May 2003
Containing sources of dust can reduce reliance on more expensive control measures.
By Mark Kestner, Ph.D.
Some 30 years ago at a coal mine in Wyoming an engineer was scratching his head trying to figure out how to control dust when a baghouse dumped fines back onto a conveyor. He’d been told by several vendors that he’d have to put in an elaborate enclosure and use surfactants or foaming agents to control these emissions — and it wasn’t going to be cheap. But he had a different idea. Being a farm-boy and a good Christian, he invented a plow that split the coal stream like Moses split the Red Sea and buried the dust at the point of return. Problem solved.
Engineered solutions to the practical problems of dust control, like the dust plow, are the best method of reducing your reliance on more expensive dust control measures, like chemical suppressants or baghouses. The prevention and containment of dust and spillage by good engineering practice comprises a large category of control measures that range from simple curtains for transfer points to landscape architectures that shelter and beautify.
Design for the worst case
New dual skirt systems can make load points leak tight. Photo courtesy of Martin Engineering.
In an average year, I visit about 60 aggregate plants and I’ve always been impressed with the fact that no two are alike. Crushing rock is a real art and this diversity reflects the nuances of geology, markets, and equipment design. Modern aggregate plants are marvels of speed and efficiency. However, the goal of more rock at lower cost can backfire when accomplished at the expense of safety and environmental protection.
I was once asked to take a look at a screening plant processing nitrates in the Atacama desert of South America. In this remote location, the engineers figured they could just let the plant rock and roll. And it did. The dust was so bad that air filters on every engine had to be changed twice daily and after six months, most of the mobile equipment was out of service with worn out motors.
Dust can bite back when engineers don’t design for the worst case. The covers and enclosures that this mine was so anxious to eliminate would have kept the plant on-line and saved a lot of money in replacement parts.
To keep costs down, engineers take shortcuts. Conveyors can be shortened if they load closer to the tail pulley and items like dust skirts, curtains, and scrapers are often absent from the final design. And these are new plants. Older plants can be so shot full of holes that operators spend hours cleaning up.
It costs lots of money for a crew to shovel, bobcat, and sweep. Just a few man-hours a day translates into thousands of dollars a year. Believe me, if your accountants had to do the shoveling, you wouldn’t get an argument about buying that new scraper.
Load points need to be enclosed
Whenever stone is loaded onto a conveyor, the potential for spillage and dust exists. Load points should be enclosed on three sides, covered, skirted, and fitted with a dust curtain. Examples of such load points are crusher and screen discharge chutes to conveyors as well as belt-to-belt transfer points. Much of this work can be done with in-house labor and materials, like scrap steel and used rubber.
Crusher discharges may require significantly more enclosure, particularly horizontal and vertical shaft impactors that move a lot of air. With machines like these there is a much greater potential that stone will spill and dust will leak.
Impact beds that ensure a flat profile and dual belt skirts are often necessary to solve the problem. New designs feature soft rubber skirting that folds against the conveyor to give an air tight seal. Increasing the cross-sectional area of the enclosure and extending it several feet can also help to slow the air stream down.
Because screens are elevated and vibrate at high frequency, dust emissions can be very visible and tend to stay suspended. Screen covers can help control emissions. All kinds of designs are available ranging from simple canvas tarps to lift-off covers. Screens are particularly sensitive to moisture, and covers can help to reduce the amount of water for dust suppression.
To control dust from stacking operations, articulated conveyors can be used to reduce the drop distance. Partial enclosure with a head box and rubber curtains helps shield the process stream from the wind. Complete enclosure is possible using telescopic chutes or stack-out tubes which contain the flow all the way onto the pile. Telescopic chutes can be equipped with sensors that maintain a fixed distance to the pile or with vacuum returns to collect dust produced by breakage when stone impacts the surface.
Feed hoppers are a final example of a load point that can benefit from improved engineering, but large hoppers are tough to completely enclose without obscuring the view of the operator or restricting the movement of hammers used to break oversize rock. Partial enclosures using elevated sidewalls, fences, or screens can help a great deal, especially when the prevailing winds are strong and predictable.
| Table 1 Good Engineering Practices |
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| DUST SOURCE | GOOD ENGINEERING PRACTICE |
| A. MATERIAL HANDLING | |
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| B. PILES | |
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| C. PAVED AND UNPAVED ROADS | |
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Berms, windbreaks, and screens can protect piles
Completely enclosing piles is generally not possible because they are so active. Instead, good engineering practice takes advantage of the topography. Locating the plant in-pit is one of the best methods of protecting piles from the wind and containing emissions on the property.
If there are not natural barriers, windbreaks or screens may be necessary. Berms and fences are solid structures that push wind over or around piles. Screens are porous structures designed to reduce wind speed across the pile surface. Windbreaks and screens can be quite effective if they are positioned in proper relationship to dust sources. However, they may aggravate emissions if they are misplaced and create turbulence. For large stationary plants with multiple sources, there are computer models to help determine the height and placement of any windbreak or screen.
Landscape architecture is an important part of plant design. Your neighbors don’t want to see your quarry, and control measures used to reduce wind erosion also keeps them out of sight. Building a berm with some trees and shrubs will improve plant appearance and show a skeptical public that your site can actually support plant life.
This producer had to completely enclose the stone falling onto the pile with a “stacking tube” and vent it to a baghouse.
Control truck traffic and road dust
Plant roads should be designed to get rock from the face and stone out of piles as directly as possible. Fewer vehicle-miles traveled means less dust, so routes should be as short as possible. Short routes also speed production and deliveries. To prevent drivers from wandering through the plant, barricades like boulders or Jersey blocks can be used to keep traffic on designated routes.
Pay attention to the gradation of the road surface. The silt content of the surface is directly proportional to its potential to emit dust. Altering the gradation of the surface can have a tremendous effect on dust emissions from unpaved roads. Putting down some chips to stabilize the surface will reduce the frequency of watering or the use of chemical suppressants.
Be careful how you lay out paved roads. Paved surfaces produce a lot less dust, but only if you keep it clean. Access to paved roads from unpaved areas should be restricted so that any trackout of mud or dirt is confined. Paving areas that permit access from several directions or allow trucks to criss-cross the pavement from one dirt road to another makes cleaning or treatment much more difficult.
Telescopic chutes contain the stone falling into trucks and reduce dust emissions.
Producers want machinery that makes less dust
All new aggregate processing plants have to meet New Source Performance Standards (NSPS) in order to obtain an operating permit. The annual fee for an operating permit is calculated based upon the potential of the plant to emit dust. Good engineering practices reduce the potential for emissions and decrease permit fees. This market-based regulatory approach implemented by recent amendments to the Clean Air Act provides a financial incentive to control emissions.
Some crusher manufacturers have already developed designs that minimize dust. One VSI manufacturer incorporates a dust recirculation system into its design that keeps fines out of the air stream. Others have equipped their machines with curtains or baffles to redirect air flow. Because of the short residence time of stone in impactors, it is especially important to introduce spray droplets directly into the crushing zone and some newer machines feature ports that offer better access and protection for nozzles.
In the future, mining companies will tend to purchase equipment that produces less dust. I think this is part of the reason high speed cone crushers have taken market share from vertical shaft impactors. As permit fees charge more and more for every ton of potential emissions, the demand for cleaner, less dusty production machinery will increase.
Engineering firms and equipment manufacturers have a special responsibility to help the aggregate industry achieve air quality standards because good dust control is nothing more than good process control. If you can appreciate this fact and translate it into good engineering practice, the only dust you’ll make is the dust you leave your competition in.
Impact beds keep a flat profile that prevents belt sag.
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.
Taking an Engine’s Breath Away
Component specifications and air-intake system design significantly affect fixed restriction and performance.
Air-intake systems on large, mobile mining equipment that operates in sometimes dusty environments require special consideration. Buying a heavy-duty filter from a reputable company is the easy part. How can you, in specifying equipment options, affect the performance of the air-intake system as a whole?
The primary functions of an air-intake system are to minimize restriction and deliver clean air. A good air filter element is designed to flow the amount of air the equipment needs and gives a low restriction when the new element is first placed in service. There is very little you can do, however, about the fact that a filter element gets more restrictive the more it loads up with dust. But you have a lot of control over the rest of the system.
Air flow itself creates restriction, measured in inches of water. .Every bend, twist, turn, step, and edge that air flows through or across causes some restriction.
When looking at optional pieces and parts for an air-intake system, therefore, keep in mind that everything adds restriction. All restrictions in an air-intake system, other than the filter element itself, are fixed restrictions that take away from the restriction allowed to the element. Each engine has a maximum recommended change-out point for the element. Some fixed restriction is inevitable, but air-intake system designers, and those who specify the parts, have a large input as to how much fixed restriction results.
A look at a well-designed system highlights sources of fixed restrictions that can become significant in more poorly designed systems. The machine in this example is a scraper with a Cat 3412TA, 750-hp engine requiring 1.875 cfm at full load.
Scrapers can be specified with smaller and larger engines, so the range on intake options is wide. A good design for heavy dust environments includes an 18-in. diameter, 27-in. long air cleaner assembly with a 10-in. inlet, 8-in. outlet, a built-in pre-cleaner for long element life, automatic dust ejection from the pre-cleaner, an inner safety filter for protection during primary element changes in the field, and horizontal mount for easy access (Figure 1).
Mounting the air cleaner assembly at the rear of the cab, at a location that minimizes exposure to debris generated by the scraper and tires, protects the housing from damage and provides easy access to element changes. The air cleaner assembly produces 6-in. restriction at rated engine flow.
The inlet is extended straight up to the top of the cab by a straight tube the same size as the inlet to the housing, connected to the housing with a straight hose and clamps. It is extended high to get away from dust generated by the operation of the equipment. The tube and connections result in 0.2 in. restriction. The inlet is topped off with a rain shield to protect the element from moisture, which extends element life but results in 1.5 in. restriction.
The housing outlet is connected to a 90° elbow, the same diameter as the housing outlet, to point toward the turbo inlet of the engine. The elbow is connected to a straight tube the same diameter as the housing outlet. The elbow adds 0.75 in. restriction; the tube adds 0.1 in. restriction. This outlet tube is connected to the turbo inlet with a reducing hose, adding 2.0 in. restriction. For this well-designed air intake, the total system restriction at the turbo inlet is 10.55 in.
Poorly designed air-intake systems for off-road equipment may suffer significantly greater restriction due to use of inadequate components. These components may include smaller air cleaner assemblies with narrower air intake and outlet; small, add-on pre-cleaners mounted to the air cleaner housing inlet; and reducing elbows and hoses that allow use of cheaper tubing (Figure 2).
The mounting location of the air cleaner also affects its exposure to dirt and impact, the length and configuration of hoses and elbows to reach the turbo inlet, and the ease of changing filter elements.
Remember: Restriction is your enemy!
This article is provided by The Association of Equipment Management Professionals (AEMP), formerly the Equipment Maintenance Council (EMC), an individual membership organization comprised of equipment maintenance and management professionals. AEMP offers a certification program for the industry, the Certified Equipment Manager (CEM). For more information, contact Stan Orr, CAE, AEMP executive director, at 970-384-0510, e-mail at ceo@equipment.org or visit AEMP’s website at www.equipment.org.
Spec Sand Sales Help Improve Yields
Plants capable of making multiple specification sand products can open new markets and boost profits.
By Bob Drake
Maximizing the yield from permitted aggregate reserves involves developing a mix of products that provide the greatest revenue and profit margins while optimizing non-renewable resources and minimizing waste. Many aggregate producers are finding opportunities to improve yields by increasing the range of specification products made from natural or manufactured sands and broadening the markets served. Beyond obvious uses for concrete and asphalt, profitable markets may be available — or can be developed — for masonry, filter, golf course, foundry, plaster, glass, or other high-priced specialty sands. Meeting multiple, tight specifications is the challenge.
Numerous methods exist to remove fines from sand products, such as fine material washers (screws) and air classifiers. However, simultaneously producing multiple specification sand products is more complex and usually more capital intensive. Methods range from single units that can produce two or three graded sand products to recipe sand plants that deliver fractionated sand sizes that are reblended to make multiple products.
Hydraulic classification is the basis for the design and operation of most sand classifying equipment with multiple-product capability. Sand classifiers, density and rising-current separators, and other devices rely on the different rates at which particles of different sizes settle in a column of water. Following are descriptions of some of the systems and controls available to simultaneously produce multiple, specification sands in a wet process. Dewatering devices — not included in this review — are used in conjunction with many of these systems.
1. allmineral
allmineral’s allflux separator has a central conical hopper to collect coarse sand and a secondary processing stage, a peripheral ring area, to recover fine-sand. An adjustable upward water current allows only coarser sand to settle in the central hopper, where it is discharged through an automatically controlled gate at the bottom. In the fine-sand section, water added through a screen plate produces hindered settling and a fluidized bed. A benefit of the two-stage process is that coarse sand particles, removed in the central hopper, cannot destroy the homogenous fluidized bed in the peripheral fine-sand section, allmineral says. Lighter particles remain above the fluidized bed and are discharged, along with most of the water, over a weir. Clean fine sand is discharged through automatically controlled cone valves in the screen plate. The allflux units, which provide classifying, thickening, and organics removal in one step, are available to handle throughputs of 440 to 11,000 gpm, according to allmineral. Recovery of ultra-fine sand can be adjusted to a degree by sizing of the unit for specific hydraulic capacities, the company says. InfoExpress 701
2. CFS Inc.
Classification and Flotation Systems says its Density Separator can make cuts on sand from 24 mesh to 140 mesh. Use of one or more units allows fractionation of sand sizes that then can be reblended to meet specs. An evenly spaced spray bar system produces a rising current of water in the upper section of the square or rectangular tank and expands the sand slurry into a “teetered” state. Coarser sand settles to the bottom (underflow) and finer material is distributed to the top (overflow). Continuous overflow and the bottom coarse-material discharge valve keep the unit in equilibrium. A force balance pressure cell on the tank coupled to the automatic valve through an electronic controller can change the separation point. CFS Density Separator sand plants have capacities up to 1,000 tph, the company says. InfoExpress 702
3. Eagle Iron Works
Eagle Iron Works’ Water Scalping-Classifying tanks are designed to remove excess water, classify minus-3/8-in. material, and where needed, retain fines to meet specifications. A specific range of particle sizes accumulate at any given point along the bottom of the classifying tank where they are discharged through a series of valves that are periodically opened by sand reblending controllers. EIW recently introduced a four-cell classifying tank with four discharge valves and two material-sensing assemblies per tank station, allowing classification of four sand products. Rising current classifiers can be installed at the feed-box end to increase the sharpness of classification, EIW says.
The company manufactures stationary, semi-portable, and portable classifiers in a range of sizes. Three sand reblending systems also are available — Dialsplit, Autospec III, and PC-based Autospec Mark V. The Mark V control continually monitors the feed to the tank and automatically updates tank settings to maximize the yield of in-spec sand and to maintain consistent Fineness Modulus, according to EIW. InfoExpress 703
4. Finlay Hydrascreens
Finlay Hydrascreens offers two models of its Hydrasander dewatering/classifying system. Hydrasanders use spiral screws and a split-level tank with two bucket wheels rotating at 1 rpm to segregate and collect specified and secondary sands. To collect coarse sands, model 152E has an 11-ft.-diameter bucket wheel with 48 buckets, and model 252E has a 15-ft.-diameter bucketwheel with 54 buckets. Both models have 7-ft. 7-in.-diameter fines bucketwheels with 12 buckets. Manual or automatic controls determine the speed of the spirals that move materials from the coarse to the fine bucketwheels. Throughput capacity ranges (coarse and fine sands combined) for the 152E and 252E are 80 to 140 tph and 120 to 220 tph, respectively. InfoExpress 704
5. GreyStone
GreyStone offers 10 models of its stationary Aggre-Spec sand classifier, ranging in size from 8 x 20 ft. to 12 x 48 ft. They are capable of producing up to three products at one time. In addition, the company manufactures 18 models of semi-portable classifiers and eight models of portable classifiers, capable of producing up to 820 tph and 350 tph, respectively, according to GreyStone.
GreyStone’s Aggre-Spec III Windows-based controls use a three-method reblending control system to automatically select the best process for maintaining sand specifications, the company says. Users can reblend up to two spec products simultaneously. Aggre-Spec III monitors feed changes and re-calculates new programmable logic controller (PLC) settings if necessary to maximize efficiency. The controls alert operators to out-of-spec conditions to help prevent stockpile contamination. InfoExpress 705
6. Kolberg-Pioneer
Kolberg-Pioneer’s Series 7000 sand classifying/blending tank systems are available in stationary, semi-portable, and portable configurations in sizes ranging from 8 x 20 ft. to 12 x 48 ft. The systems can simultaneously make two specification products and one by-product. A PLC or PC controls electric/hydraulic components to open and close the three urethane dart discharge valves at each settling station. Adjustable-height sensing paddles control the amount of material that accumulates before the valve opens. Optional rising-current classifer cells provide hindered settling at the first three stations for better control over coarse sand fractions and sharper cuts, says Kolberg-Pioneer.
Kolberg-Pioneer’s SpecSelect I, II, and III control systems are PLC- or PC-based and use touch-screen operator interfaces. A new Windows 2000-based interface enables advanced networking and allows data exchange with other Windows programs. SpecSelect I provides independent control for each settling station by a percentage method. SpecSelect II provides automatic self-compensating control and dependent control of valve activity at all stations. PC-based SpecSelect III provides all the function of SpecSelect I and II plus a color display screen, station progress screens, and optional disc download and modem hook up. InfoExpress 706
7. LPT Group
LPT Hydrosizers — hindered settling classifiers — are at the core of its recipe sand plant design. The company has constructed multiple-fraction sand plants that use a series of Hydrosizers in conjunction with dewatering bins, hydrocyclones, or dewatering screens. LPT offers two basic styles of recipe plant systems. The most versatile system uses a series of bins to store fractionated sands from which product is metered out and combined to meet specifications. LPT’s less expensive In-Line Recipe Plant uses an algorithm to blend sand fractions on the fly without use of bins. LPT says it commissioned the “first true recipe plant in the United States” in North Florida in 2000. The automated plant is designed to produce DOT and commercial-grade sands as well as glass and other specialty sands. InfoExpress 707
8. McLanahan Corp.
McLanahan says that its sand classifying tanks historically have been used for one or a combination of three conditions: where large quantities of water must be handled from pumps or dredges; where excessive amounts of intermiediate grain sizes in the 16- x 100-mesh range prohibit production of one or two spec sands; and where it is desired to make two spec products from a single sand. The company says computer-controlled classifying tanks today enable producers to expand uses, maximizing specification sand production while compensating for moderate gradation variances in the feed material. Software allows storage and retrieval of multiple sand specs and valve station gradations.
McLanahan says its newest Sand-Manager classifying tanks feature touch-screen control systems with PC software, simplified wiring and plumbing arrangements, wider and deeper flumes, Twist-Lok valve seats, upgraded Pro-Plus II software, and new control bridge and access louvers. InfoExpress 708
9. Powerscreen
Powerscreen’s Fines Master 120 sand recovery and dewatering unit can produce one or two grades of sand, recovering 96 to 98 percent of all material larger than 200 mesh and 100 percent of material larger than 100 mesh, according to the manufacturer. Key to separation of sand sizes is a 5- x 12-ft., high-frequency dewatering screen with a split deck to make two products. Twin variable-speed bucket wheels recover 80 percent of the product to the dewatering screen and twin hydrocyclones discharge the remaining 20 percent to the screen, Powerscreen says. The Fines Master 120 is available in portable, pit-portable, and stationary models. InfoExpress 709
Bob Drake is editor for Aggregates Manager.
Primary Change Relieves Secondary
Replacing a primary jaw with an impactor provides finer feed to the secondary circuit, increasing throughput.
The primary impactor is fed from both sides of the hopper by up to three, 9-cu.-yd wheel loaders in load-and-carry operations.
roduction bottlenecks — created by screens, crushers, or conveyors — are probably more the rule than the exception in many aggregate plants. Just as plants and production environments vary, however, so too do the ways of relieving these bottlenecks.
Constrained by the capacity of the secondary cone crusher at its Newport, Mich., limestone quarry about 25 miles south of Detroit, Thompson-McCully opted to make changes in the primary circuit to improve production throughout the rest of the plant. In 1998, the company replaced a pit-portable 4248 primary jaw crusher with a pit-portable Grasan KRH1620 primary impactor. The goal was to provide a smaller, more cubical feed material to the 66-in. standard secondary cone and two 66-in. fine head tertiary cones.
“Going from the jaw to the impactor has improved just about everything,” says Frank Azzopardi, quarry manager. “We got 650 to 700 tons per hour from our jaw plant. Now we get 850 tons per hour from the impactor. The maintenance cost is a little higher than a jaw, but the impact crusher gives us greater efficiency and a better, more cubical product with no slivers. We can reduce 24-in. limestone down to 6-in.-minus in one step.”
The impactor’s greater reduction ratio is key to improving the secondary cone crusher’s capacity, says Azzopardi. The old primary jaw crushed to a minus-8-in. size. The impactor produces a finer product, including more fines, but Azzopardi says the additional fines do not create a problem at the quarry.
The electric-powered Grasan plant incorporates a Hazemag 1620 APPH primary impactor with a 50-in. high, 80-in. wide inlet that accommodates limestone in sizes up to 36-in. cube, the manufacturer says. Oversize rock can be reduced in the hopper with a pedestal boom-mounted Allied Construction hydraulic hammer. Controlled blasting and careful loading, however, make use of the hammer an infrequent event, Azzopardi says.
Depending on production needs, the company uses as many as three Caterpillar 988F, 9-cu. yd. wheel loaders in a load-and-carry operation to feed the crusher from both sides of the hopper. The loaders typically haul material 200 to 600 ft. from the muckpile to the crusher. A grizzly feeder scalps plus-4-in. material to the impactor; 4-in.-minus stone bypasses the crusher and falls directly to the discharge conveyor. The primary crusher feeds a 48-in.-wide field conveyor that transports the stone about a half mile out of the 200-ft.-deep quarry to a surge pile.
“In the five plus years we’ve operated our KRH1620, we’ve crushed more than 6 million tons of shot limestone and had no unplanned downtime, just simple maintenance as we go,” says Azzopardi. “We don’t have mechanics on staff, so we rely heavily on durable, dependable equipment that runs with minimum maintenance.”
Blow bars and apron settings on the impactor are inspected daily. Aprons are adjusted about every four days using the Hazemag’s hydraulic adjustment system.
“Normally we flip the four blow bars in the crusher every 10 to 15 days to use all four sides to their fullest. So, we’re replacing the blow bars every 40 to 60 days,” says Azzopardi. The Newport quarry operates a single 10-hour shift, five days a week.
Crushing operations shut down for about three months in the winter, which allows time for more thorough service work and repairs. It is also during this time that the primary crusher may be moved to a new location and the field conveyor repositioned.
Thompson-McCully’s pit-portable primary impactor is mounted on a four-axle chassis and is relocated about once a year.
The Bottom Line
In an effort to improve production from its secondary cone crusher, Thompson-McCully replaced a primary jaw crusher with a primary impactor. The impactor’s greater reduction ratio provides a finer feed for the secondary cone while increasing throughput from the primary circuit by more than 20 percent.
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