Dust Control Handbook for Industrial Minerals Mining and Processing

Work Practices to Minimize Dust Exposure from CrushersThe following practices can serve to reduce operator exposure to airborne dust at crusheroperations.Maintain closure/locking devices such as clamps and other fasteners. Fasteners are onlyeffective when used.Where compatible with the process, wash down areas on a periodic basis. Periodicwashing prevents the accumulation of material which can eventually become too large forwash down, necessitating a dry cleanup problem with the generation of dust.Keep makeup air and recirculating air dust filtration systems on operator booths, frontendloaders or other mobile equipment in place. Ensure that only correct filters areutilized, and maintain the systems to manufacturer specifications.Wear approved respiratory protection when working in dust collectors, mills, airclassifiers, screens, and crushers.Where feasible, automate the crushing process using sensing devices and/or videocameras. This removes the operator from the crusher area and reduces the potential fordust exposure.Where crusher operation must be supervised continuously, provide an enclosed boothwith a positive-pressure, filtered air supply for the operator. See Chapter 9—OperatorBooths, Control Rooms, and Enclosed Cabs for more information.MILLINGMilling, often called grinding, is a process by which granular minerals are reduced in size bycompression, abrasion, and impaction. Mills may be classified into two broad types based onhow they operate: tumbling mills (Figure 4.12) and stirring mills (Figure 4.13). Tumbling millsgenerally operate in a horizontal orientation, and the shell of the mill rotates to impart motion tothe contents or charge. Stirring mills may be horizontal or vertical and motion is imparted to thecharge by an internal stirring element.Tumbling mills employ some type of medium to perform the size reduction, often rods or balls,usually manufactured from iron or a steel alloy, or high density ceramic material when metalcontamination of the product is a concern. Autogenous mills use the feed stock (incoming ore)itself as the medium, and semi-autogenous mills use a combination of the feed stock and balls.Stirring mills reduce the size of the feed stock between fixed and rotating mill components.Materials discharged from mills can range in size from less than 40 to 300 micrometers (µm) fortumbling mills and from 40 to less than 15 µm for stirring mills [Wills 2008].Although mineral processing milling is often done wet, the industrial minerals industry primarilyutilizes dry milling. At some operations, the grinding mill is fed with coarse rejects or one of theproducts from their dry screening operation. These milling operations are fed dried granularminerals, usually less than 12 mesh (1.7 millimeters), and produce products in the 50-micrometer (μm) range, but some fine-grind products are less than 10 μm in size, which meansthat all of the fine-grind product is in the respirable size range.122 Crushing, Milling, and Screening