320

CATHODIC CONVERSION OF CHALCOPYRITE

Shirts et al. (36) have denrcnstrated that chalco-pyrite will react cathodically by bringing it intophysical contact with a metal having a 'lower potential.Three metals, Cu, Fe, and Pb. were used. The reac-tions observed wei^e

ELECTROCHEMICAL REACTIONS AND SOLT'TION CHEMISTRY

CuFeS, + Cu + 2H+ + CuzS + Fe2* * HzS

2CuFeS, + Fe + 4H+ * Cu + 2Fe2+ + 2HZS

2CuFeS, + Pb +.6tt+ * CuzS + ?FeZ+ + Pb?+ + 3H2S (9f)

cuFes, + pb + 4H+ * cu * FeZ* * PbZ* + 2H2S (e?)

Galvanic couples are readily established because ofthe exce'llent electronic conductivity of a1l phases.The principal cathodic reaction is

2CuFeS, + 6H+ + 2e' = cuzs + 2Fe2+ + 3H2S (e3)

It is evident from Figure 3(h), )ine d-e, that there'is a large driving force for the cathodic conversionof CuFeS2 to Cu2S by copper. The half-cell reactionrnvolvlng tne copper 1s

CurS+2H++2¿-=2Cu+HzS (e4)

Hiskey and l,ladsworth (37) measured the kinetics ofthe galvanic conversion of CuFeS2 using copoer. Figure23 illustrates the galvanic CuFeS2-Cu couple. Boththe cathodic and anodic reactions produce CUZS ¡,hichwelds and establishes good electrical contact. Itwas found that at lower temperatures, stirring disrup-ted the bond and retarded the reaction. At 90"C stir-ring had less effect on the overall kinetics. Figure24 illustrates results obtained for an iñtimate mix-ture of copper powder and chalcopyrite concentrates.

30 60 90 r20TIME (MINUTES)

FIGURE 24. Fraction reacted versus time for cathodicieduction of c.halcopyrite by various sizes ofrrtallic copper.

Several s'izes of copper were used with 3251400 meshchalcopyrite with excess copper, molar Cu/CuFeS2ratio of '1.5. The extent of the reaction was rBasuredby the fraction o, indicated by the amount of ironreleased to the so'lution. The curves of Figure 24il'lustrate the importance of the copper, anodicsurface area. Initially the chalcopyrite surface areais larger than that of the copper and the rate ispredominantly anodically controlled. As the chalco-pyrite reacts, its area becomes smaller and thekinetics shift from predominant'ly anodic control topredominantly cathodic control. The Ievelling offbelow a = 1.0 was attributed to'lack of perfect mixingof the initial solid phases. Carefu'l blending in-creased the extent of reaction. In a galvanic systemit is necessary to nonnalize the data in terms of theanodic (Ar) and cathodic (A.) surface areas by therelationships I¿ = i¿A¿ and I. = i6A. where I¿ and Isrefer to the total anodic and cathodic currents res-pectively. Additional tests showed the rate of reac-tion to be essentia'lly independent of tFe III insolution. The experimental activation energy wasapproximately ll.5 kcal/mole. Figure 25 depicts theinitia'l rate versus initial hydrogen ion concentration.The rate order for hydrogen ion is in the range 0.48-0.52 or essentially one-half. Assuming single chargetransfer in the activated state, the net cathodic andanodic electrode processes may be represented by theequati ons

B-FE- -(r-B^)rE^I. = A.k.2F exptl#t - A.kiar*2F exp¡-nff¡ (95)

and

(as ¡

(e0 ) B r.oFú,lrl> 0.8zoo.0.6-.4Uo

3 o.+

Reaction (94),. having a rnre negative potential thanreaction (9¡), wlll react anodically. It is alsoevident from Figure 3(b) that metallic iron wilIreduce CuFeS2 to retallic copper as wi'll metallic Pbin acid chloride golutions.

o G325l+4OO)o l-?oot+2701

I''.tI o (-roo/+r5o)Lu r a (-60l+65)

o (- 35/1481

FIGURE 23, Schematic representation of the CuFeS"-Cuogalvanic couple in sulfuric acld. '