Productive use of steelmaking by-product in environmental applications (I): Mineralogy and major and trace element geochemistry

Minerals Engineering 35 (2012) 49–56

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Productive use of steelmaking by-product in environmental applications (I): Mineralogy and major and trace element geochemistry G.B. Douglas ⇑, L.A. Wendling, S. Coleman CSIRO Land and Water, Centre for Environment and Life Sciences, Private Bag No. 5, Wembley, WA 6913, Australia

a r t i c l e

i n f o

Article history: Received 27 January 2012 Accepted 4 April 2012 Available online 16 June 2012 Keywords: Iron ore Environmental Mineral processing Classification

a b s t r a c t Geochemical and mineralogical characterisation of steel-making by-products is essential to understand their long-term behaviour and their potential use as environmental amendments or construction materials. A steel-making by-product generated in Western Australia from iron ore smelting, HIsmelt, has been extensively characterised in terms of its major and trace element geochemistry and mineralogy. More than 95% of the HIsmelt steel-making by-product is accounted for by the major element oxides CaO (ca. 38%), SiO2 (ca. 31%), Al2O3 (ca. 13–15%), MgO (ca. 7–10%) and FeO (ca. 6%). Compared to other steel-making by-products produced worldwide, the HIsmelt steel-making by-product has a similar major element geochemistry; however, trace element concentrations in the HIsmelt by-product rarely exceed the mean concentrations reported for steel-making by-products generated elsewhere. Only Ba and V concentrations are sometimes higher in the HIsmelt by-product. There are distinct differences within in the HIsmelt by-product mineralogy based on hand specimen grain size classification and combined scanning electron microscopy/statistical analysis despite the samples having similar geochemical compositions. The majority of mineralogical variation of individual HIsmelt by-product samples can be explained by a solid-solution series involving the mineral end members monticellite–akermanite–gehlenite–merwinite [CaMgSiO4–Ca2MgSi2O7–Ca2Al2SiO7–Ca3Mg(SiO4)2]. These minerals typically constitute 70–90% of the mineralogy in coarse to medium-grained samples and only 37% in fine-grained samples, with a corresponding decline in pore volume from 10.7 to 0.3%. This mineral suite is also consistent with inferred mineral end member compositions present in ternary phase diagrams of major and trace element oxides. The coarser-grained HIsmelt by-product samples may also contain periclase (MgO) and/or enstatite (MgSiO3), the latter also inferred by X-ray diffraction analysis and derived from dolomite (CaMgCO3) decomposition during smelting. Zircon (ZrO2) is a common trace to dominant mineralogical component in the coarser-grained HIsmelt by-product. In contrast, finer-grained HIsmelt by-product generally has more diverse mineralogy and may also contain spinel (MgAl2O4), hercynite (FeAl2O4), ilmenite (FeTiO3), and perovskite (CaTiO3). Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction By-products from the smelting of iron ore are produced at numerous sites internationally with an estimated production of 50 M tonnes per annum (Proctor et al., 2000). In general, approximately 300 kg of by-products are generated for every 1000 kg of steel. In an effort to reduce the environmental footprint of the steel-making industry, numerous studies have been undertaken to characterise the physico-chemical properties of steel-making by-products with a view to understanding their behaviour under environmental conditions (e.g. Bayless and Schulz, 2003; Navarro et al., 2010) or their productive reuse (Escalante-García et al., 2003; Ochola and Moo-Young, 2004; Kostura et al., 2005; Ettler ⇑ Corresponding author. Tel.: +61 8 9333 6131; fax: +61 8 9333 6499. E-mail address: [email protected] (G.B. Douglas).

et al., 2009). The productive use of steel-making by-products as an environmental amendment is limited by the potential for negative environmental or human health impacts. In addition to leaching trials and ecotoxicological assessment (see Wendling et al., 2009), thorough characterisation of the geochemistry and mineralogy of steel-making by-products is essential to understand their long-term behaviour and inform their potential use as environmental amendments or construction materials. 1.1. General characteristics of steelmaking by-products Major chemical constituents of iron- and steel-making by-products include Al, Ca, Fe, Mg, manganese (Mn), phosphate ðPO3 4 Þ, Si and sulphur (S) (Proctor et al., 2000). As part of the steel-making process the by-products are enriched in a range of elements, in particular oxides or more complex minerals containing mostly

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Mg, Ca and Si. Consequently, these materials may have an inherently high capacity to neutralise acidity, often similar to that of calcium carbonate (Proctor et al., 2000; Bayless and Schulz, 2003; Bayless et al., 2004; Ochola and Moo-Young, 2004). Although trace element contents may be high in some steel-making by-products, extensive testing suggests that they are not readily leachable (e.g. using TCLP testing protocol, Proctor et al., 2000). Some steelmaking by-products have been shown to have a rapid and substantial trace element uptake capacity, particularly when the adsorbate was present as an oxyanion such as ðPO3 4 Þ (Agyei et al., 2000) or CrO2 (Ochola and Moo-Young, 2004). An investigation of long4 term field disposal sites of steel-making by-products indicated that the by-products were influential in limiting the mobility of trace elements, primarily through the precipitation of secondary minerals such as gypsum (Bayless and Schulz, 2003). Batch experiments using steel-making by-products have demonstrated a particularly high P adsorption capacity (>44 g P/kg slag; Sakadevan and Bavor, 1998). Similarly, column studies have shown that both crystalline and amorphous steel-making by-products removed >95% of total P from simulated wastewater (Johansson, 1999a). Some experimental evidence suggests that the dominant mechanism of ðPO3 4 Þ removal from aqueous solution by steel-making by-products may be hydroxyapatite precipitation, due to an alkaline pH and excess Ca (Johansson and Gustaffson, 2000). Examination of P interactions with steel-making by-product from an exhausted (saturated) effluent filter indicated that mechanisms for P removal from wastewater included: P adsorption onto metal oxides/oxyhydroxides throughout the by-product matrix and amorphous Fe oxide/oxyhydroxide surface films; precipitation of P, mainly as Fe–phosphates, on amorphous Fe oxide/oxyhydroxide surface films; and P sequestration by an amorphous organic resin formed on the by-product surface (Pratt et al., 2009). At high pH, steel-making by-products exhibit a net negative charge and can readily adsorb cations from solution. Amendment of metal-contaminated dredged sediments with 4% w/w blast furnace steel-making by-product resulted in a reduction in the leachability of some metals, including Ba, Ni, and Zn (Barth et al., 2007). One investigation of steel-making by-product as a potential component for a reactive permeable barrier to treat landfill leachate showed that 60–95% of the metals examined were removed from solution by the by-product (Chung et al., 2007) The pH substantially influenced metal sorption to the steel-making by-product, with sorption at pH 7 > pH 5 > pH 9 (Chung et al., 2007). In laboratory leaching trials, Simmons et al. (2002) observed sequestration of Fe, Cd, Be, Zn, and Sb from solution by a steel-making by-product; however, Ba and V concentrations were higher in effluent than in source water. Steel-making by-product composition is dependent upon both the raw materials used and type of steel-making process by which it is generated. In the present study, the physico-chemical characteristics of a steel-making by-product generated using the HIsmelt process were investigated. The HIsmelt process is an air-based direct smelting technology that uses carbon in a dissolved form and high mixing rates. In addition, the HIsmelt facility uses Western Australian iron ore and other local raw materials that would potentially result in a by-product of a distinct mineralogy and major and trace element composition. Laboratory studies using the pre-commercial HIsmelt by-product have shown that this material has a substantial ability to attenuate acidity with a particularly high capacity to adsorb Cu from a synthetic acidified waste stream used in laboratory column trials (Douglas et al., 2008). In addition, the HIsmelt by-product demonstrated a good capacity to remove natural dissolved organic carbon (DOC) from local groundwater (Wendling et al., 2009). Ecotoxicological characterisation of HIsmelt by-product leachate, and an environmental radioactivity assessment, both essential pre-requi-

sites to any widespread environmental application of this material, are discussed in Wendling et al. (2009). Depending on physico-chemical characteristics, steel-making by-products such as the one produced using the HIsmelt process may have a number of potential productive end-uses (Dippenaar, 2005; Motz and Geiseler, 2001), including such diverse applications as road-building materials (Chaurand et al., 2007), nutrient removal (Kostura et al., 2005; Shilton et al., 2006; Pratt et al., 2009), as an agricultural soil amendment (Gutierrez et al., 2010), as a acid neutralising material (Ochola and Moo-Young, 2007), or in constructed wetlands (Sakadevan and Bavor, 1998). The inherent acid neutralisation capacity of the HIsmelt by-product may have particular potential given the widespread presence of acidsulphate soils (ASS) in coastal, estuarine and some inland areas in southwest Western Australia (Degens et al., 2008). In addition, a potential exists for the use of HIsmelt by-product as an amendment to ameliorate acidity in mine pits or acid mine drainage in an in situ permeable reactive barrier configuration. 2. Materials and methods 2.1. Sample selection Three HIsmelt by-product samples (HS1, HS2 and HS3) chosen from representative stockpiled material supplied to CSIRO by the HIsmelt Corporation, Kwinana, Western Australia. The sampling criteria was based on variation in grain size distribution and byproduct morphology evident in hand specimens and represent a range of compositions produced during smelting. 2.2. Scanning electron microscopy – AutoGeoSEM The AutoGeoSEM consisted of a Philips XL40 controlled pressure scanning electron microscope (SEM) fitted with an energy dispersive analytical X-ray (EDAX) spectrometer with bespoke software for microscope automation and data processing. The SEM operated in controlled pressure mode of between 0.1 and 0.5 mBar to reduce sample charging and allow non-conductive samples to be examined without coating. For AutoGeoSEM analysis, the SEM was operated at 30 kV and 3 nA to minimise analysis time and maximise the back-scattered electron (BSE) and X-ray signals. Whole, unground HIsmelt samples were mounted in resin, sectioned and polished. The polished sections were placed in the SEM with the operator defining the analytical area and measurement parameters for each sample. The AutoGeoSEM software then drove the SEM stage to the first field of view in the area to be analysed, collected a back-scattered electron image and used image processing to distinguish the grains from the mounting resin and to separate touching grains. Image analysis was then used to calculate grain attributes such as area, width, equivalent circular diameter, perimeter, elongation, and location of the centroid. Once the individual grains were located, the AutoGeoSEM then used the EDAX to collect a spectrum from a square within each grain. The spectra and images for each field of view were then stored and the process repeated for the next field of view. For the three HIsmelt samples, 4323, 1034 and 1047 grains were analysed. The time required to move the stage to the next field of view, the image acquisition time, and the image processing/analysis time were each ca. 5 s. Images were usually acquired at a magnification of 100 to 400 and typically contained 100–200 grains per field of view. The spectrum acquisition time was ca. 0.3–0.5 s. The spectra collected were used to classify the grains into an initial grouping. Each spectrum was compared to a database of standard spectra and a ‘‘match factor’’ from 0 to 100

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was generated. For most minerals, match factors of 90–95 were typical; however, minerals that contained many elements and varied in composition had lower match factors. If a measured spectrum did not match spectra in the database a new group was created and similar spectra were assigned to this group. Match factors of <85 were typically not considered to match, and were assigned to a new group.

2.3. Mineralogical analyses Quantitative X-ray diffraction (XRD) analysis was used to characterise the mineralogical composition. Approximately 1 g of each sample was ground to <10 lm for 10 min in a McCrone micronizing mill under ethanol. The resulting slurries were oven dried at 60 °C then thoroughly mixed with an agate mortar and pestle before being lightly back pressed into stainless steel sample holders. The XRD patterns were recorded with a PANalytical X’Pert Pro Multi-purpose Diffractometer using Fe filtered Co Ka radiation, a 1=4 ° divergence slit, a ½° anti-scatter slit and a fast X’Celerator Si strip detector. Diffraction patterns were recorded in steps of 0.016°2H with a 0.4 s counting time per step, and logged to data files for analysis. Quantitative analyses were performed on XRD data using the commercial package TOPAS from Bruker. Results were normalised to 100%, and hence do not include estimates of unidentified or amorphous materials.

2.4. Chemical analyses 2.4.1. Fusion X-ray fluorescence: major elements One gram of each oven dried sample (105 °C) was accurately weighed with 4 g of 12–22 lithium borate flux. The mixtures were fused at 1050 °C in a Pt/Au crucible for 20 min then poured into a 32 mm Pt/Au mould of similar temperature. The melt was quickly cooled over a compressed air stream. Resulting glass disks were analysed on a PANalytical AXios Advanced wavelength dispersive XRF system using the CSIRO Mineralogical and Geochemical Services in-house Silicates program.

2.4.2. Pressed powder X-ray fluorescence: trace elements Four grams of each oven dried sample (105 °C) was accurately weighed with 1 g of Licowax binder and mixed well. The mixtures were pressed in a 32 mm die at 12 t pressure and the resulting pellets were analysed on a PANalytical AXios Advanced wavelength dispersive XRF system using the CSIRO Mineralogical and Geochemical Services in-house Powders program.

2.4.3. Neutron activation analysis Samples of ground powder (20 g) were examined by neutron activation analysis (NAA) at Actlabs, Canada. During irradiation, radioactive isotopes formed by neutron capture were analysed and quantified using c spectrometry to measure c-ray decay signatures (Helmke, 1996).

3. Results 3.1. X-ray diffraction analysis Mineralogical analysis indicated that >90% of the HIsmelt byproduct was accounted for by the melilite group mineral gehlenite (Ca2Al2SiO7) and related Ca–Al–Si–±Mg minerals such as merwinite (CaMgSi2O6). Minor enstatite (Mg2Si2O6) was provisionally identified in addition to minor spinel (MgAl2O4) and quartz (SiO2).

3.2. Major and trace element analysis An aggregated sample of HIsmelt by-product constituting a range of grain sizes in hand specimen was analysed by a combination of fusion XRF (major elements), pressed powder XRF (trace elements) and NAA (major and trace elements). Analyses of 56 major and trace elements are summarised in Table 1. More than 95% of the HIsmelt is accounted for by major element oxides: SiO2, Fe2O3, Al2O3 CaO and MgO (Table 1). As documented for selected trace elements in AutoGeoSEM analyses, trace element concentrations are generally low with only the alkaline earth elements Sr (1307 ppm) and Ba (468 ppm) displaying a similar enrichment to their Group II element counterparts, Mg and Ca. Apart from some rare earth elements (La, Ce, Nd, Y), most other trace elements are less than 20 ppm with 21 elements below detection limits; Ag, As, Au, Bi, Br, Cd, Co, Cs, Cu, Ga, Ge, Hg, Mo, Ni, Pb, Sb, Se, Sn, Ta, W, and Zn.

3.3. Scanning electron microscopy image and element analysis Major and trace element analysis and morphological analysis were undertaken using SEM-EDAX/AutoGEOSEM. Electron photomicrographs of samples HS1, HS2 and HS3, and a more detailed photomicrograph from near the centre of each image, are shown in Figs. 1–3. Major oxide and selected trace element oxide analysis of each of the three samples analysed by SEM-EDAX/AutoGEOSEM are summarised in Table 2. Samples HS1, HS2 and HS3 were analysed for geochemical composition at 4323, 1034 and 1047 points, respectively. The composition of all three HIsmelt by-product samples was dominated by the major element oxides CaO (ca. 38%), SiO2 (ca. 31%), Al2O3 (ca. 13–15%), MgO (ca. 7–10%) and FeO (ca. 6%). The most abundant trace element (expressed as a trace element oxide) in all three samples was ZrO2 (ca. 1%) (Table 2). Despite Table 1 Summary of major and trace element composition of HIsmelt by-product determined by X-ray fluorescence (XRF, suffix ‘‘.x’’) and neutron activation analysis (NAA, suffix ‘‘.n’’). Element oxides are expressed on a percent basis, all other elements are expressed as parts per million (ppm). Element

Concentration

Element

Concentration

SiO2.x TiO2.x Al2O3.x Fe2O3.x MnO.x MgO.x CaO.x Na2O.x K2O.x P2O5.x SO3.x Sum.x Ag.x As.n Au.n Ba.x Bi.x Br.n Cd.x Ce.n Cl.x Co.n Cr.n Cs.n Cu.x Eu.n Ga.x Ge.x

29.5 0.6 14.2 4.6 0.7 9.1 35.6 0.1 0.03 1.1 0.3 95.7 <4 <0.5 <0.002 468 <4 <0.5 <5 101 168 <1 86 <1 <2 2 <2 <2

Hf.n Hg.n Ir.n La.n Lu.n Mo.n Nb.x Nd.n Ni.x Pb.x Rb.x Sb.n Sc.n Se.n Sm.n Sn.x Sr.x Ta.n Te.n Th.n Tl.x U.n V.x W.n Y.x Yb.n Zn.x Zr.x

3 <1 0.005 46.3 0.54 <1 11 31 <2 <3 8 <0.1 14.5 <3 7.7 <4 1307 <0.5 1.1 8.2 10 7.2 117 <1 64 7 <3 164

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Fig. 1. Scanning electron photomicrographs of HIsmelt by-product sample HS1. The first image (left) is at a large scale, while the second image (right) provides a more detail of a selected area. Magnification and scale bar are shown on each image. Note the large grain size relative to other by-product samples, circular and irregular voids formed by gas bubbles and irregular crystal packing, respectively, and often irregular (sutured) grain boundaries.

Fig. 2. Scanning electron photomicrographs of HIsmelt by-product sample HS2. The first image (left) is at a large scale, while the second image (right) provides a more detail of a selected area. Magnification and scale bar are shown on each image. Note the large, bright ovoids of FeS (left, and in more detail on right). As in HS1, large circular pores formed by gas bubbles and smaller irregular pores between grain boundaries are present.

Fig. 3. Scanning electron photomicrographs of HIsmelt by-product sample HS3. The first image (left) is at a large scale, while the second image (right) provides a more detail of a selected area. Magnification and scale bar are shown on each image. Note the finer grain size and irregular spotted appearance relative to HS1 and HS2 photomicrographs. Both circular to ovoid pores (some with interstitial mineral overgrowths) and irregular pores (in between grain boundaries) are also present, as found in samples HS1 and HS2.

substantial morphological and grain size differences between HS1, HS2 and HS3 which were evident at both hand specimen and electron microscopy scales, all three samples displayed a similar major element composition. The most substantial compositional variation in trace elements occurs for Co, Cr, Cu and Ni with only Cr increasing in average abundance with decreasing grain size from HS1 to HS3 (Table 2). Analysis and compositional-based classification of 6405 points in HIsmelt samples HS1, HS2 and HS3 revealed a diversity

of mineral associations and small-scale porosity recorded as resin/pore size (Table 3). A decrease in resin/pore abundance from HS1 to HS3 from 10.7% to 0.3% mirrored a reduction in grain size noted in hand specimens and larger-scale SEM photomicrographs (data not shown) that were used to initially select HIsmelt samples for detailed SEM analysis. Classifications for each grouping were based on idealised end member mineral compositions and generally exceeded 90% in terms of a goodness-of-fit. However, there may have been substantial inherent

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Table 2 Statistical summary of major element and trace element oxides including mean, one standard deviation and range as determined by scanning electron microscopy (SEM) analysis of HIsmelt samples HS1, HS2 and HS3. The number of points analysed for each sample (n) is also shown. Oxide

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O SO3 ZrO2 Cl Cr V Cu Zn Ni Co

HS1 (n = 4323)

Range

Mean

Stdev

Min

Max

Oxide

30.64 0.37 14.28 6.55 0.55 7.45 38.26 0.11 0.13 0.63 0.75 0.01 0.00 0.00 0.00 0.00 0.00 0.00

4.69 1.94 4.75 8.11 1.05 2.16 5.74 0.33 0.17 2.80 2.54 0.17 0.01 0.01 0.04 0.03 0.01 0.06

1.87 0.00 0.00 0.80 0.00 0.00 3.76 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

43.61 25.76 51.58 84.93 12.86 27.23 59.30 5.42 4.92 46.96 32.19 9.81 0.25 0.36 1.04 1.23 0.60 1.74

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O SO3 ZrO2 Cl Cr V Cu Zn Ni Co

HS2 (n = 1034)

Range

Mean

Stdev

Min

Max

Oxide

30.46 0.29 14.89 5.94 0.43 9.20 37.30 0.23 0.14 0.16 0.88 0.00 0.03 0.00 0.00 0.00 0.02 0.00

3.63 1.23 5.22 5.06 0.53 2.51 5.10 0.22 0.12 0.73 2.08 0.02 0.07 0.01 0.02 0.00 0.05 0.01

5.27 0.00 2.73 1.45 0.00 2.31 5.17 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

39.64 18.49 43.25 83.48 4.34 23.07 59.83 1.58 0.51 10.58 25.02 0.33 0.27 0.16 0.44 0.00 0.32 0.20

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O SO3 ZrO2 Cl Cr V Cu Zn Ni Co

HS3 (n = 1047)

Range

Mean

Stdev

Min

Max

30.74 0.56 13.31 6.37 0.58 10.40 36.16 0.29 0.15 0.24 1.07 0.01 0.06 0.00 0.00 0.00 0.02 0.01

7.67 3.49 13.67 7.30 0.68 5.00 9.30 0.37 0.15 2.32 2.46 0.15 0.08 0.02 0.04 0.03 0.06 0.06

2.42 0.00 0.00 0.32 0.00 0.00 3.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

38.26 38.09 62.45 83.56 4.58 24.12 68.14 8.84 2.15 50.61 30.32 4.85 0.51 0.35 0.75 0.88 0.32 1.03

Table 3 HIsmelt by-product sample compositional groupings, percentage of each group, percentage match to groupings or pore/resin, mean major element oxide compositions, and tentative classification of each group for samples HS1, HS2 and HS3. HS1 (%)

Compositional grouping

59.6

SiO2 + CaO + Al2O3 + MgO

13.8 8.2 1.7 1.3 1.2 10.7

CaO + SiO2 + FeO + MgO + Al2O3 CaO + SiO2 + Al2O3 (42 + 31 + 14) CaO + SiO2 + SO3 + Al2O3 + FeO + MgO CaO + ZrO2 + SiO2 + FeO CaO + FeO + SiO2 + Al2O3 Resin/pore

HS2

Match (%)

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2O

ZrO2

SO3

Classification

90

32.8

0.0

16.5

2.8

0.1

7.0

40.3

0.1

0.1

0.2

0.1

13.2 4.0 9.9 8.1 23.6 0.0

1.3 0.3 0.5 1.1 2.4 0.0

9.4 6.8 6.8 5.5 9.9 0.0

34.7 41.8 33.2 41.1 28.2 0.0

0.2 0.2 0.2 0.5 0.1 0.0

0.1 0.2 0.1 0.2 0.1 0.0

2.0 1.7 1.5 13.9 2.4 0.0

0.6 0.1 7.2 0.7 1.3 0.0

Melilite–Akermanite/ Gehlenite SS Merwinite/Monticellite Gehlenite Merwinite/FeS? Zircon/Melilite Merwinite–Melilite SS Resin/Pore

91 89 89 90 90 100

28.3 30.8 28.6 22.5 23.6 0.0

0.3 0.1 0.5 0.8 0.4 0.0

9.7 14.1 11.3 5.6 7.8 0.0

(%)

65.9

SiO2 + CaO + Al2O3 + MgO

90

31.6

0.1

16.6

4.1

0.2

8.3

38.3

0.2

0.1

0.3

0.1

17.5 13.2 1.0

CaO + SiO2 + FeO + MgO + Al2O3 CaO + SiO2 + Al2O3 SiO2 + CaO + FeO + TiO2 + MgO + Al2O3

91 89 90

28.8 31.5 25.0

0.4 0.1 5.1

11.7 9.9 15.8

11.2 3.8 13.5

1.0 0.3 1.8

11.8 8.9 11.9

33.0 42.9 24.3

0.3 0.3 0.4

0.1 0.2 0.1

1.4 2.0 1.8

0.3 0.1 0.3

0.8 1.6

CaO + ZrO2 + SiO2 + FeO Resin/pore

91 100

21.2 0.0

1.1 0.0

5.8 0.0

6.4 0.0

1.0 0.0

6.9 0.0

40.9 0.0

0.3 0.0

0.2 0.0

16.2 0.0

0.1 0.0

HS3

Melilite–Akermanite/ Gehlenite SS Merwinite/Monticellite Gehlenite Merwinite/Melilite SS/Ti phase Zircon/Melilite Resin/Pore

(%)

37.2

SiO2 + CaO + Al2O3 + MgO

91

33.0

0.0

17.4

2.2

0.1

7.0

39.7

0.2

0.2

0.1

0.0

23.9 13.8 10.7 4.3 4.1 1.5 1.3 1.2

SiO2 + CaO + MgO + FeO CaO + SiO2 + Al2O3 CaO + SiO2 + FeO + MgO + Al2O3 Al2O3 + CaO + SiO2 + MgO Al2O3 + MgO + SiO2 + CaO + FeO CaO + ZrO2 + SiO2 + FeO Al2O3 + MgO + CaO + SiO2 Al2O3 + CaO + SiO2 + MgO + FeO

93 89 89 89 92 90 94 89

35.3 32.5 33.3 26.8 8.0 20.5 14.2 23.7

0.0 0.2 0.4 0.0 0.3 0.3 0.1 0.2

2.6 6.9 3.0 26.9 56.8 4.8 45.8 26.1

9.7 3.0 12.5 2.2 5.4 6.6 4.5 9.9

1.3 0.1 1.5 0.1 0.4 0.6 0.3 0.9

14.9 8.4 11.8 7.5 21.6 3.5 18.8 15.5

34.7 45.5 35.4 35.9 6.6 44.6 15.5 21.8

0.3 0.4 0.3 0.2 0.6 0.7 0.6 0.4

0.1 0.2 0.1 0.1 0.0 0.4 0.0 0.0

1.0 2.4 1.5 0.1 0.1 14.5 0.2 1.2

0.0 0.1 0.3 0.0 0.1 3.5 0.0 0.1

1.0 0.3

CaO + TiO2 + FeO Resin/pore

89 100

11.6 0.0

30.9 0.0

3.5 0.0

5.6 0.0

0.0 0.0

2.1 0.0

41.6 0.0

0.0 0.0

0.2 0.0

3.6 0.0

0.5 0.0

compositional variation due to solid solution between end members in addition to other element substitution, in particular by Fe (as FeO) given its abundance in the initial smelting composition. All three HIsmelt samples were dominated by similar SiO2– Al2O3–CaO–MgO compositional groupings with the first four most abundant groupings constituting ca. 70–90% of the mineralogy. In particular, these groupings were dominated by melilite [(Ca,Na)2(Al,Mg,Fe2+)(Si,Al)2O7] and/or its solid-solution minerals

Melilite–Akermanite/ Gehlenite SS Akermanite Gehlenite Merwinite/Monticellite Gehlenite/Merwinite? Spinel/Melilite? Zircon/Melilite Spinel/Gehlenite? Gehlenite/Spinel/ Hercynite Perovskite Resin/Pore

akermanite (Ca2MgSi2O7) and gehlenite (Ca2Al2SiO7), in addition to merwinite [Ca3Mg(SiO4)2] and monticellite (CaMgSiO4). These minerals also accounted for the majority of geochemical variation as observed in the SiO2–MgO–CaO ternary diagram (Fig. 4a). The melilite (akermanite–gehlenite solid-solution) group minerals were the most abundant in each sample, however, the absolute percentages varied between each sample. Both HS1 and HS2 had a similar melilite group mineral abundance of 59.6% and 65.9%, respectively, while HS3 contained only 37.2%.

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G.B. Douglas et al. / Minerals Engineering 35 (2012) 49–56

(a)

SiO2

(b)

Quartz, Zircon

HS1 HS2 HS3

Al2O3

Hercynite

Spinel Monticellite Akermanite Merwinite

Gehlenite

Gehlenite

MgO

Perovskite, Calcite

Spinel, Periclase

CaO

MgO

Periclase

Monticellite

CaO

Akm Merw Pervos, Cal

Fig. 4. SiO2–MgO–CaO (a) and Al2O3–MgO–CaO (b) elemental oxide ternary diagrams incorporating the 6404 point chemical analyses of HS1, HS2 and HS3 HIsmelt byproduct samples.

An MgO-rich trend was apparent for the coarsest-grained HIsmelt sample HS1 in the Al2O3–MgO–CaO ternary diagram (Fig. 4b). This potentially signified the presence of periclase (MgO) produced from the thermal decomposition of the Mgcomponent of dolomite (CaMgCO3) added during the smelting process. A zircon–melilite compositional grouping was also present in each HS1, HS2 and HS3 although this grouping only constituted 0.8–1.5%. This compositional grouping perhaps represented (partially resorbed) zircon inclusions or micro-scale admixtures within a melilite matrix HIsmelt sample HS3 was notable for its more diverse mineral assemblage compared to the two coarser-grained samples. Notable was the likely presence of spinel (MgAl2O4) and hercynite (FeAl2O4) in addition to a titanium-bearing phase with a composition resembling that of perovskite (CaTiO3). The presence of spinel,

hercynite and perovskite can also be inferred from a FeO–TiO2– ZrO2 ternary diagram (Fig. 5b). 4. Discussion 4.1. Major and trace element geochemistry Comparison of an aggregated sample of the HIsmelt by-product to the mean concentration of a variety of steelmaking by-products indicates a wide range of mean concentrations of major and trace elements that reflect not only the source of iron ore and other input materials, but also the different smelting processes used. In terms of major elements, the HIsmelt by-product has higher Al2O3 and P2O5 compared to average compositions of blast furnace (BF), basic oxygen furnace (BOF) and electric arc furnace (EAF) byproducts (Table 4). Concentrations of FeO and MnO in HIsmelt are

SiO2

(a)

HS1 HS2 HS3

FeO

Quartz, Akermanite, Merwinite, Monticellite

(b)

Hercynite, metallic Fe

Ilmenite Gehlenite

Al2O3

Spinel, Hercynite

Zircon

ZrO2

TiO2

Rutile, Perovskite

Zircon

ZrO2

Fig. 5. SiO2–Al2O3–ZrO2 (a) and FeO–TiO2–ZrO2 (b) elemental oxide ternary diagrams incorporating the 6404 point chemical analyses of HS1, HS2 and HS3 HIsmelt byproduct samples.

55

G.B. Douglas et al. / Minerals Engineering 35 (2012) 49–56 Table 4 Major (wt.%) and trace element (mg kg1) geochemistry of blast furnace (BF), basic oxygen furnace (BOF) and electric arc furnace (EAF) by-products and HIsmelt byproduct (modified after Proctor et al., 2000). ND – Not Detected, N/A – Not Analysed. Element

Units

BF

BOF

EAF

HIsmelt

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 SO3 Ag As Ba Be Cd Co Cr Cu Hg Mo Ni Pb Sb Se Sn Tl V Zn

wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.% wt.% mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1 mg kg1

36.4 n/a 7.8 2.2 0.7 11.6 38.3 n/a n/a 0.1 2.6 ND 1.3 273 8.2 ND 3 132 5.3 ND 0.8 1.4 3.57 ND 3.9 1.6 ND 54 20

12.8 n/a 4.5 23.7 4.2 9.2 39.2 n/a n/a 0.7 0.3 9.1 ND 75 0.5 2.5 3.8 1271 30 0.1 11 4.9 50 3.3 15 6.5 7.2 992 46

15.9 n/a 6.6 24.5 5.1 9.0 35.1 n/a n/a 0.4 0.5 8.4 1.9 557 1.1 7.6 4.8 3046 178 0.04 30 30 27.5 4 18 10 11 513 165

29.5 0.6 14.2 4.6 0.7 9.1 35.6 0.1 0.03 1.1 0.3 ND (<4) ND (<0.5) 468 N/A ND (<5) ND (<1) 86 ND (<2) ND (<1) ND (<1) ND (<2) ND (<3) ND (<0.1) 2 ND (<4) 8 117 ND (<3)

similar to mean BF by-product concentrations, but substantially lower that BOF or EAF by-products; however, this may reflect the efficiency of recovery of reduced Fe. The HIsmelt silica (SiO2) concentrations are less than, but most similar to those of the BF byproduct and higher than those of BOF and EAF by-products. Both MgO and CaO have similar mean concentrations for by-products produced by all four smelting methods while SO3 is lowest in the HIsmelt and BOF by-products. Although information is not available for TiO2, Na2O and K2O from other smelting methods, concentrations of these oxides are low in the HIsmelt by-product. Trace element concentrations in HIsmelt by-product rarely exceed those of the other smelting methods with only Ba higher than mean BF and BOF by-product concentrations and V higher than mean BF by-product concentrations (Table 4). In comparison to other steel-making by-products, however, the HIsmelt by-product has very low concentrations of a suite of elements including Ag, As, Cd, Co, Cu, Mo, Ni, Pb, Sn and Zn (Table 4).

4.2. Mineral classification and abundance A suite of multi-element ternary diagrams: SiO2–MgO–CaO, Al2O3–MgO–CaO, SiO2–Al2O3–ZrO2 and FeO–TiO2–ZrO2 were constructed incorporating the 6404 point analyses of HS1, HS2 and HS3 to further elucidate the relationship between geochemistry and mineralogy the HIsmelt by-product as a function of grain size (Figs. 4a and b and 5a and b). Mineral phases either formally identified by XRD in this study or thought to be present in either ore or smelting materials or the resultant slag by-product were plotted on the relevant ternary diagram. A summary of these mineral phases and their end member geochemical composition expressed as elemental oxides is given in Table 5. These mineral phases are idealised end members with compositional variation via solid solution assumed to occur, for instance, within or between the akermanite–gehlenite (melilite) group and merwinite during the smelting process/by-product formation. Distinct compositional trends as a function of grain size are apparent in the ternary diagrams. Within the SiO2–MgO–CaO compositional space, the majority of the compositional variation of individual mineral compositions can be explained by a mixing space within a monticellite–akermanite–gehlenite–merwinite solid-solution series, in particular for HS2 and lesser extent HS1 samples (Fig. 4a). Notable, however, are two substantially smaller subsets of HS3 sample compositions, one that trends strongly towards a spinel (MgAl2O4) end member and the other towards a perovskite (CaTiO3) and/or calcite (CaCO3) end member. Compositional constraints on mineralogy apparent within the SiO2–MgO–CaO ternary diagram are also similarly reflected in an Al2O3–MgO–CaO ternary diagram (Fig. 4b). Samples of HS3 show a compositional variation consistent with presence of spinel in addition to perovskite and/or calcite. In relative terms, the HS1 and HS2 samples display lesser dispersion in the monticellite– akermanite–gehlenite–merwinite solid-solution mixing space than the finest-grained sample, HS3. In the Al2O3–MgO–CaO ternary diagram, the coarsest-grained HS1 samples also trend towards a pyroxene MgO-rich (possibly enstatite) composition as identified in XRD analysis or possibly periclase. The coarser-grained HIsmelt by-product samples potentially contain periclase (MgO) and/or enstatite (MgSiO3) sourced from dolomite (CaMgCO3) decomposition. In contrast, finer-grained HIsmelt by-product generally has more diverse mineralogy and may contain spinel (MgAl2O4), hercynite (FeAl2O4) and possibly ilmenite (FeTiO3), perovskite (CaTiO3) and calcite (CaCO3) in addition to periclase (MgO) and/or enstatite (MgSiO3). Zircon (ZrO2) appears to be a common trace to dominant mineralogical component in the coarser-grained HIsmelt by-product in particular. This may reflect

Table 5 Mineral phases either formally identified by XRD analysis, in hand specimens, inferred to be present in ore or smelting materials or the resultant by-product, their general abundance, and their geochemical composition expressed as elemental oxides. MINERAL

Identified in XRD

SiO2

Akermanite (2CaO  MgO  2SiO2) Gehlenite (2CaO  Al2O3  SiO2) Merwinite (3CaO  MgO  2SiO2) Monticellite (CaO  MgO  SiO2) Spinel (MgO  Al2O3) Imenite (FeO  TiO2) Perovskite (CaO  TiO2) Rutile (TiO2) Zircon (ZrO2  SiO2) Calcite (CaO  CO2) Periclase (MgO) Sulphur (FeO  SO3) Quartz (SiO2) Metallic Fe (FeO)

YES YES YES YES YES NO NO NO NO NO NO NO YES YES

44.08 21.91 36.56 38.40

(abundant) (abundant) (common) (common) (common)

Al2O3 37.18

CaO

MgO

41.14 40.90 51.18 35.84

14.78

71.67

FeO

MnO

TiO2

SO3

12.26 25.76 28.33 47.35

41.25

52.65 58.75 100.00

32.78

67.22 100.00 100.00 47.37

(trace) (trace)

ZrO2

100.00 100.00

52.63

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G.B. Douglas et al. / Minerals Engineering 35 (2012) 49–56

its origin as an accessory mineral in Hamersley Banded Iron Formation which constitutes the major ore for the HIsmelt Kwinana smelting operation. Zircon appears to be a significant mineralogical component in sample HS1 in particular, as reflected in the SiO2–Al2O3–ZrO2 ternary diagram (Fig. 5a). Zircon is known to be an accessory mineral in Hamersley Banded Iron Formations (Trendall et al., 2004) which is the iron ore source for the Western Australian HIsmelt process. Sample HS3 also displays a substantial spinel component as reflected in the SiO2–MgO–CaO ternary diagram (Fig. 4a). Within the FeO–TiO2–ZrO2 ternary diagram, the majority of compositional variation is contained within an ilmenite–zircon–FeO space, where the FeO may potentially be contained within residual metallic Fe, hercynite (FeAl2O4) or within Fe within the monticellite–akermanite–gehlenite–merwinite solid-solution series (Fig. 5b). The element assemblages inferred from ternary diagrams of major element oxides are also confirmed by classification analysis of the 6404 point analyses of HS1, HS2 and HS3 (Table 3). The classifications, based on a goodness of fit criteria as compared to mineral end member compositions (Table 3), suggest that a solid-solution series of the minerals melilite–akermanite–gehlenite can explain approximately 60%, 66% and 37% of the compositional variation in HS1, HS2 and HS3 respectively. In addition, the mineral assemblage of merwinite–monticellite–akermanite–gehlenite can further explain 22%, 31% and 48% of the compositional variation in HS1, HS2 and HS3 respectively. This mineral classification data also indicates that the finer grained HS3 sample has not only a greater diversity of minerals or mineral associations, but also a greater spread of different minerals. 5. Conclusions A combined mineralogical and geochemical study of HIsmelt steelmaking by-product using an AutoGEOSEM technique and statistical analysis has demonstrated that there may be substantial mineralogical variation based on hand-specimen grain size, despite a similar overall major and trace element composition. The major minerals present within the HIsmelt steel-making by-product consist of merwinite–monticellite–melilite (akermanite–gehlenite) while minor zircon, quartz and metallic Fe may also be present. This mineral suite is also consistent with inferred mineral end member compositions present in ternary phase diagrams of major and trace element oxides. Acknowledgements The authors gratefully acknowledge HIsmelt Corporation Ltd. for the funding of this by-product characterisation study. Michael Verrall from CSIRO Exploration and Mining is thanked for his assistance with AutoGEOSEM analyses. References Agyei, N.M., Strydom, C.A., Potgieter, J.H., 2000. An investigation of phosphate ion adsorption from aqueous solution by fly ash and slag. Cem. Concr. Res. 30, 823– 826. Barth, E., Sass, B., Chattopadhyay, S., 2007. Evaluation of blast furnace slag as a means of reducing metal availability in a contaminated sediment for beneficial use purposes. Soil Sediment Contam. 16, 281–300.

Bayless, E.R., Schulz, M.S., 2003. Mineral precipitation and dissolution at two slagdisposal sites in northwestern Indiana. USA Environ. Geol. 45, 252–261. Bayless, E.R., Bullen, T.D., Fitzpatrick, J.A., 2004. Use of 87Sr/86Sr and d11B to identify slag-affected sediment in southern Lake Michigan. Environ. Sci. Technol. 38, 1330–1337. Chaurand, P., Rose, J., Briois, V., Olivi, L., Hazemann, J.-L., Proux, O., Domas, J., Bottero, J.-Y., 2007. Environmental impacts of steel slag reused in road construction: a crystallographic and molecular (XANES) approach. J. Hazard. Mater. 139, 537–542. Chung, H.I., Kim, S.K., Lee, Y.S., Yu, J., 2007. Permeable reactive barrier using atomizing slag material for treatment of contaminants from unsanitary landfill and contaminated site. Geosci. J. 11, 137–145. Degens, B., Fitzpatrick, R., Hicks, W. 2008. Acidification and formation of Inland ASS materials in Salt Lakes by acid drainage and regional groundwater discharge. In: Fitzpatrick, R., Shand, P. (Eds.), Inland Acid Sulfate Soil Systems Across Australia. CRC LEME Open File, Report No. 249, Avon Basin, WA Wheat, pp. 176–188 Dippenaar, R., 2005. Industrial uses of slag (the use and reuse of iron and steelmaking slags). Ironmaking Steelmaking 32, 35–46. Douglas, G., Adeney, J., Klauber, C., 2008. Comparison of mining waste by-products in a permeable reactive surface barrier for the retardation of heavy metals – column results and geochemical modelling. Progress Report No. 3. CSIRO: Light Metals National Research Flagship. Report DMR-3367, January 2008, 46p. Escalante-García, J.I., Fuentes, A.F., Gorokhovsky, A., Fraire-Luna, P.E., MendozaSuarez, G., 2003. Hydration products and reactivity of blast-furnace slag activated by various alkalis. J. Am. Ceram. Soc. 86, 2148–2153. Ettler, V., Zdenek, J., Kribek, B., Sebek, O., Mihaljevic, M., 2009. Mineralogy and environmental stability of slags from the Tsumed smelter. Namibia. Appl. Geochem. 24, 1–15. Gutierrez, J., Oh Hong, C., Lee, B.-H., 2010. Effect of steel-making slag as a soil amendment on arsenic uptake by radish (Raphanus sativa L.) in an upland soil. Biol. Fertility Soils 46, 617–623. Helmke, P.A. 1996. Neutron Activation Analysis. In: Sparks, D.L. (Ed.), Methods of Soil Analysis. Part 3. Chemical Methods – SSSA Book Series No. 5. Soil Science Society of America and American Society of Agronomy, Madison, WI, pp. 141– 159 Johansson, L., 1999. Industrial by-products and natural substrata as phosphorus sorbents. Environ. Technol. 20, 309–316. Johansson, L., Gustaffson, J.P., 2000. Phosphate removal using blast furnace slags and opoka-mechanisms. Water Res. 34, 259–265. Kostura, B., Kulveitova, H., Lesko, J., 2005. Blast furnace slags as sorbents of phosphate from water solutions. Water Res. 39, 1795–1802. Motz, H., Geiseler, J., 2001. Products of steel slags and opportunity to save natural resources. Waste Manag. 21, 285–293. Navarro, C., Diaz, M., Villa-Garcia, M., 2010. Physico-chemical characterisation of steel slag. Study of its behaviour under environmental conditions. Environ. Sci. Technol. 44, 5383–5388. Ochola, C.E., Moo-Young Jr., H.K., 2004. Establishing and elucidating reduction as the removal mechanism of Cr (VI) by reclaimed limestone residual RLR (modified steel slag). Environ. Sci. Technol. 38, 6161–6165. Ochola, C.E., Moo-Young Jr., H.K., 2007. A novel approach to remediating contaminated groundwater by reusing industrial co-products. Int. J. Environ. Waste Manag. 1, 228–248. Pratt, C., Shilton, A., Havercamp, R.G., Pratt, S., 2009. Assessment of physical techniques to regenerate active slag filters removing phosphorus from wastewater. Water Res. 43, 277–282. Proctor, D.M., Fehling, K.A., Shay, E.C., Wittenborn, J.L., Green, J.J., Avent, C., Bigham, R.D., Connolly, M., Lee, B., Shepker, T.O., Zak, M.A., 2000. Physical and chemical characteristics of blast furnace, basic oxygen furnace, and electric arc furnace steel industry slags. Environ. Sci. Technol. 34, 1576–1582. Sakadevan, K., Bavor, H., 1998. Phosphate adsorption characteristics of soils, slags, and zeolites to be used as substrates in constructed wetland systems. Water Res. 32, 393–399. Shilton, A., Elmetri, I., Drizo, A., Pratt, S., Haverkamp, R., Bilby, S.C., 2006. Phosphorus removal by an active slag filter – a decade of full scale experience. Water Res. 40, 113–118. Simmons, J., Ziemkiewicz, P., Black, D.C., 2002. Use of steel slag leach beds for the treatment of acid mine drainage. Mine Wat. Env. 21, 91–99. Trendall, A.F., Compston, W., Nelson, D.R., De Laeter, J.R., Bennett, V.C., 2004. SHRIMP zircon ages constraining depositional chronology of the Hamersley Group Western Australia. Aust. J. Earth Sci. 51, 621–644. Wendling, L., Douglas, G., Coleman, S., Petrone, K., 2009. Best management practices: investigation of mineral-based by-products for the attenuation of nutrients and DOC in surface waters from the Swan Coastal Plain. Report for the Water Foundation, Western Australian Department of Water. CSIRO: Water for a Healthy Country National Research Flagship, June 2009, 131p.