© 2001 by CRC Press LLC Chapter Six Stabilization and Solidification Technologies © 2001 by CRC Press LLC 6.1 Overview of Mixed Waste Solidification and Stabilization Methods Vincent C. Maio Idaho National Engineering and Environmental Laboratory Idaho Falls, Idaho Introduction A variety of treatment methods exist for the various mixed waste types currently existing and being generated in both the government (e.g., the Department of Energys nuclear weapons complex) and private sector (e.g., the medical and nuclear power industries). Fundamental treatment methods for mixed waste consist of thermal destruction, nonthermal destruction, chemical treatment, physical treat- ment, treatment for separating specific chemical fractions (radioactive and/or nonradioactive), and/or immobilization through stabilization that may or may not involve thermal or solidification methods. Specific technologies to accomplish these methods include, but are not limited to, incineration, plasma melting, wet air oxidation, acid digestion, pH adjustment, surface decontamination, filtration, evapora- tion, ion exchange, solvent extraction, and cement grouting. In most mixed waste treatment applications, combinations of two or more of the above methods are required to produce mixed waste forms acceptable for final disposal. However, independent of which methods are selected, a low-temperature solidification/stabilization (S/S) process is usually required to accomplish a significant step in the overall mixed waste treatment train. Its popularity is a result of its simplicity and successful treatment history, because S/S has been effectively used in general waste treat- ment since the 1950s 1 and is presently identified by the Environmental Protection Agency (EPA) as the best-demonstrated available technology (BDAT) for 57 hazardous waste streams. 2 Specifically for mixed waste, S/S methods are used as either the principal and primary treatment step, or are involved in treating the secondary waste produced by other mixed waste treatment processes. For example, incineration will destroy any hazardous or nonhazardous organics residing in the mixed waste stream, thereby greatly reducing its volume, but it will also generate potentially toxic and radioactive off- gases, fly ashes, bottom ashes, and spent off-gas scrubbing solutions. With the exception of the off-gas, these secondary streams will more than likely contain hazardous metals and radionuclides requiring further solidification/stabilization. Likewise, hazardous metals and radionuclides are routinely removed from mixed wastewaters to allow reuse of liquid streams. Such separation processes are frequently accompanied by an S/S stage to treat the concentrated mixed waste residue. Before the final land disposal of a mixed waste that has been stabilized and/or solidified, the party responsible for the waste treatment must ensure that the final waste form meets requirements and criteria that have been independently established for both the hazardous and radiological constituents residing in the waste. For most situations, the hazardous constituents (characteristic and/or listed) will be regulated © 2001 by CRC Press LLC by the EPA, and will require treatment to meet the Land Disposal Restrictions (LDRs) as defined by the Resource Conservation and Recovery Act (RCRA). In addition, some disposal sites require that the radiological constituents be controlled by meeting guidelines that have been established for commercially generated low-level waste (LLW) as recommended by the Nuclear Regulatory Commission (NRC). Treatment requirements can be further complicated by the existence of special criteria established at specific disposal sites. As such, it is highly recommended that the desired disposal site be contacted before establishing S/S treatment paths to ensure that the final mixed waste form meets the specific set of specifications applicable to that site. Regardless of the disposal site, it is usually advantageous to create both highly durable and low-volume mixed waste forms, provided the chosen treatment process is both economical and practical. Low-temperature solidification/stabilization (S/S) technologies have proven to be effective in meeting these criteria without generating either off-gas emissions or significant secondary waste streams. This subsection provides an introduction to low-temperature S/S methods as they apply to the ex situ treatment of liquid and solid mixed wastes (i.e., remediated and/or containerized wastes that are both RCRA hazardous and radioactive). Thermal stabilization processes, (e.g., vitrification), are partially omitted from the discussion. Unlike low-temperature methods, they create secondary wastes, produce off-gases, destroy organic constituents, and require complex equipment and control. It is important to note that the information provided in this subsection is only relevant and correct for low-level mixed waste or waste that has both radioactive and hazardous components. As such, some of the information and conclusions provided in regard to economics, chemistry, physics, and regulations may not hold applicable for only hazardous waste. This subsection provides a general description of S/S technology, along with definitions for the terminology associated with its use. Mixed waste applications and specific S/S technologies are discussed, as well as descriptions of the associated equipment. Sections addressing S/S produced waste form per- formance and S/S economical considerations are also provided. A reference section and bibliography are provided for those seeking additional information. S/S Technology Descriptions and Terminology Definitions Stabilization refers to the practice of employing various additives and/or binders for the primary purpose of rendering the hazardous and/or radiological constituents in the mixed waste less toxic, soluble, and/or mobile. 3 The additives and/or binders accomplish this through chemical and/or physical means. One of the most common methods of stabilization involves the addition of chemicals that lower the solubility of the hazardous and/or radioactive constituents in the mixed waste, thus substantially lowering its leachability to the environment. For example, nonhazardous sulfides, hydroxides, and phosphates are frequently used in binders and additives in an attempt to convert the highly soluble RCRA metal salts and oxides (e.g., CdCl 2 and HgSO 4 ) residing in the mixed waste to relatively insoluble metal compounds (e.g., Cd (OH) 2 , and HgS). Solidification refers to the use of additives, binders, and admixtures that transform the mixed waste from a sludge, semisolid, liquid, or particulate form into a solid (i.e., a form that holds it shape without a container) containing no free liquid. Usually, the primary goals behind mixed waste solidification are to convert the waste medium into one that is easier to handle and store, while at the same time minimizing the hazardous and radiological component leaching potential by reducing the surface area of the waste exposed to the environment. 3 In addition, solidified waste forms reduce the risk of waste particulate dispersion during handling, storage, transportation, and disposal, and therefore increase safety for both the workers and environment. Solidification also results in an increase in compressive strength and a frequent decrease in permeability relative to the original mixed waste condition. The solid formed through solidification can be either a monolithic block or a dense pellet. However, many liquid waste streams are solidified for acceptable disposal without formation of a monolith. The use of an absorbent, such as vermiculite, to bound free liquids is occasionally practiced to facilitate handling, but by definition is not considered an acceptable solidification or treatment step. © 2001 by CRC Press LLC The term chemical fixation 1 is frequently used in the literature to define solidification or stabilization, or a combination of the two methods. 3 Usually the term is used to imply stabilization, but the available literature indicates use of the term to indicate solidification as well. Collectively, the term solidification/stabilization (S/S) refers to mixed waste treatment processes that involve at least one, but preferentially both of the mechanisms described above. 2 For example, polymer- based encasement of mixed waste is considered more of a solidification method than a stabilization method, although it is frequently classified as an S/S method. Use of a molten polymer to encapsulate a mixed waste will result in a solidified waste form upon cooling; but without the use of specific additives (e.g., sulfides), the polymeric material alone does little to reduce the actual toxicity of the hazardous and/or radiological species residing in the original mixed waste. It does reduce the mobility and solubility of the contaminants by providing a physical hydrophobic barrier between the waste and the environment; however, if the barrier is compromised, a pathway is provided for leaching of the contaminants to the environment. In contrast, simple cement grouting of mixed waste is considered a solidification and stabilization method although it is more porous than polymeric substances. Hydration reactions that occur during setting and curing of the alkaline cement-waste grout mix create relatively insoluble haz- ardous and radioactive metal hydroxides, chemically lowering the leachability of some of the constituents in the waste. In addition, cement-based S/S methods reduce the mobility of inorganic compounds by forming insoluble metal carbonates and silicates, substituting the metal into a mineral structure, and physically encapsulating the waste. Thus, even if the internals of the cement waste form are exposed to the environment directly, the waste contaminants retain a resistance to any leaching. However, over time, the reserve alkalinity of the cement waste form may be comprised, which will eventually increase the leachability of some of the toxic metals. Additionally, many of the cement bonding mechanisms are limited in their ability to retain or chemically fix certain radionuclides (e.g., 60 Co). Solidification/stabilization (S/S) processes are broadly classified as either inorganic or organic, although some methods have recently been developed that incorporate both classes of materials. Traditional inorganic methods such as simple portland cement, natural/man-made pozzolans, or low-temperature ceramics involve complex hydration and/or simple acid-base cement chemistry to both solidify and stabilize. Because these methods require hydration reactions, they can be used for treating both aqueous and solid wastes. On the other hand, traditional organic-based/polymeric methods (e.g., polyethylene solidification) simply encapsulate the waste and inherently do not mix well with aqueous wastes. Thus, these methods are not generally applicable to liquid wastes because they usually involve a closed extrusion process. However, if a direct mixing process is utilized, certain organic-based methods, such as those employing polyesters, are applicable to aqueous wastes. Macroencapsulation using organic-based material results in waste forms that are coated on only the outside surface, resulting in a polymer jacket around the original solid mixed waste. The use of sealed plastic containers to contain mixed waste debris for disposal is also currently in practice, and for some specific applications is considered acceptable macroencapsulation. In contrast, microencap- sulation involves a homogeneous mix of both the polymer and the solid mixed waste. As a consequence, macroencapsulation is frequently used for large debris waste or lead monoliths, and microencapsulation is employed for mixed waste particulate, soil, sludges, and crushed debris. For mixed waste organic-based solidification applications, polymers can broadly be classified as either having thermoplastic or thermosetting properties. Thermoplastics, such as low- and high-density poly- ethylene, are usually noncross-linked linear polymers that melt and become viscous at a specific transition temperature. They can be melted, reworked, and returned to their original form upon cooling. They require no reaction to solidify, and the mixed waste material does not interact with the polymer chemistry. Basic microencapsulation operations involve simply mixing the waste with the polymer in the melting phase of an extruder, and pouring the homogenous polymer-waste mix in a disposal drum for cooling and hardening. Thermosetting polymers, on the other hand, cannot be reworked, reformed, or remelted. They are usually cross-linked, rely on polymerization reactions to solidify, and decompose upon over- heating. Unlike thermoplastic resins, the waste can react and interfere with the thermosetting resin reactions. Polyester resins, as well as epoxies, are classified as thermosetting. © 2001 by CRC Press LLC Over the past 15 years, a considerable mixed waste S/S development effort has been expended toward enhancing or replacing the baseline S/S methods of cement grouting and polymer encapsulation. The majority of these development activities have focused on increasing the actual mass-based waste loading and/or decreasing the volume of the final waste form. This objective is usually based on the perception that reduced final waste form volumes, usually produced as a consequence of increased waste loading, translate into an overall more economical S/S process. For many applications, this assumption is valid because the volume-sensitive costs associated with the handling, shipping, and especially disposal of the final mixed waste form usually have a larger impact on the total S/S life-cycle cost in comparison to other S/S expenditures (i.e., labor, capital, material, and design). However, each S/S application is unique and the economics are dependent on many variables and factors, including the original mixed waste volume. Savings (or increased costs) realized through the development of alternative S/S methods that achieve higher waste loadings and/or greater volume reductions are directly proportional to the amount of a given homogenous mixed waste inventory to be treated. Established baseline technologies may already be adequate for relatively low waste volumes because insufficient waste inventories may not achieve the savings required to recover the costs associated with developing and deploying an alternative method. Additionally, higher waste loadings are often accompanied by increased contaminant leaching over time and/or result in poor-quality waste forms with inferior physical properties. Although a decreased product quality may still be within the criteria established for disposal, the potential for decreased public trust and acceptance is also a cost that must be weighed accordingly. Some confusion and inconsistency have arisen in regard to methods used to compare the waste loading and volume reduction of competing S/S methods. Mixed waste loading is usually calculated as a dry mass/weight percent of the final waste form as determined by Equation (6.1.1): [M W /M WF ] × 100 = Waste Loading (%) m (6.1.1) where M W is the dry mass of the original mixed waste just before the solidification/stabilization step, and M WF is the mass of the final waste form containing M W . Volume reduction is the percent difference in volume between that of the original mixed waste and that of the final waste form, which for many S/S applications has experienced some level of compaction and densification. Except for situations in which the original mixed waste volume is of a particulate form with a high void fraction and low bulk density, the volume of the final waste form is rarely less than that of the original waste. Therefore, the objective is to select an S/S method that minimizes the amount of volume increase from waste to waste form. Volume reduction (i.e., the negative value of the volume increase) is normally calculated via Equation (6.1.2): 100 × [V W 0  V WF ]/[V W 0 ] = Volume Reduction (%) V (6.1.2) where V W 0 is the volume of the original, uncompacted unstabilized mixed waste before any pretreatment (including that of evaporation), and V WF is the volume of the final, usually densified and compacted, waste form. An example indicating the differences in calculating waste loading and volume reduction during S/S is provided below. Example Problem In a HEPA filtered laboratory hood permitted for waste storage and treatment, 50 kg of a dry, radiolog- ically contact-handled mixed waste sludge (characteristically RCRA hazardous for cadmium metal only) occupies 45 L in an open 55-gal standard waste barrel. In an effort to exit RCRA and dispose of the barrel in a Department of Energy (DOE) low-level waste (LLW) landfill, cement-based and pozzolanic stabilizing agents, along with the appropriate amounts of water, are added to the waste and the mix is allowed to set and cure. Toxicity Characteristic Leaching Procedure (TCLP) results on a crushed 100-g sample of the treated waste form confirm that the cadmium leach levels pass Universal Treatment Standards (UTS) for land disposal. Department of Transportation (DOT) shipping forms required for transport of the © 2001 by CRC Press LLC barrel to the LLW disposal site document that the net weight of the waste barrel is 90 kg and the cement compacted monolith inside it occupies ~ 40 L. Calculate the waste loading and waste form volume reduction. Solution: Because the waste is defined as already dry, the waste loading is simply determined as [100 × 50 kg/90 kg = 55.55 wt%], and the volume reduction is calculated as [100 × (45 L  40 L)/45 L = 11.11 vol%]. Usually mixed waste inventories that are amenable to (or are candidates for) low-temperature S/S methods have been poorly characterized and/or were generated from complex processes involving a multitude of physical, thermal, and/or chemical steps. As a consequence, the mixed waste may have unknown properties (e.g., high pH) and/or contain species (e.g., excess oils, reactives, excess salts, nuisance metals, specific radionuclides) that will interfere with the selected S/S options and/or deteriorate the waste form over time. To avoid costly large-scale S/S deployment failures, prescreening and small- scale testing of candidate S/S methods, followed by appropriate performance testing of the resultant waste form, is almost always recommended. In addition to identifying the most optimum S/S method for deployment, the prescreening and small-scale testing will establish the necessity of any pretreatment steps. If the testing is performed with small samples of the actual mixed waste, it is usually designated to the EPA as a treatability study. As detailed in 40 CFR sections 261.4 Parts e and f, the treatability study designation provides exemption from many RCRA permit requirements, and allows the prescreen- ing test to occur more readily as long as the proper notifications are made, the amount of mixed waste treated is less than 1000 kg (10,000 kg for soil waste), and the treatability study residues are properly managed. Prescreening through surrogate testing may be the only option if the radiological levels are excessive, but extreme care must be taken when using simulated wastes to determine the performance of candidate S/S methods. The omission of even the most inconsequential species in the surrogate, as a result of incomplete mixed waste characterization, can lead to failure. Low-Temperature Mixed Waste S/S Applications Numerous mixed waste types are suitable for solidification/stabilization based on operating experience and/or testing with both actual mixed waste inventories and surrogates. Low-temperature ex situ S/S methods are most suitable for inorganic, radiologically contacted-handled mixed waste consisting of a liquid or a solid medium that is homogeneous and/or particulate, such as soils and dry sludges. The classes of mixed waste that have been frequently and effectively immobilized via S/S methods include contaminated soil, baghouse dust, collected particulate, wastewater treatment sludges/residues, evapora- tor bottoms, scrubber blowdown generated from the treatment of off-gases, incinerator bottom and fly ashes, various pond sludges, concentrated aqueous wastes, and transuranic homogeneous solid waste. Specific waste streams consist of paint chips, paint sludges, ion exchange resins, mixed waste sludges resulting from the treatment of high level wastes, unconcentrated salt wastes, and previously unsuccess- fully stabilized waste forms. S/S treatment is also applicable to inorganic/organic absorbents, inorganic chemicals, low reactive metal chips and turnings, crushed glass, and crushed ceramic. Various debris waste, if properly sized, are candidates for S/S, especially micro- or macroencapsulation techniques. As a general rule, S/S methods are not applicable to mixed wastes containing significant quantities (e.g., >10% by mass) of large debris, oily sludges, organic liquids, high concentrations of salts, reactives, and/or explosives. Although limited S/S methods are available for mixed wastes containing nonhalon- genated, halongenated, semivolatile, and/or volatile organics, nonpolar and hydrophobic organics do not consistently react well with many inorganic or organic binders. The best demonstrated available tech- nology (BDAT) for organic-based mixed waste is still incineration or its equivalent. Salts (defined as the reaction product generated when a metal ion replaces the hydrogen ion of an acid) are highly soluble. As a consequence of this characteristic, mixed wastes containing appreciable salts can sometimes affect the set and cure rate of inorganic, cement-based binders, and/ or result in waste forms that are susceptible to deterioration over time due to the salt mineral expansions in the © 2001 by CRC Press LLC macropores of the waste form microstructure. This deterioration may lower the durability and strength of the stabilized waste form and create pathways for the hazardous and radiological constituents to be released from the immobilized waste over time. 4 Because a considerable number of past DOE processes involving the formulation of nuclear materials required the use of metals and acids, a significant inventory of mixed waste containing salts was produced. As introduced in the preceding subsection, considerable development has therefore occurred to validate S/S methods for this mixed waste class. Most of the successful development has centered on the use of redox chemistry during pretreatment or stabilization to help immobilize and lower the leachability of the hazardous constituents, despite the long-term detrimental effect of the salts on the waste form structure. Low-Temperature Mixed Waste S/S Methods The most common inorganic-based, low-temperature, ex situ S/S additive for mixed waste, as well as hazardous and/or low-level waste (LLW), is simple portland cement. The use of ordinary portland cement as a waste solidification and stabilization medium has been in practice for decades. Because cement hydration reactions occur at low temperatures, generate no off-gases, chemically bind aqueous wastes to the matrix, and are relatively inexpensive, they are excellent choices for providing the S/S treatment of many mixed waste types. Dry, basic, portland cement clinker is manufactured by calcining natural limestone (CaCO 3 ) and clay materials at 1400 to 1500°C and then subsequently crushing it to a powder. The resultant cement binder product usually consists of specific combinations of the following three basic oxides: silica (SiO 2 ), lime (CaO), and alumina (Al 2 O 3 ). At a minimum, the specific oxide combinations consist of the following three crystalline compounds:  Tricalcium silicate: 3CaO-SiO 2  Dicalcium silicate: 2CaO-SiO 2 Tricalcium aluminate: 3CaO-Al 2 O 3 For most cements, these three compounds make up ~80% of the dry matrix, and their proportions determine which of the eight American Society of Testing and Materials (ASTM) standard types of portland cements are formulated. Sulfur-containing gypsum (CaSO 4 ) is also added when making portland cements. The role of the gypsum is to slow down the cement setting during hydration (i.e., the addition of water). The tricalcium aluminates and gypsum react with water to form a mineral (i.e., ettringite) and gel that coats the remaining unhydrated cement clinkers. Setting is slowed because the water must diffuse through this mineral barrier to hydrate the remaining cement components. This extends the time the cement waste mix is workable before complete setting. Under normal conditions, the rate of hydration is such that after 28 days, only two thirds of the cement has reacted with water. However, the rate and extent of hydration and curing are greatly dependent on the type of contaminates in the waste, as well as the amount of water added to the waste-cement mix. The hydration reactions between the dry cement powder, the mixed waste, and any added water can be complex, and even the most simple cement hydration mechanisms are not yet fully understood. In general, the bulk of the crystalline calcium silicates in the cement reacts with water to form amorphous hydrated calcium silicates (also known as tobermorite gels or 3CaO-2SiO 2 -3H 2 O), and crystalline slaked lime [also known as calcium hydroxide (Ca(OH) 2 ) or portlandite]. The tobermorite gel, which causes setting, is the main cementing component of the waste form and the presence of the soluble slaked lime in the pores causes the cement form to be basic (i.e., pH ~11.5). As a favorable consequence of the high pH, the acetic acid solution used as an extract in the RCRA required Toxicity Characteristic Leaching Procedure (TCLP) test can be immediately neutralized and will lose its ability to leach any hazardous RCRA metals. The presence of the hydroxide will also result in the favorable conversion of any RCRA hazardous metals present in the waste to their less-soluble hydroxide form. Other portland cement-based stabilization reactions include formation of carbonates, silicates, and substitution of the metals into cement hydration products. The slaked lime is also somewhat soluble, and over long time periods in a © 2001 by CRC Press LLC wet environment will diffuse through the pores and leach out of the cement waste form. Because the Ca (OH) 2 can constitute over 30% of the waste form, its slow removal from the waste monolith over extremely long time periods can seriously degrade it. However, the benefit that Ca(OH) 2 provides in buffering the leachability strength of any acidic solutions that the waste form is exposed to substantially exceeds any long-term negative consequences of this degradation. During cement hydration and the onset of setting, some of the tobermorite gel is formed from a sol (i.e., a homogeneous dispersion of fine solids in a liquid) that follows the principles of colloidal chemistry. According to these principles, the formed tobermorite sol coagulates into floccules or gel substances. The gel then precipitates to cementing solids once the static charge is lost. Because this precipitation or setting step is greatly affected by the ionic strength of the sol, the presence of salts in the mixed waste can significantly impact the cements setting rate in either direction (i.e., accelerate or retard the set). Once all the gel has settled, it begins to dry and crystallize in what is designated as the cement hardening or curing phase. During this curing phase, crystalline slabs and needles are created that decrease the porosity and increase the strength of the waste form. 5 In addition to simple portland cement grouting, inorganic S/S methods for mixed waste include  but are not limited to  methods involving mixtures of portland cement and various other binders or additives, such as extra gypsum, clay, lime, soluble silicates, and other natural or man-made pozzolans. For some applications, these various binders and additives can also stand-alone as a S/S method. Pozzolans are siliceous and/or aluminous material. They are substances, naturally occurring or pro- duced as industry by-products, that acquire some cement-like characteristics when activated by calcium hydroxide. As such, the use of these additives is cost-effective because a special manufacturing step is not required, as in the case of portland cement. The fly ash generated from coal combustion and other pozzolans contain a glassy silica phase that reacts with the slaked lime in cements during hydration to form a calcium silicate hydrate (CSH) gel, which is the main setting agent of cement. Blast furnace slag, another popular pozzolan, is produced as a by-product of the iron and steel industry. Its earthly constituents come from iron ore processing and it consists of the same oxides as portland cement, but in different proportions. Immediately after its production, the slag is usually quenched for rapid cooling in a process known as granulation. The granulation results in a reactive amorphous glass and avoids any crystallization. Like portland cement, blast furnace slag also reacts with water (i.e., is hydrated) to form hydrated calcium silicates or tobermorite gels. However, unlike simple portland cement, it forms this critical cementing agent (tobermorite gel) by consuming the slaked lime, Ca(OH) 2 , provided by the hydration of the portland cement. Removal of some of the slaked lime is advantageous because less of it in the waste form will lead to less dissolution of the lime over time, and consequently less long- term waste form degradation. In addition, there will be less slaked lime available to potentially react with salts, and thus produce undesired expansive and destructive minerals in the future. Blast furnace slags also routinely contain sulfides that react with and lower the solubility of many hazardous metals in the mixed waste. In addition, it has been postulated that because blast furnace slag is amorphous and not crystalline, it hydrates to tobermorite gel without forming the colloidal solutions commonly observed for the calcium silicates in simple portland cement. Because sols are not formed, the ionic nature of the salts may not affect the tobermorite gel formation or set rate of the waste form. Sulfur polymer cement (SPC) is another inorganic-based S/S method recently developed for applica- tion to mixed waste. Specifically, the use of SPC as a mixed waste S/S method has been investigated by the DOEs Brookhaven National Laboratory, Oak Ridge National Laboratory, and the Idaho National Engineering and Environmental Laboratory. Originally, SPC was developed by the U.S. Bureau of Mines in an attempt to use waste sulfur in materials that may serve as commercial alternatives to construction cement. The development resulted in a high-strength, quick-setting cement substitute, which is currently only manufactured by Martin Resources of Odessa, Texas. SPC is manufactured by reacting solid sulfur with small amounts of the inexpensive chemical modifiers di- and monocyclopentadiene. 6 The presence of sulfur makes it especially attractive to mixed wastes containing mercury because sulfur tends to readily amalgamate with hazardous mercury, thus lowering its leachability. Unlike other inorganic S/S methods, © 2001 by CRC Press LLC heated SPC exhibits thermoplastic properties and thus has a low enough melting point and viscosity to be used in a manner similar to that of organic or polymeric methods (i.e., deployment equipment consisting of heated and stirred vessels). Deployment of the SPC process on the mixed waste market to date has been limited. However, a commercial mixed waste treatment and disposal facility may implement the SPC S/S method developed at Brookhaven National Laboratories. Although not as widely deployed as inorganic-based methods, organic-based S/S methods are expe- riencing increased use for mixed wastes. For more than 20 years, various organic polymers have been used to encapsulate hazardous and low-level waste materials because their chemically inert properties usually allow for higher waste loading than that achievable with cements. Polymers in general have moderate to excellent resistance to the acids, bases, and organics present in many mixed waste streams. Both the thermoplastic and thermosetting polymeric stabilizing materials are normally formed through either chain- or condensation-type polymerization reactions involving one or several types of monomers. In most waste solidification operations, the waste media is mixed with a melted thermoplastic preformed polymer or gets microencapsulated during a controlled polymerization reaction involving a thermosetting plastic. The inherent resistance of organic polymers to water favors the low leachability potential of a final waste form, but also presents challenges in encapsulating wastes with high water contents. However, the development and use of water-extendable polymers and emulsifiers has led to the encapsulation of many aqueous wastes. The most prevalent organic-based thermoplastic polymer in use for both the macro- and microen- capsulation of mixed waste appears to be polyethylene. Polyethylenes natural resistance to chemical, microbial, and solvent attacks, as well as its increased performance in high-radiation fields, makes it suitable for most dry and homogeneous mixed waste streams. Manufactured via the polymerization of ethylene gas, polyethylene is an inert crystalline/amorphous substance with a relatively low melting point. High-density polyethylene (e.g., 0.95 g/cm 3 ) contains little branching, as opposed to low-density poly- ethylene (e.g., 0.92 g/cm 3 ). Low-density polyethylene is the preferred type for waste encapsulation because it also has a lower melt temperature (e.g., 120°C) and viscosity. 6 Thermosetting microencapsulation techniques involving polyesters, urea formaldehyde, epoxies, poly- urethane, polybutadiene, ester-styrene, and polysiloxane have also been demonstrated on surrogate and/or actual mixed wastes. Bitumen, generated from the distillation of petroleum crude, has been widely used in Europe and Japan as another organic-based material for mixed waste encapsulation. However, its use in the United States has been limited as a result of fire safety concerns. Additionally, numerous tests with unique, low-temperature, microencapsulation techniques involving sol-gels (e.g., polycerams) and novel thermosetting polymers indicate that greater waste loading (i.e., greater than those achievable with conventional portland cement) may be possible with even the troublesome salt-containing mixed wastes. The sol-gel polycerams consist of inorganic and organic compounds linked together by strong silicon oxide bonds. The inorganic and organic liquid precursors are intimately mixed with the waste in a sol, which then gels to a solid waste form following a series of hydration and condensation reactions. Although potentially more costly, there are many other mixed waste S/S technologies at various stages of development and/or deployment that could be considered as competing with or as adequate as the well-established baselines of portland cement grouting or polyethylene encapsulation. Mixed waste sta- bilization methods currently in the later stages of development by the DOE include enhanced concretes using proprietary additives, phosphate-bonded ceramics, and several methods provided by commercial vendors. The chemically bonded phosphate ceramic (CBPC) S/S process, developed at Argonne National Lab- oratory, is one of several phosphate-based, low-temperature waste S/S processes available. CBPC is unique in that it is formed at room temperatures like a portland cement, but has some properties of a ceramic. The ceramic strength of CBPC is derived from its acid/base chemistry, which produces strong covalent bonds. Acid/base cements have been in existence for over 50 years, but their application as a mixed waste S/S method has only recently been evaluated. 7 As given by Equation (6.1.3) below, low-temperature stabilization of mixed waste with CBPC is based on the acid/base exothermic reaction between magnesium oxide (MgO) and monopotassium phosphate © 2001 by CRC Press LLC (KH 2 PO 4 ) binders. The binders are ground to a powder and blended. The MgO is also frequently calcined beforehand to reduce its reactivity. The reaction produces MgKPO 4 , hydrated by six moles of water. Additionally, pozzolanic Class C or F fly ash is routinely added to the binders and waste to increase waste form strength and integrity. Under most conditions, heat from the reaction causes a temperature increase up to less than 80°C, until the waste form starts cooling upon curing. MgO + KH 2 PO 4 + 5H 2 O → MgKPO 4 · 6H 2 O) → MKP·6H 2 O (6.1.3) The hard, insoluble, stable, and dense ceramic of MKP·6H 2 O acts as a crystalline host matrix for the mixed waste. The RCRA hazardous heavy metals and radioactive contaminants in the waste also react with KH 2 PO 4 to form insoluble phosphates. In addition, the phosphate minerals (e.g., monazite) formed are natural hosts to radioactive elements and are also insoluble. The final waste forms routinely have compressive strengths greater than 2000 psi and porosities less than those fabricated of cement. The density of the ceramic waste form (~1.8 g/cm 3 ) is also routinely less than that of a cement form (~2.4 g/cm 3 ). A 50 wt% concentrated phosphoric acid (H 3 PO 4 ) solution can be substituted for the KH 2 PO 4 binder to form the insoluble newberyite, MgHPO 4 ·3H 2 0, ceramic. However, the MKP system is usually perferrred over the acid system because testing has indicated that it generates less heat and improves leachability performance. Although considered high-temperature methods, alternatives involving nonvitrification, thermal sin- tering techniques may also lead to acceptable waste forms with considerably more volume reduction compared to that achievable with grouts, polymers, or low-temperature ceramics. Thermal-based stabi- lization involving sintering methods differs from vitrification in that only melting at grain phase bound- aries occur without the complete amorphous restructuring that takes place in glass formation. Like vitrification, sintering occurs at temperatures above 1000°C and can emit volatile hazardous metals. Although densification is possible for some additional volume reduction, slight volume increases usually occur. However, a waste loading as high as 80% is possible. The equipment for sintering is less complex than vitrification, but more complex than grouting or microencapsulation. For a typical sintering process, grinding, mixing, and extruding equipment are required, as well as ovens, calciners, and off-gas treatment systems. For most waste streams, sintering methods will require an extensive process development effort involving statistically designed experiments. Testing to date indicates that none of the above-described alternative S/S technologies clearly outper- forms the others or the baselines. Potential end users will need to consider factors other than waste form performance in choosing an alternative. These factors include but are not limited to  the issue of stabilization versus encapsulation, the availability of equipment, previous operating experience, the applicability of the technology to other types of waste media, the cost of development, and issues involving both safety and stakeholder concerns. 8 S/S Equipment Equipment systems for mixed waste S/S methods are not unlike those that have been used in the hazardous waste industry. Designers of mixed waste S/S processing systems must account for the presence of radioactive fields and protect the worker accordingly. Although most applications will result in contact- handled environments, advanced shielding and remote operations may be necessary. Operational systems for deploying most inorganic-based mixed waste stabilization processes involve simple, readily available equipment. Basic equipment components of any system will involve: waste feed, binder, and additive hoppers; waste feed transfer equipment; a mixer; and a mixing vessel and/or disposal containers. Either additives are preblended with other binders and fed from a designated single feed hopper, or dedicated feed hoppers for each additive and/or binder are installed and employed. Hopper weighing scales are used in most systems to ensure the mixing of accurate amounts of waste and additives. In-container or inline mixer systems are applicable for inorganic-based binder processes. Many existing systems in the non-DOE hazardous waste industry are inline, where the inorganic binders, additives, waste, [...]... nonflammability of the final waste forms, and, most importantly, (6) the ability of the process to treat a very wide variety of waste streams that cannot be handled by other methods Stabilization of Low-Level Waste The CBPC technology was deployed to treat and dispose of low-level mixed waste at the ANL-West facility. 26 Two debris wastes that contained hazardous metals and low-level fission product contaminants... demonstration and deployment of innovative technologies for mixed waste stabilization (Mayberry and DeWitt, 1983) Several of these innovative technologies are described in this chapter section Applications Aqueous waste streams (slurries and sludges), fine particulate wastes, high salt wastes, and high-volume environmental media are candidates for stabilization In practice, stabilization and the related technologies. .. Solidification and stabilization technology in Standard Handbook of Hazardous Waste Treatment and Disposal, 2nd ed., H.M Freeman, McGraw-Hill, New York, 7.31–7. 46 4 U.S Department of Energy, Oak Ridge National Laboratory, Evaluation of Final Waste Forms and Recommendations for Baseline Alternatives to Grout and Glass, ORNL/TM-13214, Oak Ridge, TN, September 1997 5 Spence, R.D., Ed., 1993, Chemistry and Microstructure... mixed wastes Specifications for the various types of Portland cements, hydraulic slags, and pozzolans manufactured in the United States are provided by the American Society for Testing and Materials standard methods, ASTM C-150, C-989, and C -6 1 8, respectively (ASTM, 1997) Portland cement Type I (general purpose), and Type V (sulfate resistant), are most often used in waste form application Type I Portland... Using Portland Cement, 2nd, EB071, Portland Cement Association, Skokie, IL American Nuclear Society 19 86 Measurement of Leachability of Solidified Low-Level Radioactive Wastes by a Short-Term Test Procedure, ANSI/ANS- 16. 1-1 9 86, The American Nuclear Society, La Grange Park, IL American Society for Testing and Materials 1997 Annual Book of ASTM Standards, Philadelphia, PA Atkins, M., Nelson, K., and Valtentine,... monthly journal Waste Management is an international journal of industrial, hazardous and radioactive waste management science and technology This journal is a forum for new developments in stabilization treatments The U.S DOE and the USEPA Office of Solid Waste and Office of Research and Development have also published many useful reports on general and specific mixed waste stabilization technologies. .. during thermal treatment, potentially causing hot spots that may require expensive control systems and equipment with demanding structural integrity.2 Some sites do not allow thermal treatments Therefore, there is a critical need for a low-temperature treatment -and- stabilization technology that will effectively treat the secondary wastes generated by high-temperature treatment processes and wastes that... performance-based standards (leaching limits) for wastes that display the RCRA characteristic of toxicity In contrast to treated listed hazardous/ mixed wastes, characteristically hazardous and mixed wastes can exit RCRA regulation if treatment (stabilization) results in the waste form no longer displaying the original characteristic(s) Treatment and disposal of the radioactive contaminants in mixed wastes... for treating hazardous, radioactive, and mixed wastes, debris, and contaminated environmental media The objective is to reduce the hazard (solubility, leaching, toxicity) of specific contaminants so that the resulting waste forms can be disposed of in approved/licensed facilities In general, the stabilization chemistry and technologies that are applied to hazardous and low-level radioactive wastes are... alpha- and gamma-radiolysis in similar waste forms They are comparable to a G(H2) value of 0 .6 molecules of H2/100 eV investigated for solidification of tritiated water, 0.095 ± 0.005 total molecules/100 eV for the total gas production in FUETAP concrete, and 0 to 0.43 total molecules/100 eV (combined alpha- and gamma-radiolysis) for simulated Hanford current acid waste and double-shell slurry wastes . organization/company: Bernan Associates, 461 1-F Assembly Drive © 2001 by CRC Press LLC Lanham, MD 2070 6- 4 391 Telephone: 1(800)27 4-4 447 (toll-free) Fax: (301)45 9-0 0 56 or 1(800) 86 5-3 450 (toll free) E-mail: query@bernan.com Web. C.C., 1998, Solidification and stabilization technology in Standard Handbook of Hazardous Waste Treatment and Disposal, 2nd ed., H.M. Freeman, McGraw-Hill, New York, 7.317. 46. 4. U.S. Department. waste storage and treatment, 50 kg of a dry, radiolog- ically contact-handled mixed waste sludge (characteristically RCRA hazardous for cadmium metal only) occupies 45 L in an open 55-gal standard