A BEST PRACTICES TOOLBOX ACKNOWLEDGMENTS The Mine Safety and Health Administration has conducted a series of Technical Support assistance visits over the last five years to determine ways of controlling miners" exposures to mercury at gold and silver mining operations. This manual "Controlling Mercury Hazards in Gold Mining: A Best Practices Toolbox," draws upon the experience gained in those visits. The objective of the manual is to limit miners" exposures to mercury by sharing information within the mining community on methods of controlling mercury . This "Best Practices manual was conceived and written by Michael Lynham, while an industrial hygienist with MSHA's Toxic Materials Division of the Denver Safety and Health Technology Center. Thanks to the personnel of the Denver Safety and Health Technology Center"s Toxic Materials and Physical Agents Divisions for their assistance in composing this document. Special thanks to Galen Trabant, who assisted Michael Lynham in designing many of the suggested control measures, as well as to Steven Kneipple and the Denver Mineral Engineers, Inc., for their design assistance. For illustrative contributions, thanks to Betty Parsons, Richard Duran, and Fred Bigio. Credit also goes to MSHA"s Metal and Nonmetal (MNM) Administration"s Margie Zalesak and to Jaime Alvarez, Kevin Good, and Richard Feehan for assistance on the Introduction, Industry Overview, and Technical Review sections. Kathy Snyder assisted in compiling information on trends in the industry. Andrea Hricko, Deputy Assistant Secretary, provided guidance in finalizing this manual, with legal review provided by Mark Malecki of the Office of the Solicitor (SOL), and other legal assistance provided by SOL"s Deborah Green, Jack Powasnik and Robert Aldrich. The report is on the internet thanks to Webmaster. Sonya Patterson, Hazel Haycroft, Bria Culp, and Joelle Hoberman assisted with word processing. Coordination of the final document was handled by Jennifer Davis. MSHA appreciates the willingness of the American Conference of Governmental Industrial Hygienists in allowing us to reprint diagrams from their publications. Finally, MSHA thanks those mine operators and miners who cooperated with Technical Support staff during these evaluations, as well as the Metal and Nonmetal mine inspectors who accompanied Technical Support staff during the evaluations. Important Note: The following figures are in color. They are best viewed when your display setting for colors exceeds 256. For Windows users check your settings via the control panel/display option to see what your system is capable of. Technical Review
No specific brand-name product is required to be used under Mine Safety and Health Administration (MSHA) regulations. MSHA provides product information solely as a service to the general public. Reference to any specific product by trade name, trademark, manufacturer, or otherwise in this report does not constitute or imply its endorsement, recommendation, or favoring by MSHA. The Mine Safety and Health Administration (MSHA) is committed to ensuring that miners are protected from health hazards at the worksite. It accomplishes this through enforcement of health standards and by working with labor, management, and other interested parties to develop and promote solutions to health problems, to share information and to provide health and safety training. Mercury remains a serious hazard in the mining industry. Although there are no current mercury mines in the United States, mercury can be naturally present in ore and is produced as a by-product of gold and silver mining. The development and growing use of cyanide technology has increased gold and silver production as well as the recovery of other by-products such as mercury. Because mercury has no warning properties, MSHA is concerned that many individuals are not fully cognizant of its hazards and the means necessary to control the risks to miners. In addition, miners may unknowingly take the contamination home and expose their families.5 In the past six years, MSHA has taken nearly 700 samples at gold and silver mines. More than 80 overexposures were found to be above MSHA"s exposure limit, with 50 percent of the overexposures measuring more than twice the exposure limit.6 The exposures ranged as high as 50 times the allowable limit. Many were found in refinery workers. Overexposures to silver have also been found in refinery workers. In addition, MSHA"s Technical Support Group has conducted nine studies involving mercury hazards at gold and silver mines in the past five years.7 This document has been developed by MSHA solely for the purpose of providing information to the gold mining community on the hazard associated with exposure to mercury. This document is not intended to establish official MSHA policy on all possible methods of compliance at every mining operation. MSHA standards applicable to occupational exposure to mercury may include, but are not limited to those set forth in Appendix D. The purpose of this publication is to present "Best Practices that can be utilized within the mining industry in controlling employees" exposures to liquid and vapor forms of mercury. This document will:
A checklist is provided for each section to summarize the key points of the respective chapters. Information from the checklists is not going to be collected; it is not required to be completed; and it will not be used for checking compliance with MSHA regulations. Rather, the checklists are an organizational method for providing information to the public by MSHA for educational purposes. U.S. gold production has increased tenfold since 1980.8 Production has been at historic record levels in the 1990's, with U.S. gold production valued at some $4.1 billion in 1996.9 The rise in U.S. gold production has coincided with a worldwide production increase attributable to rising gold prices since 1980.10 In the 1990's the United States has become the world"s second largest gold-producing nation, after South Africa.11 12 In 1996, U.S. gold mines13 produced approximately 325,000 kilograms of gold, a slight increase over 1995 production.14 From January through April 1997, U.S. mines reportedly produced 104,000 kilograms of gold.15 After rising over the period 1991 through 1996,16 gold prices registered a net decline in the first 5 months of 1997.17 However, the Gold Institute, a trade organization, has projected further production increases through at least 1998.18 In the United States, there are currently 191 gold mines and 15 silver mines which employ approximately 19,000 miners. They are distributed in twelve states: Alaska, Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, South Carolina, South Dakota, Utah and Washington. Approximately 40 percent of the mines have an associated mill. There are also 10 free standing mills which process ore from various mines. Many of the mills also have a refinery. Twenty-five percent of the gold mines list silver as a secondary commodity. Nevada has the largest number of gold and silver mines in the U.S. It has 30 percent of the total operations and 65 percent of the miners employed in gold and silver mining. Fifty-eight percent of its mines have an associated mill. California is a distant second with 19 percent of the operations and 10 percent of the miners. Forty-eight percent of its mines have an associated mill. For 1996, preliminary data indicate that with slightly over 190 operations, the U.S. gold industry employed 17,610 miners.19 A trend to increased underground gold mining is anticipated by many in the mining community. The U.S. Geological Survey has noted a trend to conversion from surface to underground gold mining as certain near-surface deposits reach depletion.20 Last year, for example, Barrick Gold Corporation opened a major new underground gold mine in Nevada, and further increases in underground gold mining are predicted.21 In the past six years, MSHA has taken 690 samples at 72 gold and silver mines and found 86 overexposures at 14 mines. (These 86 overexposures exceeded the exposure limit without considering the sampling error factor). Fifty percent of the overexposures were greater than twice the exposure limit. Many of the overexposures involved refinery workers, some of whom were also overexposed to silver. MSHA believes that these data indicate an ongoing risk to miners, which must be addressed by the mining community. TECHNICAL REVIEW I. Source of Elemental Mercury In the Western United States, low-grade gold ore deposits containing mercury have been developed and are being processed by cyanidation to recover precious metals. "Cyanidation uses cyanide solution to extract gold from the ores. These ores usually contain less than 15 parts per million of mercury.22 During the cyanidation process, 10-30 percent of the mercury in the ore is extracted along with the gold and silver. Reactions representing silver, gold, and mercury are shown below:23 Silver
2 Ag + 4 CN- + H2O2 => 2 Ag(CN)2- + 2 OH- Gold
2 Au + 4 CN- + H2O2 => 2 Au(CN)2- + 2 OH- Mercury Hg2++ + 4 CN-42- => Hg(CN)42 2 Hg + 8 CN- + O2 + 2 H2O 2 Hg(CN)-42- + 4 OH- The small amount of elemental mercury that is extracted from the host ore can pose health risks to employees who work with the gold-bearing solution purification and concentration circuit and in the refinery. The reason is that the cyanidation process involves concentrating the precious metals and the mercury by a factor of 3,000-4,000.24 This is required to produce a small volume of high grade gold solution that is suitable for final gold recovery. Elemental mercury vapor is released when the "pregnant leach solution undergoes the "Hydro/Pyro25 recovery process required to produce the dore bars. A review of the toxicology of elemental mercury will show the importance of limiting miners" exposures to this contaminant. II. Toxicology of Elemental Mercury Symptoms of Exposure Mercury is a heavy metal that can cause permanent disability and even death in miners unless it is properly controlled. It can also affect the children of miners who may become exposed when their parents bring home mercury through contamination of clothing and other items.26 Exposure to mercury can affect the nervous system and the kidneys. Mercury poisoning can result from both acute and chronic exposures. It is critical to recognize that exposure to mercury can be without warning, thus, workers may not know the extent to which they have been contaminated. It is essential that personal, environmental, and biological monitoring be done to determine the exposure hazard and evaluate symptoms, as necessary. The symptoms of acute mercury exposure generally involve the lung and central nervous system. The early symptoms include tremors, nausea, shortness of breath, chest pain, progressive pulmonary failure, and pneumonitis. Nervous system damage including hallucinations has also been reported. The symptoms can progress and deaths have resulted from severe acute poisonings within days of exposure.27 The symptoms of chronic mercury poisoning are nonspecific, making early detection of poisoning difficult. Early signs of chronic mercury poisoning are mild central nervous system dysfunction, including increased irritability, loss of memory, reduced self-confidence, insomnia, anorexia, and slight hand tremor. Chronic mercury poisoning can cause kidney damage resulting in proteinuria, which is the appearance of protein in the urine. Mercury can also adversely affect both male and female reproductive systems; in one study, following a single acute exposure, several men developed chronic and irreversibly impaired sexual functions.28 If excessive exposure to mercury is not controlled, the symptoms can become severe, resulting in permanent disability and loss of function. At any stage with symptoms, removal from exposure may not result in improvement. The symptoms of chronic mercury poisoning do not vary by route of exposure.29 Routes of Exposure
Mercury can be absorbed via the lungs, skin and gastrointestinal tract. Although the absorption rate varies for each route, it is critical for the protection of the worker that each route of exposure be controlled. Uncontrolled routes of exposure will lessen the overall protection given the worker by the controls in place. The following is a brief discussion of each absorption and elimination pathway.30 Pulmonary Route of Exposure Vapor inhalation is the main route of entry into the body. Mercury vapor diffuses across the alveolar membranes of the lungs and it is estimated that between 74 and 80 percent of the inhaled mercury is retained in the body.31 After absorption, the elemental mercury continues to exist for a short time in the metallic form, thereby enabling it to cross the blood-brain barrier or placenta. Metallic mercury is oxidized by hydrogen peroxide-catalase to divalent ionic mercury (Hg2+) which fixes to proteins. Oxidation of mercury to the ionic form, which is less likely to cross barriers, can trap it in the brain and the placenta.32 Dermal Route of Exposure Mercury vapor is absorbed through the human skin. The dermal uptake at whole-body exposure is estimated to be 2.2 percent of the pulmonary uptake.33 Dermal contact with liquid mercury could significantly increase biological levels. Metallic mercury may also cause allergic contact eczema. In addition to percutaneous exposure, contact with mercury by unprotected skin or porous work clothing can result in enhanced risk of pulmonary uptake due to the vaporization of the mercury. Gastrointestinal Route of Exposure Metallic mercury is not well absorbed when ingested unless a gastrointestinal (GI) fistula (lesion) or other GI inflammatory disease is present. The oral absorption of elemental mercury is limited and is less than 0.01 percent. 34 Elimination of Metallic Mercury Excretion of elemental and divalent mercury occurs primarily in the urine and feces. Mercury has a half-life of 40 to 60 days. The highest concentrations of absorbed mercury are found in the gray matter of the brain, in the kidney and in the liver. III. Mercury Exposure Limits MSHA standards at 30 CFR §§ 56/57.5001, for metal and nonmetal mines, incorporate by reference the 1973 edition of the American Conference of Governmental Industrial Hygienists publication "TLV"s Threshold Limit Values for Chemical Substances in Workroom Air Adopted by ACGIH for 1973. The standards require that exposure to airborne contaminants not exceed, on the basis of a time weighted average, the 1973 threshold limit values adopted by ACGIH. As a result, MSHA"s current TLV-TWA (8-hour shift) for mercury (all forms except alkyl) is 0.050 mg/m3. 35 The following is a list of other exposure limits for metallic mercury vapor established by various safety and health organizations. These limits are designed to prevent adverse central nervous system and kidney effects resulting from exposure to metallic mercury vapor. Currently, no unequivocally safe level has been ascertained to protect the reproductive functions of males and females. The ACGIH recommends that women of childbearing age should not be exposed to air concentrations of metallic mercury vapor greater than 0.010 mg/m3. American Conference of Governmental Industrial Hygienists 36
The ACGIH has also assigned a "skin designation to certain forms of mercury. This refers to the potential for significant contribution to an individual"s overall exposure to mercury by the cutaneous route. This includes mucous membranes and the eyes, either by contact with vapors or by direct contact with the mercury. The amount of mercury an individual may safely contact is not specified by the ACGIH. National Institute for Occupational Safety and Health37
Occupational Safety and Health Administration,38
World Health Organization39
IV. General Cyanidation Process The run-of-mine ore is crushed and ground to an optimum particle size to make the ore amenable to gold extraction. An oxidative process may be required as a pretreatment for sulphidic and carbonaceous ores that will permit the precious metals to be extracted by standard hydrometallurgical techniques. The next step of the cyanidation process is to leach the precious metals from the gold-bearing ore with a weak cyanide solution. In this step, the precious metals and mercury are solubilized into the cyanide solution as cyanide-metal complexes. This phase can be accomplished through heap leaching or tank agitation leaching. The "pregnant leach solution is pumped to carbon columns where the precious metals undergo further purification and concentration. There are two standard techniques by which this phase can be accomplished and they are "carbon-in-pulp and "carbon-in-leach. Each technique involves mixing the "pregnant leach solution with activated carbon in carbon columns or tanks. The metal cyanide complexes are adsorbed onto the activated carbon. The difference between the two techniques is that in the "carbon-in-leach method, the leaching and carbon adsorption phases are done simultaneously within the same vessel. With the other technique, the leaching and carbon adsorption phases are two distinct steps. The next step involves acid washing the loaded carbon to remove inorganic fouling agents to improve the efficiency of the metals desorption process. The carbon is washed using either a weak hydrochloric acid solution or a nitric acid solution. The acid-washed carbon is then sent to a carbon elution vessel where the precious metals are stripped from the carbon. The elution systems are operated at elevated temperatures and pressures. The stripped carbon is transferred to a rotary kiln for regeneration. The carbon is regenerated by heating it in the rotary kiln at approximately 700°C in a reducing atmosphere, such as steam. The re-activated carbon is returned to the carbon adsorption circuit. The carbon strip solution is pumped to the refinery to recover the precious metals. The gold and silver are recovered using either the Merrill-Crowe zinc precipitation method or through electrowinning. The silver content of the ore body usually dictates the gold recovery method that will be utilized at any one mine. Mines having an ore body containing a high percentage of silver will tend to use the Merrill-Crowe technique while others will use electrowinning. The Merrill-Crowe method involves using elemental zinc to reduce the metal cyanide complexes, such as gold and silver to their respective elemental forms. The solution is drawn through a filter press where the precipitate is collected. Electrowinning involves reducing the metal cyanide complexes to their elemental form by applying a voltage across a pair of electrodes immersed in the carbon strip solution. In each method, the neutral mercury cyanide complex will also be reduced to its elemental form. Therefore, both the zinc precipitate and the electrowin sponge must be retorted prior to smelting of the concentrate. The retorted concentrate is mixed with slag-forming fluxes and smelted in a furnace. Once smelting is completed, the slag is poured off and the precious metal alloy is poured into dore bars. A general cyanidation process flow sheet is depicted in Figure 1. V. Sources of Mercury Exposure The primary sources of mercury exposure connected with the cyanidation process stem from the following:
BEST PRACTICES: EMPLOYEE HYGIENE FACILITY Clothing worn by employees who work with or near mercury-contaminated equipment and work surfaces can become contaminated with mercury. This potentially contaminated clothing should never be worn home for laundering or stored in the same lockers used to hold employees" street clothes. MSHA has identified situations in which employees" personal motor vehicles and their homes have been contaminated with mercury because good industrial hygiene practices were not followed. The purpose of the employee hygiene facility is to limit employees" exposures to mercury and to prevent mercury from being transported from the employee"s work environment to their autos, homes and personal belongings. This is accomplished by providing a place to shower and by isolating street clothes from sources of mercury such as contaminated work clothes and tools. II. Building Materials Building materials that have a porous surface can, over time, become contaminated with mercury vapor. These contaminated surfaces will be a secondary exposure source if they are not properly cleaned or treated. Since most construction products are porous, it is best to select a material that provides a smooth finished surface that can be easily sealed and cleaned. The following considerations should be given when selecting building materials for the portions of the cyanidation plant containing mercury:
III. Location and Interior Layout A general layout of a hygiene facility is depicted in Figure 2. It consists of four separate areas:
The hygiene facility location should be such that employees enter the clean side of the facility from the parking lot. They should not be required to walk through any process area where there is potential for mercury contamination. The general flow of foot traffic for the facility is shown in Figure 2. Employees should enter directly into the work area from the "dirty side of the hygiene facility and not go through the lunch room. It is suggested that a "lunch bay, for transferring lunches to the lunch room be located in the interior wall separating the clean locker room and the lunch room. Such an arrangement is depicted in Figure 3. The clean side should be equipped with clothes lockers, and toilet facilities. Visitors and employees should remove their street clothes and store them in a locker at this location. Attention should be paid to the physical arrangement and equipment (including towels or robes) to ensure privacy. It is recommended that all jewelry be removed prior to entering the work areas. Next, employees should proceed to the "dirty room to don their work clothes. Showers should be located between the "dirty and clean locker rooms. This arrangement serves two purposes: (1) employees exit the shower directly into the clean locker room and (2) it separates the clean locker room from the "dirty one. Soap and towels should be provided. The "dirty side of the facility should also be equipped with a bathroom, clothes lockers and benches. Employees don their work clothes and enter directly into their work areas from this location. The "dirty locker room exit can be provided with an air-lock to isolate it from the adjacent work area. This will help prevent mercury vapor from infiltrating from the plant. At the end of the work shift, employees should clean their rubber boots in a wash solution containing a mercury complexing agent. Also, employees should remove their coveralls prior to entering the "dirty locker room. After entering the locker room, they should remove and store their work clothes and shower. Again, provision should be made to ensure privacy. After showering, employees should proceed to the clean room, dress, and exit directly to the parking lot. A floor drain should be installed in each area of the hygiene facility. This will permit the use of a hose to wash down each location of the facility. The floor should slope toward the drain to assist drainage. The floor drains for the clean room can be connected to the same drainage system used for the bathrooms. It would be preferable for the "dirty locker floor and shower drainage to be recycled back into the cyanidation process to prevent inadvertently introducing metallic mercury into the environment. The drains should be cleaned and treated with a mercury complexing agent to prevent the accumulation of mercury. Employees" work clothes and reusable coveralls should be laundered either on-site or by a laundry service. Clothes wash rooms should be located in the "dirty locker room. The dryer exhaust should be vented to the outside of the building and connected to a mercury vapor collection system (local exhaust ventilation). Drainage from the clothes washer should be returned to the process sump to prevent spread of contamination. Clothing should be washed on a daily basis. A mercury vapor analyzer should be used to scan employees" work clothes to avoid donning mercury contaminated clothing. If an outside laundry service is utilized, they should be informed that the clothing is contaminated with metallic mercury. This will allow them to take necessary precautions to protect their work force and to comply with State and Federal environmental regulations. IV. Visitors A policy should be established that requires all visitors to follow the entry and exiting procedures outlined above. Personal protective equipment such as respirators, boots, gloves and coveralls should be made available for use by visitors. Visitors should not be allowed to leave the property with contaminated equipment or clothing. V. Heating, Ventialtion and Air Conditioning System The hygiene facility should have its own heating, ventilation, and air conditioning (HVAC) system. The building should be kept under a slight positive static pressure with respect to the outside work areas (0.5 inches of water). The static pressure within the structure should be greatest inside the clean room and lowest inside the "dirty room. This will cause the air to flow from the clean room through the shower area to the "dirty room. Figure 4 shows the static pressure gradient and the general airflow for the hygiene faculty. The intake air vents for the HVAC system should be located in a mercury vapor-free zone. If this is not feasible, the HVAC unit should be equipped with a mercury vapor removal system. The duct work should be doubled walled so that the duct insulation is not exposed to the intake air stream. It is recommended that the space between the acoustical ceiling tiles and the concrete ceiling not be used as a return airway for the ventilation system. This could lead to contamination of the ceiling tiles. The ventilation system should be well maintained and the air quality entering the facility should be checked for contamination using a mercury vapor analyzer. VI. Maintenance Practices The hygiene facility should be used only for the purpose stated above and not as an equipment storage area. The street clothes lockers should not be used to store hand tools or other equipment. The hygiene facility and its equipment, such as the clothes washer and dryer should be cleaned on a regularly scheduled basis. At a minimum, the facility should be cleaned once a shift. Showers should also be treated to prevent the growth of bacteria and fungi. It is suggested that cloth curtains, rugs, and cloth upholstered furniture not be used in the hygiene facility, since these items can absorb mercury and cannot be satisfactorily cleaned. A mercury vapor analyzer should be used to monitor the effectiveness of clean-up procedures. Sources pinpointed by the instrument should be cleaned with a mercury complexing agent. If linoleum is laid over a concrete floor, a continuous sheet should be used in lieu of 12-inch tiles in order to minimize the number of seams in the floor. Cracks and crevices at corners and joints should be sealed with epoxy, and be flush with the surface. Chemical smoke tubes should be used to indicate the static pressure relationships with respect to the three locations within the hygiene facility. Ensure that the air movement always flows from the clean locker room towards the "dirty locker room. Test the airflow with doors and windows in both the open and closed positions. I. Building Materials
II. Location and Interior Layout
III. Heating, Ventilation and Air Conditioning System
IV. Maintenance Practices
LUNCH ROOM I. General Guidelines The lunch room should consist of a wash room (change area with wash facilities) and eating area. If the lunch room is located adjacent to the process area, an air-lock entrance is recommended to isolate the wash room from the eating area. A sketch of a lunch room is depicted in Figure 4. The HVAC system used to provide contaminant-free air to the hygiene facility can be used to supply tempered air to the lunch room. If the lunch room is going to have its own HVAC system, ensure that the air intake is situated in a mercury vapor-free area. The lunch room should be under a positive static pressure with respect to the outside work areas and slightly negative when compared with the hygiene facility. Employees should wash their rubber boots in a mercury complexing solution, remove their coveralls, and wash their hands prior to entering the eating area. Consideration should be given to having the employees remove their boots and use disposable slippers prior to entering the eating area. The same basic principles should be applied (except the air-lock entrance) to a trailer that is going to be used as a lunch facility. II. Maintenance Practices The lunch room should be used only for this purpose and not as a process control room, office or equipment storage area. This will minimize the amount of mercury transported into this room. At a minimum, the lunch room should be cleaned after each shift. After cleaning, a mercury vapor analyzer should be used to check surface areas for mercury contamination. Wipe samples can also be used to ascertain the effectiveness of clean-up procedures. It is recommended that a sign stressing the importance of good personal hygiene be posted inside the lunch room. For reasons stated above, no curtains, rugs, and cloth upholstered furniture should be used in the lunch room. The static pressure relationships with respect to the eating area and the wash room should be monitored to ensure that the air movement is always toward the wash room. I. General Considerations
CARBON ADSORPTION and REACTIVATION I. Introduction The carbon adsorption and reactivation processes are designed to: (1) produce a concentrated gold-bearing solution from which the precious metals can be recovered using either zinc precipitation or electrowinning and (2) reactivate and recycle the carbon back into the carbon adsorption circuit. The carbon adsorption circuit involves adsorbing the metal cyanide complexes onto activated carbon. The general order of preference of carbon adsorption for gold, silver and mercury cyanide complexes is as follows:41 The neutral mercury cyanide complex competes directly with the gold cyanide complex for adsorption sites on the activated carbon. The loaded activated carbon undergoes an elution step where the metals are desorbed from the carbon. The stripped carbon is sent to a rotary kiln for reactivation. The electrolyte is pumped to the refinery where the precious metals are recovered.
II. Mercury Sources and Control Techniques Carbon Adsorption Circuit The following locations within the carbon adsorption circuit are potential sources of mercury vapor:
The loaded carbon surge bins should be covered and maintained under a negative pressure using local exhaust ventilation. The air from the bins should be exhausted through an activated carbon pack prior to being discharged to the atmosphere. The carbon elution vessel should also be vented through a carbon pack. The sumps should be cleaned on a regular basis and treated with a mercury complexing agent. Carbon Reactivation Circuit The following locations within the carbon reactivation circuit are potential sources of mercury vapor as well as liquid mercury:
Since the elution step is not 100 percent efficient, the stripped carbon will contain residual amounts of mercury and precious metals. Mercury vapor can express itself when the carbon is transferred from the elution vessel to the rotary kiln feed bins. This is due to the high operating temperature of the strip vessel. If a push water system is used to move the stripped carbon to the kiln feed bins, the push water tank and the carbon fines tank can be significant sources of mercury vapor. This is primarily due to having fine carbon, that contains mercury, becoming entrained in the push water system. The kiln feed bins are sources of mercury vapor due to residual mercury contained on the carbon. The kiln is an obvious source of mercury vapor due to the heating of the carbon. The kiln exhaust ventilation system is a point source of mercury. Employees who maintain the ventilation unit can be exposed to high concentrations of mercury. The carbon is usually screened to remove the fines prior to being recycled back to the carbon adsorption circuit. These screens are significant sources of mercury vapor. A filter press is used to dewater the carbon fines that are separated out from the reactivated carbon. After dewatering, air is forced through the filter press to reduce the moisture content of the fines. This may be a source of mercury depending on the mercury content of the fines and the temperature of the compressed air. The tanks and screens connected with the carbon reactivation system should be covered and maintained under a negative pressure using a local exhaust ventilation system. The air exhausted from these pieces of equipment should pass through a carbon pack prior to being discharged to the atmosphere. The rotary kiln should also be equipped with an exhaust ventilation system to capture emissions from the kiln. The airstream should be scrubbed by passing it through a mercury removal system. The design of the exhaust ventilation system for the kiln and maintenance procedures will dictate the severity of employees"s exposures to mercury when operating and maintaining the ventilation unit. Design and maintenance procedures will be discussed later in this document. A mercury vapor analyzer should be used to determine if the filter press blow-down procedure is a significant source of mercury vapor. If so, the air should be exhausted through a mercury scrubbing device. This can consist of bubbling the air into a tank that contains a mercury complexing agent or using a carbon pack impregnated with either sulfur or iodine used in conjunction with a demister. Mercury Sources and Control Techniques
I. Introduction Once the precious metals are stripped from the loaded carbon, the strip solution is pumped to the refinery where precious metals are recovered. When mercury is present in the ore, the following processes are performed sequentially to recover the precious metals:
Each process presents mercury exposure problems to employees who are directly involved with them. The following is a discussion of each process and recommended approaches to controlling mercury vapor emitted during these operations. II. Location and Interior Layout The refinery should be located in a building separate form the carbon adsorption and reactivation processes. If a two-story building is used, mercury distillation equipment should be located on the lower floor. A refinery layout is depicted in Figure 5. It is suggested that all process equipment be aligned along an outside wall. This will simplify the installation of exhaust ventilation systems that will be required to remove gases, vapors, and fumes from the various processes. The process equipment should be laid out so that the process flows from one end of the building to the other. The mercury retort and related equipment should be isolated from the rest of the refinery. A sump system should be installed in the middle of the refinery to allow washing down of the floor and process equipment. It is recommended that a mercury trap be installed just below the drain opening to catch any droplets of mercury. A sketch of a gravity trap is depicted inFigure 6. Drainage from the refinery is funneled into a container. A grate is placed in the bottom of the container that allows the mercury to fall through and protect it from being washed out. The water overflows the bucket and falls to the bottom of the sump. The container should be checked and periodically emptied. This device will help minimize the amount of liquid mercury entering the sump. As always around water and molten metal, extreme care should be taken so there is no chance for a steam explosion. III. Heating, Ventilation and Air Conditioning System The ventilation system that supplies make-up air to the refinery should deliver a minimum of ten percent more air to the interior of the building than is removed by local exhaust ventilation systems. This will ensure that the local exhaust ventilation systems will operate as designed. The supply air plenum and diffusors should be located near the ceiling, along the wall opposite the process equipment. This will cause the air to sweep across the refinery toward the process equipment. The intake air vents should be located away from sources of mercury vapor. Avoid placing the intake air vents where mercury vapor from other processes can be entrained and introduced into the refinery. The fresh make-up air ventilation system should be properly maintained and checked for contamination using a mercury vapor analyzer. Temperature plays a key role in controlling the level of mercury within the refinery. Temperature in the refinery should be kept at or below 65°F. If evaporative coolers are used in addition to the primary HVAC system, they should also be properly maintained and monitored for mercury contamination. IV. Electrowinning Electrowinning is used to recover gold from concentrated solutions generated by the elution of gold from loaded carbon. Generally, precious metals are plated onto either a carbon steel wool cathode or a stainless steel mesh cathode housed in a polypropylene basket. In addition, expanded metal sheets are also used, in lieu of the polypropylene baskets, to sandwich and hold the stainless steel mesh in place. Mercury will be plated onto the cathodes as well as the gold and silver. The elevated temperature of the electrowinning cell solution raises the vapor pressure of the plated mercury. The refiners are exposed to this vapor when they remove the cathodes from the electrowinning cells. It should be noted that there is also potential for dermal contact when performing this task. Mercury Sources and Control Techniques (A) Removing the cathodes from the electrowinning cells Standard electrowinning tanks are equipped with an air plenum that is maintained under a negative pressure by connecting the plenum to an exhaust fan. Its primary purpose is to remove by-product gases, such as ammonia and hydrogen from the tank. An explosion proof fan should be used to exhaust air from the electrowinning cells due to the potential generation of hydrogen gas at the cell"s cathodes if a malfunction was to occur. However, the refiner is exposed to a plume of mercury vapor when the cell is initially opened. This occurs because the exhaust system is ineffective in capturing mercury vapor from the cathode as it is being pulled from the cell. This problem can be addressed by quenching the cathodes inside the electrowinning cell prior to their removal from the cell, which can be accomplished by taking the cell temporarily out of service and draining the hot solution from the tank. Introducing cool water into the tank reduces the temperature of the cathodes inside the cell. This will reduce the vapor pressure of the mercury plated on the cathode, thereby reducing the mercury vapor concentration. Effects of thermal stress on the electrowinning cells due to repeated temperature changes should be considered prior to implementation. Stainless steel electrowinning cells are recommended. When the electrowinning cells cannot be taken out of service, the lid of the cell can be converted into an air plenum and connected to an exhaust fan. Air is drawn through the plenum to the back of the cell away from a person lifting the lid. A sketch of the cell is shown in Figure 7. In addition, the cathode can be placed immediately into a quench tank located next to the electrowinning cell which will reduce the vapor pressure of the mercury plated on the cathodes. (B) Stripping the electrowon sponge from the cathodes When a carbon steel wool cathode is used, the wool must be removed from the polypropylene basket and placed in a mercury retort pan. This is a major source of mercury exposure for refiners and should be done on an epoxy-coated table that is under the influence of local exhaust ventilation. A sketch of a downdraft hood and work bench is shown in Figure 8.42 Any material that has fallen into the clean-out doors should be placed into the retort pan. When stainless steel mesh cathodes are used, they are placed in a wash tank where the plated material is removed with a high pressure water hose. The sludge is dewatered by drawing it through a filter press. The water exiting the filter press is either returned to an electrowinning cell or drained into a sump. This procedure requires the refiner to be in close proximity to the cathode. It is important to cool the cathode prior to moving it to the wash tank. The wash tank should be located adjacent to the electrowinning cells and designed to minimize handling of the cathode by refiners. A wash tank is depicted in Figure 9. (C) Placing the electrowon sponge into the mercury retort It is a common practice to store the sponge in a 5-gallon bucket or other container inside the retort room prior to placing it into the mercury retort. Water is poured into the container to cover the sponge and is intended to act as a barrier to prevent mercury vapor from escaping into the room. Water is not an effective mercury vapor barrier.43 It is the temperature of the water that controls the amount of mercury vapor being emitted from the container. As the water gets warmer, its ability to act as a "vapor barrier decreases. This source of mercury vapor can be eliminated by placing the electrowon sponge directly into the mercury retort. V. Merrill-Crowe zinc precipitation The Merrill-Crowe zinc precipitation gold recovery method involves using elemental zinc to reduce gold to its elemental form. As with electrowinning, mercury is reduced and co-precipitated with the silver and gold. Refiners are exposed to mercury vapor when the filter press leaves are separated and while the precipitate is being scraped into the retort pans, which requires the refiner to work directly with the precipitate. The best approach to controlling exposures to mercury vapor is to cool the precipitate with water prior to pulling the filter presses. It is important to place the precipitate directly into the mercury retort after it is recovered from the filter presses. When directly handling steel wool or zinc precipitate cathodes, refiners are also at risk of dermal exposure to mercury in addition to inhalation exposure. The appropriate personal protective equipment should be worn by employees when handling the sponge or precipitate. VI. Mercury Retort Materials containing significant concentrations of mercury must be treated for its removal prior to smelting. In order to minimize the release of vapor during smelting, the following general work practices should be followed when operating a mercury retort. Retort Room The mercury retort should be isolated from the rest of the refinery by placing it in a separate room. The room should be kept under a negative pressure with respect to the rest of the refinery, which can be accomplished by using either wall fans or local exhaust ventilation. A pressure differential of at least 0.5 inches of water should be maintained between the retort room and the interior of the refinery. The make-up air vents for the room"s exhaust ventilation system should be located on the wall opposite the refinery"s fresh air supply plenum. It is preferable to use local exhaust ventilation to keep the retort room under a negative pressure. The primary reason is that mercury vapor that is drawn to a wall fan will be exhausted directly to the outside of the building. This will contaminate the ground with metallic mercury in the immediate vicinity of the fan"s exhaust. In addition, the potential exists for the vented mercury to be vapor entrained in the make-up air system and introduced back into the refinery. The exhaust ventilation opening should be located near the floor of the retort room due to the density of the mercury vapor. Locating exhaust openings near the ceiling causes a portion of the "captured mercury vapor to condense inside the room before it can be collected by the exhaust system. The ventilation system should be equipped with a mercury vapor removal unit and the retort room should be cleaned daily. The following clean-up procedure is recommended:
Retort The retort should be operated and maintained in accordance with the manufacturer"s recommendations. Tests should be conducted to determine the retort"s optimum operating parameters prior to placing it into service. Once a baseline has been determined, the above tests can be performed periodically to monitor the operating efficiency of the retort. This will minimize the potential of sending mercury contaminated concentrate to the flux mixing station and furnace. The retort should be kept under a vacuum during operation and the air exiting the water condensation system should be scrubbed with a mercury vapor removal system (usually charcoal impregnated with a sulfur compound) prior to being introduced into the vacuum pump. The scrubber system is required because the water condensation unit is not 100 percent efficient. Mercury vapor losses usually vary between 0.2 percent and 0.4 percent for each distillation cycle.44 It is also recommended that the treated air be discharged to the outside of the refinery. The air should be monitored for mercury vapor to determine if the charcoal scrubber unit has become saturated with mercury vapor. This measure should be implemented for two reasons: (1) mercury vapor will not be introduced into the retort room from the condensation unit if the mercury scrubber unit becomes saturated and (2) monitoring will enable the refiners to replace the activated carbon prior to it becoming saturated with mercury vapor. The spent charcoal from the scrubber unit should be disposed by retorting. Records should be kept to determine the life expectancy of the charcoal scrubber. This will also assist the refiners in determining when the charcoal should be replaced. The doors to the mercury retort should only be open while the concentrate is being placed into or removed from the retort. Leaving the doors open needlessly allows residual mercury vapor to enter the room. Condensed mercury vapor is trapped in a condensation pot. It is a common practice to remove the liquid mercury from the pot by opening a ball valve and draining it into a container such as a 5-gallon bucket. The mercury is then poured into a flask. This procedure is messy and contributes to exposures to mercury vapor. The best means of accomplishing this task is to transfer the mercury from the condensation pot directly to the mercury flask by using water. The water used to transfer the elemental mercury to the flask should be filtered using a mercury removal system. An alternate method is depicted in Figure 10. The mercury is transferred directly into a flask under the influence of exhaust ventilation. The chances of over-filling the mercury flasks are reduced by:
Retort equipment and other items connected with the gold recovery process should be cleaned prior to storage. This should be done with a mercury complexing agent. A parts cleaning tank containing a mercury-complexing agent can be used to clean small parts and tools. The equipment should be stored inside the retort room. VII. Smelting The concentrate obtained from the filter presses and electrowinning cells is heated in a furnace in the presence of slag-forming fluxes at temperatures in excess of the melting point of all the components of the charge, typically between 1200°C and 1400°C. This is done for approximately 1.5 hours to ensure complete separation of impurities from the gold and silver. The molten gold and silver form an alloy that is heavier than the slag and sinks to the bottom of the furnace. Once the smelting is completed, the slag is poured off and the precious metal alloy is removed from the furnace. Furnace Charge Preparation Flux is added to the concentrate to remove base metals and other impurities from the precious metal alloy. The mixing of the fluxes and the concentrate should be done under the influence of local exhaust ventilation. The best approach to performing this task is to vacuum the flux materials and concentrate from the retort pans either directly into the furnace or into a hopper that will automatically feed the material into the furnace. This will limit the refiners" exposures to the flux chemicals, silver dust and any residual mercury that may be present after retorting. An alternative approach is to mix the concentrate and the fluxes at a flux mixing table that is maintained under a negative pressure using exhaust ventilation. A flux mixing table is shown in Figure 11. Smelting of the Concentrate Any residual mercury contained in the concentrate will be volatilized in the initial stages of the smelting process. An exhaust hood should be placed over the furnace to capture any mercury vapor and fumes that are generated during the smelting process. There are two types of hood design that can be used to control contaminates generated at the furnace. An annular hood mounted on top of the furnace can be used to capture emission during smelting. A cover that can be hydraulically moved can be placed over the top of the furnace to enhance the performance of the hood. A hatch can be installed in the hood to facilitate loading the concentrate into the furnace. The other design is to construct an enclosure around the furnace. An exhaust enclosure for a gas furnace is depicted in Figure 12. The furnace should be enclosed on three sides. The front of the enclosure should extend down past the furnace pour spout and protrude outward to contain fumes that are forced out of the spout. An opening should be left in the lower portion of the front of the enclosure for combustion air. A similar enclosure can be used for an electric furnace (Figure 12a). The only changes would be to extend the front of the enclosure to within 12 inches of the floor and eliminate the protrusion. Pouring of the dore bars Pouring of dore bars can be a source of mercury vapor. Depending on the silver content of the concentrate, this process can also be a significant source of silver fume. MSHA"s exposure limit for silver fumes and dust is 0.010 mg/m3. The dore bars should be allowed to cool in the molds well below the melting point of silver, which is 962°C. A graphite compound can be used to coat the molds to facilitate removal of the bars after they have cooled. An exhaust hood should be constructed so that silver fumes will be captured during the pour. The molds should be allowed to cool under the hood prior to removing the dore bars from them. Cooling of the slag pots Slag pots should be allowed to cool under an exhaust hood to capture fumes. Chipping and storage of the dore bars Dore bars are quenched in a water bath after they are poured and then chipped to remove slag. This process is accomplished using either a pneumatic chipping gun or automatic descaler which is a source of silver dust. Depending on the number of bars that need to be scaled, an automatic scaler is considered to be the best method for removing slag from dore bars. Descalers should be equipped with local exhaust ventilation. Compressed air has been used to remove moisture and steel pellets from dore bars prior to weighing them. This activity must be done in an exhaust hood. Experimentation should be performed to determine the minimum air pressure required to obtain the desired results. Lowering the air pressure will enhance the performance of the exhaust hood. If a hand-held chipping gun is to be used, the chipping process should be done at a table equipped with local exhaust ventilation. Another method is to chip the dore bar in a container of water. Either of these methods should reduce exposure to silver dust. The dore bars should be checked to determine if they are a source of metallic mercury vapor, which can be done by placing a dore bar inside a plastic trash bag and allowing it to sit for 3-4 hours. Next, use a mercury vapor analyzer to determine if any mercury has off gassed from the dore bar which can be done by inserting a probe through the bag and obtaining a reading with the mercury vapor analyzer. If mercury is present, the bars can be wrapped in plastic to reduce the amount of mercury entering the vault. In addition, the vault should be well ventilated to limit the accumulation of mercury vapor within this location. VIII. Refinery Housekeeping Practices Cleanliness in the refinery cannot be over-emphasized. The following practices will assist in keeping minimum background levels of mercury vapor in the refinery.
I. Location and Interior Layout
II. Heating, Ventilation and Air Conditioning System
III. Electrowinning
IV. Zinc Precipitation
V. Mercury Retort
VI. Smelting
VII. Housekeeping
GENERAL ENGINEERING CONTROLS
The primary engineering controls that can be used to limit mercury vapor within the carbon adsorption and reactivation building and the refinery are:
The following is a brief discussion of each control measure. It should be noted that air monitoring should be performed to check the adequacy of control measures in limiting employees" exposures to mercury and other contaminants. I. Local Exhaust Ventilation Local exhaust ventilation systems are used to control contamination at the source. The following guidelines should be applied to the design of these systems.
II. General Dilution Ventilation General dilution ventilation is used to dilute the contaminants of concern with uncontaminated air when used. Alone, it is not satisfactory because it does not control contaminants at the source of generation. Therefore, it should be used in conjunction with local exhaust ventilation systems to assist in control of exposures to toxic substances. Wall fans are commonly used to provide general ventilation inside the refinery. Wall fans are commonly used to provide general ventilation inside the above mentioned areas. The following parameters should be considered when using wall fans as an engineering control.
III. Temperature Control Temperature control within the refinery and the carbon handling building is an important factor in controlling exposures to mercury vapor. Evaporative coolers and air conditioning units can be used to provide make-up air to the above locations. Duct work should be double walled so that duct insulating material is not exposed to the intake air stream. These cooling devices must be properly maintained and the intake air monitored for the presence of mercury vapor. It is desirable to keep the above locations between 60°F and 65°F. Cooling the cathodes with water prior to stripping them is an important control technique in limiting exposures to mercury vapor. If quench tanks are used to reduce the temperature of the cathodes, they should be cleaned regularly, which can be done by pumping the contents through a filter press. The filtered material can be retorted and smelted to recover the precious metals. IV. Water Filtration with Activated Carbon Sumps and drains are well documented sources of mercury exposures. These sources can be minimized by limiting the amount of liquid mercury and contaminated debris from entering sumps and drains. In addition, filters containing activated carbon can be used for removal of mercury from waste water. This technology can be used to treat effluent from the employee hygiene facility, refinery and carbon adsorption and reactivation building, which control would reduce the amount of mercury being recycled back through the cyanidation process as well as limit the accumulation of mercury in the drains and sumps. I. Local Exhaust Ventilation
II. General Dilution Ventilation
III. Temperature Control
IV. Water Treatment
MAINTENANCE Maintenance operations Maintenance workers are especially at risk of exposure to mercury as well as other contaminants. Extra care should be taken to monitor their work activities to ensure that personal exposures to contaminants are below MSHA limits. Engineering controls must be used to reduce exposure, which often means reviewing the work prior to initation, bringing in temporary controls, and cleaning equipment. Because of the unique physical configurations of many maintenance operations, special care must be given to ensure that the employees are wearing appropriate personal protective equipment. Equipment that has come in contact with mercury can absorb it into the material pores and cracks as well as have surface contamination. Heating, cutting and welding on this equipment will volatilize the mercury causing high airborne concentrations of this contaminant. In addition, maintenance workers may be exposed to other types of contaminants such as cyanide and welding fumes. Equipment maintenance procedures should be developed for working on mercury contaminated equipment. Each protocol should focus on identifying the hazards associated with a particular job and the means of controlling all potential routes of employees" exposures to the contaminants .
PERSONAL PROTECTIVE EQUIPMENT Mercury vapor is odorless and cannot be seen or tasted. Its presence can only be determined through the use of mercury vapor analyzers and spot test kits. The major route of exposure is through inhalation. Secondary routes of mercury exposure are through skin absorption and ingestion. The following is a discussion of the types of personal protective equipment that should be worn by employees who work around elemental mercury. I. Respiratory Protection As a minimum, a respiratory protection program consistent with the American National Standards Institute (ANSI) Z88.2-1969, "Practices for Respiratory Protection should be implemented. The ANSI standard was updated in 1992. NIOSH, in its Guide to Industrial Respiratory Protection document and ANSI, standard Z88.6-1984, ". . . Respiratory Protection - Respirator Use - Physical Qualifications for Personnel provides information on medical evaluations governing respirator use. The guidelines outlined in these documents should be implemented in conjunction with ANSI standard Z88.2-1969. The purpose of this information is to provide guidance and information for examining physicians to assist them in determining the suitability of personnel for respirator use. Only NIOSH certified respirators approved for metallic mercury vapor should be used. Since mercury vapor cannot be detected by the respirator wearer, approved mercury vapor cartridges will have end-of-service indicators that change color to indicate saturation. When the indicator changes color, the cartridges must be replaced. The end-of-service indicator should be checked throughout the entire shift. NIOSH provides the following guidelines for respirator selection given various concentrations of metallic mercury vapor. Respirator users should remember that a particular metallic mercury vapor cartridge will have specific limitations on its use. These limitations take precedence over the table found below when selecting a particular type of respirator. For example, a full-facepiece PAPR that utilizes a mercury vapor cartridge that limits its use to a concentration of 0.500 mg/m3 of mercury vapor would only provide a protection factor of ten in lieu of fifty. The manufacturer should be consulted on the limitations of their mercury vapor cartridges prior to their use. It is important to ensure that employees are cleaning and checking their respirators on a daily basis to ensure that they will function as designed and that are free of contamination. A mercury vapor analyzer can be used to scan the respirators for the presence of mercury. Because mercury has no warning properties, it is essential that the respirator fit be maintained at all times. Employees must be trained to check the fit, and should be fit tested at least annually. NIOSH Respirator Selection Guide for Chemical Cartridge Respirators
II. Skin Protection Employees should be provided with and required to use coveralls, rubber or neoprene gloves and rubber boots to prevent repeated or prolonged skin contact with metallic mercury. These items should be of such material and construction as to minimize the possibility of the clothing acting as a secondary source of mercury vapor. Coveralls It is recommended that outer coveralls without pockets, belts or cuffs be worn to minimize trapping of mercury that can be a continuing source of mercury vapor in the microenvironment surrounding a person. There is some evidence that it may be more difficult to wash mercury out of dyed fabrics than out of white fabrics.46 Coveralls should be changed at least twice a shift. In addition, the coveralls should be changed immediately after an individual has worked with the concentrate or on mercury contaminated equipment. Footwear & Gloves Porous leather boots or gloves should not be worn in areas potentially exposed or contaminated with mercury. Rubber or neoprene boots or gloves need to be cleaned and monitored to ensure that they are free of mercury contamination. III. Personal Hygiene Employees who work around mercury should wash their hands thoroughly with Hgx soap and water before eating, smoking or using the toilet facilities. In addition, eating or smoking should not be permitted in areas where mercury is present. The use of good personal hygiene practices should be reinforced through the use of educational posters and periodic training. I. General Guidelines
II. Respiratory Protection
III. Skin Protection
IV. Personal Hygiene
WORKPLACE MONITORING for ELEMENTAL MERCURY There are several methods that can be used to determine and quantify the presence of elemental mercury both liquid and vapor within the workplace. The type of sampling method selected depends on the objectives of the survey. The following methods will be discussed:
Results of the sampling should be shared with all employees. I. Personal Air Monitoring The purpose of personal air monitoring is to determine an individual"s exposure to airborne metallic mercury vapor. A sampling device is placed on the employee within his/her breathing zone, which usually involves clipping the monitor to the shirt collar. The sampling time is dependent on the objectives of the survey. The sampling results are compared to either predetermined values established by the company or to regulatory standards to ascertain whether corrective action is needed to ensure the health of the employee. MSHA"s full-shift exposure limit is 0.050 mg/m3 and its fifteen minute excursion limit is 0.150 mg/m3. Exceeding either of these limits requires corrective action on the part of the mine operator. There are two sampling methods that can be used to obtain an individual"s exposure to airborne mercury vapor. Passive diffusion monitors can be used to collect full-shift samples. The primary benefit of using this sampling protocol is the ease of use. The other method is active-integrated sampling, which entails using a calibrated sampling pump and a sorbent tube. The pump draws air, at a predetermined flow rate, through a sorbent tube where the contaminant is captured. The sorbent tube is sent to a laboratory to determine the amount of the contaminant collected on the sampling media. The results for both air monitoring methods are presented in either µg/m3 or mg/m3 of mercury. A gold coil mercury dosimeter is also available for personal sampling. This is an active sampling device that uses a low flow pump to draw air past a gold coil. The mercury contained in the airstream is adsorbed onto the gold film. The sample results are obtained by burning the mercury off the gold coil using the Jerome, model 431-x mercury vapor analyzer. The advantage of this method is that the sample results can be determined on-site. II. Area Airborne Monitoring The purpose of obtaining area airborne samples is to determine specific sources of mercury vapor within the plant and to ascertain the effectiveness of clean-up and equipment decontamination procedures. Area samples should not be used as a substitute for personal exposure sampling. This type of monitoring should be used only to approximate employees" personal exposures to mercury vapor or to determine contamination of an area or objects. Area airborne monitoring can be done using the same sampling techniques described above for personal exposure monitoring. In addition, there are two types of hand-held instruments that can be used to obtain instantaneous airborne concentrations of metallic mercury vapor. Bacharach Instrument Company* makes a mercury vapor analyzer that is based on ultra-violet (UV) absorption principle. It has a sensitivity of 0.01 mg/m3 and is accurate within +/- 5 percent. Since the mercury vapor detector depends on the absorption of UV radiation by the sample, it will be affected to some extent by any substances that have a greater absorption of UV light than does normal air. Commonly encountered substances are vapors of various hydrocarbons, water vapor, sulfur compounds, and particulate, such as smoke. The advantage of the UV instrument is that it can be used continuously in relatively high concentrations of mercury vapor (less than 1.0 mg/m3) whereas the gold film instrument requires periodic regeneration of the gold film. The disadvantages of the UV instrument are that the instrument must be zeroed between samples and its sensitivity. Arizona Instruments Inc.* makes a gold film mercury vapor monitor that is based on mercury"s ability to alter the resistance of a gold film. The instrument"s sensitivity is 0.003 mg/m3 and the accuracy is +/- 5 percent at 0.100 mg/m3. Ammonia and acid gases are the principle chemicals that interfere with the operation of the instrument. The instrument is also temperature sensitive. The advantage of the gold film instrument is its sensitivity and its ability to automatically zero itself between each sample.
Wipe samples are used to determine the effectiveness of equipment decontamination procedures and clean-up protocols used for the hygiene facility and lunch room. This type of sampling involves swiping a 100cm2 area with moist filter paper. The wipe sample is placed in a vial and submitted to a laboratory for analysis. The results are presented in either micrograms or milligrams of mercury. These results should be used only to determine the presence or absence of mercury. Also, there are commercially available surface test kits for evaluating surface contamination. They can be used as indicators of the presence or absence of mercury on various surfaces. Scrape samples can be used to determine the presence of mercury on equipment and other surfaces, such as concrete and paint. These samples can be analyzed in-house using the following procedure:
Although not quantitative, this procedure will provide the necessary information to determine if the materials contain mercury. If desired, samples may be sent to a laboratory for quantitative analysis. IV. Environmental Screening Samples The purpose of this type of sampling is to determine the presence of mercury in soil. The sampling and analysis can be performed on-site by using the following approach:
This method identifies only the presence and relative amounts of mercury out-gassing from the soil surface. It should not be used to determine actual concentrations of mercury in the soil. Figure 13 illustrates this sampling procedure. I. General Considerations
BIOLOGICAL MONITORING 48 I. Biological Monitoring Biological monitoring assesses the internal exposure of an individual.49 That is, it measures the amount of an agent actually absorbed into the body. For elemental mercury, the assessment is made by measuring the amount of this constituent in the blood or urine. Measurement of mercury in urine is the recommended biological monitor for workers exposed to metallic mercury in lieu of mercury in blood. The reason is that the latter reflects exposure to organic mercury as well as metallic and inorganic mercury. Thus, blood tests can be influenced by the consumption of fish containing methyl mercury. This is not the case for urine tests. Persons without occupational or unusual environmental exposure to mercury rarely excrete more than 5µg/g creatinine in their urine.50 There are a number of variables that must be considered in the collection and interpretation of the sampling results. For example, excretion of mercury from the body fluctuates considerably, independent of exposure; therefore, individual findings cannot be used to determine the intensity of exposure or the presence or absence of mercury toxicity. The program should be under the direct supervision of a medical doctor who is familiar with the toxicity of elemental mercury. Together, the company and physician should establish criteria on which corrective action will be taken based on an individual"s biological monitoring history. The ACGIH recommends that the level of mercury in the urine not exceed 35µg adjusted for creatinine.51 Accordingly, any employee who was found to have exceeded this level (35 µg hg/Cr) should be removed from the source of mercury contamination until levels return below that level. The purpose of the biological monitoring program should be explained to all the affected employees. The employees should receive their monitoring results as soon as practical. The results should be explained to them so the employees have a complete understanding of what they mean as it relates to their health and well-being. II. Medical Surveillance Medical Surveillance measures the end health effect of an exposure to a toxic substance and offers early detection of a problem. Employees who will be working with mercury should be given an initial medical examination that can be used as a baseline for future health evaluations. The examination should consist of a complete medical history and symptom questionnaire, with emphasis on:
Signs of early mercury intoxication should be elicited and a baseline handwriting sample collected from the employee.52 The examinations should be repeated on an annual basis. The results of the examinations should be explained to the employees and remain confidential between them and their physician.
Mercury continues to represent a serious health risk to miners, which requires constant vigilance to control. As discussed in this "Best Practices toolbox there are ways to control the hazards. MSHA is sharing this information with miners, mine operators and health and safety professionals to educate and inform the minining community about the protections that are available to guard against the hazard of exposure to mercury. MSHA encourages comments on this "Best Practices toolbox. |