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INTRODUCTION

COMMONLY used in steelmaking as a fluxing agent, CaF2 reduces the slag viscosity and metal entrapment, also increases the solubility of CaO and improves the kinetics of desulfurization and dephosphorization. An attempt to replace fluoride-based fluxes with those less harmful to the environment has been underway in the past few decades, as it is known that fluorides give rise to emissions such as SiF4, HF, or NaF. The positive effect of alkali oxides, as substitutes to CaF2 in lowering slag viscosity, has been reported earlier for the production of high Al steels.[1–5] Studies have also shown that with the high concentration of Na2O or Al2O3 in the flux, the extent of dephosphorization is significantly enhanced.[6] It has been demonstrated that fluxes containing Na2O are effective in removal of B from silicon-containing melts.[7,8] Despite these, the application of Na2O-containing slags in refining of metallurgical melts has been limited due to the volatility of sodium containing compounds, leading to loss of Na and consequently reversion of impurities to the metal as well as problems in the off-gas handling.[9,10] Further, it is known that excessive amounts of alkali oxides in the slag can intensify the refractory degradation by chemical attack.[11] The number of studies on the kinetics of Na2O evaporation in molten slag is very limited. Zhang et al.[12] investigated the evaporation mechanism from CaO-SiO2-Al2O3-Na2O-B2O3 melts. They found that NaBO2 is the primary gas emitted from the slag when both Na2O and B2O3 are present at around 5 wt pct. An interest in using fluxes containing higher amounts of Na2O exists in treatment of high-purity metals, such as

solar silicon.[13,14] The effectiveness of purification depends on the content of Na2O.[14] Thus, the present study aimed at investigating the rate and extent of Na2O loss from slags containing high concentration of Na2O. Also, the effect of basicity and B2O3 content on the evaporation rate was studied and will be presented here.

II.

EXPERIMENTAL

A. Preparation of Slags Flux mixtures were prepared by blending reagent grade powders of CaO, Al2O3, SiO2, Na2O, and B2O3. The blended powders were pelletized and then heated in a resistance furnace in alumina crucibles to 1673 K (1400 C) at a rate of 15 K/min. In order to minimize volatilization of Na2O during this pre-melt step, the crucibles were covered with a lid and sealed using alumina paste. The melted flux was then air cooled and pulverized to powder in a puck mill followed by X-Ray fluoroscopy (XRF) analysis. The blend composition and XRF analysis of the melted flux are provided in Table I. As it was not possible to measure the B2O3 using XRF, its content was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The thermogravimetric analysis (TGA) results of weight loss of the individual oxides showed that pure Al2O3 and CaO experienced weight losses at high temperature, corresponding to calcination of residual carbonates and hydrates in these oxide. Similarly, the concentrations of