Introduction

With the rapidly increasing rate of crystalline silicon-based photovoltaic (PV) installations, the demand for sustainably produced, lower cost silicon is growing. Metallurgical Grade Silicon (MG-Si) (> 96 pct purity) is produced through carbothermic reduction of quartz in the submerged arc furnace and currently used as a precursor for high purity Si production. Main routes to purify the MG-Si to a suitably pure material (6N) for solar cell manufacturing include chemical routes (established Siemens and FBR) and metallurgical routes (including gas, slag and vacuum treatments).[1,2,3] In addition, direct high purity silicon production using high purity raw materials, has been considered as an alternative approach to reduce the cost of Si production. In recent years, Mg has received attention as an alternative, potentially fossil-free, reductant agent available in high purity, at a reasonable price, allowing a low temperature, exothermic reduction reaction. Although the magnesiothermic production of Si with a purity (> 99.9 wt pct) is promising, scalability of the process and low Si yield has so far limited its further development.[4,5,6,7,8,9] Further additional purification to achieve the high purities required in current high performance solar cell production would be required. As such, the overall scope of work is to produce a reduction product high in Mg2Si, which could be directly transformed into silane gas suitable for the Siemens route.

Different studies have shown that the magnesiothermic reduction of SiO2 may take place at low temperature, around the melting point of magnesium (923 K), where other reactants and products are in solid states. Since the vapor pressure of Mg (around 9.8 × 104 Pa) is relatively high above its melting point, it is expected that Mg in gaseous and liquid forms participate in the reaction. It has been demonstrated that diffusion of Mg to the product layer/SiO2 interface is the rate limiting step. In other words, the reaction path is strongly dependent on the diffusivity of Mg during reaction. Theoretically, at equilibrium, 2 mol of Mg would reduce 1 mol of SiO2 and 1 mole of Si would be produced, according to Reaction [1]. With excess Mg available during magnesiothermic reduction of SiO2, Mg2Si is produced according to Reaction [2]. Mg2SiO4 is formed as a result of Mg deficiency, according to reaction [3].[7,10,11,12,13,14,15,16,17,18,19] However, there is little information in the literature on how reaction conditions affect formation of different phases quantitatively. Moreover, high purity natural quartz, considering its abundance and low price, is an attractive raw material to produce Si with higher purity than MG-Si, especially with low content in boron (B) and phosphorus (P) that are challenging to remove metallurgically.[2] In this initial study, we hence aim at providing a better understanding of the reaction paths in magnesiothermic reduction of high purity natural quartz by investigating different parameters including Mg:SiO2 mole ratio, time, quartz particle size and quartz type.

$$ {\text{SiO}}_{2} + 2{\text{Mg}} = 2{\text{MgO}} + {\text{Si}}\quad \Delta G_{1373}^{0} = - 243.4\;{\text{kJ}} $$
(1)
$$ {\text{SiO}}_{2} + 4{\text{Mg}} = 2{\text{MgO}} + {\text{Mg}}_{2} {\text{Si}}\quad \Delta G_{1373}^{0} = - 272.1\;{\text{kJ}} $$
(2)
$$ {\text{SiO}}_{2} + {\text{Mg}} = 0.5{\text{Mg}}_{2} {\text{SiO}}_{4} + 0.5{\text{Si}}\quad \Delta G_{1373}^{0} = - 146.4\;{\text{kJ}} $$
(3)

Experimental Materials and Procedure

Materials

The reduction reaction was carried out using three types of natural quartz and metallic magnesium as raw materials. As can be seen in Table I, the quartz types A and F are highly pure and type D has a small amount of impurity elements. Moreover, quartz types A and F are hydrothermal metamorphic type while quartz type D is a quartzite.

Table I Chemical Composition of Three Types of Natural Quartz as Raw Materials Obtained from Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (the Analysis was Performed by Jusnes[20])
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The quartz lumps were crushed first with a jaw crusher and next in a tungsten carbide ring mill to smaller particles. After crushing, quartz particles were screened to produce three different particle size ranges using sieves with 45 to 53 μm, 500 to 600 μm and 3.15 to 3.35 mm opening size. After sieving, the particle size distributions of the resulting quartz particles were measured by a laser scattering particle size distribution analyzer, Partica LA-960. The median quartz particle sizes with their standard deviations are reported in Table II. As a reductant agent, metallic Mg in the form of turnings were supplied by Alfa Aesar (purity of ≥ 99.8 wt pct and in the size range of ≤ 3.2 mm).

Table II The Studied Reduction Reaction Parameters
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Experimental Set-Up

Reduction experiments were carried out in a gas-tight stainless-steel reactor. For each experiment, approximately 1 g of quartz and a selected amount of Mg turnings were weighed and added to the high purity Al2O3 crucible to obtain the desired Mg:SiO2 mole ratios indicated in Table II. At each mole ratios, about 10 wt pct of more Mg was used due to possible of Mg loss. The reactor was subsequently tightened firmly inside the glovebox with Ar atmosphere. To be able to monitor temperature during the reaction, a thermowell for accommodating a thermocouple was designed in the lid of the reactor. The assembly was placed on a platform rod at the bottom of a graphite tube furnace that has two thermocouples. The thermocouple on the side of the furnace was controlling the furnace temperature, while the thermocouple from the top of the furnace monitored the reactor temperature. After reaching the set temperature of 1373 K, the reactor was inserted into the hot zone of the furnace by pushing the rod assembly upwards. The furnace setup and reactor are shown in Figure 1. All experiments were conducted under the flow of Ar gas. After desired reaction time outlined in Table II, the reactor was removed by pulling the rod assembly into the cold zone of the furnace.

Fig. 1
figure 1

Schematic of set-up used to conduct reduction reactions

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Product Characterization

The product phases after reduction reaction were identified by powder X-ray diffraction test using Bruker D8 Focus with CuKα radiation (wavelength of 1.54 Å) and LynxEye™ SuperSpeed Detector. The X-ray diffraction tests were performed from 5 to 115 deg diffraction angle and using a step size of 0.016 deg. Quantitative phase analysis was done based on the Rietveld method by the TOPAS 5 software.[21] Furthermore, a microstructure examination of the product was performed by Field Emission Scanning Electron Microscopy (Zeiss Ultra FESEM) equipped with XFlash® 4010 Detector supplied by Bruker Corporation for Energy-Dispersive X-ray Spectroscopy (EDS). To study the microstructure more deeply, two lamellas were prepared by focused ion beam (FIB) using a Helios G4 UX dual-beam instrument. Transmission Electron Microscopy examination was done by double Cs aberration corrected cold FEG JEOL 200FC equipped with Centurio detector for Energy-Dispersive X-ray Spectroscopy (EDS) and a GIF Quantum ER for Electron Energy Loss Spectroscopy (EELS).

Results and Discussion

Effect of Mg:SiO2 Mole Ratio

To study the effect of amount of Mg available during reaction on the product phases, quartz particles were reacted with Mg at Mg:SiO2 mole ratios of 1, 2, 3 and 4. The selection of a wide range of molar ratios was to evaluate well the effect of reactant portions on the type and amount of products. The X-ray diffraction patterns for the reacted 704 μm quartz particle size of type AFootnote 1 and different Mg:SiO2 mole ratios are illustrated in Figure 2 (X-ray diffraction patterns of other quartz particle sizes and Mg:SiO2 mole ratios can be found in Figures S-1 through S-4 refer to Electronic Supplementary Material). The phase distribution obtained from Rietveld analysis of the reacted samples and the phase distribution at equilibrium condition (calculated by FactSage 7.3) are compared in Table III. At a mole ratio of 1, the quartz is partially reduced to Si and while the Si content broadly agrees with equilibrium calculations, only a small amount of Mg2SiO4 phase was measured compared to the calculated one. A significant fraction of unreduced SiO2 and MgO were, however, measured in the product. By increasing the Mg:SiO2 mole ratio to 2 and 3, equilibrium calculations yield varying mixtures of MgO, Si and Mg2Si products. The experimental result at a mole ratio of 2 shows that most of the quartz was reduced to Si and with excess available Mg in the Mg2Si phase. Compared with the mole ratio of 1, very little SiO2 and Mg2SiO4 phases were detected for the mole ratio of 2, which agrees with equilibrium calculations. At a mole ratio of 3, all quartz was reduced and consequently more Mg2Si phase was detected. At a mole ratio of 4, enough Mg is available to contain all Si in the form of Mg2Si in addition to excess unreacted Mg. In summary; except for the lower amount of Mg2SiO4 in the reaction product for the mole ratio of 1, there is generally a good agreement between calculated phase distribution at equilibrium and those established by quantitative phase analysis by the Rietveld method. This indicates that conditions in the samples with higher Mg:SiO2 mole ratios are closer to equilibrium than ones with lower Mg:SiO2 mole ratios.

Fig. 2
figure 2

X-ray diffraction patterns of 704 μm quartz particle size of type A samples at different Mg:SiO2 mole ratios

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Table III Phase Distribution at Equilibrium Condition and Experiments for 704 μm Quartz Particle Size of Type A Samples at Different Mg:SiO2 Mole Ratios
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The backscattered electron images with X-ray mapping of Si, Mg and O elements of the samples discussed above are given in Figure 3. In Figure 3(a), an initial quartz particle transformed into a four-layer product is illustrated for a Mg:SiO2 mole ratio of 1. In the core, there is unreacted quartz that is surrounded by a thin layer (the composition of this layer will be discussed in section D). The next layer is composed of an integrated mixture of MgO and Si phases. The outer layer of the particle consists of porous MgO with some Si seen as bright areas. At a mole ratio of 2, the quartz particle displays a similar four-layer structure, shown in Figure 3(e). However, smaller unreacted core and more Si as thin bright veins (in 2D) in the outer MgO layer, are observed compared to the sample in Figure 3(a). X-ray mapping of Si, Mg and O elements in Figures 3(b) through (d) and Figures 3(f) through (h) for mole ratios of 1 and 2, respectively, demonstrate that magnesiothermic reduction of SiO2 can be best described by the shrinking unreacted core model where Mg diffuses into the initial quartz particles to react with it. The product of the reaction between SiO2 and Mg forms a layer around unreacted SiO2 core. To progress reaction further, Mg has to diffuse through the product layer to reach the unreacted SiO2 in the core of the particle.[22]

Fig. 3
figure 3

(a) Backscattered electron image of sample with a Mg:SiO2 mole ratio of 1 and 120 min reaction time, (b) to (d) X-ray mapping of Si, Mg and O elements of the sample in figure (a), (e) backscattered electron image of sample with a Mg:SiO2 mole ratio of 2 and 120 min reaction time, (f) to (h) X-ray mapping of Si, Mg and O elements of the sample in figure (e), (i) backscattered electron image of sample with a Mg:SiO2 mole ratio of 3 and 120 min reaction time, (j) to (l) X-ray mapping of Si, Mg and O elements of the sample in figure (i), (m) backscattered electron image of sample with a Mg:SiO2 mole ratio of 4 and 10 min reaction time, (n) to (p) X-ray mapping of Si, Mg and O elements of the sample in figure (m), initial quartz particle size is 704 μm for all samples

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The microstructure of the reaction product is completely different when the Mg:SiO2 mole ratio is 3 or 4, as shown in Figures 3(i) and (m), respectively. In these figures, the matrix is the Mg2Si product phase enclosing two other phases. The secondary phase in the matrix is Si for the mole ratio of 3 and unreacted Mg for the mole ratio of 4. The dark particles represent the MgO phase that initially were quartz particles. It is clearly seen that original particles have fractured into smaller particles during reaction. The distribution of phases at equilibrium, listed in Table S-I, verifies that a large amount of Si–Mg liquid is formed at the reaction temperature of 1373 K. The formation of the liquid phase facilitates penetration of Mg in the liquid phase into the cracks of the quartz particles and enhances the reaction rate. Cracking of quartz particles is also observed at mole ratios of 1 and 2 (Figures 3(a) and (e)), but there is not enough liquid phase to penetrate into the cracks during the reaction at these mole ratios. During cooling, first Mg2Si precipitates from liquid. At the mole ratio of 3, the remaining liquid phase solidifies as Mg2Si and Si phases via the eutectic reaction at 1218 K, while at the mole ratio of 4, the liquid phase converts to Mg2Si and Mg at the 912 K eutectic point, as shown in Figure S-5. For lower reaction temperatures (773 K to 973 K), periodic layered structures composed of alternating layers of MgO and Mg2Si, have been previously reported.[11,19,23,24] In more recent work, it was shown that each sublayer may contain one phase as major phase and the other one as minor phase.[23] This periodic layered structure was attributed to the high mobility of Mg atoms vs low diffusion rate of Si atoms. In other words, Si atoms formed by reduction reaction diffuses outwards and accumulates outside the MgO layer as a Mg2Si layer. The MgO and Mg2Si layers grow until the MgO layer creates a barrier against Si diffusion. At this point, new sublayers of MgO and Mg2Si would be formed. In the current work, however, a different structure has been observed as the high temperature of reaction leads to Mg–Si liquid. It also worth mentioning that this periodic layered structure is only observed when Mg2Si is one of the main products of the reduction reaction.

Effect of Reaction Time and Quartz Particle Size

It is known from prior work that the rate of magnesiothermic reduction of SiO2 is controlled by the rate of Mg diffusion. As reaction time and diffusion distance are two important parameters in diffusion controlled processes, the magnesiothermic reduction experiments were performed on different quartz particle sizes and varying reaction time. The backscattered electron images of these samples against reaction time for a mole ratio of 2 are given in Figure 4. For all three quartz particle sizes, a noticeable amount of Mg2Si is observed after a short reaction time at the particle periphery. Moving towards the particle core, elemental Si is found close to the reaction front. With increasing reaction time, the Mg2Si phase is converted to Si as it is not thermodynamically stable under these reaction conditions. This observation is illustrated as a change in Mg2Si phase content against reaction time, shown in Figure 5(a) for mole ratios of 1 and 2. With, for example, the 704 μm quartz particle size and a mole ratio of 2, the Mg2Si content decreases from about 8.9 to 4.4 wt pct with increasing reaction time from 10 to 120 minutes. At the beginning of the reaction, Mg melts, evaporates, and rapidly reduces quartz at the particle periphery. The Si produced and the inwards diffusing Mg vapor may form a liquid phase according to the Si-Mg binary phase diagram, Figure S-5. With increasing reaction time, Mg in the liquid phase diffuses into the particle to further reduce quartz. Consequently, there would be less Mg2Si with longer reaction time. At each mole ratio, the highest amount of Mg2Si is obtained for the largest quartz particle size (6.9 and 14.7 wt pct for mole ratios of 1 and 2, respectively, and 3360 μm quartz particle size), which further confirms that the Mg2Si formation is related to the limitation in Mg diffusion. As seen in Figures 4(d) through (f), it is apparent that the size of the unreacted quartz core is decreasing with time for 704 μm quartz particle size. For higher mole ratios of 3 and 4, there is no significant change in the microstructure of samples with increasing reaction time, as shown in Figures S-6 and S-7, respectively.

Fig. 4
figure 4

Backscattered electron image of samples with a Mg:SiO2 mole ratio of 2 with different quartz particle sizes and reaction time, (a) to (c) 57 μm quartz particle size and 10, 20 and 120 min reaction time, respectively, (d) to (f) 704 μm quartz particle size and 10, 20 and 120 min reaction time, respectively. (g) and (h) 3360 μm quartz particle size and 10 and 120 min reaction time, respectively, (i) higher magnification of area shown in the yellow square in figure (g)

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Fig. 5
figure 5

The change in the amount of Mg2Si (a), Si (b) and Mg2SiO4 (c) phases against reaction time at Mg:SiO2 mole ratios of 1 and 2

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The comparison of total formed Si (sum of produced Si in Si and Mg2Si phases), Figure 5(b), reveals that Si was formed already after 10 minutes reaction time for all conditions (two mole ratios and different particle sizes) and increases with increasing reaction time. After 120 minutes reaction time, the Si content in the two smaller size fractions (57 and 704 μm) has reached a stable, equivalent equilibrium level, while the largest particles (3360 μm) have a lower Si content indicating incomplete reduction reaction even after 120 minutes reaction time. As summarized in Table III, the equilibrium content of Mg2SiO4 is 74.2 wt pct at the mole ratio of 1. Figure 5(c) shows that the maximum measured Mg2SiO4 content is less than half of this amount, around 31.2 wt pct for the 57 μm quartz particle size. The Mg2SiO4 content in the finest particle size initially increases for up to 20 minutes reaction time, thereafter, decreasing for the sample held up to 60 minutes and subsequently stabilizing at slightly higher contents. For the 704 μm quartz particle size, maximum Mg2SiO4 content is around 6.5 wt pct, while for 3360 μm quartz particle size is only around 2 wt pct after 120 minutes reaction time. It is clear from these results that approaching the equilibrium content of Mg2SiO4 is kinetically limited by the slow diffusion rate of elements in solid state. Hence, a higher interfacial area between the unreacted quartz and the product phase would result in more Mg2SiO4 formation for smaller particle sizes. At a mole ratio of 2, the Mg2SiO4 phase is only noticeable for the 57 μm quartz particle size. Mg2SiO4 was formed initially and is subsequently decreasing rapidly with time. As discussed in section D, the Mg2SiO4 phase is formed during the reduction of quartz by Mg as an intermediate phase at a mole ratio of 2.

Effect of Quartz Type

The properties of three quartz types have been thoroughly studied elsewhere.[20] In addition to differences in impurity elements content, these quartz types vary in grain size, thermal disintegration behavior, fluid inclusions and pre-existing cracks. However, the backscattered electron images for different quartz types, comparison Figures 3(i), 6(a) and 6(b), demonstrate no significant difference in the reduction behavior between different quartz types at a Mg:SiO2 mole ratio of 3.

Fig. 6
figure 6

Backscattered electron image of quartz type D (a) and quartz type F (b) at a mole ratio of 3, 704 μm quartz particle size and 10 minutes reaction time

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Detailed Analysis of the Mechanism of the Reaction at Lower Mg:SiO2 Mole Ratio

To investigate the reaction mechanism in more detail, High-Angle Annular Dark-Field (HAADF) images from different areas together with simultaneous Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray Spectroscopy (EDS) elemental mapping of the sample with a Mg:SiO2 mole ratio of 2 and 704 μm quartz particle size was carried out that typical results are presented in Figure 7. The exact position of the two prepared TEM lamellas can be found in Figure S-8. To discuss the different layers, we start from the interface between unreacted quartz core and product in Figure 7(a). The figure reveals that the thin layer around the unreacted core (indicated in section A) is composed of two layers. The average chemical compositions of these two layers are given in the ternary phase diagram of Mg–Si–O in Figure 8. The layer adjacent to the quartz core (layer (i)) contains Si and a Mg–Si–O phase with a stoichiometry close to MgSiO3. The second layer (layer (ii)) around quartz core contains some MgO in addition to Si and a Mg–Si–O phase. In this layer, the Mg–Si–O stoichiometry is close to Mg2SiO4. The formation of these layers is well described by the Mg–Si–O ternary phase diagram determining which phases can co-exist at equilibrium. The HAADF image from the interface between layer (iii) and outer layer (layer (iv)) exhibits in Figure 7(e). The average chemical composition shows that layer (iii) contains MgO and Si phase while layer (iv) contains more MgO phase with less Si content indicating separation of Si, Figure 8. There are also some Mg2Si and Mg phases in layer (iii) and layer (iv) that indicates the diffusion of Mg through the product layer.

Fig. 7
figure 7

HAADF STEM image and STEM-EELS-EDS element mapping of Si, Mg and O of the sample with a Mg:SiO2 mole ratio of 2 and 704 μm quartz particle size, (a) interface between unreacted SiO2 core and product, (b) to (d) element mapping, (e) interface between layer (iii) and layer (iv) (outer layer), (f) to (h) element mapping

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Fig. 8
figure 8

Ternary phase diagram of Si–Mg–O calculated by FactSage 7.1

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Based on these observations, the steps of reduction reaction are illustrated in Figure 9. The reduction of SiO2 happens through series of intermediate reactions. Firstly, SiO2 is reduced to a mixture of MgSiO3 and Si shown as layer (i) according to reaction [4]. By diffusion of more Mg, MgSiO3 reacts with Mg, according to reaction [5], to produce a Mg2SiO4 and Si mixture that forms layer (ii) (time t1 in Figure 9). Finally, reduction of Mg2SiO4 by Mg to a mixture of MgO and Si takes place according to reaction [6] (time t2 in Figure 9). The just- formed MgO and Si mixture has an integrated structure but Si will subsequently separate from the mixture with time and accumulate either at the periphery of the particles or as thin veins (in 2D image) in the MgO matrix due to the low wettability of MgO by Si.[25] Simultaneously, Mg may combine with the produced Si that is seen as a Mg2Si phase precipitating during cooling. Hence, the propagation of the reaction front into the quartz core takes place through these intermediate reactions rather than directly through reaction [1] until SiO2 converts completely to products (time t3 in Figure 9). In other words, the reaction proceeds via the mass transport of Mg through both the co-existing intermediate phases of Mg2SiO4 and MgSiO3. As mentioned previously, the reaction rate is fast until the product layer of MgO and Si introduces a barrier against Mg diffusion. The formation of the intermediate phases between SiO2 and products was also reported in other studies.[11,24] In the study by Kodentsov et al.[21] on the interaction between monocrystalline SiO2 and a Mg diffusion couple at temperatures of 500 °C and 550 °C, MgO was formed in a layer adjacent to SiO2 in a short reaction time experiment. The formation of MgSiO3 and Mg2SiO4 was seen in longer reaction time experiments, and it was concluded to be due to establishment of equilibrium in the diffusion zone in longer reaction time.

Fig. 9
figure 9

Schematic of reaction steps with formation of different layers around untreated quartz core from t0 = 0 to t3 that t3 is the reaction time for complete reduction of quartz (t0 < t1 < t2 < t3)

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The bright field images from different areas of the quartz with a Mg:SiO2 mole ratio of 2 and 704 μm particle size are seen in Figure 10. The natural quartz material is composed of large grains in the order of a few micrometers that strain contrast from dislocations inside grains is seen in Figure 10(a). Figure 10(b) shows the penetration of Mg into SiO2. Images in Figures 10(c), (d) and (e) show a nano-grained structure as well as a high density of defects. Decreasing grain size from a few micrometers to a few hundred nanometers indicates that there must be high number of nucleation sites during the reduction reaction. Moreover, a higher density of defects shown in Figure 10(e) compared to Figure 10(d) is attributed to the diffusion of Si away, leaving a porous MgO structure. Finally, the presence of dislocations and twin band in the Si, Figure 10(f), confirms that Si is formed from a liquid phase.

$$ 2{\text{Mg}} + 3{\text{SiO}}_{2} = 2{\text{MgSiO}}_{3} + {\text{Si}}\quad \Delta G_{1373}^{0} = - 293.3\;{\text{kJ}} $$
(4)
$$ 2{\text{Mg}} + 4{\text{MgSiO}}_{3} = 3{\text{Mg}}_{2} {\text{SiO}}_{4} + {\text{Si}}\quad \Delta G_{{1373\,{\text{K}}}}^{0} = - 287.6\;{\text{kJ}} $$
(5)
$$ 2{\text{Mg}} + {\text{Mg}}_{2} {\text{SiO}}_{4} = 4{\text{MgO}} + {\text{Si}}\quad \Delta G_{{1373\,{\text{K}}}}^{0} = - 173.6\;{\text{kJ}} $$
(6)
Fig. 10
figure 10

Schematic and BF images of different layers formed during reduction reaction of the sample with a Mg:SiO2 mole ratio of 2 and 704 μm quartz particle size, (a) unreacted quartz core, (b) the interface between unreacted quartz and layer (i), (c) layer (ii) with Mg2SiO4 and Si phases, (d) layer (iii) with MgO and Si phases, (e) layer (iv), (f) Si grain in layer (iv)

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Conclusions

In the current work, mechanisms and reactions governing the rate and products of magnesiothermic reduction of natural quartz particles were investigated. The conclusions of the study are:

  • The reaction is strongly dependent on the amount of Mg available in terms of both reaction mechanism and reaction rate.

  • At Mg:SiO2 mole ratios of 1 and 2, MgO and Si are the main products. After initial formation of Si, the reaction rate decreases due to the low diffusion rate of Mg through the product layer. While the reaction occurs through the formation of intermediate phases at the reaction front (MgSiO3 and Mg2SiO4 phases) around unreacted quartz particle core, Mg2Si is formed in outer areas of particles with higher concentrations of Mg. The observed sequence of phases from the unreacted core to the surface of a particle is SiO2/MgSiO3/Mg2SiO4/MgO–Si/Si–Mg2Si–MgO.

  • At Mg:SiO2 mole ratios of 1 and 2, smaller particle sizes show higher reaction rate; fastest and complete reaction for 57 μm quartz particle size and slowest for 3360 μm quartz particle size with some unreacted quartz at the examined conditions.

  • At Mg:SiO2 mole ratios of 3 and 4, the formation of a high amount of liquid and cracking of quartz particles accelerate the reduction reaction and also the largest 3360 μm quartz particles are reduced rapidly due to the higher Mg mobility/diffusion. The final products are composed of mixtures of MgO–Mg2Si–Si and MgO–Mg2Si–Mg for mole ratios of 3 and 4, respectively.

  • Different natural quartz types display the same reduction reaction mechanisms at high temperature (1373 K) and high Mg:SiO2 mole ratio.