7
Metal Casting Industry
The U.S. metal casting industry ships approximately 13 million tons of castings annually, valued at more than $23 billion (OIT, 1997c). Ferrous metal castings account for 85 percent of the tonnage (valued at $12.2 billion), and nonferrous metal castings, primarily of aluminum, account for the remaining 15 percent (valued at $7.8 billion) (American Metal Casting Consortium, 1995). The metal casting industry, which employs approximately 210,000 workers, is dominated by small businesses. Of the 3,100 metal casting establishments in the United States, 80 percent employ fewer than 100 workers (OIT, 1997c). The metal casting industry uses 0.25 quadrillion Btus of energy per year.
Separation Needs
Mould Production
The use of sands to make moulds and cores is nearly universal in the metal casting industry. A pattern (often made of wood) is used to make the mould, which establishes the external shape of the metal casting. To make a sand mould, clean sand is mixed with small amounts of hydrophilic clay, usually bentonite, and pulverized coal. Water is added to facilitate handling, and the mixture is placed in two boxes called the "cope" and the "drag." The pattern is then used to transfer the desired external shape of the metal casting in the packed sand in the cope (top of the casting) and the drag (bottom of the casting). The two boxes with the pattern impression constitute the mould.
The core is the interior piece of the mould needed to make hollow castings. In the core room of a metal casting facility, new sand is mixed with resin and sometimes a catalyst and formed into the core. The core is then placed in the mould, and the two mould boxes are assembled into a ''flask'' ready for casting.
In the mould production process for investment casting, a wax pattern is repeatedly dipped into ceramic slurry and coated with dry ceramic grit until the mould is of adequate thickness. The mould is then dried via water evaporation before it can be used. Drying is the rate-limiting step for this kind of mould production, and the metal casting industry would benefit from a separation technology that reduced drying time in investment casting mould production (Carter, 1997).
Casting Production
During casting, molten metal is poured into the mould. The flask is then set aside to cool. In the shake-out house, when the metal casting is solid but still hot, it is removed from the mould. After further cooling, the casting is transferred to the cleaning room where remnants of the core are removed. The metal casting that comes out of the mould includes metal that was cast into the "gates" (openings through which molten metal reaches the mould and through which gases are vented) and "risers" (reservoirs from which metal flows into the casting as it cools and contracts). The gates and risers are excess metal and comprise between 5 and 50 percent of the casting. They must be removed either mechanically or by hand.
During the mould production and casting production operations, dust is created. Most metal casting operations have dust control facilities to maintain high-quality ambient air, but disposal of the captured dust is a problem. Better dust removal technologies would benefit the industry as a whole.
Removal of Impurities from Molten Metal
Both ferrous and nonferrous metals are melted in furnaces prior to casting. Removal of impurities from the molten metal is an important separation issue for the metal casting industry. Technologies currently used include the addition of deoxidants, usually silicon or aluminum, and the addition of magnesium to certain alloys for the removal of oxygen and sulfur. Improved, cost-effective technologies to remove impurities from molten metal would be of great interest to the metal casting industry.
Gas inclusions, caused by gases given off during casting, are often found in the gates and risers removed from mould castings. These impurities increase the difficulty of recycling foundry-generated scrap. Methods for preventing or minimizing gas inclusions in castings would be beneficial to the industry.
Upgrading Scrap
Separation issues for the metal casting industry include the handling of foundry and nonfoundry scrap. Gates and risers removed from metal castings can sometimes be cleaned and returned directly to the furnace, but gas inclusions often limit their recyclability. Specific impurity levels determine which pieces can be reused directly and which pieces must be diverted to other products or re-alloyed and refined. Methods for efficiently and cost effectively separating gate and riser scrap by impurity level would be beneficial to the industry.
With few exceptions, foundries must have a reasonable idea of the composition of scrap in order to use it. Limitations are imposed by the scrap required quality and the cost of scrap separation. Each foundry would prefer to use only scrap produced in that foundry, but this is not generally possible.
Scrap separation is accomplished in a number of ways. The best and most cost-effective method is segregation of different scrap types by the scrap generator. With this method, different types of scrap are never mixed together in the first place, and foundries have clean scrap of known composition to reuse. Some facilities, such as large iron foundries and smaller specialty foundries, have on-site scrap separation facilities. Others, such as smaller metal casting operations that compete in more generalized markets, do not have the financial resources to operate their own separation facilities. These operations depend on intermediary scrap processors who separate and segregate scrap for resale. Separation processes used at these facilities include hand sorting, identification by visual or chemical means, magnetic separation, and eddy current separation. Currently, only the most valuable metals are separated (Carter, 1997). More economical and efficient means of separating nonfoundry scrap would increase the industry's ability to recycle scrap.
Sand Reclamation
Sand is the material most commonly used for making moulds in the metal casting industry. More than 10 million tons of sand are used for this purpose annually in the United States. The reclamation of spent sand is a function of the type of sand, its relative value, the additives mixed in with the sand, and the process it has undergone. The most widely used sand is silica, and high-silica sand is easy to reclaim. "Lake sand" from the Great Lakes region can be cheap, if locally available, but it contains carbonates and other minerals that make it more difficult to reclaim. High-value sands, such as zircon and chromite, are almost always reclaimed.
The core room of a metal casting facility is the source of about 20 percent of spent sand, which originates from rejected cores and spillage. This sand poses a serious separation problem because it is often contaminated with resins and other chemicals. Some core room sand is currently reclaimed using dry attrition scrubbing with dust removal and thermal treatment to burn or volatilize the organic
binders. Wet scrubbing, although more efficient, is used less frequently because of the difficulty and cost of drying the sand and dewatering the scrubber sludge prior to disposal. Improved technologies for drying sand and dewatering sludge would increase the metal casting industry's ability to reclaim core room sand (Wood, 1997).
The shake-out house, where castings are removed from moulds, is the source of approximately 50 percent of spent sand from metal casting operations. This spent sand is a mixture of clay-bonded mould sand and resin-bonded core sand. During the casting process, most mould sand is dried but otherwise remains unchanged. The thin layer of clay in contact with the molten metal, however, is calcined from the heat. Spent mould sand that has undergone casting therefore contains calcined clay that must be removed before reuse. Like core room sand, spent mould sand can be reclaimed using dry attrition scrubbing with dust removal. Wet scrubbing is used less frequently because of drying and dewatering problems.
Core sands are typically mixed with resins that act as binders to ensure that the shape of the core is maintained during casting despite pressure from the molten metal. Many different binders are used, and these are generally oxidized or volatilized by the heat of the casting process. If the binders are not destroyed during casting, they leave behind adherent residues that can complicate core sand reclamation. Better methods for reclaiming spent sand from the shake-out house would be beneficial to the industry.
Most sand that is reclaimed originates in the shake-out house. Shake-out sand is first subjected to lump-breaking. Metals and coarse pieces are removed using magnets and screens. Some of this sand is reused directly. Because spent mould sand cannot be used to make cores, however, reclaimed sand from the shake-out house combined with sand from the core room gradually results in a build-up of excess spent sand. Some spent sand, whether contaminated or not, must therefore be discarded to a landfill.
The remaining 30 percent of spent sand from the metal casting industry originates in the cleaning room. Currently, reclamation of this sand is not attempted. Sand is removed from metal castings primarily by shot blasting. During this process, metal flashing is removed along with sand, and some metal is smeared onto sand surfaces. In addition, metal may be abraded from the casting and become mixed in with the spent sand. Finally, sand grains may be broken into finer particles, sometimes changing the size distribution of the sand from that desired. Available sand reclamation technologies still leave too much contaminated material, which must be discarded. Cost-effective and efficient methods for reclaiming cleaning room sand would, therefore, be beneficial to the industry.
Gaseous and Aqueous Emissions
The metal casting industry faces problems of gaseous and aqueous emissions similar to those faced by the aluminum and steel industries, but on a lesser scale.
Because most foundries are small businesses, the volume of emissions is lower. However, the industry needs improved methods for removing specific components from melting and refining process emissions, for separating fine particulates from gases and liquids, and for dewatering sludge.
Separation Technologies
Sand Reclamation
Better characterization of the surface chemistry of sand and additives would improve the industry's ability to reclaim sand. Methods for low-temperature oxidation or separation of binders would improve the industry's ability to reclaim core room sand that has not been through the casting process. Methods of curing or solidifying organic binders would also be beneficial.
Gaseous and Aqueous Emissions
As for the aluminum and steel industries, inorganic membrane modules composed of cordierite honeycomb monolith coated with a microfiltration membrane and vanadium-based catalysts could be used for removing SO2 from flue gas, and oxygen-based combustion could lower emissions of NOx. Advances in membrane technology and lower-cost oxygen production would therefore benefit the metal casting industry.
Summary
The following separation needs were identified for the metal casting industry:
- technologies to reduce the drying time of investment casting mould production
- better dust removal technologies
- improved technologies to remove impurities from molten metal
- methods for preventing or minimizing gas inclusions in gates and risers
- methods for efficiently and cost-effectively separating gate and riser scrap by impurity level
- economical and efficient means of separating nonfoundry scrap
- better methods for reclaiming core room sand (contaminated with resin and other chemicals), including improved sand drying and sludge dewatering technologies
- better methods for reclaiming shake-out house sand (contaminated with calcined clay and metal pieces)
- cost-effective and efficient methods of reclaiming cleaning room sand (sand with metal smears and a changed size distribution)
- lower cost oxygen for oxy-fuel burners
- better methods for removing specific components from gaseous and liquid emissions