Engineering Controls Database

Best Practices for Dust Control in Metal/Nonmetal Mining – Mineral Processing Operations – Total Mill Ventilation Systems

Problem:	Respirable crystalline silica dust exposure has long been known to be a serious health threat to workers in many industries and occupations. Workers with high exposure to crystalline silica include miners, sandblasters, tunnel workers, silica millers, quarry workers, foundry workers, and ceramics and glass workers Overexposure to respirable crystalline silica dust can has been associated with development of silicosis, lung cancer, pulmonary tuberculosis, and airways disease. The International Agency for Research on Cancer (IARC) reviewed the published experimental and epidemiologic studies of cancer in animals and workers exposed to respirable crystalline silica and concluded that there was sufficient evidence to classify silica as a human carcinogen [IARC 1997]. Silicosis is also a fibrosing disease of the lungs caused by the inhalation, retention, and pulmonary reaction to the crystalline silica. When silicosis becomes symptomatic, the primary symptom is usually dyspnea (difficult or labored breathing and/or shortness of breath), first noted with activity or exercise and later, as the functional reserve of the lung is also lost, at rest. Once contracted, there is no cure for silicosis. The goal, therefore, is to limit worker exposure to respirable dust to prevent development of these diseases.
What this means to you:	Silica refers to the chemical compound silicon dioxide (SiO2), which occurs in a crystalline or noncrystalline (amorphous) form [NIOSH 2002]. Silica is a common component of rocks; and; throughout the mineral processing cycle, mined ore goes through a number of crushing, grinding, cleaning, drying, and product-sizing sequences as it is processed into a marketable commodity. Because these operations are highly mechanized, they are able to process high tonnages of ore. This in turn can generate large quantities of dust, often containing elevated levels of respirable crystalline silica, which can be liberated into the work environment.
Description of control:	Although LEV is the most common dust control technique used at mineral processing operations to capture and filter dust from the major dust sources, it is not possible to capture and control all the minor dust sources within a mineral processing operation using this same technique. As these minor sources continually generate and liberate dust over a work shift, they can have a cumulative effect and cause respirable dust concentrations to gradually increase to unacceptable levels. The best way to address and deal with these minor dust sources is to install a total mill ventilation system (TMVS). A TMVS provides a general purging of the plant air to minimize dust throughout the entire mill building, thus lowering respirable dust levels for all workers within the structure [Cecala and Mucha 1991; Cecala et al. 1995]. The TMVS lowers respirable dust levels by using clean outside air to sweep up through a building to clear and remove the dust-laden air. This upward airflow is achieved by placing exhaust fans at, or near, the top of the structure. The size and number of exhaust fans is based upon the initial respirable dust concentration and the total volume of the structure. All the processing equipment within a mill generates heat and produces a thermodynamic chimney effect that works in conjunction with the TMVS. To be effective, a TMVS must meet three design criteria: • Supply of clean make-up air. The TMVS must supply clean make-up air at the plant’s base. Any outside dust sources, such as bulk loading near an air inlet location, can cause outside dust-laden air to be drawn into the plant and make the problem worse. It is critical to ensure that the make-up air is clean by controlling the air’s entry location through inlets such as wall louvers, plants doors, or other openings. • Effective upward airflow pattern. The TMVS should provide an effective upward airflow pattern that ventilates the entire plant and also sweeps through dust sources, work areas, and dust-laden areas. To create the most effective airflow pattern for purging the entire mill, the exhaust fans must be properly located on the roof or high outer walls, and the make-up air inlets must be located at the base of the structure. • Competent mill shell. Because a TMVS uses exhaust fans to draw make-up air through the points of least resistance, the plant’s outer shell needs to be intact and competent. Any unwanted openings, especially near the exhaust fans, can short-circuit the ventilation system’s designed airflow pattern and reduce the effectiveness. Figure 1 shows the concept of the TMVS from an outside perspective. A normal range of airflow for a TMVS would be 10–35 air changes per hour (ACPH). During the development of this technique, two different field studies were performed to document the effectiveness of a TMVS [USBM 1993]. In the first study, with a 10-ACPH ventilation system, a 40% reduction in respirable dust concentration was achieved throughout the entire mill building. In a second field evaluation, two ventilation volumes were tested—17 ACPH and 34 ACPH—and average respirable dust reductions were recorded at 47% and 74%, respectively. The TMVS has proven to be a cost-effective system to lower respirable dust concentrations throughout an entire mineral processing structure. Not only is the initial cost of this technique inexpensive when compared to the other engineering controls, the operation and maintenance are also minimal. To further reduce their costs, operations can potentially install all the components for this system with in-house personnel. Therefore, the TMVS can be a very cost-effective system to lower respirable dust levels at mineral processing operations [Cecala 1998; Cecala and Thimons 1997; Cecala, et al. 1996]. NOTE: The above information is taken directly from the following publication: NIOSH [2010]. Information circular 9517. Best practices for dust control in metal/nonmetal mining. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-132.
References:	Cecala AB [1998]. Supplementing your dust control equipment with whole-plant ventilation. Powder Bulk Eng 12(1):19–32. Cecala AB, Thimons ED [1997]. Methods to lower dust exposures at mineral processing operations. In: Proceedings of the National Stone Association Meeting—a compliance for the 21st century. St. Louis, MO: National Stone Association, October, pp. 231–249. Cecala AB, Daniel JH, Thimons ED [1996]. Methods to lower dust exposure at mineral processing operations. Appl Occup Environ Hyg J 11(7):854–859. Cecala AB, Mucha R [1991]. General ventilation reduces mill dust concentrations. Pit Quarry 84(1):48–53. Cecala AB, Klinowski GW, Thimons Ed [1995]. Reducing respirable dust concentrations at mineral processing facilities using total mill ventilation system. Min Eng 47(6):575–576. IARC [1997]. IARC monographs on the evaluation of carcinogenic risks to humans: silica, some silicates, coal dust and para-aramid fibrils. Vol 68. Lyon, France: World Health Organization, International Agency for Research on Cancer. NIOSH [2002]. NIOSH hazard review: health effects of occupational exposure to respirable crystalline silica. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2002-129. USBM [1993]. Reducing respirable dust concentrations at mineral processing facility using total mill ventilation system. By Cecala AB, Klinowski GW, Thimons ED. Washington, DC: U.S. Department of the Interior, U.S. Bureau of Mines, RI 9469.
Keywords:	dust control metal/nonmetal mining mineral mining mineral processing miners ventilation
How well does this control work?	Figure 1 shows the concept of the TMVS from an outside perspective. A normal range of airflow for a TMVS would be 10–35 air changes per hour (ACPH). During the development of this technique, two different field studies were performed to document the effectiveness of a TMVS [USBM 1993]. In the first study, with a 10-ACPH ventilation system, a 40% reduction in respirable dust concentration was achieved throughout the entire mill building. In a second field evaluation, two ventilation volumes were tested—17 ACPH and 34 ACPH—and average respirable dust reductions were recorded at 47% and 74%, respectively. [img=1]