Reaction Mechanism and Kinetics of Boron Removal from Molten Silicon via CaO-SiO2-CaCl2 Slag Treatment and Ammonia Injection

Abstract

To improve the boron-removal efficiency of metallurgical-grade silicon by increasing the reaction rate, a combined method with the 30 mol pct CaO-23.3 mol pct SiO2-46.7 mol pct CaCl2 slag treatment and ammonia injection at 1723 K to 1823 K was proposed. For 1 hour and at 1823 K, the maximum removal efficiency of boron was 98 pct, and the final boron concentration in silicon decreased to 1.5 ppmw by the present method without the introduction of the iron catalyst. A kinetic model was also established to clarify the reaction mechanism and rate-limiting steps of this complicated boron-removal process. In this model, the rate-limiting step is the mass transfer of boron oxide at the interface between the slag and silicon phase.

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Abbreviations

T :

Kelvin temperature (K)

ρ m :

The density of silicon (kg/m3)

ρ g :

The density of ammonia (kg/m3)

c [B] :

The B concentration in Si (mol/m3)

\( c_{{{\text{SiO}}_{2} }} \) :

The SiO2 concentration in the slag (mol/m3)

c a :

The NH3 concentration in the slag (mol/m3)

γ [B] :

The activity coefficient of B

c [B], 0 :

Initial B concentration in Si (mol/m3)

k [B] :

Mass transfer coefficient of B at the interface (Si/slag) (m/s)

\( k_{{ ( {\text{BO}}_{3/2} ),{\text{s}}}} \) :

Mass transfer coefficient of BO3/2 at the interface (slag/gas) (m/s)

\( k_{{ ( {\text{BO}}_{3/2} ),{\text{b}}}} \) :

Mass transfer coefficient of BO3/2 at the interface (NH3/slag) (m/s)

p a :

The pressure of bubbles (Pa)

μ s :

The viscosity of the slag (Pa s)

u 0 :

The flow rate of NH3 in the corundum tube (m/s)

σ :

Surface tension between NH3 gas and slag (N/m)

g :

Gravitational constant (N/kg)

Sh :

Sherwood number

a [Si] :

The activity of Si

V m :

The volume of Si (m3)

V s :

The volume of the slag (m3)

\( D_{{ ( {\text{BO}}_{3/2} )}} \) :

Diffusion coefficient of BO3/2 in the slag (m2/s)

H :

Henry’s constant (Pa m3/mol)

R :

Ideal gas constant (J/(K·mol))

ρ s :

The density of the slag (kg/m3)

\( c_{{ ( {\text{BO}}_{3/2} )}} \) :

The B concentration in the slag (mol/m3)

c BOCl :

BOCl concentration in gas (mol/m3)

\( \gamma_{{ ( {\text{BO}}_{3/2} )}} \) :

The activity coefficient of BO3/2

γ a :

The activity coefficient of NH3

\( c_{{ ( {\text{BO}}_{3/2} ),0}} \) :

Initial BO3/2 concentration in the slag (mol/m3)

r :

The average radius of bubbles (m)

\( k_{{{\text{SiO}}_{2} }} \) :

Mass transfer coefficient of SiO2 at the interface (Si/slag) (m/s)

\( k_{{ ( {\text{BO}}_{3/2} ),{\text{i}}}} \) :

Mass transfer coefficient of BO3/2 at the interface (Si/slag) (m/s)

k r :

The constant for Eq. [9] (m/s)

d 0 :

The diameter of ammonia gas tube (m)

d b :

The average diameter of bubbles (m)

u b :

The rising rate of bubbles in the slag (m/s)

δ :

The thickness of the boundary layer (m)

h s :

The height of the slag (m)

Sc :

Schmidt number

\( a_{{{\text{SiO}}_{2} }} \) :

The activity of SiO2

S i :

The area of the interface (Si/slag) (m2)

S s :

The area of the interface (slag/gas) (m2)

D a :

Diffusion coefficient of NH3 in the slag (m2/s)

t :

Reaction time (s)

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This work was supported by the NSFC Project (No. 51604176), and the Chengdu Science and Technology Benefiting Project (No. 2016-HM01-00399-SF). We thank for Mr. Amit Patel (The University of Tokyo) for his linguistic assistance during the preparation of this manuscript.

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Manuscript submitted May 5, 2019.

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Chen, H., Yuan, X., Morita, K. et al. Reaction Mechanism and Kinetics of Boron Removal from Molten Silicon via CaO-SiO2-CaCl2 Slag Treatment and Ammonia Injection. Metall Mater Trans B 50, 2088–2094 (2019). https://doi.org/10.1007/s11663-019-01639-4

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