The Pollution Prevention Act (PPA, 1990) set a priority for reducing the amount of manufacturing waste through "source reduction"—preventing the generation of waste on the factory floor. The second-best option is to recycle wastes for other uses. The next option in the priority list is to recover the energy content of any wastes that are generated. The last resort is to treat the wastestream. This solid waste management hierarchy also applies to foundry solid waste. In this section, following waste characterization and preceding waste reuse, source reduction regarding solid waste generation and pollution emission is addressed. Effective measures include chemical substitution, in-plant reclamation, waste segregation, and process modifications to reduce emission.
Regulated chemicals are of particular concern to foundrymen, waste recyclers, and decision-makers. An increasingly applied measure is to substitute or at least minimize the use of these chemicals in the plant, and basically eliminate the source of the environmental threat. For instance, all resin manufacturers are reducing the free-phenol content of their products to mitigate the discharge of phenol. Water-based refractory coatings are replacing solvent-based products, leading to casting improvements, such as a reduction in the number of scrap pieces and improved cycle times, and most importantly, a lighter environmental impact.32
Targeted chemicals can also be heavy metals. Nonleaded brass castings (also described as very-low-lead alloys, because no lead is intentionally added to them) are an important new approach to reducing lead in drinking water. A variety of other approaches to meeting lead release requirements have been tried or are currently being used, including the use of organic and inorganic coatings, the chemical removal of interior surface lead, and the reduction of internal surface areas of devices by implementing design changes. Most of the lead-free alloys contain bismuth as the major alloying element. Bismuth replaces lead in copper alloys and contributes to the machinability and pressure tightness of the alloys. Bismuth, like lead, is almost completely insoluble in copper and has a low melting point. It is not known to be toxic to humans and is used as a chemical compound in a popular remedy for upset stomachs.
Chemical substitution or minimization may bring great benefits through a managed scrap charge process. Metal scraps are carefully charged, screening out heavy metal-rich scraps and avoiding a mixed metal scrap charge. Scraps containing toxic polymer materials shall be treated before being charged.
In-plant reclamation refers to the sand reclamation process in a foundry facility, which directly minimizes the generation of spent foundry sand. Sand reclamation includes physical, chemical, or thermal treatment of foundry sands so they may be safely substituted for new sand in molding and core-making mixes.
Mechanical attrition is used to remove most of the spent binder. First, dry attrition or abrasion processes crush lumps to grain size. Mechanical abrasion is then used to separate the binder from the sand grains. Sometimes, sand is pneumatically propelled against a metal target plate. The impact of the sand on the plate scrubs off the clay and resin coating from the sand grains. Fines are separated and removed by dry classification.
Depending on the binder system used, 60 to 80% of the mechanically reclaimed sand can be reconditioned satisfactorily for molding, with the addition of clean sand. The remaining 20 to 40% of the mechanically treated sand may then be thermally treated to remove the residual organic binder, restoring the sand to a clean condition. Mechanical attrition has the lowest cost. It allows lump breaking, removes and segregate metal scraps, mechanically scrubs as much binder as possible while avoiding breakage of grains, and removes dust, fines and binder residue by air classification.
In some cases, particularly for resin bonded sand, thermal treatment is used to burn the resin binder and carbonaceous residues. Thermal treatments are usually gas heated, but electric or oil heating can also be used. Sand is heated to approximately 500 to 800°C (930 to 1475°F), at which temperature sand bonded with an entirely organic binder system can be reclaimed up to 100%. The sand is then cooled and crushed to grain size by mechanical scrubbing. Binder systems containing inorganic chemicals, for example, silicate-based systems, cement systems, and phosphoric acid systems, are difficult to reclaim at high percentages because no burnout of the inorganic material occurs. Thermal reclamation is costly because of the large amount of heat and relatively expensive equipment needed. The ensured reclamation quality (sand thermal stabilization and clean up) and the need to remove resin residue, however, has led to its increasing use.
Wet reclamation, although being phased out in the U.S., has been used for silicate bonded sand. After the sand is crushed to grain size, water scrubbing using mechanical agitation is used to wash off the silicate residues, then dried. This process requires a large amount of water and also the treatment and clarification of the water before its recirculation and disposal. In addition, the capital cost of the equipment is high and it requires a large amount of floor space.
Whatever method of reclamation is used, there is always some loss of sand so that 100% reclamation can never be achieved. Sand losses include burn-on, spillage, and inefficiency in the sand system and the need to remove fines. Total sand losses of up to 10% may be expected.
Waste segregation helps separate hazardous materials from nonhazardous materials, divide recyclable materials from nonrecyclable materials, make materials largely "pure" and then consistent in physical and chemical property, and leads to managed waste disposal. There are up to 40 wastes-treams covering spent sand, slag, and dust, inclusive of spent molding sand, core sand waste, cupola slag, scrubber sludge, baghouse dusts, shotblast fines, buffing wastes, and others.32 In a facility, workers tend to group several wastestreams and discard them as a composite.12 As a result, complex properties with wide variation are identified, either rendering an assessment that the materials are hazardous, although only a minimum stream deserves the classification, or impeding the recycling program by worsening the materials' consistency.
Besides the segregation of generated wastestreams, in-plant reclamation also considers material division. In typical foundry processes, sand from collapsed molds or cores are subjected to reclamation. However, it is well known that reclamation of sand is easiest when only one type of chemical binder is used. If more than one binder is used, care must be taken to ensure that the binder systems are compatible. Shaken-out green sand and chemical bonded sand are better separated from each other to ensure their rebonding and casting quality. Waste segregation, such as separating fresh casting mixtures and core sand that have not been in contact with hot metal from the other waste-streams, also mitigates the organic compounds identified in the wastestreams.
Toxins, such as benzene, naphthalene, phenol, toluene, xylene, formaldehyde, and mercury, are found in resins and scraps and released as a result of evaporation and solvent processes and during combustion. Respiration of these emissions affects the brain and central nervous system, causes irritation to the skin, eyes, nose, and throat, breathing difficulties, lung problems, impaired memory, stomach discomfort, liver, and kidney changes. Clean-air regulations as well as workplace safety and health standards, however, have forced operators to address the issue. To meet the challenge of providing environmentally friendly core binders and melting processes, a number of suppliers have introduced technologies that are a promising step towards a new generation of binders that both reduce the amount of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) released and yet do not compromise casting quality.
There is a large amount of development work going on worldwide to improve the performance of core binders and to make them more environmentally friendly. The inorganic binder system is a green and relatively environmentally safe bonding process. For example, for silicate-based processes, there is very little in the way of fumes or smells during core manufacture, storage, or casting. A recently improved resin/CO2 process involves a water-based alkaline phenolic resin. Both the binder and sand are gas-cured with CO2 to activate the coupling agent, with low emission. Protein-based foundry sand binders are an entirely new class of sand core binders. They are made from high strength collagens with an additive to promote rapid thermal breakdown of the binder coating. This binder essentially has zero odor, contains no hazardous chemicals, and offers excellent sand reclamation. More improvements come from renewed sodium silicate, modifications of the PUCB and alkaline phenolic resins, and the introduction of new binders. The ultimate industry goal is to develop bonding systems that provide an equally good casting surface, improve the shakeout behavior, and eliminate the use of noxious gaseous catalysts and scrubbers.
The dry ice blaster is an effective and mess-free method for in-place cleaning that eliminates the need to disassemble machinery before it is cleaned. Compressed air propels tiny dry ice pellets at supersonic speeds so they flash freeze and then lift grime, paint, rust, mold, and other contaminants from metal surfaces. Pellets vaporize quickly into the air, leaving no wastewater or solvents, only the soiled contaminant to be swept up.
Special baghouse filters are designed for high-efficiency filtration with a unique three-layer construction. The dust filtration is effective for a wide range of particle sizes. The layered design includes a polypropylene prefilter layer, a melt-blown polypropylene microfiber final filter layer, and a polypropylene outer migration barrier layer, resulting in a cost-effective filter bag.
The volume of sand carried over and adhering to the castings is the biggest factor relating to shot blast costs. By stopping the sand from going into the casting cleaning department, and keeping it in the sand system where it belongs, the benefits go right to the bottom line. In addition to saving on all the shot blasting costs, other savings include less wear on the dust collectors, reduced waste-streams, no airborne silica dust, and reduced cleanup time.
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