Sulfide and sulfate solid solubility in lime, magnesia, and calcined dolomite: Part II. free energy of formation of magn
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The f r e e e n e r g y of f o r m a t i o n of m a g n e s i u m s u l f a t e and its s o l u b i l i t y in m a g n e s i a have b e e n d e t e r m i n e d . The s a l i e n t f e a t u r e of the r e s u l t s i s that t r a c e a m o u n t s of c a l c i u m i m p u r i t y in m a g n e s i a n o t i c e a b l y l o w e r the d i s s o c i a t i o n t e m p e r a t u r e and p r e s s u r e of m a g n e s i u m s u l f a t e . A h i g h - p u r i t y m a g n e s i a , which was p r e p a r e d b y oxidation of a h i g h - p u r i t y m a g n e s i u m , was u s e d to d e t e r m i n e the e q u i l i b r i u m c o n s t a n t for the f o r m a t i o n of MgSO 4. The s u l f a t e s o l u b i l ity in m a g n e s i a s a t u r a t e d with MgSO 4 was found to be h i g h e r than that in l i m e s a t u r a t e d with CaSO 4. At l l 0 0 ~ CaSO 4 and CaS f o r m in c a l c i n e d d o l o m i t e at SO2 p a r t i a l p r e s s u r e s h i g h e r than t h o s e in p u r e CaO. F r o m t h e s e f i n d i n g s it is e s t i m a t e d that at MgO s a t u r a t i o n the a c t i v i t y c o e f f i c i e n t r e l a t i v e to CaO is about 0.8.
MARCHAL*s t u d i e d d e c o m p o s i t i o n
of MgSO 4 b y m e a s u r i n g the d i s s o c i a t i o n p r e s s u r e Pso3 cc Pso~ {Poz)l/2; l a t e r Dewing and R i c h a r d s o n 2 s t u d i e d the s a m e r e a c tion b y a d i f f e r e n t i a l t h e r m a l a n a l y s i s m e t h o d in a flowing s t r e a m of SO2-O 2 m i x t u r e s . T h e r e is a l a r g e d i s c r e p a n c y of about 1700 cal b e t w e e n the two s e t s of d a t a f o r the f r e e e n e r g y of d e c o m p o s i t i o n of MgSO4. As p o i n t e d out in a r e v i e w p a p e r b y Kellogg, s the u s u a l e r r o r s in d e c o m p o s i t i o n - p r e s s u r e m e a s u r e m e n t s y i e l d f r e e - e n e r g y v a l u e s which a r e l e s s p o s i t i v e than the t r u e v a l u e s , y e t the v a l u e s o b t a i n e d b y M a r c h a l a r e m o r e p o s i t i v e than t h o s e b y Dewing and R i c h a r d s o n . In the light of the p r e s e n t work, t h i s d i s c r e p a n c y in the f r e e - e n e r g y d a t a is a t t r i b u t e d to the p r e s e n c e of t r a c e i m p u r i t i e s in MgSO 4 o r MgO. EXPERIMENTAL The s a m p l e of m a g n e s i a o r c a l c i n e d d o l o m i t e ( ~ 2 5 0 mg) was c o n t a i n e d in a p l a t i n u m t r a y and s u s p e n d e d in the u n i f o r m - t e m p e r a t u r e zone of a v e r t i c a l f u r n a c e in a flowing SO2-O2-N2 m i x t u r e (~ 1 1/min). A f t e r a r e a c t i o n t i m e of 2 to 16 h, the s a m p l e w a s w i t h d r a w n r a p i d l y f r o m the hot zone, and s u b s e q u e n t l y a n a l y z e d f o r t o t a l s u l f u r b y the i o d o m e t r i c - c o m b u s t i o n method. In a given s e r i e s of e x p e r i m e n t s a t a d e s i r e d t e m p e r a t u r e and oxygen p a r t i a l p r e s s u r e , the s u l f u r content of m a g n e s i a was
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