secondary vortex are formed, with the lighter particles carried with a majority of the fluid out the axial exit at the top of the unit, and the heavier particles removed at the bottom.

Liquid baths are even being used to separate large pieces of mixed construction and demolition debris. One such system is designed by Flo-Cait Environmental Systems, Holland, Mich. It involves a simple water bath that floats wood debris and separates it from concrete and other heavy material. Job site material can be dumped into the Flo-Cait system directly from the job site. Wood floats to the top and is directed to a side chute where it falls onto an expanded metal conveyor. Water drains through the conveyor and is cycled back into the system. A conveyor in the tank pulls out the heavy material that sinks to the bottom. From there, the collected wood can be processed in a wood hog, and the heavies can be crushed.

"It’s a very simple system," says James Cullom of Flo-Cait, who owned a demolition company for 17 years. "It’s so simple that the U.S. Patent Office could not believe that it was not yet patented."

The Flo-Cait system can process 60 to 100 tons of C&D material per hour using only 50 amps of power, and can even capture glued plywood and oriented strand board. "Plywood floats about 1 inch from the top, and OSB about 2 inches," says Cullom. "We have about an 8-inch cascading waterfall, so we can recover those items. Regular particle board, however, uses a lot of glue and it just sinks."

Another density separation technique – developed by Rutherford Light Alloys, England, and marketed in the U.S. by British Technologies Group, Gulph Mills, Pa. – works on the same principle of a wet bath but uses a fluidized bed of sand to separate material. The system uses an annular trough in which sand and scrap are continuously circulated by electrical vibrators that can be adjusted to provide optimum movement. Air is fed into a portion of the trough base creating a fluidized bed of sand, and scrap particles sink or float according to their densities.

For example, scrap particles are carried across the fluidized section of the trough by the circulating sand, and as this happens less dense material, such as aluminum, magnesium and light alloys, remain at the top and are conveyed out of the sand by a downward sloping perforated flight fixed across the width of the trough. Perforations in the flight allow sand to fall through to recirculate back into the trough.

Denser metal, such as zinc, brass, copper and stainless steel, sink deeper as they travel through the fluidized zone, and are conveyed by a second, lower flight out of the trough.

The system is designed mainly for scrap streams that contain a large amount of mixed nonferrous metal scrap particles. The specific gravity of the sand can be adjusted between 2.3 and 4.6, depending on the material.

"This system is simple to operate and has a low capital cost," says Max Lupton of Rutherford. "We believe it is more accurate than air separation, and because it is a dry process there are no effluents to contaminate the end product as with a wet process. In addition, the metal particles are cleaned by the scouring effect of the fluidized sand, and the oxidation losses that are associated with wet processes are avoided."

Although the system was designed mainly for mixed metal streams, Rutherford is performing tests on more complex streams such as chopped wire and auto shredder fluff to gauge the system’s efficiency.

An earlier system that was also developed in England, called Dryflo, used iron powder as a medium, but this system was eventually abandoned because it was not practical.

Air separation relies on material densities just like liquid or sand baths, and it is by far the most popular of the density separation techniques. It is estimated that of the approximately 100 wire chopping operations in the U.S. today, about 90 percent use air separation as the primary technique in their sorting systems.

There are two main forms of air separation systems on the market today. The first is called an air knife. In this method, chopped or ground commingled material is shot vertically along a deep corridor. At predetermined points along the corridor, streams of controlled air are directed downward, perpendicular to the trajectory of the material stream. The downward air streams serve as air "knives" cutting off lighter fractions, but letting heavier, denser ones through. Capture bins can be positioned accordingly.

In the second method, chopped commingled particles are sent over a vibrating table that is set at a few degrees’ incline with air blowing up through the materials to fluidize them. As the materials vibrate, the air blows the lighter particles up and these particles then fall downhill. The heavier particles, on the other hand, remain vibrating up the incline, achieving separation.

"That’s what we call a stoner," says Matt Mayo, application engineer for Triple S Dynamics, Dallas. "Just two fractions are separated – one light and one heavy. In another system, the table is trapezoidal and inclined in two directions. The mixed stream is fed on the side and as separation occurs, the various types of materials fan out, with the denser material still at the top, and the less dense material at the bottom."

"Flexibility in processing is the key to success on an air/gravity system that is going to process a wide spectrum of material," says Wayne Buck, an engineer at Sweed Machinery Inc., a maker of air separation systems in Golden Hill, Ore. "The key to separating different types of materials is to adjust the air flow, the frequency of the vibrations, and the angle of the deck."

While air separation has been around since the early 1900s, companies like Triple S Dynamics are still trying to tweak performance characteristics. For example, the company is developing new types of decks that allow for better air flow and help separate material better.

Whatever type of density separation is used, one company representative says that it is important to make sure as much of the material as possible is liberated before entering the density phase of separation, and that the particles are as uniform as possible. That means sometimes having two or three shredding phases, employing shredders with close knife clearances and sharp knives, and having some kind of screening phase prior to the density separation phase. "Take care of size and shape first," says Mayo, "and then the density portion will provide a better sort."

Challenges with wet systems involve the salt in the media solution that sticks to the particles being sorted. "You have to do a good job of washing the material, especially the PVC, or you will hinder the properties of the material," says Jack Milgrom, managing director of Walden Research Inc., Maywood, N.J.

Other challenges are to ensure that the specific gravity of a fluid bath is at the targeted set point. Therefor, a monitoring system has to be installed to adjust the medium as appropriate. Fine particles of dust can also easily contaminate a solution and change its specific gravity, so many manufacturers recommend that the infeed material be throughly pre-screened prior to introducing material to the bath.

Another problem faced by density separation systems is with materials that have densities that are very close together or that have a broad range of physical characteristics, making it difficult to target separation at a specific gravity. "Most operations have to deal with broad mix of materials and plastics," says Groscurth. "Also, many times the processor does not exactly know the composition of the mix, making separation even more challenging."

Milgrom says that about 50 percent of the plastics in wire is PVC. The rest are elastomers and other plastics. Elastomers such as rubber are a major concern, but because of their elasticity, they come through the chopping or shredding stage in larger particles. This means they can be screened prior to the wet separation phase. After the wet stage that is set up to recover PVC, leftover commingled material can be put into a mixer or "agglomerator" where it is slowly heated. This will enable any residual PVC to be screened out because PVC has a low melt temperature, and when it softens it becomes larger and sticks to other PVC particles. The low heat does not affect the other particles, which stay small.

Despite all the engineering and effort that goes into the types of separation methods mentioned above, "one just has to remember that a processor still cannot achieve a 100-percent sort," says Vaughn. Groscurth agrees and adds that more metal needs to be recovered in these types of operations. "This is a problem area that continually needs to be addressed," he says.

While sorting for metal is still lucrative, one processor is wary of making the effort to conduct any extra steps to try to recover plastics. "The economics are just not there to try to sort out the different plastic fractions," he says. "We are evaluating our own wire chopping operations, and I know others, both here in the U.S. and in Europe, that are simply landfilling plastic residues. We want to do the right thing and keep these plastic streams out of the landfill, but it is just not profitable to do so right now."

The author is managing editor of Recycling Today.