What is Recycled Foundry Sand?
Foundry sand is high quality silica sand with uniform physical characteristics. Recycled Foundry Sand (RFS) is a byproduct of the ferrous and nonferrous metal casting industry, where sand has been used for centuries as a molding material because of its thermal conductivity. In modern foundry practice, sand is typically recycled and reused through many production cycles. Industry estimates are that approximately 100 million tons of sand are used in production annually. Of that, six (6) to ten (10) million tons are discarded annually and are available to be recycled into other products and industries.
Sand used at foundries is of a high quality, much of it supplied by members of the National Industrial Sand Association. Stringent physical and chemical properties must be met as poor quality sand can result in casting defects. Foundries invest significant resources in quality control of their sand systems, with extensive testing done to maintain consistency. As a result, RFS from an individual facility will generally be very consistent in composition.
Although there are other casting methods including die casting, investment casting, and permanent mold casting, sand casting is by far the most prevalent casting technique. Sand is used in two different ways in metalcasting: as a molding material, which forms the external shape of the cast part, and as cores, which form internal void spaces in products such as engine blocks. Since sand grains do not naturally adhere to each other, binders must be introduced to cause the sand to stick together and hold its shape during the introduction of the molten metal into the mold and the cooling of the casting.
What types of Foundry Sand are recycled?
Two general types of binder systems are used in metalcasting: clay-bonded systems (green sand) and chemically-bonded systems (resin sands). Both types of sands are suitable for recycling and beneficial use but they have different physical and environmental characteristics.
* Green Sands are used to produce about 90% of casting volume in the U.S. Green sand is composed of naturally occurring materials which are blended together: high quality silica sand (85-95%); bentonite clay (4-10%) as a binder; a carbonaceous additive (2-10%) to improve the casting surface finish; and water (2-5%). Green sand is the most commonly used RFS for beneficial reuse. It is black in color, due to the carbon content; has a clay content that results in a percentage of the material that passes a 200 sieve; and adheres together due to the clay and water.
* Resin Sands are used both in coremaking, where high strengths are necessary to withstand the heat of the molten metal, and in mold making. Most chemical binder systems consist of an organic binder that is activated by a catalyst, although some systems use inorganic binders. Chemically bonded sands are generally light in color and coarser in texture than clay bonded sands.
Foundries produce RFS generally in proportion to their overall production volume, although there are different sand-to-metal ratios employed in different casting processes and products. Most foundries have two sand systems, one feeding the external molding lines and one feeding the internal core lines. After the metal is poured and the part is cooling, green sand is literally shaken off the castings, recovered and reconditioned for continual reuse. Used cores are also captured during this cooling and “shake out” process; these break down and are crushed and reintroduced into the green sand systems to replace a portion of sand lost in the process. Broken and/or excess cores, or those cores which do not break down when crushed, are discarded.
Depending on the projected end use, it may be important to segregate sand streams at the foundry as each stream can have different characteristics. Additionally, some sand is typically unrecoverable during the “shake out” and finishing processes. These sands may be contaminated with metal and/or very large chunks of burned cores (referred to as core “butts”) and will need to undergo some type of segregation, crushing and screening before recycling.
What are the best options for recycling Foundry Sand?
Sands from ferrous (iron, and carbon steel) and aluminum foundries constitute more than 90 percent of available sands and are the generally the best candidates for recycling. Ferrous and aluminum foundries tend to be larger than other nonferrous foundries and often have more uniform sand streams due to the nature of their production. These sands have been the subject of many research projects, as well as having a history of use in construction projects of various types. However, innovative foundries of many types have found recycling options for their sands. Other nonferrous sands, or mixed sand streams, may be recyclable if they meet the environmental testing requirements of their state regulatory agency. and the performance requirements of the marketplace.
As with any material, transportation costs are generally the highest cost factor in recycling foundry sands. The most economically sustainable options for recycling foundry sand will generally match the volume and characteristics of the foundry sand with nearby businesses and construction projects. Foundry sand is a highly engineered material whose quality control is critical for the integrity of the cast part. Foundries spend significant resources monitoring and testing their sand systems so that the casting sands are the same day after day. Because foundry sand is a uniformly graded fine aggregate, most recycling applications take advantage of those properties. Due to its consistency and engineering properties, foundry sand can be superior to other types of granular materials for certain end uses. In other instances, foundry sands will provide comparable performance at lower cost.
The following markets have the potential to absorb large volumes of foundry sands from different types of foundries:
Construction Projects
Construction applications are by far the most common uses for RFS. The greatest volumes of RFS are currently used in construction applications such as structural fills, general fills, road and building bases, and embankments. Foundry sands are designed to have high structural integrity in the foundry, where they must hold their shape in contact with metals being poured at up to 2700 degrees. Green sands have been shown to perform well in structural fills and bases where they typically exhibit higher strengths than native soils.
The Federal Highway Administration (FHWA) published the first edition of “Foundry Sand Facts for Civil Engineers” in 2004. The overview was developed with support from U.S. EPA and FIRST. This publication is a primer for public agencies, foundries, contractors and others interested in recycling foundry sands in construction projects. An electronic version of the publication is available on FHWA’s website. FIRST Members and Technical Library Subscribers can download electronic versions of the document directly from FIRST’s Technical Library. Paper copies may be ordered through FIRST’s Technical Library, as well as through the National Technical Information Service.
Both U.S. EPA and FHWA have set a goal of increasing the use of foundry sands and other industrial materials in the construction industry. Foundry sand is a priority material for both agencies. FIRST and other industry partners are working with both agencies on programs such as the Green Highways Partnership and ReSource America. Additionally, the Recycled Materials Resource Center (RMRC) is updating the foundry sand information in FHWA’s on-line “User Guidelines for Waste and Byproduct Materials.” RMRC anticipates having that update available in mid-2007.
Cement Kiln Feedstock
Foundry sand is an excellent feedstock for the manufacturing of Portland cement. According to the Portland Cement Association, foundry sand is being used by a number of North American cement kilns. The mineral content and grain size of the sands make foundry sands a good technical fit for cement production. The largest constraint on this use is the limited number of cement kilns in the United States and Canada. Shipping distances and/or limited quantities may prevent foundries from participating in the cement kiln market. The American Foundry Society is working with the Portland Cement Association to increase the use of foundry sands in Portland cement production.
Flowable Fill
Foundry sand is also an excellent fit as an aggregate substitute in the production of Controlled Low Strength Material (CLSM), a type of low strength concrete specifically designed to be excavatable. This product, which is usually produced by ready mix concrete companies, is commonly referred to as Flowable Fill. A number of research studies referenced in FIRST’s Technical Library document the technical fit for foundry sand in this application, where neither the color, clay content nor grain size is a detriment. Flowable fill is able to use large volumes of foundry sand because a ton of foundry sand can be used per cubic yard of CLSM. Foundry sand will usually be less expensive than other local aggregate sources. The principal business constraint is that many ready mix producers do not have a separate storage bin or silo for foundry sand or have not developed mix designs incorporating their local foundry sands. Smaller foundries may not be able to meet the volume requirements for a concrete ready mix plant but may be able to blend their sands with other sources to meet requirements.
The U.S. EPA has a Comprehensive Procurement Guideline in place requiring the use of flowable fill containing foundry sand and/or coal ash in projects with more than $10,000 in Federal funding CPG III can be accessed on-line at http://www.epa.gov/cpg/pdf/cpg-fr.pdf Regrettably, CPG’s are rarely enforced, and many public sector projects using flowable fill are built without the benefit of foundry sand. The FY2007 Energy Bill contains a provision requiring EPA and other Federal departments to enforce the CPG’s on the inclusion of recycled materials in cement and concrete. Foundry sand is one of the “recovered mineral components” referenced in Section 108 of the bill. U.S. EPA is currently engaged in a Congressionally-mandated 30 month study that will make recommendations as to how to increase the use of industrial materials in these products.
Manufactured Soils
An emerging use for some foundry sands is as a component in the manufacturing of topsoil. In many parts of the country, high quality topsoils for landscaping are not available in urban areas. Commercial landscapers and nursery growers frequently manufacture topsoil by blending composted materials and low quality soils. Most bagged topsoil sold in garden centers and home improvement centers is manufactured topsoil. Specialty turf applications are also a potential market for some foundry sands. Foundry green sands are of particular interest to soil blending companies because of their dark color, clay content, moisture retention, and consistency.
Few state regulatory agencies currently allow this use, out of concern about possible contaminants. Some research studies have shown that many foundry sands are “cleaner than dirt” with respect to the presence of potentially harmful metals. These studies are available through FIRST’s Technical Library. The Agricultural Research Service (ARS), an arm of the U.S. Department of Agriculture, has undertaken a multi-phase peer-reviewed research project to assess the safety and suitability of foundry sands for soil-related applications. The industry expects that ARS will issue guidelines in 2008.
Other Applications
Foundry sands can replace a portion of the fine aggregate in many other types of applications and markets, all of which exist at a commercial scale in the United States. These applications and markets include the following:
- Hot Mix Asphalt
- Ready mix concrete
- Precast concrete products
- Bricks and pavers
- Grouts and mortars
- Landill daily cover
- Landfill construction material
Information about these and other uses for RFS can be found under TECHNICAL APPLICATIONS.
What are the barriers to recycling Foundry Sand?
Education
The principal barrier to recycling Foundry Sand is lack of knowledge on the part of foundry suppliers and prospective users. Some foundries do not have reuse-ready sands because they do not have local market partners. Making reuse-ready foundry sands is a simple 3 step process involving segregating, screening and storage. Many contractors and other potential users are unfamiliar with the nature and availability of fine foundry aggregates. FIRST's workshops and website are resources for all parties interesting in recycling foundry sands.
Environmental Regulation
Solid waste regulations are frequently cited as barriers for industrial byproduct recycling. Research indicates that most RFS will meet Federal drinking water standards. With rare exceptions, the EPA considers RFS non-hazardous under the Resource Conservation and Recovery Act (RCRA). Competing granular materials – sands, gravels, and native soils – are not regulated materials although their environmental profiles may be similar to foundry sands. Because states have different environmental requirements, it is important to check with a state environmental specialist to understand how RFS may be used in your state. The Association of State and Territorial Solid Waste Management Officials (ASTSWMO) maintains a list of the beneficial use specialists in each state environmental agency. ASTWSMO can be contacted through its website at http://www.astswmo.org/
Engineering
Foundry Sand, essentially a fine aggregate, generally does not have standalone ASTM or AASHTO specifications. Properly screened foundry sands will usually meet state DOT performance specifications for fill, embankment and road base materials. Research has shown that most foundry sands meet federal Superpave guidelines for Hot Mix Asphalt aggregates. Specifications and mix designs for CLSM exist. Some States require testing and approval before use. Other States maintain lists of approved sources and accept project suppliers’ certifications of foundry sand quality. Some states have appointed either a regulatory specialist or a Market Development Specialist assigned to developing regulatory and market development tools for RFS and other industrial byproducts.
Economics
Transportation distances will usually be the largest economic factor in determining viable market options for individual foundries or contractors. Hauling costs are generally the same as for other locally sourced sands and gravels. Small foundries may not generate enough material on a weekly or monthly basis to satisfy the need for construction sands. In those cases, it may be necessary to blend foundry sands from multiple sources or to blend foundry sand with other sands in order to meet volume requirements.