Introduction
M
etallurgical silicon is produced by the reduction of silica in an electric arc furnace by means of carbonaceous materials. As alternatives to the dominating Siemens chemical route, two main direct metallurgical routes have been established for the production of low-cost solar grade silicon in Norway: the Elkem route (developed by Elkem Solar[
1]) and the Solsilc route (developed by Fesil Sunergy[
2]). The Solsilc route uses high-purity raw materials in form of pellets. It aims at “direct” high-purity silicon production, avoiding several downstream refining steps. Elkem produces a high-grade MG-Si, which is subsequently refined using pyrometallurgical and hydrometallurgical processes.
For both routes, the raw materials play an important role for the optimization of the process. The main quartz specifications are the purity chemistry, lump size, lump mechanical and thermal strength, and softening properties. The absence of fines (particles less than 2 mm in size) and a softening temperature close to the quartz melting point are desirable to maintain a high gas permeability in the furnace burden.[
3] Common impurities in quartz, like Na, K, Al, and Fe, come from minerals like mica and feldspar. Hydrothermal quartz and pegmatite core are the purest among the silica sources.
These can be purified even more by means of mineral liberation, separations, and acid treatments.[
4,
5] The purification processes must be carried out with fine particles (in the range of micrometers). To charge these materials in an electric arc furnace, agglomeration (pellets or briquettes) is necessary.
SiO reactivity[
6] and chemistry are the most important distinguishing properties of carbon. High-reactivity carbon materials are desirable as reductants, because they preserve matter and energy in the process by rapidly reacting with energy-rich SiO(g). The SiO reactivity of coal based reductants depends mainly on the ranks of the coals, on its petrographic properties,[
7] on the carbon particle size and on the SiO diffusion in pores.[
8] Coal and charcoal contain both B, P, and ash minerals mainly consisting of SiO
2, Fe
2O
3, Al
2O
3, TiO
2, CaO, and MgO. B and P originate from the plants, whereas ash containing oxides derive from clay minerals deposited onto the plant material precursors.[
9] High-purity carbon black is a synthetic carbonaceous powder material produced from pure liquefied natural gas or methane. Low B and P content in the metal produced is necessary because these elements are difficult to remove by directional solidification in the subsequent ingot production of silicon for photovoltaic purposes.
In the submerged arc furnace, silicon and carbon raw materials react under atmospheric pressure over a range temperature of 1573 K to 2273 K (1300 °C to 2000 °C). The upper low temperature part of the furnace is called the outer zone, whereas the hottest part of the furnace the inner zone. There is general agreement about the reactions taking place in the inner zone of the furnace.[
6,
10‐
13] The main reactions in the inner zone are as follows:
$$ 2 {\text{SiO}}_{{2({\text{l}})}} + {\text{SiC}}_{{ ( {\text{s)}}}} = 3 {\text{SiO}}_{{ ( {\text{g)}}}} + {\text{CO}}_{{ ( {\text{g)}}}} $$
(1)
$$ {\text{SiO}}_{{ ( {\text{g)}}}} + {\text{SiC}}_{{ ( {\text{s)}}}} = 2 {\text{Si}}_{{ ( {\text{l)}}}} + {\text{CO}}_{{ ( {\text{g)}}}} $$
(2)
$$ {\text{Si}}_{{ ( {\text{l)}}}} + {\text{SiO}}_{{ 2 ( {\text{l)}}}} = 2 {\text{SiO}}_{{ ( {\text{g)}}}} $$
(3)
The prevailing gaseous species in the furnace are SiO(g) and CO(g). In the inner crater zone silicon carbide and molten silica react with each other and form SiO(g), CO(g), and Si according to Eqs. [
1] and [
3]. In the outer zone of the furnace, carbon reacts with the ascending SiO and CO.[
14,
15] SiO(g) produced in the inner zone is recovered in the outer zone through condensation to Si and SiO
2 and through the reaction with C to form SiC. Equation [
4] is the overall reaction for the inner zone under equilibrium condition at 1 atm (10
5 Pa).[
6,
14,
15]
$$ 3 {\text{SiO}}_{2} + 2 {\text{SiC}} = {\text{Si}} + 4 {\text{SiO}}_{{ ( {\text{g)}}}} + 2 {\text{CO}}_{{ ( {\text{g)}}}} $$
(4)
Equation [
5] represents the overall reaction for the inner zone under nonequilibrium conditions when P
SiO = 0.5 and the temperature is about 2253 K (1980 °C).[
14,
16]
$$ {\text{SiO}}_{{ 2 ( {\text{l)}}}} + {\text{SiC}}_{{ ( {\text{s)}}}} = {\text{Si}}_{{ ( {\text{l)}}}} + {\text{SiO}}_{{ ( {\text{g)}}}} + {\text{CO}}_{{ ( {\text{g)}}}} $$
(5)
SiO is formed at lower temperatures at the silica-gas interface by the reaction[
12,
15,
17‐
19]
$$ {\text{SiO}}_{{ 2 ( {\text{s,l)}}}} + {\text{CO}}_{{ ( {\text{g)}}}} = {\text{SiO}}_{{ ( {\text{g)}}}} + {\text{CO}}_{{ 2 ( {\text{g)}}}} $$
(6)
According to Wiik[
12] and Sahajwalla
et al.,[
18] SiO can also be generated at the SiC–gas interface at higher temperatures. In absence of free carbon, SiC may take over the role of carbon and react with CO
2(g) to form SiO(g) and CO(g).
$$ 2 {\text{CO}}_{{ 2 ( {\text{g)}}}} + {\text{SiC}} = {\text{SiO}}_{{ ( {\text{g)}}}} + 3 {\text{CO}}_{{ ( {\text{g)}}}} $$
(7)
Danes
et al.[
13] carried out a thermodynamic study of the Si-C-O system in an isobaric reactor. The reactor was first filled with inert gas at 1 atm and the pressure was held constant by means of a regulating valve which allowed gas evacuation. When an initial complex of SiC + SiO
2 is heated, SiO + CO pressure increases until it reaches 1 atm (10
5 Pa). This condition represents the invariant points where three condensed phases (Si, SiC, and SiO
2) are in equilibrium at the specific temperature 2104 K (1811 °C) and gas composition. The reaction [1] runs at constant temperature until either SiC or SiO
2 is consumed. Above 2104 K (1811
°C) depending on the progress of the reactions [2] and [3], Si, SiC + Si, or Si + SiO
2 will not be consumed completely.
Hirasawa[
11] produced silicon in a two-stage reduction process. It was observed that more silicon was formed at 2273 K (2000 °C) than at 2223 K (1950 °C), and that a larger amount of Si was obtained for longer holding time at 2273 K (2000 °C). They found that the reduction of SiO
2 into SiO(g) (reaction [1]) is the reaction rate-controlling step.
Fruehan and Ozturk[
17] and Wiik[
12] studied the rate of formation of SiO(g) by reacting CO(g) with silica (reaction [6]). Experimental observations strongly indicated that the rate of formation of SiO(g) is controlled by chemical kinetics on the silica surface.
SiC occurs in different polytypes. According to the JANAF Thermochemical Tables,[
20] the cubic polytype (
β-SiC) is more stable than the hexagonal (
α-SiC) at all temperatures, but the difference is so small that is not important in the equilibrium evaluation of the system. Filsinger and Bourrie[
10] and Presser and Nickel[
21] stated that the reactivity of silica with SiC does not depend on the crystal structure of the SiC.
The solid-state reactivity and the surface structure of quartz changes when the surface is activated mechanically.[
22‐
25] During the mechanical treatment of quartz (grinding or milling), Si-O bonds are broken and highly disordered silica layers (thickness from 20 to 500 nm) are formed on the surface of the quartz grains.[
24] These deviations from ideal lattice are metastable defects that lead to the storage of energy and increase the reactivity of the material. According to Steinike and Tkáčová,[
23] mechanically activated quartz shows an increase in the extent of adsorption and gas diffusion into the disturbed near surface layers. Quartz seems more reactive with CO
2(g) than O
2(g). They also observed that mechanically activated quartz (low temperature form) transforms directly into crystobalite at a temperature of 1473 K (1200 °C), which is lower than the high-temperature quartz-crystobalite transformation at approximately 1673 K (1400 °C). This was confirmed by Balek
et al.[
24]
Agarwal and Pad[
26] used thermogravimetry to study the kinetics of reactions in pellets, which were made of carbon black and silica. The reaction rate increases by reducing both carbon and silica particle size, but no significant improvement occurs below a critical size of 20
μm. In general, in a powder compact, where the reaction occurs between solids through gaseous intermediates, the possible reaction controlling steps can be surface reactions or diffusion of the gases through the pores or a combination of these. When pellets made of carbon and silica mixture are heated, they react quickly to SiO(g) and CO(g). When (P
CO/(P
CO + P
SiO)) reaches the chemical equilibrium value, the reaction retards quickly. CO(g) has to diffuse out of the pellet for subsequent reaction taking place. Agarwal and Pad state that pellets porosity leads to higher rates of diffusions of CO(g) and SiO(g) from pellets and, therefore, an increase in reaction rate for the reduction of SiO
2 to SiO(g).
Jensen[
27] recorded the reactions taking place in the crater zone of a 50-kW single electrode pilot scale furnace when lumpy charge was used. From the images, the crater appears complex and dynamic. SiO
2 reacts with SiC and forms Si(l), SiO(g), and CO. The cavity expands as the reaction proceeds. The expansion of the cavity is balanced by SiO
2 and SiC entering slowly the crater. Melted viscous quartz enters in the crater zone slowly. SiC is still a solid phase and is covered by a thin Si layer. A Si bath is present on the bottom; SiC pieces and melted quartz float into it. When quartz enters in contact with SiC, it reacts violently starting bubbling and SiO(g) is produced according to reaction [1]. When SiC enters in the crater zone it disappears fast according to reaction [2]. When Si and SiO
2 enter in contact, SiO
2 starts bubbling and the reactants quickly disappear according to reaction [3].
Although there have been some theoretical suggestions[
13,
15] and experimental attempts[
11,
12,
17,
28] to produce silicon on small scale, no experiment has successfully reproduced the inner zone environment of the industrial furnace. There have also been investigations on the reactivity of lumpy silica[
12,
17,
28] and pellets.[
26] However, these studies were not performed under the conditions of silicon production and did not attempt to compare the reaction mechanisms and kinetics of the same raw materials used either as lumpy or pellet charge.
The aim of the present investigation was therefore:
(a)
To develop a small-scale experimental setup that can adequately simulate the hot zone of the industrial silicon production furnace
(b)
To study the mechanisms and kinetics of the reactions between quartz and SiC when the two compounds are present as fine powder in pellets or as lumps in different charge mixes