Technical Overview
Recycled Foundry Sand is basically a fine aggregate.
Once properly screened and segregated from other foundry discards, foundry sands are usable in most of the same applications where conventional construction sands are used. From an engineering standpoint, foundry sands have several characteristics that make them a good fit for many applications. In a handful of cases, those characteristics may limit their value in substituting for other sands.
In general, foundry sands will be finer sieve analysis than construction sands. The sands from an individual foundry will usually be a uniform grain size, as controlling the sand system is one of the most critical parts of the foundry casting process. For some applications, it may be necessary to source material from several foundries, or blend foundry sands with other aggregates to achieve the desired grain size distribution.
Most of the characteristics that determine the “fit” for foundry sand from a particular source are related to the type of molding system used in that foundry’s production. At a basic level, that relates to whether the foundry uses clay to bind the sand grains together in the manufacturing process. Many foundries generate two or more different sand streams from different places in the casting process. In determining which sand streams fit local market requirements, it is helpful to think about whether the clay is likely to be an advantage or a disadvantage in the ultimate application.
More than 80 percent of casting volume in the U.S. is produced in iron and steel foundries, collectively referred to as "ferrous" foundries. The most common molding technique used for ferrous casting is called “green” sand. Some aluminum foundries cast in green sand, although other molding systems are generally more prevalent in nonferrous foundries. In green sand systems, high grade industrial sands are bound together with bentonite clays, into which a small amount of carbonaceous material and water are added. Green sand is black in color; adheres together; and has some percentage of material passing the 200 sieve size, depending on the foundry’s internal sand handling process. Green sands look and feel much like a silty soil. Most of the large scale construction projects that have been field-tested are designed using foundry green sands because they are the most prevalent types of sands produced.
The combination of the sand, clay and moisture content makes many green sands particularly suitable for geotechnical applications such as structural fills and base courses. The presence of the clay appears to improve the performance of these sands relative to other granular materials in parts of the country subject to freeze/thaw cycles. Additionally, green sands can often be placed in construction projects in colder climates long past the point in the construction season where other granular materials can be moved and graded. Green sands generally require only screening to remove tramp metal and sand lumps in order to be market ready. In its evaluation of foundry sand as a construction material, the Federal Highway Administration also pointed out that foundry sands can be transported, placed and graded using conventional construction equipment.
Research is underway to determine whether these same green sand properties will provide a good substructure for manufactured soils, a growing need in many parts of the nation. Conversely, the presence of the clays may be a limitation in the amount of green sand that can be used as fine aggregate in hot mix asphalt or ready mix concrete.
Different foundry processes introduce different amounts of fines in the sand stream. It is sometimes possible for foundries to exclude most of the passing 200 material from their “fine foundry aggregate” – or the fines can be washed out, as is sometimes done at virgin sand quarries. Both of these fines-control techniques may introduce additional costs for foundry sand processing.
Resin sands are similar high grade industrial sands which are bound together with a variety of proprietary chemical formulas. Chemically bonded sands are always used as the “cores” in castings that have hollow void spaces. Some foundries, generally steel or non-ferrous foundries, also use chemically bonded sands as their external molds. These sands, absent the clay and the carbon, are usually lighter in color and coarser in texture than clay-bonded sands. That may, for example, make them more suitable for use in certain concrete products where the dark color of the green sands can be detrimental to the finished product. Used sand molds from resin sand systems will often require crushing before screening to produce a uniform granular material.
Resin sands are often used in manufactured products, as they may lack the hydraulic properties of green sands. Many resin sands have physical and environmental profiles that are suitable for geotechnical or even agricultural use. Restrictions on their use may be posed by the state’s environmental regulations, which vary from state to state. Some resin sands are quite environmentally benign, such as the sodium silicate binder systems. Others may leach trace amounts of organics such as phenolics. However, the detection of a trace chemical does not necessarily mean that human health or the environment is at risk. Research performed at Penn State University concluded that "excess foundry sands do not typically pose more threats to the environment than soil."
In surveying the nation as to successful implementation strategies for foundry sands, the proven market applications break down into three groups:
Manufactured Products – In these applications, recycled foundry sands are used specifically as a substitute for other fine aggregates in a final product which is bound together in some manner. The highest volume uses in manufactured products are: feedstock for Portland Cement kilns; fine aggregate in hot mix asphalt; and fine aggregate in flowable fill. Although it is less common to see full commercial-scale use, foundry sands have also been shown to be useful fine aggregates in products such as bricks, blocks and pavers; pre-cast concrete; ceramic tiles; and other products where sand is a raw material.
Geotechnical Applications -- Many types of construction projects require granular materials to level construction sites, create berms or retaining walls, build embankments, or backfill structures. Properly prepared foundry sands have been shown to perform well in all of these applications, as well as in bases under roadways, paved surfaces and structures.
Soils Based Applications -- Foundry sand is an ideal candidate for soil blending because of its composition, color and consistency. In urban areas, soil scientists in nursery and landscaping companies are manufacturing soils by blending a range of recycled materials with organic matter. It is particularly important to check with your state regulatory agency before contemplating this use as many states have different rules for soil-based applications.