Nonaqueous Solvents for Leaching CaCl₂ Flux from Calcium-Reduced Titanium Powder

Abstract

Deoxidation of titanium by a calcium reductant is a promising process for the recycling of titanium scrap. Molten calcium chloride (CaCl₂) is typically used as a flux for calcium oxide (CaO) formed as a by-product of deoxidation. At present, removal of CaCl₂–CaO can only be achieved by aqueous leaching. However, when CaCl₂ hydrate is heated, a pyrohydrolysis reaction occurs, making it difficult to reuse the CaCl₂. Therefore, in this study, we examined organic solvents as alternatives to aqueous leaching. Formamide, ethylene carbonate, propylene carbonate, dimethyl sulfoxide (DMSO), and ethylenediamine were selected as candidate organic solvents. The solubility of CaCl₂ in DMSO was found to be 9.2 g per 100 g-solvent at 69°C and that in formamide was 25.9 g per 100 g-solvent at 47°C.

Vacuum distillation and crystallization separation were examined as separation methods for the solvent and solute after leaching. A low temperature vacuum distillation, i.e., less than around 200°C for DMSO, is required to prevent thermal decomposition. However, the rate of the vacuum distillation at low temperatures was slow. We therefore devised a process combining a nonpolar solvent-induced precipitation with vacuum distillation to reduce the amount of solvent requiring distillation. Benzene was selected as a nonpolar solvent to induce precipitation from DMSO. After the precipitation, DMSO-solvated CaCl₂ was obtained and distilled under vacuum.

Fig. 9 Process flow of CaCl₂-leaching by DMSO.