- © 2016 The Mineralogical Society
Phases present in slags from Araxá (Brazil), Oka (Quebec) and Fen (Norway) resulting from the production of ferroniobium by the aluminothermic reduction of pyrochlore-group minerals are a function of composition of the pyrochlore feed and the diverse fluxes (CaO, CaF2) added to the alumina and iron oxide used in the reduction process. Absent from all slags investigated are Fe-bearing compounds. All of the slags are modally-dominated by prismatic crystals of β-alumina or hibonite, with a wide variety of Th-Ba-Ti-Nb-Al-oxide compounds and silico-aluminate glasses occurring in the interstices between these crystals. Slag from Fen consists of a framework of β-alumina (Ba and/or CaTi varieties) with a barian titanian niobate, considered to be an highly reduced Nb3+- and Ti3+-bearing anion deficient perovskite, as the first oxide phase to crystallize. This was followed by a Zr-Ti-Th-niobate (21.7–24.1 wt.% ThO2), a Nb-rich, Th-bearing zirconolite-like phase (17.7–21.2 wt.%; Nb2O5; 6.8–9.5 wt.% ThO2) and diverse Th-Nb-rich, Al-poor (< 1 wt.% Al2O3) perovskites (29.4–40.5 wt.% Nb2O5; 3.3–11.4 wt.% ThO2). The latter represents a NaNbO3–CaTiO3–Na2/3Th1/3TiO3–ThTi2O6 solid solution. Late Na-rich silicate glass contains celsian. Slag from Araxá also consists of a framework of β-alumina (Ti-poor). Interstitial compounds differ from Fen in being predominantly rare-earth element (REE)-Nb-Al-Th-perovskites belonging to a REEAlO3–Ca2AlNbO6–CaTiO3–ThTi2O6 solid solution with minor NaNbO3 and CaZrO3, and compositionally diverse barium calcium aluminates [(Ba,Ca)Al2O4] set in a F-bearing (6–7 wt.%) Na-Ba-Si-aluminate glass with minor Ba4Nb2O9, CaF2, BaF2 and NaF. Slag from Oka consists of a framework of hibonite [(Ca,REE)(Al,Ti,Mg)12O19] zoned with respect to the REE content. Interstitial compounds are CaZr4O9, Nb-Th-poor perovskites belonging to the REEAlO3–CaTiO3 series with minor CaZrO3 set in a F-bearing (5–6 wt.%) calcium aluminate glass. These data demonstrate that during ferroniobium production REE, Zr, Th, U and significant amounts of Nb are sequestered in the oxide phases in accord with their lithophile geochemical character. None of these elements are present as compounds in the siderophillic ferroniobium alloys. These data demonstrate that the aluminothermic reduction process results in the loss of significant amounts of Nb to the slag. Perovskite-group compounds are present in all of the slags but differ widely in their composition as a result of compositional differences in the feed pyrochlores and fluxes added to the smelting process. Slags originating from ferroniobium production are enriched in Th and U and could be considered as an environmental hazard if these elements are mobilized during chemical weathering of slag.
Slags produced by smelting are the most important waste materials resulting from metallurgical processes. These slags commonly contain significant amounts of toxic metals or radioactive materials (Ettler et al., 2009; Puziewicz et al., 2007; Helios-Rybicka, 1996). Typically these metallurgical wastes are abandoned at the site of their production and exposed to chemical weathering resulting in the potential release of toxic elements to surface and ground waters (Piatak et al., 2004). Mineralogical investigations of slags are essential for understanding the sequestration of toxic elements in the slags and are a necessary requirement for assessing any potential detrimental environmental effects. A further aspect of such investigations is to assess the efficiency of the smelting process. Knowing how much metal is lost to the slag and in which phases it is concentrated can assist in improving the smelting process.
The primary material from which niobium metal or niobium-iron alloys, used as additives in steel, are extracted is pyrochlore. The initial step in the production process is based upon the aluminothermic reduction of this mineral (Gupta and Suri, 1993). The process involves the addition of metallic Al and iron oxide together with other fluxing compounds, such as CaO or CaF2, to pyrochlore concentrates. Ignition of the mixture in an electric arc furnace at ∼2200°C results in an exothermic reaction and the formation of an iron-niobium alloy, termed ferroniobium, which typically contains ∼50–65 wt.% Nb and an aluminium-rich oxide slag. For every tonne of ferroniobium produced ∼1.8 tonnes of slag are generated. Typically, this slag has been disposed of simply by dumping it adjacent to the processing plant, although recently at the St. Honoré Niobec Nb mine (Québec) this material has been back-filled into mined-out galleries.
Very little is known about the compounds present in these slags although they have been recognized as a potential radiological hazard by the U.S. Nuclear Regulatory Commission (Veblen et al., 2004) and the International Atomic Energy Agency (Wymer, 2006) because of the presence in them of Th- and U- bearing compounds. Phase characterization of slags from the USA and Estonia have been reported only by Veblen et al., (2004) and Gorkunov and Munter (2007). Although a wide variety of compounds, including zirconolite, perovskite, hibonite and diverse calcium aluminates were identified, neither of these investigations provided detailed compositional or textural data for the slags investigated. This study presents the results of the first detailed ‘mineralogical’ study of three slags produced by the aluminothermic reduction of pyrochlores extracted from carbonatites mined at Fen (Norway), Araxá (Brazil) and Oka (Québec). Of these smelters only that at Araxá is currently producing ferroniobium.
Polished thin sections of slags were examined by standard petrographic methods and back-scattered electron petrography. Compositions of phases present were determined by quantitative energy dispersive X-ray spectrometry using a Hitachi FE-SU70 scanning electron microscope equipped with the Oxford Instruments AZtec software package. The electron accelerating voltage was 20 kV and beam current 300 pA. Standards used were: synthetic pyroxene glass (Si, Ca); corundum (Al); jadeite (Na,Al), BaF2 (Ba, F), Mn-hortonolite (Mg, Fe Si, Mn); synthetic rare-element phosphates (La, Ce, Pr, Nd, Sm, P); SrTiO3 (Sr, Ti), ThNb4O12 (Th, Nb); metallic Zr and U. Analytical errors for major and minor elements are considered to be ±0.2 wt.% and 0.5 wt.%, respectively. The veracity of the electron-dispersive spectroscopy (EDS) data was assessed initially by wavelength-dispersive X-ray spectrometry (WDS) of the barium titanates using a JEOL 8900 electron microprobe at the Fachbereich Geowissenschaften Universität Tübingen, Germany (acceleration voltage 20 kV and beam current 20 nA). Subsequently, combined WDS (Oxford Instruments Wave 500) and EDS (Oxford Instruments X-Max detector) data for barium titanates, including oxygen, were collected simultaneously using a Quanta 200 analytical SEM at Oxford Instruments America (Concord, MA) with an accelerating voltage of 20 kV and beam current of 10 nA.
In our EDS protocol particular attention was given to BaL and TiK analytical line overlaps by analysis of pure Ti, TiO2, FeTiO3 and SrTiO3 to assess the amount of ‘false’ Ba that might be introduced due to an analysis by AZtec spectrum stripping methods. The ‘false’ Ba introduced was found to be a linear function of Ti content, which amounted to <0.5 wt.% BaO for FeTiO3. In addition, benitoite (BaTiSi3O9) was found to give acceptable compositional data for BaO and TiO2 using our EDS analytical conditions. All Ba-titanates analysed had well-resolved Ba and Ti peaks and any accuracy errors are considered to be < 0.3 wt.% BaO or TiO2. We consider that our data provide realistic compositions for these phases as in addition they are in accord with the WDS data.
An example of the ferroniobium alloy produced at Araxá in 1976 is shown in Fig. 1. The material consists principally of round-to-ellipsoidal crystals of Nb (92.9 Nb; 4.3 Fe; 0.7 Sn; 0.6 Ti; 0.3 Al wt.%) set in a matrix of a Nb-Fe-alloy (64.1 Nb; 33.6 Fe; 0.9 Si; 0.9 Al; 0.4 Ti wt.%). Some areas of the latter are enriched in phosphorous (1–2 wt.%). Small pore spaces in the Nb-Fe-alloy contain a Nb-Ti-alloy intimately intergrown with a very small unidentified phase (<1 μm) containing significant sulfur (>10 wt.%) and minor tin (∼1–2 wt.%). The Fe-rich Nb-alloy does not correspond to the compounds Nb19Fe21 and NbFe2 reported in the binary system Fe–Nb but is close to that of the phase Nb3Fe2 (Gasik 2013; Zelaya Bejarano et al., 1991, 1993). The ferroniobium alloy does not contain any complex oxides (perovskite, zirconolite), Ba-Ca-aluminates, Th and U-bearing compounds or silicate and aluminate glasses.
Slag from the Fen Complex
Slag derived from the Fen carbonatite pyrochlore consists principally of a framework of β-alumina crystals with small amounts (<1 vol.%) of relict anhedral pure alumina. A wide variety of niobates, aluminates and titanates occupy the interstices together with minor residual celsian and feldspathic glass.
The β-alumina structure [(Na,Ba,Ca,REE)Al11O17] is a derivative of the magnetoplumbite structure (AB12O19; Townes et al.,1967) formed by replacing the [AO3] layer in that structure with an A2O layer in which the A site is 50% occupied, resulting in an effective composition of AO for this layer. The structure differs from magnetoplumbite in that the fifth layer has 75% of the oxygens missing resulting in 17 oxygens in the unit cell. The empty spaces in this layer can be occupied by a variety of large cations (Collin et al., 1986).
In Fen slag, β-alumina forms subhedral-to-euhedral crystals which exhibit a pronounced pale brown-to-blue pleochroism (Fig. 2) and strong green cathodoluminescence (CL) (Fig.3). Back-scattered electron (BSE)-imagery demonstrates that these crystals are heterogeneous (Fig.4), undoubtedly as a consequence of different cations occupying the ‘inter-layer’ regions of the crystal structure. In the cores, these regions are enriched in Ca, Ti and Ce, whereas the rims are devoid of these elements and occupied only by Ba (Table 1). The origins of the green cathodoluminescence spectrum (Fig. 3) cannot be identified because of the potentially large number of activator cations in the interlayer regions. The presence of a single emission band suggests Mn2+ or Eu2+ might be activators as similar emission bands have been observed in a variety of barium and strontium aluminates (Dejene and Kebede, 2012; Xing et al. 2006; Ravichandran et al., 1999). The single emission band rules out heavy REE such as Dy and Tb as activators which typically produce several very sharp CL emission lines over a wide range of wavelengths.
Many of the oxide compounds occupying the interstitial regions are heterogeneous with respect to their composition (Fig. 5), and consequently only representative compositions of relatively homogeneous examples (Figs 6 and 7) are presented here. Interstitial regions vary widely in size and range up to 500 μm. Individual regions differ considerably with respect to the character and proportions of the oxide compounds present. Many do not contain residual feldspathic glass. Small spheres (up to 200 μm) of metallic niobium can be found in some of the interstices and trapped in the β-alumina.
Barium titanium niobate
Barium titanium niobate was the first phase to crystallize in the interstitial regions and occurs as anhedral-to-subhedral crystals included in later-forming compounds (Figs 5 and 6). The compound does not exhibit compositional zoning in BSE-imagery. Analysis of this niobate, by both EDS and WDS methods, with the Nb and Ti contents calculated as Nb2O5 and TiO2 results in very large analytical totals (∼112 wt.% total oxides) which are considered to be erroneous but not due to analytical problems. The error results from overestimation of the oxygen content of the compound by assuming that Nb and Ti are present as Nb5+ and Ti4+, respectively. However, direct analysis of the oxygen content by both EDS and WDS methods shows that oxygen is present at levels suggesting that both Nb and Ti have been reduced to the trivalent state. For example, analysis of one crystal by EDS with total oxygen calculated from stoichiometry gives 33.28 wt.% O, in contrast to only 24.06 wt.% O and 25.17 wt.% O, when O is determined by direct analysis by EDS or WDS, respectively. Calculation of the composition of these niobates with Nb and Ti as trivalent cations results in acceptable analytical totals (Table 2). Without crystallographic studies it is impossible to identify the structural group of these niobates as calculation of a rational structural formula is not possible. However, recalculation of the compositions on the basis of 3 or 11.5 atoms of oxygen suggests that these compounds could be anion deficient perovskites similar to Ba1–xCax/2(Ti1–xNbx)O3–y (Ravez and Simon, 2000). This phase forms in the initial stages of the crystallization process and it seems that oxygen fugacity increases rapidly once the slag starts to separate and crystallize. Speculation on values is not warranted in this heterogeneous non-equilibrium system.
Zirconium titanium thorian niobate and zirconolite-like compounds
Zirconium titanium thorian niobate (Table 3) and Th-bearing zirconolite-like (Table 4) compounds crystallized subsequently to the reduced Ba-Ti-niobates (Figs 5–7). Unlike the latter, analysis of these compounds resulted in satisfactory analytical totals assuming that Nb and Ti are present as penta- and tetravalent cations, respectively. Although the compounds are complexly intergrown it is evident, on the basis of textural relationships, that the very Th-rich niobates crystallized prior to the zirconates; the latter being relatively depleted in Th and U. Calculation of structural formulae on the basis of 7 atoms of oxygen (Tables 3 and 4) suggests that both compounds are related to the zirconolite structural group [(Ca,Zr,Ce,Th,U)2(Nb,Ti,Ta, Al)2O7], although they probably have A-site cation and oxygen deficiencies. Naturally-occurring zirconolite is well-known for the sequestration of actinides (Gieré et al.,1998; Bellatreccia et al., 1999) and is a major component of SYNROC (Ringwood, 1985). Comparison of the compositions of these slag-derived zirconolites with natural examples not surprisingly shows major differences notably with respect to the paucity of rare-earth elements (REE) and Fe together with extreme enrichment in Th in Zr-Ti-Th-niobate.
Thorian niobium perovskite
The final major oxide phase to crystallize in the interstices of the beta-alumina framework is a Th-Na-Ca-Nb-titanate (Table 5; Figs 6 and 7). This compound exhibits significant compositional heterogeneity characterized by depletion in Th and increases in the content of Nb and Na. Calculation of structural formulae on the basis of 6 atoms of oxygen suggests that this is a non-stoichiometric perovskite-like compound enriched in Na and Ca relative to the earlier crystallizing zirconolite-like compounds. It is not possible to assign definitively cations to specific end members of known perovskite solid solutions as high Th contents of perovskite-group compounds result in significant A-site deficiencies (Mitchell and Chakhmouradian, 1999). However, the potential major components could be NaNbO3 (lueshite), CaTiO3 (perovskite), Na2/3Th1/3TiO3 and ThTi2O6 (thorutite). Note that the latter is not a perovskite-group compound. However, Mitchell and Chakhmouradian (1999) have shown experimentally that up to 18 – 20 mol.% ThTi2O6 can be accommodated in NaREETi2O6 (synthetic loparite) – ThTi2O6 solid solutions at 1450°C.
Other oxide compounds
Minor amounts of Al-Nb-titanates and Nb-Ba-titanates crystallized contemporaneously with the compounds described above. These show wide variations in their composition with representative examples illustrated in Figs 6 and 7.
The final stages of crystallization of the slag, interstitial to the oxide phases, results in the formation of anhedral crystals of celsian (41.4 BaO; 27.2 Al2O3; 31.2 SiO2 wt.%) set in Na-rich silicate glass (18.8–19.4 Na2O; 4.7–5.4 BaO; 1.0–2.0 CaO; 35.5–36.0 Al2O3; 36.7–38.0 SiO2 wt.%).
In common with slag from Fen, that from Araxá consists of a framework of β-alumina crystals. It differs in that the crystals are thin quench prisms (up to 3 mm × 0.5 mm) which are not pleochroic (Fig. 8), although they show a similar green cathodoluminescence. The crystals are relatively homogenous and are similar in composition (Table 1) to the Ba-Na-variety of β-alumina found in Fen slag but lack detectable Ti. The absence of blue pleochroism is ascribed to the paucity of Ti3+ which undoubtedly acts as the chromophore in β-alumina from Fen slag. Interstitial compounds (Figs 9 and 10) differ significantly from those in Fen slag and consist principally of diverse REE-Nb-Na-Th-U-bearing perovskites, Ba-Ca-aluminates [(Ba,Ca)Al2O4], together with abundant late-stage heterogeneous F-Si-Na-Ca-Ba-aluminate glass, pyrochlore and minor Na-Nb-Ba-oxides, CaF2, BaF2 and NaF. Zirconolite and celsian are absent.
The earliest interstitial compounds to crystallize were large (up to 500 μm) euhedral-to-subhedral perovskites. These consist typically of a large, relatively homogeneous core of REE-Al-Ca-Ti-perovskite with a thin (20–50 μm) rim of Na-Nb-perovskite (Figs 9 and 10). Discrete compositional zoning is rarely present (Fig. 11). Representative compositions are given in Table 6 which demonstrates that the REE-rich cores are complex solid solutions involving the REEAlO3, Ca2AlNbO6, CaTiO3 and ThTi2O6 end-member molecules. The rim perovskites are REE poor and solid solutions between Ca2AlNbO6, CaTiO3, NaNbO3 and CaZrO3 (Table 6). Anhedral perovskites of similar composition occur in complex intergrowths with late-stage glass (Fig. 12). Compositional evolution involves principally a decrease in Th, REE and Al coupled with increases in Nb, Na and Zr. Unlike the perovskites in the Fen slag these perovskites are near-stoichiometric and do not appear to be oxygen deficient and in this respect are similar to many synthetic ternary and quaternary perovskites (Mitchell, 2002). Some perovskites (Fig. 11) contain small (<50 μm) euhedral inclusions of a U-bearing thorianite [85.32 ThO2, 6.60 UO3, 3.90 Ce2O3, 1.98 ZrO2 wt.%; (Th0.82U0.06Ce0.06Zr0.04)0.98O2].
Barium calcium aluminates (MAl2O4; M=Ba, Ca)
These compounds crystallized subsequently to REE-Al-rich perovskite and prior to the glass mesostasis of the interstitial material (Fig. 10). They occur as subhedral or anhedral crystals (up to 500 μm) which show considerable variation in composition. The earliest-formed crystals are Ca-poor near-stoichiometric BaAl2O4 (Fig. 10; Table 7 comps. 1–3). In contrast, the later-forming varieties are more abundant, mantled by glass (Fig. 9) and enriched in Ca (Table 7, comps. 4–6). Typically, compositions are non-stoichiometric and exhibit M site large-cation deficiencies. Glasser and Glasser (1963) have synthesized diverse MAl2O4 (M = Ba, Ca, Sr) compounds and shown them to have a stuffed tridymite-type structure. In these compounds, AlO4 tetrahedra form a framework isostructural with tridymite with the large divalent cations occupying the interstitial sites. The M site large-cation deficiency of the Araxá slag examples (M1–xAl2O4; x ≈ 0.15) is considered to result from incomplete occupation of the interstitial channels of the AlO4 tridymite-type framework.
Late-stage F-Na-Ba-Ca-aluminate glass
Late-stage isotropic glass forms subsequently to the Ba-Ca-aluminates. BSE-images (Fig. 12) demonstrate that the glass is heterogeneous and consists of irregular patches (up to 50 μm) of aluminate with slightly different average atomic number together with minor anhedral BaF2, CaF2, NaF and interstitial oxides of irregular morphology. The majority of the latter are Na-Zr-Nb-perovskites similar in composition to the rims (Table 6) of the early-forming large REE-perovskites. Also present are an A-site cation-deficient F-bearing Na-Ca-pyrochlore [(Na0.69Ca1.20)1.89(Nb1.77Ti0.15Al0.08)2.00(O6.17F0.83)7], and minor amounts of an unidentified Na-Nb-Ba-oxide [57.9 BaO, 34.9 Nb2O5, 3.1 TiO2, 3.6 Na2O, 0.8 CaO wt.%;], that is possibly a variety of the 6H-rhombohedral perovskite Ba4Nb2O9 (Juri et al., 2013).
Representative compositions of the heterogeneous aluminate glass are given in Table 8. Regions of relatively high are slightly enriched in Ba and depleted in Si relative to those of lower . All glass is Na (12–14 wt.% Na2O) and F-rich (6 – 7 wt.% F). In contrast to the late stage glass in Fen slag Ba-bearing feldspar is absent.
Slag from Oka in contrast to that from Fen and Araxá consists of two phases of discrete texture and composition. The majority of the slag consists of an extremely fine grained, near isotropic, optically non-resolvable material which contains numerous globules of Nb-Si-Fe-alloys. The principal component of this phase is a hibonite-like compound (Fig. 13), the crystallization of which was followed by minor β-alumina, a calcium zirconate similar to CaZr4O9, REE-Al-Ca-Ti-perovskites and calcium aluminate glass. The other, and minor, part of the slag is relatively coarse grained and consists of quench prismatic crystals of anorthitic plagioclase (An78–61) set in a F-Na-REE-bearing heterogeneous aluminosilicate glass (see below).
In contrast to slag from Fen and Araxá the framework of the fine-grained slag consists principally of hibonite [(Ca,REE)(Al,Ti,Mg)12O19], a member of the magnetoplumbite structural group (Figs 13 and 14). The difference suggests that redox conditions present during the formation of the Oka slag were not as reducing as at Fen or Araxá, as β-alumina, as noted above can be considered as an oxygen deficient derivative structure of the magnetoplumbite structure. The hibonite forms hexagonal euhedral crystals (<100 μm) which exhibit discontinuous compositional zoning in BSE images (Figs 13).The zoning principally reflects variations in REE contents (Table 9).
Beta-alumina is a minor component of this slag and occurs as thin prismatic crystals which formed subsequently to hibonite (Fig.13). The compound is homogeneous and differs from Ba-bearing β-alumina at Fen and Araxá in containing significant amounts of Mg, Sr, Na and K in the ‘inter-layer’ regions (Table 9).
This compound crystallized subsequently to hibonite and prior to perovskite. It occurs as anhedral, rounded small (<20 μm) crystals commonly enclosed in perovskite (Fig. 14). The crystals are of relatively uniform composition, primarily exhibiting variations in their U contents (Table 10). The composition is close to that of CaZr4O9 a well-known compound in the CaO–ZrO2 system (Gasik, 2013), but appears to be non-stoichiometric and is possibly an oxygen deficient variety. Other compounds in the CaO–ZrO2 system (Ca-stabilized ZrO2, Ca6Zr19O44) and zirconolite are absent.
Two varieties of perovskite crystallized subsequently to hibonite and calcium zirconate. Perovskite-I occurs as round-to-irregular small crystals (10 μm) enclosed in larger (<50 µm) anhedral plates of perovskite-II (Fig. 14). Both perovskites are of broadly similar composition (Table 11) and differ principally only with respect to their REE, Al, Ti and Ca contents. Significant compositional zoning is absent. Perovskite-I is enriched in REE and Zr and deficient in Nb relative to perovskite-II. All of the perovskites are considered to be dominantly members of a REEAlO3–CaTiO3 solid solution with minor CaZrO3 and Ca2AlNbO6. Perovskites-I and II are similar to the early-forming perovskites in the Araxá slag. Oka perovskites lack detectable Na and Fe.
F-Na-Ce calcium aluminate glass
The final phase to form in the Oka slag (Fig. 15) is a calcium aluminate which contains significant contents of F, Na and Ce (5.6–6.4 F; 1.6 – 2.4 Na2O; 0.7–2.0 Ce2O3; 35.6–36.7 CaO; 55.2–58.4 Al2O3 wt.%). This phase is similar in composition to CaAl2O4 but cannot be matched with this, or any other calcium aluminate, with respect to its stoichiometry and thus is considered to be a late-stage glass.
A minor part of the Oka slag which is of distinct texture and composition is an anorthite-bearing glass (Fig. 15). This consists of quench prismatic crystals of anorthite (An78–61) set in a heterogeneous F-REE-bearing aluminosilicate glass. Representative compositions of the glass (Table 12) indicate significant contents of Ti, Zr, REE, Th and U. Crystallization of this glass explains the presence of small (< 10 μm) quench crystals of CaF2, MgAl2O4, TiO2 and unidentifiable Zr-Ti-Al-oxides adjacent to the anorthite laths (Fig. 15).
The three slags investigated have some similarities in that the bulk of the slag, or framework, is composed of aluminium-dominated compounds of related structures i.e. β-alumina and hibonite. This is not surprising given that the smelting process relies on the addition of large amounts of aluminium. However, the effects of specific smelting conditions are evident in that redox conditions at each smelter must be different with those at Fen being the most reducing and at Oka the most oxidizing. A common feature of all slags is that regardless of the significant quantities of Fe added to the pyrochlore concentrates, all of the Fe is concentrated in the ferroniobium product and none is found in any of the compounds present in the slags. In contrast, Nb separation is not as efficient and significant amounts of Nb are present in the slags sequestered in a wide variety of complex oxides. All slags contain compounds which have significant contents of Th and U due to very efficient separation and concentration of these lithophile elements from their parental pyrochlore. It is the presence of these elements which could result in potential radiological hazards when the slags are exposed to chemical weathering. Whether or not Th and U are released to ground water, precipitated at the weathering site, or adsorbed on other secondary minerals depends on the stability of the host compound. A detailed discussion of this topic is beyond the scope of this work but it should be noted that many of the Th- and U-containing compounds in the slags are also components of SYNROC (Ringwood, et al., 1981) and are considered to be relatively stable. However, perovskite is known to be susceptible to decomposition at low temperatures in ground waters (Lumpkin, 2014; Lumpkin et al. 1998), and radiation damage in the perovskites enriched in Th and U encountered in this study can be expected to enhance such decomposition. The study by Veblen et al., (2004) is instructive in that it was shown that perovskites in slags decomposed quickly under natural weathering conditions with the precipitation of released Th as ‘stable’ thorianite. In addition, 252Cf irradiated perovskite is known to be susceptible to etching in weak nitric acid and even certain propriety soft drinks (Mitchell and Gleadow, unpublished data).
It is clear that every slag so far examined, including those studied by Veblen et al., (2004), although containing similar phases, vary widely in the abundance and composition of those phases. Hence, with respect to weathering each slag must be considered as a distinct entity and considered on its own merits with respect to composition and local redox conditions. Interestingly, aluminothermic slags produced by smelting for a variety of metals (V, Cr, Nb) have not been studied in any detail with respect to radionuclide release in the natural environment with respect to the SYNROC concept.
The slags contain a variety of perovskite-group compounds with compositions that exceed those of natural perovskites in terms of their Th, U and Al contents, and many have no synthetic analogues (Mitchell, 2002). These data suggest that it might be instructive, with regard to structural changes in perovskites as a function of composition, to synthesize these compounds and determine their space groups. Similar conclusions apply for Zr-Ti-Th-niobate (Table 3) and U-Th-zirconolite (Table 4).
The observation above that many of the oxide phases in the slag are enriched in Nb indicates that the aluminothermic smelting process is not very efficient and considerable amounts of Nb are ‘lost’. Clearly, there is much scope for bench-scale experiments to improve the process and for investigations of re-processing the slag.
Data and conclusions presented in this study are based on investigation of only three slags collected from single batches of smelting. Differences are related to the composition of the pyrochlore concentrates and the fluxes added as well as the proportions of Al and Fe2O3. Thus, the compounds in the Araxá slag reflect the Ba-rich character of the pyrochlore feed-stock. It is probable that slags from a single smelter will also differ in composition and character as the nature of the feed pyrochlore and fluxes added changes over time. To date there are no published investigations of these variations.
Finally, the aluminium-rich nature of the slags and the presence of hibonite and perovskite might provide some insight into the high-temperature condensation processes responsible for the formation of the calcium-aluminium-rich (CAI) inclusions (MacPherson et al., 2005) found in some carbonaceous chondrites.
This work is supported by the Natural Sciences and Engineering Research Council of Canada, Lakehead University and Almaz Petrology. Richard McLaughlin (Oxford Instruments) and Thomas Wenzel (Tübingen University) are thanked for wavelength dispersive electron microprobe analysis of compounds in the Fen slag. Greg Lumpkin and Karen Hudsen-Edwards are thanked for reviews of this manuscript.
- Manuscript received 24 September 2014.
- Manuscript Accepted for publication 31 May 2015.