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Fundamental Studies of Metal Behavior During SolidsIncinerationERIC G. EDDINGS a & JOANN S. LIGHTY aa Department of Chemical Engineering , University of Utah , Salt Lake City, UT, 84112, U.S.A.Published online: 16 May 2007.

To cite this article: ERIC G. EDDINGS & JOANN S. LIGHTY (1992) Fundamental Studies of Metal Behavior During SolidsIncineration, Combustion Science and Technology, 85:1-6, 375-390, DOI: 10.1080/00102209208947178

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CQmbWI. Sci. and Tech., 1992, Vol. 85, pp. 375-390Photocopyingpermitted by licenseonly

© Gordon and Breach SCience Publishers S.A.Printed in United Kingdom

Fundamental Studies of lVIetal Behavior During SolidsIncineration

ERIC G. EDDINGS and JOANN S. LIGHTY Department of ChemicalEngineering. University of Utah. Salt Lake City, UT 84772. u.s.A.

(Received October 28. /99l)

Abstract-An experimental apparatus was constructed which allows investigation of the vaporizationbehavior of metal contaminants during incineration of their host substrate. Comparisons were madebetween equilibrium predictions and experimental observations for a number of different metals inchlorinated, inert, and reducing environments between 150°C and 650°C.

The equilibrium predictions for Ph vaporization were found to show the greatest deviation fromexperimental observations. Comparisons showed that a knowledge of elements associated with the initialmetal species. as well as omission of PbCI, from the calculations, can be important for the equilibriumpredictions. Experimental results showed that the formation of volatile PbCI, predicted by equilibrium wasnot kinetically favorable under the conditions studied. Subsequent vaporization studies involving PbCl,deposited on a silica substrate demonstrated an influence of initial concentration on the amount of Pbvaporization observed. The extent of vaporization appeared to be independent of a moderate increase intemperature and an increase in the time aJlowed for vaporization.

Key words: Incineration, metals, vaporization, hazardous waste, contaminated soil.

INTRODUCTION

The Hazardous and Solid Waste Act of 1984has indicated that landfilling is no longera viable solution to some solid waste problems; subsequently, generators of thesewastes are looking for more permanent methods of waste reduction or elimination.Since incineration is a permanent disposal option for many organic wastes, as well asan effective remediation technique for contaminated solids such as soils, use ofincineration as a method for solid waste reduction, remediation, or elimination hasgained considerable interest in recent years. Due to the high temperature environmentof incineration processes, it is possible that some of the trace metal species present inthe waste may unfortunately vaporize as well. There is much concern about potentialemissions of the trace toxic metals that can be found in both combustible andnon-combustible fractions of solid wastes (Korzun and Heck, 1990; Pacyna, 1987;Oppelt, 1987).

The formation of toxic heavy metal fume during incineration is a result of vapori­zation of volatile metal species and subsequent homogeneous nucleation of thesupersaturated metal vapors (Germani and Zoller, 1988; Lee, K. C., 1988;Quann andSarofim, 1982). Further condensation of these metals takes place on this fume due toits high surface area-to-volume ratio leading to highly toxic emissions ofconcentratedfine particulates which are difficult to capture and have tremendous bioavailability(Denison, 1988; Ulrich, 1978). Minimization of metal vaporization from its hostsubstrate is critical in limiting stack emissions of toxic heavy metals. The safe designand operation of incineration systems that handle metal-bearing wastes are dependenton an understanding of the rates and mechanisms involved in the vaporization of themetal contaminants (Penner et al., 1988; Vogg, 1987).

Present modeling attempts of these mechanisms have assumed thermodynamicequilibrium when calculating the partitioning of metals between the solid and gasphases (Tillman and Leone, 1990; Barton et al., 1990a; Lee, C. C., 1988). Although

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376 E. G. EDDINGS AND J. S. LIGHTY

there has been some success using this method, there is experimental evidence thatequilibrium calculations might overpredict the amount of metal vaporized (Bartonet al., I990b). Identification of the conditions under which the equilibrium assumptionmay be used and quantitation of the true vaporization rates for non-complying metalsis required for accurate predictions of the fate of metals.

In general, the vaporization ofa particular metal from a solid will depend on severalfactors:

• the local temperature experienced by the solid particle,• the form (or species) in which the metal exists, and• the presence of other species (gaseous species, the solid surface, etc.) which may

interact with the metal.The effect of temperature on the vaporization of metals has been identified as beingof tremendous importance in determining the volatility of metal species (Barton,1990a). A small change in temperature has an exponential influence on the vaporpressure of most toxic metals of interest. In addition, certain species of the sameelement are much more volatile than others; in particular, the chloride species of manyof the transition metals. The species of a given metal which will be stable in a specificenvironment can be affected by the presence of reactive gaseous species. Previousexperiments have shown that HCI in the gas stream will react with solid lead oxideto form a more volatile lead chloride and that the chloride environment was five timesmore volatile than a comparable environment contaminated with SOh CO2 , or O2

(Lighty et al., 1990a). Studies on the behavior of minerals during coal combustion(Quann et al., 1982; Smith, 1980) indicated the presence of a local reducing environ­ment for a burning particle. In the reducing environment, the relatively stable andrefractory oxides of many metals may react to produce a more volatile suboxide orbase metal due to the presence of CO and lack of°2 , The effects of both of theseenvironments, a reactive combustion gas and local reducing conditions, will beconsidered in the following.

This study is an extension of previous fundamental work investigating the remedi­ation of organic materials from solid substrates such as contaminated soils (Lightyet al. 1990a; Lighty et al. 1990b; Lighty et al., 1989). The metals used for investigationin this work were Pb, Cr, Cd, Ni, Zn, and Cu. The first four are among the top 17toxic chemicals listed by the EPA as posing the greatest threats to human health. Thenext two, copper and zinc, were included for comparison with full-scale data.

EQUILIBRIUM MODEL

The computer model used for the calculation of equilibrium partitioning of the metalspecies was the 1989 update of the program of Gordon and McBride (1976). Thismodel has gained some popularity, in part, because of its extensive database. Thermo­dynamic data for key species of most metals of interest, in addition to a compre­hensive database for organic species and radicals, are incorporated into this computerprogram, thereby allowing calculations for very complex systems. The final equi­librium system is determined by first taking the elements of the input system anddetermining all possible species that can be derived from this pool of substances(unless directed otherwise). An optimization routine is then performed to find theoverall system composition which minimizes the Gibbs free energy of the system. Thisfinal composition is the equilibrium composition.

The process to be modeled is an open system; however, the equilibrium calculationinvolves a closed-system approach. Due to this difference, a representative time

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METAL BEHAVIOR DURING INCINERATION 377

Reacting GasRow

Nitrogen GasRow

PerforatedPlate

SS Bed w/400 Mesh ::=~~~i:3~~5S Screens

Cooling Nitrogen<Pyrex or -I55 Tube

to _==~~:..,,,,.~Bubblers

FIGURE I Differential-bed reactor (DBR) used for measuring metal vaporization rates.

window was used and the integrated values of all system inputs (gases, contaminatedsolids, etc.) were used for the model inputs. In addition, since the program calculatesan optimal composition for the system as a whole, the mole fractions in the programoutput correspond to the entire system-gaseous and condensed species alike. Thefraction vaporized for a particular metal was therefore calculated by taking a ratio ofthe sum of the vapor mole fractions to the combined sum of the vapor and condensed­phase mole fractions for that metal.

EXPERIMENTAL

Differential-Bed Reactor

A recently-constructed Differential-Bed Reactor (DBR), shown in Figure I, was usedfor the collection of metal vaporization data. A thin bed (~6 mm thick) of contami­nated substrate is contained between two stainless steel screens. This bed is placedinside a 11.5em square ceramic block, shown in Figure 2, which is a cast refractorymaterial with a high capacity for thermal and mechanical shock. There is a small holein the stainless steel bed through which a small ceramic alignment tube and a 0.020"diameter thermocouple are inserted to measure particle temperatures. Inside therefractory block, there is a coiled Ni/Cr wire that is connected in series with a 20 V,70A transformer to provide direct heat to the bed of particles. The transformer iscoupled with a variac to allow for variation of the heating rate. A PID controller iswired into the ceramic block heater circuit to maintain isothermal operation and the

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METAL BEHAVIOR DURING INCINERATION 379

controller input is the solid particle temperature measured inside the stainless steelbed. Four ceramic tubes with a 3mm I.D. are cast directly into the refractory and areconnected to a liquid nitrogen tank to allow for rapid cooling of the ceramic blockafter completing a reaction.

The gas stream passing through the bed of particles is initially heated by means ofa quartz heat exchanger within an electric furnace (Figure I). An electronic solenoidat the top of the furnace is actuated by a computer through a 12V power supply andallows passage of either reacting gas or inert gas through the reactor for specifiedperiods of time. Also, an additional solenoid can be triggered which allows the gasstream to bypass the heat exchanger (for use during cooling). The gas then passes intoa short transition section, made out of stainless steel coated with ceramic paint, whichcontains a perforated plate to straighten the gas flow prior to contacting the particlebed. In addition, a tap for pressure measurement and a suction pyrometer for gastemperature measurement are included in the transition section.

After passing through the bed of particles, the gas stream enters a cooling sectionwhich consists of a porous stainless steel tube inside of a larger stainless steel pipe.Nitrogen is introduced through four fittings in the outer pipe and passes through thewalls of the porous, inner tube to prevent impingement of any particulates and to helpcool the gas stream. The gas then passes through a 0.2 J1m filter for particulatecollection and a caustic bubbler for HCI and uncondensed metals.

The overall flow schematic, Figure 3, shows the preparation of the reaction gasmixture. Metered quantities of COh °2 , and HCl are combined with N2 to simulatethe combustion gas mixture inside a rotary kiln. The N2 flow is passed through anambient temperature water bubbler prior to mixing to saturate the stream with water(~3% H20 in the final reactant flow). The concentration of water in the simulatedreaction mixture is lower than would be encountered in an incinerator; however, it isin excess with respect to the metal contaminants. The flowrate passing through theparticle bed for these experiments was 1000std. ml/rnin.

The experimental procedure involves loading the particle bed into the refractoryblock and inserting the bed thermocouple; the block is then placed in-line with thetransition section and the cooling section and the reactor is sealed by compressionusing a lab jack. Graphite gaskets are used at all sealing interfaces. The gas furnacehas been preheated to the desired temperature, and a flow of inert N2 is initiatedthrough the quartz heat exchanger. At the same time, the power to the ceramicblock heating circuit is turned on to initiate the heatup cycle for the DBR. Duringthe heatup cycle, the flows of the reacting gases can be calibrated to the desiredlevels through bypass valves. When the particle temperature has stabilized, thereaction solenoid is actuated and the reacting gas mixture is allowed to flow throughthe bed of particles. The amount of time desired for the reaction gas flow, as wellas the desired heatup time, is set by a computer interface with the electronicsolenoids.

When the reaction is complete, the reaction solenoid switches back to inert N2 flowand the heat exchanger is briefly allowed to purge (~30 seconds). The coolingsolenoid is then actuated and the nitrogen flow bypasses the gas heat exchanger. Thepower to the block heating circuit is turned off and a flow of liquid nitrogen throughthe refractory block is initiated. When the system has cooled to a manageabletemperature, the cooling is stopped and the stainless steel bed of particles can beremoved.

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METAL BEHAVIOR DURING INCINERATION

TABLE IInitial metal species of reagents and metal concentrations on the solid

381

Metal salts Initial concentrations Initial concentrationsMetal dissolved initially on the porous clay" on the silica particles"

Pb Pb (C, H,o,), 75 ppm" ± 10% 63ppm ± 3%Zn ZnSO,' 7H,O 964ppm ± 3% 524ppm ± 5%Cd CdSO, . 8H,O 128ppm ± 2% 215ppm ± 15%Cu CuSO,' 5H,o 389ppm ± 6% 233ppm ± 3%Ni NiSO,' ?H,O 74ppm ± 9% IlOppm ± 15%Cr Cr,(SO,), . 15H,O 189ppm ± 15% 115ppm ± 6%

• As determined by EPA Method 3050 and ICP analysis.•• ppm = /lg of the elemental metal per gram of contaminated solid.

Sample Preparation and Analysis

For the multiple metal tests, the solid materials used were a commercial, montmorillo­nite clay sorbent (~80m2jg) and silica gel particles (~275 m2jg). A narrow sizedistribution (1-2mm) of particles was used for the clay experiments and a slightlybroader distribution (1.5-4mm) was used for the silica gel particles. The effect of pHon the adsorption of cations varies considerably with the type of solid sorbent used(de Haan and van Riemsdijk, 1986)and, although this effect is small for the negatively­charged clay sorbents, a non-acid aqueous solution was formulated by choosing metalsalts that had reasonable solubilities in distilled H20 . The contamination methodinvolved mixing the aqueous solution of the metals of interest with either the claysorbent particles or the silica gel particles in a piecewise fashion. A rather dilutesolution was used to assist in the overall dissolution process and to provide a moreuniform distribution of the metals on the solid substrates. Because the sample size foreach experiment was small (~ 4 grams), it was important to avoid "hot spots" of highmetal concentrations. The resultant wet mixtures were thoroughly mixed and thendried at 105°C for 24 h. Table I summarizes the initial metal species used and the finalanalysis of the dried, contaminated solids. It should be mentioned that some of thesolid salts that were added did not completely dissolve in the solution; therefore, it ispossible that some of the entrained metal salts were deposited directly onto the solidsubstrates.

Results for the experiments involving the contaminated clay or silica particles wereobtained by dissolving the clay and silica gel samples according to Method 3050 (U.S.EPA, 1986) and all of the flame analyses were accomplished by Inductively-CoupledPlasma Emission Spectrometry (ICP) using Method 6010 (U.S. EPA, 1986).

Some specificexperiments involving PbO were performed using glass beads (500)lmin diameter) as well as the clay and silica particles. The contaminated solids wereprepared by making a suspension of PbO in methanol (PbO is insoluble in methanol)and combining it with the substrate and then heating the tumbling mixture to driveoff the methanol. The contaminated glass beads were analyzed by dissolving the PbOcoating in a nitric acid solution and using ICP techniques. The clay and silica particleswere analyzed by Method 30S0, as before. Other experiments involved the depositionof PbCI2 onto .the clay and silica particles and were accomplished by dissolvingquantities of PbCI2 powder in distilled, deionized water and adding an excess amountof this solution to the dry particles. After allowing for diffusion, the excess solutionwas poured off and the saturated particles were dried overnight at IOsoC.

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382

Metal

Pb

Cr

CdZnNiCu

E. G. EDDINGS AND J. S. LIGHTY

TABLE IIMelting points (0C) of various metal compounds

Elemental Chloride Sulfate Sulfide Oxide

328 501 (CI,) 1170 1114 886- 15 (Cl.)

1857 824 (CI,) 100 1550 2266-96 (O,Cl,)

321 568 1000 1750 1500420 283 600 1020 1975

1455 1001 848 797 19841083 620 (CI,) decomposes decomposes 1326

430 (CI) >200 >220

Source: CRC Handbook of Chemistry & Physics, 68th edition (1987), CRC Press, Inc., Boca Raton,Florida.

RESULTS AND DISCUSSION

Multiple Metals Experiments

The melting points of various metal species can be used as a rough indication of theirrelative volatilities. A summary of melting points for some important species is givenin Table II. As seen in this table, there is a wide range in the relative volatilities of asingle metal depending on its species; therefore, a knowledge of the nature of the metalcontaminant on the surface of a particle can be very important in terms of predictingthe metal volatility. With current analytical methods, however, it is difficult toobtain information about the species of a metal contaminant on a solid substrate,especially at trace contaminant concentrations. The following results were thereforeobtained in terms of total metal concentration (e.g., the amount of elemental Pb thatis present on the solid substrate); however, some information about speciation wasderived from both sample preparation methods and experimental results. .

Inert environment Initial vaporization experiments were performed in an inertenvironment to determine the extent of vaporization of the initial contaminant metalspecies. The results for the vaporization behavior of 6 metals from the porous claysubstrate in a nitrogen environment are shown in Figure 4. The overall vaporizationwas limited, with Zn, Pb, and Cd displaying virtually no vaporization at temperatures

1.00

FractionVaporized

0.80

0.60

0.40

0.20

0.00Cr Zn Pb· Cu

Elemcnt

Cd Ni

• ISO·C• 4OO·C• 5400C

FIGURE 4 Metal vaporization from contaminated clay sorbent particles in an inert nitrogen environment.

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METAL BEHAVIOR DURING INCINERATION

1.00,....------------,

0.80

383

0.00 ...

FractionVaporized

0.60

0.40

0.20

Cr Zn

•Pb en Cd

Element

Ni

• ExperimenraJ• Equilibrium

• equilibrium calculationswere notperformed for Cd

FIGURE 5 Experimental values vs.equilibrium predictions for a thirty minute exposure of contaminatedclay sorbent to a simulated reducing environment (3% CO, 4% CO" balance N,) at 538'C.

up to 540°C. Surprisingly, the Cr data showed vaporization only at the lowesttemperature, 150°C; however, there was a considerable amount of experimental errorin the initial concentration of this element (see Table I) and the displayed vaporizationwas within these error limits. The Ni data also displayed limited vaporization;however, these values were also on the order of the experimental error. The results forCu, however, displayed some experimental vaporization which may give some insightinto the nature of the Cu on surface. The experiment carried out at 150°Cin nitrogendid not show any appreciable vaporization of copper; however, the experimentalresults at 400°C and 540°C in nitrogen demonstrated approximately 20% and 30%vaporization, respectively. Since copper sulfate undergoes decomposition above200°C (perhaps to a more volatile species), it is possible that the copper precipitatedas a sulfate onto the clay sorbent (from the predominantly sulfate aqueous solution)and it is also possible that some of the other metals precipitated as solid sulfates aswell.

Reducing environment Experiments were also performed with the contaminated claysorbent at 540°C using an atmosphere of 3% CO and 4% CO2 in nitrogen to simulatea local reducing environment (due to oxygen depletion). Equilibrium predictions weremade as well, and these results are shown in Figure 5. The Pb was predicted tovaporize slightly but there was no vaporization observed experimentally. The copperagain displayed some experimental volatile behavior; however, this behavior was notpredicted by the equilibrium model. There was no experimental vaporization ofcadmium found in the reducing environment, even though some might have beenexpected. Kistler and co-workers (1987) found significant vaporization ( - 40%) ofcadmium sulfide while pyrolyzing sewage sludge at a slightly higher temperature of625°C with a local reducing environment resulting from char formation in theirsystem.

H'Cl environment Experiments involving the contaminated clay sorbent were alsoperformed in a simulated combustion gas environment, 7% 02, 6% COlo - 3% H20,200 ppm HCl and balance N2 • Because the chloride species of many of the contami­nant metals are relatively volatile, a reaction with the gaseous HC) to form the metalchloride was expected to increase the measurable vaporization of some of the metalsrelative to the inert nitrogen experiments.

An experimental run, shown in Figure 6, was made with the contaminated clay

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384 E. G. EDDINGS AND J. S. LIGHTY

1.00

0.80

Fraction 0.60

Vaporized0.40

0.20

0.00Cr Zn Pb Cu Cd Ni

11III ExperimentalBI Equilibrium

• equilibrium calculationswere not performed for Cd

Element

FIGURE 6 Experimental values vs.equilibrium predictions for a thirty minute exposure ofcontaminatedclay sorbent to simulated combustion gas (200 ppm Hel, 7% 0" 6% CO,. 3% H,O, balance N,) at 538°C.

sorbent exposed to the simulated incinerator gas for 30 minutes at a temperature of540°C. The equilibrium predictions are included for comparison. It is apparent fromthe figure that in this case the vaporization of Pb is greatly overpredicted by theequilibrium model. This may be demonstrating a low reactivity of the contaminantmetal species with the gaseous HCI, since the volatile species predicted by the equi­librium model were chlorides.

Experimental vaporization of Cu was again observed but was not predicted by theequilibrium model. The model can only consider as reaction products those specieswhich are found in its database. It is possible that a volatile Cu species which may beimportant under these experimental conditions has not been included in the thermo­dynamic data file of the program and subsequently is not being considered as a finalproduct.

Effect of substrate Another consideration which may affect the accuracy of equi­librium vaporization predictions may be the effect of surface interactions of the claysorbent with the contaminant metal species. At the low concentrations being investi­gated, the amount of metal contaminant may be much less than that required for acomplete molecular monolayer, or at least a monolayer covering of the most activeadsorption sites; therefore, adsorption effects might be important. In addition, theremay be potentially strong attractions to the clay particles due to the negativelycharged surface of the clay (Newman, 1987). Instead of some of the metals precipi­tating from solution as sulfates during the drying process, they may have beenionically bound to the anionic clay. Whether by binding or some form of adsorptionto the clay surface, these interactions could be a source of inconsistency with theequilibrium model (which considers pure condensed phases only). Investigations ofthe binding phenomenon of Pb on kaolinite clay to control air pollution haveindicated the possibility of such behavior (Uberoi and Shadman, 1990; Ho, et al.,1990).

To evaluate this surface behavior, silica gel (assumed to be non-ionic) was contami­nated with the same metal speciesas the clay sorbent (seeTable I) and the contaminatedsilica was also exposed to the simulated HCI combustion gas for 30 minutes at 540°C.As shown in Figure 7, the vaporization results are essentially the same as thoseobtained for the clay sorbent. Again, the only observed experimental vaporizationwas with Cu and the equilibrium model again overpredicted the volatility ofPb. It wasnot clear from these results alone whether surface-contaminant interactions, initial

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METAL BEHAVIOR DURING INCINERATION 385

1.00

0.80

Fraction 0.60

Vaporized0.40

0.20

0.00Cr Zn Pb Cu

Element

Cd Ni

11II ExperimentalII Equilibrium

• equilibrium calculationswere not performed for Cd

~ • ~ 't

FIGURE 7 Experimental values vs.equilibrium predictions for a thirty minute exposure of contaminatedsilica particles to simulated combustion gas (200 ppm HCI, 7% 0" 6% CO" 3% H,O, balance N,) at538°C.

species reactivity, or some other factor was responsible for inhibiting the predictedvaporization.

Lead Investigations

The experiments involving multiple metals demonstrated that the use of an equi­librium model to predict metal vaporization may lead to erroneous predictions in thecase of Pb and Cu. Further investigations involving Pb gave insight into what mightbe the cause of these erroneous predictions.

Reactivity ofPbSO. If the Pb was present as a sulfate on both the contaminated clayand silica particles in these experiments, there is evidence that the sulfate species couldhave a low reactivity with HCI and subsequently not form the more volatile chloridespecies. Zil'berman & co-workers (\969) investigated reactions of HCl with PbSO.and PbO, as well as with basic lead sulfates (PbSOa ' (PbO)n)' They found, using x-raydiffraction techniques, that HCI did not react with PbSO. either as the sole reactant,in combination with PbO as a basic lead salt, or in a mechanical mixture; that onlythe PbO reacted with HCI. Therefore, if the precipitated lead was present as PbSO.,it is possible that it may not have reacted significantly with HCI and thereforeremained on the solid as a less volatile lead species.

Formation of PbCI. Further investigations of the Pb/Cl system were needed;therefore, equilibrium calculations were performed for the temperature range of27-800°C. The calculations, shown in Figure 8, predicted complete vaporization ofsolid PbO in an atmosphere of excess HCI in nitrogen. At the lower temperatures, thisresult is due to the predicted formation of the extremely volatile PbCI., which meltsat a temperature of - 15°C. The equilibrium calculations were repeated excludingPbCl. as a possible reaction product and significant vaporization was only predictedabove 400°C (due to formation of PbCI2-m.p. 501°C). Experimental results usingPbO-coated, non-porous glass beads in a gaseous environment of excess HCI innitrogen are plotted in Figure 8 as well. The observed condensed product from theseexperiments was white in color, which is characteristic of PbCI2 , as opposed to theyellow, oily liquid PbCl., and the measured vaporization was minimal. With respectto vapor-phase PbCI., a previous experimental study on Pb/Cl interactions using

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386 E. G. EDDINGS AND J. S. LIGHTY

800

.

200 400 600Temperature (0C)

PbOon

G1~~r~ -I

~···_--'---7-'---

W/PbCI4 "'-

w/o PbCI4 -

o

1.00

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0.40

0.20

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FractionVaporized

FIGURE 8 Equilibrium predictions of Pb volatility, with and without PbCl, as a reaction product, forPbO in a gaseousenvironment of excessHel in nitrogen. Someexperimental valuesfor thesameconditionsarc shown for comparison.

molecular-beam mass spectrometry cited PbCl 2 as the only detected volatile reactionproduct between 27°C and 627°C (Balooch, et al., 1984). In addition, other Pb/Clkinetic studies in the literature only mention PbCl 2 as a reaction product (Chen, et al.,1989; Trichon and Feldman, 1989; Mikhail and Webster, 1982; Ball and Casson,1978). Based on these experimental observations, it is assumed that the formation ofPbCl, is not kinetically favorable under these conditions.

Equilibrium model input data A typical data set available for use in attempting topredict the vaporization of metals during incineration will only give information onthe level of elemental metal present in the waste. It is generally not possible to knowwhat species the metal contaminant might be. As was shown in the previous sections,however, the presence of certain species can have a tremendous influence on theequilibrium predictions obtained from a program such as that of Gordon andMcBride (1976). To illustrate this point, a series of trial calculations were madeutilizing different input data to the equilibrium model and these calculations aresummarized in Figure 9. The experimental values for the two temperatures, 150 and593°C, are included for comparison (Case A). The experimental data is from runs

1.00

0.80

Fraction 0.60Vaporized

0.40

0.20

0.00A B C D E

Case

Case DescriptionsA- Experimental valuesEQuilibrium Predictjons'B- using elemental inputsC- using species inputs'D. using elemental inputs and

omitting lead tetrachlorideE- using species inputs and

omitting lead tetrachloride

FIGURE 9 Comparison of experimental values of Pb vaporization at two temperatures 10 four differentequilibrium predictions.

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METAL BEHAVIOR DURING INCINERATION 387

0.00 b=......JlII_u~L

1.00

0.80

Fraction 0.60Vaporized

0040

0.20

PbO physicallydeposited on

substrates

Silica Oay Glass EquilibriumParticles Particles Beads Prediction

FIGURE 10 Effectof substrate on vaporization behavior. Solids were exposed to simulated combustiongas (200ppm HCl. 7% 0,. 6% CO,. 3% H,o, balance N,) for thirty minutes at 538°C.

involving the multiple-metal contaminated clay particles which were exposed to thesimulated combustion gas at the temperatures indicated. Case B corresponds toequilibrium calculations made using elemental metals as inputs and including PbCI.as a reaction product. Case C represents calculations made using the initial species ofthe metals dissolved in the contaminating solution as program inputs and alsoincluding PbCl. as a reaction product. Cases D and E represent predictions madeexcluding the possibility of PbCl. as a reaction product for elemental and speciesinputs, respectively. It is apparent that if species inputs are utilized as well as theomission of the unfavorable PbCl., that the equilibrium predictions more accuratelyreflect the experimental observations, as shown in Case E. The stable condensed Pbspecies predicted to be at equilibrium for the conditions of Case E was PbSO•. IfPbC1. was omitted and the presence of sulfur or sulfur species (such as sulfates) wasnot specified, then the high temperature experimental run was overpredicted (Case D)due to the formation of PbCI2 which is volatile at this temperature. Note that at thelower temperature PbCl2 is not volatile.

Effect of substrate Due to the apparent stability of PbSO. in HCI environments, analternate Pb species was required to identify potential effects resulting from substrateinteractions. Since it is known that PbO reacts readily with HCI (Lighty, et al., 1990a;Zil'berman, et al., 1969), this Pb salt was used for some additional experiments.Figure 10 compares the experimental results for PbO physically deposited on claysorbent, silica particles, and smooth glass beads (using the methanol suspensionmethod). In addition, the equilibrium prediction for these experiments was includedfor comparison (identical for all three; PbC1. was omitted). The experimental con­ditions were again a 30-minute exposure to the simulated HCI combustion gas at atemperature of 540°C. There was a noticeable difference between the experimentalresults; the vaporization from the clay sorbent and the silica particles was low (on theorder of the experimental error), but the vaporization from the glass beads wassignificant. This could be demonstrating a strong adsorption effect of the surface, asmentioned previously. The relatively low surface area of the glass beads required amultilayer coating of PbO to obtain the same total amount of Pb that was used withthe clay sorbent and the silica experiments; therefore, some vaporization of themultiple layers (after reaction with HCI) was to be expected for the glass beads.

The results from Figure 10 indicated that surface adsorption effects should beinvestigated more closely; in particular, the difference between multiple-layer and

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Final Pb 8000Concentration

(ppm) 6000

4000

2000

E. G. EDDINGS AND J. S. LIGHTY

2000 4000 6000 8000 1()()()() 12000 14000

Initial Pb Concentration (ppm)

b 30 minutes @ 538°Co 60 minutes @ 538°Co 30 minutes @ 649°C

FIGURE II Effect of initial contamination level on the vaporization behavior of PbCl, from a highlyporous silica substrate in a gaseous nitrogen environment.

monolayer coverage. More recent vaporization experiments were performed involvingonly the reaction product, PbCl, to simplify the analysis. The PbCl, was depositedon the porous silica particles and subsequently exposed to inert nitrogen gas under avariety of conditions. Three different initial concentrations were used: approximately1400,6000, and 12,000ppm Pb (as before, ppm Pb = JlgPb/gram of contaminatedsolid). The results, shown in Figure II, demonstrate a difference in volatility dependingon the extent of coverage. Significant vaporization of PbCl, from the silica particleswas only detected at high initial concentrations and the extent of vaporizationappeared to be independent of experimental variations. The base condition forexperiments having an initial concentration of 12,000ppm was a temperature of540°C maintained for 30 minutes; however, doubling the time-at-temperature to60 minutes, as well as an increase in temperature to 650°C, still resulted in a finalconcentration of approximately 8000ppm Pb. Experiments performed with initialconcentrations less than 8000ppm did not exhibit any significant vaporization. Thismay be demonstrating a contamination level comparable to or less than a monolayercovering of the most active adsorption sites. Future experiments at higher concen­trations will need to be conducted to verify the existence of a critical concentrationlevel for Pb vaporization under these experimental conditions. Equilibrium calcu­lations (excluding PbCI.) for these experimental conditions predicted completevaporization of the PbCl, at low concentrations. The equilibrium calculations, how­ever, treat condensed-phase components as pure, independent species and do notconsider interactions with the substrate.

CONCLUSIONS

A fixed-bed reactor has been constructed which is capable of performing metalvaporization experiments in a high-temperature, reactive environment. Vaporizationexperiments were performed and the results compared with equilibrium predictions.It was determined that a knowledge of the elements associated with the initial metalspecies present on the solid contaminant can be important in attempting to predictmetal behavior. More sophisticated analytical methods need to be developed to beable to distinguish between different species of the same metal as these species canexhibit varying degrees of volatility and reactivity. Evidence also indicates that in theabsence of other effects, PbSO. will probably not exhibit the same volatile behavior

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METAL BEHAVIOR DURING INCINERATION 389

as PbO in the presence ofHCI due to its low reactivity with HCI and its thermodynamicstability.

In addition, the determination of the range of species to be included in equilibriumcalculations may need to be verified experimentally. Based on our observations as wellas those of other investigators, it appears that the formation of volatile PbCI. is notkinetically favorable under the experimental conditions investigated. Considerationof this species in vaporization calculations may result in erroneous predictions. Also,there was a significant amount of Cu vaporization exhibited under nearly all of theconditions investigated that was not predicted by the equilibrium calculations. It ispossible that an important Cu species is not being considered in the equilibriummodel.

Results of vaporization experiments using PbCI2 deposited on silica particlesdemonstrated a level of contamination below which vaporization of the surfacecontaminant was not observed. Equilibrium predictions of the experimental con­ditions did not reflect this behavior. There appears to be a strong binding of somemetal species by the silica particles at low contamination levels which may be due, inpart, to surface adsorption. More research is needed addressing interactions betweensubstrates and metal contaminants.

ACKNOWLEDGEMENTS

This work was funded by the National Science Foundation through the Presidential Young InvestigatorAward and the Advanced Combustion Engineering Research Center, as well as contributions by the GasResearch Institute. The authors also wish to acknowledge the collaboration of Mr Brian R. Keyes andDr William H. McClennen with certain aspects of the experimental work as well as the assistance ofMr. Tim DeJulis with the analytical measurements.

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