metals
Article
Ammoniacal System Mechanisms for Leaching
Copper from Converter Slag
Alvaro Aracena 1, *, Andrés Valencia 1 and Oscar Jerez 2
1
2
*
Escuela de Ingeniería Química, Pontificia Universidad Católica de Valparaíso, Avenida Brasil 2162,
Valparaíso 2362854, Chile; andres.h.valencia@gmail.com
Instituto de Geología Económica Aplicada (GEA), Universidad de Concepción, Casilla 160-C,
Concepción 4070386, Chile; ojerez@udec.cl
Correspondence: alvaro.aracena@pucv.cl; Tel.: +56-32-2372614
Received: 16 April 2020; Accepted: 13 May 2020; Published: 28 May 2020
Abstract: In pyrometallurgical processes refining copper, the main source of loss in the conversion
stage is from slag. This paper reports on research work treating converter slag containing high
percentages of copper (36 wt%) using ammonium hydroxide at room temperature. Variables analyzed
are solution pH, agitation, temperature, NH4 OH concentration and particle size. Results showed that
the hydronium ion resulting from ammonium hydroxide dissociation was the main oxidant of copper
compounds in slag, such as CuO, Cu2 O and Cu, with the exception of CuFeO2 . The particles contain
a large amount of microcracks (porosity) in their refractory structure (analyzed by compositional
image capture (BSE)). Thus, the diffusion of the leaching solution through the microcracks making
contact with the copper oxides would be allowed. Leaching mechanisms were corroborated by
X-ray diffraction and scanning electron microscopy analysis. Increasing temperature and NH4 OH
concentration while decreasing particle size obtained higher copper recoveries, reaching values of
84.8%. Under the same conditions, the main impurity (iron) was minimal (<2%). Solution pH also
affected slag leaching. Agitation of the solution positively affected the rate of copper extraction.
Leaching kinetics of the leaching solution through the porosity formed in the slag was analyzed under
the intraparticle diffusion model. The reaction order was 1.2 with respect to the concentration of
ammonium hydroxide and the model was inversely proportional to the square of the particle radius.
The activation energy obtained was 42.3 kJ/mol for temperature range 283 to 333 K.
Keywords: converter slag; ammonia leaching; microcracks; kinetics
1. Introduction
Metallurgical treatment of metallic sulfides is generally carried out by means of high temperature
methods, usually pyrometallurgy. Conversion stage, there are two sequential stages—the elimination
of iron and some other impurities in the so-called "slag blow" stage; and the subsequent stage to
convert white metal (Cu2 S) to metallic copper (Cu), known as the "copper blow" stage. During the
conversion process, highly uncontrolled oxidation occurs, causing chemical copper losses through
direct contact of metallic copper with gaseous oxygen (reaction (1)), as well as through direct contact of
the white metal with the main constituent of slag, iron oxide (FeO) (reaction (2)).
2Cu(white metal ) + 1/2O2(gaseous) = Cu2 O(slag)
(1)
Cu2 S(white metal) + FeO(slag) = Cu2 O(slag) + FeS(white metal) .
(2)
Thermodynamically, reaction (1) is very favorable; however and although reaction (2) should
not occur because the activity of copper oxide in a slag is on the order of 4.25 × 10−7 , there is ample
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Metals 2020, 10, 712
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evidence of this oxidized phase. The concentration of copper in slag can vary between grades of
0.68 and 9.13 wt% [1–11].
Slag can be disposed of in landfills as long as it contains less than 1% Cu. Due to the high
concentration of useful metal in slag, a proper copper recovery pathway through the use of alternative
treatments is attractive. Since this type of copper involves oxidation, one such treatment alternative is
hydrometallurgy. Much research has been conducted on copper recovery from converter slags. In the
chemical leaching pathway, Anand [1] leached converter slag (4.03% Cu, 1.99% Ni, 0.48% Co) with
ferric chloride and obtained extractions of 92%, 28% and 24% copper, nickel and cobalt, respectively,
after 10,800 s (3 h). To increase extractions, the same author [2] pre-treated slags with furnace oil
(10 wt%) for 10,800 s (3 h). Extraction increased for all three metals, reaching values of 82%, 95% and
80% Cu, Ni and Co, respectively. Then, in a separate study, Anand [3] applied a pressurized leaching
solution of dilute sulfuric acid at temperature 403 K (130 ◦ C) and 0.59 MPa O2 , yielding recovery of
over 90% of the metals of interest (Cu, Ni and Co) and less than 0.8% of Fe (main impurity).
Research done since 2000; Cu, Co and Zn were recovered by leaching slags with diluted H2 SO4 ;
slags were first roasted [5] with concentrated sulfuric acid, causing sulfation of slag constituents.
Recoveries of 88% copper, 87% cobalt, 93% nickel and 83% iron were obtained after 7200 s (2 h) of
roasting at 423 K (150 ◦ C) and a 3:1 acid/slag ratio. Perederiy [10] applied the High Pressure Oxidative
Acid Leaching (HPOXAL) method to extract nickel, cobalt and copper. The work was carried out
in a titanium autoclave with oxygen and acid injection. The results obtained at 523 K (250 ◦ C), 90
psi PO2 and times between 900 and 1200 s (15 to 20 min) yielded between 95–97% extraction of Ni,
Co and Cu, each. In another approach, the Taguchi method was used to determine optimum conditions
for dissolving copper from converter slags [6] saturated with Cl2 gas. The experimental work was
performed in a 0.5 dm3 (500 mL) capacity glass reactor. The results showed 98.35%, 8.79% and 25.17%
extraction of Cu, Fe and Zn, respectively, at chlorine gas flow of 3.67 × 10−3 dm3 /s (220 mL/min),
solid/liquid ratio of 0.1, agitation of 550 rpm and reaction time 5400 s (90 min). Another study [7]
used potassium dichromate with sulfuric acid where they found that using the H2 SO4 -K2 Cr2 O7
mixture was more advantageous for copper extraction than using H2 SO4 alone—using only sulfuric
acid, they obtained extractions of 20.5%, 66.6%, 62.1% and 65.7% of Cu, Co, Zn and Fe, respectively;
with added potassium dichromate, extractions varied to 81.15%, 12.00%, 3.15% and 10.27% for the
same order of metals. The amount of leached copper increased, while the other metals decreased
their extractions.
In recent years, Turan [11] leached a mixed slag (BS) generated during flash fusion and conversion
using microwaves, hydrogen peroxide and acetic acid. Results showed that the use of microwaves
gave shorter leaching times. High copper (95%) and zinc (30%) recoveries were achieved, while iron
recoveries were minimal (1.6%). Beşe [8] leached converter slag in a ferric sulfate-sulfuric acid system
in the presence and absence of ultrasound. The results showed that ultrasound gave extractions of
89.28%, 51.32%, 69.87% and 13.73% of copper, zinc, cobalt and iron, correspondingly. The absence of
ultrasound showed similar extractions.
A different kind of approach from biohydrometallurgy has also been used to recover copper from
converter slags. For example, indirect bioleaching, Carranza [9] leached 9 wt% Cu slag with ferric
sulfate via the BRISA process (in Spanish—Biolixiviación Rápida Indirecta con Separación de Acciones;
in English-Fast Indirect Bioleaching with Actions Separation). That process gave recovery of nearly
93% copper over 14400 s (4 h) of work. In relation to direct bioleaching, Mehta [4] used bacterium
Thiobacillus ferrooxidans and iron oxidizing bacteria to bioleach converter slags at room temperature.
High recoveries of 99.0% Copper (over 6912 × 103 s (1920 h)), 22.0% nickel (over 6048 × 103 s (1680 h))
and 30.0% cobalt (over 4320 × 103 s (1200 h)) were obtained.
Of the above works on converter slags using acid systems, none look at alkaline system, that is,
basic solutions at pH values above 6. Our research work, then, takes the novel approach to leaching
converter slags in ammonia-based systems, with pH values of 6 to 14. Although different copper
compounds have already been tested with this leaching alternative in different ammonia media
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chloride solution [13], complex copper in ammonia-ammonium chloride solution [14], malachite ore
in ammonium
solutions [15], tenorite
and cuprite
in ammonium
mediaore
[16,17].
Ammonia
such
as malachitenitrate
in ammonia/ammonium
carbonate
system [12],
oxidized copper
in ammonium
leaching
has not[13],
been
carried copper
out forin
converter
slags.
chloride
solution
complex
ammonia-ammonium
chloride solution [14], malachite ore in
Therefore,
the
main
objective
of
this
research
work
is
to media
leach a[16,17].
slag from
a Peirce-Smith
ammonium nitrate solutions [15], tenorite and cuprite in ammonium
Ammonia
leaching
converter
using
ammonium
hydroxide
(NH
4
OH)
under
different
working
conditions
as an
has not been carried out for converter slags.
alternative
to the
leaching
sulfuric
acid
(or anywork
otherisacid).
Therefore,
main with
objective
of this
research
to leach a slag from a Peirce-Smith converter
using ammonium hydroxide (NH4 OH) under different working conditions as an alternative to leaching
2. Experimental
Design
with
sulfuric acid (or
any other acid).
Codelco Chile's Ventanas Division provided converter slag specifically from their conversion
2. Experimental Design
process in a Peirce-Smith Furnace. In brief, slag is cooled with industrial water to reduce
temperature
from 1373
K to 293
K, is then
crushed,
ground slag
and specifically
classified into
several
fractions,
Codelco Chile’s
Ventanas
Division
provided
converter
from
theirsize
conversion
(most frequently
−210 + Furnace.
149, −149In
+ 105,
and −74
+ 53
μm, withwater
average
particle
sizes of 180,
process
in a Peirce-Smith
brief,−105
slag+is74cooled
with
industrial
to reduce
temperature
127,1373
90 and
μm,
Chemical
analysis
was into
performed
obtained
at average
from
K to64293
K, respectively).
is then crushed,
ground and
classified
several on
sizesamples
fractions,
(most frequently
sizes+of
180−149
and+64105,
μm,−105
showing
36%−74
Cu,+19%
Fe and
remaining
insoluble
Converter
−210
149,
+ 74 and
53 µm,
withthe
average
particle
sizes ofparts.
180, 127,
90 and slag
64
here
presented aChemical
very highanalysis
concentration
of copper,
bibliographically
recorded
µm,
respectively).
was performed
on higher
samplesthan
obtained
at average sizes
of 180(0.68–
and
Cu). Therefore,
hydrometallurgical
treatmentinsoluble
of waste (slag)
of interest.
649.13%
µm, showing
36% Cu,
19% Fe and the remaining
parts. was
Converter
slag here presented
analyses of copper,
the solutions
by Atomic Absorption
Spectroscopy
with
a very Chemical
high concentration
higherwere
than made
bibliographically
recorded (0.68–9.13%
Cu). (AAS)
Therefore,
a
Hitachi
Z-8100
Zeeman
kit
(Hitachi
High-Technologies
Corporation,
Tokyo,
Japan).
For
the
slag
hydrometallurgical treatment of waste (slag) was of interest.
characterization
in scanning
electron were
microscopy
(SEM),
theAbsorption
sample was
covered with
coalwith
to be
Chemical analyses
of the solutions
made by
Atomic
Spectroscopy
(AAS)
Chemical
analysis
(EDS)High-Technologies
and compositionalCorporation,
image capture
(BSE)
and morphological
a conductive.
Hitachi Z-8100
Zeeman
kit (Hitachi
Tokyo,
Japan).
For the slag
(SEM) were performed
using
a Tescan®
Vega LSH
electron
microscope,
equipped
characterization
in scanning
electron
microscopy
(SEM),
the sample
was covered
withwith
coalan
to EDS
be
Bruker® 6030
detector
(TESCAN,
Czech Republic).
by X-ray
conductive.
Chemical
analysis
(EDS) Brno,
and compositional
imageSolids
capturewere
(BSE)determined
and morphological
diffraction
(XRD), using
D4 Endeavor
kit, microscope,
operated with
Cu radiation
and Bruker
a Kβ ®
Ni
(SEM)
were performed
usingaaBruker®
Tescan® Vega
LSH electron
equipped
with an EDS
radiation
filter
(Bruker AXS
D8, Bruker,
Billerica,
USA). by X-ray diffraction (XRD),
6030
detector
(TESCAN,
Brno,Advance
Czech Republic).
Solids
were MA,
determined
order®toD4
know
the chemical
species
and/or
elementsand
present
XRD AXS
of the
using aInBruker
Endeavor
kit, operated
with
Cu radiation
a Kβin
Niconverter
radiation slag,
filter an
(Bruker
original
sample
was
performed,
obtaining
the
results
observed
in
Figure
1.
Here,
peaks
indicate
Advance D8, Bruker, Billerica, MA, USA).
delafossite
(CuFeO
(Cu
2O), tenorite
(CuO), metallic
(Cu),
chalcocite
In order to
know2),thecuprite
chemical
species
and/or elements
present incopper
converter
slag,
an XRD of(CuS),
the
magnetite
(Fe3Owas
4) and
fayalite (Fe
2SiO4) compounds-in
short, copper
present
the converter
slag is
original
sample
performed,
obtaining
the results observed
in Figure
1. in
Here,
peaks indicate
mainly oxidized.
delafossite
(CuFeO2 ), cuprite (Cu2 O), tenorite (CuO), metallic copper (Cu), chalcocite (CuS),
magnetite (Fe3 O4 ) and fayalite (Fe2 SiO4 ) compounds-in short, copper present in the converter slag is
mainly oxidized.
30000
CuFeO2
Intensity, CPS
25000
Cu
Cu2O
20000
Fe3O4
Fe2SiO4
15000
CuS
CuO
10000
5000
0
10
20
30
40
50
60
2θ, degree
Figure 1. X-ray diffraction (XRD) of original converter slag sample.
Figure 1. X-ray diffraction (XRD) of original converter slag sample.
70
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The 180 µm sample was analyzed by SEM to determine the degree of fracture and morphology of
The 180 μm
was
analyzed
by SEM
to determine
the degreedegree
of fracture
and morphology
the converter
slagsample
particles
(Figure
2). The
particles
show an important
of fracture
or porosity
of
the
converter
slag
particles
(Figure
2).
The
particles
show
an
important
degree
of
fracture
or
(indicated by arrows in the images). This fracturing, called microcracks, could help diffuse leaching
porosity
(indicated
by
arrows
in
the
images).
This
fracturing,
called
microcracks,
could
help
diffuse
solutions through the particle.
leaching solutions through the particle.
100 µm
50 µm
Figure 2. Scanning electron microscopy (SEM) micrograph of original sample of converter slags (264×
Figure 2. Scanning electron microscopy (SEM) micrograph of original sample of converter slags (264×
and 546× magnification).
and 546× magnification.
Leaching experiments began by placing 1 dm3 of ammonium hydroxide leaching solution into
Leaching
experiments
began
by placing
1 dm3of
ofaammonium
hydroxide
leaching
into
3 capacity
a 2 dm
reactor. This
reactor
consisted
heating blanket,
condenser
(tosolution
decrease
thea
3 capacity reactor. This reactor consisted of a heating blanket, condenser (to decrease the
2
dm
evaporation rate), mechanical agitator of the digital type (with a teflon impeller), thermocouple and
rate),the
mechanical
agitator Then,
of the 1digital
type
a teflon slag
impeller),
thermocouple
and
aevaporation
tube to extract
liquid samples.
× 10−3
kg (with
of converter
sample
was introduced.
−3 kg of converter slag sample was introduced. For
a
tube
to
extract
the
liquid
samples.
Then,
1
×
10
For experiments run at elevated temperature, the solution was first heated to the desired temperature.
experiments
run at elevated
temperature,
the solution
was first
heated
to thebydesired
temperature.
During
the experimental
time,
liquid samples
were extracted
and
analyzed
AAS for
Cu and Fe
During
the
experimental
time,
liquid
samples
were
extracted
and
analyzed
by
AAS
for
and
Fe
analysis. In some cases, solid samples were obtained by washing and drying residues andCu
they
were
analysis.
In
some
cases,
solid
samples
were
obtained
by
washing
and
drying
residues
and
they
were
analyzed by X-ray diffraction to get the qualitative phase analysis and BSE was used for cross sections
analyzed
X-ray diffraction to get the qualitative phase analysis and BSE was used for cross
of
particle by
morphology.
sections of particle morphology.
3. Results and Discussion
3. Results and Discussion
Variables that directly affected converter slag dissolution rate, shown below, were NH4 OH
3 ), temperature
Variables (mol/dm
that directly
affected converter
slag pH,
dissolution
rate, (PS,
shown
were
NH
4OH
concentration
(K), solution
particle size
µm),below,
stirring
speed
(rpm)
3
3
concentration
(mol/dm
), temperature
(K), solution pH, particle size (PS, m), stirring speed (rpm)
and
liquid/solid
ratio (dm
/kg).
and liquid/solid ratio (dm3/kg).
3.1. Solution pH
3.1. Solution pH
The converter slag dissolution zone in ammonia media at different solution pH values was
analyzed.
The pH values
6.0, 10.5zone
and 14.0.
The testsmedia
were performed
at solution
NH4 OH concentration
of
The converter
slag were
dissolution
in ammonia
at different
pH values was
3 , temperature 298 K, particle size 64 µm, agitation speed 500 rpm and liquid/solid ratio of
0.7
mol/dm
analyzed. The pH values were 6.0, 10.5 and 14.0. The tests were performed at NH4OH concentration
3, temperature
1000/1.
To maintain
the pH value
of the
solution,
sulfuric
acid (98%speed
purity)
sodium
(98.5%
of 0.7 mol/dm
298 K,
particle
size 64
m, agitation
500orrpm
and hydroxide
liquid/solid
ratio
purity)
wasToadded,
as appropriate.
Figure
shows thesulfuric
results acid
obtained
leaching
times uphydroxide
to 2700 s.
of 1000/1.
maintain
the pH value
of the3 solution,
(98%for
purity)
or sodium
(98.5% purity) was added, as appropriate. Figure 3 shows the results obtained for leaching times up
to 2700 s.
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Copper extraction, %
80
60
pH=6.0
pH=10.5
pH=14.0
40
20
0
0
500
1000
1500
2000
2500
3000
Time, seconds
Figure 3. Effect of the pH of the leaching solution on copper extraction. Working conditions—[NH4 OH]
3. 3Effect
ofµm,
theAgitation
pH of= 500
the rpm,
leaching
=Figure
0.7 mol/dm
, PS = 64
ratio =solution
1000/1. on copper extraction. Working
3
conditions—[NH4OH] = 0.7 mol/dm , PS = 64 m, Agitation = 500 rpm, ratio = 1000/1.
It can be seen in the figure that as the experimental time increases, the extraction of copper from
It can beslags
seen increases.
in the figure
thatfor
as athe
time increases,
theCu
extraction
of copper
from
the converter
Thus,
pHexperimental
of 10.5, an extraction
of 29.9%
was obtained
at a time
the
converter
slags
increases.
Thus,
for
a
pH
of
10.5,
an
extraction
of
29.9%
Cu
was
obtained
at
a
time
of 600 s, while at 2700 s, extraction reached 69.6%. It should also be noted that copper extractions
of 600
s, while
at 2700
s, of
extraction
reached
69.6%.extractions
It should of
also
be 7.3%
notedand
that15.1%,
copper
extractions
were
lower
for pH
values
6.0 and 14.0,
reaching
only
respectively.
weredecrease
lower for
values will
of 6.0
14.0, reaching
This
in pH
extraction
beand
discussed
later. extractions of only 7.3% and 15.1%, respectively.
This decrease in extraction will be discussed later.
3.2. Effect of Agitating the Solution
3.2. Effect of Agitating the Solution
The agitation of the leaching solution was varied between a range of 100 and 950 rpm. In addition,
an agitation-free
test of
was
added.
The solution
working was
conditions
were for
pH value
10.5,
an ammonium
The agitation
the
leaching
variedused
between
a range
of 100
and
950 rpm. In
3
hydroxide
concentration
of test
0.7 mol/dm
, particle
size 64 conditions
µm, temperature
298 K
liquid/solid
addition, an
agitation-free
was added.
The working
used were
forand
pHavalue
10.5, an
3, particle
ratio
of 1000/1.
Figure 4concentration
shows time-dependent
copper
extractions
form,
thetemperature
stirring ranges
ona
ammonium
hydroxide
of 0.7 mol/dm
size 64
298used
K and
the
solution. ratio of 1000/1. Figure 4 shows time-dependent copper extractions for the stirring ranges
liquid/solid
seen in the figure, a low stirring speed (100 rpm) copper extraction was achieved close
usedAs
oncan
thebe
solution.
to 38.3%. As stirring speed increased, copper extraction increased to 84.8% (at stirring rate 950 rpm).
Above 500 rpm, the amount of copper extracted is not highly influenced. This phenomenon is due to
the role of mass transfer. Thus, the mass transfer would not be relevant to the flow of ions through the
slag particles. It was thus concluded that future experiments would be set at 500 rpm agitation.
It should be noted that working without agitation (0 rpm) an extraction value of 11.1% was achieved.
This value is important because copper extraction from converter slag still occurs, even without stirring
the solution. This is mainly due to the leaching solution diffusing throughout the slag via microcracks
(pores) [18]. These microcracks are produced by the sudden cooling of the slags in the pyrometallurgical
process, when water (at 293 K) makes contact with molten slag (1373 K).
Partially leached samples (0 rpm, 2700 s) were analyzed by BSE, which are shown in Figure 5.
It can be seen in this figure that the particles contain a large amount of microcracks in their structure,
even subdivided inside them generating a greater amount of porosity in the particle. Thus, the diffusion
of the leaching solution through the microcracks by contact with the copper oxides would be allowed.
For future work, the study of more detailed analyzes could be evaluated through the use of
techniques such as the B.E.T. (Brunauer, Emmett and Teller), which would allow to determine the
specific surface area of the solid.
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Metals 2020, 10, 712
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100
0 rpm
100 rpm
850 rpm
950 rpm
250 rpm
500 rpm
Copper extraction, %
80
60
40
20
0
0
500
1000
1500
2000
2500
3000
Time, seconds
Figure
of the
theagitation
agitationrate
rateofofthe
the
solution
a function
of time.
Working
conditions:
Figure4.4. Analysis
Analysis of
solution
as as
a function
of time.
Working
conditions:
pH =
7 of 18
3 , T = 298K, PS = 64 µm, ratio = 1000/1.
3
pH
=
10.5,
[NH
OH]
=
0.7
mol/dm
10.5, [NH4OH]4 = 0.7 mol/dm , T = 298K, PS = 64 m, ratio = 1000/1.
Metals 2020, 10, x FOR PEER REVIEW
As can be seen in the figure, a low stirring speed (100 rpm) copper extraction was achieved
close to 38.3%. As stirring speed increased, copper extraction increased to 84.8% (at stirring rate 950
rpm). Above 500 rpm, the amount of copper extracted is not highly influenced. This phenomenon is
due to the role of mass transfer. Thus, the mass transfer would not be relevant to the flow of ions
through the slag particles. It was thus concluded that future experiments would be set at 500 rpm
agitation.
It should be noted that working without agitation (0 rpm) an extraction value of 11.1% was
achieved. This value is important because copper extraction from converter slag still occurs, even
without stirring the solution. This is mainly due to the leaching solution diffusing throughout the
slag via microcracks (pores) [18]. These microcracks are produced by the sudden cooling of the slags
in the pyrometallurgical process, when water (at 293 K) makes contact with molten slag (1373 K).
Partially leached samples (0 rpm, 2700 s) were analyzed by BSE, which are shown in Figure 5. It
can be seen in this figure that the particles contain a large amount of microcracks in their structure,
even subdivided inside them generating a greater amount of porosity in the particle. Thus, the
diffusion of the leaching solution through the microcracks by contact with the copper oxides would
50 µm
100 µm
be allowed.
Figure 5. BSE images of partially leached samples (0 rpm and 2700 s) of converter slag for analysis
Figure 5. BSE images of partially leached samples (0 rpm and 2700 s) of converter slag for analysis
(455× and 656× magnification). The arrows indicate the microcracks inside the structure.
(455× and 656× magnification). The arrows indicate the microcracks inside the structure.
3.3. Effect of Temperature
For future work, the study of more detailed analyzes could be evaluated through the use of
The effect of temperature on the leaching rate of converter slag was studied for a range of 283 to
techniques such as the B.E.T. (Brunauer, Emmett and Teller),
which would allow to determine the
333 K, with NH4 OH concentration constant at 0.7 mol/dm3 , particle size at 64 µm and liquid/solid ratio
specific surface area of the solid.
at 1000/1. Figure 6 shows the results of the temperature effect.
As temperature
increases, copper extraction is faster. Thus, at time 2700 s and temperature 293 K,
3.3. Effect
of Temperature
copper extraction reached 50%; increasing temperature 1.1 times (323 K), the same extraction was
The effect
of temperature
on the leaching
obtained
after only
660 s, a reduction
of 75.5%.rate of converter slag was studied for a range of 283 to
333 K,
with
NH
4OH concentration constant at 0.7 mol/dm3, particle size at 64 m and liquid/solid
To determine converter slag leaching mechanisms, samples obtained at a temperature of 298 K,
3 NH
ratio
at 1000/1.
Figure
6 shows the results of the temperature effect.
0.7
mol/dm
4 OH and at times of 2700 and 7200 s (i.e., partially and completely leached) were
analyzed. Solid samples were analyzed by XRD. Results for both samples (2700 and 7200 s) were
similar. Therefore, only analysis performed for the sample obtained at 7200 s are presented in Figure 7.
n, %
80
60
3.3. Effect of Temperature
The effect of temperature on the leaching rate of converter slag was studied for a range of 283 to
333 K, with NH4OH concentration constant at 0.7 mol/dm3, particle size at 64 m and liquid/solid
ratio at 1000/1. Figure 6 shows the results of the temperature effect.
Metals 2020, 10, 712
7 of 16
Copper extraction, %
80
60
Metals 2020, 10,40
x FOR PEER REVIEW
8 of 18
As temperature increases, copper extraction is faster. Thus, at time 2700 s and temperature 293
K, copper extraction reached 50%; increasing temperature 1.1 times
(323 K), the 313K
same extraction was
283K
20
obtained after only 660 s, a reduction of 75.5%.
293K
323K
To determine converter slag leaching mechanisms, samples obtained
at
a
temperature
of 298 K,
298K
333K
0.7 mol/dm3 NH4OH and at times of 2700 and 7200 s (i.e., partially and completely leached) were
0 samples were analyzed by XRD. Results for both samples (2700 and 7200 s) were
analyzed. Solid
0
1000
2000
3000
4000
5000
6000
7000
similar. Therefore, only analysis performed for the sample obtained at 7200 s are presented in Figure
Time, seconds
7.
Figure
6. 6.
Results
of copper
extraction
as a function
of time for
Working conditions:
Figure
Results
of copper
extraction
as a function
of different
time fortemperatures.
different temperatures.
Working
3 , PS = 64 µm, Agitation = 500 rpm, ratio = 1000/1.
pH
=
10.5,
[NH
OH]
=
0.7
mol/dm
3
4 = 10.5, [NH4OH] = 0.7 mol/dm , PS = 64 m, Agitation = 500 rpm, ratio = 1000/1.
conditions: pH
60000
CuFeO2
50000
Intensity, CPS
Cu2O
40000
Fe3O4
Fe2SiO4
30000
CuS
20000
10000
0
0
10
20
30
40
50
60
70
2θ, degree
Figure 7. Partially reacted sample of converter slag for a time of 7200 s.
Figure 7. Partially reacted sample of converter slag for a time of 7200 s.
Copper compounds can be seen in Figure 7, with strong peaks identifying compound delafossite
Copper
compounds
can However,
be seen in
Figure
7, not
with
strong inpeaks
identifying
and a slight
presence
of cuprite.
tenorite
was
identified
this sample
neithercompound
metallic
delafossite
and
a
slight
presence
of
cuprite.
However,
tenorite
was
not
identified
in
this
copper (Cu). These results indicate that ammonia leaching occurred for oxidized copper speciessample
such
metallic
(Cu).
These
results indicate
that ammonia
leaching
occurred
for
oxidized
asneither
Cu2 O and
CuO copper
as well as
for Cu.
Conversely
and as confirmed
by XRD,
CuFeO
was
not
dissolved
2
species
as are
Cu2analogous
O and CuO
as well as
for Cu.
and as over
confirmed
XRD,
bycopper
NH4 OH.
Thesesuch
results
to previous
studies
[5],Conversely
in which recoveries
85% ofby
copper
CuFeO
2
was
not
dissolved
by
NH
4
OH.
These
results
are
analogous
to
previous
studies
[5],
in
which
were not possible due to the presence of copper ferrite (CuFeO2 ), a ferromagnetic ceramic compound
recoveries
85% to
of leach
copper
were
not
possibleproperties.
due to the presence of copper ferrite (CuFeO2), a
which
is veryover
difficult
due
to its
refractory
ferromagnetic ceramic compound which is very difficult to leach due to its refractory properties.
3.4. Converter Slag Leach Mechanism
3.4. Converter Slag Leach Mechanism
Ammonium hydroxide incorporated into the solution was immediately decomposed down to
Ammonium
hydroxide
into theion
solution
immediately
decomposed
down
form an
ammonium
ion (NH3incorporated
) and a hydronium
(H3 O+was
), following
reaction
(3). In this
andto
+
+
form anwork
ammonium
(NHdemonstrated
3) and a hydronium
ion is
(Hresponsible
3O ), following
reaction copper
(3). In this
and
previous
[16,17], ion
it was
that H3 O
for leaching
oxides
previous work [16,17], it was demonstrated that H3O+ is responsible for leaching copper oxides
through physical and electrochemical processes, while NH3 created copper complexes to maintain
cupric ions in the alkaline solution system (constant dissociation of ammonia = 1.77 × 10−5 to 298 K).
8NH4OH + 8H2O → 8NH3 + 8H3O+ + 8OH−
(3)
Metals 2020, 10, 712
8 of 16
through physical and electrochemical processes, while NH3 created copper complexes to maintain
cupric ions in the alkaline solution system (constant dissociation of ammonia = 1.77 × 10−5 to 298 K).
8NH4 OH + 8H2 O → 8NH3 + 8H3 O+ + 8OH−
(3)
Therefore, the leaching mechanism for oxide slags (Cu2 O and CuO) in the presence of ammonium
hydroxide is given by the following semi-reactions [16,17]:
2CuO + 2H3 O+ → Cu2+ + 2H2 O + Cu(OH)2
(4)
2Cu2 O + 2H3 O+ → 3Cu2+ + 2H2 O + Cu(OH)2 + 4e.
(5)
In the case of the reaction (4), the hydronium ion provides protons necessary for the dissolution
of CuO (the dissolution of this compound follows a physical process). Reaction (5), in contrast, has
the hydronium ion in an electrochemical process to provide the necessary electrons to generate the
oxide/reduction pair (using reaction (6)) and thus dissolve the Cu2 O.
4H3 O+ + 4e + O2 → 6H2 O.
(6)
With respect to the compound Cu(OH)2 generated in reactions (4) and (5), this is leached with
H3 O+ according to reaction (7). The hydronium ion competes to be able to leach cuprite and tenorite
(reactions (4) and (5)) and to dissolve Cu(OH)2 (reaction (7)).
Cu(OH)2 + 2H3 O+ → Cu2+ + 4H2 O.
(7)
Copper ions released into the solution (reactions (4) and (5)) are complexed by the ammonium
ion to generate copper tetra-amine (Cu(NH3 )4 2+ ) according to reaction (8). It should be noted that
the tetra-amine formation process is allowed through previous generation of Cu(NH3 )2 2+ ; however,
that ion is thermodynamically unstable in basic systems [13,15].
4Cu2+ + 16NH3 → 4(Cu(NH3 )4 2+ ).
(8)
To corroborate the formation of copper tetra-amine, a Pourbaix stability diagram (EH -pH)
was constructed for the Cu-NH4 OH-H2 O system for temperature 293 K, copper concentration
0.00236 mol/dm3 and ammonium hydroxide concentrations of 0.1, 0.35 and 0.7 mol/dm3 .
The thermodynamic data used for the construction of the diagram were extracted from the HSC
Chemistry program database [19]. This diagram (Figure 8) shows that solution pH was preponderant
vs. potential in obtaining the copper tetra-amine. Thus, for NH4 OH concentration 0.1 mol/dm3 ,
Cu(NH3 )4 2+ was maintained in solution at pH ranging from 7 to 14 for the entire potential range.
Outside those pH ranges, copper precipitated as an oxide (CuO), decreasing copper concentration
in solution and inhibiting further purification/concentration of this metal. Thus pH effect is shown,
here, as in Figure 3. In addition, were copper oxide to form, NH3 consumption (reaction (8)) would
stop, NH4 OH would not decompose and H3 O+ would not be generated (reaction (3)). In short,
CuO production directly and negatively affects Cu2 O and CuO leaching from the slag.
Tests with NH4 OH concentrations (0.35 and 0.70 mol/dm3 ) show that as concentration of
ammonium hydroxide increases, the stability zone of the copper tetra-amine was reduced, yielding for
NH4 OH 0.7 mol/dm3 a pH range from 8.3 to 13.4.
effect is shown, here, as in Figure 3. In addition, were copper oxide to form, NH3 consumption
(reaction (8)) would stop, NH4OH would not decompose and H3O+ would not be generated (reaction
(3)). In short, CuO production directly and negatively affects Cu2O and CuO leaching from the slag.
Tests with NH4OH concentrations (0.35 and 0.70 mol/dm3) show that as concentration of
Metals
2020, 10, hydroxide
712
9 of 16
ammonium
increases, the stability zone of the copper tetra-amine was reduced, yielding
3
for NH4OH 0.7 mol/dm a pH range from 8.3 to 13.4.
Figure 8. Stability diagram of the NH3 -Cu-H2 O system for a temperature of 278 K. NH4 OH
Figure 8. Stability diagram of the NH3-Cu-H2O system for a temperature of 278 K. NH4OH
concentrations = 0.7, 0.35 and 0.1 mol/dm33 . Copper concentration = 0.00236 mol/dm3.3.
concentrations = 0.7, 0.35 and 0.1 mol/dm . Copper concentration = 0.00236 mol/dm
The formation of the copper tetra-amine generated an imbalance between reactions (4) and (5)
(i.e., Cu2+ concentration decreased), thus causing a higher dissolution rate for the two copper oxides
CuO and Cu2 O. These two compounds delivered a greater amount of copper ions to the basic solution,
given by reactions (9) and (10).
2CuO + 4NH4 OH + 4NH3 → 2(Cu(NH3 )4 2+ ) + 2H2 O + 4OH−
(9)
2Cu2 O + 8NH4 OH +8NH3 + O2 → 4(Cu(NH3 )4 2+ ) + 4H2 O + 8OH−
(10)
It should be noted that if the concentration of ammonia in the solution is reduced, it could lead to
the formation of copper oxide.
On the other hand, metallic copper can be dissolved by hydronium ions (coming from the disiation
of NH4 OH) by an electrochemical process. Then, the dissolution mechanism would be given by the
expression 11.
Cu + 2NH4 OH + 2NH3 + 1/2O2(g) → Cu(NH3 )4 2+ + H2 O + 2OH−.
(11)
3.5. Effect of NH4 OH Concentration
Figure 9 shows the results of converter slag copper extraction as a function of NH4 OH concentration.
The working temperature was 298 K; particle size, 64 µm; and liquid/solid ratio, 1000:1.
The Figure shows that increased concentration of ammonium hydroxide increased dissolution
of converter slag. Thus, at concentration of 0.2 mol/dm3 , 36.1% was obtained at a time of 7200 s;
at concentration 0.8 mol/dm3 , a recovery of 71.3% was obtained after the same amount of time. At the
minimum NH4 OH concentration tested (0.1 mol/dm3 ), copper extraction was 15.6%. Stoichiometric
concentrations from reactions (9) and (10) are calculated at 0.5 and 1.0 mol/dm3 of NH4 OH, respectively;
however, concentrations 5 to 10 times lower than the above still gave significant copper extractions.
3.5. Effect of NH4OH Concentration
Figure 9 shows the results of converter slag copper extraction as a function of NH4OH
concentration. The working temperature was 298 K; particle size, 64 m; and liquid/solid ratio,
1000:1.
Metals 2020, 10, 712
10 of 16
100
80
Copper extraction, %
0.6 mol/dm3
0.7 mol/dm3
0.8 mol/dm3
0.1 mol/dm3
0.2 mol/dm3
0.3 mol/dm3
0.5 mol/dm3
60
40
20
0
0
500
1000
1500
2000
2500
3000
Time, seconds
Metals
2020, 10,
xEffect
FOR of
PEER
REVIEW
11 of 18
Figure
9.9.Effect
Figure
ofNH
NH
4OHconcentration
concentrationon
onconverter
converterslag
slagleach
leachrate.
rate.Working
Workingconditions:
conditions:pH
pH==10.5,
10.5,
4 OH
TT==298
500 rpm,
rpm, ratio
ratio =
1000/1.
298K,
K,PS
PS== 64
64 µm,
m, Agitation
Agitation =
= 500
= 1000/1.
Four different converter slag particle sizes were compared: 64, 90, 127 and 180 m. Working
3.6.
Particle Size
conditions
wereAnalysis
similar
to previous
experiments.
Theof
copper
extraction
curves are
given in
Figure 10.
The Figure
shows that
increased
concentration
ammonium
hydroxide
increased
dissolution
3, 36.1%
It
clear that
particle
size
is largely
determinant
ofwere
dissolution
rate.obtained
For
a of
recovery
Four
different
converter
slag particle
sizes
compared:
64,size
90,180
and
180 µm.
of isconverter
slag.
Thus,
at concentration
of 0.2
mol/dm
was
at 127
a m,
time
7200
s; of
at
3
11.3%
was
obtained
after
time
1800
s,
while
for
particle
size
64
m,
copper
extraction
reached
69.9%
Working
conditions
were similar
to previous
experiments.
The
copper
extraction
curves
are given
concentration
0.8 mol/dm
, a recovery
of 71.3%
was obtained
after
the same
amount
of time.
At the
3), copper
for
the same
This
islargely
likely
due
to two of
situations,
that
is,For
asStoichiometric
particle
size
in
Figure
10.NH
It 4time.
is
clear
thatphenomenon
particletested
size is(0.1
determinant
dissolution
rate.
size
180 µm,
minimum
OH
concentration
mol/dm
extraction
was
15.6%.
3 of
useful
metal
was released
from
thes,slag
andfor
2) particle
surface
area
increased.
adecreased—1)
recovery of 11.3%
was
obtained
after
while
size 64
copper
extraction
concentrations
from
reactions
(9)
andtime
(10)1800
are
calculated
at 0.5 and
1.0µm,
mol/dm
NH4OH,
reached
69.9%however,
for the same
time. This phenomenon
is likely
duethe
to above
two situations,
is, as particle
respectively;
concentrations
5 to 10 times lower
than
still gave that
significant
copper
size
decreased: 1) useful metal was released from the slag and 2) surface area increased.
extractions.
Copper extraction, %
3.6. Particle 80
Size Analysis
180 µm
127 µm
90 µm
64 µm
60
40
20
0
0
500
1000
1500
2000
2500
3000
Time, seconds
Figure 10. Evaluation of particle size in copper extraction rate. Working conditions: pH = 10.5,
Figure
10. Evaluation 3of particle size in copper extraction rate. Working conditions: pH = 10.5,
[NH
4 OH] = 0.7 mol/dm , T = 298 K, Agitation = 500 rpm, ratio = 1000/1.
[NH4OH] = 0.7 mol/dm3, T = 298 K, Agitation = 500 rpm, ratio = 1000/1.
3.7. Examination of the Solid/liquid Ratio
The ratio between leaching solution and slag was examined with the aim of obtaining the
highest copper extraction with the lowest amount of solution. Working conditions were similar to
those indicated in the previous points. The amount of solution was varied from 0.1 to 1.4 dm3 with
amount of slag constant at 1 × 10−3 kg. Results are shown in Figure 11.
Metals 2020, 10, 712
11 of 16
3.7. Examination of the Solid/liquid Ratio
The ratio between leaching solution and slag was examined with the aim of obtaining the highest
Metals 2020, 10, x FOR PEER REVIEW
12 of 18
copper
extraction with the lowest amount of solution. Working conditions were similar to those
indicated in the previous points. The amount of solution was varied from 0.1 to 1.4 dm3 with amount
of slag constant at 1 × 10−3 kg. Results are shown in Figure 11.
Copper extraction, %
80
60
40
20
0
0
200
400
600
800
1000
1200
1400
1600
dm3 solution/ kg particle
Figure
ratio
onon
copper
extraction.
Working
conditions:
pHpH
= 10.5,
[NH
=
Figure11.
11.Effect
Effectofofliquid/solid
liquid/solid
ratio
copper
extraction.
Working
conditions:
= 10.5,
[NH
4OH]
4 OH]
3 , T3 = 298 K, PS = 64 µm, Agitation = 500 rpm.
0.7
mol/dm
= 0.7
mol/dm
, T = 298 K, PS = 64 m, Agitation = 500 rpm.
3 /kg, an extraction
As
decreased. At
At100
100dm
dm3/kg,
Asthe
theliquid/solid
liquid/solidratio
ratio decreased,
decreased, copper
copper extraction
extraction decreased.
an extraction of
3
33
of9.3%
9.3%was
wasachieved;
achieved;atat400
400dm
dm3/kg,
/kg,60.8%.
60.8%. The
The highest
highest Cu
Cu extractions
extractions were
at
ratio
were at ratio 1000
1000dm
dm/kg,
/kg,
which
and
4 OH
2 O-Cu.
whichmarks
marksthe
themost
mostefficient
efficientreaction
reactionbetween
betweendiffusion
diffusionmedium
mediumNH
NH
4OH
andCuO-Cu
CuO-Cu
2O-Cu.
3.8.
3.8.Converter
ConverterSlag
SlagLeaching
LeachingKinetics
Kinetics
Although
and
as
previously
mentioned,
converter
slag particles
have microcracks,
Figure 6Figure
shows6
Although and as previously
mentioned,
converter
slag particles
have microcracks,
that
slag that
leaching
also be
described
by described
the intraparticle
diffusion
model,diffusion
that is, solution
shows
slagcould
leaching
could
also be
by the
intraparticle
model,leaching
that is,
through
slag
porosity.
Assuming
that
constant
reactant
concentrations
and
diffusion
rate through
solution leaching through slag porosity. Assuming that constant reactant concentrations
and
porous
layer
of
radius
r
,
the
rate
equation,
described
by
the
shrinking
core
model,
is
given
by
the
o
diffusion rate through porous layer of radius ro, the rate equation, described by the shrinking core
following
[20]:
model, is expression
given by the
following expression [20]:
2
1 − 3(1 − xCS ) 32 + 2(1 − xCS ) = kapp t.
(12)
(12)
1 − 3(1 − xCS )3 + 2(1 − xCS ) = k app t.
In Equation (11), xCS is the converted (reacted) fraction of the converter slag; apparent rate constant
xCS is the converted
(reacted) fraction
of the size.
converter
slag;kapp
apparent
rate
(kapp ) In
is aEquation
function (11),
of temperature,
NH4 OH concentration
and particle
Constant
is directly
constant
(k
app
)
is
a
function
of
temperature,
NH
4
OH
concentration
and
particle
size.
Constant
k
app
is
related to the intrinsic rate constant of the system, given by expression (13):
directly related to the intrinsic rate constant of the system, given by expression (13):
CnNH OH
−Ea
Cn
NH
OH4 −𝐸𝑎
kapp
(13)
𝑅𝑇 , ,
(13)
k app
= =k k 24 2 𝑒 e⁄RT
ro ro
n
where k is the intrinsic rate constant, CNH
, is the concentration of ammonium hydroxide and n is
4 OH
where k is the intrinsic rate constant, CnNH
, is the concentration of ammonium hydroxide and n is
4 OH
thereaction
reactionorder
orderofofNH
NHOH.
4OH. Finally, ro is the initial radius of the slag particle.
the
Finally,
ro is the initial radius of the slag particle.
4
2
2
3 + 2(1 − x
)
A
graph
of
1
−
3(1
−
x
versus
timewas
wasconstructed
constructedfor
forthe
theexperimental
experimentaldata
data
3
CS
) )versus
A graph of 1 − 3(1 − xCS ) + 2(1 − xCSCS
time
obtained
in
Figure
6
for
temperature
range
283
to
333
K
and
for
particle
size
64
m.
Results
are
obtained in Figure 6 for temperature range 283 to 333 K and for particle size 64 µm. Results are shown
shown in Figure 12. The same data treatment method was used for the results obtained on the effects
of ammonium hydroxide concentration (Figure 9) and particle size (Figure 10).
Metals 2020, 10, x FOR PEER REVIEW
13 of 18
Metals 2020, 10, 712
Metals 2020, 10, x FOR PEER REVIEW
12 of 16
13 of 18
1-3(1-x
)2/3
1-3(1-xcs)2/3
+2(1-x
) +2(1-xcs)
cscs
0.14
in Figure 12. The same data treatment method was used for the results obtained on the effects of
283K (Figure 9) and particle size (Figure 10).
ammonium hydroxide
concentration
0.12
293K
298K
0.14
0.10
313K
283K
323K
0.12
0.08
293K
333K
298K
0.10
0.06
313K
323K
0.08
0.04
333K
0.06
0.02
0.04
0.00
0
500
1000
1500
2000
2500
3000
3500
4000
0.02
Time, seconds
0.00
0 Figure
500
1000 kinetics
1500
2000 slag 2500
35006.
12. Leaching
of converter
using the 3000
data in Figure
4000
Time, seconds
The regression lines shown in Figure 12 fit very well to experimental data over the entire
Figure
12. Leaching
kinetics
of results
converter
slag using the
data
in Figure 6. of this kinetic model
temperature range
(0.95
> R2 > 0.99)
These
demonstrate
the
applicability
Figure
12.
Leaching
kinetics
of
converter
slag
using
the
data
in Figure 6.
for diffusion in a porous layer of radius ro (Equation (12)).
The regression lines shown in Figure 12 fit very well to experimental data over the entire
Similarly to the above,
though to quantify reaction order n, a graph was constructed using
The range
regression
lines
in Figure
12demonstrate
fit very well
experimental
datakinetic
over the
entire
temperature
(0.95 >
R2 >shown
0.99) These
results
thetoapplicability
of this
model
kinetic data on the effect of NH4OH concentrations. Results are shown in Figure 13, where
2 > 0.99) These results demonstrate the applicability of this kinetic model
(0.95
> Rof
for temperature
diffusion in arange
porous
layer
radius ro (Equation (12)).
experimental data on all ammonium hydroxide concentrations measured fit very well to Equation
for
diffusion
porousthough
layer oftoradius
ro (Equation
(12)).n, a graph was constructed using kinetic
Similarly
to in
thea above,
quantify
reaction order
(12). Afterwards, the slopes of each adjustment were used for Equation (13) and a graph of ln kapp
the above,
though to quantify
reaction
order
a graph
was constructed
using
data on Similarly
the effect to
of NH
OH concentrations.
Results
are shown
in n,
Figure
13, where
experimental
versus ln CNH4OH 4was constructed, with initial particle radius and temperature constant, as
data on thehydroxide
effect ofconcentrations
NH4OH concentrations.
are
Figure
13, where
datakinetic
on all ammonium
measured fitResults
very well
to shown
Equationin(12).
Afterwards,
shown in Figure 14.
on all ammonium
concentrations
measured
well
Equation
the experimental
slopes of eachdata
adjustment
were used hydroxide
for Equation
(13) and a graph
of ln kfit
versus
ln to
CNH
appvery
4 OH
Afterwards,
slopes
of each
adjustment
were used
for Equation
(13)inand
a graph
was(12).
constructed,
withthe
initial
particle
radius
and temperature
constant,
as shown
Figure
14. of ln kapp
versus ln CNH4OH was constructed, with initial particle radius and temperature constant, as
shown in0.12
Figure 14.
0.1 mol/dm3
0.2 mol/dm3
0.3 mol/dm3
3
0.5
0.1 mol/dm
mol/dm33
0.6
0.2 mol/dm
mol/dm33
0.7
0.3 mol/dm
mol/dm33
0.8
0.5 mol/dm
mol/dm3
1-3(1-x
)2/3
1-3(1-xcs)2/3
+2(1-x
) +2(1-xcs)
cscs
0.10
0.12
0.08
0.10
0.06
0.08
0.04
0.6 mol/dm3
0.7 mol/dm3
0.8 mol/dm3
0.06
0.02
0.04
0.00
0.02
0
1000
2000
3000
4000
5000
6000
Time, seconds
0.00
Figure 13. Leaching kinetics of converter slags for different NH4 OH concentrations and particle size of
0
1000
2000
3000
4000
5000
6000
64 µm.
Time, seconds
Figure 13. Leaching kinetics of converter slags for different NH4OH concentrations and particle size
of 64 m.
Metals 2020, 10, 712
13 of 16
-6.5
-7.0
ln kapp
-7.5
-8.0
-8.5
-9.0
-9.5
-10.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
ln CNH4OH
Figure 14. Analysis of the reaction order in relation to the concentration of ammonium hydroxide.
Figure 14. Analysis of the reaction order in relation to the concentration of ammonium hydroxide.
This figure shows the linear adjustment of the seven kapp values (over the whole NH4 OH
This figure
shows
theR2linear
adjustment
the seven
app values
(overusing
the whole
4OH
concentration
range)
with
of 0.90.
Reactionoforder
n was kthen
calculated
slope NH
values,
2 of 0.90. Reaction order n was then calculated using slope values, which
concentration
range)
with
R
which resulted in a reaction order n of 1.2 for NH4 OH concentration in the solution.
resulted
in a reaction
order
n ofparticle
1.2 for NH
4OHsquared
concentration
the solution.
Looking
at Equation
(13),
radius
variedininversely
with the apparent kinetic
Looking
at Equation
particle radius
squareddata
varied
inversely
the10apparent
kinetic
constant.
To corroborate
this(13),
relationship,
experimental
obtained
fromwith
Figure
were used
and
constant.
To
corroborate
this
relationship,
experimental
data
obtained
from
Figure
10
were
used
and
adjusted according to Equation (12), yielding Figure 15. Controlled diffusion through porous layer
2
adjusted
according
to
Equation
(12),
yielding
Figure
15.
Controlled
diffusion
through
porous
layer
of radius
ro (Equation
(13)),
requires that 1/(ro ) must vary linearly with ln kapp . Figure 16 shows
Metals 2020,
10, x FOR PEER
REVIEW
15 of 18
of radius
ro (Equation
requires
thatthe
1/(rln
o2)kmust
vary
linearly
with
ln
k
app. Figure
16
shows
such
such
variation,
where it(13)),
can be
seen that
data
have
a
very
good
linear
fit
to
experimental
app
variation,
where
it can bethat
seenthe
that
the ln kapp data
havemodel
a very(Equation
good linear
to experimental
data,
R2 of 0.98,
indicating
heterogeneous
kinetic
(12))fitmodels
converter data,
slag
2
R of 0.98, very
indicating
dissolution
well. that the heterogeneous kinetic model (Equation (12)) models converter slag
dissolution very well.
0.07
1-3(1-xcs)2/3+2(1-xcs)
0.06
0.05
180 m
127 m
90 m
64 m
0.04
0.03
0.02
0.01
0.00
0
10
20
30
40
Time, seconds
Figure 15. Slag leach kinetics for different particle sizes.
Figure 15. Slag leach kinetics for different particle sizes.
0.0030
0.0025
50
Time, seconds
Figure 15. Slag leach kinetics for different particle sizes.
Metals 2020, 10, 712
14 of 16
0.0030
0.0025
kapp
0.0020
0.0015
0.0010
0.0005
0.0000
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
1/r°2, 1/m2
Figure 16. Converter slag leaching kinetics for different particle sizes.
Figure 16. Converter slag leaching kinetics for different particle sizes.
To calculate the activation energy (Ea ), the intrinsic rate constant (modifying Equation (13)) was
To calculate
the
activation
energy
(Ea), (given
the intrinsic
rate11),
constant
(modifying
Equation
was
first calculated
using
apparent
rate
constants
in Figure
at temperature
range
283 to(13))
333 K,
3
first
calculated
using apparent
rate, constants
(given
at temperature
range
to µm).
333 K,
NH
concentration
0.7 mol/dm
reaction order
n in
of Figure
1.2 and11),
particle
size 67 µm
(ro =283
33.5
4 OH
3, reaction order n of 1.2 and particle size 67 m (ro = 33.5 m).
NH14OH
concentration
Table
shows
the k-values0.7
asmol/dm
a function
of the temperature range used in this study.
Table 1 shows the k-values as a function of the temperature range used in this study.
Table 1. Temperature-dependent and apparent rate constant values for converter slag leaching in
ammonia
Table 1.system.
Temperature-dependent and apparent rate constant values for converter slag leaching in
ammonia system.
T, K
283
293
298
313
323
333
T−1 /103 , K−1
kapp , s−1
−ln (s−1 µm2
(mol/dm3 )−1 )
3.534
3.413
3.356
3.195
3.096
3.003
0.30 × 10−5
1.41 × 10−5
2.09 × 10−5
3.93 × 10−5
4.35 × 10−5
6.35 × 10−5
5.265
3.718
3.324
2.693
2.592
2.213
After the k-values had been obtained, an Arrhenius graph (ln k versus 1/T) was constructed
(Figure 17). A good linear adjustment (R2 > 0.85) of the ln k data can be appreciated. The calculated
activation energy was 42.3 kJ/mol for the temperature range 283 to 333 K, which is a typical value
for a particle diffusion model (Equation (12)). Finally, then, the dissolution rate of converter slag in
an ammonia system (NH4 OH) can be represented by the following expression:
2
3
1 − 3(1 − xCS ) + 2(1 − xCS ) = 2.52 × 10
7
C1.2
NH
4 OH
r2o
e
−42.3
RT
t.
(14)
(Figure 17). A good linear adjustment (R2 > 0.85) of the ln k data can be appreciated. The calculated
activation energy was 42.3 kJ/mol for the temperature range 283 to 333 K, which is a typical value for
a particle diffusion model (Equation (12)). Finally, then, the dissolution rate of converter slag in an
ammonia system (NH4OH) can be represented by the following expression:
Metals 2020, 10, 712
15 of 16
-2.0
-2.5
ln k
-3.0
-3.5
-4.0
-4.5
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
103/T, 1/K
Figure
Figure17.
17.Arrhenius
Arrheniusgraph
graphfor
forthe
thetemperature
temperaturerange
range283
283toto333
333K.K.
Here, R = 8.314 J/mol/K, ro is given in microns, CNH
2
3
4 OH
is in mol/dm3 , t is s, T is Kelvin and k =
3 ·s).
2.52 × 107 µm2 ·mol/(dm
1 − 3(1
− xCS ) + 2(1 − xCS ) = 2.52 × 107
C1.2
NH4 OH
r2
o
𝑒
−42.3⁄
𝑅𝑇
t.
(14)
4.Here,
Conclusions
R = 8.314 J/mol/K, ro is given in microns, CNH4OH is in mol/dm3, t is s, T is Kelvin and k = 2.52 ×
7
3 s).
10 µConverter
m2۰mol/(dm
slag ۰
can be leached with NH OH at a concentration of 0.7 mol/dm3, 298 K, pH 10.5 and
4
agitation of 500 rpm. By increasing the temperature and concentration of ammonium hydroxide and
4. Conclusions
decreasing
the particle size, it was possible to obtain a high recovery of copper, reaching a value of 84.8%.
3, 298delafossite
NH
leached
copperwith
oxides
CuO and Cuof
but
however,
Converter
slag can
be leached
NHlike
4OHCu
at2aO,concentration
0.7not,
mol/dm
K, pH 10.5
4 OH directly
(CuFeO
).
This
was
corroborated
by
XRD
analyzes.
and agitation
of 500 rpm. By increasing the temperature and concentration of ammonium hydroxide
2
particlesthe
contain
a large
of microcracks
in their
refractory
structure.
Thus,
the diffusion
andThe
decreasing
particle
size,amount
it was possible
to obtain
a high
recovery
of copper,
reaching
a value
ofofthe
leaching solution through the microcracks making contact with the copper oxides would be
84.8%.
allowed.
converter
slag leaching
be described
the intraparticle
diffusion
modeldelafossite
because
NHThus,
4OH directly
leached
copper can
oxides
like Cu2O,byCuO
and Cu but not,
however,
of(CuFeO
slag porosity.
2). This was corroborated by XRD analyzes.
The
thatcontain
best represented
slag dissolution
is given
by the
following
expression:
−
Themodel
particles
a large amount
of microcracks
in their
refractory
structure.
Thus,1the
2
through
the microcracks
making
contact
withThe
thereaction
copper order
oxides
(1 − xCS ) 3of
3diffusion
+ the
2(1 leaching
− xCS ) = solution
kapp t, with
calculated
activation energy
is 42.3
kJ/mol.
would
be
allowed.
Thus,
converter
slag
leaching
can
be
described
by
the
intraparticle
diffusion
was 1.2 for the concentration of ammonium hydroxide and inversely proportional to the square of the
model radius.
because of slag porosity.
particle
Author Contributions: Conceptualization, A.A.; Formal analysis, O.J.; Funding acquisition, A.A.; Investigation,
A.A., O.J.; Methodology, A.V.; Validation, O.J.; Writing—original draft, A.A. All authors have read and agreed to
the published version of the manuscript.
Funding: The APC was financed by the Office of the Vicerrectoría de Investigación y Estudios Avanzados of the
Pontificia Universidad Católica de Valparaíso.
Conflicts of Interest: The authors declare no conflict of interest.
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