Soil washing for volume reduction of radioactively contaminated soils

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  • Soil Washing for Volume Reduction of Radioactively Contaminated Soils

    Michael C. Eagle William S. Richardson Scott S. Hay Clinton Cox

    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 &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&er working for tbe U S E?A He directs tbe safe& bealtb, and enuimmwntalpmgratms for tbe National Air and Radiation Environmental

    Montgowmy, Alabama Laboratoryin

    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.

    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.

    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

    REMEDIATION/SUMMER 1993 327

  • investigation. The soil washer employs several physical processes common to the

    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.

    TIERS OF THE VORCE PROGRAM A Superfund treatability study is designed to support the investigation,

    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:

    1. Soil chamctmzation quickly and inexpensively determines if volume reduction by physical methods is feasible.

    2. Bench-scale testing is designed to determine if volume reduction technology can meet the performance goals for site remediation.

    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.

    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.

    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.

    Soil Characterization Characterization of representative soil samples provides the initial

    information to determine if volume reduction is technically feasible. Characterization identifies physical differences in the soil constituents that

    328 REMEDIATION/SUMMER 1993

  • Son. WASHING FOR VOLUME REDUCXION OF R~DIOAC~VELY CONTAMINATED SOILS

    The simplest approach to characterization begins by separating soil particles by size and determining the distribution of radionuclides among the soil sixe fractions.

    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:

    Size Specific gravity Particle shape Magnetic properties Friability Solubility Wetability Radionuclide concentration

    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.

    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

    . products or mill tailings (Ra226, U235/238, Th230). It is important that soil characterization reports include an assessment

    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

    REMEDIATION/~UMMER 1993 329

  • Figure 1. General Flow Diagram for Bench-Scale Testing.

    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.

    General Approach to Bench-Scale Testing Bash for Volume Reduction

    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.

    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.

    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.

    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

    330 REMEDIATION/SUMMER 1993

  • Son. WASHING FOR VOLUME REDUC~ON OF RADIOAC~~VELY CONTAMINATED SOILS

    Table 1. Particle Liberation Techniques.

    Technkpe Washing m i n e -on andGrinding De-Bonding -hing Surface

    Basic water action moderate particle/ vigorous particle/ size reduction surfactant action prindple particle action particle action

    Genetal trommel, washer, trommel, trommel, mill crushers, trommei, mill Equipment screw classifier screw classifier mill grinders

    La, Test stirring units, trommel trommel crushers, trommel Equipment trommel, mill grinders

    elutriation column

    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.

    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.

    Particle Liberation For volume reduction of radioactively contaminated soils, particle

    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.

    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.

    Scrubbing employs washing and adds particle surface-to-surface

    REMEDIATION/SUMMER 1993 331

  • Attrition d i f f m j h m scrubbing in that a stronger firce ia applied to uupply the

    action between particlea.

    8Ul@Ct?-tO-SUrfa;ce

    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.

    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.

    Crushing and grinding equipment can be used to reduce the size of particles. A wide variety of size-reduction equipment, such as roll crushers, is available. Grinding will also produce additional fines that will be collected in the small, nonremediated fraction, thus reducing the percent recovery of the remediated fraction.

    To consider the feasibility of particle liberation for a given site, the soil must be properly characterized to determine if the contaminants are concentrated in particle coating and/or friable particles or aggregates. Characterization of soil particle size and contaminant distribution before and after scout liberation tests will be necessary in this case.

    As noted in the previous section, particle liberation does not always come before particle Separation. If possible, particles with few or no unattached contaminated particles should be separated before the appli- cation of a liberation step.

    Partick Separation The general unit operations for particle separation are listed in Table

    2. Those applicable to the VORCE Program are screening, wet classifica- tion, gravity separation, magnetic separation, and flotation.

    These general techniques have an extensive history of practical application. Table 2 also provides information on the equipment available to perform these processes. It does not include novel separation tech- niques or techniques for the selective separation of ultrafine particles (below twenty microns), such as dielectric separation or immiscible-liquid separation, or dry unit operations, such as electrostatic separation.

    Considerable information is available on each of these unit operations from the minerals processing industry. Each represents a number of different types of specific technologies and equipment. For example, wet classification may be performed by liquid cyclones, spiral classifiers, solid- bowl centfiges, and similar devices.

    This field equipment is commercially available, and its operational characteristics are well known. Graduation from bench scale through a PDU to a subsequent demonstration plant will likely be easier with this equipment than with technologies based on less refined methods and equipment. However, commercial-scale processing units use much larger flow rates and volumes than those of bench-scale units or PDUs. During the feasibility and development studies, it is important to remember that

    332 R.EMEDUTION/SUMMER 1993

  • Son. WASHING FOR VOLUME RBDucno~ OF RADIOACTIVELY CONIAMINATED Sons

    Table 2. Particle Separation Techniques.

    Common Name

    Basic Principle

    screening

    various diameter openings and effective particle size

    Lab Test EqUlPmat

    inexpensive

    screens can plug, fine screens are fragile, dry screens produce dust

    screens, sieves

    vacuum sieve/ screen, trommel screen

    classification sravity magnetic separation

    faster vs. slower differences in magnetic setting, particle density, size, susceptibility density, size, shape, and shape of particles weight of

    particles

    continuous economical, simple to processing, long simple to implement history, reliable, implement, inexpensive long history

    difficulty with ineffective high operating

    and humus soils clayey, sandy, for fines COStS

    mechanical, jigs,.shaking magnetic non-mechanical tables, troughs, separators hydrodynamic sluices classifiers

    elutriation columns

    jig, shaking table lab magnets

    flotation

    suspend fines by air agitation, add promoter/ collector agents, skim oil froth

    very effective for some panicle sizes

    contaminant must be small fraction of total volume

    flotation machines

    agitair laboratory unit

    test systems may operate at a smaIler scale than the minimum capabiIities of off-the-shelf equipment. Therefore, care should be taken before bypassing scale-up studies to assure that the final design will operate within the defined parameters of commercially available equipment. Concern for public acceptance of such large equipment is also a vital factor to be considered.

    Dewatering Dewatering is the removal of free water from a mixture of soil and

    water. Like liberation and solid-solid separation techniques, considerable information is available on solid-liquid separations. The equipment is classsed by the.drking force used in the separation. The main categories of dewatering equipment for volume reduction are listed in Table 3.

    For the contaminated soil fraction that requires transportation and subsequent disposal, dewatering is especially important in order to comply with disposal site moisture limits, as well as to lower the weight and volume

    REMEDIATION/SUMMER 1993 333

  • MCHAzL c. EAGLE WILLJAM s. RICHARDSON SCOm s. H A Y CLINTON C O X

    Table 3. Dewatering Techniques. Sedimentathn Expression Technique Filtration Centrffugadon

    Bask Prindple

    labTestEquipment

    passage of particles through porous medium particle size

    simple operation, more selective separation

    batch nature of operation, washing

    be poor

    drum, disk, horizon- tal (belt) filters

    vacuum filters, filter press

    dicial gravity settling: particle size, shape, density, and fluid density

    fast, large capacity

    expensive, more complicated equipment

    solid bowl sedi- mentation and centrifugal, perf^ rated basket

    bench or floor centrifuge

    gravity settling: compression with partide size, shape, density, and fluid porous filter density; flocculent aided

    liquid escape through

    simple, less handles slurries expensive equip- difficult to pump, ment, large capacity drier product

    slow high pressures required, high resistance to flow in cases

    cylindrical continu- batch and continuous ous clarifiers, rakes, pressure overflow, lamella, deep cone thickeners

    cylindrical tubes, filter press, pressure beaker, flocculents equipment

    of material. Particles larger than 4 mesh (4.75 mm) are relatively easy to dewater with vibrating screens, but unfortunately, the radioactive contami- nants are usually associated with the smaller soil particles. The selection and development of a solid-liquid separator for volume reduction pro- cesses will be as challenging as the development of the liberation and separation steps, and considerable planning and testing may be required to develop the most appropriate method.

    Bench-Scale Testing Bench-scale testing provides the first indication of the applicability of

    volume reduction unit processes to soil remediation. The tests provide the fraction of soil that does not require expensive disposal, often at a distant location. The activity of representative soil samples, as well as the activity of the clean volume, is also provided. The VORCE plant, for example, produces more than 50 percent volume reduction of 40 pCi/g Montclair soil with a clean fraction at 11 pCi/g, but some contaminated soils have the potential for remediation of over 80 percent of the total soil.

    Examples of bench-scale test equipment for each of the liberation and separation techniques are listed in Tables 1 and 2. Vendors can supply a list of additional equipment, and they will usually provide advice and, in some cases, technical assistance with bench-scale testing. Not all bench- scale equipment is available off-the-shelf, and improvisation may be

    334

  • Son. WASHING FOR VOLUME REDUCITON OF RADIOACTIVELY CONTAMINATED Sons

    Figure 2. Vertical Column Hydroclassifier.

    Soil Sample . .

    required to apply the unit operation at this level. ORIA, for example, found it difficult to obtain suitable equipment to test washing processes and, in response, developed a bench-scale device from an orbital stirring table that provided vigorous washing action. ORIA also adapted a bench-scale vertical column hydroclassifier (Hay et al., 19911, shown in Figure 2, that separates particles based on differences in settling velocities. This equip- ment was vital in the assessment of the behavior of soil streams under hydrodynamic separation conditions, and its use contributed considerably to the development of the separation process.

    During bench-scale tests, it becomes increasingly important to com- pare the performance capabilities of bench equipment to corresponding field units. Otherwise, tests may not take into account operational limitations of field equipment, possibly leading to a false conclusion about the applicability of a particular field-scale unit. Successful separation of contaminated fines with a laboratory fiIter may falsely lead to the conclusion, for example, that filtration of fines is feasible with field equipment. With careful examination of the operational parameters of the available field equipment, one might determine that field-scale filtration would not be suitable for a size separation below a given particle size, and another dewatering method would have to be selected.

    ~~ ~ ~~

    REMEDIATION/SUMMER 1993 335

  • -.it u naxsuuy to review mil char acterixdon 8tWlie8 to determine if d i f f k r e m in partick shape and den8ity are 8ignificant variables.

    Moving from separation by screening and filtration to wet classification is not only an increase in scale but also a change in separation techniques. Screens make a separation by effective particle size, whereas a hydrody- namic classifier, following Stokes Law, makes the separation based on differences in settling velocities. With many soils, these two separations will provide similar results, but significant differences in particle density, particle shape, or the amount of entrained air may give dissimiar products. Thus, it is necessary to review soil characterization studies to determine if differences in particle shape and density are significant variables. More importantly, it is necessary to conduct bench-scale separations with a hydroclassifier, similar to the one shown in Figure 2, to assess the effect of differences in these parameters.

    Process Development Unit (PDV) The development of a PDU may be considered an extension of the

    bench-scale batch process to a continuous operation that may ultimately be demonstrated on-site. Although the PDU employs equipment that is bench-scale or slightly larger, its development and demonstration are designated as a separate tier of the treatability study because it represents a significant effort that bridges the smaller to the larger process.

    The decision to develop a PDU is based on favorable results from the bench-scale testing. From the batch tests, a continuous process is devel- oped that begins to simulate a field system addressing many operational issues not addressed during bench-scale testing. The technical necessity of developing the PDU is matched by its importance in helping to obtain the publics acceptance of on-site treatment as a viable alternative to complete removal of contaminated material.

    PDU development may be divided into two general steps: off-site development followed by on-site development. The off-site development should be conducted at or very near the bench-scale testing facility in order to make effective use of the expertise, support structure (e.g., radiation counting facilities), and available bench-scale equipment. On-site devel- opment is conducted to take advantage of larger quantities of test soil and to develop the process under actual field remediation conditions.

    Pilot Plant Construction of a pilot plant is the final tier in a treatability study. It is

    the culmination of the development protocol established to produce a viable soil treatment plant and, naturally, represents the largest step, physically and fmancially, in the program. If the four-tiered development is rigorously followed, the pilot plant may, in many cases, actually become the field treatability plant. Only a decision to remediate a site in a shorter time period or to expand the remediation area significantly would require a plant with greater capacity than the pilot plant itself.

    ORIA has recently tested a pilot plant to determine its effectiveness in reducing the volume of radioactive soil at the MontclaidGlen Ridge Superfund site. Its development and construction employed most of the elements of the tiers described below. The plant operation uses several of

    336 REMEDIATION/SUMMER 1993

  • SOIL WASHING FOR VOLUME REDUCXION OF UDIOACTIVELY CONTAMINATED Sons

    ~

    The primary goal of the project was to recover a significant 80il fraction with a specific activity less than IbpCifg above

    the processes already described and was constructed primarily from off- the-shelf mechanical units.

    Soil Test Results Characterization of representative Montclair soil samples demon-

    strated that water-insoluble forms of radium-226 are distributed primarily in small-sized soil particles (Richardson et al., 1990). Wet screening revealed that the 40 pCi/g soil could be physically separated at 200 mesh (74 microns) into two categories: (1) a larger-sized particle fraction with contamination below 15 pCi/g representing over half the contaminated soil, and (2) a smaller-sized particle fraction with contamination greater than 15 pCi/g. (A 5/15 pCi/g remediation standard had been adopted for the study because cleanup criteria had not been specifically established at that time. The site record of decision (ROD) later established these criteria.)

    Bench-scale studies also demonstrated that these fractions could be effectively separated by hydroclassification, indicating that the densities of the soil particles were similar (Hay et al., 1990, a fact later confirmed by density measurements. Studies also indicated that soil pretreatment by vigorous washing (Richardson et al., 1990) was required to produce a +200- mesh fraction with activity less than 15 pCi/g and that attrition would provide an additional, significant reduction in activity of the +200-mesh fraction. Adjunct tests demonstrated that the small-sized material could be recovered by filtration after the process waterwas clarified by a polyanionic flocculating agent. The process water could be recycled because the contaminants are primarily insoluble radiobarite, unextracted carnotite, and clay particles with adsorbed radium cations (U.S. EPA, 1989a).

    Construction and Operation With off-the-shelf mining equipment available for most units used in

    the system, a pilot plant was built to test the applicability of the bench-scale processes. The primary goal of the project was to recover a significant soil fraction with a specific activity less than 15 pCi/g above background.

    Figure 3 illustrates the basic components found in the process stream of the plant. The hopper/grizzly loads the whole soil and separates rocks larger than two inches (+2 inches). The material smaller than two inches (-2 inch) is conveyed at one to five tons per hour upward into the trommel where soil particles are vigorously washed with a high-pressure water spray and separated by tumbling. From the opposite end of the trommel, the +l/.I-inch rock is collected following passage over a 1/4- inch screen, and the -1/4-inch material passes through the screen to the first screw hydroclassifier. The +1/4-inch rocks are further washed with a water spray and collected in drums.

    The first classifier performs a preliminary separation at approximately 60 mesh (250 microns). The +60 material is moved up the classifier from the bottom of the settling pond into the attrition mill where scrubbing action removes additional surface contamination from the +60 particles. At the same time, the -60 material passes over the edge of this classifiers pond and is pumped to the hydrocyclone sump.

    REMEDIATION/SUMMER 1993 337

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  • Son. WASHING FOR VOLUME REDUCTION OF R~DIOAC~~VELY CONTAMINATED Sons

    Table 4. Recovered Products.

    Wet Ra-226 productstfeafn we%ht Percat Acth.ity

    (Size) Name (Percent) Solids (pCi/gdry) ~

    +2 in R o c k s 11 33.00 6.1

    -2in / +'/4 ina Gravel 17 68.65 18.0

    -'/4 in / +140 mesh Sand 18 80.00 9.2

    16.7 Screen 57b 12.7'

    - -60 mesh / +200 mesh Oversize - 11 73.70

    -200 mesh Filter Cake 43 68.65 44.6d

    a This designation is for material smaller than 2 inches but larger than I/4 inch. Total weight percent of remediated material (+200 mesh). Combined specific activity of remediated product stream. This activity is low because 10 pCi/g soil was in the system at start-up.

    The scrubbed +60 material from the attrition mill is conveyed to the second screw hydroclassifier, which, in turn, makes a cut at 140 mesh (105 microns), depositing the +140 sand from the bottom of the pond into a drum, while the -140 material is passed to the hydrocyclone sump.

    The slurry in the hydrocyclone sump is fed to the hydrocyclone which makes a 200 mesh cut. The +200 material drops from the hydrocyclone to a vibrating screen while the -200 mesh material is recycled through the hydrocyclone sump. The vibrating screen makes a sharp cut at 200 mesh; the -200 material is pumped to the clarifier while the +200 material is collected in a drum.

    The combined -200 mesh material is mixed with an ionic polymeric flocculent to increase the particle size of this material for faster and more effective settling in the clarifier. The flocculated material is collected at the bottom of the clarifier, pumped to the filter press, and separated by filtration into a frne filter cake that is collected in a drum. The process water, having been cleaned by the flocculatiodclarification processes, is recycled through the wash system.

    Results Table 4 lists characteristics of the five product streams recovered

    during the test of 40 pCi/g Montclair soil. The remediated material consists of the first four products in the table

    (rock, gravel, sand, and oversize screen); the nonremediated material is the last product listed, the filter cake. The combined remediated product stream represent 57 percent by weight of the treated soil; the radium-226

    REMEDIATION/SUMMER 1993 339

  • The typical remedial action for mila contaminated with radioactivity is total removd and disposal,

    ~ onen at sites far I r e m o v d h m t h e

    contaminated area.

    specific activity of the material in the dried state is 12.7 pCi/g, including background. It is important to note that the recovered material in nature would be about 90 percent solids instead of those percentages reported in Table 4. If one assumes that the nonremediated filter cake would remain at 68.65 percent solids and the remediated material would dry to 90 percent solids, the recovered material is 54 percent by weight. In this case, the specific activity of the remediated soil would be 11.2 pCi/g above background.

    From the standpoint of actual volume reduction, the percentage of remediated material is determined from volume of filter cake produced relative to the total volume treated. The volume percent recovered is 54 percent of the total volume of soil treated. The remaining volume, 46 percent, is nonremediated material.

    As described above, water from the filter press is recycled through the process. Samples collected from the filtrate contain less than 86 pCi/l radium-226.

    ECONOMICS The typical remedial action for soils contaminated with radioactivity is

    total removal and disposal, often at sites far removed from the contami- nated area. This action may often be easier to justify than others, as it is well accepted by the local community and provides a seemingly permanent solution to the problem. The proposed remedy for the MontclairNest Orange Radium Site is excavation, transportation, and off-site disposal of all radium-contaminated materials exceeding the cleanup criteria.

    When it can be justified, the volume of radioactive contaminants should be reduced by concentration and/or separation for easier and less- expensive transportation and disposal. The cost of developing and implementing volume reduction technology is easily justified by the lower cost for a reduced disposal volume.

    In order to demonstrate the savings generated by volume reduction, the cost of treatment by soil washing coupled with disposal of the nonremediated soil fraction is calculated and compared to disposal only of the total soil. The illustrations presented are based on treating 200,000 yd3 at twenty tons per hour (tph) with a 50 percent volume reduction. Many contaminated sites exist with volumes ranging from 10,000 to 400,000 yd3; volume reduction can vary from 35 to 80 percent.

    Table 5 illustrates the capital investment and operating cost of the plant system in 1993 dollars. Remediation would require twenty-three months in this scenario, with a treatment cost of $21/yd3.

    Table 6 illustrates the comparisons of total disposal alone to total treatment and disposal of the nonremediated soil fraction. Three disposal options are illustrated (Waddell, 1992) with an average disposal rate of $550/yd3 used in the comparison (a conservative estimate). The 20 tph operation produces a net savings of $50,300,000 for the remediation effort. By comparison, if the local disposal rate of $135/yd3 were used in the 20 tph plant, savings would be $8,800,000. In another example, 50,000 yd3 is treated by a smaller 5 tph plant represented by the Montclair pilot plant

    340 REMEDIATION/SUMMER 1993

  • Son. WASHING FOR VOLUME REDUCTION OF RADIOAC~VELY CONTAMINATED Sons

    Table 5. Operating Cost of 20 TPH Plant.

    capital Illvestmmt Plant Construction" Site Development Site Utilities Plant Tmnsportation/Setup

    operatlngcoet Cost per Month

    Directb Indire@ Overheadd

    Opemtive Time (M0s.Y Total Operating Cost

    Cost per Yd3

    Treatment Coste

    Total Cost Per Yd3

    $1,070,000

    !i 98,380 23,000

    $137,380 23

    iG,000

    $3,173,540

    $16

    $4,243,540

    $ 2 1

    a Includes equipment. design, management, labor, and testing. Labor, electricity, water, tractors, expendables. Supervisor, security, maintenance, expendables. Administrative, field ofice, personnel, expendables. Full-time operation at 85 percent availability. Treatment of 200,000 yd3 (200,000 yd3 at 50 percent reduction).

    inflation not considered. 8 Capital investment and operating cost without taxes and insurance in 1W3 dollars,

    described above. Development and production of the plant would be less costly but similar to that of the 20 tph plant; operating cost would be virtually the same. Treatment would take twenty-four months. If the $550 average disposal cost is used in this example, disposal would require $27,500,000, whereas treatment and disposal of 50 percent of the total volume would cost approximately $18,200,000 for a net savings of $9,300,000. Clearly, treatment of a smaller volume of contaminated soil with a pilot plant is a viable option not requiring construction of a larger plant. Recovery of more than 50 percent, possible in some cases, would yield a greater savings.

    On consideration of these examples, it is clear that volume reduction by soil washing is a cost-effective remediation option that should be considered in any feasibility study. The capital cost is relatively fixed, and operating costs can be controlled by the size of the plant. Sites from less than 50,000 yd3 upward are candidates. Disposal rates must be low before

    RFMEDIATION/SUMMER 1993 341

  • Table 6. Comparison of Disposal to Volume ReductiodDisposal. (200,000 yd3 at 20 TPH)

    Disposal' Local DOE Average

    Rate (S/ydY COStb 135 $27,000,000

    1,865 373,000,000 550 $ 110,000,ooo

    Volume Reductionc/Average Disposal

    Treatment Costb Average Disposalb Backfillb

    $ 4,200,0OOd 55,000,000

    Hx)*000 Total Volume ReductiodDisposal $59,700,000

    Disposal Only $1 1 0,000,000

    Net Savings $50,300 ,OOO

    a Transportation to disposal site and disposal (Waddell, 1992). 100,OOO yd' treated and 100,OOO yd3 disposed. 50 percent volume reduction. S21/ydJ (see Table 5).

    volume reduction is eliminated from consideration. Even then, other requirements may mandate volume reduction to reduce the total disposal volume accumulating in depositories in the United States.

    CLEANUP- A clear prerequisite for developing volume reduction processes for

    I-adioactive soils is to have weildefmed cleanup criteria for the site. Unresolved criteria questions will make it much more difficult to perform a treatability study. In some cases, a study may actually be conducted to assess the practicality of acceptance criteria within the bounds of acceptable risk factors. For a ueatability study, the acceptance criteria for soil should be approved and documented for the following four general categories:

    1. Maximum specific activity of soil that can remain on- or off-site without restrictions.

    2. Maximum specific activity of soil that may be reused on- or off-site with defrned restrictions.

    3. Range of activity of soil that requires local off-site disposal. 4. Activity of soil that requires out-of-state disposal.

    The acceptance criteria for each of the above categories will likely be more complex than a simple activity level for a specific radionuclide. For

  • Son, WASHING FOR VOLUME &.DUCTION OF RADIOACI-IVELY CONTAMINATED Sons

    I I I Site characterization

    information that is wed to prevent the unintentional contamination of clean mil with contaminated soil during excavation...

    aIeoprovide8

    the MontclairNest Orange site, the ROD sets the acceptable level for the soil near the surface at 5 pCi/g above background; levels for soil distant from dwellings may not exceed 15 pCi/g. (The ROD also sets the definition of background at the site as 1 pCi/g.) This cleanup standard for radium in soil is driven by an acceptance standard for indoor radon concentrations. (Radon is a gaseous daughter of radium that enters the home by migration through the soil and permeation through basement walls and foundations.) This complicates the 6 pCi/g criteria (5 pCi/g plus 1 pCig background) because the permeability of the soil and its distance from the basement walls must also be considered. In general, ambiguities in the acceptance criteria will have a significant impact on the development and application of a volume reduction process. Considerable effort should be devoted to ensure a clear understanding of the acceptance criteria before proceeding beyond the characterization tier of a treatability study.

    SITE CHARACTERIZATION It is important initially to characterize the contaminated site to obtain

    information on the type of radiation present and the specific activity and location of radioactive contaminants. For large sites with underground contamination, it is important to collect samples from a three-dimensional matrix to evaluate their distribution. This information permits a reasonable estimate of the total volume of contaminated soil that must be treated and, in turn, an evaluation of the economic feasibility of volume reduction. Site characterization also provides information that is used to prevent the unintentional contamination of clean soil with contaminated soil during excavation and to minimize excavation of uncontaminated soil. It is important to locate the contaminated areas and to selectively excavate only those volumes for treatment or disposal. Finally, the site characterization provides representative soil samples from the site used in soil character- ization and bench-scale studies.

    CONCLUSION Physical liberation and separation methods are widely used in process-

    ing .ore and coal. These processes are well characterized and considerable information is available on their operation. These proven methods are excellent candidates for use in volume reduction of soils contaminated with low levels of radioactivity and have been demonstrated to be effective in the tests with Montclair soil. Physical separation can significantly lower the high cost of remediation of sites with radioactive soils by reducing the soil volume that must be disposed of. Therefore, physical separation technologies should be considered during the feasibility studies for Superfund and other sites. Soil characterization will provide preliminary information on the feasibility of volume reduction, liberation, separation, and collection of clean and contaminated fractions. Bench-scale test results effectively lead to a preliminary design that will correlate well with field equipment. The equipment, commonly used in the coal and ore industries, is commercially available or relatively easy to manufacture and operate.

    The VORCE demonstration plant effectively separates over 50 percent

    REMEDIATION/SUMMER 1993 343

  • of the Montclair soil contaminated with radium-226, producing a fraction with approximately 11 pCi/g activity, In a conservative estimate, volume reduction could reduce the cost of remediation by 25 percent.

    It is important to have established cleanup criteria when doing the studies and developing the processes. Site characterization is a valuable aid in planning the use of plant equipment and will greatly enhance the overall planning and development process. M

    REFERENCES

    Hay, S., W. Richardson, C. Cox, J. Stinson, and C. DuBose. 1991. 'Comparison of Wet Sieving to Vertical Column Hydroclassification for Soil Particle Sizing." U.S. Public Health Service Annual Meeting, Atlanta, GA.

    Richardson, W.S., T.B. Hudson, J.G. Wood, and C.R. Phillips. 1930. "Characterization and Washing of Radionuclide-Contaminated Sois from New Jersey." P A Workshop on R a d t ~ t f W & Contamtnated Sites. EPA 520/1-30.009.

    U.S. Environmental Protection Agency (EPA). 1989a. Cburucterlzution of Cofituminafed Soil from tbe MontckMGIen Rkige, New Jmq, S u m n d S i t e s . EPA 520/1-89-012.

    U.S. Environmental Protection Agency (EPA). 1989b. GurCie for Conducting Treatability Studies under CERCLA. EPA 540/2-89/058.

    U.S. Environmental Protection Agency (EPA). 1332. "Characterization Protocol for Radio- active Contaminated Soils." EPA 9380.1-10FS.

    Waddell, J. 1332. 'Costing Methodology Used in FUSRAP FS-EIS Documents." In F U S W Cbotcesil: ExpIotjns RemedkaIAction Alternutim. U.S. Department of Energy, Oak Ridge, TN.

    344 REMEDIATION/~UMMEX 1993

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