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Operations focus: Screen Plays

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The right approach will help operators handle the fines and dust generated from concrete and aggregate crushing.

Curt Harler November 8, 2012

Raise a little dust!” Gran ma would advise the kids cleaning up her porch.

Grandma did not have to deal with OSHA, the EPA or local regulating bodies. C&D recyclers, however, must spend almost as much time with directives as they do with dust.

“You always have to keep dust down,” warns Tom Kovesci, general manager at Stone Products, Canton, Ohio. “Creating a cloud of dust is a good way to get the EPA involved in your job.”

On many C&D job sites today, everything that is crushed has to go back onto the site. It also makes good economic sense to eliminate both the cost of hauling C&D to a landfill, not to mention ever-increasing cost of landfilling. In general, the fines produced on a job can be use as a filler-type product or used as a base rock. One of the easier materials to reuse is screened 304 concrete. In most jurisdictions, 304 can be used as a base anywhere.

While there are good uses for the material, the challenge remains of reducing the bulk material to a usable size while staying within the guidelines of local air quality rules. The most common method of reducing dust is to spray the material as it moves through the crushing process. However, there is a delicate balance between dampening material to keep the dust down and creating a mud bath.

“If the material gets too wet, you can’t screen the fines,” Kovesci says. The amount of water and the technique used to apply the water to the material will vary by material, job site and by conditions. Kovesci says the major consideration is the material being handled.

“The most common uses for fines produced by concrete crushing are base material, fill or road underlayment,” says Gary Pederson, vice president of sales for Major Wire Industries Limited, Candiac, Quebec, Canada. Screening blacktop generally is done at five-eighths/minus. Most C&D sites will take concrete to 304 limestone screened at one-and-a-half inches/minus. There, fines typically are not removed.

Pederson says there are several ways screens can be configured depending on the material requirements. It is common to have one screen deck screening material at one to one-and-a-half inches to make base material, with a second deck screening material at a quarter-inch or so to make fines.

Kovesci says the Flex-mat screens they have do a good job for screening fines.

Practical considerations dictate that all prepared material will not be used as soon as it is created. That means the fines have to be stacked.

“Typically, when dealing with fines, the best way to stockpile is through use of a telescoping stacker with a non-segregation PLC,” says Patrick Reaver, sales manager with Astec Industries, Sterling, Ill. This allows the user to keep the head pulley close to the stockpile to eliminate segregation and material blowing and drifting, he says.

“In concrete the fines are best used as infill. With RAP, the fines are best used back into an HMA product,” Reaver says.

Pederson says he finds that base material is a common use if the fines are “clean.” If the fines have a lot of dirt or foreign materials, they are commonly used for fill or road underlayment. Some of the heaviest concentrations of dirt can be eliminated from the get-go if the bulldozer and loader operators are aware of the need to produce a clean product.

By not scooping too deeply, they can keep subsoil out of the mix. Removal of the rebar or wire mesh used in the initial construction of a roadbed or other project is one of the other things a C&D recycler needs to deal with. Rebar is tougher to keep out of the mix and can be diverted at the crusher.

The final application for the fines will vary by geography. “In certain areas of the United States, where fines are needed for compaction, they will have a higher value,” Pederson says.


Proper Settings

Reducing fines is best and most efficiently achieved by getting the proper crusher setting. With impactors, speed reductions and curtain settings will help reduce the amount of fines.

“When trying to get high volumes of fines pulled out of the material we prefer to use a high frequency screen,” Reaver says. While he notes that any recommendation depends on what one is trying to achieve, he says they try to cut the fines as small as conditions will allow.

“Crusher setting is important in that you do not want the material to stay in the circuit any longer than necessary,” says Ron Griess, Yankton, S.D.-based KPI/JCI product manager. Equally important is screening, he continues, saying, if the screen is not efficient, you will also retain material longer than needed.

“Crusher speed and crusher settings need to be set for the application,” Griess says. He emphasizes that the speed needs to be set to get good penetration. “The crusher setting needs to be set to get the best efficiency for the application,” he continues.

The longer the material spends in the circuit the finer it gets.

“The more efficient you are at getting the material to the desired size the first time around the less amount of time you will spend in the circuit,” Griess says.

If a project involves large amounts of in-feed or surging in the circuit, the screen will be less efficient. The result is wasteful: material will go back to the crusher that is already to size.

On top of that, Griess says, material going back to be crushed when it is already to size will increase fines and generate unwanted dust.


Applying Water
“When crushing concrete it is very important to incorporate dust suppression,” Reaver states. He says the preferable spots are right before the impactor, right after the impactor, and at the conveyor discharge points.

Stockpiling concrete fines presents its own unique challenges, Pederson says. If the material is stockpiled too long and is exposed to high moisture (especially heavy rainfall between the time it is stacked and the time it is used) the stockpile will tend to harden or “set up,” he says.

“The best alternatives are to use the material as soon as possible after screening or, depending on the size of the pile and if it is feasible, to cover the stock piles,” Pederson says.

On the other hand, there are times when a contractor needs to apply more water to the fines to assure compliance with dust suppression regulations. There are several ways to get water onto the material being run. “Misters seem to work well,” Kovesci says. He recommends misting the material at the point where it is dumped from one conveyor to another.

“Don’t soak it down,” he advises. Turning dust and fines into a muddy slurry is a recipe for disaster. “Once it gets muddy, it has to dry out,” Kovesci says. That typically means the entire process will be slowed for at least a day as the water leaches out.

“When using dust suppression a high pressure low volume system is preferred to get the best results,” Reaver says.

Water fogging dust suppressors will allow the producer to reduce the amount of dust without making the material too wet. “Wet material will increase the chances of your screening material blinding and reduce production,” Pederson warns.

Self-cleaning screen cloth will help control the amount of blinding and help to remove more fines from the base materials being produced.


Keep Dust Out

Another option for removing dust is to use a vacuum-style dust collection unit. These work well on confined job sites where there is not a lot of room either to stockpile fines or where other buildings are close by and absolutely none of the dust can be allowed to escape.

It sounds simplistic, but the best way to reduce the amount of dust being handled is not to make dust in the first place.

The types of crushers used will make a difference on the amount of fines produced. “Compression crushers will tend to make less fines than impact crushers,” Pederson says. “The speed of HIS or VSI impact crushers will also have an effect on the amount of fines produced.

“The size of the material is important to the size of the crusher and its ability to handle the reduction ratio required to be efficient at the crusher’s CSS (closed side setting),” Griess says.

Slower speeds on impact crushers will reduce fines. The use of compression type crushers (jaws, cones) will help to reduce fines.

“Review these options with your crusher supplier or manufacturer,” Pederson concludes.

 

The author is a freelance writer living in the Cleveland area. He can be contacted at curt@curtharler.com.

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