Modern MRFs Help Operators Cut CostsModern MRFs Help Operators Cut Costs
August 1, 1995
Sydney M. Harris
Material recovery facilities (MRFs) come in all sizes, shapes and forms. Some facilities process municipal solid waste (MSW) while others handle paper, commingled containers and commercial waste.
Similarities among MRFs include the basics such as a tipping floor and processing, storage and maintenance areas. However, once the processing and operations begin, the similarities among MRFs become few and far between.
Going Back In Time Today's processing equipment has come a long way. Manufacturers used to assume that the materials' bulk density was light and could be sufficient for light duty structural members, conveyors, screens and ancillary equipment. Further, abrasiveness and the nature of the material were rarely considered. These oversights led to tipping floors being gouged by glass, failing conveyor belts and structures, excessive erosion on the ram face and box of balers and low overall plant efficiency.
Some manufacturers designed equipment according to the experiences of the mining and bulk materials handling industry and to their European counterparts. However, the extra-heavy equipment was not cost-competitive and the light-duty material led to high maintenance and operating costs.
Early MRFs also were labor intensive, compared to today's automated facilities. Today's technology helps manual laborers increase their productivity levels, according to Bill Brennan, the operations manager of Star Recycling, which is a division of ReSource NE Inc. Productivity will continue to increase as long as manufacturers work closely with operators in designing equipment, said Brennan.
Traditional ways to purchase equipment, such as selecting from a manufacturer's laundry list, are dwindling, according to Brennan. For example, as the recycling industry has matured into an intense operation with specific equipment needs, companies like Caterpillar Inc. have developed equipment specifically for solid waste operators. Cat is currently working with Star Recycling to enhance the front-end processing of C&D waste.
Now that manufacturers have overcome the earlier design barriers, today's processing equipment and systems are sturdier, more mechanized and operator-friendly. Some systems also are becoming more affordable for MRF operators.
Processing Systems Manufacturers and vendors continually evaluate processing systems. "We have learned that more automation in a system requires a more homogeneous feed," said Dick Merrill, sales vice president at Mayfran International, a supplier of conveying equipment and engineered systems. To meet this requirement, Mayfran, as well as CRInc. and RRT offer systems which use multiple feed conveyors as much as possible. This method of operation relies less on the tipping floor operator to feed the system properly.
The materials handling conveyor is the heart of any processing system. The conveyor moves materials from each sub-process point so that they can be transformed from mixed wastes to marketable commodities.
Public and private facilities have distinctly different equipment needs. For example, a large portion of the private market is demanding heavy-duty equipment to reduce maintenance costs and downtime. However, many municipalities, in their direct bids for systems, are selecting light components. Manufacturers suspect that municipalities with tight budgets are willing to override operating cost concerns in order to minimize their capital costs. Nonetheless, both privately and publicly operated processing facilities are mechanizing their MRFs and including more unit operations, which require conveyors, to reduce labor costs. Glass crushing and aluminum recovery by eddy current separators are examples of these systems.
Unless the operator prefers a specific type of conveyor, the manufacturer or system provider usually installs the equipment which they are most comfortable with and are willing to warrant. For example, the processing line in-feed conveyor can be a flexwall "box" belt or a chain-driven rubber belt. Both types offer advantages and disadvantages. For instance, the boxbelt reportedly can be difficult to repair and has arrangement limitations while the chain-driven rubber belt reportedly softens with age and allows materials to leak into conveyor pits or floors. Chain-driven belt conveyors, with either steel or rubber belting, are almost always used for baler in-feed conveyors.
Conveyors are not limited to the boxbelt or chain-driven types. For instance, the sliding pan sorting conveyor has rubber belting which slides directly on the conveyor's steel pan structure. These conveyors reportedly are durable and minimize the leakage of material under the belt. Some manufacturers provide slotted pans to automatically remove materials that drift under the belt. Also, "belly rests" have recently been developed as part of the conveyor frame. Belly rests make sorters more comfortable as they lean against the conveyor.
Some conveyors, such as troughing idler conveyors, are suited for carrying granular materials such as glass or crushed concrete, at higher speeds than sliders or chainbelt conveyors. These units reportedly are low-cost, simple to operate and easy to maintain.
Bag Openers Materials in plastic bags present some unique challenges. The bags can be manually or mechanically opened. In either case, the processing costs will be higher than those of loose materials. However, the higher operating costs may be offset by the cost savings in collection. Facility developers must determine these costs before designing and building a site.
Several types of bag breakers are available. The bag opener can be a trommel, with specially designed knives installed on the circumference of the drum. This unit is suitable for mixed waste and MSW, but generally not appropriate for commingled containers, unless glass breakage is part of the operating plan. For commingled containers, operators may choose an auger type bag breaker. Other units include heated rods or high-speed rotating teeth.
Magnetic Separation Magnets are a time-tested tool for separating ferrous materials for the infeed stream of recyclables. Depending upon the facility layout, the magnet can be installed in-line or cross-belt. Permanent or electro magnets are available, with or without a continuous belt. The permanent magnet generally is more cost-effective when the depth of pick up is low, usually seven to eight inches. The electromagnet reportedly gives more power and is more effective at depths of 10 to 14 inches.
The designer's experiences often dictate the type and arrangement. The in-line arrangement reportedly is more effective than the crossbelt, but is usually more difficult to accommodate in the layout. Since the power of the magnet decreases as the distance to the feed belt increases, it's important to establish an optimum distance from the magnet to the conveyor carrying the feed stream. Also, large, non-ferrous objects on the belt can interfere with the magnet's ability to attract ferrous materials. This is more likely to happen with a crossbelt magnet, with the resulting contamination of the ferrous stockpile.
In-line magnets allow a "free-fall"' before pick up, which reportedly prevents obstruction and reduces contamination of the recovered product. A dual magnet arrangement, where the ferrous material is picked up, then allowed to free-fall before being picked up on the second magnet, reportedly is more effective for producing products free of contamination. However, this arrangement is much more expensive than the others and is rarely used in MRFs, unless the price of scrap justifies it.
Commingled bottles and cans present some possible maintenance problems. The lids of steel cans often get caught between the magnet and its moving belt, which results in excessive wear, maintenance problems and downtime. To minimize belt wear, suppliers offer options such as urethane, heavy-duty rubber or stainless steel metal-clad belts. The metal-clad belt reportedly reduces wear, but it is another capital cost of the magnet. Once again, the MRF's designer and operator need to analyze the costs and benefits of the available options and plan accordingly. Another option is to install a magnetic head pulley on the infeed conveyor, or a transfer conveyor to the sorting belt. Typically, these are relatively ineffective at the head end of the system because they often allow unacceptable levels of trash to be captured along with the ferrous metals. The scrap metal recovered has minimal value unless the trash is subsequently removed. However, if the system does not have an eddy current separator, a magnetic head pulley can effectively remove steel and bimetal cans from the aluminum stream at the tail end of a processing line.
Screens Screens are a vital element of any recycling system. Screens remove grit, small stones, broken glass and other materials that are below a pre-determined size cut. They also divide the feed stream into size components that are easier for sorters to process.
Vibratory finger screens are often placed at the head of the plant for processing commingled containers. These screens separate the "lights" (plastics, aluminum) from the "heavies" (glass). Occasionally, a rotating screen, or trommel, is used for the same function. MRFs that process paper or mixed waste usually prefer the trommel for head-end screening. Trommels, however, are costly and require more floor space and building height as well as longer and higher conveyor runs. As a result, the conveyors will cost more and the facility structural costs are higher. Nonetheless, trommels are relatively simple, rugged pieces of equipment which reportedly provide high operating results with minimum downtime.
Mike McLemore, executive vice president of Central Manufacturing Co., said that when designing trommels to accommodate systems which process paper, mixed waste, MSW or commingled containers, it's important to consider the system's rotational speed, pitch (angle of drum to the horizontal), length of drum, screen diameter and hole pattern. Also, since trommels break most glass, they are not an appropriate screen for glass recovery.
Mechanical bar screens also are available for MRFs. These screens reportedly have a limited throughput, which is compensated by a lower capital cost and smaller footprint than trommels, according to Steve Jones, manager of business development, Rader Resource Recovery Inc.
Material Classification Mechanical or pneumatical systems are used to process commingled bottles and cans and to separate materials automatically. The system suppliers usually provide these systems as a proprietary process. Each vendor has its own design and usually warrants the separation efficiencies of the system. The air system design features include duct capacity, air velocities, dust control and cleanout. Excessive carryover of glass, for instance, may create a housekeeping problem as well as lower the market value of the recovered aluminum and plastics. Separation efficiencies reportedly can be as high as 90 percent.
CRInc., a licensee of the Bezner system, mechanically separates the lights and heavies. The process requires chain curtains, which are mounted over a sloping conveyor to limit the roll-through of materials based on the weight of the product. Separation efficiencies are expected to be similar to the air classifier.
Glass Crushers To reduce the amount of residue sent to a landfill, as well as enhance marketability, many MRFs will install glass crushing systems for either the mixed broken glass stream alone or the colored, separated streams of flint, green or amber. Typically, the crushers will reduce the aggregate to a cullet of minus 3/8 inches, or as required by the processor. If a market price for broken glass is close to the price offered for cullet, the operator may opt to install simple glass breakers, or nothing at all, rather than invest in expensive, high-maintenance crushing equipment with the necessary auxiliary equipment such as dust control. Also, glass processing equipment wears rapidly, requiring frequent replacement. If the incentive of higher prices is eliminated, crushing becomes less economical.
A few facilities find markets for mixed broken glass aggregate if it is clean and uniform. A crushing system for mixed broken makes sense when the costs for landfilling are compared to those for utilizing the aggregate for road base, asphalt or pipe bedding. The operator may be hampered, however, by the lack of legislation in many locales to allow such use.
Currently, some experimentation is being undertaken to further process mixed broken glass to recover the flint, or clear, glass from the mixture. This may increase recovery of flint glass in the mixed broken stream by 20 to 30 percent, reducing landfilling costs substantially. However, unless the per-ton disposal costs are significant, the process may only be economically feasible at this time for large installations. Each location will require cost-benefit analyses to determine applicability.
Balers And Densifiers Balers have been used in the recycling industry for many years. Through the years, they have undergone many revisions to become a marketing tool for plant operators. Today's single ram and two ram balers have been designed to handle diverse materials. Also, more powerful machines have increased bale densities, which allow for heavier payloads on trucks and containers.
"The second generation of balers is more automated than previous balers," said Fred Johnson, vice president of International Baler Co. "Equipment supplied to a MRF today does not require the constant attendance of an operator. As a result, the plant manager can save operating dollars and improve the company's bottom line." Richard Harris, vice president and general manager of American Baler Co. echoes his thoughts. "More flexibility has been built into balers," said Harris. "Automatic controls allow them to operate on multiple grades of recyclables with minimum operator attention."
Hydraulics also play a major role in effective balers. The modern hydraulic systems are "more forgiving," said Harris, who attributes the major changes in the baler industry to technological advances.
A typical MRF of the 1990s uses a single baler to bale paper, cardboard, plastic jugs, aluminum and ferrous cans. To cut operational expenses, operators must make sure that the baling wire is used efficiently. During the life span of the baler, it's possible to spend more on wire than the baler cost itself, said Joe Szany, sales manager of Lindemann Recycling Equipment Inc.
Eddy Current Separators The eddy current separator (ECS) is the latest advancement in recycling equipment. These units, which were originally developed for the automobile shredding industry, have been designed for non-ferrous metals recovery. The original units, which touted eccentric rotors and rare earth magnets, also bore hefty price tags. Current offerings have concentric rotors and some use "modified" rare earth magnets to reduce costs. They reportedly have less controls and often are sold without a feeder. Greater amounts of non-ferrous metals can be removed by rearranging the splitter bar, but more trash will contaminate the end-product.
Rolling Stock All material recovery facility operators need fork lifts to move material and bales and front end, skid-steer loaders. With numerous models to choose from, service and availability of equipment parts are very important considerations, said Steve Viny, president of Norton Environmental. "Without good dealer service, you can be in some serious trouble. Garbage is unforgiving: it comes in every day, and it also must go out every day."