Shredder Supplement -- Crushed, Ground and Mulched

Techniques for processing the stream of materials produced at demolition and construction sites are still evolving.

The sorting and separating of different materials presents one challenge, but either before or after that hurdle is cleared, reducing the materials in size presents another challenge.

Recycling equipment manufacturers offer many different types of shredders, crushers and grinders for construction and demolition (C&D) recyclers to use.

CLEARING THE LAND

When earlier generations cleared away vast portions of America’s eastern forests, the trees that were felled either found use as shelter, fencing or firewood, or were burned in open piles.

That frenzied era of deforestation in North America is over, but there are still numerous construction projects preceded by the razing of trees. Unlike earlier centuries, the trees taken down are not required for shelter, and in many areas open burning is a violation of local ordinances or clean air statutes.

What, then, to do with the resulting “green waste?” Landfilling has been one option, but states attempting to reach mandated landfill diversion rates have singled out green waste, or organic waste, as a portion of the solid waste stream that should be easily diverted.

Grinding tree limbs and branches into a usable mulch product has become one of the most common diversion techniques. Both municipalities and contractors engaged in land clearing have grown to rely on tub grinders and, more recently, horizontal grinders to process trees and brush into mulch.

Tub grinders can trace their origins back to farmers who devised the machines to grind hay and other products into animal feed. In the 1980s, manufacturers began designing and selling heavier-duty tub grinders to handle the green waste stream that municipalities hoped to divert from landfills.

According to Dan Brandon, marketing manager for Morbark Corp., Winn, Mich., tub grinders are now built in sizes up to 15 feet in diameter and powered by engines with up to 1,000 horsepower. These large grinders can produce up to 500 cubic yards of end product per hour.

“Despite all its recent design enhancements, the tub grinder remains a very simple machine,” says Brandon. “At its heart is the hammermill, a high-speed steel rotor that can be equipped with a variety of hammer configurations, numbers, sizes and shapes, depending on the machine and the application.”

The top half of the hammermill extends through the floor of the tub to encounter incoming material. The material is torn into smaller pieces until it is reduced in size sufficiently to be pulled through the grates of the tub floor and onto a conveyor belt that takes it away from the tub.

The tub itself serves two purposes, according to Brandon. “It is first a containment vessel, holding material in place until it reaches the hammermill,” he says. “Secondly, the tub functions as a feeding mechanism, with the operator using the rotation of the tub to carry material around until it passes over the mill.”

Morbark customers are primarily looking for strong high-volume tub grinders, says Brandon. “We sell more large tubs with our most popular being our Model 1300, which has a 700 to 800 horsepower engine. It seems to fit an awful lot of applications, and it has the right combination of torque, power and size.”

The private sector—primarily wood recyclers and contractors who clear land—buy the majority of tub grinders, according to Brandon. Municipalities that process their own green waste so that it is not land-filled also make up a significant percentage of the market, he notes. “With wood being up to 30% of the waste stream, it is important,” Brandon comments.

The wood recyclers “set up a yard and charge a tipping fee,” says Brandon. “They process the material and some of them color the material and screen it. Some of them are into composting.”

In the past few years, the tub grinder configuration has been challenged by the horizontal feed grinder design. Horizontal grinders contain a conveyor belt that feeds material into a solid drum that is fitted with replaceable cutters or a fixed hammer-style rotor, according to Mike Hinsey of Mac/Saturn, Grand Prairie, Texas.

Both Hinsey and Brandon note that horizontal grinders offer a safety advantage over tub grinders. “An advantage of the horizontal grinders is material containment,” says Brandon, whose company introduced a line of horizontal grinders in 1999. “One of the difficulties people have with tubs is if the operator isn’t keeping the tub completely full with material. They do have a tendency to throw material out so you have to be extremely safety conscious.”

Brandon says that manufacturers have taken steps to make tub grinders safer, but “even with deflectors and other material containment devices, they can still propel pieces of wood up in the air. Horizontal machines limit that.”

According to Hinsey, “safety problems, such as debris flying from the hopper of a tub grinder, are all but eliminated by the closed rotor housing design and the force-feed ram present on many horizontal grinders.”

Another advantage of horizontal grinders cited by Brandon is their ability to take in larger sections of trees—including the main trunk—without as much pre-cutting. “They can process long material better than a tub grinder. You can put a whole tree in there because they’re not limited by the infeed diameter of a tub. With a tub you have to cut things a little bit shorter to get things down to the hammermill.”

Hinsey believes ongoing operational costs may also be lower with horizontal feed machines. “Costs can be reduced by using the replaceable cutter tips found on horizontal feed grinders rather than hammers. They can also be powered by lower horsepower engines, saving energy costs.”

While the advantages of the horizontal grinders are beginning to be noticed by equipment buyers, tub grinders still have their loyal markets and their list of advantages. “Tubs are a little more productive in the hands of a good operator,” says Brandon. “They offer the possibility to produce more material in an hour.”

Maintenance tasks can also be easier with the tub configuration, Brandon notes. “They are easy to maintain. You can tip that tub up and get right into the hammermill and the grate pretty easily,” he remarks.

There is also a comfort level for some buyers who are more familiar with the tub configuration. “People are comfortable with them because they have been around them for awhile and they understand them,” says Brandon.

CRUSHED INTO NEW LIFE

While recycled concrete and asphalt cannot necessarily be said to be shredded, there is certainly a size reduction process occurring when slabs of pavement or building material enter a crushing machine.

Both hungry metallic jaws and fast-moving rotors are used to pulverize concrete and asphalt and break it down to a size where it can be re-used as road bed or as some other form of aggregate.

The last two decades have witnessed a staggering growth in the amount of concrete and asphalt that is recycled for a second use as opposed to being tipped into a landfill.

A 1998 study by the Portland Cement Association, Skokie, Ill., and the Construction Materials Recycling Association, Lisle, Ill., estimated that 95 million metric tons of crushed concrete is used annually, making up almost 5% of the two billion tons per year construction aggregates industry.

While figures for earlier years are hard to pin down, most industry observers agree that the relatively new industry grew rapidly in the 1990s. That growth triggered innovation by both concrete recyclers and equipment makers, with a number of crushing configurations competing for market share and attempting to make their process the most efficient.

 Impact crushers use a large rotor with hardened steel “blow bars” placed intermittently to bang against the concrete being fed in and then to throw the pieces being created against a hard steel wall to break down those pieces further into smaller pieces.

“As you put material in, the rotor and steel blades whack it into smaller pieces,” says Jeff Brace, manufacturing manager with Kurtz Manufacturing, Belvidere, Ill. “It also takes that material and throws it against a steel curtain to break the material.”

Impact crushers are built in many different sizes, with some being able to accept sizable slabs of concrete, while others may require that material be broken down first in another type of crusher.

A common preliminary crusher type is the jaw crusher, which, as the name implies, uses metal “jaws” to break down large concrete and asphalt slabs into smaller pieces. Both impact and jaw crushers can be set up to contain downstream configurations that include a belt magnet to remove any steel reinforcing bar that enters the material stream.

Sellers of impact crushers contend that their units are better configured to produce a uniformly sized recycled concrete product, and that while jaw crushers can make a good first step in breaking down slabs, machines such as impact crushers or cone crushers produce the end product. (Cone crushers, more commonly used in the mining and virgin aggregates industries, are sometimes adapted for recycled concrete uses.)

“With a jaw crusher, you’re making it smaller so you can fit what comes out through something else to size it,” says Brace. “An impact crusher lets you take that large-sized product and size it more consistently.”

As with other types of shredding or size reduction machines, crushers have wear parts that require timely replacement. “The expense can change depending on the model and the machine’s make,” says Brace.

Impact crushers are generally considered to carry higher ongoing wear parts costs, though many impact crusher manufacturers would point out that the machines are more affordable up front by a significant margin, leaving the purchaser with the ability to budget in the wear parts costs.

The wear parts on an impact crusher requiring intermittent replacement, according to Brace, include the blow bars on the rotor, the side liner plates, and the apron or curtain liners.

Calculating an average blow bar life “depends on the material you are crushing,” says Brace, citing a range of “anywhere from 5,000 tons to 20,000 tons of material processed.” He says that “anything with a high silica or high sand content usually causes more wear. So might the presence of more steel reinforcement bar.”

Brace says that large tonnage concrete crushing operators often favor the jaw crusher first/impact crusher second configuration in part to prolong the life of the impact crusher’s wear parts. “With the jaw crusher, they liberate the steel at that step, then it goes to the impactor without the steel in there to wear against the blow bars.”

Crushing equipment is made in various sizes and degrees of portability to suit different market segments. This decade, large operations devoted exclusively to crushing concrete to produce secondary aggregates have been established, and they tend to buy the largest units.

Other segments of the market—often seeking smaller machines—include demolition contractors and road construction contractors. These contractors tend to do their crushing on-site, perhaps crushing the concrete remains of one structure to be used as parking lot sub-base near the new building. Road contractors may tear up and crush slabs of old highway and use the recycled product as road base.

The portability required in such uses has become sought after, says Brace. “A lot of these contractors like to have one machine to do it all.”

A variety of factors have led concrete crushing and re-use to become more common, including the depletion of quarries located within major metropolitan areas and the desire of states and municipalities to keep concrete and asphalt out of landfills. Those factors seem unlikely to shift, meaning concrete crushing should continue to become a standard construction industry and road building practice.

Mason Brown is the founder of Big City Crushed Concrete, Dallas, a company that operates large-volume crushing units at several sites in metropolitan Dallas. He considers himself fortunate to have entered the business in the late 1980s, at a time in Dallas when it became very cost-effective for contractors to use secondary aggregates.

He is optimistic that the industry is here to stay, as the underlying reasons to recycle concrete make economic sense. “I think the industry is going to continue to grow, because landfill space is going to become more scarce and expensive,” he states. “The demand to increase the amount of material that is recycled is going to grow.”

Sidebar

More than One Way to Crush

Machines used to recycle concrete and asphalt have their origins in the mining, quarrying and aggregates industries. Rock crushing machines used in these industries have been reconfigured and often have materials separation systems added downstream to serve the concrete recycling industry.

Following are brief thoughts on the pros and cons of three crusher designs as presented by Svedala Industries in its Crushing and Screening Fact Book.

Impactors

The impactor primary crusher has a high capacity and is designed to accept large feed sizes. The primary impactor is designed to handle from 200 metric tons per hour (220 short tons per hour) to as much as 1,800 metric tons per hour (2,000 short tons per hour) and feed sizes up to 1,830 mm (71 inches) in the largest machine size. Primary impactors are generally used in non-abrasive applications where fines production is not a problem. Of all types of primary crushers, the impactor yields the most cubical product.

JawCrushers

In terms of feed acceptance capability, the customer gets more for his investment if the primary is a jaw crusher. This means less complex drilling and blasting routines since the crusher can accept large boulders. The disadvantage of this type of machine is the relatively small discharge area. This discharge area and the machine’s reciprocating motion together determine the capacity to great extent. The limited discharge area thus limits the capacity. Jaw crushers are used mostly in plants with capacities up to around 700-800 metric t/h (770-880 short t/h).

Cone Crushers

Due to their design, cone crushers are generally more expensive than impact breakers in purchase price. However—thanks to low wear in a correctly applied machine—the cone crusher is less expensive to run than a conventional impact breaker. Customers who crush abrasive or hard materials are therefore well advised to install cone crushers in the fine crushing and cubicizing stage. Cone crushers can also cubicize fine fractions, something which is not possible with conventional impact breakers. Cone crushers can be adapted to different duties—an important point when the customer’s requirements often change during the life of the machine.