Hydrometallurgy, 18 (1987) 21-38
21
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
The R e a c t i o n b e t w e e n R e d u c e d I l m e n i t e and
O x y g e n in A m m o n i u m Chloride S o l u t i o n s
J.B. FARROW*, I.M. RITCHIE**
Department of Physical and Inorganic Chemistry, University of Western Australia, Nedlands,
W.A. (Australia)
and P. MANGANO***
Associated Minerals Consolidated, Capel, W.A. (Australia)
(Received April 23, 1986; accepted in revised form September 30, 1986)
ABSTRACT
Farrow, J.B., Ritchie, I.M. and Mangano, P., 1987. The reaction between reduced ilmenite and
oxygen in ammonium chloride solutions. HydrometaUurgy, 18: 21-38.
A mechanistic study of the aeration reaction used for removing iron from a reduced ilmenite
matrix is described. Using polarization and mixed potential measurements, it was shown that the
rate of the aeration reaction in ammonium chloride solution is largely determined by the speed at
which oxygen diffuses to the reduced ilmenite surface, and that, within the limit of experimental
error, the reaction rate was independent of the Ti203 content in the reduced ilmenite. Diffusion
control was confirmed by rate measurements. Aeration rate measurements were also carried out
in other electrolytes in addition to ammonium chloride. It was concluded from these and other
studies that the ammonium chloride plays three distinct and important roles in the aeration reaction. Firstly, the ammonium ion acts as a buffer for hydroxyl ions and prevents excessively high
local pH values, which might otherwise cause precipitation of iron (II) hydroxide before the iron (II)
ions could diffuse from the rutile matrix. Secondly, the ammonia formed as a result of this buffering reaction complexes iron (II) ions until they also have moved away from the regions of high
pH, thus preventing the precipitation of iron (II) hydroxide in the pores of the rutile. Finally, the
chloride ion helps to break down any passive films which might form during aeration.
INTRODUCTION
In 1961, a two-step procedure was developed by the Western Australian Government Chemical Laboratories for upgrading the TiO2 content of ilmenite
* Present address: CSIRO Division of Minerals and Geochemistry, WAIT, Bentley, W.A.,
Australia.
** Present address: School of Mathematical and Physical Sciences, Murdoch University, Murdoch, W.A., Australia.
*** Present address: Electrolytic Zinc Company, Risdon, Tasmania, Australia.
0304-386X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
22
FEED
REDUCED
SYNTHETIC
II_MENITE
RUTILE
!EDUCTi
ERAT
O o!
FE ( METAL
)-~--~---"~~
02
ILMENITE J
Fig. 1. Schematic diagram showing the two-step upgrading process for converting a single grain
of ilmenite to reduced ilmenite and then synthetic rutile.
from mineral sands from about 60% to about 92.5% (Becher, 1963; Becher et
al., 1965 ). This process is now operated commercially (Bracanin et al., 1972).
The iron (II) and iron (III) content of the ilmenite is first reduced to metallic
iron in a rotary kiln at about 1150 °C using a bituminous coal as both reductant
and fuel for the kiln:
Ilmenite + C --, Fe + TiO2
(1)
The material resulting from this step, termed reduced ilmenite, consists of a
matrix of rutile honeycombed with metallic iron. Some reduction of the T i Q
to various reduced species also occurs (C)stberg, 1960; Jones, 1973; Grey and
Reid, 1974). In the second step, the metallic iron is removed from the reduced
ilmenite by an accelerated corrosion reaction using oxygen dissolved in an
ammonium chloride solution. The product is termed synthetic rutile, which is
suitable for use as a feedstock in the chloride route for the production of T i Q
pigment. The two-step process is schematically illustrated in Fig. 1 for a single
grain of ilmenite.
Since the original description of the upgrading process (Becher, 1963), a
number of papers have been published relating to various chemical aspects of
the reduction of ilmenite (Yamaguchi et al., 1966; Levitskii et al., 1969; EIGuindy and Davenport, 1970; Jones, 1973, Grey et al., 1973, 1974; Grey and
Reid, 1974). There have also been several studies suggesting that pre-oxidation of the ilmenite improves its subsequent reduction (Becher et al., 1965;
Khairy et al., 1966; Hussein et al., 1967; Fetisov et al., 1968; Bracanin et al.,
1972; Sinha, 1972 ), although Donnelly et al. (1970) reported that there is little
23
difference in reduction rate between natural and pre-oxidized Western Australian ilmenites.
However, up to the present time, there has been little discussion of the
mechanics of the second stage of the upgrading process known as the aeration
reaction. When this reaction is carried out industrially on a batch basis, air is
forced through a pulp consisting of 20 t of reduced ilmenite and 40 m 3 of 0.1 M
NH4C1. Heat released in the exothermic corrosion reaction results in the temperature of the system rising to about 65-75 ° C. The oxidation of the iron metal
in the reduced ilmenite is usually completed in 14-16 hours (Bracanin et al.,
1972). A variety of hydrated iron (III) oxides are formed as a by-product of
the reaction, and are removed by means of a cyclone. In this paper, the results
of a mechanistic study of the aeration reaction will be presented.
EXPERIMENTAL
1. Chemicals
All samples of reduced ilmenite used in this work were provided by Associated Minerals Consolidated Ltd. The grains, which tended to have a flattened
ovoid shape, were typically about 0.2 mm along the major axis. The surface,
which was porous like pumice, was often covered with iron globules which
seemed to have been extruded from the pores (see, for example, the micrographs of Jones and Stephens, 1973 ). A typical assay for the reduced ilmenite
was 67.3% as TiO2 and 26.2% as Fe (metal) with silicon, aluminium and manganese as major impurities.
Other chemicals were of analytical reagent grade quality. Argon and nitrogen were supplied by Commonwealth Industrial Gases.
2. Electrical equipment
The electrochemical cells, rotating discs and ancillary equipment used in
this study have been described previously by Power et al. (1981). The iron
electrode was made from some 99.99% pure iron rod supplied by Koch-Light.
The reduced ilmenite-nujol oil paste electrode was prepared by mixing the
solid with the minimum quantity of nujol oil (spectroscopic grade) to form a
coherent paste, which was pressed into the cavity of the specially designed
electrode. The polarization curves were measured potentiodynamically using
a Princeton Applied Research potentiostat (173) and a programmer (175) in
conjunction with a Bryans 26,000 A3 X-Y chart recorder. Mixed potential
measurements were made using a high impedance voltmeter. All potential
measurements are reported relative to the saturated calomel electrode ( SCE ).
24
3. Reaction apparatus
Various types of laboratory aerator were tested in preliminary work. It was
found that an inverted-cone reactor, in which air was blown into the system
via a one-way valve at the apex, gave very good turbulent mixing and reproducible kinetics. This procedure also ensured that a high level of dissolved
oxygen was maintained in the reaction solution. The vertical height of the
cone, which was made of glass, was 22 cm and the diameter of the base 10 cm.
These dimensions ensured that the sides were sufficiently steep for there to be
no quiet spots in which the reduced ilmenite could become stagnant. The base
of the cone was flanged in order to carry a Quickfit reaction vessel top
( MAF3/52 ) through which a combination glass electrode, a heater and a sampling pipette could be placed in the reaction solution. The whole of the invertedcone reactor was lagged to minimize heat loss.
In a typical experiment, 30 g of reduced ilmenite were added to 500 ml of the
reaction solution which had been preheated to the desired temperature; air was
bubbled through the solution at up to 101 m i n - 1. At appropriate time intervals,
approximately 0.2 g samples of solid were removed. The total iron in the reduced
ilmenite samples taken from the reaction was determined by atomic absorption spectrophotometry. The solution for analysis was prepared by washing
the solid samples to remove loosely adhering surface oxides, drying, weighing
and leaching for 16 hours in a known volume of 10% HC1.
4. Solubility of iron( II) hydroxide
In order to determine the solubility ofFe ( OH ) 2 in ammonium chloride solutions, it was necessary to prepare some iron (III) -free Fe (OH) 2. The first step
in this preparation was to treat a solution which contained 0.1 M NH4C1 and
0.5 M HCI with excess iron powder (Fluka, purum p.a.) under strictly anaerobic conditions (nitrogen atmosphere). Once the pH (measured in situ with
a combination glass electrode) and the temperature of the mixture (thermostatted at 25.0+0.1°C) had stabilized, fresh solid NaOH was added under
nitrogen to precipitate Fe (OH) 2- When the pH of the solution had again stabilized, indicating that precipitation of Fe (OH)2 had ended, its value was
recorded, and the suspension was stirred for a further four hours before being
filtered. The solubility of the iron (II) hydroxide at the measured pH was then
determined by analysis of the filtrate using atomic absorption spectrophotometry. To prevent precipitation of iron hydroxides in the filtrate prior to analysis, the solution was made strongly acidic with HC1.
5. Analysis
All iron analyses were carried out using a Varian 175 Atomic Absorption
Spectrophotometer.
25
I I
1
T
1
T
r
T - - I - ~
15
FE 2+ + 2E
10
c~
EM
J
-5
.I.
-0,8
1
1
-0,4
0.5
-0,2
E/V (SCE)
Fig. 2. Evans diagram for the oxidation of iron and the reduction of air on a copper electrode. In
each case, the disc rotation speed was 400 min ', the background electrolyte 0.1 M NH4CI and
the temperature 25 ° C.
The Ti203 content in the reduced ilmenite samples was determined by a wet
chemical technique involving dissolution of the sample in g 2 s o 4 followed by
the quantitative addition of Fe 3+ to oxidize the Ti 3+ to Ti 4÷. The resulting
Fe 2+ was determined by titration with standard dichromate. The results were
corrected for iron in the samples.
X-ray analyses were carried out using a Philips P W 1050 diffractometer with
an A M R curved-crystal monochromator.
R E S U L T S AND D I S C U S S I O N
1. The rate-determining step
(a) Polarization and mixed potential measurements
The reaction between a solid sample of iron and an air-saturated solution of
ammonium chloride was first studied, because this system is a good model for
the aeration reaction. The iron-oxygen reaction, being a redox process, can be
investigated by electrochemical techniques. Information about the reaction
mechanism can be obtained by constructing an Evans diagram in which the
polarization curves for the two half-reactions (the oxidation of iron and the
reduction of oxygen) are plotted on the same current-potential plot (Evans,
1960). The Evans diagram for the oxidation of a rotating iron disc in air-saturated 0.1 M NH4C1 at 25 °C is shown in Fig. 2. The iron dissolution curve,
26
which was not easy to reproduce experimentally, shows active dissolution of
iron over the potential range investigated and no effect of rotation speed. With
a freshly polished iron disc electrode, the dissolution reaction was relatively
slow, but the dissolution rate increased as successive potential sweeps were
carried out, presumably because the surface area was becoming pitted and the
active area increased. The oxygen reduction curve, measured on a copper electrode, is very irreversible, and only reaches a limiting current, which is rotation
speed dependent, at potentials more negative t h a n - 0.6 V. Copper was chosen
for these measurements since it is essentially inert over the potential range of
interest. From the point of view of the reaction mechanism, the most important feature of Fig. 2 is that the two polarization curves attain the same current
magnitude in the region in which the oxygen reduction current is near the
limiting current. This implies that the reaction rate, at least initially, will be
largely controlled by oxygen diffusion to the iron surface. Thus the use of a
copper electrode to measure the cathodic curve is justified since the limiting
current region is largely independent of electrode material.
To test the hypothesis of diffusion control further, the potential of an iron
disc electrode, reacting with oxygen in 0.1 M NH4C1, was measured as a function of the disc rotation speed. According to Power and Ritchie (1981), this
potential, known as the mixed or corrosion potential, will increase with
increasing rotation speed if the reaction is under diffusion control. It was found
that the mixed potential measured at constant rotation speed drifted progressively, at first rapidly and then more slowly, to the more negative values. The
drift is presumably a consequence of the increase in rate of the iron dissolution
half-reaction. The change in mixed potential had largely ceased after half an
hour, and the measurements shown in Fig. 3 were made after this time. The
diagram does show that the mixed potential does increase with increasing rotation speed, as expected for a diffusion-controlled reaction.
Although the above experiments do show that the reaction between bulk iron
metal and oxygen in ammonium chloride solution is controlled by the oxygen
transport step, it is still necessary to show that the reaction involving iron in
reduced ilmenite is diffusion controlled. Unfortunately, an Evans diagram cannot be constructed for the reaction involving reduced ilmenite, since the anodic
and cathodic areas are unknown. However, information about the reaction
mechanism can be obtained from mixed potential measurements using a
reduced ilmenite-nujol oil paste electrode. As before, the potential of the paste
electrode is measured as a function of disc rotation speed in an air-saturated
0.1 M NH4C1 solution at 25 ° C. The mixed potential of the paste electrode, like
that of the iron disc electrode, was found to drift extensively, and measurements were only made after it had reached some sort of equilibrium value. The
results are shown in Fig. 3. It can be seen that the mixed potential of the reduced
ilmenite electrode has much the same sort of rotation speed dependence as the
iron electrode over the range of rotation speeds investigated. This indicates
27
I
I
I
-0.60
0.61
-0.62
-0,63
-0,54
I
1
I
2.0
2.5
3.0
3,5
LOG (W/RPM)
Fig. 3. Plot of mixed potential as a function of rotation speed for the oxidation of iron ( • ) and
the oxidation of reduced ilmenite ( • ) in air-saturated 0.1 M NH4CI at 25 ° C. Precision of potential measurements _+2 mV.
that the aeration reaction is also diffusion controlled. However, in view of the
drift in potential measurements with time, the close agreement between the
two sets of results shown in Fig. 3 must be regarded as fortuitous.
(b ) Possible rutile involvement in oxygen reduction
A measurable difference between the results shown in Fig. 3 might have been
expected had the rutile, as well as the iron of the reduced ilmenite, provided
sites for the reduction of oxygen. In this case, the cathodic area would have
exceeded the anodic area, and the mixed potential at any given rotation speed
would have been somewhat larger. The fact that an increase in cathodic area
relative to the anodic area does cause a measurable increase in the mixed
potential was demonstrated by incorporating some carbon in the reduced
ilmenite paste electrode. However, pure futile has such a low conductivity that
it is unlikely to provide a suitable surface for oxygen reduction. Possibly if it
were partly reduced, its conductivity might be sufficiently high for oxygen
reduction to occur. In order to investigate this, mixed potential measurements
were made using reduced ilmenite samples with varying Ti203 content. Drifting of the potentials with time was again found to be a problem. An additional
difficulty was that the reproducibility of the measurements in the same sample
was poor, and differences of up to_+ 15 mV were observed. However, despite
these difficulties, there seemed to be no systematic change in mixed potential
as the percentage of Ti203 in the reduced ilmenite was increased from 5 to 17%.
28
It is therefore likely that the level of Ti203 in the reduced ilmenite does not
have any influence on the aeration reaction, although the irreproducibility of
the measurements may hide some slight effect.
(c ) Effect of stirring
If the aeration reaction is diffusion controlled, better mixing of reactants
should lead to faster reaction rates. The dependence of the rate of reaction on
the mixing of reactants was tested on the laboratory scale by using a cylindrical
reaction vessel in which the reactants were mixed by a paddle stirrer. The
expected increase in reaction rate was observed at low stirrer speeds, but when
the speed was increased above the minimum necessary to suspend all the
reduced ilmenite particles in solution, the reaction rate became constant, the
leaching time being approximately 6 hours. A similar observation has been
made by Sareyed-Dim and Lawson (1976) during their study of the reaction
between a copper (I!) solution and zinc metal powder. The reason for this
behaviour would seem to be that when the solid material is completely suspended, increases in stirring speed cause the suspension to move round the
reaction vessel more rapidly, but do little to change the relative velocity of the
liquid movement across the particle surface. In order to increase the reaction
rate further, a higher degree of turbulence is required. This was achieved by
using the inverted-cone reactor, and all subsequent experiments were carried
out in this apparatus.
(d) Effect of temperature
The effect of temperature on the aeration process is complex because a number of temperature-dependent parameters are involved. Since the reaction is
diffusion controlled, the most important consideration is how the flux of
molecular oxygen to the surface of the reduced ilmenite varies with the temperature. This flux is determined by two opposing factors: the solubility and
diffusion of oxygen, with the latter in turn depending on the diffusion coefficient of oxygen and the viscosity of the ammonium chloride solution. The solubility of oxygen decreases with increasing temperature and increasing ionic
strength. In contrast, the diffusion coefficient of an ammonium chloride solution increases as the temperature increases. The temperature at which the
maximum flux of oxygen to the reduced ilmenite surface occurs will be that at
which the decrease in solubility becomes more significant than the increase in
oxygen diffusion rate. In practice, the matter is further complicated by other
effects such as which oxides are formed and their effect on the "viscosity" of
the solution.
The optimum reaction temperature is therefore best determined experimentally. The variation of rate of oxidation of reduced ilmenite is shown in Fig. 4,
in which the iron content of reduced ilmenite after two hours of oxidation is
plotted as a function of temperature. It can be seen that the rate increases with
29
I
I
I
i
I
f
!4
]3
]2
9
7
6
20
I
I
I
30
40
50
I
60
I
I
70
80
9@
T/Oc
Fig. 4. The amount of iron left in reduced ilmenite after 2 hours aeration in air-saturated NH4CI
as a function of temperature.
temperature up to about 70 ° C, above which the rate decreases with temperature. For the related oxidation of iron in water, the maximum rate of reaction
has also been found to occur at about 70 °C (Speller, 1951 ).
2. The role o[ ammonium chloride
(a) Comparison with sodium chloride
It is well known that the chloride ion plays an important role in the breakdown of passive oxide films (Speller, 1951), and is therefore a desirable constituent in any electrolyte solution used for the promotion of rusting. However,
the role of the ammonium ion in the aeration reaction is far less clear, and in
order to obtain information on this point, two experiments were carried out at
70°C in the inverted-cone reactor, one using 0.1 M ammonium chloride solution and the other 0.1 M sodium chloride solution for comparison. In each case,
the total iron content of the reduced ilmenite sample and the bulk solution p H
( measured in situ) were followed during the course of the reaction.
Figure 5 shows the variation in total iron for the reactions with the two
electrolytes. It can be seen that the behaviour of the two systems is quite different. W h e n ammonium chloride is used as an electrolyte, the total iron present in the reduced ilmenite is leached rapidly, and is essentially complete after
4 ½to 5 hours of aeration. The only iron remaining in the reduced ilmenite after
this time has to be acid-leached in order to remove it, and is thought to be
present as an iron-manganese sulphide solid solution. The reaction using
sodium chloride also ceases after about five hours of aeration, but leaves some
13% of iron in the reduced ilmenite. It would seem that further leaching is
30
36
I
I
I
I
I
I
I
32
28
2(?
16
.
•
•
•
8
4
I
i
I
,--
:
]
2
3
4
5
"
I
i
6
/
m
AERATION TIME/HOURS
Fig. 5. The amount of iron left in reduced ilmenite as a function of aeration time at 70 ° C in 0.1 M
NH4CI ( • ) and0.1MNaC1 (A).
prevented by the formation of some protective fihn on the iron surface, as
discussed below.
The variation of the bulk solution pH with time is also quite different for
the reaction carried out with ammonium chloride, and that using sodium chloride. As shown in Fig. 6, the air-saturated, 0.1 M NH4C1 solution at 70°C has
an initial pH of about 5.4 which rises rapidly to about 7.5 upon addition of the
reduced ilmenite sample. Reduced ilmenite is inherently slightly basic, probably due to a trace of residual ash carried over from the reduction stage. As the
aeration reaction proceeds, the pH of the solution falls, from about 7.5 to 4.5
in the first hour, and then more slowly to a final pH of about 3.6.
The initial pH of an air-saturated 0.1 M NaC1 solution at 70 °C is 5.5, which
1
ii
i0
9
PH
8
7
6
5
j._
4
__
A
_
.
3
I
I
1
2
I
3
1
I
I
I
4
g
6
/
AERATION TIME/HOURS
Fig. 6. T h e p H o f t h e b u l k s o l u t i o n a s a f u n c t i o n o f a e r a t i o n t i m e a t 70 °C in 0.1 M NH4CI ( • )
a n d 0.1 M N a C l ( • ). A l s o s h o w n a r e t h e p H o f 0.1 M NH4C1 ( A ) a n d 0.1 M NaC1 ( [] ) s o l u t i o n s .
31
is not too different from the pH of an air-saturated 0.1 M NH4C1 solution ( see
Fig. 6 ). Upon addition of reduced ilmenite, the pH again rises rapidly to pH 8,
but unlike the NH4C1 system, continues to rise to a peak value of 10 about 1
hour after the start of the reaction, before falling slowly to stabilize at about 9.
The pH of a solution is known to be an important factor in determining the
rusting rate of iron (Pourbaix, 1973). The large difference in the aeration
rates, shown in Fig. 5 for the two electrolytes, is only to be expected given the
pH data of Fig. 6, which indicates that most of the reaction in NHnC1 takes
place at a pH of about 4, and most of the reaction in NaC1 takes place at a pH
between 9 and 10. This latter pH range lies close to the passive region for iron
at 70 ° C, which, as can be seen from the E - p H diagrams for iron at 25 °C and
100 ° C ( Biernat and Robins, 1972 ), does not change rapidly with temperature.
Alternatively, the solid products of the rusting reaction may have been retained
within the pores of the reduced ilmenite, a process which Becher et al. (1965)
termed in situ rusting.
(b) Effect o[ iron oxides on p H
As noted earlier, and also shown in Fig. 6, the pH of the bulk solution during
an aeration in 0.1 M NH4C1 at 70 °C drops approximately I pH unit below the
initial value of about 5 set by the dissociation of the ammonium ion. This drop
in pH cannot be due to the hydrolysis of the iron (II) ions produced as intermediates during the aeration reaction since appreciable hydrolysis of these
ions only occurs at pH values greater than 5 (Leussing and Kolthoff, 1953).
Nor can the pH of the bulk solution be ascribed to the hydrolysis of iron (III)
ions, since the solubility of these ions in the pH range 4-5 is very low. It is
therefore suggested that the various iron oxides and hydroxides formed during
the aeration reaction control the pH of the solution, not only when ammonium
chloride is used as an electrolyte, but also when sodium chloride is used as an
electrolyte. The suggestion is reasonable since it is known that oxides and
hydroxides have a variety of both Br~nsted acid and base sites on their surfaces
(Anderson and Rubin, 1981 ), which can cause marked pH changes when finely
divided oxides are added to unbuffered solutions.
The hypothesis was tested experimentally in the following way. The mixture
of iron oxides and hydroxides produced after an eight hour aeration in 0.1 M
NH4C1 solution at 70 °C was separated from the reduced ilmenite by decantation, filtered, washed twice with 0.1 M NaC1 solution and air dried. It was
further treated with 100 cm 3 of 0.1 M NaC1 solution, stirred for 4 hours at 20 ° C,
then filtered and air dried. The product from a sodium chloride aeration was
prepared similarly. X-ray diffraction analysis showed that the material from
the ammonium chloride aeration was a mixture of hematite (o~-Fe20~) and
lepidocrocite ( 7-FeOOH ), whereas that from the sodium chloride aeration was
composed of magnetite ( Fe304 ) and goethite ( c~-FeOOH ). Quantities of these
two dry oxide materials were then added to 100 cm 3 of 0.1 M NaC1 at 20°C,
32
AERATION TTMF/HOUR$
2
!2
4
6
'
,
]0
°
•
I
,0
0,5
I
!,0
I
i,~
2,0
MA~.S OF OXIDE ADDED/SRAMS
Fig. 7. The variation ofpH with amount of oxide added to 0.1 M NaC1at 20°C: oxide from NH4Cl
aeration ( • ) ; oxide from NaC1 aeration ( • ). Also includedfor comparisonare the results from
Fig. 6 -- the variation of pH with aeration time at 70 °C: 0.1 M NH4Cl ( • ) ; 0.1 M NaC1 ( • ).
the pH of the slurry being measured after each addition. The results from these
two experiments are shown in Fig. 7, which also shows the variation of pH
during the course of the a m m o n i u m chloride and sodium chloride aerations,
respectively.
It is clear from the results shown in Fig. 7 t h a t the residues from the two
aeration reactions have quite different effects when added to a 0.1 M NaC1
solution. The material from the a m m o n i u m chloride aeration causes an initial
sharp drop in pH, which continues to fall as more material is added, although
at a progressively slower rate. On the other hand, the addition of the material
from the sodium chloride aeration causes the pH of the bulk solution to rise
rapidly with the first addition of oxide. Further additions only caused a small
increase in pH. It is also clear from Fig. 7 t h a t the oxide material from an
aeration reaction will induce the same sort of pH change in an unbuffered
sodium chloride solution as t h a t which occurs in the original aeration reaction.
The agreement is not exact because there are m a n y differences between the
two experiments ( e.g. temperature ), but the result is certainly consistent with
the hypothesis t h a t the pH of the bulk solution during an aeration reaction is
controlled by the type of oxide formed during the reaction, which in turn
depends on the electrolyte used.
(c ) A m m o n i u m chloride as a buffer
In the case of a m m o n i u m chloride, we believe t h a t an important factor in
the reaction mechanism is the ability of the a m m o n i u m ion to buffer the surface of the reacting iron against very high surface pH values.
33
The hydroxyls responsible for high surface pH values originate during the
reduction of oxygen according to the equation
O2(aq) + 2 H 2 0 + 4 e - - , 4 O H - ( a q )
(2)
Unless these hydroxyls are removed from the surface by some mechanism, they
will react with the iron (II) ions produced during the anodic half-reaction
Fe-,Fe 2+ (aq) +2 e
(3)
to produce a precipitate of iron (II) hydroxide in the pores or near the surface
of the reduced ilmenite. This solid material, which can be readily oxidized further, would tend to hinder the passage of oxygen to the surface of the iron,
particularly in the pores of the reduced ilmenite, and so cause a reduction in
the oxidation rate. In situ rusting is undesirable anyway, because separation
of the rutile and iron has not been achieved.
The ammonium ion is thought to prevent high surface pH values and the
subsequent precipitation of oxide by the reaction
NH + (aq) + O H - (aq) -,NH3 (aq) + H 2 0
(4)
since NH~ is a weak acid with a pKa of 9.24 at 25 ° C (Burgess, 1978). There
can be no doubt that this reaction does occur, since the odour of ammonia can
often be detected above the reaction mixture. To test the importance of buffering the solution near the iron metal surface, the aeration reaction was carried
out at 70°C in 0.1 M ZnC12 and 0.1 M MgC12. Like NH~, both Zn 2+ and Mg 2+
can act as buffers against high surface hydroxyl ion concentrations through
the reactions
Zn 2+ (aq) + O H - (aq) - , Z n O H - (aq)
(5)
Mg 2+ (aq) + O H - (aq) --,MgOH- (aq)
(6)
their respective pKa values being 9.0 and 11.6 at 25 °C (Burgess, 1978). From
these pKa values, the order of effectiveness in buffering the surface against
high pH values should be
Zn 2+ > N H ~ > M g 2+ > N a +
This order should therefore be reflected in the rate of the aeration reaction in
the electrolytes containing these ions.
The results for the four systems are compared in Table 1. It can be seen from
this table that the order of effectiveness of the ions is not quite as expected.
Although
Zn z+ > M g z+ > N a +
the ammonium ion promotes not only the most rapid aeration reaction, but
also the most complete, only 0.5% of iron being left in the reduced ilmenite.
34
TABLE 1
Comparison between aeration rates and buffering properties of several electrolyte solutions
NH4C1
ZnClz
MgCI2
NaC1
Percent of iron left
after 2 hour
after 3 hour
at completion ~
6.1
2.3
0.5
13.3
8.8
4.2
11.3
9.2
8.0
18.0
14,8
13.0
K~ of hydrated cation at 25 ° C b
9.24
9.0
11.6
aReaction was defined as being complete when the iron content of the reduced ilmenite remained
unchanged over a period of 3 hours.
hFrom Burgess, 1978.
However, substantial amounts of iron are left in the pores of the reduced ilmenite when either zinc chloride or magnesium chloride are used as an electrolyte.
It is believed that this is in situ rusting rather than passivation, because in
both cases, the pH dropped steadily during the course of the reaction reaching
a final value near 3.
Although it is apparent from Table i that the buffering action of the cations
is an important factor in promoting the aeration reaction, the anomalous position of the ammonium ion in the activity series still requires explanation. One
possible reason why the catalytic effect of Zn 2÷ on the aeration reaction is less
than NH~ is that Zn 2÷ could form in the high pH regions close to the cathodic
sites an insoluble hydroxide which would block the reduced ilmenite pores,
whereas NH4~ could not. Apart from the fact that such a compound would
certainly be unstable at pH 3, this explanation does not account for the occurrence of in situ rusting in zinc chloride solution, but not in ammonium chloride
solution. A second and more likely explanation is to suppose that the iron (II)
ions generated during the oxidation reaction are stabilized by the formation of
an iron ( II ) -ammonia complex.
(d) The complexing of iron(H) by ammonia
As noted earlier, the pH close to the cathodic sites may be sufficiently high
to generate an appreciable concentration of free NH3 in solution according to
eqn. (4). Complex ions of the type [Fe(NH3)x(HzO)6_x] 2+ may be then
formed by reaction with the free ammonia
[Fe(H20)~] 2 ÷ + x N H ~ - - , [ F e ( N H 3 ) x ( H 2 0 ) 6 x] 2 + + x H 2 0
(7)
which would stabilize the iron(II) and allow it to move away from the metal
surface without being precipitated. Precipitation would subsequently take place
in the bulk of the solution where the pH is much lower, causing eqn. (4) and
35
-3
-4
-5
-6
~FROM FE(OH)?
0LUBIL[TY
I
I
I
8
9
10
11
PH
Fig. 8. Solubility of iron (II) hydroxide in 0.1 M NH4C1 as a function of pH at 25 ° C. Measurement
of Leussing and Kolthoff (1953) ( • ) . The solid line corresponds to the solubility of Fe (OH) 2 in
0.1 M NaC1 calculated from the solubility product data of Leussing and Kolthoff (1953).
then (7) to be reversed. Ammonia would be converted back to ammonium ion,
and then the iron ( I I ) - a m m o n i a complex would break up, liberating Fe 2+ for
precipitation as the hydroxide.
Iron ( II ) - a m m o n i a complexes have been proposed by several authors ( Watt
and Jenkins, 1953; Leussing and Kolthoff, 1953; Hackerman et al., 1958; Goodrich and Hackerman, 1962; Uhlig, 1962; Schick and Uhlig, 1964; Jones and
Hackerman, 1968), but most of the experimental work concerning these complexes has been in relatively concentrated ammoniacal solutions (Watt and
Jenkins, 1953; Jones and Hackerman, 1968). In order to determine whether
the i r o n ( I I ) - a m m o n i a complexes do form during an aeration reaction in 0.1
M NH4C1, some measurements have been made of the equilibrium solution
concentration of iron (II) in contact with solid Fe (OH) 2 as a function of pH
with 0.1 M NH4CI as the background electrolyte.
The results from these measurements are shown in Fig. 8, together with a
single measurement made by Leussing and Kolthoff (1953) at pH 10.44 in the
same electrolyte, which is in good agreement. Included for comparison is the
expected concentration of iron (II) in solution as a function of pH calculated
from the solubility product for Fe (OH)2 reported by Leussing and Kolthoff
(1953) at 25°C in sodium chloride. It can be seen that when the pH is less
than about 8, there is good agreement between the iron (II) concentrations
measured in this study and those calculated from the solubility product of
Leussing and Kolthoff. However, as the pH is increased above 8, the difference
between the measured and calculated values increases, reaching three orders
of magnitude by pH 10, i.e. Fe(OH)2 is appreciably more soluble in NH4C1
36
than NaC1 at high pH. This result is undoubtedly evidence for the existence of
an iron (II) -ammonia complex, and its role in promoting iron oxidation in the
aeration reaction.
CONCLUSIONS
From the results presented in this paper, it has been concluded that
(i) The reaction between air and iron in reduced ilmenite is diffusion controlled, at least in the early stages of the reaction.
(ii) The reaction rate in 0.1 M NH4C1 is a maximum at a temperature of
about 70 ° C, based on a reaction time of two hours.
(iii) Ammonium chloride acts on the reacting system in three ways: (a) the
ammonium ion buffers the system near the reacting metal surface against
high local pH values which would cause iron hydroxide precipitation
near that surface; (b) the ammonia formed at high pH values stabilizes
iron (II) and also helps to prevent precipitation near the metal surface;
(c) the chloride ion helps to break down any passive oxides which might
form.
( iv ) When the rusting reaction is carried out in ammonium chloride solution,
the iron is oxidised to a mixture of hematite and lepidocrocite, but when
a sodium chloride solution is used, a mixture of magnetite and goethite
results.
ACKNOWLEDGEMENT
The authors are most grateful to the Western Australian Mining and Petroleum Research Institute and Associated Minerals Consolidated Ltd., not only
for their strong support of this work, but also for their permission to publish
this paper.
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