Abstract: Recess portions each comprising one or a plurality of transverse grooves or a large number of dimples are disposed below the lowermost position of a meniscus under a steady operation state within a distance of 200 mm on the inner peripheral surface of the casting mold so that a solidified shell is gradually cooled and the cooling capacity of each inner surface of the casting mold is substantially inform.

Abstract: To provide a continuous casting method for a billet capable of stable casting without causing rhomboidity or side periphery deformation in the billet produced by continuous casting, in a casting mold used for this method, recess portions each comprising one or a plurality of transverse grooves or a large number of dimples are disposed below the lowermost position of a meniscus under a steady operation state within a distance of 200 mm on the inner peripheral surface of the casting mold so that a solidified shell is gradually cooled and the cooling capacity of each inner surface of the casting mold is substantially uniform.

Abstract: SiF.sub.4 gas containing oxygen-containing silicofluoride(s) typified by (SiF.sub.3).sub.2 O as impurity can be refined to extremely high purity by making the SiF.sub.4 gas contact with HF in the presence of a liquid medium having strong affinity for water such as sulfuric acid or phosphoric acid. By reaction with HF, the impurity such as (SiF.sub.3).sub.2 O is converted to SiF.sub.4, while the liquid medium absorbs water formed by the reaction to thereby prevent a reverse reaction between SiF.sub.4 and H.sub.2 O to form (SiF.sub.3).sub.2 O.

Abstract: A process of preparing silicon tetrafluoride by introducing hydrogen fluoride gas into a dispersion of powdery silicon oxide material, which needs not to be pure SiO.sub.2, in sulfuric acid not lower than 65% in the concentration of H.sub.2 SO.sub.4. The reaction takes place even at room temperature. By using amorphous silicon oxide material, the rate of reaction can be enhanced with better yield. Preferably, the concentration of H.sub.2 SO.sub.4 in the liquid phase of the reaction system is maintained above 80% to obtain SiF.sub.4 containing little (SiF.sub.3).sub.2 O.