Kinetics of copper slag oxidation under nonisothermal conditions

Kinetics of copper slag oxidation under nonisothermal conditions

Stoyko Gyurov • Diana Rabadjieva •

Daniela Kovacheva • Yoanna Kostova

Received: 12 April 2013 / Accepted: 27 November 2013

� Akademiai Kiado, Budapest, Hungary 2014

Abstract The kinetics of air copper slag oxidation under

nonisothermal conditions is studied using simultaneous TG–

DTA at a varying heating rate of slag and flow rate of the

oxidizing gas flux. The values of the kinetic parameters,

activation energy and pre-exponential factor, have been

determined based on: data from DTA by the methods of

Kissinger and Ozawa; data from TG using an isoconversion

method and the computation procedures of Ozawa–Flynn–

Wall and Kissinger–Akahira–Sunose. No relationship

between the kinetic parameters and the oxidation gas flow rate

has been established. The changes of the phase composition

with temperature are investigated by X-ray powder diffraction

analysis on the basis of data obtained for the products formed

at the different stages of the oxidation process. The mor-

phology of the oxidized slag as well as the elements distri-

bution is studied by electron microscopy and EDS analysis.

Keywords Copper slag � Oxidation � Nonisothermal

kinetics � Simultaneous TG/DTA � Arrhenius parameters

Introduction

The slag being a waste product from the pyrometallurgical

production of copper contains mixed or individual oxides

of iron, silicon, aluminum, calcium, copper, nickel, cobalt,

etc. It has been calculated that about 2.2 tons of slag are

generated per each ton of produced copper and approxi-

mately 24.6 million tons of slag are accumulated every

year [1–3]. Dumping or disposal of this slag causes wast-

age of certain amount of metal available in the raw material

and causes environmental problems [4]. Many efforts have

been focused on developing methods for the recycling of

slag and the reduction of its quantity [5–7]. Various

methods have been proposed for extracting copper and

other precious metals from slag by treating it with solutions

of acids, bases, and salts under atmospheric pressure and

high pressure as well [8–23]. Application of these methods

does not result in a significant reduction of the disposed

slag because the quantity of the extracted copper and pre-

cious metals is negligible in comparison to the amount of

slag [1–3, 5]. A new approach has been proposed in the EU

patent No. 2 331 717 B1 [24], which is based on the

decomposition of the fayalite (2FeO�SiO2) present in the

slag into iron oxide and silicate phases by oxidation and

subsequent hydrometallurgical treatment. As a result, use-

ful products could be obtained in the form of amorphous

silicon dioxide and iron oxide and the slag can be com-

pletely utilized. Therefore, it is important to know thor-

oughly the process of oxidation at the temperatures of

fayalite decomposition.

There are a restricted number of articles addressing

the oxidation of fayalite as a chemical compound. Oxidation

of fayalite (2FeO�SiO2) single crystals involved mainly

its transformation to hematite (Fe2O3), magnetite (Fe3O4),

and amorphous silica [25–27]. The reaction equation is as

follows:

2FeO�SiO2 þ 1=2 O2 ! Fe2O3 þ SiO2 ð1Þ2FeO�SiO2 þ 1=3 O2 ! Fe3O4 þ SiO2 ð2Þ

S. Gyurov (&) � Y. Kostova

Institute of Metal Science, Equipment and Technologies ‘‘Acad.

A. Balevsci’’ with Haydroaerodinamics centre Bulgarian

Academy of Sciences, 67 ‘‘Shipchenski prohod’’ str., 1574 Sofia,

Bulgaria

e-mail: [email protected]

D. Rabadjieva � D. Kovacheva

Institute of General and Inorganic Chemistry, Bulgarian

Academy of Sciences, ‘‘Acad. Georgi Bonchev’’ str. bld.11,

1113 Sofia, Bulgaria

123

J Therm Anal Calorim

DOI 10.1007/s10973-013-3569-2

Sanders and Gallagher [28] reported that the magnetite

is oxidized in a two-stage process; the first stage is the

formation of the metastable spinel c-Fe2O3 (maghemite),

and second stage is the formation of the equilibrium phase

a-Fe2O3 according to the reactions:

4Fe3O4 þ O2 ! 6c-Fe2O3 ð3Þc-Fe2O3 ! a-Fe2O3: ð4Þ

During high temperature annealing in air atmosphere

coarse magnetite powder undergoes oxidation only above

773 K [29]. Magnetite prepared at relatively low

temperatures from freshly precipitated precursors, however,

has a much greater surface area and defects concentration and

therefore oxidizes at much lower temperatures [30]. The

process starts at temperature of about 473 K and ends in the

interval 648–673 K. The beginning of the conversion is at

temperature of about 648 K and the end is at 798–933 K.

Thermal behavior of copper slag obtained from rever-

beratory furnace was investigated using simultaneous

thermal analysis (STA), dilatometry, and X-ray diffraction

(XRD) techniques [31]. The authors reported two exo-

thermic peaks in DTA curve, which could be related to the

magnetite–maghemite and maghemite–hematite transfor-

mations, respectively. The isothermal decomposition of

copper slag in synthetic air was investigated by Gyurov

et al. [32]. However, the process has not been sufficiently

studied. In order to develop an appropriate technology for

industrial utilization of copper slag, additional investiga-

tions are needed, especially with respect to the heating rate

and the parameters of the oxidizing gas flux.

The aim of the present work is to study the processes of

nonisothermal oxidation of copper slag by using the simul-

taneous TG–DTA, XRD, and SEM. The obtained data from

TG–DTA were used to calculate the kinetic parameters

(activation energy and pre-exponential factor), while the

data obtained from the XRD and SEM were used to deter-

mine the phase composition and morphological character-

istics of the formed solid phases at the stages of oxidation.

Experimental

The copper slag used in the experiments is a waste product

from the pyrometallurgical production of copper in Aurubis

Bulgaria AD. The slag was sifted through a 100 mesh sieve

and the fraction with particle sizes under 100 meshes was

used in the experiment.

XRD analysis

The XRD analysis was carried out using an automatic

Bruker D8 Advance powder X-ray diffractometer with

CuKa radiation (Ni filter) and registration of LynxEye

solid-state position-sensitive detector. The X-ray spectra

were recorded in the range from 5.3� to 80� 2h with a step

of 0.02� 2h. The qualitative phase analysis was made using

the PDF-2(2009) database of the International Center for

Diffraction Data (ICDD) by means of the DiffracPlusEVA

software package. Refinement of the unit cell parameters

was made with Topas4.2.

SEM analysis

The slag compositions as well as the distribution of the

chemical elements were determined using a JEOL JSM 35

CF scanning electron microscope with a TRACOR

NORTHERN TN-2000 X-ray microanalyzer applying

JEOL standards.

Simultaneous TG–DTA

The nonisothermal oxidation of copper slag was carried out

using the computerized combined thermal analysis appa-

ratus LABSYSEvo, SETARAM Company, France, at

atmospheric pressure in a flow of synthetic air (MESSER

CHIMCO GAS-OOH 1056, ADR 2, 1A) in the temperature

range 298–1,273 K. Synthetic air was used as an oxidant in

order to maintain a constant oxygen partial pressure in all

experiments. Corundum crucibles with a volume of 100 ll

were used. The sample mass in all tests was 80 ± 1 mg.

The experiments were carried out under dynamic condi-

tions, with heating rates of 4, 8, 12, 16, and 20 K min-1

and oxidizing gas flow rates of 20, 35, and 50 mL min-1.

Additional experiments for identification of the interme-

diate solid phases were carried out under the following

conditions: heating up to 523, 573, 623, 673, 723, 773, 873,

973, 1,073, 1,173, and 1,273 K with heating rate of

12 K min-1 and maintaining the desired temperature for

2 h. The flow rate of the oxidizing gas was 20 mL min-1.

These oxidized samples were used in XRD analysis.

Experimental results

XRD analysis

XRD powder diffraction patterns are shown in Fig. 1. The

initial slag is composed of fayalite Fe2SiO4; a cubic phase

with a spinel type structure and an unit cell parameter of

8.37 A; quartz–SiO2; calcite (Ca, Mg)CO3; and calcium

mono-carboaluminate Ca4Al2O6CO3�11H2O. The phases

presented are typical for copper slag but the amount of

different components may vary depending on the manu-

facturers. In any case, fayalite is dominant phase in this

type of slags [6, 33]. The value of unit cell parameter at

S. Gyurov et al.

123

room temperature of the spinel iron oxide (8.37 A) is lower

than the reported values for pure magnetite: 8.3958 A [34],

8.3965 A [35], and 8.3970 A [36] showing that magnetite

phase is slightly oxidized. The change of the unit cell

parameter of the spinel iron oxide with the temperature is

presented in Fig. 2.

SEM analysis

Chemical composition of the slag is (in mass%): FeO

50.93, MgO 1.43, CaO 1.39, MnO \0.01, SiO2 31.26,

Al2O3 6.93, Na2O 3.02, K2O 1.74, TiO2 0.22, and CuO 0.6,

respectively. The SEM image of the cross-section of slag

oxidized to 1,173 K and the distribution of Fe, Si, and O2 is

shown in Fig. 3. Three types of microstructure are

observed in the SEM image of the cross-section of slag

oxidized to 1,173 K: particles with dendrites, particles with

lamellar structure, and particles with eutectic-like structure,

which is probably a grain section, perpendicular to the

lamellae, Fig. 3. The particles contain three phases: iron

oxide, silicon oxide and iron silicate (residual fayalite).

TG–DTA

The obtained TG–DTA curves are similar in shape to the

one shown in Fig. 4. It shows an endothermic effect within

the temperature range 298 to about 400 K and a broad

exothermal effect in the temperature interval 400–1,273 K,

where four peaks could be distinguished. The general view

of the exothermic effect revealed the simultaneous run of

several (oxidation or phase transition) processes. The endo-

effect is characterized by a minimum decrease of mass and

is due to the released residual moisture in the slag. The

exo-effect is characterized by gradual increase of mass and

is due to the oxidation processes in the slag. The peak

temperatures Tp from the DTA curves are summarized in

Table 1. They follow, with some exceptions, the general

trend of shifting toward higher temperature values with the

increase of the heating rates. It can be observed that the

sample mass increases continuously between 693 and

1,273 K and is associated with the oxidation processes of

the slag.

Discussion

The XRD data presented in Fig. 1 show that in the temper-

ature range 293–673 K no changes of the slag’s phase

composition are observed. From 673 to 773 K, changes of

the unit cell parameter of magnetite are observed due to its

partial oxidation to maghemite, Fig. 2. The first diffraction

peaks of hematite phase are observed at temperatures

between 673 and 723 K. It is difficult to identify the process

leading to hematite formation: transformation of maghemite

or decomposition of fayalite. Actually, the oxidation and

transformation of the slag magnetite into hematite take place

in parallel with the oxidation and decomposition of the slag

fayalite into magnetite, hematite, and amorphous silicate

phase. At temperatures in the interval 673–1,273 K, the

processes of oxidation and decomposition of the fayalite and

transformation of magnetite phase become more intensive.

In the temperature range 1,073–1,273 K, a gradual increase

of the unit cell parameter of the spinel phase is observed as a

result of reduction of part of the iron ions. The size of the

crystallites of magnetite in the slag, oxidized at temperatures

below 873 K is greater than that of the samples oxidized at

temperatures in the interval 973–1,273 K. The smaller

grains (the broader peaks) of magnetite phase observed at

higher temperatures indicate that magnetite phase here is an

intermediate product from decomposition of fayalite.

1273K

1173K

1073K

873K

973K

773K

723K

623K

673K

573K

HS

S

SS H

HHHH

H

H

FF SSS

S

FFF

F

FFFF

F

F

F

Inte

nsity

/a.u

.

FS

H

298K

523K

H-hematite

F-fayalite

S-spinel

20 25 30 35 40 45 50 55 60 65

2θ /°

Fig. 1 X-ray powder diffraction patterns of the copper slag at

different stages of the oxidation process

8.34

8.35

8.36

8.37

8.38

8.39

8.40

Uni

t cel

l par

amet

er/A

Temperature/K300 400 500 600 700 800 900 1000 1100 1200 1300

Fig. 2 Variation of the unit cell parameter of the spinel iron oxide

with the temperature

Kinetics of copper slag oxidation

123

Oxidized to a temperature of 1,273 K slag contains hema-

tite, amorphous silicate phase, and residual magnetite.

According to our SEM observations, the decomposition

of the fayalite phase of the slag to hematite and silicate

phase is accompanied by mass redistribution. There are

different opinions concerning the problem which of the

components will be diffused—Fe, Si, or O2. If oxidation

occurs by transport of oxygen via fayalite, oxides of iron

and silicon will be observed at the boundary gas/solid

phase surface. If oxidation takes place by diffusion of iron

or silicon from the interior to the surface, the latter will

be covered by a layer of the respective oxide. According

to Mackwell [27], iron is diffused from the interior of the

fayalite single crystal to the surface and the kinetics obeys

a parabolic law. According to Gaballah et al. [26], the

kinetics is pseudo-parabolic with formation of a silicon

oxide and an iron oxide layer. Oxygen is diffused through

the latter and hematite crystals grow on it. All particles

shown in Fig. 3 have on their surface a layer of iron

oxide where under layers of fayalite and silicate phases

follow. This presumes diffusion of iron from the particle

core to the surface and of oxygen via the boundary sur-

face toward the interior of the particle. According to

Fisler and Mackwell [37], the experiments with a plati-

num tracer show that fayalite grows at the quartz–fayalite

boundary and the process is controlled by iron and oxy-

gen diffusion.

446

357

268

179

90

1

416

L1: O Ka Sika FeKa

L2: O Ka Sika FeKa

L3: O Ka Sika FeKa 200 μm/DIV

200 μm/DIV

200 μm/DIV

333

250

167

84

1

344

258

172

86

0

Fig. 3 SEM image of the

cross-section of slag oxidized to

1,173 K and the distribution of

Fe, Si, and O2

99

100

101

102

103

104

105

Mas

s/%

DT

A/a

.u.

Temperature /K

1149

856

691582

Exo

400 600 800 1000 1200

Fig. 4 DTA–TG curves of copper slag oxidation at heating rate

8 K min-1 and 35 mL min-1 oxidizing gas flow rate: dashed line TG

curve, line DTA curve

S. Gyurov et al.

123

Kinetic analysis

The collected TG–DTA data were used to determine the

kinetic parameters (the activation energy and the pre-

exponential factor) of the oxidation processes as described

elsewhere [38–51]. The kinetic analysis is based on the

assumption that the rate constant (k) is described by the

Arrhenius equation:

k ¼ A exp �E=RTð Þ; ð5Þ

where A is the pre-exponential factor, E is the activation

energy, R is the universal gas constant, and T is the abso-

lute temperature.

The DTA data (Tp—peak temperature) were processed

and the kinetic parameters of the oxidation processes were

obtained using the methods of Kissinger and Ozawa

[48, 49]. All obtained data for activation energy (E), the

pre-exponential factor (lnA), and regression coefficient (r2)

are presented in Table 2. Figure 5 presents samples of

Kissinger and Ozawa plots, respectively, for the tempera-

tures of third peaks of the DTA curves. The activation

energy E is calculated from the slope of the obtained

straight line and the pre-exponential factor lnA from the

intercept. It is seen that there is no correlation between the

amount of oxidizing gas and the values of kinetic param-

eters. The activation energy is a constant value for a given

process and can be influenced by the gas flux only if the

reaction produces or consumes a gas component [44].

During our DTA experiments the partial pressure of oxy-

gen at the reaction surface remains constant and the amount

of gas does not affect the reaction rate.

The values of activation energy calculated by the first

and second peaks are substantially equal. They are close to

the average values reported by Sanders and Gallagher

(about 120 kJ mol-1) by the oxidation of magnetite [28].

In the temperature range 293–673 K, XRD data (Figs. 1, 2)

indicate only the oxidation of magnetite to the maghemite

as to 673 K does not exhibit peaks of hematite, further-

more, mass effect is very small as could be seen in Fig. 4.

This is due to the morphology of the slag. The magnetite

grains in copper slag occur as small inclusions in fayalite or

glass phases [32]. Oxidation of individual small magnetite

grains will produce a negligible amount of hematite.

The third peak of the DTA curves is associated with the

fayalite oxidation and decomposition to iron oxides and

amorphous silicate mass. The values of activation energy

calculated by the third maximum are in the range

164–195 kJ mol-1, which are substantially different from

those reported by Gaballah et al. [26] for the oxidation of

single crystal synthetic fayalite. The nature of the forth

peaks was not revealed in the present study. It may be due

to some structural transitions of the silicate mass as sup-

posed by Gaballah et al. [26] or to some effects due to the

grain size distribution.

In addition to fayalite, the slag also contains small

quantities of other phases, which also undergo changes

during the heat treatment. The process is complex and the

calculated values of the kinetic parameters may differ from

those of pure single-phase components of the slag due to the

fact that the processes can interfere. This explains the

observed differences of the calculated values of the activa-

tion energy in some of the samples studied. In case of a

complex process, the effective activation energy of a solid-

state reaction is generally a composite value determined by

the activation energies of various processes and by their

influence on the overall reaction rate [43–46, 52, 53]. Since

the oxidation develops in several steps with a different

activation energy, the total rate of the process varies with

temperature and degree of conversion and the effective

activation energy will be a function of those two parameters.

Kissinger and Ozawa methods have intrinsic limitation.

For example, they are not able to detect reaction com-

plexities over the course of the reaction. In more compli-

cated case, Kinetics Committee of the International

Confederation for Thermal Analysis and Calorimetry (IC-

TAC) for carrying out kinetic computations on thermal

analytical data recommended the use of isoconversional

methods [52]. The TG data were processed using the iso-

conversional principle. The kinetic parameters are deter-

mined according to the computation procedures of Ozawa–

Table 1 The peaks temperatures Tp of the DTA curves (K)

Heating rate b/K min-1 Gas flow rate 20 mL min-1 Gas flow rate 35 mL min-1 Gas flow rate 50 mL min-1

I II III IV I II III IV I II III IV

4 – 694 852 1,123 560 669 846 1,117 582 678 847 1,120

8 – 702 867 1,159 582 691 856 1,149 585 689 854 1,157

12 – 709 882 1,182 588 708 874 1,176 588 711 873 1,178

16 588 718 888 1,206 595 715 886 1,200 – 717 883 1,183

20 601 724 902 1,218 597 722 900 1,216 616 735 897 1,199


123

Flynn–Wall (OFW) [50, 51] and the procedures of Kis-

singer–Akahira–Sunose (KAS) [48, 54].

The changes of degree of conversion (a) with tempera-

ture are shown in Figs. 6, 7, and 8. The activation energy

Ea and pre-exponential factor are calculated by the equa-

tions of OFW and KAS. Single OFW and KAS plots are

shown in Fig. 9, respectively. The dependences of the

activation energy Ea on the degree of conversion are pre-

sented in Figs. 10 and 11. Values of Ea obtained at dif-

ferent gas flows are approximately equal; i.e., the variation

of the gas flow rate does not affect the reaction rate of the

process. Four stages of slag oxidation can be distinguished

on the Ea curves depending on conversion progress: the

first one with constant Ea at conversion from 0.2 to 0.3; the

second with decreasing Ea at conversion from 0.3 to 0.5;

third with increasing Ea at conversion from 0.5 to 0.7; and

fourth with constant Ea at conversion from 0.7 to 0.8. As

can be seen from Figs. 10 and 11, the values of the acti-

vation energy are constant in temperature range 800–950 K

(a increased between 0.2 and 0.3). The Ea values are about

122 kJ mol-1 calculated by KAS equation and about

139 kJ mol-1 calculated by OFW equation. They are close

to Ea values calculated by second peak of DTA curves and

it could be assumed that transformation of magnetite to

hematite is the rate-controlling process of the copper slag

oxidation up to temperatures of 950 K. The second stage of

oxidation is characterized by decreasing of the calculated

activation energy to 76 kJ mol-1 (KAS) and 94 kJ mol-1

(OFW). Maximum values of the calculated activation

energy of about 170 kJ mol-1 (KAS) and 180 kJ mol-1

(OFW) remain constant until the end of the process. They

Table 2 Activation energies (E/kJ mol-1), the pre-exponential factors (lnA/s-1), and regression coefficients (r2) obtained in Kissinger and

Ozawa methods by the data from Table 1

Gas flow rate/mL min-1 I peak II peak III peak IV peak

E lnA r2 E lnA r2 E lnA r2 E lnA r2

Kissinger

20 201 26.47 0.97 193 18.68 0.97 170 8.62 0.99

35 108 14.33 0.96 107 10.01 0.99 164 14.37 0.94 161 7.69 0.99

50 101 8.83 0.93 173 15.72 0.94 215 14.18 0.99

Ozawa

20 213 38.45 0.98 170 25.72 0.95 191 21.88 0.99

35 117 26.49 0.97 118 22.63 0.99 178 26.74 0.96 242 27.44 0.99

50 112 21.46 0.94 187 21.54 0.99 230 26.1 0.98

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

0.81 0.83 0.85 0.87 0.89

103/Tp /K–1

ln(H

eatin

gra

te,

/K

min

–1)

–1

–1

–1

b

β

20 ml min

35 ml min

50 ml min–12.8

–12.6

–12.4

–12.2

–12

–11.8

–11.6

–11.4

–11.2

–11

0.81 0.83 0.85 0.87 0.89 0.91

103/Tp /K–1

ln( β

/Tp2 )

–1

–1

20 ml min

35 ml min

50 ml min–1

aFig. 5 Relationship between

ln(b/Tp2) and 103/Tp—Kissinger

plot (a) and between ln(b) and

103/Tp—Ozawa plot (b) for

copper slag oxidation for the III

maximum of DTA curves,

obtained at different gas flow

rates

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Temperature /K

Con

vers

ion

4K min-1

8K min-1

12 K min-1

16 K min-1

20 K min-1

550 650 750 850 950 1050 1150 1250

Fig. 6 Temperature dependence of the degree of conversion, a,

during a nonisothermal oxidation of copper slag at flow rate of

oxidizing gas 20 mL min-1. Data were obtained from TG analysis at

different heating rates

S. Gyurov et al.

123

are close to E values calculated by the forth peak of DTA

curves. A more detailed study in temperature range

950–1,120 K is difficult because several simultaneous and

complex processes can occur. Khawam and Flanagan [55]

have shown that variation of activation energy with the

reaction progress is a result of the complex nature of the

solid-state reaction. The slag oxidation includes decom-

position of fayalite, adsorption of oxygen, diffusion of iron

and other species, growth of layers of iron oxide and sili-

cate phase. Therefore, the interpretation of kinetic data is

difficult and could not point unambiguously rate-control-

ling process.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Temperature /K

Con

vers

ion

4K min–1

8K min–1

12K min–1

16K min–1

20K min–1

550 650 750 850 950 1050 1150 1250





0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Con

vers

ion

4 K min–1

8 K min–1

12 K min–1

16K min–1

20 K min–1

Temperature /K550 650 750 850 950 1050 1150 1250





1

1.5

2

2.5

3

3.5

0.7 0.9 1.1 1.3

103/Tα /K-1

ln( β

)

0.2

0.4

0.6

0.8

a

-13

-12.5

-12

-11.5

-11

-10.5

-10

0.7 0.9 1.1 1.3

103/Tα /K-1

ln( β

/Tα2 )

0.2

0.4

0.6

0.8

bFig. 9 Relationship between

ln(b) and 103/Ta—OFW plot

(a) and between ln(b/Ta2) and

103/Ta—KAS plot

(b) calculated from TG data at

conversion 0.2, 0.4, 0.6, and 0.8

80

100

120

140

160

180

200

0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85

Conversion (α)

Eα

/kJ

mol

–1

–1

–1

20 mL min

35 mL min

50 mL min–1

Fig. 10 Dependence of activation energy (Ea) on the degree of

conversion (a) (Ea–a curve) for nonisothermal oxidation of copper

slag, calculated from TG data by KAS method

60

80

100

120

140

160

180

0.150 0.250 0.350 0.450 0.550 0.650 0.750 0.850Conversion (α)

Eα

/kJ

mol

–1

–1

–1

20 mL min

35 mL min

50 mL min–1

Fig. 11 Dependence of activation energy (Ea) on the degree of

conversion (a) (Ea–a curve) copper slag Ea depending on the degree

of conversion, calculated from TG data by OFW method


123

Conclusions

The simultaneously performed thermal (TG/DTA), XRD,

and electron-microscopic analyses (SEM) of air oxidized

slag prove that the process is complex and takes place in

four stages. The first two stages take place in the temper-

ature range from 673 to 873 K, where the cubic phase with

a spinel type structure firstly oxidizes to metastable spinel

c-Fe2O3, and consequently to a-Fe2O3. The third stage is

the oxidation and decomposition of the fayalite phase with

the formation of two phases—hematite and amorphous

silicate at temperatures up to 873 K. Finally, the forth stage

is the decomposition of residual fayalite with simultaneous

polymorphic transitions in the silicate and iron phases.

These elementary processes overlap to a certain degree

but the use of methods, independent of the reaction model,

makes it possible to calculate the activation energy for the

single stages, as well as the changes of the activation

energy with the degree of conversion. The values for the

first and second stage are close to these, obtained by

Sanders and Gallagher, and for the fourth stage to those

obtained by Gaballah et al. The process of slag oxidation is

accompanied by structural transformation and formation of

structure of ordered platelike crystals of hematite, amor-

phous silicate phase, and residual fayalite.

Acknowledgements This research is funded by EU Operational

Program ‘‘Development of the Competitiveness of the Bulgarian

Economy’’ 2007–2013 under Contract No. BG16PO003-1.1.01.-

0224-C0001. The authors wish to thank Aurubis Bulgaria AD for the

supplied slag.

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Kinetics of copper slag oxidation under nonisothermal conditions