September 2003
Both can be effective means of controlling dust. Here’s how to decide which is best for you.
By Mark Kestner, Ph.D.
Spray systems can be more cost-effective than baghouses, especially at large sources like truck hoppers that are difficult to control.
The subjects of the last two installments in this series, good operating and engineering practices, only go so far in the battle to control dust (see February and May 2003 issues). Most plants have to use spray systems or baghouses as the primary method of controlling dust from crushing and screening operations.
Spray systems suppress, baghouses collect
Water spray systems control dust in two ways. First, spray droplets knock down dust particles in the air. The higher the pressure and the smaller the droplet, the greater the suppressive effect. Secondly, spray systems add moisture to the process that prevents dust from occurring downstream. How much moisture depends primarily on spray pressure and the number of nozzles in service. A good commercial high-pressure spray system will add no more than about 0.5 percent moisture to the process — just about a gallon per ton. However, a garden hose at 40 psi will add a lot more because it lacks the atomization necessary to suppress airborne dust and the power to penetrate a material stream.
Baghouses control dust by collecting it. They are used to filter contaminated air from a “point source”, transport the collected dust to a central hopper for disposal, and return clean air to the environment. Their efficiency depends primarily on intake air velocity and the degree of source enclosure. Large open sources like truck hoppers are more difficult to enclose than a screen deck for example. And, unlike spray systems, baghouses do not prevent dust emissions from downstream sources because the amount of airborne dust they collect is only a small portion of all the fine particles in the process stream.
Both spray systems and baghouses can effectively control dust. Indeed, many modern crushing plants use a combination of the two. Whether you use one, the other, or both depends on how your permit is written, the effect of moisture on your production process, and your budget.
All new aggregate processing plants must obtain an operating permit. This permit is a license to emit a fixed amount of particulate pollution calculated using estimates of emission factors and control efficiencies. If your emissions exceed New Source Performance Standards (NSPS), you may be required to restrict operating hours or limit production rates. Once the state is assured that the facility will comply with NSPS, it will issue the permit and assess an annual fee based on the amount of total potential controlled emissions calculated in tons/year. This market-based approach is designed to charge the producer for the right to pollute and to provide a financial incentive to use cleaner production machinery and more efficient control measures.
Most states allow aggregate plants to use reasonably available control measures (RACM). However, if you are in a non-attainment area for particulate, the permitting agency may require more stringent control that uses best available control measures (BACM). Traditionally, regulatory agencies have considered source enclosure and ventilation to a baghouse BACM and the most efficient method for particulate control. However, baghouses are not practical for every source and most, if not all, state EPAs now also include spray systems as a BACM. Even the California South Coast Air Quality Management District (SCAQMD) now has permitted spray systems for crusher dust control.
The effect of moisture on your operations has much to do with the decision to go wet or dry. Can you maintain the process flow at an optimal moisture content — just enough to kill the dust, but not enough to reduce screen efficiency or throw products off-spec? I often ask plant managers if there aren’t just a few days during the year, maybe a day or two after a rainfall, when the plant runs just right — no visible dust and all products in spec. If the answer is yes, it’s a pretty good indicator that the plant should be able to operate in compliance at design capacity using a wet suppression system.
If the answer is no, dry collection with a baghouse is the alternative. Some rock just reacts badly to water. For example, if there is a lot of clay or shale in the stone, fines may tend to pack in crushers or plug chutes even when only slightly wet. The problem may also be compounded by defects in plant design, like insufficient screen area or undersized bins. In either case, it may not be possible to use wet suppression, particularly when processing finer sizes. It is not unusual to find a primary plant equipped with a spray system while the finish plant uses a dry collector — especially if air classifiers are used for product separations. Dry collection may be the only option for dust sources that would bottleneck production if moisture is introduced.
The cost of control is also an important factor. While less efficient in smaller size ranges, wet suppression systems are much less capital intensive. Indeed, the cost of spray system equipment for a large stationary aggregate plant may be less than 2 percent of the cost of the total facility. Baghouses are significantly more expensive. The cost of the collector and source enclosure may be 10 times that of a spray system. You have to determine whether the additional dollars will be offset by higher production rates over the lifetime of the plant.
Spray systems for visible dust, baghouses for respirable particulate
Spray system control efficiency falls with decreasing particle size. A typical high-pressure spray system will produce a majority of droplets in the 20- to 40-micron range. Very small particles, especially PM2.5, are tough to knock down with droplets that are much larger. Baghouses can collect these fine particles better than most spray systems can suppress them. As a result, baghouses are the preferred method for control of hazardous or toxic dusts.
Wet suppression system technology has grown to include the use of chemicals, higher pressures, or air-atomization in an attempt to improve control efficiency. Surfactants, or wetting agents, have been used since the 1950s to boost the performance of water spray systems on dusts that are hard to wet, like coal. Surfactants increase the rate at which a water droplet wets dust particles, but are of limited use on materials like stone that are much easier to wet than coal or coke.
Foaming agents for dust control were developed in the 1970s and have become an important commercial method for dust control in the coal industry. Foam bubbles are under a great deal of stress and, when they are burst by mechanical contact, they shatter into hundreds of tiny droplets. This results in improved moisture distribution at reduced water consumption rates, which made them attractive to coal-fired power plants. Because these chemicals have BTU value, utilities have been able to pass their costs along to ratepayers. In the aggregate industry, treatment costs of 5 to 15 cents per ton have limited their market.
Water spray systems have employed higher and higher pressures to reduce droplet size and improve performance. Commercial water spray systems that operate in excess of 200 psi can achieve control efficiencies at water consumption rates on par with foam systems. While less efficient than baghouses for the finest dusts, lower-cost water spray systems will remain the workhorse of visible dust control in the aggregate industry.
New, more stringent regulation of respirable silica-containing dust has provided greater impetus to the use of dry collection and air filtration systems for personnel protection. Silica is a known carcinogen and the evidence linking death rates to fine particulate exposure is no longer disputed. Of special concern are particles of soot produced by combustion processes, hence the new regulations targeting diesel exhaust. Dry collection is the best method of protecting workers from hazardous dust in control rooms, mobile equipment cabs or enclosed work areas.
Advantages and disadvantages
Wet suppression systems suffer from two main disadvantages. First, the addition of too much moisture or additive can hurt production. Excessive moisture can reduce screen efficiency and throw products out of spec. Chemical additives may upset water chemistry in clarifiers or alter physical properties. Foaming agents, for example, can affect the air-entraining properties of concrete.
Secondly, wet suppression systems are tough to use in cold weather. When temperatures drop into the low 20s, spray nozzles may freeze on-line and any water frozen in piping can cause them to burst. Heat-tracing or the use of anti-freeze agents is usually impractical because of their higher cost and there are very few examples of spray systems that operate continuously in subfreezing temperatures. Baghouses are much less susceptible to cold weather operating problems. The greatest danger is condensation in ductwork or on filter bags that can lead to corrosion or performance losses.
While baghouses offer the distinct advantages of dry processing and better cold-weather performance, they too have their disadvantages. First of all, they are not practical for all sources. Large truck hoppers are tough to enclose and vent to a collector, and many operators do not like to block their view of screen decks or other critical areas with covers or enclosures. Stacking conveyors are also extremely difficult to adapt to dry collection systems which employ telescopic spouts with vacuum returns. These are much more expensive and difficult to maintain than the alternative wet suppression system.
Secondly, disposing of the collected dust can create another major source of emissions. Putting the dust back into the process may simply move the dust problem downstream. The successful application of fabric filtration systems requires that the collected dust is either taken out of the process altogether for disposal or returned in a manner that prevents re-entrainment.
Conclusion
Good operating and engineering practices, wet suppression and dry collection systems are the four horsemen of dust control. Good operating and engineering practices are the most cost-effective controls because they rely on in-house labor and material. Wet suppression systems are inexpensive and easy to maintain making them the primary control measure for the majority of aggregate plants. Dry collection is the best control strategy for hazardous dusts or processes that cannot tolerate moisture. Picking the horses for your team and making them run together in a comprehensive dust control plan that doesn’t sacrifice productivity for compliance is the real challenge to aggregates managers.
Mark Kestner, Ph.D., is president of National Environmental Service Co., which designs and manufactures wet-suppression systems for the mining and material handling industries. He has almost 25 years of experience controlling fugitive emissions from utility, industrial, and mining operations.
Small Secondary Pays Huge Dividends
A new twist on the primary/secondary crushing circuit pays big dividends for California recycling operation.
Although small in size, the secondary impact crusher allowed for major improvements in production capacity.
Fresno, Calif.-based Kroeker Inc. Demolition and Environmental makes a living providing custom crushing services with its portable, open-circuit Eagle Crusher UltraMax 1400-45 impact crushing plant and separate 6- x 20-ft. triple-deck screening plant. For years, this single circuit has profitably delivered primary/secondary crushing of recycled concrete, aggregate, and recycled asphalt.
However, a recent contract for recycling 352 concrete barracks at the Fort Ord military base in Monterey, Calif., posed unprecedented challenges for Kroeker, Inc. The lightweight, 70 lbs. per cu. ft. concrete found in the walls and ceilings of the barracks included an extraordinarily high amount of reinforcing material. “On typical jobs, we will fill one 40-yd. bin with rebar in about a week and a half. With this material, we were filling two 40-yd. bins every day,” says Jason Strickland, Kroeker Inc.’s superintendent.
This forced Strickland to open the primary and secondary apron settings on the UltraMax 1400-45 and production suffered. The wider settings delivered approximately 60 percent passing spec the first pass, increasing the amount of returns to the primary. Running only about 30 percent of feeder speed capacity, the returns limited Kroeker Inc.’s daily spec product average production to 1,500 tons. Already encountering a three-month delay of environmental issues stemming from a potential lead-based paint on the interior walls, Kroeker Inc. turned to Eagle Crusher Company for a solution to boost production.
Unique Secondary Solution
Eagle Crusher recommended adding a skid-mounted UM 04 horizontal shaft impact (HSI) crusher to the circuit, the smallest impactor the company manufactures. The secondary UM 04 features a 27- x 32-in. feed opening, solid steel three-bar rotor, and up to 100-tph production capacities, while requiring only 60 to 100 hp for operation. The reason such a small impactor could be employed was due to Kroeker Inc. using a primary HSI impactor, rather than a jaw crusher, for initial reduction.
“With a primary impactor, much more product passes spec the first time through the circuit than with a jaw primary,” explains Jay Giltz, applications manager for Ohio-based Eagle Crusher. “If we were dealing with a jaw, more material would have been sent to the secondary crusher, and Kroeker would have needed a much larger, costlier secondary crusher for final reduction.”
A conveyor carries oversized material from the upper and middle screen decks to the UM 04 for final reduction. Upon installation of the secondary impactor, Kroeker Inc. immediately experienced a substantial increase in production. According to Strickland, “Our average daily production increased to 2,200 tons.” He estimates when crushing normal California concrete recycle material, rather than the lightweight concrete at Fort Ord, production will increase to approximately 3,000 tons per day.
Lower Costs, Higher Profits
After inserting the UM 04 into the circuit, Strickland opened the settings on the UltraMax 1400-45 plant impactor to 4 in. for the primary and 2 in. for the secondary apron. The UM 04 impactor’s aprons were set at 2.5 in. and 1 in. respectively for final reduction. The end result was more than 90 percent passing the Class II A, B 3/4-in. minus spec the first time through the circuit from feed material ranging up to 36-in. wide.
The cost for the nearly 47-percent increase in productivity at Fort Ord was a relatively low, $118,000 for the base skid-mounted impactor with diesel power and an additional 4 gallons of fuel per hour. However, blow bar wear life in the UltraMax 1400-45 increased by 40 percent to 42,000 tons of processed material per set of bars, lowering Kroeker Inc.’s wear costs.
According to Strickland, increased production is where the real payoff lies. “We are making an extra $3,000 per day off the pay belt by adding the UM 04 to the circuit.” With the addition of the UM 04 secondary, Kroeker Inc. was able to finish the Fort Ord recycling job much faster than if it had continued to use only the UltraMax 1400-45.
Kroeker Inc.’s application of coupling a large to a small impactor represents a new, more profitable way of thinking about the primary/secondary crushing circuit. Since primary impactors deliver a much higher percentage of material passing spec than that of a jaw crusher, producers can add a much smaller secondary crusher. This saves producers not only on the initial purchase price of the secondary crusher but on long term wear and fuel costs as well.
The Bottom Line
An unusually high amount of rebar in a concrete recycle application limited production and increased wear costs for Kroeker Inc.’s primary crushing plant. A small secondary impact crusher was added to the crushing circuit and boosted production by nearly 47 percent and blow bar wear life by 40 percent. Kroeker Inc. realized an additional $3,000 per day in spec product off the pay belt.
To submit a suggestion for a Success in the Field or for more information about any of these stories, contact Aggregates Manager at 330-966-2454, Fax: 330-966-2454 or email at bob@aggman.com
Dust Control for Enhanced Profit
Technologies for point and non-point dust sources can cut equipment wear and maintenance problems.
By Bob Drake
Dust is inevitable at aggregate operations. Uncontrolled, it can incite neighbors and regulators, impact employee safety and health, and negatively affect equipment operation, maintenance, and efficiency — not to mention profits.
“There isn’t much good that can be said about dust,” according to a Metso Minerals brochure. “It’s an acknowledged health hazard. It also accelerates wear on machinery and equipment. It makes maintenance and repair more difficult and more time consuming.”
Material handling and processing, storage, and traffic are the major processes that create dust, according to Neil Archer, president of Arch Environmental Equipment. For stationary or point sources such as conveyors, crushers, and screens he recommends a “total concept” approach that comprises three phases: confine, seal, and suppress. Dust collection also may be appropriate.
For non-point sources such as haul roads and stockpiles, prevention and suppression are the focus. This can include engineered solutions for road design, paving, surface stabilization, and wind screens.
Following are brief descriptions of a variety of dust control technologies categorized by dust source. Additional information from each manufacturer is available using the appropriate InfoDirect number and the link on Aggregates Manager’s web site (www.aggman.com).
Control for non-point sources
1. Arr-Maz Products
Arr-Maz Products’ Road Oyl, applied with a standard spray truck, penetrates and binds aggregate roads into a surface stronger than asphalt, according to the company. A concentrate is diluted from 4:1 to 15:1 with water to create a non-corrosive, non-hazardous, non-flammable, and non-toxic liquid. Road Oyl’s effective life depends on dilution rate, traffic, degree of penetration, grade and condition of the road, track-on, and spillage. Applications are cumulative, so reapplications should become more dilute and less frequent until a maintenance level is reached, Arr-Maz says. Road Oyl also can be used on stockpile surfaces and for erosion control.
InfoDirect 701
2. Klein Products
Klein Products of Kansas manufactures 4,000- to 30,000-gallon water tanks and spray systems for mounting on haul trucks. The tanks fit a number of models of Caterpillar, Euclid, Komatsu, Terex, and Payhauler trucks. Options include extra front, side, and rear sprayheads; high-pressure pumps; suction loading systems; electronic tank level gauges; chemical agent injection systems; and epoxy tank linings.
InfoDirect 702
3. Martin Engineering
Martin Engineering’s Fog Cannon is designed to suppress dust at open-air sites, such as stockpiles or blasting sites. A high-powered fan and plug-free nozzles provide a continuous jet of 77- to 110-micron particles in an air/water/additive mist with a range of up to 800 ft., depending on the model, according to Martin Engineering. The Fog Cannon can be used with chemical additives or water alone. Options include permanent or portable mounts and manual, automatic, and radio remote controls.
InfoDirect 703
4. Midwest Industrial Supply
Midwest Industrial Supply says its family of products, laboratory testing, site and traffic analysis, training, and a Guaranteed Annual Haul Road Program help put together the aggregate operations puzzle and favorably impact operating costs. EK35 is a penetrating, continuously active suppressant that is resistant to rain and heavy traffic and acts as a reworkable binder, according to the company. HR51 is an acrylic co-polymer installed with Midwest’s GCP Application Process — scarify and grade the haul road, mix HR51 with a site-specific amount of water into the road surface, and compact the road. Soil-Sement is a polymer emulsion with a strong molecular structure used to bond with surface materials for dust and erosion control and soil stabilization, the company says.
InfoDirect 704
5. NESCO
National Environmental Service Co.’s (NESCO) DirtSquirt is a portable wheel wash system for sites where equipment and truck tires cause tracking problems. It includes a 200-psi pump module, dual spray bars, a utility bump, presence detector, connecting hoses, and replacement spray nozzles. |
InfoDirect 705
Control for point sources
6. Arch Environmental
Arch Environmental Equipment Inc. says its Tri-Return training idler can prevent the misalignment of conveyor belts that can wear belt edges, damage conveyor structures, and cause material spillage. Spillage can be a source of dust as well as a safety issue. The Tri-Return design incorporates three idler rolls to fit the full cup of the belt. Trainer rolls are covered in rubber to reduce buildup, increase roll life, and speed reaction time, the company says.
InfoDirect 706
7. BHA Group
BHA Group offers baghouse filter bags made from DuPont Teflon resins. The BHA-TEX membrane’s surface sheds even highly sticky dust with minimal pulse-cleaning pressure and operates with virtually no dustcake, the company says. Easier cleaning of the surface of the bags allows a consistently low pressure drop, resulting in reduced fan energy consumption, higher process throughput, and reduced compressed air costs, according to BHA.
InfoDirect 707
8. Cyclonaire
Cyclonaire’s Cyclolok is a gravity-loaded, bottom-discharge dense-phase conveyor suitable for low-volume dust reclaim systems. The unit can handle up to 2 tons per hour of any abrasive, friable material at temperatures up to 450° F, the company says.
InfoDirect 708
9. Dustex Corp.
Dustex Corp. introduces a line of dust collectors with standard filter media ratings from 21 to 563 sq. ft. The dust collectors feature both bottom and top bag orientations; galvanized bag cages, cups, and venturis; 16-oz. singed polyester filter bags; and a sequential timer. The units are available in carbon steel or stainless steel construction.
InfoDirect 709
10. Dust Solutions Inc.
Dust Solutions Inc.’s fogger produces a dense fog of 1- to 10-micron size droplets by passing water through a field of high-frequency sound waves without using high-pressure water pumps, the company says. In addition to atomizing the water droplets, the sound waves clean the fogger similar to a sonic laboratory cleaner, according to Dust Solutions.
InfoDirect 710
11. Farr Air Pollution Control
Farr Air Pollution Control’s Big Round Filter reverse-air dust collector has a self-contained cleaning system that does not require plant compressed air. Cleaning of the fabric bags is performed by a rotating arm. A perferated inner cone reduces can velocities, resulting in high filtration efficiency, high dust loads, and extended filter life, according to Farr. The Big Round Filter is available in 21 standard models with a choice of 8-, 10-, or 12-ft. fabric bags or pleated filters.
InfoDirect 711
12. Metso Minerals
Metso Minerals offers the Trellex dust-sealing system for screens, feeders, chutes, and other equipment. The elasticity of Trellex rubber enables it to be configured to fit almost any type of equipment, Metso says. The combination of Trellex dust-sealing rubber and standard metals (STMs), enables construction of easily opened inspection covers and tight seals, according to the company. The systems are based on three components: Trellex dust-sealing cloth and fabric, rubber grip strips, and STM steel profiles. The rubber cloth or fabric is clamped in place by the grip strips to form an enclosure.
InfoDirect 712
13. Midwest Industrial Supply
Midwest Industrial Supply’s recent purchase of the Dust-Buster product line from Martin Marietta Magnesia Specialties included its line of Simplex control units for applying instantaneous and residual foam dust suppressants at transfer points, crushers, and stack-out and reclaim areas. The Simplex units are complete with water and chemical pumps; water and air flow meters; air, water, and chemical pressure gauges; pre-wired control panel; and foam accumulator in a painted steel environmental enclosure. The systems are designed for manual or automatic operation.
InfoDirect 713
14. NESCO
National Environmental Service Co. (NESCO) offers two high-pressure water-spray systems — Dust Pro and Dustboy. Dust Pro, for plants processing more than 500 tph, uses less than 1 gallon of water per ton of material. The compact Dustboy is designed for plants handling less than 500 tph. Both systems have high-pressure (200 psi) pumps and all brass plumbing. The Dust Pro has a remote control panel, running time meter, and optional automation package and trailer-mounting.
InfoDirect 714
15. Pyott-Boone Electronics
Pyott-Boone Electronics’ Model 234 Dust Suppressor System is an electronically controlled water spraying system for use at conveyor transfer points. When a sensor detects material movement on the belt, an electric solenoid valve activates the water spray system subject to an adjustable start/stop delay.
InfoDirect 715
16. Siemens
Siemens Energy & Automation’s Sitrans AS100 acoustic sensor, operating with a Sitrans CU 02 control unit, detects flow conditions and can be used to detect and instantly react to equipment failure, such as a burst filter bag, the company says. Two fully programmable and independent relays can be used to operate an alarm or control device.
InfoDirect 716
Bob Drake is editor for Aggregates Manager.