Environ. Sci. Technol. 1996, 30, 2155-2167
Catalytic Extraction Processing: An Elemental Recycling Technology
Department of Environmental Protection (MADEP) recycling certifications.
CHRISTOPHER J. NAGEL,* CLAIRE A. CHANENCHUK, AND ESTHER W. WONG
Chemical and other industrial manufacturers are facing unprecedented regulatory pressure in areas of pollution prevention, toxic use reduction, waste minimization, and hazardous waste disposal. The Clean Air Act (CAA) Amendments of 1990, the promulgation of the Land Disposal Restrictions (LDRs), and the upcoming reauthorization of the Resource Conservation and Recovery Act (RCRA) are providing industry with immense incentives to convert residual materials into salable products or to recycle on-site. Innovative technologies that target waste minimization, environmental performance, and economic issues will be essential in order to meet the increasing environmental standards for a clean environment. Catalytic extraction processing (CEP) is a patented technology that allows organic (1, 2), organometallic, inorganic, and ash (3-5) waste streams, which can be hazardous, nonhazardous, or radioactive (6), to be manufactured into marketable commercial products. CEP has been demonstrated to achieve and surpass existing U.S. Environmental Protection Agency (EPA) waste treatment standards [i.e., best demonstrated available technology (BDAT) standards] while also meeting EPA’s goals under important initiatives, such as the Combustion Strategy and the Pollution Prevention Policy.
Molten Metal Technology, Inc., 400-2 Totten Pond Road, Waltham, Massachusetts 02154
ROBERT D. BACH Department of Chemistry, Wayne State University, Detroit, Michigan 48202
Catalytic Extraction Processing (CEP) is an innovative Elemental RecyclingTM technology that converts organic, organometallic, and inorganic waste, byproduct, or process streams into marketable commercial products: industrial gases, metal alloys, and ceramics (e.g., inorganic oxides, halides, sulfides). Feed materials are injected into a molten metal bath where dissociation of molecular entities to their respective elements and reaction of these dissolved elemental intermediates to form products occur. Process chemistry is driven by reaction thermodynamics, solution equilibria, and metal catalysis, which allow specific partitioning of elements and conversion of feed materials to the desired products as predicted. Experimental results demonstrate CEP’s capabilities for waste minimization (e.g., product formation) and environmental performance (e.g., minimum emissions). Examples include the processing of highly toxic toluene diisocyanate production wastes (EPA RCRA listed waste K027), chlorinated organics (EPA RCRA listed waste F024), and mixed metallic, plastic, and inorganic wastes (weapon componentry). Synthesis gas, hydrogen chloride, ceramic, and metal products were manufactured from these waste materials. CEP consistently demonstrates destruction removal efficiencies (DREs) exceeding 99.9999%, NOx and SOx below detection limits (typically 1 ppm to 100 ppm), and dioxins/furans nondetectable to the targeted regulatory limit of 0.1 ng/Nm3 2,3,7,8 TCDD toxicity equivalent (TEQ). Condensed-phase environmental quality was verified by ceramic phases passing toxicity characteristic leaching procedure (TCLP) and the absence of hazardous organic constituents in both metal and ceramic phases. CEP’s environmental performance has been validated by the U.S. EPA’s best demonstrated available technology (BDAT) equivalency designations, while its manufacturing capabilities have been confirmed by Massachusetts
S0013-936X(95)00545-1 CCC: $12.00
1996 American Chemical Society
Background
Process Overview Figure 1 shows a schematic diagram of a typical CEP system. Under conventional CEP operation, solid iron (or other metal) is first loaded into the reactor and melted, typically by induction heating. Fluxes (lime, silica, etc.) are added to form an initial ceramic layer. Once both the metal and ceramic phases are molten, feed processing can begin. CEP can accept feeds in various physical forms. Gases, fine solids, pumpable liquids, and slurries can be fed in the bottom of the reactor through tuyeres, which are cylindrical concentric metal tubes. Shreddable solids can be added through submerged lances entering the top of the reactor and extending into the liquid metal reaction medium. Nonshreddable and bulk solids can be fed through zoned reactor configurations (e.g., baffle, lances) designed to ensure sufficient contact of the bath metal with the feed to effect complete dissociation/dissolution of the feed. Feed material first dissociates into its respective elements in the bath metal (elemental dissociation/dissolution) and proceeds to product formation (product synthesis). Consider a feed consisting of chlorobenzene (C6H5Cl) contaminated with cobalt oxide (CoO) entering the iron bath:
elemental dissociation/dissolution: C6H5Cl + CoO f 6C(l) + 5H(l) + Cl(l) + Co(l) + O(l) * Principal author to whom all correspondence should be addressed.
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Theory
FIGURE 1. Schematic diagram of CEP system.
product synthesis: C + O f CO(g) 2H f H2(g) Cl + H f HCl(g) or yCl + xM f MxCly (ceramic or gas phase product) Co(l) f Co-MB (metal alloy product) where M is metal, MB is bath metal, and X is dissolved intermediates of element X. The molten metal bath is a catalyst that participates in the conversion of discrete compounds into dissolved elements but is not consumed. It provides the environment for metal-induced dissociation/ dissolution of the feed into respective elements comprising the feed (C, H, Cl, Co, O). The catalytic effect of the molten metal causes complex compounds in the feed to be dissociated into their elements, which readily dissolve in the liquid metal solution to form dissolved elemental intermediates as a consequence of the binding energy between elements comprising the feed and the bath metal.
FIGURE 2. Gibbs free energy diagram for select oxides.
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Reaction Thermodynamics. CEP product synthesis is dependent on manipulation of the reaction pathways of feed elements through the addition of co-reactants and the variation in operating conditions. Reaction thermodynamics are important in determining the control of potential reaction pathways, assessing their relative likelihood of occurrence, and enabling the process robustness. Consider a Gibbs free energy diagram (or Ellingham diagram) for selected oxidation reactions (as shown in Figure 2). The left vertical axis is the free energy change for formation of oxides (per mole of oxygen), which is plotted versus temperature for a series of reactions. Each line on the diagram represents the reaction of a diatomic molecule or an elemental metal (M) with oxygen to form an oxide (MxOy):
2x/yM + O2 f 2/yMxOy In order to assess the feasibility of a reduction or an oxidation reaction, the Gibbs free energy changes accompanying these reactions at various temperatures and pressures are considered. Gibbs free energy (∆G) change is defined as
∆G ) ∆H - TδS where ∆G is the standard Gibbs free energy change, ∆H is the standard enthalpy change, ∆S is the standard entropy change, and T is the temperature. A negative Gibbs free energy change indicates the reaction is thermodynamically feasible. Using diagrams such as Figure 2, one can compare the oxidation Gibbs free energy change for various elements at a particular temperature and determine which reaction is more thermo-
formation is at equilibrium (7). The theoretical CO/CO2 ratio of 10 000:1 and H2/H2O ratio of 2000:1 (7) in CEP synthesis gas makes CEP distinct from open-flame organic feed processing systems such as coal gasification (8). Figure 3 compares experimentally observed CEP CO selectivity to that of other synthesis gas-producing technologies (9). CEP results were obtained from coal processing in a 6-t liquid iron capacity sealed reactor at the MEFOS (The Foundation of Metallurgical Research) facility in Lulea, Sweden. CEP was demonstrated to operate under a highly reducing environment (i.e., oxygen deficient), producing a marketable gaseous product with minimal impurities (e.g., carbon dioxide). FIGURE 3. Comparison of CO selectivity for CEP with various gasification technologies.
dynamically favorable. Consider the reactions of Co, C, Al, Si, Ba, and Ca at a temperature of 1500 °C, which is a representative operating temperature in CEP systems. Using carbon as the reducing agent, oxidation reactions with a Gibbs free energy change more negative than that for CO formation [2C + O2(g) f 2CO(g), ∆Gf ) -530.9 kJ/mol of O2] will be favorable. Hence, aluminum [4/3Al + O2(g) f 2/3Al2O3, ∆Gf ) -740 kJ/mol of O2] and calcium [2Ca + O2(g) f 2CaO, ∆Gf ) -896 kJ/mol of O2], which exhibit a more negative Gibbs free energy change than that for oxidation of carbon, will undergo oxidation and be recovered in the ceramic phase as metal oxides. The other metals, including nickel [2Ni + O2(g) f 2NiO, ∆Gf ) -167 kJ/mol of O2] and cobalt [2Co + O2(g) f 2CoO(g), ∆Gf ) -226 kJ/mol of O2], which exhibit a less negative Gibbs free energy change than that for oxidation of carbon, will remain in the metal and be recovered as ferroalloy. Similarly, carbon dioxide [2CO(g) + O2(g) f CO2(g), ∆Gf ) -260 kJ/mol of O2; C + O2(g) f 2CO2(g), ∆Gf ) -396 kJ/mol of O2] and water [2H2 + O2(g) f 2H2O, ∆Gf ) -297 kJ/mol of O2] formation are thermodynamically unfavorable under these conditions, as is the formation of undesirable byproducts such as NOx and SOx. This thermodynamic analysis, used for illustrative purposes, assumes simple, three-component reactions and does not account for the solution chemistry, atomic interactions, and non-idealities of a molten metal bath. Experimental results (i.e., partitioning of elements, minimal byproduct formation, etc.) have, however, verified the accurate prediction of the behavior of elements in CEP systems using thermodynamics. The theoretical partial pressure of oxygen in typical CEP systems is in the range of 10-16 atm when carbon monoxide
Solution Thermodynamics. Organic compounds and reducible materials including metals and certain radionuclides can be dissolved in the bath metal with the proper choice of operating conditions [e.g., temperature (T), pressure (P), moles of species (Ni ... Nn)). The metal bath actively participates in the dissolution of the feed material when the feed comes into direct contact with the bath metal and dissolves into the liquid melt (catalytic dissociation). The initial chemical entities of the feed disappear as the materials become uniform dissolved elemental intermediates. For example, the formation of a dissolved elemental carbon intermediate [e.g., Fe(3+γ)C] provides a discrete stage of material between organic feed and products, preventing the presence of partially converted feeds and therefore eliminating the bypass of feed materials. This solution property of the CEP system (formation of elemental intermediates) affords complete conversion of feed constituents and predictability in subsequent product generation. The favorability of carbon dissolution in iron is demonstrated by its highly negative free energy of solution (Table 1, columns 2 and 3) under CEP operating conditions (-53 kJ/mol). This thermodynamic driving force for the iron bath to act as a catalyst in the dissociation of elements (catalytic dissociation) is present not only for hydrocarbons (i.e., C containing) but also for more complex organic and inorganic components, as further demonstrated by negative Gibbs free energies of solution for oxygen, sulfur, and phosphorous in molten iron. Both hydrogen and nitrogen are liberated from the bath as molecular gases, consistent with their low solubilities in iron (i.e., positive free energies of solution) (13). By contrast, carbon and other elements exhibit low solubility in other mediums, such as molten salt, molten glass, and plasma. Hence, in the absence of a driving force for dissolution, these processes (i.e., molten salt technology, vitrification, and plasma processing) oper-
TABLE 1
Select Gibbs Free Energy of Elemental Intermediate Formation with and without Liquid Solvent liquid solvent (iron)
absence of solvent
elemental intermediate formation
Gibbs free energy of solutiona,b (J/mol)
Gibbs free energy of solution at 1500 °C (J/mol)
dissociated element
C(g) f Cd 1/ H (g) f H 2 2 1/ O (g) 2 2 1/ N (g) f N 2 2 1/ S (g) f S 2 2 1/ P (g) f P 2 2
22 594 - 42.256 T(K) 36 485 + 30.460 T(K) -117 152 - 2.887 T(K) 3598 + 23.891 T(K) -135 060 + 23.430 T(K) -122 173 - 19.246 T(K)
-52 336 90 495 -122 271 45 960 -93 519 -156 299
1/ H (g) f H 2 2 1/ O (g) f O 2 2 1/ N (g) f N 2 2 1/ S (g) f S 2 2 1/ P (g) f P 2 2
C(g) f C
Gibbs free energy Gibbs free energy of dissociation (J/mol)c of dissociation at 1500 °C (J/mol) 718 567 - 157.69 T(K) 222 132 - 57.47 T(K) 252 358 - 65.11 T(K) 476 310 - 64.81 T(K) 218 650 - 59.93 T(K) 246 671 - 58.46 T(K)
438 959 120 2291 136 908 361 392 112 385 143 013
a In liquid iron. b Extracted from Rao, Y. K. Stoichiometry and Thermodynamics of Metallurgical Process; Cambridge University Press: Cambridge, 1985. c Extracted from HSC Chemistry Version 2.0, Copyright Outokumpu Research Oy, Pori, Finland, A Roine. d C is the dissolved elemental C in the iron bath.
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FIGURE 4. Effect of carbon concentration on conversion performance.
ate largely in a thermal free radical mode, afford no distinct intermediate stage between feed and product (i.e., dissolved elemental intermediate), and may allow feed bypassing. Consider the Gibbs free energy of dissociation of graphite and several selected diatomic molecules that are relevant to CEP (Table 1, columns 4 and 5). The positive Gibbs free energy indicates that molecular dissociation is unfavored in the absence of a solution media. The use of dissolved elemental intermediates is a unique feature in CEP that is distinct from typical thermal treatment technologies. The CEP unit is not acting as a thermal treatment device in that temperature is not the primary means to change the physical and chemical composition of the feed materials as shown in Figure 4. Since the CEP reactions proceed through dissolved species, the solubility/concentration of elements can affect the rate of reactions. The affinity of metal for carbon dissolution can thereby enhance the conversion efficiency as exemplified in Figure 4. The lower the carbon concentration relative to saturation, the higher the forces driving the formation of the dissolved elemental carbon intermediate [i.e., Fe(3+γ)C(l)]. Figure 4 summarizes DRE data for polystyrene (composition: C, 60%; H, 40%; particle size: 0.5-1.5 cm) processing. The experiment was designed to study the effect of the bath carbon concentration on the off-gas composition. The experiment was processed at the temperature of 1500 °C with a constant pressure of 1 atm. In this experiment, a relatively low feed rate of approximately 100 kg/h was used. The carbon concentration in the metal was measured using LECO analysis (a standard commercial carbon concentration analyzer), and the offgas was analyzed by EPA Method TO-14 (10). The DRE calculation was determined using the concentration of the polystyrene monomer (styrene) in the TO-14 protocol. Results verified that at constant operating conditions, the lower the carbon concentration relative to saturation ([C]eq
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∼ 4.27% in molten iron; 11), the higher the forces driving the formation of the dissolved intermediate, and hence more efficient feed conversion per unit time is achieved. This finding illustrates the utility of solvation effects as a control parameter for the conversion of organic compounds into synthesis gas (CO and H2). This experiment was optimized to operate at a moderate to low [C]/[C]eq ratio (i.e.,
