Soluble Catalysts for the Oxygen Reduction Reaction, and Their

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Kinetics, Catalysis, and Reaction Engineering

Soluble Catalysts for the Oxygen Reduction Reaction (ORR), and their Application to Becher Aeration. Stephen Fletcher, Warren J. Bruckard, Carmen Calle, Keri ConstantiCarey, Michael D. Horne, Roman Ruzbacky, and Graham J. Sparrow Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01085 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on June 4, 2019

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Industrial & Engineering Chemistry Research

Soluble Catalysts for the Oxygen Reduction Reaction (ORR), and their Application to Becher Aeration. Stephen Fletcher1, Warren J. Bruckard*2, Carmen Calle2, Keri Constanti-Carey2, Michael D. Horne2, Roman Ruzbacky2, Graham J. Sparrow2 1

The Fletcher Consultancy, Loughborough, Leicestershire LE11 3LU, United Kingdom.

2

CSIRO Mineral Resources, Private Bag 10, Clayton South, VIC 3169, Australia.

Orcid identifiers: (S Fletcher): https://orcid.org/0000-0002-3255-9015 (M Horne): https://orcid.org/0000-0002-0218-7249 Corresponding author: Warren J. Bruckard CSIRO Mineral Resources, Private Bag 10, Clayton South, VIC 3169, Australia. Email: warren.bruckard @csiro.au Phone: +61 3 9545 8500

Email addresses: [email protected];

[email protected];

[email protected];

[email protected];

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[email protected];

Page 2 of 51

[email protected];

[email protected]


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Graphical Abstract


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Abstract

It is shown that two water-soluble redox catalysts, namely methyl viologen dichloride and diquat dibromide, can accelerate the rate of the oxygen reduction reaction on metallic iron in aqueous solutions. It is also shown that the same catalysts can speed the removal of metallic iron from reduced ilmenite in the aeration step

of

the

Becher

process.

Under

industrial

conditions

(2%

ammonium chloride and 70 °C) aeration times can be decreased from 10 h to less than 3 h without irreversible adsorption of the catalysts. In all cases where catalysts are present, the dissolved iron

precipitates

as

magnetite

(Fe3O4).

This

environmentally

preferred product is easier to separate and faster to dewater than conventional mixtures of iron(II, III) oxide-hydroxides.

Abbreviations

Oxygen reduction reaction (ORR); reduced ilmenite (RI); synthetic rutile (SR); methyl viologen (V); diquat (D); X-ray fluorescence (XRF); (SEM);

X-ray

diffraction

Saturation

(XRD);

Magnetization

scanning Analyser

electron

microscopy

(SATMAGAN);

9,10-

anthraquinone-2-sulfonic acid (AQ-2). Commonwealth Scientific and


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Industrial Research Organization (CSIRO); International Centre for Diffraction Data (ICDD). Key Words

Oxygen reduction reaction (ORR), spin allowed, redox catalyst, Becher aeration, viologen, diquat, ilmenite, magnetite, synthetic rutile. 1. INTRODUCTION

Australia is a world leader in the extraction of mineral sands and has some of the world’s largest verified resources of ilmenite (FeTiO3), rutile (TiO2) and zircon (ZrSiO4). Currently, it produces more than 20% of the world’s supply of these minerals(1). Most of Australia's rutile is exported to the USA, Western Europe and China, where it is processed into pigment grade titanium dioxide for

use

in

paints,

plastics,

paper,

and

cosmetics.

As

an

environmentally friendly material, titanium dioxide has superseded toxic lead carbonate (2PbCO3·Pb(OH)2) in many surface coatings(2). Industrially, rutile is also used as a feedstock for titanium metal production in the aerospace industry. The Becher process(3,4) was developed more than 50 years ago to remove iron from ilmenite, and thereby produce a “synthetic rutile” (SR) that could supplement diminishing global stocks of natural rutile(5,6). In the Becher process a carbothermic reduction of ilmenite is first carried out in a rotary kiln, yielding small


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nuclei of metallic iron embedded inside grains of titania. This mixture is widely referred to as “reduced ilmenite” (RI). A lowash coal performs the function of both fuel and reductant, and 95% metallisation is achieved in about nine hours. After cooling, the metallic iron nuclei are dissolved out by aeration into an ammonium chloride

solution

at

70

°C.

During

this

process,

which

is

kinetically complex, the metallic iron is converted into a variety of soluble iron(II, III) species which are able to diffuse away from

the

titania

grains.

Ultimately,

they

precipitate

from

solution as mixtures of iron(II, III) oxide-hydroxides(7). Any remaining iron is then removed by acid leaching. In current industrial practice, the aeration step of the Becher process can take as long as 22 h to complete. Many parallel pathways are possible, and the dominant pathway is determined by the prevailing conditions. Important factors include temperature, pH, activity of reduced ilmenite, ammonium chloride concentration, oxygen concentration, stirring rate, and pulp rheology(8,9). In current practice, the temperature, pH and RI activity are all open variables, making it difficult to predict the exact course of reaction. Consequently, there is poor control over the phase composition of the iron(II, III) oxide-hydroxide mixtures that are formed, even though they must be removed to waste cost-effectively. Given the slowness and unpredictability of the aeration process,


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speeding up the reaction rate and generating phase-pure products are both desirable. In Australian commercial operations, Becher aeration is carried out in 2% w/v NH4Cl solutions. It is known that ammonium chloride enhances the rate of dissolution of metallic iron in at least three different ways(8). Firstly, the chloride ions help to break down any passivating films that exist on the iron surface. Secondly, ammonium ions form soluble ammine complexes with the dissolving iron(II) species. Thirdly, the ammonium chloride helps to maintain the solution pH in the range 4.6 to 6.0, where iron(II) species are

moderately

soluble.

The

rutile

and

iron(II,

III)

oxide-

hydroxides are also self-buffering in the same range. In

the

scientific

literature,

a

few

compounds

that

form

coordination complexes with iron have been tested to see if they can accelerate Becher aeration or influence the phase composition of the iron(II, III) oxide-hydroxides formed. These test compounds include organic acids such as acetic, tartaric and citric acids(1013),

multidendate ligands such as ethylenediammonium dichloride(14),

various

phenolic

and

aldehydic

compounds

such

as

pyrogallol,

saccharin, starch and formaldehyde(15) and sugars such as glucose and sucrose(16). One group has even suggested the addition of ferrous chloride(17). However, while some short-term benefits may accrue from these additives, none of them has been found to exert a genuinely catalytic effect.


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The Becher aeration process is fundamentally an electrochemical reaction in which metallic iron is oxidised by molecular oxygen from air(18). The reaction is kinetically slow for two reasons. Firstly,

ion

transfer

from

metallic

iron

into

solution

is

inherently slow, due to the large number of metal-metal bonds that must be broken. Secondly, the electronic ground state of molecular oxygen is a triplet diradical stabilized by a very large resonance, which causes the triplet state energies to fall below the singlet state energies by ~90 kJ mol−1

(19).

This “inverted” electronic

structure causes molecular oxygen to be inert towards molecules that have fully paired electrons, i.e. most “ordinary” molecules. In the jargon of quantum mechanics, such reactions are “spinforbidden”. It follows that if one wishes to contrive a rapid reaction between molecular oxygen and metallic iron, then one must insert a free radical intermediate into the system that can be oxidized and reduced many times by the target species. An example of this general principle is shown in Figure 1, which illustrates the catalysis of iron dissolution by anthraquinone sulfonate. This system was first described by Bruckard et al. in 2004(18). The key step is the electron transfer reaction between the semiquinone radical and oxygen. This is spin-allowed and therefore has a very high probability of occurrence. Indeed, the second order rate constants for reactions between semiquinone radicals and molecular


8

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oxygen have been tabulated(20) and they are typically of the order of 108 M ―1s ―1 (i.e. almost diffusion controlled).

Figure 1 Catalysis of iron dissolution by anthraquinone sulfonate. Although the catalyst is thermodynamically less powerful than oxygen, it reacts much faster with iron.

The first product of the oxygen reduction reaction is superoxide (O2∙ ― ), which is the conjugate base of the hydroperoxyl radical (HO2⋅ ). The latter is a weak acid (aqueous pKa 4.8) so it is 99% ionized at pH 7(21). Other products include hydrogen peroxide. In a previous research program in CSIRO, a family of soluble anthraquinone sulfonates was identified as catalysts for Becher aeration(22).

Addition

of

these

compounds

to

aeration

pulps

routinely halved aeration times without damaging the quality of the

synthetic rutile, and

also

assured

the

formation

of the


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preferred black

Page 10 of 51

oxide (magnetite). However, the

anthraquinone

sulfonates were negatively charged in solution. Coupled with the fact that the isoelectric point of magnetite occurs at pH 8(23), this meant that the catalysts were immobilized on the positivelycharged magnetite surfaces at pH values below 8. Although this didn’t prevent the catalysts from working, it did prevent them from being recycled in subsequent aerations. In the present work, we report the identification of a new family of

Becher

aeration

catalysts

whose

molecules

are

positively

charged in solution. These are electrostatically repelled by the magnetite surfaces, and so can be recycled sustainably from batchto-batch.

2. NATURE OF THE CATALYSTS

2.1 Molecular structure and electrochemistry The

new

redox

catalysts

for

Becher

aeration

are

multi-ring

heterocycles containing two quaternary nitrogen atoms. The first, referred to as catalyst V, is 1,1'-dimethyl-4,4'-bipyridinium dichloride, commonly known as methyl viologen [CAS: 4685-14-7]. The second, referred to as catalyst D, is 6,7-dihydrodipyrido[1,2a:2',1'-c]pyrazinediiumdibromide, commonly known as diquat [CAS: 85-00-7].


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The molecular structures of V and D are shown in Figs 2 and 3, respectively. With two positively charged nitrogen atoms in their structures, V and D can both be redox cycled via semiquinoid intermediates, as shown in Figs 4 and 5.

Figure 2. Molecular structure of methyl viologen, catalyst V.

Figure 3. Molecular structure of diquat, catalyst D.

Figure 4. One and two electron transfer reactions for catalyst V.


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Page 12 of 51

Figure 5. One and two electron transfer reactions for catalyst D.

In order to speed up electron transfer between two different redox couples, such as Fe(0)/Fe(II) and O2/O2•−, a redox catalyst must have a reversible potential that falls somewhere in between the reversible potentials of both couples. Unfortunately, at the present time, little is known about the reversible potentials of the Fe(0)/Fe(II) and O2/O2•− couples under industrial conditions, and the only data available are at best approximate. However, the standard potentials (relative to the standard hydrogen electrode) at 25°C provide a rough guide. The standard potential of the Fe(0)/Fe(II) couple is –0.44 V and the standard potential of the O2/O2•− couple (assuming one molar concentration of O2) is –0.18 V(24). With due allowance for discrepancies between the standard conditions

and

the

actual

conditions

(especially

the

local

temperature and pH) one can crudely estimate that a functioning catalyst should have a standard potential in the range 0 to –0.60


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V at 25°C. This is indeed the case for V and D. According to the scientific literature, the standard potential of the V2+/V•+ couple at 30°C is –0.45 V(25) and the standard potential of the D2+/D•+ couple

is

V(26).

−0.35

Likewise,

the

standard

potential

of

a

previous catalyst (9,10-anthraquinone-2,6-disulfonic acid) is – 0.18 V(27), and the standard potential of yet another catalyst (9,10-anthraquinone-2-sulfonic acid) is –0.22 V. We conclude from these observations that a moderately negative standard potential is a requisite feature of Becher aeration catalysts. This is useful information which may help guide the discovery of future catalysts.

2.1 Use as herbicides Catalysts V and D are used in agriculture as broad-spectrum herbicides, Accordingly,

and

both

are

poisonous

some care must be

taken

at

high

concentrations.

to avoid skin

contact,

ingestion and inhalation. However, safe disposal is relatively straightforward,

because

both

compounds

adsorb

strongly

onto

common clay minerals (such as kaolinite and montmorillonite) and soil(26,28). They are also rapidly removed from natural waters by retention on sediment. 3. EXPERIMENTAL

3.1. Analytical methods Three RI samples sourced from commercial operations in Western Australia were studied in the presence and absence of V and D.


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Page 14 of 51

Individual samples and final products were characterized by XRF, XRD and SEM. Solution concentrations of the redox catalysts were determined by UV-Vis spectrometry. Metallic iron concentrations were assayed by SATAMAGAN. Head samples, reaction products, and leach residues were analyzed for TiO2, iron(II, III) oxide-hydroxides, SiO2, Al2O3, MgO and Mn3O4 by XRF using a Philips PW 2404 X-ray spectrometer. XRD powder diffraction intensity profiles were recorded on a Philips PW 1050 goniometer with a Philips PW 1710 diffraction controller using Cu Kα radiation. Crystalline phases were identified by comparison of peak positions and intensities with calibrated standards published by the ICDD. SEM micrographs were taken using a JEOL 25S SEM. The metallic iron concentrations of withdrawn samples were determined using

a

model

135

SATMAGAN

from

Corrigan

Instrumentation

(Georgetown, Canada). The concentrations of redox catalysts in the aeration liquors were determined spectrophotometrically using a model 330 Perkin-Elmer UV–Vis. spectrophotometer by comparing the intensity of their characteristic absorption peaks at 255 nm for catalyst V and 310 nm for catalyst D, with freshly prepared standards. Both wet and dry screening methods were used to size the starting materials.


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3.2. Aeration conditions Bench scale aerations were carried out in 2 L round-bottomed flasks fitted with stainless steel stirrers, gas inlet tubes, thermocouples, and double condensers. Large scale aerations took place in a 20 L cylindrical plastic vessel fitted with four vertical baffles and a Rushton turbine, as described by Bruckard et al.

(18,29).

Accuracy and precision of experimental data required

multiple replicate measurements on both the lab scale and the pilot-plant scale which took several years to complete. In the 2 L reactors, the aeration suspensions contained 250 g of RI in 500 ml of NH4Cl heated to 70 °C. High purity compressed air was bubbled through the suspensions at 2.5 L min−1 throughout the reaction. In the 20 L reactors, 9 kg of RI was added to 18 L of ammonium chloride at room temperature and air was injected at various rates through a small tube below the turbine. The redox catalysts were dissolved in the aeration liquors immediately prior to the start of each test. Samples of solid phases were withdrawn at regular intervals and the

iron(II,

III)

oxide-hydroxides

were

removed

by

washing.

Residues from these samples were then returned to the reactor and make-up water was added to compensate for any losses caused by evaporation and entrainment. Aerations were continued until the metallic iron content of the RI was less than 1 wt% or until it was clear that the reaction had otherwise terminated. At the


15


Page 16 of 51

completion of each test, the total solids were wet-screened to remove the fine iron(II, III) oxide-hydroxides. The remaining +75 μm fraction, predominantly SR, was washed and dried and split into subsamples for analysis. The iron(II, III) oxide-hydroxides were also split for analysis. For the catalyst tests, powdered methyl viologen from Sigma Chemicals (98% pure) was used. Alternatively, a concentrated solution of diquat dibromide (268 g/L) from ICI Australia (now Orica Ltd) was diluted with aeration liquor as required. 4. RESULTS AND DISCUSSION

In what follows, the Becher aeration process is discussed in terms

of

the

ammonium

chloride

concentration,

the

catalyst

concentration, the batch-to-batch recyclability of each catalyst, and the phase composition of the products. The key measure of catalytic activity is the rate of oxidation of metallic iron from reduced ilmenite. This rate is inversely proportional to the reaction time.

4.1. Sample characterisation XRF analyses of the RI starting materials (Table 1) indicate that they

are

typical

of

material

produced

by

Western

Australian

companies and contained 64-70 wt% TiO2 and 24-28 wt% Fe(0). XRD analyses showed that the major phases present were metallic iron, rutile, reduced rutile phases of general formula TiO2−x, and an M3O5


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ferrous-pseudobrookite phase with extensive substitution of Fe by Mg and Mn. Table 1. Sample

Analytical data (wt%) for RI samples. A

B

C

TiO2

70.4

64.4

68.5

Fetotal* Femetallic† SiO2 Al2O3 MgO Mn3O4

26.1 24.0 0.66 0.87 0.22 1.31

26.5 24.1 1.12 0.94 0.52 1.40

28.0 27.8 0.70 1.20 0.20 1.50

* Total iron determined by XRF. † Metallic iron determined by SATMAGAN.

The size distributions of the RI grains were similar to those reported previously(18) with negligible amounts finer than 106 μm or coarser than 300 μm. Backscatter electron micrographs of RI and SR in Figure 6 clearly show the presence of metallic iron (bright spots) in the RI and their absence in SR.


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Figure 6. Backscatter electron micrograph of polished sections of a reduced ilmenite (RI) particle (left) and a synthetic rutile (SR) particle (right). The brightness of each pixel is proportional to the average atomic number, so metallic iron appears white, while titanium dioxide appears grey. Pores in the particles are black.

4.2. Baseline concentration of ammonium chloride A series of preliminary experiments was carried out to decide if the baseline concentration of ammonium chloride (2% w/v) should be changed in the presence of a catalyst. Accordingly, concentrations of 0.5, 1, 2, and 4% w/v NH4Cl were tested in the presence of 0.2%


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Page 19 of 51

w/v catalyst D. The results are shown in Figure 7. It can be seen that the aeration rate increased monotonically with increasing ammonium chloride concentration. Furthermore, the aeration rate in 1.0% w/v NH4Cl with catalyst was faster than the aeration rate in 2% w/v NH4Cl without catalyst. Clearly, less ammonium chloride was needed when a catalyst was present. Nevertheless, to avoid needless complication,

it

was

concentration

of

2%

decided w/v

in

to

retain

order

to

an

ammonium

preserve

the

chloride baseline

comparison with previous work and with industrial best practice.

30 2% AC, no D

25

0.5% AC, 0.2% D Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

1% AC, 0.2% D 2% AC, 0.2% D

15

4% AC, 0.2% D

10 5 0 0

2

4

6

8

10

12

Time/h

Figure

7.

Effect

of

ammonium

chloride

concentration

on

the

concentration

on

the

aeration of RI with 0.2% w/v catalyst D.

4.3. Effect of initial catalyst concentration To

determine

reaction

rate,

the

effects

aerations

of

catalyst

were

carried

out

with

initial


19


concentrations of catalysts V and D of 0.1, 0.2, 0.4 and 0.8% w/v. The results are shown in Figs 8 and 9, respectively. Clearly, increasing

the

concentration

of

both

catalysts

increased

the

aeration rate. With catalyst V, initial concentrations of 0.1, 0.2 and 0.4% w/v decreased the aeration time from 10 h to 5-6 h, while 0.8% w/v decreased the aeration time to 3-4 h. Catalyst D was more effective, with aeration taking only 3 h for additions of 0.2% and 0.4% w/v.

30 no catalyst

25

0.1% V Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 51

20

0.2% V 0.4% V

15

0.8% V

10 5 0 0

2

4

6

8

10

12

Time/h

Figure 8. Effect of initial concentration of catalyst V on the aeration of RI.


20

Page 21 of 51

30 no catalyst

25

0.1% D

Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0.2% D 0.4% D

15

0.8% D

10 5 0 0

2

4

6

8

10

12

Time/h

Figure 9. Effect of initial concentration of catalyst D on the aeration of RI. The data in Figure 10 illustrate the different catalytic powers of catalyst D and catalyst V. It can be seen that an initial concentration of 0.4% w/v of catalyst V gives a slower rate than 0.2% w/v of catalyst D. Likewise, the previous anthraquinone catalyst

AQ-2

(9,10-anthraquinone-2-sulfonic

acid,

monosodium

salt) is also only about half as effective as catalyst D.


21


30 no catalyst

25

Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 51

0.4% V

20

0.2% D 15

0.2% AQ-2

10

0.4% AQ-2

5 0 0

2

4

6

8

10

12

Time/h

Figure 10. Comparison of the catalytic powers of V, D and AQ-2.

4.4. Recycle of catalysts V and D Previous

work(29)

had

revealed

that

anthraquinone

sulfonate

molecules were gradually being depleted from the bulk of solution during the Becher aeration process; and that when the liquor was filtered at the end of a single run it had lost its catalytic power. The explanation of these effects was that the anthraquinone sulfonate molecules were adsorbing on the magnetite surfaces. This was due to their strong coulombic attraction: the anthraquinone sulfonate molecules were negatively charged in solution, while the magnetite surfaces were positively charged. Accordingly, it was logical to screen for catalysts that were positively charged in solution, so they would be repelled from the magnetite surfaces under the same conditions. This led to the discovery of V and D.


22

Page 23 of 51

To confirm that catalysts V and D were not adsorbed, they were added at the 0.4% w/v level to the start of a first aeration, and at the end of that process the remnant liquor was filtered and recycled for a second aeration. This was repeated a further two times to give a total of four aerations. The resulting iron dissolution rates are shown in Figures 11 and 12, and the measured concentrations of catalysts are given in Table 2. 30 no catalyst

25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

1st recycle 2nd recycle

15

3rd recycle 10 5 0 0

2

4

6 Time/h

8

10

12

Figure 11. Recycle aerations with an initial concentration of 0.4 w/v catalyst V.


23


30 no catalyst

25

0.4% D Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 51

20


15

3rd recycle 10

5 0 0

2

4

6 Time/h

8

10

12

Figure 12. Recycle aerations with an initial concentration of 0.4 w/v catalyst D.

Table 2.

Catalyst concentration

in the

liquor during recycle

tests*. Aeration

Catalyst conc. (% w/v) Initial

Final

0.40 0.30 0.19 0.09

0.30 0.19 0.09 0.01

Aeration time (h)

Catalyst V Initial Recycle 1 Recycle 2 Recycle 3

5 6 8 >10

Catalyst D Initial 0.40 Recycle 1 0.20 Recycle 2 0.11 Recycle 3 0.04 * All aerations were in a

0. 20 3 0.11 4 0.04 4 0.003 >8 2 L reactor with 2 w/v% NH4Cl initially.


24

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In both cases the catalysts exerted a persistent effect over three successive aerations, only failing on the fourth aeration (third recycle). For catalyst V, the aeration rate gradually diminished over the four aerations, whereas for catalyst D, the aeration rate remained consistently high over three batches, and then

suddenly

failed

on

the

fourth

batch

(third

recycle).

Regardless of the detailed explanation of these data, the overall results

confirm

successfully

that

recycled

the

cationic

from

batch

catalysts

V

to batch. The

and

D

cause

can

be

of the

eventual fall-off remains unsolved, but is most likely due to the gradual loss of filtrate volume (physical entrainment of solution) during multiple sampling events on the small laboratory scale. On the large industrial scale this would be less significant.

4.5. Composition of the iron(II, III) oxide-hydroxides In commercial operations the phase compositions of the iron(II, III) oxide-hydroxides formed during Becher aeration are important because they impact the settling, filtering, and disposal of the iron-containing waste. In current plant practice, mixtures of iron(II,

III)

oxide-hydroxides

such

as

hematite

(α-Fe2O3),

lepidocrocite (γ-FeOOH), maghemite (γ-Fe2O3), goethite (α-FeOOH) and

magnetite

(Fe3O4),

are

formed

in

varying

proportions

as

sludges. Many of these phases are slow to settle, difficult to pump, and generally costly to de-water and transport.


25


Page 26 of 51

Magnetite is the industry’s preferred product because of its superior settling properties. Remarkably, the use of aeration catalysts V and D guarantees the formation of this oxide. XRD analyses confirm that in

all tests conducted

with the redox

catalysts in 2% w/v NH4Cl, magnetite (Fe3O4) is the only oxide detected in the final product. By contrast, the iron(II, III) oxide-hydroxides produced during catalyst-free aerations in the laboratory are mainly mixtures of lepidocrocite and magnetite, with magnetite as the minor phase. Analytical data presented in Table 3 show that the addition of catalysts produced no detrimental effects on the SR, with grades similar to those achieved in standard practice. It should also be noted that these SR samples were not given an acid wash to remove remnant

iron,

so

even

higher-grade

products

are

potentially

achievable. Table 3. Catalyst

Analytical data (wt%) for SR produced from standard RI. No catalyst

V (0.4 w/v)

D (0.2 w/v)

TiO2 91.7 90.7 Fe2O3 4.65 4.77 SiO2 0.95 0.81 Al2O3 1.12 1.09 MgO 0.30 0.24 Mn3O4 1.61 1.66 * All aerations were in a 2

D (0.4 w/v)

92.5 92.2 3.10 3.31 0.81 1.01 1.07 1.21 0.29 0.35 1.61 1.63 L reactor with 2 w/v% NH4Cl.


26

Page 27 of 51

4.6. Hard-to-aerate specimens One specimen of RI was classified by the industry as ‘hard to aerate’, and this challenging material was also investigated. Our laboratory tests confirmed the factory diagnosis. The rate of metallic iron removal was indeed very slow in the absence of catalyst, even in 4% w/v NH4Cl (Figure 13). The results were consistent with the iron particles remaining passive inside the ilmenite grains (so-called ‘in-situ rusting’). However, with the addition of catalyst D, the situation improved dramatically. With 4% w/v NH4Cl and 0.8% w/v catalyst D, the rate of iron removal became

comparable

with

that

achieved

by

standard

run-of-mine

material. Such encouraging results provide hope that, in future, redox catalysts may enable the efficient processing of ‘hard to aerate’ RI. Analytical data for some hard-to-aerate specimens of RI are given in Table 4. 30 4% AC, no catalyst

25

Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2% AC, 0.2% D 2% AC, 0.4% D

20

4% AC, 0.2% D 4% AC, 0.4% D

15

4% AC, 0.8% D

10 5 0 0

1

2

3

4

5

6

7

8

9

Time/h


27


Page 28 of 51

Figure 13. Aeration of a ‘hard to aerate’ RI sample.

Table 4.

Analytical data (wt%) for SR produced from hard-to-

aerate RI. Ammonium chloride (2 w/v %) Catalyst

D (0.2 w/v)

D (0.4 w/v)

TiO2 84.9 Femetallic† 2.8 Fe2O3 9.14 SiO2 1.26 Al2O3 1.13 MgO 0.43 Mn3O4 1.40 *All aerations were

Ammonium chloride (4 w/v %) D (0.2 w/v)

D (0.4 w/v)

81.6 86.0 3.2 1.7 11.3 7.70 1.10 1.06 1.01 1.07 0.36 0.34 1.22 1.17 in a 2 L reactor.

86.1 1.6 7.0 1.17 1.09 0.36 1.17

† Metallic iron determined by SATMAGAN.

4.7. Performance of catalyst D in a pilot scale reactor In view of the successful results obtained with catalyst D in the 2 L laboratory reactors, additional work was carried out in a 20 L pilot scale reactor with a charge of 18 L of ammonium chloride and 9 kg of RI. The construction of this reactor was designed to simulate the performance of 80 m3 plant reactors. To ensure in-


28

Page 29 of 51

situ rusting was not a factor on this scale, a 5% w/v NH4Cl solution was used throughout. 30 no catalyst, low air

25

no catalyst, high air Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0.2% D, low air

15

0.2% D, high air

10 5 0 0

2

4

6 Time/h

8

10

12

Figure 14. Aerations in 5% w/v NH4Cl with 0.2% w/v catalyst D in the 20 L pilot scale reactor. On the pilot scale the air flow rate became a significant parameter. Some results for two different air flow rates are shown in Figure 14. The lower air flow rate was 9 L/min and the higher air flow rate was 16 L/min. The fact that the iron dissolution rate could be speeded up by increasing the air flow rate (both in the presence and absence of catalyst) was clear proof that the pilot

scale

reactor

was

not

oxygen-saturated.

But

the

iron

dissolution kinetics were not fully diffusion controlled, either. We know this because fully diffusion-controlled reactions cannot be catalysed, and clearly some catalysis was occurring. It follows that, on the 20 L pilot scale, the reaction kinetics were under mixed interfacial and diffusional control.


29


Page 30 of 51

Similar considerations apply to full-size commercial reactors. On the 80 m3 industrial scale, present-day air flow rates are only just adequate to keep pace with the reaction rates. If, in future, the reaction rates are catalyzed by factors of two or three, then the air flow rates will need to be increased proportionately.

5. Conclusions Two heterocyclic compounds containing quaternary nitrogen atoms have

been

identified

as

redox

catalysts

for

the

oxidative

dissolution of metallic iron from reduced ilmenite. They are methyl viologen and diquat, referred to as catalysts V and D. The crucial feature of these catalysts is their ability to form free-radical intermediates that can react rapidly with molecular oxygen. In all cases where V and D are used, the dissolved iron reports as magnetite (Fe3O4). This environmentally friendly product is easier to separate and faster to dewater than traditional mixtures of iron(II, III) oxide-hydroxides. Remarkably, catalysts V and D can be recycled from batch-tobatch.

Being

positively

charged,

they

are

electrostatically

repelled from the surface of the precipitating magnetite. This stands

in

anthraquinone therefore

sharp

contrast

sulfonates,

electrostatically

to

which

the are

attracted

previously-discovered

negatively to

the

charged

surface

of

and the

magnetite.


30

Page 31 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimentally, it is found that the dissolution rate of metallic iron increases monotonically with catalyst concentration (Figures 8 and 9). In small reactors this presents no problems, because there is always sufficient flow of air to maintain the higher rate. But in large reactors, there is a problem. There is presently not enough air supply to maintain the catalyzed rates, so the overall reaction falls under mixed interfacial and diffusional control. Extra air will therefore need to be supplied to take full advantage of the catalysts in commercial operations. Like any new technology, the use of redox catalysts involves some level of increased technological complexity and some element of ecological risk. As noted above, higher flow rates of air will certainly

need

to

be

retro-fitted

to

existing

aerators,

and

environmental concerns will compel a closed-loop control of the catalyst flow. Finally, we draw attention to the fact that activated carbons containing quaternary nitrogen atoms have recently been reported to be catalysts for the oxygen reduction reaction (ORR) in fuel cells(30). However, the mechanism is a mystery. Given the results of the present work, it seems likely that semiquinoid moieties (similar to those shown in Figure 4) exist on the surfaces of Ndoped carbon, which act as loci for spin-allowed oxygen reduction. Acknowledgements


31


Page 32 of 51

The authors gratefully acknowledge the staff of the Clayton Chemical Analysis Unit of CSIRO Mineral Resources for chemical analyses, and the late Mr James Thorpe Woodcock for valuable discussions. Iluka Resources Ltd and Tronox Management Pty Ltd are also thanked for providing the reduced ilmenite samples, for assistance

with

the

pilot

scale

reactor,

and

for

granting

permission to publish. Competing Interests

The authors declare no competing interests.


32

Page 33 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References

(1)

U.S. Geol. Surv. Mineral Commodity Summaries, Titanium Mineral Concentrates. January 2017 (https://minerals.usgs.gov/minerals/pubs/commodity/titaniu m/mcs-2017-timin.pdf)

(2)

Hernberg, S.; Lead poisoning in a historical perspective. Am. J. Ind. Med. 2000, 38, 244.

(3)

Becher, R.G. Improved process for the beneficiation of ores containing contaminating iron. Australian Patent 247110, 16th September, 1963.

(4)

Becher, R.G.; Canning, R.G.; Goodheart, B.A.; Uusna, S. New process for upgrading ilmenitic sands. Proc. Australasian Inst. Min. Metall. 1965, 214, 21.

(5)

Bracanin, B.F.; Cassidy, P.W.; MacKay, J.M.; Hocking, H.W. The development of a direct reduction and leach process for ilmenite upgrading. Light Metals 1972; Rotsell, W.C. (Ed.). Metallurgical Society of AIME: New York, 1972, p. 209.

(6)

Lakshmanan, V.I.; Bhowmick, A.; Halim, Md. A. Titanium dioxide: Production, properties, and applications. Chem. Phys. Res. J. 2014, 7, 37.


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Ward, C.B.; Gibbons, S.L.; Ritchie, I.M.; Muir, D.M. Transformations of iron oxide by-products during the Becher process. Proc. Australas. Inst. Min. Metall. 1990, 295, 53.

(8)

Farrow, J.B.; Ritchie, I.M.; Mangano, P. The reaction between reduced ilmenite and oxygen in ammonium chloride solutions. Hydrometallurgy. 1987, 18, 21.

(9)

Ward, C.B.; Muir, D.M.; Ritchie, I.M. Recovery of byproduct iron oxides from the upgrading of ilmenite to synthetic rutile. Extraction Metallurgy ’89; The Institution of Mining and Metallurgy: London, 1989, p. 1019.

(10

Hollitt, M.J.; McClelland, R.A.; Liddy, M.J. Wimmera

)

titanium minerals developments – earth, wind, fire and water. Mervyn Willis Symposium and Smelting & Refining Course; Nilmani, M., Rankin, W.J. (Eds.), University of Melbourne: Australia, 1992, p. 19:1.

(11

Ward, J.; Bailey, S.; Avraamides, J. Aeration catalysts

)

for upgrading reduced ilmenite to synthetic rutile. Diversity – The Key to Prosperity. The AusIMM 1996 Annual Conference. The Australasian Institute of Mining and Metallurgy: Melbourne, 1996, p. 231.


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(12

Nguyen, T.T.; Truong, T.N.; Duong, B.N. 2016. Impact of

)

organic acid additions on the formation of precipitated iron compounds. Acta Metall. Slovaca. 2017, 22, 259.

(13

Truong, T.N.; Nguyen, T.T.; Duong, B.N. Acetic acid and

)

sodium acetate mixtures as an aeration catalyst in the removal of metallic iron in reduced ilmenite. Acta Metall. Slovaca. 2017, 23, 371.

(14

Ward, J.; Bailey, S.; Avraamides, J. The use of

)

ethylenediammonium chloride as an aeration catalyst in the removal of metallic iron from reduced ilmenite. Hydrometallurgy, 1999, 53, 215.

(15

Kumari, E.J.; Koshy, P.; Mohan Das, P.N. Investigations on

)

the effect of certain carbonyl compounds on the removal of iron during the rusting of reduced ilmenite. Trans. Indian Inst. Met. 2000, 3, 573.

(16

Kumari, E.J.; Bhat, K.H.; Sasibhushanan, S.; Mohan Das,

)

P.N. Catalytic removal of iron from reduced ilmenite. Miner. Eng. 2001, 14, 365.

(17

Geetha, K.S.; Surender, G.D. Intensification of iron

)

removal rate during oxygen leaching through gas-liquid mass transfer enhancement. Metall. Mater. Trans. B. 2001, 32B, 961.


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Bruckard, W.J.; Calle, C.; Fletcher, S.; Horne, M.D.;

)

Sparrow, G.J.; Urban, A.J. The application of

Page 36 of 51

anthraquinone redox catalysts for accelerating the aeration step in the becher process. Hydrometallurgy. 2004, 73, 111. (19

Kaim, W.; Schwederski, B. Bioinorganic Chemistry:

)

Inorganic Elements in the Chemistry of Life. John Wiley & Sons Ltd, Chichester, UK, 1994, p 84.

(20

Song, Y.; Buettner, G.R. Thermodynamic and kinetic

)

considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide. Free Radical Biol. Med. 2010, 49, 919.

(21

Bielski, B.H.J.; Arudi, R.L. Preparation and stabilization

)

of aqueous/ethanolic superoxide solutions. Anal. biochem. 1983, 133, 170.

(22

Fletcher, S., Horne, M.D. 1996. Treatment of titanium-

)

containing material.

Australian Patent 717006, 15th

August, 1996. (23

Tombácz, E.; Majzik, A.; Horvát, Z.S.; Illés, E. Magnetite

)

in aqueous medium: coating its surface and surface coated with it. Rom. Rep. Phys. 2006, 58, 281.


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(24

Armstrong, D.A.; Huie, R.E.; Koppenol, W.H.; Lymar, S.V.;

)

Merényi, G.; Neta, P.; Ruscic, B.; Stanbury, D.M.; Steenken, S.; Wardman, P. Standard electrode potentials involving radicals in aqueous solution: inorganic radicals (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1139.

(25

Michaelis, L.; Hill, E.S. The viologen indicators. J. Gen.

)

Physiol. 1933, 16, 859.

(26

Summers, L.A., The Bipyridinium Herbicides; Academic

)

Press: London, 1980.

(27

Schröder, U. Anodic electron transfer mechanisms in

)

microbial fuel cells and their energy efficiency. Phys. Chem. Chem. Phys. 2007, 9, 2619.

(28

Weber, J.B.; Perry, P.W.; Upchurch, R.P. The influence of

)

temperature and time on the adsorption of paraquat, diquat, 2,4-D and prometone by clays, charcoal, and an ion-exchange resin. Soil Sci. Soc. Am. Pro. 1965, 29, 678.

(29

Bruckard, W.J.; Calle, C.; Constanti-Carey, K.; Fletcher,

)

S.; Horne, M.D.; Ruzbacky, R.; Sparrow, G.J. Redox catalysts for Becher aeration. Heavy Minerals Conference 2003; The South African Institute of Mining and Metallurgy: Johannesburg, 2003, p. 33.


37


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Ferrero, G.A.; Fuertes, A.B.; Sevilla, M.; Titirici, M.M.

)

Efficient metal-free N-doped mesoporous carbon catalysts

Page 38 of 51

for ORR by a template-free approach. Carbon. 2016, 106, 179.


38

Page 39 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure.

1.

sulfonate.

Catalysis Although

List of Figures and Tables

of

the

iron

dissolution

catalyst

is

by

anthraquinone

thermodynamically

less

powerful than oxygen, it reacts faster with iron. Figure 2. Molecular structure of methyl viologen, catalyst V. Figure 3. Molecular structure of diquat, catalyst D. Figure 4. One and two electron transfer reactions for catalyst V. Figure 5. One and two electron transfer reactions for catalyst D. Figure 6. Backscatter electron micrograph of polished sections of a reduced ilmenite (RI) particle (left) and a synthetic rutile (SR) particle (right). The brightness of each pixel is proportional to the average atomic number, so metallic iron appears white, while titanium dioxide appears grey. Pores in the particles are black. Figure 7. Effect of ammonium chloride concentration on the aeration of RI with 0.2% w/v catalyst D. Figure 8. Effect of initial concentration of catalyst V on the aeration of RI. Figure 9. Effect of initial concentration of catalyst D on the aeration of RI. Figure 10. Comparison of the catalytic powers of V, D and AQ-2.


39


Page 40 of 51

Figure 11. Recycle aerations with an initial concentration of 0.4 w/v catalyst V. Figure 12. Recycle aerations with an initial concentration of 0.4 w/v catalyst D. Figure 13. Aeration of a ‘hard to aerate’ RI sample. Figure 14. Aerations in 5% w/v NH4Cl with 0.2% w/v catalyst D in the 20 L pilot scale reactor. Table 1. Analytical data (wt%) for various RI samples. Table 2. Catalyst concentration in the liquor during recycle tests. Table 3. Analytical data (wt%) for SR produced from standard RI. Table 4. Analytical data (wt%) for SR produced from hard-to-aerate RI.


40

Page 41 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure

1.

sulfonate.

Catalysis Although

of the

iron

dissolution

catalyst

is

by

anthraquinone

thermodynamically

less

powerful than oxygen, it reacts much faster with iron.

Figure 2. Molecular structure of methyl viologen, catalyst V.


41


Page 42 of 51

Figure 3. Molecular structure of diquat, catalyst D.

Figure 4. One and two electron transfer reactions for catalyst V.

Figure 5. One and two electron transfer reactions for catalyst D.


42

Page 43 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



43


Page 44 of 51

Figure 6. Backscatter electron micrograph of polished sections of a reduced ilmenite (RI) particle (left) and a synthetic rutile (SR) particle (right). The brightness of each pixel is proportional to the average atomic number, so metallic iron appears white, while titanium dioxide appears grey. Pores in the particles are black.


44

Page 45 of 51

30 2% AC, no D

25

Metallic iron/wt%

0.5% AC, 0.2% D 20

1% AC, 0.2% D 2% AC, 0.2% D

15

4% AC, 0.2% D

10 5 0 0

2

4

6

8

10

12

Time/h

Figure 7. Effect of ammonium chloride concentration on the aeration of RI with 0.2% w/v catalyst D.

30 no catalyst

25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0.2% V 0.4% V

15

0.8% V

10 5 0 0

2

4

6

8

10

12

Time/h

Figure 8. Effect of initial concentration of catalyst V on the aeration of RI.


45


30 no catalyst

25

Metallic iron/wt%

0.1% D 20

0.2% D 0.4% D

15

0.8% D

10 5 0 0

2

4

6

8

10

12

Time/h

Figure 9. Effect of initial concentration of catalyst D on the aeration of RI.

30 no catalyst

25

Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 51

0.4% V

20

0.2% D 15

0.2% AQ-2

10

0.4% AQ-2

5 0 0

2

4

6

8

10

12

Time/h

Figure 10. Comparison of the catalytic powers of V, D and AQ-2.


46

Page 47 of 51

30 no catalyst

25

Metallic iron/wt%

0.4% V 20


15

3rd recycle 10 5 0 0

2

4

6 Time/h

8

10

12

Figure 11. Recycle aerations with an initial concentration of 0.4 w/v catalyst V.

30 no catalyst

25

0.4% D Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20


15

3rd recycle 10

5 0 0

2

4

6 Time/h

8

10

12

Figure 12. Recycle aerations with an initial concentration of 0.4 w/v catalyst D.


47


30 4% AC, no catalyst

Metallic iron/wt%

25

2% AC, 0.2% D 2% AC, 0.4% D

20

4% AC, 0.2% D 4% AC, 0.4% D

15

4% AC, 0.8% D

10 5 0 0

1

2

3

4

5

6

7

8

9

Time/h

Figure 13. Aeration of a ‘hard to aerate’ RI sample.

30 no catalyst, low air

25

no catalyst, high air Metallic iron/wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 51

20

0.2% D, low air

15

0.2% D, high air

10 5 0 0

2

4

6 Time/h

8

10

12

Figure 14. Aerations in 5% w/v NH4Cl with 0.2% w/v catalyst D in the 20 L pilot scale reactor.


48

Page 49 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical Abstract (Un-numbered)

Table 1. Sample

Analytical data (wt%) for RI samples. A

B

C

TiO2

70.4

64.4

68.5

Fetotal* Femetallic† SiO2 Al2O3 MgO Mn3O4

26.1 24.0 0.66 0.87 0.22 1.31

26.5 24.1 1.12 0.94 0.52 1.40

28.0 27.8 0.70 1.20 0.20 1.50

* Total iron determined by XRF. † Metallic iron determined by SATMAGAN.


49


Table 2.

Catalyst concentration

in the

Page 50 of 51

liquor during recycle

tests*. Aeration

Catalyst conc. (% w/v) Initial

Final

0.40 0.30 0.19 0.09

0.30 0.19 0.09 0.01

Aeration time (h)

Catalyst V Initial Recycle 1 Recycle 2 Recycle 3

5 6 8 >10

Catalyst D Initial 0.40 Recycle 1 0.20 Recycle 2 0.11 Recycle 3 0.04 * All aerations were in a

Table 3. Catalyst

0. 20 3 0.11 4 0.04 4 0.003 >8 2 L reactor with 2 w/v% NH4Cl initially.

Analytical data (wt%) for SR produced from standard RI. No catalyst

V (0.4 w/v)

D (0.2 w/v)

TiO2 91.7 90.7 Fe2O3 4.65 4.77 SiO2 0.95 0.81 Al2O3 1.12 1.09 MgO 0.30 0.24 Mn3O4 1.61 1.66 * All aerations were in a 2

D (0.4 w/v)

92.5 92.2 3.10 3.31 0.81 1.01 1.07 1.21 0.29 0.35 1.61 1.63 L reactor with 2 w/v% NH4Cl.


50

Page 51 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4.

Analytical data (wt%) for SR produced from hard-to-

aerate RI. Ammonium chloride (2 w/v %) Catalyst

D (0.2 w/v)

D (0.4 w/v)

TiO2 84.9 † Femetallic 2.8 Fe2O3 9.14 SiO2 1.26 Al2O3 1.13 MgO 0.43 Mn3O4 1.40 *All aerations were

Ammonium chloride (4 w/v %) D (0.2 w/v)

D (0.4 w/v)

81.6 86.0 3.2 1.7 11.3 7.70 1.10 1.06 1.01 1.07 0.36 0.34 1.22 1.17 in a 2 L reactor.

86.1 1.6 7.0 1.17 1.09 0.36 1.17

† Metallic iron determined by SATMAGAN.


51

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