kmu - dissolution of steelmaking slags in acetic acid for...

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Energy 32 (2007) 528–539

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Dissolution of steelmaking slags in acetic acid for precipitated calciumcarbonate production

Sebastian Teira,�, Sanni Elonevaa, Carl-Johan Fogelholma, Ron Zevenhovenb

aTeknillinen korkeakoulu (Helsinki University of Technology), Laboratory of Energy Engineering and Environmental Protection, P.O. Box 4400,

FIN-02015 TKK, FinlandbAbo Akademi University, Heat Engineering Laboratory, Biskopsgatan 8, FIN-20500 Abo/Turku, Finland

Received 27 October 2005

Abstract

A promising option for long-term storage of CO2 is to fixate carbon dioxide as magnesium- and calcium carbonates. Slags from iron

and steel works are potential raw materials for carbonation due to their high contents of calcium silicates. Precipitated calcium carbonate

(PCC) is used as filler and coating materials in paper. If slag could be used instead of limestone for producing PCC, considerable energy

savings and carbon dioxide emissions reductions could be achieved. In this paper, the leaching of calcium from iron and steel slags using

acetic acid was investigated. Thermodynamic equilibrium calculations at atmospheric gas pressures showed that extraction of calcium is

exothermic and feasible at temperatures lower than 156 1C, while the precipitation of calcium carbonate is endothermic and feasible at

temperatures above 45 1C. The formation of calcium- and magnesium acetate in the solution was found to be thermodynamically

possible. Laboratory-scale batch experiments showed that iron and steel slags rapidly dissolve in acetic acid in a few minutes and the

exothermic nature of the reaction was verified. While silicon was successfully removed by filtration using solution temperatures of

70–80 1C, further separation methods are required for removing iron, aluminum and magnesium from the solution.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Mineral carbonation; CO2 storage; CO2 utilization; Steel slag; Calcium carbonate; Calcium silicate

1. Introduction

The increasing carbon dioxide, CO2, content in theatmosphere and its long-term effect on the climate has ledto increasing interest and research in options for capture,utilization and long-term storage of carbon dioxide. Oilrefineries, coal-fired power plants, iron and steel works,cement, lime and natural gas production are the largestconcentrated sources of anthropogenic CO2 emissions. Thecurrent potential to reuse CO2 in industry is limited, somost of the captured CO2 would have to be stored.Although CO2 can be stored in aquifers and utilized indepleted oil and gas fields, the distances to the CO2

producer site can be thousands of kilometers, which raisethe overall storage costs significantly.

e front matter r 2006 Elsevier Ltd. All rights reserved.

ergy.2006.06.023

ing author. Tel.: +358 9 4513631; fax: +358 9 4513418.

ess: [email protected] (S. Teir).

The disposal of CO2 as solid carbonates is anotherpotential option for long-term storage of CO2. Thismethod accelerates the natural weathering of silicateminerals, where these minerals react with CO2 and formcarbonate minerals and silica [1]. Suitable carbonates forstoring CO2 are magnesium- and calcium-based carbo-nates, since they are hard to dissolve in water. Whilemagnesium- and calcium oxides and hydroxides carbonatefaster, the availability of magnesium- and calcium silicatesis much better [2]. Although calcium silicate has beensuccessfully carbonated at temperatures and pressuresrelevant for industrial processes [3], natural calcium silicateresources are too small and expensive to be of practicalinterest [2,4]. Therefore, current research activities focusmostly on carbonation of magnesium silicates [5]. How-ever, industrial by-products, such as iron and steel slagsand cement-based materials, can have very high contentsof calcium and magnesium oxides, and could thereforebe carbonated for CO2 storage and hazardous waste

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ARTICLE IN PRESSS. Teir et al. / Energy 32 (2007) 528–539 529

neutralization [6–8]. At the same time, calcium carbonate(CaCO3) and precipitated calcium carbonate (PCC) iscurrently far more industrial uses than magnesiumcarbonate.

We are currently studying the possibility to producecalcium carbonates by leaching calcium from calciumsilicates using acetic acid, and precipitating calciumcarbonate out of the solution by injection of CO2. In thispaper, we have focused on the extraction of calcium fromiron and steel slag in aqueous solutions of acetic acid.

2. Concept

Currently, PCC is manufactured by carbonating calcinedlimestone (natural calcium carbonate). However, thecalcination process produces more CO2 than is boundduring the carbonation process. A process that producesPCC from a carbonate-free mineral would therefore be amore environmentally sustainable method for producingPCC, since no calcination step would be required.However, producing calcium carbonates from calciumsilicates requires a more complex process and no commer-cial process is currently available. Natural calcium silicates,such as wollastonite, are too rare and expensive to be usedfor producing PCC [4]. Instead, iron and steel slag couldprovide a suitable feedstock for PCC production, sinceslags are free from carbonates and have high contents ofcalcium.

2.1. Iron and steel slags

Iron and steel slags (or steelmaking slags) are non-metallic by-products from iron and steel manufacturing,and consist primarily of calcium, magnesium, and alumi-num silicates in various combinations. Slag is formed whenlimestone reacts with silicon dioxide and other impuritiesof iron ore at high temperatures. Different blast furnaceslag types are produced depending on cooling techniqueused: air-cooled, expanded or foamed, granulated, orpelletized slag. Iron and steel slags are highly variable withrespect to their composition, even for the same plant andfurnace [9]. The amount of slag produced is largely relatedto the overall chemistry of the raw material. It has beenestimated that the world output in 2003 was 160–200Mt ofiron slag and 96–145Mt of steel slag [10]. Iron and steelslags are mainly utilized as a concrete aggregate and invarious applications in highway construction.

In Finland, there are four steel plants in operation thatproduce in total 1.4Mt of slag/year [11]. Most of theFinnish steelmaking slags have a high concentration ofcalcium oxide (CaO) similar to that of wollastonite. Severalslags have a high content of magnesium oxide (MgO) aswell. Iron and steel slag also contains several traceelements, which may be freed if the slags are dissolved ina carbonation process.

Steel slags have recently been found to carbonate easilyunder mild process conditions. Huijgen et al. [8] reached

calcium conversions of 70% in 30min with steel slag indistilled water at 20 bar CO2 pressure and a temperature of200 1C. However, carbonating iron and steel slags in asingle step will produce a slurry containing all carbonates,silica and other impurities from the slag. Multi-stepprocesses, such as those suggested by Yogo et al. [12] andKakizawa et al. [3], may be better alternatives forseparating various elements at different stages and produ-cing a purer carbonate product.

2.2. Current PCC production process

Most PCC is currently produced from lime, which hasbeen manufactured by calcining limestone in a lime kiln attemperatures over 900 1C:

CaCO3! CaOþ CO2. (1)

The lime (calcium oxide) is hydrated with water into acalcium hydroxide slurry:

CaOþH2O! CaðOHÞ2. (2)

CO2 is injected into the slurry, where it reacts withcalcium hydroxide (Ca(OH)2) and forms calcium carbo-nate, which precipitates out:

CaðOHÞ2ðaqÞ þ CO2! CaCO3 # þH2O: (3)

The process can utilize process or power plant flue gasesas such to satisfy its CO2 requirements. Although CO2 isbound during the PCC production process, a largeramount of CO2 is released from the lime production, dueto the fuel combusted to provide heat to the endothermiccalcination reaction (Eq. 1) [4].

2.3. Alternative carbonation process

One of the most promising multi-step process ideas forproducing calcium carbonate from calcium silicates is theacetic acid carbonation process suggested and studied byKakizawa et al. [3]. The process consists of two main steps,where the reactions occur (Fig. 1). First, calcium ions areextracted from a natural calcium silicate mineral byleaching in acetic acid (CH3COOH):

CaSiO3 þ 2CH3COOH! Ca2þ þ 2CH3COO�

þ SiO2 þH2O: ð4Þ

After filtrating silica out of the solution, CO2 is pumpedinto the solution, forming calcium carbonate that pre-cipitates from the solution:

Ca2þ þ 2CH3COO� þ CO2 þH2O

! CaCO3ð#Þ þ 2CH3COOH: ð5Þ

Acetic acid is recovered in this step and recycled for usein the extraction step. According to Kakizawa et al., theGibbs free energy change of each step is negative, andshould therefore not require large amounts of energy input.However, the second step may require pressurized CO2 inorder to achieve a higher carbonation efficiency.

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Fig. 1. Multi-step process for carbonating calcium silicates using acetic acid.

S. Teir et al. / Energy 32 (2007) 528–539530

Kakizawa et al. studied the process with batch experi-ments. Using an aqueous solution of acetic acid (27% acidand 72% water at 60 1C) to extract calcium ions fromwollastonite (CaSiO3) an extraction ratio of 48% wasachieved in 250min. Using CO2 pressures of 5–50 bar andtemperatures of 40–80 1C, a carbonate conversion of 20%from a solution of calcium acetate and water was achievedin approximately 1 h [3]. However, the composition of theprecipitate was not studied.

This process could possibly be used for carbonating ironand steel slags as well, since the composition of several slagtypes resemble wollastonite [11]. However, iron and steelslag can also have high contents of magnesium silicates(and many other compounds), for which it is possible thatsimilar reactions might occur in contact with acetic acid:

MgSiO3 þ 2CH3COOH!Mg2þ

þ 2CH3COOH� þ SiO2 þH2O; ð6Þ

Mg2 þ 2CH3COO� ! CO2 þH2O

!MgCO3ð#Þ þ 2CH3COOH: ð7Þ

For instance, 3.1 kg of blast furnace slag can store 1 kg ofCO2 assuming a stoichiometric conversion of the CaOcontained in the slag [11]. If also the MgO components ofthe blast furnace slag could be carbonated, the slagrequirements would be reduced to 2.3 kg/kg CO2. How-ever, there are also small contents of many othercompounds in iron and steel slag (such as heavy metals)which may be released by acetic acid.

2.4. Process comparison

We have studied the feasibility of producing calciumcarbonates using natural calcium silicates by processmodeling, and compared it with the current PCC produc-tion route from limestone in a previous paper [4]. The netCO2 emissions from the current PCC production methodwere according to our model 0.21 kgCO2/kg CaCO3

produced, with the CO2 bound in the carbonation processaccounted for, and the energy need for the calcinationfacility included. The alternative carbonation process binds0.44 kgCO2/kg CaCO3 produced, but the power demandfor the process, modeled to run at 30 bar CO2 pressure,

causes indirectly 0.10 kgCO2/kgCaCO3 from (fossil fuel-based) power generation, reducing the net sequestration ofCO2 to 0.34 kgCO2/kgCaCO3, which is still a significantreduction (according to Kakizawa et al. the best precipita-tion rate was achieved at 30 bar CO2 pressure). If thealternative carbonation process could be developed toproduce synthetic CaCO3 the emissions from energyintensive lime kilns producing CaO for PCC productioncould also be omitted. This would reduce the total CO2

emissions of the PCC production chain by a further 0.21 kgCO2/kg CaCO3 to a total of 0.55 kg CO2/kg CaCO3

produced. Therefore, the acetic acid process seems to havea high potential for simultaneously reducing CO2 emissionsand producing PCC. Although natural calcium silicateswere found to be too expensive for use as feedstock for theprocess [4], steelmaking slags were found to be muchcheaper [11]. However, the possibility to use steelmakingslags as feedstock for this process has not previously beenstudied.

2.5. Potential and profit margins

We have recently studied the potential for reducing CO2

emissions by carbonating iron and steel slag [11]. Usingiron and steel slag from Finnish steel mills, 0.26–0.53 kg ofCO2 could be stored/kg of slag carbonated (taking both theCaO and MgO content of the slags into consideration),reducing the CO2 emissions of the steel mills by 8–21%.The maximum potential for reducing CO2 emissions byslag carbonation for the four steel mills in Finland wascalculated to be 550 kt/a CO2, while the world-widepotential was estimated to be 70–180Mt CO2/a (calculatedusing composition data of Finnish slags). Production ofPCC from carbonation of iron and steel slag could be amore profitable refining method for the slag products, if thepurity required for commercial PCC could be achieved. InFinland, granulated blast furnace slag can be purchased for10 h/t, which is approximately the same price as forlimestone lumps used for producing lime for PCCmanufacturing (11 h/t), while the cheapest available PCCtype has a price tag of 120 h/t. Since only the CaO-component of the slag is used for PCC production, the CO2

reduction potential is lower than for carbonation of both


CaO and MgO in slag. Using the alternative carbonationprocess for PCC production, roughly 290 kt CO2 emissionsper year could be avoided from the steel factories inFinland, while the annual output of PCC would be 850 kt[11].

Although calcium carbonate is a stable compound,which is thermodynamically favored over CO2 andsparingly soluble in water, waste incineration of productscontaining PCC may eventually release the CO2 stored ascarbonate. However, if PCC produced from slag couldreplace other PCC products produced from lime, theomission of using a lime kiln for calcination would reduceCO2 emissions and save natural mineral resources regard-less of the means for disposal of PCC.

3. Methods

The alternative carbonation process had not beenpreviously tested for steelmaking slags. Also, the elementalcomposition of the filtered solids from the crystallizationexperiments by Kakizawa et al. had not been reported.Therefore, we performed simple thermodynamic equili-brium calculations prior to laboratory experiments todetermine if calcium could be extracted from steelmakingslags using acetic acid. We also wanted to verify thatcalcium carbonate and magnesium carbonates are theore-tically possible products of the process. The thermody-namic equilibrium of reaction equations Eqs. (4)–(7) andthe solution equilibrium of blast furnace slag wascalculated using Outokumpu HSC 5.1. The program usesminimization of total Gibbs energy (G) of the compoundsinvolved for determining chemical compositions at thermo-dynamic equilibrium. The calculations were only per-formed for carbonation at atmospheric pressure using themain elements of blast furnace slags (data of average blastfurnace composition supplied by Ruukki) as input data.

Cooling water in

Cooling water out

T, pH, samples

Temperature bath

Fig. 2. Experim

The iron and steel slags used in our experiments wereprovided by Raahe steel works (Ruukki) and Tornio steelworks (Outokumpu). Wollastonite mined near Lappeen-ranta (Nordkalk) was also used for comparison. Materialswith a large size distribution were sieved to 125–500 mmand only the sieved fractions were studied, while materialsin powder form were used as such. The composition of thecalcium silicate-based materials used in the experimentswere analyzed using X-ray fluorescence spectroscopy(XRF). Crystallite orientations of the samples weredetermined by X-ray diffraction (XRD). In order todetermine more exact elemental concentrations of steelslags and their variation, three samples of each materialwere separately dissolved in a standardized solution ofhydrochloric acid, hydrofluoric acid, and phosphoric acid.Saturated boric acid was later added to each batch ofsolution for complete dissolutions of the materials. Thesolutions were analyzed using Inductively coupled plasma-atomic emission spectroscopy (ICP-AES).The extraction of calcium ions from various iron and

steel slags in acetic acid was studied using batch experi-ments. The experimental setup is displayed in Fig. 2. Aceticacid solutions of various concentrations were heated to aspecified temperature in a glass reactor of 250ml. The glassreactor was surrounded by a water bath, which was heatedusing a separate closed temperature-controlled water bathwith an external water flow connected to the open waterbath containing the reactor. Nitrogen was continuously fedto the reactor (above the surface of the solution) at 1 l/minto prevent CO2 in air from interfering with the experi-ments. The solution was stirred using a magnetic stirrer atapproximately 600–700 rpm. When the temperature for thesolution had stabilized after heat-up to the desiredtemperature, 4.2 g of slag was added to the solution (inorder to be comparable with previous experiments madewith 300ml solution and 5 g of slag [13]). The temperature

Gas in

Batch feed

Gas out

Magnetic stirrer

ental setup.

ARTICLE IN PRESSS. Teir et al. / Energy 32 (2007) 528–539532

and pH of the solution were recorded with electrodesinline, and were not regulated after the addition of thebatch. The solution was stirred for 2 h, and four samples of10ml each were extracted 5, 20min, 1, and 2 h after theaddition of the slag using a syringe. The samples wereimmediately filtered through a syringe membrane filter of0.45 mm pore size and the liquid samples were sent toanalysis. In order to estimate the losses due to evaporation,the weight change of the solution was calculated bymeasuring the reactor weight before and after the experi-ment run. The mass changes from addition of the slagbatch and the sample extraction were also accounted for.The weight change of the solution is referred to as ‘‘massbalance check’’ in the result section.

Based on the results from the XRF-analysis theconcentrations of the five main common elements of thematerials in the solution samples were measured after thetests using ICP-AES and atomic absorption spectrophoto-metry AAS. Two different dilutions were prepared fromeach solution sample, and the concentrations of theselected elements in each dilution were measured usingtwo separate frequencies. The accuracy of the ICP-AESand AAS analyses were estimated to72%. The solutionwas assumed to be free from these elements prior to theaddition of the batch of slag, being prepared from onlydistilled water and pure acetic acid.

4. Results and discussion

4.1. Thermodynamic equilibrium calculations

Gibbs free energy calculations of the extraction reactions(Eqs. (4) and (6)) using HSC 5.1 showed that the extractionof Ca+2-ions from CaSiO3 is thermodynamically possibleat temperatures lower than 156 1C, while the extraction ofMg+2-ions from MgSiO3 proceeds at temperatures lowerthan 123 1C (Fig. 3). Gibbs free energy calculations of thecarbonation reactions (Eqs. (5) and (7)) showed that thecarbonation of Ca+2-ions proceeds already at tempera-tures over 45 1C, while the carbonation of Mg+2-ionsshould only be possible at temperatures over 144 1C. The

Extraction (Equations 4 and 6)

-300

-200

-100

0

100

200

300

0 100 200 300

Temperature (°C)

del

taH

(kJ

)

-12

-8

-4

0

4

8

12

Lo

g(K

)

Eq.4, deltaH (kJ)Eq.6, deltaH (kJ)Eq.4, Log(K)Eq.6, Log(K)

Fig. 3. Equilibrium constant K and DH calculated for the reactions involved in

and (7)).

calculations also show that the extraction reactions areexothermic (DHo0), while the carbonation reactions areendothermic (DH40). The net reactions of calcium silicatecarbonation (Eqs. (4) and (5)) and magnesium silicatecarbonation (Eqs. (6) and (7)) are both exothermic.The dissolution of the various species of blast furnace

slag in acetic acid was studied in more detail by calculatingthe chemical composition at thermodynamic equilibriumusing HSC. The input parameters were set to simulate theextraction experiments carried out later on, using 4.2 g ofblast furnace slag in a 250ml aqueous solution of aceticacid (33.3wt% CH3COOH, 66.7wt% H2O). Only the sixlargest species in blast furnace slag were used as input data,to simplify the results. All the compounds in the databaseof HSC 5.1 were used as potential products, except for C,CxHy, and all carbonates, which are unlikely products fromthe extraction process performed in absence of CO2. Theresults and input data are summarized in Table 1. Theresults show that all compounds (except for Ti) areexpected to be dissolved in the solution. A large part ofthe metals dissolved are expected to form acetates, such asmagnesium acetate, calcium acetate, and iron(II) acetate.Although Fe, S and Ti are in oxide states in slags, havingthe input of these in elemental form does not notably affectthe outcome of the calculation, since HSC merelycalculates the equilibrium composition. Although allcompounds are not likely to be formed due to slowkinetics, the modeling results give a hint upon thefeasibility of the extraction.

4.2. Characterization of calcium-based materials used in the

experiments

The XRF-analyses of the sieved fractions of the calciumsilicate-based materials (Table 2) are similar to thecompositional data supplied by the manufacturers. There-fore, the sieved fractions used in the experiments shouldwell represent the materials in terms of composition.Based on the XRF-analyses Ca, Mg, Al, Fe, and Si wereselected for analysis by ICP-AES. Table 3 shows resultsfrom the ICP-AES analyses of the selected elements in the

Carbonation (Equations 5 and 7)

-100

0

100

200

300

400

0 100 200 300

Temperature (°C)

del

taH

(kJ

)

-2

0

2

4

6

8

Lo

g(K

)

Eq.5, deltaH(kJ)Eq.7, deltaH(kJ)Eq.5, log(K)Eq.7, log(K)

acetic acid carbonation for CaSiO3 (Eqs. (4) and (5)) and MgSiO3 (Eqs. (6)

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Table 1

Thermodynamic equilibrium composition of solution of blast furnace slag (six main species) in acetic acid (33.3wt%) and water (66.7wt%)

Input Composition (g) Output Element Composition as conversion rates of element

At 30 1C (%) At 50 1C (%) At 70 1C (%)

H2O(aq) 166.7 CaCH3COO+ Ca 62 65 70

CH3COOH(aq) 83.3 Ca2+ Ca 38 35 30

CaO 1.70 Si(OH)3� Si 100 100 100

SiO2 1.46 Mg(CH3COO)2(aq) Mg 76 70 69

MgO 0.43 Mg2+ Mg 24 30 31

Al2O3 0.39 Al3(OH)45+ Al 100 100 100

Ti 0.042 TiO2 Ti 100 100 100

Fe 0.026 Fe(CH3COO)2(aq) Fe 91 89 89

S 0.008 Fe2+ Fe 9 11 11

S4O3�2 S 99 99 98

S5O3�2 S 1 1 2

The results are presented as equilibrium conversion rates of various elements.

Table 2

Composition of calcium silicate-based materials used in the experiments as determined by XRF-analysis (units: wt%)a

Element Blast furnace slag

(350–500mm)

Steel converter slag

(350–500mm)

Electric arc furnace

slag (125–350mm)

AOD process slag

(o125mm)

Wollastonite

(o250mm)

CaO 40.6 43.6 40.8 60.7 44.5

SiO2 34.1 13.9 26.6 27.6 52.2

Fe2O3 0.901 24.1 1.59 0.210 0.297

MgO 10.7 1.44 7.21 5.83 0.541

Al2O3 9.40 1.77 8.36 1.21 1.48

F 0.07 0 0.11 5.5 0.02

Cr 0.003 0.232 5.07 0.228 0.001

Ti 1.03 0.512 2.64 0.356 0.012

Mn 0.376 2.39 2.29 0.076 0.005

S 1.73 0.086 0.092 0.273 0.008

aOnly elements present at 41wt% are shown.

Table 3

Concentrations of selected elements in calcium silicate-based materials used in the experiments as determined by dissolution and ICP-AES-analysis (units:

wt%)a

Blast furnace slag

(350–500mm)


(350–500mm)

Electric arc furnace slag

(125–350mm)

AOD process slag

(o125mm)

Wollastonite (o250mm)

Average Std.dev. Average Std.dev. Average Std.dev. Average Std.dev. Average Std.dev.

CaO 39.0 1.58 41.4 2.21 38.9 0.583 69.4 1.07 38.8 5.20

SiO2 25.7 0.682 11.0 0.830 21.2 0.166 22.0 0.598 32.2 4.01

Fe2O3 0.404 0.028 26.0 1.69 3.82 0.766 0.252 0.110 0.201 0.024

MgO 11.9 0.370 1.42 0.054 6.05 0.033 6.51 0.010 0.506 0.023

Al2O3 8.64 0.486 1.88 0.293 6.34 0.129 1.22 0.064 1.17 0.159

aResults expressed in terms of oxides. The standard deviation was calculated based on the results from dissolutions of three samples of each material.

S. Teir et al. / Energy 32 (2007) 528–539 533

dissolutions of the calcium-based materials. In order tocompare the crystal structures of iron and steel slags withwollastonite, XRD analyses of the sieved fractions weremade. The results from the XRD analyses have beensummarized in Table 4. Phases could be identified from allmaterials except for the blast furnace slag sample, since itscrystal structure is mostly amorphous. Calcium silicate inthe form of Ca2SiO4 was identified both in argon–oxygen

decarburization (AOD) process slag and steel converterslag.

4.3. Extraction experiments

To compare the potential for leaching calcium out fromiron and steel slags, various slags were dissolved in aqueoussolutions of acetic acid (33.3wt% glacial acetic acid and

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Table 4

Minerals in calcium silicate-based materials identified by XRD

Material analyzed Highest peak (counts) Phases identified

Wollastonite 2500 Wollastonite CaSiO3, quartz SiO2

AOD process slag 1000 Fluorite CaF2, periclase MgO, calcium silicate Ca2(SiO4)

Electric arc furnace slag 350 Gehlenite Ca2Al(AlSiO7), merwinite Ca3Mg(SiO4)2,

magnesiochromite (Mg, Fe)(Cr,Al)2O4

Steel converter slag 300 Srebrodolskite Ca2Fe2O5, lime CaO, iron Fe, calcium

silicate Ca2SiO4, calcium iron oxide Ca2Fe15.6 O25

Blast furnace slag Mostly amorphous phases

0

0.5

1

1.5

2

2.5

3

0:00 0:20 0:40 1:00

Time (h:mm)

So

luti

on

pH

AOD process slag



Blast furnace slag

Wollastonite49

50

51

52

53

54

0:00 0:10 0:20 0:30

Time (h:mm)

So

luti

on

tem

pera

ture

(°C

)

AOD process slag

Blast furnace slag



Wollastonite

Fig. 4. Inline pH and temperature recordings of extraction experiments in aqueous solution of acetic acid (33.3%) at 50 1C (batch of 4.2 g slag added at

0:00). Peaks in pH curves at 0:20 and 0:42 were caused by temporary electrode disturbances.


66.7% distilled water, pH ¼ 1.5 at 25 1C) at a solutiontemperature of 50 1C. When a batch of slag was added to asolution of acetic acid, the solution temperature rose brieflyto 1.2–3.1 1C, indicating a reaction occurring and verifyingthe exothermic nature of the reaction (Fig. 4). The highesttemperature peak was observed with AOD process slag.The pH of the solution rose also in 10min after the slagaddition from 0.8–1.1 to 2.4–2.7. However, no significantincrease in solution temperature could be observed whenadding wollastonite to a similar solution of acetic acid. ThepH of the solution containing wollastonite rose very slowlyfrom 0.8 to 1.7 during 2 h, indicating that the extraction ofcalcium from wollastonite is slow. The result from the AASand ICP-AES analyses (Fig. 5) verifies that only 51% of thecalcium fixed in wollastonite could be extracted during 2 h.This is similar to the results achieved by Kakizawa et al. [3],who reported that 48% of the calcium fixed in wollastonitewas extracted during 250min when dissolving 13.26 g ofwollastonite in an aqueous solution of acetic acid (aceticacid/water ¼ 13.72 g/50 g). We found the extraction ofcalcium from iron and steel slags to be much faster: almostall the calcium from iron and steel slag could be dissolved(Fig. 6–9), which was also predicted by thermodynamic

modeling. The extraction efficiency numbers were calcu-lated by comparing the concentration of selected elementsin the filtered solution samples with concentrations of theelements in the raw materials (Table 3) and accounting forthe loss of solution volume due to sample extraction.However, the analyses also show that other elements of theslag, such as silicon, dissolve as well. Steel converter slaghas, besides calcium, high contents of iron, while the otherslags tested have high contents of magnesium. Electric arcfurnace slag and blast furnace slag contain aluminum aswell, which was released into the solution during theexperiments. The extraction efficiency of blast furnace slag(Fig. 6) was found to exceed 100%. Since the mass balanceshows only a loss of 3.7% (due to evaporation of thesolution), the result indicates that the concentration ofCaO of the blast furnace slag sample used in thatexperiment was higher than that of the samples analyzedin Table 3.In order to determine the influence of acetic acid

concentration in the solution, additional extraction experi-ments with blast furnace slag were performed with aqueousacetic acid solutions of varying concentrations: 0wt%,0.04wt%, 4wt%, 10wt%, and 33.3wt% acetic acid

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0

2000

4000

6000

8000

10000

12000

0:00 0:20 0:40 1:00 1:20 1:40 2:00

Time (h:mm)

Co

nce

ntr

atio

n (

mg

/l)

Si (mg/l)

Fe (mg/l)

Mg (mg/l)

Ca (mg/l)

40 % of Ca

41 % of Ca51 % of Ca 51 % of Ca

Mass balance check: -4.2 %

Fig. 5. Dissolution of wollastonite in an aqueous solution (250ml) of acetic acid (33.3wt%) at 50 1C (batch of 4.2 g added at 0:00).

0

2000

4000

6000

8000

10000

12000

0:00 0:20 0:40 1:00 1:20 1:40 2:00

Time (h:mm)

Co

nce

ntr

atio

n (

mg

/l)

94 % of Ca

113 % of Ca 105 % of Ca106 % of Ca


Si (mg/l)

Fe (mg/l)

Al (mg/l)

Mg (mg/l)

Ca (mg/l)

Fig. 6. Dissolution of blast furnace slag in an aqueous solution (250ml) of acetic acid (33.3wt%) at 50 1C (batch of 4.2 g added at 0:00).


(pH ¼ 6.2, 3.4, 2.6, 2.3, 1.5, respectively, measured at25 1C). The other experiment parameters were kept similarto the previous experiments. The results from the AAS andICP-AES analyses (Fig. 10) show that the acetic acidconcentration of the solution has a dramatic effect uponextraction of calcium in the range 0–10wt% acetic acid.Only 30mgCa/l was extracted after 2 h of residence time inwater, while 4.0 gCa/l was extracted already in 5min usingan aqueous solution of 10wt% acetic acid. Using asolution of 33.3wt% acetic acid produced a result similarto the experiment performed at 10wt% acetic acid. Almostall of the calcium in the batch of slag was extracted in asolution of only 10wt% acetic acid, which indicates thatroughly 6–7ml glacial acetic acid is required/gram of blastfurnace slag for adequate calcium extraction.

The effect of temperature upon the extraction efficiencywas investigated by performing additional extractionexperiments of blast furnace slags at 30 and 70 1C. Asolution concentration of 4wt% acetic acid was selectedfor study, since calcium was only partially extracted usingthis solution concentration at 50 1C (Fig. 10). The resultsfrom the AAS and ICP-AES analyses (Fig. 11) showed thattemperature has a significant effect upon the solubility ofcalcium (and other elements as well) from blast furnaceslag. At 30 1C the extraction is significantly slower than at50 1C, but instead a better calcium extraction can beachieved. At 70 1C the extraction process is faster than at50 1C, but less calcium can be extracted. Apparently thesolubility of blast furnace slag decreases with highertemperatures. However, this can be compensated for by

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74 % of Ca

84 % of Ca 88 % of Ca 90 % of Ca


Fig. 7. Dissolution of steel converter slag in an aqueous solution (250ml) of acetic acid (33.3wt%) at 50 1C (batch of 4.2 g added at 0:00).

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89 % of Ca 94 % of Ca 93 % of Ca


Fig. 8. Dissolution of electric arc furnace slag in an aqueous solution (250ml) of acetic acid (33.3wt%) at 50 1C (batch of 4.2 g added at 0:00).


increasing the acetic acid concentration in the aqueoussolution.

In our previous extraction experiments we found that thedissolved silicon content of the extraction solution wassignificantly reduced after 1 h dissolution of blast furnaceslag or steel converter slag at 70 and 80 1C in an aqueoussolution of 33.3% acetic acid [13]. We observed that thesilica dissolved in the solution forms a gel under theseconditions, which can be mechanically filtered from thesolution. To produce a solution suitable for precipitationof CaCO3, i.e. with high concentration of calcium but lowconcentration of silica, 50 g of blast furnace slag wasdissolved in a mixture of 100ml of acetic acid and 200ml ofdistilled water (500ml total reactor volume) at 70 1C for

2 h. The composition of the produced calcium-rich solutionis shown in Table 5. As can be seen from the table, it ispossible to minimize the content of silicon in the solutionsby leaching slag at 70 1C in an aqueous solution of33.3wt% acetic acid, and removing the formed silicon-rich gel by mechanical filtration. However, other separa-tion measures are needed to remove aluminum, iron,magnesium and other elements released from the slag.

5. Conclusions

Carbonation of calcium silicate-rich slags is an interest-ing option for simultaneously reducing CO2 emissions andrefining by-products from the industry. If the produced

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Fig. 9. Dissolution of AOD process slag in an aqueous solution (250ml) of acetic acid (33.3wt%) at 50 1C (batch of 4.2 g added at 0:00).

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106 % of Ca103 % of Ca

70 % of Ca

Fig. 10. Dissolution of blast furnace slag in aqueous solutions with various concentrations of acetic acid at 50 1C (batch of 4.2 g added at 0:00).


calcium carbonate could reach the purity specifications ofPCC, the economic value of slags could be increased ten-fold, while also reducing CO2 emissions and preservingnatural mineral resources. Although the global CO2

storage potential by carbonating slags is low in comparisonwith other CO2 storage options, it can reduce the annualCO2 emissions for an individual steel plant with severalhundreds of kilotons.

The experiments performed with extracting calcium fromiron and steel slag shows that iron and steel slags dissolvemore completely than wollastonite in an aqueous solutionof acetic acid. While iron and steel slags dissolve poorly inwater, calcium can be rapidly leached out of iron and steelslags using acetic acid. However, other elements of the

slags (such as magnesium, aluminum, iron, and silica)dissolve as well, which was predicted by thermodynamicmodeling. Silicon in iron and steel slag was found to form agel in strong acetic acid solutions (33.3 wt% acetic acid) attemperatures over 70 1C, and can be removed by mechan-ical filtration. Since slags dissolve easily in acetic acid,additional measures for separating other dissolved ele-ments from acetic acid are required to reuse the acetic acidin the process. The disposal of the generated wastes willdepend on in which form they can be removed from theprocess.While extraction of calcium from steelmaking slags was

successfully performed in acetic acid, the feasibility of theprocess depends on successful precipitation of calcium

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21 % of Ca 25 % of Ca


70 °C

Fig. 11. Dissolution of blast furnace slag in aqueous solutions (250ml) of acetic acid (4wt%) at 30, 50, and 70 1C (batch of 4.2 g added at 0:00).

Table 5

Elemental composition of a filtered solution prepared at 70 1C from blast

furnace slag

Al (mg/l) Mg (mg/l) Fe (mg/l) Ca (mg/l) Si (mg/l)

1218 4120 292 17000 129


carbonate. Preliminary thermodynamic equilibrium calcu-lations predict that precipitation of CaCO3 is thermo-dynamically possible over 45 1C, while the magnesium inthe solution should not form carbonates at atmosphericpressures. However, to improve the conditions for success-ful precipitation additional methods may be required, suchas pressurization or the use of additives. Future work willconcentrate on the precipitation of CaCO3 from calcium-rich aqueous solutions, as well as the removal and disposalof other elements and trace elements from the solution.

Acknowledgements

This article is based on work presented at the ECOS2005 conference 20–22 June 2005 at Trondheim, Norway.The project was funded by Nordic Energy Research, the

Finnish Funding Agency for Technology and Innovation(TEKES), UPM, Ruukki Productions, Wartsila and theFinnish Recovery Boiler Committee. We thank the peopleworking at the Laboratory of Energy Engineering andEnvironmental Protection and the Laboratory of AppliedThermodynamics for facilitating this work. We thank RitaKallio at Ruukki for fast analysis services, and PetriKobylin at the Chemical Department for his support withthe HSC modeling. We also thank Hannu Revitzer at theChemical Analysis Centre of our university for his ideas,service and support, and Ruukki, Nordkalk and Out-okumpu for providing us with samples of calcium silicate-based materials. Ron Zevenhoven was Academy ResearchFellow for the Academy of Finland and is currently withAbo Akademi University.

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http://minerals.usgs.gov/minerals/pubs/commodity/myb/

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