fixing chromium and lead in ceramic materials: a...

ABSTRACT. — Industrial wastes rich in toxicheavy metals are produced in high amounts yearly.As other hazardous wastes, they require special,expensive disposal, and they may represent a seriousenvironmental and health issue in case of heavymetal dispersion. Petrology may offer useful tools toeffectively inertize hazardous industrial wastes andeventually recycle them back in industry. Mineralphases like oxides and aluminosilicates, occurring innatural rocks and employed in the ceramic industry,have a high potential for long-term bonding a widevariety of refractory as well as low-melting heavymetals. These resistant phases may then be used toincorporate a wide range of hazardous metalcomponents in waste during inertization treatments.Experimental work was done on petrological modelslike the MgO-SiO2-Al2O3 (MAS) system and itslittle known extensions to Cr2O3 (MASCr) as wellas to the low-melting PbO (PMASCr). Experimentswere planned with different bulk compositions andon a wide range of temperatures in order to testfeasibility and efficiency of a petrology-basedinertization of highly chromiferous and Cr-Pb-richindustrial wastes, as those from tanneries and fromgalvanic processes. Run products exploring therefractory MASCr system between 1250°C and1560°C showed that the addition of Cr contributes tostabilize the refractory, Cr-rich phases of the MAS

system and to lower the thermal minimum of thesystem by approximately 100°C. Different Cr-bearing phases are dominant in the different portionsof the system, from spinels in the most Mg-rich bulkcompositions to sapphirine and mullite in the leastMg-rich ones. Glass occurs in all runs and is Cr-poor. Cr2O3 content of Cr-hosting phases in the runproducts may vary between 100 wt% in pureeskolaite to 60 wt% maximum in spinel, 30 wt% insapphirine down to 12-23 wt% in mullite. Spinelsand sapphirine and the most abundant and mostinteresting phases in the view of inertization.Experiments in the unknown PMASCr system wereplanned (a) to cope with compositions of galvanicsludge (Crtot + Pb oxides > 30 wt%, molar Cr:Pbabout 1:1), and (b) to induce simultaneouscrystallization of Pb-feldspar and Cr-bearing spinelbetween 950°C and 1050°C, from subsolidus andfrom melt conditions (after short high-temperaturetreatment at 1350°C). The planned association of Pbfeldspar and Cr-rich spinel (with Cr2O3 up to 60wt%) occurs in all runs, even at low temperature,and in association with terms of the eskolaite-corundum solid solution. Spinel and Pb feldspar arethe most abundant phases crystallized except in theruns treated at high temperature, where Cr-poor, Pb-bearing glass can overcome Pb feldspar. Cr and Pbare therefore completely separated in distinct phases.Both groups of experiments are highly encouragingfor waste-oriented applications: heavy metals like Crand Pb can be efficiently bondend in crystalline

Per. Mineral. (2004), 73, 99-111 http://go.to/perminSPECIAL ISSUE 3: A showcase of the Italian research in applied petrology

Fixing chromium and lead in ceramic materials: a petrological approach toinertization and recycling of toxic industrial waste

MARILENA MORONI1*, CRISTIANO BRIGIDA2, STEFANO POLI1 and MASSIMILIANO VALLE3

1 Dipartimento di Scienze della Terra «A. Desio», Università degli Studi di Milano, Via Botticelli 23, 20133 Milano, Italy2 Dipartimento di Ingegneria Chimica e di Processo (DICHEP), Università degli Studi di Genova, P.le Kennedy 1,

16129 Genova, Italy3 Petroceramics SRL c/o Dipartimento di Scienze della Terra «A. Desio», Università degli Studi di Milano,

Via Botticelli 23, 20133 Milano, Italy

* Corresponding author, E-mail: [email protected]

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archaeometry and Cultural Heritage

phases and they are sharply fractionated, therebyallowing further recovery of the economicallyinteresting Cr-rich phases for recycling and safedisposal of the remaining inertized waste.

RIASSUNTO. — Ogni anno vengono prodottegrandi quantità di scarti industriali ricchi di metallipesanti ad alta tossicità. Come rifiuti pericolosi, peressi è prescritto uno smaltimento speciale, e costoso,poiché la dispersione dei metalli tossici in essicontenuti può diventare un serio problemaambientale e sanitario. La petrologia può fornirestrumenti utili per l’inertizzazione di vari rifiutitossici e perfino per il loro riciclo nell’industria.Minerali come ossidi ed allumosilicati, presenti nellerocce e utilizzati variamente nell’industria ceramica,hanno un alto potenziale per incorporare a lungotermine un’ampia gamma di metalli refrattari ebassofondenti. Queste fasi resistenti sono quindiutilizzabili per immobilizzare molti metallipericolosi tramite un processo d’inertizzazione deirifiuti industriali. Sono state svolte indaginisperimentali su modelli petrologici come il sistemaMgO-SiO2-Al2O3 (MAS) e le sue poco noteestensioni al componente refrattario Cr2O3 (MASCr)e a quello bassofondente PbO (PMASCr).Esperimenti con diverse composizioni di partenza esu un’ampia gamma di temperature hanno testato lafattibilità e l’efficienza di processi di inertizzazionesu base petrologica di fanghi industriali pericolosi adalto Cr (ad esempio, da concerie) e a Cr-Pb (adesempio, fanghi galvanici). Gli esperimenti chehanno esplorato il sistema MASCr tra 1250°C e1560°C, hanno mostrato che l’aggiunta di Cr alsistema MAS, sebbene ne abbassi di 106°C ilminimo termico, contribuisce a stabilizzarne le fasirefrattarie e cromifere. Diverse fasi cromiferedominano le diverse porzioni del sistema MASCr,da spinelli nelle composizioni a più alto Mg asaffirina e mullite in quelle a più basso Mg. In tutti iprodotti di sintesi è presente una fase vetrosa, maquesta è povera in Cr. Il contenuto di Cr2O3 dellefasi cromifere sintetizzate varia tra 100 wt%nell’eskolaite pura ad un massimo di 60 wt% nellospinello, 30 wt% nella saffirina fino a 12-23 wt%nella mullite. Spinello e saffirina sono le fasi piùabbondanti ed interessanti per l’inertizzazione difanghi cromiferi. Gli esperimenti nel sistemaPMASCr sono stati progettati (a) sulla base dellacomposizione dei fanghi galvanici (con ossidi diCrtot e Pb >30 wt% e Cr:Pb = 1:1 molare), e (b)nell’intento di indurre cristallizzazione simultanea difeldspato di Pb e Cr-spinello tra 950°C e 1050°C incondizioni di subsolidus e da fusione (brevetrattamento di alta temperatura a 1350°C).L’associazione Pb-feldspato + Cr spinello (con

Cr2O3 fino a 60 wt%) ricorre in tutti i prodotti disintesi, anche in quelli di minor temperatura, inassociazione a termini della soluzione solidaeskolaite-corindone. Spinello e Pb-feldspato sono lefasi più abbondanti eccetto che nei prodotti trattatiad alta temperatura, dove la fase vetrosa, ricca in Pbe povera in Cr, diventa abbondante. Cr e Pb vengonoquindi completamente separati in fasi distinte.Entrambi i gruppi di esperimenti sono moltoincoraggianti per applicazioni all’inertizzazione:metalli pesanti come Cr e Pb vengono efficacementeimmobilizzati in fasi cristalline e nettamentefrazionati, permettendo quindi il successivo recuperoe riciclo delle fasi cromifere, economicamenteinteressanti, e lo smaltimento sicuro del rimanenterifiuto inertizzato.

KEY WORDS: Cr, Pb, waste, spinel, feldspar,inertization, petrology.

INTRODUCTION: WASTE MANAGEMENT AND

FEASIBILITY OF INERTIZATION

Various types of toxic wastes are producedin high amounts (thousands of tonnes) yearly inall industrialized areas. Common types ofhighly hazardous wastes, produced bymetallurgical and chemical factories andlaboratories, are characterized by high contentsin toxic heavy metals such as Cr, Pb, Cu, Zn,Cd, Ni, Hg, Sn, Se, etc.. In these wastes, oftenin form of sludges, total contents of heavymetals in solution or in aqueous suspension canreach tens of units percent in weight. Amongthe most dangerous industrial wastes are theCr-rich sludges produced by leather treatmentin tanneries and the Cr-Pb-rich sludges derivedfrom galvanic (electroplating) processes inmetallurgical plants. Hazard in these sludges iseven enhanced by the massive presence of thehighly poisonous CrVI (in chromatecompounds) together with the less dangerousCrIII. National and international legislations onenvironmental issues recognize the danger ofthe heavy metal-rich sludges by classing themas highly hazardous waste for which carefulhandling and safe storage in sealed containersin special dumps is to be prescribed. The costof waste disposal for toxic wastes is high, over

M. MORONI, C. BRIGIDA, S. POLI and M. VALLE100

0.5 Euro/kg: managing of thousands of tonnesof waste can cost millions of Euros tocompanies which adhere to the lawprescriptions. Storage in special dumps doesnot guarantee neutralization of the wastehazard on a long term basis, as it depends onthe resistance of the containers duringprolonged interaction with weathering agents.

A way to effectively prevent serious heavymetal pollution is to declass the wastes bymeans of inertization treatments which havealso the consequence of lowering disposal costs.

Attempts were made for inertizing sludgesby incorporating heavy metals in glass or inconcrete (e.g., Espinosa and Ténorio, 2000,Kim et al., 2000). However the moderately tohighly reactive nature and the relatively easydeterioration of these materials do not preventadequately toxic metal dispersion in theenvironment on a long term basis. Moreoverinertization treatments represent an additionalcost which would weigh on the shoulders ofeither local communities or waste-producingindustries. Hence the effectiveness of theinertization process should be the primaryrequirement justifying its adoption. Thepossibility of safely recycling in someapplication would also add economic value tothe inertized waste, therefore makinginertization a profitable affair for at least sometypes of waste.

The issue of inertization of heavy metal-bearing sludges may find a profitable solutionin the adoption of treatments able to tightlybond heavy metals into the lattice of highlyresistant crystalline phases such as naturaloxides and aluminosilicates. Natural mineralassemblages contain mineral phases which actas reservoirs of heavy metals. Some of thesenatural phases possess structures able toincorporate a wide variety of metals inresponse to changing environmental conditions(e.g., temperature, pressure, fluid activity, etc.).For example, spinel crystallized in ultramaficintrusions will be highly enriched in Cr, andaffine metals, relative to spinel in metamorphicrocks, which may be richer in Al, Zn etc..Natural feldspar is known to accept large ion

metals like the light metals Rb and Ba as wellas the chalcophile, heavy metals Pb and Tl inpegmatites and in metasomatized granites.Experimental work on such heavy metalacceptors is intended to define systematicallyand maximize these natural phenomena inorder to apply them to practical problemsrelated to industry (e.g., ceramics) and wastemanagement. Inertization may therefore beinduced by thermal treatments after addition ofinorganic additives to the waste according towaste composition, with the aim of inducingcrystallization of metal-bonding minerals.These additives, such as SiO2, Al2O3, MgO,etc., are typical components of natural rock-forming minerals. The process would not onlyensure safe immobilization of heavy metals butmay as well turn the treated waste itself to aresource, an economic material with a potentialfor recycling in the ceramic industry as asecondary raw material according to wastetype. This is a way to link petrology, materialscience and waste treatment processes.

A BASIC TOOL FOR INERTIZATION OF HEAVY

METAL-RICH SLUDGES: THE MGO-SIO2-AL2O3SYSTEM AND ITS EXTENSIONS TO CR AND PB

In Earth Sciences the MgO-SiO2-Al2O3system (MAS; Osborn and Muan, 1960;Schreyer and Schairer, 1961; Eitel, 1965) is avaluable model for the construction ofthermodynamic databases describing phaseequilibria in mafic-ultramafic systems atintermediate to high pressures. At atmosphericpressure the MAS system is a reference systemfor the sectors of Material Science involved inthe production of refractory ceramics (thermalminimum at 1355°C), as well as structural andelectrotechnical ceramic materials. This systemdescribes associations of high temperaturesilicates and oxides, such as mullite, enstatite,olivine, corundum, spinel, etc.. Most of thesephases show wide Al substitution with Cr andmetals affine to it (Zn, Ni, Sn, Cu, etc.).(Phillips et al., 1963; Azad and Min, 2001 andreferences therein).

Fixing chromium and lead in ceramic materials: a petrological approach to inertization and recycling ... 101

The extension of the MgO-SiO2-Al2O3system to Cr2O3 (quaternary MASCr system) isbeing used in Material Science for theproduction of special Cr-bearing technicalrefractories (e.g., furnace linings). Howeverthis quaternary system has been only limitedlyexplored. The detailed experimental researchreported in Brigida (2002) and Brigida et al.(2001, 2002a-b, and in prep.), heresummarized, has brought a notable increase ofinformation about this system, with strongimplications for treatment of highlychromiferous waste.

Galvanic sludges contain high amounts of bothCr and Pb (several tens of wt%; Table 1), often

in similar amounts, together with minorquantities of other metals (Sn, Ba, Fe, etc.). Pb isreleased by consumption of Pb-Sn electrodesemployed in the process. For these toxic wastes acost-effective inertization should immobilize Crand Pb, and affine metals, contemporaneously,i.e. in a single step treatment. Petrology-orientedinertization modeling should therefore considerthe extension of the refractory MASCr system tolow-melting Pb, with the aim of inducingresistant Pb-bonding crystalline phases (e.g.feldspar) together with the Cr-bonding phases.No data are available in literature on the unusualPbO-MgO-SiO2-Al2O3-Cr2O3 (PMASCr)system. The first results were obtained by ourgroup and are described in detail in Mastrovito(2001), Mastrovito et al. (2002) and Moroni etal. (2001, 2003, and in prep.) and summarized inthis work.

EXPERIMENTAL SETTINGS: SAMPLE PREPARATION, EXPERIMENTAL

APPARATUS AND ANALYTICAL TECHNIQUES

Experimental work was done by means ofpowder pellets tightly wrapped in Pt foil andheated in a vertical furnace according todifferent temperature programs for the MASCrand the PMASCr experiments. For each systembulk compositions were chosen in order toexplore different portions of the system or totest the effectiveness of metal incorporation ofa specific phase assemblage. The temperatureprogram therefore mirrors the nature of thephase assemblages wanted. In the refractoryMASCr system the equilibration temperaturesduring the thermal treatment were between1250°C and 1560°C, whereas in the PMASCrsystem the equilibration temperatures werelower (950°C and 1050°C) due to the presenceof the low-melting Pb. The equilibrationtemperatures were reached both fromsubsolidus and from higher-temperature,melting conditions, in order to test attainmentof chemical and textural equilibrium amongphases. With high temperature treatmentenhanced heavy metal volatilization was


TABLE 1 Representative bulk chemical composition

of dry galvanic sludge.

major elements trace elementswt% ppm

SiO2 0.47 Cu 204Al2O3 4.78 Zn 195Fe2O3 1.71 Cd 0.9MgO 0.07 Ni 43CaO 0.11 Sr 145Na2O 0.06 Zr 72K2O 0.06 Y 9TiO2 0.16 Nb 18P2O5 0.3 Sc 1MnO 0.11Cr2O3 11.60CrO3 20.46LOI 9.8CO2 0.73SO3 0.49BaO 1.66PbO 5.58SnO2 3.95tot trace % 0.08

Tot% (*) 60.58

(*) balance to 100% is 39.42 wt% NO3, corresponding to8.91 wt% N.

however taken into account. The duration ofthe experiments varied between 1 month (forMASCr experiments) and 1 week (forPMASCr experiments). Quenched run productswere fully characterized in their mineralogical,chemical and textural features by means of X-ray powder diffraction (XRPD), scanningelectron microscope (SEM) and electron probemicro-analysis (EPMA). Structural and indirectchemical data were also gained by means of X-ray data refinement with the Rietveld method.Details on the experimental and analyticalprocedures can be found in Brigida (2002),Brigida et al. (2001, 2002, and in prep.),Mastrovito (2001), Mastrovito et al. (2002) andMoroni et al. (2001, 2003, and in prep.).

EXPERIMENTS IN THE MASCR

AT ATMOSPHERIC PRESSURE

Liquidus surfaces of systems MgO-Al2O3-SiO2 (MAS), MgO-SiO2-Cr2O3 (MSCr) andSiO2-Al2O3-Cr2O3 (SACr) are represented in

Figure 1. In spite of the extensive use of spinelsin ceramic production, no exhaustivedescriptions are available for MgO-Al2O3-Cr2O3 liquidus in literature. Phase relations at 1atm were studied only in some ternary sub-systems containing Cr2O3 (Greskovich andStubican, 1968; Keith, 1954; Roeder et al.,1968), and no information is available on thewhole quaternary system.

MAS is the reference compositional plan.Phases forming in MAS involve the twoternary compounds cordierite and sapphirine,which have limited range of solid solution, andthe binary compounds spinel and mullite, bothshowing solid solutions. Cordierite has alimited range of ternary solid solution. Ternaryliquidus surface is dominated by a wideprimary phase field of spinel, but the mostimportant topologies are related to the primaryphase field of cordierite, which is bounded bysix univariant curves and six invariant points,including the system minimum eutectic of1355°C. Along such curves liquid phaseequilibria relative to lowest temperatures are


Fig. 1 – Liquidus surfaces of the systems MgO-Al2O3-SiO2 (MAS), MgO-SiO2-Cr2O3 (MSCr) and SiO2-Al2O3-Cr2O3(SACr) at atmospheric pressure. Dashed lines are boundaries of two-liquid fields. Small crosses labelled A to F indicatebulk compositions of experiments in the MASCr system as projections onto the MAS liquidus.

MgO

1578

Sapphirine

Pichrochromite

Enstatite

Spinel ss

Mullite ss

Two Liquids

2090

2120

1860

1550

1546 1355

1365

1440

1850

1840

1720

EskolaiteCristobalite/TridymiteEskolaite

Periclase Corundum

1580

Picrochromite

Chromic oxide

Forsterite

Spinel

Corundum

x

x

x

x

x

x

E

C

A

B

F

D

Periclase

Mullite

Cordierite ssEns

Crd

Cor-Esk+ Liquidus

Cri

Tri

Cr O2 3 Cr O2 3

2Al O3

SiO2

1925

1482

1482

1460

1453

sapph

Cor-Esk+ 2 Liquids

Forsterite

therefore observed. The SiO2-Al2O3 side of thesystem is characterized by the presence of thetwo tapering fields of corundum and mullite.Sapphirine primary phase field is a smalltriangle on the liquidus, directly in contact withcordierite field and delimited by the threeperitectics of 1453°, 1460° and 1482°C. Majorfeatures of system SACr are the wide field oftwo liquid immiscibility and the narrowprimary field of mullite, separated by a verysteep thermal region of the liquidus surface.The minimum of the system (1580°C) islocated at the intersection with the smallprimary field of cristobalite, where liquidequilibrates with cristobalite and solidsolutions of corundum and mullite. No ternarycompounds are present at liquidustemperatures.

The ternary system MgO-SiO2-Cr2O3(MSCr) is characterized by the fields of threemajor primary phases, periclase,pichrochromite and eskolaite. The thermalminimum of the system is approximately at1546°C, at which equilibrium betweencristobalite, pichrochromite, protoenstatite anda silica-saturated liquid exists.

Bulk compositions for experiments in theMASCr system (A to F) are shown in Fig. 1.They have a fixed chromium content asCr2O3:Al2O3 = 1:9 in moles, whereasAl2O3:SiO2 ratio is ~ 1 (A, C, E) or ~ 2:3 (B,D, F) in weight percent. Relative MgO contentsvary accordingly from lower (A-B), tointermediate (C-D) and higher (E-F). Whenreferred to the MAS system, the experimentscover the most complex region of the liquidussurface, where primary phase fields of highlyrefractory phases, such as cordierite,sapphirine, mullite and spinel, lie.

The phase relations in the quaternary systemare dominated by important binary (corundum-eskolaite solid solution, Al2O3-Cr2O3), ternary(mullite-Cr mullite; spinel-pichrochromite,enstatite-Al enstatite; cordierites) as well asquaternary solid solutions (Cr-richsapphirines). The experiments better definedthe high potential for Cr incorporation ofvarious refractory phases which show a

significant extension of their stability fieldstowards Cr-rich bulk compositions. Theaddition of the fourth component Cr2O3 to theMAS system lowers the system thermalminimum of 1355°C ± 3° of at least 106°C. Soglass is formed in all runs. However glass isCr-poor as Cr invariably shows a strongpartitioning for the solid phases (oxides andsilicates). Different Cr-bearing phases aredominant in the different portions of thesystem, from spinels in the most Mg-rich bulkcompositions to sapphirine and mullite in theleast Mg-rich ones.

Figure 2a-b shows examples of spinel-dominated assemblages from different runproducts. Glass produced during synthesisstrongly influences crystal growth and texturalfeatures: large amounts of liquid duringisothermal treatment at high temperature favoureuhedral crystallization and coarse grain size,and allow achievement of maximumhomogeneity and both chemical and texturalequilibrium (Fig. 2a-b).

Cr2O3 content of Cr-hosting phases in thesystem may vary between 100 wt% in pureeskolaite to 60 wt% maximum in spinel, 30wt% in sapphirine down to 12-23 wt% inmullite. Figure 3a-b shows compositionaltrends for spinel and sapphirine. Spinel andcorundum compositions cover the entire rangeof possible solid solutions. Spinel compositionsrun all over the join spinel (MgO*Al2O3)-picrochromite (Cr2O3*MgO). High Cr contentin spinel is favoured by high equilibrationtemperatures. Sapphirine is the only realquaternary compound in the MASCr system. Inthe experiments the Cr2O3 content insapphirine can be as high as 30 wt%: this wasan unexpected result as Cr-bearing sapphirinesin granulites containing only up to 4.9 wt%Cr2O3 were known (Herd, 1973). Quantitativeanalysis of powder diffraction data by theRietveld method allowed refinement of thelattice parameters of Cr-rich sapphirine andevaluated relative amounts of sapphirineexperimentally produced to be around 35 wt%.This has important consequences on quaternarytopologies of the MASCr system but also



(a)

(b)

Fig. 2a-b – Back-scattered electron (BSE) micro-images of run products from MASCr experiments showing euhedraloxide-silicate assemblages in glass-rich matrix: (a) spinel (white) and forsterite (grey) crystals, with forsterite bearing fine-grained spinel inclusions; (b) zoned spinels (white), with Cr-rich cores, reacting with liquidus to form prismatic Al-enstatite(grey).

Fig. 3a-b – Compositional variations in Cr-rich phases. (a) Al vs. Cr content in sapphirine from runs at differenttemperatures. Sapphirine formula on the basis of 20 oxygens. (b) Al vs. Cr content in spinel. Spinel formula on the basis of4 oxygens. Modified after Brigida et al. (2002).

makes sapphirine a very interesting phase forceramic applications in Cr-bearing systems,considering that in the subsolidus it cancrystallize from a wide variety of bulkcompositions and in a large thermal range.

EXPERIMENTS IN THE PMASCR

AT ATMOSPHERIC PRESSURE

If relatively little information was availablefor the MASCr system, nothing was knownabout the effects of the introduction of the lowmelting element Pb in the refractory MASCrsystem. But exploration of such a system wasurged by the Pb- and Cr-rich nature of thegalvanic waste.

Figure 4 is intended as a simplifiedrepresentation of the system MgO-SiO2-Al2O3-Cr2O3 extended to PbO (PMASCr). In addition

to the topologies of the MASCr, this 5-component system relies also on phaserelations of the PbO-Al2O3-SiO2 (PAS) system.The ternary PbO-Al2O3-SiO2 (PAS) system(right side of Figure 4) highlights the fluxingaction of Pb in silicate systems at best. ThePAS system and its sub-systems were mainlyapplied to the glass industry to produce Pb-rich«crystal», highly refractive glass, glazes andfrits (see the review by Nordyke, 1984). Inparticular the PbO-Al2O3-SiO2 (PAS) systemwas mostly investigated in the Pb-rich portion(>40 %mol PbO), characterized by eutectictemperatures as low as 700°C. Pb-bearing fritsand glazes are now almost completelyabandoned due to the easy mobilization of Pbfrom glass into organic fluids even at lowtemperature. So Pb incorporation in glass is farfrom being an adequate inertization. Highercontents in Si and, over all, Al, diminish the


Fig. 4 – Simplified 3-D representation of the 5-component PbO-MgO-Al2O3-SiO2-Cr2O3 (PMASCr) system built up fromthe graphical combination of the MAS and SACr sketches with the liquidus surface in the PbO-MgO-Al2O3-SiO2 (PAS)system (the latter modified after Chen et al., 2001). The projection of the bulk compositions of the experiments in thePMASCr system is not possible in this diagram.

2020°

2800°

2275°

1723°

886°

SiO2

MgO

Al O2 3

Cr O2 3

PbO

low-T°C PbO-SiO and

PbO-Al O -SiO phases2

2 3 2

fluxing effect of Pb and give way to steeperliquidus surfaces and strong Pb partitioning inthe liquid. However, the central part of the PASsystem hosts the wide stability field of the Pbfeldspar (Chen et al. 2001). Pb feldspar,PbAl2Si2O8, is able to accommodate over 40wt% PbO in its lattice and is the mostimportant, high-temperature Al-silicate in thePAS system and is resistant to acid and baseattack. Pb feldspar is easily nucleated at about950°C and should be stable up to 1200°C(Sorrell, 1962; Scheel, 1970; Bruno andGazzoni, 1970; Benna et al., 1996; Chen et al.,2001; Mastrovito, 2002). It shows a completemiscibility with the Ba-rich end-member,celsian, as well as thallium-bearing varieties(Sorrell, 1962; Bruno & Pentinghaus, 1974).These and the above characters make the Pbfeldspar structure highly interesting for theproblem of effective inertization of Pb andaffine metals in waste.

Therefore the PbO-MgO-SiO2-Al2O3-Cr2O3system (PMASCr) was investigated by several

experiments (hereafter labelled VD, VDN,MRS, MNV), whose bulk compositions wereplanned for associations including Pb feldsparplus Cr-rich spinel with decreasing Cr content(from VD to MNV). For the VD bulkcomposition, the Cr:Pb mole ratio was kept 1:1as in the real galvanic waste. Experiments wereequilibrated at low temperature, between950°C and 1050°C, from subsolidus and after ashort step at 1350°C. Experiments were neverlonger than one week in order to limitexcessive volatilization of Pb and Cr. Theplanned association of Pb feldspar and Crspinel occurs in all runs, even at lowtemperature, and in association with crystals ofthe eskolaite-corundum solid solution. Spineland Pb feldspar are the most abundant phasescrystallized except in the runs after the hightemperature step, where Pb-bearing glassovercomes Pb feldspar. Figure 5a-b showscharacteristic textural features for the runproducts. Spinels, eskolaite and corundumoccur as anhedral to euhedral crystals with a


Fig. 5a-b - Back-scattered electron (BSE) micro-images of run products from PMASCr experiments. (a) Loose aggregatesof euhedral spinels (black) bordering patches with tabular Pb-feldspar crystals (light grey) in Pb-bearing glass matrix (darkgrey) – equilibration at 1050°C from subsolidus; (b) euhedral spinels and spindle-like eskolaite crystals (both in black) inPb-bearing glassy matrix (white) – equilibration at 1050°C from the high temperature step (melting conditions). Greyshadows are caused by the presence of spinel and eskolaite crystals below the polished surface of the transparent glassmatrix.

(a) (b)

fine to very fine (to sub-mm scale) grain size.Spinel and eskolaite form loose aggregates inPb feldspar matrix or bordering coarse-grainedsubhedral feldspar crystals and glass patches.In the subsolidus runs micrometric corundumgrains may locally occur as clots in the core ofPb feldspar crystals.

Silicate glass is Pb-rich and Cr-poor. It isalways present in phase associations except inthe subsolidus run at 950°C. Pb feldspar isalways stoichiometric and does not host Cr inits structure. Spinel is the phase showing themost significant shifts in composition which isgenerally compatible with corresponding bulkcomposition (Figure 6a-b): the higher is thebulk Cr, the richer is the spinel in Cr. In themost Cr-rich bulk compositions (more similarto galvanic sludges) spinels can incorporate upto about 60 wt% Cr2O3. Spinel stoichiometry

varies between MgCr1.8Al0.2O4 andMgCr0.8Al1.2O4. Notably, for a given bulkcomposition the Cr enrichment in spinel ismaximum in the lowest temperature (950°C),glass-free runs.

IMPLICATIONS FOR WASTE INERTIZATION

The experiments on the highly complexMASCr system definitively showed that theaddition of Cr to the MAS system contributesto stabilize the refractory phases of the MASsystem as well as to lower the thermalminimum of the system by approximately100°C. These experiments bear importantimplications for both ceramic applications andinertization of Cr-bearing wastes. The presenceof wide Cr-bearing solid solutions in the


Fig. 6a-b – Compositional variations in Cr-rich spinel in the PMASCr experiments. (a) Trend in the Al vs. Cr content inspinel from all the experiments. Spinel formula on the basis of 4 oxygens. (b) Variation of Cr content in spinel inrelationship with Cr content of the starting bulk composition. For each bulk composition maximun Cr content in spinel isobtained at the lowest temperature (950°C).

950°C

VDNVD MNV MARS

1050°C

1050°C

0.25 0.5 0.75 1 1.25 1.5 1.75 20.0

0.2

0.3

0.4

0.5

Cr/(Cr + Al) in spinel

Cr/

(Cr

+ A

l) i

n b

ulk

spinel - bulk

0.1

b)

0.25 0.5 0.75 1 1.25 1.5 1.75 2

0.25

0.5

0.75

1

1.25

1.5

1.75

2

Cr (a.p.f.u.)

Al

(a.p

.f.u

.)

spinela)

MASCr phase assemblages is highlyencouraging due to the highly refractorycharacter and high chemical stability of theseCr-bonding phases. Similarly the strong Crpartitioning to solids and the consequent, verylow Cr contents in glass are of interest forinertization due to the chemical instability ofglass. In particular, the topology of the centralpart of the MASCr system, where thesapphirine-bearing quaternary assemblagesdominate, is of high applicative interest and isto be further investigated. As a matter of factsapphirine often occurs in moderate amounts inindustrial MAS ceramics (e.g., Chi et al., 2001)and may play an important role, together withspinel, in Cr inertization.

As far as the PMASCr system is concerned,the experiments performed were not plannedfor thoroughly exploring the complexities of a5-component system but to test the potentialapplication of such an unusual petrologicalsystem to a specific problem, i.e. theinertization of Cr- and Pb-rich galvanic waste.

The most favourable and satisfactory resultswere obtained for the low temperature (950°C)treatment conditions: (a) high system reactivitycausing ready and contemporaneous Pbfeldspar and Cr-rich spinel crystallization; (b)sharp heavy metal separation in distinctcrystalline phases; (c) absence of glass; (d)maximum incorporation of Cr in the most Cr-rich and interesting phases for ceramicapplications, i.e. spinel and eskolaite; (e)minimization of heavy metal volatilization. Thedrawback of low temperature treatment doesnot lie in reaction kinetics but in the very finegrain size of products. In this system Pb seemsto have a catalyzing effect on Cr spinelcrystallization at low (<1000°C) temperature,whereas at high temperature the presence of Pbseems to enhance metal volatilization. Theexperimental results so far obtained show thatthis system is of high interest for application totreatment of waste materials containing heavymetals with contrasting chemical behavior(refractory vs. low-melting).

Therefore the successful, industrialapplication of petrological models, like the

MASCr and PMASCr, to inertization of realCr-bearing wastes is dependent on variousfactors, on which research is to be continued:

– reduction of all the hazardous CrVI in thewaste to CrIII (treatment in controlled, reducingatmosphere). This would have two advantages:(a) to avoid the crystallization of undesiredchromate phases bonding CrVI and Pb together(like phoenicochroite, Pb2CrO5); (b) toinfluence the quality of gas emissions duringtreatment as CrIII is less volatile than CrVI.

– minimization of less stable phases, such asglass, and control of heavy metal volatilizationduring treatment: this is best done by keepingtreatment temperatures as low as possiblecompatibly with the phase association required.

– evaluation of the reaction kinetics of theimmobilizing phases during treatment: long-lasting treatments, as those often performedduring base petrological research, may not beeconomically viable in industrial applications.Preliminary kinetic experiments in thePMASCr system indicate a rapid reactionkinetics (Moroni et al., 2003).

– possibility of immobilization of minorheavy metal components of the waste into thebonding phases. Crystalline structures likespinel and feldspar are indeed able toincorporate a wide range of heavy metalsnormally accompanying Cr and Pb in wastesand with chemical affinities either to Cr (e.g.Ni, Sn, Zn, Cd) or to Pb (Ba, Tl), as shown bytest experiments on real galvanic sludges(Moroni et al., 2003; Moroni, unpublisheddata).

– extension of the effectiveness of theinertization treatment from small volumes(research laboratory) to big volumes (industrialscale).

In the view of recycling the inertized waste,or part of it, as a secundary raw material forsome industrial applications, it is important forthe most economically interesting components,e.g. Cr, to be separated from the least valuableones, e.g. Pb. Heavy metal separation can beperformed already during the thermalinertization, as shown in the experiments on thePMASCr system: Cr and Pb are bonded by


different phases, provided that all Cr is reducedto CrIII. Recover of Cr-bearing phases may thenbe operated by means of techniques ofmechanical separation normally used for orebeneficiation. The efficiency of recovery maybe then evaluated on the basis of texturalfeatures of the treated waste.

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fixing chromium and lead in ceramic materials: a...

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