Figure 1. HGMS can be used to separate very fine particles of radioactive material from solid, liquid, or gas waste streams, magnetic particles trapped in steel wool or nickel foam. It reduces the volume of waste to be treated and thus reduces the volume of acids needed for treatment processes as well.
Introduction
Decontamination of materials such as soils or wastewater that contain radioactive isotopes, heavy metals, or hazardous components is a subject of great interest as indicated by the growth of waste remediation and minimization efforts. Conventional treatments of these wastes include both chemical and physical methods. Magnetic separation is a physical separation process that segregates materials on the basis of their magnetic susceptibility. Magnetic separation of radioactive wastes has been shown to reduce the volume of these wastes as well as to reduce the chemical reagents necessary for further remediation.
The HGMS method can be used to separate magnetic components from solids, liquids, or gases. A diagram of the method is shown in Figure 1. Usually, contaminated material is slurried with water and passed through a magnetized volume. Field gradients are produced in the magnetized volume by a ferromagnetic matrix material such as steel wool or nickel foam. Ferromagnetic and paramagnetic particles are extracted from the slurry by the ferromagnetic matrix. The diamagnetic fraction and fluid pass through the magnetized volume. The extracted particulates are then flushed from the matrix when the magnetic field is turned off.
In many situations radioactive contaminants are found concentrated in the fine-particle-size fraction of less than 20 microns. For effective decontamination of the fine-particle-size range most conventional operations resort to expensive chemical dissolution methods for treatment. HGMS is able to separate out particles in the range of 90 to ~0.1 microns effectively without chemicals. (The technology is currently used on a commercial scale in the kaolin clay industry.)
HGMS work at Los Alamos National Laboratory (LANL) is being developed for soil remediation, wastewater treatment, and treatment of actinide chemical processing residues. LANL and Lockheed Environmental Systems and Technologies Company (LESAT) have worked on a co-operative research and development agreement (CRADA) to develop HGMS for radioactive soil decontamination. The program is designed to transfer HGMS from the Laboratory to industry for the treatment of radioactively contaminated materials on a commercial scale.
High-gradient Magnetic Separation Equipment
The development of HGMS at LANL has included the design, purchase, and installment of several magnet units. LANL currently has one conventional coil and three supercon-ducting, high-gradient separators. The conventional coil (1-inch bore diameter) and one 3-inch bore superconducting magnet are installed in a vented hood in the plutonium facility. These two magnetic systems comprise the radioactive soil treatment laboratory that became operational in 1993. The second 3-inch bore superconducting magnet is installed in a glove box. It is used for testing highly contaminated material such as processing residues or radioactive wastewater. LANL also has a 6-inch bore magnet that is being used for nonradioactive tests and prototype development for soil decontamination work under the CRADA.
HGMS Results
We have completed a comprehensive series of HGMS experiments with nonradioactive surrogates and have progressed to routine testing of radioactive materials. Our results on spiked material have been used to develop an analytical model that describes the HGMS process. The model provides guidance in selecting the appropriate bench-scale experiments to perform and assists in analyzing the resulting data. A validated analytical model also supports prototype design and process scale-up. It takes into account variables in the material characteristics (such as particle size and magnetic susceptibility) and the separator parameters (such as the magnetic field strength).
In studies involving soil remediation, we are developing and testing HGMS as part of an integrated system with LESAT's soil- washing and gravity-based separation equipment. The goal is to replace an expensive chemical leach-unit operation with magnetic separation for treatment of the fine-particle-size fraction. HGMS tests have been conducted on uranium- and plutonium-contaminated soils and soil-washing residues from Fernald, China Lake, Johnston Atoll, Rocky Flats Plant (RFP), Idaho National Engineering Laboratory, LANL, and the Nevada Test Site (NTS). Test results to date are promising. For example, tests on Fernald's contaminated uranium soils indicate that HGMS can effectively reduce the uranium concentration in nearly 75% of the soil mass to below 70 ppm. Further testing continues on Fernald soils to demonstrate the technology on a pilot scale.
HGMS is also being developed for magnetic filtration of fluid waste streams at the waste treatment facility or those generated during actinide chemical processing. The caustic liquid waste generated by chloride processing operations at TA-55 can produce up to 15,000 liters (l) of liquid effluent annually, with an average alpha activity of 1010 dpm/l. The TA-55 caustic liquid effluent is transferred to the LANL Wastewater Treatment Facility (TA-50), where precipitation and filtration operations are conducted until the effluent meets the industrial waste disposal criteria (<0.52 µCi/l). This effluent is then combined with all other LANL liquid waste for continued treatment. The goal at TA-55 is to reduce actinide activity in the process waste streams to <0.52 µCi/l, which could eliminate the generation of transuranic waste at TA-50, and meet the acceptance level for disposal as an industrial liquid waste.
Several series of tests have been performed on caustic waste samples. Figure 2 illustrates the HGMS test results on a TA-55 caustic waste stream effluent. After processing 500 ml of the pH-unadjusted solution through the separator, only 70% of the activity was removed. The lower activity limit reached was 34 µCi/l. In three other tests the pH was adjusted to 9.5, and the solution was processed three times at 6.5 tesla. In addition to undergoing pH adjustment, two solutions were aged at 70 ºC for 6 and 24 hours, respectively. Controlled heating is well documented in the generation of controlled crystallization and specific particle-sized species. In all three solutions the results indicated that it is possible to reduce activity to levels acceptable to the industrial waste discard limits. After two passes through the separator, more than 99% of the activity was removed. These results are significant and illustrate the flexibility of HGMS for treating a variety of caustic waste samples.
Figure 2. HGMS of TA-55 waste effluent showing the effect of the aging
and pH adjustment.
Benefits of Magnetic Separation We have shown that HGMS can be used to concentrate plutonium and uranium in waste streams and contaminated soils. One of the major benefits of this technology is that it only partitions the existing waste volume. It does not create additional waste. The ability to concentrate the actinides from extraneous materials before further processing reduces the volume of waste to be processed and yields more efficient recovery or treatment operations. For example, it reduces the volume of chemical reagents (acids) necessary for subsequent operations, it allows a more efficient ion exchange or solvent extraction, and it reduces the chemicals in subsequent waste treatment operations.
The principal developers of this project are Laura A. Worl and
Dennis D. Padilla Advanced Technology Group Nuclear Materials
Technology Division
F. Coyne Prenger and Dallas D. Hill Energy and Process
Engineering Group, Engineering Sciences and Applications Division
Thomas L. Tolt Lockheed Environmental Systems and Technology
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