Indirect mineral carbonation of blast furnace slag with

Journal of Energy Chemistry 26 (2017) 927–935

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Indirect mineral carbonation of blast furnace slag with (NH4 )2 SO4 as a recyclable extractant Jinpeng Hu, Weizao Liu, Lin Wang, Qiang Liu, Fang Chen, Hairong Yue, Bin Liang, Li Lü, Ye Wang, Guoquan Zhang, Chun Li∗ College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China

a r t i c l e

i n f o

Article history: Received 11 April 2017 Revised 17 June 2017 Accepted 19 June 2017 Available online 30 June 2017 Keywords: Blast furnace slag CO2 Mineral carbonation CO2 sequestration

a b s t r a c t Large quantities of CO2 and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO2 emission reduction and comprehensive utilisation of the solid waste. In this study, a recyclable extractant, (NH4 )2 SO4 , was used to extract calcium and magnesium from blast furnace slag (main phases of gehlenite and akermanite) by using low-temperature roasting to fix CO2 through aqueous carbonation. The process parameters and efficiency of the roasting extraction, mineralisation, and Al recovery were investigated in detail. The results showed that the extractions of Ca, Mg, and Al can reach almost 100% at an (NH4 )2 SO4 -to-slag mass ratio of 3:1 and at 370 °C in 1 h. Adjusting the pH value of the leaching solution of the roasted slag to 5.5 with the NH3 released during the roasting resulted in 99% Al precipitation, while co-precipitation of Mg was lower than 2%. The Mg-rich leachate after the depletion of Al and the leaching residue (main phases of CaSO4 and SiO2 ) were carbonated using (NH4 )2 CO3 and NH4 HCO3 solutions, respectively, under mild conditions. Approximately 99% of Ca and 89% of Mg in the blast furnace slag were converted into CaCO3 and (NH4 )2 Mg(CO3 )2 •4H2 O, respectively. The latter can be selectively decomposed to magnesium carbonate at 10 0–20 0 °C to recover the NH3 for reuse. In the present route, the total CO2 sequestration capacity per tonne of blast furnace slag reached up to 316 kg, and 313 kg of Al-rich precipitate, 10 0 0 kg of carbonated product containing CaCO3 and SiO2 , and 304 kg of carbonated product containing calcium carbonate and magnesium carbonate were recovered simultaneously. These products can be used, respectively, as raw materials for the production of electrolytic aluminium, cement, and light magnesium carbonate to replace natural resources. © 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction Carbon dioxide capture (utilisation) and storage (CCS/CCUS) is recognised as one of the options for tackling the increase in the atmospheric concentration of CO2 for climate change mitigation [1]. Although a huge amount of CO2 can be stored through geological and oceanic sequestration, proper geological conditions, such as depleted oil and gas fields, and geographical conditions, e.g. close to deep sea for the CO2 sources, as well as the inevitable use of expensive monitoring equipment to ensure that CO2 does not escape into the atmosphere for a very long time are required. Mineral carbonation is a potential CO2 sequestration method in which calcium and magnesium oxides in various minerals, particularly sil-

∗

Corresponding author. E-mail address: [email protected] (C. Li).

icate minerals, react with CO2 and form thermodynamically stable carbonates. Silicate rocks are abundant in nature to the extent that, in theory, the potential for CO2 storage by mineral carbonation is higher than those of other CO2 storage methods [2]. Mineral carbonation consists of direct and indirect methods. The former mimics the weathering process in nature but proceeds at a much faster rate which can be enhanced using a variety of methods, including activation pre-treatment of minerals [3], the use of a high temperature and pressure [4,5], and high-gravity methods [6]. However, the products of direct mineral carbonation are generally mixtures, which are difficult to separate for further utilisation or for selling as valuable products. Indirect CO2 mineralisation ordinarily comprises two successive steps: (1) extraction of Ca and Mg from minerals with an acidic or weakly acidic additive and (2) carbonation of Ca- and Mg-rich solutions or solids with CO2 in a basic or weakly basic environment. This method is now receiving widespread attention because of its relatively mild

http://dx.doi.org/10.1016/j.jechem.2017.06.009 2095-4956/© 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

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reaction conditions, high carbonation conversions of Ca and Mg, and purer, thus more valuable, by-products. The key issues of the indirect mineral carbonation method that need to be solved are the regeneration of the chemical additives for reuse with low energy consumption and the improvement of the economy of the process. Thus far, the extractions of calcium and magnesium from silicate minerals have been attempted using a variety of acidic or weakly acidic additives including acetic acid [7], sulfuric acid [8–10], nitric acid and hydrochloric acid [11], ammonium nitrate, ammonium chloride, and ammonium acetate [12], ammonium bisulfate [13–15], and ammonium sulfate [16–18]. The extracted Ca and Mg were then carbonated with the aid of sodium hydroxide [11,19] and ammonia [20,21]. However, the recycling of most of these acidic and basic chemicals is either difficult or involves high energy consumption. When ammonium sulfate (AS) is used, it will decompose into acidic ammonium bisulfate and basic NH3 at over 250 °C and be regenerated after the subsequent mineral carbonation [22]. Eqs. (1) and (2) show the total reactions for the extraction and CO2 mineralisation, respectively.

(Mg, Ca)x Siy Ox +2y+z H2z (s ) + x(NH4 )2 SO4 (s ) → x(Mg, Ca)SO4 + ySiO2 (s ) + (z + x )H2 O(g ) + 2xNH3 ( g ) (1) MgSO4 /CaSO4 (s )+ CO2 (g )+ H2 O(l )+ 2NH3 (g ) → MgCO3 /CaCO3 (s ) + (NH4 )2 SO4 (aq )

(2)

Therefore, there have been several studies which have involved the use of easily recyclable ammonium sulfate or ammonium bisulfate + NH3 as additives to treat various silicate minerals, including serpentine [15–17], olivine [14], amphibole [14] and pyroxene [14], steel slag [23] and concrete aggregate [23]. The iron and steel industry is one of the largest industrial sources of emissions of CO2 and solid wastes. The main solid wastes include blast furnace (BF) slag and steel slag emitted in the iron-making and steel-making processes, respectively. Currently, approximately 30 0–10 0 0 kg of BF slag is discharged per tonne of iron produced depending on the grade of the iron ores and the process conditions employed [24]. BF slag is a Ca- and Mg-rich silicate mineral, with CaO, MgO, and Al2 O3 contents of 34%–52%, 6%–10%, and 10%–14%, respectively [19,25]. In 2015, the global output of iron was approximately 1.6 billion tonnes [26]. If all of the Ca and Mg elements in the BF slag are utilised to fix CO2 , theoretically, approximately 200–680 million tonnes of CO2 can be safely sequestered annually. Although the amounts are quite small in comparison to the capacity of natural mineral resources, using BF slag to store CO2 on site could be an inexpensive way to significantly reduce CO2 emissions for individual iron and steel plants. Meanwhile, a high-added-value by-product, namely Al(OH)3 , an Al-rich precipitate, and carbonation products could be obtained for further use, which may improve the economy of the process. Al(OH)3 is mainly used as a raw material for the production of electrolytic aluminium. The global output of electrolytic aluminium was approximately 57.7 million tonnes in 2015 [27], and its production consumed approximately 120 million tonnes of natural bauxite (main constituents were Al(OH)3 or AlOOH) (calculated as Al2 O3 ). If worldwide aluminium resources contained in BF slag could be recovered effectively, nearly the same amount of bauxite would be saved, thus leading to more sustainable use of natural resources. Based on the above analysis, a process for combined carbonation of Ca and Mg extracted from BF slag using recyclable AS and recovery of high-added-value Al(OH)3 was proposed, and its schematic flowsheet is shown in Fig. 1. The present study focuses on the process parameters and efficiency of the roasting extrac-

Table 1. Chemical composition of blast furnace slag used (wt%). CaO

MgO

Al2 O3

SiO2

Fe2 O3

MnO

TiO2

S

38.95

10.58

13.90

33.29

1.47

0.29

0.78

0.74

tion, mineral carbonation, and recovery of aluminium. In addition, a preliminary economic analysis is also presented. 2. Experimental 2.1. Materials The water-quenched BF slag used in this research was provided by Zenith Steel Group Company Limited (Changzhou, China). The density and the specific surface area of the milled BF slag (D50 = 50.43 μm) are 1.3 g/cm3 and 0.512 m2 /g respectively. The chemical composition analysed using X-ray fluorescence (XRF) is shown in Table 1. X-ray diffraction (XRD) analysis showed that the as-received water-quenched BF slag was amorphous and could be converted into the crystalline state after annealing at 800 °C for an hour as shown in Fig. 2. The main constituents were gehlenite (Ca2 Al2 SiO7 ) and akermanite (Ca2 MgSi2 O7 ) with contents of 31% and 66%, respectively. Ammonium sulfate ((NH4 )2 SO4 ), ammonium bicarbonate (NH4 HCO3 ), and ammonium carbonate ((NH4 )2 CO3 ) employed in this study were of analytical grade. 2.2. Methods In each roasting experiment, a mixture of the BF slag and (NH4 )2 SO4 at a certain mass ratio (AS/slag) in a crucible was heated at a rate of 10 °C/min to the required temperature and annealed for a certain period of time in a horizontal tube furnace (GSL-1600X, Hefei Kejing Materials Technology Co. Limited, Hefei, China). The roasted slag was leached with deionised water at a solidto-liquid mass ratio (S/L) of 1:4 at 80 °C for 1 h. The slurry thus acquired was filtered to obtain a leaching residue rich in CaSO4 and SiO2 and leaching solution rich in MgSO4 , Al2 (SO4 )3 , and (NH4 )2 SO4 . Because of the slight solubility of CaSO4 in water, a small amount of CaSO4 appeared in the leaching solution. The concentrations of Ca, Mg, and Al ions in the leaching solution and the CaSO4 content in the leaching residue were measured in order to calculate the extraction ratios of Ca, Mg, and Al elements under different roasting conditions. Extraction ratio of Mg or Al:

R1 =

c1 × V1 × 10 m1 × w1

−3

× 100%

Extraction ratio of Ca:



R2 =

−3

c2 × V1 × 10 + m2 × w3 m1 × w2

(3)

 × 100%

(4)

In the two formulas above, m1 represents the mass (g) of BF slag used in the roasting experiment; w1 and w2 are the mass fractions (wt%) of Mg (or Al) and Ca in the BF slag, respectively; V1 is the volume (mL) of the leaching solution; c1 and c2 are the concentrations (g/mL) of Mg2+ (or Al3+ ) and Ca2+ in the leaching solution, respectively; m2 is the mass (g) of the leaching residue; and w3 is the mass fraction (wt%) of Ca in the form of CaSO4 in the leaching residue. For the recovery of Al, gaseous NH3 was injected into the leaching solution to investigate the effect of pH on the selective precipitation of aluminium at room temperature. After each precipitation test, the slurry was filtered and the concentrations of Mg and

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Fig. 1. Schematic illustration of the mineral carbonation with BF slag.

was used to calculate the amount of fixed CO2 for the leaching residue. The CO2 sequestration capacity, E1 (kg/t), of the leaching residue per tonne of BF slag can be calculated using Eq. (5):

E1 =

m 1 m1

× 10

3

(5)

The carbonation of Mg requires a higher pH value than that of Ca [30]. Thus, a solution of (NH4 )2 CO3 or NH4 HCO3 + NH3 was used during the carbonation of the Al-depleted leaching solution. Since a small amount of CaSO4 was dissolved in the leaching solution, these Ca ions also participated in the carbonation reaction together with Mg. The reaction proceeded at 40 °C for 1 h at different solution concentrations and (Ca + Mg)/(NH4 )2 CO3 molar ratios. After filtration, a solid carbonate mixture and a filtrate rich in (NH4 )2 SO4 were obtained. The amount of CO2 (m2 ) fixed into the calcium carbonate and magnesium carbonate mixture was measured. The CO2 sequestration capacity, E2 (kg/t), of the leaching solution per tonne of BF slag can be calculated using Eq. (6): Fig. 2. XRD pattern of the water-quenched BF slag annealed at 800 °C.

Al in the filtrate were measured. The precipitation ratios of both Mg and Al at different pH values were calculated to determine the optimal process conditions. The precipitation product was analysed and characterised. The carbonations of the leaching residue and the Al-depleted leaching solution, respectively, with NH4 HCO3 and (NH4 )2 CO3 , which can be obtained by using NH3 to capture CO2 from flue gases, were assessed. The carbonation reaction of the leaching residue with a saturated NH4 HCO3 solution was conducted at an NH4 HCO3 /CaSO4 molar ratio of 3:1 and at 55 °C for 1 h [28]. After filtration, a solid mineralisation product and a filtrate rich in (NH4 )2 SO4 were obtained. XRD analysis indicated that the only new phase which appeared in the solid product was CaCO3 . Like in our previous study [29], the amount of CO2 fixed into CaCO3 was determined by using thermogravimetric analysis. The mass loss (m1 ) between 400 °C and 850 °C resulted from the thermal decomposition of CaCO3 and

E2 =

m 2 m1

× 10

3

(6)

Therefore, the total CO2 sequestration capacity, E (kg/t), per tonne of BF slag can be calculated as follows:

E = E1 + E2 2.3. Analysis and characterisation The Ca, Mg, and Al concentrations in the leaching solution were measured using inductively coupled plasma optical emission spectroscopy (ICP–OES, Spectro ARCOS ICP, Germany). In order to measure the CaSO4 content in the leaching residue, 1 g of the residue was dissolved in 500 mL of deionised water and then the concentration of Ca in the solution was measured using ICP–OES. To measure the Al content in the precipitation product, a certain amount of the solid product sample was mixed thoroughly with Na2 O2 and NaOH and fused at 750 °C, and then leached in 20 wt% hydrochloric acid. The concentration of Al in the solution thus obtained was measured using ICP–OES. Loss on ignition (LOI) for a solid sample

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Fig. 3. Effect of roasting temperature on extraction of Ca, Mg, and Al (AS/slag mass ratio: 4/1; roasting time: 1 h).

is the ratio between the mass loss of the solid during calcinations at 10 0 0 °C for 2 h and its initial mass. The phase constituents of solid products were determined using an X-ray diffraction spectrometer (DX-2007, Danton, China) operating with a Cu Kα radiation source filtered with a graphite monochromator at a frequency of λ = 1.54 nm. The voltage and anode current were 40 kV and 30 mA, respectively, and the scanning range (2θ ) was from 5 ° to 90 °. The surface morphologies of solid samples were observed using scanning electron microscopy (SEM, JSM-7500F, JEOL) at an accelerating voltage of 5 kV. The relative elemental contents of the samples were analysed with a combined energy-dispersive X-ray spectrometer (EDS, IS250, Oxford, Japan). An X-ray fluorescence spectrometer (XRF, XRF-180, Shimadzu, Japan) was used for the total analysis of BF slag with a Rh Kα radiation source. The vibration spectra of the aluminium-rich precipitation product were measured using a Fourier transform infrared (FTIR) spectrometer (NEXUS 67, Nicolet, USA) with a resolution of 2 cm−1 and 32 scans in the 40 0 0–40 0 cm−1 wave number range. The contents of elements C, N, H, and S in the precipitation product were measured using an element analyser (Euro EA 30 0 0, Leeman Labs Inc., USA). The thermal analysis of the carbonation product from the leaching residue was performed using a thermogravimetric (TG) and differential scanning calorimetry (DSC) analyser (HTG-2, Heaven, China) under a nitrogen atmosphere at a flow rate of 30 mL/min and heating rate of 10 °C/min. A thermogravimetric analyser (TGA/DSC, 2/1600, Mettler Toledo, Switzerland) and a mass spectrometer (GSD-320-T3, Pfeiffer Vacuum, Germany) were combined for the investigation of the thermal decomposition behaviour of the carbonation product from the Al-depleted leaching solution under a nitrogen atmosphere at a flow rate of 30 mL/min and heating rate of 10 °C/min. The amount of CO2 fixed in the carbonation product in the forms of calcium carbonate and magnesium carbonate was analysed using a high-frequency infrared-ray carbon and sulfur analyser (TL851-6, Lida, China). 3. Results and discussion 3.1. Roasting extraction 3.1.1. Effects of roasting conditions on extraction of Mg, Al, and Ca The extraction ratios of Ca, Mg, and Al at different roasting temperatures are shown in Fig. 3. Clearly, all of the extraction ra-

Fig. 4. Effect of reactant mass ratio (AS/slag) on extraction of Ca, Mg, and Al (roasting time: 1 h; roasting temperature: 370 °C).

Fig. 5. Effect of roasting time on extraction of Ca, Mg, and Al (roasting temperature: 370 °C; AS/slag mass ratio: 3/1).

tios of the three elements increased monotonically with increasing roasting temperature, and beyond 350 °C, the reactions had been complete. It is well known [31] that the reaction of (NH4 )2 SO4 with silicates can be divided into two steps: (1) decomposition of (NH4 )2 SO4 into NH3 and NH4 HSO4 and (2) further digestion of the silicates by NH4 HSO4 . The decomposition reaction occurs at over 250 °C [22]. At higher temperatures, more NH4 HSO4 will be produced, and this is beneficial to the conversion of BF slag. The extraction ratios of Ca, Mg, and Al at different AS/slag mass ratios are shown in Fig. 4. The extraction ratio increased with increasing mass ratio owing to an increase in both the equilibrium conversion of the slag and reaction rate resulting from increased contact between the surfaces of the two reactants. At the mass ratio of 2:1, the extractions of Ca, Mg, and Al were only 80%, 85%, and 65%, respectively, while these values all increased rapidly to nearly 100% upon increasing the mass ratio to ≥3:1. The extraction ratios of Ca, Mg, and Al at different roasting times are shown in Fig. 5. It can be seen that almost all of the Ca, Mg, and Al in the BF slag can be extracted after a roasting time of 1 h.

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Fig. 6. XRD patterns of the roasting products obtained at different roasting temperatures (AS/slag mass ratio: 4/1; roasting time: 1 h).

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Fig. 7. XRD patterns of the roasting products obtained at different mass ratios (roasting time: 1 h; roasting temperature: 370 °C).

On the basis of the above results, the optimal extraction conditions for the BF slag were determined to be an AS/slag mass ratio of 3/1, a roasting time of 1 h, and a roasting temperature of 350–370 °C. Under these conditions, the extraction ratios of Ca, Mg, and Al can all reach up to ∼100%. It was noticed that the extraction ratio of Al was almost always lower than that of Mg when the roasting reaction was not complete, as shown in Figs. 3–5. This is because the reaction of gehlenite with (NH4 )2 SO4 is more difficult than that of akermanite, and the reason for this will be discussed further hereinafter. Moreover, since the extraction of total Ca in the BF slag was from both gehlenite and akermanite, it is reasonable that the extraction ratio of Ca was always between those of Mg and Al. 3.1.2. Evolution of phase constituent during roasting XRD patterns of the roasting products at different roasting temperatures, AS-to-slag mass ratios, and roasting times are shown in Figs. 6–8, respectively. Fig. 6 shows that there was still a large amount of (NH4 )3 H(SO4 )2 in the roasting product at low roasting temperatures, indicating that the rate-limiting step in the roasting extraction may be the reaction between the slag and (NH4 )3 H(SO4 )2 (essentially a digestion reaction). With increasing roasting temperature, the intensity of the diffraction peaks for (NH4 )3 H(SO4 )2 decreased while those of the diffraction peaks for the sulfates of Ca, Mg, and Al increased gradually, indicating that the digestion reaction was enhanced. This result is consistent with the increasing extraction ratios of Ca, Mg, and Al shown in Fig. 3. It was noticed that both Mg and Al would form compound sulfates combined with AS. These sulfates were anhydrous at high temperatures and became the ones with crystal water at low temperatures. The occurrence of the crystal water may stem from the existence of a large amount of (NH4 )3 H(SO4 )2 with strong hygroscopicity in the roasted slag, which would absorb ambient moisture during sample storage, preparation, and testing. Fig. 7 shows that (NH4 )3 H(SO4 )2 and NH4 HSO4 began to appear in the roasted product in addition to the compound sulfates when the AS-to-slag mass ratio exceeded 3:1. This result, combined with the result in Fig. 4, indicates that the dosage of (NH4 )2 SO4 was excessive at this time. Fig. 8 shows that at a roasting temperature of 370 °C and AS-to-slag mass ratio of 3:1, there was still a lot of (NH4 )3 H(SO4 )2 in the roasted slag when the roasting time was shorter than 40 min. However, the amount decreased clearly

Fig. 8. XRD patterns of the roasting products obtained at different roasting times (roasting temperature: 370 °C; AS/slag mass ratio: 3/1).

at 60 min, indicating more complete conversion of the BF slag. The result was consistent with that in Fig. 5. Furthermore, the compound sulfates in the slag roasted for 60 min were all anhydrous, while those in the slag roasted for less than 60 min contained water of crystallisation for the same reason mentioned above. In addition, the presence of compound sulfates was observed in the roasted slag with different NH4 + -to-Mg2+ and NH4 + -to-Al3+ molar ratios. (NH4 )3 Al(SO4 )3 and (NH4 )2 Mg(SO4 )2 were formed when the AS-to-slag mass ratio was high or the roasting reaction was incomplete. However, under the opposite conditions, the compound sulfates were NH4 Al(SO4 )2 and (NH4 )2 Mg2 (SO4 )3 . According to the phase constituent in the roasted slag obtained under the optimal roasting conditions, the reaction of BF slag with AS can be written as Eqs. (7) and (8), and the theoretical mass ratio of AS to BF slag can be calculated to be 2:1 (g/g). Therefore, the optimised mass ratio in this study was 1.5 times the theoretical value.

Ca2 MgSi2 O7 (s ) + 3.5(NH4 )2 SO4 (s ) → 2CaSO4 (s ) + 0.5(NH4 )2 Mg2 (SO4 )3 (s ) + 2SiO2 (s ) + 6NH3 (g ) + 3H2 O ( g )

(7)

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Table 2. Changes in Gibbs free energy of reactions of akermanite and gehlenite with AS under the standard condition. Reaction

G0 250 °C (kJ/mol)

G0 350 °C (kJ/mol)

G0 400 °C (kJ/mol)

Ca2 MgSi2 O7 +AS Ca2 Al2 SiO7 +AS

−228.3 −94.3

−347.4 −237.5

−406.0 −301.7

Fig. 9. Precipitation of Al and Mg at different pH values.

Fig. 10. FTIR pattern of the Al-rich precipitation product.

Table 3. Chemical composition of the precipitation product (wt%).

Table 4. Chemical composition of the leaching residue (wt%).

Al2 O3

MgOa

SiO2 a

TiO2 a

Fe2 O3 a

Sa

LOI

44.44

0.27

7.29

1.93

2.26

7.15

43.8

The Al2 O3 content was analysed using chemical analysis. a the contents were measured using EDS.

SiO2 b

SO3 b

LOI

27.88

25.01

39.83

7.283

a b

Ca2 Al2 SiO7 (s ) + 6(NH4 )2 SO4 (s ) → 2CaSO4 (s ) + 2NH4 Al(SO4 )2 (s ) + SiO2 (s ) + 10NH3 (g ) + 5H2 O(g )

CaOa

(8)

The changes in Gibbs free energy of Eqs. (7) and (8) under the standard condition of 0.1 MPa of partial pressure for each gaseous product and at different temperatures were calculated using the HSC 5.0 commercial software. Because of the lack of thermodynamic data for NH4 Al(SO4 )2 (s), they were estimated by simply adding the data of MgSO4 (s) and (NH4 )2 SO4 (s) together. The results are listed in Table 2. Clearly, the reaction tendency of akermanite with AS is greater than that of gehlenite over the entire roasting temperature range, which can explain why the extraction of Al was always lower than that of Mg when the reaction was incomplete, as shown in Figs. 3–5. 3.2. Recovery of Al The leaching solution of the roasted slag was acidic, with a pH value of 1–2, because of the existence of a small amount of (NH4 )3 H(SO4 )2 . Owing to the large difference in hydrolysis pH values between Mg and Al, selective precipitation for the recovery of Al is possible. The NH3 produced in the roasting unit in the process can be used to adjust the pH value of the leaching solution. The precipitation of Al from the leaching solution obtained under the optimal roasting conditions was investigated, and the result is shown in Fig. 9. When the pH value was 5.5, the precipitation ratio of Al reached up to 99.6%, while co-precipitation of Mg was only 1.7%. Thus, this was chosen as the optimal pH value. The chemical composition of the Al-rich precipitation product at pH 5.5 is shown in Table 3.

The CaO content was analysed using chemical analysis. The SiO2 and SO3 contents were measured using EDS.

XRD analysis showed that the precipitation product was amorphous; hence, the FTIR spectrum of the precipitation product was measured to determine its phase constitution. The result is shown in Fig. 10. It is clear that there are four absorption peaks in the spectrum. The peak at 3446 cm−1 is attributed to the O–H stretching vibration. The peak at 1642 cm−1 is attributed to the bending movements of crystal water. The strong absorption peak at 1139 cm−1 is related to the antisymmetric stretching vibration of SO4 2− . The weak peak at 607 cm−1 is the overlapping peak of the SO4 2− antisymmetric stretching vibration and the Al–O stretching vibration [32]. Therefore, it is speculated that the precipitation product was amorphous aluminium hydroxide sulfate.

3.3. Mineral carbonation 3.3.1. Carbonation of the leaching residue The XRD pattern of the leaching residue obtained under the optimal roasting conditions is shown in Fig. 11(1). The main crystal constituent was CaSO4 . The chemical composition of the leaching residue dried at 105 °C for 1 h is shown in Table 4. CaSO4 occupied 67.7% in mass, while the remaining was inert SiO2 or H2 SiO3 and crystal water. The carbonation of the leaching residue was carried out as described in the Experimental section. The XRD pattern of the carbonation product is shown in Fig. 11(2). Clearly, the CaSO4 in the leaching residue transformed completely into CaCO3 . Chemical analysis also indicated that the conversion ratio was close to 99%. The amount (m1 ) of CO2 fixed by the leaching residues gained under different roasting conditions (temperature, mass ratio, and time), which corresponded to the extraction results in Figs. 3–5,

J. Hu et al. / Journal of Energy Chemistry 26 (2017) 927–935

Fig. 11. XRD patterns of the leaching residue before (1) and after (2) carbonation.

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Fig. 13. SEM image of carbonation product from the leaching residue.

Fig. 14. XRD pattern of mineralisation product from the Mg-rich leaching solution. Fig. 12. CO2 sequestration capacities of the leaching residues obtained under different roasting conditions.

respectively, was measured and used to calculate the CO2 sequestration capacity, E1 (kg/t), of the leaching residue per tonne of BF slag. The results are shown in Fig. 12. The results indicated that the CO2 sequestration capacity increases with the improvement of roasting conditions. Based on the optimal roasting conditions, the maximum mass of CO2 fixed by the leaching residue was approximately 260 kg per tonne of BF slag. The morphologies of the carbonation products are shown in Fig. 13. Combined with the EDS analysis, it can be determined that the small porous particles with a particle size of ≤10 μm are CaCO3 while the smooth lump particles with a particle size between 10 and 40 μm are SiO2 . The CaCO3 and SiO2 contents (wt%) in the carbonation product were measured to be approximately 60% and 33%, respectively. 3.3.2. Carbonation of the Mg-rich leaching solution After the selective precipitation of Al, a solution of (NH4 )2 CO3 was added to the Al-depleted leaching solution for the carbon-

ation reaction. The effects of the molar ratio (R) of (NH4 )2 CO3 to (Ca + Mg) and the total concentration (mol/L) of Ca and Mg ions on the precipitation of Ca and Mg were investigated and the results are listed in Table 5. The results indicated that the calcium ion was always precipitated preferentially under any conditions employed while the precipitation of magnesium depended significantly on the reaction conditions. A higher total concentration of Mg and Ca ions (thus, a higher Mg ion concentration) corresponded to a lower R required to achieve a high Mg precipitation ratio. When the total ion concentration was 0.3 mol/L, the precipitation of Mg could reach up to 90% with the minimum mole ratio, which was the optimal carbonation condition. The XRD pattern of the carbonation product thus obtained is shown in Fig. 14. As can be seen, the main crystal phase was (NH4 )2 Mg(CO3 )2 •4H2 O, while CaCO3 was not observed, probably owing to low crystalline degree or a relatively low content in the product. Thermogravimetry-mass spectrometry (TG-mass) was employed to analyse the thermal stability of the carbonation product and the result is shown in Fig. 15. Fig. 15(a) shows that there are three obvious temperature ranges of weight loss: 30–200 °C,

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Table 5. Dependence of the precipitation of Ca and Mg on the molar ratio of (NH4 )2 CO3 to (Ca + Mg) and the total concentration of Ca and Mg ions. Num.

Total concentration of Mg and Caa (mol/L)

Rb

Precipitation percentage of Mg (%)

Precipitation percentage of Ca (%)

1 2 3 4 5

0.074 0.074 0.13 0.13 0.307

5:1 3:1 3:1 2:1 2:1

77.6 16.3 73.3 31.5 89.8

99.8 99.6 99.8 99.7 99.9

a b

The total concentration of Mg and Ca ions was adjusted by adding deionised water to the leaching solution. R: molar ratio of (NH4 )2 CO3 to (Ca + Mg).

Fig. 15. TG-mass curves for the carbonation product from the leaching solution: (a) TG-mass profiles and (b) magnified mass profiles.

20 0–50 0 °C, and 500–750 °C. In the first temperature range, large amounts of NH3 , CO2 , and H2 O appeared, as shown in Fig. 15(b). Clearly, this is related to the decomposition of (NH4 )2 CO3 and removal of adsorbed and crystal water. Therefore, low-temperature decomposition can be adopted to recover the ammonia from the carbonation product for reuse, during which an amorphous magnesium carbonate was produced. The appearance of CO2 and a small amount of H2 O in the second temperature range was attributed to the decomposition of amorphous magnesium carbonate and removal of residual crystal water. The results in the first two temperature ranges were in good agreement with the literature [33]. In the last temperature range, a distinct CO2 peak, which was attributed to the decomposition of calcium carbonate [6], was observed. It is noteworthy that the amorphous magnesium carbonate is nonstoichiometric, i.e. its molecular formula can be summarised as MgO•CO2( x ) •H2 O( y ) [33]. Based on the results of TG-mass analysis, the amount of CO2 fixed in the carbonation product from the Mg-rich leaching solution in the form of magnesium carbonate and calcium carbonate was measured using a carbon and sulfur analyser following the decomposition of (NH4 )2 CO3 at 200 °C. Under the optimal roasting and carbonation conditions, the CO2 sequestration capacity, E2 , of the leaching solution was calculated to be 56 kg-CO2 /t-slag. The morphology of the carbonation product from the Mg-rich leaching solution was observed and is shown in Fig. 16. This figure, combined with the EDS analysis results, indicates that the majority of precipitates were compact, column-like (NH4 )2 Mg(CO3 )2 •4H2 O crystals of dozens of microns, while a small amount of minute particles of several microns was CaCO3 . The morphology of the car-

Fig. 16. SEM image of the carbonation product from the Mg-rich leaching solution.

bonation product after calcination at 200 °C is shown in Fig. 17. Clearly, the surface of the column-like crystals became rough compared to the surfaces of those without calcinations owing to the escape of the gases produced by the decomposition of (NH4 )2 CO3 . The MgO and CaO contents (wt%) in the calcined product were measured to be approximately 31% and 19%, respectively.

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used to capture CO2 from flue gases to obtain NH4 HCO3 and (NH4 )2 CO3 . Carbonation of the leaching residue of the roasted slag with NH4 HCO3 can enable the stable fixation of CO2 with a CO2 sequestration capacity of up to ∼260 kg-CO2 per tonne of blast furnace slag. By adjusting the pH value of the leaching solution to 5.5 with the NH3 , complete separation of Mg and Al can be achieved. Carbonation of the Al-depleted leaching solution with (NH4 )2 CO3 can further increase CO2 fixation by 56 kg-CO2 per tonne of blast furnace slag. Therefore, a total of 316 kg of CO2 can be sequestered with 1 tonne of BF slag in this process. The two carbonation products and Al-rich precipitate all have economic value. Preliminary economic analysis showed that the sale revenue of these products can reach up to 913 RMB Yuan. As a conclusion, the process we proposed is feasible since it can not only couple greenhouse gas (CO2 ) reduction with industrial waste (BF slag) treatment perfectly, but also create high economic value. A scale-up experiment for treatment of 300 tonne BF slag per annum based on the present study is under way. Acknowledgments Fig. 17. SEM image of the carbonation product from the Mg-rich leaching solution after calcination at 200 °C.

4. Preliminary economic assessment Based on the above experimental results, a preliminary analysis of the further utilisation and market values of the obtained carbonation products and Al-rich by-product was conducted. Approximately 10 0 0 kg of product consisting of 60 wt% CaCO3 and 33 wt% SiO2 could be produced through carbonation of the leaching residue from 1 tonne of BF slag. This product could be used as a raw material for cement production to replace 600 and 330 kg of natural limestone and silica, respectively. Based on the unit prices of limestone and silica of 400 and 600 ¥/t, respectively, the marketing income would be 438 RMB Yuan. Approximately 304 kg of product consisting of 31 wt% MgO and 19 wt% CaO could be produced through carbonation of the Aldepleted leaching solution from 1 tonne of BF slag. This product could be used as a raw material for the production of light magnesium carbonate or magnesium oxide instead of 400 kg of natural dolomite. Its market value would be 200 RMB Yuan, based on the unit price of dolomite of 500 ¥/t. Alternatively, the carbonation of the leaching solution could be divided into two steps to obtain pure CaCO3 and MgCO3 with high added value separately based on the large difference in the pH values required for carbonation of Ca and Mg, as implied in Table 5. The product value would increase to 754 RMB Yuan. Approximately 313 kg of the Al-rich precipitate, with 44 wt% Al2 O3 and an Al2 O3 -to-SiO2 mass ratio of 6, could be produced as a by-product from 1 tonne of BF slag. This product could be used as a raw material for electrolytic aluminium production to replace 230 kg of natural bauxite. Its market value would be 275 RMB Yuan, based on the unit price of Al2 O3 of 20 0 0 ¥/t. Therefore, an income of 913 RMB Yuan can be obtained from the entire process by selling the carbonation products and by-product. 5. Conclusions In the present study, a novel, facile process was proposed; in this process, BF slag is roasted with recyclable AS for the extraction of calcium, magnesium, and aluminium at an AS-to-slag mass ratio of 3:1 and at 370 °C. The extraction ratios of the three elements approached 100%. The NH3 released in the roasting was

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