See It, Sort It, Sell It

September/October 2008

Optical sorting systems can transform mixed paper streams into cleaner, higher-value commodities, but processors must weigh many issues before pursuing this high-tech route. 

By Marc Hequet 

Decades ago, paper processors who needed to sort mixed paper had only one choice—do it by hand. Then, almost 20 years ago, mechanical screening systems hit the scrap paper market. Those machines took care of the "rough-cut" sorting, such as culling OCC or ONP from a mixed material stream, saving a lot of manual labor in the process. Then, about a decade ago, optical paper sorting technology emerged, giving processors some high-tech options for their separation tasks. Such systems are designed for "fine-cut" sorting, such as distinguishing white from colored office paper or further separating different paper grades.

Today, paper processors continue to use all three sorting approaches, depending on the types of paper they handle, their end markets, and other factors. So how do packers decide whether optical sorting technology is right for their processing operations? Here's some background on the technology and a review of factors to consider. 

The Optical Advantage
Packers sort mixed paper for two main reasons—to remove contaminants or prohibitives and to separate the material into clean, homogenous grades that will fetch higher prices than the original mixed material. Though the emergence of mixed paper as a hot export grade has reduced the need for extensive sorting in some cases, many mixed paper streams have plenty of nonpaper items—such as plastics, aluminum cans, and waste—that must be removed, even if the processor plans to sell the material as mixed paper. Further, many paper mills still require highly sorted fiber for their operations, prompting processors to look for the best sorting strategy to meet the market demands.

Mechanical screening systems—the step before optical sorters—are now considered mature technology, with more than 1,000 units installed in the United States, according to Nat Egosi, president of recycling-center builder RRT Design & Construction (Melville, N.Y.). Mechanical screens reportedly are effective at separating specific paper grades from mixed material based on factors such as size and rigidity. The Star Screen from Lubo Screening & Recycling Systems (Emmen, Netherlands), for instance, removes OCC from mixed materials, while its single-deck and double-deck news screen systems separate ONP from other papers.

For finer sorting tasks, processors now can turn to a variety of optical sorting systems (see "Sorting It All Out" for information on vendors in this niche). All of these systems use an optical-sensor array to "see" recyclable paper as the mixed material stream passes under it on a conveyor belt. Depending on the sorting technology they feature, sensors can detect the paper's color, material type, gloss, and even lignin (wood) content. Operators can adjust the equipment's software to identify and sort by size and shape, too, though paper on a belt tends to overlap, making it difficult for sensors to distinguish shape.

Many optical sorters available today use near-infrared spectroscopy because most materials reflect a unique "fingerprint of light" in the near-infrared wavelength spectrum when they are illuminated. Some sorters use visual spectrometry to sort material by color. Others have a sensor that can identify paper that was printed with four-color process (cyan, magenta, yellow, and black) technology. Regardless of the recognition technology, when an optical sensor detects an item that the processor has programmed it to remove—such as a piece of corrugated in a stream of newspaper—the sensor triggers an air jet aimed at the spot on the conveyor with the unwanted item and blows it off the belt.

Optical paper sorters are most effective after mechanical screens do the first sort, cutting unwanted items to less than 15 percent of the paper stream, says Jeffrey Van Galder, national sales manager for Karl W. Schmidt & Associates (Commerce City, Colo.), which makes conveyors and sorting systems for material recovery facilities, paper processing plants, and other applications and is the exclusive North American representative for the Redwave optical sorting technology. "The optical sorting is going to be most beneficial if you're making a primary cut with your mechanical screens and then further want to clean or upgrade your material," he says.

The throughput capacity for optical sorters depends on the type of material on the belt and the width of the belt itself. MSS (Nashville, Tenn.), a manufacturer of automated sorting systems for a variety of materials, says its MultiWave system can sort 6 to 8 tons an hour of light, single-sheet office paper, 12 to 14 tons an hour of ONP from single-stream operations, and 20 tons an hour of ONP from dual-stream collections in which mechanical preprocessing hasn't "fluffed" the paper too much. Redwave offers optical paper sorting systems in three widths—48 inches, 64 inches, and 96 inches—with sorting capacities of 6 tons an hour, 8 tons an hour, and 12 tons an hour. And the PaperSort system by TiTech (Oslo, Norway) comes in conveyor widths of 28 inches to 112 inches, with throughput capacities up to 9 tons an hour. The company also offers a system that measures 160 inches wide—comprising two 80-inch-wide conveyors—with a processing capacity of 12 tons an hour.

Over the years, optical paper sorters have become faster and more selective. In MSS' MultiWave system, material drops onto a 96-inch-wide conveyor moving at 1,200 feet per minute—almost 14 miles an hour. TiTech's PaperSort system runs at 700 feet per minute. In comparison, conveyor belts on manual sorting lines run at 75 feet to 150 feet a minute.

In addition to their speed advantage, optical sorters can replace or reduce manual labor and provide greater uniformity in the final paper product, manufacturers say. Indeed, some sorting of white office paper from mixed office paper is all but impossible to do manually in a cost-effective way. Workers can barely pick enough to cover their wages, says Felix Hottenstein, sales director for MSS, asserting that optical sorting is the answer for creating that higher-value output. "Machines bring a lot more consistency into the output product," he says. "They show up every day, work the same all the time, and don't have bad-hair days."

The sorting accuracy of optical systems depends on such factors as the customer's infeed material and how well the material is presented on the conveyor belt. Some vendors claim that their systems have recovery accuracy rates as high as 95 percent. 

Assessing the Challenges
Overall, optical sorters sound great, right? So what's the catch? According to Steve Miller, president of Bulk Handling Systems (Eugene, Ore.), which makes mechanical screen sorters for paper, it's "technically feasible" for optical technology to identify scrap paper among containers, brown fiber in a newspaper stream, and various colors of paper among other colors. "They all work. They're all able to identify material," he says, with one caveat—"it is not possible to control the mechanical movement of material such that optical technology would be a cost-effective alternative to mechanical screening or manual sorting."

The biggest mechanical challenge for optical sorting technology is that it requires "singulation"—distribution of recyclable items in a single layer on the conveyor belt, with no item hiding another. Optical sensors "rely on line of sight," Miller explains. "The technology puts a lightwave down and takes a reflection from it. Everything has to be laid on the belt as an individual piece." Without such singulation, he says, "optical sorters produce too many false positive ejections and miss the target pieces that are covered." In addition, if an unwanted item hides beneath an item the system is programmed to recover, that item might remain in the stream—probably to be removed the old-fashioned way, by hand.

Singulation is indeed the hard part for optical sorting systems, Jeffrey Van Galder says. To work properly, the system must spread out material upstream using a sequence of mechanical separations. The mixed material must be "metered through the use of the screens and belt speeds throughout the system," Van Galder says. "Even the initial infeed of the system can have an impact on the flow of material downstream when it meets the optical sorters." This control of the incoming material is "critical to the success of the optical sorters," he says. "If material is overlapped or buried within the stream to be sorted, it's difficult or impossible for the optical sorter to make accurate identifications and take appropriate action with the read that it makes."

The high operating speed of optical sorting systems can create another drawback. Once the belt is traveling fast enough to generate respectable throughput, the paper on the belt has a tendency to flutter. "Paper tends to fly when it's moved along a belt at high speeds," BHS' Miller notes.

Among other challenges of optical sorting systems, a voltage surge in the building could throw off the system's software, or the equipment might fail to go through its proper startup or shutdown sequence, says RRT's Egosi. In addition, the equipment can get out of calibration, its sensor can get dirty, its air compressor can malfunction, and its air ejectors can clog or fail. What's more, the general vibration of the system can cause errors. 

Making the Decision
Paper processors should weigh several factors before purchasing an optical paper sorting system, in part because such systems are still relatively new. "Nobody can say with any certainty how well they work [because] their operating history has been quite short and the technology is still evolving," Egosi says.

Prospective buyers should evaluate the potential labor savings, projected increase in product quality and consistency, and estimated increase in value for the final product against the capital and operational cost of the equipment. One vendor says its optical sorting systems can cost from $750,000 to $2 million, depending on the model and the system's features, such as the width of its conveyor belt and any options.

In terms of operating costs, the largest is electricity to power the conveyor and the compressor for the air jets. Power requirements can vary: A clean infeed stream means fewer ejections and lower power requirements, drawing perhaps 30 kilowatts, Hottenstein says. A more contaminated stream will work the air jets harder and require more power for the compressor, using up to 60 kW.

A single-stream facility that purchases an optical sorting system should expect a payback period of two or three years, Hottenstein says, though the precise payback "depends strongly on the circumstances." For instance, is the operation running one shift or two? What are the quality requirements? Operators who handle a higher-grade product such as office paper can find that the system pays for itself in as little as 18 months. When sorting white office paper from mixed commercial material, the extracted white paper is so much more valuable than the mixed office paper that it justifies the outlay, Hottenstein says.

Cost per ton of output also varies dramatically depending on the material processed. With high-grade material such as office paper, which includes many thin and light paper sheets, the throughput is lower, driving up cost per processed ton. Lower-grade material such as ONP, which is thicker and heavier, means more throughput by weight, lowering the cost per processed ton.

When processors are considering optical sorters, what often persuades them is a desire to expand their capacity, says RRT's Egosi. They want to process more material with the same number of—or fewer—employees. Processors also turn to optical sorters when their incoming material changes or when a customer's specifications become tighter, necessitating a finer sort to make the final paper product marketable.

As a final word of advice, Egosi counsels prospective buyers to shop for optical sorting systems that require little or no maintenance, though he concedes that such equipment might be hard to find now. Current optical systems work reasonably well, he says, but they will almost certainly get better. Manufacturers continue to make "gradual improvements all the time" in the hardware and software used in their optical sorting systems, Hottenstein notes. "The more we keep running those machines, the more [we learn] what's causing the issues," Egosi says, "and the more the machines will be modified and upgraded." • 

Marc Hequet is a writer based in Minneapolis.