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I. Mihailova, D. Mehandjiev

317

Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010, 317-326

CHARACTERIZATION OF FAYALITE FROM COPPER SLAGS

I. Mihailova1, D. Mehandjiev2

1 University of Chemical Technology and Metallurgy

8 Kl. Ohridski, Sofia 1756, Bulgaria E-mail: [email protected] 2 Institute of Catalysis, Bulgarian Academy of Sciences,

Acad. G. Bonchev St., Bldg. 11,Sofia 1113, Bulgaria

ABSTRACT

The aim of the present study was to characterize the fayalite crystal phases in two types of copper slag flash

smelting furnace slag (FS) and converter (CS) slag, given their possible use in adsorption and catalytic processes. Two

typical slag compositions different in chemical composition and way of production were selected, in order to clarify how

these differences affect fayalite crystallization and composition. The main mineral phase found in copper slags is fayalite.

The samples of the slag were characterized using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy

(FTIR), Light optical microscopy (LOM) in transmitted and reflected light.

Keywords: fayalite, copper slags, flash smelting furnace slag, converter slag.

Received 05 March 2010

Accepted 12 June 2010

INTRODUCTION

The chemical, phase and granulometric compo-

sition of metallurgical slags has always attracted a great

deal of interest in view of their reuse in the process of

production or use for other purposes, such as the ex-

traction of certain metals from them or for the produc-

tion of construction materials. Being a waste product,

the slag recovery or disposal in designated places is

closely related to the issue of environmental protec-

tion. During the cooling of the slag, crystallisation pro-

cesses occur in it and certain phases are formed. The

main phases formed during each crystallization process

are known, and they depend on the cooling conditions

of the metallurgical slag at certain stages in the produc-

tion process. Since slag-forming additives (fluxes) are

based on silicon, aluminium, calcium and magnesium,

compounds of mineral analogues are formed, a process

similar to the process of crystallization in natural sili-

cate melts - volcanic lava or magma. For example, the

slag from processing and production of ferrous metals

is characterized by phases of the melilite group -gehlenite and akermanite. Some studies, however, have

shown that these slags have adsorption properties for

heavy metals, such as lead, copper, etc., and the increased

content of these phases enhance the slag adsorption prop-

erties [1, 2]. In such cases, slag can be successfully used

as an adsorbent in purification of water containing heavy

metals [3]. It was also found that metals adsorbed on

the slag surface, for example copper, make it catalyti-

cally active in processes of complete oxidation and cata-

lytic removal of carbon monoxide and harmful organic

substances [4]. The flash smelting furnace slag and con-

verter slags in copper production contain mainly fayalite

due to the nature of the process of production.

Fayalite is a common member of the olivine

group of minerals. Olivine is a name for a series of

silicate minerals with the formula M2SiO

4, where M is

most commonly Fe or Mg. Fayalite (Fe2SiO

4) and

forsterite (Mg2SiO

4) form a substitutional solid solu-

tion where the iron and magnesium atoms can be sub-

stituted for each other without significantly changing

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Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010

318

the crystal structure. The complete solid solution be-

tween these two elements is observed in olivine. Com-

positions in the MgFe series commonly are identified

by the molar percentages of forsterite (Fo) and fayalite

(Fa) (e.g., Fo90

Fa10

), or in shortened form, by just their

forsterite number, where Fo#= Mg/(Mg+Fe)×100.

The crystal structure of Fe2SiO

4has been refined

using X-ray methods [5-7]. Olivine has an orthorhombic

structure. The structure consists of isolated SiO44- tetra-

hedra, which are held together by M cations occupying

two types of octahedral site (M1 and M2). The isolated

tetrahedra point alternately up and down along rows par-

allel to the z -axis. The magnetic spin structure has been

determined using powder neutron diffraction at 4 and 58

K by Santoro et al., who have also measured the powder

magnetic susceptibility between 4 and 300K [8].

Olivine with up to one formula unit of calcium

(CaMgSiO4) is referred to as monticellite. The iron ana-

logue, kirschsteinite (CaFeSiO4), is rare in nature. There

also is a manganese end member, tephroite (Mn2SiO

4).

The metal cations occupy two positions in the crystal

structure, M1 and M2, which are defined by their sym-

metry; commonly, Fe2+ displays a preference for the

M1 site. The members of the olivine group are pre-

sented in Table 1. M. W. Schaefer has reported large

amounts of Fe3+ ions in fayalites (members of the olivine

group) from various localities. Such data are contrary to

the generally held belief that fayalite is incapable of ac-

commodating much Fe3+ ions in its crystal structure [9].

Members of the olivine group (olivines) are im-

portant rock-forming minerals in terrestrial, planetary,

and astronomical geomaterials. Olivines are the major

component of Earths mantle, they are common to many

types of meteorites, and have been identified on the sur-

faces of planetary bodies and in the spectra of astronomical

targets. Olivine phases are indicators of low-silica envi-

ronments, they crystallize at high temperatures, and they

generally break down readily in the presence of weather-

ing agents such as water. As such, their identification and

characterization is a subject of considerable interest to a

wide variety of researchers [10]. Olivines are, quite liter-

ally, universally relevant minerals.

On Earth, olivine compositions in the forsterite

fayalite series are common in mafic to ultramafic rocks,

in which their compositions typically range from about

Fo8595

. Compositions between Fo80

and Fo50

are typical

of gabbroic rocks. Olivine group minerals can be formed

in natural and in technological processes - in certain

mineral aggregates (rocks), in slags of the Blast furnace

and agglomerates [11]. Fayalite also has considerable

technological importance in copper metallurgical slags.

Ivanov et al. carried out interesting research on

phase composition and structure of copper slag from

Bulgarian plants [12-16].

Our previous research [17, 18] on the phase com-

position and structure of flash smelting furnace slag and

converter slag from copper production found that the

chemical compositions of fayalite included Al, Mg, Zn,

Cu, Al, Ca and other elements as isomorphic impuri-

ties. The phases in the samples were characterized by a

variable and non-stoichiometric composition. However,

the preference of the various elements contained in the

slag to the different crystalline or amorphous phases

was clearly manifested.

The aim of the present study was to characterize

the fayalite crystal phases in two types of copper slag

flash smelting furnace slag and converter slag, given their

possible use in adsorption and catalytic processes. Two

typical slag compositions different in chemical compo-sition and way of production were selected, in order to

clarify how these differences affect fayalite crystalliza-

tion and composition.

EXPERIMENTAL

Two kinds of copper slags were examined slag

from flash-smelting furnace process and converter slag.

Samples were taken from the slag fields used for crys-

Mineral names Chemical

composition

Forsterite Mg2SiO4

Fayalite Fe2SiO4

Tephroite Mn2SiO4

Liebenbergite Ni2SiO4

Co-olivine Co2SiO4

Ca-olivine Ca2SiO4

Monticellite CaMgSiO4

Kirschsteinite CaFeSiO4

Glaucochroite CaMnSiO4

Table 1. Olivine End-Members.

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tallization of the liquid slag by spraying with water. The

samples of copper flash smelting furnace slag and con-

verter slag are labeled as FS and CS respectively. The

samples of the slag were characterized using X-ray dif-

fraction (XRD), Fourier transformation infrared spec-

troscopy (FTIR), Light optical microscopy (LOM) in

transmitted and reflected light. Quantitative determi-

nation of elements in the samples was carried out by

Gravimetric analysis, Complexonometry, Atomic ab-

sorption spectrometry (AAS) and Inductively coupled

plasma (ICP-OES). The XRD characterization of the

different samples of slag was conducted using a DRON

3M X-ray diffractometer with a CoKá radiation (40

kV, 25 mA) as an X-ray source (λ = 0.17903 nm). The

samples were continuously scanned over an angular

range of 2è 8 90° using a step size of 0.05°. PowderCell

2.4 software [19] was used to determine the unit cell

parameters on the basis of experimental X-ray data.

FTIR spectroscopy has been applied. Infrared transmit-

tance spectra were recorded by using pressed pellet tech-

nique in KBr. The measurements were made with a FTIR

spectrophotometer Bruker EQUINOX 55 in the fre-

quency range from 4000 to 400 cm-1. In order to have a

good evaluation about the structures and textures in-

cluded of each sample as well as to make a comparison

between different samples, light optical microscopy in

transmitted and reflected light was used. In this study

we have used polarizing microscopes Laboval pol u

Carl Zeiss Jena.

RESULTS AND DISCUSSION

Chemical composition results

Table 2 shows the chemical composition of

samples of slag.

X-ray diffraction

Fig. 1 shows X-Ray diffraction data for samples

of slag.

Fayalite (Fe2SiO

4) and magnetite (Fe

3O

4) are two

dominant phases in the samples. The two presented X-

ray diffraction patterns show amorphous halo at lower

values of 2è, associated with the presence of amorphous

phase. The converter slag sample contains larger amount

of magnetite in comparison with the sample of flash

smelting furnace slag. Divergence between experimen-

tal data and reference pattern data for the intensity of

diffraction peaks may be observed. The reflections of

the fayalite phase with the indices (120) (240) (020)

(130) have a higher intensity than expected. This is due

Fig. 1. XRD patterns of samples of copper slags

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320

to textures in the samples related to fayalite crystals

morphology. The presence of additional crystalline

phases could not be ascertained from the presented XRD

patterns. Peaks with negligible intensity remain after

juxtaposing diffraction maxima with the reference pat-

terns for fayalite and magnetite. Diffraction maxima at

34.8 ° 2è (d = 3.00) in the XRD pattern of flash smelt-

ing furnace slag sample and 31.0 ° 2è (d = 3.34) in the

XRD pattern of converter slag sample are an exception.

Their presence is likely due to copper sulphide phases.

The cell parameters of fayalite and magnetite

found in slag were calculated using the program Pow-

der Cell 2.4. They are given in Table 3.

As seen in Table 3, the values of the unit cell

parameters of fayalite and magnetite in copper slags are

very close to literature values for pure phases.

FTIR analysis

Fayalite is an especially important phase for dif-

ferent types of spectroscopy because, by definition, it

contains an equal distribution of Fe2+ cations between

the M1 and M2 octahedral sites. Thus, features associ-

ated with each of the two sites must represent equal

numbers of Fe2+ cations, removing the uncertainties

associated with assumptions about order/disorder of Fe2+

and other cations [24].

Thermal infrared band assignments for MgFe

olivines were determined early on [25-26] from trans-

mission spectra and many studies have resulted in re-

fined and extended assignments. Symmetry analysis in-

dicates that there are a total of 84 normal modes of

vibration [27], 35 of which are infrared active (but which

may not be individually observable in unpolarized spec-

tra, or may be too weak to be observed). These modes

represent internal stretching and bending modes as well

as lattice modes (cation translations and rigid rotation

translation) of the SiO4

tetrahedra. In spectra of ran-

Table 2. Chemical composition of slag samples.

Cell parameter of

magnetite (cubic)

Cell parameters of fayalite (orthorhombic) Sample

= = > ?

Flash smelting

furnace slag (FS)

8.385 4.824 10.477 6.089

Converter slag

(CS)

8.412 4.825 10.502 6.120

Magnetite

[20]

8.3958

Magnetite

[21]

8.3965

Magnetite

[22]

8.3970

Fayalite

[5]4.818 10.471 6.086

Fayalite

[23]4.818 10.470 6.086

Table 3. Cell parameters of the crystalline phases in the slags. Comparison with literary data.

Content, %massElements/ Oxides Flash

smelting

furnaceslag (FS)

ConverterSlag (CS)

SiO2 25.56 22.45

CaO 5.94 1.52

MgO 1.59 0.48

K2O 0.78 0.44

Na2O 0.76 0.62

Fe 46.63 47.71

Cu 1.14 5.07

Pb 0.06 0.66

Zn 0.33 1.05

Ni 0.017 0.036

Co 0.043 0.160As 0.02 0.02

Sb 0.021 0.064

Bi 0.020 0.007

Cd 0.016 0.002

Cr 0.004 0.043

S 0.18 0.74

Ó 83.111 81.072

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domly oriented samples, the three principal bands be-

tween : 1000 and 850 cm-1 are the result of splitting of

degenerate SiO asymmetric stretching vibrations of the

í3mode in SiO

4. A feature near 825 cm-1 results from

the í1mode (symmetric stretch), and three or four fea-

tures in the 610460 cm-1 region can be attributed to split-

ting of the degenerate í4 asymmetric bending vibration.Features in the 500200 cm-1 region are attributable to

the í2symmetric bending mode, rotations and translation

of the SiO4

tetrahedron, and translations of one of the

divalent cations. In the far infrared (20085 cm-1), addi-

tional translational modes and combinations of SiO4

and cation translations are observed [10].

Fig. 2 shows FTIR spectra of samples.

As can be seen the presented spectra are mainly

due to the fayalite phase in the samples.

The vibrational modes in olivine series minerals

vary slightly as a function of solid solution composi-tion, leading to shifts in band position and changes in

band strength, including the presence or absence of some

features. Regardless of whether the measured olivines

are natural or synthetic, their infrared spectra exhibit a

Fig. 2. FTIR spectra of the samples of copper slags

Sample / Fo# Band

1

Band

2

Band

3

Band

4

Band

5

Band

6

Band

7

Band

8

FS (this study) 948 914 874 826 562 502 472 425

CS (this study) 947 911 873 826 564 504 475 413

Fo31 956 927 883 833 582 516 482 (417)

Fo29 955 926 878 831 573 513 482 411

Fo18 951 918 874 829 567 511 480 411

Fo14 950 920 883 831 570 506 479 (407)

Fo11 947 920 874 829 563 507 476 409

Fo9 949 920 876 831 570 506 478 (413)

Fo1 945 916 874 828 563 505 475 (419)

Fo0 945 917 875 830 566 507 478 –

Fo0 946 917 879 830 569 505 479 (418)

Table 4. Olivine transmittance band positions, cm-1.

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generally linear decrease in the wavenumber position

of each band as the molar proportion of Fe increases,

and this trend is attributed to the increasing radius and

mass of the cation linking the SiO4tetrahedra. The lin-

earity of band positions as a function of composition in

the forsterite fayalite series indicates a nearly random

distribution of Mg2+ and Fe2+ in the M1 and M2 sites

and that any significant deviation from linearity would

indicate ordering of the Mg2+ and Fe2+ ions [10].

In Table 4, experimental data for fayalite in cop-

per slag is compared to literature data [10] for olivines

with a forsterite number (Fo#) in the range of 31-0.

There is a good correspondence between experimental

data and literature data for olivines with Fo# below 11.

Due to the low content of magnesium in slag, the

forsterite number of the fayalite phase formed in slag

should be small. It is interesting to make a theoretical

estimate of the expected forsterite number on the basis

of the available data for the content of the main com-

ponents involved in fayalite structure formation. Mag-

nesium, iron and silicon content in FS and CS samples

are given in Table 1.

On the basis of these data using the formula

Fo#

= a.100/a + 0,435.b, where a and b denote respec-

tively the percentage content of magnesium and iron,

the forsterite number Fo#

was calculated, assuming that

all magnesium and iron participate in the fayalite phase

formation in both slags. The calculated values are as fol-

lows: Fo#= 4.5 (FS) and Fo

#= 1.4 (CS). The forsterite

number value of fayalite in flash smelting furnace slag is

higher than that in converter slag, which should be ex-

pected given the higher content of magnesium in the phase.

Experimental values for the forsterite number of

fayalite phases may also be calculated using the linear

relationship between the band position and the forsterite

number. In Fig. 3 is shown the dependence of position

of the band 1 at : 950 cm-1 on the forsterite number,

according to literature data [28, 29].

In Fig. 3 with straight horizontal lines are also

given the values derived by us for the same band of the

fayalite phase in both slags. The intersection points of

these lines with the straight line give the forsterite num-

ber value. Thus the calculated value for FS sample is 8.3,

Fig. 3. The dependence of position of the band 1 at ~ 950

cm-1 on the forsterite number (The equation of the straight

line is: Y= 945.088 + 0.34776 X. Then Fo#= 8.3 for the sample

of flash smelting furnace slag (FS) and Fo#

= 5.5 for the

sample of converter slag (CS).

Fig. 4. Microstructure of sample of flash smelting furnace

slag. Elongate aggregates of fayalite and black magnetite

crystals in a groundmass of glass. 120x optical magnification.

Plane polarized (a) and cross polarized (b) transmitted light

photomicrographs.

0 5 10 15 20 25 30 35

944

946

948

950

952

954

956

B a n d c m

- 1

Fo#

CS

FS

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and that for CS sample is 5.5. There are certain differ-

ences between calculated and found values of the forsterite

number, though the theoretical estimate suggesting a

higher forsterite number of flash smelting furnace slag

compared to that of converter slag was confirmed.

However, it should be kept in mind that silicon

also plays a determining role. Calculations based on its

content show that fayalite formation phase, in which all

magnesium and iron participate can not be expected.

Taking into consideration the above limitation in the

calculation of the forsterite number, forsterite number

values obtained are 4.7 for flash smelting furnace slag

and 1.6 for converter slag. The calculation assumes that

all the magnesium participates as well as some iron,

inasmuch as allowed by the presence of silicon. From

this follows that the excess iron should form a new phase

and presence of magnetite in both slags was actually

Fig. 5. Typical morphology of fayalite crystals in the sample

of flash smelting furnace slag. 120x optical magnification.Plane polarized (a) and cross polarized (b) transmitted light

observed. Since magnetite is clearly distinguished under

X-rays as a separate phase, it appears that there is less

iron participating in the fayalite phase. At the same time

silicon-containing phases are additionally found that could

increase the value of the forsterite number, which was

actually observed. Some deviations from data for the iso-

morphic series forsterite - fayalite are inevitable and may

be also due to possible inclusion in the olivine crystal

structure of other cations present in the slag.

Light optical microscopy

Olivine group minerals crystallize in the same

system, both in rocks and in slags, but their morpholo-

gies are different. The most frequently occurring forms

of olivine in rocks have been analyzed by optical mi-

croscopy. These forms are crystal and rounded. Olivine

group minerals from slags show skeletoidal or crystal

Fig. 6. Inclusions of magnetite and silicate glass in the

fayalite crystals from flash smelting furnace slag. 120x opticalmagnification. Plane polarized (a) and cross polarized (b)

transmitted light photomicrographs.

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Fig. 7. Allotriomorphic fayalite crystal in the converter slag. 120x optical magnification. lane polarized (a) and cross polarized

(b) transmitted light photomicrographs

Fig. 8. Fayalite displaying skeletal crystal form in the converter slag. 120x optical magnification. Plane polarized (a) and cross

polarized (b) transmitted light photomicrographs

Fig. 9. Typical morphology of fayalite crystals in the sample of converter slag. 120x optical magnification. Plane polarized (a) and

cross polarized (b) transmitted light photomicrographs.

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form under microscope. Morphology of olivine crystal

forms in slags differs from those in natural aggregates

(rocks) [11].

Fig. 10. Fayalite (1), magnetite (2), sulphide inclusions (3)

and copper (4) in silicate glass a) and b) – sample of flash

smelting furnace slag; c) – sample of converter slag. 120x

optical magnification. Reflected light photomicrographs.

Fayalite crystals occurring in experimentally ob-

tained copper converter slags with variable SiO2

con-

tents are described [15]. The crystals differ in the de-

gree of skeletal growth which is explained in terms of

hindrances in heat transfer brought about by the in-

creasing viscosity of the melt upon increase of the SiO2

content. The formation of tubular inclusions in crystals

elongated along the b -axis is related to the influence of

sulphide droplets at the surface of the crystals [15].

In Figs. 4-10 are given the typical structural re-

lationships between phases present in the examined

samples of slag. It was confirmed that fayalite is the

dominant crystal phase, followed by magnetite and cop-

per sulphide inclusions, with the glass phase present

as a main mass. Fayalite crystals when viewed in trans-

mitted light were pleochroit with shades ranging from

green to pink. For FS sample were typical dendritic

growth and subparallel orientation in space as well as

not fully formed skeletal crystals. The relations be-

tween fayalite and the magnetite phase in the samples

differed. In fayalite crystals themselves there were small

grains of magnetite inclusions. In some cases the lat-

ter formed elongated zones in skeletal fayalite crystals

and showed simultaneous crystallization of both phases.

Heterogeneous xenomorphic crystals were typical

for CS sample. They included many impurities of crys-

talline phases and glass within their boundaries (Fig. 9).

Specific skeletal forms were also observed. The magne-

tite content was definitely higher compared to that in

FS sample and the amorphous phase was more hetero-

geneous.

CONCLUSIONS

From the present study of fayalite crystal phases

in two types of copper slag - flash smelting furnace slag

and converter slag the following conclusions can be

drawn:• Despite the complex composition of metallur-

gical slag and the possibility of inclusion of other ele-

ments as isomorphic impurities in the fayalite phase

Fe2SiO

4, the latters crystal lattice was not substantially

altered, according to XRD data. The values of the unit

cell parameters of fayalite and magnetite in copper slags

were very close to literature values for pure phases. This

may have been due to isomorphic substitutions of Fe2+,

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both by cations with smaller ionic radii, such as Mg2+,

and by cations with larger ionic radii.

• The obtained FTIR spectra of copper slag

samples corresponded almost entirely to the spectrum

of the fayalite phase. This made possible the use of FTIR

to determine the composition of fayalite through the

established dependency of spectrum bands positions on

fayalite composition.

• Dendritic growth as well as not fully formed

skeletal crystals were common features relevant to the

morphology of fayalite in the examined samples of cop-

per slag. Fayalite crystals included many impurities of

crystalline phases and glass within their boundaries. Spe-

cific crystal forms were observed in both types of slag.

• On the base of presented data the copper slags

may be used as adsorbents and support for catalysts.

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