Soil washing for volume reduction of radioactively contaminated soils

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<ul><li><p>Soil Washing for Volume Reduction of Radioactively Contaminated Soils </p><p>Michael C. Eagle William S. Richardson Scott S. Hay Clinton Cox </p><p>Micbael C Eagle is a cbemical engineer for tbe U S EPAk Omce of Radialion and Indoor Air, and is also tbe project manager for several engineering projects witbin tbe US. EPA. rrmUarnS Richardson, Pb.A, is a prqfessor of cbemistty at Auburn Uuiversity in Montgomery9 Alabama and an assaciate witb S Coben &amp;Associates, Inc., McEea% Vi@niu, wbere be is a consultant in tbe daelapment of cbemical andpbysical metbods for site remediatioa Scott S Hay is a pbysical scientist witb expetience in radiocbemistty and ettvinmnaeutac remediatioa He is presently a senior radioaualyst for S Coben 6. Assdates, I=., in Montgomery9 Alabama. Clinton Cox is a U S Public Healtb Service o&amp;er working for tbe U S E?A He directs tbe safe&amp; bealtb, and enuimmwntalpmgratms for tbe National Air and Radiation Environmental </p><p>Montgowmy, Alabama Laboratoryin </p><p>7he 0,Dce of Radiation and Indoor Air of the U.S. Environmental Protection Agency has demonstrated a soil washingplant for the treatment of radioactively contaminated soils from two Supwfund sites in New Jmey. Theplant mploys unit operations that are widely used in theprocessing of minerals and coal. These operations were examined and tested to deter- mine how they would apply to volume reduction of these contaminated soils. In this context, they are considered to be innovative candidates for remediation of othersites with large volumes ofsoil contaminated with low- level radioactivity. Laboratory testing ofsoil characteristics and behavior in unit processes is used to assess the applicability of volume reduction/ chemical extraction (VORCE) technology to spcil;:c sites. </p><p>One of the missions of the U.S. Environmental Protection Agencys (EPA) Office of Radiation and Indoor Air (ORIA) is to support EPA regional offices in the remediation of Superfund sites contaminated with radioactive material. Responding to that mission, ORIA initiated the VORCE (Volume ReductiodChemical Extraction) Program in 1989 to perform site charac- terization and treatability studies and, using the results of those studies, to develop site-specific processes for reducing high volumes of soils contami- nated with low concentrations of radionuclides. The resulting program consists of an innovative laboratory protocol for soil characterization, bench-scale testing for process development, and testing both process development units (PDUs) and site-scale plants to demonstrate the field capability of the developed process. </p><p>ORlA has recently demonstrated a soil washing plant for the volume reduction of radium-contaminated soils from two Superfund sites in New Jersey. These sites, on the National Priorities List (NPL), are the Montclaid West Orange Radium Site and the Glen Ridge Radium Site, both located in Essex County. An estimated 323,000 cubic yards of soil are contaminated to varying degrees with radioactive waste materials, allegedly originating from a radium-ore processing plant or utilization facility that operated in the vicinity during the early part of this century. High disposal cost was the impetus for ORIA to investigate volume reduction technology as a potential method for remediation. The VORCE soil washing plant is the result of that </p><p>REMEDIATION/SUMMER 1993 327 </p></li><li><p>investigation. The soil washer employs several physical processes common to the </p><p>coal and mineral industries, including attrition, screening, and wet classification, which can be applied to the remediation of not only the Montclair site, but also other sites with soil similarly contaminated by low- level radioactivity. These processes are performed as unit mechanical operations in which soil particles that contain most of the radioactivity are liberated and subsequently separated from particles with little or no radioactivity. </p><p>TIERS OF THE VORCE PROGRAM A Superfund treatability study is designed to support the investigation, </p><p>evaluation, and ultimate implementation of treatment alternatives at CERCLA sites (U.S. EPA, 1989b). A treatability study for radioactive soils under the VORCE Program generally comprises a four-tiered testing program to assess physical separation as a viable option for remediation and to develop the treatment protocol if the assessment indicates that contaminant removal can be achieved by these processes. The four tiers consist of the following: </p><p>1. Soil chamctmzation quickly and inexpensively determines if volume reduction by physical methods is feasible. </p><p>2. Bench-scale testing is designed to determine if volume reduction technology can meet the performance goals for site remediation. </p><p>3. Process development units (PDUs) are designed to demonstrate the applicability of and develop the volume reduction system; it is usually developed in a laboratory setting, but may include on-site testing and demonstration. </p><p>4. fifotplunt development is designed to provide detailed cost, design, and performance data on a field-scale system. In some cases, the system may become the actual plant used in site remediation, as small sites may not require a larger plant. </p><p>The results of each tier are used to decide whether to proceed with the following tier or to end the treatability study. The cost for a complete study may be estimated by assuming that the expense for each tier is approxi- mately three times that of the preceding one. For example, if soil characterization cost $50,000, then the total cost to demonstrate a pilot plant might be $50,000 plus $150,000 plus $450,000 plus $1,350,000, for a total development cost of $2 million. A field-scale system of about twenty tons per hour may cost between $2 million and $2.5 million to develop and demonstrate from tier one to tier four and about $225 an hour to operate. Actual construction would be about half the total cost. </p><p>Soil Characterization Characterization of representative soil samples provides the initial </p><p>information to determine if volume reduction is technically feasible. Characterization identifies physical differences in the soil constituents that </p><p>328 REMEDIATION/SUMMER 1993 </p></li><li><p>Son. WASHING FOR VOLUME REDUCXION OF R~DIOAC~VELY CONTAMINATED SOILS </p><p>The simplest approach to characterization begins by separating soil particles by size and determining the distribution of radionuclides among the soil sixe fractions. </p><p>can be exploited to separate contaminated soil particles from clean particles. Thus, soil characterization should provide information about both the contaminated and uncontaminated soil particles. For volume reduction, the VORCE Program may employ one or more exploitable differences between contaminated and clean particles: </p><p>Size Specific gravity Particle shape Magnetic properties Friability Solubility Wetability Radionuclide concentration </p><p>The simplest approach to characterization begins by separating soil particles by size and determining the distribution of radionuclides among the soil size fractions. Mineral content, physical form, specific gravity, and other physical properties of the Contaminated and the clean particles are determined by petrographic, physical, and radioanalytical methods (U.S. EPA, 1992). If the radioactivity is largely associated with a certain size fraction or mineral, unique properties of these constituents can be exploited for volume reduction. For example, if the radioactive contami- nation is associated with the mineral monazite, then either monazites high density or intermediate magnetic susceptibility might be employed to remove the particles from the soil. A highly contaminated size fraction might be separated by screening or hydroclassification methods. </p><p>Indeed, the most important soil characteristic used in the plant designed for the New Jersey soils was radionuclide distribution among the various sized particles. Most of the radioactivity was found to be concen- trated in specific size fractions that could be separated from other clean fractions. Because small soil particles, such as silt or clay, have a much greater surface area per unit volume than larger soil particles, such as pebbles or sand, many soil contaminants concentrate on the surface of the smaller particles simply because a given volume of these particles provides a greater surface area for absorption than an equal volume of larger particles. More importantly, clay minerals, which are generally less than two microns in size, have a high cation exchange capacity and often provide good adsorption sites for cations found in most fission products (Cs137, SrgO, Se79), activation products (0, Ni59), and ore-processing </p><p>. products or mill tailings (Ra226, U235/238, Th230). It is important that soil characterization reports include an assessment </p><p>of the technical feasibility of volume reduction based on specific soil characteristics determined during the study. If volume reduction is considered feasible, the report should also include a conceptual flow diagram for a proposed volume reduction process accompanied by supporting technical information necessary for planning the next phase of the study. The soil characterization study/feasibility assessment would, in </p><p>REMEDIATION/~UMMER 1993 329 </p></li><li><p>Figure 1. General Flow Diagram for Bench-Scale Testing. </p><p>turn, be used to decide whether to proceed with bench-scale testing or to conclude the study. Therefore, those making the decision must be experienced in soil washing techniques in order to make an informed decision about the feasibility of volume reduction by these methods. </p><p>General Approach to Bench-Scale Testing Bash for Volume Reduction </p><p>Bench-scale testing is used to develop and test a volume reduction process that has been selected, based on the results obtained during soil characterization. Bench-scale testing employs, on a small scale and in a batch sequence, the general techniques of particle liberation, particle separation, and dewatering. A general flow chart for the sequence of bench-scale testing is shown in Figure 1. </p><p>Particle separation processes divide a mixture of soil particles into two or more volumes. Particle separation is typically the first step in the process and is used to separate part of the clean soil particles from the bulk soil. This first step is often dry screening with a grizzly or similar device to separate rocks and other large material that may contain little contamina- tion and, due to their size, may damage the process equipment down- stream. Wet screening and hydroclassification are other examples of particle separation techniques commonly used downstream from the rough screening techniques. </p><p>During particle liberation, contaminated soil particles are released from clean particles, resulting in a mixture of both unattached contaminated and clean particles. Attrition is one example of a particle liberation process, typically performed in an attrition mill, that removes contaminated coatings from soil particles. After the liberation step, particle separation is again used to segregate the mixture of liberated contaminated coatings from clean particles. </p><p>Liberation and separation are almost always performed in water, because aqueous processes are generally more effective than dry pro- cesses. They have the added advantage of minimizing the suspension of small radioactive soil particles in the air, which would otherwise pose an </p><p>330 REMEDIATION/SUMMER 1993 </p></li><li><p>Son. WASHING FOR VOLUME REDUC~ON OF RADIOAC~~VELY CONTAMINATED SOILS </p><p>Table 1. Particle Liberation Techniques. </p><p>Technkpe Washing m i n e -on andGrinding De-Bonding -hing Surface </p><p>Basic water action moderate particle/ vigorous particle/ size reduction surfactant action prindple particle action particle action </p><p>Genetal trommel, washer, trommel, trommel, mill crushers, trommei, mill Equipment screw classifier screw classifier mill grinders </p><p>La, Test stirring units, trommel trommel crushers, trommel Equipment trommel, mill grinders </p><p>elutriation column </p><p>airborne inhalation hazard. Dewatering the contaminated volume be- comes an important unit operation because there are restrictions on the amount of free water in disposed waste. </p><p>The flow chart shown in Figure 1 is quite simple but will likely grow in complexity and specificity as the bench-scale testing progresses toward the design of a PDU. Other steps will likely enter into the process. Recycled wash water may require decontamination from a buildup of dissolved and/ or suspended radionuclides, or soil streams may require recycling in order to produce different separations and liberations that will further concen- trate the contaminant or bring the stream to an acceptable level of remediation. Monitoring streams are necessary in these cases, and methods to assay the streams must be evaluated for each design. Soil reconstitution may also be necessary to restore the clean fraction to the original volume and consistency required to fill the excavated space. </p><p>Particle Liberation For volume reduction of radioactively contaminated soils, particle </p><p>liberation is used to remove small particles from larger ones and to break up aggregates of particles. Soil particles smaller than twenty microns in size, which may contain a major part of free contamination, tend to adhere to each other, as well as form friable coatings around larger clean particles. Liberation techniques may be employed to break up the aggregates and to remove clay-sized particles from larger clean particles. The common unit operations for liberation are illustrated in Table 1. ORIA employs four of the methods: washing, scrubbing, attrition, and crushing and/or grinding. Because use of surfactants tends to increase the solubility of many radionuclide contaminants in water, it is normally not applicable. </p><p>Washing employs water action to provide a mild force to detach one particle from another. Use of water sprays to remove slime from gravel-size soil particles is an example of washing. </p><p>Scrubbing employs washing and adds particle surface-to-surface </p><p>REMEDIATION/SUMMER 1993 331 </p></li><li><p>Attrition d i f f m j h m scrubbing in that a stronger firce ia applied to uupply the </p><p>action between particlea. </p><p>8Ul@Ct?-tO-SUrfa;ce </p><p>action with other particles or equipment surfaces to provide a moderate force for liberation. Tumbling action, taking place in a rotating trommel or an auger that is used for wet soil transportation, is one example of scrubbing. </p><p>Attrition differs from scrubbing in that a stronger force is applied to supply the surface-to-surface action between particles. Such a force is generated as opposite-pitched blades turn in an attrition mill. Because the action may remove particle surface coatings, care must be taken not to generate excessive amounts of noncontaminated fines, as they will reduce the overall recovery by adding clean fmes to the radioactive ones. Also, dewatering fines requires a significant effort. </p><p>Crushing and grinding equipment can be used to reduce the size of particles. A wide variety of size-re...</p></li></ul>