(This week's post is from Federal Highway Administration Research and Technology. It provides an excellent overview of green sand content and recycled use in paving technologies.)
Foundry sand consists primarily of clean, uniformly sized, high-quality silica sand or lake sand that is bonded to form molds for ferrous (iron and steel) and nonferrous (copper, aluminum, brass) metal castings. Although these sands are clean prior to use, after casting they may contain Ferrous (iron and steel) industries account for approximately 95 percent of foundry sand used for castings. The automotive industry and its parts suppliers are the major generators of foundry sand.
The most common casting process used in the foundry industry is the sand cast system. Virtually all sand cast molds for ferrous castings are of the green sand type. Green sand consists of high-quality silica sand, about 10 percent bentonite clay (as the binder), 2 to 5 percent water and about 5 percent sea coal (a carbonaceous mold additive to improve casting finish). The type of metal being cast determines which additives and what gradation of sand is used. The green sand used in the process constitutes upwards of 90 percent of the molding materials used.(1)
In addition to green sand molds, chemically bonded sand cast systems are also used. These systems involve the use of one or more organic binders (usually proprietary) in conjunction with catalysts and different hardening/setting procedures. Foundry sand makes up about 97 percent of this mixture. Chemically bonded systems are most often used for "cores" (used to produce cavities that are not practical to produce by normal molding operations) and for molds for nonferrous castings.
The annual generation of foundry waste (including dust and spent foundry sand) in the United States is believed to range from 9 to 13.6 million metric tons (10 to 15 million tons).(2) Typically, about 1 ton of foundry sand is required for each ton of iron or steel casting produced.
CURRENT MANAGEMENT OPTIONS
In typical foundry processes, sand from collapsed molds or cores can be reclaimed and reused. A simplified diagram depicting the flow of sand in a typical green sand molding system is presented in Figure 7-1. Some new sand and binder is typically added to maintain the quality of the casting and to make up for sand lost during normal operations.(3)
Figure 7-1. Simplified schematic of green sand mold system.
Little information is available regarding the amount of foundry sand that is used for purposes other than in-plant reclamation, but spent foundry sand has been used as a fine aggregate substitute in construction applications and as kiln feed in the manufacture of Portland cement.
Most of the spent foundry sand from green sand operations is landfilled, sometimes being used as a supplemental cover material at landfill sites.
Foundry sand can be obtained directly from foundries, most of which are located in midwestern states, including Illinois, Wisconsin, Michigan, Ohio, and Pennsylvania.
Foundry sand, prior to use, is a uniformly graded material. The spent material, however, often contains metal from the casting and oversized mold and core material containing partially degraded binder. Spent foundry sand may also contain some leachable contaminants, including heavy metals and phenols that are absorbed by the sand during the molding process and casting operations. Phenols are formed through high-temperature thermal decomposition and rearrangement of organic binders during the metal pouring process.(4) The presence of heavy metals is of greater concern in nonferrous foundry sands generated from nonferrous foundries.(5) Spent foundry sand from brass or bronze foundries, in particular, may contain high concentrations of cadmium, lead, copper, nickel, and zinc.(3)
HIGHWAY USES AND PROCESSING REQUIREMENTS
Asphalt Concrete and Flowable Fill Aggregate
Foundry sand has been used as a substitute for fine aggregate in asphalt paving mixes. It has also been used as a fine aggregate substitute in flowable (or controlled density) fill applications.
Prior to use, spent foundry sand requires crushing or screening to reduce or separate oversized materials that may be present. Stockpiles of sufficient size typically need to be accumulated so that a consistent and uniform product can be produced (i.e., day-to-day variations in the material characteristics can be overcome by blending in a comparatively large stockpile).
Since only small quantities of spent foundry sand are generated at small foundries, it will generally be necessary for these operators to transport their spent sand to a central storage area that receives sand from a group of plants before transferring it to an end user.
Typical physical properties of spent foundry sand from green sand systems are listed in Table 7-1.
The grain size distribution of spent foundry sand is very uniform, with approximately 85 to 95 percent of the material between 0.6 mm and 0.15 mm (No. 30 and No. 100) sieve sizes. Five to 12 percent of foundry sand can be expected to be smaller than 0.075 mm (No. 200 sieve). The particle shape is typically subangular to rounded. Waste foundry sand gradations have been found to be too fine to satisfy some specifications for fine aggregate.
Spend foundry sand has low absorption and is nonplastic. Reported values of absorption were found to vary widely, which can also be attributed to the presence of binders and additives.(3) The content of organic impurities (particularly from sea coal binder systems) can vary widely and can be quite high. This may preclude its use in applications where organic impurities could be important (e.g., Portland cement concrete aggregate).(4) The specific gravity of foundry sand has been found to vary from 2.39 to 2.55. This variability has been attributed to the variability in fines and additive contents in different samples.(3) In general, foundry sands are dry, with moisture contents less than 2 percent. A large fraction of clay lumps and friable particles have been reported, which are attributed to the lumps associated with the molded sand, which are easily disintegrated in the test procedure.(3) The variation in permeability, listed in Table 7-1, is a direct result of the fraction of fines in the samples collected.
Spent foundry sand consists primarily of silica sand, coated with a thin film of burnt carbon, residual binder (bentonite, sea coal, resins) and dust. Table 7-2 lists the chemical composition of a typical sample of spent foundry sand as determined by x-ray fluorescence.
Silica sand is hydrophilic and consequently attracts water to its surface. This property could lead to moisture-accelerated damage and associated stripping problems in an asphalt pavement. Antistripping additives may be required to counteract such problems.
Depending on the binder and type of metal cast, the pH of spent foundry sand can vary from approximately 4 to 8.(7) It has been reported that some spent foundry sands can be corrosive to metals.(5)
Because of the presence of phenols in foundry sand, there is some concern that precipitation percolating through stockpiles could mobilize leachable fractions, resulting in phenol discharges into surface or ground water supplies. Foundry sand sources and stockpiles must be monitored to assess the need to establish controls for potential phenol discharges.(4,6,7)
Table 7-1. Typical physical properties of spent green foundry sand.
|Specific Gravity(3)||2.39 - 2.55||ASTM D854|
|Bulk Relative Density, kg/m3 (lb/ft3)(7)||2590 (160)||ASTM C48/AASHTO T84|
|Absorption, %(1,3,7)||0.45||ASTM C128|
|Moisture Content, %(3)||0.1 - 10.1||ASTM D2216|
|Clay Lumps and Friable Particles(1,3)||1 - 44||ASTM C142/AASHTO T112|
|Coefficient of Permeability (cm/sec)(3)||10-3 - 10-6||AASHTO T215/ASTM D2434|
|Plastic limit/plastic index(7)||Nonplastic||AASHTO T90/ASTM D4318|
Table 7-2. Foundry sand sample chemical oxide composition, %. (1)
|LOI||5.15 (0.45 to 9.47)(1)
2.1 - 12.1(3)
Typical mechanical properties of spent foundry sand are listed in Table 7-3. Spent foundry sand has good durability characteristics as measured by low Micro-Deval abrasion(8) and magnesium sulfate soundness loss(9) tests. The Micro-Deval abrasion test is an attrition/abrasion test where a sample of the fine aggregate is placed in a stainless steel jar with water and steel bearings and rotated at 100 rpm for 15 minutes. The percent loss has been determined to correlate very well with magnesium sulfate soundness and other physical properties. Recent studies have reported relatively high soundness loss, which is attributed to samples of bound sand loss and not a breakdown of individual sand particles.(3) The angle of shearing resistance (friction angle) of foundry sand has been reported to be in the range of 33 to 40 degrees, which is comparable to that of conventional sands.(3)
Table 7-3. Typical mechanical properties of spent foundry sand.
|Micro-Deval Abrasion Loss, %(5)||< 2||–|
|Magnesium Sulfate Soundness Loss, %||5 - 15(1,5)
6 - 47(3)
|Friction Angle (deg)(3)||33 - 40||–|
|California Bearing Ratio, %(3)||4 - 20||ASTM D1883|
American Foundrymen's Society. Alternative Utilization of Foundry Waste Sand. Final Report (Phase I) prepared by American Foundrymen's Society Inc. for Illinois Department of Commerce and Community Affairs, Des Plaines, Illinois, July, 1991.
Collins, R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction, National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, DC, 1994.
Javed, S. and C. W. Lovell. Use of Foundry Sand in Highway Construction. Joint Highway Research Project No. C-36-50N, Department of Civil Engineering, Purdue University, July, 1994.
MOEE. Spent Foundry Sand - Alternative Uses Study. Report prepared by John Emery Geotechnical Engineering Limited for Ontario Ministry of the Environment and Energy and the Canadian Foundry Association, Queen’s Printer for Ontario, July, 1993.
MNR. Mineral Aggregate Conservation - Reuse and Recycling. Report prepared by John Emery Geotechnical Engineering Limited for Aggregate and Petroleum Resources Section, Ontario Ministry of Natural Resources, Queen’s Printer for Ontario, February, 1992.
Ham, R. K., W. C. Boyle, E. C. Engroff and R. L. Fero. "Determining the Presence of Organic Compounds in Foundry Waste Leachates," Modern Casting. American Foundrymen’s Society, August, 1989.
Johnson, C. K. "Phenols in Foundry Waste Sand," Modern Casting. American Foundrymen’s Society, January, 1981.
Ontario Ministry of Transportation. Resistance of Fine Aggregate to Degradation by Abrasion in the MicroDuval Apparatus, LS-619, Ontario Ministry of Transportation, Ontario, Canada, 1996.
- American Association of State Highway and Transportation Officials. Standard Method of Test, "Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate," AASHTO Designation: T104, Part II Tests, 14th Edition, 1986.
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