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.
• Chemically Bonded Sands are used both in coremaking, where high strengths are necessary, 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.   

Metalcasting facilities produce FS 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 casting is cooling, green sand is literally shaken off the castings, recovered and reconditioned for 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.  Some sand streams may need crushing and screening to reduce all the mold and core pieces to grain size, but this is easily done with commonly available aggregate screening equipment.