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Boron Removal From Molten Metallurgical Grade Silicon via Flux-Powder Injection

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Abstract

This work assessed boron removal from molten metallurgical grade silicon (MG-Si) using highly basic CaO–CaF2 flux-powder together with the injection of gaseous oxygen. The dynamics of both the oxygen bubbles and the injected flux particles were examined in detail. An elevated oxygen partial pressure was achieved at the flux–O2–Si interface and boron removal occurred in a non-equilibrium state. This technique was found to reduce the boron concentration in MG-Si to a level on the order of 1 mass ppm. The effects of the flux particle dynamics at the flux–O2–Si interface close to the exit of the injection nozzle on boron removal were also evaluated. The rate of boron removal was determined based on the flux particle kinetic energy, which in turn depended on the oxygen gas flow rate and the particle size in the injected flux-powder. Increasing the oxygen flow rate increased the kinetic energy of the flux, allowing it to penetrate the O2–Si interface at the exit of the nozzle. This effect produced a new reaction field for further removal of boron from molten MG-Si. In contrast, lower flow rates did not allow penetration through the interface. In this case, reaction field formation was inhibited and the boron removal rate was independent of the flux injection rate. Eight consecutive runs for 60 seconds using optimized parameters decreased the boron concentration in MG-Si from 14 to 1.5 mass ppm.

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Abbreviations

A :

Reaction area over which boron removal occurs (m2)

A gas-exit :

Cross-sectional gas exit area (opening area of injection nozzle) (m2)

\(A^{\prime}_{{\text{gas-exit}}}\) :

Projected cross-sectional gas exit area at Si–O2 interface (maximum area of reaction field formed by supplying flux-powder to single bubble) (m2)

c :

Sonic velocity in oxygen injected into molten silicon at 1773 K (m s−1)

d f :

Diameter of flux-powder particle (μm)

d f,cr. :

Critical diameter of flux-powder particle (μm)

D N,I :

Inner diameter of nozzle used to inject flux-powder and oxygen (mm)

D N,O :

Outer diameter of nozzle used to inject flux-powder and oxygen (mm)

f 1(x):

Correction factor for deviation from spherical particle shape (−)

F buoyancy :

Buoyancy force (N m−3)

F drag :

Drag force imparted to flux particle by silicon melt (N m−3)

F gravity :

Gravitational force acting on flux particle (N m−3)

F interfacial tension :

Interfacial tension force acting on flux particle (N m−3)

\(g\) :

Gravitational acceleration (9.80 m s−2)

k :

Friction factor related to movement of flux particle through silicon melt (−)

\(\overline{k}_{{\text{B}}}\) :

Apparent rate constant for boron removal (m s−1)

[mass ppm B]0 :

Initial boron concentration in MG-Si melt (mass ppm)

[mass ppm B]:

Boron concentration in refined silicon melt (mass ppm)

\({\text{Ma}}^{\prime}\) :

Nominal Mach number (−)

\(\dot{m}_{{\text{f}}}\) :

Flux injection rate (g s−1)

\(\dot{m}_{{{\text{f,cr}}{.}}}\) :

Critical value of flux injection rate (g s−1)

\(Q_{{\text{g}}}\) :

Oxygen flow rate at nozzle-exit in molten silicon at 1773 K (m3 s−1)

\(Q_{{\text{g}}}^{{\text{s}}}\) :

Flow rate of oxygen at a temperature and pressure of 298 K and 101.325 kPa, respectively (SCCM: cm3 min−1)

Re:

Reynolds number (−)

T :

Absolute temperature (K)

t :

Reaction time (seconds)

u g,0 :

Linear flow velocity of oxygen injected into molten silicon at 1773 K through nozzle (m s−1)

V :

Volume of refined silicon melt (m3)

v f :

Velocity of flux particle relative to silicon melt (m s−1)

v f,0 :

Initial velocity of flux particle relative to silicon melt (m s−1)

x :

Penetration depth (m)

μ :

Mass ratio of injected powder to gas (−)

π :

Circumference ratio (−)

ρ f :

Density of flux particle (g m−3)

ρ Si :

Density of silicon melt (g m−3)

σ f -Si :

Flux–Si interfacial tension (J m−2)

σ g -f :

Gas–flux interfacial tension (J m−2)

σ Si -g :

Si–gas interfacial tension (J m−2)

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Acknowledgments

The author would like to acknowledge the advice provided by Professors Emeriti Chikabumi Yamauchi and Masamichi Sano of Nagoya University during the present work. The assistance of Messrs. Shoji Takai, Hideo Nakahigashi and Yutaka Kadoya with regard to improving the experimental flux-powder injection apparatus used for the experiments is also gratefully acknowledged.

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  1. Department of Mechanical Systems Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama, 939-0398, Japan

    Mitsuru Tanahashi

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Tanahashi, M. Boron Removal From Molten Metallurgical Grade Silicon via Flux-Powder Injection. Metall Mater Trans B 54, 3010–3022 (2023). https://doi.org/10.1007/s11663-023-02884-4

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