January 2002
Pit Sense—Long-term Mine Planning: The Key to a Secure Future
Reducing Silica Exposure in Aggregate Operations
Success in the Field Mist System Reduces On-Site Dust
Maintenance Matters Emission Regulations to Affect Fuel Cost, Lubricant Choice and Engine Design
Long-term Mine Planning: The Key to a Secure Future
Nothing good happens quickly in the aggregates business, but a good rendition of a long-term mine plan can benefit a company, from permitting to public relations, in the short-term.
Cut-a-way renditions of long-term mine plans help people understand what's going on, both inside and outside the gates.
Nothing happens quickly in the aggregates business except breakdowns and disasters. The failure of a critical piece of equipment or a shot that goes wrong can cause an aggregate producer to scramble to keep production moving. You do whatever it takes to get the stone out the gate. It doesn’t take long for critical benches to be consumed and for narrow, steep haul roads to appear, as the quarry supervisor plays an increasingly tough game of catch-up. In a relatively short period of time, an operator can find his back up against a very high wall.
This illustrates a point: the need for the guidance of a mine plan is often precipitated by an immediate crisis. No matter how great the immediate need, however, the future health of your operation depends upon identifying and meeting long-term production goals. A quick one- or two-year extraction plan might get you out of a short-term predicament, but long-term concerns must be satisfied to provide the kind of options that can keep these predicaments from reoccurring.
For the sake of clarity, let’s define long-term planning as looking ahead anywhere from five years to the completion of mining at a site. Anything less addresses short-term needs and will probably involve existing equipment and readily accessible reserves. Long-term planning involves identifying and analyzing all options for the operating life of a mine. If a detailed reserve analysis and estimate has been performed (see the previous Pit Sense column, AggMan, November 2001, p. 22), then it is clear where the reserves are, and at least a basic life-of-mine pit configuration has been established. A significant part of any long-term mine plan is the development of an ultimate site configuration plan that maximizes recoverable reserves. This configuration is the destination; the trick is to figure out how you’re going to get there from here.
Now is the time to assemble the team of technical, operations and sales personnel that will assist in the planning process and start asking questions. What do you think the market will do over the next 10 to 20 years? What products will be produced, and in what percentages? Will new products be required, either to meet demand or to use material at hand? Will this create more or less waste material? Can the existing plant handle the task? Can the existing fleet get the required material to the plant? Come to think of it, in 10 years, where is the plant going to be? How about the stockpiles? Are additional reserves needed, or do existing reserves require permitting? This is serious strategic planning and has probably already been performed to some extent by most organizations, although maybe not with a mine plan in mind.
The design of the aggregate processing plant and the mine plan are closely interrelated. Plant design is a function of the type and amount of material produced. Plant location is a function of reserve location and material-handling choices. The mine plan is, in turn, driven by production demands and material transport requirements. Answering the tough questions like those above determines the goals a long-term mine plan must meet. The key is anticipating major changes in the production circuit, taking advantage of the lead time to evaluate alternative equipment and operating options, and then incorporating the best combinations in the mine plan.
Most sizable aggregate companies find themselves, willingly or not, in the real estate business. Analysis of a company’s real estate holdings and property requirements can be a significant component of a long-term plan. If a company is fortunate enough to be close to a major metropolitan market, development near the mining site is probably a concern as well. Can mining and reclamation be sequenced to realize gains from the leasing or sale of some of this property? For many operations, mining must drive the land-use schedule, but for some sites allowing property development to set the mining timetable may prove advantageous. Benefits might include choosing who your nearest neighbors will be, or the public relations windfall of demonstrating the productive use of a reclaimed mine site.
To paraphrase an earlier statement, nothing good happens quickly in the aggregates business. Reserve analysis and strategic planning may have highlighted some areas of concern that could take quite some time to remedy. Acquiring and permitting new reserve properties can take several years if everything works perfectly and might take decades if it doesn’t. Acquisition of additional property without geologic reserves might be beneficial for accessory uses such as overburden and waste disposal, stockpile areas, processing plant locations, groundwater and stormwater management, and buffer zones. If acquiring new properties is part of the strategic plan, analysis of available options can become part of the long-term planning process.
Long-term mine planning is perhaps the toughest component of a comprehensive mine plan. It requires thoughtful consideration and detailed analysis of all phases of operation of an aggregate facility. In return, the plan paves the way for greater operating efficiency and acts as the compass that leads the way into a more certain future.
Larry Bolling, P.G., is a geologist with the Industry & Environment Group of Morris & Ritchie Associates, Inc. He can be contacted at LBolling@mragta.com.
Reducing Silica Exposure in Aggregate Operations
By Andrew B. Cecala, John A. Organiscak, Steven J. Page and Edward D. Thimons
Workers at surface mining operations are often exposed to high levels of silica and other types respirable dust. In an effort to lower these respirable dust exposures, the National Institute for Occupational Safety and Health (NIOSH) has been conducting research to address this problem in a practical and economically viable manner.
Lowering the dust exposure of equipment operators in enclosed cabs has been a major focus of research efforts over the past several years. Many types of operations utilize enclosed cabs to protect equipment operators from dust exposure. Normally when the equipment is new, the cabs are fairly airtight. These tightly sealed cabs, combined with good filtration systems, generally provide the operator with good dust protection. However, at most operations, the equipment is older. As aging occurs, many components of the enclosure deteriorate, the structural integrity of the cab diminishes and the effectiveness of the air filtration system fails. NIOSH has been successfully researching cost-effective methods to improve both filtration effectiveness and cab integrity of these older cabs in order to provide a healthier work environment for equipment operators.
Drill operators and helpers have the highest dust exposure of all workers at surface mining operations based upon dust sampling records. Since much of the overburden contains a high percentage of silica, the health hazard associated with this dust can be even more serious. NIOSH research is addressing techniques to lower respirable dust levels at surface drilling operations.
Figure 1. Job Classification for Surface Mining Exceeding MSHAšs Dust Compliance Permission Exposure Limit (PEL) for a 10-Year Period (1991-2001).
NIOSH Research
NIOSH’s mission is to assure a safe and healthy work environment for the working men and women of this nation. The primary emphasis of NIOSH’s Pittsburgh Research Laboratory (PRL) is mining health and safety research. This article focuses on two areas of research performed at PRL to lower miners’ exposure to silica and other respirable dust at surface operations. The first area deals with enclosed cabs. A significant number of miners work in enclosed cabs at surface operations, including drill, dozer, loader and scraper operators, as well as a vast array of different haulage vehicles and trucks. Secondly, this article discusses methods to lower dust levels at surface drills. The dust generated during surface drilling exposes the drill operator, drill helper, explosive crew, as well as any other individuals working in and around the drill to high respirable dust levels. Figure 1 shows the relevance of this research based upon the Mine Safety and Health Administration’s dust compliance sampling records for the metal/nonmetal mining industry. This chart indicates that the highest exposure categories at surface operations involve these job classifications. The intent of this article is to provide mine operators with a number of techniques to help lower the dust exposure of workers at surface operations.
Dust Control Research
Enclosed Cab Dust Control Research. Many types of heavy equipment used in the aggregate industry have the equipment operator located inside an enclosed cab. There has been a significant amount of recent research investigating how to improve the protection to miners working in enclosed cabs. This has included a number of cooperative efforts with mining companies, heating and air conditioning companies, and cab filtration manufacturers. Many of these studies have investigated retrofitting older cabs with new filtration and pressurization systems. These studies have encompassed a full spectrum of different types and conditions of equipment and have included evaluating enclosed cabs that were not structurally sound, as well as ones that were very sound.
From this research, we have identified a number of significant factors that determine how effective an enclosed cab will be at protecting a worker. A term called “protection factor” is commonly used for comparing the cab effectiveness and measures the ratio of outside verses inside respirable dust levels. The higher the protection factor value, the more protection afforded to the machine operator, or the lower the worker’s personal dust exposure.
A brief summary of some of these studies highlights the importance of these significant factors. One cab evaluated was a very old Davey M8B surface drill, in which it was not physically possible to seal the cab.1# This cab had large holes in the enclosure where control linkages entered the cab, as well as a loose-fitting bifold door that was on the drill table side of the drill. A new air-conditioning/heater pressurization and filtration unit was installed on the roof of the cab, but because of the numerous gaps and holes in the cab enclosure, positive pressure was not achievable. The discharge of the new filtration system was directed down over the drill operator in an attempt to provide him with a clean-air zone within the cab. Dust measurements taken before and after the implementation of the new unit indicated very minor changes to the drill operator’s respirable dust exposure.2# Because of this, we do not believe it is cost-effective to install an air cleaning unit on surface mining equipment that is not capable of being sealed to some minor level of pressurization.
Figure 2. Schematic for an effective filtration and pressurization system on an enclosed cab.
If even a very minimal amount of pressurization is attainable inside these cabs, totally different results can be achieved. At this same operation, a substantial reduction to an enclosed cab operator’s respirable dust exposure was achieved with very minimal pressurization. A CAT980B front-end loader was equipped with a Red Dot Corp. and Clean Air Filter unit located on top of the cab. Both of the companies cost-shared this research effort with NIOSH.
In addition to the installation of the new filtration unit, visible cab enclosure cracks were sealed with silicon and the door jams were sealed with dense foam weather stripping. Because of these sealing efforts, a positive static pressure of 0.01 to 0.015 inches water gauged (in.w.g.) was achieved inside the enclosed cab. The front-end loader operator’s protection factor went from 1:1.1 during baseline testing to 1:10.1 with the new dust filtration system and other improvements to the cab integrity, allowing pressurization to be achieved. The cost for the Red Dot Corp. unit was $2,300, but this did not include the cost for the compressor for the air conditioning unit. The Clean Air Filter pressurization and filtration component was an additional $1,600.
A similar study was performed on an Ingersol Rand DM45E drill at a different operation. Three days of baseline testing were performed, followed by the installation of a new Air International Transit/Sigma Air Conditioning Co. filtration and pressurization unit. After determining that the unit was working properly, three additional days of post-testing was performed. Sigma Air Conditioning Co. cost-shared this research effort. Their unit was comprised of three different components: a filter/heater/air conditioning main unit, a condenser unit for air conditioning and a pressurizer unit. This unit delivered up to a maximum of 450 cfm and pressurization inside the cab ranged from approximately 0.20 to 0.40 in.w.g. The cost for this unit was approximately $10,000, plus the cost for installation. Respirable dust concentrations inside the cab went from 0.64 mg/m3 during pre-testing to 0.05 mg/m3 during post-testing with the new system, representing a 92-percent reduction in respirable dust levels in the drill cab. The average protection factor measured with the new system was 1:52.
In addition to the above research, another study is currently being performed at a surface mine evaluating the performance of a new pressurization system on a DrilTech D40KII rotary percussion drill. Baseline measurements were taken when outside temperatures ranged from 60 to 70° F. A new Clean Air Filter Co.y cab filtration and pressurization system was installed to an existing and older Red Dot AC unit. Immediately after the installation, the static pressure inside the cab was 0.01 in.w.g.
Time was spent improving cab integrity by installing new door gaskets and plugging and sealing cracks and holes in the shell of the cab. This increased the cab pressure to approximately 0.1 in.w.g. Since the post-testing on this cab was performed in the winter months when outside air temperatures were low, a floor heater in the cab was being used. The results from this study showed that this radiator type floor heater inside the cab actually caused dust levels to be approximately 17 times higher during post-testing than for pre-testing.3# It was believed that baseline measurements were assisted by the air-conditioning unit being used during pre-testing, which lowered dust levels as the re-circulated air in the cab traveled through the condenser unit.
Testing the drill verified that the floor heater increased dust levels in the cab as a result of dust from the drill operator’s clothing and work boots, and from product that had accumulated on the floor. Because of the significant increase in dust levels with the floor heater, NIOSH recommends that they not be used. Heaters should be positioned high in the cab where they are less prone to pick up dust from the floor and operator’s clothing.
For an enclosed cab to be effective from a dust control standpoint, there are two key components that are necessary: 1) effective filtratio, and 2) cab integrity. From the various field evaluations, it was obvious that both of these components are vitally important for the systems effectiveness.
For effective cab filtration, a system should be composed of both a re-circulation and clean outside-air system. Approximately 75 percent of the air inside an enclosed cab should be re-circulated through a good grade filter. This allows air to be conditioned to the cab operator’s comfort without major air changes that would significantly affect the size and the cost for conditioning the cab air. Another consideration is to have separate fans for makeup and recirculating air. A major component in an effective system is to have the makeup air positively pressurize the enclosed cab. This results in any system leakage to be from the inside the cab to outside avoiding dusty air from entering the cab. It is also highly recommended that the makeup air be positively pressurized after being filtered to eliminate any possibility of dust laden air being drawn into the system.
Also, the inlet for the makeup air should be located on the cab the furthest distance from the dust sources (where practical).4# This reduces the amount of loading on the filters and increases the time between cleaning and/or replacement. The discharge for clean air into an enclosed cab should be high in the enclosure, preferably at the roof. This allows the clean air to be blown down over the equipment operator’s breathing zone without becoming contaminated by any in-cab dust sources.
Many systems have the intake and discharge for the re-circulation air located in the roof unit. Although this is acceptable, the most beneficial design would be to draw the re-circulated air from the bottom of the cab. This would provide a one directional flow of clean air from the top to the bottom of the cab. We do not recommend the discharge of clean air low in the cab because as we observed, this can entrain a significant amount of dust from soiled work clothes, boots and a dirty floor. Figure 2 is the recommended schematic for an effective filtration and pressurization system on an enclosed drill cab.
The second factor for dust control effectiveness in enclosed cabs is cab integrity. Cab integrity is necessary in order to achieve some level of pressurization. Field testing has shown installing new door gaskets and plugging and sealing cracks and holes in the shell of the cab has a major impact on increasing cab pressurization. To prevent dust laden air from infiltrating into the cab, the cab’s static pressure must be higher than the wind’s velocity pressure.5# Although higher static pressure requirements have an advantage for overcoming wind speeds, a major drawback is that this necessitates that more air must be delivered by the outside air unit, and this causes more loading on the filters. Another drawback is that it creates more air conditioning (heat and cooling) requirements for operator comfort which increase the size and cost for this component. We have a number of field studies that provided very good protection to the cab operator with minimum cab pressurization.
Figure 3. Air Ring Seal (AIRRS) location and operation using compressed air to produce high velocity jets along donut shaped ring.
Surface Drill Dust Control Research
The following section provides control technology that has been effective in reducing the dust exposure of drill operators, drill helpers and other personnel working in and around the drilling process. There are typically three major dust sources associated with these drills:
- Dust generated from dust collectors;
- Dust from drill skirt leakage; and
- Dust from leakage around the drill stem and drill table.
Controlling dust generated from the dust collector dump cycle. This technique was developed by the Bureau of Mines and is composed of a barrier or shroud placed around the hopper discharge doors extending to the ground.6# This shroud confines the dust collector fines during dumping to an enclosed space, thus reducing airborne dust entrainment into the surrounding work environment. Although this dust control technique was developed for surface mine drills, it can be applied to any mobile rock drill.
During testing of this technique, a temporary shroud was installed around the hopper doors to measure respirable dust reductions. The shroud was made of a brattice material and mounted by large magnets for easy installation and removal during testing. Two flaps were cut in the shroud to allow the operator access to open and close the hopper doors. Average airborne respirable dust concentrations at the hopper discharge during dumping were reduced by 81 percent when using the shroud. Considering the very minimal cost associated with the material, supplies and manpower required to install this brattice shrouding around the hopper discharge doors on the dust collector, it should be implemented on all drills in which this technique is applicable.
Controlling drill skirt dust leakage. The use of an exhaust ventilation collector system to capture dust at the drill site is a common control technique. This is normally accomplished by enclosing the area where the drill stem enters the ground by hanging a rubber or cloth “skirt” or “shroud” from the underside of the drill deck. The dust is removed by the collector filtering media and the clean air is exhausted to the environment.
The integrity of the drill stem shroud, including how well it seals to the ground, is probably the single most important factor contributing to the effectiveness of a dry collection system.7,8 Generally, the shroud volume should be 1.8 times the volume of the hole and there should be at least 0.2 in.w.g. of negative pressure inside the shroud. The length and width of the shroud should be 2.5 times its height. The air is ducted out of the drill stem shroud either from the top of the shroud near the outside edge or from the side of the shroud near the top. The most common open area in this shroud is the gap between the bottom of the device and the ground, which is called the shroud height. During field tests, the dust reductions varied from 31 to 99 percent over a height range of 27 in. down to 0 in. With a shroud height of 6 to 9 in. or lower, it was apparent that the dust control system worked very well. However, as the height increased, the control efficiencies decreased.
Most decks shrouds were rectangular and constructed of four separate pieces of rubber belting attached to the drill deck.9 Because of this design, there was a measurable amount of dust escaping from the open seams as well as the open area between the shroud and the ground. In additional testing#,9 this technique was further optimized. This work showed that circular and slightly conical shroud design, without any seams, was superior to the previous design. The shroud is capable of being hydraulically raised to nearly flush with the drill deck and lowered to make contact with the ground after leveling the drill. The shroud has a small trap door which can be manually raised/lowered so that the cuttings can be shoveled from inside the shroud without losing the dust capture efficiency.
Testing on this technique consisted of comparisons with the shroud in fully operable condition and with the shroud partially raised to simulate a leakage condition. Respirable dust concentrations were less than 0.5 mg/m3 with the shroud lowered and 52 mg/m3 with the shroud raised. Dust reduction efficiencies greater than 99 percent were achieved. This compares to typical efficiencies for square shrouds in the 95-percent range. For the minor changes to the shroud arrangement, it only makes sense to use the improved circular design.
Controlling dust leakage around the drill stem and drill table. Another significant source of dust on a drill rig is the dust leakage around the drill stem and drill table. The best technique found to control this dust leakage is a device called the Air Ring Seal (AIRRS), which has been designed and tested by NIOSH.
Leakage around the drill stem most likely occurs because of excessive wear in the mechanical donut-type rubber seal which is used on many drills. This rubber seal is normally a high-maintenance area because the drill stem is constantly rotating against it. The AIRRS was designed specifically to reduce respirable dust emissions coming from the drill stem, but a secondary benefit is the elimination of a high-wear item on the drill.
The AIRRS is a donut-shaped compressed air ring with closely spaced holes along the inside perimeter of the ring, (Figure 3). High-velocity air jets are produced as this compressed air exits through these drill holes in the donut-shaped ring. This AIRRS is located immediately below the drill table with the air jets directed downward around the drill steel to impede movement of dust particles flowing up through this opening.
In addition to the reduction in respirable dust concentrations, there were also a number of other benefits with the AIRRS. The new system visually eliminated all the large cuttings on this drill from depositing on the drill table. Second, it eliminated the use of a rubber bushing underneath the deck that was frequently damaged and required a lot time and money to keep operating. The AIRRS is a virtually maintenance-free, non-mechanical seal. It was determined through testing that using the AIRRS at a lower bailing velocity should also improve its performance.
The AIRRS was successfully field tested and shown to be a low-maintenance, nonmechanical seal to reduce dust emissions from the drill pipe and deck bushing gap. The low cost and simplicity of the device provides a viable means to drill operators to reduce dust emissions as well as reduce housekeeping requirements on drills.
Controlling Dust By Wet Suppression
Previous tests conducted in the field at U.S. surface coal mines showed that wet suppression systems can significantly control respirable dust. The critical factor affecting the efficiency of the wet systems is the amount of water pumped into the bail air. Since no data were available on optimal water flow rates for wet suppression systems, a field study was designed to examine the relationship of respirable dust emission rate versus water flow rate. Various water flow rates were tested for a number of holes. Each hole was drilled at a specific and constant water flow rate. A flow meter equipped with a needle valve was mounted in the cab of the drill. Flows were controlled and recorded by one of the test team members from inside the cab. A recording flow meter was mounted in the water line near the control system pump. Uncontrolled emission rates ranged from 3.8 to 9.3 g/ft. and control efficiencies ranged from 9 percent at a flow of 0.2 gpm to 96 percent at a flow of 1.2 gpm.
Very practical and simple operational guidelines can be provided to all mine operators who perform wet drilling and which automatically accounts for various operating conditions such as different drills, changes in bit size and different strata. In order to operate at close to the optimum water flow rate, the operator should slowly increase the amount of water just to the point where visible dust emissions are abated. Due to the initial sharp increase of dust control effectiveness, the visible dust abatement point will be easy to identify. Addition of more water beyond this point will not provide any significant improvement in dust control, but will most likely create operational problems. It is important that the water be increased slowly to account for the lag time as the air/water/dust mixture travels from the bottom to the top of the hole.
Conclusions
This article provides operators with a number of methods and techniques to lower respirable dust levels to workers at surface operations. Many different types of surface equipment use enclosed cabs to house the equipment operators. These enclosures have many advantages to protect workers from various health and safety concerns at mine sites. Various field studies have shown that operator’s respirable dust exposure inside these enclosed cabs can be significantly reduced through improved air pressurization and filtering systems, along with having a competent cab structure with integrity to achieve some level of pressurization. A number of different commercial systems have been shown to significantly lower respirable dust levels inside these enclosed cabs in a very economical manner. In addition to the enclosed cab research, a number of other dust control techniques were discussed to help lower dust levels around drilling machines. This would impact lowering the drill operators, drill helper, explosive crew and any other personnel working in and around this area. Research is continuing in a number of different areas to further improve designs and control technology in this area to lower worker exposure down to lowest levels in a cost-effective manner.
Andrew B. Cecala# and John A. Organiscak# are mining engineers for the National Institute for Occupational Safety and Health (NIOSH). Steven J. Page# is a research physicist for NIOSH. Edward D. Thimons# is the branch chief for NIOSH’s Pittsburgh Research Laboratory.
Footnotes
#1 Mention of any company name or product does not constitute endorsement by the National Institute for Occupational Safety and Health.
#2 J.A. Organiscak, A.B. Cecala, W.A. Heitbrink, E.D. Thimons, M. Schmitz, and E. Ahrenholtz. Field Assessment of Retrofitting Surface Coal Mine Equipment Cabs with Air Filtration Systems. Proceedings of Blacksburgh Mine Conference, 2000.
3 Cecala, A.B., J.A. Organiscak, & WA Heitbrink. Dust Underfoot—Enclosed Cab-Floor Heaters Can Significantly Increase Operator’s Respirable Dust Exposure. Rock Products. Vol. 104, No. 4, April 2001, pp. 39-44.
#4 Technology News 485, Improved Cab Air Inlet Location Reduces Dust Levels and Air Filter Loading Rates. February 2001.
#5 Heitbrink, W.A., E.D. Thimons, JA Organiscak, A.B. Cecala, M. Schmitz, and E. Ahrenholtz. Static Pressure Requirement for Ventilated Enclosures. Progress in Modern Ventilation, Vol. 2, Proceedings of the Ventilation—2000. 6th International Symposium on Ventilation for Contaminant Control. June 4-7, 2000, Helsinki, Finland.
6# Technology News 447, Dust Collector Discharge Shroud Reduces Dust Exposure to Drill Operators at Surface Coal Mines. March 1995.
7# Technology News 286, Optimizing Dust Control on Surface Coal Mine Drills. September 1987.
8# Technology News 447, Dust Collector Discharge Shroud Reduces Dust Exposure to Drill Operators at Surface Coal Mines. March 1995.
9# NIOSH Hazard Control 27. New Shroud Design Controls Silica Dust from Surface Mine and Construction Blast Hole Drills. November 1998. DHHS Publication No. 98-150.
A Variety of
Dust-Suppression Techniques Are Available to Help
Producers Reduce Dust at Their Operations
By Steven Frank
The dust-suppression control box in this photo is located not far beneath one of the orange spray nozzles. The nozzle is positioned near a transfer point so aggregate material dropping from one belt to another can be spritzed with water or water and chemical to keep dust down.
The chemical proportional feed pump system shown here takes in water and a chemical, and mixes the two in the blue canister shown in the far right control box. From there, the solution is pumped through the yellow hose to the two boxes on the left. Flow regulators in those control panels pump the solution, through one hose, and air, through another hose, to a foaming container above. The air and solution are combined in a foaming process, then injected into a material chute to intercept dust particles.
The spray nozzles in this picture deliver water, or a water and chemical solution, into the material chute where aggregate passes. As the liquid is spritzed into the chute, droplets coat the dust particles, weighing them down. This weight prohibits dust from rising up into the air as aggregate travels through the plant.
Dust control turned from a good idea to a necessity in the past few years. Industries whose members thought they were immune to dust control restrictions now face the fact that new laws are not just enforced, but also apply to them. These industries have to take an active approach to dust control. In many cases, the active approach has been in response to a visit from the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA) or the Mining Safety and Health Administration (MSHA). The aggregate industry realized years ago that a proactive approach to dust control could not only keep plant owners on the good side of these agencies and surrounding neighbors, but could enhance the goal of environmental excellence.
Dust control can be broken down into three areas—containment, collection and suppression. For this article, we will concentrate on suppression, as the combined topics are enough to fill several hundred pages.
In the aggregate industry, most dust arises from crushing and screening operations. Transport roads and the re-handling of material also contribute to the issue of dust-control and should be dealt with separately.
Water it down
Whatever source you address first, dust suppression involves the application of water and/or chemicals. Fluid is applied either directly to the body of material in order to prevent fines from being carried off into the air or to the air above the material in order to return airborne fines to the material bed.
One big advantage of dust suppression is that the material does not have to be handled again. The suppressed dust returns to the main body of conveyed material, without requiring additional material handling equipment.
There are several systems used for this purpose, ranging from “garden hose” technology, up through water and surfactant sprays, foams and fog generation systems. These various suppression technologies call for adding different amounts of moisture to the material in different ways.
Suppress with water
One of the oldest methods for controlling fugitive dust is the application of water on the material. By wetting the fines, either as they lay in the material body or as they are being picked up into the air, the weight of each dust particle is increased so each is less likely to become airborne. The moisture also increases the cohesive force of the material body itself, creating larger, heavier groups of particles, and making it more difficult for air movement to carry off fines. This can be done by applying the water through a series of properly sized spray nozzles at a point where the material expands and takes in air, such as during discharge from the head pulley in a transfer chute.
Water can also be applied to create a “curtain” around a transfer point, so any dust fines that become airborne come into contact with the water sprays surrounding the open area around the chute. The water droplets make contact with the dust fines, increasing their mass to remove them from the air stream.
The most effective sprays are low-velocity ones. High-velocity sprays can add energy, and a corresponding velocity increase, to the air and the dust particles. This energy is counterproductive to the task of keeping (or returning) the dust with the material body. The high-velocity air movements can make matters worse by keeping dust particles in suspension.
The effectiveness of water spray systems depends on the velocity of the applied water, the size of the nozzle opening and the location of the spray nozzles. The techniques to improve plain water spray dust suppression include reducing droplet size, increasing droplet quantities, increasing the droplet’s velocity or decreasing the droplet’s surface tension, which makes it easier to merge with dust particles.
Plain water spray application systems are relatively simple to design and operate. The challenge arises because water has only a minimal residual effect. Once the water evaporates, the dust suppression effect is gone. Water is generally inexpensive, usually easy to obtain and safe for the environment and for workers who come into contact with it.
Unfortunately, the application of water has several liabilities. For instance, a plain water spray may appear to be the least-expensive form of dust control available. The water is available almost free in many operations (such as mines), and it can be applied through low technology systems. But this cost justification may be deceptive. Many bulk solids are hydrophobic. In other words, they have a high surface tension and repel water. In an effort to achieve effective suppression, the amount of water is increased, but because the material does not mix well with water, there will be some particles that remain dry and others that become very wet. This can lead to material build-up on chute walls, screen plugging and conveyor belt carryback.
When applying water to conveyor systems, a good axiom is “less is more.” For material handling in general, adding excess moisture prior to screening can cause screen blinding. Excess water can also promote belt slippage and increase the possibility of wet, sticky fines accumulating within chutes and around the transfer point. Adding moisture can cause material to stick together, complicating the flow characteristics of the material being conveyed.
Suppress with fog
Fog suppression is one method to optimize the application of water to dusty materials. These systems use special nozzles to produce extremely small water droplets in a dispersed mist. These droplets mix and agglomerate with dust particles of similar size, with the resulting larger, combined particles falling back to the material body.
Fog systems are based on the concept that a wet suppression system’s water droplets must be kept within a specific size range to control dust effectively. If the water droplets are too large, the smaller dust particles typically just “slipstream” around them, pushed aside by the air around the droplets.
The fog system breaks water down into very fine particles that join with the similarly sized particles of airborne dust. The increased weight of combined particles allows them to settle back to the main material stream. Fog systems supply micron-sized droplets that maximize the capture potential of the water, while minimizing the amount of water added to the product.
Atomization is designed to reduce the surface tension of the water droplets, while increasing the number of droplets in a given area, and eliminating the need for the addition of surfactants or other additives. The low level of water added through fog systems—typically at 0.01 to 0.05 percent by weight of the material—generally will not degrade the performance of the material.
Fog systems provide highly effective dust capture combined with low installation and operating costs. Because of the small orifice size of the nozzles, potable (drinking) water is typically required for fog suppression systems so filtration to remove suspended solids from the water supply is required. Nozzles can plug quickly if the water supply is contaminated or if the water treatment system is not serviced at required intervals.
Another consideration before choosing a fog system is the air volume and velocity at the open area surrounding the transfer point or chute. Hydraulic fog systems that don’t require compressed air tend to be more compatible with the need to control the air movement through transfer points. For the most effective performance, fog dust suppression systems require tight enclosures that minimize turbulent, high-velocity air movement through the system. Since the fog droplets are very small, both the fog droplets and the dust can be carried out of the treatment area onto surrounding equipment by air exiting the chute.
Disadvantages of fog generation systems are that the system design and installation requires some degree of experience and some degree of customization—both of which can add to the system’s price tag. Another potential drawback of a fogging application is that treatment is site-specific. Dust control is achieved only at the point of application. Several fogging devices may be required for a complex conveyor system. The overall capital expenditure may preclude fogging if the conveyor system is too extensive.
Add a chemical
To improve the wetting characteristics of water, reduce overall water use and minimize the drawbacks associated with excessive moisture addition, it is a common practice to enhance the water by adding chemical surfactants—surface-acting agents. The purpose of adding surfactants is to improve the dust suppression performance of the water.
If dust from some materials falls onto a puddle of water, the dust particles can lay on top of the water, sometimes for hours. This phenomenon takes place because these materials are hydrophobic—they do not mix well with water. Since it is not possible to alter the nature of the dust particles to give them a greater affinity for water, chemicals are added to alter the water particles so they attract or join with the dust particles they touch. By adding chemicals (usually surfactants), the surface tension of the water is reduced, allowing the dust particles to become wet.
Objections to chemically enhanced water suppression systems include the continuing cost of the chemical additive, typically ranging from 0.5 to 3¢ per ton of material, and the ongoing maintenance of the system. Although the use of a surfactant reduces the amount of water added to the dusting material, water/surfactant sprays may still add more water than is acceptable.
Suppress with foam
The use of surfactants with water will improve the likelihood that fines will stick to the droplets and will result in suppression of the dust. It stands to reason that the next objective would be to maximize the surface area of the available water droplets. This makes more contact with the dust fines possible while limiting the amount of water added.
Some suppliers offer dust suppression systems that create a chemical foam. Given the proper mixing of material and foam, these systems provide effective dust control. As the water is in the form of foam, its surface area is greatly increased, improving the chance for contact between dust and water. Some foam bubbles attract and hold dust particles together through agglomeration. Other bubbles implode on contact with dust particles, wetting them to increase their weight and, at the same time, create an additional droplet, which is then available to contact additional dust particles.
With moisture addition of approximately 1 quart per ton, foam suppression systems typically add less than 10 percent of the water used by a water-only spray suppression system. Consequently, these systems are a good choice where water supplies are limited, or where excess water can degrade material. In addition, the reduced water means fewer problems with screen clogging and material sticking to mechanical components and walls.
Foam is created by adding air to the surfactant and passing this solution through a mixing device. Adjustment of the air/water/chemical ratio allows the application engineer to generate foam ranging from very wet to “shaving cream” dry, depending on the needs. A well-established foam can expand the surface area of a given quantity of water 60 to 80 times. This allows for much lower rates of water addition.
Water quality plays an important role in the effectiveness of a foam dust control program. In order to ensure that foam generation can take place using plant site water, it is wise to analyze the water for the following characteristics:
- pH;
- Conductivity;
- Total suspended solids; and
- Calcium hardness.
While many applications can benefit from foam technology, there are some liabilities to the process. Foam generation requires compressed air. If compressed air is not available at the application site, a compressor must be installed and maintained. The foam application equipment is slightly more expensive than conventional spray equipment and requires somewhat more maintenance. In some applications, the surfactant chemicals may cause too much sand/fines to stick to the rock being used. In applications where the material is being washed before processing, this is not a problem. In applications where the material is not being washed before processing, testing should be performed to ensure the final product will meet the necessary spec. If there is too much sand/fines, foam application rates can be reduced to the absolute minimum that will control the dust. This may vary from day to day.
Finally, the amount of surfactant required to generate foam is somewhat greater than the amount required for wet spray. The volume of surfactant to a given body of water is higher; however, due to the foam’s expansion, the amount of mix applied is lower, as shown in Figure 1.
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Apply Appropriate Moisture to Suppress Dust Different suppression technologies apply different amounts of moisture in different methods. This chart shows approximate amounts of moisture to be applied for currently available suppression technologies.
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Choose your location wisely
In fog, foam, water and water/chemical spray applications, the sites chosen for nozzle placement and suppressant delivery patterns are as important, if not more important, than the selection of the material to be applied. Even the best-designed program will fail if the suppressant material is not delivered to the correct location to allow mixing with the dust fines.
The success of the suppression effort relies on the proper mixing of the material and the suppressant at the transfer point. When applying dust suppression, it is best to locate the suppression system as close to the beginning of the transfer point or dusting source as possible. This allows for maximum mixing of the suppressant with the material.
The installation of fog systems is a little different since fog systems are designed to treat the air around the material, rather than the material itself. Therefore, the application point for the fog mist is generally near the end of the transfer point. This allows the material load to settle, and any pick-ups for active or passive dust collection systems to remove dust-laden air without risk of blinding the filtration media with wet dust particles. Fog generation nozzles are installed to cover the full width of the conveyor’s skirted area. It is recommended that skirtboard height be at least 24 inc., to allow the cone of the nozzle output to reach optimum coverage.
Control suppression techniques
Applying dust-suppression water and/or chemicals at transfer points must be controlled automatically so that water and/or chemical is applied only when the conveyors are running and there is material present.
Control systems can be as simple as an operator opening a valve on a “garden hose” system. At the other end of the spectrum can be a stand-alone computer system dedicated solely to monitoring and controlling the dust control system.
As the control systems become more sophisticated, the operating cost of the dust control system goes down. More sophisticated control systems allow greater latitude in the selection and operation of sensors that can detect material motion, material throughput, instantaneous dust loading in the area, chemical use, air and water flows, and even percent moisture being added to the material. Sensors that can differentiate between two different materials can allow the suppressant type or flow rate to be changed automatically as the material changes.
Maintain suppression equipment
Just as with operating an automobile, dust suppression systems require routine maintenance. Without a doubt, the most common cause of dust suppression system failures is a lack of maintenance.
Nozzles must be checked and cleaned, flow rated checked, filters cleaned, chemical levels checked, water and air flow rates checked on a routine basis or even the best system is doomed to failure. Some dust suppression companies are now offering this routine service as a part of their system package.
Dust suppression is just one piece of the puzzle
There is no such thing as 100-percent dust control. Even “clean rooms” in labs have some dust in them. Be realistic in expectations for dust control and look for specific percentages of reduction instead of adopting a “no-dust” approach.
Dust suppression alone cannot be the complete answer to the control of fugitive material. But carefully chosen, effectively engineered and properly maintained, a dust suppression system can improve the efficiency and minimize the risk of overloading its counterparts, the material containment, and dust collection systems.
Steven Frank is a project engineer of dust suppression for Martin Engineering, Neponset, Ill.
Mist System Reduces On-Site Dust
A custom-designed misting system was designed with nearly a dozen water nozzles at dust-generating points such as the hopper and the end of conveyors.
Operating in an environmentally sensitive area such as Long Beach, Calif., required contractor Romero Construction to implement a water-mist system to control dust.
Visit any quarry or demolition site across North America and you’ll find one common problem stirring up trouble—dust. The occurrence of airborne particles is one of the most frequent complaints from an operation’s neighbors. And more likely than not, those neighbors are taking action.
Move that thought into California, where air quality standards are tougher than anywhere else in the world, and you have a situation that challenges even the most creative producer.
Such was the situation facing Romero Construction when it was awarded a contract to crush and remove material from a 10-acre shopping center demolition site in downtown Long Beach. “Normally we’d set up three baghouse operations on a job like this to collect the dust,” said Don Lablanco, general manager of the Materials Division of Romero Construction.
But Lablanco had other ideas in mind. “We hired a private consultant, George Koch, to help us eliminate the dust at its sources, rather than collecting it. It just made good sense to us to prevent the situation from happening in the first place, instead of looking for ways to correct it later.”
Enlisting the aid of Cooley Equipment, a local Nordberg and NESCO dealer, Lablanco and Koch set out to design and recommend a water mist system at 10 or 12 major dust-generating points along the material handling route. Romero set up its crushers, a Nordberg Boss Jaw Crusher and HP400 Cone Crusher, and screen directly adjacent to material stockpile to minimize handling. The debris included both concrete and asphalt from the old shopping center structure and parking lots. A pair of Caterpillar 988F loaders moved the material from stockpile to hopper.
The NESCO mist system is custom designed for each application and is used to control environmentally-sensitive, dust-generating sites. The system includes nozzles strategically placed at known dust generating points, such as the hopper and conveyor ends. The system is automatic but may also be manually activated from the control panel.
Prior to use, the Romero site had to be approved by the South Coast Air Quality District since this type of operation calls for a baghouse system. Prior to the Long Beach project, a spray mist system had never been employed for this type of project in the state of California, which is now re-evaluting the merits of this type of system, both in terms of production as well as environmental friendliness.
From an environmental standpoint and in terms of mobilization and operating costs, the NESCO system is far easier to set up and dismantle and travels along with the plant to the next jobsite. “Compared with a typical baghouse operation,” said Lablanco, “the NESCO mist system basically pays for itself over the length of a job like this.” All engineering and set-up charges for this system were included in the cost.
Romero Construction is a major player in the California construction arena, specializing in fine-grading, paving and curb and gutter projects. The material being crushed and sized in Long Beach is being recycled as a base material for a Romero fine-grading contract at the multi-million dollar Pier 400 project a few miles away at the port of Los Angeles.
In order to meet deadlines and budgets, productivity is also important to Lablanco. Normal operating procedures enforced by the South Coast Air Quality District dictate maximum capacity of 500 tons per hour. With the NESCO mist system, capacity was increased to 800 tph, an 80-percent increase in productivity. Lablanco anticipates that this figure will be increased to 1,000 tph in the near future.
With the Long Beach City Hall, and the Long Beach newspapers literally overseeing this project from their offices surrounding the site, dust control was a true concern in Long Beach. Proof positive that a little ingenuity has gone a long way in bringing both environmental and construction concerns together.
the bottom line...
Romero Construction opted for a water-mist system over its traditional use of baghouses when working at a highly sensitive site in Long Beach, Calif. The project represented the first time a spray mist system had been used in this type of project in the state. According to the contractor, the system offered a higher throughput and was easier to move from job to job.
To submit a suggestion for a Success in the Field or for more information about any of these stories, contact AggMan at (717) 337-0027, Fax: (717) 337-9337 or email at bill@aggman.com
Emission Regulations to Affect Fuel Cost, Lubricant Choice and Engine Design
Editor’s Note: This monthly column is supplied exclusively for AggMan by The Equipment Maintenance Council (EMC).
While the diesel engine has undoubtedly proven itself to be the most efficient source of power for heavy-duty on-highway trucks and off-highway equipment, it also is a major contributor to air pollution.
“In 2002, emission controls will take the next step in a long journey to minimize the release of harmful particulates, hydrocarbons and nitrogen oxides (NOx) into the atmosphere,” said Brian Jacoby, field formulation engineer for Castrol Heavy Duty Lubricants Inc. “Compared to current requirements, NOx is targeted to be reduced by 50 percent to 2.5 g/bhp-hr. This new Environmental Protection Agency (EPA) requirement was originally scheduled for 2004. However, under a 1998 consent decree agreed to by all major engine original equipment manufacturers (OEMs), its timeline was moved up two years.”
Also on the horizon is the next regime of emissions reductions planned for 2007. It is anticipated that additional changes to hardware, fuels and lubricants will be needed to meet this more stringent requirement.
As for 2002, most engine OEMs will be utilizing exhaust gas recirculation (EGR) as a primary component to help control emissions. EGR technology works by directing a portion of exhaust gas into the combustion mixture to cool it. The cooled combustion mixture results in emissions with lower levels of NOx. However, according to Jacoby, the balance of control with EGR can be challenging for OEM engine makers because reducing NOx can lead to increases in particulates, which also need to be controlled. Considered to be the most significant base change for some engine makers, EGR will play an important role in achieving successful results. Others insist that new technology and advances in the fuel combustion process will allow 2002 emissions targets to be met without the use of EGR. Obviously, many improvements made to reduce emissions have come from advances in diesel engine design. However, as 2007 emissions targets draw closer, a more systematic approach that includes the use of aftertreatment devices and improved fuel quality will be necessary.
Indeed, aftertreatment devices will play a critical role in meeting the 2007 emissions targets. The need to integrate this technology into future engine design and its potential success at more sizable emissions reductions will rely heavily on a balanced program to improve fuel quality. One drawback to aftertreatment is that higher fuel sulfur limits can poison the device, especially where the use of the most promising technologies have been found to be effective. Higher sulfur levels, such as up to 500 parts per million (ppm) in the fuel used today, can also increase the total output of particulates by the engine. Because of this, the use of ultra-low-sulfur fuel will be needed. Aftertreatment devices work more effectively with ultra-low-sulfur fuel, which is defined as having 15 ppm or less sulfur.
All these proposed changes probably will come at an increased cost. In order for engine OEMs to meet the new hardware requirements, it is speculated that engine costs may increase by $2,000 to $3,000. Also, there may be increases in warranty costs due to the addition of EGR and aftertreatment devices. Under the new EPA ruling, these devices need to remain functional for up to 435,000 miles or 10 years. Over time, the goal is to provide catalyst control that will last for the life of the engine, similar to that of the passenger car.
Jacoby says that he believes fuel will also be a source of increased costs.
“While supply and demand have always been the key drivers of fuel costs, refining costs and additive treating of the fuel are also contributing their fair share. There is probability to see a 4¢- to 6¢-increase in fuel costs for ultra-low-sulfur fuel alone, along with possible future increases as additive technology becomes more involved,” said Jacoby. “There is also a cost impact at the fleet operator’s level. For an operator with a mix of on-highway and off-highway equipment, what fuel should he buy? Should he inventory two fuels? This will be similar to the questions fleet managers faced in 1993 when fuel sulfur was reduced to comply with newly enacted EPA mandates.”
It is expected that most operators with mixed fleets will only buy one fuel, hoping for improved performance with the added cost. Whatever the choice, rising fuel costs will continue to play an important role in many key decisions at the shop level.
Fleet operators’ maintenance programs also will be impacted. Inventory costs might rise with fuel and hardware accessories for the aftertreatment devices, along with increases in service time for each vehicle. In addition, technicians will have to be trained to properly maintain the components of the aftertreatment devices. It will be important for fleet managers to take a proactive role in controlling costs and preventing excessive equipment downtime.
Lubricants, too, will be affected by new emissions targets. When API category CH-4 was officially licensed in 1998, it was anticipated to last through 2004, running concurrent with EPA emissions changes. However, the sweeping change by the EPA to a 2002 timeline, along with interim changes in engine performance, have prompted the need for yet another lubricant specification—Proposed Category 9 (PC-9) which will be API category CI-4. With changes to electronic controls and small design changes to other engine components, OEMs have been able to comply with current emissions targets. For lubricants, though, this has come at a price. Specifically, engine oils are being forced to handle increases in contamination—mainly soot. It has been a major challenge for most lubricant manufacturers to control soot-related wear and handle viscosity increases and oil filtration without going through significant reformulation.
If not handled properly by engine oil, soot contamination can damage engine components. Increases in valve train wear and ring and liner wear are critical areas of concern to engine OEMs and end users. Some engines have seen reported soot levels skyrocket in the last two years to 4 percent to 6 percent by volume. For most, this is double the allowable limit the OEM specifies.
With soot volumes that high, the oil is challenged to maintain viscosity control. This is most important at low operating temperatures, where increases in viscosity could affect start-up lubrication, filtration and, subsequently, wear.
The filtration issue is very important. Concerns about filter plugging and poor performance at lower temperatures have caused fleet managers to evaluate the types of filters being used and make necessary adjustments. This has opened the door for new filter technology and aftermarket add-on filtering devices. Proper filter selection can be critical to the long life of a component.
Maintenance levels on the lubricant also should be considered. While many OEMs have discouraged extending the service life of the lubricant, citing risk to engine life, most fleet operators have enjoyed some level of drain interval extension using CH-4 oils. However, some are reconsidering their approach to drain intervals with PC-9 because the key elements in their decision continue to change. Operators don’t want to sacrifice what they’ve gained and OEMs do not want “carte blanche” for engine users.
What is needed is a balanced approach considering lubricant performance, controlled engine performance, filter selection and performance, and a tightly controlled maintenance program. Just as it is important for engines, fuels and aftertreatment devices to work together, it is equally important to search for lubrication solutions to achieve performance goals.
Another issue with regard to lubricants is backward compatibility to older engines. In the past, lubricant specification upgrades usually meant an across-the-board improvement in performance. Jacoby said PC-9 might be different.
“PC-9 is being driven by changes to the on-highway engine market, where emissions related hardware changes will dictate engine performance. But what about off-highway engines in use and older on-highway engines in service?” he asked. “The new PC-9 oil specifications are targeting these new engines with higher TBN and higher ash engine oil candidates. While they may be what is needed for the new engine, it has raised concerns with users of older engines, who fear increased wear and reduced performance.”
Off-highway, many diesel engines can have a useful life of 20 years or more, which operators do not want to put at risk. Some predict that very few of the new engines will be in use over the next few years and wonder why broad changes are necessary. Undoubtedly, this issue will be a major challenge for lubricant manufacturers. They have to meet the needs of the new engines without compromising the performance of the majority of engines in the marketplace.
Many key issues must be considered as a result of 2002 emissions targets. Like the ripple effect when a rock is thrown into a pond, the resulting changes in the future will be more significant as time goes on. Success will depend on the ability to adapt, understand and implement new technology to its fullest extent.
The Equipment Maintenance Council (EMC) is an individual membership organization comprised of equipment maintenance professionals. Its members are responsible for the purchase, maintenance, employee training, shop facilities and parts management of leading corporations and government entities that utilize heavy, off-road equipment. Its members also represent the major manufacturers and suppliers of the heavy equipment industry. EMC provides end users with cutting-edge education, and it is the only organization to offer a certification program for the industry, the Certified Equipment Manager (CEM). For more information, contact Stan Orr, CAE, EMC executive director, at (970) 384-0510, e-mail at ceo@equipment.org, or visit EMC’s web site at www.equipment.org.