Proceedings of the 7th World Congress on Mechanical, Chemical, and Material Engineering (MCM'21)
Prague, Czech Republic Virtual Conference– August, 2021
Paper No. MMME 102
DOI: 10.11159/mmme21.102

Improving the Process of Metallic Fuels Production through the Solar
Carbothermal Reduction of Magnesia
Youssef Berro, Jean Puig, Marianne Balat-Pichelin

Laboratoire PROcédés, Matériaux et Energie Solaire PROMES-CNRS, UPR 8521
7 rue du four solaire, 66120 Font-Romeu, France
youssef.berro@promes.cnrs.fr; jean.puig@promes.cnrs.fr; marianne.balat@promes.cnrs.fr
Abstract - Recent studies focused on the carbothermal reduction of magnesia as a possible production process of metallic Mg
powders that can be used as transportation fuels due to their high energetic value, absence of greenhouse gas emissions, and the ability
to regenerate them (through reduction/combustion cycles). Herein, we investigated the development of the reduction process, undervacuum, in the Sol@rmet reactor using concentrated solar energy and charcoal reducing agent as sustainable sources. We proved thatthe reduction is improved by controlling various parameters as the argon flow, the heating rate, the retention time, the type of filter
collector, and the binder used to form the C/MgO pellets. In fact, a circulating swirl flow inside the reactor allows to prevent thecondensation of the produced Mg inside the reactor and to purge out the produced CO, thus reducing its partial pressure andaccelerating the reaction. Moreover, using a metallic filter has improved the collection of produced magnesium powders. Finally, we
found that poly vinyl alcohol (PVA) and bentonite binders have a catalytic effect on the reaction with the best Mg yield of around 96%,
with 96% Mg purity, reached when the temperature is raised progressively over 22 min. and using 5% starch + 5% bentonite binders.
Keywords: metallic fuels, carbothermal reduction, magnesia, concentrated solar energy, Sol@rmet reactor.

1. Introduction

Today, the debate continues worldwide considering two related critical issues, the depletion of fossil fuels and theglobal warming [1]. The interest of using metallic powders as transportation fuels comes from their high energetic value,
the absence of greenhouse emissions during their combustion, and the ability to regenerate them [2]. The prospect ofregenerating metallic fuels, through the reduction of their combustion oxide products, using concentrated solar energy
comes from the production of sustainable energy-carrying materials through combustion/reduction cycles.
Previous thermodynamic studies proved that the reduction of magnesia can be assisted using carbon reducing agents as
the equilibrium temperature is decreased from 3700 to 2130 K [3], [4]. The use of vacuum-assisted solar furnace isadvantageous as it reduces the reduction temperature with lower exergy cost [5]. In fact, the onset temperature of themagnesia carbothermal reduction decreases from 1800 to 1300 K as the pressure decreases from Patm to 10 Pa [6]. Xiong etal. [7] proved that under low pressure and considerable flow rate, the CO partial pressure is reduced, which limits the gassolid reaction and promotes the solid-solid MgO-C phase boundary reaction. Further, operating under vacuum conditions
will prevent the possible oxidation (through the backward reaction with CO) of the produced metallic powders during theircondensation, which allows to obtain a high purity product [7], [8]. Moreover, a slow preheating will decrease the starting
and average temperatures of the reaction, which will necessitate longer reaction times meaning more MgO sintering.
Hence, the use of concentrated solar energy is advantageous as high temperatures can be reached rapidly [9], [10].
Early experimental research by Fruehan et al. [11] in 1996, proved that coconut charcoal is more reactive than graphite
during magnesia reduction as it have higher surface area (of 1050 m2/g) allowing a higher contact area with MgO. In
addition, they confirmed that the reaction rate declines as the carbon particle size and the carbon content decrease. In fact,
an excess of carbon (C/MgO = 2) will improve the reduction yield [11]. Later kinetic studies proved that the reduction rateis faster using charcoal than with graphite (thermal gravimetric) [12] or carbon black (solar-driven) [13], and then it
follows the same trend. This behavior is attributed to the two-step reaction where at low temperature, the rate is favored by
the MgO/C contact (solid-solid reaction affected by the carbon surface area). Whereas at high temperature (or longer time),due to the MgO sintering, the rate is determined by the gas-solid reactions independently from the carbon type. Recently,
Puig et al. [14] proved that a C/MgO molar ratio of 1.25 is sufficient and that biochar carbon gives better yield than carbon

MMME 102-1

black (by around 5%) with a product purity higher than 90%. Moreover, they confirmed that the progressive rise of thetemperature, during alumina carbo-reduction under 900 Pa, allows to limit the formation of undesirable by-products as COemission is spread out over time, and thus to obtain higher yield and metal content [15]. Similar conclusion was obtained
during the magnesia reduction, showing that reducing the heating rate improves both the reaction rate and yield [16], [17].
One important parameter during the carbothermal reduction of magnesia is the milling of the C/MgO reactants as theweight loss rises from 24% without milling to 40, 51, 58, and 67% with 1, 2, 4, and 8 h of milling respectively. Increasing
the milling time affects the size of MgO particles (decrease from 50 to 15 nm) and the specific surface area of the MgO/C
sample (increase from 5 to 110 m2/g) [18], [19]. Additionally, CaF2 binder proved to have a catalytic effect during thecarbothermal reduction of magnesia at 1723 K under 30-100 Pa, as the reduction rate increases with an optimal quantity of
5% CaF2 [20]. Moreover, mechanical tests showed that pellets with 5% bentonite / 5% starch binders have the best strength
where the bentonite conserves the strength under heat while the starch is decomposed into char, which might be profitable
for the later-on reduction process [21].
Herein, we investigated the amelioration of the carbothermal reduction of magnesia in the Sol@rmet reactor using
concentrated solar energy for the production of metallic Mg powders. This can be achieved by adjusting the circulation of
the argon flow (controlling Preactor and PCO), increasing the temperature progressively and adding various binders.

2. Experimental Setup

The carbothermal reduction of magnesia was performed in the Sol@rmet reactor using a 1.5 kW solar furnace thatconcentrate the solar power up to 11000 suns. The reaction temperature was controlled by opening the shutter placed
between the heliostat and the parabolic mirror as shown in Fig. 1. The Sol@rmet reactor consists of a cooled metallic partand a glass dome allowing the passage of the solar radiations (see Fig. 2). An optical monochromatic (5 μm) pyrometer(Heitronics K15.42 II) was used to measure the temperature of the sample and supposing that the emissivity of the sample
is of 0.95 at this wavelength. However, modelling results showed that the temperature of the pyrometer is 100-200 K
higher than the real temperature of the sample surface [14]. Vacuum was applied using a dry primary pump (Edwards
nXDS15i) having a maximum pumping rate of 15 m3/h. The operating pressure inside the reactor was regulated bycontrolling the argon flow using mass flow controllers. Pure MgO (Sigma-Aldrich, 325 mesh, > 99%) and birch charcoal(Carbon Terra, C > 94%, Ø < 12μm), with a C/MgO ratio of 1.25, were milled mechanically using Fritsch Pulverisette 4
mill. Various binders as poly vinyl alcohol (PVA), bentonite, and starch (Avedex 36 LAC 14, Avebe) were added withdifferent percentages. Cylindrical pellets (Ø = 8 mm, t = 2-3 mm) were then formed by dry pressing powders at one ton.

Fig. 1: Scheme of the experimental set-up including the concentrating solar system and the Sol@rmet reactor.

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All experiments were performed when the direct normal irradiation (DNI) is of 900-1020 W/m2 with a change less of
3% during each experiment. The produced powders were collected on a porous stainless-steel filter (98% retention of 0.2
μm particles) placed on the entry of the pumping tube (with a small quantity on the reactor walls), while the produced CO
and CO2 were analysed during the reaction using the Xstream gas analyser. The collected powders were analysed by X-ray
diffraction (PANanalytical X’Pert Pro) and the percentages of Mg and MgO were quantified using Highscore Plus software(comparing to International Centre for Diffraction Data ICDD files using reference intensity ratio technique).

Fig. 2: Photos of the Sol@rmet reactor during the reaction (left), and seconds after the end of the reaction (right).

The performance of the magnesia reduction was determined by computing the magnesium metallic yield (𝑦𝑦𝑀𝑀𝑀𝑀 )according to equation (1), where 𝑚𝑚𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 is the maximum quantity of Mg that can be produced taking into account theinitial molar quantity of MgO. %𝑀𝑀𝑀𝑀𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 and %𝑀𝑀𝑀𝑀𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 are the percentages of Mg in the collected powders on the filter
and the reactor walls, respectively. The accuracy of the Mg quantification method was estimated to be lower than 10% foran Mg percentage higher than 90% [14]. Thus, a relative error of about ± 5% is considered for the results of the Mg yield.
𝑦𝑦𝑀𝑀𝑀𝑀 (%) = 100 ∗ (𝑚𝑚𝑓𝑓𝑓𝑓𝑙𝑙𝑡𝑡𝑡𝑡𝑡𝑡 ∗ %𝑀𝑀𝑀𝑀𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 + 𝑚𝑚𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ∗ %𝑀𝑀𝑀𝑀𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 )/𝑚𝑚𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

3. Results and Discussion(1)

3.1. Effect of Circulating Argon Flow
Preliminary experiments for the carbothermal reduction of magnesia in the Sol@rmet reactor were performed byincreasing the temperature progressively to optimize the process. During these experiments (A1-A9), the carrier gas was
injected either from the upper part only or from the upper/lower parts of the reactor. Under the primary vacuum, the reactor
pressure was adjusted to 900-1000 Pa by controlling the argon flow. Results, presented in Table 1, show the importance of
creating a swirl circulation flow in the reactor, which prevents the deposition of produced metal powders on the glass dome
and allows purging the produced CO from the reactor and thus reducing its partial pressure and accelerating the reaction.
Comparing tests A1 and A2 proved that the calcination of MgO reactant (at 1273 K for 3 h) improves the reaction
yield. Furthermore, adding PVA binder, as in test A3, increases the metallic yield by around 10%. However, it appears
from test A4 (no binder) that lowering the reaction time, and thus increasing the pellet temperature gives a yield of 52%
suppressing the beneficial effect of PVA. During all previous tests (A1-A4), the main problem is the deposition of the

MMME 102-3

produced magnesium on the reactor walls and glass dome. Thus, in the following tests (A5-A9), argon is injected fromboth the upper and lower parts of the reactor, which creates a swirl flow inside the reactor and improves the reduction. Thisis observed in test A5, where using a non-controllable upper/lower argon flow allows to achieve a total yield of 67%. The
textural properties of the produced powders, collected on the filter, after test A5 are determined by scanning electron
microscopy (SEM) measurements and the corresponding images (Fig. 3) showed that 10-100 μm agglomerates of micronand submicron grains were obtained.

Fig. 3: Scanning electron microscopy images of the produced Mg agglomerates (left) of submicron grains (right) collected on thefilter after test A5.

Changing the carrier gas into 90%Ar-10%CO, in test A6, decreases the yield to around 30%, thus it is notadvantageous to use 10%CO as it promotes the reverse reaction. During test A7, pure MgO is used to reduce the steps ofthe process. The argon flow is injected from the upper and lower parts of the reactor at 0.5 and 3 L/min respectively. Hightemperatures, up to 2300 K, are reached giving 54% Mg yield (92% Mg purity on the filter). However, this value is much
lower than that obtained in test A5 using calcined MgO. In test A8, the ceramic filter is replaced by a metallic filter, which
improves the collection of metallic products and increases the yield to 68%. Increasing the pellet weight, in test A9, isadvantageous giving highly pure Mg powders (95%) with the maximal yield of 69% and temperatures attaining 2550 K.
Table 1: Tests for the improvement of the solar carbothermal reduction of magnesia: operating conditions, Mg yield and purity.

Test

MgO

A1
A2
A3
A4
A5
A6
A7
A8
A9

pure
calcined
calcined
calcined
calcined
calcined
pure
pure
pure

pellet
(mg)
145.3
139.3
141.5
139.4
138.7
139.7
137.2
139.7
397.8

T
(K)
pure MgO
1610-1800
calcined MgO
1920-2190+ PVA binder
1790-2170
lower reaction time
1920-2230
non-controllable upper/lower flow 1930-2100
90%Ar-10%CO flow
1940-2090
controllable upper/lower flow
2160-2290
use of metallic filter
1970-2130
higher pellet mass
1780-2550
observations

time(min.)
prog 28 + held 2prog 28 + held 7
prog 28 + held 5prog 15 + held 9prog 15 + held 9prog 16 + held 9prog 15 + held 5prog 15 + held 2prog 26 + held 3

Mg yield*
(%)
32
39
48
52
67
29
54
68
69

* The magnesium yield percentage accounts the powders collected on the filter and the reactor walls (see equation 1).

MMME 102-4

Purity(%Mgfilter)
60
60
82
87
84
87
92
87
95

3.2. Effect of Milling and Binders
The following experiments (B1-B6) were performed using a double argon flow, of 1 and 3.5 L/min from the upper andlower parts of the reactor respectively, giving an operating pressure of 835-845 Pa. They are performed by increasing thetemperature progressively (opening shutter) each two minutes, and the reaction is ended when the CO production becomes
lower than 200 ppm (around 2-3 min). Thus, the total time of the reaction is around 22-23 min. Table 2 presents theconditions (temperature and DNI) and the gas production (CO and CO2) progress with time during the reduction of the
C/MgO + 10% PVA pellet (test B1).
Table 2: Parameters progress with time during the reduction of the C/MgO + 10% PVA pellet (test B1): conditions (T and DNI) andgas production (CO and CO2).

Time
(min)
0
2
4
6
8
10
12
14
16
18
20
22

Shutter
opening (%)
10
15
20
30
40
50
60
70
80
90
100
100

DNI
(W/m2)
917
919
919
922
922
925
925
926
929
927
929
929

T (K)
940
1300
1520
1800
2020
2130
2250
2340
2480
2530
2570
2560

CO peak
(ppm)
28
434
376
911
1942
6426
3784
2105
1468
407
405
180

CO2 peak
(ppm)
228
142
37
*
*
*
*
*
*
*
*
*

The temperature profile during the reduction of the C/MgO + 10% PVA pellet (test B1) is illustrated as a function of
the reaction time and shutter opening in Fig. 4. The collected powders (on the filter or in the reactor) are analysed by XRD
and quantified to determine their purities. The XRD pattern of the powders collected on the filter after the reduction of the
C/MgO + 10% PVA pellet is presented in Fig. 5, where only Mg and MgO peaks are detected and quantified.

Fig. 4: Temperature profile as a function of time and shutter opening during the reduction of C/MgO + 10% PVA pellet (test B1).

MMME 102-5

Fig. 5: XRD pattern of the powders collected from the filter after the reduction of the C/MgO + 10% PVA pellet (test B1).

The performance of various C/MgO pellets, depending on the binder type (PVA, starch, and/or bentonite), is examined
and compared in Table 3. When using 10% PVA (test B1), most of the products are collected on the metallic filter with a
93% purity and a total Mg yield (filter + reactor) of 83%. This value is much better than those obtained during thepreliminary experiments (tests A1-A9) in the first section where a maximum of 69% is reached. This difference highlights
the effect of the mechanical milling of the carbon and magnesia particles together before forming the pellets. This could be
attributed to the decrease of the size of the C and MgO particles and the increase of the C/MgO surface contact, thus
accelerating the phase boundary reaction that proved to be the rate-controlling step during the high temperature reduction.
When using 5% starch + 5% bentonite as binders (tests B2 and B3), a metal yield up to 96% with 96% Mg purity isachieved with only 5 mg of the pellet remains. The difference between the two tests can be attributed to the DNI (B2: 951962 W/m2 and B3: 998-1004 W/m2), as the temperature at 15% shutter opening during B2 is of 1250 K compared to 1300
K during B3.
Table 3: Performance of various pellets depending on the binder type during the carbothermal reduction of magnesia.

TestbinderB1

10% PVA
5% starch +5% bentonite
5% starch +5% bentonite
5% starch +5% bentonite
10% starch
5% starch

B2
B3
B4
B5
B6

mpellet
(mg)
252

Tmax
(K)
2570

mremain(mg)
21.1

mfilter
(mg)
82.7

93

mreactor
(mg)
10.1

83

Mg yield
(%)
84.7

248

2480

10.1

77.3

96

21.4

58

87.4

248

2480

5

85.7

96

21.8

58

95.8

208

2390

4.2

69.1

96

15.4

58

90.6

250
253

2540
2600

17.4
12

74.7
79.6

93
93

9
10.7

74
74

76.2
77.4

%Mgfilter*%Mgreactor** XRD analysis are performed for tests B1, B3 (same value admitted for B2 and B4), and B6 (same value for B5).

MMME 102-6

When the pellet mass is reduced to 200 mg (test B4), the reaction yield decreases slightly to around 91%, which isconsistent with previous results (comparing tests A8 and A9) proving that increasing the pellet weight will improve thereduction rate. During tests B5 and B6, starch is used as a binder with 10% and 5% respectively, giving yields of about 7677% with a smaller preference when less binder is used. Thus, comparing all experiments proved that PVA and bentonite
binders act as catalysts for the carbothermal reduction of magnesia. Around 12 mg of the pellet did not react during test B6,
and are characterized by XRD proving that the remaining part consists of a mix of magnesia (54%), magnesium carbonate
Mg(CO3) (around 29%), and carbon (17% including 9% graphite).

4. Conclusion

The advantageous properties of the metallic powders when used as transportation fuels make it necessarily to develop
new sustainable processes for their production. Hence, we investigated the improvement of the vacuum-assisted
carbothermal reduction of magnesia in the Sol@rmet reactor using concentrated solar energy and charcoal reducing agent
as renewable sources. Experiments proved that creating a good circulating swirl flow of the carrier gas inside the reactor,
through the injection from the upper and lower parts of the reactor, would reduce the CO partial pressure through its
purging and thus accelerating the reaction rate. Moreover, increasing the reaction temperature progressively over a reduced
retention time allows achieving higher temperatures and thus higher reduction yields. Further, using a metallic filter
improves the collection of the produced magnesium powders. Finally, results showed that PVA and bentonite binders havea catalytic effect during the reaction with a highest Mg yield value, of 96% with 96% Mg purity, reached using a C/MgO
molar ratio of 1.25 with 5% starch + 5% bentonite binder (maximal temperature around 2500 K). However, despite thathigher temperatures (up to 2600 K) are reached when using 5% starch, lower Mg yield is obtained (77%).

Acknowledgements

This study in funded through the STELLAR project by the “Agence Nationale de la Recherche (ANR)” contract n°
ANR-18-CE05-0040-02.

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