328

Previous investigations have demonstrated thecatalytic effect of silyer additions in the ferricsulfate leaching of chalcopyrite.

+'+cuFeS, + qre 3 "1 cu++ + 5Fe** + zso

The enhanced rate of leaching vlas found to be due tothe formation of an intermediate AgrS film which formson the CuFeS, surface by an exchangÉ reaction. Underthese conditlons, unlike the uncatalysed ferric sul-fate leach, the elemental sulfur forms a nonprotectivereaction product on the AgrS crystallites. As aresult, the rate appears t6 become lÍmited by anintermediate electrochemical reaction in the Ag,Sfilm rather than by transport through the elemefitalsulfur reaction product.

Ag2slsurface cuFesr) + 2Fe+3 * 2Ag+ + zFe++ + so

The reaction rale wag.found to.[ave a complex depen-dence on the Ag', Fe'-, and Fe'' concentrations, butindependent of the Cu" concentration. Electro-chemical ¡neasurenents with a AgrS-coated chalcopyriteelectrode provide furtherinforñation regarding thenature of the reaction mechanism. The kinetics ofthe rate limiting, intermediate electrochemical reac-.tion are analyzed in terms of the Butler-Volmer equa-tion.

INTRODUCTION

In sulfuric acid solutions of ferric sulfate,chalcopyrite dissolves according to the followingreacti on ,

+? +) ¿¡CuFeSr + 4Fe'" * Cu'' + 5Fe" + 25" (l)

This reaction stoichiometry has been observed in anumber of investigat'ions. (l-5) Exceptions to thisconsensus are the early work of Sullivan (6) and themore recent results reported by Jones and Peters. (7)The topochemical nature of the leaching reaction isc)ear1y illustrated in Figure I which shows the crosssection of partial)y reacted chalcopyrite particlesurrounded by a dense, tenacious sulfur layer.

The formation of an elemental sulfur layer on thechalcopyrite surface may significantly influence thereaction kinetics by establishing a diffusion barrier.The details of rate control for this particulaíreaction have not been well established. Some inves-tigators (1,7) attribute rate control to a surfacereaction. 0ther investigators (2-5,8,9) report thatthe reaction rate is limited by transport in thechalcopyrite lattice or through the elementa'l sulfurreaction product layer, Differences in leachingbehavior reported in the literature (5) may be áue tovariations in the intrinsic electrochemical properties

ELECTROCHEMICAL REACT¡ONS AN D SOLUTION CH EMISTRY

ELECTROCHEMISTRY IN SILVER CATALYSED FERRIC SULFATELEACHING OF CHALCOPYRITE

J.D. Miller, P.J. McDonough and H.Q. Portillo

Department of l4etallurgy and Metallurgical EngineeringUniversity of Utah

Figure l. SEM photograph of the cross section of a

paÉtially leached chalcopyrite particle (10 um) show-ing the sulfur layer and topochemical nature of theunóatalysed ferric sulfate )each. (5)

of the chalcopyrites used in the various studies.These electrochemical differences are illustrated bythe anodic polarization curves presented in Figure 2.(to)

Figure 2. Anodic polarizationsix different locations in I M

25"C. (ro)

culves for CuFeS^ fromH2SO4, 30 nv/ninl

-

TRANSVAL,SO AFRICA

r--l Pr lsl' ausrnlrtr z-t-

=:-- -4É{-=-

to-5 rda