SOLAR-GRADE SILICON: CURRENT AND ALTERNATIVE
PRODUCTION ROUTES
Md Saiful Islam, Muhammad A. Rhamdhani, Geoffrey A. Brooks
Faculty of Engineering and Industrial Sciences
Swinburne University of Technology, Melbourne, Victoria, Australia
[email protected]
ABSTRACT
Silicon is an important semiconducting and photovoltaic material. It is also widely used for
chemical and metallurgical applications. The rapid growth in the demand of solar
photovoltaic (PV) cell results in the shortage of solar-grade (SOG) silicon feedstock.
Expensive scrap electronic grade (EG) silicon (99.9999999% Si) is commonly used as the
raw material to produce SOG-Si (99.9999% Si). Many researchers have reported that
relatively inexpensive metallurgical grade (MG) silicon (98-99% Si) can be used as an
alternative raw material. The Siemens process, which is based on the hydrogenous reduction
of trichlorosilane SiHCl3, is the current dominating production method of SOG-Si. In
Australia, there is no available commercial production of SOG-Si production, but the
abundance of raw materials such as quartz, combined with the expected growth in
photovoltaic silicon demand, has fuelled research in this area. This paper evaluates the
progress of SOG-Si production in the world, identifies problems and technical challenges
associated with the current and alternative techniques.
Keywords: Silicon; Solar grade silicon; Metallurgical grade silicon.
INTRODUCTION
Silicon is a non-metallic semiconducting element, which makes up 25.7 mass% of the earth’s
crust and is the second most abundant element on earth, after oxygen. Silicon is usually found
in nature as silicon dioxide (silica) and silicate. Silicon is used as an alloying element in the
aluminium industry and as a reducing element in the steel industry. The purity of silicon used
directly in metal industry is 98% and commonly called metallurgical-grade silicon (MG-Si).
A small portion of silicon is used in the electronic/semiconductor industry as electronic chips
such as transistors, liquid crystal displays, diodes, etc. The purity of the silicon used in this
industry is 99.999999% (eight nines) or higher, and referred to as electronic-grade silicon
(EG-Si). Another application of silicon is for solar photovoltaic (PV) panel wafers. For this
application silicon must be purified to 99.9999% (six nines) purity, and is usually called
solar-grade silicon (SOG-Si).
In recent years, PV power generation has increased significantly. In 2007, approximately 2.3
GW of PV power generator was installed worldwide and bringing the total global capacity to
7.8 GW. Germany (49%), Japan (24%) and USA (11%) were dominating the installed solar
capacity as shown in Figure 1(a) (2011a). While the current global solar capacity has yet to
have significant impact on the world’s electricity consumption, it does represent the
tremendous growth opportunities for solar power generation in the future. According to the
latest estimates from Jeffries and Energy Information Administration (2011a), solar power
generation is projected to be 11% of the total world capacity demand by 2030, as shown in
Figure1(b).
Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
Fig. 1: (a) Installed Capacity by Country reported to International Energy agency in 2007;
(b) Solar power generation as percentage of world electricity consumption(2011a). (Source:
IEA-PVPS Trends in photovoltaic Applications 2011)
Tab. 1: Global polycrystalline silicon market data, (in tons) (2007)
Year
2005
2006
2007
2008
2009
2010
2011
2012
2013
Available
polycrystalline-Si
30,680
33,390
37,500
51,000
73,500
96,500
115,200
142,000
168,000
Demand of
polycrystalline-Si
33,850
39,520
46,900
62,940
81,340
103,440
121,560
148,150
173,200
Stock polycrystalline-Si
-3,170
-6,130
-9,400
-11,940
-7,840
-6,940
-6,360
-6,150
-5,200
About 95% of the current solar PV cell module market is for silicon based solar cells, i.e.
using silicon as raw material, of which 60% is polycrystalline silicon and 30% is single
crystal silicon (Kawamoto and Okuwada, 2007). Table 1 shows the global polycrystalline
silicon production and demand data(2007). It can be seen from Table 1 that there is a
shortage of stock of polycrystalline silicon. The production of polycrystalline Si for
photovoltaic solar wafer is mainly relying on the off-spec high-purity scrap of electronicgrade silicon from the semiconductor industry. With the increase demand of polycrystalline
silicon (and depletion of world reserves of EG-Si scrap), it is imperative to develop a new
process that is more sustainable with lower environmental impact. This paper evaluates the
progress of SOG-Si production in the world, identifies problems and technical challenges
associated with the current and alternative techniques.
SILICON PRODUCTION ROUTES
In general there are two principle methods for producing polycrystalline SOG-Si; namely
chemical/metallurgical and electrochemical routes. Examples of chemical/metallurgical route
include thrichlorosilane process (Siemens process), chorine-free silane process, fluoride
process and thermal reduction of silicon halides. Electrochemical routes include dissolution
of quartz in fluoride and three-layer electro-refining of silicon. Other types of refining
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Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
processes for MG-Si include etching/acid leaching process, slag process, electron beam and
plasma processes, and directional solidification process. A list of selected SOG-Si production
routes and refining methods are given in Table 2. The next sub-sections explain each in more
details.
Tab. 2: Selected solar grade polycrystalline silicon production and refinement processes
Process Route
Trichlorosilane
based1
Monosilane based2
Chemical Reactions / Temperature /
Raw material
Pressure
Chemical/Metallurgical Approach
Metallurgical grade
silicon
Si(s) + 3 HCl(g) → HSiCl3(g); (10000C)
2 HSiCl3 (g)+ H2(g)→ Si(s)+ SiCl4 (g)+HCl(g)
Hemlock HSC, Siemens,
Wacker, Tokuuyama, etc
Metallurgical grade
silicon
3SiCl4(g)+2H2(g)+Si(s)→4SiHCl3(g); (5000C,
30 MPa, catalyst)
4SiHCl3→3SiCl4 + SiH4; (600C, 0.3 MPa)
SiH4 (g)→ Si(s) + 2H2 (g); (800-8500C)
REC, MEMC, Joint Solar
silicon GmbH
H2SiF6 (l)+ 2NaF(s)→ Na2SiF6(s)+ 2HF(l)
Na2SiF6 (s)→ SiF4 (g)+ 2NaF(s); (6500C)
SiF4(g)+ 4Na (l)→ Si (s)+ 4 NaF(s);(5000C)
Wacker-chemie and Dow
croning
Zincothermic
reduction4
Aluminothermic
reduction5
By-products of
phosphoric acids
and phosphoric
fertilizers
Silicon halide, pure
Zinc
Silicon halide, Pure
Aluminium
Reduction by
Alkali/alkaline
earth metals6
Silicon halide, Pure
Sodium
Fluoride processes3
Reduction By
Hydrogen7
Hallidothermic
reduction8
Carbothermic
reduction9
Gas phase
reduction of Pure
SiO210
Developer
3SiCl4 (g) + 2Zn (l, g) = 2ZnCl2 (g) + Si (s);
(8000C, 1 atm)
3SiCl4 (g) + 4Al (l) = 4AlCl3 (g) + 3Si (s);
(400-12000C, 1 atm)
SiCl4 (g) + 4Na/K (g) = 4NaCl/KCl (l) + Si (s);
(800-8800C,1 atm)
SiCl4 (g) + 4Na (l, g) = 4NaCl (g) + Si (l);
(1727-19270C, 1 atm)
Umicore, Shientsu,
Chisso etc
Shinetsu Chemicals,
Sumitomo Chemicals etc
Silicon halide,
Hydrogen
Silicon halide,
Aluminium
subchloride
SiCl4 (g)+ 2H2 (g)= 4HCl (g)+ Si(l);(>14120C)
City Solar AG
Corporation etc.
Ultra pure quartz,
carbon black. Oil
black, gas black
SiO2(s)+2C(s) =Si(l) + 2CO(g);( >19000C, 1
atm)
Quartz, Carbon
black, Hydrogen
SiO2(s)+ C(s) → Si(l)+ CO2(g);(1300-15000C,
10-3-1 torr)
SiO2(s) + Si(l) → SiO(g)
SiO(g) + H2(g) → Si(l) + H2O(g);(170019000C, 1 atm)
SiCl4 (g) + AlClx (g) → Si (l) + AlCl3 (g);
(10000C, 1 atm)
Electrochemical Approach
Three layer
Electrorefining11
Metallurgical grade
silicon
Me (Na, K, Ca, Ba, etc) → Me
;(1700K) (Anode)
Si4+ + 4e- → Si(l); (Cathode)
Direct electrolytic
reduction of SiO212
Pure silica
SiO2 (s)+4e- → Si(s) + 2O2-(1123K)
n+
SOLSILC and SPURT,
Dow Corning, Elkem,
Siemens, Kawasaki,
Heliosil, etc.
+ neNTNU, SINTEF
Refinement Methods of MG-Si
Refinement of MGSi: Etching/Acid
leaching13
Metallurgical grade
silicon
Crushing → Sieving → Acid Leaching (HCl,
HF, HNO3) with Stirring (mechanical or
Ultrasonic) at 500C for 8 hrs → Washing &
Drying
Hemlock, Elkem ASA,
Heliotronic, etc
Refinement of MGSi :Slag process14
Metallurgical grade
silicon
[B] + (3/2)O2- + (3/4)O2→(BO33-);15000C
[P] + (3/2)O2- + (3/4)O2→(PO43-)
Nippon Steel, Crystal
Systems, etc
Refinement of MGSi : electron beam
and Plasma 15
Metallurgical grade
silicon
Electron beam melting (Phosphorus removal)
→Plasma(Ar-O2, Ar-H2O etc) treatment(Boron
removal); (20270C) →Directional solidification
JFE, Univ. Tokyo,
Kawasaki steel
Corporation, UNICAMP
Refinement of MGSi : Solidification
from Si-Al alloy16
Metallurgical grade
silicon
Directional solidification of Si-Al alloy to
remove Boron (1273-1473K)
Univ. Tokyo
References: 1(Woditsch and Koch, 2002), 2(Zadde et al., 2002), 3(Sancier, 1985), 4(Sakaguchi, 2007), 5(Saegusa and
Yamabayashi, 2007), 6(Aries, 1962, Frosch et al., 1980), 7(Harvey and Francis, 1978),8(Yasuda et al., 2011),9(Geerligs et al.,
2002),10(Gribov and Zinov’Ev, 2008), 11(Olsen and Rolseth, 2010),12(Yasuda et al., 2007)13(Ma et al., 2009)14(Johnston and
Barati, 2010)15(Yuge et al., 2001)16(Yoshikawa and Morita, 2006)
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Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
Production of Metallurgical-Grade Silicon (MG-Si)
MG-Si, is the precursor for solar grade and electronic grade silicon, which is commercially
produced through reaction between high-purity silica and charcoal, wood or coal in an
electric arc furnace with graphite electrodes at temperature over 19000C (Fishman, 2008).
Molten silicon collects at the bottom of the furnace through reduction of silica with carbon.
The MG-Si produced by this process is 98-99% pure with major impurities of impurities of
iron, aluminium, titanium, vanadium, boron, and phosphorus.
Chemical/Metallurgical Productions of Solar-Grade Silicon (SOG-Si)
Trichlorosilane Routes (Siemens Process)
The current dominant process to produce solar grade polycrystalline silicon is the Siemens
process which is based on the hydrogenous reduction of trichlorosilane SiHCl3. The
major problems associated with this process are that it involves the production of
chlorosilanes and reactions with hydrochloric acid which is toxic and corrosive. Chlorine
emissions by this route are estimated to amount 0.002 kg of chlorine per square meter of cell
(Braga et al., 2008). Furthermore, this process is high cost and has low productivity. Figure 3
shows the flowsheet of the process. The processing starts with a reaction between low grade
MG-Si and hydrochloric acid yielding trichlorosilane, from which impurities are removed via
distillation and refinement. Hydrogen is then reacted with the refined trichlorosilane at high
temperatures yielding high purity silicon deposits (O"Mara et al., 1990).
Fig. 3: Schematic diagram of
Siemens process (Zadde et al., 2002).
Fig. 4: Schematic diagram of Silane
process (Zadde et al., 2002).
Hemlock HSC is one of the largest semiconductor manufacturing companies which convert
trichlorosilane into high-purity polycrystalline-Si using a chemical vapour deposition (CVD)
reactor technology process (Woditsch and Koch, 2002). At present, most of the
polycrystalline-Si produced is obtained mainly by this old technology and the reduction in
the cost of production is obtained by simplifying the stages of purification of trichlorosilane,
increasing the deposition rate for silicon, and reducing the power consumption. Wacker
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Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
Chemie AG, a German company, developed a technology which based on a reduction reaction of trichlorosilane with hydrogen on pure silicon rods in a fluid-bed reactor, which offers
the following advantages: shorter deposition time; and growth of polycrystalline-Si in Si <
300 µm grains (seeds) (Dornberger, 2005). The Tokuyama Corporation, a Japanese company,
has developed a technology for production of polycrystalline using an alternative route called
VLD (vapour-to-liquid deposition), which is based on chlorosilane decomposition on a
silicon liquid film at 1500°C and allows a 10-fold higher deposition rate than that of from the
Siemens process but the silicon contains a large amount of carbon (Müller et al., 2005, 2009).
Researchers are concentrating on the purification of metallurgical grade silicon with a direct
route without the stages that involve formation of chlorosilanes, which claimed to be five
times more energy efficient than the conventional Siemens process that uses more than 200
KWh/kg Si (Øvrelid et al., 2006).
Silane Processes
Silane process starts with the synthesis of trichlorosilane via the reaction of SiCl4 and H2 with
MG-Si. The second step involves disproportionation of dichlorosilane to silane, followed by
rectification of the intermediate and final products (Figure 4). The third step is silane
pyrolysis with the formation of high-purity silicon (Block and Wagner, 2000). Another route
to optimize the process is to produce monosilane by preparing alcoxysilanes via direct
reaction between ethanol and metallurgical grade silicon. It is claimed that the alcoxysilane
technology is safer ecologically compared to the chlorosilane technology, since products of
intermediate stages are not toxic (Yamada and Harada, 1993).
The REC Company has proposed a process, whereby SOG-Si is produced in an inverse Ushape hot filament CVD reactor from thermal decomposition of SiH4. This is a continuous
process, which recycles chlorides and hydrogen (Braga et al., 2008). REC has also developed
the Fluidized Bed Reactor (FBR) silicon refining process, which can produce SOG-Si at a
lower cost, while using 80-90 percent less energy than the traditional Siemens method for
converting silane gas to high purity silicon (2011c). FBR is more energy efficient because it
produces more silicon per cubic meter of reactor space. This is mainly because the silicon
crystals have a larger total surface area compared to the rods used in the Siemens process;
FBR is also a continuous process. MEMC, a USA company, is producing granular
polycrystalline-Si by decomposing silane (SiH4) in a fluidized bed reactor at lower
temperature, where the silane decomposes around seed particles increasing their average size
to approximately 1,000 microns (2011b). Another Company, Joint Solar Silicon GmbH,
developed a production route of polycrystalline-Si by decomposing monosilane and
depositing it onto the heated walls of silicon tubes (Müller et al., 2005).
Fluoride Processes
Fluoride processes are regarded as a separate group because the raw materials for these processes are low-cost byproducts of the production of phosphoric acids and phosphoric
fertilizers containing hexafluorosilicic acid (H2SiF6) and SiF4. Silicon is typically
concentrated in the form of Na2SiF6 in waste processing, which serves as the precursor for
SiF4, which is then reduced by sodium metal and produce metallic Si and solid NaF in a gas
phase reaction (Sancier, 1985). Since solid sodium fluoride (NaF) is produced as the
byproduct, the resultant mixture is separated by leaching in an aqueous solution or by heating
to above the melting point of silicon (Sanjurjo, 1988). This process is believed to be
promising in lowering the cost of polycrystalline-Si but it has drawbacks involving certain
problems with the final purification of silicon and requiring a rather complex reduction appa4
Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
ratus (Sancier, 1985). The fluoride routes have a certain potential for the utilization of siliconcontaining waste.
Zincothermic Reduction
In the Zincothermic reduction process, solid silicon is produced with ZnCl2 byproduct in a
gas-phase reaction at 1223 K from SiCl4 and Zn vapors (Sakaguchi, 2007). During 1951,
DuPont was the first company to commence the commercial production of high-purity silicon
by zincothermic reduction but it suffered from drawbacks with regard to impurity control
(especially boron) and the large amount of ZnCl2 byproduct generation (Lyon et al., 1949).
Battelle Columbus Laboratories investigated the zincothermic reduction of SiCl4 using
fluidized bed reactor technology by reusing of the ZnCl2 byproduct generated in the reduction
step (Blocher Jr. et al., 1977). The ZnCl2 recovered in the reduction step is electrolyzed to
produce zinc metal and chlorine (Cl2) gas. With the growth of the PV industry, SOG-Si
production through zincothermic reduction has been researched by many researchers in
recent years. Chisso, in cooperation with Nippon Mining Holdings and Toho Titanium, has
developed a continuous SOG-Si production process involving the reaction of SiCl4 and zinc
metal by solving the previous technical problems such as recycling and purity control of the
zinc reductant, morphological and purity control of the silicon products, enhancing the
production of high quality polycrystalline-Si by utilizing zinc chloride byproducts (Honda et
al., 2007, 2008).
Aluminothermic Reduction
SOG-Si production by the reduction of SiCl4 with aluminum metal has also been widely
investigated by various researchers. In aluminothermic reduction process, solid silicon is
produced with aluminum trichloride (AlCl3) byproduct by reaction of SiCl4 and liquid
aluminum at 550-6500C (Woditsch et al., 1985). The reductant metal (Al) is supplied in solid
or liquid form because the vapor pressure of aluminum metal is only 10-7 atm at 1300 K
(Barin and Platzki, 1989). The reaction byproduct, aluminum trichloride (AlCl3) is removed
as a vapor and it can be regenerated into reductant metal and Cl2 gas by electrolysis. In the
recent years, high-purity silicon production by aluminothermic reduction using a closed AlCl3
cycle has also been proposed by Woditsch et al, Sumitomo Chemicals and Shinetsu
Chemicals (Sakaguchi, 2007, Woditsch et al., 1985).
Reduction by Alkali/Alkaline Earth Metals
Metallothermic reduction reaction of silicon halides by alkali metals such as sodium and
potassium and alkaline earth metals such as magnesium can be written as,
SiX4 (g, X = F, Cl, Br) + M (l, g , M = Na, K, Mg) → Si (s, l) + MXy (s, l, g)
(1)
In this reduction process, the reductant metals can be supplied in vapor phase and the reaction
products such as KF are mostly deposited as solids or liquids because of their low vapor
pressure. The condensed byproduct must be separated from the silicon products after
reduction by washing/leaching treatment (Aries, 1962). The halide byproducts (salts) can be
recycled and the recovered salts are electrolyzed into high-purity reductant metals and halogen gases. National Aeronautics and Space Administration (NASA) developed a process of
reduction at ultrahigh temperatures (2000 K) in plasma, aimed to achieve simultaneous
production of silicon and removal of the by-product as a vapor (Frosch et al., 1980).
Silicon Halides Reduction by Hydrogen
The process involves a reaction between silicon halide and hydrogen by the employment of
electric arc or plasma to produce liquid silicon and gaseous co-products. The process is
characterized by the following steps: halogenation of silica bearing material in the presence
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Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
of carbon to produce gaseous silicon halide, separation of the product gas stream to produce
gaseous silicon halide by fractional distillation, arc heater reduction of the silicon halide by
hydrogen to produce liquid silicon and gaseous hydrogen halides, electrolysis of the
hydrogen halide held to produce gaseous halide and the hydrogen (Harvey and Francis,
1978). City Solar AG, a German company, is developing a process of production of
polycrystalline-Si by direct reduction of SiCl4 to silicon in hydrogen plasma.
Halidothermic Reduction
(Yasuda et al., 2011) demonstrated that halidothermic reduction of SiCl4 by gaseous
aluminum subchloride reduction (AlClx; x = 1, 2) at 10000C can be achieved. The reactions
involved in this route are:
SiCl4 (g) + 2AlCl (g) = Si(s) + 2 AlCl3 (g); ∆G0= -229 kJ at 10270C
SiCl4 (g) + 4AlCl2 (g) = Si(s) + 4 AlCl3 (g); ∆G0= -292 kJ at 10270C
(2)
(3)
In the reduction of SiCl4, the aluminum subchlorides can be supplied in the form of vapor
phase. The other species involved in the reaction are in the form of vapor phase and only the
silicon product exists as a solid. The silicon products can therefore be easily separated from
the byproducts (AlCl3, SiCl2, and SiCl3) and the unreacted reductants (AlCl, AlCl2); therefore
contamination of the silicon can be avoided.
Carbothermic Reduction of Silica
In principle, carbothermal reduction of pure silica can be used to produce SOG-Si and MGSi. In 1978, production of low-cost SOG-Si by reduction of quartz sand with purified carbon
materials (coal, carbon black, or oil coke) in an arc furnace was first implemented (Dosaj et
al., 1978) but the impurity concentration in the resultant silicon was above the level of SOGSi. The process is difficult to control because of reduction of SiO2 to silicon proceeds in
several stages in the furnace in different layers of the charge (Katkov, 1997) and there are
losses of silicon in the form of SiO and Si vapors in the course of melting.
SOLSILC and SPURT projects proposed such route for production of SOG-Si (Geerligs et
al., 2002). In the first stage, high purity raw materials (quartz, carbon black) have to be
pelletized and highly active SiC is synthesized under optimal conditions in rotary plasma
furnace. In the second stage, Si is to be produced from this SiC in a submerged arc furnace.
SOG-Si can be obtained using the plasma-assisted refinement of silicon.
Gas Phase Reduction of Pure SiO2
There have been a number of researches focusing on the possibility of gas-phase reduction of
SiO2 to produce solar silicon. In 1961(Aries, 1961), gas phase reduction of silica was first
carried out by two steps: the preparation of extremely pure silicon monoxide by reacting
silica and MG-Si at 1200-14000C under vacuum, and the conversion of the pure silicon
monoxide to elemental silicon by hydrogen reduction at 13000C at 20 atm. (Demin et al.,
2004) also demonstrated the possibility of reduction of silica with hydrogen and a mixture of
hydrogen with hydrocarbons at temperatures higher than 2500°C. (Gribov and Zinov’Ev,
2008) performed reduction of high-purity quartzite with carbon black at temperature between
1300-15000C under vacuum (10-3 – 1 Torr) to obtain SiO vapor and in the second stage
reduction of SiO was carried out in hydrogen atmosphere as high as 17000C.
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Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
Electrochemical Productions of Solar-Grade Silicon (SOG-Si)
Three Layer Electrorefining
The principle of the three layer electro-refining of silicon in molten oxide is similar to the
super-purity aluminium production by the three-layer refining process (Pearson and Phillips,
1957). This is an electrochemical process comprising three molten layers in which impure
silicon alloy (e.g. with a heavy, noble metal copper) placed in the bottom of a reactor as an
anode of an electrochemical cell. Over this layer is a liquid layer of molten salt electrolyte
(CaF2/BaF2 at 14120C) with intermediate density. The top layer is the high purity liquid Si.
By providing an electric current through the system, silicon from the bottom layer is
dissolved anodically into the electrolyte and transported through the electrolyte as a complex
ion and deposited as high purity silicon at the top layer (Figure 5). This method has been
shown to be able to remove all contaminating elements from silicon with the exception of
boron, which has thermodynamic properties very similar to silicon (Olsen and Rolseth,
2010). SINTEF and NTNU are working with the development of this electro-refining route
(Øvrelid et al., 2006).
Fig. 5: A schematic diagram of three-layer
electrorefining of Si (Olsen and Rolseth, 2010).
Direct Reduction of SiO2
In this process, SOG-Si is produced by combining the electrochemical process and the
conventional metallurgical process as follows; low purity SiO2 is purified to SiO2 powder
having low boron and phosphorus contents by acidic and basic treatments (Aulich et al.,
1984). In the first process the low purity silica is purified by fusing it with glass forming
oxides to form a melt from which thin glass fibres are drawn and subsequent treatment of the
fibres with hot HCl leads to remove all non-silicious oxides from the glass network. This
higher purity silica powder is then reduced to silicon by the direct electrolytic reduction in
molten salt (CaCl2) and finally SOG-Si is produced by removing metal impurities with
directional solidification process (Yasuda et al., 2007).
Other Refinement Techniques of Metallurgical Grade Silicon
Metallurgical grade silicon is produced by conventional carbothermal processes on a
commercial scale using electric arc furnace (Fishman, 2008). The production of low-cost
SOG-Si from metallurgical-grade material has long attracted wide research interests in
replacing the complex, expensive trichlorosilane process by simpler, cheaper and sufficiently
efficient purification processes. In general, metallurgical processes offer the advantages of
high production rates and low cost. The technology of obtainment of solar silicon from
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Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
metallurgical silicon is based on the methods of removal of impurities. All the impurities
under consideration can be divided into two groups. Most metallic impurities have very small
segregation coefficients; therefore methods involving directional solidification are commonly
used to effectively remove these impurities. The second group includes impurities with
sufficiently large segregation coefficients (B, P, C, Al, Cu, and Ni), which can be removed
from molten silicon by oxidation. Since metallurgical processes are rather selective in
removing impurities, various combinations of such processes are employed in a certain
sequence. The possible combinations can be hydrometallurgical refining, liquid extraction
and gas extraction or recrystallization from the Al-Si system, liquid extraction and directional
solidification (Dietl et al., 1981).
Etching/Acid Leaching Process
Hydrometallurgical refining of MG-Si is an important step of the metallurgical method,
which enables production of MG-Si from 98% to more than 99.9% in purity, with the
advantages of simple equipment, low cost, and dealing with the large quantity (Ma et al.,
2009). The process starts with the crushing of MG-Si and breakage occurs mostly at grain
boundaries due to their low strength. If MG-Si lumps crushed to particle size equivalent to
the size of multi-crystalline grains, a major portion of the metallic impurities can be removed
by acid leaching as most metallic impurities are concentrated along grain boundaries because
of their low segregation coefficient in polycrystalline-Si. Previous researchers have tested
many combinations of acids (HCl, HF, H2SO4, and aqua regia) in different sequences and
under different conditions (varied temperature, concentration, and treatment duration,
stirring) to optimize the process (Dietl, 1983, Ma et al., 2009). Elkem ASA, the largest
worldwide manufacturer of MG-Si, has commenced a metallurgical route based production
route on pyrometallurgical refinement (Smith et al., 1998) and on chemical treatment using
acid solutions (Ceccaroli and Friestad, 2005).
Slag Process
The slag treatment for SOG-Si production is based on the principle of liquid-liquid extraction
in steel industry (Figure 6). The slag used for the extraction of impurities must dissolve
individual impurities better than molten silicon does, the solubility of silicon in the slag must
be low, and the slag must be nonreactive with molten silicon and must differ markedly from
it in density. The impurities with a higher oxygen affinity in comparison with silicon oxidize
and pass into the slag. The process is particularly attractive for reducing the B and P content
in the production of SOG-Si. In this method, liquid silicon is treated with CaO-SiO2 , CaOSiO2-CaF2, CaO-SiO2-Al2O3, CaO-SiO2-Al2O3-MgO and other molten slags (Teixeira and
Morita, 2009, Johnston and Barati, 2010). It is suggested that a suitable slag for purifying
MG-Si would be CaO or MgO-SiO2 based with relatively high SiO2 because B and P showed
the greatest removal from silicon with highly oxidizing slags at constant basicity. Refining by
this process is dependent on several parameters; reaction kinetics, diffusion of impurities and
partitioning coefficients. MG silicon was refined using slags, gas and moisture with a focus
on reducing B and P in a modified Heat Exchanger Method (HEM) furnace followed by
directional solidification (Figure 7) (Khattak et al., 2002).
8
Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
Fig. 6: Schematic diagram of Slag
process.
Fig. 7: A Schematic flow diagram of SOG-Si
production (Khattak et al., 2002).
Electron Beam Melting and Gas Blowing (Plasma)
The principle of the electron beam melting technique is the generation of a beam of free
electrons that are accelerated towards MG-Si granules and an interaction occurs at the point
of action of the beam with the atoms of the material and converts the electron beams kinetic
to evaporate the impurities which have higher vapor pressure than silicon (Pires et al., 2005).
(Ikeda and Maeda, 1992) reported that 90% carbon, 93% phosphorus, 89% calcium and 75%
aluminum were removed by this technique under 10-2 Pa for 30 minutes but the content of
boron was unchanged because its vapor pressure is much lower than silicon. Oxidizing
refining using gas blowing is among the most important pyrometallurgical processes
employed in the purification of MG-Si. The most widely used procedures are bubbling of
active gases (oxygen, water vapor and their mixtures) diluted with an inert gas through
molten silicon and exposure of the melt surface to flowing gases and reactions with oxygen
lead to the formation of oxides (Yuge et al., 2001). Kawasaki Steel Corporation, a Japanese
company, uses both electron beam furnace with plasma torches (mixture of argon and water)
with the objective of eliminating P and B, respectively. At the first stage, electron beam
reduced the phosphorus content to 0.1 ppm and in the second stage boron content decreased
from 10 ppm to 0.1 ppm (Yuge et al., 2001).
Solidification from Si-Al Alloy
Si-Al liquid alloy, which melts at a relatively low temperature, can be used for refining of
silicon. Boron and phosphorus are difficult to remove from pure silicon, can be easily removed from the Si-Al alloy by solidification process. Calculated results of directional
solidification of the Si-Al alloy revealed the removal fraction of boron can be as much as 90
percent; and with addition of titanium, boron can be removed more than 94 percent
(Yoshikawa and Morita, 2006). Removal of boron and phosphorus impurities by utilizing SiAl alloy is an advantage but alloy formation is problematic because of the necessity of
additional purification step for removal of residual aluminum in the silicon products
(Yoshikawa et al., 2005).
9
Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
SILICON IN AUSTRALIA
Australia has abundant silicon mineral resources such as lump silica (quartzite), quartz
pebble, sandstone or as unconsolidated sand, spread around the continent. In South Australia,
the most significant silica mines are the Glenshera sand pit near Mount Compass, 50 km
south of Adelaide, and the 23-Mile lump silica deposit near Whyalla (2010). Unimin
Australia Ltd now operates the sand pit at Glenshera to supply Australia’s largest container
glass plant at Croydon, a western suburb of Adelaide. The deposit near Whyalla is now
operated by OneSteel Manufacturing Pty Ltd. Queensland has at least 200 Mt of defined
silica sand resources, with additional potential of about 1500 Mt (Cooper et al., 1996). New
South Wales and Victoria also have extensive deposits of silica sand dune (McHaffie and
Buckley, 1995, 2011d).
Simcoa Operations Pty Ltd is an Australian-based silicon company which is producing high
purity metallurgical grade silicon with producing in excess of 33,000 tonnes of high purity
silicon annually and 85% of its allocated to a diverse range of overseas markets. Simcoa's
production of high purity silicon is particularly suitable to the metallurgical industry, and also
provides some unique advantages for the silicones and semi-conductor industries by
maintaining the level of grade control.
Australia currently does not produce solar polycrystalline-Si feedstock for solar panel wafer,
but the abundance of raw materials such as quartz, metallurgical grade silicon, combined with
the expected growth in photovoltaic solar demand, has fuelled research in this area.
DISCUSSION
Research involving old technologies associated with the Siemens process is more advanced
and is already operating in industries. High purity silicon is obtainable through these
processes and these processes are highly resistance to contamination. The main factor
responsible for the shortage in SOG-Si supply by these processes (Siemens process,
Monosilane process) is the low-productivity, batch type process, low energy efficiency and
complicated process. Furthermore, the major problems are that it involves the production of
chlorosilanes and reactions with hydrochloric acid, which is toxic and corrosive and emits
chlorine to the environment (Braga et al., 2008, Yasuda and Okabe, 2010). Several
companies (REC silicon, Wacker, Tokuyama) are making an effort to economize the
chemical route by introducing fluidized bed reactor; filament reactor (VLD) by increasing
energy efficiency and production rate with semi-continuous process (2011c). The other
chemical route (fluoride route) is not efficient enough to provide SOG-Si, but it has certain
potential for the utilization of silicon-containing waste. Hydrogen reduction of silicon halides
can produce high purity silicon but the problems involving with that are; batch type process, a
large amount of halogen and silicon halide produced as a byproduct and slow production
speed. The metallothermic (zinc, aluminum, alkali/alkali earth metal) reduction process is
semi-continuous and simple process but the byproducts are difficult to remove.
Halidothermic reduction is the production of high purity silicon via a gas-phase reaction
where contamination by byproducts and unreacted reductants can be avoided but aluminum
remains in silicon. One metallurgical route, the carbothermal reduction of high purity silica,
can minimize the concentration of the most harmful and difficult-to-remove impurities (B, P)
owing to an appropriate choice of raw materials. But there are problems associated with this
process; i.e. losses of silicon as SiO vapor and further refining step is needed to remove
carbon from silicon. The gas phase reduction of pure silica can produce high purity silicon
but it needs complex apparatus and high temperature. The refinement of MG-Si is a
promising and cost effective way to produce SOG-Si. The refining of metallurgical grade
10
Md. S. Islam, M. A. Rhamdhani, and G. A. Brooks
silicon is highly scalable, semi-continuous and simple process and better environment
friendlier than chemical routes but is difficult to obtain high purity silicon at this stage. The
combination of different cleaning processes is principally successful to produce SOG-Si on
batches up to 200 kg but diffusion controlled reaction steps are necessary the up-scaling is
difficult: longer time, higher losses of silicon, equipment contamination, surface to volume
ratio becomes cost relevant (Woditsch and Koch, 2002). As with other metals (Fe, Mg, Al,
Cu, Ni, etc) production processes, it is clear that the metallurgical routes of Si provide high
production output with sufficient purity (99% +). However, to obtain ultra high purity Si,
further processes are necessary. These may include the electrolytic route, directional
solidification and their combination.
CONCLUSION
The shortage of low-cost SOG-Si is the main factor preventing environmentally friendly solar
energy from playing a major role in the energy market. It is shown the significant advances in
developing all the approaches to the preparation of SOG-Si. At this stage, it is difficult to
conclude which process will become the standard SOG-Si production process in the next
generation so further research and development of these processes are required. The chemical
route also requires a considerable amount of energy and the handling and emissions of toxic
chemical compounds leading to the need for responsible research for solutions enabling the
energy generated to be really clean and low cost. The metallurgical route makes more sense
from the environmental standpoint, low cost and high productivity, since the high growth of
the PV industry is related directly to the quest for renewable and clean forms of generating
energy.
ACKNOWLEDGEMENT
The author would like to acknowledge financial support from Swinburne University of
Technology Postgraduate Research Scholarship.
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Brief Biography of Presenter
Md Saiful Islam is currently PhD student in the Faculty of Engineering and Industrial
Sciences, Swinburne University of Technology. This research is part of the PhD project titled
Electrically Enhanced Slag-Metal Reactions.
15