Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 2 3 Fig. 4 − Flow diagram for titanium sponge production using magnesium reduction. between 925° and 1010°C in the reaction: TiO 2 + 2C + 2Cl 2 → 2CO + TiCl 4 ; ∆ G 1300°C = −125 kJ (30 kcal) Volatile chlorides are collected and unreacted solids and nonvolatile chlorides are discarded. Titanium tet- rachloride (TiCl 4 ) is separated from the other chlorides by double distillation [23] . Vanadium oxychloride (VOCl 3 ), which has a boiling point close to that of TiCl 4 , is separated by complexing with min- eral oil, reducing with H 2 S to VOCl 2 , or complexing with copper. TiCl 4 is finally oxidized at 985°C to TiO 2 and the chlo- rine gas is recycled [21,24] . TiCl 4 for metal production, which must be of very high purity, requires a greater effort to re- move impurities, especially oxygen and carbon-containing compounds. MAGNESIUM-REDUCTION (KROLL) PROCESS Nearly all sponge is produced us- ing the magnesium-reduction process (Fig. 4) in the reaction: TiCl 4 (g) + 2Mg (l) → Ti (s) + 2MgCl 2 (l) ∆ G 900°C = −301 kJ (−72 kcal) Gaseous TiCl 4 ismetered into a car- bon-steel or 304 stainless-steel reaction vessel containing liquid magnesium. An excess of 25%magnesium over the stoi- chiometric amount ensures that the lower chlorides of titanium (TiCl 2 and TiCl 3 ) are reduced to metal. The highly exothermic reaction [ DH 900°C = − 420 kJ / mol, or − 100 kcal / mol] is controlled by the feed rate of TiCl 4 at 900°C. The re- action atmosphere is helium or argon. Molten magnesium chloride is tapped