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Shredding Myths

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An expert in shredding equipment explains how and why preshredding and residuals shredding can be incorporated into a C&D recycling operation.

Kristin Smith May 14, 2013

Incorporating shredding equipment into a C&D recycling facility is enabling recyclers to reduce material size for easier processing down the line or as a way to create valuable end products. Shedder configurations can vary from facility to facility depending on material stream and desired material size, among other factors.

SSI Shredding Systems Inc., Wilsonville, Ore., designs and manufactures low-speed, high-torque industrial shredders and size reduction systems for a variety of solid waste recycling and scrap shredding applications. Terri Ward, a one-time SSI customer, is responsible for solid waste market sales and for product management of SSI’s Pri-Max™ primary reducers. She has spent 20 years with the company. Ward helps answer some questions posed by Construction & Demolition Recycling about shredding equipment that C&D recyclers can apply to their operations.
 

Construction & Demolition Recycling (CDR):What is the primary purpose of a shredder used at a C&D recycling facility and what are some benefits of including a shredder as part of a C&D processing system?

Terri Ward (TW): From the front end to the back end, there are a number of ways low-speed shredders are used in C&D recycling facilities. Whether a shredder can be justified within an operation—as well as what type and configuration of shredder—depends on a number of factors. The size of the operation (tons per day), character of incoming material, end market opportunities and transportation and disposal costs unique to each operation dictate how a shredder might be incorporated successfully into an operation. That said, two common applications are preshredding and residuals shredding.


Preshredding.
Preshredding is where a primary reducer style shredder is positioned as “Step 1” in the process to accept material from the tipping floor and produce a coarse shred with a top size between 18 inches and 36 inches—with minimal fines or dust. It’s a lot trickier than it sounds to shred bigger than smaller. But in this case, the shredder’s main function is to prepare material for the downstream equipment to be as effective as possible. This means making material more consistent and flowable, without making it too small to recover—whether mechanically or by hand. Not every shredder can do this by design, but the benefits are substantial:

  • Safety and consistency – Loading mixed C&D on a sort-line without preparing it in some fashion means long, unwieldy and/or heavy objects routinely encountered by sorting personnel. This can compromise both safety and efficiency, as this debris has to be dealt with quickly or the line stopped frequently. To overcome this, most operators use an excavator—sometimes a landfill compactor—to crush material. However, this is a slow process that limits throughput, is hard on equipment not inherently designed for the task and relies on expensive diesel fuel to operate. And the results are less than consistent.
  • Capacity – Even using an excavator to crush material, most sort lines suffer from frequent “slugs” of material followed by empty spaces or “black belt” due to the irregularity of material. Primary reducers are able to produce at very high rates—up to 100 to 150 tons per hour—of mixed C&D. And they are able to discharge material that’s consistently sized and on a metered basis, dampening the slugs and eliminating black belt.
  • Sorting efficiency – Whether the downstream system is simply a manual sort line or a more automated system of screens, density separators and/or optical sorters, size-reduced C&D is more homogenous and optimizes the flow of material for all of these methods to be most effective.
  • Metal liberation – In the process of sizing mixed C&D, commingled materials are often separated from one another, liberating individual objects—such as metals—so they can be more effectively and cleanly recovered. A magnet can recover ferrous metals from a flow of size-reduced C&D much better than manual sorters can from an inconsistent flow of unsized C&D.
  • Layout flexibility – Space constraints are less problematic and equipment layouts become more flexible as size reduced material can more easily make 90-degree transitions, flow through chute work and bidirectional conveyors, etc., without the jamming or bridging that is common to unprocessed C&D.


Residuals shredding.
Residuals are the leftovers or material remaining after all of the higher-value commodities (i.e., metals, wood, cardboard, aggregate) or lower-cost disposal elements (i.e., fines) have been removed. This can be anything from bulky waste removed at the tipping floor (mattresses, tires, carpet, plastic tarps, buckets, etc.) to the smaller bits of treated wood, insulation, vapor barrier, etc., coming off the end of a sort line. For many customers, these materials represent significant transportation and disposal costs. Depending on local market conditions, a facility may choose a low-speed shredder to process these for the following reasons:

  • Densification – Shredding bulky residual wastes can increase the bulk density by two to four times, minimizing transportation costs to haul to landfill. Depending on the distance hauled and volume of residual waste, this can easily justify the cost of a shredder. Further, in some cases problematic wastes, such as tires and mattresses, can’t be cost-effectively landfilled without being shredded first. Likewise, some landfills will cut their disposal rates for shredded waste, as it is so much easier to spread and compact than unshredded waste.
  • Daily cover – Some landfills can accept shredded residuals as ADC (alternative daily cover), which can reduce disposal costs substantially. The type of shredder varies widely, as requirements for ADC can be as small as three-fourths to 1 inch to as las much as 12-inch munus.
  • Alternative fuel – Due to the potential high Btu value of C&D residual streams—particularly the lights fraction from a density separator—some customers are able to shred a substantial portion of their residuals stream for use as an alternative fuel by cement kilns, lime kilns, paper mills, biomass power plants and the like.


CDR: What different equipment configurations work best when incorporating a shredder into a C&D processing system?

TW: Unsorted C&D is loaded with a wide variety of materials challenging to any shredder—abrasive materials like concrete, sand, and carpet; nonshreddables, such as I-beams and manhole covers; and large bulky objects that are hard to grab and want to bridge, such as plywood, water heaters and treated posts. Counting on a primary reducer to be Step 1 in a large-scale recycling plant means performance and reliability are of utmost importance.

In order to deliver this, primary reducers, such as SSI’s Pri-Max™, incorporate high-efficiency hydraulic drives for maximum shock-load protection for nonshreddables; large openings for small abrasives and nonshreddables to pass through; large surface areas of abrasion-resistant wear material to minimize maintenance efforts; and huge claw-like cutters mounted on bidirectional cutting shafts to effectively grab bulky material and prevent bridging.

While portability still has its place, most modern C&D recycling lines are housed within a building, running off site power, in stationary configurations. Even if being retrofitted to an existing recycling line, the most cost-effective and highest performance approach for a primary reducer is also to be a fixed, electric-powered machine. Portable units are designed for over-the-road travel. This generally means lighter-weight materials, tight clearances and diesel power units—all things inappropriate for processing C&D on a high volume basis. Heavy-duty processors, large transitions to heavy-duty conveyors and electric power are all preferable for C&D pre-shredders. Running the same machine off a diesel power unit versus an electric power unit can cost $100,000 or more annually.


CDR: When would preshredding of material make the most sense?

TW: There are always exceptions, but a primary reducer to preshred material generally makes the most sense in recycling plants that wish to exceed roughly 40-ton-per-hour line capacity and where there’s a high percentage of construction waste or industrial and commercial waste, which is lighter and bulkier than demolition waste. Preshredding also makes sense in small buildings that rely on making 90-degree turns with material, as well as in systems planning to substantially automate the sorting process, rather than relying mostly on manual sorting.


CDR: Is some sorting of material required prior to preshredding, such as removal of metals?

TW: Generally speaking, most low-speed shredders used for preshredding C&D can handle most anything in the waste stream without damage. That doesn’t mean everything tipped on the floor should go through the shredder. Hazardous or flammable materials (propane tanks, chemicals, etc.) should always be removed on the tipping floor to maximize safety.

To avoid slowing production and minimize unnecessary wear, it’s advisable to remove nonshreddables, difficult to shred items and large abrasives, such as heavy I-beams, bundles of wire/cable/rebar and concrete blocks and slabs. Finally, some customers will remove matresses, carpet rolls, tires, large tarps, etc. If the downstream sorting equipment can’t handle it, there’s no value to shredding it or the material retains a higher value by not shredding it.


CDR: How does material composition factor into the size and speed of the shredder installed at a C&D recycling facility? Can the same shredder be used for a variety of materials, such as concrete and wood?

TW: With regard to preshredding C&D, the physical size of the shredder needs to be large and easy to load or there’s no point in preshredding. If the raw material is already relatively small because it’s coming from demolition sites in populated areas where contractors are incentivized to crunch up material on site to minimize transportation and disposal costs, then the case for a preshredder is difficult to make.

It’s rare to see a high-speed shredder/grinder preshredding C&D for sorting—mainly because they aren’t as robust as a low-speed shredder for handling heavy tramp metal; but, also because they tend to shatter or pulverize material to where sorting becomes nearly impossible.

Most low-speed shredders are very versatile and can be used to process a variety of materials. Not only mixed C&D, but bulky wastes, tires, appliances, clean wood streams, mattresses and most other material streams C&D recyclers often encounter. That said, they’re still geared toward making a relatively coarse product—not a small product that would typically meet an end market “spec.” Usually a secondary low-speed shredder or high-speed grinder will be utilized to make a finished product, depending on the material to be converted and the finished product size specification.

As far as concrete goes, it’s generally not advisable to process high volumes of concrete through a shredder because of the high wear costs. While there’s often concrete mixed in with C&D that goes through a shredder, segregated loads and/or large blocks, chunks or slabs are really more cost-effectively processed by crushing, not shredding.


CDR: How can a shredder buyer weigh the claims made by equipment makers? Do you recommend on-site visits to installed units?

TW: Prospective buyers need to go beyond a “quick-hit” site demo on a portable piece of equipment. They need to do more homework and get themselves educated about the different types of shredder designs to be sure they can truly do the job required for an extended period of time without frequent disruption or downtime.

If you’re not mechanically inclined, have your mechanic or plant engineer dig in deeper with the manufacturer regarding design, performance, maintenance, trouble-shooting, etc. Few if any shredders do everything well; there are always trade-offs. Be sure you ask a lot of questions—including what the shredder’s weaknesses are.

Make sure the salesperson you’re dealing with has experience in the industry, understands the application and is really listening to your goals and preferences. If not, find someone who does. It’s not that uncommon for otherwise well-designed equipment to be misapplied. A combination of site visits, factory visits and talking with existing shredder users can help ensure you’re buying the right piece of equipment for you.


CDR: What kind of maintenance schedule, including wear parts replacement, is recommended to keep the shredder running properly?

TW: Beyond the routine inspections, lubrication and filter changes, there’s no question that given the nature of C&D, in the case of preshredding, wear is the most significant cost over time. Most manufacturers recommend periodically hard-facing (welding) wear surface areas—much like one would do to loader buckets or excavator teeth—in order to maintain optimal processing.

Depending on the shredding technology, make up of process material and operating hours, hard-facing frequency can range anywhere from weekly to annually. The more diligently hard-facing is done, the longer more extensive work can be deferred. At some point, though, cutter tips/rotors/shafts and fixed anvils/combs and other wear surfaces will require replacement. When considering what technology to use, it’s important to look to the long-term and worst-case service requirements—do shafts/rotors have to be removed to replace parts? How long will it take to remove rotors or shafts when it becomes necessary? How much work can be done on site versus at a fabrication or machine shop?

 

Terri Ward is responsible for solid waste market sales at SSI Shredding Systems Inc., Wilsonville, Ore. More information is available at www.ssiworld.com.

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