Challenges of carbothermic route of solar silicon synthesis
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Challenges of carbothermic route of solar silicon synthesis
M.A. Arkhipov, A.B.Dubovskiy, A.A. Reu,V.A. Mukhanov, S.A. Smirnova
Quartz Palitra Ltd.
1, Institutskaya St., Alexandrov, Vladimir Region 601650, Russia
Email: [email protected]
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Traditional route for silicon synthesis
MG: SiO2 + 2C = Si+ 2CO 2N, B, P = 20-40 ppm
Si + 3HCl = SiHCl3 + H2
SiHCl3 + H2 = Si + 3HCl 9N, B, P = 0.001–
0.1 ppm
SOLAR
&
SEMI
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World production of solar grade silicon
Production: 25 000 -30 000 tonnes/year
Demand: over 50 000 tonnes/year
Booking up to Y 2019
Main drawbacks
• Ecoligical threats – due to chlorine use
• Machinery - absence of “turnkey” suppliers.
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Alternative route
SiO2 + 2C = Si + 2CO 4N, B, P ~ 1 ppm
Purification by Direct Solidification and Chemical etching to 6N, B, P = 1 ppm
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MG carbo process
Solar carbo process
Quartz Quartzite 2N-3N Quartz 4N5
Carbon Charcoal, coke 2N-3N
Thermal black
4N
Electrode Carbon 4N Graphite 4N
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Si SiC
Si drops
Electrode
Arc furnace before stocking
Raw materialOxide lining
Carbon lining
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1. SiO2 + C = SiO + CO2. SiO + 2C = SiC + CO3. SiC + SiO = 2Si + CO4. 2SiO = SiO2 + Si5. 2SiC + SiO2 = 3Si +2CO6. 2SiO2 + SiC = 3SiO + CO
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Equilibrium SiO pressures after Schei, Tuset and Tveit.
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SiO +2C = SiC +CO2SiO = SiO2 +Si
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For carbon important: pores, surface area diffusivity
Ideal: upper zone SiC formation
lower zone SiC → Si
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SiO2 + C(1+x) = x Si + (1-x)SiO + (1+x)CO
x – yield
x = 0.8-0.9 for MG silicon
x = 0.6-0.85 for solar silicon
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Silicon move in high temperature zone
T
X
Si
Energy stored inliquid-solid surface isdecreased strongly with temperature rise
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Si SiC
SiC + quartz chargeArc is strong
Silicon is collectedunder electrode
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Si SiC
SiC + quartz
current
Too big concentrationof SiC or too highconductivity of charge
Uniform heating
Silicon remains atsintering place
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AC arc DC arc
t1 – arc absent because of low voltage
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+_
High electrode consumptionand contamination
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High puritymaterials
Low reaction ability
SiC formation near bottom
SolutionCatalyst thatcan be removed during process
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Carbon-powderCharcoal-foam use glue
Briquette: quartz, carbon, glue
Quartz 10% - 75% weight
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Reaction in briquette (upper zone)
1. SiO2 + C = SiO + CO
2. SiO + 2C = SiC + CO
Sources SiO: a) reaction#1 b) from bottom zone
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Optimum gas flow inside briquette
Stage 1: SiC formation
Stage 2: binder lose cementing ability
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Weak cementing force or low density briquette
C
CC
SiO2SiO2
SiO SiO
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Strong cementing force or high density briquette
CC SiO2
SiO2
C
SiO2
SiCC
C
SiO
SiC
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150 kW DC arc furnaceV = 28-65 VI = 1500-3600 AGraphite liningGraphite electrode
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Average batch purity: 99.98%
B = 0.4 ppmP = 2 ppmNa = 20 ppmAl = 60 ppmCa = 10 ppmTi = 15 ppmFe = 50 ppmMn = 1 ppmMg =1.5 ppmCu = 1.5 ppmZr = 2 ppm
Main impurities
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Maximum batch weight: 15 kg
Energy consumption: 35 kW*h/kg
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CONCLUSIONS:
1. Carbothermic arc technology presuppose SiC sintering below 1900 °C.To meet the requirement with high purity components efficient to use catalyst.
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2. DC arc furnace is more efficient than AC:a) less electrode consumption (if electrode is cathode)b) less contaminationc) less loss of energy through electrode
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3.Binder (cement), chemical composition of briquette and method of its preparation are to guarantee:
a) SiC formation in upper zone
b) High resistivity
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4. After SiC formation it’s important to avoid losing SiO by reaction:
SiC + 2SiO2 = 3SiO + CO
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5. Important to keep top of furnace “cold” and bottom “hot” to provide condensation of SiO gas to get capsulation of crater.
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The present work was done under the contract with Big Sun Energy TechnologyCo., Ltd.