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INTRODUCTION

Fifteen years ago a distinguished electrochemistreferred to electrochglslfX as an "underdevelopedscience. " In=JñE-;;¡T;IEJñ question, ProfessorBockris (1) stated that many areas of natural pheno-mena and applied technology are not perceived asbeing electrochemically based. At that time" thiswas especially true for much of non-ferrous extrac-tive netallurgy and was a particularly accuratedescription of hydrometal1urgy. During the Iastdecade, there has been considerable advancement inapplying the theory of electrochemistry to ourknowledge of hydrometaiTurgTGTpiúes§es and inunderstanding the mechanisms of underlying electrodereactions. Impressive as these advances are, thereis stilI a growing need for electrochemica'l researchof mineral systems and electrochemical engineeringto appiy thisrv',ork in practicai situations.

In a broad sense electrochemistry is defined asthe physical chemist[ñTr-iñ§-Tn-!o-1 ution and ofphenomena which occur at the electrified interface.Bockris and Reddy (2) refer to elerctrochemigryas being divided into two fundanreTÍáT-ii6ñIi§ITonicsand electrodics. Ionics is restricted to thebehavior of ions in so'lutions, examples being manyfamiliar redox processes. E'lectrodics represents themain part of electrochemistry, and is the study ofcharged interfaces and the conditions governingcharge transfer reactions across them. Consideringhydrometal lurgical applications, electrodics suggeststhe following relationship for a conducting solid(bulk or surface)

Electrodics: Solid § Ions in solution+ Electrons in solid

An exce'llent example of the field of electrodics isthe study of metal corrosion in aqueous solution.

The electronic properties of most metal sulfidesand of certain oxides are such that corrosion andgalvanic couples are read'ily established in aqueoussystems. This is true for p-type, n-type or metallicIike compounds for which ta = l. where t. is theelectronic transport numbei. As a resuli of theirelectronic conductivity, certain minerals can parti-cipate in coupled charge transfer processes ana'logousto a metal corroding in an electrolyte, and thekinetics of leaching can be related to the potentialof the solid in contact with the aqueous electrolyte.From a thermodynamic point of view most hydrornetal-'lurgical dissolution reactions are strongly favored

ELECTROCH EMICAL REACTIONS AN D SOLUTION CHEM ISTRY

ELECTROCHEMICAL PROCESSES IN THE LEACHING OF

: METAL SULFIDES AND oXTDES

J. .Brent Hiskey and Milton E. Wadsworth

J.. .Brent. Hiskey is Senior Scientist, Kennecott Minerals Company Process TechnologyM.E. Wadsworth is Professor of Metallurgy and Metal'lurgical Engineeiing, University ofUtah

' Sa'l t Lake City, Utah

such that back reacticn kinetics normally are unim-portant. Build up on ions in solution, however, maymarkedly affect the kinetics by influencing thepotential at the mineral (electrode) solut'ioni nterface.

Properties of some sulfide and ox'ide minerals

Several investigators have presented informationon the electróchemical behavior of su'lfide and oxideminerals. Metal sulfides have received the mostattention because they are recognized as va'luablesources of non-ferrous metal. Koch (3) has reviewedthe e'lectrochemistry of sulfide ininerals and Shuey (4)has reviewed the e'lectronic properties of both oxideand sulfide minerals. Included in Table I areresistivities, electronic, and structural informationfor se'lected sulfide and oxide minerals. The cova-'lent character of most sulfides provides non-locali-zation of charge resulting in appreciable intrinsicelectronic conductivity. The ability of many sui-fides to form non-stoichiometric compounds results inincreased conduction via electron holes or excesselectrons. Oxides in general are more ionic innature and usually have h'igher resistivities comparedto sulfide minera'ls.

A fundamental property of semiconducting minerals'is the characteristic "rest" potential. For inter-facia'l electrode processes, the "rest" potentialcoresponds to the equilibrium (no net anod'ic orcathodic current) electrode potential. Rest poten-tials for severa'l hetal súlfides qre shown inTable II. It is important to remember that a mineralelectrode system will establish and maintain a cer-tain equi'librium potentiai that depends not only onthe solution composition but also on the compositionof the solid phase. Figure 'l illustrates resultsfor the e'lectrochemical oxjdation of Cu2S to CuS inan acid solution (10). The steps in the data havebeen explained on the basis of the sequential forma-tion of a series of CUS compounds with correspondingrest ootentials as follows:

CutS * Crl.93S * Crl.83S * Cr.l.67 + Cu.,.OS + CuS

From the work of Etienne and Peters (ll), the stan-dard free energy of formation of Cu¡-93S and Cu1.g3Swere calculated. At 25"C, the standard free energyof formation values are

aG"rrr"*(Cu.,.9SS) = -1g,2BZ t640 cal mole-l