Know the environmental impact of your additives: step one in pollution prevention is reviewing the additives in your green sand molds.
Melting Organics 4% Core/Mold Making Organics 2% Melting Metallics 3% Pouring, Cooling and Shakeout Metallics 1% Pouring, Cooling and Shakeout 90% Fig. 1. The pouring, cooling and shakeout operations contribute the most emissions in a green sand casting facility. Note: Table made from pie chart.
One of the first steps of pollution prevention is understanding your molding media materials, why they are used and their effect on emissions. For the past four years, members of the metalcasting industry have been conducting a literature review to compile a database that illustrates the additives used in green sand molding and the correlating environmental issues that occur during pouring, cooling and shakeout (Table 1). The table database was developed from various sources of information, including research, technical development and plant applications. Most of the references have been published in the last 15 years, and the literature database is updated on a regular basis.
Table 1. Binders, Additives and Environmental Issues Present for Green Sand Cast Steel, Iron, Aluminum and Copper-Base Alloys Factors Steel Iron Brass Aluminum Binder Sodium Bentonite Predominant Predominant Yes Yes Calcium Bentonite Rare Yes Predominant Predominant Bentonite With No Yes Yes Yes Processed Carbon Bentonite Processed Yes Yes Yes Yes With Magnesium Fireclay No Yes No Yes Carbon Additives Seacoal No Predominant Predominant No Gilsonite No Yes Yes No Causticized Lignite No Yes No No Cellulose No Yes Yes No Cereal/Starch Predominant Yes No Yes Polymers or Yes Yes Yes Yes Surfactants Inorganic Additives Soda Ash No Yes Yes No Advanced Oxidation No Yes Yes No Core Sand Additions Cold box Yes Yes Yes Yes Hotbox n/a Yes Yes n/a Shell n/a Yes Yes Yes Sodium Silicate Yes n/a n/a Yes Other Additives Dust Collector No Yes Yes Yes Fines Release Agents Yes Yes Yes Yes Environmental Issues Emissions (Pouring, Lowest Highest Medium Lowest Cooling, Shakeout) Without Cores Emissions (Pouring, Medium Highest Highest Lowest Cooling, Shakeout) With Cores Greenhouse Gases Lowest Highest Medium Lowest (CO & [CO.sub.2]) Without Cores Emission Characteristics as Related to the 1,800F LOI Molding Sand % >5 LOI or >1.25 Highest Highest Highest Highest Volatiles 5-3 LOI or Medium Medium Medium Medium 1.25-0.75 Volatiles <3 LOI or <0.75 Lowest Lowest Lowest Lowest Volatiles Reduction of Emissions as Related to LOI & Volatiles in Molding Sand Reduce LOI & Medium Highest Highest Medium Volatile Matter With Less Carbonaceous Additives Change Carbon Form No Highest Medium No From Gilsonite to Reference Reference Causticized Lignite Change Carbon From No Medium Medium No Gilsonite to Reference Reference Seacoal Change Carbon From No Lowest Lowest No Seacoal to Reference Reference Seacoal/Causticized Lignite Blend Reading the Table If a factor is marked "predominant," that binder or additive is frequently used in sand casting of that particular metal. If a factor is marked "rare," it is rarely used. If a binder or additive is marked "no," it is not used in casting that metal type. If "yes," then it is used. For the environmental factors, "lowest," "medium" and "highest" refers to the extent an emission characteristic is present in conjunction with a metal type. The abbreviation n/a means there is no information available.
This article presents the findings of the review and discusses how to apply the results to your green sand facility.
Molding Sand Additives
The database includes two areas of review--additives and environmental issues. The additives section examines five primary additive types: binders, carbon additives, inorganic additives, core sand additions and "other" additives. Since the metal types poured in a metalcasting facility have an impact on the selection of additives used in the green sand molding process, the database was developed to show how the additives are used by primary metal type, including steel, iron, copper-base and aluminum alloys.
The practice of using one molding sand with multiple metal types (pouring aluminum and copper-base with the same sand system) is becoming popular in the casting industry for both ferrous and nonferrous facilities.
Recognizing the differences in additives required in the preparation of the molding sand is important. For example, the database shows that calcium bentonite is the predominant binder used when pouring aluminum and copper-base alloys. However, copper-base casting facilities also use a quantity of sodium bentonite or bentonite with processed carbon to increase dry and hot compression strength because of the higher pouring temperatures. Also, copper-base plants prefer to put a small quantity of seacoal or other organic additives in the prepared molding sand for clean separation from the castings at shakeout, but seacoal is not required for aluminum castings. When casting both metals using the same sand system, a balance in binder selection and carbon additives is required to achieve a molding blend that is appropriate for both metal types.
The addition of seacoal into molding sand has been shown to increase the emission characteristics of the mold. Casting facilities that pour only aluminum do not observe the increase in emission characteristics because they do not use seacoal, but aluminum casting facilities that add copper-base alloys to their capabilities--and therefore seacoal to their sand--will see an increase in emissions.
Core Sand Additions
Returned core sand is another contributor to emissions at pouring, cooling and shakeout. Core sand additions should be monitored as a raw material additive similar to binders and other green sand additives.
The environmental issues section of the database shows the addition of returned core sand can increase the loss on ignition (LOI) and volatiles. The LOI listed in the table refers to the 1,800? LOI test; volatiles refer to the 1.200F volatiles test.
Dust collector fines and release agents also can affect a facility's emissions. References on dust collector fines go back over the past 50 years in the industry. Many metalcasting facilities have tried (with various levels of success) to utilize dust collector fines as an addition to molding sand in both dry and wet (sometimes known as black water) applications. Dust collector fines consist of the binders (bentonite) and carbon additives that have been removed from the metalcasting process at cooling and shakeout and are returned back into the molding sand in a controlled method. Some dust collector fines can contain up to 40% binder and carbon additives. (On average, the fines contain 30% binder and carbon additives.) As a rule of thumb, when dust collector fines are added to prepared molding sand in a controlled method using various mechanical techniques, they can help reduce the amount of binder added, among other benefits.
Green sand release agents are used to achieve a smooth and even separation between the prepared mold and the pattern on the molding machine. These release agents are formulated predominately from petroleum or vegetable-based oils. The references on this subject have shown that these release agents contribute to emissions during pouring, cooling and shakeout. Since all metal types use release agents in the metalcasting process, a reduction in their use or selection of the base oil will have an impact on pollution prevention.
Application of the Database
Different metals can be cast via various green sand molding methods, so producers of each metal type will have unique opportunities for pollution prevention. Green sand binders (clays) have minimal impact on pollution prevention since they are mostly inorganic. The greatest impact on pollution prevention will be the selection of the type of green sand additives (organic or inorganic), core sand dilution and green sand release agents.
Pollution prevention can take many directions, as the database suggests. A critical area is the quantity and type of organic materials added into green sand for all metal types. Many metalcasting facilities have successfully reduced their emissions by replacing carbon materials in their green sand additives. Iron casting facilities have reduced their quantity of seacoal and/or gilsonite (materials that have higher emission characteristics) by up to 50% in the preblend formulation with the addition of processed carbon or causticized lignite (materials that have little or no emission characteristics).
Nonferrous casting facilities, which use olivine or silica sand-based green sand systems, also have been able to reduce or eliminate seacoal and/or gilsonite from their preblend with the addition of processed carbon or causticized lignite. This reduction in carbon additives in nonferrous plants has resulted in the lower emissions required to meet government regulations.
Another area for review is greenhouse gases (CO and [CO.sub.2]). Seacoal (and other organic compounds referenced in the database) has a large impact on the development of CO during pouring, cooling and shakeout (Fig. 2). As the database shows, seacoal is not used in steel and aluminum casting facilities and therefore does not contribute CO in their molding sand. However, since iron and copper-base casting facilities do utilize seacoal, its reduction at those facilities will lead to pollution prevention.
Other Sources 10% Cast Iron 22% Phenolic Urethane Cores 12% Seacoal 56% Fig. 2. Seacoal is a main contributor to greenhouse gas emissions at green sand casting facilities during pouring, cooling and shakeout. Note: Table made from pie chart.
As referenced in the database, the widely accepted methods for detecting the quantity of organic additives in green sand molding systems is the LOI test and the volatile test. It has been well documented that as the LOI test and volatile test decrease, the emissions characteristics of green sand during pouring, cooling and shakeout decrease.
About the Authors
Vic LaFay is research and technical development manager and Stephen Neltner is technical service engineer for S&B industrial Minerals SA, Cincinnati. Dave Carroll is regional sales manager for American Colloid Co., Elgin, III. D.J. Couture is senior manufacturing process engineer for General Motors Corp., Pontiac, Mich.
For More Information
"Know Your HAPs," J. Schifo, MODERN CASTING, October 2005, p. 33.