Solid-Phase Separation

Description

Solid-phase separation techniques concentrate contaminants, such as metals and radionuclides, through physical and chemical processes. There are several types of techniques for separating solids from soil. These processes are described below.

• Gravity Separation. Gravity separation capitalizes on the density difference between metals and soil. When the metals-contaminated soil is suspended in water, denser materials such as metals sink and are removed.
• Sieving/Physical Separation. Sieving and physical separation are elementary separation processes. They use sieves and screens to separate different size particles. Most organic and inorganic contaminants tend to bind to the fine fraction of a soil (i.e., clay and silt). Thus, separating the fine clay and silt particles from the coarser sand and gravel soil particles concentrates the contaminants into a smaller volume of soil that can then be further treated or disposed.
• Dry Soil Separation. Dry soil separation separates radioactive particles from clean soil particles. In this process, radionuclide-contaminated soil is excavated and screened to remove large rocks. Large rocks are crushed and placed with soil on a conveyor belt, which carries the soil under radiation detectors that measure and record the level of radiation in the material. Radioactive particles are tracked and mechanically diverted through automated gates, which separate the soil into contaminated and clean segments.
• Magnetic Separation. In magnetic separation, a magnetized medium such as magnetized steel wool is used to separate slightly magnetic radioactive particles from soil. All uranium and plutonium compounds are slightly magnetic while most soil is nonmagnetic. Soil is mixed with water to form a slurry, and the slurry is passed through the magnetic medium.
• Chemical Leaching. Chemical leaching processes use weak acids such as vinegar to dissolve and wash the metals from the soil. The metals recovered by the process can possibly be recycled.

Limitations and Concerns

Fugitive dust from dry soil screening and separation is a major concern.

Gravity separation processes rely on a difference in particle density. The specific gravity of particles will affect the settling rate and process efficiency. Additionally, settling velocity is dependent on the viscosity of the suspending fluid, which must be known to estimate process efficiency and to size equipment.

Special measures may be required to mitigate odor problems resulting from organic sludge that undergoes septic conditions.

If it is intended for soil to be returned to its original location, a rigorous testing system should be established to ensure that the soil does not contain low-levels of contamination.

Magnetic separation may leave a small suspension of radioactive materials in a slurry, which may be more difficult to remediate than the original soil contamination.

Applicability

Separation techniques are used to treat soil. The target contaminant groups for separation processes are semi-volatile organic compounds (SVOCs), fuels, and inorganics (including radionuclides). Dry soil separation effectively treats soils contaminated with gamma emitting radionuclides, including Th, U, Cs-137, Co-60, Pu-239, Am-241 and Ra-222. Separation is also useful when heavy metal contaminants occur as particulates (e.g., in small-arms ranges).

Technology Development Status

Physical separation techniques are generally proven. Dry soil separation can substantially reduce the volume of radioactive waste (by more than 90%), and it has been used on a commercial scale both at Johnston Atoll in the South Pacific and at the Energy Department’s Savannah River site in South Carolina. Magnetic separation is a newer, experimental separation process. Chemical leaching is in the field-test stage.

Web Links

http://enviro.nfesc.navy.mil/erb/restoration/technologies/remed/phys_chem/phc-21.asp

http://www.frtr.gov/matrix2/section4/4_20.html

Other Resources and Demonstrations

See Precipitation, Vapor-Phase Membrane Separation, and Soil Washing.

See http://www.estcp.org/documents/techdocs/199513.pdf for the use of different separation technologies at small arms ranges.