Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 31 of 707

Buffing processes propel dust particles, cotton lint, abrasive dust, and metallic dust into the air. Microcrystalline silica, or tripoli, which is used in buffing com- pounds, is a good example of such dust. According to OSHA permissible exposure limits, exposure to airborne crystalline silica shall not exceed an 8-hour time- weighted average limit as stated in 29 CFR Part 1910 1000 Table Z-3 for Mineral Dusts, specifically "Silica: Crystalline: Quartz (respirable)." The threshold limit val- ue and biological exposure indices for the 1987-1988 American Conference of Governmental Industrial Hygienists is 0.1 mg/m3 (respirable dust). Excessive inhalation of dust may result in respiratory disease including silicosis, pneumoconiosis, and pulmonary fibrosis. The International Agency for Research on Cancer (IARC) has evaluated Monographs on the Evaluation of the Carcinogenicity Risk of Chemicals to Humans, Silica and Some Silicates (1987, Volume 42), that there is "sufficient evidence for carcinogenicity of crys- talline silica to experimental animals" and "limited evidence" with respect to humans. A conventional particulate respiratory protector is required based on consider- ations of airborne concentrations and duration of exposure. Refer to the most recent standards of the American National Standard Institute (ANSI Z.88.2), the Occupational Safety and Health Administration (OSHA) (29 CFR Part 1910 134), and the Mine Safety and Health Administration (MSHA) (30 CFR Part 56). The use of adequate ventilation and dust collection is also required. Grinding, polishing, or buffing operations that generate airborne contami- nants in excess of exposure limits into the breathing zones of employees should be hooded and exhausted as necessary to maintain legal exposure limits. A hood used for the control of contaminants from a grinding, polishing, or buffing oper- ation should be connected to an exhaust system that draws air through the hood to capture air contaminated by the operation and to convey the contaminated air through the exhaust system. Where large quantities of exhaust air cause negative pressures that reduce the effectiveness of process exhaust systems or cause a carbon monoxide hazard due to back-drafting of flues of heating devices, provisions shall be made to supply clean make-up air to replace the exhausted air. The make-up air supply, where necessary, should be adequate to provide for the combined exhaust flows of all exhaust ven- tilation systems, process systems, and combustion processes in the workplace without restricting the performance of any hood, system, or flue. Dust collection equipment is available in numerous designs utilizing a number of principles and featuring wide variation in effectiveness, first cost, operating and maintenance costs, space, arrangement, and materials of construction. Consultation with the equipment manufacturer is the recommended procedure in selecting a col- lector for any problem where extensive previous plant experience on the specific dust problem is not available. Factors influencing equipment selection include: 1. Concentration and particle size of contaminant 2. Degree of collection required 3. Characteristics of air or gas stream 4. Characteristics of contaminant 5. Method of disposal under Federal, State, and Local Regulations. There are many other aspects of buffing and polishing than these briefly discussed here. Though this very important contributor to the metal-finishing industry is more of an art than a science, basic engineering principles can be applied to this opera- tion. With the proper melding of buff and compound, applied in a controlled fashion, optimum finish and maximum economy can be achieved. For questions or comments, contact the author at 30

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