Investigation of ancient lead slags from "Ari" at Lavrion and related metallurgical recycling...

Errata

In page 869, 5th line: instead of “a shallow furnace” read “a shaft furnace”.

Investigation of ancient lead slags from «Αri»at Lavrion and related metallurgical

recycling activities

Μελέτη αρχαίων σκωρίων μολύβδου από το Αρύστο Λαύριο και σχετικές δραστηριότητες

μεταλλουργικής ανακύκλωσης

P. E. Tsakiridis, C. Tsaimou, P. Oustadakis, G. D. Papadimitriou

Laboratory of Physical Metallurgy, School of Mining and

Metallurgical Engineering, National Technical Univeristy of Athens,

Greece, [email protected].

ABSTRACT

The aim of the present investigation is to study the chemical and miner-alogical composition of lead slags collected during the excavation cam-

paign of ancient furnaces at “Ari” - Lavrion by C. Tsaimou, in order to define the nature of pyrometallurgical activities related to the excavated furnac-es. Chemical analysis, X-ray diffraction and scanning electron microscopy in conjunction with energy dispersive spectroscopy-EDS were used to iden-tify the mineral species, consisting of CaO rich silicates, oxides and metal-lic phases. The primary slag phases include melilite-group minerals, spinel-group minerals, clinopyroxene, olivine-group minerals and glass. The pres-ence of CaF2 and BaSO4 in all slags, which constitute the typical gangue of the primary ore, as well as of Ca-rich silicates indicate the use of bar-

868 ΠΡΑΚΤΙΚΑ 5ου ΣΥΜΠΟΣΙΟΥ ΕΛΛΗΝΙΚΗΣ ΑΡΧΑΙΟΜΕΤΡΙΚΗΣ ΕΤΑΙΡΕΙΑΣ

ren local ore (tailings of the ore enrichment stage) as slag forming materi-al. The examination of the finds (slags, silver-poor litharge and metallic lead without silver) in association with the old smelting blast furnaces, indicate that the excavated plants have served for production of lead from litharge rather than for primary silver production. This corresponds to recycling of old byproducts, probably at the latest stage of metallurgical activities in Lavrion.

InTRODuCTIOn

The mining and metallurgical activity that was developed in the Lavreo-tiki peninsula took place since the Early Bronze Age and was continued un-til the 20th century. Ancient Greeks intensively exploited the argentifer-ous lead ores, in order to recover not only silver, but also metallic lead (Co-nophagos 1980, Papadimitriou 2000). Silver was a valuable commodity in an-cient times and until the decline of the Roman Empire, lead was principally obtained as a by-product of silver production.

There is evidence of lead workings and artefacts from very early peri-ods, in excavations dating well before the time of the Roman Empire. Early (Copper Age) production of lead was mainly located in Asia Minor, howev-er later centres of lead production moved east to the Aegean and Greece (from 2100-1200 BC) and the Iberian Peninsula (1200 BC to 500 AD), through the successive Greek, Carthaginian and Roman Empires. Lead was used on a large scale, as a useful material in its own right, for alloying, building and a range of miscellaneous uses. The use of lead declined after the collapse of the Roman Empire, but in the Middle Ages in Europe, lead began to be used again for many applications. Some of these applications, such as in water pipes and paints, have recently been phased out, or at least greatly reduced, in view of the potential health risks; however, some continue to this day (Nriagu 1983).

There are many reports of lead and silver mines in the classical literature. Within the Aegean area four main centres are attested by ancient authors: Lavrion in Attica, the island of Siphnos, the island of Thasos and various dis-tricts in Macedonia and Thrace. Finds of silver-poor litharge and lead at Thor-ikos in Lavrion, suggest that silver mining and the cupellation of argentifer-ous ores was practised at least from the end of the Middle Bronze Age; but

INVESTIGATION OF ANCIENT LEAD SLAGS FROM «ARI» AT LAVRION 869

the ancient authors record that the richest and deepest levels were exploit-ed fully since 483 B.C.

During the smelting process, the argentiferous lead ore was melted in a shallow furnace and exposed to a blast of air. Two molten fluids were tapped from the furnace and allowed to solidify in a small basin shaped in the ground or in finely-broken slag. The lead-silver metal alloy, which was the most dense, was settled on the bottom, to be overlain by the slag, a semi-glassy largely silicate material. The silver metal was then separated from the lead by cupellation, a process already ancient in Roman time. The metal was placed in a shallow open basin or ceramic bowl (the cupel) in a fur-nace and a strong blast of air was blown across the surface of the molten metal. The lead oxidized to form PbO (litharge, from Greek for the “spume of silver”) and the relatively pure silver remained as metal (Conophagos 1980, Papadimitriou and Kordatos 2001).

The litharge produced from the cupellation process, could be recycled through the smelting furnace to recover the lead metal. Lead oxide can be reduced in charcoal or wood fires below 900°C. A series of reactions took place in the furnace, but overall the effect is:

2PbO + C => 2Pb + CO2 (1)

The lead was molten and was tapped off from the bottom of the fur-nace. The fluxes with gangue formed a molten slag, a material composed of metal oxides/silicates. This process was very simple, but presented the problem of relatively low lead metal recovery. A large quantity of lead was lost to the slag, which was tapped or allowed to flow from the furnaces and solidify outside in round shallow molds.

Rather than a mere waste, slag was a technological by-product inten-tionally designed to optimise metal yield. The mineral charge was smelted approaching eutectic compositions, namely compositions able to produce low temperature melts. Effective metal-silicate segregation in the furnace was achieved through the design of low viscosity melts. The starting ore, furnace temperature, flux and procesing technology are all recorded within the slags.

The density and viscosity of the molten slag had to be kept low enough to ensure the gravity separation of metal-rich liquids, containing elevat-ed amounts of the desired metal. Silicate slag floated on the surface and concentrated the oxide components of the silicate gangue and additives.


Slag also contained metal-rich droplets unable to decant in time during the smelting process. In contrast with the other metals, most of the zinc was vaporized or dissolved in the liquid silicate slag - up to 20% ZnO (Ettler et al. 2000). The vaporized zinc could subsequently condense, forming oxides trapped by the molten slag.

Most of the slags are dense and dark gray to black and appear stony or visibly crystalline (Navarro et al. 2008). The slag generally contains a rel-atively high amount of lead and most of any zinc present. On the other hand the lead obtained from the above process may contain small amounts of some metallic impurities also contained in the ore. These can include the metals: copper, arsenic, antimony, bismuth, tin and silver. The tracing of these elements, particularly the volatile ones like arsenic and antimony, could serve as ore provenance indicators (Farquhar et al 1995, Manasse and Mellini 2002). It should be noted that, mineralogically, the furnace by-prod-ucts of metal smelting show the following primary phases: melilite, Zn-rich Ca–Fe silicates, spinels, Zn-rich fayalite, metal oxides, metallic Pb and glass (Kucha et al 1996).

Figure 1: Excavated complex of ore enrichment from Ari site, Lavrion


The aim of the present research work is the mineralogical and micro-structure examination of archaeometallurgical lead slags, from the area of Lavrion, excavated by the School of Mining and Metallurgical Engineering of Athens, Greece, under the directorship of Prof. C. Tsaimou, in order to de-fine the nature of pyrometallurgical activities related to the excavated fur-naces. Chemical analysis, X-ray diffraction and scanning electron microsco-py were used in order to identify the composition and the microstructure of the selected slag samples.

Figure 2: An excavated flat sluice from Ari site, Lavrion

Figure 3: An uncovered lead smelting furnace from Ari site, Lavrion


ExPERImEnTAL

Six samples of lead slag (identified as S1, S2 … S6), resulting from the sys-tematic excavation in the Ari site (Lavrion region), have been investigated. Ari is located at approximately 10 km north west from the modern city of Lavri-on. The site was in the antiquity one of the largest fields of mining and metal-lurgical activities, as is actually testified by numerous mining pits and galleries, washing and ore dressing plants and metallurgical furnaces. The presence of large quantities of metallurgical remains, such as slags, minerals and litharge (a by product of silver production), suggest that the ancient plants have prob-ably served at first for production of silver from lead bearing ores, extracted from several mine pits a few kilometers away and at a later period for lead re-covery from litharge by reduction in metallurgical furnaces (Figs. 1-3).

Seven major elements (Si, Pb, Ca, Fe, Zn, Al, Mg,) were determined using an atomic absorption spectrophotometer (Perkin Elmer 4000). The analysis of Pb, Ca, Fe, Zn and Mg was done using air–acetylene flame. In the case of Al and Si, nitrous oxide–acetylene flame was used. Ag, As, K, Na and Mn were determined by inductively coupled plasma mass spectrometry (ICP-MS, X Series II, Thermo Scientific). XRD tests were conducted with a Bruker D8 Focus diffractometer by a powder diffraction method, with nickel-filtered CuKα radiation (=1.5406 Å), 40 kV voltage and 40 mA current. The microstructure of the slag samples was evaluated by scanning electron microscopy (SEM) using a Jeol 6380 LV SEM. Experimental conditions involved 15 kV accelerating voltage at low vacuum (30 Pa), using a backscattered electron detector. SEM was performed in pol-ished sections. Polished sections were produced by vacuum impregnation, of the selected sample, in a low viscosity epoxy resin. After removing a small sur-face by cutting in a micro-saw, the sample was grinded and polished with 1μm diamond paste, on a lapping disk. Microanalysis was performed by an Oxford INCA Energy Dispersive Spectrometer (EDS) connected to the SEM.

RESuLTS AnD DISCuSSIOn

A total of six lead slag samples, the waste product of smelting incorpo-rating most of the unwanted elements in the ores, were examined. They consisted of pieces 5-15 cm thick with wrinkled or bubble-pocked upper surfaces, the largest pieces being up to 30 cm long (Fig. 4).


The textural features of the slags depend upon the cooling rates. They are well crystallized and show only a few large gas bubbles. None of the slag samples present magnetism to a handheld magnet. They are all grey-black in colour and contain small metallic inclusions of Pb inside their matrix.

The chemical composition of the examined slags is reported in Table 1. The slags are composed of up to 65% of SiO2, CaO, Fe2O3 and Al2O3 and up to 35% of PbO and ZnO. The measurements showed that silica (24-26%) and lead oxide (20-22%) are the main constituents, followed by calcium ox-ide (13-16%) and iron oxide (10-18 %) and smaller amounts of other gangue elements.

The composition is consistent with a Ca-rich silicate slag, which float-ed on the surface and concentrated the oxide components of the silicate gangue and additives. Lead, which is contained mostly as metal-rich drop-lets of Pb, was unable to decant in time during the smelting process. Its high content confirmed the relatively low lead metal recovery, a fact that indicated an ancient smelting process. Except lead, all slags contain 9-22 % ZnO, 0.05-0.97 % arsenic and 3-15 ppm silver. The very low content of sil-ver is an indication that the slags result from the production of lead from litharge rather than from primary silver production. The presence of arse-nic, silver and zinc, which derived from the initial ore, is an indication of us-ing the barren ore (after the ore enrichment stage) as a flux, for the for-mation of molten slag.

Figure 4: Slags from Ari site, Lavrion


Table 1: Chemical analysis of slags

(%) S1 S2 S3 S4 S5 S6

SiO2 26.1 26.1 24.3 24.3 24.7 25.4

CaO 15.6 15.5 16.2 14.2 13.6 13.3

Al2O3 4.70 4.65 4.59 3.02 4.63 4.40

Fe2O3 14.1 15.5 17.0 10.2 11.6 18.7

MgO 2.45 3.10 2.27 2.04 2.85 2.87

K2O 1.17 1.14 1.04 0.80 1.22 1.30

Na2O 0.61 0.54 0.47 0.40 0.61 0.54

ZnO 9.26 10.1 9.29 22.0 18.5 9.68

PbO 22.5 20.3 21.2 19.5 18.5 20.0

MnO 0.66 1.05 1.06 1.16 1.19 1.16

Ag (ppm) 15.2 3.3 8.8 6.9 3.8 5.7

As 0.19 0.07 0.08 0.97 0.05 0.05

Much of the arsenic must have been vaporized in the smelting process, hence there is relatively little in the furnace products. On the other hand, most of the zinc was dissolved and remained in the liquid silicate slag. Minor elements are MgO, MnO, K2O and Na2O. The total of the chemical analyses is below 100%. This observation should be attributed to the fact that parts of the metals such as Pb are present in the oxidic state and sometimes FeO can be present in the trivalent and MnO in its tri- or tetravalent form. Slags con-tain a quantity of iron oxide which depends on variables such as the tem-perature and atmosphere inside the furnace, and also on the quality of the ore used and the presence of materials such as silica and lime.

The X-ray diffraction spectra of the examined slags, from Ari are given in Figure 5. They are composed essentially of CaO rich silicate (melilite group and clinopyroxene), olivines, oxides (spinels), metallic phases and glass. Aker-manite (Ca2MgSi2O7) and hardystonite (Ca2ZnSi2O7) are the predominant phases. They belong mineralo-gically to a complex solid solution with the general formula (Ca,Na,K)2(Mg,Fe2+,Fe3+,Al)(Si,Al)2O7 (Deer et al 1992). In iron-rich silicate slag produced by the old iron metallurgy, melilite group


phases normally occur only as an accessory constituent. But melilite is usu-ally the main component in the Ca-Mg slags (Ettler et al 2000).

The detection of ZnFe2O3, in addition to Fe3O4, indicates that the triva-lent iron was able to combine with zinc to produce the spinel phase upon cool-ing from the melt. In contrast with the other metals, most of the Zn is vapor-ized or dissolved in the liquid silicate slag. The vaporized Zn can subsequently condense, forming mixed oxides trapped by molten slag. During the cooling of the melt, zinc enters first into the spinel structure. Melilite starts to nucleate simultneously with spinel and also concentrates zinc (Saffarzadeh et al 2006).

Phase occurrence obviously depends on chemical composition. For in-stance, the detection of calcic clinopyroxenes esseneite (CaFeAlSiO6) indi-cates Ca-rich slags. Esseneite usually occurs with anorthite or melilite solid solutions and glass. The formation of olivine is due to the presence of mag-nesium oxide in the slag.

The dissolution of MgO into most slags usually results in increasing the liquidus temperature of the slag with respect to an MgO containing phase, such as magnesia wustite, spinel or olivine phase. It should be noticed that slags with lower Mg content would crystallize only pyroxenes instead of ol-ivine (Devic and Marceta 2007).

The strong peak of metallic Pb confirms the fact that, because of the smelting process, a large quantity of lead was lost to the slag, which was

Figure 5: XRD pattern of the examined slags


allowed to flow from the furnaces and solidify.The occurrence of a small amount of glassy phase (slow rate of cooling) is denoted by the small back-ground and particularly by the characteristic small hump in the 25° 2θ and 35° 2θ range of the diffraction pattern.

Wollastonite (Ca2.87Fe0.13(SiO3)3) has probably been stabilized by the pres-ence of impurity ions (Fe3+, Al3+) and a relatively slow rate of cooling. Part of the iron minerals are selectively oxidized and fluxed as fayalite (Fe2SiO4), a phase that was detected in all slags at lower percentages (a second part of the iron minerals are oxidized and fluxed as Fe3O4 which increase the viscos-ity of the slag). The predominance of zinc ferrite and magnetite indicates only slightly reducing conditions, while the presence of fayalite together with the heterogeneity of the samples show that the operating conditions are likely to have fluctuated considerably during the process (Kucha et al 1996).

It should also be noted that the Zn-bearing fayalite (Zn2SiO4) was detect-ed in slags S4 and S5. Zn2+ substitutes for Fe2+ in the fayalite crystal struc-ture. Zinc enters into the crystal structures of oxides and silicates (spinel, melilite, clinopyroxene and olivine), due to the substitution for Fe2+ in octa-hedral sites (Ericsson and Filippidis 1986).

It is likely that CaF2 is a relic of the primary ore, while the presence of cerrusite should be attributed to the carbonation of the PbO content in slags. The occurrence of illite ((K,H3O)Al2Si3AlO10(OH)2) is due to the sam-pling procedure, where clay minerals were not fully removed from the slag samples. A minor amount of quartz is also present.

All slags were subjected to scanning electron microscopy (SEM) using a backscatter detector, in conjunction with energy dispersive spectroscopy (EDS). The minerals crystallization is the result of the smelting process and they are always euhedral fully developed crystals or skeletal crystals with hopper structures. Sizeable euhedral crystals and elongated laths and pris-matic crystals are typical for the silicate phases. Rectangular and trellis pat-terns and isolated octahedra or cubic forms are typical of the oxide phases. Within the isotropic glasses, complex silicate crystalline phases are the most abundant and oxide crystalline phases are less common (Fig. 6). Microscopic examination showed that akermanite (Ca2MgSi2O7) and hardystonite (Ca-2ZnSi2O7), of the melilite group, are the most abundant crystalline phases in all slags and are crystallized with the form of planar slabs. Melilite-type phases crystallize only in Ca-rich melts and form in both slowly cooled and quenched slag.


Zinc ferrite and magnetite (spinels) developed through crystallization from the melt into idiomorphic individuals, but exist also in eutectic growth with melilite phases. Τhe spinel-group phase is amongst the first to crystal-lize from the silicate melt. While the silicates nucleate simultaneously with the oxides, small crystals of spinel appeared to be embedded in the melilite phase. The magnetite and zinc ferrite spinel oxides occur in all of the slags and appear most frequently as cubic to octahedral euhedral crystals. Some-times spinel crystals exhibited skeletal form where the cores are hollow. In back-scattered electron mode, the zinc ferrite grains presented an optical zoning, with a bright core and a significantly darker grey rim. The brighter internal parts are enriched in zinc, and the darker external parts are Fe-rich.

All slags from Ari contain clinopyroxene. This is due to the high CaO-con-tent in these slags. A precondition for the crystallization of an iron-rich pyrox-ene is the existence of CaO, because ferrosilite (FeSiO3) is not stable. Clinopy-roxene was detected with the form of euhedral prismatic long crystals. EDS analysis proved that it was actually a Ca-Fe-Al clinopyroxene, which was iden-tified as esseneite (CaFeAlSiO6) by X-ray diffraction. Its crystal size, as well as that of the melilite (the two dominant phases in all slags), indicate a relatively slow rate of cooling (crystallization). The amount of Zn bound in clinopyrox-ene was low (1.5 wt.% ZnO), a fact that suggests that the clinopyroxene can-not accommodate more zinc under the conditions of its crystallization.

Sizeable euhedral crystals of ferroan wollastonite were detected in all slags. They were typically embedded in glass with higher silica content. Their precise determination was made possible by interpreting the X-ray diffraction pattern and EDS analysis.

Olivine-group phases, which represent an isomorphic mixture of forster-ite Mg2SiO4 and fayalite Fe2SiO4, probably are the last silicates crystallizing from the liquid in all slags, filling spaces between the earlier-crystallized sili-cates. They form small needles or dendrites, often difficult to analyze by EDS, due to their size. They are sometimes detected with the form of skeletal phenocrysts, which appear to have crystallized directly from the host melt.

In slags S4 and S5, the chemical composition of the olivines corresponds to Zn2SiO4 (willemite). Its presence supports the suggestion that Zn2+ (ionic radius 0.74Å) can enter into octahedral sites of the structure and substitute for Fe2+ (ionic radius 0.76Å) (Ericsson and Filippidis 1986). The glass phase usually appears colorless but in places is light brown to yellow in color, al-most always containing complex silicate mineral phases. It is chemically het-


erogeneous and is composed mostly of silicon, calcium, iron, aluminum and magnesium. It also exhibits elevated concentrations of zinc, whereas in com-parison with the crystalline silicates, it presents higher percentages of lead.

Lead metallic phases are easy to distinguish in the material as they have very higher reflectance when compared with the other ore minerals. They have various forms: drop-like, vermicular and oval. Lead behaves as incom-patible element since it has been markedly characterized as part of polyme-tallic alloys in the form of spherical metallic inclusions associated with trac-er amounts of other elements such as Sb, Sn, As, Zn, Al, and Si. In some cases “drops” of metallic Ag were also detected.

COnCLuSIOnS

According to the results presented in this paper, the following notewor-thy conclusions have been reached:

― The surface finds (slags, silver-poor litharge, metallic lead) together with the old smelting blast furnaces and the low silver content in the slags, indicate that the excavated plants have served for production of lead from litharge rather than for primary silver production. This corresponds to recycling of old by-products, probably at a later stage of metallurgical activities in Lavrion.

― All the examined lead slags were compositionally similar and so produced by a similar smelting technology. Their composition is characterized by relatively low contents of silica and high contents of the divalent cations. The lead concentration ranges from 16 to 20%, a fact that confirms that because of the smelting process, a large quantity of lead was lost to the slag.

― Primary slag phases include: melilite-group minerals (akermanite and hardystonite), spinel-group minerals (magnetite and zinc ferrite), clinopyroxene (esseneite), olivine-group minerals (olivine, fayalite and willemite) and glass.

― The identification of Ca-rich silicates (melilite, clinopyroxene) in all slags suggests the presence of CaCO3 or lime as melting agent. The presence of CaF2 and BaSO4 in all samples, which are derived from the primary ore, indicates the use of the barren local ore (from the ore enrichment stage) as a flux for the formation of the molten slag.


― The crystal size of the melilite and clinopyroxene (the two dominant phases in all slags) reveals a relatively slow rate of cooling of the initial slag melt. This fact was also confirmed by the occurrence of a small amount of glassy phase in the 25° 2θ and 35° 2θ range of the X- ray diffraction pattern, which is denoted by the small background and particularly by the characteristic small hump.

― The slag was rich in zinc, probably derived from the local ore. Zn is almost totally dissolved in the silicate melt. During the cooling of the melt, it enters into the spinel and melilite structure. However, Pb is mainly concentrated in the residual glass and Pb-rich metallic inclusions are trapped in the glass.

(a) (b)

(d)(c)


REfEREnCES

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Figure 6: Backscattered scanning electron micrographs of polished section of Pb-metallurgical slag from Ari site-Lavrio: Prismatic crystals of melilite (Mel) and columnar clinopyroxene (Cli). Euhedral crystals of zoned spinel (Sp) embedded in glassy matrix (Gl) and in eutectic growth with melilite. Dendritic olivine-group crystals (Ol) in the amorphous glass (Gl) phase. Spherules composed of lead (Pb). Distinct particles of Fluorite (Fl) dispersed in matrix. Euhedral crystals of wollastonite (Wo) embedded in glass (Gl) phase. Tabular

plates of barites (Bar) together with octahedral fluorite (Fl) inside large pore

(e) (f)


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Investigation of ancient lead slags from "Ari" at Lavrion and related metallurgical recycling...

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