The Kroll process consists of the reduction process in which TiCl₄ is reduced by Mg and the distillation process in which residual Mg and MgCl₂ in the titanium sponge are removed by evacuation. It is well known that the higher the feeding rate of TiCl₄ in the reduction process, the higher the chlorine content in the titanium sponge after the distillation process.
Microscopic study on the residual chlorine in titanium sponge was conducted usingEPMA and EDX. It was found that there were three types ofresidual chlorine: 1) MgCl₂ in micro-cavities in the titanium grains, 2) MgCl₂ in the closed space between the titanium particles and 3) TiCl₂ on the titanium particle surface. Among them, the second type MgCl₂ is ascribed the residual chlorine content. Two routes to from such MgCl₂ in the Kroll process were discussed. One was trapping of MgCl₂ produced by the reduction of TiCl₄ and the other one was MgCl₂ formation via TiCl₂. The correlation between the residual chlorinecontent and the ratio of the TiCl₄ feeding rate to amount of Mg in the reduction vessel could be explained by the both hypothetical mechanisms. It was concluded that theamount of Mg in the vessel is one of the key factors for producing low chlorine content titanium sponge.