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4 A High-Valent Bis-Fe(IV) Intermediate in MauG

4 A High-Valent Bis-Fe(IV) Intermediate in MauG

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C H A P T E R 3 2





MOSSBAUER

SPECTROSCOPY

IN THE STUDY OF LATERITE

MINERAL PROCESSING



EAMONN DEVLIN1 , MICHAIL SAMOUHOS2, AND CHARALABOS ZOGRAFIDIS2

1

2



Institute of Materials Science, N.C.S.R. “Demokritos,” Attiki, Athens, Greece

School of Mining and Metallurgical Engineering, National Technical University of Athens, Athens, Greece



32.1 INTRODUCTION

Nickel trends in the world market have shown a rising demand over the last decade, with global annual nickel consumption

growth greater than 3%, and a constrained supply. World primary refined nickel production of 1.44 Mt in 2010 was

anticipated to increase to 1.60 Mt in 2011 and 1.74 Mt in 2012 to satisfy projected increases in consumption [1]. Although a

decline in global consumption and an overcapacity is currently predicted for 2013, long term demand for Nickel is expected

to rise. While nickel is used in a range of industries including engineering, electrical and electronics, automobile and

automobile components, and batteries, most nickel, 65%, is used in the manufacture of stainless steels, with a further 20% in

other steel and nonferrous alloys, often for high-specification applications.

Primary nickel is produced from two different ores, lateritic and sulfidic. Lateritic ores are normally extracted by

opencast mining of ore deposits in layers at varying depths below the surface, while sulfidic ores are mined from

underground. Approximately 70% of global land-based nickel resources are contained in laterites, but they are

responsible for only about 40% of the global nickel production. Production has been dominated by sulfide ores, as

they are easier to process, through conventional mining, smelting, and refining, compared to laterite ores that require

intensive hydrometallurgical processing. Thus, laterite ores require substantially more energy and chemicals to produce

than the sulfide nickel. Nevertheless, as the global demand for nickel is rising, and many of its uses limit high rates of

recycling, a progressive shift to nickel laterite projects in the global nickel industry is underway despite the high

environmental cost that it entails. Offsetting this disadvantage are the facts that these ores are found close to the surface

and are cheaper to access, and that they contain the valuable element cobalt [2]. The processing costs do mean that major

sustainability challenges such as energy and greenhouse emissions remain of paramount importance to the nickel sector

[3]. In connection with these challenges, new processing techniques are being sought. One of these techniques is the use

of microwave radiation [4]. In this chapter, we will look at the use of M€

ossbauer spectroscopy (MS) as a tool in

monitoring traditional and microwave processing of laterite ores in Greece.

M€

ossbauer spectroscopy has been used as a characterization tool in a wide range of mineral processes including

ilmenites, chalcopyrites, pyrrhotites, bauxites, as well as laterites to some extent [5–9]. The M€

ossbauer technique yields

detailed information on the phases present, their composition, structure, and their relative amounts. In the case of

laterite processing in particular, in addition to the above useful data, it gives critical information on the overall degree of

metal reduction.

M€

ossbauer Spectroscopy: Applications in Chemistry, Biology, and Nanotechnology, First Edition.

Edited by Virender K. Sharma, G€

ostar Klingelh€

ofer, and Tetsuaki Nishida.

Ó 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.



608



32.2 CONVENTIONAL PROCESSING



609



To get some background, let us quickly review laterite ore processing. The three types of nickel laterite deposits of

economic interest for nickel are limonitic, intermediate, and saprolitic (or garnieritic). The main mineralogical phase

of the saprolitic ores is garnierite (Ni,Mg)6Si4O10(OH)8, which has a high magnesia and low iron content, with a Ni grade

of 1.5–3.5%. Limonitic laterites contain iron oxides as the main mineralogical phase—goethite (a-FeOOH), hematite

(Fe2O3), or magnetite (Fe3O4)—thus having a higher iron content and a lower Ni grade (1–2%). Laterite ores

are classified into three classes: (i) Class A—garnieritic laterites (Fe < 12% and MgO > 25%); (ii) Class B—limonitic

laterites (high Fe content, 15–32% or >32% and MgO < 10%); and (iii) Class C—intermediate laterite ores that lie

between garnieritic and limonitic ores (Fe 12–15% and MgO 25–35% or 10–25%). Greek nickeliferous laterite deposits

represent 90% of nickel laterite reserves in the European Union and are classified as either type B or C laterites.

Reviews of pyrometallurgical processing of laterite ores have been carried out by Bergman [10], Taylor [11], Simons

[12], and Diaz [13], among others. The two main stages of standard pyrometallurgical treatment of Greek nickeliferous

laterites for ferronickel production are (1) drying, preheating, and controlled prereduction of laterite ores with solid fuel

reducing agents and fuel oil in rotary kilns (R/Ks) for the production of a calcine; and (2) smelting reduction of the calcine

in open-bath submerged-arc electric furnaces (E/Fs) for the production of Fe–Ni alloy with 12–16% Ni [14]. The

reductive reactions of iron, nickel, and cobalt oxides (Fe2O3, FeO, NiO, and CoO) in the calcine with the remaining

carbon in the E/Fs are endothermic, and the energy consumed is about 16% of the total energy in the smelting reduction

step [14]. Thus the calcine’s reduction degree is an extremely significant parameter for the electric energy requirements

in the E/Fs. The chemical characteristics and the granulometry of the calcine also constitute critical parameters for the

E/Fs’ energy requirements.

It is in stage 2 that microwave technology can be applied, providing an alternative energy source for the laterite

calcination and prereduction [15,16]. The use of microwave energy minimizes the duration of the calcination–

prereduction process, and can produce a more reduced calcine product, reducing the smelting process energy demand.

A range of characterization techniques must be used to monitor the key variables in the processes (conventional

processing and microwave heating), in addition to M€

ossbauer spectroscopy, which gives us critical information on the

reduction degree.



32.2 CONVENTIONAL PROCESSING

To monitor the daily variations in the calcination process in a standard industrial pyrometallurgical process, daily 2–3 kg

samples were obtained over a period of 20 days. The calcine samples were produced by a selected R/K and taken from

the charging pipes of the E/F that was fed exclusively by the R/K. The calcine samples were immediately cooled to room

temperature using nitrogen gas at a high flow rate to avoid reoxidation phenomena. In addition, daily slag samples of

approximately 0.5 kg were taken over the same time period from the same E/F. The following parameters were

monitored:

 Reduction degree (RD) of the calcine, calculated based on iron speciation determined by M€

ossbauer spectroscopy

[17].

 Chemical analysis of the calcine and slag samples by atomic absorption spectrometry (AAS) and LECO induction

furnace for the determination of carbon and sulfur content.

 Temperature values of calcine and slag, measured by laser pyrometer.

 Melting temperature of slag samples, determined by triangular diagrams.

 Grain size analysis of the calcine samples, experimentally determined by the use of standard test sieves.

 Fe and SiO2 content (%) of the laterite feed in the R/K within the examined time period based on the R/K

operational data.

 E/F energy consumption index (kWh TÀ1 of calcine).

 Chemical analysis of nickel in the Fe–Ni alloy.

 E/F electrode consumption index (millimeters of slipping per megawatthour per 24 h).

We will present the M€

ossbauer data to show its usefulness in monitoring various stages of the process. Transmission

M€

ossbauer spectroscopy was carried out on powder samples at room temperature with a Co57(Rh) source in constant

acceleration mode using a thin iron foil for calibration.



610





32 MOSSBAUER

SPECTROSCOPY IN THE STUDY OF LATERITE MINERAL PROCESSING



0.0



Absorbtion (%)



–0.5

–1.0

–1.5

Fit

Data

Fe2.5+ Ox

Fe2+ NM

Fe2.5+ NM

Fe3+ Ox



–2.0

–2.5

–3.0

–10



–5



0

Velocity (mm s–1)



5



10



0.0



FIGURE 32.1

€ ssbauer

Room-temperature Mo

spectra of samples 4 (upper) and

14 (lower). Ox: Magnetic iron

oxides. NM: nonmagnetic iron

containing phases. (Reproduced

from Ref. 21 with permission of

Wiley.)



Absorbtion (%)



–0.5



–1.0



–1.5



Fit

Data

Fe2.5 Ox

Fe 2+ NM

Fe2.5 NM

Fe3+ Ox



–2.0



–10



–5



0

Velocity (mm s–1)



5



10



Previous laboratory studies conducted on the roasting reduction of Greek laterite samples have revealed that iron in

the calcine is mainly found in the form of hematite (a-Fe2O3), nonstochiometric magnetite (Fe3O4), and complex iron–

silica phases, such as fayalite (2FeO.SiO2) [18].

In Fig. 32.1, the M€

ossbauer spectra of two representative samples from days 4 and 14 (#4, #14), with different

degrees of reduction, are presented. The significantly higher nonmagnetic ferrous content of sample 4 is clearly visible.

Ferrous and ferric components are observed along with intermediate states (Fe2.5ỵ) that represent iron sites such as the

B site in magnetite, which is a mixture of Fe2ỵ and Fe3ỵ iron states. Nonmagnetic phases and magnetic oxides (hematite,

nonstochiometric magnetite) are observed. No metallic a-iron is detected. The different subspectral components are

graphically distinguished in Fig. 32.1 on the basis of the iron oxidation state. Thus, the M€

ossbauer spectroscopy yields an

accurate quantitative analysis of the content of the mineral phases present that allows us to determine the degree of

reduction (RD ẳ (Fe2ỵ/Fetot)%), Fig. 32.2 [19].



FIGURE 32.2

Reduction degree of the calcine

samples. (Reproduced from

Ref. 21 with permission of Wiley.)



611



32.2 CONVENTIONAL PROCESSING



FIGURE 32.3

Reduction degree of calcine

versus temperature. (Reproduced

from Ref. 21 with permission of

Wiley.)



Correlations between the reduction degree and other parameters such as the calcine temperature, determined by

the use of an optical pyrometer, can be obtained (Fig. 32.3). It should be noted that the temperature measurements of the

calcine samples are made at the E/F freeboard and correspond to the temperature of the calcine fed into the charging

pipes of the E/F. The temperature of the calcine fed into the E/F is about 100  C lower than that of the calcine exiting the

R/K. It is clear that an increase of the calcine temperature favors the reduction.

Previous work on the reducibility of Greek nickeliferous laterites has shown the importance of the calcine

temperature in rotary kiln roasting reduction [20]. As hematite reduction can take place above 570  C, increasing the R/K

temperature improves both the reduction degree and the reduction rate. We also see, in Fig. 32.4, that when the calcine

reduction degree increases, we obtain a decrease in the calcine’s carbon content. Both temperature and calcine carbon

content are critical parameters for the smelting reduction procedure in the E/F. A higher calcine temperature

corresponds to reduced electric energy requirements both for smelting and for the endothermic reductive reactions

of iron and nickel oxides of the calcine in the E/F. Thus, we see the crucial role that MS can play in generating RD values

that can be correlated with various control parameters to obtain better process control. For example, limiting the calcine

carbon contents results in fewer operational problems and a quieter smelting reduction process, as smaller volumes of

reduction gases are generated through the slag in the E/F.

Examining the variation of reduction degree with Nickel content reveals another important correlation. A lower

calcine carbon content due to higher reduction results in a lower reduction degree of the iron oxides inside the E/F,

Fig. 32.5. This results in an increased nickel content in the Fe–Ni alloy and an increased iron content in the slag.

Furthermore, increasing the iron content in the slag reduces the slag resistivity, with the result that the electrodes are

lifted more out of the slag and the electrode current increases [21]. The decreased contact between the electrode and

the slag leads to decreased electrode consumption, which is a crucial factor in the economics of the smelting reduction

process. In Fig. 32.6, we clearly see the effect of reduction degree on electrode slipping.

Thus, we see how the reduction degree, obtained from M€

ossbauer data, yields information bearing on critical

economic parameters. The optimization of open-bath submerged-arc E/F operation for laterite smelting reduction, in

terms of energy consumption, electrode consumption, and limitation of the operational problems, is a multiparametric

problem. The temperature and the reduction degree of the calcine feeding material, the iron and silica content of the

calcine and slag, the ore grain size of the calcine, and the temperature of the slag, all constitute critical parameters.

M€

ossbauer spectroscopy can yield vital information to help with the understanding and modeling of such systems.



FIGURE 32.4

Reduction degree of calcine

versus residual carbon content.

(Reproduced from Ref. 21 with

permission of Wiley.)



612





32 MOSSBAUER

SPECTROSCOPY IN THE STUDY OF LATERITE MINERAL PROCESSING



FIGURE 32.5

Ni content in the Fe–Ni produced

in E/F versus calcine reduction

degree. (Reproduced from

Ref. 21 with permission of Wiley.)



FIGURE 32.6

Electrodes slipping versus calcine

reduction degree. (Reproduced

from Ref. 21 with permission of

Wiley.)



Mathematical modeling of the roasting reduction–smelting reduction process using comprehensive sets of such raw data

should be the goal for optimization of the metallurgical method.



32.3 MICROWAVE PROCESSING

The use of microwaves has huge potential for mineral processing arising from several clear advantages of the microwave

technique. These include the direct transfer of heat to the target, with little thermal loss to the surrounding area, and the

reduction of heat transfer problems associated with poor thermal conductivity. Microwaves are a clean and controllable

energy source that facilitate continuous materials processing. Implementation of this technique means addressing

technical problems that sometimes occur, for example, inhomogenous sample heating and financial issues associated with

energy and equipment costs.

Microwave radiation generates heat rapidly inside absorptive materials, and the heat then spreads via conduction

through the material. The prime absorption mechanisms are dipolar rotation and electron/anion resistivity that produce

ohmic heating (i.e., conductive currents due to the movement of ion or electron charges) [22].

In nonmagnetic media, microwave heating at 2.45 GHz is determined by the temperature-dependent complex

permittivity [eà (T)] and, in particular, by its imaginary part [e00r ðT Þ], which is a measure of the electromagnetic energy

conversion into heat.

e T ị ẳ e0r ðT Þ À je00r ðT Þ;



(32.1)



where eà ðT Þ is the complex permittivity, e0r ðT Þ is the relative real permittivity or relative dielectric constant, and e00r ðT Þ is

the relative imaginary permittivity or relative dielectric loss.

The average power (PL) absorbed by a material per unit volume is estimated by





1

PL ẳ ve0 e00r T ịjEint j2 W cm3 ;

2



(32.2)



613



32.3 MICROWAVE PROCESSING



Microwave Drying at 750 W

Conventional Drying at 140°C



50

40



Drying rate (g mm



–2



–7



sec x 10 )



60



FIGURE 32.7



30



Microwave drying rates at 750 W

and conventional drying rates of

limonitic nickeliferous laterite

ores as a function of moisture

fraction. (Reproduced from

Ref. 23 with permission of the

Canadian Institute of Mining,

Metallurgy, and Petroleum.)



20

10

0

0.0



0.1



0.2



0.4 0.5 0.6 0.7



Moisture content (x)



0.3



0.8



0.9 1.0



where v is the angular frequency (2pf, in rad sÀ1), Eint is the internal electric field in the sample (V mÀ1), and e0 is the

permittivity of free space.

It is clear from the foregoing equations that the dielectric properties, and especially e00 , determine the rate of

microwave heating and can be used to predict the successful application of microwave heating for a pyrometallurgical

process.

In the last decade, a great deal of work has been published on the application of this technique to the

pyrometallurgical processing of oxide ores. A review of this work by Pickles gives a good indication of the range of

applications of the technique at all stages of processing: drying, calcination, sintering, reduction and smelting, slag

reduction, segregation, and the processing of electric arc furnace dust [4].

A single example of the power of this technique can be seen in its use in the drying stage (Fig. 32.7). Greatly improved

drying rates are achieved due to the different heating and moisture transport properties associated with the microwave

method, for example, direct microwave heating of the moisture content rather than conventional heating of the entire

sample mass.

For the microwave processing study, we used hematitic nickeliferous laterite ore from the Lokrida area, one of the

main laterite sources in Greece. A sample mass of about 50 kg was collected from the homogenized ore heaps and was

pulverized using an LM2 Labtechnics pulverizing mill. A 5 kg charge grinding with 2 min of milling was sufficient to mill the

total mass of laterite to a À16 mesh (À1.18 mm) particle size. The reducing agent applied was lignite (30.1% Cfix). The

bulk laterite sample was analyzed by X-ray fluorescence using a Xepos Spectro bench instrument (Table 32.1). Iron oxide

(40.7 mass%) and silica (30.6 mass%) are the major chemical phases with noticeable amounts of MgO, Al2O3, and Cr2O3

also present. The metallic nickel content is about 1 wt%.

XRD of the raw bulk laterite and the carbothermic reduced laterite samples was carried out using a Bruker D8 Focus

analyzer. The XRD pattern of the raw bulk laterite ore is presented in Fig. 32.8. It shows that the iron oxide is present

almost entirely in the form of hematite. In addition to quartz, the other important phases are clinochlore, montmorillonite, and calcite. The dominant hematite phase, together with the absence of nickel–silicate minerals, classifies the

laterite ore as type C.

The carbothermic reductions of the laterite samples were done using a Ceralink ThermWave 1.3 microwave furnace

working at 2.45 GHz. Temperature measurements of the top surface of the powder samples were done using an optical

pyrometer Raytek MX4, located at the end of a 20 cm long and 4 cm in diameter bronze 2.45 GHz choke tube, attached to

the center of the metal furnace wall. The optical pyrometer was connected to a PC and measurements were recorded

every 0.1 s. A real-time temperature versus time diagram was produced using the appropriate software. The variation of



TABLE 32.1 XRF Chemical Analysis of Bulk Hematitic Laterite Ore (wt%)

Species



SiO2



Fe2O3



MgO



Cr2O3



CaO



Al2O3



NiO



K2O



L.O.I



30.6



40.7



4.5



4.0



3.2



4.8



1.3



0.8



8.0



Source: Reproduced from Ref. 19 with permission of Elsevier.





32 MOSSBAUER

SPECTROSCOPY IN THE STUDY OF LATERITE MINERAL PROCESSING



614

200



1. SiO2 - Quartz

2. Fe2O3 - Hematite

3. (Mg,Fe,Al)6(Si,Al)4O10(OH)8 - Clinochlore

4. (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2 xH2O Montmorillonite

5. FeOOH - Goethite

6. KAl2Si3AlO10(OH)2 - Muscovite

7. CaCO3 - Calcite

8. Mg3(OH)2Si4O10 - Talc



1



Intensity (cps)



150



100

1

2



50



3



3



2

1



4



2



334



66 8



3



12



33



5



1

1,2



1



1



7



2 2



2



1 2



3



1



2



1



3



0

5



10



15



20



25



30



35



40



45



50



55



60



65



70







FIGURE 32.8

X-ray pattern of the bulk hematitic laterite ore. (Reproduced from Ref. 19 with permission of Elsevier.)



microwave power was achieved employing a proportional power controller connected to the furnace magnetron.

Powder samples were placed, without applying pressure, in a cylindrical alumina crucible with r ¼ 4 cm, h ¼ 6 cm and d

(thickness) ¼ 3.3 mm, (r ¼ 7.5, h ¼ 10, and d ¼ 3.3 mm crucible was used in the case of the 477 g sample), which was

positioned at the center of the furnace on a low-density porous alumina platform. Measurements of the real and

imaginary permittivities of hematitic laterite samples were done using the cavity perturbation technique at the Microwave

Properties North laboratory in Canada [24]. The apparatus consists of a cylindrical cavity connected to a network

analyzer and a conventional resistance furnace. The materials, in powder or pellet form, are placed in a long, slim quartz

sample holder tube, and are then loaded vertically into the system Fig. 32.9, whereupon the furnace temperature is

increased in steps. After a soak at each temperature step, the samples are rapidly moved down into the cavity for

measurement ($2 s duration), and then rapidly returned to the furnace. The method is based on measuring the difference

in the cavity resonant frequency and Q factor between the empty sample holder and the holder with sample. The

dielectric constants are derived from the resonant frequency and Q factor changes produced by the sample, and are



FIGURE 32.9

Schematic diagram of the TM0n0

cavity system (in cross section)

showing the linear actuator with

the quartz sample holder and a

sample located on the y axis in

the center of the cavity.

(Reproduced from Ref. 19 with

permission of Elsevier.)



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