Abstract from the Massachusetts Institute of Technology thesis by Stephanie Dalquist.
Conventional manufacturing techniques have not been subject to much scrutiny by industrial ecologists to date. Many newer techniques and products draw more attention as they rise quickly from research to global scales, accentuating their environmental consequences. Despite the presence of new technologies and increased overseas production, casting is not being displaced from the US, and represents a stable component in the national economy. Data from the US government, US industry groups, and UK mass balance profiles facilitate an understanding of sand casting and comparison across manufacturing processes. The figures in the US and UK are similar in terms of diversity of metals (where the US is 76%, 13%, 8% and the UK 72%, 13%, 10% for iron, aluminum, and steel, respectively), energy per ton of metal (10.1 and 9.3 MBtu/ton in the US and UK), and overall emissions, with notable similarities in benzene and particulate emissions. One notable discrepancy is in sand use, where the US sends to waste 0.5 tons of sand per ton of cast metal, whereas the UK sends 0.27 tons.
Although we live in an age where new technologies demand exotic manufacturing techniques, most products still require traditional manufacturing processes and carry along their inherent environmental ramifications. Developing countries entering mass production, in particular, are taking on an increased environmental burden in manufacturing. Complex products like semiconductors (Williams, 2002) and cars are frequently subjected to life cycle assessments as a part of or in conjunction with environmental impact analyses. For conventional processes like sand casting, such an evaluation is uncommon. Although sand casting has reached a stable market size in the United States, international production is growing (Modern Casting, 2000). China alone increased shipments 60% over the years 1997 to 2002, and in 2002 shipped 16.2 million tonnes of cast metal. Global casting production showed a 3% overall increase in 2002. Each kilogram of cast material requires substantial energy, often in the form of fossil fuel generated electricity or direct firing of coke or natural gas. Most of this energy is used to melt the metal for casting, but increased quantities of energy and materials are required to meet customer demands of surface specifications. The highest material demand, besides the metal which forms the final product, is the sand used to create the mold. Organic compounds are used as binders, and burned out as gaseous releases during mold formation. More organic compounds are used in cleaning and finishing. As in many processes being assessed, there is little consensus on the magnitude of the impacts. Some firms keep relatively good information, but publicly available aggregate data and sector analyses are scarce. The lack of evidence that the environmental impact is well understood or well addressed exemplifies the suitability of this sector and need for life cycle research.
System Boundaries: Process Materials and Energy Use
The system boundary outlines the sand casting manufacturing process and the boundaries of this inventory. The resources considered (Figure 1) are the material and energy inputs and outputs for mold preparation, metal preparation, casting, and finishing stages and their subprocesses.
Note that the manufacture of materials (e.g., extraction of metals or sand and water purification) is not within the system boundary. Also, equipment manufacture is not included. Furnace service life is long enough that manufacturing impacts are in the distant past and end-of-life impacts are yet in the distant future. Therefore, they can be neglected as the environmental impact of dealing with them decreases over a long time frame, much like the future value of currency in economics.
The limited body of literature available suggests greater concern about the energy implications of sand casting than the material challenges of reducing sand and water consumption. The US Department of Energy (DOE) has published an energy and environmental profile of the casting industry, as has the US Environmental Protection Agency (EPA). Some sand casting foundries must also file a Toxic Release Inventory (TRI) with the EPA.
TRI applies to companies with manufacturing operations in Standard Industrial Classification (SIC) codes 20 through 39 – casting is in 33 – TRI applies to companies with more than 10 employees that use 25,000 pounds of approximately 600 designated chemicals or use more than 10,000 pounds of any designated chemical or chemical category (EPA, 2004). 654 of some 2,800 US foundries were required to file a TRI in 1995 (EPA, 1999) – 33% of foundries do not have 10 employees, and the other 44% who do have more than 10 employees do not use enough regulated material to be required to file a TRI.
Energy data is available from the Energy Information Administration (EIA) Manufacturing Energy Consumption Survey (MECS). Although the results are obtained from self-reporting questionnaires, the numbers tend to agree with other surveys and with approximations calculated from basic information. Even though sand casting is relatively uniform in concept, the myriad parameters complicate the derivation of accurate systematic estimations on a national scale from individual foundries and equipment. Consequentially, determining concordance between individual foundry data and reported data is an exercise of coincidence.
At least three gaps in the known literature on sand casting must be addressed. Two of these are in common with those which have not been addressed in semiconductors (Williams, 2002), demonstrating an issue in life cycle analysis which extends beyond this process. There is a noticeable lack of process data, like input and output materials. This is particularly true of the cleaning process and the components in and reactions of sand binders. Second, of the process data that does exist, no comparisons have been made between the self-reported numbers and experimentally-derived models. Finally, there is a lack of data which is not self-reported. Addressing this issue is beyond the scope of this study, but critical in attaining confidence in the life cycle analysis of a process.
The manufacturing process (Figure 2) begins with the formation of a single-use mold from sand and binders, which hold the sand mold together. A lot of the sand comes from the sand reclamation process, where sand from previous molds gets reclaimed for use in new molds. Cores are also made at this point for parts that have internal cavities.
In the green sand casting process, molds are made from a mixture of sand, clay, water, and carbonaceous additives (e.g. bituminous seacoal, anthracite, or ground coke). About 85% of the mold (EPA, 1998), by mass, is sand. The clay (4 to 10%) and water (2 to 5%) act as the binder system from which the mixture derives its strength. Although the carbonaceous additives are a very small component by mass, they are needed to prevent the metal from oxidizing as it solidifies. They burn off on contact with the molten metal, creating an assortment of hazardous air pollutants (HAPs).
Green sand casting is often accompanied by the use of chemical binding systems. Many parts require cores, internal cavities that must be strong enough to hold together as the metal falls in around it. Therefore, binders other than clay are used, including synthetic resins. Many of these binders have to be cured at high temperatures, though new techniques are being adopted to allow curing at room temperature. The resulting core is harder and stronger than the green sand mold. A few foundries use chemical binder systems in molds, too, but this is uncommon because the green sand process is inexpensive, relatively clean, and flexible.
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