IN DEGREE PROJECT MATERIALS SCIENCE AND ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2020

Sequestration of carbon dioxide in steel slag

SUSHANTH KOMBATHULA

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

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Abstract

Although Iron and steel industry is essential for the development of society, the industry is

responsible for a large portion of CO2 emissions. The industry also produces by-products like

metallurgical slag in orders of million tons. The slag is alkaline in nature and rich in Ca and

Mg oxides. Upon use the oxide interact with atmospheric CO2 and form carbonates, making

them unstable. Storing CO2 in the slag would make it more stable, enhances its life cycle and

promotes further usage in various applications.

CO2 sequestration can be done through carbonation of steel slag. Carbonation of slag can be

achieved through direct and indirect carbonation. Direct carbonation is performed either in a

gaseous or an aqueous state in a single step. Indirect carbonation involves multiple steps as it

activates the Ca/Mg ions in the slag before they interact with CO2. For an industrial process

the direct route would be more viable as it involves lesser steps, easier to scale up. Since there

are no solvents to activate the Ca/ Mg the cost involved is also less.

This thesis focuses on developing an industrial process to sequester CO2 in metallurgical slag.

Sequestration through a combination of gaseous and aqueous has been attempted while

studying the effect of carbonation time, carbonation temperature and shape of slag used.

Carbonation of the slag is performed using CO2 and steam. The results show that carbonation

yield increases with carbonation time and decreases with increase in temperature. The effect of

the shape of slag used for carbonation was studied by performing carbonation test in a slag

pellet. Diffusion plays a significant role in carbonation process. Powdered slag showed higher

carbonation yield compared to the pellet. CO2 uptake as high as 53g of CO2/kg of slag at

200 oC for 6 hr has been achieved. The results indicate the possibility for an industrial

carbonation process.

Keywords: Carbon dioxide, Steel slag, Carbonation

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Sammanfattning

Även om järn- och stålindustrin är avgörande för samhällets utveckling, är industrin ansvarig

för en stor del av koldioxidutsläppen. Industrin producerar också biprodukter som metallurgisk

slagg i order på miljoner ton. Slaggen är alkalisk till sin natur och rik på Ca- och Mg-oxider.

Vid användning interagerar oxiden med atmosfärisk CO2 och bildar karbonater, vilket gör dem

instabila. Att lagra koldioxid i slaggen skulle göra den mer stabil, förbättra livscykeln och

främja ytterligare användning i olika applikationer.

CO2-bindning kan göras genom kolsyrning av stålslagg. Kolsyrning av slagg kan uppnås

genom direkt och indirekt karbonatisering. Direkt karbonatisering utförs antingen i ett

gasformigt eller vattenhaltigt tillstånd i ett enda steg. Indirekt kolsyrning involverar flera steg

eftersom den aktiverar Ca/Mg-jonerna i slaggen innan de interagerar med CO2. För en

industriell process skulle den direkta vägen vara mer livskraftig eftersom den innebär mindre

steg, lättare att skala upp. Eftersom det inte finns några lösningsmedel för att aktivera Ca/Mg

är kostnaden också mindre.

Denna avhandling fokuserar på att utveckla en industriell process för att binda koldioxid i

metallurgisk slagg. Sekvestrering genom en kombination av gasformig och vattenhaltig har

försökt under undersökning av effekten av kolsyratid, kolsyratemperatur och form av den

använda slaggen. Kolsyringen av slaggen utförs med CO2 och ånga. Resultaten visar att

karbonatiseringsutbytet ökar med kolsyratiden och minskar med temperaturökningen. Effekten

av formen på slagg som användes för karbonatisering studerades genom att utföra

karbonatiseringstest i en slaggpellet. Diffusion spelar en viktig roll i

karbonatiseringsprocessen. Pulveriserad slagg visade högre karbonatiseringsutbyte jämfört

med pelleten. CO2-upptag så högt som 53 g CO2/kg slagg vid 200 oC under 6 timmar har

uppnåtts. Resultaten indikerar möjligheten för en industriell karbonatiseringsprocess.

Nyckelord: Koldioxid, Stålslagg, Karbonering

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Table of Contents1. INTRODUCTION 1

1.1 INTRODUCTION 1

1.2 OBJECTIVES 3

1.3 SOCIAL AND ETHICAL ASPECTS 3

2. BACKGROUND 4

2.1 STEEL SLAG 4

2.2 MINERAL CARBONATION 5

2.2.1 Direct Carbonation 5

2.2.1.1 Gas-solid carbonation 5

2.2.1.2 Aqueous carbonation 6

2.2.2 Indirect Carbonation 6

2.2.3 Parameters affecting carbonation yield 8

2.2.4 By-products for mineral carbonation 9

2.2.4.1 Metallurgical slag for mineral carbonation 10

2.2.5 Prior attempts in carbonation of steel slag 12

3. MATERIALS, METHODS AND EXPERIMENTAL PROCEDURES 13

3.1 RAW MATERIALS 13

3.2 METHODOLOGY 14

3.3 EXPERIMENT FACILITIES 15

4. RESULTS AND DISCUSSION 16

4.1 THEORETICAL CALCULATION 16

4.2 INFLUENCE OF CARBONATION TIME 16

4.3 INFLUENCE OF CARBONATION TEMPERATURE 17

4.4 INFLUENCE OF SLAG SHAPE 17

5. CONCLUSION 18

6. RECOMMENDATIONS FOR FUTURE WORK 19

7. ACKNOWLEDGMENTS 20

8. REFERENCES 21

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Chapter 1

Introduction

1.1 Introduction

The energy sector and industries such as cement, iron and steel manufacturing plants account

for significant portion of anthropogenic CO2 emissions. The iron and steel industry stands out

in particular as the largest emitter of CO2 [1]. 36% of industrial green house gas (GHG)

emissions in Sweden comes from iron and steel plants. Emissions from blast furnaces make up

86% of the total emissions from the plants. To improve the efficiency of the plants and to

reduce the amount of CO2 emissions, several efforts are being taken – scrap steel recycling,

transmaterialisation, dematerialisation. However, said measures are slow and take time to show

their effect. Sweden has set itself to have zero net GHG emissions by 2045 [2]. Therefore, there

is a need for strategies that show faster results.

Increase in atmospheric CO2 concentration gave rise to global mean temperature by 1.5 oC [3].

This increase in temperature is believed to have a significant impact on a magnitude of world-

wide events like food availability, human health, ecosystems, coastlines and biodiversity [4].

Acknowledging the impact of the rising CO2 levels governments around the world have vowed

to take necessary steps to regulate and limit their own emissions. To tackle said problem some

countries have even enacted legislation like carbon tax [5] and carbon cap-and-trade

schemes [6]. One strategy that could show an immediate reduction in CO2 emissions is carbon

capture and storage (CCS).

CCS is a three-step process: (i) separation of CO2 from gaseous waste streams, (ii) transport of

CO2 to a storage facility and (iii) long term isolation of CO2 from atmosphere. As the society

strives to transition from fossil fuels to renewable sources of energy CCS could prove to be a

valuable bridging technology [7]. CCS is not a new concept. There are a number of works

world-wide including research and development projects [8], [9]. However efforts for a fully

integrated CCS plant are still in infancy [10]. Sequestration of CO2 can be accomplished

through several means – geological storage, ocean storage, industrial use and mineral storage.

Among the methods for CO2 sequestration summarised in Table 1 mineral carbonation (MC)

is most suitable for adoption into an industrial process.

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Table 1 CO2 sequestration methodologies

CO2 sequestration

method Advantages Disadvantages

Geological storage

• Feasible on a large

scale

• Substantial storage

capacity

• Low cost

• Careful monitoring

• Possibility of

leakage

Ocean storage • Large storage capacity

• Significant

possibility of

leakage

• Harmful to

ecosystem

Industrial Use • Incorporating CO2 into

other products

• Limited storage

capacity

• Short storage time

Mineral storage

• Permanent storage

• Carbonated products

are environmentally

benign

• Energy intensive

• High cost

Accelerated mineral carbonation has also been proposed as a potentially effective carbon

sequestration method to tackle CO2 emissions by Intergovernmental Panel on Climate Change

(IPCC) [11]. The process mimics the natural weathering of silicates when contact with

atmosphere. The reaction between CO2 and the alkaline materials produces thermodynamically

stable carbonates storing CO2 permanently.

Swedish iron and steel industries produce 1-1.5 Mton of residual products [12]. 70% of the

total residuals is metallurgical slag. Slag is produced during the steel manufacturing process as

well steel refining processes. Steel making slags in general are alkaline materials composing

primarily of Ca, Mg and Al silicates. Slag compositions vary based on the initial feedstock and

processing conditions. There are some slag compositions where a non-negligible release of Cr

is observed [13]. However, most slag compositions are relatively stable non-hazardous waste

and are used for various applications – water treatment facilities, fertilizers, cement and

constructions [12]. Engineering applications of steel slag require the composition to be within

a threshold of free CaO and MgO as they are prone to absorb moisture and change to carbonates

which make structures unstable. Hence making steel slag viable for carbonation which stores

CO2 permanently as carbonates and stabilises the slag for further usage.

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1.2 Objectives

The overall aim of the thesis is to develop a process to sequester CO2 in steel slag which can

be adopted as industrial process. Specifically:

• To study the effects of parameters including carbonation temperature, carbonation time

and shape of slag on the carbonation speed and efficiency.

• To propose a process to sequester CO2 by using steel slags based on the results form

investigation.

1.3 Social and Ethical aspects

With increasing CO2 levels, sequestration of CO2 through carbonation is being a viable route

to help tackle the need for fossil fuels and rising CO2 emissions. Carbonation of metallurgical

slag means that there is a lesser need for mining of alkaline minerals and lesser cost of

carbonation over the mining costs as the minerals are no longer being transported.

Steel industries are one of the major CO2 emitters among the industries. Along with CO2 they

also produce by-products like slag. This slag is used in treating water, as pesticides and in

concrete industries. However, not all compositions of slag can be re-used in such manner. Slags

that are re-used end up in landfills and often also cause of pollution due to chemical leaching.

Carbonation of slag would mean rise in value of slag as it would make the slag more stable.

Carbonated slag can be used as a raw material in concrete industry, plastic and rubber industry

etc., This leads to reduced waste and pave way for more sustainable solutions especially in

developing countries where infrastructure development is still in infancy.

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Chapter 2

Background

This chapter provides general information about steel slag and carbonation process, providing

motivation and corresponding literature review as and when required.

2.1 Steel Slag

Slag is the by-product from steel making either from blast furnace (BF), basic oxygen furnace

(BOF) or electric arc furnace (EAF) or the refining processes like argon oxygen decarburization

(AOD). It is formed from the non-metallic constituents of the raw material ores, coke and fluxes

used during production of steel. The slag is separated from the liquid metal at the end of the

metallurgical treatments due to its low density.

During the primary stage of steel production, the steel slag produced is referred to as furnace

slag. This is the major source of steel slag. The molten steel is then transferred in a ladle for

further refining by removal of impurities within the steel. This process of refining is called

ladle refining and is completed in the transfer ladle. Due to addition of fluxes to the ladle,

additional steel slag is formed – ladle slag.

Basic oxygen process is one the primary steel production routes. Molten metal from BF, scrap

and fluxes (lime, dolomite) are charged into a furnace and injected with high pressure oxygen

through a lance. the oxygen reacts with the silicon, manganese, phosphorous and some iron to

form oxides which combine with lime and dolomite to form the steel slag.

Steel can also be produced by re-melting iron and steel scraps in an EAF. EAF utilizes high

voltage current to melt the scraps and remove excess carbon, silicon and other elements from

molten iron. Lime is added as a part of the flux to refine the molten iron, the slag thus produced

is called EAF slag.

AOD process is used to refine the steel when the melt has oxidizable elements like Cr and Al.

This is achieved in three steps: decarburization, reduction and desulfurization. Often the melt

is also de-siliconized for proper blowing in the furnace. For reduction, a mix of lime and

fluorspar are used as they have higher affinity towards oxygen than Cr/Al. finally

desulfurization is achieved by maintaining high lime concentration in the slag and low oxygen

activity in the metal bath. This leads to formation of CaS in the slag.

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2.2 Mineral Carbonation

Mineral carbonation (MC) is an accelerated form of weathering that occurs in silicate rocks

when exposed to atmospheric elements. MC occurs when a metal oxide reacts with CO2 to

form insoluble carbonate. The reactions are exothermic and spontaneous although they occur

on a larger geological time scale.

MC can be performed in situ – underground in nodes which oil and gas previously occupied.

However, carbonation of industrial by-products like steel slag are done ex-situ in an industrial

setting. The following sections will provide basic understanding in how ex-situ mineral

carbonation is carried out.

2.2.1 Direct Carbonation

Direct carbonation is the simplest method of MC. It is accomplished by reaction of an alkaline

mineral with CO2 either in a gaseous or aqueous phase as shown in figure 1. The process is

sometimes paired with a pre-treatment procedure which does not include extraction of reactive

components of the mineral like Ca or Mg ions. Direct MC is a simple process and has an added

advantage of being minimal in chemical usage.

2.2.1.1 Gas-solid carbonation

Gas-solid carbonation is the basic method for direct carbonation where gaseous CO2 reacts

with metal oxide at suitable temperature and pressure [4]. For better reaction kinetics elevated

temperature and pressures are used during the process. However, there is an upper limit for

how high the temperature can be increased to. As temperature is increased due to high entropy

of CO2 the equilibrium is shifted towards free CO2. Even under ideal conditions gas-solid

carbonation is slow to be adopted as an industrial process for CO2 sequestration.

Carbonation

Mineral CO2

MCO3

Figure 1 Direct Carbonation is accomplished in a single step. M denotes Ca/Mg

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2.2.1.2 Aqueous carbonation

Aqueous carbonation is another method for direct carbonation where CO2 reacts with alkaline

minerals in an aqueous suspension in a single stage [14]. CO2 reacts with water and forms

bicarbonate and H+. H+ liberates the metal ion from the mineral which reacts with the

bicarbonate to for carbonate. Direct aqueous carbonation of olivine is shown in reactions (1)-

(3).

CO2 + H2O → H2CO3 → H + HCO3− (1)

Mg2SiO4 + 4H+ → 2Mg2+ + SiO2 + 2H2O (2)

Mg2+ + HCO3− → MgCO3 + H

+ (3)

As with direct gas-solid carbonation elevated pressure and temperature enhance direct aqueous

carbonation. Increased pressure helps greater dissolution of CO2 pushing the reaction forward.

However, increase in temperature means lower stability of carbonates which helps formation

of bicarbonates aiding in carbonation. However further increase of temperature decreases the

solubility of CO2 into the aqueous suspension. This puts an upper limit on how high the

temperature can be increased. Direct aqueous carbonation of olivine was done at 170 bar and

150 oC [15].

At times the mineral is pre-treated before either of the direct MC processes to promote and

accelerate reaction rate of carbonation. There procedures include mechanical grinding,

chemical leaching, thermal and mechano-chemical pre-treatments. Although pre-treatment

procedures prove to have beneficial effect on the yield of carbonation [16] they are energy

intensive and expensive for a large scale carbonation plant [14].

2.2.2 Indirect Carbonation

Any MC process that occurs in more than a single stage is an indirect MC as shown in figure

2. Indirect carbonation process begins by extracting the reactive components (Ca2+, Mg2+) from

the minerals with the help of acids or other solvents. The extracted components are then reacted

with CO2 either in a gaseous or an aqueous phase. Solvents also help in producing pure

carbonates as impurities such as iron and silica can be removed before carbonate precipitation

[17]. There are numerous methods used for extraction of the reactive ions from the minerals –

hydrochloric acid (HCl) extraction, molten salt process, other acid extraction, bioleaching,

ammonia extraction and caustic extraction.

HCl extraction was developed for extraction of magnesium from serpentine [18]. Although the

process was effective it is energy intensive. It also posed problems with contaminations due to

extraction and precipitation of iron [19]. The molten salt process was developed as an answer

to the HCl extraction process. It was much less energy intensive and used molten salt,

MgCl2.3.5H2O instead of HCl. MgCl2.3.5H2O was also used in direct carbonation process

while reacting with the minerals at 300 oC and 30 bar CO2 pressure [19]. Although the molten

salt process had lesser energy requirements it was difficult to work with owing to the corrosive

nature of the solvent.

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Acids like nitric acid (HNO3), sulphuric acid (H2SO4), acetic acid (CH3COOH),formic acid

(HCOOH) have also been investigated to extract the reactive ions from the minerals [20]. This

method also had its disadvantages. The acidic conditions in the aqueous suspension make it

harder for dissolution of CO2 [21]. To tackle this the pH had to be increased at a later step to

prevent precipitation of iron. Teir et al. developed a process addressing this same problem [22]

where the acids were removed and the solids were recombined with water to increase the pH

to 7. The solids were then treated with NaOH to increase the pH furthermore. This process

produced pure carbonates however proved to be drastically more expensive for an industrial

process. Although acid extraction methods are effective, they are not as feasible as other

sequestration methods.

Bioleaching employs the help of bacteria to extract metals from minerals [23]. This process is

environmentally friendly and innovative however, it falls short in reaction rate when compared

with other methods of extraction. Ammonia extraction is similar to the acid extraction method

but it uses ammonia salts like ammonium bisulphate (NH4HSO4) [24] and NH4Cl [25]. This

method seems better than the acid extraction as there is no need for another solvent to increase

the pH later. Use of caustic soda to extract the reactive ions from minerals was investigated

[26], [27] however there were no favourable results nor particular interest shown in this

process.

Carbonation

Mineral

CO2

MCO3

Extraction

Acid/Base/

Solvent

Figure 2 Indirect carbonation occurs in more than a single stage. M denotes Ca/Mg.

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2.2.3 Parameters affecting carbonation yield

The yield of carbonation following either of the methods depends on primarily on the following

parameters.

• Pressure o Dissolution of CO2 increases with increase in pressure following Henry’s law.

Higher dissolution of CO2 means lower pH in the system which inhibits

carbonates perception [28].

o The yield is also affected by partial pressure of CO2 used for carbonation. Too high concentrations of CO2 in the gas might inhibit formation of precipitates

however, lower concentrations require more severe operating conditions to

achieve adequate results [29].

• Temperature o Accelerates dissolution of oxide and silicates while hindering dissolution of

CO2 into the aqueous suspension [30], [31].

o Also affect other characteristics of the system like viscosity, CO2 diffusion rate and rate of evaporation of liquid phase [32].

o The nature of the carbonates formed is also temperature dependant. Metastable phases/polymorphs [33] or compositions that are not favourable at lower

temperatures have been reported to exist at higher temperatures [34].

• System pH o While lower pHs may improve the dissolution of the sparingly soluble silicates,

they may be unfavourable for the precipitation of carbonate minerals [28].

o The morphology of carbonate crystals is affected by pH [35]. • Presence of water

o Water affects hydration, dissolution, diffusion and the mixing conditions in the suspension phase.

o Increasing liquid to solid (L/S) ratio enhances the hydration, dissolution and mixing in the suspension phase. However, excess water could be a mass transfer

barrier for CO2 diffusion [36]–[38].

o Lower L/S ratio reduces the heating requirements for the carbonation process. • Solid material characteristics

o Reaction rate is affected by particle size and intro-particle porosity. o Increased fineness of the particles translates to higher dissolution of minerals

and exposure to CO2 [36], [39]. Particle size and porosity are expected to change

during the carbonation process.

o Formation of passivating layers of either precipitated CaCO3 or depleted Si may prevent further reaction with the unreacted core [35], [40].

• Mineralogy o The yield varies depending on the composition and chemical properties of

silicates and aluminosilicates in the mineral.

o When using industrial by-products however, the mineral phases can be altered including particle size, presence of impurities and degree of ion substitutions

[41] as well as accessibility of reactive components within the solid [42].

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2.2.4 By-products for mineral carbonation

Although MC started out with using geological minerals for CO2 sequestration, using solid by-

products which are generated from industrial process such coal/oil shale fired power plant,

solid waste incinerator, cement plant, iron and steel plant, paper industry would be more

beneficial. This approach has the following advantages:

• The industries producing the by-products are often large emitters of CO2.

• The by-products from the industrial processes are generally less stable compared to the geological minerals. Hence reduce the degree of pre-treatment and are less energy

intensive operations.

• They would be a stable source of Ca/Mg mineral matter negating any need for mining.

• Hazardous wastes would be more manageable after pH-neutralization and mineral transformation.

• The final carbonated product could be employed for further use in a different form.

Although the quality of by-products is not great compared to natural minerals, they can offset

a significant amount of CO2 emissions especially from the industries which are the largest

contributors. Given how cost effective they are and the ease of availability, they can be adopted

as an industrial process with ease [4].

Criteria that applied to the natural minerals also apply when selecting by-products for MC.

• By-products contain substantial amount of alkaline earth metals like Ca/Mg and have high degree of alkalinity. This is important as Ca/Mg are the major carbonate former.

• Lesser presence of mineral phases (which are not oxides) is more favourable. This is because it is harder for silicates to carbonate compared to oxide.

• The by-product is well investigated and is easy to gather information about. Information like process economics, scalability and life cycle analysis ease the decision-making

process easier as each region might be more suitable for a specific by-product.

A large variety of alkaline wastes have been investigated for MC, including cement

wastes [43], waste ashes [44], alkaline paper mill wastes [45], mine tailings [46] and

metallurgical slag. The cases with metallurgical slag will be discussed in further later in the

thesis.

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2.2.4.1 Metallurgical slag for mineral carbonation

Slag from iron and steel industries is a viable option for mineral carbonation. The slags are

generally a combination of Ca-, Mg-, Al-silicates which could be used for carbonate formation.

It is to be expected that slags containing free CaO/MgO are more suitable than the ones that

have silicates. There have been numerous investigations into the viability of slag as a CO2

sequestration medium, some of them are summarised in table 2.

Table 2 Prior attempts at CO2 sequestration in steel slag

Slag

Process

BOF

DC: T = 100 oC, P = 10 bar, L/S = 2, t = 30 min [36]

IC: (1) T = 30 oC, CH3COOH (2) T = 30 oC, P = 1 bar, NaOH [47]

EAF

DC: T = 20 oC, P = 1 bar, L/S = 10, t = 40 hr [48]

Ladle

DC: T = 20 oC, P = 1 bar, L/S = 10; t= 40 hr [48]

AOD

DC: T = 90 oC, P = 9 bar, L/S = 16, t = 120 min [49]

Although these methods have shown varying results, they are not suitable for an industrial

process - usage of solvents like CH3COOH and NaOH quickly becomes expensive over huge

quantities, shorter durations are generally favoured over processes that take longer time.

In 2015 alone Sweden’s iron and steel industry has produced 2 Mton of residuals, 71% of which

is metallurgical slag [12]. As seen in figure 3, more than 80% of the slag produced is used after

production. However, some slags compositions are not being used and are often sent to

landfills. These slag composition however, are suitable for CO2 sequestration as they have high

Ca/Mg concentration as seen in table 3.

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Table 3 General slag compositions for different slag types

Slag

Al2O3

(mass %)

CaO

(mass%)

FeO

(mass%)

MgO

(mass%)

SiO2

(mass%)

BOF

[50]–[52]

1-6 30-55 10-35 5-15 8-20

EAF

[52]–[54]

2-9 35-60 15-30 5-15 9-20

Ladle

[52], [55]

5-35 30-60 0-15 1-10 2-35

AOD[56]

2.06 58-41 0.20 2.06 26.35

Based on usage and composition EAF and AOD slags are best suited for MC. AOD slags could be

better suitable than EAF as the phases like CaS is less stable.

0

100

200

300

400

500

Blast furnaceslag

LD-slag AOD-slag Low-alloyedEAF-slag

High alloyedEAF-slag

Ladle slag Other slag

Produced

Valorised

Figure 3 Slag quantities produced and valorised in Sweden, 2015 (in kton)

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2.2.5 Prior attempts in carbonation of steel slag

Attempts have been made to sequester CO2 in various steel slags, as seen in Table 4. The

process routes however were not streamlined for an industrial process. Higher pressures and

use of solvents can be hard and expensive. In some cases, although not so intensive, the

processes take long time for completion which is not viable for an industrial process. A simple

process with less intense parameters which takes less carbonation times have to be designed.

Table 4 Prior attempts in carbonation of steel slag

Slag Process

BOF

DC: T = 100 oC; P = 19 bar; L/S = 2; t = 30 min [36]

IC: (1)T = 30 oC; CH3COOH (2) T = 30 oC; P = 1 bar; NaOH [47]

EAF

DC: T = 20 oC; P = 1 bar; L/S = 10; t = 40 hr [48]

Ladle

DC: T = 20 oC; P = 1 bar; L/S = 10; t = 40 hr [48]

AOD

DC: T = 90 oC; P = 9 bar; L/S = 16; t = 120 min [49]

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Chapter 3

Materials, Methods and Experimental

procedures

This chapter deals with materials used, methodology and experimental information used in the

thesis.

3.1 Raw materials

All the experiments are based on EAF slag provided by Höganäs Sweden AB. The slag has

high concentration of CaO as seen in table 5.

Table 5 EAF slag composition

Mass %

Al2O3 8.27

C 0.362

CaO 38

Cr2O3 0.47

F 0.5

FeO 26

K2O 0.03

MgO 13.2

MnO 2.57

Na2O 0.04

NbO 0.06

NiO 0.02

P2O5 0.38

S 0.055

SiO2 13.3

TiO2 1.21

V2O5 0.12

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3.2 Methodology EAF slag from Höganäs is mechanical ground and particles less than 0.25 mm are separated to use for the carbonation tests. The ground powder is then calcinated at 650 oC for 3 hrs. This ensures that there are no volatile solvents in the slag before they are carbonated. The result of the processes can be seen in figure 4.

The tests were done to study the effect of specific parameters on the carbonation process. They were done in 3 experiments. All the tests were performed in the presence of CO2 at 3 l/min and steam at 8 ml/min. Experiment 1 – This test was done to observe how the yield of carbonation process

changes with time. The test was run for 1, 2, 4, 6 hr at 200 oC as seen in table 6. Experiment 2 – This test was done to investigate how temperature affects the yield of

carbonation process. Two different temperatures were investigated – 200 oC, 360 oC as shown in table 7.

Experiment 3 – This test was performed to investigate how the shape of slag would affect the yield of carbonation. The shapes tested were powder and pellet form as shown in table 8. The slag pellet was formed by mechanically pressing the slag powder.

Table 6 Experiment 1 - to observe the effect of time on carbonation process

Shape Temperature (oC) Time (hr) Powder 200 1 Powder 200 2 Powder 200 4 Powder 200 6

Figure 4 Raw slag (left), ground slag (middle), slag after calcination (right)

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Table 7 Experiment 2 - To observe effect of temperature on carbonation process

Shape Temperature (oC) Time (hr)

Powder 200 2

Powder 360 2

Table 8 Experiment 3 - To observe the effect of shape on the carbonation process

Shape Temperature (oC) Time (hr)

Powder 200 2

Pellet 200 2

3.3 Experiment Facilities

The carbonation tests were performed in a tubular furnace as seen in figure 5b. The slag was

put in a hollow metallic tube (as seen in figure 5a) which supported it and ensured maximum

exposure to CO2 and steam.

CO2,

steam

Figure 5 (a) Metallic tube which hosts the slag powder/pellet (b) tubular furnace (c) schematic for

the carbonation process

(a)

(b)

(c)

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Chapter 4

Results and Discussion The following chapter shall discuss the results from the experiments.

4.1 Theoretical calculation Theoretical carbonation % (tCO2) is calculated based on reaction (4), (5). The amount of CaO

in the slag may not exist wholly as free CaO. If all 38% of CaO were to exist as free CaO, tCO2

should be 300g of CO2/kg of slag. However, if the 13.3% SiO2 in the slag were to exist as

CaSiO3 it would be hard to carbonate with the conditions of the experiments conducted in this

thesis. Therefore, after considering free CaO that would be available for carbonation – 24.7%,

tCO2 would be 190gCO2/kg of slag. This range of tCO2 shall be compared with observed

carbonation % after experiment (ECO2).

CaO + CO2 → CaCO3 (4) SiO2 → CaSiO3 (5)

4.2 Influence of carbonation time Weight change from experiment 1 as seen in table 9, indicates that the yield from carbonation

is higher the longer they are exposed to CO2 at least up to 6 hr. It is expected for the yield to

increase if the experiment is further continued. Since there is continuous movement of CO2 to

and away from the slag there is no problem of pH reducing due to increased CO2 concentration.

Table 9 Experiment 1- Longer exposure shows higher yield

Shape Temperature (oC) Time (hr) ECO2

(gCO2/kg of slag)

tCO2

(gCO2/kg of slag)

Powder 200 1 0

190-300 Powder 200 2 40

Powder 200 4 44

Powder 200 6 53

This could be the case of the more CaO interacting with CO2 resulting in higher yield.

Therefore, the yield is expected to increase when the carbonation duration is increased over

6 hr. Prolonged exposure could result in reduced yield as pH of the system drops. Lower pH is

not suitable for carbonate precipitation.

17

4.3 Influence of carbonation temperature Table 10 shows the yield decreasing when temperature is increased. This is to be expected

because the carbonation reaction is an exothermic reaction.

Table 10 Experiment 2 - yield decreases with increase in temperature

Shape Temperature (oC) Time (hr) ECO2

(gCO2/kg of slag)

tCO2 (gCO2/kg of slag)

Powder 200 2 40 190-300

Powder 360 2 20

4.4 Influence of slag shape Table 11 shows that there is no change in weight of the pellet after carbonation. However,

there is visible change in the pellet – white coloured patches as seen in figure 6. Further characterization is required to understand this phenomenon. The indifference in weight

could also be a case of there not being enough diffusion into the interior of the pellet. The

pellet was also considerably harder after carbonation. This could probably be a signal towards

slag being used for construction perhaps in tandem with concrete or maybe even replace it.

Table 11 Experiment 3 - no change in weight of slag pellet

Shape Temperature (oC) Time (hr) ECO2

(gCO2/kg of slag)

tCO2 (gCO2/kg of slag)

Powder 200 2 40 190-300

Pellet 200 2 0

Both experiment 1 and 3 show deficiency of diffusion into the slag. Stirring could improve the

yield of carbonation as there is a high possibility for all particles to get exposed to CO2. This

could be achieved by using a different reactor like a screw reactor or by use of a stirrer. Stirring

in an aqueous carbonation route might even reduce carbonation times drastically.

Figure 6 Slag before carbonation (left), powder slag after carbonation for 2 hr at 200 oC

(middle), slag pellet after carbonation for 2 hr at 200 oC (right)

18

Chapter 5

Conclusion

• Longer durations of carbonation time, preferably over 6hr showed better yield.

• Increasing temperature doesn’t translate to increase in yield.

• From the amount of carbonation that has taken place in experiment 1 and the results from experiment 3, it is reasonable to assume that diffusion has significant impact on

carbonation.

Based on the results obtained, an industrial carbonation process is certainly plausible. But to

make the effort viable the carbonation yield must be improved significantly. Major industries

like steel and concrete are affected by this development the most. Even energy sector has a

stake in the technology. After all, sequestration of carbon dioxide in by products such as steel

slag would be a profitable venture at the same time improving their environmental standards.

19

Chapter 6

Recommendations for future work

• There is still a lot to be uncovered about the carbonation process. Characterization of the carbonated products would help in understanding the present process and designing

a better process.

• AOD slag is relatively less stable than EAF slag. Carbonation of AOD slag could be more efficient.

• Diffusion of CO2 into the slag seems difficult. A new methodology which enhances

diffusion could increase carbonation yield drastically.

• Studying how carbonated slag can be applied in various industries.

20

Chapter 7

Acknowledgments

I would like to express my gratitude to supervisors Weihong Yang from the department of Materials Science and Engineering at KTH for giving me the opportunity to do my thesis on this project. Weihong’s research group have been very welcoming most importantly Tong Han. Without the continuous support and constructive criticism from Weihong and Tong the project would not have been fruitful. I would like to thank Jernkontoret for the financial support, especially Robert Eriksson and Björn Haase for provided essential information and insight. Special thanks to Karnica and BTS for giving me constant motivation and mental support. Your cheers have been most important for me to relax during the duration of my studies at KTH. Lastly, I would like to thank my family for their understanding and support. Your video calls and messages have been a source of happiness and inspiration for me.

Sushanth Kombathula

21

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