Cement Fly Ash Spend Foundry Sand Water
Minimum 25 334 818 291
Maximum 94 463 1264 504
Source: From Deng, A., Excess Foundry Sand Characterization and Experimental Investigation in Controlled Low-Strength Material, PhD Dissertation of the Pennsylvania State University, University Park, PA, August 2004.
The primary characteristic necessary for a liner, cover, or cutoff wall is low permeability, which essentially enables them to slow down the seepage or diffusion of chemicals. Clay is therefore the main material used to construct these containment systems. The thickness and chemical compatibility of containment systems are of concern in assessing the performance of a system. For example, clay liners are constructed as a simple liner that is 2 to 5 ft thick. In composite and double liners, the compacted clay layers are usually between 2 and 5 ft thick, depending on the characteristics of the underlying geology and the type of liner to be installed. Regulations specify that the clay used can only allow water to penetrate at a rate of less than 1.2 in./yr. However, the effectiveness of clay liners can be reduced by fractures induced by freeze-thaw cycles, drying out, and the presence of some chemicals.
Most spent foundry sand discarded is green sand. The primary components of green sand are silica and bentonite. Thus, green sands are essentially a sand-bentonite mixture, which makes them potentially useful as a liner and cover materials, that is, for hydraulic barrier layers.
The critical properties of green sand affecting its performance as a hydraulic barrier material include grain size distribution, compaction curves, and hydraulic conductivity when compacted. In general, hydraulic conductivity in use should be less than 1 x 10-7 cm/s, which is the criterion for a conventional clay barrier. Sometimes, if aggressive leachate might penetrate through the barrier, the chemical compatibility and durability of a green sand barrier must be researched. As zero-valence iron, clay, and carbonaceous materials help the containment of chemicals, green sand is thought to be an active containment media in the subsurface cleaning domain.
An emerging domain for some spent foundry sand reuse is as a component in the manufacture of topsoil and growing amendments or composites. In many parts of the globe, high-quality topsoils for landscaping are not available in urban areas. Commercial landscapers and nursery growers frequently manufacture topsoil and composite by blending composted materials and low-quality soils, which not only exhausts natural resource, but also increases manufacture cost. Spent foundry sand has been reported to be amended into a product matching the characteristics of topsoil and amendments; this could be another competitive and vast market for both the metal casting industry and horticultural professionals.
Topsoil is the uppermost layer of the Earth's surface, ranging in depth from a few inches to many feet. Topsoil has been created over long time by the physical and chemical action of climate, weather on the Earth's parent rock materials, and decay of plants. As a result, a considerable accumulation of decaying organic matter is found in topsoil. Topsoil is the base for gardening and landscaping activities, where plants are grown by gardening efforts. Unfortunately, genuine topsoil created by natural forces is often unavailable because it is so scarce, and when it can be obtained it is often very expensive.
Topsoil should have a loose and open structure so that it drains fast to keep the ground surface dry. At the same time, it must be able to retain enough moisture in order that plants growing in it are not constantly subjected to drought stress. The properties of interest include particle gradation, clay content, nutrient content, and retention capacity.
Spent foundry green sand is of particular interest to soil blending companies that produce topsoil, because of its dark color, clay content, moisture retention, and consistency. A high sand content is required in topsoil, so spent foundry sand could be a major component. Spent sand reduces the formation of clumps and prevents the mix from compacting, which allows air to circulate within the topsoil and to stimulate decomposition. The U.S. Agricultural Research Service is leading some pilot studies conducted to investigate the feasibility of beneficially using foundry sand as a topsoil replacement.
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