Solid Products Characterization in a Multi-Step

1
Solid Products Characterization in a Multi-Step
2
Mineralization Process
3
Azadeh Hemmatia,b, Jalal Shayeganb, Paul Sharratta, Tze Yuen Yeoa, Jie Bu∗ a
a
4
b
5
A*Star, Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833, Singapore
Chemical and Petroleum Engineering Department, Sharif University of Technology, Azadi Street, Tehran, Iran
6
7
Abstract
8
In this paper, we describe a carbon dioxide mineralization process and its associated solid products.
9
These solid products include amorphous silica, iron hydroxides and magnesium carbonates. These
10
products were subjected to various characterization tests, and the results are published here. It was
11
found that the iron hydroxides from this process can have different crystalline properties, and their
12
formation depended very much on the pH of the reaction conditions. Different forms of magnesium
13
carbonate were also obtained, and the type of carbonate precipitated was found to be dependent on the
14
carbonation temperature. Hydromagnesite was obtained mainly at low temperatures, while dypingite
15
was obtained at higher temperatures. Near ambient conditions, nesquehonite was the predominant
16
form of magnesium carbonate obtained. The process mass balance shows that 3.1 tonne of hydrated
17
magnesium carbonate with 99.82 wt % purity is obtained for each tonne of CO2 sequestrated. And for
18
each tonne of carbon dioxide sequestrated, about 3.74 tonnes of mineral ore are needed as a source of
19
magnesium.
20
21
Keywords: Carbon Utilization, Mineralization, Magnesium Carbonate, Characterization
22
23
24
Research highlights
Corresponding author, Tel/fax: +65 6316 6188
E-mail address: Bu_Jie@ices.a-star.edu.sg.
∗
1
1
A multi-step pH-swing process makes value- added products from captured CO2.
2
High yields of carbonate precipitate are obtained from carbonation of the extracted
magnesium.
3
Different forms of magnesium carbonate can be obtained depending on reaction
4
temperature.
5
6
Different forms of iron hydroxide can be obtained depending on reaction pH.
7
The precipitate has a purity of 99.9% based on MgO content.
8
Contents
9
1.
Introduction ................................................................................................................... 3
10
2. Material and methods ........................................................................................................ 6
11
2.1. Process description ..................................................................................................... 6
12
2.1. 1. Mineral pre-treatment .......................................................................................... 7
13
2.1.2. Dissolution experiments ....................................................................................... 7
14
2.1.3. Purification experiments ....................................................................................... 7
15
2.1.4. Carbonation experiments ...................................................................................... 8
16
2.2. Material characterisation ............................................................................................. 8
17
3. Results and discussion ..................................................................................................... 10
18
3.1. Characterization of raw materials .............................................................................. 10
19
3.2. Acid extraction of mineral ore................................................................................... 11
20
3.3. Removal of impurities in leachate ............................................................................. 12
21
3.4. Precipitation of magnesium carbonate ....................................................................... 14
22
3.4.1. The effect of precipitation temperature on morphology....................................... 15
23
3.5. Process mass balance ................................................................................................ 18
24
4. Conclusions ..................................................................................................................... 20
25
Acknowledgements ............................................................................................................. 21
26
References .......................................................................................................................... 21
27
28
29
2
1
1. Introduction
2
The world’s reliance on using fossil fuels as a cheap and convenient energy source has led to
3
the emission of large amounts of carbon dioxide [1]. As a consequence, CO2 concentrations
4
in the atmosphere have been on an upward trend, with it currently being 30% above pre-
5
industrial levels. This increase is very likely to lead to irreversible harmful consequences
6
[2,3]. As such, there exists a pressing need to develop carbon dioxide mitigation technologies
7
to address this problem.
8
The accelerated weathering of minerals, also called mineral carbonation, provides an option
9
for carbon sequestration. This method may be particularly useful in regions where suitable
10
underground geological formations for CO2 injection are unavailable, the risk of CO2 leakage
11
from an underground site is considered unacceptable, or where mineral sources for
12
carbonation abound in the vicinity of large carbon emission sources (such as coal-fired power
13
plants) [4]. The main advantage of mineral carbonation is that the CO2 is stored as carbonates
14
that are stable over geological time periods (millions of years) [5]. The capacity of mineral
15
carbonation is also huge, and it can potentially store billions of tonnes of carbon dioxide per
16
year permanently [6,7]. However, its typically high energy requirements and poor process
17
economics are two limiting factors for mineral carbonation [8].
18
One of the more promising routes for mineral carbonation involves aqueous reactions. This
19
process is similar to the natural alteration of ultramafic rocks, also called serpentinization [9].
20
Currently there are two main approaches for aqueous carbonation of calcium and magnesium
21
silicates: (a) direct carbonation, where mineral dissolution and carbonation take place
22
simultaneously in an aqueous solution, and (b) indirect pH-swing processes which includes
23
the extraction of alkaline-earth metals from a mineral and separate precipitation of the
24
carbonate in subsequent steps [10,11]. In a direct approach, the entire process is
25
thermodynamically favoured. As a result, energy inputs are theoretically not required during
3
1
the chemical processing of the material. This method is currently regarded as the most
2
economical mineral-carbonation pathway [12]. However, mass transfer limitations arise from
3
the low solubility of CO2 in the acidic environment which is necessary for mineral
4
dissolution. As a result the direct process may be less favourable kinetically.
5
On the other hand, if process efficiency and high product purities are desirable, an indirect,
6
multi-step mineral carbonation process appears to be more attractive than a direct single step
7
process [13]. The indirect aqueous approach to CO2 mineralization has several advantages:
8
the process is kinetically favoured and simple, the products are of high purity and the reaction
9
takes place in a very short time [14,15]. Most indirect methods involve the use of HCl, H2SO4
10
or NaOH as reagents in the process [16,17,18,19,20]. Typically the magnesium contained in
11
serpentine dissolves quickly in acid, forming magnesium salts in solution and leaving behind
12
silica:
13
14
HX + Mg3Si2O5(OH)4
MgX2 + SiO2 + H2O
(eq.1)
15
16
The magnesium in the resulting leachate is filtered from the silica residue, and precipitated as
17
magnesium carbonate in a separate unit operation downstream in the process [21]. However,
18
due to the relatively high cost associated with carbon mineral sequestration, commercial
19
implementations of this technology will not be possible without considering the utilization of
20
solid products [12,22,23,24]. In order for the products to be marketable, the quality and purity
21
of the products must be sufficiently high [25].
22
The magnesium carbonate products from the indirect process can find use in the paper,
23
polymer, environmental protection and fertilizer industries [26,27]. However, potential lower
24
value applications may include utilizing the product for mine reclamation or soil modification
25
[28]. The disposal of carbonate solids near the surface or underground has low environmental
4
1
risks. The magnesite product could possibly replace conventional soil liming agents, while
2
the porous silica residue could improve soil water retention in arid environments [29]. Bai et
3
al [30] showed that the silica residue can also be further processed to produce a purified
4
nanoporous SiO2. The process includes extraction with NaOH followed by CO2 bubbling in
5
solution to reprecipitate the silica. At higher purities, magnesite has also been used as a fire
6
retardant and as filler material for the manufacturing of paint, bricks, and ceramics [31,32].
7
Magnesium carbonate can also be used as structural material for electric arc furnaces, blast
8
furnaces, basic oxygen furnace and housing construction [33].
9
It is also possible to obtain different morphs of magnesium carbonate from a mineralization
10
process. The properties of magnesium carbonate can vary according to its different
11
morphologies. Therefore it is valuable to be able to control morphologies according to the
12
requirements, and controlled synthesis is important for the practical application of materials
13
[34,35]. Two different morphologies of MgCO3·3H2O have been reported in literatures by
14
adjusting the reaction conditions in the precipitation process (such as reaction temperature,
15
stirring time, and aging time) to produce either agglomerates consisting of very thin sheets or
16
well-formed needles [35].
17
The different morphologies of magnesium carbonate usually involve different degrees of
18
hydration or carbonation, for example nesquehonite (Mg(CO3)·3H2O) which is thermally
19
stable up to 100 ºC [36]. Nesquehonite forms as a mineral in nature at relatively low-
20
temperatures, and the synthetic product can be used in a large number of industrial
21
applications, such as in the production of eco-cement and corrosion-resistant protective
22
coatings [14]. The specifications of different grades of magnesium carbonate for various
23
industrial uses are shown in Table 1 [37].
24
25
Table 1
5
1
Other metal cations are also present in magnesium-silicate minerals, the most abundant of
2
these being iron [38,39].
3
The industrial applications of the solid products from carbon dioxide mineralization are
4
summarized in Table 2.
5
Table 2
6
7
8
In this article, we detail the characteristics of the products obtained from a novel carbon
9
mineralization process [41]. In this context, we describe the quantities, purity and
10
morphology of products from a closed loop multi-step mineralization process under different
11
conditions. Moreover, we present the synthesis of different kinds of magnesium carbonates
12
and by-products from a magnesium-silicate mineral with a view to investigate the quality of
13
these products in the CO2 sequestration process.
14
2. Material and methods
15
2.1. Process description
16
A general scheme of the novel mineralization process is presented in Figure 1. In the
17
extraction step, magnesium carbonate is reacted with acid to give a magnesium-rich leachate.
18
The residual silica and magnetite is filtered off. The leachate contains mainly magnesium, as
19
well as trivalent and divalent iron. The trivalent iron is removed as a precipitate (P1) via the
20
careful addition of base to adjust the solution pH to a preset value (pHI). The remaining
21
solution is then transferred to a second precipitation reactor, where the solution pH is
22
adjusted to another preset pH value (pHII) to remove the divalent iron as a precipitate (P2).
23
After filtering of P2, the impurities-free solution is transferred to a third precipitation reactor,
24
where the magnesium is reacted with sodium carbonate to give magnesium carbonate
6
1
precipitates (P3) at an even higher pH (pHIII). The sodium carbonate used to produce P3 is
2
obtained from the scrubbing of flue gas CO2 by NaOH. The resultant sodium chloride salt in
3
the end solution is recycled to a bipolar membrane electro-dialyser, where it is regenerated
4
into HCl and NaOH for re-use in the process.
5
Figure 1.
6
7
8
2.1. 1. Mineral pre-treatment
9
Pre-treatment of the mineral was carried out by milling the magnesium silicate rock with a
10
ball mill to decrease the size particles of the ore (purchased from Quebec) to < 56 µm.
11
12
2.1.2. Dissolution experiments
13
4 g of milled mineral ore with 10–56 µm particle size fractions were added into 100 mL 1M
14
HCl solution. The dissolution experiments were carried out in a three-neck round bottom
15
flask at 80 ºC and atmospheric pressure. The mixture was agitated with a magnetic stirrer at
16
a constant rate of 600 rpm. A condensation apparatus was used to prevent vapour losses of
17
HCl and water. After 6 h of dissolution, the mixture was cooled down to ambient temperature
18
and filtered with a 0.45 µm membrane filter paper.
19
20
2.1.3. Purification experiments
21
To obtain P1, 100 ml of the filtered leachate was prepared and transferred into a conical flask.
22
The initial pH of the leachate was measured and recorded using a METTLER TOLEDO
23
Seven Easy pH meter (S20) with accuracy +/- 0.01 at 25°C. 1M NaOH was added dropwise
24
to the leachate at room temperature and pressure. The solution was constantly stirred at 600
25
rpm. Addition of NaOH was stopped when the pH of the solution reached a value of 5. The
7
1
resulting slurry was then filtered using 0.45 µm membrane filter paper to give a filtrate and
2
the P1 solids.
3
After removal of P1, whole amount of filtrate 1 was transferred to a second conical flask. In
4
this step, 1M NaOH was added until the pH of the solution reached a value of 9. The
5
temperature, pressure and stirring conditions for this step are identical to the previous step
6
where P1 was obtained. The precipitate in this step (P2) was filtered using 0.45 µm membrane
7
filter paper to give a second filtrate and the P2 solids.
8
2.1.4. Carbonation experiments
9
1M Na2CO3 solution was prepared and used as the CO2 source for the carbonation step.
10
In order to test the effect of carbonation temperature on the morphology of the precipitated
11
carbonates, individual carbonation experiments were carried out at 0, 10, 20, 30, 40, 50 and
12
60ºC. For the carbonation experiments, 100 ml filtrate from the second purification step was
13
prepared in a conical flask. The temperature of the filtrate was maintained at the preset
14
temperature, and the previously prepared Na2CO3 solution was added dropwise until the pH
15
of the solution reached a value of 10 (pHIII). The reaction was carried out at atmospheric
16
pressure, and the solution was constantly stirred at 600 rpm. The precipitate in this step (P3)
17
was filtered using 0.45 µm membrane filter paper to give a third filtrate and the P3 solids.
18
2.2. Material characterisation
19
The raw minerals were analyzed by XRD X-Ray powder diffraction (Bruker AXS powder
20
diffractometer D8 Advance) with Cu-Kα1 source (λ=1.541Å) over the range 2θ=2.000-
21
105.008° at step size of 0.017° to determine the composition of mineral phases in the raw
22
material.
23
The starting minerals and products were also analyzed by XRF (XRF, S4 Explorer) to
24
determine the elemental oxide composition in the materials. The minerals were also subjected
8
1
to TGA (SDT 2960 Simultaneous DSC-TGA) analysis to investigate the water and hydroxide
2
content of the rock.
3
The silica residue from the extraction stage, P1, P2 and P3 were washed and dried for 24 h at
4
80ºC and then characterised by XRD analysis. The indexing of the crystalline phases of the
5
products was accomplished using a DIFFRAC plus EVA auxiliary software. The results were
6
compared with the ICDD database. Morphological analyses were also characterized by means
7
of a scanning electron microscopy (SEM).
8
The carbonate content in P2 and P3 was investigated via elemental analysis as well as TG-
9
FTIR. In this laboratory, a Setaram Setsys 12 TG unit is linked to a Digilab Excalibur series
10
FTIR spectrometer fitted with a KBr beamsplitter and a mid-range (liquid N2-cooled) MCT
11
detector. The interface unit comprises a transfer line and 10 cm pathlength gas cell, both
12
thermostatted at 220°C. In a typical experiment, 40–50 mg of sample was loaded in an
13
alumina crucible in dry air (30 ml min-1, 1 : 1 purge : auxiliary flow) and the furnace ramped
14
at 10°C min-1 to a target temperature and held isothermal for 1 h. Time-resolved FTIR data
15
acquisition was made using the kinetics mode, in which a spectrum was recorded every 20 s,
16
representing an average of 60 coadded scans at 4 cm-1 resolution. Evolution profiles specific
17
to H2O vapour (band centre at 3900 cm-1) and CO2, (2340 cm-1) were tracked using pre-
18
existing spectral ‘‘markers’’.
19
Thermogravimetric and differential thermal analyses (TG-DTA) for representative product
20
samples were also conducted, with an air flow rate of 20 mL/min at a heating rate of 5 °C
21
min−1. The sample weight was in the range of 5–10 mg and the temperature range was from
22
ambient temperature to 800 °C.
23
ICP–OES (Varian Vista-MPX inductively-coupled plasmaoptical emission spectrometer) was
24
used to characterize the aqueous solution of Mg and Fe ions sampled at regular intervals in
25
ore dissolution process. Each 100 µl of samples were diluted in 100 ml flux with DI water.
9
1
The Mg and Fe removal efficiency in the leachate solution was calculated on the basis of the
2
MgO content of the extraction process by using the following formulae:
3
4
5
6
7
8
where Mg% and Fe% are concentrations of elements in solid products. Mproduct is the weight
9
of product and V is the leachate volume after 6 hours acid dissolution. CFe and CMg are the
10
elements concentration in leachate.
11
The particle sizes of the precipitated magnesium carbonate products were measured using
12
Marlven Master sizer MS 2000.
13
3. Results and discussion
14
3.1. Characterization of raw materials
15
Natural minerals are not homogeneous in their composition. The XRD pattern of the mineral
16
(Figure 2) shows that the ore sample contains 46.04 wt % of forsterite [Mg2Fe2SiO4] and
17
26.49 wt % of lizardite [Mg3FeSi2O5(OH)4]. Chrysotile [Mg3(Si2O5)(OH)4] (13.89 wt %) and
18
magnetite [Fe3O4] (13.59 wt %) also show up as weak additional lines.
19
20
Figure 2.
21
22
The XRF and elemental analysis of raw ore listed in table 3 showed that the dry mineral
23
contains 46.72 wt/wt % of MgO and 42.70 wt/wt % of SiO2. Some other metals also exist in
10
1
the mineral, such as Fe, Al, Cr, Ca, Ni and Mn. The most abundant impurity is FeO which is
2
8.04 wt/wt % and other elements were detected at less than 1% in solid.
3
Table 3
4
TGA results show that when the mineral sample was heated from 30 to 800 ºC, 87.54 wt % of
5
the mineral ore remained in solid phase, indicating that sample moisture accounted for about
6
12.46 wt % of the weight. These results suggest that the theoretical total magnesium and iron
7
can be extracted from each tonne of this mineral source would be about 245 and 51.5 kg
8
respectively.
9
3.2. Acid extraction of mineral ore
10
The initial colour of the slurry containing the leachate and solid residue was dark gray, and
11
the colour of the slurry became lighter as the dissolution proceeded. After filtration, the
12
filtrate appears to be a yellowish green, due to dissolved ferric chloride in solution.
13
The magnesium concentration in the leachate after 6 hours of extraction was found to be 9.2
14
g/l while the iron concentration was about 1.9 g/l. A maximum extraction yield of 93 wt %
15
for both Mg and Fe was obtained after 6 hours.
16
A grey powder was obtained after acid extraction of ore (compare with ground ore, which
17
was dark gray with a blend of purple). The residual solids were found to be amorphous silica
18
at 82.87 wt% purity detected by XRF. It also contained 14.91 wt % of MgO and less than 1.5
19
wt % of iron, presumably trapped between silica sheets.
20
Figure 3 shows the SEM image of a raw ore particle and the amorphous silica remains after
21
acid extraction.
22
23
Figure 3.
24
11
1
The solid residue from the acid leaching step consists of residual unreacted mineral rock and
2
amorphous silica. Some magnetite was also found to be magnetically separated from the
3
extraction reactor in the dissolution experiment. The magnetite is may find potential
4
application in steel industries.
5
3.3. Removal of impurities in leachate
6
The filtrate from the acid dissolution step not only contains magnesium ions, but also other
7
co-extracted metal species such as Fe, Ni, Al, Mn, Ca and etc.
8
If a pure magnesium carbonate product is desired, the other impurities must be removed from
9
the solution prior to the carbonation step. The particularly low Ksp for Fe(OH)3 leads to it
10
precipitating preferentially from a mixture of Fe2+ and Fe3+ ions as the pH is raised to a
11
sufficiently high level [42]. Therefore, by increasing (OH)− concentration in solution, the Fe3+
12
species can be precipitated according to the following equation:
13
14
15
16
In the first precipitation reactor the pH was increased from 0.56 to 5. Some precipitation
17
started to occur at pH = 2.5. By the time the pH value reached 5, practically all the Fe3+ in the
18
aqueous phase were converted to brown crystals of Fe(OH)3. The filtrate obtained from this
19
step was clear.
20
The P1 solids appear to be a brown precipitate consisting mainly of Fe(OH)3 and Fe2O3.
21
About 76.68% of total Fe extracted from ore was precipitated in this step. Precipitation of
22
other elements in the solution also occurred as well.
23
The solid product after filtration, washing and drying was analysed. Figure 4 shows the SEM
24
image of Fe(OH)3 and Fe2O3 crystals. After drying in oven and loosing bound water, some of
12
1
the hydroxides were partially converted to the oxide. The cubes in the image show the
2
metastable structure of γ-Fe2O3, while ferric hydroxide appears as hexahedral crystals.
3
Figure 4.
4
5
6
XRF analysis shows that the main metal present in P1 was Fe (91.28 wt % as Fe2O3). It also
7
detected the presence of the trace elements (4.24 wt % of Al2O3, 1.07 wt % of Cr2O3, 1.04 wt
8
% MgO and 0.8 wt % of NiO) that were co-extracted from acid dissolution step.
9
The ferrous species present in the solution can also be precipitated at a sufficiently high pH
10
level, according to the following equation:
11
12
13
14
After addition of sufficient amount of NaOH to raise the pH to 9, a dark green precipitate (P2)
15
was obtained. P2 appears to gradually turn brown upon exposure to air, due to the oxidation
16
of ferrous species to ferric oxides and hydroxides. The browned P2 solids also appear to
17
contain mainly Fe(OH)3 and Fe2O3 after drying. The filtrate from this second purification step
18
also appears to be a clear liquid.
19
Interestingly, in stark contrast with P1, XRD analysis of P2 showed that it was an amorphous
20
oxide as there is no crystalline phase found in the second step solid product after drying. XRF
21
analysis showed that the major component of the solid was Fe2O3 (~80 wt %). A small
22
quantity of magnesium (~10 wt %) also exists in P2. Minor quantities of other impurities
23
extracted from mineral solid such as Ni, Mn, Co, Ca and Zn were also present.
24
Therefore we infer from these results that separately precipitating the ferrous and ferric
25
fractions of the dissolved iron is important. In P1 , the iron precipitate was more crystalline
13
1
and has a higher purity than P2. P1 is thus more valuable economically than P2 for the iron
2
and steel industries [3]. The amorphous iron product (P2) may also have other applications,
3
such as use in pigments and coagulation materials. Iron oxide-hydroxide can also be used in
4
aquarium water treatment as a phosphate binder.
5
6
3.4. Precipitation of magnesium carbonate
7
It is known that the Mg2+− CO3- −OH- system has the potential to produce metastable
8
compounds in different forms. XRF analysis of the carbonate product shows that all
9
precipitates across different temperatures contain more than 99 wt % of MgO. The remaining
10
impurities detected were mainly CaO.
11
For the carbonation step, the precipitation of the carbonate took place spontaneously upon the
12
addition of sodium carbonate to the purified leachate solution. Precipitation of the product
13
was considered to be complete when pHIII reached 10.
14
The first set of precipitations was carried out at room temperature (~23°C). After aqueous
15
washing, a white product was obtained and identified as pure nesquehonite crystals by XRD.
16
In the diffraction pattern presented in Figure 5, the first two main intense peaks for
17
nesquehonite at 2θ=13.7° and 23.4° are completely fitted to experimental counts. The short
18
peaks are also virtually identical to nesquehonite from the database.
19
20
Figure 5.
21
22
The thermal behaviour of nesquehonite is shown in Figure 6.
23
24
Figure 6.
25
14
1
Nesquehonite is a type of hydrated magnesium carbonate which is composed of 29 wt %
2
MgO, 39 wt % H2O and 32 wt % CO2. The weight loss of the parent sample P3 produced at
3
room temperature occurred in three stages where the first weight loss (25–150 °C)
4
corresponds to adsorbed water, the second (150–200 °C) corresponds to the evolution of
5
water of crystallization, and the final loss (<350 °C) is the evolution of hydroxyl groups. In
6
the weight loss derivative curve, the first peak appears before 100ºC, and can be attributed to
7
the moisture content in the product. Carbon dioxide resulting from thermal decomposition is
8
released from 350°C to 550°C. The theoretical mass ratio of MgO:CO2 in nesquehonite is
9
about 0.916. The weight loss from the TGA curve shows a mass ratio of 0.917, which further
10
supports the XRD characterization of the solids as nesquehonite.
11
The first stage weight loss (33%) roughly matches the expected weight loss (39%) from water
12
evolution. As seen in Figure 6, most of the carbonate was thermally stable; CO2 did not
13
evolve until at least 350 ºC. The weight loss in the temperature region of 350–500 ºC was
14
determined to be decomposition of carbonate ions releasing carbon dioxide. The second stage
15
weight loss (which was complete by 550ºC) was 35%, close to the value expected for
16
nesquehonite decarbonation (32%).
17
3.4.1. The effect of precipitation temperature on morphology
18
The precipitation of the magnesium carbonates was conducted under different temperatures
19
to observe its effect on carbonate morphology. A maximum purity of 99.89 wt % of MgO
20
was obtained at a precipitation temperature of 50 °C. It was observed that at different
21
temperature the purity were always higher than 97.66 wt % of MgO while the impurity of P3
22
was just CaO in all cases.
23
SEM images of the various precipitated carbonates are shown in Figure 7. Image A shows
24
that when precipitated at T=0 ºC, the magnesium carbonate crystals appear as porous spheres,
25
and are texturally similar to previous documented account of hydromagnesite [11]. Images B,
15
1
D, E, F and G are the hydrated magnesium carbonate crystals precipitated at T= 10,20,30,40
2
and 50 ºC respectively. Image C shows the crystals of hydrated magnesium carbonate
3
precipitated in room temperature (~23°C). They appear to be needle shaped crystals, which
4
are consistent with the crystalline structure of nesquehonite [40]. Image H in figure 7 shows
5
the carb onate products obtained at T=60°C. The images show flaky rods, which correspond
6
to the structure of dypingite [36]. The occurrence of nesquehonite across a wide range of
7
temperatures suggests that it is the most likely phase to be formed when the carbonate
8
precipitation is carried out at ambient temperature.
9
Figure 7.
10
11
12
XRD analysis was conducted to confirm the identities of the obtained carbonate products.
13
Figure 8 shows the XRD scan results of the precipitates obtained at different temperatures.
14
The results show that the carbonate products obtained at precipitation temperatures of T= 10
15
– 50°C have the same crystalline structure, which is nesquehonite. The XRD patterns of the
16
carbonate products precipitated at T=0°C and T=60°C are markedly different from those of
17
T= 10 – 50°C. This observation is consistent with the SEM results mentioned earlier. The
18
XRD pattern for the P3 sample precipitated at 0ºC shows a majority of hydromagnesite
19
(Mg5(CO3)4(OH)2·4H2O) in the sample. According to analysis, the precipitated carbonates at
20
0ºC contained 92.57% of hydromagnesite, and the rest appear to be nesquehonite. The XRD
21
pattern for the carbonate product precipitated at T=60°C is mainly consistent with that of
22
dypingite. This carbonation product appears to contain 88 wt % of dypingite and 12 wt % of
23
nesquehonite as detected by XRD.
24
25
Figure 8.
16
1
2
Figure 9 shows the TG-FTIR curves for the thermal decomposition of the carbonate product
3
precipitated at T=0°C. The decomposition proceeds via dehydration of the sample at low
4
temperatures (up to 350°C) and, above that threshold, loss of CO2. All of the bound CO2 in
5
the sample was completely liberated at 550°C). The first of the dehydration steps takes place
6
at 60°C. Dehydroxylation and further dehydration are observed at 150°C and 190°C. Two
7
peaks showing the loss of CO2 are observed at 410°C and 480°C. The total amount of H2O
8
liberated appears to be 28 wt % of the sample, which is higher than the amount of water
9
stoichiometrically available in hydromagnesite (19.25 wt%). This difference is attributed to
10
the adsorbed humidity in the solid sample.
11
12
Figure 9.
13
14
At temperatures (from 10ºC to 50ºC), magnesium carbonate hydrates display needle-like
15
morphology. Nesquehonite was found to be the best expected product for mineralization
16
process because the C/Mg ratio is higher than other forms of magnesium carbonate crystals.
17
With the increase of temperature, the viscosity of the initial solution gradually decreases,
18
which accelerates the collision rate of the nuclei. A higher collision rate brings about a higher
19
number of nucleated particles, so this results in more narrow particles.
20
Figure 10 shows the TG-FTIR curves for the thermal decomposition of the carbonate product
21
obtained at T=60°C. Compared with Figure 9, we can see that the decomposition profile of
22
this carbonate product is dramatically different from that in Figure 10, suggesting a different
23
material being obtained. Compared with hydromagnesite, dypingite appears to be less
24
thermally stable. Thermogravimetric analysis of this sample shows that dypingite starts to
17
1
gradually lose carbon dioxide at 200 ºC. After losing all crystalline water at 350°C, dypingite
2
is observed to follow a similar degradation pattern to nesquehonite.
3
Figure 10.
4
5
6
Figure 11 compares the CO2 FTIR absorbance of the three different magnesium carbonates
7
obtained. The decomposition patterns of nesquehonite and dypingite have similar profiles and
8
the binary CO2 peaks are similar. In contrast to the other two forms of magnesium carbonate,
9
a smaller peak is observed before the main peak for hydromagnesite.
10
Figure 11.
11
12
13
It is often desirable for an industrial process to achieve uniformity in the quality of materials
14
that are used. As such, the ability to control the particle size and morphology of magnesium
15
carbonate for downstream use is of great importance. Figure 12 shows the particle size
16
distribution of the precipitated magnesium carbonates obtained at different temperatures. It is
17
observed that the particle sizes fall within a very narrow range of sizes, with the majority of
18
them sized at 1-20 microns. The precipitated dypingite also appears to have a narrower range
19
of sizes than the other carbonates.
20
Figure 12.
21
22
3.5. Process mass balance
23
A mass balance was recorded for the room temperature (~23°C) carbonation step. The
24
efficiency of the CO2 mineralization process was measured by comparing the amount of
25
obtained nesquehonite versus the amount of the magnesium available in the leachate from the
18
1
first step. Of the total magnesium that exists in the leachate solution, 0.85 wt% is co-
2
precipitated with iron in the first purification step, and 2.62 wt % is lost together with the iron
3
in the second precipitation step. The majority of the extracted magnesium (96.53 wt %) were
4
utilized in the carbonation reaction.
5
The overall process efficiency, in other words the conversion of magnesium in the mineral to
6
magnesium carbonates, is about 73.6%. Approximately 93% of the CO2 captured with NaOH
7
was converted into P3 in the solid phase.
8
Practically all the iron that was extracted from mineral ore was removed in the first two pH
9
adjustment steps. Roughly 76.68 wt % of total iron precipitated as Fe(OH)3 in the first
10
purification step, and the rest was precipitated as amorphous hydroxides in the second
11
purification step.
12
A summary of the distribution of iron and magnesium in the three precipitates (P1, P2 and P3)
13
are shown in Table 4. It can be seen that all Fe were removed from the leachate solution prior
14
to the carbonation step. This allows the formation of a highly pure carbonation product.
15
Unfortunately a small fraction of magnesium is also lost in the purification steps, likely due
16
to adsorption on the formed iron precipitates.
17
18
Table 4.
19
20
Table 5 shows the amounts of raw material required, as well as the amount of solid products
21
obtained for every tonne of carbon dioxide mineralized.
22
23
Table 5.
24
25
19
1
As it is possible to control the formation of various types of carbonates by manipulating the
2
reaction temperature, the productivity of the process is approximately uniform across
3
different carbonation temperatures. The amount of the P3 produced from each 100 ml of
4
liquid solution in final precipitation reactor is from 2 to 3 gram in different temperatures. Due
5
to the different amounts of water of crystallization in each type of magnesium carbonate
6
formed, the mass obtained in 100ml of solution varies accordingly as well.
7
4. Conclusions
8
In this work, we describe a carbon dioxide mineralization process and its associated solid
9
products. The products from this process include amorphous silica, iron hydroxides and
10
various magnesium carbonates. Material characterization tests were performed on both the
11
raw materials and solid products; these include XRF, XRD, SEM, TG-FTIR and particle size
12
analysis.
13
It was found that the first precipitate (P1) from the process mainly consisted of crystalline
14
Fe(OH)3. The second precipitate (P2) consisted mainly of amorphous Fe(OH)2, which quickly
15
oxidized into trivalent iron hydroxide after contact with air. Other co-extracted metals such as
16
nickel, aluminium, calcium etc are also removed from the solution together with the
17
precipitated iron hydroxides.
18
The third precipitate (P3) is magnesium carbonate, which may be of different crystalline
19
structures depending on the carbonation step temperature. Hydromagnesite was found to be
20
the main product when the carbonation was carried out at T=0°C, while dypingite was the
21
main carbonate product at T=60°C. At T=10 – 50°C, nesquehonite was predominantly formed
22
in the carbonation step. The multiple characterization tests confirmed the occurrence of these
23
materials.
20
1
Also, an indicative mass balance of the carbon dioxide mineralization process is given. 3.74
2
tonnes of ore are required for the mineralization of one tonne of carbon dioxide. As a result,
3
0.78, 0.27 and 3.1 tonnes of P1, P2 and P3 are produced respectively. The overall process
4
efficiency was found to be 73.6%.
5
6
Acknowledgements: We gratefully acknowledge the financial support from the Science &
7
Engineering Research Council (SERC), A*STAR.
8
9
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1
Solid Products Characterization in Multi-Step Mineralization Process
2
3
Azadeh Hemmati, Jalal Shayegan, Paul Sharratt , Tze Yuen Yeo, Jie Bu
4
5
LIST OF FIGURES
6
7
Figure 1. Scheme of staged CO2 mineralization process
8
9
10
Figure 2. XRD diffractogram of serpentine ore used in this study with the list of crystalline
phase peaks detected
11
12
Figure 3. Raw serpentine crystal (Left) and remained silica residue after extraction (Right)
13
14
Figure 4. Fe3+ oxide and hydroxide crystals
15
16
Figure 5. XRD pattern of final product of carbonation process in ambient temperature
17
18
Figure 6. TG-FTIR analysis of magnesium carbonate precipitated in room temperature
19
20
Figure 7. The scanning electron microscope images of experimental precipitates.
21
22
Figure 8. XRD pattern of final products of carbonation process at different temperatures
23
24
Figure 9. TG-FTIR analysis of magnesium carbonate precipitated in 0 °C
25
27
1
Figure 10. TG-FTIR analysis of magnesium carbonate precipitated in 60 °C
2
3
Figure 11. CO2 absorbance of degraded magnesium carbonate detected by FTIR
4
5
Figure 12. Particle size distribution of magnesium carbonate at different temperatures
6
28
1
2
3
4
Figure 1. Scheme of staged CO2 mineralization process
5
29
1
2
3
4
Figure 2. XRD diffractogram of mineral ore used in this study with the list of crystalline
5
phase peaks detected
6
30
1
2
3
4
5
Figure 3. Raw mineral crystal (Left) and remained silica residue after extraction (Right)
6
31
1
2
3
4
Figure 4. Fe3+ oxide and hydroxide crystals
5
32
1
2
3
4
Figure 5. XRD pattern of final product of carbonation process in ambient temperature
5
33
1
2
3
4
Figure 6. TG-FTIR analysis of magnesium carbonate precipitated in room temperature
5
34
1
A
B
2
C
D
3
4
E
F
5
G
H
6
7
8
Figure 7. The scanning electron microscope images of experimental precipitates
9
35
1
2
3
4
Figure 8. XRD pattern of final products of carbonation process at different temperatures
5
36
1
2
3
4
Figure 9. TG-FTIR analysis of magnesium carbonate precipitated in 0 °C
5
6
37
1
2
3
4
Figure 10. TG-FTIR analysis of magnesium carbonate precipitated in 60 °C
5
38
1
2
3
4
Figure 11. CO2 absorbance of degraded magnesium carbonate detected by FTIR
5
39
1
2
3
4
5
6
Figure 12. Particle size distribution of magnesium carbonate at different temperatures
7
8
40
1
Solid Products Characterization in Multi-Step Mineralization Process
2
3
Azadeh Hemmati, Jalal Shayegan, Paul Sharratt , Tze Yuen Yeo, Jie Bu
4
5
LIST OF TABLES
6
Table 1- Different application requirements match for magnesium carbonate product
7
Table 2- Different applications of products and by-products of the mineralization process
8
Table 3 - Mineral composition of investigated mineral ore, given as oxide wt.%
9
Table 4- Mg and Fe elimination amount from leachate
10
Table 5- Process mass balance
11
41
1
2
Table 1- Different application requirements match for magnesium carbonate product
Application
Cosmetics
Fireproofing
Production of magnesium
refractory brick and magnesium
metal
process cosmetics, toothpaste and
dusting powder
fragrance retainer and as anti-caking
agents
daily consumption to revitalize the
body minerals
Heavy and light pharma grade
MgCO3
purity
99.9%
Particle
size
1-5
Micron
Ca
Fe
NA
NA
Heavy
metals
NA
NA
NA
<0.2%
<0.004%
Nil
NA
NA
<0.75%
NA
<20
NA
50 Microns
<50ppm
<10ppm
3
42
<0.35%
ppm
1
2
3
Table 2- Different applications of products and by-products of the mineralization process
Product
Magnetite
(Fe3O4)
Applications
Reference
recording tapes production
reinforcing fillers in rubber, tires, plastic and paints;
free-flow or anti-caking agents in powder materials;
carrier for liquid active ingredients in human and
Silica
animal nutrition; consumer products such as
(SiO2)
toothpaste or food; feed products as well as technical
rubber goods contain amorphous silica; construction
materials, cosmetics, and some foods; laundry
detergents
iron and steel industries; pigment and coagulation
Iron hydroxide II and III
industries; cosmetics and tattoo inks; aquarium water
(Fe(OH)2 and Fe(OH)3)
treatment as a phosphate binder
mine reclamation or soil amendments; smoke and fire
retardant; smoke suppressant in plastics, fire
extinguishing compositions, fireproofing, filler
material during the manufacturing of paint, paper,
plastics; dusting powder and toothpaste
additive in bricks; reinforcing agent in neoprene
rubber; drying agent
soil enhancers, roadfill or filler for mining operations
Magnesite
pharmaceuticals, cosmetic manufacturing, rubber
(MgCO3)
industry, lithographing inks; precursors for
magnesium-based chemicals
flooring, blocks, mortars and other building materials
electric arc furnaces, blast furnaces, basic oxygen
furnace, housing
land reclamation
laxative to loosen the bowels; colour retention in
foods; antacid; additive in table salt to keep it free
flowing.
Nesquehonite
eco-cement and corrosion-resistant protective
(MgCO3·3H2O)
coatings
flame-retardant in electric and electronic parts;
constructional materials, waste pipes, gutter,
Hydromagnesite
automobile parts; cabinets for televisions, computers
(Mg5(CO3)4(OH)2·4H2O)
and similar equipments; profiles and fittings such as
fittings for cables and electric switches, sealants,
plasters, paints or mixtures thereof
4
5
43
[30]
[3]
[31]
[32]
[28]
[35]
[40]
[33]
[3]
[14]
1
2
3
Table 3- Mineral composition of investigated mineral ore, given as oxide wt.%
Component
MgO
Al2O3
SiO2
CaO
Cr2O3
MnO
Fe2O3
NiO
Content (wt.%)
46.72
0.61
42.70
0.4
0.45
0.14
8.40
0.29
4
5
6
7
44
1
2
Table 4- Mg and Fe elimination amount from leachate
P1
P2
FeO
76.68% 23.32%
MgO
0.85%
2.62%
3
4
45
P3
0%
96.53%
1
2
Table 5- Process mass balance
3
Mineral ore sequestrated CO2
Tonne
3.74
1
P1 (91.28% Fe) P2 (80.6% Fe) P3Hydrated MgCO3
0.78
4
5
46
0.27
3.1
1
2
Research highlights
3
A multi-step pH-swing process makes value- added products from captured CO2.
4
High yields of carbonate precipitate are obtained from carbonation of the extracted
5
magnesium.
6
Different forms of magnesium carbonate can be obtained depending on reaction
7
temperature.
8
Different forms of iron hydroxide can be obtained depending on reaction pH.
9
The precipitate has a purity of 99.9% based on MgO content.
10
11
47