36 Applications and Needs of Thermodynamics in Electrometallurgy
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T. J. O'KEEFE and R. A. NARASAGOUDAR Graduate Center for Materials Research, University of Missouri—Rola, Rolla, MO 65401 I.
Introduction. Historically, there has always been a concerted effort to provide a firm thermodynamic base for the development of processes involving metal extraction and refining. There can be no argument that equilibrium principles can and have been applied to advantage in aqueous processing. Certain inherent characteristics in the thermodynamic approach have prevented its use as extensively as it might be used in certain areas of electrometallurgy. The initial science of electrochemistry was almost exclusively thermodynamic in nature, and explanations of cell behavior were sought using the familiar equation of Nernst:
where Ε and E° are the equilibrium and the standard potentials re spectively, F is the Faraday constant, R the gas constant, Τ the absolute temperature, n the moles of electrons and a is the acti vity. The Ε or E° could be used in determining the spontaneity of a reaction, depending on the state of reactants and products, by the expression: AG = -nFE
(2)
The use of these expressions is effectual only in cases where there is no extensive deviation in the system behavior due to charge transfer overpotential or other kinetic effects. (l_) The calculated threshold or thermodynamic energy requirement {2) (AG in the previous equation) is often much lower than actually en countered, but is s t i l l useful in estimating an approximate or theoretical minimum energy required for electrolysis. Part of th e difficulty in applying thermodynamics to many systems of indus trial interest may reside in an inability to properly define the activities or nature of the various species involved in the 0-8412-0569-8/80/ 47-133-701$05.00/0 © 1980 American Chemical Society In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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702
T H E R M O D Y N A M I C S OF
AQUEOUS SYSTEMS W I T H INDUSTRIAL
APPLICATIONS
r e a c t i o n s . Many e l e c t r o c h e m i c a l r e a c t i o n s i n v o l v i n g metals, such as cathodic r e d u c t i o n , cementation, gaseous r e d u c t i o n from s o l u t i o n or c o r r o s i o n , are very s u s c e p t i b l e t o changes caused by the presence o f t r a c e c o n c e n t r a t i o n s o f c e r t a i n chemicals i n s o l u t i o n . T h e i r e f f e c t s can be h e l p f u l or h a r m f u l , depending on the p a r t i c u l a r s , but the c o n d i t i o n s r e s u l t i n g from the presence o f the t r a c e i m p u r i t i e s are most o f t e n d i f f i c u l t t o c o n t r o l and p r e d i c t . One of the o b j e c t i v e s o f t h i s paper w i l l be t o show some spec i f i c examples o f these e f f e c t s i n e l e c t r o l y s i s and i l l u s t r a t e the s u b s t a n t i a l need f o r a b e t t e r understanding o f the thermodynamics o f the s o l u t i o n chemistry i n v o l v e d i n e l e c t r o d i c s . Some o f these needs are more obvious and have been i n d i c a t e d p r e v i o u s l y (3,) and i n c l u d e such items as AG°, K and Cp data on the systems o f i n t e r e s t . However, much more e x t e n s i v e i n f o r m a t i o n i s necessary on a d s o r p t i o n phenomena, complex i o n formation and the e q u i l i b r i u m concentrations of these i n f l u e n t i a l s p e c i e s . This need has always e x i s t e d but i t i s even more important now i f the c u r r e n t c h a l l e n ges b e i n g imposed by energy and m a t e r i a l s shortages and e n v i r o n mental c o n t r o l are t o be met. s o
II.
E x t r a c t i o n Processes.
A. Flowsheet Design. The b a s i c process flowsheet f o r the e x t r a c t i o n and r e f i n i n g o f metals when e l e c t r o l y s i s i s i n v o l v e d i s s i m i l a r i n many r e s p e c t s . The s p e c i f i c s may change t o s u i t the p h y s i c a l needs and chemistry o f the p a r t i c u l a r metal system i n q u e s t i o n , but the o b j e c t i v e s o f the u n i t operations and u n i t processes are comparable i n general terms. As an example, the proc e s s i n g scheme f o r the p r o d u c t i o n o f e l e c t r o l y t i c z i n c i s shown i n F i g u r e 1. Z i n c i s a good example because i t i n c o r p o r a t e s many o f the items t h a t must be addressed by the e x t r a c t i v e m e t a l l u r g i s t i f a pure metal i s t o be produced e f f i c i e n t l y . The very a c t i v e standard p o t e n t i a l o f z i n c forces c o n s i d e r a b l e care t o be taken d u r i n g p r o c e s s i n g t o i n s u r e t h a t troublesome i m p u r i t i e s , ones t h a t can reduce c u r r e n t e f f i c i e n c y , are removed from the system. The f i n a l q u a l i t y o f the metal i s a l s o important, f o r many a p p l i c a t i o n s r e q u i r e s p e c i a l h i g h grade metal o f 99·99% p u r i t y . Time does not permit an e x t e n s i v e e v a l u a t i o n of the r o a s t , l e a c h , p u r i f i c a t i o n and e l e c t r o l y s i s steps i n d i v i d u a l l y . B r i e f l y , the obj e c t i v e i s t o convert the ZnS t o an oxide which can be leached i n s u l f u r i c a c i d e l e c t r o l y t e . I m p u r i t i e s such as Cu, N i , Co, Sb and As are then removed by cementation u s i n g z i n c powder p r i o r t o e l e c t r o l y s i s . Throughout the procedure a c l o s e s c r u t i n y i s made o f the types and c o n c e n t r a t i o n s o f v a r i o u s chemical species p r e sent. A t y p i c a l chemical a n a l y s i s range r e q u i r e d f o r s a t i s f a c t o r y e l e c t r o l y s i s i s given i n Table I . The very low contents f o r c e r t a i n t r a c e metals, such as n i c k e l , c o b a l t , copper, antimony, and germanium are n o t a b l e , and i f these l e v e l s aren't a t t a i n e d then decreased c u r r e n t e f f i c i e n c y can r e s u l t . Elements such as l e a d and cadmium must be minimized t o i n s u r e proper metal p u r i t y .
In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
O'KEEFE AND NARASAGOUDAR
Thermodynamics in Electrometallurgy 703
Zn Flow Sheet Block Diagram
Downloaded by PENNSYLVANIA STATE UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch036
Zinc Cone. (ZnS) Ψ Roasting
S 0 •> H2SO4 Prod. 2
Leaching -> Fe(0H)3 P r e c i p i t a t e (Zn + Impurities) + 2
Zinc Dust -> P u r i f i c a t i o n Pure Z n
+ 2
Solution Ψ Electrolysis Ψ Cathode Zn
F/gare 1.
Zinc flow sheet block
diagram
Table I . A n a l y s i s o f N e u t r a l P u r i f i e d S o l u t i o n f o r Zn E l e c t r o l y s i s . ( k ) (mg/£)
Element Zn Mn Cd Sb As Sb+As Ge Co Ni Fe Cu F~ ci-
S o l u t i o n Prepared i n Laboratory
Actual Plant Solution
130,000
