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Lithium and Aluminum Coal Fly Ash .pdf



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Titre: An Efficient Approach for Lithium and Aluminum Recovery from Coal Fly Ash by Pre-Desilication and Intensified Acid Leaching Processes
Auteur: Shenyong Li, Shenjun Qin, Lianwei Kang, Jianjun Liu, Jing Wang and Yanheng Li

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metals
Article

An Efficient Approach for Lithium and Aluminum
Recovery from Coal Fly Ash by Pre-Desilication and
Intensified Acid Leaching Processes
Shenyong Li

ID

, Shenjun Qin *, Lianwei Kang, Jianjun Liu, Jing Wang and Yanheng Li

Key Laboratory for Resource Exploration Research of Hebei Province, Hebei University of Engineering,
Handan 056038, China; shenyong360@hebeu.edu.cn (S.L.); kanglianwei@126.com (L.K.);
liujianjun1060@hebeu.edu.cn (J.L.); wangjing081824@126.com (J.W.); liyh1982@gmail.com (Y.L.)
* Correspondence: qinsj528@hebeu.edu.cn; Tel.: +86-0310-857-7902
Received: 26 June 2017; Accepted: 11 July 2017; Published: 14 July 2017

Abstract: A novel technique was developed for the recovery of lithium and aluminum from coal
fly ash using a combination of pre-desilication and an intensified acid leaching process. The main
components of the high-aluminum fly ash were found to be Al2 O3 and SiO2 , and the Al/Si ratio
increased from 1.0 to 1.5 after desiliconization. The lithium content of the coal fly ash met national
recycling standards. The optimal acid leaching conditions, under which the leaching efficiencies of
lithium and aluminum were 82.23% and 76.72%, respectively, were as follows: 6 mol/L HCl, 1:20
solid to liquid ratio, 120 ◦ C and 4 h. During the hydrochloric acid pressure leaching process, spherical
particles of desilicated fly ash were decomposed into flakes. Part of the mullite phase was dissolved,
and most of the glass phase leached into the liquor. The generation of the silicates hindered lithium
transport, which decreased the leaching rate of lithium. This work suggests that the preprocessing is
a promising option for effective recovery of high-aluminum and fly ash-associated lithium.
Keywords: coal fly ash; pre-desilication; acid leaching; lithium; high-aluminum

1. Introduction
Coal fly ash (CFA) is an industrial waste residue formed from organic matter, clay, and associated
minerals after the high-temperature combustion and cooling of coal in coal-fired power plants. CFA can
cause serious dust pollution [1], and the toxic elements contained in CFA can leach into soil and cause
serious secondary pollution [2–4]. Millions of tonnes of CFA, containing high proportions of aluminum
and lithium, are released each year from coal-fired power stations in the Inner Mongolia, Ningxia,
and Shanxi provinces in northern China [5–8], which is illustrated in Figure 1. Typically, the CFA in
these areas contains 40–50% aluminum and more than 0.2% lithium. The lithium content of this CFA
generally exceeds the comprehensive industrial recycling index of the Chinese national standard for
Specifications for Rare Metal Mineral Exploration (DZ/T/0203-2002) regarding pegmatite-associated
lithium. Considering its high aluminum and lithium content [5,9], this CFA can be utilized as
a substitute for bauxite and allophytin, high quality sources of which are scarce in China. The fly ash
covered in this study was markedly enriched in a number of trace elements in comparison with that of
the other regions shown in Figure 2 [10,11]. Furthermore, the lithium contained in the CFA should be
recycled. Therefore, the development of an effective extraction technique for aluminum and lithium
from CFA is of great significance to resources recycling [12].
Previously, aluminum has been recovered from CFA using hydrometallurgy techniques such as
solvent extraction [13], acid leaching [14–18], alkaline leaching [19–21], and acid/alkali combination
methods [22]. The CFA may be pre-processed before leaching using auxiliary reinforcement methods,
such as lime stone sintering [6], soda lime sintering [23], and ammonium sulfate sintering [14,24].
Metals 2017, 7, 272; doi:10.3390/met7070272

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combination
combination methods
methods [22].
[22]. The
The CFA
CFA may
may be
be pre-processed
pre-processed before
before leaching
leaching using
using auxiliary
auxiliary
reinforcement
methods,
such
as
lime
stone
sintering
[6],
soda
lime
sintering
[23],
and
ammonium
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2 of 12
reinforcement methods, such as lime stone sintering [6], soda lime sintering [23], and ammonium
sulfate
sintering
[14,24].
Each
process
has
its
own
advantages
and
disadvantages.
For
instance,
large
sulfate sintering [14,24]. Each process has its own advantages and disadvantages. For instance, large
quantities
quantities of
of residue
residue (8–10
(8–10 times
times greater
greater than
than the
the original
original quantity
quantity of
of CFA)
CFA) are
are generated
generated during
during
Each process
has
its ownammonium
advantagessulfate
and disadvantages.
For instance,
large
quantities
of residue
sintering
[25].
Moreover,
sintering
also
produces
toxic
gases,
such
sintering [25]. Moreover, ammonium sulfate sintering also produces toxic gases, such as
as ammonia.
ammonia.
(8–10 times
greater than the more
original quantity
of CFA) arethan
generatedleaching,
during sintering
[25]. Moreover,
Alkaline
Alkaline leaching
leaching requires
requires more energy
energy and
and material
material than acid
acid leaching, and
and exhibits
exhibits aa lower
lower
ammoniumextraction
sulfate sintering
also produces
toxic
gases, such as
ammonia.
Alkaline leaching
requires
aluminum
efficiency.
With
the
development
of
corrosion-resistant
materials,
aluminum extraction efficiency. With the development of corrosion-resistant materials, acid
acid
more energy
and material
than acid leaching,
and exhibits
a lower aluminum
extraction efficiency.
leaching
leaching has
has become
become increasingly
increasingly feasible
feasible and
and efficient.
efficient. However,
However, CFA
CFA contains
contains various
various minerals
minerals
Withare
the development
of corrosion-resistant
materials,
acid leaching
has become increasingly
feasible
that
that arenot
notreadily
readilydissolved
dissolvedin
insolutions
solutionsof
ofmineral
mineralacids,
acids,such
suchas
asmullite,
mullite,quartz,
quartz,alpha-aluminum,
alpha-aluminum,
and efficient.
However, CFA contains
variousleaching
minerals that
are not readily dissolved
in solutionsthe
of
and
and aluminosilicate.
aluminosilicate. Hence,
Hence, the
the direct
direct acid
acid leaching of
of CFA
CFA isis impracticable,
impracticable, and
and destroys
destroys the
mineral
acids,
such
as
mullite,
quartz,
alpha-aluminum,
and
aluminosilicate.
Hence,
the
direct
acid
structure
structure of
of the
the CFA.
CFA. The
The addition
addition of
of fluoride
fluoride can
can significantly
significantly improve
improve the
the acid
acid leaching
leaching of
of CFA
CFA
leaching
of
CFA
is
impracticable,
and
destroys
the
structure
of
the
CFA.
The
addition
of
fluoride
can
[18].
Moreover,
a
mixture
of
ammonium
bisulfate
and
aqueous
sulfuric
acid
was
used
for
[18]. Moreover, a mixture of ammonium bisulfate and aqueous sulfuric acid was used for the
the
significantly
improve
the
acid
leaching
of
CFA
[18].
Moreover,
a
mixture
of
ammonium
bisulfate
and
high-efficiency
high-efficiency leaching
leaching of
of aluminum
aluminum from
from high-aluminum
high-aluminum CFA
CFA [14].
[14]. However,
However, these
these techniques
techniques
aqueous large
sulfuric acid was
used and
for the high-efficiency
leaching of aluminum from high-aluminum
consume
consume largequantities
quantitiesof
ofacid
acid andcause
causesecondary
secondarypollution.
pollution.
CFA [14]. However, these techniques consume large quantities of acid and cause secondary pollution.

Figure
Figure1.1.Location
Locationof
ofthe
thehigh-aluminum
high-aluminumcoalfields
coalfieldsin
innorthern
northern China.
China.

Figure 2. Relative
concentrations
of a number
of trace elements
in fly
ash in Shuozhou,and
Yunnan,
Figure
Figure2.2.Relative
Relativeconcentrations
concentrationsof
ofaanumber
numberof
oftrace
traceelements
elementsin
infly
flyash
ashin
inShuozhou,
Shuozhou,Yunnan,
Yunnan, andSpetzugli.
Spetzugli.
and Spetzugli.

In
Inthe
thepresent
presentstudy,
study,CFA
CFAwas
waspretreated
pretreatedusing
usingdesilication
desilicationto
todisrupt
disruptsilicon-aluminum
silicon-aluminumbonds
bonds
In
the
present
study,
CFA
was
pretreated
using
desilication
to
disrupt
silicon-aluminum
bonds
and
in
and increase
increase the
the extraction
extraction efficiency
efficiency of
of lithium
lithium and
and aluminum
aluminum during
during acid
acid leaching.
leaching. As
As shown
shown
in
and
increase
the
extraction
efficiency
of
lithium
and
aluminum
during
acid
leaching.
As
shown
in
Figure
3,
the
complete
aluminum
and
lithium
extraction
process
consists
of
pre-desilication,
Figure 3, the complete aluminum and lithium extraction process consists of pre-desilication,
Figure
3, the
complete
aluminum
and
lithium extraction
process
consists of
pre-desilication,
intensified
intensified
acid
leaching,
filtration,
decontamination,
and
precipitation
recycling.
The
study
intensified
acid
leaching,
filtration,
decontamination,
and
precipitation
recycling.
The present
present
study
acid leaching, filtration, decontamination, and precipitation recycling. The present study focused
on the pressure acid leaching of lithium and aluminum from CFA in a hydrochloric acid solution.

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focused on the pressure acid leaching of lithium and aluminum from CFA in a hydrochloric acid
solution.
effects of
leaching
processon
conditions
on aluminum
lithium and
aluminum
were
The
effectsThe
of leaching
process
conditions
lithium and
extraction
wereextraction
systematically
systematicallyand
investigated,
and mechanisms
the correlative
mechanisms
were characterized.
investigated,
the correlative
were
characterized.

Figure
of aluminum
aluminum and
and lithium
lithium from
from coal
coal fly
fly ash
ash (CFA).
(CFA).
Figure 3.
3. A
A flow
flow diagram
diagram of
of the
the extraction
extraction of

2. Materials
Materials and
and Methods
Methods
2.
2.1. Raw
Raw Materials
Materials
2.1.
CFA was
was obtained
obtained from
from the
the pulverized-coal-fired
pulverized-coal-fired boilers
boilers of
of thermal
thermal power
power plants
plants in
inShuozhou,
Shuozhou,
CFA
Shanxi Province,
Province, China.
China. The
The material
material samples
samples had
had an
an average
averageparticle
particlesize
sizeof
of33.2
33.2µm
μm(d(d5050)) and
andaadd9090
Shanxi
value
of
about
73.9
μm.
The
chemical
composition
of
the
CFA
was
analyzed
using
X-ray
value of about 73.9 µm. The chemical composition of the CFA was analyzed using X-ray fluorescence
0
fluorescence
(XRF)
spectroscopy
(ARL
Perform′
X
4200,
Thermo
Scientific,
Boston,
MA,
USA).
(XRF) spectroscopy (ARL Perform X 4200, Thermo Scientific, Boston, MA, USA). Inductively-coupled
Inductively-coupled
plasma(ICP-MS;
mass spectrometry
X Series 2,Boston,
Thermo
Scientific,
Boston,
plasma
mass spectrometry
X Series 2, (ICP-MS;
Thermo Scientific,
MA,
USA) was
usedMA,
for
USA)
was
used
for
the
elemental
analysis.
The
morphology
and
phase
mineralogy
analyses
were
the elemental analysis. The morphology and phase mineralogy analyses were carried out using
carried out
using microscopy
scanning electron
microscopy
UHR
FE-SEM
SU8220,
Hitachi,
Japan) and
scanning
electron
(SEM; UHR
FE-SEM(SEM;
SU8220,
Hitachi,
Japan)
and X-ray
diffractometry
X-ray diffractometry
(XRD; D/Max-2200,
Rigaku, Japan), respectively.
(XRD;
D/Max-2200, Rigaku,
Japan), respectively.
All
the
reagents
used
in
this
study
were
of analytical
grade,
forHHCl,
H2SO4, HF, and
All the reagents used in this study were
of analytical
grade,
exceptexcept
for HCl,
2 SO4 , HF, and HNO3 ,
HNO3were
, which
were of reagent
guaranteed
grade.
water
used for allAllofof the
the
which
of guaranteed
grade.reagent
Deionized
waterDeionized
was used for
all ofwas
the experiments.
experiments.
All
of
the
chemicals
were
purchased
from
commercial
sources
and
used
as
received
chemicals were purchased from commercial sources and used as received without further purification.
without further purification.
2.2. Pre-Desilication Experiments
2.2. Pre-Desilication Experiments
The experiments were carried out in a 1 L batch autoclave fitted with a stirring device and
Theelectrical
experiments
were
carried
out in a CFA
1 L batch
autoclave
fitted
with a stirring
device
and
external
heater
system.
Typically,
sample
(0.2 kg) and
a sodium
hydroxide
solution
◦ C for
external
Typically,
a CFA sample
(0.2 kg)
andata 120
sodium
hydroxide
(6
× 10−4electrical
m3 , 150 heater
kg/m3system.
) were mixed
at a moderate
agitation
speed
1 h. The solution
stirring
−4 300
3) were to
(6 × 10of
m3, rpm
150 kg/m
mixed
moderate
agitationforspeed
°C for 1 h.The
Theselective
stirring
speed
was applied
keep at
theaCFA
in suspension
all of at
the120
experiments.
speed ofof300
rpm wassilicon
applied
toachieved.
keep the After
CFA alkaline
in suspension
forthe
alldesiliconized
of the experiments.
The
leaching
amorphous
was
leaching,
CFA (DCFA)
selective
leaching
was
achieved.
After alkaline
leaching,
desiliconized
was
separated
fromof
theamorphous
solution bysilicon
vacuum
filtration.
Deionized
water was
used tothe
remove
all of the
CFA (DCFA) was separated from the solution by vacuum filtration. Deionized water was used to
remove all of the residual liquor and increase the final volume to 1 L. The chemical composition of

Metals 2017, 7, 272

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residual liquor and increase the final volume to 1 L. The chemical composition of CFA and DCFA are
shown in Table 1. The separated DCFA samples were used in the intensified acid leaching processes.
Table 1. Main chemical composition of CFA and the desiliconized coal fly ash (DCFA) samples (mass
fraction, wt %). Abbreviation: L.O.I, loss of ignition.
Content/%

SiO2

Al2 O3

CaO

Fe2 O3

TiO2

P2 O5

MgO

Li2 O

Na2 O

MnO

L.O.I

CFA
DCFA

44.12
34.30

42.17
49.88

2.44
1.98

2.43
3.07

1.67
1.81

0.69
0.08

0.68
0.81

0.20
0.22

0.14
8.74

0.02
0.02

1.41
0.22

2.3. Intensified Acid Leaching Processes
The main objective of the present study was to improve the leaching of aluminum and lithium
from CFA, thereby facilitating the downstream recovery of solid aluminum and lithium products.
The acid leaching experiments were conducted in 0.5 L sealed hydrothermal reaction kettles fitted
with polytetrafluoroethylene-lined pots. These kettles were placed into an incubator heated to the
target temperature. The examined acid leaching variables were the acid species, acid concentration,
temperature, solid to liquid (S/L) ratio, and time (Table 2). The aluminum in the filtrates was analyzed
as detailed in the Chinese national standard GB/T9734-2008; the lithium was analyzed using atomic
absorption spectrometry (AAS; Zeenit 700, Analytik Jena AG, Germany). The air-dried residues were
analyzed using XRD and SEM. Intensified acid leaching experiments were carried out for both CFA
and DCFA.
Table 2. Intensified acid leaching conditions. Abbreviations: S/L, solid to liquid.
Leaching Condition
Factors

Acid Concentration
(mol/L)

Leaching
Temperature (◦ C)

Acid species
Acid concentration
Temperature

6
1, 2, 4, 6, 8
6

120
120
30, 60, 90, 120, 150

S/L ratio

6

120

Time

6

120

S/L Ratio

Leaching Time (h)

1:20
1:20
1:20
1:5, 1:10, 1:15, 1:20,
1:30, 1:40
1:20

4
4
4
4
0.5, 1, 2, 4, 6, 8

3. Results and Discussion
3.1. CFA Analysis
The chemical composition analysis showed that the CFA samples contained 42.17 wt % aluminum
and 0.2 wt % lithium oxide (Table 1), which is equivalent to mid-grade bauxite ores. The combined
aluminum and silica content in the CFA was more than 85%, and the mass ratio of aluminum to silicon
dioxide (A/S) was approximately 1; thus, the Bayer process is not suitable for the direct recovery of
the CFA. However, the concentration of lithium is unusually high in the studied CFA, and close to that
of some lithium-rich ores.
The XRD analysis (Figure 4) indicated that the CFA was primarily composed of mullite, corundum,
and quartz, which is consistent with previously reported research [14]. Moreover, the lithium phase
consisted of minor lithium silicate. SEM images show that the granular features of the CFA consisted
of microspherical beads, some of which were smooth, while others were pitted with cracks and
subsidence holes. The loss of ignition (L.O.I) of the CFA was determined to be 2.13, which accounts for
carbon residue and indicates that some compositions were readily resolved.

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Figure
XRDpattern
patternof
ofraw
raw CFA
CFA sample.
sample.
Figure
4.4.XRD
Figure 4. XRD pattern of raw CFA sample.
3.2. Pre-Desilication of the CFA
3.2. Pre-Desilication of the CFA
The chemical
composition
and XRD pattern of the DCFA samples are shown in Table 1 and
3.2. Pre-Desilication
of the
CFA
The
chemical
composition
and
XRD pattern of the DCFA samples are shown in Table 1 and
Figure 5, respectively. After pre-desilication, the A/S ratio of the DCFA increased to 1.5. During the
The5,chemical
andwas
XRD
pattern
ofA/S
thethe
DCFA
are
shown in
1 andand
Figure
respectively.
pre-desilication,
thefrom
ratio
of samples
the
DCFA
increased
toTable
1.5.
During
the
alkali
reaction, composition
the After
free silica
removed
main
phase.
However,
most
mullite
Figure
5,
respectively.
After
pre-desilication,
the
A/S
ratio
of
the
DCFA
increased
to
1.5.
During
the
alkali
reaction,
the
free
silica
was
removed
from
the
main
phase.
However,
most
mullite
and
corundum
corundum is stabile in concentrated sodium hydroxide solution, as shown in Table 3. After
alkali
reaction,
the
free silica
was
removed
frominthe
phase.
However,
most
mullite
and
is stabile
in concentrated
sodium
hydroxide
solution,
shown
in Table
3. After
pretreatment,
lithium
pretreatment,
lithium
silicate
was
still evident
theasmain
XRD
pattern.
Hydroxy
sodalite
was produced
corundum
is
stabile
in
concentrated
sodium
hydroxide
solution,
as
shown
in
Table
3.
After
silicate
wasreaction
still evident
in the
XRD
pattern.
Hydroxy
sodalite
was produced
by the
reaction
of excess
by the
of excess
alkali
with
mullite.
Some of
the corundum
diffraction
peaks
disappeared
pretreatment,
lithium
silicate
wascorundum
still little
evident
in the XRD
pattern.
Hydroxy
sodalite
was
produced
alkali
with
mullite.
Some
of the
diffraction
peaks
after
pretreatment;
however,
after
pretreatment;
however,
very
aluminum
could
bedisappeared
detected
in the
alkali
leaching
solution.
by
the
reaction
of
excess
alkali
with
mullite.
Some
of
the
corundum
diffraction
peaks
disappeared
veryHence,
little aluminum
could be
detectedcorundum
in the alkali
solution.
Hence, in
complex
chemistry
complex chemistry
involving
andleaching
NaOH may
have resulted
the formation
of
after
pretreatment;
however,
very
little
could
detected
in the
alkaliasleaching
solution.
involving
and
NaOH
may
have
in be
the
formation
of NaAlO
, which
then(2)
reacted
NaAlO
2corundum
, which then
reacted
with
Naaluminum
2SiO3 resulted
to generate
hydroxy
sodalite
[20],
reactions
and
2per
Hence,
complex
chemistry
involving
corundum
and NaOH
may than
havethat
resulted
in the
formation
of
Na
2O generate
content of
the DCFA
was significantly
greater
of CFA,
which
is consistent
with(4).
NaThe
hydroxy
sodalite
[20], as per reactions
(2) and (4).
The Na
of the
2 SiO
3 to
2 O content
NaAlO
2, which then reacted with Na2SiO3 to generate hydroxy sodalite [20], as per reactions (2) and
with
the
data
shown
in
Table
1.
The
possible
reactions
occurring
during
pre-desilication
are
as 1.
DCFA was significantly greater than that of CFA, which is consistent with the data shown in Table
(4). follows:
The Na2O content of the DCFA was significantly greater than that of CFA, which is consistent
The possible reactions occurring during pre-desilication are as follows:
with the data shown in Table 1. The possible reactions occurring during pre-desilication are as
NaOH + SiO2 → Na2SiO3 + H2O
(1)
follows:
NaOH + SiO2 → Na2 SiO3 + H2 O
(1)
NaOH
+
Al
2
O
3

NaAlO
2
+
H
2
O
(2)
NaOH + SiO2 → Na2SiO3 + H2O
(1)
NaOH
+
Al
O

NaAlO
+
H
O
(2)
2
3
2
2
3Al2O3·2SiO
2 + NaOH
Na
8(AlSiO
4)H
6 (OH)
2 + NaAl(OH)4
NaOH
+ Al2O→
3 →
NaAlO
2 +
2O
(2) (3)
3Al
O
NaOH+→
(3)
3 ·22SiO
8 (AlSiO
4 )6 (OH)
2 + NaAl(OH)
4
Na
SiO32 2+++NaAlO
H
2Na
O

Na48)(AlSiO
2 + NaOH
3Al22O
3·2SiO
NaOH 2→
Na
8(AlSiO
6 (OH)42)6+·(OH)
NaAl(OH)
4
(3) (4)

Na
NaAlO2 2+ +HH
O → Na8 (AlSiO
4 )6 ·(OH)
2 + NaOH
Na22SiO
SiO33 ++NaAlO
2O2 → Na8(AlSiO
4)6·(OH)
2 + NaOH

Figure 5. XRD pattern of DCFA.
Figure
5. XRD
pattern
of DCFA.
Figure
5. XRD
pattern
of DCFA.

(4)

(4)

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6 of 12

Table 3. Mineralogical analysis of CFA and DCFA (wt %).

Table 3. Mineralogical
analysis of CFA
and DCFA
(wt %).
Lithium
Glass
Hydroxy
Amphodelite
Quartz Corundum
Silicate
Phase
Sodalite
Lithium
Glass
Hydroxy
Phase
Mullite
Quartz
Corundum
Amphodelite
CFA
65.3
13.2
0.9
12.4
3.7
Silicate
Phase
Sodalite
DCFA
12.1 13.2
0.8 0.9
-12.4
9.12.6CFA 63.965.3. 2.4 3.7
DCFA
63.9
2.4
12.1
0.8
9.1
2.6

Phase

Mullite

6 of 12

Calcite
Calcite

1.2

1.6
1.2
1.6

3.3. Intensified Acid Leaching Processes

3.3. Intensified Acid Leaching Processes

3.3.1. Effect of Acid Species

3.3.1. Effect of Acid Species

The effects
of different acid species (hydrochloric acid, nitric acid, sulfuric acid, and hydrofluoric
The effects of different acid species (hydrochloric acid, nitric acid, sulfuric acid, and
acid) on
the
leaching
of aluminum
and
areand
shown
in Figure
6. After
intensified
hydrofluoric acid)rates
on the
leaching rates
of lithium
aluminum
lithium
are shown
in Figure
6. Afteracid
leaching,
most
of
the
components
of
the
DCFA
were
dissolved.
All
of
the
acids
accelerated
intensified acid leaching, most of the components of the DCFA were dissolved. All of the acids the
pressured
leaching
of aluminum
andoflithium.
hydrofluoric
acid exhibited
the lowest
accelerated
the pressured
leaching
aluminumHowever,
and lithium.
However, hydrofluoric
acid exhibited
the lowest
aluminum
and lithium
extraction
It may be
that aluminum
fluoride
aluminum
and lithium
extraction
efficiencies.
It mayefficiencies.
be that aluminum
fluoride
sediment was
formed,
sediment
was
formed,
which
was
sparingly
soluble
in
water
and
the
mineral
acids,
but
reacted
with
which was sparingly soluble in water and the mineral acids, but reacted with silicate; the extraction of
silicate;
the extraction
of lithium
fromofDCFA
was higher
than that
of aluminum.
mullite,
lithium
from DCFA
was higher
than that
aluminum.
The mullite,
corundum,
and The
lithium
silicate
corundum, and lithium silicate phases were dissolved into the bulk solution. The non-volatility of
phases were dissolved into the bulk solution. The non-volatility of sulfuric acid is not conducive to
sulfuric acid is not conducive to the leaching of liquid concentrate; therefore, the extraction of
the leaching of liquid concentrate; therefore, the extraction of aluminum using sulfuric acid required
aluminum using sulfuric acid required higher steam temperatures and pressures than those for the
higherother
steam
temperatures and pressures than those for the other acids tested. The lithium leaching
acids tested. The lithium leaching efficiency of hydrochloric acid was higher than that of nitric
efficiency
of
hydrochloric
acid acid
was was
higher
than as
that
nitric acid.
Hence, and
hydrochloric
acid was
acid. Hence, hydrochloric
selected
theofoptimal
acid species
used in further
selected
as the optimal acid species and used in further experiments.
experiments.

◦ C, S/L ratio of 1/20, 4 h).
Figure 6.
Effects
of different
acid acid
species
on the
acid
leaching
120°C,
Figure
6. Effects
of different
species
on the
acid
leachingofofDCFA
DCFA(6
(6mol/L,
mol/L, 120
S/L ratio of 1/20, 4 h).

3.3.2. Effect
of Hydrochloric
Acid
Concentration
3.3.2. Effect
of Hydrochloric
Acid
Concentration
Hydrochloric acid concentration has a significant effect on the leaching rates of lithium and
Hydrochloric
acid concentration has a significant effect on the leaching rates of lithium and
aluminum [16]. Figure 7 shows that the aluminum and lithium extraction efficiencies increased
aluminum [16]. Figure 7 shows that the aluminum and lithium extraction efficiencies increased with
with hydrochloric acid concentration from 1 to 8 mol/L, with maximal rates of 79.5% and 83.6%,
hydrochloric
acid concentration from 1 to 8 mol/L, with maximal rates of 79.5% and 83.6%, respectively.
respectively. At higher concentrations, the extraction efficiencies decreased; this was attributed to
At higher
concentrations,
the extraction
efficiencies
decreased;
this the
wasresistance
attributedoftosome
the mass
the mass transfer resistance
of aluminum
and lithium
ions and
phasestransfer
to
2−
+
resistance
of
aluminum
and
lithium
ions
and
the
resistance
of
some
phases
to
decomposition
as a of
result
decomposition as a result of the formation of silicic acid from SiO3 and H . The lithium content
2

+
of the DCFA
formation
of silicic
acidoffrom
SiO3 and
andthe
Hextraction
. The lithium
contentisofmore
DCFA
is lower
is lower
than that
aluminum,
of aluminum
efficient
thanthan
thatthat
of of
lithium.
This
corresponds
the low observed
lithium,
which does
diffuse
aluminum,
and
the
extractionwith
of aluminum
is moreconcentrations
efficient thanofthat
of lithium.
Thisnot
corresponds
welllow
in acidic
solutions.
Subsequently,
a dynamicwhich
experimental
analysis
of well
lithium
leaching
was
with the
observed
concentrations
of lithium,
does not
diffuse
in acidic
solutions.
conducted.
The
aluminum
and
lithium
extraction
efficiencies
obtained
in
the
present
study
were
Subsequently, a dynamic experimental analysis of lithium leaching was conducted. The aluminum
higher than those obtained in previous acid leaching studies [16,21] under the same condition. This
and lithium extraction efficiencies obtained in the present study were higher than those obtained in
was attributed to the desilication process and the high-pressure conditions. The results indicate that
previous acid leaching studies [16,21] under the same condition. This was attributed to the desilication
process and the high-pressure conditions. The results indicate that the optimum hydrochloric acid

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Metalsthe
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7, 272

hydrochloric acid concentration was 6 mol/L. Slightly higher extraction efficiencies
7 of 12
thewere
optimum
hydrochloric
acid HCl;
concentration
6 mol/L.
Slightly
higher extraction
efficiencies
obtained
with 8 mol/L
however,was
using
6 mol/L
HCl significantly
reduces
the acid’s
were
obtained with
8 mol/L HCl; however, using 6 mol/L HCl significantly reduces the acid’s
consumption
and pollution.
consumption
and
pollution.
concentration
was
6 mol/L.
Slightly higher extraction efficiencies were obtained with 8 mol/L HCl;
however, using 6 mol/L HCl significantly reduces the acid’s consumption and pollution.
%Extraction efficiency
%Extraction efficiency

90
80
70
60
50
40
30

90
80
70
60
Aluminium
Aluminium
Lithium
Lithium

50
40
30
0

0

1

1

2

2

3
4
5
6
3 Acid concentration(mol/L)
4
5
6
Acid concentration(mol/L)

7

7

8

8

9

9

7. Effect
of hydrochloric
acid concentration
theleaching
acid leaching
of DCFA
(120
°Cratio
, S/Lof
ratio
of 1/20,
FigureFigure
7. Effect
of hydrochloric
acid concentration
on theon
acid
of DCFA
(120 ◦ C,
S/L
1/20,
4 h). 4 h).
Figure 7. Effect of hydrochloric acid concentration on the acid leaching of DCFA (120 °C, S/L ratio of 1/20, 4 h).

3.3.3.
Effect
of Temperature
3.3.3.
Effect
of Temperature
3.3.3.
Effect
of Temperature
The effects of temperature on the hydrochloric acid leaching extraction efficiencies of
The
effects
of temperature
on the on
hydrochloric
acid leaching
efficiencies
ofefficiencies
aluminum of
The
effects
temperature
the
hydrochloric
acidextraction
leaching
extraction
aluminum
and of
lithium
from DCFA
in
a sealed
hydrothermal
reaction kettle
are shown
in Figure 8.
and
lithium
from
DCFA
in
a
sealed
hydrothermal
reaction
kettle
are
shown
in
Figure
8.
The
aluminum
aluminum
and lithium
from DCFA
in a sealed
hydrothermal
reaction kettle
shown
in Figure 8.
The aluminum
and lithium
extraction
increased
with◦ temperature.
At 120are
°C,
the aluminum
and
and
lithium
extraction
with temperature.
At
120temperature.
C, the aluminum
extraction
The
aluminum
and increased
lithium
extraction
increased
with
At 120and
°C,lithium
the
aluminum
and
lithium
extraction
efficiencies
were 76.7%
and 82.3%,
respectively, which
were
maintained
at higher
efficiencies
were 76.7%
and 82.3%, respectively,
whichrespectively,
were maintained
at
higher
temperatures.
lithium
extraction
76.7% and
were
maintained
higher
temperatures.
Atefficiencies
30 °C, thewere
aluminum
and82.3%,
lithium
extraction which
efficiencies
were
38.2% at
and
41.1%,
At temperatures.
30 ◦ C, the aluminum
and
lithium
extraction
efficiencies
were
38.2%
and
41.1%,
respectively.
At
30
°C,
the
aluminum
and
lithium
extraction
efficiencies
were
38.2%
and
41.1%,
respectively. At approximately
103
°C,
the
extraction
efficiency
of
aluminum
was
higher
than
◦ C, the extraction efficiency of aluminum was higher than that of lithium. that
At respectively.
approximately
103
At
approximately
103 °C, the and
extraction
efficiency
of aluminum
wasofhigher
than that
of lithium. With increasing temperature
pressure,
the diffusion
velocities
hydrochloric
acid
With
increasing
temperature
and
pressure, the
diffusion
velocities
of hydrochloric
acid
and activated
of and
lithium.
With
increasing
temperature
and
pressure,
the
diffusion
velocities
of
hydrochloric
activated molecules increases rapidly, resulting in higher ion collision probabilities, acid
which
molecules
increases
rapidly, resulting
higher ion
collisioninprobabilities,
which
increases
the rate and
and
activated
increasesinrapidly,
resulting
higher
ionextraction
collision
probabilities,
which
increases
themolecules
rate and effectiveness
of leaching.
Above 120
°C, the
efficiency
of aluminum

effectiveness
of
leaching.
Above
120
C,
the
extraction
efficiency
of
aluminum
was
higher
than
that
increases
the rate
and
effectiveness
leaching.
120 to
°C,the
thelow
extraction
efficiency
aluminum
was higher
than
that
of lithium, of
which
was Above
attributed
concentration
of of
lithium
and its
of lithium,
which
was
attributed
to the
low was
concentration
of
lithium
and
its mass transfer
resistance.
was
higher
than
that
of
lithium,
which
attributed
to
the
low
concentration
of
lithium
mass transfer resistance. In addition, it is difficult to destroy the mullite structureand
at its
low
In addition,
it is difficult
to destroyaddition,
the mulliteit structure
at low
somestructure
of the leavings
mass
transfer
resistance.
is difficult
to temperatures,
destroy
the and
mullite
at low
temperatures,
and some In
of the leavings
were
not dissolved.
Considering
extraction
efficiency
and
were
not dissolved.
efficiency
energyConsidering
consumption,
the optimal
leaching
temperatures,
andConsidering
somethe
of optimal
theextraction
leavings
weretemperature
not and
dissolved.
extraction
efficiency
and
energy consumption,
leaching
is 120 °C.
◦ C.
temperature
is
120
energy consumption, the optimal leaching temperature is 120 °C.
% Extraction efficiency
% Extraction efficiency

90
80
70
60
50
40
30
20

90
80
70
60
50

Aluminium
Aluminium
Lithium
Lithium

40
30
20

80
100
120
140
160
80
100
120
140
160
Temperature(℃)
Temperature(℃)
FigureFigure
8. Effect
of temperature
on the on
acid
of DCFA
(6 mol/L
HCl, S/L
1/20,
4 h). 4 h).
8. Effect
of temperature
theleaching
acid leaching
of DCFA
(6 mol/L
HCl,ratio
S/L of
ratio
of 1/20,
Figure 8. Effect of temperature on the acid leaching of DCFA (6 mol/L HCl, S/L ratio of 1/20, 4 h).
20

20

40

40

60

60

3.3.4.3.3.4.
EffectEffect
of S/L
of Ratio
S/L Ratio
3.3.4. Effect of S/L Ratio
The leaching
experiments
werewere
carried
out under
the following
conditions:
hydrochloric
acid acid
The leaching
experiments
carried
out under
the following
conditions:
hydrochloric

The
leaching
experiments
were
carried
out
under
the
following
conditions:
hydrochloric
acid in
concentration
=
6
mol/L,
temperature
=
120
C,
time
=
4
h.
The
effect
of
the
S/L
ratio
is
shown
concentration = 6 mol/L, temperature = 120 °C, time = 4 h. The effect of the S/L ratio is shown
concentration
=
6
mol/L,
temperature
=
120
°C,
time
=
4
h.
The
effect
of
the
S/L
ratio
is
shown
in Figure 9. The aluminum extraction efficiency decreased from 76.7% to 22% when the S/L was in

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Figure 9.
9. The
The aluminum
aluminum extraction
extraction efficiency
efficiency decreased
decreased from
from 76.7%
76.7% to
to 22%
22% when
when the
the S/L
S/L was
was
Figure
increased from 1/40
1/40 to 1/5.
1/5. The
The lithium
lithium extraction efficiency
efficiency decreased linearly when the
the S/L
S/L ratio
increased
from 1/40 to 1/5.
The lithium extraction
extraction efficiency decreased linearly when the
S/L ratio
was decreased from
from 1/20
1/20 to
to 1/5.
1/5. The
The S/L
S/L ratio
ratio refers
refers to
to the
the mass
mass of
of solids
solids to
to the
the volume
volume of
of acid;
acid; the
the
was decreased
1/20 to 1/5. The S/L ratio refers to the mass of solids to the volume of acid;
volume
of
acid
is
relative,
and
the
mass
of
solids
is
constant.
Increased
acid
volume
enhances
the
volume
of acid
is relative,
andand
the the
mass
of solids
is constant.
Increased
acidacid
volume
enhances
the
the volume
of acid
is relative,
mass
of solids
is constant.
Increased
volume
enhances
leaching conditions,
conditions, and
and ensures
ensures that
that the
the DCFA
DCFA can be
be thoroughly mixed
mixed in the
the hydrothermal
leaching
the leaching
conditions, and
ensures that
the DCFAcan
can bethoroughly
thoroughly mixed in
in
hydrothermal
reaction kettle. Violent
Violent chemical reactions occurred in the heterogeneous systems when the surfaces
reaction
kettle.
chemical reactions occurred in the heterogeneous systems when the surfaces
of
the
reactant
particles
were covered with hydrochloric
hydrochloric acid solution, which promoted aluminum
of the reactant particles were
covered with hydrochloric acid solution, which promoted aluminum
and
lithium
dissolution.
Furthermore,
the
presence
of more hydronium ions implies an increased
and lithium dissolution. Furthermore,
Furthermore, the
the presence
presence of
of more hydronium ions implies an increased
accumulation
of
silicic
acid,
which
increases
the
viscosity
of the
the leach
leach liquor,
liquor, such
such that
that itit obstructs
obstructs
accumulation of
of silicic
silicicacid,
acid,which
whichincreases
increasesthe
theviscosity
viscosity
ofof
the leach
liquor, such
that it obstructs
the
the mass transfer
transfer of
of lithium
lithium and
and results in
in lower
lower extraction
extraction efficiency.
efficiency. This
This corresponds
corresponds with
with the
the
the
massmass
transfer of lithium
and resultsresults
in lower extraction
efficiency. This corresponds
with the decrease
decrease
in
lithium
extraction
efficiency
observed
at
S/L
ratios
greater
than
1/20.
Hence,
the
optimal
decrease
lithium extraction
observed
at S/L
ratios
greater
1/20.
Hence,
theS/L
optimal
in lithiuminextraction
efficiency efficiency
observed at
S/L ratios
greater
than
1/20.than
Hence,
the
optimal
ratio
S/L
ratio
was
determined
to
be
1/20.
S/L ratio
was determined
to be 1/20.
was
determined
to be 1/20.

Figure 9. Effect of S/L ratio on the acid leaching of DCFA (6 mol/L HCl, 120 ◦°C, 4 h).
Figure9.9. Effect
Effectof
ofS/L
S/L ratio
DCFA (6
(6 mol/L
mol/L HCl,
Figure
ratio on
on the
the acid
acid leaching
leaching of
of DCFA
HCl, 120
120 °C,
C,44h).
h).

%%Extraction
Extractionefficiency
efficiency

3.3.5. Effect of
of Time
3.3.5.
3.3.5. Effect
Effect of Time
Time
The
effects
of time
time on
on lithium
lithium and
and aluminum
aluminum extraction
extraction efficiencies
efficiencies are
are depicted
depicted in
in Figure
Figure 10.
10.
The
of
The effects
effects
of time
on lithium
and
aluminum
extraction
efficiencies
are
depicted
in Figurewith
10.
In
general,
previous
studies
have
indicated
that
the
extraction
ratio
of
aluminum
increases
In
general,
previous
studies
have
indicated
that
the
extraction
ratio
of
aluminum
increases
with
In
general, reaction
previoustime
studies
haveleaching
indicated
that the extraction
ratio of
aluminum
with
increasing
in acid
acid
conditions.
A similar
similar trend
trend
was
observedincreases
in the
the present
present
increasing
reactiontime
timeinin
leaching
conditions.
A
was
observed
in
increasing
reaction
acid
leaching
conditions.
A
similar
trend
was
observed
in
the
present
study.
study. An
An aluminum
aluminum extraction
extraction efficiency
efficiency of
of 36.6%,
36.6%, 76.8%,
76.8%, 78.5%,
78.5%, and
and 79.5%
79.5% were
were obtained
obtained after
after 11 h,
h,
study.
An
aluminum
extraction
efficiency
of
36.6%,
76.8%,
78.5%,
and
79.5%
were
obtained
after
1
h,
4
h,
6
h,
4 h,
h, 66 h,
h, and
and 88 hh of
of acid
acid leaching,
leaching, respectively.
respectively. The
The lithium
lithium extraction
extraction efficiency
efficiency did
did not
not significantly
significantly
4and
8 h of
acid
leaching,
respectively.
The longer
lithiumthan
extraction
efficiency
did not
significantly
increase
at
increase
at
acid
leaching
reaction
times
4
h.
As
the
reaction
progresses,
the
aluminum
increase
at acid
leaching
reaction
times
longer
than
4 h. As the
reaction the
progresses,
theand
aluminum
acid
leaching
reaction
times
longer
than
4
h.
As
the
reaction
progresses,
aluminum
lithium
and lithium
lithium phases
phases are
are gradually
gradually dissolved,
dissolved, and
and the
the reaction
reaction substrates
substrates become
become depleted.
depleted. The
The
and
phases
are
gradually
dissolved,
and
the
reaction
substrates
become
depleted.
The
decrease
in
decrease in
in the
the lithium
lithium extraction
extraction ratio
ratio at
at long
long reaction
reaction times
times may
may be
be attributed
attributed to
to the
the production
productionthe
of
decrease
of
lithium
extraction
ratio
at
long
reaction
times
may
be
attributed
to
the
production
of
orthosilicic
acid,
orthosilicic
acid,
and
the
degradation
of
the
lixivium
filtering
behavior.
Thus,
optimal
aluminum
orthosilicic
acid, andofthe
degradation
of the
lixivium
filtering
behavior.
Thus,
and
the
degradation
the
lixivium
filtering
behavior.
Thus,
optimal
aluminum
andoptimal
lithiumaluminum
extraction
and
lithium
extraction
was
achieved
after
4
h
at
a
reaction
temperature
of
120
°C.
and
lithium
extraction
was
achieved
after
4
h
at
a
reaction
temperature
of
120
°C.

was achieved after 4 h at a reaction temperature of 120 C.
90
90
80
80
70
70
60
60

Aluminium
Aluminium
Lithium
Lithium

50
50
40
40
30
30

00

11

22

33

44

55

Time(hours)
Time(hours)

66

77

88

99

◦ C, S/L ratio of 1/20).
Figure
Figure10.
10.Effect
Effectofoftime
timeon
onthe
theacid
acidleaching
leachingofofDCFA
DCFA(6(6mol/L
mol/LHCl,
HCl,120
120 °C,
S/L
1/20).
Figure 10. Effect of time on the acid leaching of DCFA (6 mol/L HCl, 120 °C, S/L ratio of 1/20).

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3.4. Characterization of Lithium and Aluminum Extraction under Optimum Conditions
3.4. Characterization of Lithium and Aluminum Extraction under Optimum Conditions
3.4.1. Extraction Efficiency of Lithium and Aluminum from CFA and DCFA

90

90

80

80

70

70

60
50
40

CFA
DCFA

30
20

% Extraction efficiency

% Extraction efficiency

3.4.1.The
Extraction
Efficiencyprocess
of Lithium
and Aluminum
from CFA
and
DCFA of pre-desilication and
novel recovery
developed
in the present
study
consisted
intensified
acidrecovery
leaching.process
The single
factor experiments
in the following
optimized acid
The novel
developed
in the presentresulted
study consisted
of pre-desilication
and
leaching
conditions
for
extracting
aluminum
from
DCFA:
a
hydrochloric
acid
concentration
of 6
intensified acid leaching. The single factor experiments resulted in the following optimized acid
mol/L,
a
leaching
temperature
of
120
°C,
pressure
of
1.8
MPa,
an
S/L
ratio
of
1/20,
and
a
leaching
leaching conditions for extracting aluminum from DCFA: a hydrochloric acid concentration of 6 mol/L,
of 4 h.temperature
Figure 11 shows
aluminum
CFA andtime
DCFA
◦ C,lithium
atime
leaching
of 120the
pressureand
of 1.8
MPa, anextraction
S/L ratio efficiencies
of 1/20, andfor
a leaching
of
under
these
optimized
conditions.
The
extraction
ratios
of
lithium
and
aluminum
from
the
DCFA
4 h. Figure 11 shows the lithium and aluminum extraction efficiencies for CFA and DCFA under these
nearly doubled
after The
4 h.extraction
Figure 11a
indicates
thatand
thealuminum
CFA desilication
processnearly
enhances
the
optimized
conditions.
ratios
of lithium
from the DCFA
doubled
leaching
lithium.
lithiumthat
extraction
efficiency
of CFA
is similar
to that
DCFA inofthe
initial
after
4 h. ofFigure
11aThe
indicates
the CFA
desilication
process
enhances
theofleaching
lithium.
stage
of
pressure
acid
leaching.
However,
the
amount
of
lithium
extracted
from
the
DCFA
increases
The lithium extraction efficiency of CFA is similar to that of DCFA in the initial stage of pressure
dramatically
the latter
leachingof
stage.
For extracted
DCFA, 82.3%
theDCFA
total lithium
wasdramatically
extracted at
acid
leaching.during
However,
the amount
lithium
fromofthe
increases
4
h,
which
then
decreased
slowly
with
increased
leaching
time;
the
lithium
extraction
CFA
during the latter leaching stage. For DCFA, 82.3% of the total lithium was extracted at 4from
h, which
increased
slowly
with
time.
Figure
11b
compares
the
aluminum
extraction
efficiencies
from
CFA
then decreased slowly with increased leaching time; the lithium extraction from CFA increased
and DCFA
the same
conditionsthe
(10aluminum
g, 120 °C,extraction
6 mol/L efficiencies
HCl, 1/20 S/L
a DCFA
sealed
slowly
with under
time. Figure
11b compares
fromratio,
CFA in
and
hydrothermal
reaction
kettle).
The
aluminum
extraction
efficiency
increased
with
leaching
time
for

under the same conditions (10 g, 120 C, 6 mol/L HCl, 1/20 S/L ratio, in a sealed hydrothermal
both
CFA
and
DCFA.
The
aluminum
extraction
ratio
of
DCFA
was
significantly
higher
than
that
of
reaction kettle). The aluminum extraction efficiency increased with leaching time for both CFA and
CFA after
min. This
result indicates
a significant
proportion
the mullite,
which
highly
DCFA.
The10
aluminum
extraction
ratio of that
DCFA
was significantly
higherofthan
that of CFA
afteris10
min.
stable
and
acid-resistant,
was
dissolved
or
transformed
into
soluble
substances.
In
the
This result indicates that a significant proportion of the mullite, which is highly stable and acid-resistant,
pre-desilication
intensifiedinto
acid
leaching
processes,
hydroxide
hydrochloric
was
dissolved orand
transformed
soluble
substances.
Inthe
the sodium
pre-desilication
andand
intensified
acid
acid
are
present
in
excess,
based
upon
stoichiometric
calculations;
thus,
the
lithium
and
aluminum
leaching processes, the sodium hydroxide and hydrochloric acid are present in excess, based
upon
could theoretically
be completely
leached
the desilication
stage, sodium
aluminate
was
stoichiometric
calculations;
thus, the
lithiumout.
andDuring
aluminum
could theoretically
be completely
leached
generated
from
excess alkali,
transformed
intowas
aluminum
hydroxide.
wasalkali,
dissolved
out.
During
thethe
desilication
stage,and
sodium
aluminate
generated
from theThis
excess
and
under
acidic
conditions,
which
resulted
in
a
rapid
increase
in
aluminum
extraction
during
early
transformed into aluminum hydroxide. This was dissolved under acidic conditions, which the
resulted
acid
leaching
stage.inThe
results show
that the
leaching
reaction
rate
may bestage.
controlled
by chemical
in
a rapid
increase
aluminum
extraction
during
the early
acid
leaching
The results
show
reactions
and diffusion
transfer
at the early
of acid
leaching,
that the
extraction
of
that
the leaching
reactionmass
rate may
be controlled
by stage
chemical
reactions
and and
diffusion
mass
transfer at
aluminum
occurred
mainly
via
chemical
reactions;
lithium
extraction
was
mainly
controlled
by
the early stage of acid leaching, and that the extraction of aluminum occurred mainly via chemical
diffusion mass
transfer
in thewas
later
stagescontrolled
of leaching.
agreesmass
withtransfer
the results
of previous
work
reactions;
lithium
extraction
mainly
byThis
diffusion
in the
later stages
of
[26].
Hence,
the
desilication
of
CFA
is
feasible
and
necessary
for
the
efficient
extraction
of
lithium
leaching. This agrees with the results of previous work [26]. Hence, the desilication of CFA is feasible
and necessary
aluminum.
as is
shown in
4, and
the aluminum.
main chemical
compositions
distribution
and
forHowever,
the efficient
extraction
of Table
lithium
However,
as is shown
in Table of
4,
CFA,
DCFA,
and
the
residues
demonstrates
that
most
of
the
lithium
and
aluminum
in
the
DCFA
the main chemical compositions distribution of CFA, DCFA, and the residues demonstrates that most
was
withaluminum
the intensified
leaching.
of
thedissolved
lithium and
in theacid
DCFA
was dissolved with the intensified acid leaching.

10

60
50
40
CFA
DCFA

30
20
10

0

0

0

30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480

0

30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480

Time(minutes)

Time(minutes)

(a)

(b)

Figure 11.
11. Extraction
Extraction efficiency
efficiency of
of CFA
CFA and
and DCFA
DCFA under
under optimum
optimum conditions,
conditions, (a)
(a) lithium
lithium extraction;
extraction;
Figure
(b)
aluminum
extraction
(6
mol/L,
120
°C,
S/L
ratio
of
1/20,
4
h).

(b) aluminum extraction (6 mol/L, 120 C, S/L ratio of 1/20, 4 h).
Table 4. Chemical compositions of CFA, DCFA, and the residues (mass fraction, wt %).
Content/%
CFA
DCFA
Leaching residue

SiO2
44.12
34.30
76.81

Al2O3
42.17
49.88
9.84

CaO
2.44
1.98
0.31

Fe2O3
2.43
3.07
0.26

TiO2
1.67
1.81
1.31

MgO
0.68
0.81
0.12

Li2O
0.20
0.22
0.02

Metals 2017, 7, 272

10 of 12

Table 4. Chemical compositions of CFA, DCFA, and the residues (mass fraction, wt %).
Content/%
CFA
DCFA
residue
Metals 2017, Leaching
7, 272

SiO2

Al2 O3

CaO

Fe2 O3

TiO2

MgO

Li2 O

44.12
34.30
76.81

42.17
49.88
9.84

2.44
1.98
0.31

2.43
3.07
0.26

1.67
1.81
1.31

0.68
0.81
0.12

0.20
0.22
0.02

10 of 12

3.4.2. Morphological Analysis
3.4.2.
Representativesamples
samplesprocessed
processed
under
different
conditions
prepared
and examined
Representative
under
different
conditions
werewere
prepared
and examined
using
using Figure
SEM. Figure
12 the
shows
the microscopic
topographies
CFA,and
DCFA,
and leaching
the acid
SEM.
12 shows
microscopic
surface surface
topographies
of CFA, of
DCFA,
the acid
leaching Glazed
residue.microspheres
Glazed microspheres
and porous
spongy bodies
porouscan
bodies
can beseen
clearly
seenCFA
in the
CFA
residue.
and spongy
be clearly
in the
sample
sample
(Figure
12a).
The
major
components
of
these
microspheres
were
found
to
be
aluminum
(Figure 12a). The major components of these microspheres were found to be aluminum oxide and
oxide and
silicon
dioxide,
which to
corresponds
to previous
the results
of previous
studies cenospheres
[20]. Generally,
silicon
dioxide,
which
corresponds
the results of
studies
[20]. Generally,
are
cenospheres
are smaller
and darker
than
thethe
desilication
CFA, the
smaller
and darker
than ferrospheres.
After
theferrospheres.
desilication ofAfter
the CFA,
surfaces of of
thethe
microspheres
surfacescoarse
of the
and collapsed,
and the
main
components
become
andmicrospheres
collapsed, andbecome
the maincoarse
components
were gradually
eroded
(Figure
12b). Thiswere
was
gradually for
eroded
(Figure 12b).
Thisions
wasinto
beneficial
for theoftransport
acid ionsparticles.
into the A
interiors
of
beneficial
the transport
of acid
the interiors
the largeofirregular
majority
the
large
irregular
particles.
A
majority
of
the
spherical
bodies
were
decomposed,
becoming
smaller
of the spherical bodies were decomposed, becoming smaller and irregularly shaped (Figure 12c,d).
and irregularly
shapedwith
(Figure
12c,d).and
Hydrogen
ions
reacted with
mullite
corundum
to
Hydrogen
ions reacted
the mullite
corundum
to generate
smallthe
holes,
whichand
is also
consistent
generate
smallobservations
holes, which [14].
is also
with of
previous
observations
[14].
A dissolved
large quantity
of
with
previous
A consistent
large quantity
the abundant
alumina
was
by the
the abundant
alumina
dissolved
by theisacid,
as waswith
the lithium
silicate.
This result
is extraction
consistent
acid,
as was the
lithiumwas
silicate.
This result
consistent
the observed
increase
in the
with the observed
increase
in the extraction
efficiencies of lithium and aluminum with time.
efficiencies
of lithium
and aluminum
with time.

Figure 12.
12. SEM
SEM images
imagesofofsamples
samplesprocessed
processed
under
optimum
conditions.
(a) CFA;
(b) DCFA;
Figure
under
optimum
conditions.
(a) CFA;
(b) DCFA;
(c,d) (c,d)
acid

acid
leaching
residue
(6
mol/L
HCl,
120
°C,
S/L
ratio
of
1/20,
4
h).
leaching residue (6 mol/L HCl, 120 C, S/L ratio of 1/20, 4 h).

4. Conclusions
In the present study, a two-step process, based on pre-desilication and intensified acid leaching,
was developed for the efficient extraction of aluminum and lithium from CFA. The factors
influencing the extraction of aluminum and lithium under pressure acid leaching were investigated.
The optimal pre-desilication conditions were experimentally determined to be a 150 kg/m3 sodium
hydroxide solution, an S/L ratio of 1/3, a temperature of 120 °C, and a reaction time of 1 h. In the
intensified acid leaching process, aluminum and lithium extraction efficiencies of 76.7% and 82.3%,

Metals 2017, 7, 272

11 of 12

4. Conclusions
In the present study, a two-step process, based on pre-desilication and intensified acid leaching,
was developed for the efficient extraction of aluminum and lithium from CFA. The factors influencing
the extraction of aluminum and lithium under pressure acid leaching were investigated. The optimal
pre-desilication conditions were experimentally determined to be a 150 kg/m3 sodium hydroxide
solution, an S/L ratio of 1/3, a temperature of 120 ◦ C, and a reaction time of 1 h. In the intensified acid
leaching process, aluminum and lithium extraction efficiencies of 76.7% and 82.3%, respectively, were
obtained from DCFA under optimum acid leaching conditions: a hydrochloric acid concentration of
6 mol/L, a leaching temperature of 120 ◦ C, an S/L ratio of 1/20, and a leaching time of 4 h.
The experimental results show that the A/S ratio of the CFA was increased from 1 to 1.5 after
desiliconization. Overall, the extraction ratio of lithium was higher than that of aluminum. Compared
with using only acid leaching, the pre-desilication process significantly increased the extraction
efficiencies of lithium and aluminum from the CFA. Further studies are needed to assess the viability
of recycling process of HCl and the economy, and the separation and purification of lithium and
aluminum products will be investigated in the next stage.
Acknowledgments: Special thanks are given to the editor and reviewers for their careful reviews. We gratefully
acknowledge financial support from the National Natural Science Foundation of China (No. 41472133), the
Science Foundation of Hebei (No. D2014402046), and the Scientific Research Foundation of the Higher Education
Institutions of Hebei Province (No. QN2016049).
Author Contributions: Shenyong Li and Shenjun Qin conceived and designed the experiments; Shenyong Li and
Lianwei Kang performed the experiments; Jianjun Liu and Jing Wang analyzed the data; Yanheng Li contributed
sample fabrication; Shenyong Li wrote the paper. All authors contributed to the data analysis and discussion.
Conflicts of Interest: The authors declare no conflict of interest.

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