Expert from the NIOSH publication that provides recommended guidelines for the health and safety of foundry workers.
IV. Engineering Controls
The foundry environment may be a potential source of numerous toxic air contaminants such as silica, CO, and thermal decomposition products; physical hazards such as noise, heat, and vibration; and safety hazards, including contact with molten metal. The short- and long-term health and safety effects fo the potential hazards, in general, and illustrated how they may affect foundry workers are reviewed in Chapter III. To reduce worker exposure, foundry hazards must be adequately identified and evaluated, and engineering controls, administrative controls, work practices, and, when appropriate, personal protective clothing and equipment should be applied. Ventilation, enclosures, barriers, and substitution of less toxic materials and hazardous processes can be utilized to help control safety and health hazards in different foundry operations.
To improve working conditions in foundries, proper consideration should be given to controlling dust and fumes, especially silica dust, by engineering methods. A plant that is well-designed from environmental and production standpoints will have a substantially reduced need for dust control. However, when a plant design is not adequate to eliminate the dust and fume hazards, retrofit control procedures must be introduced.
A. Preparation of Mold Materials
The preparation of mold materials involves recovering sand and other materials from the shakeout and adding new binder materials and sand for mold production. The addition and recovery of sand and binders are major contributors to the crystalline silica and other dust hazards in the foundry air. In addition to crystalline silica, other hazards may result during mold material preparation. For example, hot green sand may produce steam when passing through the sand preparation system, or smoke may result from high sand temperatures and the presence of organic core binding materials.
Data from NIOSH Health Hazard Evaluations (HHEs) confirm that crystalline silica is a health hazard in sand preparation areas of ferrous and non-ferrous foundries. In a 1974 NIOSH HHE of semiautomated foundry, concentrations of respirable free crystalline silica dust in 14 of 17 personal samples taken exceeded the NIOSH recommended 10-hour TWA of 50 µg/m³. The major sources of atmospheric contamination in the sand preparation area were leakage of dust from containing bins, inadequate containment of hot sand at shakeout operations, inadequate exhaust ventilation, and sand spillage at transfer points.
In a brass foundry surveyed by NIOSH in 1975, potentially toxic respirable crystalline silica dust concentrations were found in all the sampled areas. Utility workers assigned to sand pile and sand spillage cleanup, in areas where ventilation was minimal, were exposed to silica in concentrations of 0.07 to 1.05 µg/m³ during a 6-7 hour sampling time. Improving control of conveyor and muller leakage and enclosing and mechanizing the transfer of materials from the conveyor pit would reduce the environmental crystalline silica concentrations.
In a steel foundry surveyed by NIOSH, the molding sand (72% crystalline silica) was prepared in a muller loaded by a mechanical bucket lift but filled manually. After mixing, the sand was delivered to each work location by wheelbarrow. Used sand was recycled by processing the shakeout wastes through a riddle, which removed slag and solid wastes, and then the reusable sand was shot 10-20 feet (3-6 meters) through the air into a storage bin. Personal respirable crystalline silica exposure concentrations for mullers and laborers during an 8-hour work shift in the sand preparation area ranged from 0.10 to 0.82 µg/m³, exceeding the NIOSH recommended TWA of 50 µg/m³.
In sand reclamation systems, the sand is usually dry from the knockout or shakeout process to the point at which binders and other materials are added. To eliminate dust in the greensand systems, this dry part of the cycle must be controlled as much as possible.
The basic foundry principle, that the temperature of a foundry sand system varies with the sand-to-metal ratio of the molding operation, was applied in developing the Schumacher process. At normal molding ratios of 3 to 7 sand: 1 metal, the sand forming the mold becomes hot when the molten metal is poured into the mold cavity; therefore, a higher sand-to-metal ratio will result in a cooler sand temperature resulting in less dust. Management generally prefers a low sand-to-metal ratio because it permits more castings per mold; but the hot dry sand produces more dust during shakeout and subsequent sand-handling operations than do the low sand-metal ratios.
The Schumacher system may solve the problems of hot sand and resultant high dust exposure while still allowing high metal loading without sacrificing a low sand ratio in the sand system. Moist sand from the mixer is diverted into two streams: about one-fourth of the total amount is transported to molding operations and the remaining three-fourths bypasses the molding operation and rejoins the used molding sand at the casting shakeout. The mass of cool, moist sand that bypasses the molding and pouring operations cools the molding sand. Thus, a foundry can pour a high number of casting in each mold with little regard for the heat build-up in the low sand-to-metal ratio molds. The mixture of used sand and cool-damp sand, which was added at the shakeout, quenches dust and heat. Foundry sand that contains more than about 2% moisture evenly distributed is unlikely to be a significant source of dust.