ICRI / Dust Control Technology White Paper on Silicosis
SILICA IN THE REPAIR ENVIRONMENT:
What you should know about the Issues and Solutions
Co-Authored by Edwin Peterson, President
Dust Control Technology
Sources for all the information used in this paper are cited in the references section at the end of the paper.
Every year millions of workers in the U.S. are exposed to crystalline silica. Occupational exposure to crystalline silica dust causes or contributes to the development of silicosis, a disabling, irreversible, and sometimes fatal lung disease. Silica is a natural constituent of the earth's crust and is a major component of sand and granite. Crystalline silica, or free silica, is a term for the chemical compound silicon dioxide (Si02) when it occurs as a crystalline structure. Crystalline silica occurs naturally in many different forms, but the three most common forms are quartz (which is the most abundant), cristobalite and tridymite. The most serious exposures result from quartz in the form of respirable dust produced by grinding, sandblasting, and mixing operations. Activities such as jack hammering, rock drilling, concrete mixing, concrete drilling, brick and concrete block or slab cutting and guniting are also associated with potential exposure to crystalline silica dust.i
WHAT IS DUST?
Dust is the general name for minute solid particles with a diameter of less than 500 micrometers. Dust is generated from a wide range of activities, both industrially and domestically. Activities such as mining, construction, demolition and agriculture are industries that are large contributors to general atmospheric dust levels. Dust is produced in a very wide range of sizes. Larger, heavier particles tend to settle out of the air and onto surfaces; smaller, lighter particles have a tendency to hang indefinitely in the air. Dust exposure is contingent on the amount of dust emitted and the physical characteristics of the material, in addition to the methods of handling of the material. Dust exposure generally occurs from, but is not limited to, coming in contact with dust from one or more of the following tasks:
Abrasive blasting involving harmful materials
Demolition of concrete and masonry structures
Breakage or crushing of ore
Release of previously generated dust through the loading or dumping of materials
WHAT ARE THE HAZARDS OF DUST?
In construction, demolition, and renovation situations, dust from a variety of sources poses serious and recognized health risks to workers, causing acute and chronic respiratory diseases such as silicosis, sarcoidosis, asbestosis, coal miner’s pneumoconiosis, and other pneumoconiosis-type ailments. In addition to potential health problem for workers, dust emissions in some sectors also create another threat by increasing the probability of fires or explosions.
Health risks occur when workers are exposed to excessive amounts of harmful dust. The harmfulness of dust is based on the composition of the dust (i.e. chemical or mineralogical), the size and shape of the particle (i.e. fibrous or spherical), the concentration of the dust (either by weight or quantity of dust particles), and lastly, the exposure time.
For occupational health purposes, dust is categorized by its composition. There are two main types of dust that exist on a worksite. The first of the two is fibrogenic dust. Fibrogenic dust has fiber-like qualities, making it biologically toxic. If retained in the lungs, fibrogenic dust can impair the lungs’ ability to function properly. Examples of this kind of dust include asbestos dust and free-crystalline silica.
The second type of dust is inert dust, which essentially any dust containing less than 1% of quartz. Typically, health effects caused by inert dust are potentially reversible, as opposed to the more permanent effects of fibrogenic dust. However, inert dust has the potential to obscure visibility, cause unpleasant deposits in exposed bodily orifices, and potentially injure mucous membranes or the skin through chemical action.
Additionally, dust is classified by size into three categories: respirable dust, inhalable dust, and total dust. Respirable dust is small enough to penetrate deep into the lungs and bypasses the nose, throat, and upper respiratory tract. It is defined as being less than or equal to 5µm, which is about 1/12th the width of the average human hair. Inhalable dust has a median size of 10 µm and, when inhaled, becomes trapped in the nose, throat, and upper respiratory tract. Total dust includes all airborne particles, without regard to size or composition.
Long-term exposure to certain harmful respirable dusts can cause a condition known as pneumoconiosis. Pneumoconiosis is a general name for dust-related respiratory diseases that are categorized by a tissue response to the buildup of mineral and/or metallic dust particles in the lungs.
There are three varieties of pneumoconiosis that are much more prominent and common in industrial situations. The first—and most prevalent ailment in the concrete industry—is silicosis, which is a chronic, irreversible disease resulting in shortness of breath and eventually, death, due to scarring of the lung tissue. Crystalline silica is naturally present in the following materials used in construction:
Many abrasives used for blasting
Brick and refractory brick
Concrete, concrete block, cement and mortar
Granite, sandstone, quartzite and slate
Rock and stone
Sand, fill dirt and top soil
Asphalt containing rock or stone
Next is the form of pneumoconiosis called Coal Worker’s Pneumonia (CWP), more commonly known as black lung. In this disease, respirable dust particles from coal mining and/or refining collect in the lungs and darken the tissue, causing breathing difficulties and potentially, death.
Thirdly is the form of pneumoconiosis called asbestosis, which is caused by the inhalation of asbestos fibers. This is an irreversible disease that causes shortness of breath and an increased risk of cancer, including a very rare type of cancer known as mesothelioma. Typical exposure occurs in mining and demolition sites.ii
What is Crystalline Silica?
Crystalline silica is a basic component of soil, sand, granite, and many other minerals. Quartz is the most common form of crystalline silica. Cristobalite and tridymite are two other forms of crystalline silica. All three forms may become respirable-sized dust particulate when workers chip, cut, drill, or grind objects containing crystalline silica.
What are the Hazards of Crystalline Silica?
Silica exposure remains a serious threat to nearly 2 million U.S. workers, including more than 100,000 workers performing high-risk jobs, such as abrasive blasting, foundry work, stonecutting, rock drilling, quarry work and tunneling. Crystalline silica has been classified as a human lung carcinogen. Additionally, breathing crystalline silica dust can cause silicosis, which in severe cases can be disabling or even fatal. The respirable silica dust enters the lungs and causes the formation of scar tissue, thus reducing the lungs’ ability to take in oxygen. There is no cure for silicosis. Since silicosis affects lung function, it increases susceptibility to lung infections like tuberculosis. In addition, smoking causes lung damage and adds to the damage caused by breathing silica dust.iii
Dust control can be accomplished by simply enclosing the dusty area inside a structure. Sides and sometimes the top of the structure must be closed to accomplish complete dust control. This method is a permanent solution. Enclosures are expensive to both build and to maintain. Each site is unique and determines the size and location of the structure.
Silicosis is classified into three types: chronic/classic, accelerated and acute.
Chronic/classic silicosis, the most common, occurs after 15–20 years of moderate to low exposures to respirable crystalline silica. Symptoms associated with chronic silicosis may or may not be obvious; therefore, workers need to have a chest x-ray to determine if there is lung damage. As the disease progresses, the worker may experience shortness of breath upon exercising. In the later stages, the worker may experience fatigue, extreme shortness of breath, chest pain, or respiratory failure.
Accelerated silicosis can occur after 5-10 years of high exposures to respirable crystalline silica. Symptoms include severe shortness of breath, weakness, and weight loss. The onset of symptoms takes longer than in acute silicosis.
Acute silicosis occurs after a few months or as long as 2 years following exposures to extremely high concentrations of respirable crystalline silica. Symptoms of acute silicosis include severe disabling shortness of breath, weakness, and weight loss, which often leads to death.
History of Silicosis
Silicosis is a disabling, non-reversible and sometimes fatal disease caused by the inhalation of dust that contains silica. There are reports of workers succumbing to silicosis dating back to ancient Greece. During the Industrial Revolution of the late 1800’s and early 1900’s, farmers and immigrants took jobs in dusty foundries, mills and mines. Because exposures to silica significantly increased during this time so, too, did the disease’s prevalence.
Studies in the mid-1900’s showed high rates of silicosis among miners, specifically among lead and zinc miners in Oklahoma, Kansas and Missouri, where rocks and ores can consist of more than 95% silica. One study showed more than half of the 700 miners sampled had silicosis.
In the 1930’s, during a hydroelectric project through a West Virginia mountain, workers were exposed to dust containing very high levels of silica. Although it was known at the time that exposure to this type of dust was potentially fatal, sponsors of the project did nothing—not even providing respiratory protection to the workers. This event, known as the “Hawk’s Nest Incident,” has been called America’s worst industrial disaster and it is estimated this exposure resulted in as many as 700 deaths.
Soon after, at a national conference in 1936, Frances Perkins (then Secretary of Labor) declared war on silicosis. In 1940, a film entitled “Stop Silicosis” was issued to help advance efforts to prevent silicosis on a national level.
More recently, in the late 1960’s and early 1970’s, several new federal laws were passed, adopting rules to limit the exposure to silica dust. Since that time, deaths due to exposure have declined; however, silicosis still claims the lives of more than 250 American workers each year, in addition to disabling many others.iv
Statistics concerning Silicosis
The National Institute for Occupational Safety and Health (NIOSH) estimates that two million workers in the U.S. are exposed to crystalline silica on an annual basis, and more than 100,000 of those workers are exposed to dangerous levels of silica through abrasive blasting and rock drilling operations. In fact, data from OSHA SEP investigations show that employees in 22% of inspected workplaces were overexposed to silica dust between January 1, 2006 and April 10, 2007.v Hundreds of workers die of silicosis each year in the U.S. and hundreds more become disabled and are unable to take care of themselves or their families. Since 1968, more than 14,000 workers in the U.S. have died as a result of silicosis.vi
Workers in the construction industry, and more specifically, those who are working with concrete in tasks such as sawing, chipping, tuck pointing, milling, polishing, needle gunning, drilling, clean up, scarifying, grinding and/or crushing are much more likely to be exposed to silica dust and become ill or die as a result of this exposure. The construction industry has the highest number of deaths due to silicosis. As a result, some states are taking legal measures to reduce these statistics. For example, New Jersey law (N.J.S.A. 34:5–182) prohibits dry cutting and dry grinding of masonry materials.
OHSA STANDARD FOR RESPIRABLE CRY STALLINE SILICA
OSHA has an established Permissible Exposure Limit, or PEL, which is the maximum amount of crystalline silica to which workers may be exposed during an 8-hour work shift (29 CFR 1926.55,1910.1000). OSHA also requires hazard communication training for workers exposed to crystalline silica and requires a respirator protection program until engineering controls are implemented. Additionally, OSHA has a National Emphasis Program (NEP) for Crystalline Silica exposure to identify, reduce and eliminate health hazards associated with occupational exposures.vii
OHSA suggests employers and employees take measures to protect against exposure to crystalline silica, including:
Replacing crystalline silica materials with safer substitutes, when possible.
- Recognize when silica dust may be generated and plan ahead to eliminate or control dust at the source.
- Knowing how to protect against exposure to crystalline silica dust.
- Providing engineering or administrative controls, where feasible, such as local exhaust ventilation with dust collectors, blasting cabinets and/or wet methods to prevent dust from becoming airborne.
- Using all available work practices to control dust exposures, such as water spray.
- Routinely maintain dust control systems to keep them in good working condition.
- Wearing only an N95 NIOSH-certified respirator, if respirator protection is required.
- Wearing only a Type-CE abrasive-blast supplied-air respirator for abrasive blasting.
- Wearing only disposable or washable work clothes and shower if facilities are available.
- Vacuuming the dust from clothes or changing into clean clothing before leaving the worksite.
- Participating in training, exposure monitoring, health screening and surveillance programs to monitor any adverse health effects caused by crystalline silica exposure.
- Providing workers with training that includes information on health effects, work practices and protective equipment to protect against respirable crystalline silica.
- Being aware of the health hazards related to crystalline silica exposures and the additional lung damage that may occur from smoking.
- Refraining from eating, drinking, smoking and/or applying cosmetics in areas where crystalline silica dust is present. Taking care to wash hands and face outside of dusty areas before performing these activities.
- Preventing dust from becoming airborne during clean up by using water hoses or wet wiping rather than compressed air. Using vacuums with hi-efficiency particulate air (HEPA) filters and wet sweeping rather than dry sweeping.vii,ix
There are a number of engineering controls that may be installed and maintained in an effort to eliminate or reduce the amount of silica in the air and the build-up of dust on equipment and surfaces. Examples of these controls include: exhaust ventilation and dust collection systems, water sprays, wet drilling, enclosed cabs, and drill platform skirts. The extreme abrasiveness of silica dust can damage the systems put in place, therefore it is imperative employers practice preventative maintenance.x
Dust collection systems (see Fig.1) utilize ventilation principles to carry a dust-filled air stream through ductwork to a collection and isolation point. The system relies on four main components in order to effectively capture and collect the dust. These four main components are: an exhaust hood to capture dust emissions at their source, a system of ducts to transport the captured dust to the dust collector, a dust collector in which to store the removed dust, and a fan and motor system to provide the required suction for the system. Careful attention must be given to the selection and implementation of each component, as poor performance of one component will adversely affect the entire system.
Fig. 1: Dust collection system
Ventilation systems (see Fig. 2) are often used in concrete repair and restoration projects to remove the respirable dust from the work area. These typically consist of fan and motor systems to direct the airflow from the work area to a containment system, as described above, or exhausted from the work area to the exterior of the building. These systems may also utilize negative air machines with HEPA filters to clean the air prior to exhausting it from the work area. These systems should be designed for a minimum of 20 air changes per hour. If the silica-containing dust is to be exhausted from the work area, it is important to consider the effects on the outside environment prior to designing such a system.
Fig. 2: Negative air machine
Equipment-mounted dust collection systems (see Fig. 3) typically consist of a dust-collecting hood and vacuum attachment mounted to the cutting and drilling piece of the equipment. During operation, dust and small particles are captured by a shroud and routed through the vacuum hose into a dust collection system. These dust-controlling tools provide greater visibility when cutting. No dust is allowed to become airborne and, because these systems are designed to cut dry, no slurry is produced to block visibility.
Fig. 3: Equipment-mounted dust collection system
Dust control can also be accomplished by simply enclosing the dusty area inside a structure. The sides and sometimes the top of the structure may be closed to achieve complete dust control. Each worksite is unique and therefore determines the size and location of the structure. Enclosures can be expensive to both build and to maintain and, due to their stationary nature, are permanently fixed.
There are two different types of wet dust suppression; one wets the dust before it is airborne (surface wetting), the other wets the dust after it becomes airborne.
The principle behind surface wetting is to prevent dust from even forming and becoming airborne. This method is based on the effective wetting of the material, either through static spreading (wetting material while it is stationary) or dynamic spreading (wetting material while it is moving).
The principle of surface wetting is used in wet cutting and wet blasting procedures. Wet cutting involves the use of water to suppress airborne dust during the cutting process and is often associated with concrete and masonry cutting procedures (see Fig. 4a & 4b).
Fig. 4a: Wet cutting hand-held saw
Fig. 4b: Walk-behind concrete saw
Water spray/misting systems (see Fig. 5) spray very small water droplets into airborne dust, which collide with the airborne dust particles and cause them to stick to each other, falling out of the air onto the ground. This collision is optimized when the droplets of water and the dust particles are of similar sizes.
Fig. 5: Fine water spray/misting system
When the water droplet size and the dust particle size are not similar, the Slipstream Effect™ occurs (see Fig. 6). In this scenario, the two never collide but are instead swept around each other, due to electrostatic force on the water droplets and the effect this has on the path of the dust particles. Just as with magnets, similarly charged particles repel each other. Thus, it is advantageous to have the particles either both neutrally charged (so that they neither repel nor attract one another) or oppositely charged (so that they attract one another), in order to increase the likelihood of a water and particle collision.
Fig. 6: The Slipstream Effect™
In some cases, it may be beneficial to add surfactants, tactifying agents or chemical foams to water spray/misting systems to improve dust-controlling performance. Surfactants lower the surface tension of the water droplets and allow them to spread further over the spray area and penetrate deeper into the material being sprayed. Tactifying seal the ground and prevent latent dust from being stirred up into the air. Chemical foams increase the surface area and the efficiency of the wetting.xi
Hydrodemolition is another method to prevent worker exposure to crystalline silica-containing dusts that are created during impact methods of concrete surface preparation, removal or sawing. By both eliminating mechanical impact and introducing water, hydrodemolition bypasses the only mode of entry and damage from silica by preventing the creation of respirable dusts. Once operations are completed, standardized wash down and disposal practices must be followed to prevent the drying of the demolition by-product into respirable dust and to avoid subsequent cement fines. Through the combination of hydrodemolition and good operational practices, employee exposure to silica dusts can be virtually eliminated.xii
PPE (Personal Protective Equipment)
Employers are required by the OSHA Standard 1910.132 to conduct a Personal Protective Equipment (PPE) Hazard Assessment and corresponding training of affected employees. PPE is the last resort level of protection employers should rely on to protect against worker exposure to contaminants. If PPE fails, is not worn or is fitted improperly, or is of the wrong type, employees will be exposed. Documentation that PPE is effective is required, especially in the case of airborne contaminants, like silica dust.
When engineering controls cannot keep silica exposures below the National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL) of 0.05 mg/m3 (as a 10-hour time-weighted average), proper respiratory protection should be used. Respirators should not be the primary method of protection, but can be an important element in the defense against silicosis. Beards and mustaches can interfere with the fit of the respirator to the face, rendering most of them ineffective.
Respiratory Protection Programs must be established by employers when respirators are used. A comprehensive respiratory protection program is required by the OSHA Respiratory Protection Standard, and outlined in the NIOSH Guide to Industrial Respiratory Protection. A respiratory program must cover the following basic elements, as applicable:
Periodic environmental training
- Regular training of personnel regarding exposure and respirator use
- Selection of appropriate NIOSH-approved respirators
- A medical evaluation of the worker’s ability to perform the work while wearing a respirator
- Respirator fit testing (annually)
- Maintenance, inspection, cleaning and story of respiratory protection equipment
- Procedures for regularly evaluating the effectiveness of the program
Filtering Face Piece Respirators (see Fig. 7) are half-mask, air-purifying respirators containing N-95 Type or higher filters to provide minimal protection (up to 0.5 mg/m3). NOTE: Although acceptable by NIOSH, an N-95 filtering face piece (disposable respirator) provides minimum protection. A tight-fitting half-mask air-purifying respirator is much more effective at protecting against silica exposure.
Fig. 7: Disposable respirators
Half- or Full-Mask Respirators with Replaceable Filters (see Fig. 8) provide better protection against respirable dust, up to 1.25 mg/m3. These contain, at a minimum, N-95 filters that can be replaced.
Fig. 8: Half-mask respirators
Supplied-Air Respirators (see Fig. 9) can provide protection up to 25 mg/m3. These respirators have full-face protection and utilize high-efficiency filters to give a higher level of protection. Some use battery-powered motors to filter the air; others function via pressure-demand or other positive pressure mode.
Fig. 9: Supplied-air respirators
Type CE Abrasive-Blasting Respirators (SAR) (see Fig. 10) are full-faced, hooded masks with draping that extends over the neck and shoulders. They are operated in a pressure-demand or other positive pressure mode and a tight-fitting mask must be worn under the blasting hood. These are the only respirators that should be worn during abrasive blasting.
Fig. 10: Abrasive blasting hood
Having workers engage in the practice of vacuuming clothing after exposure to silica-containing dust can further protect against inhaling residual dust that may cling to clothing garments. If this is not possible, it is advisable to wear disposable or washable clothes to the worksite and shower, where available, before changing into clean clothes prior to leaving the worksite.
Washing Hands and Face
Workers who wash their hands and face before eating, drinking, smoking or applying cosmetics may also reduce their risk of ingesting silica-containing dust.xiv
Blasting Medium Substitution
Many blasting mediums, especially sand, quartz and slag, used in abrasive blasting contain high levels of silica. Rocks, mineral sands containing less than 2% silica and some metal slags (based on content) could be substituted to reduce levels of exposure. Alternatively, non-silica blasting mediums are available and could reduce risks to workers even more.
Non-Silica Blasting Medium
Alternative blasting mediums that do not contain silica, such as ilmenite, aluminum oxide, garnet, metal shot, steel grit, crushed glass and sodium bicarbonate may additionally reduce exposure. However, it is imperative that all environmental requirements are followed and protective equipment is utilized, even with the use of these safer blasting mediums.xv
Practicing general rules of worksite cleanliness will help remediate silica dust on worksites, possibly preventing the dust from becoming airborne and potentially inhaled. Many of the strategies, such as avoiding the use of compressed air in cleaning, following guidelines for spill and debris clean up, and using filters when vacuuming, are easy and inexpensive to follow.
Avoiding Compressed Air for Cleaning
The use of compressed air when cleaning creates more respirable dust by stirring up latent dust and causing it to become airborne. Although this was at one time common practice, it is now advised against. Methods of removing dust, like wet hosing or wet wiping, are just as effective at removing dust, if not more, but also reduce inhalation hazards.
Cleaning up Spills and/or Debris
When dealing with spills and/or debris clean up, it is best to avoid any type of activity that will stir up the dust and cause it to become airborne. The ideal methods for dealing with incidents involving silica-containing dust would incorporate the use of water. As with normal cleaning, wet hosing or wet wiping would be suitable, as would wet sweeping. For those individuals involved in the clean up, the use of a respirator would also be advisable.
Vacuums that are used on the worksite, whether in clean up or to remove dust from workers’ clothing, should be equipped with high-efficiency particulate air (HEPA) filters. The use of the filters will capture the dust and prevent it from being recirculated into the air.xvi
Controlling Access to the Work Area
Silica dust exposure can be minimized by controlling access to work areas. By preventing unprotected and/or uninformed individuals from entering areas where dust is being generated, accidental exposures can be avoided.
Marking and posting the boundaries of work areas where exposure to airborne dust can occur will help alert individuals to the potential hazard. Warning signs should also be posted to mark areas contaminated with respirable crystalline silica (see Fig. 11).
Fig. 11: Warning sign
Cordoning off or fencing in areas where dust is present reduces the possibility of unnecessary exposure. The physical presence of a barrier is an even more effective tool in controlling which individuals can enter the worksite.
EMPLOYEE TRAINING PROGRAMS
In addition to establishing workplace guidelines for employees to follow, employers should provide workers with training that includes information about health effects, work practices and protective equipment for respirable crystalline silica. Training in the proper use, fit, care, cleaning and maintenance of respirators is also advisable.xvii
EMPLOYEE MONITORING PROGRAMS
Monitoring, not only of the air quality to measure exposure, but also to gauge the health of the workers is recommended. These steps will help ensure that workers are being protected by the controls that are in place and will determine if additional worker awareness, education and/or training is necessary. Most importantly, annual health screening could prevent additional damage to the lungs of workers who may have already been exposed to silica dust, potentially saving lives. xviii
The ICRI Environmental Safety and Health Committee was established in 2006 in response to the industry-sponsored Vision 2020 initiative. The committee mission, as stated in the Vision 2020 documents is to “develop environmentally and worker friendly repair methods, equipment and materials that will greatly reduce the adverse effects on the workers, the public and the earth’s ecosystem.” To support this goal, a number of strategies were developed. The first objective was to create recommendations for minimizing the effects of hazardous airborne particulates on the workers, the public and the environment. This paper provides the most current understanding of the hazards related to dust created in the concrete and masonry repair environment. It is important that the members of ICRI appreciate the severity of the hazards and implement mitigation and control strategies to ensure the safety and health of those exposed to the dust-generating work. The primary recommended strategies are 1) to use engineering controls to minimize the amount of dust generated, and 2) to control and exhaust dust that is produced. In addition to these controls, the proper selection and use of personal protective equipment, including various forms and styles of respirators, are an important strategy for protecting the workers performing the tasks.
It is the responsibility of our industry leadership to ensure that we are proactive in addressing these health hazards. In addition to this paper, the committee will soon publish a new guideline document, “Guideline and Recommendations for Safety in the Concrete Repair Industry.” This guideline will be an illustrative document to help management and workers better understand the hazards associated with concrete repair work, the strategies available to minimize those hazards, and how to provide a safer environment for all the stakeholders in the concrete repair project.
- i Virginia workers newsletter
- ii Dust Control Technology
- iii OSHA 3177
- iv NYS PowerPoint Presentation
- v Connecticut Labor Dept., CONN-OHSA Division, Spring 1997
- vi N.J. Dept. of Health & Senior Services, Occupational Health Surveillance Program
- vii OHSA Fact Sheet, 2002
- viii OHSA Fact Sheet, 2002
- ix N.J. Dept. of Health & Senior Services, Occupational Health Surveillance Program
- x NIOSH brochure
- xi Dust Control Technology
- xii Rampart Hydro
- xiii CONN-OSHA
- xiv OHSA Fact Sheet, 2002
- xv Queensland Government, Dept. of Employment and Industrial Relations, 2004
- xviN.J. Dept. of Health & Senior Services, Occupational Health Surveillance Program
- xvii NIOSH, August 2004
- xviii NIOSH, August 2004
Finely Atomized Water Sprays
Finely atomized water sprays are normally used at transfer points without excessive turbulence or when the velocity of dust dispersion is less than 200 ft/min. The optimum droplet size, water usage, relative velocity, and number and location of nozzles depend on the conditions at individual transfer points.
Electrostatically Charged Fogs
Electrostatically charged fog uses charged water droplets to attract dust particles, which increases collision. The atomized water droplets are charged by induction or direct charging.
Design of a Water Spray System
The most integral part of a water spray system is of course the nozzle. The nozzle?s physical characteristics affect such factors as droplet size, droplet distribution, droplet velocity, spray pattern of the droplets, angle of the droplets, water flow rate, and water pressure.
The spray nozzle is the heart of a water-spray system. Therefore, the physical characteristics of the spray are critical. Factors such as droplet size distribution and velocity, spray pattern and angle, and water flow rate and pressure all vary depending on the nozzle selected. In general, a fogging type nozzle producing droplets of a size of 10-150µm is used for an airborne dust control system and a flat-spray nozzle producing coarse droplets of a size of 200-500µm at high pressure is used for a surface wetting situation. Typically, higher droplet velocity is desirable for both types of dust controls through water sprays as this allows for more dust particles at a time to be suppressed. The spray angle is necessary to know because it determines the amount of coverage that a certain nozzle with give and thus will let one know how many nozzles are necessary for a given amount of desired coverage area.
Comparison of some different dust control techniques
The following comparison chart between pure water dust control and electrostatically charged dust control is taken directly from Chapter 3 of the OSHA Dust Control Handbook cited in the references section at the end of this paper.
Finely Atomized Water Sprays
|» Water requirements are low-typically 5 to 20 gal/h per nozzle.
||» Tight enclosures are needed for effective system operation.
|» Moisture addition to the product is quite low-typically less than 0.1% of the material weight.
||» The system may not be effective either in highly turbulent environments or when the dust dispersion rate is more than 200 ft/min.
|» The material is not chemically contaminated.
||» Requires good droplet to particle size match for effective control.
|» The system can be economical.
Electrostatically Charged Fogs
|» Electrostatic fogs can be effective if the dust cloud carries predominantly positive or negative charges.
||» These systems are not recommended for underground coal mines or other gassy applications where explosions can be triggered by sparks.
|» The material does not become chemically contaminated.
||» Capital costs are high.
|» Moisture addition to the product is generally less than 0.5% by weight.
||» These systems require high-voltage equipment.
||» Maintenance of electrical insulation is critical for safe working conditions.
Information for this paper was taken from the following sources:
Bureau of Mines: U.S. Department of the Interior. Dust Control Handbook for Minerals Processing. Authored by Vinit Mody and Raj Jakhete, 1987.
Center for Applied Energy Research. Environmental and Coal Technologies Research
LaFarge North America. Material Data Safety Sheet: Slag
New Jersey Department of Health and Senior Services Occupational Health Surveillance Program. Stop Silicosis in Sandblasters: Use Silica Substitutes
Occupational Health and Safety Administration. Secondary Lead Smelter
North Carolina Department of Environmental and Natural Resources: Division of Pollution Prevention and Environmental Assistance. Primary Metals
Workers Health Centre. Fact Sheet: Silica and Silicosis