Effective CO2-specific sequestration capacity of steel slags and variability in their leaching behaviour in view of industrial mineral carbonation

Minerals Engineering 23 (2010) 262–269

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Effective CO2-specific sequestration capacity of steel slags and variability in their leaching behaviour in view of industrial mineral carbonation Frédéric J. Doucet * Laboratory of Applied Mineralogy and Industrial Chemistry, Council for Geoscience, 280 Pretoria Road, Private Bag X112, Pretoria 0001, South Africa

a r t i c l e

i n f o

Article history: Received 28 May 2009 Accepted 9 September 2009 Available online 9 October 2009 Keywords: Waste processing Leaching

a b s t r a c t Industrial mineral carbonation of alkaline wastes, an increasingly promising component of carbon capture and storage, may play an important role as a CO2 mitigation strategy in the context of climate change. Steelmaking slags are of particular interest owing to their high content of calcium. The cumulated ‘effective’ CO2-specific sequestration capacity (calculated on the basis of calcium and magnesium extracted to a 0.5 M HNO3 solution) of three basic oxygen and one electric arc furnace slags generated at steel mills in South Africa was 253 kt CO2 per annum, which was 25.2% lower than their cumulated ‘theoretical’ capacity (estimated on the basis of total calcium and magnesium content in slags). The mineralogical composition and solubility characteristics of slags conferred very distinct leaching behaviours to the slags, including differences in: (i) the amount of heat generated during their dissolution, (ii) their buffering capacity, (iii) the rate and extent of calcium and magnesium extraction from the slags, and (iv) the mineralogical composition of the non-dissolved residues. These findings suggest that separate leaching processes may need to be developed for slags with largely distinct mineralogical compositions and structural features. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The mineral carbonation of steel slag with anthropogenic CO2 (Huijgen et al., 2005; Teir et al., 2007; Bonenfant et al., 2008; Lekakh et al., 2008) is an increasingly promising concept that may play an important role as an alkaline waste management strategy and a CO2 mitigation option in the context of climate change. It involves the reaction of calcium and magnesium from industrial alkaline wastes with CO2 to form stable mineral carbonates. Worldwide the iron and steel industry accounts for 6–7% of the total CO2 emissions (Kim and Worrell, 2002; House and van der Walle, 2007) whilst it generates about 350-million tons of iron and steel slag per annum (Miklos, 2000). Admittedly, CO2 sequestration in steel slag is unlikely to have a substantial impact on CO2 emissions on a global scale, but estimates suggest that electric arc (EAF) and basic oxygen (BOF) furnace slags have the potential to sequester 35–45% and 6–11% of CO2 generated from EAF and BOF furnaces, respectively (Lekakh et al., 2008). This would amount to a saving of about 170-million tons of CO2 per annum (Eloneva, 2008). Mineral carbonation has therefore the potential to provide substantial CO2 emissions reduction for individual steel

* Tel.: +27 (0) 12 841 1300; fax: +27 (0) 86 611 8838. E-mail address: [email protected] 0892-6875/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2009.09.006

plants, assuming that economically-viable industrial carbonation processes can be developed. A number of laboratory-scale processes have been developed in recent years (e.g. acetic acid process route; Eloneva et al., 2008a), although they suffer from excessive energy requirements and/or solvent regeneration costs. The prospect of mineral carbonation of steel slag to become an economically-feasible CO2 mitigation option is likely to rely on the identification of affordable and recyclable solvents/additives for effective calcium (Ca) and magnesium (Mg) extraction, and the successful development of auxiliary process routes that can supply products with marketable value (e.g. precipitated calcium carbonate (PCC: Teir et al., 2005), carbonated construction aggregates). This replacement of, for instance, some of the PCC used in industry by slag-derived products would contribute towards a more sustainable use of a country’s natural resources. For auxiliary process routes to be developed, aqueous mineral carbonation must follow a sequence of several inter-related steps such as: Step 1: Chemical extraction of Ca and Mg from steel slag using an effective selective solvent. Step 2: Separation of Ca and Mg from other elements which were also leached out of steel slag (optional). Step 3: Carbonation of Ca- and Mg-enriched solution with CO2 to form calcium carbonate (CaCO3) of varying degrees of purity.

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Step 4: Energy-efficient and environmental-friendly recovery of solvent chemicals for re-use. The primary objective of this paper, which focuses on Step 1 above, was to provide ‘theoretical’ and ‘effective’ quantifications of the intrinsic CO2-specific sequestration potential of four steel slags generated in South Africa and to evaluate the selective dissolution behaviour of their Ca- and Mg-bearing phases under different leaching environments. 2. Experimental 2.1. Materials preparation and characterisation Representative samples of three basic oxygen furnace (BOF) and one electric arc furnace (EAF) slags were collected from steel manufacturing plants in South Africa. Samples were then crushed (if necessary) and ground until all material passed a predetermined sieve size (d < 150 lm). Homogeneous sub-samples of the prepared batches were obtained with a sample splitter for future experiments. Particle size distribution (PSD) for each batch was obtained by laser diffraction (Malvern Mastersizer, 2000 fitted with a

Hydro 2000G dispersion unit, Malvern Instrument Ltd. Worcester, UK) to confirm the effectiveness of the milling procedure. Scattered light data were recorded from 2000 to 5000 snapshots of 10 ls. A polydisperse mode of analysis and a refractive index of 1.533 with an adsorption of 0.1 were chosen. Size data collection was performed at constant obscuration in the range 15–20%. The elemental and mineralogical compositions of each slag were analysed using XRF (PANanalytical Axios X-ray fluorescence spectrometer equipped with a 4 kW Rh tube) and XRD (Bruker D8 Advance X-ray diffractometer), respectively. The untreated samples were micronized to a particle size <30 lm for increased accuracy prior to XRD analysis. The ‘theoretical’ maximum CO2 specific sequestration capacity per kilo of waste (SSCCO2ðmaxÞ ; expressed in g CO2 kg waste) was calculated for each BOF and EAF slag from its total Ca and Mg concentrations. This method is based on the assumption that the total amount of Ca and Mg can be extracted from the waste and subsequently carbonated (Huijgen et al., 2005). This sequestration capacity is therefore an intrinsic property of the waste. While Fe can also form carbonates, it was not considered in these calculations since Fe is a valuable metal. SSCCO2ðmaxÞ and the annual production of the waste in South Africa were then used to calculate its

Table 1 Chemical composition of selected BOF and EAF steel slags generated in and outside South Africa. Waste group

Concentration (wt.%)

a

SiO2

CaO

MgO

Fetotal

MnO

P2O5

Al2O3

S

BOF slag

USA Chinaa Japana Europea Finlanda

10–15 9–15 13.8 12–18 13.9

40–50 34–48 44.3 42–55 43.6

5–10 2.5–0 6.4 <3–8 1.4

15–30 17–27 17.5 14–20 24.1

5–10 1.5–6 5.3 <5 2.4

1–3 0.9 n.d. <2 n.d.

2 0.9–2.8 1.5 <3 1.8

n.d. 0.2 0.07 n.d. 0.09

BOFSA1 BOFSA2 BOFSA3

South Africa

18.8 16.9 13.9

41.2 49.9 38.1

8.0 7.7 9.4

16.5 17.3 26.9

4.3 1.2 3.2

1.3 0.3 1.0

3.4 2.0 7.5

n.d. n.d. n.d.

EAF slag

USAa Japana Europea Finlanda Swedena Canadaa

19.4 19.0 10–17 26.6 34.0 14.6

32.1 38.0 25–40 40.8 47.0 32.8

9.4 6.0 4–15 7.2 6.0 10.0

26.4 15.2 18–29 24.2 25.3 34.2

6.8 6.0 <6 2.3 n.d. 2.5

n.d. n.d. <1.5 n.d. n.d. 0.3

8.6 7.0 4–7 8.4 2.3 5.1

0.6 0.38 n.d. 0.09 n.d. 0.07

EAFSA

South Africa

18.0

32.3

9.5

30.1

4.5

0.6

5.5

n.d.

n.d. = Not disclosed. a Data extracted from Fregeau-Wu et al. (1993), Okumura (1993), Bi and Lin, (1999), Proctor et al. (2000), Motz et al. (2001), Moosberg–Bustnes, (2004), Teir et al. (2007), Bonenfant et al. (2008), Lekakh et al. (2008).

Table 2 ‘Theoretical’ CO2-specific sequestration capacity of selected steel slags generated in and outside South Africa. Waste group BOF slag

BOFSA1 BOFSA2 BOFSA3 EAF slag

EAFSA

a

USA Chinaa Japana Europea Finlanda South Africa

USAa Japana Europea Finlanda Swedena South Africa

CaO (wt.%)

MgO (wt.%)

SSCCO2ðmaxÞ (g/kg)

ASCCO2ðmaxÞ (kt/y)

40–50 34–48 44.3 42–55 43.6 41.2 49.9 38.1 32.1 38.0 25–40 40.8 47.0 32.3

5–10 2.5–10 6.4 <3–8 1.4 8.0 7.7 9.4 9.4 6.0 4–15 7.2 6.0 9.5

369–501 294–486 417 <362–518 357 411 475 402 354 364 240–477 399 434 357

n.c.a n.c. n.c. n.c. n.c. 64 6 217 n.c. n.c. n.c. n.c. n.c. 51

Cumulated theoretical capacity for the four slags: 338 kt CO2 per annum. Cumulated theoretical capacity for the four slags: 2418 kt CO2 for the last decade. a Not calculated since the amount of waste generated in these countries was not reported in the published literature.

‘theoretical’ annual maximum CO2 (ASCCO2ðmaxÞ ; expressed in g CO2 year).

sequestration

potential

10.1 0.2 – –

Quartz Si(OH)2

F.J. Doucet / Minerals Engineering 23 (2010) 262–269

Other

264

4.2 4.4 5.6 6.0 24.3 16.6 35.5 38.3 – – 4.2 – – – 11.8 – – – 2.6 18.7 – – – 2.5 BOFSA1 BOFSA2 BOFSA3 EAFSA

13.8 44.2 37.8 18.2

14.5 14.2 2.6 14.1

–

5.9 4.5

23.4 8.7 – 2.2

3.9 7.3 – –

Magnetite Fe3O4 Wuestite FeO Lime CaO Calcite CaCO3 Brownmillerite (Mg,Si-exchanged) Ca2Fe1.4Mg0.3Si0.3O5 Larnite ß-Ca2SiO4

Ca-containing phases identified Wastegroup

Table 3 Mineralogical composition of selected steel slags generated in South Africa (%).

Portlandite Ca(OH)2

Ca silicate a-Ca2SiO4

Akermanite Ca2Mg(Si2O7)

Katoite Ca3Al2(OH)12

Gypsum CaSO42H2O

Fe-containing phases identified

2.2. Calcium and magnesium chemical extraction from slags Acidic, neutral and alkaline leaching experiments were performed on each sample.

2.2.1. Baseline leachability testing A standard baseline testing method was used to assess and compare the reactivity of BOF and EAF wastes under acidic condition. For this purpose, the existing NEN 7341 standard leaching test (NEN 7341:1995, 1995) was selected since it was originally developed and designed to estimate the maximum overall elemental amount that may be leached when waste materials are exposed to extreme environmental conditions, such as geological timeframe, after disintegration of the materials, after complete oxidation, and/or after the loss of their acid neutralization capacity. The test was performed on milled samples (d < 150 lm). The moisture content of each sample was determined by oven-drying at 105 °C over 18 h and calculated as an average value of triplicates. Waste samples (5.00 ± 0.02 g; dry weight) were placed in closed high-density polyethylene bottles. Ultra-pure water (electrical conductivity <1 lS cm1) was added to give treatment solutions characterised by a liquid to solid ratio (L/S) of 10 (kg/kg). All treatment solutions were continuously stirred to maintain slag particles in suspension throughout the experiment. The first step involved holding the pH of the suspension constant at 7.00 ± 0.05 for 3 h by automatic addition of aliquots of 1 M HNO3 using a TritraLabÒ 856 auto-titration workstation (Radiometer analytical, Villeurbanne, France). At the end of this leaching period, the leachant was filtered under reduced pressure through 0.1 lm membranes filters (Whatman NucleoporeÒ Track-Etched polycarbonate, Whatman UK Ltd.) in closed polycarbonate filtration vessels (Sterifil, 47 mm Millipore) to give Eluate A. The second step involved the immersion of the particles retained on the filter into 250 ml ultra-pure water. Solution pH was held at 4.00 ± 0.05 using the above HNO3 solution for a further 3 h. Filtration of the second leachant provided Eluate B. The concentrations of dissolved Ca and Mg in Eluates A and B were determined by ICP-MS (Elan DRC II, PerkinElmer Ltd. Waltham, USA) and the composition of the non-dissolved residues was determined by ICP-OES (Liberty 200, Varian, Palo Alto, USA) and XRD.

2.2.2. Dissolution experiments Chemical extraction of Ca and Mg from steel slag was investigated by dissolving BOF and EAF slags (<150 lm) in several aqueous solutions with L:S of 10 kg/kg at 22 °C: (1) ultra-pure H2O, (2) 0.5 M NaOH, (3) 0.5 M H2SO4 or (4) 0.5 M HNO3. Progress of the reaction was monitored by measuring the heat produced, if any, as leaching progresses, and by sampling small aliquots from treatment solutions at regular intervals (5, 20, 40 and 60 min upon addition of the slag). The aliquots were filtered using the procedure described in Section 2.2.1 and their elemental composition was determined by ICP-MS. At completion of the experiment (i.e. 60 min), the non-dissolved residues were separated from the supernatant, washed thoroughly with ultra-pure water, and oven-dried at 40 °C up to constant weight. Their elemental and mineralogical compositions were analysed by ICP-OES and XRD, respectively. The total calcium and magnesium content of leachates was then used to calculate the ‘effective’ CO2-specific sequestration capacity, which provides a more realistic indication of the maximum amount of CO2 that can be sequestered in steel slag than previously-published ‘theoretical’ capacity.

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3. Results and discussion

0.16

3.1. Slag characterisation and theoretical sequestration potential

0.14

BOFSA2

pH 4 BOFSA1

0.12 BOFSA1

0.1 BOFSA2

BOFSA3

0.08

EAF

0.06 EAF

0.04 BOFSA3

0.02 0 0

10

20 30 40 Percent of Ca released (%)

50

Fig. 2. Correlation between the proportion of Ca released from slags in solution and the concentration of HNO3 required to maintain the solution pH constant at 7.00 ± 0.05 followed by 4.00 ± 0.05 for 3 h (pHstat experiment with L/S = 10 kg/kg; N = 2; slag particles < 150 lm).

6 Temperature increase during leaching (C)

The bulk chemical composition of BOF and EAF steel slags used in this study was similar to that of equivalent slags generated in other countries (Table 1). The slags were rich in Ca (32–50 wt.% CaO), a result of the addition of limestone as a fluxing agent in the manufacturing process of steel for the removal of undesirable impurities (Huijgen et al., 2005). This compositional characteristic, coupled to their elevated alkalinity (BOFSA: 11.9–12.4; EAFSA: 11.3), make them suitable candidates for the industrial sequestration of CO2. The calculated ‘theoretical’ maximum CO2-specific sequestration capacity of the slags ranged from 357 to 475 g of CO2/kg of steel slag (Table 2), which is in agreement with published data (250–509 g of CO2/kg of slag; Huijgen and Comans, 2005; Huijgen et al., 2005). When the annual production of these four wastes was taken into account, it was found that up to 338 kt of cumulated CO2 could in principle be sequestered in these four slags per annum, provided that an economically-viable industrial mineral carbonation process is developed. This cumulative sequestration volume was estimated on the basis of fresh slag production in a single year and did not include previously dumped material which can be measured in millions of tons. Based on the total amount of the four slags which was generated over the last 10 years, an estimated 2418 kt of CO2 could theoretically be sequestered in existing slag dams. Since most Ca and Mg in steel slag was tied up as silicates or in other poorly-soluble mineral phases (Table 3), it can be assumed that little Ca or Mg contained in dumped material would have undergone extensive natural carbonation or other forms of weathering over the last decade. Whilst ‘theoretical’ CO2-specific sequestration capacity is a useful variable for the compilation of a preliminary short-list of potentially-suitable wastes, the so-called ‘effective’ CO2-specific sequestration capacity provides a more realistic assessment of the possible extent of CO2 mitigation by industrial mineral carbonation. ‘Effective’ capacity was estimated on the basis of measured leached Ca and Mg and was therefore lower than the ‘theoretical’ capacity calculated on the basis of the total elemental composition. By its nature, this parameter strongly depends on a number of vari-

Increase in [HNO3] for pH stability

pH 7

A

5

4

3

2

1

0 0

10

20

30 40 Time (minutes)

50

60

30 pH4

6 Temperature increase during leaching (C)

pH 7

Cumulative HNO3 volume (ml)

25 BOFSA1

20 EAFSA

BOFSA2

15

BOFSA1

BOFSA2

10

EAFSA

5

BOFSA3

BOFSA3

B 5

4

3

2

1

0 0

40

80

120

160 200 240 Time (minutes)

280

320

360

Fig. 1. pHstat leaching curves for four untreated steel slag samples (<150 lm) illustrating differences in the volume of added HNO3 required to maintain solution pH constant at 7.0 ± 0.05 for 3 h followed by constant solution pH of 4.0 ± 0.05 for 3 h (L/S = 10 kg/kg; N = 2; solution pH was maintained constant at selected values by addition of 1 M HNO3).

0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Time (minutes)

1

1.1

Fig. 3. Temperature recordings of dissolution experiments of four steel slags in 0.5 M HNO3 at an ambient temperature of 22 °C (L/S = 10 kg/kg; N = 2; slag particles <150 lm; slag samples added at t = 0 min) over time (A: 60 min; B: 1 min). Markers used: N for BOFSA1,  for BOFSA2, d for BOFSA3, j for EAFSA.

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F.J. Doucet / Minerals Engineering 23 (2010) 262–269

ables, including leaching conditions (e.g. type and concentration of solvent/additive used, leaching duration and temperature) and the structural properties and the solubility of complex Ca- and Mgcontaining oxide phases present in slags. For instance, wastes containing a high proportion of reactive portlandite (Ca(OH)2; Bonenfant et al., 2008; Pérez-López et al., 2008) or reactive free lime (CaO or MgO) as sources of Ca and/or Mg are especially desirable since their highly reactive, easily-leachable Ca and Mg are more likely to contribute towards ‘effective’ capacity than Ca and Mg originating from mineral phases such as brownmillerite. However, only a minor amount of CaO, MgO and Ca(OH)2, if any, is generally present in steel slag, with Ca and Mg being primarily tied up as silicates and/or aluminates (Lekakh et al., 2008). This was confirmed by XRD since CaO and Ca(OH)2 were only found in small amount (<8%) in BOFSA1 and BOFSA2 (Table 3) and MgO was not detected. Untreated slag samples showed large differences in their mineralogical composition (Table 3), which consisted of an average of 7–8 phases (56–79% Ca-containing phases, 21–44% Fe-containing phases, 610% quartz), although the type of phases identified differed greatly between sample types. The content of calcite in BOFSA1 was fairly high (23.4%) and the leaching and subsequent carbonation of Ca from this phase defies the objective of the CO2 sequestration exercise since its dissolution would liberate gaseous

CO2. The contribution of the calcite phase to the total Ca content of this slag was approximately 16%, suggesting that BOFSA1 may still be suitable for mineral carbonation if most of the remaining Ca can be efficiently extracted and carbonated at competitive costs to achieve a positive net CO2 yield. Whilst elemental compositions of the four slags were similar (Table 1), the variability in their mineralogical composition (Table 3) and in the solubility characteristics of the phases present in the slags was a strong indication that their leaching behaviour may be substantially different. This has surprisingly not been reported in the published literature treating the mineral carbonation of steel slag. For instance, Ca (hydr)oxide phases are expected to leach more readily (on the basis of their solubility) than Ca silicates, although the dissolution reactions may be controlled by other factors, such as surface reaction limitations, e.g. the inclusion of reactive phases within insoluble minerals preventing them from being exposed to leaching environments. If large variability in the leaching behaviour between slags is confirmed, separate leaching processes may need to be developed for slags with largely distinct mineralogical compositions and structural features. Differences in processes may include the use of various solvents, concentrations, temperature conditions, or leaching times.

6

A

Temperature increase during leaching (C)

Temperature increase during leaching (C)

6

5

4

3

2

1

5

4

3

2

1

0

0 0

10

20

30 40 Time (minutes)

50

0

60

10

20

30

40

50

60

50

60

Time (minutes)

6

6

C

Temperature increase during leaching (C)

Temperature increase during leaching (C)

B

5

4

3

2

1

0

D 5

4

3

2

1

0 0

10

20

30

40

Time (minutes)

50

60

0

10

20

30 40 Time (minutes)

Fig. 4. Temperature recordings of dissolution experiments of four steel slags (A: BOFSA1; B: BOFSA2; C: BOFSA3; D: EAFSA) under acidic and alkaline conditions at an ambient temperature of 22 °C (L/S = 10 kg/kg; N = 2; slag particles <150 lm; slag samples added at t = 0 min) over a 60 min period. Markers used: N for 0.5 M HNO3, j for 0.5 M H2SO4, d for 0.5 M NaOH.

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F.J. Doucet / Minerals Engineering 23 (2010) 262–269

3.2. Ca and Mg chemical extraction from slags 3.2.1. Baseline leachability testing Differences in the mineralogical properties of the slags elicited distinct chemical behaviours between slags in a dual pHstat system (Fig. 1). For instance, upon addition of 1 M HNO3 to maintain a constant slag suspension pH of 7.00, BOFSA1 exhibited a stronger buffering action than BOFSA2, BOFSA3 and EAFSA which all showed

100 90

similar buffering properties, whereas their resistance to HNO3 addition toward pH 4.00 diverged significantly (P 60.01). These neutralizing actions were directly related to the number of ions (e.g. Ca) which were extracted from slags to solution (Fig. 2) and were controlled by the leaching of anions neutralising the hydronium ions ðH3 Oþ Þ originating from HNO3 addition. The correlation between the difference in proton consumption and the amount of extracted Ca between slag samples at two pH conditions provided preliminary evidence of substantial differences in the hydrolysis/ dissolution properties of the minerals constituting the slag samples. It also suggested that regardless of the solvent used for promoting slag mineral dissolution, its acidity must be sufficiently strong to exhaust most slag buffering minerals and to induce accelerated dissolution rates by a proton-promoted mechanism.

80 Ca dissolved (%)

70

3.2.2. Dissolution experiments Extraction of Ca and Mg from the four slags was evaluated under several leaching environments: (1) H2O, (2) 0.5 M HNO3, (3) 0.5 M H2SO4, and (4) 0.5 M NaOH.

60 50 40

(a) H2O – Dissolution of the four slags in ultra-pure water over a 60 min period was minimal. In particular, during this period, the temperature of ultra-pure water had remained unchanged upon addition of slag samples, little Ca (1.4– 8.7%) and Mg (0.01–0.02%) had been released to solution, and the mineralogical composition of untreated and treated slags was identical. (b) Acidic and alkaline treatments – Suspending the slags in acidic or alkaline solutions caused a substantial rapid increase in solution temperature (between 3.7 °C and 5.6 °C; Figs. 3 and 4) for all samples, except for BOFSA1 and EAFSA which had been dispersed in NaOH (temperature increase <1.6 °C). The exothermic nature of slag dissolution under acidic conditions and the absence of heat production when slags were dispersed in water have also been reported elsewhere (e.g. Eloneva et al., 2008b). The rate and extent of heat production of the four slags were dissimilar (Figs. 3 and 4), which provided further evidence of significant differences in the hydrolysis/dissolution properties of the slags already identified in Section 3.2.1.

30 20 10 0 0

10

20

30

40

50

60

Time (minutes) 100 90 80 Mg dissolved (%)

70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

Time (min) Fig. 5. Differences in the leaching rate of (a) Ca and (b) Mg between several BOF and EAF slags in 0.5 mol dm3 HNO3 (L/S = 10 kg/kg; N = 2; d BOFSA1; N BOFSA2;  BOFSA3; j EAFSA).

While the slags were fairly ‘inert’ in NaOH (as found by ICP-MS and XRD analysis of leachates and leached residues, respectively), most Ca and Mg leached rapidly (<20 min) from slags that had been suspended in 0.5 M HNO3 (Fig. 5), although Ca extraction was generally more efficient (71–89%) than that of Mg (35–83%). An exception was BOFSA1 where Ca and Mg followed a similar pattern of dissolution. The incomplete elemental extraction under the present experimental conditions indicated that the ‘theoretical’ sequestration capacity reported earlier in this paper as well as in the published literature (Huijgen et al., 2005) is unrealistic. Whilst this approach is useful in drawing a preliminary short-list of poten-

Table 4 ‘Effective’ CO2-specific sequestration capacity of selected steel slags generated in South Africa. Waste group

Leached Ca (g/kg)

Leached Mg (g/kg)

‘Effective’ sequestration capacity (g/kg) for leaching in 0.5 M HNO3 over 60 min

Annual ‘Effective’ sequestration capacity (kt/y) for leaching in 0.5 M HNO3 over 60 min

% Decrease in comparison to ‘theoretical’ capacity

BOF slag 257.3 BOFSA1 298.9 BOFSA2 201.8 BOFSA3

42.3 30.8 37.0

359 384 288

56 5 156

12.5 16.7 28.1

EAF slag EAFSA

26.4

257

37

27.5

190.4

Cumulated ‘theoretical’ capacity for the four slags: 338 kt CO2 per annum. Cumulated ‘effective’ capacity for the four slags: 253 kt CO2 per annum.

268

F.J. Doucet / Minerals Engineering 23 (2010) 262–269 100 Mineralogical composition (%)

90

A

80 70 60 50 40 30 20 10 0 3 4 4 O )2 iO2 O7) .3O5 tite FeO SiO e3O aCO (OH e Ca rtz S g(Si2 C a2 s .3Si0 ite F Lim ite C e Ca Qua Wue Magnet rnite ite Ca2M 1.4Mg0 ndit Calc a la L t r Fe an Po 2 a m r C Ake ed) ang exch g,Si(M e it ler nmil Brow

100 Mineralogical composition (%)

90

B

80 70 60 50 40 30 20 10 0 3 aCO ite C Calc

4 4 O )2 iO2 FeO .3O5 SiO (OH e Ca Fe3O rtz S Ca2 stite .3Si0 Lim e Ca tite Qua Wue Mg0 rnite ndit gne a a .4 la L t 1 M r e Po a2F d) C nge c ha i-ex ,S g ite(M iller wn m Bro

100 Mineralogical composition (%)

90

C

80 70 60 50 40 30 20 10 0

) 12 O4 iO4 iO2 FeO H2O .3O5 i2O7 OH) Fe3 a2S rtz S O4.2 stite .3Si0 Al2( tite Mg(S ite C Qua CaS Wue Mg0 Ca3 gne Ca2 a m .4 e e Larn u 1 M it it s e n a2F Kato Gyp rma d) C Ake nge cha i-ex ,S g ite(M iller wnm Bro

100 Mineralogical composition (%)

90

D

80 70 60 50 40 30 20 10 0 ) 3 iO4 iO4 3O4 FeO .3O5 i2O7 aCO a2S a2S e Fe stite .3Si0 ite C Mg(S te C ite C netit Wue Mg0 Ca2 Calc ilica Larn Mag e1.4 nite mS F a 2 iu a m r C Calc Ake ed) ang exch g,SiM ( e it iller wnm Bro

Fig. 6. Mineralogical composition of (A) BOFSA1, (B) BOFSA2, (C) BOFSA3 and (D) EAFSA (histogram: untreated slag; j slag residue upon leaching in 0.5 M HNO3 solution at 22 °C for 60 min).

F.J. Doucet / Minerals Engineering 23 (2010) 262–269

tially-suitable wastes, it is recommended to calculate a so-called ‘effective’ sequestration capacity on the basis of measured dissolved Ca and Mg upon the completion of leach tests. The cumulative ‘effective’ sequestration capacity for the four slags when using the nitric process was 253 kt CO2 per annum, which was about 25.2% lower than the ‘theoretical’ capacity, whereas the ‘effective’ sequestration capacity of individual slags was 12.5–28.1% lower than their ‘theoretical’ counterpart (Table 4). The ‘real’ sequestration capacity can be expected to be even lower since the conversion of leached Ca and Mg to carbonated products is also likely to be an incomplete process. Similarly to heat production, the rate and extent of Ca and Mg extraction differed substantially between samples. The more reactive Ca-bearing phases were lime, portlandite, larnite and calcite, which were found to dissolve completely within 20 min in all acidic treatments, whereas brownmillerite was generally the most insoluble Ca phase in the nitric environment (Fig. 6). However, XRD analysis of EAFSA before and after nitric treatment suggested total disappearance of brownmillerite, suggesting that the environment surrounding this mineral in EAF slag may be different from that in the three BOF slags. Since the non-dissolved leached slag residues were essentially composed of Fe-containing phases (wuestite and magnetite), it is conceivable that brownmillerite in BOF slags had been present as inclusions within the iron phases and had therefore not been directly exposed to the corrosive solution. This explanation was indirectly supported by XRD analysis of slag residues that had been treated under harsher conditions (0.5 M H2SO4), which showed that for EAF total brownmillerite dissolution had occurred and had been accompanied by partial dissolution of wuestite and to a lesser extent magnetite. However, the use of H2SO4 as solvent for the purpose of CO2 sequestration is probably less attractive than HNO3 since most of the dissolved Ca fraction (>95%) had rapidly reacted with sulphate ions to form crystalline gypsum, which represented over 84% of the final solid residues, and was therefore not easily available for subsequent reaction with CO2. 4. Conclusions Industrial mineral carbonation of steelmaking slags could represent a viable dual strategy for the mitigation of CO2 and the reexploitation of wastes from industry. This approach has the potential of reducing the annual CO2 emissions from individual steel manufacturing plants by several hundreds of tons per annum, depending on local plant conditions (e.g. Ca and Mg content of slag, annual amount of slag generated). Rapid extraction of Ca and Mg from BOF and EAF slags was successfully performed in HNO3 at room temperature. Future work will focus on the removal of other leached elements from slag-derived leachates with the subsequent carbonation of the ‘purified’ calcium-rich solutions and the formation of calcium carbonate products with varying degrees of purity, particle sizes, shapes and brightness. An anticipated difficulty arising from the application of a nitric process is the resulting acidity of the Ca-rich leachate. The pH of the leachate would need to be adjusted in a cost-effective way since a solution pH in excess of 10 is required for the precipitation of Ca into Ca(OH)2 and its subsequent carbonation. Some species are also known to prevent Ca(OH)2 precipitation (e.g. acetic acid (Eloneva et al., 2008b)) although this problem is not anticipated in the HNO3 system presented in this paper. Innovative cost-effective approaches for pH adjustment are currently being tested. A key issue for industrial

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