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Mechanism of Vanadium Leaching During Surface Weathering of Basic Oxygen Furnace Steel Slag Blocks: A µXANES and SEM Study Andrew James Hobson, Douglas I. Stewart, Andrew William Bray, Robert J.G. Mortimer, William Matthew Mayes, Michael Rogerson, and Ian T. Burke Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00874 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017
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Mechanism of Vanadium Leaching During Surface Weathering of
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Basic Oxygen Furnace Steel Slag Blocks: A µXANES and SEM Study
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Andrew J. Hobson1, Douglas I. Stewart2, Andrew W. Bray1, Robert J. G. Mortimer3, William M.
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Mayes4, Michael Rogerson4 and Ian T. Burke1*
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School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK.
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School of Civil Engineering, University of Leeds, Leeds, LS2 9JT, UK.
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School of Animal, Rural and Environmental Sciences, Nottingham Trent University, Brackenhurst Campus,
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Southwell, Nottinghamshire NG25 0QF, UK
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*Email for correspondence:
[email protected], phone: +44 113 3437532, fax: +44 113 3435259.
School of Environmental Sciences, University of Hull, Hull, HU6 7RX, UK.
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Prepared for: ES&T
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Word Count: 4507 (+ 2100 in tables and figures)
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TOC artwork
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Abstract
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Basic oxygen furnace (BOF) steelmaking slag is enriched in potentially toxic V which may become
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mobilised in high pH leachate during weathering. BOF slag was weathered under aerated and air-
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excluded conditions for 6 months prior to SEM/EDS and µXANES analysis to determine V host phases
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and speciation in both primary and secondary phases. Leached blocks show development of an
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altered region in which free lime and dicalcium silicate phases were absent and Ca-Si-H was
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precipitated (CaCO3 was also present under aerated conditions). µXANES analyses show that V was
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released to solution as V(V) during dicalcium silicate dissolution and some V was incorporated into
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neo-formed Ca-Si-H. Higher V concentrations were observed in leachate under aerated conditions
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than in the air-excluded leaching experiment.
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Ca3(VO4)2 solubility which demonstrate an inverse relationship between Ca and V concentrations.
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Under air-excluded conditions Ca concentrations were controlled by dicalcium silicate dissolution
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and Ca-Si-H precipitation, leading to relatively high Ca and correspondingly low V concentrations.
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Formation of CaCO3 under aerated conditions provided a sink for aqueous Ca, allowing higher V
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concentrations limited by kinetic dissolution rates of dicalcium silicate. Thus V release may be
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slowed by the precipitation of secondary phases in the altered region, improving the prospects for
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slag reuse.
Aqueous V concentrations were controlled by
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Introduction
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Basic oxygen furnace (BOF) steelmaking is a primary method of steelmaking which accounts
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for ~2/3 of worldwide steel production [1, 2]. In a basic oxygen converter low carbon steel is
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produced by blowing oxygen through molten pig iron and recycled scrap steel to remove carbon.
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BOF slag is the primary by-product of BOF steelmaking and is produced when limestone (or
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dolomite) is added to the molten iron as a fluxing agent to draw out impurities [3]. Due to the large
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quantities of steel slag produced worldwide (170-250 million metric ton/year [4]) reuse has become
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increasingly important in order to comply with environmental regulations limiting disposal of wastes.
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Currently steel slag that cannot be recycled in blast furnaces is primarily reused as aggregate in road
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surfacing and construction due to its high stability and skid resistance [5, 6], whilst other uses
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include neutralisation of acidity in soils or mine wastes and possibly for CO2 sequestration [7-10].
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Depending on its precise mineralogy, some steel slag may be unsuitable for reuse, particularly in
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engineering applications. Steel slag may be enriched in phases such as free lime (CaO) and periclase
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(MgO) which expand on hydration, resulting in significant volume change [11]. In situations where
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slag cannot be reused, or where supply exceeds after-use demand, slag is generally disposed to
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landfill [12].
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Steel slag formed during primary steelmaking (i.e. from BOF or electric arc furnace (EAF)
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processes) typically comprises a mixture of Ca oxides, Fe oxides and silicates. The precise chemical
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composition varies by iron source and processing, however, the bulk chemical composition is
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relatively consistent between locations worldwide (Table 1). Typically BOF slag is dominated by Ca,
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Fe and Si, with minor amounts of Mg, Mn and Al [3, 13, 14]. The mineralogical composition can be
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complex but typically contains a range of calcium-containing silicate phases (e.g. Larnite, β-Ca2SiO4;
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Merwinite Ca3Mg(SiO4)2), calcium and aluminium ferrite phases (e.g. Brownmillerite, Ca2FeAlO5,
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Srebrodolskite, Ca2Fe2O5), free Mg and Ca oxides (e.g. Lime, CaO; Periclase, MgO) and a refractory
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oxide solid solution of Fe-Mg-Mn-Ca- oxides (e.g. Wüstite, FeO) [3, 11, 15-17].
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When stored in contact with water, CaO and Ca-silicates in BOF slag readily react with to generate a high pH leachate (typically pH 10 – 12.5), e.g. via equations 1 and 2 [18, 19]:
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CaO + H2O → Ca2+ + 2OH-
(Eqn. 1)
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Ca2SiO3 + H2O → 2Ca2+ + H2SiO42- + 2OH-
(Eqn. 2)
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The release of Ca2+ and H2SiO42- to solution can lead to oversaturation with respect to
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calcium silicate hydrates (Ca-Si-H; Eqn. 3), and secondary carbonates in the presence of atmospheric
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CO2 (Eqn. 4; [18]). Spinels (e.g. Magnetite; Eqn. 5) and hydroxide phases can also form, where di-
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and trivalent metal ions are released[20]. 3Ca2+ + 2H2SiO42- + 2OH- → 3CaO·2SiO2·3H2O
(Eqn. 3)
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Ca2+ + CO2 + 2OH- → CaCO3 + H2O
(Eqn. 4)
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Fe2+ + 2Fe3+ + 4H2O → Fe3O4 + 8H+
(Eqn. 5)
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`
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Steel slags usually contain a variety of trace elements from the primary ore that become
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concentrated by processing. At high pH, several potential toxic metals are solubilised and become
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mobile in leachate including Al, Fe, and V [18-21]. In recent years V leaching has received significant
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attention due to its relative enrichment in BOF slag (0.04-1.48 wt%; [14, 21]) and the potential
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mobility and toxicity of V(V) species in alkaline leachates. V is present in steel slag in multiple
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oxidation states (V(III), V(IV) and V(V) [21, 22]), however, there is uncertainty about the exact host
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phases and the V oxidation state within those phases, making evaluation of V leaching behaviour
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difficult [23]. This uncertainty is compounded by the use of leaching data acquired from powdered
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specimens, which may be unrepresentative of slag aggregate weathering in civil engineering
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applications [8, 20]. As a result regulatory bodies, such as the UK Environment Agency, are adopting
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worst case scenario assumptions when considering V leaching from steel slags [24]. This will have an
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adverse effect on beneficial reuse of slag, and could lead to overly stringent long-term monitoring
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requirements for landfills.
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Previous research has shown that under air-excluded leaching conditions up to 1.7% of the V
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in BOF slag was released to solution [20]. This is due to the relatively high reactivity of the V hosting
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phase, and the prediction that V(III) and V(IV) can be oxidised to V(V) during slag weathering [17, 20,
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22]. No data are available concerning leaching of steel slags under aerated conditions that will be
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more favourable to V oxidation. Equally, the role of secondary phase formation in controlling V
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release is currently unknown, although the potential for aqueous V to be incorporated into Ca-Si-H
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has been noted [20].
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A thorough understanding of weathering processes in BOF slag is essential to enable long-
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term, cost effective management and use of steel slag whilst protecting both the environment and
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human health from its potentially hazardous components. This study used scanning electron
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microscopy (SEM) and X-ray absorption near-edge spectroscopy (XANES) to determine the
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distribution and speciation of V within BOF steel slag, and to determine whether leachate
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equilibrium atmosphere affects V release during slag leaching. This research provides new insights
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into the distribution of V within both unweathered and weathered BOF slag, and investigates the
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mechanisms underpinning enhanced V leaching and its fate in neo-formed phases.
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Methods
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Sample collection and characterisation
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Basic oxygen furnace (BOF) steel slag was collected in May 2013, within one week of its
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deposition, from British Steel’s Yarborough Landfill (Scunthorpe, UK; LAT 53°35′22.24″ LONG
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0°35′41.52″). The sample consisted of 50-500g blocks (~100kg total). Sub-samples were either cut
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into smaller 8g blocks (20x10x10mm) using a diamond saw for use in leaching experiments, or
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ground to
