1

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: arkh8@yahoo.com

2

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

3

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.

4

Alternative route

SiO2 + 2C = Si + 2CO 4N, B, P ~ 1 ppm

Purification by Direct Solidification and Chemical etching to 6N, B, P = 1 ppm

5

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

6

Si SiC

Si drops

Electrode

Arc furnace before stocking

Raw materialOxide lining

Carbon lining

7

1. SiO2 + C = SiO + CO2. SiO + 2C = SiC + CO3. SiC + SiO = 2Si + CO4. 2SiO = SiO2 + Si5. 2SiC + SiO2 = 3Si +2CO6. 2SiO2 + SiC = 3SiO + CO

8

Equilibrium SiO pressures after Schei, Tuset and Tveit.

9

SiO +2C = SiC +CO2SiO = SiO2 +Si

10

For carbon important: pores, surface area diffusivity

Ideal: upper zone SiC formation

lower zone SiC → Si

11

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

12

Silicon move in high temperature zone

T

X

Si

Energy stored inliquid-solid surface isdecreased strongly with temperature rise

13

Si SiC

SiC + quartz chargeArc is strong

Silicon is collectedunder electrode

14

Si SiC

SiC + quartz

current

Too big concentrationof SiC or too highconductivity of charge

Uniform heating

Silicon remains atsintering place

15

AC arc DC arc

t1 – arc absent because of low voltage

16

+_

High electrode consumptionand contamination

17

High puritymaterials

Low reaction ability

SiC formation near bottom

SolutionCatalyst thatcan be removed during process

18

Carbon-powderCharcoal-foam use glue

Briquette: quartz, carbon, glue

Quartz 10% - 75% weight

19

Reaction in briquette (upper zone)

1. SiO2 + C = SiO + CO

2. SiO + 2C = SiC + CO

Sources SiO: a) reaction#1 b) from bottom zone

20

Optimum gas flow inside briquette

Stage 1: SiC formation

Stage 2: binder lose cementing ability

21

Weak cementing force or low density briquette

C

CC

SiO2SiO2

SiO SiO

22

Strong cementing force or high density briquette

CC SiO2

SiO2

C

SiO2

SiCC

C

SiO

SiC

23

150 kW DC arc furnaceV = 28-65 VI = 1500-3600 AGraphite liningGraphite electrode

24

25

26

27

28

29

30

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

31

Maximum batch weight: 15 kg

Energy consumption: 35 kW*h/kg

32

CONCLUSIONS:

1. Carbothermic arc technology presuppose SiC sintering below 1900 °C.To meet the requirement with high purity components efficient to use catalyst.

33

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

34

3.Binder (cement), chemical composition of briquette and method of its preparation are to guarantee:

a) SiC formation in upper zone

b) High resistivity

35

4. After SiC formation it’s important to avoid losing SiO by reaction:

SiC + 2SiO2 = 3SiO + CO

36

5. Important to keep top of furnace “cold” and bottom “hot” to provide condensation of SiO gas to get capsulation of crater.

37

The present work was done under the contract with Big Sun Energy TechnologyCo., Ltd.