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The r a t e of salt r e m o v a l f r o m t i t a n i u m sponge was d e t e r m i n e d u s i n g a t h e r m o g r a v i m e t r i c technique. Both m a g n e s i u m - and s o d i u m - r e d u c e d t i t a n i u m sponge were used in the i n v e s tigation. M e a s u r e m e n t s were made of the r a t e of salt r e m o v a l by v a c u u m e v a p o r a t i o n for a r a n g e of t e m p e r a t u r e , bulk density, and sponge s u r f a c e a r e a s . R e m o v a l of m a g n e s i u m chloride was c h a r a c t e r i z e d by the p r e s e n c e of a long p e r i o d of s u r f a c e evaporation, or c o n s t a n t - r a t e drying, d u r i n g which the m a j o r p o r t i o n of the salt was evaporated. Sodiumr e d u c e d sponge showed no such c o n s t a n t - r a t e period.

RAW

t i t a n i u m sponge is f o r m e d c o m m e r c i a l l y by r e ducing p u r i f i e d t i t a n i u m t e t r a c h l o r i d e with e i t h e r m a g n e s i u m , in a s i n g l e - s t e p r e d u c t i o n , or sodium, in a t w o - s t e p r e d u c t i o n . The r e s u l t i n g chloride, along with a s m a l l e x c e s s of r e d u c i n g metal, m u s t be s e p a r a t e d to obtain a t i t a n i u m sponge product which can be m e l t e d to f o r m a u s a b l e ingot. In the m a g n e s i u m p r o c e s s , r e duction is c a r r i e d out above the m e l t i n g point of the salt and p a r t of the salt can be d r a i n e d f r o m the bottom of the r e a c t i o n v e s s e l ; however, some salt r e m a i n s within the sponge m a s s and m u s t be r e m o v e d e i t h e r by e v a p o r a t i o n or by leaching. 1 Leaching has the m a j o r advantage of e a s e of r e m o v a l of l a r g e q u a n t i t i e s of salt at a r e l a t i v e l y low cost. However, r e s i d u a l hydrated s a l t f r o m the Ieaching p r o c e s s d e c o m p o s e s d u r i n g s u b s e q u e n t a r c m e l t i n g , and volatile p r o d u c t s of d e e o m p o sltton c a u s e Ingot p o r o s i t y and c o m p l i c a t e the m e l t i n g p r o c e s s . Removal of salt by e v a p o r a t i o n yields sponge g e n e r a l l y c o n s i d e r e d to be of higher quality, but evapor a t i o n is a s l o w e r , m o r e costly p r o c e d u r e . The r e l a t i v e l y open s t r u c t u r e of t i t a n i u m sponge p r o duced by m a g n e s i u m r e d u c t i o n f a c i l i t a t e s salt r e m o v a l by e v a p o r a t i o n . In c o n t r a s t , s o d i u m - r e d u c e d sponge has a much f i n e r g r a i n s t r u c t u r e than m a g n e s i u m - r e duced m a t e r i a l . In addition, s o d i u m - r e d u c e d sponge contains a g r e a t e r m a s s of NaC1 than the c o r r e s p o n d Ing MgC12 In the m a g n e s i u m - r e d u c e d m a t e r i a l , and NaC1 has a lower v a p o r p r e s s u r e . Thus, salt r e m o v a l f r o m s o d i u m - r e d u c e d m a t e r i a l has u s u a l l y b e e n a c c o m p l i s h e d by an aqueous leach. However, t h e r e ts some i n t e r e s t in kno